Colorimetric determination of the activity of alkaline phosphatase based on the use of Cu(II)-modulated G-quadruplex-based DNAzymes

An acid-responsive DNA hydrogel-mediated cascaded enzymatic nucleic acid amplification system for the sensitive imaging of alkaline phosphatase in living cells

Alkaline phosphatase (ALP) is a class of hydrolase that catalyzes the dephosphorylation of phosphorylated species in biological tissues, playing an important role in many physiological and pathological processes. Sensitive imaging of ALP activity in living cells is contributory to the research on these processes. Herein, we propose an acid-responsive DNA hydrogel to deliver a cascaded enzymatic nucleic acid amplification system into cells for the sensitive imaging of intracellular ALP activity. The DNA hydrogel is formed by two kinds of Y-shaped DNA monomers and acid-responsive cytosine-rich linkers. The amplification system contained Bst DNA polymerase (Bst DP), Nt.BbvCI endonuclease, a Recognition Probe (RP, containing a DNAzyme sequence, a Nt.BbvCI recognition sequence, and a phosphate group at the 3'-end), and a Signal Probe (SP, containing a cleavage site for DNAzyme, Cy3 and BHQ2 at the two ends). The amplification system was trapped into the DNA hydrogel and taken up by cells, and the cytosine-rich linkers folded into a quadruplex i-motif in the acidic lysosomes, leading to the collapse of the hydrogel and releasing the amplification system. The phosphate groups on RPs were recognized and removed by the target ALP, triggering a polymerization-nicking cycle to produce large numbers of DNAzyme sequences, which then cleaved multiple SPs, restoring Cy3 fluorescence to indicate the ALP activity. This strategy achieved sensitive imaging of ALP in living HeLa, MCF-7, and NCM460 cells, and realized the sensitive detection of ALP in vitro with a detection limit of 2.0 × 10-5 U mL-1, providing a potential tool for the research of ALP-related physiological and pathological processes.

Magnetically actuated microfluidic chip combined with a G-quadruplex DNAzyme-based fluorescent/colorimetric sensor for the dual-mode detection of ochratoxin A in wheat

In this work, a portable fluorescent/colorimetric sensor based on G-quadruplex DNAzyme was constructed to achieve rapid and dual-mode detection of ochratoxin A (OTA) in wheat. OTA aptamers coupled with magnetic beads (MBs) can self-assemble with two segments of DNA and hemin to form a G-quadruplex DNAzyme structure that can catalyze the oxidation of Amplex Red (ADHP) with H2O2, making the solution red and producing strong fluorescence in solution. However, in the presence of OTA, the structure of the G-quadruplex DNAzyme was damaged, resulting in reduced catalytic activity. According to the principle of detection, a magnet-controlled chip integrating the reaction, washing, and detection was designed in this study. Shuttling the MB-DNAzyme probes onto a magnetically controlled chip considerably reduced the background signal and improved the detection efficiency and sensitivity. In addition, a portable fluorescence and colorimetric detection platform was built for on-site OTA detection.

Tunable metal-organic frameworks assist in catalyzing DNAzymes with amplification platforms for biomedical applications

Various binding modes of tunable metal organic frameworks (MOFs) and functional DNAzymes (Dzs) synergistically catalyze the emergence of abundant functional nanoplatforms. Given their serial variability in formation, structural designability, and functional controllability, Dzs@MOFs tend to be excellent building blocks for the precise "intelligent" manufacture of functional materials. To present a clear outline of this new field, this review systematically summarizes the progress of Dz integration into MOFs (MOFs@Dzs) through different methods, including various surface infiltration, pore encapsulation, covalent binding, and biomimetic mineralization methods. Atomic-level and time-resolved catalytic mechanisms for biosensing and imaging are made possible by the complex interplay of the distinct molecular structure of Dzs@MOF, conformational flexibility, and dynamic regulation of metal ions. Exploiting the precision of DNAzymes, MOFs@Dzs constructed a combined nanotherapy platform to guide intracellular drug synthesis, photodynamic therapy, catalytic therapy, and immunotherapy to enhance gene therapy in different ways, solving the problems of intracellular delivery inefficiency and insufficient supply of cofactors. MOFs@Dzs nanostructures have become excellent candidates for biosensing, bioimaging, amplification delivery, and targeted cancer gene therapy while emphasizing major advancements and seminal endeavors in the fields of biosensing (nucleic acid, protein, enzyme activity, small molecules, and cancer cells), biological imaging, and targeted cancer gene delivery and gene therapy. Overall, based on the results demonstrated to date, we discuss the challenges that the emerging MOFs@Dzs might encounter in practical future applications and briefly look forward to their bright prospects in other fields.

