A gold nanoparticles-based colorimetric assay for alkaline phosphatase detection with tunable dynamic range

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.

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.

Overview on the Development of Alkaline-Phosphatase-Linked Optical Immunoassays

The drive to achieve ultrasensitive target detection with exceptional efficiency and accuracy requires the advancement of immunoassays. Optical immunoassays have demonstrated significant potential in clinical diagnosis, food safety, environmental protection, and other fields. Through the innovative and feasible combination of enzyme catalysis and optical immunoassays, notable progress has been made in enhancing analytical performances. Among the kinds of reporter enzymes, alkaline phosphatase (ALP) stands out due to its high catalytic activity, elevated turnover number, and broad substrate specificity, rendering it an excellent candidate for the development of various immunoassays. This review provides a systematic evaluation of the advancements in optical immunoassays by employing ALP as the signal label, encompassing fluorescence, colorimetry, chemiluminescence, and surface-enhanced Raman scattering. Particular emphasis is placed on the fundamental signal amplification strategies employed in ALP-linked immunoassays. Furthermore, this work briefly discusses the proposed solutions and challenges that need to be addressed to further enhance the performances of ALP-linked immunoassays.

Alkaline phosphatase (ALP) activatable small molecule-based prodrugs for cancer theranostics

Highly water-soluble small molecule-based prodrugs (5-FUPD and SAHAPD) are formulated. They comprise a phosphate group to lock the active drug payload (5-fluorouracil and SAHA) along with a turn-on fluorophore consisting of a glutathione (GSH) depletory feature. Installation of the phosphate group along with purification of final product has been accomplished in an operationally facile manner. Activation of the prodrugs is facilitated by alkaline phosphatase (ALP)-mediated hydrolysis of the phosphate group followed by 1,8-elimination. The prodrugs were found to be highly effective against ALP flared human cervical cancer (HeLa) and liver cancer (HepG2) cell lines. Most notably, they were found to be innocuous to normal liver cells (WRL-68).

Fluorescence Turn-on Detection of Alkaline Phosphatase Activity and Al3+ Using Vitamin B6 Cofactor Conjugated GSH Capped Mn-doped ZnS Quantum Dots

The glutathione (GSH) functionalized Mn-doped ZnS quantum dots (GSH_Mn_ZnS QDs) was conjugated with pyridoxal 5'-phosphate (PLP). The -CHO group of vitamin B₆ cofactor PLP interacted with the -NH₂ group of GSH functionalized Mn_ZnS QDs. The conjugation of PLP quenched the fluorescence emission of GSH_Mn_ZnS QDs at 601 nm. Addition of alkaline phosphatase (ALP) catalytically dephosphorylated the PLP into pyridoxal that restored the fluorescence emission of GSH_Mn_ZnS QDs. With a sensitivity of 0.035 U/L, the PLP conjugated GSH_Mn_ZnS QDs was applied to quantify ALP activity in human serum and plasma. Further, the developed nanoprobe PLP conjugated GSH_Mn_ZnS QDs was also applied to detect Al³⁺. The complexation-induced fluorescence enhancement was observed at 492 nm upon the interaction of Al³⁺ with the PLP conjugated GSH_Mn_ZnS QDs. Without any interference from other tested metal ions, this nanoprobe can be employed to detect Al³⁺ down to 2.30 µM.

Rational synthesis of carbon dots with phosphate ester group for direct mapping of endogenous alkaline phosphatase and polarity monitoring in living cells

Carbon dots (CDs) have been extensively employed in biomolecule imaging. However, the imaging of biological enzymes with CDs has not been reported, which greatly limits their application in biological imaging. Herein, for the first time, a new type of fluorescent CDs is elaborately designed to realize the direct mapping of alkaline phosphatase (ALP) in cells. The obtained phosphorus and nitrogen co-doped CDs (P, N-CDs) generate specific structures including xanthene oxide and phosphate ester, thereby enabling P, N-CDs to be exclusively cleaved by ALP without auxiliary media. The fluorescence intensity of P, N-CDs can be specifically turned on in the presence of ALP, making them powerful probes for sensitive sensing of ALP activity with a detection limit of 1.27 U·L^-1. Meanwhile, P, N-CDs possessing electron deficiency structure fulfill sensitive responding to polarity variations. The excellent photo-bleaching resistance and biocompatibility of the P, N-CDs are taken for directly mapping the intracellular endogenous ALP via turned-on fluorescence imaging, as well as real-time monitoring the polarity fluctuation in cells through ratiometric fluorescence imaging. The present work offers a new way to design and synthesize functional CDs for direct imaging of intracellular enzymes.

Analyte-mediated formation and growth of nanoparticles for the development of chemical sensors and biosensors

The cornerstone of nanomaterial-based sensing systems is the synthesis of nanoparticles with appropriate surface functionalization that ensures their stability and determines their reactivity with organic or inorganic analytes. To accomplish these requirements, various compounds are used as additives or growth factors to regulate the properties of the synthesized nanoparticles and their reactivity with the target analytes. A different rationale is to use the target analytes as additives or growth agents to control the formation and properties of nanoparticles. The main difference is that the analyte recognition event occurs before or during the formation of nanoparticles and it is based on the reactivity of the analytes with the precursor materials of the nanoparticles (e.g., metal ions, reducing agents, and coatings). The transition from the ionic (or molecular) state of the precursor materials to ordered nanostructured assemblies is used for sensing and signal transduction for the qualitative detection and the quantitative determination of the target analytes, respectively. This review focuses on assays that are based on analyte-mediated regulation of nanoparticles' formation and differentiate them from standard nanoparticle-based assays which rely on pre-synthesized nanoparticles. Firstly, the principles of analyte-mediated nanomaterial sensors are described and then they are discussed with emphasis on the sensing strategies, the signal transduction mechanisms, and their applications. Finally, the main advantages, as well as the limitations of this approach, are discussed and compared with assays that rely on pre-synthesized nanoparticles in order to highlight the major advances accomplished with this type of nano-sensors and elucidate challenges and opportunities for further evolving new nano-sensing strategies.

