Colorimetric determination of the activity of alkaline phosphatase by exploiting the oxidase-like activity of palladium cube@CeO2 core-shell nanoparticles

Pyrroloquinoline quinone exhibiting peroxidase-mimicking property for colorimetric assays

BACKGROUND

Small molecule mimics offer the advantages of easy preparation, good thermodynamic stability, and reproducible catalytic activity. However, most of the reported organic artificial mimics face challenges including low catalytic activity, oxidative self-destruction, and auto-aggregation into inactive dimers. Therefore, novel organic mimics with high catalytic activity as well as good thermal and environmental stability are highly desirable.

RESULTS

We found that pyrroloquinoline quinone (PQQ), a nutritionally important growth factor, displayed significant peroxidase-like activity to catalyze the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue oxidized TMB (oxTMB) by H2O2. Its catalytic activity surpassed that of other organic small molecules reported previously, including fluorescein derivatives and adenosine/guanosine triphosphate. Based on the reduction of oxTMB by the enzymatically generated ascorbic acid (AA), the PQQ/TMB-based system was used to monitor alkaline phosphatase (ALP) activity with ascorbic acid 2-phosphate (AAP) as the substrate. This sensing system allowed for the determination of ALP with a detection limit of 1 mU/mL. Furthermore, we discovered that PQQ can coordinate with Cu2+ to form metal-organic nanoparticles. The building blocks of two active cofactors endowed the nanoparticles with impressive functionalities for biocatalytic applications. With Cu-PQQ nanoparticles as signal labels, colorimetric immunoassays of prostate specific antigen (PSA) were achieved with a lowest detectable concentration down to 0.1 pg/mL.

SIGNIFICANCE

This work is valuable for the design of novel biosensors by utilizing PQQ as an artificial enzyme because of its high catalytic activity, good stability, ease of storage, and simple chemical structure.

Application of gold nanoclusters in fluorescence sensing and biological detection

Gold nanoclusters (Au NCs) exhibit broad fluorescent spectra from visible to near-infrared regions and good enzyme-mimicking catalytic activities. Combined with excellent stability and exceptional biocompatibility, the Au NCs have been widely exploited in biomedicine such as biocatalysis and bioimaging. Especially, the long fluorescence lifetime and large Stokes shift attribute Au NCs to good probes for fluorescence sensing and biological detection. In this review, we systematically summarized the molecular structure and fluorescence properties of Au NCs and highlighted the advances in fluorescence sensing and biological detection. The Au NCs display high sensitivity and specificity in detecting iodine ions, metal ions, and reactive oxygen species, as well as certain diseases based on the fluorescence activities of Au NCs. We also proposed several points to improve the practicability and accelerate the clinical translation of the Au NCs.

Construction of the fluorescence sensing platform with a bifunctional Cu@MOF nanozyme for determination of alkaline phosphatase and its inhibitor

In this work, a novel and sensitive fluorescence sensing system for alkaline phosphatase (ALP) was constructed using a bifunctional copper metal-organic framework (Cu@MOF) nanozyme, which had excellent oxidase-mimetic activity and fluorescence properties. Owing to the presence of 2-amino-1,4-benzenedicarboxylic acid (1,4-BDC-NH2) ligand, Cu@MOF displays excellent fluorescence performance at 444 nm. Additionally, Cu2+ endows the oxidase-like activity of Cu@MOF, which could trigger p-phenylenediamine (PPD) to be oxidized to a brown product (PPDox) and quench the photoluminescence of Cu@MOF through the inner filtration effect (IFE). As the preferential affinity of ATP for Cu2+, the catalytic activity of Cu@MOF was significantly reduced once ATP was added, thus PPD could not be oxidized and fluorescence was recovered. In the presence of ALP, ATP was hydrolyzed to adenosine and Pi, which allowed Cu@MOF to regain its catalytic activity and continued to catalyze the generation of PPDox. The fluorescence of Cu@MOF was therefore weakened once again. The ALP activity was directly proportional to the degree of decrease in fluorescence intensity. Thus, this novel fluorescence sensing strategy had a linear range of 0.5-60 U/L and the limit of detection was 0.14 U/L. The established sensing method could also be used to for ALP inhibitors screening, and achieved satisfactory results in determining the level of ALP activity in human serum.

