How are MIPs made?
Molecularly imprinted polymers are made by first building a complex of a target molecule and associated attached binding molecules that possess the ability to be incorporated into a polymer (see Figure). The complex is usually dissolved in a larger amount of other polymerizable molecules. The bulk of the other molecules for the polymer is made with special molecules called crosslinking monomers. These molecules have two places to bind to the polymer chain to form a rigid three dimensional structure. The crosslinkers are necessary to hold the complexing molecules in place after the target molecule or "template" is removed. It is also usual to add a solvent to the mixture. The solvent molecules get caught up in the growing polymer and leave gaps and pores in the structure to make the target complexes more accessible after the polymer is formed. Typically, after polymerization, a chunk of plastic is obtained. This chunk is ground up into a powder and the target molecule is removed by washing it out with the right solvent. The powder is left with special holes that have a memory for the target molecule are ready to recapture that specific molecule the next time it comes along.
The key step in making a MIP is to form a complex that will survive the polymerization process and leave behind a suitable set of binding sites when the target is removed. If this doesn't happen the final product won't have any memory, or like many of us, it's memory will be blurred and inexact and so the polymer will also bind the wrong molecules. Much of this procedure was mapped out by Professor Wulff in his early experiments. A few variations on this procedure have appeared recently directed at having surface active polymers where porosity is avoided. This is to obtain an increase in the speed of binding with a concomitant loss in capacity for binding in order to make fast responding sensors.