Over the past few decades, researchers have established artificial enzymes as highly stable and low-cost alternatives to natural enzymes in a wide range of applications. A variety of materials including cyclodextrins, metal complexes, porphyrins, polymers, dendrimers and biomolecules have been extensively explored to mimic the structures and functions of naturally occurring enzymes. Recently, some nanomaterials have been found to exhibit unexpected enzyme-like activities, and great advances have been made in this area due to the tremendous progress in nano-research and the unique characteristics of nanomaterials. To highlight the progress in the field of nanomaterial-based artificial enzymes (nanozymes), this review discusses various nanomaterials that have been explored to mimic different kinds of enzymes. We cover their kinetics, mechanisms and applications in numerous fields, from biosensing and immunoassays, to stem cell growth and pollutant removal. We also summarize several approaches to tune the activities of nanozymes. Finally, we make comparisons between nanozymes and other catalytic materials (other artificial enzymes, natural enzymes, organic catalysts and nanomaterial-based catalysts) and address the current challenges and future directions (302 references).

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes

Iron oxide nanoparticles have been widely used in many important fields due to their excellent nanoscale physical properties, such as magnetism/superparamagnetism. They are usually assumed to be biologically inert in biomedical applications. However, iron oxide nanoparticles were recently found to also possess intrinsic enzyme-like activities, and are now regarded as novel enzyme mimetics. A special term, "Nanozyme", has thus been coined to highlight the intrinsic enzymatic properties of such nanomaterials. Since then, iron oxide nanoparticles have been used as nanozymes to facilitate biomedical applications. In this review, we will introduce the enzymatic features of iron oxide nanozyme (IONzyme), and summarize its novel applications in biomedicine.

Iron Oxide Nanozyme: A Multifunctional Enzyme Mimetic for Biomedical Applications.

Günther Jutz,†,§ Patrick van Rijn,†,‡,§ Barbara Santos Miranda,‡ and Alexander Böker*,† †DWI Leibniz-Institut für Interaktive Materialien e.V., Lehrstuhl für Makromolekulare Materialien und Oberflac̈hen, RWTH Aachen University, Forckenbeckstrasse 50, D-52056 Aachen, Germany ‡Department of Biomedical Engineering-FB40, W.J. Kolff Institute for Biomedical Engineering and Materials Science, University of Groningen, University Medical Center Groningen, A. Deusinglaan 1, 9713 AV Groningen, The Netherlands

Ferritin: a versatile building block for bionanotechnology.

Due to possessing an extremely small size and a large surface area per unit of volume, nanomaterials have specific characteristic physical, chemical, photochemical, and biological properties that are very useful in many new applications. Nanoparticles' catalytic activity and intrinsic ability in generating or scavenging reactive oxygen species in general can be used to mimic the catalytic activity of natural enzymes. Many nanoparticles with enzyme-like activities have been found, potentially capable of being applied for commercial uses, such as in biosensors, pharmaceutical processes, and the food industry. To date, a variety of nanoparticles, especially those formed from noble metals, have been determined to possess oxidase-like, peroxidase-like, catalase-like, and/or superoxide dismutase-like activity. The ability of nanoparticles to mimic enzymatic activity, especially peroxidase mimics, can be used in a variety of applications, such as detection of glucose in biological samples and waste water treatment. To study the enzyme-like activity of nanoparticles, the electron spin resonance method represents a critically important and convenient analytical approach for zero-time detection of the reactive substrates and products as well as for mechanism determination.

Enzyme-like activity of nanomaterials.

While magnetic nanoparticles offer exciting possibilities for stem cell imaging or tissue bioengineering, their long-term intracellular fate remains to be fully documented. Besides, it appears that magnetic nanoparticles can occur naturally in human cells, but their origin and potentially endogenous synthesis still need further understanding. In an effort to explore the life cycle of magnetic nanoparticles, we investigated their transformations upon internalization in mesenchymal stem cells and as a function of the cells’ differentiation status (undifferentiated, or undergoing adipogenesis, osteogenesis, and chondrogenesis). Using magnetism as a fingerprint of the transformation process, we evidenced an important degradation of the nanoparticles during chondrogenesis. For the other pathways, stem cells were remarkably “remagnetized” after degradation of nanoparticles. This remagnetization phenomenon is the direct demonstration of a possible neosynthesis of magnetic nanoparticles in cellulo and could lay some foundation to understand the presence of magnetic crystals in human cells. The neosynthesis was shown to take place within the endosomes and to involve the H-subunit of ferritin. Moreover, it appeared to be the key process to avoid long-term cytotoxicity (impact on differentiation) related to high doses of magnetic nanoparticles within stem cells.

https://www.pnas.org/content/pnas/116/10/4044.full.pdf

Biosynthesis of magnetic nanoparticles from nano-degradation products revealed in human stem cells.

Magnetic birefringence of natural and synthetic ferritin

Measurements of the Faraday rotation at room temperature over the light wavelength range of 300–680 nm for horse spleen ferritin (HSF), magnetoferritin with different loading factors (LFs) and nanoscale magnetite and Fe2O3 suspensions are reported. The Faraday rotation and the magnetization of the materials studied present similar magnetic field dependences and are characteristic of a superparamagnetic system. The dependence of the Faraday rotation on the magnetic field is described, excluding HSF and Fe2O3, by a Langevin function with a log-normal distribution of the particle size allowing the core diameters of the substances studied to be calculated. It was found that the specific Verdet constant depends linearly on the LF. Differences in the Faraday rotation spectra and their magnetic field dependences allow discrimination between magnetoferritin with maghemite and magnetite cores which can be very useful in biomedicine.

http://iopscience.iop.org/article/10.1088/0957-4484/23/35/355704/pdf

The Faraday effect of natural and artificial ferritins.

Magnetoferritin is a spherical biomacromolecule with a diameter of about 12 nm. It consists of a protein shell composed of apoferritin that is surrounding magnetic nanoparticles of magnetite (Fe3O4) or maghemite (γ-Fe2O3). Magnetoferritins with various iron content (loading factor) were synthetically prepared and their peroxidase-like activities studied via the oxidation of the chromogenic substrate N,N-diethyl-p-phenylenediamine sulfate by hydrogen peroxide to give a purple product with an absorption maximum at 551 nm. Magnetoferritin with higher loading factor exhibits a higher peroxidase-like activity. The catalytic activity was successfully applied to the determination of hydrogen peroxide in the 5.8 to 88.2 mM concentration range.

Peroxidase-like activity of magnetoferritin

Effect of iron oxide loading on magnetoferritin structure in solution as revealed by SAXS and SANS.

Micro-Raman spectroscopy (-RS) was used to studies of naturally occurring biogenic ferritin, synthetic ferritin with magnetic core (magnetoferritin) as well as their several mimetics. We demonstrate the ability of -RS to discriminate between ferritin and magnetoferritin used in the studies. The results are promising for further use of Raman spectroscopy as potential tool to distinguish between the forms of iron present in biogenic materials and biological tissues. PACS numbers: 87.15.-v, 87.64.Je

/pdf/micro-raman-spectroscopy-of-natural-and-synthetic-ferritins-1ynydr70mk.pdf

L. Melníková

Papers

Magnetic birefringence of natural and synthetic ferritin

The Faraday effect of natural and artificial ferritins.

Peroxidase-like activity of magnetoferritin

Effect of iron oxide loading on magnetoferritin structure in solution as revealed by SAXS and SANS.

Micro-Raman Spectroscopy of Natural and Synthetic Ferritins and Their Mimetics