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Despite significant progress in solution‐processing of 2D materials, it remains challenging to reliably print high‐performance semiconducting channels that can be efficiently modulated in a field‐effect transistor (FET). Herein, electrochemically exfoliated MoS₂nanosheets are inkjet‐printed into ultrathin semiconducting channels, resulting in high on/off current ratios up to 10³. The reported printing strategy is reliable and general for thin film channel fabrication even in the presence of the ubiquitous coffee‐ring effect. Statistical modeling analysis on the printed pattern profiles suggests that a spaced parallel printing approach can overcome the coffee‐ring effect during inkjet printing, resulting in uniform 2D flake percolation networks. The uniformity of the printed features allows the MoS₂channel to be hundreds of micrometers long, which easily accommodates the typical inkjet printing resolution of tens of micrometers, thereby enabling fully printed FETs. As a proof of concept, FET water sensors are demonstrated using printed MoS₂as the FET channel, and printed graphene as the electrodes and the sensing area. After functionalization of the sensing area, the printed water sensor shows a selective response to Pb²⁺in water down to 2 ppb. This work paves the way for additive nanomanufacturing of FET‐based sensors and related devices using 2D nanomaterials.

 

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. 

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. 

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.