Uptake and release of cargo molecules from responsive surfaces

Abstract

The development of smart surfaces is of interest for a wide range of applications such as drug-delivery, anti-fouling and lubrication. These layers are stimuli-responsive enabling the controlled uptake and release of a cargo molecule. The aim of this work is to develop methods for monitoring such processes in thermoresponsive systems. To do so, it is required to produce responsive surfaces and design methods to probe their responsiveness as well as the binding of cargos.

I present synthetic strategies to graft poly(N-isopropylacrylamide) (pNIPAM), a pNIPAM-based diblock copolymer and polyvaline from silica substrates as well as
a procedure for generating mixed surfactant layers of didodecyldimethylammonium bromide (DDAB), and deuterated sodium dodecylsulfate (d-SDS) with defined composition.
The main technique for investigating these films is Total Internal Reflection (TIR) Raman spectroscopy. This chemically specific and surface selective technique
allows the monitoring of phase transitions inside the layers. It also provides insight on the accumulation of cargo molecules or changes in the layers' composition. A novel Raman imaging technique provides insight into the uniformity of the layers and cargo distribution within them.

First, I studied the potential of the cationic surfactant DDAB to transport the anionic surfactant d-SDS onto a silica surface. I show that the adsorption was possible
only when the mixture is on the cationic side implying the electrostatic interactions between the vesicles and the surface govern the adsorption. Once the coadsorption had taken place, I triggered the gel-liquid phase transition of DDAB in order to induce a phase separation on the surface. No preferential desorption of d-SDS was observed.

Second, I looked at grafted pNIPAM at the silica-water interface and present data on the phase transition showing a change in the hydration of the polymer. I then
introduce three cargo molecules (benzamide, d-malonamide and potassium thiocyanate) which have the potential to bind onto one state of pNIPAM via hydrophobic interactions, hydrogen bonding or specific ion interactions. TIR Raman spectroscopy
was shown to be able to detect small quantities of the molecules but ultimately showed no selective binding.

Third, the focus shifts to grafted polyvaline layers since they are less hydrophobic than pNIPAM. I determine the secondary structure of the grafted chains and present
the impact of temperature. All adsorption measurements with the cargo molecules introduced in the previous chapter showed no selective binding.

Fourth, I report on a diblock copolymer with an inner cationic block poly((2-dimethylamino) ethyl methacrylate) (pDMAEMA) and an outer neutral thermoresponsive
block, pNIPAM. The copolymer was thermoresponsive. Cargo adsorption measurements showed accumulations of each molecule inside the film. The electrostatics driven
adsorption of KSCN was stronger at higher temperatures.