Nanophosphors, inorganic luminescent nanoparticles, have attracted much attention in recent decades for their potential industrial uses and fundamental scientific interest. 1, 2 Nanosizing the phosphor particles induces some interesting features that are not observed in bulk materials, such as optical transparency, quantum effects, and surface activities due to their high surface areas. 3− 5 In particular, dynamic physicochemical phenomena occurring at nanophosphor surfaces lead to environmentally responsive photoluminescence (PL), eg, PL enhancement and quenching by the surrounding media, 6, 7 fluorescent resonance energy transfer (known as FRET) with surface species, 8− 10 and PL switching by surface redox reactions. 11, 12 These interactive PL behaviors have opened up new methodologies for imaging and sensing. 13, 14 However, most of the previous studies have focused on biomedical applications because they mainly used colloidal nanophosphors, which interact efficiently with solute species in vitro and in vivo but not with gases in a dry environment. To realize an efficient interaction with gas molecules, nanophosphors need to be homogeneously dispersed and fixed in a transparent and highly gas permeable matrix without losing surface activities, as if they were “free-standing in the air.” In this study, we have succeeded in synthesizing a structurally new monolithic nanocomposite close to this imaginary structure by fixing Eu3+-doped YVO4 (YVO4: Eu3+) nanoparticles in an ultralight three-dimensional network of chitosan nanofibers. To fabricate a free-standing monolith containing functional nanoparticles, a supporting matrix is practically essential as a physical scaffold. Most of the nanoparticle composites reported so far are composed of inorganic nanoparticles embedded in a dense polymer resin, which provides not only mechanical toughness and processability, but also physicochemical stability. 15 In other words, the dynamic chemical activities of nanoparticle surfaces are blocked by the surrounding resin. For example, we previously discovered photobleaching behavior of a YVO4: Bi3+, Eu3+ nanophosphor/polyurethane composite: The nanophosphor surface exhibited photoredox reactions with the resin, resulting in oxidative decomposition of the resin accompanied by reductive photobleaching of the nanophosphor. 16− 18 This effect can be applied to interactive PL devices, eg, a photooxidative cleaner for pollutant gases combined with a sensing function, but only if we can utilize the nanoparticle surfaces without them being blocked by the surrounding resin.