Single quantum dot fluorescence enhancement by tunable nanoporous gold

Nanoengineered metal materials with the property of surface enhanced fluorescence (SEF) are attracting considerable attention due to their promising contribution to the fields such as biosensing and photonics by lowering the limit of detection (LOD) and enhancing the illumination efficiency. A variety of SEF materials such as nanoantennae [1, 2] and nanoparticles [3-8] have demonstrated significant fluorescence enhancement on organic fluorophores. Despite numerous advantages, such nanomaterials have their own limitations. Fabrication of SEF materials often involves complicated lithography processes, limiting their ubiquitous application. In addition, most SEF materials are designed to enhance fluorescence at certain local “hot spots” [4]; however, strong enhancement over a wide area is preferred in many applications. Herein we demonstrate a tunable three-dimensional (3D) free-standing nanoporous gold (NPG) film which greatly enhances the fluorescence of semiconductor quantum dots (QDs). QDs on the ligament experience near-field enhanced excitation and the decay of magnetic field, emitting more photons in a given time period. The NPG film is fabricated by simple chemical etching process and its nanopore size can be flexibly tuned by controlling the etching time and temperature. We demonstrate a high enhancement factor of ~50-fold of QDs fluorescence achieved by NPG films. The superior fluorescence enhancement greatly improves imaging of single QDs using regular fluorescence microscopy.