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Although photothermal imaging was originally designed to detect individual molecules that do not emit or small nanoparticles that do not scatter, the technique is now being applied to image and spectroscopically characterize larger and more sophisticated nanoparticle structures that scatter light strongly. Extending photothermal measurements into this regime, however, requires revisiting fundamental assumptions made in the interpretation of the signal. Herein, we present a theoretical analysis of the wavelength-resolved photothermal image and its extension to the large particle scattering regime, where we find the photothermal signal to inherit a nonlinear dependence upon pump intensity, together with a contraction of the full-width-at-half-maximum of its point spread function. We further analyze theoretically the extent to which photothermal spectra can be interpreted as an absorption spectrum measure, with deviations between the two becoming more prominent with increasing pump intensities. Companion experiments on individual 10, 20, and 100 nm radius gold nanoparticles evidence the predicted nonlinear pump power dependence and image contraction, verifying the theory and demonstrating new aspects of photothermal imaging relevant to a broader class of targets.more » « less
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Abstract Single entity electrochemistry experiments are typically motivated by the need to reveal how heterogeneity affects performance within inherently diverse nanoparticle populations. Here we show that a commonly used supporting electrode, tin‐doped indium oxide (ITO), can also play a significant role in creating heterogeneity in nanoparticle electrochemical responses. To investigate the impact of the substrate, we optically monitored the electrodissolution kinetics of gold nanoparticles on ITO thin films with similar resistivity from two different suppliers. The ITO from the two suppliers showed marked differences in the gold electrodissolution kinetics, with ITO from one of the suppliers even producing poor sample‐to‐sample reproducibility across substrates within the same lot number. The role of nanoparticle size and surface effects were accounted for in our analysis to validate that the observed heterogeneity is dominated by the ITO electrodes. The results show that the role of the supporting electrode cannot be ignored when performing single entity structure‐function studies.