Colorimetric determination of glucose in solution and via the use of a paper strip by exploiting the peroxidase and oxidase mimicking activity of bimetallic Cu-Pd nanoparticles deposited on reduced graphene oxide, graphitic carbon nitride, or MoS2 nanosheets

Extraction, Purification, Sulfated Modification, and Biological Activities of Dandelion Root Polysaccharides

In this study, polysaccharides were extracted at a rate of 87.5% ± 1.5% from native dandelion roots, and the dandelion root polysaccharides (DRPs) were then chemically modified to obtain sulfated polysaccharides (SDRPs) with a degree of substitution of 1.49 ± 0.07. The effects of modification conditions, physicochemical characterizations, structural characteristics, antioxidant properties, hypoglycemic activity, and proliferative effects on probiotics of DRP derivatives were further investigated. Results showed that the optimum conditions for sulfation of DRPs included esterification reagents (concentrated sulfuric acid: n-butanol) ratio of 3:1, a reaction temperature of 0 °C, a reaction time of 1.5 h, and the involvement of 0.154 g of ammonium sulfate. The DRPs and SDRPs were composed of six monosaccharides, including mannose, glucosamine, rhamnose, glucose, galactose, and arabinose. Based on infrared spectra, the peaks of the characteristic absorption bands of S=O and C-O-S appeared at 1263 cm-1 and 836 cm-1. Compared with DRPs, SDRPs had a significantly lower relative molecular mass and a three-stranded helical structure. NMR analysis showed that sulfated modification mainly occurred on the hydroxyl group at C6. SDRPs underwent a chemical shift to higher field strength, with their characteristic signal peaking in the region of 1.00-1.62 ppm. Scanning electron microscopy (SEM) analysis indicated that the surface morphology of SDRPs was significantly changed. The structure of SDRPs was finer and more fragmented than DRPs. Compared with DRPs, SDRPs showed better free radical scavenging ability, higher Fe2+chelating ability, and stronger inhibition of α-glucosidase and α-amylase. In addition, SDRPs had an excellent promotional effect on the growth of Lactobacillus plantarum 10665 and Lactobacillus acidophilus. Therefore, this study could provide a theoretical basis for the development and utilization of DRPs.

Mn3O4 nanoparticles decorated porous reduced graphene oxide with excellent oxidase-like activity for fast colorimetric detection of ascorbic acid

Mn₃O₄ nanoparticles composed of porous reduced graphene oxide nanosheets (Mn₃O₄@p-rGO) with enhanced oxidase-like activity were successfully fabricated through an in-situ approach for fast colorimetric detection of ascorbic acid (AA). The residual Mn²⁺ in the GO suspension of Hummers method was directly reused as the manganese source, improving the atom utilization efficiency. Benefiting from the uniform distribution of Mn₃O₄ nanoparticles on the surface of p-rGO nanosheets, the nanocomposite exhibited larger surface area, more active sites, and accelerated electron transfer efficiency, which enhanced the oxidase-like activity. Mn₃O₄@p-rGO nanocomposite efficiently activate dissolved O₂ to generate singlet oxygen (¹O₂), leading to high oxidation capacity toward the substrate 3,3',5,5'-tetramethylbenzidine (TMB) without the extra addition of H₂O₂. Furthermore, the prominent absorption peak of the blue ox-TMB at 652 nm gradually decreased in the presence of AA, and a facile and fast colorimetric sensor was constructed with a good linear relationship (0.5-80 μM) and low LOD (0.278 μM) toward AA. Owing to the simplicity and excellent stability of the sensing platform, its practical application for AA detection in juices has shown good feasibility and reliability compared with HPLC and the 2, 4-dinitrophenylhydrazine colorimetric method. The oxidase-like Mn₃O₄@p-rGO provides a versatile platform for applications in food testing and disease diagnosis.

