A molecular design towards sulfonyl aza-BODIPY based NIR fluorescent and colorimetric probe for selective cysteine detection

Cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) are essential biothiols for cellular growth, metabolism, and maintenance of a biological system. Thus, the detection of biothiols is highly important for early diagnosis and evaluation of disease progression. In this article, a series of sulfonyl aza-BODIPYs was synthesized, characterized, and examined by ¹H-NMR, ¹³C-NMR, crystallization, photophysical properties and DFT calculation. Among these structures, a fluorescent probe, BDP-1, exhibited selective detection of Cys among various biothiols via nucleophilic aromatic substitution and typical size of Cys molecules. BDP-1 showed color change and near-infrared (NIR) fluorescence enhancement after reaction with Cys to generate BDP-OH, confirmed by HRMS. The red shift of absorption wavelength showed a similar tendency resulting in time-dependent density functional theory (TD-DFT). Furthermore, the calculated detection limit of BDP-1 toward Cys was 5.23 μM. This probe facilitates the colorimetric and fluorescent detection of Cys over other biothiols.

A new fluorescent and colorimetric probe based-on sulfonyl aza-BODIPY (BDP-1–3) are designed and synthesized for selective cysteine detection.     

A highly selective TPE-based AIE fluorescent probe is developed for the detection of Ag+

The detection of Ag⁺ in the environment is very important to determine the level of pollution from silver complexes, which have caused various human health problems. Herein, an aggregation-induced emission (AIE) chromophore (tetraphenylethane, TPE) attached to a benzimidazole group (tetra-benzimidazole, TBI–TPE) is synthesized and utilized to detect Ag⁺ in the environment. The strong chelating effect between the benzimidazole group and Ag⁺ leads to the formation of aggregates, and strong yellow fluorescence signals were observed after adding Ag⁺ into a TBI–TPE solution. The stoichiometry of the complex of TBI–TPE and Ag⁺ was established to be 1 : 2 using photochemical and mass spectra measurements. The detection limit of the Ag⁺ assay is 90 nM with a linear range from 100 nM to 6 μM. This study provides a facile method to determine Ag⁺ in real environmental samples with satisfactory results.

We develop a highly selective TPE-based AIE fluorescent probe containing a benzimidazole group for the detection of Ag⁺.     

A dual responsive fluorescent probe for selective detection of cysteine and bisulfite and its application in bioimaging

A coumarin-based dual responsive fluorescent probe with a simple structure was developed for the detection of Cys and HSO₃⁻. Under simulated physiological conditions, Cou-F displayed an on–off fluorescence response to Cys at 521 nm and an off–on fluorescence response to HSO₃⁻ at 500 nm. Furthermore, Cou-F had the advantages of high sensitivity, strong specificity and rapid response. The detection limits of Cou-F toward Cys and HSO₃⁻ were 0.54 μM and 0.65 μM, respectively. Cou-F enabled high selective responses to Cys and HSO₃⁻ over other biologically related species. The response times of Cou-F toward Cys and HSO₃⁻ were 80 s and 100 s. The fluorescence imaging of Cys and HSO₃⁻ was achieved in living RAW246.7 cells.

A coumarin-based dual responsive fluorescent probe with a simple structure was developed for the detection of Cys and HSO₃⁻.     

A highly selective colorimetric fluorescent probe for detection of Hg2+ and its application on test strips

An efficient fluorescent probe Pyr-Rhy based on pyrazole was developed, which can detect Hg²⁺ in water. Its fluorescence properties were studied by UV-vis and fluorescence spectroscopy, and the study results indicated that this probe can selectively detect Hg²⁺via complexation reaction, and then cause a remarkable color change from colorless to pink and a strong fluorescence enhancement can be observed. Furthermore, this probe showed high sensitivity with the detection limit down to 2.07 × 10⁻⁸ M, and its stoichiometric ratio toward Hg²⁺ ions was 1 : 1. The sensing mechanism was investigated by Job''s plot ¹H NMR titrations, and FT-IR spectra analysis, which demonstrated a chelation-enhanced fluorescence (CHEF) mechanism. More importantly, obvious color changes of sensor Pyr-Rhy can be observed when it was impregnated on filter paper testing strips and immersed in Hg²⁺ solution (water as solution), indicating its potential application for trace Hg²⁺ detection in environmental samples.

