STUDIES ON DEVELOPMENT OF MEMBRANE SENSORS AND THEIR ANALYTICAL APPLICATIONS

Abstract

The development of high performance membrane sensors (ISEs) is a fast growing area and the electrodes now take their place among the several recent developments in analytical chemistry which can be claimed as spectacular. Thedevices find application in the analysis of raw materials, in quality control of the products, monitoring of environment and numerous other situations due to fast speed, sensitivity, cost, reliability and non consumption of sample in the process. ISEs are specially useful in field applications and also in clinical studies where a large number of samples need a rapid, cheap and reliable method of analysis. The device can be used irrespective of colour, viscosity etc., of the test solution and does not require any sample preparation. Due to major advancement made in this field, efforts have continuously been directed to explore more selective ligands and there have been relatively few notable advances. Asurvey of literature as well as market reveals the availability of electrodes for mono and bivalent cations. Sensors for polyvalent cations and anions are still not commercially available. Even those commercially available do not display ideal selectivity. As such investigations for tailor made electro active phase, having specific selectivity for a particular ion specially polyvalent ions are called for. The development of biosensors is an outcome of the efforts of analytical, biological and clinical chemists involved in perfecting instruments and techniques capable of determining the identity and concentration of living things. Biosensors incorporate a biological sensing element either - intimately connected to or integrated (i) within a transducer. Biosensing applications are manifold - these devices apart from being concerned with clinical chemistry also find growing applications in fermentation monitoring and control, assessment of industrial and natural environment and in food industry for rapid methods of estimating shelf life, deterioration and contamination. Research interest in this direction is evident in terms of excellent review articles and monographs available on the subject. In spite of a significant development in various aspects of this technique - the device is yet to be commercially exploited and utilized and the field provides a rich area of further investigations. The work included in this thesis deals widi the development of sensors with the help of inorganic gels and bacterial strains. Inorganic gels taken up for investigations were selective to both cations as well as anions. Two inorganic gels viz., titanium ferricyanide and a mixed oxide of titanium-iron doped with cerium were found to exhibit promising cation exchange characteristics and their membranes have been tried for the estimation of potassium and barium ions. Thorium tellurite and stannic tellurite gels, prepared under specified conditions, were anion selective and their membranes were used for the estimation of sulphate and chromate ions. Besides this a glucose sensorhas been developed by immobilising lactobacillus bulgaricus and a phenol sensor is fabricated with the help of pseudomonas cruciviae. Before going into the detailed investigations with a membrane sensor, the amount of membrane ingredients was first optimized so that it demonstrates best performance in terms of detection limit and slope. The time and concentration of equilibrating solution were also ascertained so that the membrane develops

