Two-dimensional Hyper-branched Gold Nanoparticles Synthesized on a Two-dimensional Oil/Water Interface

Two-dimensional (2D) gold nanoparticles can possess novel physical and chemical properties, which will greatly expand the utility of gold nanoparticles in a wide variety of applications ranging from catalysis to biomedicine. However, colloidal synthesis of such particles generally requires sophisticated synthetic techniques to carefully guide anisotropic growth. Here we report that 2D hyper-branched gold nanoparticles in the lateral size range of about 50 ~ 120 nm can be synthesized selectively on a 2D immiscible oil/water interface in a few minutes at room temperature without structure-directing agents. An oleic acid/water interface can provide diffusion-controlled growth conditions, leading to the structural evolution of a smaller gold nucleus to 2D nanodendrimer and nanourchin at the interface. Simulations based on the phase field crystal model match well with experimental observations on the 2D branching of the nucleus, which occurs at the early stage of growth. Branching results in higher surface area and stronger near-field enhancement of 2D gold nanoparticles. This interfacial synthesis can be scaled up by creating an emulsion and the recovery of oleic acid is also achievable by centrifugation.


Chemicals and Materials
All chemicals including Hydrogen tetrachloroaurate(III) hydrate (HAuCl 4 •3H 2 O, 99.999%), Hydroxylamine hydrochloride (NH 2 OH•HCl, 99.9999%), 4-Chlorobenzenethiol (CBT, 97%) and Oleic acid (90%) were purchased from Sigma Aldrich Inc. and without further purification. Gold nanospheres were purchased from BBI Solutions. Deionized (DI) water with a resistivity of 18 M Ω-cm was used in all cases. All glassware was treated with a piranha solution (H 2 SO 4 : H 2 O 2 = 7:3 v/v, this solution is a very harmful and strong acidic oxidant) for 30 min, and rinsed with DI water for several times.
Synthesis of two-dimensional (2D) gold nanoparticles at an oleic acid/water interface 2D gold nanoparticles were synthesized in a 30 ml glass vial. 0.850 ml hydrogen tetrachloroaurate(III) hydrate solution (HAuCl 4 •3H 2 O, 1.2 mg/ml) was mixed with 12.8 ml DI water, and 37.5 μl of aqueous hydroxylamine hydrochloride (NH 2 OH•HCl, 0.05 M) was added to the solution. After homogeneous mixing, 2.8 ml of oleic acid was slowly introduced to form the oleic acid-water interface. Within a few minutes, color was observed at the interface after the addition of oleic acid. 1 ml of the aqueous phase just below the oleic acid-water interface was collected using a pipette at 4 min, 4 min 30 s, 5 min, 6 min and 7 min.

Characterization of 2D gold nanoparticles
The morphological properties of the synthesized 2D gold nanoparticles were characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM). 20 μl of each solution was dropped onto a carbon-coated 300 mesh TEM grid (Inc. Ted Pella) and allowed to remove the solution by using filter paper within 30 s. TEM images were obtained with a JEOL JEM 1010 electron microscope operating at an acceleration voltage of 80 kV. AFM images were obtained with a SPA-400 (Seiko Instrument, Japan).

High resolution TEM analysis of 2D gold nanodendrimer
High resolution TEM studies were performed in a JEOL JEM 3010 electron microscope operating at an acceleration voltage of 300 kV. TEM samples were prepared by carefully dropping 20 μl of dispersion collected at 4min onto a carbon-coated 300 mesh TEM grid (Inc. Ted Pella).
Synthesis of 2D gold nanoparticles in oleic acid-in-water emulsions 1.53 ml of hydrogen tetrachloroaurate(III) hydrate solution (HAuCl 4 •3H 2 O, 0.715 mg/ml) was diluted with 162.27 ml DI water in a 1-neck round-bottom flask, followed by the addition of 450 μl of hydroxylamine hydrochloride aqueous solution (NH 2 OH•HCl, 0.05 M) with stirring. Under vigorous stirring, 33.6 ml of oleic acid was quickly added to the solution to form oleic acid-in-water emulsions. The reaction in the emulsion mixture was allowed to proceed for 20 min while it was stirred continuously with a magnetic stirrer. After 30 s the stirring was stopped and the mixture was separated into water and oleic acid, and 150 ml of the aqueous phase was collected. The synthesized nanoparticles in the aqueous solution were isolated by centrifugation (5000 rpm, 10 min) and subsequently re-dispersed in water. 20 μl of colloidal gold nanodendrimer solution was dropped onto a carbon-coated 300 mesh TEM grid (Inc. Ted Pella). TEM images were obtained with a JEOL JEM 1010 electron microscope operating at an acceleration voltage of 80 kV and the UV-VIS extinction spectra were taken on a JASCO V530 spectrophotometer.

Growth mechanism simulation
We used a phase field crystal model to simulate the growth of 2D gold nanodendrimer. This model relies on dimensionless density, , where and are time averaged particle density and a reference solution of particle density , respectively. The dynamical evolution is expressed by , where is the phenomenon constant. The term is the Gaussian noise represented by reaction fluctuation, and and are the noise amplitude and Gaussian random number, respectively. The parameters used are = -0.75, = 0.00004, and (initial concentration) = -0.50320. In our simulation, the whole domain size is 2000 x 2000. The equations are discretized in both space and time, and we used Δx = Δy = 1 and Δt = 0.25.

Near-Field Calculation
The near-field optical properties were calculated using a commercially available finite-difference time-domain (FDTD) package (OptiFDTD 8.0). The simulation has used the Drude-Lorentz model which is adopted to investigate the metallic dispersion. The permittivity was set to the values of bulk gold, and we assumed that the 2D gold nanoparticles are embedded in surrounding medium of air. The 785 nm laser source (plane wave, amplitude: 1V/m) is used to excite the particle. The direction of 785 nm laser source is from left to right. The mesh size and simulation space volume are chosen so that further changes in them do not affect the simulation results.

Particle Preparation and functionalization for SERS measurements
For SERS measurements, the gold nanodendrimer and gold nanosphere were functionalized with a 10 mM ethanolic solution of 4-chlorobenzenethiol for 3 h under magnetic stirring at room temperature. Functionalized particles were re-dispersed in water after being centrifuged thrice (8000 rpm, 10 min) in order to eliminate the remaining ethanol and 4-chlorobenzenethiol. The concentrations of functionalized gold nanodendrimer and gold nanospheres in solution, obtained by ICP-MS, were converted to the number per unit volume for each sample.

Raman and SERS measurements
A Raman spectrometer QE65000 from Ocean Optics Inc. and 785 nm laser module I0785MM0350MS from Innovative Photonic Solution Inc. were used for Raman and SERS measurements. Raman measurements were carried out for 4-chlorobenzenethiol powders, 10 mM ethanolic solution of 4-chlorobenzenethiol, ethanol solutions, and the silicon substrate. The Raman measurement was conducted using a 785 nm laser at a power of 250 mW and an integration time of 10 s. SERS measurements were conducted with 50 μl of functionalized gold nanodendrimers and 50 μl of functionalized gold nanospheres on a silicon substrate. The SERS measurement also used a 785 nm laser at a power of 250 mW and an integration time of 10 s. The baseline of the SERS spectrum was corrected before normalization.