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A systematic study of electrochemically roughened (ECR) thin film platinum (Pt) microelectrodes for glutamate, GLU (a major excitatory neurotransmitter) detection is presented. Scanning electron microscopy, energy dispersive spectroscopy, surface profilometry, electrochemical impedance spectroscopy and amperometry techniques were applied to investigate the effect of high-frequency electrical pulses on Pt microelectrode roughness, electroactive area, charge transfer resistance, and sensitivity and selectivity to hydrogen peroxide, a by-product of enzymatic biosensors and GLU. An increase in the mean surface roughness from 9.0 ± 0.5 to 116.3 ± 7.4 nm (n = 3) was observed which resulted in a 55 ± 2% (n = 3) increase in the electroactive area. An ECR microelectrode treated at +1.4 V and coated with a selective coating produced a GLU selectivity value of 342 ± 34 (n = 3) vs ascorbic acid and the highest GLU sensitivity of 642 ± 45 nAμM⁻¹cm⁻²(n = 3) when compared to other surface-treated Pt microelectrodes reported in the literature. An impedance model was created to elucidate the microstructural and electrochemical property changes to the ECR microelectrodes. The ECR surface comprises of uniformly distributed homogenous pores with very low impedance, which is ∼6-times lower when compared to a methanol cleaned electrode. The model could lay a foundation for the rational designing of biosensors for enhanced neurotransmitter detection.

 

The brain is a complex network that accounts for only 5% of human mass but consumes 20% of our energy. Uncovering the mysteries of the brain’s functions in motion, memory, learning, behavior, and mental health remains a hot but challenging topic. Neurochemicals in the brain, such as neurotransmitters, neuromodulators, gliotransmitters, hormones, and metabolism substrates and products, play vital roles in mediating and modulating normal brain function, and their abnormal release or imbalanced concentrations can cause various diseases, such as epilepsy, Alzheimer’s disease, and Parkinson’s disease. A wide range of techniques have been used to probe the concentrations of neurochemicals under normal, stimulated, diseased, and drug-induced conditions in order to understand the neurochemistry of drug mechanisms and develop diagnostic tools or therapies. Recent advancements in detection methods, device fabrication, and new materials have resulted in the development of neurochemical sensors with improved performance. However, direct in vivo measurements require a robust sensor that is highly sensitive and selective with minimal fouling and reduced inflammatory foreign body responses. Here, we review recent advances in neurochemical sensor development for in vivo studies, with a focus on electrochemical and optical probes. Other alternative methods are also compared. We discuss in detail the in vivo challenges for these methods and provide an outlook for future directions. 

Electrical microstimulation has enabled partial restoration of vision, hearing, movement, somatosensation, as well as improving organ functions by electrically modulating neural activities. However, chronic microstimulation is faced with numerous challenges. The implantation of an electrode array into the neural tissue triggers an inflammatory response, which can be exacerbated by the delivery of electrical currents. Meanwhile, prolonged stimulation may lead to electrode material degradation., which can be accelerated by the hostile inflammatory environment. Both material degradation and adverse tissue reactions can compromise stimulation performance over time. For stable chronic electrical stimulation, an ideal microelectrode must present 1) high charge injection limit, to efficiently deliver charge without exceeding safety limits for both tissue and electrodes, 2) small size, to gain high spatial selectivity, 3) excellent biocompatibility that ensures tissue health immediately next to the device, and 4) stable in vivo electrochemical properties over the application period. In this review, the challenges in chronic microstimulation are described in detail. To aid material scientists interested in neural stimulation research, the in vitro and in vivo testing methods are introduced for assessing stimulation functionality and longevity and a detailed overview of recent advances in electrode material research and device fabrication for improving chronic microstimulation performance is provided.