Facile colorimetric detection of alkaline phosphatase activity based on the target-induced valence state regulation of oxidase-mimicking Ce-based nanorods

Sensitive and quick electrochemiluminescence biosensor for the detection of reactive oxygen species in seminal plasma based on the valence regulation of gold nanoclusters

BACKGROUND

Gold nanoclusters (AuNCs) obtained by electroreduction have excellent electrochemiluminescence (ECL) properties, and its ECL intensity is regulated by the valence state. In addition, their ECL signals can be rapidly quenched by reactive oxygen species (ROS). Based on this observation, a sensitive ROS biosensor was designed based on valence regulation of AuNCs. Excessive ROS in seminal plasma can lead to male infertility, and the short half-life and instability of ROS pose a challenge for their detection. Since valence regulation can be done quickly and is very sensitive, this ECL biosensor holds promise to address this issue.

RESULTS

The ECL mechanism of AuNCs and the quenching mechanism of AuNCs by ROS were explored, mainly because ROS change the valence state of AuNCs. The ECL signals of the biosensor have a linear relationship with logarithm of the target concentration in the range of 1.0 × 10-8 to 1.0 × 10-3 M and 1.0 × 10-3 to 1.0 × 10-1 M, with a detection limit of 0.75 × 10-10 M (S/N = 3). The biosensor enables rapid one-step detection of ROS and has the advantage of being stable and reusable. More notably, the results of 57 real samples showed that the biosensor can be used to accurately assess the concentration of seminal plasma ROS, guiding the monitoring of sperm quality and the diagnosis of male infertility.

SIGNIFICANCE

Compared with the traditional strategy of applying AuNCs only as a luminescent body, this strategy of regulating the valence state of AuNCs to achieve sensitive and rapid detection broadens the application of AuNCs in the field of analysis. Compared with other ROS detection strategies, the one-step immediate detection method effectively avoids the inaccuracy caused by the short half-life and natural dissipation of ROS, and is expected to improve the accuracy and efficiency of clinical diagnosis.

Polymer-enhanced peroxidase activity of ceria nanozyme for highly sensitive detection of alkaline phosphatase

Nanoceria have demonstrated a wide array of catalytic activity similar to natural enzymes, holding considerable significance in the colorimetric detection of alkaline phosphatase (ALP), which is a biomarker of various biological disorders. However, the issues of physiological stability and formation of protein corona, which are strongly related to their surface chemistry, limit their practical application. In this work, CeO2 nanoparticles characterized by enhanced dimensional uniformity and specific surface area were synthesized, followed by encapsulation with various polymers to further increase catalytic activity and physiological stability. Notably, the CeO2 nanoparticles encapsulated within each polymer exhibited improved catalytic characteristics, with PAA-capped CeO2 exhibiting the highest performance. We further demonstrated that the PAA-CeO2 obtained with enhanced catalytic activity was attributed to an increase in surface negative charge. PAA-CeO2 enabled the quantitative assessment of AA activity within a wide concentration range of 10 to 60 μM, with a detection limit of 0.111 μM. Similarly, it allowed for the evaluation of alkaline phosphatase activity throughout a broad range of 10 to 80 U/L, with a detection limit of 0.12 U/L. These detection limits provided adequate sensitivity for the practical detection of ALP in human serum.

Hydroxyl radical-mediated synthesis of carbonyl functionalized graphene quantum dots-like as enzyme mimics with tunable fluorescence emission

