Triarylboron Anchored Luminescent Probes: Selective Detection and Imaging of Thiophenols in the Intracellular Environment

Article abstract of DOI:10.1021/acs.langmuir.8b01036

The advances in boron incorporated organics have captured overwhelming interest on account of their outstanding properties and promising applications in various fields. Mostly, triarylborane compounds (TAB) are exploited as sensors of F^- and CN

Full text of DOI:10.1021/acs.langmuir.8b01036

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge
Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage
Triarylboron anchored luminescent probes: Selective detection
and imaging of thiophenols in the intracellular environment
SUDHAKAR PAGIDI, Neena K Kalluvettukuzhy, and Pakkirisamy Thilagar
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01036 • Publication Date (Web): 20 Jun 2018
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Triarylboron anchored luminescent probes: Selective detection and
imaging of thiophenols in the intracellular environment
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Sudhakar Pagidi, Neena K Kalluvettukuzhy and Pakkirisamy Thilagar*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangaloreꢀ560012, India. Email:
thilagar@iisc.ac.in
KEY WORDS. boron, sensor, thiophenol, fluorescence, cell imaging
ABSTRACT: The advances in boron incorporated organics has captured overwhelming interest on account of their outstanding
properties and promising applications in various fields. Mostly, triarylborane compounds (TAB) are exploited as sensors of Fꢀ and
CNꢀ anions at the expense of intrinsic Lewis acidic nature of boron. New molecular probes 1 and 2 for detection of toxic thiophenol
were designed by conjugating highly fluorescent borylanilines with the luminescent quencher 2,4ꢀdinitrobenzene based sulfonaꢀ
mides (DNBS), wherein the electrophilicity of DNBS moiety have been modulated by fineꢀtuning the intrinsic Lewis acidity of
boron. The interplay between PET (photoinduced electron transfer) and ICT have been employed for developing the TAB tethered
turnꢀon fluorescent sensor for thiophenol with high selectivity for the first time. The newly developed probes showed very fast reꢀ
sponse towards thiophenol (within ~ 5 min) with limits of detection (LOD) lies in the micromolar ranges, clearly points to their
potentiality. Further, compounds 1 and 2 were explored for detecting thiophenol in the intracellular environment by discriminating
biothiols. DFT and TDꢀDFT calculations were performed to support the sensing mechanism.
1. Introduction
Organic thiols play key roles in chemical, biological and
environmental sciences. Aliphatic thiols such as cysteine, hoꢀ
mocysteine and glutathione are associated with a variety of
physiological processes for example, redox homeostasis. Variꢀ
ations in the concentration of these thiols in biological fluids
have been associated with a number of diseases such as liver
damage, Alzheimer’s disease and cardiovascular diseases.1ꢀ11
Chart 1. Chemical structures of the TAB based thiol
probes (
1 and 2) and the luminophores (1a and 2a)
On the other hand aromatic thiols (thiophenols) play an imꢀ
portant role in organic synthesis and are widely used as preꢀ
cursors for the preparation of agrochemicals, pharmaceuticals,
and various industrial products. Despite their synthetic utility,
thiophenols are highly toxic to human beings and environꢀ
ment. Prolonged exposure to thiophenols induces a burning
sensation, coughing, wheezing, laryngitis, shortness of breath,
headache, nausea and vomiting and at the extreme, it could be
fatal.12ꢀ20 Studies have shown that thiophenols possess a mediꢀ
an lethal dose (LC50) of 0.01 to 0.4 mM for fish.21ꢀ24 Thus,
selective detection and discrimination of thiophenols from
biologically important aliphatic thiols is a pivotal area of reꢀ
search.25ꢀ42 Numerous thiol sensors have been developed by
exploiting their strong nucleophilicity and affinity towards
transition metal ions.28ꢀ36 Most of the probes are reaction based
thiol dosimeters and are constructed by conjugating an elecꢀ
generated upon addition of thiophenol.