A novel electrochemical platform for assay of alkaline phosphatase based on amifostine and ATRP signal amplification

Alkaline phosphatase (ALP), an important hydrolase involved in dephosphorylation, is a common clinical indicator of many diseases. In the present study, we constructed a novel electrochemical sensor using amifostine as the substrate of ALP and activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) as a signal amplification strategy for sensitive determination of ALP activity. In particular, in the presence of ALP, the phosphate group of amifostine was hydrolyzed to form a sulfhydryl group, which could attach to a gold electrode via a sulfur-gold bond. Then, the initiator α-bromophenylacetic acid (BPAA) was linked to the hydrolysis product of amifostine through an amide bond, resulting in the production of electroactive polymer chains on the gold electrode by the monomer ferrocenylmethyl methacrylate (FMMA) via ARGET ATRP. Under optimal parameters, the electrochemical sensor demonstrated a limit of detection (LOD) of 1.71 mU mL^-1 with a linear range of 5-100 mU mL^-1. In addition to satisfactory selectivity, the potential application of this approach for ALP activity detection in human serum samples was demonstrated. Due to its efficiency, simplicity of operation, and cost-effectiveness, the proposed electrochemical sensor has great promise as a universal method for ALP assays and inhibitor screening.

Recent Advancements in Developments of Novel Fluorescent Probes: In Cellulo Recognitions of Alkaline Phosphatases

Alkaline phosphatase (ALP) is one of the vital phospho-ester bond cleaving biocatalysts that has inevitable significance in cellular systems, viz., early-stage osteoblast differentiation, cell integrity in tissues, bone mineralization, cancer biomarker, liver dysfunction, cellular osmotic pressure, protein folding and many more. Variation from optimal levels of ALP in intra and extracellular fluids can cause severe diseases, including death. Due to these reasons, ALP is considered as a vital biomarker for various preclinical and medical diagnosis. Fluorescence image-based diagnosis is the most widely used method, owing to its simplicity, robustness, non-invasive properties and excellent spatio-temporal resolution (up to the nM/pM level), as compared to conventional analytical techniques, such as the electroanalytical method, nuclear magnetic resonance (NMR) and high-performance liquid chromatography (HPLC). Most of the reviews reported for ALP’s recognition in the literature scarcely explain the structurally related, photophysical and biophysical parameters; and the sub-cellular localizations. Considering these facts, in order to enhance the opto-analytical parameters of fluorescence-based diagnostic materials at the cellular level, herein we have systematically documented recent developments in the opto-analytical capabilities of quencher-free probes for ALP, used in in vitro (biological buffers) to in cellulo conditions, along with in vivo models.

Fast and sensitive near-infrared ratiometric fluorescent probe with a self-immolative spacer for imaging of endogenous alkaline phosphatase activity in cells and in vivo

Alkaline phosphatase (ALP), a vital hydrolase widely distributed in organisms, is regarded as a critical biomarker strongly associated with many physiological and pathological processes. Therefore, fast and efficient detection of ALP activity in vivo is of great value for clinical diagnosis. Herein, a novel near-infrared (NIR) ratiometric fluorescent probe (HP) was designed based on ESIPT for trapping ALP activity in cells and in vivo. Notably, incorporating a self-immolative spacer dramatically reduces the response time (8.5 min) of HP. Moreover, the probe exhibits excellent water solubility, large Stokes shift (147 nm), the ratiometric determination of ALP at 570 nm and 689 nm, low detection limit (3.98 U L^-1). More importantly, the probe was also successfully applied to detect and monitor variations in endogenous ALP activity in zebrafish due to the drug (APAP) induced organ damages.

A cationic AIEgen and hexametaphosphate based simple and convenient fluorometric assay for alkaline phosphatase and its inhibitor

Alkaline phosphatase (ALP) is an important biomarker to diagnose a number of diseases, such as anaemia, hepatobiliary diseases, chronic nephritis, and hypothyroidism. Therefore, the development of simple and convenient assays to monitor levels of ALP is highly desirable. In the present study, an aggregation-induced emission based simple, real-time, and direct fluorescence detection platform has been developed, by using a tetracationic pyridinium derivative of tetraphenylethylene (TPy-TPE) and anionic sodium hexametaphosphate (HMP) as component units. The sensing system, based on the TPy-TPE-HMP assembly, is highly responsive to the ALP dependent disintegration of the TPy-TPE-HMP aggregation complex, owing to HMP digestion by ALP. The sensing platform has an ALP detection limit of 16 mU mL^-1 and linear range of 0-742 mU mL^-1, respectively. The enzyme kinetic parameters, K_m and V_max, have been evaluated. In addition, the potential applicability of the TPy-TPE-HMP sensing system has also been shown with diluted human serum samples. Moreover, the TPy-TPE-HMP probe system is also useful for screening inhibitors of ALP.

