Silicon Microcuvette-Based Testing System for Optical Absorption Spectroscopy of Vegetable Oils

Abstract

A fast and simple testing method for micro-nano liter volumes of agricultural products can be very advantageous in the food industry for quality control purposes. In this work, optical transmittance spectra of olive oils, heated to different temperatures, were compared using a custom-built test system. Results indicate that testing at two specific wavelengths of 370 nm and 470 nm is sufficient to determine their state of oxidation and to distinguish them from other oils. A novel microcuvette (μCUV) was designed with integrated heating as well as optical detection capabilities for this purpose. Silicon micromachining techniques were used to fabricate an inverted pyramidal structure, with an opening 750 μm × 750 μm, sidewall slope of 54.7° and an optical window of 30 μm × 30 μm at the bottom. To carry real-time spectroscopic measurements, while maintaining a liquid under test at controlled elevated temperatures, microheater structures surrounding the μCUV were designed. The design parameters were the dimensions of the silicon structure, microheater geometry, thickness of the heating element, and inter-element spacing. While several geometries were compared, simulation results indicate that due to the sloped sidewalls of the μCUV and presence of corners, multiple specially designed heaters at the top as well as the bottom of the structure are necessary to obtain a temperature variation of less than 8% across the inner surface of the μCUV. The silicon-based system can be directly integrated into on-chip control electronics catering to a wide range of applications. We propose a fast and versatile liquid test system based on our results.