“Secondary doping” methods to significantly enhance the conductivity of PEDOT:PSS for its application as transparent electrode of optoelectronic devices

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the most successful conducting polymer in terms of the practical application. It can be dispersed in water and some polar organic solvents, and high-quality PEDOT:PSS films can be readily prepared through solution processing. In addition, PEDOT:PSS is highly transparent in the visible range and has excellent thermal stability. Nevertheless, PEDOT:PSS has a problem of low conductivity. The as-prepared PEDOT:PSS films from its aqueous solution have a conductivity of lower than 1 S cm⁻¹, which severely impedes the application of PEDOT:PSS in various aspects. It has been discovered that the conductivity of as-prepared PEDOT:PSS from its aqueous solution can be significantly enhanced by adding organic compounds like high-boiling point polar organic solvents, ionic liquids and surfactants or through a post-treatment of PEDOT:PSS films with organic compounds, including high-boiling point polar solvents, salts, zwitterions, cosolvents, organic and inorganic acids. Conductivity of more than 3000 S cm⁻¹ was recently observed on PEDOT:PSS films after treated with sulfuric acid. This conductivity is comparable to that of indium tin oxide (ITO), the conventional transparent electrode material of optoelectronic devices. In addition, PEDOT:PSS has high mechanical flexibility while ITO is a brittle material. Thus, PEDOT:PSS is very promising to be the next-generation transparent electrode material. This article reviews the methods to enhance the conductivity of PEDOT:PSS, the mechanisms for the conductivity enhancements and the application of the highly conductive PEDOT:PSS films in polymer light-emitting diodes and polymer solar cells.     

Electrochemical deposition of polypyrrole/graphene oxide composite on microelectrodes towards tuning the electrochemical properties of neural probes

By exploiting the electrostatic interaction between positively charged pyrrole cation radicals and negatively charged graphene oxide (GO) sheets, we prepared polypyrrole/graphene oxide (PPy/GO) composite films by a one-step electrochemical process. We studied the effects of the polymerization current density and the GO content in electrolyte on the formation of PPy/GO coatings onto platinum neural microelectrode sites. As compared with pure PPy film, PPy/GO coatings show a rougher surface feature with micrometer-scale bulges. The impedance of the PPy/GO coated Pt electrode is only about 10% of the bare Pt electrode at the biological relevant 1 kHz, while the charge capacity density is more than two orders of the magnitude of the bare Pt electrode. Moreover, the PPy/GO coated Pt electrodes show higher performance than the PPy coated electrodes for the application of neural probe.     

In situ three-dimensional analysis of the linear actuation of polypyrrole micro-rod actuators using optical microscopy

Polypyrrole (PPy) micro-rod actuators were electrochemically prepared into a porous polycarbonate (PC) by template synthesis. The linear actuation of the PPy micro-rod actuators at various potentials was consecutively monitored with in situ mode using an optical microscope (OM) and converted into 3-dimesional images. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and a potentiostat-galvanostat were used to confirm the actuation of PPy micro-rods during the redox process. Linear stretching actuation in the longitudinal dimension was successfully demonstrated.     

SnO2 thin film sensor with enhanced response for NO2 gas at lower temperatures

Semiconducting SnO₂ thin films having higher value of electrical conductivity have been deposited using RF sputtering technique in the reactive gas environment (30% O₂ + 70% Ar) using a metallic tin (Sn) target for detection of oxidizing NO₂ gas. The effect of growth pressure (12-18 mTorr) on the surface morphology and structural property of SnO₂ film was studied using Atomic force microscopy (AFM), Scanning electron microscopy (SEM) and X-ray Diffraction (XRD) respectively. Film deposited at 16 mTorr sputtering pressure was porous with rough microstructure and exhibits high sensor response (∼2.9 × 10⁴) towards 50 ppm NO₂ gas at a comparatively low operating temperature (∼100 °C). The sensor response was found to increase linearly from 1.31 × 10² to 2.9 × 10⁴ while the response time decrease from 12.4 to 1.6 min with increase in the concentration of NO₂ gas from 1 to 50 ppm. The reaction kinetics of target NO₂ gas on the surface of SnO₂ thin film at the Sn sites play important role in enhancing the response characteristics at lower operating temperature (∼100 °C). The results obtained in the present study are encouraging for realization of SnO₂ thin film based sensor for efficient detection of NO₂ gas with low power consumption.     

