Study Of Dynamic "Gap" Of SERS-active Substrate Assembled By Thermo-sensitive Hydrogel Templates

With the development of Surface-enhanced Raman Spectroscopy (SERS) technology, signals can be detected with the sensitivity at the level of signal-molecule level. It is believed that one of the reasons is owing to the enormous electromagnetic enhancement of the plasmonic coupling effect among metal nanoparticles. So the progress of SERS technique is largely dependent on the development of preparation techniques of new SERS active substrates, especially for the controllable "gap" between metal nanoparticles of SERS substrates. In fact, challenges are still remained for fabricating of uniform and efficient SERS substrates. It is necessary to develop simple methods to fabricate SERS substrates which can provide high signal enhancement and uniform signals. Importantly, it is required to optimize the Raman enhancement factor and control the nano-gaps easily. In most cases, once metal nanoparticles assemblies were formed, the spatial distributions of metal nanoparticles building blocks within the composite substrate have been fixed. So, it is very important to develop a new method for more effective SERS substrates.Poly (Nisopropylacrylamide)(PNIPAM) has been extensively studied in regard to its well-known phase behavior in aqueous solutions which has the sharpest transition among the class of thermosensitive alkylacrylamide polymers. Indeed, it undergoes a reversible phase transition at a lower critical solution temperature (LCST) of about32℃in pure water from a swollen state to a shrunken state by increasing temperature due to dissociation of the hydrophobic interaction between NIPAM segments and water. Below the LCST, PNIPAM is hydrated and the chains are in an extended conformational state. Above the LCST, PNIPAM is in a collapsed conformational state due to the breaking down of the delicate hydrophilic/hydrophobic balance in the network structure. Dehydration takes place in the PNIPAM which results in the subsequent aggregation of the PNIPAM chain and leads to the shrinking of the hydrogel. In this thesis, we will use thermo-sensitive template to assemble responsive Ag or Au nanoparticles, in which the overall composite dimensions and interparticle spatial distances can respond and adapt to external temperature stimulus. Au or Ag nanoparticles with different shapes were assembled on or inside the swollen hydrogel template, where the gaps between nanoparticles were so big that "hot spots" of these substrates were not obvious. While, if the probe molecules were mixed with these substrates, they would enter the gaps between nanoparticles. Upon SERS detection, the template can shrink due to temperature increase and this will make the gaps between nanoparticles decreasing significantly. Not only the "hot spots" on the substrates will increase instantly, but also the uniform of the whole substrate can also be improved. High sensitivity and reproducibility SERS signals can thus be obtained. This thesis work includes the followings:(1) At first, the processes and principles of free radical polymerization for thermo-sensitive microgel polymer PNIPAM were introduced. We give the explanation for the factors affecting synthesis of the optimal microgel template, such as the temperature, the number of the cross-linking agent, kinds of initiators and the reason for the microgel swelling and shrinking et al. At the same time, the Raman spectra of all mononers and SERS spectra of the composite substrates were measured, which are served as blank control to study SERS signals of the probe molecules.(2) PNIPAM/Ag composite substrates were synthesized through decorating Ag nanoparticles on the PNIPAM template. PNIPAM nanosphere templates with a positive surface charge were grown using a cationic initiator (AAPH). Ascorbic acid (AA)-reduced silver colloid with negative surface charge was assembled on the surface of the template. With increase of temperature, the largest shrinking ration of the composite was achieved at18%. Dynamic gaps from30.4nm to less than5nm were obtained. These changes led to an increasing of the uniformity and "hot spots" of the substrate. The SERS enhancement for multiple of CV and R6G molecules was about1000times with the template shrinking, consistent with the results estimated by DDA theoretical computation. These novel stimuli-responsive systems offer obvious advantage:a reversible near-field coupling between the Ag nanoparticles, depending on the external temperature and leading to a control over the SERS intensities of probes adsorbed on the substrate.(3) Based on the principle of the dissolution in the similar material structure, Au core was covered by PNIPAM shell and then its growth can be controlled in situ. The obtained bigger size Au core would have strong electrostatic attraction to the Ag+(H2O)n (n=1-4) in aqueous solution. This would be followed by the reductive reaction and further growth of vast Ag+(H2O)n attracted into porosity of the gel network. Then, individual Au@PNIPAM/Ag composite with core-satellite structure which can generate plasmon resonance was obtained. Similar to the study in the third chapter, the gaps between metal nanoparticls can decrease because of the PNIPAM template shrinking from wet to dry state. Especially this kind of individual Au@PNIPAM/Ag composite can be found through Raman optical microscope. Uncertain effects on SERS signals resulting from variability of the configurations are minimized because these individual substrates are uniform relatively. The individual substrate can also be used for inspecting pesticide residues accurately and rapidly. The RSD of4-ATP and Sumithion SERS signals were all less than15%.(4) PNIPAM nanosphere templates with a negative surface charge were grown using an anionic initiator (KPS). Au nanorods stabilized by CTAB with positive surface charge with different aspect ratio (AR) values were decorated on the thermo-sensitive hydrogel template because of electrostatic attractive force. The surface plasmon bands of Au nanorods were observed an obvious red-shift with increasing temperature due to strong plasmon coupling. This clearly demonstrated that the shrinking of the template drives the decorated Au nanorods closer to each other. Moreover, when the laser was785nm, the SERS enhancement for multiple of the same molecule was the strongest among the different lasers were used. This demonstrated that not only "hot spots" were increased but also SPR of Au nanorods matched more with the laser because of the shrinking of the template. The results showed that Thiram can be detected accurately in10-9M level. It is therefore promising to apply the PNIPAM/Au nanorods substates in protable Raman instrument (the laser is generally785nm) and detect the residual pesticide on the vegetables or apples.