Light is an appealing stimulus for molecular transformations because it can be delivered to a sample with high spatiotemporal resolution and control of exposure energy and intensity. In conjunction with light stimulation, photoresponsive molecules can be used to produce molecular motors, alter material properties, unmask reactive moieties, and activate fluorescence in displays and super-resolution imaging experiments. Light functions as an input in all of these applications and as an output in the form of fluorescence in some of them. Many super-resolution microscopy techniques, for example, require fluorescent probes that can be stochastically switched between non-emissive and emissive states using a light signal. Other types of photoresponsive molecules such as photoremovable protecting groups (PPGs) use a light input to obtain a chemical output. PPGs have been used to protect specific functional groups in synthesis and to “mask” or “cage” biologically active molecules and pro-fluorophores. The Harbron Lab focuses on enhancing and controlling the reactivity of photoinduced molecular transformations.
The ubiquitous presence of light in our environment distinguishes it from chemical reagents that are present only when deliberately added to a reaction mixture. The ideal photoresponsive molecule reacts efficiently with light and yet not so efficiently that it responds to ambient light in an uncontrolled fashion. Achieving this balance in reactivity is one of the major challenges in the field, along with conferring water solubility and tuning light responsiveness to the visible to near-infrared region. These challenges can be addressed within individual molecular families by time-consuming structural modification. We seek a more modular approach that yields balanced reactivity and aqueous compatibility without laborious molecular tuning.
Pairing photoresponsive dyes with conjugated polymer nanoparticles (CPNs or Pdots) holds great promise for addressing these challenges. CPNs are exceptionally bright, photostable fluorophores that absorb and emit in the visible to near-infrared and are stably suspended in water. When doped with dyes, CPNs act as powerful light harvesters, funneling the energy of hundreds of conjugated polymer chromophores to a single dye via fluorescence resonance energy transfer (FRET). We recently determined that pairing CPNs’ highly efficient excitation and FRET processes with a dye’s inefficient photochemical reaction amplifies the reactivity of the dye in a controlled fashion. We term this concept "controlled amplification."
Our current research focuses on 3 projects involving controlled amplification:
1) The development of PPG-doped CPNs that release reagents with high efficiency and control in response to visible light stimulation.
2) Photophysical studies of dye-doped CPNs to determine optimal CPN properties for controlled amplification.
3)The creation of self-reporting CPNs that generate superoxide (O2•?) and report its relative concentration via fluorescence