The synthesis of graphene quantum dots-like enriched with specific oxygenated groups (o-GQDs) exhibiting great catalytic performance offers a promising tool for diagnosis and biomedicine, but introducing specific oxygen groups remains a challenge. Here, we propose a mild synthetic protocol for producing regulated fluorescence emission (from blue to yellow) carbonyl functionalized GQDs with double catalytic function through Fe3O4-catalyzed hydroxyl radical (·OH) oxidation the precursors like graphene oxide, polyaniline (PANI) and polydopamine (PDA). The method can be carried out at room temperature than the traditional high-temperature oxidation in concentrated acid. Interestingly, o-GQDs exhibit excellent peroxidase (POD)- and ascorbate oxidase-like activity. XPS characterization showed a significant increase in carbonyl content in o-GQDs compared to the precursor, even a 14-fold increase in blue-emitting iron-doped GQDs (b-Fe-GQDs). The introduction of Fe3O4 during the synthesis process results in a minor degree of Fe doping, which enhances the catalytic activity of b-Fe-GQDs through coordination with N. Based on this feature, highly sensitive single-signal and ultra-selective dual-signal methods for alkaline phosphatase detection were developed. This low cost and safe synthesis strategy paves the way for practical usage of o-GQDs.

Nanozyme-Enabled Biomedical Diagnosis: Advances, Trends, and Challenges

As nanoscale materials with the function of catalyzing substrates through enzymatic kinetics, nanozymes are regarded as potential alternatives to natural enzymes. Compared to protein-based enzymes, nanozymes exhibit attractive characteristics of low preparation cost, robust activity, flexible performance adjustment, and versatile functionalization. These advantages endow them with wide use from biochemical sensing and environmental remediation to medical theranostics. Especially in biomedical diagnosis, the feature of catalytic signal amplification provided by nanozymes makes them function as emerging labels for the detection of biomarkers and diseases, with rapid developments observed in recent years. To provide a comprehensive overview of recent progress made in this dynamic field, here an overview of biomedical diagnosis enabled by nanozymes is provided. This review first summarizes the synthesis of nanozyme materials and then discusses the main strategies applied to enhance their catalytic activity and specificity. Subsequently, representative utilization of nanozymes combined with biological elements in disease diagnosis is reviewed, including the detection of biomarkers related to metabolic, cardiovascular, nervous, and digestive diseases as well as cancers. Finally, some development trends in nanozyme-enabled biomedical diagnosis are highlighted, and corresponding challenges are also pointed out, aiming to inspire future efforts to further advance this promising field.

Multienzyme-Like Nanozymes: Regulation, Rational Design, and Application

Nanomaterials with more than one enzyme-like activity are termed multienzymic nanozymes, and they have received increasing attention in recent years and hold huge potential to be applied in diverse fields, especially for biosensing and therapeutics. Compared to single enzyme-like nanozymes, multienzymic nanozymes offer various unique advantages, including synergistic effects, cascaded reactions, and environmentally responsive selectivity. Nevertheless, along with these merits, the catalytic mechanism and rational design of multienzymic nanozymes are more complicated and elusive as compared to single-enzymic nanozymes. In this review, the multienzymic nanozymes classification scheme based on the numbers/types of activities, the internal and external factors regulating the multienzymatic activities, the rational design based on chemical, biomimetic, and computer-aided strategies, and recent progress in applications attributed to the advantages of multicatalytic activities are systematically discussed. Finally, current challenges and future perspectives regarding the development and application of multienzymatic nanozymes are suggested. This review aims to deepen the understanding and inspire the research in multienzymic nanozymes to a greater extent.

A sea urchin-shaped nanozyme mediated dual-mode immunoassay nanoplatform for sensitive point-of-care testing histamine in food samples

Rapid detection of histamine remains a challenge due to the complexity of food matrices. Based on the high peroxidase-like activity of sea urchin-shaped Pt@Au NPs (SU-Pt@Au NPs), a novel dual-mode nanoplatform is developed for the sensitive detection of histamine utilizing an indirect competitive enzyme-linked immunosorbent assay. According to the colorimetric-based UV-vis nanoplatform, histamine is sensitively detected with a liner range from 0.5 to 100 ng/mL and a limit of detection (LOD) as low as 0.3 ng/mL. Then, a smartphone-loaded color picker APP can intelligently detect histamine in point-of-care testing (POCT) based on the R/B ratio of the color channels, with a detection range of 0.5 to 1000 ng/mL and a LOD as low as 0.15 ng/mL, significantly expanding the detection range. Such an easy-to-use and sensitive detection system is employed to quantify histamine in Pacific saury, crab, and pork samples, indicating outstanding application potential in protein-rich meat food safety.

