Silver-doped microcrystalline cellulose as a material for simple detection of urinary cysteine using a smartphone

Thiol-bearing microcrystalline cellulose was demonstrated for the first time as a substrate for colorimetric detection of urinary cysteine based on the aggregation of silver nanoparticles (AgNPs) on the cellulose surface. The cellulose was functionalized with 3-mercaptopropyl trimethoxysilane and doped with Ag(NH₃)₂⁺ (Ag-MCC). The obtained Ag-MCC was used to extract cysteine from samples, given the strong affinity of the thiol group of cysteine toward silver species. After treating the material with NaBH₄ solution, AgNPs were produced on the material surface and the aggregation of AgNPs was induced by cysteine. The material color changed from yellow to orange and purple with increasing cysteine concentration. The color intensities were observed using a smartphone and ImageJ software. Under the optimized conditions, the linear working range for cysteine determination was in the range of 0 – 25 µM with the limit of detection (LOD) and limit of quantification (LOQ) of 0.25 and 1.0 µM, respectively. The method was further applied to detect cysteine in human urine samples, and the recovery was found in the range of 81.2 to 110%. This cellulose showed potential as a green material for sensing applications with easy surface modification. The detection of micromolar-level cysteine was achievable with this concept using only a smartphone camera.