Fluorometric Method for Inorganic Pyrophosphatase Activity Detection and Inhibitor Screening Based on Click Chemistry

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 %.

Thermophilic Inorganic Pyrophosphatase Ton1914 from Thermococcus onnurineus NA1 Removes the Inhibitory Effect of Pyrophosphate

Pyrophosphate (PP_i) is a byproduct of over 120 biosynthetic reactions, and an overabundance of PP_i can inhibit industrial synthesis. Pyrophosphatases (PPases) can effectively hydrolyze pyrophosphate to remove the inhibitory effect of pyrophosphate. In the present work, a thermophilic alkaline inorganic pyrophosphatase from Thermococcus onnurineus NA1 was studied. The optimum pH and temperature of Ton1914 were 9.0 and 80 °C, respectively, and the half-life was 52 h at 70 °C and 2.5 h at 90 °C. Ton1914 showed excellent thermal stability, and its relative enzyme activity, when incubated in Tris-HCl 9.0 containing 1.6 mM Mg²⁺ at 90 °C for 5 h, was still 100%, which was much higher than the control, whose relative activity was only 37%. Real-time quantitative PCR (qPCR) results showed that the promotion of Ton1914 on long-chain DNA was more efficient than that on short-chain DNA when the same concentration of templates was supplemented. The yield of long-chain products was increased by 32-41%, while that of short-chain DNA was only improved by 9.5-15%. Ton1914 also increased the yields of UDP-glucose and UDP-galactose enzymatic synthesis from 40.1% to 84.8% and 20.9% to 35.4%, respectively. These findings suggested that Ton1914 has considerable potential for industrial applications.

Triazol-Methanaminium-Pillar[5]arene-Functionalized Single Nanochannel for Quantitative Analysis of Pyrophosphate in Water

Inorganic pyrophosphate (PPi) is an important biological functional anion and plays crucial roles in life science, environmental science, medicine, and chemical process. Quantification of PPi in water has far-reaching significance for life exploration, disease diagnosis, and water pollution control. The label-free quantitative detection of PPi anions with a nanofluidic sensing device based on a conical single nanochannel is demonstrated. The channel surface is functionalized with a synthetic PPi receptor, triazol-methanaminium-functionalized pillar[5]arene (TAMAP5), using carbodiimide coupling chemistry. Due to the specific binding between TAMAP5 and PPi, the functionalized nanochannel can discriminate PPi from other inorganic anions with high selectivity through ionic current recording, even in the presence of various interfering anions. The current response exhibits a linear correlation with PPi concentration in the range from 1 × 10^-7 to 1 × 10^-4 M with a limit of detection of 6.8 × 10^-7 M. A spike-and-recovery analysis of PPi in East Lake water samples indicates that the proposed nanofluidic sensor has the ability to quantitate micromolar concentrations of PPi in environmental water samples.

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%.

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.

Homogeneous assay based on the pre-reduction and selective cation exchange for detection of multiple targets by atomic spectrometry

In view of the high sensitivity and good selectivity, chemical vapor generation atomic spectrometry (CVG-AS) and inductively coupled plasma mass spectrometer (ICP-MS), especially low-cost atomic fluorescence spectrometry (AFS) have been widely used in bioassay. However, the existing AS method is mostly based on heterogeneous strategies, and can't detect multiple targets in one system. In this study, we present the discovery and mechanism study of a phenomenon of Hg²⁺ pre-reduction that the concentration of Hg²⁺ decreased when it was mixed with the reductants (ascorbic acid (AA), SnCl₂, or NaBH₄/KBH₄) over long-time reaction (hours) by CVG-AFS and ICP-MS. A homogeneous Cu²⁺ assay method was developed based on the competition reaction of Cu²⁺ and Hg²⁺ for consuming AA, and its application in the detection of pyrophosphate (PPi) and alkaline phosphatase (ALP) was investigated based on the PPi complexation with Cu²⁺, and ALP hydrolyzation of PPi using CVG-AFS as a representative detector. Subsequently, in order to further verify the applicability of the system, cation exchange reaction (CER) was utilized here based on the selectively recognize Ag⁺ and C-Ag⁺-C by CuS nanoparticles (NPs). As the exchanged Cu²⁺ from CuS NPs can be sensitively and selectively detected via above-mentioned Cu²⁺ assay method, this strategy can be extended for the Ag⁺, DNA and prostate specific antigen (PSA) detection based on base complementary pairing and the specific recognition of aptamer. Under the optimal experimental conditions, the system showed high sensitivity for the detection of Cu²⁺, PPi, ALP, Ag⁺, DNA, and PSA, with limit of detections (LODs) of 0.12 nmol L^-1, 25 μmol L^-1, 0.025 U/L, 0.2 nmol L^-1, 0.05 nmol L^-1, and 0.03 ng/mL, respectively. The method was successfully used to determination Cu²⁺, ALP, and PSA in human serums, showing similar results with those of ICP-MS and kits methods.

