Abstract
D.Phil. (Chemistry)
The problem of maintaining good quality of water for domestic use and for aquatic
life remains a challenge. Water sources are often contaminated with pollutants
from natural sources such as volcanic eruptions and by human activities such as
manufacturing industries, mining, water-purification processes, agricultural
activities and a vast number of other activities. Water-purification processes used
by municipal authorities are designed to remove most of the pollutants but some
trace amounts will always remain and have been detected in drinking water and
treated waste water reservoirs. These trace amounts pose a threat to human
health and the well-being of aquatic life. The detection of these trace amounts of
pollutants is often carried out by laboratory-based techniques that require
sophisticated, expensive instruments and often require extensive sample
preparation and pre-concentration. Simple, quick and in-field detection methods
are necessary especially for remote small communities with limited or no access to
laboratories. Optical detection systems offer hope as a solution to this problem.
In this work newly developed fluorescence-based molecular sensors for the
detection of pollutants in water were developed, characterised and tested for their
sensing abilities towards organic and inorganic pollutants. The fluorescent probes
for organic pollutants were designed based on the host-guest chemistry of the
cyclodextrin molecule. Azo dye-modified β-cyclodextrins were synthesised and
linked via ethylene glycol and epichlorohydrin to produce the sensors that were
then tested for their sensing response towards chlorophenols and small aliphatic
chlorinated alkanes which are often formed during the disinfection of water in the
purification process. The sensor molecules were characterised by UV-Vis, FT-IR
and 1D and 2D NMR spectroscopy. The amount of cyclodextrin in each sensor
molecule was quantified using the anthrone method (67%) as well as by 1H-NMR
spectroscopy (72%). To demonstrate the host-guest interaction of the sensor
molecules, isothermal titration calorimetry (ITC) was used. ITC measurements
showed that modifying β-cyclodextrin and using linkers did not alter its host-guest interaction with guest molecules as demonstrated by the stoichiometry, n, stability
(or binding or association) constant (K) and thermodynamic parameters of the
interaction.
The sensor molecule linked via ethylene glycol showed selectivity towards 4-
chlorophenol among the chlorophenols investigated and has the potential to be
used in a sensor for the detection of 4-chlorophenol. The sensor molecule linked
via epichlorohydrin showed sensitivity towards chloroform, a typical disinfection
by-product. These experimental results showed that the sensor molecules could
be used for quick on-field detection of chlorinated organic compounds in water.
Sensor molecules for inorganic pollutants were based on the complex formation of
crown ethers with metal ions. The sensor was formed by modifying a dibenzo-18-
crown-6 ether molecule with an azo dye. The sensor was then characterised using
UV-Vis spectrophotmetry, FT-IR and NMR spectroscopies as well as mass
spectrometry and CHNS elemental analysis. The sensor molecule was then
subjected to different metal ions and the fluorescence change of the probe
observed. Interestingly, the sensor was highly sensitive and selective to mercury
(II) and Cu (II) ions in water. Mercury (II) is one of the most hazardous heavy
metals among the heavy-metal ions found in environmental waters and its early
detection in water sources is important. The synthesised molecular sensor can
therefore be incorporated into a simple hand-held gadget with a light source and
be used for on-field detection of mercury (II) ions in remote areas.