One-Dimensional Synergistic Core–Shell Nanozymes with Superior Peroxidase-like Activity for Ultrasensitive Colorimetric Detection of Blood Cholesterol

Self-synergy-powered Ni/Fe nanocube-based cholesterol detection with dual modes

Nanoenzyme-leveraged multimode detection would benefit enhancing sensitivity and mitigating detection error. Moreover, multienzyme-like nanozymes hold tremendous potential in sensing by offering synergistic effects and cascaded catalysis. Herein, cost-effective multienzymic Ni/Fe nanocubes (Ni/FeNCs) were synthesized via a facile co-precipitation, and verified to catalyze H2O2 decomposition as peroxidase (POD) and catalase (CAT) mimics. Thereby, a dual-mode sensing platform based on Ni/FeNC and cholesterol oxidase (ChOx) was developed for cholesterol detection. Utilizing the H2O2 produced via the oxidation of cholesterol catalyzed by ChOx, OH•/O2•- radicals and O2 were formed efficiently via Ni/FeNCs-based H2O2 decomposition, facilitating the generation of chemiluminescence (CL) and fluorescence signals. For CL assay, an Ni/FeNC-luminol-H2O2 CL system was fabricated, where both POD-mimic-mediated radical decomposition of H2O2 and ferricyanide ions in Ni/FeNCs could induce CL reaction with respective mechanism. Notably, these two CL processes were both deduced to be enhanced by in-situ generated O2. This dual-catalyzed luminol CL system, involving self-cascade catalysis of ferricyanogen and CAT mimic as well as the self-synergy between POD-like and CAT-like activities of Ni/FeNCs, was proposed for the first time, and able to boost CL signal. To generate fluorescent signal, o-phenylenediamine was introduced, and oxidized by both OH•/O2•- and O2 produced via POD/CAT-mimic-mediated H2O2 decomposition to 2,3-diaminophenazinc, which could quench the fluorescence of WS2 quantum dots via internal filtration effect. The Ni/FeNC-based dual-mode assay is applicable and flexible for cholesterol detection. Particularly, the low-cost Ni/FeNC is a promising candidate of luminol-H2O2 CL system due to its dual-CL-mechanism involving self-cascade and synergistic catalysis.

Dual enzyme-mimicking bimetallic MOF for selective SERS detection of L-DOPA in human serum based on cascade catalytic reaction

Surface-enhanced Raman scattering (SERS) in conjunction with nanozymes has emerged as a powerful technique for bioanalysis due to their signal conversion and amplification effect. However, clinical applications continue to encounter challenges such as inadequate selectivity. Herein, we proposed a novel approach for the selective SERS detection of L-DOPA in human serum with dual enzyme-mimicking MOF-919(Fe-Cu). This material exhibits both catechol oxidase-like and peroxidase-like activities, enabling it to catalyze catecholic compounds to o-quinones and subsequently oxidize 3,3',5,5'-tetramethylbenzidine (TMB) molecules into SERS-active ox-TMB. Ultrasonic-assisted synthesis ensures uniform and dense loading of AgNPs within MOF-919(Fe-Cu), thereby enhancing SERS activity. A sensitive and selective method for SERS detection of L-DOPA was developed. The catechol oxidase-like activity of MOF-919(Fe-Cu) provides ideal selectivity and anti-interference to L-DOPA, while the cascade catalytic system transforms weak SERS signals of L-DOPA into amplified SERS signals of ox-TMB. The mechanism of dual enzyme-mimicking activity was further discussed. The detection process was optimized to achieve high sensitivity towards L-DOPA with a linear range 0.5-100 μmol/L and low limit of detection (LOD) of 0.16 μmol/L. Furthermore, it has been applied in serum sample of Parkinson's patients, demonstrating excellent recovery and low relative error compared to HPLC-UV. This approach offers an economical, efficient and accurate strategy for the detection of L-DOPA and provides valuable insights for the integration of selective multifunctional nanozyme and sensitive SERS detection method.

