Advances in modifying fluorescein and rhodamine fluorophores as fluorescent chemosensors

Synergistic integration of a rhodamine-labelled tripeptide into AIE-active fluorogenic probe: Enabling nanomolar detection of Al3+ ions through test strips, thin films, and Arduino-assisted optosensing platform

Peptide-fluorophore conjugates (PFCs) have been expeditiously utilized for metal ion recognition owing to their distinctive characteristics. Selective detection and quantification of aluminum is essential to minimize health and environmental risks. Herein, we report the synthesis and characterization of a new chemoprobe with aggregation-induced emission characteristics by chemically conjugating rhodamine-B fluorophore with a tripeptide. The probe revealed β-sheet secondary conformation in both solid and solution states, as confirmed by FT-IR, PXRD, and CD experiments. AIE characteristics of the probe in water-MeCN mixtures revealed the formation of spherically shaped nanoaggregates with an average size of 353 ± 7 nm, as confirmed by SEM, TEM, and DLS studies. The probe exhibited a large stokes shift (175 nm) and displayed selective colorimetric and fluorometric responses towards Al3+ ions with an extremely low detection limit (51 nm) and a fast response time (≤15 s). Comparative NMR studies confirmed the cleavage of spirolactam ring upon aluminum binding. The probe's practicality was enhanced through integration into test strips and thin films, allowing solid-phase detection of Al3+ ions. Furthermore, an RGB-Arduino enabled optosensing device has been developed to enable instant quantifiable analysis of aluminum concentrations in real-time conditions.

Exploring the Te(II)/Te(IV) Redox Couple of a Tellurorosamine Chromophore: Photophysical, Photochemical, and Electrochemical Studies

A tellurorosamine dye [Te(II)] undergoes aerobic photooxidation. Although Te(IV) species have been used in a number of oxidations, key Te(IV)-oxo and Te(IV)-bis(hydroxy) intermediates are challenging to study. Under aerobic irradiation with visible light, Te(II) (λmax = 600 nm) transforms into a Te(IV) species (λmax = 669 nm). The resultant Te(IV) species is not stable in the dark or at -20 °C, decomposing back to Te(II) and other byproducts over many hours. To eliminate the structural ambiguity of the Te(IV) photoproduct, we used spectroelectrochemistry, wherein the bis(hydroxy) Te(IV)-(OH)2 was electrochemically generated under anaerobic conditions. The absorption of Te(IV)-(OH)2 matches that of the Te(IV) photoproduct. Because isosbestic points are maintained both photochemically and electrochemically, the oxo core formed photochemically must rapidly equilibrate with Te(IV)-(OH)2. Calculations on the bis(hydroxy) versus oxo species further corroborate that the equilibration is rapid and the spectra of the two species are similar. To further explore Te(IV) cores, two novel compounds, Te(IV)-Cl2 and Te(IV)-Br2, were synthesized. Characterization of Te(IV)-X2 was simplified because these cores have no analogue to the Te(IV)-(O)/Te(IV)-(OH)2 equilibrium. This work provides insights into the photophysical and electrochemical behavior of Te analogues of chalcogenoxanthylium dyes, which are relevant for a broad range of photochemical applications.

A Ratiometric Fluorescent Probe Dye-Functionalized MOFs Integrated with Logic Gate Operation for Efficient Detection of Acetaldehyde

Volatile organic compounds (VOCs) are a class of hazardous gases that are widely present in the atmosphere and cause great harm to human health. In this paper, a ratiometric fluorescent probe (Dye@Eu-MOFs) based on a dye-functionalized metal-organic framework was designed to detect VOCs, which showed high sensitivity and specificity for acetaldehyde solution and vapor. A linear correlation between the integrated fluorescence intensity (I510/I616) and the concentration of acetaldehyde was investigated, enabling a quantitative analysis of acetaldehyde in the ranges of 1 × 10-4~10-5 μL/mL, with a low detection limit of 8.12 × 10-4 mg/L. The selective recognition of acetaldehyde could be clearly distinguished by the naked eye under the excitation of UV light. The potential sensing mechanism was also discussed. Significantly, a molecular logic gate was constructed based on the whole system, and finally, a molecular logic network system for acetaldehyde detection connecting basic and integrated logic operations was realized. This strategy provided an effective guiding method for constructing a molecular-level logic gate for acetaldehyde detection on a simple platform.

Antimicrobial, antioxidant, cell imaging and sensing applications of fluorescein derivatives: A review

Fluorescein itself is a synthetic organic compound and a prominent member of the xanthene dye family. It exhibits strong fluorescence under ultraviolet (UV) or blue light excitation, making it widely used in various applications, including fluorescence microscopy, flow cytometry, immunoassays, and molecular biology techniques. One of the reasons fluorescein derivatives are highly valuable is their tunable fluorescence properties. Through chemical modifications of the fluorescein structure, different functional groups or substituents can be introduce, altering the compound's fluorescence characteristics such as emission wavelength, intensity, and photo stability. This flexibility allows for tailoring of fluorescent probes to specific experimental requirements, enhancing their utility in a range of scientific disciplines. Fluorescein derivatives also possess excellent antimicrobial and antioxidant activity. This review sheds light on the significant impact of fluorescein derivatives as biological active compounds, highlighting their potential in designing new therapeutic agents with antimicrobial properties. Additionally, their role as antioxidants is discussed. A major aspect covered in the review is the application of fluorescein derivatives as powerful cell imaging probes. Their unique fluorescent properties make them valuable tools for visualizing cellular structures and processes, opening up new possibilities for studying cellular dynamics and interactions.

