Organosilanes and their magnetic nanoparticles as naked eye red emissive sensors for Ag+ions and potent anti-oxidants

Article abstract of DOI:10.1039/d1nj00242b

The present article reports the synthesis and structural characterization of a triazole appended isatin linked silane-based sensor using a highly efficient click chemistry approach. The yellow-coloured solution of the highly selective chemosensor turned red in the presence of Ag⁺ions and a red shift in the absorption spectra was also observed during the addition of these ions. IR spectroscopic studies support the metal ion binding with organosilanes in the cavity created by the carbonyl group of isatin, the nitrogen atom of the triazole ring and the Si-O bond of silane. Furthermore, the sensing application was enhanced by the immobilization of the ligand on the surface of iron oxide silica nanoparticles and the resulting silica surfaced nanocomposites were characterized using various techniques. B-H plots and calibration curves were employed to measure the association constants and limits of detection, respectively, of the synthesized ligands and their magnetic nanoparticles. The nanohybrids demonstrate a greater binding potential to Ag⁺ions. In addition, the synthesized organosilanes were investigated for cytotoxicity using the MTT assay, and the antioxidant activity was measured by decolouration of ABTS˙⁺

Full text of DOI:10.1039/d1nj00242b

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Organosilanes and their magnetic nanoparticles as
naked eye red emissive sensors for Ag+ ions and
potent anti-oxidants†
Cite this: New J. Chem., 2021,
45, 5517
Gurjaspreet Singh, *a Diksha,a Akshpreet Singh,b Pinky Satija,a Pawan,a Mohit,a
D. Gonzalez-Silvera,c Cristobal Espinosa-Ruızc and Marıa Angeles Esteban*c
´
´
´
´
The present article reports the synthesis and structural characterization of a triazole appended isatin
linked silane-based sensor using a highly efficient click chemistry approach. The yellow-coloured
solution of the highly selective chemosensor turned red in the presence of Ag+ ions and a red shift in
the absorption spectra was also observed during the addition of these ions. IR spectroscopic studies
support the metal ion binding with organosilanes in the cavity created by the carbonyl group of isatin,
the nitrogen atom of the triazole ring and the Si–O bond of silane. Furthermore, the sensing application
was enhanced by the immobilization of the ligand on the surface of iron oxide silica nanoparticles and
the resulting silica surfaced nanocomposites were characterized using various techniques. B–H plots
and calibration curves were employed to measure the association constants and limits of detection,
respectively, of the synthesized ligands and their magnetic nanoparticles. The nanohybrids demonstrate
Received 15th January 2021,
Accepted 17th February 2021
DOI: 10.1039/d1nj00242b
a greater binding potential to Ag+ ions. In addition, the synthesized organosilanes were investigated for
+
cytotoxicity using the MTT assay, and the antioxidant activity was measured by decolouration of ABTSꢀ
.
rsc.li/njc
techniques, for example, X-ray fluorescence spectroscopy (XRF),
atomic absorption spectroscopy (AAS), ion selective electrodes,
Introduction
In the cycles of evolution, living organisms and synthesis, metal electrochemical processes and inductively coupled plasma with
ions play a key role; hence, there is an urgent need for the mass spectroscopy and atomic emission spectroscopy, has the
production of cost-effective and simple molecular receptors for disadvantages of the lack of portability, the need of skilled
their detection. Detection of Ag+ ions is imperative as silver is operators, and being time consuming and expensive.
commonly used in medicinal products,1 electrical imaging2
An organosilane with the molecular formula of Y(CH2)3Si(OR)3
and cosmetics.3 Industries release waste materials containing is an assimilation of hybrid silica precursors with organic
silver into the environment every year and pollute nature. functionalization and consists of hydrolysable alkoxy groups.
