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Web Date: August 27, 2012

Nanopore Measures Nanoparticles’ Surface Charges

Analysis: With small sample volumes and high precision, method could help manufacturers check quality of particle suspensions
Department: Science & Technology
News Channels: Analytical SCENE, Nano SCENE, Materials SCENE
Keywords: nanopore, zeta potential, nanoparticle, colloid, dynamic light scattering
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Nanoparticle Race
A microfluidic cell helps researchers measure charge on nanoparticles’ surfaces. Researchers put the particles (yellow) in the front chamber of the two-chambered cell and watch as the particles pass through a nanosized pore (gray square) in a silicon film (green). A voltage produced by two electrodes (gray rectangles at top) drives the particles’ movement.
Credit: Anal. Chem.
Schematic of microfluidic device that measures nanoparticle charge.
 
Nanoparticle Race
A microfluidic cell helps researchers measure charge on nanoparticles’ surfaces. Researchers put the particles (yellow) in the front chamber of the two-chambered cell and watch as the particles pass through a nanosized pore (gray square) in a silicon film (green). A voltage produced by two electrodes (gray rectangles at top) drives the particles’ movement.
Credit: Anal. Chem.

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.

 
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Comments
Matthew (Wed Sep 19 14:00:23 EDT 2012)
Hi Alexander,

This method does not measure the actual charge on the nanoparticles, it measures their effective charge, or their zeta potential. This is not the true charge of the nanoparticles because the true surface charge gets screened by the addition of salt and hydrogen and surfactant added to the solution. Either way, it's pretty cool.

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