A surface plasmon resonance (SPR) expression after hybridization of chitosan–gold nanoparticle‐antibody ( CS ‐ AuNPs‐Ab ) based on: i) metal‐free click chemistry, and, ii) in water system as an approach for a rapid antigen sensing, is proposed. The chitosan‐hydroxybenzyl triazole complex enables us to carry out the conjugation of mPEG and trifluoromethylated oxanorbornadiene ( OND ) in water. CS‐mPEG‐OND further allows metal‐free click to hybridize chitosan ( CS ) with azido‐modified gold nanoparticles ( azido‐AuNPs ) in aqueous solution at room temperature. The CS‐mPEG‐OND conjugated with LipL32 antibody ( Ab ) not only effectively binds with LipL32 antigen ( Ag ) but also performs the cycloaddition with azido‐AuNPs to display a change in color within 2 min. The phenomenon leads to a simple and efficient naked‐eye antigen detection technique.
