The development of quantum dot thin films for sensing organic pollutants in water

Abstract

Organic pollutants in water are a significant environmental problem, as they can have harmful effects on both human health and the ecosystem. Atrazine and phenanthrene are two common organic pollutants that are often found in water sources. In this study, the aim was to develop a fluorescence sensor that could detect these pollutants in water using a combination of quantum dots (QDs) and polydimethylsiloxane (PDMS) polymer. The study involved the synthesis of quantum dots, which were then interacted with the target organic molecules in solution to confirm proof of concept. The QDs were then immobilized into PDMS, and the thin film synthesis methodology was optimized. The films were then interacted with the target analytes in synthetic standard solutions as well as an inlet water sample obtained from Rand Water. Hydrophobic QDs were successfully synthesized by the organometallic hot-injection method. The hydrophobic nature of the QDs was maintained to facilitate their optimal interaction with the PDMS. The QDs were synthesized twice and the second batch resulting in CdSeTe core QDs and CdSeTe/ZnS core/shell (C/S) QDs were used further in the study. The maximum emission wavelength peak of the core QDs was 588 nm and upon the addition of the shell this red-shifted to 594 nm after 60 min of the reaction. The size of the QDs was determined using transmission electron microscopy (TEM) and it was found that core and C/S QDs had average particle diameters of 3.06 and 4.02 nm, respectively. A size increase was observed as the shell layer had been coated over the QD core. There was also a slightly larger size distribution for the C/S QDs than what was observed with the core QDs with full width half maximum (FWHM) values of 31.0 and 49.5 nm respectively. The optimal excitation wavelength for the QDs in chloroform solution was 420 nm and an emission range of 430-800 nm was used to measure changes in fluorescence with 4 nm slit widths. When the QDs in chloroform solution were interacted with atrazine for 1 min, quenching was observed. This correlated to literature and served as proof of concept for the rest of the experiments. The photoluminescence quantum yield (PLQY) of the QDs was measured by comparison to Rhodamine 6G in ethanol and was calculated to be 47%. The calculated PLQY of the QDs was lower than expected as the fluorescence of the C/S QDs was very bright even when they were immobilized in the PDMS to produce QD@PDMS thin films. Various film preparation parameters were tested and optimised, including those relating to the spin coater, the QD concentration in the film, film curing temperatures, as well as fluorescence measurement parameters. It was found that spin coating the PDMS at a speed of 500 rpm (300 rpm/s) for 10 s and then curing the films on a hot plate at a temperature of 80 oC for 15 min produced the best quality films with optimal, evenly distributed QDs and no bubbles within the material or striations over the surface. These were the optimized film spin coating parameters used further in the study. The QD concentration within the films was also varied where two different concentrations were tested. Films with low QD concentration (LQD) and high QD concentration (HQD) were prepared by dissolving QDs in chloroform to form solutions with concentrations of 0.004 g/mL and 0.008 g/mL, respectively. In each case, the QD@chloroform solution was then sonicated for 30 min to fully disperse the QDs, whereafter the curing agent and PDMS were added using a ratio of 1 mL QD@chloroform solution: 1 g curing agent: 10 g PDMS. The excitation wavelength was optimized for the QD@PDMS films, and a wavelength of 400 nm was used with an emission range of 410-700 nm and slit widths of 2 nm. The film thicknesses were measured by breaking the films under liquid nitrogen and examining them with a scanning electron microscope (SEM). The thicknesses ranged from 250.44 µm for HQD films to 437.68 µm for a film with no QDs. These values, along with the film masses, were used to calculate the concentration of QDs within the films. The concentrations were determined to be 0.28 and 0.51 mg/mL for LQD and HQD films, respectively. The thin films were then interacted with atrazine. Atrazine is a widely used herbicide that belongs to the class of triazine herbicides. It is an organic, polar compound which is highly soluble in water and other polar solvents. The interaction between QD@PDMS films and various concentrations of atrazine in ethanol for 1 min was tested. The LQD films exhibited quenching (as expected from literature based on measurements in solution), while the HQD films showed enhancement. The effect of various solvents on the fluorescence of the films was tested, by immersing the films in various solvents (100% H2O, 100% ethanol and a mixture of the two, H2O: ethanol (2:1)) for 24 hours. Enhancement was seen for both LQD and HQD films with 100% H2O and 100% ethanol, whilst slight quenching was seen with H2O: ethanol (2:1). It was found that the solvents used did not have a significant effect on the fluorescence properties of the films, therefore any changes observed in fluorescence were attributed to the interaction of an organic molecule with the films. The LQD and HQD thin films were then interacted with phenanthrene (Phe), which is a polycyclic aromatic hydrocarbon (PAH) that is composed of three fused benzene rings and is non-polar. It therefore has a strong affinity for non-polar PDMS surfaces, due to the intermolecular van der Waals forces that exist between the two non-polar entities. Three films each of optimized LQD and HQD were immersed in 4 mL phenanthrene solutions containing H2O: ethanol (2:1) solvent at concentrations of 0, 5.61 × 10-6, and 5.61 × 10-5 M for a duration of 24 hours. The fluorescence of the films was measured before and after the interaction with the solutions, and again after 21 days of standing on the laboratory bench. Enhancement was observed for LQD and HQD films that were interacted with Phe, where films tested with solvent blanks showed quenching. For LQD films, the F/F0 value increased as the Phe concentration increased, which correlated to literature where it was previously seen that L-cysteine capped CdSeTe/ZnSe/ZnS QDs conjugated to graphene oxide (GO) showed fluorescence enhancement with increasing concentrations of phenanthrene (Adegoke & Forbes, 2016). The LQD and HQD films were then tested with a water sample obtained from Rand Water (RW) from the inlet to a water treatment plant. No significant results were observed with unconcentrated, filtered water sample. The pre-concentration of 400 mL inlet water was achieved through solid-phase extraction (SPE). When the films were immersed and interacted with a 3 mL concentrated extract in ethanol and deionized water for 24 hours, quenching was observed for both LQD and HQD films with F/F0 values of 0.75 and 0.83 respectively, which likely indicated the presence of organic pollutants in the water sample. It was observed that when the films were exposed to ambient light over time, there was enhanced fluorescence, whereafter storing films in complete darkness reduced the enhanced illumination of the films close to the original values before exposure to ambient light. Thus, it was found that the films are very sensitive to ultraviolet and ambient light, and the effect that light has on the QD fluorescence emission should thus be investigated further. It is thus recommended that the films be stored in the dark after synthesis and prior to use. Additional research into the influence of variations in exposure to light on the fluorescence emission of QD@PDMS is crucial for future development of this material as a fluorescence sensor. This study has shown the potential of the QD@PDMS thin films in the sensing of non-polar organic pollutants (specifically PAHs) in water and has successfully developed a facile means to reproducibly produce thin films for fluorescence sensing.