An expedient ‘click’ approach for the synthetic evaluation of ester-triazole-tethered organosilica conjugates

Article abstract of DOI:10.1002/aoc.4028

The present work articulates the synthesis of a new series of organo-functionalized triethoxysilanes derived from versatile carboxylic acids and 3-azidopropyltriethoxysilane in excellent yields. A proficient and convenient route implicating the Cu(I)-catalysed 1,3-cycloaddition of organic azide with terminal alkynes, labelled as click silylation, has been developed for the generation of ester-triazole-linked alkoxysilanyl scaffolds (4a–f). All the synthesized compounds have been thoroughly characterized using elemental analysis and Fourier transform infrared, ¹H NMR and ¹³C NMR spectroscopic techniques. Importantly, the fabricated alkoxysilanes are potentially amenable for an in situ sol–gel condensation reaction with silica nanospheres leading to the incorporation of organic functionality via covalent grafting onto the nanostructured particle system. As a proof of concept, a one-pot preparation of organic–inorganic hybrid nanoparticles is presented using bis-silane 4?f. The efficiency and selectivity of the prepared nanocomposite towards metal ions is highlighted using adsorption experiments, and the immobilized nanoparticles present a high sensing efficiency towards Cu²⁺ and Pb²⁺ ions while demonstrating better response than that of the bulk material.

Full text of DOI:10.1002/aoc.4028

Received: 15 May 2017
DOI: 10.1002/aoc.4028
Revised: 14 July 2017
Accepted: 17 July 2017
F U L L P A P E R
An expedient ‘click’ approach for the synthetic evaluation of
ester‐triazole‐tethered organosilica conjugates
Gurjaspreet Singh
| Sunita Rani
Department of Chemistry, Panjab
The present work articulates the synthesis of a new series of organo‐functional-
ized triethoxysilanes derived from versatile carboxylic acids and 3‐
azidopropyltriethoxysilane in excellent yields. A proficient and convenient
route implicating the Cu(I)‐catalysed 1,3‐cycloaddition of organic azide with
terminal alkynes, labelled as click silylation, has been developed for the gener-
ation of ester‐triazole‐linked alkoxysilanyl scaffolds (4a–f). All the synthesized
compounds have been thoroughly characterized using elemental analysis and
University, Chandigarh 160014, India
Correspondence
Gurjaspreet Singh, Department of
Chemistry, Panjab University, Chandigarh
160014, India.
Email: gjpsingh@pu.ac.in
Funding information
Fourier transform infrared, H NMR and 13C NMR spectroscopic techniques.
1
Science and Engineering Research Board,
Grant/Award Number: 132/2014; Univer-
sity Grants Commission, Grant/Award
Number: 26532
Importantly, the fabricated alkoxysilanes are potentially amenable for an in situ
sol–gel condensation reaction with silica nanospheres leading to the incorpora-
tion of organic functionality via covalent grafting onto the nanostructured par-
ticle system. As a proof of concept, a one‐pot preparation of organic–inorganic
hybrid nanoparticles is presented using bis‐silane 4 f. The efficiency and selec-
tivity of the prepared nanocomposite towards metal ions is highlighted using
adsorption experiments, and the immobilized nanoparticles present a high
sensing efficiency towards Cu2+ and Pb2+ ions while demonstrating better
response than that of the bulk material.
KEYWORDS
1,2,3‐triazole, adsorption, click silylation, organic–inorganic hybrid nanoparticles, triethoxysilane
1 | INTRODUCTION
stability, hydrogen bonding capability and high dipole
moment, which altogether allow its binding to biomolec-
Ever since the discovery of Cu(I) catalysed azide–alkyne
[3 + 2] cycloaddition (the CuAAC reaction) by Sharpless
and co‐workers in 2001, there has been a notable renais-
sance of chemical transformations in a wide variety of
research areas, ranging from advanced material science,
to bio‐conjugation, dendrimers, catalysis, polymer sci-
ences, etc.[1] This cyclization concept has the privilege of
being the flagship reaction of click chemistry, as the syn-
thesis is modular, regioselective and proceeds without
involving any side products, coupled to an enormous
versatility of reaction conditions and substrate scope.[2]
The click‐generated 1,2,3‐triazole scaffold is bestowed
with significant physicochemical features like metabolic
ular targets and aqueous miscibility as well.[3] Conse-
quently, the triazole architecture possesses great
biological potency as an interconnector between two
functional entities, the technique labelled as symbiosis,
which gives a platform for further functionalization lead-
ing to anti‐HIV, anti‐parasitic, anti‐allergic, anti‐bacterial
and anti‐fungal activities.[4]
Despite the importance of transition metal ions in
several biological and catalytic domains, their detection
in the environment has been of great concern since an
amount above a threshold value can lead to fatal issues.[5]
Although a large number of sophisticated techniques,
for instance atomic absorption spectrometry, inductively
Appl Organometal Chem. 2017;e4028.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4028

