Small-molecule fluorescent probes based on covalent assembly strategy for chemoselective bioimaging

β-galactosidase-activated red fluorescent probe assists in the diagnosis of pancreatitis

Acute pancreatitis (AP) is an acute inflammatory disease resulting from abnormal digestion of itself and surrounding organs by pancreatic enzymes caused by a variety of pathogenic factors. When the tissue of a biological organism is afflicted with pancreatitis and experiences swelling, bleeding, and necrotic injuries, the abnormal expression of β-galactosidase (β-Gal) activity becomes one of the main indicators for assisting in the diagnosis of pancreatitis. In this study, a highly specific red fluorescent probe designed for the detection of β-Gal activity has been developed. β-galactoside is used as the enzyme activating group, and the long-wavelength luminescent water-soluble organic molecule NBDOH is used as the luminophore to construct the fluorescent probe NBD-gal. NBD-gal is activated by abnormally overexpressed β-Gal, releasing a strong red fluorescent signal. The new fluorescent probe developed in this study is used to assist in the diagnosis of pancreatitis by detecting abnormal expression of β-Gal activity in vivo. In in vivo imaging experiments, NBD-gal can effectively differentiate between normal nude mice and pancreatitis nude mice. In addition, NBD-gal can be effectively localized in real-time to the pancreas and intestines, which are rich in β-Gal in nude mice. Therefore, NBD-gal exerts great potential in the early diagnosis, treatment, and prevention of biomedical clinical pancreatitis.

Fluorogenic detection of cyanide ions in pure aqueous media through an intramolecular crossed-benzoin reaction: limitations unveiled and possible solutions

Reaction-based fluorogenic sensing of lethal cyanide anions in aqueous matrices remains a big challenge. We have revisited the reported approach about an intramolecular crossed-benzoin reaction leading to the release of a phenol-based fluorophore. Fluorescence assays and RP-HPLC-MS analyses have helped us to highlight its limitations related to poor aqueous stability of probes and impossibility to achieve molecular amplification despite the assumed catalytic activation mechanism. Traceless cleavable linker strategies were considered to obtain usable cyanide-responsive chemodosimeters and statistical analyses of fluorescence data have been conducted in depth to accurately delineate their sensing performances, especially the limit of detection (LOD).

Copper and nitrogen-doped carbon quantum dots as green nano-probes for fluorimetric determination of delafloxacin; characterization and applications

BACKGROUND

Carbon quantum dots (CQDs) have gained much interest recently for being efficient probes. Their cost-effectiveness, eco-friendliness, and unique photocatalytic activities made them distinctive alternatives to other luminescent approaches like fluorescent dyes and luminous derivatization. Meanwhile, delafloxacin (DLF) is a recently approved antibacterial medicine. DLF has been authorized for the treatment of soft-tissue and skin infections as well as pneumonia. Therefore, new eco-friendly, cost-effective, and sensitive tools are needed its estimation in different matrices.

RESULTS

In the proposed study, green copper and nitrogen carbon dots (Cu-N@CDs) were synthesized from a green source (plum juice with copper sulphate). Cu-N@CQDs were then characterized using multiple tools including X-ray photon spectroscopy (XPS), FTIR and UV-VIS spectroscopy, Zeta potential measurements, High-resolution transmission electron microscopy (HRTEM), and fluorescence spectroscopy. After gradually adding DLF, the developed quantum dots' fluorescence was significantly enhanced within the working range of 0.5-100.0 ng mL-1. The limits of detection and quantification were 0.08 and 0.27 ng mL-1, respectively. The accuracy of the proposed method ranged from 96.00 to 99.12 % in recovery%, when recovered from milk and plasma samples.

SIGNIFICANCE

Cu-N@CDs were utilized and validated for selectively determining DLF in several matrices including pharmaceutical forms, human plasma and in milk samples using spectrofluorimetric technique. The bio-analytical method is simple and could be used in content uniformity testing as well as in therapeutic drug monitoring in human plasma.

