Download SPR (Surface Plasmon Resonance) Chemical

An Introduction to Chemical and Biological Sensors
Denise Wilson, Associate Professor
Department of Electrical Engineering
University of Washington
NSF/RISE Workshop/Short Course
Development and Study of Advanced Sensors
and
Sensor Materials
July 10, 2006
Outline
• Classes of Chemical and Biological Sensors
–
–
–
–
Single Stage Transduction (Solid State I)
Multiple Stage Transduction (Solid State II)
Single Stage Transduction (Optical I)
Multiple Stage Transduction (Optical II)
• Performance Metrics
– Steady-state, short term
– Steady-state, long term
– Transient
Single Stage Transduction
(Solid State I)
• Class Definition: Biochemical Activity/Energy is
directly converted to Electrical Energy
– Electrochemical Sensors: Ion interaction (chemical
signal) with a functionalized (partially or fully selective)
material induces a separation of charges (electrical signal)
• Basic Electrochemical Cell:
– Potentiometric: voltage is measured at zero current draw
– Amperometric: current is measured at constant voltage
• Variations on the Electrochemical Cell:
– FET (Field Effect Transistor) Based Devices
– LAPs (Light addressable potentiometric sensors)
– Conductivity Based Sensors (Chemiresistors): adsorption
or absorption of molecules onto a functionalized (partially
or fully selective) material induces a change in resistance
Single Stage Transduction
(Solid State I)
• Basic Electrochemical Cell
Working
Electrode
Measurement
Circuit
Reference
Electrode
Basic Operation:
• Ions exchange electrons with the metal electrode:
• With the reference electrode: in a controlled and constant way
• With the working electrode: in a manner dependent on analyte concentration
• The separation of charges is the electrical signal that represents the (bio) chemical
signal at the electrode(s)
Single Stage Transduction
(Solid State I)
• Basic Electrochemical Cell: Signal Measurement
– Potentiometric: In the presence of zero current, the voltage induced
by the separation of charges (between working and reference
electrode) is measured.
• Governed by the Nernst Equation
• Voltage = E1o - E2o + (RT/nF) ln ([C1]/[C2])
– E1o = standard electrode potential of ion interacting with electrode 1
– E2o = standard electrode potential of ion interacting with electrode 2
– C1 = concentration of ion 1 at electrode 1
•
•
•
•
– C2 = concentration of ion 2 at electrode 2
Demonstrates no scaling penalty (same sensitivity for smaller devices)
Reference electrode materials and surrounding solutions are chosen for
stability (eg. Ag/AgCl)
Working electrode materials are designed or chosen for analyte selectivity
Selectivity controlled through: electrode design or permeable membranes
Single Stage Transduction
(Solid State I)
• Basic Electrochemical Cell: Signal Measurement
– Amperometric: In the presence of a controlled voltage, the current
between working and reference electrodes is measured.
• Ions are consumed in the measurement process (non-equilibrium)
• Current:
– Dependent on conductivity of solution
– Responds only to ions whose redox potential < applied potential
Current
Applied Potential
• Possesses inherent scaling (miniaturization) penalty that reduces
sensitivity and sensor performance.
• Reference and working electrode materials chosen on same basis as
potentiometric operation
• Selectivity controlled through:
– Electrode design or permeable membranes or applied voltage
Single Stage Transduction
(Solid State I)
• Variations on the Basic Electrochemical Cell: ChemFET
–
–
–
–
–
Potentiometric measurement is inherent in FET structure (zero gate current)
Ions interact with interface between gate and insulator layer
Induced charge is “seen” as field effect in underlying FET channel
Conductive gate is used to provide an “electrical” reference electrode
Example:
• NMOSFET
• Analyte of interest = negative ion
• Increased concentration = higher induced negative surface charge which must be
overcome for FET to conduct current = higher threshold voltage
Conductive Gate
FET Insulator
n+
Source
Negative Ions
++++
p
n+
Drain
Positive (induced) charge
Single Stage Transduction
(Solid State I)
• Variations on the Basic Electrochemical Cell: ISFET
– Gate/Insulator combination layers that provide desired selectivity are
limited to a few analytes (e.g. palladium/oxide for measuring hydrogen)
– A wider selection of insulator layers with desired selectivity is possible.
