Probing the dynamic fluorescence properties of single water-soluble quantum dots

The influence of surface modifications, including carboxyl-, amine-, and mercaptoundecanoicacid-functionalized chemistries, on the dynamic fluorescence properties of single quantum dots (QDs) are probed using a combined confocal fluorescence microscopy and spectroscopy system. Water-soluble QDs with carboxyl- or amine-functionalized surfaces prepared through a linker arm show significant fluorescence intermittency and appear statistically identical. In contrast, water-soluble QDs functionalized via mercaptoundecanoicacid, an electron-donating thiol group, demonstrate suppressed fluorescence intermittency suggesting the photostability of water-soluble QDs is strongly dependant upon the method of surface functionalization. In addition, the long-term photostability of the mercaptoundecanoicacid-functionalized QD demonstrates an excitation-time-dependent blue-shift in the fluorescence emission spectra. Our measurement and analysis techniques promise a wealth of insight in elucidating the photophysics of bio-conjugated QDs and assessing and designing bio-conjugated QDs for biomedical applications requiring long-term photostabililty.