A universal approach for synthesis of copper nanoclusters templated by G-rich oligonucleotide sequences and their applications in sensing

Herein, five common G4 sequences have been selected, including three different length of telomere DNA, hemin aptamer, and thrombin aptamer, to synthesize Cu nanoclusters (Cu NCs) in-situ. All G4s are proper templates for Cu NCs with low temperature treatment. The particles (G4-Cu NCs) smaller than 3 nm in diameter were obtained and showed light green fluorescence. This is the first report of metal clusters templated by G4s in-situ. As proof of the concept, hemin and alkaline phosphatase (ALP) were used as the targets to test whether the system can monitor the interaction between G4s and its substrate. The results suggest that G4-Cu NCs can indicate the behavior of G4 and its interaction with hemin, and sensing ALP is achieved with the aid of ATP. The linear ranges of hemin and ALP are 300-4000 nM and 10-500 U/L, respectively, and the corresponding limits of detection as low as 97 nM for hemin and 2.8 U/L for ALP. Moreover, this present system has been successfully applied for the detection of ALP in human serum samples with satisfactory recoveries. This synthesis approach is universal, and it can be easily extended to evaluating the formation of G4, or monitoring the interaction between G4 and its substrate, or selective targeting individual G4, or sensitive detection of other important biomarkers by changing template G4 sequence.

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.

Recent Developments in G-Quadruplex Binding Ligands and Specific Beacons on Smart Fluorescent Sensor for Targeting Metal Ions and Biological Analytes

The G-quadruplex structure is crucial in several biological processes, including DNA replication, transcription, and genomic maintenance. G-quadruplex-based fluorescent probes have recently gained popularity because of their ease of use, low cost, excellent selectivity, and sensitivity. This review summarizes the latest applications of G-quadruplex structures as detectors of genome-wide, enantioselective catalysts, disease therapeutics, promising drug targets, and smart fluorescence probes. In every section, sensing of G-quadruplex and employing G4 for the detection of other analytes were introduced, respectively. Since the discovery of the G-quadruplex structure, several studies have been conducted to investigate its conformations, biological potential, stability, reactivity, selectivity for chemical modification, and optical properties. The formation mechanism and advancements for detecting different metal ions (Na⁺, K⁺, Ag⁺, Tl⁺, Cu⁺/Cu²⁺, Hg²⁺, and Pb²⁺) and biomolecules (AMP, ATP, DNA/RNA, microRNA, thrombin, T4 PNK, RNase H, ALP, CEA, lipocalin 1, and UDG) using fluorescent sensors based on G-quadruplex modification, such as dye labels, artificial nucleobase moieties, dye complexes, intercalating dyes, and bioconjugated nanomaterials (AgNCs, GO, QDs, CDs, and MOF) is described herein. To investigate these extremely efficient responsive agents for diagnostic and therapeutic applications in medicine, fluorescence sensors based on G-quadruplexes have also been employed as a quantitative visualization technique.

A fluorescent assay for alkaline phosphatase activity based on phosphorylation protection and DNAzyme-assisted amplification

Alkaline phosphatase is one of the most important tool enzymes and diseases indicator, monitoring ALP activity with convenient, precise, efficient and sensitive methods plays a fundamental role in modern life and healthcare industries. In this study, we described a novel method for ALP analysis based on Pb²⁺ dependent DNAzyme. By modifying DNAzyme sequence with terminal phosphate group and introducing exonuclease I (exo I), we managed to analyze ALP by utilizing its causal function of DNAzyme probe from exo I mediated degradation and function of triggering the subsequent cleavage of the hairpin reporting probe. Other than one amplificative strategy by DNAzyme mediated cleavage and cycle, this system also involved an exo I mediated degradation to further reduce the background noise. Combining stepwise fluorimetry and electrophoresis, we verified the detective mechanism of this proposed method. Further, after selectivity demonstration, this method achieved a considerable LOD of 0.0017 U L^-1 and linear range of 0.0025 U L^-1 to 250 U L^-1. For potential of practical application, this method also exhibited excellent performances in inhibitor screening and intracellular ALP assay, both with a linear fitting equation. Based on these results, this method should be highly committed for improving ALP analysis in modern life industry.