A Highly Sensitive Electrochemiluminescence Biosensor for Pyrophosphatase Detection Based on Click Chemistry-Triggered Hybridization Chain Reaction in Homogeneous Solution

The abnormal expression of pyrophosphatase (PPase) is closely related to many diseases and malignant tumors, so the detection for PPase is of great significance in clinical diagnosis, disease monitoring, and other biomedical aspects. In this study, a sensitive and specific electrochemiluminescence (ECL) biosensor combined highly specific Cu⁺-catalyzed azide-alkyne cycloaddition (CuAAC) with high efficiency of hybridization chain reaction (HCR) for the purpose of detecting pyrophosphatase has been designed. Highly efficient hybridization chain reaction amplification processed in homogeneous solution and the amplification products were connected to the electrode surface in one step, which solved the problem of low DNA amplification efficiency on the electrode surface because of the steric hindrance. Ru(phen)₃²⁺ was embedded into the dsDNA and functioned as ECL probes; the enhanced ECL intensity of the system had a linear relationship with the logarithm of PPase concentration in the range of 0.025-50 mU with a detection limit of 8 μU. The method was proved to be of good specificity, repeatability, and stability that could be used for screening and quantitatively determining pyrophosphatase inhibitor sodium fluoride. The practicability of this method in clinical application has been proved through the detection of serum from the clinical arthritis patients. Moreover, the method can be used to monitor PPase activity of arthritis patients before and after administration to provide reference for the effect of drug treatment.

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.

Dual-Modal Imaging-Guided Precise Tracking of Bioorthogonally Labeled Mesenchymal Stem Cells in Mouse Brain Stroke

Noninvasive and precise stem cell tracking after transplantation in living subject is very important to monitor both stem cell destinations and their in vivo fate, which is closely related to their therapeutic efficacy. Herein, we developed bicyclo[6.1.0]nonyne (BCN)-conjugated glycol chitosan nanoparticles (BCN-NPs) as a delivery system of dual-modal stem cell imaging probes. Near-infrared fluorescent (NIRF) dye Cy5.5 was chemically conjugated to the BCN-NPs, and then oleic acid-coated superparamagnetic iron oxide nanoparticles (OA-Fe₃O₄ NPs) were encapsulated into BCN-NPs, resulting in Cy5.5-labeled and OA-Fe₃O₄ NP-encapsulated BCN-NPs (BCN-dual-NPs). For bioorthogonal labeling of human adipose-derived mesenchymal stem cells (hMSCs), first, hMSCs were treated with tetra-acetylated N-azidoacetyl-d-mannosamine (Ac₄ManNAz) for generating azide (-N₃) groups onto their surface via metabolic glycoengineering. Second, azide groups on the cell surface were successfully chemically labeled with BCN-dual-NPs via bioorthogonal click chemistry in vitro. This bioorthogonal labeling of hMSCs could greatly increase the cell labeling efficiency, safety, and imaging sensitivity, compared to only nanoparticle-derived labeling technology. The dual-modal imaging-guided precise tracking of bioorthogonally labeled hMSCs was tested in the photothrombotic stroke mouse model via intraparenchymal injection. Finally, BCN-dual-NPs-labeled hMSCs could be effectively tracked by their migration from the implanted site to the brain stroke lesion using NIRF/T₂-weighted magnetic resonance (MR) dual-modal imaging for 14 days. Our observation would provide a potential application of bioorthogonally labeled stem cell imaging in regenerative medicine by providing safety and high labeling efficiency in vitro and in vivo.

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.

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.

