Nanoporous gold (npAu) electrodes prepared by electrochemical dealloying are perfectly suited platforms for sensing applications due to their self-standing, well-conducting structure and high surface-to-volume ratio. By tuning the pore size and modifying their surface with covalently bound self-assembled monolayers (SAMs) that hold specific functional groups (FG), it is possible to create fast-responding electrochemical electrodes that are highly selective and sensitive to targeted chemical species. Suitably selected SAMs also enable the binding of active enzymes for biosensing and catalysis on the biocompatible npAu. Two different examples of this electrode design approach are presented here: (i) the detection of fluoride ions through surface potential changes of the npAu-SAM system¹ and (ii) the immobilization of the well-known enzyme lactase oxidase (LOx) from Aerococcus viridans. LOx catalyses the oxidation of L-lactate while reducing the electron mediating species, which can further be detected electrochemically at the electrode’s surface, making the system useful for bio-applications.
The detection of fluoride ions in water (i) is a complex topic due to their redox-inactive nature. However, it is crucial to monitor the quality of drinking water and wastewater treatment in-situ due to the adverse health effects of fluoride. The modification of npAu with a boronic acid (BA) terminated SAM allowed for the creation of a fast-responding potentiometric system that is sensitive to stepwise addition of F- to water. This sensitivity is based on the change in the charge state of the BA moiety induced by fluoride binding, which further alters the electrode’s potential. The modified npAu electrodes exhibit favourable regenerability in alkaline media and demonstrate a highly sensitive response in reproducible, well-defined potential steps with a detection limit of 0.2 mM.
The second approach (ii) exploits the high selectivity of enzymes immobilized on npAu for biosensing applications. For this purpose, a fundamental understanding of the immobilization behaviour of enzymes on the nanoporous metal is crucial. Therefore, the well-studied enzyme LOx was chosen as a reference. Immobilization on several SAMs provided insights into the effects of the different FGs (such as sulfonic acid, carboxylic acid, or amine groups) and the nanoporous metal as carrier material on catalytic effectiveness, immobilization yield, and stability of the biomolecule. These findings provide a promising basis to develop a future L-lactate sensing platform.
This work was supported by the Lead Project Porous Materials @ Work for Sustainability at TU Graz as well as NAWI Graz.
References:
1.    L.M. Novak, E.-M. Steyskal, RSC Adv., 13 (2023) 6947-6953.