Nanoconfined Microenvironment-Regulated Photochromic Kinetics in Gold Nanocluster-Based Photoswitching Fluorescent Nanoparticles

Photoswitchable fluorescent nanoparticles (PF NPs) have attracted significant attention for their promising applications in sensing, imaging, and anticounterfeiting. However, the behavior of photochromic motifs on the nanoscale surface remains poorly understood, limiting the rational design of robust PF NPs. In this study, we address this issue by disclosing the distinct role of nanoconfined microenvironments on the photochromic properties of PF NPs composed of fluorescent gold nanoclusters and photochromic spiropyran using a combined experimental and simulation approach. To artificially modulate the microenvironment of spiropyran within PF NPs, positively charged stearyl triphenyl phosphonium (STPP) is introduced into the lipid bilayers. Photochromic kinetic analysis reveals that increasing the STPP content significantly alters the first-order photochromic kinetics of spiropyran. Accordingly, we propose a charge-confinement model to elucidate how the microenvironment of spiropyran modulates the photochromic properties of PF NPs. Molecular dynamics simulations further confirm that STPP incorporation can induce directional motion of the zwitterionic merocyanine-state spiropyran, leading to obvious changes in the local nanoconfinement features on the surface of PF NPs. Leveraging their dual-colored emission, highly reversible photoswitching capability, and good biocompatibility, we demonstrate the potential of these PF NPs as robust probes for cell imaging applications. This study not only provides a nanoconfined microenvironment-based strategy for regulating the photochromic property of PF NPs but also offers alternative avenues for designing advanced optical probes toward versatile applications.