Digital Processing for Single Nanoparticle Electrochemical Transient Measurements

We demonstrate the use of digital frequency analysis in single nanoparticle electrochemical detection. The method uses fast Fourier transforms (FFT) of single entity electrochemical transients and digital filters. These filters effectively remove noise with the Butterworth filter preserving the amplitude of the fundamental processes in comparison with the rectangle filter. Filtering was done in three different types of experiments: single nanoparticle electrocatalytic amplification, photocatalytic amplification, and nanoimpacts of single entities. In the individual nanoparticle stepwise transients, low-pass filters maintain the step height. Furthermore, a Butterworth band-stop filter preserves the peak height in blip transients if the band-stop cutoff frequencies are compatible with the nanoparticle/electrode transient interactions. In hydrazine oxidation by single Au nanoparticles, digital filtering does not complicate the analysis of the step signal because the stepwise change of the particle-by-particle current is preserved with the rectangle, Bessel and Butterworth low pass filters, with the later minimizing time shifts. In the photocurrent single entity transients, we demonstrate resolving a step smaller than the noise. In photoelectrochemical setups, the background processes are stochastic and appear at distinct frequencies that do not necessarily correlate with the detection frequency (f_p), of TiO₂ nanoparticles. This lack of correlation indicates that background signals have their characteristic frequencies and that it is advantageous to perform filtering a posteriori. We also discuss selecting the filtering frequencies based on sampling rates and f_p. In experiments electrolyzing ZnO, that model nanoimpacts, a band-stop filter can remove environmental noise within the sampling spectral region while preserving relevant information on the current transient. We discuss the limits of Bessel and Butterworth filters for resolving consecutive transients.