Anisotropic gold nanoparticles are chemically stable and serve as localized surface plasmon resonance (LSPR) sensors operatable in the first biological optical window (650-950 nm). However, alternative materials are awaited because they are expensive and somewhat complicated to prepare. Here we employ CuS (covellite) nanoplates, which consist of earth-abundant elements and exhibit LSPR in the near-infrared region, as materials for LSPR sensors. The CuS nanoplates respond to refractive index changes of the surrounding medium in the second biological optical window (1000-1350 nm). The refractive index sensitivity (160-600 nm RIU-1) and the operation wavelength (1100-1250 nm) of the CuS nanoplates can be controlled by simply changing the composition of reaction suspension for nanoplate synthesis. (C) 2016 Optical Society of America

Fast and reversible control of the plasmonic properties of compound nanoparticles (i.e. CuS nanoplates) was achieved through electrochemical redox reactions. Their electrochemical tunability can be applied to fast-switching near infrared electrochromic devices, whose visible appearance is not changed by switching.

Metal nanoparticles absorb and scatter light strongly on the basis of resonance of free electron oscillation with incident light, which is denoted as localized surface plasmon resonance (LSPR). Especially, gold and silver nanoparticles are known to exhibit LSPR in the visible and near-infrared (NIR) regions and applied to sensors,1 photovoltaics and photocatalysis,2 and photochromic materials.3 The LSPR properties such as resonant wavelength and intensity depend significantly on particle size and shape, local refractive index, and interparticle distance. Those are also dependent on electron density of the nanoparticles, which can be controlled electrostatically.4-6 However, it is difficult to induce large changes in resonant wavelength and intensity by the electrostatic means. Here, we focus on plasmonic copper sulfide nanoparticles as an electrochromic material. Nanoparticles of copper sulfide, represented by Cu2-x S, have sufficient holes to exhibit LSPR in the NIR region.7 In addition, copper valence can be controlled reversibly by chemical redox reactions,8 suggesting that reversible electrochemical tuning of their LSPR properties is possible. However, such a redox reaction-based control of LSPR has not been reported so far. Covellite (CuS) nanoparticles were prepared by heating copper(I) chloride and elemental sulfur in a mixture of oleylamine and oleic acid under N2 atmosphere9 and deposited on an ITO electrode. The figure shows absorption spectra of the electrode at different potentials (vs. Ag|AgCl) in deuterated water containing 0.1 M Na2S. Broad absorption due to LSPR is observed in the NIR region at -0.6 V while that gradually disappears as the applied potential is shifted negatively, indicating that plasmonic copper(II) sulfide is reduced to non-plasmonic copper(I) sulfide.10 It is noteworthy that reductive disappearance and oxidative reappearance of LSPR can be repeated at least 12 cycles and the response time is a few seconds. These results indicate that copper sulfide nanoparticles can be applied to fast NIR electrochromic devices that are driven in a narrow potential range (~0.4V). 1. K. M. Mayer, J. H. Hafner, Chem. Rev. 2011, 111, 3828. 2. Y. Tian, T. Tatsuma, J. Am. Chem. Soc. 2005, 127, 7632. 3. Y. Ohko, T. Tatsuma, T. Fujii, K. Naoi, C. Niwa, Y. Kubota, A. Fujishima, Nat. Mater. 2003, 2, 29. 4. T. Ung, M. Giersig, D. Dunstan, P. Mulvaney, Langmuir 1997, 13, 1773. 5. A. H. Ali, R. J. Luther, C. A. Foss, Jr, Nanostruct. Mater. 1997, 9, 559. 6. H. Nishi, S. Hiroya, T. Tatsuma, ACS Nano 2015, 9, 6214. 7. J. M. Luther, P. K. Jain, T. Ewers, A. P. Alivisatos, Nature 2011, 10, 361. 8. P. K. Jain, K. Manthiram, J. H. Engel, S. L. White, J. A. Faucheaux, A. P. Alivisatos, Angew. Chem. Int. Ed. 2013, 52, 13671. 9. H. Nishi, K. Asami, T. Tatsuma, Opt. Mater. Express 2016, 6, 1043. 10. I. Kriegel, C. Jiang, J. Rodríguez-Fernandez, R. D. Schaller, D. V. Talapin, E. da Como, J. Feldmann, J. Am. Chem. Soc. 2012, 134, 1583. Figure 1