Sensing and analysis of soluble phosphates in environmental samples: a review

Excess phosphate levels in water can lead to increased algal growth, eutrophication and reduced water quality. Phosphate levels in water are regulated by the EU through the Urban Waste Water Treatment Directive (annual mean total phosphorus concentrations of 1-2 mg/l) and the Water Framework Directive that will enforce "good ecological and chemical status" by 2015. Legislation is therefore driving the need for increased monitoring of soluble phosphate in water, escalating the desire for a direct, label free approach that could provide remote, continuous monitoring in real-time. The standard method for measuring soluble phosphate in water is a colourimetric technique developed in the 1960s. This colourimetric approach is difficult to adapt for on-line measurements, uses specific reagents which require safe disposal and thus incurs significant costs to the water industry when carried out on a large scale. This review considers optical and electrochemical sensors plus recent advances with synthetic receptors and molecularly imprinted polymers. Progress in the development of phosphate sensors, designed for use in a variety of disciplines, is highlighted with a view to adapting successful approaches for use in the water sector. Additional considerations include the need for long term stability, low maintenance, specificity for phosphate and the capability of measuring total phosphorus concentrations down to at least 1 mg/l, as required by legislation. A sensor that could directly measure soluble, inorganic phosphate concentrations would draw significant interest from the environment sector and other disciplines, including the agricultural, detergent and bio-medical industries.