A turn-on fluorescent method for determination of the activity of alkaline phosphatase based on dsDNA-templated copper nanoparticles and exonuclease based amplification

The development of biosensors for alkaline phosphatase activity detection based on a phosphorylated DNA probe

Alkaline phosphatase (ALP) is a zinc-containing metalloprotein that shows very great significance in clinical diagnosis, which can catalyze the hydrolysis of phosphorylated species. ALP has the potential to serve as a valuable biomarker for detecting liver dysfunction and bone diseases. On the other hand, ALP is an efficient biocatalyst to amplify detection signals in the enzyme-linked assay. It has always been a major research focus to develop novel biosensors that can detect ALP activity with high selectivity and sensitivity. There have been numerous reports on the development of biosensors to determine ALP activity using a phosphorylated DNA probe. Among them, various beneficial strategies, such as λ exonuclease-mediated cleavage reaction, terminal deoxynucleotidyl transferase-triggered DNA polymerization, and Klenow fragment polymerase-catalyzed elongation, are employed to generate amplified and more intuitive signal. This review discusses and summarizes the development and advances of biosensors for ALP activity detection that use a well-designed phosphorylated DNA probe, aiming to provide some guidelines for the design of more sophisticated sensing strategies that exhibit improved sensitivity, selectivity, and adaptability in detecting ALP activity.

Paying Comprehensive Attention to the Temperature-Dependent Dual-Channel Excited-State Intramolecular Proton Transfer Mechanism of Fluorescence Ratio Probe BZ-DAM

The mechanism of fluorescence detection of diethyl chlorophosphate (DCP) based on 2-substituted benzothiazole (BZ-DAM) was studied by a theoretical calculation method. It should not be ignored that both the BZ-DAM and the detection product BZ-CHO have two excited-state intramolecular proton transfer (ESIPT) channels. Density functional theory (DFT) and time-dependent DFT (TDDFT) theory were used to study the photophysical mechanism of two compounds in two channels in (acetonitrile) ACN solvent, and the temperature dependence of the two channels was given. Channel 1 is more likely to exist at low temperatures and channel 2 is more likely to exist at high temperatures. By theoretical analysis of the constructed potential energy curve, the hydrogen bond energy and electron-hole analysis, we confirmed that both molecules undergo ESIPT and intramolecular charge transfer (ICT) processes in channel 1 and ESIPT and twisted intramolecular charge transfer (TICT) coupling processes in channel 2. The formation of product BZ-CHO molecules led to a significant fluorescence blue-shift phenomenon and inhibited the ICT process, which confirmed that BZ-DAM could be used as a fluorescence probe for fluorescence detection. We sincerely hope that this work will not only help to clarify the excited-state dynamics behavior of the BZ-DAM probe but also provide a new idea for designing and optimizing a new chemical dosimeter.

A turn-on fluorescent strategy for alkaline phosphatase detection based on enzyme-assisted signal amplification

As a representative biochemical indicator, alkaline phosphatase (ALP) is of great importance in indicating and diagnosing clinical diseases. Herein, we developed a signal-on fluorescence sensing method for sensitive ALP activity detection based on the enzyme-assisted target recycling (EATR) technique. In this method, a two-step signal amplification process is designed. In the presence of ALP, the 3' phosphate group of an ss-DNA is removed explicitly by ALP, thus releasing free 3'-OH. Terminal deoxynucleotidyl transferase (TdT) can subsequently extend this substrate to generate poly(A) tails, converting the trace-level ALP information into multiple sequences and achieving the first-time amplification. A poly(T) Taqman probe labeled with FAM and BHQ1 provides the second one under the assistance of T7 exonuclease (T7 Exo) through alternate hybridization and degradation of ds-DNA regions. The previously quenched fluorescence is recovered due to the departure of FAM/BHQ1 during the cleavage of T7 Exo. Thus, taking advantage of template-free TdT-mediated polymerization and T7 Exo-based EATR, this strategy shows a sensitive LOD at 0.0074 U/L (S/N = 3) and a linear range of 0.01-8 U/L between ALP concentration and fluorescence intensity. To further verify the specificity and accuracy in practical application, we challenged it in a set of co-existing interference and biological environments and have gained satisfying results. The proposed method successfully quantified the ALP levels in clinical human serum samples, suggesting its applicability in practical application. Moreover, we have used this method to investigate the inhibition effects of Na₃VO₄. Above all, the proposed assay is sensitive, facile, and cost-effective for ALP determining, holding a promising perspective and excellent potential in clinical diagnosis and drug screening.

