Highlights • Detection and quantification of AgNP at ppb levels in natural seawater samples. • The use of long path cells (up to 200 cm) in UV–visible spectrophotometry is proposed. • Knowledge of the molar attenuation coefficient of the NP under study in the sample matrix is required. Abstract Silver nanoparticles (AgNPs) are emerging contaminants that are difficult to detect in natural waters. UV–visible spectrophotometry is a simple technique that allows detection of AgNPs through analysis of their characteristic surface plasmon resonance band. The detection limit for nanoparticles using up to 10 cm path length cuvettes with UV–visible spectrophotometry is in the 0.1–10 ppm range. This detection limit is insufficiently low to observe AgNPs in natural environments. Here we show how the use of capillary cells with an optical path length up to 200 cm, forms an excellent technique for rapid detection and quantification of non-aggregated AgNPs at ppb concentrations in complex natural matrices such as seawater.