Literature studies reveal that silver in drinking water at Isatin represents a significant class of heterocyclic compounds
concentrations greater than 0.9 mmol Lꢁ1 is toxic to living and is known to possess various biological properties.6 Isatin
beings.4 Ag+ ions affect the enzyme functioning by combining derivatives have high potential for the therapeutic treatment of
with various functional groups of metabolites.5 Thus, it is very cancer, and several tests have already demonstrated that the
necessary to develop a selective and sensitive sensor for the anticancer activity of isatin functionalized drugs is more
detection of silver ions. The widespread use of chemosensors effective than that of cis-platinum.7 Furthermore, due to the
for the investigation of Ag+ ions has added benefits of sensitivity, presence of keto functional groups in the isatin moiety, it
being free from dangerous chemicals, a fast response, portability, shows strong binding during metal complex formation.8
simplicity and selectivity, whereas the use of traditional Moreover, isatin derived compounds possess potent antioxidant
activity
to
inhibit
reactive
oxygen
species
(ROS)
establishment.9,10 These species such as free radicals (super-
oxide and hydroxyl) and non-radicals (hydrogen peroxide and
ozone) are generated by biochemical reactions and damage
biomolecules. The 1,2,3-triazole ring can be easily synthesized
via a copper catalysed click reaction. This ring system has great
scope in the remedial field due to its high binding ability toward
various macromolecules such as proteins, DNA and nucleic
a Department of Chemistry, Panjab University, Chandigarh, 160014, India.
E-mail: gjpsingh@pu.ac.in
b Department of Chemistry, Goswami Ganesh Dutta Sanatan Dharma College,
Sector-32C, Chandigarh, India
c Department of Cell Biology & Histology, Faculty of Biology, University of Murcia,
30100 Murcia, Spain. E-mail: aesteban@um.es
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj00242b
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acids. According to the literature, the incorporation of a triazole field range of ꢁ10 000 to 10 000 Oe at room temperature.
ring into the heterocyclic moiety was found to introduce a vast Thermogravimetric analysis of the samples was done on SDT Q
variety of activities like antibacterial, antifungal, anticancer and 600 V20.9 Build 20 using the TGA instrument. X-Ray Diffraction
antiprotozoal activities.11,12
(XRD) was conducted using a PANalytical X’pert PRO spectro-
Nanoscience has developed as an inventive area of study that photometer with Cu-Ka radiation.
has applications in a variety of scientific and technical fields
General procedure for the synthesis of polycyclic-substituted-
isatin alkynes (7a–7b)
including materials science, telecommunication and
bioscience.13 Almost, many user products, like remedial,14
electronic component15 and antimicrobial drug16 contain The isatin alkyne (3) and Schiff bases (6a–6b) were synthesized
nanomaterials. Magnetic nanoparticles have gained interest according to the procedures reported in the literature.18,19 In a
due to their substantial use in several scientific fields, including 100 ml single neck round bottom flask containing 40 ml of
nature and biomedical applications, for example, the distribution ethanol, alkyne (2.7 mmol) was added and a clear solution was
of magnetic drugs in which the deposition of drugs on the target obtained after stirring for 30 min. To this solution, compound
tissue can be controlled using an external magnetic field.17 Taking 6a (2.67 mmol) was added and the mixture was refluxed for 6 h.
into consideration the public health issues, the development of a The reaction was monitored using TLC and, on completion,
naked eye molecular sensor has attracted great interest because ethanol was evaporated to yield the solid product which was
they demonstrate easy detection over fluorometric methods dried under reduced pressure.
without the aid of expensive instruments. Herein, we emphasize
Synthesis of (Z)-3-(2-(anthracen-10-ylmethylene)hydrazono)-
the development of a naked eye probe for the detection of Ag+ ions 1-(prop-2-ynyl) indolin-2-one 7a. Red solid, yield: 89%, m.p.:
by using a powerful synthetic tool, click chemistry, and their 182–184 1C, IR (neat cmꢁ1): 1180 (N–CH2), 1719 (CQO), 2153
1
magnetic nanoparticles show much better results due to the (CRC), 3044 (CQC–H), 3264 (CRC–H). H NMR (400 MHz,
presence of a large surface area. The use of absorption spectro- CDCl3) d 8.88 (d. J = 3.3 Hz, 1H, H7), 8.47 (dd, J = 7.9, 1.2 Hz, 1H,
scopy for detection studies has added advantages of speed and H11), 8.34–8.25 (m, 6H, H1, H4, H14, H17, H18, H21), 8.25–8.06
easy operation, while the present investigation also has benefits of (m, 4H, H15, H16, H19, H20), 7.51 (td, J = 7.8, 1.3 Hz, 1H, H3),
sensitivity, selectivity and lower LOD values. In addition, the 7.15 (d, J = 7.9 Hz, 1H, H2), 4.68 (d, J = 2.5 Hz, 2H, –N–CH2–),
antioxidant and cytotoxicity drug profile of the synthesized com- 2.32 (t, J = 2.4 Hz, 1H, –CRCH).
pounds has been measured which gave promising results.
