Development of nanomaterial chemosensors for toxic metal ions sensing

Article abstract of DOI:10.1002/aoc.4868

Heavy metal ions are harmful to aquatic life and humans owing to their high toxicity and non-biodegradability, so their removal from wastewater is an important task. Therefore, this work focuses on designing suitable, simple and economical nanosensors to detect and remove these metal ions with high selectivity and sensitivity. Based on this idea, different types of mesoporous materials such as hexagonal SBA-15, cubic SBA-16 and spherical MCM-41, their chloro-functionalized derivatives, as well as 4-(4-nitro-phenylazo)-naphthalen-1-ol (NPAN) azo dye have been synthesized, with the aim of designing some optical nanosensors for metal ions sensing applications. The mentioned azo dye has been anchored into the chloro-functionalized mesoporous materials. The designed nanosensors were characterized using scanning and transmission electron microscopy as well as Fourier transform infrared and UV–visible spectral analysis, nitrogen adsorption–desorption isotherms, low-angle X-ray diffraction and thermogravimetric analyses. Their optical sensing to various toxic metal ions such as Cd (II), Hg (II), Mn (II), Fe (II), Zn (II) and Pb (II) at different values of pH (1.1, 4.9, 7 and 12) was investigated. The optimization of experimental conditions, including the effect of pH and metal ion concentration, was examined. The experimental results showed that the solution pH had a major impact on metal ion detection. The optical nanosensors respond well to the tested metal ions, as reflected by the enhancement in both absorption and emission spectra upon adding different concentrations of the metal salts and were fully reversible on adding ethylene diamine tetra acetic acid or citric acid to the formed complexes. High values of the binding constants for the designed nanosensors were observed at pHs 7 and 12, confirming the strong chelation of different metals to the nanosensor at these pHs. Also, high binding constants and sensitivity were observed for NPAN-MCM-41 as a nanosensor to detect the different metal ions. From the obtained results, we succeeded in transforming the harmful azo dye into an environmentally friendly form via designing of the optical nanosensors used to detect toxic metal ions in wastewater with high sensitivity.

Full text of DOI:10.1002/aoc.4868

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Received: 23 November 2018
DOI: 10.1002/aoc.4868
Revised: 20 January 2019
Accepted: 25 January 2019
F U L L P A P E R
Development of nanomaterial chemosensors for toxic metal
ions sensing
Tarek A. Fayed
| Marwa N. El‐Nahass
| Hosny A. El‐Daly | Amal A. Shokry
Department of Chemistry, Faculty of
Science, Tanta University, 31527 Tanta,
Egypt
Heavy metal ions are harmful to aquatic life and humans owing to their high
toxicity and non‐biodegradability, so their removal from wastewater is an
important task. Therefore, this work focuses on designing suitable, simple
and economical nanosensors to detect and remove these metal ions with high
selectivity and sensitivity. Based on this idea, different types of mesoporous
materials such as hexagonal SBA‐15, cubic SBA‐16 and spherical MCM‐41,
their chloro‐functionalized derivatives, as well as 4‐(4‐nitro‐phenylazo)‐
naphthalen‐1‐ol (NPAN) azo dye have been synthesized, with the aim of
designing some optical nanosensors for metal ions sensing applications. The
mentioned azo dye has been anchored into the chloro‐functionalized mesopo-
rous materials. The designed nanosensors were characterized using scanning
and transmission electron microscopy as well as Fourier transform infrared
and UV–visible spectral analysis, nitrogen adsorption–desorption isotherms,
low‐angle X‐ray diffraction and thermogravimetric analyses. Their optical sens-
ing to various toxic metal ions such as Cd (II), Hg (II), Mn (II), Fe (II), Zn (II)
and Pb (II) at different values of pH (1.1, 4.9, 7 and 12) was investigated. The
optimization of experimental conditions, including the effect of pH and metal
ion concentration, was examined. The experimental results showed that the
solution pH had a major impact on metal ion detection. The optical
nanosensors respond well to the tested metal ions, as reflected by the enhance-
ment in both absorption and emission spectra upon adding different concentra-
tions of the metal salts and were fully reversible on adding ethylene diamine
tetra acetic acid or citric acid to the formed complexes. High values of the bind-
ing constants for the designed nanosensors were observed at pHs 7 and 12,
confirming the strong chelation of different metals to the nanosensor at these
pHs. Also, high binding constants and sensitivity were observed for NPAN‐
MCM‐41 as a nanosensor to detect the different metal ions. From the obtained
results, we succeeded in transforming the harmful azo dye into an environ-
mentally friendly form via designing of the optical nanosensors used to detect
toxic metal ions in wastewater with high sensitivity.
Correspondence
Marwa N. El‐Nahass, Department of
Chemistry, Faculty of Science, Tanta
University, 31527 Tanta, Egypt.
Email: marwacu@yahoo.com; marwa.
elnahas@science.tanta.edu.eg
Funding information
Scientific Research Fund at Tanta Uni-
versity, Egypt
KEYWORDS
on‐line analysis, optical nanosensors, toxic metal ions
Appl Organometal Chem. 2019;e4868.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 23
https://doi.org/10.1002/aoc.4868

