2, NASA Glenn Research Center, Cleveland, Ohio, United States
Nanoplasmonics has enabled a new class of highly sensitive, on-chip biomarker recognition devices, especially relevant to point-of-care (POC) medical applications. However, the sensing properties have largely been studied and exploited at room temperature. In order to translate the significant benefits of on-chip nanoplasmonic sensing technology to more extreme environments, such as emerging exo-life detection applications, the basic transduction principle must be well-characterized in relevant environments. In this work, we study the optical response and sensing properties of nanoplasmonic structures over a wide range of temperatures, from ambient to cryogenic, relevant to exo-life detection. We explore both Au nanoantennas fabricated using electron beam lithography (EBL), and rough Au ‘nano-island’ surfaces fabricated via wafer-scale sputter deposition. The latter provides a facile means to fabricate surfaces with tailored optical resonances (by adjusting the sputtering rate and time), suitable for basic materials investigations as a function of temperature, while the former allows more control of the nanosensor geometry, to further elucidate material- and temperature-dependent effects. In order to measure the temperature-dependent sensing properties, we have designed and built a custom epi-fluorescence brightfield/darkfield microspectroscopy setup coupled to a cryogenic stage, capable of recording the spectra from an area less than 50 square microns and over a temperature range from ambient to 80 K. Through both experiment and simulation, we show how the plasmonic coupling and sensing properties (resonance quality factor, refractive index sensitivity, and sensor figure of merit) change with temperature and geometry, and discuss implications for cryogenic nanoplasmonic sensor application. Preliminary experimental results show that for the Au nano-island films, exposure to cryogenic temperature results in both a red-shift and reduced plasmon resonance quality factor, consistent with theoretical predictions for geometries supporting propagating surface plasmons. Isolation of individual nanoresonators is expected to reverse this trend, and improve sensing performance, as localized surface plasmon resonances exhibit opposite behavior at cryogenic temperature. This is, to our knowledge, the first time nanoplasmonic sensing properties have been studied at temperatures applicable to in-situ exo-life detection scenarios.