reproducible, noise free and stablepotentials. These investigations were performed with all the gel membranes included in this dissertation. For obtaining the inorganic gels possessing good exchange capacity and selectivity some preliminary experiments were performed to determine the optimum conditions of precipitation viz., the concentration of reactants, mixing ratio, mode of mixing, pH etc. The parameters were set to provide a material which should be stable and possess best exchange characteristics. Titanium ferricyanide was obtained by mixing titanium tetrachloride (102 mol dm"3) to ferricyanide (10 2 mol dm"3) at pH~l and 90° temperature. The mixing ratio was 2.5:1. The black coloured product could be sieved to desired mesh size and was stable in acid and salt solutions but decomposes in alkaline medium. The exchange capacity of material for K+ is 0.18 meg g"1. The chemical formula of the compound, as determined by chemical and T.G. analysis etc., is K(TiO) [Fe(CN)6]. Polystyrene based membrane of this compound has moderate swelling, water content and electrolyte uptake. Specific conductivity data of the membrane in various cationic forms exhibits the sequence: Na+> K+ > Li+ > Tl+ > Rb+ > Cs+ and Zn2+ > Ca2+ >Mg2+ Membranes equilibrated with KC1 solution of 0.1 M concentration for 3 days were used for potential measurements with 10"' to 10"5 M KC1 solutions. 10"1 M KC1 was used as reference solution. The proposed sensor can measure K+ in the range of concentration 10"' to 10"4 M although it is non-Nernstian in behaviour. The response time is 20 s and the potentials remain constant for three minutes. The sensor can be (iii) used in the pH range 5 to 9 and also in non-aqueous medium up to 30% non-aqueous content. Selectivity of the proposed sensor was estimated by Fixed interference mediod. Bi and trivalent cations do not interfere at all in the working of this electrode assembly. Among the monovalent ions Rb+, Cs+ and Ag+ may cause some interference if the same are present at concentrations higher than 10~3 M. Na+ and NH4+ interfere even at 10~4 Mconcentration. Changing anions do not cause any disturbing influence. Other K+ selective membrane sensors proposed in literature record very limited selectivity and life time of one month only whereas the one under consideration can be used up to four months time. Mixed hydrous oxides are known to posses better sorption characteristics than simple oxides. Mixed oxides prepared with suitable dopants exhibit even better exchange capacity and selectivity. Ti(IV) - Fe(III) mixed and Ce(IV) doped oxide was prepared by using fresh solutions of ferric chloride and titanium tetrachloride 0.2 M each and 0.01 Msolution of eerie ammonium nitrate. Coprecipitation of the oxide was performed with 1MNH4OH at pH ~ 6.0. The brownish black product was stable in acid and salt solutions. DTA, TGA and IR has been used to characterise the product. Exchange capacity ofthe product was 0.57 meg g"1 ofthe material. Polystyrene based membranes were prepared and the values of porosity, water content and electrolyte uptake were found to be maximum for Ba2+. The magnitude of these parameters is more for membranes loaded with monovalent ions. The conductance data of the membranes in various cationic forms can be arranged in the following sequence:

Cs+>Tl+>Rb+>K+and Ba2+ >Ca2+>Mg2+>Zn2+ It was observed that the doped - oxide membranes have higher conductance values than the respective undoped ones. This particular membrane was found to exhibit a much better selectivity for Ba2+ as compared to other mono - bi or trivalent ions. These membranes were equilibrated with 0.1 M BaCl2 solution for 48 h and a linear plot of potentials vs concentration was observed in 10"' to 10"4 M concentration. The membrane sensor can measure Ba2+ in the range 7 x 10"5 to 10"1 M concentration as per IUPAC recommendations. The response time is 30 s over the entire working concentration range and the functional pH range is 5 to 9. The electrode system can be used in non-aqueous medium also having a maximum non-aqueous content equal to 25 per cent. Stable potentials are not obtained in solutions having more non-aqueous content. Most of the monovalent, bi and trivalent ions do not interfere widi the working of the electrode assembly except Li+, NH4* and Cu2+ ions. These ions may interfere if the same are present at concentrations higher than 10~3 M. The membrane sensor has also been used as an indicator electrode in the titration of BaCl2 widi K2S04. Thorium tellurite was obtained by mixing thorium nitrate solution (0.033 M) to potassium tellurite (0.067 M) at pH < 1. Mixing ratio of thorium/tellurium was 0.49. White gel was washed and dried at 60°C and analyzed. Anion uptake capacity of the product, measured in terms of 36C1 uptake was 1.12 meq per gram of the compound. On the basis of chemical analysis data the following formula was assigned to thorium salt: Th(TeO,)2, 4H,0 (v) Functional properties of polystyrene based thorium tellurite membrane are higher than those of titanium ferricyanide and lesser than the hydrous oxide membranes. Conductivity measurements of the membrane in various anionic forms present the following sequence: Cr04" > CI > S042" > Br" = N03" > Mo04" > P04~ > Fe(CN)46" This membrane was found to be quite selective to Cr042" ions and so the potentials were measured by fixing the membrane in contact with K2Cr04 orK2Cr207. Before measuring the potentials the membrane was equilibrated with chromium ion solution (0.1 M) for 48 h. The electrode assembly can be used for the estimation of chromium as chromate/dichromate in the concentration range 5 x 10"5 to 10"' M. Response time of the membrane sensor was 20 s and the potentials remain constant for five minutes. The working pH range of the assembly is 3 to 6 if Cr6+ is taken as dichromate and 8 to 11 if it is taken as chromate. Electrode can be used in partially non-aqueous solvents also. Selectivity coefficient values for various anions at a level of interference of 10"3 M indicate that only CI", Br" and N03" would interfere if present in large amounts. Among the various cations Pb2+, Ag+ and Ba2+ would interfere at levels at which these decrease the chromium concentration by precipitation. Apart from this the effect of anionic detergent on the electrode response has also been observed. Even small addition of the anionic detergent causes some shift in potentials. It is possible to over come this interference by treating the membrane with the surfactant solution (10~5 M) for one hour. The treatment conditions the membrane and it becomes immune to the disturbing effect of anionic detergent.

