Halochromic Isoquinoline with Mechanochromic Triphenylamine: Smart Fluorescent Material for Rewritable and Self-Erasable Fluorescent Platform

Halochromic isoquinoline attached mechanochromic triphenylamine, N-phenyl-N-(4-(quinolin-2-yl)phenyl)benzenamine (PQPBA) and tris(4-(quinolin-2-yl)phenyl)amine (TQPA), smart fluorescent materials exhibit thermo/mechanochromism and tunable solid state fluorescence and their unusual halochromic response in PMMA matrix have been used for fabricating rewritable and self-erasable fluorescent platforms. PQPBA and TQPA showed strong fluorescence in solution (Φ_f = 0.9290 (PQPBA) and 0.9160 (TQPA)) and moderate solid state fluorescence (Φ_f = 20 (PQPBA) and 17% (TQPA). Interestingly, they exhibited a rare temperature (0-100 °C) dependent positive fluorescence enhancement via activating radiative vibrational transition. The deaggregation of PQPBA and TQPA in PMMA polymer matrix lead to the enhancement of fluorescence intensity strongly and fabricated strong blue fluorescent thin films (Φ_f = 58% (PQPBA) and 54% (TQPA). The halochromic isoquinoline has been exploited for demonstrating reversible off-on fluorescence switching by acid (TFA (trifluoroacetic acid)/HCl) and base (NH₃) treatment in both solids as well as PMMA thin films. Importantly, rewritable and self-erasable fluorescent platform has been achieved by make use of unusual fluorescence responses of PQPBA/TQPA with TFA/HCl after exposing NH₃. Single crystal and powder X-ray diffraction (PXRD) studies provided the insight on the solid-state fluorescence and external stimuli-induced fluorescence changes.

Solvatochromic fluorescent probes for recognition of human serum albumin in aqueous solution: Insights into structure-property relationship

Human serum albumin (HSA) as the most abundant protein in human blood plasma, serves many physiological functions. The dysregulation of HSA in serum or in urine is associated with various diseases, such as cirrhosis of liver, multiple myeloma, and cardiovascular disease. Therefore, to quantify HSA in body fluids with high selectivity and sensitivity is of great significance for disease diagnosis and preventive medicine. We herein developed a series of amide-functionalized flavonoids probes, 1-3, for recognition of human serum albumin. All flavonoids could be easily prepared by a Claisen-Schmidt condensation and Algar-Flynn-Oyamada reaction, and showed positive solvatochromism on their dual emissions. The chemical structure of flavonoids played an important role on their HSA-sensing abilities. Among three probes, the compound 1 showed the highest sensitivity, the remarkable selectivity, and the quantitive response for HSA in aqueous solution. Together with its high tolerance of environmental pH, anti-interference properties, and time-insensitivity, thus it provides a promising sensing method for HSA.

Hydroxybenzo[b]quinolizinium Ions: Water-Soluble and Solvatochromic Photoacids

It is shown by photometric and fluorimetric analysis, along with supporting theoretical calculations, that hydroxy-substituted benzo[b]quinolizinium derivatives display the characteristic features of organic photoacids. Specifically, the experimental and theoretical results confirm the strong acidity of these compounds in the excited state (pK_a* < 0). The combination of the prototropic properties of 8- and 9-hydroxybenzo[b]quinolizinium with the particular solvent-solute interactions of the excited acid and its conjugate base leads to a pronounced fluorosolvatochromism, hence the emission maxima shift from 468 nm (8-hydroxybenzo[b]quinolizinium) or 460 nm (9-hydroxybenzo[b]quinolizinium) in CH₃CN to 507 and 553 nm in DMF, respectively. This novel type of photoacid represents several features that may be used for applications as water-soluble fluorescent probes or as a source for the photoinduced supply of acidity.

