S.C. Thorpe

Crowned and liquid-crystalline phthalocyanines as gas-sensor materials

Sem iconducting films o f crown-ether substituted m etal-free phthalocyanines are shown to have unique properties o f fast response and reversal to N O 2 at room temperature. Increasing the size of the crown ring or com pleting the phthalocyanine with Cu(II) gives slower responses. C om pleting the crown-ether rings with sim ple metal salts leads to vciy rapid reversible conductivity decreases in 1-5 ppm N 0 2 at room temperature. D iscotic liquid-crystal phases o f octaoctyl phthalocyanine also respond rapidly and reversibly to NO j. T he origins o f these effects are discussed with reference to reduced local polarizability, increased molecular separation and improved molecular assembly, and experim ents exploring the influence o f m olecular design and crystal engineering on these sensing properties are described.

Gas-sensing properties of semiconducting films of crown-ether-substituted phthalocyanines

The effects of nitrogen dioxide and ammonia on the semiconductivity properties of solution-deposited thin films of the tetra-substituted 15-crown-5, 18-crown-6 and 21-crown-7 metal-free and copper phthalocyamnes are reported. The 15-crown-5 films showed good reversible conductivity changes at room temperature in NO2 concentrations up to 5 ppm. The sensor characteristics worsen as the size of the crown-ether ring increases and this is consistent with the effect of greater separation of adjacent adsorbed species as molecular size increases. Lateral repulsions, which are believed to play a dominant role in controlling the response and reversal processes, are reduced by this increase. Treatment of the films with aqueous KCl solution led to dramatic changes in gas-sensing properties and, for the 15-crown-5 derivative, in film morphology. After KCl treatment the film conductivity decreased very rapidly and reversibly on exposure to NO2, even at room temperature, and the 15-crown-5 film changed from a polycrystalline needle structure to an extremely smooth structure. These effects are tentatively ascribed to pronounced changes in the molecular assembly induced by the interaction of the potassium ions with the crown-ether moieties, and to consequent changes in the porosity of the film to small gas molecules. Above 3 ppm NO2 the response begins to saturate, suggesting depletive chemisorption on an n-type material. However, similar responses are also observed for the electron-donor gas ammonia, indicating the presence of both donor and acceptor impurities in the materials. The gas-sensing properties of the KCl treated films at room temperature are the best of any organic semiconductor film yet reported.

Sol-gel materials for gas-sensing applications

Abstract High temperature sintering of porous pellets of metal oxides to form flammable gas sensors remains one of the most striaghtforward and popular fabrication methods in use today, but is unsatisfactory in may wasys including lack of control over structure and morphology of the pellets. In addition, the chemical and physical role of catalytic dopants is often unclear and further sintering during operation can lead to long-term drift and unreliable behaviour. We have been exploring the use of alternative fabrication techniques, for the development of highly sensitive flammable gas detectors, based on the sol-gel process. The main advantage of this technique over conventional processing technologies is the use of homogenous, multi-component systems which can be prepared to a high degree of purity by mixing the molecular precursor solutions, and the reduction in fabrication temperature leading to unusual galsses or ceramics with better-defined properties. Several advantages in the development of highly sensitive flammable gas sensors are to be expected. Deliberate addition of impurities can be carefully controlled and low temperature fabrication should allow greater control over the structure, stoichiometry and morphology of the sensors. Both of these factors will contribute towards low background conductivity and high purity for high sensitivity. Finally the high porosity and large surface of glassy materials produced by sol-gel methods should enhance sensitivity in mechanims dominated by surface phenomena. In this paper we present details of the preparation of novel tin oxide thin-film sensors and initial results of response to a variety of organic solvents and common flammable gases. A comparison with conventional, commercially available tin dioxide (Taguchi) flammable gas sensors will be given.

Hydrophobic membrane sensors for the optical determination of hydrogen chloride gas

The acid-base complexation reaction between the metal free porphyrin, H2TPP, and hydrogen chloride gas has been used as the basis for a chemical gas sensor. The porphyrin is incorporated into a thin silicone rubber membrane which provides the hydrophobic conditions needed for the operation of the sensor. Porphyrin fluorescence emission provides an optical probe for the measurement of sorbed hydrogen chloride gas. A kinetic method of operation achieves low ppm (at least 2 ppm) detection. Factors such as long term stability and humidity effects limit the use of the membranes and these are considered in detail. Good agreement has been obtained between the experimental response and the response generated by a diffusion controlled model. The membranes have also been found to show sensitivity to chlorine and nitrogen dioxide but not to sulphur dioxide.

