Dual Fluorophore Containing Efficient Photoinduced Electron Transfer Based Molecular Probe for Selective Detection of Cr3+ and PO43- Ions through Fluorescence " Turn-On-Off" Response in Partial Aqueous and Biological Medium: Live Cell Imaging and Logic Application

Article abstract of DOI:10.1021/acs.analchem.8b02570

The present work describes a new photoinduced electron transfer (PET) based molecular probe in which naphthalimide (NPI) and anthracene (AN) chromophores are linked through a molecular bridge of piperazine and triazole units by the Click reaction. A typical meaningful structural variation has made the present probe highly selective for Cr³⁺ ion (limit of detection (LOD), 5.567 × 10^-8 M) that displayed enhanced, "turn-On" emission (due to the PET-Off photophysical mechanism) and naked-eye sensitive bright green color fluorescence in the environment of interfering and competitive ions, in Tris-HCl buffer. The minimum energy structure obtained through theoretical calculations (density functional theory (DFT) and time-dependent (TD)-DFT) revealed a "tub" shape structure for probe 10. Upon complexation, the conformation of piperazine fragment changes from chair to boat in which the triazole and piperazine units create a cavity to tether Cr³⁺. Moreover, the probe showed excellent biocompatibility and cell permeability to sense Cr³⁺ sensitively in live cells and, thus, holds great promise for application in biological and environmental sciences. Additionally, the sensitive "Off-On-Off" sensing behavior of probe 10 providing two chemical inputs (Cr³⁺ and PO₄^3-) helps to construct an INHIBIT logic gate. Also the probe has been utilized as printing material to decode secret information through the Cr³⁺ ion containing "marker ink" under UV light.

