Preparation of Transparent-Type Plasmonic Sensors By the Sol-Gel Process and Electrodeposition

Surface-enhanced Raman scattering spectroscopy (SERS)¹ is a powerful method for molecular-level characterization of various surfaces and interfaces, featuring high detection ability compared to normal Raman spectroscopy. We have developed plasmonic sensors for the SERS, which could achieve very high enhancement effect, and succeeded to analyze extremely small amount of materials². The plasmonic sensor has a patterned nanostructure with concentric circular grooves so as to have high electric field at its center. The sensors are made of metals such as Ag, Au, and Cu, which are active for the plasmonic enhancement. In the present study, we proposed a new type of the sensor, “transparent” plasmonic sensor, which enables the measurements of various types of specimens, regardless of the substrate materials. In order to fabricate these sensors, we applied a sol-gel process to form SiO₂ films and the formation of Au–Ag nanodot patterns embedded in the sol–gel SiO₂ films using electrodeposition.  We also evaluated the performance of the fabricated sensors.

The transparent-type plasmonic sensors were fabricated by the following process. Indium tin oxide (ITO) was sputtered on a glass substrate. SiO₂ thin films were prepared by the sol-gel spin-coating technique on the ITO-glass surface, using tetraethoxysilane as the precursor material and polyethylene glycol (PEG) as a thickener. Polydimethylsiloxane (PDMS), which exists as fine needles, was used as a template. The template, consisting of fine PDMS needles, was pressed into the SiO₂ thin film and then heated, thus forming and transferring nanodot patterns into the film. The residual SiO₂ film on the ITO was removed by reactive ion etching (RIE). Then, Au–Ag films were electrodeposited at −0.7 V vs. Ag/AgCl on the SiO₂ nanodot patterns. Electrodeposition and electrochemical measurements were performed with an electrochemical analyzer (HZ-7000, Hokuto Denko). The surface morphology was observed by scanning electron microscopy with a field-emission source (FE-SEM, S-4800, Hitachi High-Technologies Corp.) and focused ion beam–scanning electron microscopy (FIB-SEM, NB-5000, Hitachi). SERS measurements were performed using Raman microscopy (Nanofinder 30, Tokyo Instruments, Inc.).

30-nm-depth nanodot patterns in sol-gel SiO₂ films were confirmed using PDMS template. And, after the removal of the residual SiO₂ film on the ITO, we could embed the Au-Ag films in the nanodot patterns by electrodeposition. And, we evaluate the behavior of PEG in the sol-gel SiO₂ process. By 350 °C annealing, it was confirmed that the Raman peak of PEG disappeared and then the use of PEG in sol-gel SiO₂ process didn’t affect the property of transparent-type plasmonic sensors. Using these sensors, we evaluated the outermost surfaces of electrodeposited Cu films and the behavior of organic additives in electrodeposition to confirm the ability of transparent-type plasmonic sensor. From the examination, the status of oxidation of Cu and a small amount of organic additives was detected. It was confirmed that the developed transparent-type plasmonic sensors worked well.

 We have developed a process to prepare transparent-type plasmonic sensors which were constituted from Au–Ag nanodot patterns embedded in sol–gel SiO₂ films. From these results, it is clear that these developed plasmonic sensors are effective for the in situ analysis and detection of the interfaces between solids and liquids in water or electrolyte.

Acknowledgments

This research was partially supported by "Nanotechnology Network Project", "Development of Systems and Technology for Advanced Measurement and Analysis", and "Grant-in-Aid for Scientific Research, 23656470 and 25630041” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

References 

1. M. Fleishmann, P. J. Hendra, and A. J. McQuillan, Chem. Phys. Lett., 26, 163 (1974).

2. M. Yanagisawa, N. Shimamoto, T. Nakanishi, M. Saito, and T. Osaka, ECS Trans., 16, 397 (2008).