In this paper, mechanoresponsive fluorescent molecules with high “off–on” contrast ratios that undergo molecular packing transformation from amorphous phase to crystalline by scratching are first reported. Three 2-aminobenzophenone derivatives 1, 2, and 3, which possess crystallization-induced fluorescence characteristics, exhibited bright cyan, blue, and green fluorescence colors with “off–on” contrast ratios as high as 175-fold by scratching. The scratching induced transformation from amorphous phase to crystalline of 1, 2, and 3 were directly observed by XRD and SEM studies. In addition, the fluorescence of the scratched sample can readily convert back into the initial nonfluorescent state by fuming in ethanol or heating. Letters with three fluorescence emission colors of 1, 2, and 3 in ethanol solution smeared on a glass surface can be easily written by scratching and erased by fumigation, demonstrating the potential value as reversible mechanoresponsive fluorescent materials.

The development of red fluorophores with efficient solid-state emission is still challenging. Herein, a red fluorophore 1 with aggregation-induced emission (AIE) and excited-state intramolecular proton transfer (ESIPT) characteristics is rationally designed and facilely synthesized by attaching an electron-donor diethylamine and an electron-acceptor maleonitrile group to salicyladazine. In contrast to many red fluorophores which undergo serious aggregation-caused quenching (ACQ), compound 1 emits bright red fluorescence (λem = 650 nm, ΦF = 24.3%) in the solid state with a large Stokes shift of 174 nm. Interestingly, control compounds 2 and 3, which have similar structures as 1, exhibit obvious aggregation-caused quenching (ACQ) characteristics. The difference in the crystal structures of 1, 2, and 3 reveals that the interplanar spacing among molecules plays a decisive role in realizing the AIE characteristics of 1. Moreover, when the hydroxyl group of 1 was substituted by an esterase reactive acetoxyl, a fluorescence light-up probe 4 was developed for sensing of esterase based on the selective reaction between 4 and esterase to generate the AIE and ESIPT active molecule 1. The linear range for in vitro quantification of esterase is 0.01–0.15 U/mL with a detection limit of 0.005 U/mL. Probe 4 was also successfully applied to image esterase in mitochondria of living cells.

Triphenylmethanol (TPOH) is a non-fluorescent compound. Interestingly, after UV irradiation at 254 nm, TPOH showed a strong blue fluorescence on a solid surface. Moreover, the fluorescence can be quenched by UV irradiation at 365 nm, making the photoactivatable fluorescence reversible. TPOH is thus applied for rewritable photopatterning tuned by UV light.

A novel fluorescent probe SA-βGal is reported here with light-up response to β-galactosidase. SA-βGal possesses the β-galactopyranoside group to react with β-galactosidase and releases the fluorescent salicylaldehyde azine with both aggregation induced emission (AIE) and excited-state intramolecular proton transfer (ESIPT) characteristics. The linear fluorescent response enables the in vitro quantification of β-galactosidase activity in a range of 0–0.1 U mL−1 with a detection limit of 0.014 U mL−1. The probe exhibits significant advantages, such as no self-quenching at high concentrations, a large Stokes shift (190 nm) and high specificity to β-galactosidase with an excellent light-up ratio of 820 fold. Moreover, thanks to its good retention in living cells, the application of SA-βGal for the imaging of cellular β-galactosidase was also achieved with high contrast.

•We developed a design of fluorescent DNA aptazyme sensors based on the target-induced unfolding of DNA hairpins.•Our approach required no change in the active sites of any DNAzyme or aptamer.•The new aptazyme sensors showed high selectivity and sensitivity for the detection of their targets.•The aptazyme sensor for interferon-gamma was the first example of an aptazyme to it.DNA aptazymes are allosteric DNAzymes activated by the targets of DNA aptamers. They take the advantages of both aptamers and DNAzymes, which can recognize specific targets with high selectivity and catalyze multiple-turnover reactions for signal amplification, respectively, and have shown their great promise in many analytical applications. So far, however, the available examples of DNA aptazyme sensors are still limited in utilizing only several DNAzymes and DNA aptamers, most likely due to the lack of a general and simple approach for rational design. Herein, we have developed such a general approach for designing fluorescent DNA aptazyme sensors. In this approach, aptamers and DNAzymes are connected at the ends to avoid any change in their original sequences, therefore enabling the general use of different aptamers and DNAzymes in the design. Upon activation of the aptazymes by the targets of interest, the rate of fluorescence enhancement via the cleavage of a dually labeled substrate by the active aptazymes is then monitored for target quantification. Two DNAzymes and two aptamers are used as examples for the design of three fluorescent aptazyme sensors, and they all show high selectivity and sensitivity for the detection of their targets. More DNA aptazyme sensors for a broader range of targets could be developed by this general approach as long as suitable DNAzymes and aptamers are used.A simple and general approach for rational design of fluorescent DNA aptazyme sensors based on the target-induced unfolding of DNA hairpins.