An origami-based colorimetric sensor for detection of hydrogen peroxide and glucose using sericin capped silver nanoparticles

The hydrogen peroxide (H₂O₂) measurement is considered highly important in industrial wastewater quality assessment, environmental protection, and disease detection. Here, a simple high-performance paper-based sensor is proposed for rapid and in situ detection of H₂O₂. To this end, 3,3',5,5'-tetramethylbenzidine is embedded in the sensor to act as a color indicator, whose reaction with hydrogen peroxide is catalyzed by a silver nanozyme modified by sericin. The result of the reaction clarified by the appearance of blue color in the sensor detection zone is received by a portable scanner, while also calculating its intensity by image analysis software. This method is sensitive to hydrogen peroxide in the concentration range of 0.5‒240 mg/dL, providing a detection limit of 0.15 mg/dL. The ability of the sensor to determine glucose is also evaluated by adding a layer containing glucose oxidase enzyme to the sensor structure. A desirable response is obtained in the range of 1.0‒160 mg/dL, together with a detection limit of 0.37 mg/dL. Accordingly, the proposed sensor shows satisfactory results compared to clinical methods for monitoring the amount of glucose in biological samples such as serum and saliva.

Dual-Loading of Fe3O4 and Pd Nanoparticles on g-C3N4 Nanosheets Toward a Magnetic Nanoplatform with Enhanced Peroxidase-like Activity for Loading Various Enzymes for Visual Detection of Small Molecules

Enzyme mimics now play a significant role in biochemistry. Especially, peroxidase mimics have been widely used for developing colorimetric sensors of blood glucose. The peroxidase mimics previously reported could not be recycled for reusing and may generate scattering to cause unwanted optical interference when it was used for fabricating colorimetric sensors. We herein prepared a broad-applicable and reusable magnetic enzyme-loading nanoplatform with enhanced peroxidase-like activity by simultaneously loading Fe₃O₄ nanoparticles (Fe₃O₄NPs) and palladium nanoparticles (PdNPs) on graphitic carbon nitride (g-C₃N₄) nanosheets (Fe₃O₄NPs/PdNPs/g-C₃N₄). The prepared Fe₃O₄NPs/PdNPs/g-C₃N₄ possesses stable and enhanced peroxidase-like activity and good enzyme-loading capacity and can be used to load various natural enzymes to form highly-efficient and stable double-active nanozyme for fabricating colorimetric sensors for the visual detection of small molecules. Especially, the magnetic feature facilitates the magnetic separation of Fe₃O₄NPs/PdNPs/g-C₃N₄ from sample solution, which is in favor of recycling and eliminating the optical interference caused by nanozyme in colorimetric sensors. The prepared Fe₃O₄NPs/PdNPs/g-C₃N₄ has been successfully used to load glucose oxidase (GOx) and cholesterol oxidase (Chox) to form magnetic peroxidase-GOx and peroxidase-Chox double-active nanozymes, which can be used to fabricate colorimetric methods for the detection of glucose and cholesterol, respectively, with a visual detection limit of 15 μM and a spectrometry detection limit of 1.0 μM. With the developed glucose and cholesterol detection methods, we have successfully detected glucose and cholesterol in serum with a recovery of 98-104% and a RSD (n = 5) < 5%. With high peroxidase-like activity, good stability, reusable features, and broad applicability of loading enzyme, the developed magnetic Fe₃O₄NPs/PdNPs/g-C₃N₄ provided a promising approach for fabricating cost-effective, sensitive, and simple colorimetric sensors for the visual detection of various small molecules.

Nanozyme-based colorimetric biosensor with a systemic quantification algorithm for noninvasive glucose monitoring

Diabetes mellitus accompanies an abnormally high glucose level in the bloodstream. Early diagnosis and proper glycemic management of blood glucose are essential to prevent further progression and complications. Biosensor-based colorimetric detection has progressed and shown potential in portable and inexpensive daily assessment of glucose levels because of its simplicity, low-cost, and convenient operation without sophisticated instrumentation. Colorimetric glucose biosensors commonly use natural enzymes that recognize glucose and chromophores that detect enzymatic reaction products. However, many natural enzymes have inherent defects, limiting their extensive application. Recently, nanozyme-based colorimetric detection has drawn attention due to its merits including high sensitivity, stability under strict reaction conditions, flexible structural design with low-cost materials, and adjustable catalytic activities. This review discusses various nanozyme materials, colorimetric analytic methods and mechanisms, recent machine learning based analytic methods, quantification systems, applications and future directions for monitoring and managing diabetes.

