(E-183) Characterization of a label free and low-cost optical cavity-based biosensor with different silver thicknesses

Introduction:: In recent years biosensor development for healthcare applications has seen significant advancements. We have proposed an optical cavity-based biosensor (OCB) as a point-of-care (POC) platform. The OCB is based on an optical cavity structure (OCS), a simple optical resonator structure created by two partially reflective surfaces separated by a small cavity. Due to its resonance characteristics, a small change within the resonator causes measurable optical signal changes. When the target analytes reach the immobilized receptors within the OCS, the small change in the local refractive index (RI) shifts the resonance transmission spectrum. Previously we were able to achieve the detection of this small change using low-cost components (two laser diodes and a CCD or CMOS camera) by employing a differential detection method [1-4].

However, like other biosensor types, OCB still requires careful optimization of the OCS and surface chemistry. The most important part of the OCS is the partially reflective surfaces because their reflectance is directly related to the quality (Q) factor of the resonator. With a high Q-factor, a small shift of the resonance curve can cause a bigger intensity change for a certain wavelength. In this work, we characterized how the different silver thickness affects the differential value (η) of the OCB using RI measurements. Different silver thicknesses change not only their reflectance but also the loss due to the absorption. We will present the best silver thickness based on the LOD for the RI measurements out of 4 different silver layer thicknesses we attempted.


Materials and Methods:: A silver layer was sputter-deposited on drilled and plain 3-in glass substrates at 15, 22.7, 30, and 37.5s. The silver layer on the drilled substrate was patterned using a photolithography process. Spin-on-Glass (SOG) was then spin-coated and cured on drilled and plain substrates. On top of the SOG layer of the plain glass substrate, a SU8-2010 layer was patterned using a photolithography process to define the microfluidic channel. Finally, both drilled and plain substrates were stamped together using the UV glue and cured for 5 min under 5 mW/cm2 UV light. The microfluidic channel has a length of 61 mm, a height of 9.5±0.05 µm, and a width of 1 mm. A circular area with a diameter of 160 μm within the microfluidic channel was used for the data processing, calculating the average intensities and η (Figure 1a).

The setup consisted of two 10mA laser diodes at λ=880nm: 904nm, beam splitter, neutral filter, mirror, and camera. The sample was placed between the camera and incoming laser lights. Initially, the channel was filled with deionized (DI) water (RI=1.3329) to get the baseline data. Then, 5 different RI solutions (5µl each) were injected into the inlet port of the channel, followed by rinsing with DI water, and corresponding intensity variations were recorded by the camera (shutter time: 1.5ms). (Figure 1b). The outlet port was connected to a low-cost micro vacuum pump which was used to generate the flow at 2-3 µl/min.

Results, Conclusions, and Discussions:: The silver layer thicknesses were as follows 22.5 nm (deposition time: 15s), 44.6 nm (22.5s), 64 nm (30s), and 76.2 nm (37.5s), as measured by the profilometer. Experimental validation was conducted by measuring the response to different RI. Each RI injection induced a change in RI within the optical cavity and caused an increase in η. The OCS with a 44.6 nm silver shows significantly larger differential value changes for each RI compared to that of the OCS with a 22.5 nm silver layer (Figure 1c). From 44.6 nm to 64 nm, there were some improvements in the differential value change while the error bar, which indicates the sample-to-sample variation of the measured differential values, is increased. The average standard deviations for measured RIs were 0.07474 and 0.22457 for the silver thickness of 44.6 nm and 64 nm, respectively. This larger variation for the 64 nm silver (3 times larger) between samples could be due to the high Q-factor with a thicker silver. With a high Q-factor cavity, a small fabrication error can cause a larger variation in the measured intensities. The OCS with 76.2 nm silver shows a reduced differential value change for the RI measurement compared to that with 44.6 nm and 64 nm due to the absorption loss of the silver layers. Considering the overall differential value change for the RI in the range from 1.3329 to 1.3334 and the sample-to-sample variation, we concluded that the silver thickness of 44.6 nm is the best out of those 4 different silver thicknesses we compared. The LOD of the RI in this range for the OCS with a 44.6 nm silver layer was determined to be 2.63 x 10^-6 RIU. We are currently working to finely optimize the optical cavity structure and the silver thickness using RI measurements.


Acknowledgements (Optional): : Research reported in this presentation was supported by NIGMS of the National Institutes of Health under award number R15GM146233.

References (Optional): : 1. Rho, D.; Breaux, C.; Kim, S. Demonstration of a Low-Cost and Portable Optical Cavity-Based Sensor through Refractive Index Measurements. Sensors (Basel) 2019, 19, doi:10.3390/s19092193.

2. Rho, D.; Kim, S. Large dynamic range optical cavity based sensor using a low cost three-laser system. In Proceedings of the 2017 39th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC), 11-15 July 2017, 2017; pp. 1393-1396.

3. Rho, D.; Kim, S. Label-free real-time detection of biotinylated bovine serum albumin using a low-cost optical cavity-based biosensor. Opt. Express 2018, 26, 18982-18989, doi:10.1364/OE.26.018982.

4. Rho, D.; Kim, S. Demonstration of a Label-Free and Low-Cost Optical Cavity-Based Biosensor Using Streptavidin and C-Reactive Protein. Biosensors 2021, 11, 4.