Real-Time Colorimetric Assay of Inorganic Pyrophosphatase Activity Based on Reversibly Competitive Coordination of Cu2+ between Cysteine and Pyrophosphate Ion

Ultrasensitive NIR fluorometric assay for inorganic pyrophosphatase detection via Cu2+-PPi interaction using bimetallic Au-Ag nanoclusters

BACKGROUND

Inorganic pyrophosphatase (PPase) is key enzyme playing a key role in biochemical transformations such as biosynthesis of DNA and RNA, bone formation, metabolic pathways associated with lipid, carbohydrate and phosphorous. It has been reported that lung adenocarcinomas, colorectal cancer, and hyperthyroidism disorders can result from abnormal level of PPase. Therefore, it is of notable significance to develop simple and effective real time assay for PPase enzyme activity monitoring for screening of many metabolic pathways as well as for early disease diagnosis.

RESULT

The fluorometric detection of PPase enzyme in near infrared region-1 (NIR-1) has been carried out using bimetallic nanoclusters (LA@AuAg NCs). The developed sensing strategy was based on quenching of fluorescence intensity of LA@AuAg NCs upon interaction with copper (Cu2+) ions. The off state of LA@AuAg_Cu2+ ensemble was turned on upon addition of pyrophosphate anion (PPi) due to strong binding interaction between PPi and Cu2+. The catalytic conversion of PPi into phosphate anion (Pi) in the presence of PPase led to liberation of Cu2+ ions, and again quenched off state was retrieved due to interaction of free Cu2+ with LA@AuAg NCs. The ultrasensitive detection of PPase was observed in the linear range of 0.06-250 mU/mL with LOD as 0.0025 mU/mL. The designed scheme showed good selectivity towards PPase enzyme in comparison to other bio-substrates, along with good percentage recovery for PPase detection in real human serum samples.

SIGNIFICANCE

The developed NIR based assay is ultrasensitive, highly selective and robust for PPase enzyme and can be safely employed for other enzymes detection. This highly sensitive nature of biosensor was result of involvement of fluorescence-based technique and synergistic effect of dual metal in NIR based bimetallic NCs. Moreover, owing to the emission in NIR domain, in future, these nanoclusters can be safely employed for many biomedical applications for In vivo studies.

Stimuli-responsive aggregation-induced emission of molecular probes by electrostatic and hydrophobic interactions: Effect of organic solvent content and application for probing of alkaline phosphatase activity

We suggest that aggregation-induced emission (AIE) molecular probes with single charged/reactive group can exist in the formation of nanostructures but not monomers at extremely low organic solvent content. The nanoaggregates show good dispersivity and emit week emission. Stimuli-responsive assembly of nanoaggregates by electrostatic interactions can turn on the fluorescence, facilitating the design of biosensors with single-charged molecular probes as the AIE fluorogens. To prove the concept, tetraphenylethene-substituted pyridinium salt (TPE-Py) was used as the AIE fluorogen for probing of alkaline phosphatase (ALP) activity with pyrophosphate ion (PPi) as the enzyme substrate. The dynamic light scattering and transmission electron microscope experiments demonstrated that TPE-Py probes existed in aqueous solution at nanometer size and morphology. Stimuli such as the negatively charged PPi, citrate, ATP, ADP, NADP and DNA could trigger the aggregation of the positively charged TPE-Py nanoparticles, thus enhancing the fluorescence via AIE effect. ALP-enzymatic hydrolysis of PPi into two phosphate ions (Pi) limited the aggregation of TPE-Py nanoparticles. The strategy was used for the assay of ALP with a low detection limit (1 U/L) and wide linear range (1-200 U/L). We also investigated the effect of organic solvent content on the AIE process and found that high concentration of organic solvent can prevent the hydrophobic interaction between AIE molecules but show no essential influence on the electrostatic interaction-mediated assembly. The work should be evaluable for understanding AIE phenomenon and developing novel, simple and sensitive biosensors using a molecular probe with single charged/reactive group as the signal reporter.