A Fluorescent Biosensor for Streptavidin Detection Based on Double-Hairpin DNA-Templated Copper Nanoparticles

In this paper, we developed a sensitive, label-free and facile fluorescent strategy for detecting streptavidin (SA) based on double-hairpin DNA-templated copper nanoparticles (CuNPs) and terminal protection of small molecule-linked DNA. Herein, a special DNA hairpin probe was designed and synthesized, which contained two poly T single-stranded loops and a nick point in the middle of the stem. Inspired by the concept of the terminal protection interaction, the specific binding of SA to the biotinylated DNA probe can prevent the exonuclease degradation and keep the integrity of DNA probe, which can be used for synthesizing fluorescent CuNPs as a template. Conversely, the DNA probe would be digested by exonucleases and therefore, would fail to form CuNPs without SA. After systematic optimization, the detection range of SA concentration is from 0.5 to 150 nM with a low detection limit of 0.09 nM. Additionally, the proposed method was also successfully applied in the biological samples. Finally, the proposed method is sensitive, effective and simple, and can be potentially applied for predicting diseases and discovering new drugs.

Redox-Responsive Breakup of a Nucleic Acids@CoOOH Nanocomplex Triggering Cascade Recycling Amplification for Sensitive Sensing of Alkaline Phosphatase

Alkaline phosphatase (ALP), an essential hydrolase with crucial roles in living organisms, has widely been regarded as a biomarker for various human diseases in clinical diagnoses. Herein, taking advantage of cobalt oxyhydroxide (CoOOH) nanoflakes and nonenzymatic cascade recycling amplification (CRA), a highly sensitive and label-free fluorescence biosensing strategy for the determination of ALP activity is introduced. In our design, ALP can promote the dephosphorylation of l-ascorbic acid 2-phosphate (AAP) to reduce ascorbic acid (AA), which is then able to decompose CoOOH in a nucleic acids@CoOOH nanocomplex into Co²⁺ cofactors. Further, enzyme-free CRA was rapidly initiated by integrating DNAzyme recycling amplification and catalytic hairpin assembly, resulting in the generation of an abundance of G-quadruplex structure-contained DNA duplexes. In the presence of thioflavin T (ThT), analytical target ALP was converted in an amplified and activatable fluorescence signal. The experimental results show that this method can be applied for the quantitative analysis of ALP activity with a low detection limit of 0.027 mU/mL. Moreover, this developed biosensing approach exhibits excellent specificity, and the evaluation of ALP activity in the complex human serum samples was successfully realized, indicating that it can afford a reliable, robust, and cost-effective nanoplatform for an ALP-based clinical diagnosis and for biomedical research.

Theoretical Investigation on the ESIPT Process and Detection Mechanism for Dual-Proton Type Fluorescent Probe

Recently, a new fluorescent probe AE-Phoswas reported to detect the activity of alkaline phosphatases (ALP) in different living cell lines. Here, we present an in-depth computational analysis of the mechanism and source of the fluorescence of the AE-Phos probe. There is an intermediate product (AE-OH-Phos) in the experiment as well as a different configuration of products that may emit fluorescence. It is essential to investigate the origin of fluorescence and the detection mechanism of the probe, which could help us eliminate the interference of other substances (including an intermediate product and possible isomers) on fluorescence during the experiment. According to the change of geometric parameters and Infrared spectra, we deduce that the dual intramolecular hydrogen bonds of salicylaldehyde azine (SA) were enhanced at the excited state, while AE-OH-Phos was attenuated. Considering the complex ESIPT behavior of the dual proton-type probe, the potential energy surfaces were further discussed. It can be concluded that the single proton transfer structure of SA (SA-SPT) is the most stable form. Both the concerted double proton transfer process and stepwise single proton transfer process of SA were forbidden. The fluorescence for SA was 438 nm, while that of SA-SPT was 521 nm, which agrees with the experimentally measured fluorescence wavelength (536 nm). The conclusion that single proton transfer occurs in SA is once again verified. In addition, the distribution of electron-hole and relative index was analyzed to investigate the intrinsic mechanism for the fluorescence quenching of the probe and the intermediate product. The identification of the origin of fluorescence sheds light on the design and use of dual-proton type fluorescent probes in the future.

