Abstract

Development of quantum dot fluorescence sensors for detection of targeted pesticides and polycyclic aromatic hydrocarbons

Pesticides and polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants that continue to be of great concern due to their potential toxic and human health effects. Human activities play a significant role in the release of these compounds into the environment. The occurrence of these pollutants in low concentration and the need for their continuous monitoring presents a challenge for the environmental analytical chemist. Sensitive analytical techniques that can selectively detect these compounds without being affected by the environmental matrix are required. Advances in nanotechnology have allowed for the design of sensitive and selective nanomaterials which provide alternative analytical tools for such challenges.

In this work, therefore, extensive reviews on the current application of quantum dots (QDs) and nanoparticles in designing probes for detection of pesticides and PAHs were conducted. Through these reviews, the current status on the application of QDs sensors for these compounds was assessed. Knowledge gaps were identified where it was noted that there were no QD-based sensors for atrazine and GQD sensors for PAHs. The reviews further assessed the advantages and challenges for coupling QDs to various analyte receptors to enhance selectivity and how these impact on the scaling of such sensors for routine applications.

QDs are highly fluorescent nanoparticles with interesting optical and structural properties that could be explored for designing sensitive fluorescence probes. Thus, QD-based fluorescence probes were designed for the detection of targeted pesticides and PAHs in water samples. Atrazine and pyrene were chosen as representative pesticide and PAH compounds, respectively. The structure and properties of the QD probes were studied using a range of techniques including fluorescence, UV-vis, Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), Raman, and powered X-ray diffraction (XRD).

A quantum dot coupled to molecularly imprinted polymer (QD@MIP) sensor for atrazine detection was developed and optimized. Under optimal conditions, the sensor showed excellent sensitivity and selectivity for atrazine due to the size and shape specificity of the molecularly imprinted polymer as a receptor. The developed sensor showed excellent sensitivity and could detect atrazine down to 0.80 x 10^-7 mol/L which is below the World Health Organization (WHO) guideline for atrazine in water (4.6 x 10^-7 mol/L).

A graphene quantum dot (GQD) based sensor was also developed for the detection of PAHs in water samples. GQDs offer a safer alternative to cadmium-based QDs in designing fluorescence probes. In this work, GQDs were synthesized using bottom-up and top-down approaches and a fluorescence “turn-off-on” strategy that uses GQDs with ferric ions was developed and used for rapid detection of PAHs in water, using pyrene as a case study. The probe showed a linear response to pyrene concentration within the 2 - 10 x 10^-6 mol/L range and was able to detect pyrene down to 0.325 x 10^-6 mol/L and 0.242 x 10^-6 mol/L for GO-GQDs and CA-GQDs, respectively. The potential application of the developed probe was tested using real environmental water samples spiked with known pyrene concentrations, and recoveries between 97 – 107% were obtained. This part of the thesis demonstrated, for the first time, the promising application of GQDs for environmental monitoring of PAHs in water samples.

This work demonstrated how nanotechnology advances could be harnessed and used to solve environmental challenges. The main objectives were achieved and future research recommendations to enhance the robustness of the developed sensors were proposed. These recommendations include (i) immobilizing the QD sensor materials on solid supports and investigating their re-usability, (ii) further optimization of the GQD sensor for PAHs to improve its sensitivity, (iii) exploring the possibility of using such sensors for multi-analyte detection, and lastly (iii) standardizing the QD-based sensing methods as analytical protocols that could be used in by regulators in environmental monitoring.