It is of great significance to sense cysteine (Cys) in a simple and fast way because of its potential applications in biological processes. In this contribution, we proposed a Cu2+-mediated fluorescence switchable strategy to detect Cys by designing a dual-signal fluorescence-enhanced sensor. Two organic dyes immobilized onto layered double hydroxides (LDHs) as novel sensors have been fabricated. It was found that the two fluorescence emissions of the obtained ultrathin films (donated as calcein@NFR/LDHs UTFs) could be effectively quenched by Cu2+ (off state), then reversibly recovered by Cys (on state) owing to the specific coordination of Cys and Cu2+. Under the optimal conditions, there were good linear relationships between the off–on efficiency (off–on) and the amount of Cys in the range of 1.0 × 10−7–1.2 × 10−6 M, with lower detection limits of 5.0 × 10−8 M at 520 nm and 2.3 × 10−8 M at 607 nm, respectively. There is little interference by common ions, amino acids and some excipients. The dual-signal fluorescence-enhanced response of the sensor contributes to its high sensitivity and selectivity. The proposed strategy is label-free, fast and selective, which is applicable to the analysis of Cys in medicine with satisfactory results.

This paper reports the fabrication of ultrathin films (UTFs) of silver nanoclusters functionalized by chromotropic acid (CTA) and layered double hydroxide nanosheets (CTA-AgNCs/LDH) via the layer-by-layer (LBL) assembly technique, and their fluorescence turn on response for the detection of melamine has been demonstrated. UV-vis absorption and fluorescence spectra show the uniform growth of the UTFs and the enhancement of interlayer CTA-AgNCs upon increasing the deposition cycle. The SEM image and XRD results indicate that the (CTA-AgNCs/LDH)n UTFs possess long-range order stacking in the normal direction of the substrate, with CTA-AgNCs accommodated between the LDH nanosheets as a monolayer arrangement. Compared with CTA-AgNCs solution, the fluorescence intensity of UTFs has been improved significantly resulted from the localization and confinement effects of LDH nanosheets. Moreover, the (CTA-AgNCs/LDH)10 displays sensor performance for melamine with a linear response range (0.03–0.1 μM), low detection limit (4 nM) and good stability. A high selectivity of UTFs toward melamine over the other amino acids, metal ions and some saccharides has also been demonstrated. The sensitive assay for melamine could be attributed to the interaction based on the recognition of triple hydrogen bonds between melamine and (CTA-AgNCs/LDH) films. Therefore, this work provides a convenient way to immobilize Ag nanoclusters using the LDH nanosheet as a 2D matrix, which has great potential for the development of melamine detection.

In this study, new facile, economical and fluorescent ultrathin films (UTFs) for detection of tiopronin are developed. The UTFs have been fabricated by combining the alizarin red S–boric acid adduct (ARS–H3BO3) with MgAl layered double hydroxide (LDH) nanoparticles through the layer-by-layer (LBL) assembly technique. UV-vis, fluorescence spectroscopy, XRD, SEM and AFM have been adopted to monitor the assembly process. The UTFs display a uniform morphology and a periodic layered structure. Using ARS as a probe, which does not emit fluorescence by itself but displays fluorescence when complexed with boron, the assembled ARS–H3BO3/LDH UTFs display a high luminescence response to tiopronin. In the presence of Cu2+, the fluorescence of UTFs was quenched, which is attributed to the complexation between Cu2+ and ARS. Upon adding tiopronin to the UTF–Cu system, tiopronin would form a complex with Cu2+ preferentially, leading to the increased fluorescence of UTFs. Based on the above mechanism, a turn-on fluorescent ensemble for tiopronin is developed. A linear response was obtained in the range of 0–80 ng mL−1, with a detection limit of 0.47 ng mL−1. Compared with ARS and other analytes, the tighter binding of tiopronin to Cu2+ led to an assay with high specificity. Therefore, this work provides a facile LBL strategy for the fabrication of a solid state sensor based on ARS dye for detection of tiopronin.

