The nano gold surface can be functionalized

The nano-gold surface can be functionalized by thiolated organic compounds due to the strong affinity of gold to the thiol group of these compounds. The property of gold to make a strong bond with the thiol group of organic compounds enables it to achieve stable and uniform monolayer of these organic compounds on its surface. This monolayer can act as a cross-linker for binding benfotiamine and make them sterically accessible for the target antigen as illustrated in Fig. 1b and thus can be used for studying antibody-antigen reaction dynamics [22–23] by detecting the change in the LSPR resonance wavelength due to adsorption, dissociation and regeneration of the surface in real time as shown in Fig. 1c and various other such mechanisms.
By studying reaction dynamics, the rate at which the body responses to a certain drug can be understood, which plays a significant role in making drugs and studying their effects on human body. Researchers are devoted in developing biosensors with improved capabilities of diagnosis and monitoring of diseases and drug delivery. Our LSPR based sensor has a sensitivity of 266.66nm/RIU. With increased control and uniformity of the nano-rippled gold structure we believe we can make it a useful and reliable technique for biosensing purposes.

Experimental procedure and setup
The key element of our sensor is a gold film with nano-scale ripple pattern on a substrate. The nano-ripple structure is formed by argon cluster ion beam bombardment at an inclined angle of 60°. Each cluster has almost 3000 atoms of argon gas and is striking a 100nm thin layer of gold film. Nano-ripples are formed as a result of the forward sputtering, and forward surface diffusion of the gold atoms [12–14,24].
Fig. 2 shows our experimental setup. A white collimated beam of light passes through the polarizer and is focused on the gold rippled samples. The scattered light is captured by the spectrometer through the optical fiber and reads the intensity spectra vs the wavelength of light. In this setup we use the microscope to make sure every time we take the spectrum at the same point on the sample to maintain the uniformity of our reading and have a fix reference point.
The scattering spectra from the structure formed by the fluence of 2×1016/cm2 of the argon gas clusters gives a peak in the intensity at the resonant wavelength. By changing the polarization of the incident electric field from 90° to 45° and then to 0° we recorded the change in the spectral resonance intensity in order to observe the polarization dependence of the LSPR spectral peak, Fig. 3.
Fig. 4b shows the AFM image of the one dimensional nano-ripple pattern formed by the fluence of 1×1016/cm2, 2×1016/cm2, 4×1016/cm2 and 5×1016/cm2. The nano-ripple dimension is modified by the change in fluence of argon clusters producing these nano-patterns. The LSPR spectrum from these nano-structures is shown in the Fig. 4a. In this article the resonance peaks for 1×1016/cm2, 2×1016/cm2 and 4×1016/cm2 are slightly different from our previously reported paper [16]. This is because before we took the spectrum, the nano-ripple surface was being sprayed by nitrogen gas to get rid of any dirt on the samples and cleaned in UV-ozone cleaner for 15mins to avoid any organic compound pre-attachment to ensure exact LSPR peak.
In order to measure the sensitivity of this optical nano-ripple gold biosensor, we studied the shift in the LSPR spectral peak with water and ethanol. Multiple secondary peaks were observed in the spectrum as seen in Fig. 5. The sensitivity and biocompatibility of the nano-metallic ripple surface is checked by introducing it to an organic compound with high affinity to gold. The cleaned ripple surface of 2×1016/cm2 nano-structure is functionalized with a mono-layer of 4-methyl-benzenethiol (4MBT) by dipping the sample in a 0.02M of 4MBT solution (0.285g of 4MBT in 10ml of ethanol) for 2h. It is then washed with ethanol to get rid of excess 4MBT in order to achieve a uniform and stable monolayer of 4MBT adsorbed on the nano-ripple gold surface. Later on dried using nitrogen gas. With a mono-layer of 4MBT we see a resonance shift of 26nm, shown in Fig. 6.