The membrane sensor has also been used for the titration of chromate ions with barium acetate. Stannic tellurite gel has also been used for the fabrication of membranes selective to anions. This compound was prepared by slowly mixing 0.05 Msolution of stannic chloride to 0.10Msolution of potassium tellurite at pH ~ 1. The product was analysed and the compound can be expressed as Sn (Te03)2.3H20 on the basis of chemical analysis data. Water content was obtained by the pyrolysis curve. Anion uptake capacity of the product is 1.26 meq per gram of the material (obtained by the uptake of 36C1). The membrane of this compound could be prepared with the help of araldite as binder. Except conductivity data, other characteristics of the membrane could not be determined due to the presence of araldite. The conductivity order of membrane in different anionic forms is: S022" > CI" > N03" > Br" > Cr04" > Mo04" > P04" > Fe(CN)46" This membrane can be used for the estimation of S04" in the concentration range 10"' to 7.5 x 10"5 M. The response time of the membrane is between 30 to 40 seconds and the useful pH range is 7 to 10. It can also be used in non-aqueous solvents. Among the various anions CI", Br" and N03~ may cause some interference. Cations like Na+, K+, Cd2+, Zn2+ etc. do not interfere. Alkaline earth metal ions interfere only when these are present in significant amount. Ag+ and Pb2+ affect the electrode assembly at all concentrations.

An anionic detergent SDS, if present, in the system would cause interference. The membrane can be conditioned to tolerate the presence of anionic surfactant by treating it with 5 x 10"4 M SDS for two hours. The electrode assembly could also be used to indicate the end point in the titration of S04" with BaCl2. The two membrane sensors thorium tellurite and stannic tellurite have been successfully used for the estimation of Cr6+ and S04~ in waste water from plating, tannery and pulp and paper industry. Lactobacillus bulgaricus degrades glucose in very short duration of time and this is accompanied with a change in hydrogen ion concentration due to the production of lactic acid. Entrapment technique was used to immobilize the cells of above mentioned strain, for the preparation of membrane. The membrane was coated over a pH electrode with an O-ring and covered with a nylon net. Electrode response was measured with a pH meter. The biosensor can measure up to 2500 mg l"1 of glucose and the functional pH range is 5 to 7. The optimum temperature for the working of the electrode is 31°C. Among the various sugars, sucrose, fructose and lactose cause interference if the same are present at concentrations more than 1500 mg l"1. The biosensor remains stable for eight days under operational conditions. During the course of investigations to identify a suitable bacterial strain for the development of phenol biosensor, it was observed that Pseudomonas Cruciviae is able to degrade phenol in the presence of oxygen and the decrease in oxygen is found to be proportional to phenol concentration. As such the membrane was prepared by (viii) immobilizing the cells with acrylamide, TEMED and ammonium persulphate etc. Membrane was attached to the surface of a gas permeable membrane on the oxygen electrode using an O-ring and was covered with a dialysis membrane. The electrode responds to phenol concentration in the range 0.25 to 4 millimols l"1. Response time of the sensor is four minutes and maximum activity is in pH range 5.5 to 8.0. This sensor also records a positive response to m-cresol, chlorophenols and catechol. Thus these compounds would interfere in the working of this biosensor. Other substituted phenols do not interfere. Stability of the biosensor is one week under operational conditions.