Surfactant assemblies encapsulating fluorescent probes as selective and discriminative sensors for metal ions

A distinctive and proficient fluorescent switch for ratiometric recognition of the menacing cyanide ion: biological studies on MDA-MB-231 cells

A new fluorescent ratiometric switch (BOHB) was developed for swift and selective detection of cyanide ions in aqueous media without any interference from other competitive anions. Upon gradual addition of cyanide ions into the probe solution, a prominent fluorescence color change from yellow to cyan was observed under a UV chamber. The fluorescence changes thus observed were ratiometric, and the detection limit of this new probe was found to be (22.1 ± 0.89) μM, suggesting that the efficiency of BOHB for the detection of cyanide ions is brilliant even at a minute level. The blue shift in fluorescence intensity upon the addition of cyanide ions was attributed to the deprotonation mechanism of acidic protons present in BOHB. This phenomenon was further explored using 1H-NMR study, which supported the mechanism. Further, stability study was performed over a period of 5 days to prominently establish the stability of BOHB. The probe is also highly capable of recognizing CN- within a very short time-span (almost 15 seconds), thereby making it a brilliant fluorescent switch for the swift recognition of CN-. Furthermore, BOHB was employed for real water sample analysis to display its practical application. Besides, the easy-to-prepare dipstick experiment provides a simple, reusable and recyclable protocol for the suitable qualitative identification of CN-. Lastly, triple negative breast adenocarcinoma (MDA-MB-231) cells were made susceptible to CN- sensing in a biological system, thereby making BOHB a biomarker tool.

Visualizing plant salt stress with a NaCl-responsive fluorescent probe

Salt stress is an adverse environmental condition that harms plant growth and development. The development of salt stress probes is critical for tracking the growth dynamics of plants, molecular breeding or screening of growth regulators. The sodium chloride (NaCl)-responsive fluorescent probe Aza-CyBz is designed based on the tenet that NaCl induces formation of ordered aggregates, and the sensitive fluorescence response can enable the visualization of plant salt stress in root tip tissues and live plants. Herein, we describe a detailed three-step route for synthesis of Aza-CyBz and applications to monitoring salt stress in Arabidopsis thaliana. The procedures for operating fluorescence imaging under various stresses are also listed to eliminate interference from the oxidative mechanism of salt stress. Compared with conventional invasive approaches such as inductively coupled plasma emission spectrometry and flame photometer, our protocol can real-time monitor salt stress experienced by plants, which demands simple pretreatment procedure and staining technique. Due to near infrared fluorescence, this method provides direct visual observation of salt stress at both tissue and live plant levels, which is superior to conventional noninvasive approaches. The preparation of probe Aza-CyBz takes ~2 d, and the imaging experiments for assessing salt stress experienced by plants, including the preparation of stressed plant samples takes ~9-11 d for root tip tissues and ~23 d for live plants. Notably, acquisition and analysis visual images of salt stress in plants can be completed within 2 h and they require only a basic knowledge of spectroscopy and chemistry.

Single-Probe-Based Sensor Array for Fingerprint Recognition of Trivalent Metal Ions and Application in Water Identification

Abnormal concentration levels of trivalent metal ions (M3+) might hinder their natural biological activities in physiological processes and cause severe health hazards. Herein, a dual-chromophore probe (RhB-TPE) composed of rhodamine and tetraphenylethene (TPE) units was synthesized and explored for discriminating M3+ ions. It exhibited special aggregation and AIE properties in aqueous media. Its ensemble with anionic surfactant SDBS assemblies (RhB-TPE/SDBS) could be utilized as fluorescent sensors for selective and sensitive detection of M3+ ions such as Fe3+, Al3+, and Cr3+ by illustrating quenched TPE emission and switched-on rhodamine emission. Moreover, the use of SDBS assemblies at two concentrations could provide a single-probe-based sensor array and realize four-signal pattern recognition of different concentrations of the three M3+ ions and identify M3+ mixtures or unknown samples. The cross-reactive fluorescence variation was attributed to the M3+ influence on the FRET process from TPE to open-ring form rhodamine in the two ensemble sensors. With the coexistence of Al3+, the optimized RhB-TPE/SDBS ensemble sensor array was successfully applied to differentiate commercially available brand mineral water and purified water, as well as tap water. The present work provides a novel strategy to generate a single-probe-based sensor array and realizes fingerprint recognition of three trivalent metal ions and efficient discrimination of different types of water. The modulation FRET process of a dual chromophore in different surfactant ensembles inspires the future construction of novel and effective sensing platforms.

