Nanoporous gold electrodes for application in trace metal sensors, biosensors and biofuel cells

Nanoporous Gold (NPG) is a material of emerging interest for immobilization of biomolecules and especially enzymes. NPG materials provide a high surface area onto which biomolecules can either be directly physisorbed, covalently linked after first modifying the NPG with a self-assembled monolayer (SAM) or entrapped in a polymer matrix. The immobilization of enzymes while using NPG substrate material is being pursued for applications in sensors, assays, supported synthesis, catalysis and biofuel cells. NPG materials can be prepared by using many different approaches. However, the most common method used is the dealloying of a low carat gold alloy containing between 20-50 atomic % gold in a strong acid (70% HNO3), which oxidizes the least noble metal, removing it from the alloy. The rapid rearrangement of the gold atoms at the solid/liquid interface leaves behind the characteristic surface morphology. The resultant structure consists of interconnected ligaments and pores with typical widths between 5-200 nm. The surface area of these materials can be up to 500 times higher than their geometric area. Surface addressability of NPG is crucial for functionalization and surface modification for the use in sensors, biosensors and biofuel cells. Full addressability of the surface area of NPG was observed with small molecules such as sulphuric acid. The surfaces could also be modified using bulky anthraquinone functional groups attached on activated diazonium salts throughout the whole structure. Surface modification of NPG has been achieved using a variety of strategies, such as through SAM formation of thiol compounds, electro-reduction of in situ synthesized diazonium compounds and the drop-casting or electro-polymerization of osmium redox polymers and hydrogels. Surface functionalized NPG could be used for a variety of applications. Bulky negatively charged sulfonate groups could therefore attract positively charged free trace metal ions (such as Cu2+) in solutions for direct detection at the electrode surface. The sensor displayed a detection range from 0.2 to at least 25 µM which is within the legal concentration limit of 20.5 µM (1300 ppb) in drinking water (United States, EPA). The sensitivity and limit of detection (LOD) were found to be 8.18 µA cm-2 µM-1 and 18.9 nM (~1.2 ppb) respectively. The BDS surface functionalization was also capable of blocking biofouling material from the electrode surface, making it possible to measure in complex media such as artificial human serum. Fructose dehydrogenase (FDH) could be covalently attached to carboxylic acid terminated diazonium compounds for the precise detection of D-fructose concentrations in a range of natural sweeteners and beverages. The sensor was able to give accurate readings within 5 seconds with a linear range of 0.05 - 0.3 mM D- fructose concentration, a sensitivity of 3.7 ± 0.2 μA cm-2 mM-1 and a LOD of 1.2 μM. When combining anodic enzymes, such as glucose dehydrogenase (GDH) and FDH, with cathodic enzymes such as bilirubin oxidase (BOD), enzymatic biofuel cells with considerable power outputs can be obtained. GDH/MvBOD EFCs generated power densities of up to 17.5 and 7.0 μW cm-2 in PBS and artificial serum, respectively, at an OCV of ~0.45 V (vs Ag/AgCl) with a concentration of 5 mM D-glucose. These EFCs retained over 60% of their initial power density after 8 hours of continuous operation. FDH/BpBOD EFCs generated power densities of up to 13 µW cm-2 at an operating potential of 0.18 V vs Ag/AgCl at a concentration of 10 mM D-fructose. The half-life was found to be ca. 19 h.