Abstract
Surface plasmon resonance (SPR) is the collective oscillation of free electrons at the interface between metal and dielectric layers. Since the resonant condition of SPR is dominated by refractive indices of metal and dielectric layer, it is possible to distinguish the refractive index change of dielectric layer through the shifts of resonant condition. Considering the system resolution of SPR system nowadays, the sensing resolution up to 10-6 RIU (refractive index change) is available for a general SPR system. Accordingly, SPR provides a label-free platform to detect bio-interactions. However, for pursuing trace measurement, rapid detection, precise diagnosis, versatile functions, or lower cost, researchers devote themselves to develop different interrogations for diverse applications. For the most general four interrogations: angle, wavelength, intensity, and phase are well applied in current literatures. In this study, we focus on the sensitivity enhancement and potential applications of SPR interrogated by different routes, in order to expend its applying fields. For the non-complicated and efficient sensing platform, intensity interrogation, we discussed the relation between sensitivity and thickness of metal sensing layer, and further derived a generalized sensitivity model for intensity interrogation SPR system. In addition, we found that the optimized metal thickness in intensity interrogation is different from the thickness for best coupling efficiency, which is the most commonly used in research. This difference can be explained through two damping factors in SPR. For the sophisticated and ultra-sensitive sensing platform, phase interrogation, we applied such interrogation on LSPR. LSPR is characterized by coupler-free excitation, nano-scale, and diverse resonant modes; nevertheless, its sensitivity is over 10 folds lower than SPR. In contrast with other researchers who modified the resonant modes, we used phase interrogation instead of extinction spectra to enhance the sensitivity. Our results show that the sensitivity was enhanced over 80 folds at the same LSPR structures under the near-field excitation. This study not only confirms the feasibility of exploiting LSPR by optical phase, but also complements its insufficiency on sensitivity. For the low-cost and large linear detection range sensing platform, wavelength interrogation, we expect to have the potential application on point-of-care test. We proposed a color SPR system based on wavelength interrogation, in which we directly observed the color change of reflection rather than identified the resonant wavelength through spectrometer. Moreover, by using Ag/Au bi-metallic film to replace the general used Au film, we are able to enhance the sensitivity and color contrast without losing linear detection range. In short, we hope to promote SPR sensing system through the studies on intensity, phase, and wavelength interrogations.