Recent Progress and Prospect of Metal-Organic Framework-Based Nanozymes in Biomedical Application

A nanozyme is a nanoscale material having enzyme-like properties. It exhibits several superior properties, including low preparation cost, robust catalytic activity, and long-term storage at ambient temperatures. Moreover, high stability enables repetitive use in multiple catalytic reactions. Hence, it is considered a potential replacement for natural enzymes. Enormous research interest in nanozymes in the past two decades has made it imperative to look for better enzyme-mimicking materials for biomedical applications. Given this, research on metal-organic frameworks (MOFs) as a potential nanozyme material has gained momentum. MOFs are advanced hybrid materials made of inorganic metal ions and organic ligands. Their distinct composition, adaptable pore size, structural diversity, and ease in the tunability of physicochemical properties enable MOFs to mimic enzyme-like activities and act as promising nanozyme candidates. This review aims to discuss recent advances in the development of MOF-based nanozymes (MOF-NZs) and highlight their applications in the field of biomedicine. Firstly, different enzyme-mimetic activities exhibited by MOFs are discussed, and insights are given into various strategies to achieve them. Modification and functionalization strategies are deliberated to obtain MOF-NZs with enhanced catalytic activity. Subsequently, applications of MOF-NZs in the biosensing and therapeutics domain are discussed. Finally, the review is concluded by giving insights into the challenges encountered with MOF-NZs and possible directions to overcome them in the future. With this review, we aim to encourage consolidated efforts across enzyme engineering, nanotechnology, materials science, and biomedicine disciplines to inspire exciting innovations in this emerging yet promising field.

Specializing Carbon Nanozyme Active Sites for Sensitive Alkaline Phosphatase Activity Metal-Free Detection

As biological enzymes regulate metabolic processes, alkaline phosphatase (ALP) is a critical diagnostic indicator associated with many diseases. To accurately measure the enzyme activity, nanozymactic materials can offer sensitive strategies for ALP detection. However, nanozymes often lack specific target binding sites, and the presence of common co-components, e. g., metal ions, may cause false-positive or false-negative results in enzyme activity determination. Herein, we developed a colorimetric assay for ALP detection using metal-free nanozymatic carbon dots (CDs). The ALP hydrolysis of pyrophosphate ions (PPi) to phosphate ions (Pi) induces a "turn-on" response based on the nanozyme activity. This PPi-induced inhibition mechanism is extensively studied via the Michaelis-Menten model, revealing that PPi acts as a noncompetitive inhibitor for CDs at a binding site distinct from the common nanozyme active site. With superior responses to ALP substrates, a highly sensitive and selective method is established for sensing ALP activity with a linear range of 0.010-0.200 U/L and a detection limit of 0.009 U/L. This finding explores the recognition and binding behavior of nanozymes, allowing for precise and reliable measurements even in complex samples, and represents a significant breakthrough for nanozyme-based assays in biological analysis.

Ceria-Based Nanozymes in Point-of-Care Diagnosis: An Emerging Futuristic Approach for Biosensing

In recent years, there has been a notable increase in interest surrounding nanozymes due to their ability to imitate the functions and address the limitations of natural enzymes. The scientific community has been greatly intrigued by the study of nanoceria, primarily because of their distinctive physicochemical characteristics, which include a variety of enzyme-like activities, affordability, exceptional stability, and the ability to easily modify their surfaces. Consequently, nanoceria have found extensive use in various biosensing applications. However, the impact of its redox activity on the enzymatic catalytic mechanism remains a subject of debate, as conflicting findings in the literature have presented both pro-oxidant and antioxidant effects. Herein, we creatively propose a seesaw model to clarify the regulatory mechanism on redox balance and survey possible mechanisms of multienzyme mimetic properties of nanoceria. In addition, this review aims to showcase the latest advancements in this field by systematically discussing over 180 research articles elucidating the significance of ceria-based nanozymes in enhancing, downsizing, and enhancing the efficacy of point-of-care (POC) diagnostics. These advancements align with the ASSURED criteria established by the World Health Organization (WHO). Furthermore, this review also examines potential constraints in order to offer readers a concise overview of the emerging role of nanoceria in the advancement of POC diagnostic systems for future biosensing applications.

