## 1 Introduction  
Nanomaterials, including metals, semiconductors, and insulators, have been intensively investigated for their applications in medicine, energy, and electronics due to their superior chemical and physical properties [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. Among the different nanomaterials, metallic nanoclusters with tunable fluorescence, easy surface modification, and precise structure have been extensively applied in bioimaging, sensing, and therapy [13, 14, 15, 16, 17, 18, 19, 20]. For example, gold nanoclusters (AuNCs) conjugated with the ligand of glucose have been developed as a fluorescent molecular contrast agent for the specific targeting of glucose transporters that are overexpressed in brain cancer cells [21]. Hemoglobin-conjugatedAuNCs have been explored for the rapid sensing of three haptoglobin phenotypes in plasma, namely, Hp 1-1, Hp 2-1, and Hp 2-2, because the fluorescent AuNCs combined with various haptoglobin phenotypes have caused their fluorescence changes in the intensity and the peak position [22]. Nanoclusters composed of gold or copper as a metal core and ovalbumin, pepsin, trypsin, or glutathione as the ligands have been used to create an ultrafast sensor that provides a clear distinction between Gram-positive and Gram-negative bacteria [23]. Radiofrequency-responsive AuNCs that are obtained via the reduction of Au(I)-thiolate complex using L-glutathione under the template polymer have been manufactured to improve the synergistic therapy of tumor ablation and artery embolization [24]. Silver nanoclusters (AgNCs) that are conjugated with specific DNA sequences as the highly fluorescent probes have been proposed for the specific, sensitive, and one-step immunoassay of bivalent anti-digoxigenin antibodies [25]. Achievements in the designs of fluorescent metal nanoclusters with various surface ligands as the probes have shown enormous potential for biomedical applications. In comparison with traditional organic fluorescent dyes, fluorescent metal nanoclusters produce broad emission spectra, large Stokes shifts, and long imaging times for bioimaging [26; 27; 28; 29; 30]. However, recent studies of ultra-small nanoclusters have been confirmed to produce antimicrobial activity [31; 32; 33; 34; 35]. For example, 6-mercaptoethanol-acid-conjugated AuNCs have displayed wide-spectrum antibacterial activity against both Gram-positive and Gram-negative bacteria due to their high interaction with bacteria to induce intracellular reactive oxygen species (ROS) generation to consequently kill bacteria [36]. Quaternary-ammonium-conjugated AuNCs have been prepared as antibacterial agents to combat methicillin-resistant _Staphylococcus aureus_ (_S. aureus_) and multidrug-resistant _Escherichia coli_ (_E. coli_) because AuNCs with a positive surface charge can improve their electrostatic adsorption onto the bacterial cell membrane and further enter into the bacteria to increase ROS production to kill bacteria [37]. Moreover, brightly fluorescent melamine-DNA-conjugated AgNCs have exhibited higher antimicrobial activities against Gram-negative _E. coli_ and Gram-positive _S. aureus_ than those of DNA-AgNCs alone [38]. The great advancements for the uses of fluorescent metal nanoclusters as antibacterial agents have importantly impacted the challenge against bacteria due to their increases in ROS generation in bacteria. The bactericidal effects of fluorescent metal nanoclusters have obstructed their bioimaging applications in bacteria owing to the cause of bacterial death. To address the issue of ROS production induced by metal nanoclusters in bacteria, the ROS scavenger cysteine was utilized as a ligand for the syntheses of cysteine-conjugated gold nanoclusters (Cys-AuNCs) and cysteine-conjugated silver nanoclusters (Cys-AgNCs) using a facile hydrothermal approach.