Analysis of Carbon-Based Microelectrodes for Neurochemical Sensing

Abstract

The comprehensive microscopic, spectroscopic, and in vitro voltammetric analysis presented in this work, which builds on the well-studied properties of carbon-based materials, facilitates potential ways for improvement of carbon fiber microelectrodes (CFMs) for neuroscience applications. Investigations by both, scanning electron microscopy (SEM) and confocal Raman spectroscopy, confirm a higher degree of structural ordering for the fibers exposed to carbonization temperatures. An evident correlation is also identified between the extent of structural defects observed from SEM and Raman results with the CFM electrochemical performance for dopamine detection. To improve CFM physico-chemical surface stability and increase its mechanical resistance to the induced compressive stress during anticipated in vivo tissue penetration, successful coating of the carbon fiber with boron-doped diamond (BDD) is also performed and microspectroscopically analyzed here. The absence of spectral shifts of the diamond Raman vibrational signature verifies that the growth of an unstrained BDD thin film was achieved. Although more work needs to be done to identify optimal parameter values for improved BDD deposition, this study serves as a demonstration of foundational technology for the development of more sensitive electrochemical sensors, that may have been impractical previously for clinical applications, due to limitations in either safety or performance.

Keywords: boron-doped diamond thin film; carbon fiber; confocal Raman spectroscopy; fast-scan cyclic voltammetry (FSCV); scanning electron microscopy (SEM).

Figure 1

(a–c) Side view and cross-sectional SEM images of carbon fibers. For cross-sectional images mounting on Si substrates and fiber trimming were performed.

Figure 2

Cross-sectional SEM images, at different resolutions, of carbon fibers: (a,b) sample 1 exposed to a thermal stabilization process (225–285 °C); (c,d) sample 2 carbonized to low temperatures (450–950 °C); and (e,f) sample 3 carbonized to high temperatures (1000–1500 °C).

Figure 3

(a,b) High resolution SEM images of samples 2 and 3 enabling visualization of nanoscale defects near CF surfaces.

Figure 4

(a,b) Surface confocal Raman mapping images of samples 1 and 3, respectively. (c,d) Cross-sectional confocal Raman mapping images of samples 1 and 3, respectively. (e) Integrated Raman spectra of the three samples, as labeled. For clarity of vibrational assignments, the spectra were deconvoluted with Lorentz fitting lines.

Figure 5

(a,b) Cyclic voltammograms at 200, 400, 600, 800 and 1000 nM dopamine (DA) for carbon fiber microelectrodes (CFMs) fabricated from samples 2 and 3 CFs, respectively, as labeled. (c) Shape and amplitude of the background current for the CFMs associated with (a,b). (d) Average detected oxidation peak current showing a linear increase for both CFMs, with a correlation coefficient of about 1.0 based upon three trials at each DA concentration.

Figure 6

(a) Optical image of a boron-doped diamond (BDD) coated CF. (b–d) Confocal Raman mapping images of diamond (red), boron incorporation (blue), and sp² carbon content (green), respectively. (e) Confocal Raman mapping image of the BDD coated CF performed with Cluster Analysis software. (f) Raman spectra associated with each cluster in image (e) and only in vibrational regions of interest. The same color code is maintained for the image and for the spectra.