Uwe Kaiser from Philipps-University Marburg talking about tuning dye fluorescence lifetimes with doped semiconductor quantum dots (13.6 MB MP3)

“The problem with dye molecules is that the emission band is very broad, so the most you can distinguish spectrally is three, maybe four, different dyes,” says Uwe Kaiser, a postdoctoral researcher at Philipps-University Marburg in Germany, and first author of this latest research. “With the doped quantum dot you can get different temporal behaviour, and in the long term you can imagine a combination of the two, with spectral and temporal multiplexing to detect maybe eight different dye molecules at once.”

More research and development is needed before such sophisticated sensing capabilities are realised. However, this latest work demonstrates substantial fluorescence lifetime extensions of over six orders of magnitude, marking a significant step towards this goal.

The collaboration of researchers from Phillips-University Marburg, CIC biomaGUNE in Spain and CONICET in Argentina synthesised quantum dots with a cadmium sulphide core and a doped zinc sulphide shell. Previous work had demonstrated a minor increase in lifetime within the nanosecond time regime by conjugating the dye to just the semiconductor quantum dot. However, the long radiative lifetime of the manganese states and an efficient energy transfer to the dye allow a much greater extension of the lifetime.

In addition, the energy transfer processes are sensitive to the distance between the manganese ions in the shell and the cadmium sulphide core. As a result, manipulating the core-shell structure could potentially tune the lifetime of the fluorophore.

“Next we want to make quantitative measurements - take the dye, the dye with the quantum dot and the dye with the doped quantum dot, put them all together and see if we can get the concentration of each by time-resolved measurements,” says Kaiser. If they can then use dye molecules with different functionalities - such as sensitivity to calcium ions and sensitivity to potassium ions - they will have a powerful tool for quantitative detection of multiple biomarkers in one measurement. “We hope to promote the collaboration of biological and physical research with this work,” adds Kaiser.

Too much of a good thing

Spectral overlap is key for efficient energy transfer, but the researchers found that too much is a hindrance. “If the overlap is too good, both donor and acceptor decay very fast and then you haven’t won anything,” says Kaiser.

In addition the time-resolved measurements revealed the interplay between the three energy states, highlighting the dual role of the manganese ions as both acceptors and donors. This came as a surprise to Kaiser and his colleagues. While energy transfer from the quantum dot to manganese ions is far less efficient than transfer from the manganese ions to the dye, it will affect how the system responds to an increase in manganese ion concentrations.

Ever elusive ions

The low concentrations of ions pose one of the main challenges in producing the doped core-shell quantum dots with any control, since transmission electron microscopy does not detect them. Photoluminescence spectra can also be misleading because the emission of the manganese ions matches that of the inevitable residual defects in the quantum-dot shell.

Since the energy transfer processes are far less efficient when the manganese ions are on the surface, the time-resolved measurements give a good hint as to whether they have been incorporated in the shell. As Kaiser points out, “If the lifetime is milliseconds, you can be quite sure.”

Full details are reported in Nanotechnology 23 495201.