Download Electric Fields for Surface Design and Chemical Analysis Christian Ulrich

Linköping Studies in Science and Technology Dissertation No. 1206 Electric Fields for Surface Design and Chemical Analysis Christian Ulrich Department of Physics, Chemistry and Biology Linköpings universitet, SE-581 83 Linköping, Sweden Linköping 2008 The cover picture shows SPR response as a function of electrode potential, thickness variations of different surface gradients, and the variations in potential and current density in a solution containing a bipolar electrode. During the course of the research underlying this thesis, Christian Ulrich was enrolled in Forum Scientium, a multidisciplinary doctoral programme at Linköping University, Sweden. c Copyright 2008 Christian Ulrich, unless otherwise noted. All rights reserved. Christian Ulrich Electric Fields for Surface Design and Chemical Analysis ISBN 978–91–7393–819–8 ISSN 0345–7524 Published online at www.ep.liu.se Linköping Studies in Science and Technology Dissertation No. 1206 Printed in Sweden by LiU-Tryck, Linköping 2008 Work. Finish. Publish.† – Michael Faraday (1791-1867) † Advice to the young William Crookes. Abstract This thesis deals with the use of electric fields for evaluation and control of chemical systems. An electric field can result in the flow of charge across an interface between a metal and a solution, by means of chemical reactions. This interplay between electricity and chemistry, i.e. electrochemistry, is a field of crucial importance both within research and industry. Applications based on electrochemical principles encompass such diverse areas as batteries and fuel cells, pH electrodes, and the glucose monitor used by people suffering from diabetes. A major part of the present work concerns the use of static electric fields in solutions containing a non-contacted metal surface. In such a setup it is possible to control the extent of electrochemical reactions at different positions on the metal. This allows the formation and evaluation of various types of gradients on electrodes, via indirectly induced electrochemical reactions. This approach is a new and simple way of forming for instance molecular gradients on conducting surfaces. These are very advantageous in biomimetic research, because a gradient contains a huge amount of discrete combinations of for example two molecules. The basis for the technique is the use of bipolar electrochemistry. Briefly, a surface can become a bipolar electrode (an electrode that acts as both anode and cathode) when the electric field in the solution exceeds a certain threshold value, thereby inducing redox reactions at both ends. In our experiments, the driving force for these reactions will vary along the electrode surface. Since the result of an electrochemical reaction can be the deposition or removal of material from an electrode, bipolar electrochemistry can be used to create gradients of that material on a surface. In order to gain a deeper understanding of these processes, the potential and current density distributions at bipolar electrodes were investigated with different methods. Especially the use of imaging techniques was important for the visualization and analysis of the gradients. Using this knowledge, the formation of more complex gradients was facilitated, and the results were further compared to simulations based on simple conductivity models. These simulations also provided us with means to predict the behavior of new and interesting setup geometries for pattering applications. The other major part is more application driven and deals with the use of alternating electric fields for chemical analysis, a technique known as electrochemical impedance spectroscopy (EIS). In this work, EIS has been applied for the analyv sis of engine oils and industrial cutting fluids. Emphasis was placed on practical aspects of the measurement procedure, and on the evaluation of the results using statistical methods. It was for example shown that it was possible to simultaneously determine the amount of different contaminants in low conducting solutions. Generally, EIS is used to measure the impedance of a solution or a solid, often as a function of the frequency of the alternating electric field. The impedance of a system is closely correlated to its complex dielectric constant, and EIS can therefor be used to examine many chemical and physical processes. It is further well suited for characterizing low conducting media with little or no redox-active species. The evaluation of impedance data is often a quite complex task, which is why we have made use of statistical methods that drastically reduce the effort and quickly reveal significant intrinsic parameters. vi Populärvetenskaplig sammanfattning Denna avhandling handlar i grunden om hur elektriska fält kan användas för att utvärdera och kontrollera olika kemiska system. Området inom kemin som rör sambandet mellan elektricitet och kemiska föreningar kallas elektrokemi, och används flitigt inom både industriella sammanhang och inom forskning. Det finns många exempel på kommersiella produkter som är baserade på elektrokemiska principer, till exempel batterier, pH-elektroder och glukosmätare som används av diabetiker. En stor del av det här arbetet rör användandet av statiska elektriska fält i en lösning innehållande en okontakterad metallyta. Med en sådan uppställning är det möjligt att kontrollera omfattningen av elektrokemiska reaktioner på olika positioner på ytan. På det här sättet är det möjligt att skapa och utvärdera olika kemiska gradienter. Tillvägagångssättet bygger på bipolär elektrokemi, och har här för första gången utnyttjats till att skapa en proteingradient på en bipolär guldyta. En sådan ytgradient kommer att uppvisa en gradvis variation i en eller flera egenskaper. En av de stora fördelarna är alltså att en gradient innehåller en stor mängd kombinationer av till exempel två kemiska egenskaper. Metoden är snabb och enkel, och gör det möjligt att skapa avancerade gradienter av molekyler, som sedan skulle kunna användas inom forskning och utveckling av exempelvis biosensorer. En bipolär elektrod, det vill säga en elektrod som agerar som både anod och katod, kan skapas genom att placera en elektriskt ledande yta i ett parallellt elektriskt fält. Detta kommer att ske under förutsättning att det elektriska fältet överstiger ett visst tröskelvärde och därmed inducerar redoxreaktioner vid ytans båda ändar. Drivkraften för dessa reaktioner kommer att variera utmed ytan, parallellt med det elektriska fältet. Eftersom resultatet av en elektrokemisk reaktion kan vara både borttagande och deponering av material på en elektrod, kan bipolär elektrokemi användas till att skapa gradienter av detta material. För att få en djupare förståelse för fenomenet har mätningar gjorts för att utvärdera hur potentialen och strömmen fördelar sig kring en bipolär elektrod. Särskilt kombinationen av elektrokemi och avbildande optiska tekniker, huvudsakligen ellipsometri och ytplasmonresonans (SPR), har också varit en viktig del av arbetet. Detta eftersom olika processer som sker vid en elektrod då kan avbildas och studeras i realtid. vii Resultaten har vidare jämförts med simuleringar baserade på grundläggande konduktivitetsmodeller. Den andra stora delen av arbetet rör mer direkta analytiska applikationer av elektrokemi, och speciellt användandet av varierande elektriska fält. Denna mätteknik kallas för elektrokemisk impedansspektroskopi (EIS), och har här utnyttjats för analys av motoroljor och industriella skärvätskor. Tonvikt har lagts vid att optimera mättekniken och att utvärdera resultaten med hjälp av statistiska verktyg. Resultaten visar till exempel på möjligheten att samtidigt bestämma halter av olika föroreningar i lågledande lösningar. Generellt används EIS för att mäta impedansen för både fasta och flytande ämnen, ofta som funktion av frekvensen för det applicerade elektriska fältet. Denna impedans är tätt förknippad med systemets komplexa dielektricitetskonstant, och EIS kan användas till att undersöka många kemiska och fysikaliska processer. Därmed är tekniken till exempel mycket lämpad för att karaktärisera lågledande medier. Utvärderingen av impedansdata kan dock vara mycket komplex, något som vi visat kan underlättas genom att använda statistiska verktyg. viii List of Publications This thesis is based on six papers (included at the end). In the text, they will be referred to by their roman numerals (Papers I–VI). Paper I Formation of Molecular Gradients on Bipolar Electrodes Christian Ulrich, Olof Andersson, Leif Nyholm, and Fredrik Björefors Angewandte Chemie International Edition, 47:3034–3036, 2008. Author’s contribution Was responsible for the planning, and performed all experimental work together with Olof Andersson. Wrote the manuscript. Paper II Potential and Current Density Distributions at Electrodes Intended for Bipolar Patterning Christian Ulrich, Olof Andersson, Leif Nyholm, and Fredrik Björefors Submitted to Analytical Chemistry Author’s contribution Was responsible for the planning, and performed all experimental work and simulations. Olof Andersson was responsible for the iSPR experiments. Wrote the manuscript. Paper III Imaging SPR for Detection of Local Electrochemical Processes on Patterned Surfaces Olof Andersson, Christian Ulrich, Fredrik Björefors, and Bo Liedberg Sensors and Actuators B, doi:10.1016/j.snb.2008.05.042, 2008. Author’s contribution Contributed during the planning, evaluation, and writing. ix Paper IV Current Oscillations During Chronoamperometric and Cyclic Voltammetric Measurements in Alkaline Cu(II)-citrate Solutions Jonas Eskhult, Christian Ulrich, Fredrik Björefors, and Leif Nyholm Electrochimica Acta, 53:2188–2197, 2008. Author’s contribution Was responsible for the ellipsometric experiments, and performed the optical simulations. Contributed to the writing. Paper V Simultaneous Estimation of Soot and Diesel Contamination in Engine Oil Using Electrochemical Impedance Spectroscopy Christian Ulrich, Henrik Petersson, Hans Sundgren, Fredrik Björefors, and Christina Krantz-Rülcker Sensors and Actuators B, 127:613–618, 2007. Author’s contribution Responsible for the planning, performance, and evaluation of the experimental work, including the cell design. Wrote the manuscript. Paper VI Quality Evaluation of Industrial Cutting Fluids Using Electrochemical Impedance Spectroscopy Christian Ulrich, Dan Louthander, Per Mårtensson, André Kluftinger, Michael Gawronski, and Fredrik Björefors In manuscript Author’s contribution Responsible for the planning, performance, and evaluation of the experimental work, including the cell design. Wrote the manuscript. x Acknowledgements It is with a sense of sadness and sentimentality I write this acknowledgement, because in thanking all the people who have helped me through the years, I realize that my time at IFM is nearing its end. It has been an inspirational and rewarding time, but now new things await. I am forever grateful to my supervisor Fredrik Björefors. You have given me invaluable help through the years, and further managed to provide both excellent supervision and support. My co-supervisor Ingemar Lundström is, by his mere presence, a source of inspiration. Special thanks go to my former