As an emerging class of fluorescent probes, metal–organic frameworks (MOFs) have recently received great interest owing to their fascinating functional properties, intriguing tunable structures, high selectivity and good sensitivity. Herein, we present a novel strategy based on the application of a facile water-stable MOF {[Cd(μ3-abtz)·2I]}n (Abtz–CdI2–MOF, abtz = 1-(4-aminobenzyl)-1,2,4-triazole) using powerful solvo-thermal synthetic techniques, which can serve as an “off–on” fluorescent switch for the label-free detection of dopamine (DA) without any additional surface modification and functionalization. The fluorescence signal of Abtz–CdI2–MOF can be efficiently quenched by KMnO4, and then restored by DA in an “off–on” mode. The possible quenching mechanism should be static quenching resulting from the ground state recombination between Abtz–CdI2–MOF and MnO4−. Further when DA is added, MnO4− can be gradually released from the hybrid system, and bring about ground state complex decomposition, which is triggered by competitive interactions between Abtz–CdI2–MOF and the reduced iodine ions. Therefore, the fluorescence signal can be gradually turned “on”. The relative intensity of restored fluorescence is proportional to the concentration of DA in the wide linear range of 0.25–50 μM with a low detection limit of 57 nM (S/N = 3). Additionally, some potentially foreign interfering substances, such as ascorbic acid (AA) and uric acid (UA), glucide, amino acids and metal ions did not affect DA determination significantly. The proposed strategy was also successfully applied for DA determination in biological samples with satisfactory recoveries from 94.5% to 102%.

•A facile and green strategy.•Synthesis of the 5 nm sized UCNPs.•Enhanced up-conversion green emission.•The as-prepared UCNPs in pure β phase.•Functionalization via folic acid coating.•Application for Hela cancer cell targeting imaging.5 nm intense green emission NaYF4:Yb/Er upconversion nanoparticles (UCNPs) with pure β phase was synthesized with a simple “green” strategy for the first time. Traditional organic solvothermal method is often applied to prepare the high-quality and uniform UCNPs, but the preparation of lanthanide-oleate complexes is laborious as heating and multistep post-treatment for purification are often required. The water-alcohols solvothermal method is environmentally friendly, but the fabricated UCNPs have big size, poor biocompatibility and high cytotoxicity, which limited their application for cell imaging. Herein, NaYF4:Yb/Er UCNPs were prepared with rare-earth nitrates RE(NO3)3 (RE=Y0.80 Yb0.18 Er0.02) as precursors and diethylene glycol (DEG)/ethylene glycol (EG)/water as the solvent. A facile green solvothermal method with the temperature being controlled at 300 °C was developed. The as-prepared NaYF4:Yb/Er UCNPs were characterized and were found to have enhanced UC emission and controllable particle size. The as-prepared UCNPs were further functionalized via folic acid coating for the targeted imaging and improved bio- compatibility. It was made the UCNPs potential for upconversion bioimaging of living cells by the strong upconversion luminescence, the excellent biocompatibility, and the super-small size. The good colloidal stability and low cell cytotoxicity of the as-prepared UCNPs and the developed synthesis protocol might advance both the fields of UCNPs and biomolecule-based nanotechnology for future studies.A super small 5 nm sized Yb3+/Er3+ doped fluoride UCNPs was prepared with intense upconversion emission by enhancing the solvothermal reaction temperature and using DEG/EG/Water as the solvent.

•First MOF-Cd-abtz fluorescent probe for direct detection of ascorbic acid.•The static quenching mechanism of MOF-Cd-abtz as fluorescent probes.•Detection of ascorbic acid by MOF-Cd-abtz fluorescent probe in biological liquid.Nowadays metal-organic frameworks (MOFs) have aroused great interest because of their fascinating tunable structures and unusual functional properties. In the recent years, MOFs have been used as new sensing materials to effectively detect cations, anions and nitroaromatic complexes due to their high selectivity and good sensitivity. In this work, we explored a unique fluorescent metal-organic framework {[Cd(abtz)2(NCS)]·(ClO4)}n (MOF-Cd-abtz; abtz = 1-(4-aminobenzyl)-1,2,4-triazole) for highly selective and sensitive detection of ascorbic acid in biological liquid (such as human urine and serum samples). Because both the fluorescent material MOF-Cd-abtz and ascorbic acid can absorb the excitation light at 250 nm, thereby ascorbic acid can quantitatively quench fluorescent signals of MOF-Cd-abtz in biological liquid. This MOF-Cd-abtz based fluorescent method exhibits good liner response to ascorbic acid in the range of 0.1–140 μM. The relative standard deviation (RSD) for three times replicate detections of 25 μM ascorbic acid was 0.94%, and the detection limit was 75 nM (S/N = 3). The developed method can be applied for the determination of ascorbic acid in human urine and serum samples with quantitative spike recoveries from 93% to 103%.The MOF-Cd-abtz probe was developed for highly sensitive determination of AA based on the static quenching effect caused by relatively high AA absorbance at 250 nm.

