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Fluorescent nitrogen-doped carbon dots (CDs) were prepared via hydrothermal method at 190 °C for 10 h using rhizobium from soy as the carbon and nitrogen source. Their optical properties, structure, morphology, and functional groups were characterized in detail and the results showed that they possess unique excitation-dependent fluorescence behavior, with average diameter 4.5 ± 2.0 nm and good water dispersibility. Due to the overlap of the UV–vis absorbance of chlortetracycline hydrochloride (CCH) and the fluorescence excitation band of CDs, the fluorescence of the prepared CDs can be quenched by CCH selectively and sensitively. The changes of the fluorescence intensity of CDs have a good linear relationship with the concentration of CCH in a wide concentration range of 5–100 μM, with a detection limit of 0.254 μM. This present method has been successfully applied to determine the CCH in water with recovery ranging from 96.0% to 100.7%. [ABSTRACT FROM AUTHOR]

• Up-conversion nanozyme NaYF 4 :Yb/Er@PVP@MnO 2 (UCNP@PVP@MnO 2) was prepared. • The combination of up-conversion fluorescence of NaYF 4 :Yb/Er with enzyme-mimicking properties of MnO 2 has been utilized. • A fluorescent and colorimetric dual-signal mode based on nanozyme for the detection of mancozeb. In this work, a fluorescence/colorimetric dual-mode detection method based on MnO 2 nanoflower-decorated upconversion nanoparticles: NaYF 4 :Yb/Er@polyvinylpyrrolidone@MnO 2 (UCNP@PVP@MnO 2) was proposed to detect the presence of mancozeb (MB). In this detection system, the MnO 2 nanoflowers in the nanocomplex of UCNP@PVP@MnO 2 would quench the fluorescence of the UCNP. With the addition of H 2 O 2 and 3,3′,5,5′-tetramethylbenzidine (TMB), the reaction between MnO 2 and H 2 O 2 resulted in the dissolution of MnO 2 and the dissolution of the MnO 2 layer contributed to the fluorescence recovery of UCNP. Simultaneously, MnO 2 oxidized the colorless TMB to a blue product oxidized 3,3′,5,5′-tetramethylbenzidine (oxTMB). The blue solution was able to quench the recovered fluorescence of UCNP due to the fluorescence inter filter effect (IFE) between the UCNP and blue oxTMB. Finally, with the addition of MB, the oxTMB was reduced to TMB by MB and the color of the solution became lighter while the fluorescence intensity of the solution increased. The detection method had a good linear range of 5–120 μM and 0.5–60 μM for fluorescence and colorimetric detection, respectively, and the limits of detection (LOD) were 2.34 and 0.245 μM, respectively. [ABSTRACT FROM AUTHOR]

In this paper, a β-cyclodextrin (CD)-modified citric acid (Cit)-stabilized blue-emitting upconversion nanocomposite NaYF 4 :Yb/Tm@NaYF 4 :Yb-Cit-CD (abbreviated as Y/Yb/Tm@Y/Yb-Cit-CD) has been prepared. Under 980 nm laser excitation, the fluorophore Y/Yb/Tm@Y/Yb of the nanocomposite emits blue fluorescence, and the spectral peak is approximately 482 nm. The emission spectrum partly overlaps with the absorption spectrum of procyanidins, which will lead to the upconversion fluorescence intensity of Y/Yb/Tm@Y/Yb decreasing by the fluorescence resonance energy transfer between Y/Yb/Tm@Y/Yb and procyanidins. Thus, the Y/Yb/Tm@Y/Yb-Cit-CD nanocomposite can be used as an upconversion fluorescence probe to detect procyanidins. The quantitative analysis of procyanidins has displayed a linear range of 0.1–1000 μM, with a detection limit of 0.06718 μM (S/N = 3). Furthermore, this probe has shown potential for detection of procyanidins in biological samples and upconversion luminescence imaging analysis in vivo. Under 980 nm laser excitation, NaYF 4 :Yb/Tm@NaYF 4 :Yb-Cit-CD emits blue fluorescence at about 482 nm. The emission spectrum just partly overlap with the absorption spectrum of procyanidins, which will lead to the up-conversion fluorescence quenching of NaYF 4 :Yb/Tm@NaYF 4 :Yb caused by the absorption of procyanidins based on the mechanism of fluorescence resonance energy transfer. In view of the above, NaYF 4 :Yb/Tm@NaYF 4 :Yb-Cit-CD nanocomposite can be used as an optical sensing probe to detect procyanidins. [Display omitted] • NaYF 4 :Yb/Tm@NaYF 4 :Yb-Cit-CD was used as fluorescence probe for procyanidins. • Upconversion fluorescence imaging was used to detect procyanidins in vivo. • Wide linear range of 0-1000 μM and low LOD of 0.06718 μM for procyanidins. [ABSTRACT FROM AUTHOR]

