A Review on Advanced Sensing Materials for Agricultural Gas Sensors.

This work is a comprehensive review of sensing materials, which interact with several target gases pertinent to agricultural monitoring applications. Sensing materials which interact with carbon dioxide, water vapor (relative humidity), hydrogen sulfide, ethylene and ethanol are the focus of this work. Performance characteristics such as dynamic range, recovery time, operating temperature, long-term stability and method of deposition are discussed to determine the commercial viability of the sensing materials considered in this work. In addition to the sensing materials, deposition methods are considered to obtain the desired sensing material thickness based on the sensor's mechanism of operation. Various material classes including metal oxides, conductive polymers and carbon allotropes are included in this review. By implementing multiple sensing materials to detect a single target analyte, the issue of selectivity due to cross sensitivity can be mitigated. For this reason, where possible, it is desirable to utilize more than one sensing material to monitor a single target gas. Among those considered in this work, it is observed that PEDOT PSS/graphene and TiO2-coated g-C3N4 NS are best suited for CO2 detection, given their wide dynamic range and modest operating temperature. To monitor the presence of ethylene, BMIM-NTf2, SWCNTs and PtTiO2 offer a dynamic range most suitable for the application and require no active heating. Due to the wide dynamic range offered by SiO2/Si nanowires, this material is best suited for the detection of ethanol; a gas artificially introduced to prolong the shelf life of the harvested crop. Finally, among all other sensing materials investigated, it observed that both SWCNTs and CNTs/SnO2/CuO are most suitable for H2S detection in the given application.

Artificial Olfactory Signal Modulation for Detection in Changing Environments.

Animals have evolved to sense in complex environments through both modulation behavior including sniffing as well as sophisticated neural processing including memory and neuromodulation. Here, we explore thermal modulation of chemically diverse sensor arrays, where response patterns are based on partitioning of odorants across the array. The differential response patterns contain information about the chemical nature of the odorant for identification. By transitioning away from well-defined concentration modulation, traditionally used in the field, to thermal modulation, it is possible to capture both diagnostic patterns as well as intensity information in complex environments. This performance is demonstrated with carbon-black based, chemically diverse sensor arrays, that are thermally modulated with light at 25 mHz exposed to different analytes of varying concentrations.

Self-assembly of alkylthiosulfates on gold: role of electrolyte and trace water in the solvent.

Spontaneous self-assembly of alkylthiosulfates on gold produce monolayers similar to the corresponding alkanethiols. Alkylthiosulfate self-assembly from THF solutions is inhibited in the presence of tetrabutylammonium tetrafluoroborate electrolyte. The mechanism of alkylthiosulfate self-assembly and the role of electrolyte and trace water in the solvent are investigated using open-circuit potential measurements, contact angle goniometry and redox electron transfer blocking experiments to explore the hypothesis that trace water present in the solvent facilitates monolayer formation on gold. Furthermore, the unique behavior of tetrabutylammonium tetrafluoroborate, compared to other tetrabutylammonium electrolytes, on the inhibition of alkylthiosulfate self-assembly has been explained.

Electrochemically assisted self-assembly of alkylthiosulfates and alkanethiols on gold: the role of gold oxide formation and corrosion.

Electrochemically directed self-assembly of alkylthiosulfates enables the selective formation of monolayers on gold surfaces. These monolayers are identical to those formed from the corresponding alkanethiols. However, the mechanistic details of monolayer formation under electrochemical conditions as well as the role of other variables and residual water in the solvent have not been extensively studied. A systematic investigation shows that self-assembly is not a result of an outer-sphere one-electron oxidation of alkylthiosulfate. Voltammetry and electrochemical quartz crystal microbalance techniques reveal that self-assembly involving alkylthiosulfates as well as alkanethiols under oxidative conditions proceed through direct reaction with gold oxide and in some cases is accompanied by corrosion. X-ray photoelectron spectroscopy indicates that monolayers can undergo rapid exchange with molecules in solution under electrochemically directed self-assembly conditions.

Field enhanced charge carrier reconfiguration in electronic and ionic coupled dynamic polymer resistive memory.

