Direct observation of rapid discrete spectral dynamics in single colloidal CdSe-CdS core-shell quantum dots.

We measure the anomalous spectral diffusion of single colloidal quantum dots over eight temporal decades simultaneously by combining single-molecule spectroscopy and photon-correlation Fourier spectroscopy. Our technique distinguishes between discrete and continuous dynamics and directly reveals that the quasicontinuous spectral diffusion observed using conventional spectroscopy is composed of rapid, discrete spectral jumps. Despite their multiple time scales, these dynamics can be captured by a single mechanism whose parameters vary widely between dots and over time in individual dots.

Direct probe of spectral inhomogeneity reveals synthetic tunability of single-nanocrystal spectral linewidths.

The spectral linewidth of an ensemble of fluorescent emitters is dictated by the combination of single-emitter linewidths and sample inhomogeneity. For semiconductor nanocrystals, efforts to tune ensemble linewidths for optical applications have focused primarily on eliminating sample inhomogeneities, because conventional single-molecule methods cannot reliably build accurate ensemble-level statistics for single-particle linewidths. Photon-correlation Fourier spectroscopy in solution (S-PCFS) offers a unique approach to investigating single-nanocrystal spectra with large sample statistics and high signal-to-noise ratios, without user selection bias and at fast timescales.

Intraoperative detection and removal of microscopic residual sarcoma using wide-field imaging.

Background: The goal of limb-sparing surgery for a soft tissue sarcoma of the extremity is to remove all malignant cells while preserving limb function. After initial surgery, microscopic residual disease in the tumor bed will cause a local recurrence in approximately 33% of patients with sarcoma. To help identify these patients, the authors developed an in vivo imaging system to investigate the suitability of molecular imaging for intraoperative visualization.

Extracting spectral dynamics from single chromophores in solution.

Fluorescence spectroscopy of single chromophores immobilized on a substrate has provided much fundamental insight, yet the spectral line shapes and dynamics of single chromophores freely diffusing in solution have remained difficult or impossible to measure with conventional linear spectroscopies. Here, we demonstrate an interferometric technique for extracting time dependent single chromophore spectral correlations from intensity correlations in the interference pattern of an ensemble fluorescence spectrum. We apply our technique to solutions of colloidal quantum dots and explore the spectrum of single particles on short time scales not feasible with conventional fluorescence measurements.

Compact biocompatible quantum dots via RAFT-mediated synthesis of imidazole-based random copolymer ligand.

We present a new class of polymeric ligands for quantum dot (QD) water solubilization to yield biocompatible and derivatizable QDs with compact size (approximately 10-12 nm diameter), high quantum yields (>50%), excellent stability across a large pH range (pH 5-10.5), and low nonspecific binding. To address the fundamental problem of thiol instability in traditional ligand exchange systems, the polymers here employ a stable multidentate imidazole binding motif to the QD surface.

Narrow-band absorption-enhanced quantum dot/J-aggregate conjugates.

We report narrow-band absorption enhancement of semiconductor nanocrystals via Förster resonance energy transfer from cyanine J-aggregates. These J-aggregated dyes associate electrostatically with short quantum-dot (QD) surface ligands in solution. Energy transfer efficiencies approach unity for this light sensitization and result in a 5-fold enhancement in the QD excitation near the J-aggregate absorption maximum.

Monovalent, reduced-size quantum dots for imaging receptors on living cells.

We describe a method to generate monovalent quantum dots (QDs) using agarose gel electrophoresis. We passivated QDs with a carboxy-terminated polyethylene-glycol ligand, yielding particles with half the diameter of commercial QDs, which we conjugated to a single copy of a high-affinity targeting moiety (monovalent streptavidin or antibody to carcinoembryonic antigen) to label cell-surface proteins. The small size improved access of QD-labeled glutamate receptors to neuronal synapses, and monovalency prevented EphA3 tyrosine kinase activation.