Co-dispersion of cellulose nanofibers and 3,3',5,5'-tetramethylbenzidine in water: Different strategies for colorimetric probes.

3,3',5,5'-tetramethylbenzidine (TMB), insoluble in water, is known to change color in the presence of strong oxidizers. Responsive aqueous dispersions of TMB were obtained with anionic cellulose nanofibers (CNFs) as the only stabilizing agent. A Pickering emulsion approach and the use of a miscible co-solvent were also explored, combining an aqueous CNF suspension with a solution of TMB in either chloroform or ethanol, respectively. The minimum CNF consistency to attain visual homogeneity was 0.18-0.30 wt%, depending on the strategy. A stability study showed that the co-solvent approach (with ethanol) was the best at protecting TMB under common storage conditions. Then, dispersions were used to detect iron(III) in water by their optical response: from colorless or whitish to blue (1-electron oxidation). In this regard, emulsions of TMB/chloroform in water outperformed the other systems. After 30 min of reaction, their limit of detection (LOD) for iron(III) was 1.5 mg/L. Although lowering the pH to 4 via acetate buffer allowed for lower LOD and faster kinetics, stability was compromised. Furthermore, TMB dispersions were also apt for paper impregnation, resulting in visually responsive dipsticks. It is concluded that the advantages of nanocellulose stabilization could be extrapolated to other colorimetric systems involving TMB.