Turn-on Coumarin Precursor: From Hydrazine Sensor to Covalent Inhibition and Fluorescence Detection of Rabbit Muscle Aldolase

Hydrazine, a highly toxic compound, demands sensitive and selective detection methods. Building upon our previous studies with pre-coumarin OFF-ON sensors for fluoride anions, we extended our strategy to hydrazine sensing by adapting phenol protecting groups (propionate, levulinate, and γ-bromobutanoate) to our pre-coumarin scaffold. These probes reacted with hydrazine, yielding a fluorescent signal with low micromolar limits of detection. Mechanistic studies revealed that hydrazine deprotection may be outperformed by a retro-Knoevenagel reaction, where hydrazine acts as a nucleophile and a base yielding a fluorescent diimide compound (6,6'-((1E,1'E)-hydrazine-1,2diylidenebis(methaneylylidene))bis(3(diethylamino)phenol, 7). Additionally, our pre-coumarins unexpectedly reacted with primary amines, generating a fluorescent signal corresponding to phenol deprotection followed by cyclization and coumarin formation. The potential of compound 3 as a theranostic Turn-On coumarin precursor was also explored. We propose that its reaction with ALDOA produced a γ-lactam, blocking the catalytic nucleophilic amine in the enzyme's binding site. The cleavage of the ester group in compound 3 induced the formation of fluorescent coumarin 4. This fluorescent signal was proportional to ALDOA concentration, demonstrating the potential of compound 3 for future theranostic studies in vivo.

"Covalent-Disassembly"-Based Approaches For Sensing Applications

The detection of analytes with small molecular probes is crucial for the analysis and understanding of chemical, medicinal, environmental and biological situations as well as processes. Classic detection approaches rely on the concept of molecular recognition and bond formation reactions. Bond breakage reactions have been less explored in similar contexts. This concept article introduces metal-salen and metal-imine complexes as "covalent-disassembly"-based (DB)-probes for detecting polyoxophosphates, thiols, amino acids, HCN and changes in pH. It discusses the roles, importance and combinations of structurally functionalized molecular building blocks in the construction of DB-probes. Applications of optimized DB-probes for analyte detection in live cells and foodstuff are also discussed. Furthermore, the mechanism of the disassembly of a Fe(III)-salen probe upon pyrophosphate binding is presented. Extraordinary selectivity for this analyte was achieved by a multistep disassembly sequence including an unprecedented structural change of the metal complex (i. e. "induced-fit" principle). Design principles of probes for sensing applications following the "covalent-disassembly" approach are summarized, which will help improving current systems, but will also facilitate the development of new DB-probes for challenging analytic targets.

Fluorimetric Reusable Polymeric Sensor for Hydrogen Sulfide Detection

In this study, with the help of reactive monomers, crosslinkers, and photoinitiator that detect H2S in various matrices, an H2S sensitive fluorescence sensor polymerizes under ultraviolet (UV) light was developed. To this goal, a polymeric membrane was prepared, and the characterization of the membrane was carried out with Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) methods. Afterward, appropriate conditions were identified, the excitation wavelength was determined as 370 nm, and the emission wavelength was determined as 425 nm. It was established that the fluorescence intensity of the prepared polymeric membrane decreased in the presence of H2S. A detailed analysis was executed to determine the sensor's most suitable pH value and time. It was found that the optimum pH was 8.0, and the optimal duration was 15 s. It has been calculated that the linear range of the developed method is 2.19 × 10-8- 6.25 × 10-7 M, and the detection limit (LOD) is 7.37 × 10-9 M. The effect of some possible interfering ions was investigated, and it determined that the sensor had excellent selectivity. In addition, the sensor used to determine H2S can be used at least 100 times. The recovery percentages were 102.1%-103.2%, and 104.6%, using tap water samples. In terms of providing reliable, fast results, high sensitivity, reusable, low cost, and ease of use, the developed fluorimetric sensor, compared to standard methods, has become more advantageous.

