A tissue‐based amperometric biosensor for xanthine determination has been developed. The biosensor uses a carbon paste electrode (CPE) which contains buttermilk (BTM) as source of xanthine oxidase (XOD) (EC.1.1.3.2.2.) and cobalt phthalocyanine (CoPc) as mediator. The system is also mediated with ferricyanide in solution to obtain a double‐mediated biosensor. The effect of various variables upon the response was studied. Linearity was observed over the concentration range of 1–15 mM xanthine. The optimum operational pH range for the electrode is 8.0–9.0. Reproducibility was studied and a relative standard deviation (rsd) of 10.26 % was found. After 60 days the normalized response of the BTM‐modified electrode was 60 % of the first day.

The use of mushroom tissue as a source for polyphenol oxidase for the determination of phenolic compounds is described. The enzymatic reaction between mushroom polyphenol oxidase and phenolic compounds is coupled with the use of cobalt phthalocyanine dispersed in carbon paste. The effects of applied potential to the responses are investigated. Cobalt phthalocyanine dispersed electrodes give shorter response times and a lower applied potential compared to conventional tissue biosensors.

A tissue-based amperometric biosensor for xanthine determination has been developed. The biosensor uses a carbon paste electrode (CPE) which contains buttermilk (BTM) as source of xanthine oxidase (XOD) (EC.1.1.3.2.2.) and cobalt phthalocyanine (CoPc) as mediator. The system is also mediated with ferricyanide in solution to obtain a double-mediated biosensor. The effect of various variables upon the response was studied. Linearity was observed over the concentration range of 1-15 mM xanthine. The optimum operational pH range for the electrode is 8.0-9.0. Reproducibility was studied and a relative standard deviation (rsd) of 10.26 % was found. After 60 days the normalized response of the BTMmodified electrode was 60 % of the first day.Keywords: Xanthine oxidase, Biosensor, Buttermilk, Cobalt phthalocyanine, MediatorCoupling tissues and pure enzymes as sensitive agents for biosensors to enable detection of chemicals of agricultural and pharmaceutical interest has been the subject of a considerable research effort. Tissue-based amperometric biosensors use tissues as enzyme sources while having the advantage of low cost and simplicity. One drawback of tissue-based CPEs is the low reproducibility. This can be addressed with the use of mediators and some careful qualititave work. Thus different successful applications of tissue-based CPEs have been carried out [1][2][3][4][5][6][7][8][9][10][11][12]. Simple operation and less susceptibility to interference can be achieved by incorporating mediators such as ferricyanide and its derivatives with the biosensing element [13][14][15][16][17][18][19][20]. Electrochemical characteristics of some metallophthalocyanines have been tested to be employed as mediators [21][22][23]. CoPc modified enzyme electrodes have been previously designed [7,[24][25][26][27] for amperometric detection. Amperometric, mediated biosensors have been successfully used often in the detection of hypoxanthine and xanthine [28][29][30][31][32][33][34][35][36]. Among these some employed double-mediated biosensor systems [20] with higher response characteristics compared to single mediator systems. Xanthine, an important endogenous molecule, can be detected by different methods [37][38][39], including amperometric biosensors.In the present study we designed a biosensor system which uses BTM tissue as source of XOD (EC.1.1.3.2.2.) and couples the CoPc and ferricyanide mediators for the enhanced detection of xanthine. The utility of BTM comes from the fact that BTM tissue contains XOD [40][41][42][43][44]. It has been shown that BTM XOD activity increased during cold storage (5 o C, 24 h.) or heat treatment (60 o C, 5 min) [42]. On the other hand BTM XOD inhibitory effects of various drugs and chemicals such as salicylic, acetylsalicylic, gentisic and azodisalicylic acid, sulfalazine, folate analogues have also been reported [43,44]. The present approach to xanthine biosensor design differs from traditional amperometric XOD electrodes by not involving monitoring of hydrogen peroxide. Such a doubleme...

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