Reusable ring-like Fe3O4/Au nanozymes with enhanced peroxidase-like activities for colorimetric-SERS dual-mode sensing of biomolecules in human blood

Ultra-small gold nanoparticle-coupled MOF-808 enabled sensitive detection of bacteria at neutral pH

Metal-organic framework (MOF)-based mimics are considered star materials to replace natural enzymes. However, their activity is generally limited to acidic conditions, which severely restricts their applications in biological systems where neutral pH is commonly required. Herein, a Zr(IV)-based MOF (MOF-808)/gold nanoparticle (AuNP) hybrid (called Hybrid-60) that shows superior peroxidase-like (POD-like) activity in both acidic and neutral media was prepared by in-situ growth of ultra-small AuNPs (UsAuNPs, ∼3.5 nm) on MOF-808. In comparison with the conventional AuNPs and MOF-808 nanozymes, Hybrid-60 demonstrated ∼8.04- and ∼6.74-time enhanced POD-like activities and superior high activity under neutral conditions, which broke the pH limitation. Furthermore, Hybrid-60 exhibited good tolerance to extreme pH value, concentrated salinity, and high-temperature environments. Taking Staphylococcus aureus (S. aureus) as a model analyte, we developed a simple immune sandwich assay using Hybrid-60 as colorimetric nanotags (ISAHC). A dual recognition strategy using anti-S. aureus antibody and concanavalin A-labeled Hybrid-60 was proposed to specifically capture and high-affinity label the target S. aureus. Then, leveraging the high POD-like activity of Hybrid-60, a simple and specific detection of S. aureus at nearly neutral pH was realized with a wide linear range (1 × 102-1 × 105 CFU/mL) and a low detection limit (32 CFU/mL). Moreover, the ISAHC method enabled one to detect the target S. aureus in human urine and serum with satisfactory recoveries from 93.8 % to 111.0 %, which indicates its clinical applicability. This study provides a new approach to develop neutral nanozymes and facilitate the point-of-care detection of bacteria.

Ligand-Driven Annular-Epitaxial Growth of CuS-Au Heterostructures as Trinity Plasmonic Nanozyme for Multimode Diagnosis of Pathogenic Bacteria

This study presents a novel method to control the site-selective growth of Au nanostars on CuS nanodisc substrate, it indicates that the surfactant ligands play a key role in the architecture control, only CTAC and homologous series with appropriate affinity to CuS can direct the annular-epitaxial growth of Au nanoparticles on the CuS, which demonstrates superior peroxidase (POD)-mimic and SERS activity. Mechanistic studies indicate that plasmon-enhanced catalytic and SERS activity can be attributed to the spatially separated CuS-Au heterostructure, which supports the light-triggered hot electron-hole pairs production and localized surface plasmon resonance hotspots. For practical biosensing, the CuS-Au heterostructures assembled lateral flow assay (LFA) was used for SERS/catalytic colorimetric/photothermal three-mode detection of Streptococcus pneumoniae and Klebsiella pneumoniae, with visually colorimetric mode at 103 CFU/mL and quantitative SERS/photothermal modes at 2-102 CFU/mL within 15 min, 15 clinical samples were used to validate the assay, the result was 100% concordant to the results of quantitative real-time PCR. This study provides a unique avenue to controllably produce plasmon-enhanced nanozyme, which can provide multi-mode signals for LFA application and meet the requirements of different scenarios.

Recent progress of noble metal-based nanozymes: structural engineering and biomedical applications

Due to their tunable catalytic activity, high chemical stability, and favorable electronic structure, noble metal-based nanozymes that can mimic important biocatalytic processes have attracted great attention. Rational structural design of noble metal-based nanozymes can endow them with excellent enzyme-like activities, enhanced sensitivity and stability, as well as unique physicochemical functionalities towards various biomedical applications such as sensing, diagnostics, and disease treatment. This review summarizes the recent progress in structural engineering of noble metal-based nanozymes and emphasizes the relationship between key structural factors of nanozymes and their enzyme-like properties in various enzyme-mimicking reactions. The diverse applications of noble metal-based nanozymes in biosensors, antibiosis, and disease treatment are further introduced. Finally, current challenges and future research directions in noble metal-based nanozymes are discussed. This review could offer scientific guidance to design and fabricate advanced nanozymes with enhanced functionality and performance towards clinical, environmental and biomedical applications.

Magnetic-plasmonic nanoparticle-based surface-enhanced Raman scattering for biomedical detection

Surface-enhanced Raman scattering (SERS) is a powerful spectroscopic technique that enables rapid, non-destructive, and susceptible detection of biological samples. The magnetic-plasmonic composite materials composed of magnetic and plasmonic nanoparticles have attracted extensive attention as SERS substrates in the biomedical field because of their ability to enrich, separate, and selectively identify biomolecules. In this review, the state-of-art progress of magnetic-plasmonic nanoparticle (MPNP)-based SERS substrates for biomedical detection is highlighted, covering the design and construction of MPNPs with different morphologies, organic and inorganic surface functionalization strategies adopted to improve the adaptability and applicability in biological systems for MPNPs, application development of MPNPs in biomedical detection, as well as the future challenges and issues to be addressed. It is highly expected that this review will help to fully understand the research status of MPNP-based SERS substrates and facilitate their further development and wider application in biological systems.

Microfluidic Synthesis of Magnetic Nanoparticles for Biomedical Applications

Magnetic nanoparticles have attracted great attention and become promising candidates in the biomedicine field due to their special physicochemical properties. They are generally divided into metallic and non-metallic magnetic nanoparticles, according to their compositions. Both of the two types have shown practical values in biomedicine applications, such as drug delivery, biosensing, bioimaging, and so on. Research efforts are devoted to the improvement of synthesis strategies to achieve magnetic nanoparticles with controllable morphology, diverse composition, active surface, or multiple functions. Taking high repeatability, programmable operation, precise fluid control, and simple device into account, the microfluidics system can expand the production scale and develop magnetic nanoparticles with desired features. This review will first describe different classifications of promising magnetic nanoparticles, followed by the advancements in microfluidic synthesis and the latest biomedical applications of these magnetic nanoparticles. In addition, the challenges and prospects of magnetic nanoparticles in the biomedical field are also discussed.

A simple colorimetric and paper-based-smartphone sensing platform based on the enhanced peroxidase-like activity of Al doping Prussian blue for point-of-care detection of GSH

Glutathione (GSH) is a very important antioxidant and also participates in many important physiological processes, accurate determination of GSH in food and blood fluid is crucial for human health. Herein, we introduce a simple colorimetric and paper-based-smartphone dual signal output platform for point-of-care (POC) detection of GSH in garlic and human serum. The experiment revealed that the peroxidase-like activity of Aluminum-doped Prussian blue (AlPB) could be enhanced by the doping of Al element on Prussian blue. AlPB exhibited excellent peroxidase-like activity and efficiently catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2 to generate blue oxidized TMB, resulting in an absorbance and color-based-smartphone dual signal readouts. Enzyme kinetic studies also indicated that the AlPB showed high affinity towards TMB. Hence, a reliable colorimetric and color-based-smartphone dual signal readouts assay was constructed based on AlPB-mediated the peroxidase-like activity, it was used for highly sensitive colorimetric and POC detection of GSH in food and human serum within 7 min with a wide range of linear response from 1 to 20 μM, a low detection limit of 0.1748 μM. This work demonstrates a novel and versatile strategy to develop superior peroxidase mimics and holds great potential for rapid and portable detection of GSH in food, healthcare and clinical diagnosis, and also open promising avenues for more powerful paper-based POC tests.

