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论文范文
1.Introduction Saccharides play a relevant role in the food industry and the research for the development of reliable and low-cost biosensors is always very active [1]. Lactose and glucose are two of the most important saccharides and the determination of their concentration is very important for different reasons. In particular, lactose is a disaccharide that consists of one molecule of galactose and one of glucose bonded through a β-1→4 glycosidic linkage. It is present in milk in a percentage varying from 2 to 8% (by weight) depending on different species and individuals [2]. In industry, the quantitative determination of lactose in milk and in dairy products is mandatory because lactose content is a fundamental parameter for evaluating milk quality [3, 4]. Moreover, in the last years, the determination of this parameter in foods has acquired a significant role since many individuals manifest lactose intolerance and therefore cannot consume milk and dairy products without suffering and gastrointestinal disorders [5]. This intolerance is related to lack of the required enzyme lactase (β-galactosidase) in the digestive system that inhibits the metabolization of lactose into galactose and glucose [6]. Then the determination of lactose concentration is important for industry and for medical applications. As far as glucose is concerned, it is a monosaccharide and it is the simplest form of sugar that can be absorbed into the bloodstream. It is naturally present in foods such as grains, fruits, and vegetables and it is also one of the major ingredients in many sweeteners and processed foods. The control of glucose level especially in soft drinks is nowadays mandatory due to their role in caloric overconsumption and body weight gain [7]. Various methods have been used for determining the concentration of these two saccharides such as spectrophotometry [8, 9], polarimetry [10], high performance liquid chromatography [11], and infrared spectroscopy [12]. All the above-mentioned techniques are time-consuming, expensive, and they require sample pretreatment and considerable technical skills. Therefore, enzyme-based biosensors can offer a rapid, simple, and robust method to quantify lactose and glucose in food. These devices are particularly useful since they combine the high specificity of the enzymes with the versatility of the transducer. In the last years, different types of enzymatic biosensors for lactose [13–19] and glucose [4, 20–24] determination have been developed for application in food industry, but few of them have been reported for the determination of both sugars. One of the first similar devices used different enzymes (invertase, β-galactosidase, amyloglucosidase, mutarotase, and glucose oxidase) covalently immobilized on a cellulose membrane [25]. Lactose and glucose concentrations were also simultaneously determined by using a measuring cell containing lactose and glucose electrodes made by mixing galactosidase/glucose oxidase and glucose oxidase, respectively, with carbon paste [26]. Another biosensor sensitive to glucose and lactose was developed using coimmobilized ferrocene, glucose oxidase, β-galactosidase, and mutarotase on β-polymer. The ferrocene was entrapped in the β-cyclodextrin polymer and glucose oxidase, β-galactosidase, and mutarotase were cross-linked with the β-cyclodextrin polymer. In this case, cyclic voltammetry and amperometric measurements were employed in order to show the efficacy of electron transfer between immobilized glucose oxidase and a glassy carbon electrode via ferrocene included in the cavities of the β-cyclodextrin polymer [27]. More recently, Soldatkin et al. [28] reported the development of an array of biosensors for the simultaneous determination of four carbohydrates in solution. Several enzyme systems selective to lactose, maltose, sucrose, and glucose were immobilized on the surface of four conductometric transducers and served as biorecognition elements of the biosensor array. The main working characteristics of these devices are reported in Table 1. 
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