Water soluble sulfur-rich nanoparticles (S-NPs) having the sulfur content of more than 80 wt% were synthesized using a self-assembled block copolymer structure comprising 4-bromophenyl vinyl sulfide (BPVS) and N-isopropyl acrylamide (NIPAM), and a site-selective cross-linking reaction in selective aqueous solution. The core cross-linking reaction of sodium polysulfide with 1,2,3-trichloropropane as a cross-linker in the presence of poly(BPVS-b-NIPAM) was conducted in water, in which the site selective reaction takes place inside the micelles. DLS analysis demonstrated the formation of stable S-NPs with uniform sizes, which could be controlled by the nature of the cross-linking agents and reaction conditions (D-h = 140-170 nm in CHC13). IJV-vis, fluorescence, and cyclic voltammetry measurements led to the optical and electrochemical properties derived from polysulfide units and incorporated cross-linked units in the resulting products, supporting the successful preparation of desired S-NPs. The S-NPs exhibited LCST behavior around 32 degrees C, which stems from the poly(NIPAM) shell. (C) 2017 Elsevier Ltd. All rights reserved.