Analysis of diffusion in solute-polymer systems and ion sensing using fluorescence techniques

Abstract

Monitoring the mechanisms and various effects of solute, gas vapors or very tiny particulate samples with their surrounding media typically involve multiple collections and analyses with various instruments, which is time-consuming and instrumental dependent. The development of rapid, nondestructive, in-situ and precise methodologies are essential. Fluorescent probes allow us to visualize the interactions and activity of target reagents in, for examples, biochemistry, environmental systems and materials physical behaviors. By monitoring luminescent changes in the structure of the samples or the interactions of the probes, the science or mechanisms could be understood. In this thesis, three works were established; with the use of fluorescence to 1) monitor the solute transport in various polymeric systems and to 2) trace ions and 3) quantify on ligand-ions bounded and unbounded complexes. It is fundamentally important to understand the aqueous solution transport across polymer films. Transport is widely occurred in separation techniques, biological transport, drug delivery, food barrier and flow through gels etc. A new method based on confocal laser scanning microscopy (CLSM) has been developed for in situ visualizing and detecting solute dynamics in polymeric film in a non-invasive way. The method was successfully applied to investigate the effects of (1) the presence of impermeable fillers; (2) cross-link density; and (3) film thickness on solute transport in polymeric-nanocomposite films. An aqueous solution of fluorescence dye (Rhodamine-6G) was used as penetrant. Its transport across montmorillonite (MMT) reinforced polyvinyl alcohol (PVA) nanocomposite films were observed by monitoring the thickness fluorescence intensity (the transport profile) against time. For neat PVA films, the results show that the crosslinking ratio governs the sorption dynamics and the surface hydrophilicity of these films. The rate of solute transport increases with thinner films. The addition of MMT significantly alters the transport properties of PVA films by changing the surface wettability, and by creating obstacles and forcing the solutes to take a tortuous path through the thickness of the film. The effect of MMT concentration was shown to be the dominant factor that governs the transport property of PVA-MMT nanocomposite films. The way of monitoring will be crucial to our understanding on the physics of diffusion and flow of small molecules across membrane and hence will be very beneficial to packaging and filtration technology. The second work was on developing a sensor membrane for the determination of Zn(II) ions. An optical sensor for highly sensitive and selective determination of toxic metal ions is beneficial to wastewater purification and food safety. The sensing membrane consisted of PVA and fluorescent ligands 8-hydroxyquinoline-5-sulphonic acid (8HQS) which selectively read zinc(II) ions. First, the fluorescence properties of the 8HQS ligand and its ion complex were characterized. Under the exposure of UV light, a strong fluorescence was exhibited when the 8HQS ligand dissolved in water complexed with the exterior zinc(II) ions, and the ligand itself was very poorly emissive. In membranes, the resultant ligand-polymer without zinc(II) ions also exhibited weak fluorescence. Upon sessile drops of zinc(II) solution on the membrane surface, a significant change in fluorescence intensity could be instantaneously visualized, its intensity increased with the increase of the concentration of zinc ions complexed. Several parameters of membranes were studied. In addition, this sensor also showed good selectivity over common metal ions. The polymer film was successfully adopted to screen and quantify the amount of zinc(II) ions rapidly in water with satisfactory results. Furthermore, a preliminary result on the lifetime characteristics of ligand-metal ions system was reported. We have demonstrated that a time resolved fluorescence spectrometry measurement technique provided an assessment on the contributions between the interacted or non-interacted form of the metal-chelated ligand, towards its responsive zinc ions in water and PVA medium. The decay in both media consisted of a short lifetime term with an increasing amplitude ratios and a long lifetime term with decreasing amplitude ratios due to two physical reasons 1) Zn(II)-bounded ligand and 2) free unbounded ligand. This quantification on ligand-ions complexes could be used for tracing molecular dynamics in biological and solid polymeric materials.

Date of Award	2 Oct 2013
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwok Yiu Robert LI (Supervisor)