4-Chloro-2-(((2-hydroxybenzylidene)hydrazono)methyl)phenol (1) was reported to exhibit typical aggregation-induced emission (AIE) characteristics in our previous work. Here we introduce an acryloyl group onto the hydroxyl moiety of 1 and develop 2 (4-chloro-2-(((2-hydroxybenzylidene)hydrazono)methyl)phenyl acrylate) for fluorescence turn-on detection of cysteine (Cys). 1H-NMR and mass spectrometry data of the products revealed that the reaction between 2 and Cys resulted in the formation of 1 with excited-state intramolecular proton transfer (ESIPT) and AIE properties. With fluorescence enhancement detection by 2, the linear range and detection limit for Cys were obtained to be 0–30 μM (R2 = 0.998) and 0.46 μM respectively with satisfactory selectivity over homocysteine (GSH), glutathione (Hcy) and other amino acids. The method was also used for Cys detection in a serum sample.

5-Chlorosalicylaldehyde azine (1) has been reported to exhibit a typical aggregation-induced emission (AIE) feature in our previous work. In this work, we further discovered that AIE fluorescence of 1 could be selectively quenched by Cu2+, hence 1 and Cu2+ complex was applied as a turn-on fluorescence probe for pyrophosphate (PPi) detection in aqueous solution. 1H-NMR data of the isolated products revealed that the addition of PPi underwent displacement of 1 in 1–Cu2+ complex, which led to the release of free 1 to aqueous solution, resulting in recovered AIE fluorescence. With fluorescence enhancement detection by the 1–Cu2+ complex, the linear range and detection limit for PPi were obtained as 0–15 μmol L−1 (R2 = 0.997) and 0.064 μmol L−1, respectively, with satisfactory selectivity over other anions. The method was also used for PPi detection in serum samples.

DNA–protein conjugates are very useful in analytical chemistry for target recognition and signal amplification. While a number of methods for conjugating DNA with proteins are known, methods for purification of DNA–protein conjugates from reaction mixture containing unreacted proteins are much less investigated. In this work, a simple and efficient approach to purify DNA–invertase conjugates from reaction mixture via a biotin displacement strategy to release desthiobiotinylated DNA–invertase conjugates from streptavidin-coated magnetic beads was developed. The conjugates purified by this approach were utilized for quantitative detection of cocaine and DNA using a personal glucose meter through structure-switching DNA aptamer sensors and competitive DNA hybridization assays, respectively. In both cases, the purified DNA–invertase conjugates showed better performance compared to the same assays using unpurified conjugates. The approach demonstrated here can be further expanded to other DNA and proteins to generate purified DNA–protein conjugates for analytical and other applications.

•The chemosensor 1 developed here performed a ratiometric response to Al3+.•Al3+ detection can be undertaken in aqueous solution at neutral pH.•The sensor design was transformed into test papers for the detection of Al3+.•The application of 1 for Al3+ imaging in live cells was also achieved.A ratiometric fluorescent chemosensor 1 was developed for the detection of Al3+ in aqueous solution based on aggregation-induced emmision (AIE). The chemosensor showed the fluorescence of its aggregated state and Al3+-chelated soluble state in the absence and in the presence of Al3+, respectively, and resulted in a fluorescence ratio (I461/I537) response to Al3+ in neutral aqueous solution at a detection limit as low as 0.29 μmol L−1. The method was also highly selective to Al3+ over other physiological relevant metal ions investigated in this study. Taking advantage of its AIE characteristics, the chemosensor was successfully applied on test papers for simple and rapid detection of Al3+. Moreover, the application of 1 for the imaging of Al3+ in living cells by ratiometric fluorescence changes was also achieved.