•Phosphorescent “off–on” switch based on Mn:ZnS QDs for sensitive detection of GSH.•Avoiding the complex modification/immobilization of QDs.•Applying for detection of GSH in food, wine and biological samples.•The possible mechanism for the restored phosphorescence.•The better analytical performance including wide linear range, low detection limit, high selectivity.The phosphorescent 3-mercaptopropionic acid (MPA) capped Mn-doped ZnS quantum dots (MPA-Mn:ZnS QDs)-KMnO4 hybrid system was developed to be an “off–on” phosphorescent switch based on chemical redox mechanism for detection of glutathione (GSH) in food, wine, and biological samples. By employing KMnO4 as the quencher to the phosphorescence of MPA-Mn:ZnS QDs, GSH recovered the emission for its determination by the use of its reducibility. The possible mechanism for the restored phosphorescence was elucidated that SO42− oxidized by KMnO4 on the surface of the Mn:ZnS QDs was reduced to S2− with the addition of GSH. The phosphorescent hybrid system allowed highly sensitive detection of GSH in aqueous solution with a detection limit of 97 nM and a wide linear range of 0.3–280 μM. The precision for 11 replicate detection of 200 μM GSH was 3.5% (relative standard deviation, RSD). The developed method was applied for the detection of GSH in food, wine, and biological samples with the quantitative spike recoveries from 95% to 104%.Glutathione was selectively detected based on an “off–on” phosphorescent switch of MPA-Mn:ZnS QDs in food, wine and biological samples.

•Facile synthesis of red emitting copper nanoclusters.•Synthesis of copper nanoclusters without adjusting the pH value.•The first study to apply metal nanoclusters to detect glycoproteins.As an emerging class of fluorescent probes, copper nanoclusters (Cu NCs) have been considered as an intriguing candidate for detecting biomoleculars due to their outstanding fluorescent properties, excellent biocompatibility and low cost. Herein, we fabricated bovine serum albumin (BSA) protected Cu NCs (BSA-Cu NCs) and further functionalized them with 3-aminophenylboronic acid (APBA) for selectively discerning glycoproteins. In aqueous solution, Cu2+ ions were directly reduced into BSA-Cu NCs by hydrazine hydrate (N2H4·H2O) at room-temperature using BSA as the capping agent. The synthetic process was very rapid, simple and easy for controlling due to the lack of any other complicated procedure such as heating and adjusting the pH value of the reactive mixture. The APBA-Cu NCs showed strong fluorescent emission at 630 nm in the red range. So it can effectively avoid the disturbance of auto-fluorescence in biosamples. The fluorescence of the APBA-Cu NCs was obviously quenched by glycoprotein samples. Then, the APBA-Cu NCs were employed as a probe for selective capture and sensitive detection of glycoproteins with a wide linear range of 5–220 nM and a low detection limit of 2.60 nM owing to the covalent reaction between the boric acid group of APBA and the cis-glycol groups of the glycoproteins. The developed method was also successfully applied to determine glycoproteins in egg white of chickens and human urine samples with quantitative spike recoveries from 95% to 104%.

The phosphorescent 3-mercaptopropionic acid (MPA) capped Mn-doped ZnS quantum dots (MPA–Mn:ZnS QDs)–Fenton hybrid system was developed for a highly sensitive detection of Fe2+ in environmental samples and biological fluids. The phosphorescence of the MPA–Mn:ZnS QDs can be effectively quenched by hydroxyl radicals produced from the Fenton reaction between Fe2+ and H2O2 at a low concentration level. However, the phosphorescence of the MPA–Mn:ZnS QDs cannot be quenched by either Fe2+ or H2O2 individually at the same concentration level. Thus, Fe2+ can be indirectly detected using the phosphorescence quenching caused by hydroxyl radicals based on the Fenton reaction. A possible mechanism for the quenching effect of Fe2+ was elucidated as electron transfer from the conduction band of the MPA–Mn:ZnS QDs to the unoccupied band of hydroxyl radicals. The phosphorescent hybrid system allowed a highly sensitive detection of Fe2+ in an aqueous solution with a wide linear range of 0.01–10 μM and a detection limit of 3 nM, and the precision for 11 replicate detections of 0.1 μM Fe2+ was 1.5% (relative standard deviation, RSD). The developed method was applied to determine Fe2+ in environmental samples and biological fluids with quantitative spike recoveries from 95% to 104%.

Thiourea (TU) has been widely used in agriculture, industry and other application fields. Due to its serious toxicity and hazard to the environment, it is necessary to find a sensitive and selective method for detecting thiourea. Herein, a facile, fast, and specific colorimetric method was established for the determination of thiourea with high sensitivity based on leaching of gold nanoparticles (Au NPs) by thiourea. In hydrochloric acid media, the absorbance of the Au NPs at 519 nm decreased with the increase in the concentration of thiourea. The color change of the Au NPs with different concentrations of thiourea could make it convenient to be observed by the naked eye. This method allowed the determination of thiourea in the range of 5–350 nM with a detection limit of 2.14 nM. Various foreign species did not interfere with the determination. This method was found to be sensitive and selective and was applied for the determination of thiourea in natural water, industrial waste water and fruit juices.

