Separation of Magnetic Microparticles With Different Molecular Surface Functionalizations by Close‐to‐Surface Traveling‐Wave Magnetophoresis

Magnetic microparticles (MPs) are at the core of a magnetic lab-on-a-chip platform, where they can be used for liquid stirring, diffusion increase, and uptake, transport, concentration, and detection of analytes. A simple idea for analyte detection is to measure their change in magnetophoretic mobility upon analyte uptake. As typical biomolecular analytes are in the nanometer size range, they do not significantly increase the size of the MPs and, therefore, do not change their mobility away from any wall. Here, we show that MPs transported close to an underlying surface exhibit significantly different mobilities depending on their chemical surface properties. Specifically, traveling-wave magnetophoresis leads to different average velocities for MPs with different molecular surface coverages despite having the same size and magnetic susceptibility. This effect is attributed to surface-coverage-dependent interactions between particle and substrate, mediated by the surrounding liquid, leading to different average distances between the substrate and MP. This, in turn, leads to different drag forces for their close-to-surface motion. We found that MPs of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>2</mml:mn> <mml:mspace></mml:mspace> <mml:mi>μ</mml:mi> <mml:mi>m</mml:mi></mml:mrow> <mml:annotation>$2 \,{\mathrm\umu} \mathrm{m}$</mml:annotation></mml:semantics> </mml:math> diameter covered by a polymer with carboxyl ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>COOH</mml:mi> <mml:annotation>$\mathrm{COOH}$</mml:annotation></mml:semantics> </mml:math> ) end groups and a mixture of carboxyl and amino ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>NH</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:annotation>$\mathrm{NH}_2$</mml:annotation></mml:semantics> </mml:math> ) groups showed a large difference in their average close-to-substrate transport velocities in water at high driving frequency.