•A fluorescent turn-on probe can react with thiols to release a fluorescent product.•The product showed aggregation-induced emission characteristics.•The probe does not suffer from aggregation-induced fluorescence quenching.•The probe can be applied as solid materials for thiols detection.•The fluorescence turn-on response is applied for imaging cellular thiols.A turn-on fluorescent probe DNBS-CSA is reported here with selective detection of thiols over other amino acids. Probe DNBS-CSA possesses the widely-used thiol-selective 2,4-dinitrobenzenesulfonyl (DNBS) group to react with thiols and release the fluorescent salicylaldehyde azine with aggregation-induced emission (AIE) characteristics. The use of AIE dye prevented the sensor from aggregation-induced fluorescence quenching which is challenging for other thiols sensors. Upon the addition of cysteine (Cys) to DNBS-CSA in aqueous solution (10 mM PBS buffer at pH 7.4 containing 30% DMSO), an intense fluorescence enhancement was observed at 558 nm with a large stokes shift of ∼170 nm. Under the optimal condition, the fluorescence intensity linearly increased with the concentration of Cys in the range of 8–18 μM with a correlation coefficient of R2 = 0.991. The fluorescence enhancement of DNBS-CSA upon addition of Cys on test paper was also observed, enabling DNBS-CSA function as solid materials (such as test papers) for thiols detection. The detection of thiols in serum samples was successfully performed. With good cell permeability, the probe was applied to the imaging of thiols in HEK 293T cells.

A new method for caspase 3 activity assay has been developed based on HRP-mimicking DNAzyme–peptide conjugates. The mechanism of detection was based on the specific cleavage of DEVD-peptides by active caspase 3 for recognition and the catalytic properties of HRP-mimicking DNAzymes for signal amplification. Under optimal conditions, the detection limit of caspase 3 was 0.89 nM. The proposed method was also successfully applied for the detection of caspase 3 in apoptosis cell lysates.

•A fluorescent probe for heparin based on aggregation-induced emission was reported.•The probe could aggregate on heparin through electrostatic interactions.•Highly sensitive and selective with rapidly responsive fluorescence detection of heparin was achieved.•The method was applied for heparin detection in diluted horse serum samples.A facile, rapidly responsive fluorescence turn-on probe for heparin with high selectivity and sensitivity was reported in this paper. The probe could aggregate on the negatively charged heparin template through electrostatic interactions and then display intense fluorescence due to its aggregation-induced emission (AIE) characteristics. Under optimal condition, the probe showed high selectivity to heparin over chondroitin sulfate(ChS), hyaluronic acid (HA), dextran (DeX) and other substances, with a linear range of 0.2–14 μg/mL, and a detection limit of 57.6 ng/mL. In diluted serum, it also showed good performance.

•A “turn-on” fluorescent chemosensor for the detection of Zn(II) was facilely synthesized.•Detection of Zn(II) can be performed in water at neutral pH with high sensitivity.•The chemosensor exhibits remarkably selective to Zn(II) over other metal ions.•The chemosensor could be efficiently delivered to live cells for bioimaging of Zn(II).Compound 1 was facilely synthesized through a one step reaction from commercially available materials. As a sensitive and selective “turn-on” fluorescent chemosensor for Zn(II), 1 exhibits a 40-fold fluorescence enhancement response to Zn(II) over other physiological relevant metal ions in aqueous solution at neutral pH. Furthermore, 1 could be efficiently delivered to live cells for bioimaging of Zn(II).

A novel fluorescent probe 1 is reported here with ratiometric response to hydrophobic proteins (casein) or proteins with hydrophobic pockets (BSA, HSA) through hydrophobic interaction. Probe 1 underwent deprotonation in aqueous solution at pH 7.4 and emitted blue fluorescence at 436 nm. Upon the addition of BSA, HSA or casein, the aggregation-induced emission fluorescence of 1 at 518 nm was turned on. The fluorescence intensity ratio, I518/I436 was linearly related to the concentrations of these proteins. The detection limits for BSA, HSA and casein based on IUPAC (CDL = 3Sb m−1) were 16.2 μg mL−1, 10.5 μg mL−1 and 5.7 μg mL−1, respectively.