Synthesis of (Z)-1-(prop-2-ynyl)3-(2-(pyren-1-ylmethylene)
hydrazono)-indolin-2-one 7b. Orange solid, yield: 85%, m.p.:
185–187 1C, IR (neat cmꢁ1): 1178 (N–CH2), 1718 (CQO), 2193
1
(CRC), 3047 (CQC–H), 3241 (CRC–H). H NMR (400 MHz,
Experimental section
Chemicals and reagents
CDCl3) d 9.03 (d. J = 3.3 Hz, 1H, H13), 8.89–8.83 (m, 3H, H2, H7,
H23), 8.29 (d, J = 2.9 Hz, 2H, H21, H22), 8.22–8.09 (m, 6H, H1,
H4, H14, H15, H19, H20), 7.51–7.47 (m, 1H, H3), 7.19 (dd, J =
7.6, 0.9 Hz, 1H, H2), 4.66 (d, J = 2.6 Hz, 2H, –N–CH2–), 2.18 (d,
J = 7.1 Hz, 1H, –CRCH).
Reagents, namely, propargyl bromide (80% in toluene) (Sigma
Aldrich), isatin (Avra), hydrazine hydrate (Avra), potassium
carbonate (Avra), 1-pyrenecarboxaldehyde (Sigma Aldrich),
9-anthracenecarboxyaldehyde (Acros Organics) and bromotris
(triphenylphosphine)copper(I) (Sigma Aldrich) were used as
received. Solvents were purified and dried before use.
3-Azidopropyltrietoxysilane (AzPTES) was synthesized using a
well-known procedure in the literature.
General procedure for the synthesis of organosilanes (8a–8b)
In a two-neck round-bottomed flask containing 1 : 1 THF/Et3N,
compound 7a (0.25 mmol) was added and the mixture was
stirred for 15 min at room temperature under nitrogen atmosphere.
3-Azidopropyltriethoxysilane (0.24 mmol) was slowly added to the
resulting solution, followed by the catalyst [CuBr(PPh3)3] (0.01 mmol)
Instrumentation
FT-infrared spectra were obtained using a Thermo fisher addition. The reaction mixture was stirred at 60 1C for 5 h.
scientific spectrophotometer. An advanced-II Bruker spectro- The reaction was then brought to room temperature. The solvents
1
meter was used to record the NMR spectra; H (400 MHz) and were evaporated under reduced pressure, followed by the addition
13C (100 MHz). CDCl3 was used as the internal reference and of diethyl ether. The contents were kept under stirring overnight
chemical shifts were reported relative to tetramethylsilane. after which the product was isolated as a light orange solid which
J values are given in Hz. Melting points were measured with a was filtered under nitrogen and dried under vacuum.
Mel Temp II instrument using sealed capillaries. The mass
Synthesis of (Z)-3-(2-(anthracen-10-ylmethylene)hydrazono)-
spectral observation (the ESI† source with a capillary voltage of 1-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-
3000 V) was performed using a WATERS Q-TOF micro-MASS yl)methyl)indolin-2-one (8a). Crimson red solid, yield: 82%,
spectrometer. A Hitachi SU8010 ultra-high-resolution scanning m.p.: 250–254 1C, IR (neat cmꢁ1): 734, 1071 (Si–O), 956 (C–C),
electron microscope was used for FE-SEM analysis. UV-Vis 1170 (O–CH2), 1254 (N–CH2), 1464 (CH3–C), 1719 (CQO), 2925
1
analysis was done by using a V-530 double-beam spectrophoto- (CQC–H). H NMR (400 MHz, CDCl3) d 8.66 (s, 1H, H7), 8.27
meter. By using a vibrating sample magnetometer (VSM) (155, (t, J = 7.5 Hz, 1H, H11), 8.12 (d, J = 8.5 Hz, 6H, H1, H4, H14,
PAR) magnetic measurements were performed in the magnetic H17, H18, H21), 7.67 (d, J = 7.9 Hz, 6H, H3, H15, H16, H19,
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H20, –Tz–H–), 6.96 (q, J = 9.2, 8.5 Hz, 1H, H2), 5.16 (d, J = 6.0 Hz, plates and incubated overnight in 5% CO2 at 37 1C. This cell