An efficient fluorescent probe Pyr-Rhy based on pyrazole was developed, which can detect Hg²⁺ in water.     

A simple two-photon turn-on fluorescent probe for the selective detection of cysteine based on a dual PeT/ICT mechanism

Herein, a simple two-photon turn-on fluorescent probe, N-(6-acyl-2-naphthayl)-maleimide (1), based on a dual PeT/ICT quenching mechanism is reported for the highly sensitive and selective detection of cysteine (Cys) over other biothiols. The probe was applied in the two-photon imaging of Cys in cultured HeLa cells, excited by a near-infrared laser at 690 nm.

N-(6-acyl-2-naphthayl)-maleimide (1) is a simple two-photon fluorescent probe with selectivity for cysteine, based on a thiol-Michael-addition-transcyclization cascade and dual PeT/ICT quenching mechanism.

Cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) are structurally similar biothiols, but their biological functions are quite different from one another.^1–6 Among these biothiols, Cys functions as one of the twenty-one amino acids for peptide and protein synthesis, and Cys deficiency is also associated with certain disease symptoms.^7–10 Methods for the selective detection and differentiation of Cys among different biothiols, including high performance liquid chromatography (HPLC),¹¹ capillary electrophoresis,¹² electrochemical assay,¹³ UV-vis spectroscopy,¹⁴ and fluorescence-based methods,^15–17 are important for its biological studies. Recently, fluorescent probes have attracted much attention as vital chemical biology tools due to their high sensitivity, convenient operation, and real-time imaging capabilities.^18–20 A number of Cys-selective fluorescent probes have been reported,²¹ which utilize Cys-selective recognition groups such as aldehydes,^11,22 acrylates,²³ thioesters,²⁴ and electron-deficient aromatic halides^25–27 in their structures. However, many of them have relatively long response times and low sensitivity due to a slow cyclization process. In addition, fluorescent probes with high selectivity for Cys over Hcy are difficult to achieve because they differ by only one methylene group.²⁸ Recently, we reported that N-(N′-butyl-1,8-naphthalimide-4-yl)-maleimide, containing a single maleimide group as the recognition group, is a fast, sensitive, and selective fluorescent probe for Cys based on a dual photo-induced electron transfer (PeT) and photo-induced intramolecular charge transfer (ICT) quenching mechanism.²⁸ Different from many other maleimide-based fluorescent probes that only undergo a PeT mechanism,¹⁵ the additional ICT quenching mechanism keeps the 1,8-naphthalimide (NAP) fluorophore in the thiol-Michael adduct in a low fluorescence emission state due to the strong electron-withdrawing effects of the succinimide group at its 4-position. Then, a subsequent transcyclization step, involving the formation of a six-membered thiomorpholinone ring and cleavage of a five-membered succinimide ring, converts the non-fluorescent thiol-Michael adduct into the major fluorescent product, in which the ICT quenching is removed, resulting in a drastic fluorescence turn-on response.²⁸ A similar transcyclization process and the simultaneous removal of ICT quenching allowed us to design a NAP-based turn-on fluorescent probe for γ-glutamyltranspeptidase²⁹ and a coumarin-based turn-on fluorescent probe with dual recognition groups and dual cyclization for the selective detection of Cys.³⁰ In addition, another NAP-based dual PeT/ICT probe was recently reported by Meka and Heagy for the detection of hydrogen sulfide, although two recognition groups instead of one were adopted in their probe to achieve the dual quenching mechanism.³¹Our previous work and that of other groups has demonstrated that the combination of PeT and ICT mechanisms is particularly suitable for the design of fluorescent probes with a significant fluorescence turn-on response.^30–33 However, many of these probes have a short excitation wavelength in the UV or visible range, which is not optimal for biological applications due to enhanced phototoxicity and/or autofluorescence.