A green synthesis of gold–palladium core–shell nanoparticles using orange peel extract through two-step reduction method and its formaldehyde colorimetric sensing performance

Gold–palladium core–shell nanoparticles (Au@PdNPs) have been successfully synthesized through a two-step facile and green method. HAuCl₄ and H₂PdCl₄ were used as the precursors, while orange peel extract (OPE) was used as both reducing and stabilizing agent. Initially, OPE was used to synthesize the gold core nanoparticles (AuNPs), followed by the reduction of PdCl₄²⁻ solution to produce the Pd shell. The synthesized nanoparticles showed a strong visible absorbance at 534 nm indicated the formation of AuNPs at an optimum water/OPE ratio of 16.67, while the formation of Au@PdNPs was indicated by the disappearance of the peak at 534 nm. Further characterization by using IR spectroscopy indicated that –OH groups of OPE plays the main role in the nanoparticles formation, while TEM images observed a core–shell structure of the Au@PdNPs with an average core diameter of 40 nm and an average shell thickness of 7 nm. In the presence of different formaldehyde concentrations, the core–shell nanoparticles showed a clear color change from light to dark brown. Linearity of the absorbance responses from the formaldehyde concentration range of 36.3 mM to 3.63 M (R² = 0.991) was achieved with an estimated LOD of 304.9 mM, suggested that the developed nanoparticles are promising to be applied for formaldehyde sensing.