A Study of the Drift Phenomena of Gate-Functionalized Biosensors and Dual-Gate-Functionalized Biosensors in Human Serum.

In this paper, we study the drift behavior of organic electrochemical transistor (OECT) biosensors in a phosphate-buffered saline (PBS) buffer solution and human serum. Theoretical and experimental methods are illustrated in this paper to understand the origin of the drift phenomenon and the mechanism of ion diffusion in the sensing layer. The drift phenomenon is explained using a first-order kinetic model of ion adsorption into the gate material and shows very good agreement with experimental data on drift in OECTs. We show that the temporal current drift can be largely mitigated using a dual-gate OECT architecture and that dual-gate-based biosensors can increase the accuracy and sensitivity of immuno-biosensors compared to a standard single-gate design. Specific binding can be detected at a relatively low limit of detection, even in human serum.

Polymeric Semiconductor in Field-Effect Transistors Utilizing Flexible and High-Surface Area Expanded Poly(tetrafluoroethylene) Membrane Gate Dielectrics.

Organic field-effect transistors (OFETs) were fabricated using three high-surface area and flexible expanded-poly(tetrafluoroethylene) (ePTFE) membranes in gate dielectrics, along with the semiconducting polymer poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl)-alt-(2,2':5',2″:5″,2‴-quaterthiophen-5,5‴-diyl)] (PDPP4T). The transistor behavior of these devices was investigated following annealing at 50, 100, 150, and 200 °C, all sustained for 1 h. For annealing temperatures above 50 °C, the OFETs displayed improved transistor behavior and a significant increase in output current while maintaining similar magnitudes of Vth shifts when subjected to static voltage compared to those kept at ambient temperature. We also tested the response to NO2 gas for further characterization and for possible applications. The ePTFE-PDPP4T interface of each membrane was characterized via scanning electron microscopy for all four annealing temperatures to derive a model for the hole mobility of the ePTFE-PDPP4T OFETs that accounts for the microporous structure of the ePTFE and consequently adjusts the channel width of the OFET. Using this model, a maximum hole mobility of 1.8 ± 1.0 cm2/V s was calculated for the polymer in an ePTFE-PDPP4T OFET annealed at 200 °C, whereas a PDPP4T OFET using only the native silicon wafer oxide as a gate dielectric exhibited a hole mobility of just 0.09 ± 0.03 cm2/V s at the same annealing condition. This work demonstrates that responsive semiconducting polymer films can be deposited on nominally nonwetting and extremely bendable membranes, and the charge carrier mobility can be significantly increased compared to their as-prepared state by using thermally durable polymer membranes with unique microstructures as gate dielectrics.

Selective Detection of Functionalized Carbon Particles based on Polymer Semiconducting and Conducting Devices as Potential Particulate Matter Sensors.

This paper reports a new mechanism for particulate matter detection and identification. Three types of carbon particles are synthesized with different functional groups to mimic the real particulates in atmospheric aerosol. After exposing polymer-based organic devices in organic field effect transistor (OFET) architectures to the particle mist, the sensitivity and selectivity of the detection of different types of particles are shown by the current changes extracted from the transfer curves. The results indicate that the sensitivity of the devices is related to the structure and functional groups of the organic semiconducting layers, as well as the morphology. The predominant response is simulated by a model that yielded values of charge carrier density increase and charge carriers delivered per unit mass of particles. The research points out that polymer semiconductor devices have the ability to selectively detect particles with multiple functional groups, which reveals a future direction for selective detection of particulate matter.

Blended Conjugated Host and Unconjugated Dopant Polymers Towards N-type All-Polymer Conductors and High-ZT Thermoelectrics.

