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Long-wavelength emission with high contrast toward species of interest is highly desired for fluorescence imaging in living objects. Herein, we proposed an exquisite enzyme-regulated "aggregation-induced emission (AIE) and intramolecular charge transfer (ICT)" strategy to optimize the signal-to-noise ratio for highly sensitive detection of enzymes. Benefiting from the synergistic effect, probes ABT-Gal-1 and ABT-Gal-2 exhibited faint background signals but dramatically enhanced near-infrared fluorescence upon exposure toward β-galactosidase (β-gal). Notably, both probes could be accessible in aqueous media. Preferably, the fluorescence intensity of ABT-Gal-1 exhibited a satisfactory linear relationship in the wide concentration range of 0.5–30 U/L with a very low detection limit to 0.24 U/L. Further, by monitoring endogenous β-gal in ovarian cancer cells, a precious merit of resistance to multiple washes was identified, which was facilitated by the nanoaggregate formation in situ upon activation. This work proposes a guiding strategy for manipulating various lighting mechanisms to develop attractive probes and, equally importantly, it provides an ideal practical tool for elucidating the biological role of β-gal in biological scenarios. [Display omitted] • Comprehensive insights into the ICT and AIE characteristic of amino-type ESIPT-based fluorophore are proposed. • An exquisite enzyme-regulated "AIE and ICT" strategy to optimize the signal-to-noise ratio is reported for the first time. • Debut of activatable AIE probe for β-gal with emission in near-infrared window is well presented. • Probe ABT-Gal-1 was allowed in aqueous media with the lowest detection limit (0.24 U/L) compared with reported probes. • The nanoaggregation-allowed probe was firstly disclosed with resistance to multiple washes during cell imaging. [ABSTRACT FROM AUTHOR]

Fluorescent nanoprobes for carbon monoxide (CO) imaging emerged as an excellent tool for cancer diagnosis, but they are activated by single-biomarker. Herein, we developed a dual-locking fluorescent nanoprobe (HA-DMC) for HAase-triggered carbon monoxide imaging in the mitochondrion of the cancer cells. The fluorescent nanoprobes have "double locks" structures made of acetyl hyaluronic acid chains (hyaluronidase-triggered, first lock) as a barrier to form nanoparticles and the nitro group (CO-triggered, second lock) as intramolecular charge transfer (ICT) groups to quench the fluorescence of coumarin-pyridine derivative dyes. When the "locks" were only unlocked in sequence (from hyaluronidase to CO), a strong green fluorescence appeared for achieving cancer imaging. Meanwhile, the HA-DMC exhibited some satisfactory properties, such as mitochondria-targeting properties, excellent selectivity and sensitivity (24-fold), good water dispersibility, low detection limit (225 nM), and low cytotoxicity. Such features ensured that the HA-DMC could be successfully utilized for fluorescently imaging exogenous CO in the mitochondrion of living cells. In addition, the HA-DMC was successfully used to monitor exogenous and endogenous CO in vivo. We thus envision that HA-DMC is a promising platform for CO imaging in the mitochondrion of cancer cells and would promote the progress of fluorescent nanoprobes. [Display omitted] • A dual-locking fluorescent nanoprobe (HA-DMC) for CO imaging was well developed. • HA-DMC was used for HAase-triggered mitochondrial CO imaging in cancer cells. • This work provides the first dual-locking probe for HAase and CO in tumor cells. [ABSTRACT FROM AUTHOR]

Hypochlorous acid (HOCl) is a crucial reactive oxygen species in biological systems, and its abnormal metabolism is associated with various diseases. Therefore, it is essential to accurately detect HOCl to understand its biological function and develop related therapeutic strategies for diseases. In this work, we developed a novel fluorescence probe, BCy-S , based on the twisted intramolecular charge transfer (TICT) mechanism for rapid and sensitive detection of HOCl in an aqueous solution. The probe exhibited a significant turn-on fluorescence response to HOCl, with high selectivity, rapid kinetics (less than 40 s), large Stokes shift (about 170 nm), and low detection limit (35.2 nM), based on the specific oxidation reaction of HOCl with sulfur atoms. Additionally, the outcomes of the bioimaging experiment showed that BCy-S was capable of real-time fluorescence visualization of HOCl in living HeLa cells and zebrafish. The development of BCy-S presents a new tool for investigating the physiological function of HOCl and its pathological role in diseases. [Display omitted] • A new TICT-based HOCl fluorescent probe has been designed and developed. • The probe exhibits an excellent sensitivity, large Stokes Shift and low detection limit to HOCl. • The probe could be applied for real-time fluorescence imaging of HOCl in live HeLa cells and zebrafish. [ABSTRACT FROM AUTHOR]

