Enhanced Electron Transfer Rates by AC Voltammetry for Ferrocenes Attached to the End of Embedded Carbon Nanofiber Nanoelectrode Arrays

The effect of the interior structure of carbon nanomaterials on their electrochemical properties is not well understood. We report here the electron transfer rate (ETR) of ferrocene (Fc) molecules covalently attached to the exposed end of carbon nanofibers (CNFs) in an embedded nanoelectrode array. The ETR in normal DC voltammetry was found to be limited by the conical graphitic stacking structure interior of CNFs. AC voltammetry, however, can cope with this intrinsic materials property and provide over 100 times higher ETR, likely by a new capacitive pathway. This provides a new method for high‐performance electroanalysis using CNF nanoelectrodes.     

Solid-state pH nanoelectrode based on polyaniline thin film electrodeposited onto ion-beam etched carbon fiber

An ultramicro pH sensor has been constructed based on a thin polyaniline film that was electrochemically deposited onto a carbon fiber nanometer-size electrode. The substrate nanoelectrodes were fabricated using ion-beam conically etched carbon fibers with tip diameters ranging ca. from 100 to 500 nm. The polyaniline film was deposited from HCl solution containing the aniline monomer by cycling the potential between −0.2 and +1.0 V. The electromotive force (emf) signal between the pH sensitive polyaniline-coated nanoelectrode and an Ag/AgCl reference electrode was linear over the pH range of 2.0-12.5 with a slightly super-Nernstian slope of ca. −60 mV/pH unit. Response times ranged from several sec at pHs around 7 up to 2 min at pH 12.5. The proposed pH nanoelectrode displayed high ion selectivity with respect to K⁺, Na⁺, Ca²⁺, and Li⁺, with log K_H,M values around −12 and has a working lifetime of about 20 days. Key parameters important for the pH nanoelectrode performance, including polyaniline film preparation, selectivity, response time, temperature dependence, relative coating thickness, stability, and reproducibility, have been characterized and optimized. The performance of the pH nanoelectrode was examined by measuring the pH of several real samples including body fluids (serum, urine) and low ionic strength water samples (rain, deionized and tap water). The results agreed very well with those obtained by using commercial glass pH electrodes. The proposed pH nanoelectrode demonstrated attractive properties and seems particularly promising for use under physiological conditions.     

The Convenient Determination of Palladium at a Solid Electrode via Adsorptive Stripping Voltammetry at a Glassy Carbon Electrode Modified with a Random Array of Mercury Nanodroplets

The detection of palladium using adsorptive stripping voltammetry reported by Wang et al. (J. Wang, K. Varughese Anal. Chim. Acta 1987, 199, 185 [3]) at a hanging mercury drop electrode is extended to a more convenient solid electrode. To this end a random array of 3.5×10⁸ mercury nanodroplets per cm² (65 nm average diameter) was electrodeposited on a glassy carbon substrate. Adsorptive stripping voltammetry was performed using 2×10^?4 M dimethylglyoxime as a chelating agent for the Pd(II) ion, with accumulation at ?0.20 V vs. SCE for 120 s and a linear detection range of 5–150 μM was determined with a limit of detection of 1.6 μM.     

Fabrication of nanometre-sized platinum electrodes by controllable electrochemical deposition

A new and simple method for fabricating controllable insulated nanometer-sized platinum electrodes is presented. Electrochemical etching of platinum wire is employed, and then a repeated process of cycle voltammetric deposition of electrophoretic paint and heat curing for shrink film follows which effectively controls the size of the nanoelectrodes, which is different from previous DC electrolysis deposition. This technique allows complete insulation of the whole body of the etched platinum wire, except for the very tip with the shrink film during heat curing of the film, leaving an electrochemical active area with effective diameters of nanometers. The process overcomes the pinhole formation resulting from the electrophoretic paint deposition process. The size of the platinum electrodes and the thickness of the deposed paint for insulation can be properly controlled and reproduced. The fabricated electrodes show ideal steady-state voltammetric behaviors from which the effective areas of the nanoelectrodes are measured. The effective radius of the prepared nanoelectrodes ranges from 3.1 nm to hundreds of nanometers.     

Preparation of Nanoelectrode Ensembles by Assembly of Nano—Silver Colloid on Gold Surface

A novel method for preparing silver nanoelectrode ensembles(SNEEs) and gold nanoelectrode ensembles (GNEEs) has been developed. Silver colloid particles were first absorbed to the gold electrode surface to form a monolayer silver colloid. N-hexadecyl nercaptan was then assembled on the electrode to form a thoil monolayer on which hydrophilic ions cannot be transfered. The SNEEs was prepared by removing thiol from silver colloid surface through applying and AC voltage with increasing frequency at 0.20V(vs.SCE). Finally,GNEEs was obtained by immersing a SNEEs into 6 mol/L HNO3 to remove the silver colloid particles. By comparison with other methods such as template method ect., this method enjoys some advantages of lower resistance, same diameter,easy preparation,controllable size and density.     

