Resonant profile shift resulting from a change of resonant conditions is classically used for sensing, either liquid refractive index or immobilized biological layer effective thickness. Resonant waveguide gratings (RWG) allow sensing over a large spectral domain, depending on the materials and geometrical parameters of the grating. Profiles measurements usually involve scanning instrumentation. We recently demonstrated that direct imaging multi-assay RWGs sensing may be rendered more robust using spatial Fano profiles from “chirped” RWG chips. The scheme circumvents the classical but demanding scans: instead of varying angle or wavelength through fragile moving parts or special optics, a RWG structure parameter is varied. Our findings are illustrated with resonance profiles from nanostructured silicon nitride waveguide on glass. A sensitivity down to Δn=2x10-5 or biomolecules mass density of 10 pg/mm2 is demonstrated through theory and experiments. To assess different sensing wavelength, the period might also vary within the same chip support. We discuss guiding properties and sensing sensitivities of RWG sensing over the whole visible spectral range. Resonant profiles are analyzed using a correlation approach, correlating the sensed signal to a zero-shifted reference signal. This analysis was demonstrated to be more accurate than usual fitting, for analyzing signals including noise contribution. The current success of surface plasmon imaging suggests that our work could leverage an untapped potential to extend such techniques in a convenient and sturdy optical configuration. Moreover, extended spectral range sensing can be addressed by dielectric waveguide structures. This allows sensitive sensing of small volumes of analyte, which can be circulated close from the resonant waveguide. Together with the demonstration of highly accurate fits through correlation analysis, our scheme based on a “Peak-tracking chip” demonstrates a new technique for multispectral sensitive sensing through nanostructured chip imaging.
One of the most significant challenges facing physical and biological scientists is the accurate detection and identification of single molecules in free-solution environments. The ability to perform such sensitive and selective measurements opens new avenues for a large number of applications in biological, medical and chemical analysis, where small sample volumes and low analyte concentrations are the norm. Access to information at the single or few molecules scale is rendered possible by a fine combination of recent advances in technologies. We propose a novel detection method that combines highly sensitive label-free resonant sensing obtained with high-Q microcavities and position control in nanoscale pores (nanopores). In addition to be label-free and highly sensitive, our technique is immobilization free and does not rely on surface biochemistry to bind probes on a chip. This is a significant advantage, both in term of biology uncertainties and fewer biological preparation steps. Through combination of high-Q photonic structures with translocation through nanopore at the end of a pipette, or through a solid-state membrane, we believe significant advances can be achieved in the field of biosensing. Silicon microrings are highly advantageous in term of sensitivity, multiplexing, and microfabrication and are chosen for this study. In term of nanopores, we both consider nanopore at the end of a nanopipette, with the pore being approach from the pipette with nanoprecise mechanical control. Alternatively, solid state nanopores can be fabricated through a membrane, supporting the ring. Both configuration are discussed in this paper, in term of implementation and sensitivity.
The asymmetric Fano resonance lineshapes, resulting from interference between background and a resonant scattering, is archetypal in resonant waveguide grating (RWG) reflectivity. Resonant profile shift resulting from a change of refractive index (from fluid medium or biomolecules at the chip surface) is classically used to perform label-free sensing. Lineshapes are sometimes sampled at discretized “detuning” values to relax instrumental demands, the highest reflectivity element giving a coarse resonance estimate. A finer extraction, needed to increase sensor sensitivity, can be obtained using a correlation approach, correlating the sensed signal to a zero-shifted reference signal. Fabrication process is presented leading to discrete Fano profiles. Our findings are illustrated with resonance profiles from silicon nitride RWGs operated at visible wavelengths. We recently demonstrated that direct imaging multi-assay RWGs sensing may be rendered more reliable using “chirped” RWG chips, by varying a RWG structure parameter. Then, the spatial reflectivity profiles of tracks composed of RWGs units with slowly varying filling factor (thus slowly varying resonance condition) are measured under monochromatic conditions. Extracting the resonance location using spatial Fano profiles allows multiplex refractive index based sensing. Discretization and sensitivity are discussed both through simulation and experiment for different filling factor variation, here Δf=0.0222 and Δf=0.0089. This scheme based on a “Peak-tracking chip” demonstrates a new technique for bioarray imaging using a simpler set-up that maintains high performance with cheap lenses, with down to Δn=2×10-5 RIU sensitivity for the highest sampling of Fano lineshapes.