– Removal of the “gate” reintroduces need for a reference electrode
– Reference electrode in same sensing solution introduces instability
– Vulnerability of insulator layer to trapped charge introduces drift.
– Example:
• N-ISFET with a negative ion of interest (same basic operation as ChemFET)
Reference Electrode
FET Insulator
Source
Negative Ions
n+
++++
p
n+
Drain
Positive (induced) charge
Single Stage Transduction
(Solid State I)
• Variations on the Basic Electrochemical Cell: ENFET
– Immobilized enzyme attached to surface of insulator in ISFET structure:
• Increases sensitivity (through catalytic enzyme activity)
• Increases selectivity (through inherent functionalization of selected enzyme)
– Insulator is partially protected from trapped charge (by enzyme layer)
– ENFET retains many of the disadvantages of the ISFET structure
Reference Electrode
FET Insulator
Source
n+
Ions of interest
Immobilized Enzyme
++++
p
n+
Drain
Induced Charge
Single Stage Transduction
(Solid State I)
• Variations on the Basic Electrochemical Cell: LAPs
– Light, rather than the FET, structure is used to amplify the induced charge
from electrochemical interactions at the sensor surface
Requires an external light source and optics for operation
Eliminates need for drain and source electrodes
A single “gate” is required for an entire array of sensors
Active sensing area is determined by illumination area.
I
p
Reference Electrode
Insulator
Induced Charge
Conductive Gate
PhotoCurrent
•
•
•
•
pH 7
pH 5
Bias Voltage
Transparent Substrate
AC Light Source
pH 9
Single Stage Transduction
(Solid State I): pH ChemFETs
pH
Sensor
(8 Total)
The sensors are designed and
fabricated in a standard CMOS
process (1.5 micron AMI). Various
combinations of Silicon Nitride and
Aluminum oxide, native to the
CMOS process are used for pH
sensitivity.
These designs are extensions of
designs and results initially
presented by:
J. Bausells, J. Carrabina, A.
Errachid, A. Merlos, “Ionsensitive field-effect
transistors fabricated in a
commercial CMOS
technology,” Sensors &
Actuators B – Chemical, 7
Sept 1999, B57(1-3), 56-62.
Single Stage Transduction
(Solid State I): pH ChemFETs
M2FET
The M2FET uses the second layer of
aluminum in a standard CMOS
process for pH sensitivity.
Aluminum oxide grows on top of the
aluminum layer when exposed to an
oxygen environment, providing a pH
sensitive layer that is then measured
via the underlying field effect (MOS)
structure
Single Stage Transduction
(Solid State I): pH ChemFETs
M2FET Characteristic Curve (Vgs = 1.2)
1.4
Channel Current (mA)
1.2
1
0.8
pH = 5
pH = 6
0.6
0.4
0.2
Difference in Channel Current between pH5 and pH6
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0
Drain Voltage (V)
-0.0001
2
3
4
5
6
-0.0002
M2FET:
-0.0003
Current (mA)
The results show sub-microamp differences
between the drain currents of the FET when
operated in the linear (ohmic) region.These
changes in drain current reflect changes in
threshold voltage of the FET induced by
accumulation of charge (OH- ions) on top of
the aluminum oxide layer.