2H, –NCH2–), 4.35 (t, J = 7.4 Hz, 2H, –N3CH2–), 3.80 (th, J = 14.3, concentration was previously calculated to achieve adequate
7.1 Hz, 6H, –OCH2–), 2.07–1.95 (m, 2H, –CCH2C–), 1.22 (h, J = absorbance values in the cytotoxic assay and to prevent over-
7.5 Hz, 9H, –OCH2CH3), 0.61 (t, J = 8.2 Hz, 2H, SiCH2). 13C NMR growth of the cells. The control contains only the reactant with
(101 MHz, CDCl3): d 172.71, 167.98, 163.59, 162.20, 158.33, compounds without cells. After this, culture medium was
144.85, 142.40, 138.21, 133.70, 132.37, 131.43, 129.11, 127.83, replaced by 200 mL wellꢁ1 of the substance and examined at
127.34, 125.25, 124.99, 116.80, 116.18, 115.45, 111.94, 110.18, different dilutions (200, 100, 50 and 20 mg mLꢁ1). After 24 h of
60.01, 58.54, 52.62, 21.39, 18.29, 16.77. MS: m/z 635.39 (M + H)+. substance exposure, the standard MTT was added to the EMEM
Synthesis of (Z)-3-(2-(pyren-1-ylmethylene) hydrazono)-1-((1- culture medium without fetal bovine serum and phenol red.
(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methyl)indolin-2-one 100 mL of MTT solution of culture medium was then added to each
(8b). Rust solid, yield: 77%, m.p.: 260–264 1C, IR (neat cmꢁ1): 721, cell and incubated for 4 h, the plates were then centrifuged and the
1113 (Si–O), 957 (C–C), 1164 (O–CH2), 1252 (N–CH2), 1440 (CH3– supernatant was discarded, followed by the addition of 200 mL of
C), 1671 (CQO), 2925 (CQC–H). 1H NMR (400 MHz, CDCl3) d 8.76 DMSO to solubilize the formazan crystals formed. Furthermore,
(s, 1H, H13), 8.41 (d, J = 7.8 Hz, 3H, H7, H12, H23), 8.28 (d, J = 9.1 to avoid any substance interference, the supernatant was placed
Hz, 4H, H14, H15, H21, H22), 8.11 (s, 2H, H19, H20), 7.99 (s, 1H, onto a new plate. A plate reader (FLUOstar Galaxy, BMG) was
H4), 7.33 (s, 1H, H1), 7.06 (s, 1H, H3), 6.89 (s, 1H, –Tz–H–), 6.81 (s, used to calculate the optical density values at 570 and 690 nm.21
1H, H2), 5.11 (s, 2H, –NCH2–), 3.21 (d, J = 7.2 Hz, 2H, –N3CH2–),
3.09 (d, J = 7.4 Hz, 6H, –OCH2–), 2.10 (s, 2H, –CCH2C–), 1.15–1.12
(m, 9H, –OCH2CH3), 0.61 (s, 2H, SiCH2). 13C NMR (101 MHz,
CDCl3): d 179.27, 161.15, 151.27, 150.23, 145.28, 142.66, 140.01,
132.17, 131.29, 129.03, 127.50, 126.51, 123.62, 117.84, 114.32,
113.46, 113.41, 102.27, 72.72, 60.18, 45.00, 29.70, 25.12, 17.81
MS: m/z 661.75 (M + 3H)+.
Analysis of UV-Vis spectra
A stock solution of 10ꢁ4 M concentration of the sensor and 10ꢁ5 M
of all metal cations (Ag+, Rb+, Zn2+, Cu2+, Co2+, Ni2+, Ba2+, Al3+,
Sn2+, Ca2+, K+, Hg2+) in methanol was prepared for the detection
studies. To determine the selectivity of the ligand, 10 mL of
methanolic solution of each metal cation was added to the sensor
solution and UV-Vis spectra were recorded. Of all the tested ions,
only the addition of Ag+ ions resulted in a significant amount of
change in the spectrum, while other ligands produced only a small
change in the absorption band (Fig. 1). For the calorimetric
detection, the concentration of Ag+ ions was increased from
10 to 100 mL and the absorption spectra were recorded. The
detection limit and association constants were calculated by using
the calibration curve and B–H plot, respectively.
