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Research Centre Spain
In the past decades the interest in the environmental problems caused by atmospheric contamination has increased. The emissions of NOX, COX, organic vapours and SO2 have reached high levels giving rise, among others, to acid rain and greenhouse effect. One of the most toxic gases among the major ones in atmospheric contamination is sulphur dioxide; it is strong irritant and causes several damages on eyes, mucous membranes, skin and respiratory tract. Moreover, its combination with water vapour in the atmosphere to form sulphuric acid is one of the principal causes of acid rain. There are multiple sources of SO2, which ones include volcanism, biomass burning, wine industry, smelting of sulphidic ores or burning of fossil fuels for vehicles and energy production. However, the specific detection and determination of SO2 is often limited to liquid phase. Usually the determination is made by iodometric titration, procedure known as Ripper method (1). For detection in gas phase several devices based on nanostructured metallic oxides have been developed, but these devices have some problems as high working temperatures, lack of selectivity, high response times or low sensitivity (2).
In order to improve the performance parameters of the gas sensors, different nanostructured materials have been studied as sensing part for these devices in the past years. These materials include metallic particles (3, 4), organic polymers (5) and carbon nanotubes (6) among others. The devices made with the new materials have provided lower working temperatures, a decrease in response and recovery times and higher sensitivities than using traditional gas sensing devices. However, they still have very low selectivity since their response to several gaseous analytes. For this reason, several functionalization processes for the sensing materials have been proposed as a possible solution to increase the selectivity. The aim of these processes is to incorporate specific receptors for each target analyte on the surface of sensors.
Carbon nanotubes (CNTs) are one of these new materials, and they have been used as the conducting channel of field effect transistor (FET) devices, which have been successfully used for the detection of NH3 (7), NOx (8), COx (9), organic vapour (10), etc. CNTs are an interesting material for sensing devices for their semiconducting nature and because they offer
many possibilities to be functionalized. Two main strategies can be followed: covalent and non-covalent functionalization.
In the covalent functionalization (3), the functional groups are covalently bound to the structure of the CNTs, and each new covalent bond implies a defect on the surface that changes the electronic properties of the CNTs. The non-covalent functionalization (4) consists of coating the CNT surface with a material (e.g. a polymer, protein, metallic films or nanoparticles, among others). In this way, the molecules are adsorbed onto the CNTs and do not break their electronic structure. These methodologies have shown an increase in the selectivity for some substances, specially for some biosensors like for instance for DNA sequences or proteins (Immunoglobulins E and G) (11-13).
We employ FETs based on networks of CNTs to selectively detect SO2 in air at room temperature. The selectivity of the device is acquired in a two-steps functionalization process of the CNTs. First, the CNTs are coated with a thin layer of polyethilenimine (PEI). Since PEI coats all the surface of CNTs it avoids any interaction of CNTs with the atmosphere, either interactions with SO2 or with any interference in the surrounding environment. Then, a specific receptor for SO2, a platinum (II) complex [PtI(4-E-2,6-{ CH2N(CH3)2}2-C6H2] (14), is covalently bonded to the amino groups of PEI. The complex (sensing material) acts as a selective pincer for SO2, binding the molecule in a reversible reaction by coordination bonds. The specific detection is improved by the use of a blocking molecule (N-acryloxysuccinimide) which prevents interferences due to the possible response of the remaining free amino-groups of the polymer that have not reacted with the Pt (II) complex, which can have acid-base reactions with gases with acid properties. The response of the device to SO2 was monitored through changes of the current flowing through the CNTs. The device was selective to SO2 and the lowest concentration detected was 0.7%. |
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