In recent times, boron compounds have attracted a lot of atꢀ
tention owing to their potential applications in light emitting
diodes and displays, solar cells and energy harvesting systems,
nonꢀlinear optical materials and cell imaging.43ꢀ56 Among variꢀ
ous boron compounds, triarylborane (TAB) based compounds
holds a special position, in view of their numerous applicaꢀ
tions in materials and biology.57ꢀ62 Due to their bright and tunꢀ
able luminescence features, TAB compounds are widely exꢀ
ploited as sensors and external stimuli responsive materials.63ꢀ
68 However, boron based sensors have not been explored exꢀ
clusively for the detection of thiols.69,70
We are interested in the design and development of strongly
luminescent materials and efficient molecular probes for
detecting various ions and neutral molecules.71ꢀ76 Recently, we
demonstrated that borylanilines, the simplest TAB based DꢀA
systems, are highly luminescent and a small change in the
dihedral angle between donor amine unit (ꢀNH2) and acceptor
boryl moiety (ꢀBMes2) has a dramatic effect on their optical
properties.75,76 It was envisioned that the availability of
reactive primary amine (ꢀNH2) group on borylanilines can be
conveniently exploited for the attachment of an electron
deficient fluorescence quencher, DNBS via a sulfonamide
bond. The attachment DNBS to boryl aniline would quench
tron
deficient
fluorescence
quencher
2,4ꢀ
dinitrobenzenesulfonylchloride (DNBS) with a luminophore
via sulfonamide bond. The high electron deficiency of DNBS
moiety acts as an electron sink and induces photoinduced elecꢀ
tron transfer (PET), resulting in the quenching of fluorescence
from luminophore.33ꢀ42
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the fluorescence of borylanilines via PET process. The
addition of thiols could regenerate the borylanilines by
cleaving the sulfonamide bond; consequently, a turnꢀon
fluorescence response can be realized. Depending upon the
electrophilic and nucleophilic nature of respective thiol probe
and thiols (thiophenols or biothiols), selective turnꢀon
fluorescence detection would be possible. Accordingly, TAB
based thiol probes 1 and 2 have been designed and synthesized
and their selective turnꢀon luminescence discrimination of
thiophenol over biothiols were investigated. Further, the newly
developed probes showed a selective turnꢀon fluorescence
response toward thiophenol in presence of biothiols in the
intraꢀcellular environments.
raphy over silica gel with EtOAc/hexane (1:2) as eluent to
obtain 1 as yellow solid. Yield: 70%. H NMR (400 MHz,
1
1
2
3
4
5
6
7
8
CDCl3, 25 °C): δ (ppm) 8.66 (d, J = 2.24 Hz, 1H), 8.39 (dd, J1
= 8.50 Hz, J2 = 2.28 Hz, 1H), 8.06 (d, J = 8.64 Hz, 1H), 7.15
(s, 1H), 6.75 (s, 1H), 6.73 (s, 2H), 6.71 (s, 2H), 2.25 (s, 6H),
1.94 (s, 6H), 1.93 (s, 6H), 1.81 (s, 6H); 13C NMR (100 MHz,
CDCl3, 25 °C): δ (ppm) 150.4, 148.9, 146.9, 143.8, 142.9,
141.2, 140.5, 140.2, 137.8, 134.5, 133.9, 129.3, 129.2, 126.7,
121.9, 121.0, 53.9, 23.3, 23.1, 21.6. Anal. Cald. for
C32H34BN3O6S: C, 64.11; H, 5.72; N, 7.01. Found C, 64.08; H,
5.69; N, 7.05. ESIꢀMS (positive ion mode): Mcalc. = 599.2261
Da; found: 622.2595 Da [M+Na]+.
