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Zinc Phosphate Glass as an Amperometric Sensor in the Detection of Hydrazine

Introduction Among the II-VI group compounds, the oxides of the II group elements are very well studied, particularly ZnO. ZnO is a wide band gap (3.37 eV) semiconductor and in its nano dimensions, finds application in optoelectronic devices [1-5]. Zinc oxide has the ability to form glasses in a wide composition with glass formers such as silicates [6-8], borates [9-11] and phosphates [12-14]. Zinc phosphate glasses are of particular interest, because of its low transition temperature and high thermal expansion coefficient [15-20] and its application in lasers [21-23], as detectors, receivers, wave guides and so on [24-26] are well known. Phosphate glasses are known to have high proton conduction [27-29] and are being used as electrolyte in batteries [30-32], as a sensor for moisture and hydrogen detection [33-36]. However, to our knowledge we do not find any reports in literature, the use of phosphate based glasses as electrodes, which can function as an electrochemical sensor for the detection of gases, organic molecules etc. Hydrazine and it derivatives find application as rocket fuel [37], as hydrazine fuel cells [38] and so on. It is highly soluble in water and is known to be health hazardous on exposure [39, 40]. Hence, effective detection and quantification of hydrazine in solution is an important task. Several reports are available in the use of nanostructured ZnO for the detection of hydrazine [41-44]. Here, we report perhaps for the first time the zinc phosphate glass as a sensor for the detection of hydrazine. The zinc phosphate glass has the advantage over the nanostructured ZnO in the ease of preparation and is also environmentally benign and has reasonably good sensitivity towards the detection of hydrazine. The zinc phosphate glasses having composition (ZnO)60(P2O5)40, (ZnO)65 (P2O5)35, (ZnO)70(P2O5)30 ( mol % ) are synthesized by conventional melt quench method. The characterisation was done using powder XRD, TGA/DTA and spectroscopic techniques such as UV-visible, IR, Raman and 31 P NMR. Au electrodes modified with the synthesised glasses are tested for the electrochemical detection of hydrazine. The Au electrode modified with the glass of composition (ZnO)60 (P2O5)40 showed better current response for the detection of hydrazine compared to other two compositions of the glass modified electrodes and hence it is further selected for a detailed electrochemical studies. A detection limit of 0.3 μM (S/N = 5), and an average sensitivity of 0.95 μA μM -1 cm -2 with a wide linear concentration range of 0.3506 μM are obtained. 2. Experimental 2.1 Materials Analytical grade zinc oxide (99.9%), ammonium dihydrogen ortho phosphate (99.9 %) and hydrazine hydrate solution (99 %) were purchased from S.D Fine Chemicals, India. Gold disc of diameter 2 mm (geometrical area 0.0314 cm 2 ) supplied by CH Instruments was used as working electrode and a platinum strip as the counter electrode. Phosphate buffer (PBS) having pH 7.4 was prepared using potassium hydrogen phosphate and potassium dihydrogen phosphate in millipore water by a standard procedure. 2.2 Preparation of Zinc phosphate glasses Glasses having composition (ZnO)60(P2O5)40, (ZnO)65(P2O5)35, (ZnO)70(P2O5)30 mol % were prepared by conventional melt quench technique. In a typical procedure, zinc oxide and ammonium dihydrogen phosphate of definite composition were taken and grounded well, and heated to melting in a porcelain crucible at 1150 o C in an electrical furnace for about 30 minutes. When the melt was perfectly homogeneous, it was poured on to a brass metal plate then immediately pressed by another brass metal plate to obtain the required transparent glass. The sample thus prepared was further annealed at 200 o C (below Tg) for 2 hours and allowed