Controlled Electrochemical Synthesis of Polypyrrole Nanoparticle Thin Film and Its Redox Transition to a Highly Conductive and Stable Polypyrrole Variant

We demonstrated here a unique method to produce a highly stable and conductive polypyrrole (PPY) nanoparticle film. The procedure entails controlling the redox switching and the electrochemical synthesis of PPY. PPY was synthesized at a very low forming potential or reaction rate in nonaqueous CH2Cl2 solvent to promote the PPY nanoparticle formation. Then its property was further optimized by first electrochemically reducing it at a hydrogen evolution potential in a neutral 0.1 M NaClO4, then in a slightly acidic 0.05 M asparagine electrolyte. The PPY nanoparticle thin film was characterized by AFM, UV−vis and EQCM. The procedures described here have proven to be reproducible. The data provided by the EQCM shows a reversible doping and undoping mechanism of asparagine indicating the presence of a highly conductive PPY variant. Both UV−vis and electrochemical characterization suggest that the PPY film made using our approach has excellent redox activity as well as high stability when characterized in asparagine solution. The reversible doping and undoping of asparagine during redox switching shows great potential of these PPY nanoparticle films as biological membranes for a broad range of biological applications

Dynamics of Redox Processes in Ionic Liquids and Their Interplay for Discriminative Electrochemical Sensing

Motivated by the use of ionic liquids (ILs) as green replacers of traditional electrolytes, a mechanistic study has been systematically conducted to comprehend various design principles responsible for electrochemical profiling of redox-active species in ILs. The full spectrum of properties associated with ILs is exploited to assess the viability of this platform, thus revealing the correlation between the redox properties and the physiochemical parameters of the species involved. This includes the evaluation of (1) the variation of redox responses toward analytes with similar molecular structures or functionalities of ILs, (2) the influence in terms of physical criteria of the system such as viscosity and conductivity as well as chemical structure of ILs, and (3) the sustainability in harsh conditions (high temperature or humidity) and interferences. The principle is exemplified via trinitrotoluene (TNT) and dinitrotoluene (DNT) with inherent redox activity as analytes and IL membranes as solvents and electrolytes using glassy carbon (GC) electrodes. A discrete response pattern is generated that is analyzed through linear discriminant analysis (LDA) leading to 100% classification accuracy even for the mixture of analytes. Quantitative analysis through square wave voltammetry (SWV) gave rise to the detection limits in liquid phase of 190 and 230 nM for TNT and DNT, respectively, with a linear range up to 100 μM. Gas-phase analysis shows strong redox signals for the estimated concentrations of 0.27 and 2.05 ppm in the gas phase for TNT and DNT, respectively, highlighting that ILs adopt a role as a preconcentrator to add on sensitivity with enhanced selectivity coming from their physiochemical diversity, thus addressing the major concerns usually referred to most sensor systems

Study of Ionic Liquid Immobilization on Polyvinyl Ferrocene Substrates for Gas Sensor Arrays

In this report, the effects of conductive polymer oxidation states and structures on the design and development of ionic liquid (IL)/conductive polymer (CP) composite films for gas sensing are systematically characterized. Four different polyvinyl ferrocene (PVF) films synthesized by varying the conditioning potential (0.7 vs 0.0 V) and the electrolyte are tested for their gas-sensing properties (e.g., sensitivity, selectivity, response time, linearity, and dynamic range against various gas analytes such as dichloromethane, ethanol, natural gas, methane, formaldehyde (37%), and benzene) utilizing the quartz crystal microbalance (QCM) and ATR-FT-IR. The best available film is further studied as a substrate for the immobilization of various ILs that enhanced both the sensitivity and selectivity. Finally, two arrays, each comprising four sensors with the following scheme are developed and characterized for their ability to classify the four target analytes by using linear discriminant analysis: (1) the highest sensitivity PVF film immobilized with four different ILs and (2) the highest sensitivity IL immobilized in four different PVF films. Array 2 is proven to be much better than array 1 in discriminating the analytes, which is very significant in establishing the fact that a diverse set of PVF redox states allow the rational development of a PVF/IL composite-based sensor array in order to analyze complex mixtures utilizing structural differences and the extent of intermolecular interactions

Characterization of the Native and Denatured Herceptin by Enzyme Linked Immunosorbent Assay and Quartz Crystal Microbalance Using a High-Affinity Single Chain Fragment Variable Recombinant Antibody