In this work, (2-hydroxy-4-methoxyphenyl)(phenyl)methanone azine (1) was found to exhibit aggregation-induced emission (AIE) and tunable solid fluorescence upon alternate annealing/melting treatments. According to the characterizations by X-ray crystallography, X-ray powder diffraction, and differential scanning calorimetry, the switching between the two different polymorphs was responsible for the tunable solid fluorescence as a consequence of polymorph-dependent excited-state intramolecular proton transfer (ESIPT) fluorescence, while the thermochromism was contributed by the conformational flexibility of rotary phenyl rings. The change in the tightness of packing upon annealing or melting thermal treatments resulted in the emission at different wavelengths. Therefore, polymorph-dependent ESIPT fluorescence could be utilized as a new strategy to develop efficient AIE-active materials in response to external stimuli.

A series of salicylaldehyde-2-pyridinehydrazone derivatives were found to exhibit interesting “turn-on” fluorescence response toward Zn2+ in water at neutral pH, which could be prepared readily from commercially available materials through a one-step reaction in ethanol at room temperature. Among these compounds, 1 showed a 22-fold fluorescence enhancement in 99% water/DMSO (v/v) at pH 7.0 with a linear range of 0.10–1.00 μmol/L and detection limit of 30 nmol/L Zn2+. Especially, 1 was not affected by Cd2+ which usually induced a comparable fluorescence response to that of Zn2+ in other reported works. Moreover, live cell imaging showed that 1 could be useful for monitoring Zn2+ within biological samples.

In this work, two methods with complementary features, catalytic and molecular beacon (CAMB) and label-free fluorescent sensors using an abasic site, have been combined into new label-free CAMB sensors that possess advantages of each method. The label-free method using a dSpacer-containing molecular beacon makes CAMB more cost-effective and less interfering with the catalytic activity, while CAMB allows the label-free method to use true catalytic turnovers for signal amplifications, resulting in a new label-free CAMB sensor for Pb2+ ion, with a detection limit of 3.8 nM while maintaining the same selectivity. Furthermore, by using CAMB to overcome the label-free method’s limitation of requiring excess enzyme strands, a new label-free CAMB sensor using aptazyme is also designed to detect adenosine down to 1.4 μM, with excellent selectivity over other nucleosides.

Coumarin derivative 1 was synthesized as an efficient ratiometric chemodosimeter for the detection of Cu(II) in 99% water/DMSO (v/v) at pH 7.0. Mechanism studies suggested that 1 formed a complex with Cu(II) at 2:1 ratio accompanied by quenching of green fluorescence at 524 nm; when the solution was heated to 50 °C for 30 min, Cu(II)-promoted hydrolysis of coumarin lactone moiety of 1 occurred with bright blue fluorescence at 451 nm emerged. With fluorescence intensity ratio detection at 451 nm and 524 nm, 1 features an excellent sensitivity with the detection limit of 15 nmol L−1 toward Cu(II) and a good selectivity over other metal ions.Graphical abstractHighlights► A coumarin-based ratiometric chemodosimeter for the detection of Cu(II) is facilely synthesized by three steps reactions. ► Cu(II) catalyzes the hydrolysis of chemodosimeter with ratiometric fluorescence signal greatly amplified. ► Detection of Cu(II) can be performed in water at neutral pH with high sensitivity. ► The chemodosimeter exhibits remarkably selective fluorescence enhancement to Cu(II) over other metal ions.

A ratiometric fluorescent pH probe, 4-carboxylaniline-5-chlorosalicylaldehyde Schiff base (1) was synthesized via a facile reaction and used for pH sensing in live cells. While exhibiting weak fluorescence when dispersed in solution, 1 displayed aggregation-induced emission enhancement (AIEE) characteristics in its aggregate/solid state, as a result of the restriction of free intramolecular rotation of a C–N bond and the non-planar configuration in the aggregate/solid state. The integration of hydroxyl and carboxyl groups provided 1 with a ratiometric fluorescent response to pH based on AIEE and the pH-dependent spectral characteristics of compound 1, with proton dissociation constants pKa1 and pKa2 of 4.8 ± 0.1 and 7.4 ± 0.1, respectively. The probe exhibited a significant fluorescence color change from orange to green with an intensity ratio (I516 nm/ I559 nm) enhanced when the pH increased from 5.0 to 7.0 in aqueous solution. Confocal fluorescence imaging of intracellular pH through ratiometric response was successfully achieved in live HepG2 cells. The results demonstrate that probe 1 is a good candidate for monitoring pH fluctuations in live cells with good selectivity, stability, and excellent membrane permeability.