Construct a Magnetic Pt/Ru Alloy Peroxidase Mimic As a Reusable and Cost-Effective "Signal-Off" Sensing Platform for Sensitive and Wide-Linear-Range Assay

"Signal-off" nanozyme sensing platforms are usually employed to detect analytes (e.g., ascorbic acid (AA) and alkaline phosphatase (ALP)), which are mostly based on oxidase (OXD) nanozymes. However, their drawbacks, like dissolved oxygen-dependent catalysis capability, relatively low enzyme activity, limited amount, and kind, may not favor sensing platforms' optimization. Meanwhile, with the need for sustainable development, a reusable "signal-off" sensing platform is essential for cutting down the cost of the assay, but it is rarely developed in previous studies. Magnetic peroxidase (POD) nanozymes potentially make up the deficiencies and become reusable and better "signal-off" sensing platforms. As a proof of concept, we first construct Fe3O4@polydopamine-supported Pt/Ru alloy nanoparticles (IOP@Pt/Ru) without stabilizers. IOP@Pt/Ru shows high POD activity with Vmax of 83.24 × 10-8 M·s-1 for 3,3',5,5'-Tetramethylbenzidine (TMB) oxidation. Meanwhile, its oxidation rate for TMB is slower than the reduction of oxidized TMB by reducers, favorable for a more significant detection signal. On the other hand, IOP@Pt/Ru possesses great magnet-responsive capability, making itself be recycled and reused for at least 15-round catalysis. When applying IOP@Pt/Ru for AA (ALP) detection, it performs better detectable adaptability, with a linear range of 0.01-0.2 mM (0.1-100 U/L) and a limit of detection of 0.01 mM (0.05 U/L), superior to most of OXD nanozyme-based ALP sensing platform. Finally, IOP@Pt/Ru's reusable assay was demonstrated in real blood samples for ALP assay, which has never been explored in previous studies. Overall, this study develops a reusable "signal-off" nanozyme sensing platform with superior assay capabilities than traditional OXD nanozymes, paves a new way to optimize nanozyme-based "signal-off" sensing platforms, and provides an idea for constructing inexpensive and sustainable sensing platforms.

Ultrasensitive determination of α-glucosidase activity using CoOOH nanozymes and its application to inhibitor screening

In this work, a novel method for the colorimetric sensing of α-glucosidase (α-Glu) activity was developed based on CoOOH nanoflakes (NFs), which exhibit efficient oxidase-mimicking activity. Colorless 3,3',5,5'-tetramethylbenzidine (TMB) can be oxidized by CoOOH NFs into blue-colored oxidized TMB (oxTMB) in the absence of H₂O₂. L-Ascorbic acid-2-O-α-D-glucopyranose (AAG) can be hydrolysed by α-glucosidase to produce ascorbic acid, resulting in a significant decrease of catalytic activity of CoOOH NFs. Thus, a colorimetric α-glucosidase activity detection method was designed with a limit of detection of 0.0048 U mL^-1. Furthermore, the designed sensing platform exhibits favorable applicability for the α-glucosidase (α-Glu) activity assay in real samples. Meanwhile, this method can be expanded to study the inhibitors of α-Glu. Finally, the as-proposed method combined with a smartphone would be a color recognizer, which was successfully applied for the determination of α-Glu activity in human serum samples.

CeO2–Co3O4 nanocomposite with oxidase-like activity for colorimetric detection of ascorbic acid†

A CeO₂–Co₃O₄ nanocomposite (NC) was prepared and characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The obtained CeO₂–Co₃O₄ NC displayed biomimicking oxidase-like activity, which can catalytically oxidize the 3, 3′, 5, 5′-tetramethylbenzidine (TMB) substrate from colorless to the blue oxidized TMB (ox-TMB) product with a characteristic absorption peak at 652 nm. When ascorbic acid (AA) was present, ox-TMB would be reduced, resulting in a lighter blue and lower absorbance. On the basis of these facts, a simple colorimetric method for detection of AA was established with a linear relationship ranging from 1.0 to 500 μM and a detection limit of 0.25 μM. When this method was used to detect AA in human serum and commercially available vitamin C tablet samples, a good recovery of 92.0% to 109.0% was obtained. Besides, the catalytic oxidation mechanism was investigated, and the possible catalytic mechanism of CeO₂–Co₃O₄ NC can be described as follows. TMB is adsorbed on the CeO₂–Co₃O₄ NC surface and provides lone-pair electrons to the CeO₂–Co₃O₄ NC, leading to an increase in electron density of the CeO₂–Co₃O₄ NC. An increased electron density can improve the electron transfer rate between TMB and the oxygen absorbed on its surface to generate O₂˙⁻ and ˙O₂, which further oxidize TMB.

