Nanoparticle-Free Single Molecule Anti-Stokes Raman Spectroscopy

In the absence of large, plasmon-supporting nanoparticles, biocompatible dendrimer- and peptide-encapsulated few-atom Ag nanoclusters produce scaffold-specific single molecule (SM) Stokes and anti-Stokes Raman scattering. The strong SM vibrational signatures are enhanced by the ${\mathrm{A}\mathrm{g}}_n$ transitions in nanoparticle-free samples and cannot arise from plasmon enhancement. Characteristic SM-Raman intermittency is observed, with antibunching of the underlying ${\mathrm{A}\mathrm{g}}_n$ emission directly confirming the SM nature of the emissive species.

Figure 1

(a) Dark-field and (b) Stokes-shifted emission images from individual peptide-encapsulated 2–8 atom Ag NCs within the same field of view. (c) Emission spectrum of a single peptide-encapsulated ${\mathrm{A}\mathrm{g}}_n$ NC positioned between the monochromator slits [vertical white lines in (a) and (b)] excited at 514.5 nm, $30\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. (d) PAMAM-encapsulated ${\mathrm{A}\mathrm{g}}_n$ dark-field and (e) Stokes-shifted emission image of the same field of view. (f) Emission spectrum of a dendrimer-encapsulated Ag NC. Dark-field scattering arises only from glass imperfections and is completely uncorrelated with emissive features.

Figure 2

(a) Summed Raman spectra from 100 PAMAM- (○) and 100 peptide-encapsulated (▵) ${\mathrm{A}\mathrm{g}}_n$ NCs. Spectral subtractions remove the similar NC fluorescent background and yield characteristic (b) dendrimer and (c) peptide Raman lines when only positive peaks are retained for each subtraction order.

Figure 3

Single PAMAM-encapsulated ${\mathrm{A}\mathrm{g}}_n$ coincidence counts (inset) with 6-ps, 532-nm excitation, collected longer than 570 nm, and (main) with 200-fs, 445-nm excitation, collected longer than 590 nm. Shorter excitation pulses and larger spectral shifts from the excitation frequency enable NC antibunching to be more clearly observed at zero delay (*). The reduced peak only at zero delay indicates a sub-Poissonian source, with greatly reduced coincidences signifying emission of approximately one photon at a time—proof of single molecule emission.

Figure 4

(a) Stokes-shifted (right, positive shifts to lower energy) and anti-Stokes–shifted (left, negative shifts to higher energy) Raman spectra for an individual dendrimer-encapsulated Ag NC. AS-shifted frequencies match exactly with their Stokes counterparts. Lower (gray) Stokes-shifted spectrum shows fluorescence observed from the dendrimer-encapsulated NC when Raman scattering has blinked off. (b) With the same spectral axis as (a), three successive 10-s frames show intermittency of the AS emission. (c, inset) Expanded AS spectrum from (a).