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SINGH AND RANI
coupled plasma mass spectrometry, surface plasmon reso-
nance and atomic fluorescence spectrometry, have been
used for monitoring metal ions, these are constrained by
the shortcomings of lengthy and protracted protocols,
long pre‐concentration steps, complex and costly instru-
mentation, etc.[6] Hence, instigation of a feasible, rapid
and cost‐effective method for metal ion detection is a sub-
ject of serious consideration. In this scenario, chelating
ligand‐based molecular chemosensing that responds via
modifications of optical signals upon specific binding with
a guest analyte is the method par excellence.[7] In essence,
a sensory probe mainly consists of a coordination site
fused with a signalling subunit generally through covalent
bonding, and the specific interaction of a target analyte
with the binding sites changes the electronic properties
of the signalling subunit resulting in recognition of the
target via colour or absorption/emission modulations
(Figure 1).
The rationalized designing of an appropriate receptor
for binding metal ions is of interest in modern coordina-
tion chemistry since the nature of the ligands plays a piv-
otal role in governing the stability of a complex, and
significant modifications in the properties of complexes
can be achieved by substituting the donor atom in a
ligand with other atoms of different size, electronegativity
and π‐bonding capacity.[8] It has been ascertained
recently that 1,2,3‐triazole scaffolds can function as strong
N donor ligand architectures for diverse metal ions; while
stable metal complexes with esters are well known.[9]
With such important considerations in mind, in the work
reported herein, we embarked upon the designing of a
series of click triazole‐based chemosensors that were
coupled with aromatic esters.
the inorganic component with the flexibility and spe-
cific functioning of the organic moiety resulting in a
plethora of applications in optics, biosensors, metal
ion sensing and removal, chromatography, drug deliv-
ery, supported catalysis, etc.[12] A thorough literature
survey indicates that hybrid nanocomposites featuring
immobilized ligand scaffoldings on silica matrices are
on the verge of finding use in cation recognition owing
to their sensitive, selective, efficient and cost‐effective
natures.[13]
Keeping all these aspects in mind and expanding our
research on the applications of click triazole‐containing
siloxy derivatives in metal ion recognition,[14] various
conjugated building blocks, i.e. benzene‐, biphenyl‐, thio-
phene‐, pyridine‐ and pyrazine‐based esters, have been
utilized and the prospects of the synthesized compounds
have also been appraised for the organic sheathing of sil-
ica nanospheres. Following this, a metal ion sensing sys-
tem covalently grafted onto the silica surface was
constructed, fully characterized and then scrutinized fur-
ther for adsorptive removal of metal ions.
2 | EXPERIMENTAL
Caution! Azide compounds are sensitive to heat and
shock. Hence, care and protection are required in han-
dling of these compounds.
2.1 | Materials and equipment
All carboxylic acids, 3‐chloropropyltriethoxysilane (3‐
ClPTES), sodium azide, bromotris(triphenylphosphine)
copper(I) (CuBr(PPh3)3), tetraethyl orthosilicate (TEOS)
and propargyl alcohol were used directly as received. 3‐
Azidopropyltriethoxysilane (3‐AzPTES) was prepared
from substitution reaction of 3‐ClPTES with sodium
azide.[15] 1H NMR and 13C NMR spectra were recorded
with a JEOL (AL 400 MHz) spectrometer using CDCl3
as an internal reference and chemical shifts were mea-
sured relative to tetramethylsilane. Electronic spectral
measurements were carried out using a JASCO V‐530
UV–visible spectrophotometer. Fourier transform infra-
red (FT‐IR) spectra were recorded as neat spectra with a
Thermo Scientific Nicolet IS50 spectrophotometer. Ele-
mental analysis was conducted with a PerkinElmer model
2400 CHNS elemental analyser and a Thermo Scientific
Flash 2000 organic elemental analyser. The morphology
of hybrid composites was examined using transmission
electron microscopy (TEM) at 80 kV (Hitachi H‐7500).
Energy‐dispersive X‐ray (EDX) analysis was performed
with a JEOL JSM‐6610 LV using a voltage of 15 kV. X‐
ray diffraction (XRD) was employed using a PANalytical
Organo‐functionalized triethoxysilanes (OfTES) con-
sist of a siloxy group (Si(OR)3), an inorganic reactive
centre prone to hydrolysis in conjunction with an
organic substituent which gives structural adaptability
for specific functioning.[10] These modified silanes are
useful materials for applications in diverse fields such
as cross‐coupling reactions, HPLC packing, medicines,
optics, adhesion, coatings and catalysis.[11] OfTES are
organo‐precursors in the synthesis of organic–inorganic
hybrid nanocomposites via anchoring of organic func-
tionality on an inorganic metal oxide surface. This inte-
grates the mechanical, chemical and thermal stability of
FIGURE 1 Overview of complexation of receptor–target binding