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FAYED ET AL.
SBA‐15, cubic SBA‐16 and spherical MCM‐41. The inves-
tigated compounds have been characterized using differ-
ent spectroscopic techniques. Their optical sensing
response to various metal ions such as Cd (II), Hg (II),
Fe (II), Pb (II), Zn (II) and Mn (II) was performed
quantitatively using electronic absorption and emission
techniques. Their reversibility on adding ethylene
diamine tetra acetic acid (EDTA) or citric acid was inves-
tigated. Also, the binding constants of the formed com-
plexes were calculated. Finally, the effect of ionic radius
of metal ions on the binding ability has been evaluated.
1 | INTRODUCTION
Water treatment has become one of the most important
issues as wastewater. It contains many toxic pollutants,
such as heavy metal ions, which could damage the envi-
ronment.[1,2] Heavy metal ions, such as Cd (II), Hg (II),
Zn (II), Mn (II), Fe (II) and Pb (II) are non‐biodegradable
pollutants and can accumulate in the environment and in
living tissues, causing various diseases and disorders in
living organisms, even at trace levels.[3–8] These metals
are released into the environment through both natural
and anthropogenic sources from various industries,
resulting in their finding way into natural water bodies
and environment.[9–13] Several techniques, such as induc-
tively coupled plasma mass spectrometry and atomic
fluorescence spectroscopy, have been developed.[14] These
techniques are often expensive and time‐consuming and
require tedious sample pretreatment or enrichments,
sophisticated instrumentation and personnel training.[15]
Nanostructured materials are powerful molecular tools
for the analysis of many metal ions and anions.[16–18]
These materials are attracting much attention, owing to
their high stability. They efficiently detect toxic ions in
terms of sensitivity and selectivity with real‐time moni-
toring, while being non‐destructive, inexpensive and
having a low detection limit and fast response time.
These features are necessary to analyze ultra trace levels
of environmental pollutants. It is noted that the anchor-
ing of the different organic dyes into these nanomaterials
can be used for selective diverse metal ions capture from
water samples with high sensitivity.[19–22] Among these
nanomaterials, mesoporous silica has significant advan-
tages for use in detection systems. It exhibits highly
uniform channels, large surface area, narrow pore‐size
distribution and tunable pore sizes over a wide range.
These excellent properties exert a significant influence
in improving the electrochemical signal and enhancing
the charge transport to understand the metal coordina-
tion with specific color optimization. In addition, the
mesoporous materials can be used broadly in different
nanotechnological applications such as catalysis, adsorp-
tion, separation, sensing, medicine, lasers, drug, gene
delivery and intrapore inclusion chemistry.[23–27]
2 | EXPERIMENTAL
2.1 | Materials
Cetyltrimethyl ammonium bromide (Sigma‐Aldrich),
triblock copolymers pluronic P123 (EO20PO70EO20)
and pluronic F127 (EO106PO70EO106) (BASF, USA),
tetraethyl orthosilicate (TEOS, Sigma‐Aldrich), 3‐
chloropropyltriethoxysilane (CPTS, Aldrich), p‐nitro ani-
line, α‐naphthol, EDTA and citric acid were used. Tolu-
ene, acetone and butanol were purchased from Tedia.
Cadmium (II) chloride, iron (II) sulfate heptahydrate,
mercury (II) chloride, lead (II) nitrate, zinc (II) chloride
and manganese (II) chloride were purchased from Wako
Pure Chemicals, Osaka, Japan. Sodium hydroxide,
sodium nitrite, aqueous ammonia solution (25% wt,
Fluka) and hydrochloric acid (35%) were purchased from
Beijing Chemical Int. All of the chemicals were used as
received without further purification. Distilled water
was used for the preparation of all aqueous solutions.
Buffer solutions of 0.2 M HCl–KCl were used to adjust
the pH at 1.1, as well as, 0.2 M CH3COOH–CH3COONa
was used to adjust the pH at 4.9, and 0.066 M
Na2HPO4–KH2PO4 was used to adjust the pH at 7 and 12.
2.2 | Synthesis of 4‐(4‐nitro‐phenylazo)‐
naphthalen‐1‐ol azo dye
The investigated azo dye was synthesized as follows;[29] in
a conical flask, 0.01 mol of p‐nitro aniline (1.38 g) in 5 ml
H2O was completely dissolved in 5 ml of hydrochloric
acid (35%). The amine was diazotized by adding 0.7 g of
sodium nitrite in 5 ml H2O dropwise with cooling at
0–5 °C. In another beaker, 1.44 g of α‐naphthol was
dissolved in 10 ml of 10% sodium hydroxide, then
cooled with vigorous stirring. After that, the solution of
α‐naphthol/sodium hydroxide was added to the diazo-
nium salt dropwise with stirring, while the temperature
was kept at 0–5 °C until the red dye was formed. The
obtained dye, Scheme 1, was filtered, washed with cold
Azo dyes offer an extensive range of colours which are
sensitive to their environment, so they can be used in
designing optical nanosensors for metal ion detection in
wastewater, via being anchored into the pores of the
safely nanostructured materials, SBA‐15, SBA‐16 and
MCM‐41.[28]
Herein, we report on the synthesis of optical
nanosensors for toxic metal ions detection via anchoring
of an azo dye namely 4‐(4‐nitro‐phenylazo)‐naphthalen‐
1‐ol, NPAN, into the nanostructured materials, hexagonal

