Design, construction and validation of an open-source gold nanoparticle concentration tracking device

Gold nanoparticles (GNPs) exhibit unique optical properties governed by localized surface plasmon resonance, enabling applications in biomedicine, sensing, catalysis, and photonics. Accurate concentration tracking during fabrication is essential, since particle density strongly affects colloid stability and functional performance. Pulsed laser ablation in liquids (PLAL) has gained recognition as a sustainable route for nanoparticle synthesis, ligand-free, high-purity colloids without chemical by-products. However, monitoring GNP concentration during PLAL typically relies on UV-VIS spectrophotometers, that can be costly, bulky, and difficult to integrate into fabrication workflows. In this work, we present the design, construction, and validation of a compact, open-source alternative for real-time nanoparticle tracking. The device combines a 405 nm laser, a photodiode, and an ESP32-S3 microcontroller for data-acquisition and processing. All design files, firmware, and 3D-printed casing are openly released. Validation experiments show strong agreement with conventional UV-VIS absorption, demonstrating that the proposed device can replace commercial spectrophotometers. By providing a concentration sensor that can be easily integrated in a production chain, this approach reduces the physical footprint and cost of GNPs fabrication facilities, while maintaining reliable monitoring. With a cost below 40 € and modular architecture, the system enables reproducible, accessible, and sustainable monitoring for PLAL and other nanomaterial synthesis processes.