Engineering the penetration depth of nearly guided wave surface plasmon resonance towards application in bacterial cells monitoring

Photonic techniques based on evanescent waves sensing (such as the surface plasmon resonance (SPR) method) using plasmonic and nanostructured metallic/semiconductor materials hold huge potential in biosensing and associated analysis of biomolecular interactions. However, conventional SPR suffers from low penetration depths (<300 nm), limiting the applications for the surface interactions and analysis of larger biomolecules, such as for bacteria cells with a typical size of ∼1 μm. These cases result in the measured signal being non-monotonic with concentration, making the technique unreliable for high concentrations. Infrared wavelengths can be used, but then signal contrast suffers, and the instruments required for mid-infrared or longer wavelengths are prohibitively expensive. With this in mind, we developed a “nearly” guided SPR (NGWSPR) structure to enhance the performance of these sensors by increasing penetration depth and figure of merit using wavelengths in the optical telecommunication window where off-the-shelf instruments are available at low cost. The use of this technique for monotonic detection of cultured live Escherichia coli bacterial cells is demonstrated, thus opening a pathway to utilize and promote the approach for biosensing, biomedical research and industrial applications.