Molecularly Engineered Nitrogen-Enriched and Solid-State Emissive Carbon Dots for Visual Zeptomole Explosive Tracing

Abstract

Carbon dots (Cdots) are widely studied for their excellent photoluminescence, stability, and optoelectronic properties, and solid-state fluorescence (SSF) of Cdots is still under active exploration to enhance their potential in practical optoelectronic applications. The carbon source used in the Cdots synthesis method plays a significant role in carbonization, influencing the structure, properties, and applications. Herein, nitrogen-doped Cdots (N-Cdots) was strategically synthesized using a solvothermal route employing melamine derivative and terephthalaldehyde precursors. The synthesized N-Cdots, comprehensively characterized by various spectroscopic and microscopic techniques, displayed strong solution-phase emission along with solid-state fluorescence without incorporation into any matrix. The presence of amine and aldehyde groups on the surface played a key role in the generation of SSF in N-Cdots. The emission of N-Cdots was highly sensitive to solvent polarity, which demonstrates a solvatochromic effect across a wide range of solvents. The exciting luminescent features of N-Cdots were applied for the selective recognition of nitroaromatic explosives in both solution and solid states. Notably, N-Cdots exhibited exceptional sensitivity via inner filter fluorescence quenching mechanisms, achieving ultralow detection limits of 2.565 fM in solution and 100 zeptomole (2.29 ppq) in the solid state. In addition, N-Cdots can effectively detect PA in complex and diverse environmental conditions. Remarkably, this zeptomole-level detection surpasses earlier techniques based on expensive noble-metal constructs, providing a highly economical platform for ultra-trace explosive sensing. This study advances the development of molecularly engineered Cdots that exhibit solid-state luminescence, thereby paving the way for future electronic devices capable of ultrasensitive, on-site detection of explosives, which will enhance public safety and environmental protection.