Cluster Nanozymes with Optimized Reactivity and Utilization of Active Sites for Effective Peroxidase (and Oxidase) Mimicking

Abstract

activity (2007), [1] a wide range of nanomaterials (i.e., nanozymes) has been found to exhibit this property. [2][3][4][5] However, the mechanism behind those transition metal-based mimetics is not clear until recently. [6][7][8] As illustrated in Figure 1a, the redox between their surface M n+ (oxidation) and H 2 O 2 (reduction) first generates OH radicals (step (a)), which later oxidize substrates (e.g., tetramethylbenzidine (TMB)) giving a color change for the evaluation of activity (step (b)). To complete the catalytic cycle, OH radicals must oxidize other H 2 O 2 to produce HO 2 radicals (step (c)) for the regeneration of surface M n+ (step (d)). Since the later three steps are two orders of magnitude faster than step (a), the redox rate of surface M n+ with H 2 O 2 is thus the key step affecting the peroxidase-like activity of nanozymes. [6] Our recent study also suggests that the activity of a given nanozyme can be further enhanced by tuning the initial concentration of surface M n+ and its local structure for H 2 O 2 activation. [8] This mechanism is distinct from that of peroxidases. Taking horseradish peroxidase (HRP) as an example (Figure 1b), [9] its active site consists of a Fe(III)-protoporphyrin IX with His-42 and Arg-38 in the reaction pocket coactivating the OO bond of H 2 O 2 . The adsorbed H 2 O 2 first oxidizes Fe(III) (i.e., the resting state) to Fe(IV)-oxo with a radical retained on the porphyrin ring (the compound I). This reactive Single-atom catalysts have attracted attention in the past decade since they maximize the utilization of active sites and facilitate the understanding of product distribution in some catalytic reactions. Recently, this idea has been extended to single-atom nanozymes (SAzymes) for the mimicking of natural enzymes such as horseradish peroxidase (HRP) often used in bioanalytical applications. Herein, it is demonstrated that those SAzymes without constructing the reaction pocket of HRP still undergo the OH radical-mediated pathway like most of the reported nanozymes. Their positively charged single-atom centers resulting from support electronegative oxygen/nitrogen hinder the reductive conversion of H 2 O 2 to OH radicals and hence display low activity per site. In contrast, it is found that this step can be facilitated over their metallic counterparts on cluster nanozymes with much higher site activity and atom efficiency (cf. SAzymes with 100% atom utilization). Besides the mimicking of HRP in glucose detection, cluster nanozymes are also demonstrated as a better oxidase mimetic for glutathione detection.

Maximizing the peroxidase-like activity of Pd@PtxRu4−x nanocubes by precisely controlling the shell thickness and their application in colorimetric biosensors

“…S13†). 46 In addition, our DFT results showed that the Gibbs free energy changes at the 2 HO* adsorbed on the surface of pure Pt, pure Ru, and Pt–Ru alloy were −2.731, −0.207, and −0.189 eV, respectively. Taken together, incorporating Ru atoms into the Pt lattice to form a Pt–Ru alloy could directly dissociate H 2 O 2 into 2 HO*, which indicates that the catalytic site should be ascribed to the Pt–Ru alloy as a whole rather than a single element.…”

“…4f). 46 The UV-Vis absorption spectra illustrate that the absorbance of oxTMB decreased gradually (Fig. 4g), with the absorbance at 652 nm showing a linear relationship between the absorbance and GSH concentration from 1 to 100 × 10 −6 M (Fig.…”

Surface Science of Nanozymes and Defining a Nanozyme Unit

“…For the latter, radicals stay on the Fe-protoporphyrin ring. Similar surface science work was also performed on single-atom nanozymes, where the OH•-mediated pathway was also identified …”

Shape Regulation of CeO2 Nanozymes Boosts Reaction Specificity and Activity

“…[4] Up to date, many transition metal oxides with switchable oxidation states (e. g., V 2 O 5 , [5] Mn 3 O 4 , [6] Fe 2 O 3 , [7] Co 3 O 4 , [8] NiO, [9] Cu 2 O, [10] and CeO 2 [11][12][13][14][15] ) have been reported to exhibit this peroxidase (POD)-like activity even in their single atom form. [16][17][18][19][20][21][22] Interestingly, CeO 2 is the only reducible transition metal oxide that can mimic alkaline phosphatase (ALP) in catalyzing the hydrolysis of phosphorylated molecules [23][24][25][26][27] as that of non-reducible ZrO 2 . [28] Given that these two enzyme-like reactions undergo different catalytic mechanisms, it would be very interesting if one could precisely manipulate the reaction specificity on a pristine CeO 2 .…”