Quantitative sensing of nanoscale colloids using a microchip Coulter counter

We have fabricated a microchip Coulter counter on a quartz substrate, and have used it to detect individual nanoscale colloidal particles with a sensitivity proportional to each particle's size. We demonstrate the ability of this device to sense colloids as small as 87 nm diameter, and to distinguish between colloids whose diameters differ by less than 10%. Further reductions in the pore size, easily done with current nanofabrication techniques, make our device applicable to measuring biological macromolecules, such as DNA and proteins.