N-Type thermoelectrics typically consist of small molecule dopant+polymer host. Only a few polymer dopant+polymer host systems have been reported, and these have lower thermoelectric parameters. N-type polymers with high crystallinity and order are generally used for high-conductivity ( σ ${\sigma }$ ) organic conductors. Few n-type polymers with only short-range lamellar stacking for high-conductivity materials have been reported. Here, we describe an n-type short-range lamellar-stacked all-polymer thermoelectric system with highest σ ${\sigma }$ of 78 S-1 , power factor (PF) of 163 µW m-1 K-2 , and maximum Figure of merit (ZT) of 0.53 at room temperature with a dopant/host ratio of 75 wt%. The minor effect of polymer dopant on the molecular arrangement of conjugated polymer PDPIN at high ratios, high doping capability, high Seebeck coefficient (S) absolute values relative to σ ${\sigma }$ , and atypical decreased thermal conductivity ( κ ${\kappa }$ ) with increased doping ratio contribute to the promising performance.

Permittivity Threshold and Thermodynamics of Integer Charge-Transfer Complexation for an Organic Donor-Acceptor Pair.

Molecular charge doping involves the formation of donor-acceptor charge-transfer complexes (CTCs) through integer or partial electron transfer; understanding how local chemical environment impacts complexation is important for controlling the properties of organic materials. We present steady-state and temperature-dependent spectroscopic investigations of the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) complexed with the electron donor and hole transport material N,N'-diphenyl-N,N'-di-p-tolylbenzene-1,4-diamine (MPDA). Equilibrium formation constants (KCT) were determined for donor-acceptor pairs dissolved in a series of solvents covering a range of values of permittivity. A threshold for highly favorable complex formation was observed to occur at ϵ ∼ 8-9, with large (>104) and small (<103) values of KCT obtained in solvents of higher and lower permittivity, respectively, but with chloroform (ϵ = 4.81) exhibiting an anomalously high formation constant. Temperature-dependent formation constants were determined in order to evaluate the thermodynamics of complex formation. In 1,2-dichloroethane (ϵ = 10.36) and chlorobenzene (ϵ = 5.62), complex formation is both enthalpically and entropically favorable, with higher enthalpic and entropic stabilization in the solvent with higher permittivity. Complexation in chloroform is exothermic and entropically disfavored, indicating that specific, inner-shell solvent-solute interactions stabilize the charge-separated complex and result in a net increase in local solution structure. Our results provide insight into how modification to the chemical environment may be utilized to support stable integer charge transfer for molecular doping applications and requiring only modest changes in local permittivity.

Sensitive organic electrochemical transistor biosensors: Comparing single and dual gate functionalization and different COOH-functionalized bioreceptor layers.

We developed new measurement configurations based on organic electrochemical transistors (OECTs). Three types of COOH-functionalized bioreceptor layers were deposited on indium tin oxide (ITO) electrodes on poly(ethylene terephthalate) (PET) substrates and their performance was tested using single gate functionalization organic electrochemical transistor (S-OECT) and dual gate functionalization organic electrochemical transistor (D-OECT) configurations. The three layers included one p-type semiconductor, one insulator, and one self-assembled layer, and the dual gates were connected in series through buffer solutions, so the solution-electrode interfaces had the opposite polarities. We investigated the sensitivities of these systems using the human IgG antigen-human IgG antibody receptor pair for main experiments, and drifts of antibody-functionalized gates without analytes as control experiments. Drifts without analyte can obscure the real sensitivity. We show that the D-OECT has the capability to cancel the drifts, and is also beneficial for showing the sensitivity more exactly. This configuration has the ability to increase the accuracy of antibody-antigen interaction detection, and further decrease or eliminate the effect of ions in the buffer solution. We also prove that the D-OECT can work well with different bioreceptor materials, which indicates that the system can be further applied to different conditions.

A New Polystyrene-Poly(vinylpyridinium) Ionic Copolymer Dopant for n-Type All-Polymer Thermoelectrics with High and Stable Conductivity Relative to the Seebeck Coefficient giving High Power Factor.

