102
S. E. H. Murph and A. Goriounova
surface functionalization. Information is summarized in Table 1. The same experimental procedure, including the concentration of L-cysteine was used for incubation
with the gold nanoparticle stainless-steel wool filters.
Heavy Metal Uptake in Solution
Development of Colorimetric Sensor Based on Au Nanoparticle
L-cysteine Aggregation
Nanoparticles prepared in solution are capped with a variety of surfactant or capping
agents used during the preparation procedures, such as citrate. The presence of
the capping agents provides surface passivation and helps prevent aggregation of
nanoparticles. Nanoparticles could, however, assemble into larger structures through
spontaneous or deliberate molecular interactions upon addition of analytes. Through
specific surface chemistries, the interparticle forces could be manipulated giving the
ability to tune the individual nanoparticle and nanoparticle assemblies’ properties.
The collective properties of various NP assemblies have been exploited in applications such as molecule sensing, nano-thermometry, imaging, electronics, or catalysis
[4, 9, 21]. Analyte-mediated gold nanoparticles’ chemical sensing has been reported
for detection of DNA, protein, antibodies, metal ions, and others [21].
The unique optical properties of gold nanoparticles in the visible region of the
spectrum make them suitable for use as analyte-mediated colorimetric sensors [4, 9].
The coupling of plasmon resonances of gold nanoparticles that are within 1 diameter
length apart upon the addition of analytes can be exploited as a colorimetric sensing
technology.
The optical properties of the colorimetric sensor are monitored via ultraviolet–
visible spectroscopy (Fig. 5a, b). The instant aggregation of functionalized gold
nanoparticles upon addition of zinc ions can be easily documented. The ultraviolet–
visible spectra of aqueous individual gold nanoparticles with dimensions of 20 nm
in diameter exhibit a plasmon band around 524 nm. Subsequent addition of metal
ions to the gold nanoparticles aqueous solution generates a decrease in the intensity
of the transverse plasmon band at 520 nm and appearance of a new plasmon band
(600–800 nm). The appearance of the new plasmon bands indicates the formation
of aggregated nanostructures. The gold nanoparticles plasmon band also became
broader and less intense, which indicates that the zinc (II) was captured on the gold
nanoparticles surface and causing them to agglomerate. Moreover, as the concentration of zinc (II) increases, significant color changes can be observed with the naked
eye; color changes from red to increasingly darker shades of purple were observed
(Fig. 5c). This is in agreement with previous published studies [16, 18]. Typically,
when gold nanoparticles are closer than 1 nanoparticle diameter to each other, their
plasmon bands begin to couple, resulting in broadening and redshifting. If more than
one plasmon band appears, it means that various size gold nanoparticle clusters are
formed. The recorded colorimetric changes are linear with the zinc ion concentration
S. E. H. Murph and A. Goriounova
surface functionalization. Information is summarized in Table 1. The same experimental procedure, including the concentration of L-cysteine was used for incubation
with the gold nanoparticle stainless-steel wool filters.
Heavy Metal Uptake in Solution
Development of Colorimetric Sensor Based on Au Nanoparticle
L-cysteine Aggregation
Nanoparticles prepared in solution are capped with a variety of surfactant or capping
agents used during the preparation procedures, such as citrate. The presence of
the capping agents provides surface passivation and helps prevent aggregation of
nanoparticles. Nanoparticles could, however, assemble into larger structures through
spontaneous or deliberate molecular interactions upon addition of analytes. Through
specific surface chemistries, the interparticle forces could be manipulated giving the
ability to tune the individual nanoparticle and nanoparticle assemblies’ properties.
The collective properties of various NP assemblies have been exploited in applications such as molecule sensing, nano-thermometry, imaging, electronics, or catalysis
[4, 9, 21]. Analyte-mediated gold nanoparticles’ chemical sensing has been reported
for detection of DNA, protein, antibodies, metal ions, and others [21].
The unique optical properties of gold nanoparticles in the visible region of the
spectrum make them suitable for use as analyte-mediated colorimetric sensors [4, 9].
The coupling of plasmon resonances of gold nanoparticles that are within 1 diameter
length apart upon the addition of analytes can be exploited as a colorimetric sensing
technology.
The optical properties of the colorimetric sensor are monitored via ultraviolet–
visible spectroscopy (Fig. 5a, b). The instant aggregation of functionalized gold
nanoparticles upon addition of zinc ions can be easily documented. The ultraviolet–
visible spectra of aqueous individual gold nanoparticles with dimensions of 20 nm
in diameter exhibit a plasmon band around 524 nm. Subsequent addition of metal
ions to the gold nanoparticles aqueous solution generates a decrease in the intensity
of the transverse plasmon band at 520 nm and appearance of a new plasmon band
(600–800 nm). The appearance of the new plasmon bands indicates the formation
of aggregated nanostructures. The gold nanoparticles plasmon band also became
broader and less intense, which indicates that the zinc (II) was captured on the gold
nanoparticles surface and causing them to agglomerate. Moreover, as the concentration of zinc (II) increases, significant color changes can be observed with the naked
eye; color changes from red to increasingly darker shades of purple were observed
(Fig. 5c). This is in agreement with previous published studies [16, 18]. Typically,
when gold nanoparticles are closer than 1 nanoparticle diameter to each other, their
plasmon bands begin to couple, resulting in broadening and redshifting. If more than
one plasmon band appears, it means that various size gold nanoparticle clusters are
formed. The recorded colorimetric changes are linear with the zinc ion concentration