Fluorescence imaging is a versatile tool in biomedicine e.g. in the investigation of diseased cells and tumour tissues. Dr. Michael Schäferling from the Biophotonics Division develops modified nanoprobes that make the procedure relatively gentle and efficient.
Fluorescent imaging, like contrast agents, enables colour identification by fluorescent substances whose distribution in the organism can be visualised. For example, the distribution of cancer tumours in biological tissues can be detected three-dimensionally. Methods such as magnetic resonance tomography (MRT) have already been used for tumour tissue detection. Fluorescence imaging is more suitable for visualising and investigating changes in various chemical parameters such as pH, oxygen supply and ion concentrations (e.g. Ca2+) in diseased cells and for drawing conclusions about cell behaviour and changes in cell metabolism.
Dyes react with chemical parameters and visualise them
The imaging method uses fluorescence markers to selectively stain certain biomolecules such as proteins or DNA, cell components or tissues. In addition, fluorescent dyes (fluorescence indicators) that react sensitively to their chemical environment are needed to determine and control vital chemical parameters such as oxygen saturation or pH. They are necessary since the parameters investigated such as oxygen concentration and pH fail to show any inherent colour or self-fluorescence when irradiated by visible-range light. The monitoring of oxygen and pH in cells and tissues is an important tool in tumour research since tumour cells are usually hypoxic i.e. they are in a state of oxygen deficiency and pH is low.
Research within a scholarship sponsored by the German Research Foundation
In BAM’s Biophotonics Division, Dr. Michael Schäferling, a Heisenberg Research Fellow of the German Research Foundation (DFG), deals with developing fluorescent molecular probes and nanosamples for chemical sensors and bioanalytics – with the primary aim of determining oxygen (or pressure), pH, temperature, anions, H202 and temperature. In the nanoprobes, fluorescent indicator dyes are linked with carrier particles which themselves measure only between 10 and 100 nanometres in size. This results in special properties such as increased signal intensity and reduced susceptibility to interference. The nanoprobes are therefore more sensitive and can provide more detailed information. Polymer nanosensors for the determination of oxygen in tumour cells have already been produced and successfully tested in vivo in cooperation with Dr. Ute Resch-Genger, Head of the Biophotonics Division.
Optimised nanoprobes enable relatively gentle and efficient fluorescence imaging
Dr. Michael Schäferling's current work focuses on inorganic nanocrystals with ions of rare earths (e.g. ytterbium or erbium) incorporated into them. These can be produced to have a diameter of approx. 20-25 nm. The nanoparticles exhibit upconversion luminescence due to the incorporation of two different rare earth ions, which means that they emit visible light when excited by near-infrared (NIR) light. In contrast to classical fluorescent dyes, these photon-upconverting nanoparticles emit energy-richer light than they require for excitation – an effect also called anti-Stokes shift.
This phenomenon offers new and promising options for imaging applications: since NIR light is not absorbed by biological materials, it can penetrate deeper into the tissues. Tumour cells can thus become also clearly visible in deeper tissues. Moreover, NIR light irradiation is much gentler for living cells and tissues than visible or ultraviolet light.
In the three-year research project "Self-referenced photon-upconverting nanosensors for imaging applications" funded by the DFG, these nanomaterials will be functionalised and combined with sensitive dyes to be used for imaging determination of oxygen, pH or Ca2+ in biological samples.
BAM – part of an international research network
The international basic and applied research being carried out into upconverting nanoparticles has been coordinated throughout Europe under the COST (European Cooperation in Science and Technology) initiative "European Upconversion Network", which also includes BAM’s Biophotonics Division and Dr. Michael Schäferling. A nanoprobe that enables imaging of pH distribution in different areas of living cells has already been presented within such an international cooperation. Options for amplifying luminescence of upconversion nanoparticles are currently being investigated within a European M-era.Net network funded by DFG in cooperation with Dr. Ute Resch-Genger and the Biophotonics Division. BAM plays a key role in the design and spectroscopic characterisation of the particles. The final aim is to make fluorescence imaging even more contrast-rich and gentle for living cells and tissues by penetrating deeper into the tissue using NIR light. This will provide otherwise inaccessible chemical information from the interior of cells or diseased tissues such as tumours.