Following the thin-layer electrochemistry behavior, (37) the CA responses in CNPs can be well fitted by the exponential decay equation i t = i 0 e – t/τ (panel D), with the fitted parameters shown in Figure S2. Note that the total transferred charges enclosed in CA and CV peaks are the same. The time duration for CV peak is ∼1 s at 0.2 V/s, while it is only ∼20 ms for the CA peaks. This could be explained by the different time duration of these two techniques. Interestingly, the resulting peak current from CA increase linearly with K 4Fe(CN) 6 concentration and is ∼100 times larger than the CV peak current in the same solutions, as compared in panel C. With a step potential waveform from 0 to 0.5 V, 50 μM K 4Fe(CN) 6 results in ∼8 nA peak current, and the small current peak (∼1 nA) at 0 M K 4Fe(CN) 6 should be from the capacitive charging at the carbon/solution interface. Figure 1īesides CV measurements, the CA techniques are also applied to the same CNP, and the resulting i– t responses are recorded and analyzed. (35) Note that the diffusion-limiting current (i.e., E > 0.4 V) at these concentrations is too small to be resolved. The peak current would increase linearly with respect to the scan rate, while asymmetrical oxidation/reduction peaks can be a result at higher scan rates, due to the coupled mass transport processes. As shown in Figure 1A, symmetrical oxidation/reduction peaks are displayed, and 5–50 μM K 4Fe(CN) 6 can produce 5–75 pA peak currents at 0.2 V/s. Owing to the thin-layer electrochemical processes, the CNPs can be used for sensitive detection of analytes based on the current peaks in CVs. The solution volume can be estimated from the enclosed charges in the oxidation/reduction peaks, and the solution depth is typically around 1000–3000 a. When immersing CNPs in solution, the solution would enter into the CNPs and gradually become stable, as evidenced by the overlapped CV responses ( Figure S3). (36) Briefly, the quartz nanopipets are prepared by a laser puller, and then a carbon layer is deposited at the interior wall of the pipets, which was then characterized by electrochemical methods and transmission electron microscopy ( Figure S1). The employed CNPs probes with a tip radius ( a) of 30–100 nm is fabricated by the chemical vapor deposition method, following previous reports. In addition, a small amount of electroactive species from the substrate can also be effectively collected at the tip and produce nanoampere current spikes. Interestingly, both the experiments and simulation show the feedback responses featuring the transient peak current following SICM theory, and mediator-free approach curves can be realized based on the charging/discharging currents. With the large transient peak currents in CNPs, the feedback, generation/collection (G/C) and the constant-height imaging experiments could then be conducted with a very dilute redox mediator in the solution. Note that the electron transfer (ET) processes at the carbon layer are coupled with the ion transport (IT) processes through the CNPs, (34,35) and thus the resulting current signals are determined by the in-series R IT and R ET. The fundamental charge transport processes and equivalent circuit can be seen in Scheme 1. Herein, we introduce a new high-sensitivity SECM method based on the transient current responses from CNPs.
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