Washing-Free and Label-Free Onsite Assay for Inorganic Pyrophosphatase Activity Using a Personal Glucose Meter

In this study, we demonstrated a personal glucose meter-based method for washing-free and label-free inorganic pyrophosphatase (PPase) detection, which relies on the cascade enzymatic reaction (CER) promoted by hexokinase and pyruvate kinase. In principle, the absence of target PPase enables adenosine triphosphate sulfurylase to catalyze the conversion of pyrophosphate (PPi) to ATP, a substrate of CER, which results in the significant reduction of glucose levels by the effective CER process. In contrast, the PPi cleavage activity works in the presence of target PPase by decomposing PPi to orthophosphate (Pi). Therefore, the CER process cannot be effectively executed, leading to the maintenance of the initial high glucose level that may be measured by a portable personal glucose meter. Based on this novel strategy, a quantitative evaluation of the PPase activity may be achieved in a dynamic linear range of 1.5-25 mU/mL with a detection limit of 1.18 mU/mL. Compared with the previous PPase detection methods, this method eliminates the demand for expensive and bulky analysis equipment as well as a complex washing step. More importantly, the diagnostic capability of this method was also successfully verified by reliably detecting PPase present in an undiluted human serum sample with an excellent recovery ratio of 100 ± 2%.

Detection of Pyrophosphate and Alkaline Phosphatase Activity Based on PolyT Single Stranded DNA - Copper Nanoclusters

The determination of pyrophosphate and alkaline phosphatase activity plays a significant role in medical diagnosis. In this work, a label-free "ON-OFF-ON" fluorescence strategy is developed for the analysis of pyrophosphate and alkaline phosphatase activity. Using PolyT single strand DNA as templates to synthesize fluorescent copper nanoparticles, the coordination effect of pyrophosphoric acid on Cu²⁺ inhibited the generation of fluorescence. Afterwards, the addition of alkaline phosphatase into hydrolyze pyrophosphoric acid resulted in the release of Cu^2+, whereby the fluorescence intensity could be recovered. Thereupon enhanced-sensitivity for alkaline phosphatase was obtained (0.1 mU/L), much better than previously reported methods. Meanwhile, it could be performed directly in homogeneous solution, which was very close to the actual activity level of alkaline phosphatase under physiological conditions. Likewise, satisfactory results were also obtained in specificity assessment, which demonstrated its potential application in clinical diagnosis. Notably, a new, sensitive, low-cost, short-time, and high-sensitivity platform for alkaline phosphatase detection was constructed, and the design of biosensor using DNA-templated Copper nanoclusters (CuNCs) was instructed in this study.

Plasmonic Nanomaterials for Colorimetric Biosensing: A Review

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

A Simple Colorimetric Assay for Sensitive Cu2+ Detection Based on the Glutathione-Mediated Etching of MnO2 Nanosheets

In this paper, we developed a quick, economical and sensitive colorimetric strategy for copper ions (Cu²⁺) quantification via the redox response of MnO₂ nanosheets with glutathione (GSH). This reaction consumed MnO₂ nanosheets, which acted as a catalyst for the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to a blue product (oxTMB). In the presence of Cu²⁺, the GSH was catalyzed to GSSG (oxidized glutathione), and the solution changed from colorless to deep blue. Under the optimum conditions, the absorption signal of the oxidized product (oxTMB) became proportional to Cu²⁺ concentration in the range from 10 to 300 nM with a detection limit of 6.9 nM. This detection system showed high specificity for Cu²⁺. Moreover, the system has been efficaciously implemented for Cu²⁺ detection in actual tap water samples. The layered-nanostructures of MnO₂ nanosheets make it possess high chemical and thermal stability. TMB can be quickly oxidized within 10 min by the catalyzing of MnO₂ nanosheets with high oxidase-like activity. There is no need of expensive reagents, additional H₂O₂ and complicated modification processes during the colorimetric assay. Therefore, the strategy primarily based on MnO₂ nanosheets is promising for real-time, rapid and highly sensitive detection of Cu²⁺ under practical conditions.

DNAzyme-Amplified Label-Free Biosensor for the Simple and Sensitive Detection of Pyrophosphatase

The level of pyrophosphatase (PPase) expression has been suggested as a potential biomarker of various cancers, and its prognostic value has been evaluated in patients suffering from lung cancer, colorectal cancer, and hyperthyroidism. However, the detection of PPase usually needs specific materials that require complicated, time-consuming reactions with restricted linear range and sensitivity, limiting their application in early clinical diagnosis. Herein, we developed a DNAzyme-based biosensor for the detection of PPase. In the presence of PPase, pyrophosphate (PPi) and Cu2+ ions released from the PPi-Cu2+-PPi complex induce the cleavage of the DNAzyme and the corresponding substrate. An apurinic/apyrimidinic (AP) site was elaborately designed within substrates that could encase the fluorophore 2-amino-5,6,7-trimethyl-1,8-naphthyridine (ATMND). The fluorescence of ATMND was initially quenched but restored when the DNAzyme/substrate complex was hydrolyzed with the release of ATMND. In this way, the PPase activity can be estimated by detecting the increased fluorescence of the released ATMND. Under optimized conditions, the activity of PPase could be analyzed at concentrations from 0.5 to 1000 mU, with the lowest detectable concentration being 0.5 mU. This work lays a foundation for developing a DNAzyme-amplified fluorescent biosensor with a high sensitivity, a wide linear range, and single-step operation for use as an easy diagnostic for PPase analysis.