A highly selective and sensitive coumarin-based chemodosimeter 1 for Cu2+ in water is reported in this work. 1 was designed and facilely synthesized by a one-step reaction with coumarin as a fluorophore and 2-picolinic acid as the binding moiety, which showed very week fluorescence in buffer solution, and its fluorescence was considerably enhanced by the addition of Cu2+ at room temperature in 5 min. Mechanism study suggested that Cu2+ promoted the hydrolysis of 1 via the catalytic sensing cycle, generating a highly fluorescent product 7-hydroxycoumarin with fluorescence signal greatly amplified. The probe exhibited remarkably selective fluorescence enhancement to Cu2+ over other metal ions at 454 nm, with a detection limit of 35 nM Cu2+. Under optimal condition, 1 was successfully used for the determination of Cu2+ in fetal equine serum and two water samples.Graphical abstractHighlights• A coumarin-based fluorescence turn on chemodosimeter for Cu2+ is facilely synthesized by a one-step reaction. • Cu2+ catalyzes the hydrolysis of chemodosimeter with fluorescence signal greatly amplified. • The chemodosimeter exhibits remarkably selective fluorescence enhancement to Cu2+ over other metal ions. • Detection of Cu2+ can be performed at room temperature with high sensitivity.

A Schiff base (SB) immobilized hybrid mesoporous silica membrane (SB-HMM) was prepared by immobilizing a Schiff base onto the pore surface of mesoporous silica (pore size = 3.1 nm) embedded in the pores of a porous anodic alumina membrane. In contrast to the non-fluorescent analogous SB molecule in homogeneous solutions, SB-HMM exhibited intense fluorescence due to emission enhancement caused by aggregation of SB groups on the pore surface. The high quantum efficiency of the surface SB groups allows SB-HMM to function as a fluorescent sensor for Cu(II) ions in an aqueous solution with good sensitivity, selectivity and reproducibility. Under the optimal conditions described, the linear ranges of fluorescence intensity for Cu(II) are 1.2–13.8 (M (R2 = 0.993) and 19.4–60 (R2 = 0.992) (M. The limit of detection for Cu(II) is 0.8 μM on basis of the definition by IUPAC (CLOD = 3.3Sb/m).Graphical abstractHighlights► A hybrid mesoporous membrane (SB-HMM) functionalized by Schiff base fluorophores was fabricated. ► SB-HMM showed strong fluorescence with aggregation-induced emission enhancement properties. ► SB-HMM was applicable for the detection of Cu(II) in an aqueous solution with good reversibility and reproducibility.

In this study, p-carboxyl-N-salicylideneaniline (pCSA) derivatives were simply synthesized by condensing salicylaldehyde and primary amine. With two planar π-conjugated moieties linked by rotatable C–N single bond in the molecular structure, pCSA derivatives were characteristic of aggregation-induced emission enhancement (AIEE) properties, exhibiting strong fluorescence in solid state. Facilely recrystallized from organic solvent, one-dimensional (1D) microrods of pCSA were prepared with green, yellow and orange AIEE colors depending on substituents on salicylaldimine moiety. The obtained 1D microrods could be utilized as potential optical waveguides for propagating and manipulating light in miniaturized devices.Highlights► p-carboxyl-N-salicylideneaniline (pCSA) with AIEE properties were simply prepared. ► Green, yellow and orange AIEE colors could be tuned by substituents on salicylaldimine moiety. ► 1D microrods of pCSA could be facilely recrystallized from organic solvent and utilized as potential optical waveguides.

A salicylaldehyde azine-based derivative, which exhibited switchable solid-state fluorescence in response to external thermal and mechanical grinding stimulus, was reported in this study. Diffuse reflectance solid-state spectroscopy, fluorescence spectroscopy, solid-state 13C and 1H NMR spectroscopy, single crystal XRD analyses, powder X-ray diffractometry, and differential scanning calorimetry were used for investigating the changes of molecular packing modes. The multistimuli fluorescence originated from two distinctive crystalline lattices via different π–π interactions. The local dipole coupling among molecules with donor–planar–acceptor structure was evidenced to be the essential factor for the effective solid luminescence-switching properties.

