Tyranny of the langmuir binding isotherm
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. [@walt2013Optical Methods for Single Molecule Detection and Analysis]
This is one of the reasons why tools like Elisa test will never work at low concentrations. The same is true for iSCAT, for example, which requires binding/unbinding to a substrate in order to measure optical-mass.
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 Simoa does, for example.
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