Unnatural nucleotide-based rkDNA probe combined with graphene oxide for detection of alkaline phosphatase activity

In this study we developed a fluorescent double-stranded DNA, incorporating an unnatural dU^rk nucleotide, that we used as a probe for the detection of alkaline phosphatase (ALP) based on enzymatic cleavage of the non-fluorescent complementary strand. Primer extension performed using the unnatural nucleotide triphosphate dU^rkTP and the natural deoxynucleotide triphosphates dATP, dCTP, and dGTP provided a simple fluorescent DNA strand that hybridized with the 5́-monophosphate non-fluorescent complementary strand. When applying the 5́-phosphate recognition and cleavage properties of lambda exonuclease (λ-exo), this probe could bind to graphene oxide (GO) and quench the fluorescence (in the absence of ALP) or not bind to GO and retain its fluorescence (in the presence of ALP). We obtained strongly fluorescent DNA strands through simple incorporation of multiple A sites in the complementary sequence, thereby increasing the number of dU^rk residues during primer extension. This unnatural nucleotide-based rkDNA probing system exhibited high fluorescence differentiation for discriminating the status of ALP. This rkDNA-GO probing system appears to be a promising tool for monitoring the activity of disease-associated enzymes.

Single-Molecule Biosensing of Alkaline Phosphatase in Cells and Serum Based on Dephosphorylation-Triggered Catalytic Assembly and Disassembly of the Fluorescent DNA Chain

Alkaline phosphatase (ALP) is a valuable biomarker and effective therapeutic target for the diagnosis and treatment of diverse human diseases, including bone disorder, cardiovascular disease, and cancers. The reported ALP assays often suffer from laborious procedures, costly reagents, inadequate sensitivity, and large sample consumption. Herein, we report a new single-molecule fluorescent biosensor for the simple and ultrasensitive detection of ALP. In this assay, the ALP-catalyzed dephosphorylation of detection probe can protect the detection probe against lambda exonuclease-mediated digestion, and the remaining detection probes can trigger ceaseless hybridization between two Cy5-labeled hairpin probes through toehold-mediated DNA strand displacement, generating a long fluorescent DNA chain, which can be subsequently separated from unhybridized hairpin probes and disassembled into dispersed Cy5 moieties upon NaOH treatment. The free Cy5 moieties indicate the presence of ALP and can be directly quantified via single-molecule counting. This biosensor enables efficient amplification and transduction of the target ALP signal through enzyme-free assembly and disassembly processes, significantly simplifying the experimental procedure and improving the assay accuracy. The proposed biosensor allows specific and ultrasensitive detection of ALP activity with a detection limit down to 2.61 × 10^-6 U mL^-1 and is suitable for ALP inhibition assay and kinetic analysis. Moreover, this biosensor can be applied for endogenous ALP detection in human cells and clinical human serum, holding the potential in the ALP biological function study and clinical diagnosis.

Plasmonic Nanomaterials for Colorimetric Biosensing: A Review

In the last few decades, plasmonic colorimetric biosensors raised increasing interest in bioanalytics thanks to their cost-effectiveness, responsiveness, and simplicity as compared to conventional laboratory techniques. Potential high-throughput screening and easy-to-use assay procedures make them also suitable for realizing point of care devices. Nevertheless, several challenges such as fabrication complexity, laborious biofunctionalization, and poor sensitivity compromise their technological transfer from research laboratories to industry and, hence, still hamper their adoption on large-scale. However, newly-developing plasmonic colorimetric biosensors boast impressive sensing performance in terms of sensitivity, dynamic range, limit of detection, reliability, and specificity thereby continuously encouraging further researches. In this review, recently reported plasmonic colorimetric biosensors are discussed with a focus on the following categories: (i) on-platform-based (localized surface plasmon resonance, coupled plasmon resonance and surface lattice resonance); (ii) colloid aggregation-based (label-based and label free); (iii) colloid non-aggregation-based (nanozyme, etching-based and growth-based).

Colorimetric detection of alkaline phosphatase activity based on pyridoxal phosphate-induced chromatic switch of polydiacetylene nano-liposomes

A colorimetric assay based on polydiacetylenes (PDA) nano-liposomes is reported for facile and sensitive detection of alkaline phosphatase (ALP) activity. The critical basis of this method is that the interaction of pyridoxal phosphate (PLP) with nitrogenous group functionalized PDA nano-liposomes induces distinct blue-to-red color changes of PDA nano-liposomes. In the presence of ALP, as a nature substrate, PLP is enzymatically hydrolyzed to form pyridoxal, which cannot interact with PDA nano-liposomes. As a result, the concentration of PLP is reduced and the color change of PDA nano-liposomes is retarded, which is associated with ALP level. Under optimal conditions, the proposed method showed good linear relationship with ALP activity in the range 10-200 U/L with a limit of detection of 2.8 U/L. The detection process could be vividly observed with the naked eye. Additional attempts by using the method for the evaluation of inhibitor efficiency were also achieved with satisfying results. The method was further challenged with real human serum samples, showing consistent results when compared with a commercial standard assay kit. Such simple and easy-to-use approach may provide a new alternative for clinical and biological detection of ALP.

