## 3 Results and Discussion
### Analysis of Berries  
In order to eliminate the matrix effect, the multiple standard addition method was used in analysis with the developed TYR biosensor at optimal conditions, where a volume of 1.0 mL of berries extract was added to 9.0 mL of supporting electrolyte and subsequently 3 or 4 standard additions of 0.01 mol-L\({}^{-1}\) Trolox (each of 0.5 mL) were applied. A representative amperogram of raspberries analysis with standard addition curve \(I\) (\(\mu\)A) = \(-\)2.0082 \(c\) (mmol-L\({}^{-1}\)) - 1.5137 (\(\mu\)A) and \(R^{2}\) = 0.9978 is shown in inset of Figure 5. As reference method, DPPH assay was used with these parameters of calibration curve for standard compound Trolox \(A\) = \(-\)0.0103 \(c\) (mmol L\({}^{-1}\)) + 0.3516 and \(R^{2}\) = 0.9943. The biosensor provided constant current response with stable base line after 120 s. In amperometric techniques, response time is one of the most important parameters describing the performance and quality of biosensors, because it reflects kinetics of electrode reaction. The response time differed with each kind of berries. It is interesting to note that a shorter response time was observed for raspberries and cranberries (<10 s) than for standard addition of Trolox (>25 s). It can be attributed to polyphenolic compounds present in berries, which are more suitable for TYR active site (simpler substrate) in comparison to Trolox. In the case of lifetime, the long-term stability of the biosensor (stored dry in a fridge at 5 \({}^{\circ}\)C) was tested for up to two weeks by monitoring of 150 \(\upmu\)mol L\({}^{-1}\) Trolox solution (see Figure S3) every 2 days (results not shown). TYR biosensor usually provides a constant current response for one week (signal drop by 19%). The fabrication method based on Nafion(r) is relatively simple, but potential leaching of  
Figure 5: Representative TEAC analysis of raspberries using multiple standard additions method with CPE/MWCNTs/TYR/Nafion® biosensor at constant working potential -0.25 V and stirring speed of 400 rpm. the enzyme should be taken in account. Moreover, a probable polymerisation of Trolox could shorten the lifetime of TYR biosensor. Final TEAC values of individual dried berries obtained at the TYR biosensor and DPPH assay are shown in Table 1. It is obvious that TEAC values are different for the both used analytical methods. Generally, measurements with DPPH radical offer satisfactory repeatability of more than 95%. The repeatability of obtained results at TYR biosensor was comparable with DPPH assay for all studied berries except strawberries (90%). It can be caused by the high content of nonphenolic antioxidant e.g., AA, carotenoids, minerals with reduction properties, etc. and by presence of insoluble fiber. For example, the AA content in strawberries pulp is up to 90 mg\(\cdot\)100 g\({}^{-1}\) depending on variety, which is at a similar level as in citrus fruits. A presence of AA in pulp of berries is known and its content taken from the National Nutrient Database (USDA) (U.S. Department of Agriculture, 2010); for standard reference, the release available can be found in Table 1. Presented data are also comparable with information from the literature [24; 25]. Moreover, the studied amperometric CPE/MWCNTs/TYR/Nafion(r) biosensor can be useful for determination of total phenolic content (TPC) [26] and also total antioxidant capacity (TAC) [27], as Trolox equivalent for samples with low concentration levels of other nonphenolic antioxidants. For dried berries (blueberries, cranberries and raspberries), the values of TEAC for the presented biosensor are in good correlation with reference method of DPPH assay as shown in Table 1. One of the discrepancies can be observed in the case of blueberries where, especially in pulp, the value of TEAC for the biosensor is lower than the value from the reference method. This is probably due to the presence of a higher amount of anthocyanins which exhibit anti-TYR activity [28].