A Co-based MOF as nanozyme with enhanced oxidase-like activity for highly sensitive and selective colorimetric differentiation of aminophenol isomers

Nanozyme with the merit of excellent and adjustable catalytic activity, outstanding stability and low cost is a promising alternative for natural enzymes widely applied in a variety of fields. In the present study, a new two-dimensional cobalt-based MOF nanocomposite designated as MVCM@β-CD was synthesized. Combined with the strategies of increasing the ratio of Co(Ⅲ)/Co(Ⅱ) and modifying with small molecule β-cyclodextrin (β-CD), MVCM@β-CD displayed remarkably enhanced oxidase-mimicking activity, which was attributed to synergistic effect from large surface area of two dimensional Co-MOF nanosheet, numerous exposed active sites, high-proportioned trivalence of cobalt and regulating action of β-cyclodextrin. The addition of aminophenol isomers inhibited the catalytic oxidation process, resulting in different color change of the solution and UV-Vis absorption behaviors, based on which a sensitive ratiometric colorimetry for m-aminophenol (m-Ap) and a simple colorimetric p-aminophenol (p-Ap) detection method were developed with the detection limit of 0.16 μM and 1.01 μM, respectively. This method realized the colorimetric differentiation of aminophenol isomers, which provided a simple, accurate and low-cost approach for visual discrimination without complicated instrument and procedure, especially appropriate for on-site detection.

Advances in metal graphitic nanocapsules for biomedicine

Metal graphitic nanocapsules have the advantages of both graphitic and metal nanomaterials, showing great promise in biomedicine. On one hand, the chemically inert graphitic shells are able to protect the metal core from external environments, quench the fluorescence signal from the biological system, offer robust platform for targeted molecules or drugs loading, and act as stable Raman labels or internal standard molecule. On the other hand, the metal cores with different compositions, sizes, and morphologies show unique physicochemical properties, and further broaden their biomedical functions. In this review, we firstly introduce the preparation, classification, and properties of metal graphitic nanocapsules, then summarize the recent progress of their applications in biodetection, bioimaging, and therapy. Challenges and their development prospects in biomedicine are eventually discussed in detail. We expect the versatile metal graphitic nanocapsules will advance the development of future clinical biomedicine.

Hydrogel-Involved Colorimetric Platforms Based on Layered Double Oxide Nanozymes for Point-of-Care Detection of Liver-Related Biomarkers

Monitoring the liver status in a convenient and low-cost way is significant for obtaining a warning about drug-indued liver diseases promptly. Herein, we designed a novel colorimetric point-of-care (POC) platform for the determination of three liver-related biomarkers─aspartate transaminase (AST), alanine transaminase (ALT), and alkaline phosphatase (ALP). This platform integrated agarose hydrogels into a portable device, where hydrogels were loaded with nanozymes and different reaction substances for triggering specific reactions and generating colorimetric signals. Typically, Au-decorated CoAl-layered double oxide (Au/LDO) was for the first time developed as the nanozyme with peroxidase (POD) mimic activity, which can accelerate the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxTMB with the coexistence of hydrogen peroxide (H₂O₂). The detection mechanism of AST and ALT is based on the fact that they can cause individual cascade reactions to generate H₂O₂, and H₂O₂ further activates the Au/LDO nanozyme to catalyze the chromogenic reaction of TMB. As for ALP, it can catalytically hydrolyze l-ascorbic acid-2-phosphate to ascorbic acid. The latter then discolored the oxTMB that was produced with the assistance of Au/LDO. Teaming up with a smartphone, the color information of hydrogels can be converted to hue values, which allow quantitative analysis of ALT, AST, and ALP with detection limits of 15, 10, and 5 U/L, respectively. Moreover, the simple and cost-effective platform was successfully applied for the simultaneous determination of the three analytes in human plasma. Additionally, since the hydrogel is disposable and can be replaced by new ones loaded with different reaction regents, the platform is expected to serve the POC testing of various chem/bio targets.