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Compound 2. A similar procedure described for 1 was used
for the synthesis of 2. Quantities used for the reaction were; 2a
(200 mg, 0.56 mmol), 2,6ꢀlutidine (85µL, 0.73 mmol), 2,4ꢀ
dinitrobenzenesulfonyl chloride (194 mg, 0.73 mmol). Comꢀ
2. Experimental Section
2.1 Materials and methods
1
pound 3 was obtained as a yellow color solid. Yield: 85%. H
Chemicals required were purchased from dealers (Sigmaꢀ
Aldrich, USA; Merck, Germany; SDFCL, India), and used as
such without any further purification, unless otherwise menꢀ
tioned. Standard schlenkꢀline techniques were used for perꢀ
forming all the reactions under inert atmosphere of argon.
THF was refluxed over sodiumꢀbenzophenone until to get
deep purple in color and then distilled under argon atmosꢀ
phere. DCM is distilled from calcium hydride and stored over
activated 4Å molecular sieves prior to use. Bruker Avance 400
NMR (400 MHz, CDCl3, 25 °C): δ (ppm) 8.61 (d, J = 2.0 Hz,
1H), 8.33 (dd, J1 = 8.88 Hz, J2 = 2.4 Hz, 1H), 7.93 (d, J = 8.8
Hz, 1H), 7.38ꢀ7.31 (m, 3H), 7.23 (s, 1H), 7.05 (s, 1H), 6.76 (s,
4H), 2.30 (s, 6H), 1.85 (s, 12H); 13C NMR (100 MHz, CDCl3,
25 °C): δ (ppm) 150.3, 148.7, 141.4, 141.0, 139.8, 138.4,
135.1, 134.8, 133.7, 130.5, 130.0, 128.7, 127.9, 127.1, 121.0,
23.7, 21.6. Anal. Cald. for C30H30BN3O6S: C, 63.05; H, 5.29;
N, 7.35. Found C, 63.06; H, 5.25; N, 7.32. ESIꢀMS (positive
ion mode): Mcalc. = 571.1948 Da; found: 594.3358 Da
[M+Na]+.
1
MHz NMR spectrometer was used for recording H and 13C
NMR spectra with operating frequency of 400 and 100 MHz
respectively, and the corresponding spectra were referenced
internally to the deuterated solvent signals (TMS signal at 0.00
ppm for 1H and CDCl3 signal at ~ 77.6 ppm for 13C). Chemical
shift multiplicities are denoted as singlet (s), doublet (d) and
doublet of doublets (dd). Micromass QꢀTOF High Resolution
Mass Spectrometer by electrospray ionization (ESI) method
using argon (6kV, 10 mA) as the FAB gas was used for reꢀ
2.3 Reactivity of probes with thiols
Reactivity of compounds 1 and 2 towards different thiols
were carried out by titrating 10 µM solution of compounds in
4:1 methanolꢀwater mixture with thiols (12 ꢁM, 4:1 methanolꢀ
water mixture). Thiophenol sensing studies were carried out
by titrating 10 µM solution of compounds in 4:1 methanolꢀ
water mixture with thiophenol (12 ꢁM, 4:1 methanolꢀwater
mixture). Then emission spectral measurements were carried
out with different time intervals. Further, emission spectral
measurements were carried out by gradually increasing the
concentration of thiophenol and spectra were collected ~ 5 min
after the addition of thiophenol. The detection limit was calcuꢀ
lated from the fluorescence titration curve in presence of thioꢀ
phenol. The detection limit was calculated using the equation,
limits of detection (LOD) = 3ꢀ/k; where ꢀ is the standard deꢀ
viation of the blank measurements and k is the slope of the
curve obtained by plotting relative fluorescence intensity and
analyte concentration.
