Development of a rhodamine-benzimidazol hybrid derivative as a novel FRET based chemosensor selective for trace level water

Article abstract of DOI:10.1039/c4ra02585g

A newly designed rhodamine-benzimidazol hybrid molecule has been developed as a FRET-based chemosensor for the selective detection of trace level water in both polar protic and aprotic organic solvents. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02585g

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Development of a rhodamine–benzimidazol hybrid
derivative as a novel FRET based chemosensor
selective for trace level water†
Cite this: RSC Adv., 2014, 4, 21608
Received 24th March 2014
Accepted 1st May 2014
Siddhartha Pal, Manjira Mukherjee, Buddhadeb Sen, Somenath Lohar
*
and Pabitra Chattopadhyay
DOI: 10.1039/c4ra02585g
www.rsc.org/advances
A newly designed rhodamine–benzimidazol hybrid molecule has been
To the best of our knowledge, an optical water sensor based
developed as a FRET-based chemosensor for the selective detection on a FRET mechanism (not hydrogen bonding) is a new
of trace level water in both polar protic and aprotic organic solvents. approach that has not yet been reported. The utilization of this
novel strategy for water detection has several advantages. The
selectivity of this rhodamine-based probe (L) towards water is
extremely high (Fig. S1†) and independent of solvent type (non-
polar, polar, aprotic and protic). This probe also responds
During the past few decades, considerable attention has been
paid to the detection and quantitative analysis of trace water in
through FRET, which provides a good detection limit.11 Fluo-
rescence techniques have become powerful tools for sensing
different analytes in trace amounts because of their simplicity,
high sensitivity and real-time monitoring with a short response
time.12 Among them, FRET, a distance-dependent radiationless
energy transfer from an excited donor uorophore to a suitable
acceptor uorophore, is now an important physical phenom-
enon with considerable interest for studying molecular level
interactions in biological systems. FRET also has potential
applications in optoelectronic and thin lm device development
due to its sensitivity to distance and intramolecular phenomena,
which is resistant to outside perturbation. The distance over
which energy can be transferred is dependent on the spectral
characteristics of the uorophores, but is generally in the range
organic solvents because of their indispensable applications in
food, textile, ceramics, electronic, pharmaceutical, petroleum,
and environmental monitoring industries.1 Previously, water
content in samples was measured up to the z1 ppm level using
the Karl Fischer coulometric titration method.2 Although this
method has been improved through modication, it still has
several disadvantages including a long measuring time, the use
of toxic and expensive chemical reagents, difficulty in endpoint
determination and interference by other common species.
Nowadays, optical water sensors based on uorescence intensity
and naked eye detection are particularly attractive on account of
their high sensitivity and selectivity, quick, inexpensive, and easy
fabrication and non-destructive properties as well as their
capability of remote and in situ monitoring of optical measure-
ments.3–10 In previous reports, several approaches have been
used to facilitate sensitive water detection in many polar aprotic
and polar protic solvents, including intramolecular charge
transfer,4 proton transfer,5 photo-induced electron transfer,6
water-induced decomplexation of dyes with anions,7 water-
induced interpolymer p-stacking aggregation,8 solvatochrom-
ism,9 etc.10 In most cases, however, the strong polarity and
hydrogen bonding of the sensing medium are key in the deter-
mination of water by these sensors as well as in lowering their
LOD. As a result, there is still an increasing demand for highly
sensitive optical sensors for the detection of water in polar protic
solvents including a new detection principle for the develop-
ment of new optical water sensors.
˚
of 10–100 A. However, the efficiency of energy transfer also
depends on the relative orientation of transition dipoles of both
the donor and acceptor, and the extent of spectral overlap
between the donor emission and acceptor absorption.11
To synthesize the probe L, rhodamine B acid was rst con-
verted to its activated acid chloride form by the conventional
method.13 The mixture of the produced rhodamine B acid
chloride and added 2-amino phenyl benzimidazole (2-APBZ) was
then reuxed for 32 h (Scheme 1 and ESI†). The crystallized
product (L) obtained from dry acetonitrile was characterised
using physico-chemical and spectroscopic tools (Fig. S2–S4†).
