Pd/Au Thin Film: Design, Fabrication and Characterization for Their Biosensing Applications
Authors: Fathi Fatemeh , Zeng Xiangqun 
Published: July 2021

Biosensors are typically based on the chemical/biological reactions of the analytes or the interactions/binding of the analytes with biological recognition elements to quantitatively measure the analytes with sufficient sensitivity and selectivity. In recent decades, biosensor research has experienced a revolution due to their significant potential impact on a broad range of applications including health care, the environment, food safety etc. However, in order for biosensors to provide quantitative information about an analyte/es for real world applications, the more crucial need is the robust chemical and biointerfaces that allow highly sensitive and selective detection of bioanalytes in a miniaturized and low-cost platform such as point of care diagnostics. Additionally, real-time and continuous-use biosensors are essential for in vivo sensing to reflect the dynamic concentrations of an analyte in vivo. Compared to other chemical sensors (e.g., optical or solid state-based sensors), electrochemical biosensors possess an impressive capacity to achieve the analytical requirements of high sensitivity and selectivity as well as continuous monitoring of analytes within a small footprint, are constructed with low-cost materials and fabrication techniques, and are controlled by compact low-power microelectronic circuits. However, there are some challenges associated with real-time electrochemical including: 1) some bioanalytes are typically at very low concentrations with co-present of many high concentration interference species; 2) the bioanalytes are not redox active or if they are redox active, they have high oxidation or reduction potentials that could result in side-reactions and electrolyte decomposition. These challenges necessitate the need for the development of new robust electrocatalysts and electrochemical sensing mechanisms.
Rather than relying on the analytes own redox reactions for detection, our recent work shows that a bimetallic Pd/Au thin film can be used to sensitively and selectively detect hydrogen peroxide and 3-nitrotyrosine based on the unique surface electrochemistry of palladium hydride (PdHx), palladium oxides (PdOx) and palladium hydrous oxide in the presence of these analytes. The Pd/Au thin film was fabricated using underpotential deposition (UPD) of copper followed by redox replacement of the copper by palladium on the gold surface in palladium solution at open circuit potential. Our preliminary work found that the quantities of hydrogen and oxygen incorporated to from PdHx and PdOx depends on the Pd thickness, the surface coverage of Pd vs. Au, the trace amount of copper that remained, the morphology of Pd, the adsorbate from the electrolytes, and the gold substrate properties. In this work, we systematically investigate the electrochemical properties of Pd/Au thin film electrodes using this fabrication method at various conditions to understand the parameters that impact Pd/Au thin film properties in order to further optimize the fabrication methods. We studied Pd/Au thin film thickness and electrolytes effects (0.5 M sulfuric acid, 0.1 M phosphate buffer solution with three different pH, etc.) on the resulting Pd/Au film properties. Also, the electrode electrochemical properties particularly PdHx and PdOx redox processes at various conditions (such as various electrochemical potential windows, scan rate) were thoroughly studied and characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy and electrochemical quartz Crystal microbalance. The results indicate that dominant phosphate anions at different pH greatly influence PdHx and PdOx redox properties, as do different potential windows and scan rate. Furthermore, the electrode reproducibility at the same experimental conditions were investigated. Since the Au substrate electrode areas vary in these experiments, surface oxide reduction charge method was used to calibrate the electrode areas for Pd/Au electrodes.
Finally, the Pd/Au electrode fabricated at optimum conditions were tested for biosensing of most common neurotransmitters: dopamine (DA), gamma aminobutyric acid (GABA), and acetylcholine (Ach) using in situ electrochemical and quartz crystal microbalance (EQCM) techniques at phosphate buffer solution (pH=7.4). Our results show that the difference in the potential dependent adsorption of these bioanalytes at catalytic Pd/Au surface depends on their chemical and physical properties which can be used to sensitively and selectively detect these analytes in real-time with multimodal EQCM methods. From the QCM signal, a high sensitivity of 3, 0.2 and 0.13 ng.nM-1 for detection of DA, Ach, and GABA, respectively, broad detection range of 1–10000nM, and low detection limit up to 1nM was obtained. Besides, the CV results show that the corresponding PdOx reduction and PdHx oxidation current peaks decrease with increase in analyte concentrations as expected. Additionally, the Pd/Au thin film demonstrated good stability (up to 7 days), reproducibility, reusability (ca. 70% after extensive use) and selectivity and showed promising results for sensing these neurotransmitters simultaneously.
The Pd/Au thin film electrochemistry-based method reported in this work is a powerful method allowing the synergistic integrating of two different metallic catalysts, into a surface structure on nanoscale. Thus, its unique surface electrochemistry opens up new opportunities to develop useful electrocatalytic materials and innovative biosensing methods for highly sensitive and selective detection of a broad range of bioanalytes based on the surface chemistry and electrochemistry of noble metal electrode catalysts.