supervisor Tina Krantz-Rülcker, who gave me the opportunity to explore the mysteries of experimental research. My former co-supervisor Fredrik Winquist was also the supervisor on my diploma work, which started this journey. Thanks to Bo Liedberg for welcoming me into his group. Hans Sundgren has been essential to many of my undertakings. I could always knock on his door to discuss things, big as well as small. Bo Thunér and Ingemar Grahn are acknowledged for valuable assistance with all things practical; Stefan Klintström for guidance whenever needed and for managing Furum Scientium; Agneta Askendal for the laboratory assistance; the administrative staff at IFM for all the help, and especially Susann Årnfelt, Kerstin vestin, Pia Blomstedt, and Anna Maria Uhlin. All the people I have collaborated with, who have contributed to this thesis. In no particular order I would like to thank the following. Olof Andersson for the successful team work, his positive attitude, and nice company; Henrik Petersson for the data evaluations and general support; Jonas Eskhult and especially Leif Nyholm for the successful cooperation and rewarding discussions; representatives of the industrial partners through the years, Jaco Visser, Claes Frennfelt, Per Mårtensson, Michael Gawronski, and André Kluftinger; Dan Louthander, for the collaboration in the end, but foremost for all the Friday morning breakfasts at Lanemos. Without naming anyone in particular, I have very much valued the company of my former room-mates, the coffee-group, the lunch-group, all the colleagues within Forum Scientium, and all former and present members of S-SENCE and the Sensor Science and Molecular Physics group. xi Last but not least, I wish to thank my friends and family for their never ending love and support, which has made it possible for me to cope with all the hardships of life, especially towards the end. Christian Ulrich, Linköping, 2008. xii Contents 1 General introduction 1 2 Electrochemistry 2.1 Introduction to electrochemical reactions . 2.2 The electrical double layer . . . . . . . . . 2.3 Electrochemical cells and cell resistance . 2.4 Mass transfer and the diffusion layer . . . 2.5 Voltammetry . . . . . . . . . . . . . . . . 2.6 Electrochemical impedance spectroscopy . 2.6.1 Theory . . . . . . . . . . . . . . . 2.6.2 General applications . . . . . . . . 2.7 Bipolar electrochemistry . . . . . . . . . . 2.8 Limitations of electrochemistry . . . . . . . . . . . . . . . . 3 3 8 10 11 12 14 14 18 18 20 . . . . . . . 21 21 21 21 22 23 23 24 4 Electrode surface design and analysis 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Surface gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 27 27 29 5 Alternating electric fields for 5.1 Introduction . . . . . . . . . 5.2 Practical considerations . . 5.3 Applications . . . . . . . . . 35 35 36 37 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Imaging optical methods and electrochemistry 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . 3.2 Surface plasmon resonance . . . . . . . . . . . . . 3.2.1 Theory . . . . . . . . . . . . . . . . . . . 3.2.2 SPR and electrochemistry . . . . . . . . . 3.3 Ellipsometry . . . . . . . . . . . . . . . . . . . . 3.3.1 Theory . . . . . . . . . . . . . . . . . . . 3.3.2 Ellipsometry and electrochemistry . . . . chemical . . . . . . . . . . . . . . . . . . xiii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Future outlook 41 Bibliography 43 Paper I 51 Paper II 59 Paper III 71 Paper IV 79 Paper V 91 Paper VI 99 xiv 1 General introduction In 1834, Michael Faraday published his two laws of electrolysis, relating the amount of charge passed in a circuit to the extent of electrochemical reactions taking place at the interface between an electrode and a solution. This fundamental discovery might seem somewhat self-evident today, but one has to remember that Faraday’s work was done in a time when electricity was poorly understood, and the nature of the atom was still unknown. His contributions to this field credited him as one of the founders of modern electrochemistry. Electrolysis was, however, only a small part of his collected work. Faraday also introduced the idea of electric fields into 19th century science. Furthermore, he defined the dielectric constant as a result of his studies on capacitances. He is regarded as one of history’s greatest experimentalists and, needless to say, his impact on modern science is unmistakable. A more recent pioneer was Jaroslav Heyrovsky, who has been called the father of electroanalytical chemistry. He received the Nobel Prize in chemistry in 1959 “for his discovery and development of the polarographic methods of analysis”. Polarography is an electroanalytical technique where a dropping mercury electrode is used as the working electrode in voltammetric experiments. Basically, a current response is recorded when a potential difference (and hence a resulting electric field) is applied across the mercury/solution interface. This, for example, facilitates the control and evaluation of the amount of electrochemical reactions at 1 2 General introduction an electrode. Similar techniques can also be used to establish a significant electric field in the entire solution between two electrodes. These types of electric fields have been extensively used in the work described in this thesis. Electric fields can be utilized to affect, control, and analyze different physical and chemical parameters in a material or solution. This is the basis of electrochemistry, which deals with the interaction between electricity and chemical compounds. What separates electrochemistry from many other techniques or methods is the fact that it allows both the manipulation and probing of a sample at the same time. Voltammetry is perhaps the most well known example, where information about an analyte is obtained as it is oxidized or reduced at an electrode/solution interface under potential control. Important parameters such as reaction kinetics, adsorption processes, as well as mass transfer characteristics can be studied. Many of the electrochemical techniques employed today are based on the use of static electric fields. It is also possible to make use of alternating electric fields, which for example allows the possibility to evaluate the impedance of the system of interest. This technique is known as electrochemical impedance spectroscopy (EIS), and is widely used in science and industrial applications. Impedance data often contains information on many different parameters of the investigated system, and EIS has therefore been applied in many diverse areas. Some examples include corrosion studies on pipelines and reinforcements in concrete, body impedance measurements, and fundamental research related to sensor science and batteries. The evaluation of the data can, however, be quite a complex task. In this work, statistical methods were therefore used to more easily extract relevant information. 2 Electrochemistry 2.1 Introduction to electrochemical reactions A very simple electrochemical setup can consist of a battery, two metal plates, and a solution (i.e. an electrochemical cell). The battery is of course also an electrochemical cell, but is here treated as an arbitrary voltage source. The metals are immersed in the solution and connected to the battery as in Figure 2.1, resulting in a current in the circuit if the voltage is sufficiently high. The metals (and wires) are in this case electronic conductors and charge is here transported by electrons. In the solution, charge is carried by the movement of charged species. An imperative demand for the passage of a current in this circuit is that charge can cross from the metal to the solution and vice versa. This is in most cases enabled by the uptake or loss of electrons by species in the solution. These electrochemical reactions are one of the most fundamental connections between electricity and chemistry. It is also possible to make use of spontaneous electrochemical reactions to convert chemical energy into electrical energy. In electrochemistry, as in most other fields, a well defined terminology is essential. In his publication from 1834,1 Faraday (after discussions with colleagues) defined many of the electrochemical terms we still use today. First of all, the electronic conductors in contact with the solution were called electrodes. Typical electrode materials today include solid metals (e.g. Pt and Au), carbon (graphite), 3 4 Electrochemistry - + i Figure 2.1: A simple electrochemical setup. and semiconductors (e.g. indium-tin oxide and Si). The solution was called an electrolyte, and the most common electrolytes contain charged species such as H+ , Na+ , and Cl – , in order to increase the conductivity, in either water or organic solvents. Faraday defined the substances that can pass a current in a solution as ions, and divided them into negative anions and positive cations. Further, he termed the electrode at which oxidation occurs the anode and that where reduction occurs the cathode. Oxidation (Red −−→ Ox + ne – ) is the removal of electrons from a species and reduction (Ox + ne – −−→ Red) is the addition of electrons. A redox reaction is consequently a reaction in which there is a transfer of electrons from one species to another. The main concern in electrochemical systems is processes and factors that affect the transport of charge across the interface between two phases, e.g. between an electrode and an electrolyte.2 To chemists, Faraday is perhaps best known for his work on electrolysis, which involves passing a current through a solution and thereby inducing reactions at the metal/solution interfaces. To proceed with a real example, Figure 2.2 shows an electrochemical setup where two electrodes are immersed in an electrolyte. This electrolyte contains 1 M of Cl – and equal concentrations of ferri and ferrocyanide ions. One electrode is a piece of platinum and the other is a silver wire coated with solid silver chloride. The composition of this cell allows two reactions to occur; Fe(CN)63 – + – 4– −− * −− * e– ) − − Fe(CN)6 at the platinum electrode and AgCl + e – ) − − Ag + Cl at the silver electrode. These are called the half-reactions of the cell, and together they will constitute the overall chemical reaction. If the power supply is disconnected, the high impedance voltmeter will show the open-circuit of the cell. This potential, measured in volts (V), is related to the energy available in the cell to force an external current in the circuit. By using an amperemeter, this current can be recorded. Associated with every electrochemical half-reaction is a potential, called the 2.1 Introduction to electrochemical reactions 5 Power Power supply supply Ai V V Ag Pt AgCl 1M ClFe(CN)63-/4- Figure 2.2: An electrochemical cell with two electrodes immersed in an electrolyte. A power supply together with the voltmeter and amperemeter provides the possibility to obtain current-potential curves. standard reduction potential (denoted E 0 ). For the two reactions 4− −− * Fe(CN)63− + e− ) − − Fe(CN)6 and − −− * AgCl + e− ) − − Ag + Cl E 0 is 0.361 V and 0.222 V, respectively. These values are valid under certain conditions (i.e. standardized temperature and pressure, and unit activity). In practice, the values of the electrode potentials will depend on the temperature and the activities of the species present in the solution. It is important to note that the standard reduction potentials are defined with respect to something called the standard hydrogen electrode, which will be further explained below. For nonstandard conditions, electrode potentials can be calculated using the Nernst equation, which includes two terms. The first is the standard reduction potential and the second term involves the activities of the species in the reaction. For the half-reaction 4− −− * Fe(CN)63− + e− ) − − Fe(CN)6 the Nernst equation giving the half-cell potential, E, is E = E0 − AF e(CN )4− RT ARed RT 6 ln = E0 − ln nF AOx F AF e(CN )3− (2.1) 6 where R = molar gas constant (8.31447 JK−1 mol−1 ), T = temperature (K), n = number of electrons in the half-reaction, F = Faraday constant (96485.3 C), and Ai = activity of species i. The activity can often be approximated by the concentration of the species. Solids and solvents are normally omitted from equation because their activities are unity (or close to unity). When all activities are unity, the logarithmic term in the equation is zero, thus giving E = E 0 . 