Several pH-dependent processes and reactions take place in the human body;hence,the pH of body fluids is the best indicator of disturbed health conditions.However,accurate and real-time diagnosis of the pH of body fluids is complicated because of limited commercially available pH sensors.Hence,we aimed to prepare a flexible,transparent,disposable,user-friendly,and economic strip-based solid-state pH sensor using palladium nanoparticles (PdNPs)/N-doped carbon (NC)composite material.The PdNPs/NC composite material was synthesized using wool keratin (WK) as a precursor.The in-situ prepared PdNPs played a key role in the controlled switching of protein structure to the N-doped carbon skeleton with π-π arrangement at the mesoscale level,which mimics the A-B type polymeric structure,and hence,is highly susceptible to H+ ions.The optimized carbonization condition in the presence of PdNPs showed that the material obtained using a modified Ag/AgC1 reference electrode had the highest pH sensitivity with excellent stability and durability.The optimized pH sensor showed high specificity and selectivity with a sensitivity of 55 mV/pH unit and a relative standard deviation of 0.79％.This study is the first to synthesize PdNPs using WK as a stabilizing and reducing agent.The applicability of the sensor was investigated for biological samples,namely,saliva and gastric juices.The proposed protocol and material have implications in solid-state chemistry,where biological material will be the best choice for the synthesis of materials with anticipated performance.