In this study, new facile, economical and fluorescent ultrathin films (UTFs) for detection of tiopronin are developed. The UTFs have been fabricated by combining the alizarin red S–boric acid adduct (ARS–H3BO3) with MgAl layered double hydroxide (LDH) nanoparticles through the layer-by-layer (LBL) assembly technique. UV-vis, fluorescence spectroscopy, XRD, SEM and AFM have been adopted to monitor the assembly process. The UTFs display a uniform morphology and a periodic layered structure. Using ARS as a probe, which does not emit fluorescence by itself but displays fluorescence when complexed with boron, the assembled ARS–H3BO3/LDH UTFs display a high luminescence response to tiopronin. In the presence of Cu2+, the fluorescence of UTFs was quenched, which is attributed to the complexation between Cu2+ and ARS. Upon adding tiopronin to the UTF–Cu system, tiopronin would form a complex with Cu2+ preferentially, leading to the increased fluorescence of UTFs. Based on the above mechanism, a turn-on fluorescent ensemble for tiopronin is developed. A linear response was obtained in the range of 0–80 ng mL−1, with a detection limit of 0.47 ng mL−1. Compared with ARS and other analytes, the tighter binding of tiopronin to Cu2+ led to an assay with high specificity. Therefore, this work provides a facile LBL strategy for the fabrication of a solid state sensor based on ARS dye for detection of tiopronin.

It is of great significance to sense cysteine (Cys) in a simple and fast way because of its potential applications in biological processes. In this contribution, we proposed a Cu2+-mediated fluorescence switchable strategy to detect Cys by designing a dual-signal fluorescence-enhanced sensor. Two organic dyes immobilized onto layered double hydroxides (LDHs) as novel sensors have been fabricated. It was found that the two fluorescence emissions of the obtained ultrathin films (donated as calcein@NFR/LDHs UTFs) could be effectively quenched by Cu2+ (off state), then reversibly recovered by Cys (on state) owing to the specific coordination of Cys and Cu2+. Under the optimal conditions, there were good linear relationships between the off–on efficiency (off–on) and the amount of Cys in the range of 1.0 × 10−7–1.2 × 10−6 M, with lower detection limits of 5.0 × 10−8 M at 520 nm and 2.3 × 10−8 M at 607 nm, respectively. There is little interference by common ions, amino acids and some excipients. The dual-signal fluorescence-enhanced response of the sensor contributes to its high sensitivity and selectivity. The proposed strategy is label-free, fast and selective, which is applicable to the analysis of Cys in medicine with satisfactory results.

This paper reports the fabrication of ultrathin films (UTFs) of silver nanoclusters functionalized by chromotropic acid (CTA) and layered double hydroxide nanosheets (CTA-AgNCs/LDH) via the layer-by-layer (LBL) assembly technique, and their fluorescence turn on response for the detection of melamine has been demonstrated. UV-vis absorption and fluorescence spectra show the uniform growth of the UTFs and the enhancement of interlayer CTA-AgNCs upon increasing the deposition cycle. The SEM image and XRD results indicate that the (CTA-AgNCs/LDH)n UTFs possess long-range order stacking in the normal direction of the substrate, with CTA-AgNCs accommodated between the LDH nanosheets as a monolayer arrangement. Compared with CTA-AgNCs solution, the fluorescence intensity of UTFs has been improved significantly resulted from the localization and confinement effects of LDH nanosheets. Moreover, the (CTA-AgNCs/LDH)10 displays sensor performance for melamine with a linear response range (0.03–0.1 μM), low detection limit (4 nM) and good stability. A high selectivity of UTFs toward melamine over the other amino acids, metal ions and some saccharides has also been demonstrated. The sensitive assay for melamine could be attributed to the interaction based on the recognition of triple hydrogen bonds between melamine and (CTA-AgNCs/LDH) films. Therefore, this work provides a convenient way to immobilize Ag nanoclusters using the LDH nanosheet as a 2D matrix, which has great potential for the development of melamine detection.