好二的标题
However, special treatment can be done on the silica helices to prevent aggregation of GNPs so that we can get even larger GNPs [7]. When APTES is used as silica surface function group, the amine group in it is positively charged in neutral pH, and the repulsion between the charges can effectively prevent the GNPs from aggregation. Using this method, we are able to grow GNPs to about 14 nm in diameter [7].
Small GNPs (<2 nm) do not show a localized SPR band owing to the quantum size effect observed for nanoparticles < 3 nm. The color of the GNPs suspension as well as those adsorbed at the surface of nanohelices is brown [7]. When the samples were grown in water, no significant absorbance was observed. However, for the GNPs grown in HFIP, a broad absorbance peak appeared at 526 nm after 2 h. As the GNPs grew larger (~5 nm), this local SPR band increased in intensity, became sharper and the color gradually turned red [7].
While being used in SERS, it’s found that GNPs of diameter 14 nm gives the strongest enhancement (shown in Figure 4 (Tamoto, R.; Lecomte, S.; Si, S.; Moldovan, S.; Ersen, O.; Delville, M.; Oda, R.23148)). Raman spectroscopy for benzene is done three times, without substrates, with 14 nm free GNPs, and with 14 nm GNPs on silica helices. For the two trials using GNPs, the concentration of benzenethiol is 10-6M and for the trial without any substrates, 10-2M benzenethiol was used. However, even though the concentration of sample is 10000 times diluter, the resulting spectrum has much more significant peaks representing benzenethiol [7]. Thus, we prove that the substrate has a strong enhancement for SERS. Besides, silica helices with different size GNPs are tested with SERS, and the 14 nm diameter GNPs are found to have the greatest enhancement on SERS. Using gold/silica substrates, we are able to perform SERS with concentration level of 10-6 M [7].
Figure 4. SERS of benzenethiol enhanced by GNPs
In the synthesis of gold nanoparticles on silica nanohelices, the effectiveness of the substrate is greatly dependent on the size of GNPs; however, the shape of the nanoparticles is also of tremendous importance, and it can be demonstrated by a sandwich structure substrate made by silver nano particles. It involves two monolayers of metal nanoparticles with nano-scale gaps prepared by the close-packed arrangement of nanoparticles, and the two monolayers are linked by functional probes. It is considered that the “metal-probe-metal” sandwich structure formed a “cavity” in which the localized electromagnetic field can be greatly enhanced [2]. Consequently, controllable “hot spots” construction can be accomplished and high quality SERS spectra can be expected.
The synthesis of the substrate largely depend on the self-assembly of metal nanoparticles. At first, a quartz slide was cleaned and close-packed polystyrene nanospheres was self-assembled on the slide. Then, silver film was deposited onto the polystyrene monolayer using vacuum evaporation deposition method and the polystyrene was removed by ultra-sonication. The periodical silver nanoprisms arrays were obtained, the size of which is dependent on the size of polystyrene nanospheres. Those silver nanoprisms are immersed into 1, 4-Benzenedithiol (BDT) solution and the 1,4-BDT molecules selectively adsorb on the metal nanoprisms only, as a result of the different surface properties between the nanoprisms and the pits from the removed polystyrene [2]. Figure 5 (Guo, H.; Xu, W.; Zhou, J.; Xu, S.; Lombardi, J. R. 3.) shows the synthesis process.
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