Paper immobilized BSA-decorated gold nanoclusters for single-step optical sensing of glucose and cholesterol without cross-reactivity

Minimally invasive methods for detecting glucose, cholesterol and hydrogen peroxide are crucial for monitoring the nutritional and health status of humans and animals. The peroxidase mimetic activity by nanozymes is one of the versatile methods for detecting glucose, cholesterol, hydrogen peroxide, and other biomolecules. However, the strict requirement of acidic pH limits their sensing and interfacing ability with natural enzymes. The present study developed bovine serum albumin (BSA) coated gold nanoclusters (AuNC) immobilized on paper fabric to enable single-step visual detection of glucose, cholesterol and hydrogen peroxide in complex biological fluids like serum and milk. The BSA-AuNC suspension and immobilized paper fabric synergistically interface with the natural oxidative enzymes, glucose oxidase or cholesterol oxidase, at physiological pH. The concomitant loss in the fluorescent intensity of BSA-AuNC-loaded paper fabric exposed to the generated hydrogen peroxide (glucose/glucose oxidase or cholesterol/cholesterol oxidase) was directly proportional to the concentration of glucose or cholesterol. These reactions enabled simple visual detection as well as quantification of hydrogen peroxide, glucose and cholesterol using Image-J software and common smartphone-based mobile applications. The detection ability of BSA-AuNC-embedded paper fabric is specific and remains unaltered in the presence of similar oxidase enzymes or similar substrate analogues. With these unique features, the BSA-AuNC embedded paper fabric stands out as a prominent analytical device with enormous potential as a simple, user-friendly detection tool for monitoring biomolecules that are important to health, nutrition, and environmental safeguarding.

Highly sensitive and accurate detection of cholesterol based on a single red upconversion biosensor

Cholesterol (CHOL) is an important clinical biochemical indicator that plays an important role in the regulation of the fluidity, permeability, and microstructure of cell membranes. Therefore, it is necessary to accurately monitor CHOL levels in biological samples for the early prevention and diagnosis of various diseases. The single-band red upconversion nanoparticle (UCNP) emits light within the optical transmission window of biological tissues, and can penetrate deeper biological tissues and cause less energy loss due to scattering and thus have higher sensitivity and accuracy. Here, using the nontoxic, sensitive, and photochemically stable 3,3',5,5'-tetramethylbenzidine (TMB) as the quenching agent and single red UCNP as the fluorescent donor, a dual-readout colorimetric and fluorescent sensor was developed to detect CHOL. The detection mechanism and feasibility were discussed in detail, and experimental conditions such as Fe2+ concentration, TMB concentration and reaction time were explored. Under optimal conditions, the limits of CHOL detection by colorimetry and fluorescence were 0.85 μM and 0.63 μM. The sensing system was used to measure CHOL in serum samples and the values obtained by these two modes were close, and the spiked recoveries were 97.2-102.2% and 97.1-103.7%, respectively, which holds great potential in clinical diagnosis and health management.

A smartphone-assisted one-step bicolor colorimetric detection of glucose in neutral environment based on molecularly imprinted polymer nanozymes

In order to improve the specificity and the peroxidase-like activity of nanozyme at the neutral pH as well as to facilitate the naked-eye visual detection of the analyte concentration, a nanozyme based on molecularly imprinted polymers (MIPNs) for selective and bicolor colorimetric detection of glucose in neutral environment was developed. Compared with free nanozyme, the synthesized MIPNs showed a better catalytic capability, with a catalytic efficiency (kcat/Km) 9.5 times higher than that of free nanozyme. The kinetics experiment showed that the MIPNs demonstrated a fast kinetic feature and the kinetic data fitted a pseudo-first-order model. In practical application, the color of the detection system changed gradually from pink to blue as the glucose concentration increased in a broad linear range from 0 to 3 mM, with a detection limit of 6.22 μM. The colorimetric visualization of glucose concentration was read with a smartphone and no other instrument was needed. Therefore, a manageable and highly efficient method for the MIPNs-catalyzed visualization at the neutral pH and the one-step bicolor visual detection was constructed. This newly established method may also provide a new idea for further development and application of nanozymes.