Enhancing the peroxidase-like activity of MIL-88B by ligand exchange with polydopamine

Metal-organic frameworks (MOFs) as nanozymes have been widely used in biosensing. However, MOFs have inherent defects of easy agglomeration, leading to the stacking of active surfaces. In addition, the low conductivity of MOFs is not conducive to the electron migration in the Fenton-like reaction, which leads to a further decrease in catalytic activity and severely restricts their application. In response to the above problems, it makes sense to develop a method to improve both the dispersion and conductivity of MOFs. Here, a simple ligand exchange method with polydopamine (PDA) was used to synthesize MOF PDA-MIL-88B. PDA-MIL-88B shows stronger peroxidase-like activity than MIL-88B. One reason is the good dispersibility of PDA-MIL-88B, which is conducive to exposing the active surface. In addition, the reduced electrochemical impedance of PDA-MIL-88B increases its electrical conductivity, which is favorable for electron migration in the Fenton-like reaction. As a result, PDA-MIL-88B can better catalyze 3,3',5,5'-tetramethylbenzidine to achieve redoximorphic color changes. PDA-MIL-88B can be used to detect glucose in human serum with good sensitivity and selectivity. This work can provide a strategy for MOFs to enhance the enzyme-like activity.

A colorimetric and ratiometric photometric sequential assay for ascorbic acid and alkaline phosphatase in serum based on valence states modulation

The photometric method is widely used in real clinical tests due to its simple operation, low cost and convenient. Many of the reported colorimetric ALP assays so far are non- ratiometric because the detection was based on changes in absorbance at a single wavelength. The development of novel colorimetric and ratiometric assay is of importance for quantitatively measuring target with high accuracy. The challenge in the design of ratiometric photometric assay is that the chromophore must have a significant spectral shift before and after binding to the target. Here, we report a colorimetric and ratiometric photometric sequential assay of AA and ALP based on the complexation between ARS and Cu²⁺ and redox reaction between AA and Cu²⁺. The absorption band of ARS centered at 425 nm (yellow color), which could be shifted to 510 nm (red color) upon Cu²⁺ binding. However, as far as we know, this classic color reaction has not been used to develop a ratiometric photometric method to sequentially detect AA and ALP, although photometric methods based on the regulation of other color reagents with oxidizing metal ions have been reported. The proposed sensing system shows a limit of detection for ALP at 0.24 U L^-1 and could be applied for detecting ALP in newborn calf serum. The established sensing system makes a useful contribution to the detection of ALP in complex clinical samples.

Dual-mode fluorescence and colorimetric detection of pesticides realized by integrating stimulus-responsive luminescence with oxidase-mimetic activity into cerium-based coordination polymer nanoparticles

The great threat of pesticide residues to the environment and human health has drawn widespread interest to explore approaches for pesticide monitoring. Compared to commonly developed single-signal pesticide assays, multi-mode detection with inherent self-validation and self-correction is expected to offer more reliable and anti-interference results. However, how to realize multi-mode analysis of pesticides still remains challenging. Herein, we propose a dual-mode fluorescence and colorimetric method for pesticide determination by integrating stimulus-responsive luminescence with oxidase-mimetic activity into cerium-based coordination polymer nanoparticles (CPNs(Ⅳ)). The CPNs(Ⅳ) exhibit good oxidase-like activity of catalyzing the colorless 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to its blue oxide, offering a visible color signal; by employing acid phosphatase (ACP) to hydrolyze ascorbic acid 2-phosphate (AAP), the generated ascorbic acid (AA) can chemically reduce the CPNs(Ⅳ) to CPNs(Ⅲ), which exhibit a remarkable fluorescence signal but lose the oxidase-mimicking ability to trigger the TMB chromogenic reaction; when pesticides exist, the enzymatic activity of ACP is restrained and the hydrolysis of AAP to AA is blocked, leading to the recovery of the catalytic TMB chromogenic reaction but the suppression of the fluorescence signal of CPNs(Ⅲ). According to this principle, by taking malathion as a pesticide model, dual-mode 'off-on-off' fluorescence and 'on-off-on' colorimetric detection of the pesticide with good sensitivity was realized. Excellent interference-tolerance and reliability were verified by applying it to analyze the target in real sample matrices. With good performance and practicability, the proposed dual-mode approach shows great potential in the facile and reliable monitoring of pesticide residues.

