Detection and Spectroscopy of Gold Nanoparticles Using Supercontinuum White Light Confocal Microscopy

We combine confocal microscopy using supercontinuum laser illumination and an interferometric detection technique to identify single nanoparticles of diameter below 10 nm. Spectral analysis of the signal allows us to record the plasmon resonance of a single nanoparticle. Our results hold great promise for fundamental studies of the optical properties of single metal clusters and for their use in biophysical applications.

Figure 1

(a) A spectrum of the supercontinuum white-light laser source. (b) Schematics of the experimental arrangement illustrating the interaction between a gold nanoparticle and the optical field. (c) A white-light confocal scan of a sample containing 60 nm particles. (d) Cross section marked in (c), showing that the supercontinuum light beam can be tightly focused.

Figure 2

Normalized plasmon spectra of single gold nanoparticles of various diameters (as specified by the manufacturer): (a) $60\pm 12\mathrm{n}\mathrm{m}$, (b) $31\pm 6\mathrm{n}\mathrm{m}$, (c) $20\pm 4\mathrm{n}\mathrm{m}$, (d) $10.3\pm 1.0\mathrm{n}\mathrm{m}$, and (e) $5.4\pm 0.8\mathrm{n}\mathrm{m}$. The symbols show the experimental data while the solid curves are fits that consider spheroidal particles. The fit outcome for the ratio of the spheroid axes $a:b$ is shown in the legend of each figure.

Figure 3

Normalized signal $\sigma$ from nanoparticles of various sizes averaged over the wavelength region of 480–700 nm. Because of the large signal dynamics, we have plotted the positive values on a logarithmic axis and the negative values linearly. The horizontal error bars show the uncertainty in the particle diameter as specified by the manufacturer. The vertical bars were determined from the spread in $\sigma$ measured for several tens of particles. See the text for details.

Figure 4

(a) A raw confocal image of a sample containing nominal 5 nm nanoparticles. (b) An AFM image of the same area. Three arrows in (a) and (b) point to a particle that is missing in the optical image and to two others that give very weak signals. An area of small aggregation or dirt is also seen in both images (a) and (b). (c) A histogram of the signal strength from more than 110 particles with a nominal diameter of 5 nm. (d) Cross sections marked in (a) show the variation of SNR.