“Three-in-One” Nanocomposite as Multifunctional Nanozyme for Ultrasensitive Ratiometric Fluorescence Detection of Alkaline Phosphatase

Nanozymes, as a unique class of nanomaterials with enzyme-like properties, have attracted significant interests due to their potential applications in many significant fields. Great endeavours have been devoted to improving...

Midas Touch: Engineering Activity of Metal-Organic Frameworks via Coordination for Biosensing

The ever-increasing attention on the highly sensitive biosensors pushes people to explore functional nanomaterials for signal amplification. To endow inert metal-organic frameworks (MOFs) with enzyme mimicking activity, a simple strategy of introducing Cu²⁺ via coordination with 2,2'-bipyridine ligands of Zr-MOF, just like "Midas touch," is proposed. More details on the coordination environment of Cu active sites in Zr-MOF-Cu are disclosed via electron paramagnetic resonance and synchrotron-radiation-based X-ray absorption fine structure analyses. The as-prepared Zr-MOF-Cu exhibits unparalleled catalytic ability, which can catalyze ascorbic acid (AA) to dehydroascorbic acid and further stimulate the reaction with o-phenylenediamine to produce fluorescent signal probes with 8-fold signal amplification. On the basis of catalyzing the dephosphorylation process of l-ascorbic acid-2-phosphate to yield AA via alkaline phosphatase (ALP) and AA-dependent signal responses, a universal fluorescent system has been successfully constructed for quantitative measurement of the activity of ALP and the ALP-related enzyme-linked immunosorbent assay with carcinoembryonic antigen as a model. Moreover, the stable loading of Cu active sites endows the sensing platform with anti-inference capacity and enables its reuse without loss of catalytic activity after 6 months.

Sensitive ratiometric fluorescence probe based on chitosan carbon dots and calcein for Alkaline phosphatase detection and bioimaging in cancer cells

Alkaline phosphatase (ALP) is a commonly used marker in clinical practice, and this enzyme is a key indicator for diagnosing various diseases. In this study, we describe the development of a reliable and novel fluorescent assay for ALP detection based on chitosan carbon dots (C-CDs, peak emission, 412 nm) and calcein (peak emission, 512 nm). In the presence of Eu³⁺ (which binds calcein), the fluorescence intensity of calcein is quenched. Utilizing the ALP-triggered generation of phosphate ions (PO₄^3-) from the substrate p-nitrophenyl phosphate (pNPP), the Eu³⁺ ions bind PO₄^3- (which shows a higher affinity toward Eu³⁺ than calcein), and the fluorescence of calcein is recovered. As a consequence, C-CDs fluorescence is decreased by inner filter effect (IFE). Exploiting these changes in the fluorescence intensity ratio of C-CDs and calcein, we developed a high sensitivity, accurate, and easily synthesized ratiometric fluorescence probe. Our novel fluorescent bioassay demonstrates good linear relationship in the 0.09-0.8 mU mL^-1 range, with a low detection limit of 0.013 mU mL^-1. The excellent applicability of this novel assay in HepG2 cells and human serum samples demonstrates that our novel method has excellent biomedical research and disease diagnosis prospects.

Electrochemical detection of alkaline phosphatase activity via atom transfer radical polymerization

Alkaline phosphatase (ALP) activity is a diagnostic indicator for a variety of clinical diseases. In this study, an electrochemical method for detecting ALP activity through activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) was developed. Specifically, 3-mercaptopropionic (MPA) was firstly fixed on the electrode through sulfur-gold bonding. Subsequently, α-bromophenylacetic acid (BPAA) as initiator was attached to MPA through the recognized carboxylate-Zr⁴⁺-phosphate chemistry. Finally, in the existence of ALP, L-Ascorbic acid 2-phosphate sesquimagnesium salt hydrate (AAPS) was hydrolyzed to produce ascorbic acid (AA) which participated in the ARGET ATRP reaction, grafting polymer containing plenty of ferrocene electroactive probes on the surface of electrode. Under optimal experimental conditions, this method had a linear scope of 20-200 mU mL^-1, and a limit of detection (LOD) of 1.64 mU mL^-1. In addition, the proposed method had good selectivity as well as anti-interference capability, with satisfactory results in inhibition rate and human serum experiments. By merits of good analytical performance, easy operation, and low cost, such a method for ALP activity detection has promising applications in ALP-related disease detection and inhibitor screening.

Grafting of polymers via ring-opening polymerization for electrochemical assay of alkaline phosphatase activity

As an important hydrolytic enzyme, abnormal activity of alkaline phosphatase (ALP) is closely associated with a variety of diseases. It has been identified as an important diagnostic indicator for clinical hepatobiliary and bone diseases. Herein, a novel electrochemical sensor based on signal amplification strategy through ring-opening polymerization (ROP) has been developed to assay of ALP activity. First of all, 3-mercaptopropanoic acid (MPA) was employed as a cross-linking agent to attach O-phosphoethanolamine to the electrode surface via amide bond. Then, ALP catalyzed the hydrolysis of phosphate monoester structures to hydroxyl groups, which could initiate ROP reaction. The polymer grafted on the electrode surface contains a large number of ferrocene electroactive molecules, which effectively increased the signal output of the electrochemical sensor and improved the sensitivity of ALP activity detection. Under optimum conditions, this electrochemical sensor rendered a satisfactory linear dependence over the range from 20 to 120 mU mL^-1, with a low detection limit of 0.66 mU mL^-1. Furthermore, this strategy presented satisfactory selectivity and interference resistance in human serum sample, and compared with clinical data, the relative error of the results obtained by this method was less than 5%. Thus, this method showed considerable potential for the detection of ALP activity in clinical application.