A Novel Rhodamine-Phenolphthalein Architecture for Selective Mercury Ion Detection in Aqueous Media

In this study, the primary objective is to synthesize a novel fluorescent Rh-PP-Rh compound and explore its extensive range of photochemical behaviors. Initially, the synthesis of the novel Rh-PP-Rh was carried out for this purpose. Subsequently, UV-Vis and fluorescence spectroscopy were employed to investigate the interactions between Rh-PP-Rh and a diverse array of ions in aqueous solvent systems. Through fluorescence and UV-Vis studies, it was observed that Rh-PP-Rh demonstrated turn-on sensor properties in the presence of Hg2+ ions. Furthermore, the limits of detection (LOD) and association constant (Ka) values for Rh-PP-Rh/Hg2+ were determined as 334 nM and 9.13×1011 M-2, respectively. Additionally, the reversible studies demonstrated a switchable on/off response upon alternate addition of HgCl2 and [Bu4N]F to Rh-PP-Rh. These findings suggest that the probe Rh-PP-Rh also possesses specific sensor properties for F- ions in the presence of mercury. In addition, the investigation encompassed an assessment of the visual analysis of the color alterations of Rh-PP-Rh both on filter paper and in an EtOH/H2O solution. The findings demonstrated that Rh-PP-Rh can be successfully utilized in solutions containing mercury, as it generates significant color transformations.

Self-indicating polymers: a pathway to intelligent materials

Self-indicating polymers have emerged as a promising class of smart materials that possess the unique ability to undergo detectable variations in their physical or chemical properties in response to various stimuli. This article presents an overview of the most important mechanisms through which these materials exhibit self-indication, including aggregation, phase transition, covalent and non-covalent bond cleavage, isomerization, charge transfer, and energy transfer. Aggregation is a prevalent mechanism observed in self-indicating polymers, where changes in the degree of molecular organization result in variations in optical or electrical properties. Phase transition-induced self-indication relies on the transformation between different phases, such as liquid-to-solid or crystalline-to-amorphous transitions, leading to observable changes in color or conductivity. Covalent bond cleavage-based self-indicating polymers undergo controlled degradation or fragmentation upon exposure to specific triggers, resulting in noticeable variations in their structural or mechanical properties. Isomerization is another crucial mechanism exploited in self-indicating polymers, where the reversible transformation between the different isomeric forms induces detectable changes in fluorescence or absorption spectra. Charge transfer-based self-indicating polymers rely on the modulation of electron or hole transfer within the polymer backbone, manifesting as changes in electrical conductivity or redox properties. Energy transfer is an essential mechanism utilized by certain self-indicating polymers, where energy transfer between chromophores or fluorophores leads to variations in the emission characteristics. Furthermore, this review article highlights the diverse range of applications for self-indicating polymers. These materials find particular use in sensing and monitoring applications, where their responsive nature enables them to act as sensors for specific analytes, environmental parameters, or mechanical stress. Self-indicating polymers have also been used in the development of smart materials, including stimuli-responsive coatings, drug delivery systems, food sensors, wearable devices, and molecular switches. The unique combination of tunable properties and responsiveness makes self-indicating polymers highly promising for future advancements in the fields of biotechnology, materials science, and electronics.

A general fluorescence off/on strategy for fluorogenic probes: Steric repulsion-induced twisted intramolecular charge transfer (sr-TICT)

Various control strategies are available for building fluorogenic probes to visualize biological events in terms of a fluorescence change. Here, we performed the time-dependent density functional theory (TD-DFT) computational analysis of the twisted intramolecular charge transfer (TICT) process in rhodamine dyes. On the basis of the results, we designed and synthesized a series of rhodamine dyes and established a fluorescence quenching strategy that we call steric repulsion-induced TICT (sr-TICT), in which the fluorescence quenching process is greatly accelerated by simple intramolecular twisting. As proof of concept of this design strategy, we used it to develop a fluorogenic probe, 2-Me PeER (pentyloxyethylrhodamine), for the N-dealkylation activity of CYP3A4. We applied 2-Me PeER for CYP3A4 activity-based fluorescence-activated cell sorting (FACS), providing access to homogeneous, highly functional human-induced pluripotent stem cell (hiPSC)-derived hepatocytes and intestinal epithelial cells. Our results suggest that sr-TICT represents a general fluorescence control method for fluorogenic probes.

A series of viologen complexes containing thiophene and Br- dual fluorescent chromophores for continuous visual sensing of pH and Hg2

The mercury ion (Hg2+) is a typical high-toxicity substance that can cause severe damage to the environment and human bodies. For the detection of Hg2+, there are still significant challenges in the detection range and limit of detection (LOD). In this study, three viologen-based fluorescent probes are developed, CdCl4(Btybipy) (1), ZnBr4(Btybipy) (2), CdBr4(Btybipy) (3) (Btybipy = bis-1-thiophen-3-ylmethyl-[4,4']-bipyridinyl) through conventional solvent methods for detecting pH and Hg2+. Reversible discoloration and fluorescence response behaviour in the pH range of 4-12.8 is demonstrated by viologen-based fluorescent probes, which exhibit "ON-OFF-ON" signal changes. Compared with complex 1, it is surprising to find that complexes 2-3 display both fluorescence enhancement and fluorescence quenching simultaneously with the addition of different concentrations of Hg2+ (0-20 and 25-400 μM). There is broad linearity in the range of 0-20 and 50-300 μM with LODs of 2.14 and 3.13 nM, respectively. This occurrence of dual-signal modes is attributed to the participation of Br- and the thiophene S atom as dual chromophores in the coordination reaction of Hg2+. Dual-signal mode output, high sensitivity, wide detection range, and low LODs are exhibited by these fluorescent probes. The unique coordination reaction between Br- and the thiophene S atom with Hg2+ can provide a potential strategy for the exploitation of promising sensing platforms for monitoring Hg2+.

