A newly-approved project is aiming to develop new nanostructured biosensors to measure harmful substances in water. Researchers at the Helmholtz Centre for Environmental Research (UFZ) are using aptamers (oligonucleic acid or peptide molecules) as a basis for the biosensors because of their ability to identify and bind to target molecules with great precision. The term aptamer means "fitting pieces" (from the Latin word aptus, meaning to fit, and the Greek word meros, meaning piece). Aptamers consist of nucleic acids and have a three-dimensional structure that enables them to identify and bind certain target molecules. These binding abilities allow for tracing, detecting and measuring certain substances, which means they can be used, for example, in biosensors. Graphical model of the ethanolamine binding aptamer. The small ethanolamine molecule will be bound to a G-rich sequence region of the aptamer, which is able to form a quadruplex structure. (Image: Ronny Jesse) Biosensors are a simple, quick, low-cost way of taking measurements. At the heart of all biosensors there is a biologically active component. This bioreceptor has the ability to interact with its target substance, producing a signal in the process. Signal transducers in the sensor make the signal measurable and visible. Once measurement has taken place, the biosensor is returned to its original state. In other words, it can be regenerated. To design biosensors, scientists need suitable bioreceptors, such as aptamers, that can identify the target substance. The scientists first have to identify the right aptamer for a particular target molecule. Such target molecules can be very complex structures, like whole cells or organisms, or tiny molecules consisting of just a few atoms. Using an in vitro method, researchers search for and select the best binding partners for the target molecule from a huge pool of 10 million x 100 million nucleic acids with different sequences. This evolutionary selection method is called Systematic Evolution of Ligands by EXponential enrichment (Selex), and because it is performed in a ‘test tube’ the use of animals, plants or cell cultures is not required during the process. Beside ethical benefits, another advantage of this is the possibility of aptamer selection even for toxic substances. Once suitable aptamers for a target molecule have been found by the Selex process and their sequences have been defined, aptamers can be produced very accurately and reliably at any time by chemical synthesis. Accordingly, no biologically induced variations have to be taken into account during synthesis, as they would if natural systems were used. Biosensor principle: The biological receptor element recognizes the analyte in a complex sample. The receptor is in intimate contact to the transducer to transform the recognition signal into an electrical signal. The connected data processing unit directly generates the measuring value. (Image: Verena Helbig) In the UFZ's biosensor laboratory headed by Dr. Beate Strehlitz, Dr. Regina Stoltenburg has developed two different modifications of the Selex method. One of these is known as FluMag Selex. The 'Flu' stands for fluorescence and refers to the fact that a fluorescence molecule is added to the nucleic acids during the Selex procedure to make them visible. In this manner the molecules can always be found again and researchers can measure the enrichment of those which exhibit best binding and detecting abilities to the given target. The 'Mag' refers to magnetic beads. These are dust-mote-sized magnetic beads onto which the scientists 'stick' the even smaller target molecules to make them more manageable. Many other modifications of the Selex procedure have been developed by teams in research institutes around the world enabling aptamer selections for a wide range of different applications. In a new book titled, "Aptamers in Bioanalysis" (M. Mascini, Wiley-Interscience) Strehlitz and Stoltenburg describe the Selex procedure and its many variants in a review chapter. By use of the FluMag Selex procedure aptamers for a wide range of target molecules can be selected. Thus it has been used successfully to generate aptamers for a protein, a peptide and for ethanolamine, the smallest molecular aptamer target so far. The ethanolamine-binding aptamers have been patented. Additionally Dr. Christine Reinemann succeeded in selecting aptamers for soluble constituents of Penicillium expansum spores (mold spore extract). Based on this aptamers she hopes to develop a detection method for mold fungi within a project with third-party funding from Saxony's Office for Environment, Agriculture and Geology (LfULG). Together with PhD student Sören Linkorn and researchers from the Institute of Food Technology and Bioprocess Engineering at TU Dresden, Stoltenburg wants to select aptamers that can recognize pathogenic bacteria. These aptamers are supposed to develop a biosensor-based detection method for pathogens in water. This research is being conducted within the International Water Research Alliance Saxony (IWAS). The German Federal Ministry of Education and Research (BMBF) recently approved a joint project under leadership of Forschungszentrum Dresden-Rossendorf (FZD) and in collaboration with the University of Rostock, proaqua GmbH & Co. KG in Mainz and the UFZ. The project is part of the BIONA (Bionic Innovations for Sustainable Products and Technologies) research program. It will use the natural nanostructures of bacterial coat proteins to fix aptamers onto sensor surfaces in a controlled manner. It is hoped that the UFZ will develop aptamers that are capable of detecting certain organic substances, such as undesirable pharmaceutical residues, that enter the environment through wastewater. If the UFZ researchers are successful, biosensors will soon be able to help with the prompt identification of potential health risks, such as mold fungi in rooms or germs and pharmaceutical residues in water. For more information, visit: www.ufz.de