Metal nanoclusters (NCs) as a new type of fluorescent material have been extensively explored because of their attractive set of features such as their ultrafine size, low toxicity, and excellent photostability. However, little progress has been made in producing water-soluble, homogeneous, and ultrabright metal NCs. In this study, gold NCs (AuNCs) with a photoluminescence quantum yield (QY) as high as 65% are synthesized in water through a simple blending route. Weak emission is observed from the 6-aza-2-thiothymine-protected AuNCs (ATT-AuNCs); however, the fluorescence intensity can be prominently enhanced by introducing L-arginine (Arg) into the capping layer. The fluorescence enhancement mechanism is systematically investigated by the measurements of ultraviolet−visible absorption spectroscopy, photoluminescence spectroscopy, fluorescence lifetime spectroscopy, transmission electron microscopy, dynamic light scattering, X-ray photoelectron spectroscopy, 1 H nuclear magnetic resonance, and calculations from density functional theory, with results isolating the vital role of the ligand shell and ruling out the effect of the gold core. The supramolecular host−guest assemblies formed between ATT capped on the gold core and the guanidine group of Arg make the capping ligands of ATT rigid. Subsequently, the intramolecular vibration and rotation of ATT are greatly suppressed, which reduce the nonradiative relaxation of excited states and, as a result, predominantly increase the luminescence QY of ATT-AuNCs. Further experiments demonstrate that a small change in guanidine substituents can arouse obvious changes in the photoluminescence features of NCs. We envision that this work will substantively contribute to the process of developing efficient synthetic routes to high-quality metal NCs.

Ligand‐protected gold nanoclusters (AuNCs) have emerged as a new class of electrochemiluminescence (ECL) luminophores for their interesting catalytic and emission properties, although their quantum yield (ΦECL) in aqueous medium is low with a poor mechanistic understanding of the ECL process. Now it is shown that drying AuNCs on electrodes enabled both enhanced electrochemical excitation by an electrocatalytic effect, and enhanced emission by aggregation‐induced ECL (AIECL) for 6‐aza‐2‐thiothymine (ATT) protected AuNCs with triethylamine (TEA) as a coreactant. The dried ATT‐AuNCs/TEA system resulted in highly stable visual ECL with a ΦECL of 78 %, and a similar enhancement was also achieved with methionine‐capped AuNCs. The drying enabled dual‐enhancement mechanism has solved a challenging mechanistic problem for AuNC ECL probes, and can guide further rational design of ECL emitters.

Gold nanoclusters (AuNCs) are attractive electrochemiluminescence (ECL) emitters because of their excellent stability, near IR emission, and biocompatibility. However, their ECL quantum yield is relatively low, and our limited fundamental understanding has hindered rational improvement of this parameter. Herein, we report drastic enhancement of the ECL of ligand‐stabilized AuNCs by on‐electrode pre‐oxidation with triethylamine (TEA) as a co‐reactant. The l‐methionine‐stabilized AuNCs resulted in a record high ECL yield of 66 %. This strategy was successfully extended to other AuNCs, and it is more effective for ligand shells that allow more effective electron transfer. In addition, excitation of the pre‐oxidized ECL required a lower potential than conventional methods, and no additional instrument was required. This work opens avenues for solving a challenging problem of AuNC‐based ECL probes and enriches fundamental understanding, greatly broadening their potential applications.

An easily prepared platinum nanoparticle (PtNP) probe for the sensitive and selective detection of Hg(2+) ions is developed here. The PtNPs with an average size of approximately 2.5 nm were prepared by a reduction method with sodium borohydride and trisodium citrate serving as reductant and stabilizer, respectively. The resulting PtNPs could catalyze the reduction of Hg(2+) by surface-capping citrate. The effect of Hg(2+) uptake implies amalgam formation, which leads to remarkable inhibition of the peroxidase-like activity of citrate-capped PtNPs. On the basis of this effect, a colorimetric mercury sensor was established through the use of citrate-capped PtNPs to catalyze the colorimetric system of 3,3',5,5'-tetramethylbenzidine (TMB) and H2O2. The high specificity of the Hg-Pt interaction provides the excellent selectivity for Hg(2+) over interfering metal ions. The sensitivity of this smart probe to Hg(2+) is extremely excellent with a limit of detection (LOD) as low as 8.5 pM. In view of these advantages, as well as the cost-effectiveness, minimized working steps, and naked-eye observation, we expect that this colorimetric sensor will be a promising candidate for the field detection of toxic Hg(2+) ions in environmental, biological, and food samples.