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.

Sensitive Fluorescence Assay for the Detection of Alkaline Phosphatase Based on a Cu2+-Thiamine System

The authors describe a novel, facile, and sensitive fluorometric strategy based on a Cu²⁺-thiamine (Cu²⁺-TH) system for the detection of alkaline phosphatase (ALP) activity and inhibition. The principle of the method is as follows. Under a basic conditions, TH, which does not exhibit a fluorescence signal, is oxidized into fluorescent thiochrome (TC) by Cu²⁺. Ascorbic acid 2-phosphate (AAP), which is the enzyme substrate, is hydrolyzed to produce ascorbic acid (AA) by ALP. The newly formed AA then reduces Cu²⁺ to Cu⁺, which prevents the oxidation of TH by Cu²⁺; as a result, the fluorescent signal becomes weaker. On the contrary, in the absence of ALP, AAP cannot reduce Cu²⁺; additions of Cu²⁺ and TH result in a dramatic increase of the fluorescent signal. The sensing strategy displays brilliant sensitivity with a detection limit of 0.08 U/L, and the detection is linear in the concentration range of 0.1 to 100 U/L. This approach was successfully applied to ALP activity in human serum samples, indicating that it is reliable and may be applied to the clinical diagnosis of ALP-related diseases.

A path-choice-based biosensor to detect the activity of the alkaline phosphatase as the switch

Alkaline phosphatase (ALP), which converts the phosphate group (-PO₄) in the substrate to the hydroxyl group (-OH), is a useful tool in the biological analysis, a good indicator of dissolved inorganic phosphorus levels and an important biomarker for several diseases. In conventional designs for ALP detection, both the interferent with a -PO₄ and the target with a -OH will go into the sensing path and give out the undesired background and the desired signal respectively. This limited the sensitivity of the method and required the complicated design to achieve a satisfying limit of detection (LOD) of ALP. Here, we provided a new sensing strategy for ALP detection design. We designed a path-choice-based biosensor with two DNA tracks in which ALP works as the switch to guide the reaction path of lambda exonuclease (λ exo). The path-choice character enlarged the difference between signal and background by separating the interferent removing path and the target sensing path. The substrate preference of ALP and λ exo was studied to optimize the structure of DNA tracks. The path-choice-based biosensor achieved simple, fast (30 min), sensitive (LOD 0.014 U L^-1) and selective detection of the activity of ALP. The method has been applied to detect the activity of ALP in cell lysates, which shows the potential application in ALP-related biological research.

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.

Alkaline phosphatase determination via regulation of enzymatically generated poly(thymine) as a template for fluorescent copper nanoparticle formation

We propose a new fluorometric method for alkaline phosphatase (ALP) determination. This method is based on the regulation of enzymatically generated poly(thymine) for the preparation of copper nanoparticles (CuNPs). 2'-Deoxythymidine 5'-triphosphate (dTTP) serves as the source for polymerization mediated by terminal deoxynucleotidyl transferase (TdT). This process generates poly(thymine), which acts as the template for synthesis of fluorescent CuNPs. However, if ALP catalyzes the hydrolysis of dTTP, the TdT-mediated polymerization will be disabled. This prevents the formation of CuNPs and causes a drop in fluorescence. The findings were used to design a sensitive and selective fluorometric method for ALP determination. A linear response in the activity range from 0.1 to 20 U L^-1 and a limit of quantification of 0.3 U L^-1 were obtained. The results indicate that the proposed method can be successfully applied to ALP assay in spiked diluted serum. This demonstrates the method's reliability and practicability. Graphical abstract A fluoromoetric method for alkaline phosphatase assay has been developed based on regulation of enzymatically generated poly(thymine) as template for the formation of fluorescent CuNPs.

