Thanks to their small sensing volume, nanosensors based on localized surface plasmon resonances (LSPR) allow the detection of minute amounts of analytes, down to the single-molecule limit. However, the detected analytes are often large molecules, such as proteins. The detection of small molecules remains largely unexplored. Here, we use a hybrid photonic-plasmonic nanosensor to detect a small target molecule (pyridine). The sensor's design is based on a dielectric photonic microstructure acting as an antenna, which efficiently funnels light toward a plasmonic transducer and enhances the detection efficiency. This sensor exhibits a limit of detection as small as 10-14 mol L-1. Using a calibration procedure based on electrodynamical numerical simulations, we compute the number of detected molecules. This yields a limit of detection in mass of 4 zeptograms (1 zg = 10-21 g), a record value for plasmonic molecular sensors. Our system can hence be seen as an optical molecular weighing scale, enabling room temperature detection of mass at the zeptogram scale.
Keywords: limit of detection; molecular weighing scales; nanosensors; optical antennas; pyridine; surface plasmons.