Diverse and ever growing in importance, nanoparticle characterization covers a wide range of challenges including:
- Determination of size, concentration, and conformation of functionalized nanobiotherapeutics and diagnostic nanolabels
- Stability assessments of colloidal suspensions or emulsions for the inhaled delivery of controlled-release drugs
- Detection and identification of small quantities of industrial additives that may leach into the environment
- Analysis of size distributions, zeta potential, and aggregation behavior dependent on solution conditions, including pH and ionic strength, of nanoparticles, emulsions and colloids utilized in food, cosmetics, semiconductor processing and a host of other consumer or industrial products
Wyatt’s Light Scattering Toolkit provides essential analytical instrumentation for nanoparticles combined with unparalleled levels of automation, sensitivity and resolution. By turning commonly utilized, low-throughput manual measurements into automated, high-throughput workflows, a plethora of new possibilities are opened to scientists and engineers working in this field.
The measurement techniques applied to nanoparticle and colloid characterization in Wyatt instruments are Dynamic Light Scattering (DLS), Multi-Angle static Light Scattering (MALS), Field-Flow Fractionation (FFF) and Massively Parallel Phase Analysis Light Scattering (MP-PALS).
DLS is a common means for estimating the size of nanoparticle and other colloidal suspensions, and even obtaining a coarse size distribution, in all kinds of solvents and in relatively short time.
However, the majority of DLS instruments require you to manually fill a cuvette with sample, insert the cuvette into the instrument, operate the software to begin data collection, then wash the cuvette and repeat the entire procedure with the next sample – a slow and laborious process. Wyatt offers two methods of automating DLS so you can set and forget dozens or even hundreds of samples, freeing you up to do other things and increasing productivity ten-fold.
The Mobius provides automated, simultaneous zeta potential and dynamic light scattering measurements.
- The DynaPro Plate Reader performs DLS measurements in situ in standard microwell plates containing 96, 384 and even 1536 samples, completing hundreds or thousands of samples per day.
- The Mobius, combining simultaneous DLS and electrophoretic mobility measurements to determine size and zeta potential, can be connected to standard autosamplers and HPLC pumps in order to automate dozens of size and zeta potential measurements. The Mobius works with organic and aqueous solvents.
- If automation is not needed, the DynaPro NanoStar offers convenient cuvette-based DLS measurements over a very wide temperature range and with sample volume as small as 1.25 µL.
When accurate, high-resolution size distributions of nanoparticle suspensions are necessary, the analytical tool to use is Field-Flow Fractionation (FFF) using an Eclipse FFF system, coupled to a DAWN or miniDAWN MALS detector with an optional integrated WyattQELS DLS module. The NanoStar can also provide on-line DLS detection in conjunction with a Wyatt MALS detector, then easily be switched back to cuvette-based DLS.
FFF is conceptually related to size exclusion chromatography, yet provides separation performance well beyond that of SEC, spanning the range of 1 – 1000 nm with excellent resolution. Whereas unfractionated DLS at best distinguishes particles that differ in size by 4-5x, FFF-MALS and FFF-DLS can characterize accurately particles that differ in size by a few percent!
DLS data are often mined to calculate estimates of particle counts. While this analysis is certainly possible and well-supported in the DYNAMICS and ASTRA software packages, it is not meant to be an absolute or highly reliable estimate. In all honesty (and we have no problem telling you the real value of the analyses we help you carry out), the algorithm behind the DLS number calculation relies on a variety of weak assumptions and is not a very accurate means of obtaining particle number densities (# particles/mL over a given size range).
A much more reliable method of determining number densities is FFF-MALS, which combines a size-based separation step with light scattering detection and analysis. MALS can determine the numbers of nanoparticles in solution at each size range, as long as the refractive index of the nanoparticle is known. FFF-MALS has been shown to provide number densities of viruses, liposomes, and other nanoparticles with results similar to TEM counting to within a few percent.
While light scattering does not provide high-resolution images of nanoparticles, it is an effective means of learning about the distributions of shapes or other conformational properties in particle ensembles. This is accomplished using three devices: an Eclipse DualTec FFF separation system, a DAWN MALS detector, and a WyattQELS DLS module integrated into the DAWN.
FFF separates the particles by size. As each elution volume enters the DAWN, light scattering makes two independent measurements: the rms radius Rg, by means of MALS, and the hydrodynamic radius Rh, by means of DLS. The ratio Rg:Rh, known as the 'shape factor,' provides an indication of a primary structural parameter.
Some a priori information about the expected shape is required for proper interpretation of the shape factor. If the particles are known to be elliptical and of uniform composition, the shape factor indicates the degree of deviation from sphericity and the axial ratio. If the particles are known to be spherical but may be hollow shells or uniform spheres, the shape factor indicates the actual conformation, and likewise for a variety of simple, well-defined structures.