Herceptin/Trastuzumab is a humanized IgG1κ light chain antibody used to treat some forms of breast cancer. A phage-displayed recombinant antibody library was used to obtain a single chain fragment variable (scFv, designated 2B4) to a linear synthetic peptide representing Herceptin’s heavy chain CDR3. Enzyme linked immunosorbent assays (ELISAs) and piezoimmunosensor/quartz crystal microbalance (QCM) assays were used to characterize 2B4-binding activity to both native and heat denatured Herceptin. The 2B4 scFv specifically bound to heat denatured Herceptin in a concentration dependent manner over a wide (35–220.5 nM) dynamic range. Herceptin denatures and forms significant amounts of aggregates when heated. UV–vis characterization confirms that Herceptin forms aggregates as the temperature used to heat Herceptin increases. QCM affinity assay shows that binding stoichiometry between 2B4 scFv and Herceptin follows a 1:2 relationship proving that 2B4 scFv binds strongly to the dimers of heat denatured Herceptin aggregates and exhibits an affinity constant of 7.17 × 10¹³ M^–2. The 2B4-based QCM assay was more sensitive than the corresponding ELISA. Combining QCM with ELISA can be used to more fully characterize nonspecific binding events in assays. The potential theoretical and clinical implications of these results and the advantages of the use of QCM to characterize human therapeutic antibodies in samples are also discussed

General Signal Amplification Strategy for Nonfaradic Impedimetric Sensing: Trastuzumab Detection Employing a Peptide Immunosensor

A label-free and reagent-free peptide mimotope capacitive biosensor has been developed for cancer drug (trastuzumab) quantification based on nonfaradic readout. The low sensitivity issue of capacitive biosensors was overcome with two innovations: peptide mimotope mixed self-assembled monolayer (SAM) biointerface and dilution of the analysis buffer. Signal amplification was achieved through dilution of phosphate-buffered saline (PBS) to tune <i>C</i>_dl to dominate the overall capacitance change upon target binding, which contribution is often negligible without dilution. After 1000× dilution, the limit of detection was lowered 500-fold (0.22 μg/mL) and the sensitivity was increased 20-fold [0.04192 (μg/mL)⁻¹] in comparison with undiluted PBS. The proposed signal amplification strategy is more straightforward and practical compared to biorecognition element engineering and other strategies. The proposed method was further applied to planar electrodes for optimizing sensing response time to less than 1 min

Single Chain Fragment Variable Recombinant Antibody as a Template for Fc Sensors

A label free immunosensor for detection of Fc receptors expressed on cell surface was developed and characterized using a Quartz Crystal Microbalance (QCM) transducer. Taking advantage of the characteristics of single chain fragment variable (scFv) recombinant antibody and the multivalency of an antibody, the engineered recombinant scFv was immobilized onto preformed functionalized self-assembled monolayers (SAMs) template surface. The monomeric scFv can bind with the CH1 region of any rabbit IgG to form a highly oriented IgG layer with its Fc portion pointing toward a solution phase. This results in a highly oriented Fc sensor that can be used to study the thermodynamics and kinetics of binding between the Fc portion of immunoglobulin and the cell surface Fc receptor (FcR), an important area of the immune system. The Fc sensor was used to study the binding between Staphylococcus aureus and the Fc receptor on macrophage. Parallel characterization of cell surface Fc receptors in the same samples by ELISA was also performed

Antimicrobial Susceptibility Assays Based on the Quantification of Bacterial Lipopolysaccharides via a Label Free Lectin Biosensor

A label free lectin biosensor developed in our laboratory that can quantitatively measure the binding between the lectin immobilized at the carbohydrate sensor surface and the lipopolysaccharide (LPS) on Gram-negative bacteria was demonstrated for an antibiotic susceptibility assay. The biosensor utilizes a polythiophene interface containing fused quinone moieties glycosylated to form a carbohydrate platform for the immobilization of Concanavalin A (Con A) and is capable of LPS binding measurements via orthogonal quartz crystal microbalance and electrochemical readouts (EQCM). Such orthogonal transduction provides cross-validation, better sensor sensitivity, and a large dynamic range of the measurements. We have applied this label free lectin biosensor for a new antibiotic susceptibility assay by characterizing the antimicrobial activities of various antibiotics (i.e., ciprofloxacin, ceftriaxone, and tetracycline) against Escherichia coli W1485 as a model system. The label free biosensor allows both end point and real time measurements of antibiotic effects on the bacterial cell surface LPS, which is shown to correlate to their antibiotic effects. At the end point, after 18 h incubation of bacterial cells with these three antibiotics respectively, the bacterial LPS binding signal was reduced to 23%, 27%, and 38%, respectively, for the three antibiotics, indicating that ciprofloxacin is the most effective against this E. coli strain. Real time measurements at the 1 h time point showed a similar trend with a reduction of binding to 91%, 93%, and 95%, respectively. From the binding kinetics of these measurements, the relaxation time (τ) was obtained, where higher τ value means slow binding interactions between the lectin and the bacterial LPS. The obtained order of τ, (i.e., τ_{ciprofloxacin} > τ_ceftriaxone > τ_tetracycline) again indicated that ciprofloxacin has more bactericidal activity than the other two antibiotics with the same concentrations. Thus, we are able to establish that the reduction in the binding of LPS with the lectin Con A sensor upon exposure to various antibiotics has a direct relation with the antibiotic dosages making this label free biosensor assay promising for therapeutic management of these drugs as well as for applications in antibiotic research and development

