Optical sensors based on whispering gallery modes in fluorescent microbeads: response to specific interactions

Abstract

Whispering gallery modes (WGMs) in surface-fixated fluorescent polystyrene microbeads are studied in view of their capability of sensing the formation of biochemical adsorption layers on their outer surface with the well-established biotin-streptavidin specific binding as the model system. Three different methods for analysis of the observed shifts in the WGM wavelength positions are applied and used to quantify the adsorbed mass densities, which are then compared with the results of a comparative surface plasmon resonance (SPR) study.

Keywords: biosensing; cavity modes; label-free detection; optical sensing; refractive index sensing; whispering gallery modes.

Figure 1.

Illustration of the origin of cavity mode shifts in a WGM resonator upon biomolecular adsorption: (a) Effect of resonator size increase due to an adsorbate layer on whispering gallery mode (WGM) formation; (b) WGM wavelength shift due to formation of the adsorbate layer; (c) Definition of the two states of polarization for WGMs: k⃗—wave vector, E⃗—electric field, H⃗—magnetic field, n_s—refractive index of the resonator, n_e—refractive index of the environment.

Figure 2.

Surface plasmon resonance (SPR) response to a sequence of biomolecules adsorbed onto a polyelectrolyte-modified gold surface.

Figure 3.

WGM spectra obtained from a single surface-adsorbed fluorescent polystyrene bead with a nominal diameter of 10 μm in dependence of the same sequence of biomolecules adsorbed onto the particle surface as used in the SPR study; the inset shows the overview over the entire emission wavelength range of the fluorescent dye applied, while the main figure displays a close-up of the most intense modes. The latter are labeled according to their respective polarization (TM/TE) and mode number m. It should be noted that under the given conditions, only 1st order modes are observable. PBS—phosphate buffered saline, BSA—bovine serum albumin, StrA—streptavidin.

Figure 4.

Results of the evaluation of WGM positions as determined after the indicated surface treatments; PBS—phosphate buffered saline, BSA—bovine serum Albumin, EA—ethanolamine hydrochloride, StrA—streptavidin, TM_RO/TE_RO—TM/TE mode determined by ray optics model, n_e free—simultaneous fitting for n_e and R by means of Airy model, n_e fixed—fitting only for R by means of Airy model, keeping n_e fixed to values obtained by SPR.

Figure 5.

Same evaluation as shown in Figure 4b for two other sensor beads exposed to a series of surface treatment steps similar to the one shown in Figure 4. In contrast to the latter, the sensors were exposed to fibrinogen (Fibr) before injection of streptavidin; ΔR = R_i − R_j, where R_i and R_j are sensor bead radii obtained from two subsequent treatment steps, is the incremental radius increase and ΔR₀ = R_i − R₀, where R₀ is the initial sensor bead radius, is the total radius increase, respectively.

Figure 6.

Surface mass densities adsorbed in the different steps of the biomolecular adsorption experiment as determined by SPR and the WGM sensor. For evaluation of the WGM sensor data (cf., Table 1), three different models were applied: RayOpt—ray optics model, Airy—Airy model, PrtbTh—perturbation theory.