‘Naked-eye’ colorimetric/fluorimetric detection of F? ions by biologically active 3-((1H-indol-3-yl)methyl)-4-hydroxy-2H-chromen-2-one derivatives

Article abstract of DOI:10.1039/c6ra24597h

An improved synthetic protocol has been developed for the construction of highly functionalized heterologous alkyl and benzyl indolyl-coumarin derivatives through a rapid, catalyst-free and solvent-free one-pot three-component reaction of indole, aldehyde

Full text of DOI:10.1039/c6ra24597h

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‘Naked-Eye’ Colorimetric/Fluorimetric Detection of F− Ion by
Biologically Active 3-((1H-Indol-3-yl)methyl)-4-hydroxy-2H-
chromen-2-one Derivatives
`Received 00th January 20xx,
Accepted 00th January 20xx
Shailya,b*, Ajay Kumarb, Sumit Kumara and Naseem Ahmeda*
DOI: 10.1039/x0xx00000x
An improved synthetic protocol has been developed for the construction of highly functionalized heterologous alkyl and
benzyl indolyl-coumarin derivatives through a rapid, catalyst-free and solvent-free one-pot three component reaction of
indole, aldehyde and 4-hydroxy-coumarin. The synthesized compounds display a high selectivity and sensitivity towards F−
ion. The detection of F− ion was accompanied by fluorescence quenching with noticeable colour change, which is evident
from Stern-Volmer plot. The Stern-Volmer plot shows association constant of 6.2625 × 104 M-1 with lowest detection limit
www.rsc.org/
1
(LOD) of 0.50 μM for F− ion. The interaction mechanism was studied by H-NMR titration and also supported by advanced
TD-DFT calculations. The designed chemosensor has been utilized as a mini colorimetric kit for F− ion on test paper strips
and also capable in detecting F− ion from commercially available toothpaste and mouth rinser. The chemosensors also
exhibited good antifungal and antibacterial activity.
the synthesis of relevant heterologous indolyl-coumarin
scaffolds.
1. Introduction
Synthesis of biologically and pharmaceutically
In spite of the gargantuan potential interest, there is no data
available in the literature describing the optical properties of
effective sensory molecules remains key objectives of current
research in view of their potential applications in biology,
medical and sensing material. Coumarin and indole derivatives
play an important role in human life due to their valuable
α-benzylaminocoumarins
and
benzylindolyl
coumarin
derivatives. Although a very few reports have been published
describing the optical behaviour of bis(coumarin)methylene.13-
15
biological, medical and optical properties.1 Therefore,
a
Anions play a noteworthy role in a wide range of biological,
conjugation of indole and coumarin is expected to be more
efficient than individual form because of their several
applications such as optical brighterners,2 photosensitizers,3
fluorescent and laser dyes4 as well as additives in food,
perfumes, cosmetics and pharmaceuticals.5 The 4-
hydroxycoumarin-indole based receptor brings two
fluorescent chromophores into remarkable proximity,
possessing an enolic-OH group and indolyl-NH group
(hydrogen bond donor position due to which the electronic
properties of the receptor is changed allowing the subsequent
environmental, medicinal and chemical processes. Among
various anions, F− ion is one of the most essential ions in the
human body with its significant applicability in dental care and
potential use for the treatment of osteoporosis, psychiatric
and hypnotics.16 However, excess F− ion is also harmful to
human health. It is easily absorbed and causes acute gastric,
fluorosis, cancer and kidney problems.17, 18 According to World
Health Organization (WHO) F− ion in drinking water is
considered beneficial at level of about 5.3 × 10−3 mol L−1 but
harmful once it exceeds 7.9 × 10−3 mol L−1.19–21 Thus, it is
necessary to add appropriate concentration of F− ion in
drinking water.22 At present, a large population in the world
still drink water containing inappropriate level of F− ion which
leads to several types of pathologies. In India alone about
seven regions of West Bengal (WB) are predicted to be
endemic for fluorosis and almost 66 million common people in
these regions are at high risk of F− contamination.23 Therefore,
considering these obvious health concerns and to minimize the
possibility of excessive F− ion intake, a simple, economical and
selective assay is highly desirable for practical purposes.24
Several efforts have been made to monitor F− ion by
traditional analytical methods such as Ion chromatography25
and ion-selective electrodes.26 However, the major
disadvantages of these techniques are that they are time
consuming and require costly instruments. Alternatively, the
fluorimetric and colorimetric techniques have attracted more
attention over other available techniques for detecting cations
and anions due to high selectivity, quick response, real-time
detection of anions), which leads to
a highly sensitive
chemosensor.6 In the past two decade, several reports have
been published on the synthesis of α-benzylaminocoumarins
and benzylindolyl coumarin derivatives.7-12 Despite the
intrinsic worth of these methods, some of them suffer with
several limitations such as long reaction time, use of catalysts,
toxic reagents, carcinogenic solvents, limited substrate scope,
low to moderate yields, tedious work-up procedure and
purification steps. Therefore, it is a challenging task to develop
a novel, efficient, economical and eco-friendly approach for
a.
