Scheme 1. Conjugation strategies involving AuNCs and CTX reported in this paper. (a) CTX immobilization by using a PEG-NHS spacer; (b) direct immobilization of HC-CTX exploiting the thiol groups derived from the rupture of disulphide bridges between the mAb heavy chains; (c) CTX immobilization by using a spa-BC3 biolinker; (d) HC-CTX conjugation using a PEG-mal spacer; (e) CTX immobilization by using spa-BC3 with a PEG-mal spacer.