Surface encapsulation of ZIF-8 on Ag nanoparticles modified cotton swab for highly rapid and selective surface-enhanced Raman spectroscopy analysis of glucose and lactic acid in human sweat

Herein, ZIF-8 shell encapsulated Ag nanoparticles decorated cotton swab (CS@Ag@ZIF-8) was firstly designed and prepared for highly rapid and selective surface-enhanced Raman spectroscopy (SERS) analysis of glucose and lactic acid in human sweat. The CS not only act as support matrix for Ag modification and ZIF-8 encapsulation, but also provide great potential in-situ analysis of human sweat with low cost. The as-developed CS@Ag@ZIF-8 shows high SERS activity owing the good adsorption of ZIF-8 shell and electromagnetic enhancement of AgNPs. The 4-mercaptophenylboronic acid (4-MPBA) and 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) with limits of detection (LOD) of 1.0 and 10.0 ng/L can be reached, as well as enhancement factor of 108 level. In addition, the good stability and repeatability of CS@Ag@ZIF-8 can be obtained in various conditions. The recognition probes based on 4-MPBA and DTNB modified CS@Ag@ZIF-8 were fabricated for rapid and selective detection of glucose and lactic acid in human sweat. The promising linearity in range of 0.1-100.0 μmol/L and 0.1-50.0 mmol/L with LOD of 0.04 μmol/L and 0.03 mmol/L for glucose and lactic acid were achieved, respectively. The detection errors between commercial meter and developed method was in range of -6.4 to 6.0 %. Our results provide a promising strategy in fabrication of portable SERS substrates with satisfied performance for rapid, selective and in-situ quantification of biomolecules in complex biological samples.

A competitive dual-mode for tetracycline antibiotics sensing based on colorimetry and surface-enhanced Raman scattering

Tetracycline antibiotics (TCs) are extensively used as broad-spectrum antimicrobials. However, their excessive use and misuse have led to serious accumulation in foods and environments, posing a significant threat to human health. To solve such public issue, we have designed a novel dual-mode detection method, integrating colorimetric sensing with surface-enhanced Raman scattering (SERS) technology, for sensitive and rapid evaluation on TCs. In this method, an Ag@NH2-MIL-101(Al)-Ag nanoprobe that leverages the synergistic effect between silver nanoparticles (AgNPs) and NH2-MIL-101(Al), resulting in remarkable peroxidase-like activity and enhanced SERS performance, was proposed. Moreover, for the instant colorimetric sensing, a competitive oxidative-reaction between TCs and 3,3',5,5'-tetramethylbenzidine (TMB) was developed based on the peroxidase-like activity of nanoprobe. Our innovative colorimetric-SERS dual-mode approach demonstrated high sensitivity accompanied by robust selectivity with limits of detection (LODs) for TCs of 10-3 ppm in colorimetric mode and 10-5 ppm in SERS mode. Furthermore, TCs can be reliably detected in honey samples with recoveries ranging from 87.69% to 120.49%. This dual-mode sensing strategy effectively combining the ease of colorimetric detection with high sensitivity of SERS provides significant values for rapid, direct, and multiplexed evaluation on various hazards in environment and food chains.

A multifunctional Pt/DMSN nanozyme-based colorimetric-fluorescence sensing platform for breast cancer detection

Nanozyme-linked immunosorbent assay has emerged as a promising strategy for sensitive biosensing. However, the catalytic activity and stability of nanozymes affect the accuracy of immunosorbent assays. In this study, we synthesized a Pt/DMSN nanozyme with peroxidase-mimicking activity, which effectively catalyzed the oxidation of peroxidase substrate 3,3',5,5'-tetramethylbenzidin (TMB) in the presence of hydrogen peroxide. Capitalizing on its peroxidase-like activity, the Pt/DMSN nanozyme was functionalized with dual-fluorescent recognition elements (HER2-mAbs and sk6Ea aptamers) to establish a nanozyme-linked immunosorbent assay platform, which exhibited catalytic stability and substrate affinity comparable to horseradish peroxidase. The resulting Multi-Pt/DMSN platform was used to selectively distinguish HER2-positive breast cancer cells from luminal A, triple-negative breast cancer subtypes, and non-neoplastic cells, achieving a detection limit of 50 HER2-positive cells within 30 min. The combination of robust enzyme-like activity and tumor-targeting properties enables fluorescence imaging, providing dual-mode diagnostic functionality. This work presents a prospective platform for differentiating breast cancer subtypes in early diagnosis.

Recent Progress of Round-the-Clock Photocatalytic System in Environmental and Energy Applications: A Review

The prevailing overdependence on fossil fuels contributes to energy supply instability and pronounced price volatility. The combustion of these fuels emits considerable greenhouse gases and pollutants, further deteriorating the climate and environment. In response, Round-the-Clock Photocatalytic Systems (RTCPs) have emerged as a viable technological solution, attracting significant research interest due to their convenience, sustainability, and environmental benefits etc. which act as "photo-batteries" facilitate catalytic processes in the absence of light, offering continuous operation. Given the considerable potential of RTCPs, a timely examination of recent advancements is essential to optimize efforts. This review delineates the fundamental mechanisms of RTCPs, explores innovative strategies and current developments, and addresses the challenges of scaling up production. It aims to provide new insights and serve as a foundational reference for future research on RTCPs.

Coupling of an Au@AgPt nanozyme array with an micrococcal nuclease-specific responsiveness strategy for colorimetric/SERS sensing of Staphylococcus aureus in patients with sepsis

Rapid and ultrasensitive detection of pathogen-associated biomarkers is vital for the early diagnosis and therapy of bacterial infections. Herein, we developed a close-packed and ordered Au@AgPt array coupled with a cascade triggering strategy for surface-enhanced Raman scattering (SERS) and colorimetric identification of the Staphylococcus aureus biomarker micrococcal nuclease (MNase) in serum samples. The trimetallic Au@AgPt nanozymes can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) molecules to SERS-enhanced oxidized TMB (oxTMB), accompanied by the color change from colorless to blue. In the presence of S. aureus, the secreted MNase preferentially cut the nucleobase AT-rich regions of DNA sequences on magnetic beads (MBs) to release alkaline phosphatase (ALP), which subsequently mediated the oxTMB reduction for inducing the colorimetric/SERS signal fade away. Using this "on-to-off" triggering strategy, the target S. aureus can be recorded in a wide linear range with a limit of detection of 38 CFU/mL in the colorimetric mode and 6 CFU/mL in the SERS mode. Meanwhile, the MNase-mediated strategy characterized by high specificity and sensitivity successfully discriminated between patients with sepsis (n = 7) and healthy participants (n = 3), as well as monitored the prognostic progression of the disease (n = 2). Overall, benefiting from highly active and dense "hot spot" substrate, MNase-mediated cascade response strategy, and colorimetric/SERS dual-signal output, this methodology will offer a promising avenue for the early diagnosis of S. aureus infection.

Rational Design of Nanozymes for Engineered Cascade Catalytic Cancer Therapy

Nanozymes have shown significant potential in cancer catalytic therapy by strategically catalyzing tumor-associated substances and metabolites into toxic reactive oxygen species (ROS) in situ, thereby inducing oxidative stress and promoting cancer cell death. However, within the complex tumor microenvironment (TME), the rational design of nanozymes and factors like activity, reaction substrates, and the TME itself significantly influence the efficiency of ROS generation. To address these limitations, recent research has focused on exploring the factors that affect activity and developing nanozyme-based cascade catalytic systems, which can trigger two or more cascade catalytic processes within tumors, thereby producing more therapeutic substances and achieving efficient and stable cancer therapy with minimal side effects. This area has shown remarkable progress. This Perspective provides a comprehensive overview of nanozymes, covering their classification and fundamentals. The regulation of nanozyme activity and efficient strategies of rational design are discussed in detail. Furthermore, representative paradigms for the successful construction of cascade catalytic systems for cancer treatment are summarized with a focus on revealing the underlying catalytic mechanisms. Finally, we address the current challenges and future prospects for the development of nanozyme-based cascade catalytic systems in biomedical applications.

