An in vivo probe based on mechanically strong but structurally small carbon electrodes with an appreciable surface area

Echem methods and electrode types of the current in vivo electrochemical sensing

For a long time, people have been eager to realize continuous real-time online monitoring of biological compounds. Fortunately, in vivo electrochemical biosensor technology has greatly promoted the development of biological compound detection. This article summarizes the existing in vivo electrochemical detection technologies into two categories: microdialysis (MD) and microelectrode (ME). Then we summarized and discussed the electrode surface time, pollution resistance, linearity and the number of instances of simultaneous detection and analysis, the composition and characteristics of the sensor, and finally, we also predicted and prospected the development of electrochemical technology and sensors in vivo.

Microelectrochemical Techniques for Probing Kinetics at Liquid/Liquid Interfaces

We provide an overview of recent advances in microelectrochemical approaches to investigate the kinetics of various physicochemical processes that occur at the interface between two immiscible electrolyte solutions (ITIES). To place the advances in context, background material on the structure of the ITIES, derived from both experimental studies and computer simulation, is also provided. The main focus of the article is micro-ITIES techniques, single droplet measurements, microelectrochemical measurements at expanding droplets (MEMED) and scanning electrochemical microscopy (SECM). Recent developments in a combined SECM-Langmuir trough technique for probing diffusion processes across Langmuir monolayers at the water/air (W/A) interface are also highlighted, by considering an organic monolayer at a water surface as a special case of a liquid/liquid interface.

Advanced electroanalytical chemistry at nanoelectrodes

Nanoelectrodes, with dimensions below 100 nm, have the advantages of high sensitivity and high spatial resolution. These electrodes have attracted increasing attention in various fields such as single cell analysis, single-molecule detection, single particle characterization and high-resolution imaging. The rapid growth of novel nanoelectrodes and nanoelectrochemical methods brings enormous new opportunities in the field. In this perspective, we discuss the challenges, advances, and opportunities for nanoelectrode fabrication, real-time characterizations and high-performance electrochemical instrumentation.

One-step electrochemical detection of cholesterol in the presence of suitable K₃Fe(CN)₆/phosphate buffer mediator by an electrochemical approach

One-step approach of cholesterol biosensor was fabricated onto smart micro-chips based on cholesterol oxidase (ChOx) co-immobilized thioglycolic acid self-assembled monolayer (TGA-SAM) for biomedical applications. The selective cholesterol biosensor was investigated with modified tiny micro-chip (Au/SAM/ChOx) by the facile and reliable cyclic voltammetric (CV) method in a K3Fe(CN)6/phosphate buffer (PB) system. The modified micro-chip displayed a large dynamic range (1.0 nmol L(-1) to 1.0 mmol L(-1)), lower detection limit (~0.49 nmol L(-1), based on S/N~3), higher sensitivity (~93.75 µA µmol L(-2) cm(-2)), good linearity (correlation coefficient r(2), 0.9995), lower sample volume (<50.0 μL), and good stability as well as reproducibility. The Au/TGA system was implemented for a facile and simple approach to the immobilization of ChOx onto micro-chip, which can offer analytical access to a large group of enzymes for a wide range of bio-molecule applications in health-care and biomedical fields. This integrated microchip provides a promising low-cost platform for the sensitive and rapid detection of biomolecules using miniatured samples.

Reusable and mediator-free cholesterol biosensor based on cholesterol oxidase immobilized onto TGA-SAM modified smart bio-chips

A reusable and mediator-free cholesterol biosensor based on cholesterol oxidase (ChOx) was fabricated based on self-assembled monolayer (SAM) of thioglycolic acid (TGA) (covalent enzyme immobilization by dropping method) using bio-chips. Cholesterol was detected with modified bio-chip (Gold/Thioglycolic-acid/Cholesterol-oxidase i.e., Au/TGA/ChOx) by reliable cyclic voltammetric (CV) technique at room conditions. The Au/TGA/ChOx modified bio-chip sensor demonstrates good linearity (1.0 nM to 1.0 mM; R = 0.9935), low-detection limit (∼0.42 nM, SNR∼3), and higher sensitivity (∼74.3 µAµM⁻¹cm⁻²), lowest-small sample volume (50.0 μL), good stability, and reproducibility. To the best of our knowledge, this is the first statement with a very high sensitivity, low-detection limit, and low-sample volumes are required for cholesterol biosensor using Au/TGA/ChOx-chips assembly. The result of this facile approach was investigated for the biomedical applications for real samples at room conditions with significant assembly (Au/TGA/ChOx) towards the development of selected cholesterol biosensors, which can offer analytical access to a large group of enzymes for wide range of biomedical applications in health-care fields.

