Nanozyme
Introduction of Nanozymes
Natural enzymes play vital roles in biological reactions in living systems. However, some intrinsic drawbacks, such as ease of denaturation, laborious preparation, high cost, and difficulty of recycling, have limited their practical applications. To tackle these problems, intensive efforts have been devoted to developing natural enzymes' alternatives called "artificial enzymes". As an emerging research area of artificial enzymes, nanozymes, the catalytic nanomaterials with enzyme-like characteristics, have attracted researchers' enormous attentions.
History of Nanozymes Research
Natural enzymes are ubiquitous biocatalysts that play central roles in virtually all the biological reactions in living systems. Since they catalyze the reactions with remarkable efficiency and extraordinary specificity at mild conditions, natural enzymes have been extensively explored for various applications beyond living systems. On the other hand, natural enzymes are proteins or ribonucleic acids, which inevitably have several intrinsic drawbacks, such as ease of denaturation, laborious preparation, high cost, difficulty of recycling, etc. These drawbacks have in turn limited their practical applications.
To tackle these drawbacks, intensive efforts have been devoted to developing natural enzymes' alternatives called "artificial enzymes" (or "enzyme mimics") since 1950s. Artificial enzymes aim at "imitating the catalytic processes that occur in living systems". Initially, scientists have used cyclodextrins and their derivatives to mimic varieties of enzymes, ranging from thiamine pyrophosphate and pyridoxal phosphate to hydrolytic enzymes and even cytochrome P-450. Inspired by the success of these studies, researchers have investigated numerous types of materials like metal complexes, polymers, supramolecules, and biomolecules (such as nucleic acids, catalytic antibodies, and proteins) for mimicking various kinds of natural enzymes. Over the past two decades, along with the remarkable achievements made in the field of nanotechnology, varieties of functional nanomaterials have been discovered to possess unexpected enzyme-mimicking catalytic activities. These emerging functional nanomaterials are now collectively termed as "nanozymes". The term "nanozymes" was coined by Pasquato, Scrimin, and their coworkers in 2004 to describe the gold nanoparticle-based transphosphorylation mimics resulting from the self-assembly of triazacyclonane-functionalized thiols onto the surface of gold nanoparticles. Later, in their comprehensive review published in 2013, Wei and Wang defined "nanozymes" as "nanomaterials with enzyme-like characteristics".
Classification of Nanozymes
Since peroxidase nanozymes were reported, more and more nanozymes have emerged one after another. These nanozymes can be divided into three categories:
(1) Fe-based nanozymes. The initial studies focused on the peroxidase catalytic activity of ferromagnetic nanomaterials, and studied the effects of the size, morphology and surface modification of Fe3O4 and Fe2O3 nanomaterials on their catalytic activity. Subsequently, it was found that the oxides formed by Fe and other nanomaterials also had peroxidase-like catalytic activity, such as Fe-Bi oxide nanoparticles, Fe-Co oxide nanoparticles and Fe-Mn oxide nanoparticles.
(2) Non-Fe metal-based nanozymes. In addition to Fe-based nanozymes, many other metal-based nanozymes were also found. For example, cerium dioxide nanoparticles, manganese dioxide nanoparticles, copper oxide nanoparticles, and cobalt tetroxide nanoparticles, which are all have peroxidase catalytic activity. Copper sulfide nanoparticles and cadmium sulfide nanoparticles also have similar catalytic activity.
(3) Non-metal-based nanozymes. Many non-metallic materials also have peroxidase activity, especially carbon-based nanomaterials, such as carbon nanotubes, graphene oxide, and carbon nanodots. In addition, porous polymer nanomaterials also have mimic enzyme activity. The discovery of these new nanozymes is of great significance, which further shows that many nanomaterials have potential peroxidase catalytic activity, and on this basis to expand their scope of application.