Highly sensitive colorimetric detection and effective adsorption of phosphate based on MOF-808(Zr/Ce)

MOF-808(Zr/Ce) has been successfully used for the sensitive and rapid detection of phosphate and phosphate removal by effective adsorption.

Prussian blue nanoparticles-enabled sensitive and accurate ratiometric fluorescence immunoassay for histamine

Herein, a nanozyme-mediated ratiometric fluorescence immunoassay for histamine (HA) has been developed. Prussian blue nanoparticles (PBNPs) with outstanding peroxidase-like activity were labelled with goat anti-mouse IgG via a facile electrostatic adsorption to yield the nanozyme-antibody conjugate which acted as a bridge to link the ratiometric fluorescence readout with HA concentration. As substrate, o-phenylenediamine (OPD) was oxidized into 2,3-diaminophenazine (oxOPD) by H2O2 under the catalysis of PBNPs, producing a novel emission at 570 nm and quenching the fluorescence of carbon dots (CDs) at 450 nm simultaneously. Under optimal conditions, the ratio of fluorescence intensity at 570 nm and 450 nm (I570/I450) linearly correlated with HA concentration ranging from 1.6 ng/mL to 125 μg/mL, with a detection limit (LOD) of 1.2 ng/mL. In addition, analytical performances including specificity, accuracy and applicability were evaluated, which revealed that this ratiometric fluorescence immunoassay affords an effective platform for sensitive and accurate detection of HA.

Synthesis-temperature-regulated multi-enzyme-mimicking activities of ceria nanozymes

Ceria (CeO2) nanozymes have drawn much attention in recent years due to their unique physiochemical properties and excellent biocompatibility. It is therefore very important to establish a simple and robust guideline to regulate CeO2 with desired multi-enzyme-mimicking activities that are ideal for practical bioapplications. In this work, the multi-enzyme-mimicking activities of CeO2 were regulated in a facile manner by a wet-chemical method with different synthesis temperatures. Interestingly, a distinct response in multi-enzyme-mimicking activities of CeO2 was observed towards different synthesis temperatures. And the regulation was ascribed to the comprehensive effect of the oxygen species, size, and self-restoring abilities of CeO2. This study demonstrates that high-performance CeO2 can be rationally designed by a specific synthesis temperature, and the guidelines from radar chart analysis established here can advance the biomedical applications of ceria-based nanozymes.

Target-induced synergetic modulation of electrochemical tag concentration and electrode surface passivation for one-step sampling filtration-free detection of acid phosphatase activity

As a biomarker of several diseases, the activity of acid phosphatase (ACP) is generally used to assistantly diagnose these diseases. Thus, developing reliable ACP activity analytical methods becomes quite significant. Herein, we recommend a one-step sampling filtration-free electrochemical method for ACP activity determination based on the target-induced synergetic modulation of tag concentration and surface passivation. Mn₃O₄ microspheres with favorable oxidase-mimicking activity are synthesized to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to its product TMBox, resulting in a remarkable re-reduction signal of TMBox to TMB recorded by an integrated electrochemical system consisting of screen-printed electrode (SPE) and 3D-printed holder. When hexametaphosphate ions (HMPi) with rich negative charges are employed to interact positively charged TMBox, the formed flocculent precipitate TMBox-HMPi automatically sedimentates onto SPE surface, and both the decreased concentration of free TMBox in solution and the increased electrode surface passivation triggered by TMBox-HMPi sedimentation synergistically reduce the re-reduction signal of TMBox. When ACP is present, it hydrolyzes the HMPi substrate, greatly relieving the formation of the TMBox-HMPi precipitate and its sedimentation onto SPE surface. As a result, the electrochemical re-reduction signal of TMBox becomes remarkable again. With the strategy of using one stimulus to generate two-fold signal change, highly sensitive ACP activity detection was realized, with a wide linear range from 0.05 to 50 U/L and a limit of detection down to 0.024 U/L. Reliable monitoring of ACP activity in clinical serum was also demonstrated.

