Surface Plasmon Resonance (SPR) is currently considered a highly sensitive sensing technique for optical sensors that enables real-time, fast, and very accurate detection^[1]. The most known and effective architecture configuration (Kretschmann Reather) of SPR sensor is the arrangement of a metallic thin film deposited onto a glass substrate and attaching this substrate onto the base of a right-angled glass prism. This configuration allows for the interrogation of several properties of light including wavelength, incidence angle, or optical power. To generate SPR on the metallic thin-film surface, a p-polarized and collimated light beam is directed toward one of the sides of a glass prism^[2]. The SPR sensors count the small changes in the refractive index (RI) of a sensing medium above a thin-film metallic layer of gold or silver^[3][4]. The details of literature about the background and on-going improvement of SPR sensor structures have been reported in Ref^[1].

A number of hybrid structures containing SPR thin films covered with metallic nanostructures with different shapes, including periodic T-shape rods^[5],gratings^[6][7], nano spots^[8], and nanowires^[9][10], have been reported to enhance the SPR sensitivity using Kretschmann Reather configuration but none of them is easy and cost-effective. However, the glancing angle deposition technique (GLAD) is a versatile single step, cost-effective, and time efficient method to fabricate nanostructures that can enhance the sensitivity of conventional SPR sensors. In literature it was found about the effectiveness of GLAD nanostructures that improve the sensitivity of surface-enhanced Raman scattering (SERS) and metal-enhanced fluorescence (MEF) substrates^[11][12]. The SPR sensor performance including the configuration of the hybrid nanostructures, were investigated using the rigorous coupled wavelength analysis (RCWA) method built into the commercially available Diffract Mod software package (R Soft Design Synopsys. Inc., Mountain View, CA, USA). Figure 1 which is imported from the Ref^[1], shows the schematic illustrations of the configuration of the hybrid nanostructures and optimized performance in terms of SPR CAW (angular difference between the maximum and minimum reflectance).

Fig. 1 Schematic diagram of the simulated model (a), and response curves of the conventional SPR sensor (b)^[1].

The GLAD process to fabricate the silver (Ag) nanostructures on top of the metallic thin layers already deposited onto a glass substrate were practically performed at Department of Mechanical Engineering, Chung-Ang University, South Korea, and  Electron Science Research Institute (ESRI), Edith Cowan University, Australia.

Fig. 2 Schematic illustration of the fabricated hybrid nanostructures using the GLAD process^[1].

The developed sensor configuration enables the measurement of the effective RI changes due to changing the concentration of compounds in a solution of water. The effect of Ag nanostructures geometry on SPR sensor sensitivity was deeply investigated in this study. The viability and sensitivity of the hybrid nanostructured SPR configuration for detecting changes in RI of water was experimental performed.  Fig. 3 shows the experimental set up to examine the variations in the reflected intensity of a broadband light source.

Fig. 3 Schematic diagram of the experimental setup used to measure the sensitivity of different types of SPR sensor chips ^[1].

The hybrid GLAD nanostructured SPR surface was found to be more sensitive to the variations of the solution RI. The experimental results plotted against the simulated results (Fig. 4) confirmed that the hybrid GLAD nanostructured sensor enhances the sensitivity of the SPR sensor.

Fig. 4 Measured and simulated sensitivity of fabricated surface for measuring the RI variations^[1].

We have successfully demonstrated a hybrid nanostructured SPR sensor fabricated a single and simple area GLAD process, which displayed enhanced sensitivity to changes in the RI of solutions. The cost-effective GLAD hybrid nanostructured SPR sensor displayed a measured detection resolution of 3.81× 10−6 RIU and a measured sensitivity of 230 a.u/RIU, which were in good agreement with the simulated results. Since GLAD is a simple and efficient fabrication process and does not require time-consuming fabrication steps, such as lithography, the GLAD process will make easy to construct high-sensitivity SPR sensors for future applications in various industries and research fields.