Sensors and Actuators Ll, 18-19 (1994) 511-514 511

Detection of nitro compounds by organic semiconductor sensors

Abstract

A sensor-based ~st~ment for dete~ination of the ~m~sition and ~ncentrations of vapours of organic nitro compounds and nitrogen dioxide in the atmosphere is described. Four organic semi~nductor sensors based on fluoroaluminium phthalocyanine (AlpcF), chlorogallium phthalocyanine, chloroahtminium naphthalocyanine and the copper complex of tetraazadibenzo-14-annulene are used in conjunction with platinized platinum precon- centrators. The ‘sensors and preeoncentrators are located in separate concentric annular sections of a sample chamber. These sections are rotated by a microprocessor-controlled stepper motor to permit loading of the preeoncentrator, relocation to a position directly opposite a sensor, evacuation, and preeoncentrator triggering and sensor-response measurement. The importance of the evacuation stage for obtaining fast response, and the need to energize the pre~n~ntrator thr~ghout the meas~ement stage to prevent readsorption problems, have been demonstrated. The performance of the A&F sensor has been evaluated, showing sensitivity to ppm levels of nitrobenzene.

Organic nitro compounds such as trinitrotoluene (TNT) and ethylene glycol dinitrate (EGDN) are well- known explosives. The presence of electron-withdrawing nitro groups suggests that materials of this class might be detectable using semi~n~~ing phthal~an~e fiims which are capable of detecting nitrogen dioxide down to ppb levels [l]. However, their low volatility and tendency to be tenaciously adsorbed on a wide range of materials, together with the fact that their electron affinities are substantially smaller than that of NOz, present fo~idable problems in the development of such a detection system. In an extensive materials evaluation programme in which sensors were tested in various concentrationsof nitrobenzene (NB) and EGDN in an evacuable glass apparatus in which all sections were fully thermally-jacketed to prevent vapour con- densation, Tamizi and Wright [Z] demons~ated the feasibility of this approach. From many compounds tested, fluoroaluminium phthalocyanine (AlpcF) was found to give conductivity increases on exposure to NB but not on exposure to EGDN. Conversely, chloro- aluminium naphthalocyanine was shown to respond to EGDN but not to NB, while chlorogallium phthalo- cyanine gave a response to both these value. It was also shown that the only way to obtain reasonable sensitivity and fast response was to collect the analyte on a platinized platinum preconcentrator, rapidly evacu- ate the chamber in which the sensor and preconcentrator

were positioned close to each other, and trigger the preconcentrator to release the collected vapour directly onto the sensor with minimum competition from oxygen or background NOz. It was suggested that a four-sensor instrument using these three materials together with one sensor based on the copper complex of tetraa- zadl~~-l~annuiene (Cu-TADA) [3], which responds to NO2 but not to any of the explosives vapours, might be able to provide information on both the composition and concentrations of explosive vapours [2]. In this paper we report the design, construction, and evaluation of a prototype instrument of this type.

