Ask an Expert: I am trying to choose the right gold nanoparticle for my project. What is the difference between Citrate, CTAB, PEG, Silica and Silica-PEG coatings?
Use of gold nanoparticles has rapidly expanded over the last decade as several applications take advantage of their excellent optical properties, which can be fine-tuned by varying their shape and size. However, uncoated gold nanoparticles can aggregate in solution. Laser irradiation can cause unmodified particles to melt resulting in significant changes in their optical properties. Passivating their surface with specialized coatings, helps preserve their optical properties and resist aggregation and shape change under a wide range of biological, physical and environmental conditions.
Examples of coatings include:
Citrate: Highly negatively charged, small ion, easily displaced on the gold surface by other thiol or amine containing molecules. Citrate-capped gold nanoparticles can be easily conjugated to antibodies and other molecules of interest.
CTAB (Cetrimonium Bromide): Positively charged with a positive end attached to an aliphatic chain. Can be displaced on the gold surface by other thiol or amine containing molecules to enable conjugation to antibodies, proteins and other moieties. However, CTAB can be toxic to cells due to its chemical properties.
PEG (Polyethylene Glycol): Polymer attached to the surface of gold typically with chain lengths from 2 kDa to 10 kDa. PEG can displace CTAB or citrate and form a brush border on the gold nanoparticle surface. This limits opsonization in the blood stream and thereby slows RES uptake and evades the immune system. Due to the steric stabilization provided by PEGylation, these gold nanoparticles are stable in a range of pH and ionic strength solutions.
Silica: Silica is a mesoporous siloxane polymer coating that forms a dense branched matt on the surface of gold. Silica coatings enhance the thermodynamic stability of gold nanoparticles and help particles resist melting when hit with high energy lasers. The silica coating can limit or negate the effects of plasmon coupling. The silane chemistry can be used for surface bioconjugations and other applications. Silica-coated gold nanoparticles are stable in high pH solutions. Learn more about the benefits of silica-coated gold nanoparticles.
Silica-PEG: PEG coating can be placed over the silica coating to render it more biocompatible (less immunogenic) and keep it stable in a range of pH and ionic strength solutions.
Here is a comprehensive summary of the differences between these coatings:
| Citrate | CTAB | PEG | Silica | Silica-PEG | |
|
Binding strength to np surface |
Low | Moderate | Very Strong | Very Strong | Very Strong |
|
Ability to modify surface |
Very Easy |
Easy | Hard | Moderate | Hard |
| Coating thickness | ~1 nm | 1-2 nm |
>2 nm Controllable |
>5 nm Controllable |
>5 nm Controllable |
| Stabilizing factor |
Charge |
Charge/ Micelle |
Sterics |
Charge |
Sterics |
| Zeta Potential |
< -15 mV |
> +30 mV |
- 5 - +5 mV |
< - 30 mV |
-5 - +5 mV |
| Cytotoxicity |
No |
Yes |
No |
No |
No |
| Citrate | CTAB | PEG | Silica | Silica-PEG | |
| Stability in PBS |
None |
None |
Stable |
Moderate |
Stable |
| Heat stability | Good | Excellent | Poor | Good | Good |
| Ability to retain shape when used with high powered lasers | Good | Fair | Poor | Excellent | Excellent |
|
Products available with this coating |
Spheres |
Rods |
Spheres &Rods |
Spheres &Rods |
Spheres &Rods |
|
Why choose this coating |
Planning further modifications |
Planning further modifications |
In vivo or In vitro applications |
Planning further modifications |
In vivo or In vitro modifications |
| Applications include |
Lateral flow assays |
Lateral flow assays |
Histology Microscopy Photoacoustics Optical Imaging |
Photoacoustics SERS Microscopy Optical Imaging Histology Drug-Loading in Silica Theranostics |
Photoacoustics SERS Microscopy Optical Imaging Histology Drug-Loading in Silica Theranostics |
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Related links:
Conjugated gold nanorods & custom conjugation services
Plasmonic properties of gold nanorods
Ask an expert: What is CTAB and why is it toxic?
Ask an expert: Understanding Gold Nanoparticle Aggregation
Our products:
CTAB-stabilized and PEGylated Gold NanoRods
