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.

Carbon nanotube dispersion in nematic liquid crystals: An overview

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.

The low temperature dynamics of a magnetic nanoparticle system Ni3[Cr(CN)6]2 with an average nanoparticles size of 4 nm was studied. Using different temperature and field protocols memory phenomena were studied by the DC magnetization and magnetic relaxation measurements of the system at temperatures below Tm = 19 K. Aging experiments show an absence of any waiting time dependence in the magnetization relaxation due to a field change after zero field and field cooling. This observation discriminates the dynamics of the system from the behaviour of a classical spin-glass.

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Magnetic Relaxation and Memory Effect in Nickel-Chromium Cyanide Nanoparticles

The doping of liquid crystals (LCs) with carbon nanotubes and magnetic nanoparticles have attracted wide interest in many areas of science, technology and medicine. LCs occur as additional, thermodynamically stable states of matter between the liquid state and the crystal state in some materials. They can be characterized by a long-range orientational order of the molecules and, as a consequence, by an anisotropy in their physical properties. LCs can be oriented under electric or magnetic fields due to the anisotropy of dielectric permittivity or diamagnetic susceptibility. Carbon nanotubes are molecular scaled tubes of graphitic carbon with outstanding properties. The simplest nanotube is composed of a single sheet of a network of carbon atoms, called graphene, which is rolled up into a tubular form. Because of the small value of the anisotropy of the diamagnetic susceptibility of liquid crystals, the idea of doping them with fine magnetic particles was also introduced. Nanotubes can undergo functionalization, including by magnetic particles, to produce novel hybrid materials potentially suitable for applications.

SAW study of structural changes in liquid crystals doped with carbon nanotubes induced by electric and magnetic fields

Magnetic properties of Prussian blue analogues, the molecule-based ferrimagnet Pr[Fe(CN) 6 ]5H 2 0, are presented The zero-field-cooled magnetisation curves deviate from the field-cooled magnetisation measurements M(T) below certain temperature T f = 145 K, and show a broad maximum at about T = 12 K Such a behaviour of M(T) is typical for the small particles system or for non-interacting magnetic clusters The saturation magnetic moment μ s = 202 μ B , determined from M(1/H) curves, agrees very well with the theoretical value The Curie temperature T c = 148 K were determined from M 2 (T) curves The M(H) curve shows hysteresis behaviour at T = 45 K Magnetic susceptibility χ obeys Curie-Weiss law very well above T = 30 K with μ eff = 38 μ B and θ = -186 K The changes in shapes of low-temperature 1 H-solid-echo spectra reflect the changes in the magnetic state of the sample

Magnetic properties of Pr[Fe(CN)6] · 5H2O

Adsorption of photodynamic active dyes (hypericin, chlorin e6, methylene blue, ALA and fluorescein) and they release from clinoptilolite type of zeolite (CZ) in the presence of biomolecules, such as collagen, albumin and hemoglobin, were quantitatively investigated by absorption and fluorescence spectrometry. Using multiphoton microscopy, we demonstrated that effective inducing of two-photon excited luminescence and second harmonic generation signals in nano/micro-particles of CZ by femtosecond near-infrared laser excitation can be successfully utilized in multiphoton imaging of CZ particles and the dye adsorption processes. In addition, CZ magnetic (MCZ) particles were fabricated and proposed as a promising material for drug delivery and controlled release in biological systems. It is shown that the temperature of the aqueous suspension of MCZ of 2 cm3 can be increased by 7 ºC for 3 minutes by an induction heater. Furthermore, it was demonstrated that MCZ particles have strong inhibition and destruction effects on insulin and lysozyme fibrillization, and the protein amyloid formation is inhibited in a dose-dependent manner. Optical spectrometry and multiphoton microscopy are effective approaches that may reveal potential of magnetic zeolites in drug delivery and biomedical imaging of cancer photodynamic therapy and hyperthermia.

Zeolite magnetic nano/micro-particles for adsorption, delivery and release of photodynamic dyes

The 1 H NMR spectroscopic analysis and magnetization measurements were performed on TM22+[MoIV(CN)8].nH2O compounds (TM=Mn, Fe, Co, Ni, Cu, VO). The 1 H NMR signals reflect the magnetic moment of the TM ions (μP). The decay rates of FID signals increase as μP and the applied static rf field increase. The spin-lattice relaxation times at 27.7 MHz vary from 0.0187 ms (Mn) up to 0.45 ms (Cu). Magnetization measurements indicate ferrimagnetic ordering in Mn- and Co-compounds, and short range magnetic ordering in Ni-compound. Susceptibility data follows the Curie Weiss law above T=20 K.

Z. Mitróová

Papers

Magnetic Relaxation and Memory Effect in Nickel-Chromium Cyanide Nanoparticles

SAW study of structural changes in liquid crystals doped with carbon nanotubes induced by electric and magnetic fields

Magnetic properties of Pr[Fe(CN)6] · 5H2O

Zeolite magnetic nano/micro-particles for adsorption, delivery and release of photodynamic dyes

Magnetic Properties and 1H NMR Study of TM 2 2+ [MoIV(CN)8].nH2O