Smart probe for simultaneous detection of copper ion, pyrophosphate, and alkaline phosphatase in vitro and in clinical samples

Squaraine dye as a fluorescent probe for highly sensitive detection of pyrophosphate and alkaline phosphatase

We synthesized a squaraine dye (F-0) to develop a method for detecting pyrophosphate (PPi) and alkaline phosphatase (ALP) by modulating the fluorescence of F-0. The fluorescence intensity of the F-0 system was quenched upon the addition of Cu2+ ions; however, it was restored when PPi was introduced due to the formation of a complex between PPi and Cu2+. Since ALP can hydrolyze PPi, the fluorescence of the system was quenched again upon the addition of ALP. Based on these principles, we established a fluorescent probe that exhibits an "off-on-off" fluorescence response. The detection limits of this method for PPi and ALP were 103 nmol dm-3 and 0.18 U dm-3, respectively. Moreover, this method demonstrates good selectivity and specificity and can be applied to the detection of PPi in actual samples.

A Ratiometric Fluorescence Biosensor for Detection of Alkaline Phosphatase Via an Advanced Chemometric Model

In this paper, a ratiometric fluorescence biosensor was introduced for alkaline phosphatase (ALP) detection based on 2-aminopurine (2-Amp) and thioflavin T (ThT)-G-quadruplex system. We designed a special DNA (5'-AGGGTTAGGGTTAGGGTTAGGGAAA/i2-Amp/AAAA-PO4-3', AP) modified with a phosphate moiety at the 3'-end, G-quadruplex at the 5'-end, and a fluorophore (2-Amp) in the middle. In the absence of ALP, the G-rich AP strand could be prone to fold into G-quadruplex structures in the presence of K+. Then, ThT combined with G-quandruplex, resulting in the enhancement of fluorescence emission peak at 485 nm. However, ALP-mediated hydrolysis of the 3'-phosphoryl end promoted the cleavage of AP by the exonuclease I (Exo I), releasing 2-Amp which displayed a strong fluorescence emission peak at 365 nm. Moreover, the quantitative fluorescence model (QFM) was derived for the analysis of the fluorescence measurements obtained by the proposed ratiometric fluorescent biosensor. With the aid of the advanced model, the proposed ratiometric fluorescent biosensor possessed satisfactory results for the detection of ALP in the human serum samples, with accuracy comparable to that of the reference method-the commercial ALP assay kit. Under the optimized experimental conditions, this method exhibited good selectivity and higher sensitivity, and the detection limit was found to be as low as 0.017 U/L. Therefore, it is reasonable to expect that the method had a great potential to detect ALP quantitatively in clinical diagnosis.

Fluorescent Probes for Disease Diagnosis

The identification and detection of disease-related biomarkers is essential for early clinical diagnosis, evaluating disease progression, and for the development of therapeutics. Possessing the advantages of high sensitivity and selectivity, fluorescent probes have become effective tools for monitoring disease-related active molecules at the cellular level and in vivo. In this review, we describe current fluorescent probes designed for the detection and quantification of key bioactive molecules associated with common diseases, such as organ damage, inflammation, cancers, cardiovascular diseases, and brain disorders. We emphasize the strategies behind the design of fluorescent probes capable of disease biomarker detection and diagnosis and cover some aspects of combined diagnostic/therapeutic strategies based on regulating disease-related molecules. This review concludes with a discussion of the challenges and outlook for fluorescent probes, highlighting future avenues of research that should enable these probes to achieve accurate detection and identification of disease-related biomarkers for biomedical research and clinical applications.