Department of Chemistry, Indian Institute of Technology, Roorkee-
247667, Uttarakhand, India
Department of Chemistry, D. B. S. (P.G.) College, Dehradun-248001,
b.
Uttarakhand, India. E-mail: shailyrke@gmail.com,
nasemfcy@iitr.ac.in
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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28
detection and cost effectiveness.27,
In literature, a great the multiplicities: br = broad, s = singlet, d = doublet, dd =
deals of excellent colorimetric and fluorescent sensors for F− double of doublet, dt = doublet of triplet, t = triplet, q =
ion have been reported,29-31 while, most of these sensors have quartet, m = multiplet. The UV–vis. absorption spectra were
complicated structure and synthesized by multi-step recorded on a Shimadzu UV-2450 spectrophotometer and the
procedures.32–34 The development of chemosensor for the fluorescence emission spectra were recorded on a Shimadzu
selective and sensitive detection of F− ions is crucial. RF-5301 PC Spectrofluorophotometer with a 3.0 cm standard
Therefore, design and development of synthetic receptors and quartz cell. Melting points were recorded on Optimelt
chemosensor for the selective and sensitive detection of F− ion automated melting point system. Quantum yield were
have garnered considerable attention for researchers due to obtained by using FLS 980 Fluorescence spectrometer
toxicity of the F− ion.
(Edinburgh Instruments). The IR spectra were recorded on a
In continuation of our research interest on the Nicolet 6700 FTIR Thermoscientific in the range 4000–400 cm−1
synthesis of various functionalized molecules, acid catalysis.35- with KBr pellet. Fluorescence life time spectra were obtained
37
Furthermore, suitable supramolecular sensing probe for by using HORIBA Jobin Yvon, Fluorocube Fluorescence Lifetime
cations have been developed and recently published.38 System.
Influenced by the results of these studies, herein we
synthesized
heterologous
indole-4-hydroxycoumarin 2.2 General procedure for the synthesis compounds 4a–4n:
derivatives via one-pot three component reactions of
substituted 4-hydroxycoumarin, aldehyde and indole at 50–60 A mixture of 4-hydroxycoumarin 1 (1.0 mmol), aldehyde 2 (1.0
oC under grinding and solvent-free conditions (scheme 1). The mmol) and indole 3 (1.0 mmol) was taken in mortar and mixed
current method is significantly improved from earlier thoroughly with pestle and ground at 50–60 °C. The reaction
described protocols in terms of applicability of wide range of was monitored by thin layer chromatography. After
aldehyde, better yield, shorter reaction time, simplistic workup completion, the reaction mixture was cooled to room
and essentially no need of column chromatography. The temperature and washed with ethanol and dried to obtain
synthesized compounds selectively detect F− ion by naked eye, pure desired product.
exhibit intense hypochromic shift in absorption and significant
fluorescence quenching in emission spectra upon interaction
with F− ion in acetonitrile. Interestingly, synthesized series of
3. Results and discussion
compounds also exhibited significantly antifungal and 3.1 Synthesis, substrate scope and characterization of compounds
antibacterial response.
4a–4n:
In the design of anion sensor, we were interested in
developing heterologous scaffold bearing –OH and –NH groups
in the same molecule; a 4-hydroxycoumarin moiety as enolic
scaffold and an indole as an –NH entity to detect anions
through deprotonation mechanism. At the outset 4-
hydroxycoumarin 1a, benzaldehyde 2a and indole 3a were
chosen as the model substrate to optimize the reaction
conditions. In
a preliminary attempt, the reaction was
performed in the presence of 20 mol% of I2 at room
temperature, the proposed reaction not proceed under these
conditions to give the desired product 4a (Table 1, entry 1).
