ORIGINAL PAPER Silver-doped CdS quantum dots incorporated into chitosan-coated cellulose as a colorimetric paper test stripe for mercury Nutthaya Butwong 1 & Pimpanitpa Kunthadong 1 & Phimpha Soisungnoen 1 & Chatrachatchaya Chotichayapong 1 & Supalax Srijaranai 2 & John H. T. Luong 3 Received: 6 January 2018 /Accepted: 10 January 2018 # Springer-Verlag GmbH Austria, part of Springer Nature 2018 Abstract Silver-doped CdS quantum dots capped with mercaptoacetic acid were incorporated into a chitosan-coated filter paper to fabricate a colorimetric test stripe for mercury. Chitosan assisted in the stabilization of CdSAg QDs on cellulose and enhanced the analyte diffusion in the test stripe. Hg(II) ions were captured by mercaptoacetic acid on CdSAg, resulting in a visualized color change from yellow to deep brown. For enhanced sensitivity and quantitation with good reproducibility, the color intensity was recorded, transmitted to a smartphone camera and computed by the ImageJ software to provide a digital readout. The assay was rapid, simple, and selective for mercury as several potential interfering species provoked no signal response. The applicability of the sensing approach was demonstrated for the analysis of spiked Hg(II) ions in cosmetic cream with a detection limit of 124 μM. Keywords Ag-doped CdS quantum dots . Mercaptoacetic acid . Visual detection Introduction Skin whitening is a popular cosmetic technique, especially in Asian countries in order to circumvent melamine formation and hyperpigmentation of aging spots or dark spots related to preg- nancy [1]. Among all skin bleaching agents, mercury is efficient in suppressing the synthesis of melamine [1]. The use of mer- cury is forbidden or significantly restricted in numerous coun- tries because of their adverse effects on human health [2]. Nevertheless, commercial creams with over 2.5 wt.% ammoni- ated mercury [1] are still available, which might impair the brain, nervous system and kidneys for long-term and repeated uses [2]. Albeit many hyphenated techniques can detect mercu- ry below 0.1 nM [3], a sample preparation step associated with expensive instrumentation is often mandatory. Thus, an inex- pensive and user-friendly method for mercury in skin-bleaching cosmetics or other complex matrices is of interest for field/spot testing or the consumer level. To date, various colorimetric sensors for determination of Hg 2+ have been widely advocated [4]. Considerable efforts have focused on nanoparticles (NPs), especially Au NPs, Ag NPs, and carbon NPs (carbon dots and graphene), based on particle-surface interactions and optical properties [4–8]. These methods provide very low detection limits, however, most of these sensors must be operated in an aqueous solution, e.g., drinking water and lake water and sen- sitive to the solution pH. Thus, a colorimetric sensing scheme with low cost, simplicity, ease of operation, and rapid response is of importance, particularly for the field or spot testing. In this context, paper-based sensing can fulfill such requirements be- cause low-cost cellulose provides an easy and accessible test stripe. Paper-based colorimetric test stripes incorporation with nanomaterials offer enhanced detection sensitivity and im- proved long-term photostability [9]. High surface-to-volume nanomaterials with remarkable optoelectronic and chemical properties have been attempted Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00604-018-2671-3) contains supplementary material, which is available to authorized users. * Nutthaya Butwong nbutwong@yahoo.com 1 Applied Chemistry Department, Faculty of Sciences and Liberal Arts, Rajamangala University of Technology Isan, 744, Suranarai Rd., Nakhon Ratchasima 30000, Thailand 2 Materials Chemistry Research Unit, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand 3 Innovative Chromatography Group, Irish Separation Science Cluster (ISSC), School of Chemistry and the Analytical & Biological Chemistry Research Facility (ABCRF), University College Cork, Cork, Ireland Microchimica Acta https://doi.org/10.1007/s00604-018-2671-3