SINGH AND RANI
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X'pert PRO diffractometer with Cu Kα radiation. Thermo-
gravimetric analyses were carried out with an SDT Q
600 V20.9 Build 20 instrument. Each sample was loaded
in an alumina pan and ramped at 10 °C min−1 from 25
NMR (100 MHz, CDCl3, 25 °C, δ, ppm): 54.45 (OCH2),
75.31 (C≡CH), 76.54 (C≡CH), 128.50 (C2,6), 126.34
(C3,5), 132.56 (C1), 135.54 (C4), 165.87 (C═O).
to 1000 °C in dry air at 60 ml min−1
.
2.3.2 | Prop‐2‐yn‐1‐yl [1,1′‐biphenyl]‐4‐car-
boxylate (3b)
2.2 | General procedure for synthesis of
The reactants used were as follows: 2b (0.5 g,
2.31 mmol), Et3N (0.97 ml, 6.93 mmol) and propargyl
alcohol (0.13 ml, 2.31 mmol). Yield: 0.4 g, 74%. Anal.
Calcd for C16H12O2 (%): C, 81.34; H, 5.12. Found (%):
C, 81.19; H, 4.99. FT‐IR (neat, cm−1): 1259 (v OCH2),
1650 (v C═O), 2115 (v C≡C), 3203 (v C≡CH). 1H
NMR (400 MHz, CDCl3, 25 °C, δ, ppm): 3.29 (t, 1H,
C≡CH, J = 2.4 Hz), 4.78 (d, 2H, OCH2, J = 2.4 Hz),
7.27–7.31 (m, 3H, H9,10,11), 7.50–7.56 (m, 4H, H3,5,8,12),
7.94 (d, 2H, H2,6, J = 8.3 Hz). 13C NMR (100 MHz,
CDCl3, 25 °C, δ, ppm): 52.31 (OCH2), 76.12 (C≡CH),
76.58 (C≡CH), 126.85(C2,6), 127.18 (C10), 127.43 (C8,12),
127.88 (C3,5), 128.87 (C9,11), 136.64 (C1), 142.18 (C4,7),
165.57 (C═O).
acid chlorides
All the acid chloride derivatives were prepared by follow-
ing a previously reported method involving acylation
reaction with thionyl chloride. Typically, to 1.00 g of car-
boxylic acid in a 100 ml round‐bottom flask, a catalytic
amount of N,N‐dimethylformamide (three drops) was
added. Then addition of thionyl chloride (10 equiv.;
Table T1, supporting information) was carried out and
the reaction mixture was refluxed for 8 h. After comple-
tion of the reaction, excess thionyl chloride was removed
azeotropically with toluene (25 ml) using a Dean–Stark
assembly. The final product was isolated after drying
under vacuum and was stored in a sealed vessel for subse-
quent use.
2.3.3 | Prop‐2‐yn‐1‐yl thiophene‐2‐carbox-
ylate (3c)
2.3 | General procedure for synthesis of
aromatic Ester‐containing terminal
alkynes (3a–f)
The reactants used were as follows: 2c (0.5 g,
3.41 mmol), Et3N (1.43 ml, 10.23 mmol) and propargyl
alcohol (0.19 ml, 3.40 mmol). Yield: 0.44 g, 77%. Anal.
Calcd for C8H6O2S (%): C, 57.81; H, 3.64; S, 19.29.
Found (%): C, 57.68; H, 3.53; S, 19.03. FT‐IR (neat,
cm−1): 1265 (v OCH2), 1648 (v C═O), 2118 (v C≡C),
To a suspension of acid chloride (1.00 equiv.) in 25 ml
of chloroform at 0 °C, triethylamine (3.00 equiv.) was
added dropwise and the reaction mixture was gently
stirred for 10 min. Propargyl alcohol (1.00 equiv.) was
slowly added afterwards and the resulting suspension
was allowed to reflux for 6 h. After cooling to room
temperature, the mixture was poured into brined water
followed by extraction with ethyl acetate (15 ml). The
organic extracts were repeatedly washed with brine to
ensure the complete removal of by‐products, and subse-
quently the organic layer was dried over anhydrous
sodium sulfate. The solvent was then removed in vacuo
leading to the isolation of pure ester‐containing termi-
nal alkynes (3a–f).
1
3216 (v C≡CH). H NMR (400 MHz, CDCl3, 25 °C, δ,
ppm): 3.35 (t, 1H, C≡CH, J = 2.4 Hz), 4.81 (d, 2H,
OCH2, J = 2.4 Hz), 7.08 (t, 1H, H3, J = 4.5 Hz), 7.78
(d, 1H, H4,
J
=
5.0 Hz), 7.87 (d, 1H, H2,
J = 3.7 Hz). 13C NMR (100 MHz, CDCl3, 25 °C, δ,
ppm): 52.16 (OCH2), 76.35 (C≡CH), 76.87 (C≡CH),
127.32 (C3), 132.13 (C4), 133.54 (C2), 134.94 (C1),
164.17 (C═O).
2.3.4 | Prop‐2‐yn‐1‐yl pyridine‐2‐carboxyl-
ate (3d)
2.3.1 | Prop‐2‐yn‐1‐yl benzoate (3a)
The reactants used were as follows: 2a (0.5 g, 3.56 mmol),
Et3N (1.49 ml, 10.68 mmol) and propargyl alcohol
(0.21 ml, 3.56 mmol). Yield: 0.45 g, 78%. Anal. Calcd for
C10H8O2 (%): C, 74.99; H, 5.03. Found (%): C, 74.88; H,
4.96. FT‐IR (neat, cm−1): 1250 (v OCH2), 1648 (v C═O),
2114 (v C≡C), 3204 (v C≡CH). 1H NMR (400 MHz, CDCl3,
25 °C, δ, ppm): 3.28 (t, 1H, C≡CH, J = 2.4 Hz), 4.67 (d, 2H,
OCH2, J = 2.4 Hz), 7.42 (dd, 1H, H4, J = 20.2, 13.1 Hz),
7.53 (m, 2H, H3,5), 7.94 (d, 2H, H2,6, J = 8.0 Hz). 13C
The reactants used were as follows: 2d (0.5 g, 2.81 mmol),
Et3N (1.18 ml, 8.42 mmol) and propargyl alcohol
(0.16 ml, 2.81 mmol). Yield: 0.41 g, 74%. Anal. Calcd
for C9H7NO2 (%): C, 67.07; H, 4.38; N, 8.69. Found (%):
C, 66.98; H, 4.12; N, 8.47. FT‐IR (neat, cm−1): 1263 (v
OCH2), 1656 (v C═O), 2118 (v C≡C), 3234 (v C≡CH).
1H NMR (400 MHz, CDCl3, 25 °C, δ, ppm): 3.39 (t, 1H,
C≡CH, J = 2.4 Hz), 4.67 (d, 2H, OCH2, J = 2.4 Hz),
7.57 (m, 1H, H4), 8.06 (m, 1H, H3), 8.49 (d, 1H, H2,

