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A ratiometric fluorescent hybrid nanoprobe CDs-1 for arginine (Arg), exhibiting high sensitivity (the limit of detection, LOD, being 6.5 × 10−8 mol/L) and excellent selectivity and anti-interference ability, was fabricated through fluorescence resonance energy transfer (FRET) and the electrostatic attraction between positively-charged hemicyanine molecules and negatively-charged carbon dots (CDs). Arg can be quantitatively detected in the concentration range from 6.0 × 10−5 mol/L to 2.7 × 10−4 mol/L. Further, due to its ability to target mitochondrion and low cytotoxicity, intracellular Arg was successfully tracked through ratiometric fluorescence imaging.

Near-infrared (NIR) fluorescent sensors have been actively pursued due to organisms exhibiting little autofluorescence in near-infrared regions. Capable of emitting luminesence under 980 nm excitation, upconversion nanoparticles have shown promise as optical agents for in vivo bioimaging. Based on luminescence resonance energy transfer (LRET), a NIR luminescent Arg-nanoprobe was developed by the decoration of upconversion nanoparticles (UCNPs) (NaYF4:Yb,Er) with organic molecules. The as-developed nanoprobe could quantitatively detect concentrations within the range of 528–1250 μM, and had high sensitiviy (the limit of detection, LOD, being 15.6 μM) and good photostability. Because the probe could target lysosomes, intracellar detection of Arg behavior was demonstrated via upconversion luminescence (UCL) imaging.

Based on photo-induced electron transfer, 1,8-naphthalic anhydride-based organic molecular fluorescent pH-sensor (1) was designed and synthesized. Hydrophobic, fluorescent hybrid nanosensor (1-PS35-b-PAA30) encapsulated with the hydrophobic pH responsive fluorophore for sensing intracellular pH has been fabricated based on the self-assembly of amphiphilic diblock copolymer PS35-b-PAA30 and 1. The as-synthesized 1-PS35-b-PAA30 sensor exhibits excellent photo-stability, good anti-disturbance ability, and enhanced fluorescence intensity under acidic environment with respect to the corresponding free dye in highly polar aqueous system because of the encapsulation of 1 inside nanoparticle cores with weak polarity environment. The fluorescence intensity of 1-PS35-b-PAA30 is enhanced by 7.3-fold upon changing from base (pH = 9.0) to acid (pH = 4) in aqueous system, which can exactly meet the physiological pH range in cells. Moreover, its linear fluorescent response from pH 5.2 to 7.4 makes this sensor suitable for the practical tracking of pH fluctuation in live cells. The fluorescence imaging and TEM experiments indicated that the sensor permeated into cells and could not be observed because of aggregation in acidic condition.

Accurate and efficient detection of ClO− was extremely important due to the harm of ROS in the environment and organism. In this paper, yellow fluorescent N,O-CDs were successfully prepared by the solvothermal method. The microscopic size of the N,O-CDs was approximately spherical with an average particle size of 4.8 ± 0.8 nm. The fluorescence quantum yield in ethanol solution was calculated as 10.5 % using fluorescein as the standard reference. The as-fabricated N,O-CDs had high sensitivity and low detection limit (7.5 µM) for quantitatively detecting ClO− with a linear range from 0.07 mM to 0.16 mM. The probe not only shows good selectivity and anti-interference to metal ions, anions and amino acids but also has excellent light stability and thermal stability. Also, a wide selection range for pH was demonstrated.

As momentous reactive oxygen species (ROS), it is necessary to develop high-sensitivity and high-specificity fluorescent probes for tracking hypochlorite anion (ClO−) in environmental and biological systems. Herein, a kind of red luminescent carbon dots (NS-dots) was synthesized by one-step solvothermal method to detect ClO− in PBS buffer solution (VPBS:VEtOH = 100:1, pH = 7.4). The NS-dots has high sensitivity and low detection limit (13.3 μmol/L) for detecting ClO− with linear range from 6.7 × 10−5 mol/L to 26.7 × 10−5 mol/L. Using Rhodamine B (31% at 520 nm in water) as a reference, the NS-dots have a fluorescence quantum yield of 7.2%. Intracellular photostability, mitochondrial targeting properties and the fluorescence imaging towards intracellular ClO− were demonstrated.