Monoamine oxidase A (MAO-A) is involved in various diseases such as heart failure (HF), prostate cancer (PCa), psychiatric disorders, etc. The precise sensing of MAO-A is important to elucidate its function in various biological processes, as well the diagnosis and treatment of related diseases. In this research, we created a brand-new fluorescence probe ANET for the specific detection of MAO-A. Propylamine moiety of ANET was specifically recognized for MAO-A and then the product EHBT was released. A well-calculated p K a value of 8.01 ± 0.02 was achieved for EHBT by pH-dependent spectra. The fluorescence at 700 nm of EHBT was observed at pH 8.0 due to the enhanced intramolecular charge transfer (ICT). The fluorescence intensity of EHBT at 559/700 nm was in a linear relationship with MAO-A concentration in the 0.375–1.50 μg/mL range. The detection limit was 19.8 ng/mL. It is well known that the environment of mitochondria is microalkaline (pH 8.0), however, other organelles are neutral or acidic. So the fluorescence at 700 nm of ANET could be notably turned on by MAO-A in mitochondria. In addition, ANET can selectively accumulate in the mitochondria through powerful electrostatic attractions. Finally, ANET was applied for specific imaging of MAO-A in mitochondria of SH-SY5Y, PC-3, and glucose-deprived (GD) H9c2 cells. [Display omitted] • A probe was developed for Monoamine Oxidases A (MAO-A) based on ICT. • ANET can selectively accumulate in the mitochondria through strong electrostatic interactions. • Since the p K a of EHBT matches the pH in mitochondria, fluorescence of EHBT can be specifically turned on at 700 nm. • ANET was capable of differentiating MAO-A overexpressing cells from other cells. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A new N-oxide based and turn on fluorescent probe for Fe2+ was developed. • The probe possessed a high selectivity for Fe2+ among various biologically relevant species and fast rapid response. • Monitoring Fe2+ with TPM fluorescence imaging for the first time. Abstract Iron is an essential element for human body, involved in important biochemical processes in the body, especially in biological oxidation. Misregulation of iron levels in the cell is concerned with serious diseases. Here, a novel two photon and turn on NIR-fluorescent probe based ICT for detection of Fe2+ is first reported. The probe exhibited brilliant selectivity and excellent sensitivity (DL = 4.5 μM), rapid response (15 min) and "naked-eye colorimetric sensor" for monitoring of Fe2+. More importantly, the probe was successfully applied to real-time monitor Fe2+ in living cell through two-photon microscopy fluorescence imaging with excitation 680 nm. [ABSTRACT FROM AUTHOR]

Herein we report a new carbazole-benzo[e]indolium dye (CEBI) as a ratiometric fluorescent and chromogenic probe for sulfite (SO 3 2− ) detection based on intramolecular charge transfer (ICT) mechanism. Upon treatment with SO 3 2− , the probe CEBI displayed a remarkable fluorescence ratiometric response (∼560-fold fluorescenc enhancement) together with a color change from red to weak red and a large shift (135 nm) in emission wavelength. The experimentally observed changes in the spectral features of the probe upon addition of SO 3 2− were systematically studied using high resolution electrospray ionization mass spectrometry (HR-ESI–MS), proton nuclear magnetic resonance ( 1 H NMR), and time-dependent density functional theory (TDDFT) calculations. We found a linear relationship between the ratio of fluorescence intensities at 465 nm and 600 nm ( F 465 /F 600 ) and SO 3 2− concentration over the range of 0–40 μM with a detection limit of 0.025 μM. Moreover, the novel probe demonstrated fluorescence imaging of SO 3 2− in living cells and zebrafish, suggesting its potential in clinical diagnostics. [ABSTRACT FROM AUTHOR]

