Low-cost portable photometer using wi-fi–enabled microcontroller and silicon photodiode for iron and phosphorus determination

Abstract

A low-cost and portable photometer was developed based on the ESP32 microcontroller integrated with a BPW34 silicon photodiode for the colorimetric determination of iron (Fe) and phosphorus (P) in fertilizer samples. The BPW34 sensor provided broad spectral responsivity in the 400–1100 nm range, rapid response (<100 ns), and low dark current (<2 nA), making it well suited for visible-light detection in colorimetric assays. Calibration studies showed linear responses for Fe(II)–phenanthroline and phosphomolybdate blue complexes within the ranges of 1.14–10.0 mg L⁻¹ and 0.085–1.0 mg L⁻¹, with coefficients of determination (R²) of 0.981 and 0.9966, respectively. The limits of detection (LOD) were 0.34 mg L⁻¹ for Fe and 0.025 mg L⁻¹ for P. Validation against standard methods showed good agreement between the developed photometer and the reference spectrophotometer, with Bland–Altman mean bias 0.00 mg L⁻¹ for Fe and −2.25 mg L⁻¹ for P, along with acceptable spike recoveries of 93–105% (Fe) and 90–112% (P). These results confirm that the developed photometer offers reliable analytical performance and practical applicability for quantitative colorimetric determination of nutrients in fertilizer samples, representing an affordable alternative to conventional spectrophotometric instruments.