A novel n-type copolymer dopant polystyrene-poly(4-vinyl-N-hexylpyridinium fluoride) (PSpF) with fluoride anions is designed and synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. This is thought to be the first polymeric fluoride dopant. Electrical conductivity of 4.2 S cm-1 and high power factor of 67 µW m-1 K-2 are achieved for PSpF-doped polymer films, with a corresponding decrease in thermal conductivity as the PSpF concentration is increased, giving the highest ZT of 0.1. An especially high electrical conductivity of 58 S cm-1 at 88 °C and outstanding thermal stability are recorded. Further, organic transistors of PSpF-doped thin films exhibit high electron mobility and Hall mobility of 0.86 and 1.70 cm2 V-1 s-1 , respectively. The results suggest that polystyrene-poly(vinylpyridinium) salt copolymers with fluoride anions are promising for high-performance n-type all-polymer thermoelectrics. This work provides a new way to realize organic thermoelectrics with high conductivity relative to the Seebeck coefficient, high power factor, thermal stability, and broad processing window.

Stabilization and Specification in Polymer Field-Effect Transistor Semiconductors.

The strong and varied chemical interactions between polymer semiconductors and small molecules, and the electronic consequences of these interactions, make polymer organic field-effect transistors (OFETs) attractive as vapor sensing elements. Two hindrances to their wider acceptance and use are their environmental drift and the poor specificity of individual OFETs. Approaches to addressing these two present drawbacks are presented in this Spotlight on Applications. They include the use of semiconducting polymers with greater inherent stability, circuits that add further stability, and arrays that generate patterns that are much more specific to analyte vapors of interest than the individual responses.

3,4,5-Trimethoxy Substitution on an N-DMBI Dopant with New N-Type Polymers: Polymer-Dopant Matching for Improved Conductivity-Seebeck Coefficient Relationship.

Achieving high electrical conductivity and thermoelectric power factor simultaneously for n-type organic thermoelectrics is still challenging. By constructing two new acceptor-acceptor n-type conjugated polymers with different backbones and introducing the 3,4,5-trimethoxyphenyl group to form the new n-type dopant 1,3-dimethyl-2-(3,4,5-trimethoxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (TP-DMBI), high electrical conductivity of 11 S cm-1 and power factor of 32 µW m-1 K-2 are achieved. Calculations using Density Functional Theory show that TP-DMBI presents a higher singly occupied molecular orbital (SOMO) energy level of -1.94 eV than that of the common dopant 4-(1, 3-dimethyl-2, 3-dihydro-1H-benzoimidazol-2-yl) phenyl) dimethylamine (N-DMBI) (-2.36 eV), which can result in a larger offset between the SOMO of dopant and lowest unoccupied molecular orbital (LUMO) of n-type polymers, though that effect may not be dominant in the present work. The doped polymer films exhibit higher Seebeck coefficient and power factor than films using N-DMBI at the same doping levels or similar electrical conductivity levels. Moreover, TP-DMBI doped polymer films offer much higher electron mobility of up to 0.53 cm2 V-1 s-1 than films with N-DMBI doping, demonstrating the potential of TP-DMBI, and 3,4,5-trialkoxy DMBIs more broadly, for high performance n-type organic thermoelectrics.

Maximized Hole Trapping in a Polystyrene Transistor Dielectric from a Highly Branched Iminobis(aminoarene) Side Chain.

