Abstract: This application relates to compositions comprising cellulose nanocrystals (CNCs) and photosensitizer molecules conjugated to the CNCs. In some embodiments the compositions generate reactive oxygen when exposed to light. Methods of preparing the compositions and using the compositions or formulations containing the compositions as biocidal disinfectants are also described. In some embodiment the photosensitizer molecules comprise cationic dyes with acidic protons, such as azure A. In some embodiments the CNC and photosensitizer molecules are coupled together using a pH mediated synthetic protocol whereby the photosensitizer molecules are first dispersed in an acidic suspension of oxidized CNCs and the suspension is then adjusted to an alkaline pH to facilitate photosensitizer fixation. In some embodiments, photobactericidal potency of the composition is significantly more toxic to a broad spectrum of bacteria than the light-activated photosensitizer molecules in a non-conjugated free form.