Controlled cellulose disassembly affords many variants including amorphous micro-sized cellulose crystals (MCC) and cellulose nano-sized crystals (CNC), which have emerged recently as green and sustainable nanomaterials. Unfortunately, their lower reactivity and thermal and chemical instability constitute an impediment for multifaceted nanoscience and nanotechnology usage. Herein, MCC and CNC are functionalised using two phosphorus derivatives (phosphoryl chloride and hexachlorocyclotriphosphazene), under acid-free, urea-free and corrosive-free, gentle experimental conditions. Notably, CNC can accommodate more phosphorus species compared to MCC, with the bulky phosphazene precursor being less reactive in both cases. In contrast, CNC is less prone to post-grafting titanium alkoxide species compared to MCC. Alternatively, phosphorylation circumvents this drawback and enhanced post-grafting reactivity was observed for P-MCC and P-CNC modified cellulose. The presence of phosphorus fragments provides a way of anchoring ultrastable metal oxide clusters on the nanofibrillar surface through metal–phosphonate (P–O–Ti) bridges. Direct condensation of soluble phosphorus reagents with metal alkoxide bearing acetylacetonate fragments constitutes another alternative to grow a substantial amount of metal oxide clusters on both MCC and CNC. The beneficial effect of phosphorus incorporation is convincingly illustrated by their efficiency to scavenge representative anionic and cationic dye pollutants compared to native, non-modified cellulose and its carboxymethylcellulose analogues.