Plasmonic effects on the photophysics studied at the single nanoparticle single molecule level

PEDRO F. ARAMENDÍA

Snatiago

Congreso; 25th Inter-American Photochemical Society Meeting; 2016

Inter-American Photochemical Society

Metallic nanoparticles (MNP) display collective electron oscillation modes that are calledplasmons. Plasmons work as an electric antenna at the visible frequency range and thereforethey are able to direct the electric field in the nanometric scale. The concentration of theelectric field around a MNP has a profound effect on electronic states of nearby molecules,which experience a modification in their dynamics. As a consequence, absorption crosssection, emission and internal conversion rates are changed in a way that depends on thematerial, size and shape of the MNP, the nature of the surrounding medium, the absorptionand emission spectra of the molecule, and its distance and orientation relative to the MNP andthe polarization direction of the incident photon. The most explored effects were theabsorption and emission enhancement, which jointly lead to an increase in brightness. It isalso well known that the close approach of a molecule to the metal surface results in anoverall quenching of emission due to radiationless energy transfer to the MNP. Less exploredis the fact that the molecular interaction with a plasmonic structure can result in aphotoprotection of the molecule against photobleaching. As a consequence, the molecule canemit an increased number of photons before photobleaching or can be monitored a longertime.All of the appointed effects: increased brightness, longer monitoring times, increased numberof emitted photons, are profitable features for fluorescence microscopy, including superresolution optical microscopy.In this work, we use single-molecule (SM) fluorescence microscopy to investigate theinteraction of a photochromic fluorescent system: the well known spiropyran-merocyanine(SP ↔ MC), with spherical gold nanoparticles (AuNPs). The photochrome is embedded in athin polymer layer covering AuNPs spread on a coverslip. The absorption of the MC formstrongly overlaps the extinction spectrum of 70 nm diameter AuNPs. Spontaneous thermalactivation of the SP ↔ MC conversion on films of adequate concentration suffice to providesingle molecule conditions. We observe a significant increase in the brightness of theemission of the MC form, in the duration of its ON time, and in the total number of emittedphotons when the molecules are located in the vicinity of a AuNP. The spatial distribution ofSMs with improved photophysical performance was obtained with 40 nm precision relative tothe nearest AuNP by using sub diffraction limited localization. In this way we could map thelocation of the brightest and longest lived molecules relative to a generic single NP withnanometric precision. We demonstrate that even photochromic systems with poorphotochemical performance for SM can become suitable for long time monitoring and superresolution microscopy by interaction with metallic NP.25st