Nanozymes: Classification and Analytical Applications - A Review

The recent discovery of a new class of nanomaterials called nanozymes, which have the action of enzymes and are thus of tremendous significance, has altered our understanding of these previously believed to be biologically inert nanomaterials. As a significant and exciting class of synthetic enzymes, nanozymes have distinct advantages over natural enzymes. They are less expensive, more stable, and easier to work with and store, making them a viable substitute. This practical advantage of nanozymes over natural enzymes reassures us about the potential of this new technology. Peroxidase-like nanozymes have been investigated for the purpose of creating adaptable biosensors via the use of molecularly imprinted polymers (MIPs) or particular bio recognition ligands, including enzymes, antibodies, and aptamers. This review delves into the distinctions between synthetic and natural enzymes, explaining their structures and analytical applications. It primarily focuses on carbon-based nanozymes, particularly those that contain both carbon and hydrogen, as well as metal-based nanozymes like Fe, Cu, and Au, along with their metal oxide (FeO, CuO), which have applications in many fields today. Analytical chemistry finds great use for nanozymes for sensing and other applications, particularly in comparison with other classical methods in terms of selectivity and sensitivity. Nanozymes, with their unique catalytic capabilities, have emerged as a crucial tool in the early diagnosis of COVID-19. Their application in nanozyme-based sensing and detection, particularly through colorimetric and fluorometric methods, has significantly advanced our ability to detect the virus at an early stage.

Nanozyme-based sensing of dopamine using cobalt-doped hydroxyapatite nanocomposite from waste bones

Dopamine is one of the most important neurotransmitters and plays a crucial role in various neurological, renal, and cardiovascular systems. However, the abnormal levels of dopamine mainly point to Parkinson's, Alzheimer's, cardiovascular diseases, etc. Hydroxyapatite (HAp), owing to its catalytic nature, nanoporous structure, easy synthesis, and biocompatibility, is a promising matrix material. These characteristics make HAp a material of choice for doping metals such as cobalt. The synthesized cobalt-doped hydroxyapatite (Co-HAp) was used as a colorimetric sensing platform for dopamine. The successful synthesis of the platform was confirmed by characterization with FTIR, SEM, EDX, XRD, TGA, etc. The platform demonstrated intrinsic peroxidase-like activity in the presence of H2O2, resulting in the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). The proposed sensor detected dopamine in a linear range of 0.9-35 μM, a limit of detection of 0.51 µM, limit of quantification of 1.7 µM, and an R2 of 0.993. The optimization of the proposed sensor was done with different parameters, such as the amount of mimic enzyme, H2O2, pH, TMB concentration, and time. The proposed sensor showed the best response at 5 mg of the mimic enzyme, pH 5, 12 mM TMB, and 8 mM H2O2, with a short response time of only 2 min. The fabricated platform was successfully applied to detect dopamine in physiological solutions.

Paper-Based Colorimetric Detection of miRNA-21 Using Pre-Activated Nylon Membrane and Peroxidase-Mimetic Activity of Cysteamine-Capped Gold Nanoparticles

Irregular expression of MicroRNA-21 (miRNA-21) is considered as a promising biomarker for early cancer diagnosis. In this paper, a new genosensor based on paper and nanozyme activity of cysteamine-capped gold nanoparticles (Cys/AuNPs) was developed to detect picomolar concentrations of miRNA-21. Such nanozyme catalyzes the colorimetric reaction of hydrogen peroxide (H₂O₂) and 3,3',5,5' tetramethylbenzidine (TMB), to produce a blue color measurable by a smartphone. Due to their positive charge, Cys/AuNPs were attached to the negative phosphate groups of the DNA strand backbone via electrostatic interactions, leading to the quantitative determination of miRNA-21 concentration by the peroxidase-like activity of Cys/AuNPs. Furthermore, a paper-based assay was carried out on nylon disk devices to allow fast immobilization of DNA_probe. After performing the paper-based assay, a good linear range was observed between 1 pM and 1 nM (Y = 0.080 [MiRNA-21]/pM + 13.846, R² = 0.993) with a detection limit of 0.5 pM. The developed method was effective, selective, and sensitive for the miRNA-21 detection. The application of the proposed method for miRNA-21 detection was examined in a human serum sample, and a recovery rate of 90.0-97.6% was obtained showing the acceptable accuracy of the developed approach.