Capping molecules on the surface of nanomaterials not only enhance the dispersion and stability of nanomaterials but also greatly facilitate their surface modification and biological applications. However, most capping molecules can severely block the active sites of the catalytic core, thereby decreasing the enzymatic activity of nanomaterial-based enzyme mimics. This work demonstrates the superiority of chitosan (Ch) as a capping molecule for synthesizing catalytic platinum nanoparticles (PtNPs). The experimental results show that Ch simultaneously exhibits an excellent stabilizing effect and enhances the oxidase-like activity of PtNPs. Kinetic studies indicate that Ch-PtNPs have a higher affinity for 3,3',5,5'-tetramethylbenzidine (TMB) than other kinds of oxidase mimics. Furthermore, the TMB chromogenic reaction catalyzed by Ch-PtNPs is found to be much faster in an acidic medium, thus adapting well to the optimal pH for acid phosphatase (ACP). Therefore, a novel colorimetric approach for ACP determination is developed for the first time, which is based on the Ch-PtNP-catalyzed oxidation of TMB, the inhibitory effect of ascorbic acid (AA) on the oxidase-like activity of Ch-PtNPs, and the ACP-catalyzed hydrolysis of AA 2-phosphate (AAP) into AA. The linear range for ACP is 0.25-2.5 U L and the limit of detection is measured to be 0.016 U L. This new colorimetric method is utilized to detect ACP in real biological samples and to screen ACP inhibitors. We believe that these new PtNPs, which exhibit high colloidal stability, excellent catalytic performance, good biocompatibility, simple preparation, and easy modification, can be promising candidates for a broad range of applications in optical sensing, environmental monitoring, clinical diagnosis, and drug discovery.

This work elucidated the valence states effect on the electrogenerated chemiluminescence (ECL) performance of gold nanocluster (AuNC). The N-acetyl-l-cysteine-AuNCs (NAC-AuNCs) and the electrochemical reduction method for reducing the AuNCs were first employed to this study. Results demonstrate that the electrochemical reduction degree of the AuNCs depended on the reduction potential, and the enhancement of the ECL signals was positively correlated with the reduction degree of AuNCs, which indicated that the valence state of Au plays a vital role in the ECL performance of AuNCs. Furthermore, the proposed method has been successfully extended to the chemical reduction technique and other nanoclusters. Therefore, an excellent AuNC-based ECL method with various advantages, such as simple preparation, lower toxicity, high sensitivity, and Φ, and excellent stability, has been proposed. This approach not only opens up a new avenue for designing and developing ECL device from other functional-metal based NCs, but also extends the huge potential application in the ECL sensing.

Although it has been demonstrated that rareearth elements (REEs) disturb and alter the catalytic activity of numerous natural enzymes, their effects on nanomaterialbased artificial enzymes (nanozymes) have been seldom explored. In this work, the influence of REEs on the peroxidase-like activity of bare gold nanoparticles (GNPs) is investigated for the first time, and a new type of Ce 3+ -activated peroxidase mimetic activity of GNPs is obtained. The introduced Ce 3+ can be bound to the bare GNP surface rapidly through electrostatic attraction, after which it donates its electron to the bare GNP. As H 2 O 2 is a good electron scavenger, more • OH radicals are generated on the surfaces of the bare GNPs, which can considerably enhance TMB oxidation. Due to its redox cycling ability, the activation effect of Ce 3+ is proved to be more efficient in comparison to those of the other reported metal ion activators (e.g., Bi 3+ , Hg 2+ , and Pb 2+ ). In addition, it is determined that Ce 3+ should directly contact with the gold core to trigger its activation effect. When the surface states of the bare GNPs are altered, the Ce 3+ -stimulated effect is strongly inhibited. Furthermore, a novel colorimetric method for Ce 3+ is developed, on the basis of its enhancing effect on the peroxidase mimetic activity of bare GNPs. The sensitivity of this newly developed method for Ce 3+ is excellent with a limit of detection as low as 2.2 nM. This study not only provides an effective GNP-based peroxidase mimic but also contributes in realizing new applications for nanozymes.

Herein, we reported for the first time a facile synthetic process of gold nanoclusters (AuNCs) by using N-acetyl-L-cysteine both as a reducing agent and as a protection ligand. Based on the pH stimuli-responsive properties of the as-prepared AuNCs, we constructed a pH-sensing platform for the detection of urea, urease, and urease inhibitors.