Metal ions and anions play significant roles in biological systems and industrial processes, therefore it is important to develop good fluorescent probes to detect metal ions and anions. Here, N,O-co-doped carbon quantum dots (CDs) that could detect Zn2+ via a ratiometric fluorescence method were fabricated. The reaction between the as-prepared CDs and zinc acetate gave the composite CDs-Zn, in which fluorescence changed ratiometrically upon addition of S2 O8 2- . With change in excitation light, the emission peaks of the CDs and CDs-Zn were kept fixed while intensity changed. CDs and CDs-Zn exhibited good photostability, thermal stability, selectivity, and strong anti-interference ability. In addition, CDs and CDs-Zn displayed low dark toxicity under physiological temperatures. Ratiometric fluorescence imaging of intracellular Zn2+ and S2 O8 2- was carried out in living HeLa cells for both of these probes. Compared with reported ratiometric fluorescent hybrid nanosensors based on organic dyes and inorganic nanomaterials, the as-prepared CDs and CDs-Zn had low toxicity, and were easy to synthesize.

Sulfur dioxide (SO2) is an environmental pollutant in the atmosphere which is easily absorbed by the human body. After being inhaled in the body, SO2 is quickly converted into bisulfite (HSO3-), forming a balance in which SO2 and HSO3- coexist in the body status. A large number of epidemiological studies have shown that abnormal levels of sulfite and bisulfite are related to the appearance of numerous diseases such as atherosclerosis, essential hypertension, and lung tissue fibrosis. Therefore, it is essential to develop an effective method to detect bisulfite. In this work, starting from 4-bromonaphthalene-1-carbonitrile, three uncomplicated but efficient HSO3- sensitive ratiometric fluorescent probes HNIC, CIVP and HVIC were designed and synthesized through ICT mechanism and the Michael-type addition reaction. The probes can image HSO3- in living cells. The probes not only have good fluorescence stability and strong anti-interference ability, but also display mitochondrial targeting ability.

A ratiometric fluorescent probe 1 was synthesized, which exhibited high sensitivity and excellent selectivity for hydrazine recognition. Probe 1 can quantitatively detect hydrazine in the concentration range from 30 to 180 μM with a LOD of 2.1 μM. Furthermore, it displayed excellent selectivity, anti-interference over many nucleophilic species, biological species, metal ions and anions, and a fast response time (4 min). Due to its ability to target mitochondria, the response of hydrazine to this probe in a living cell was successfully tracked in a ratiometric manner via fluorescence imaging.

SO2 and its derivatives (HSO3−, SO32−) play a significant important role in many industries and organisms. Based on coumarin and benzopyranose, a promising NIR ratiometric fluorescent probe BAOA (11-oxo-2,2′,3,3′,6,7,7′,8′-octahydro -1H,1′H,5H,6′H,11H- [10,12′-bipyrano [2,3-f] pyrido [3,2,1-ij] quinolin]-13′-ium perchlorate) was designed and developed to detect HSO3− rapidly and sensitively. The sensing mechanism was Michael addition reaction, in which, strongly nucleophilic HSO3− attacked carbon-carbon double bonds and BAOA-HSO3 was formed. A superior linear calibration curve between the fluorescence ratio I490/I722 and concentrations of HSO3− was obtained in the range of 1.25–8.75 μmol/L and the LOD was figured out as 63.0 nmol/L. Cell experiments showed that BAOA could not only locate mitochondria, lysosomes, ER and Golgi but also detect exogenous HSO3− in living cells.

It is crucial to develop highly sensitive and selective probes toward hydrazine because it is a class of highly toxic and pollutant compound. Herein, using fracture of carbon carbon double bond and dissociation of amide by hydrazine, a novel off-on fluorescent probe was developed for hydrazine. The probe can quantitatively detect hydrazine in concentration range from 0 to 20 μM with the LOD of 140 nM. Further, it displayed excellent selectivity and anti-interference ability over many neutral molecules, metal ions, anions, and biological species. The ability to target lysosome and the response of hydrazine to this probe in a living cell was successfully tracked via fluorescence imaging.

