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View of the negative influence of metal ions on natural environment and human health, fast and quantitative detection of metals ions in water systems is significant. Ultra-small grain size CdS quantum dots (QDs) modified with N-acetyl-L-cysteines (NALC) (NALC-CdS QDs) are successfully prepared via a facile hydrothermal route. Based on the changes of fluorescence intensity of NALC-CdS QDs solution after adding metal ions, the fluorescence probe made from the NALC-CdS QDs is developed to detect metal ions in water systems. Among various metal ions, the fluorescence of NALC-CdS QDs effectively quenched by the addition of Cu2+, the probe shows high sensitivity and selectivity for detecting Cu2+ in other interferential metal ions coexisted system. Importantly, the fluorescence intensity of NALC-CdS QDs changes upon the concentration of Cu2+, the probe displays an excellent linear relationship between the fluorescence quenching rate and the concentration of Cu2+ in ranging from 1 to 25 μM. Besides, the detected limitation of the probe towards Cu2+ as low as 0.48 μM. The measurement of Cu2+ in real water sample is also carried out using the probe. The results indicate that NALC-CdS QDs fluorescence probe may be a promising candidate for quantitative Cu2+ detection in practical application. [ABSTRACT FROM AUTHOR]

In this work, an efficient and stable fluorescent probe for Al3+ was established. The fluorescent probe based on the fluorescence 'turn-on' mode of zinc sulfide crystal composite zinc oxide quantum dots (ZnS/ZnO QDs). The ZnS/ZnO QDs were synthesized via two-step method using L-Cysteine (L-Cys) as a sulfur source and stabilizer. In the synthesis of ZnS/ZnO QDs, the fluorescence of zinc oxide quantum dots (ZnO QDs) decreased and its stability increased in aqueous solution after the addition of L-Cys. In addition, the as-synthesized ZnS/ZnO QDs shows fluorescent enhancement to Al3+. The ZnS/ZnO QDs based fluorescence 'turn-on' probe presented wide linear ranges (1 nM–8 μM and 8–100 μM). The availability of as-established sensing probe was also estimated by real water sample tests. Furthermore, the fluorescent enhancing mechanism was carried out by recording the fluorescent lifetime of samples, which might be related to the QDs dispersion and charge transfer weaken. [ABSTRACT FROM AUTHOR]

A new high performance fluorescence assay for detection of Ag+ based on CdS quantum dots (QDs) using L-Aspartic acid (L-Asp) as a stabilizer was proposed in this work. The CdS quantum dots conjugation with L-Aspartic acid (L-Asp@CdS QDs) were successfully synthesized via a simple hydrothermal process. The QDs have a fluorescence emission band maximum at 595 nm with a quantum yield of 11%. The obtained CdS QDs exhibit a particle size of 1.63 ± 0.28 nm and look like quantum dot flowers. Basically, the fluorescence intensity of L-Asp@CdS QDs can be enhanced only upon addition of Ag+ and a redshift in the fluorescence spectrum was observed. Under optimum conditions, the fluorescence enhancement of L-Asp@CdS QDs appeared to exhibit a good linear relationship in between 100–7000 nM (R2 = 0.9945) with the Ag+ concentration, with a detection limit of 39 nM. The results indicated that the L-Asp@CdS QDs were well used in detection for Ag+ as fluorescence probe in aqueous solution with high sensitivity and selectivity. Moreover, the sensing system has been applied in detection Ag+ in real water samples. The recovery test results were 98.6%~113%, and relative standard deviation (n = 5) is less than 3.6%, which was satisfactory. [ABSTRACT FROM AUTHOR]

Thioglycolic acid-functionalized ZnSe quantum dots (QDs) as a colorimetric nanoprobe were prepared and applied to the determination of cobalt(II) and iron(III). Test strips were obtained by a dipping-drying process. On exposure to Co(II), they undergo a color change from white to brown, and on exposure to Fe(III) from white to pink. The limits of detection (LOD) are 2.6 mg L−1 for Co(II) and 2.2 mg L−1 for Fe(III). Test strips introduce a low-cost, portable, rapid and convenient tool for determination of Co(II) and Fe(III). In addition, two other analytical methods have been studied for detection of Co(II) and Fe(III) at low concentration. The first is UV-vis spectrometry which has a LOD as low as 0.14 mg L−1 for Co(II) (at 412 nm) and 0.12 mg L−1 for Fe(III) (at 400 nm). The second is dynamic light scattering (DLS) with a LOD of 3.0 μg L−1 for Co(II) and 2.5 μg L−1 for Fe(III). [ABSTRACT FROM AUTHOR]