Doxycycline (DOX) is widely used as feed additive and may remain in animal food. Thus, the detection of DOX in food field has become important. This work has reported a simple and rapid strategy to detect DOX by ratiometric fluorescence method without any sensitization materials based on bovine serum albumin (BSA) protected Au nanoclusters (BSA-Au NCs). Under excitation at 390 nm, the BSA-Au NCs has displayed two distinct emission peaks at 510 nm and 640 nm which are respectively originated from BSA and Au NCs, respectively. The fluorescence intensity ratio of I 510 / I 640 of BSA-Au NCs probe has presented a good linear range (0.1–30 μM) and 0.006 μM limit of detection for DOX. This method was used successfully to detect DOX in food samples and the good recoveries reached 96.7–103.7%. The research has showed that potential application of BSA-Au NCs in food field because of its convenient synthesis scheme and excellent selectivity and sensitivity. • A ratiometric fluorometric probe for doxycycline was developed. • BSA-Au NCs has displayed two distinct fluorescence emission peaks at 510 nm and 640 nm. • The method has good selectivity and sensitivity for doxycycline. [ABSTRACT FROM AUTHOR]

In this work, the carbon dots (CDs) were prepared by one-pot hydrothermal method with chitosan as a carbon source and hydrogen peroxide as an oxidant, this route is a novel, simple and green. The prepared CDs has a relatively high photoluminescence (PL) quantum yield (QY) (18.9%) and good water solubility. CdTe quantum dots (CdTe QDs) possess the characteristic of responding to pH rapidly. So a ratiometric fluorescent probe for measuring pH was constructed using the prepared CDs and CdTe QDs. Under the excitation of 330 nm, the probe shows dual (blue and red) emission with peaks at 430 and 600 nm, respectively. When the pH of the solution changes, the blue fluorescence intensity of the CDs remains stable as a reference signal, while the red fluorescence intensity of CdTe QDs changes accordingly. This method has obvious pH-sensitive feature, stable fluorescence properties, and a great linear relationship with pH values in the range of pH 3.0–11.0. What is more, the method can monitor proton-producing enzyme-catalyzed reactions by acting as a ratiometric pH probe. Carbon dots were prepared by one-pot hydrothermal method with chitosan as a carbon source and hydrogen peroxide as an oxidant, and a ratiometric fluorescence was constructed for determination of pH and enzyme reactions based on carbon dots and CdTe quantum dots. Unlabelled Image • Carbon dots were prepared by one-pot hydrothermal method with chitosan as a carbon source and hydrogen peroxide as an oxidant. • A ratiometric fluorescence was constructed for determination of pH based on carbon dots and CdTe quantum dots. • The pH sensor was used to monitor proton-producing enzyme-catalyzed reactions by acting as a ratiometric pH probe. [ABSTRACT FROM AUTHOR]

A dual-emission fluorescent probe to detect ethyl thioureas (ETU) was constructed by the use of the nanocomposite formed with sulfur quantum dots (SQDs) and glutathione stabilized gold clusters (GSH-AuNCs). [Display omitted] • The composite of Sulfur quantum dots and glutathione stabilized gold clusters had dual emission peaks. • Ethyl thioureas significantly affected on one of the fluorescence emissions for the composite. • A new ratio fluorescence determination of Ethyl thioureas was established. • Conditions for promoting and inhibiting hydrolysis of mancozeb to Ethyl thioureas were analyzed and derived. A nanocomposite-based dual-emission fluorescent probe was constructed to detect ethylene thioureas (ETU). The nanocomposite SQDs@GSH-AuNCs was formed by the combination of sulfur quantum dots (SQDs) and glutathione stabilized gold clusters (GSH-AuNCs). After the combination, the fluorescence intensities at 450 nm and 570 nm decreased due to the aggregation fluorescence quenching effect. However, the addition of ETU resulted in a partial recovery of fluorescence at 570 nm. A possible reason was the occurrence of a ligand reaction between the zero- and one-valence gold and ETU to form a complex, which weakened the aggregation fluorescence quenching effect. Under the optimal reaction conditions, the concentration of ETU in the range of 2.5 μM to 100 μM had a good linear relationship with the ratio of fluorescence intensity (I 570 /I 450), and the detection limit was 2.08 μM. The experimental results showed that the method had high selectivity and sensitivity, and had a good recovery of ETU in agricultural products. In addition, the hydrolysis process of mancozeb (Mz) was simulated, and by the use of SQDs@GSH-AuNCs, the conditions for promoting and inhibiting the hydrolysis of Mz to produce ETU were analyzed and obtained, which provided an experimental basis for the storage of Mz and a new method for analyzing the hydrolysis process of Mz. [ABSTRACT FROM AUTHOR]