Dynamic resistive memory devices based on a conjugated polymer composite (PPy(0)DBS(-)Li(+) (PPy: polypyrrole; DBS(-): dodecylbenzenesulfonate)), with field-driven ion migration, have been demonstrated. In this work the dynamics of these systems has been investigated and it has been concluded that increasing the applied field can dramatically increase the rate at which information can be 'written' into these devices. A conductance model using space charge limited current coupled with an electric field induced ion reconfiguration has been successfully utilized to interpret the experimentally observed transient conducting behaviors. The memory devices use the rising and falling transient current states for the storage of digital states. The magnitude of these transient currents is controlled by the magnitude and width of the write/read pulse. For the 500 nm length devices used in this work an increase in 'write' potential from 2.5 to 5.5 V decreased the time required to create a transient conductance state that can be converted into the digital signal by 50 times. This work suggests that the scaling of these devices will be favorable and that 'write' times for the conjugated polymer composite memory devices will decrease rapidly as ion driving fields increase with decreasing device size.

Compensation doping in conjugated polymers: engineering dopable heterojunctions for modulating conductivity in the solid state.

Compensation doping of conjugated polymers is used to create a composite that contains a sufficient level of ions to support the doped form of the polymer upon removal of the compensating ion. Interfacing this form of conjugated polymer with another semiconductor capable of becoming doped upon uptake of the compensating ion allows a field-driven change in conductivity to be achieved in the solid state. As a result, this system is capable of rectification as well as the storage of charge and can be highly tunable. The electrodeposition of the heterojunctions is scalable to the nanometer range and provides a means for creating devices on existing crossbar structures outside strictly controlled fab lines.

Catalytic, Conductive Bipolar Membrane Interfaces through Layer-by-Layer Deposition for the Design of Membrane-Integrated Artificial Photosynthesis Systems.

In the presence of an electric field, bipolar membranes (BPMs) are capable of initiating water disassociation (WD) within the interfacial region, which can make water splitting for renewable energy in the presence of a pH gradient possible. In addition to WD catalytic efficiency, there is also the need for electronic conductivity in this region for membrane-integrated artificial photosynthesis (AP) systems. Graphene oxide (GO) was shown to catalyze WD and to be controllably reduced, which resulted in electronic conductivity. Layer-by-layer (LbL) film deposition was employed to improve GO film uniformity in the interfacial region to enhance WD catalysis and, through the addition of a conducting polymer in the process, add electronic conductivity in a hybrid film. Three different deposition methods were tested to optimize conducting polymer synthesis with the oxidant in a metastable solution and to yield the best film properties. It was found that an approach that included substrate dipping in a solution containing the expected final monomer/oxidant ratio provided the most predictable film growth and smoothest films (by UV/Vis spectroscopy and atomic force microscopy/scanning electron microscopy, respectively), whereas dipping in excess oxidant or co-spraying the oxidant and monomer produced heterogeneous films. Optimized films were found to be electronically conductive and produced a membrane ohmic drop that was acceptable for AP applications. Films were integrated into the interfacial region of BPMs and revealed superior WD efficiency (≥1.4 V at 10 mA cm-2 ) for thinner films (<10 bilayers≈100 nm) than for either the pure GO catalyst or conducting polymer individually, which indicated that there was a synergistic effect between these materials in the structure configured by the LbL method.

Transparent Bipolar Membrane for Water Splitting Applications.

This study describes the use of a benzimidazolium-based anion exchange membrane for creating bipolar membranes and the assessment of their suitability for solar-driven water splitting. Bipolar membranes were prepared by laminating anion exchange membrane with Nafion NR-211 membrane without modification of the interface. Under acidic and basic conditions, proton and hydroxide ion conductivities of 103 and 102 mS cm-1 were obtained for Nafion and benzimidazolium-based membranes, respectively. The fabricated bipolar membranes have an average thickness of 90 µm and show high transmittance, up to 75% of the visible light. The findings suggest that the two membranes create a sharp hydrophilic interface with a space charge region of only a few nanometers, thereby generating a large electric field at the interface that enhances water dissociation.

Band gap modulation in polythiophene and polypyrrole-based systems.

