l-Arginine-Functionalized Mild Etchant-Derived Nb₂CT_x MXene for Electrochemical Dual Nucleotide Sensing

5'-Ribonucleotides, such as guanosine 5'-monophosphate (GMP) and adenosine 5'-monophosphate (AMP), are major contributors to the umami flavor of meat, serve as freshness indicators, and are central to physiological processes, including energy metabolism, immune modulation, and cardiovascular regulation. Conventional chromatographic methods (HPLC-MS/MS and LC-MS) provide accurate measurements but are constrained by high cost, complex workflows, and poor portability, limiting their use in rapid food quality assessment. This work aims to develop a sensitive, portable, and low-cost electrochemical platform for simultaneous GMP and AMP detection. For the first time, Nb₂CT_x MXene was synthesized via rapid, HF-free scalable production by using a molten salt etching with NH₄HF₂ at 130 °C. MXene was then functionalized with poly-l-arginine (pARG) by electropolymerization, which generated a cationic, hydrogen-bonding network to overcome charge repulsion with nucleotides and promote efficient electron transfer. The density functional theory (DFT) adsorption energy calculations confirmed pARG as the optimal functional monomer, showing stronger binding to GMP/AMP compared to other monomers. The resulting pARG/Nb₂CT_x-modified screen-printed carbon electrode (SPCE) represents the first ever reported electrochemical sensor capable of simultaneously detecting GMP and AMP. The developed sensor exhibited linear ranges of 100-1000 nM with detection limits of 84.5 nM (for GMP) and 43.3 nM (for AMP), alongside excellent reproducibility, stability, and reliable operation in real meat tissue samples. This study establishes the first molten salt synthesis method of Nb₂CT_x and the first dual-sensing platform for GMP/AMP. In addition, the HF-free, low-temperature synthesis route, together with a cost-effective sensing design, underscores the sustainability of this approach, offering a scalable, selective, and eco-friendly strategy for food freshness monitoring and nucleotide biomarker analysis.