Combined Scattering Confocal and Multiphoton Luminescence Imaging of Gold Nanospheres

Noble metal nanoparticles are characterized by a strong peak in the scattering and absorption spectrum, termed the plasmon resonance. Researchers have taken advantage of this to create a new label for biological molecules. A disadvantage of techniques based on scattering and absorption is that the detected signal is at the same wavelength as the incident light, making it more challenging to discriminate between signal and background. Gold nanoparticles also luminesce, suggesting an alternate method for their detection. A tightly focused ultra-short pulse laser beam can be used to achieve multiphoton excitation of the particles; the resulting luminescence exhibits a peak in the same region of the spectrum as the plasmon resonance. Because excitation is nonlinear, significant luminescence is only observed when the particle is in the focus, permitting localization with both high lateral and axial resolution. The physical mechanism underlying multiphoton luminescence in gold is still the subject of debate. Here, we present a systematic study in single gold nanospheres with diameters between 15 nm and 100 nm using peak laser intensities between 10 and 350 GW/cm2. A scattering confocal microscope incorporated in the setup was used to distinguish single particles from clusters. We observed that not all gold nanospheres have a detectable multiphoton luminescence signal; however, laser intensities above an exposure-time dependent threshold can alter such particles so that they do. In addition, we found that gold nanoparticles exposed to laser intensities above about 150 GW/cm2 can exhibit behavior reminiscent of the bleaching and blinking of conventional fluorophores.