Surface plasmon resonance of self-assembled phthalocyanine monolayers: possibilities for optical gas sensing

A diphthalocyanine disulfide (Pc) molecule has been deposited as a monolayer on gold-coated substrates through the process of self-assembly. To establish the molecular orientation of the Pc molecule on the gold surface the two complementary techniques of transmission IR and reflection absorption IR (RAIR) spectroscopies were used. The appearance of IR absorption bands associated with the Pc nucleus in the transmission spectrum, and their absence in the RAIR spectrum, suggests that the Pc self-assembled monolayer (SAM) is orientated with the macrocycle parallel to the metal surface. The Pc SAM has been used in conjunction with surface plasmon resonance (SPR) to establish the utility of combining these techniques for optical gas sensing. The SPR reflectivity curves for the gold substrate and the Pc SAM on the gold substrate have been obtained. On exposure of the Pc SAM to the environmentally important NO2 gas, changes of the reflectivity signal were obtained in proportion to the concentration of the analyte gas. The results obtained show that the monolayer deposition technique of self-assembly is an ideal method for the production of chemically sensitive substrates which can be combined with surface plasmon resonance for the optical sensing of gaseous species.

Formation and characterisation of a self-assembled phthalocyanine monolayer suitable for gas sensing

Abstract The formation and characterisation of a novel self-assembled monolayer film for gas sensing are reported. The phthalocyanine 1,1′4,4′,8,8′,11,11′,15,15′,18,18′,22,22′-tetradecakishexyl-225,25′-(3,3′-dithiodipropyl)diphthalocyanine, has been synthesised from its hydroxypropyl precursor, via a two-step reaction. The disulfide functionality of the molecule has been used to produce a self-assembled phthalocyanine monolayer on a gold substrate. The phthalocyanine monolayer has been characterised using reflection absorption infrared spectroscopy (RAIRS).

Substituted phthalocyanine gas sensors

Abstract Gas sensors are playing an increasingly important role in the monitoring of workplace environments. Sublimed films of metal phthalocyanines have been successfully used as NO2 sensors, but do suffer certain limitations. Langmuir—Blodgett films of many substituted phthalocyanines have proved difficult to prepare reproducibly. An asymmetrically substituted phthalocyanine, 1,4-bis(4-hydroxybutyl) 8,11,15,18,22,25-hexaoctylphthalocyanine, designed specifically to form good-quality Langmuir—Blodgett films, has been successfully deposited. Prepared devices show a useful room-temperature response to NO2. The reproducibility, lifetime, shelflife, specificity and effects of water vapour on these devices have been investigated.

Langmuir-Blodgett films of an asymmetrically substitute phthalocyanine: Improved gas-sensing properties

Abstract 1,4-bis(4-hydroxybutyl)-8,11,15,18,22,25-hexahexylphthalocyanine has been deposited as Langmuir-Blodgett (LB) films on to glass substrates bearing platinum-interdigitated electrode patterns. The prepared devices exhibit a useful room temperature response on exposure to NO 2 gas in the occupational hygiene range. The magnitude of the response is proportional to the gas concentration and is also dependent upon the LB dipping direction relative to the axes of the electrodes. There is a lack of response to Cl 2 and CO gases, exposure to which does not affect the response of the device on subsquent exposure to NO 2 gas. The device can tolerate at least 65% relative humidity without detrimental effects.

A kineto-optical method for the determination of chlorine gas

Abstract The redox and optical properties of porphyrins make them particularly interesting and useful as potential chemical sensors for electron-accepting gases such as chlorine. Our preliminary investigations using vacuum sublimed films of 5,10,15,20-tetraphenylporphin (H 2 TPP) have shown that a fluorescence decrease can be observed when the films are exposed to chlorine gas at low concentrations (ppm). Unfortunately vacuum sublimed films of H 2 TPP produce only a weak fluorescence emission. H 2 TPP dissolved in silicone rubber films exhibits a strong fluorescence emission which makes the films more suitable for optical measurement. In addition, the non-polar environment favours a protonation reaction in which H 2 TPP is converted to the diacid dication, H 4 TPP 2+ . This is the sensing interaction used to detect and measure chlorine gas which can be followed either by monitoring the fluorescence decrease from H 2 TPP or the fluorescence increase from H 4 TPP 2+ . Equilibration of the films does not occur, however, the interaction is suited to a kineto-optical method of measurement. The films respond quickly, reversibly and reproducibly with the kineto-optical method requiring only a short exposure period.

Effects of heat treatment on chemical, morphological and NO2-sensing properties of lead phthalocyanine films

Thick sublimed films of lead phthalocyanine which have been heated in air at 360 °C for 1 h show rapid conductivity changes on exposure to NO2, with some slow components remaining. These slow components are more evident for high NO2 concentrations, and increase in magnitude on prolonged exposure to the gas. Electron probe microanalysis (EPMA), scanning electron microscopy (SEM) and X-ray diffraction (XRD) experiments to characterise the effects of heat treatment on the chemical and structural nature of the films are reported, and both morphological and chemical mechanisms for the rapid response are discussed. These results, together with previous secondary-ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), infrared (IR) spectroscopy and calorimetric studies, and the absence of corresponding effects for planar copper phthalocyanine, strongly support selective reaction of strong adsorption sites as the most likely origin of the kinetic effects. The advantages and limitations of sensors using this material are discussed.