POLYANILINE RETAINED GLASS TEMPLATESAS SENSORS FORACIDIC/ BASIC AND/OR REDOX GASES

Polyaniline is a conducting polymer that can exist in six diftrent stiuctural forms depending upon the extent of' oxidation and the protonation of the polymer. These forms have their own cha~.acteristic physical properties sucll as colour and electronic conductivity. Tlle different forms can he interconverted lly suitnhle acid/ 17ase andlcw reclox treatments. The use ofthis fact to devise sensors for acidhase and/ or redox Rases using an array of different h r m s of polyaniline retained glass plates is descfibed in this nrlicle.


INTRODUCTION
Polyaniline (PANI) has received a great deal of academic and. industrial scientific interest as one of the promising conductive polymers that is attractive for several technological applications. These include their use as electrodes in rechargeable batteries, electrochromic devices, information memories, electrocatalysts and liquid crystal display devices."Vhis polymer can be prepared by electropolymerisation or by chemical polymerisation from an aqueous or non-aqueous acidic solution. The polymer can be chemically attached to glass surfaces to give materials with a surface electronic ond duct ion.'''^ The polymer can e x~s t in SIX different structural forms with the stability of a given form depending upon the extent of protonation and the oxidation of the polymer chains. The six forms are inter-convertible and their physical characteristics are summarised in Table 1. As can be clearly seen from this table, the different forms of the polymer are stable under hfferent acidichasic and redox conditions. Thus, by exposing a selected form of the polymer to gases with acidic, basic, reducing, oxidjsing, acidic and reducing or oxidising and acidic properties, i t should be possible to convert the chosen form of the polymer to different forms with a concomitant change in colour and conductivity. This property of the polymer has already been utilised in devising pH ~e n s o r s ,~ sensors'for various other ionsb-" and also for NH, g a~,~J " However, we show below that this polymer can be used to sense not only a single ~onic or gaseous species but can also be used to devise a sensor for various species havlng a c~d h a s e andlor redox properties. The different species can be distinguished and measured separately, In our work, we have exposed different forms .of polyaniline attached on to s u r f a~e s of glasses to gases such as Cl,, HC1, H,S, NH,, and the changes of colour ~n d conductivity upon exposure were determined. In this way, i t is possible to qualjtatively identify the different types of gases using the polyaniline sensor. Furthermore, the extents of changes in each case were dependent upon the amount, of a given gas to which the polyaniline plate was exposed. Hence, the same material may also be used to quantitatively measure the above gases.

METHODS AND MATERIALS
All chemicals used were of Analar grade and were purchased from Aldrich. The aniline used was distilled under nitrogen. The glass plates used to attach polyaniline on their surfaces were of high-quality pre-cleaned glasses. Polyaniline-attachment to glass surfaces was done as described el~ewhere.",~ The as-prepared glass plates according to the method" were green in colour with a resistivity of 120 kR cm-' while those prepared by the other method4 were also green in colour with an improved conductivity (resistivity = 12 kQ cm-I). The polymer in these cases was therefore, in its emeraldine salt form. The other forms of the polymer retained glass plates were prepared by necessary acidbase or redox treatment of the emeraldine salt retained glass plates. Polyaniline retained glass plates were dried for several days in a dry desiccator. The electrical connections to the glass plates were through two tight copper clips fured to the plates within a 1 cm distance, each soldered to a copper wire. The desiccator was connected to a vacuum manometer and evacuated. Measured volumes of the selected gas were introduced to the desiccator through a drying tube. The conductivity and the colour of the polyaniline retained glass plates were determined as a function of the partial pressure of the.gas in each case. Table 2 summarises the qualitative results obtained when the polyaniline plates. were exposed to the above gases. The quantitative estimation of NH,, HC1, H,S and Cl,, when they are alone, using different forms of polyaniline are shown in figures 1-4 respectively. Exposure of emeraldine salt retained glass plates to NH, gas resu1.t~. in the increase of resistivity as depicted in figure 1. When the emeraldine saltretained glass plates prepared according to the procedurehnd subsequently immersed in a dilute alkali solutjion to give a greenish-blue colour with a high resistivity are exposed' to HC1, the conductivity is i'ncreased in a systematic way as shown in figure 2. In the case of an acidic and reducing gas such as H,S, i t would be better to use a fully oxidised violet pernigraniline base form of polyaniline since only gases with these properties can change the conductivity and the colour of the polymer. Thus, the polyaniliile coated glass plates were first deprotonated by treating with NaOH until they became blue and non-conducting. The materials were then further oxidised using'an aqueo,us Fe:", and the resulting violet polymer was washed several times with double distilled water and dried in a dry desiccator. Figure 3 depicts the results ' obtained. For an oxidising and neutral gas such as C1, the best form of polyaniline that can be used for quantitative estimation of the gas is emeraldine salt. The results obtained for such polymer-retained glasses when exposed to different doses of C1, are shown in figure 4.