The synthesis of Pt doped WO3 nanosheets and application on colorimetric detection of cysteine by naked eye using response surface methodology for optimization

We present a simple, sensitive, and specific colorimetric using the peroxidase properties method based on Pt doped WO₃ nanosheets to detect the cysteine. Pt@WO₃NSs were synthesized by hydrothermal method and characterized by Fourier transform infrared (FTIR), Transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction patterns (XRD) methods. The response surface methodology (RSM) method based on the central composite design (CCD) was used to optimize test parameters such as pH, nanosheet concentration, and temperature. When cysteine is present in the environment due to its competition with 3,3', 5,5'-Tetramethylbenzidine (TMB) in the use of hydrogen peroxide, the blue discoloration is reduced compared to the absence of cysteine and leads to its detection. We have favorably created a peculiar approach for sensing cysteine based on the colorimetric method in solution and paper with linear range 0.01-15 μM, 0.005-14 μM and R² = 0.9887 and R² = 0.9871 respectively. The detection limit for solution-based is 1.2 nM and for paper-based is 1 nM.

Recent advances in biomedical applications of 2D nanomaterials with peroxidase-like properties

Significant progress has been made in developing two-dimensional (2D) nanomaterials owing to their ultra-thin structure, high specific surface area, and many other advantages. Recently, 2D nanomaterials with enzyme-like properties, especially peroxidase (POD)-like activity, are highly desirable for many biomedical applications. In this review, we first classify the types of 2D POD-like nanomaterials and then summarize various strategies for endowing 2D nanomaterials with POD-like properties. Representative examples of biomedical applications are reviewed, emphasizing in antibacterial, biosensing, and cancer therapy. Last, the future challenges and prospects of 2D POD-like nanomaterials are discussed. This review is expected to provide an in-depth understanding of 2D POD-like materials for biomedical applications.

Green and facile synthesis of iron-doped biomass carbon dots as a dual-signal colorimetric and fluorometric probe for the detection of ascorbic acid

A new method based on biomass Fe-CDs with fluorescence properties and simulated oxidase activity colorimetric and fluorometric dual-readout assay for highly effective detection of AA was established.

Biomimetic two-dimensional nanozymes: synthesis, hybridization, functional tailoring, and biosensor applications

Biological enzymes play important roles in mediating the biological reactions in vitro and in vivo due to their high catalytic activity, strong bioactivity, and high specificity; however, they have also some disadvantages such as high cost, low environmental stability, weak reusability, and difficult production. To overcome these shortcomings, functional nanomaterials including metallic nanoparticles, single atoms, metal oxides, alloys, and others have been utilized as nanozymes to mimic the properties and functions of natural enzymes. Due to the development of the synthesis and applications of two-dimensional (2D) materials, 2D nanomaterials have shown high potential to be used as novel nanozymes in biosensing, bioimaging, therapy, logic gates, and environmental remediation due to their unique physical, chemical, biological, and electronic properties. In this work, we summarize recent advances in the preparation and functionalization, as well as biosensor and immunoassay applications of various 2D material-based nanozymes. To achieve this aim, first we demonstrate the preparation strategies of 2D nanozymes such as chemical reduction, templated synthesis, chemical exfoliation, calcination, electrochemical deposition, hydrothermal synthesis, and many others. Meanwhile, the structure and properties of the 2D nanozymes prepared by conjugating 2D materials with nanoparticles, metal oxides, biomolecules, polymers, ions, and 2D heteromaterials are introduced and discussed in detail. Then, the applications of the prepared 2D nanozymes in colorimetric, electrochemical, fluorescent, and electrochemiluminescent sensors are demonstrated.

