A surface plasmon resonance (SPR) expression after hybridization of chitosan-gold nano-particle-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.