Electrostatic height modulation of bound particle labels by buffer exchange in an evanescent wave biosensor

We introduce a technique to electrostatically modulate the height of bound particle labels on a biosensor surface by exchanging the buffer. In an evanescent wave biosensor, height modulation leads to a modulation of the scattered and reflected light intensity. We measured a lower scattering and, therefore, a higher reflection for a decreasing ionic strength, which can be explained by an increasing electrostatic force repelling bound particles from the surface. By comparing bonds with troponin, 105 base pair (bp) DNA and 290 bp DNA, we found that the signal change for an ensemble of bound particles was related to the length of the analyte. Additionally, we observed for individual particles that the thermal fluctuations of scattered light intensity became smaller for decreasing ionic strength and that the average intensity shifted toward lower values, corresponding to larger particle heights. A quantitative model comprising electrostatic repulsion and van der Waals interaction could fit the measured height displacements for four different DNA lengths (105 bp to 590 bp) as analytes. Height manipulation of bound particle labels thus reflects analyte-specific properties and may lead to biosensors with enhanced specificity.