^34,35 Considering that two-photon fluorescence imaging has advantages such as the excitation process being carried out by a near-infrared (NIR) laser that has a reduced cell toxicity and low fluorescence background,³⁶ in this work, we aimed to introduce a similar dual PeT/ICT quenching mechanism to the known two-photon fluorophore 6-acyl-2-naphthylamine^37–39 in order to design a simple maleimide-based two-photon fluorescent probe, 1, for the selective detection of Cys over Hcy and GSH. It was also tested to determine whether it is a turn-on fluorescent probe with high sensitivity and selectivity, which reacts with Cys via a fast two-step thiol-Michael addition and transcyclization cascade reaction.²⁸ The structure of probe 1 is shown in Fig. 1. It has a maleimide group at its 2-position, which promotes the PeT quenching effect. It also has an additional electron-withdrawing methylcarbonyl group at its 6-position to ensure a pull–pull ICT quenching effect.Open in a separate windowFig. 1Design rationale of the fluorescent probe 1 for the selective turn-on detection of Cys over Hcy and GSH.Probe 1 was conveniently synthesized from 6-acyl-2-naphthylamine (3)³⁹ in a two-step process with a total yield of 38% (see Scheme S1 in the ESI^†). First, the amine 3 was reacted with maleic anhydride to form the maleic amide acid 4. Then, the amide acid 4 was cyclized to afford the maleimide 1 in the presence of acetic anhydride (see the ESI^† for more details).We then investigated the absorption and fluorescence emission response of the probe towards just 1 equiv. of Cys. The time-dependent absorption spectra upon the addition of 1 equiv. of Cys are shown in Fig. 2a. Probe 1 has a maximum absorption peak at 292 nm. Upon addition of Cys, the maximum absorption peak shifts to 314 nm, a red-shift of 22 nm. Notably, an isosbestic point can be seen at 295 nm after 2 min, indicating the formation of an intermediate within 2 min, which is then converted into the final product. The UV spectral changes supported the presence of a proposed cascade reaction sequence for the fast formation of a thiol-Michael adduct intermediate, which then underwent a relatively slow intramolecular transcyclization process to give the final product. From time-dependent fluorescence emission studies (Fig. 2b), probe 1 was found to have almost no fluorescence emission due to dual PeT and ICT quenching effects. Upon the addition of 1 equiv. of Cys, a drastic turn-on fluorescence response (a >3000 fold increase) was observed at 446 nm (see Fig. S1b in the ESI^†). The fluorescence intensity at 446 nm reached its maximum value after around 30 min indicating that the cascade reaction finished in about 30 min (Fig. 2b, and S2a in the ESI^†). The pseudo-first-order reaction kinetic constant based on the fluorescence enhancement was calculated as 0.123 min⁻¹ (half-time = 5.64 min, Fig. S2b in the ESI^†), indicating an overall fast cascade reaction. Fluorescence titration experiments using an increasing amount of Cys from 0 to 4.0 equiv. over 30 min showed a steady increase in the fluorescence intensity and the maximum intensity was reached at exactly 1.0 equiv. of Cys. Further Cys addition did not increase the fluorescence intensity, indicating that probe 1 reacts with Cys in a 1 : 1 molar ratio (Fig. 2c and S3 in the ESI^†), which was also supported by the Job plot (see Fig. S4 in the ESI^†). From the linear relationship of the fluorescence intensity at 446 nm versus the Cys concentrations, the detection limit of probe 1 (2 μM) for Cys was calculated as 1.4 nM (S/N = 3, Fig. 3d), indicating that 1 is a highly sensitive probe for Cys. Moreover, the probe showed excellent selectivity for the detection of Cys over many other species (Fig. 2e, and S5 in the ESI^†), including the structurally similar thiols Hcy, GSH, and N-acetylcysteine (NAC). The fluorescence intensity at 446 nm for 1 equiv. of Cys was significantly higher (12.2-fold, 9.1-fold, and 17.7-fold, respectively) than that of 10 equiv. of Hcy, GSH, or NAC. To further confirm the reaction mechanism, the reaction product, 2, from the reaction of probe 1 with Cys, was isolated and its structure was confirmed using ¹H nuclear magnetic resonance (NMR) spectroscopy, ¹³C NMR spectroscopy, 2D-rotating-frame nuclear Overhauser effect spectroscopy (ROESY), and high-resolution mass spectrometry (HRMS) (see the ESI for more details^†). The fluorescence quantum yields of probe 1 and product 2 were measured as 0.002 and 0.782, respectively (see the ESI for more details^†). Therefore, the formation of the transcyclization product 2 was determined to be responsible for the observed fluorescence turn-on response. For the other thiols, the transcyclization steps of the thiol-Michael adducts were much slower, resulting in the observed high selectivity. Overall, we have shown here that probe 1 is a highly sensitive and selective turn-on fluorescent probe for Cys.Open in a separate windowFig. 2(a) Time-dependent UV-vis spectra of probe 1 (10 μM) upon the addition of 1 equiv. of Cys (a spectrum was recorded every 2 minutes); (b) time-dependent fluorescence emission spectra of probe 1 (2 μM) upon the addition of 1 equiv. of Cys (a spectrum was recorded every 3 minutes); (c) time-dependent fluorescence emission intensity at 446 nm of probe 1 (2 μM) upon addition of Cys (0 to 4 equiv.); (d) a linear relationship of the fluorescence intensity at 446 nm versus the Cys concentration (0.2–2.0 μM); (e) fluorescence response of probe 1 (2 μM) at 446 nm toward various species in PBS buffer (10 mM, pH 7.4): (1) blank; (2) Cys; (3) Hcy; (4) GSH; (5) NAC; (6) valine; (7) glycine; (8) isoleucine; (9) lysine; (10) leucine; (11) histidine; (12) asparagine; (13) methionine; (14) proline; (15) serine; (16) alanine; (17) threonine; (18) arginine; (19) glutamine; (20) aspartic acid; (21) glutamic acid; (22) tyrosine; (23) tryptophan; (24) phenylalanine; (25) glucose; (26) H₂O₂; (27) Na⁺; (28) K⁺; (29) Ca²⁺; (30) Mg²⁺; (31) Fe³⁺; (32) Fe²⁺; (33); Cu²⁺; (34) Zn²⁺ (All measurements were made in 10 mM PBS buffer, pH 7.4, 25 °C, and λ_ex = 314 nm).Open in a separate windowFig. 3Two-photon fluorescence images (b, e, h, k) of HeLa cells collected at 410–510 nm (blue to cyan-blue, λ_ex = 690 nm), the corresponding bright field view (a, d, g, j), and overlap of the fluorescence channel and the bright field view (c, f, i, l) after different treatments: (a–c) the cells were pretreated with 0.5 mM of N-ethylmaleimide (NEM) for 30 min and then incubated with 10 μM of probe 1 for 30 min; (d–f) cells were first pretreated with 0.5 mM of NEM for 30 min, then after addition of 1 mM of Cys were incubated for 30 min, and finally, incubated with 10 μM of probe 1 for 30 min (scale bar = 10 μm); the conditions for (g–i) and (j–l) were similar to those of (d–f), except that 10 μM of Hcy and 10 μM of GSH were used instead of 10 μM of Cys.Encouraged by the fast, selective, and sensitive in vitro fluorescence response of probe 1 for the detection of Cys, we further evaluated its potential use as a two-photon imaging agent for Cys in biological systems, such as in living cells. The fluorescence response of probe 1 towards Cys at different pH values was evaluated and a suitable pH range for Cys detection was determined to be 7.0 to 10.0, which is a good range for cell imaging applications because physiological conditions have a pH of around 7.4 (see Fig. S7 in the ESI^†). HeLa cells were then pretreated with N-ethylmaleimide (NEM, 0.5 mM) for 30 min to remove the endogenous cellular thiols, and incubated with Cys (1 mM), Hcy (1 mM), or GSH (1 mM), respectively for 30 min to increase the specific thiol levels. The samples were then further incubated with probe 1 (10 μM) for 30 min and were then washed with PBS buffer before two-photon fluorescence cell images and the corresponding bright-field view images were taken (Fig. 3(d–l)). Control images were also taken for samples pretreated with NEM (0.5 mM) and then incubated with probe 1 (10 μM) (Fig. 3a–c). Only cells pretreated with NEM and then Cys showed a distinctive blue fluorescence (Fig. 3e). The above cell imaging studies clearly demonstrated that probe 1 is capable of the selective detection and imaging of intracellular Cys over Hcy and GSH in living cells by two-photon fluorescence imaging with low background fluorescence interference.     

