Laser Treated Carbon Nanotube Yarn Microelectrodes for Rapid and Sensitive Detection of Dopamine in Vivo

Abstract

Carbon nanotube yarn microelectrodes (CNTYMEs) exhibit rapid and selective detection of dopamine with fast-scan cyclic voltammetry (FSCV); however, the sensitivity limits their application in vivo. In this study, we introduce laser treatment as a simple, reliable, and efficient approach to improve the sensitivity of CNTYMEs by three fold while maintaining high temporal resolution. The effect of laser treatment on the microelectrode surface was characterized by scanning electron microscopy, Raman spectroscopy, energy dispersion spectroscopy, and laser confocal microscopy. Laser treatment increases the surface area and oxygen containing functional groups on the surface, which provides more adsorption sites for dopamine than at unmodified CNTYMEs. Moreover, similar to unmodified CNTYMEs, the dopamine signal at laser treated CNTYMEs is not dependent on scan repetition frequency, unlike the current at carbon fiber microelectrodes (CFMEs) which decreases with increasing scan repetition frequency. This frequency independence is caused by the significantly larger surface roughness which would trap dopamine-o-quinone and amplify the dopamine signal. CNTYMEs were applied as an in vivo sensor with FSCV for the first time and laser treated CNTYMEs maintained high dopamine sensitivity compared to CFMEs with an increased scan repetition frequency of 50 Hz, which is five-fold faster than the conventional frequency. CNTYMEs with laser treatment are advantageous because of their easy fabrication, high reproducibility, fast electron transfer kinetics, high sensitivity, and rapid in vivo measurement of dopamine and could be a potential alternative to CFMEs in the future.

Keywords: Ascorbic acid; CNT yarn; Dopamine; Fast-scan Cyclic Voltammetry; In vivo; Laser treatment.

Figure 1

Comparison of the response to 1 µM dopamine at an unmodified disk CNTYME (black line) and the same electrode after 15 pulses laser treatment (red line). Electrochemical response was measured with scan rate of 400V/s and scan repetition frequency of 10Hz. (A) Background subtracted cyclic voltammograms to 1 µM dopamine, (B) measured oxidation current versus time for a flow injection analysis experiment (dopamine bolus injection and changing back to PBS buffer are marked as black arrows), and (C) background currents in PBS buffer.

Figure 2

Example cyclic voltammograms of 200 µM ascorbic acid on an (A) unmodified CNTYME and (B) laser-treated CNTYME after 15 min equilibration in PBS buffer solution using a waveform of −0.4 to 1.3 V and back at 400 V/s, with sample scanning frequency of 10Hz. (C) The ratio of oxidation current of 1 µM dopamine over 200 µM ascorbic acid at unmodified CNTYME (0.15 ± 0.01) and laser-treated CNTYME (0.43 ± 0.11). * indicates the ratios are significant different (unpaired t-test, p < 0.05, n = 6).

Figure 3

Effect of scan repetition frequency for 1 µM dopamine detection at CFMEs, unmodified and laser treated CNTYMEs: (A) peak oxidation current at CFMEs (green dot, n = 5), unmodified CNTYMEs (black dot, n = 5), and laser treated CNTYMEs (red dot, n = 5) with −0.4 – 1.3 V waveform and scan rate of 400 V/s. Peak currents were normalized to the current at 10 Hz, and error bars represent the standard error of the mean. (B) Example normalized CVs of 1 µM dopamine bolus injection at unmodified CNTYME, laser treated CNTYME, and a CFME at 10 Hz (blue line) and 50 Hz (orange line) scan repetition frequency. CVs are normalized to the 10 Hz signal.

Figure 4

Comparison of the effect of FSCV scan repetition frequency in vivo. Example normalized CVs of stimulated dopamine release at (A) a CFME and (B) a laser etched CNTYME in vivo, with typical FSCV waveform scanning from −0.4V to 1.3V and back at 400V/s, and scan repetition frequency of 10 Hz (blue line) and 50 Hz (orange line). (C) The ratio of oxidation current for stimulated release with 50 Hz to 10 Hz scan repetition frequency at CFMEs (n = 6) and laser-treated CNTYMEs (n = 6). Stimulated dopamine release was detected in the caudate putamen with a stimulation pulses train of 120 pulses at 60 Hz. Error bars are standard error of mean. *** p < 0.001 (unpaired t-test)

Figure 5

SEM images of (A) unmodified CNTYME and (B) laser treated CNTYME. Scale bar: 500nm.

Figure 6

Representative three-dimensional laser confocal profile image of (A) CFME, (B) unmodified CNTYME, and (C) the same microelectrode after laser treatment. Insets indicate the associated line plot from each sample. Note the scales are different for the different panels.