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Lipoic Acid contains both a carboxyl group and a dithiol group. The dithiol group strongly binds to the metal nanoparticle surface after reduction while the carboxylic acid group is available for further chemical derivatization. The acid group also provides a highly negatively charged surface at appropriately high pH.
Carboxyl surfaces can be used to covalently bind molecules with free amines (e.g. antibodies) to the surface of the particles. An amide bond between the acid surface and the free amine is formed using EDC / NHS chemistry.
At nanoComposix we use two different molecules to provide a surface with chemically accessible carboxylic acids. One is lipoic acid and the other is the lipoic acid derivative, lipoic-dPEG12-COOH, which is similar to lipoic acid but contains a PEG spacer between the thiol and carboxylic acid functional groups.
For BioReady gold nanoparticles with diameters less than 100 nm, the lipoic-dPEG12-acid is used. For larger diameter BioReady nanoparticles (e.g. 150 nm gold nanoshells), lipoic acid is used.
- Provides a stable particle surface in a variety of different solvents. Due to the strong binding affinity of the thiols present in the lipoic acid to the surface of the particles, this ligand is not displaceable
- Allows for covalent linkages to molecules with primary amines
Representative Source: Lipoic acid (Alfa Aesar, L04711)
Molecular Weight: 206.3
Displaceable: Not displaceable – strong binding affinity to the particle surface via the thiol groups
- Negatively charged
Isoelectric Point: ~4
Salt stability: Stable in many salt solutions
Toxicity: Generally regarded as safe, low toxicity
Solvent compatibility: Water, ethanol, chloroform, many other polar solvents
- Lateral Flow
- Further functionalization through -COOH terminal chemistry
See above for a representative zeta potential-pH, or Isoelectric Point (IEP) curve for lipoic acid-capped 40 nm gold nanoparticles.This data was generated by manual titration using HCl and NaOH and subsequent zeta potential measurement.
Lipoic acid capped nanoparticles have low IEP’s, which means that they remain negatively charged at all but the most acidic of pH ranges (< 4).The magnitude of the negative charge steadily increases as the pH becomes more basic until around pH 9, when it starts to become more neutral due to likely 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.
In the presence of a high enough salt concentration the surface charge of particles in solution can be shielded by the dissolved ions, leading to reduced colloidal stability. The ions in solution prevent the like charges from repelling one another as readily. For each particle type the salt concentration at which this colloidal destabilization occurs can be different.
The above chart provides UV-vis spectra of lipoic acid-capped 40 nm gold nanoparticles in varying concentrations of sodium chloride (NaCl) solution. The samples were prepared by spiking solutions of nanoparticles with NaCl at the listed concentrations and allowing the resulting solutions to incubate for 10 minutes prior to UV-Vis measurement.
If the nanoparticles are stable at the given salt concentration, we would expect the spectrum to remain the same as that of the particles without salt with a strong plasmon resonance optical absorption 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).
Significant destabilization of the particles becomes apparent at 50 mM NaCl. At this concentration, a decrease in absorbance at 520 nm is observed, and a broad secondary peak at higher wavelengths arises due to the presence of aggregates.
Silver nanoparticles (of a given surface) can generally be expected to have lower salt stability than their gold counterparts