Zhu et al. Nanoscale Research Letters 2012, 7:613 http://www.nanoscalereslett.com/content/7/1/613 fabricate sensitive GNP films with high mechanical adhe- sion strength. It is found that the GNP deposition density on the films can be adjusted by changing the electrophor- esis time, and the sub-10-nm gaps can be formed between the neighboring particles on the film for the relatively longer electrophoresis time. Finally, we demonstrate the excellent signal enhancement ability of these GNP thin films as substrates in SERS measurements and find that rhodamine 6 G (R6G) molecules can be detected at very low concentrations using these films. Methods Figure 1 shows the scheme of the fabrication process of the GNP thin films. The process contains two steps: In the first step, the GNP sol, as an electrophoretic solution for GNP thin films, was synthesized by a seed-mediated method [20,21]. As shown in Figure 1a, the GNP sol was synthesized by adding the seed solution (aqueous solution of cetyltrimethylammonium bromide (CTAB), NaBH4, and HAuCl4) into the growth solution (aqueous solution of CTAB, ascorbic acid, and HAuCl4). The resulting solu- tion was gradually transformed into red color. In this syn- thesis procedure, bilayers of CTAB, which was used as surfactant, were formed on the GNPs' surface, and these GNPs kept a net positive charge to prevent random aggre- gation between particles. After growing over one night, the GNP sol was centrifuged at 14,000 rpm for 10 min and then redispersed into deionized water (>18.0 MΩ cm) Page 2 of 7 to remove redundant reactants in the solution. In the sec- ond step, electrophoretic deposition method was employed for film preparation. As illustrated in Figure 1b, two indium tin oxide (ITO) glass substrates (30 × 10 × 0.6 mm) were immersed into the GNP sol (approximately 1015 particles L−1) and used as cathode and anode, with an interelectrode distance of 3 mm, for electrophoresis. Dur- ing the experiment, electrophoresis was carried out by ap- plying a voltage of 4.5 V at room temperature. Under otherwise equal conditions, three film samples, S1, S2, and S3, were prepared with different electrophoresis times, i.e., 8, 2, and 1 min, respectively. When the electrophoresis process was finished, the samples were rinsed with deio- nized water (>18.0 MΩ cm) and then baked on a hot plate at 60°C for 30 min to remove the residual solution. Results and discussion Firstly, we characterized the micromorphology of the GNPs in the electrophoresis solution using a transmission elec- tron microscope (TEM). Figure 2 shows the TEM image of GNPs in the sol. It is found that the GNPs are single crys- tals with a nearly uniform spherical shape and well dis- persed in the solution due to the Coulomb repulsion effect, which was introduced by the bilayers of CTAB on GNPs' surfaces [22]. The average size of GNP is between 43 and 47 nm, which is dependent on the amount of gold seeds added during the synthesis process. Then, we also exam- ined the plasmon resonance characteristic of GNP sol using Figure 1 Schematic illustration of (a) preparation of GNP sol and (b) fabrication process of GNP thin film.