Study Of Noble Metal Nanoparticles Loading Intelligent Hybrid Microgels Used As The Substrate Of Surface Enhanced Raman Spectroscopy

Noble metal nanoparticles (NMNPs), such as silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs), are able to absorb the light with specific wavelength to produce localized surface plasmon resonance (LSPR) effect and strong electric fields on their surfaces. If surface plasmonic coupling interactions between NMNPs are generated due to their approaching, the surface electric fields will be further enhanced. It is because of the property that NMNPs can be used as the substrates for surface enhanced Raman spectroscopy (SERS), so that the sensitivity of SERS analysis can even be elevated to the level of testing single molecule. However, pure NMNPs have very high surface energy, so they are easy to aggregate to form noble particles with larger size. Even though colloidal NMNPs are stably dispersed in aqueous medium, they can aggregate due to the absorption of analytes on their surfaces in the SERS analysis process. Hence, pure NMNPs have no obvious application value, and they are needed to be loaded into suitable carriers to avoid the possibility of aggregating. If NMNPs are loaded into intelligent microgels and the formed intelligent hybrid microgels are used as SERS substrates, they cannot aggregate after absorbing analytes, since they are immobilized into the polymer network of the microgels. In addition, the surface plasmonic interactions between the NMNPs inside the hybrid microgels can be tuned by external stimulus, so the SERS signal strength of the analytes can be increased. The volume phase transition of the hybrid microgels in response to external stimulus can be used to catch trace hydrophobic organic compounds in the aqueous solution, so SERS method can be used to analyze the organic compounds which cannot be analyzed for common SERS substrates and SERS analysis scope is enlarged. Therefore, design and preparation of new type of intelligent hybrid microgels and investigating the relationship between their structure and composition and their properties, and studying their applications in SERS substrates, are of obivious scientific value for developing novel intelligent hybrid materials and remarkable practical significance for elevating the sensitivity of SERS method and enlarging its application scope. Temperature and pH value are common environmental conditions, and their variations are the external stimuli which are easily carried out. Additionally, surface enhancement effect of AgNPs is the strongest among NMNPs, so it is most suitable for application in SERS substrate. Thus, three new types of AgNPs loading intelligent hybrid microgels which can respond to temperature or/and pH value change were prepared. In addition, AuNR@PNIPAM hybrid microgels based on Au nanorod (AuNR) as core and crosslinked poly(N-isopropylacrylamide)(PNIPAM) as shell were prepared according to the related references, and then they were assembled to produce the assembly product with stable structure by centrifugation followed by post-crosslinking reaction. Three new types of AgNPs loading intelligent hybrid microgels and the assembly product were used as SERS substrates, and the effect of temperature and pH value on the SERS signal strength of the selected model analytes was investigated. The research results of this thesis are described as the following four aspects:(1) AgNPs loading hybrid microgels (AgNPs/PNIPAM microgels) were prepared by in-situ reduction of the Ag+ions incorporated within PNIPAM microgels by NaBH4. The AgNPs inside the hybrid microgels are of face centered cubic crystal structure, and their average particle diameter is between8and13nm and their mass content can top23.1%. The hybrid microgels have spherical shape and relatively good monodispersity and reversible temperature stimulus responsiveness. The LSPR wavelength of the hybrid microgels is red-shifted with temperature raising, and the red-shifting magnitude is increased with AgNPs content within the hybrid microgels. Using AgNPs/PNIPAM hybrid microgels as substrate, the SERS spectra of4-mecaptobenzoic acid (PMBA) as model analyte were measured at various temperatures. It was found that the SERS signal strength of PMBA is markedly elevated as the temperature is raised from20℃to50℃, with about three times increase of its enhancement factor (EF). Again, the change of the SERS signal strength with temperature is well reversible. The temperature range within which the SERS signal strength of PMBA is enhanced the most rapidly is consistent with the one of volume phase transition of the hybrid microgels, indicating that the’spot hot’resulting from the surface plasmonic coupling between the AgNPs after shrinkage of the hybrid microgels is the main reason for enhancement of SERS signal strength of the analyte.(2) The pH stimulus responsive hybrid microgels were prepared by loading AgNPs into the microgels with semi-interpenetrating polymer network (semi-IPN) structure composed of linear poly(acrylic acid)(PAA) and crosslinked PNIPAM by in-situ reduction. As PAA content within the microgels is increased, in-situ produced AgNPs content is accordingly raised, but their particle size is not changed noticeably. The hydrodynamic diameter of the hybrid microgels is sharply lowered with the pH value of the aqueous medium decreasing from6.0to4.0, and their LSPR wavelength is pronouncedly red-shifted, with these changes reversible. Using the hybrid microgels as substrate, the SERS spectra of PMBA aqueous solution of10-6M concentration were measured, and the results revealing that1587cm-1peak strength is obviously and reversibly dependent on the pH value of the solution. With the solution pH value lowering from7.0to3.0, the signal strength of the peak is increased about six times. The surface plasmonic coupling between AgNPs stemming from volume shrinkage of the hybrid microgels in response to pH variation is the main reason for the SERS signal enhancement of PMBA.(3) The pH/temperature dual stimuli responsive hybrid microgels with little interference between two stimulus-responsive components were prepared by loading AgNPs into the microgels with IPN structure composed of crosslinked PAA and crosslinked PNIPAM by in-situ reduction. AgNPs are homogeneously distributed within the hybrid microgels, and their particle diameters are within the range of11to15run and their mass content can top38%. The LSPR property of the hybrid microgels is reversibly responsive to both temperature and pH stimulus. As the pH value of the hybrid microgels dispersion is reduced from6.0to4.0, the SERS signal of PMBA of10-6M concentration in the dispersion is remarkably enhanced, with EF increasing about two times. As the temperature is raised from20℃to45℃, the SERS signal is noticeably elevated, with EF increasing about three times. This is the first time when NMNPs loading dual stimuli responsive hybrid microgels are used as SERS substrate, and it is realized that SERS signal strength can be enhanced by both temperature and pH value change.(4) The well core-shell structured and monodisperse AuNR@PNIPAM hybrid microgels were first prepared according to the related references. The transverse LSPR wavelength of the hybrid microgels with reversible temperature stimulus responsiveness is little changed with temperature, but their longitudinal one is sharply red-shifted with temperature raising, with red-shifting magnitude topping60nm and well reversibility. The surfaces of AuNR@PNIPAM hybrid microgels were modified by precipitation copolymerization to introduce the amino groups on their surfaces, which can react with aldehyde groups. Then, the hybrid assembly product with high stability was produced by centrifugation followed by post-crosslinking reaction. Using AuNR@PNIPAM microgels and their assembly product as substrates, the SERS signal of the aqueous solution of1-naphthol can be detected by raising temperature to make the hybrid microgels undergo volume phase transition, which cannot be detected by SERS using conventional metal nanomaterials as substrates. This is due to the fact that the hybrid microgels and their assembly product can catch trace1-naphthol in the aqueous solution during their temperature change triggered volume phase transition. Furthermore, the SERS signal strength of1-naphthol based on the hybrid microgels assembly as substrate is stronger than that based on the hybrid microgels dispersion as substrate. This is because the amount of AuNR inside the assembly product is much bigger than the one in the dispersion under the same laser spot in the process of measuring SERS spectra.