FRET processes of bi-fluorophoric sensor material containing tetraphenylethylene donor and optical-switchable merocyanine acceptor for lead ion (Pb2+) detection in semi-aqueous media

Designing Multifunctional AIEgens by Molecular Engineering of Imidazo[1,2-a]pyridine For Color Tunable Molecular Salts, Anti-Counterfeit Applications and Sensing of Mn2+, Ag+, and Fe3

Mechanochromic materials, known for their ability to change color in response to mechanical stimuli such as pressure, stretching, grinding, or rubbing, hold significant importance due to their diverse applications. In this study, we synthesized and characterized two novel pyridine-tethered imidazo[1,2-a]pyridine mechanoresponsive luminogens with appended tetraphenylethene, named GBY-10 and GBY-11. GBY-10 exhibited reversible mechanofluorochromism, while GBY-11 did not revert to its original color after solvent fuming. The photophysical properties of these luminogens were significantly influenced by the position of the terminal pyridine. Additionally, we created color-tunable mechanoresponsive molecular salts by co-grinding GBY-11 with various aryl acid derivatives. Co-grinding GBY-11 with pentafluorobenzoic acid resulted in a significant bathochromic shift of the emission maxima by 66 nm, compared to 37 nm and 12 nm shift for benzoic acid and para-nitrobenzoic acid, respectively. This counter-ion-dependent luminescence suggests strong electronic interactions between the counter ions. These luminogens also demonstrated reversible pH-responsive behaviour, making them suitable for anti-counterfeiting applications. Furthermore, the pyridine-functionalized luminogen, GBY-10, showed metal ion detection (Mn2+, Ag+, Fe3+) ability in water, with detection limits as low as 0.0043, 0.015, and 0.0029 mM, respectively. This report opens new avenues for designing promising AIE-active materials for potential applications in anti-counterfeiting, sensing, optoelectronics, and biomedicine. A de-novo approach of engineering imidazo[1,2-a]pyridine scaffold to mechanoresponsive AIE-active molecules for anti-counterfeiting and metal sensing applications. By co-grinding the designed luminogens with various aryl acid derivatives, color tuneable mechanoresponsive molecular salts can be further developed.

A chitosan-based sensing membrane for on-site and sensitive dual-channel portable detection and efficient adsorption of Pb2+ in groundwater

The presence of lead ion (Pb2+) in groundwater poses a serious risk to human health, even at low levels. Therefore, it is essential to develop a new strategy for both selective detection and effective removal of Pb2+ in groundwater, which has been rarely reported. Here, we developed a multi-functional chitosan-based fluorescent sensing membrane (CM-L/CG) by using a casting method for the sensitive/selective detection and removal of Pb2+ in groundwater. The CM-L/CG membrane can be integrated into the portable laser-induced fluorescence spectrometer (LIFs) for on-site detection of Pb2+, with a low detection limit of 1.02 ppb. Moreover, the CM-L/CG membrane demonstrates an outstanding 99 % removal rate and an adsorption capacity of 247.6 mg g-1 for Pb2+ and the adsorption process is mainly controlled by chemisorption. Importantly, the CM-L/CG membrane enables the real-time and on-site detection of Pb2+ in groundwater samples via a smartphone-based RGB (Red Green Blue) color analysis-assisted portable platform and portable LIFs-based platform, achieving acceptable results. This dual-functional fluorescent sensing membrane represents a breakthrough in environmental monitoring technology, offering a comprehensive solution for sensitive detection and efficient removal of Pb2+ from groundwater.

Gold nanoclusters-based fluorescence resonance energy transfer for rapid and sensitive detection of Pb2

Lead pollution has remained a significant global concern for several decades due to its detrimental effects on the brain, heart, kidneys, lungs, and immune system across all age groups. Addressing the demand for detecting trace amounts of lead in food samples, we have developed a novel biosensor based on fluorescence resonance energy transfer (FRET) from fluorescein R6G to gold nanoclusters (AuNCs-CCY). By utilizing polypeptides as a template, we successfully synthesized AuNCs-CCY with an excitation spectrum that overlaps with the emission spectrum of R6G. Exploiting the fact that Pb2+ induces the aggregation of gold nanoclusters, leading to the separation of R6G from AuNCs-CCY and subsequent fluorescence recovery, we achieved the quantitative detection of Pb2+. Within the concentration range of 0.002-0.20 μM, a linear relationship was observed between the fluorescence enhancement value (F-F0) and Pb2+ concentration, characterized by the linear equation y = 2398.69x + 87.87 (R2 = 0.996). The limit of detection (LOD) for Pb2+ was determined to be 0.00079 μM (3σ/K). The recovery rate ranged from 96 % to 104 %, with a relative standard deviation (RSD) below 10 %. These findings demonstrate the potential application value of our biosensor, which offers a promising approach to address the urgent need for sensitive detection of heavy metal ions, specifically Pb2+, in food samples.