Synthesis of complex (8a-Ag)
In a round bottom flask, equimolar amounts of organosilane
(8a) and anhydrous silver chloride were taken, to which 50 ml
of dry methanol was added, and the mixture was stirred for 4 h
at room temperature. The solid formed was filtered, washed
with dry methanol, and vacuum-dried to obtain a brown
product. Yield: 82%. M.p.: 4300 1C. IR (neat, cmꢁ1): 1707
(CQOꢂ ꢂ ꢂAg+), 1016 (Si–Oꢂ ꢂ ꢂAg+).
Total antioxidant activity (TAA)
Synthesis of organosilane functionalized iron oxide
nanoparticles
TAA was calculated using a procedure based on the ability of
anti-oxidants to suppress 2,20-azino-bis-3-(ethylbenzothiazoline- Firstly, iron oxide (Fe3O4) nanoparticles were synthesized for
6-sulphonic acid) (ABTS) radical cation. The inhibition of free which, in a single neck RBF, FeCl2ꢂ4H2O and FeCl3ꢂ6H2O were
radical production by the presence of antioxidants was deter- dissolved in deionized water in a 1 : 2 molar ratio. After this,
mined by decolouring the radical cation of ABTS and absorbance 25% ammonium solution was poured and stirring was continued
quenching was measured at 730 nm. A solution that includes for 30 min. Black colour precipitates were formed and the mixture
ABTS 2 mM, horseradish peroxidase 0.25 mM and H2O2 35 mM was again stirred for 1 h. The required nanoparticles were
was prepared. The synthesized compounds were dissolved in collected by washing with ethanol and distilled water
DMSO or chloroform, and 50 mL of the suspension, containing accompanied by drying in the oven. In the second step, Fe3O4
1 mg 100 mLꢁ1 was added to 950 mL of the ABTS solution and the nanoparticles were coated with
a silica layer to avoid
change in absorbance peak was observed at 730 nm for 5 min. aggregation of bare iron oxide nanoparticles. For this, 0.3 g of
This effect is determined by comparing the sample values with a Fe3O4 nanoparticles in ethanol were stirred for 30 min. After
regular ascorbic acid curve and is expressed as ascorbic acid stirring, 20 mL of ammonia solution and 0.2 mL of tetraethy-
equivalents (mmol) per milligram particle. All specimen was lorthosilicate were added and the solution was refluxed for 4 h.
estimated in triplicate.20
Fe3O4@SiO2 brown-colored nanoparticles were obtained which
were isolated magnetically and washed with absolute ethanol
and oven-dried. In the final step, hybrid nanoparticles were
Cytotoxicity assay
The cytotoxicity assay of the synthesized compounds was synthesized by dissolving Fe3O4@SiO2 and organosilane 8a in
performed by using the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5- dry toluene and the solution was refluxed for 24 h. By using an
diphenyl tetrazolium bromide] viability test. The HeLa cell lines external magnet, Fe3O4@SiO2@Silane nanoparticles were
were detached from culture with trypsin and aliquots of 200 mL separated and washed with chloroform and dried under
containing 104 cells wellꢁ1 were placed in 96-well tissue culture reduced pressure (Fig. 2).
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Fig. 1 UV-visible absorption spectra of the receptors (a) AC and (b) PC in the presence of different metal ions in methanolic solutions.
respectively. The shifting of alkyne protons from d = 4.66–
4.68 ppm to d = 5.11–5.16 ppm confirms the synthesis of the
triazole ring. In the 13C NMR of the synthesized compounds,
signals in the range d = 144–146 ppm correspond to the imine
group of the Schiff base. 13C signal due to –OCH2, –NCH2 and
–OCH2CH3 of organosilanes was observed in the ranges of 58–60,
45–52 and 17–19 ppm, respectively. The mass spectra of receptors,
+
8a and 8b, contain base peak at 635.39 (M + H) and 661.75
+
(M + 3H) , respectively.