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FAYED ET AL.
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water then dried and crystallized from ethanol. Its purity
was confirmed by mass and Fourier transform infrared
(FT‐IR) spectral measurements (Figures 1 and 2). The
mass spectra of NPAN gave molecular ion peaks at the
characteristic positions, m/z = 293, which is equivalent
to the molecular weight of the proposed compound. The
FT‐IR spectra showed sharp bands at vibrational frequen-
cies 3457, 3062, 1625, 1584, 1405 and 1321 cm−1, charac-
teristic for the aromatic system O–H, C–H, C=C, N=N,
C–N and C–NO2 stretching vibrations, respectively.
solution with constant stirring for 1 h to obtain a homo-
geneous solution at 450 °C, after that 12 g of TEOS was
added dropwise. The stirring of the mixture was contin-
ued for another 24 h for mesostructure formation.
Hydrothermal treatment was carried out by aging the
mixture for another 24 h. The synthesized mesoporous
material was filtered, dried and calcinated at 550 °C
for 6 h.[31]
2.3.3 | Synthesis of spherical MCM‐41
2.3 | Synthesis of mesoporous materials
2.3.1 | Synthesis of hexagonal SBA‐15
Mesoporous spherical silica, MCM‐41, was synthesized as
follows: 205 ml of ammonium hydroxide (25 wt% solu-
tion) was mixed with 270 ml of distilled water. A 2.0 g,
5.5 mmol aliquot of the surfactant, cetyltrimethyl ammo-
nium bromide, was added into the solution with stirring.
When the solution became homogenous, 10 ml, 45 mmol
of TEOS was introduced, giving rise to a white slurry.
Subsequently, the reaction temperature was maintained
at 50 °C for 2 h. After that the resulting product was fil-
tered, washed with distilled water, dried for 24 h at ambi-
ent temperature and followed by calcination at 550 °C for
4 h to remove the organic template and give the spherical
mesoporous MCM‐41.[32]
Mesoporous hexagonal silica, SBA‐15, was synthesized as
follows: 8.0 g of triblock copolymer Pluronic P123 were
dissolved in 12 g of HCl (35%) and 60 g H2O with stirring
until a clear solution was obtained. A 17 g aliquot of
TEOS was added, stirred for 24 h at 40 °C and then
heated at 100 °C for another 24 h under static conditions.
Finally, the product was filtered, washed with distilled
water, dried in air at room temperature and then calci-
nated at 550 °C for 6 h to remove the surfactant.[30]
2.3.2 | Synthesis of cubic SBA‐16
Mesoporous body‐centered cubic silica, SBA‐16, was syn-
thesized as follows: 1.6 g of F127 was dissolved in 120 g
of H2O and 5 g of hydrochloric acid (35%) was added
dropwise. A 7.5 g aliquot of butanol was added to this
2.4 | Chloro‐functionalized mesoporous
materials
Chloro‐functionalized mesoporous materials SBA‐15,
SBA‐16, as well as MCM‐41, were synthesized as follows:
1.0 g of the synthesized mesoporous materials was added
to 40 ml of toluene in a 100 ml round‐bottom flask.
The suspension was stirred vigorously for 5 min, then
9.07 mmol (2.18 ml) of 3‐chloropropyltriethoxysilane
was added and the solution was refluxed for 6 h.
The solids were then filtered, washed with toluene and
acetone to remove the unreacted silane, and dried
overnight.[33]
SCHEME 1 Structure of the investigation azo dye, NPAN
FIGURE 1 Mass spectrum of the
investigated azo dye, 4‐(4‐nitro‐
phenylazo)‐naphthalen‐1‐ol (NPAN)