Xanthene-based Hg2+ fluorescent probe for detection of Hg2+ in water/food samples, as well as imaging of live cells, zebrafish and tobacco seedlings

In this paper, an Hg2+ detection probe, HOS, was prepared with a xanthene as the parent fluorophore. Hg2+-initiated thioacetal deprotection reaction is the detection mechanism of this probe. After testing, the probe HOS was able to accurately determine Hg2+ with a detection limit of 36 nM. It was successfully applied to the detection of Hg2+ in different water samples and shrimp samples, meanwhile, the filter paper strips prepared by HOS were obviously changed from light yellow to dark yellow under daylight, and from green to yellow under 365 nm UV light. Furthermore, probe HOS enabled Hg2+ bioimaging experiments on HepG2 cells, zebrafish and tobacco seedlings under laser confocal microscopy.

The syntheses of fluorescein-based conjugated microporous polymers by direct arylation polymerization and fluorescence sensing Fe3+ in aqueous solutions

Determination of ferri ions in environment and human bodies is very important for environmental protection and disease diagnosis. Recently, conjugated microporous polymers (CMPs) used for fluorescence sensing metal ions have attracted much attention, but this technique is done in organic solvents. In this study, the two new fluorescein-based CMPs named FLEDOT and FLBTh were synthesized by "greener method", direct arylation polymerization, with tetraiodofluorescein sodium salt (TIFS) and 3,4-ethylenedioxy thiophene or 2,2'-bithiophene. Pleasely, the prepared fluorescein-based CMPs can fluorescently sense for Fe3+ in water with high sensitivity and selectivity. The quenching constants (KSV) of FLEDOT and FLBTh are 1.51 × 104 and 1.09 × 104 L mol-1, and the limits of detection (LODs) as low as 1.99 × 10-10 and 2.75 × 10-10 mol L-1, which are comparable to the sensitivity found in organic solvents' dispersions such as N,N-dimethylformamide (DMF)' dispersions. UV-Vis absorption spectra show that the fluorescence quenching mechanisms of Fe3+ are absorption competition quenching process and energy transfer process.

A Rhodamine-coumarin Triazole Conjugate as a Fluorescent Chemodosimeter for Cu(II) Detection and its Application in Live Cell Bioimaging

A rhodamine-triazole fluorescent probe bearing a coumarin moiety RTC was synthesized using the Cu(I)-catalyzed click reaction. The rhodamine-triazole conjugate was highly selective to Cu2+ among other metal ions, including Ca2+, Co2+, Cu2+, Cd2+, Mg2+, Fe2+, Fe3+, Hg2+, Zn2+, Ni2+, Pd2+ and Pb2+ in physiological conditions. Upon the addition of Cu2+, the colorless RTC solution turned pink and exhibited a significant fluorescence emission centered at 578 nm. The binding of Cu2+ induced a hydrolysis reaction, leading to a release of the coumarin unit from the rhodamine probe, as confirmed by mass spectrometric data. From the fluorescence titration, the detection limit of RTC for Cu2+ was determined to be 21 nM (1.3 ppb). The sensor was responsive to Cu2+ in a wide pH range and successfully applied to monitor Cu2+ in HEK293T cells by confocal fluorescence imaging.

A naphthalimide functionalized fluoran with AIE effect for ratiometric sensing Hg2+ and cell imaging application

The pollution caused by mercury ions (Hg²⁺) poses a potential threat to public health. Therefore, monitoring Hg²⁺ concentration in the environment is necessary and significant. In this work, a naphthalimide functionalized fluoran dye NAF has been prepared, which shows a new red-shift in emission at 550 nm with the maximum intensity in a mixture of water-CH₃CN (v/v = 7/3) due to aggregating induced emission (AIE) effect. Meanwhile, NAF can be employed as a Hg²⁺ ions sensor, which displays a selective and sensitive response to Hg²⁺ ions by the reduced fluorescence of naphthalimide fluorophore and increased fluorescence of fluoran group, respectively, showing ratiometric fluorescence signal changes with more than 65-fold emission intensity ratio increase and naked eyes visible color change. In addition, the response time is fast (within 1 min) and the sensing can be conducted in a wide pH range (4.0-9.0). Moreover, the detection limit has been evaluated to be 5.5 nM. The sensing mechanism may be attributed to the formation of a π-extended conjugated system due to the Hg²⁺ ions-induced conversion of spironolactone to the ring-opened form, partially accompanied by the fluorescence resonance energy transfer (FRET) process. Significantly, NAF exhibits suitable cytotoxicity to living HeLa cells, which allows it to be utilized for ratiometric imaging of Hg²⁺ ions assisted by confocal fluorescence imaging.