Turn-on detection of Al3+ ions using quinoline-based tripodal probe: mechanistic investigation and live cell imaging applications

In this study, an easily synthesizable Schiff base probe TQSB having a quinoline fluorophore is demonstrated as a fluorescent and colorimetric turn-on sensor for Al3+ ions in a semi-aqueous medium (CH3CN/water; 4 : 1; v/v). Absorption, emission and colorimetric studies clearly indicated that TQSB exhibited a high selectivity toward Al3+, as observed from its excellent binding constant (Kb = 3.8 × 106 M-1) and detection limit (7.0 nM) values. TQSB alone was almost non-fluorescent in nature; however, addition of Al3+ induced intense fluorescence at 414 nm most probably due to combined CHEF (chelation-enhanced fluorescence) and restricted PET effects. The sensing mechanism was established via Job's plot, NMR spectroscopy, ESI-mass spectrometry, and density functional theory (DFT) analyses. Furthermore, to evaluate the applied potential of probe TQSB, its sensing ability was studied in real samples such as soil samples and Al3+-containing Digene gastric tablets as well as on low-cost filter paper strips. Fluorescence microscopy imaging experiments further revealed that TQSB can be used as an effective probe to detect intracellular Al3+ in live cells with no cytotoxicity.

Molecular design and architectonics towards film-based fluorescent sensing

The past few decades have witnessed encouraging progress in the development of high-performance film-based fluorescent sensors (FFSs) for detecting explosives, illicit drugs, chemical warfare agents (CWAs), and hazardous volatile organic chemicals (VOCs), among others. Several FFSs have transitioned from laboratory research to real-world applications, demonstrating their practical relevance. At the heart of FFS technology lies the sensing films, which play a crucial role in determining the analytes and the resulting signals. The selection of sensing fluorophores and the fabrication strategies employed in film construction are key factors that influence the fluorescence properties, active-layer structures, and overall sensing behaviors of these films. This review examines the progress and innovations in the research field of FFSs over the past two decades, focusing on advancements in fluorophore design and active-layer structural engineering. It underscores popular sensing fluorophore scaffolds and the dynamics of excited state processes. Additionally, it delves into six distinct categories of film fabrication technologies and strategies, providing insights into their advantages and limitations. This review further addresses important considerations such as photostability and substrate effects. Concluding with an overview of the field's challenges and prospects, it sheds light on the potential for further development in this burgeoning area.

The complexometric behavior of selected aroyl-S,N-ketene acetals shows that they are more than AIEgens

Using the established synthetic methods, aroyl-S,N-ketene acetals and subsequent bi- and multichromophores can be readily synthesized. Aside from pronounced AIE (aggregation induced emission) properties, these selected examples possess distinct complexometric behavior for various metals purely based on the underlying structural motifs. This affects the fluorescence properties of the materials which can be readily exploited for metal ion detection and for the formation of different metal-aroyl-S,N-ketene acetal complexes that were confirmed by Job plot analysis. In particular, gold(I), iron(III), and ruthenium (III) ions reveal complexation enhanced or quenched emission. For most dyes, weakly coodinating complexes were observed, only in case of a phenanthroline aroyl-S,N-ketene acetal multichromophore, measurements indicate the formation of a strongly coordinating complex. For this multichromophore, the complexation results in a loss of fluorescence intensity whereas for dimethylamino-aroyl-S,N-ketene acetals and bipyridine bichromophores, the observed quantum yield is nearly tripled upon complexation. Even if no stable complexes are formed, changes in absorption and emission properties allow for a simple ion detection.

A novel "turn-on" fluorescent probe based on thiocarbamoyl-DHP for Hg2+ detection in water samples and living cells

In this study, two fluorescent sensing probes, dihydropyridine (DHP) derivatives (DHP-CT1 and DHP-CT2) bearing phenoxy thiocarbonyl group, have been developed for Hg2+ detection. The tandem trimerization-cyclization of methylpropiolate with ammonium acetate gave 1.4-DHP and 1,2-DHP derivatives, which were reacted with O-phenylcarbonochloridothioate to produce DHP-CT1 and DHP-CT2, respectively. DHP-CT1 exhibits superior sensitivity and selectivity of fluorescence enhancement towards Hg2+ in aqueous media. The fluorescence intensity shows a good linear relationship with the concentration of Hg2+ in the range of 0-10 µM providing the extremely low LOD of 346 nM (69.4 ppb). The fluorescence enhancement is caused by the Hg2+ promoted hydrolysis of the thioamide bond releasing the fluorescent 1,4-DHP that was confirmed by NMR and HRMS. The quantitative analysis of Hg2+ in water samples using DHP-CT1 probe was demonstrated in aqueous solution and paper-based sensing strips. Furthermore, DHP-CT1 was also applied for monitoring intracellular Hg2+ in living RAW264.7 macrophages through fluorescence cell imaging.

