Upcycled Lupinus luteus peel-derived carbon dots for copper-assisted fluorometric sensing of alendronate

Osteoporosis affects millions worldwide, and alendronate sodium (ALD) remains a first-line therapy. However, ALD has a narrow therapeutic index and lacks a chromophore, making its routine monitoring in patients and quality control laboratories challenging. This work addresses the need for a simple, sensitive, and eco-friendly analytical method by introducing a fluorescent probe derived entirely from agricultural waste. Nitrogen and sulfur co-doped carbon dots (N, S-CDs) were synthesized from yellow lupin (Lupinus luteus) peel biomass using a rapid 5 minutes microwave-assisted solvothermal method. The resulting N, S-CDs exhibit strong blue fluorescence (λ _em = 430 nm, λ _ex = 340 nm) with a quantum yield of 13.07%. Upon addition of Cu²⁺ ions, the fluorescence is effectively quenched. Subsequent introduction of ALD restores the fluorescence due to preferential chelation between ALD and Cu²⁺, displacing Cu²⁺ from the carbon dot surface. The fluorescence enhancement ratio (F/F ₀) shows a linear response to ALD concentration from 0.9 to 45 µg mL^-1, with a limit of detection of 0.60 µg mL^-1. The method was successfully applied to pharmaceutical tablets (recoveries 95.5-97.0%) and spiked human serum (98.8-102.1%) and urine (99.0-103.1%) samples. By valorizing lupin peel waste-an abundant, low-cost, and naturally nitrogen/sulfur-rich biomass-this work demonstrates a sustainable pathway for converting agricultural byproducts into functional analytical tools. The proposed N, S-CDs/Cu²⁺ probe offers a green, rapid, and cost-effective alternative to conventional HPLC methods for ALD monitoring in both pharmaceutical and clinical settings.