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.

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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.

Donor−linker−acceptor (D-L-A)-based photoinduced electron transfer (PET) has been frequently used for the construction of versatile fluorescent chemo/biosensors. However, sophisticated and tedious processes are generally required for the synthesis of these probes, which leads to poor design flexibility. In this work, by exploiting a Schiff base as a linker unit, a covalently bound D-L-A system was established and subsequently utilized for the development of a PET sensor. Cysteamine (Cys) and N-acetyl-L-cysteine (NAC) costabilized gold nanoclusters (Cys/ NAC-AuNCs) were synthesized and adopted as an electron acceptor, and pyridoxal phosphate (PLP) was selected as an electron donor. PLP can form a Schiff base (an aldimine) with the primary amino group of Cys/NAC-AuNC through its aldehyde group and thereby suppresses the fluorescence of Cys/NAC-AuNC. The Rehm−Weller formula results and a HOMO−LUMO orbital study revealed that a reductive PET mechanism is responsible for the observed fluorescence quenching. Since the pyridoxal (PL) produced by the acid phosphatase (ACP)-catalyzed cleavage of PLP has a weak interaction with Cys/NAC-AuNC, a novel turn-on fluorescent method for selective detection of ACP was successfully realized. To the best of our knowledge, this is the first example of the development of a covalently bound D-L-A system for fluorescent PET sensing of enzyme activity based on AuNC nanoprobes using a Schiff base.

The Os nanozyme possesses a high peroxidase-like activity and negligible oxidase-like activity freeing the assays from the O2 and color interference.