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 carbon quantum dot-based fluorescent nanosensor for selective detection of flumioxazin in real samples

Herein, a carbon quantum dot-based highly selective luminescent probe has been designed for the detection of the pesticide flumioxazinviathe alkyne azide click reaction.

Rhodamine B derivatives-modified upconversion nanoparticles as a fluorescent turn-off–on sensor for the highly sensitive detection of Cu2+ and pyrophosphate

Rhodamine B derivatives (RBP)-modified UCNPs (UCNPs@mSiO₂–RBP) were developed as a fluorescent turn-off–on sensor based on FRET and IFE to detect Cu²⁺ and pyrophosphate (PPi) with a wide linear response range (0–10 μM for Cu²⁺ and 5–35 μM for PPi, much wider than that reported previously) and high sensibility (117 nM for Cu²⁺ and 70 nM for PPi). The MTT experiments and the bioimaging experiments show its promising prospect in tissue imaging.

A new fluorescent turn-off–on sensor was developed based on the the rhodamine B derivatives (RBP) modified UCNPs to detect Cu²⁺ and pyrophosphate (PPi).

ADP-ribosyl-N₃: A Versatile Precursor for Divergent Syntheses of ADP-ribosylated Compounds

Adenosine diphosphate-ribose (ADP-ribose) and its derivatives play important roles in a series of complex physiological procedures. The design and synthesis of artificial ADP-ribosylated compounds is an efficient way to develop valuable chemical biology tools and discover new drug candidates. However, the synthesis of ADP-ribosylated compounds is currently difficult due to structural complexity, easily broken pyrophosphate bond and high hydrophilicity. In this paper, ADP-ribosyl-N₃ was designed and synthesized for the first time. With ADP-ribosyl-N₃ as the key precursor, a divergent post-modification strategy was developed to prepare structurally diverse ADP-ribosylated compounds including novel nucleotides and peptides bearing ADP-ribosyl moieties.

High-throughput and ultratrace naked-eye colorimetric detection of Au3+ based on the gold amalgam-stimulated peroxidase mimetic activity in aqueous solutions

Herein, we present a catalysis-based, label-free, and efficient strategy for a rapid, high-throughput, highly selective and ultrasensitive naked-eye colorimetric assay of Au³⁺ in aqueous solutions, based on the gold amalgam-stimulated peroxidase mimetic activity.

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.

A fluorescent graphitic carbon nitride nanosheet biosensor for highly sensitive, label-free detection of alkaline phosphatase

Graphitic C3N4 (g-C3N4) nanosheets provide an attractive option for bioprobes and bioimaging applications. Utilizing highly fluorescent and water-dispersible ultrathin g-C3N4 nanosheets, a highly sensitive, selective and label-free biosensor has been developed for ALP detection for the first time. The developed approach utilizes a natural substrate of ALP in biological systems and thus affords very high catalytic efficiency. This novel biosensor is demonstrated to enable quantitative analysis of ALP in a wide range from 0.1 to 1000 U L(-1) with a low detection limit of 0.08 U L(-1), which is among the most sensitive assays for ALP. It is expected that the developed method may provide a low-cost, convenient, rapid and highly sensitive platform for ALP-based clinical diagnostics and biomedical applications.

A novel fluorescence assay for inorganic pyrophosphatase based on modulated aggregation of graphene quantum dots

A simple and highly sensitive fluorometric method has been developed for inorganic pyrophosphatase (PPase) activity detection based on the disaggregation and aggregation of graphene quantum dots (GQDs).

Tb-MOF: a naked-eye and regenerable fluorescent probe for selective and quantitative detection of Fe3+and Al3+ions

A unique Tb-MOF fluorescent probe has features that are visible to the naked-eye and can be regenerated; it presents high selectivity and sensitivity to the quantitative detection of Fe³⁺and Al³⁺ions.

Imine-functionalized thioether Zn(ii) turn-on fluorescent sensor and its selective sequential logic operations with H2PO4−, DFT computation and live cell imaging

Thioether Schiff base (H₂L), a nontoxic Zn²⁺-sensor (LOD, 0.050 μM) has shown selective ON–OFF emission following INHIBIT logic circuit with H₂PO₄⁻and useful agent for the identification of Zn²⁺and H₂PO₄⁻in intracellular fluid in living cells.