Aggregation-Induced Emission of Au/Ag Alloy Nanoclusters for Fluorescence Detection of Inorganic Pyrophosphate and Pyrophosphatase Activity

The development of sensitive and accurate detection of inorganic pyrophosphate (PPi) and pyrophosphatase activity (PPase) is important as they play vital roles in biological systems. However, it is still not satisfactory for most of the analytical methods for PPi and PPase because of their Cu²⁺-dependence and poor accuracy. Although the metal ion triggered aggregation-induced emission (AIE) of metal nanoclusters (NCs) offers a new approach to design a Cu²⁺-free strategy for the accurate determination of PPi and PPase recently, current methods are all focused on utilizing pure metal NCs. Alloy NCs incorporating the advantages of diverse metal usually can achieve improved behaviors in the application, such as enhanced sensitivity and stability. In this work, glutathione stabilized alloy Au/Ag NCs were synthesized via a simple method and used for the fluorescence detection of PPi and PPase based on a Zn²⁺-regulated AIE strategy. The controlled release of Zn²⁺ by PPi and PPase could regulate the AIE of Au/Ag NCs and be employed to response PPi concentration and PPase activity. This method processes simple procedure, high sensitivity and stability, and low toxicity. In addition, we also studied the AIE behaviors of this Au/Ag NCs and offer some fundamental understanding of the AIE properties of water-soluble alloy NCs. This study not only provides a straightforward and new approach for PPi and PPase determination but a basis for further study on the AIE properties of alloy NCs and their application.

Visual and dual-fluorescence homogeneous sensor for the detection of pyrophosphatase in clinical hyperthyroidism samples based on selective recognition of CdTe QDs and coordination polymerization of Ce3+

A visual / dual fluorescent strategy based on selective recognition of QDs and coordination polymerization of Ce³⁺ was developed for pyrophosphatase detection.

Single-strand DNA-scaffolded copper nanoclusters for the determination of inorganic pyrophosphatase activity and screening of its inhibitor

A fluorescence method for the determination of inorganic pyrophosphatase (PPase) activity has been established based on copper nanoclusters (CuNCs). The polythymine of 40 mer (T40) acts as a template for the reduction reaction from Cu²⁺ to Cu⁰ by ascorbic acid (AA). This reaction leads to the formation of fluorescent CuNCs with excitation/emission peaks at 340/640 nm. However, the higher binding affinity between inorganic pyrophosphate (PPi) and Cu²⁺ hinders the effective formation of CuNCs. This shows low fluorescence intensity. PPase catalyzes the hydrolysis of PPi into Pi during which free Cu²⁺ ions are produced. This facilitates the formation of fluorescent CuNCs. Thus, the fluorescence intensity was restored. The fluorescence enhancement of the system has a linear relationship with PPase activity in the range 0.3 to 20 mU·mL^-1, and the detection limit is0.2 mU·mL^-1. The relative intensity (I/I₀) at 640 nm for the analytical solution versus system is also employed to screen the inhibitor for PPase with high efficiency. Graphical abstract Schematic representation of a fluorescent assay for the determination of inorganic pyrophosphatase activity and screening its inhibitor based on single-strand polythymine-scaffolded copper nanoclusters.

An improved structure-switch aptamer-based fluorescent Pb2+ biosensor utilizing the binding induced quenching of AMT to G-quadruplex

An improved aptamer-based fluorescent Pb²⁺ biosensor utilizing the binding induced quenching of AMT to G-quadruplex has been rationally designed with a LOD of 3.6 nM. The utility of the developed biosensor was demonstrated by the successful detection of Pb²⁺ in real complex clinical samples with satisfactory recovery and good reproducibility.

