Luminescent PbS and PbS/CdS quantum dots with hybrid coatings as nanotags for authentication of petroleum products

Abstract

There is an urgent need to tag some valuable liquid products, such as petroleum products, for authentication. However, it is a challenging task because of the strong autofluorescence of liquid petroleum products in the visible range and the chemically active and harsh medium. Therefore, strongly luminescent, near-infrared (NIR) fluorophores with long-term stability are needed. The use of NIR quantum dots (QDs), such as lead chalcogenides, seems to be the best approach; however, most widely used coatings do not provide enough stability, and QDs are quenched in a short time in liquid petroleum products. Here, we report for the first time the synthesis of highly luminescent, ultrasmall, NIR-emitting PbS and PbS/CdS QDs with a hybrid coating consisting of oleylamine (OLA), 1-dodecanethiol (DT), and poly(methacrylic acid) (PMAA), adopting a simple, greener synthetic method. The photoluminescence (PL) emission wavelengths of these QDs were tuned between 700 and 1100 nm for detection with low-cost, widely used silicon detectors, which allows easy translation of such QDs as luminescent nanotags to serve as a means for the authentication of goods, such as petroleum. In the nanoparticle design, a thin layer of a CdS shell deposited by a cation-exchange process was adopted to enhance the emission intensity and stability of PbS QDs. The influence of postsynthetic ligand exchange of OLA with DT on the stability is also shown. PMAA in the coating provided a significant blue shift in the peak maxima, enhanced the luminescence intensity, and, most importantly, improved the long-term stability of QDs, especially in liquid petroleum products (oil, gasoline, and diesel). Such stability and size tunability was utilized to create binary barcodes. Hence, these QDs are shown as promising luminescent nanotags for liquid petroleum products. The development of such stable QD-based nanotags offers an invaluable use of nanotechnology for optical barcode generation.