Hydrogel, aerogel and film of cellulose nanofibrils functionalized with silver nanoparticles

• Ag nanoparticles were incorporated into hydrogel, aerogel and film of NFC.
• Addition of AgNO3 triggered hydrogelation of NFC.
• The gelation mechanism of NFC by monovalent cations was explored.
• The NFC-Ag aerogel had less BET surface area than the NFC aerogel.
• Presence of Ag species reduced coalescence of NFC in the film formation.

In this work, we describe hydrogels, aerogels and films of nanofibrillated cellulose (NFC) functionalized with metal nanoparticles using silver as an example. The TEMPO process used to produce NFC generates negatively charged surface carboxylate groups that provide high binding capability to transition metal species such as Ag+. The gelation of NFC triggered by transition monovalent metal ions was revealed for the first time. The interaction was utilized to bind Ag+ on the NFC surface and simultaneously induce formation of NFC-Ag+ hydrogels, where Ag+ was slowly reduced to Ag nanoparticles by hydroxyl groups on NFC without additional reducing agent. The NFC-Ag+ hydrogel was initiated by strong association of carboxylate groups on NFC with Ag+ and sufficient NFC surface charge reduction. The stiff hydrogel has a storage modulus leveled off at a plateau value of ~6800 Pa. Porous aerogels and flat thin films comprising a continuous matrix of NFC were decorated with Ag nanoparticles through freeze-drying or solution-casting of NFC-Ag+ dispersions with low contents of Ag+, respectively, followed by UV reduction. The presence of Ag species on NFC reduced coalescence of nanofibrils in the film formation as revealed from AFM phase images.

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