Department of Physics, Fudan University The measurements of the longitudinal photoluminescence band of gold nanorods in aqueous solutions and living cells Yan Cen, Xiao Huang, Ji-Yao Chen Department of Physics, Fudan University The gold nanorods (AuNRs) have two absorption bands due to the transverse and longitudinal surface Plasmon resonances (SPRs). However only the transverse photoluminescence (PL) band was reported under the single-photon excitation (SPE) with the wavelength shorter than 520 nm, and the longitudinal PL band became dominate one under two-photon excitation (TPE) of a near-infrared laser pulse. In this work, the 633 nm and 514 nm were used as SPE sources to separately study the longitudinal and transverse PL properties of various AuNRs with different aspect ratio and corresponding longitudinal absorption peaks of 650, 680, 720 and 750 nm. Under the 514 nm SPE, the sole transverse PL band centered at 550 nm was found for all AuNRs, whereas the longitudinal PL bands of AuNRs were in the region of 660-720 nm and red-shifted with the increased aspect ratio under the 633 nm SPE. The PL quantum yields of transverse and longitudinal bands of AuNRs were measured to be on the orders of 10-4 and 10-5, respectively. The longitudinal PL band of cellular AuNRs was further detected in liver cancer cells under 633 nm SPE when these cells have been incubated with AuNRs (0.02 nM), with the features of broadened PL spectra and peak shifts as compared to that of AuNRs in aqueous solutions. These data may provide useful information for AuNR detections with TPE in biological systems particularly in cell imaging, because the longitudinal PL band contributes a main role in these cases. 1.Motivation 2. Result FIG 2. The absorption spectra of different AuNRs with the rod length about 50-70 nm and aspect ratios of about 2、2.5、3 and 3.5 in aqueous solutions. Their longitudinal band peaks are at 650, 680, 720 and 750 nm, respectively. These gold nanorods were stabilized by cetyltrimethylammomium bromide (CTAB) FIG 1. TPE PL images of Au nanorods in QGY cells under the irradiation of 800 nm fs laser. APPLIED PHYSICS LETTERS 98, 213704 (2011) by Xi Wu Ji-Yao Chen etc The longitudinal PL band of AuNRs certainly contributed a lot to the image detection. FIG 5. Images of QGY cells. Cells have been incubated with AuNRs (0.02 nM) for 20 minutes. Left: the confocal reflectance image; Middle: the DIC image; Right: the merged image. A 488 nm laser was used in imaging measurements. The bar is 20 μm. FIG 4 PL spectra of gold nanoparticle aqueous solutions under TPE of 800nm. ACS NANO 4,112(2010) by Xi Wu Ji-Yao Chen etc FIG 6. The PL spectra of AuNRs in QGY cells, comparing with that of AuNRs in aqueous solutions. Excitation: 632 nm. TPL band LPL band 650nm 2.4×10-4 1.6×10-5 680nm 1.5×10-4 2.3×10-5 720nm 1.1×10-4 3.8×10-5 750nm 2.1×10-5 2.9×10-5 3. Conclusion We confirmed the longitudinal PL band of AuNRs in aqueous solutions and living cells under the 633 nm SPE. The longitudinal PL band would play a main role in TPE imaging of AuNRs in biological systems. Although the quantum yield of the longitudinal PL band is only 10-5, the huge two-photon absorption cross section make the AuNRs become a powerful PL probes under TPE. The results obtained here may provide the data to better understand the process of TPE imaging of AuNRs in living systems. FIG 3. The PL spectra of AuNRs with different aspect ratios in aqueous solutions. A): Transverse PL spectra under excitation of 514 nm; B): Longitudinal PL spectra under excitation of 632 nm. Table1. The quantum yield of transverse PL band and longitudinal PL band of AuNRs