Selective and sensitive CQD-based sensing platform for Cu2+ detection in Wilson's disease

Excessive Cu2+ intake can cause neurological disorders (e.g. Wilson's disease) and adversely affect the gastrointestinal, liver, and kidney organs. The presence of Cu2+ is strongly linked to the emergence and progression of Wilson's disease (WD), and accurately measuring the amount of copper is a crucial step in diagnosing WD at an early stage in a clinical setting. In this work, CQDs were fabricated through a facile technique as a novel fluorescence-based sensing platform for detecting Cu(II) in aqueous solutions, and in the serum samples of healthy and affected individuals by WD. The CQDs interact with Cu(II) ions to produce Turn-on and Turn-off states at nano-molar and micro-molar levels, respectively, with LODs of 0.001 µM and 1 µM. In fact, the Cu2+ ions can act like a bridge between two CQDs by which the charge and electron transfer between the CQDs may increase, possibly can have significant effects on the spectroscopic features of the CQDs. To the best of our knowledge, this is the first reported research that can detect Cu(II) at low levels using two different complexation states, with promising results in testing serum. The potential of the sensor to detect Cu(II) was tested on serum samples from healthy and affected individuals by WD, and compared to results obtained by ICP-OES. Astonishingly, the results showed an excellent correlation between the measured Cu(II) levels using the proposed technique and ICP-OES, indicating the high potential of the fluorimetric CQD-based probe for Cu(II) detection. The accuracy, sensitivity, selectivity, high precision, accuracy, and applicability of the probe toward Cu(II) ions make it a potential diagnostic tool for Wilson's disease in a clinical setting.

Fluorescent Turn-On Sensing of Zinc(II) and Alkaline Phosphatase Activity Using a Pyridoxal-5'-Phosphate Derived Schiff Base

A novel Zn2+ ion and alkaline phosphatase (ALP) selective fluorescence turn-on sensor L was developed by reacting pyridoxal 5'-phosphate (PLP) with hydrazine. Sensor L shows significant flurescence enhancement at 476 nm due to the formation of a L-Zn2+ complex in 1:1 binding stoichiometry with the association constant of 3.1⋅104 M- 1. Using L, the concentration of Zn2+ can be detected down to 2.34 µM, and the practical utility of L was validated by quantifying Zn2+ in real water samples. Additionally, the receptor L was applied to mimic the dephosphorylation reaction catalysed by the enzyme ALP and the resulted fluorescence change was monitored to detect the ALP activity.

Rolling circle transcription/G-quadruplex/QnMorpholine probe for highly selective and sensitive detection of alkaline phosphatase activity

In this study, we combined a rolling circle transcription (RCT) system producing 22AG G-quadruplex RNA with a QnMorpholine (QNM) fluorescent probe for the selective and sensitive detection of alkaline phosphatase (ALP). ALP is involved in various biological functions, with monophosphate cleavage being one of its characteristic properties. Here, we developed a padlock RCT probing system in which a large amount of RCT 22AG RNA G-quadruplex was produced in the absence of ALP, providing a high fluorescence signal. In contrast, no RNA G-quadruplex was produced in the presence of ALP, with minimal fluorescence. This huge deviation in signal intensity allowed us to identify the presence or absence of ALP in a test sample. Under practical conditions, our system allowed the differentiation for ALP even when it was present at an extremely low concentration (0.0085 U/L), along with very high specificity. The simplicity and efficiency of this approach for ALP detection suggest its potential for use as a reliable diagnostic tool.

A new enzymatic assay to quantify inorganic pyrophosphate in plasma

Inorganic pyrophosphate (PPi) is a crucial extracellular mineralization regulator. Low plasma PPi concentrations underlie the soft tissue calcification present in several rare hereditary mineralization disorders as well as in more common conditions like chronic kidney disease and diabetes. Even though deregulated plasma PPi homeostasis is known to be linked to multiple human diseases, there is currently no reliable assay for its quantification. We here describe a PPi assay that employs the enzyme ATP sulfurylase to convert PPi into ATP. Generated ATP is subsequently quantified by firefly luciferase-based bioluminescence. An internal ATP standard was used to correct for sample-specific interference by matrix compounds on firefly luciferase activity. The assay was validated and shows excellent precision (< 3.5%) and accuracy (93-106%) of PPi spiked into human plasma samples. We found that of several anticoagulants tested only EDTA effectively blocked conversion of ATP into PPi in plasma after blood collection. Moreover, filtration over a 300,000-Da molecular weight cut-off membrane reduced variability of plasma PPi and removed ATP present in a membrane-enclosed compartment, possibly platelets. Applied to plasma samples of wild-type and Abcc6-/- rats, an animal model with established low circulating levels of PPi, the new assay showed lower variability than the assay that was previously in routine use in our laboratory. In conclusion, we here report a new and robust assay to determine PPi concentrations in plasma, which outperforms currently available assays because of its high sensitivity, precision, and accuracy.