A highly sensitive and selective fluorescent probe without quencher for detection of Pb2+ ions based on aggregation-caused quenching phenomenon

Lead is a highly toxic heavy metal, and various functional nucleic acid (FNA)-based biosensors have been developed for the detection of Pb²⁺ in environmental monitoring. However, most fluorescence biosensors that have been reported were designed on the basis of a double-labeled (fluorophore and quencher group) DNA sequence, which not only involved an inconvenient organic synthesis but also restricted their wider use in practical applications. Here, we utilized a G-rich DNA sequence as a recognition probe and conjugated fluorene (CF) to develop a fluorescence sensor without a quencher based on the aggregation-caused quenching (ACQ) effect. In the presence of Pb²⁺, the degree of aggregation of CF was reduced because Pb²⁺ induced the formation of a G-quadruplex structure of the CF-DNA probe, and the fluorescence signal increased with the concentration of Pb²⁺ (0–1 μM), with a limit of detection of 0.36 nM. This fluorescent probe without a quencher enables the sensitive and selective detection of Pb²⁺. On the basis of these advantages, the CF-DNA probe represents a promising analytical method for detecting Pb²⁺.

Fluorescent probe with only a fluorophore but no quencher for detecting Pb²⁺ on the basis of the aggregation-caused quenching (ACQ) phenomenon.     

A novel and fast responsive turn-on fluorescent probe for the highly selective detection of Cd2+ based on photo-induced electron transfer

A novel, highly sensitive and fast responsive turn-on fluorescence probe, 2,2′-((1E,1′E)-((1,10-phenanthroline-2,9-diyl)bis(methanylylidene)) bis(azanylylidene)) diphenol (ADMPA), for Cd²⁺ was successfully developed based on 2,9-dimethyl-1,10-phenanthroline and o-aminophenol. ADMPA showed a remarkable fluorescence enhancement toward Cd²⁺ against other competing cations, owing to the suppression of the photo-induced electron transfer (PET) and CH N isomerization. A good linear relationship (R² = 0.9960) was obtained between the emission intensity of ADMPA and the concentration of Cd²⁺ (0.25–2.5 μM) with a detection limit of 29.3 nM, which was much lower than that reported in literature. The binding stoichiometry between ADMPA and Cd²⁺ was 2 : 1 as confirmed by the Job''s Plot method, which was further confirmed by a ¹H NMR titration experiment. Moreover, the ADMPA probe was successfully applied to detect Cd²⁺ in real water samples with a quick response time of only 6.6 s, which was about 3–40 times faster than the reported cadmium ion probe.

A novel, highly sensitive and fast responsive turn-on fluorescence probe ADMPA for Cd²⁺ was successfully developed based on 2,9-dimethyl-1,10-phenanthroline and o-aminophenol.     

A highly sensitive and selective fluorescent probe for quantitative detection of Al3+ in food,water, and living cells

Three novel β-pinene-based fluorescent probes 2a–2c were designed and synthesized for the selective detection of Al³⁺. Probe 2a showed higher fluorescence intensity toward Al³⁺ than the other two compounds. Probe 2a determined the concentration of Al³⁺ with a rapid response time (45 s), wide pH range (pH = 1–9), excellent sensitivity (LOD = 8.1 × 10⁻⁸ M) and good selectivity. The recognition mechanism of probe 2a toward Al³⁺ was confirmed by ¹H NMR, HRMS and DFT analysis. Probe 2a was successfully used as a signal tool to quantitatively detect Al³⁺ in food samples and environmental water samples. Furthermore, probe 2a was successfully utilized to label intracellular Al³⁺, indicating its promising applications in living cells.

