Disposable Smart Sensor Embedded with Reduced Graphene Quantum Dots for Multimodal Detection of Epinephrine

The reduced graphene oxide quantum dots developed using a biobased reducing agent from pristine graphene oxide exhibit a broad-spectrum fluorescence emission, attributed to the incorporation of multiple active functional moieties derived from the bioextract of <i>Lawsonia inermis</i>. These surface functional groups serve as key binding sites for selective analyte interaction. In this work, we have synthesized bioreduced graphene quantum dots for the sensing of the epinephrine hormone, which is important for regulating respiratory function, physiological stress response function, and blood redistribution function of the human body. In their zero-dimensional form, graphene-based materials exhibit both fluorescence and electrical conductivity arising from surface plasmon resonance and a high surface-to-volume ratio. We have harnessed both of these properties to develop a multimodal sensor system that demonstrates a ratiometric fluorescence response due to the inner filter effect alongside an enhanced electrical conductivity driven by ionic motion. A flexible cellulose filter-paper strip has been used as a platform for efficient charge transport through facile adsorption fluorescence color detection. For stable and reliable current conduction, the paper strip is coated with a biopolymer composite of optimized proportions. For practical application, we have fabricated a mobile-phone-assisted portable sensor system employing cost-effective materials and common electrical components. This smart sensor can visually distinguish variations in the RGB color intensity of the sensing solution at different epinephrine concentrations. The system exhibits consistent and high-sensitivity performance across various biological fluids, including blood serum, urine, and sweat with limits of detection of 0.23, 0.17, and 44.6 nM for fluorescence, electrical, and smart sensing methods, respectively. Overall, this study introduces a novel, multifunctional biosensing platform that integrates eco-friendly materials, portability, and real-time biomonitoring, paving the way for next-generation, accessible healthcare diagnostics.