The fluorescence intensity of N, S co-doped graphene quantum dots (N, S-GQDs) can be quenched by Fe3+ and Hg2+. Density functional theory (DFT) simulation and experimental studies indicate that the fluorescence quenching mechanisms for Fe3+ and Hg2+ detection are mainly attributed to the inner filter effect (IFE) and dynamic quenching process, respectively. The electronegativity difference between C and doped atoms (N, S) in favor to introduce negative charge sites on the surface of N, S-GQDs leads to charge redistribution. Those negative charge sites facilitate the adsorption of cations on the N, S-GQDs' surface. Atomic population analysis results show that some charge transfer from Fe3+ and Hg2+ to N, S-GQDs, which relate to the fluorescent quenching of N, S-GQDs. In addition, negative adsorption energy indicates the adsorption of Hg2+ and Fe2+ is energetically favorable, which also contributes to the adsorption of quencher ions. Blue fluorescent N, S-GQDs were synthesized by a facile one-pot hydrothermal treatment. Fluorescent lifetime and UV-vis measurements further validate the fluorescent quenching mechanism is related to the electron transfer dynamic quenching and IFE quenching. The as-synthesized N, S-GQDs were applied as a fluorescent probe for Fe3+ and Hg2+ detection. Results indicate that N, S-GQDs have good sensitivity and selectivity on Fe3+ and Hg2+ with a detection limit as low as 2.88 and 0.27 nM, respectively. [ABSTRACT FROM AUTHOR]

Citric acid capped CdS quantum dots (CA-CdS QDs), a new assembled fluorescent probe for copper ions (Cu2+), was synthesized successfully by a simple hydrothermal method. In this work, the fluorescence sensor for the detection of heavy and transition metal (HTM) ions has been extensively studied in aqueous solution. The results of the present study indicate that the obtained CA-CdS QDs could detect Cu2+ with high sensitivity and selectivity. It found that the existence of Cu2+ has a significant fluorescence quenching with a large red shifted (from greenish-yellow to yellowish-orange), but not in the presence of 17 other HTM ions. As a result, Cu2S, the energy level below the CdS conduction band, could be formed at the surface of the CA-CdS QDs and leads to the quenching of fluorescence of CA-CdS QDs. Under optimal conditions, the copper ions detection range using the synthesized fluorescence sensor was 1.0 × 10‒8 M to 5.0 × 10‒5 M and the limit of detection (LOD) is 9.2 × 10‒9 M. Besides, the as-synthesized CA-CdS QDs sensor exhibited good selectivity toward Cu2+ relative to other common metal ions. Thus, the CA-CdS QDs has potential applications for detecting Cu2+ in real water samples. [ABSTRACT FROM AUTHOR]

Abstract Water-soluble rhodamine B assisted graphene quantum dots (RhB-GQDs) were synthesized by a facile one-pot hydrothermal method, using rhodamine B and ethylenediamine as nitrogen source. The as-synthesized RhB-GQDs have few layers configuration with an average size of 4.5 nm and good distribution. The fluorescence quenching approach was applied to the sensing of mercury ions in water. Rodamine B absorb on the surface of RhB-GQDs, and it can prevent RhB-GQDs from contacting with interfering cations. The presence of N atoms reduces the ability of interfering cations to form stable complex with carboxyl groups, and gives the RhB-GQDs good selectivity to mercury ions. In the case of low concentration, the fluorescence quenching efficiency of as-synthesized RhB-GQDs still has a good liner relationship with concentration. Due to the strong affinity of Hg2+ to amino and carboxyl on the surface of RhB-GQDs, the fluorescence of RhB-GQDs is quenched by nonradiative electron transfer between Hg2+ and the excited state of RhB-GQDs. Based on the fluorescence turn off process, the as-synthesized RhB-GQDs can be served as fluorescent platform for the sensitive and selective detection of Hg2+ with a detection limit of 0.16 nM. Graphical abstract Rhodamine B assisted graphene quantum dots (RhB-GQDs) synthesized a facile one-pot hydrothermal method using rhodamine B and ethylenediamine as nitrogen source exhibit the excellently selective and sensitive response to Hg2+ as flourescent sensor. fx1 [ABSTRACT FROM AUTHOR]