cording High resolution mass spectra (HRMS). UVꢀvis abꢀ
sorption spectra were recorded on a Perkin Elmer LAMBDA
750 UV/visible spectrophotometer. Fluorescence measureꢀ
ments were carried out on Horiba JOBIN YVON Fluoromaxꢀ4
spectrometer. Spectrophotometric grade solvents, microbalꢀ
ance (± 0.1 mg precision) and standard volumetric glass were
used for preparing the solutions of compounds for spectral
measurements. For solution state spectral measurements,
quartz cuvettes with sealing screw caps were used. DFT calcuꢀ
lations for all the molecules were carried out using the Gaussiꢀ
an G09 program employing hybrid B3LYP (hybrid Becke 3ꢀ
LeeꢀYangꢀParr) exchangeꢀcorrelation functional with 6ꢀ31G
(d) basis set. Ground state geometry optimizations followed by
subsequent frequency test is carried out to make sure that molꢀ
ecule has minimum energy on the potential energy surface.
TDꢀDFT calculations were performed on the ground state opꢀ
timized structures.77ꢀ80
2.4 Cell culture and fluorescence imaging
HeLa cells were grown in Dulbecco’s modified Eagle Meꢀ
dium (DMEM) supplemented with 10% fetal calf serum, 1%
penicillin, and 10000 units/mL of streptomycin at 37 °C in a
humidified air containing 5% CO2. The cells were incubated
with 1 and 2 (10 ꢁM in 1:200 DMSOꢀDMEM v/v pH = 7.4)
for 30 min at 37 °C, and used directly for fluorescence imagꢀ
ing as a control experiment. For another control experiment,
cells were treated with thiolphenol (1.25 ꢁM, 1:1000 DMSOꢀ
DMEM v/v pH = 7.4) for 30 min at 37 °C and used directly
for fluorescence imaging. The cells treated with thiophenol
was washed with PBS three times to remove the remaining
thiophenol and then coꢀincubated with compounds 1 and 2 (10
ꢁM in 1:200 DMSOꢀDMEM v/v pH = 7.4)) for 30 min at 37
°C. Finally cells were washed with PBS to remove the remainꢀ
2.2 Synthetic procedures
Borylanilines 1a and 2a are synthesized using our recently
reported methodology.75,76
Compound 1: 2,4ꢀDinitrobenzenesulfonyl chloride (174
mg, 0.65 mmol) was added to dichloromethane (15 mL) soluꢀ
tion of compound 1a (200 mg, 0.54 mmol) and 2,6ꢀlutidine
(76 µL, 0.65 mmol) at 0 °C under stirring. The reaction mixꢀ
ture was slowly warmed to room temperature and stirring was
continued for another 24 hrs. The solvent was removed under
vacuum and the residue was subjected to column chromatogꢀ
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ing compounds and images were taken under an inverted fluoꢀ
matched with the energy of the fluorescence peaks observed
for respective borylanilines, which directly corroborate the
above inference (Figure S11).
1
2
3
rescence microscope (Olympus, 1X71).
3. Results and discussion
4
3.1 Synthesis and Characterization
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8
Compounds 1 and 2 were synthesized by lutidine mediated
condensation of 2,4ꢀdinitrobenzenesulfonylchloride and the
corresponding borylaniline (1a and 2a for 1 and 2, respectiveꢀ
ly, Scheme 1). Compounds 1a and 2a were prepared using our
recently reported methodology.75,76 Compounds 1 and 2 were
characterized by multiꢀnuclear NMR spectroscopy and high
resolution mass (HRMS) spectrometry. The 1H resonances
corresponding to –NH2 group (~ 3.61 and ~ 3.59 ppm for 1a
and 2a, respectively)75.76 disappeared and a new peak correꢀ
sponding to sulfonamide appeared around ~ 8.66 ppm and ~
8.61 ppm, respectively for 1 and 2, confirming the formation
of the target molecules. Compounds were stable under ambiꢀ
ent conditions.
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R4
R4
R3
R3
SO2Cl
NO2
R2
R1
R2
R1
2,6-lutidine
CH2Cl2, r.t.,
NH
S
O
B
B
+
O
NO2
DNBSA =
NO2
NO2
1a. R1 = R2 = CH3, R3 = H, R4 = -NH2
2a. R1 = R2 = R4 = H, R3 = -NH2
1. R1 = R2 = CH3, R3 = H, R4 = -DNBSA
2. R1 = R2 = R4 = H, R3 = -DNBSA
Scheme 1. Synthesis of TAB based molecules 1 and 2.
