β-Cyclodextrin capped ZnS nanoparticles for CER-assisted colorimetric and spectrophotometric detection of Pb2⁺, Cu2⁺, and Hg2⁺ in an aqueous solution

Abstract

Herein, simple, low-cost, and room-temperature synthesis of beta-cyclodextrin (β-CD) stabilized zinc sulfide nanoparticle (ZnS NP) through the chemical precipitation method has been reported for cation exchange reaction (CER) based colorimetric sensing of Pb²⁺, Cu²⁺, and Hg²⁺. Formation of β-CD stabilized ZnS NPs (ZnS@β-CD) was verified by physicochemical characterization techniques such as XRD, XPS, FE-SEM, and TEM. ZnS@β-CD NPs showed color change selectively for the metal ions Pb²⁺, Cu²⁺, and Hg²⁺ among the various metal ions including Sn²⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, and Cd²⁺. The solubility product of reactants and the transformed products are the reason for selective CER of ZnS@β-CD NPs towards Pb²⁺, Cu²⁺, and Hg²⁺ ions. ZnS@β-CD NPs dispersion revealed rapid color change from white to orange, black, and bright yellow on the addition of higher concentrations of Pb²⁺, Cu²⁺, and Hg²⁺ respectively. This color change is due to the formation of complete CER-transformed nanostructures such as PbS, CuS, and HgS in higher concentrations (10⁻¹- 10⁻³ M) of corresponding metal ions. The partial CER altered products Zn_1-x,Pb_xS, Zn_1-xCu_xS and Zn_1-xHg_xS were detected due to the appearance of pale color in the lower metal ions concentrations of 10⁻⁴ - 10⁻⁶ M. This CER assisted transformation was also monitored through spectrophotometric methods. Moreover, infrared spectroscopic analysis was used to testify the structure of CER transformed product. The synthesized ZnS@β-CD NPs act as an efficient CER-based sensor for distinguishing and determining Pb²⁺, Cu²⁺, and Hg²⁺ at different level concentrations in the aqueous solution.

Keywords: Cation exchange reactions; Colorimetric sensor; Heavy metal ions; ZnS nanoparticles; β-Cyclodextrin.

Graphical abstract

Scheme 1

Schematic diagram for the synthesis of ZnS@β-CD NPs.

Fig. 1

XRD patterns of ZnS@β-CD NPs.

Fig. 2

The XPS analysis of ZnS@β-CD NPs (a) Wide scan XPS (b) Zn 2p deconvoluted peaks (c) S 2p deconvoluted peaks (d) C 1s deconvoluted peaks, (e) O 1s deconvoluted peaks.

Fig. 3

(a) SEM Image of β-CD, (b) SEM image of ZnS@β-CD NPs, (c) EDX spectra of ZnS@β-CD NPs, (d) Elemental mapping images of ZnS@β-CD NPs.

Fig. 4

HR-TEM images of ZnS@β-CD NPs at various scale (a) 50 nm, (b) 10 nm, (c) Particle size distribution histogram, (d) SAED pattern.

Fig. 5

(a) UV–Visible absorption spectrum and (b) Tauc plot of ZnS@β-CD NPs.

Fig. 6

(a) Colorimetric and (b) UV–Visible spectroscopic analysis of ZnS@β-CD NPs towards different metal ions.

Fig. 7

(a) Colorimetric and (b) UV–Visible spectroscopic analysis of ZnS NPs (without β-CD) towards different metal ions.

Fig. 8

Colorimetric responses of ZnS@β-CD NPs towards Pb²⁺, Cu²⁺, and Hg²⁺ in presence of other heavy metal ions.

Fig. 9

(a) Colorimetric and (b) UV–Visible spectroscopic analysis of ZnS@β-CD NPs towards different concentrations of Pb²⁺. Time-resolved interactions of ZnS@β-CD NPs with Pb²⁺: (c) Photographs of 10⁻² M and 10⁻¹ M of Pb²⁺ at 0 min and after time t (50 min for 10⁻² M and 20 min for 10⁻¹ M). (d) Time-based UV–Visible spectroscopic analysis of 10⁻² M and 10⁻¹ M of Pb²⁺ metal ions.

Fig. 10

(a) Colorimetric and (b) UV–Visible spectroscopic measurements of ZnS@β-CD NPs towards different concentrations of Cu²⁺, (c) XRD pattern of CuS NS.

Fig. 11

(a) Colorimetric and (b) UV–Visible spectroscopic measurements of ZnS@β-CD NPs towards different concentrations of Hg²⁺. Time-resolved interactions of ZnS@β-CD NPs with Hg²⁺: (c) Photograph of 10⁻¹ M of Hg²⁺ at 0 min and after 10 min. (d) Time-based UV–Visible spectroscopic responses of 10⁻¹ M of Hg²⁺.

Fig. 12

(a) Colorimetric and (b) UV–Visible spectroscopic measurements of ZnS@β-CD NP added with multiple metal ions (10⁻³ M of Pb²⁺, Cu²⁺, &Hg²⁺, Pb²⁺ & Hg²⁺, Cu²⁺ & Hg²⁺, and Pb²⁺ & Cu²⁺) at 0 min and 10 min.

Fig. 13

Colorimetric responses of ZnS@β-CD towards 10⁻³ M concentrations of Pb²⁺, Cu²⁺, and Hg²⁺ metal ions (a) at 40 °C and (b) at 50 °C. UV–Visible spectroscopic measurements of ZnS@β-CD NPs towards 10⁻³ M concentrations of Pb²⁺, Cu²⁺, and Hg²⁺ metal ions (c) at 40 °C and (d) at 50 °C.

Fig. 14

(a) Colorimetric and (b–g) UV–Visible spectroscopic measurements of ZnS@β-CD NP in different pH medium in presence of 10⁻³ M concentrations of Pb²⁺, Cu²⁺ and Hg²⁺.

Fig. 15

FT-IR spectra of ZnS@β-CD NPs and its CER converted products.