Highly selective enrichment and direct determination of imazethapyr residues from milk using magnetic solid-phase extraction based on restricted-access molecularly imprinted polymers

Restricted access media magnetic molecularly imprinted polymers (RAM-MMIPs) were prepared as magnetic solid phase extraction (M-SPE) material by reversible addition fragmentation chain transfer (RAFT) technique. The resulting RAM-MMIPs had a uniform, imprinted, hydrophilic layer (63 nm), good binding capacity (34.85 mg g-1) and satisfactory selectivity. In addition, these RAM-MMIPs had a robust ability to eliminate the interference of protein macromolecules. These RAM-MMIPs were then coupled with HPLC/UV to identify imazethapyr (IM) residues in untreated milk samples. Several major factors would affect M-SPE extraction efficiency, such as the amount of RAM-MMIPs, pH, extraction time of the sample solution, and the volume ratio of the elution solvent. Under the optimal conditions, the developed method had good linearity (R2 > 0.9993), low detection limit (2.13 μg L-1), and low quantitative limit (7.15 μg L-1). These results indicated this proposed approach is an efficient method for direct enrichment and detection of IM herbicides in milk and other biological samples.

A NiAg-graphene quantum dot-graphene hybrid with high oxidase-like catalytic activity for sensitive colorimetric detection of malathion

This paper reports the synthesis of a nickel-silver-graphene quantum dot-graphene hybrid.

Salmonella typhimurium detector based on the intrinsic peroxidase-like activity and photothermal effect of MoS2

A multimode dot-filtration immunoassay (MDFIA) was established for rapid and accurate detection of the target (Salmonella typhimurium), which was based on the intrinsic color, peroxidase-like activity and photothermal effect of molybdenum disulfide (MoS₂). Obviously, multimode detection can improve detection accuracy compared to the direct visual detection in test strips. A thermal imaging camera was used as detector to record the temperature change (ΔT) of MoS₂ and establish the standard curve of ΔT and the concentration of Salmonella typhimurium to realize quantitative determination. The main parameters that affect the analytical performance of MDFIA were optimized. Under the optimal experimental conditions, the limit of detection (LOD) of photothermal detection reached 10² CFU mL^-1 and was one order of magnitude lower than the limit of direct visual detection and catalytic color development detection (10³ CFU mL^-1). The accuracy and analytical sensitivity were enhanced by intrinsic peroxidase-like activity and the huge photothermal effect of MoS₂. Moreover, this method exhibited high selectivity, good repeatability, and acceptable stability and the entire process was simple to be accomplished in 30 min, which generally meets the need of rapid detection. The successful implementation in real samples with the recovery being between 99.5 and 119.2% showed that it could be used as a promising quality control strategy for detection of other foodborne pathogens. The peroxidase-like activity and excellent photothermal effect of MoS₂ was used to develop a multimode dot-filtration immunoassay for rapid detection of Salmonella typhimurium.

Bimetallic PtCu nanoparticles supported on molybdenum disulfide-functionalized graphitic carbon nitride for the detection of carcinoembryonic antigen

A molybdenum disulfide based graphite phase carbon nitride (MoS₂/g-C₃N₄) which is supported by a platinum-copper nanoparticle (PtCu) Z-type catalyst was created in this study. The catalyst exploits optoelectronic synergistic effect with large surface area, good catalysis, and biocompatibility to amplify the signal. The electrode impedance of the synthesized MoS₂/g-C₃N₄-PtCu was reduced five times in visible light compared with dark conditions, thereby improving the detection of carcinoembryonic antigen (CEA). At a voltage of - 0.4 V, the immunoprobe constructed with this material is used for CEA detection. A linear relationship between 100 fg mL^-1 and 80 ng mL^-1 concentrations was achieved with a minimum detection limit of 33 fg mL^-1 (S/N = 3). The recovery rate was 103-104%, and the relative standard deviation was 2.9-3.8%. This implies that the sandwich immunosensors have good reproducibility, selectivity, and stability and can be used in various applications. Graphical Abstract.

Colorimetric strategy for ascorbic acid detection based on the oxidase-like activity of silver nanoparticle single-walled carbon nanotube composites

A colorimetric assay for the determination and quantification of ascorbic acid (AA) is presented using silver nanoparticle (AgNP) single-walled carbon nanotube (AgNP/SWCNT) nanocomposites prepared using a microwave-assisted method. The AgNP/SWCNT nanocomposites possessed oxidase-like properties toward 3,3',5,5'-tetramethylbenzidine (TMB) and could catalyze the oxidation of TMB to form a blue oxidation product (λ_max = 652 nm) in the absence of H₂ O₂ . AA can specifically inhibit the oxidation of TMB, resulting in a decline of the absorbance value and blue colour fading. As such, amounts of AA can be assessed easily by the unaided eye and quantitatively using an ultraviolet-visible light spectrophotometer. Under the optimal reaction conditions, this strategy showed a good linearity ranging from 0.4 μM to 5.0 μM for AA detection, and the limit of detection was 130 nM. This assay was also applied for AA measurement in vitamin C tablets and juice samples that yielded satisfactory results.