3.2 Optical Properties – Studies on the fluorometric
response of probes toward thiophenol
Figure 1. Emission spectral changes of probe 1 (top) and 2
(bottom) upon addition of different thiols (concentration of thiol
probes = 10 μM, concentration of different thiol analytes = 12 μM
and λex = 350 nm) in methanolꢀwater (v/v = 4:1) mixture.
Compounds 1 and 2 showed a broad absorption in the reꢀ
gion 275ꢀ400 nm in MeOH solution and were weakly fluoresꢀ
cent (Figure S7). The efficient PET between DNBS and TAB
moieties could be the reason for the nonꢀemissive nature of
these compounds. The observations are in line with the optical
properties of donorꢀacceptor systems having DNBS moiety.33ꢀ
42 The above inference was further supported by the TDꢀDFT
calculations (vide-infra). To check the applicability of 1 and 2
as luminescent sensors for compounds containing thiol (ꢀSH)
functional groups, they were allowed to react with various
thiols (cysteine, homocysteine, glutathione and thiophenol)
and the reaction was monitored by fluorescence spectroscopy.
Compounds 1 and 2 showed a turnꢀon fluorescence response
with a green (λem = ~ 520 nm) and cyan blue (λem = ~ 490 nm)
color intense luminescence towards thiophenol and did not
show any changes with other biothiols (Figure 1). Further, the
selectivity of probes towards thiophenols was checked in the
presence of 10 times excess of biothiols (50 ꢁM) compared to
thiophenol (5 ꢁM). The compounds 1 and 2 did not show any
response in presence of excess of biothiols and showed a fluoꢀ
rescence response only when thiophenol were added (Figure
S10). This clearly inferred us that the developed TAB based
probes are selective turnꢀon fluorescent sensors for thiophenol
over biothiols. The insensitivity of probes towards aliphatic
thiols could be due to their higher pKa values (ca. 8.5) comꢀ
pared to that of thiophenols (ca. 6.5). To prove that, the lumiꢀ
nescence response of 1 and 2 towards thiophenol are due to
the regeneration of respective borylanilines 1a and 2a, by
cleaving the sulfonamide bond, the luminescence spectra of 1
and 2 after addition of thiophenol were compared with that of
freshly prepared 1a and 2a. The energy of the fluorescence
peaks of 1 and 2 after addition of thiophenol were exactly
To get further insight into the reaction between the comꢀ
pounds and thiophenol, the fluorescence of the reaction mixꢀ
ture was monitored at different time intervals (Figure 2).
Compounds 1 and 2 display an instantaneous fluorescence
response (within seconds) toward thiophenol and reach a platꢀ
eau within ~ 4 and ~ 5 min, respectively (Figure S8). This
could be due to the enhanced electrophilic nature of the sulꢀ
fonamide bond owing the presence of highly Lewis acidic
TAB substituent. Further, the fluorescence intensity of the
probes increased progressively with concomitant increase in
the concentrations of thiophenol (Figure 3). Upon addition of
2 ꢁM (0.2 equivalents) of thiophenol, the fluorescence intensiꢀ
ty of 1 and 2 showed ~ 6 and ~ 262 fold increase in their fluoꢀ
rescence intensity, respectively, compared to the solutions of
corresponding compounds without adding thiophenol. The
changes in the fluorescence spectra of 1 and 2 cease after addiꢀ
tion of 13 ꢁM (1.3 equivalents) and 15 ꢁM (1.5 equivalents) of
thiophenol, respectively (Figure S9). The limits of detection
(LODs) of thiophenol for 1 and 2 were calculated using the
fluorescence titration data and the values were found to be ~
0.08 ꢁM and ~ 0.15 ꢁM, respectively (Figure S12). Very small
reaction time and lower detection limits in the micromolar
ranges clearly indicate the potential of newly developed TAB
based compounds as luminescent probes for sensing thiopheꢀ
nol in practical applications such as intracellular environments
and contaminated (or polluted) samples like water or soils.