Monitoring inorganic pyrophosphatase activity with the fluorescent dizinc(ii) complex of a macrocycle bearing one dansylamidoethyl antenna

The dizinc(ii) complexes of L were used for the recognition of anions by fluorescence spectroscopy (L is a heteroditopic hexaazamacrocycle with two diethylenetriamine coordination heads with 2-methylpyridyl and dansylamido ethyl arms, and m-xylyl spacers). The protonation of L and stability constants of its zinc(ii) complexes were determined in aqueous solution, at 298.2 ± 0.1 K and I = 0.10 ± 0.01 M in KNO₃. At a 2 : 1 Zn²⁺/L ratio, the dinuclear complexes clearly dominate. The ligand alone does not display fluorescence changes upon increasing the pH value, but in the presence of Zn²⁺ the emission reaches a maximum at pH ≅ 7.5, at which 95% of the ligand is in the dinuclear complex form. The emission appears concomitantly with the [Zn₂H_-1L]³⁺ species formation, which supports that the latter complex corresponds to the metal-promoted deprotonation of dansylamide NH. The [Zn₂H_-1L]³⁺ complexes were used for the recognition of phosphate and polyphosphate anions in aqueous solution buffered at pH 7.5 with 2 mM PIPPS, at 298.2 K. The binding of anions causes a decrease of the emission. The association constant determination revealed that HPPi^3- is the strongest bound anion (log K_app = 5.57), followed by HATP^3- (two times weaker), and the remaining anions show lower binding constants, with HPO₄^2- having the weakest uptake by the receptor. The observed selectivity of the [Zn₂H_-1L]³⁺ receptor for PPi in relation to HPO₄^2-, and the fact that the formation of the [Zn₂H_-1L]³⁺ complex is not disturbed by the presence of Mg²⁺, allowed monitoring of the PPi hydrolysis by using inorganic pyrophosphatase in real-time.

Gold nanozyme as an excellent co-catalyst for enhancing the performance of a colorimetric and photothermal bioassay

Advanced oxidation processes (AOPs) have recently proposed for advancing colorimetric sensing applications, owing to their excellent performance of sensitive color readout that generated from the oxidation of chromogenic substrates like 3,3',5,5'-tetramethylbenzidine (TMB) by reactive oxygen species (ROS) of AOPs such as ·OH and ·O₂^- radicals. However, the efficiency of ROS generation and the related H₂O₂ decomposition in most AOPs is quite low especially at neutral pH, which greatly hampered the practical sensing applications of the AOPs. We herein communicated that β-cyclodextrin (β-CD)-capped gold nanoparticles (β-CD@AuNPs) can promote catalysis at neutral pH for AOP as an excellent co-catalyst. In this strategy, inorganic pyrophosphate (PPi) ions was first used to coordinate with Cu²⁺ and form Cu²⁺-PPi complex. In the presence of hydrogen peroxide, target inorganic pyrophosphatase (PPase) can hydrolyze PPi into inorganic phosphate (Pi) and release free Cu²⁺ simultaneously, resulting in a Cu²⁺-triggered Fenton-like AOP reaction. The introduced β-CD@AuNPs acts as a co-catalyst, analogous to mediators in the most co-catalyzed system, to enhance the rate-limiting step of Cu²⁺/Cu⁺ conversion in Cu²⁺/H₂O₂ Fenton-like AOP and resulting in an efficient generation of ·OH and ·O₂^- radicals, which further producing an intense blue color by oxidizing TMB into its oxidation product (TMBox) within a short time. Finally, this reaction system was used to simply detecting target PPase with the colorimetric and photothermal readout based on the in-situ generated TMBox indicator. More significantly, we successfully demonstrated nanozyme can serve as a co-catalyst to promote the AOP catalysis at neutral pH, and inspire other strategies to overcome the pH limitation in the AOP catalysis and expand its colorimetric and photothermometric application.

Cu(II)-Regulated On-Site Assembly of Highly Chemiluminescent Multifunctionalized Carbon Nanotubes for Inorganic Pyrophosphatase Activity Determination

A novel signal-on chemiluminescence (CL) assay for pyrophosphatase (PPase) activity determination was innovatively developed based on the Cu(II)-regulated on-site assembly of highly chemiluminescent Cu(II), N-(aminobutyl)-N-(ethylisoluminol) (ABEI), gold nanodot, and chitosan multifunctionalized carbon nanotubes (Cu(II)/ABEI-Au/cs-CNTs). First, ABEI-functionalized gold nanodots (ABEI-Au) were assembled on the surface of chitosan-modified carbon nanotubes (cs-CNTs) via the reduction of HAuCl₄ with ABEI in a cs-CNT suspension to form ABEI-Au/cs-CNTs. Then, it was found that the catalyst Cu(II) can be selectively, efficiently, and quickly adsorbed onto ABEI-Au/cs-CNTs via the high-affinity interactions between Cu(II) and cs-CNTs to form novel hybrid nanomaterials Cu(II)/ABEI-Au/cs-CNTs. The CL intensity of Cu(II)/ABEI-Au/cs-CNTs was enhanced by about 2 orders of magnitude compared with that of ABEI-Au/cs-CNTs. Furthermore, it was found that in the presence of pyrophosphate ions (PPi), PPi could coordinate with Cu(II) to form a stable PPi-Cu(II) complex and block the assembly of Cu(II)/ABEI-Au/cs-CNTs. After the addition of PPase, PPase could catalyze the hydrolysis of PPi into Pi and release Cu(II) from the PPi-Cu(II) complex. The released free Cu(II) could trigger the on-site assembly of highly chemiluminescent Cu(II)/ABEI-Au/cs-CNTs, resulting in an enhanced CL intensity. The enhanced CL intensity had a good linear relationship with the activity units of PPase ranging from 0.025 to 0.5 U, with a detection limit of 9 mU. The method was employed to monitor the PPase inhibitor efficiently. Cu(II)/ABEI-Au/cs-CNTs with excellent CL may also find more applications in the development of novel CL analytical methods.

