Sub-micrometric reflectometry for localized label-free biosensing

Abstract

In this work we present an optical technique for characterizing sub-micrometric areas based on reflectivity of the light as a function of angle of incidence for the two pure polarizations s and p, covering a range of angles of incidence from −71.80° to 71.80° with a resolution of 0.1°. Circular areas with a diameter in the order of 600 nm can be characterized, and the spectra for the two polarizations can be obtained with a single measurement. For biosensing purposes, we have fabricated several Bio Photonic Sensing Cells (BICELLs) consisting of interferometers of 1240 nm of SU-8 polymer over silicon. An indirect immunoassay is performed over these BICELLs and compared experimentally with FT-VIS-NIR spectrometry and theoretical calculations. The Limit of Detection (LoD) achieved is comparable with standard high resolution spectrometry, but with the capability of analyzing sub-micrometric domains for immunoassays reactions onto a sensing surface.

Micron‐scale wedge thin films prepared by plasma enhanced chemical vapor deposition

“…The optical tool used for analysis is based on a combination of two previously developed optical techniques, Beam Profile Reflectometry and Beam Profile Ellipsometry, used for the characterization of thin films in the semiconductor industry. The tool was adapted to characterize sub‐micrometric areas for localized label‐free biosensing, but the same principle can be used for determining the evolution of the refractive index of the wedge thin films prepared in the present work.…”

“…Subsequently, we present a morphological characterization of the deposited wedged‐shaped films, both at micro and nanoscales, and a micrometer‐resolved study of their optical properties. For this purpose we utilize a recently developed sub‐micrometric optical tool that, based on the principles of beam profile reflectometry and ellipsometry, utilizes a highly focalized laser to measure light reflectometry as a function of angle of incidence and polarization over small areas with a spot diameter smaller than 1 μm . Thanks to this specific instrument, micron size resolved refractive index and birefringence values have been obtained along the wedge zone.…”

Engineering vertically interrogated interferometric sensors for optical label-free biosensing

“…By adapting these techniques, we can reach even a lower spot size with a microscope objective of × 150, these are the so-called reflectometry and ellipsometry at profile level (RPL and EPL). These techniques allow characterizing the sensing surface with a sub-micrometric spot [55]. The system uses a laser source emitting at 637 nm with a high numerical aperture microscope objective (× 0.95).…”

“…The possibility of using a sub-micrometric spot allows localized label-free biosensing in small areas, such as submicrometric holes [20] or pillars. We used these techniques for biorecognizing anti-BSA over circular area of SU-8 over silicon functionalized with BSA with a limit of detection comparable with high resolution spectrometry [55]. The characterization of such small sensing areas can be interesting for increasing the integration of the sensing cell, or to measure variations of biofunctionalization or recognition on smaller areas.…”

Optofluidic liquid sensing on electromicrofluidic devices

“…Although this method only reveals differences in liquids with different RI, the availability of procedures relying on a microfluidic approach (i.e., handling minute amounts of liquid) may be of interest for certain niche applications (e.g., to monitor solute concentration in solutions or mixture of liquids) where the scarcity of samples is a bottleneck for a conventional RI detection [1]. Optofluidic RI sensors based on plasmonics [2][3][4][5][6], photonic crystals [7][8][9][10][11] or photonic crystals fibers [12] are primarily composed of periodic metallic or dielectric structures that can be used to confine and guide light. However, even if these sensing techniques enable detection of liquids with extremely small volumes, in practice, the rudimentary character of the currently available liquid delivery systems makes that still relatively large liquid volumes are required for detection.…”