Carbon-ring microelectrode arrays for electrochemical imaging of single cell exocytosis: fabrication and characterization

Fabrication of carbon microelectrode arrays, with up to 15 electrodes in total tips as small as 10-50 μm, is presented. The support structures of microelectrodes were obtained by pulling multiple quartz capillaries together to form hollow capillary arrays before carbon deposition. Carbon ring microelectrodes were deposited by pyrolysis of acetylene in the lumen of these quartz capillary arrays. Each carbon deposited array tip was filled with epoxy, followed by beveling of the tip of the array to form a deposited carbon-ring microelectrode array (CRMA). Both the number of the microelectrodes in the array and the tip size are independently tunable. These CRMAs have been characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, and electrogenerated chemiluminescence. Additionally, the electrochemical properties were investigated with steady-state voltammetry. In order to demonstrate the utility of these fabricated microelectrodes in neurochemistry, CRMAs containing eight microring electrodes were used for electrochemical monitoring of exocytotic events from single PC12 cells. Subcellular temporal heterogeneities in exocytosis (i.e. cold spots vs hot spots) were successfully detected with the CRMAs.

Investigation of Localized Catalytic and Electrocatalytic Processes and Corrosion Reactions with Scanning Electrochemical Microscopy (SECM)

Scanning electrochemical microscopy (SECM) has developed into a very versatile tool for the investigation of solid-liquid, liquid-liquid and liquid-gas interfaces. The arrangement of an ultramicroelectrode (UME) in close proximity to the interface under study allows the application of a large variety of different experimental schemes. The most important have been named feedback mode, generation-collection mode, redox competition mode and direct mode. Quantitative descriptions are available for the UME signal, depending on different sample properties and experimental variables. Therefore, SECM has been established as an indispensible tool in many areas of fundamental electrochemical research. Currently, it also spreads as an important new method to solve more applied problems, in which inhomogeneous current distributions are typically observed on different length scales. Prominent examples include devices for electrochemical energy conversion such as fuel cells and batteries as well as localized corrosion phenomena. However, the direct local investigation of such systems is often impossible. Instead, suitable reaction schemes, sample environments, model samples and even new operation modes have to be introduced in order to obtain results that are relevant to the practical application. This review outlines and compares the theoretical basis of the different SECM working modes and reviews the application in the area of electrochemical energy conversion and localized corrosion with a special emphasis on the problems encountered when working with practical samples.

Carbon fibre-based microbiosensors for in vivo measurements of acetylcholine and choline

This report describes technical improvements to the manufacture of a carbon fibre electrode for the stable and sensitive detection of H2O2 (detection limit at 0.5 microM). This electrode was also modified through the co-immobilisation of acetylcholinesterase (AChE) and/or choline oxidase (ChOx) in a bovine serum albumin (BSA) membrane for the development of a sensor for in vivo measurements of acetylcholine and choline. Amperometric measurements were performed using a conventional three-electrode system forming part of a flow-injection set-up at an applied potential of 800-1100 mV relative to an Ag/AgCl reference electrode. The optimised biosensor obtained was reproducible and stable, and exhibited a detection limit of 1 microM for both acetylcholine and choline. However, due to the high operating potential used, the biosensor was prone to substantial interference from other electroactive compounds, such as ascorbic acid. Therefore, in a further step, a mediated electron transfer approach was used that incorporated horseradish peroxidase into an osmium-based redox hydrogel layered onto the active surface of the electrode. Afterwards, a Nafion layer and a coating containing AChE and/or ChOx co-immobilised in a BSA membrane were successively deposited. This procedure further increased the selectivity of the biosensor, when operated in the same flow-injection system but at an applied potential of -50 mV relative to an Ag/AgCl reference electrode. The sensor exhibited good selectivity and a high sensitivity over a concentration range (0.3-100 microM) suitable for the measurement of choline and acetylcholine in vivo.