Advances and Future Trends in Nanozyme-Based SERS Sensors for Food Safety, Environmental and Biomedical Applications

Nanozymes, a kind of nanoparticles with enzyme-mimicking activities, have attracted considerable attention due to their robust catalytic properties, ease of preparation, and resistance to harsh conditions. By combining nanozymes with surface-enhanced Raman spectroscopy (SERS) technology, highly sensitive and selective sensors have been developed. These sensors are capable of detecting a wide range of analytes, such as foodborne toxins, environmental pollutants, and biomedical markers. This review provides an overview of recent advancements in the synthesis and surface modification of nanozymes, highlighting their ability to mimic multiple enzymes and enhance catalytic performance. In addition, we explore the development and applications of nanozyme-based SERS sensors in food contaminants, environmental pollutants, and biomedical markers. The review concludes with perspectives and challenges facing the field, involving the need for deeper understanding of nanozyme principles and mechanisms, development of standardized systems for characterization, and the engineering of nanozymes with tailored properties for specific applications. Finally, we discuss the potential for integrating various techniques with nanozymes to create multi-modal detection platforms, paving the way for the next generation of analytical tools in the fields of food safety, environmental monitoring, and biomedical diagnostics.

A colorimetric strategy and smartphone-based test strip for the detection of glucose based on the peroxidase activity of a hemin-derived nanozyme

Nanozymes are nanomaterials with catalytic properties similar to enzymes and offer advantages over natural enzymes such as lower cost, ease of storage, and convenience for large-scale preparation. Thus, they have received increasing attention and are extensively applied in fields such as chemistry, sensing, food, environment, and medicine. Herein, a hemin-derived nanozyme (Hemin-CDs) was prepared using hemin as the precursor and used to substitute the natural horseradish peroxidase (HRP) for colorimetric detection. The prepared Hemin-CDs exhibit excellent water dispersibility, stability and superior peroxidase-like activity. They catalyze the oxidative coupling of colorless 4-aminoantipyrine (4-AAP) with N-ethyl-N-(2-hydroxy-3-sulfopropyl)-3-methylaniline sodium salt (TOOS) in the presence of H2O2, forming a purple quinone diimine dye with an absorption peak at 554 nm, which enables the quantification of H2O2 by measuring the absorbance. Quantitative detection of glucose was further demonstrated under physiological conditions. The detection of H2O2 and glucose takes only 10 minutes, with linear ranges of 1-150 μM and 2.5-300 μM, and LODs of 0.867 μM and 1.026 μM, respectively. Glucose in human serum was successfully detected with satisfactory recoveries (87.5-104.0%). Furthermore, a test strip was developed, and a smartphone was utilized to recognize the color of the test position, enabling rapid and accurate quantification of glucose concentrations within 5 minutes, which promises to enhance the accessibility and convenience of glucose testing.

Bioinspired synergy strategy based on the integration of nanozyme into a molecularly imprinted polymer for improved enzyme catalytic mimicry and selective biosensing

Nanozymes offer many advantages such as good stability and high catalytic activity, but their selectivity is lower than that of enzymes. This is because most of enzymes have a protein component (apoenzyme) for substrate affinity to enhance selectivity and a non-protein element (coenzyme) for catalytic activity to improve sensitivity. The synergy between molecularly imprinted polymers (MIPs) and nanozymes can mimic natural enzymes, with MIP acting as the apoenzyme and nanozyme as the coenzyme. Despite researchers' attempts to associate MIPs with nanozymes, the full potential of this combination remains not well explored. This study addresses this gap by integrating Fe3O4-Lys-Cu nanozymes with peroxidase-like catalytic activities within appropriate MIPs for L-DOPA and dopamine. The catalytic performance of the nanozyme was improved by the presence of Cu in Fe3O4-Lys-Cu and further enhanced by MIP. Indeed, the exploration of the pre-concentration property of MIP has increased twenty-fold the catalytic activity of the nanozyme. Moreover, this synergistic combination facilitated the template removal process during MIP production by reducing the extraction time from several hours to just 1 min thanks to the addition of co-substrates which trigger the reaction with nanozyme and release the template. Overall, the synergistic combination of MIPs and nanozymes offers a promising avenue for the design of artificial enzymes.

Advancements in reusable SERS substrates for trace analysis applications

Surface Enhanced Raman Spectroscopy (SERS) technique is an effective analytical technique in which fingerprint information about analytes can be obtained, can provide detection limit performance at the single molecule level, and analyzes are performed in a single step without any intermediate steps. SERS technique offers additional benefits rather than other analytical techniques including high selectivity, ultrasensitive detection, uncomplicated protocols, in situ sampling, on-set capability and cost-effectiveness. As a result of the combination of developments in materials and nanotechnology science with the SERS analysis technique, this technique strengthens its use advantage day by day. The most important factor that limited the use of this technique was the fact that the solution containing the desired analyte(s) was dropped onto the SERS substrate and the same substrate could not be reused in subsequent analyses. To solve this problem, scientists have focused on developing reusable SERS substrates in recent years. In these studies, scientists basically used three SERS substrate cleaning applications (1) washing the SERS substrate with a suitable solvent that can elute the analyte from SERS surface after analysis, (2) cleaning the SERS substrate with catalytic degradation of analytes after analysis by modifying them with catalytic active materials and (3) Applying plasma cleaning procedure to SERS substrate after analysis and (4) applying adsorption and desorption procedure prior to SERS analysis. Herein, the aim of this review article is to evaluate the reusable SERS substrates-based methods based on their level of development and their potential to recycle. This review offers a coherent discussion on a wide range of sensing schemes employed in fabricating the SERS substrates. We utilized a critical approach in which elaborative examples were selected to highlight key shortcomings of various experimental configurations. In the same vein, there is a discussion of the advantages and limitations concerning the key instrumental advances and the expansion of the recent methods developed in this area.

Apple polysaccharide stabilized palladium nanoparticles for sensitive detection of organophosphorus pesticide

The widespread application of organophosphorus pesticides (OPs) has inflicted significant damage on human well-being and food security. Hence, it is imperative to develop a friendly and accessible biosensor for the detection of OPs. Herein, apple polysaccharide (AP) stabilized palladium nanoparticles (AP-PdnNPs) with a particle size of 2.75-5.95 nm were prepared using AP as a stabilizer and reducing agent. AP-Pd30NPs exhibited good peroxidase-like activity and effectively decomposed H2O2 to ·OH, which catalyzed the 3,3',5,5'-tetramethylbenzidine system to become blue. The catalytic kinetics of AP-Pd30NPs conformed to the typical Michelis-Menten equation. Furthermore, OPs directly inhibited the peroxidase-like activity of AP-Pd30NPs. Thus, a highly effective colorimetric biosensor was developed for the detection of OPs. The detection range of the biosensor was 0.050 μg/L - 200 mg/L, and the limit of detection was extremely low to 0.010 μg/L. Compared with other nanomaterials, the detection platform based on AP-Pd30NPs can effectively detect organophosphorus pesticides without coupling natural enzymes；this method is more economical and practical. Therefore, this established method explores good perspective for the detection of OPs.

Copper (II)-catalyzed polydopamine mediated photothermal sensors for visual quantitative point-of-care testing

BACKGROUND

Temperature sensing is commonly used in point-of-care (POC) detection technologies, yet the portability and convenience of use are frequently compromised by the complexity of thermosensitive processes and signal transduction. Especially, multi-step target recognition reactions and temperature measurement in the reaction vessel present challenges in terms of stability and integration of detection devices. To further combine photothermal reaction and signal readout in one assay, these two processes enable to be integrated into miniaturized microfluidic chips, thereby facilitating photothermal sensing and achieving a simple visual temperature sensing as POC detection.

