Nano- and Micro-sized Molecularly Imprinted Polymer Particles on Solid Surfaces

University dissertation from Department of Physics, Lund University

Abstract: Popular Abstract in English Nanoscience and nanotechnology have existed in this world before scientists actually coined the terms. Nature around us and human beings themselves are all made of nanomaterials (1-100 nm), such as DNA, proteins, liposomes, and enzymes. These building blocks possess molecular recognition properties, resulting in their self-assembly to form various two- and three-dimensional nanostructured materials useful in the field of science and technology. Scientists from different disciplines, including physics, chemistry, biology, and material science, use these nanostructured materials for advanced applications in medicine, energy, etc. The detection of drugs in therapeutic applications requires selective molecular interaction with the target material. Thus, researchers have designed biosensors for detecting and sensing molecules in biological materials, e.g., glucose biosensors, which are quite important for sensing the glucose levels in the body. Biosensors can work when the biomolecules or the recognition materials are perfectly integrated with the transducer surface. These transducer surfaces are a very important element of the sensor fabrication as they convert the biological recognition event into a detectable signal. The basic recognition mechanism, the analyte binding by the immobilized biomolecule, depends on the famous “lock and key method” first reported by the Nobel-Prize laureate Emil Fischer in 1894. The interaction between the lock and key should be reversible and stable to ease the operation and for reuse of the chemical sensor. Due to the poor stability of the biological recognition material in different physical and chemical environments, biosensors may give false readings. Further, the binding event is sometimes irreversible, which makes these biosensors suitable for only one-time use, making them more expensive. To solve this problem, the biomaterials may be replaced by “artificial receptors” or “plastic antibodies,” referred to as molecularly imprinted polymers (MIPs). These polymer materials are synthesized in the presence of template molecules. After the polymerization, the template molecules are extracted, leaving behind “cavities” or “memory sites” with high selectivity and specificity for the template. The feature of molecular “memory” imprinted into the polymers enables them to selectively rebind the templates multiple times, which is similar to the “lock and key mechanism.” The template molecules can be any biomolecule, such as antibodies, enzymes, nucleic acids, microorganisms (bacteria, viruses) or drugs. This makes the MIPs suitable for the detection of drugs or for food-based sensing application studies with their ease of quality control, easy handling and cost effectiveness. The most important criteria to develop a chemical sensor based on MIPs is their stable and uniform integration with the transducer surface. Therefore, this research investigates different immobilization approaches of spherical and hydrophobic MIP particles on a solid transducer surface. Different covalent and electrostatic chemistries have been studied for the dense, uniform and stable attachment of polymer spheres to chemically functionalized inorganic supports, such as glass, silica, and gold wafers. A detailed investigation of the MIP-integrated surfaces using different surface analytical techniques demonstrated the presence of polymer spheres on the transducer surface with stability in both physical and chemical environments. The MIP surfaces retain their characteristic template-binding property after immobilization on the transducer surface, thus making them suitable for sensing applications. Ease of fabrication, low cost, sensitivity, selectivity and stability have made MIP sensors popular in physical, chemical, and biological sensing applications, e.g., the detection of various poisonous materials in the environment, food, and humans. Optical-based sensing systems are well known due to their ease, low cost and sensitivity. In principle, MIP-based optical sensors require fluorescent labels to detect the analyte binding, which makes the sensing process long and complicated. In my research, I have developed an easy, reproducible and label-free optical sensor that can be used for direct real-time measurement of analytes at different concentrations. MIP spheres imprinted against two model compounds, nicotine and propranolol, were used. Nicotine is found in cigarettes, and propranolol is a drug used to control anxiety and depression. MIP-based optical sensors were developed by the covalent immobilization of MIP spheres on an optically active transducer surface. The obtained results clearly demonstrated that the MIP surfaces are selective and specific towards analyte (propranolol and nicotine) sensing, even in the biological samples. The reported immobilization and sensing approach can be used in biomedicine to perform both in vivo and in vitro investigations of molecules and organisms that cause health hazards. Further, these optical sensors have a wide range of applications, such as the identification of chemical warfare agents and of different dyes useful in the defense and art industries. Different immobilization approaches reported in this thesis can also be used to attach large-sized soft polymer spheres onto solid substrates for numerous material-based applications.