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FAYED ET AL.
FIGURE 2 Fourier transform infrared
(FT‐IR) spectrum of the investigated azo
dye, NPAN
functionalized ones and their loaded forms were recorded
within the range 4000–400 cm−1 from a Jasco FT/IR‐4100
spectrophotometer as KBr pellets. Small‐angle powder X‐
ray diffraction (XRD) measurements were studied at
room temperature using a GNR, APD 2000 PRO step
scans X‐ray diffractometer, with Cu Kα radiation
(40 kV, 30 mA), and a scanning range of 0–8° (2θ) with
2.5 | Synthesis of the optical nanosensors,
NPAN‐SBA‐15, NPAN‐SBA‐16 and
NPAN‐MCM‐41
The investigated optical nanosensors, NPAN‐SBA‐15,
NPAN‐SBA‐16 and NPAN‐MCM‐41, Scheme 2, were
synthesized as follows:[34] 1.0 g of chloro‐functionalized
SBA‐15 or SBA‐16 or MCM‐41 was suspended in 20 ml
xylene at 140 °C and stirred for 1 h, then 2 mmol
(0.26 g) from the synthesized azo dye was added with
constant stirring at the same temperature. After 12 h,
the solid product was filtered. The removal of hydrochlo-
ric acid was tested by adding a silver nitrate solution to
the filtrate where a white precipitate was formed. After
that the formed product was washed with water and
ethanol, many times, and dried at 120 °C in air for 12 h
to yield the investigated optical nanosensors.
a
step of 0.02°. The surface morphology of the
mesostructured materials was investigated using a Jeol,
JSM‐52500LV scanning electron microscope. The pore
shape and ordering of mesostructured materials were
examined and photographed using a Jeol‐JEM‐100 SX
transmission electron microscope. Thermogravimetric
analysis was performed using the simultaneous thermal
analyzer (TGA/DTA) Perkin Elmer STA 6000 under
nitrogen with a heating rate of 15 deg/min. Nitrogen
adsorption–desorption experiments were carried out at
77.40 K on a Nova 3200e surface area and pore size ana-
lyzer (Quanta Chrome instrument). The Brunauer–
Emmett–Teller (BET) surface area (SBET) was calculated
from the linearity of the BET equation. The pore volume
was calculated using the density functional theory
method. Also, the pore radius was obtained from the pore
2.6 | Instrumentation and
characterization
The FT‐IR spectra of the azo dye, mesoporous materials,
SBA‐15, SBA‐16 and MCM‐41, their chloro‐
SCHEME 2 Synthesis of the
investigated optical nanosensors