The aggregation enhanced photoluminescence of gold nanorods in aqueous solutions.

The photoluminescence (PL) properties of single gold nanorod (AuNR) under one-photon excitation (OPE) have been reported recently. In this work, the PL of AuNRs in aqueous solutions were studied with OPE of 514 or 633 nm to characterize the emissions of transverse and longitudinal surface Plasmon resonance (TSPR and LSPR) bands, because the AuNRs aqueous solution was frequently used in bio-medical applications. We found that under 514 nm OPE the TSPR emissions of four groups of AuNRs with different aspect ratios in aqueous solutions were all strong dominating the PL emission with the quantum yield (QY) of 10(-4), which is at least three orders of magnitude higher than that of single AuNR. We further found that the aggregate was the basic form of AuNRs in aqueous solution and living cells, measured by the elastic light scattering and transmission electron microscopy measurements. The Plasmon coupling particularly the TSPR coupling between the neighbored AuNRs in aggregates enhanced the PL and increased the QY, because the conjugation of the rod side to side was a main aggregate mode. Under 633 nm OPE, only LSPR emissions of AuNRs aqueous solutions occurred with the QY level of 10(-5) which is very similar to that of singe AuNR, because of the negligible LSPR coupling.