Microelectrode Designs for Oxidase-Based Biosensors. In: Marinesco S, Dale N, editors. Microelectrode Biosensors. Totowa: Humana Press; 2013. pp. 3-25. [DOI: 10.1007/978-1-62703-370-1_1]

Neurotransmitter Release of Reprogrammed Cells Using Electrochemical Detection Methods

The detection of neurotransmitter release from reprogrammed human cell is an important demonstration of their functionality. Electrochemistry has the distinct advantages over alternative methods that it allows for the measuring of the analyte of interest at a high temporal resolution. This is necessary for fast events, such as neurotransmitter release and reuptake, which happen in the order of milliseconds to seconds. The precise description of these kinetic events can lead to insights into the function of cells in health and disease and allows for the exploration of events that might be missed using methods that look at absolute concentration values or methods that have a slower sampling rate. In the present chapter, we describe the use of constant potential amperometry and enzyme-coated multielectrode arrays for the detection of glutamate in vitro. These biosensors have the distinct advantage of "self-referencing," a method providing high selectivity while retaining outstanding temporal resolution. Here, we provide a step-by-step user guide for a commercially available system and its application for in vitro systems such as reprogrammed cells.

Electrochemical Nitric Oxide Microsensors Based on a Fluorinated Xerogel Screening Layer for in Vivo Brain Monitoring

Nitric oxide (NO) is an important free radical synthesized and released by brain cells. At low (nanomolar) levels, it modulates synaptic transmission and neuronal activity, but at much higher levels mediates neuronal injury through oxidative stress. However, the precise concentrations at which these biological actions are exerted are still poorly defined. Electrochemical detection of NO in vivo requires rigorous exclusion of endogenous redox molecules such as ascorbate or nitrite. A fluorinated xerogel composed of trimethoxymethylsilane and heptadecafluoro-1,1,2,2-tetrahydrodecyl silane has been proposed to create a screening layer around NO sensors, protecting against such chemical interference in vitro. Here we detected NO in the living brain using carbon fiber microelectrodes covered with nickel porphyrin and this fluorinated xerogel. These microsensors were insensitive to interfering redox molecules and surpassed similar microelectrodes coated with a Nafion screening layer. In vivo, in the rat parietal cortex, these electrodes could detect brain NO released by local microinjection of the glutamatergic agonist N-methyl-d-aspartate (NMDA). NMDA-evoked NO release peaked at 1.1 μM and lasted more than 20 min. This fluorinated xerogel screening layer can therefore be applied in vivo, allowing for the fabrication of highly specific microsensors to study NO physio-pathological actions in the brain.

Minimally Invasive Microelectrode Biosensors Based on Platinized Carbon Fibers for in Vivo Brain Monitoring

The ability to monitor the chemical composition of brain interstitial fluid remains an important challenge in the field of bioanalytical chemistry. In particular, microelectrode biosensors are a promising resource for the detection of neurochemicals in interstitial fluid in both animals and humans. These biosensors can provide second-by-second temporal resolution and enzymatic recognition of virtually any redox or nonredox molecule. However, despite miniaturization of these sensors to 50-250 μm in diameter to avoid vascular and cellular injury, inflammation and foreign-body reactions still occur following their implantation. Here, we fabricated microelectrodes with platinized carbon fibers to create biosensors that have an external diameter that is less than 15 μm. Platinization was achieved with physical vapor deposition, and increased sensitivity to hydrogen peroxide and improved enzymatic detection were observed for these carbon fiber microelectrodes. When these devices were implanted in the brains of rats, no injuries to the parenchyma or brain blood vessels were detected. In addition, these microelectrodes provided different estimates of basal glucose, lactate, and oxygen concentrations compared to conventional biosensors. Induction of spreading depolarization in the cerebral cortex further demonstrated the greater sensitivity of our microelectrodes to dynamic neurochemical changes. Thus, these minimally invasive devices represent a major advance in our ability to analyze brain interstitial fluid.

Glucose and Lactate Miniaturized Biosensors for SECM-Based High-Spatial Resolution Analysis: A Comparative Study

With the aim of developing miniaturized enzymatic biosensors suitable for in vitro diagnostic applications, such as monitoring of metabolites at single cell level, glucose and lactate biosensors were fabricated by immobilizing enzymes (glucose oxidase and lactate oxidase, respectively) on 10 μm Pt ultramicroelectrodes. These electrodes are meant to be employed as probes for scanning electrochemical microscopy (SECM), which is a unique technique for high-spatial-resolution electrochemical-based analysis. The use of enzymatic moieties improves sensitivity, time scale response, and information content of the microprobes; however, protein immobilization is a key step in the biosensor preparation that greatly affects the overall performance. A crucial aspect is the miniaturization of the sensing, preserving their sensitivity. In this work, we investigated the most common enzyme immobilization techniques. Several fabrication routes are reported and the main figures of merit, such as sensitivity, detection limit, response time, reproducibility, spatial resolution, biosensor efficiency, permeability, selectivity, and the ability to block electro-active interfering species, are investigated and compared. With the intent of using the microprobes for in vitro functional imaging of single living cells, we carefully evaluate the spatial resolution achieved by our modified electrodes on 2D SECM imaging. Metabolic activity of single MCF10A cells were obtained by monitoring the glucose concentrations in close proximity of single living cell, using the UME-based biosensor probes prepared. A voltage-switch approach was implemented to disentangle the topographical contribution of the cells enabling quantitative measurements of cellular uptakes.