Ligand and Gold(I) Fluorescein-AIEgens as Photosensitizers in Solution and Doped Polymers

The synthesis of fluorescein propargyl diether (L) and two different dinuclear gold(I) derivatives containing a water-soluble phosphane [1,3,5-triaza-7-phosphatricyclo[3.3.1.13.7]decane (PTA) for complex 1 and 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA) for complex 2] has been successfully performed. All compounds display intrinsic emission from fluorescein, being less intense for gold(I) complexes due to the heavy-atom effect. All compounds aggregate in acetonitrile/water mixtures with the formation of larger aggregates for those samples containing more water content, as evidenced by dynamic light scattering and small-angle X-ray scattering experiments, in agreement with the absorption and emission data. The emission of the samples increases when they are used to obtain luminescent materials with four different organic matrices [poly(methyl methacrylate, polystyrene (PS), cellulose, and Zeonex]. The compounds display very high values of singlet oxygen (¹O₂) production in dichloromethane. Singlet oxygen production was also evaluated in the doped matrices, being the highest in PS and with an exciting increase on PS microspheres. Density functional theory (BP86-D3) and GFN2-xTB calculations were used to model the assembly of L and complexes 1 and 2 with the different organic matrices and rationalize the experimental findings based on the geometries, molecular electrostatic potential surfaces, and complementarity and HOMO-LUMO gaps.

A general highly efficient synthesis of biocompatible rhodamine dyes and probes for live-cell multicolor nanoscopy

The development of live-cell fluorescence nanoscopy is powered by the availability of suitable fluorescent probes. Rhodamines are among the best fluorophores for labeling intracellular structures. Isomeric tuning is a powerful method for optimizing the biocompatibility of rhodamine-containing probes without affecting their spectral properties. An efficient synthesis pathway for 4-carboxyrhodamines is still lacking. We present a facile protecting-group-free 4-carboxyrhodamines' synthesis based on the nucleophilic addition of lithium dicarboxybenzenide to the corresponding xanthone. This approach drastically reduces the number of synthesis steps, expands the achievable structural diversity, increases overall yields and permits gram-scale synthesis of the dyes. We synthesize a wide range of symmetrical and unsymmetrical 4-carboxyrhodamines covering the whole visible spectrum and target them to multiple structures in living cells - microtubules, DNA, actin, mitochondria, lysosomes, Halo-tagged and SNAP-tagged proteins. The enhanced permeability fluorescent probes operate at submicromolar concentrations, allowing high-contrast STED and confocal microscopy of living cells and tissues.

A Selective and ''Off-On'' Fluorescent Chemosensor Based on Fluorescein for Al3+: Synthesis, Characterization, Spectroscopy Analyses, and DFT Calculation

An efficient fluorescent cation chemosensor based on fluorescein L4 was well prepared and identified with spectroscopy analyses. UV-vis and fluorescence measurements examined the analyte complexation of the L4 with various cations, demonstrating a clear tendency to Al³⁺ ion. In the Job plot study, a stoichiometry ratio of a complex between L4 and Al³⁺ ion was determined to be 1: 2 (L4: Al³⁺). A stoichiometry ratio of complex between L4 and Al³⁺ ion was determined to be 1: 2 (L4: Al³⁺) using the Job plot. The association constant (K_a) of the L4-Al³⁺ complex was found 2.8 × 10⁷ M^-2. The obtained limit of detection (LOD) value (1.37 × 10^-6 M for Al³⁺) exhibited the considerable sensitivity of the chemosensor L4 to Al³⁺ ion. DFT/TD-DFT calculations have also been employed to support the binding mode and photophysical properties of the complexation of chemosensor L4 to Al³⁺ ion and also to investigate the enhancement of L4 fluorescence by Al³⁺ ion.

Homoadamantane-Fused Tetrahydroquinoxaline as a Robust Electron-Donating Unit for High-Performance Asymmetric NIR Rhodamine Development

Rhodamines have emerged as a useful class of dye for bioimaging. However, intrinsic issues such as short emission wavelengths and small Stokes shifts limit their widespread applications in living systems. By taking advantage of the homoadamantane-fused tetrahydroquinoxaline (HFT) moiety as an electron donor, we developed a new class of asymmetric NIR rhodamine dyes, NNR1-7. These new dyes retained ideal photophysical properties from the classical rhodamine scaffold and showed large Stokes shifts (>80 nm) with improved chemo/photostability. We found that NNR1-7 specifically target cellular mitochondria with superior photobleaching resistance and improved tolerance for cell fixation compared to commercial mitochondria trackers. Based on NNR4, a novel NIR pH sensor (NNR4M) was also constructed and successfully applied for real-time monitoring of variations in lysosomal pH. We envision this design strategy would find broad applications in the development of highly stable NIR dyes with a large Stokes shift.

A Novel Indole Derivative with Superior Photophysical Performance for Fluorescent Probe, pH-Sensing, and Logic Gates

An indole-related molecules have been considered as the potential fluorescent probes for biological and electrochemical sensing. However, most of the indole probes have been usually used in a single detection mode. Indolium probes that enable accurate detection in complex environments are rarely reported. Here, four novel indole derivatives including the phenyl group substituted with different functional moieties were designed on the basis of the donor-π-acceptor (D-π-A) concept. These derivatives exhibit positive solvatochromism owing to their varied molecular conformations upon contacting to various solvents and the different HOMO-LUMO gaps caused by the difference in electronic push-pull capability of the substituents. Their solid-state fluorescence emissions and multiple chromisms are observed due to the inherent twisted geometries and aggregation modes. In addition, these derivatives show dramatic color and fluorescence responses due to the protonation of the nitrogen and oxygen containing groups, and thus novel colorimetric pH sensors, fluorescent papers and logic gates have been designed.