1
-0.0004
-0.0005
Difference
-0.0006
-0.0007
-0.0008
-0.0009
-0.001
Drain Voltage
Single Stage Transduction
(Solid State I): pH ChemFETs
These sensors use the overlying
passivation layer (Silicon Nitride) as the
pH sensitive layer (the bottom structure is
an interdigitated version of the top
structure)
Single Stage Transduction
(Solid State I)
• Conductivity-Based Sensors (Chemiresistors)
– Composite Polymer
• Combination of stable, conductive material (e.g. carbon black) and
• Chemically sensitive, insulating material
– Molecule in sensing environment adsorbs into polymer sensor
– Carbon black molecules “stretch”
– Conductivity decreases; resistance increases
• Highly sensitive to humidity (caused polymer to swell) http://nsl.caltech.edu/resnose.html
Single Stage Transduction
(Solid State I)
• Conductivity-Based Sensors (Chemiresistors)
– Metal-Oxide Sensors
• Composed of metal-oxide and a catalyst to enhance selectivity
–
–
–
–
–
–
–
Oxygen naturally binds to the sensor surface
Binding processes extract electrons from the bulk
Analytes of interest in the sensing environment bind with Oxygen
Electrons are re-injected into the bulk
Conductivity increases (resistance decreases)
Type of oxygen (and resulting sensitivity) is dependent on temperature
Sensors are highly sensitive to humidity (which blocks binding sites)
O2- O2- O2- O2- O2- O2-
-
-
Tin-Oxide with no Analyte Present
O2-
-
O2- - - -
Tin-Oxide with Analyte Present
Multiple Stage Transduction
(Solid State II)
• Class Definition: Biochemical Activity/Energy is converted to an
intermediate form of energy before being converted to Electrical
Energy
– Example: Acoustic Wave Devices
• Basic Operation: Input path
– an AC voltage is used to piezoelectrically convert
– Electrical energy to
– Mechanical energy (a pressure wave) propagating through a material
• Basic Operation: Output path
– Loss (attenuation) and
– Delay (phase changes) of mechanical energy (the pressure wave) are affected by:
• Viscoelastic properties of the material and
• Overall mass of the active sensing area which is
• Responsive to Biochemical binding and events on the surface of the sensor
– Altered mechanical energy is converted back to Electrical Energy
Multiple Stage Transduction
(Solid State II)
• Classes of Acoustic Wave Sensors
• Bulk Acoustic Wave: basic operation
• Wave moves parallel to the surface of the sensor and
• Permeates through the entire material
• Bulk Acoustic Wave: characteristics
•
•
•
•
Poor Sensitivity: only surface wave responds to biochemical activity
Stable: on a quartz substrate
Relatively Inexpensive
Strong cross-sensitivity to temperature
Direction of Propagation
Electrodes
Direction of Motion
Multiple Stage Transduction
(Solid State II)
• Classes of Acoustic Wave Sensors
• Surface Acoustic Wave: basic operation
• Wave moves perpendicular to the surface of the sensor and
• Propagates only on the surface of the material
• Surface Acoustic Wave: characteristics
•
•
•
•
Frequencies of operation on the order of 100’s of MHz and GHz
High Sensitivity: surface wave is large part of overall activity
Poorly suited to measurement in liquid (due to perpendicular propagation)
Relatively Expensive (high measurement overhead)
Direction of Motion
Electrodes
Direction of Propagation
Multiple Stage Transduction
(Solid State II)
• Classes of Acoustic Wave Sensors
• Shear Horizontal Surface Acoustic Wave
•
•
•
•
Wave moves parallel to the surface of the sensor and
Propagates only on the surface of the material
Difficult to fabricate: requires careful control of cutting piezoelectric material
Can be used in liquids
• Shear Horizontal Acoustic Plate Mode
•
•
•
•
Electrodes on one side of a plate generate a
Pressure wave that is guided through the bulk of the material
To the other side of the plate where the sensing environment is located
Across the plate and back down to the electrodes on the opposite side of the plate
•
•
•
•
Wave moves parallel to the surface of the sensor
Electrodes and electronics are protected from the sensing environment
Moderate sensitivity: active area is not “optimal” percentage of overall sensor volume
Sample Application: detect mercury levels in drinking water
Single Stage Transduction
(Optical I)
• Class Definition: Optical energy (which is directly related to biochemical
behavior) is directly converted to Electrical Energy
– Luminescence Spectroscopy: light emitted by a system is a function of
biochemical activity/energy
• Chemiluminescence: emission light represents biochemical reactions
• Fluorescence: emission light quickly (on the order of ns) indicates
presence of particular molecules
– Electrons are excited into singlet state (paired with opposite spin
electron) by incoming light; radiate energy in the form of light upon
returning to ground state
• Phosphorescence: emission light slowly (on the order of ms) indicates
presence of particular molecules
– Electrons are excited into triplet states (paired with same spin electron) by
incoming light; radiate energy in the form of light on returning to ground state
but take longer because the triplet state is forbidden.
– Fluorescence/Phosphorescence Spectroscopy
• Input Energy: optical
• Output Energy: optical converted to electrical through a photodetection system
Single Stage Transduction
(Optical I)
5uG Spectral Results with 3mm Slit
• Fluorescence Analysis
Systems
500
Xenon
LED 20mA
LED 50mA
400
Relative Intensity
– Regardless of the
“color” of light
input, the output
(emission) spectrum
is the same.
