Dynamic light scattering is increasingly important for determining nanoparticle size, but researchers also want to know about zeta potential.
Nanoparticles occupy an important middle ground between bulk materials and atomic or molecular structures. Generally considered to be any object under 100 nm in diameter, nanoparticles are so small that they exhibit the properties of large, bulk materials, but are heavily influenced by the dynamics of molecules and atoms, where different forces exert greater influence. As such, they are seen as a bridge and a potential tool for manipulating systems at a variety of size levels.
But the learning curve for nanoparticle production has been steep. They are difficult to visualize, and their properties are often unknown until they are subjected to extensive experimentation. Some nanoparticle types, such as gold, have been studied for years before their strengths could be harnessed. But it’s well known that size and surface area play a large role in how nanoparticles behave. In response to this, a wide variety of techniques have been developed to gauge size. For analysis of specific particles, few options can outdo electron microscopy, which can accurately determine structures down to just a few nanometers or less. Other options include atomic force microscopy, nuclear magnetic resonance, and various forms of spectroscopy. But for fast and non-destructive determination of nanoparticles in solution, few methods can compare to the compactness and simplicity of use offered by dynamic light scattering (DLS).
Horiba Instruments, Irvine, Calif., manufactures a wide range of scientific instruments geared for particle characterization, including those based on laser diffraction. But until recently, it had not released a dynamic light scattering tool that included the ability to measure zeta potential. According to Ian Treviranus, the product line manager for Horiba’s particle characterization instrument business, the increased capability and popularity of the technique eventually convinced the company to develop their own approach to DLS with zeta potential, which debuted in 2011 in the nanoPartica SZ-100 desktop nanoparticle analyzer.
In addition to the sizing capabilities of DLS, the SZ-100 featured another capability: the ability to find zeta potential. A measurement that is becoming highly prized among researchers involved in nanoscale research, zeta potential is determined through electrophoretic light scattering, or ELS. Just as particle size and surface area play a major role in how a concentration of nanoparticles behaves, so too, does zeta potential, which describes the stability of a particle concentration.
The DLS advantage
As compared to the wavelength of visible light, nanoparticles are small enough to conform to the phenomenon known as Rayleigh scattering, in which light is scattered in all directions. Use of coherent light, typically a laser, can then produce a signal that contains information about changes in scattering intensity. The changes stem from Brownian motion, which describes the constant movement experienced by nanoparticles.
Interference patterns, which describe intensity fluctuations, are generated by the laser and information about how the particles are changing position over time can be gathered. Signal processing algorithms can then be applied to the raw signal data to produce a chart showing peaks and average particle size.
"The technology of dynamic light scattering has been around for decades, but the ability of companies like Horiba to develop advanced designs has been driving costs down," says Treviranus.
DLS, as a technique, has been available for more than 30 years. But only in the last 10 years, he says, has the popularity of DLS accelerated. The critical mass of acceptance was achieved in several ways. First, the instruments themselves improved markedly, becoming more reliable and predictable, says Treviranus. As researchers paid more and more attention to particles 100 nm in size and below, they became more skilled and more comfortable with the technique. This has helped results because DLS requires a skill set for preparing samples and interpreting results. Researchers began to trust results through experience.
Additionally, standards authorities such as NIST began releasing reference materials that could be directly tested on DLS instruments. This allowed laboratory researchers to achieve a greater degree of process control.
The other factor, says Treviranus, is a regulation. Horiba has been watching European Union regulations, for example, which are beginning to determine what kind of nanoparticles constitute a potential health risk. These impending rules represent both a challenge and an opportunity.
"We want to meet the need or demand whether it’s the market or the regulations themselves stipulating a demand or need," he says.
As a result of this expectation of rules regarding nanoparticle content for various commercial products—as common as sunscreen, or as rarified as functionalized gold nanoparticles—sales of particle sizing tools have been increasing in recent years.
In addition to the compact, bench-top footprint and the approachable, reliable operation of DLS instruments, the attraction to the technique is its strong ability in the "sweet spot" of nanoparticle size, from about 10 to 100 nm.
"That’s the size where most applications are looking, and other modern sizing technologies tend to peter out when you start talking about sizes below 100 nm," says Treviranus. Theoretically, DLS can pick out particle sizes from as small as 1 nm to as large as several micrometers in diameter. But realistically, he says, the useful results will emerge from somewhere in the middle. One of Horiba's other particle sizing tools, the LA950 laser diffraction analyzer, can measure "peaks", or particle signatures, well down to 30 nm. But DLS can reliably go much smaller.
Finally, real-world applications for these instruments are beginning to proliferate. In the carbon black market, for example, the standard of size not too long ago was 2 µm. Now the carbon black industry is examining materials deep into the nano regime.
"People are increasingly asking about how product quality can change and the process can change at the nanoscale," says Treviranus. "One of the clearest aspects of that is the pharmaceutical drug delivery research taking place with regard to gold nanoparticles."
