Although some ions, due to their unique chemical properties, can regulate the enzyme-like activity of nanomaterials, it is still a huge challenge to explore the mechanism of regulation. Herein, we found that Cr 6+ (CrO 4 2− ) as a smart switch can significantly increase the peroxidase-like (POD-like) activity of silver nanoparticles (Ag NPs), which were anchored efficiently on carbon spheres (Cal-CS/PEG/Ag) using amino-modified poly(ethylene glycol) (PEG) as a bridge. Density functional theory (DFT) calculations demonstrated that the addition of Cr 6+ can not only adjust the surface electronic redistribution of Ag atoms but also improve the geometric structure of the adsorbed intermediate, which resulted in the optimization of free energy and change of bond lengths in the catalytic reaction process, increasing the POD-like activity of Cal-CS/PEG/Ag. Based on the Cr 6+ -increased POD-like activity of Cal-CS/PEG/Ag, we successfully constructed a visual sensor of Cr 6+ along with quantitative analysis by the UV spectrum. The sensor has good selectivity for other 29 interfering ions and molecules with a detection limit of 79 nM. In this work, the detailed mechanism of the Cr 6+ -increased POD-like activity of Ag NPs was studied and a new possibility for the rational design of ion visual sensors using nanomaterials was proposed.

Compared with natural enzymes, nanozymes have advantages such as high stability, low cost, and a broad application prospect. However, due to the low catalytic activity of conventional nanozymes, synthesizing nanozymes with excellent enzyme-like activity will be long-term work. Herein, we synthesized single-atom nanozymes centered on Mn with N, P, and S doped (Mn-SAzyme) with enhanced peroxidase-like (POD-like) activity. We found that the presence of the Mn metal center significantly increased the POD-like activity of the nanozyme, exhibiting catalytic properties consistent with Michaelis–Menten kinetics. The POD-like activity was measured to be 17.48 U/mg, which was higher than that of the sample without the metal center. Using methylene blue (MB) as the organic dye model, the degradation efficiency of Mn-SAzyme could reach 90.1% within 300 min at pH 6.5, within the H2O2 concentration of 0.1 M and 25 mg/L of nanozyme dosage. Free radical quenching experiments and electron paramagnetic resonance tests showed that •O2 – and •OH were the key to degrading organic dyes. This work aims to explore more methods for synthesizing highly active nanozymes and successfully applying the synthesized nanozymes to the degradation of organic dyes.

Developing a peroxidase-like activity modulator to accurately regulate peroxidase-like activity has always been the primary challenge to be addressed. External stimulation can effectively modulate the electronic structure of metal-centered active sites, which optimizes the chemical reaction path and changes the peroxidase-like activity. Herein, in a weak acidic environment (pH = 6.0), we found that the electron transfer may occur between silver nanoparticles (Ag NPs) anchored on the surface of reduced graphene oxide/poly(ethylene glycol) (rGO/ PEG) nanosheets and added mercury(II) (Hg 2+ ), which can induce the formation of mercurous ions (Hg(I), Hg 2 2+ ). Hg(I) with regulatory catalytic performance can stimulate the expression of the active sites of Ag NPs via optimizing the electronic structure so that the adsorbed H 2 O 2 molecules can efficiently produce reactive oxygen intermediates ( • OH and • O 2 − ) and be stabilized on the surface of Ag NPs. Meanwhile, density functional theory (DFT) calculations showed that the generated Hg(I) significantly reduced the Gibbs free energy of the chemical reaction and provided a pathway for electron transfer, which effectively enhanced the peroxidase-like activity. Based on this finding, a visual colorimetric sensor was constructed to detect Hg 2+ with a detection limit of 19.7 nM in the range of 0.02−5.0 μM. Finally, we confirmed that the developed rGO/PEG/Ag nanohybrids can quantitatively detect Hg 2+ in the environment. This method is reliable and has potential advantages in the environmental application field.

As a typical biomarker for clinical diagnosis of diseases, alkaline phosphatase (ALP) plays a crucial role in regulating the dephosphorylation of biomolecules. Consequently, it is necessary to develop an effective and simple method for the determination of ALP activity. In this work, we report a catalytic reaction-based colorimetric method to monitor ALP activity by using a nanostructured MoO2@NP-doped carbon nano-octahedron (MoO2@NPC) as a favorable oxidase mimetic. Because the C–O bond generated in the nano-octahedron MoO2@NPC regulates the electronic valence state of Mo between +5 and +6, the as-prepared MoO2@NPC regular octahedron with good peroxide-like (POD-like) properties and colorless 3,3′,5,5′-tetramethylbenzidine dihydrochloride hydrate (TMB) can be catalyzed to dark-blue oxTMB. When ALP hydrolyzes the substrate AAP, the produced AA with a certain reducing ability can not only reduce oxTMB to TMB but also consume hydroxyl radicals (•OH) generated in the MoO2@NPC/H2O2/TMB system, which significantly inhibits the color reaction of TMB. On this basis, a simple and efficient colorimetric method for ALP was constructed based on carbon-based nanozymes. Furthermore, the system goes for the detection of ALP with high sensitivity for the first time. The linearity is good in the range of 0.5–30 U/L, and the detection limit was 0.02 U/L and had good stability and reproducibility. Reliable and accurate measurements of analytes in real samples were also validated by our analytical method, showing that it can provide a reliable, convenient, and efficient nanoplatform for clinical diagnosis of ALP activity.

The Co3O4 hollow dodecahedron (Co3O4@CN) was prepared by calcining ZIF-67 in N2 and then air atmosphere. In the Co3O4@CN compound material, Co3O4 nanoparticles (Co3O4 NPs) are anchored in the carbon skeleton, solving the aggregation problem of Co3O4 NPs. Because Co3O4@CN retains the porous structure of ZIF-67, and the presence of Co2+ and Co3+ favors electron transfer between the enzyme substrates, it has excellent peroxidase-like (POD-like) activity. Enzymatic kinetic tests show that Co3O4@CN has approximately six times more affinity for TMB than HRP and the maximal reaction rate is approximately three times that of HRP. The cholesterol sensor was constructed with Co3O4@CN, and its linear range was 20-1000 μM, and the detection limit was 14.33 μM. The application of Co3O4@CN to the detection of human serum cholesterol will be a valuable research direction.