The combination of Raman spectroscopy and Optical Tweezers has been used to trap living cells and collect information
about their biochemical state. Cells can continue living in such traps for periods of hours, allowing acquisition of time
resolved Raman spectra. However no spatial information can be acquired as the cells continue to rotate and move in the
single beam trap.
Here we describe the development of Holographic Optical Tweezers (HOT) for the controlled movement of floating cells
in order to construct their Raman images. Instead of a single trap, rapidly programmable multiple trapping points can be
produced around the periphery of the cells to impede the rotational motion of the cell. By trapping and scanning the cell
using HOT relative to a fixed Raman exciting laser, a point by point image of the cell can be constructed. We use an
interactive program that permits us to position the trapping points relative to the live image feed we see from the
microscope, using point and click. To demonstrate the possibilities of this technique images are shown of floating Jurkat
Living cells initiate a stress response in order to survive environmentally stressful conditions. We monitored changes in the Raman spectra of an optically trapped <i>Saccharomyces cerevisiae</i> yeast cell under normal and hyperosmotic stress conditions. When the yeast cells were challenged with a high concentration of glucose so as to exert hyperosmotic stress, it was shown that two chemical substances - glycerol and ethanol - could be monitored in real time in a single cell. The volume of the detection area of our confocal microspectrometer is approximately 1 <i>fL</i>. The average quantities of detected glycerol and ethanol are about 300 <i>attomol</i> and 700 <i>attomol</i> respectively. This amounts to the detection of approximately 10<sup>8</sup> glycerol molecules and 4 X 10<sup>8</sup> ethanol molecules after 36 min of hyper osmotic stress. Besides this, we also optically trapped a single yeast cell for up to three hours under normal conditions and monitored the changes in the Raman spectra during the lag phase of its growth and the <i>G</i><sub>1</sub> phase of its cell cycle. During the lag phase the cell synthesises new proteins and the observed behavior of the peaks corresponding to these proteins as well as those of RNA served as a sensitive indicator of the adaptation of the cell to its changed environment. The changes observed in the Raman spectra of a trapped yeast cell in the late <i>G</i><sub>1</sub> phase or the beginning of S phase corresponded to the growth of a bud.
We report on ultrafast pump and probe studies of biological systems, in the form of polynucleotide and calf thymus DNA complexes. Molecules for study are bound to the polynucleotides and probed in the visible region to observe changes in the absorption over time. Various dipyridophenazine metal complexes are studied alone and complexed with DNA or synthetic polynucleotides to investigate changes occurring in their excited states upon interacting with nucleobases. Transient absorption measurements are performed pumping at 400nm and probing from 450-700nm with pulse duration of 400fs.
We report on ultrafast (200fs pulse durations) pump (267nm) and probe (430-700nm) studies of dipyrido -[3,2-a:2’,3’-c] phenazine (dppz) derivatives.These compounds are of interest as their metal polypyridyl complexes are used as photophysical probes for duplex DNA, where the dppz ligand can intercalate between the base pairs. Dppz, Me2dppz and F2dppz were studied to investigate changes occurring in their excited states over 1ns. In each case a transient decay and formation of a new species are seen in the 10 ®100ps region. The nature of these transients is discussed.
The process of excitation energy migration (EEM) in conjugated polymers, which is relevant both for light emitting diodes and laser applications is probed by doping the blue light emitting methyl-substituted ladder-type poly(para-phenylene) with small concentrations of a highly fluorescent (pi) -conjugated macromolecule. The experimentally attained temperature and concentration dependence of the steady state photoluminescence are modeled and discussed by means of a two step EEM process: (1) a thermally activated migration within the host and (2) transfer from the host to the guest. In particular we show that a Forster type mechanism alone cannot account for the experimental facts in such a guest host system. We have used the same materials to tune the emission color of organic- light-emitting diodes (OLEDs). Electroluminescence (EL)- current characteristics of the fabricated devices are presented. We found that the EEM process in OLEDs is more efficient for the EL process due to charge carrier trapping on the guest material compared to PL. For dopand concentrations as low as 1 w% one observes an increase of the photoluminescence as well as the electroluminescence quantum efficiency ((eta) <SUB>EL</SUB>) by approximately 50% with respect to the pure host polymer efficiencies. We realized a white light emitting diode with an external (eta) <SUB>EL</SUB> of up to 1.8% with the emission color being independent of the applied bias.