Characterizing the optical properties of single palladium-coated core-shell gold nanorods as multifunctional orientation probes.

Bimetallic core-shell gold nanorods are promising multifunctional orientation probes due to the synergetic effect of two different metals, as examined under an optical microscope. Herein, we performed a single particle study to characterize the optical properties of palladium-coated core-shell gold nanorods (Pd-AuNRs) using scattering-based dark-field (DF) and differential interference contrast (DIC) microscopy, and to examine their use as multifunctional orientation probes in biological and physical studies. The longitudinal surface plasmon resonance (SPR) was strongly damped with the increased SPR linewidth for single Pd-AuNRs due to Pd metals coated on the AuNR surface under single particle scattering spectroscopy. Despite the strong plasmon damping with much reduced scattering intensity, DIC microscopy allowed us to detect single Pd-AuNRs with much higher sensitivity. We found polarization-dependent DIC images and intensities of single Pd-AuNRs, which enabled the determination of their orientation angle under DIC microscopy. Furthermore, we found that DIC polarization anisotropy can be obtained from the orthogonally polarized bright and dark intensities for more accurate and reliable angle measurements of the Pd-AuNR probes. The results support the idea that bimetallic Pd-AuNRs can be used to develop multifunctional orientation probes.