6 Electrochemistry For a complete cell, the voltage (Ecell ) is the difference between the potentials of the cathode and anode: Ecell = Ecathode − Eanode (2.2) If the cell voltage is positive, the net cell reaction is spontaneous in the forward direction. If the cell voltage is negative, the reaction is spontaneous in the reverse direction. Going back to the example in Figure 2.2, AgCl and Ag are both solids, and hence their activities are unity. The activity of Cl – can be found from the concentration in solution. The potential of the Ag/AgCl electrode will in this case be 0.235 V due to the concentration of chloride ions (1 M). The potential of the Pt electrode will be E ≈ E 0 = 0.361 V , since the activities of the ferri/ferrocyanide ions are approximately equal. Often in electrochemical experiments, only one of the two half-reactions is of interest. The electrode at which this reaction occurs is called the working electrode.2–4 In order to study only one reaction, the other half of the cell is standardized using a reference electrode, which is made up of phases having essentially constant chemical composition regardless of the experimental conditions. The internationally accepted primary reference is the standard hydrogen electrode (SHE), which has all components at unit activity: Pt/H2 (p = 1 bar)/H+ (a = 1, aq) where a slash represents a phase boundary. The potential of the SHE is defined as being equal to zero, i.e. the standard reduction potential (E 0 ) for the reduction of two protons to form hydrogen is 0.00 V. All electrochemical half reactions can now be assigned a standard reduction potential with respect to the SHE. Potentials are often measured and quoted with respect to other reference electrodes than the SHE, since the latter is not very convenient from an experimental point of view. A commonly used reference is the Ag/AgCl electrode shown in Figure 2.2, called the silver-silver chloride electrode, Ag/AgCl/KCl (saturated in water) which for a saturated chloride solution has a potential of 0.197 V vs. SHE. In an electrochemical experiment, the potential of the working electrode is said to be observed or controlled with respect to the reference. This is equivalent to observing or controlling the energy of the electrons within the working electrode. The energy can be raised by driving the electrode to more negative potentials. When the electrons reach a level sufficient to transfer into vacant electronic states on species in the electrolyte, a flow of electrons from electrode to solution (a reduction current) occurs, see Figure 2.3. Similarly, the energy of the electrons can be lowered by imposing a more positive potential on the electrode. At some 2.1 Introduction to electrochemical reactions Energy level of electrons Electrode 7 Electrode Solution Solution e- Vacant MO - Reduction Potential Occupied MO + Energy level of electrons - Electrode Electrode Solution Solution Vacant MO Oxidation Potential + A + e- → A- Occupied MO A → A+ + e- e- Figure 2.3: Reduction (top) and oxidation (bottom) of species A in solution. Vacant MO is the lowest vacant molecular orbital of species A and occupied MO is the highest occupied. point electrons on species in the electrolyte will find a more favorable energy level on the electrode, resulting in a flow of electrons from solution to electrode, i.e. an oxidation current. The potentials at which these processes occur are related to the standard reduction potentials, E 0 , for each electrochemical reaction in the system. Continuing with the example in Figure 2.2, we are now ready to change the potential of the Pt electrode with respect to the Ag/AgCl reference electrode. If the potential is made sufficiently positive, electrons cross from the solution phase into the electrode, resulting in the oxidation of Fe(CN)64 – to Fe(CN)63 – . While this reaction occurs at the Pt electrode, AgCl is reduced to Ag, and Cl – is liberated into the solution. The potential here will be nearly constant because the composition of the Ag/AgCl/Cl – interface is almost unchanged with the passage of modest currents, due to the high Cl – concentration. Thus, it works as an excellent reference electrode in this setup. Indeed, the essential characteristic of 8 Electrochemistry a reference electrode is that its potential remains practically constant with the passage of small currents. Hence, when a potential is applied between the two electrodes, nearly all of the potential change occurs at the Pt/solution interface. The number of electrons that cross an interface is related stoichiometrically to the extent of the chemical reaction, and is measured in terms of the total charge Q. Charge is expressed in units of coulombs (C), where 1 C is equivalent to 6.24 · 1018 elementary charges. The relationship between charge and amount of reactions is given by Faraday’s law. This law states that the passage of 96485.3 C causes 1 equivalent of reaction (e.g. consumption of 1 mol of reactant or generation of 1 mol of product in a one-electron reaction). The current, i, is the rate of flow of charges (or electrons), where 1 ampere (A) is equivalent to 1 C/s. If the current is plotted as a function of the potential, a current-potential (i vs. E) curve is obtained. Such curves are very informative about the nature of both the solution and the electrodes, and about the reactions that occur at the interfaces. 2.2 The electrical double layer Charge transfer reactions take place in a region very close to the electrode surface. As described above, the driving force for these reactions is the potential difference between the electrode and electrolyte. However, other processes are also important in this region due to the polarization of the electrodes. If the solution in a setup like the one in Figure 2.1 only contains an inert supporting electrolyte (e.g. KNO3 ), no charge transfer reaction will take place if the applied potential difference is moderate. The left electrode is in this case made negative by the battery and will hence have an excess of electrons at the surface. This electrode will thus attract positive ions and dipoles present in the solution (see Figure 2.4). If the potential (and thereby the charge) of the electrode is changed, the composition of the solution side will also change to maintain electroneutrality. In this respect, the electrode/solution interface behaves like a capacitor, which is governed by the equation q =C (2.3) E where q is the charge stored on the capacitor, E is the potential across the capacitor and C is the capacitance (in farads, F). When the electrode potential is changed, charge will accumulate until q satisfies Equation 2.3. The resulting movement of charged species in the solution will give rise to a current, called the charging current. A transient current can thus flow in the circuit without any charge transfer reactions at the interface. This is a very important effect that has to be considered in electrochemical experiments whenever the potential of an electrode is changed. The whole array of charged species at the metal/solution interface is called the electrical double layer.2 The interface can be characterized by a double-layer capacitance, Cdl , which typically is in the range of 10–40 µF/cm2 . Cdl is, however, 2.2 The electrical double layer 9 Electrode Solution Figure 2.4: Illustration of a negatively charged electrode and the corresponding abundance of positive charges in the solution. unlike real capacitors often a function of potential. Normally the existence of the double-layer capacitance and the presence of a charging current cannot be neglected in electrochemical experiments. The charging current can in reality be much larger than the charge transfer current if the concentrations of electroactive species are very low, or if the potential change is very rapid. The solution side of the double layer is itself made up of at least two layers. The one closest to the electrode is called the inner layer and contains solvent molecules and sometimes other species (ions or molecules) that are said to be specifically adsorbed (Figure 2.5). This inner layer is also called the compact or Helmholtz layer. The locus of the electrical centers of the specifically adsorbed ions defines the inner Helmholtz plane (IHP), which is at a distance x1 from the electrode surface. The inner layer extends out from the electrode to the outer Helmholtz plane (OHP), which is defined as passing through the locus of centers of the nearest solvated ions, at a distance x2 from the surface. The solvated ions interact with the charged metal only through long-range electrostatic forces. They are said to be non-specifically adsorbed and are distributed in a region called the diffuse layer, which extends from the OHP into the bulk of the solution. The thickness of the diffuse layer is determined by the potential difference and the total ionic concentration, and for concentrations greater than 10−2 M, the thickness is less than ∼100 Å. The double layer capacitance is a very important parameter when evaluating and modeling results from electrochemical experiments. It can give important information on the structure of the double layer, and also on the composition of the electrolyte. If an electrode surface is modified by a layer of some material, the thickness and dielectric properties of that material will greatly affect Cdl . 10 Electrochemistry IHP OHP Electrode - Diffuse layer + + + Solvated cation + + + = Solvent molecule = Specifically adsorbed ion or molecule x1 x2 Figure 2.5: Schematic representation of the double layer for a negatively charged electrode. IHP is the inner Helmholtz plane and OHP is the outer. 