Cu-MOF derived CuO@g-C3N4 nanozyme for cascade catalytic colorimetric sensing

The use of peroxidase mimics has great potential for various real applications due to their strong catalytic activity. Herein, a facile strategy was proposed to directly prepare CuO@g-C3N4 by Cu-MOF derivatization and demonstrated its efficacy in constructing a multiple enzymatic cascade system by loading protein enzymes onto it. The resulting CuO@g-C3N4 possessed high peroxidase-like activity, with a Michaelis constant (Km) of 0.25 and 0.16 mM for H2O2 and 3,3',5,5'-tetramethylbenzidine (TMB), respectively. Additionally, the high surface area of CuO@g-C3N4 facilitated the loading of protein enzymes and maintained their activity over an extended period, expanding the potential applications of CuO@g-C3N4. To test its feasibility, CuO@g-C3N4/protein oxidase complex was prepared and used to sense the ripeness and freshness of fruits and meat, respectively. The mechanism relied on the fact that the ripeness of fruits increased and freshness of food decreased with the release of marked targets, such as glucose and xanthine, which could produce H2O2 when digested by the corresponding oxidase. The peroxidase mimics of CuO@g-C3N4 could then sensitively colorimetric detect H2O2 in present of TMB. The obtained CuO@g-C3N4/oxidase complex exhibited an excellent linear response to glucose or xanthine in the range of 1.0-120 μmol/L or 8.0-350 μmol/L, respectively. Furthermore, accurate quantification of glucose and xanthine in real samples is achieved with spiked recoveries ranging from 80.2% to 120.0% and from 94.2% to 112.0%, respectively. Overall, this work demonstrates the potential of CuO@g-C3N4 in various practical applications, such as food freshness detection.

Ni3V2O8 nanoflower mimetic enzyme constructs a dual-signal system for ascorbic acid detection

Ascorbic acid is a nutritional small molecule essential to human life activities and health, playing a vital role in many physiological processes. Fresh fruits and beverages can provide abundant AA to maintain human metabolic balance. Therefore, it is of great significance to develop a nanomaterial with superior nanozyme activity for rapid and convenient detection of ascorbic acid (AA) in fruits and beverages. Herein, a dual-signal sensing platform based on UV-vis absorption and test strip chromaticity for the quantitative determination of AA is presented. The sensing platform is based on the horseradish peroxidase-like activity of Ni3V2O8 nanoflowers, which catalyzes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by hydrogen peroxide to the blue oxide TMB (ox TMB). The ox TMB produced by the oxidation has a characteristic absorption peak at 650 nm. In the presence of AA, the blue ox TMB is reduced to colorless TMB, and the quantitative detection of AA can be achieved by detecting the decrease in intensity of the absorption peak by UV-Vis spectrophotometry. Under the optimal experimental conditions, the sensing platform exhibited excellent sensitivity and selectivity. A wide linear range of 0.1 μM to 40 μM with a detection limit of 0.032 μM was obtained. The linear equation is ΔA = 0.02513c + 0.1164 with a correlation coefficient of 0.9979. It showed excellent properties in the detection of real samples of fruit juices and beverages, meanwhile, a method for the rapid detection of AA based on chromaticity change of test strips was constructed with high sensitivity and convenience. The linearity range for the ascorbic acid was 1-50 μM with LOD of 0.42 μM. The developed sensing platform has the capability to quickly and accurately detect ascorbic acid (AA) in fresh fruits and beverages. This proposed method offers a new and promising approach for the rapid and cost-effective detection of ascorbic acid, which has a wide range of potential applications.

Tailoring of peroxidase mimetics bifunctional nanocomposite: Dual mode electro-spectroscopic screening of cholesterol and hydrogen peroxide in real food samples and live cells

A simple and rapid screening of biomarkers in clinical and food matrices is urgently needed to diagnose cardiovascular diseases. The cholesterol (Chol) and hydrogen peroxide (H₂O₂) are critical bio-indicators, which require more inventive detection techniques to be applied to real food, and bio-samples. In this study, a robust dual sensor was developed for Chol and H₂O₂ using hybrid catalyst. Bovine serum albumin (BSA)-capped nanocatalyst was potentially catalyzed 3,3',5,5'-tetramethylbenzidine (TMB), and H₂O₂. The enzymatic nanoelectrocatalyst delivered a wide range of signaling concentrations from 250 nM to 3.0 mM and 100 nM to 10 mM, limit of detection (LOD) of 53.2 nM and 18.4 nM for Chol and H₂O₂. The cholesterol oxidase-BSA-AuNPs-metal-free organic framework (ChOx-BSA-AuNPs-MFOF) based electrode surface effectively operated in live-cells and real-food samples. The enzymatic sensor exhibits adequate recovery of real-food samples (96.96-99.44%). Finally, the proposed system is a suitable choice for the potential applications of Chol and H₂O₂ in clinical and food chemistry.