A novel biosensor for detecting toxicity in water using sulfur-oxidizing bacteria

A novel toxicity detection methodology based on sulfur-oxidizing bacteria (SOB) has been developed for the rapid and reliable detection of toxic chemicals in water. The methodology exploits the ability of SOB to oxidize sulfur particles in the presence of oxygen to produce sulfuric acid according to the following equation: S + H₂O + 1.5O₂ → SO₄²⁻ + 2H⁺, ΔG°′ = −587.1 kJ/reaction. The reaction results in an increase in electrical conductivity (EC) and a decrease in pH as SOB convert insoluble sulfur particles to sulfate and protons. The proposed technique is validated using EC and pH data. Using a synthetic stream water (EC = 0.12 mS/cm and pH 7.2), the baseline steady-state EC and pH values were ∼1.0 mS/cm and ∼2.5 over 30 days of testing when hexavalent chromium (Cr⁶⁺) was not added to the system. When Cr⁶⁺ was added to the system, the effluent EC decreased and the pH increased due to inhibition of SOB. We found that the system can detect Cr⁶⁺ at a concentration of 5 ppb which is lower than any method to date.     

Electrosynthsis of large polypyrrole films by multi-potential steps method

Polypyrrole (PPy) films were prepared by multi-potential steps polymerization in an aqueous pyrrole solution, with lithium perchlorate and oxalic acid as supporting electrolytes. Morphology and structure of PPy films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Electrochemical behaviors of PPy films were studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results show that multi-potential steps polymerization improves the conducti...     

PPy/ATP化学修饰电极的制备及其性能研究

采用多步掺杂与脱掺杂的电化学方法制备掺杂ATP的PPy化学修饰电极(PPy/ATP),利用紫外-可见光谱(UV)、傅立叶红外光谱(FTIR)、循环伏安曲线(CV)等对ATP的掺杂进行表征,并对PPy/ATP化学修饰电极的CV曲线及交流阻抗曲线(EIS)进行测试发现,掺杂ATP后,PPy膜的电化学性能仍然较好.利用紫外-可见光谱(UV)对ATP掺杂稳定性进行测试,结果发现,随浸泡时间延长,在260 nm指纹处的吸光度Abs变得越来越小,吸收峰几乎完全消失;但是将电极置于新的氯化钠溶液中电位恒定为负值时,仍然会有ATP脱掺杂出来,说明ATP的掺杂比较稳定,浸泡虽然会使部分脱掺杂,但仍有一部分较稳定地掺杂在PPy中.     

Polyphenol-modified glassy carbon electrodes for copper detection

The present work reports the preparation of electrochemically polymerized flavone - luteolin and flavonol - kaempferol modified glassy carbon (GC) electrodes (PolyLut/GC and PolyKae/GC, respectively). Electrochemical polymerization was performed by electrochemical oxidation of luteolin and kaempferol by potential cycling in aqueous media. Cyclic voltammograms of luteolin on the GC electrode indicated one clear oxidation peak at +475 mV, which can be assigned to the oxidation of 3′-hydroxyl and 4′-hydroxyl groups in the B-ring of the luteolin molecule. The cyclic voltammograms of kaempferol on the GC electrode contained two oxidation peaks, one at about +390 mV, which is assigned to the oxidation of 4′-hydroxyl and 3-hydroxyl groups of the B-ring and C-ring of the kaempferol molecule, and second oxidation peak at about +710 mV, which is assigned to the 7-hydroxyl group of the A-ring. The interaction of PolyLut/GC and PolyKae/GC electrodes with copper(II) (Cu(II)) ions were investigated by differential pulse voltammetry (DPV). It was determined that PolyLut/GC and PolyKae/GC electrodes showed sensitivity towards Cu(II) with good reproducibility and stability of analytical signal. The effect of the interfering ions on the voltammetric measurements of Cu(II) was examined.     

