# Lambda Theta Reflectometry: A New Technique for Measuring Optical Film Thickness in Planar Protein Arrays

## TL;DR

Lambda Theta Reflectometry (LTR) is a new method for measuring protein layers on surfaces with high precision, without using labels.

## Contribution

LTR introduces a novel reflectometric technique that measures film thickness via null reflectivity at varying wavelengths and angles.

## Key findings

- LTR achieves milli-Ångström precision in measuring SiO2 layer thickness.
- LTR results align well with spectroscopic ellipsometry for film thicknesses between 1390 to 1465 Å.
- LTR enables label-free biosensing across a wide range of protein concentrations without strict manufacturing requirements.

## 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.