Highly sensitive fluorescent probes for selective detection of hypochlorite: Applications in blood serum and cell imaging

Being one of the vital reactive oxygen species (ROS), abnormal level of hypochlorite ion (ClO^-) may pose detrimental threats to living organisms. Therefore, highly selective, and rapid monitoring of ClO^- in living system is of prime importance to protect living organisms from its harmful effects. In this regard, design of synthetic fluorescent probes for ClO^- has garnered considerable attention. However less fluorescence emission in aggregated state and less photostability of several existing probes for ClO^- inspired us to design aggregation induced emission (AIE) active fluorescent probes SH1 and SH2. Probes were rationally designed by introducing thiourea moiety that selectively reacted through desulfurization reaction and resulted in highly selective detection of ClO^-. Hypochlorite induced desulfurization reaction was validated through ¹H NMR titration and DFT studies. Fine tuning of probes SH1 and SH2 prompted highly sensitive nanoscale (55 nM and 77 nM) and rapid (15 and 35 sec) detection of ClO^-. Probe SH1 displayed less cytotoxic effect to live cells before it was successfully applied for bioimaging of ClO^- in live MCF-7 cells. Moreover, probes displayed excellent sensing potential for ClO^- in blood serum and real water samples. Advantageously, probe coated portable fluorescent films were fabricated for the easy and fast monitoring of ClO^-. Of note, this work offers excellent design strategy for highly selective detection of ClO^- that may lead to clinical trials.

FluorMango, an RNA-Based Fluorogenic Biosensor for the Direct and Specific Detection of Fluoride

Nucleic acids are not only essential actors of cell life but also extremely appealing molecular objects in the development of synthetic molecules for biotechnological application, such as biosensors to report on the presence and concentration of a target ligand by emission of a measurable signal. In this work, FluorMango, a fluorogenic ribonucleic acid (RNA)-based biosensor specific for fluoride is introduced. The molecule consists of two RNA aptamer modules, a fluoride-specific sensor derived from the crcB riboswitch which changes its structure upon interaction with the target ion, and the light-up RNA Mango-III that emits fluorescence when complexed with a fluorogen. The two modules are connected by an optimized communication module identified by ultrahigh-throughput screening, which results in extremely high fluorescence of FluorMango in the presence of fluoride, and background fluorescence in its absence. The value and efficiency of this biosensor by direct monitoring of defluorinase activity in living bacterial cells is illustrated, and the use of this new tool in future screening campaigns aiming at discovering new defluorinase activities is discussed.

Dual Response Site Fluorescent Probe for Highly Sensitive Detection of Cys/Hcy and GSH In Vivo through Two Different Emission Channels

Much research has demonstrated that metabolic imbalances of biothiols are closely associated with the emergence of different types of disease. In view of the significant effect of biothiols, quantitative evaluation and discrimination of intracellular Cys/Hcy and GSH in complex biological environments is very important. In this study, probe CDS-NBD, synthesized by attaching 2,4-dinitrobenzenesulfonate (DNBS, site 1) and nitrobenzoxadiazole (NBD, site 2) as the highly sensitive and selective dual response site for thiols onto the coumarin derivative 7-hydroxycoumarin-4-acetic acid, exhibited large separation of the emission wavelengths, fast response, notable fluorescence enhancement, excellent sensitivity and selectivity to Cys/Hcy and GSH over other biological species. Additionally, CDS-NBD could make a distinction between two different fluorescent signals, GSH (an obvious blue fluorescence) and Cys/Hcy (a mixed blue-green fluorescence). Further study on imaging of Cys/Hcy and GSH in vivo by employing probe CDS-NBD could also be successfully achieved.

Spectroscopic properties and fluorescent recognition of dye sensitized layered lutetium-terbium hydroxides