A novel colorimetric immunoassay based on enzyme-regulated instant generation of Turnbull's blue for the sensitive determination of ochratoxin A

The aim of this study was to develop a novel colorimetric sensing method based on enzyme-regulated instant generation of Turnbull's blue, serving as a chromogenic agent, for a sensitive immunoassay for the determination of ochratoxin A (OTA).

Zinc ion-triggered aggregation induced emission enhancement of dual ligand co-functionalized gold nanoclusters based novel fluorescent nanoswitch for multi-component detection

Herein, a zinc ion (Zn²⁺)-triggered aggregation induced emission enhancement (AIEE) fluorescence "on-off-on" nanoswitch was fabricated for inorganic pyrophosphate (PPi) and inorganic pyrophosphatase (PPase) activity detection. Dual ligand functionalized Au NCs were utilized as the substrate of the AIEE nanoswitch. The introduction of Zn²⁺ can cause Au NCs aggregated along with the enhanced fluorescence. After the addition of PPi, aggregated Au NCs disaggregated along with decreased fluorescence due to the competitive combination between PPi and Zn²⁺ (on-off). When PPase was introduced, PPi was hydrolyzed and release Zn²⁺, resulting in aggregated Au NCs along with enhanced fluorescence again (off-on). On the basis of this, highly selective and sensitive detection PPi (liner range from 0.1 to 300 μM) and PPase activity (liner range from 0.1 to 10 mU) can be achieved. The detection limits are 0.04 μM for PPi and 0.03 mU for PPase, respectively. Furthermore, the as-prepared Zn²⁺-triggered AIEE nanoswitch was successfully used for quantitative analysis of PPase activity in human serum with satisfactory spiked recoveries, and applied for the inhibitors screening.

Coordination polymers of Tb3+/Nucleotide as smart chemical nose/tongue toward pattern-recognition-based and time-resolved fluorescence sensing

The abundant functional groups on guanosine monophosphate (GMP) make it possible to interact with various metal ions. The subtle difference in the structure of GMP and deoxy-guanosine monophosphate (dGMP) coupled with Tb3+ can be readily exploited to form two coordination polymers, which have been unveiled as two time-resolved fluorescence (TRF) sensing reporters (Tb-GMP and Tb-dGMP) in our study. Based on this finding, herein, we have proposed a novel TRF orthogonal sensing array (Tb-GMP/dGMP) for pattern-recognition-based sensing of various metal ions. In addition, upon integration of some thiol-affinity metal ions, Tb-GMP/dGMP can be further extended to construct two metal ion-involved pattern-recognition-based sensor arrays (Tb-GMP/dGMP-Cu, Tb-GMP/dGMP-Ag) for the TRF sensing different levels of disease-relevant biothiols in biofluids, illustrating the powerful and multifunctional capabilities of the Tb-GMP/dGMP system and would inspire simpler and more widespread designs of chemical nose/tongue-based applications.

Fluorometric determination of the activity of inorganic pyrophosphatase and its inhibitors by exploiting the peroxidase mimicking properties of a two-dimensional metal organic framework