A nonfluorescent coumarin derivative (1) was synthesized as an efficient “turn-on” fluorescent chemodosimeter for Cu2+ in water. Mechanism studies suggested that 1 formed a complex with Cu2+ in a 1:2 metal-to-ligand ratio, and a 50-fold fluorescence enhancement was observed when the complex simultaneously underwent Cu2+-promoted hydrolysis at ambient temperature.

A novel fluorescence turn-on method for sensitive and selective detection of protamine was developed based on aggregation-induced emission enhancement (AIEE) characteristics of 4-(6′-carboxyl)hexyloxysalicylaldehyde azine (CHSA) induced by electrostatic interaction between protamine and CHSA−. Under optimal conditions described, a large Stokes shift of ∼198 nm could be observed, and the fluorescence enhancement at 538 nm was linearly related to the concentration of protamine in the range of 0.6∼18 μg mL−1 with the relative correlation coefficient of R2 = 0.9996 (n = 11) and a detection limit as low as 43 ng mL−1. The relative standard deviation (R.S.D.) was 2.0% (n = 5). The proposed method was successfully utilized in quantifying protamine in diluted horse serum. In addition, due to the special electrostatic association between protamine and heparin, CHSA could also be employed as a probe to study the interaction between them.

A novel fluorescence turn-on detection method of human serum albumin (HSA) and bovine serum albumin (BSA) in aqueous solution is investigated using 2,4-dihydroxyl-3-iodo salicylaldehyde azine (DISA). Upon the addition of DISA to HSA/BSA solution, a fluorescence turn-on effect at 529 nm can be observed with a large stokes shift of ∼129 nm based on hydrophobic binding-mode between protein and dye. Under the optimal condition, the linear ranges of fluorescence intensity for HSA and BSA are 0.1–30 μg mL−1 with the relative correlation coefficient of R2 = 0.991 (n = 10) and 0.3–50 μg mL−1 with R2 = 0.997 (n = 10); and the detection limits for HSA and BSA based on IUPAC (CDL = 3Sb/m) are 20 ng mL−1 and 50 ng mL−1, respectively.

An abasic site called dSpacer has been introduced into duplex regions of the 8−17 DNAzyme and adenosine aptamer for label-free fluorescent detection of Pb2+ and adenosine, respectively. The dSpacer can bind an extrinsic fluorescent compound, 2-amino-5,6,7-trimethyl-1,8-naphthyridine (ATMND), and quench its fluorescence. Addition of Pb2+ enables the DNAzyme to cleave its substrate and release ATMND from DNA duplex, recovering the fluorescence of ATMND. Similarly, the presence of adenosine induces structural switching of the aptamer, resulting in the release of ATMND from the DNA duplex and a subsequent fluorescence enhancement. Under optimized conditions, this label-free method exhibits detection limits of 4 nM for Pb2+ and 3.4 μM for adenosine, which are even lower than those of the corresponding labeled-DNAzyme and aptamer sensors. These low detection limits have been obtained without compromising any of the selectivity of the sensors. Finally, the dynamic range of the adenosine sensor has been tuned by varying the number of hybridized base-pairs in the aptamer duplex. The method demonstrated here can be applied for label-free detection and quantification of a broad range of analytes using other DNAzymes and aptamers.

Salicylaldehyde hydrazones of 1 and 2 were synthesized and their potential as fluorescent probes for zinc ion was investigated in this paper. Both of the probes were found to show fluorescence change upon binding with Zn2+ in aqueous solutions, with good selectivity to Zn2+ over other metal ions such as alkali/alkali earth metal ions and heavy metal ions of Pb2+, Cd2+ and Hg2+. They showed 1:2 metal-to-ligand ratio when their Zn2+ complex was formed. By introducing pyrene as fluorophore, 2 showed interesting ratiometric response to Zn2+. Under optimal condition, 2 exhibited a linear range of 0–5.0 μM and detection limit of 0.08 μM Zn2+ in aqueous buffer, respectively. The detection of Zn2+ in drinking water samples using 2 as fluorescent probe was successful.

