Nanometer scale gaps of metal nanostructures provide unique photonic features such as surface enhanced Raman scattering (SERS) due to a plasmon coupling. SERS-active substrates have been studied for highly sensitive measurements of Raman spectroscopy or imaging. Narrower gap basically can induce stronger signals and at the same time it becomes hard to insert target molecules, in particular larger targets like proteins, into the gaps. Dynamic control of the gap distance could provide highly enhanced Raman signals due to both high insertion efficiency of targets on large gaps and high enhancing effects on small gaps. In our previous study, we have developed a fabrication method of a thin metal film on the gels through transferring them from a solid substrate onto a water-swollen gel . In this study, we aimed to create a novel nanoblock array structure of which gap distance can be changed by external stimuli.
Two-dimensional array structure was prepared by a self-assembly or electron beam lithography techniques. Gold nanoparticles (AuNPs) thin films and gold arrays with sub-100 nm dots were prepared on glass and silicon substrates, respectively. Poly-(acrylic acid) (PAA) hydrogel, which is well known as a stimuli responsive hydrogel, was prepared through the polymerization of the monomer solution on those substrates. Then the AuNPs thin film and sub-100 nm dots were transferred onto the surfaces of PAA hydrogel .
The volume change of PAA hydrogel was induced by the change of NaCl concentration. The change of the interparticle distance through expansion and shrinkage of the hydrogel was evaluated by the peak shift of absorption spectra due to a plasmon coupling. Absorption spectra also showed reversible change of gaps.
When dye molecules were absorbed on this gel under swollen condition and then the hydrogel was shrunk by high concentration salt solution, further enhancement of Raman signal intensities were observed in comparison with those without a gap control. This protocol was applied for SERS of macromolecules for biosensing .
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