Characteristics of Nanozymes
As a new type of promising artificial enzymes, nanozymes have attracted considerable attentions, particularly in recent years. Since the seminal work on fullerene derivatives-based DNase mimics in the early 1990s, incredible growth has been witnessed in the field of nanozymes by the exponential number of publications. The growing interests in nanozymes can be attributed to their unique characteristics over natural enzymes and even conventional artificial enzymes. Nanozymes are unique in several aspects, such as their size- (shape-, structure-, composition-) tunable catalytic activities, large surface area for modification and bioconjugation, multiple functions besides catalysis, smart response to external stimuli, etc.
Table 1. Characteristics comparison between nanozymes and natural enzymes.
| Nanozymes | Natural enzymes |
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Applications of Nanozymes
To date, plenty of nanomaterials have been investigated to mimic diverse natural enzymes, which have already found many interesting applications.
The emergence of nanozymes provides a new idea for tumor diagnosis. For example, the researchers coupled antibodies on the surface of magnetic nanoparticles to become nanoprobes for both tumor recognition and chromogenic tumor. The results are similar to the traditional HRP enzyme-labeled antibody immunohistochemical method, which has potential application prospects.
In the field of tumor therapy, it is interesting to find that iron oxide nanoparticles can directly kill tumor cells through its peroxidase mimic enzyme in the presence of hydrogen peroxide. It has also been found that when ferromagnetic nanoparticles are in contact with living cells as drug carriers or contrast agents, the presence of hydrogen peroxide will trigger a catalytic reaction to produce free radicals, and even trace magnetic nanoparticles can kill 80% of HeLa cells. On the one hand, this phenomenon provides a new idea for the treatment of tumor. On the other hand, it also suggests that when magnetic nanoparticles are used in vivo, the effect of their catalytic activity, that is, biosafety, must be carefully considered.
In the detection of blood glucose and uric acid, colorimetry is a routine method to detect the concentration of glucose, and its principle is realized by the color reaction produced by two enzyme systems-horseradish peroxidase and glucose oxidase. Because ferric tetroxide nanozyme has the catalytic function of peroxidase, it can not only replace horseradish peroxidase in colorimetric method, but also immobilize glucose oxidase directly to the surface of nanoparticles. While glucose oxidase catalyzes glucose to produce hydrogen peroxide, the nanozyme can directly exert its peroxidase catalytic activity and then produce color reaction with substrate TMB or ABTS. This method is not only simple and convenient, but also can detect the content of glucose more quickly.
The antibacterial activity of nanozymes has been discovered recently. We know that hydrogen peroxide is a commonly used germicidal disinfectant, which is because hydrogen peroxide can be decomposed to produce free radicals, thus destroying the active components of bacteria, such as cell membrane, protein, nucleic acid and so on. However, the efficiency of producing free radicals is low, and the addition of catalysts will accelerate the production of free radicals. Nanomaterials with peroxidase mimic enzyme activity can be used as such catalysts to improve the efficiency of hydrogen peroxide to produce free radicals and enhance the efficacy of sterilization and disinfection.
One of the important contents of environmental monitoring is to monitor the content of peroxide. Nanozymes can be used to monitor the environment instead of natural enzymes. For example, the nitrogen-containing and sulfur-containing compounds in rainwater will be oxidized by hydrogen peroxide, thus increasing their acidity and forming acid rain. Using the catalytic activity of peroxidase nanozyme, scientists can quickly detect the content of hydrogen peroxide in rainwater and realize the monitoring of acid rain.
The catalytic function of nanozymes can also be used in sewage treatment. We know that phenol is one of the most harmful carcinogens in sewage, and how to remove phenol from sewage is an important part of sewage treatment. The researchers found that peroxidase nanozymes can catalyze hydrogen peroxide to produce a large number of free radicals, which can degrade phenol in sewage to produce carbon dioxide, water and small molecular organic acids. Compared with the limitations of natural enzymes, such as stringent reaction conditions and easy denaturation and inactivation, nanozymes have good stability, low cost, can be recycled, environmentally friendly and harmless, and can degrade a variety of pollutants. Therefore, nanozymes have extensive application value in sewage treatment.
Reference
- Wang, Xiaoyu, Guo, Wenjing, Hu, Yihui, et al. Nanozymes: Next Wave of Artificial Enzymes [J]. SpringerBriefs in Molecular Science. 10.1007/978-3-662-53068-9.