Propagating surface plasmon resonance on microhole arrays.

Metallic thin films patterned with micrometer size triangle or hole arrays present plasmonic properties when excited in the Kretschmann configuration, that are improved in comparison to conventional thin film surface plasmon resonance (SPR). These optical properties can be tuned by varying the physical aspects of the microplasmonic structures. Triangles and microhole arrays were prepared with modified nanosphere lithography (NSL) using latex spheres of 0.65, 0.82, 1.0, 1.5, or 3.2 microm in diameter. This allowed the preparation of triangles with edge lengths between 275 to 2000 nm and microhole arrays of various periodicities, diameters, and hole depths. These microstructures were studied to understand the relationship between the physical aspects and the optical properties, such as the sensitivity, working refractive index range, spectral width of the plasmonic peaks, spectral noise, and refractive index resolution. Microhole arrays with a hole diameter equal to half the periodicity were found to combine the advantages of both localized surface plasmon resonance (LSPR) on nanoparticles and SPR on a thin film. These microhole arrays exhibited high sensitivity to refractive index (>3000 nm/RIU), sensitivity to monolayer formation (2-fold improvement compared to thin films), and excellent refractive index resolution (10(-6) RIU). Finally, a biosensor for the detection of 10 nM of immunoglobulin G (IgG) exhibited a greater response with microplasmonic materials compared to conventional thin Au films. Hence, these novel plasmonic materials exhibit a strong potential as an SPR sensing platform. They can be implemented on existing instrumentation and use detection protocols developed for current SPR sensors.