functionalised Co(II) based MOF for the detection of Hg(II) and MnO 4 (-I) ions.
The MOFs contains a nitro-functionalised ligand (NPDC) which is synthesized by
solvothermal method. The 3D structure of MOF consists of bipyridine linkers and
NPDC ligand coordinated to Co(II). Upon the addition of ions to the MOF containing solution, change in intensity of color can easily be observed by naked eyes
[11].
3.2 Nanomaterials Based Platforms
Nanomaterials such as quantum dots, and metallic nanoparticles offers large surface
area and simplicity in sensor design which makes them suitable for the sensing
applications. Quantum dots (QDs) are the zero-dimensional semiconductor
nanocrystals with the size usually less than 12 nm. Due to this, they have substantial advantages in the field of sensing process as an outcome of their exceptional
optical and electronic properties. These properties include high fluorescence
quantum yield, broad absorption spectra, tuneable composition, high photostability,
etc. Also, QDs can overcome issues including pH dependence, self-quenching at
high concentrations, and photobleaching with the traditional fluorescent probes
[12]. Upon realizing these advantages, Zhou et al., synthesized nitrogen doped
graphene quantum dots for the colorimetric sensing of Fe(III). Here, these QDs
prepared by hydrothermal treatment and ethylenediamine (EDA) is used as nitrogen
source change in intensity of blue color w.r.t. change in concentration Fe(III) ions is
observed by naked eyes under the UV light [13]. Further, Wang et al. developed a
HgS/ZnS core/shell quantum dots for the visual sensing of Cr(III) ions [14].
Metallic nanoparticles are like gold nanoparticles (AuNPs), silver nanoparticles
(AgNPs), and copper nanoparticles(CuNPs) are another promising candidates
which are used to described nanosized metals with dimensions (length, width or
thickness) within the size range 1–100 nm. The main characteristics of these particles are large surface-area-to-volume ratio as compared to the bulk equivalents,
large surface energies the transition between molecular and metallic states providing specific electronic structure (local density of states LDOS), plasmon excitation, quantum confinement, short-range ordering, increased number of kinks, a
large number of low‐coordination sites such as corners and edges, having a large
number of ˝dangling bonds˝ and consequently specific and chemical properties and
the ability to store excess electrons. Using these benefits, Biswas et al. reported the
use of polyethylene glycol (PEG) functionalised AgNPs for the colorimetric
detection of arsenic with the detection limit of 1 ppb [15]. PEG is a biocompatible
polymer contains various hydroxyl groups which acts as the binding sites for the As
(III) and AgNPs are having its characteristic property of Localized surface plasmon
resonance which depends upon the dielectric constant of the surrounding environment. Here, red shift in localized surface plasmon resonance and color change
from pale yellow to bluish color is observed upon the addition of arsenic(III).
Further schematic representation of sensing of As(III) is shown in Fig. 7 [15].
Materials in Colorimetric Detection of Water Pollutants
135
The MOFs contains a nitro-functionalised ligand (NPDC) which is synthesized by
solvothermal method. The 3D structure of MOF consists of bipyridine linkers and
NPDC ligand coordinated to Co(II). Upon the addition of ions to the MOF containing solution, change in intensity of color can easily be observed by naked eyes
[11].
3.2 Nanomaterials Based Platforms
Nanomaterials such as quantum dots, and metallic nanoparticles offers large surface
area and simplicity in sensor design which makes them suitable for the sensing
applications. Quantum dots (QDs) are the zero-dimensional semiconductor
nanocrystals with the size usually less than 12 nm. Due to this, they have substantial advantages in the field of sensing process as an outcome of their exceptional
optical and electronic properties. These properties include high fluorescence
quantum yield, broad absorption spectra, tuneable composition, high photostability,
etc. Also, QDs can overcome issues including pH dependence, self-quenching at
high concentrations, and photobleaching with the traditional fluorescent probes
[12]. Upon realizing these advantages, Zhou et al., synthesized nitrogen doped
graphene quantum dots for the colorimetric sensing of Fe(III). Here, these QDs
prepared by hydrothermal treatment and ethylenediamine (EDA) is used as nitrogen
source change in intensity of blue color w.r.t. change in concentration Fe(III) ions is
observed by naked eyes under the UV light [13]. Further, Wang et al. developed a
HgS/ZnS core/shell quantum dots for the visual sensing of Cr(III) ions [14].
Metallic nanoparticles are like gold nanoparticles (AuNPs), silver nanoparticles
(AgNPs), and copper nanoparticles(CuNPs) are another promising candidates
which are used to described nanosized metals with dimensions (length, width or
thickness) within the size range 1–100 nm. The main characteristics of these particles are large surface-area-to-volume ratio as compared to the bulk equivalents,
large surface energies the transition between molecular and metallic states providing specific electronic structure (local density of states LDOS), plasmon excitation, quantum confinement, short-range ordering, increased number of kinks, a
large number of low‐coordination sites such as corners and edges, having a large
number of ˝dangling bonds˝ and consequently specific and chemical properties and
the ability to store excess electrons. Using these benefits, Biswas et al. reported the
use of polyethylene glycol (PEG) functionalised AgNPs for the colorimetric
detection of arsenic with the detection limit of 1 ppb [15]. PEG is a biocompatible
polymer contains various hydroxyl groups which acts as the binding sites for the As
(III) and AgNPs are having its characteristic property of Localized surface plasmon
resonance which depends upon the dielectric constant of the surrounding environment. Here, red shift in localized surface plasmon resonance and color change
from pale yellow to bluish color is observed upon the addition of arsenic(III).
Further schematic representation of sensing of As(III) is shown in Fig. 7 [15].
Materials in Colorimetric Detection of Water Pollutants
135