Thiolate-capped PbS nanocrystals in water: sensitivity to O2, pH and concentration, an alternate pathway for crystal growth and a top-down synthesis

Abstract

The mechanism of formation of water-soluble, thiolate-capped PbS nanocrystals has been investigated using dynamic light scattering. An evaluation of the potential for use of these species as optical contrast agents in biological imaging applications has also been performed in terms of the responses of the nanocrystals to O₂ and to perturbations in pH and concentration. In the synthesis of PbS colloids, addition of sodium sulfide to a mixture of lead acetate, thioglycerol and dithioglycerol in water at high pH gave not only the intended nanocrystals, but also a small concentration of micron-sized particles. Over time, both Ostwald ripening of the nanocrystals and ligand etching of these large particles were at play, with the breakdown of large particles being accelerated by light. Exposure of solutions containing PbS nanocrystals to air resulted in aggregation as a consequence of oxidation of coordinated thiolate to disulfide, but the nanocrystals did not catalyze this process. Lowering the pH of solutions containing the nanocrystals from 11.5 to 8.4 also brought about aggregation through protonation of the surface-bound thiolate ligands. A ten-fold dilution of PbS nanocrystals into H₂O at pH 11 resulted in size focusing, but no change in the average size of the particles, whereas a ten-fold dilution into H₂O containing excess thiolate at pH 11 caused degradation of the particles into very small clusters. The demonstrated chemical instability of thiolate-capped PbS nanocrystals to acid, O₂ and changes in concentration make it unlikely that they will serve as effective near infrared fluorophores in biological imaging applications. A “top-down” protocol for the synthesis of PbS nanocrystals is also reported: bulk PbS was dissolved in aqueous solutions containing thioglycerol and dithioglycerol at high pH to give particles whose mean hydrodynamic diameters could be tuned between 18 and 27 nm by varying the PbS ∶ dithioglycerol ratio.