Polarization Resolved Measurements of Individual DNA‐Stabilized Silver Clusters

Abstract

hybrid materials at truly nanoscale dimensions, enabling functionality at enormously high spatial densities. [ 11,12 ] Given their nanometer size scale, such clusterbased materials may also combine desirable properties usually associated with the molecular regime, such as high fl uorescence quantum yields and large Stokes shifts, with emergent near-fi eld interactions arising from the polarizability of free electron systems, as currently exploited in surface-enhanced Raman spectroscopy [ 13 ] (SERS), and plasmonic coupling schemes with metal nanoparticles. [ 12 ] Despite the numerous Ag:DNA [ 3,[14][15][16][17][18][19] studies, little is known about the mechanism by which fl uorescent excitation and emission occur. It was previously established in the literature on small metal clusters that the excitation energies are considerably up-shifted from the particle-in-box energies, due to Coulomb interactions that lead to collective, phased oscillation of the cluster's valence electrons. [ 20,21 ] Recent experimental works [ 22,23 ] indicate that fl uorescent Ag:DNA contains a neutral silver core that is surrounded by base-bound silver ions. This arrangement is analogous to the known structure of gold clusters that are stabilized by small organic ligand molecules, [24][25][26] which possess both transitions associated with the gold core, and transitions involving charge transfer between the gold core and the surrounding ligands plus their directly attached gold atoms. [ 25,26 ] For Ag:DNAs, the specifi c mode of cluster-DNA binding is unknown, but given the existence of a neutral silver core one might expect both core-centered transitions and charge transfer transitions between the core and baseattached silver ions.In the case of such silver core-ligand transitions, small variations in the specifi c conformation of the DNA might be expected to affect directions of transition dipole moments. Thus it is of great interest to study individual Ag:DNAs under conditions that minimize environmental fl uctuations, using techniques that can probe such orientational effects.In this paper, we present single-cluster optical studies of Ag:DNAs that investigate both their response as a function of the polarization of the excitation light and the polarization of the light they emit. Single particle polarization studies have previously been used to investigate individual fl uorescent molecules, providing insight into the orientation of the emitters in the surrounding medium [ 27,28 ] as well as photophysical events of single molecules, such as rotational jumps of a single Polarization-resolved excitation and emission measurements on individual 10-15 atom silver clusters formed on DNA are presented. The emission is highly linearly polarized, typically around 90% for all emitters, whereas the polarization dependence of the excitation strongly varies from emitter to emitter. These observations support the hypothesis that the luminescence arises from collective electron oscillations along rod-shaped silver clusters, whereas the excitation ...

Ultrafast coherence transfer in DNA-templated silver nanoclusters

“…1 , overlaid with the laser spectrum used in the ultrafast spectroscopy experiments. The absorption band is near-Gaussian, broad and largely featureless even at cryogenic temperatures, with a maximum at 770 nm, in agreement with earlier work 7 16 27 . No other bands with appreciable oscillator strength are observed in the visible range.…”

DNA-Protected Silver Clusters for Nanophotonics

“…This qualitative difference from conventional molecular behavior is another metal-like aspect of clusters, expected to arise from dephasing processes of collective excitations [ 56 , 57 ]. Our cryogenic microscopy also found a strong polarization dependence of individual Ag N -DNA, as expected for cluster rods ( Figure 4 g) [ 58 ].…”

“…The Ag° rod (gray) is shown attached to DNA bases via Ag + (blue) [ 14 ]; ( d – g ) Single Ag 15 -DNA at 2K (excited at 590 nm) [ 55 ]; ( d ) Wide field image shows bright spots from individual Ag 15 -DNA; ( e ) Observation of 1-step blinking between bright and dark states confirms emission from a single Ag 15 -DNA; ( f ) Spectra for individual Ag 15 -DNA remain spectrally broad at 2K, consistent with collective transitions [ 55 ]; ( g ) Dependence of emission on polarization for 2 individual Ag 15 -DNA. The high modulation index is expected for rod clusters [ 58 ].…”

Single‐Molecule Characterization of Near‐Infrared‐Emitting Silver Nanoclusters

“…In this case, we focus on the nearinfrared emitters using 633 nm as the excitation wavelength. Although a number of single molecule studies have been performed on AgNC to investigate specifi c photophysical parameters, [ 5,6,8,10,14,25,[27][28][29][30][31][32][33] we simultaneously measure several photophysical properties for each individual cluster, including decay times, emission spectra, and antibunching. This allows us to compare and correlate these photophysical properties within a sample to better understand the photophysical behavior and heterogeneity of these complex as-synthesized AgNC samples.…”