Solvents exhibit different color reactions via regulation of the structure, kinetics, and charge distribution of organic compound molecules. In recent years, solvatochromic probes have good application prospects in the fields of chemistry and biology. In this work, we synthesized three fluorescent probes (PBIC–C, PAIC-C, PPIC-C) sensitive to solvent polarity. We have studied the photophysical properties (spectra, quantum yield, and fluorescence lifetime under different solvents) of these probes. The isomerization nature was verified by nuclear magnetic titration. In order to further confirm our guess, we also performed theoretical calculations, and the results were roughly the same as the experimental results. In addition, cell imaging tests revealed that the designed probes could be used as dual intracellular mitochondria/lysosome-targeting fluorescent probes for biological applications.

It is significant for cell physiology to keep the homeostasis of pH, and it is highly demanded to develop ratiometric fluorescent sensors toward pH. In this work, under mild condition, through the electrostatic interaction between carbon nanodots (CDs) and organic molecules, two novel ratiometric fluorescence hybrid nanosensors were fabricated for sensing acidic pH. These nanohybrid systems possess dual emission peaks at 455 and 527 nm under a single excitation wavelength of 380 nm in acidic pH condition. With the increasing of pH, the fluorescence of the 1,8-naphthalimide derivative completely quenches, while the blue fluorescence of CDs keeps constant. Furthermore, the CDs−organic molecular nanohybrids exhibit excellent anti-disturbance ability, reversible pH sensing ability, and a linear response range in wide pH range respectively. Besides the ability to target lysosome, with one of the nanosensor, stimulated pH change has been successfully tracked in a ratiometric manner via fluorescence imaging.

A Near-Infrared (NIR) ratiometric and on-off fluorescent probe 1 was synthesized, which exhibited high sensitivity and excellent selectivity in arginine (Arg) recognition. 1 can quantitatively detect Arg in concentration range from 0 to 150 μM with the LOD of 138 nM. Further, it displayed excellent selectivity and anti-interference over many biological species, metal ions and anions. Due to its ability to target lysosome, the response of Arg to this probe in a living cell was successfully tracked in a ratiometric manner via fluorescence imaging.

It is significant to develop probes for rapid, selective, and sensitive detection of the highly toxic hydrazine in both environmental and biological science. In this work, under mild condition, a novel colorimetric and ratiometric fluorescent probe (probe 1) was synthesized. Probe 1 with a chemoselective reaction mediated by hydrazine in which the diethyl malonate moiety was transformed into an aldehyde group exhibited high sensitivity and good selectivity in hydrazine recognition. Probe 1 can quantitatively detect hydrazine in concentration range from 0 to 120 μM with the LOD of 1.6 μM. Further, it displayed good selectivity and anti-interference over many species including some nucleophilic species, anions and metal ions. The ratiometric fluorescence change of this probe upon addition of hydrazine in living cells has been successfully demonstrated. Furthermore, the probe displays low dark toxicity under physiological temperature.

It is important to develop fluorescent probes for rapid, selective, and sensitive detection of highly toxic hydrazine in both environmental and biological science. In this text, under mild conditions, a novel near-infrared (NIR) ratiometric and on–off fluorescent probe 1 was synthesized based on hemicyanine and coumarin derivatives, which can detect hydrazine with high selectivity and anti-interference over other amines, biological species, anions and metal ions. The limit of quantification (LOQ) value was 0–400 μM and the detection limit could be as low as 560 nM.

It is significant to develop probes for rapid, selective, and sensitive detection of highly toxic hydrazine in both environmental and biological science. In this work, under mild conditions, through the electrostatic attraction between negatively charged CDs and positively charged hemicyanine molecules, a novel ratiometric fluorescent probe containing CDs and a hemicyanine derivative was fabricated for reliable, selective, and sensitive sensing of hydrazine. This nanohybrid system possesses dual emission peaks at 550 and 610 nm under a single excitation wavelength of 530 nm. The addition of hydrazine to CDs–hemicyanine nanohybrid solution results in complete fluorescence quenching of the hemicyanine derivative, while the orange fluorescence of CDs remains constant. Furthermore, the CDs–hemicyanine nanohybrid system shows high selectivity toward hydrazine over other various species, including some nucleophilic species, metal ions and anions. The limit of quantification (LOQ) was 0–1 mM and the detection limit was as low as 8.0 μM.

