Multi-analyte needle-type sensor for measurement of pH, phosphate, and redox potential in soil

This work reports on development of a multi-analyte needle-type sensor for measurement of pH, phosphate, and oxidation reduction potential (ORP) in soil. The sensor was fabricated using meniscus etching and selective electrodeposition of sensing surfaces (cobalt oxide for phosphate, iridium oxide for pH, and gold for ORP). The cobalt-oxide based phosphate sensor showed ~53 mV/decade sensitivity in the 1-10000 μM range, while the iridium oxide based pH sensor showed ~62 mV/pH sensitivity in the pH 2~12 range. The key advantage of the needle-type sensors is the ability to penetrate samples for in situ measurements. Without sampling and placing samples in a fluidic analysis system, microscale measurements can be done by simply inserting needle-type sensors into samples at measurement site. This study is the first application of a needle type sensor to microscale probing of soil, with an ultimate goal of microprofiling the plant root environment (rhizosphere) for remediation applications.