Field-deployable measurement of soil extracellular enzyme activity using surface-enhanced Raman spectroscopy

Abstract

Soil is essential for maintaining ecological function, productivity, and environmental balance, thereby supporting human society. As an important indicator of soil health, extracellular enzyme activities provide valuable insights into the biological transformation of soil organic matter. However, conventional colorimetric methods are time-consuming, labor-intensive, and impractical for large-scale or field-based monitoring. In this study, we demonstrate for the first time the use of surface-enhanced Raman spectroscopy (SERS) to monitor the oxidation kinetics of L-3,4-dihydroxyphenylalanine (L-DOPA), catalyzed by two representative enzymes: horseradish peroxidase (HRP) and polyphenol oxidase (PPO). By coupling SERS detection with partial least squares (PLS) modeling, we established quantitative correlations between Raman spectral data and enzyme concentration (R² = 0.983 for HRP; R² = 0.865 for PPO). This model was further applied to 117 soil samples collected from diverse ecosystems in Nashville, TN, to correlate SERS spectra with enzyme activity measured by standard laboratory assays. Despite the inherent heterogeneity of field soil samples, SERS-based predictions of enzyme activity showed strong agreement with conventional measurements (R² = 0.753). All SERS measurements were performed using a handheld Raman spectrometer, underscoring the potential for rapid, in situ deployment. This study introduces a scalable, cost-effective paradigm for real-time monitoring of soil enzyme activity, providing a foundation for advanced environmental sensing and enabling data-driven assessments of soil biogeochemical processes in the field.