Schematic illustration of functionalized GNR ligand with CTAB, UDT, and MUA (d). To ensure the MAPK inhibitor integrity of each specimen and the formation of Au-S bond on GNR after MUA modification, we measured the characteristic extinction spectra,

the XPS, and the zeta potential of as-synthesized GNR, GNR-MUA, and 1-undecanethiol modified gold nanorods (GNR-UDT) (Figure  1c). The LSPR spectral position is expected to be strongly affected by various factors such as the composition, formation and distribution of linkages, size, or shape of nanoparticles, as well as the refractive index of dielectric medium around them [26]. The as-synthesized GNR exhibited an absorption band centered at 850 nm. After the surface functionalization, a redshift of the extinction spectra was observed between GNR-MUA and GNR-UDT, at wavelengths 864 and 854 nm, respectively. The

intensity of LSPR peak was found to be constant, but the FWHM of the peaks became broader for GNR-MUA and GNR-UDT as the gold-thiol bond formed [27]. XPS spectra measurement can confirm the ATM Kinase Inhibitor mouse formation of thiols bond to the Au surface. The XPS spectra shows that thiolates have S 2p binding energies of about 162.40 eV, whereas unbound thiols have those of 164 to 165 eV (Additional file 1: Figure S1). This result is identical with the results of Zhao et al. [28]. Here, the C 1s peak at 284.88 eV was used as an internal standard calibration peak. The results also indicated that MUA

was successfully bound to the surface of GNR. We further certified the degree of this replacement through zeta potential of GNR-MUA (Table  1). GNR displayed a very positive zeta potential (58.08 ± 0.6 eV) when CTAB dispersed on the metal surface. It has been noticed that there was an apparently decrease of zeta potential GNR-MUA (29.4 ± 0.6 eV) when surface GNR was modified with MUA. Besides, as the pH of GNR-MUA was Tau-protein kinase adjusted from acid to base this website condition, the zeta potential becomes almost neutral. This result supports that CTAB coverage of GNR is partially displaced (Table  1). Table 1 Zeta potentials and pH of GNR, GNR-MUA, and GNR-MUA after adding 30 μL NaOH   Zeta potential pH GNR 58.07 ± 0.55 3.92 GNR-MUA (0.03 M) 29.4 ± 0.6 7.49 GNR-MUA (+NaOH 30 μL) 8.69 ± 1.3 10.16 The face-selective modifications had been widely used in understanding and controlling the dynamics of self-assembled gold nanoparticles [29]. However, the mechanism of replacing CTAB is still an open question [30]. Here, the partially displaced surface can be explained by the following: First, according to the synthesis method of GNR by Sau et al., the GNR made in the presence of silver ions are single crystalline, with 111 facets on the long side of the rods [15]. On the other hand, it was reported that the surface energy of different facets generally increases in the order γ111 < γ100 < γ110 [31].