Keywords
Citation
(2011), "Nanosensors for organic vapours", Sensor Review, Vol. 31 No. 1. https://doi.org/10.1108/sr.2011.08731aab.003
Publisher
:Emerald Group Publishing Limited
Copyright © 2011, Emerald Group Publishing Limited
Nanosensors for organic vapours
Article Type: Nanosensor update From: Sensor Review, Volume 31, Issue 1
Another example of where existing gas-sensing technology is deficient is the selective detection of volatile organic compounds (VOCs). This is a key topic of gas sensor research, as many such compounds pose a risk to health. Further, when present in human breath, certain VOCs can indicate the presence of an illness.
A group from the Swiss Federal Institute of Technology (ETH) has developed a novel nanosensor for detecting trace levels of acetone in exhaled breath. Acetone is a key biomarker for Type 1 diabetes and also indicates the associated complication of diabetic ketoacidosis. The sensor consists of films of pure and SiO2-doped WO3 nanoparticles, 10-13 nm in diameter (Figure 2), directly deposited and in situ annealed onto interdigitated gold electrodes by flame aerosol technology on an alumina substrate. A unique innovation is that the films consist of ε–WO3, a metastable phase that has a high selectivity to acetone vapour. The sensor’s resistance falls as a function of the acetone vapour concentration and can selectively detect levels down to 20 ppb (Figure 3). The technology should allow the real time, non-invasive diagnosis of diabetic conditions, as shown in Figure 4. Formaldehyde is another VOC with health implications. It can occur inside buildings and arises from plastics and wood coating and may cause eye irritation and trigger allergies and contributes to poor indoor air quality. The US EPA regards it as a probable human carcinogen and several countries stipulate maximum permitted indoor levels. Researchers from China’s Donghua University have developed a formaldehyde sensor based on a coated quartz crystal microbalance. The coating comprised a nanofibrous membrane of polyethyleneimine (PEI)/polyvinyl alcohol, fabricated by electrospinning and with fibre diameters of 40 nm−1.8 μm. The response is due to the reversible interaction between formaldehyde molecules and amine groups of the PEI. The sensor showed a linear and reversible response over the concentration range 10−255 ppm at room temperature. VOC-responsive nanosensors are also being developed for homeland security applications. A group from the University of Pennsylvania has recently reported the use of SWCNTs, functionalised with DNA, in a resonant sensor to detect explosives’ vapours and nerve agents. SWCNTs were grown by chemical vapour deposition directly on top of two aluminium nitride contour-mode resonators, fabricated on the same chip and operating at two different frequencies. At 287 and 450 MHz the resonators showed sensitivities of 17.3 and 28.0 KHz μm2/fg, respectively. The two devices were functionalised with two distinct sequences of single-stranded DNA. Single-stranded DNA-decorated SWCNTs can selectively adsorb different vapours and gases (depending upon the particular DNA sequence used) and cause a shift in the fundamental frequency of the resonator. The sensors were tested with 2,6-dinitroluene (DNT), an explosive vapour, and dimethyl-methylphosphonate (DMMP), a simulant for the nerve agent sarin at concentrations which represented 10, 25 and 50 per cent of their saturated vapour pressures. At 10 per cent the sensors showed a response (adsorbed mass density) of ∼0.9 fg/μm2 for DNT and ∼0.7 fg/μm2 for DMMP, illustrating the ability to distinguish between these two compounds.
The selective detection of VOC species is the topic of recently reported work by the Department of Chemical Engineering at MIT. Sensors have been developed which are based on conducting polymer-metal nanoparticle hybrid structures, fabricated by assembling nanoparticles (e.g. Ni, Pd) on top of functionalised conducting polymer film surfaces using conjugated linker molecules (Figure 5). Selectivity is achieved by assembling different metals on the same polymer film which eliminates the need to develop either different polymer chemistries or device configurations for each compound. Chemisorption of the analyte vapour induces charge redistribution in the metal nanoparticles and changes their work function. The linker molecule causes this change to affect the electronic states in the underlying conducting polymer film, leading to a readily measurable change in resistance. By varying the metals, the sensors have been shown to exhibit selective responses to acetone and toluene vapours at low ppm concentrations and it is anticipated that the use of other metals such as Ag (Figure 6), Au and Cu will confer selectivity to other VOCs. This work would appear to represent a significant step towards the realisation of a generic sensing technique for the selective determination of VOCs.