In this work, selective colorimetric sensors for simple and rapid detection of Hg(II) ions based on the
monitoring of an unmodified silver nanoparticles (AgNPs) reduction were developed. The average
diameter of synthesized AgNPs was 8.3 ± 1.4 nm which was characterized by transmission electron
microscopy (TEM). The abrupt change in absorbance of the unmodified AgNPs was observed which progressively
decreased and slightly shifted to the blue wavelength as the concentration of Hg(II) increased,
indicating the oxidation of Ag(0) to Ag(I) occurred. It appears that the AgNPs were oxidized by Hg(II),
resulting in disintegration of the AgNPs into smaller particles as well as mediating the reduction of Hg(II)
to Hg(0) adsorbed onto the surface of AgNPs. The adsorption of Hg(0) resulted in the lack of sufficient
charges on AgNPs surfaces due to the decrease in the surface coverage of negatively charged citrate molecules,
which then leaded to enlargement of AgNPs. The calibration curve of this technique was demonstrated
from 0.5 to 7 ppm (r
2 = 0.995), the limit of detection (LOD) was 0.06 ppm (SDblank/slope of
calibration curve) with the precision (RSD, n = 4) of 3.24–4.53. Interestingly, the results show a significant
enhance in the Hg(II) analytical sensitivity when Cu(II) is doped onto the unmodified AgNPs, which
improves the quantitative detection limit to 0.008 ppm. In addition, greater selectivity toward Hg(II)
compared with the other metal ions tested was observed. Furthermore, the percentage recoveries of
spiked drinking water, tap water and SRM1641d (mercury in water) were in acceptable range with a good
precision (RSD) which were in agreement with the values obtained from graphite furnace atomic absorption
spectrometer (GFAAS). The technique proposed in this study provides a rapid, simple, sensitive and
selective detection method for Hg(II) in water samples.