Experimental

A cylindrical sensor holder surrounded by a close- fitting annulus ~n~ining four pr~on~en~ators was constructed from ~~-de~i~ polythene. Exposure of this material to the target vapours followed by thermal desorption into a mass spectrometer and principal component analysis of the resulting data proved that the material had a low affinity for the vapours and that the mass spectrum of the desorbed material gave pencil ~m~nents well removed from those for the target materials. The sensor and preconcentrator sec- tions could be rotated relative to each other using a stepper motor controlled using a single-chip micro- controller (TDS Computers ILtd., London). A series of

~~~~5~4~~7.~ 0 1994 Elsevier Sequoia. All rights reserved SSDI 09254005(93)01061-8

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channels machined in the two sections permitted a four-stage operating cycle as follows:

(i) a pump draws the sample over the preconcentrators for a predetermined time;

(ii) the sample inlet is cut off and the preconcentrator moved towards the sensor;

(iii) the sensor and preconcentrator are briefly evacu- ated, and

(iv) the evacuation is stopped and the preconcentrator rapidly heated electrically, with the sensor current monitored.

Low concentrations of NB in dry air were generated by passing cylinder air via a Dylan mass-flow controller over a PTmE permeation vial of NB maintained at 60 “C in a glass chamber, with short glass connections to the sensor/preconcentrator unit. The sensors were prepared by slow (< 1 A s-l) vacuum sublimation of the materials onto platinum interdigitated electrodes on alumina substrates fitted with platinum heater/re- sistance thermometer coils, as previously described [2].

Results

The performance of the system has been tested using the AlpcF sensor with a range of NB concentrations and diierent system operating conditions. Since it is well established that the response kinetics of organic semiconductor sensors are determined by surface struc- ture and in particular by the presence of residual strongly-bound electron acceptor species on the fihn surface at the commencement of sampling [4], the freshly sublimed AlpcF film was first ‘initialized’ by exposure to NOz in a normal sensor test rig. The film, operating at 150 “C, was exposed to 13 cycles of 150 pbb NO, in dry air for periods of N 12 000 s, interspersed with reversal in clean dry air at 180 “C for the same time. During this process the typical increase in response rate was observed, and following the cycles reproducible calibration data were obtained for exposures to a range of concentrations of NOz. The initialized sensor was then mounted in the in- strument chamber described above and immediately used for experiments with nitrobenzene vapour.

An initial experiment in which a clean preconcentrator filament was rapidly heated to 190 “C in its normal discharge position opposite the sensor (maintained at 150 “C) in clean air gave no measurable sensor response, proving that any effects due to the semiconductor sensor being heated by the hot preconcentrator were negligible. Dry air containing 3 ppm nitrobenzene was then allowed to flow over the preconcentrator for a period of 30 min before measuring sensor responses. (This time was selected to ensure that the preconcentrator had adequate time to equilibrate with the vapour. In a final

practical device a significantly shorter preconcentrator charging time would be possible, but the objective of the present work was to evaluate the system response under optimum conditions rather than to minimize the cycle time.) In the first experiment, the preconcentrator was moved to its position directly opposite the sensor and the preconcentrator filament was switched on after a 100 s delay, with no evacuation of the chamber. As seen in Fig. 1 the sensor current increased a little as the preconcentrator was triggered, then continued to rise at a slower rate until the filament was switched off some 1500 s later. The chamber was then evacuated to reverse the sensor current to near its original value and the preconcentrator was recharged as previously. In a second experiment, the charged preconcentrator was moved into position opposite the sensor and the chamber was evacuated for 100 s before switching on the filament. As seen in Fig. 2 (upper trace) both the initial rise as the filament was switched on, and the subsequent rise, were faster than when there was no evacuation. Also, the increase in sensor current stopped abruptly when the filament was de-energized, to be replaced by a slow decay. Finally, after reversal as described above, clean air was passed over the pre- concentrator for 30 min and the same experiment was repeated, giving the result shown in Fig. 2 (lower trace) in which there is an initial step as the filament is switched on, but no significant rise thereafter and also no detectable change as the filament is switched off.

These results show that the system response is sig- nificantly improved when the chamber is evacuated before the filament is energized. To adsorb on the AlpcF film, the nitrobenzene must compete with other electron acceptor molecules, notably oxygen in the air. This process is believed to be the rate-limiting step in

