Ligand stabilized ultra-small coinage metal clusters have emerged as a new class of functional material with unique physico-chemical properties and have tremendous application potential specially in the areas of trace level sensing, bio-imaging, light emitting diodes and catalysis [1–5]. These small clusters show molecular like optical absorption properties, high quantum yield and tuneable luminescence behaviour with good photo stability, selective catalytic activity and good bio-compatibility which make them superior than the organic dye/fluorophores [6–8]. The origin of emission from these clusters are broadly decided by the nature of metal(s) and their oxidation state, size of the cluster, atomic arrangement of the cluster, type of ligand and metal-ligand interfacial structure [7–10]. Strong luminescence, large stokes shift, high photo-thermal stability, good biocompatibility, availability of easy surface modification routes and their high stability under ambient conditions are reasons behind their potential applications [7–11]. The use of stabilizing ligands with various surface functional groups, e.g., ‘thiol’, ‘alkyne’, ‘halide’, ‘carbene’, ‘amine’ and ‘carboxylic acids’ have been studied to synthesize a variety of emissive clusters [12–18]. Although various ligands have been successfully used, but still the ‘thiol’ based ligands including small thiol molecules, amino acids, peptides and proteins are found most intriguing in terms of synthesizing highly emissive metal clusters. L-Glutathione is a tripeptide with a free-SH group and have been used extensively to synthesize a variety of metal clusters and nanoparticles [19–26]. The highly fluorescent Au22(SG)18 cluster with strong red emission is one of the most prominent early example with a quantum yield of 16% [20, 21]. It was revealed that aggregation of Au(I)-SG oligomers on Au(0) kernel induced ligand to metal charge transfer (LMCT) transitions for showing such strong emission [20, 21]. In pursuit of isolating more emissive cluster, alloying of two metals were tested as the resulting bimetallic cluster may achieve higher quantum yield through better geometrical rigidity, smaller kernel size and efficient electron transfer dynamics [27–37]. Thus, alloying of Ag or Cu with Au have resulted a number of highly luminescent bimetallic clusters in recent years with high quantum yield up to 71% [33].
Trace level detection of various analytes including metal ions, explosives, specific biomolecules, toxic molecules and food contaminants by luminescent noble metal clusters is a promising approach [38–43]. The detection of trace quantity of metal ions is important as their contamination in food, drinking water and in body fluids is very important from human health safety and diagnostic prospective. The advantages of luminescence sensing of analytes includes very high sensitivity and selectivity, low cost, on the spot testing feasibility and fast response time [38] Luminescence quenching (Turn-OFF) is the most dominant pathway of sensing analytes demonstrated by various metal clusters and we may ascribe its origin through electron/energy transfer from metal cluster to analyte through a static or dynamic adduct formation mechanism [44–48]. Luminescence enhancement (Turn-ON) based sensing of analytes is more advantageous but is less common and it follows a distinct mechanism than the previous one [49–51]. In metal ion sensing, the ions may interact with surface ligands, or it may interact with surface metal atoms of the cluster and may also form a different alloy resulting a luminescence response. Thus, we were interested to understand the metal ion sensing ability of a monometallic (Aun) and a bimetallic nanocluster (AunAgm) stabilized by a sufficiently polar thiol ligand {L-Glutathione (L-GSH)}. We anticipated that various metal ions will interact differently with mono and bimetallic nanocluster surface atoms or with the ligand functional groups and may result different responses. Thus, the present work reports synthesis, characterization and luminescence sensing behaviour of Glutathione stabilized Au and AuAg clusters for various metal ions with a major focus on their photo physical aspects. Interestingly, we could find that while gold clusters showed selective luminescence quenching based sensing of Fe3+ and Hg2+, the AuAg cluster showed luminescence quenching for Fe3+, Hg2+ and Cu2+ but luminescence enhancement in presence of Cd2+. Detailed steady state and time correlated single photon counting (TCSPC) analysis were performed along with hydrodynamic diameter (Dh) to understand this different sensing behaviour for two different clusters. The present work is also significant for the observed turn-on luminescence response of the bimetallic cluster and may find application as a real world sensor for the detection of trace level metal ions. In addition to it the work suggests that it may be feasible to synthesize a tri-metallic cluster with high phosphorescence quantum yield at room temperature by combining Au, Ag and Cd.