A copper(II)-based two-dimensional metal-organic framework with nanosheet structure (CuBDC NS) that possesses peroxidase (POx) mimicking activity was prepared. In the presence of hydrogen peroxide, the system catalyses the oxidation of terephthalic acid to a blue-fluorescent product (excitation = 315 nm; emission = 425 nm). Pyrophosphate has a very strong affinity for Cu²⁺ ion and blocks the POx-mimicking activity of the CuBDC NS. If, however, inorganic pyrophosphatase is present, the POx mimicking activity is gradually restored because pyrophosphate is hydrolyzed. The findings were used to design a method for the determination of the activity of inorganic pyrophosphatase by fluorometry. Fluorescence increases linearly in the 1-50 mU·mL^-1 inorganic pyrophosphatase activity range. The limit of detection is 0.6 mU·mL^-1 (S/N = 3). Graphical abstract A copper(II)-based two-dimensional metal-organic framework (CuBDC NS) is described that possesses POx-mimicking activity. Inorganic pyrophosphate (PPi) was hydrolyzed to phosphate in the presence of inorganic pyrophosphatase (PPase). Hence, it cannot coordinate with Cu²⁺ in CuBDC NS, its structure was well-conserved to catalyses the oxidation of terephthalic acid (H₂BDC) to produce a blue fluorescent product (oxBDC) in the presence of hydrogen peroxide (H₂O₂).

Quantum Dot Doping-Induced Photoluminescence for Facile, Label-Free, and Sensitive Pyrophosphatase Activity Assay and Inhibitor Screening

Development of simple, convenient, and sensitive assay methods for pyrophosphatase (PPase) activity is of importance, for disease diagnosis and drug discovery. Herein, a simple, rapid, label-free, and sensitive fluorescence sensor for PPase activity assay is developed, using Cu²⁺ doping-induced quantum dot (QD) photoluminescence as a signal reporter. The Cu²⁺ doping of ZnSe QD can induce a dopant-dependent emission response, which will be inhibited after the premixing of Cu²⁺ with pyrophosphate (PPi), to form a Cu²⁺-PPi complex. Then, the hydrolysis of PPi into phosphate (Pi), specifically catalyzed by PPase, liberates the free Cu²⁺ to regain the QD doping for the fluorescence response, which is highly dependent on the PPase activity. The PPase can be sensitively and selectively assayed, with a detection limit of 0.1 mU/mL. The developed sensing strategy can be also employed for the PPase inhibitor screening. Thus, the current QD doping-based sensing strategy offers an efficient and promising avenue for Cu²⁺, PPi, or PPase-related target analysis, and might hold great potential for the further applications in the clinical disease diagnosis.

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.

Color-Coded Single-Particle Pyrophosphate Assay with Dark-Field Optical Microscopy

In this work, we demonstrate a convenient yet sensitive color-coded single-particle detection method for the quantification of pyrophosphate (PPi) by using single gold nanoparticle (GNP) as the probe. The design is based on GNP-dependent catalytic deposition of Cu onto the surface of GNPs with reduced nicotinamide adenine dinucleotide (NADH). Without PPi, Cu²⁺ can be directly reduced to Cu⁰ through the gold-catalyzed oxidization of NADH. In the presence of PPi, the coating process is impeded due to the strong coordination capability of PPi with Cu²⁺. The selective coating of Cu shell onto the GNPs surface results in the extraordinary red-shift of localized surface plasmon resonance from individual GNPs. By quantitatively counting the fraction of yellow particles with color-coded dark-field optical microscopy, the trace amounts of PPi in solution can be accurately quantified. The limit-of-detection is as low as 1.49 nM with a linear dynamic range of 0-4.29 μM, which is much lower than the spectroscopic measurements in bulk solution. In artificial urine sample, good recovery efficiency was achieved. As a consequence, the method demonstrated herein will find promising applications for the ultrasensitive detection of target biomolecules under biological milieu in the future.

Multiplexed ratiometric photoluminescent detection of pyrophosphate using anisotropic boron-doped nitrogen-rich carbon rugby ball-like nanodots

B-doped N-rich carbon rugby ball-like nanodots have been synthesized by a heterophase polymerization route and can act not only as a ratiometric sensor for Hg²⁺ but also as a dual-mode PPi-selective sensor.

Gold nanoparticle-based colorimetric biosensors

Gold nanoparticles (AuNPs) provide excellent platforms for the development of colorimetric biosensors as they can be easily functionalised, displaying different colours depending on their size, shape and state of aggregation. In the last decade, a variety of biosensors have been developed to exploit the extent of colour changes as nano-particles (NPs) either aggregate or disperse, in the presence of analytes. Of critical importance to the design of these methods is that the behaviour of the systems has to be reproducible and predictable. Much has been accomplished in understanding the interactions between a variety of substrates and AuNPs, and how these interactions can be harnessed as colorimetric reporters in biosensors. However, despite these developments, only a few biosensors have been used in practice for the detection of analytes in biological samples. The transition from proof of concept to market biosensors requires extensive long-term reliability and shelf life testing, and modification of protocols and design features to make them safe and easy to use by the population at large. Developments in the next decade will see the adoption of user friendly biosensors for point-of-care and medical diagnosis as innovations are brought to improve the analytical performances and usability of the current designs. This review discusses the mechanisms, strategies, recent advances and perspectives for the use of AuNPs as colorimetric biosensors.

