Label-free detection of antibody-antigen interactions on (R)-lipo-diaza-18-crown-6 self-assembled monolayer modified gold electrodes

Novel (R)-diaza-18-crown-6 has been prepared by a simple two-step synthetic method and characterized for its ability to form a uniform self-assembled monolayer (SAM) on gold as well as to immobilize proteins using atomic force microscopy, quartz crystal microbalance, and electrochemical impedance spectroscopy (EIS) experiments. The (R)-lipo-diaza-18-crown-6 was shown to form a well-defined SAM on gold, which subsequently captures the antibody (Ab) molecules that in turn capture the antigen (Ag) molecules. The Ab molecules studied include antibody C-reactive protein (Ab-CRP) and antibody ferritin (Ab-ferritin) along with their Ag's, i.e., CRP and ferritin. Quantitative detection of the Ab-Ag interactions was accomplished by EIS experiments with a Fe(CN)6(3-/4-) redox probe present. The ratios of the charge-transfer resistances for the redox probe on the SAM-antibody-covered electrode to those with the antigen molecules attached show an excellent linearity for logAg] with lower detection limits than those of other SAMs for the electrochemical sensing of proteins.