We synthesized highly branched and electron-donating side chain subunits and attached them to polystyrene (PS) used as a dielectric layer in a pentacene field-effect transistor. The influence of these groups on dielectric function, charge retention, and threshold voltage shifts (ΔVth) depending on their positions in dielectric multilayers was determined. We compared the observations made on an N-perphenylated iminobisaniline side chain with those from the same side chains modified with ZnO nanoparticles and with an adduct formed from tetracyanoethylene (TCNE). We also synthesized an analogue in which six methoxy groups are present instead of two amine nitrogens. At 6 mol % side chain, hopping transport was sufficient to cause shorting of the gate, while at 2 mol %, charge trapping was observable as transistor threshold voltage shifts (ΔVth). We created three types of devices: with the substituted PS layer as single-layer dielectric, on top of a cross-linked PS layer but in contact with the pentacene (bilayers), and sandwiched between two PS layers in trilayers. Especially large bias stress effects and ΔVth, larger than those in the case of the hexamethoxy and previously studied dimethoxy analogues, were observed in the second case, and the effects increased with the increasing electron-donating properties of the modified side chains. The highest ΔVth was consistent with a majority of the side chains stabilizing the trapped charge. Trilayer devices showed decreased charge storage capability compared to previous work in which we used less donating side chains but in higher concentrations. The ZnO and TCNE modifications resulted in slightly more and less negative ΔVth, respectively, when the side chain polystyrene was not in contact with the pentacene and isolated from the gate electrode. The results indicate a likely maximum combination of molecular charge stabilizing activity and side chain concentration that still allows gate dielectric function.

Suppression of Ionic Doping by Molecular Dopants in Conjugated Polymers for Improving Specificity and Sensitivity in Biosensing Applications.

Ionic doping effects in conjugated polymers often cause nonspecific signaling and a low selectivity of bioelectronic sensing. Using remote-gate field-effect transistor characterization of molecular and ionic doping in poly(3-hexylthiophene) (P3HT) and acid-functionalized polythiophene, poly[3-(3-carboxypropyl) thiophene-2,5-diyl] (PT-COOH), we discovered that proton doping effects on the interfacial potential occurring in P3HT could be suppressed by sequentially doping P3HT by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). To be specific, intrinsic pH sensitivity shown by pure P3HT (18 mV/pH in a range from pH 3 to 9) was fully dissipated for doped P3HT:F4TCNQ. However, F4TCNQ sequential doping instead increases pH sensitivity of acid-functionalized polythiophene, PT-COOH (40 mV/pH), compared to that of a pure PT-COOH (30 mV/pH). Interactions between polythiophene backbone and side chains, which constrain the activity of COOH, are weakened by stronger F4TCNQ doping leaving behind responsive COOH groups exposed to aqueous solutions. This is supported by the reduced pH sensitivity of PT-COOH sequentially doped by a weaker dopant, tetracyanoethylene (TCNE) (21 mV/pH). Thus, doping is shown to stabilize a nonpolar conjugated polymer to pH-induced fluctuations on one hand, and to activate a COOH side chain to pH-induced response on the other.

Design and Synthesis of Air-Stable p-Channel-Conjugated Polymers for High Signal-to-Drift Nitrogen Dioxide and Ammonia Sensing.

The development of high-performance-conjugated polymer-based gas sensors involves detailed structural tailoring such that high sensitivities are achieved without compromising the stability of the fabricated devices. In this work, we systematically developed a series of diketopyrrolopyrrole (DPP)-based polymer semiconductors by modifying the polymer backbone to achieve and rationalize enhancements in gas sensitivities and electronic stability in air. NO2- and NH3-responsive polymer-based organic field-effect transistors (OFETs) are described with improved air stability compared to all-thiophene conjugated polymers. Five DPP-fluorene-based polymers were synthesized and compared to two control polymers and used as active layers to detect a concentration of NO2 at least as low as 0.5 ppm. The hypothesis that the less electron-donating fluorene main-chain subunit would lead to increased signal/drift compared to thiophene and carbazole subunits was tested. The sensitivities exhibited a bias voltage-dependent behavior. The proportional on-current change of OFETs using a dithienyl DPP-fluorene polymer reached ∼614% for an exposure to 20 ppm of NO2 for 5 min, testing at a bias voltage of -33 V, among the higher reported NO2 sensitivities for conjugated polymers. Electronic and morphological studies reveal that introduction of the fluorene unit in the DPP backbone decreases the ease of backbone oxidation and induces traps in the thin films. The combination of thin-film morphology and oxidation potentials governs the gas-absorbing properties of these materials. The ratio of responses on exposure to NO2 and NH3 compared to drifts while taking the device through repeated gate voltage sweeps is the highest for two polymers incorporating electron-donating linkers connecting the DPP and thiophene units in the backbone, in this category of organic semiconductors. The responses to NO2 were much larger than that to NH3, indicating increased susceptibility to oxidizing vs reducing gases, and that the capability of oxidizing gases to induce additional charge density has a more dramatic electronic effect than when reducing gases create traps. This work demonstrates the capability of achieving improved stability with the retention of high sensitivity in conjugated polymer-based OFET sensors by modulating redox and morphological properties of polymer semiconductors by structural control.