A new method to investigate the catalytic mechanism of YhdE pyrophosphatase by using a pyrophosphate fluorescence probe

YhdE is a Maf (multicopy associated filamentation) proteins from Escherichia coli which exhibits pyrophosphatase activity towards selected nucleotides, although its catalytic mechanism remains unclear. Herein we used a novel fluorescence probe (4-isoACBA–Zn(II) complex) to characterize the enzymatic properties of YhdE and its mutant, establishing a new method for assaying pyrophosphatase catalytic function. Our results reveal for the first time that the new fluorescence sensor confers high sensitivity and specificity and pyrophosphate (PPi) is the direct catalytic product of YhdE. Crystal structures of a mutant in the active-site loop (YhdE_E33A) show conformational flexibility implicated in the catalytic mechanism of YhdE. ITC experiments and computational docking further reveal that Asp70 and substrate dTTP coordinate Mn²⁺. Quantum mechanics calculations indicate that YhdE hydrolysis appears to follow a stepwise pathway in which a water molecule first attacks the α-phosphorus atom in the substrate, followed by the release of PPi from the pentavalent intermediate.

A Colorimetric Sensor for the Highly Selective Detection of Sulfide and 1,4-Dithiothreitol Based on the In Situ Formation of Silver Nanoparticles Using Dopamine

Hydrogen sulfide (H₂S) has attracted attention in biochemical research because it plays an important role in biosystems and has emerged as the third endogenous gaseous signaling compound along with nitric oxide (NO) and carbon monoxide (CO). Since H₂S is a kind of gaseous molecule, conventional approaches for H₂S detection are mostly based on the detection of sulfide (S^2-) for indirectly reflecting H₂S levels. Hence, there is a need for an accurate and reliable assay capable of determining sulfide in physiological systems. We report here a colorimetric, economic, and green method for sulfide anion detection using in situ formation of silver nanoparticles (AgNPs) using dopamine as a reducing and protecting agent. The changes in the AgNPs absorption response depend linearly on the concentration of Na₂S in the range from 2 to 15 μM, with a detection limit of 0.03 μM. Meanwhile, the morphological changes in AgNPs in the presence of S^2- and thiol compounds were characterized by transmission electron microscopy (TEM). The as-synthetized AgNPs demonstrate high selectivity, free from interference, especially by other thiol compounds such as cysteine and glutathione. Furthermore, the colorimetric sensor developed was applied to the analysis of sulfide in fetal bovine serum and spiked serum samples with good recovery.

A novel dual-functional biosensor for fluorometric detection of inorganic pyrophosphate and pyrophosphatase activity based on globulin stabilized gold nanoclusters

A novel ultrasensitive dual-functional biosensor for highly sensitive detection of inorganic pyrophosphate (PPi) and pyrophosphatase (PPase) activity was developed based on the fluorescent variation of globulin protected gold nanoclusters (Glo@Au NCs) with the assistance of Cu²⁺. Glo@Au NCs and PPi were used as the fluorescent indicator and substrate for PPase activity evaluation, respectively. In the presence of Cu²⁺, the fluorescence of the Glo@Au NCs will be quenched owing to the formation of Cu²⁺-Glo@Au NCs complex, while PPi can restore the fluorescence of the Cu²⁺-Glo@Au NCs complex because of its higher binding affinity with Cu²⁺. As PPase can catalyze the hydrolysis of PPi, it will lead to the release of Cu²⁺ and re-quench the fluorescence of the Glo@Au NCs. Based on this mechanism, quantitative evaluation of the PPi and PPase activity can be achieved ranging from 0.05 μM to 218.125 μM for PPi and from 0.1 to 8 mU for PPase, with detection limits of 0.02 μM and 0.04 mU, respectively, which is much lower than that of other PPi and PPase assay methods. More importantly, this ultrasensitive dual-functional biosensor can also be successfully applied to evaluate the PPase activity in human serum, showing great promise for practical diagnostic applications.

