Formaldehyde sensing with anchored porous bead microarrays

Air pollution surrounds us outdoors as well as indoors, with significant health consequences. Addressing this issue starts with a widespread reporting of ambient pollutants concentrations, which inevitably relies in part on miniaturized sensors that are affordable, light and easy to deploy and operate. Besides widespread electrochemical or CMOS miniaturized sensors, optical sensors based on functional porous materials represent an interesting alternative. We show in this article the fabrication and use of a miniaturized colorimetric sensor thanks to a microfluidic method known as anchored droplet generation. By capillary trapping of sub-millimeter sol droplets at predefined position within a microfluidic chamber, we obtain a silica porous bead array. This method is compatible by design with operando monitoring of key parameters of the sol-gel process, such as gelation or shrinkage. This monitoring capability was used to seek optimal fabrication conditions of microarrays with a density of ~125 beads/cm². Microarrays of porous beads functionalized with a formaldehyde probe were produced and a low-cost homemade system was used to monitor beads color, directly within the microfluidic chip, upon exposure to formaldehyde. Graphical Abstract: [Figure not available: see fulltext.]