It is highly demanded and challenging to develop small-molecular fluorescent sensors for the ratiometric detection of pH fluctuation. A ratiometric fluorescent pH sensor was constructed by integrating a pH-sensitive fluorophore with a pH-insensitive hemicyanine group. Its linear and reversible ratiometric fluorescent response from pH 7.0 to 8.8 and 6.0 to 9.0 respectively makes this sensor suitable for the practical tracking of pH fluctuation in live cells. With this sensor, stimulated pH fluctuation has been successfully tracked in a ratiometric manner via fluorescence imaging with low dark toxicity.

Rare earth upconversion nanomaterials have the advantages of near-infrared region excitation, almost zero interference background, large anti-Stokes shifts without broad emission peaks, which can overcome parts of the defects of organic fluorescent probes. In this work, core-shell rare earth upconversion nanoparticles NaYF4:Yb,Tm:20 %Yb,0.35 %Tm@NaYF4 (UCNPs) were synthesized, and then water-soluble nanoparticles DMSA-UCNPs and PAA-UCNPs were subsequently fabricated. Meanwhile a naphthalimide-based colormetric Hg2+-probe was synthesized, and assembled with DMSA-UCNPs and PAA-UCNPs to give composite nanoprobes A-DMSA-UCNPs and A-PAA-UCNPs toward mercury ions. Both nanoprobes use the upconversion luminescence ratio at 451 nm and 361 nm (UCL451/UCL361) as the detection signal, and can quickly respond to Hg2+ ions within 1–2 min. The detection limits of A-DMSA-UCNPs and A-PAA-UCNPs were calculated to be 2.47 μmol/L and 8.15 μmol/L respectively. A-DMSA-UCNPs and A-PAA-UCNPs both could be employed as ratiometric nanoprobes for Hg2+ imaging in living cell and lysosomal localization.

A novel 1,8-naphthalimide-based chemosensor was designed and synthesized for rapid recognition of Cr 3+ . The desired sensor showed off-on fluorescent and lifetime-based response upon Cr 3+ and S 2− in solution, on test paper and in cells. With the intensity-based method, the limit of quantification (LOQ) value was 0–5.5 × 10 −5 M and the detection limit could be as low as 0.60 ppm. The ' in situ ' prepared Cr 3+ complex can recognize S 2− among a series of common anions with high selectivity and sensitivity, the LOD can be as low as 307 nM. The lifetime of the sensor changes from 4.95 to 4.89 and further to 5.88 ns upon addition of Cr 3+ and S 2− in turn.

Based on 8-hydroxyquinoline, a 1,8-naphthalimide-derived colorimetric and fluorescent probe was synthesized for naked-eye and fluorescent recognition of F− (from both tetrabutylammonium fluoride and NaF) with high sensitivity and excellent selectivity. The probe can be employed to detect F− quantitatively within certain concentration range, and the detection limit could be as low as 1.8 μM for tetrabutylammonium fluoride and 9 μM for NaF. With this probe, the tests for fluorine contents in tap water, water sample from Eyebrow Lake and several brands of toothpastes were successfully performed.

A 8-hydroxyquinoline-based derivative was designed and synthesized, which is shown to act as a highly sensitive fluorescent probe for formaldehyde in aqueous solution. Based on the photo induced electron transfer (PET) process, addition of formaldehyde greatly enhanced the fluorescence of the probe. Formaldehyde can be detected quantitatively in the concentration range from 900 nM to 1.3 × 10 −4 M and the detection limit on fluorescence response of the probe can be as low as 900 nM. The proposed method was successfully employed for preliminary application in several commercially available foods. Moreover, it has been successfully blended with a host polymer polyvinyl alcohol and the polymer containing the probe has been made in nanofiber form via electrospinning. The fluorescence of the nanofibers could also be enhanced after they were immersed into water solution of formaldehyde.

A dual-model fluorescent probe for formaldehyde has been developed. With excitation at 365 and 400 nm respectively and upon addition of formaldehyde, the fluorescent probe displays off-on fluorescence responses at 415 and 505 nm. Formaldehyde can be detected quantitatively in the concentration range from 0 to 2.7 × 10 −2 M and the detection limit on fluorescence response of the probe can be as low as 6 μM. The proposed method was successfully employed for preliminary application in several commercially available foods.

