1. 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
Sensors for Measuring Carbon Dioxide, Bicarbonate, and pH in the Ocean

2. Comprehensive Solutions Require Big Data
Argo floats: examples of a step toward Trillion Sensors
Spatial and temporal variability in concentrations of chemical
Under-sampling due to limitation of sensors is problematic

3. Low Power Sensors with Minimal Complexity
Satlantic SeaFETTM
Honeywell Durafet®
Argo, UCSD
High operational complexity or cost of maintenance
Dr. Kenneth Johnson is the inventor of SeaFET
“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.

4. 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

5. Differential Amplifier for Stability/Sensitivity
Current Mirror
Chemically-active (Sense) and ocean reference (Ref) electrodes
Minimal electronics, current-based transduction to reduce noise

6. 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

7. 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:

8. Receptor Designed to Bind Carbonate
Redox active scaffold
Recognition domain
Cl- receptor
HCO3- receptor
Challenge: differentiation between ions in sea water
Dual-binding site leads to selective binding of bicarbonate (HCO3-)

9. Receptor Designed to Bind Carbonate
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

10. 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
(left) Two prospective phenols that can be attached as shown to graphene and are expected to ionize at around pH ≈ 7. (right) Illustration how ionization of the units creates negative charge on the aromatic ring. The resonance structure reveals that there will be significant charge density at the carbon attached to the graphene and thereby raise its Fermi level.
To produce sensors that respond to pH we will pursue a two-pronged approach. In one case we will attach molecules with the proper acidity to ionize at pH ≈ 7 to graphene sheets (Figure 11). These can be attached by a direct linkage between the aromatic rings so that they can optimally influence the Fermi level with ionization. Upon ionization of the OH attached to the aromatic ring, the system will donate electron density into the graphene as shown. Attachment of these functional systems to graphene will be afforded by new methods developed at MIT.

11. 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
We will also make use of a system previously developed by Swager based upon polypyrrole (Figure 12). Although simple forms of polypyrrole, and another polymer polyaniline, are both known to be sensitive to pH effects, they display a highly irreversible pH responses that will not allow for continuous monitoring. In contrast the elaborated structure shown in Figure 12 displays perfect reversibility over many protonation and deprotonation cycles. One reason for this behavior are that the polymer has a more precise backbone that is incapable of undergoing crosslinking reactions that are prevalent in many forms of polypyrrole and polyaniline. Additionally, the 3-D canopied structure, by design, creates an environment wherein there is sufficient porosity in the polymer network for facile water and proton diffusion. Polypyrrole is an intrinsic organic semiconductor that can be oxidized to have a large amount of charge. Specifically, the polymer can be oxidized to levels where there is as much as charge on each repeating unit. The acidity (pKa) of the H-N groups in the polymer is a function of the degree of oxidation. Protonation and deprotonation processes also result in dramatic changes in the conductance of the material as well as changes in its Fermi levels. Hence, these materials will be investigated in chemiresistive sensor modes as well as pH responsive coatings on gate electrodes. The polymers are best deposited directly on electrodes by an electrodeposition process and are intrinsically insoluble, which will prevent delamination. Our previous studies showed the resulting films to be highly robust and did not fatigue even with extended pH cycling between pH 3-9. It is possible to entrain polymeric counter anions in the films during the synthesis and polymer dispersions may also be prepared in this way in a similar process used to produce commercial products CleviosTM and BaytronTM, dispersions based on PEDOT (Figure 5) used widely in the organic electronics area. We expect that we can vary the pKa to be maximally responsive by preparing materials in a fixed oxidation state. A polymeric counterion can further assist in locking this state and produce a material that will resist oxidation or reduction from external sources (i.e. microbes or biological materials) that may exist in the ocean environment

12. 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)

13. Thank you!
Questions?