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SINGH AND RANI
J = 4.7 Hz), 8.89 (m, 1H, H5). 13C NMR (100 MHz,
CDCl3, 25 °C, δ, ppm): 52.51 (OCH2), 76.39 (C≡CH),
76.84 (C≡CH), 128.13 (C2), 133.43 (C4), 134.71 (C3),
148.19 (C5), 149.98 (C1), 166.32 (C═O).
2.4.1 | (1‐(3‐(Triethoxysilyl)propyl)‐1H‐
1,2,3‐triazol‐4‐yl)methyl benzoate (4a)
The reactants used were as follows: 3a (0.5 g, 3.12 mmol)
and 3‐AzPTES (0.77 ml, 3.12 mmol). Yield: 1.18 g, 93%.
Anal. Calcd for C19H29N3O5Si (%): C, 56.00; H, 7.17; N,
10.31. Found (%): C, 56.21; H, 7.38; N, 10.09. FT‐IR (neat,
2.3.5 | Prop‐2‐yn‐1‐yl pyrazine‐2‐carboxyl-
ate (3e)
cm−1): 1090 (v Si─O), 1654 (v C═O), 2956 (v C═CH). H
1
NMR (400 MHz, CDCl3, 25 °C, δ, ppm): 0.49 (m, 2H,
SiCH2), 1.10 (t, 9H, OCH2CH3, J = 7.0 Hz), 1.93 (m, 2H,
CCH2C), 3.69 (q, 6H, OCH2CH3, J = 7.0 Hz), 4.26 (t, 4H,
CH2N3, J = 7.3 Hz), 5.14 (s, 2H, OCH2), 7.58 (dd, 1H,
H4, J = 20.2, 13.2 Hz), 7.68 (m, 2H, H3,5), 7.70 (s, 1H,
Tz‐H), 8.08 (d, 2H, H2,6, J = 8.0 Hz). 13C NMR
(100 MHz, CDCl3, 25 °C, δ, ppm): 7.15 (SiCH2), 17.97
(OCH2CH3), 23.89 (CCH2C), 52.21 (N3CH2), 58.21
(OCH2CH3), 62.98 (OCH2), 123.90, 141.88 (Tz‐C), 128.28
(C2,6), 131.70 (C3,5), 136.87 (C1,4), 164.81 (C═O).
The reactants used were as follows: 2e (0.5 g,
3.51 mmol), Et3N (1.47 ml, 10.52 mmol) and propargyl
alcohol (0.20 ml, 3.50 mmol). Yield: 0.4 g, 70%. Anal.
Calcd for C8H6N2O2 (%): C, 59.26; H, 3.73; N, 17.28.
Found (%): C, 59.13; H, 3.67; N, 17.25. FT‐IR (neat,
cm−1): 1264 (v OCH2), 1648 (v C═O), 2110 (v C≡C),
1
3263 (v C≡CH). H NMR (400 MHz, CDCl3, 25 °C, δ,
ppm): 3.46 (t, 1H, C≡CH, J = 2.4 Hz), 4.84 (d, 2H,
OCH2, J = 2.5 Hz), 8.75 (m, 1H, H4), 8.98 (t, 1H, H3,
J = 7.8 Hz), 9.16 (d, 1H, H2, J = 4.7 Hz). 13C NMR
(100 MHz, CDCl3, 25 °C, δ, ppm): 52.51 (OCH2), 76.74
(C≡CH), 77.14 (C≡CH), 143.17 (C2), 146.35 (C1),
147.65 (C2), 148.95 (C4), 167.59 (C═O).
2.4.2 | (1‐(3‐(Triethoxysilyl)propyl)‐1H‐
1,2,3‐triazol‐4‐yl)methyl [1,1′‐biphenyl]‐4‐
carboxylate (4b)
2.3.6 | Di(prop‐2‐yn‐1‐yl) pyridine‐2,6‐
dicarboxylate (3f)
The reactants used were as follows: 3b (0.5 g,
2.12 mmol) and 3‐AzPTES (0.52 ml, 2.12 mmol). Yield:
0.94 g, 92%. Anal. Calcd for C25H33N3O5Si (%): C,
62.09; H, 6.88; N, 8.69. Found (%): C, 62.13; H, 6.59;
N, 8.47. FT‐IR (neat, cm−1): 1098 (v Si─O), 1641 (v
C═O), 2968 (v C═CH). 1H NMR (400 MHz, CDCl3,
25 °C, δ, ppm): 0.61 (m, 2H, SiCH2), 1.21 (t, 9H,
OCH2CH3, J = 7.0 Hz), 2.05 (m, 2H, CCH2C), 3.81
(q, 6H, OCH2CH3, J = 7.0 Hz), 4.37 (t, 4H, CH2N3,
J = 7.3 Hz), 5.50 (s, 2H, OCH2), 7.38–7.47 (m, 3H,
H9,10,11), 7.60–7.65 (m, 4H, H3,5,8,12), 7.73 (s, 1H, Tz‐
The reactants used were as follows: 2f (0.5 g, 2.45 mmol),
Et3N (2.05 ml, 14.70 mmol) and propargyl alcohol
(0.28 ml, 4.90 mmol). Yield: 0.42 g, 70%. Anal. Calcd for
C13H9NO4 (%): C, 64.20; H, 3.73; N, 5.76. Found (%): C,
64.02; H, 3.65; N, 5.78. FT‐IR (neat, cm−1): 1278 (v
1
OCH2), 1657 (v C═O), 2138 (v C≡C), 3256 (v C≡CH). H
NMR (400 MHz, CDCl3, 25 °C, δ, ppm): 3.33 (t, 1H,
C≡CH, J = 2.4 Hz), 4.81 (d, 2H, OCH2, J = 2.4 Hz), 7.93
(t, 1H, H3, J = 9.1 Hz), 8.18 (d, 2H, H2,4, J = 7.9 Hz). 13C
NMR (100 MHz, CDCl3, 25 °C, δ, ppm): 52.57 (OCH2),
76.43 (C≡CH), 76.92 (C≡CH), 126.98 (C2,4), 139.54 (C3),
149.17 (C1,5), 168.11 (C═O).
H), 8.11 (d, 2H, H2,6
, J =
8.3 Hz). 13C NMR
(100 MHz, CDCl3, 25 °C, δ, ppm): 8.85 (SiCH2),
18.04 (OCH2CH3), 23.13 (CCH2C), 50.58 (N3CH2),
57.58 (OCH2CH3), 62.40 (OCH2), 123.55, 148.40 (Tz‐
C), 126.10 (C2,6), 128.47 (C10), 130.60 (C8,12), 127.88
(C3,5), 132.92 (C9,11), 137.13 (C1), 143.48 (C4,7), 165.68
(C═O).
2.4 | General procedure for synthesis of
aromatic Ester‐Triazole dyad‐embellished
triethoxysilanes (4a–f)
To the ester‐based terminal alkynes 3a–f (1.00 equiv.)
dissolved in tetrahydrofuran–Et3N (3 ml each) solvent
system, slow addition of 3‐AzPTES (1.00 equiv.) was
2.4.3 | (1‐(3‐(Triethoxysilyl)propyl)‐1H‐
1,2,3‐triazol‐4‐yl)methyl thiophene‐2‐car-
boxylate (4c)
carried out followed by
a
catalytic amount of
CuBr(PPh3)3 (0.01 mmol). The mixture was allowed to
reflux for 6 h, after which the solvents were removed
in vacuo followed by the addition of dry ether (5 ml)
to separate out the catalyst by vacuum filtration. The fil-
trate was then concentrated leading to the formation of
click‐generated triethoxysilane derivatives with excellent
yields in each case.
The reactants used were as follows: 3c (0.5 g, 3.01 mmol)
and 3‐AzPTES (0.74 ml, 3.01 mmol). Yield: 1.18 g, 95%.
Anal. Calcd for C17H27N3O5SSi (%): C, 49.37; H, 6.58; N,
10.16; S, 7.75. Found (%): C, 49.34; H, 6.84; N, 10.26; S,
7.83. FT‐IR (neat, cm−1): 1095 (v Si─O), 1667 (v C═O),
1
2954 (v C═CH). H NMR (400 MHz, CDCl3, 25 °C, δ,
ppm): 0.53 (m, 2H, SiCH2), 1.13 (t, 9H, OCH2CH3,

