Phosphate (Pi) is a necessary element for growth and energy transport in humans, animals and plants. However, the excess amount of this element could cause severe problems such as joint pain and skin rash for humans and eutrophication in nature. Eutrophication is the enrichment of nutrients, especially phosphate, in aquatic systems, with harmful effects on surface water bodies, such as lakes and creatures in the water. 
This thesis investigated the applicability of gold nanoparticles to detect phosphate using different methods including paper-based and tablet-based sensors. A novel and highly sensitive tablet-based colorimetric sensor is developed to detect phosphate (Pi) in water using mercaptoacetic acid-gold nanoparticles (MA-AuNPs). The principle of this sensor is based on the aggregation (blue colored) and disaggregation (red-colored) mechanism of AuNPs due to surface plasmon resonance, where europium ions (Eu3+) act as an aggregating agent. Herein, dextran tablets encapsulated the Eu3+ ions to make the detection system user friendly. Hence, the sensor just requires dissolving the Eu3+-dextran tablet into the water sample and subsequently adding MA-AuNPs for the colorimetric quantification of phosphate. This assay is very sensitive, with a calculated lower detection limit of 3.779 µg/L and upper detection limit of 0.328 mg/L. The assay does not interfere with common ions in water, thus being Pi-specific, and the display results were stable up to three weeks. Overall, this new approach provides a simple, stable, rapid, low cost and promising device for Pi detection in water.