2.3 Electrochemical cells and cell resistance In the electrolyte, charge is transported by the movement of ions (anions in one direction and cations in the other). The total resistance between the electrodes will thus depend on the charge, mobility, and concentration of the species present in the electrolyte. This solution resistance, Rs , will behave as a true resistance over a wide range of conditions. If a current, i, is passed through a solution, a potential drop equal to iRs will arise. When controlling the potential of an electrode (with respect to a reference electrode), this potential drop often has to be taken into account. The actual potential of the electrode will be less than the applied potential by and amount equal to iRs . This is shown by Equation 2.4 Eappl (vs. SHE) = E(vs. SHE) + iRs (2.4) where Eappl is the voltage applied by the external power supply and E is the potential of the working electrode. If both the current and the solution resistance are reasonably small, the potential drop can often be disregarded. However, in situations where this is not true, the problem can be minimized by an alteration of the electrochemical setup. When a working electrode and a reference electrode are used to perform electrochemical measurements, the setup is called a two-electrode cell (see Figure 2.6A). The potential of the working electrode will in this case be given by Equation 2.4. Many electrochemical instruments offer the possibility to use a third electrode, called a counter electrode, resulting in a three-electrode cell. As can be seen in Figure 2.6B, the current will now be passed between the working and the counter 2.4 Mass transfer and the diffusion layer (A) 11 (B) Power supply Ai WE Power supply RE Eappl V V WE V V Ai CE RE EWE vs. RE Figure 2.6: A two-electrode cell (A) and a three-electrode cell (B). WE is the working electrode, RE the reference electrode, and CE the counter electrode. electrodes. The behavior of the working electrode is not affected by the electrochemical properties of the counter electrode and it often consists of an inert conducting material. A high input impedance device is used to measure the potential difference between the reference and working electrodes, and the current passed in this circuit will hence be negligible. The result is thus a very low potential drop in the solution, and the measured voltage will be representative of the true potential of the working electrode. The existence of a potential drop can however be useful in many situations. Large potential drops can for instance be used to perform electrophoresis, and gel separations of proteins and DNA. We have utilized the potential drop in a solution containing an isolated conducting surface, to form reaction gradients. This will be further discussed in section 4.3. 2.4 Mass transfer and the diffusion layer As previously discussed, the electrolyte is far from static during electrochemical experiments. The majority of processes occurring here are based on the movement of different species. This movement of material from one location in solution to another is called mass transfer and can arise from the following:2 1. Migration – movement of charged species in an electric field. 2. Diffusion – movement of species as a result of a concentration gradient. 3. Convection – movement of species caused by stirring or by a density (or heat) gradient. In electrochemical experiments, it is often desirable to keep one or more of these contributions to mass transfer to a minimum. For example, by the addition of a supporting electrolyte the migration of the redox species can be made negligi- 12 Electrochemistry COx COx* 1 2 δN x Figure 2.7: Concentration profiles at two different electrode potentials: (1) where ∗ COx (x = 0) is about half of COx and (2) where COx (x = 0) ≈ 0. ble. The diffusion, however, is more complicated to control, and it often has a considerable impact on electrochemical experiments. In an experiment where species Ox is reduced at a cathode, the concentration at the electrode surface, COx (x = 0), will become smaller than the bulk concen∗ . This will give rise to a concentration profile that extends into the tration, COx solution near the electrode by an amount δN , the Nernst diffusion layer thickness.3 The magnitude of δN is defined by the intersection of the tangent to the concen∗ tration profile with the horizontal line COx (Figure 2.7). δN increases with time until it reaches a constant value, termed the stationary diffusion layer thickness. If the solution is unstirred, δN of up to ∼0.5 mm is possible, and reaching steady state can take minutes. In a stirred solution, δN is well defined and much smaller (in the order of 1 µm), and steady state will usually be reached within seconds. If the potential is sufficiently high, COx (x = 0) will decrease to an extremely small value and, provided that the chemical reactions are rapid compared to the mass transfer processes, the current will be limited by the rate at which new species can reach the electrode. Increasing the potential further will hence not affect the current, since the diffusion is a function of the concentration gradient, only. 2.5 Voltammetry There are many ways to use electrochemistry as an analytical tool. One widely used method is to let the potential of a working electrode vary linearly with time and record the current. A result from such a linear potential sweep can be seen in Figure 2.8. In this example, the potential of a working electrode was swept between E1 and E2 in an electrolyte containing only the reduced form of an analyte. 2.5 Voltammetry 13 Current 0 E1 E0 E2 Potential Figure 2.8: Current potential curve resulting from a linear potential scan. In the beginning of the sweep (near E1 ) no reaction can occur, but as the potential gets closer to E 0 an oxidation starts, resulting in the current increase. The surface concentration of the reduced analyte now begins to decrease, leading to a flux of species to the surface. When the potential has moved past E 0 , the surface concentration will be almost zero, and the flux of redox species will reach a maximum value. This rate of mass transfer can however not be maintained, because the diffusion layer continues to grow. The result will be a current-potential curve with a peak, positioned somewhat positive of E 0 for the analyte. This method is referred to as linear sweep voltammetry, and is just one of many electroanalytical techniques. Voltammetry2–5 is actually a collection of techniques where current is measured as a function of applied potential. Two of these techniques will be further described, i.e. cyclic voltammetry and pulse voltammetry. Cyclic voltammetry is an extension of linear sweep voltammetry, and is often used to study the redox behavior of electroactive species. The word cyclic comes from the fact that when the potential has been swept between E1 and E2 , the direction of the sweep is reversed. The resulting triangular waveform can be seen in Figure 2.9A. The potentials E1 and E2 are chosen to incorporate the redox behavior of the analyte, and is often centered around its E 0 -value. Another important parameter is the sweep rate (or scan rate), normally ranging from 10 mV/s to a few V/s. The current-potential curve obtained is called a cyclic voltammogram. Figure 2.9B shows the result of a measurement in a solution containing a reversible redox couple. During the reversed scan, the redox behavior of the species oxidized during the forward scan can be evaluated. The concentration of an analyte can be determined, since the current is proportional to this concentration. Also, the value of E 0 can be estimated by the position of the oxidation and reduction peaks. In fact, several important parameters such as reaction kinetics, surface adsorption, and mass transfer are frequently studied with cyclic voltammetry. In pulse voltammetry, the potential is instead changed in a stepwise manner. There are many pulse parameters that can be changed, naturally giving different 14 Electrochemistry (A) Potential E2 E1 Time Current (B) Oxidation 0 Reduction E1 E0 E2 Potential Figure 2.9: (A) Triangular potential waveform applied to the working electrode in cyclic voltammetry. (B) A schematic cyclic voltammogram for a reversible redox couple. current responses. This technique provides the same general information as cyclic voltammetry, but is especially known for low limits of detection, and better sensitivity and resolution. This is due to the fact that the influence of the charging current can be minimized using an appropriate sampling of the current. Also, short potential pulses will counteract the build-up of thick diffusion layers, and redox species close to the electrode will consequently not be depleted. Both of these effects will result in a better signal to noise ratio. 2.6 2.6.1 Electrochemical impedance spectroscopy Theory Electrochemical impedance spectroscopy (EIS)2, 3, 6 is a powerful tool for examining many chemical and physical processes in solutions as well as solid materials. It is a non-destructive technique capable of separating different contributions to the overall electrochemical processes. The basis for this is that the impedances associated with different processes have varying frequency-dependencies. EIS can therefore be used to study electrode kinetics, adsorption rates, corrosion processes, battery properties, ageing of sensors, and many more. Impedance (Z) is generally defined as the total opposition a device offers to the flow of an alternating current at a given frequency. Impedance can be measured in 2.6 Electrochemical impedance spectroscopy 15 E(t) = E0sin(ωt) I(t) = I0sin(ωt+θ) Time Phase shift Figure 2.10: Sinusoidal potential excitation and current response. many different ways. Usually, a sinusoidal potential or current excitation signal is applied to a device and the resulting current or potential is recorded, respectively. If a potential excitation signal (E(t)) is used, expressed as a function of time, it will have the form E(t) = E0 sin (ωt) (2.5) where E0 is the amplitude of the signal and ω (rad/s) is the radial frequency. The resulting current can be expressed as I(t) = I0 sin (ωt + θ) (2.6) where It is the amplitude and θ is the phase difference between the potential and current, see Figure 2.10. For a pure resistance, the phase difference is zero, and for a pure capacitor it is 90◦ . In the time domain, the relation between the system properties and the response is, however, very complex. If many capacitive and inductive elements are present, a solution of a system of differential equations is required. By using Fourier transformation of the signals into the frequency domain, the mathematical treatment is significantly simplified. The Fourier transforms of the voltage, E(jω), and current, I(jω), become E0 π and I0 π · exp(θj), respectively. The impedance function can thus be defined as Z(jω) = E(jω) I(jω) (2.7) and Z(jω) can be now be related to different circuit elements. For a resistance it is simply equal to R, and for a capacitance and an inductance, Z(jω) is 1/(Cωj) 16 Electrochemistry Im(Z) Z’’ |Z| θ Z’ Re(Z) Figure 2.11: Vector representation of the complex impedance. and Lωj, respectively. Usually, Z(jω) is for simplicity written as just Z(ω). Now, the impedance of a system with multiple elements can be calculated in the same way as for multiple resistors. Since the complex quantity Z(ω) contains information on both the magnitude and the phase of the impedance, it is convenient to express it as a complex number. Z = Re(Z) + jIm(Z) = Z 0 + jZ 00 (2.8) The impedance is easily depicted in a plot of the imaginary part of the impedance vs. the real part, as in Figure 2.11. The impedance is here represented by a vector of length |Z|, and the angle between this vector and the x-axis is the phase shift θ (=argZ). The real and imaginary parts of the impedance are termed the resistance (R) and the reactance (X), respectively. It is worth noticing that the original time variations of the applied and measured signals have disappeared in the representation of the impedance. Some important relationships are X (2.9) R To display the variation of the impedance with frequency, a Bode plot is often used. Here, log |Z| and θ are both plotted against log ω or log f . Another representation is a Nyquist plot, where X is plotted against R for different values of ω. Examples of both these types of plots can for a series and parallel RC circuit be seen in Figures 2.12 and 2.13, respectively. Sometimes it is mathematically convenient to use the reciprocal of the impedance, in which case |Z| = p R2 + X 2 and θ = arctan 1 1 = = Y = G + jB (2.10) Z R + jX where Y is the admittance, G the conductance, and B the susceptance. The unit of impedance is ohm (Ω), and the unit of admittance is siemen (S). Two more 2.6 Electrochemical impedance spectroscopy 17 -Im(Z) ω R C ∞ Re(Z) -θ log |Z| log ω log ω Figure 2.12: A series RC circuit (top left) and the resulting Nyquist plot (top right). Bottom left and right are the corresponding Bode plots. -Im(Z) C ∞ ω R Re(Z) -θ log |Z| log ω log ω Figure 2.13: A parallel RC circuit (top left) and the resulting Nyquist plot (top right). Bottom left and right are the corresponding Bode plots. 