Ultra Compact , High Sensitivity Label - Free Biosensor Using VCSEL

We report a new label-free biosensor system for protein interaction characterization The system consists of a VCSEL, a plastic guided mode resonant filter, and a pin detector. The sensor has high sensitivity and ultra compactness, consumes low power, and can be fabricated in 2D arrays. 02003 Optical Society of America OCIS codes: (170.0170) Medical Optics and Biotechnology, (230.0250) Optoelectronics. One of the greatest challenges in molecular biology is to understand how the many protein targets encoded by DNA interact with other proteins, pharmaceutical candidates, and a large host of enzymes and inhibitors [l , 21. The different sensing techniques can be put into 2 groups: labeling with fluorescent compounds and direct molecule identification. Label attachment increases assay complexity and possibly alters the functionality of molecules through conformational modification or epitope blocking. Hence, various label-free biosensor technologies have einerged. Among the optical techniques are surface plasmon resonance, ellipsometry and reflectance interference spectroscopy. These sensors, though exhibit high sensitivities, are bulky, slow and expensive. Recently, a guided mode resonant (GMR) filter has been optimized to work as a label-free biosensor [3,4]. The GMR consists on a sub-wavelength grating surrounded by a region with smaller index of refraction (Fig. 1). Light incident onto the GMR in the normal direction can couple via the grating structure under a certain resonance condition This coupling results in a reflectance peak with sharp spectral distribution. The resonant wavelength (hpeak) is varied by the attachment of biomolecules, cells, and bacteria to its surface and depends critically on the thickness &io) and refractive index (&io) of those specimens for hi, < 50nm. Hence, it is a high sensitive tool to track moleculadprotein interactions once desirable receptors are coated on the top layer. Since the wavelength regime is determined by the grating period, it has been conveniently chosen to be 850nm based on available low cost optoelectronic components. Finally, the device is built in plastic [4], and thus it is not only low cost but also disposable with minimum environmental impact. The rest of the readout system, including the optoelectronic components, is obviously reusable and measurements can be repeated to improve accuracy. Fig. lb shows typical GMR optical reflectance spectra using a white light source, a spectrometer and an APD array, as described in ref. 3. Continuous deposition changes the thio thickness, which in turn changes he*. This system yields a very high resolution of 0.1 nm in protein thckness change [3]. In this paper, we report a new compact readout system using a vertical cavity surface emitting laser (VCSEL) and a pin detector to replace the light source, spectrometer and detector m y . Fig. 2 shows the experimental setup. The VCSEL emits a single mode at low bias currents at 850 nm. By temperature tuning the VCSEL, its wavelength is varied from 849 to 854 nm. As VCSEL has high spatial coherence, the optical coupling efficiency of the entire system is increased, which reduces the power requirement. The reflection wavelength of the GMR is polarization sensitive. In this experiment, the VCSEL is coupled into a multimode fiber and hence a polarizer is needed in front of the GMR filter. In the future, this system will be further miniaturized when the single mode, single polarization VCSEL is free space coupled into the GMR filter. Fig. 3 shows the reflectance spectra of a GMR filter coated with different refractive index fluids. In this experiment we measure hpe* shift as a function of refractive index of the attached fluid. The thickness of the coating is mych larger than 50nm in all cases and hence is not the cause for &* shift. Fig. 3a was measured with a whte light source and an optical spectrum analyzer as reference traces, while Fig. 3b was with the new compact tunable VCSEL and pin detector. As can be seen, hpek increases with increased index of refraction. The reflection spectra using VCSEL has a full width at half maximum of lnm, about 1/3 of that using white light. This is attributed to the spatial coherence of VCSEL. We believe the narrowed spectra could effectively increase the