The merits of nanostructures in sensing may seem obvious, yet playing these attributes to their maximum advantage can be a work of genius. As fast as sensing technology is improving, expectations are growing, with demands for cheaper devices with higher sensitivities and an ever increasing range of functionalities and compatibilities. At the same time tough scientific challenges like low power operation, noise and low selectivity are keeping researchers busy. This special issue on sensing at the nanoscale with guest editor Christofer Hierold from ETH Zurich features some of the latest developments in sensing research pushing at the limits of current capabilities.

Cheap and easy fabrication is a top priority. Among the most popular nanomaterials in sensing are ZnO nanowires and in this issue Dario Zappa and colleagues at Brescia University in Italy simplify an already cheap and efficient synthesis method, demonstrating ZnO nanowire fabrication directly onto silicon substrates[1]. Meanwhile Nicolae Barson and colleagues in Germany point out the advantages of flame spray pyrolysis fabrication in a topical review[2] and, maximizing on existing resources, researchers in Denmark and Taiwan report cantilever sensing using a US$20 commercial DVD-ROM optical pickup unit as the readout source[3]. The sensor is designed to detect physiological concentrations of soluble urokinase plasminogen activator receptor, a protein associated with inflammation due to HIV, cancer and other infectious diseases.

With their extreme properties carbon nanostructures feature prominently in the issue, including the demonstration of a versatile and flexible carbon nanotube strain sensor[4] and a graphene charge sensor with sensitivities of the order of 1.310 3eHz 1/2[5]. The issue of patterning for sensing devices is also tackled by researchers in the US who demonstrate a novel approach for multicomponent pattering metal/metal oxide nanoparticles on graphene[6].

Changes in electrical properties are an important indicator for sensing. In search of a better understanding of these systems Zhang etal from Southern Illinois University inspect the role of Joule heating, exothermal reactions and heat dissipation in gas sensing using nanowires[7]. The mechanisms behind electrical chemical sensors are also further scrutinized in a kinetics study by Joan Ramon Morante from the University of Barcelona in Spain. 'In spite of the growing commercial success many basic issues remain still open and under discussion limiting the broad use of this technology,' he explains. He discusses surface chemical reaction kinetics and the experimental results for different representative gas molecules to gain an insight into the chemical to electrical transduction mechanisms taking place[8].

Perhaps one of the most persistent targets in sensing research is increasing the sensitivity. Gauging environmental health issues around the commercial use of nanomaterials places high demands on low-level detection and spurred a collaboration of researchers in the UK, Croatia and Canada to look into the use of particle-impact voltammetry for detecting nanoparticles in environmental media[9]. At the University of Illinois Urbana-Champaign in the US, researchers have applied wave transform analysis techniques to the oscillations of an atomic force microscopy cantilever and tailored a timefrequency-domain filter to identify the region of highest vibrational energy[10]. The approach allows them to improve the signal to noise ratio by a factor 32 on current high-performance devices. In addition, researchers in Korea report how doping NiO nanofibres can improve the sensitivity to a number of gases, including ethanol, where the response was enhanced by as much as a factor of 217.86[11].

Biomedicine is one of the largest industries for the application of nanotechnology in sensing. Demonstrating the state of the art, researchers in China use silicon wafers decorated with gold nanoparticles for label-free detection of DNA at concentrations as low as 110fM, a sensitivity comparable to the best signal amplification-assisted electrochemical sensors reported[12]. In another study actin-conjugated gold and silver nanorods are used to detect ATP, a common indicator of cell viability[13]. They show how aggregation induced by ATP-induced polymerization of the G-actin gives rise to a measurable change in the plasmon resonance absorbance of the nanorods. A review of the use of fluorescent silica nanoparticles for biomedical applications is provided by researchers at Dublin City University in Ireland[14].

The first scanning tunnelling microscope in the early 1980s and subsequent scanning probe developments brought the world of nanoscale structures into view in a manner that gorged the imaginations of scientists and the public. New ways of probing structures at this scale revealed a wealth of curious properties that triggered a surge of research activity in nanotechnology, now a multibillion dollar industry. One good turn deserves another and in fact nanostructures provide the perfect tools for the type of sensing and imaging applications that brought such widespread research interest to nanotechnology. This special issue highlights just how broad and innovative the range of sensing nanotechnologies has grown.

References

[1] Zappa D, Comini E and Sberveglieri G 2013 Thermally-oxidized zinc oxide nanowires chemical sensors Nanotechnology 24 444008

[2] Kemmler J A, Pokhrel S, Mdler L, Weimar U and Barsan N 2013 Flame spray pyrolysis for sensing at the nanoscale Nanotechnology 24 442001

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