Palladium nanoparticles/wool keratin-assisted carbon composite-modified flexible and disposable electrochemical solid-state pH sensor

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/AgCl 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.