Rapid industrial progress has led to the elevation of heavy metal concentration in the
environment, which raised tacit concerns. Among these metals, mercury (II) ion, Hg2+ is
one of the most detrimental to the human health due to its neurotoxic and nephrotoxic
properties. To date, methods to determine the presence of Hg2+ ions require expensive
equipment and sophisticated operation. This is impractical for a constant monitoring of
Hg2+ in water sources. Thus, it is necessary to develop a simple yet effective bio sensing
method to monitor Hg2+ level in our environment. Recently, gold-nanoparticles (AuNPs)
have attracted considerable amount of attention due to their nature of strong plasmonic
resonance which can be exploited for a simple and rapid colorimetric assay. As the
distance between nanoparticles decreases, the plasmon energy band is lowered and
consequently it turns the colour of AuNPs from red to blue. The key of utilizing this
phenomenon as a bio sensing mechanism is to control the AuNPs aggregation via surface
chemistry approach. Here, a sensitive and practical colorimetric assay for in-situ
detection of Hg2+ ions in water using cysteine functionalized gold nanoparticles (Cys-
AuNPs) together with the addition of polyamidoamine (PAMAM), G2 dendrimer is
reported. In this study, Cys-AuNPs and PAMAM dendrimer specifically capture Hg2+
ions and induce colour changes of the solution. The mechanism of Hg2+ interaction with
the system was investigated using UV-vis spectrophotometer, dynamic light scattering
(DLS) and transmission electron microscope (TEM). To increase the practicality value
of the system, a custom-made chamber to be integrated with a mobile phone was
fabricated. Colour changes can be monitored using a mobile app. The newly developed
detection system can detect Hg2+ as low as 0.5 part per billion (ppb) in a laboratory
prepared sample or in a real river water sample. The results were validated using direct
thermal decomposition mercury analyser. This cost-effective colorimetric approach is
practical to determine the presence of Hg2+ water sources
