Two-Dimensional Self-Assembly of Au@Ag Core–Shell Nanocubes with Different Permutations for Ultrasensitive SERS Measurements

Different self-assembly methods not only directly change the arrangement of noble metal particles on the substrate but also indirectly affect the local electromagnetic field distribution and intensity of the substrate under specific optical excitation conditions, which leads to distinguished different enhancement effects of the structure on molecular Raman signals. In this paper, first, the gold species growth method was used to prepare the silver-coated gold nanocubes (Au@Ag NCs) with regular morphology and uniform size, and then the two-phase and three-phase liquid–liquid self-assembly and evaporation-induced self-assembly methods were used to obtain the substrate structure with different NC arrangement patterns. The optimal arrangement of NCs was found by transverse comparison of Raman signal detection of probe molecules with the same concentration. Subsequently, surface-enhanced Raman scattering (SERS) measurements of Rhodamine (Rh6G) and aspartame (APM) were carried out. Furthermore, the finite element method (FEM) was employed to calculate the local electromagnetic fields of the substrates with different Au@Ag NC arrangements, and the calculated results were in agreement with the experimental results. The experimental results show that the SERS-active substrate was largely associated with the different arrangements of Au@Ag NCs, and the island membrane Au@Ag NCs array substrate obtained by evaporation-induced self-assembly can generate a strong local electromagnetic field due to the edge and corner bonding gap between the tightly arranged NCs; this endows the substrate with benign sensitivity and reproducibility and has great potential in molecular detection, biosensing, and food safety monitoring.


Sample preparation Au NP synthesis
We used a seed-mediated growth method to synthesize the Au NPs synthesis, which is modified by Xia's group (1) . Firstly, HAuCl 4 , NaBH 4 and CTAB were used to prepare CTAB-coated gold seed solution with 3 nm, then placing the solution in a 30 ℃ incubator for incubation to remove excess NaBH 4 . Secondly, CTAC, AA, gold precursor solution and HAuCl 4 were successively added into a beaker, and the gold seeds were grown twice under the condition of slow stirring to obtain about 30 nm Au NPs. After that, the CTAC in the solution was removed by centrifugation at 8000 rpm once for 15 S2 minutes. We can get the desired size of Au NPs by changing the amount of AA and HaCL 4 or by multi-step growth, which can get a more uniform size distribution of Au NPs.

Au@Ag NCs synthesis
The Au@Ag NCs were prepared according to the method of Lin's group (2) . In a typical synthesis, the above solution was re-dispersed in 50 mL of deionized water and heated to 60 ℃ in a water bath under stirring after centrifugation, then 0.32 g of CTAC was added to the beaker for a concentration of 20 mM, subsequently, AgNO 3 solution (50 mM) of 0.5 mL were mixed with Au NPs seed solution and stirred at 60 ℃ for 20 min. Then, the AA solution(500 mM) of the same volume as the silver nitrate solution was added to the above solution and the mixture was gently stirred for 3 hours under a water bath at 60 ℃. We used an ice-water bath to stop the reaction, and the reactants were centrifuged at 8000 RPM for 10 minutes to remove CTAC from the solution. Finally, we quantified the solution to 6 mL with deionized water.

Two-phase liquid-liquid self-assembly of Au@Ag NCs
First of all, we took 6 ml of freshly prepared Au@Ag NCs solution and placed it in a new beaker, 2 mL of cyclohexane solution was drawn with pipette gun and slowly extended to the cup wall and added to the surface. Delamination phenomenon was observed due to the properties of the two solutions, then, we used a needle to absorb the alcohol solution rotated the needle along the cup wall slowly, and added the alcohol into the colloidal solution. With the increase of alcohol content in the solution, a gold film will gradually form at the oil-water interface, after that, a needle was used to remove the most surface of cyclohexane, and the gold particles were concentrated to form a dense film due to the surface pressure at the interface. Finally, the metal film was transferred to the cleaned monocrystalline silicon surface by a pullout method to form a single layer of dense Au@Ag NCs substrate (3) .

Three-phase liquid-liquid self-assembly of Au@Ag NCs
First of all, 1 ml of Au@Ag NCs solution was taken into the test tube, and the surface ligand was replaced by PVP, 1 ml dichlorothane and 1 ml ethanol were added into solution respectively. After shaking the test tube vigorously for 3-5 s, we could observe the generation of NCs in the test tube wall, and then we dropped 2 ml n-hexane into the test tube using a pipette. Then the test tube is placed at an angle with slow rotation of the tube, the Au@Ag NCs in the inner wall of the test tube will move to the interface where the Helmholtz free energy is lowest due to the difference in interfacial surface tension, and then, following the same transfer process as described above, a single layer of dense Au@Ag NCs substrate was formed (4) .

Evaporation-induced self-assembly of Au@Ag NCs
We firstly prepared a large beaker with 25 ml deionized water and placed the supporting substrate in the beaker without allowing the substrate to float or overturned, after that, the ceaned monocrystalline silicon substrate is placed on the substrate. Then 5 µl freshly prepared Au@Ag NCs solution is dropped on the silicon wafer with a pipettor gun. Taked a medium beaker and turned it upside down. The purpose is to place the substrate in a confined space with constant temperature and humidity of 27 ℃ and 75 % for slowly evaporation for 72 h. Finally, the Au@Ag NCs island film mounting substrate was obtained (5) (6) , the substrate surface will be cleaned with an alcohol solution to remove excess surfactants and dust to avoid interference with subsequent spectral measurements.

Characterization
We used a variety of instruments to characterize and test the samples, the Ultraviolet (UV) visible (Vis) absorption spectra of the solution were obtained with UV-5900PC (Shanghai Yuanli Instrument Co. Ltd.). The size and morphologies of the Au@Ag NR horizontal array were identified using a scanning electron microscope (SEM, Renishaw Changers). Transmission electron microscopy (TEM) images were acquired with a JEM-2800 TEM at an acceleration voltage of 200 kV. Raman spectral measurements were conducted with a confocal Raman microscope (Renishaw inVia Raman Microscope). The wavelength of the excitation light was 532 nm and 785 nm. The objective for laser illumination was 50×, and the laser power was 2.5 mw and 50 mw. All of the spectra were measured in a dark environment.