Dual detection of Hg2+ and Pb2+ by a coumarin-functionalized Schiff base in environmental and biosystems

Fluorescent chemosensors are often preferred for tracking toxic ions because of their non-destructive measurement and ease of use in environmental real samples and biosystems. Exploring high selectivity, great sensitivity, and biocompatible fluorophores with facile, accessible and dual-responsive features is currently highly demanding. A coumarin-based naphthol hydrazone Schiff base chemosensor, NaChro, is designed and synthesized in a two-step process to detect toxic metal ions with strong emission. Fluorescence spectra analysis demonstrates that the probe binds to Hg2+ and Pb2+ ions with a 1:1 and a 2:1 stoichiometry, respectively, with high sensitivity, short response time and minimal interference from other metal ions. The observed reversible turn-on reaction was attributed to the inhibition of C = N isomerization and excited-state intramolecular proton transfer (ESIPT) processes once the ions were introduced. The practical applications of NaChro are successfully addressed in paper strips, various water samples, HeLa cells and Zebrafish, demonstrating that the probe can detect and track Hg2+ and Pb2+ ions in environmental samples and biosystems.

Recent Progress on Fluorescent Probes in Heavy Metal Determinations for Food Safety: A Review

One of the main challenges faced in food safety is the accumulation of toxic heavy metals from environmental sources, which can sequentially endanger human health when they are consumed. It is invaluable to establish a practical assay for the determination of heavy metals for food safety. Among the current detection methods, technology based on fluorescent probes, with the advantages of sensitivity, convenience, accuracy, cost, and reliability, has recently shown pluralistic applications in the food industry, which is significant to ensure food safety. Hence, this review systematically presents the recent progress on novel fluorescent probes in determining heavy metals for food safety over the past five years, according to fluorophores and newly emerging sensing cores, which could contribute to broadening the prospects of fluorescent materials and establishing more practical assays for heavy metal determinations.

Designs and Applications of Multi-stimuli Responsive FRET Processes in AIEgen-Functionalized and Bi-fluorophoric Supramolecular Materials

Materials capable of displaying strong ratiometric fluorescence with Förster resonance energy transfer (FRET) processes have attracted much research interest because of various chemosensor and biomedical applications. This review highlights several popular strategies in designing FRET-OFF/ON mechanisms of ratiometric fluorescence systems. In particular, the developments of organic and polymeric FRET materials featuring aggregation-induced emission-based luminogens (AIEgens), supramolecular assemblies, photochromic molecular switches and surfactant-induced AIE/FRET mechanisms are presented. AIEgens have been frequently employed as FRET donor and/or acceptor fluorophores to obtain enhanced ratiometric fluorescences in solution and solid states. Since AIE effects and FRET processes rely on controllable distances between fluorophores, many interesting fluorescent properties can be designed by regulating aggregation states in polymers and supramolecular systems. Photo-switchable fluorophores, such as spiropyran and diarylethene, provide drastic changes in fluorescence spectra upon photo-induced isomerizations, leading to photo-switching mechanisms to activate/deactivate FRET processes. Supramolecular assemblies offer versatile platforms to regulate responsive FRET processes effectively. In rotaxane structures, the donor-acceptor distance and FRET efficiency can be tuned by acid/base-controlled shuttling of the macrocycle component. The tunable supramolecular interactions are strongly influenced by external factors (such as pH values, temperatures, analytes, surfactants, UV-visible lights, etc.), which induce the assembly and disassembly of host-guest systems and thus their FRET-ON/FRET-OFF behavior. In addition, the changes in donor or acceptor fluorescence profiles upon detections of analytes can also sufficiently alter the FRET behavior and result in different ratiometric fluorescence outputs. The strategies and examples provided in this review offer the insights and toolkits for future FRET-based material developments.

