Gold nanoparticle conjugates have been widely used in biological research and biosensing applications. For example, gold nanoparticles can function as probes in light or electron microscopy, lateral flow immunoassays and immunoblotting protocols such as dot-blots and Western blots.
There are two ways of preparing gold conjugates, i.e. passive absorption and covalent coupling via a linker. Although a relatively simple preparation process, passive absorption of proteins to gold nanoparticles does not provide a permanent attachment of the coating because molecules may desorb from the surface over time. In addition, in some cases, the proteins lose their properties after being absorbed to the surface, which can be caused by changes in tertiary structure or binding of the active site/antigen binding site to the gold surface rendering it inaccessible.
Despite these drawbacks passive adsorption of proteins to gold nanoparticles remains popular and is a simple way of generating a protein gold conjugate. Preparation of gold nanoparticle protein conjugates through passive adsorption to standard spherical gold nanoparticles can be optimized with our convenient Gold Conjugation Optimization Kits for Passive Adsorption.
In comparison to passive adsorption, covalent coupling permanently immobilizes molecules of interest to functionalized gold nanoparticles (e.g. with NHS, carboxyl or amine groups). This approach provides much improved stability of the protein coating over the passive absorption method. Covalent coupling uses chemical linkers that react with specific chemical groups on the molecule to be conjugated. Therefore, the method is more specific and controllable than the passive absorption method and the number of covalently conjugated ligands can be optimized for a particular application. Covalent coupling through a linker also have the advantage of minimizing steric hindrance and impact on the tertiary structure of the conjugated protein. All in all this results in less deleterious effects on the properties of a conjugated protein.
Our Carboxyl Gold Nanoparticles, Carboxyl Gold NanoUrchins and Carboxyl Gold Nanorods all rely on EDC/NHS chemistry for conjugation. EDC and NHS "activate” the carboxyl groups on the particle surface to form an intermediate that can subsequently react with primary amine groups on the specific protein or other ligand to be conjugated. The efficiency of EDC conjugation is typically low and sensitive to pH and covalent coupling protocols requires some degree of optimization to achieve desired performance or stability.
The following protocol provides general guidelines for coupling biomolecules to our carboxylated gold ganoparticles, with conjugation of a standard IgG to our 20nm Carboxyl Gold Nanoparticles given as an example. For conjugation of other biomolecules or nanoparticle types, the optimal conjugation conditions may vary. To obtain maximum conjugation to the particle surface, the amount of protein for conjugation is about 1 to 10X excess that of its theoretical quantity needed for full coverage.
Note: It is imperative that the protein to be conjugated is pure and free of contaminating proteins (e.g. BSA) and other amine containing components. Any other molecules containing primary amines may compete with the protein to be conjugated and may result in a significant decrease in conjugation efficiency. The protein should also have enough accessible primary amine groups for conjugation. Lysine residues are the primary target sites for EDC conjugation.
* The concentration of protein may vary depending on the particle size and protein to be conjugated. In general, the amount of protein should be 1 to 10X excess of the amount of full surface coverage. The total surface area of particles and the docking area should be estimated to calculate the optimal amount of protein. The following table is a general reference for conjugating IgG to our Carboxyl Gold Particles of different sizes. The calculation for other proteins are similar but the docking area needs to be calculated for your protein of interest.
Table I. Suggested quantities of IgG for Carboxyl Gold Nanoparticles of different sizes during EDC conjugation. The docking area of an IgG molecule is estimated to be 45 nm2. "N X full coverage amount” means the excess ratio between the incubation amount and the amount needed for full coverage of particle surface.
In many cases there is no direct measurement to validate successful conjugation and the best approach is therefore to subject the conjugated particles to a functional test. This can either be a direct test in the assay or application of interest or alternatively if an antibody has been conjugated, functionality can be validated with an immuno-dot blot assay. We also recommend to always include a separate positive control when performing EDC/NHS conjugations and two examples are given below.
This assay uses HRP as a positive control for EDC conjugation, and TMB assay for signal readout. It is a simple, fast and sensitive method. Briefly, after conjugation, mix 5 µL of conjugated particles with 100 µL of TMB solution and develop for 1 to 2 minutes. The solution turns blue if there is successful conjugation of HRP.
Figure 1. TMB assay on HRP-conjugated gold nanoparticles. Particles are treated with or without EDC and then incubated with HRP for conjugation. Particles are then incubated with TMB. The solution containing HRP-conjugated gold nanoparticles (right) turns blue, indicating successful conjugation.
This assay uses IgG as a positive control for EDC conjugation, and a dot-blot assay for signal read out. This assay may be a better control for confirmation of IgG functionality after conjugation onto particles.
Figure 2. Dot-blot assay of Goat anti-human IgG antibody using human IgG-conjugated gold particles. Particles are treated with or without EDC and then incubated with human IgG for conjugation. There is concentration-dependent positive signal only when the particles are treated with EDC (lower row), suggesting successful conjugation.
Q: what is the optimal conjugation pH for conjugation?
A: Activation of the carboxylated gold nanoparticles using EDC/NHS is generally performed in slightly acidic conditions while the conjugation reaction is optimally performed around pH 7-8.
Figure 3. Effect of conjugation time on the performance of dot-blot assay. Particles are incubated with human IgG for 1 hour, 2 hours, 3 hours and overnight. 2 hours of incubation shows the best dot-bot signal among all conjugation times.
Q: what are the pros and cons of 1-step conjugation?
A: The advantage of 1-step conjugation is its simple procedure. However, 1-step conjugation comes with a risk of crosslinking proteins among themselves, instead of conjugating them onto the particles. This risk is dependent on the structure of the protein to be conjugated, which determines the ratio of the accessible carboxyl groups between the proteins and particles. Also, excess amounts of EDC/NHS will increase the chance of protein crosslinking. It is recommended to carry out titration experiments of protein and EDC/NHS concentrations to find the conditions of maximal conjugation/performance with minimal protein crosslinking.
Q: what other factors can influence conjugation results?
A: if the conjugation pH and conjugation time are suitable, but there is no positive result, it is necessary to make sure EDC/NHS is freshly prepared just before conjugation. EDC should always be stored at -20 degrees. Effective removal of excess EDC/NHS after activation is important to prevent them from crosslinking proteins.