Single molecule fluorescence techniques [1-3] are key for several applications including DNA sequencing [4, 5], molecular and cell biology [6, 7], and early diagnosis [8]. Unfortunately, observation of single molecules by diffraction-limited optics is restricted to detection volumes in the femtolitre range and imperatively requires pico-or nanomolar concentrations, far below the micromolar range where most biological reactions occur [2]. This limitation can be overcome using plasmonic nanostructures, and confinement of light down to nanoscale volumes has been reported [9-13]. While these nanoantennas enhance fluorescence brightness [14-20], large background signals [20-22] and/or unspecific binding to the metallic surface [23-25] has hampered the detection of individual fluorescent molecules in solution at high concentrations. Here we introduce a novel "antenna-in-box" platform that is based on a gap-antenna inside a nanoaperture. This design combines fluorescent signal enhancement and background screening, offering high single molecule sensitivity (fluorescence enhancement up to 1100 folds and microsecond transit time) at micromolar sample concentrations and zeptolitre-range detection volumes. The antennain-box device can be optimized for single molecule fluorescence studies at physiologicallyrelevant concentrations, as we demonstrate using various biomolecules. Our antenna-in-box design is shown in Figure 1a and b. The rationale behind our design is that in any nanoantenna experiment on molecules in solution, the observed fluorescence signal is a sum of two contributions: the enhanced fluorescence from the few molecules in the nanoantenna gap region (hot