Excitation mechanisms revealed by single particle spectroscopy

15 Sep 2016. NUS scientists have discovered the mechanism of aggregation enhanced two-photon photoluminescence (2PPL) of metal nanoparticles by studying 2PPL properties of Au nanorod-nanosphere (NR-NS) heterodimers on the single particle level.

Noble metal nanoparticles, such as gold (Au) and silver (Ag), display a unique property known as localised surface plasmon resonance (LSPR), which has been utilised for many important applications. In particular, LSPR can significantly modulate the optical responses of nearby chromophores, such as surface enhanced Raman scattering and metal enhanced fluorescence. Prof XU Qing Hua’s research group recently discovered that originally non-fluorescent metal nanoparticles display strong 2PPL upon formation of nanoparticle aggregates with 2PPL enhanced by up to 800-fold in a colloid solution. As many biologically important species can cause aggregation of metal nanoparticles, this phenomenon of aggregation enhanced 2PPL can potentially find many biological applications such as two-photon sensing, imaging and phototherapy. This method also has the unique advantages of deeper tissue penetration, three-dimensionally confined excitation, and less photo-damage to the sample.

Aggregation enhanced 2PPL has been successfully demonstrated in metal nanoparticles of different compositions and morphologies. However, the enhancement mechanisms still remain unclear. Most previous studies are based on conventional ensemble-based measurements, which are averaged results and complicated by sample inhomogeneity. A team led by Prof Xu, together with his Ph.D. student, Ms GARAI Monalisa, from the Department of Chemistry in NUS has performed single-particle spectroscopic studies on 2PPL properties of Au NR-NS heterodimers with two different spatial orientations (end-linked and side-linked). Single-particle studies are expected to provide a more direct morphology-property relationship and is advantageous to explore aggregation induced 2PPL enhancement mechanisms.

The research team demonstrated that the 2PPL properties of the dimers are dominated by the coupling interactions between the metal nanoparticles instead of individual nanoparticles. For example, 2PPL of both end-linked and side-linked Au NR-NS are polarised along the coupling directions. The plasmon coupling induced local electric field enhancement is the key factor in determining the 2PPL properties of the heterodimers. Plasmon coupling enhanced 2PPL is generally believed to arise from enhanced two-photon excitation efficiency through two effects: (i) improved resonance effects due to plasmon coupling induced red-shifted LSPR mode and (ii) plasmon coupling induced giant local electric field amplification. Stronger 2PPL signals have been observed for coupled nanostructures with excitation along the coupling direction, but with less favourable overlap between the LSPR spectra and excitation wavelength. The findings unambiguously support that plasmon coupling induced local electric field amplification is the dominant effect responsible for the big difference in 2PPL intensities of different nanostructures. This conclusion has been further confirmed by theoretical calculations.

The researchers anticipate that these mechanistic studies are very important for fundamental understanding of plasmon coupling enhanced optical properties in aggregated nanoparticle clusters, which provide insights on designing nanostructures with tailored optical properties. The huge amplification of local electric field in the interparticle gap region can be further explored for various biomedical and energy harvesting applications.

Figure above shows schematic of polarised 2PPL of coupled gold nanorod-nanosphere heterodimers.

Reference

Garai M, Zhang T, Gao N.Y., Zhu H, Xu Q.-H. “Single Particle Studies on Two-Photon Photoluminescence of Gold Nanorod−Nanosphere Heterodimers”. J. Phys. Chem. C 120 (2016) 11621.