Cost-effective and sensitive anthocyanin-based paper sensors for rapid ammonia detection in aqueous solutions

In this work, we developed a cost-effective and environmentally friendly anthocyanin-based paper sensor with high sensitivity and optical visibility for the rapid detection of ammonia in aqueous solutions. The detection principle is based on a color change upon ammonia exposure to an anthocyanin-containing paper, which can be recorded simply via a smartphone. The paper sensors were fabricated by extracting anthocyanin from different sources (i.e., red cabbage, blueberry, and blackberry) and immersing pre-cut paper in anthocyanin extracts. Anthocyanin was extracted from different sources into water and aqueous ethanolic solution (80%) using solid-liquid extraction (SLE) and sonication assisted extraction (SAE) methods. The sensor sensitivity and optical visibility were improved by selecting a suitable combination of anthocyanin source, extraction technique, and solvent and controlling the ammonia release from the samples via alkalinization using a suitable base. Sensors fabricated with anthocyanin extracted from red cabbage (Red-C) into water using the SLE method and samples alkalinized with NaOH showed higher sensor sensitivity and better optical visibility. The Red-C anthocyanin sensors also exhibited a visible color change from dark to light blue for ammonia samples with concentrations as low as 2 mg NH₃-N/L. Moreover, the spike recovery results of the sensors (101.9-109.4%) were in good agreement with those of the standard spectrophotometry method (105.4-112.2%), which suggest that these biosensors are a promising analytical tool as a replacement for time-consuming and environmentally unfriendly standard spectrophotometry methods for the on-site screening of ammonia.