Based on 1,8-naphthalimide, a novel multifunctional colorimetric and fluorescent probe was designed and synthesized for basic pH and F− naked-eye and fluorescence recognition with high sensitivity and excellent selectivity. The probe can detect F− quantitatively in a concentration range of 0–16.7 μM and 0–20 μM in different methods, and the detection limit could be as low as 25 nM for F−. The detection of F− on chromatography plates and the preliminary application of the probe to detect fluoride contents in toothpaste have been successfully demonstrated.

It is important to develop probes for rapid, selective, and sensitive detection of highly toxic benzenethiols in both environmental and biological science. In this work, based on the selective cleavage reaction by benzenethiols under mild conditions, a novel naphthalimide-based fluorescent probe was designed and synthesized for the rapid recognition of benzenethiols with excellent selectivity and anti-interference over other various species including some nucleophilic species, aliphatic thiols, anions and metal ions. The limit of quantification (LOQ) value was 0–4.0 μM and the detection limit could be as low as 10.3 nM. The fluorescence enhancement of this probe upon addition of benzenethiol on test paper and the application of the probe for selective detection in water samples and living cells have been successfully demonstrated.

A novel naphthalimide-based fluorescent probe was designed and synthesized for thiol recognition with high sensitivity and excellent selectivity. The probe can detect thiol quantitatively in a concentration range of 0–6.0 μM and the detection limit could be as low as 1.6 nM. The fluorescence enhancement of this probe upon addition of Cys on test paper and the application of the probe for selective detection of intracellular Hcy, GSH and Cys have been successfully demonstrated.

Previous reports of fluorescent sensors for alcohols based on charge-transfer character of their excited state are based on mono-, di-, and tetra-phosphonate cavitands, which are capable of selecting analytes through shape/size selection and various specific H-bonding, CH-π, and cation-dipole interactions. To contrast, color changes based on absorption properties of the ground state are more suitable for direct observation with the naked eye. Three sensitive and selective colorimetric sensors for C1-C4 alcohols have been developed on the basis of alcohol-mediated ground-state intramolecular proton transfer. Reverse proton transfer induced by water achieves a fully reversible reaction. In addition, the solvent color indicates alcohol concentration.

A series of isomers were synthesized and identified, two isomers of which were developed as the dual-channel fluorescent probe toward Cd2+. BF2 dissociates from the probe upon reaction with CdCl2, demonstrating a new approach for sensing Cd2+.

Based on Rhodamine B, two Al 3þ /Cu 2þ -selective chemosensors were synthesized, which display a high selectivity for Al 3þ and Cu 2þ among environmentally and biologically relevant metal ions. A 1,8Naphthyridine modified rhodamine derivative can detect Al 3þ in ethanol with offeon fluorescence behavior, and detect Cu 2þ in aqueous solution with colorimetric method. The 8-Hydroxyquinoline-based rhodamine sensor can detect Al 3þ in ethanol with both offeon fluorescence and colorimetric methods. The detection mechanism involves a ring-opening process as a consequence of metal complex formation. Job’s plots and a time-of-flight mass spectral study indicated that each chemosensor chelated Al 3þ /Cu 2þ with 1:1 stoichiometry. The binding of the chemosensors and Al 3þ /Cu 2þ is chemically reversible by the addition of F � /EDTA solution, respectively.

Compared with other fluorescent probes, ratiometric fluorescence responses are more attractive because the ratio between the two emission intensities can be used to measure the analyte concentration and provide a built-in correction for environmental effects. A highly selective and sensitive ratiometric fluorescent probe for Fe 3+ was synthesized, which exhibits an enhanced fluorescence with a large red-shift in emission from 361 to 455 nm upon addition of Fe 3+ . The red-shift of the emission peak can be ascribed to the reformed orbital, and the increase of emission intensity may be ascribed to the inhibition of the rotation of C–C bonds between each two aromatic rings.

