Another application that we use our nanomaterials for is that of energy transfer. In our case, this is both resonance energy transfer (RET), as well as electron (e-) and hole (h+) transfer. In each of these cases, we precisely design and synthesize the nanomaterials to order. For RET applications, we utilize the developments in Self-Assembly, to bring two nanomaterials, or nano-bio conjugates, in close proximity, and measure energy transfer via Forster Resonance Energy Transfer (FRET), and Bioluminescence Resonance Energy Transfer (BRET). For e- and h+ transport, the nanomaterials are brought into delicate contact with titanium dioxide (TiO2) and conductive polymers. These topics are discussed below.
When two fluorescent molecules, or when two nanomaterials, come in close proximity to each other (1-10 nm), their dipole moments begin to interact. This is especially the case when one of the molecule is “excited” and an e- is at a previously unoccupied energy level. The dipole-dipole interaction between the excited “donor” and an “acceptor” in close proximity leads to the non-radiative resonance energy transfer, commonly shown as FRET. The energy transfer is related to the dipole interactions, which is actually the extent of energy levels involved. Conveniently, these energy levels can be easily diagnosed by measuring the spectral overlap of the materials. A key to effective energy transfer is a high overlap (manifested as a high overlap integral, J ), and donor-to-acceptor distances (r ) that are comparable to the Forster distance (R0). Thus, we use synthesis to tailor J, and self-assembly to tailor and program r.
Led by graduate students Hyunjoo Han, Rabeka Alam, and Josh Zylstra we have made a lot of recent progress on using FRET to analyze our bio interfaces, as well as to optimize RET to record levels. In our recent Chemistry of Materials report we used fluorescently tagged ssDNA as acceptor, and used FRET analysis to determine the surface coverage and structure of ssDNA bound to a QD interface after histidine phase transfer. As discussed in the Self-Assembly section. Future studies involve using these FRET interactions to study the energy transfer between multiple QDs in different architectures and symmetries.
Traditionally, the quantum dot (QD) or quantum rod (QR) is used as the energy donor, due to the ease of excitation. A fluorescent dye or fluorescent protein is used as the acceptor, due in large part to the narrower excitation band width and shorter lifetimes. However, we have recently showed that QRs are ideal energy acceptors when bound to firefly luciferase enzymes (PPy). In these cases, the PPy undergo bioluminescence and emit colors typical of fireflys (yellow-green) when bound with the substrate luciferin. However when the PPy are bound to the QR via the approaches developed in the Self-Assembly section, the energy from PPy bioluminescent is transferred to the QR donor, which subsequently emits light in the orange, red, and far-red colors. As shown by recently by graduate student Rabeka Alam in Nano Letters, energy transfer efficiencies of up to 44 are now possible when PPy is linked to QR with rod-in-rod morphology. Ongoing studies are underway, and focused on further understanding of the energy transfer as it related to rod morphology.
In addition to resonance energy transfer, we are also exploring charge transfer (e- and h+) in a number of ways. Using QDs prepared via the MWI mediated hydrothermal method discussed in the Synthesis section, we are integrating them into dye-sensitized solar cells (DSSCs) like morphologies using TiO2. Here, both the QD and TiO2 are prepared in a similar manner under similar conditions, which may aid in the preparation of QD/TiO2 hybrids that will be suitable for both fundamental studies related to e- transfer as well as device fabrication.
Finally, we recently prepared custom built CdSe/ZnS QDs with tailored ZnS shell thickness and surface chemistry for a novel study related to h+ transfer in QD-Polymer systems. Many thin film solar cell devices are designed to incorporate soft components, such as conductive polymers, and quantum dots. While electron (e-) transfer is often studied, we recently contributed to a study that focused on hole (h+) transport from a QD to a hole-conducting polymer. Led by graduate student Corey Hine and Prof. Maye at SU with collaborators at Brookhaven National Laboratory, the h+ tunneling through the ZnS shell was studied for the first time by state of the art single molecule spectroscopy studies. These studies were recently reported in ACS Nano.