Bimetallic structures to enhance the performance of surface plasmon resonance sensors
Authors:   , Satheesh Babu Roshni 
Published: December 2021

<p><b>Surface plasmon resonance (SPR) sensing is a label−free and rapid detection method and has extensive applications in the field of medical diagnostics, food control, and environmental monitoring. However, the lack of sensitivity to detect small molecules is a continuing concern in the application of this technique. Past research has explored different plasmonic structures such as metal nanoparticles, metallic nanoslits, nanoholes, colloidal Au nanoparticles, 2D nanomaterials, and multilayer structures as the sensing layer to improve the sensitivity of these sensors. However, the sensitivity improvement could be realised only with the cost of the increased complexity of optical configuration and sensor chip fabrication. Silver (Ag) is a very good candidate as the metallic layer for the sensor chip due to its higher electrical conductivity as compared to gold (Au). Besides cost−effectiveness, Ag thin film based sensors have better sensitivity with a sharp resonance dip and a high signal−to−noise ratio. However, the poor chemical stability of Ag thin films prevents their use in practical applications. Noble metals such as Au and platinum (Pt) offer greatly enhanced chemical stability. This work investigated the development of SPR sensors composed of a silver−noble metal bilayer structure to utilize both the sensitivity of silver and the chemical stability of the noble metal.</b></p>
<p>To enable this research, an automated experimental SPR testbed for sensor characterisation was designed and constructed. This testbed is based on the Kretschmann configuration, using a He−Ne laser source at 632.8 nm. SPR sensor consisting of multilayer metal structures was fabricated using standard microelectronic fabrication techniques.</p>
<p>The influence of the relative thickness of a noble metal capping layer on the SPR response and sensitivity from the Ag layer was systematically optimised, using both theoretical and experimental approaches. A theoretical analysis of the performance of the bimetallic SPR sensors was done using the transfer matrix method (TMM) by assuming a five−layer configuration. In the case of an Au capping layer, these simulations indicate an optimised thickness of 45 nm for Ag and 5 nm for Au. The observation from experimental analysis of different thickness combinations of Ag and Au matched the simulated results. However, the results of the stability studies exclude the practical use of 45 nm Ag/5 nm Au structures, as long−term degradation of the Ag layer occurs. A structure of 40 nm Ag/10nm Au was thus selected as the best composition for sensor applications. It is showed that sensors fabricated with this structure showed enhanced sensitivity compared to single−layer Au sensors, with a sensitivity 50% higher than that of the single−layer Au sensor. In the case of Ag/Pt structures, simulations indicated enhanced sensitivity from a 10 nm Ag/16 nm Pt structure. However, experimental measurements did not show any evidence for SPP excitation of Pt at the measured wavelength of 632.8 nm, making it unsuitable as a capping layer in our studies.</p>
<p>The application of 40 nm Ag/10 nm Au bimetal layers as biosensors was done by the immobilization of thiol−terminated vitamin B12 aptamers on the Au sensor surface. However, the results were not reproducible, and more work on the binding kinetics of this aptamer will need to be performed to use this in a biosensor structure.</p>