Optofluidic formaldehyde sensing: Towards on-chip integration

Abstract

Formaldehyde (HCHO), a chemical compound used in the fabrication process of a broad range of household products, is present indoors as an airborne pollutant due to its high volatility caused by its low boiling point (T =-19 °C). Miniaturization of analytical systems towards palm-held devices has the potential to provide more efficient and more sensitive tools for real-time monitoring of this hazardous air pollutant. This work presents the initial steps and results of the prototyping process towards on-chip integration of HCHO sensing, based on the Hantzsch reaction coupled to the fluorescence optical sensing methodology. This challenge was divided into two individually addressed problems: (1) efficient airborne HCHO trapping into a microfluidic context and (2) 3,5-diacetyl-1,4-dihydrolutidine (DDL) molecular sensing in low interrogation volumes. Part (2) was addressed in this paper by proposing, fabricating, and testing a fluorescence detection system based on an ultra-low light Complementary metal-oxide-semiconductor (CMOS) image sensor. Two three-layer fluidic cell configurations (quartz-SU-8-quartz and silicon-SU-8-quartz) were tested, with both possessing a 3.5 μL interrogation volume. Finally, the CMOS-based fluorescence system proved the capability to detect an initial 10 μg/L formaldehyde concentration fully derivatized into DDL for both the quartz and silicon fluidic cells, but with a higher signal-to-noise ratio (SNR) for the silicon fluidic cell (SNRsilicon = 6.1) when compared to the quartz fluidic cell (SNRquartz = 4.9). The signal intensity enhancement in the silicon fluidic cell was mainly due to the silicon absorption coefficient at the excitation wavelength, a(λabs = 420 nm) = 5 x 104 cm-1, which is approximately five times higher than the absorption coefficient at the fluorescence emission wavelength, a(λem = 515 nm) = 9.25 x 103 cm-1.