Development of coumarins and their composites as optical sensors for Cu2+ contaminants in water and soil

Heavy metal species are non-biodegradable, and potentially toxic molecules. Due to the potential health hazards and environmental damage that heavy metals can cause, the detection, monitoring and ultimately remediation of heavy metals are important for the sustainable management of heavy metals in the environment. Among others, optical sensing methods, especially those using fluorescence-based approaches have been developed for the sensing of heavy metals where sensing is facilitated by small organic molecular sensors or nanomaterials. These sensors allow for the detection of trace-amounts of the analytes of interest using a relatively small sample amount. In this work, transition metal Cu2+ (along with its unique electronic properties) is kept at the focus as the analyte, and coumarin derivatives will be designed, synthesised, and assessed for Cu2+ sensing in aqueous liquid media. The aim of this study was to design a systematic series of novel coumarin derivatives, also, to study their interactions with Cu2+ and the resulting optical properties. The potential use of such coumarins in combination with nanomaterials was also assessed. Testing the Cu2+-sensing performance of these coumarins will also inform the development of enhanced molecular Cu2+ sensing platforms. Towards this goal, this thesis includes the following chapters. Chapter I gives an overview of the impact of heavy metals on our daily life and health, and an introduction of various molecular design principles that allow for selective Cu2+ sensing mainly focusing on fluorescent coumarin derivatives. A brief summary of the driving forces behind Cu2+ coordination by molecular probes is given as well. Finally, methods that combine multiple material types such as nanomaterials, hydrogels and polymers will be introduced. In Chapter II, a small series of structurally related coumarin derivatives were designed, synthesised, and evaluated in aqueous media containing Cu2+ to obtain a primary understanding on how molecular structure can tailor the response of these optical probes in Cu2+-sensing applications, where selectivity and sensitivity to the analyte is crucial. This Chapter is presented in the form of a peer-reviewed and published research article in MDPI Molecules (Molecules, 24(2019) 3569.). In Chapter III, based on the findings discussed in Chapter II, a more detailed and systematic study is presented on the assessment of the structural requirements towards the Cu2+ recognition site of coumarin-derived molecular probes. This is aimed at the comparison and identification of Cu2+ selective sensing probes by the screening of an extended range of coumarin derivatives under the exact same test conditions. The results of this chapter clarified what molecular structures could support the design of Cu2+ selective molecules. In Chapter IV, the combinatorial use of a selected coumarin (from Chapters II and III) with non-toxic SiO2 nanoparticles (NPs) is evaluated and described. The integration of SiO2 NPs with molecular probes is aimed at the enhancement of Cu2+-sensing performance, especially sensitivity and sensing range, in aqueous media (without the use of other solvents) to meet the required measurement range for both water and soil monitoring. In Chapter V, the application of the coumarins (developed in Chapter III) for Cu2+ sensing in tap water is discussed first. Afterwards, 3 selected coumarins from Chapter III that exhibited good sensitivity to Cu2+, but interference to Ni2+ and Hg2+, were selected and integrated into a multiplex assay. This is to create a proof-of-concept method, and to show how these non-selective coumarins can be used in a simultaneous assay for accurate quantification of several metal ions at the same time.