When reaction was carried out at elevated temperature (40-
100 oC), the desired product 4a was obtained in 20% yield
along with the traces of side product 5a (Table 1, entry 2–4).
Furthermore, when the reaction was carried out in absence of
I2 under solvent free condition at room temperature for 1h,
the reaction underwent smoothly to give the desired product
4a with 50% yield along with a significant amount of 5a (40%)
(Table 1, entry 5). In order to improve the yield of
heterodimeric product 4a, reaction was explored at different
temperatures under neat condition, the optimum 88% yield
was obtained at 50 °C (Table 1, entry 7). Thus the reaction at
50 °C under solvent-free conditions emerged as the optimum
for subsequent studies. After having the optimized reaction
conditions, we further investigated the substrate scope and
generality of the reaction. A number of structurally diverse
aldehydes, substituted indole and 4-hydroxycoumarin are used
Scheme 1: One pot green synthesis of alkyl/benzylindolylcoumarin derivatives.
2. Experimental details
2.1 Reagents and instrumentation:
All common reagents and solvents were of AR grade,
purchased from Sigma Aldrich and Himedia, India and used as
1
received otherwise mentioned. H-NMR and 13C-NMR spectra
of compounds were recorded in DMSO-d6 on a Brüker (500
MHz) spectrometer and chemical shifts were reported as part
per million (ppm) in δ scale downfield from TMS (as internal
standard). The following abbreviations were used to explain
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Table 1. Optimization of Reaction Condition for the synthesis of benzylindolylcoumarin 4a.a
OH
CHO
OH
Catalyst
Temperature
N
O
O
H
NH
HN
O
O
NH
1a
2a
3a
4a
(desired product)
5a
(side product)
Yieldb (%)
Entry
Catalyst
Temperature (oC)
4a
5a
40
35
Trace
--d
40
10
5
c
1
2
3
4
5
6
7
8
9
I2
I2
I2
I2
RT
40
50
100
RT
30
50
60
100
--
c
Trace
20
c
c
--d
-
-
-
-
-
50
60
88
88
5
-- d
-- d
a The reaction was carried out using 4-hydroxycoumarin 1 (1.0 mmol), aldehyde 2 (1.0 mmol) and indole 3 (1.0 mmol) in mortar and mixed thoroughly with pestle. b isolated yield, c
catalyst 20 mol % used, d reactants decomposed.
Table 2. Summarized results of substrate scope under optimized reaction conditions.a
S. No.
1
2
3
4
5
6
7
8
Comp. R1
R2
R3
H
H
H
H
H
H
H
H
H
H
CH3
CH3
CH3
CH3
R4
H
H
H
H
H
H
H
H
H
OCH3
H
H
H
Time(min)
Yield b(%)
88
85
86
85
90
88
85
88
90
90
88
80
82
81
4a
4b
4c
4d
4e
4f
H
H
H
H
H
H
H
H
H
H
CH3
H
H
H
Phenyl
5
5
5
5
8
10
5
9
5
6
6
8
8
8
4-Bromophenyl
4-Chlorophenyl
2-Chlorophenyl
4-Methylphenyl
3-Methoxyphenyl
4-Nitrophenyl
4-Fluorophenyl
Ethyl
Ethyl
Ethyl
4-Bromophenyl
4-Chlorophenyl
2-Chlorophenyl
4g
4h
4i
9
10
11
12
13
14
4j
4k
4l
4m
4n
H
a
The reaction was carried out using 4-hydroxycoumarin 1 (1.0 mmol), aldehyde 2 (1.0 mmol) and indole 3 (1.0 mmol) in mortar and mixed thoroughly with pestle and ground at
50–60 °C. b isolated yield.