Immobilized imidazolium-based ionic liquids in C18 for solid-phase extraction

In this work, two solid-phases based on imidazolium-based ionic liquids were obtained and characterized for solid-phase extraction of fluoroquinolones. The process of immobilization was performed replacing a toxic reagent by UV-irradiation to get a harmless process. The obtained solid-phases were characterized by nuclear magnetic resonance spectroscopy and elemental analysis. Each solid-phase was packed in a cartridge and was used in solid-phase extraction processes for norfloxacin and ciprofloxacin, after the optimization of some parameters such as the elution solvent, the eluent volume and, the sample volume to be used during the loading step. The developed solid-phases with immobilized ionic liquids were successfully implemented for the studied compounds and indicate high probabilities to be useful in solid-phase extractions of other fluoroquinolones.

Ginkgo biloba leaf polysaccharide stabilized palladium nanoparticles with enhanced peroxidase-like property for the colorimetric detection of glucose

Sensitive glucose detection based on nanoparticles is good for the prevention of illness in our bodies. However, many nanoparticles lack stability and biocompatibility, which restrict their sensitivity to glucose detection. Herein, stable and biocompatible Ginkgo biloba leaf polysaccharide (GBLP) stabilized palladium nanoparticles (Pd n -GBLP NPs) were prepared through a green method where GBLP was used as a reducing and stabilizing agent. The results of Pd n -GBLP NPs characterized by UV-visible spectroscopy (UV-Vis), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and X-ray photoelectron spectra (XPS) confirmed the successful preparation of Pd n -GBLP NPs. TEM results indicated that the sizes of Pd NPs inside of Pd n -GBLP NPs (n = 41, 68, 91 and 137) were 7.61, 9.62, 11.10 and 13.13 nm, respectively. XPS confirmed the successful reduction of PdCl4 2- into Pd (0). Dynamic light scattering (DLS) results demonstrated the long-term stability of Pd n -GBLP NPs in different buffer solutions. Furthermore, Pd91-GBLP NPs were highly biocompatible after incubation (500 μg mL-1) with HeLa cells for 24 h. More importantly, Pd91-GBLP NPs had peroxidase-like properties and followed a ping-pong mechanism. The catalytic oxidation of substrate 3,3',5,5'-tetramethylbenzidine (TMB) into blue oxidized TMB (oxTMB) by Pd91-GBLP NPs was used to detect the glucose concentration. This colorimetric method had high selectivity, wide linear range from 2.5 to 700 μM and a low detection limit of 1 μM. This method also showed good accuracy for the detection of glucose concentrations in blood. The established method has great potential in biomedical detection in the future.

Nanomagnet-Silica Nanoparticles Decorated with Au@Pd for Enhanced Peroxidase-Like Activity and Colorimetric Glucose Sensing

Nanomagnet-silica shell (Fe₃O₄@SiO₂) decorated with Au@Pd nanoparticles (NPs) were synthesized successfully. The characterization of Fe₃O₄@SiO₂-NH₂-Au@PdNPs was achieved using several spectroscopic and microscopic techniques. The quantitative surface analysis was confirmed using X-ray photoelectron spectroscopy. The Fe₃O₄@SiO₂-NH₂-Au@Pd_0.30NPs exhibited excellent peroxidase-like activity by effectively catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂. The absorption peaks at 370 and 652 nm confirmed the peroxidase-like activity of the Fe₃O₄@SiO₂-NH₂-Au@Pd_0.30NPs. The Michaelis-Menten constant (K_m) of 0.350 and 0.090 mM showed strong affinity toward H₂O₂ and TMB at Fe₃O₄@SiO₂-NH₂-Au@Pd_0.30NPs. The mechanism of the peroxidase-like activity was found to proceed via an electron transfer process. A simple colorimetric sensor based on glucose oxidase and Fe₃O₄@SiO₂-NH₂-Au@Pd_0.30NPs showed excellent selectivity and sensitivity towards the detection of glucose. The fabricated glucose biosensor exhibited a wide linear response toward glucose from 0.010 to 60.0 μM with an limit of detection of 60.0 nM and limit of quantification of 200 nM. The colorimetric biosensor based on Fe₃O₄@SiO₂-NH₂-Au@Pd_0.30NPs as a peroxidase mimic was also successfully applied for the determination of glucose concentrations in serum samples. The synthesized Fe₃O₄@SiO₂-NH₂-Au@Pd_0.30NPs nanozymes exhibited excellent potential as an alternative to horseradish peroxidase for low-cost glucose monitoring.

