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. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3184573͔The microarray is a common method for quantifying molecular species present in a test sample, with applications in life science research and clinical diagnostics enabled by detection of single-strand DNA sequences for gene expression analysis 1 or detection of disease biomarker proteins. 2The utility of microarrays is attributed to the ability to dispense thousands of specific biological capture molecules in a spatially defined pattern. By allowing fluorescently labeled biomolecules in a sample to be captured by their complementary binding partners within the array, researchers can identify the presence and quantity of specific analytes in a high throughput fashion. However, information about low abundance molecules within samples may be lost because the fluorescence measurement systems lack the sensitivity to detect a small number of fluorescent tags. By enhancing the signal-to-noise ratio ͑SNR͒ observed from surface-bound fluorescent molecules, lower concentrations of analyte molecules may be detected than is possible on an ordinary glass slide microarray substrate. The SNR from fluorescent spots is enhanced by replacing the common glass slide substrate with a photonic crystal ͑PC͒ supporting resonant reflections.3 These PCs are nanostructures comprised of a periodically modulated low refractive index SiO 2 surface structure coated with a high refractive index dielectric layer of TiO 2 . A resonant reflection is observed when evanescent diffracted orders couple to modes of an effective high refractive index layer, and are radiated through diffraction in-phase with the reflected zeroth-order wave and out-of-phase with the transmitted zeroth-order wave.4 Under broadband illumination a highly efficient reflection is observed at a specific wavelength. PC surfaces have been demonstrated with resonances that coincide with the wavelength of a laser that is used to excite fluorophores, leading to increased local energy density which strongly excites fluorophores ͑enhanced excitation͒. 5,6 PCs with resonances that coincide w...

Fluorescence enhancement by heterostructure colloidal photonic crystals with dual stopbands

“…Fortunately, Cunningham et al realized remarkable enhancement of fluorescence (ca. 108-fold) from quantum dots on the two-dimensional PC slabs with dual band gaps due to a combination of highintensity near fields and strong coherent scattering effects of guided resonance [20,21]. Enlightened by the work, we develop a facile approach on self-assembled heterostructure PCs with dual stopbands to enhance the fluorescent signal.…”

Optimization of photonic crystal enhanced fluorescence by excitation laser angle scanning

“…In previous work, a confocal laser scanner, incorporating a focused laser beam, was used to good effect for PCEF [5,15]. However, due to the angle selectivity of the PCEF surface, only a portion of the excitation energy can be coupled into the resonance mode and contribute to enhanced fluorescence emission.…”

“…While the effects of PC enhanced excitation and enhanced extraction can be combined with multiplicative effects [5,7,8], this work is primarily concerned with optimization of enhanced excitation through the interaction of the PC resonant mode characteristics and the degree of collimation of the illumination that is provided to the PC.…”

“…Their optical resonance characteristics can be exploited to provide not only a heightened excitation field (resulting in a phenomenon called "enhanced excitation"), but also control over the photonic dispersion, providing a powerful mechanism to redirect the emitted light into certain preferred directions, where it can be detected with greater efficiency ("enhanced extraction"). The simultaneous implementation of these two techniques has been shown to boost the radiation detected from quantum dots and fluorescent dye molecules by two orders of magnitude [5][6][7][8].…”

Enhanced fluorescence emission using a photonic crystal coupled to an optical cavity

“…In previous work, we have shown that an optimal design for PC enhanced fluorescence utilizes transverse magnetic (TM) polarized light (polarization perpendicular to grating direction) for normal excitation and transverse-electric (TE) polarized light (polarization parallel to grating direction) for extraction of the emitted fluorescence signal. 8 Therefore, the PC resonance was designed to be at normal-incidence excitation (h ¼ 0 ) for TM polarized light from a k ¼ 632.8 nm, He-Ne laser for fluorescence excitation. The geometry of the structure and the indices of the surrounding dielectric media determine the resonance wavelength of the PC.…”