Highly Sensitive Alkaline Phosphatase Biosensor Based on Internal Filtration Effect between G-Quadruplex/N-methylmesoporphyrin IX and p-Nitrophenol

In this work, an alkaline phosphatase (ALP) biosensor was established based on G-quadruplex/N-methylmesoporphyrin IX (G4/NMM) and p-nitrophenol (PNP). Because the absorption of PNP was close to the excitation wavelength of G4/NMM, PNP could reduce the fluorescence of G4/NMM. Meanwhile, PNP was the hydrolysis product of p-nitrophenylphosphate (PNPP) by ALP. Therefore, ALP could be detected. This ALP biosensor had a linear analytical range from 2.5 to 25 U/L a the detection limit of 0.81 U/L. Moreover, it showed a satisfactory selectivity and recovery rates.

A novel sensing platform for the determination of alkaline phosphatase based on SERS-fluorescent dual-mode signals

Alkaline phosphatase (ALP), as an important biomarker, is closely associated with various diseases. Multi-mode sensing platforms can combine the advantages of different technologies and solve their inherent or practical limitations. Herein, we developed a sensing platform for the determination of alkaline phosphatase (ALP) in human serum based on SERS-fluorescent dual-mode assay. Based on the fact that ALP can trigger the in-situ reaction between o-phenylenediamine (OPD) and ascorbic acid (AA), we connected gold nanoparticles (AuNPs) to 3,4-diaminobenzene-thiol (OPD(SH)) through an Au-S covalent bond to synthesize a nanoprobe (OPD(S)-AuNPs). The nanoprobe provides a unique interactive ammonium group for the diol group of AA, which was then used to generate an N-heterocyclic compound that can exhibit good SERS and fluorescence signals without adding SERS reporter and fluorophores or quantum dots (QDs). When being excited at different wavelengths as 360 nm and 785 nm, the fluorescence and SERS signals can be separately generated, which can avoid the disturbance from each other. The response of the fluorescence system was linear from 1.0 to 20 mU mL^-1 (R² = 0.994) with a detection limit of 0.3 mU mL^-1, while that of the SERS system was linear from 0.5 to 10 mU mL^-1 (R² = 0.998) with a detection limit of 0.2 mU mL^-1. The sensing platform developed was further employed in ALP inhibitor evaluation.

One-component nano-metal-organic frameworks with superior multienzyme-mimic activities for 1,4-dihydropyridine metabolism

Although a number of nanozymes have been developed, it is still difficult to develop single-component nanozyme with overall high multienzyme-like activities. In this study, the nanosized metal-organic frameworks (nano-MOFs) FePCN (PCN stands for porous coordination network) was synthesized by integrating zirconium and iron ions with different catalytic property on single-component MOFs and exhibited superior intrinsic multienzyme-like activities, namely oxidase-, peroxidase- and phosphatase-mimicking activity. The catalytic active sites of oxidase- and peroxidase-, and phosphatase-like activity of FePCN were Fe-centers and Zr-O clusters, respectively. Based on the intrinsic oxidase-like activity and the similarity of molecular structures between cytochrome P450 oxidase (CYP) cofactors and the organic linker in FePCN, FePCN exhibited high CYP-like activity to catalyze the oxidation of hypotensive drug 1,4-dihydropyridine (1,4-DHP) into diethyl 2,6-dimethylpyridine-3,5-dicarboxylate (DDPD) and the yield of DDPD reached over 80%. Moreover, as peroxidase- and phosphatase-mimics, FePCN was successfully applied to detecting H₂O₂ under neutral condition and catalyzing the dephosphorylation of adenosine triphosphate (ATP), respectively. This study provides a feasible way for rational design one-component nanomaterials as multienzyme-mimics.

Alkaline Phosphatase-Triggered Etching of Au@FeOOH Nanoparticles for Enzyme Level Assay under Dark-Field Microscopy

In clinical diagnosis, the level of biological enzymes in serum has been generally regarded as markers of human diseases. In this work, a kind of simple and sensitive plasmonic probe (indicated as Au@FeOOH) has been synthesized with the guidance of plasmonic imaging and subsequently developed for the alkaline phosphatase (ALP) level detection under dark-field microscopy (DFM). As a kind of hydrolysis enzyme, ALP can promote the hydrolysis of l-ascorbic acid 2-phosphate to ascorbic acid (AA). AA further acts as a strong reduction reagent for the decomposition of the FeOOH shell, which results in a blue shift of localized surface plasmon resonance spectra and an obvious color change under DFM. RGB analyses show that using a ΔR/G value instead of scattering wavelength or R/G value as the analytical signal, the deviation attributed to the size distribution of the initial Au NPs is greatly suppressed, and a linear range from 0.2 to 6.0 U/L (R² = 0.99) and a limit of detection of 0.06 U/L are acquired with various concentrations of ALP during the detection. Besides, this approach exhibits excellent selectivity in complex biological serum samples, which is expected to be applied for the early diagnosis of clinical diseases by monitoring various biomarkers in the future.