– The spectrum
(“color”) of the input
light influences only
efficiency of the
emission
600
300
200
100
0
450
470
490
510
530
Wavelength (nm)
550
570
590
Single Stage Transduction
(Optical I)
Traditional Fluorescence Analysis Systems
White Light
Source
Conventional
Optics
Optical
Dispersion
Sample
Photomultiplier
Tube
Conventional
Optics
Signal
Processing
Single Stage Transduction
(Optical I)
Portable Fluorescence Analysis Systems
LED Array
Fiber Optic
Coupling
Optical
Dispersion
Sample
Photodiode
Array
Fiber Optic
Coupling
Signal
Processing
Single Stage Transduction
(Optical I): Fluorescence Analysis
Optimization Schemes for LED-based (miniaturized) systems
4.6
2
4.5
1.8
4.4
1.6
4.3
1.4
In this case, input light is
transmitted (inadvertently) to
the output path; thus
optimization involves reducing
the overlap between excitation
(input) light and the emission
spectrum. Optimization
criteria depend on system
configuration and
experimental constraints.
4.2
1.2
1
4
0.8
3.9
0.6
3.8
0.4
3.7
0.2
1
1.5
2
2.5
3
3.5
4
4.5
5
1
Ileak
Iemitted
1.2
4.1
3.6
1.4
Ileak
Iemitted
0.8
0.6
0.4
0.2
0
350
400
450
500
550
600
650
700
(a) “Best” Configuration
0
350
400
450
500
550
600
650
700
(b) “Worst” Configuration
Multiple Stage Transduction
(Optical II)
• Class Definition: Biochemical Activity/Energy is converted to
Optical Energy before being converted to Electrical Energy
– Surface Plasmon Resonance
• Biochemical events/analytes of interest influence refractive index of the
sensing environment very close (within 100nm) of the SPR surface (in the
evanescent field).
• Refractive index influences how input light is reflected back to the output
path (as a function of incidence angle or wavelength, depending on the
system configuration.
• Transduction Path:
–
–
–
–
Light Energy is converted to
Surface Charge Oscillations (surface plasmons)
The unconverted Light Energy is then converted to
Electrical Energy (via Photodetection) in the output path
Multiple Stage Transduction
(Optical II): SPR
Sample
Metal
Surface Plasma Wave
Evanescent
Wave
θinc
Substrate
Incident
Light
Optical Fiber w/ Cladding
Reflected
Light
Gold Coating
Exposed Core
When the wave vector
from a white light source
closely matches that of
the surface plasma wave
at the metal-sample
interface on the probe,
reflected light is
significantly attenuated
(compared to the
attenuation in the
reference media where
no analyte is present)
Multiple Stage Transduction
(Optical II): SPR
• The Big Picture
– Why SPR?
•
•
•
•
•
Highly sensitive (10-4 to 10-6 RI units)
Very local (10-100nm from sensing surface)
Directly indicative (of interactions between sensor and environment)
Relatively unencumbered by sampling overhead (e.g. tagging, mixing, etc)
Readily referenced to compensate for background fluctuations (e.g. drift)
– How is it used (SPR = transduction mechanism)?