In this paper, the structural and electronic properties of polythiophene and polyprrrole-based systems have been investigated using first-principles calculations both in periodic and oligomer forms. Of particular interest is the band gap modulation through substitutions and bilayer formation. Specifically, S has been substituted by Se and Te in polythiophene, leading to polyseleophene and polytellurophene, respectively, and N has been substituted by P and As in polypyrrole. The values obtained of the binding energy suggest that all the systems studied can be realized experimentally. Stacking (bilayer formation) of pure polythiophene, polypyrrole and their derivatives leads to linear suppression of the band gap or HOMO-LUMO gap as a function of the stacking. Mixed bilayers, including one formed from polythiophene on top of polypyrrole, have also been considered. Overall, a wide range of band gaps can be achieved through substitutions and stacking. Hybrid (B3LYP) calculations also suggest the same trend in the band gap as PBE calculations. Trends in the binding energy are similar for both periodic and molecular calculations. In addition, Γ-point phonon calculations were performed in order to check the stability of selected systems.

Electric and Photoelectric Properties of 3,4-Ethylenedioxythiophene-Functionalized n-Si/PEDOT:PSS Junctions.

Organic/inorganic solid-state junctions play a critical role in tandem artificial photosynthetic devices supported by conducting polymer membranes. Recent work with n-Si/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT: PSS) hybrid junctions has shown that the electrical behavior is dominated by the passivating groups present on the silicon surface. In this work, the photovoltaic behavior of n-Si/ PEDOT: PSS was investigated with methyl, thiophene, and 3,4-ethylenedioxythiophene (EDOT) groups covalently attached to the Si(111) surface. X-ray photoelectron spectroscopy results demonstrated that complete monolayer coverage was achieved in 3 h and that the organic passivating groups were retained over two months of exposure to ambient conditions with minimal silicon oxidation. All surfaces investigated exhibited similar light-limited photocurrents and bulk-limited open-circuit voltages, and thiophene produced a dramatic reduction of the fill factor attributed to the formation of trap states at the interface. Furthermore, shunt behavior observed near the power-producing regions for the thiophene and EDOT surfaces is indicative of increased recombination events under forward bias and suggests that hole transport across the interface is enhanced. Thus, thiophene- and EDOT-functionalized Si(111) offer similar stabilities and efficiencies to those of methylated surfaces as well as enhanced hole transport to the PEDOT: PSS interface from the n-Si surfaces.

Reduced Graphene Oxide Bipolar Membranes for Integrated Solar Water Splitting in Optimal pH.

The integration of light absorbers and catalysts for the water splitting process requires a membrane capable of both ion and electron management and product separation to realize efficient solar fuels systems. Bipolar membranes can maintain a pH gradient for optimal reaction conditions by the dissociation of water. Such membranes that contain graphene in the interfacial layer are fabricated by the chemical reduction of a uniformly deposited graphene oxide layer to convert sp(3) catalyst regions to sp(2) conductive regions. The resulting electrical and water dissociation properties are optimized by adjusting the exposure conditions, and treatments of less than 5 min render an interface that exceeds the conductivity requirements for integrated solar water splitting and increases the overpotential by <0.3 V. Integration with photoelectrodes is examined by characterizing the electrical interface formed between graphene and Si microwires, and we found that efficient Ohmic junctions are possible.

Electrical Characteristics of the Junction between PEDOT:PSS and Thiophene-Functionalized Silicon Microwires.

Thiophene moieties have been attached to Si microwires (Si MWs) by a two-step chlorination/alkylation reaction method. X-ray photoelectron spectroscopy indicated that saturation of the surface occurred after 30 min of reaction time. Electrical measurements using a standard probe station indicated that the junction between individual thiophene-functionalized Si MWs and the conducting polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) ( PEDOT: PSS) became more ohmic as more thiophene was added to the MW surface. Under a light-limited current of 20 nA, representative of operation of Si MWs under 1 Sun illumination conditions, the iR loss of thiophene-n-Si MW/PEDOT-PSS contacts was 20 mV, representing an order of magnitude reduction compared with PEDOT-PSS junctions formed with methyl terminated n-Si MWs. Such iR losses are much less than typical catalytic overpotentials for fuel formation, and hence the thiophene-functionalized Si MW contacts will not limit the performance of a Si MW array-based solar fuels device under 1 Sun illumination.

