Zhu et al. Nanoscale Research Letters 2012, 7:613 http://www.nanoscalereslett.com/content/7/1/613 Page 4 of 7 Figure 3 Comparison of top-view FESEM images of the three samples: (a) S3, (b) S2, (c) S1. (d) High-resolution FESEM image of sample S1. The positions (red circles) indicate that sub-10-nm gaps have been formed between the neighboring particles on this film. directly depositing a droplet of the GNP sol onto an ITO glass substrate, and GNPs remained on the surface of the substrate after evaporating the sol under ambient condi- tions. FESEM observations (not shown in figure) were car- ried out at the same positions on the substrates both before and after applying 5-min ultrasonication on the GNP thin films and the reference one. We found that the GNPs on thin films fabricated by electrophoretic depos- ition method were resistant to the ultrasonication process. The micromorphology had little change on the surface of the thin film after applying ultrasonication. In contrast, GNPs on the reference sample were not able to resist the ultrasonication process, and a retention rate below 35% was observed. This contrasting result indicates that the GNP thin films fabricated by electrophoretic deposition method possess superior mechanical adhesion strength. Raman spectroscopy is not only a powerful analytical technique in composition analysis, but also a testing tool to examine the LSPR effect of GNP thin films. The Raman spectra have been intensively used in the field of identification of organic molecules from their vibration spectra at very low concentrations. In the past two dec- ades, many studies have revealed that noble metal nano- particle films with a strong LSPR effect can amplify the Raman signals in biological detection process [9-11]. In order to evaluate the Raman-enhancing capability of the GNP thin films fabricated by electrophoretic deposition method, various R6G solutions with different concentra- tions were prepared because they have been extensively studied in prior literatures [1,24]. R6G is a strongly fluorescent xanthene derivative that is a yellowish het- erocyclic compound and shows a molecular resonance Raman effect when excited into its visible absorption band. In this experiment, a confocal Raman microscope/ spectrometer with a × 50 objective was used for the Raman measurements. An Ar laser with a wavelength of 514 nm was employed for the excitations. The laser power focused on the samples was measured to be 1.5 mW/cm2. The laser beam size focused on the sam- ples was 2 μm in diameter, and the acquisition time was 10 s. For comparison, R6G in methanol solutions with different concentrations were dropped onto GNP thin films and bare ITO glass substrates with no nanoparti- cles. Raman spectra were recorded after drying the solv- ent. For bare ITO glass substrates, the resulting spectra show no discernable Raman peaks of R6G molecules at the concentration of 10−4 M (not shown in figure). Until the concentration increased to 10−3 M, a few weak peaks can be observed. Therefore, when the R6G concentra- tion is equal to or less than 10−4 M, peaks associated to R6G molecules cannot be detected in the Raman measurement.