A CeO₂–Co₃O₄ nanocomposite (NC) was prepared and characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction.

Recent Advances in Construction and Application of Metal-Nanozymes in Pharmaceutical Analysis

Nanozymes, made of emerging nanomaterials, have similar activity to natural enzyme and exhibit promising applications in in the fields of environment, biology and medicine, and food safety science. In recent years, with the deep finding and research to nanozymes by researchers, its application in field of pharmaceutical analysis has emerged gradually, possessing great significance in drug safety evaluation and quality control. This review summarizes the construction of metal nanozymes, strategies to improve their performance and their application in pharmaceutical detection and analysis, especially in detection of target analytes consisting of small molecule medicine macromolecule, toxic and others, which proposes theoretical foundation for development of nanozymes in this field. At the same time, it also provides opportunities and challenges for the construction and application of new nanozymes.

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.

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.

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.

Dual-readout performance of Eu3+-doped nanoceria as a phosphatase mimic for degradation and detection of organophosphate

Eu³⁺-Doped nanoceria (Eu:CeO₂) with self-integrated catalytic and luminescence sensing functions was synthesized by a simple and gentle one-pot method to build a dual-readout nanozyme platform for organophosphate compound (OPC) sensing in this work. The catalytic degradation of the model substrate of OPC, p-nitrophenyl phosphate (p-NPP), by as-prepared Eu:CeO₂ can be completed in 2 min with little influence of temperature and pH values, highlighting the advantages of Eu:CeO₂ as an artificial enzyme for dephosphorylation. Most importantly, the characteristic red emission of Eu³⁺ (592 nm) from Eu:CeO₂ can be quenched by p-NPP, accompanied by a color change from colorless to yellow. Based on this, linear ranges of 4-50 μM with a detection limit of 3.3 μM and 1-20 μM with a detection limit of 0.6 μM for p-NPP were obtained by colorimetric and fluorescence methods, respectively. Furthermore, the fluorescence strategy was effectively applied to the determination of ethyl para-nitrophenyl (EPN), one of the most commonly used pesticides, with a detection limit of 5.86 μM. The proposed strategy was also successfully applied to the assay of p-NPP and EPN in real water samples, showing great application prospects in detecting OPC in the environment.

A Review on Metal- and Metal Oxide-Based Nanozymes: Properties, Mechanisms, and Applications

Since the ferromagnetic (Fe3O4) nanoparticles were firstly reported to exert enzyme-like activity in 2007, extensive research progress in nanozymes has been made with deep investigation of diverse nanozymes and rapid development of related nanotechnologies. As promising alternatives for natural enzymes, nanozymes have broadened the way toward clinical medicine, food safety, environmental monitoring, and chemical production. The past decade has witnessed the rapid development of metal- and metal oxide-based nanozymes owing to their remarkable physicochemical properties in parallel with low cost, high stability, and easy storage. It is widely known that the deep study of catalytic activities and mechanism sheds significant influence on the applications of nanozymes. This review digs into the characteristics and intrinsic properties of metal- and metal oxide-based nanozymes, especially emphasizing their catalytic mechanism and recent applications in biological analysis, relieving inflammation, antibacterial, and cancer therapy. We also conclude the present challenges and provide insights into the future research of nanozymes constituted of metal and metal oxide nanomaterials.

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.

A simple enzyme-catalyzed reaction induced "switch" type fluorescence biosensor based on carbon nitride nanosheets for the assay of alkaline phosphatase activity

An enzyme-catalyzed fluorescence "switch" type sensor was constructed for the determination of alkaline phosphatase (ALP) activity by combining the fluorescence quenching effect of Ag+ on ultrathin g-C3N4 nanosheets (CNNSs) with the simple redox reaction of AA and Ag+. Briefly, Ag+ exhibits a significant quenching effect on the fluorescence of CNNSs. Thus the fluorescence signal of the CNNS-Ag+ system is extremely weak even in the presence of l-ascorbic acid-2-phosphate (AAP) ("off" state). When ALP coexists in the system, the enzyme can specifically catalyze the hydrolysis of AAP to form ascorbic acid (AA), which reduces Ag+ to Ag0. In this case, the fluorescence signal of the system is recovered ("on" state). Based on this principle, a signal-enhanced CNNS fluorescence sensor was developed to determine the activity of alkaline phosphatase. The experimental results show that the detection range of alkaline phosphatase is 0.5-20 U L-1, and the detection limit is 0.05 U L-1 (S/N = 3). Meanwhile, this method was used to assay ALP in serum samples.

