In Situ IRRAS And SERS Spectra Studies Of Molecular Recognition At The Interface

Molecular recognition is one of the most important processes for the construction of biological assemblies. The mutual recognition of complementary components by means of multiple hydrogen bonds is not only a vital biological process but also the basis for the construction of a variety of sensors. However, the hydrogen bonding interactions cannot yield significant driving forces for molecular recognition in bulk water due to strong competitive binding of water molecules. Langmuir monolayers at the air-water interface could provide unique microenvironments for molecular interactions. Infrared reflection absorption spectroscopy (IRRAS) has been a leading structural method for the in situ characterization of the Langmuir monolayers at the air-water interface on the molecular level. As an ultrasensitive analytical technique, surface-enhanced Raman scattering (SERS) spectroscopy has been widely applied in surface science and molecular recognition due to high sensitivity and excellent selectivity. Hydrogen bonding patterns and molecular orientations of the molecular recognition between barbituric acid lipids monolayer and melamine at the air-water interface could be studied by IRRAS in detail. When noble metal nanoparticles assembled substrate was applied in molecular recognition between barbituric acid lipid monolayers and melamine, false SERS signals might be obtained for molecular recognition. Ag nanoparticles are commonly used SERS substrates in detection of proteins and biomolecules due to the excellent Raman activity. The irreversible aggregation would happen when Ag nanoparticles were apparent under physiological environments containing high-concentration salts, but silica-coated Ag@SiO2core-shell nanoparticle SERS tags displayed excellent stability. Ag@SiO2SERS tags could be used in protein recognition and detection after being modified with functional molecules. Magnetic separation provides a convenient method for separation of magnetic nanomaterials by applying an external magnetic field. After coating with silica, the Fe3O4@SiO2core-shell magnetic microspheres not only displayed better stability but also could be applied in bioseparation and detection if functionalized with specific molecules.1. Metal Coordinations and Photoisomerization in Monolayers of Azobenzene-Containing Lipids at the Air-Water InterfaceAzobenzene-containing alanine derivative lipids, C12AzCONH, were synthesized. The metal coordinations of C12AzCONH monolayers and corresponding Langmuir-Blodgett (LB) films at the air-water interface have been investigated by infrared reflection absorption spectroscopy (IRRAS) and FTIR spectroscopy. On pure water surface, the dimmers of carbonyl groups were formed in the C12AzCONH monolayer and the long axes of azobenzene were oriented almost perpendicular to the monolayer surface. C12AzCONH molecules formed a condensed monolayer on pure water, so that the photoisomerization from trans form to cis one was almost inhibited. In the presence of Ag+and Zn2+ions, the condensing effects of the metal ions gave rise to an increase of intermolecular hydrogen bonding, the condensed molecular arrangement did not facilitated the photoisomerization of azobenzene. In the cases of Cu2+, Ca2+, and Pb2+ions, the monolayers were expanded to a certain extent favorable to the photoisomerization from trans form to cis one.2. In Situ IRRAS Studies of Molecular Recognition of Barbituric Acid Lipids to Melamine at the Air-Water InterfaceMolecular recognition of barbituric acid lipids to melamine at the air-water interface has been investigated in detail using in situ infrared reflection absorption spectroscopy (IRRAS). Hydrogen bonding patterns and molecular orientations of the molecular recognition have been revealed. Prior to molecular recognition, the barbituric acid moieties in the monolayers were hydrogen bonded with a flat-on fashion at the air-water interface, and the alkyl chains were preferentially oriented with their C-C-C planes perpendicular to the water surface. After molecular recognition, the NH2stretching bands of recognized melamine were clearly observed at the air-water interface as well as other characteristic bands, the barbituric acid moieties underwent a change in orientation with non-hydrogen bonded C4=O bonds almost perpendicular to the water surface and other C=O bonds involved in hydrogen bonds with melamine, and the alkyl chains were preferentially oriented with their C-C-C planes parallel to the water surface. The monolayers of barbituric acid lipids exhibited selective recognition of melamine even in the presence of nucleobases containing complementary donating and accepting groups. 3. SERS Studies of Molecular Recognition of Barbituric Acid Lipids to Melamine at the InterfaceA method for monolayer-isolated nanoparticle-enhanced Raman scattering has been developed, the molecular recognition and recognition selectivity of barbituric acid lipid monolayers were investigated with smooth substrates, and the SERS signals of melamine from true molecular recognition were obtained. The versatility of LB films was exploited.4. Synthesis and Application of Surface Enhanced Raman Scattering (SERS) Tags of Ag@SiO2Core/Shell Nanoparticles in Protein Detection at the InterfaceA simple method has been developed to prepare silica-encapsulated Ag (Ag@SiO2) Raman tags, embedded with the reporter molecules, using environmental-friendly solvents (mixed solvents of ethanol and water) without vitreophilic pretreatment. The Ag@SiO2Raman tags exhibited excellent stability resistant to high-concentration salts and long-term storage. Myoglobin (Mb)-conjugated Ag@SiO2Raman tags specifically bound to thioethyl-iminodiacetic acid (IDA)-functionalized wafers through the coordination interactions of IDA-Cu2+-histidine residues available on the Mb surfaces for SERS detection. Mb could be directly detected by means of the formation of the Mb-sandwiched structures via Cu2+coordination between the IDA-functionalized wafers and IDA-modified Ag@SiO2-Au composite nanoparticles, where Au nanoparticles were adsorbed onto aminated Ag@SiO2Raman tags followed by modifying with IDA. It is clear that the prepared Ag@SiO2Raman tags can be applied for assays of other proteins or biomolecules if corresponding specific ligands are used.5. Synthesis and Application of Fe3O4@SiO2Core/Shell Magnetic Microspheres in Protein Separation and DetectionSuperparamagnetic Fe3O4nanoparticles were synthesized with tunable sizes by varying concentration of iron source FeCl3·6H2O, and different stabilizers had an influence on morphology and stability of Fe3O4nanoparticles. The Fe3O4nanoparticles were first pretreated with hydrochloric acid and sodium citrate and then coated with silica followed by modifying with mannose moieties. Concanavalin A (Con A) could be selectively detected with mannose-functionalized Fe3O4@SiO2microspheres in combination with mannose-functionalized Ag nanoparticles by SERS technology after magnetic separation. It is clear that the prepared Fe3O4@SiO2magnetic microspheres can be applied for separation of other proteins or biomolecules if corresponding specific ligands are used.