The 1,5 sigmatropic shi14 is responsible for obtaining the nal
rearranged cyclic organic moeity (L), which is well supported by
1
IR, HNMR and 13CNMR data.
Department of Chemistry, Burdwan University, Golapbag, Burdwan, 713104, India.
E-mail: pabitracc@yahoo.com; Fax: +91-342-2530452; Tel: +91-342-2558554 ext. 424
† Electronic supplementary information (ESI) available: Experimental section,
Fig. S1–S8 Table S1, other electronic format. See DOI: 10.1039/c4ra02585g
The addition of water to a solution of L in dry organic
solvents facilitates the formation of the uorescent zwitterionic
structure L1 through the protonation of the imidazole nitrogen
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deviation (szero) of seven replicate (of each dry solvent)
measurements at the zero level, the DL and QL were esti-
mated using as 3s/S and 10s/S, respectively. These values
are quite signicant compared to previous reports (viz.
Table 1).
Fluorescence and absorption spectra of L were measured in
dry organic solvents (acetonitrile, methanol, DMSO and THF)
containing various concentrations of water. A new peak gener-
ated at approximately 575 nm increased in intensity (ꢁ10 times)
with increasing water content in acetonitrile, methanol or
DMSO. An isoemissive point at around 547 nm (viz. Fig. 1a, c
and e) along with the steady decrease in uorescence intensity
due to the benzimidazole unit were also observed. In the low
water content region below 11 (v/v)%, as shown in Fig. S7,† the
uorescence intensities increased almost linearly with
increasing water content in the organic solvent. In contrast, the
corresponding absorption spectra exhibited signicant changes
Scheme 1 Synthesis of fluorescence FRET sensor L.
of the 2-amino phenyl benzimidazole (2-APBZ) unit and depro-
tonation of the alcoholic hydrogen (Fig. S5 and S6†) Thus, a
spirolactone ring opening15 occurred via extended conjugation in absorbance; the spectra showed a ꢁ20 nm blue shi (Fig. 1b,
from the NEt2 lone pair to the 2-APBZ unit, causing FRET to d and f), which is signicant evidence supporting the occur-
operate in the molecule (Scheme 2, ESI†) and creating red rence of FRET.16 In the absence of water, the rhodamine moiety
uorescence. A peak at ca. 3449 cmꢀ1 attributable to nOH was adopted a closed, non-uorescent spirolactam form corre-
observed in the IR spectrum of L in dry acetonitrile, but was
absent in the spectrum of L in 20% (v/v) water in acetonitrile.
Alternatively, the peak at ca. 3158 cmꢀ1 attributable to nNH was
obtained in later case but not in the former case (Fig. S2†).
sponding to a weak spectral overlap between the 2-amino phenyl
benzimidazole (2-APBZ) emission and the rhodamine absorp-
tion. Eventually, FRET was suppressed, and only the blue
emission of the donor was observed upon excitation of the 2-
1HNMR titration was carried using D2O (Fig. S5 and S6†). Here, APBZ chromophore at 350 nm. The water induced ring opening
the signal at ca. d ¼ 12.64 ppm was observed in the spectrum of
L in dry DMSO-d6 but was absent when the spectrum was
acquired by adding a few drops of D2O into DMSO-d6. Other-
wise, some corresponding peaks shied slightly towards
downeld and some remained unaffected. The peak at
approximately 73 ppm due to the sp3 hybridized carbon was
observed in the 13CNMR spectrum of L in dry DMSO-d6, which
also supported the existence of –OH attached to sp3 carbon
instead of carbonyl carbon (Fig. S4b†).
To calculate the detection limit (DL) and quantitation
limit (QL),6 the calibration curves11 (Fig. 2) in the lower region
were obtained. From the curve slope (S) and the standard
Fig. 1 (a) Fluorescence (lex ¼ 350 nm) and (b) absorption spectra of L
(c ¼ 2.0 ꢂ 10ꢀ5 M) in acetonitrile containing water (1.43–20% v/v). (c)
Fluorescence (lex ¼ 350 nm) and (d) absorption spectra of L (c ¼ 2.0 ꢂ
10ꢀ5 M) in methanol containing water (1.43–25.7% v/v). (e) Fluores-
Scheme 2 Probable mechanistic pathways of sensor L for detection cence (lex ¼ 350 nm) and (f) absorption spectra of L (c ¼ 2.0 ꢂ 10ꢀ5 M)
of water in organic solvents.
in DMSO containing water (1.43–28.6% v/v).
