Nano‐Gold as Artificial Enzymes: Hidden Talents

Mesoporous silica-encapsulated gold nanoparticles as artificial enzymes for self-activated cascade catalysis

Nanozyme-catalyzed cascade reactions for high-sensitive glucose sensing and efficient bacterial killing

Nanomaterials with enzyme-like activity, generally referred to as ‘nanozymes’, find myriad potential in various biomedical fields. More importantly, the nanoparticles that can functionally mimic the activity of cellular antioxidant enzymes attract tremendous interest owing to their possible therapeutic candidature in oxidative stress-mediated disorders. Oxidative stress culminating due to excess reactive oxygen species (ROS) level and dysregulated cellular antioxidant machinery is implicated in the development and progression of various pathophysiological disorders such as cancer, diabetes, cardiovascular and neurodegenerative diseases. Moreover, the optimum essentiality of ROS due to its pivotal role in cell signaling evokes the requirement of novel artificial antioxidant enzymes that can circumvent the detrimental effects of enhanced ROS levels without perturbing the basal redox status of cells. In recent years, the fast emanating artificial enzymes, i.e. nanozymes with antioxidant enzyme-like activity, has made tremendous progress with their broad applications in therapeutics, diagnostic medicine, bio-sensing, and immunoassay. Among various antioxidant nanoparticles reported till-date, the metal oxide nanozymes have emerged as the most efficient and successful candidates in mimicking the activity of first-line defense antioxidant enzymes, i.e. superoxide dismutase, catalase, and glutathione peroxidase. This review intends to exclusively highlight the development of representative metal oxide-based antioxidant nanozymes capable of maintaining the cellular redox homeostasis and their potential therapeutic significance.

Nanomaterials with intrinsic enzyme-like activities (nanozymes), have been widely used as artificial enzymes in biomedicine. However, how to control their in vivo performance in a target cell is still challenging. Here we report a strategy to coordinate nanozymes to target tumor cells and selectively perform their activity to destruct tumors. We develop a nanozyme using nitrogen-doped porous carbon nanospheres which possess four enzyme-like activities (oxidase, peroxidase, catalase and superoxide dismutase) responsible for reactive oxygen species regulation. We then introduce ferritin to guide nitrogen-doped porous carbon nanospheres into lysosomes and boost reactive oxygen species generation in a tumor-specific manner, resulting in significant tumor regression in human tumor xenograft mice models. Together, our study provides evidence that nitrogen-doped porous carbon nanospheres are powerful nanozymes capable of regulating intracellular reactive oxygen species, and ferritinylation is a promising strategy to render nanozymes to target tumor cells for in vivo tumor catalytic therapy.

An artificial smart bifunctional enzyme with both superoxide dismutase (SOD) and glutathione peroxidase (GPx) activities was successfully constructed by the self-assembly of a porphyrin core with four suspensory adamantyl moieties and β-cyclodextrin-terminated temperature-sensitive copolymer through host–guest interaction in aqueous solution. The Mn(III) porphyrin, Mn(III)meso-tetra[1-(1-adamantyl methyl ketone)-4-pyridyl] porphyrin (MnTPyP-M-Ad), was designed as both a “supramolecular linker” and an efficient active site of SOD and the temperature-responsive block copolymer (β-CD-PEG-b-PNIPAAm-Te) with incorporated tellurium moieties in the PNIPAAm chain as GPx active sites was synthesized by ATRP. As a new type of artificial bifunctional enzymes, it exhibited stable SOD-like activity of 0.137 μM (IC50) at 37 °C, and high GPx catalytic efficiency with temperature-responsive dependence characteristic, and the new artificial enzyme was found to exhibit the highest GPx catalytic efficiency (v0 = 8.11 μM min−1) in close to physical temperature.

Enzymes are truly outstanding biological catalysts with the ability to accelerate the rate of chemical reactions up to 1019 times for specific substrates and reactions. New approaches based on amino acids or peptides as characteristic molecular moieties have led to a significant expansion of the field of artificial enzymes or enzyme mimics, catalyzing various reactions with rate increases up to 103. A number of possibilities now exist for the construction of artificial enzymes. These are generally synthetic polymers or oligomers with enzyme-like activities, often called synzymes. This review has basic information about Ideal Requirement for Artificial Enzymes Environment, Design Approach for Artificial Enzyme and their Significance for further development in this field.

