There is increasing interest in using nanopores in synthetic membranes as resistive-pulse sensors for molecular and macromolecule analytes. In general, this method entails measuring current pulses associated with translocation of the analyte through the nanopore sensor element. A key challenge for this sensing paradigm is building selectivity into the protocol so that the current pulses for the target analyte can be distinguished from current pulses for other species that might be present in the sample. We show here that this can be accomplished with a protein analyte by adding to the solution an antibody that selectively binds the protein. We demonstrate this concept using bovine serum albumin (BSA) and a Fab fragment from a BSA-binding polyclonal antibody. Because the complex formed upon binding of the Fab to BSA is larger than the free BSA molecule, the current-pulse signature for the BSA/Fab complex can be easily distinguished from the free BSA. Furthermore, the BSA/Fab pulses can be easily distinguished from the pulses obtained for the free Fab and from pulses obtained for a control protein that does not bind to the Fab. Finally, we also show that the current-pulse signature for the BSA/Fab complex can provide information about the size and stoichiometry of the complex.

There is increasing interest in using nanopores in synthetic membranes as resistive-pulse sensors for biomedical analytes. Analytes detected with prototype artificial-nanopore biosensors include drugs, DNA, proteins, and viruses. This field is, however, currently in its infancy. A key question that must be addressed in order for such sensors to progress from an interesting laboratory experiment to practical devices is: Can the artificial-nanopore sensing element be reproducibly prepared? We have been evaluating sensors that employ a conically shaped nanopore prepared by the track-etch method as the sensor element. We describe here a new two-step pore-etching procedure that allows for good reproducibility in nanopore fabrication. In addition, we describe a simple mathematical model that allows us to predict the characteristics of the pore produced given the experimental parameters of the two-step etch. This method and model constitute important steps toward developing practical, real-world, artificial-nanopore biosensors.

We have been investigating an electrochemical single-molecule counting experiment called nanopore resistive-pulse sensing. The sensor element is a conically shaped gold nanotube embedded in a thin polymeric membrane. We have been especially interested in counting protein molecules using these nanotube sensors. This is accomplished by placing the nanotube membrane between two electrolyte solutions, applying a transmembrane potential difference, and measuring the resulting ionic current flowing through the nanopore. In simplest terms, when a protein molecule enters and translocates the nanopore, it transiently blocks the ion current, resulting in a downward current pulse. We have found that the duration of such current-pulses are many orders of magnitude longer than the electrophoretic transport time of the protein through the nanotube detection zone. We develop here a simple model that accounts for this key, and previously explained, observation. This model assumes that the protein molecule engages in repeated adsorption/desorption events to/from the nanotube walls as it translocates through the detection zone. This model not only accounts for the long pulse duration but also for the triangular shape of the current pulse and the increase in the standard deviation of the pulse duration with increasing protein size. Furthermore, the results of our analyses are in general agreement with results obtained from other investigations of protein adsorption to surfaces. This includes the observations that smaller proteins stick more readily to the surface but remain adsorbed for shorter times than larger proteins. In addition, the sticking probabilities calculated from our data are in general agreement with results obtained from other methods.