Bimetallic Ru–Pd Nanoparticles Anchored on Carbon Nanotubes for High-Performance Electrochemical Detection of Tryptophan: Experimental and DFT Insights

Abstract

A highly sensitive electrochemical sensor for the determination of tryptophan was developed using a bimetallic ruthenium–palladium catalyst supported on carbon nanotubes (Ru–Pd/CNT). The Ru–Pd/CNT nanocomposite was synthesized via a sodium borohydride (NaBH4) reduction method, enabling the uniform deposition of Ru and Pd nanoparticles onto the CNT surface. Comprehensive physicochemical characterization using X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), inductively coupled plasma mass spectrometry (ICP-MS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed techniques confirmed the successful formation of a well-dispersed bimetallic system with strong metal–support interactions and mixed oxidation states. The Ru–Pd/CNT catalyst was used to modify a glassy carbon electrode (Ru–Pd/CNT@GCE) and was evaluated as an electrochemical sensor for tryptophan using cyclic voltammetry (CV), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS). The modified electrode exhibited strong electrocatalytic activity toward tryptophan oxidation and provided a large linear dynamic range (LDR) of 1–1200 μM, a low limit of detection (LOD) of 0.05 μM, and a sensitivity of 0.0023 A M–1. Density functional theory (DFT) calculations provided theoretical insight into the enhanced sensing performance. Comparative DFT calculations for CNT, Pd/CNT, Ru/CNT, and Ru–Pd/CNT showed that the HOMO–LUMO energy gap followed the order CNT > Pd/CNT > Ru/CNT > Ru–Pd/CNT, decreasing from 7.498 eV for pristine CNT to 0.466 eV for Ru–Pd/CNT. This trend indicates enhanced electronic activation and improved charge-transfer potential in the bimetallic Ru–Pd/CNT system. Molecular electrostatic potential (MEP), electron localization function (ELF), and localized orbital locator (LOL) analyses also indicated favorable electron redistribution and balanced localization–delocalization behavior at the metal–CNT interface. Based on the combined experimental and theoretical results, the Ru–Pd/CNT@GCE sensor represents a promising platform for sensitive and selective tryptophan detection under controlled analytical conditions.