Alkaline phosphatase-triggered self-assembly of near-infrared nanoparticles for the enhanced photoacoustic imaging of tumors

In this chapter, we discuss the need for the development of enzyme-activatable probes in the field of tumor-targeted photoacoustic (PA) imaging, then we give a brief description of the innovation of designing alkaline phosphatase (ALP)-activatable probes for PA imaging. After that, we provide detailed protocols for the syntheses and characterizations of a near-infrared photoacoustic imaging probe, 1P, developed in our research group. With this tool, 1P could form nanoparticles 1-NPs under the catalysis of ALP and thus could be used to enhance PA imaging both in vitro and in vivo.

Tunable Dual-Effector Allostery System for Nucleic Acid Analysis with Enhanced Sensitivity and an Extended Dynamic Range

In the last few years, studies have demonstrated the existence of dual-effector allosteric cooperativity in nature and the mechanism underlying enhanced activation/inhibition performance. In this work, we design an artificial dual-effector allostery system for the construction of a dynamic biosensor that can achieve nucleic acid detection with superior sensitivity and across an extraordinary broad detection range. Our dual-effector allostery-regulated biosensor is based on the multibranched hybridization chain reaction (mHCR) involving three hairpins (H1, H2, and H3). In the presence of the target nucleic acid, the mHCR is initiated via cascading strand displacement events. The products of mHCR are then captured on the electrode surface based on the mechanism of the multivalent proximity ligation assay (mPLA) and the multivalent binding assay (mBA). The subsequent conjugation of streptavidin-modified horseradish peroxidase (SA-HRP) can lead to an increase in the electrochemical signal. Importantly, two distinct allosteric activation sites and two distinct allosteric inhibition sites in H1 are designed to fine-tune the nucleic acid detection sensitivity and the dynamic range. Using this new dual-effector allostery tool, we report the detection of nucleic acid at a dynamic range spanning 10-10¹² aM, 11 orders of magnitude showing the broadest dynamic range reported to date with an allosteric regulation biosensor construct.

Restoring the Oxidase-Like Activity of His@AuNCs for the Determination of Alkaline Phosphatase

In this paper, we propose a simple colorimetric method for the sensitive and selective detection of alkaline phosphatase (ALP) activity based on the turn off/turn on oxidase mimic activity of His@AuNCs. His@AuNCs/graphene oxide hybrids (His@AuNCs/GO) were easily obtained using the self-assembly method with poly (diallyldimethylammonium chloride) (PDDA)-coated GO and showed high oxidase-like activity compared with His@AuNCs. We found that the pyrophosphate ion (P₂O₇^4-, PPi) could effectively inhibit the oxidase mimic activity of His@AuNCs/GO, and the hydrolysis of PPi by ALP restored the inhibited activity of His@AuNCs/GO, enabling them to efficiently catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to generate the blue oxidized product oxTMB. The intensity of the color showed a linear dependency with the ALP activity. ALP was detected in the linear range of 0-40 mU/mL with a low detection limit (LOD) of 0.26 mU/mL (S/N = 3). The proposed method is fast, easy, and can be applied to monitor the ALP activity in serum samples accurately and effectively, which suggests its practicability and reliability in the detection of ALP activity in clinical practice.

An Exonuclease I-Aided Turn-Off Fluorescent Strategy for Alkaline Phosphatase Assay Based on Terminal Protection and Copper Nanoparticles

As an important DNA 3'-phosphatase, alkaline phosphatase can repair damaged DNA caused by replication and recombination. It is essential to measure the level of alkaline phosphatase to indicate some potential diseases, such as cancer, related to alkaline phosphatase. Here, we designed a simple and fast method to detect alkaline phosphatase quantitively. When alkaline phosphatase is present, the resulting poly T-DNA with a 3'-hydroxyl end was cleaved by exonuclease I, prohibiting the formation of fluorescent copper nanoparticles. However, the fluorescent copper nanoparticles can be monitored with the absence of alkaline phosphatase. Hence, we can detect alkaline phosphatase with this turn-off strategy. The proposed method is able to quantify the concentration of alkaline phosphatase with the LOD of 0.0098 U/L. Furthermore, we utilized this method to measure the effects of inhibitor Na₃VO₄ on alkaline phosphatase. In addition, it was successfully applied to quantify the level of alkaline phosphatase in human serum. The proposed strategy is sensitive, selective, cost effective, and timesaving, having a great potential to detect alkaline phosphatase quantitatively in clinical diagnosis.

Study of the interaction mechanism between hydrophilic thiol capped gold nanoparticles and melamine in aqueous medium

In the last years, intense efforts have been made in order to obtain colloidal-based systems capable of pointing out the presence of melamine in food samples. In this work, we reported about the recognition of melamine in aqueous solution, using gold nanoparticles stabilized with 3-mercapto-1-propanesulfonate (AuNPs-3MPS), with the aim of deepening how the recognition process works. AuNPs were synthesized using a wet chemical reduction method. The synthesized AuNPs-3MPS probe was fully characterized, before and after the recognition process, by both physicochemical (UV-vis, FT-IR, ¹H-NMR, DLS and ζ-potential) and morphostructural techniques (AFM, HR-TEM). The chemical and electronic structure was also investigated by SR-XPS. The sensing method is based on the melamine-induced aggregation of AuNPs; the presence of melamine was successfully detected in the range of 2.5-500 ppm. The results achieved also demonstrate that negatively charged AuNPs-3MPS are potentially useful for determining melamine contents in aqueous solution. SR-XPS measurements allowed to understand interaction mechanism between the probe and the analyte. The presence of sulfonate groups allows a mutual interaction mediated by electrostatic bonds between nanoparticles surface thiols and positively charged amino groups of melamine molecules.

