One of the applications that we use our synthesized nanomaterials for is “self-assembly.” Future devices made of nanomaterials need each particle to perform independently, and for this, the particles should be organized in some rational, functional, and programmable way. For example, can nanomaterials be assembled with controlled interparticle distances, symmetry, and coordination number?
Conveniently, each of the nanomaterials are prepared with an encapsulating ligand, which forms a chemical ligand, or self-assembled monolayer (SAM) shell. While during synthesis this encapsulation is used to arrest nucleation and growth and provide stability, after synthesis this “surface chemistry” is used to provide a driving force for nanoparticles to ‘see each other’ and assemble.
Led by graduate students Josh Zylstra and Hyunjoo Han, we have developed a number of chemical routes to modify the surface of semiconductive quantum dots (QDs) and quantum rods (QRs). For example, we have used to small molecule, amino acid, histidine to chelate to the Zn+ -rich interface of CdSe/ZnS core/shell QDs. Using this approach, the qdots are phase transferred from their as-synthesized non-polar solvents to aqueous buffers. Interestingly, this ligand/monolayer exchange is extremely effective, and we used 1D and 2D NMR to investigate the binding mechanism, as reported recently in Langmuir.
Once in buffers, we next attach biomacromolecules, such as single-stranded DNA, peptides, and modified proteins to the interface of the QDs. Here, the idea is to endow the nanomaterials with the molecular recognition properties of the biomaterials. For instance, the base-pair encoding of DNA can be used to self-assemble ‘complementary’ nanomaterials. Our ability to both control the nanomaterial synthesis, as well as the surface chemistry, allows us to more directly attach the biomaterial to the QD interface. For instance, we have found that the histidine surface of the QDs described above can be place exchanged with thiolated ssDNA, and engineered proteins. Led by graduate student Hyunjoo Han, we have shown that ssDNA can be directly attached to CdSe/ZnS QDs, as well as polyhistidine tagged proteins, as reported recently in Chemistry of Materials. The interface was probed via spectroscopy using FRET, as described in the Energy Transfer section.
In addition to oligonucleotides (ssDNA), graduate student Rabeka Alam has additionally shown that engineered luciferase enzymes can be attached to quantum rods (QR) with tailorable loading. For instance, 1 protein can be bound to the surface, or 50 can, depending on rod length. This was recently reported in Nano Letters, and involved our collaborated Prof. Branchini at Connecticut College. These bio-nanomaterials are then used to study bioluminescence resonance energy transfer (BRET), as described in the Energy Transfer section.