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emission position (420 nm) with increasing water content in
dry DMSO from 4.05 ns to 3.51 ns, which is in good agree-
ment with the FRET process. A signicant spectral overlap
was also observed between 2-APBZ and the rhodamine-B
¨
moeity (Fig. 4). Again, the Forster distance between donor
˚
and acceptor was also calculated (ca. 52 A; ESI†), and
supports the occurrence of FRET.
In summary, a new rhodamine–benzimidazol hybrid
molecule has been established as a dual channel chemo-
sensor for the detection of trace level water in both protic and
aprotic organic solvents through a FRET pathway. This probe
is highly selective and sensitive towards water because
water has the maximum dielectric constant among all polar
protic solvents. For this reason, only water is able to create a
dipole in the molecule through zwitterion formation (viz.
Scheme 2). This dipolar charge is responsible for ring
opening (through extended conjugation), favouring the FRET
process.
Fig. 2 Calibration curves in the lower concentration range, with error
bars for calculating the DL and QL of L as a function of water content in
(a) acetonitrile, (b) methanol, (c) DMSO and (d) THF.
Table 1 Comparison of the detection limit of this FRET-based probe
(L) with previously reported water sensors
Detection limit (DL)
Quantitation limit (QL)
Ref.
0.038 v/v% (MeCN)
0.02 v/v% (THF)
0.006 v/v% (MeCN)
0.002 v/v% (MeCN)
0.0035 v/v% (THF)
0.1 wt% (MeCN)
—
—
—
—
4a
4b
4c
4d
5a
6a
8
—
0.4 wt% (MeCN)
0.3 v/v% (EtOH)
0.07 v/v% (MeCN)
0.03 v/v% (DMF)
0.0085 v/v% (MeOH)
0.0108 v/v% (MeCN)
0.0147 v/v% (DMSO)
0.0105 v/v% (THF)
0.1 v/v% (EtOH)
0.02 v/v% (MeCN)
0.008 v/v% (DMF)
0.0026 v/v% (MeOH)
0.0032 v/v% (MeCN)
0.0044 v/v% (DMSO)
0.0033 v/v% (THF)
This work
Fig. 3 Fluorescence lifetime decay profiles of L at 420 nm (donor
emission position) with increasing water content.
of the uorescent rhodamine moiety, generated the intense
absorption in the 2-APBZ emission region. As a result, the
spectral overlap was improved, and excitation of the 2-APBZ
chromophore resulted in the intense red emission of rhoda-
mine owing to FRET.
In the case of L, the uorescence quantum yield was calcu-
lated in HPLC grade organic solvents in presence of water. From
these measurements, it is clear that the uorescence quantum
yield increases from 0.77 to 10.21 (14 times, for acetonitrile, at
l ¼ 575 nm), from 0.77 to 9.32 (12 times, for methanol, at
l ¼ 575 nm) and from 0.77 to 8.28 (10 times, for DMSO, at l ¼
575 nm) upon the addition of 30% (v/v) water. For this reason,
the probe (L) exhibits uorescence and visual colour changes
with increasing water content (Fig. S8†).
In support of this FRET process, a uorescence life time
experiment was performed. In this study, the probe (L) was
placed in dry DMSO (10 mM) and excited at the donor exci-
tation position at 350 nm (from UV-Vis peak of 2-APBZ at 345
Fig. 4 Overlap spectra of donor emission and acceptor absorbance of
nm)17. Fig. 3 showed a decrease trend in lifetime at the donor L (10 mM) in dry acetonitrile.
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Y. H. Kim, M. G. Choi, H. G. Im, S. Ahn, W. Shim and
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Financial assistance from CSIR, New Delhi, India is gratefully
acknowledged. We sincerely acknowledge Prof. Samita Basu
and Mr Ajay Das, Chemical Science Division, SINP, Kolkata for
enabling TCSPC instrument. Thanks to the honorable reviewers
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