943, 758; 1H-NMR (DMSO-d6, 500 MHz) δ (ppm): 11.6 (s,
broad, D2O exchangeable, 1H), 10.9 (s, D2O exchangeable, 1H),
for this purpose. There were no significant effects on the
reaction yield by substituents on indole and 4-
hydroxycoumarin. However, substituents on aldehydes
exhibited marginal effects on both yield as well as in reaction
rate. The aldehyde bearing electron withdrawing substituent
afforded the product 4g with 85% yield, whereas aldehyde
having electron rich group like 4-methyl and 3-methoxy
afforded the product with relatively better yield (Table 2, entry
8.02 (d, J = 7.5 Hz, 1H), 7.60 (t, J = 7.0 Hz, 1H), 7.37-7.28 (m,
7H), 7.10–7.03 (m, 4H), 6.90 (t, J = 7.5 Hz, 1H); 6.08 (s, 1H);
13C-NMR [125 MHz (DMSO-d6)] δ (ppm): 162.2, 160.7, 152.6,
143.1, 136.4, 132.3, 128.6, 128.2, 127.7, 126.1, 124.7, 124.3,
123.9, 121.3, 118.9, 118.7, 116.7, 116.6, 114.6, 111.9, 108.9
37.5.
5 and 6). The assigned structures of the products 4a–4n were
well characterized by using analytical tools such as 1H, 13C-
NMR and IR spectroscopy (S1-S27).
3.2 Anion sensing: colorimetric and fluorimetric studies:
The UV–vis absorption and fluorescence emission spectra of all
of the compounds were investigated in acetonitrile and the
results are summarised in Table S2, ESI†. Out of the 14
3.1.1 3-((1H-indol-3-yl)(phenyl)methyl)-4-hydroxy-2H-
chromen-2-one (4a):
compounds 4a-4n
, for the complete study of optical
behaviour, 4a was selected. The chemosensor 4a (1.0 × 10−5
M) exhibited a strong absorption band centred at 485 nm in
acetonitrile which can be assigned to the π–π* transition (Fig.
Yield: 323 mg (88%) as white solid; MP = 191 °C; IR νmax (KBr,
cm-1): 3417, 3345, 2940, 2830, 1689, 1621, 1563, 1417, 1278,
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2a). Upon gradual addition of F− ion (TBAF, 0–20 equiv.) to the deprotonation possibility of –OH and –NH protons as per
above solution of 4a, the absorption band at 485 nm regularly proposed mechanism, absorption and emission titration of 4a
decreased and a new absorption band appeared at 415 nm was also performed with a relatively strong base [Bu4N]OH,
(blue shift of 70 nm) (Fig. 3a). A clear isosbestic point was which leads to deprotonation. Similar changes in absorption
observed at 455 nm, indicating the strong interaction between and emission spectra were obtained as those observed with F−
4a and F− ion. The absorption intensity at 415 nm reaches a ion (Fig. S29, ESI†). The similar anion sensing properties of 4a
plateau after the addition of exactly 20 equivalent of F− ion were also observed in the more polar solvent DMSO. It was
and there is no significant enhancement of the absorption reported as the protic solvents43 such as water would compete
intensity on further addition of F− ion. Moreover, a light with the anions for the bonding sites of chemosensor and
orange colour obtained after the addition of F− ion to the could therefore disturb their hydrogen bonding interactions.
solution of 4a which was initially colourless. This colour change To investigate the applicability of chemosensor 4a for F− ion
could be easily observed via “naked eye” (Fig. 1). On the detection in drinking water similar experiments were carried in
contrary, absorption spectrum of 4a was also studied in the DMSO-H2O media and was observed that the receptor was
presence of various anions (20 equiv.) such as I−, Br−, OAc−, Cl−, able to demonstrate noticeable changes in both absorption
−
−
−
−
−
ClO4 , BF4 , HSO4 , NO3 and PF6 (tetrabutylammonium salt), and fluorescence spectra when water content was not more
however, no considerable change in the colour of the solution than 10 (v/v) in DMSO (Fig. S30, ESI†). Thus the
as well in the UV–vis spectra was observed.
chemosensor could be applied for F− ion detection in
%
As shown in Figure 3b, the selectivity of 4a to F− ion was environmental water samples and biological samples.
also investigated by fluorescence emission spectral titration in
acetonitrile. The chemosensor 4a exhibited an intense
emission band at 507 nm (absolute quantum yield, ɸf = 0.47),
upon the addition of F− ion (0-20 equiv.) to the solution of 4a
(10 μM) fluorescence intensity at 507 nm decreases gradually
and shifted to 534 nm (red shift of 27 nm, absolute quantum
yield, ɸf = 0.049) (Fig. 3b). Finally the reaction gets saturated
with F− ion and demonstrated a remarkable quenching effect,
which is attributed to the formation of receptor-anion (host–
guest) hydrogen bonded complex. The result suggested that 4a
can act as a “switched on-off” sensor for F− ion particularly. As
Fig. 1. (a) Colourimetric and (b) fluorimetric naked-eye detection of 4a in the
mentioned earlier also, selectivity of chemosensor 4a towards
absence and presence of different anions (20 equiv.) in CH3CN.
