Sean Mathai
Room B69
Phone: 8344 8162
E-mail: s.mathai@pgrad.unimelb.edu.au
The main area of reasearch that I am undertaking is to do with a novel mode of cancer therapy. Photodynamic therapy (PDT) is offering great promise as an alternative in the treatment of cancer that is more efficient and effective than currently available therapies. PDT involves the use of a dye (photosensitiser) that is injected into the body. The dyes used are normally potential specific on uptake so will accumulate in the cancerous cells rather than the normal cells due to a higher membrane potential observed around cancerous cells. The dose of the dye can be changed so as to only “mark” the cancerous cells. Light irraditation on the body, normally with the use of a laser (which adds to the selectivity of this therapy) enables the dye to convert this light energy into a form that oxygen is able to use. Oxygen, which is readily found within the body, is able to use this energy to excite itself. It is thought that it is this excited oxygen that is responsible for carrying out the observed cytotoxic effect. Once the dye has carried out its role on the cancerous cells, it is important that the dye can be washed out of the body system so that it is unable to aid in further cell killing of the healthy cells.
Obviously, excited oxygen is crucial in this therapy, so one of the areas that I am investigating is determining the optimal environments in which oxygen can use the dyes’ energy most efficiently. The most direct method of doing this is to measure the luminesence that is given off by excited (or singlet) oxygen when it relaxes. This luminescence has been found to have a maximum intensity at 1272nm in solution. Therefore, to measure this radiation, we use a liquid nitrogen cooled infrared germanium detector. This set-up has enabled the collection of the phosphorescence signal from singlet oxygen. Quenching of the phosphorescent signal occurs in aqueous media via absorption of the infrared radiation by the O-H vibrational bonds in water. With the use of a highly sensitive Ge detector however, it is hoped that spectra within aqueous media can be obtained.
One of the factors that is limiting the introduction of PDT as a more prominent alternative is the minimal penetration of light through the skin. It is known that light of a longer wavelength, that is of a lower energy, is able to penetrate the skin the deepest. However, the light still needs to be of a sufficient energy to allow satisfactory excitation of the dye so that there is enough energy to be transfered to the oxygen. One way around this problem is to use two photon excitation. Two photons of longer wavelength light can be absorbed “instantaneously” by a sensitiser to be almost as if there was a single photon absorbed at half the wavelength, that is to say, two photons of 800nm light can be used to be absorbed by the 400nm band of the sensitiser. Two photon excitation results from a sufficiently high photon intensity (achievable with focussing a laser beam) making it possible for a third-order nonlinear process to occur- two photon excitation. Once the sensitiser is excited, the subsequent processes are thought to be the same as would occur had the sensitiser been excited with one photon.
To investigate how the environment effects the two photon absorption ability of certain sensitisers, we will use the Z-scan experiment. This experiment involves monitoring the change in the transmission through a sample of the sensitiser as the sample moves through a focused laser beam. Being sure that all the radiation is being collected at the detector, the resulting transmission vs distance of sample from the focal point relationship is able to yield the two photon cross section (ability to absorb two photons) of the sensitiser. The results from these experiments will then aid in the imaging of cells by two photon microscopy. Time resolved two photon fluorescence microscopy can be used to investigate the different environments within the cell to which the sensitisers accumalate, which will aid in the understanding of where the singlet oxygen is produced in PDT and possible ways in which the therapy could be improved by targetting sections of the cell where optimal singlet oxygen production may be achieved.
Figure 1. Set-up of the Z-scan experiment showing a detector monitoring the change in transmission as a sample on a micrometer stage moves through a focussed beam of a laser beam from a Ti:Sapph laser.
