Solvent Selection for PVP-Coated Nanopowders
NanoComposix PVP-stabilized Au and Ag nanopowders can be fully dispersed into a variety of solvents without particle agglomeration. The particle surface is capped with polyvinylpyrrolidone (PVP) and excess PVP is added as an excipient to facilitate the drying process. The PVP prevents particle sintering while they are stored in the dried state and allows the particles to be redispersed into a compatible solvent at a later time.
Table of contents:
- Solvent Selection
- Effect of Solvent on Nanoparticle Optical Properties
- Effect of Solvent on Nanoparticle Hydrodynamic Radius
To redisperse the particles, we recommend adding an appropriate solvent from the table below to the dried powder and bath sonicating for 30 seconds. After redispersion, samples should be stored at 4°C and away from light, as described in our Storage and Handling instructions.
|Solvent Refractive Index, n||Nanopowder Solubility|
Effect of Solvent on Nanoparticle Optical Properties
UV-visible spectroscopy can be used to detect the presence of aggregation or changes in particle size; such effects are readily observed in the absorption spectrum as a change in the width of the plasmon peak and/or the appearance of a secondary peak that is red-shifted from the plasmon peak. Dispersions of PVP-coated silver nanoparticles with diameters of 10 nm, 50 nm and 100 nm were dried into powder form and resuspended in a variety of solvents. The absorption spectrum of the resuspended sample is compared with that of the original aqueous dispersion in the figures below.
As shown above, the redispersion of the nanopowders into water and methanol is nearly perfect with no discernable difference in the optical spectra before and after drying. For other solvents there are differences in the optical spectrum. However, the majority of the difference in the spectrum of the redispersed particles is due to the difference in refractive index between water and the new solvent and is predicted well by theory. For example, Mie scattering calculations of the predicted optical spectra for 10, 50, and 100 nm diameter silver nanoparticles in water (solvent refractive index, n = 1.33) and in chloroform (n = 1.45) are shown below.
In each case, the shifts in peak position and the broadness of the plasmon features observed in Figure 1 due to solvent transfer agree well with theory. Only the very small secondary feature at ~650 nm in the 50 nm sample and the slight broadening of the tail in the 100 nm sample are indicative of low levels of agglomeration.
In general, as the refractive index of the solvent increases, the nanoparticle extinction spectrum shifts to longer wavelengths (known as red-shifting). The figure below compares the plasmon peak wavelength for 10 nm, 50 nm and 100 nm diameter Ag nanoparticles in different solvents (symbols) with the predicted value from Mie theory (dashed lines). There is excellent agreement between theory and experiment, demonstrating the linear shift in plasmon peak wavelength with increasing solvent refractive index.
Effect of Solvent on Nanoparticle Hydrodynamic Radius
In addition to UV-visible spectroscopy, dynamic light scattering (DLS) can be used to check the agglomeration state of a nanoparticles dispersion. The table below lists the average hydrodynamic diameter determined via DLS for the 50 nm diameter Ag nanopowder redispersed into different solvents. While the size of the Ag colloid measured via TEM is 50 nm in diameter, the hydrodynamic diameter measured by DLS gives an apparent particle size, which is influenced by the PVP coating and the interaction between the polymer coating and the solvent.
|Hydrodynamic Diameter (nm)|