Small Au nanorods-MnO2 sheet aggregation with enhanced photoacoustic imaging for tumor

• Ultra-small Au nanorods can be assembled by MnO 2 sheet. • Self-assembled ultra-small Au nanorods result in strong near infrared absorption. • Photoacoustic intensity of small Au nanorods aggregation increase by 2.7 times. • Self-assembled ultra-small Au nanorods can be used tumor imaging in vivo. Small size gold nanorods (diameter < 6 nm) are limited to use as photoacoustic imaging agents due to its poor performance. To address this limitation, local surface plasmon resonance (LSPR) coupling effect produced by the aggregation of Au nanoparticles is a commonly used strategy to enhance its performance. Herein, a small Au NRs aggregation assembled by MnO 2 nanosheet (Au NRs-MnO 2 ) was explored. As expected, the Au NRs-MnO 2 exhibited obviously enhanced NIR absorption compared with small Au NRs and the PA intensity of the Au NRs-MnO 2 dispersed in water was 2.7 times that of Au NRs. More importantly, the Au NRs-MnO 2 exhibited excellent tumor imaging in vivo. Therefore, the obtained Au NRs-MnO 2 aggregation will provide an effective strategy for small size Au NRs to enhance PA imaging.