−
−
−
−
different anions (I−, Br−, OAc−, Cl−, ClO4 , BF4 , HSO4 , NO3 and
−
PF6 ) was also investigated which displayed no change in the
optical properties. Moreover, no significant fluorescence
quenching was observed upon the addition of any other anion.
Hence, individual response of 4a against these anions revealed
a remarkable selectivity only for F− ion binding (Fig. 2b). The
selective response of 4a towards F− ion was evaluated by
adding 5 equiv. of various anions and result showed that there
was no interference from other anionic species (Fig. 4a).
The time resolved fluorescence study of chemosensor 4a
was also analysed in the absence and presence of F− ions,
obtained at excitation frequency of 455 nm. As shown in
Figure 5b, the fluorescence lifetime decreased from 4.65 ns to
2.47 ns after the addition of F− ions, indicates strong
interaction of receptor 4a with F− ions. The binding
stoichiometry was obtained from the Job’s plot measurement
on the basis of fluorescence. The maximum fluorescence
intensity appeared at 0.5 mole fraction which clearly
expressing 1:1 binding stoichiometry of 4a with F− ion (Fig.
4b).39 The association constant was calculated by using S.V.
plot which was found to be 6.2625 × 104 M-1 (Fig. 5a) with 0.50
μM detection limit (Fig. 5c).40–42
Fig. 2. (a) Absorption and (b) fluorescence emission spectra of 4a (1.0 × 10−5
M) in the absence and presence of 20 equiv. of different anions in CH3CN. λex
= 455 nm.
The obvious hypsochromic shift in the absorption band
from 485 to 415 nm and remarkable quenching in fluorescence
spectra suggests that the π-conjugation was inhibited by the
addition of F− ion to 4a which may be due to the
deprotonation of –OH and –NH protons. To test the
Fig.3. (a) Absorption and (b) fluorescence emission spectral responses of 4a
(1.0 × 10−5 M) towards varying F ion concentrations (0 to 20 × 10−5 M) in
−
CH3CN. λex = 455 nm.
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HOMO-4→LUMO whereas in the 4a+F− originated from
HOMO→LUMO and HOMO-1→LUMO. We also calculated the
UV-Vis absorption spectra of chemosensor 4a and 4a+F- in
gaseous state by TD-DFT. The absorption spectra indicates a
systematic blue shift from λmax = 478 nm to λmax = 321 nm, in
chemosensor 4a when binding with F− ion. The blue shift in
absorption maxima of chemosensor 4a after binding with F−
ion could be understand in terms of increases energy gap
between HOMO and LUMO of 4a+F− (Fig. 7). For chemosensor
4a, the HOMO was distributed on the indole scaffold while the
LUMO was distributed on the coumarin moiety with energy
deference ΔE = 3.432 eV. Whereas, for 4a+F−, the HOMO was
distributed on the whole indole and coumarin system while
LUMO was distributed on coumarin scaffold with energy
deference ΔE = 4.220eV. This clearly indicates the sensing of
the fluoride ion. Therefore, the interaction of F− ion with -OH
of chemosensor 4a was result of deprotonation due to charge
transfer, which also support the blue shift in the UV-Vis.
absorption spectra of 4a with F− ion and naked eye colour
change.
−
Fig.4. (a) Interference studies of F with 4a (1:5), black bars represent the
−
emission intensity of [4a-other anions] system and red bars show [4a-F -
other anions] system (b) Jobs plot shows 1:1 stoicheometry in CH3CN.
NH
NH
NH
−
- HF2
+ F-
O
O
O
O
O
O
O
O
O
H
H
-F
I
II
III
Scheme 2: Proposed sensing mechanism of 4a through deprotonation in the
presence of F- ion.
