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Particle size determination of standard particles using nanoparticle tracking analysis
introduction
Standard size particles of the calibrated size (Figure 1) provide third parties with verification methods for new equipment and new technologies. Considering that the sphere is the only shape that can be accurately described by a single value (ie its radius), it avoids the ambiguity of the result and is the ideal object for calibration.
Figure 1: SEM image of a sample of Duke Science 2 calibrated lactic acid particles used throughout the following experiments.
background
The NanoSight instrument has the unique ability to directly visualize and size nanoparticles in liquid suspensions. Particle visualization overcomes the inherent problems of techniques such as photon correlation spectroscopy (PCS or dynamic light scattering) for the simultaneous sizing of each particle. The intensity of the scattered light produced by the nanoparticles is related to the radius of the particle. The radius is increased by 10 times and the scattered light intensity is 106 times larger. Therefore, the average particle size produced by PCS (measuring the total scattered light produced by the collection of particles) is highly dependent on a small amount of large particles, which may be contaminants. On the other hand, electron microscopy requires a lot of time for sample preparation and imaging, and only a small area can be viewed, so there is a problem of sampling representativeness.
As shown in Figure 2, NanoSight's view makes it easy to distinguish particles based on the intensity of the scattered light. Nonetheless, the particle size requirement is determined from light scattering to determine the refractive index of the particles. NanoSight technology calculates the equivalent hydrodynamic radius of a sphere based on the Brownian motion of each particle tracked on multiple frames, and is therefore completely independent of the refractive index (Figure 3). Tracking the performance of each particle one by one helps to better analyze the polydisperse system (Figure 4).
Figure 2: Image of 100 nm and 400 nm diameter standard particles observed on the NanoSight LM10 system.
Figure 3: Typical distribution of the NanoSight LM10 generation. The above distribution is generated by 100nm latex Duke reference material (depending on the sample).
Figure 4: (a) Scatter plot (top) and particle size distribution (bottom) for 100 nm + 200 nm Duke reference material and (b) 200 nm + 400 nm Duke reference material. The scatter plot gives a graph of particle scattering intensity versus particle size. The particle size distribution may be difficult to distinguish between systems with very similar particle sizes. The relative intensity map clearly distinguishes between the two populations and helps to resolve the particle population.
Sample Preparation
Depending on the sample type and particle size, the only required preparation is to dilute the sample to 106 and 109 particles per ml. During the dilution process, the Brownian motion of each particle can be observed for analysis. The optimum concentration depends on the particles and solvent.
Hydrodynamic radius
Since this technique is used to determine the hydrodynamic radius of the particles (ie, the particle radius plus a few nanometers of water), the sample should be prepared in a 1 mM salt solution to reduce the size of the tightly wrapped water layer around the particles. Since the sample measures the hydrodynamic radius, the measured value will always be a few nanometers larger than the results measured by the transmission electron microscope listed by the manufacturer of the reference material.
Temperature measurement
Proper temperature measurement is important because incorrect sample temperature readings can cause viscosity calculation errors. A temperature reading error of 1 °C causes a 2.4% dimensional error for aqueous solutions. Because the LM10 requires less analysis (<500 μl), it takes only a few minutes to achieve temperature equilibration and can be read directly during the analysis.
Main characteristics
â— Particles can be measured in the natural state (no drying/vacuum conditions required)
â— Suitable for detecting the size of polydisperse samples
â— Sample preparation process to visualize individual particles
â— Ability to analyze quickly (see Table 1)
Table 1: Table of the measured values ​​of the particle size versus TEM for a set of Duke standard latex spheres
Specific contact information
For further information, please contact Malvern.