A new colorimetric assay for sensitive detection of glucose-6-phosphate dehydrogenase deficiency based on silver nanoparticles

Glucose-6-phosphate dehydrogenase (G6PD) is the principal enzyme in the pentose phosphate pathway that plays a fundamental role in the production of nicotinamide adenine dinucleotide phosphate, which is very important in preventing the oxidation of cells, especially red blood cells. This enzyme deficiency was associated with many disorders, the most common of which were hemolysis episodes. In the last decade, nanoparticles have been used to design optical and electronic sensors due to their unique properties. This report presents a new colorimetric method that used silver nanoparticles to detect glucose 6-phosphate dehydrogenase activity directly. The glucose-6-phosphate dehydrogenase detection mechanism was based on an aggregation of silver nanoparticles, leading to increased nanoparticle size, which causes discoloration. In the presence of the enzyme, the color of the solution was yellow, and when the enzyme was not present, the color of the solution was grayish. Utilizing this method, colorimetric sensing of glucose 6-phosphate dehydrogenase was gained with a detection limit of 0.009 U ml^-1and a linear range of 0-16.0 U ml^-1. In this way, the presence or absence of the enzyme can be easily detected with the naked eye during one step.

Nanoparticle-based colorimetric sensors to detect neurodegenerative disease biomarkers

Neurodegenerative disorders (NDDs) are progressive, incurable health conditions that primarily affect brain cells, and result in loss of brain mass and impaired function. Current sensing technologies for NDD detection are limited by high cost, long sample preparation, and/or require skilled personnel. To overcome these limitations, optical sensors, specifically colorimetric sensors, have garnered increasing attention towards the development of a cost-effective, simple, and rapid alternative approach. In this review, we evaluate colorimetric sensing strategies of NDD biomarkers (e.g. proteins, neurotransmitters, bio-thiols, and sulfide), address the limitations and challenges of optical sensor technologies, and provide our outlook on the future of this field.

Detection of dopamine by a minimized electrochemical sensor using a graphene oxide molecularly imprinted polymer modified micropipette tip shaped graphite electrode

A simple and efficient electrochemical sensor based on a homemade reshaped micropipette tip carbon paste electrode is reported. Molecularly imprinted polymer membranes of graphene oxide and polypyrrole are synthesized and modified on the surface of micropipette tip carbon paste electrode. The merit of the method is evaluated under optimized conditions via differential pulse voltammetrics. The prepared sensor exhibits remarkable sensitivity toward dopamine with a linear range of 6.4 × 10−8–2 × 10−4 M, with a limit of detection as low as 1 × 10−8 M. The proposed method is applied for the determination of dopamine in urine samples by the standard addition route. A range of 1 × 10−7–1 × 10−4 M is obtained from these samples. The relative recoveries are in the range of 95.2%–104%. The proposed method has acceptable performance for the determination of dopamine in real samples with excellent sensitivity and selectivity.

Magnetic nanomaterials with unique nanozymes-like characteristics for colorimetric sensors: A review

Colorimetric sensors for the rapid detection of numerous analytes have been widely applied in many fields such as biomedicine, food industry and environmental science due to their highly sensitive and selective response, easy operation and visual identification by naked eyes. In this review, the recent progress of the colorimetric sensors based on the magnetic nanomaterials with unique nanozymes-like catalytic activity (magnetic nanozyme) and their colorimetric sensing applications are presented. Emerging magnetic nanozyme-based colorimetric sensors, such as metal oxide/sulfides-based, metal-based, carbon-based, and aptamer-conjugated magnetic nanomaterials, offer many desirable features for target analytes detection. And due to the unique nanoscale physical-chemical properties, magnetic nanozymes have been used to mimic the catalytic activity of natural enzymes such as peroxidases, oxidases and catalases. This review also highlights the catalytic mechanisms of enzyme-like reactions, and promising colorimetric sensing system for the detection of chemical compounds like H₂O₂, pesticide, ascorbic acid, dopamine, tetracyclines, perfluorooctane sulfonate, phenolic compounds, heavy metal ion and sulfite have been deeply discussed. In addition, the remaining challenges and future directions in utilizing magnetic nanozyme for colorimetric sensors are addressed.

