Self-Standing Thin Films of Cellulose Nanocrystals and Graphene Quantum Dots for Detection of Trace Iron(III)

Cellulose nanocrystal (CNC) thin films have gained attention for application in green optoelectronics devices. However, the dependence on plasticizers, external cross-linkers, or auxiliary polymers can impair the full exploration of the nanomaterials' properties. Herein, we report on the evaluation of a surface-oxidized CNCs ultrathin film, made from a formulation that excludes the use of auxiliary compounds, to be used as a sensing platform. The physicochemical characterizations using UV–vis, Fourier transform infrared , and Raman spectroscopies, nuclear magnetic resonance, zeta potential, transmission electron and field emission scanning electron microscopies, X-ray diffraction, thermogravimetric analysis, and contact angle measurement showed that the adopted procedure led to (i) the modification of the CNC surface and (ii) the formation of smaller CNCs. The production of casting films from highly diluted CNC-modified aqueous suspensions showed that a very thin, transparent, self-supporting, and manipulable film could be obtained, which was not the case for diluted unmodified CNC suspension. Additionally, the CNC thin film has been demonstrated to be a suitable platform for hosting graphene quantum dots, based on which an optochemical sensor was successfully employed to detect iron(III) in a dynamic range from 1 fM to 2 pM, yielding a limit of detection of 0.8 fM.