Gold nanobipyramids (GNBP) as spectral OCT contrast agents in the NIR-II window. (a) TEM images of GNBPs with three different resonance peaks at 1225, 1315 and 1450 nm, respectively. (b) The Vis-NIR spectra of GNBP1225, GNBP1315 and GNBP1450. (c) Spectral contrast OCT image of a mouse ear after subcutaneous injections of GNBP1225, GNBP1315 and GNBP1450. The spectral contrast signals of GNBP1225, GNBP1315 and GNBP1450 are shown in red, green and blue colors. Figures reprinted with permission from [105]. Copyright (2020) American Chemical Society.