Disulfide-linked gold nanoparticles (AuNP) were synthesized by reacting dithiobis[succinimidylpropionate] (DSP) coated nanoparticles with glutathione disulfide. AuNP-cross-linking was monitored by the red shift and broadening of the AuNP's localized surface plasmon absorption resonance (LSPR) spectrum. The exposure of the disulfide-linked AuNPs to a variety of free thiols with systematically varying molecular weight revealed a AuNP-disulfide stability to reduction by thiols up to a critical molecular weight, M-c, of >300 Da thus making the disulfide-linked AuNP the first reagent that can discriminate thiols based on their size.