Limonin is a biomolecule which is responsible for the bitter element in many citrus fruits and vegetables. It has proven to be antioxidants which are biologically active and exhibit anticarcinogenic activity. It is formed from a tasteless precursor called limonite β ring lactone, which is found in the juice sacs. During juice extraction from the fruit, the precursor encounters the acidic compound of the juice and is gradually lactonized to form bitter limonin. This bitterness is generally not desired, therefore quantification and detection of limonin is very crucial for improving fruit quality in the citrus industry. The current techniques available for detecting limonin in fruits are time consuming, expensive and not fit for active monitoring of limonin levels.

In this study, we have shown the applicability of ceria nanoparticles (CNPs) to be used as a transducer integrated in silk fibroins that is coupled with an organic electrochemical transistor to detect limonin. CNPs possess antioxidant properties which is attributed to the redox nature of dual oxidations states, Ce3+ and Ce4+, switching on the surface. This constant switching of oxidation states on the surface produce oxygen vacancies that act like hot spots for targeting electroactive entities like limonin. Therefore, in the present work, the electrochemical interaction between CNPs and limonin has been exploited to detect limonin using a transistor platform. Multiple characterization techniques were done to confirm the presence of CNPs in silk fibroins. The SOD and the catalase mimetic activity was assessed using SOD assay kit and Amplex red kit which showed the preservation of CNPs antioxidant properties in silk. Immobilizing and embedding CNPs in silk fibroins was studied to provide the most sensitive and efficient transducer for the OECT. In order to find the limit of detection of the OECT, limonin concentrations were varied and tested at different gate voltage (V_G). The limonin OECT prepared in this study showed a detection limit of 10nM with a constant increase in the I_D current with limonin concentration. A range of interfering species which are commonly available in citrus fruits, were also tested to investigate the selectivity of the sensor. It was observed that none of the analytes, other than the limonin, produced a noticeable change in current. Thus, the CNPs-OECT limonin sensor has potential applications in sensing limonin on field applications. Future field and research application of the CNPs-OECT sensor could include real time monitoring of the amount of limonin in fruit juices or to study and reduce the amount of limonin in citrus fruits and to quantify the dosage of limonin as a potential antioxidant material.