No abstract

For many years, investigators have sought after high-resolution fluorescence imaging in centimeter-deep tissue because many interesting in vivo phenomena—such as the presence of immune system cells, tumor angiogenesis, and metastasis—may be located deep in tissue. Previously, we developed a new imaging technique to achieve high spatial resolution in sub-centimeter deep tissue phantoms named continuous-wave ultrasound-switchable fluorescence (CW-USF). The principle is to use a focused ultrasound wave to externally and locally switch on and off the fluorophore emission from a small volume (close to ultrasound focal volume). By making improvements in three aspects of this technique: excellent near-infrared USF contrast agents, a sensitive frequency-domain USF imaging system, and an effective signal processing algorithm, for the first time this study has achieved high spatial resolution (~ 900 μm) in 3-centimeter-deep tissue phantoms with high signal-to-noise ratio (SNR) and high sensitivity (3.4 picomoles of fluorophore in a volume of 68 nanoliters can be detected). We have achieved these results in both tissue-mimic phantoms and porcine muscle tissues. We have also demonstrated multi-color USF to image and distinguish two fluorophores with different wavelengths, which might be very useful for simultaneously imaging of multiple targets and observing their interactions in the future. This work has opened the door for future studies of high-resolution centimeter-deep tissue fluorescence imaging.

A donor-acceptor dyad comprised of BF2-chelated dipyrromethene (BDP or BODIPY) and fullerene connected with a pyrrole ring spacer, 1 has been newly synthesized and characterized. Due to -carbon substitution and extended conjugation offered by the pyrrole ring, the optical absorbance and emission spectra of BDP macrocycle was found to be red-shifted significantly.Electrochemical studies provided information on the redox potentials while computational studies performed at the B3LYP/6-31G* level yielded an optimized geometry of the dyad that was close to that reported earlier for a BDP-C60 dyad covalently connected through the central boron atom, 2. The HOMO of the dyad was found to be on the BDP macrocycle, extended over the pyrrole bridging group, a property that is expected to facilitate electronic communication between the BDP and fullerene entities. The established energy level diagram using spectral, redox and optimized structural results predicted possibility of photoinduced electron transfer in both benzonitrile and toluene, representing polar and nonpolar solvents. However, such energy diagram suggested different routes for the charge recombination processes, that is, direct relaxation of the radical ion-pair in polar solvent while populating the triplet level of the sensitizer ( 3 BDP* or 3 C60*) in nonpolar solvent. Proof for charge separation and solvent dependent charge recombination processes were established from studies involving femto-and nanosecond pump-probe spectroscopy. The measured rate of charge separation, kCS for 1 was higher in both solvents compared to the earlier reported values for 2 due to electronically well-communicating pyrrole spacer. The charge recombination in toluene populated 3 BDP* as an intermediate step while in benzonitrile it yielded directly ground state of the dyad. The present findings reveal the significance of pyrrole spacer between the donor and acceptor to facilitate charge separation and solvent polarity dependent charge recombination processes.