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Figure 2. Time dependent emission spectra of 1 (left) and 2
(right) upon addition of thiophenol (concentration of thiol probes
= 10 ꢁM, concentration of thiophenol = 12 ꢁM and λex = 350 nm)
in methanolꢀwater (v/v = 4:1) mixture.
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Figure 4. Emission spectra of compounds 1 (top) and 2 (bottom)
with different analytes (concentration of thiol probes = 10 ꢁM,
concentration of different analytes = 20 ꢁM and λex = 350 nm) in
methanolꢀwater (v/v = 4:1) mixture.
3.3 Detection of thiophenol in the intracellular
environment
To determine the cell permeability as well as the use of 1 and
2 for the detection of thiophenol in intracellular environments,
preliminary in vitro studies were carried out using HeLa cells.
MTT assay experiment was performed on HeLa cells to check
the cytotoxicity of probes 1 and 2 (Figure S13). The results
indicated that 1 and 2 are biocompatible and showed low cytoꢀ
toxicity for living cells. Further, we also checked the viability
of cells in the presence of thiophenol of concentration 1.25
µM and found that cells are healthy under the experimental
conditions (87 % of viability after 3 hrs of incubation). When
cells were treated only with 1 and 2 (10 µM in 1: 200 DMSOꢀ
DMEM v/v pH = 7.4) at 37 °C for 30 min, no significant
change in fluorescence intensity was observed (Figure 5A and
B). The insensitivity of 1 and 2 towards intracellular aliphatic
thiols could be the reason for the absence of fluorescence.
Upon coꢀincubation of 1 and 2 for 30 min at 37 °C (10 µM, 1:
200 DMSOꢀDMEM v/v pH = 7.4) with the cells preꢀtreated
with thiophenol (1.25 µM, 1: 1000 DMSOꢀDMEM v/v pH =
7.4 for 30 min at 37 °C), showed green and blue colour fluoꢀ
rescence, respectively (Figure 5E and F). This clearly indicates
that 1 and 2 are cell permeable and can selectively react with
thiophenol in the intracellular environment. Thus, the TAB
based thiol probes 1 and 2 are useful for the selective detection
of highly toxic thiophenol in the intracellular environment by
discriminating biothiols.
Figure 3. Emission spectra of 1 (top) and 2 (bottom) upon addiꢀ
tion of increasing concentration of thiophenol (concentration of
thiol probes = 10 ꢁM and λex = 350 nm, inset shows the photoꢀ
graphs of compounds before and after addition of thiophenol) in
methanolꢀwater (v/v = 4:1) mixture.
Selectivity is always important for designing a good probe.
To evaluate the selectivity of probes 1 and 2 towards thiopheꢀ
nol in the presence of other competing thiols and other nucleꢀ
ophiles, the PL spectra of these probes were recorded in comꢀ
peting environments (Figure 4). No prominent changes in the
luminescence spectra of 1 and 2 were observed even in the
presence of excess aliphatic thiols and other competing nucleꢀ
ophiles. The fluorescence intensity was enhanced only when
thiophenol was added. Thus, these probes are very selective
for thiophenol even in the competing media. Hence the utility
of these probes for the detection of thiophenol in the intracelꢀ
lular environment was evaluated.
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Figure 5. Fluorescence imaging of thiophenol in living cells: (A
and B) cells treated with probes 1 and 2 (10 µM) for 30 min at 37°
C; (C and D) cells treated with thiophenol (1.25 ꢁM) for 30 min at
37° C; and (E and F) then coꢀincubated with probes (10 µM) 1
and 2 for 30 min at 37° C.
