The ability to capture the chemical signatures of biomolecules (i.e., electron-transfer dynamics) in living cells will provide an entirely new perspective on biology and medicine. This can be accomplished using nanoscale optical antennas that can collect, resonate and focus light from outside the cell and emit molecular spectra. Here, we describe biologically inspired nanoscale optical antennas that utilize the unique topologies of plant viruses (and thus, are called gold plant viruses) for molecular fingerprint detection. Our electromagnetic calculations for these gold viruses indicate that capsid morphologies permit high amplification of optical scattering energy compared to a smooth nanosphere. From experimental measurements of various gold viruses based on four different plant viruses, we observe highly enhanced optical cross-sections and the modulation of the resonance wavelength depending on the viral morphology. Additionally, in label-free molecular imaging, we successfully obtain higher sensitivity (by a factor of up to 10 6 ) than can be achieved using similar-sized nanospheres. By virtue of the inherent functionalities of capsids and the plasmonic characteristics of the gold layer, a gold virus-based antenna will enable cellular targeting, imaging and drug delivery. Keywords: molecular sensor; nanophotonics; optical antenna; optical spectroscopy; plant virus; plasmonics; plasmonic resonant energy transfer (PRET); surface enhanced Raman scattering (SERS)
INTRODUCTIONIn current nanotechnology, the development of nanoscale optical antennas that are capable of receiving and transmitting light in unique light-electron interactions called 'surface plasmons' 1 is of considerable interest because of the potential applications of such antennas in molecular detection, 2-4 multiple harmonic generation, 5,6 plasmonic photovoltaics, 7 optoelectronics, 8 negative-index materials 9 and more. In particular, the use of an optical antenna as a molecular sensor permits the dynamic detection of chemical signatures in a non-invasive and label-free manner. Moreover, the detection sensitivity can reach the single-molecule level. 3,10 These powerful characteristics distinguish the optical antenna as a potential method for unraveling the inherent complexity of biological systems (i.e., cells), which is a difficult task using current techniques.To be suitable for use as a biomolecular sensor in cellular systems, an optical antenna should not interfere with cellular components and should be efficiently accessible (or mobile) to subcellular regions. These characteristics can be achieved by exploiting the nanoscale size and chemical stability of non-fixed, biochemical-moiety-containing nanoparticles. In this context, gold nanospheres have been successfully demonstrated to be capable of actively targeting subcellular regions and carrying imaging agents and drugs. 11,12 However, the smooth surfaces of chemically synthesized gold nanoparticles do not impart these nanoparticles with ideal functionality for molecular fingerprint detect...