In-situ measurement of magnetic nanoparticle quantity in a microfluidic device

In this article we demonstrate a method for the accurate in situ determination of the quantity of the entrapped magnetic nanoparticles in the reaction chamber of a microfluidic device. The measurement is based on the resonance frequency shift of a passive electrical resonant circuit where a flat inductor coil integrated in a silicone elastomer film acts as a sensor. The particle amount inside the chamber affects the inductance therefore the resonance frequency is changed. The method also enables the on-line monitoring of the actual particle quantity in the chamber. The effect of particle size and arrangement on the catalytic activity of the reaction was investigated on the basis of the results gathered by the method. It was shown that the arrangement of the particles in the chamber obeys the sphere-packing theory therefore the volumetric ratio of the particles and the chamber is independent on the particle size. In the experiments phenylalanine ammonia-lyase enzyme was immobilized as a biocatalyst on the surface of magnetic nanoparticles. It was demonstrated that decreasing the particle size resulted in increasing catalytic activity due to the increased area to volume ratio. Further improvement could be achieved by using a binary mixture of particles with two different particle sizes. In this case a 40 % increment of the catalytic activity and 17 % increment of the entrapped particle mass could be observed.