Staff Scientist, Biological Nanostructures
Bruce Cohen graduated from Princeton University with an A.B. in Chemistry and earned his Ph.D. in Chemistry from University of California Berkeley, working with Daniel Koshland at the interface of photochemistry and biophysics. He moved to UCSF, where he was a Howard Hughes Medical Institute postdoctoral fellow in the neurobiology laboratory of Lily Y. Jan, developing novel fluorescent probes of protein function.
Optical microscopy is the primary means of studying complex living systems, enabling real-time analysis of individual cellular components at high spatial and temporal resolution. Inorganic nanocrystals have shown promise as transformative probes for these imaging studies, with exceptional optical properties not found in organic or protein-based probes. Along these lines, we are developing novel nanocrystals as biosensors and single-molecule probes, improving bioconjugation and targeting chemistries, and imaging live cells with these reagents. We aim to integrate the development of novel luminescent nanomaterials into multidisciplinary efforts to address significant biological questions of cell function.
Lanthanide-doped upconverting nanoparticles (UCNPs) sum the energies of 2 incident NIR photons to emit one at visible wavelengths, an unusual property unlike anything found in the cell. UCNPs have significant advantages over other luminescent reporters, including an absence of on–off blinking, single-molecule multiphoton NIR excitation at powers approaching those used for standard one-photon confocal imaging, no overlap with cellular autofluorescence, and no measurable photobleaching under prolonged single-particle excitation. Our synthetic efforts have established control over UCNP size to produce smaller nanocrystals more compatible with many imaging applications. Current efforts are aimed at optimizing single nanocrystal brightness for extended single-molecule imaging in live cells, and developing UCNPs capable of chemical sensing for studying cellular biochemistry.
Quantum Dots and UCNPs
Many nanocrystals exhibit brightness and stability far superior to conventional fluorophores, making them ideal as the bases for biosensing reagents. We have developed quantum dot and UCNP energy transfer-based systems for the sensitive detection of cellular chemistry and protein motions. Our current work focuses on developing bright, selective sensors for studying neuronal activity, including voltage sensors, sensors of neurotransmission in the brain, and ion sensors.
Improved bio-compatibility and targeting
Broadening the scope of nanocrystal surface conjugation chemistry is essential to expand their reach for imaging applications. High-quality UCNPs and quantum dots are synthesized in hydrophobic solvent and must be transferred to water and made biocompatible to have any utility as imaging probes or biosensors. An ongoing challenge in applying nanocrystals as probes for cellular imaging is improving their aqueous passivation and developing new reactions that work on nanocrystal surfaces. A growing area of research for us is development of new bioconjugation chemistries, including click reactions, protein self-ligation, and bioorthonganol reactions for immunotargeting.