18 Electrochemistry quantities related to impedance are the modulus function M = jωCc Z = M 0 + jM 00 , and the complex dielectric constant ε = M −1 = Y /(jωCc ) = ε0 + jε00 . Here, Cc = ε0 Ac /l is the capacitance of the empty cell of electrode area Ac and electrode separation length l. ε0 is the dielectric permittivity of free space, 8.854·10−12 F/m. All four of these functions are valuable in EIS due to their different dependence on, and weighting with, frequency. An electrochemical cell can often be modeled as an equivalent circuit using electrical components. Examples of circuit elements include a solution resistance, a double layer capacitance, a charge transfer resistance, and an impedance term representing mass transfer. Using such an equivalent circuit, impedance data can now be evaluated in order to determine the physical and chemical properties of the electrochemical system. 2.6.2 General applications During the last couple of decades, EIS has been used for many different applications. Here, a brief summary of some important examples will be given. For a thorough insight into the theory and applications of EIS, see the excellent text by Barsoukov and Macdonald.6 The characterization of the interfacial region is very important in optimization of electroanalytical sensors, and EIS is very useful for evaluating the processes occurring at the electrode/solution interface. The sinusoidal excitation will give rise to an alternating charging and discharging of the double layer, as well as influencing redox reactions and mass transfer. Hence, a lot of information may be obtained since these processes can have different frequency-dependencies. For example, Janek et al. used EIS to investigate the interfacial impedance of self-assembled monolayers (SAMs) on gold surfaces.7 Surface modifications with DNA have also been studied, for example the electron transfer through monolayers of thiol-labeled DNA duplexes,8 as well as oligonucleotide-DNA interactions.9 Further, EIS has been used for surface characterization of conductive polymer-modified electrodes,10 and solid polymer electrolytes.11 Corrosion research is an enormous field and EIS has been extensively used to gain information on the corrosion processes at metals and metal-coated surfaces.12 Also, reaction mechanisms can be deduced, as shown by Keddam et al. who studied iron dissolution in acidic media.13, 14 2.7 Bipolar electrochemistry If the word bipolar is used to describe an object, the meaning will be that is has two poles. In an electrochemical setup the poles are the anode and cathode. A bipolar electrode is hence a single electrode, acting as both anode and cathode. A requisite for this is that an oxidation and a reduction must take place at this 2.7 Bipolar electrochemistry 19 Voltage source E Reduction Oxidation i1 Oxidation Reduction i2 Feeder electrode Feeder electrode i1+i2 Bipolar electrode Figure 2.14: Experimental setup showing the current paths when electrochemical reactions are taking place on the bipolar electrode. A variable voltage source is used to control the total current between two feeder electrodes. electrode simultaneously. Furthermore, these two reactions must be separated in space to give rise to the two poles. The driving force for these reactions is an electric field in the solution parallel or perpendicular to the electrode surface. In Figure 2.14, an example of a bipolar setup is shown. Here, a voltage (or current) source and two feeder electrodes are used to establish an electric field in the solution. It should be noted that the isolated conducting surface has no physical connection to the power source, and its potential is hence floating. The potential difference between a point on the equipotential surface and the solution will now vary laterally along the surface. This potential difference can induce charge transfer reactions at both ends of the surface, provided that the electric field exceeds a certain threshold value. At this point, the surface becomes a bipolar electrode. A potential gradient across the electrode is thus induced by the electric field in the solution. In practice, the bipolar electrode will act as an additional, less resistive, path for the total current. As apposed to a regular electrochemical setup, it is the applied external electric field that is used to control the interfacial potentials at the electrode of interest. The overall potential drop across the cell will now be non-linear when a floating substrate acts as a bipolar electrode, and the current density distribution will also be affected. Since the potential difference between a point on the bipolar electrode and the solution will vary laterally along the surface, the rates of the reactions will vary accordingly. Thus, electrochemical reaction gradients are present on bipolar electrodes. The use of bipolar electrochemistry can be very advantageous, for instance when connecting stacks of individual cells, as a result of the reduced internal resistance. Bipolar electrochemistry is a quite established field. For example, in 1973 Eardley et al.15 investigated the conductivity of fixed arrays of particles dispersed in a continuous electrolyte medium. In recent years several new and interest- 20 Electrochemistry ing applications have been reported. The phenomenon has been used in battery applications,16, 17 in proton exchange membrane based fuel cells,18 for energy storage and load leveling,19, 20 as well as to carry out electrochemical reactions in poorly conducting media.21 Further, Bradley et al. have in several publications studied the use of bipolar electrochemistry for the directional growth of copper wires between micrometer-sized particles,22 and the electrodeposition of palladium catalysts.23, 24 Moreover, Said et al.25 used bipolar electrochemistry to visualize the magnitude and direction of an electric field in an electrolyte. This type of electrochemistry has also been used as an external electric field driven in-channel detection technique in microfluidic PDMS channels.26 Finally, the use of bipolar electrochemistry provides the possibility of studying chemical reactions without any physical contact to the electrode, for instance in electrochemiluminescence applications.27–29 Considerable theoretical contributions to the understanding of bipolar electrochemistry have been made by Duval et al.30–35 Here, simulations were made of the electrochemical behavior of parallel plate electrodes in a lateral electric field, in conjunction with electrokinetic experiments. It was shown how the potential and current distribution can be evaluated for different setups. 2.8 Limitations of electrochemistry Even though electrochemical techniques are very versatile, voltammetry and EIS are more widely utilized as analytical tools within research than in day-to-day use. This is related to some of the inherent limitations with electrochemical methods. First, there is the issue of stability of the electrode surface. The activity and morphology of the electrodes frequently change over time, often as a result of the adsorption of species from the solution. This is, however, something that can be circumvented by the use of disposable electrodes, or by careful polishing and cleaning procedures. Second, the limit of detection is normally in the micromolar range. Although this is sufficient for many applications, electrochemistry cannot compete with other high-performance techniques. Third, is the issue of selectivity. It is sometimes difficult to separate the response from a single analyte from the total current. For instance, if two compounds are to be distinguishable in voltammetry, their standard potentials must be sufficiently separated. Finally, a rather important limitation is the fact that regular electrochemistry offers no lateral resolution of the reactions and processes at electrode surfaces. Apart from scanning techniques such as scanning electrochemical microscopy, a collective current response from the entire surface is always obtained. However, this limitation can be overcome by combining electrochemistry with imaging optical methods, such as ellipsometry and surface plasmon resonance. 3 Imaging optical methods and electrochemistry 3.1 Introduction One of the major advantages of electrochemistry is that it is easily combined with other techniques. Successful combinations have been made with, for example, ultraviolet and visible spectroscopy, vibrational spectroscopy, acoustic resonance (i.e. quartz crystal microbalance), mass spectrometry, chromatography, and electrophoresis. The combination of imaging optical techniques and electrochemical measurements is a very interesting area, because it allows the detection of electrochemical processes with lateral resolution in the micrometer range. This chapter focuses on the combination of electrochemistry with imaging surface plasmon resonance and imaging ellipsometry. 