Covalent Attachment of Ferrocene to Silicon Microwire Arrays.

A fully integrated, freestanding device for photoelectrochemical fuel generation will likely require covalent attachment of catalysts to the surface of the photoelectrodes. Ferrocene has been utilized in the past as a model system for molecular catalyst integration on planar silicon; however, the surface structure of high-aspect ratio silicon microwires envisioned for a potential device presents potential challenges with respect to stability, characterization, and mass transport. Attachment of vinylferrocene to Cl-terminated surfaces of silicon microwires was performed thermally. By varying the reaction time, solutions of vinylferrocene in di-n-butyl ether were employed to control the extent of functionalization. X-ray photoelectron spectroscopy (XPS) and electrochemistry were used to estimate the density and surface coverage of the silicon microwire arrays with ferrocenyl groups, which could be controllably varied from 1.23 × 10(-11) to 4.60 × 10(-10) mol cm(-2) or 1 to 30% of a monolayer. Subsequent backfill of the remaining Si-Cl sites with methyl groups produced ferrocenyl-terminated surfaces that showed unchanged cyclic volammograms following two months in air, under ambient conditions, and repeated electrochemical cycling.

Graphene oxide as a water dissociation catalyst in the bipolar membrane interfacial layer.

Bipolar membranes are formed by the lamination of an anion- and cation-exchange layer. Upon a sufficient applied reverse bias, water molecules at the layer junction dissociate, generating OH(-) and H(+), which can be useful in electrodialysis and electrosynthesis applications. Graphene oxide has been introduced into bipolar membrane junctions (illustrated in the adjacent graphic) and is shown to be an efficient new water dissociation catalyst, lowering the overpotential by 75% compared to a control membrane. It was found that adjusting deposition conditions changes the nature of the graphene oxide films, leading to tunable membrane performance. Additionally, it is shown that their low overpotentials are stable, making for industrially viable, high-performance bipolar membranes.

Use of bipolar membranes for maintaining steady-state pH gradients in membrane-supported, solar-driven water splitting.

A bipolar membrane can maintain a steady-state pH difference between the sites of oxidation and reduction in membrane-supported, solar-driven water-splitting systems without changing the overall thermodynamics required to split water. A commercially available bipolar membrane that can serve this purpose has been identified, its performance has been evaluated quantitatively, and is demonstrated to meet the requirements for this application. For effective utilization in integrated solar-driven water-splitting systems, such bipolar membranes must, however, be modified to simultaneously optimize their physical properties such as optical transparency, electronic conductivity and kinetics of water dissociation.

Controlling volatility in solid-state, redox-based memory devices using heterojunction barriers to ion transport.

In this work, we show the ability to tune the volatility of redox-based memory by designing barriers to ion drift. By changing the nature and properties of the barrier material, the key performance metric (ratio of retention time to read/write time) could be altered to yield ratios in the range of 1 to 10(9).

Fabrication and optimization of a conducting polymer sensor array using stored grain model volatiles.

During storage, grain can experience significant degradation in quality due to a variety of physical, chemical, and biological interactions. Most commonly, these losses are associated with insects or fungi. Continuous monitoring and an ability to differentiate between sources of spoilage are critical for rapid and effective intervention to minimize deterioration or losses. Therefore, there is a keen interest in developing a straightforward, cost-effective, and efficient method for monitoring of stored grain. Sensor arrays are currently used for classifying liquors, perfumes, and the quality of food products by mimicking the mammalian olfactory system. The use of this technology for monitoring of stored grain and identification of the source of spoilage is a new application, which has the potential for broad impact. The main focus of the work described herein is on the fabrication and optimization of a carbon black (CB) polymer sensor array to monitor stored grain model volatiles associated with insect secretions (benzene derivatives) and fungi (aliphatic hydrocarbon derivatives). Various methods of statistical analysis (RSD, PCA, LDA, t test) were used to select polymers for the array that were optimum for distinguishing between important compound classes (quinones, alcohols) and to minimize the sensitivity for other parameters such as humidity. The performance of the developed sensor array was satisfactory to demonstrate identification and separation of stored grain model volatiles at ambient conditions.

