First report of silver ion recognition via a silatrane-based receptor: excellent selectivity, low detection limit and good applicability

Article abstract of DOI:10.1039/C9NJ00288J

The development of selective and sensitive complex molecular systems for the detection of Ag⁺ ions is of great importance because the extensive use of the toxic silver containing materials has adverse effects on the human health and the environment. Here, we report a new chemosensor for silver ion recognition in aqueous solutions, which was constructed by integrating three moieties: silatrane, 1,2,3-triazole and barbituric acid, using Knoevenagel condensation and Cu(i) catalyzed [2+3] azide-alkyne cycloaddition reaction. The sensing assay based on silatrane exhibited high selectivity and high specificity towards silver ions. Overall, the results show that this strategy to develop new functional materials can be employed for the efficient sensing of Ag⁺ ions. Further, the benefit of the existence of silicon in the receptor lies in the extension of this molecular system in the field of nanochemistry, where silica-based nanomaterials can have more significant outcomes. Moreover, the ion detection ability in aqueous solution makes the sensor suitable to be employed for detection in real samples.

Full text of DOI:10.1039/C9NJ00288J

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First report of silver ion recognition via silatrane based receptor:
selectivity, low detection limit and good applicability
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C9NJ00288J
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Gurjaspreet Singh , Akshpreet Singh , Pinky Satija , Geetika Sharma , Shilpy , Jandeep
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Singh , Jasbhinder Singh , Kamal Nain Singh , Amarjit kaur
aDepartment of Chemistry and Centre of Advanced Studies, Panjab University, Chandigarh,
60014, India
1
bDepartment of Chemistry, Lovely Professional University, Phagwara-144411, India
*Corresponding Author
Dr. Gurjaspreet Singh
Associate Professor
Department of Chemistry and Centre of Advanced Studies
Panjab University, Chandigarh, India
Email: gjpsingh@pu.ac.in
Abstract: The development of selective and sensitive complex molecular systems for
+
detection of Ag ion is of great importance due to extensive use of toxic silver containing
materials having effect on human health and environment. We report here a new
chemosensor for silver ion recognition in aqueous solution, which is constructed by
integrating three moieties; silatrane, 1,2,3-triazole and barbituric acid, using knoevenagel
condensation and Cu (I) catalyzed [2+3] azide-alkyne cycloaddition reaction. The sensing
assay based on silatrane exhibited high selectivity and high specificity towards silver ion.
Overall, the results show that this strategy to develop new functional material can be
+
employed for efficient sensing of Ag ion. Further, the benefit of existence of silicon in the
receptor lies in the extension of this molecular system in the field of nanochemistry, where

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silica based nanomaterials can have more significant outcomes. Moreover, the use of aqueous
solution can make the sensor to be employed for detection in real samples.
Keywords: Knoevenagel condensation, Receptor, Silane, Silatrane, Silver ion, Triazole
Introduction
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Silatranes are hypervalent silicon compounds featuring tricyclic caged structure with
trialkanolamine ligand, N-[(CR R ) O] SiR. The molecule contain pentacoordinated silicon
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2 n
3
atom having distorted trigonal bipyramidal geometry which resides on the bridgehead
position of the cage, holding three endocyclic oxygens surrounded equatorially and a
substituent group (R) enclosed axially. A feature of interest in these atrane compounds is the
transannular dative bond between silicon atom and nitrogen of the trialkanolamine ligands
that facilitate it into hypercoordinated molecule and impart some peculiar properties such as
1
-2
architectural beauty, structural stability and uniqueness . The biological applications of
silatranes include anti-pilotropic, anti-microbial, anti-cancer, anti-tumor and anti-
inflammatory activity and its other practical relevance is in the field of agriculture, sol-gel
3
processes and protective coating . Moreover, it is also employed as strong reducing agent in
some organic conversions and as a precursor for the synthesis of meso and micro porous
4
materials . Silatranes are known to be hydrolytically more stable and less susceptible towards
hydrolysis than its open chain trialkoxysilane analogue5.
The development of new molecular receptors for the detection of metal ions is the current
focus of research interest among scientists for monitoring environmental and biological
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+
processes . In particular, silver ion (Ag ) sensing has received significant attention owing to
its applications in electronics, pharmaceutical industries, food preservatives and many other
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industrial consumer products . Silver containing materials has a negative impact on human
health and environment. It is a hazardous pollutant and can bind with the functional groups of
numerous metabolites, inactivate the enzymes such as sulfhydryl, and can displace calcium
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and zinc ions present in hydroxyapatite of bones to induce many disorders . Moreover, the
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excessive exposure to Ag in human body can result in cardiac enlargement, damage to eyes
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and skin, degenerative changes in liver and growth retardation . So, a need to synthesize
selective, sensitive and portable receptors for recognition of silver ion arises. The use of