Probe 2a exhibiting high sensitivity, good selectivity, wide pH range, lower detection limit, and rapid detection for Al³⁺, probe 2a was applied for the successful detection of Al³⁺ in water samples, food samples and HeLa cells.     

A selective and sensitive near-infrared fluorescent probe for real-time detection of Cu(i)

The disruption of copper homeostasis (Cu⁺/Cu²⁺) may cause neurodegenerative disorders. Thus, the need for understanding the role of Cu⁺ in physiological and pathological processes prompted the development of improved methods of Cu⁺ analysis. Herein, a new near-infrared (NIR) fluorescent turn-on probe (NPCu) for the detection of Cu⁺ was developed based on a Cu⁺-mediated benzylic ether bond cleavage mechanism. The probe showed high selectivity and sensitivity toward Cu⁺, and was successfully applied for bioimaging of Cu⁺ in living cells.

The disruption of copper homeostasis (Cu⁺/Cu²⁺) may cause neurodegenerative disorders.     

A novel near-infrared fluorescent probe for highly selective recognition of hydrogen sulfide and imaging in living cells

A novel near-infrared fluorescent probe (L) based on a 1,4-diethyl-1,2,3,4-tetrahydro-7H-pyrano[2,3-g]quinoxalin-7-one scaffold has been synthesized and characterized. Probe L displays highly selective and sensitive recognition to H₂S over various anions and biological thiols with a large Stokes shift (125 nm) in THF/H₂O (6/4, v/v, Tris–HCl 10 mM, pH = 7.4). This probe exhibits turn-on fluorescence for H₂S through HS⁻ induced thiolysis of dinitrophenyl ether. Confocal laser scanning micrographs of MCF-7 cells incubated with L confirm that L is cell-permeable and can successfully detect H₂S in living cells.

A novel “off–on” fluorescent probe (L) for H₂S detection with NIR emission and imaging H₂S in living cells.     

Green synthetic nitrogen-doped graphene quantum dot fluorescent probe for the highly sensitive and selective detection of tetracycline in food samples

Tetracycline (TC) is a broad-spectrum antibiotic. When humans consume too much food containing tetracycline residues, it can be a serious health hazard. Therefore, it is essential to develop a strategy to detect TC. In this study, we prepared light blue-green luminescent nitrogen-doped graphene quantum dots (N-GQDs) by a hydrothermal method using the natural products potato straight-chain starch and urea as precursors; the fluorescence quantum yield of the prepared N-GQDs was 5.2%. We investigated the detection of tetracycline (TC) by this N-GQD fluorescent sensor based on the internal filtration effect (IFE) of TC on N-GQDs. The reaction is green, simple and no other contaminating products are present. A good linear relationship was established between the relative fluorescence intensity ratio of the system and the logarithm of the TC concentration of 2.5 × 10⁻¹⁰ to 5 × 10⁻⁶ M (R² = 0.9930), with a detection limit of 9.735 × 10⁻¹³ M. The method has been used to analyze TC in three real food samples (whole milk, skim milk, honey) with low detection limits (3.750 × 10⁻¹¹ to 2.075 × 10⁻⁹ M), wide linear range, and satisfactory recoveries of 93.80–109.20% were obtained. In conclusion, the proposed method is a green, rapid, highly sensitive and selective method for the detection of tetracycline in real food samples, demonstrating the potential application of N-GQDs in food detection.

Tetracycline (TC) is a broad-spectrum antibiotic.     

