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.