RESULTS

A copper ion (Cu2+)-catalyzed photothermal sensing system integrated onto a microfluidic distance-based analytical device (μDAD), enabling the visual, portable, and sensitive quantitative detection of multiple targets, including ascorbic acid, glutathione, and alkaline phosphatase (ALP). The polydopamine nanoparticles (PDA NPs) were synthesized by the regulation of free Cu2+ through redox or coordination reactions, facilitating the transduction of distinct photothermal response signals and providing the versatile Cu2+-responsive sensing systems. Promoted by integration with a photothermal μDAD, the system combines PDA's photothermal responsiveness and thermosensitive gas production of ammonium bicarbonate for improved sensitivity of ALP detection, reaching the detection limit of 9.1 mU/L. The system has successfully achieved on-chip detection of ALP with superior anti-interference capability and recoveries ranging from 96.8 % to 104.7 %, alongside relative standard deviations below 8.0 %.

SIGNIFICANCE AND NOVELTY

The μDAD design accommodated both the photothermal reaction of PDA NPs and thermosensitive gas production reaction, achieving the rapid sensing of visual distance signals. The μDAD-based Cu2+-catalyzed photothermal sensing system holds substantial potential for applications in biochemical analysis and clinical diagnostics, underscored by the versatile Cu2+ regulation mechanism for a broad spectrum of biomarkers.

The Exploration of Nanozymes for Biosensing of Pathological States Tailored to Clinical Theranostics

The nanozymes (NZs) are the artificial catalyst deployed for biosensing with their uniqueness (high robustness, surface tenability, inexpensive, and stability) for obtaining a better response/miniaturization of the varied sensors than their traditional ancestors. Nowadays, nanomaterials with their broadened scale such as metal-organic frameworks (MOFs), and metals/metal oxides are widely engaged in generating NZ-based biosensors (BS). Diverse strategies like fluorescent, colorimetric, surface-enhanced Raman scattering (SERS), and electrochemical sensing principles were implemented for signal transduction of NZs. Despite broad advantages, numerous encounters (like specificity, feasibility, stability, and issues in scale-up) are affecting the potentialities of NZs-based BS, and thus need prior attention for a promising exploration for a revolutionary outcome in advanced theranostics. This review includes different types of NZs, and the progress of numerous NZs tailored bio-sensing techniques in detecting abundant bio analytes for theranostic purposes. Further, the discussion highlighted some recent challenges along with their progressive way of possibly overcoming followed by commercial outbreaks.

Nanozymes and their biomolecular conjugates as next-generation antibacterial agents: A comprehensive review

Antimicrobial resistance (AMR), the ability of bacterial species to develop resistance against exposed antibiotics, has gained immense global attention in the past few years. Bacterial infections are serious health concerns affecting millions of people annually worldwide. Therefore, developing novel antibacterial agents that are highly effective and avoid resistance development is imperative. Among various strategies, recent developments in nanozyme technology have shown promising results as antibacterials in several antibiotic-sensitive and resistant bacterial species. Nanozymes offer several advantages over corresponding natural enzymes, such as inexpensive, stable, multifunctional, tunable catalytic properties, etc. Although the use of nanozymes as antibacterial agents has provided promising results, the specific biomolecule-conjugated nanozymes have shown further improvement in catalytic performance and associated antibacterial efficacy. The exclusive design of functional nanozymes with theranostic potential is found to simultaneously inhibit the growth and image of AMR bacterial species. This review comprehensively summarizes the history of nanozymes, their classification, biomolecules conjugated nanozyme, and their mechanism of enzyme-mimetic activity and associated antibacterial activity in antibiotic-sensitive and resistant species. The futureneeds to effectively engineer the existing or new nanozymes to curb AMR have also been discussed.

Mobile phone-assisted imprinted nanozyme for bicolor colorimetric visual detection of erythromycin in river water and milk samples

The residues of erythromycin (ERY) may have negative impacts on the ecological environment, health, and food safety. How to detect ERY effectively and visually is a challenging issue. Herein, we synthesized a molecularly imprinted polymer based nanozymes for selective detection of erythromycin (ERY-MIPNs) at neutral pH, and developed a mobile phone-assisted bicolor colorimetric detection system. This system produced a wide range of color changes from blue to pinkish purple as the ERY concentration increased, making it easy to capture the visualization result. Also, the system showed good sensitivity to ERY ranging from 15 to 135 μM, with a detection limit of 1.78 μM. In addition, the system worked well in the detection of ERY in river water and milk, with the recoveries of 95.57% ∼ 103.20%. These data suggests that this strategy is of considerable potential for practical applications and it provides a new idea for visual detection with portable measurement.

Recyclable Au@R-Fe3O4/g-C3N4 substrates for rapid SERS detection and degradation of multiple pollutants

Developing a Surface-enhanced Raman spectroscopy (SERS) method with excellent detecting ability, good recyclability and analyzing multiple pollutants rapidly are critical for evaluation of water quality in emergency pollution affairs. While constructing a multifunctional substrate with these characteristics to realize the application of SERS in water quality monitoring remains a challenge. In this work, a reusable Au@R-Fe3O4/g-C3N4 SERS substrate is prepared by loading Au nanoparticles (Au NPs) on Fe3O4 nanorings (R-Fe3O4) and the formed Au@R-Fe3O4 is further combined with g-C3N4 nanosheets through a simple electrostatic assembly method. The Au@R-Fe3O4/g-C3N4 nanocomposite presents multifunction of magnetic enrichment, SERS signal enhancement, multiple pollutants analyzing, and photocatalytic activity, which achieves quantitative detection of rhodamine B (RhB), tetracycline hydrochloride (TC), and 4-chlorophenol (4-CP), with detection limits of 5.30 × 10-9, 7.50 × 10-8, 7.69 × 10-8 mol/L, respectively. Furthermore, the recyclable detection capability of Au@R-Fe3O4/g-C3N4 for multi components is demonstrated by the strong SERS signal after 9 cycles of "detection-degradation" processes. Combined with good uniformity and stability, this SERS method based on Au@R-Fe3O4/g-C3N4 substrate provides a new strategy for the multi-pollutants detection and degradation in water environment.

Breaking the pH Limitation of Nanozymes: Mechanisms, Methods, and Applications

Although nanozymes have drawn great attention over the past decade, the activities of peroxidase-like, oxidase-like, and catalase-like nanozymes are often pH dependent with elusive mechanism, which largely restricts their application. Therefore, a systematical discussion on the pH-related catalytic mechanisms of nanozymes together with the methods to overcome this limitation is in need. In this review, various nanozymes exhibiting pH-dependent catalytic activities are collected and the root causes for their pH dependence are comprehensively analyzed. Subsequently, regulatory concepts including catalytic environment reconstruction and direct catalytic activity improvement to break this pH restriction are summarized. Moreover, applications of pH-independent nanozymes in sensing, disease therapy, and pollutant degradation are overviewed. Finally, current challenges and future opportunities on the development of pH-independent nanozymes are suggested. It is anticipated that this review will promote the further design of pH-independent nanozymes and broaden their application range with higher efficiency.

A label-free colorimetric biosensor utilizing natural material for highly sensitive exosome detection

Exosomes, extracellular vesicles secreted by cells, play a crucial role in intercellular communication by transferring information from source cells to recipient cells. These vesicles carry important biomarkers, including nucleic acids and proteins, which provide valuable insights into the parent cells' status. As a result, exosomes have emerged as noninvasive indicators for the early diagnosis of cancer. Colorimetric biosensors have garnered significant attention due to their cost-effectiveness, simplicity, rapid response, and reproducibility. In this study, we employ sporopollenin microcapsules (SP), a natural biopolymer material derived from pollen, as a substrate for gold nanoparticles (AuNPs). By modifying the SP-Au complex with CD63 aptamers, we develop a label-free colorimetric biosensor for exosome detection. In the absence of exosomes, the SP-Au complex catalyzes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), resulting in a color change from colorless to blue. However, the addition of exosomes inhibits the catalytic activity of the SP-Au complex due to coverage of exosomes on AuNPs. This colorimetric biosensor exhibits high sensitivity and selectivity for exosome detection, with a detection limit of 10 particles/μL and a wide linear range of 10 - 108 particles/μL. Additionally, the SP-Au biosensor demonstrates remarkable resistance to serum protein adsorption and excellent catalytic stability even in harsh environments, making it highly suitable for clinical diagnostics.

