Cellulose nanofibers as effective mediator for optical chemical sensors for heavy metal ions detection

Two forms of nanocellulose-based sensing materials were developed for heavy metal ions (HMIs) detection: all-solid-state and suspension. In these materials, cellulose nanofibers (CNF), isolated from cellulose bleached pulp via homogenization, were employed as a support matrix. For all-solid-state optodes development free-base 5,10,15,20-tetraphenylporphyrin (TPP) and zinc-porphyrin derivative (ZnPC) were deposited on CNF support. Moreover, CNF suspension enriched with silver nanoparticles (AgNPs) bearing 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPP(SO₃)₄) was tested for HMIs assessment in solution. The solid-state platform enabled fast and low-cost optical detection of multiple HMIs in individual and multicomponent solutions; a smartphone digital camera was used for optical signal acquisition, while chemometric methods were employed for data analysis. Among tested HMIs, the highest attention was focused on Cd²⁺ and Pb²⁺ ions detection, for which the low detection limit (LDL) was estimated 0.0018 and 0.0033 mgL^-1, respectively. These values are lower than the WHO provisional guideline values of 0.003 and 0.01 mgL^-1 for natural waters. CNF suspension was characterized by SEM, IR and UV-vis spectroscopy, showing a successful loading of TPP(SO₃)₄ and incorporation of AgNPs into CNF. The effective assessment of HMIs in individual solutions through the naked eye detection, and by UV-vis tests demonstrate the potential of CNF-based sensing materials for environmental monitoring applications.