Electrochemical detection of alkaline phosphatase activity through enzyme-catalyzed reaction using aminoferrocene as an electroactive probe

As a nonspecific phosphomonoesterase, alkaline phosphatase (ALP) plays a pivotal role in tissue mineralization and osteogenesis which is an important biomarker for the clinical diagnosis of bone and hepatobiliary diseases. Herein, we described a novel electrochemical method that used aminoferrocene (AFC) as an electroactive probe for the ALP activity detection. In the condition with imidazole and N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC), the AFC probe could be directly labeled on single-stranded DNA (ssDNA) by one-step conjugation. Specifically, thiolated ssDNA at 3'-terminals was modified to the electrode surface through Au-S bond. In the condition without ALP, AFC could be labeled on ssDNA by conjugating with phosphate groups. In the presence of ALP, phosphate groups were catalyzed to be removed from the 5'-terminal of ssDNA. The AFC probe cannot be labeled on ssDNA. Thus, the electrochemical detection of ALP activity was achieved. Under optimal conditions, the strategy presented a good linear relationship between current intensity and ALP concentration in the range of 20 to 100 mU/mL with the limit of detection (LOD) of 1.48 mU/mL. More importantly, the approach rendered high selectivity and satisfactory applicability for ALP activity detection. In addition, this method has merits of ease of operation, low cost, and environmental friendliness. Thus, this strategy presents great potential for ALP activity detection in practical applications. An easy, sensitive and reliable strategy was developed for the detection of alkaline phosphatase activity via electrochemical "Signal off".

A turn-on near-infrared fluorescent probe for visualization of endogenous alkaline phosphatase activity in living cells and zebrafish

Alkaline phosphatase (ALP) is an essential hydrolase and widely distributed in living organisms. It plays important roles in various physiological and pathological processes. Herein, a turn-on near-infrared (NIR) fluorescent probe (DXMP) was developed for sensitive detection of ALP activity both in vitro and in vivo based on the intramolecular charge transfer (ICT) mechanism. Upon incubation with ALP, DXMP exhibited a strong fluorescence increment at 640 nm, which was attributed to the fact that ALP-catalyzed cleavage of the phosphate group in DXMP induced the transformation of DXMP into DXM-OH. The probe exhibited prominent features including outstanding selectivity, high sensitivity, and excellent biocompatibility. More importantly, it has been successfully used to detect and image endogenous ALP in living cells and zebrafish.

Visual detection of alkaline phosphatase based on ascorbic acid-triggered gel-sol transition of alginate hydrogel

Stimuli-responsive hydrogel has been emerged as a popular tool for chemical sensing due to its unique mechanical properties. In this work, we fabricated an ascorbic acid (AA)-responsive alginate hydrogel for the visual detection of alkaline phosphatase (ALP). This alginate hydrogel (RhB@Alg/Fe³⁺) was crosslinked with Fe³⁺, and rhodamine B (RhB) was encapsulated into the hydrogel as an indicating reagent to assistant visual detection. Because of the weak affinity of Fe²⁺ to alginate, the presence of reductive AA can trigger the dissolution of RhB@Alg/Fe³⁺ to give an observable red color in the sol solution. On this basis, by using ascorbic acid 2-phosphate as a substrate of ALP, which can be hydrolyzed by ALP to produce AA, the gel-sol transition process of RhB@Alg/Fe³⁺ was further modulated by ALP. This finding leads to a simple visual method for ALP detection with a low detection limit of 0.37 mU/mL and an excellent selectivity over other proteins. Compared with conventional colorimetric assays, this visual sensor shows the distinct advantages of simple fabrication, cost-effectiveness and easy to implement. We believe that this study can provide a new insight into the fabrication of responsive alginate hydrogel for promising applications in chemical sensing and biomedical fields.

Real-time imaging of alkaline phosphatase activity of diabetes in mice via a near-infrared fluorescent probe

A novel water-soluble near-infrared fluorescent probe named QX-P with simple synthesis is developed for detecting ALP. The probe can not only visualize ALP activity in four cell lines, but also real-time image ALP activity of diabetes in mice.

Applications of fluorescent materials in the detection of alkaline phosphatase activity

Alkaline phosphatase (ALP) is important in the diagnosis of many diseases. Because ALP is used to detect biomarkers for many diseases, many researchers conduct investigations to develop ALP detection strategies. The use of fluorescent material has attracted attention because of the technique's high sensitivity and the low sample volume required. Herein, we review and discuss the working mechanisms and advantages of four main categories:DNA fluorescent probes, molecular fluorescent probes, chemical coordination-based probes, and nanoparticle probes. Development prospects and trends are also discussed.

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.