A carbon dot doped lanthanide coordination polymer nanocomposite as the ratiometric fluorescent probe for the sensitive detection of alkaline phosphatase activity

The development of sensitive methods for alkaline phosphatase (ALP) activity analysis is an important analytical topic. Based on the stimulus-responsive lanthanide coordination polymer, a simple ratiometric fluorescence sensing strategy was proposed to detect ALP activity. A carbon dot (CD) doped fluorescent supramolecular lanthanide coordination polymer (CDs@Tb-GMP) was prepared with Tb³⁺ and the ligand guanine single nucleotide (GMP). To construct a ratiometric fluorescence biosensor, the fluorescence of Tb-GMP was used as a response signal, and the fluorescence of CDs was used as a reference signal due to its good stability. When excited at 290 nm, the polymer network Tb-GMP emits characteristic fluorescence at 545 nm, while the CDs encapsulated in the polymer network emit fluorescence at 370 nm. After adding ALP to the system, the substrate GMP can be hydrolyzed by ALP, resulting in the destruction of the polymer network. Accordingly, the fluorescence of Tb-GMP significantly decreased, while the fluorescence of CDs slightly increased due to their release from the polymer network. By comparing the relationship between the fluorescence intensity ratio of the two signals and the concentration of ALP, sensitive detection of ALP could be achieved with the linear range from 0.5 to 80 U L^-1 and a detection limit of 0.13 U L^-1. Furthermore, the proposed ratiometric sensing system was applied to the detection of ALP in human serum samples with desirable results, indicating potential application in clinical diagnosis.

A dual-mode resonance Rayleigh scattering and colorimetric alkaline phosphatase assay based on in situ ascorbic acid-induced signal generation from manganese dioxide nanosheets

Multimode sensing has attracted extensive attention because they provide more than one transduction channel, thus improving accuracy and sensitivity. Due to the structural diversity, MnO2 nanosheets and nanoneedles were successively obtained via one-step redox reaction and different self-assembly methods. MnO2 nanosheets possess outstanding optical properties including extremely strong resonance Rayleigh scattering (RRS) and absorbance signal, and were selected as a dual-mode sensing material. Inspired by the selectivity of alkaline phosphatase (ALP) towards dephosphorylate ascorbic acid 2-phosphate (AAP) to generate ascorbic acid (AA), which has the ability to decompose MnO2 nanosheets along with optical signals and colour change, a dual-mode optical ALP sensing platform has been designed. Therefore, MnO2 nanosheets can serve as colorimetric probes by directly visualizing the colour variation with bare eyes. Moreover, the detection limit obtained by the RRS sensing mode can be as low as 0.17 mU L-1, which is far superior to that obtained by previously reported methods. The strategy not only has good feasibility and sensitivity, but also shows good prospects for a series of ALP-extended sensing applications.

A novel alkaline phosphatase activity sensing strategy combining enhanced peroxidase-mimetic feature of sulfuration-engineered CoOx with electrostatic aggregation

Given alkaline phosphatase (ALP) takes part in the phosphorylation/dephosphorylation processes in the body, its activity is universally taken as an important indicator of many diseases, and thus developing reliable and efficient methods for ALP activity determination becomes quite important. Here, we propose a new sensing strategy for ALP activity by integrating the improved peroxidase-mimicking catalysis of sulfuration-engineered CoO_x with the hexametaphosphate ion (HMPi)-mediated electrostatic aggregation. After sulfuration engineering, the CoO_x composite coming from the pyrolysis of ZIF-67 exhibits enhanced peroxidase-mimetic catalytic ability to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to its oxide TMBox, offering a remarkable color change from colorless to mazarine; with the presence of HMPi, the rapid electrostatic assembly of negatively charged HMPi and positively charged TMBox leads to the aggregation of the latter, resulting in a color fading phenomenon; when ALP is added in advance to hydrolyze the HMPi mediator, the aggregation procedure is significantly suppressed, and such that the solution color can be recovered. Based on this principle, efficient determination of ALP activity was gained, giving a wide detection scope from 0.8 to 320 U/L and a detection limit as low as 0.38 U/L. Reliable analysis of the target in serum samples was also achieved, verifying the feasibility and practicability of our strategy in measuring ALP activity for clinical applications. Graphical abstract.

