Functionalization of oxidized cellulosic materials via versatile azetidinium chemistry

Paper-based analytical devices (PADs) have been widely considered a cost-effective and convenient solution for biosensing. The controlled functionalization of paper is a key requirement to implement sensitive, reproducible and robust PADs. The present work leverages an azetidinium bifunctional coupler for the modular grafting of chemical functionalities onto carboxymethyl cellulose (CMC), which can be used to easily incorporate functionality to paper sheets. Two approaches were developed: (i) CMC was first adsorbed onto paper and azetidinium derivatives were then grafted onto the carboxylate groups to confer the desired chemical functionality (i.e., alkyl, alkyne and azide), and (ii) the functionalities were first grafted onto CMC, which was then irreversibly adsorbed onto cellulose fibers. The modified CMC and paper sheets were characterized by NMR, FTIR, conductometric titration, and fluorescence microscopy. The degree of modification of the paper surfaces was quantitively assessed by reacting alkyne-bearing paper with FAM-azide and compared to paper surfaces where carboxylate groups were introduced through alternate routes. These experiments showed that grafting azetidinium derivatives onto CMC before or after adsorption onto cellulose fibers can introduce accessible reactive groups onto paper, albeit with different efficiency. As proof of concept for the introduction of biological functionalities onto paper, alkynylated biotin was reacted with paper functionalized with azide-CMC, followed by the specific binding of fluorescent streptavidin. The method developed to functionalize cellulosic materials via azetidinium derivatives is simple, cost-effective, versatile, and provides broad flexibility to graft a range of chemical and biological functionalities. We anticipate this method will aid in the development of value-added functional paper and PADs.