An exonuclease-assisted fluorescence sensor for assaying alkaline phosphatase based on SYBR Green I

In this report, we propose a fast, reliable and convenient approach to determine the alkaline phosphatase (ALP) activity based on a label-free fluorescence strategy. Upon catalysis of ALP, dephosphorylated dsDNA hampers the λ exonuclease (λexo) cleavage, shows high affinity to SYBR Green I (SG I), resulting in a strong fluorescence emission peak at 520 nm. In the absence of ALP, the dsDNA with 5'-phosphoryl-termini could be employed as a substrate of λexo. After cleavage, a weak fluorescence emission peaks at 520 nm could be observed. The assay was both selective and sensitive, and the detection limit was found to be as low as 3 U/L. This method was utilized to evaluate Na₃VO₄ as ALP inhibitor. The method was successfully applied to the determination of the activity of ALP in spiked human serum samples.

A fluorescent aptasensor for ochratoxin A detection based on enzymatically generated copper nanoparticles with a polythymine scaffold

A fluorescence enhancement method is presented for the determination of ochratoxin A (OTA). The interaction of OTA with its aptamer causes structural changes which, in turn, change fluorescence of enzymatically generated polythymine-coated copper nanoparticles (CuNPs) (with excitation/emission maxima at 340/625 nm). The OTA-binding aptamer was immobilized on magnetic beads. When it binds OTA, it is partially released and exposes a region with a partly complimentary DNA strand (cDNA). After magnetic separation, the cDNA was employed as a primer to trigger the terminal deoxynucleotidyl transferase-mediated polymerization. This process generates polythymine which act as a template for synthesis of the CuNPs. The method is sensitive in having a 2.0 nM detection limit for OTA. It was successfully applied to the determination of OTA in spiked diluted red wine. Graphical abstract Schematic presentation of a fluorometric enhancement method for ochratoxin A assay based on ochratoxin A inducing structure switching of its aptamer and enzymatically generated polythymine for copper nanoparticles formation.

Photoelectrochemical determination of alkaline phosphatase activity based on a photo-excited electron transfer strategy

A sensitive photoelectrochemical (PEC) biosensor for determination of alkaline phosphatase (ALP) activity was constructed based on a photo-excited electron transfer strategy. Immobilization of CdTe quantum dots (QDs) on TiO₂ nanotube arrays (TNAs), addition of iron (III) and adenosine triphosphate (ATP) in turn can effectively adjust the photocurrent response of TNAs under visible light irradiation due to a photo-excited electron transfer process, and alkaline phosphatase (ALP) activity can be determined for its catalysis toward dephosphorylation of ATP. The preparation of CdTe QDs, construction of TNA/QD PEC biosensor and the mechanism of photo-excited electron transfer are investigated in the present work. Under the optimal experimental conditions, the TNA/QD PEC biosensor shows a low limits of detection (LODs) (0.05 U L^-1) and limits of quantification detection (LOQs) (0.15 U L^-1), wide linear range from 0.2 to 15 U L^-1, and good selectivity towards ALP determination, which has been successfully applied for human serum analysis with good precision (RSD ≤ 5.4%) and high accuracy (recovery rate, 91-112%).

Photometric and fluorometric alkaline phosphatase assays using the simplest enzyme substrates