Spiropyran-Merocyanine Based Photochromic Fluorescent Probes: Design, Synthesis, and Applications

Due to ever-increasing insights into their fundamental properties and photochromic behaviors, spiropyran derivatives are still a target of interest for researchers. The interswitching ability of this photochrome between the spiropyran (SP) and merocyanine (MC) isoforms under external stimuli (light, cations, anions, pH etc.) with different spectral properties as well as the protonation-deprotonation of its MC form allows researchers to use it suitably in sensing purposes by developing different colorimetric and fluorometric probes. Selective and sensitive recognition can be achieved by little modification of its SP moiety and functional groups. In this review, we emphasize the recent advancements (from 2019 to 2022) of spiropyran-merocyanine based fluorogenic and chromogenic probes for selective detection of various metal ions, anions, neutral analytes, and pH. We precisely explain their design strategies, sensing mechanisms, and biological and environmental applications. This review may accelerate the improvements in designing more advanced probes with innovative applications in the near future.

Rapid and Highly Sensitive Detection of Target DNA Related to COVID-19 Virus With a Fluorescent Bio-conjugated Probe via a FRET Mechanism

A novel cyanine 3 (Cy3)-based bio-conjugated sensor has been developed to detect target DNA or extracted RNA from COVID -19 samples using the fluorescence resonance energy transfer (FRET) experiment. A special sequence of the COVID -19 genome was selected as a complementary DNA (target DNA) part. The opposite chain of this target sequence was designed in 2 parts; one part was attached to the Cy3 organic dye (capture DNA or Cy3- DNA), and the other part was attached to the BHQ₂ molecule (quencher DNA or BHQ₂- DNA). The Cy3 molecule acts as a donor pair, and BHQ₂ acts as an acceptor pair in the FRET experiment. The capture DNA and quencher DNA can form a sandwiched complex in the presence of target DNA. The formation of the entitled sandwiched hybrid causes the decrement of emission intensity of the Cy3 donor in bio-conjugated Cy3-DNA via energy transfer from Cy3 (as a donor) to BHQ₂ (as an acceptor). Indeed, in the presence of non-complementary DNA, the pairing of DNA strands does not occur, the FRET phenomenon does not exist, and therefore fluorescence intensity of Cy3 does not decrease. Moreover, this biosensor was successfully applied to analyze real samples containing extracted RNA of COVID -19 prepared for the reverse transcriptase-polymerase chain reaction (RT-PCR) test, and the results were promising.

Rapid FRET-based homogeneous immunoassay of procalcitonin using matched carbon dots labels

A novel method for the detection of procalcitonin in a homogeneous system by matched carbon dots (CDs) labeled immunoprobes was proposed based on the principle of FRET and double antibody sandwich method. Blue-emitting carbon dots with a strong fluorescence emission range of 400-550 nm and red-emitting carbon dots with the best excitation range of 410-550 nm were prepared before they reacted with procalcitonin protoclone antibody pairs to form immunoprobes. According to the principles of FRET, blue-emitting carbon dots were selected as the energy donor and red-emitting carbon dots as the energy receptor. The external light source excitation (310 nm) could only cause weak luminescence of CDs. However, once procalcitonin was added, procalcitonin and antibodies would be combined with each other quickly (≤20 min). Here, blue-emitting carbon dots acquired energy could be transferred to red-emitting carbon dots efficiently, causing the emitted fluorescence enhancement of red-emitting carbon dots. The fluorescence detection results in PBS buffer solution and diluted rabbit blood serum showed that the fluorescence intensity variation was linear with the concentration of procalcitonin. There was a good linear relationship betweenF/F0 and procalcitonin concentrations in PBS buffer solution that ranged from 0 to 100 ng ml^-1, and the linear equation wasF/F0 = 0.004 *C_pct + 0.98359. Detection in the diluted rabbit serum led to the results that were linear in two concentration ranges, including 0-40 ng ml^-1and 40-100 ng ml^-1, and the detection limit based on 3σK^-1was 0.52 ng ml^-1. It is likely that this matched CDs labeled immunoprobes system can provide a new mode for rapid homogeneous detection of disease markers.