A novel hydrazone-based fluorescent "off-on-off" probe for relay sensing of Ga3+ and PPi ions

A novel hydrazone-based fluorescent probe (E)-3-((2-(benzo[d]thiazol-2-yl)hydrazono)methyl)-4H-chromen-4-one (BTC) has been rationally designed and synthesized. BTC can subsequently detect Ga³⁺ and PPi ions through the absorption and emission off-on-off response with high specificity. Importantly, fluorescent probe BTC can well discriminate Ga³⁺ from Al³⁺ and In³⁺. The association constant (K) was calculated as 2.06 × 10⁴M^-1, and the limit of detection (LOD) was calculated as 4.88 × 10^-2μM. Competitive binding studies also illustrated good results of the probe BTC towards Ga³⁺. Job's plot and HRMS results substantiated the 1:1 stoichiometry between BTC and Ga³⁺ ion. The interaction binding mode of BTC with Ga³⁺ was proposed by HRMS, ¹H NMR spectral titration, UV-vis absorption and fluorescence spectral measurements. The combination of the restraint of the photo-induced electron transfer (PET) process and the chelation enhanced fluorescence (CHEF) process is responsible for the fluorescence enhancement of this probe. The in situ chelated BTC-Ga³⁺ could further monitor pyrophosphate ion (PPi) by demetallization process with quenching fluorescence emission. Additionally, the BTC and BTC-Ga³⁺ showed good cell permeability and could detect Ga³⁺ and PPi ions in onioninner epidermal cells, respectively.

Copper (II) Ion-Modified Gold Nanoclusters as Peroxidase Mimetics for the Colorimetric Detection of Pyrophosphate

Copper (II) ions have been shown to greatly improve the chemical stability and peroxidase-like activity of gold nanoclusters (AuNCs). Since the affinity between Cu²⁺ and pyrophosphate (PPi) is higher than that between Cu²⁺ and AuNCs, the catalytic activity of AuNCs-Cu²⁺ decreases with the introduction of PPi. Based on this principle, a new colorimetric detection method of PPi with high sensitivity and selectivity was developed by using AuNCs-Cu²⁺ as a probe. Under optimized conditions, the detection limit of PPi was 0.49 nM with a linear range of 0.51 to 30,000 nM. The sensitivity of the method was three orders of magnitude higher than that of a fluorescence method using AuNCs-Cu²⁺ as the probe. Finally, the AuNCs-Cu²⁺ system was successfully applied to directly determine the concentration of PPi in human urine samples.

An Exonuclease I-Aided Turn-Off Fluorescent Strategy for Alkaline Phosphatase Assay Based on Terminal Protection and Copper Nanoparticles

As an important DNA 3'-phosphatase, alkaline phosphatase can repair damaged DNA caused by replication and recombination. It is essential to measure the level of alkaline phosphatase to indicate some potential diseases, such as cancer, related to alkaline phosphatase. Here, we designed a simple and fast method to detect alkaline phosphatase quantitively. When alkaline phosphatase is present, the resulting poly T-DNA with a 3'-hydroxyl end was cleaved by exonuclease I, prohibiting the formation of fluorescent copper nanoparticles. However, the fluorescent copper nanoparticles can be monitored with the absence of alkaline phosphatase. Hence, we can detect alkaline phosphatase with this turn-off strategy. The proposed method is able to quantify the concentration of alkaline phosphatase with the LOD of 0.0098 U/L. Furthermore, we utilized this method to measure the effects of inhibitor Na₃VO₄ on alkaline phosphatase. In addition, it was successfully applied to quantify the level of alkaline phosphatase in human serum. The proposed strategy is sensitive, selective, cost effective, and timesaving, having a great potential to detect alkaline phosphatase quantitatively in clinical diagnosis.

