Detection of dissolved CO(2) based on the aggregation of gold nanoparticles

A sensitive colorimetric assay of dissolved CO2 (dCO2) was developed based on the plasmon shift of gold nanoparticles (AuNPs). A water-soluble random copolymer poly(dimethyl acrylamide-co-(N-amidino)ethyl acrylamide), or P(DMA-co-NAEAA), containing amidine groups was synthesized. In the presence of dCO2, the amidine groups in the NAEAA block protonate and convert the polymer from a neutral to a positive-charged state, hence triggering the negative-charged AuNPs to aggregate by the electrostatic interaction. The degree of AuNP aggregation is dependent on the charge density of polymer, which is related to dCO2 concentration. The aggregation of AuNPs results in a red shift of the AuNP plasmonic spectrum, or a color change from red to blue. In addition, dCO2 concentration can be quantitatively measured by the UV absorbance change of the AuNP solution. A linear relationship between 0.264 and 6.336 hPa of dCO2 with a limit of detection (LOD) of 0.04 hPa can be acquired. This is the first report to detect dCO2 using the optical properties of nanoparticles.