Unusually Conductive Organic-Inorganic Hybrid Nanostructures Derived from Bio-Inspired Mineralization of Peptide/Pi-Electron Assemblies.

Supramolecular materials derived from pi-conjugated peptidic macromolecules are well-established to self-assemble into 1D nanostructures. In the presence of KOH, which was used to more fully dissolve the peptide macromolecules prior to triggering the self-assembly by way of exposure to HCl vapor, we report here an unexpected mineralization of KCl as templated presumably by the glutamic acid residues that were present along the backbone of the peptide macromolecules. In order to decouple the peptidic side chains from the central pi-electron unit, three-carbon spacers were added between them on both sides. The assembled structures that resulted from the collective formation of ß-sheets, π-orbital overlaps, and mineralization resulted in highly interconnected dendritic structures under suitable KOH concentrations. Electrical measurements indicated that when well-interconnected, these dendritic structures maintained conductivities comparable to those of metals at around 1800 S/cm. About 50 mA current was measured for 0.5 V/37.5 µm. Varying the gate voltage in a transistor configuration had no effect on the current levels, indicating a conductive instead of a semiconducting pathway. Control experiments without the peptide, measurements of conductivity over time, and conductivity quenching by ammonia suggested the conductivity of these dendritic networks was derived from proton doping of the central π-electron units in a strong acid environment and was facilitated by closely spaced chromophores, as suggested in the literature, leading to facile π-electron transfer along the interconnected dendritic pathways. Our findings suggest that mineralization templated by appropriate amino acids combined with peptide/π-electron self-assembly can lead to highly conductive dendritic macrostructures as well as control of nanowire growth in specific directions.

Correction: Solid-state electrical applications of protein and peptide based nanomaterials.

Correction for 'Solid-state electrical applications of protein and peptide based nanomaterials' by Sayak Subhra Panda et al., Chem. Soc. Rev., 2018, 47, 3640-3658.

A Humid-Air-Operable, NO2-Responsive Polymer Transistor Series Circuit with Improved Signal-to-Drift Ratio Based on Polymer Semiconductor Oxidation.

A subparts per million-sensitive nitrogen dioxide (NO2) sensing circuit with improved humid air stability was realized incorporating UV-ozone treatment on a poly(bisdodecylquaterthiophene)/polystyrene blend film. The circuit consisted of a pair of organic field-effect transistors (OFETs) in series, one OFET with and one without this treatment. In contrast to most previous OFET sensors, the readout was obtained from the voltage Vout at a point between the OFETs. The circuit showed a low detection limit (200 ppb) toward NO2 and greatly reduced the voltage drift in the humid environment compared to the current drift of the circuit or the individual OFETs because of the balance of conductance drifts on either side of the readout point, which differs from the existing OFET-based sensors. By using Vout as the detection parameter, the sensitivity of the circuit approaches 25 and 400% for NO2 concentrations of 200 ppb and 20 ppm, respectively. Moreover, the Vout is substantial enough to be easily measured by a voltmeter, which could remove the need for complex equipment (semiconductor analyzer system) for the sensing test. We thus demonstrate a simplified approach to stabilized OFET circuits that could be used in printable, flexible, or wearable sensors.

Analytical Platform To Characterize Dopant Solution Concentrations, Charge Carrier Densities in Films and Interfaces, and Physical Diffusion in Polymers Utilizing Remote Field-Effect Transistors.

