Electrospinning for High Performance Sensors

A strategy to improve the sensing features of chemical sensors is to increase the specific surface of the interacting material: the higher the specific surface area of a sensing material the higher its sensor ability to interact, such as biological sensing structures do. Indeed, in nature, surfaces and receptors are in essence a macroscopical extension of the molecular structure of a material, where the properties of surfaces and receptors are directly related to their structure down to the molecular level. Similar structures can be reconstructed 'in vitro' for sensor and detecting systems of exceptional sensitivity and remarkable specificity. As a consequence, many techniques have been used to augment the surface of sensing layers with fine structures, especially to form controlled nanostructures, as it happens in natural systems, taking advantage of the large specific area of nanostructured materials. Accordingly nanostructured sensors, when compared to the conventional ones, showed desired properties like faster adsorption and minimized bulk effects (i.e. long diffusion-desorption time, analyte entrapment, etc.). From recent literature, electrospinning has been confirmed to be one of the best candidates among the various nanotechnologies for designing and developing smart and ultra-sensitive sensing systems, both for the uniqueness of the resulting nanostructures and for production rate and cost. Parameters like the extremely rapid formation of the nanofibres structure, which occurs on a millisecond scale, the large coverage in continuous mode, the easy tuning of size and shape, and the nanofibres assembling in situ have raised great scientific interest, confirmed from the number of publications over the last 10 years and reported in the following figure. Since the dimension of fibres is roughly comparable to that of the interacting molecules, people may exploit the tiny size with some size effects, such as quantization, and the singlemolecule sensitivity. About the morphology of the fibres, it depends on the solution properties (system parameters), process conditions (operational parameters) and environmental conditions. The resulting aligned or non-woven nanofibres, arranged in 2D- or 3D-fibrous structures with tuneable porosity and high specific surface area, can be placed directly onto suitable transducers, often without further expensive refinement. Developments of electrospun nanomaterials have allowed chances to fabricate more efficient interfaces with electronic components also due to their compatibility with semiconductor processes. Since electrospinning is a technique capable of continuously creating polymeric fibres, i.e. with no interruption during the process, it sounds appropriate for the production of huge quantities of nanofibres (micron size yarns consisting of nanofibres can be produced at high rates, up to 70 m/min), then also potentially appealing to the sensor market. Electrospinning apparatus, using multiple nozzles, as well as needleless electrospinning processes using a range of spinnerets, is able to increase further the production rate and to control jet formation, jet acceleration and the collection of nanostructures. The further opportunity to customise and functionalise these micro-nanofibres on a large-scale enables the electrospinning technique to match a wide range of requirements for specific sensing applications, giving a benefit over other methods commonly used for the production of micro-nanostructures. Another advantage of this top-down nano-manufacturing process is the relatively low cost of the equipment and its functioning compared to that of most bottom-up methods. Despite the increased interest in sensors from scientists and the industrial potentials of the technology, the percentage of