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traditional techniques such as Atomic Absorption spectroscopy (AAS), inductively coupled
plasma/mass spectroscopy (ICP MS), inductively coupled plasma/atomic emission
spectroscopy (ICP AES), X-ray fluorescence spectroscopy (XRF), potentiometric
methodology, ion selective electrodes and electrochemical processes have been widely
explored for silver ion identification but these methodologies are highly expensive, have
time-consuming pre-treatment steps, requires skilled operators and lack portability facility,
which limit their practical advantages10-11. However, the investigation of Ag by chemo-
sensors has abundant benefits, for example, cost effectiveness, sensitivity, selectivity,
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portability, free from dangerous materials, rapid response and simplicity . The design of
such sensors that are easy to use and allow visual detection is the growing area of research
interest. There are many recognized chemo-sensors for the determination of silver ions but
still there is a need to find its effortless and practical sensors.
The present investigation is about the synthesis of a silatrane based complex functional
system for the determination of silver ion in aqueous phase. The inclusion of triazole ring
through biocompatible click chemistry reaction into the molecular system added the
coordination sites to the receptor. 1,2,3-triazole moiety have proficient binding unit that can
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be used for recognition of various cations and anions . The synthetic path used to conjugate
triazole motif into the silatrane was based on Cu (I) catalyzed [2+3] azide-alkyne
cycloaddition reaction (CuAAC reaction). Further, the utilization of knoevenagel
condensation approach was done to attach donor groups containing barbituric acid, the
motive of which was to enhance the complexation ability of the sensor. It is the first time
when knoevenagel condensation approach has been used in the synthesis of silatrane
containing molecules. Since the recognition ability of the silatrane was achieved using
aqueous solution, the synthesized sensor can be practically applied to the real samples.
Experimental Section:
Materials and Methods
The schlenk technique was employed to perform reactions under nitrogen atmosphere.
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Commercially purchased solvents were dried before use by the known standard procedures .
p-Hydroxybenzaldehyde (avra), Propagyl bromide (sigma aldrich), Potassium carbonate
(
thomas), 1,3-Dimethylbarbituric acid (alfa aesar), Bromotris(triphenylphosphine)Copper(I),
(CuBr(PPh ) ) (sigma adrich), Sodium azide (avra), (3-Chloropropyl)triethoxysilane (sigma
3
3

