Surface-functionalized CdSe quantum dots for the detection of hydrocarbons

CdSe semiconductor quantum dots (QDs) were synthesized with surface modifying groups to enhance their response towards the detection of aromatic hydrocarbons. The sensing characteristics of unmodified QDs using trioctyl phosphine oxide (TOPO) plus stearic acid (SA) surface groups were compared with QDs with additional benzoic (BA) or pentafluorobenzoic acid (FBA) surface modifying groups. The unmodified and surface-modified QDs were encapsulated into poly(methyl methacrylate) (PMMA) thin films by drop coating QD-PMMA solutions onto silicon substrates. Reversible enhancement and quenching of the photoluminescence (PL) upon film exposure to toluene and xylene vapors were both observed for the QD-SA, BA and FBA systems. PL enhancement was observed at low target gas concentrations, with the onset of the quenching process for each of the films being dependent on the target gas type and its concentration. Unmodified QD/PMMA films had detection limits for xylenes and toluene of 250 and 500 ppm, respectively in a balance of nitrogen. While detection limits of 15 ppm xylenes and 50 ppm toluene in a balance of nitrogen were achieved with the FBA- and BA-modified QD/PMMA films. The surface-modified QDs were found to provide a factor of 16 and 10 sensing enhancement, respectively, for the detection of xylenes and toluene, over the unmodified QDs.