Europium oxide modified reduced graphene oxide composite for trace detection of hydrogen phosphate ions in soil samples.

The phosphate (PO) ion is a constituent of the environment, soil, plants, and animals. There should be a real-time and portable phosphate detection sensor. Herein we propose a colorimetry based sensitive method for hydrogen phosphate (HPO) ions detection using europium oxide modified reduced graphene oxide composite (EuO-RGO) and gold nanoparticles (Au NPs). We detect the HPO by observing the anti-aggregation of gold nanoparticles. In the presence of a EuO-RGO composite, the Au NPs underwent an aggregation process, causing a colour change of Au NPs from wine red to wine blue. Once Eu-modified RGO was pre-mixed with HPO ions and introduced into Au NPs, the Eu nanoparticles in the Eu-modified RGO were attracted to the HPO ions. Because of this, the aggregated Au NPs started to anti-aggregate, and the colour of Au NPs changed from wine blue to wine red. The calibration curve of the sensor goes from 0 nM to 500 nM concentration of HPO ions. Our sensor has a detection limit of 0.08 nM, which is lower than the reported values. This improved lower detection limit is probably due to the use of RGO, which according to the literature review, can adsorb phosphate ions onto its surface. We optimized the incubation time and europium oxide (EuO) nanoparticle concentration to improve the sensor's sensitivity. Lastly, we tested an agricultural sample using our developed method.