A supramolecular fluorescent probe based on double-cavity nor-seco -cucurbit[10]uril (ns -Q[10]) was constructed using the non-covalent host-guest interactions between ns -Q[10] and positively charged Astrazon Pink FG (APFG). The intramolecular charge transfer (ICT) effect induces the APFG@ ns -Q[10] probe to emit strong orange-red fluorescence. APFG@ ns -Q[10] exhibits desirable optical properties and low toxicity, and has been successfully applied to the imaging of HK2 and HeLa cells. When APFG@ ns -Q[10] interacts with ClO-, the C C double bond in APFG is oxidized and the molecule is cleaved, which leads to the fluorescence is quenched. The specificity and sensitivity of APFG@ ns -Q[10] to ClO- allows it to be used for the detection and tracking of ClO- in cells. APFG@ ns -Q[10] demonstrates good biocompatibility, which is an excellent foundation for its application in cell imaging. • A supramolecular fluorescent probe based on ns -Q[10])was constructed using the non-covalent host-guest interactions. • APFG@ ns -Q[10] exhibits desirable optical properties and low toxicity, and was applied to the cell imaging. • The specificity and sensitivity of APFG@ ns -Q[10] to ClO- allows it to be used for the detection of ClO- in cells. [ABSTRACT FROM AUTHOR]

A new naphthalimide-based probe was designed and synthesized for the rationmetric detection of hydrazine. The probe is designed in such a way that the intramolecular charge transfer (ICT) of the 4-hydroxynaphthalimide was masked by the 4-bromobutyrate moiety, and the selective deprotection of the 4-bromobutyrate group of the probe by hydrazine through a substitution-cyclisation-elimination process results in the recovery of the ICT of 4-hydroxynaphthalimide. Thus the probe can realize both colorimetric and fluorimetric detection of hydrazine in a ratiometric manner. The probe is highly selective toward hydrazine over other abundant interferences. The LOD (limit of detection) of the probe for N 2 H 4 was calculated to be 0.27 μM which is well below the safety level (10 ppb, 0.31 μM) set by EPA. Notable, the probe loaded thin-layer-chromatography (TLC) plate could be successfully used to detect gaseous hydrazine. Moreover, the cell imaging studies demonstrated that the probe can be exploited to detect hydrazine in biological systems. [ABSTRACT FROM AUTHOR]

A new simple quinoline-based chemosensor 1 was synthesized for Zn 2+ . 1 showed the selective fluorescence enhancement in the presence of Zn 2+ with a 1:1 stoichiometry in a near-perfect aqueous solution (bis-tris buffer:DMSO = 999:1), which was reversible with the addition of ethylenediaminetetraacetic acid (EDTA). The detection limit (0.6 μM) of 1 for Zn 2+ was much lower than World Health Organization guideline (76 μM) in drinking water. 1 was successfully applied to quantify and image Zn 2+ in water samples, test kit and living cells. The sensing mechanism of Zn 2+ by 1 via the intramolecular charge transfer was explained by theoretical calculations. [ABSTRACT FROM AUTHOR]

We report a novel turn-on two-photon fluorescent probe ( TPNF ) for detecting formaldehyde based on aza-Cope rearrangement and intramolecular charge transfer (ICT). The probe, containing a 1,8-naphthalimide dye with a homoallylamino group, has was shown with high selectivity, sensitivity and low cytotoxicity and it was capable of detecting formaldehyde in live cells. Moreover, a time-dependent density functional theory (TD-DFT) calculation was performed to demonstrate the turn on mechanism of the probe via ICT effect. Finally, a zebrafish larvae imaging experiment reveals that TPNF displayed a significant signal change when treated with FA in vivo. These results indicated that TPNF is a promising tool in more biomedical researches. [ABSTRACT FROM AUTHOR]