A novel microporous Tb-MOF fluorescent sensor for highly selective and sensitive detection of picric acid

A new three-dimensional metal–organic framework (MOF) sensor with molecular formula (C₂H₆NH₂)₂[Tb₂(ptptc)₂(DMF)(H₂O)]·DMF·6H₂O (complex 1) has been constructed from terphenyl-3,3′,5,5′-tetracarboxylic acid (H₄ptptc) and terbium nitrate under solvothermal conditions. The structure of complex 1 was characterized by single-crystal X-ray diffraction analysis (XRD), elemental analysis, IR spectroscopy and thermogravimetric (TG) analysis, and the purity was further confirmed by powder X-ray diffraction (PXRD) analysis. XRD analysis reveals that complex 1 crystallizes in a triclinic system P$\bar{1}$ space group and consists of a three-dimensional anionic network which has one-dimensional channels. Fluorescence titration experiments showed that complex 1 displayed real-time, highly selective and sensitive fluorescence quenching behavior towards picric acid with a nanomolar scale experimental detection limit (100 nM). Recycling titration experiments suggested that the as-synthesized probe has good reversibility and can be used for at least five cycles in fluorescence titration experiments without obvious fluorescence intensity reduction or framework structure destruction. Furthermore, the high selectivity and sensitivity as well as good recyclability of complex 1 make it a potential fluorescent sensor for picric acid.

A new three-dimensional reusable Tb-MOF fluorescent sensor for the highly selective and sensitive detection of picric acid was reported.     

Highly selective and sensitive fluorescent probe for the rapid detection of mercury ions

Mercury (Hg) is one of the major toxic heavy metals, harmful to the environment and human health. Thus, it is significantly important to find an easy and quick method to detect Hg²⁺. In this study, we designed and synthesized a simple fluorescent probe with excellent properties, such as high sensitivity and selectivity, rapid response, and outstanding water solubility. When Hg²⁺ (5 μM) was added to the probe solution, it exhibited a very large fluorescent enhancement (about 350-fold stronger than the free probe) with the help of hydrogen peroxide (H₂O₂). Probe HCDC could quantitatively detect Hg²⁺ in the range of 0–10 μM using the fluorescence spectroscopy method and the detection limit was measured to be about 0.3 nM (based on a 3σ/slope). Analytical application was also studied, and the probe HCDC exhibited excellent response to Hg²⁺ with the addition of H₂O₂ in real water samples. So, our proposed probe HCDC provided a practical and promising method for determining Hg²⁺ in the environment.

A novel water-soluble, highly selective and sensitive rapid-response fluorescent probe was developed to monitor mercury ions in real water samples.     

An anthracenecarboximide-guanidine fluorescent probe for selective detection of glyoxals under weak acidic conditions

An anthracenecarboximide-guanidine based turn-on fluorescent probe ANC-DCP-1 for selective detection of glyoxals (methylglyoxal and glyoxal, GOS) over formaldehyde under weak acidic conditions around pH 6.0 was reported. The probe showed great potential in studying relative GOS levels in weak acidic biological fluids such as in urine for diabetic diagnosis and prognosis, and also found application in the food industry such as for fast unique manuka factor (UMF) scale determination of Manuka honey.

Formation of 5-membered dihydroxyimidazolidines with increased deprotonation at around pH 6.0 and enhanced intramolecular charge transfer for turn-on fluorescence detection of glyoxals.     

A novel fluorescent off–on probe for the sensitive and selective detection of fluoride ions

A highly sensitive and selective fluorescent probe for fluoride ions has been developed by incorporating the dimethylphosphinothionyl group as a recognition moiety into the fluorophore of coumarin. The detection mechanism is based on the fluoride ion-triggered cleavage of the dimethylphosphinothionyl group, followed by the release of coumarin, which leads to a large fluorescence enhancement at 455 nm (λ_ex = 385 nm). Under the optimized conditions, the fluorescence enhancement of the probe is directly proportional to the concentration of fluoride ions in the range of 0–30 μM with a detection limit of 0.29 μM, which is much lower than the maximum content of fluoride ions guided by WHO. Notably, satisfying results have been obtained by utilizing the probe to determine fluoride ions in real-water samples and commercially available toothpaste samples. The proposed probe is rather simple and may be useful in the detection of fluoride ions in more real samples.