In2O3 + xBaO (x = 0.5-5 at.%) - A novel material for trace level detection of NOx in the ambient

Indium oxide (In₂O₃) doped with 0.5-5 at.% of Ba was examined for their response towards trace levels of NO_x in the ambient. Crystallographic phase studies, electrical conductivity and sensor studies for NO_x with cross interference for hydrogen, petroleum gas (PG) and ammonia were carried out. Bulk compositions with x ≤ 1 at.% of Ba exhibited high response towards NO_x with extremely low cross interference for hydrogen, PG and ammonia, offering high selectivity. Thin films of 0.5 at.% Ba doped In₂O₃ were deposited using pulsed laser deposition technique using an excimer laser (KrF) operating at a wavelength of (λ) 248 nm with a fluence of ∼3 J/cm² and pulsed at 10 Hz. Thin film sensors exhibited better response towards 3 ppm NO_x quite reliably and reproducibly and offer the potential to develop NO_x sensors (Threshold limit value of NO₂ and NO is 3 and 25 ppm, respectively).     

Polypyrrole/phytase amperometric biosensors for the determination of phytic acid in standard solutions

Amperometric biosensors based on the physical immobilization of phytase (PhyA) into polypyrrole (PPy) films were prepared in aqueous medium. The PPy/PhyA films were characterized by cyclic voltammetry, and surface and structural characterization techniques, SEM and FTIR. Both voltammetric and amperometric transduction methods were used in order to detect phytic acid in acetate buffer at pH 5.5 at room temperature. The biosensors exhibited a detection limit of 0.15 mmol L⁻¹ and a linear range of phytic acid content from 0.5 to 2.0 mmol L⁻¹, which are adequate values for typical analyses of phytic acids in most seeds, grains, and vegetables.     

Nanostructured spinel ZnCo2O4 for the detection of LPG

Nanostrucutred spinel ZnCo₂O₄ (∼26-30 nm) was synthesized by calcining the mixed precursor (consisting of cobalt hydroxyl carbonate and zinc hydroxyl carbonate) in air at 600 °C for 5 h. The mixed precursor was prepared through a low cost and simple co-precipitation/digestion method. The transformation of the mixed precursor into nanostructured spinel ZnCo₂O₄ upon calcinations was confirmed by X-ray diffraction (XRD) measurement, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM). To demonstrate the potential applicability of ZnCo₂O₄ spinel in the fabrication of gas sensors, its LPG sensing characteristics were systematically investigated. The ZnCo₂O₄ spinel exhibited outstanding gas sensing characteristics such as, higher gas response (∼72-50 ppm LPG gas at 350 °C), response time (∼85-90 s), recovery time (∼75-80 s), excellent repeatability, good selectivity and relatively lower operating temperature (∼350 °C). The experimental results demonstrated that the nanostructured spinel ZnCo₂O₄ is a very promising material for the fabrication of LPG sensors with good sensing characteristics. Plausible LPG sensing mechanism is also discussed.     

Two-dimensional molecular imprinting approach for the electrochemical detection of trinitrotoluene

In this work, we demonstrated a sensitive and selective electrochemical sensing protocol for the detection of TNT prepared from alkanethiols self-assembled on AuNPs modified glassy carbon (GC) electrode with preadsorbed templates of TNT. It demonstrated that the 2D molecular imprinting monolayers (MIMs) can provide a better site accessibility and lower mass-transfer resistance, while the AuNPs can enhance electrode conductivity, facilitate the electron transfer and increase the amount of TNT-imprinted sites. The prepared sensor showed not only high selectivity toward TNT in comparison to other similar nitroaromatic compounds (NACs), but also a wide linear range over TNT concentration from 4.0 × 10⁻⁸ to 3.2 × 10⁻⁶ M with a detection limit of 1.3 × 10⁻⁸ M (S/N = 3). Moreover, the imprinted sensor has been applied to the determination of TNT in spiked environmental water samples and shows promise for fast and sensitive measurement of trace levels of TNT in real samples.     

