Xplora microspectrometer

Extinction spectroscopy was used to study the aggregation properties of the BSA-NR complexes by monitoring their time- and concentration-dependent plasmon resonances. Extinction spectra were acquired using the internal white light source embedded in the XploRA microspectrometer (Horiba Jobin Yvon, Kyoto, Japan) for excitation. Collection of the light transmitted through the microcell containing the NRs-protein solution was performed with a 10× microscope objective (Olympus M-Plan, NA 0.2) [61 (link)]. Detection of the transmitted intensity ( $I_T$ ) was accomplished with a Peltier cooled CCD (Sincerity). We considered the absorbance profile to assign the plasmon resonance, calculated as $-{\log }_{10}\left(I_T/I_0\right)$ , where $I_0$ is the excitation intensity. For these experiments, NR-BSA solutions were prepared in a 1 mL eppendorf and 80 µL aliquots were pipetted to the glass microcell for analysis at defined time intervals.

SEM images were acquired by means of a Zeiss CrossBeam 540 apparatus equipped with EDX detector. Raman spectroscopy was carried out with a XploRA micro-spectrometer (Horiba Scientific), with excitation at 638 nm. The laser beam was focused with a 100X objective (NA 0.9, WD 0.21 mm) on a diffraction limited spot. The Raman signal was collected through the same objective in a backscattering configuration and dispersed by an 1800 lines/mm grating onto a charge-coupled device (CCD) detector (Syncerity, Horiba Scientific). The laser power was set at 0.2 mW in order to avoid sample damaging and the signal was integrated over 30 s.