Ag–ZnO Nanocomposites as a 3D Metal-Enhanced Fluorescence Substrate for the Fluorescence Detection of DNA

Fluorescence enhancement, including metal-enhanced fluorescence (MEF), has been widely explored in the field of biosensor technology. Two- and three-dimensional (2D and 3D) MEF substrate approaches have been developed for biosensors to detect target biomarkers such as genes and proteins. However, several issues remain, not least poor reproducibility caused by limited recognition and diffusion of biomolecules among target molecules, biological probes, and MEF substrates in aqueous environments. Here, an Ag–ZnO nanocomplex as a 3D MEF substrate was developed in two steps: hydrothermal synthesis of ZnO nanowire (NW) as a building block of Ag nanoparticles (NPs), and photochemical deposition of AgNPs on the ZnO NW. Fluorescence enhancement on Ag–ZnO nanocomposites (NCs) compared with the glass substrate was 14.8-, 16.9-, 11.4-, and 14.2-fold in the presence of 10, 20, 40, and 60 base pair (bp) double-stranded (ds) DNA, respectively. The MEF effects on glass, ZnO NW, and Ag–ZnO NCs were analyzed by measuring the fluorescence lifetime of fluorescein (FAM)-labeled dsDNA, and the average lifetimes were 9.76, 9.34, and 7.86 ns, respectively. Ag–ZnO NCs showed a 50-fold higher sensitivity for the detection of dsDNA than Ag nanoisland film (AgNIF) as a 2D MEF substrate. Therefore, Ag–ZnO NCs may have potential as a 3D MEF substrate. The MEF phenomenon might involve broad-range interactions between the surface of the metal nanostructure on Ag–ZnO NCs and fluorophores in various scenarios. Our Ag–ZnO NCs could facilitate the development of sensitive MEF-based biosensors for DNA detection.