Carbon Nanotubes Based Nanoelectrode Arrays: Fabrication,Evaluation, and Application in Voltammetric Analysis

Fabrication, electrochemical characterization, and applications of low‐site density carbon nanotubes based nanoelectrode arrays (CNTs‐NEAs) are reported in this work. Spin‐coating of an epoxy resin provides a new way to create the electrode passivation layer effectively reducing electrode capacitance and current leakage. Cyclic voltammetry showed the sigmoidal shape curves with low capacitive current and scan‐rate‐independent limiting current. Square‐wave voltammetry showed well‐defined peak shapes in voltammograms of K₃Fe(CN)₆ and 4‐acetamidophenol (acetaminophen) and the peak currents to be proportioned to their concentrations, demonstrating the feasibility for voltammetric analysis of the CNTs‐NEAs. The CNTs‐NEAs were also used successfully for voltammetric detection of trace concentrations of lead(II) at ppb level at first‐time. The CNTs‐NEAs provide an excellent platform for ultra sensitive electrochemical sensors for chemical and biological sensing.     

Microfluidic-based electrochemical genosensor coupled to magnetic beads for hybridization detection

This paper describes the development of a rapid and sensitive enzyme-linked electrochemical genosensor using a novel microfluidic-based platform. In this work, hybridization was performed on streptavidin-coated paramagnetic micro-beads functionalized with a biotinylated capture probe. The complementary sequence was then recognized via sandwich hybridization with a capture probe and a biotinylated signaling probe. After labeling the biotinylated hybrid with a streptavidin-alkaline phosphatase conjugate, the beads were introduced in a disposable cartridge composed of eight parallel microchannels etched in a polyimide substrate. The modified beads were trapped with a magnet addressing each microchannel individually. The presence of microelectrodes in each channel allowed direct electrochemical detection of the enzymatic product within the microchannel. Detection was performed in parallel within the eight microchannels, giving rise to the possibility of performing a multiparameter assay. Quantitative determinations of the analyte concentrations were obtained by following the kinetics of the enzymatic reaction in each channel. The chip was regenerated after each assay by removing the magnet and thus releasing the magnetic beads. The system was applied to the analytical detection of PCR amplified samples with a RSD% = 6. A detection limit of 0.2 nM was evaluated.     

Monitoring of Intracellular Vesicles in Cultured Neurons at Different Growth Stages Using Intracellular Vesicle Electrochemical Cytometry

The exploration of intracellular vesicles plays crucial roles for neuronal activity assessment. Neurons at different growth stages may possess distinct neuronal activity, including vesicular content and release kinetics. Here, we monitored the vesicular content and its release kinetics in neurons at different growth stages by intracellular vesicle electrochemical cytometry. We found that the neurotransmitters content of vesicles changed to be increased and the vesicle release process became longer as the neurons grew. Further, we demonstrated that the vesicular adsorption and rupture modes changed from the dominant simple event to simple and complex event coexisting mode. We speculate that this work provides a new strategy for the neuronal activity assessment or real- time cell activity analysis.     

Recent advances in sensing and biosensing with arrays of nanoelectrodes

This review deals with recent advances in the field of electrochemical sensing and biosensing with nanoelectrode ensembles (NEEs) and nanoelectrode arrays (NEAs), focusing mainly on articles published since 2015. At first, a brief introduction on the properties and possible advantages which characterize electroanalytical signals at the NEE/NEA is presented, followed by an overview on the most recent theoretical advances concerning the modeling of relevant electrochemical signals. Novel nanofabrication methods and nanoelectrode materials are discussed together with original (bio)funtionalization procedures, suitable to obtain more sensitive and reliable sensors. Advanced applications of NEE/NEA-based sensors in the biological and biomedical field are presented, including their integration with living cells and application for neurochemical studies. Advances, present limits, and prospects for research in the area are finally discussed. As far as future research trends are concerned, on the one hand, there is a need for development of theoretical models which take into account specific effects that can rule electrochemistry with arrays of nanosized electrodes, such as double layer and quantum mechanical effects. On the other hand, frontier studies concerning the application of the NEE/NEA to the biomedical and neurochemical fields can open new tracks both to fundamental knowledge and application.     

One step fabrication of nanoelectrode ensembles formed via amphiphilic block copolymers self-assembly and selective voltammetric detection of uric acid in the presence of high ascorbic acid content

A novel one-step approach to glassy carbon nanoelectrode ensembles (NEEs) with the pores of 20-120 nm in radii has been developed using an amphiphilic block copolymer [polystyrene-block-poly (acrylic acid)] self-assembly. This procedure is simple and fast, and requires only conventional, inexpensive electrochemical instrumentation. Electrochemical methods were used to characterize the NEEs prepared using this new procedure. The NEEs drastically suppressed the response of ascorbic acid (AA) and resolved the overlapping voltammetric response of uric acid (UA) and AA into two well-defined peaks with a large anodic peak difference (ΔE_pa) of about 310 mV. The peak current obtained from differential pulse voltammetry (DPV) was linearly dependent on the UA concentration in the range of 0.25-50 μM at neutral pH (PBS, pH 6.86) with a correlation coefficient of 0.999, and the detection limit was 0.04 μM (S/N = 3). The NEEs has also been demonstrated to be applicable in the detection of UA in serum and urine samples with excellent sensitivity and selectivity. The NEEs will hopefully be of good application for further sensor development.