# Design of Inverted Nano-Cone Arrayed SERS Substrate for Rapid Detection of Pathogens
## 4 Discussion and Conclusions  
A novel SERS substrate design is proposed for rapid, culture-free diagnosis of bacterial sepsis. At the present time, SERS sensors have two ways to make contact with the target SERS substrate--by providing colloidal nano particles which adhere to the targets or flat SERS surface on which targets are expected to sit. Both methods have limitations to get right sensitivity unless a high enough number of target cells is available. In the colloidal method, plasmonic nano particles bind with target bacteria in solution which can be heavily affected by the zeta potential and other test conditions. The flat surface approach is to have a nanostructured metal layer on the substrate to be covered with target cells. They both have the following issues and limitations to be improved by our design. Firstly, loose contact between plasmonic metal and target bacteria is degrading the sensitivity. The Raman signal enhancement by plasmonic metal nanostructures happens when the plasmonic hot spots and the target pathogen come to a very short distance, typically less than 10 nm. There is no long-distance force that can let plasmonic metal and cell wall touch in close contact and generate enhanced signal, resulting the arrival of cells to plasmonic structures is limited by slow diffusion. Our design suggests binding of pathogenic cells to magnetic nanoparticles, and then using a magnetic field to attract magnetic particles down to the 3D metal nanostructures, forcing the target cells to get very close proximity to the hot spots formed on the top of nano-cones instantaneously. Secondly, the incident light cannot illuminate the plasmonic metal under the target cells at the maximum strength. In colloidal method, target bacteria and nano metal particles are randomly bound to each other, and incident light can be blocked by the target cells quite often if they are behind the target cells. For flat surface approach, bacteria would sit at the top of plasmonic surface, and incident light will come from top and large portion of incident light will be blocked/ absorbed by the target bacteria, and small number of photons can reach the plasmonic metal at the maximum strength. Our design allows light comes from the bottom through the transparent nanostructures, thereby has no photon blocking problem. Thirdly, the limited light field enhancement of the current colloidal and flat surface approach which lingers around \(10^{6}\) can be improved by the transparent nanocones where the radius decreases along the light path. A high reflective and low loss layer is deposited outer surface of the cone. When this higher concentration of \"photon gas\" reaches the plasmonic metal nanorod at the top of the cone array, the Raman signal enhancement can be as high as \(10^{8}\). The high sensitivity originates from the combination of  
Figure 7: The comparison of \(E^{4}\) factor between hexagonal dense packing cone array and flat surface. photon funneling, dense packing, and optimized cone shape to ensure the close contact of the target to the hot spots. The Raman enhancement factor can be increased by two orders of magnitude while only reducing the contact area by one order and makes overall sensitivity to be one order higher compared with the conventional flat nanosurface approach. The Si-coated inverted cone structure proposed in this paper has the advantages of low-cost and large-scale production, and it is easy to incorporate an immunomagnetic pathogen collecting workflow. The proposed design can use existing Raman configuration with the incident light entering the substrate from the bottom, funneling through the transparent PDMS medium which has a conical shape. The same backscattering configuration of the existing Raman instrument can be used by switching the incident light from top to bottom. This design is similar to the TERS (Tip Enhanced Raman Spectroscopy) system which has one tip with very high electric field hot spot.