Electrochemiluminescence resonance energy transfer immunoassay for alkaline phosphatase using p-nitrophenyl phosphate as substrate

Research progress on preparation methods and sensing applications of molecularly imprinted polymer-aptamer dual recognition elements

The aptamer (Apt) and the molecularly imprinted polymer (MIP), as effective substitutes for antibodies, have received widespread attention from researchers because of their creation. However, the low stability of Apt in harsh detection environment and the poor specificity of MIP have hindered their development. Therefore, some researchers have attempted to combine MIP with Apt to explore whether the effect of "1 + 1 > 2" can be achieved. Since its first report in 2013, MIP-Apt dual recognition elements have become a highly focused research direction in the fields of biology and chemistry. MIP-Apt dual recognition elements not only possess the high specificity of Apt and the high stability of MIP in harsh detection environment, but also have high sensitivity and affinity. They have been successfully applied in medical diagnosis, food safety, and environmental monitoring fields. This article provides a systematic overview of three preparation methods for MIP-Apt dual recognition elements and their application in eight different types of sensors. It also provides effective insights into the problems and development directions faced by MIP-Apt dual recognition elements.

Colorimetric detection of alkaline phosphatase based on the off-on effect of light-responsive oxidase mimicking activity of covalent organic framework (Cu-TpBpy-COF) under near-neutral condition

A colorimetric strategy has been developed for the detection of alkaline phosphatase (ALP) activity based on the off-on effect of the catalytic activity of light-responsive oxidase mimics covalent organic framework (Cu-TpBpy-COF) in near-neutral condition. Cu-TpBpy-COF can effectively catalyze the oxidation of the colorless substrate 3,3',5,5'-tetramethylbenzidine (TMB) by oxygen to form a blue oxidized product (oxTMB) with an absorption peak at 652 nm. Cu2+ is the active center of Cu-TpBpy-COF and pyrophosphate (PPi) can form a complex with Cu2+ to weaken the catalytic activity of Cu-TpBpy-COF. In the presence of ALP, PPi is hydrolyzed into orthophosphates (Pi) with low affinity to Cu2+, thus resulting in absorbance restoration. The absorbance at 652 nm is related to ALP activity in the linear range 10-150 U·L-1 with a detection limit of 7.17 U·L-1. The recoveries of ALP in serum samples are in the range 94.7~107.0% with relative standard deviations (RSD) lower than 5%. The decisive role of Cu2+ on the enhancing catalytic activities of Cu-TpBpy-COF in neutral condition was verified by TpBpy-COF and TpBD-COF as controls, in which the main difference between them is that TpBpy-COF contains pyridine nitrogen. Upon Cu2+ modification, Cu-TpBpy-COF has better catalytic activity than TpBpy-COF in a broader pH range because of the in situ generation of Cu+ under irradiation.

Fluorescent assay for alkaline phosphatase by integrating strand displacement amplification with DNAzyme-catalytic recycling cleavage of molecular beacons

A fluorescence sensing method for the quantification of alkaline phosphatase (ALP) was developed by integrating the strand displacement amplification with DNAzyme-catalytic recycling cleavage of molecular beacons. ALP can hydrolyze a 3'-phosphoralated primer into a 3'-hydroxy primer which can initiate the strand displacement amplification to produce the Mg2+-dependent DNAzyme. The DNAzyme can then catalyze the cleavage of the DNA molecular beacon labeled with FAM fluorophore at its 5'-end and BHQ1 quencher at its 3'-end, turning on the fluorescence of FAM fluorophore. The content of ALP in a sample can be deduced from the measured fluorescence intensity. Due to the cascading nature of its amplification strategy, the proposed method achieved sensitive and specific ALP detection in human serum samples. Its results were in good consistent with the corresponding values obtained by a commercial ALP detection kit. The limit of detection of the proposed method for ALP is about 0.015 U/L, lower than some methods recently reported in literature, demonstrating its potential for ALP detection in biomedical research and clinical diagnosis.