A facile fluorescent method for the determination of hydrazine in aqueous solution with excellent sensitivity was developed. 5-Chlorosalicylaldehyde (CS), a readily commercially available compound, was applied as the derivatization reagent in this work. Under the addition of CS to hydrazine aqueous solution (ethanol/water/acetic acid = 30/66/4), an intense fluorescence enhancement was observed at 570 nm with a large stokes shift of ∼170 nm. Upon the optimal condition, the fluorescence intensity linearly increased with the concentration of hydrazine in the range of 0.2 and 9.3 μM with a correlation coefficient of R2 = 0.9995 (n = 10) and a detection limit of 0.08 μM. The R.S.D. was 2.0% (n = 5). Determination of hydrazine in river and drinking water samples was successfully performed. Hydrazine vapor sensing by the proposed method was also reported.

A new ratiometric and selective fluorescent chemosensor (1) for quantification of zinc ions in aqueous ethanol has been synthesized and investigated in this work. In an environmentally friendly media of 30% (v/v) water/ethanol and 10 mM Tris–HCl neutral buffer (pH 7.03), 1 displayed selective Zn2+ ratiometric fluorescence response, with a dynamic working range of 1.0–8.0 μM and a detection limit of 0.5 μM Zn2+. The determination of Zn2+ in synthesized water sample was also successful.

A highly selective and sensitive rhodamine-based colorimetric chemosensor (1) for quantification of divalent copper in aqueous solution has been investigated in this work. It was designed using salicylaldehyde hydrazone and rhodamine 6G as copper-chelating and signal-reporting groups, respectively. In environmentally friendly media (50% (v/v) water/ethanol and 10 mM NaAc–HAc neutral buffer (pH 7.0)), the sensor exhibited selective absorbance enhancement to Cu2+ over other metal ions at 529 nm, with a dynamic working range of 0.05–5.00 μM and a detection limit of 10 nM Cu2+, respectively. To achieve fluorometric determination of Cu2+, the Cu2+-induced absorbance enhancement of 1 was efficiently converted to fluorescence quenching by fluorescence inner filter effects using rhodamine B (RB) as a fluorophore. The selectivity and sensitivity of fluorescence analysis were similar to those of absorptiometric measurement. Both absorptiometric and fluorometric methods were successfully applied to the detection of Cu2+ in three water samples.

Two 10-mer DNA probes, or one 20-mer DNA probe, respectively, hybridize with a 21-mer target DNA to form a vacancy or bulge opposite the target nucleotide. The former double-DNA-probe method and the latter bulge form method are applicable to the detection of single-nucleotide polymorphisms (SNPs). A small fluorescent dye enters into the vacancy or bulge and binds with a target nucleotide via a hydrogen bonding interaction, which causes fluorescence quenching. The interaction between fluorescent dye and the target nucleotide is confirmed by measuring the melting temperature and fluorescence spectra. The fluorescent dye, ADMND (2-amino-5,7-dimethyl-1,8-naphthyridine), is found to selectively bind with C over A or G. The methods proposed here are economic, convenient, and effective for the fluorescence detection of SNPs. Finally, the double-DNA-probe method and bulge form method are successfully applied to the detection of C/G and C/A mutations in the estrogen receptor 2 gene and progesterone receptor gene using ADMND.

A new fluorogenic method for the selective and sensitive determination of chromium(VI) in acidic water using rhodamine B hydrazide was developed. This method was based on the oxidation of non-fluorescent rhodamine B hydrazide by potassium dichromate in acidic aqueous conditions to give rhodamine B, which was highly fluorescent, as a product. With the optimum condition described, the fluorescence enhancement at 585 nm was linearly related to the concentration of chromium(VI) in the range of 5.0 × 10−8 to 2.0 × 10−6 mol L−1 (2.60–104 ng mL−1) with a correlation coefficient of R2 = 0.9993 (n = 18) and a detection limit of 5.5 × 10−9 mol L−1 (0.29 ng mL−1). The R.S.D. was 2.2% (n = 5). The proposed method was also applied to the determination of chromium(VI) in drinking water, river water and synthetic samples.

A new fluorescent reagent 2-amino-5,7-dimethyl-1,8-naphthyridine (ADMND) was proposed for the determination of trace nitrite. The reaction is based on the diazotization of naphthyridine amine with nitrite to form a diazonium salt that hydrolyzed when boiling to give hydroxyl group substituted naphthyridine. Fluorescence quenching degree of ADMND by nitrite ion is linear in the nitrite concentration range of 1 × 10−7 to 2.5 × 10−6 mol l−1 with a detection limit of 4.06 × 10−8 mol l−1. Reaction and determination acidity for nitrite is the same which made the method much simpler compared with the widely accepted fluorescence method with DAN as a fluorescence reagent.