Figure 6. Frontier molecular orbitals and electronic transitions of
TAB based thiophenol probe 1 (top right) and 2 (bottom right)
and the corresponding borylaniline 1a (top left) and 2a (bottom
left) furnished upon the addition of thiophenol; illustrating the
mechanism of turnꢀon fluorescence response towards thiophenol.
3.4 Rationalization of the fluorescence Off–On sensing
mechanism using DFT/TDDFT calculations
To gain insights into the electronic properties and to account
for the “off-on” fluorescence response of 1 and 2 towards thiꢀ
ophenol, DFT/TDꢀDFT calculations have been performed
using B3LYP method and 6ꢀ31G (d) basis set incorporated in
Gaussian 09 software (Figure 6). In case of 1 and 2, highest
occupied molecular orbital (HOMO) is delocalized on the aryl
groups attached to B and lowest unoccupied molecular orbital
(LUMO) is localized only on the DNBS moiety. The oscillator
strength (f) obtained from TDꢀDFT calculations corresponding
to the lowest energy transitions was found to be very small,
the values are f = 0.0001 and 0.0004 for 1 and 2, respectively.
This negligibly small value of oscillator strength and the nonꢀ
crossing nature of the frontier molecular orbitals clearly indiꢀ
cate that the S1 state of 1 and 2 is a “dark state” and the transiꢀ
tion from S1 to S0 is forbidden. As discussed in earlier secꢀ
tions, 1 and 2 upon treatment with thiophenol generate the
respective borylanilines 1a and 2a. HOMO of 1a and 2a is
delocalized mostly on aniline fragment; on the other hand,
LUMO of 1a and 2a is completely delocalized over the entire
molecule, indicating the charge transfer characteristics of a
donorꢀacceptor system.14ꢀ16,20 TDꢀDFT calculations for the
borylanilines showed that their S1 state is an emissive state,
confirmed by the considerably higher f values such as 0.1730,
and 0.0503, respectively for 1a and 2a. The allowed S1 to S0
transition is characterized as HOMOꢀLUMO transition and
confirms borylanilines as fluorescent species.
4. Summary and Conclusions
The intrinsic Lewis acid characteristics of triarylborane
(TAB) have been judiciously employed to modulate the
electrophilicity of 2,4ꢀdinitrobenzene based sulfonamides
(DNBS) for the development of a new class of “offꢀon”
fluorescent probes for selective detection of thiophenol over
biothiols, for the first time. The probes were constructed by
conjugating the TAB based luminophore borylanilinies with
the luminescent quencher 2,4ꢀdinitrobenzenesulfonyl (DNBS)
moiety. The limits of detection (LOD) of 1 and 2 towards
thiophenol lie in the range of fractions of micromoles; ~ 0.08
ꢁM and ~ 0.15 ꢁM, respectively. The preliminary in vitro cell
imaging studies shows that the organoboron based thiol probes
1 and 2 can detect highly toxic thiophenol selectively in the
intracellular environment by discriminating biothiols. Very
small reaction time (~ 5 min), lower detection limits in the
micromolar ranges and the selective thiophenol detection in
the intracellular environment clearly demonstrate the potential
of the newly developed TAB based compounds and would
pave the way for the use of these probes in practical applicaꢀ
tions.
ASSOCIATED CONTENT
Supporting Information
Additional spectral characterization and spectroscopic data are
given in the Supporting Information and “This material is availaꢀ
ble free of charge via the Internet at http://pubs.acs.org.”
AUTHOR INFORMATION
Corresponding Author
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* thilagar@iisc.ac.in
1
2
3
4
5
6
7
8
Funding Sources
Science and Engineering Research Board (SERB), New Delhi,
India.
ACKNOWLEDGMENT
PT and PS thanks Science and Engineering Board (SERB), New
Delhi, India for financial support and KKN thanks IISc for
research fellowship.
9
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