Cryogenic Fluorescence Spectroscopy of Ionic Fluorones in Gaseous and Condensed Phases: New Light on Their Intrinsic Photophysics

Fluorescence spectroscopy of gas-phase ions generated through electrospray ionization is an emerging technique able to probe intrinsic molecular photophysics directly without perturbations from solvent interactions. While there is ample scope for the ongoing development of gas-phase fluorescence techniques, the recent expansion into low-temperature operating conditions accesses a wealth of data on intrinsic fluorophore photophysics, offering enhanced spectral resolution compared with room-temperature measurements, without matrix effects hindering the excited-state dynamics. This perspective reviews current progress on understanding the photophysics of anionic fluorone dyes, which exhibit an unusually large Stokes shift in the gas phase, and discusses how comparison of gas- and condensed-phase fluorescence spectra can fingerprint structural dynamics. The capacity for temperature-dependent measurements of both fluorescence emission and excitation spectra helps establish the foundation for the use of fluorone dyes as fluorescent tags in macromolecular structure determination. We suggest ideas for technique development.

Progress in the Development of Imidazopyridine-Based Fluorescent Probes for Diverse Applications

Different classes of Imidazopyridine i.e., Imidazo[1,2-a]pyridine, Imidazo[1,5-a] pyridine, Imidazo[4,5-b]pyridine, have shown versatile applications in various fields. In this review, we have concisely presented the usefulness of the fluorescent property of imidazopyridine in different fields such as imaging tools, optoelectronics, metal ion detection, etc. Fluorescence mechanisms such as excited state intramolecular proton transfer, photoinduced electron transfer, fluorescence resonance energy transfer, intramolecular charge transfer, etc. are incorporated in the designed fluorophore to make it for fluorescent applications. It has been widely employed for metal ion detection, where selective metal ion detection is possible with triazole-attached imidazopyridine, β-carboline imidazopyridine hybrid, quinoline conjugated imidazopyridine, and many more. Also, other popular applications involve organic light emitting diodes and cell imaging. This review shed a light on recent development in this area especially focusing on the optical properties of the molecules with their usage which would be helpful in designing application-based new imidazopyridine derivatives.

A pyridyl functionalized rhodamine chemodosimeter for selective fluorescent detection of mercury ions and cell imaging

Rhodamines gain sustained attention owing to their great potential for probe design applications. Herein, the facile preparation of a new pyridyl functionalized rhodamine dye PR is reported, which has stable fluorescence signal in water with maximum emission peak at 594 nm and Stokes shift of 81 nm. Based on dye PR, a new fluorescent probe PRHg has been developed by modifying the spirolactone of PR with hydrazine hydrate so as to produce spirolactam recognizing group for sensing of Hg²⁺. PRHg exhibits high selectivity and sensitivity towards Hg²⁺ in water/ethanol (v/v = 4/1, pH = 7.0) by a specific Hg²⁺-binding promoted spirolactam ring opening and hydrolyzing process. And, the detection limit for Hg²⁺ is evaluated to be 8.5 nM. Besides, the probe can respond to Hg²⁺ within 40 min and over a wide pH range from 4.0 to 10.0. Moreover, PRHg (40 µM) performs low cytotoxicity to HeLa cells (over 91.0 % cell survival rate), which allows the probe to be employed for tracing intracellular Hg²⁺ by fluorescence imaging.

DNA-enabled fluorescent-based nanosensors monitoring tumor-related RNA toward advanced cancer diagnosis: A review

As a burgeoning non-invasive indicator for reproducible cancer diagnosis, tumor-related biomarkers have a wide range of applications in early cancer screening, efficacy monitoring, and prognosis predicting. Accurate and efficient biomarker determination, therefore, is of great importance to prevent cancer progression at an early stage, thus reducing the disease burden on the entire population, and facilitating advanced therapies for cancer. During the last few years, various DNA structure-based fluorescent probes have established a versatile platform for biological measurements, due to their inherent biocompatibility, excellent capacity to recognize nucleic and non-nucleic acid targets, obvious accessibility to synthesis as well as chemical modification, and the ease of interfacing with signal amplification protocols. After decades of research, DNA fluorescent probe technology for detecting tumor-related mRNAs has gradually grown to maturity, especially the advent of fluorescent nanoprobes has taken the process to a new level. Here, a systematic introduction to recent trends and advances focusing on various nanomaterials-related DNA fluorescent probes and the physicochemical properties of various involved nanomaterials (such as AuNP, GO, MnO2, SiO2, AuNR, etc.) are also presented in detail. Further, the strengths and weaknesses of existing probes were described and their progress in the detection of tumor-related mRNAs was illustrated. Also, the salient challenges were discussed later, with a few potential solutions.

A quinoline-rhodamine hybrid probe for ratiometricly sensing of Hg2+ in water and cell imaging application

To develop efficient tools for monitoring toxicant Hg²⁺ in aqueous solution attracts great attention because the abnormal distribution of Hg²⁺ in environment poses great threat to human health. We here report the preparation of a novel quinoline-rhodamine hybrid fluorescent probe P7RHg for ratiometricly sensing of Hg²⁺ in water, with a spirolactam-thiosemicarbazide reaction group. Upon treatment by Hg²⁺, the ratio of fluorescence intensity (F₆₀₀/F₄₆₀) exhibits nearly 90-fold enhancement, presenting two well-resolved emission peaks (140 nm). Meanwhile, the specific Hg²⁺-induced desulfurization provides probe P7RHg an excellent selectivity to Hg²⁺, with a detection limit of 8.6 nM. Moreover, the low cytotoxicity allows P7RHg to be employed for tracing Hg²⁺ in living cells by confocal fluorescence imaging.