Aptamer-based colorimetric determination of early-stage apoptotic cells via the release of cytochrome c from mitochondria and by exploiting silver/platinum alloy nanoclusters as a peroxidase mimic

An enzyme-free aptameric nanosensor is presented for apoptosis assay. The method exploits the peroxidase-mimicking property of silver/platinum alloy nanoclusters (Ag/Pt NCs) and uses a Cyt c binding ssDNA aptamer. An extra-strand polycytosine (C₁₄) aptamer was designed as a template for synthesis of the Ag/Pt NCs. If cell lysate or purified Cyt c is placed in a polystyrene microplate, Cyt c will bind to the surface of the wells of a microtiterplate. On addition of Apt@Ag/PtNCs, it will associate with Cyt c and then catalytically oxidize colorless tetramethylbenzidine (TMB) in the presence of H₂O₂ to give a blue colored oxidation product (TMB_ox) due to the peroxidase-mimicking property of the Ag/Pt NCs. Under optimal conditions, the absorbance of TMB at 660 nm is linearly enhanced as the concentration of Cyt c increases from 50.0 fM to 500 nM, and the detection limit is ~10 pM. The assay is simple, sensitive and cost effective in that it is enzyme-free, antibody-free and label-free. Graphical abstractSchematic diagram of the apoptosis assay on the basis of microplate well-coated mitochondrial cytochrome c releasing by using Aptamer@Ag/Pt NCs.

C3N4 nanosheet-supported Prussian Blue nanoparticles as a peroxidase mimic: colorimetric enzymatic determination of lactate

Prussian Blue nanoparticles were deposited on g-C₃N₄ nanosheets. The resulting nanocomposite possesses peroxidase-like (POx) activity and can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine at room temperature in the presence of H₂O₂. This leads to formation of a blue product with an absorption maximum at 650 nm. The formation of the Prussian Blue nanoparticles on the g-C₃N₄ nanosheets, and the POx-like activity of the composite were characterized in detail. The POx mimic was used for determination of L-lactic acid via detection of H₂O₂ that is produced by the enzyme lactate oxidase (LOx). The assay has a linear range that extends from 5 to 100 μM, and the detection limit is 2.2 μM. The method was successfully applied to the determination of L-lactic acid in spiked human serum. Graphical abstract Ultra-small Prussian Blue (PB) nanoparticles were used to modify g-C₃N₄ nanosheets, and their peroxidase-like activity was explored for detection of L-lactic acid. LOx represent L-lactate oxidase, and TMB represents 3,3',5,5'-tetramethylbenzidine.

Non-enzymatic detection of serum glucose using a fluorescent nanopolymer probe

A fluorescent probe is described for the determination of serum glucose after hepatotoxin-induced liver injury. The probe is based on the use of a water-soluble polymer and has been prepared from a multi-functional azlactone polymer as the linker, amino boronic acid, and Alizarin Red as the signalling moiety. The excitation/emission peaks of the polymeric fluorescent probe are at 468/567 nm. Fluorescence is reduced on addition of glucose. Intensity drops linearly in the 0.1 mM to 14 mM glucose concentration range. The probe was applied to non-enzymatic detection of glucose in rat serum after CCl₄-induced liver damage. Graphical abstract A polymer based fluorescent probe has been constructed and applied for non-enzymatic monitoring of serum glucose following hepatotoxin induced liver injury.