Towards biocompatible NIR-II nanoprobes - transfer of hydrophobic Ag2S quantum dots to aqueous solutions using phase transfer catalysed hydrolysis of poly(maleic anhydride-alt-1-octadecene).

Development of protocols for transferring of hydrophobic quantum dots (QDs) into aqueous solution is of special importance for their biomedical applications. Particularly, hydrophilization of Ag2S without quenching of photoluminescence is a great challenge. Application of standard protocol for amphiphilic polymer coating of hydrophobic Ag2S nanocrystals failed, whereas ligand exchange and direct synthesis of Ag2S in aqueous solution leads to poorly emitting materials. In this paper we present the facile method for transferring of hydrophobic Ag2S QDs into aqueous solution employing the phase transfer catalysed hydrolysis of a commercially available polymer - poly(maleic anhydride-alt-1-octadecene), (PMAO) in the presence of tetramethylammonium hydroxide. Because the original surface ligands are retained and we do not use solvents (like THF) detrimental for Ag2S emission, the modification does not deteriorate photoluminescence properties and quantum yield of modified QDs in aqueous solution reaches 60% of that for hydrophobic QDs in chloroform. The hydrodynamic diameter of modified, water soluble Ag2S QDs is about 10 nm and is only slightly larger than their original size. Moreover, the polymer coated nanocrystals are not aggregated and are stable for months. Surface characterization of QDs by NMR and IR spectroscopy indicates that polymer chains intercalate alkyl chains of dodecanethiol (DDT) bound to the surface of Ag2S. The cytotoxicity studies indicate that the presented method should be regarded as a notable progress towards the biocompatible Ag2S NIR-II emitting nanoprobes.