Carbon Nanotubes Grown on Metal Microelectrodes for the Detection of Dopamine

Abstract

Microelectrodes modified with carbon nanotubes (CNTs) are useful for the detection of neurotransmitters because the CNTs enhance sensitivity and have electrocatalytic effects. CNTs can be grown on carbon fiber microelectrodes (CFMEs) but the intrinsic electrochemical activity of carbon fibers makes evaluating the effect of CNT enhancement difficult. Metal wires are highly conductive and many metals have no intrinsic electrochemical activity for dopamine, so we investigated CNTs grown on metal wires as microelectrodes for neurotransmitter detection. In this work, we successfully grew CNTs on niobium substrates for the first time. Instead of planar metal surfaces, metal wires with a diameter of only 25 μm were used as CNT substrates; these have potential in tissue applications due to their minimal tissue damage and high spatial resolution. Scanning electron microscopy shows that aligned CNTs are grown on metal wires after chemical vapor deposition. By use of fast-scan cyclic voltammetry, CNT-coated niobium (CNT-Nb) microelectrodes exhibit higher sensitivity and lower ΔEp value compared to CNTs grown on carbon fibers or other metal wires. The limit of detection for dopamine at CNT-Nb microelectrodes is 11 ± 1 nM, which is approximately 2-fold lower than that of bare CFMEs. Adsorption processes were modeled with a Langmuir isotherm, and detection of other neurochemicals was also characterized, including ascorbic acid, 3,4-dihydroxyphenylacetic acid, serotonin, adenosine, and histamine. CNT-Nb microelectrodes were used to monitor stimulated dopamine release in anesthetized rats with high sensitivity. This study demonstrates that CNT-grown metal microelectrodes, especially CNTs grown on Nb microelectrodes, are useful for monitoring neurotransmitters.

Figure 1

SEM images of (A) bare niobium, (B) bare tantalum, (C) bare CF, (D) CNT-grown niobium, (E) CNT-grown tantalum, and (F) CNT-grown CF. Scale bar: 500 nm.

Figure 2

Electrochemical response of bare metal or carbon fibers with scan rate of 400 V/s and repetition frequency of 10 Hz. (Left) Background current in PBS solution for (A) niobium, (B) tantalum, and (C) carbon fiber microelectrodes. (Right) Background-subtracted cyclic voltammograms for 1 μM dopamine at bare (D) niobium, (E) tantalum, and (F) carbon fiber microelectrodes.

Figure 3

Comparison of electrochemical response at CNT-grown niobium, tantalum, and carbon fiber microelectrodes: background current at (A) CNT-Nb, (B) CNT-Ta, and (C) CNT-CF and background-subtracted cyclic voltammograms for 1 μM dopamine at (D) CNT-Nb, (E) CNT-Ta, and (F) CNT-CF microelectrodes. Measurements were taken after (—) 15 min and (---) 160 min of equilibration in PBS solution with a waveform of −0.4 to 1.3 V and back at 400 V/s, 10 Hz.

Figure 4

Plot of normalized anodic current to corresponding dopamine concentration. The fitting curve is modeled on the basis of eq 3, where C_DA is the x-axis and fractional surface coverage is the y-axis. An equilibrium value, β_DA, is fit for each curve. (A) CNT-coated Nb microelectrode; (B) CNT-coated Ta microelectrode; (C) CNT-coated CFME; (D) CFME (n = 5 per electrode material; error bar is standard error of mean and sometimes is so small as to be less than the size of the point).

Figure 5

Detection of other neurochemicals at CNT-Nb microelectrodes. (Upper row) CVs of (A) 200 μM AA, (C) 20 μM DOPAC, (E) 1 μM serotonin, (G) 1 μM adenosine, and (I) 1 μM histamine in PBS buffer. Red dashed line is CV of 1 μM dopamine obtained from the same CNT-Nb electrode. For AA, DOPAC, and serotonin, the electrode was scanned to 1.3 V; for adenosine and histamine, the electrode was scanned to 1.45 V. (Lower row) Column plots show the ratio of oxidation current for (B) 200 μM AA, (D) 20 μM DOPAC, (F) 1 μM serotonin, (H) 1 μM adenosine, and (J) 1 μM histamine compared to the corresponding oxidation current of dopamine at CNT-Nb microelectrode (black, n = 5) and CFMEs (gray, n = 5). The oxidation current ratios at CNT-Nb microelectrodes are significantly different than CFMEs for the measurement of ascorbic acid (paired t test, p < 0.0001) and histamine (paired t test, p < 0.05).

Figure 6

Dopamine detection in vivo at CNT-Nb microelectrodes. (A) Sample CVs depicting stimulated dopamine release detected from a CNT-Nb microelectrode placed in the caudate putamen with stimulation pulse trains of 120, 60, 24, and 12 pulses at 60 Hz. (B) Associated concentration vs time plot. (C) Averaged dopamine concentration at different pulses detected at CNT-Nb microelectrodes (n = 4). The electrode was scanned from −0.4 to 1.3 V and back at 400 V/s at 10 Hz.