Orientation-Controlled Bioconjugation of Antibodies to Silver Nanoparticles

Here we report on the use of heterobifunctional cross-linkers (HBCLs) to control the number, orientation, and activity of immunoglobulin G antibodies (Abs) conjugated to silver nanoparticles (AgNPs). A hydrazone conjugation method resulted in exclusive modification of the polysaccharide chains present on the fragment crystallizable region of the Abs, leaving the antigen-binding regions accessible. Two HBCLs, each having a hydrazide terminal group, were synthesized and tested for effectiveness. The two HBCLs differed in two respects, however: (1) either a thiol or a dithiolane group was used for attachment to the AgNP; and (2) the spacer arm was either a PEG chain or an alkyl chain. Both cross-linkers immobilized 5 ± 1 Abs on the surface of each 20-nm-diameter AgNP. Electrochemical results, obtained using a half-metalloimmunoassay, proved that Abs conjugated to AgNPs via either of the two HBCLs were 4 times more active than those conjugated by the more common physisorption technique. This finding confirmed that the HBCLs exerted orientational control over the Abs. We also demonstrated that the AgNP-HBCL-Ab conjugates were stable and active for at least 2 weeks. Finally, we found that the stability of the HBCLs themselves was related to the nature of their spacer arms. Specifically, the results showed that the HBCL having the alkyl chain is chemically stable for at least 90 days, making it the preferred cross-linker for bioassays.