Employing two distinct photonic crystal resonances to improve fluorescence enhancement

Abstract

Surface-bound fluorescence assays such as microarrays have emerged as a prominent technology in current life sciences research and are currently performed on optically passive substrates such as glass microscope slides. We present an alternative approach using photonic crystal substrates exhibiting resonant reflections. In this work, we design and fabricate a photonic crystal with a TM-polarized resonance at the cyanine-5 excitation wavelength and a TE-polarized resonance spectrally overlapping this fluorophore's emission spectrum. The former resonance increases the excitation of the fluorophore through enhanced electric field intensities, while the latter resonance redirects a proportion of emitted light toward the detection instrumentation. Spots of cyanine-5 conjugated streptavidin on the photonic crystal demonstrate a 60-fold increase in fluorescence intensity and a 42-fold increase in signal-to-noise ratio relative to a glass slide.

Figure 1

(a) Schematic of PC designed in this work. The dimensions are as follows: period Λ=360 nm, grating depth d=60 nm, SiO₂ thickness t_SiO2=300 nm, and TiO₂ thickness t_TiO2=160 nm. The grating width is 50% of the period. (b) Atomic force micrograph of PC fabricated by nanoreplica molding, showing the measured period of Λ=368 nm and a grating depth of d=60 nm, in good agreement with the simulated structure.

Figure 2

(a) Simulated and (b) measured transmission efficiencies as a function of wavelength for white light illumination of the PC at normal incidence with either TM-polarized incident light or TE-polarized incident light.

Figure 3

Intensity profile as a function of distance for a line of fluorescent image pixels profiling three spots of cyanine-5 conjugated streptavidin for both the glass slide and the PC under normal incidence illumination. Fluorescence intensity is plotted on a logarithmic scale.