3.3 Nature of interaction between chemosensor 4a and F−
ions:
To gain an insight about the mechanism of interaction
between 4a and F− ion, 1H-NMR titration experiment was
1
performed in DMSO-d6. The H-NMR spectra of 4a show NMR
peaks as singlet at δ 11.65, 10.91 and 6.09 ppm which are
attributable to the –OH, –NH and –CH proton, respectively and
all aromatic protons appeared within the region δ 6.89–8.03
ppm. Upon the addition of 0.5 equiv. of [Bu4N]F to the solution
of 4a in DMSO-d6, the most noticeable change was observed in
–OH proton. The singlet at δ 11.65 ppm corresponding to –OH
proton completely disappeared due to deprotonation. This
was confirmed by the appearance of broad signal
−
corresponding to HF2 at around 16.0 ppm in presence of
excess F− ions.44 Alternatively a slight upfield shift of -NH
proton was noticed (from δ 10.91 to 10.84 ppm), reflecting the
increase in electron density (Fig. 6). Thus, the 1H-NMR
experiment clearly supported the hypothesis that the F− ions
preferably interacts with the receptor –OH rather than –NH.
We also carried out computational DFT and TD-DFT studies in
gas phase, to further appreciate the binding procedures of F−
Fig.5. (a) Stern-Volmer plot (b) Life time decay plot (c) Limit of detection for
−
binding of F ion with chemosensor 4a.
ion with chemosensor 4a 45
The DFT and TD-DFT calculations
.
were performed for the optimized structure of chemosensor
4a and 4a+F− to the B3LYP/6-31G(d) level using Gaussian 03
program. DFT calculation results showed that the –OH bond
length was changed by 0.975 Å to 1.446 Å after addition of F−
ions. So, one conclude from these data deprotonation of the -
OH protons of chemosensor 4a. TD-DFT calculations were
performed, to interpret further the absorption properties of 4a
and 4a+F− complexes. As shown in Table S1, Fig. S34 and Fig.
S35 ESI†, the lowest energy transitions of 4a arise from the
HOMO→LUMO, HOMO-2→LUMO, HOMO-3→LUMO and
Fig. 6. 1H-NMR titrations of 4a with F− ion in DMSO-d6.
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0.78 mg/ml) and, compounds 4d and 4m (MIC: 1.56 mg/ml)
show equal activity to the reference drugs. Compounds 4d 4g
4i and 4l show higher antibacterial activity (MIC: 3.12 mg/ml)
compared to Ampicillin and Cefadroxil against Shigella flexneri
Thus, the electron releasing group (CH3, OCH3 and 2-methyl
indole moiety) containing compounds such as 4e 4f and 4n
has shown excellent (MIC: 0.39 mg/ml) to good (MIC: 1.56
mg/ml) activity against B. subtilis. Against S. aureus also, the
electron donating group (2-methyl indole moiety and 4-
substituted phenyl) carrying compounds (4l and 4m) exhibit
equal (MIC: 1.56 mg/ml) antibacterial activity compared to
4. Application
,
,
4.1 Visual colour changes on test papers:
.
To investigate the potential application of 4a as an anion
sensor in solid state, we prepared a test kit using Whatman-40
filter paper coated with acetonitrile solution of 4a (10 µM)
followed by drying in air. The colour of the test strip changed
to bright orange only with F− ions supporting the practical
applicability of the receptor 4a (Fig. 8). This experiment shore
up that compound 4a has the potential to detect F− ions in
solid state.
,
Cefadroxil. Against E. coli, compounds 4d, 4g, 4i, 4j, 4k, 4l and
4m show excellent activity compared to Ampicillin and
Cifadroxil regardless of presence of electron releasing or
donating group. Against Shigellaflexneri, all compounds exhibit
lower activity compared to the standard drugs without
depending on the presence of any functional group.
4.2 Ocular Colorimetric fluoride detection from mouth rinser
and toothpaste:
Inspired by the results of test kit, we were moved to test its
capacity in detecting F− ions from commercially available
toothpaste (131 mg in 1.0 g) and mouth rinser (0.01 % w/v).
The chemosensor 4a (1.0 × 10−5 M) can effectively sense
fluoride from mouth rinser and toothpaste by appearance of
light orange colour to the colourless solution (Fig. 9). We have
directly used commercially available mouth rinser and in case
of toothpaste a 10 mL 10−6 (M) water solution was prepared to
conduct F− ions detection.
