Development of a gold nanoparticle based colorimetric sensor kit for the analysis of sodium chloride levels in seawater
Authors:   , Mbambo Andile Truelove 
Published: June 2021

In this study, colorimetric assay was used to develop a gold nanoparticle enabled optical sensor for the analysis of sodium chloride in seawater. Sodium chloride has a major effect in the seawater’s salinity level, thus, this has led to the design of a colorimetric device that is selective to sodium chloride, which can be used for both seawater and the nearby estuarine water. Flocculation assay based on gold nanoparticles was used to optimise the colorimetric response of the selected sodium chloride concentration range (5-40 ppT) for the developed kit. Polydispersed and stable gold nanoparticles were synthesized via the Turkevich method using tri-sodium citrate as a reducing and capping agent. The optical properties of the as-synthesized gold nanoparticles were characterized by Ultra-Violet- Visible spectroscopy where the existence of the Surface Plasmon Resonance (SPR) absorption peak was observed at 525 nm. Transmission Electron Microscopy (TEM) image revealed the morphology of the gold nanoparticles to be isotropic/ spherical with a calculated average size of gold nanoparticles which was found to be 7 nm. The introduction of sodium chloride to gold nanoparticles solution resulted in aggregation which was indicated by the change of colour from red to blue. Scanning Electron Microscopy (SEM) images confirmed the spherical morphology of the gold nanoparticles. The gold nanoparticles with sodium chloride image showed crystals of the salt covering the aggregated gold nanoparticles inferring that the tri- sodium citrate barrier on the surface of the nanoparticles had been broken. In the Fourier-transform Infrared Spectroscopy spectrum (FTIR), the existence of hydroxyl was observed in the gold nanoparticles solution at 3540 cm-1 due to the deionised water used in preparation of all solutions. The carbonyl groups observed at 1782 cm−1, 1520 cm-1 and 1290 cm-1 in the gold nanoparticles spectrum were due to the citrate capping the nanoparticles. Ultraviolet-Visible Spectroscopy (UV-Vis) spectroscopy was also used to validate the colorimetric method as the application of gold nanoparticles is based on solution colour convergence from wine red to blue upon analyte introduction. The colorimetric assay of the sodium chloride concentration range gave distinguishable colour shades which were visually observed. The intensity of the colour in standard solutions increased as the concentration of NaCl was increased. A red shift was observed in the UV-Vis spectrum due to the increase in sodium chloride concentration. A decrease in the 525 nm SPR peak was observed with an increase of another SPR peak at longer wavelengths around 660 nm. Images of the colorimetric assay were analysed using the Colorgrab application. Results of this analysis showed that the colour intensities in the Greyscale percentages were decreasing with increasing sodium chloride concentration and were in the range of 42.33-57.66%. The red green blue (RGB) colour model analysed by ImageJ software revealed that the red colour of the gold nanoparticles gradually disappeared with the development of the blue colour as the concentration of sodium chloride was increased in solution. Commission Internationale d’Eclairage (CIE) Lab colour management method validated the colour similarities and differences of the sodium chloride colorimetric assay. The sodium chloride quantification colour wheel/disk was then fabricated using the RBG colour model values. The developed colorimetric device was applied to screen the level of salinity along the coastal seawater and estuaries of Durban in KwaZulu-Natal. Results of this colorimetric method had a linearity value of 0.9980, a detection and quantification limits of 1.18 ppT and 3.57 ppT, respectively. The concentrations of NaCl in the tested seawater and estuary samples were in the ranges of 30-35 ppT and 5-30 ppT, respectively. The measured concentrations of sodium chloride in water samples using the proposed colorimetric method were in agreement with those observed when using the traditional methods such as ion chromatography and titration. Overall, gold nanoparticles based colorimetric sensor used for the analysis of sodium chloride in seawater and estuarine water was rapid, cost effective, accurate, precise, sensitive, and selective.