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Surface-Enhanced Raman Scattering Sensors
Surface-Enhanced Raman Scattering (SERS) sensors continue to attract attention as an analytical technique for chemical sensing and biomedical applications. SERS sensors offer multiple analysis advantages, including easy operation without complicated sample preparation, single-molecule sensitivity, high throughput, and point-of-care applications from commercially available portable Raman spectroscopes. Electric and magnetic field enhancements from localized surface plasmon (LSP) effects can reach several orders of magnitude higher than the incident field; this enhancement is preferred in most SERS sensors. The design and fabrication of plasmonic nanostructures is the key for high-performance SERS sensors, since the maximum field enhancement determines the sensors’ sensitivity, reproducibility, and applicability. [1]
One of the unique features of SERS sensors is that an analyte can be identified by its unique Raman spectrum, providing a route for label-free detection. Unfortunately, Raman scattering itself is inefficient because of the small scattering cross section. However, the analyte’s low scattering cross section can be overcome by designing plasmonic structures to either enhance the intrinsic SERS signal of the analyte, or use an extrinsic design where the SERS signal of a reporter molecule is only enhanced in the presence of the analyte. Figure 1 demonstrates the ability of this type of biosensor to measure the level of vascular endothelial growth factor (VEGF) in clinical blood plasma samples taken from breast cancer patients. [2]
Figure 1. Schematic illustration of SERS immuno-sensor for biomarker operating detection
We have created a system to imitate this type of sensor to show surface plasmonic nanostructure enhanced Raman effects, as shown in Figure 2. A thin layer of Au is deposited on a Si substrate. A hexagonal array of air holes in the Au film forms bow-tie-shaped sharp metal edges, which will create a strong LSP EM field when illuminated. A hexagonal array of silver plasmon nanoprobes are placed on top of the Au film. The nano probe is used as the analyte, which can effectively adjust the scattering peak spectrum (see "RSoft Application Gallery Note: Plasmon Nano Probe"). A nanorod array is used as the “reporter molecule” in this structure. Its index, shape, and material properties will directly report the outputs detected.
Figure 2. A mimic structure for surface-enhanced Raman sensor
We have performed CW simulations for this structure with the RSoft FDTD tool, FullWAVE, at a wavelength of 0.54um with periodic boundary conditions. The period is that of the LSP triangular array. The third-order nonlinear responses χ(3) of gold and silver are on the order of 8×10-19 (m/V)2 and 2×10-19 (m/V)2, respectively. A 5x109 V/m strength electric field is incident on the structure. The reflected power is monitored and its spectrum, derived from FFT calculation, is outputted.
Figure 3. Simulation results show strong Raman scattering with LSPs
Simulation results are shown in Figure 3. Significant Raman scattering peaks appear for the patterned gold film due to the strong LSP fields created. On the other hand, the flat gold film doesn’t show any Raman scattering. Different indexes of the reporter material shift the Raman scattering peaks.
Note that we used Method 3 described in Appendix D of the FullWAVE manual to set up the nonlinear simulation. The normalized χ(3) is related to the incident electric field.
References
- Ming Li, Scott K Cushing and Nianqiang Wu. “Plasmon-Enhanced Optical Sensors: A Review”, Analyst. 2015 Jan 21; 140(2): 386-406.
- Ming Li, Scott K Cushing,etc. “Three-Dimensional Hierarchical Plasmonic Nano-Architecture Enhanced Surface-Enhanced Raman Scattering Immunosensor for Cancer Biomarker Detection in Blood Plasma”, ACS Nano, 2013, 7 (6), pp 4967–4976.