Silica-coated triangular gold nanoprisms as distance-dependent plasmon-enhanced fluorescence-based probes for biochemical applications

Plasmon-enhanced fluorescence (PEF)-based anisotropic nanostructures are considered extremely promising tools for improving the inherent problems of traditional fluorophores and for detecting important biomolecules with high sensitivity. Herein, a novel triangular gold nanoprism (AuNPR)-based fluorescence probe, AuNPR@SiO₂@12,17-tetramethyl-3-dihydro-(2s-trans)-thyl-7(Ce6), was developed for PEF by virtue of multiple "hot spots" of AuNPRs. Fluorescence enhancement of fluorophores can be realized owing to the larger and stronger electromagnetic fields located at the sharp tips of AuNPRs than those on spherical particles and nanorods. A silica shell was employed as a rigid spacer to precisely adjust the distance between the AuNPR and Ce6 for optimal PEF. Owing to the improved fluorescence signal, core-shell PEF-based AuNPRs can be applied as a turn-on probe for highly selective and sensitive detection of pyrophosphate (PPi) with a desirable detection limit of 0.2 μM using a displacement approach. Meanwhile, we demonstrated that these nanomaterials have great potential for real-time monitoring of polymerase chain reaction (PCR) products, successfully revealing an approximately 240 times higher detectable fluorescence response than that of traditional gel electrophoresis. Furthermore, cell imaging indicates the potential applications of PEF-based probes in living cells.

d-PET-controlled "off-on" Polarity-sensitive Probes for Reporting Local Hydrophilicity within Lysosomes

Polarity-sensitive fluorescent probes are powerful chemical tools for studying biomolecular structures and activities both in vitro and in vivo. However, the lack of "off-on" polarity-sensing probes has limited the accurate monitoring of biological processes that involve an increase in local hydrophilicity. Here, we design and synthesize a series of "off-on" polarity-sensitive fluorescent probes BP series consisting of the difluoroboron dippyomethene (BODIPY) fluorophore connected to a quaternary ammonium moiety via different carbon linkers. All these probes showed low fluorescence quantum yields in nonpolar solution but became highly fluorescent in polar media. BP-2, which contains a two-carbon linker and a trimethyl quaternary ammonium, displayed a fluorescence intensity and quantum yield that were both linearly correlated with solvent polarity. In addition, BP-2 exhibited high sensitivity and selectivity for polarity over other environmental factors and a variety of biologically relevant species. BP-2 can be synthesized readily via an unusual Mannich reaction followed by methylation. Using electrochemistry combined with theoretical calculations, we demonstrated that the "off-on" sensing behavior of BP-2 is primarily due to the polarity-dependent donor-excited photoinduced electron transfer (d-PET) effect. Live-cell imaging established that BP-2 enables the detection of local hydrophilicity within lysosomes under conditions of lysosomal dysfunction.

A new ratiometric fluorescent probe for rapid, sensitive and selective detection of endogenous hydrogen sulfide in mitochondria

We have developed a new ratiometric fluorescent probe composed of a coumarin-merocyanine dyad based on the FRET mechanism. The probe showed clear dual-emission signal changes in blue and red spectral windows upon addition of H2S in a dose dependent manner under a single wavelength excitation. The probe targeted mitochondria with high selectivity and sensitivity toward H2S.