4.3 Biological Evaluation:
Synthesized compounds were evaluated for their in vitro
antibacterial activity against Gram-positive bacteria (B. subtilis
and S. aureus), Gram-negative bacteria (E. coli and S. Flexneri
)
and antifungal potential against Candida albicans fungal strain.
The results are summarized in Table 3.
4.3.1 Antibacterial activity:
Fig. 7. DFT optimised structure and HOMO-LUMO band gap of chemosensor
The compounds 4d, 4e, 4f, 4l, and 4n (MIC: 3.12, 0.39, 1.56,
−
4a and 4a+F .
3.12 and 1.56 mg/ml respectively) show excellent antibacterial
activity against B. subtilis compared to the standard drugs.
Compound 4e (MIC: 0.39 mg/ml) and, compounds 4d and 4l
(MIC: 3.12 mg/ml each) exhibit equal antibacterial activity than
positive controls against B. subtilis. In case of S. aureus, the
compounds show equal or lower activity than Ampicillin and
Cefadroxil drugs. Compound 4d (MIC: 0.78 mg/ml) and,
compounds 4l and 4m (MIC: 1.56 mg/ml) were found equally
potent as the standard drugs against S. aureus. However,
compounds 4i and 4j (MIC: 0.39 mg/ml) show better activity
than positive controls against E. coli while compound 4l (MIC:
Fig. 8. Colour change of test paper containing 4a (10 µM) with different
anion.
Table 3. Minimum inhibitory concentration (MIC, mg/ml-1) of synthesized compounds 4a-n against bacterial and fungal strains.
Gram-positive
Gram-negative
E.coli
Fungus
C.albicans
25
B.subtilis
>100
25
S.aureus
>100
25
S.flexneri
50
50
4a
4b
4c
4d
>100
12.5
3.12
50
3.12
25
0.78
>100
1.56
12.5
3.12
50
6.25
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ARTICLE
4e
4f
4g
4h
4i
4j
4k
4l
0.39
1.56
>100
6.25
50
6.25
12.5
3.12
6.25
1.56
0.39
3.12
-
12.5
25
12.5
25
3.12
6.25
3.12
1.56
1.56
>100
0.78
1.56
-
50
25
12.5
12.5
3.12
6.25
3.12
6.25
>100
3.12
12.5
50
12.5
12.5
25
25
25
0.39
3.12
12.5
25
3.12
-
0.78
12.5
0.39
0.39
1.56
0.78
1.56
>100
1.56
0.78
-
4m
4n
Ampicillin
Cefadroxil
Fluconazole
0.78
1.56
-
-
3.12
4.3.2 Antifungal activity:
while compounds 4j and 4k exhibited highest activity towards
Gram-negative bacteria, even as compound 4k exhibited the
Compounds were subjected to MIC (minimum inhibitory highest activity against pathogenic fungi C. albicans in the MIC
concentration) determination by microdilution method46 and method.
the results are summarized in Table 3. The standard reference
drugs Fluconazole was selected as positive control in
Acknowledgements
antifungal activity assay. Compound 4j exhibits excellent
antifungal activity (MIC: 0.39 mg/ml) against C. albicans
.
We thank Ministry of Human Resource and Development
(MHRD), New Delhi, India [grant number MHR01-23-200-428],
for financial supports.
Moreover, compounds 4b, 4k and 4n show similar antifungal
activity (MIC: 3.12 mg/ml) as Fluconazole against C. albicans
while others show lower activity. Thus, compound 4j (carrying
electron releasing group -OCH3 on the indole moiety) shows
excellent antifungal activity against C. albicans compared to
standard drug.
Notes and references
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Graphical Abstract
‘NakedꢀEye’ Colorimetric/Fluorimetric Detection of F− Ion by Biologically
Active 3ꢀ((1HꢀIndolꢀ3ꢀyl)methyl)ꢀ4ꢀhydroxyꢀ2Hꢀchromenꢀ2ꢀone Derivatives
Shailya,b*,Ajay Kumar b, Sumit Kumara and Naseem Ahmeda*
aDepartment of Chemistry, Indian Institute of Technology, Roorkee-247667, Uttarakhand,
India
bDepartment of Chemistry, D. B. S. (P.G.) College, Dehradun-248001, Uttarakhand, India
E-mail: shailyrke@gmail.com, nasemfcy@iitr.ac.in