Polyvinylpyrrolidone (PVP) Surface

PVP (polyvinylpyrrolidone) is a large polymer that associates with the particle surface through Van der Waals forces and metal ligand charge transfer. The 40 kDa PVP molecule is not easily displaced by other molecules and offers excellent steric stability. It’s a great choice for particles that may be exposed to a broad range of salt, pH, and solvent conditions. PVP is made from the monomer n-vinylpyrrolidone. At nanoComposix we typically use a 40 kDa version that helps prevent particles from directly contacting and aggregating when solution conditions change or when the particles are dried down onto a substrate or thin film.

Many of our PVP-stabilized materials are also available as dried powders that can be readily dispersed in wide variety of solvents. See list of compatible solvents.

Advantages

• Provides a stable particle surface in a variety of different solvents. While it is possible to displace PVP with molecules that contain thiols or amines, other surfaces such as citrate are more suitable for ligand exchange.

Representative Source:  Polyvinylpyrollidone (Millipore Sigma / Calbiochem, 5295)

Property Highlights

• Displaceable: PVP is less displaceable than other surfaces such as citrate, carbonate or tannic acid.
• Negatively charged
• Isoelectric Point: < 3
• Excellent salt stability: Stable in highly saline solutions
• Toxicity: Generally regarded as safe, low toxicity
• Solvent compatibility: Water, ethanol, chloroform, many other polar solvents

Applications

• Color engineering
• Lyophilization/drying

Surface Charge

See above for a representative zeta potential-pH, or Isoelectric Point (IEP) curve for PVP-capped 40 nm gold nanoparticles. This data was generated by manual titration using HCl and NaOH and subsequent zeta potential measurement.

PVP capped nanoparticles have very low IEPs, which means that they remain negatively charged at all but the most acidic of pH ranges (< 3). The magnitude of the negative charge steadily increases as the pH becomes more basic until around pH 10, when it starts to become more neutral likely due to electrical double layer suppression from high ionic content.

• We have demonstrated that for 40 nm gold and silver bPEI and citrate capped particles, the IEP curves are very similar. This should enable a reasonable basis for comparison of zeta potential for silver nanoparticles with the above data based on gold nanoparticles.
• For more information about zeta potential and IEP theory, click here.

Salt Stability

If the nanoparticles are stable at the given salt concentration, we would expect the spectrum to remain the same with an equivalent optical absorbance at the 520 nm gold plasmon resonance as the pure particle solution without salt. If the particles have begun to aggregate, we would expect this the be reflected in the spectrum with a decrease in the surface plasmon peak at 520 nm and an increase at the longer wavelengths at which aggregates absorb (700–1100 nm).

If the nanoparticles are stable at the given salt concentration, we would expect the spectrum to remain the same as for the nanoparticles in pure water, with a strong optical plasmon absorbance at 520 nm. If the particles have begun to aggregate, we would expect this the be reflected in the spectrum with a decrease in the surface plasmon peak at 520 nm and an increase in absorbance at the longer wavelengths at which aggregates absorb (700–1100 nm)

The particles are stable in saturated salt solution, this is one of the most salt stable surfaces offered at nanoComposix.

Silver nanoparticles (of a given surface) can generally be expected to have lower salt stability than their gold counterparts.

Solvent Selection

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.

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.

UV-visible spectra of Ag-PVP nanopowders redispersed in water (n=1.33)

UV-visible spectra of Ag-PVP nanopowders redispersed in methanol (n=1.33)

UV-visible spectra of Ag-PVP nanopowders redispersed in ethanol (n=1.36)

UV-visible spectra of Ag-PVP nanopowders redispersed in isopropanol (n=1.38)

UV-visible spectra of Ag-PVP nanopowders redispersed in dimethylformamide (n=1.43)

UV-visible spectra of Ag-PVP nanopowders redispersed in chloroform (n=1.45)

UV-visible spectra of Ag-PVP nanopowders redispersed in dimethyl sulfoxide (n=1.48)

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.

Calculated absorbance spectra for Ag nanoparticles dispersed in water (index of refraction n = 1.33) or chloroform (n = 1.45). The plasmonic features shift and broaden due to an increase in the solvent refractive index.

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.

Comparison of 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.