Fluorescent Probes for Sensing and Imaging within Specific Cellular Organelles

Fluorescent probes have become powerful tools in biosensing and bioimaging because of their high sensitivity, specificity, fast response, and technical simplicity. In the last decades, researchers have made remarkable progress in developing fluorescent probes that respond to changes in microenvironments (e.g., pH, viscosity, and polarity) or quantities of biomolecules of interest (e.g., ions, reactive oxygen species, and enzymes). All of these analytes are specialized to carry out vital functions and are linked to serious disorders in distinct subcellular organelles. Each of these organelles plays a specific and indispensable role in cellular processes. For example, the nucleus regulates gene expression, mitochondria are responsible for aerobic metabolism, and lysosomes digest macromolecules for cell recycling. A certain organelle requires specific biological species and the appropriate microenvironment to perform its cellular functions, while breakdown of the homeostasis of biomolecules or microenvironmental mutations leads to organelle malfunctions, which further cause disorders or diseases. Fluorescent probes that can be targeted to both specific organelles and biochemicals/microenvironmental factors are capable of reporting localized bioinformation and are potentially useful for gaining insight into the contributions of analytes to both healthy and diseased states. In this Account, we review our recent work on the development of fluorescent probes for sensing and imaging within specific organelles. We present an overview of the design, photophysical properties, and biological applications of the probes, which can localize to mitochondria, lysosomes, the nucleus, the Golgi apparatus, and the endoplasmic reticulum. Although a diversity of organelle-specific fluorescent stains have been commercially available, our efforts place an emphasis on improvements in terms of low cytotoxicity, high photostability, near-infrared (NIR) emission, two-photon excitation, and long fluorescence lifetimes, which are crucial for long-time tracking of biological processes, tissue and body imaging with deep penetration and low autofluorescence, and time-resolved fluorescence imaging. Research on fluorescent probes with both analyte responsiveness and organelle targetability is a new and emerging area that has attracted increasing attention over the past few years. We have extended the diversity by developing organelle-specific responsive probes capable of detecting changes in biomolecular levels (reactive oxygen species, fluoride ion, hydrogen sulfide, zinc cation, thiol-containing amino acids, and cyclooxygenase-2) and the microenvironment (viscosity, polarity, and pH). Future research should give more considerations of the "low-concern" organelles, such as the Golgi apparatus, the endoplasmic reticulum, and ribosomes. In addition, given the tiny sizes of subcellular organelles (20-1000 nm), we anticipate that clearer visulization of the cellular events within specific organelles will rely on super-resolution optical microscopy with nanoscopic-scale resolution.

Bacteriochlorin Dyads as Solvent Polarity Dependent Near-Infrared Fluorophores and Reactive Oxygen Species Photosensitizers

Symmetrical, near-infrared absorbing bacteriochlorin dyads exhibit gradual reduction of their fluorescence (intensity and lifetime) and reactive oxygen species photosensitization efficiency (ROS) with increasing solvent dielectric constant ε. For the directly linked dyad, significant reduction is observed even in solvents of moderate ε, while for the dyad containing a 1,4-phenylene linker, reduction is more parallel to an increase in solvent ε. Bacteriochlorin dyads are promising candidates for development of environmentally responsive fluorophores and ROS sensitizers.

Reaction-Driven Self-Assembled Micellar Nanoprobes for Ratiometric Fluorescence Detection of CS2 with High Selectivity and Sensitivity

The detection of highly toxic CS2, which is known as a notorious occupational hazard in various industrial processes, is important from both environmental and public safety perspectives. We describe here a robust type of chemical-reaction-based supramolecular fluorescent nanoprobes for ratiometric determination of CS2 with high selectivity and sensitivity in water medium. The micellar nanoprobes self-assemble from amphiphilic pyrene-modified hyperbranched polyethylenimine (Py-HPEI) polymers with intense pyrene excimer emission. Selective sensing is based on a CS2-specific reaction with hydrophilic amino groups to produce hydrophobic dithiocarbamate moieties, which can strongly quench the pyrene excimer emission via a known photoinduced electron transfer (PET) mechanism. Therefore, the developed micellar nanoprobes are free of the H2S interference problem often encountered in the widely used colorimetric assays and proved to show high selectivity over many potentially competing chemical species. Importantly, the developed approach is capable of CS2 sensing even in complex tap and river water samples. In addition, in view of the modular design principle of these powerful micellar nanoprobes, the sensing strategy used here is expected to be applicable to the development of various sensory systems for other environmentally important guest species.

A viscosity sensitive fluorescent dye for real-time monitoring of mitochondria transport in neurons

We present here a viscosity sensitive fluorescent dye, namely thiophene dihemicyanine (TDHC), that enables the specific staining of mitochondria. In comparison to the common mitochondria tracker (Mitotracker Deep Red, MTDR), this dye demonstrated its unique ability for robust staining of mitochondria with high photostability and ultrahigh signal-to-noise ratio (SNR). Moreover, TDHC also showed high sensitivity towards mitochondria membrane potential (ΔΨm) and intramitochondria viscosity change. Consequently, this dye was utilized in real-time monitoring of mitochondria transport in primary cortical neurons. Finally, the Two-Photon Microscopy (TPM) imaging ability of TDHC was also demonstrated.

