New approaches in optical imaging techniques are greatly desired to obtain point resolutions reaching nanometric dimensions. Predominantly the realm of electron microscopy, the intricate information such optical imaging techniques could elicit (of the molecular dipoles) are immense. Despite the promise of multi-photon excited fluorescence (mPEF) near-field scanning optical microscopy (NSOM), aperture-based mPEF NSOM was unable to obtain point resolutions below ~175 nm using organic chromophores. This difficulty can be partly attributed to the low photo-stability and relatively low two-photon absorption cross sections of the fluorophores. Au25 nanoclusters have shown unusually large two-photon absorption cross sections (4.27 x 105 GM) in the solution phase suggesting their potential future applications in super-resolution imaging and sensing. With the use of single Au25 nanoclusters as strong two-photon absorbers we report the observation of two-photon excited fluorescence (2PEF) NSOM resulting in a point resolution of 37 nm (<?/21). Interestingly, a ~3-fold enhancement (1.31 x 106 GM) of the two-photon absorption cross section in solid is observed for these single nanoclusters. We attribute this enhancement to a local field enhancement effect. According our knowledge this is the first time that aperture-based 2PEF NSOM has been able to obtain point resolutions below 100 nm. Therefore, this study introduces gold nanoclusters as a fluorophore that can be utilized both in multiphoton imaging and that can potentially be used to study spectroscopy at molecular-scale dimensions using super-resolution imaging techniques.

Multi-photon excited fluorescence near-field scanning optical microscopy with monolayer protected Au<sub>25</sub> nanoclusters reaches super-resolution

248th National Meeting (2014)

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Multi-photon excited fluorescence near-field scanning optical microscopy with monolayer protected Au25 nanoclusters reaches super-resolution

Multi-photon excited fluorescence near-field scanning optical microscopy with monolayer protected Au₂₅ nanoclusters reaches super-resolution