Photochromic aerogels based on cellulose and chitosan modified with WO$_3$ nanoparticles

In this work, we report the first synthesis of photochromic aerogels and films based on TEMPO-oxidized cellulose and chitosan modified with tungsten trioxide nanoparticles. The blue coloring of aerogels based on WO$_3$-modified biopolymers under UV light occurs due to the reduction of WO$^{+6}$ to WO$^{+5}$. The coloration of films of the same composition occurs due to the reduction of W$^{+6}$ to W$^{+5}$ and W$^{+4}$. The photochromic properties of aerogels and films are reversible; oxidation by atmospheric oxygen leads to bleaching of materials. At that, films become colorless within a few days while aerogels with a high specific surface area (200 m$^2$/g) become colorless within several minutes. The antioxidant properties of the WO$_3$ sol and the WO$_3$/TEMPO-oxidized cellulose/chitosan composite were studied by luminol-activated chemiluminescence method. The antioxidant capacity of WO$_3$/TEMPO-oxidized cellulose/chitosan gel is 1.5 times higher than that of the commercially available antioxidant mexidol.