Highly solvatochromic and tunable fluorophores based on a 4,5-quinolimide scaffold: novel CDK5 probes

Novel 4,5-quinolimide-based fluorophores are more solvatochromic and red-shifted than known naphthalimide analogues. Conjugation of one of these fluorophores to a peptide derived from CDK5 kinase demonstrated its sensitivity for monitoring the interaction with its regulatory partner p25. Introduction of the quinolimide-labelled peptide into living glioblastoma cells probed the interaction with endogenous p25.

A FRET probe with AIEgen as the energy quencher: dual signal turn-on for self-validated caspase detection

The accurate detection of biological substances is highly desirable to study various biological processes and evaluate disease progression. Herein, we report a self-validated fluorescent probe which is composed of a coumarin fluorophore as the energy donor and a fluorogen with aggregation-induced emission characteristics (AIEgen) as the energy quencher linked through a caspase-3 specific peptide substrate. Unlike the traditionally widely studied fluorescence resonance energy transfer (FRET) probes, our new generation of FRET probe is non-fluorescent itself due to the energy transfer as well as the dissipation of the acceptor energy through the free molecular motion of AIEgen. Upon interaction with caspase-3, the probe displays strong green and red fluorescent signals synchronously due to the separation of the donor-quencher and aggregation of the released AIEgen. The fluorescence turn-on with dual signal amplification allows real-time and self-validated enzyme detection with a high signal-to-background ratio, providing a good opportunity to accurately monitor various biological processes in a real-time manner.

Bridging the gap between in vitro and in vivo RNA folding

Deciphering the folding pathways and predicting the structures of complex three-dimensional biomolecules is central to elucidating biological function. RNA is single-stranded, which gives it the freedom to fold into complex secondary and tertiary structures. These structures endow RNA with the ability to perform complex chemistries and functions ranging from enzymatic activity to gene regulation. Given that RNA is involved in many essential cellular processes, it is critical to understand how it folds and functions in vivo. Within the last few years, methods have been developed to probe RNA structures in vivo and genome-wide. These studies reveal that RNA often adopts very different structures in vivo and in vitro, and provide profound insights into RNA biology. Nonetheless, both in vitro and in vivo approaches have limitations: studies in the complex and uncontrolled cellular environment make it difficult to obtain insight into RNA folding pathways and thermodynamics, and studies in vitro often lack direct cellular relevance, leaving a gap in our knowledge of RNA folding in vivo. This gap is being bridged by biophysical and mechanistic studies of RNA structure and function under conditions that mimic the cellular environment. To date, most artificial cytoplasms have used various polymers as molecular crowding agents and a series of small molecules as cosolutes. Studies under such in vivo-like conditions are yielding fresh insights, such as cooperative folding of functional RNAs and increased activity of ribozymes. These observations are accounted for in part by molecular crowding effects and interactions with other molecules. In this review, we report milestones in RNA folding in vitro and in vivo and discuss ongoing experimental and computational efforts to bridge the gap between these two conditions in order to understand how RNA folds in the cell.

A photochromic-acidochromic HCl fluorescent probe. An unexpected chloride-directed recognition

Non-classical protomerism of Schiff bases offers several advantages; for example, specific interactions in the -C[double bond, length as m-dash]N- linkage can be controlled and differentiated because the interactions are not governed by keto-enol tautomerism. Herein, the pH sensing properties of a new protomeric Schiff base probe () are reported. In particular, among several acids, the probe displays significant optical responses upon interaction with hydrochloric acid (HCl). X-ray structural analysis confirmed the existence of an intermolecular interaction with HCl through a -C[double bond, length as m-dash]NH-ClO- linkage. Moreover, an optical response via a second channel is manifested as photochromic fluorescence behavior. The properties of were investigated by UV-vis and fluorescence spectroscopy in a solution and the solid state. Its strong acidofluorochromic behavior was analyzed and its pKa and values were determined, which revealed a photobasic character. Positive solvatochromism that resulted from specific interactions taking place in was studied using four different solvent scales, namely, Lippert-Mataga, Kamlet-Taft, Catalán and the recently proposed scale of Laurence et al., which yielded consistent results. Finally, theoretical calculations were conducted to analyze the mechanism of the probe in terms of natural transition orbitals (NTOs) and the spatial extent of charge transfer excitations.

