Fabrication of novel nanozyme based colorimetric probes for the rapid, sensitive and selective screening of restricted adulterants in consumer products: a case- study of mercury in cosmetics and thiocyanate in milk

Abstract

This thesis presents novel nanomaterials known as nanozymes that were synthesized and applied as simple, easy-to-use colorimetric probes for the rapid, sensitive and selective detection of restricted low quality, cheap, and toxic substances usually added fraudulently to consumer products especially in food and cosmetics as adulterants; mercury in cosmetics and thiocyanate in milk. We synthesized nanomaterials known as nanozymes, which possess intrinsic catalytic activity toward certain colorimetric reactions. In the presence of model adulterants, mercury and thiocyanate, the catalytic-like activities of the nanozymes on these reactions were suppressed and the intensity of the accompanying color changes which were distinct to the naked eye were adulterant concentration dependent. The instant color changes allowed for the prepared materials to be explored as rapid colorimetric probes for the detection of restricted substances, mercury and thiocyanate in cosmetics and milk, respectively. Both materials exhibited excellent analytical performance as marked by very low limits of detection (LoDs); 2.34 nM for the mercury nanozyme and 2.89 μM for the thiocyanate nanozyme which were well below acceptable levels of 4.99 μM for mercury in cosmetics and 0.24 mM for thiocyanate in milk as set by the US Food and Drug Administration (FDA) and International Dairy Federation (IDF) respectively, and within the linear dynamic ranges of 0-10nM for mercury and 2-50 μM for thiocyanate. High recoveries ranging from 89.4 to 118.1 % for both nanozymes accompanied by excellent selectivities toward Hg 2+ and SCN− over potentially interfering species were obtained. The excellent characteristics of the developed nanozymes presented them as simple, fast, sensitive, selective, visual, equipment free point-of-need diagnostic tools of high throughput for adulterant analysis in consumer products.