Smartphone-Integrated Ultra-Sensitive Colorimetric Detection of Cysteine Using Palladium Nanozyme

L-cysteine plays a vital role in protein synthesis and regulates the metabolism of phospholipids. However, the deficiency of L-cysteine within the body is associated with severe diseases like cancer, rheumatoid arthritis, and Alzheimer's. Conventional L-cysteine detection methods inherit limitations in accuracy and cost. In this research, a highly sensitive and specific colorimetric assay is developed using biogenic palladium nanoparticles (PdNPs). The biogenic PdNPs exhibite peroxidase mimetic activity, which oxidizes 3,3′,5,5′-tetramethylbenzidine in the presence of hydrogen peroxide (H₂O₂). The catalytic activity is determined by Michaelis–Menten and Lineweaver–Burk plots, yielding a K_m of 3.65 × 10⁻⁵ m, V_max of 9.71 × 10⁻⁷ M s⁻¹, and k_cat of 2.37 × 10⁻⁴ s⁻¹, with a catalytic efficiency (k_cat/K_m) of 6.493 M⁻¹ s⁻¹. The colorimetric assay exhibits a low limit of detection of 0.12  μM and a limit of quantification of 0.36  μM. Smartphone-based analysis enhances the method's efficiency for precise detection of L-cysteine. A smartphone-based method successfully detects L-cysteine in real samples, showing high precision and excellent recovery rates ranging from 99.41% to 106.65%. This developed technique overcomes the limitations of traditional methods, exhibiting strong potential for rapid and specific L-cysteine detection in future biosensing applications.