The asymmetric Fano resonance lineshape, resulting from interference between background and a resonant scattering, is archetypal in resonant waveguide grating (RWG) reflectivity. Resonant profile shift resulting from a change of refractive index (from fluid medium or biomolecules at the chip surface) is classically used to perform label-free sensing. Lineshapes are sometimes sampled at discretized “detuning” values to relax instrumental demands, the highest reflectivity element giving a coarse resonance estimate. A finer extraction, needed to increase sensor sensitivity, can be obtained using a correlation approach, correlating the sensed signal to a zero-shifted reference signal. Correlation approach is robust to asymmetry of Fano lineshapes and allows more accurate determination than usual fitting options such as Gaussian or Lorentz shape fitting. Our findings are illustrated with resonance profiles from silicon nitride “chirped” RWGs operated at visible wavelengths. The scheme circumvents the classical but demanding spectral or angular scans: instead of varying angle or wavelength through fragile moving parts or special optics, a RWG structure parameter is varied. Then, the spatial reflectivity profiles of tracks composed of RWGs units with slowly varying filling factor (thus slowly varying resonance condition) are measured under monochromatic conditions. Extracting the resonance location using plain images of these “pixelated” Fano profiles allows multiplex refractive index based sensing with a sensitivity down to 2×10-5 RIU as demonstrated experimentally. This scheme based on a “Peak-tracking chip” demonstrates a new technique for bioarray imaging using a simpler set-up that maintains high performance with cheap lenses.
In the frame of biological threat, security systems require label free biochips for rapid detection. Biosensors enable to
detect biological interactions, between probes localized at the surface of a chip, and targets present in the sample
solution. Here, we present an optical transduction, enabling 2D imaging, and consequently parallel detection of several
reactions. It is based on the absorption of biological molecules in the UV domain. Thus, it is based on an intrinsic
property of biological molecules and does not require any labelling of the biological molecules. DNA and proteins
absorb UV light at 260 and 280 nm respectively. Sensitivity is a major requirement of biosensing devices.
Configurations leading to enhancement of the interaction between light and biological molecules are of interest. For a
better sensitivity, resonant grating structures are then studied. They enable to confine the electric field close to the
biological layer. Imaging of resonant grating is not largely studied, even for visible wavelengths, but it results in good
sensitivity. The protein used in this study is the methionyl-tRNA synthetase. Its absorption is representative of protein
absorption, and it can then serve as a model for immunological detection. The best experimental contrast due to a
monolayer of proteins is 40%. With data processing currently employed for biochip imaging: average on several
acquisitions and on all the pixels imaging the biological spots, the device is able to detect a surface density of proteins in
the 10 pg/mm range.
DNA and protein absorption at 260 and 280 nm can be used to reveal theses species on a biochip UV
image. A first study including the design and fabrication of UV reflective multilayer biochips designed for
UV contrast enhancement (factor of 4.0) together with spectrally selective AlGaN detectors demonstrated the
control of chip biological coating, or Antigen/Antibody complexation with fairly good signals for typical
probe density of 4x1012 molecules/cm2.
Detection of fractional monolayer molecular binding requires a higher contrast enhancement which
can be obtained with structured chips. Grating structures enable, at resonance, a confinement of light at the
biochip surface, and thus a large interaction between the biological molecule and the lightwave field. The
highest sensitivity obtained with grating-based biochip usually concerns a resonance shift, in wavelength or
diffraction angle. Diffraction efficiency is also affected by UV absorption, due to enhanced light-matter
interaction, and this mechanism is equally able to produce biochip images in parallel.
By adjusting grating parameters, we will see how a biochip that is highly sensitive to UV absorption
at its surface can be obtained. Based on the Ewald construction and diffraction diagram, instrumental
resolution and smarter experimental configurations are considered. Notably, in conjunction with the 2D UV-sensitive
detectors recently developed in-house, we discuss the obtainment of large contrast and good signals
in a diffraction order emerging around the sample normal.
This paper describes the design of biochips enhancing absorption in UV (and more particularly at 260-280 nm). These
contrast-enhancing multilayers structures are developed in order to image biological compounds immobilized at a
surface. The measurement set-up is a reflection set-up, including a previously developed AlGaN detector, spectrally
selective at the wavelength of biological compounds absorption (260 nm for DNA and 280 nm for proteins). A contrast
study is carried out using an inorganic absorber (titanium dioxide), enabling an optimized electromagnetic interaction
with the absorber. The biochip design is based on an aluminum mirror covered with a transparent dielectric. To account
for roughness and oxidation, the material is modeled by a thin layer of effective medium defined in the Bruggeman
approximation. We discuss the absorption expected from various biological compounds, and the capability of our set-up
for detection of monolayers of DNA or protein molecules.
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