Sequential detection of vitamin B6 cofactors and nitroaromatics by using albumin-stabilized fluorescent copper nanoclusters

Luminescent Cu nanoclusters-encapsulated ZIF-8 as on-off-on fluorescent probe for efficient and selective quantification of E. coli

Rapid and accurate detection of Escherichia coli (E. coli) is critical for maintaining water quality, and protecting aquatic ecosystems and public health. This research focuses on the development of a Förster resonance energy transfer (FRET)-based "turn-on" fluorescent nanosensor for real time, sensitive detection of E. coli. Copper nanoclusters-encapsulated metal organic frameworks (CuNCs@ZIF-8) were sythesized as a fluorescent donor with excellent luminescence properties. Further, MnO2 nanospheres were synthesized as a receptor with good adsorption and quenching abilities. This novel nanoconjugate (CuNCs@ZIF-8@ MnO2) was employed for the construction of a sensitive, accurate, and rapid sensing platform against E. coli in water on the basis of p-benzoquinone/hydroquinone (p-BQ/HQ) redox pair formation. Fluorescence is quenched by energy transfer when MnO2 nanospheres are added to CuNCs@ZIF-8. Upon contact with E. coli, NADH-quinone reductase converts p-BQ to HQ, which reduces MnO2 to Mn2+, releasing the nanospheres and restoring fluorescence in the composite. Based on this FRET ON-OFF-ON fluorescent probe, E. coli can be detected across a broad concentration range (5 × 101 to 5 × 105 CFU/mL), with a detection limit as low as 8 CFU/mL within 50 min. The sensor's practicality was verified through the investigation of E. coli in real water samples, with recoveries in the range 94.3 to 106.5%. This approach offers an efficient method for on-site detection and quantification of E. coli in both environment and food safety domains.

Ionic liquid-based yellow-emitting carbon dots for fluorescence-smartphone dual-mode detection of vitamin B6 in milk

The determination of vitamin B6 (VB6) in food is of great significance due to its vital role in maintaining health and its necessity for ingestion through dietary sources. Therefore, based on ionic liquid-based yellow-emitting carbon dots (Y-CDs), a novel fluorescence-smartphone dual-mode method was first developed. The present method was applied to the detection of VB6 in milk. In the fluorescence method, the formation of complexes between VB6 and Y-CDs results in a significant decrease of the fluorescence intensity of Y-CDs. VB6 in milk samples was successfully determined according to this method, which exhibited a low detection limit (5 × 10-5 mg/mL) and excellent recoveries (98.80%-103.80%), demonstrating its feasibility in real sample analysis. In addition, the smartphone-based analysis method was established by researching the correlation between different VB6 concentrations and the (R + B) values of Y-CDs. When this method was applied, the detection process of VB6 was simplified. By combining the two methods, the possibility of incorrect analysis results can be effectively reduced, and the reliability of detection results can be improved through cross-validation of the two methods. Compared with traditional chromatography and electrochemical methods, the dual-mode method was more rapid, convenient, accurate, and suitable for the detection of VB6.

Tellurium Containing Long Lived Emissive Fluorophore for Selective and Visual Detection of Picric Acid through Photo-Induced Electron Transfer

A novel tellurium (Te) containing fluorophore, 1 and its nickel (2) and copper (3) containing metal organic complex (MOC) have been synthesized to exploit their structural and optical properties and to deploy these molecules as fluorescent probes for the selective and sensitive detection of picric acid (PA) over other commonly available nitro-explosives. Furthermore, density functional theory (DFT) and single crystal X-ray diffraction (SCXRD) techniques revealed the inclusion of "soft" Tellurium (Te) and "hard" Nitrogen (N), Oxygen (O) atoms in the molecular frameworks. Owing to the presence of electron rich "N" and "O" atoms along with "Te" in the molecular framework, 1 could efficiently and selectively sense PA with more than 80 % fluorescence quenching efficiency in organic medium and having detection limit of 4.60 μM. The selective detection of PA compared to other nitro-explosives follows a multi-mechanism based "turn-off" sensing which includes photo-induced electron transfer (PET), electrostatic (π-π stacking and π-anion/cation) interaction, intermolecular hydrogen bonding and inner filter effect (IFE). The test strip study also establishes the sensitivity of 1 for detection of PA.