• Non-functionalized = bulk refractive index
• Functionalized = specific analytes
– The Full Spectrum of SPR-based instruments
• User-Intensive, Single Measurements: Biacore
• User-Intensive, Single Field Measurements: TI Spreeta (Chinowsky/Yee)
• Distributed and Autonomous, Multiple Measurements:
–
–
–
Insertion-based probes
Compact signal processing
Streamlined, robust optical path
Multiple Stage Transduction
(Optical II): SPR
• Point of resonance can be
detected at a
– Particular angle (constant
wavelength interrogation)
– Particular wavelength (constant
angle interrogation)
• Constant Angle
– Polychromatic light source at
constant angle of incidence
• Constant Wavelength
– Monochromatic light source at
different angles of incidence
Constant Angle is chosen here for: inexpensive light source, easy
alignment, and simpler, more compact configuration (= less overhead)
Multiple Stage Transduction
(Optical II): SPR
Sampling Options:
• In-line
• “Dip” insertionbased probe
The probe configuration is :
• easily replaced, easy to use
• Less prone to sensor layer blocking,
but can be
• more sensitive to ambient fluctuations
• more susceptible to fouling
Multiple Stage Transduction
(Optical II): SPR
Air
Increasing RI
Raw Data (background overwhelms resonance)
Referenced Data (Resonance is evident)
Multiple Stage Transduction
(Optical II): SPR
Multivariate
Calibration
Approach #1 (Traditional)
Software
High Resolution
Photodetection
Communication/
ADC Overhead
Measurement to
Reference Ratio
Approach #2 (Voltage-Mode, Partially Integrated)
Low Resolution
Photodetection
Integration Time
Programming
“Flatlining”
Reference Ratio
High Resolution
Regression
Multivariate
Calibration
Software
Low Resolution
Regression
Multiple Stage Transduction
(Optical II): SPR
Approach #3 (Pulse-Mode, Fully Integrated)
Multivariate
Calibration
Software
Low Resolution
Photodetection
“Flatlining”
Current Scaling
Conversion to
Pulse Mode
Approach #4 (Current-Mode, Fully Integrated)
Low Resolution
Regression
Multivariate
Calibration
Software
Low Resolution
Photodetection
Dark Current
Compensation
“Flatlining”
Current Scaling
Low Resolution
Regression
Multiple Stage Transduction
(Optical II): SPR
Approach #2
All Designs are
mixed signal,
fabricated in
standard CMOS
Approach #4
6 /12...
6 /6
6 /6
6 /12...
6 /6
6 /6
4 /4
Sp_0
Sp_1
4 /4
Sp_7
Vdd
6 /9
18/6
18/6
18/6
18/6
Hold
6/6
6 /9
Vbuff
4 /4
6/6
6/12
Chold
Vi Σ
Prec harg e
ViΥ
6/12
18/6
18/6
18/6
6/6
6/6
18/6
Vi
Vi
Vref
6/6
Vcomp
6/6
Vbias
6/6
Approach #3
Vset
Idark
Mdark*Idark
(a)
C
15/6
6/6
6/6
6/6
6/6
Multiple Stage Transduction
(Optical II): SPR
Pixel
Analog
Sampling
Digital
Control
2mm
Phototransistor
15 pixel array
fabricated on a
1cm2 die in the
1.5 micron AMI
process through
MOSIS
Multiple Stage Transduction
(Optical II): SPR
Approach
SOC
Integration
Size
(l X l )
Traditional
None
Big
Voltage Mode
Partial
200 X 1800
Pulse Mode
Full
200 X 1200
Current Mode
Full
200 X 1000
Current Mode
Approach
Traditional
Voltage Mode
Pulse Mode
Prediction Error
6.07%
6.05%
7.8%
RI Resolution
5 X 10-4
2 X 10-4
6 X 10-4
Performance Metrics
Chemical and Biological Sensors
• Steady State, Short Term
– Sensitivity: Change in output/Change in Input
• Chemiresistor; DR/DC
• SPR: Dl/DRI
– Resolution:
• Input: smallest detectable change in input parameter
• Output: smallest detectable change in output parameter
– Offset: output in the presence of no input
– Detection limit: minimum detectable change
– Dynamic range: maximum - minimum detectable (input) signal
– Signal to Noise Ratio: ratio of useful information to non-useful information at
any given signal level
– Selectivity: practical ratio of sensor response to (a) analyte of interest divided
by response to (b) most significant interferent.
Performance Metrics
Chemical and Biological Sensors
• Steady State, Long Term
– Stability: Change in output for a constant input over a period of time several
orders of magnitude greater than the response time of the sensor.
– Drift: Loose term often used in place of stability -- means the non-monotonic
change in sensor output over time in response to the same input conditions.
• Transient
– Hysteresis: maximum difference in output obtained when sensor input is
increased as compared to sensor input decreased.
– Response time:
• Propagation delay: how long before a change in the sensing environment
is reflected in a change in the sensor output (delay between a 50% change
in the input and a 50% change in the output of the sensor is typically
chosen)
• Rise/Fall time: time required for the sensor output to rise (or fall) from
10% of its final output to 90% of its final output.