3.2 3.2.1 Surface plasmon resonance Theory Surface plasmon resonance (SPR) spectroscopy is an optical method for measuring the refractive index of very thin layers of material on a metal. SPR is a very 21 22 Imaging optical methods and electrochemistry Evanescent field Glass prism Metal film Ambient Figure 3.1: Schematic illustration of the Kretschmann setup. sensitive technique for real-time, label-free monitoring of events taking place at a surface/liquid interface, and has been exploited for biosensing36 for over 20 years. A surface plasmon is a p-polarized surface bound electromagnetic wave propagating at the interface between a plasma and a dielectricum. Since a metal has nearly free electrons, it can be regarded as a plasma. Associated with the surface plasmon is an evanescent field, which is a non-propagating wave that decays exponentially in the direction orthogonal to the direction of the interface. A surface plasmon can, under certain conditions, be excited by photons. In Figure 3.1 a common SPR setup, called the Kretschmann configuration, is shown. Here, a light beam is totally reflected at the glass/metal interface, exciting a surface plasmon and an evanescent field. If the metal film is thin enough, the evanescent field will penetrate into the ambient on the other side. The optical properties of the interface region will affect the excitation resonance condition. Thereby, a link is established between the ambient and the reflected light. Examples of SPR biosensing applications include the study of biomolecular interactions and binding properties, environmental monitoring, and medical diagnostics.37 Imaging SPR38 (also known as SPR microscopy) is attained when the detector is a replaced by a charge-coupled device (CCD) camera. The lateral resolution, typically in the µm range, is a function of the wavelength of the light, the objective, and the CCD resolution. See Brockman et al.39 for an extensive review on the theory and applications of imaging SPR. 3.2.2 SPR and electrochemistry If the metal substrate in SPR also acts as a working electrode in an electrochemical experiment, these techniques can be employed simultaneously. Many electrochemical reactions will change the optical properties of the electrode surface and the nearby solution, and hence change the SPR response. This fact has been used for several different investigations of surface interactions. Iwasaki et al.40 used the combination of SPR and cyclic voltammetry to investigate oxide 3.3 Ellipsometry 23 film formation and ion adsorption on gold electrodes. They also showed that it was possible to separate the diffusion and adsorption processes. Further, Xiang et al.41 showed that SPR in conjunction with scanning electrochemical microscopy could be used to determine local variations in thin film thicknesses. The setup was also used to study conformational changes of cytochrome c molecules attached to a surface. Electropolymerization is a convenient way of forming a polymer film on an electrode. This has been investigated by Knoll and co-workers for poly(3,4ethylenedioxythiophene) (PEDOT) films of different thicknesses.42, 43 Finally, Ertl and co-workers used imaging SPR to obtain images of the potential distribution at electrodes in order to study spatiotemporal patterns.44 The same setup was also used to study Turing-type patterns on electrode surfaces.45 In this work Papers I, II, and III are based on the use of imaging SPR and electrochemistry. It is especially the visualization of electrochemical reactions with lateral resolution that has been explored. 3.3 3.3.1 Ellipsometry Theory Ellipsometry is an optical technique for characterization of surfaces, thin films, and multilayers.46 The basic idea is to analyze the polarization changes when light is reflected by a surface or film. Ellipsometry is non-destructive, making it suitable for real-time in situ measurements. Furthermore, it is essentially insensitive to drift in light intensity, since it is the polarization change that is evaluated. Ellipsometry is very sensitive to surface changes, and can have sub-Ångström resolution in film thickness determinations. The electric field, E, associated with a wave of light, can be divided into two components, Ep and Es . Ep is the p-polarization (or TM-polarization) and is the component of the electric field that lies in the plane of incidence. Es is the spolarization (or TE-polarization) and its component is perpendicular to the plane of incidence. It is the correlation between Ep and Es that determines the state of polarization of the light. In order to perform an ellipsometric measurement, an incident light beam with known polarization is required. After reflection at a sample, the change in the polarization of the light is determined. In Figure 3.2, a common setup, the PCSA (polarizer compensator sample analyzer) ellipsometer, can be seen. The light source is often a laser, emitting monochromatic, collimated light. The polarizer and analyzer are both linear polarization filters, and the compensator is a quarter wave plate, with which an arbitrary elliptical polarization can be obtained by inducing a phase shift between the components of the light. As the beam travels through the polarizer, it becomes linearly polarized. The compensator, which is fixed at ±45◦ , will then produce elliptically polarized light. The reflection 24 Imaging optical methods and electrochemistry Light source Detector Rotating polarizer Fixed compensator Rotating analyzer Sample Figure 3.2: Schematic representation of a PCSA ellipsometer. at the sample will induce an additional phase shift, denoted 4, along with a relative amplitude change, denoted tan Ψ. The beam then passes through the analyzer before reaching the detector, where the intensity of the transmitted light is measured. The strategy is to adjust the optical components so that the light is extinguished at the detector. This will be possible if the light is linearly polarized after the reflection at the surface, which means that it can be quenched by the analyzer at a certain angle. By rotating the polarizer and the analyzer until zero intensity is detected, 4 and tan Ψ can be determined using the angles of the optical components. Now, using the two parameters together with a model of the system, different surface characteristics can be estimated. For instance, the thickness of a film on the surface can be calculated with the McCrackin algorithm,47 if the refractive indices of the substrate and the film are known. Imaging ellipsometry48 is an interesting development of ellipsometry, where the regular detector is replaced by a CCD camera and a focusing objective. This allows the simultaneous determination of for instance film thicknesses over the entire surface area observed with the CCD. The lateral resolution will be determined by the wavelength of the light, the CCD resolution, and the objective. Typical values for the resolution are in the µm range. Imaging ellipsometry has for example been used for biosensor applications, as shown by Jin et al.,49 where antigen-antibody interactions were visualized. 3.3.2 Ellipsometry and electrochemistry In combination with electrochemistry, ellipsometry is perhaps most widely used to evaluate film formation and growth on electrode surfaces. This in situ combination was pioneered by Tronstad50 in 1931, and has since gained popularity, see for instance Kruger51 and Kruger and Calvert.52 Here, the thickness and optical properties of passive iron films were evaluated, as the electrochemical potential was either controlled or monitored. 3.3 Ellipsometry 25 In a review by Christensen and Hamnett,53 the in situ evaluation of electrodeposited polymer films was discussed. The influence of potential, and hence the film thickness, on the ellipsometric angles and reflected intensity was shown. Another example is the in situ study of a self-assembled ferrocenylalkylthiol monolayer on a gold electrode performed by Abrantes et al.54 They showed that the monolayer thickness increased 1 to 2 Å as the ferrocene was oxidized to ferricinium, due to a reversible orientation change. Further, Yu and Jin55 used imaging ellipsometry to investigate the effect of the potential of a gold electrode on the adsorption of fibrinogen. In this thesis, imaging ellipsometry was used in Paper I to obtain thickness maps of a protein gradient, formed via bipolar patterning. It proved to be a very convenient way of visualizing the results from different preparation steps. In Paper IV, ellipsometry was used in situ during the electrodeposition of nanostructured Cu and Cu2 O materials. This gave valuable information on the deposition process, and the structure of the resulting film. 26 Imaging optical methods and electrochemistry 4 Electrode surface design and analysis 4.1 Introduction Enormous efforts have been put into the modification of electrodes to alter or produce specific functionalities. Such modifications can be very advantageous, and examples of application areas include fuel cells and batteries, the protection from corrosion, and the use of modified electrodes as analytical sensors. Some 30 years ago, interest arose regarding the modification of electrode surfaces by covalent attachment of monolayers of different species.2, 56 Later, thicker polymeric films and inorganic layers started to be used, as well as conducting polymers and organic metals. This chapter focuses on the use of electrochemistry for both the design and evaluation of modified electrodes. 4.2 Materials and methods In this section, a summary of some important methods and materials used to modify electrodes will be given, with some emphasis on self-assembly of alkanebased thiols on gold surfaces. 27 28 Electrode surface design and analysis Polymers. A polymer is a macromolecule built up by repeating monomer units. There is an abundance of different polymers available, and new ones can readily be synthesized. Polymer films can be formed on an electrode from a solution of dissolved polymer by, e.g., cast or dip coating, spin coating, covalent attachment or electrodeposition. If the solution contains the monomer, polymerization on the surface can be induced via different means, e.g., thermal, photochemical, or electrochemical. Conducting polymers contain partially delocalized electrons, and are typically classified as semiconductors. Electroactive polymers contain electroactive components linked to the polymer backbone, while ion-exchange polymers (polyelectrolytes) can attract ions from the solution to charged sites in the film via ion-exchange processes. Some electrochemical applications include work by Smela et al.,57 who used a conducting polymer (polypyrrole) to fabricate microactuators. Further, the ellipsometric characterization of electropolymerized polypyrrole was performed by for instance Kim et al.58 For an extensive review on conducting polymers, see Simonet and Rault-Berthelot.59 Inorganic films. The most common inorganic material formed on electrodes are metal oxides. By anodization of a metal electrode, an oxide film can easily be grown, where the thickness is influenced by the potential and the time. Interesting work in this area has been done by Iwasaki et al.40 and Stevenson et al.60 In Paper IV, electrodeposition was used to form nanostructured Cu and Cu2 O layers. Electrochemistry was here combined with a gravimetric technique (quartz crystal microbalance, QCM) and ellipsometry to investigate the potential and local pH dependence on the nature of the deposited materials. Self-assembled monolayers (SAMs). The formation of a self-assembled monolayer is a spontaneous process, leading to a molecular film with an often large degree of order, as a result of the lateral interactions between the adsorbed molecules. One of the most investigated systems is the self-assembly of organosulfur (thiol) compounds. In 1983, Nuzzo and Allara showed that gold surfaces could be easily functionalized by the spontaneous self-assembly of organic disulfides.61 The tail-group of a thiol can be chosen to display different