Progress in Electrochemical Immunosensors with Alkaline Phosphatase as the Signal Label

Electrochemical immunosensors have shown great potential in clinical diagnosis, food safety, environmental protection, and other fields. The feasible and innovative combination of enzyme catalysis and other signal-amplified elements has yielded exciting progress in the development of electrochemical immunosensors. Alkaline phosphatase (ALP) is one of the most popularly used enzyme reporters in bioassays. It has been widely utilized to design electrochemical immunosensors owing to its significant advantages (e.g., high catalytic activity, high turnover number, and excellent substrate specificity). In this work, we summarized the achievements of electrochemical immunosensors with ALP as the signal reporter. We mainly focused on detection principles and signal amplification strategies and briefly discussed the challenges regarding how to further improve the performance of ALP-based immunoassays.

Fluorescence recovery based on synergetic effect for ALP detection

Alkaline phosphatase (ALP) is an important biomarker associated with diabetes, liver dysfunction, bone diseases, and breast cancer. Here we developed a method based on synergetic fluorescence recovery for the sensitive detection of ALP. Cadmium-zinc-selenium (CdZnSe) quantum dots (QDs) were prepared by one-pot water bath method without any complicated and rigorous conditions. CdZnSe QDs displayed high luminous efficiency, good stability, and good biocompatibility. KMnO₄ and ascorbic acid phosphate (AAP) can dynamically quench the fluorescence of CdZnSe QDs. Ascorbic acid, produced by ALP-catalyzed hydrolysis of AAP, reacted with KMnO₄, causing the synergetic fluorescence recovery of CdZnSe QDs. The synergetic recovery efficiency correlates well with the logarithmic ALP concentration in the range of 2.5-250 U/L with a detection limit of 0.21 U/L. In addition, good recoveries were obtained in the detection of ALP in human serum. This method provided a new research idea to improve the detection sensitivity and selectivity of ALP detection.

A Novel Electrochemical Sensing Strategy Based on Poly (3, 4-ethylenedioxythiophene): Polystyrene Sulfonate, AuNPs, and Ag+ for Highly Sensitive Detection of Alkaline Phosphatase

Alkaline phosphatase (ALP) is a crucial marker for the clinical analysis and detection of many diseases. In this study, an accurate signal amplification strategy was proposed for the sensing and quantification of alkaline phosphatase using poly (3, 4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), gold nanoparticles (AuNPs), and Ag⁺. Signal amplification was achieved by the modification of PEDOT:PSS and AuNPs on glassy carbon electrodes. Atomic force microscopy was performed to characterize the morphology of the modified nanomaterials. To detect ALP, 1-naphthyl phosphate (1-NP) was used as the substrate, and alkaline phosphatase catalyzed 1-NP into 1-naphthol (1-N), which resulted in the reduction of Ag⁺ to Ag⁰ on the surface of the modified electrode (AuNPs/PEDOT:PSS/GCE). The deposition of Ag drastically enhanced the detection signal. Differential pulse voltammograms of 1-N, which is the enzymatic product from the ALP reaction with 1-NP, were recorded. In the linear range of 0.1-120 U L^-1, a quantitative analysis of alkaline phosphatase was achieved, with high sensitivity and a low detection limit of 0.03 U L^-1. Stable, selective, and reproducible electrochemical sensors were designed. Moreover, the proposed electrochemical sensor exhibited a prominent sensing performance in the spiked diluted human serum. Thus, the sensor can be used in numerous applications in alkaline phosphatase or other analyte detection.

A highly sensitive colorimetric sensing platform based on silver nanocomposites for alkaline phosphatase

Alkaline phosphatase (ALP) plays significant roles in regulating intracellular processes and is an important biomarker connected to several diseases. In this work, one facile and sensitive sensing platform based on CQD-silver nanocomposites (CQD-silver NPs) for colorimetric detection of alkaline phosphatase (ALP) was introduced. ALP triggers the removal of the phosphate group of ascorbic acid 2-phosphate (AA2P), which is then transformed into ascorbic acid (AA). The as-obtained AA can easily cause significant aggregation of monodispersed NPs and cause the system color to turn from bright yellow to gray. Based on the color change of the ratio of 490 nm/630 nm, ALP was sensitively and selectively detected. Under the optimum, the established method showed linearity for ALP in the range of 0.1-50 U L^-1 and the detection limit was low at 0.035 U L^-1, and it was subjected to ALP inhibitor screening from goji berry extract. These results indicated that the colorimetric system can be used as a simple tool for visual and fast evaluation of ALP activity as well as providing an alternative to screen ALP inhibitors.