A sensitive mitochondrial thermometry 2.0 and the availability of thermogenic capacity of brown adipocyte

The temperature of a living cell is a crucial parameter for cellular events, such as cell division, gene expressions, enzyme activities and metabolism. We previously developed a quantifiable mitochondrial thermometry 1.0 based on rhodamine B methyl ester (RhB-ME) and rhodamine 800 (Rh800), and the theory for mitochondrial thermogenesis. Given that the synthesized RhB-ME is not readily available, thus, a convenient mitochondrial thermometry 2.0 based on tetra-methyl rhodamine methyl ester (TMRM) and Rh800 for the thermogenic study of brown adipocyte was further evolved. The fluorescence of TMRM is more sensitive (∼1.4 times) to temperature than that of RhB-ME, then the TMRM-based mito-thermometry 2.0 was validated and used for the qualitatively dynamic profiles for mitochondrial thermogenic responses and mitochondrial membrane potential in living cells simultaneously. Furthermore, our results demonstrated that the heterogenous thermogenesis evoked by β3 adrenoceptor agonist only used overall up to ∼46% of the thermogenic capacity evoked by CCCP stimulation. On the other hand, the results demonstrated that the maximum thermogenesis evoked by NE and oligomycin A used up to ∼79% of the thermogenic capacity, which suggested the maximum thermogenic capacity under physiological conditions by inhibiting the proton-ATPase function of the mitochondrial complex V, such as under the cold activation of sympathetic nerve and the co-release of sympathetic transmitters.

Fluorescent labeling as a strategy to evaluate uptake and transport of polymeric nanoparticles in plants

The use of biodegradable nanopolymers in agriculture offers an excellent alternative for the efficient delivery of agrochemicals that promote plant protection and development. However, tracking of these systems inside plants requires complex probe tagging strategies. In addition to providing a basis for better understanding such nanostructures to optimize delivery system design, these probes allow monitoring the migration of nanoparticles through plant tissues, and determine accumulation sites. Thus, these probes are powerful tools that can be used to quantify and visualize nanoparticle accumulation in plant cells and tissues. This review is an overview of the methods involved in labeling nanocarriers, mainly based on polymeric matrices, for the delivery of nanoagrochemicals and the recent advances in this field.

A Ratiometric Afterglow Response of Aluminium Ions in Methanol-Water

An afterglow is beneficial as an emission signal in the field of displays and imaging probes. Here, boronic acid-appended and spirolactam ring-containing rhodamine dye 1 was synthesized and grafted onto the surface of room-temperature phosphorescence-active boronate nanoparticles (BPs), composed of polymeric 3-benzo-2,4,8,10-tetraoxa-3,9-diboraspiro[5.5]undecane. The resultant ensemble, 1@BP, exhibited a greenish afterglow. However, the addition of Al³⁺ into the dispersion solution with 1@BP led to a change in the afterglow to grass green as a result of Förster-type energy transfer from the phosphorescent BP to the Al³⁺ -interacting rhodamine dye 1 on the surface. Based on the ratio of the two emission intensities, a linear response in the concentration range of 3.8-15.2 μM was observed, with a detection limit of 4.2 μM for Al³⁺ . A metal ion-dependent discernable color in afterglow was observed on a 1@BP-coated filter paper, which would be useful for not only film-based afterglow chemosensors but also encryption application.

Thermal Stability of Fluorescent Chitosan Modified with Heterocyclic Aromatic Dyes

Fluorescent biopolymer derivatives are increasingly used in biology and medicine, but their resistance to heat and UV radiation, which are sterilizing agents, is relatively unknown. In this work, chitosan (CS) modified by three different heterocyclic aromatic dyes based on benzimidazole, benzothiazole, and benzoxazole (assigned as IBm, BTh, and BOx) has been studied. The thermal properties of these CS derivatives have been determined using the Thermogravimetric Analysis coupled with the Fourier Transform Infrared spectroscopy of volatile degradation products. The influence of UV radiation on the thermal resistance of modified, fluorescent chitosan samples was also investigated. Based on the temperature onset as well as the decomposition temperatures at a maximal rate, IBm was found to be more thermally stable than BOx and BTh. However, this dye gave off the most volatile products (mainly water, ammonia, carbon oxides, and carbonyl/ether compounds). The substitution of dyes for chitosan changes its thermal stability slightly. Characteristic decomposition temperatures in modified CS vary by a few degrees (<10 °C) from the virgin sample. Considering the temperatures of the main decomposition stage, CS-BOx turned out to be the most stable. The UV irradiation of chitosan derivatives leads to minor changes in the thermal parameters and a decrease in the number of volatile degradation products. It was concluded that the obtained CS derivatives are characterized by good resistance to heat and UV irradiation, which extends the possibilities of using these innovative materials.