Enhanced room temperature response of SnO2 thin film sensor loaded with Pt catalyst clusters under UV radiation for LPG

The room temperature response characteristics of SnO₂ thin film sensor loaded with platinum catalyst clusters are investigated for LPG under the exposure of ultraviolet radiation. The SnO₂-Pt cluster sensor structures have been prepared using rf sputtering. Combined effect of UV radiation exposure (λ = 365 nm) and presence of Pt catalyst clusters (10 nm thick) on SnO₂ thin film sensor surface is seen to lead to an enhanced response (4.4 × 10³) for the detection of LPG (200 ppm) at room temperature whereas in the absence of UV illumination a comparable response (∼5 × 10³) could be obtained but only at an elevated temperature of 220 °C. The present study therefore investigates the effect of UV illumination on LPG sensing characteristics of SnO₂ sensors loaded with Pt clusters of varying thickness values. Results indicate the possibility of utilizing the sensor structure with novel dispersal of Pt catalyst clusters on SnO₂ film surface for efficient detection of LPG at room temperature under the illumination of UV radiations.     

Analyte selective response in solution-deposited tetrabenzoporphyrin thin-film field-effect transistor sensors

Organic thin film transistor (OTFT) chemical sensors rely on the specific electronic structure of the organic semiconductor (OSC) film for determining sensor stability and response to analytes. The delocalized electronic structure is influenced not only by the OSC molecular structure, but also the solid state packing and film morphology. Phthalocyanine (H₂Pc) and tetrabenzoporphyrin (H₂TBP) have similar molecular structures but different film microstructures when H₂Pc is vacuum deposited and H₂TBP is solution deposited. The difference in electronic structures is evidenced by the different mobilities of H₂TBP and H₂Pc OTFTs. H₂Pc has a maximum mobility of 8.6 × 10⁻⁴ cm² V⁻¹ s⁻¹ when the substrate is held at 250 °C during deposition and a mobility of 4.8 × 10⁻⁵ cm² V⁻¹ s⁻¹ when the substrate is held at 25 °C during deposition. Solution deposited H₂TBP films have a mobility of 5.3 × 10⁻³ cm² V⁻¹ s⁻¹, which is consistent with better long-range order and intermolecular coupling within the H₂TBP films compared to the H₂Pc films. Solution deposited H₂TBP also exhibits a textured film morphology with large grains and an RMS roughness 3-5 times larger than H₂Pc films with similar thicknesses. Despite these differences, OTFT sensors fabricated from H₂TBP and H₂Pc exhibit nearly identical analyte sensitivity and analyte response kinetics. The results suggest that while the interactions between molecules in the solid state determine conductivity, localized interactions between the analyte and the molecular binding site dominate analyte binding and determine sensor response.     

Magnetic active-valve micropump actuated by a rotating magnetic assembly

We report on a high-efficiency and self-priming active-valve micropump consisting of a microfluidic chamber structure in glass that is assembled with a polydimethylsiloxane (PDMS) elastic sheet. The latter comprises two valving membranes and a central pumping chamber actuation membrane, having each an integrated permanent magnet that is magnetically actuated by arc-shaped NdFeB permanent magnets mounted on the rotation axis of a DC minimotor. The choice of this actuation principle allows very low-voltage (0.7 V) and low power (a few 10 mW) operation of the micropump. For the realisation, we use affordable powder blasting glass micropatterning and PDMS molding technologies. A flow rate of 2.4 mL/min and up to 70 mbar backpressure are obtained at the micropump resonance frequency of around 12 Hz, values that are much higher than reported so far for such type of micropump.     

Electrical conductivity dependence of Ni doped Sm0.95Ce0.05FeO3−δ on surface morphology and composition