A sensitive and selective fluorescent off–on probe is developed for fluoride ion detection, and its applicability has been demonstrated by determining fluoride ions in real-water samples and toothpaste samples.     

A selective hybrid fluorescent sensor for fructose detection based on a phenylboronic acid and BODIPY-based hydrophobicity probe

Fructose is widely used in the food industry. However, it may be involved in diseases by generating harmful advanced glycation end-products. We have designed and synthesized a novel fluorescent probe for fructose detection by combining a phenylboronic acid group with a BODIPY-based hydrophobicity probe. This probe showed a linear fluorescence response to d-fructose concentration in the range of 100–1000 μM, with a detection limit of 32 μM, which is advantageous for the simple and sensitive determination of fructose.

We have designed and synthesized a novel fluorescent probe for fructose detection through hydrophobic interactions by combining a phenylboronic acid group and a BODIPY-based hydrophobicity probe with a detection limit of 32 μM.     

A fluorescent microsensor for the selective detection of bifenthrin

Based on the fluorescence quenching phenomenon, a smart fluorescent microsensor was synthesized. The bifenthrin (BI) microsensor inherited the high selectivity of molecular imprinted polymers (MIPs) and the excellent fluorescence properties of aqueous CdTe quantum dots (QDs). Aqueous CdTe QDs are functionalized by octadecyl-4-vinylbenzyl-dimethyl-ammonium chloride (OVDAC). A type of functional monomer, 4-vinylphenylboronic acid (VPBA), was used and its boronic acid groups could covalently combine with a cis-diol compound for direct imprinting polymerization. The OVDAC-functionalized aqueous CdTe QDs were used as solid supports and auxiliary monomers. Under optimal conditions, experimentation showed that BI had a linear detection range of 10 to 300 μmol L⁻¹ with a correlation coefficient of 0.9968 and a high imprinting factor (IF) of 4.53. In addition, the prepared MIP-OVDAC/CdTe QDs were successfully used to detect BI in water samples. Therefore, this work provided a highly selective and sensitive fluorescence probe for the detection of BI. In addition, the fluorescence probe could be used to detect other targets by changing the functional monomers.

Based on the fluorescence quenching phenomenon, a smart fluorescent microsensor was synthesized.     

Triple-FRET multi-purpose fluorescent probe for three-protease detection

A new, robust and reliable methodology for three-protease screening in a single-enzyme mode has been developed and verified, employing a multi-purpose peptide probe with three selectively cleavable sites furnished with four fluorophores. A triple-FRET-based single-excitation quadruple-emission concept for unambiguous sensing of trypsin, chymotrypsin and caspase-8 in the lowest detectable concentrations of 0.5 ng mL⁻¹, 0.2 μg mL⁻¹, and 2 U mL⁻¹, respectively, has been applied and graphically depicted. Then the developed 4-dye probe has been also studied from the perspective of simultaneous two-protease screening, which was found only partially feasible, primarily due to unselective chymotrypsin cleavage.

A triple-FRET four-dye system for detection of three proteases has been developed and verified.     

A novel quinoline derivative as a highly selective and sensitive fluorescent sensor for Fe3+ detection

In this research, a novel selective and sensitive fluorescent sensor for detecting Fe³⁺ was designed and synthesized; it revealed an obvious fluorescence quenching effect upon addition of Fe³⁺, and possessed the quantitative analysis ability on account of the formation of a 1 : 1 metal–ligand complex. Furthermore, the density functional theory calculations were utilized to study the molecular orbitals as well as the spatial structure. Simultaneously, the cell experiments and zebra fish experiments verified the application value of the sensor in the biological field.

A novel selective and sensitive fluorescent sensor for detecting Fe³⁺ was designed and synthesized; it revealed obvious fluorescence quenching effect upon addition of Fe³⁺, and possessed the quantitative analysis ability on account of the formation of a 1 : 1 metal–ligand complex.