After storing the sensor for four weeks at ambient conditions, no drastic change in signal intensity upon biosensing could be monitored. also examine the recent success of aptasensor technology and how these findings pave the way for the analysis of small molecules in POCT and other health-related applications. Finally, the current major limitations of aptamers are discussed, and possible approaches for overcoming these challenges are presented. species), ochratoxins (produced by and species) and Fusarium toxins (produced by over 50 species of Fusarium) [8,9]. Alongside these naturally occurring compounds, many small molecules are anthropogenic. Among these are polychlorinated biphenyls (PCBs), which have been widely used in BTF2 industrial applications. In the 1980s, the use of PCBs was mostly banned in all countries, since the compounds exhibit moderate toxic potential in animals and humans and cannot be degraded naturally [10,11]. There are also small molecules which act as pesticides and are used in 20-HETE agriculture. In this context the most prominent compound is Glyphosate which was extensively discussed in the media over the past year due to its potential carcinogenic effects [12,13]. Many pharmaceuticals are defined as small molecules. For drug application this could be extremely attractive, since small molecules can pass the bloodCbrain barrier due to their size [14,15,16]. Modern medicine is unimaginable without pharmaceuticals and they are used all around the world. They are ubiquitous and can also be found in the environment as 20-HETE a 20-HETE pollutant through human excretion. The most analyzed pharmaceuticals in the environment are antibiotics, followed by analgesics and hormones. In 713 analyzed water samples 20-HETE originating from all around the world, 631 were found to contain these pharmaceutical compounds [16]. Moreover, small molecules often play important roles in regulatory pathways in the human body. Vitamins, hormones, messenger molecules and cofactors are different groups of small molecules which regulate the metabolism. The ever-growing interest in monitoring these compounds in the environmentas well as in the human bodyled to a rising interest in the development of a variety of sensors for the detection of small molecules. Due to their ubiquitous nature and important functions, small-molecule targets are of high interest and extensive research has been carried out in this area in the past years. In the last ten years, 17,912 research articles dealing with small molecules were published (Web of Science, keyword small molecules, 07/31/20). Commonly, small molecules are detected via chromatographic techniques, such as high-pressure liquid chromatography (HPLC) and gas chromatography (GC). However, these methods are often expensive, require trained technicians and are time consuming. A promising alternative for detection and monitoring of small molecules is the use of biosensors. 3. Biosensors Biosensors are bioanalytical devices containing a biological element (such as, cells, antibodies, enzymes or oligonucleotides [17]) which selectively reacts/binds with the target of interest. The resulting biological recognition events are converted into a measurable signal by the transducer. The first use of a biosensor, reported by Clark and Lyons in 1962, was for the detection and quantitation of glucose concentration in blood [18]. Glucose oxidase was immobilized on a semi-permeable membrane, which encased an oxygen electrode, and a decrease in the measured oxygen concentration oxygen concentration was directly correlated to the glucose concentration [18]. In general, there are four groups of transduction methods which are usually used in biosensors. Optical Piezoelectric Calorimetric Electrochemical In optical transduction methods, recognition binding events are converted into measurable changes in various optical properties, such as fluorescence, refractive index and diffraction. Piezoelectric transducers are based on changes of the molecular weight upon target binding. Since enzyme catalyzed reactions are usually exothermic, these changes in heat can be monitored by calorimetric transducers. Finally, electrochemical transducers, such as the mentioned Clarke-biosensor, induce changes of current, impedance or ion concentrations [19]. 4. Aptasensors In 1990, three laboratories independently announced the establishment of a new in vitro selection method for nucleic acid sequences which bind their target in a highly selective manner. This technique, termed as SELEX (systematic evolution of ligands by exponential enrichment), resulted in the discovery of aptamers [1,2]. During the SELEX process, a library of random oligonucleotide sequences with up to 1018 individual nucleic acid sequences is exposed to the desired target. A small percentage of the librarys sequences binds to the target and subsequently separates. The latter are amplified via polymerase chain reaction (PCR) and the selection process is typically repeated for 8C15 rounds [20]. To increase selectivity, counter selection may be performed (addition of molecular structures similar to the target) and the sequences binding to the nontarget structures are removed. Due to their stringent selection process, aptamers offer a feasible alternative to antibodies 20-HETE and they offer several prominent advantages. Aptamers are chemically synthesized at.