Anaerobic Oxidation of Methane to Methyl Radical in NTf₂‑Based Ionic Liquids

In this report, we show that at the ionic liquid (IL)/Pt electrode interface, the C–H bond in methane can be activated and abstracted in situ to form methyl radical by electrochemically generated bis­((trifluoromethyl)­sulfonyl)­amide ([NTf₂]) radicals from the ionic liquid, 1-butyl-1-methylpyrrolidinium bis­((trifluoromethyl)­sulfonyl)­amide ([Bmpy]­[NTf₂]). DFT modeling supports the abstraction of hydrogen from CH₄ by the NTf₂• radical resulting in the formation of a methyl radical that then reacts with the IL to form the CF₃• radical and molecular products, CF₃CF₃ and CF₃H. This work thus provides a new pathway of controlled activation of the C–H bond of methane anaerobically at low potential in a neutral condition at room temperature which can have important applications such as organic electrochemistry and preparative-scale organic electrochemical synthesis

Nitrogen-Doped 4H Silicon Carbide Single-Crystal Electrode for Selective Electrochemical Sensing of Dopamine

In this work, we designed, fabricated, and characterized the first nitrogen (N)-doped single-crystalline 4H silicon carbide (4H-SiC) electrode for sensing the neurotransmitter dopamine. This N-doped 4H-SiC electrode showed good selectivity for redox reactions of dopamine in comparison with uric acid (UA), ascorbic acid (AA), and common cationic ([Ru­(NH3)6]3+), anionic ([Fe­(CN)6]3–), and organic (methylene blue) redox molecules. The mechanisms of this unique selectivity are rationalized by the unique negative Si valency and adsorption properties of the analytes on the N-doped 4H-SiC surface. Quantitative electrochemical detection of dopamine by the 4H-SiC electrode was achieved in the linear range from 50 nM to 10 μM with a detection limit of 0.05 μM and a sensitivity of 3.2 nA.μM–1 in a pH = 7.4 phosphate buffer solution. In addition, the N-doped 4H-SiC electrode demonstrated excellent electrochemical stability. This work forms the foundation for developing 4H-SiC as the next-generation robust and biocompatible neurointerface material for a broad range of applications such as the in vivo sensing of neurotransmitters

Characterization of the Ionic Liquid/Electrode Interfacial Relaxation Processes Under Potential Polarization for Ionic Liquid Amperometric Gas Sensor Method Development

Electrochemical amperometric sensors require a constant or varying potential at the working electrode that drives redox reactions of the analyte for detection. The interfacial redox reaction(s) can result in the formation of new chemical products that could change the initial condition of the electrode/electrolyte interface. If the products are not inert and/or cannot be removed from the system such that the initial condition of the electrode/electrolyte interface cannot be restored, the sensor signal baseline would consequently drift, which is problematic for the continuous and real-time sensors. By setting the electrode potential with the periodical ON–OFF mode, electrolysis can be forestalled during the off mode which can minimize the sensor signal baseline drift and reduce the power consumption of the sensor. However, it is known that the relaxation of the structure in the electrical double layer at the ionic liquid/electrode interface to the steps of the electrode potential is slow. This work characterized the electrode/electrolyte interfacial relaxation process of an ionic liquid based electrochemical gas (IL-EG) sensor by performing multiple potential step experiments in which the potential is stepped from an open circuit potential (OCP) to the amperometric sensing potential at various frequencies with different time periods. Our results showed that by shortening the sensing period as well as extending the idle period (i.e., enlarge the ratio of idle period versus sensing period) of the potential step experiments, the electrode/electrolyte interface is prone to relax to its original state, and thus reduces the baseline drift. Additionally, the high viscosity of the ionic liquids is beneficial for electrochemical regeneration via the implementation of a conditioning step at zero volts at the electrode/electrolyte. By setting the working electrode at zero volts instead of OCP, our results showed that it could further minimize the baseline drift, enhance the sensing signal stability, and extend the functioning lifetime of a continuous IL-EG oxygen sensor