Water-dispersible ZnS:Mn nanocrystals (NC) were synthesized by capping the surface with mercaptoacetic acid (MAA) molecules at three different pH conditions. The obtained ZnS:Mn-MAA NC products were physically and optically characterized by corresponding spectroscopic methods. The UV-Visible absorption spectra and PL emission spectra showed broad peaks at 310 and 590 nm, respectively. The average particle sizes measured from the HR-TEM images were 5 nm, which were also supported by the Debye-Scherrer calculations using the X-ray diffraction (XRD) data. Moreover, the surface charges and the degrees of aggregation of the ZnS:Mn-MAA NCs were determined by electrophoretic and hydrodynamic light scattering methods, indicating formation of agglomerates in water with various sizes (50–440 nm) and different surface charge values accordingly the preparation conditions of the NCs (−7.59 to −24.98 mV). Finally, the relative photocatalytic activities of the ZnS:Mn-MAA NCs were evaluated by measuring the degradation rate of methylene blue (MB) molecule in a pseudo first-order reaction condition under the UV-visible light irradiation. As a result, the ZnS:Mn-MAA NC prepared at the pH 7 showed the best photo-degradation efficiency of the MB molecule with the first-order rate constant (kobs) of 2.0 × 10−3·min−1.

The development of selective and sensitive sensors to detect heavy transition-metal ions in an aqueous environment is of great interest due to these ions' significant effects on the ecosystem and human health.1 In this regard, there has been research performed using semiconductor nanocrystals as fluorescence sensors for the detection of heavy transition-metal ions such as cadmium and mercury in water. 2,3 However, the most commonly used CdS and CdSe nanocrystals contain environmentally hazardous components. Therefore, development of a nanocrystal having no biological toxicity and high colloidal stability in water is needed for such applications. Previously, we have reported the synthesis and surface characterization of the water-dispersible mercaptoacetic acid (MAA)-capped ZnS:Mn nanocrystal. 4 In addition, more recently, significantly low biological toxicity and high colloidal stability of the ZnS:Mn-MAA nanocrystal (NC) have also been demonstrated. 5 In this article, we describe a novel application of the ZnS:Mn-MAA NC as a selective and convenient fluorescence sensor material to detect Zn 2+ and Cd 2+ ions in aqueous solution. The experimental details regarding the preparation and characterization of the obtained ZnS:Mn-MAA NC products are provided in Supporting Information.To study the surface-related properties of the ZnS:Mn-MAA NC, we measured the surface charge of the nanocrystal using an electrophoretic method. 6 The obtained zeta potential of the ZnS:Mn-MAA NC at ambient temperature was −22.38 mV, which was directly related to the pH condition of the nanocrystal preparation (pH = 10). The negatively charged nanocrystal surfaces were presumed to be suitable for further coordination by positively charged transition-metal ions such as Mn, and Cd 2+. As a result, fast luminescence quenching was observed by the addition of the aqueous solution containing most divalent transition-metal ions to that of ZnS:Mn-MAA NC, except for Zn 2+ and Cd 2+ions, as shown in Figure 1. Additionally, Figure 2(a) and (b) presents the photoluminescence (PL) emission spectra before and after addition of metal ions to the ZnS:Mn-MAA NC-containing solution, respectively. As can be seen in these spectra, the strong emission peaks from the ZnS:Mn-MAA NC remained only when Zn 2+ and Cd 2+ ions were added, while other metals caused almost complete fluorescence quenching. This unique result led us to preliminarily conclude that the ZnS:Mn-MAA NCs can be used as a selective metal ion photosensor for these two metal ions in water. The sensing limit of the concentration of the added molar concentration of the metal ions ([M]) was 2.50 μM over 1.0 mg/L of the ZnS: Mn-MAA NCs. There have been similar attempts to use the surface-modified ZnS:Mn NCs with other ligands for the detection of heavy transition-metal ions in aqueous solutions. 7,8 However, their fluorescence quenching effects were totally different from that of the ZnS:Mn-MAA NC.The most plausible explanation for our results is that the added metal ions were coordinated by the MAA l...

Water-dispersible ZnS:Mn nanocrystals (NCs) were synthesized by capping the surface with polar L-aspartic acid (Asp) molecules. The obtained ZnS:Mn-Asp NC product was optically and physically characterized using the corresponding spectroscopic methods. The ultra violet-visible (UV-VIS) absorption spectrum and photoluminescence (PL) emission spectrum of the NCs showed broad peaks at 320 and 590 nm, respectively. The average particle size measured from the obtained high resolution-transmission electron microscopy (HR-TEM) image was 5.25 nm, which was also in accordance with the Debye-Scherrer calculations using the X-ray diffraction (XRD) data. Moreover, the surface charge and degree of aggregation of the ZnS:Mn-Asp NCs were determined by electrophoretic and hydrodynamic light scattering methods, respectively. These results indicated the formation of agglomerates in water with an average size of 19.8 nm, and a negative surface charge (−4.58 mV) in water at ambient temperature. The negatively-charged NCs were applied as a photosensor for the detection of specific cations in aqueous solution. Accordingly, the ZnS:Mn-Asp NCs showed an exclusive luminescence quenching upon addition of copper (II) cations. The kinetic mechanism study on the luminescence quenching of the NCs by the addition of the Cu2+ ions proposed an energy transfer through the ionic binding between the two oppositely-charged ZnS:Mn-Asp NCs and Cu2+ ions.

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