Unraveling the true sensing species in 4-nitrophenol derivatives detection: influence of carbon dot surface chemistry

• Fabrication of sustainable carbon dots (CDs) from coffee waste through the circular economy concept. • Identified multimodal and outstanding selectivity of 4-nitrophenol (4-NP) and its derivatives. • Experimental CD probe systems with pollutant molecules developed using theoretical calculations. • CD-optimized surface functional groups enhance the adsorption of contaminant molecules. • Advanced theoretical calculations confirm the hydrogen bonding between 4-NP and CD. The accurate and selective detection of 4-nitrophenol (4-NP) is a critical analytical challenge due to its toxicity, persistence, and widespread presence in industrial effluents. The established detection mechanisms to explain the carbon dots (CDs) interactions are based on their inner filter effect (IFE), Förster resonance energy transfer (FRET), and electron transfer. However, the role of surface chemistry of the CDs in 4-NP detection, the formation of surface complexes, and whether these mechanisms coexist are still under debate. To address this, we vary the surface functional groups of CDs and show which functional group is more specific for the 4-NPs detection. In this context, integrating experimental techniques with computational methods such as density functional theory (DFT) provides critical insights for elucidating and optimizing molecular-level interactions. This study, which uses carbon nanomaterials, specifically sustainable CDs synthesized from coffee waste, to detect 4-NPs. It is demonstrated that CDs exhibit stable absorbance and photoluminescence properties, and their high selectivity towards compounds with para -position nitro groups reassures the accuracy of the detection. Oxygenated functional groups, such as hydroxyl and carboxyl, facilitate their interaction with these contaminants on their surfaces. By integrating waste valorization, rational nanomaterial design, and target-specific sensing, this study demonstrates a scalable strategy for the optical detection of 4-NPs with improved accuracy and mechanistic clarity.