Noble metal nanoparticles have unique properties, such as localized surface plasmon resonance (LSPR). The LSPR band can be modified by interparticle coupling, which significantly enhances the local electric filed, which again enhances optical signals in two-photon photoluminescence (TPPL). Gold nanospheres (Au NSs) are especially suited for TPPL, as they exhibit very weak TPPL signals in a discrete state, but much stronger signals in an aggregated state.

Qing-Hua Xu, National University of Singapore and colleagues demonstrate that cationic Au NSs form aggregates on the surface of Gram-positive Staphylococcus aureus bacteria, as well as on the surface of Gram-negative Escherichia coli; the most common pathogens responsible for hospital-acquired infection infections. Due to the enhancement in TPPL by interparticle coupling, bacteria of skin infections could easily be detected.

A further advantage is that Au NSs work as „light-directed nanoheaters“. The radiation of the near infra-red (NIR) area of the spectrum is transfered into heat, and this heat is sufficient to damage bacterial cells by cell membrane disruption and enzyme denaturation. The Au NSs show high selectivity towards the pathogenic bacterial cells, and show low cytotoxicity against mammalian cells. As more and more bacteria are non-responsive to antibiotics, this photothermal effect is of interest as new and effective therapeutic.

• Plasmon-Coupled Gold Nanospheres for Two-Photon Imaging and Photoantibacterial Activity
  Peiyan Yuan, Xin Ding, Zhenping Guan, Nengyue Gao, Rizhao Ma, Xiao-Fang Jiang, Yi Yan Yang, Qing-Hua Xu
  Adv. Healthcare Mat. 2014.
  DOI: 10.1002/adhm.201400524