Free-standing sensors

Free-standing sensors—These sensors, which include active infrared, passive infrared, bistatic microwave, monostatic microwave, dual-technology, and video motion detection (VMD) sensors, consist of individual sensor units or components that can be set up in a variety of configurations to meet a user s needs. They are installed aboveground, and depending on how they are oriented relative to each other, they can be used to establish a protected perimeter or a protected space. More details on each of these sensor types are provided below. 

Fig. 7.2 (a) SEM view of thermally decoupled membrane array. [Reprinted with permission from Splinter et al. (2001). Copyright 2004 Elsevier], (b) SEM view of a free-standing sensor platform fabricated using a sacrificial layer of porous silicon [Reprinted with permission from Furjes et al. (2004). Copyright 2004 Elsevier], (c) Optical microscope photograph of a three-section heater sensor array. [Reprinted with permission from Frandoso et al. (2008). Copyright 2008 Elsevier], (d) Suspended porous silicon micro-hotplate with a Pt heater. The thickness of the membrane is 4 pm. The depth of the cavity under the membrane is more that 60 pm [Reprinted with permission from Tsamis et al. (2003). Copyright 2003 Elsevier]... 

SENSORS BASED ON FREE-STANDING MOLECULARLY IMPRINTED POLYMER MEMBRANES. COMPUTATIONAL MODELLING OF SYNTHETIC MIMICKS OF BIORECEPTORS... 

Fig. 2.16 (a) A schematic drawing of polymer microring resonator couple to a side polished optical fiber, (b) A microscope image of the fabricated EO polymer electric field sensor, (c) SEM image of resonators fabricated on the polished flat of a free standing fiber. The scale bar in the picture represents 100 pm. Reprinted from Ref. 15 with permission. 2008 Institute of Electrical and Electronics Engineers... 

From the vantage point of microfluidics, the structures developed by Petersen et al [33] are the most appropriate. More recently, Baltes and coworkers combined CMOS circuitry with the microfabrication of sensors to construct a thermal mass flow system based on thin-film pyrometers [66]. As free standing mass flow sensors, they have attractive features. However, all of these silicon-based devices operate at relatively high temperatures in the 100-200 °C range. This elevated temperature limits their potential application in more complex microfluidic systems. The ideal flow sensor would be a very-low-temperature element that could be used on the walls of the microchannel. 

Element Six Ltd, http //www.e6.com/wps/wcm/connect/E6 Content EN/Home/Applications/sensors/ DIAFILM EP (electrochemical processing) grade Microcrystalline free-standing 500 pm thick Boron concentration 3 x 10 Resistivity 45 mil cm... 

Much of the work done with conductive polymers follows the same trends as that with non-conductive polymers, so will not be described here. There will be important roles for conductive polymers as sophisticated microstructures are designed. One example of early work in this area is the use of poly(pyrrole) as a support for montmorillonite at the surface of electrodes, and in free-standing films (119). Also Kittlesen, White and Wrighton reported in 1984 that electropolymerized poly(pyrrole) and poly(N-methylpyrrole) could be prepared at electrodes with widths of only 1.4 microns (120). These electrodes were part of an ultramicroelectrode array used to demonstrate the possibility of combining surface chemistry with microelectronics technology to prepare microsensors. Since the conductivity of poly(pyrrole) (and many other conductive polymers) depends on redox state, the authors suggest that miniaturized redox sensors may be prepared from systems such as theirs. 

Systems capable of effecting the detection of lanthanide and actinide ions using electrochemical methods have generally consisted of a receptor or ionophore immobilized within a membrane to form an ion-selective electrode (ISE). This electrode is then pertmbed when a cation guest is boimd. On the other hand, optical sensors or optodes are based on a molecule that is either free standing or attached to a polymer matrix. In both cases, the expectation is that an optical response will be produced when the system is exposed to a particular anal5de. Needless to say, for either approach it is beneficial to have a receptor that is well tuned for the lanthanides and actinides. In practice, this has translated to the use of macrocycles rich in O, N, S, or P donor atoms as will be clear from the summary of recent work provided in this article. This article focuses on ISE- and optode-based approaches to lanthanide and actinide cation sensing. 

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