Portably colorimetric paper sensor based on ZnS quantum dots for semi-quantitative detection of Co2+ through the measurement of grey level

Cobalt ion (Co2+) is known to be harmful for human beings, and effectively portable sensor is one of the key for its prophylaxis and treatment. In this work, Zn(CH3COO)(2)center dot 2H(2)O, Na2S center dot 9H(2)O and thioglycolic acid were used to prepare ZnS quantum dots through a facile water-bath method. The structure and morphology of the ZnS quantum dots were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. Scanning electron microscope (SEM) was also used to observe the surface morphology of the test paper prepared by a simple dipping-drying process. The Co2+ sensing properties of as-prepared test paper were investigated. And it was found that the color of as-fabricated test paper could change from white to brown within 1 min with addition of Co2+ aqueous solution, which makes it possible for qualitative detection of Co2+ through naked eyes. In this mode, a widely detectable range from 10 to 1000 mg L-1 is reached by the test paper. In addition, through measuring the grey level of the test paper by ordinary smart phone and laptop, a semi-quantitative function of the test paper is realized. Moreover, the as-fabricated paper sensor can resist the interference of a serious cations and anions, and suits for a wide pH surrounding from 1.08 to 9.65, which makes it a promising Co2+ sensor for practical use. In addition, through X-ray diffraction and UV-vis spectra, the sensing properties of as-fabricated test paper are attributed to the chemical reaction between as-prepared ZnS quantum dots and Co2+ ions. And the formation of CoS2 should be responsible for the color change from white to brown. (C) 2018 Elsevier B.V. All rights reserved.