An FFF-MALS/DLS system can be further coupled to ICP-MS for elemental analysis. In this approach, the size and relative distribution of the eluting particles are correlated with elemental composition as confirmed by mass spectrometry. This combined technique has been used to examine the size and chemical composition of nanoparticles found in foods as well as the type of metals associated with nanoparticulate fractions recovered from natural sources such as lakes and streams.
Figure 1. The gold content of particles of different sizes. Size determined by on-line DLS upstream of the ICP-MS system
High-throughput screening of aggregation and stability of nanoparticles, in a plethora of permutations of solvents and solvent conditions, is essential for productive formulation efforts. The DynaPro Plate Reader is a true workhorse for formulators as it performs DLS screening on hundreds of samples, in situ and unattended, in standard microwell plates to assess size, polydispersity and aggregation. The DynaPro readily integrates with liquid handling robotics for even higher productivity.
If high-throughput is not needed, the cuvette-based DynaPro NanoStar is a versatile instrument which can also provide on-line DLS (e.g., FFF-DLS) when coupled to a Wyatt MALS detector.
In addition to size and aggregation analyses, zeta potential is considered an important stability-indicating indicator for nanoparticle and colloidal suspensions. Since zeta potential is affected by solvent conditions and excipients, it also benefits from a high degree of automation when optimizing formulations. The Mobius combines simultaneous DLS sizing and MP-PALS electrophoretic mobility determination in order to calculate zeta potential with no user intervention (no need to switch between DLS and PALS modes). Uniquely, mobility measurements in the Mobius may be made manually, or automatically by means of a standard HPLC autosampler using the flow cell. The Atlas accessory adds to the utility of the Mobius by enabling measurements even under conditions of high ionic strength.
Controlled-release drug delivery
Light scattering and field-flow fractionation provide characterization capabilities essential in developing novel nanoparticle drug delivery systems (nanoDDS), whether the carrier vehicle is a liposome, polymer micelle or some other manufactured nanoparticle.
Basic sizing of nanoDDS is usually carried initially in a batch (unfractionated) dynamic light scattering (DLS) instrument which makes measurements in a microcuvette, such as a DynaPro NanoStar, but far more accurate size distributions are obtained using a separation step e.g. SEC-MALS for particles up to ~ 100 nm in size or SEC-MALS for particles up to 1000 nm in size. Both of these techniques are sensitive enough to resolve particles that differ by just a few nm in radius, amongst a heterogeneous distribution.
One of the most important analytical challenges in nanoDDS development is the determination of whether a drug, therapeutic peptide or RNA is incorporated into the carrier, bound to the outside or free of the carrier. The combination of SEC or FFF with a DAWN MALS detector and WyattQELS online DLS module can help answer these questions by identifying the particle's conformation, and both detecting and identifying unbound molecules.
For stable formulations, nanoDDS constructs may be evaluated in dozens or even hundreds of formulations in the DynaPro Plate Reader to minimize aggregation. The Mobius is ideal for measuring the zeta potential of drug-delivery nanoparticles in a series of formulation buffers, automated by an autosampler, as well as high-salt buffers that disrupt conventional zeta potential measurements when bubbles are formed at the electrodes by electrolysis.
Afonin, K. A.; Kasprzak, W.; Bindewald, E.; Puppala, P. S.; Diehl, A. R.; Hall, K. T.; Kim, T. J.; Zimmermann, M. T.; Jernigan, R. L.; Jaeger, L.; Shapiro, B. A. Computational and experimental characterization of RNA cubic nanoscaffolds. Methods 2014, 67, 256-265.
Dhayal, S. K.; Gruppen, H.; de Vries, R.; Wierenga, P. A. Controlled formation of protein nanoparticles by enzymatic cross-linking of α-lactalbumin with horseradish peroxidase. Food Hydrocolloid. 2014, 36, 53-59.
Loeschner, K.; Navratilova, J.; Legros, S.; Wagner, S.; Grombe, R.; Snell, J.; von der Kammer, F.; Larsen, E. H. Optimization and evaluation of asymmetric flow field-flow fractionation of silver nanoparticles. J. Chromatogr. A 2013, 1272, 116-125.
Perevyazko, I. Y.; Delaney, J. T.; Vollrath, A.; Pavlov, G. M.; Schubert, S.; Schubert, U. S. Examination and optimization of the self-assembly of biocompatible, polymeric nanoparticles by high-throughput nanoprecipitation. Soft Matter 2011, 7, 5030-5035.
Wyatt, P. Submicrometer particle sizing by multiangle light scattering following fractionation. J. Colloid Interf. Sci. 1998, 197, 9-20.