Core-Labeled Reverse Micelle-Based Supramolecular Solvents for Assisted Quick and Sensitive Determination of Amitriptyline in Wastewater

In recent years, the issue of pharmaceutical contaminants in water bodies has emerged as a significant environmental concern owing to the potential negative impacts on both aquatic ecosystems and human health. Consequently, the development of efficient and eco-friendly methods for their determination and removal is of paramount importance. In this context, the development of a surfactant ensemble sensor has been explored for hard-to-sense amphiphilic drug, i.e., amitriptyline. Herein, a pyrene-based amphiphile chemoreceptor was synthesized and characterized through various spectroscopic techniques such as 1H, 13C NMR, single-crystal XRD, FTIR, and ES-mass spectrometry. Then, dodecanoic acid (DA) and a pyrene-based receptor in a THF/water solvent system were used to generate reverse micelle-based self-aggregates of SUPRAS (SUPRAmolecular Solvent). The structural aspects, such as morphology and size, along with the stability of the SUPRAS aggregates were unfolded through spectroscopic and microscopic insights. The present investigation describes a synergistic approach that combines the unique properties of premicellar concentration of supramolecular solvent with the promising potential of pyrene-based receptor for enhanced amitriptyline extraction with simultaneous determination from water (LOD = 12 nM). To evaluate the effectiveness of the developed aggregates in real-world scenarios, experiments were conducted to determine the sensing efficiency among various pharmaceutical pollutants commonly found in water sources. The results reveal that the synergistic nanoensemble exhibits remarkable sensing ability, toward the amitriptyline (AMT) drug outperforming conventional methods.

Turn-on detection of Al3+ and Zn2+ ions by a NSN donor probe: reversibility, logic gates and DFT calculations

An efficient dual functional naphthalene-derived Schiff base NpSb probe has been synthesised and evaluated for its fluorescence and chromogenic response towards metal ions. The NpSb probe was capable of selectively recognising Al3+ and Zn2+ ions when they were excited at the same wavelength in an aqueous organic solvent system. Almost non-fluorescent NpSb displayed a 'turn-on' fluorescence response when treated with Zn2+ (λem = 416 nm) and Al3+ (λem = 469 nm) ions due to the chelation-enhanced fluorescence (CHEF) effect. The limit of detection (LoD) values for Al3+ and Zn2+ have been determined to be 38.0 nM and 43.0 nM, respectively. The binding constants for Al3+ and Zn2+ were found to be 1.18 × 106 M-1 and 3.5 × 105 M-1, respectively. The NpSb also acted as a colorimetric sensor for Al3+ as the colour of the probe's solution turned to pale green from colourless upon Al3+ addition. The binding mechanism between NpSb and Zn2+/Al3+ was supported by the ESI-MS, Job's plot, NMR, and DFT studies. The reversibility experiments were carried out with an F- ion and EDTA with the development of corresponding logic gates. Moreover, NpSb could be applied to detect Al3+ ions in real samples such as tap water, distilled water and soil samples.

Interaction mode of hydroxypropyl-β-cyclodextrin with vaccine adjuvant components Tween 80 and Triton X-100 revealed by fluorescence increasing-quenching analysis

The nonionic surfactants Tween 80 (Tw80) and Triton X-100 (TX100), which are used as components of adjuvants, were used with bovine serum albumin (BSA) and hydroxfypropyl-β-cyclodextrin (HP-β-CD) as model antigens. The interaction patterns of Tw80 and TX100 with the hydrophobic cores of the model antigens were investigated. The fluorescence of 8-anilinonaphthalene-1-sulfonic acid (ANS), a hydrophobic fluorescent probe, was used to evaluate the effect of surfactants on each model antigen. A Hanes Woolf plot was used to analyze the adsorption of ANS to BSA, and an activator-inhibitor model was used to analyze the concentration-dependent increase and decrease of ANS fluorescence intensity. For BSA, TX100 occupies the ANS binding site inside the BSA hydrophobic core, while Tw80 does not contribute to the ANS binding site in the hydrophobic core. For HP-β-CD, the ANS concentration required for analyzable fluorescence intensity extended to the range where ANS concentration-dependent quenching was not negligible. Using the activator inhibitor model, we were able to separate the activators and inhibitors of ANS fluorescence and evaluate the affinity of ANS for HP-β-CD and surfactants. The results obtained showed that TX100 provided a hydrophobic environment to the ANS while being encapsulated by HP-β-CD, while Tw80 did not interact with HP-β-CD and provided a hydrophobic environment to the ANS independently of each other. The interpretations obtained were corroborated by the determination of the CMC of TX100 and Tw80, the effect of salt on ANS fluorescence, and ¹H-NMR and ROESY. In summary, the results showed that the large hydrophilic head of Tween, composed of sorbitan and PEG chains, floated in the aqueous phase like a balloon, while Triton pierced the hydrophobic core of the antigen like a spear. In both BSA and HP-β-CD model antigens, TX100 impinged on the hydrophobic core.