SINGH AND RANI
5 of 13
J = 7.0 Hz), 1.95 (m, 2H, CCH2C), 3.73 (q, 6H, OCH2CH3,
J = 7.0 Hz), 4.29 (t, 4H, CH2N3, J = 7.3 Hz), 5.36 (s, 2H,
OCH2), 7.48 (t, 1H, H3, J = 4.5 Hz), 7.64 (s, 1H, Tz‐H),
7.73 (d, 1H, H4, J = 5.0 Hz), 7.77 (d, 1H, H2, J = 3.7 Hz).
13C NMR (100 MHz, CDCl3, 25 °C, δ, ppm): 8.26 (SiCH2),
19.19 (OCH2CH3), 23.86 (CCH2C), 50.94 (N3CH2), 58.94
(OCH2CH3), 63.93 (OCH2), 126.85, 143.14 (Tz‐C), 128.26
(C3), 131.31 (C4), 132.76 (C1,2), 162.11 (C═O).
2.4.6 | Bis((1‐(3‐(triethoxysilyl)propyl)‐1H‐
1,2,3‐triazol‐4‐yl)methyl) pyridine‐2,6‐
dicarboxylate (4f)
The reactants used were as follows: 3f (0.5 g, 2.06 mmol)
and 3‐AzPTES (1.02 ml, 4.13 mmol). Yield: 1.37 g, 91%.
Anal. Calcd for C31H51N7O10Si2 (%): C, 50.45; H, 6.97; N,
13.29. Found (%): C, 50.31; H, 6.94; N, 13.21. FT‐IR (neat,
1
cm−1): 1079 (v Si─O), 1671 (v C═O), 2983 (v C═CH). H
NMR (400 MHz, CDCl3, 25 °C, δ, ppm): 0.53 (m, 2H,
SiCH2), 1.20 (t, 9H, OCH2CH3, J = 7.0 Hz), 1.91 (m, 2H,
CCH2C), 3.79 (q, 6H, OCH2CH3, J = 7.0 Hz), 4.19 (t, 4H,
CH2N3, J = 7.3 Hz), 5.31 (s, 2H, OCH2), 7.57 (s, 1H, Tz‐
H), 7.77 (m, 1H, H4), 8.71 (s, 1H, H3), 9.25 (s, 1H, H2).
13C NMR (100 MHz, CDCl3, 25 °C, δ, ppm): 8.52 (SiCH2),
18.34 (OCH2CH3), 25.05 (CCH2C), 51.10 (N3CH2), 57.61
(OCH2CH3), 62.67 (OCH2), 126.42, 147.13 (Tz‐C), 129.23
(C2,4), 138.50 (C3), 150.18 (C1,5), 166.41 (C═O).
2.4.4 | (1‐(3‐(Triethoxysilyl)propyl)‐1H‐
1,2,3‐triazol‐4‐yl)methyl pyridine‐2‐carbox-
ylate (4d)
The reactants used were as follows: 3d (0.5 g,
3.10 mmol) and 3‐AzPTES (0.77 ml, 3.12 mmol). Yield:
0.96 g, 93%. Anal. Calcd for C18H28N4O5Si (%): C,
52.92; H, 6.91; N, 13.71. Found (%): C, 52.78; H, 6.92;
N, 13.68. FT‐IR (neat, cm−1): 1085 (v Si─O), 1656 (v
C═O), 2973 (v C═CH). 1H NMR (400 MHz, CDCl3,
25 °C, δ, ppm): 0.62 (m, 2H, SiCH2), 1.22 (t, 9H,
OCH2CH3, J = 7.0 Hz), 2.05 (m, 2H, CCH2C), 3.81 (q,
6H, OCH2CH3, J = 7.0 Hz), 4.39 (t, 4H, CH2N3,
J = 7.3 Hz), 5.51 (s, 2H, OCH2), 7.40 (m, 1H, H4),
7.77 (s, 1H, Tz‐H), 8.31 (d, 1H, H2, J = 4.7 Hz), 8.79
(s, 1H, H3, J = 4.7 Hz), 9.24 (m, 1H, H5). 13C NMR
(100 MHz, CDCl3, 25 °C, δ, ppm): 7.38 (SiCH2), 18.25
(OCH2CH3), 24.18 (CCH2C), 52.44 (N3CH2), 58.45
(OCH2CH3), 62.06 (OCH2), 123.04, 145.05 (Tz‐C),
129.71 (C2), 130.70 (C4), 135.03 (C3), 139.50 (C5),
145.05 (C1), 168.11 (C═O).
2.5 | Fabrication of hybrid nanoparticles
(H‐NPs)
For the immobilization of bis‐silane 4f onto the surface of
silica nanoparticles, a modified Stöber method involving
one‐pot chemical grafting was employed as shown in
Scheme 1. To a solution of TEOS (1 ml) in EtOH
(30 ml), 5 ml of aqueous ammonia was added and the
solution was stirred vigorously for 1 h. After 10 min of
stirring, the solution became turbid, indicating the forma-
tion of silica nanoparticles. After 1 h, in situ addition of 4f
(0.3 g) was carried out to eliminate the extra step of syn-
thesis and separation of native silica nanoparticles. An
immediate colour alteration from white to cream was
noticed upon addition. The mixture was then stirred for
8 h and the resultant H‐NPs were separated via centrifu-
gation and washed with chloroform to remove any physi-
cally adsorbed silane molecules.
2.4.5 | (1‐(3‐(Triethoxysilyl)propyl)‐1H‐
1,2,3‐triazol‐4‐yl)methyl pyrazine‐2‐carbox-
ylate (4e)
The reactants used were as follows: 3e (0.5 g, 3.09 mmol)
and 3‐AzPTES (0.76 ml, 3.08 mmol). Yield: 1.15 g, 91%.
Anal. Calcd for C17H27N5O5Si (%): C, 49.86; H, 6.65; N,
17.10. Found (%): C, 49.71; H, 6.85; N, 17.14. FT‐IR
(neat, cm−1): 1095 (v Si─O), 1665 (v C═O), 2990 (v
C═CH). 1H NMR (400 MHz, CDCl3, 25 °C, δ, ppm):
0.53 (m, 2H, SiCH2), 1.13 (t, 9H, OCH2CH3,
J = 7.0 Hz), 1.96 (m, 2H, CCH2C), 3.73 (q, 6H,
3 | RESULTS AND DISCUSSION
3.1 | Synthesis
Classically, the synthesis of organosilica precursors
has been achieved primarily by cross‐coupling,
hydrosilylation, alcoholysis and transmetalation reac-
tions.[16] However, these methodologies involve the use
of expensive metals, liberation of volatile toxic gases and
limited functional group tolerance and lead to only mod-
erate product yields, along with a very poor control of
regio‐selectivity and separation and purification difficul-
ties. Undeterred we set out to explore a comprehensive
method for the generation of a new series of alkoxysilane
derivatives with enhanced properties via the chemical
OCH2CH3,
J
=
7.0 Hz), 4.30 (t, 4H, CH2N3,
J = 7.3 Hz), 5.42 (s, 2H, OCH2), 7.66 (s, 1H, Tz‐H),
7.77 (m, 1H, H4), 8.71 (s, 1H, H3), 9.25 (s, 1H, H2).
13C NMR (100 MHz, CDCl3, 25 °C, δ, ppm): 8.72
(SiCH2), 19.06 (OCH2CH3), 25.25 (CCH2C), 51.30
(N3CH2), 57.81 (OCH2CH3), 62.62 (OCH2), 126.62,
149.03 (Tz‐C), 138.70 (C2), 143.83 (C1), 146.61 (C2),
152.16 (C4), 167.65 (C═O).