Synthesis of a fluorinated pyronin that enables blue light to rapidly depolarize mitochondria

Fluorinated analogues of the fluorophore pyronin B were synthesized as a new class of amine-reactive drug-like small molecules. In water, 2,7-difluoropyronin B was found to reversibly react with primary amines to form covalent adducts. When this fluorinated analogue is added to proteins, these adducts undergo additional oxidation to yield fluorescent 9-aminopyronins. Irradiation with visible blue light enhances this oxidation step, providing a photochemical method to modify the biological properties of reactive amines. In living HeLa cells, 2,7-difluoropyronin B becomes localized in mitochondria, where it is partially transformed into fluorescent aminopyronins, as detected by spectral profiling confocal microscopy. Further excitation of these cells with the blue laser of a confocal microscope can depolarize mitochondria within seconds. This biological activity was only observed with 2,7-difluoropyronin B and was not detected with analogues such as pyronin B or 9-methyl-2,7-difluoropyronin B. This irradiation with blue light enhances the cellular production of reactive oxygen species (ROS), suggesting that increased ROS in mitochondria promotes the formation of aminopyronins that inactivate biomolecules critical for maintenance of mitochondrial membrane potential. The unique reactivity of 2,7-difluoropyronin B offers a novel tool for photochemical control of mitochondrial biology.

Functionalization of Rhodamine Platforms with 3-Hydroxy-4-pyridinone Chelating Units and Its Fluorescence Behavior towards Fe(III)

Functionalization of xanthene fluorophores with specific receptor units is an important topic of research aiming for the development of new analytical tools for biological sciences, clinical diagnosis, food and environmental monitoring. Herein, we report a new dihydrorosamine containing two active amino groups, which was functionalized with 3-benzyloxy-1-(3′-carboxypropyl)-2-methyl-4-pyridinone through an amide coupling strategy. Benzylated mono- and di-functionalized dihydrorosamine derivatives (H in position 9 of the xanthene) were obtained, but with modest reaction yields, requiring long and laborious purification procedures. Looking for a more efficient approach, rhodamine 110 was selected to react with the carboxypropyl pyridinone, enabling the isolation of the corresponding mono- and di-functionalized derivatives in amounts that depend on the excess of pyridinone added to the reaction. The structure of all compounds was established by ¹H and ¹³C NMR, MS (ESI) and their absorption and emission properties were evaluated in dichloromethane. The fluorescence behavior of the debenzylated mono-rhodamine 110 derivative in the presence of Fe(III) was studied, making it an interesting fluorogenic dye for future optical sensing applications.

Selenium-Based Fluorescence Probes for the Detection of Bioactive Molecules

Chemistry of organoselenium reagents have now become an important tool of synthetic organic and medicinal chemistry. These reagents activate the olefinic double bonds and used to archive the number of organic transformations under mild reaction conditions. A number of organoselenium compounds have been identified as potent oxidants. Recently, various organoselenium species have been employed as chemical sensors for detecting toxic metals. Moreover, a number of selenium-based fluorescent probes have been developed for detecting harmful peroxides and ROS. In this review article, the synthesis of selenium-based fluorescent probes will be covered including their application in the detection of toxic metals and harmful peroxides including ROS.

A persistent luminescence resonance energy transfer-based molecular beacon probe for the highly sensitive detection of microRNA in biological samples

Herein, a time-resolved luminescence resonance energy transfer (TR-LRET) molecular beacon (MB) probe employing persistent luminescence nanoparticles (PLNPs) as the energy donors was first constructed, and further designed for microRNA21 (miR21) sensing. This probe (named as PLNPs-MB) was facilely fabricated by covalent bioconjugation between poly-(acrylic acid) (PAA) modified near-infrared (NIR) emissive PLNPs i.e. ZnGa₂O₄:Cr³⁺ and functionalized MB oligonucleotide (5'-NH₂ and 3'-BHQ3). Accordingly, PLNPs and BHQ3 were in close proximity to each other, leading to the occurrence of LRET and obvious persistent luminescence (PL) quenching. In the presence of miR21, loop of the PLNP-MB was hybridized, accompanying BHQ3 away from PLNPs and the restraint of LRET process. As a result, PL of the PLNPs was recovered, which built the foundation of miR21 quantification. The probe provided a linear response range from 0.1 to 10 nM for miR21 detection. Quantification limit of this probe was competitive and about 1-2 orders of magnitude lower than that of other reported MB probes for nucleic acid. Moreover, the proposed probe was successfully adopted for miR21 detection in biological fluids (human serum, cell extraction). This work also provided a sensitive detection nanoplatform for other targets through modifying diverse MBs onto the surface of PLNPs.

A rhodamine–TPE scaffold-based fluorescent probe for visualizing phosgene with a portable smartphone via test TLC strips

A rhodamine–TPE scaffold-based ratiometric signal readout probe was developed for sensitive, rapid, and in situ phosgene determination.

Development of Efficient One-Pot Methods for the Synthesis of Luminescent Dyes and Sol-Gel Hybrid Materials

This paper presents a comparison of the simultaneous preparation of di-O-alkylated and ether-ester derivatives of fluorescein using different methods (conventional or microwave heating). Shortening of the reaction time and increased efficiency were observed when using a microwave reactor. Moreover, described here for the first time is the application of a fast, simple, and eco-friendly ball-assisted method to exclusively obtain ether-ester derivatives. We also demonstrate that fluorescein can be effectively functionalized by O-alkylation carried out under microwave or ball-milling conditions, saving time and energy and affording the desired products with good yields and minimal byproduct formation. All the synthesized products as well as pH-dependent (prototropic) forms trapped in the SiO2 matrix were examined using UV-Vis and fluorescence spectroscopy.