Poly(ferrocenylsilane)-Based Redox-Active Artificial Organelles for Biomimetic Cascade Reactions

Artificial organelles serve as functional counterparts to natural organelles, which are primarily employed to artificially replicate, restore, or enhance cellular functions. While most artificial organelles exhibit basic functions, we diverge from this norm by utilizing poly(ferrocenylmethylethylthiocarboxypropylsilane) microcapsules (PFC MCs) to construct multifunctional artificial organelles through water/oil interfacial self-assembly. Within these PFC MCs, enzymatic cascades are induced through active molecular exchange across the membrane to mimic the functions of enzymes in mitochondria. We harness the inherent redox properties of the PFC polymer, which forms the membrane, to facilitate in-situ redox reactions similar to those supported by the inner membrane of natural mitochondria. Subsequent studies have demonstrated the interaction between PFC MCs and living cell including extended lifespans within various cell types. We anticipate that functional PFC MCs have the potential to serve as innovative platforms for organelle mimics capable of executing specific cellular functions.

"Three-in-One" Plasmonic Au@PtOs Nanocluster Driven Lateral Flow Assay for Multimodal Cancer Exosome Biosensing

Exploiting the multiple properties of nanozymes for the multimode lateral flow assay (LFA) is urgently required to improve the accuracy and versatility. Herein, we developed a novel plasmonic Au nanostar@PtOs nanocluster (Au@PtOs) as a multimode signal tag for LFA detection. Based on the PtOs bimetallic nanocluster doping strategy, Au@PtOs can indicate both excellent SERS enhancement and nanozyme catalytic activity. Meanwhile, Au@PtOs displays a better photothermal effect than that of Au nanostars. Therefore, catalytic colorimetric/SERS/temperature three-mode signals can be read out based on the Au@PtOs nanocomposite. The Au@PtOs was combined with LFA and applied for breast cancer exosome detection. The detection limit for the colorimetric/SERS/temperature mode was 2.6 × 103/4.1 × 101/4.6 × 102 exosomes/μL, respectively, which was much superior to the common Au nanoparticles LFA (∼105 exosomes/μL). Moreover, based on the fingerprint molecular recognition ability of the SERS mode, exosome phenotypes derived from different breast cancer cell lines can be discriminated easily. Based on the convenient visual colorimetric mode and sensitive SERS/temperature quantitative modes, Au@PtOs driven LFA can satisfy the requirements of accurate and flexible multimodal sensing in different application scenarios.

Antibiotic-Fe3O4 nanoparticles with highly efficient catalytic activity for enhanced chemiluminescence detection of tetracyclines residues in foods

Tetracyclines (TCs) are the most commonly antimicrobial agents that used in livestock production worldwide. It is important to supervise tetracyclines residues in food for environmental monitoring and food safety. In this study, a novel, label-free chemiluminescence (CL) assay without antibody was established. Fe3O4 NPs could facilitate the CL interaction between luminol and H2O2. Interestingly, TCs could enhance the catalytic ability of Fe3O4 NPs and result in a further amplification of the CL intensity. The CL intensity varied linearly with the concentration of tetracycline (TC), oxytetracycline (OTC), chlortetracycline (CTC), and ranging from 10-2400, 10-2800, and 5-2100 nmol/L, respectively; The limits of detection were 4 nmol/L for TC, 6 nmol/L for OTC, and 2 nmol/L for CTC. This CL strategy was applied successfully in testing three TCs residues in milk, eggs and honey samples with more sensitive results, which provided an alternative strategy for monitoring the correct use of TCs.

An Au bipyramids@CuZn MOF core-shell nanozyme enables universal SERS and a colorimetric dual-model bioassay

In this study, a new type of gold nano-bipyramids@CuZn bimetallic organic framework (AuNBPs@CuZn MOF) nanozyme with high peroxidase (POD)-like activity and surface enhanced Raman scattering (SERS) activity was constructed with a special core-shell structure, which can catalyze the oxidation of TMB (colourless and Raman-inactive) into ox-TMB (blue and Raman-active). An AuNBPs@CuZn MOF-enabling universal SERS and colorimetric dual-model bioassay was thus developed for biomolecules with excellent performance, and has promising application prospects in the biosensing fields.

Ag-carbon dots with peroxidase-like activity for colorimetric and SERS dual mode detection of glucose and glutathione

Currently, nanozymes have made important research progress in the fields of catalysis, biosensing and tumor therapy, but most of nanozymes sensing systems are single-mode detection, which are easily affected by environment and operation, so it is crucial to construct nanozymes sensing system with dual-signal detection to obtain a more stable and reliable performance. In this paper, Ag-carbon dots (Ag-CDs) bifunctional nanomaterials were synthesized using carbon dots as reducing agent and protective agent by a facile and green one-step method. A simple and sensitive colorimetric-SERS dual-mode sensing platform was constructed for the detection of glucose and glutathione(GSH) in body fluids by taking advantage of good peroxidase-like and SERS activities of Ag-CDs. Ag-CDs catalyzes H2O2 to hydroxyl radicals(•OH), which oxidized TMB to form ox-TMB blue solution with characteristic absorption peak at 652 nm and Raman characteristic peak at 1607 cm-1. Ag-CDs sensing method exhibited high performance for glucose and GSH with detection limits for colorimetric and SERS as low as 11.30 μM and 3.54 μM, 0.38 μM and 0.24 μM respectively (S/N = 3). In addition, Ag-CDs have good stability and uniformity, ensuring long-term applicability of catalytic system. This colorimetric-SERS dual-mode sensing platform can be used for the determination of glucose and GSH in saliva and urine, and has the advantages of simple, low cost, rapid, and high accuracy, which has a potential application prospect in biosensor and medical research.

Dual-Mode Lateral Flow Immunoassay Based on "Pompon Mum"-Like Fe3O4@MoS2@Pt Nanotags for Sensitive Detection of Viral Pathogens

Lateral flow immunoassay (LFIA) has been widely used for the early diagnosis of diseases. However, conventional colorimetric LFIA possesses limited sensitivity, and the single-mode readout signal is easily affected by the external environment, leading to insufficient accuracy. Herein, multifunctional Fe3O4@MoS2@Pt nanotags with a unique "pompon mum"-like structure were triumphantly prepared, exhibiting excellent peroxidase (POD)-like activity, photothermal properties, and magnetic separation capability. Furthermore, the Fe3O4@MoS2@Pt nanotags were used to establish dual-mode LFIA (dLFIA) for the first time, enabling the catalytic colorimetric and photothermal dual-mode detection of severe acute respiratory syndrome coronavirus 2 nucleocapsid protein (SARS-CoV-2 NP) and influenza A (H1N1). The calculated limits of detection (cLODs) of SARS-CoV-2 NP and H1N1 were 80 and 20 ng/mL in catalytic colorimetric mode and 10 and 8 ng/mL in photothermal mode, respectively, demonstrating about 100 times more sensitive than the commercial colloidal Au-LFIA strips (1 ng/mL for SARS-CoV-2 NP; 1 μg/mL for H1N1). The recovery rates of dLFIA in simulated nose swab samples were 95.2-103.8% with a coefficient of variance of 2.3-10.1%. These results indicated that the proposed dLFIA platform showed great potential for the rapid diagnosis of respiratory viruses.