6 of 13
SINGH AND RANI
SCHEME 1 Synthesis of organo‐functionalized triethoxysilanes (OfTES)
combination of triethoxysilyl moiety with various chal-
lenging substrates derived from carboxylic acids. Since
alkoxysilanes are known to have a propensity for mois-
ture, the synthetic scheme has to be designed in such a
way that the silyl group is introduced in the final step so
that the organic substituents endure compatibility with
the silicon core.[17] Recently, a revolutionary approach
to silylation categorized under click reactions was insti-
gated by Cattoёn and co‐workers and further extended
by Singh et al. for the cycloaddition reaction of azide
and alkyne functionalities in a highly efficient manner
resulting in a pendant 1,2,3 triazole group‐decorated
OfTES.[18]
substrates which include benzyl‐ as well as heterocyclyl‐
substituted carboxylic acids. Noticeably, the click‐silylated
products were generated in excellent yields under benign
reaction conditions allowing simple isolation. The
method not only improves the synthetic efficiency for pre-
paring OfTES but also extends this family of silyl deriva-
tives and potentially enables the rapid synthesis of
meticulously functionalized organic–inorganic H‐NPs.
3.2 | Characterization of OfTES (4a–f)
The structures and purity of all silane derivatives have
1
Herein, ester and 1,2,3‐triazole moieties are combined
as the spacer between aromatic ring and siloxyl part
(Scheme 1). Firstly, the carboxylic group is converted into
its more reactive form, i.e. carbonyl chloride, as direct
conversion of acid into ester is not feasible. The ester‐con-
taining terminal alkyne is then fabricated by the treat-
ment of acid chloride with propargyl alcohol. This
esterification reaction is facilitated by the addition of an
external base, triethylamine. Meanwhile, 3‐ClPTES was
made to undergo substitution reaction with sodium azide
yielding precursor 3‐AzPTES. After this, both alkyne and
azide substrates were allowed to click together for the for-
mation of ester‐triazole‐linked silane derivatives (4a–f) in
the presence of Cu(I) catalyst and tetrahydrofuran–Et3N
as solvent under refluxing conditions for 6 h. For the iso-
lation of final products, only addition of dry ether was
required to separate the catalyst, and any further purifica-
tion like distillation or crystallization was not required
since extremely pure products were generated as corrobo-
rated by spectral analysis (see Section 2). The scope of this
reaction was then examined using six different types of
been deduced from their FT‐IR, H NMR and 13C NMR
spectra and elemental analysis data. The results are given
in Section 2, while the NMR spectra along with atom
numbering are in the supporting information (Figures
S1–S12).
The FT‐IR spectra of functionalized alkoxysilanes con-
tain the characteristic stretching vibrational peaks of
C═C─H at 2954–2990 cm−1 confirming the formation of
triazole ring by the successful cyclization of terminal
alkynes with 3‐AzPTES. This is also corroborated by the
absence of any vibrational peak centred around 2100
and 3200 cm−1 which might arise due to residual alkyne
functionality of intermediates 3a–f. Also, highly intense
peaks at 1641–1671 and 1079–1095 cm−1 attributed to
C═O and Si─O stretching vibrations, respectively, are
also observed which are in accordance with the structures
of synthesized alkoxysilanes.
1H NMR spectra of OfTES derivatives 4a–f show the
characteristic peaks of ethoxy group as triplets and quar-
tets in the ranges 1.10–1.22 and 3.69–3.81 ppm, respec-
tively. The cyclization of terminal alkynes into 1,2,3‐

SINGH AND RANI
7 of 13
triazole scaffolds is inferred by comparing the signals of
C≡C─H moiety which experience a major shift from
3.28–3.46 ppm for 3a–f (a triplet) to 7.64–7.77 ppm (a sin-
glet), due to transformation into C═C─H of triazole ring
in 4a–f. Also, all the compounds contain well‐resolved
signals from the aromatic rings, corroborating the struc-
tures as proposed.
group (R) as the organic source.[23] The interface con-
tact of the bridging organic group which is simulta-
neously hooked between two silica matrices is
maximized for such materials, whereas the intervening
propyl chains function to minimize the steric hindrance
between the organic sites and the silica surface. In addi-
tion, these structures are elaborately constructed by the
self‐assembly of bis‐alkoxysilane precursors without any
external structure‐directing template leading to many
advantageous features like enhanced mechanical stabil-
ity, uniformly dense distribution and easier accessibility
of the organic units.
Depending upon the sequence of steps involved in
condensation, there are two different synthetic
approaches.[24] The first one is the loading of the
organic group onto the pore walls of the mesoporous sil-
ica in a stepwise grafting via successive modifications of
the silica nanoparticles (Scheme Sc1, supporting infor-
mation). Though there are many previous reports of this
method and a large number of different organic groups
has been encompassed using this pathway, there are
two major problems: (i) incomplete product formation
at any step of the synthesis will lead to a complex mix-
ture of products with inhomogeneous distribution of the
organic groups and (ii) there is a possibility of organic
functionality occupying the channel volume which
reduces the free pore space and can cause total pore
blocking. Hence, in the present case, the alternative
route that follows a one‐pot co‐condensation reaction
of silica source, TEOS, together with organo‐precursor,
4f, without any structure‐directing agent or template
has been employed (Scheme 2). Presumably, the stoi-
chiometric incorporation of organic group covalently
bridged between two silica shells has been achieved.
Similar results are obtained from 13C NMR spectral
interpretation. The most deshielded signal corresponds
to carbonyl carbon in each case which is observed at
162.11–168.11 ppm. Meanwhile, the carbon signals of
alkynyl group (3a–f) appearing in the range 75.31–76.74
and 76.54–77.14 ppm get shifted to 123.04–126.85 and
141.88–149.03 ppm, respectively, in the spectra of cyclized
products (4a–f). This confirms the successful synthesis of
final products as outlined in Scheme 1.
3.3 | Evaluation of synthesized
Alkoxysilanes (4a–f) in anchoring on silica
surface
Pristine silica nanoparticles are of considerably use for
diverse applications in numerous advanced fields.[19]
Nonetheless, the surface modification of mesoporous silica
via functional group immobilization provides the opportu-
nity to engineer the interfacial properties of the solid
matrix along with the retention of basic geometry and
mechanical strength. However, direct loading of organic
functionalities on the inorganic surface is not convenient
on account of the relative inertness of the surface in its
ground state.[20] Hence, an alternative pathway involving
surface activation or modification needs to be developed
which can be achieved either via physical adsorption or
via chemical grafting where organosilanes featuring
trialkoxysilyl groups are key organo‐precursor substrates.
The latter technique involving chemical tethering of func-
tional groups offers strong covalent bonding with the inor-
ganic oxide and consequently is preferred over the former
since the leaching of grafted molecules is minimized.[21]
It has been observed that due to the purity and
instability constraints of conventional alkoxysilanes,
the immobilization generally involves agglomeration
and polymerization throughout deposition, ensuing an
irregular surface morphology.[22] Further, depending
upon the extent of steric hindrance and hydrophobicity
of the alkoxysilanes, it has been observed that only par-
tial coverage of the inorganic matrix can be accom-
plished and the rest of the material network results
only from the hydrolysis of source TEOS, yielding
purely inorganic or non‐modified siliceous material. To
overcome this, a new methodology has been developed
which involves the utilization of bis‐alkoxysilanes of
the type (R'O)3Si─R─Si(OR′)3 having a bridging organic
3.3.1 | Characterization of nanocomposite
In the present endeavour, bis‐alkoxysilane 4f was
grafted onto the supporting matrix provided by silica
nanospheres, which was identified and characterized
using electron microscopy (SEM with EDX and TEM),
XRD, FT‐IR spectroscopy and thermogravimetric analy-
sis. In particular, FT‐IR spectroscopy is an indispensable
technique for confirming the grafting of organic groups
onto the pristine inorganic structure. The spectrum of
unmodified silica nanoparticles (Figure 2a) contains
mainly two absorption peaks which correspond to
Si─O─Si asymmetric stretching vibrations at 1049 cm
−1
and O─H vibrations at 3289 cm−1, respectively. After
the subsequent coating with the bis‐silane, the FT‐IR
spectrum gets enriched with additional vibrational
bands arising due to the pyridine ring (1542 and
1445 cm−1) and carbonyl groups at 1661 cm−1, while

