Semiconductor nanocrystals, also known as quantum dots (QDs) have attracted a lot of interest over the past few years [1][2][3][4][5] particularly because of their unique physical properties, which differ from their bulk counterparts. Compared with traditional organic dyes, QDs display several advantages as efficient biolabels, as they emit light at a variety of precise wavelengths depending on their size. They also have long fluorescence lifetimes, possess high photobleaching thresholds, [6,7] and have a larger Stokes shift between the absorption and luminescence bands. As a consequence, these materials have been used in numerous biological studies and have been efficiently conjugated to a wide variety of biomolecules in an effort to enhance their biolabelling capabilities and to investigate the mechanism of their cellular interaction. For example, enzymes, [8] antibodies, [9] peptides, [10] and oligonucleotides [11,12] have all been bound to the surface of QDs and investigated in an assortment of biological buffers.Synthetic molecule-particle conjugates have also been developed such as folic acid coated magnetic nanoparticles [13] and modified serotonin [14,15] bound to CdSe QDs. These strategies generally involve replacement of organic stabilising ligands with thiol-containing molecules via an exchange mechanism in an effort to improve aqueous solubility [16,17] or biological activity. In relation to these studies, our aims were to conjugate the nonsteroidal anti-inflammatory drug (NSAID) naproxen to aqueous CdTe QDs and investigate their photophysical properties and biological behaviour. We chose this particular NSAID primarily because it possesses well-defined anti-inflammatory properties [18] and because it displays a luminescence emission that does not overlap with that of the CdTe QDs. For example, emission is observed at 356 nm (l ex = 317 nm) for the naproxen moiety in buffer solution.Thioglycolic acid (TGA) capped CdTe QDs were prepared according to published procedures. [19,20] Aqueous conjugation of naproxen to TGA-functionalised QDs required the use of a water-soluble NSAID bearing a free amine functionality. To fulfill these demands, we decided to improve the solubility of naproxen by linking it to a small water-soluble diamine ethyl-A C H T U N G T R E N N U N G enedioxy linker [16,17] via a short synthetic route (Scheme 1, see also Supporting Information). These reactions led to the desired derivative with imparted water solubility on the naproxen molecule while also providing an amino group for reaction. Further conjugation of the naproxen derivative to the carboxylic acid functionality of the thiol capping groups on the QD surface was performed using an 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) coupling reaction (Scheme 2) with a known concentration of QDs (QD diameter is~3 nm). [21] This reaction was undertaken in borate buffer at pH 7.5 to prevent aggregation of the QDs that occurs at low pH values (< 4). [22] In our experiments, QDs (l em = 566 nm) were prepared by using an optimis...
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