Bifunctional Mo2N Nanoparticles with Nanozyme and SERS Activity: A Versatile Platform for Sensitive Detection of Biomarkers in Serum Samples

The combined application of nanozymes and surface-enhanced Raman scattering (SERS) provides a promising approach to obtain label-free detection. However, developing nanomaterials with both highly efficient enzyme-like activity and excellent SERS sensitivity remains a huge challenge. Herein, we proposed one-step synthesis of Mo2N nanoparticles (NPs) as a "two-in-one" substrate, which exhibits both excellent peroxidase (POD)-like activity and high SERS activity. Its mimetic POD activity can catalyze the 3,3',5,5'-tetramethylbenzidine (TMB) molecule to SERS-active oxidized TMB (ox-TMB) with high efficiency. Furthermore, combining experimental profiling with theory, the mechanism of POD-like activity and SERS enhancement of Mo2N NPs was explored in depth. Benefiting from the outstanding properties of Mo2N NPs, a versatile platform for indirect SERS detection of biomarkers was developed based on the Mo2N NPs-catalyzed product ox-TMB, which acts as the SERS signal readout. The feasibility of this platform was validated using glutathione (GSH) and target antigens alpha-fetoprotein antigen (AFP) and carcinoembryonic antigen (CEA) as representatives of small molecules with a hydroxyl radical (·OH) scavenging effect and proteins with a low Raman scattering cross-section, respectively. The limits of detection of GSH, AFP, and CEA were as low as 0.1 μmol/L, 89.1, and 74.6 pg/mL, respectively. Significantly, it also showed application in human serum samples with recoveries ranging from 96.0 to 101%. The acquired values based on this platform were compared with the standard electrochemiluminescence method, and the relative error was less than ±7.3. This work not only provides a strategy for developing highly active bifunctional nanomaterials but also manifests their widespread application for multiple biomarkers analysis.

An intelligent readable and capture-antibody-independent lateral flow immunoassay based on Cu2-xSe nanocrystals for point-of-care detection of Escherichia coli O157:H7

Escherichia coli (E. coli) O157:H7 is a common foodborne pathogen which can cause serious harm. It is particularly important to establish a simple and portable method to achieve on-site pathogen detection. In this study, a capture-antibody-independent lateral flow immunoassay (LFIA) was constructed based on Cu2-xSe nanocrystals (Cu2-xSe NCs) for rapid detection of E. coli O157:H7. Cu2-xSe NCs can not only be regarded as the "nano-antibody" for the recognition of E. coli O157:H7 through electrostatic adsorption, but also as nanozymes that show good peroxidase-like catalytic activity. The formed compound of E. coli O157:H7 and Cu2-xSe NCs would be captured by a detection antibody on the T line due to the specific recognition of the antibody and E. coli O157:H7. Then, Cu2-xSe NCs could catalyze the oxidation of TMB by H2O2 to generate oxTMB, thereby generating blue bands. Meanwhile, we developed a mobile app for rapid data analysis. Under the optimal reaction conditions, E. coli O157:H7 could be detected within 70 min. The detection limit of this method was 2.65 × 105 CFU mL-1 with good specificity and stability. Additionally, it could achieve on-site rapid detection of E. coli O157:H7 in environmental water samples, providing a promising biosensor for portable pathogen detection.

Dual-Signal Imaging Mode Based on Fluorescence and Electrochemiluminescence for Ultrasensitive Visualization of SARS-CoV-2 Spike Protein

Accurate and reliable detection of SARS-CoV-2 is critical for the effective prevention and rapid containment of COVID-19. Current approaches suffer from complex procedures or a single signal readout, resulting in an increased risk of false negatives and low sensitivity. Here, we developed a fluorescence (FL) and electrochemiluminescence (ECL) dual-mode imaging platform based on a self-powered DNAzyme walker to achieve accurate surveillance of SARS-CoV-2 spike protein at the single-molecule level. The specific activation of the DNAzyme walker by the target protein provides the power for the system's continuous running, enabling the simultaneous recording of the reduction in fluorescence spots and the appearance of ECL spots generated by the Ru-doped metal-organic framework (MOF) emitter. Therefore, the constructed imaging platform can achieve dual-mode detection of spike protein via reverse dual-signal feedback, which could effectively eliminate false-positive or false-negative signals and improve the detection accuracy and sensitivity with a low detection limit. In particular, the dual-mode accuracy of spike protein diagnosis in samples has been significantly improved compared to single-signal output means. In addition, this dual-mode imaging platform may become a prospective diagnostic device for other infectious viruses.

CRISPR/Cas12a and G-quadruplex DNAzyme-driven multimodal biosensor for visual detection of Aflatoxin B1

Aflatoxin B1 (AFB1) contamination severely threatens human and animal health, it is thus critical to construct a strategy for its rapid, accurate, and visual detection. Herein, a multimodal biosensor was proposed based on CRISPR/Cas12a cleaved G-quadruplex (G4) for AFB1 detection. Briefly, specific binding of AFB1 to the aptamer occupied the binding site of the complementary DNA (cDNA), and cDNA then activated Cas12a to cleave G4 into fragments. Meanwhile, the intact G4-DNAzyme could catalyze 3, 3', 5, 5'-tetramethylbenzidine (TMB) to form colourimetric/SERS/fluorescent signal-enhanced TMBox, and the yellow solution produced by TMBox under acidic conditions could be integrated with a smartphone application for visual detection. The colourimetric/SERS/fluorescent biosensor yielded detection limits of 0.85, 0.79, and 1.65 pg·mL-1, respectively, and was applied for detecting AFB1 in peanut, maize, and badam samples. The method is suitable for visual detection in naturally contaminated peanut samples and has prospective applications in the food industry.

Rapid and sensitive smartphone non-enzymatic colorimetric assay for the detection of glucose in food based on peroxidase-like activity of Fe3O4@Au nanoparticles

A low-cost and reliable analytical method based on the combination of a newly designed Fe3O4@Au as peroxidase mimetics, supported on smartphone analysis software package was proposed for the determination of glucose content in food samples. The nanocomposite was prepared by self-assembling technique, and the characterization was carried out using transmission electron microscopy (TEM), Fourier transforms infrared, and X-ray diffractometer. Record the color change of the solution with a smartphone camera and optimize the operation parameters and reaction conditions. A smartphone with a free self-developed app was accustomed live the RGB (red-greenblue) values of color intensity within the Fe3O4@Au system and were processed with Image J software before computationally convert them glucose concentrations. At the optimization experiment, reaction temperature of 60 °C, reaction time of 50 min and the amount of addition of Fe3O4@Au 0.0125 g was the optimal combination of detecting glucose smartphone color detection system. Hereon, the accuracy of the proposed method was evaluated by comparison between smartphone colorimetry and UV-vis spectrophotometer, a linear calibration in the range of 0.25 ∼ 15 mmol/L glucose was obtained with minimum detection limit of 1.83 and 2.25 μmol/L, respectively. The proposed method was applied effectively to the detection of glucose in actual samples. The results were in accordance with the conventional UV-vis spectrophotometer method.

Signal Amplification Strategy Design in Nanozyme-Based Biosensors for Highly Sensitive Detection of Trace Biomarkers

Nanozymes show great promise in enhancing disease biomarker sensing by leveraging their physicochemical properties and enzymatic activities. These qualities facilitate signal amplification and matrix effects reduction, thus boosting biomarker sensing performance. In this review, recent studies from the last five years, concentrating on disease biomarker detection improvement through nanozyme-based biosensing are examined. This enhancement primarily involves the modulations of the size, morphology, doping, modification, electromagnetic mechanisms, electron conduction efficiency, and surface plasmon resonance effects of nanozymes for increased sensitivity. In addition, a comprehensive description of the synthesis and tuning strategies employed for nanozymes has been provided. This includes a detailed elucidation of their catalytic mechanisms in alignment with the fundamental principles of enhanced sensing technology, accompanied by the presentation of quantitatively analyzed results. Moreover, the diverse applications of nanozymes in strip sensing, colorimetric sensing, electrochemical sensing, and surface-enhanced Raman scattering have been outlined. Additionally, the limitations, challenges, and corresponding recommendations concerning the application of nanozymes in biosensing have been summarized. Furthermore, insights have been offered into the future development and outlook of nanozymes for biosensing. This review aims to serve not only as a reference for enhancing the sensitivity of nanozyme-based biosensors but also as a catalyst for exploring nanozyme properties and their broader applications in biosensing.