8 of 13
SINGH AND RANI
SCHEME 2 Hydrolyzation of 4f at the
silica surface
FIGURE 2 FT‐IR spectra of (a) silica nanoparticles and (b) hybrid nanoparticles
the peak due to Si─O─Si unit remains (Figure 2b).
Indeed, the FT‐IR spectrum of the hybrid allows
highlighting of both the components (silica support
and organic loading).
In order to determine the elemental composition of
the H‐NPs, EDX analysis was carried out and the
resulting spectrum is shown in Figure 3(b). Characteristic
peaks assigned to carbon, silicon, oxygen and nitrogen
elements are present in the spectrum, which unambigu-
ously indicate the successful coating of pyridine ester‐
based triazole framework on the surface of the silica
nanoparticles.
Further, selected SEM micrographs for hybrid silica
nanoparticles are shown in Figure 4 to demonstrate
the surface topography. From the images, it can be eas-
ily inferred that a homogenized particle system is fabri-
cated during the hydrolysis of 4f with silica
nanospheres. The nanoparticles are composed uniformly
during the hydrolysis process and the two phases
exhibit their distinct properties together and no phase
separation is evident. Compared with the non‐modified
To get a better understanding of the configuration
and structural distinctiveness of the silica nanoparticles
after undergoing functionalization, powder XRD mea-
surements were made. The XRD pattern of silica nano-
particles is characterized by a broad diffraction hump
centred on 2θ = 23° (Figure 3a). Upon organic modifi-
cation by the synthesized bis‐silane, a similar diffraction
pattern is seen which confirms the structural integrity of
the silica support after the sol–gel processing, while
slight decline in the reflection intensity indicates the
organic tethering.[25] Furthermore, the pattern does not
contain any other diffraction peak, pointing to the
purity of the hybrid.

SINGH AND RANI
9 of 13
FIGURE 3 (a) XRD patterns of silica
and hybrid nanoparticles and (b) EDX
spectrum of hybrid nanoparticles
silica nanoparticles, the hybrid nanoparticles possess a
roughened surface, due to the deposition and growth
of silica occurring on a molecular scale caused by the
sol–gel process.[26]
mechanical stability of silica matrix and also its tenacity in
the hybrid composite.
To investigate the thermal stability of the silica
nanospheres, the thermogravimetric technique was
employed. Interestingly, native silica nanoparticles show
a two‐step degradation profile (Figure S13, supporting
information). The first one, taking place in the range
25–200 °C, corresponds to the thermodesorption of
physically adhered water or ethanol molecules, indicat-
ing the hydrophilic nature of the silica surface.[28] The
second major decomposition occurs from 600 °C
onwards, which can be traced to the condensation of
the free silanol groups present at the surface. The
siloxyl‐coated hybrid nanomaterial, however, shows dif-
ferent degradation behaviour. The corresponding weight
loss up to 200 °C turns out to be smaller, revealing that
on organo‐conjugation, the hydrophilicity of the surface
To deduce exact information about the organic coating
and the size of synthesized particles, TEM analysis was
conducted and selected images are depicted in Figure 4(c,
d). Importantly, the successful immobilization of the bis‐
silane on the bare silica surface is evident from the TEM
analysis. Firstly, the size of non‐modified silica nano-
spheres lies in the range 90–100 nm, whereas the hybrid
nanoparticles exhibit an average diameter of 265 nm. The
growth of silica nanospheres can be accounted for by the
sheathing of the bis‐silane unit over their surfaces.[27] Sec-
ondly, the morphology and uniformity of silica nanoparti-
cles are preserved after organic immobilization and any
separate silica clogs were not observed, hinting at the good

