Abstract
Quantitative protein measurements provide valuable information about biological pathways, immune system functionality, and the mechanisms of disease. The most accurate methods for detecting proteins are label-free and preserve native protein-binding interactions. Label-free biomolecular interaction analysis includes reflectometry, a group of techniques that detect proteins by measuring the reflectance properties of a thin film on a substrate. Most of these techniques are limited in some way by instrument complexity, sensitivity, or consumable manufacturing requirements. To address these issues, we introduce Lambda Theta Reflectometry (LTR), a new reflectometric technique that measures changes in film thickness by determining the point of null reflectivity as a function of wavelength (lambda) and angle of incidence (theta). The substrate is simultaneously illuminated with a range of angles and wavelengths, and reflected light is resolved both angularly and spectrally. Our prototype LTR reflectometer can measure SiO2 layer thickness with milli-Ångström precision. LTR measurements of Si/SiO2 oxide films are in excellent agreement with spectroscopic ellipsometry for film thicknesses ranging from 1390 to 1465 Å. This technique enables label-free biosensing measurements across a range of biological analyte concentrations (0.5 ng/mL to μg/mL) without requiring stringent control over probe deposition thickness or substrate manufacturing.
Keywords:
immunoassay; optical thickness measurement; reflectometry; spectroscopy; thin film.