Fluorescence Sensing of pH and p-Nitrophenol Using an AIEE Active Pyridoxal Derived Schiff Base

An easy-to-prepare aggregation-induced emission enhancement (AIEE) active Schiff base NPY was synthesized by condensing vitamin B6 cofactor pyridoxal with 3-hydroxy-2-naphthoic hydrazide, and employed for the fluorescent sensing of pH and p-nitrophenol (p-NP). The AIEE phenomenon of NPY was investigated in mixed DMSO/H2O medium. The weakly yellow-fluorescent NPY (λem = 535 nm) in pure DMSO turned to a bright cyan-fluorescent NPY (λem = 490 nm) upon addition of poor solvent water. The DLS and SEM analyses supported the self-aggregation of NPY that restricted the intramolecular rotation and activated the excited state intramolecular proton transfer (ESIPT) process. The AIEE luminogen (AIEEgen) NPY containing 90% of water fraction (fwater) was employed for the fluorescent sensing of pH. AIEEgen NPY displays three distinct fluorescent pH windows: non-fluorescent below pH 3.0 and above pH 10.0, cyan fluorescent between pH 3.0 to 8.0, and yellow fluorescent between pH 8.0 to 10.0. AIEEgen NPY was also applied for the detection of nitroaromatics in HEPES buffer (10% DMSO, 10 mM, pH 7.0). The addition of p-NP selectively quenched the fluorescent intensity of AIEEgen NPY with an estimated detection limit of 1.73 µM. The analytical utility of AIEEgen NPY was examined by quantifying p-NP in different real water samples.

Preparation of blue fluorescent copper nanoclusters for sensitive and selective sensing of apigenin in pharmaceutical samples

One-pot means was performed for the rapid preparation of copper nanoclusters (Cu NCs), which were employed as a fluorescence system for the sensitive apigenin measurement in pharmaceutical samples. Herein, CuCl₂ aqueous solution was reduced to Cu NCs through ascorbic acid and the Cu NCs were protected through trypsin under 65 ℃ for 4 h. The entire preparation process was rapid, facile and environmentally friendly. The trypsin-capped Cu NCs were demonstrated through ultraviolet-visible spectroscopy, fluorescence spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and fluorescence lifetime, respectively. The Cu NCs revealed blue fluorescence with emission wavelength around 465 nm under the excitation wavelength of 380 nm. The fluorescence weakening feature of Cu NCs with apigenin was observed. On this basis, a facile and sensitive turn-off fluorescent nanoprobe for the sensing of apigenin in real samples was developed. The logarithm of relative fluorescence intensity revealed a good linear relationship with apigenin contents from 0.5 μM to 300 μM with the detection limit of 0.079 μM. The Cu NCs-based fluorescent nanosensor have been employed to measure the apigenin amounts in real samples such as medical saline, bovine and human serum. The results revealed excellent potential of this Cu NCs-based fluorescent nanoprobe for the convention computation of apigenin amounts in real samples.

Fluorescence Turn-on Detection of Alkaline Phosphatase Activity and Al3+ Using Vitamin B6 Cofactor Conjugated GSH Capped Mn-doped ZnS Quantum Dots

The glutathione (GSH) functionalized Mn-doped ZnS quantum dots (GSH_Mn_ZnS QDs) was conjugated with pyridoxal 5'-phosphate (PLP). The -CHO group of vitamin B₆ cofactor PLP interacted with the -NH₂ group of GSH functionalized Mn_ZnS QDs. The conjugation of PLP quenched the fluorescence emission of GSH_Mn_ZnS QDs at 601 nm. Addition of alkaline phosphatase (ALP) catalytically dephosphorylated the PLP into pyridoxal that restored the fluorescence emission of GSH_Mn_ZnS QDs. With a sensitivity of 0.035 U/L, the PLP conjugated GSH_Mn_ZnS QDs was applied to quantify ALP activity in human serum and plasma. Further, the developed nanoprobe PLP conjugated GSH_Mn_ZnS QDs was also applied to detect Al³⁺. The complexation-induced fluorescence enhancement was observed at 492 nm upon the interaction of Al³⁺ with the PLP conjugated GSH_Mn_ZnS QDs. Without any interference from other tested metal ions, this nanoprobe can be employed to detect Al³⁺ down to 2.30 µM.

Development of fluorescence sensors with copper-based nanoclusters via Förster resonance energy transfer and the quenching effect for vanillin detection

Two kinds of copper-based metal fluorescent nanoclusters were successfully prepared by the chemical reduction method; one of them (CuNCs) was synthesized by direct reduction of copper sulfate, and the other (CuAuNCs) was synthesized by the stepwise addition of copper salt and chloroauric acid. CuNCs were used to establish the fluorescence resonance energy transfer (FRET) system with neutral red (NR) due to the supramolecular effect of β-cyclodextrin (β-CD) modified on the surface of CuNCs. NR could enter the hydrophobic cavity of β-CD and narrow the distance between CuNCs and NR, which could lead to FRET. Fluorescence was transferred from CuNCs to NR, resulting in amplification of the NR fluorescence signal, which could be used to detect vanillin. In addition, CuAuNCs with strong fluorescence were used as fluorescent probes to detect vanillin through the quenching mechanism. By comparison, the simplicity of CuNC synthesis and the high selectivity of β-CD made the FRET method more practical, which may provide a new strategy for assaying vanillin.