Fluorescence enhanced lab-on-a-chip patterned using a hybrid technique of femtosecond laser direct writing and anodized aluminum oxide porous nanostructuring

Abstract

In this paper, we demonstrate a simple yet effective hybrid method to fabricate lab-on-a-chip devices on aluminum (Al) foil. Instead of using conventional photoresists and lithography methods, an array of square units is first produced by femtosecond laser direct writing, followed by generating highly ordered anodized aluminum oxide (AAO) nanoporous structures within each unit. The AAO treated area becomes hydrophilic. Next, we functionalize the surrounding area outside the square units to superhydrophobic by electrochemical deposition and further chemical modification. This hydrophilic and hydrophobic pattern allows us to confine the liquid samples to be detected within the hydrophilic AAO detection area. We use rhodamine 6G (R6G) as a probe, and obtain a fluorescence intensity enhancement from R6G by 70 times over a flat surface. This leads to the detection sensitivity of R6G molecules to a concentration as low as 10⁻¹⁷ mol L⁻¹. By mixing R6G with RhB molecules, the fluorescence emission bands shift significantly due to the addition of RhB molecules, showing a significantly improved spectral resolution compared to traditional fluorescence spectrometers for liquid samples. This phenomenon can be attributed to the energy transfer between R6G and RhB under laser excitation, which was enhanced by the AAO nanostructures. The array-based LOC device demonstrated in this paper is simple and convenient to fabricate, has low sample consumption and dramatically enhances the fluorescence yield with improved spectral resolution.