Self-Assembled Fluorescent Block Copolymer Micelles with Responsive Emission

Responsive fluorescent materials offer a high potential for sensing and (bio-)imaging applications. To investigate new concepts for such materials and to broaden their applicability, the previously reported non-fluorescent zinc(II) complex [Zn(L)] that shows coordination-induced turn-on emission was encapsulated into a family of non-fluorescent polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer micelles leading to brightly emissive materials. Coordination-induced turn-on emission upon incorporation and ligation of the [Zn(L)] in the P4VP core outperform parent [Zn(L)] in pyridine solution with respect to lifetimes, quantum yields, and temperature resistance. The quantum yield can be easily tuned by tailoring the selectivity of the employed solvent or solvent mixture and, thus, the tendency of the PS-b-P4VP diblock copolymers to self-assemble into micelles. A medium-dependent off-on sensor upon micelle formation could be established by suppression of non-micelle-borne emission background pertinent to chloroform through controlled acidification indicating an additional pH-dependent process.

A benzothiazole-based new fluorogenic chemosensor for the detection of CN- and its real-time application in environmental water samples and living cells

Since the cyanide ion is used in a wide range of industries and is harmful to both human health and the environment, a number of research efforts are dedicated to creating fluorescence sensors for the detection of cyanide (CN-). Herein, for the fluorescence detection of CN-, a new highly selective and sensitive sensor 2-(3-(benzo[d]thiazol-2-yl)-4-hydroxybenzylidene)-1H-indene-1,3(2H)-dione (BID) was created by conjugating a benzothiazole moiety with 1H-indene-1,3(2H)-dione. The donor and acceptor components of this hybrid receptor were covalently connected through a double bond. The nucleophilic addition of a cyanide anion to the BID inhibits the intramolecular charge transfer (ICT) transition, resulting in spectral and colour alterations in the receptor. When the solvent polarity was increased from n-hexane to methanol, this molecule exhibited a bathochromic shift in the emission wavelength (610 to 632 nm), suggesting the presence of a solvatochromic action. The sensor BID has shown strong specificity towards CN- by interrupting its internal charge transfer (ICT), resulting in a significant change in the UV-vis spectrum and a notable blue shift in the fluorescence emission spectrum. The cyanide anion (CN-) is responsible for the optical alterations observed by BID, as opposed to the other anions examined. The detection limit was 5.97 nM, significantly less than the WHO's permitted amount of CN- in drinking water. The experimental findings indicate that BID's fluorescence response to CN- is pH insensitive throughout a wide pH range of 6.0 to 12.0. The interaction mechanism between the BID and CN- ions has been studied by HRMS, 1H-NMR titration experiments, FT-IR, and DFT, which confirmed the nucleophilic addition of CN- on vinylidene and subsequent disturbance of ICT. Additionally, we demonstrated the real-time detection application of CN- in environmental water samples and live-cell imaging.

Fluorescent Ensemble Sensors and Arrays Based on Surfactant Aggregates Encapsulating Pyrene-Derived Fluorophores for Differentiation Applications

Surfactant assemblies have drawn great attention in fabricating fluorescent sensors as they can provide advantages such as easy preparation, low cost, aqueous detection, high fluorescence stability, high sensitivity to external stimuli, etc. We have devoted the past few years to fluorescent cross-reactive sensors and arrays that are advantageous in differentiating similar analytes and analyzing mixed samples. In this Spotlight on Applications, we introduce our recent advances in developing surfactant assembly-based fluorescent sensors and arrays for discrimination applications. Besides using surfactant assemblies encapsulating fluorophores to fabricate multiple-element-based sensor arrays, we particularly proposed to take advantage of modulation effect of dynamic surfactant assemblies on the photophysical properties of encapsulated fluorophores to construct single-system-based discriminative sensors, which have been successfully applied in differentiation of multiple metal ions and various proteins. The applications of surfactant assembly-based sensors for the detection and discrimination of thiols, amino acids, and explosives are also introduced. Finally, the prospects of further efforts for improving surfactant ensemble sensors and their challenges are discussed.