Human welfare depends on the function of natural systems. This idea is paradigmatic to ecologists and has been the theme of a growing branch of applied ecology. I examine the narrative of human dependence on nature by considering the literature on crop pollination by animals and its importance for food production. Making the connections between human welfare and natural systems is seen as a way to better motivate society to make better decisions, but the debate around crop pollination has been surprisingly contentious. There have been confusing messages, disagreements on the facts, an unfortunate focus on dire projections for the future and a lesser focus on solutions. Most of these problems arise not from poor science but instead from poor communication of complex ideas and differences in perspective, such as the deep disciplinary gap between agricultural scientists and ecologists. By understanding these problems, we can improve the way we do our science and communicate our ideas. I argue that ecologists should continue to communicate the principle that human welfare depends on the function of natural systems and discuss how we can do so in a way that is more genuinely connected to society's needs, such as growing food. If we succeed, we will be changing an intellectually interesting conversation into a dialogue that influences how society interacts with nature.

To develop nanomaterials as artificial enzymes, it is necessary to better understand how their physicochemical properties affect their enzyme-like activities. Although prior research has demonstrated that nanomaterials exhibit tunable enzyme-like activities depending on their size, structure, and composition, few studies have examined the effect of surface facets, which determine surface energy or surface reactivity. Here, we use electron spin-resonance spectroscopy to report that lower surface energy {111}-faceted Pd octahedrons have greater intrinsic antioxidant enzyme-like activity than higher surface energy {100}-faceted Pd nanocubes. Our in vitro experiments found that those same Pd octahedrons are more effective than Pd nanocubes at scavenging reactive oxygen species (ROS). Those reductions in ROS preserve the homogeneity of mitochondrial membrane potential and attenuate damage to important biomolecules, thereby allowing a substantially higher number of cells to survive oxidative challenges. Our computations of molecular mechanisms for the antioxidant activities of {111}- and {100}-faceted Pd nanocrystals, as well as their activity order, agree well with experimental observations. These findings can guide the design of antioxidant-mimicking nanomaterials, which could have therapeutic or preventative potential against oxidative stress related diseases.

Novel gold nanozyme regulation strategies facilitate analytes detection

Poly (N-isopropylacrylamide) (PNIPAAm) is one of the most popular thermoresponsive polymers, which shows dramatic and reversible phase transition behavior in water. Therefore two strategies for the design of temperature-sensitive enzyme model based on PNIPAAm derivatives are presented: 1) A temperature-sensitive block copolymer (PAAm-b-PNIPAAm-Te) with a glutathione peroxidase-like active site was synthesized via ATRP. 2) An artificial bifunctional enzyme with both superoxide dismutase (SOD) and glutathione peroxidase (GPx) activities was constructed by the self-assembly of a porphyrin core with four suspensory adamantyl moieties and b-cyclodextrin-terminated temperature-sensitive copolymer (β-CD-PEG-b-PNIPAAm-Te) through host–guest interaction in aqueous solution. With increasing temperature, the PNIPAAm chain becoming hydrophobic leads to a change in the self-assembly structure of the polymer, which plays a key role in modulating the catalytic activity. And two new artificial enzymes were found to exhibit the highest enzymatic catalytic efficiency in close to physical temperature.

Several diseases and disorders, including cancer are endorsed by excessive oxidative stress caused due to the incomplete removal of reactive oxygen species (ROS) by the antioxidant defense system of the body. Therefore, present interest among the scientific community lies in the development of a highly stable, biocompatible artificial enzymatic system that possesses a high ROS scavenging activity over a period of time. In recent years, catalytic nanoparticles emerged as a potential candidate in the field of nanomedicine. Due to their inherent catalytic properties, they are exploited as an artificial enzyme (nanozyme), to reinstate or correct aberrant enzymatic activities in patients. Among them, cerium oxide nanoparticles/nanoceria (CNPs) emerged as a potent artificial redox enzyme, mimicking the activity of superoxide dismutase (SOD) and catalase and endure a tremendous ROS scavenging potential as depicted in a surfeit of human cell lines and animal models. In the present article, a facile synthesis of biocompatible nanoceria encapsulated albumin nanoparticles (BCNPs) via desolvation technique that lead to the abatement of intracellular ROS is reported. Physico-chemical characterizations of as-prepared BCNPs corroborate the formation of a highly monodispersed, spherical and stable aqueous delivery system. Interestingly, such entrapment does not affect the enzyme mimetic activity of CNPs, as demonstrated by SOD assay. The biocompatibility and ROS scavenging potential of BCNPs were further assessed in vitro against human lung epithelial cells by cell viability assay and flow cytometric analysis, respectively. The quantitative and qualitative assessments of cellular uptake of BCNPs were done by inductively coupled plasma mass spectrometry (ICP-MS), transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM) analysis. Furthermore, the BCNPs preserve the cell's antioxidant defense system and protect them from oxidant-mediated apoptosis as confirmed by semi-quantitative RT-PCR analysis. Thus, the as-prepared BCNPs could provide an opportunity to be utilized as a potential candidate against ROS induced diseases and disorders.