Dipolar Dyes with a Pyrrolo[2,3-b]quinoxaline Skeleton Containing a Cyano Group and a Bridged Tertiary Amino Group: Synthesis, Solvatofluorochromism, and Bioimaging

Two strongly polarized dipolar chromophores possessing a cyclic tertiary amino group at one terminus of the molecule and a CN group at the opposite terminus were designed and synthesized. Their rigid skeleton contains the rarely studied pyrrolo[2,3-b]quinoxaline ring system. The photophysical properties of these regioisomeric dyes were different owing to differing π conjugation between the CN group and the electron-donor moiety. These dipolar molecules showed very intense emission, strong solvatofluorochromism, and sufficient two-photon brightness for bioimaging. One of these regioisomeric dyes, namely, 11-carbonitrile-2,3,4,5,6,7-hexahydro-1H-3a,8,13,13b-tetraazabenzo[b]cyclohepta[1,2,3-jk]fluorene, was successfully utilized in two-photon imaging of mouse organ tissues and showed distinct tissue morphology with high resolution.

A high-content screening platform with fluorescent chemical probes for the discovery of first-in-class therapeutics

Phenotypic screening has emerged as a promising approach to discover novel first-in-class therapeutic agents. Rapid advances in phenotypic screening systems facilitate a high-throughput unbiased evaluation of compound libraries. However, limited sets of phenotypic changes are utilized in high-content screening, which require extensive genetic engineering. Therefore, it is critical to develop new chemical probes that can reflect phenotypic changes in any type of cells, especially primary cells, tissues, and organisms. Herein, we introduce our continuous efforts in the development of fluorescent bioprobes and their application to phenotypic screening. In addition, we emphasize the importance of the phenotype-based approach in conjunction with target identification at an early stage of research to accelerate the discovery of therapeutics with new modes of action.

Glycosylation enhances the aqueous sensitivity and lowers the cytotoxicity of a naphthalimide zinc ion fluorescence probe

With this research we demonstrate that glycosylation of a naphthalimide zinc ion probe, using click chemistry, leads to an improvement of the aqueous sensitivity, working pH range and targeting ability for specific cells, together with significantly reduced cytotoxicity.

Imaging phase separation in model lipid membranes through the use of BODIPY based molecular rotors

In order to fully understand the dynamics of processes within biological lipid membranes, it is necessary to possess an intimate knowledge of the physical state and ordering of lipids within the membrane. Here we report the use of three molecular rotors based on meso-substituted boron-dipyrrin (BODIPY) in combination with fluorescence lifetime spectroscopy to investigate the viscosity and phase behaviour of model lipid bilayers. In phase-separated giant unilamellar vesicles, we visualise both liquid-ordered (Lo) and liquid-disordered (Ld) phases using fluorescence lifetime imaging microscopy (FLIM), determining their associated viscosity values, and investigate the effect of composition on the viscosity of these phases. Additionally, we use molecular dynamics simulations to investigate the orientation of the BODIPY probes within the bilayer, as well as using molecular dynamics simulations and fluorescence correlation spectroscopy (FCS) to compare diffusion coefficients with those predicted from the fluorescence lifetimes of the probes.

Influence of the Near Molecular Vicinity on the Temperature Regulated Fluorescence Response of Poly(N-vinylcaprolactam)

A series of new fluorescent dye bearing monomers, including glycomonomers, based on maleamide and maleic esteramide was synthesized. The dye monomers were incorporated by radical copolymerization into thermo-responsive poly(N‑vinyl-caprolactam) that displays a lower critical solution temperature (LCST) in aqueous solution. The effects of the local molecular environment on the polymers' luminescence, in particular on the fluorescence intensity and the extent of solvatochromism, were investigated below as well as above the phase transition. By attaching substituents of varying size and polarity in the close vicinity of the fluorophore, and by varying the spacer groups connecting the dyes to the polymer backbone, we explored the underlying structure⁻property relationships, in order to establish rules for successful sensor designs, e.g., for molecular thermometers. Most importantly, spacer groups of sufficient length separating the fluorophore from the polymer backbone proved to be crucial for obtaining pronounced temperature regulated fluorescence responses.