Combining CeVO4 oxidase-mimetic catalysis with hexametaphosphate ion induced electrostatic aggregation for photometric sensing of alkaline phosphatase activity

In the present work, a novel alkaline phosphatase (ALP) activity colorimetric assay is proposed by integrating the oxidase-mimicking catalytic characteristic of CeVO₄ nanoparticles with the hexametaphosphate ion (HMPi) mediated electrostatic aggregation. The CeVO₄ nanoparticles exhibit good oxidase-mimetic catalytic ability to promote the 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to TMBox, offering a significant change from colorless to blue. After a small amount of HMPi is added, the strong electrostatic interaction between the negatively charged HMPi species and the positively charged TMBox product leads to the aggregation of the latter, generating an aubergine HMPi-TMBox agglomerate. After the agglomerate is filtered out, the reaction solution turns to be almost colorless. When ALP is used to hydrolyze the HMPi species in advance, the electrostatic aggregation process is remarkably restrained, thus retaining the blue color of the CeVO₄ catalyzed TMB solution. According to the new sensing strategy, highly selective and sensitive analysis of ALP activity was realized, providing a wide detection range from 1 to 210 U/L and a detection limit of 0.68 U/L. Accurate measurement of ALP activity in clinical serum matrices was also validated, demonstrating the practicability of the proposed assay in practical applications.

ATP induced alteration in the peroxidase-like properties of hollow Prussian blue nanocubes: a platform for alkaline phosphatase detection

Breaking the pH limitation of the enzyme-like activity of nanomaterials is of great importance for extending their applications in environmental and biomedical fields. Herein, to mimic the role of histidine residues in horseradish peroxidase (HRP), adenosine 5'-triphosphate (ATP) is reported to improve the peroxidase-like activity of hollow Prussian blue nanocubes (hPBNCs). Due to the inherited porous structures, hPBNCs can expose all the binding sites as far as possible to ATP to significantly amplify their catalytic activity and broaden their applicable pH range up to pH 12. Introduction of ATP provides the possibility of realizing efficient catalytic reactions under alkaline conditions. Upon binding with hPBNCs, ATP can enhance the stability of hPBNCs, increase the affinities of the catalysts towards substrates and improve the conductivity of hPBNCs as well as change the decomposed product from H₂O₂. Moreover, on the basis of the different catalytic activities of hPBNCs towards ATP, adenosine 5'-diphosphate and adenosine 5'-monophosphate, hPBNCs-ATP is utilized to construct a novel colorimetric sensor for the detection of alkaline phosphatase (ALP) activity in biological fluids, which is significantly important for the clinical diagnosis of ALP-related diseases.

Metal–organic framework based nanozymes: promising materials for biochemical analysis

Metal–organic frameworks with enzyme-like catalytic features (MOF nanozymes) exhibit great promise in detecting various analytes with amplified signal outputs.

Phosphate-responsive 2D-metal–organic-framework-nanozymes for colorimetric detection of alkaline phosphatase

Phosphate-responsive peroxidase-mimicking two-dimensional-metal–organic-framework nanozymes were employed to develop alkaline phosphatase assays with tunable dynamic ranges and colorimetric logic gates.

Fluoride capped V6O13–reduced graphene oxide nanocomposites: high activity oxidase mimetics and mechanism investigation

A novel high activity of the oxidase-like nanozyme of V₆O₁₃–rGO NCs, whose activity can be significantly enhanced by fluoride capping through surface chemistry.