This material is part of our BioReady product line for use in lateral flow. To buy these particles, click here.
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.
Our 150 nm Gold Nanoshells use the lipoic acid molecule shown to provide a surface with chemically accessible carboxyl acid groups. BioReady gold nanoparticles with diameters less than 100 nm are carboxyl-capped with lipoic-dPEG12-COOH, a molecule with similar functionality that contains a PEG spacer between the thiol and carboxylic acid functional groups.
Advantages
- 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
Property Highlights
- Not displaceable: Strong binding affinity to the particle surface via the dithiol group
- Negatively charged
- Isolectronic Point: ~3
- Salt stability: Stable in a variety of salt solutions
- Low toxicity: Generally regarded as safe
- Solvent compatibility: Water, ethanol, chloroform, & many other polar solvents
Applications
- Lateral Flow Immunoassays
- Bioconjugation
- Using ‒COOH terminal group for subsequent functionalization
Surface Charge
The graph above shows representative data for zeta potential measured as a function of pH, otherwise known as an Isoelectronic Point (IEP) curve, for carboxyl-functionalized 150 nm Gold Nanoshells. These data were generated by manual titration using HCl and NaOH and subsequent zeta potential measurement.
Lipoic acid capped nanoshells have low IEP’s, 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 7, when it starts to become slightly more neutral due to likely electrical double layer suppression from high ionic content.
For more information about zeta potential and IEP theory, click here.
Salt Stability
In the presence of a sufficiently high salt concentrations, 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 on nanoparticle surfaces from repelling one another as readily. For each particle type, the salt concentration at which this colloidal destabilization occurs can be different.
The graph above provides UV-Vis spectra of lipoic acid-capped 150 nm gold nanoshells 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 a given salt concentration, we expect the spectrum to remain the same as that of the particles without salt, with a strong plasmon resonance optical absorption at 800 nm. As the particles begin to aggregate, this is reflected in the spectrum by a decrease in the surface plasmon peak at 800 nm.
The particles remain relatively stable at salt concentrations below 20 mM NaCl. At 20 mM and above, however, significant destabilization of the particles becomes apparent, evidenced by a decrease in absorbance at 800 nm and an overall decrease in optical intensity at all wavelengths. These spectral changes correspond to nanoshell aggregation and precipitation out of solution.