A novel fluorescent probe for Cr(3+) based on rhodamine-crown ether conjugate and its application to drinking water examination and bioimaging

A trivalent chromium (Cr(3+)) fluorescence probe (RhC) was designed and synthesized via Schiff base reaction based on rhodamine-crown ether conjugate. This probe displayed a favorable selectivity for Cr(3+) over a range of other common metal ions in DMF/H2O (3:7, v/v; PBS buffer 50 mmol L(-1); pH=6.8) solution, leading to prominent fluorescence "OFF-ON" switching of the rhodamine fluorophore. The limit of detection was calculated to be 1.5 μmol L(-1) (S/N=3). The binding ratio of RhC-Cr(3+) complex was determined to be 1:2 according to the Job's plot and HR-MS. The probe was successfully applied to examination of Cr(3+) in drinking water spiked samples. The average recoveries ranged from 104.9% to 106.9% at spiked concentration level of 10.00 μmol L(-1), and the obtained results were consistent with those obtained using atomic absorption spectrometry (AAS). Moreover, bioimaging experiments showed that RhC can sense the Cr(3+) in living cells with a fluorescence enhancement signal.

Nucleic acid-selective light-up fluorescent biosensors for ratiometric two-photon imaging of the viscosity of live cells and tissues

Rational design of specific ratiometric viscosity probes with small molecular weight is a challenge in practical biotechnology applications. Herein two novel water-soluble, small-molecular ratiometric probes, bearing N-methyl benzothiazolium moiety (DSF and DBF), are designed for two-photon fluorescent imaging as a functional of local viscosity. The dye DSF, a light-up fluorescent probe, is sensitive to local viscosity and selectively stains nuclear DNA, which can be used to inspect asynchronous cells under confocal microscopy. While the dye DBF as a molecular rotor displays strong fluorescence enhancement in viscous media or binding to RNA. It exhibits dual absorption and emission as well, and only the red emission is markedly sensitive to viscosity changes, providing a ratiometric response and selectively imaging nucleolic and cytosolic RNA. Interestingly it is shown, for the first time, that the intracellular targeting and localization (DNA and RNA) of the two dyes are entirely realized simply by modifying the substituent attached to the benzothiazolium.

A bodipy based fluorescent probe for evaluating and identifying cancer, normal and apoptotic C6 cells on the basis of changes in intracellular viscosity

The applications of a bodipy based probe 1 for the identification of diseased cell population out of normal cells on the basis of changes in intracellular viscosity have been explored. Probe 1 works on the principle of restriction of rotation in viscous medium and the molecular rotor nature of probe 1 is supported by low temperature ¹H NMR and variable dihedral angle DFT and TD-DFT studies. More importantly, probe 1 is the first probe which shows its practical application in monitoring micro-viscosity changes in a cell based model system of undifferentiated, differentiated and apoptotic C6 glial cells. Further, probe 1 can effectively monitor the apoptosis pathway by showing an increase in fluorescence intensity from cancerous cells to apoptotic cells via real time live-cell video imaging. Moreover, the viscosity changes in living cells were proved by fluorescence lifetime imaging (FLIM) studies, flow cytometry using Annexin-V and Bcl-xl expression by immunocytofluorescence (ICC) and western blot analysis.

