Application of the Water‐Dispersible ZnS:Mn Nanocrystal as an Effective and Convenient Photosensor Material for the Detection of Zn²⁺ and Cd²⁺ Ions in Aqueous Solution

Abstract

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...

Application of L-Aspartic Acid-Capped ZnS:Mn Colloidal Nanocrystals as a Photosensor for the Detection of Copper (II) Ions in Aqueous Solution

“…However, it should also be noted that the carboxyl end is not so strongly acidic in water compared with halide acids; thus, not all of the protons in the carboxyl (–COOH) end were ionized in this environment. A very similar phenomenon was observed for the previously-mentioned MAA-capped ZnS:Mn NCs, in which the reported zeta potential of the NCs was −22.38 mV under the preparation conditions of pH 2 [ 22 ]. A plausible explanation for obtaining a much smaller negative charge value for the ZnS:Mn-Asp NCs is that a portion of the ionized protons were attracted to the neighboring amine (NH 2 ) end of the Asp molecules to form NH 3 + moieties, in order to cancel out the negative charge caused by the (COO − ) ends.…”

“…Previously, we have reported a similar study of the use of surface-modified ZnS:Mn NCs with a different ligand (mercaptoacetic acid, MAA) for the detection of heavy transition metal ions in aqueous solution [ 22 ]. However, their fluorescence quenching effects were totally different from those of the ZnS:Mn-Asp NCs.…”

“…Therefore, the development of a novel type of semiconductor nanocrystal having no biological toxicity and high colloidal stability in water is needed for such applications. Previously in our lab, the synthesis and surface characterization of water-dispersible ZnS:Mn nanocrystals by capping with mercaptoacetic acid (MAA), and their application as a photosensor for the detection of Zn 2+ and Cd 2+ ions in aqueous solution, have been reported [ 22 ].…”

Differential Surface Capping Effects on the Applications of Simple Amino-Acid-Capped ZnS:Mn Nanoparticles

“…The limits of detection of the molar concentration [M] of copper (II) metal ions using ZnS:Mn nanoparticles capped with amino acids were 1.8 × 10 −7 M (Gly-ZnS:Mn), 1.5 × 10 −7 M (Ala-ZnS:Mn), and 1.1 × 10 −7 M (Val-ZnS:Mn), and the concentration of the nanocrystals was 10.0 mg/L in all cases. The ion selectivity as an optical sensor of the ZnS:Mn nanocrystals capped with these three amino acids was completely different from that of the ZnS:Mn nanocrystals capped with structurally similar organic molecules such as mercaptoacetic acid (MAA) [ 35 ]. In the case of the MAA-ZnS:Mn nanocrystals, the addition of Zn 2+ metal ions did not affect the fluorescence light, whereas all other divalent transition metal ions almost completely quenched the orange fluorescence light of the nanocrystals.…”