A new fluorescent probe, rhodamine B hydrazide oxalamide (RBHO), which shows very weak fluorescence, was synthesized, and its fluorescence could be substantially enhanced by the addition of copper ion. The probe shows a high selectivity and sensitivity to copper ion by forming a 1:1 complex in acetonitrile, and the chelating is reversible. Limit of detection for copper ion in acetonitrile was found to be 3.7 × 10−8 mol L−1. It was also found that copper ion could catalyze the hydrolysis of the probe in 50% (v/v) buffered (10 mM Tris–HCl, pH 7.0) water/acetonitrile giving a highly fluorescent product, and the fluorescence detection of copper ion was developed in this neutral buffered media with a detection limit of 6.4 × 10−7 mol L−1. Determination of copper ion in water and synthetic samples in the presence of different interfering metal ions was successfully carried out with the new probe RBHO.

4-Chloro-2-(((2-hydroxybenzylidene)hydrazono)methyl)phenol (1) was reported to exhibit typical aggregation-induced emission (AIE) characteristics in our previous work. Here we introduce an acryloyl group onto the hydroxyl moiety of 1 and develop 2 (4-chloro-2-(((2-hydroxybenzylidene)hydrazono)methyl)phenyl acrylate) for fluorescence turn-on detection of cysteine (Cys). 1H-NMR and mass spectrometry data of the products revealed that the reaction between 2 and Cys resulted in the formation of 1 with excited-state intramolecular proton transfer (ESIPT) and AIE properties. With fluorescence enhancement detection by 2, the linear range and detection limit for Cys were obtained to be 0–30 μM (R2 = 0.998) and 0.46 μM respectively with satisfactory selectivity over homocysteine (GSH), glutathione (Hcy) and other amino acids. The method was also used for Cys detection in a serum sample.

A nonfluorescent coumarin derivative (1) was synthesized as an efficient “turn-on” fluorescent chemodosimeter for Cu2+ in water. Mechanism studies suggested that 1 formed a complex with Cu2+ in a 1:2 metal-to-ligand ratio, and a 50-fold fluorescence enhancement was observed when the complex simultaneously underwent Cu2+-promoted hydrolysis at ambient temperature.

A novel fluorescent probe SA-βGal is reported here with light-up response to β-galactosidase. SA-βGal possesses the β-galactopyranoside group to react with β-galactosidase and releases the fluorescent salicylaldehyde azine with both aggregation induced emission (AIE) and excited-state intramolecular proton transfer (ESIPT) characteristics. The linear fluorescent response enables the in vitro quantification of β-galactosidase activity in a range of 0–0.1 U mL−1 with a detection limit of 0.014 U mL−1. The probe exhibits significant advantages, such as no self-quenching at high concentrations, a large Stokes shift (190 nm) and high specificity to β-galactosidase with an excellent light-up ratio of 820 fold. Moreover, thanks to its good retention in living cells, the application of SA-βGal for the imaging of cellular β-galactosidase was also achieved with high contrast.

A ratiometric fluorescent pH probe, 4-carboxylaniline-5-chlorosalicylaldehyde Schiff base (1) was synthesized via a facile reaction and used for pH sensing in live cells. While exhibiting weak fluorescence when dispersed in solution, 1 displayed aggregation-induced emission enhancement (AIEE) characteristics in its aggregate/solid state, as a result of the restriction of free intramolecular rotation of a C–N bond and the non-planar configuration in the aggregate/solid state. The integration of hydroxyl and carboxyl groups provided 1 with a ratiometric fluorescent response to pH based on AIEE and the pH-dependent spectral characteristics of compound 1, with proton dissociation constants pKa1 and pKa2 of 4.8 ± 0.1 and 7.4 ± 0.1, respectively. The probe exhibited a significant fluorescence color change from orange to green with an intensity ratio (I516 nm/ I559 nm) enhanced when the pH increased from 5.0 to 7.0 in aqueous solution. Confocal fluorescence imaging of intracellular pH through ratiometric response was successfully achieved in live HepG2 cells. The results demonstrate that probe 1 is a good candidate for monitoring pH fluctuations in live cells with good selectivity, stability, and excellent membrane permeability.