10 of 13
SINGH AND RANI
FIGURE 4 (a, b) SEM and (c, d) TEM micrographs of hybrid nanoparticles at different magnifications
decreases considerably. In addition, the weight loss
above 100 °C, in this case, shows a large peak arising
due to the decomposition of the organic fragment of
bis‐silane (Figure S13). Since mass loss is much larger
than that of bare silica nanoparticles, this not only evi-
dences the covalent anchoring of silane at the surface,
but also provides a method to estimate the amount of
grafting via comparison method and the value turns
few. Consequently, many research groups have
pioneered the introduction of advanced techniques such
as ion exchange resins, coagulation, electro‐deposition,
solvent extraction, hydrogels, biomimetic compounds,
biopolymers, activated carbon and adsorbents for the
removal of these toxic metal ions.[30] Of all these
methods, liquid–solid phase adsorption is indeed the
most common method in practical use since the rest of
them have severe drawbacks, such as complex working
apparatus, non‐recyclability, low efficiency, sensitivity
and selectivity, and cost.[31] Additionally, amongst vari-
ous adsorbents like activated carbons, zeolites, lingo
celluloses, clays and nanoparticles, mesoporous silica
nanoparticle‐based adsorbents are predominantly used
in chemical probes and pre‐concentrators since they
have high surface areas, tunable pore sizes with narrow
distributions, quick response, easy separability, recycla-
bility, cost‐effectiveness, high scalability and low toxic-
ity.[32] The adsorption affinity can be further enhanced
by the silanization process involving chemical immobili-
zation of metal chelating ligands over the siliceous
surface. Such surface modification enables specific metal
out to be 0.324 mmol g−1 [29]
On further heating, i.e.
.
above 600 °C, the condensation and cross‐linking of
silanol groups commences to form stable inorganic silica
at the end of the analysis.
3.4 | Application of fabricated hybrid
nanomaterial for recognition of metal ions
The eradication of toxic transition metal ions accumu-
lated in the biosphere and aqueous milieu is an impor-
tant issue since long‐term exposure to them can lead to
cardiovascular deficiency, lung neurological infection,
bone lesion, hypertension and cancer, just to name a

SINGH AND RANI
11 of 13
complexation by providing (i) a better surface for ionic
interactions and (ii) binding sites for metal ions. It is
anticipated that the organic chelates which tend to be
involved in metal complexation, when adsorbed on a
suitable support material, play an imperative role in sep-
aration of ions,[33] and there are numerous literature
reports of the inclusion of different functionalities such
as amine, thiol, sugar, etc., grafted on silica nanoparti-
cles for these processes.[34]
extending reaction time from 5 to 25 min and later
levelled off. Hence, the nano‐adsorbent was reacted with
the ions for 25 min prior to analysis. The quantity of metal
ions adsorbed at equilibrium can be assessed using the
following equation:
ðCi−CeÞV
Qe ¼
(1)
M
In the work presented here, we were particularly
interested in developing modular synthetic approaches
which offer the possibility of swiftly constructing sensor
molecule libraries. We were encouraged by the CuAAC
reaction methodology and metal ion binding potential of
1,2,3‐triazoles in a range of mono‐, bi‐, tri‐ and
polydentate ligand architectures in forming stable coordi-
nation complexes with Pt2+, Pd2+, Cu2+, Ru2+ and Ag+
ions and considering that the ester group is ubiquitously
involved in a wide variety of metal complexes, while
metal complexes with pyridine ring are also well known.
Assembled together, the pyridine ring functionalized at
2,6‐position by the ester‐triazole dyad‐conjugated
organosilica precursor (4f) can be envisaged as a perfect
chelator for an assorted range of metal ions.[35] To the best
of our knowledge, the triazole‐encapped pyridine ester
immobilized on silica nanoparticles has not been employed
in the adsorptive removal of transition metal ions.
where Qe represents the amount of ions adsorbed per unit
mass of adsorbent (mmol g−1) at equilibrium, Ci and Ce are
initial concentration (mmol l−1) and concentration at equi-
librium (mmol l−1), respectively, of the metal ions, V is the
volume of metal ion solution and M is the amount of adsor-
bent (g). For calculating the saturated amount of metal
To evaluate the performance of hybrid nanoparticles
as an ion‐selective sensing probe, the UV–visible absorp-
tion changes were investigated in the presence of various
cations such as Na+, K+, Mg2+, Al3+, Ni2+, Pb2+, Co2+
,
Pd2+, Cd2+, Zn2+, Cu2+, Hg2+, Fe2+ and Fe3+ (Figure
S14). All the recognition studies were executed at 25 °C
and abundant time was provided for the uniformity of
the solution before recording any spectrum. Evidently,
the absorption spectra were mainly perturbed only in
the presence of copper and lead ions, while all other ions
remained silent even at high concentrations.
Based upon changes in absorption in the presence of
copper and lead ions, both these ions were tested for their
adsorptive removal. To start with, the amount of adsor-
bent was optimized and the most appropriate amount of
H‐NPs required for maximum adsorption was found to
be 0.025 g. Hence, 0.025 g of H‐NPs was added into aque-
ous solutions (10 ml) of Cu2+ and Pb2+ ions of concentra-
tion 0.5–2.0 mmol l−1. To study the adsorption kinetics,
UV–visible spectroscopy was utilized since the absorption
intensity is directly proportional to the concentration of
metal ions. The adsorbent was made to react with each
ion separately, after which the nanoparticles were sepa-
rated by filtration and the concentration of ions in the
supernatant was measured using absorption plots. It is
worth pointing out that adsorption of ions increased on
FIGURE 5 Langmuir adsorption isotherms for (a) Cu2+ and (b)
Pb2+ ions
TABLE 1 Significant parameters of Langmuir fitting
Adsorbed metal ion R2
Qm (mmol g−1) Ka (l mmol−1
)
Cu2+
Pb2+
0.9905 0.66
0.9927 0.45
2.06
1.49

12 of 13
SINGH AND RANI
ions adsorbed on the adsorbent, Qm (mmol g−1), the
DST‐PURSE II), New Delhi, for providing financial
Langmuir isotherm model is utilized:
assistance.
Ce
Ce
1
¼
þ
(2)
ORCID
Qe Qm KaQm
Gurjaspreet Singh http://orcid.org/0000-0002-8627-420X
where Ka (l mmol−1) is the Langmuir isotherm constant
of the system. According to this hypothesis, monolayer
adsorption of ions occurs with equal energy and
enthalpy for all adsorption sites which are homoge-
neously distributed over the adsorbent surface. Mathe-
matically, a plot of Ce/Qe versus Ce results in a
straight line (Figure 5) and Qm is calculated as the
reciprocal of the slope of such a plot in each case. The
obtained results are presented in Table 1. Evidently,
the high fitting parameter (R2) values verify that the
adsorption phenomenon follows the Langmuir princi-
ple.[36] It is evident from the results in Table 1 that
the affinity of functionalized organosilica for Cu2+ is
higher than for Pb2+. The different adsorption capacities
for different metal ions are attributed to the different
complexation interactions between the metal ions and
the grafted ligands.[37]
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