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aldrich) and Triethanolamine (Merck) were commercially purchased and directly used as
received. 3-Azidopropyltriethoxysilane (AzPTES) was prepared from sodium azide and (3-
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chloropropyl)triethoxysilane according to the known procedure in the literature . Further, the
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silane (5) was synthesized by our group in our previous work . The FT-IR Spectra was
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recorded as neat on a Thermo Scientific NICOLET IS50 Spectrophotometer. H and C
NMR spectra of 400 MHz and 100 MHz respectively, were taken on a sophisticated
multinuclear FT NMR spectrometer model Advance II (Brucker). Absorption spectra were
recorded by using JASCO V-530 double beam spectrophotometer.
5-(4-((1-(3-(2,8,9-trioxa-5-aza-1-sila-bicyclo[3.3.3]undecan-1yl)propyl)-1H-1,2,3-triazol-
4
-yl)methoxy)benzylidene)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione
(6):
A
mixture of silane 5 (0.54g, 1.00 mmol, 1 equiv.) and triethanolamine (0.13 ml, 1.00 mmol, 1
equiv.) dissolved in dry toluene (20 ml) were added to a 50 ml double neck RBF fitted with a
Dean-stark trap under nitrogen atmosphere and a catalytic amount of potassium hydroxide
0
was further added to it. The reaction mixture was refluxed for 5 hours at 100 C that resulted
in the azeotropical removal of methanol formed as byproduct. Then the solvent was
evaporated in vacuum and the left residue was extracted by adding hexane (3ml). The
contents were stirred for 2 hours and the desired solid (6) was filtered and dried. Yield: 76%.
o
-1
Mp: 192-194 C. IR (Neat, υ cm ): 543 (N→Si), 1085 (Si-O), 1671 (C=O). NMR (δ in ppm):
1
H (CDCl , 400 MHz) 6.88-7.84 (m, 4H, Ar-H), 7.26 (s, 1H, C=CH-N), 7.10 (s, 1H, C=CH),
3
5.13 (s, 2H, O-CH ), 4.33 (t, 2H, J = 20 Hz, N-CH -C), 3.67-3.76 (m, 6H, Si-O-CH ), 2.83 (s,
2
2
2
6
H, N-CH ), 2.73-2.82 (m, 6H, O-C-CH ), 1.94-2.00 (m, 2H, C-CH -C), 0.40 (t, 2H, J = 16
3 2 2
Hz, Si-CH2). 13C (CDCl , 100 MHz) 132.04, 115.16, 62.37, 59.35, 57.52, 56.78, 56.05,
3
5
3.46, 50.6, 26.33, 13.22.
Result and Discussion
Synthesis and characterization:
The molecular design for the synthesis of silatrane based receptor is based on the efficient
binding ability of three moieties, namely barbituric acid, 1,2,3-triazole and silatrane. These
three electron rich donor groups have been conjugated together by employing four step
synthetic route to obtain a proficient chemosensor (6) for the selective recognition of silver
ion (Scheme 1). The synthesis of silane (5) was done using knoevenagel condensation
1
6
reaction and click chemistry approach . In the final step of the synthesis, equimolar amount

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of the obtained silane (5) and triethanolamine were made to undergo transesterification
reaction in the presence of potassium hydroxide (KOH) as base and toluene as solvent to
afford desired hydrolytically stable silatrane based chemosensor (6). The reaction was done
by employing Dean stark apparatus to azeotropically remove by-product ethanol so as to
drive the conversion to completion.
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O
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CHO
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6 hr stirring
Br
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Water
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(
i)
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i) AzPTES, Cu(PPh ) Br, Et N, THF, 5h, 60 C
3 3 3
Scheme 1: Synthetic pathway for the synthesis of silatrane based receptor
FT-IR spectrum of the synthesized silatrane (6) was obtained and it gave the characteristic
-1
peak of the transannular bond between nitrogen and silicon at 543 cm which confirm the
structure of the desired receptor. Also, the spectrum of (6) showed an absorption band at
-1
1085 cm due to Si-O bond. Further characterization of the sensor was done by multinuclear
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NMR spectroscopy and the signals obtained were in accordance with its structure. H NMR
spectra showed two signals due to the protons of OCH and NCH at δ = 3.67-3.76 ppm and
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.73-2.82 ppm respectively that corresponds to the silatranyl cage of the synthesized silatrane

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DOI: 10.1039/C9NJ00288J
(
6). The existence of three signals of equal intensity and integration at 4.33 ppm, 1.94-2.00
ppm and 0.40 ppm confirm the presence of the propyl chain in the compound. Moreover, the
aromatic protons and a proton in the triazole ring was observed in the region δ = 6.88-7.84
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ppm. In C NMR spectrum of (6), –CH – signal next to silicon of the propyl chain appeared
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as most shielded at δ = 13.22 ppm. The spectra exhibited resonance at 50.96 ppm and 57.52
ppm which corresponds to NCH and OCH of the atrane cage.
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Recognition studies:
The recognition studies were carried out at 25 C and the concentration of sensor (6) was
o
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fixed at 10 M for all spectral experiments. The use of methanol:water (9:1) as solvent
system was employed and sufficient time was given before recording the spectra so as to
ensure complete uniformity of the solutions. The absorption spectrum recorded reveals that
receptor analyte (6) has a maximum centered at 268 nm which can be assigned to π-π*
electronic transition. On addition of silver ion, there was an appreciable change in both
intensity as well as wavelength maxima (Fig. 1). This change in the energy band can be
+
attributed as follows: addition of Ag to the aqueous solution of receptor resulted in the
decrease in the conjugation of the silatrane (6) due to its complexation with the cation and
consequently increases π-π* transition energy difference, the outcome of which was
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hypsochromic shift. Further, it was observed that upon concomitant addition of Ag , the
intensity of absorption band increases on increasing the concentration of cation (0 to 3equiv.)
along with a blue shift in λmax value (Fig. 2). Since the absorption change occurred in the UV
region, no colour change was observed throughout the titrations.

