Upconverting nano-paste in 3D-printed phone camera setup for soil phyto-iron sensing

Iron (Fe) is an essential micronutrient for plant growth, but the conventional DTPA soil analysis method for detecting available iron has notable limitations, requiring advanced instruments and lengthy preparation time. Developing a more affordable, user-friendly, and efficient method for iron detection in soil could greatly improve crop nutrition management. Here, a facile nanoscopic method was developed to quantify available Fe²⁺ ions in the soil by forming a luminescence quenching complex in chelation with bathophenanthroline disulphonic acid disodium salt (Fe²⁺/BPDS complex).

This complex readily enters luminous polyvinyl alcohol (PVA) based upconverting smart nano-paste and quenching green range without affecting the red emission. A portable 3D-printed smartphone accessory was made to have the sensing apparatus in position to minimize error and maximize reproducible on-site analysis. Thus, the above closed ratiometric duplex emission (I_R/I_G) processing allows to determine the Fe²⁺ with a limit of detection (LOD) of 3.62 μM and a linear range between 10 μM and 500 μM. Available iron (Fe²⁺) analyses in soil sample performed using the novel nanoscopic method and the conventional Inductive Coupled Plasma-Mass Spectrometry (ICP-MS) analyses were consistent demonstrating the applicability of the method to deliver highly precise Fe²⁺ results.

Phytoavailable Fe²⁺ concentration in soil is required to be between 1 μM and 100 μM so that Fe-deficiency and Fe-toxicity in plants can be avoided. The system developed above have the capacity to screen concentration. To our knowledge this is the first system for the soil phytoavailable iron measurement using upconversion nanoparticles with precision and convenience to measure using smartphone camera.