Application of the Manganese (II) Ion Doped Zinc Sulfide Nanocrystals for Dual Detection of Cyanide and Nitrite Ions in Aqueous Solution

Abstract

Transition metal ion doped zinc sulfide nanocrystals (NCs) are one of the hottest subjects in the semiconductor nanomaterials research area. 1-3 These materials have been widely used in various electronic sophisticated devices since they exhibit optimal physical and chemical properties for these applications. [4][5][6][7] In this article, we described the synthesis of the water-dispersible and positively surface charged ZnS:Mn NCs by using glycolic acid (GA) as a polar surface capping agent at low pH condition. In addition, the prepared NCs were evaluated as a photo-sensor for the detection of specific anion species in water. Particularly, the selected GA molecule has two different polar functional groups: hydroxyl (−OH) and carboxyl (−COOH) groups, which can simultaneously coordinate to the ZnS: Mn-GA NCs and the detecting ions. The synthetic condition was adjusted to low pH to impart positive charges to the surface of the NCs by protonation of the functional end groups. Additionally, to investigate the surfaces properties of the ZnS:Mn-GA NCs, the surface charge and the degree of aggregation in aqueous solution were measured, which are critical factors of the NCs for practical applications for sensors or catalysts. The detailed characterization works for the NC products were described in Appendix S1, Supporting Information. The main purpose of this study is to estimate the potential of the ZnS:Mn-GA NCs as a specific anion photo-sensor in water.The positively charged ZnS:Mn-GA NCs were tested as photo-chemical sensors for various anions with various oxidation states by further addition of:sulfate (SO 4 2− ), sulfide (S 2− ), and citrate (C 6 H 5 O 7 3− ) ions to the NCs at ambient temperature. The fluorescence images presented in Figure 1(a) were obtained from the ZnS:Mn-GA NCs upon addition of various anions, which were taken using He-Cd laser (λ max = 325 nm) as a light source, and Figure 1(b) presents histograms showing the actual changes in photoluminescence intensity upon the addition of the corresponding anions compared with the original ZnS:Mn-GA NCs (blank) solution. Figure 1 (b) shows that the intensities of the emission peaks for the ZnS:Mn-GA NCs were not much changed by the addition of most anions, while NO 2 − ions caused significant emission quenching (95%) compared to the blank sample (NCs only). Moreover, the Figure 1(a) obviously shows that the color of the emission light from the ZnS:Mn-GA NCs was completely changed (from yellow-orange to purple-blue) after addition of CN − ions. Based on these unique and interesting results, we preliminarily concluded that the ZnS:Mn-GA NCs can be used as effective anion photo-sensors for the simultaneous detection of NO 2 − and CN − ions in water. Both of the Figure 1(a) and (b) were obtained by adjusting the molar concentrations of the added anions at 50.0 μM over 10.0 mg/L of the ZnS:Mn-GA NCs. In addition, the measured limit of detection (LOD) for the added molar concentration of nitrite ion [NO 2 − ] was 1.0 μM over 10.0 mg/L of the ZnS:Mn-GA NCs in this...