Characterizing doping effects in a conductive polymer and physical diffusion in a passive polymer were performed using a remote-gate field-effect transistor (RG FET) detection system that was able to measure the electrical potential perturbation of a polymer film coupled to the gate of a silicon FET. Poly(3-hexylthiophene) (P3HT) film doped using various concentrations of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) solutions imposed additional positive potentials on the P3HT RG, resulting in a lower threshold voltage ( Vth) on the n-channel silicon FET. Changes in Vth were related to the induced hole concentrations and hole mobility in P3HT films by using our Vth shifting model for the RG FET. We discovered that the electron-donating P3HT and even inorganic materials, indium tin oxide and gold, showed similar electrical potential perturbations dependent on the concentration of F4TCNQ in overlying solutions as the dopant radical anions maximally covered the surfaces. This suggests that there are limited electroactive sites for F4TCNQ binding on electron donor surfaces which results in a similar number of positive charges in film materials forming dipoles with the F4TCNQ radical counteranions. The effect of electron acceptors such as 7,7,8,8-tetracyanoquinodimethane and tetracyanoethylene was compared to that of F4TCNQ in terms of Vth shift using our analytical tool, with differences attributed to acceptor size and reduction potential. Meanwhile, this FET analysis tool offered a means of monitoring the physical diffusion of small molecules, exemplified by F4TCNQ, in the passive polymer polystyrene, driven by concentration gradients. The technique allows for nondestructive, nonspectroscopic, ambient characterization of electron donor-acceptor interactions at surfaces.

Chemical and Biomolecule Sensing with Organic Field-Effect Transistors.

The strong and controllable chemical sensitivity of organic semiconductors (OSCs) and the amplification capability of transistors in circuits make use of OSC-based field-effect transistors compelling for chemical sensors. Analytes detected and assayed range from few-atom gas-phase molecules that may have adverse health and security implications to biomacromolecules (proteins, nucleic acids) that may be markers for physiological processes and medical conditions. This review highlights recent progress in organic field-effect transistor (OFET) chemical sensors, emphasizing advances from the past 5 years and including aspects of OSC morphology and the role of adjacent dielectrics. Design elements of the OSCs and various formats for the devices are illustrated and evaluated. Challenges associated with the present state of the art and future opportunities are also discussed.

Spray coating of the PCBM electron transport layer significantly improves the efficiency of p-i-n planar perovskite solar cells.

The p-i-n structure for perovskite solar cells has recently shown significant advantages in minimal hysteresis effects, and scalable manufacturing potential using low-temperature solution processing. However, the power conversion efficiency (PCE) of the perovskite p-i-n structure remains low mainly due to limitations using a flat electron transport layer (ETL). In this work, we demonstrate a new approach using spray coating to fabricate the [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) ETL. By creating a rough surface, we effectively improve the light trapping properties inside the PCBM ETL. We reveal that the spray coated PCBM can form a cross-linked network, which may facilitate better charge transport and enhance extraction efficiency. By improving the contact between the perovskite film and the PCBM ETL, a reduction in the trap states is observed resulting in a PCE increase from 13% to >17%.

Electronic Cortisol Detection Using an Antibody-Embedded Polymer Coupled to a Field-Effect Transistor.

A field-effect transistor-based cortisol sensor was demonstrated in physiological conditions. An antibody-embedded polymer on the remote gate was proposed to overcome the Debye length issue (λD). The sensing membrane was made by linking poly(styrene- co-methacrylic acid) (PSMA) with anticortisol before coating the modified polymer on the remote gate. The embedded receptor in the polymer showed sensitivity from 10 fg/mL to 10 ng/mL for cortisol and a limit of detection (LOD) of 1 pg/mL in 1× PBS where λD is 0.2 nm. A LOD of 1 ng/mL was shown in lightly buffered artificial sweat. Finally, a sandwich ELISA confirmed the antibody binding activity of antibody-embedded PSMA.