Surface-Enhanced Raman Spectroscopy-Machine Learning for Multiplex Naphthenic Acid Profiling in Water

Naphthenic acids (NAs) contribute to the toxicity of vast industrial effluents and present a considerable risk to aquatic ecosystems. This study presents a sensitive, data-driven approach to the detection and quantification of diverse NAs in water, leveraging surface-enhanced Raman spectroscopy (SERS) and machine learning (ML). Our methodology employs highly uniform silver (Ag) nanoparticles, dispersed in NA-containing water with a cationic surfactant added to enhance acid-nanoparticle interactions and boost SERS signals. The detection limits were as low as 10^-4 to 10^-5 M for eight distinct NA types across three groups, encompassing classical linear, cyclic, and heteroatom-containing NAs (sulfur and nitrogen). SERS spectral data were rigorously utilized to train machine learning models. For single acid identification, a random forest (RF) model demonstrated an 86.3% classification accuracy via 7-fold cross-validation. Furthermore, ridge regression models, trained on fast Walsh-Hadamard transformed (FWHT) spectra, scaling, and principal component analysis (PCA), yielded remarkable average R² values of 99.5% for the concentration prediction of most acids. To address the complexities of acid mixture samples, a Siamese convolutional neural network (SNN) was developed to accurately identify multiple acid types within complex samples by comparing mixture spectral fingerprints with individual acid reference fingerprints, achieving an overall identification accuracy of 95%. The model's reliability in multilabel acid detection is further corroborated by averaged F1 scores of approximately 95%. This work emphatically demonstrates the suitability of SERS spectroscopy, utilizing colloidal silver nanoparticles and machine learning algorithms, for the simultaneous identification and quantification of multiple NAs in complex samples. This method eliminates the need for extraction or separation, offering a proof of concept for the sensitive detection of naphthenic acids in environmental samples.