Biologically Augmented Suture for Ligament Bracing Procedures Positively Affects Human Ligamentocytes and Osteoblasts In Vitro

PURPOSE

The purpose was to evaluate the response of human ligamentocytes and osteoblasts after biological augmentation with thrombin, concentrated bone marrow aspirate (cBMA), or platelet-rich plasma (PRP) on two different types of nonresorbable flat braided suture used for ligament bracing.

METHODS

Uncoated (U) and collagen-coated (C) flat braided suture material was augmented with either thrombin (T), cBMA (B), PRP (P), or a combination of these three (A), while platelet-poor plasma was used as a source for fibrin (F) in each assay. Previously cultured ligamentocytes and osteoblasts were added with a defined density and assayed after the required time period for adhesion, proliferation, and alkaline phosphatase activity.

RESULTS

Biological augmentation of uncoated [(UFT, UFBT, UFA; P < .001), (UFPT; P = .017)] and collagen-coated suture (CFT, CFPT, CFBT, CFA; P < .001) led to a significantly higher ligamentocyte adhesion. Significantly higher adhesion was also observed for osteoblasts (UFT, UFPT, UFBT, UFA; P < .001; CFT, CFPT, CFBT, CFA; P < .001). Similarly, ligamentocyte proliferation was significantly higher [(UFT, UFPT, UFA; P = .009), (UFBT; P = .001), (CFT; P = .009), (CFBT; P = .001), and (CFA; P = .01)]. Osteoblasts showed significantly higher proliferation as well [(UFT, UFPT, UFA; P = .002), (UFBT; P = .001); (CFT: P = .003), and (CFPT, CFBT, CFA; P = .001)]. Augmentation with thrombin, PRP, and BMA for uncoated (UFT; P = .006, UFPT; P = .035, UFBT; P = .001) and BMA for coated suture (CFBT; P = .027) led to significantly higher alkaline phosphatase activity.

CONCLUSION

Biological enhancement of suture used for ligament bracing significantly increased ligamentocyte and osteoblast adhesion and proliferation, as well as alkaline phosphatase activity of osteoblasts in an in vitro model. After biological augmentation, cellular adhesion, proliferation, and alkaline phosphatase activity changed up to 1,077%, 190%, and 78%, respectively. Furthermore, no overall superiority between uncoated or collagen-coated suture material was observed for cellular adhesion, proliferation, or alkaline phosphatase activity.

CLINICAL RELEVANCE

This study provides in vitro data on a new treatment concept of biologic augmentation for acute ligamentous lesions treated with ligament bracing that has not been widely described. This concept may improve the healing of injured ligaments, in addition to providing immediate biomechanical stabilization.

Target-induced inner-filter effect-based ratiometric sensing platform by fluorescence modulation of persistent luminescent nanoparticles and 2, 3-diaminophenazine

A ratiometric fluorescence sensing platform with smartphone was designed for highly sensitive detection of glutathione (GSH), ascorbic acid (AA), and alkaline phosphatase (ALP) activity by modulation the inner filter effect...

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.

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.

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.

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.

Reducing background absorbance via a double-lock strategy for detection of alkaline phosphatase and α-fetoprotein

Lowering the background signal for more sensitive analysis of determinands is as important as amplifying the target signal. The photoinduced oxidase of fluorescein has been reported, which can catalyze the oxidization of common substrates in a few minutes. As a metaphor for locks and keys, we designed double locks confining the activity of fluorescein to reduce the background absorbance during colorimetric detection. The first lock inhibits the main activity of fluorescein by phosphating. The second lock almost completely deactivates fluorescein by forming coordination nanoparticles (CNPs) via the self-assembly of cerium chloride and fluorescein diphosphate (FDP). The Ce-FDP CNPs are characterized by scanning electron microscope (SEM), dynamic light scattering (DLS), Fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy dispersive spectrum (EDS), which show electrostatic formation and amorphous character in the morphology. Alkaline phosphatase (ALP), the key to release fluorescein, can destroy Ce-FDP CNPs along with decomposing FDP by degrading phosphate groups. Therefore, a novel colorimetric strategy for sensitive detection of ALP is established. The detection of α-fetoprotein (AFP) is further succeeded by labeling AFP antibody with ALP. By dramatically reducing the background absorbance, the detection limits of ALP and AFP are as low as 0.014 mU/mL and 0.023 ng/mL, respectively. This convenient, brief, sensitive assay provides a promising prospect for clinical diagnosis. Graphical abstract.

