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A composite material was prepared from graphene and palladium nanoparticles (PdNP) by layer-by-layer deposition on gold electrodes. The material was characterized by absorption spectroscopy, scanning electron microscopy, Raman spectroscopy and surface plasmon resonance. Cyclic voltammetry demonstrated the presence of electrocatalytic centers in the palladium decorated graphene. This material can serve as a sensor material for hydrogen at levels from 0.5 to 1% in synthetic air. Pure graphene is poorly sensitive to hydrogen, but incorporation of PdNPs increases its sensitivity by more than an order of magnitude. The effects of hydrogen, nitrogen dioxide and humidity were studied. Sensor regeneration is accelerated in humid air. The sensitivity of the nanocomposite depends on the number of bilayers of graphene–PdNPs.
Changes in the electrical conductance at 240 °C of a number of copper(I) compounds including CuI, CuBr, Cu2Te, Cu12Sb4S13, (CuI)2P14, and of NiO were measured following the exposure to the gases ammonia, nitrogen dioxide, nitrous oxide, carbon monoxide, carbon dioxide, and propane. Analytical sensitivity and kinetic parameters were compared quantitatively. The high diversity of sensitivity patterns makes these compounds promising candidates for use in sensor arrays. Principal component analysis of the data obtained with a virtual sensor array enabled highly selective sensing of five of the six analytes studied using only two principle components. Activation energies of the charge transport in the materials were determined.
The usual applications of capacitive detection in chemo- and biosensors are based on changes in effective thickness of insulating layers due to adsorption of analyte onto receptors. Ultrathin chemosensors based on molecularly imprinted polymerization enable a realization of another capacitive approach that exploits changes in electrical capacitance due to modification of the dielectric constant of the polymer. Such chemosensors were prepared by photografted molecularly imprinted polymerization on the surface of gold electrodes. An adsorbed layer of hydrophobic photoinitiator (benzophenone) provided grafted polymerization on the surface of the alkanethiol-modified gold electrode. The chemosensors were characterized by cyclic voltammetry, impedance spectroscopy, and scanning electron and atomic force microscopy. Binding of analyte was detected by measurements of electrical capacitance. The results indicate a decrease of the dielectric constant of the polymer layer due to analyte binding up to 20%.
Complete system for combinatorial synthesis and functional investigation of conductive polymers
(2005)
Complete system for combinatorial electrochemical synthesis and functional characterization of combinatorial libraries formed by different polymers and polymeric structures will be presented. Control of electrical potential of single electrode groups consisting of four electrodes (designed for 4-point measurements ) on the array comprising 96 such electrode groups provides addressable electrochemical polymer synthesis on the defined electrodes. After polymerization, electrical characteristics of polymers or/and polymer/electrode contacts and influence of potential analytes on these characteristic are measured. Final data analysis provides complete analytically relevant information on the investigated thin polymer layers or multilayer polymer structures: absolute and relative sensitivity, response time, reversibility, reproducibility, regeneration kinetics, linearity or binding constant. Examples of applications for the development of gas-sensitive (co)polymers and optimization of their sensitive properties and polymer/metal contacts will be presented.
Reduction on the spot: Planarly adsorbed thiolated porphyrin molecules (purple) incorporated in an insulating alkanethiol monolayer (blue) function as nanoelectrodes for the local reduction of metals (gray). This approach was applied for the in situ synthesis of nanoparticles of a predetermined size.
A new technique for on-line monitoring of analyte binding to sensor surfaces by surface plasmon resonance (SPR) detection is described. It is based on differential measurements using two wavelengths provided by two diode lasers. The technique is as simple and robust as the conventional SPR detection measuring the reflected radiation at fixed incidence angle, but it has the advantage of being nonsensitive to variations of the resonance width and providing essentially higher signal/noise ratios. The paper presents the first four channel prototype system for parallel 2D-monitoring at four different spots. One channel is always used as a reference to compensate temperature fluctuations and nonspecific adsorptions. Calibration with sucrose solutions revealed an absolute sensitivity of Δn ∼ 5 × 10-6. The new technique is tested with a biotin−streptavidin binding and with hybridization/denaturation of DNA. Biotin binding to a streptavidin monolayer is detected with a signal/noise ratio of about 5, which demonstrates the high potential of the new technique for applications in drug discovery. Applications to gene analysis are tested with short oligonucleotides of the sequences used for genotyping human hepatitis C viruses. A selective response to complementary oligonucleotides is observed. The high reproducibility in subsequent cycles of hybridization/denaturation (by formamide or by heating) points out potential applications of the technique in medical diagnostics, food industry, genomics, and proteomics too.
Combinatorial electropolymerization with electrical addressing was realized on the array of 96 electrode groups, each from four electrodes. The polymer synthesis was combined with subsequent high-throughput investigation of analytical properties of synthesized polymers. Two-and four-point techniques were applied simultaneously to measure electrical properties of synthesized polymers and contact resistances between electrodes and polymers and modifications of these parameters on addition of analyte. The system was used for development of sensitive materials for detection of gaseous hydrogen chloride.
The optical properties of electrochemically polymerized N-methylaniline are changed in the presence of gaseous hydrogen chloride. This effect was used for preparation of chemical sensors with transduction based on surface plasmon resonance. The interaction of hydrogen chloride with a two nm layer of poly-(N-methylaniline) on gold leads to the shift of the surface plasmon resonance. The analysis of the resonance spectra demonstrates that the effect is caused by an increase of the imaginary component of the refractive index; a minor contribution is also provided by film condensation leading to an increase of the real component of the refractive index and a decrease in the thickness. The effect is selective and quasi-reversible. The concentration dependence of the gas effect obeys Langmuir's adsorption isotherm with a reciprocal value of the binding constant of 850 ± 160 ppm.
