Heat-induced solution mixing in thermo-responsive polymer-coated microchannels for the fluorometric determination of polyamines in saliva

We developed a simple and easy method for solution mixing based on the heat-induced regulation of capillary action in thermo-responsive polymer-coated microchannels. The channels having two T-junctions were fabricated on a glass plate by a sand-blast technique and then coated with a poly(N-isopropylacrylamide) film. The polymer-coating was performed by the modification with allyltrimethoxysilane and the subsequent radical polymerization of N-isopropylacrylamide and N,N′-methylenebisacrylamide. When the channel was warmed by a Peltier device, a capillarity-based solution flow completely stopped because of the water-repellency of channel surfaces. On the other hand, the cooling of the channels allowed the restart of the solution flow through hydrophilic channels. Solution mixing downstream a T-junction was readily conducted by a Peltier device that had placed at the junction. The technique was applied to the fluorometric analysis of polyamines in saliva. The saliva sample was mixed with nickel(II) chloride solution at the first junction to mask amino acids and then mixed with o-phthalaldehyde solution at the second junction to form the fluorometric derivatives of polyamines. Blue fluorescence was observed downstream the second junction. Linear correlation was obtained between the emission intensity and the spermine concentration in the range of 20-100 μM. No mechanical pump or valve was required for the fluid manipulation.