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 novel fluorescent assay based on DNAzyme-assisted detection of prostate specific antigen for signal amplification

Prostate specific antigen (PSA) is one of the most common biomarkers for the management of prostate cancer. However, it still remains urgent to develop highly sensitive, cost-effective and selective strategies for PSA assay. In this paper, we developed a low-cost, highly sensitive and specific analytical strategy for the detection of PSA by using a fluorescence sensor based on Pb²⁺-dependent DNAzyme. We designed a DNA sequence called cmMB with a hairpin structure, containing PSA-specific aptamers and Pb²⁺-dependent DNAzyme chains. Also, a fluorophore-labelled DNA sequence called Sub-FAM, which contains a cleavage site of Pb²⁺-dependent DNAzyme and serves as substrate, is also designed for the signal generation. In the presence of PSA, interaction between aptamer and PSA blocks the hairpin structure of cmMB, resulting in the formation of Pb²⁺-dependent DNAzyme with Pb²⁺. Then, Pb²⁺-dependent DNAzyme can cleavage Sub-FAM and produce a high fluorescence. In the absence of PSA, since Sub-FAM remains to be ssDNA and can be absorbed by GO, only low fluorescence can be detected. Under optimal experimental conditions, a good linear relationship in the range of 1-100 pg mL^-1 was exhibited, with a limit of detection (LOD) of 0.76 pg mL^-1. In addition, the proposed method has potential value in the diagnosis and monitoring of prostate cancer because of its good selectivity and practical application in biological samples.

A review on emerging principles and strategies for colorimetric and fluorescent detection of alkaline phosphatase activity

Alkaline phosphatase (ALP) is a natural enzyme that is able to catalyze the dephosphorylation of phosphate esters. It participates in a great number of biological processes ranging from various metabolisms to signal transduction and cellular regulation. Since the abnormality of ALP activity in body is closely associated with many diseases, it has become an important biomarker for clinical diagnosis and treatment. Besides, it is often utilized in enzyme-linked immunosorbent assays. Given these demands, in the last few years considerable interest has been focused on exploring new materials and methods for ALP activity detection. In this review, we first made a clear classification on the principles that could be used for ALP activity determination. After that, emerging colorimetric and fluorescent strategies designed on the basis of these principles were systematically summarized. Finally, some perspectives on ALP activity analysis were discussed, hoping to inspire future efforts in the field.

Amplified colorimetric detection of tetracycline based on an enzyme-linked aptamer assay with multivalent HRP-mimicking DNAzyme

Tetracycline (TC) is widely used to treat bacterial infections in humans and animals due to its low price and good antibacterial properties. The abuse of tetracycline has led to TC residues in daily food that could seriously affect human health. Thus, it is imperative to develop highly sensitive and selective methods for TC detection. In this work, we developed a colorimetric method for TC detection based on an enzyme-linked aptamer assay (ELAA) with multivalent HRP-mimicking DNAzyme. An aptamer was used as an alternative recognition element in the enzyme-linked immunosorbent assay (ELISA). Multivalent HRP-mimicking DNAzyme, assembled via hybridization chain reactions (HCR), was used for catalytic substrate color rendering in ELAA. The multivalent HRP-mimicking DNAzyme exhibited enhanced catalytic capacity and improved the detection sensitivity greatly. The limit of detection was 8.1 × 10-2 ng mL-1 with a linear range from 1.0 × 10-2 ng mL-1 to 1.0 × 104 ng mL-1 toward TC in buffer. To challenge the practical application capability of this strategy, the detection of TC in milk samples was also investigated and showed similar linear relationships. Due to the introduction of an aptamer, this ELAA strategy shows high selectivity towards TC and has potential for the detection of a wide spectrum of analytes.

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.

Visual detection of Fusarium proliferatum based on asymmetric recombinase polymerase amplification and hemin/G-quadruplex DNAzyme

A one-step and instrument-free visual method was established based on asymmetric recombinase polymerase amplification coupled with hemin/G-quadruplex DNAzyme for the detection of Fusarium proliferatum.

Asymmetric recombinase polymerase amplification and hemin/G-quadruplex DNAzyme-based visual detection of F. proliferatum is demonstrated.

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%).

