Refine
Has Fulltext
- yes (2) (remove)
Document Type
- Doctoral Thesis (2)
Language
- English (2) (remove)
Keywords
- responsive polymer (2) (remove)
Institute
In this thesis, a route to temperature-, pH-, solvent-, 1,2-diol-, and protein-responsive sensors made of biocompatible and low-fouling materials is established. These sensor devices are based on the sensitivemodulation of the visual band gap of a photonic crystal (PhC), which is induced by the selective binding of analytes, triggering a volume phase transition.
The PhCs introduced by this work show a high sensitivity not only for small biomolecules, but also for large analytes, such as glycopolymers or proteins. This enables the PhC to act as a sensor that detects analytes without the need of complex equipment.
Due to their periodical dielectric structure, PhCs prevent the propagation of specific wavelengths. A change of the periodicity parameters is thus indicated by a change in the reflected wavelengths. In the case explored, the PhC sensors are implemented as periodically structured responsive hydrogels in formof an inverse opal.
The stimuli-sensitive inverse opal hydrogels (IOHs) were prepared using a sacrificial opal template of monodispersed silica particles. First, monodisperse silica particles were assembled with a hexagonally packed structure via vertical deposition onto glass slides. The obtained silica crystals, also named colloidal crystals (CCs), exhibit structural color. Subsequently, the CCs templates were embedded in polymer matrix with low-fouling properties. The polymer matrices were composed of oligo(ethylene glycol) methacrylate derivatives (OEGMAs) that render the hydrogels thermoresponsive. Finally, the silica particles were etched, to produce highly porous hydrogel replicas of the CC. Importantly, the inner structure and thus the ability for light diffraction of the IOHs formed was maintained.
The IOH membrane was shown to have interconnected pores with a diameter as well as interconnections between the pores of several hundred nanometers. This enables not only the detection of small analytes, but also, the detection of even large analytes that can diffuse into the nanostructured IOH membrane. Various recognition unit – analyte model systems, such as benzoboroxole – 1,2-diols, biotin – avidin and mannose – concanavalin A, were studied by incorporating functional
comonomers of benzoboroxole, biotin and mannose into the copolymers. The incorporated recognition units specifically bind to certain low and highmolar mass biomolecules, namely to certain saccharides, catechols, glycopolymers or proteins.
Their specific binding strongly changes the overall hydrophilicity, thus modulating the swelling of the IOH matrices, and in consequence, drastically changes their internal periodicity. This swelling is amplified by the thermoresponsive properties of the polymer matrix. The shift of the interference band gap due to the specific molecular recognition is easily visible by the naked eye (up to 150 nm shifts). Moreover, preliminary trial were attempted to detect even larger entities. Therefore anti-bodies were immobilized on hydrogel platforms via polymer-analogous esterification. These platforms incorporate comonomers made of tri(ethylene glycol) methacrylate end-functionalized with a carboxylic acid. In these model systems, the bacteria analytes are too big to penetrate into the IOH membranes, but can only interact with their surfaces. The selected model bacteria, as Escherichia coli, show a specific affinity to anti-body-functionalized hydrogels. Surprisingly in the case functionalized IOHs, this study produced weak color shifts, possibly opening a path to detect directly living organism, which will need further investigations.
LCST-type synthetic thermoresponsive polymers can reversibly respond to certain stimuli in aqueous media with a massive change of their physical state. When fluorophores, that are sensitive to such changes, are incorporated into the polymeric structure, the response can be translated into a fluorescence signal. Based on this idea, this thesis presents sensing schemes which transduce the stimuli-induced variations in the solubility of polymer chains with covalently-bound fluorophores into a well-detectable fluorescence output. Benefiting from the principles of different photophysical phenomena, i.e. of fluorescence resonance energy transfer and solvatochromism, such fluorescent copolymers enabled monitoring of stimuli such as the solution temperature and ionic strength, but also of association/disassociation mechanisms with other macromolecules or of biochemical binding events through remarkable changes in their fluorescence properties. For instance, an aqueous ratiometric dual sensor for temperature and salts was developed, relying on the delicate supramolecular assembly of a thermoresponsive copolymer with a thiophene-based conjugated polyelectrolyte. Alternatively, by taking advantage of the sensitivity of solvatochromic fluorophores, an increase in solution temperature or the presence of analytes was signaled as an enhancement of the fluorescence intensity. A simultaneous use of the sensitivity of chains towards the temperature and a specific antibody allowed monitoring of more complex phenomena such as competitive binding of analytes. The use of different thermoresponsive polymers, namely poly(N-isopropylacrylamide) and poly(meth)acrylates bearing oligo(ethylene glycol) side chains, revealed that the responsive polymers differed widely in their ability to perform a particular sensing function. In order to address questions regarding the impact of the chemical structure of the host polymer on the sensing performance, the macromolecular assembly behavior below and above the phase transition temperature was evaluated by a combination of fluorescence and light scattering methods. It was found that although the temperature-triggered changes in the macroscopic absorption characteristics were similar for these polymers, properties such as the degree of hydration or the extent of interchain aggregations differed substantially. Therefore, in addition to the demonstration of strategies for fluorescence-based sensing with thermoresponsive polymers, this work highlights the role of the chemical structure of the two popular thermoresponsive polymers on the fluorescence response. The results are fundamentally important for the rational choice of polymeric materials for a specific sensing strategy.