Cerium oxide nanoparticles (nanoceria) are known to exhibit enzymelike activity, such as biological catalase, oxidase, superoxide dismutase, and peroxidase enzymelike activities. Catalytically active nanoceria offer several advantages over natural enzymes, such as controlled synthesis at low cost, tunable catalytic activities, as well as high stability against stringent physiological conditions. Exploiting these properties, several biomedical applications, such as biosensing, immunoassay, drug delivery, radiation protection, and tissue engineering, have been maneuvered. This review article provides a comprehensive summary of reported biological enzymelike activities of nanoceria and the possible mechanism of catalysis. It is also discussed that what physicochemical properties (surface charge, size, surface chemistry, and enzymelike activity) of nanoceria are altered when exposed to biologically relevant buffers and even in the mammalian cell cytoplasm. Based on various reports provided in the literature, these important issues need to be addressed. It has also been deliberated that in mammalian cells variations in properties of nanoceria observed are due to the intrinsic catalytic activity or as a result of downstream effects that could have emerged after the particles have interacted with biologic/environmental components. Such studies could be helpful in providing a better understanding the nanoceria's multienzyme-like activities and potential applications in the biomedical fields with special reference to damaging effects of free radicals in biological systems.

Nanozymes as artificial enzymes that mimicked natural enzyme-like activities have received great attention in cancer diagnosis and therapy. Biomimetic nanozymes require more consideration regarding complicated tumor microenvironments to mimic biological enzymes, thus achieving superior nanozyme activity in vivo. Here we report a biomimetic hybrid nanozyme (named rMGB) which integrates natural enzyme glucose oxidase (GOx) with nanozyme manganese dioxide (MnO2) by mutual promotion for maximizing the enzymatic activity of MnO2 and GOx. Under hypoxia environment, we observed that MnO2 could react with endogenous H2O2 to produce O2 for enhancing the catalytic efficiency of GOx for starvation therapy. Meanwhile, we confirmed that glucose oxidation generated gluconic acid and further improved the catalytic efficiency of MnO2 subsequently. The biochemical reaction cycle, consisting of MnO2, O2, GOx, and H+, was triggered by the tumor microenvironment and accelerated each other so as to achieve self-supplied H+ and accelerate O2 generation, enhancing the starvation therapy, alleviating tumor hypoxia and accelerating the reactive oxygen species generation in photodynamic therapy. This biomimetic hybrid nanozyme would further facilitate the development of biological nanozymes for cancer treatment.

Self-assembled artificial enzyme from hybridized porous organic cages and iron oxide nanocrystals

Despite advancements in artificial enzymes (AEs) for acute kidney injury (AKI) therapy through renal redox homeostasis modulation, the trade-off between organ-specific targeting and high multienzyme-mimicking catalytic efficiency is still a big challenge. Herein, we address this challenge through chirality-engineered Cu2+-phenolic AEs (l-phen@Cu-TA) via incorporation of l-phenylalanine (l-phen) to synergize stereoselective recognition and catalytic activity. Chiral l-phen induces electron density redistribution from phenolic ligands to Cu2+ in the cocatalytic center, significantly enhancing H2O2 adsorption while reducing catalytic energy barriers, thereby amplifying catalase (CAT)-mimicking activity and superoxide dismutase (SOD)-mimetic performance, with concurrent scavenging of secondary radicals (•OH, ONOO-, etc.). Critically, the stereoselective recognition of l-phen@Cu-TA AEs, based on the l-type amino acid transporter 1 (LAT1), enhances the renal tubular cell uptake by 5-fold over pristine Cu-TA AEs, respectively, thereby driving renal pathological-site enrichment and internalization to boost therapeutic bioavailability. In vivo investigation reveals that the l-phen@Cu-TA treatment can restore the physiological homeostasis of AKI via ROS-scavenging by SOD-CAT enzymatic cascades and remodel the renal microenvironmental stability, thereby achieving satisfactory AKI treatment. This work pioneers chirality-modulated AEs for organ-specific antioxidant therapy, establishing a transformative paradigm for precision intervention in oxidative stress-related pathologies.