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Fig. 1: UV-Vis absorption spectra change on addition of Ag to receptor
2.5
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Fig. 2: UV-Vis spectra upon addition of increasing amount of Ag (0.2, 0.4, 0.6, 0.8,
1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0 equiv. respectively)
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The practical applicability of a chemo-sensor depends upon its selectivity to recognize a
particular metal and so the sensing ability of the silatrane (6) was examined by adding
+
+
2+
2+
2+
2+
2+
2+
2+
2+
2+
following group of cations, Ag , Na , Co , Fe , Hg , Ni , Pb , Mn , Zn , Cd , Cu
3+
and Cr . For this, two equivalents of each metal ion were added to the 1µM aqueous solution
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of receptor. Among all the tested ions, addition of only Ag ion caused significant alteration
in the spectrum of (6) while treatment with other cations gave only nominal change in the
absorption band (Fig. 3). Further demonstration of high selectivity towards silver ion is
depicted from the bar diagram in Fig. 4. Moreover, competition studies were done to check
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about interference in the detection of Ag ion in the presence of any other metal ions and the
results indicated that there was no significant interference in this recognition study.
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+Na +Co +Fe +Hg +Ni
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Pb +Mn +Zn +Cd +Cu +Cr
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Wavelength (nm)
+
+
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Fig. 3: Absorption spectrum changes of 6 in the presence of Ag , Na , Co , Fe , Hg , Ni ,
2
+
2+
2+
2+
2+
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Pb , Mn , Zn , Cd , Cu and Cr .

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3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
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5
5
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6
0
1
2
3
4
5
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7
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9
0
1
2
3
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5
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9
0
1
2
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7
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9
0
1
2
3
4
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6
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8
9
0
1
2
3
4
5
6
7
8
9
0
0.2
.0
0
+
+
2+
2+
2+
2+
2+
2+
2+
2+
2+
3+
Ag Na Co Fe Hg Ni Pb Mn Zn Cd Cu Cr
Cations
Fig. 4: The change in absorbance shown through a bar diagram at λmax 260nm
+
The evaluation of the stoichiometry of the complex between sensor (6) and Ag was done by
employing Job’s continuous variation method (Fig. 5). The plot showed maxima at 0.5 which
suggests that the complex formed between them would be 1:1 stoichiometry. To further
explore the stoichiometry of complexation and measure the strength of interaction between
+
17
receptor and Ag ion, the method of Benesi-Hildebrand was used . From the UV-Vis
-1
+
titrations profile, linearity of regression in the plot of [A-A ] vs [Ag ] was obtained, where
0
[
A-A0]-1 represent absorption intensity change of metal in the presence of receptor at
particular wavelength (Fig. 6). The graph came out to be linear with high value of regression
2
coefficient (R =0.995) which indicates the stoichiometry of complexation as 1:1. The
measure of interaction between analyte and metal ion was obtained by calculating association
-
1
+
constant (K ), which was attained through the slope of [A-A ] vs [Ag ] plot. The value
a
0
5
turned out to be 6x10 M which suggests a strong interaction between the synthesized
silatrane and silver ion. Moreover, Limit of detection (LOD) was calculated from the
standard deviation of the responses and slope of the plot and the value obtained was 0.141
µM which is well below the limit assigned by U.S. EPA (0.46 µM).

