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Journal Articles ChemPhysChem Year : 2018

Bulky Counterions: Enhancing the Two-Photon Excited Fluorescence of Gold Nanoclusters


Increasing fluorescence quantum yields of ligand-protected gold nanoclusters has attracted wide research interest. The strategy consisting in using bulky counterions has been found to dramatically enhance the fluorescence. In this communication, we push forward this concept to the nonlinear optical regime. We show that by an appropriate choice of bulky counterions and of solvent, a 30-fold increase in two-photon excited fluorescence (TPEF) signal at ~600 nm for gold nanoclusters can be obtained. This would correspond to a TPEF cross section in the range of 0.1 to 1 GM. The quest for high resolutions and penetration depths with spectroscopic tools has led to innovative approaches in optical microscopy of biological systems. Two-photon excited fluorescence (TPEF) microscopy is probably the most important advance in optical microscopy of biological systems since the introduction of confocal imaging. 1 The successful application of TPEF is partly due to the fact that commonly used fluorescent molecules can be efficiently excited by the simultaneous absorption of 2 photons. 2, 3 However, to reach ultrahigh detection sensitivity, newly engineered nonlinear optical (NLO) chromophores, endowed with large two-photon absorption cross-sections, have to be developed. Liganded silver and gold quantum clusters represent a promising direction in the quest to develop novel nonlinear optical nanomaterials. Pioneering studies have clearly demonstrated that ligated noble metal nanoclusters (NCs), with few to hundred atoms exhibit remarkable optical nonlinearities. 4-6 Shortly afterwards, strong two-photon emission was reported from uncharacterized gold clusters, making them a promising alternative as contrast agents for live cells imaging. 7 Thus, these ligated nanoclusters may provide a valuable route within the nonlinear optical regime. We have described in our combined experimental and theoretical investigations how to improve two-photon absorption (TPA) properties in such quantum nanoclusters, through the concept of "ligand-core" NLO-phores. 8 The structure of the metallic core, its charge and symmetry (influencing the NLO cross sections), and the ligands stabilizing the core play a major role in TPA efficiencies. 9 However, the main conclusion so far is that liganded silver and gold quantum clusters are excellent two-photon absorbers but are rather poor two-photon excited emitters. 10 For example, the two-photon absorption cross-section at 780 nm for Au15 is ~65 700 GM, while its TPEF cross section at 475 nm is only ~0.022 GM. 11 Concerning nonlinear emissive properties, both experimental and theoretical progresses are needed in order to produce not only large TPA but also large TPEF cross sections. On the one hand, alloying is a promising strategy. Indeed, Le Guével et al 12 reported enhanced fluorescence of gold nanoclusters by silver doping. Likewise, two-photon excited fluorescence emission was also found to be enhanced. 13 Unfortunately, the exact stoichiometry of such AuAg nanoclusters remains unknown and has prevented the report of any quantitative estimation of TPEF cross-sections. On the other hand, it is well known that the ligand shell rigidity 14, 15 influences the emissive properties of Au NCs. A simple way to increase rigidity of the protective shell is to use as ligands proteins instead of thiolated molecules. Xie et al. 16 have developed a facile, one-pot, "green" synthetic technique based on bovine serum albumin (BSA). It was shown that BSA-Au25 clusters also exhibit an efficient two photon absorption followed by red to near-infrared luminescence. 17 However, no two photon absorption cross section was reported due to the difficulty in calculating the precise concentration of the clusters in the solution. Using (bio)organic thiolate templates, further enhancement can be achieved by the increased rigidity of the metal-thiolate shell. Pyo et al. 18 have shown that it is possible to achieve one-photon excited luminescence quantum yields >60% by rigidifying the gold shell with the binding of bulky groups (e.g. tetraoctylammonium (TOA) cations). In this communication, we push forward this concept to the nonlinear optical regime. We show that by an appropriate choice of bulky counterions and solvents, a dramatic enhancement of TPEF cross sections for Au15(SG)13 and Au18(SG)14 can be obtained. [a] Dr.
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hal-02108534 , version 1 (02-11-2020)



Franck Bertorelle, Christophe Moulin, Antonin Soleilhac, Clothilde Comby-Zerbino, Philippe Dugourd, et al.. Bulky Counterions: Enhancing the Two-Photon Excited Fluorescence of Gold Nanoclusters. ChemPhysChem, 2018, 19 (2), pp.165-168. ⟨10.1002/cphc.201701186⟩. ⟨hal-02108534⟩


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