DISCUSSION
. . Table 2 may be explained as follows. Since the protonated salt form and the deprotonated base form of the leuco-emeraldine have the same colour (yellow) and both forms are electrical insulators, when leuco-emeraldine salt form attached glass plates are exposed to acidic (HCl), basic (NH,) or reducing and acidic (H,S) gases there will be no change in colour or measurable electronic conductivity. However, when the same form of the polymer is exposed to an oxidising gas such as Cl,, the polymer gets oxidised. The results also show that C1,gas being a powerful oxidising agent, oxidises the polymer to its electronically conducting green emeraldine salt form if exposed to a low dose but to an insulating violet pernigraniline salt form if exposed to a sufficiently high dose. This suggests that the oxidatioh of the polymer chains by a chemical oxidant takes place throughout the polymer chains as in a homogeheous reaction as opposed to electrochemical oxidation in which the propagation of conductive zones along the polymer chain starting from the electrode surface has been proposed." Thus, from an array of glass plates containing different forms of polyaniline, the plate with leuco-emeraldine salt form is capable of distinguishing the oxidising gases from reducing, acid or acidic and reducing gases.

The results summarised in
The colour changes observed with emeraldine salt retained glass plates are different for the four different types of gases. Hence, using the emeraldine salt form retained glass plates, the gases may be identified when they are in isolation.   Figure 4 : Resistivity of emeraldine salt retained g l a s~ plates as a fuxiction of the partial pressure of exposed C1 gas,p.
A drastic increase in conductivity of the emeraldine base (blue) retained plates is only obtained if the plates are exposed to an acidic non-redox gas such as HCI fumes. Thus, the non-redox acidic gases may be distinguished from others using an emeraldine base retained glass plates. However, it should be mentioned that the polyaniline layers retained on glass plates by the above are stable only under acidic or neutral condtions. When the glass plate containingpolyaniline f rmly retained is treated with a concentrated solution of NaOH for a relatively longer period (more than 6 hours) the polymer layer begins to peel off. Thus if one wants to use the emeraldine base retained glass plates (blue) one has to start with the emeraldine salt retained plates (green) and. treat with an alkali just prior to use. The peel off of the polymer when treated with NaOH may be due to the base hydrolysis of Si-N bonds that are likely to be the polymerlglass attachment site.
Only the gases with both acidic and reducing properties, such as H,S, can change the conductivity of the pernigraniline base. Thus the pernigraniline base coated glass plates are capable of' distinguishing the reducing and acidic gases from other gases. In the case of NH,?, detectable conductivity changes and colour changes of polyaniline retajned glass plates upon exposure to different doses of the gas can be seen clearly if the green emeraldine salt form of the polymer is used. As seen from Fi,oure 1, a systematic increase in resistivity, i.e., decrease in electronic conductivity, can be observed when the partial pressure of ammonia is increased. The plot mag be used as a calibration curve to determine the partial pressure and heilce the amount of ammonia. The colour change is froni greenish violet to bluea When partial.1~ deprotonated emeraldine salt form is exposed to an acidic gas such as HC1 fumes, the conductivity of the pol.gmer will be increased.. This is due to the fact that the polymer fj.lms in this state can get protonated. upon exposure t,o 2111 acidic and non-redox gas. In fact, when the emeraldine chloride form ret,ained glass plates are d.ried for several days, the colour changes gradually from green to greenish blue with simultaneous reduction in conductivity due to the removal of protons as HCl fumes. Thus, the dry film of emeraldine chlori.de can get protonated when exposed to HC1 giving a higher conductivity and the trend seen in figure 2 is due to this reason.
The exposure of the pernigraniljne base form of polganiline to various doses of H,S gas results in an initial decrease followed by an increase of the resistance showing that the initial protonation of the emeraldine base gives enleraldine salt a t low doses but the reduction also takes place to give leuco-emeraldine salt form.when higher doses are administered.
As shown in figure 4, when emeraldine salt is exposed to a n oxidising gas such as C1, the polymer gets oxidised resulting in the reduction of its conductivity. At a sufficiently high dose, the polymer turns violet and then its cond.uctjvity cannot be measured.
The exposure of sensory materials t o a mixture of gases is next considered. I t is worth mentioning here that most gases, which are active towards polyaniline sensor, are mutually reactive and hence it is very unlikely that they would coexist. For example, detection of components in a mixture containing acidic (e.g. HC1) and basic (e.g., NH,,) would not arise in reality. The situation is the same for reducing and oxidising gases. So the problem now reduces to the practical situation of mixtures having non-reactive gaseous components. These components would most probably show similar behaviour towards the polymer and hence t h e identification of components would not be realistic. Consider, as an example, a mixture that could. contain Cl,, Br, and I, gases. Both C1, and Br, are capable of oxidising both leuco-emeraldine salt and emeraldine salt to their highest oxidj.sed pernigraniline state. However, the redox potential of I, (0.536 V a t 25 OC) is such that it can only oxidise leuco-emeraldine into emeraldine salt but its further oxidation into pernigraniline is not possible. However an array of sensors having different forms of polymers would not distinguish the components. Nevertheless, the sensors are attractive for both qualitative identification and quantitative determination of individual gases.