Measuring nanoparticles

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2010

A new technique called induced grating allows accurate measurement of particles below 200 nanometers.

KEY CONCEPTS
• Dynamic light scattering has difficulty measuring small nanoparticles with sizes of 20 nanometers or lower.
• A new technique known as induced grating has been developed that can measure different nanoparticle sizes with the same signal intensity.
• Induced grating achieves this result by taking advantage of the fact that smaller particles diffuse faster than larger particles.

The continuing interest in using nanoparticles to improve the performance of lubricants is leading to the development of some interesting technologies. For example, TLT highlighted a nanoparticle-based lubricant package that imparts both lubricant and extreme pressure characteristics to lubricant fluids and greases (1). The nanoparticle contained molybdenum disulfide combined with canola oil.

A second technology focuses on the use of a dispersion of a potassium borate nanoparticle in a fatty ester (2). This nanoparticle-based lubricant additive has shown promise in automotive and industrial lubricant applications.

One of the challenges in working in this area is to accurately determine the size of the nanoparticles and see how this affects performance. The current technology for nanoparticle measurement is dynamic light scattering (DLS). In this technique, light hits small particles and gets scattered in all directions (Rayleigh Scattering). Due to the fact that small particles in solutions are undergoing Brownian Motion, a time-dependent fluctuation in the scattering intensity is then recorded and computed to generate the particle size and distribution information.

Yanyin Yang, product specialist at Shimadzu Scientific Instruments, Inc., in Columbia, Md., says, “DLS suffers from a major disadvantage in that intensity of the scattered light is proportional to the sixth power of the particle diameter. This means that DLS has difficulty measuring small particles of 20 nanometers or even lower.”

Yang adds, “For large particles, the underlying assumption of Brownian motion in DLS does not apply since they tend to become sediment and sink to the bottom.”

Moreover, when the sample has a broad distribution or contains contaminants and agglomerates, DLS becomes ineffective in a way that signals from larger particles or contaminants and agglomerates can overwhelm those from targeted smaller particles due to the restriction of Raleigh Scattering. High signal-to-noise ratio in this case cannot be realized.

A new test method is thus needed to address the issues of DLS and accurately measure the size and distribution of nanoparticles. Such a technique has not been available until now.

INDUCED GRATING
Shimadzu has developed a new approach to the measurement of nanoparticles known as induced grating. This technique accurately measures the size and distribution of nanoparticles with much less sensitivity dependence on particle size than that with DLS and is now commercially available as the IG-1000 single nonparticle size analyzer.

Yang explains, “In this analyzer, an array of electrodes is immersed in a sample of dispersed nanoparticles. Upon applying AC voltage, particles will be drawn toward the electrode array because of dielectrophoresis. Some areas will have a higher density of particles while others will have a lower density of particles. Therefore a special density grating is formed. When light hits this grating, it gets diffracted. The diffracted light intensity and pattern are then recorded.”

Yang continues, “Once the voltage is turned off, the nanoparticles begin to diffuse away from the electrodes. Smaller particles will diffuse faster than larger particles. Hence the change in the diffracted light intensity and pattern is computed to generate the data of particle size and distribution.”

The correlation between diffracted light intensity and nanoparticle diffusion time is demonstrated in Figure 3 for sizes ranging from 5 to 100 nanometers. Note that the light intensity decay speed is a function of particle size.


Figure 3. The induced grating technique accurately measures the size and distribution of nanoparticles by taking advantage of the fact that nanoparticles of different sizes diffuse at different rates. As shown, the diffusion rate of 5 nanometer particles is faster than that of 100 nanometer particles. (Courtesy of Shimadzu Scientific Instruments, Inc.)

Grating-based spectroscopy also is used in several analytical instruments that evaluate lubricant samples. Among the techniques are atomic absorption, inductively coupled plasma and ultraviolet-visible light spectroscopy.

The induced grating approach enables particles of different sizes to be measured with relatively the same signal intensity. As a result, the signal from a group of 1 nanometer particles is equivalent to a group of 100 nanometer particles given the same volume. On the other hand in the DLS technique, instead of 1:1, the ratio becomes 1 to 1 million, which means accurate measurement of 1 and 100 nanometer particles simultaneously cannot be realized.

Particle sizes below 200 nanometers can be accurately measured by this technique. Yang says, “We can determine the size down to 0.5 nanometers. On the high-end, particles up to 500 nanometers can be tolerated. Above that size, the particles diffuse too slowly and do not show up in the final data for the sample.”

This technique involves minimum sample preparation other than dilution to the appropriate concentration. In addition, the installation environment requires nothing but a standard laboratory environment. No vacuum and no liquid nitrogen are required. The instrument is very compact and weighs only 15 kilograms. Compared to other nanoparticle analytical tools such as TEM (Transmission Electron Microscopy), it offers the users more convenience, and much less time and labor are needed for sample preparation. The analysis results are comparable and have been justified in the single nanosize by TEM, according to Yang.

A key strength of the induced grating analysis technique is that effects of contaminants or agglomerates are minimized due to the greatly reduced single sensitivity on particle size. This also allows mixed samples to be accurately measured.

The use of dielectrophoresis means that metal nanoparticles may interfere with the electric field and cannot be measured. Yang provides an idea for circumventing this problem. She says, “The metal nanoparticles can be coated with an insulating layer that is compatible with the electric field. In addition, at the nanoscale, most metal particles are found to be oxidized already. For instance, important industrial nanoparticles made of metal oxides like titanium oxide and zinc oxide can be measured by induced grating without any problem.”

Another concern is that nonpolar solvents may not be used mainly because dielectrophoresis force is proportional to the dielectric permittivity. Nonpolar solvents have low dielectric permittivity leading to weak dielectrophoresis forces. In addition, the sample cell is made of Pyrex glass, which needs to be considered when choosing the solvent.

Yang comments, “We suggest that samples only compatible with Pyrex glass and solvents of relatively high dielectric permittivity be analyzed by the induced grating technique. Future work will involve evaluation of the possibility of using blended nonpolar and polar solvents using the induced grating technique.”

This method marks a step forward in the evaluation of nanoparticle size in a standard lab setting with minimum work spent on installation and maintenance.

Further information can be found in a recent article in American Laboratory (3) or by contacting Yang at yayang@shimadzu.com. 

REFERENCES
1. Canter, N. (2009), “EP Nanoparticles-Based Lubricant Package,” TLT, 65 (4), pp. 12-13.
2. Canter, N. (2009), “Boron Nanotechnology-Based Lubricant Additive,” TLT, 65 (8), pp. 12-13.
3. Yang, Y., Clifford, R., Vial, G. and Shimaoka, H. (2009), “A Novel Approach to Particle Size Measurement in the Single-Nanometer Range,” American Laboratory, 41 (9), pp. 18-20.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.