Ammonia sensor based on poly (vinyl alcohol) cryogel.

Patachia, Silvia ; Croitoru, Catalin ; Florea, Claudia 等

1. INTRODUCTION

PVA based polymers or PVA composite materials are often employed in capacitive humidity sensor assays due to their relative fast response (e.g. swelling degree modification) with relative humidity. Based on the fact that PVA has the ability to swell or collapse selectivity, depending on the electrolyte's nature (cation and anion type) and concentration, a novel type of electrolyte sensors could be developed (Patachia et al. 2006). The influence of inorganic aqueous salt solutions on synthetic or natural hydrophilic polymeric materials can be deduced from the Hofmeister series. According to Hofmeister series, ions are either kosmotropic (they tend to destabilize the hydrogen bonds between water molecules or between water and polymeric chains leading to a collapsing of the polymeric network) or chaotropic (they tend to stabilize the hydrogen bonds in hydrophilic polymers leading to polymeric matrix swelling) (Bagger et al. 2007). Although sensors can be outfitted today with biological molecules, sometimes those are not robust enough to survive in more rugged environments than laboratory confines (Bunde et al. 1998; Dickert & Hayden 2002; Hayden & Dickert 2001). N[H.sub.3] is the principal form of toxic ammonia and it can be found both freely dissolved or in the form of the N[H.sub.4.sup.+] ion in wastewaters from chemical plants. It has been reported toxic to fresh water organisms at concentrations ranging from 0.53 to 22.8 mg/L. Hatching and growth rates of fishes may be affected. In the structural development, changes in tissues of gills, liver, and kidneys may also occur. Toxic concentrations of ammonia in humans body may cause loss of equilibrium; convulsions, coma, and death. That's why ammonia concentration level monitoring is an important task. Up to this extent there are some variants of ammonia sensors on the market, such as: electrochemical, solid state and capacitive sensors. All of them are suitable for determining the ammonia content from aqueous solutions, from approx. 2 to 20000 ppm (Bollman & Revsbech 2005). However they have certain drawbacks, such as: short sensor life, decreased sensitivity over time, increased calibration frequency, over 10% error in determining ammonia concentration bellow 200 ppm level, poor linearity of analytical response versus concentration, high cost and so forth (Hart & Shea 2001). In this work we have obtained a PVA based material with potential use as sensor. PVA is a non-toxic, non-carcinogenic, biocompatible, biodegradable, water-soluble polymer, in consequence easy to handle and friendly for the environment (Patachia 2003). Physical crosslinking using freezing-thawing cycles has been used for the PVA cryogel obtaining. The method of physical crosslinking of PVA is often employed in pharmacy and medicine (Patachia 2003), as an alternative to chemical crosslinking which uses potentially toxic reagents. One of our aims was to achieve good linearity of the cryogels response (e.g. swelling degree variation) as a function of the external stimulus (N[H.sub.4]OH concentration variation).

2. EXPERIMENTAL

2.1 Materials

PVA 120-98 (1200 polymerization degree and 98% hydrolysis degree) was purchased from Chemical Enterprises Rasnov, Romania. Ammonium hydroxide (25% wt solution) has been purchased from Sigma.

2.2 PVA cryogel obtaining

PVA solution has been prepared by dissolving the polymer powder in Milli-Q water, under magnetic stirring at room temperature, followed by heating at 75[degrees]C for 4h. The solid content of the obtained solution was 11%.wt to a certain amount of this solution, an aqueous N[H.sub.4]OH solution has been added so as the PVA: N[H.sub.4]OH ratio to be 12% wt. The PVA cryogel has been prepared by introducing a specific volume of PVA/ N[H.sub.4]OH mixture in a PVC cylindrical recipient and submitting it to freezing at -20[degrees]C for 12 hours, followed by thawing at room temperature (26[degrees]C) for 12 hours. The above mentioned freezing-thawing procedure has been repeated four times. Alternative, a PVA cryogel without ammonia has been prepared following the same procedure.

