Green Synthesis of Cellulose Nanofibrils Decorated with Ag Nanoparticles and Their Application in Colorimetric Detection of l-Cysteine

In this study, we reported an environmentally benign method for the synthesis of silver nanoparticles (AgNPs) by using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (CNFs) as the only reducing agent and stabilizer under mild aqueous conditions. The AgNPs were tightly adhered to the CNF surface, and the resulting stable CNF–AgNP suspension can be used for rapid and selective detection of l-cysteine (Cys). The efficacy of the process relies upon the surface plasmon resonance properties of AgNPs and the aggregation effect of AgNPs in the presence of Cys. Under optimal ionic strength provided by NaCl, Cys interacted with AgNPs, leading to a change in yellow color of the CNF–AgNP to purple. The colloidal suspension showed high sensitivity (limit of detection of 49 nM) and rapid (5 min) and selective detection of Cys among 20 amino acids that make up human proteins, hence demonstrating great potential for analyzing practical samples.