A novel fluorometric method for inorganic pyrophosphatase detection based on G-quadruplex-thioflavin T

In this paper, we propose a fluorometric approach for the highly sensitive detection of inorganic pyrophosphatase (PPase) based on G-quadruplex-thioflavin T (ThT). In the absence of PPase, Cu²⁺ can coordinate with pyrophosphate (PPi) to generate a Cu²⁺/PPi complex. Then the G-rich sequence folds into the G-quadruplex structure, which can combine with ThT to generate a remarkable fluorescent signal. In the presence of PPase, the coordinated compound can be destroyed by the PPase catalyzed hydrolysis of PPi into inorganic phosphate (Pi). The subsequent release of Cu²⁺ can compete with ThT to induce a tighter G-quadruplex structure, causing the release of ThT and a sharp fluorescence decrease. Based on this mechanism, a facile and quantitative strategy for PPase detection was developed. The fluorescence intensity of the system shows a linear relationship with the PPase activities in the range of 0.5-30 U/L with a detection limit as low as 0.48 U/L. The proposed strategy for fluorescence spectrometric PPase detection is convenient, cost effective, and sensitive. This can be utilized to evaluate the inhibition effect of NaF on PPase as well as diagnose PPase-related diseases.

A novel label-free colorimetric detection of l-histidine using Cu2+-modulated G-quadruplex-based DNAzymes

We proposed a colorimetric method for l-histidine detection based on Cu²⁺-mediated DNAzyme and G-quadruplex-hemin complex catalyzed oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS). In this system, after the addition of l-histidine, the formation of G-quadruplex-hemin complex will be disturbed, thus the colorimetric signal intensity conversely corresponds to the concentration of histidine. In this assay, a lower detection limit of l-histidine (50 nM) is addressed comparing to previously reported colorimetric methods. The cost is extremely low as the proposed design is both label-free and enzyme-free. All the more vitally, the colorimetric detection procedure is substantially straightforward with no further modification processes. By and large, the sensor can provide a promising plan for the detection of l-histidine.

A Label-Free Fluorescent Assay for the Rapid and Sensitive Detection of Adenosine Deaminase Activity and Inhibition

Adenosine deaminase (ADA), able to catalyze the irreversible deamination of adenosine into inosine, can be found in almost all tissues and plays an important role in several diseases. In this work, we developed a label-free fluorescence method for the detection of adenosine deaminase activity and inhibition. In the presence of ADA, ATP has been shown to be hydrolyzed. The ATP aptamer was shown to form a G-quadruplex/thioflavin T (ThT) complex with ThT and exhibited an obvious fluorescence signal. However, the ATP aptamer could bind with ATP and exhibited a low fluorescence signal because of the absence of ADA. This assay showed high sensitivity to ADA with a detection limit of 1 U/L based on an SNR of 3 and got a good linear relationship within the range of 1–100 U/L with R² = 0.9909. The LOD is lower than ADA cutoff value (4 U/L) in the clinical requirement and more sensitive than most of the reported methods. This technique exhibited high selectivity for ADA against hoGG I, UDG, RNase H and λexo. Moreover, this strategy was successfully applied for assaying the inhibition of ADA using erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA) and, as such, demonstrated great potential for the future use in the diagnosis of ADA-relevant diseases, particularly in advanced drug development.

Highly sensitive fluorometric determination of thrombin by on-chip signal amplification initiated by terminal deoxynucleotidyl transferase

The article describes an on-chip amplification scheme initiated by a terminal deoxynucleotidyl transferase (TdT) for highly sensitive fluorometric determination of protein. Two thrombin-binding aptamers were designed to capture thrombin as they can form a sandwich structure for improved specificity. An amino-modified aptamer (TBA29) was first immobilized on a silicon chip. After capture of thrombin, a second aptamer (TBA15) was conjugated to the second binding site of thrombin. The 3'-terminal of aptamer TBA15 is exposed on the chip surface, and then fluorescein-labeled 12-dATP associates to the 3'-terminal with the help of TdT. This results in signal amplification, and eventually leads to highly sensitive detection. Under optimal conditions, fluorescence intensity is linearly related to the logarithm of thrombin concentration in the range of 100 fM - 0.1 μM, and the detection limit is as low as 2.0 fM. The assay is sensitive and selective even over potentially interfering proteins and in the presence of human serum. Graphical abstract Schematic strategy for thrombin detection. Two thrombin-binding aptamers were designed to capture thrombin to form a sandwich structure for improved specificity. The protein detection is based on TdT initiated on-chip fluorescent amplification.

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 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.