Carbon Nanostructure Based Platform for Enzymatic Glutamate Biosensors

Hydrogen peroxide (H₂O₂) is an important molecule produced in various enzymatic reactions. It is especially important in electrochemical, enzymatic biosensors detecting electroinactive analytes, such as glucose, cholesterol, and glutamate. Thus, there is a strong need for materials that have high affinity for H₂O₂ oxidation or reduction as well as enable immobilization and sustain enzyme activity without any additional polymer layers. Carbon nanofibers (CNFs) directly grown on tetrahedral amorphous carbon (ta-C) are feasible candidates for this purpose as they possess a reasonably wide water window (1.8 V) and good activity for H₂O₂ reduction in physiological pH and contain innately large amounts of suitable functional groups for enzyme immobilization. Here we show their use in ultrafast (<0.05 s) detection of H₂O₂ with the limit of detection of 26 μM and sensitivity of 0.221 A M^–1 cm^–2. Moreover, we show that ta-C/CNF hybrids can be used directly without the mass-transfer limiting polymer layers as effective immobilization platforms for glutamate oxidase for further applications in ultrafast (<0.05 s) glutamate detection. Finally, rat glial cells cultured on CNFs grown from ta-C without any additional coatings, such as polylysine, showed good adhesion on CNFs and no signs of cytotoxicity, indicating suitability of the material for future in vivo applications. This simplified and miniaturized structure provides an extremely interesting platform for various different enzyme-based electrochemical sensors

Partially Reduced Graphene Oxide Modified Tetrahedral Amorphous Carbon Thin-Film Electrodes as a Platform for Nanomolar Detection of Dopamine

In this study we present for the first time tetrahedral amorphous carbon (ta-C)a partially reduced graphene oxide (PRGO) hybrid electrode nanomaterial platform for electrochemical sensing of dopamine (DA). Graphene oxide was synthesized with the modified Hummer’s method. Before modification of ta-C by drop casting, partial reduction of the GO was carried out to improve electrochemical properties and adhesion to the ta-C thin film. A facile nitric acid treatment that slightly reoxidized the surface and modified the surface chemistry was subsequently performed to further improve the electrochemical properties of the electrodes. The largest relative increase was seen in carboxyl groups. The HNO₃ treatment increased the sensitivity toward DA and AA and resulted in a cathodic shift in the oxidation of AA. The fabricated hybrid electrodes were characterized with scanning electron microscopy (SEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and electrochemical impedance spectroscopy (EIS). Compared to the plain ta-C electrode the hybrid electrode was shown to exhibit superior sensitivity and selectivity toward DA in the presence of ascorbic acid (AA), enabling simultaneous sensing of AA and DA close to the physiological concentrations by cyclic voltammetry (CV) and by differential pulse voltammetry (DPV). Two linear ranges of 0–1 μM and 1–100 μM and a detection limit (S/N = 3.3) of 2.6 nM for DA were determined by means of cyclic voltammetry. Hence, the current work provides a fully CMOS-compatible carbon based hybrid nanomaterial that shows potential for <i>in vivo</i> measurements of DA

What determines the electrochemical properties of nitrogenated amorphous carbon thin films?

Abstract Linking structural and compositional features with the observed electrochemical performance is often ambiguous and sensitive to known and unknown impurities. Here an extensive experimental investigation augmented by computational analyses is linked to the electrochemical characterization of in situ nitrogen-doped tetrahedral amorphous carbon thin films (ta-C:N). Raman spectroscopy combined with X-ray reflectivity shows nitrogen disrupting the sp3 C–C structure of the reference ta-C, supported by the observations of graphitic nitrogen substitution in X-ray absorption spectroscopy. The surface roughness also increases, as observed in atomic force microscopy and atomic-level computational analyses. These changes are linked to significant increases in the hydrogen and oxygen content of the films by utilizing time-of-flight elastic recoil detection analysis. The conductivity of the films increases as a function of the nitrogen content, which is seen as a facile reversible outer-sphere redox reaction on ta-C:N electrodes. However, for the surface-sensitive inner-sphere redox (ISR) analytes, it is shown that the electrochemical response instead follows the oxygen and hydrogen content. We argue that the passivation of the required surface adsorption sites by hydrogen decreases the rates of all of the chemically different ISR probes investigated on nitrogenated surfaces significantly below that of the nitrogen-free reference sample. This hypothesis can be used to readily rationalize many of the contradictory electrochemical results reported in the literature