Simultaneous amperometric detection of ascorbic acid and antioxidant capacity in orange, blueberry and kiwi juice, by a telemetric system coupled with a fullerene- or nanotubes-modified ascorbate subtractive biosensor

Four fullerenes- or nanotubes-modified graphite sensor-biosensor systems (SBs), coupled with a dual-channel telemetric device, based on an ascorbate oxidase (AOx) biosensor, were developed for on line simultaneous amperometric detection of ascorbic acid (AA) and antioxidant capacity in blueberry, kiwi and orange juice. Fullerene C60 (FC60), fullerene C70 (FC70), single-walled carbon nanotubes (SWCN) and multi-walled carbon nanotubes (MWCN) increased the sensitivity of graphite toward AA and phenols 1.2, 1.5, 5.1 and 5.1 times respectively. Fullerenes combined with AOx improved the selectivity toward AA more than nanotubes, being able to hold a higher number of AOx molecules on the biosensor surface. The SBs work at an applied potential of +500 mV, in a concentration range between the LOD and 20 μM, with a response time of two minutes. The LOD is 0.10, 0.13, 0.20 and 0.22 μM for SBs modified with FC60, FC70, SWCN and MWCN respectively. Biosensors register lower AA currents than the sensors due to the enzyme capability to oxidize AA before it reaches the transductor surface. Phenols currents registered by sensors and biosensors did not differ. Based on the difference between sensor and biosensor recorded currents a AA selectivity index was developed as an indicator of specificity toward AA and of the capacity to distinguish between AA and phenols contribution to the antioxidant capacity. This value is almost zero for fullerene-modified SBs, 0.13 and 0.22 for SWCN- and MWCN-modified SBs respectively. The results of juices analysis performed with SBs were in accordance with reference methods.