Flexible Printed Biosensor for Selective and Enhanced Sensing of Histidine by a Pyrolysis-Free Green Nitrogen-Doped Graphene Quantum Dots

In this study, we report a pyrolysis-free synthesis of nitrogen-doped graphene quantum dots (N-GQDCF) from a green precursor. This approach eliminates the need for high-temperature processes, resulting in an environmentally friendly synthesis pathway, and achieves a higher nitrogen-doping percentage (14%) compared to the traditional high-temperature pyrolysis (N-GQDCFPYR) method (1.5%) and also the N-GQD synthesized from PANI (N-GQDPANI) (10%). This enables the N-GQDCF to exhibit specific sensing toward the amino acid histidine (HIS), whereas this sensing behavior was not observed in N-GQDCFPYR. A glassy carbon electrode (GCE) modified with N-GQDCF exhibited higher sensitivity and selectivity, with a limit of detection (LOD) of 0.1 nM. Further validation of the sensor’s specificity and applicability using real blood and urine samples was carried out and achieved a recovery rate of ∼100%. To ensure real-time monitoring capability, the bulkier GCE setup was replaced with miniature screen-printed flexible electrodes (SPE). The SPE was made up through a stepwise printing process, developing an all-printed SPE that incorporates reference, working, and counter electrodes. In addition to miniaturization, the transition from GCE to SPE resulted in enhanced sensing performance further, with an ∼6.6-fold increase in current response and a 10-fold reduction in the LOD to 0.01 nM. This successful transfer from GCE to SPE, with improved electrochemical detection performance, marks a significant advancement in the development of practical, real-time wearable sensing devices. This work highlights the potential use of bioderived materials in biomarker applications, paving the way for further advancements in the field of biosensing and diagnostics.