Ligand-mediated modulation of enzyme- mimetic activities of cerium oxide nanoparticles

Abstract

Nanotechnology refers to items that are small enough to be seen by the naked eye. "Nanoparticles" or "nanomaterials" are materials with a diameter of 1 to 100 nanometers. Because of their unique qualities, such as a high surface area to volume ratio, greater thermal and mechanical stability, improved catalytic, optical, and electrical capabilities, and so on, nanoparticles (NPs) are favored over their bulk counterparts. The size, chemical content, shape, origin, aggregation state, synthesis, and aspect ratio of NPs may all be used to classify them. NPs have a wide range of applications in environmental cleaning, healthcare products, water, and air purification, agriculture, and biological applications due to their unique features. NPs can also behave like enzymes

these NPs are known as nanozymes. Nanozymes are artificial enzymes that have been chosen as a replacement for natural enzymes due to their strong resilience to extreme environments, longevity, ease of recycling, mass manufacturing, and low cost. Gold, iron oxide, cerium oxide, zirconium oxide, and other NPs can imitate natural enzymes. Cerium oxide nanoparticles (CeONPs) have been found to have enzyme-mimetic functions [superoxide dismutase (SOD), catalase, peroxidase, oxidase, phosphatase, phosphotriesterase, nuclease, and others]. The oxides of the rare earth element "cerium" are known as CeONPs. Cerium, unlike other transition elements, has the unusual feature of being able to exist in two oxidation states (3+ and 4+). The cerium atom forms a face-cantered cubic shape when it combines with oxygen. CeONPs have a variety of flaws on their lattice surface, which makes them extremely reactive and adds to their catalytic properties. CeONPs can serve as both antioxidants (free radical scavengers) and pro-oxidants (free-radical producing agents). Free radicals are highly unstable molecules produced as a by-product of normal physiological processes. They are an essential component of many biological processes, but an excess of them can be dangerous. Antioxidants are naturally produced in the biological system to manage the level of free radicals, but when there is a disparity between the number of free radicals produced and the number of antioxidants produced, a state known as “oxidative stress” develops. CeONPs antioxidant or pro-oxidant properties are said to be influenced by their redox state. SOD-mimetic activity is found in CeONPs with a prominent "3+" oxidation state, whereas catalase, oxidase, and peroxidase-mimetic activity is found in CeONPs with a predominant "4+" oxidation state. CeONPs are strong antioxidants with a unique redox-switching capability, but their biological research applications are currently limited. Loss of activity in the biological system, colloidal instability, non-specific contact, and toxicity within the biological system are the main causes. Surface coating is an effective way to avoid these problems. CeONPs with varied surface coatings have improved enzyme-mimetic activity and occasionally impart new characteristics to the NPs. In this quest, we selected three different types of ligands i.e., Keggin ions or polyoxometalates (POMs), polymers, and phosphines to interact with CeONPs. The selection of these ligands was based on their binding affinity with CeONPs and stability. The POMs selected were Phosphotungstic acid (PTA) and Phosphomolybdic acid (PMA). The binding of POMs ligands significantly improved the catalase as well as peroxidase mimetic activities of CeONPs. PTA binding with CeONPs (3+) improved the SOD mimetic activity, whereas binding of PMA with CeONPs (3+) inhibited the SOD-mimetic activity. The obtained results specified that the active sites of CeONPs are different for different enzymatic reactions. Based on the enhanced enzymatic activities, CeONPs (4+) incubated with PTA were analyzed for their ROS scavenging potential in excessive hydrogen peroxide conditions. Results indicated that PTA-bound CeONPs (4+) effectively scavenge the ROS generated by hydrogen peroxide in human hepatic cells (WRL-68). Polymers have been reported to impart the colloidal stability and biocompatibility of CeONPs but the effect of polymers on different enzyme-mimetic activities was not well studied. Hence, in this work, six different polymers [sodium polyacrylate (PAA), polyethylene glycol (PEG), and their derivatives] were selected to coat the surface of CeONPs. These polymers were selected for coating as they have been found to block the adsorption of proteins on the surface of NPs and provide colloidal stability to MONPs. After binding with polymers to CeONPs, the oxidase and catalase-mimetic activities got suppressed and left SOD-mimetic activity unaltered. The peroxidase-mimetic activity of polymer-bound CeONPs got significantly enhanced (best results were obtained with PEG-coated CeONPs) and on that basis, their application has been shown in glucose sensing in human serum. The auto regeneration of surface redox potential by CeONPs remains unaltered after binding with polymers. Hence polymer binding not only improves the stability and biocompatibility of CeONPs but also can selectively tune the enzyme-mimetic properties. Further, phosphines were selected to tune the enzymatic activities and to alter the surface oxygen vacancies of CeONPs. Two different phosphines i.e., Tris(2,4,6-trimethoxyphenyl) phosphine (TTMPP) and triethylphosphines (TEP) are phosphorus-supporting ligands that were selected based on their binding affinity with metal oxides, their reducing nature (electron-rich nature), and also, they have been reported to impart colloidal stability in biological milieus. Phosphines have a central lone pair, which can be donated to transition metals or oxides, and hence they form stable complexes with metal oxides. The binding of TEP with CeONPs (3+) did not result in any significant changes in the activity or morphology of NPs but altered the redox state of CeONPs (4+). After binding with TEP, CeONPs (4+) showed SOD-mimetic behavior instead of catalase-mimetic. Nevertheless, TTMPP binding altered the redox state of CeONPs (3+) and hence showed catalase-mimetic behavior instead of SOD-mimetic, which is opposite to their nature. Hence, we can conclude that phosphines were selectively binding with the surface of CeONPs and can tune the enzymatic activities of CeONPs. The regeneration of the surface redox potential of CeONPs (3+) was inhibiting after binding with TTMPP, which indicated that TTMPP is imparting permanent alteration in the redox state of CeONPs. TEP after binding CeONPs showed an increase in the concentration of Ce (3+) atoms on lattice surface and oxygen defects. Phosphines have a major disadvantage: they are unstable in the air and easily oxidize. But after binding with CeONPs, the formulation attained good stability. CeONPs (4+) conjugated with TEP were tested for their antibacterial and antiparasitic behavior against Escherichia coli and Leishmania donovani Ag83 promastigotes respectively. TEP-bound CeONPs (4+) resulted in ~60% cell death after 24 hours and ~ 84% cell death after 48 hours in Leishmania donovani Ag83 promastigotes by ROS generation inside cells. On the other hand, CeONPs (4+)-TEP also showed ~50% cell death in Escherichia coli after 12 hours.