Known for their benign and neutral interaction with biological systems, gold is a popular choice for biotechnology researchers who aim to decorate the particles with various therapeutics and use them as a delivery vehicle. Gold nanoparticles in the size concentration considered useful for this type of application can’t be quickly measured with other techniques.
This application is also good example of how DLS has become a reliable laboratory tool. NIST has established three standards for gold nanoparticles, and these references can be checked against the SZ-100s results repeatedly.
The SZ-100 is Horiba’s first DLS instrument to incorporate zeta potential measurement capability. Zeta potential can be thought of as a stability measurement. In solution, nanoparticles not only have their inherent characteristics based on material type and surface area. They also interact with other particles in various ways that researchers are increasingly keen on observing.
"I like to use this analogy. When you go to the grocery store and you see salad dressing on the shelf with their oils and ingredients all separated, it's the opposite of what you want when you pour the dressing. You want for that stuff to stay mixed up and the same is true of pharmaceutical drugs and nanoparticle formulations. You do not want your material to aggregate or phase separate," says Treviranus.
In the SZ-100, the zeta potential measurement occurs in a disposable plastic cell. A field is created between two electrodes, and as the materials begin to accelerate, the difference in charge between the particles and surrounding medium can be measured using electrophoretic light scattering. Scattered light is mixed into an unscattered reference beam to determine velocity. The direction of the particle motion determines if the charge is negative or positive, and the mobility of the particles in the applied electric field determines the magnitude of the zeta potential. This charge magnitude, or electron mobility, of the particles is described by the zeta potential. The result indicates the sample’s dispersion stability.
When developing their zeta potential cells, Horiba discovered that traditional electrode materials and cell designs suffered from several shortcomings that were not well suited to studying nanoparticles or macromolecules. One significant problem involved Joule heating near the electrode surface, which often "cooked" the sample and changed its nature. The burning effect hurt results and lowered the lifetime of the cells.
"The early cells never helped people believe that zeta potential measurements were helpful," says Treviranus. Inspiration for a solution may have come from an unexpected source, he continues. During development, particle characterization instrument engineers at Horiba’s Kyoto, Japan, headquarters were located not far from the medical division engineers, who had already developed a carbon, or graphite, electrode for use in clinical settings. This non-metallic electrode was adapted to the SZ-100 with positive results. Resistant to both burning and fouling, the new zeta potential cells generate more reproducible results and last up to 20 times longer.
"That’s really cut down significantly on the consumable cost of these measurements, and was essentially one of the biggest innovations on the SZ-100," says Treviranus.
Horiba also improved on the cell’s basic design, using optically clear plastic and other measures to minimize electro-osmotic flow.
An average magnitude of zeta potential greater than 30 mV (in water-based systems) typically denotes a stable system, which means that pharmaceutical companies want to build up a certain amount of charge on the particles in use. This is a crucial step during formulation so that a product does not go bad, either during processing or after sale.
Released in 2011, Horiba’s nanoparticle SZ-100 is the company’s first dynamic light scattering instrument, and has been engineered to provide reliable zeta potential measurement results.
The future of DLS
As a standalone technology, dynamic light scattering is mature. The developments and innovations, says Treviranus, are incremental and now mostly have to do with the special approach of the manufacturer. In the SZ-100, for example, Horiba has its own method for processing the signal, which collected by the correlator. Each manufacturer programs their tools to collect data in a specific, and sometimes patented, pattern. Data may be collected as quickly as every few nanoseconds for single-nanometer sized particles. This forms the autocorrelation function from which size information is extracted.
The future of DLS, then, is partly determined by the proliferation of products that use nanoparticles. Horiba has already published a variety of application notes in which the SZ-100 is used to study biopolymers, food industry particles like maltose, and industrial nanoparticles like antimony trioxide. Treviranus feels comfortable with future demand.
But its long-term success also depends on the capability of competing techniques.
"What we see with DLS that’s really apparent right now is that four or five new organizations are out there trying to develop technologies that will complement DLS size results, or, for some applications, supplant DLS size results," says Treviranus. These new technologies have their uses, he says, but the downside with many of them is the limitation in capacity, or that they might not do well in reporting the size regime that is frequently demanded by researchers.
In SEM and TEM, the downside is sample preparation, and the fact that electron microscopes have sample size restrictions and sample preparation limitations with regard to delicate organic specimens. While several particles may be viewed by a TEM in high detail, the sample may not be giving enough information about the particle distribution of the colloid as a whole.
"Companies are also trying to visualize nanoparticles, but still with light scattering," says Treviranus. Some of these emerging techniques, which include technology from NanoSight Inc., Amesbury, U.K., that relies on an optical approach, have been covered in R&D Magazine previously.
Nanoparticle counting through resonance techniques is also in its nascent stages, and some centrifuges have been developed that spin particles through a measurement zone, where their size is determined by relative speed. These methods may improve and become more widespread. But so far they lack good market recognition.
The other factor that will help determine DLS is zeta potential. While DLS as a technology is relatively mature, says Treviranus, ELS is still a developing technique that could benefit from future improvements, particularly in cost of ownership and ease-of-use.