## 1 Introduction
The actual sample preparation and chromatographic analysis may also be time-consuming. In contrast, electrochemical biosensors have been developed based on screen-printing technology [9; 10]. These devices have the advantage that they can be mass-produced in a wide range of geometries at low cost, particularly when carbon materials are employed to construct the electrodes. We explored the possibility of developing a screen-printed biosensor incorporating glycerol dehydrogenase (GLDH) as the biorecognition element. This enzyme requires the cofactor NAD\({}^{+}\) for the oxidation of glycerol, which results in the formation of NADH. Consequently, it was considered possible to measure glycerol indirectly by monitoring the generation of NADH during the oxidation reaction. Our group was the first to develop an electrochemical strategy for the measurement of NADH based on a Meldolas Blue-Screen-printed Carbon Electrode (MB-SPCE) [11]. Meldolas Blue acts as an electrocatalyst for NADH oxidation, and the resulting anodic current constitutes the analytical response [11; 12]. The main advantage of this approach is that the amperometric detection of NADH can be performed at potentials of 0.0 V vs. Ag/AgCl, whereas potentials in excess of +0.5 V vs. Ag/AgCl are required to obtain the limiting oxidation current at plain carbon electrodes [13]. Consequently, the selectivity is expected to be greatly improved with the former, which is highly beneficial for the analysis of real samples that may contain electroactive interferences. We have exploited the above approach, employing biosensors based on MB-SPCEs to measure a variety of target analytes in different sample types, and all of the following examples have employed this platform. A lactate biosensor was fabricated by coating the surface of the screen-printed electrode surface with lactate dehydrogenase and the cofactor NAD\({}^{+}\). This device was successfully applied to the measurement of lactic acid in serum [14]. This analysis could be performed by simply depositing 20 \(\mu\)L aliquots of solution onto the biosensor strip for chronoamperometric measurements. The resulting data demonstrated the possibility of applying the lactate biosensor to biomedical studies. In the area of environmental monitoring, a disposable screen-printed ammonium ion biosensor was constructed by immobilising the enzymes glutamate dehydrogenase, 2-oxoglutate, and NAD\({}_{+}\) onto the electrode surface and was successfully applied to river water analysis [15]. This biosensor was subsequently incorporated into a commercial hand-held instrument for the measurement of ammonium ions in sewage effluent [16]. Additionally, we explored the possibility of adapting the MB-SPCE platform for the fabrication of amperometric biosensors for applications in the food industry. An amperometric glutamate biosensor was constructed by immobilising glutamate dehydrogenase onto the electrode surface after it had been modified with the biopolymer chitosan [17]. This was successfully applied to the determination of glutamate in a stock cube following a simple dissolution step in buffer solution, to which was added NAD\({}^{+}\). In order to circumvent the need to add NAD\({}^{+}\) to the sample, an improved glutamate biosensor was subsequently developed. This involved incorporating GLDH and NAD\({}^{+}\) into a hydrogel formed by chitosan and carbon multi-walled nanotubes (MWNTs), which were deposited onto the electrode surface using a layer-by-layer approach [18]. This novel biosensor was successfully applied to the determination of glutamate in a stock cube. In recent years, we have investigated the possibility of developing a biosensor for androstone, as this compound, together with skatole, is responsible for an off-taint in pig meat known as boar taint [19]. We immobilised the enzyme hydroxysteroid dehydrogenase (HSD) onto the screen-printed electrode by an adsorption procedure.