Izon qNano User Manual page 33

4 / Suspension stability
Particle aggregation
The aggregation of small nanoparticles (< 100 nm) in solution is a constant
process, involving a complex array of forces and factors. However, there
are a few sample preparation and storage techniques that can reduce the
rate of aggregation.
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Particle settling
As the particle size and density increases, the rate of settling will also
increase. Always vortex samples (except delicate biologicals, which should
be mixed with a pipette) before diluting, to avoid concentration errors. For
particles larger than 500 nm, we recommend vortexing particles before each
recording, and sonicating samples for 5-10 minutes if necessary.
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Ionic strength: Generally, 1 x PBS (0.15 M) is recommended for suspending
particles 100-600 nm in diameter. For nanopores larger than NP600, 1/10th
PBS is recommended. For small nanopores (NP80, NP100), using 2 x PBS
can significantly improve the signal-to-noise ratio. Higher ionic strength
conditions shrink the ion clouds that surround particles in solution, allowing
particles to cling closer to each other, where the attractive chemical forces
are stronger. By using a more moderate ionic strength, there is a larger
cushion of space between particles, and the rate of aggregation is lower.
Temperature: The rate of aggregation for particles < 100 nm is directly
proportional to the temperature-dependent rate of diffusion. Keep
samples below 25 °C to reduce the diffusion rate, and do not freeze
calibration particles. EV samples may be snap frozen and kept at –80 °C.
Zeta potential: Like charges repel each other – the higher the effective
particle surface charge (zeta potential), the stronger the repulsive force
between particles, and the slower the aggregation rate. Often, balancing
suspension stability and maintaining sample integrity can be difficult, as
occasionally the pH needs to be adjusted to increase surface charge in
order to reduce particle aggregation.
Surfactants: Adding non-ionic surfactants like Polysorbate-20
to a suspension helps to keep particles physically separated and
suspended in solution.
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