UV-Vis spectral study
UV-Vis spectroscopy was used to evaluate the sensing potential
of 8a (AC), 8b (PC) and H–Np. The receptors (8a–8b) exhibited
sharp absorption bands centered at 458 and 446 nm, respectively,
due to the presence of a conjugation system of substituents and
p-electron cloud of polycyclic rings. The high selectivity of ligands
toward Ag+ ions over other competitive species was tested by the
addition of 10ꢁ4 M methanolic solution of various metal ions
Fig. 2 Schematic diagram of the synthesis of Fe3O4@SiO2@silane.
such as Rb+, Ba2+, Al3+, Ag+, Zn2+, Cu2+, Ca2+, Sn2+, Co2+, K+, Ni2+
and Hg2+ in their chloride form. It can be seen in Fig. S13 (ESI†)
that the yellow coloured solution of receptor turns to red colour in
Result and discussions
Synthesis aspects and characterization
the presence of Ag+ ions which suggests that this ligand can be
used as a naked eye sensor. Upon increasing the concentration of
Ag+ metal ions from 10 to 100 mM, the absorption band of 8a and
The synthetic route for the synthesis of efficient chemosensors
and antioxidants is shown in Scheme 1 in which the isatin
moiety is functionalized with the materially important organo-
silane. Firstly, alkyne 3 and Schiff bases (6a–6b) were synthesized
following the method reported in the literature22,23 from the
commercially available isatin and carboxyaldehyde, respectively.
These intermediates were then used for the synthesis of new
polycyclic substituted isatin alkynes (7a–7b) by condensation
reaction. In the final step, prepared alkynes were clicked with
3-azidopropyltriethoxysilane to synthesize 1,2,3-triazole bound
organosilanes (8a–8b) (Table 1). The IR spectra reveal that the
characteristic bands of the alkyne group (CRC, 2100–2260 cmꢁ1
8b at 458 and 446 nm, respectively, undergo red shift and also a
decrease in the intensity of the band was observed (Fig. 3). The
investigation for the change in the absorbance by calculating the
energy gap is also shown in Fig. 4 and 5. Furthermore, the B–H
plot shows that the graph of [A ꢁ Ao]ꢁ1 vs. [Ag+]ꢁ1 was linear with
high regression coefficient (R) value which concludes that the
stoichiometry of the sensor–Ag+ complex is 1 : 1 (Fig. 6). The
association constant was also determined from the B–H plot
and it was found that 8a has a strong binding ability compared
to 8b. Moreover, by using the calibration curve, the limit of
detection of ligands was calculated by using the equation;
LOD = 3ssꢁ1, where s = standard deviation and s = slope of the
curve obtained. It was observed that the detection limit of 8a is
much better than that of 8b (Fig. 7). The above results suggest that
as the number of aromatic ring increases, there is increase in
repulsion due to which association as well as limit of detection of
and 3240–3270 cmꢁ1) and azido group (–NQNQN, 2091 cmꢁ1
)
had disappeared after the click reaction, which prove the com-
plete formation of the 1,2,3-triazole ring. The 1H NMR spectra
exhibit signals at d = 1.12–1.22 ppm and d = 3.09–3.80 ppm
corresponding to –CH3 and –OCH2– of the triethoxysilyl moiety,
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Scheme 1 Synthetic approach for the synthesis of a polycyclic isatin derivative based sensor.
Biological assay
Table 1 Synthesized compounds (7a–7b) and (8a–8b)
The synthesized organosilanes (AC and PC) were studied for
their antioxidant activity and cytotoxicity assay (Tables 4 and 5).
The total antioxidant effect has been determined as the ability
to retard the oxidation of proteins, DNA and other biological
species.28 It was observed that compound PC shows a good
antioxidant activity value (Fig. S14, ESI†). The cytotoxicity test of
organosilanes was also done using MTT assay against HeLa
cells in order to calculate their toxicity level. It was observed
that compound AC shows good result (Fig. S15, ESI†).
