Preparation of polydimethylsiloxane thin films containing immobilized CdSeTe/ZnS quantum dots for water sensing applications

Abstract

Pollutants in water systems are a significant environmental problem, as they can have harmful effects on both human health and the ecosystem. Here we developed a robust method for preparing CdSeTe/ZnS core/shell (C/S) quantum dots (QDs) immobilized into polydimethylsiloxane (PDMS) thin films for pollutant sensing applications. Highly fluorescent hydrophobic QDs were first synthesized using the hot-injection organometallic approach with the use of hydrophobic trioctylphosphine oxide and oleic acid ligands as capping agents. The C/S QDs had an average particle diameter of 4.0 nm and had broad absorbance in the ultraviolet-visible region. Their maximum fluorescence was at 594 nm and the photoluminescence quantum yield of the C/S QDs relative to rhodamine 6G was 47%. The C/S QDs were then immobilized into PDMS (to form QD@PDMS) by means of an optimized spin coating procedure onto glass slides at a speed of500 rpm and acceleration of 300 rpm/s for 10 s, followed by curing at 80 ºC for 15 min. This thin film format enabled direct use and analysis of the thin films by submersion into water samples followed by fluorescence spectroscopy. The homogeneity of the QD@PDMS thin films thus produced was excellent, as determined by visual inspection under an ultraviolet lamp and by fluorescence spectroscopy. An excitation wavelength of 400 nm led to the highest fluorescence intensity of the thin films and repeated exposure to this excitation wavelength did not have a major negative impact on the fluorescence emission of the QD@PDMS thin films, as the difference between the maximum and minimum fluorescence emission intensity was 8% over 60 repeated measurements. A mixture of water and ethanol (2:1) had the smallest effect on the fluorescence of the films and was thus the most effective matrix for sensing of organic pollutants. The interaction of the QD@PDMS films with individual organic pollutants (polar atrazine pesticide and non-polar phenanthrene) and with real water samples, resulted in a change in fluorescence of the thin films indicating that they are a promising candidate for sensing pollutants in water. The relative uptake and interaction of organic compounds by the thin films would need to be determined and their selectivity towards target analytes investigated for a particular application, in order to assess the viability of use of the QD@PDMS thin films to screen for the presence of the analytes and to semi-quantify them using fluorescence.