Poly(ethylene glycol)-functionalized hydrophilic carbon clusters (PEG-HCCs) are internalized by RA-FLS and reduce intracellular superoxide. (A) Example flow cytometric histograms of human RA-FLS treated with PEG-HCCs for various times, permeabilized, and stained against PEG (shaded) or unstained (white). (B) Quantification of PEG-HCCs in RA-FLS treated for various times with PEG-HCCs, left either intact or permeabilized, and stained for PEG expression (n = 3). (C) Measure of cytotoxicity through 7-AAD staining of RA-FLS (●, n = 4) and OA-FLS (□, n = 4) treated with staurosporine or PEG-HCCs for 72 h. Representative flow cytometry histograms are shown in Supplemental Figure S1. (D,E) DCFDA fluorescence of RA-FLS and OA-FLS treated with or without 1 μg/mL PEG-HCCs for 2 h (n = 5 RA-FLS and 6 OA-FLS; each data point represents cells from a different donor). * p < 0.05, ** p < 0.01, *** p < 0.001. Data are presented as mean ± SEM.