We report a simple, economical method for generating water-soluble, biocompatible nanocrystals that are colloidally robust and have a small hydrodynamic diameter. The nanocrystal phase transfer technique utilizes a low molecular weight amphiphilic polymer that is formed via maleic anhydride coupling of poly(styrene-co-maleic anhydride) with either ethanolamine or Jeffamine M-1000 polyetheramine. The polymer encapsulated water-soluble nanocrystals exhibit the same optical spectra as those formed initially in organic solvents, preserve photoluminescence intensities, are colloidally stable over a wide pH range (pH 3-13), have a small hydrodynamic diameter, and exhibit low levels of nonspecific binding to cells.

Charged quantum dots provide an important platform for a range of emerging quantum technologies. Colloidal quantum dots in particular offer unique advantages for such applications (facile synthesis, manipulation and compatibility with a wide range of environments), especially if stable charged states can be harnessed in these materials. Here we engineer the CdSe nanocrystal core and shell structure to efficiently ionize at cryogenic temperatures, resulting in trion emission with a single sharp zero-phonon line and a mono exponential decay. Magneto-optical spectroscopy enables direct determination of electron and hole g-factors. Spin relaxation is observed in high fields, enabling unambiguous identification of the trion charge. Importantly, we show that spin flips are completely inhibited for Zeeman splittings below the low-energy bound for confined acoustic phonons. This reveals a characteristic unique to colloidal quantum dots that will promote the use of these versatile materials in challenging quantum technological applications.

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Analytical ultracentrifugation (AUC) was used to characterize the size distribution and surface chemistry of quantum dots (QDs). AUC was found to be highly sensitive to nanocrystal size, resolving nanocrystal sizes that differ by a single lattice plane. Sedimentation velocity data were used to calculate the ligand packing density at the crystal surface for different sized nanocrystals. Dihydrolipoic acid poly(ethylene glycol) was found to bind between 66 and 60% of the surface cadmium atoms for CdSe nanocrystals in the 1.54-2.59 nm radius size regime. The surface ligand chemistry was found to affect QD sedimentation, with larger ligands decreasing the sedimentation rate through an increase in particle volume and increase in frictional coefficient. Finally, AUC was used to detect and analyze protein association to QDs. Addition of bovine serum albumin (BSA) to the QD sample resulted in a reduced sedimentation rate, which may be attributed to an associated frictional drag. We calculated that one to two BSA molecules bind per QD with an associated frictional ratio of 1.2.