title: Fluorescence depletion properties of insulin-gold nanoclusters.
abstract: Insulin-gold nanoclusters exhibit outstanding biocompatibility, photostability, and fluorescence quantum efficiency. However, they have never been used in superresolution microscopy, which requires nonlinear switching or saturation of fluorescence. Here we examine the fluorescence and stimulated emission depletion properties of gold nanoclusters. Their bleaching rate is very slow, demonstrating superior photostability. Surprisingly, however, the best depletion efficiency is less than 70%, whereas the depletion intensity requirement is much higher than the expectation from a simple two-level model. Fluorescence lifetime measurement revealed two distinct lifetime components, which indicate intersystem and reverse intersystem crossing during excitation. Based on population dynamic calculation, excellent agreement of the maximal depletion efficiency is found. Our work not only features the first examination of STED with metallic clusters, but also reveals the significance of molecular transition dynamics when considering a STED labeling.
authors: Department of Physics, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan.
authors: Po-Fu
authors: PF
authors: Chen
authors: Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan.
authors: Chien-Liang
authors: CL
authors: Liu
authors: Department of Physics, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan.
authors: Wei-Kuan
authors: WK
authors: Lin
authors: Department of Physics, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan.
authors: Kuan-Chieh
authors: KC
authors: Chen
authors: Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan.
authors: Pi-Tai
authors: PT
authors: Chou
authors: Department of Physics, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan ; NTU Molecular Imaging Center, No. 81, Changxing St., Taipei 10672, Taiwan.
authors: Shi-Wei
authors: SW
authors: Chu
keywords: (160.1435) Biomaterials; (160.2540) Fluorescent and luminescent materials; (160.4236) Nanomaterials; (180.1790) Confocal microscopy; (250.6715) Switching