Hydrolysis study
Additionally, the hydrolytic stability of the synthesized compounds
was determined by using IR spectroscopy technique. For this,
compound 8a was exposed to air and after every 10 min interval
infrared spectra were recorded. There was no significant change
observed in the spectra up to 170 min, which means that the
propyl chain of azidopropyltriethoxysilane and electron cloud of
the polycyclic ring protect the d orbitals of Si atom from attack of
water molecules (Fig. 7).
the receptor decreases. Additionally, UV-vis spectroscopy analysis
of magnetic nanoparticles of AC was performed. The spectra show
that the association constant (Ka) of H-Nps is very high compared
Binding mode
to that of the ligand which can be attributed to the fact that The metal complex was prepared to find the mode of binding of
magnetic nanoparticles provide high surface area for the binding Ag+ ions with the ligand. The organosilane (AC) was stirred with
of Ag+ ions. Furthermore, limit of detection of H-Nps is much silver metal salt solution in methanol for 2 h. The solution was
better than that of organosilanes (Table 2).
vacuum dried under reduced pressure to afford the solid
Table 3 shows the comparison of sensing ability of the product (Scheme 2). Significant changes were observed in the
present sensor with the previously reported sensors and the IR spectra of the complex. Upon addition of Ag+ ions to the
values of LOD clearly pave the way for its use as receptor for receptor, vibrational frequency got decreased from 1719 to
Ag+ ions.
1707 cmꢁ1 due to coordination of the CQO group of isatin
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Fig. 3 Changes in the UV-vis spectra of ligands (1) AC (2) PC and (3) AC+ Np upon the addition of increasing concentration of Ag+ in methanolic solution.
Fig. 4 Graphs showing the band gap energy (Eg) of (a) AC, AC + Ag+ (b) PC, PC + Ag+ and (C) ACNp, ACNp + Ag+.
Fig. 5 B–H plot for 1 : 1 complexation between (4) AC (5) PC (6) AC+ Np and Ag+. Error bars represent standard deviations from three repeated expected
experiments.
with Ag+. Furthermore, changes were observed in the IR region resulted in a new peak at 1058 cmꢁ1 due to the Si–O–Si stretching
near 1400 and 1100 cmꢁ1 which support the role of N atom of vibrations. In the final process, layering of AC over iron silica surface
triazole ring and Si–O bond of silane in binding with silver ion was done and additional peaks were observed in the range of 1352–
(Fig. S16, ESI†).
1732 cmꢁ1 along with the Si–O bond vibrational peak (Fig. 8).
Furthermore, to determine the thermal stability of the
synthesized iron oxide nanohybrids, TGA was performed. All
Characterization of nanoparticles
FT-IR technique is known to be the most predominant examination the compounds were heated under a nitrogen atmosphere in
method for the characterization of nanoparticles obtained at various the temperature range of 15 1C to 1000 1C at a heating rate of
synthesis levels. Firstly, the synthesis of Fe3O4 nanoparticles was 10 1C minꢁ1. The thermal decomposition profile of Fe3O4@SiO2
confirmed by the intense characteristic peak at 1388 cmꢁ1 which is shown in Fig. S17 (ESI†). It can be seen that there is a 10.36%
relates to the stretching vibration of the Fe–O bond. In the second weight loss up to 200 1C corresponding to the loss of water
phase, silica coating was done on the iron nanoparticles which molecules present on the surface. The remaining residue of the
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Fig. 6 Calibration curve shows variation in the absorbance intensity of (7) AC (8) PC and (9) AC+ Np with increase in Ag+ metal ion concentration. Error
bars represents standard deviation from three repeated expected experiments.
Table 4 Total antioxidant activity expressed in equivalent of ascorbic acid
(mM) per mg of substances; Samples dissolved in chloroform (Nd: – non
detected)
Sample code
TAA (eq asc.) mM per mg of substance
8a
8b
Nd
6.17 ꢄ 0.87
Table 5 DL 50 of substances in the HeLa cell lines after 24 h of exposure
Sample code
DL50 (mg mLꢁ1
)
R2
8a
8b
248.2 ꢄ 3.32
6.697 ꢄ 1.44
0.82
0.89
Fig. 7 Time dependent hydrolysis study of organosilane (AC) by using IR
spectroscopy.