Lessons in Organic Fluorescent Probe Discovery

Fluorescent probes have gained profound use in biotechnology, drug discovery, medical diagnostics, molecular and cell biology. The development of methods for the translation of fluorophores into fluorescent probes continues to be a robust field for medicinal chemists and chemical biologists, alike. Access to new experimental designs has enabled molecular diversification and led to the identification of new approaches to probe discovery. This review provides a synopsis of the recent lessons in modern fluorescent probe discovery.

Thiophene-fused boracycles as photoactive analogues of diboraanthracenes

The construction of thiophene-fused analogues of diboraanthracenes with different aryl substituents through boron-mercury exchange followed by the nucleophilic replacement of the chlorines of dichlorodiboradithiophene 2 with Grignard reagents is reported. These diboradithiophenes exhibited unusual photophysical and electrochemical properties. They all undergo photoisomerisation, which can be traced using photophysical and ¹H NMR spectroscopy studies.

Highly Selective and Sensitive Aggregation-Induced Emission of Fluorescein-Coated Metal Oxide Nanoparticles

We report the synthesis, characterization, and photophysical properties of novel metal oxide nanoparticles (NPs) coated with specially designed fluorescein substituents which are capped with electron-withdrawing groups. The fluorescein-coated nanoparticles were synthesized in excellent yields, and their structures were confirmed using various advanced spectroscopic, instrumental, and surface analysis techniques, revealing the formation of the target functionalized nanoparticles (FNPs) which show superior chemical and thermal stabilities. In addition, the photophysical properties of the FNPs were examined using UV-visible absorption and fluorescence spectroscopy. These latter techniques disclosed aggregation-induced emission (AIE) properties for most of the target FNPs, namely those which are soluble in common organic solvents at selective concentration ranges of water fractions in the solvent mixture.

Dual-channel colorimetric fluorescent probe for determination of hydrazine and mercury ion

Hydrazine and mercury (Hg) poisoning represented a serious hazard to human health. So, developing method to detect and recognize them is highly desirable. Here, we prepared a multifunctional colorimetric and fluorescent probe (PI-Rh) consisting of a phenanthroimidazole (PI) dye conjugated with a Rhodamine (Rh) group for the effective recognition of hydrazine and Hg²⁺, induvidually and collectively, with different colorimetric and fluorescence outputs. Probe PI-Rh displays low detection limits measured to be 0.0632 μM (~2 ppb) and 0.0101 μM (~2 ppb) respectively for hydrazine and Hg²⁺ with high selectivity and excellent sensitivity. Moreover, the experimental results indicated that the superiority of this probe lied in its wide applications, for example, successful response in real water, and soil analysis. Interestingly, an visual, rapid, and real-time detection of gaseous hydrazine can be realized with 0.2793 μM detection limit using the facile PI-Rh-impregnated test paper.

Fluorescein Based Fluorescence Sensors for the Selective Sensing of Various Analytes

Fluorescein molecules are extensively used to develop fluorescent probes for various analytes due to their excellent photophysical properties and the spirocyclic structure. The main structural modification of fluorescein occurs at the carboxyl group where different groups can be easily introduced to produce the spirolactam structure which is non-fluorescent. The spirolactam ring opening accounts for the fluorescence and the dual sensing of analytes using fluorescent sensors is still a topic of high interest. There is an increase in the number of dual sensors developed in the past five years and quite a good number of fluorescein derivatives were also reported based on reversible mechanisms. This review analyses environmentally and biologically important cations such as Cu2+, Hg2+, Fe3+, Pd2+, Zn2+, Cd2+, and Mg2+; anions (F-, OCl-) and small molecules (thiols, CO and H2S). Structural modifications, binding mechanisms, different strategies and a comparative study for selected cations, anions and molecules are outlined in the article.

Fluorescein Based Three-channel Probe for the Selective and Sensitive Detection of CO32- Ions in an Aqueous Environment and Real Water Samples

We have constructed a novel fluorescein-based fluorescent chemosensor, FL-In, functionalised with an indole moiety and capable of sensing by both the optical "turn-on" and electrochemical detection of carbonate ions (CO₃^2-) in aqueous media. The probe exhibits excellent selectivity and a low detection limit (0.27 µM) regarding carbonate ions by a possible coordination and hydrolysis reaction mechanism. The developed probe successfully detected CO₃^2- ions in different samples of water. Also, in a simple filter paper experiment, we documented its ability to allow the monitoring of CO₃^2- with the naked eye.

Unraveling the Mechanism for Tuning the Fluorescence of Fluorescein Derivatives: The Role of the Conical Intersection and nπ* State

Although a large number of fluorescein derivatives have been developed and applied in many different fields, the general mechanisms for tuning the fluorescence of fluorescein derivatives still remain uncovered. Herein, we found that the fluorescence quenching of neutral form of fluorescein derivatives in acidic medium resulted from a dark nπ* state, whereas the fluorescence of the anionic form of fluorescein derivatives in the gas phase and alkaline solutions was tuned by minimal energy conical intersection (MECI). The formation of MECI involved significant rotation of benzene ring and flip-flop motion of xanthene moiety, which would be restricted by intermolecular hydrogen bonding and lowering temperature. The energy barrier for reaching MECI depended on the substituents in the benzene moiety in accordance with experimentally observed substituent effects. These unprecedented mechanisms would lead to a recognition of fluorescein derivatives and could provide a correct and instructive design strategy for further developing new fluorescein derivatives.