Quantum dot - molecularly imprinted polymer nanomaterials for the fluorescence sensing of selected pharmaceutical and personal care products

Abstract

This research was aimed at fabricating and applying cadmium based core/shell quantum dots (QDs) as fluorescence probes for the detection in aquatic compartments of triclosan (TCS) and acetaminophen (AC) which fall within the pharmaceutical and personal care products (PPCPs) subclass of emerging chemical pollutants (ECPs). Semiconductor quantum dots possess significant advantages such as high fluorescence intensity, long-term photostability, narrow emission and broad absorption as well as high photoluminescence quantum yields, and they have thus attracted considerable research attention for environmental monitoring and sensing applications. Core/shell CdSe/ZnS QDs were synthesized through the one-pot organometallic approach in order to explore the fluorescence sensing of the QDs towards triclosan. A ligand exchange reaction was also performed with glutathione (GSH) as a surface capping agent to convert hydrophobic QDs to water soluble QDs, where the mercapto moiety of GSH binds onto the surface of the QDs and their carboxyl groups provide water solubility. The resultant quantum dots were characterized using various state of the art instruments including fluorescence and UV/Vis spectroscopy, powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), high resolution transmission electron microscopy (HRTEM), and energy dispersive X-ray spectroscopy (EDS). Characterization techniques confirmed the successful synthesis of QDs and the formation of a GSH coating thereon. The synthesized GSH-CdSe/ZnS QDs had a high photoluminescence quantum yield of 89% and an average particle size of 3.3±1.3 nm, as determined by HRTEM analysis, which agreed well with the size determined from the first excitonic absorption peak (3.7 nm). In addition, the QDs exhibited strong and narrow fluorescence intensity with an estimated full width at half maximum of 32.7 nm. The presence of triclosan in the proximity of the QDs enhanced the fluorescence intensity of the QDs. The experimental results showed that under optimum conditions (i.e. 1.5 mg GSH-CdSe/ZnS QDs in 3.0 mL of Millipore water with an incubation time of 5 min and excitation and emission wavelengths of 300 nm and 598 nm, respectively), the calibration plot of F-F0/F0 of the GSH-CdSe/ZnS QDs was proportional to triclosan concentration in the range of 10-300 nmol L-1 (F and F0 are the fluorescence intensity of the QDs in the presence and absence of triclosan respectively). The mechanism was likely based on Förster resonance energy transfer (FRET) from the analyte to the QDs which provided a fluorescence “turn-on” nanosensor for the sensing of triclosan. The Förster distance in the system of GSH-CdSe/ZnS QDs-TCS was found to be approximately 4.0 nm which meets the distance required (<10 nm) for FRET to occur. The developed QDs were found to be a sensitive probe for the determination of triclosan with a very low detection limit of 3.7 nmol L-1 in the presence of other structurally related compounds. The applicability of the probe was evaluated for the determination of triclosan in South African tap and river water matrices which indicated good recoveries of 94%-117.5%. In the next study, the performance of three QD-ligand systems were tested to find the optimum system with high sensitivity to acetaminophen. In this regard, three water-soluble QDs were synthesized through the use of hot, coordinating solvents of octadec-1-ene and oleic acid. In addition, ligand exchange reactions using L-cysteine (L-cys), N-acetyl-L-cysteine (NAC) and glutathione (reported earlier for triclosan detection) as thiol capping agents were performed. L-cys-CdSe/ZnS QDs produced enough sensitivity and were utilized as the optimized system for the determination of acetaminophen. Therefore, characterizations and analytical performance of the sensor for acetaminophen detection were investigated by using aqueous L-cys-CdSe/ZnS QDs. L-cys bound efficiently to the surface of the QDs which was confirmed by different characterisation techniques such as Fourier-transform infrared spectroscopy, high resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy. The photoluminescence properties of the synthesized L-cys-CdSe/ZnS QDs included a quantum yield of 77% with a low full width at half maxima value of 38.5 nm for L-cys-CdSe/ZnS QDs, which indicated the formation of a uniform and homogenous particle size distribution. The average particle size of the L-cys-CdSe and L-cys-CdSe/ZnS QDs determined from HRTEM were 2.1±0.5 and 5.1±0.8 nm, respectively which correlated well with the size from the first excitonic absorption peaks (2.8 nm and 4.0 nm respectively). The utilization of the L-cys-CdSe/ZnS QDs as fluorescent probes for the determination of acetaminophen in water samples was optimised for various parameters including concentration of L-cys-CdSe/ZnS QDs and incubation time. The optimum amount of L-cys-CdSe/ZnS QDs was found to be 1.0 mg in 3.0 mL Millipore water with an incubation time of 5 min. The reaction mechanism proved that the fluorescence “turn-on” effect of the L-cys-CdSe/ZnS QDs in the presence of acetaminophen was due to Förster resonance energy transfer from acetaminophen (donor) to the QDs (acceptor) with a Förster distance of 6.1 nm. Experimental investigation showed that the FRET-sensitized QD emission intensity at 595 nm was enhanced with the introduction of acetaminophen over the linear range of 3.0-100 nmol L-1. The applicability of the QDs to determine AC concentrations in the presence of some related pharmaceuticals including epinephrine hydrochloride, L-ascorbic acid, uric acid, dopamine hydrochloride and 4-aminophenol were tested and no significant interferences were found. In addition, the probe could be applied for the detection of acetaminophen in water matrices at concentrations as low as 1.6 nmol L-1 with recoveries of 90-108% which is environmentally relevant.