Table 2 Ka values and LOD of AC, PC, and AC + Np from the B–H plot
and calibration curve, respectively
AC
PC
AC-Nps
LOD (M)
Ka (Mꢁ1
3.18 ꢃ 10ꢁ6
34.55 ꢃ 10ꢁ6
4.96 ꢃ 10ꢁ7
)
9.14 ꢃ 104
1.16 ꢃ 102
2.81 ꢃ 106
Scheme 2 Synthesis of metal complex.
magnetic silica nanoparticle is due to the strong binding of
silica over Fe3O4 surface. In the case of only silane, there are
two steps of thermal decomposition, the first step corresponds
to the release of ethanol in the temperature range of 25–300 1C
and in the second step all the organic moieties decompose in
the form of gases up to a temperature of 430 1C. The rest is the
formation of stable inorganic material i.e., SiO2. When the
organosilane was immobilized on the magnetic surface, initially
a similar two-step weight loss was observed in the temperature
range of 25–430 1C. The residual silicate percentage left in the case
of magnetic nanoparticles is much more than that of pure silane,
which indicates the formation of a highly stable cross-linked
Table 3 Comparison of few reported sensors for Ag+ ions
Sensors
Measured signal
LOD
Ref.
Ratiometric fluorescent chemosensor
3,4-Bis-triazole Bodipy
Fluorescent trinuclear zinc coordination cluster
Dynamic chemistry based
AC-Np’s
Fluorescence
Fluorescence
Fluorescence
Fluorescence
Absorbance
14 ꢃ 10ꢁ6
0.172 ꢃ 10ꢁ6
2.500 ꢃ 10ꢁ6
0.496 ꢃ 10ꢁ6
24
25
26
27
1.5 ꢃ 10ꢁ6
Present work
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a bathochromic shift in the absorption peak was observed.
The infrared spectroscopy findings support the binding of
silver metal ions with the nitrogen of the triazole ring, the
Si–O bond of silane and the carbonyl group of isatin. Furthermore,
the synthesized ligand was immobilized on the surface of the
magnetic silica nanoparticles and the obtained magnetic nano-
composites were well characterized by IR, TGA, EDX, XRD and
FESEM. It was observed from the B–H plot and calibration
curve that the limits of detection and association constants of
the hybrid nanoparticles exhibit higher values than those of the
organosilanes. Moreover, the linearity of the plot with a high
regression coefficient (R) confirms the 1 : 1 stoichiometry ratio
of the metal complex. Additionally, the total antioxidant and
cytotoxicity profiles of the synthesized compounds were
determined which gave promising results. The present work
provides a naked eye, highly selective and cost-effective sensor
for the detection of Ag+ ions.
Fig. 8 FT-IR spectra of Fe3O4; Fe3O4@SiO2; and Fe3O4@SiO2@silane.
structure of silane moiety on the surface of iron oxide nano-
particles (Fig. S17, ESI†).
Fig. S18 (ESI†) shows X-ray diffraction (XRD) pattern of
synthesized iron oxide and silane coated iron oxide nano-
particles. In the XRD pattern of Fe3O4@SiO2@Silane, new
characteristic peaks were obtained at low 2y = 21.4, 18.1 along
with Fe3O4 peaks at 2y = 30.2, 35.6, 43.3, 57.4 and 63.0 which
confirm the association of silane moiety with the iron oxide
nanoparticles.
FESEM analysis provides the information about the surface
morphology of the synthesized organosilane iron oxide nano-
particles. Images provided by SEM are extremely magnified and
of high-resolution compared to that of optical microscopy. The
figure displays the formation of uniform spherical shape nano-
particles. In addition, the prepared nanoparticle tends to have
adopted the shape of a ‘heart’ on deep focusing (Fig. S19, ESI†).
For elemental analysis of Fe3O4@SiO2@AC nanoparticles,
EDX spectroscopy was performed. The spectra shown in
Fig. S20 (ESI†) have characteristic peaks of Fe, Si, and O atoms
which support the absorption of silica over iron oxide nano-
particles. Remnants of impurities are not detected which
substantiate the H-Np’s purity.
Additionally, VSM technique was used to examine the
magneto structural properties of the synthesized nanoparticles.
The hysteresis loop of the hybrid nanoparticles at room
temperature is given in Fig. S21 (ESI†). From the curve, calculated
saturated magnetization (Ms) is found to be 17.7 emu gꢁ1 which is
a remarkably high value indicating the paramagnetic character of
the synthesized H-Nps. The existence of sufficient magnetism in
silane nanoparticles makes it possible to isolate and recycle them
by simply using an external magnetic field.
Conflicts of interest
There is no conflict of interest.
Acknowledgements
The authors would like to thank UGC, DST-PURSE II and CSIR
for providing necessary financial assistance.
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