Ar 310

Monodisperse silica particles with average diameters of 145, 175, 190, 226, and 250 nm (Sukgyung AT) were weighed and subsequently dispersed in ethanol by sonication overnight. Carbon black nanoparticles with an average diameter of 24 nm (Hiblack 420B, Korea Carbon Black Co.) were dispersed in resins of ethoxylated trimethylolpropane triacrylate (M_n ~ 428 g mol⁻¹, Sigma-Aldrich), poly(ethylene glycol) PEA (M_n ~ 324 g mol⁻¹, Sigma-Aldrich), and polyurethane acrylate (PU2100, M_n ~ 1400 g mol⁻¹, Miwon Chemicals) containing 1% (w/w) 2-hydroxy-2-methylpropiophenone (Sigma-Aldrich) photoinitiator at a concentration of 0.05% (w/w) in a silica-free resin basis. The resin was added to the ethanolic dispersions of silica particles and sonicated for 10 min. The mixture was kept in a convection oven overnight to completely evaporate ethanol. The resultant dispersions were vigorously mixed using a planetary centrifugal mixer (AR-310, Thinky). For white inks, silica particles with an average diameter of 250 nm were used. Black and brown inks were formulated by dispersing 1% (w/w) carbon black nanoparticles and 0.28% (w/w) polydopamine nanoparticles with an average diameter of 91 nm in the white inks, respectively.