Diazo-reaction based dual-mode colorimetric-electrochemical sensing of nitrite in pickled food

Development of an effective and convenient sensor for sensitive detection of nitrites is of great concern since excessive amounts of nitrites can be harmful to both human health and the environment. In this work, Cu-MOF modified exfoliated graphite paper (EGP) was employed as a signal reporter to enable the visual and electrochemical dual-mode sensing of nitrites. Cu-MOFs were in situ synthesized on EGP, which exhibited an excellent oxidase enzyme-like activity to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into its oxidation product (oxTMB). The multi-layer structure and the superior electrical conductivity of EGP not only facilitated the loading of the Cu-MOF nanozyme for colorimetric sensing but also enabled its use as an underlying backbone to support electroanalysis. Based on the recognition of nitrite through a highly specific diazo reaction between nitrite and oxTMB, the addition of nitrite caused the colorimetric sensing solution to change color from blue to green, which allowed for the colorimetric sensing of nitrite with a limit of detection (LOD) of 8.5 × 10-6 mol L-1. Meanwhile, the Cu-MOF/EGP electrochemical platform was employed for ratiometric detection of nitrite based on the electrochemical oxidation of nitrite and TMB. Compared with the colorimetric mode, the electrochemical mode possessed higher sensitivity with a LOD of 5.4 × 10-7 mol L-1, indicating the high sensitivity and accuracy of the proposed dual-mode sensing strategy. Furthermore, the determination of nitrite in different pickled food samples is demonstrated.

Research Progress of SERS Sensors Based on Hydrogen Peroxide and Related Substances

Hydrogen peroxide (H2O2) has an important role in living organisms, and its detection is of great importance in medical, chemical, and food safety applications. This review provides a comparison of different types of Surface-enhanced Raman scattering (SERS) sensors for H2O2 and related substances with respect to their detection limits, which are of interest due to high sensitivity compared to conventional sensors. According to the latest research report, this review focuses on the sensing mechanism of different sensors and summarizes the linear range, detection limits, and cellular applications of new SERS sensors, and discusses the limitations in vivo and future prospects of SERS technology for the detection of H2O2.

Research Progress in Iron-Based Nanozymes: Catalytic Mechanisms, Classification, and Biomedical Applications

Natural enzymes are crucial in biological systems and widely used in biology and medicine, but their disadvantages, such as insufficient stability and high-cost, have limited their wide application. Since Fe₃O₄ nanoparticles were found to show peroxidase-like activity, researchers have designed and developed a growing number of nanozymes that mimic the activity of natural enzymes. Nanozymes can compensate for the defects of natural enzymes and show higher stability with lower cost. Iron, a nontoxic and low-cost transition metal, has been used to synthesize a variety of iron-based nanozymes with unique structural and physicochemical properties to obtain different enzymes mimicking catalytic properties. In this perspective, catalytic mechanisms, activity modulation, and their recent research progress in sensing, tumor therapy, and antibacterial and anti-inflammatory applications are systematically presented. The challenges and perspectives on the development of iron-based nanozymes are also analyzed and discussed.

SERS detection of Hg2+ and aflatoxin B1 through on-off strategy of oxidase-like Au@HgNPs/carbon dots

In this work, cerium-doped carbon dots (Ce-CDs) both as a reducing agent and template hybrid gold nanoparticles (AuNPs) with weak oxidase-like (OXD) activity was synthesized for the detection of Hg²⁺ and aflatoxin B₁ (AFB₁). The AuNPs can catalyze efficiently mercury ion (Hg²⁺) reduction to the metallic (Hg⁰) to form Au-Hg amalgam (Au@HgNPs). The obtained Au@HgNPs with strong OXD-like activity oxidize without Raman-active leucomalachite green (LMG) into the Raman-active malachite green (MG) and simultaneously as the SERS substrates by the formed Raman "hot spot" through MG-induced Au@HgNPs aggregation. While AFB₁ was introduced resulting in the SERS intensity decreasing due to Hg²⁺ with AFB₁ via carbonyl group to inhibit the aggregation of Au@HgNPs. The work paves a new path for the design of a nanozyme-based SERS protocol for tracing Hg²⁺ and AFB₁ residues in foodstuff analysis.

Interfacial deposition of Ag nanozyme on metal-polyphenol nanosphere for SERS detection of cellular glutathione

The low polarization and low Raman cross section characteristics of glutathione (GSH) make it challenging to directly detect GSH molecules through surface enhanced Raman scattering (SERS) technology. Development of nanostructures for indirect detection of GSH applied to the SERS platform is of great interest. Herein, silver nanoparticles (Ag NPs)/copper-polyphenol colloidal spheres (denoted as CuTA@Ag) with adjustable Ag NPs coverage are prepared by deposition of Ag NPs on the metal-polyphenol colloidal spheres via an interfacial polyphenol reduction method. The size and density of the Ag NPs deposited on the out layer can be readily adjusted by tailoring the concentrations of silver precursor. It leads to activity difference for the nanozyme and SERS characteristics. The SERS properties of the obtained CuTA@Ag are studied using oxTMB, catalytic products of nanozyme, as the probing molecules. They provide satisfactory SERS performance with a low detection limit of 10^-7 M (S/N = 3) and linear determination in the 1-100 μM range for GSH. Moreover, it is further able to detect the glutathione content in cancer cells with well accurate and reproducible capability, catching the signs of rising cancer marker levels. This work proposes structurally tunable nanomaterials platform for a catalytic-based SERS assay, which is expected to utilize the high sensitivity of SERS tool for GSH detection in the cellular environment.

Platinum Nanoparticles Loaded Graphitic Carbon Nitride Nanosheets with Enhanced Peroxidase-like Activity for H2O2 and Oxidase-Based Sensing

Platinum nanoparticles (PtNPs) are classical peroxidase-like nanozyme; self-agglomeration of nanoparticles leads to the undesirable reduction in stability and catalytic activity. Herein, a hybrid peroxidase-like nanocatalyst consisting of PtNPs in situ growing on g-C₃N₄ nanosheets with enhanced peroxidase-mimic catalytic activity (PtNP@g-C₃N₄ nanosheets) was prepared for H₂O₂ and oxidase-based colorimetric assay. g-C₃N₄ nanosheets can be used as carriers to solve the problem of poor stability of PtNPs. We observed that the catalytic ability could be maintained for more than 90 days. PtNP@g-C₃N₄ nanosheets could quickly catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), and the absorbance of blue color oxidized TMB (oxTMB) showed a robust linear relationship with the concentration of H₂O₂ (the detection limit (LOD): 3.33 μM). By utilizing H₂O₂ as a mediator, this strategy can be applied to oxidase-based biomolecules (glucose, organophosphorus, and so on, that generate or consume hydrogen peroxide) sensing. As a proof of concept, a sensitive assay of cholesterol that combined PtNP@g-C₃N₄ nanosheets with cholesterol oxidase (ChOx) cascade catalytic reaction was constructed with an LOD of 9.35 μM in a widespread range from 10 to 800 μM (R² = 0.9981). In addition, we also verified its ability to detect cholesterol in fetal bovine serum. These results showed application prospect of PtNP@g-C₃N₄ nanosheets-based colorimetry in sensing and clinical medical detection.

