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Sensors for Measuring Carbon Dioxide, Bicarbonate, and pH in the Ocean Prof. Timothy M. Swager (Chemistry) Prof. Jeffery Lang (EECS) Dr. Suchol Savagatrup Ms. Vera Schröder Massachusetts Institute of Technology MIT Sea Grant November 29, 2016 Comprehensive Solutions Require Big Data http://www.argo.ucsd.edu/ Spatial and temporal variability in concentrations of chemical Under-sampling due to limitation of sensors is problematic 2 Low Power Sensors with Minimal Complexity Honeywell Durafet® Satlantic SeaFETTM Argo, UCSD High operational complexity or cost of maintenance “Simple sensor systems that are capable of operating without drift for long periods of time and that are sufficiently selective and sensitive are not yet available…” Johnson et al. Chem. Rev. 2007, 107, 623. 3 Ion-Selective Field-Effect Transistors (i-FETs) Chemiresistors afford the simplicity i-FET Sensors offer additional dimensions of sensing Highly specific molecular recognition (significant departure from Ion Sensitive FETS) and simplicity in operation 4 Differential Amplifier for Stability/Sensitivity Current Mirror Chemically-active (Sense) and ocean reference (Ref) electrodes Minimal electronics, current-based transduction to reduce noise 5 Change in the Fermi Level due to Ion Binding Changes in Redox potential, not just electrostatic Anion binding (e.g., HCO3–) raises the Fermi level Protonation lowers the Fermi level 6 Receptor Design to Bind Carbonate Transduction materials composed of two elements: Redox active scaffold: Recognition domain with high degree of preorganization designed to bind the ion of interest: 7 Receptor Designed to Bind Carbonate Redox active scaffold Cl- receptor Recognition domain HCO3- receptor Challenge: differentiation between ions in sea water Dual-binding site leads to selective binding of bicarbonate (HCO3-) 8 Receptor Designed to Bind Carbonate O O Cl NH 2 NH 2 O R N S or NH 2 -2 Cl S S Cl CO3H-H+ NH 2 Organized geometry arranged to bind carbonate ions Binding of carbonate (CO32-) will favor oxidation of thianthrene unit; differentiation over nitrate (NO3-) R-group offers opportunity for immobilization of receptor through polymerization 9 Measuring pH – functionalized Graphene Prospective pH active units Resonance structure of ionized unit Two examples of phenol groups that are expected to ionize around pH ≈ 7 Ionization of the OH-group attached to the aromatic ring leads to donation of electron density into graphene/CNT 10 Measuring pH – Conducting Polymer Canopied polypyrrole Highly reversible pH response in conductivity and electroactivity. Can be used as chemiresistor or pH responsive coating Insoluble and stable under harsh conditions resulting in highly robust thin films 11 Challenges and Possible solutions Harsh conditions and drift (immobilization of receptor on electrode through polymerization) Drifts and fouling of the sensors (Each i-FET has a internal reference, with the current mirror in the circuit) Selectivity towards specific ions in the complex electrolyte system (receptor with high degree of pre-organization) 12 Thank you! Questions? 13