2.3 Removal of N[H.sub.4]OH molecule from the PVA cryogel

N[H.sub.4]OH has been removed from the cryogel by washing with distilled water at room temperature. The ammonia removal has been monitored conductometrically, using a Radelkis OK-112 conductometer and by swelling studies. For the swelling studies, the cryogel samples have been soaked in distilled water at room temperature. At different time intervals, each sample was taken out, wiped with filter paper and weighted until a constant mass was observed. The degree of swelling (SD) was determined by using Eq. (1)

SD = [m.sub.s]/[m.sub.d] x 100 (1)

where ms is the mass of the cryogel swollen in water and [m.sub.d] the mass of the dried cryogel [xerogel].

The same procedure has been followed for the PVA/ N[H.sub.4]OH cryogel [[C.sub.1]] as well as for the reference [[C.sub.2]].

2.4 Determination of cryogel sensitivity to N[H.sub.4]OH solution

The weighted cryogel samples (after swelling equilibrium reaching) have been immersed in a determined volume of distilled water. To the amount of distilled water containing the sample, 25% wt ammonia solution has been added from a biurette, and the amount of N[H.sub.4]OH in contact with the cryogel has been calculated. After 10 minutes, the sample was taken out, wiped with filter paper and weighted. Then, the cryogel sample has been reimersed in the ammonia solution, and a new volume of N[H.sub.4]OH solution has been added from the biurette. The above procedure has been repeated for different N[H.sub.4]OH concentrations (from 0.6 to 6% wt) for [C.sub.1] and [C.sub.2].

3. RESULTS AND DISCUSION

The elimination of N[H.sub.4]OH from the polymeric matrix of the PVA/ N[H.sub.4]OH cryogel and for the reference is plotted in Fig.1. The swelling kinetic of both cryogels is presented in Fig. 2:

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

As it can be seen from Fig. 1 and 2, elimination of N[H.sub.4.sup.+] and OH- occurred. The conductivity of the washing solution for [C.sub.1] is higher than that of the reference. The swelling degree of [C.sub.1] is higher, due to the kosmotropic effect of N[H.sub.4.sup.+] ion (according to the Hofmeister series) which tends to break the hydrogen bonds between the water molecules, water and polymer molecules and between the polymeric chains, leading to an increased swelling of the cryogel.

Linear response of the swelling degree with increasing of ammonia concentration has been obtained for both cryogels (Fig. 3). The slope of the linear dependence in the case of the PVA/N[H.sub.4]OH cryogel is higher, which means that the sensitivity of the PVA/N[H.sub.4]OH sample is higher than that of the reference sample.

4. CONCLUSION

PVA 120-98 cryogels with or without N[H.sub.4]OH in composition have been prepared by physical crosslinking and characterized, in terms of their swelling degree sensitivity to aqueous ammonia solution concentration range of 0.6 to 6 % wt.

The obtained cryogels are soft, opaque and have the property to swell selectively when immersed in aqueous media without dissolving.

Linear dependence of the swelling degree has been obtained. The response of the PVA/ N[H.sub.4]OH cryogel to N[H.sub.4]OH solution concentration is better than that of the cryogel reference, for the whole N[H.sub.4]OH concentration range studied.

Use of the proposed PVA/N[H.sub.4]OH material is limited to a temperature range of 4 to 35[degrees]C

Based on the obtained results and on the fact that PVA is an environmental friendly material, frequently used in the field of pharmacy, medicine and advanced purifications, new sensor assay applications based on the mentioned material could be developed in our further studies.

5. REFERENCES

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Patachia, S. (2003). Blends based on poly (vinyl alcohol) and the products based on this polymer, In: Handbook of Polymer blends and composites, Vasile, C, Kulshreshtha, A.K., (Ed), pp.230-256, RAPRA Technology Ltd, ISBN 1-85957-201-4, Shrewsbury, UK.

Patachia S., Valente A. J. M., Baciu, C. (2006). Effect of non-associated electrolyte solutions on the behaviour of poly (vinyl alcohol)-based hydrogels. European Polymer Journal, Vol. 43, No. 2 (July 2006) pp. 460-467, ISSN 0014-3057.