We report here how the electrochemical impedance spectra change as (i) electrode size is reduced to nanometer scale and (ii) spacing between vertically aligned carbon nanofiber (VACNF) electrodes is varied. To study this, we used three types of electrodes: standard microdisks (100 microm Pt, 10 microm Au, and 7 microm glassy carbon), randomly grown (RG) VACNFs where spacing between electrodes is not fixed, and electron beam patterned VACNF nanoelectrode arrays (pNEAs) where electrode spacing is fixed at 1 microm. As the size of the microdisk electrode is reduced, the spectrum changed from a straight line to a semicircle accompanied by huge noise. Although a semicircle spectrum can directly indicate the electron transfer resistance (R(ct)) and thus is useful for biosensing applications, the noise from electrodes, particularly from those with diameters < or =10 microm, limits sensitivity. In the case of VACNFs, the electrode spacing controls the type of spectrum, that is, a straight line for RG VACNFs and a semicircle for pNEAs. In contrast to microdisks, pNEAs showed almost insignificant noise even at small perturbations (10 mV). Second, only pNEAs showed linearity as the amplitude of the sinusoidal signal was increased from 10 to 100 mV. The ability to apply large amplitudes reduces the stochastic errors, provides high stability, and improves signal-to-noise (S/N) ratio. This new class of nanoelectrochemical system using carbon pNEAs offers unique properties such as semicircle spectra that fit into simple circuits, high S/N ratio, linearity, and tailor-made spectra for specific applications by controlling electrode size, spacing, and array size.
Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter that is essential for normal brain function. It is involved in multiple neuronal activities, including plasticity, information processing, and network synchronization. Abnormal GABA levels result in severe brain disorders and therefore GABA has been the target of a wide range of drug therapeutics. GABA being non-electroactive is challenging to detect in real-time. To date, GABA is detected mainly via microdialysis with a high-performance liquid chromatography (HPLC) system that employs electrochemical (EC) and spectroscopic methodology. However, these systems are bulky and unsuitable for real-time continuous monitoring. As opposed to microdialysis, biosensors are easy to miniaturize and are highly suitable for in vivo studies; they selectively oxidize GABA into a secondary electroactive product (usually hydrogen peroxide, H2O2) in the presence of enzymes, which is then detected by amperometry. Unfortunately, this method requires a rather cumbersome process with prereactors and relies on externally applied reagents. Here, we report the design and implementation of a GABA microarray probe that operates on a newly conceived principle. It consists of two microbiosensors, one for glutamate (Glu) and one for GABA detection, modified with glutamate oxidase and GABASE enzymes, respectively. By simultaneously measuring and subtracting the H2O2 oxidation currents generated from these microbiosensors, GABA and Glu can be detected continuously in real-time in vitro and ex vivo and without the addition of any externally applied reagents. The detection of GABA by this probe is based upon the in-situ generation of α-ketoglutarate from the Glu oxidation that takes place at the Glu microbiosensor. A GABA sensitivity of 36 ± 2.5 pA μM-1cm-2, which is 26-fold higher than reported in the literature, and a limit of detection of 2 ± 0.12 μM were achieved in an in vitro setting. The GABA probe was successfully tested in an adult rat brain slice preparation. These results demonstrate that the developed GABA probe constitutes a novel and powerful neuroscientific tool that could be employed in the future for in vivo longitudinal studies of the combined role of GABA and Glu (a major excitatory neurotransmitter) signaling in brain disorders, such as epilepsy and traumatic brain injury, as well as in preclinical trials of potential therapeutic agents for the treatment of these disorders.
This paper reports the recent development and applications of conductive borondoped ultrananocrystalline diamond (BD-UNCD). The authors have determined that BD-UNCD can be synthesized with an H-rich gaseous chemistry and a high CH 4 /H 2 ratio, which is opposite to previously reported methods with Ar-rich or H-rich gas compositions but utilizing very low CH 4 /H 2 ratio. The BD-UNCD has a resistivity as low as 0.01 ohm·cm, with low roughness (down to several nm) and a wide deposition temperature range (450-850¼C).The properties of this BD-UNCD were studied systematically using resistivity characterization, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and roughness measurements. Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy confirms that up to 97% of the UNCD is deposited as sp3 carbon.These series of measurements also reveal additional unique properties for this material, such as an ÒMÓ shape Raman signature, line-granular nano-cluster texture and high C-H bond surface content. A hypothesis is provided to explain why this new deposition strategy, with H-rich/Ar-free gas chemistry and CH 4 /H 2 ratio, is able to produce high sp3-content and/or 2 heavily doped UNCD. In addition, a few emerging applications for BD-UNCD in the field of atomic force microscopy, electrochemistry and biosensing are reviewed here.
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