Tyranny of the langmuir binding isotherm

<blockquote> <p>The biggest problem with capturing analyte molecules on a surface is the long time required to capture molecules at low concentration. [...] For $1\,\textrm{fM}$ analyte concentration binding to a capture antibody immobilized on a surface, it takes nearly a year to come to equilibrium. [<a class="litnote tooltip" href="/literature/@walt2013">@walt2013<span class="tooltiptext">Optical Methods for Single Molecule Detection and Analysis</span></a>]</p> </blockquote> <p>This is one of the reasons why tools like <a class="wikilink" href="/elisa_test/">Elisa test</a> will never work at low concentrations. The same is true for <a class="wikilink" href="/iscat/">iSCAT</a>, for example, which requires binding/unbinding to a substrate in order to measure <em>optical-</em>mass. </p> <p>One way of overcoming the challenge of the binding to a surface is to use beads diffusing in a solution, coated to a capturing antibody. This is what <a class="wikilink" href="/simoa/">Simoa</a> does, for example.</p>