Highly Active Electrochemiluminescence of Ruthenium Complex Co-assembled Chalcogenide Nanoclusters and the Application for Label-Free Detection of Alkaline Phosphatase

Rational design of electrochemiluminescence (ECL) reagents is essential for the development of ECL biosensors with superior performances. In this work, the assembly of tris(1,10-phenanthroline)ruthenium(II) [Ru(phen)₃²⁺] and tetrahedral chalcogenide nanoclusters of [Cd₃₂S₁₄(SC₆H₅)₃₈]^2- in the formation of complex nanoclusters (CdS-Ru) was developed, in which Ru(phen)₃²⁺ was uniformly encapsulated and dispersed at a molecular level in the chalcogenide nanocluster via multiple noncovalent interactions. It was observed that the promoted ECL emission was realized by the charge transfer between the tetrahedral CdS nanocluster and Ru(phen)₃²⁺ by the formation of the assembly complex, which was elucidated by cyclic voltammetry curves, ECL-potential curves, and in situ dynamic ECL spectra. Taking advantages of the facile charge transfer in the open framework CdS-Ru, a high ECL efficiency has been achieved with remarkable stability. Moreover, a solid-state ECL sensor based on the CdS-Ru modified electrode was fabricated for label-free detection of alkaline phosphatase (ALP) activity with a detection limit as low as 0.35 U/L and superior reproducibility. This solid-state ECL sensor also displayed favorable selectivity among various interferences and was applied for ALP activity analysis in human serum samples. These results implicated the potential applications of CdS-Ru for sensitive ECL analysis in complicated reaction systems and enlightened the rational design for self-enhanced and highly efficient ECL materials.

Distance-Based Paper Device for a Naked-Eye Albumin-to-Alkaline Phosphatase Ratio Assay

The albumin-to-alkaline phosphatase ratio (AAPR) has been a cancer prognostic indicator. This paper presents the concept of a dual-color change distance-based paper device (dPAD) for albumin (Alb) and alkaline phosphatase (ALP) detection to evaluate this cancer prognostic index. Whereas Alb interacts with the bromocresol green (BCG) indicator to form a bluish-green complex, ALP hydrolyzes l-ascorbic acid-2-phosphate (AAP) to produce ascorbic acid (AA), which reacts with KIO₃ to generate I₂ and I^-. I₂/I^- reacts with silver hexagonal nanoprisms (purple color) in the presence of Cu²⁺, resulting in a color change from purple to colorless. The distance of the color change from yellow to the bluish-green and purple to colorless correlates to Alb and ALP concentration, respectively. The angle index for the AAPR is then defined by drawing a straight line that connects the tops of the two changed band lengths in the detection area. The highest bluish-green color band length on the Alb region is the midpoint, which is the position set of the protractor at 0°, and the angle is measured using a simple protractor. The results indicate that an AAPR below 0.57 will have an angle greater than 40° and correlates with a risk factor for lung cancer. The naked-eye detection limits for Alb and ALP were found to be 0.8 g/L and 5 U/L (n = 10), respectively. The practical application of the developed dPAD was successfully demonstrated by Alb and ALP analysis in human serum and validated against standard methods. The proposed method does not require incubation conditions for the ALP assay, which strongly reduces the overall analysis steps and time. Moreover, our device provides a low-cost, simple, sensitive, selective, accurate, and precise determination of the AAPR.

