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 the number of patents about electrospinning for sensor was still about 0.3 % (2011, source EspaceNET) of all the patents related to electrospinning. An apparent lack of interest of manufacturers, of both chemical sensors and electrospun fibrous products, in this application is noticed. Some of the challenges are supposed to be related to the difficulty in exploring the application without significant funding, the perceived danger of working with nanofibres, which is worsened by the lack of clearly defined terms, the lack of control over material supply as a result of the relatively small volume requirements for electrospinning compared with other industries and the lack of involvement of manufacturers in nanofibre development projects until the final stages of laboratory-scale optimisation. However, nowadays the technology sounds mature. It is progressing going beyond the laboratory and towards industrial setting, with the rise of start-ups and the interest of large multinationals. Increasing dissemination and involving even further industries urgently need to proceed into the next step, that is the actual manufacturing of high performing electrospinning based sensors for answering to real requirements. In this book, thanks to the significant cooperation of scientists and manufacturers involved in a European Concerted Research Action, designed as COST MP1206 and entitled "Electrospun Nano-fibres for bio inspired composite materials and innovative industrial applications", and after the successful participation in the first "Electrospinning for High Performance Sensors" workshop held in 2014 in Rome, we collected some recent progress and predominant developments of several electrospinning sensing approaches, including gravimetric, resistive, photoelectric, optical, electrochemical sensors and biosensors. Thus the present book is the first collection of chapters focusing on the potentials of electrospinning in several specific sensing applications, through the usage of several transduction mechanisms based on organic, inorganic and composite materials. Their crucial role in quickly revealing molecules in traces, which are often the urgent needs in health, security, environment and food monitoring, will be described in detail. Finally, the situation of the electrospinning industry is discussed, including recent advances in commercial-scale electrospinning. The opening shortly describes the aim of the COST Action MP1206 that currently is the greatest platform of European scientists, young researchers and industrials for fabricating, investigating and sharing the potentials of such a technology. This Action is joined too by scientists, institutions and companies from COST Near Neighbour Countries and International Partner Countries thereby increasing the chances of succeeding. The book starts with a detailed description about the manufacturing of facile and ultrasensitive sensors based on synergic layers of electrospun polymer fibres added with electrospunnanonets and investigated through gravimetric and optical transducers. Then, an accurate description of the polymer dynamics during the ejection process follows, trying to analyze, through the image analysis and mathematical models, the distribution and orientation of the charged molecules in the various segments of the fibres, ranging between the Taylor cone and the grounded substrate. Therefore the unique features of chemical sensors based on nanocomposite fibres are described, where the performance of conventional to electrospun sensors have been compared to detect gases in traces. The conductivity of the fibres has been explored too, both in individual and crossed nanofibres, supposing that the improved gas sensing performance were determined by diode effect. The photoconductivity of ceramic fibres to develop gas sensors operating at room temperature represents a further chance to get low cost sensors with low power consumption. Optical properties of nanofibrous sensors with a collection of specific applications in security, health and food are the main topic of the following three chapters. They have been introduced by an overview of the properties of fluorescent electrospun nanofibres reporting illustrative examples of their application as optical sensors for the detection of heavy metal ions, explosive compounds and bio-systems. Some of the latest advances in bio-mimicking and biological sensors have been summarized, dealing with the preparation, bio-immobilization and the role of nanostructures for electrochemical and gravimetric biosensing systems for protein biomarkers and DNA detection. Two tremendously innovative and intriguing applications of electrospinning have also been reported, dealing with smart nanofibrous textiles and a brain mimicking system, respectively, before concluding with an overview on electrospinning from a manufacturer's perspective. Here the current state of the electrospinning industry is debated, including recent advances in commercial-scale electrospinning and other competing technologies. The challenges that are faced by commercial electrospinning companies are highlighted, helping to explain the limited commercialisation and uptake of electrospinning by industry. To finish, some examples of nanofibre products that are currently available or being commercialised are given, with an outlook on what the future may hold for electrospun nanofibre technologies in the years to come, comprising smart sensing systems.