Fig. 5.

Fluorescence decay fit curves of ZnO NP: (a) fast decaying photoluminescence collected over 10 ns and (b) direct lifetime measurement using phosphorescence lifetime imaging (PLIM) collected over $10\mu \mathrm{m}$. Reproduced with permission. (c) The enhanced background signal seen at $\text{time}=0\mathrm{ns}$ is related to the ZnO NP concentration where the signal has accumulated due to the repeated laser pulsing to create an “offset” that results in a more sensitive multicomponent fit of the data. The multicomponent fit of PLIM is less sensitive because there is loss of photons over a longer period, up to $10\mu \mathrm{s}$ when the decay returns to background conditions. (d) Schematic of the ZnO NP lifetimes as quantified by a quasicontinuous accumulation of steady-state photoluminescence. (d) Representative multiphoton images of uncoated and coated ZnO NP in vitro and topically applied to in vivo human skin for 6 h for signal isolation and quantification. “Offset” values $<5$ counts are shown in grayscale and include cellular autofluorescence, while “offset” values above the autofluorescence- “offset” level were assigned to ZnO NP (red). $\text{Scale bar}=40\mu \mathrm{m}$. With permission from Refs. ¹¹ and ⁴⁶.