I.1

:iLl 0 500 1000 1500

Elapsed time/s Fig. 1. Response of AlpcF film to nitrobenzene. The precon- centrator filament was enabled after 100 s and disabled after MCI s.

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75" a 5uo 1000 IS00

Elapsed time/s

Fig. 2. Response of AlpcF film to 3 ppm nitrobenzene with an evacuation stage of 100 s (upper trace) and to blank with 100 s evacuation (lower trace).

0 loo0 Time I s

Fig. 3. Typical response of AlpcF sensor to 3 ppm nitrobenzene in modified apparatus with improved seals and lower deadvolume.

the response of all phthalocyanine semiconductor sensor Iilms [4]. Thus the effect of evacuation is a substantial reduction in competition for sensor surface sites by oxygen, and hence a more rapid response to the NB released when the tilament is energized. The initial rapid step in sensor current as the filament is energized is greater after evacuation than in air, and is similar in the former case whether or not the preconcentrator had been pre-loaded with NB. This feature is therefore not considered to be a reflection of the NB concen- tration, but rather of a thermal effect (for example, higher initial temperature rise due to radiant heating when the sensor is in the poor heat-transfer conditions of a vacuum, i.e., when it cannot lose the incoming radiant heat so easily). The slow response, which shows no signs of equilibrating over 1500 s, suggests that the adsorption rate is still limited by desorption of oxygen from the sensor surface. This is to be expected since NB is a considerably weaker electron acceptor (elec- tron affinity 1.00 eV [5]) than NO, (electron afhnity 2.2 eV), and even NO1 is normally unable to achieve equilibrium adsorption with AlpcF within this time scale [4]. The signal decay following de-energixation of the preconcentrator probably reflects re-adsorption of NB onto the preconcentrator, and confirms that it is nec- essary to heat the filament continuously until the sensor reading has been recorded, rather than just applying a short thermal desorption pulse.

To check these interpretations, the sensor and pre- concentrator were mounted in a glass chamber with a very small dead volume, which also permitted the preliminary charging of the preconcentrator but with much improved evacuation due to the use of vacuum taps instead of the sliding O-ring seals on the previous chamber. Figure 3 shows a typical result, with faster

equilibrating response and more rapid reversal after de-energizing the filament. This indicates the level of performance which should be obtainable from this system with improved chamber design.

Attempts to obtain a calibration curve for NB using the original system were abandoned due to the difficulty in achieving reproducible background sensor currents at the beginning of each exposure (without which the kinetics cannot be reproducible [4]) and reproducible chamber vacuum for the measurement period (which again will prevent attainment of reproducible kinetics). However, measurable responses have been observed for NB concentrations as low as 3 ppm, suggesting that with improved chamber engineering the system could be of use as a portable explosives detection device. The advent of improved explosives of different chemical constitution has currently reduced the strategic moti- vation to invest in such improvements.

Discussion and conclusions

This work shows that selected phthalocyanines can be used to detect organic nitro compounds with high sensitivity. It has also revealed the considerable di5- cnlties involved in detection of this class of compounds. The reasons why the particular phthalocyanines and naphthalocyanines used are the only ones found to be effective are not yet understood. The extensive spec- troscopic and semiconductivity studies leading to their identification as suitable materials will be reported separately elsewhere. Similar development of the other sensor materials, with the improvements in chamber design indicated above together with additional use of the microprocessor in a simple signal-processing mode, should provide an instrument capable of identifying

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and quantifying mixtures of several explosive vapours of this class.

Acknowledgements

This work has been carried out with the support of MOD.

References

1 J.D. Wright, Gas adsorption on phthalocyanines and its effects on electrical properties, Prog. Surfl Sci., 31 (1989) l-60.

2 M. Tan&i and J.D. Wright, Gas-sensingconductometric device useful for explosives, LIK Patent Applic. (1991).

3 C.L. Honeybourne, R.J. Ewen and C.A.S. Hill, Use of thin films of conjugated organic macrocycles as the active elements in toxic gas-sensors operating at room temperature, J. Chem. Sot., Faraday Trans. 1, SO (1984) 851-863.

4 P.B.M. Archer, A.V. Chadwick, J.J. Miasik, M. Tamizi and J.D. Wright, Kinetic factors in the response of organometahic semiconductor gas sensors, Sensors and Actuators, 16 (1989) 379-392.

5 E.C.M. Chen, E.S. Chen, M.S. Milhgan, W.E. Wentworth and J.R. Wiley, Experimental determination of the electron- affinities of nitrobenzene, nitrotoluenes, pentafluoronitroto- luene and isotopic nitrobenzenes and azulenes,l. Phys. Chem., 96 (1992) 2385-2390, and refs. therein.