Boosting the oxidase mimicking activity of nanoceria by fluoride capping: rivaling protein enzymes and ultrasensitive F⁻detection

Abstract

Nanomaterial-based enzyme mimics (nanozymes) are currently a new forefront of chemical research. However, the application of nanozymes is limited by its low catalytic activity and low turnover numbers. Cerium dioxide nanoparticles (nanoceria) is among the few with oxidase activity. Herein, we report an interesting finding addressing its limitations. The oxidase activity of nanoceria is improved by over 100-fold by fluoride capping, rendering it more close to real oxidases (e.g., cytochrome P450). The turnover number reached 700 in 15 min, drastically improved from ~15 turnovers for the naked particles. The mechanism is attributed to surface charge modulation and facilitated electron transfer by Fcapping based on ζ-potential and free radical measurement. Ultrasensitive sensing of fluoride was achieved with a detection limit of 0.64 µM F -in water and in toothpastes, while no other tested anions can achieve the activity enhancement.Developing nanomaterial-based enzyme mimics, known as nanozymes, is a new frontier in inorganic, materials and analytical chemistry. [1][2][3][4][5][6][7] Nanozymes are a subset of nanoparticle-based catalysts that catalyze enzyme-like reactions at ambient conditions. Nanozymes are attractive not only for their high stability and lowcost, but also for studying fundamental processes at nanoscale surfaces. 2 Many nanoparticles such as gold, 4,8,9 graphene oxide, 10,11 and various metal oxides 1,3,5,12-19 possess oxidase, peroxidase, and/or catalase like activities. It is interesting to note that while many peroxidase nanozymes were reported (i.e., using H2O2 as a cosubstrate), very few have oxidase activity. 9,[19][20][21][22][23] The best known examples are gold nanoparticles for glucose oxidation, 9 and CeO2 nanoparticles (nanoceria) that oxidize a diverse range of substrates. 20 Oxidase nanozymes are important since they do not require unstable H2O2 as a co-substrate.Nanoceria is a versatile nanozyme, displaying oxidase, 16,20 catalase, 24,25 superoxide dismutase, 26,27 and phosphatase 28 activities under different conditions. In 2009, Asati and co-workers first reported the oxidase-like activity of nanoceria. 20 Since then, nanoceria has become an attractive system receiving extensive studies. The catalytic activities of nanoceria were attributed to the mixed oxidation states of Ce 3+ and Ce 4+ , and related oxygen vacancies. 29,30 The role of redox states, 24 surface coating, 20 pH, 31 buffer composition, 32 and nucleotide triphosphate 16 were investigated. We hypothesize that surface chemistry plays a critical role in modulating substrate/product adsorption and electron transfer, thus influencing catalytic efficiency. A challenge in the nanozyme field is poor catalytic activity and low turnover, which is in sharp contrast to their highly efficient protein counterparts.

Self‐limited Phosphatase‐mimicking CeO2 Nanozymes

“…When the surface active sites are consumed, the nanozyme became inactivated. A similar kinetic behavior was also observed for reacting CeO 2 with ABTS (CeO 2 being an oxidase‐mimic), and it was attributed to the adsorption of the oxidation product of ABTS inhibiting the adsorption of new substrates . A similar mechanism could also take place here such as by the products as an inhibitor.…”

“…For CeO 2 and Ce(IV), aside from the strong inhibition by phosphate, we only observed a weak inhibition effect of fluoride (Figure A, B), which is a reflection of its binding to cerium. Interestingly, F − boosted the oxidase‐like activity of CeO 2 , but it inhibited the oxidation ability of free Ce(IV) . For the phosphatase reaction, the accessibility of the surface Ce(IV) sites are apparently very important.…”

“…Nanozymes refer to nanoparticle‐based enzyme mimics, and they have attracted extensive interest recently due to their low cost, high stability and versatile catalytic activities . Nanoceria is an interesting and important nanozyme, and it has been studied mainly for its redox activity to mimic oxidase, peroxidase, catalase, and superoxide dismutase (SOD) . Since cerium is a hard metal, it also has a strong affinity to phosphate.…”

Ceria Nanoparticles as Enzyme Mimetics

“…[17][18][19] In previous studies, ceria nanoparticles (nanoceria) and nanoceria-supported materials have been reported to have multi-enzyme mimetic activities like those of superoxide oxidase, [20,21] catalase [22,23] and oxidase. [24,25] That means nanoceria have enzymatic activities to catalyze the scavenging of reactive oxygen species (ROS). ROS are highly reactive and unstable compounds, with the ability to capture electrons from cellular macromolecules to damage proteins and DNA and to modify membrane fatty acids, thus impairing major cell functions.…”

Adsorption of Selenite and Selenate by Metal Oxides Studied with Fluorescent DNA Probes for Analytical Application

“…We took advantage of the competitive adsorption of DNA and arsenate for its detection [24][25][26]. Similarly, hydrogen peroxide can displace DNA from cerium oxide [27], while fluoride can enhance its catalytic activity [28]. These specific interactions are useful for biosensor development.…”