A fluorescent color/intensity changed fluoranthene derivative chemosensor for Fe3+ has been prepared and confirmed by 1H-NMR, 13C-NMR, HRMS, and crystal data, which displays a high selectivity and antidisturbance for Fe3+ among environmentally and biologically relevant metal ions. Fluorescence studies show that fluorescent emission peak blue shifts about 100 nm with fluorescent intensity enhancing 75-fold, indicating a Fe3+-selective dual-emission behavior. Further study demonstrates the detection limit on fluorescence response of the sensor to Fe3+ is down to 10−7 M range. The fluorescence signals of chemosensor can be restored with o-phenanthroline, showing the binding of chemosensor and Fe3+ is really chemically reversible.

A ‘switching-on’ fluorescent chemosensor for the selective and sensitive signaling of intraceullar Cr3+ has been designed and synthesized exploiting the guest-induced prohibition of the photoinduced electron transfer process between naphthyridine moiety and 7,10-diphenylfluoranthene moiety, the system shows a Cr3+-selective chelation-enhanced fluorescence response not only in ethanol but in cell.

A fluoranthene based fluorescent chemosensor 2-(7,10-diphenyl-fluoranthen-8-yl)-pyridine ( B ) which shows excellent selectivity towards Fe 3+ is described. Upon addition of FeCl 3 into the ethanol solution of B , it exhibits a great decrease of emission intensity against environmental and biological samples. Further experiments demonstrate that Fe(NO 3 ) 3 has the similar influence as FeCl 3 on the emission spectra of B while Fe 2 (SO 4 ) 3 has no influence on that of B .

A highly fluorescent chemosensor based on 1,8-naphthyridine with high sensitivity and selectivity toward Ni(2+)/Cu(2+) over other cations both in aqueous solution over a wide pH range (4-10) and in cellular environments was developed. Counteranions such as acetate, sulfate, nitrate, and perchlorate have no influence on the detection of such metal ions. Ethylenediamine showed high selectivity toward the in situ-prepared Cu(2+) complex over the Ni(2+) complex, which can be applied to distinguish Ni(2+) and Cu(2+). The Ni(2+)-induced fluorescence on-off mechanism was revealed to be mediated by intramolecular charge transfer from the metal to the ligand, while that by Cu(2+) involves intramolecular charge transfer from the ligand to the metal, as confirmed by picosecond time-resolved fluorescence spectroscopy and time-dependent density functional theory calculations.

A novel fluorescent chemodosimeter based on 1,8-naphthyridine exhibits high selectivity to Zn(2+) and Cu(2+). When 1-(7-acetamino-1,8-naphthyridyl)-2-(6-diacetaminopyridyl)ethene was mixed with CuCl2, hydrolysis of the acetamino group catalyzed by Cu(2+) complex was first observed. Resulting from coordination and hydrolysis catalyzed by the corresponding complex of Zn(2+) or Cu(2+), the highly effective fluorescent detection of Zn(2+) and Cu(2+) is realized with Zn(2+)-selective dual-emission and Cu(2+)-selective ON-OFF behavior.

A multifunctional fluorescent chemosensor for Al3+ and Cu2+ has been developed, which displays a dual-model and on–off fluorescence response upon addition of Al3+ and Cu2+ respectively. The fluorescence signals of the chemosensor can be restored with F− and ethylenediaminetetraacetic acid (EDTA) disodium for Al3+ and Cu2+, showing that the binding of the chemosensor and Al3+/Cu2+ is chemically reversible. The ‘in situ’ prepared Al3+ complex (1·Al) showed high selectivity toward F−, which can be applied to distinguish F− from Cl−, Br− and I−. Moreover, the fluorescence emission intensity of the ‘in situ’ prepared Cu2+ complex (1·Cu) is increased with the addition of Al3+, demonstrating that 1·Cu could be a good off–on Al3+ sensor candidate.

An "off-on" Zn(2+) and "on-off" Cu(2+) fluorescent chemosensor C was designed and synthesized. The binding of C and Zn(2+)/Cu(2+) is chemically reversible by the addition of EDTA disodium solution; moreover, the fluorescence emission signal of ZnC decreased with the addition of Cu(2+), demonstrating that ZnC could detect Cu(2+)via metal displacement.

A highly sensitive and selective 1,8-naphthyridine-based fluorescent chemodosimeter for Zn(2+) has been designed, synthesized, and applied to the detection of intracellular Zn(2+).