Two different strategies are commonly used for conjugation of proteins to gold nanoparticles, i.e.
classic passive adsorption, and covalent conjugation. Cytodiagnostics offers convenient kits for conjugation using either one of these strategies, see product categories below.
The classic passive adsorption strategy utilizes the property of non-functionalized (e.g. citrate coated) standard gold nanoparticles to readily bind proteins when the pH of the solution is at or close to the isoelectric point of the protein to be conjugated. This binding is "non-specific” and is governed by ionic, van der Waals, and hydrophobic forces. Although fairly straightforward, this strategy has several drawbacks such as desorption of the conjugated protein over time, and in some cases, partial denaturation or disturbances to the tertiary protein structure which can impact the gold conjugate functionality.
In contrast, NHS-activated gold nanoparticles allows for covalent conjugation of a protein while also minimizing many of the drawbacks encountered with the passive conjugation strategy discussed above. In many cases, a gold conjugate with an increased functionality and stability is thus obtained.
Covalent conjugation to NHS-activated gold nanoparticles is achieved by the reaction between primary amines available on proteins and the N-hydroxysuccinimide ester functionalized gold surface. Conjugation is quick and straightforward and only requires mixing of your protein of interest with the pre-activated gold nanoparticles supplied with our kits.