A gold nanoparticle-intercalated mesoporous silica-based nanozyme for the selective colorimetric detection of dopamine

Highly dispersed aggregation-free gold nanoparticles intercalated into the walls of mesoporous silica (AuMS) were synthesized using thioether-functionalized silica as a nanozyme, which exhibited an excellent peroxidase mimic activity. The AuMS material was characterized via XRD, N₂ adsorption-desorption, FESEM, SEM-EDS particle mapping, TEM, and XPS. The peroxidase-like activity of the AuMS material was studied thoroughly, and the effect of pH and temperature was evaluated. The reproducibility of the peroxidase mimic activity and long-term stability of the AuMS catalyst were also studied. Furthermore, the AuMS catalyst was successfully utilized for the detection and quantification of dopamine, an important neurotransmitter, colorimetrically with a linear range of 10-80 μM and a limit of detection (LOD) value of 1.28 nM. The determination of dopamine concentration in commercially available dopamine hydrochloride injection showed high accuracy, good reproducibility, and high selectivity in the presence of uric acid, ascorbic acid, glucose, tryptophan, phenylalanine, and tyrosine.

Paper based colorimetric detection of miRNA-21 using Ag/Pt nanoclusters

Abnormal expression of MicroRNA-21 (miRNA-21) is considered to be a reliable biomarker for the early diagnosis of cancer. In this work, a novel paper based biosensor was fabricated to detect sub-micro molar concentrations of miRNA-21 based on peroxidase mimetic activity of DNA-templated Ag/Pt nanoclusters (DNA-Ag/Pt NCs), which could catalyze the reaction of hydrogen peroxide and 3,3',5,5' tetramethylbenzidine (TMB), to produce a blue color. The Mechanism of reaction was based on the inhibition effect of miRNA-21 on peroxidase-like activity of nanosensor which resulted to quantitative determination of miRNA-21 concentration. It was found that miRNA-21 could be linearly detected in the range from 1-700 pM (A₆₅₂ = 0.16x-0.96, R² = 0.99; x = -log [miRNA-21]) with a detection limit of 0.6 pM. Moreover, a paper assay was carried out on a Y-shaped paper-based microfluidic device in order to use the distinctive features of micro-channels such as short response time, very low reagent volume, low fabrication cost, etc. After performing paper based assay, a good linear range was observed between 10-1000 pM (y = 0.06x+147.48, R² = 0.99; x = [miRNA-21]) with detection limit of 4.1 pM. The practical application of proposed method for detection of miRNA-21 in real sample was assayed in the human urine sample and indicated the colorimetric method had acceptable accuracy.

Pt-Decorated Boron Nitride Nanosheets as Artificial Nanozyme for Detection of Dopamine

