Among bioactive papers, many multiplexed assays implement methods incompatible with the conventional lateral flow immunoassay (LFIA) carrier material, nitrocellulose. Consequently, its replacement by cellulose has to be considered. This technological breakthrough requires a surface chemistry which ensures both the biomolecules covalent grafting to cellulose and the conservation of their biological activity. To comply with these requirements, the process elaborated in this study implements compounds and methods compatible with biological material. While cellulose chemical modification is usually operated under harsh conditions in organic solvents, the diazonium-based functionalization procedure presented here was performed onto cellulose sheets in water and at room temperature. Paper sheets have been successfully modified and bear different chemical functions which enable grafting of biomolecules by common bioconjugate techniques and to perform LFIAs. More generally, the chemical ways developed in this study are suitable for many biomolecules and would be helpful for any sensitive molecule immobilization onto cellulose sheets.