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.

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After removing the coverslip, the substrate is soaked in Amtex CCR (Amtex Chemical Co.) for several hours to remove silicone residue, then rinsed in de-ionized water. Remnant colloids are removed by briefly sonicating the substrate in toluene, then rinsing in methanol. Finally, all remaining particulate residue is removed using the RCA SC1 cleaning procedure, which consists of a heated bath of 5:1:1 H2O:NH4OH:H2O2. The substrate is then ready to be resealed.
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We have seen events that clearly correspond to two colloids simultaneously inhabiting the pore. Such two-particle events are easily differentiated from one-particle events by their anomalous pulse heights and widths.
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