A novel on-site analytical approach for the determination of trace metal ions and nanoparticles in environmental water is presented, utilizing a self-developed amphiphilic material that enables rapid enrichment and precipitation of analytes. This material, consisting of a hydrophobic polyhedral oligomeric silsesquioxane (POSS) core covalently linked to functional hydrophilic ligands, spontaneously self-assembles in aqueous solutions to form micron-scale aggregates, thereby facilitating efficient extraction and phase separation without heating, pH adjustment, or auxiliary agents. After enrichment, the precipitate was readily redispersed and subjected to rapid spectrophotometric analysis directly in the field. The method achieved outstanding enrichment factors (up to 495 for metal ions and 475 for nanoparticles), high extraction efficiencies (95–99%), and excellent selectivity for analytes such as Cu2?, Pb2?, Ag nanoparticles, and ZnO nanoparticles. Notably, the detection limits and analytical performance of the on-site spectrophotometric method after preconcentration were significantly improved and comparable to those of laboratory-based atomic absorption spectrometry (AAS). For example, the limit of detection for Cu2? was reduced to 0.63 μg/L (24-fold improvement), for Pb2? to 11.25 μg/L (10-fold improvement), for Ag nanoparticles to 0.056 μg/mL, and for ZnO nanoparticles to 0.065 μg/mL, all with substantial increases in sensitivity after enrichment. The method also demonstrated high precision and accuracy with recoveries ranging from 91% to 108% and relative standard deviations below 5% for real water samples. The results obtained using this approach were in excellent agreement with those obtained using AAS, confirming its reliability. This study established a simple, rapid, and environmentally friendly platform for on-site spectrometric analysis of both metal ions and nanoparticles, providing a practical alternative to conventional laboratory instrumentation for environmental monitoring and analytical chemistry.