Bioinspired Coassembly of Copper Ions and Nicotinamide Adenine Dinucleotides for Single-Site Nanozyme with Dual Catalytic Functions

Nanozymes can imitate the catalytic properties of natural enzymes while overcoming the limitations of natural enzymes such as high cost, poor robustness, and difficulty in recycling. However, rational design and facile preparation of nanozymes are still in demand. Inspired by the chemical structure of laccase, we report an amorphous metal-organic coordination nanocomposite named CuNAD, which is composed of copper ions and nicotinamide adenine dinucleotide (NAD⁺) via a simple coordinating coassembly process. As a single-site nanozyme, CuNAD exhibits excellent robustness under extreme conditions, significantly stronger catalytic activity for phenolic compounds, and 4.02-fold higher sensitivity for epinephrine detection than laccase. Furthermore, by breaking through the functional constraints of laccase, CuNAD is also able to activate H₂O₂ at neutral pH, benefiting a one-step chromogenic detection platform for cholesterol. This facile approach demonstrates the potential to develop single-site nanozymes by biomimicking natural enzymes and may boost more insights into the structure-function relationship of nanozymes.

Modulating the Biomimetic and Fluorescence Quenching Activities of Metal-Organic Framework/Platinum Nanoparticle Composites and Their Applications in Molecular Biosensing

Nanoscale metal-organic frameworks (nMOFs) have gained considerable attention with significant potential applications. Although great efforts have been devoted to designing and fabricating nanoscaffold structures, approaches of deliberately regulating the intrinsic functionality of nMOFs have been poorly explored. Herein, we report a simple and novel strategy to regulate the catalytic and fluorescence quenching behaviors of nMOFs through coordination-driven self-assembly. As a proof-of-concept, we synthesized a synergistic and stable MOF-metal nanocomposite by loading platinum nanoparticles (PtNPs) on a commonly used Fe-MOF, i.e., MIL-88B-NH₂/Pt, as a MOF composite model for exploration. On one hand, the complexation with ATP effectively broke the pH limitation of the peroxidase-mimicking MIL-88B-NH₂/Pt nanozyme, bringing a 10-fold increased catalytic activity under alkaline condition. Based on the distinct catalytic enhancement between ATP and other nucleotides, real-time monitoring of apyrase activity as well as colorimetric detection of alkaline phosphatase (ALP) was performed. On the other hand, interactions of MIL-88B-NH₂/Pt with fluorescent DNA were tolerant of different nucleic acids and, more importantly, were further manipulated by inorganic molecules. As a result, H₂O₂ could only trigger the release of a G-rich sequence, while phosphates could readily induce desorption of various DNA molecules with varying lengths, sequences, and fluorescent dyes. Accordingly, fluorescent DNA and MIL-88B-NH₂/Pt as functional probe-quencher pairs were proposed, allowing the establishment of a fluorescence bioassay for ALP and PPase detection and Boolean logic calculations. This work offers a means to tune the intrinsic activities of nMOFs by surface engineering, benefiting design of functional nanomaterials and development of advanced biosensing systems.

N, S-co-doped carbon/Co1-xS nanocomposite with dual-enzyme activities for a smartphone-based colorimetric assay of total cholesterol in human serum

We fabricated a novel N,S-co-doped carbon/Co_1-xS nanocomposite (NSC/Co_1-xS) using a facile sol-gel approach, which featured a multiporous structure, abundant S vacancies and Co-S nanoparticles filling the carbon-layer pores. When the Co_1-xS nanoparticles were anchored onto the surface of N,S-co-doped carbon, a synergistic catalysis action occurred. The NSC/Co_1-xS nanocomposites possessed both peroxidase-like and oxidase-mimetic dual-enzyme activities, in which the oxidase-mimetic activity dominated. By scavenger capture tests, the nanozyme was demonstrated to catalyze H₂O₂ to produce h⁺, •OH and •O₂^-, among which the strongest and weakest signals were h⁺ and •OH, respectively. The multi-valence states of Co atoms in the NSC/Co_1-xS structure facilitated electronic transfer that enhanced redox reactions, thereby improving the resultant color reaction. Based on the NSC/Co_1-xS's enzyme-mimetic catalytic reaction, a visual colorimetric assay and Android "Thing Identify" application (app), installed on a smartphone, offered detection limits of 1.93 and 2.51 mg/dl, respectively, in human serum samples. The selectivity/interference experiments, using fortified macromolecules and metal ions, demonstrated that this sensor had high selectivity and low interference potential for cholesterol analysis. Compared to standard assay kits and previously reported visual detection, the Android smartphone-based assays provided higher accuracy (recoveries up to 93.6-104.1%), feasibility for trace-level detection, and more convenient on-site application for cholesterol assay due to the superior enzymatic activity of NSC/Co_1-xS. These compelling performance metrics lead us to posit that the NSC/Co_1-xS-based nanozymic sensor offers a promising methodology for several practical applications, such as point-of-care diagnosis and workplace health evaluations.