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".

Synergistic effects between polyvinylpyrrolidone and oxygen vacancies on improving the oxidase-mimetic activity of flower-like CeO2 nanozymes

Both oxygen vacancies and surface chemistry can affect the enzyme-like catalytic activities of CeO2-based nanozymes. However, the mechanism of the enzyme-mimetic process is not yet clearly elucidated, which is of great importance to guide the synthesis of high-performance nanozymes with desirable properties. Herein, we report a facile one-pot solvothermal method for the preparation of polyvinylpyrrolidone (PVP)-capped CeO2 nanoflowers with adjustable oxygen vacancies by changing appropriate solvothermal reaction parameters. Oxygen vacancies effectively increase under a higher precursor concentration, extended solvothermal time, and proper reaction temperature. The maximum content of surface Ce(iii) cations is up to 50% for 31.1 nm CeO2 nanoflowers, which exhibit 0.07 mM apparent Michaelis constant towards 3,3',5,5'-tetramethylbenanozymeidine and show a higher binding affinity than the other CeO2-based catalysts. Theoretical results indicate that the synergy between PVP and oxygen vacancies can significantly promote the adsorption of O2 and TMB on CeO2, which directly enhances the oxidase-mimetic activity of flower-like CeO2 nanozymes. This work can shed light on a new perspective on the enzyme-like performance promotion of CeO2-based catalysts and surface engineering of nanozymes.

Biocompatible fluorescent probe for detecting mitochondrial alkaline phosphatase activity in live cells

Alkaline phosphatase (ALP) is an enzyme that actively plays a significant role in the various metabolic processes by transferring a phosphate group to the protein, nucleic acid, etc. The elevated level of ALP in blood plasma is the hallmark of inflammation/cancer. The hyperactive mitochondria in cancer cells produce an excess of ATP to fulfill the high energy demand. Thus, we have developed a fluorescent probe Mito-Phos for ALP, which can detect phosphatase expression in mitochondria in live cells. The probe Mito-Phos has shown ~15-fold fluorescence intensity increments at 450 nm in the presence of 500 ng/mL of ALP. It takes about 60 min to consume the whole amount of ALP (500 ng/mL) in physiological buffer saline. It can selectively react with ALP even in the presence of other probable cellular reactive components. It is highly biocompatible and nontoxic to the live cells. It has shown ALP expression in a dose-dependent manner by providing concomitant fluorescence images in the blue-channel region. It has localized exclusively in the mitochondria in live cells. The probe Mito-Phos is highly biocompatible with the ability to assess ALP expression in mitochondria in live cells.

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.

A smartphone-based platform for point-of-use determination of alkaline phosphatase as an indicator of water eutrophication

A smartphone-based detection platform for the determination of alkaline phosphatase (ALP) is described. The method is based on the rational design of the stimulus-response of 7-methoxycoumarin-3-carboxylic acid (7-MC-3-COOH)-functionalized Eu-AMP infinite coordination polymer (ICP) nanoparticles. The blue fluorescence of 7-MC-3-COOH at 403 nm was suppressed, while the red fluorescence of Eu³⁺ at 615 nm was sensitized after the formation of 7-MC-3-COOH@Eu-AMP ICP. Upon exposure to ALP, the dephosphorylation of AMP resulted in the destruction of 7-MC-3-COOH@Eu-AMP ICP, and thereby, the blue fluorescence of 7-MC-3-COOH recovered; in the meantime, the sensitized red fluorescence was quenched. With the fluorescence intensity ratio F₆₁₅/F₄₃₀ as the signal readout, ALP can be detected within a concentration range 0.001 to 0.15 U mL^-1, and the limit of detection (LOD) was 0.00035 U mL^-1. Moreover, fluorescence color changes from red to blue could also be recognized by a portable device with the smartphone as a signal reader, and direct point-of-use testing (POUT) for ALP within a concentration range 0.005 to 0.7 U mL^-1 could be realized, with LOD of 0.0015 U mL^-1. Endowed with high sensitivity and superior reliability, the assay enabled direct monitoring of P-related water eutrophication in a freshwater lake with ALP as an indicator. Graphical abstract A smartphone-based platform for point-of-use determination of alkaline phosphatase.