Performance evaluation of ethylenediamine-modified surface plasmon resonance platforms for methylparaben detection

Despite the widespread use of ethylenediamine (EDA) as a surface modifier, linking agent, or dopant in complex sensing systems, its potential to function as a stand-alone recognition layer for surface plasmon resonance (SPR) detection remains unexplored. Here, EDA, a small bifunctional molecule with two reactive amine groups, is demonstrated for the first time as an active SPR interface, offering a simple approach to detect methylparaben (MP), a widely used preservative and an emerging endocrine-disrupting compound of growing concern. When EDA was introduced onto gold thin film, the resulting SPR response showed a clear rise in effective refractive index from 1.450 to 1.548 across the 0 to 1 µM of MP. This trend produced a sensitivity of 0.00299° M-1 with R2 at 0.94271, while Langmuir–Freundlich modeling indicated a strong binding interaction with an affinity constant of 1.93608 × 10–11 M-1 with R2 of 0.95283. To further clarify the interaction between EDA and MP, Fourier-transform infrared spectroscopy was employed to examine changes in the chemical environment of the modified surface. The spectra showed a shift in the N–H vibrational region at 3282.33 cm-1 together with the appearance of new C–O features at 1108.89 cm-1 and 106.30 cm-1. Complementary morphological evidence was obtained through atomic force microscopy, which revealed that the initially uneven EDA surface, displaying a root-mean-square (RMS) roughness of 0.782 nm, transformed into a film with finer and more uniformly distributed features after interaction with MP, resulting in RMS roughness of 0.871 nm.

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