Highly Sensitive Homogeneous Electrochemiluminescence Biosensor for Alkaline Phosphatase Detection Based on Click Chemistry-Triggered Branched Hybridization Chain Reaction

Alkaline phosphatase (ALP) has been used as a diagnostic index of clinical diseases since its expression level is closely related to many pathological processes. In this work, a highly sensitive electrochemiluminescence (ECL) method for the determination of ALP based on a click chemistry-induced branched hybridization chain reaction (BHCR) for signal amplification and ultrafiltration technology for the separation of homogeneous amplification products is introduced. ALP can release copper ions from a Cu²⁺/PPi complex by hydrolyzing pyrophosphoric acid, which initiates click chemistry in the system. A BHCR amplification is triggered afterward by the long single-stranded DNA (ssDNA) generated by click chemistry, resulting in a three-dimensional double-stranded DNA (dsDNA) with a large molecular weight. Based on the characteristic that Ru(phen)₃²⁺ can stably insert into the groove of dsDNA, a large amount of Ru(phen)₃²⁺ is retained together with the amplified product after ultrafiltration, and therefore a significantly enhanced ECL signal can be obtained. The test results show that this method can be used for the quantitative determination of ALP ranging from 0.002 to 50 U/L, with a detection limit of 0.7 mU/L. This method has also been confirmed to have good selectivity and anti-interference, and the results of the analysis of the ALP content in the diluted serum samples are satisfactory, showing great application potential in clinical diagnosis.

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.

Portable smartphone device-based multi-signal sensing system for on-site and visual determination of alkaline phosphatase in human serum

To provide the basis for clinical diagnosis in an emergency case, a portable smartphone device-based multi-signal sensing system for on-site determination of alkaline phosphatase (ALP) is introduced. In this system, cobalt hydroxide (CoOOH) nanoflakes can oxidize O-phenylenediamine (OPD) to produce 2,3-diaminophenazine (OxOPD), resulting in a strong fluorescence at 565 nm and an absorbance at 420 nm, respectively. The ascorbic acid 2-phosphate (AAP) can be hydrolyzed by alkaline phosphatase (ALP) to yield ascorbic acid (AA). Then, AA reduces the CoOOH nanoflakes to produce Co²⁺, and AA is oxidized to form dehydroascorbic acid (DHAA), thereby inhibiting the formation of OxOPD. The reaction product DHAA further combines with OPD to yield 3-(1,2-dihydroxyethyl)furo[3,4-b]quinoxalin-1(3H)-one (DFQ) accompanied by a strong fluorescence at 430 nm. Based on this, the fluorometric assay for ALP has a wide linear range from 0.8 to 190 U/L with a low detection limit of 0.16 U/L, and the colorimetric assay from 3 to 130 U/L with a detection limit of 1.94 U/L. Moreover, a portable smartphone sensing platform integrated with fluorescent and colorimetric signals was established for rapid determination of ALP without spectrometers. Recoveries of 97-104% for spiked samples and relative standard deviations (RSD) of less than 2% (n = 3) confirmed the feasibility of the developed platform in complicated samples, opening up new horizons for on-site evaluation in the biomedical field.

A novel copper ion sensing fluorescent probe for fast detection of pyrophosphate and alkaline phosphatase

A Cu²⁺ sensing fluorescent probe is synthesized via a Mannich reaction and is applied in the fluorescence detection of pyrophosphate and alkaline phosphatase.

A dual-mode strategy for sensing and bio-imaging of endogenous alkaline phosphatase based on the combination of photoinduced electron transfer and hyperchromic effect

Benefit from the additional correction of the output signal in dual-mode detection, traditional dual signal readout strategies are performed by constructing the ratiometric fluorescent probe through excitation energy transfer (EET) or fluorescence resonance energy transfer (FRET). To avoid the complicated modification process and obtain the results rapidly, a simple dual-mode sensing strategy based on the electronic effects of p-nitrophenol (PNP) is described to monitor the activities of alkaline phosphatase (ALP). In the sensing platform, p-nitrophenylphosphate was used as a substrate to produce the PNP using ALP as the catalyst. Due to the PNP possesses negative effect of induction and conjugation, photoinduced electron transfer and hyperchromic effect have been achieved between PNP and polyethyleneimine-protected copper nanoclusters (PEI-Cu NCs), which caused the changes of the fluorescence intensity and UV-visible absorption. The dual-mode signal sensing system showed the satisfactory linear results of ALP from 1 to 100 U/L for fluorescent sensing strategy and 1-70 U/L for the absorption method with a competitive LOD of 0.27 and 0.87 U/L (signal-to-noise ratio of 3). This strategy detected biological ALP in human serum and bio-imaging of endogenous ALP in A549 cells successfully, which verifies a certain potential of the strategy for the practical applications.