Nanopore Measures Nanoparticles’ Surface Charges

When manufacturers produce suspensions of nanoparticles for paints, inks, and some pharmaceuticals, they must check the charge on the particles’ surfaces. If the charges are too low, the particles will clump together, destroying the suspension and making the product ineffective. Researchers have now developed a fast method that measures nanoparticles’ charge by passing them through a nanopore (Anal. Chem., DOI: 10.1021/ac300705z).

Currently, researchers use a method called dynamic light scattering to measure nanoparticle charge. In the method, researchers watch how quickly the particles move in an electric field. One disadvantage is that the technique requires a relatively large sample volume, typically a few hundred microliters, to produce accurate data.

Nima Arjmandi of Imec, a nanoelectronics research center in Belgium, and his colleagues wanted to use much smaller volumes. They chose to watch how long the particles take to pass through a 720-nm-long nanopore.

Using hydrogen fluoride, they etched the nanopore, which can range from 20 to 500 nm in diameter, in a silicon film that separates two chambers of a microfluidic cell. The researchers add nanoparticles to one chamber and then apply a voltage across two electrodes, one placed in each cell compartment. The voltage drives the nanoparticles through the nanopore. When a particle enters the nanopore, the current passing through the electrodes decreases. The duration of the current drop is proportional to the particle’s surface charge.

Because the cells are only millimeters on a side, the nanopore technique requires only small sample sizes. “We can work with tens of microliters” says Arjmandi. He hopes to shrink that volume further.

Comparing their method’s measurements of various nanoparticles, including gold nanoparticles, with measurements using dynamic light scattering, the scientists found similar results. However, the precision of the nanopore technique was twice that of dynamic light scattering, Arjmandi says.