Metal Nanoparticle Modified Carbon-Fiber Microelectrodes Enhance Adenosine Triphosphate Surface Interactions with Fast-Scan Cyclic Voltammetry

Abstract

Adenosine triphosphate (ATP) is an important rapid signaling molecule involved in a host of pathologies in the body. Historically, ATP is difficult to directly detect electrochemically with fast-scan cyclic voltammetry (FSCV) due to limited interactions at bare carbon-fibers. Systematic investigations of how ATP interacts at electrode surfaces is necessary for developing more sensitive electrochemical detection methods. Here, we have developed gold nanoparticle (AuNP), and platinum nanoparticle (PtNP) modified carbon-fiber microelectrodes coupled to FSCV to measure the extent to which ATP interacts at metal nanoparticle-modified surfaces and to improve the sensitivity of direct electrochemical detection. AuNP and PtNPs were electrodeposited on the carbon-fiber surface by scanning from -1.2 to 1.5 V for 30 s in 0.5 mg/mL HAuCl₄ or 0.5 mg/mLK₂PtCl₆. Overall, we demonstrate an average 4.1 ± 1.0-fold increase in oxidative ATP current at AuNP-modified and a 3.5 ± 0.3-fold increase at PtNP-modified electrodes. Metal nanoparticle-modified surfaces promoted improved electrocatalytic conversion of ATP oxidation products at the surface, facilitated enhanced adsorption strength and surface coverage, and significantly improved sensitivity. ATP was successfully detected within living murine lymph node tissue following exogenous application. Overall, this study demonstrates a detailed characterization of ATP oxidation at metal nanoparticle surfaces and a significantly improved method for direct electrochemical detection of ATP in tissue.

Figure 1

The carbon-fiber microelectrode is significantly less sensitive to ATP compared to dopamine. (A) A false-color plot demonstrates potential on the y-axis, time on the x-axis, and current in false color for 5 μM ATP. (B) A false-color plot for 1 μM DA. (C) The representative CV of 5 μM ATP and (D) 1 μM DA. The secondary peak for ATP (0.8 V) is almost negligible on this electrode. Data shown were collected on the same electrode.

Figure 2

ATP oxidative current increases at both Au and Pt nanoparticle modified carbon-fiber microelectrodes. Electrodes were electrodeposited with either Au or Pt nanoparticles by applying a waveform that scans from −1.2 to 1.5 at 5 V/s. Significantly higher increases in current are observed for ATP compared to DA at AuNP (A) and PtNP (B) modified electrodes (n = 7–12). Representative CVs are shown for bare carbon (CF, black), AuNP-modified (orange), and PtNP-modified (blue) for ATP (C) and DA (D).

Figure 3

Metal nanoparticle electrode modification enhances the electrocatalytic conversion of ATP oxidation products. The oxidation scheme for ATP (A) and a comparison of the color plots and representative CVs of 5 μM ATP over time are shown for bare carbon-fiber (B), AuNP-modified carbon-fiber (C), and PtNP-modified carbon-fiber (D). ATP was manually injected at approximately 5 s and washed away at approximately 9 s. Dashed lines indicate at different time points (time points 1, 2, and 3) CVs were analyzed to visualize the oxidation reaction over time, and the oxidation peaks are labeled on the CVs. At line 1 (5 s, black line), only the primary oxidation product is observed. By line 2 (7 s, red line), the secondary oxidation product is observed, and by line 3 (9 s, purple line), the tertiary oxidation product is clearly observed. The representative CVs at each of these lines (1, 2, and 3) demonstrate oxidation peaks growing over time.

Figure 4

Metal nanoparticle-modified electrodes are stable with repeated injections of ATP. 5 μM ATP was repeatedly injected at the electrode 25 times. The current for the primary oxidation peak for ATP was normalized to the first injection and compared over time. Deviation from 100% indicates instability of detection. ATP current decreased by 54% by the 25th injection at bare carbon-fiber microelectrodes (open circles), with only a 29% and 13% decrease in ATP current at AuNP (orange squares) and PtNP (blue circles), respectively. (n = 5–8).

Figure 5

Scanning electron microscopy (SEM) images of bare and metal nanoparticle modified carbon-fibers. (A) Bare carbon fiber, (B) 0.5 mg/mL AuNP-modified carbon-fibers (30 s deposition), and (C) 0.5 mg/mL PtNP-modified carbon-fibers (30 s deposition). Images on the right are at higher magnification to reveal NPs on the surface. Scale bars are shown on the image.

Figure 6

Metal nanoparticles change the interaction of ATP at the electrode surface. The oxidative current for ATP was recorded as a function of scan rate. Scan rates tested ranged from 50 to 800 V/s. The log of the peak oxidative current (i_p) is plotted vs the log of scan rate. A slope of the line describes the dominating electrode interaction. A slope closer to 1.0 indicates adsorption-controlled processes, and a slope closer to 0.5 indicates diffusion-controlled processes. The slope of the line changes from 0.634 to 0.855 (AuNP-modified carbon) and 0.815 (PtNP-modified carbon), indicating changes in the surface interaction to more adsorption-limited processes (n = 6–8).

Figure 7

Higher sensitivity, surface coverage, and stronger adsorption strength are observed for ATP at metal-nanoparticle modified carbon-fiber microelectrodes. (A) The oxidative current for ATP was measured for concentrations ranging from 1 μM to 100 μM (B). The linear region of the curve spanned from 1 μM to 10 μM ATP and was used to compare the sensitivity of each electrode type for ATP (r² Bare CF = 0.9384, r² AuNP = 0.9588, r² PtNP = 0.9864). (C) Langmuir isotherms for ATP at the bare carbon-fiber (CF, black), AuNP modified (orange), and PtNP modified (blue) electrodes (n = 6–8).

Figure 8

Exogenous detection of ATP within mesenteric lymph node slices at both AuNP and PtNP-modified electrodes. (A) Example color plot, current vs time (top) and CV (bottom) for exogenous ATP at a AuNP-modified electrode. The blue arrow indicates when ATP was pressure-ejected into the tissue. Both the primary (1°, 1.2 V) and secondary (2°, 1.0 V) oxidation peaks are present on the CV for ATP. (B) Example color plot, current vs time, and CV for exogenous ATP at a PtNP-modified electrode. The primary (1°, 1.2 V) and secondary (2°, 1.0 V) peaks are visible on the CV for ATP.