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.

CRISPR/Cas12a Collateral Cleavage Activity for Sensitive 3'-5' Exonuclease Assay

This study presents a technique for detecting 3'-5' exonuclease activity through the use of CRISPR/Cas12a. These enzymes, including 3'-5' exonuclease (Exo III), perform crucial roles in various cellular processes and are associated with life expectancy. However, imbalances in their expression can increase susceptibility to diseases such as cancer, particularly under prolonged stress. In this study, an activator sequence of CRISPR/Cas12a was constructed on the 5'-end of a hairpin probe (HP), forming a blunt end. When the 3'-end of the HP was hydrolyzed with Exo III activity, the activator sequence of Cas12a was exposed, which led to collateral cleavage of the DNA signal probe and generated a fluorescent signal, allowing sensitive and highly specific Exo III detection. This detection principle relied on the fact that Exo III exclusively cleaves the 3'-end mononucleotide of dsDNA and does not affect ssDNA. Based on this strategy, Exo III activity was successfully assayed at 0.0073 U/mL, demonstrating high sensitivity. In addition, this technique was used to screen candidate inhibitors of Exo III activity.

Assay of inorganic pyrophosphatase activity based on a fluorescence "turn-off" strategy using carbon quantum dots@Cu-MOF nanotubes

A highly sensitive and selective sensor for the quantitative assay of inorganic pyrophosphatase (PPase) activity was developed based on a fluorescence "turn-off" strategy. Carbon quantum dots@Cu(II)-based metal-organic framework nanotubes (CQDs@Cu-MOF) with length less than 300 nm and width less than 20 nm were synthesized. CQDs in the nanotubes exhibited weak fluorescence owing to static quenching. The coordination reaction between pyrophosphate ion (PPi) and Cu(II) decomposed CQDs@Cu-MOF and led to the release of CQDs, of which the fluorescence recovered. In the presence of PPase, the hydrolysis of PPi generated phosphate ion (Pi). CQDs@Cu-MOF remained their structural stability and the fluorescence turned off. The fluorescence intensity difference of the mixture of CQDs@Cu-MOF and PPi in the absence and presence of PPase (-ΔF) was proportional to the PPase concentration from 0.1 to 5 mU mL^-1 and that from 5 to 50 mU mL^-1, and a limit of detection at 0.03 mU mL^-1 was obtained. PPase activity in human serum was analyzed using the proposed fluorescence sensor and the recovery values were found to vary from 95.0% to 104 %.

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%.

One-step synthesis of fluorescence-enhanced carbon dots for Fe (III) on-off-on sensing, bioimaging and light-emitting devices

Carbon quantum dots (CDs), as one of the most potential fluorescent sensing materials, have attracted lots of attention in recent years. However, the low quantum yields, complicated separation and purification procedures have limited its application and large-scale production. In this paper, a facile and universal method was successfully developed to synthesize CDs-silica (SiO₂) spheres composites (CS composites), which not only demonstrates 10-fold fluorescence enhancement compared with single CDs but also enables the purification with a simple centrifugation step. Meanwhile, fabricated composites also presented superior photoluminescence (PL) stability and high PL intensity, even in some tough environments such as acid/alkali aqueous solution, high/low temperature, and high ion concentration aqueous solution. This simple, fast, low-cost, and efficient synthesis method would potentially expand the application of CDs for clinical analysis, optical sensing (ferric ion (Fe³⁺) and pyrophosphate), bioimaging and light-emitting diodes (LEDs).

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.

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.

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.

Recent progress in two-dimensional inorganic quantum dots

This review critically summarizes recent progress in the categories, synthetic routes, properties, functionalization and applications of 2D materials-based quantum dots (QDs).

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.

Ultrasensitive Profiling of Metabolites Using Tyramine-Functionalized Graphene Quantum Dots

Graphene quantum dots (GQDs) are emerging fluorescence reporters attractive for optical sensing, owing to their high photostability, highly tunable photoluminescence, molecular size, atomically thin structure, biocompatibility, and ease of functionalization. Herein, we present a fluorometric sensing platform based on tyramine-functionalized GQDs, which is able to detect a spectrum of metabolites with high sensitivity and specificity. Furthermore, multiparametric blood analysis (glucose, cholesterol, L-lactate, and xanthine) is demonstrated. This convenient metabolite profiling technique could be instrumental for diagnosis, study, and management of metabolic disorders and associated diseases, such as diabetes, obesity, lactic acidosis, gout, and hypertension.