Novel conducting polymer-heteropoly acid hybrid material for artificial photosynthetic membranes.

Artificial photosynthetic (AP) approaches to convert and store solar energy will require membranes capable of conducting both ions and electrons while remaining relatively transparent and chemically stable. A new approach is applied herein involving previously described in situ chemical polymerization of electronically conducting poly(3,4-ethylenedioxythiophene) (PEDOT) in the presence of proton conducting heteropoly acid (HPA) phosphomolybdic acid (PMA). The electrochemical behaviour of the PEDOT/PMA hybrid material was investigated and it was found that the conducting polymer (CP) is susceptible to irreversible oxidative processes at potentials where water is oxidized. This will be problematic in AP devices should the process occur in very close proximity to a conducting polymer-based membrane. It was found that PEDOT grants the system good electrical performance in terms of conductivity and stability over a large pH window; however, the presence of PMA was not found to provide sufficient proton conductivity. This was addressed in an additional study by tuning the ionic (and in turn, electronic) conductivity in creating composites with the proton-permselective polymer Nafion. It was found that a material of this nature with near-equal conductivity for optimal chemical conversion efficiency will consist of roughly three parts Nafion and one part PEDOT/PMA.

Porous conducting polymer/heteropolyoxometalate hybrid material for electrochemical supercapacitor applications.

A porous conducting polymer/heteropolyoxometalate hybrid material that displays high specific capacitance and low ionic resistance has been prepared for electrochemical supercapacitor applications. Polypyrrole/phosphomolybdate composite films were chemically synthesized in tetrahydrofuran in the presence of sodium sulfate, which acts as a porogen. While the phosphomolydic acid could be removed from the film upon rinsing with pure tetrahydrofuran or acetone, rinsing with water or methanol resulted in retention of the heteropolyoxometalate at a level high enough to easily observe its electrochemistry. The retained phosphomolybdate exhibits fast and reversible redox behavior, adding a significant amount of pseudocapacitance to the polymer. Porous films were obtained by leaching out the sodium sulfate porogen from the films using water. The morphology obtained using this method is altered by varying the monomer-to-porogen ratio. Increasing the porosity increases the rate at which the hybrid material can be charged/discharged (i.e., oxidized/reduced) by increasing the ionic conductivity and in turn lowering the resistor-capacitor time constant of the material. The ability to tune the porosity of the material allows the optimization of performance characteristics for use in supercapacitor applications. Impedance measurements indicate that the ionic conductivity of these porous structures can be increased more than an order of magnitude over that observed for standard conducting polymer films and that the hybrid material displays peak specific capacitance of around 700 F/g as well as excellent reversibility and cyclability.

A novel layer-by-layer approach for the fabrication of conducting polymer/RNA multilayer films for controlled release.

Poly(anilineboronic acid) (PABA)/ribonucleic acid (RNA) multilayer films were prepared under neutral condition using a layer-by-layer deposition of PABA and RNA. RNA was used both as a polyelectrolyte for multilayer formation as well as dopant for PABA. Photoelastic modulated infrared reflection absorption spectroscopy measurements suggest that PABA interacts covalently with RNA through the formation of a boronate ester, a boron-nitrogen dative bond, as well as electrostatic interactions of anionic phosphates with cationic amines. The deposition procedure was monitored with UV-vis absorption spectroscopy, showing a linear dependence of absorbance with the number of PABA/RNA bilayers deposited. The multilayer films were further characterized using X-ray photoelectron spectroscopy and ellipsometry, which yielded a PABA/RNA bilayer thickness of approximately 10 nm. The PABA/RNA multilayer films are redox-active at neutral pH, consistent with the formation of a self-doped polymer. Electrochemical control of PABA under these conditions allows potential-induced controlled release of RNA from a multilayer at neutral pH, suggesting that this may serve as a novel method for controlled release of RNA under physiological conditions.