Electron transfer between metal electrodes and ferro/ferricyanide is completely suppressed at low ionic concentration. We describe here a new phenomenon related to this reaction: an immobilisation of thiolated single-stranded DNA on gold electrodes retains this activity at low ionic strength up to the level corresponding to the high ionic strength. In contrast, a hybridisation of the complementary DNAwith the thiolated single-stranded DNA followed by a binding onto the electrodes, attenuated the electrocatalytic effect. These effects can be used for discrimination between single-stranded DNA and double-stranded DNA and for semi-quantitative measurement of complementary DNA in a sample.
An approach for high-throughput analysis of bulk and contact conductance of polymer layers is described and evaluated. The approach, based on s24-technique (simultaneous two- and four-point conductance measurements), was realized as a high-throughput method and applied for investigation of conductive polymers on an array of interdigital platinum electrodes. Several examples demonstrate distinctive influence of combinatorially varied conditions of polymer synthesis (polymerization charge, content of copolymers) as well as chemical treatment of the synthesized polymers in bulk and contact resistance of metal/polymer/metal systems. The developed high-throughput s24-technique can be widely applied to material research for investigation of bulk and contact electrical properties.
The ozone-sensitive visible/near-IR absorbance changes of poly(2-chloroaniline) (P(2ClANI)), poly(N-methylaniline) (P(NMeANI)) and polyaniline (PANI) films were investigated. These three polymer films, which were assumed to be present partially in a protonated form and partially in a reduced form, were prepared by chemical oxidation of the monomers followed by treatment with the reducing agent disodium disulfite. The P(2ClANI) film was sensitive to ozone at concentrations of down to 2.5 ppm, but saturation of sensitivity started to appear above 50 ppm. By contrast, the PANI and P(NMeANI) films were sensitive to ozone in a concentration range of 50–100 ppm without showing saturation of sensitivity.
Chiroselective binding sites have been created on thin gold films by application of the spreader-bar approach.
Impedometric techniques and surface plasmon resonance were applied to detect binding. (R)-(+)-1,1 '-Binaphthyl-2,2 '-diol (R-BNOH) and (S)-(-)-1,1
'-binaphthyl-2,2'-diol (S-BNOH) were used as model analytes. The artificial receptors were prepared by co-adsorption of 16-mercaptohexadecane (matrix) with a thiol-modified chiral selector (template). The conjugates of d, l-thioctic acid and (R)-(+)- or (S)-(-)-1,1'-binaphthyl-2,2'-diamine were used as templates. Different concentration ratios of the matrix and template were tested. No chiral selectivity of surfaces formed by either the matrix or the template alone was observed. The use of alkylthiols shorter than 16-mercaptohexadecane led to the formation of surfaces with no chiral selectivity. The gold electrodes coated by the spreader-bar technique displayed an enantioselectivity of up to 4.76 or up to 2.55 as measured by the capacitive and SPR methods, espectively.
High throughput characterization of combinatorial libraries demands a compromise between completeness of the study and reasonable investigation time. This compromise can be found by the development of test protocols consisting of the minimal number of the most informative measurements. Multiparameter high throughput characterization of sensitive materials is described. The parameters include response time, regeneration time, reversibility, reproducibility, sensitivity, linearity. The approach was applied for characterization of conductometric gas-sensitive polymer materials. Chemical microarrays prepared by electrochemical synthesis of polymers on platinum electrodes.
The reversible chemosensors to creatinine and desmetryn based on artificial chemoreceptors are described. A grafted photopolymerization combined with a technique of molecular imprinting was used for the receptor layer preparation. After polymer deposition and intensive washing from templates, the electrodes were investigated by impedance spectrometry. Creatinine and desmetryn binding were detected as a decrease in the electrode capacitance. Sensor response to creatining was highly selective. No response to the addition of sodium chloride, creatine, urea or glucose were observed. The electrodes coated with the molecularly imprinted polymers selective to desmetryn displayed specific binding of this herbicide: only small capacitive effects were observed to addition of terbumeton or atrazine, while metribuzine displayed capacitance decrease similar to desmetryn.
The technique of grafting polymerization has been used for preparation of thin films of molecularly imprinting polymers on the surface of polypropylene membranes and on hydrophobized gold electrodes. The herbicide desmetryn was used as a template. The solid supports used were hydrophobic, while the polymer was hydrophilic. The adsorbed layer of benzophenone, irradiated by UV-light, initiated a radical polymerization near the surface. Polymer films were characterized by weighing, contact angle measurements and impedance spectroscopy. The electrodes coated with the molecularly imprinted polymers displayed fairly specific binding of desmetryn, as detected by the decrease in the capacitance of the electrode. Only small capacitive effects were observed on addition of terbumeton or atrazine, while metribuzine displayed capacitance decrease similar to desmetryn.
Electropolymerized molecularly imprinted polymers as receptor layers in capacitive chemical sensors
(1999)
The first application of molecularly imprinted polymers to chemical sensors with capacitive detection is described. The sensitive layer was prepared by electropolymerization of phenol on gold electrodes in the presence of the template (phenylalanine). The insulating properties of the polymer layer were studied by electrochemical impedance spectroscopy. Electrical leakages through the polymer layer were suppressed by deposition of a self-assembled monolayer of mercaptophenol before polymerization and of alkanethiol after polymerization. At the final stage of sensor preparation, the template was removed. The multilayer system obtained displayed a decrease in electrical capacitance on addition of phenylalanine. Only a low response was observed toward other amino acids and phenol.