Colorimetric Logic Gate for Pyrophosphate and Pyrophosphatase via Regulating the Catalytic Capability of Horseradish Peroxidase

By regulating the catalytic capability of horseradish peroxidase (HRP), an artful colorimetric assay platform for pyrophosphate (PPi) and pyrophosphatase (PPase) was unprecedentedly designed. In this work, Cu(I), generated by reducing Cu(II) in the presence of ascorbate, could inhibit HRP's catalytic capability of transforming colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue oxidized TMB (oxTMB). The robust coordination between PPi and Cu(II) is able to discourage the reduction of Cu(II) to Cu(I) effectively, thus restoring the original catalytic capability of HRP and regenerating blue-colored oxTMB. Upon PPase introduction, PPi would be hydrolyzed into orthophosphate, which could release Cu(II) free from the Cu(II)-PPi complex, and thus in turn allows the catalytic capability of HRP to be inhibited by Cu(I). HRP was activated or deactivated to different degrees depending on PPi or PPase levels, which could be indicated by using HRP-triggered catalytic system as a signal amplifier, thus paving a way for PPi and PPase sensing. Based on the colorimetric sensor for PPi and PPase, an "INH" logic gate was rationally constructed. With the merits of high sensitivity and selectivity, cost-effectiveness, and simplification, our proposed analytical system has also been verified to have potential to be utilized for enzyme inhibitor screening and diagnosis of PPase-related diseases.

Construction of a Turn Off-On-Off Fluorescent System Based on Competitive Coordination of Cu2+ between 6,7-Dihydroxycoumarin and Pyrophosphate Ion for Sensitive Assay of Pyrophosphatase Activity

The detection of pyrophosphatase (PPase) activity is of great significance in diagnosing diseases and understanding the function of PPase-related biological events. This study constructed a turn off-on-off fluorescent system for PPase activity assay based on PPase-regulated competitive coordination of Cu²⁺ between a water-soluble fluorescent probe 6,7-dihydroxycoumarin (DHC) and pyrophosphate (PPi). The probe DHC can coordinate with Cu²⁺ and consequently display on-off type fluorescence response. Furthermore, the in situ formed nonfluorescent Cu²⁺-DHC complex can act as an effective off-on type fluorescent probe for sensing PPi due to the higher coordination reactivity between Cu²⁺ and PPi than that between Cu²⁺ and DHC. The subsequent addition of PPase to the mixture containing Cu²⁺, DHC, and PPi leads to the fluorescence requenching of the system again (an off state) because PPase catalyzes the hydrolysis of PPi into orthophosphate in the reaction system. Under the optimum conditions, the decrease of the fluorescence intensity of DHC-Cu²⁺-PPi system was linear with the increase of the PPase activity in the range from 0.1 to 0.3 U. The detection limit was down to 0.028 U PPase (S/N = 3). Moreover, the as-established system was also applied to evaluate PPase inhibitor. This study offers a simple yet effective method for the detection of PPase activity.

Steric hindrance regulated supramolecular assembly between β-cyclodextrin polymer and pyrene for alkaline phosphatase fluorescent sensing

We herein report a strategy for sensitive alkaline phosphatase (ALP) fluorescent sensing based on steric hindrance regulated supramolecular assembly between β-cyclodextrin polymer (polyβ-CD) and pyrene. The fluorescence of pyrene was enhanced more than 10 times through supramolecular assembly with polyβ-CD. The 5'-phosphorylated dsDNA probe with pyrene attached on the 3'-terminal could be cleaved by λ exonuclease (λ exo), yielding pyrene attached on mononucleotides. Pyrene attached on mononucleotides could easily enter the cavity of polyβ-CD, resulting in fluorescence enhancement. When ALP was introduced, it could remove 5'-phosphate groups from dsDNA and then prevented the cleavage of dsDNA. Pyrene attached on dsDNA was difficult to enter the cavity of polyβ-CD because of steric hindrance, resulting in an inconspicuous fluorescence enhancement. Owing to the excellent fluorescence enhancement during steric hindrance regulated supramolecular assembly, excellent performance of the assay method was achieved for ALP with a detection limit of 0.04 Um L(-1). The detection limit was superior or comparable with the reported methods. Besides, this method was simple in design, avoiding double-labeling of probe.

An anionic conjugated polymer as a multi-action sensor for the sensitive detection of Cu(2+) and PPi, real-time ALP assaying and cell imaging

A Cu(2+) ensemble polyfluorene derivative, poly[5,5'-(((9H-fluorene-9,9-diyl)bis(hexane-6,1-diyl))bis(oxy))diisophthalate] sodium salt (PFT), displays unprecedented selectivity for PPi (LOD = 2.26 ppb) in aqueous solution as well as in random urine samples at physiological pH vis-a-vis monitoring ALP activity. Furthermore, intracellular imaging of Cu(2+) and PPi in mouse macrophage (J774A.1) and human breast cancer cells (MDA-MB231) was achieved to confirm the viability of PFT in biological systems.