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.40
.35
.30
.25
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.15
.10
.05
.00
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9
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-
0.05
0
.0
0.2
0.4
0.6
0.8
1.0
+
6/6+Ag
+
Fig. 5: Job's plot for the complexation of receptor 6 with Ag showing 1:1 stoichiometry
2
.4
2.2
.0
1.8
2
1
.6
.4
1
1.2
.0
1
0
.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
+
6
1
/Ag x10
Fig. 6: B-H Plot of silatrane 6 with Ag+

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DOI: 10.1039/C9NJ00288J
Table 1 shows the comparison with the previously synthesized chemosensors for the
detection of silver ion and the observations pave the way for silatrane based receptors for
+
recognition of Ag ion.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
Table 1. Comparison of few reported sensors of Ag ion
Sensors
Tween 20-AuNPs
Measured Signal
Absorption
LOD (nM)
100
Reference
18
DNA
Fluorescence
Absorption
80
19
Coumarin-based new Schiff base
Carbon Quantum Dots
Hydrazine carbothioamide
Silatrane based receptor
0.86
20
Fluorescence
Fluorescence
Absorbance
5093
590
21
22
141
Present work
Mode of binding:
The metal complex of silatrane can coordinate through Nitrogen and oxygen present in the
system. The C=O in the barbituric acid and N in the triazole ring has significant coordination
ability and so this can be the predominant binding mode (Fig. 7). The proposed binding
approach is supported by the vibrational spectroscopic tool. The IR spectrum of the complex
reveals that there is shift in the characteristic peak of one of the C=O bond of barbituric acid
-1
+
to 1613 cm which can be attributed to coordination of Ag ion to C=O. Further, the
-1
vibrations in the region 1400 to 1600 cm which can be ascribed to the triazole ring, also
showed positive shifts suggesting the role of N of this ring in the binding of silver.

New Journal of Chemistry
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DOI: 10.1039/C9NJ00288J
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0
O
Ag+
O
N
Si
N
O
O
N
N
O
Fig. 7: Plausible mode of interaction
Conclusion:
In this study, we have prepared a new silatrane bearing barbituric acid and 1,2,3-triazole
moieties by employing knoevenagel condensation and click reaction. The synthesized
compound was utilized for the detection of silver ion with one of the nitrogen in the triazole
ring and one of the carbonyl oxygen in the barbituric acid as the potential binding site which
is supported by IR spectroscopy study. The sensor was examined with different metal ions
+
and the results show that it is selective only towards Ag ion. Further, the stoichiometry for
binding in the complex was observed to be 1:1 as suggested by the Job’s Plot and B-H Plot.
Moreover, the chemosensing activity of the analyte in the aqueous solution increases its
applicability in real sample analyses.
Conflicts of Interest
There are no conflicts to declare.
Acknowledgements
The authors are thankful to UGC for proving scholarship and CSIR, DST-PURSE and DST-
SERB for giving other financial assistance.

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First report of silver ion recognition via silatrane based receptor:
selectivity, low detection limit and good applicability
D
E
O
x
I: 1
c
0
e
.10
l
3
l
9
e
/
n
C9NJ00288J
t
a,*
a
a
a
a
Gurjaspreet Singh , Akshpreet Singh , Pinky Satija , Geetika Sharma , Shilpy , Jandeep
b
b
a
a
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Singh , Jasbhinder Singh , Kamal Nain Singh , Amarjit kaur
aDepartment of Chemistry and Centre of Advanced Studies, Panjab University, Chandigarh,
60014, India
1
bDepartment of chemistry, Lovely Professional University, Phagwara-144411, India
*Corresponding Author
Dr. Gurjaspreet Singh
Associate Professor
Department of Chemistry and Centre of Advanced Studies
Panjab University, Chandigarh, India
Email: gjpsingh@pu.ac.in

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The current study involves synthesis of silatrane based receptor using knoevenagel
condensation and click chemistry approach for recognition of silver ion in aqueous solution.
0
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0
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0
O
N
O
N
Silver ion
O
Ag+
O
N
Si
N
O
N
N
O
O
Knoevenagel
condensation
Click reaction