Fabrication of Adjustable Au/Carbon Hybrid Nanozymes with Photothermally Enhanced Peroxidase Activity and Ultra-sensitivity for Glutathione Detection

Au nanozymes are extensively researched for their photothermal effect and catalytic performance, but overcoming the inherent defects of poor dispersibility and thermal stability through complementary materials will expand their prospects for biological applications. Herein, several novel CAu nanozymes were fabricated by in situ reduction of chloroauric acid on hollow carbon nanospheres (HCNs). Through regulating the number of reductions, sesame ball-shaped CAu (sCAu) with highly dispersed Au nanoparticles and diversity-shaped CAu (dCAu) were obtained. The number and morphology of loaded Au nanoparticles, absorption spectra, and hydrophilicity of CAu nanozymes were systematically characterized to demonstrate the flexibility of this novel method. The high-efficiency peroxidase-like sCAu0.3 nanozyme with hyperthermia-activated property was then screened for later bio-application. It is worth mentioning that its photothermal-promoted peroxidase-like activity could be achieved under near-infrared laser irradiation. Moreover, sCAu0.3 could specifically achieve glutathione detection in human blood samples. This method will provide a protocol for the regulation of CAu nanozymes to adapt to bio-detection applications.

Nanozymes towards Personalized Diagnostics: A Recent Progress in Biosensing

This review highlights the recent advancements in the field of nanozymes and their applications in the development of point-of-care biosensors. The use of nanozymes as enzyme-mimicking components in biosensing systems has led to improved performance and miniaturization of these sensors. The unique properties of nanozymes, such as high stability, robustness, and surface tunability, make them an attractive alternative to traditional enzymes in biosensing applications. Researchers have explored a wide range of nanomaterials, including metals, metal oxides, and metal-organic frameworks, for the development of nanozyme-based biosensors. Different sensing strategies, such as colorimetric, fluorescent, electrochemical and SERS, have been implemented using nanozymes as signal-producing components. Despite the numerous advantages, there are also challenges associated with nanozyme-based biosensors, including stability and specificity, which need to be addressed for their wider applications. The future of nanozyme-based biosensors looks promising, with the potential to bring a paradigm shift in biomolecular sensing. The development of highly specific, multi-enzyme mimicking nanozymes could lead to the creation of highly sensitive and low-biofouling biosensors. Integration of nanozymes into point-of-care diagnostics promises to revolutionize healthcare by improving patient outcomes and reducing costs while enhancing the accuracy and sensitivity of diagnostic tools.

A Cascade Enzyme System Integrating Peroxidase Mimic with Catalase for Linear Range Expansion of H2 O2 Assay: A Mechanism and Application Study

Peroxidase (POD) Nanozyme-based hydrogen peroxide (H₂ O₂ ) detection is popular, but hardly adapt to high concentration of H₂ O₂ owing to narrow linear range (LR) and low LR maximum. Here, a solution of combining POD and catalase (CAT) is raised to expand the LR of H₂ O₂ assay via decomposing part of H₂ O₂ . As a proof of concept, a cascade enzyme system (rGRC) is constructed by integrating ruthenium nanoparticles (RuNPs), CAT and graphene together. The rGRC-based sensor does perform an expanded LR and higher LR maximum for H₂ O₂ detection. Meanwhile, it is confirmed that LR expansion is closely associated with apparent K_m of rGRC, which is determined by the relative enzyme activity between CAT and POD both in theory and in experiment. At last, rGRC is successfully used to detect high concentration of H₂ O₂ (up to 10 mm) in contact lens care solution, which performs higher assay accuracy (close to 100% recovery at 10 mm of H₂ O₂ ) than traditional POD nanozymes. This study brings up a kind of POD/CAT cascade enzyme system and provides a new concept for accurate and facile H₂ O₂ detection. Additionally, it replenishes a new enzyme-substrate model of achieving the same pattern with competitive inhibition in enzyme reactions.

Contractile Hairpin DNA-Mediated Dual-Mode Strategy for Simultaneous Quantification of Lactoferrin and Iron Ion by Surface-Enhanced Raman Scattering and Fluorescence Analysis

DNA-mediated self-assembly technology with good sensitivity and affinity ability has been rapidly developed in the field of probe sensing. The efficient and accurate quantification of lactoferrin (Lac) and iron ions (Fe³⁺) in human serum and milk samples by the probe sensing method can provide useful clues for human health and early diagnosis of anemia. In this paper, contractile hairpin DNA-mediated dual-mode probes of Fe₃O₄/Ag-ZIF8/graphitic quantum dot (Fe₃O₄/Ag-ZIF8/GQD) NPs were prepared to realize the simultaneous quantification of Lac by surface-enhanced Raman scattering (SERS) and Fe³⁺ by fluorescence (FL). In the presence of targets, these dual-mode probes would be triggered by the recognition of aptamer and release GQDs to produce FL response. Meanwhile, the complementary DNA began to shrink and form a new hairpin structure on the surface of Fe₃O₄/Ag, which produced hot spots and generated a good SERS response. Thus, the proposed dual-mode analytical strategy possessed excellent selectivity, sensitivity, and accuracy due to the dual-mode switchable signals from "off" to "on" in SERS mode and from "on" to "off" in FL mode. Under the optimized conditions, a good linear range was obtained in the range of 0.5-100.0 μg/L for Lac and 0.01-5.0 μmol/L for Fe³⁺ and with detection limits of 0.14 μg/L and 3.8 nmol/L, respectively. Finally, the contractile hairpin DNA-mediated SERS-FL dual-mode probes were successfully applied in the simultaneous quantification of iron ion and Lac in human serum and milk samples.

Peroxidase-Mimicking Hierarchically Organized Gold Particles for Glucose Detection

In this work, we synthesize a series of hierarchically organized gold nanoparticles (Au HOPs-X) with peroxidase (POD)-like catalytic activity by the in situ reduction of Au-thiolate hierarchically organized particles (Au HOPs). The initial Au HOPs show little POD-like catalytic activity. However, after the reduction of the particles, the Au HOPs-X showed enhanced POD-like catalytic activity, where X represents the reduction degree of Au HOPs. The reasons are as follows: (1) the Au-thiolate complexes on the surface of the Au HOPs-X were reduced into Au nanoparticles, and the active Au⁰ content increases with the increase of the reduction degree; (2) the specific surface area of Au HOPs-X becomes larger. Based on this, the Au HOPs-10 with the highest catalytic activity were combined with glucose oxidase to obtain a standard curve as a function of glucose concentrations. The color of the solutions was captured by mobile phone photos to determine their saturation, and the rapid detection of glucose was achieved through the standard curve of glucose concentration and saturation determined in this study.

Highly sensitive and selective serotonin (5-HT) electrochemical sensor based on ultrafine Fe3O4 nanoparticles anchored on carbon spheres

Neurotransmitter serotonin (5-HT) is involved in various physiological and pathological processes. Therefore, its highly sensitive and selective detection in human serum is of great significance for early diagnosis of disease. In this work, employing iron phthalocyanine as Fe source, ultrafine Fe₃O₄ nanoparticles anchored on carbon spheres (Fe₃O₄/CSs) have been prepared, which exhibits an excellent electrochemical sensing performance toward 5-HT. With carbonecous spheres turned into conductive carbon spheres under the heat treatment in N₂ atmosphere, iron phthalocyanine absorbed on their surfaces are simultaneously pyrolysised and oxidized, and finally transformed into ultrafine Fe₃O₄ nanoparticles. Electrochemical results demonstrate a high sensitivity (5.503 μA μM^-1) and a low detection limit (4 nM) toward 5-HT for as-prepared Fe₃O₄/CSs. In combination with the morphology and physicochemical property of Fe₃O₄/CSs, the enhanced sensing mechanism toward 5-HT is disscussed. In addition, the developed electrochemical sensor also displays a good sensing stability and an anti-interferent ability. Further applied in real human serum samples, a satisfactory recovery rate is achieved. Promisingly, the developed electrochemical sensor can be employed for the determination of 5-HT in actual samples.