The rod-shaped Au25 nanocluster possesses a low photoluminescence quantum yield (QY=0.1%) and hence is not of practical use in bioimaging and related applications. Herein, we show that substituting silver atoms for gold in the 25-atom matrix can drastically enhance the photoluminescence. The obtained Ag(x)Au(25-x) (x=1-13) nanoclusters exhibit high quantum yield (QY=40.1%), which is in striking contrast with the normally weakly luminescent Ag(x)Au(25-x) species (x=1-12, QY=0.21%). X-ray crystallography further determines the substitution sites of Ag atoms in the Ag(x)Au(25-x) cluster through partial occupancy analysis, which provides further insight into the mechanism of photoluminescence enhancement.

Two series of Ag‐doped AgxAu25‐x nanoclusters are prepared by two different routes.

The rod-shaped Au 25 nanocluster possesses a low photoluminescence quantum yield (QY = 0.1 %) and hence is not of practical use in bioimaging and related applications. Herein, we show that substituting silver atoms for gold in the 25-atom matrix can drastically enhance the photoluminescence. The obtained Ag x Au 25Àx (x = 1-13) nanoclusters exhibit high quantum yield (QY = 40.1 %), which is in striking contrast with the normally weakly luminescent Ag x Au 25Àx species (x = 1-12, QY = 0.21 %). X-ray crystallography further determines the substitution sites of Ag atoms in the Ag x Au 25Àx cluster through partial occupancy analysis, which provides further insight into the mechanism of photoluminescence enhancement.Fluorescent nanomaterials are of major importance in many fields.[1-4] Different types of fluorescent nanomaterials have been developed, such as quantum dots (QDs), [5] lanthanide nanoparticles, [6,7] and carbon nanodots. [8,9] Metal nanoclusters (Ag, Au) have emerged as a new class of nanomaterial. [10][11][12][13][14][15][16][17][18][19][20][21][22][23] Compared to QDs, [24] Au and Ag nanoclusters (NCs) are more biocompatible and can be readily bioconjugated; other advantages include their extrememly small size, good photostability, and low toxicity; thus, fluorescent noble-metal NCs have been recognized as a promising candidate for cell labeling, biosensing, and photo-therapy applications. [25][26][27][28][29][30] However, a general issue lies in the lower quantum yield (QY) of metal NCs compared to QDs and organic dyes, which significantly limits the applications of metal NCs. Various approaches have been developed to synthesize noble metal NCs with enhanced fluorescence, for example: 1) engineering the particle surface by using different ligands, such as DHLA, [31] dendrimers, [32] polymers, [33] DNA, [34,35] peptides, and proteins; [36][37][38][39][40][41][42][43][44] 2) controlling the metal core size [45] or doping the core with other metal atoms. Doping atomically precise nanoclusters is highly desired and allows atomic-level insight into the origin of fluorescence, which is of major importance. [46,47] Recently, several doped gold NCs with conserved core size have been successfully synthesized; [48][49][50][51][52][53] however, these thiolate-protected nanoclusters, as well as the phosphine-protected gold NCs, [13,16,54] are of low fluorescence. Herein, we report the discovery of drastic fluorescence enhancement in gold nanoclusters doped with 13 Ag atoms. Interestingly, gold nanoclusters of the same structure, but doped with fewer Ag atoms, are only weakly fluorescent. Thus, the 13th Ag atom triggers strong fluorescence in the doped gold nanocluster.Two synthetic methods were devised for obtaining Agdoped, 25-metal-atom nanoclusters (see the Experimental Section and the Supporting Information for details). In the first route, polydisperse Au nanoparticles [16] were first made and then used as the precursors to react with an Ag I thiolate complex, which gave rise to Ag-doped gold nanocl...

In this work, we synthesized Au25 nanoclusters protected by 2-(naphthalen-2-yl)ethanethiolate. Our experiments revealed that the luminescence of this nanocluster consists of two bands, namely, band I centered at 740 nm and band II centered at 680 nm. Compared with 2-phenylethanethiolate protected Au25 nanoclusters, this new nanocluster has a much higher QY (quantum yield) value (6.5 times higher). Fluorescence lifetime measurements showed multiple components, i.e. 0.15 ns, ∼20 ns and ∼150 ns. With an increase in the electropositivity of the nanocluster, the fluorescence intensity of the nanocluster exhibits a significant enhancement. Since the 2-(naphthalen-2-yl)ethanethiolate protected Au25 nanocluster shares the same Au13/Au12 core-shell structure as the 2-phenylethanethiolate protected nanocluster, the band II fluorescence implies that the surface ligands play a major role in the origin of the fluorescence.

We report two synthetic routes for concurrent formation of phenylmethanethiolate (-SCH2Ph)-protected Au20(SR)16 and Au24(SR)24 nanoclusters in one-pot by kinetic control. Unlike the previously reported methods for thiolate-protected gold nanoclusters, which typically involve rapid reduction of the gold precursor by excess NaBH4 and subsequent size focusing into atomically monodisperse clusters of a specific size, the present work reveals some insight into the kinetic control in gold-thiolate cluster synthesis. We demonstrate that the synthesis of -SCH2Ph-protected Au20 and Au24 nanoclusters can be obtained through two different, kinetically controlled methods. Specifically, route 1 employs slow addition of a relatively large amount of NaBH4 under slow stirring of the reaction mixture, while route 2 employs rapid addition of a small amount of NaBH4 under rapid stirring of the reaction mixture. At first glance, these two methods apparently possess quite different reaction kinetics, but interestingly they give rise to exactly the same product (i.e., the coproduction of Au20(SCH2Ph)16 and Au24(SCH2Ph)20 clusters). Our results explicitly demonstrate the complex interplay between the kinetic factors that include the addition speed and amount of NaBH4 solution as well as the stirring speed of the reaction mixture. Such insight is important for devising synthetic routes for different sized nanoclusters. We also compared the photoluminescence and electrochemical properties of PhCH2S-protected Au20 and Au24 nanoclusters with the PhC2H4S-protected counterparts. A surprising 2.5 times photoluminescence enhancement was observed for the PhCH2S-capped nanoclusters when compared to the PhC2H4S-capped analogues, thereby indicating a drastic effect of the ligand that is merely one carbon shorter.