The crosslinking of glutaraldehyde (GA) and nano-selenium coated chitosan (CS-SeNPs) afforded composites (GA–CS–SeNPs) which exhibit new properties. In addition to having mimic enzyme activity of CS-SeNPs, these composites exhibit fluorescence, which CS-SeNPs did not possess. The GA–CS–SeNPs can catalyze the chromogenic process of oxidation of substrate TMB at 20 °C and pH of 3.5, and under the excitation wavelength of 440 nm, the composites can emit strong fluorescence. The presence of mercury (II) (Hg2+) inhibited the color process of catalytic oxidation of 3,3,5,5, -tetramethylbenzidine (TMB) and enhanced the fluorescence intensity of the composites to some extent. Therefore, using the fluorescence and mimicking enzyme properties of GA–CS–SeNPs, a fluorescent and colorimetric (dual-mode) determination of Hg2+ was developed with good selectivity and high sensitivity. The linear determination ranges of Hg2+ were 0–55 μM and 0–50 μM for fluorescent and colorimetric assays, respectively, and the limits of detection (LOD) for the dual-mode assay were as low as 0.34 μM and 0.40 μM, respectively. In addition, this new method was successfully implemented for dual-mode determination of Hg2+ in real samples. A fluorescent turn-on and colorimetric (dual-mode) determination of Hg2+ was developed based on composites formed by crosslinking of glutaraldehyde and chitosan-nano-selenium. [Display omitted] • The crosslinking of GA improved the properties of selenium nanomaterials. • The obtained composites GA–CS–SeNPs had oxidase-like activity and strong fluorescence emission. • A new dual-mode (colorimetric and fluorometric) determination of mercury (II) was established. [ABSTRACT FROM AUTHOR]

• The effects of iron (II) and H 2 O 2 on fluorescence and color of SQDs were explored. • A new dual-mode (colorimetric and fluorometric) determination of iron (II) and H 2 O 2 was established. A colorimetric and fluorescence dual-signal method based on sulfur quantum dots (SQDs) was established for determination of iron (II) (Fe2+) and H 2 O 2 in foods. Due to the complexation of Fe2+ with SQD, Fe2+ can cause fluorescence quenching of SQDs, and the color of the mixed solution changed from light yellow to deep green. By use of Fenton reaction, H 2 O 2 can restore the quenched fluorescence of SQDs, and the color of the mixture changed from green to colorless. The concentration of Fe2+ and H 2 O 2 has a good linear relationship with the fluorescence intensity and absorbance in the range of 2.5–55 μM and 1.25–500 μM, and the detection limits were 1.41 μM and 0.54 μM, respectively. For determination of H 2 O 2 , the linear ranges were 1.17–1.97 mM and 0.867–1.50 mM, and the detection limits were 0.03 μM and 0.06 μM, respectively. [ABSTRACT FROM AUTHOR]

• A colorimetric and fluorescence turn-on dual-signal method for the detection of ascorbic acid was developed. • The established probe was used successfully for ascorbic acid detection in living cells and beverages. • A linear detection range for ascorbic acid is 1–200 μM and the LOD is 0.15 µM and 0.023 µM by fluorescence method and colorimetric, respectively. A colorimetric and fluorescence turn-on dual-signal assay was developed for the determination of ascorbic acid (AA). Because the ultraviolet absorption of the oxidized 3,3′,5,5′-tetramethylbenzidine (oxTMB) overlapped with the fluorescence emission of glutathione stabilized Au nanoclusters (AuNCs), the fluorescence of AuNCs can be quenched by oxTMB. When AA was added, the blue oxTMB was reduced to colorless TMB, and the fluorescence of AuNCs was restored simultaneously. The decrease in absorbance and increase in fluorescence signal depended on the concentration of AA. In the determination range of 0.5 to 200 μM, the detection limits (LOD) for AA were as low as 0.15 µM and 0.22 µM for fluorometric and colorimetric, respectively. The established probe was used successfully for AA detection in living cells and beverages. [ABSTRACT FROM AUTHOR]

Zinc doping made the response of fluorescence intensity of carbon nanomaterials to pH-dependent, and Zn-doped carbon nanomaterials was coupled with ZnCdS quantum dots to form a ratiometric fluorescence pH sensor. [Display omitted] • Zinc doping changed the response of fluorescence intensity of carbon nanomaterials to pH-dependent. • The possible causes of the difference of optical properties were deduced through spectral analysis. • A ratiometric fluorescence pH sensor based on Zn-doped carbon nanomaterials was fabricated. Fluorescence intensity of carbon nanomaterials (CNMs) synthesized with ethanolamine hardly changed with pH value. Doping Zinc into CNMs improved optical properties of CNMs. On the one hand, the fluorescence quantum yield of CNMs increased, on the other hand, the fluorescence intensity of Zn-doped CNMs decreased with pH value. Coupling Zn-doped CNMs with ZnCdS quantum dots (QDs) as a dual emission system, the blue fluorescence intensity of CNMs (I 450) decreased while the orange fluorescence intensity of QDs (I 580) enhanced with pH value increasing. The ratio (I 450 /I 580) varied linearly with pH value in the range of 1–9 and correlation coefficient (R2) was 0.991. Furthermore, using the system as a ratiometric fluorescence pH sensor to determine ascorbic acid, a good linear relationship (R 2 = 0.998) was found when the concentration of ascorbic acid ranged from 0–1600 μM and limit of detection (LOD) was as low as 0.082 μM. [ABSTRACT FROM AUTHOR]