Fluorescence imaging plays an increasingly important role in biomedicine field due to its superior sensitivity and selectivity along with dynamic, real-time, in situ monitoring the occurrence and development of biological events. Therefore, it is crucial to construct a near-infrared (NIR) fluorescent molecule with excellent optical properties. Herein a NIR robust out mitochondria-targeting dye NN has been designed, synthesized and characterized, whose fluorescence properties are easily adjusted by changing the substitution position of the nitro group at the benzene ring in this fluorescent dye. NN was a unique fluorescence-enhanced nitro dye with high fluorescence quantum yields∼0.17, a large Stokes shift∼120 nm and stable chemical structure. Encouraged by this, a novel fluorescent probe NN - Sec based on intramolecular charge transfer (ICT) was constructed by grafting NN with a selenocysteine (Sec) recognition group 2,4-dinitrobenzenesulfonyl (DNBS), and it exhibited a targeted ability with great sensitivity and selectivity. Subsequently, the probe was used for NIR imaging in living cells and mice, and deep imaging results further illustrate its potential application in biosystems. The successful design of NN and NN - Sec offer a valuable theoretical basis and technical support for investigating the occurrence and development of Sec-related diseases. • A novel NIR fluorescent dye NN bearing nitro was reported with an enhanced fluorescence quantum yields. • The recognition group 2,4-dinitrobenzenesulfonyl of Sec was grafted with dye NN to synthesize fluorescent probe NN-Sec. • ANN-Sec can detect Sec in live cells and mice by high sensitivity and selectivity, accompanied by bright NIR emission. [ABSTRACT FROM AUTHOR]

A ratiometric fluorescent pH probe 4-((1E, 3E)-4-(benzo[d]thiazol-2-yl)buta-1, 3-dienyl)- N , N -dimethylbenzenamine ( BTDB ) was synthesized via ethylene bridging of thiazol and cinnamaldehyde for extremely acidic sensing. BTDB exhibits ratiometric fluorescence emission ( I 425 nm / I 595 nm ) characteristics with p K a of 2.34 and a large Stokes shift of 177 nm under acidic conditions. The linear response to pH is in the range of 2.3–4.0. Quantum chemical calculations with the B3LYP exchange functional theory demonstrated that the ratiometric response of the probe to acidic pH was due to H + binding with the N of cinnamaldehyde and the induced decreases of the intramolecular charger transfer (ICT) process. The as-prepared probe displayed excellent cell membrane permeability was further applied successfully to monitor pH fluctuations in live cells with excellent lysosomal targeting ability, and extreme acidity in Escherichia coli cells. [ABSTRACT FROM AUTHOR]

A novel fluorescent probe (cys-pH) based on intramolecular charge transfer (ICT) mechanism for pH detection targeting mitochondria was developed. cys-pH has excellent mitochondrial targeting, a suitable pKa (8.53), a good linear range in the pH range of 6.50–10.00, and a sensitive response to fluctuations in mitochondrial pH. cys-pH has been succeeded in applying to monitor the pH of mitochondria in living cells and the pH detection of water samples in environmental systems. In addition, pH test papers and standard colorimetric cards of cys-pH are prepared. We reasonably explained the double absorption peak of cys-pH UV–vis spectrum in the long wavelength region. [Display omitted] • A new fluorescent probe for detecting pH was designed by ICT mechanism. • Probe cys-pH can monitor pH in mitochondria. • Probe cys-pH has been applied in detecting pH in environmental water samples. • pH test papers and standard colorimetric cards were made by cys-pH. • The double absorption peak of UV–vis spectrum of cys-pH was explained reasonably. [ABSTRACT FROM AUTHOR]

Hydrazine is a widely used industrial material, and yet it is also a highly toxic and possibly carcinogenic substance which is likely to induce physiological dysfunction. Here, we report a ratiometric fluorescent probe (NA-N 2 H 4 ) for hydrazine detection based on the intramolecular charge transfer (ICT) mechanism. As a naphthalimide derivative with an acetate group at the 4th position of its naphthalic ring, this probe can realize both colorimetric and ratiometric detection for hydrazine. The probe NA-N 2 H 4 exhibits a very low mean detection limit of 9.40 ± 0.12 nM, which is well below the safety level (10 ppb, or 312 nM) as regulated by EPA. The probe's selectivity and anti-interference performance is found desirable, and the probe works well in quantitative detection toward hydrazine in real samples (drinking water and river water), as well as in the qualitative detection of gaseous hydrazine. Moreover, it can be employed to image hydrazine in living cells and in vivo (zebrafish). [ABSTRACT FROM AUTHOR]

