Bigger Isn’t Always Better A new technique addresses the increasing demand for a particle size analyzer that can overcome the limitations of dynamic light scattering. by Yanyin Yang, Ph.D., Shimadzu Scientific
Nanoparticles are defined as particles with a diameter of 200 nm or less. Their application can range from sophisticated biomedical research to standard industrial QA labs. For instance, due to their unique optical properties, quantum dots made of ZnS nano crystals are used as a powerful dye in cellular imaging to achieve high sensitivity and high resolution.[1] This has generated significant interest in fields like medical/cancer diagnostics and pathogenic bacteria analysis. The fluorescence spectrum of those quantum dots is directly related to their particle size.
In addition, nanoparticles are gaining increasing popularity in consumer products, including electronics, cosmetics and medical products.[2] In sunscreens, nanoparticles made of zinc or titanium dioxide can block out harmful rays without creating a white clown-face. For quality control, characterizing the incorporated zinc or titanium dioxide nanoparticles is important.
These are just a few examples of why bigger is not always better with today’s technology. Therefore, developing a technique that allows faster and more accurate measurement of the smallest particle sizes, even as small as 0.5 nm, is critical.
Past techniques
The existing technique for measuring nanoparticles is dominated by dynamic light scattering (DLS, also known as photon correlation spectroscopy or quasi-elastic light scattering), which is based on measuring the fluctuation in scattered light intensity due to the Brownian motion of the particles.
A notable disadvantage of DLS involves the signal intensity, which is proportional to the sixth power of particle size (or third power of particle size given the same volume). This causes a drastic reduction in detection sensitivity when particles are 20 nm or smaller.
Conversely, when particles are relatively bigger, sedimentation may occur instead of Brownian motion, leading to false results. Moreover, when the sample has a broad distribution, or when the sample contains agglomerates or contaminants, a high signal-to-noise ratio is unlikely due to the strong dependency of signal intensity on particle size. Scattered light from those bigger particles or agglomerates/contaminants can overwhelm that from the targeted smaller particles.
For particles in the nano-range or even single nano, a TEM (transmission electron microscope) is also frequently used due to its high resolution and magnification.[4] TEM can provide information like particle shape and morphology in addition to particle size. However, TEM sample preparation and data analysis can be time- and labor-consuming. In addition, there are strict requirements with respect to the installation environment and maintenance, such as a high vacuum and handling of liquid nitrogen. Furthermore, the imaging process to analyze particle size may cause biased information; the embedding, sectioning, staining, and then viewing can take weeks, and the assay for TEM is destructive.
Measuring with induced grating
To overcome the restrictions of DLS, Shimadzu developed a new technique based on “induced grating” and dielectrophoresis.[3] Using the IG-1000 technique, a specially designed electrode array is immersed in a solution of dispersed particles (the sample). Upon impressing the AC voltage, particles are drawn toward the electrodes due to dielectrophoresis, forming a density grating.
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Figures A, B and C. Comparing measurement results of bovine serum albumin (BSA) by the IG-1000 and by dynamic light scattering (DLS) reveals (a) particle size and distribution data on the IG-1000; (b) particle size and distribution data on a DLS; and (c) raw data of light intensity against particle size for measuring BSA by DLS. Noise on the DLS from scattered light of other larger particles in (c) is significant, with resulting data that’s unreliable. |
 | | One area has a high density of particles while the other has a low density. When a laser beam comes through this grating, it gets diffracted. The intensity of primary diffracted light is then detected by the sensor. When the AC voltage is off, dielectrophoresis is off, and particles start to diffuse away from the electrode array. As a result, the detected intensity of primary diffracted light begins to decay while particles are getting farther away from the electrodes. In general, smaller particles will diffuse more quickly than larger particles. The diffracted light intensity time dependency behavior is then computed, and the particle size and distribution information is obtained.
The major advantage of the IG-1000 over DLS is the suppression of sensitivity dependence on particle size. Not only can particles down to the single nano digital range or sub-nano be registered accurately, but the influence from agglomerates or contaminants is reduced greatly; therefore, nanoparticles with a relatively broad distribution can be measured with a high signal-to-noise ratio.
In other words, the IG-1000 measurement method is much more resilient than DLS due to its increased tolerance of broad distribution, mixture, and bigger particles. Moreover, input of refractive index and high transmittance requirements are not necessary. The IG-1000 can also easily measure nanoparticles like silica, which have a refractive index close to water, and dark inks or other systems with low transmittance, all of which pose problems for DLS.
Application examples
To demonstrate the IG-1000’s capability, a common protein, bovine serum albumin (BSA), which has numerous biomedical applications, was measured by both the IG-1000 and a brand DLS. Figures A, B and C list comparison results, including the raw data. Six measures were performed using the IG-1000 with a result of 3.21, ±0.18 nm in particle diameter. Consistent, seamless measurement was further demonstrated in the raw data, in which diffracted light intensity was plotted against the particle diffusion time.
On the other hand, the same BSA sample measured by DLS three times cannot generate the same quality data. False results were produced on the report as 103.4, ±0.588 nm. Although the Size Distribution by Volume plot shows that the average particle diameter could be ~3 nm, the Size Distribution by Intensity plot shows that the noise of the high-intensity scattered light from other larger particles is significant. This makes the results unreliable.
With DLS, because larger particles have higher signal intensity, their effects can overwhelm those from the smaller targeted particles, even though their volume may not be significant. In this particular case, scattered light intensity from ~552.2-nm particles accounts for 42%, while that from the targeted BSA particles (~3.481 nm) accounts for only 36.5%.
Conclusion
As more nanotechnology-based products are being developed, there is an increasing demand for a particle size analyzer that can overcome the limitations of DLS. The IG-1000 helps scientists achieve greater reliability with its ability to suppress particle size sensitivity dependence and its increased tolerance of broad distribution, mixture and bigger particles.
For more information, please visit http://www.ssi.shimadzu.com/products/product.cfm?product=IG-1000.
References
1. Walling, M.A, J.A. Novak and J.R. Shepard. 2009. Quantum dots for live cell and in vivo imaging. Int. J. Mol. Sci. 10: 441-491.
2. Schimidt, K.F. March 2007. Nanofrontiers visions for the future of nanotechnology. Project on emerging nanotechnologies, Woodrow Wilson international center for scholars. Available from www.nanotechproject.org.
3. Wada, Y., S. Totoki, M Watanabe, N. Moriya, Y. Tsunazawa, and H. Shimaoka. 2006. Nanoparticle size analysis with relaxation of induced grating by dielectrophoresis. OPTICS EXPRESS. 14: 5755-5764.
4. Bakry, R., R.M. Vallant, M. Najam-ul-Haq, M. Rainer, Z. Szabo, C.W. Huck, and G.K. Bonn. 2007. Medicinal applications of fullerenes. Int. J. Nanomedicine. 4: 639-649.
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