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  1. Abstract

    The sensing properties of poly 3‐(3‐carboxypropyl) thiophene‐2,5‐diyl (PT‐COOH) and hydroxylated polythiophene (PT‐OH) as bioreceptor layers were studied and are discussed in this paper. The polymer films cover the channel region of the OECT devices and anti‐human IgG was immobilized on the polymer films. We use threshold voltage (Vth) change as a sensing signal to detect the interaction between anti‐human IgG and human IgG. By adding different concentrations of human IgG, Vth difference can be observed on anti‐human IgG immobilized polymer films, with optimized detection from a blend of the two polymers. Open circuit potential (OCP) measurement was also done on the OECT devices based on the same anti‐human IgG and human IgG interaction pair to help us understand the mechanism behind the antibody functionalization and the interaction between antibody and antigen. Importantly, the observed positive OCP change for the PT‐OH system was self‐consistent with the negative OECT Vth change that was obtained, since the latter is applied to the gate while the former is measured at the channel.

     
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  2. Abstract

    A systematic analysis is used to understand electrical drift occurring in field‐effect transistor (FET) dissolved‐analyte sensors by investigating its dependence on electrode surface‐solution combinations in a remote‐gate (RG) FET configuration. Water at pH 7 and neat acetonitrile, having different dipoles and polarizabilities, are applied to the RG surface of indium tin oxide, SiO2, hexamethyldisilazane‐modified SiO2, polystyrene, poly(styrene‐co‐acrylic acid), poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), and poly [3‐(3‐carboxypropyl)thiophene‐2,5‐diyl] (PT‐COOH). It is discovered that in some cases a slow reorientation of dipoles at the interface induced by gate electric fields causes severe drift and hysteresis because of induced interface potential changes. Conductive and charged P3HT and PT‐COOH increase electrochemical stability by promoting fast surface equilibrations. It is also demonstrated that pH sensitivity of P3HT (17 mV per pH) is an indication of proton doping. PT‐COOH shows further enhanced pH sensitivity (30 mV per pH). This combination of electrochemical stability and pH response in PT‐COOH are proposed as advantageous for polymer‐based biosensors.

     
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  3. We investigated the enhanced vapor responses and altered response ratios of a series of thiophene (co)polymers with oxygenated side chains (CH 2 OH, linear polyethylene glycol, and crown ether), including the novel poly(3-hydroxymethylthiophene) (PTOH) and other newly synthesized polymers. Hydroxymethyl-containing copolymers had higher mobility compared to poly(3-hexylthiophene) (P3HT). The larger crown ether moiety promotes transistor characteristics of P3HT while the smaller one impairs them. Incorporating different oxygen bearing functionalities increased responses of thiophene polymers to NO 2 , NH 3 , and acetone. For example a polyether side chain increases the NO 2 response sensitivity of copolymers of both P3HT and PTOH, but sensitivity towards gas analytes was more prominent for glycol-based functionalities rather than the crown ethers. PTOH is very sensitive to NO 2 and the response likely includes a contribution from conductive protons on the OH group. The lack of correlation among the rank-ordered gas sensitivities imparted by each functional group was found to be useful for designing a selective sensor array. We specifically showed high classification accuracy for all the polymer responses to NO 2 and acetone vapors, both of which gave increased device currents but with response ratios different enough to allow highly classifying discriminant functions to be derived. 
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  4. Polythiophenes with differently functionalized side chains (alkyl, oligoethylene oxide, ester, hydroxy, and carboxylic acid) and varied counterions of potassium salt electrolytes were investigated in organic electrochemical transistors (OECTs). In addition, mixed blends were investigated to evaluate any synergistic effects between functionalities. Depending on the functional moiety attached, a large shift to lower potentials of Vth, an increase in drain current, and increase in transconductance can be observed compared to the base combination of alkyl side chain and Cl-. The newly designed and synthesized hydroxy polymer displayed stability to large shifts in VTH, slight increase in drain current, and little or no increase in transconductance when an ionic radius of the dopant is increased until a much larger anion, large polarizability, and low hydration number such as TSFI- was used. The acid-functionalized polymer, on the other hand had the same magnitude in shift with respect to any anion that is larger than Cl-. The polymers were characterized by spectroscopy, x-ray diffraction, thermal analysis, and cyclic voltammetry. This work demonstrates that side-chain engineering can have substantial difference in the level of interaction in the electrolyte which would require tailoring the ion for specific polymer interactions. 
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  5. A carboxylated thiophene polymer-based chemiresistive device in a field-effect transistor (FET) configuration with unusual and enhanced responses to the widespread pollutants nitrogen dioxide (NO 2 ) and ammonia (NH 3 ) is described. The device based on a polymeric thiophene carboxylic acid showed a dramatic and superlinear increase in drain current ( I D ) of over 15 000% to a ramped exposure to 10 ppm NO 2 over several minutes, while its ethyl ester counterpart had significantly lower response. Devices incorporating either an ester or carboxylic acid displayed comparable and previously unreported increases in I D from 10 ppm ramped NH 3 exposure of 200–300%. Conventional poly(alkylthiophenes) showed the expected current decreases from similar NH 3 exposures. Using threshold voltage shifts in silicon transistors coupled to our recently reported remote gate (RG) platform with thiophene polymer coatings, we determined that two differing response mechanisms are associated with the two gas exposures. By calculating the charge density induced in the polymers by NO 2 exposure using the silicon transistor voltage shifts, we conclude that proton conduction contributes significantly to the high sensitivity of the carboxylic acid to NO 2 , in addition to doping that was observed for all four polymers. Furthermore, hydrogen bonding moieties of the carboxylic acid and ester may be able to physisorb NH 3 and thus alter the charge distribution, rearrange polymer chains, and/or create a proton transfer network leading to the I D increase that is the opposite of the response obtained from non-carboxylated thiophene polymers. 
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  6. We summarize our recent results on material, device, and circuit structures for detection of volatile analytes in the atmosphere and proteins in aqueous solution. Common to both types of sensing goals is the design of materials that respond more strongly to analytes of interest than to likely interferents, and the use of chemical and electronic amplification methods to increase the ratio of the desired responses to the drift (signal/noise ratio). Printable materials, especially polymers, are emphasized. Furthermore, the use of multiple sensing elements, typically field-effect transistors, increases the selectivity of the information, either by narrowing the classes of compounds providing the responses, distinguishing time-dependent from dose-dependent responses, and increasing the ratio of analyte responses to environmental drifts. To increase the stability of systems used to detect analytes in solution, we sometimes separate the sensing surface from the output device in an arrangement known as a remote gate. We show that the output device may be an organic-based or a silicon-based transistor, and can respond to electrochemical potential changes at the sensing surface arising from a variety of chemical interactions. 
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