physical and chemical properties at the ambient interface. SAMs can also be used as linkers for the attachment of biomolecules to surfaces, which is usually done in order to prepare electrochemical sensors. The most developed application is a surface modified with an enzyme, such as glucose oxidase.62 Further, patterns of SAMs can be formed by microcontact printing,63 giving linewidths ranging from 30 nm to 500 µm. Informative reviews on the subject are given by Ulman64 and Whitesides and co-workers.65 Extensive work has been done in order to characterize and optimize self- 4.3 Surface gradients 29 assembly of thiol based monolayers. The most commonly used substrate is gold, but SAMs can also be formed on other metals.66 Self-assembly from a solution containing two different thiol species is also possible. Such co-adsorbtion will result in SAMs having various degrees of phase separation.67–73 One important application of SAMs is the formation of protein resistant surfaces. Li et al.74 concluded that SAMs of oligo(ethylene glycol) thiols with different lengths displayed different surface morphology and thereby different protein resistance. A SAM on a surface will have a thickness of the same order of magnitude as the diffuse layer. This means that the total capacitance of a modified electrode will be greatly influenced by the dielectric properties of the surface film. Therefore, electrochemical techniques are well suited for studying thin self-assembled systems. An early example is an investigation by Porter et al.,75 focusing on the influence of alkanethiol chain length on heterogeneous electron transfer and interfacial capacitance. Voltammetry has further been used to characterize a mixed monolayer of thiol analogues of cholesterol and fatty acids on gold,76 as well as to examine the chemistry of the bound alkanethiol head group.77 In the latter work, it was also shown that the adsorbed thiols could be desorbed from the surface by both oxidation and reduction. In a recent study, Chan and Yousaf 78 demonstrated that the ligand activity of an electroactive monolayer could be controlled by the electrochemical potential. The degree of organization of a SAM on electrode surfaces has a huge impact on its optical and electrical properties. Different packing densities and porosities will for instance affect the ability of the SAMs to impede charge transfer reactions. This was investigated in Paper III, where a pattern of microcontact printed SAMs of thiocholesterol and 1-hexadecanethiol was used to investigate the possibility of detecting local electrochemical processes in situ on chemically modified electrodes. 4.3 Surface gradients A surface with a gradually changing chemical or physical property is said to contain a surface gradient. Many types of gradients are found in nature, for instance concentration gradients across cell membranes and biomolecular gradients guiding the motility of cells. The mimicking of these in vivo gradients in the lab can be a valuable tool to better understand biological processes. Also, by having gradually changing properties on a biosensor surface, it is possible to perform highthroughput and cost effective analysis. Since all electroanalytical devices based on biomolecular interactions have to be thoroughly optimized and investigated, the access to a surface gradient of the interacting species would be very advantageous. 30 Electrode surface design and analysis i1+i2 Reduction Oxidation i1 Oxidation Reduction i2 Cathodic side Figure 4.1: Illustration of alkanethiol desorption from the cathodic side of a bipolar electrode. Two good examples of the use of molecular gradients are the study of surface tension effects79, 80 and protein adsorption.81, 82 Recent reviews on the subject are given by Kim et al.,83 Morgenthaler et al.,84 and Genzer and Bhat.85 Several gradient-forming approaches have been demonstrated to date; these approaches are based, for example, on diffusion,86, 87 electric fields88 ,89 microfluidic systems,90 and immersion procedures.91, 92 Bohn and co-workers proposed an electrochemical method to form gradients on very thin metal substrates (about 50 nm) by using in-plane electrochemical potentials.93–105 In that method, a potential gradient was generated across the surface by passing a current through the resistive film using a bipotentiostat. Gradients with varying widths and positions could thus be created on the surface. This method has also been used to deposit copper gradients in nanoporous alumina membranes,106 to manipulate droplet transportation by the spatiotemporal control of a wetting gradient,107 and to electrodeposit polymer films in various patterns.108 Based on the use of bipolar electrochemistry, we have developed a new technique to produce surface gradients, which we call bipolar patterning. Here, we use the fact that the driving force for electrochemical reactions will vary laterally across a bipolar electrode. If the result of such reactions on either end of the electrode is the adsorption or desorption of a specific molecule, a gradient of that molecule can be created on the surface. An example of this is seen in Figure 4.1. In Paper I, this technique was used to form a biomimetic gradient on a gold surface. Briefly, self-assembled monolayers and protein immobilization procedures were used to realize a protein gradient. The first step involved the reductive desorption of a SAM based on methoxy terminated poly(ethylene glycol)-containing alkanethiols. After backfilling with carboxyl terminated poly(ethylene glycol)- 4.3 Surface gradients 31 (A) Thickness (nm) 5 3 4 ~0.6 nm 3 ~2.4 nm 2 2 1 1 0 0 0.1 0.2 0.3 0.4 0.5 x (mm) 0.6 0.7 0.8 0.9 Thickness (nm) (B) 4.5 4 3.5 3 2.5 0 1 0.2 0.4 x (mm) 0.6 0.8 0 0.5 m) y (m Figure 4.2: (A) Line profiles obtained from imaging ellipsometry measurements, in which the thicknesses of the gradients after the different preparation steps are shown. Line 1 shows the result of the desorption of HS−C2 H4 −(O−C2 H4 )6 −OCH3 , line 2 shows that obtained after backfilling with HS−C2 H4 −(O−C2 H4 )8 −COOH, and line 3 represents the resulting protein gradient. (B) Thickness map of the protein gradient (the solid line shows the region from which the line profile was taken). See Paper I for further details. containing alkanethiols and activation of the carboxyl groups, the surface was incubated in a lysozyme solution. By employing imaging ellipsometry, the results of the preparation steps and the protein gradient could be evaluated (Figure 4.2). The resulting protein gradients formed with this specific setup were about 0.5–1 mm wide. Bipolar patterning is a fast and straightforward technique with many possibilities for improvements, for instance the generation of gradients of different geometries. The substrate can basically consist of any conducting material and the technique does not require access to advanced laboratory equipment. Gradients with widths in the millimeter range are sufficient for many applications, but sometimes wider gradients and other spatial geometries are required. A clear knowledge of the effects of the electric field in a solution containing a conducting substrate will make it possible to optimize this gradient forming technique. The distributions of the potential and current densities in a bipolar setup can be evaluated by rather simple methods involving the use of ordinary reference electrodes. Also, the possibilities offered by simple conductivity models was evaluated to perform simulations of new and exciting geometries. In Paper II, the potential Electrode surface design and analysis 1.4 20 1.2 10 1 0 0.8 -10 0.6 -20 0.4 -20 -10 0 10 20 Position (mm) 30 40 -30 1.6 30 (B) 1.4 1.2 20 10 1 0 0.8 -10 0.6 -20 0.4 -20 -10 0 10 20 Position (mm) 30 Solution potential (mV) 30 (A) Relative current density Relative current density 1.6 Solution potential vs. surface (mV) 32 -30 40 Figure 4.3: Solution potentials and relative current densities for different positions with respect to the bipolar electrode (0.5 mm above the surface). The shaded areas represent the position of the surface. (A) Experimental results as 1 mA was passed in a solution consisting of 10 mM [Fe(CN)6 ] 4 – and 10 mM [Fe(CN)6 ] 3 – in 500 mM KNO3 . (B) Simulated results using COMSOL Multiphysics. A simple conductive media DC model of the experimental setup was utilized. See Paper II for further details. and relative current density distributions in a solution containing a bipolar electrode was investigated, see Figure 4.3A. It is clear that the presence of a bipolar electrode has changed both distributions, which would otherwise be linear. As can be seen from the current density, the amount of electrochemical reactions varies laterally across the surface. In Figure 4.3B, results from a simulation of the setup used to obtain the experimental results are shown. The important result is that a good qualitative agreement was obtained between the measured and simulated values. This demonstrates the value of the model to predict important parameters in bipolar setups. Further, the use of imaging SPR provided the possibility to evaluate the amount of reactions occurring at the bipolar electrode. Since the extent of these reactions are governed by the potential difference between the bipolar electrode and the solution, the SPR response will be a function of this potential difference. By comparing the SPR responses with those obtained in a regular three-electrode setup (including a standard reference electrode), the potential distribution very close to the surface can be determined (Figure 4.4). To demonstrate the relevance of bipolar patterning for biomimetic and biosensor applications, a spherical electric field was used to selectively remove thiols from a surface. Here, one feeder electrode was a platinum rod, placed above the monolayer. The resulting effective layer thickness is shown in Figure 4.5. This method could now be further used to create an array of such gradient regions on a single substrate. 4.3 Surface gradients 33 0.05 (A) (B) 10 mA 1 5 mA 0.5 1 mA 0 0 5 mA (in KNO3) i (mA) ∆RTM/RTE 1.5 -0.5 -1 -1.5 -0.05 2 2.5 3 3.5 Position (mm) 4 -0.1 0.1 0.3 E vs. Ag/AgCl (V) 0.5 d (nm) Figure 4.4: (A) The SPR response for different currents passed through the electrolyte, simultaneously showing the reduction (left side) and oxidation (right side). The electrolyte consisted of 10 mM [Fe(CN)6 ] 4 – and 10 mM [Fe(CN)6 ] 3 – in 500 mM KNO3 . (B) SPR response for different potentials when the sensor surface acted as a working electrode in a three-electrode setup. The solid line was calculated using the Nernst equation. The scan rate used to obtain the cyclic voltammogram was 50 mV/s. See Paper II for further details. 2.5 2 1.5 0.5 y (mm) 0 0 0.5 1 x (mm) 1.5 2 Figure 4.5: Results from imaging null-ellipsometry measurements, showing the effective layer thickness, d, of the patterned thiol layer. The inset shows a larger image and the region from where the thickness map was taken. See Paper II for further details. 