Smartphone-Based Microfluidic Colorimetric Sensor for Gaseous Formaldehyde Determination with High Sensitivity and Selectivity

Formaldehyde is one of the most dangerous air pollutants, which can cause sick building syndrome. Thus, it is very crucial to precisely determine formaldehyde with a low cost and simple operation. In this paper, a smartphone-based microfluidic colorimetric sensor is devised for gaseous formaldehyde determination with high sensitivity and selectivity. Specifically, a novel microfluidic chip is proposed based on the 4-aminohydrazine-5-mercapto-1,2,4-triazole (AHMT) method to determine formaldehyde; the chip consists of two reagent reservoirs, one reaction reservoir and a mixing column. In this design to prevent the fluid from flowing out while letting the gas molecule in, a hydrophobic porous poly tetra fluoroethylene (PTFE) membrane is put on the top of the reaction reservoir. Using the microfluidic chip sensor, a smartphone-based formaldehyde determination system is developed, which makes the measuring process automated and simple. As per the experiment results, the limit-of-detection (LOD) of the system is as low as 0.01 ppm, which is much lower than the maximum exposure concentration (0.08 ppm) recommended by the World Health Organization (WHO). Moreover, the sensor is hardly affected by acetaldehyde, volatile organic compounds (VOCs) or acidic-alkaline, which shows great selectivity. Finally, the performance of the proposed sensor is verified by using it for the determination of formaldehyde in a newly decorated house.


Reagent Reservoir Reaction Reservoir
. The fabrication process.
First, the two parts of polydimethylsiloxane (PDMS), cross-linker/curing agent A and siloxane B, were mixed in a 1:10 ratio. Upon mixing, the PDMS was degassed in a vacuum chamber to remove any bubbles. Two kinds of PDMS casting molds were fabricated. One was made by machining a polymethyl methacrylate (PMMA) board; the other one was made by means of lithography using SU-8 resist. The casting process was as simple as placing the mold in a heat-tolerant plastic tray and pouring PDMS onto the molds. The thickness of the PDMS was controlled to be 2 mm. Placing the resulting PDMS in an oven at 95 °C for 1 h cured the PDMS, which was then peeled away from the mold and cut into a rectangle of 10 mm × 2 mm.
In order to bond the two parts together to form the final microfluidic chip, they were treated with oxygen plasma for 2 min using plasma surface treatment equipment (Europlasma CD400, Belgium; Power: 230 W; Vacuum: 13 Pa with a flow of 20 ml/min). To aid with alignment, a Firstly, a tiny hole is made in the reagent reservoir. Secondly, the reagent is injected into the reservoir through the hole using an injection syringe. Because of the hydrophobic property of the micro-channel, the reagent can hardly go into the reaction reservoir if the injecting speed is low. Thirdly, a tiny drop of PDMS is dropped onto the hole. The chip is then put in a plastic petri dish and the dish is sealed using parafilm. After 24 h at room temperature, the PDMS on the hole can be solidified, and the chip is finally obtained.

Part 3. Hydrophilic Porous PTFE Membrane Testing
A hydrophilic porous PTFE membrane was firstly put on the bottom of a plastic pipe.
Then, the water was injected into the pipe. The initial height of the water was 10 cm. The water leakage was tested after 30 min. The height of the water was then increased with a step of 10 cm until the water leaked. The pressure applied to the membrane (P) was calculated using the following equation: = ℎ where h is the height of the water, ρ is the density of the water and g is the acceleration of gravity.
The final height of the water obtained was 200 cm, which means that even when 20 kPa pressure was further applied to the solution, the solution could still not flow out.

Water
Hydrophilic Porous PTFE Membrane Figure S3. Testing the ability of a hydrophilic porous PTFE membrane to prevent water from flowing out.

Part 4. The Mixing Process
A B C D E Figure S4. Diagrams of the mixing step using pumping. At the beginning, the extruding bar is moving slowly down (A, B, C). Then the extruding bar is moving up (D, E, B). The mixing is realized by repeating the steps of B-C-D-E-B.
A, B and C show the extruding bar moving slowly down from the initial situation and no bubbles being generated due to the fact that the reagent reservoirs are filled with solution without air. D, E and B show that the extruding bar is moving up and no bubbles are being generated due to the fact that the microfluidic channel was at the bottom of the reservoirs and immerged in the solution.