A colorimetric indicator-displacement assay based on stable Cu2+ selective carbon dots for fluorescence turn-on detection of pyrophosphate anions in urine

Determination of PPi levels in urine represents a measurable factor for diagnostic, treatment, and monitoring of urolithiasis. Owing to the quenching ability of Cu²⁺ on fluorescent carbon dots (CDs) and strong binding affinity between Cu²⁺ and PPi, we develop a new off-on assay for PPi detection using newly BPHA CDs (BPHA: N,N-bis(pyridin-2-ylmethyl)hexan-1-amine). The fluorescence intensity of BPHA CDs was significantly quenched by Cu²⁺ ("off") through forming BPHA CDs/Cu²⁺ complexes and the fluorescence intensity of BPHA CDs /Cu²⁺ system was completely resumed by PPi ("on") owing to the release of free Cu²⁺. The fluorescence turn-off/on approach showed a highly selective response to PPi over the large family of other anions. The detection limits were 0.094 μM for Cu²⁺ and 0.025 μM for PPi, respectively. A wide linear range for PPi was up to 4400 μM. The indicator displacement assay (IDAs) using pyrocatechol violet (PV) as a colorimetric indicator was carried out to detect PPi with the naked eyes. The "off-on" fluorescent sensor based on BPHA CDs shows many merits, including convenient operation, cost-saving, high sensitivity, selectivity, stability and wide detecting range, which is applied to PPi detection in human urine sample.

A novel nucleic acid amplification system based on nano-gap embedded active disk resonators

Recent advances in nucleic acid based testing using bio-optical sensor approaches have been introduced but most are based on hybridization between the optical sensor and the bio-molecule and not on an amplification mechanism. Direct nucleic acid amplification on an optical sensor has several technical limitations, such as the sensitivity of the temperature sensor, instrument complexity, and high background signal. We here describe a novel nucleic acid amplification method based on a whispering gallery mode active resonator and discuss its potential molecular diagnostic application. By implanting nanoclusters as active compounds, this active resonator operates without tapered fiber coupling and emits a strong photoluminescence signal with low background in the wavelength of low absorption in an aqueous environment that is typical of biosensors. Our method also offers an extremely low detection threshold down to a single copy within 10 min due to the strong light-matter interaction in a nano-gap structure. We envision that this active resonator provides a high refractive index contrast for tight mode confinement with simple alignment as well as the possibility of reducing the device size so that a point-of-care system with low-cost, high-sensitivity and simplicity.

Submicromolar copper (II) ions stimulate transretinal signaling in the isolated retina from wild type but not from Cav2.3-deficient mice

BACKGROUND

So far, only indirect evidence exists for the pharmacoresistant R-type voltage-gated Ca²⁺ channel (VGCC) to be involved in transretinal signaling by triggering GABA-release onto ON-bipolar neurons. This release of inhibitory neurotransmitters was deduced from the sensitivity of the b-wave to stimulation by Ni²⁺, Zn²⁺ and Cu²⁺. To further confirm the interpretation of these findings, we compared the effects of Cu²⁺ application and chelation (using kainic acid, KA) on the neural retina from wildtype and Ca_v2.3-deficient mice. Furthermore, the immediately effect of KA on the ERG b-wave modulation was assessed.

METHODS

Transretinal signaling was recorded as an ERG from the superfused murine retina isolated from wildtype and Ca_v2.3-deficient mice.

RESULTS

In mice, the stimulating effect of 100 nM CuCl₂ is absent in the retinae from Ca_v2.3-deficient mice, but prominent in Ca_v2.3-competent mice. Application of up to 3 mM tricine does not affect the murine b-wave in both genotypes, most likely because of chelating amino acids present in the murine nutrient solution. Application of 27 μM KA significantly increased the b-wave amplitude in wild type and Ca_v2.3 (-|-) mice. This effect can most likely be explained by the stimulation of endogenous KA-receptors described in horizontal, OFF-bipolar, amacrine or ganglion cells, which could not be fully blocked in the present study.

CONCLUSION

Cu²⁺-dependent modulation of transretinal signaling only occurs in the murine retina from Ca_v2.3 competent mice, supporting the ideas derived from previous work in the bovine retina that R-type Ca²⁺ channels are involved in shaping transretinal responses during light perception.