Environmentally Robust Rhodamine Reporters for Probe-based Cellular Detection of the Cancer-linked Oxidoreductase hNQO1

We successfully synthesized a fluorescent probe capable of detecting the cancer-associated

NAD(P)H

quinoneoxidoreductase isozyme-1 within human cells, based on results from an investigation of the stability of various rhodamines and seminaphthorhodamines toward the biological reductant NADH, present at ∼100-200 μM within cells. While rhodamines are generally known for their chemical stability, we observe that NADH causes significant and sometimes rapid modification of numerous rhodamine analogues, including those oftentimes used in imaging applications. Results from mechanistic studies lead us to rule out a radical-based reduction pathway, suggesting rhodamine reduction by NADH proceeds by a hydride transfer process to yield the reduced leuco form of the rhodamine and oxidized NAD(+). A relationship between the structural features of the rhodamines and their reactivity with NADH is observed. Rhodamines with increased alkylation on the N3- and N6-nitrogens, as well as the xanthene core, react the least with NADH; whereas, nonalkylated variants or analogues with electron-withdrawing substituents have the fastest rates of reaction. These outcomes allowed us to judiciously construct a seminaphthorhodamine-based, turn-on fluorescent probe that is capable of selectively detecting the cancer-associated, NADH-dependent enzyme

NAD(P)H

quinoneoxidoreductase isozyme-1 in human cancer cells, without the issue of NADH-induced deactivation of the seminaphthorhodamine reporter.

A unique approach toward near-infrared fluorescent probes for bioimaging with remarkably enhanced contrast

Near-infrared (NIR) fluorescent probes are attractive molecular tools for bioimaging because of their low autofluorescence interference, deep tissue penetration, and minimal damage to sample. However, most previously reported NIR probes exhibit small Stokes shift, typically less than 30 nm, and low fluorescence quantum yield, strictly limited contrast and spatial resolution for bioimaging. Herein, by expanding the π-conjugated system of rhodamine B, while, at the same time, keeping its rigid and planar structure, we reported an efficient NIR dye, HN7, with large stokes shift of 73 nm and fluorescence quantum yield as high as 0.72 in ethanol, values superior to those of such traditional cyanine NIR dyes as Cy5. Using HN7, living cells, tissues and mice were imaged, and the results showed significantly enhanced contrast, improved spatial resolution, and satisfactory tissue imaging depth when compared to Cy5. Moreover, the nonfluorescent spirocyclic structure of rhodamine B is an inherent component of HN7; therefore, our strategy provided a universal platform for the design of efficient NIR turn-on bioimaging probes for various targets. As a proof-of-concept, two different NIR probes, HN7-N2 and HN7-S for NO and Hg²⁺, respectively, were designed, synthesized, and successfully applied for the imaging of NO and Hg²⁺ in living cells, tissues and mice, respectively, demonstrating the potential bioimaging applications of the new probes. In sum, this new type of dye may present new avenues for the development of efficient NIR fluorescent probes for contrast-enhanced imaging in biological applications.

Synthesis of ester-substituted dihydroacridine derivatives and their spectroscopic properties

Ester substituted dihydroacridine derivatives exhibit a noticeable fluorescence solvatochromism.

Hypoxia-responsive nanocarriers for cancer imaging and therapy: recent approaches and future perspectives

This review focuses on hypoxia-responsive nanocarriers, which can be disintegrated by recognizing the hypoxic microenvironment of cancer cells, and their utilization in cancer imaging and therapy.

A water-soluble colorimetric two-photon probe for discrimination of different palladium species and its application in bioimaging

A novel water-soluble colorimetric two-photon chemodosimeter for discrimination of different palladium species and its application in bioimaging.

Bioreductive fluorescent imaging agents: applications to tumour hypoxia

The development of new optical chemosensors for various reductases presents an ideal approach to visualise areas of tissue hypoxia.

Pyrrolic molecular rotors acting as viscosity sensors with high fluorescence contrast

Pyrrolic viscosity sensors exhibit one order of magnitude higher fluorescence contrast compared to that of the conventional phenolic analogues due to the viscosity-sensitive rotation of the rotational pyrrole group.

Optical properties of two fluorene derived BODIPY molecular rotors as fluorescent ratiometric viscosity probes

Two fluorescent ratiometric fluorene derived BODIPY probes present a sensitive response to microviscosity changes.

Solvent-induced multicolour fluorescence of amino-substituted 2,3-naphthalimides studied by fluorescence and transient absorption measurements

Amino-substituted 2,3-naphthalimide derivatives showed marked positive solvatofluorochromism, and the fluorescence emission was effectively quenched in methanol via the internal conversion process.