Zhu et al. Nanoscale Research Letters 2012, 7:613 http://www.nanoscalereslett.com/content/7/1/613 Page 3 of 7 Figure 2 TEM image of GNPs synthesized by seed-mediated method. The inset gives the measured extinction spectrum of the GNP sol. a UV–vis-NIR spectrophotometer. The extinction spectrum of the GNP sol (the inset in Figure 2) indicates that the sol has a single narrow plasmon resonance peak at 531 nm, and this result indirectly proved the excellent control of micromorphology of the GNPs in the synthesis process. In this experiment, a simple principle was utilized to form GNP films: because the CTAB-wrapped nanoparti- cles in the solution possess a net positive charge, under an external electric field condition, the particles moved to the cathode and attached onto the ITO glass. To in- vestigate the micromorphology of GNP thin films depos- ited on the ITO glass substrates, field emission scanning electron microscopy (FESEM) was performed. Figure 3 shows AuNP depositions from dispersed distribution to multilayered close packing which are achieved by vary- ing the electrophoresis time. For a short electrophoresis time (1 or 2 min), the particles on samples S2 and S3 were well uniformly separated over the surface of the ITO glass (Figure 3a,b). When the electrophoresis time was prolonged to 8 min (Figure 3c,d), the GNP thin film on sample S1 was gradually transformed into a multi- layered close-packed structure in the microscale. Despite the same electrophoresis solution in the fabrication process for the three films, the GNPs deposited on the three samples have different sizes with different electro- phoresis times. Most particles on samples S2 and S3 fall in the size range of 45 to 50 nm, which is close to the particle size in the original deposition solution. For sam- ple S1, however, the typical size of GNP on the film is 61 nm, which is both larger than that of the original par- ticles in the GNP sol and the ones on samples S2 and S3. This particle size change may be attributed to the recrystallization of the Au+ ions that resided in the GNP sol. We suspect that in the electrophoresis process, an electrochemical deposition process also occurred simul- taneously and the particle size gradually increased be- cause some Au+ ions were reduced into Au atoms and attached on the original GNPs on the cathode surface. From the high-resolution FESEM image (Figure 3d), we found that the particles nearly retain the shape of the original ones in the deposition solution (see Figure 2). Therefore, this shape maintenance property gives us a message that the shape and size of GNPs on films can be changed by altering the original GNPs, e.g., nanorod, nanotriangular, and nanocubic, which can be easily synthesized by adjusting the concentrations of reactants in a seed-mediated method [20,22]. From the high- resolution image, it is also found that the sub-10-nm gaps (the red circles in Figure 3d), which play an import- ant role in SERS applications [17,23], were formed be- tween the neighboring particles on sample S1. The mechanical adhesion strength of the GNP thin films was tested by ultrasonication in deionized water (>18.0 M Ω cm). For comparison, an extra sample was prepared by