[Display omitted] • Water soluble Ox-L was synthesized easily using 3H-indolium dye in high yield. • Ox-L exhibited a large Stokes shift of 145 nm. • Ox-L possessed favorable cell membrane permeability and low cytotoxicity. • Ox-L was used in imaging acidic HeLa cells and monitoring pH fluctuations in E. coli. Acid-base balance is associated with various cellular functions, and its disorders could cause several diseases. The real-time detection of intracellular pH fluctuation is of vital importance for the prevention of multiple diseases. Therefore, the development of pH-sensitive probes has always been a considerable requirement. Here, we report a ratiometric fluorescent probe Ox-L based on the 3H-indolium dye, which enables highly sensitive detection of pH fluctuations via the intramolecular charge transfer (ICT) process. The probe exhibits a large Stokes shift at 145 nm, excellent photostability, reversibility and low cytotoxicity— all which are conducive for intracellular pH imaging. Based on these properties, Ox-L has been suitably applied to visualize pH fluctuation in the living cell, and to monitor the periplasmic pH of E. coli. These results showed that the probe possesses great potential in biological systems. [ABSTRACT FROM AUTHOR]

A molecule ANB can undergo phtochromism based on AIEE property, as so to be a perfect fluorescence probe for detecting cancer cell lipid droplets. We successfully distinguished cancer cells from normal cells in cell and tissue imaging by using ANB. • A fluorescence turn-on lipid droplets (LDs) probe ANB was prepared with better photostability than commercial probes. • The bright fluorescence LDs was rational explained by AIEE, which was proved FONs (fluorescent organic nanoparticles). • The bright fluorescence LDs revealed photochromism to construct a photoinduced emission enhancement (PIEE) LDs. • We successfully distinguished tumor xenograft tissues from normal tissue by using ANB staining assays. • The phenomena of above were illustrated by PIEE-LDs. A molecule ANB (6-(4-aminostyryl)-1,3-dioxo-1H-benzo[de]isoquinolin -2(3 H)-yl)-n-butane), was developed for fluorescence turn-on imaging of cancer cells lipid droplets (LDs). ANB exercised synergistically aggregation induced emission enhancement and photochromism, which are essential to construct a photoinduced emission enhancement (PIEE) LDs. The fluorescent nanostructures PIEE-LDs extremely improve intracellular imaging contrast with high emission intensity, high emission contrast to cytosol and high photostability. Following these unique optical properties, we statistically analyzed the differences of LDs between cancer and normal cells as well as successfully observed LDs progression and fusion in living cells over a long period. Consequently, the xenograft tumor tissues were favorably distinguished from normal based on this superior PIEE-LDs. It is known that ANB exhibited excited-state conformational conversion between the twist intramolecular charge transfer (TICT) and local excited (LE) states. Based on the results of our studies, we found that LE state became dominate and presented aggregation induced emission enhancement when ANB stay in LDs, which allows us to build a fluorescent nanovesicle model to explain the distinct fluorescence imaging of ANB in LDs. Furthermore, photochromism occurred when irradiation that could promoted emission behavior of bright nanovesicles LDs to further level, and achieve PIEE-LDs. [ABSTRACT FROM AUTHOR]

• A fluorescent probe based on perylene can detect cysteine with acryl groups as recognition sites. • Acryl group on perylene can modulate the fluorescence and increase the fluorescence quantum yield to 41 % in PBS. • The probe displays high reaction speed, high sensitivity, low detection limit, and wide linear range. • The TD-DFT calculations are in good agreement with the experimental values and reveal intramolecular charge transfer. A novel fluorescence probe (TIA) is designed and synthesized by grafting acryl on hydroxyl and diazabenzo[ ghi ]perylene, respectively. The acryl groups on hydroxyl act as a recognition site and fluorescence quencher. After a conjugated addition/cyclization reaction of Cys with TIA, the acryl groups on hydroxyl fall off and acryl conjugated perylene (TIA−OH) is released as the fluorophore with 70-fold increment of fluorescence in PBS. The short response time (less than 5 min) and high signal-noise-ratio endow the probe with fast detection, high sensitivity, low detection limit (0.11 μM) and wide linear range (10−100 μM). Interestingly, compared with the parent dihydroxy 1,2-diazabenzo[ ghi ]perylene, the introduction of acryl group on the perylene can modulate the fluorescence and increase the fluorescence quantum yield of TIA−OH by more than 10-fold (41 % in PBS). The TD-DFT calculations also reveal that the acryl group conjugated on the perylene can result intramolecular charge transfer and the calculated fluorescence is in good agreement with the experimental values. What's more, the high cell membrane permeability and specificity for Cys of the probe ensure the application in fluorescence imaging of living cells to detect intracellular Cys. [ABSTRACT FROM AUTHOR]