Sm_0.95Ce_0.05Fe_1−xNi_xO_3−δ materials are considered as candidates for sensing reducing gases. The total electrical conductivity of Ni doped Sm_0.95Ce_0.05FeO_3−δ perovskite materials is discussed in terms of Ni concentration, surface morphology and relative surface atomic ratios. Powders of formula Sm_0.95Ce_0.05Fe_1−xNi_xO_3−δ (x = 0-0.10) were prepared from citrate precursors by using a sol gel method and were then pressed uniaxially and sintered at 1350 °C for 4 h to form pellets. In fresh pellets the relative surface atomic ratios of Sm and Ni increased while that of Fe and Ce decreased as a function of nickel concentration, showing the segregation of samarium species. In contrast, the chemically reduced pellets show Fe rich surfaces. The electrical conductivity of fresh, partially reduced (700 °C under 5% (v/v) H₂/N₂ for 1 h) and fully reduced (1000 °C under 5% (v/v) H₂/N₂ for 1 h) pellets was measured by the four probe DC method.Under air, x = 0.07 and x = 0.10 showed the highest electrical conductivity in the series. Interestingly the x = 0.01-0.05 materials were n-type conductors while x = 0.07-0.10 exhibited p-type behaviour. The reduction treatment at 1000 °C enhanced electrical conductivities up to ∼5000 fold due to changes associated with surface morphology and surface elemental composition. While phase separations are usually detrimental, in this case the reduced sensors are more sensitive without sacrificing reproducibility.     

Effect of La2CuO4 electrode area on potentiometric NOx sensor response and its implications on sensing mechanism

The intent of this work is to look at the effects of varying the La₂CuO₄ electrode area and the asymmetry between the sensing and counter electrode in a solid state potentiometric sensor with respect to NO_x sensitivity. NO₂ sensitivity was observed at 500-600 °C with a maximum sensitivity of ∼22 mV/decade [NO₂] observed at 500 °C for the sensor with a La₂CuO₄ electrode area of ∼30 mm². The relationship between NO₂ sensitivity and area is nearly parabolic at 500 °C, decreases linearly with increasing electrode area at 600 °C, and was a mixture of parabolic and linear behavior 550 °C. NO sensitivity varied non-linearly with electrode area with a minima (maximum sensitivity) of ∼−22 mV/decade [NO] at 450 °C for the sensor with a La₂CuO₄ electrode area of 16 mm². The behavior at 400 °C was similar to that of 450 °C, but with smaller sensitivities due to a saturation effect. At 500 °C, NO sensitivity decreases linearly with area.We also used electrochemical impedance spectroscopy (EIS) to investigate the electrochemical processes that are affected when the sensing electrode area is changed. Changes in impedance with exposure to NO_x were attributed to either changes in La₂CuO₄ conductivity due to gas adsorption (high frequency impedance) or electrocatalysis occurring at the electrode/electrolyte interface (total electrode impedance). NO₂ caused a decrease in high frequency impedance while NO caused an increase. In contrast, NO₂ and NO both caused a decrease in the total electrode impedance. The effect of area on both the potentiometric and impedance responses show relationships that can be explained through the mechanistic contributions included in differential electrode equilibria.     

Polypyrrole thin film fiber optic chemical sensor for detection of VOCs

A fiber optic reflectance sensor (FORS) using Polypyrrole (PPy) conducting polymer to detect volatile organic compounds (VOCs) is demonstrated. The conventional interfacial polymerization method is used to synthesize a sensitive polypyrrole membrane, which shows relatively low roughness and high reflectivity. In general the changes in electrical properties of conductive polymers are explored in sensing applications, however their optical properties have been less explored. In the present study, we evaluate the optical properties of PPy and transfer on the end face of polymer optical fiber (POF) for the detection of VOCs. The change in the reflected optical signal from PPy upon interaction with the VOCs is systematically evaluated. The fabricated PPy FORS shows the excellent sensitivity to the VOCs under test with the detection limit up to 1 ppm.     

Electrochemical behavior of graphene doped carbon paste electrode and its application for sensitive determination of ascorbic acid

In present paper, the graphene doped carbon paste electrode (CPE) was firstly prepared with the addition of graphene into the carbon paste mixture. Compared with conventional CPE, an improved electrochemical response of graphene doped CPE toward the redox couple of Fe(CN)₆^3−/4− was demonstrated owing to the excellent electrical conductivity of graphene. The graphene doped CPE was further used for the successful determination of ascorbic acid (AA), and it showed an excellent electrocatalytic oxidation activity toward AA with a lower overvoltage, pronounced current response, and good sensitivity. Under the optimized experimental conditions, the proposed electrochemical AA sensor exhibited a rapid response to AA within 5 s and a linear calibration plot ranged from 1.0 × 10⁻⁷ to 1.06 × 10⁻⁴ M was obtained with a detection limit of 7.0 × 10⁻⁸ M.