Photonic Crystal Effects on Upconversion Enhancement of LiErF4:0.5%Tm3+@LiYF4 for Noncontact Cholesterol Detection

Cholesterol is a vital compound in maintenance for human health, and its concentration levels are tightly associated with various diseases. Therefore, accurate monitoring of cholesterol is of great significance in clinical diagnosis. Herein, we fabricated a noncontact biosensor based on photonic crystal-enhanced upconversion nanoparticles (UCNPs) for highly sensitive and interference-free cholesterol detection. By compounding LiErF₄:0.5%Tm³⁺@LiYF₄ UCNPs with poly(methyl methacrylate) (PMMA) photonic crystals (OPCs), we were able to selectively tune the coupling of the photonic band gap to the excitation field and modulate the upconversion (UC) luminescence intensity, given the unique multi-wavelength excitation property of LiErF₄:0.5%Tm³⁺@LiYF₄. A 48.5-fold enhancement of the monochromatic red UC emission was ultimately achieved at 980 nm excitation, ensuring improved detection sensitivity. Based on the principle of quenching of the intense monochromic red UC emission by the oxidation products of 3,3',5,5'-tetramethylbenzidine (TMB) yielded from the cholesterol cascade reactions, the biosensor has a detection limit of 1.6 μM for cholesterol with excellent specificity and stability. In addition, the testing results of the as-designed biosensor in patients are highly consistent with clinical diagnostic data, providing a sensitive, reliable, reusable, interference-free, and alternative strategy for clinical cholesterol detection.

Colorimetric detection of glucose by a hybrid nanomaterial based on amplified peroxidase-like activity of ferrosoferric oxide modified with gold–platinum heterodimer

An easy and sensitive colorimetric sensor based on the Fe3O4@Au–Pt hybrid nanomaterial was constructed for H2O2 and glucose detection.

Mimicking Peroxidase-Like Activity of Nitrogen-Doped Carbon Dots (N-CDs) Coupled with a Laminated Three-Dimensional Microfluidic Paper-Based Analytical Device (Laminated 3D-μPAD) for Smart Sensing of Total Cholesterol from Whole Blood

This work presents a simple hydrothermal synthesis of nitrogen-doped carbon dots (N-CDs), fabrication of microfluidic paper-based analytical device (μPAD), and their joint application for colorimetric determination of total cholesterol (TC) in human blood. The N-CDs were characterized by various techniques including transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD), and the optical and electronic properties of computational models were studied using the time-dependent density functional theory (TD-DFT). The characterization results confirmed the successful doping of nitrogen on the surface of carbon dots. The N-CDs exhibited high affinity toward 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)-diammonium salt (ABTS) with the Michaelis-Menten constant (K_M) of 0.018 mM in a test for their peroxidase-like activity. Particularly, since hydrogen peroxide (H₂O₂) is the oxidative product of cholesterol in the presence of cholesterol oxidase, a sensitive and selective method of cholesterol detection was developed. Overall, the obtained results from TD-DFT confirm the strong adsorption of H₂O₂ on the graphitic N positions of the N-CDs. The laminated three-dimensional (3D)-μPAD featuring a 6 mm circular detection zone was fabricated using a simple wax screen printing technique. Classification of TC according to the clinically relevant criteria (healthy, <5.2 mM; borderline, 5.2-6.2 mM; and high risk, >6.2 mM) could be determined by the naked eye within 10 min by simple comparison using a color chart. Overall, the proposed colorimetric device serves as a low-cost, rapid, simple, sensitive, and selective alternative for TC detection in whole blood samples that is friendly to unskilled end users.