Stimuli-responsive nanogels for environmental and pharmaceutical applications

Abstract

The term microgel has been widely used to describe particles that swell but do not dissolve in a solvent. Traditionally they can be anything from 100 nm – 100 μm in size. This project is devoted principally to investigation of the swelling/deswelling properties of largely submicron poly(N-isopropylacrylamide) [PNIPAM] microgel particles and its derivatives and also poly(2-vinylpyridine) [PVP] microgel particles. PNIPAM microgel particles are temperature-responsive because of the hydrophobic isopropyl group and the hydrophilic amide group present in its side chains. PVP microgel particles are pH-responsive due to the pyridine groups. Surfactant free emulsion polymerization (SFEP) and emulsion polymerization techniques were employed in order to copolymerize PNIPAM with acrylic acid (AA), with 3-acrylamidophenylboronic acid (3-APB) and (3-acrylamidopropyl)trimethylammonium bromide (ATMA) and with 1-vinylimidazole (VI). The resultant microgel particles exhibited multi-responsive behaviour being sensitive to changes in temperature, pH and the PNIPAM-co-3-APB-ATMA microgels were sensitive to concentration of glucose, whilst the PNIPAM-co-VI microgels were sensitive to certain metals, copper in particular. These microgel particles were characterized using dynamic light scattering (DLS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The behaviour of the particles under various conditions of temperature, pH, glucose and metal ion are described and discussed in this work and several observations, such as swelling/deswelling transitions of PNIPAM-co-VI and PNIPAM-co-3-APB-ATMA with increase of concentration of added copper (II) and glucose respectively, were reported for the first time. The microgel containing AA exhibit characteristic temperature-sensitive behaviour with volume phase transition temperature (VPTT) being in the range of 35o-40oC and showed pH-sensitive features as the particles collapsed at low and swelled at high pHs. The PNIPAM-co-VI microgels undergo swelling before the concentration of Cu2+ reaches 0.3 g/L due to adsorption of the cations inside the particle which leads to charging up the internal phase of the microgel. Hence, the repulsion forces of positively charged Cu (II) ions are dominating over contraction forces of complex binding. However, at higher concentrations of copper (II) ions the binding forces of complexation between Cu2+ and imidazole groups of the microgels are leading to conformation of the microgel backbone, and hence weaker polymer-solvent interactions. Therefore, it is favourable that solvent would be forced out of the particle resulting into the collapse. In addition, the copper (II) uptake was calculated and the uptake was found to be well described by the Langmuir adsorption isotherm. The impact of other metal ions, such as nickel, zinc, iron and silver, was also investigated. The microgels swelled upon addition low concentrations of corresponding metal ions, however aggregation has been observed at higher concentrations. The microgels containing various concentration of VI were also examined on sensitivity to the temperature and pH changes. The investigation of such microgels with increasing temperature showed similar behaviour to those containing AA as the microgel particles shrunk continuously and the LCST has been shifted to higher temperatures (in the range of 35o-45oC). The particle size of these microgels was also investigated as a function of pH; the microgel particles swelled at low and collapsed at high pHs. The particle size of the PNIPAM-co-3-APB-ATMA microgels was investigated both as a function of temperature and glucose concentration. The microgels showed typical behaviour of the PNIPAM microgels copolymerized with functional monomer, which were 3-APB and ATMA, by continuous shrinking with increasing temperature and shifted LCST towards higher temperatures. Additionally, these microgels showed swelling behaviour with the increase of glucose concentration at physiological conditions, i.e. particles swelled in the range of glucose concentration between 0.1 and 10 mmol/L at 35oC and pH 7.5. The behaviour of these microgels was also investigated at 35oC and pH 8.5 as a function of glucose concentration. Although the swelling of the particles was slightly larger at pH 8.5, considerable swelling was also observed at pH 7.5 making them the first microgel system to be glucose sensitive at physiological pH and temperature.