34 Electrode surface design and analysis 5 Alternating electric fields for chemical analysis 5.1 Introduction It is often desirable to monitor the state, or quality, of a liquid. This is sometimes a difficult task when relying on techniques based on static electric fields, due to the often varying conductivity of the samples and the complexity of the solution. These problems can be circumvented by using EIS, which is based on alternating electric fields. We have utilized EIS in two different applications; the simultaneous determination of soot and diesel contamination in engine oil, and the determination of concentration and pH in an industrial cutting fluid. Both of these liquids contain very low amounts of electroactive species, and they can further have very different conductivities. Evaluations of these can however still be performed with EIS due to the possibility to separate different processes based on their frequencydependencies. Since this technique is very sensitive, it is of crucial importance to use a carefully designed measurement setup. 35 36 5.2 Alternating electric fields for chemical analysis Practical considerations When performing EIS measurements, several practical aspects have to be considered. A lot can be gained by using a correctly optimized experimental setup. There are several commercially available test fixtures and cells, but the prices are often quite high. Sometimes it is hence worth the time and effort to design and optimize an application-specific measurement setup. When evaluating a solid sample in order to gain information on the bulk properties, the role of the electrodes is simply to provide electrical contact to the material. For liquids, electrochemical reactions and other surface interactions can take place at the electrode/solution interfaces. In these cases a parallel plate setup is often used, where the plates consist of an inert metal. The electrode area and separation can be varied depending on the application. Listed below are some other important parameters that need to be considered. System. Most often a potentiostat is used together with a frequency response analyzer (FRA) for EIS measurements. When potentiostatic control is not required, as is the case for most materials impedance tests, a self-contained impedance analyzer can be used instead. Many different systems are commercially available, with varying specifications regarding sensitivity and frequency range. Terminal configuration. There are several connection configurations available, and the choice depends on the expected impedance range and the frequencies used. Most modern equipments have four terminals; high and low potential and high and low current. It is between the high and low current terminals that the excitation signal is applied, and the resulting signal is measured between the high and low potential terminals. If the two high and the two low terminals are shorted, a two-terminal configuration is obtained, which is sufficient for many high impedance measurements. A configuration providing a very wide measurement range is the four-terminal configuration. This ensures that only the potential drop across the cell is measured, and eliminates the effects of the cables connected to the high and low current terminals. Cables. The cables for connecting the cell to the measuring equipment need to be carefully designed. Preferably a short, static, and well shielded cable assembly should be used. It is also very important to make sure that all connections are good in order to not introduce resistive loads outside the sample volume. Otherwise these will be included in the total impedance. Shielding. For sensitive measurements, it is always recommended to shield the cell from external interference by placing it in a Faraday cage. This is true 5.3 Applications 37 Coaxial cable Hc Hp Shielding box Triaxial cable Electrode 2 Sample Lp ~200µm Electrode 1 Lc Figure 5.1: Schematic representation of the measurement set-up showing the impedance meter, the four-terminal cable configuration, the shielding box, and the electrodes. Hc: High current, Hp: High potential, Lp: Low potential, Lc: Low current. for both low and high impedances. The cage should be connected to the shield of the instrument input terminals. Temperature. The impedance is generally a function of temperature. In order to compare results from different samples, it is very important to be able to control or at least to measure the temperature of the sample. A schematic representation of the setup used in Papers V and VI can be seen in Figure 5.1. In both cases, the electrodes were separated by approximately 200 µm, and consisted of stainless steel in a parallel plate cell configuration. A four-terminal configuration was employed together with a coaxial/triaxial cable assembly, to be able to perform measurements over a large impedance range. 5.3 Applications General applications of EIS were given in section 2.6. Here, the focus will be on the chemical analysis of lubricants and cutting fluids. These liquids are abundant in industrial applications and there is presently a huge demand for on-line measurement techniques. Smiechowski and Lvovich have made several interesting EIS studies on industrial lubricants. Some examples include the detection of water leaks in engine oil,109 the analysis of colloidal dispersions,110 and the investigation of the relationship between lubricant chemical composition and EIS data.111 Further, Allahar et al.112 studied the impedance of steels in new and degraded jet engine oil, while Wang and Lee113 focused on the glycol contamination in regular engine oil. Wang and co-workers have also employed fast scan voltammetry to determine the condition of similar samples.114–117 Here, the maximum current output was measured and correlated to degradation processes in the oil. The obvious drawback was the use of only one parameter to evaluate the condition of the sample. The quality of the engine oil greatly affects the performance of an engine, and the need for on-board sensors for real-time monitoring is increasing. In Paper V, 38 Alternating electric fields for chemical analysis 8 1S1D 4S1D 3S3D 1S5D 4S5D 7 6 Diesel (%) 5 4 3 2 1 0 -1 1 1.5 2 2.5 3 Soot (%) 3.5 4 4.5 5 Figure 5.2: The result from soot and diesel predictions made with impedance data and PLS modeling. The cross-hair markers represent the true concentrations of soot and diesel for the five samples. The samples are labeled according to their levels of soot (S) and diesel (D). See Paper V for further details. the simultaneous estimation of soot and diesel in engine oil was shown to be feasible. In these experiments, a carefully planned measurement cell was employed, consisting of two stainless steel electrodes in a parallel plate configuration. Real engine oil samples with varying amounts of soot were spiked with diesel and then evaluated using a frequency range of 600 kHz to 20 Hz. Due to the low conductivity of the samples, the voltage amplitude was set to 20 Vrms . The data was then evaluated with multivariate data analysis, namely partial least squares (PLS). The use of statistical tools for the interpretation of impedance data can be very informative, since it quickly provides information on the correlation between the data and the sought parameters. It also allows the classification of the samples into categories. In Figure 5.2, the results of predictions made with two PLS models are shown, one for soot and one for diesel. As can be seen, samples with different soot concentrations are easily classified, with good accuracy. The diesel variations were harder to predict. The analysis would most likely be improved if lower frequencies could have been used. One drawback of the cell used in Paper V was the lack of temperature control. Therefore, a new cell was designed, including heaters and a temperature sensor. To simplify the sample application procedure, it was further designed as a flow cell. In Paper VI, this cell was used for quality evaluation of industrial cutting liquids. In this study, the frequency range was expanded (1 MHz–10 mHz), and the amplitude was much lower (40 mVrms ). The correlation between EIS data 5.3 Applications 39 1 Nitrite Oil pH Correlation 0.5 θ |Z| 0 -0.5 -1 10 6 -2 6 10 10 Frequency (Hz) 10 -2 Figure 5.3: Correlation of EIS data to cutting fluid (oil) concentration, pH, and nitrite level. The left part represents |Z| and the right part θ. See Paper VI for further details. 11 pH Concentration (% (w/w)) (A) 15 10 (B) 9.5 8 5 Samples Samples Figure 5.4: PLS training data for the cutting fluid concentration (A) and the pH level (B). Real values (no markers) are also shown. See Paper VI for further details. and the cutting fluid concentration, the pH, and the nitrite level is shown in Figure 5.3. It is evident that the concentration and the pH correlate well with the measured variables, and more importantly, that both parameters have different frequency dependencies. Interesting is also the fact that when the correlation for one parameter is zero, the correlation for the other is often quite high. This should provide the means to separate their individual contributions to the impedance. The nitrite level, however, correlated poorly to the impedance and was therefore not further evaluated. In Figure 5.4, the PLS training data for the concentration and pH is shown. The figures indicate the possibility of predicting both parameters simultaneously. If more samples were evaluated, model stability would most likely improve. This would also permit validations to be performed. However, the next step should in our opinion be an on-line analysis of real samples. 40 Alternating electric fields for chemical analysis 6 Future outlook One of the major conclusions reached during the work underlying this thesis, is that electric fields in their various forms are excellent tools for surface design. Static and alternating, they also allow the analysis of surface and interfacial properties, as well as chemical analysis of the bulk of a solution. The use of bipolar electrochemistry for the formation of surface gradients is a new and exciting method. In fact, in the last decade or so, many interesting applications of bipolar electrochemistry have been reported. It is especially the lack of an electrical contact to the electrode that makes this technique quite unique. We have shown that bipolar patterning can be used to create surface gradients of many different materials. Interestingly, the width, position, and geometry of the gradients can be controlled. The possibility to predict the bipolar behavior by simulations also allows the optimization of the experimental setup. This knowledge can be used to form new and exciting gradients, with varying functionalities and geometries. Ongoing work hence deals with bipolar patterning with spherical electric fields. Here, one of the feeder electrodes is placed above a substrate, inducing a circular reaction region on the bipolar electrode. With this, it is possible to create several local gradients on one surface. This approach could for instance be used to form an array of circular gradient spots on a single substrate. There are several ways to increase the widths of the gradients. If the bipolar electrode, for instance, is placed at an angle relative to the feeder electrodes, 41 42 Future outlook the gradient will be widened. Another interesting concept is to use two bipolar electrodes, connected to an external voltage source. 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