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Smart Nanomaterials for Sensor Application, 2012, 163-176 163 1D Nanostructures for Sensing Purposes Alessio Giuliani 1,2* and Yi Ge 2 Songjun Li, Yi Ge and He Li (Eds) All rights reserved - © 2012 <strong>Bentham</strong> <strong>Science</strong> Publishers CHAPTER 10 1 SINTEA PLUSTEK Srl, Via E. Fermi 44, 20090 Assago (MI), Italy and 2 Cranfield Health, Vincent Building, Cranfield University, Bedfordshire, MK43 0AL, UK Abstract: Nanowires, Nanorods and Nanotubes are well-known monodimensional (1D) nanostructures. They exhibit a very high surface to volume ratio as well as unique magnetic and electrical properties which make them ideal candidates for new and innovative sensing devices. One of the key aspects in new biosensor developments is the alignment of monodimensional nanostructures that can be obtained by employing Langmuir Blodgett techniques as well as electrical and magnetic external fields. There recently have been many successful applications of 1D nanostructures in high performance sensors, such as field biomedical sensors that are able to detect very low concentrations of DNA and novel gas sensors for the detection of ultra-low concentration of toxic gases. The well-improved sensors employing 1D-nanostructures have a profound impact on healthcare and safety, thereby playing a more and more important role in the near future. Keywords: Monodimentional (1D) nanostructures; nanomaterials; sensors. 1. INTRODUCTION Monodimensional nanostructures are metallic or semiconducting nanoparticles some tens of microns long, characterized by a cross section
164 Smart Nanomaterials for Sensor Application Giuliani and Ge Figure 2. Image of nanorods [5]. Reproduced with permission. Figure 3. Image of nanotubes. [6] Reproduced with permission. The properties of NWs largely depend on the method adopted in their synthesis as demonstrated by Xu et al. [10], who produced, in a non-acqueous bath, CdSe uniformly oriented NWs with the c-axis normal to the substrate. These results were further agreed by some other researchers [11] while on the contrary Peng et al. [12], showed that NWs deposited in an aqueous bath were oriented randomly. Field Emission Transistor (FET) has attracted considerable interests and investigations in the last decade. It is characterized by some drain electrodes and a p-type silicon semiconductor connected to a metal source. A gate electrode coupled by a thin dielectric, allows the on/off switching of conductance between the other two electrodes. For example, the application of a positive gate voltage leads to carriers depletion and thus results the reduction of conductance. This mechanism has been exploited successfully in biomedical sensors, where the binding of charged molecules to the gate causes an electrical field analogous to the application of a voltage. NWs-based FET sensors are characterized by the accumulation of carriers in the bulk of the nanomaterial [13] and they show a much higher sensitivity enabling the detection of even single viruses [14]. It’s worth noting that Zheng and his co-workers [15] successfully developed a silicon NWs based sensor platform with an objective to elucidate the role of these structures in sensing devices. They demonstrated the importance of such nanostructures by showing all electrical signals obtained in the experiments were produced just by those phenomena occurred at the silicon NWs surface. The sensors market is very big and growing fast especially due to the share and importance of biomedical sensing devices. In addition, there is a great demand for new and innovative devices that are able to effectively and simultaneously detect the high variety of pathogens and biomolecules presented in the environment. As a result, by taking the advantages of 1D nanostructures, 1D nanostructures based-sensors have make a considerable impact on patient recovery and public health monitoring [16]. 2. SYNTHESIS OF 1D NANOSTRUCTURES Nanomaterials can be produced by using either bottom-up or top-down techniques. The bottom-up approach starts from single molecules which then assemble together in order to obtain more complex
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