Cavity Carbon-Nanopipette Electrodes for Dopamine Detection

Abstract

Microelectrodes are typically used for neurotransmitter detection, but nanoelectrodes are not because there is a trade-off between spatial resolution and sensitivity that is dependent on surface area. Cavity carbon-nanopipette electrodes (CNPEs), with tip diameters of a few hundred nanometers, have been developed for nanoscale electrochemistry. Here, we characterize the electrochemical performance of CNPEs with fast-scan cyclic voltammetry (FSCV) for the first time. Dopamine detection using cavity CNPEs, with a depth equivalent to a few radii, is compared with that using open-tube CNPEs, an essentially infinite geometry. Open-tube CNPEs have very slow temporal responses that change over time as the liquid rises in the CNPE. However, a cavity CNPE has a fast temporal response to a bolus of dopamine that is not different from that of a traditional carbon-fiber microelectrode. Cavity CNPEs, with tip diameters of 200-400 nm, have high currents because the small cavity traps and increases the local dopamine concentration. The trapping also leads to an FSCV frequency-independent response and the appearance of cyclization peaks that are normally observed only with large concentrations of dopamine. CNPEs have high dopamine selectivity over ascorbic acid (AA) because of the repulsion of AA by the negative electric field at the holding potential and the irreversible redox reaction. In mouse-brain slices, cavity CNPEs detected exogenously applied dopamine, showing they do not clog in tissue. Thus, cavity CNPEs are promising neurochemical sensors that provide spatial resolution on the scale of hundreds of nanometers, which is useful for small model organisms or for locations near specific cells.

Figure 1.

TEM image of (A) a cavity carbon nanopipette electrodes with the cavity depth of about 500 nm and the orifice diameter of about 200 nm, and (B) an open-tube CNPE with the orifice diameter of about 200 nm and a long depth.

Figure 2.

Electrochemical response to 5 μM dopamine at a cavity CNPE (A-C) and an open tube CNPE (D-F). Measurements were obtained at scan rate of 400 V/s and scan repetition frequency of 10 Hz. (A, D) Background currents in PBS buffer, (B, E) background subtracted cyclic voltammogram to 5 μM dopamine, and (C, F) 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).

Figure 3.

Numerical simulation of the dopamine oxidation/reduction with cavity CNPEs. (A) FSCV waveform showing potentials where concentrations were modeled. (B) Modeled cyclic voltammogram for dopamine. Symmetric peaks show the thin layer cell effects. (C) Modeled concentrations of dopamine inside the pipette. The rectangle is the reservoir of 1 μM dopamine. Half of a nanopipette is shown. On the anodic ramp, at 0.3 V, dopamine starts to be oxidized and by 1.3 V, there is complete oxidation of all DA in the CNPE. On the cathodic ramp, dopamine is being reformed by reduction at 0.3 V and by −0.4 V all of the dopamine has been redox recycled back from dopamine-o-quinone. For all simulations, the scan rate is 400 V/s, σ = −0.01 C/m², radius=200 nM, H = 20.

Figure 4.

(A) Effect of switching potential. The plot shows average oxidation current for 1 μM dopamine at cavity CNPEs (black dot, n = 5) and CFMEs (red circle, n = 5) for each switching potential (1.0 V to 1.6 V, with the interval of 0.1 V) with a triangle waveform from −0.4 V and 400 V/s scan rate. Peak currents were normalized to the current using the 1.6 V waveform. (B) Effect of scan repetition frequency. Peak oxidation current at cavity CNPEs (black dot, n = 4) and CFMEs (red circle, n = 5) with −0.4 to 1.3 V waveform and scan rate of 400 V/s. Peak currents were normalized to the current at 10 Hz. (C) Two-hour stability test of cavity CNPEs with constant waveform application (−0.4 to 1.3V, 400 V/s, 10 Hz) (black dot, n = 3) compared to CFMEs (red circle, n = 3). Oxidation current to 1 μM dopamine was normalized to the signal observed after 10 minutes equilibration. Error bars are the standard error of the mean.

Figure 5.

(A) CVs of 200 μM ascorbic acid (red line) and 1 μM dopamine obtained from the same cavity CNPE and CFME. (B) Column plots show the ratio of oxidation current for 200 μM ascorbic acid compared to the corresponding oxidation current of dopamine at cavity CNPEs (black, n = 5) and CFMEs (gray, n = 5). The oxidation current ratio at cavity CNPEs is significantly smaller than CFMEs for the measurement of ascorbic acid (t test,p ≤ 0.0001).

Figure 6.

Electrochemical response of cavity CNPEs to exogenous dopamine application. Measurements were obtained at a scan rate of 400V/s and at a scan rate frequency of 10 Hz. (A) Background subtracted CVs of the same electrode with varying the time for dopamine application (pressure kept constant). The pressure-ejection times (0.02 to 1.5 seconds) were converted to molar quantities released by the picospritzing pipette using the initial concentration of the dopamine solution (150 μM) and the volume of solution released for each duration. (B) The oxidative current versus time for a different electrode with a 1 second puff of dopamine (27.0 pmol). The dopamine was ejected at the arrow.

Scheme 1.

Dopamine oxidation scheme.