Over the past decade, nanosized metal oxides, metals, and bimetallic particles have been actively researched as enzyme mimetic nanomaterials. However, the common issues with individual nanoparticles (NPs) are stabilization, reproducibility, and blocking of active sites by surfactants. These problems promote further studies of composite materials, where NPs are spread on supports, such as graphene derivatives or dichalcogenide nanosheets. Another promising type of support for NPs is the few-layered hexagonal boron nitride (hBN). In this study, we develop surfactant-free nanocomposites containing Pt NPs dispersed on chemically modified hydrophilic hBN nanosheets (hBNNSs). Ascorbic acid was used as a reducing agent for the chemical reduction of the Pt salt in the presence of hBNNS aqueous colloid, resulting in Pt/hBNNS nanocomposites, which were thoroughly characterized with X-ray diffraction, transmission electron microscopy, dynamic light scattering, and X-ray photoelectron and infrared spectroscopies. Similar to graphene oxide binding the metal NPs more efficiently than pure graphene, hydrophilic hBNNSs well stabilize Pt NPs, with particle size down to around 8 nm. We further demonstrate for the first time that Pt/hBNNS nanocomposites exhibit peroxidase-like catalytic activity, accelerating the oxidation of the classical colorless peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) to its corresponding blue-colored oxidized product in the presence of H₂O₂. Kinetic and mechanism studies involving terephthalic acid and isopropanol as a fluorescent probe and an ^•OH radical scavenger, respectively, proved that Pt/hBNNSs assist H₂O₂ decomposition to active oxygen species (^•OH), which are responsible for TMB oxidation. The Pt/hBNNS nanocomposite-assisted oxidation of TMB provides an effective platform for the colorimetric detection of dopamine, an important biomolecule. The presence of increased amounts of dopamine gradually inhibits the catalytic activity of Pt/hBNNSs for the oxidation of TMB by H₂O₂, thus enabling selective sensing of dopamine down to 0.76 μM, even in the presence of common interfering molecules and on real blood serum samples. The present investigation on Pt/hBNNSs contributes to the knowledge of hBN-based nanocomposites and discovers their new usage as nanomaterials with good enzyme-mimicking activity and dopamine-sensing properties.

A colorimetric paper sensor for citrate as biomarker for early stage detection of prostate cancer based on peroxidase-like activity of cysteine-capped gold nanoclusters

Citrate is currently considered a preferred biomarker for the early stage detection of prostate cancer. In the present work, based on the highly efficient catalytic properties of gold nanoclusters, a novel system for optical determination of citrate was successfully established under optimized conditions. Cysteine-capped gold nanoclusters (Cys-AuNCs) are shown to have an intrinsic peroxidase-mimetic activity. In the presence of H₂O_2, Cys-AuNCs nanostructures are able to catalyse the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with high efficiency to produce a blue dye (with an absorbance maximum at 650 nm). Citrate has carboxylic and hydroxyl groups that can bind with free amino and free carboxyl cysteine groups via hydrogen bonds, thus creating a coating on the surface of the gold nanocluster and inhibiting the cluster oxidation activity. Accordingly, a visual, sensitive and simple colorimetric method using Cys-AuNCs as peroxidase mimetic was developed for detecting citrate. A suitable linear relationship for citrate was obtained for the range of 0.5 to 1000 μM. The limit of detection (LOD) of the proposed method was calculated as 0.1 μM and the relative standard deviation (RSD) was obtained to be less than 4.0%. Moreover, the biosensor was used to perform a paper assay on a Y-shaped microfluidic device and make use of the distinctive features of microchannels such as short response time, very low reagent volume required, low fabrication cost etc. A detection limit of 0.4 μM was achieved through the paper test and a good linear range was observed between 1.0 μM-10 mM. The proposed method was further applied to citrate detection in the human urine sample.

Competitive method for fluorescent dopamine detection in cerebrospinal fluid based on the peroxidase-like activity of ficin

Dopamine (DA), a catecholamine neurotransmitter, is considered to be an important indicator for mental diseases detection in the clinic. In this study, a novel fluorescent sensing platform consisting of the ficin-H₂O₂-tyramine system for determining DA in cerebrospinal fluids (CSF) was established. The proposed method is based on the fact that ficin, a mimetic peroxidase, can catalyze H₂O₂ decomposition into OH radicals, which can oxidize non-fluorescent tyramine into fluorescent dityramine. When DA was introduced, DA can compete with tyramine for OH and resulting in the oxidation reaction of tyramine inhibited along with the fluorescence intensity of the system decreased, which provides a unique strategy for fluorescence detection of DA. Under optimal conditions, the fluorescence intensity decreased linearly with the DA level over a wide concentration range from 0.05 to 12.0 μM (R² = 0.995) with a detection limit of 46 nM (3σ/k). More importantly, the proposed sensing approach exhibits high sensitivity, good selectivity and has been successfully applied to DA sensing in complex biological samples, which made it hold great potential for DA determination in chemical and biological analytical applications.

