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Abstract
On the origin of photoluminescence of noble metal NCs, there are always hot debates: metal-centered quantum-size confinement effect VS ligand-centered surface state mechanism. Herein, we provided solid evidence that structural water molecules (SWs) confined in the nanocavity formed by surface-protective-ligand packing on the metal NCs are the real luminescent emitters of Au-Ag bimetal NCs. The Ag cation mediated Au-Ag bimetal NCs exhibit the unique pH-dependent dual-emission characteristic with larger Stokes shift up to 200 nm, which could be used as potential ratiometric nanosensors for pH detection. Our results provide a completely new insight on the understanding of the origin of photoluminescence of metal NCs, which elucidates the abnormal PL emission phenomena, including solvent effect, pH-dependent behavior, surface ligand effect, multiple emitter centers, and large-Stoke’s shift.
Highlights
HAuCl4 solution was added into the mixed solution and the mixtures were further stirred for 12 h and incubated overnight at room temperature; the obtained samples were denoted as AgxAu@DT NCs, in which the x represented the feeding ratio of Ag to Au
The incorporation of silver and gold to form AgxAu@DT NCs was evidenced through the thermogravimetric analysis (TGA) and inductively coupled plasma (ICP) atomic emission spectroscopy (Supplementary Figure S1 and Supplementary Table S1), when increasing the feeding ratio of Ag-Au from 0.25, 0.5, 1, 2, and 4, the actual ratio of Ag to Au in the obtained NCs analyzed through Inductively coupled plasma (ICP) was determined as 0.11, 0.23, 0.47, 1.02, and 1.25, respectively, and the ligand weight loss (Supplementary Figure S1) was gradually increased from 47.57 to 61.80%
The newly boosted blue emission is the consequence of the doping of silver atom as Ag(I)-thiolate into the staple motif site of Au NCs, which is confirmed by the X-ray photoelectron spectra (XPS) and TEM characterization
Summary
Quantum-sized metal nanoclusters, which bridge the gap between organometallics and nanocrystals, exhibit dramatically unique electronic and optical properties, such as molecule-like energy gaps (Schaaff et al, 1997; Lee et al, 2004; Negishi et al, 2005; Walter et al, 2008; Zhu et al, 2008; RussierAntoine et al, 2014; Bertorelle et al, 2017; Chakraborty and Pradeep, 2017), intense photoluminescence (Zheng et al, 2004; Zheng et al, 2007; Xie et al, 2009; Luo et al, 2012; Musnier et al, 2019) and catalytic properties (Tsunoyama et al, 2009; Zhu et al, 2010; Li et al, 2016a; Cai et al, 2019; Lv et al, 2019; Lv et al, 2020a; Yang et al, 2020a; Lv et al, 2020b; Cai et al, 2020; Li et al, 2021). Luminescent thiolated-protected Au and Ag NCs in particular have attracted tremendous interest due to their wide applications in bio-imaging, bio-medicine, sensing, and catalysis (Jin, 2010; Díez and Ras, 2011; Bonacic-Koutecky et al, 2012; Zheng et al, 2012; Luo et al, 2014; Sun and Jin, 2014; Jin et al, 2016; Lei et al, 2018; Yan et al, 2018; Ungor et al, 2021) Various strategies, such as heteroatom doping (Wang et al, 2014), aggregation-induced-emission (AIE) (Luo et al, 2012), and assembly induced emission enhancement (Wu et al, 2019a), have been developed to prepare highly luminescent Au and Ag NCs. Since the crystal structure of ligand-protected Au NCs, which are usually comprised of metallic core and peripheral gold(I)-thiolate staple motifs, have been revealed at atomic resolution (Jadzinsky et al, 2007), heteroatom substitution of specific native sites could give an in-depth way to understand the structure/composition-correlated properties and provide an efficient way to diversify and tailor the physicochemical properties of metal NCs (Hirai et al, 2020; Kang et al, 2020). The origin of PL or the nature of emitters of heteroatom-doped metal NCs remain unclear and even controversial, which limits the rational design of metal nanoclusters with improved and tailored optical and catalytic properties
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