Facile fabrication of self-assembled nanostructures of vertically aligned gold nanorods by using inkjet printing

We demonstrated that the vertically aligned gold nanorods (AuNRs) were quickly and easily formed by using inkjet printing when aqueous dispersion of AuNRs containing a small amount of ethylene glycol (EG) was employed as an ink. It was observed that the content of EG in water suppressed rapid drying and convection in the droplets, which is favorable for the formation of the nanostructures.


Synthesis of gold nanorods
Synthesis of gold nanorods (AuNRs) was carried out using a CTAB capped seed mediated growth method. Specifically, seed solution is first prepared by addition of 250 μL HAuCl 4 (0.01 M) to 9.75 mL CTAB solution (0.1 M). After preparation of a fresh ice cold NaBH 4 solution (0.01 M), 600 μL are injected to the seed mixture under rapid stirring. Following several minutes of stirring to remove any potential concentration gradient the seed solution is left to stand for at least 2 hours at 30 ℃.
Growth solutions for the AuNRs synthesis were then prepared. To 40 mL of CTAB solution

Inkjet Printing Process
We used electrostatic inkjet printing (Microjet FemtoJet-2000HB, MICROJET Corp.). The diameter of the nozzle was 40 μm, and the gap between the substrate and the nozzle was 40 μm.
AC voltage of 1 kV at 10 Hz was applied between the substrate and the nozzle for 500 ms for each ejection. In order to draw the lines, each droplet was fused by ejecting at intervals of 25 μm.
A droplet with a volume of about 100 pL is ejected for each ejection. The width of the line was about 100 μm. The interval between the lines was set to 200 μm. The printed area was 2 mm x 2 mm.

Scanning electron microscopy (SEM) observation
For SEM observation, we used a field emission scanning electron microscope (JSM-6700F, JEOL ltd.). The accelerating voltage was 10 kV and the emission current was 10 μA.

Particle image velocimetry (PIV) analysis
For PIV analysis of water and EG / water mixture, we dropped 0.5 μL of each droplet containing tracer particles (fluorescent pigment, particle size 3~5 μm, JUJO chemical co., ltd.) onto a substrate. The droplet was irradiated with a 532 nm laser (Seika Digital Image corp.), and the motions of the tracer particles were observed with a high-speed camera (FASTCAM Mini AX, Photron co. ltd.). The motions of particles were analyzed using PIV software (KoncertoII, Seika Digital Image corp.). In order to obtain high spatial resolution, multi-grid Interrogation was used as the analysis method.

Surface-enhanced Raman scattering (SERS) measurement
SERS measurements were carried out using a Raman spectrometer (Raman station 400F, Perkin Elmer co., ltd.). Raman spectra were collected in a back-scattering configuration using normal light incidence. Samples were prepared by immersing the substrate with vertically aligned AuNRs in 1 nM BBP ethanol solution for several seconds, then allowing them to dry completely in ambient conditions. Once completely dried the SERS measurement was carried out with 30 collections of 10 s laser exposure (λ = 785 nm). Laser power at the sample was set to 10 mW. The SERS spectrum of a pure BBP liquid was also measured by dropping 10 μL droplets onto Si.

Zeta-potential measurement
The zeta-potentials of the AuNRs were measured by using a zeta-potential measurement system (Zetasizer ZS, Malvern Panalytical co., ltd.) with a 173° scattering angle at a temperature of 25 °C. The cuvettes used in the measurements were disposable DTS 1070 capillary cells. For each AuNRs dispersion, the measurements were carried out three times to calculate the mean and corrected sample standard deviation of the zeta potential. Figure   Fig. S1. The electrostatic scheme for inkjet printing. ΔV can be applied with AC current.