To understand the mechanism by which individual DNA molecules enter nanometer-scale pores, we studied the concentration and voltage dependence of polynucleotide-induced ionic-current blockades of a single alpha-hemolysin ion channel. We find that the blockade frequency is proportional to the polymer concentration, that it increases exponentially with the applied potential, and that DNA enters the pore more readily through the entrance that has the larger vestibule. We also measure the minimum value of the electrical potential that confines a modified polymer inside the pore against random diffusion and repulsive forces.

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It was recently shown that naturally occurring, genetically engineered or chemically modified channels can be used to detect analytes in solution. We demonstrate here that the overall range of analytes that can be detected by single nanometer-scale pores is expanded using a potentially simpler system. Instead of attaching recognition elements to a channel, they are covalently linked to polymers that otherwise thread through a nanometer-scale pore. Because the rate of unbound polymer entering the pore is proportional to its concentration in the bulk, the binding of analyte to the polymer alters the latter's ability to thread through the pore, and the signal that results from individual polymer translocation is unique to the polymer type; the method permits multianalyte detection and quantitation. We demonstrate here that two different proteins can be simultaneously detected with this technique.

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