Metal–organic framework MIL-100(Fe) has high surface area, mesoporous cages, metal active sites and excellent water stability. These fascinating properties endow MIL-100(Fe) with the potential to extract organic dyes from environmental water samples. In the present work, MIL-100(Fe) is synthesized and applied as an adsorbent to remove three different types of dyes from aqueous solution in view of assessing the adsorption isotherms, kinetics and thermodynamics, desorption, and adsorbent regeneration. The adsorption for methylene blue and methyl blue follows a pseudo-second-order kinetics and fits the Freundlich model. The adsorption is a spontaneous process, but is controlled by different thermodynamic parameter changes. Moreover, MIL-100(Fe) could hardly adsorb isatin. Variables influencing the adsorption efficiency, such as adsorption time, temperature, solution pH, adsorbent dosage and salt concentration, are investigated. No remarkable effects of pH and ionic strength are observed for the adsorption of methylene blue and methyl blue on MIL-100(Fe). The used MIL-100(Fe) could be regenerated effectively and recycled without a significant loss of adsorption ability. The adsorption efficiency is about 100% for methylene blue and 52.1% for methyl blue, and the developed method is applied to remove two types of dyes in local water samples.

With the extensive use of rare earth elements (REEs) in agriculture as fertilizer and feed additives, the concentration of REEs has increased in environmental and biological samples and finally impaired human health by food chain accumulation. The determination of trace REEs has gained considerable importance because of their toxicity and increasing occurrence. In this work, walnut shell has been used as the green adsorbent in online preconcentration and detection of REEs in food and agricultural products coupled with inductively coupled plasma mass spectrometry (ICP-MS). Because of the porous surface and abundant −COO– groups on the walnut shell surface, the walnut shell-packed microcolumn provides high adsorption efficiency and high tolerable capacity for coexisting ions. Under the optimized conditions, the adsorption efficiency of the walnut shell packed microcolumn was as high as 98.9% for 15 REEs, and the tolerable concentration ratios were between 2000 and 80 000 000 for 37 kinds of coexisting interfering ions. The enhancement factors achieved were 79–102 for 15 REEs with a sample loading volume of 4.7 mL. The detection limits were in the range of 2–34 pg g–1. The relative standard deviation for 11 replicate preconcentrations of 2.5 ng L–1 REEs solution ranged from 0.5 to 2.0%. The present method was successfully applied to selective determination of REEs in 4 environmental and biological certified reference materials and 18 locally collected food and agricultural products.

Thiourea (TU) has been widely used in agriculture, industry and other application fields. Due to its serious toxicity and hazard to the environment, it is necessary to find a sensitive and selective method for detecting thiourea. Herein, a facile, fast, and specific colorimetric method was established for the determination of thiourea with high sensitivity based on leaching of gold nanoparticles (Au NPs) by thiourea. In hydrochloric acid media, the absorbance of the Au NPs at 519 nm decreased with the increase in the concentration of thiourea. The color change of the Au NPs with different concentrations of thiourea could make it convenient to be observed by the naked eye. This method allowed the determination of thiourea in the range of 5–350 nM with a detection limit of 2.14 nM. Various foreign species did not interfere with the determination. This method was found to be sensitive and selective and was applied for the determination of thiourea in natural water, industrial waste water and fruit juices.

As an emerging class of fluorescent probes, metal–organic frameworks (MOFs) have recently received great interest owing to their fascinating functional properties, intriguing tunable structures, high selectivity and good sensitivity. Herein, we present a novel strategy based on the application of a facile water-stable MOF {[Cd(μ3-abtz)·2I]}n (Abtz–CdI2–MOF, abtz = 1-(4-aminobenzyl)-1,2,4-triazole) using powerful solvo-thermal synthetic techniques, which can serve as an “off–on” fluorescent switch for the label-free detection of dopamine (DA) without any additional surface modification and functionalization. The fluorescence signal of Abtz–CdI2–MOF can be efficiently quenched by KMnO4, and then restored by DA in an “off–on” mode. The possible quenching mechanism should be static quenching resulting from the ground state recombination between Abtz–CdI2–MOF and MnO4−. Further when DA is added, MnO4− can be gradually released from the hybrid system, and bring about ground state complex decomposition, which is triggered by competitive interactions between Abtz–CdI2–MOF and the reduced iodine ions. Therefore, the fluorescence signal can be gradually turned “on”. The relative intensity of restored fluorescence is proportional to the concentration of DA in the wide linear range of 0.25–50 μM with a low detection limit of 57 nM (S/N = 3). Additionally, some potentially foreign interfering substances, such as ascorbic acid (AA) and uric acid (UA), glucide, amino acids and metal ions did not affect DA determination significantly. The proposed strategy was also successfully applied for DA determination in biological samples with satisfactory recoveries from 94.5% to 102%.