Self-Assembled\nPlasmonic Structural Color Colorimetric\nSensor for Smartphone-Based Point-Of-Care Ammonia Detection in Water

Monitoring\nchemical levels is crucial for safeguarding both the\nenvironment and public health. Elevated levels of ammonia, for instance,\ncan harm both humans and aquatic ecosystems, often indicating contamination\nfrom agriculture, industry, or sewage. Developing portable, high-resolution,\nand affordable methods for in situ monitoring of ammonia is thus imperative.\nPlasmonic sensors offer a promising solution, detecting ammonia by\ncorrelating changes in their optical response to the target analyte’s\nconcentration. While they are highly sensitive and can be fabricated\nin a variety of portable and user-friendly formats, some still require\nreagents or expensive optical equipment, which hinder their widespread\nadoption. Here, we present a self-assembled nanoplasmonic colorimetric\nsensor capable of directly detecting ammonia concentrations in aqueous\nmatrices. The proposed sensor exploits the plasmonic resonance of\nthe nanostructures to transduce changes in the chemical environment\ninto alterations in color, offering a label-free method for real-time\nanalysis. The sensor is fabricated using a self-assembling technique\ncompatible with low-cost mass production based on aluminum and aluminum\noxide, ensuring affordability and avoiding the use of other toxic\nchemicals. We developed a model to predict ammonia concentrations\nbased on visible color change of the sensor, achieving a detection\nlimit of 8.5 ppm. Furthermore, to address the need for on-site detection,\nwe integrated smartphone technology for real-time color change analysis,\neliminating the need for expensive, bulky optical instruments. Indeed,\nour approach offers a cost-effective, portable, and user-friendly\nsolution for ammonia detection in water without the need for chemical\nreagents or spectrometers, making it ideal for field applications.\nInterestingly, this platform extends its applicability beyond ammonia\ndetection, enabling the monitoring of various chemicals using a smartphone,\nwithout the need for any additional costly equipment.