Label-free biosensing using a photonic crystal structure in a total-internal-reflection geometry

Abstract

A novel optical biosensor using a one-dimensional photonic crystal structure in a total-internal-reflection geometry (PC-TIR) is presented and investigated for label-free biosensing applications. This simple configuration forms a micro Fabry-Perot resonator in the top layer which provides a narrow optical resonance to enable label-free, highly sensitive measurements for the presence of analytes on the sensing surface or the refractive index change of the surrounding medium in the enhanced evanescent field; and at the same time it employs an open sensing surface for real-time biomolecular binding detection. The high sensitivity of the sensor was experimentally demonstrated by bulk solvent refractive index changes, ultrathin molecular films adsorbed on the sensing surface, and real-time analytes binding, measuring both the spectral shift of the photonic crystal resonance and the change of the intensity ratio in a differential reflectance measurement. Detection limits of 7x10(-8) RIU for bulk solvent refractive index, 6x10(-5) nm for molecular layer thickness and 24 fg/mm(2) for mass density were obtained, which represent a significant improvement relative to state-of-the-art surface-plasmon-resonance (SPR)-based systems. The PC-TIR sensor is thus seen to be a promising technology platform for high sensitivity and accurate biomolecular detection.

Fig. 1

(a) Schematic of PC-TIR structure on a prism surface; (b) Electric field intensity distribution inside the PC-TIR structure and the evanescent field region.

Fig. 2

(a) The relationship between the minimum reflectance and extinction coefficient of the defect layer; (b) A typical PC-TIR optical spectrum that shows a primary resonance dip at the resonant wavelength.

Fig. 3

(a) PC-TIR sensor structure; (b) Reflectance spectrum when medium is air and incident angle is 60.50°; (c) Experimental, Lorentz fitting and simulated reflectance spectra when medium is PBS and incident angle is 63.47°.

Fig. 4

Experimental setup for spectral and differential reflectance measurements. OL1–OL5: objective lenses, SM fiber: single mode fiber, PH1–PH2: pinhole, PL: polarizer, NPBS: Non-polarizing beam splitter, M1–M9: reflecting mirrors, D1–D2: photodiode detectors.

Fig. 5

(a) Fluctuations of the signal channel intensity and of intensity ratio; (b) Fluctuation of intensity ratio low to 2.2×10⁻⁵.

Fig. 6

The resonant wavelength shifts with incident angle changing

Fig. 7

(a) Bulk solvent refractive index for different concentrations of ethylene glycol solution in DI-water (0%, 1%, 2%, 3%, 4%, 5%); (b) Resonance wavelength shifts with different refractive indices solution on the sensing surface.

Fig. 8

(a) Normalized differential ratio with different conc. of ethylene glycol solution; (b) Transformed resonant wavelength shift with flowing 0.50% ethylene glycol solution.

Fig. 9

(a) Resonance dip wavelength shifts with the binding of adlayer; (b), Reflectance ratios at 632.8 nm from differential reflectance measurements for a PC-TIR sensor without treatments and with APTES monolayer.

Fig. 10

(a) Detection of SA binding to biotinlyated surface; (b) Biotin-conjugated IgG binding to SA-adsorbed surface.