In this contribution a highly cost-effective flow analysis system for determination of alkaline phosphatase (ALP) activity is presented. This fully-mechanized bioanalytical system is based on economic solenoid micropumps and microvalves (powered and actuated by Arduino microcontroller) and extremely cheap dedicated optoelectronic flow-through detectors allowing absorbance and fluorescence measurements. The detection schemes for ALP assaying realized in this system are based on orthophosphate determination. For the detection of these ions formed in the course of enzymatic conversion a molybdate method requiring only common and inexpensive chemicals is utilized. Thus, for the enzymatic assays the simplest not-chromogenic/not-fluorogenic ALP substrates can be applied. Such approach results in the use of low-cost reagents for ALP assays, whereas the mechanization of assay causes low consumption of reagents as well as samples. In the course of reported investigations six ALP substrates were examined and the most promising results have been obtained for the inorganic compound - monofluorophosphate (MFP). The obtained linear ranges of absorbance and fluorescence measurements are 100-600 U L^-1 and 30-30/30-100 U L^-1, with sensitivities of 0.7 mV L^-1 U^-1 and 2.3/1.0 mV L^-1 U^-1, respectively. The calculated limits of detection are 5.1 U L^-1 (photometry) and 0.9 U L^-1 (fluorometry). The throughputs of the developed system are 13 and 12 samples/h for photometric and fluorometric detections, respectively. To demonstrate the practical utility of the developed bioanalytical system the ALP assays in complex matrix samples have been carried out. The results of ALP activity determination in serum samples are well-correlated with those obtained using reference method recommended for routine clinical analysis.

Polydopamine nanodots are viable probes for fluorometric determination of the activity of alkaline phosphatase via the in situ regulation of a redox reaction triggered by the enzyme

The authors describe an environmentally friendly and fast (~14 min) method for the synthesis of homogeneously distributed fluorescent polydopamine nanodots (PDA-NDs) using KMnO₄ as the oxidant. Alkaline phosphatase (ALP) catalyzes the hydrolysis of ascorbic acid 2-phosphate to release free ascorbic acid which undergoes an in-situ redox reaction with KMnO₄. Depending on the activity of ALP, more or less KMnO₄ is consumed, and this affects the formation of the PDA-NDs. Based on this finding, a sensitive method was worked out to quantify the activity of ALP via real-time formation of fluorescent PDA-NDs. The fluorometric signal (best measured at excitation/emission peaks of 390/500 nm) is linear in the 1 to 50 mU·mL^-1 ALP activity range, and the limit of the detection is as low as 0.94 mU·mL^-1 (based on 3 σ/m). The method was successfully applied to the determination of ALP activity in spiked human serum and in MCF-7 cell lysates. It was also applied in a method to screen for inhibitors of ALP. Graphical abstract Schematic of a fluorometric method for the determination of alkaline phosphatase (ALP) activity. The method is based on the in-situ regulation of the formation of fluorescent polydopamine nanodots (PDA-NDs) through the competition between the KMnO₄-induced polymerization of dopamine and ALP-directed ascorbic acid 2-phosphate (Asc-2P) hydrolysis. AA: Ascorbic acid.

A sensitive detection method of carcinoembryonic antigen based on dsDNA-templated copper nanoparticles

A label-free fluorescence assay has been developed for the detection of carcinoembryonic antigen based on dsDNA-templated copper nanoparticles.

A turn-on fluorescence assay of alkaline phosphatase activity using a DNA–silver nanocluster probe

A label-free fluorescence assay has been developed for the detection of alkaline phosphatase based on DNA–silver nanocluster probes.

Inner filter effect based fluorometric determination of the activity of alkaline phosphatase by using carbon dots codoped with boron and nitrogen

Boron and nitrogen codoped carbon dots functionalized with cyclodextrin (β-CD-N/B-C-dots) were obtained from β-cyclodextrin. The material displays strong fluorescence (with excitation/emission peak wavelengths of 400/500 nm) and was characterized by UV-vis, transmission electron microscopy and FTIR. If the substrate p-nitrophenylphosphate is enzymatically cleaved by alkaline phosphatase (ALP), a yellow product is formed whose absorption overlaps the excitation spectrum of the β-CD-N/B-C-dots. Hence, fluorescence is reduced due to an inner filter effect. In additon, the β-CD cavity offers a pocket for substrate recognition. The findings were used to design a method for the determination of the activity of ALP. It has a working range that extends from 0.003 to 5.5 U·L^-1, with a 0.3 mU·L^-1 detection limit. The method is fast, simple, inexpensive, and highly sensitive and selective. Graphical abstract Schematic of an inner filter effect based probe for alkaline phosphatase based on the use boron and nitrogen co-doped carbon dots (N/B-C-dots) modified with β-cyclodextrin (β-CD). PNPP: p-Nitrophenylphosphate; PNP: p-Nitrophenol anion.