Functional assemblies of nanocrystal quantum dots on microtubule scaffolds.

Abstract

We assembled semiconductor nanocrystal quantum dots (NQDs) using microtubule (MT) fibers as nanoscale scaffolds and characterized structure and function of the assemblies. MTs were chosen as the biotemplate as these biomolecules, in conjunction with kinesin motor proteins, offer the potential for dynamic transport of nanoscale cargo. Ultimately, the ability to control and direct the <italic>reconfigurable</italic> assembly of nanomaterials from the nano- to the macro-scales will likely depend on the degree to which the specific interactions between artificial inorganic/organic nano-components and natural, biological components are understood. Thus, the work here addressed two fundamental issues key to this understanding: (1) The extent to which NQD stability and photophysical properties are affected by being rendered biocompatible and (2) The impact on biofunction when biomolecules are coupled with artificial nanomaterials. These issues were addressed by first investigating several strategies for transforming hydrophobic nanocrystals into water-soluble, biocompatible materials. The resulting particles were compared in terms of their optical properties (e.g., retention of high emission quantum yields) and chemical stability (using a novel fluorescence resonance energy transfer, FRET, method to study particle aggregation). Second, a previously described strategy for rendering nanocrystals water soluble by exchanging as-prepared surface ligands with bifunctional thiol ligands was studied by investigating the effect of thiols on the optical properties of NQDs as a function of time, concentration, pH and moiety. Thiolate anions were found to both passivate existing electron traps (enhancing emission) and introduce new hole traps (decreasing emission). Third, NQDs were assembled onto MT supports by way of streptavidin-biotin and electrostatic interactions. Interdot communication through interparticle energy transfer was then used to determine the nanoscale structure of the assemblies <italic>in situ</italic>, providing distinct advantages compared to traditional characterization approaches (e.g., AFM). Finally, successful MT polymerization from nanocrystal-tubulin subunits was achieved by understanding the structure-function relationships in tubulin as impacted by NQD conjugation. Specifically, a critical limit to NQD-tubulin ratios was established that ultimately permitted robust polymerization. In addition to the fundamental understanding gained, this progress represents a novel approach to a specific, dynamic assembly process.