Copper, as an essential element in human body, can have adverse impact on environment and healthy individuals if it is excessive. So it is necessary to establish a rapid and effective method for detecting Cu2+. In this work, we describe a method for determination of Cu2+ based on water-soluble ZnO quantum dots (QDs) modified with (3-aminopropyl)triethoxysilane (APTEs). The ZnO QDs functionalized with APTEs (NH 2 –ZnO QDs) synthesized by a simple sol-gel method and displayed strong yellow-green fluorescence with a peak at 535 nm under 350 nm excitation. High-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, luminescence, and UV–visible absorption spectroscopy were used to characterize the NH 2 –ZnO QDs. In addition, the emission from NH 2 –ZnO QDs was selectively quenched upon addition of Cu2+. Therefore, this finding was used to design a fluorescent probe based on NH 2 –ZnO QDs to detect Cu2+ in water solution, and the linear relationships were 2–20 nM and 1–100 μM respectively, with detection limit for Cu2+ at 1.72 nM (on the basis of 3σ/slope criterion). This fluorescent probe had also been applied in real water sample to testify its availability in drinking water. Furthermore, the quenching mechanism was studied by measurements of UV–visible absorption spectra and fluorescent lifetime of ZnO QDs, which may be attributed to the aggregation induced by Cu2+ and the dynamic quenching existing energy transfer between QDs and Cu2+. (3-aminopropyl)triethoxysilane modified water-soluble ZnO quantum dots synthesized by simple sol-gel method exhibit the excellently selective and sensitive fluorometric determination to Cu2+. Image 1 • Water-soluble NH 2 –ZnO QDs were synthesized by facile hydrothermal method. • Fluorometric determination of Cu2+ shows higher sensitivity and wider detection limitation. • Quenching mechanism associates to the coordination interaction between NH 2 –ZnO QDs and Cu2+. • NH 2 –ZnO QDs will be also helpful for the design of fluorometric determination of Cu2+. [ABSTRACT FROM AUTHOR]

• A highly selective ratiometric fluorescent sensor N-CQDs/AuNCs for carbendaizm was developed. • The sensor has good selectivity and sensitivity to carbendazim with wide dynamic response range. • Sensor avoids the influences caused by probe concentration and drifts of optoelectronic system. • The ratiometric fluorescent sensor is a promising candidate for carbendazim detection. A novel fluorescence "turn on" ratiometric fluorescent sensor was employed to determine carbendazim. The sensing process was achieved through the strong fluorescence resonance energy transfer (FRET) between nitrogen doped carbon quantum dots (N-CQDs) and gold nanocluster (AuNCs). The photoluminescence intensity of N-CQDs can be deactivated by AuNCs through FRET effect and recovered by the addition of carbendazim. The ratiometric detection of carbendazim is achieved by recording the photoluminescence and second-order Rayleigh scattering (SRS) signal of N-CQDs/AuNCs system. With the introduction of carbendazim to the sensing platform resulted in the photoluminescence and SRS signal of N-CQDS/AuNCs enhancing. UV–vis absorption, Zeta potential and fluorescence lifetime analyses indicate that the fluorescence turn on process can be attributed to the aggregation of AuNCs breaks the FRET process and increases SRS intensity. N-CQDs/AuNCs probe present a good sensitivity and selectivity for carbendazim detection, with two linear response ranges (1−100 μM, 150−1000 μM), low detection limit of 0.83 μM and 37.25 μM. Furthermore, real sample analyses indicate that the as-presented sensor has potentials in carbendazim determination in real sample analyses. [ABSTRACT FROM AUTHOR]

In this study, l -aspartic acid stabilized CdS quantum dots (L-Asp-CdS QDs) were synthesized via a one-step hydrothermal method and used for the qualitative identification of Cr3+. Cr3+ could be identified rapidly based on the formation of precipitated floccules using this method. Qualitative assessment indicated the resistance of this method to interference from various metal ions and it can be applied over a wide pH range with a qualitative determination limit of 1 μM. Moreover, we demonstrated that the L-Asp-CdS QDs can be employed for the practical detection of Cr3+ in water samples. L-Asp-CdS QDs synthesized via a simple hydrothermal method for the first time. L-Asp-CdS QDs exhibited high selectivity for Cr3+ ions and they are suitable for detecting Cr3+ in solutions containing interfering metal ions in practical applications with real water samples. Image 1 • l -aspartic acid stabilized CdS quantum dots (L-Asp-CdS QDs) synthesized. • One-step hydrothermal method employed for synthesis of L-Asp-CdS QDs. • L-Asp@CdS QDs used for qualitative detection of Cr3+. • L-Asp@CdS QDs exhibited good selectivity for Cr3+ and low detection limit. • L-Asp@CdS QDs can be used to detect Cr3+ in real water samples. [ABSTRACT FROM AUTHOR]