A PA-coated glass microfiber with a coating thickness of 7 μm from Polymicro Technologies (Phoenix, AZ) was obtained by courtesy of Dr. Michiel Jonker, Utrecht University. This fiber was from the same lot as used in our previous studies. The PA fiber was cut into 3 cm pieces, immersed in methanol containing 1% (v/v) formic acid for 1 h, rinsed with excess water, and stored in water until use. A coating volume of 2.7 μL/m was used to derive the volume-based concentration in PA.