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

An updated comprehensive review to help researchers understand nanozymes better and in turn to advance the field.

Colorimetric aptasensor for Campylobacter jejuni cells by exploiting the peroxidase like activity of Au@Pd nanoparticles

The authors describe a method for colorimetric determination of Campylobacter jejuni (C. jejuni) in milk samples. It is based on the interaction of a specific DNA aptamer with surface protein in the cell membranes of C. jejuni. Specific binding of the aptamer with the cell membrane leads to an uptake of aptamer from solution. As a result, the concentration of aptamer floating in the solution is reduced. In the presence of large quantities of aptamer, the surface of added Au@Pd nanoparticles (NPs) is covered with aptamer via electrostatic interactions. Hence, they cannot act as a peroxidase mimic and oxidize the substrate 3,3^',5,5^'-tetramethylbenzidine (TMB) to give a blue product. However, when the aptamer is bound by the target cells, the surface of the NPs is not blocked by aptamer and the NPs exert a strong peroxidase -like activity. Under defined experimental conditions, the intensity of the blue color increases with the concentration of C. jejuni, and as little as 100 CFU·mL^-1 can bedetected in milk. Graphical abstract A colorimetric aptasensor for assay Campylobacter jejuni whole cell in food samples was investigated. This assay was designed based on interaction of specific DNA aptamer with surface protein in c. jejuni cell membrane without any modification of aptamer.

Naked-eye detection of potassium ions in a novel gold nanoparticle aggregation-based aptasensor

In this work, we studied the feasibility of interaction among gold nanoparticles (AuNPs) and a cationic dye in an aptasensor system for the detection of potassium ions. The presence and absence of potassium in the solution was distinguishable by different colors (between orange and green) appeared after reaction. Cationic dye (Y5GL) acts as a new aggregator for AuNP-based sensors which changes the aggregated AuNP solution color from blue-purple to green. In the presence of K⁺ ions, the aptamer dissociated from the surface of the AuNP so that free AuNPs and cationic dye make the solution green. The aptasensor showed that the analytical linear range was from 10 nM to 50 mM and the detection limit was 4.4 nM. Also, we examined the practicality of this method on a simple paper based platform. The linear range of the colorimetric paper sensor covered of K⁺ concentration from 10 μM to 40 mM and the detection limit of 6.2 μM was obtained. The selectivity of AuNP aggregation-based sensor improved by the use of cationic dye. Rapidity, simplicity, high sensitivity and excellent selectivity made this assay suitable for practical determination of K⁺ in real urine samples.

Synthesis of hierarchical Co3O4@NiO core-shell nanotubes with a synergistic catalytic activity for peroxidase mimicking and colorimetric detection of dopamine

Fabrication of core-shell nanostructured catalyst is a promising way for tuning its catalytic performance due to the highly active interface and rich redox properties. In this work, hierarchical Co₃O₄@NiO core-shell nanotubes are fabricated by the deposition of NiO shells via a chemical bath treatment using electrospun Co-C composite nanofibers as templates, followed by a calcination process in air. The as-prepared Co₃O₄@NiO core-shell nanotubes exhibit a uniform and novel hollow structure with Co₃O₄ nanoparticles attached to the inner wall of NiO nanotubes and excellent catalytic activity toward the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂. Due to the synergistic effect, the peroxidase-like activity of the Co₃O₄@NiO core-shell nanotubes is much higher than that of individual Co₃O₄ and NiO components. Owing to the superior peroxidase-like activity, a simple and rapid colorimetric approach for the detection of dopamine with a detection limit of 1.21µM and excellent selectivity has been developed. It is anticipated that the prepared Co₃O₄@NiO core-shell nanotubes are promising materials applied for biomedical analysis and environmental monitoring.