Thermodynamics of nanocrystal–ligand binding through isothermal titration calorimetry

Manipulations of nanocrystal (NC) surfaces have propelled the applications of colloidal NCs across various fields such as bioimaging, catalysis, electronics, and sensing applications.

A Tale of Tails: Thermodynamics of CdSe Nanocrystal Surface Ligand Exchange

The surface ligands of semiconductor nanocrystals (NCs) are central for determining their properties and for their flexible implementation in diverse applications. Thus far, the thermodynamic characteristics of ligand exchange reactions were attained by indirect methods. Isothermal titration calorimetry is utilized to directly and independently measure both the equilibrium constant and the reaction enthalpy of a model ligand exchange reaction from oleate-capped CdSe NCs to a series of alkylthiols. Increased reaction exothermicity for longer chains, accompanied by a decrease in reaction entropy with an overall enthalpy-entropy compensation behavior is observed, explained by the length-dependent interchain interactions and the organization of the bound ligands on the NCs' surface. An increase in the spontaneity of the reaction with decreasing NC size is also revealed, due to their enhanced surface reactivity. This work provides a fundamental understanding of the physicochemical properties of the NC surface with implications for NC surface ligand design.

Lighting Up AIEgen Emission in Solution by Grafting onto Colloidal Nanocrystal Surfaces

Aggregation-induced emission luminogens (termed AIEgens) have emerged as an important class of light-emitting materials with many potential applications. As implied by their names, AIEgens typically exhibit strong emission in the aggregated form for which the intramolecular motions that dissipate excited-state energy are inhibited. Here we demonstrate that AIEgens, using carboxylated tetraphenylethylene (TPE-CA) as an example, can exhibit strong emission in the solution after being grafted onto colloidal nanocrystal (NC) surfaces. Using ultrafast transient absorption spectroscopy, we found that the intramolecular motions of TPE-CAs were strongly suppressed, with rates retarded by 2 orders of magnitude, when they were grafted onto ZnS NCs. As a result, the emission quantum yield (QY) of TPE-CA-NC hybrids in solution was also enhanced by 2 orders of magnitude compared to free TPE-CA molecules in solution. This opens a new avenue for constructing multifunctional hybrid materials capitalizing on the emission of AIEgens. In addition, this methodology can be extended to suppress aggregation-caused quenching (ACQ) of ACQphores in the solid-state form.

Probing Bioluminescence Resonance Energy Transfer in Quantum Rod-Luciferase Nanoconjugates

We describe the necessary design criteria to create highly efficient energy transfer conjugates containing luciferase enzymes derived from Photinus pyralis (Ppy) and semiconductor quantum rods (QRs) with rod-in-rod (r/r) microstructure. By fine-tuning the synthetic conditions, CdSe/CdS r/r-QRs were prepared with two different emission colors and three different aspect ratios (l/w) each. These were hybridized with blue, green, and red emitting Ppy, leading to a number of new BRET nanoconjugates. Measurements of the emission BRET ratio (BR) indicate that the resulting energy transfer is highly dependent on QR energy accepting properties, which include absorption, quantum yield, and optical anisotropy, as well as its morphological and topological properties, such as aspect ratio and defect concentration. The highest BR was found using r/r-QRs with lower l/w that were conjugated with red Ppy, which may be activating one of the anisotropic CdSe core energy levels. The role QR surface defects play on Ppy binding, and energy transfer was studied by growth of gold nanoparticles at the defects, which indicated that each QR set has different sites. The Ppy binding at those sites is suggested by the observed BRET red-shift as a function of Ppy-to-QR loading (L), where the lowest L results in highest efficiency and furthest shift.

Quantifying "Softness" of Organic Coatings on Gold Nanoparticles Using Correlated Small-Angle X-ray and Neutron Scattering

Small-angle X-ray and neutron scattering provide powerful tools to selectively characterize the inorganic and organic components of hybrid nanomaterials. Using hydrophobic gold nanoparticles coated with several commercial and dendritic thiols, the size of the organic layer on the gold particles is shown to increase from 1.2 to 4.1 nm. A comparison between solid-state diffraction from self-assembled lattices of nanoparticles and the solution data from neutron scattering suggests that engineering softness/deformability in nanoparticle coatings is less straightforward than simply increasing the organic size. The "dendritic effect" in which higher generations yield increasingly compact molecules explains changes in the deformability of organic ligand shells.