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1. Enhanced and unconventional responses in chemiresistive sensing devices for nitrogen dioxide and ammonia from carboxylated alkylthiophene polymers

   https://doi.org/10.1039/D0MH00049C  

   Wagner, Justine ; Jang, Hyun-June ; Han, Jinfeng ; Katz, Howard E. ( May 2020 , Materials Horizons)

   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.more »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.« less
2. Mixed-flow design for microfluidic printing of two-component polymer semiconductor systems

   https://doi.org/10.1073/pnas.2000398117  

   Wang, Gang ; Feng, Liang-Wen ; Huang, Wei ; Mukherjee, Subhrangsu ; Chen, Yao ; Shen, Dengke ; Wang, Binghao ; Strzalka, Joseph ; Zheng, Ding ; Melkonyan, Ferdinand S. ; et al ( July 2020 , Proceedings of the National Academy of Sciences)

   The rational creation of two-component conjugated polymer systems with high levels of phase purity in each component is challenging but crucial for realizing printed soft-matter electronics. Here, we report a mixed-flow microfluidic printing (MFMP) approach for two-componentπ-polymer systems that significantly elevates phase purity in bulk-heterojunction solar cells and thin-film transistors. MFMP integrates laminar and extensional flows using a specially microstructured shear blade, designed with fluid flow simulation tools to tune the flow patterns and induce shear, stretch, and pushout effects. This optimizes polymer conformation and semiconducting blend order as assessed by atomic force microscopy (AFM), transmission electron microscopy (TEM), grazing incidence wide-angle X-ray scattering (GIWAXS), resonant soft X-ray scattering (R-SoXS), photovoltaic response, and field effect mobility. For printed all-polymer (poly[(5,6-difluoro-2-octyl-2H-benzotriazole-4,7-diyl)-2,5-thiophenediyl[4,8-bis[5-(2-hexyldecyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-2,5-thiophenediyl]) [J51]:(poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}) [N2200]) solar cells, this approach enhances short-circuit currents and fill factors, with power conversion efficiency increasing from 5.20% for conventional blade coating to 7.80% for MFMP. Moreover, the performance of mixed polymer ambipolar [poly(3-hexylthiophene-2,5-diyl) (P3HT):N2200] and semiconducting:insulating polymer unipolar (N2200:polystyrene) transistors is similarly enhanced, underscoring versatility for two-componentπ-polymer systems. Mixed-flow designs offer modalities for achieving high-performance organic optoelectronics via innovative printing methodologies.
3. High resolution voltammetric and field-effect transistor readout of carbon fiber microelectrode biosensors

   https://doi.org/10.1039/D2SD00023G  

   Cho, Whirang ; Rafi, Harmain ; Cho, Seulki ; Balijepalli, Arvind ; Zestos, Alexander G. ( May 2022 , Sensors & Diagnostics)

   Rapid and sensitive pH measurements with increased spatiotemporal resolution are imperative to probe neurochemical signals and illuminate brain function. We interfaced carbon fiber microelectrode (CFME) sensors with both fast scan cyclic voltammetry (FSCV) and field-effect transistor (FET) transducers for dynamic pH measurements. The electrochemical oxidation and reduction of functional groups on the surface of CFMEs affect their response over a physiologically relevant pH range. When measured with FET transducers, the sensitivity of the measurements over the measured pH range was found to be (101 ± 18) mV, which exceeded the Nernstian value of 59 mV by approximately 70%. Finally, we validated the functionality of CFMEs as pH sensors with FSCV ex vivo in rat brain coronal slices with exogenously applied solutions of varying pH values indicating that potential in vivo study is feasible.
4. Low frequency electrochemical noise in AlGaN/GaN field effect transistor biosensors

   https://doi.org/10.1063/5.0014495  

   Paul Bertani, Yuji Wang ( July 2020 , Applied physics letters)

   Little has been studied on how the electrochemical noise impacts the limit of detection of field effect transistor (FET) biosensors. Herein, we investigate low frequency noise associated with phosphate-buffered saline (PBS) solutions at varying ionic strengths (Ni) under both weak and strong gate biases corresponding to saturation and sub-threshold regimes, respectively, in AlGaN/GaN heterojunction FET biosensors. We show that the electrochemical noise is strongly dependent on the ionic strength and gate biasing conditions. In the saturation regime (low bias), varying the ionic strength (a range of 10−6× PBS to PBS 1 × stock solutions used for testing) has little to no effect on the characteristic frequency exponent 𝛽(𝛽=1), indicating a predominately diffusion-based process. Conversely, under higher biases (sub-threshold regime), the β parameter varies from 1 to 2 with ionic strength exhibiting both diffusion and drift characteristics, with a “cut point” at approximately 10−5× PBS (𝑁𝑖≈9×1014/mL). Under a high bias, once the PBS concentration reaches 10−3×, the behavior is then drift dominant. This indicates that the higher bias likely triggers electrochemical reactions and by extension, faradaic effects at most physiologically relevant ionic strengths. The signal-to-noise ratio (SNR) of the device has an inverse linear relationship with the low frequency current noise. The device exhibits amore »higher SNR in the sub-threshold regime than in the saturation regime. Specifically, within the saturation regime, an inversely proportional relationship between the SNR and the ionic concentration is observed. The electrochemical noise induced from ionic activities is roughly proportional to 𝑁−1/2𝑖.« less
5. Determining the hydronium pKα at platinum surfaces and the effect on pH-dependent hydrogen evolution reaction kinetics

   https://doi.org/10.1073/pnas.2208187119  

   Zhong, Guangyan ; Cheng, Tao ; Shah, Aamir Hassan ; Wan, Chengzhang ; Huang, Zhihong ; Wang, Sibo ; Leng, Tianle ; Huang, Yu ; Goddard, William A. ; Duan, Xiangfeng ( September 2022 , Proceedings of the National Academy of Sciences)

   Electrocatalytic hydrogen evolution reaction (HER) is critical for green hydrogen generation and exhibits distinct pH-dependent kinetics that have been elusive to understand. A molecular-level understanding of the electrochemical interfaces is essential for developing more efficient electrochemical processes. Here we exploit an exclusively surface-specific electrical transport spectroscopy (ETS) approach to probe the Pt-surface water protonation status and experimentally determine the surface hydronium pK a = 4.3. Quantum mechanics (QM) and reactive dynamics using a reactive force field (ReaxFF) molecular dynamics (RMD) calculations confirm the enrichment of hydroniums (H 3 O + * ) near Pt surface and predict a surface hydronium pK a of 2.5 to 4.4, corroborating the experimental results. Importantly, the observed Pt-surface hydronium pK a correlates well with the pH-dependent HER kinetics, with the protonated surface state at lower pH favoring fast Tafel kinetics with a Tafel slope of 30 mV per decade and the deprotonated surface state at higher pH following Volmer-step limited kinetics with a much higher Tafel slope of 120 mV per decade, offering a robust and precise interpretation of the pH-dependent HER kinetics. These insights may help design improved electrocatalysts for renewable energy conversion.