Redox-modulated colorimetric detection of ascorbic acid and alkaline phosphatase activity with gold nanoparticles

Gold nanoparticles (AuNPs) exhibit characteristic absorption peaks in the ultraviolet visible region due to their special surface plasmon resonance effect. This characteristic absorption peak would change with the relative colour varying from wine red to orange-yellow upon sequential addition of ascorbic acid (AA) into the mixture of AuNPs and Ag(I). Similar observations also could be found when the hydrolysis product of sodium l-ascorbyl-2-phosphate with alkaline phosphatase (ALP) was used as an alternative to AA. Results of structure characterization confirmed that the phenomena were due to the reduction of Ag(I) to Ag(0) on the surface of AuNPs and the formation of core-shell AuNPs@Ag. Therefore, a colorimetric assay for rapid visual detection of AA and ALP based on redox-modulated silver deposition on AuNPs has been proposed. Under the optimal experimental conditions, the absorbance variation ΔA_{522 nm} /A_{370 nm} of AuNPs was proportional to the concentration of AA (5-60 μmol/L) and ALP (3-18 U/L) with the corresponding detection limit of 2.44 μmol/L for AA and 0.52 U/L for ALP. The assay showed excellent selectivity towards AA and ALP. Moreover, the assay has been applied to detect AA and ALP activity in real samples with satisfying results.

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.

Fluorometric determination of the activity of alkaline phosphatase based on a system composed of WS2 quantum dots and MnO2 nanosheets

A fluorometric method is described for the detection of alkaline phosphatase (ALP) activity. It is based on the use of the product of hydrolysis of the drug amifostine (a thiophosphoester) by ALP. It is known that MnO₂ nanosheets quench the blue fluorescence of tungsten disulfide quantum dots (WS₂ QDs) which have excitation/emission wavelengths of 320/448 nm. However, in the presence of ALP and amifostine, the product of hydrolysis [2-(3-aminopropylamino)ethanethiol] triggers the decomposition of the MnO₂ nanosheets. This results in the recovery of fluorescence. Based on this finding, an assay for ALP activity was developed that works in the 0.09-1.6 U L^-1 range, with a 40 mU L^-1 detection limit. The relative standard deviation is 1.87% for five repeated measurements of 0.8 U L^-1 ALP. The method was applied to the analysis of ALP in real samples and gave satifactory results. Graphical abstractSchematic representation of a fluorometric method for determination of the activity of alkaline phosphatase (ALP). The fluorescence of a system composed of WS₂ quantum dots and MnO₂ nanosheets is quenched. Hydrolysis of the cytoprotective adjuvant amifostine (a phosphothioester) by ALP leads to a thiol that causes the decomposition of the MnO2 nanosheets. As a result, the blue fluorescence of the system becomes increasingly restored.

Fluorescent-Magnetic-Catalytic Nanospheres for Dual-Modality Detection of H9N2 Avian Influenza Virus

The outbreak of H9N2 avian influenza virus (H9N2 AIV) brings high mortality and huge economic losses every year. Sensitive and reliable detection methods are essential to timely diagnosis and treatment. Herein, a dual-modality immunoassay is proposed for H9N2 AIV detection by employing fluorescent-magnetic-catalytic nanospheres (FMCNs) as labels and alkaline phosphatase (ALP)-induced metallization as a signal amplification strategy. The excellent magnetic properties of FMCNs endow the assay a potential application in complex samples. Also, the excellent fluorescence properties of FMCNs enable fluorescence modality readout. The antibodies on the FMCN surface can achieve efficient capture and separation of targets. Amplified electrochemical modality readout can be obtained through ALP-catalyzed silver deposition. Dual-modality immunoassay combined the advantages of electrochemical assay with fluorescence assay and provides accurate detection results to meet different testing needs. With two quantitative analysis forms, H9N2 AIV can be detected by electrochemical signals with a quantitation range of 0.1 to 1000 ng/mL and a detection limit of 10 pg/mL. The linear range is 300 to 1000 ng/mL with a detection limit of 69.8 ng/mL by the fluorescence signal readout. Moreover, the specificity, anti-interference ability, accuracy, and diversity of the proposal have unlimited potential for early diagnosis of suspect infections.

Ratiometric detection of alkaline phosphatase based on aggregation-induced emission enhancement

Alkaline phosphatase (ALP) is an important enzyme that is associated with many human diseases, so the quantitative detection of ALP is vital from a clinical perspective. Nevertheless, most fluorescent assays for monitoring ALP depend on aggregation-induced quenching (ACQ), single-signal modulation, or a "signal off" mode, which suffer from poor sensitivity, a "false positive" problem, and low signal output. In this work, we utilized the electrostatically driven self-assembly of glutathione-capped gold nanoclusters (GSH-AuNCs, which show aggregation-induced emission, AIE) and amino-modified silicon nanoparticles (SiNPs) to create a hybrid probe (SiNPs@GSH-AuNCs). This nanohybrid probe showed emission from the SiNPs at around 470 nm as well as aggregation-induced emission enhancement (AIEE) of the GSH-AuNCs at 580 nm. The AIEE of the GSH-AuNCs was quenched in the presence of KMnO₄, but the AIEE was recovered by adding ascorbic acid as an oxidation-reduction reaction occurred between KMnO₄ and the ascorbic acid. The fluorescence of the SiNPs remained constant whether the AIEE was quenched or not, meaning that the fluorescence of the SiNPs could be used as an internal reference. In a typical enzymatic reaction, ascorbic acid 2-phosphate is hydrolyzed by ALP to produce ascorbic acid. Therefore, the hybrid probe was shown to allow the ratiometric detection of ALP, with a linear range of 0.5-10 U L^-1 and a limit of detection (LOD) of 0.23 U L^-1. Finally, the proposed analytical strategy was successfully applied to detect ALP in human serum samples and to determine the concentration of an ALP inhibitor. Graphical Abstract.