Nanoisozymes: The Origin behind Pristine CeO₂ as Enzyme Mimetics

Abstract

It is known that the interplay between molecules and active sites on the topmost surface of a solid catalyst determines its activity in heterogeneous catalysis. The electron density of the active site is believed to affect both adsorption and activation of reactant molecules at the surface. Unfortunately, commercial X‐ray photoelectron spectroscopy, which is often adopted for such characterization, is not sensitive enough to analyze the topmost surface of a catalyst. Most researchers fail to acknowledge this point during their catalytic correlation, leading to different interpretations in the literature in recent decades. Recent studies on pristine Cu2O [Nat. Catal. 2019, 2, 889; Nat. Energy 2019, 4, 957] have clearly suggested that the electron density of surface Cu is facet dependent and plays a key role in CO2 reduction. Herein, it is shown that pristine CeO2 can reach 2506/1133 % increase in phosphatase‐/peroxidase‐like activity if the exposed surface is wisely selected. By using NMR spectroscopy with a surface probe, the electron density of the surface Ce (i.e., the active site) is found to be facet dependent and the key factor dictating their enzyme‐mimicking activities. Most importantly, the surface area of the CeO2 morphologies is demonstrated to become a factor only if surface Ce can activate the adsorbed reactant molecules.

Shape Regulation of CeO2 Nanozymes Boosts Reaction Specificity and Activity

“…For the POD‐like reaction associated with the redox between OPD and H 2 O 2 (Figure 1e), the formation of diaminophenazine (DAP) with an emission peak centered at 557 nm was monitored. Interestingly, CeO 2 shapes displayed an opposite trend in their POD‐like activity (i. e., cube>sphere>octahedron) similar to that of using TMB as the substrate (i. e., cube>octahedron) [13–15] . Note that their oxidase‐like activity in the absence of H 2 O 2 is negligible (Figure S5).…”

“…Previous studies on the activity of CeO 2 nanozymes often relied on colorimetric substrates such as nitrophenyl phosphate (NPP) for the ALP-like reaction [18][19][20] and tetramethylbenzidine (TMB) for the POD-like reaction. [11][12][13][14][15] Considering that fluorescent probes have become indispensable for bio/molecular imaging with much higher sensitivity compared to that of colorimetric probes, we, therefore, evaluated the ALP-like and POD-like activity of CeO 2 using fluorogenic substrates, 4-methylumbelliferyl phosphate (4-MUP, at pH = 7.2) and o-phenylenediamine (OPD, at pH = 4.5). See Figure S4 for the buffer pH effect on both substrates.…”

“…Interestingly, CeO 2 shapes displayed an opposite trend in their POD-like activity (i. e., cube > sphere > octahedron) similar to that of using TMB as the substrate (i. e., cube > octahedron). [13][14][15] Note that their oxidase-like activity in the absence of H 2 O 2 is negligible (Figure S5).…”

“…[1] Among them, nanomaterials with enzyme-like characteristics (also called nanozymes) have received increasing attention [2,3] since Fe 3 O 4 nanoparticles (NPs) was found to oxidize a given substrate with H 2 O 2 as that of horseradish peroxidase (2007). [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 .…”

An Atomic Insight into the Confusion on the Activity of Fe3O4 Nanoparticles as Peroxidase Mimetics and Their Comparison with Horseradish Peroxidase

“…Pioneered by Yan’s group (2007), inorganic Fe 3 O 4 nanoparticles (NPs) were found to outperform horseradish peroxidase (HRP) based on parameters from Michaelis–Menten equation. Since then, various peroxidase nanozymes have been developed for a wide range of fields such as biosensing, biomedicine, wastewater treatment, and so on. − However, no standard protocol was provided for the quantitative comparison of activity between them (or with HRP). Although the Michaelis constant ( K m ) and the maximal reaction rate ( V max ) can be simply obtained by the Michaelis–Menten curve, the V max is very sensitive to the amount of nanozyme used (cf.…”