The layered rare earth hydroxides have attracted increasing interests due to their diverse chemical composition and tunable spectroscopic properties. In this paper, a novel Tb³⁺ activated layered lutetium hydroxide (LLuH:Tb) was fabricated, in which the inorganic NO₃^- ions were ion-exchanged with organic (ibuprofen or dodecylsulfonate) anions. After the ion-exchange reaction, the organic anions intercalated LLuH:Tb showed the distinct lamellar structure with the interlayer distance of about 2.56 nm, confirming the formation of inorganic/organic hybrid assembly. The dye ibuprofen-intercalated hybrid effectively promoted the characteristic ⁵D₄ → ⁷F₅ green emission of Tb³⁺ in the host but failed to be exfoliated into nanosheet colloid. On the contrary, the dodecylsulfonate-intercalated hybrid was readily to be exfoliated into nanosheet colloid by dissolving in formamide solvent, but the green emission of Tb³⁺ was too weak to be observed. To take advantage of their respective merits and explore the practical uses, certain amounts of dye ibuprofen were directly added to the dodecylsulfonate-intercalated hybrid colloid. Excited with the ultraviolet light, the characteristic green fluorescence of Tb³⁺ was dramatically enhanced, indicating that the dye was a superior light-harvesting antenna to sensitize the activator Tb³⁺. The dye sensitized hybrid colloid was very stable at ambient temperature and exhibited excellent fluorescent recognition for Cu²⁺ ions over other metal ions in aqueous solution due to the large fluorescence quenching. The detection limit for Cu²⁺ ion reaches 7.63 × 10^-7 mol/L, which is far lower than the limitation of Cu²⁺ in drinking water recommended by the World Health Organization (1.57 × 10^-5 mol/L). The fluorescence enhanced/quenched sensor with excellent stability exhibits a high potential for the detection of Cu²⁺ in routine environmental water.

Fluorescence Sensors Based on Hydroxycarbazole for the Determination of Neurodegeneration-Related Halide Anions

The environmental presence of anions of natural origin or anthropogenic origin is gradually increasing. As a tool to tackle this problem, carbazole derivatives are an attractive gateway to the development of luminescent chemosensors. Considering the different mechanisms proposed for anion recognition, the fluorescence properties and anion-binding response of several newly synthesised carbazole derivatives were studied. Potential anion sensors were designed so that they combined the native fluorescence of carbazole with the presence of hydrogen bonding donor groups in critical positions for anion recognition. These compounds were synthesised by a feasible and non-expensive procedure using palladium-promoted cyclodehydrogenation of suitable diarylamine under microwave irradiation. In comparison to the other carbazole derivatives studied, 1-hydroxycarbazole proved to be useful as a fluorescent sensor for anions, as it was able to sensitively recognise fluoride and chloride anions by establishing hydrogen bond interactions through the hydrogen atoms on the pyrrolic nitrogen and the hydroxy group. Solvent effects and excited-state proton transfer (ESPT) of the carbazole derivatives are described to discard the role of the anions as Brönsted bases on the observed fluorescence behaviour of the sensors. The anion-sensor interaction was confirmed by 1H-NMR. Molecular modelling was employed to propose a mode of recognition of the sensor in terms of complex stability and interatomic distances. 1-hydroxycarbazole was employed for the quantitation of fluoride and chloride anions in commercially available medicinal spring water and mouthwash samples.

Simultaneous Discrimination of Cys/Hcy and GSH With Simple Fluorescent Probe Under a Single-Wavelength Excitation and its Application in Living Cells, Tumor Tissues, and Zebrafish

Owing to the important physiological sits of biothiols (Cys, Hcy, and GSH), developing accurate detection methods capable of qualitative and quantitative analysis of biothiols in living systems is needed for understanding the biological profile of biothiols. In this work, we have designed and synthesized a 4'-hydroxy-[1,1'-biphenyl]-4-carbonitrile modified with NBD group-based fluorescent probe, BPN-NBD, for sensitive detection of Cys/Hcy and GSH by dual emission signals via a single-wavelength excitation. BPN-NBD exhibited an obvious blue fluorescence (λmaxem = 475 nm) upon the treatment with GSH and reacted with Cys/Hcy to give a mixed blue-green fluorescence (λmaxem = 475 and 545 nm). Meanwhile, BPN-NDB performed sufficient selectivity, rapid detection (150 s), high sensitivity (0.011 µM for Cys, 0.015 µM for Hcy, and 0.003 µM for GSH) and could work via a single-wavelength excitation to analytes and had the ability to image Cys/Hcy from GSH in living MCF-7 cells, tumor tissues, and zebrafish by exhibiting different fluorescence signals. Overall, this work provided a powerful tool for thiols visualization in biological and medical applications.