Full text of DOI:10.1021/acs.analchem.8b02570

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Article
Dual Fluorophore containing efficient PET based molecular
probe for selective detection of Cr3+ and PO43- ions through
fluorescence “Turn–On–Off” response in partial aqueous and
biological medium: Live cell imaging and Logic application
Sushil Kumar Dwivedi, Ramesh C. Gupta, Priyanka Srivastava,
Priya Singh, Biplob Koch, Biswajit Maiti, and Arvind Misra
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02570 • Publication Date (Web): 14 Aug 2018
Downloaded from http://pubs.acs.org on August 15, 2018
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Analytical Chemistry
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Dual Fluorophore containing efficient PET based molecular probe for selective detection of
Cr3+ and PO43- ions through fluorescence “Turn–On–Off” response in partial aqueous and
biological medium: Live cell imaging and Logic application
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Sushil K. Dwivedi†, Ramesh C. Gupta†, Priyanka Srivastava†, Priya Singh‡, Biplob Koch‡,
Biswajit Maiti† and Arvind Misra†*
†Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi – 221 005
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UP INDIA
‡Department of Zoology, Institute of Science, Banaras Hindu University, Varanasi –221005
UP INDIA
Corresponding Author E-mail: amisra@bhu.ac.in; arvindmisra2003@yahoo.com. Phone : +91-
542-6702503; Fax: +91-0542-2368127, 2368175.
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Abstract
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The present work describes a new PET based molecular probe in which naphthalimide (NPI) and
anthracene (AN) chromophores are linked through a molecular bridge of piperazine and triazole
units by Click reaction. A typical meaningful structural variation has made the present probe
highly selective for Cr3+ ion (LOD, 5.567x10-8 M) that displayed enhanced, “turn – on” emission
(due to PET–Off photophysical mechanism) and naked-eye sensitive bright green color
fluorescence in the environment of interfering and competitive ions, in Tris-HCl buffer. The
minimum energy structure obtained through theoretical calculations (DFT and TD-DFT)
revealed a “tub” shape structure for probe 10. Upon complexation the conformation of piperazine
fragment change from chair to boat in which, the triazole and piperazine units create a cavity to
tether Cr3+. Moreover, the probe showed excellent biocompatibility and cell permeability to
sense Cr3+ sensitively in live cell thus, holds great promise for application in biological and
environment sciences. Additionally, the sensitive “Off–On–Off” sensing behavior of probe 10
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3−
providing two chemical inputs (Cr3+ and PO4 ) helps to construct an INHIBIT logic gate. Also
the probe has been utilized as printing material to decode secret information through the Cr3+ ion
containing “marker-ink” under UV light.
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Introduction
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Modern day research outcome encompasses very categorically about the vital role of metal ions
in many fundamental biological processes. In living organism the transition metal ions are
generally present in trace amount and often serve as important cofactors in diverse enzymatic
and redox reactions. A slight disturbance in metal ion homeostasis may lead to acute and long-
term diseases, including heart disease, cancer and neurodegeneration.1 Specifically, the trivalent
metal ions such as, Fe3+, Al3+, Cr3+ have their own biological importance and are directly
involved in the cell functions where there is a critical control of M3+ levels.2,3 Among them, the
chromium ions (Cr3+) (recommended daily intake for an adult; 50-200mg)4 help to activate
certain enzymes and stabilizing factors associated with important biological events like,
metabolism of carbohydrate, fat, protein and nucleic acids.5-9 The deficiency of Cr3+ may impair
immune function and also increase the chance of diabetes and cardiovascular disease by
influencing the level of circulating insulin, glucose, triglycerides, and total cholesterol.10 The
mechanism by which Cr3+ affects human metabolism is based on modulation in insulin action
through glucose tolerance factors (GTF).11-13 Moreover, the excess of Cr3+ is carcinogenic.14 At
elevated temperature Cr3+ binds with DNA and make an adverse change in cellular structure or
even damage the cellular components to cause mutation and cancer.15,16 Cr3+ oxidizes to more
toxic Cr(VI) that can easily penetrate cell membrane and if ingested in excess death of an
organism is possible due to oxidization of DNA and some proteins.17,18
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Moreover, metallic chromium or its compounds have wide industrial applications such as,
chromo-plating, leather tanning, batteries, anodizing operation, dye, paint, welding, corrosion,
wood preservative etc. therefore, industrial runoff of Cr3+ is on high risk of severe environment
pollution.19,20 The chromium content in natural water is normally at µgL-1 level and there are
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severe matrix interference which cannot be minimized.21 The direct sensitive determination of
chromium in water is costly and involve expensive analytical methods.5,22,23 Thus, the
recognition of ions is important in the area of supramolecular chemistry and chemical sensor
technology and fluorescence spectroscopy is the simplest and cost effective tool to achieve
information of such events in real time. 24,25
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A limited number of molecular probes are known for selective and quantitative detection of
Cr3+.26 However, most of them worked on fluorescence “turn-off” strategy because paramagnetic
Cr3+ is known to induce strong fluorescent quenching. Till today only few probes are explored to
monitor Cr3+ in solution and live cells through fluorescence “turn-on” response.27-28 Nam et.al29
utilized cyclometalated Ir(III) complex to detect Cr3+ through modulation in PET and ILCT
processes. Duan et.al30 reported dansyl derivatives for Cr3+ ion. Li et. al31,32 reported some
rhodamine derivatives to detect Cr3+ in solution and live cells. Similarly, some naphthalimide
derivatives have been developed for Cr3+ ions with some limitations.13, 33-35 Thus, there is a great
need of some sensitive chemosensors to monitor Cr3+ selectively in environment and biological
samples.
Therefore, motivated with the challenges and our ongoing research interest in the design of some
smart scaffolds for selective detection of biologically important species, we deepen our interest
to develop an efficient probe for recognition of Cr3+. To have an efficient sensor probe the
structural motif, choice of chromophore and ionophore and/or binding site are crucial. For
instance, we previously reported some molecular probes based dansyl, benzhydryl-anthracene
and thiophenyl-naphthalimide chromophores36-38 in which, N atoms of flexible piperazine unit
and other coordination sites in the form of O and S atoms could able to tether the metal ions like,
Hg2+ and Fe3+ effectively. In continuation to that, the present work describes a new probe in
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which NPI and AN units are linked through a slightly rigid piperazine-triazole molecular bridge
by Click reaction. The typical structural modification notably made the present probe PET
sensitive that could detect Cr3+ ion selectively in solution and live cell without interference of
biologically important competing metal ions such as, Hg2+, Fe3+, Cd2+, Ni2+, Co2+, Al3+.
Experimental Section:
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Sample preparation: In pure water probe 10 is partially soluble but the optimum solubility of
the probe was found in 60% aqueous-THF, v/v = 4:6. Therefore, for photophysical studies the
stock solution of probe (c = 1 x 10-3 M) was prepared in tetrahydrofuran (THF). Then, the 15 µL
of stock solution was diluted in Tris-HCl buffer (5.0 µM; 60% aqueous THF, v/v = 4:6; pH, 7.2)
to make the final concentration 5 µM in a 3 mL solution. For metal ion interaction studies 0.1 M
solution of different ions were used. For 1H NMR titration experiment the solution of probe (c =
1 x 10-2 M) and Cr(NO3)3 were prepared in CDCl3. All the measurements including biological
and live cell studies have been performed in THF/Tris-HCl buffer (THF:H2O, v/v 4:6; pH, 7.2)
gradient system.
Estimation of LOD: The calibration curve was obtained by diluting probe solution (2 µM) to
0.10 µM concentration and emission spectra acquired in the concentration range 0.9 to 0.1 µM.
To estimate calibration sensitivity (m) fluorescence plot was obtained between (I − I0) (where I0
and I are intensities of probe in the presence and absence of Cr3+) and Cr3+ ion concentrations.
From the slope of fluorescence curve the calibration sensitivity (m) was estimated.
Quantum Chemical calculations: The geometry optimization and quantum chemical
calculations of probe 10 and their probable complex, 10.Cr3+ were performed by density
functional theory (DFT) method as implemented in Gaussian 03 suits of program employing
basis set B3LYP/6-31G* and for metal ion B3LYP/LANL2DZ* respectively. A frequency
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calculation for the metal complex was performed to make sure that the optimized structure is a
real minimum. The quantum chemical calculations using the time-dependent density functional
theory (TD-DFT; specifically, uB3LYP TD-FC/6-31G (d, p)) have been performed using
GAUSSIAN 09 suits of program.
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MTT Assay: Human cervical cancer cell line (HeLa) was cultured in 25-cm2 flask at 37˚C in a
humid incubator under 5% CO2 environment. The cells were grown in Dulbecco's modified
Eagle's medium (DMEM) containing 10% FBS, penicillin, and streptomycin. To maintain
exponential growth cells were split when reached at 80% confluency. Prior to exposure to probe
HeLa cells were seeded in a 96-well plate (1x105 cells per well) and incubated for 24h for
complete adherence. After that cells were incubated with different concentration of probe 10 (0,
5, 10, 50, 100, 150, 200 µM) for 24h. Then, MTT solution (100 µL; 5 mg/mL in PBS) was added
into each well. After incubation for 2h the residue was treated with DMSO (100 µL) to dissolve
purple color formazan crystals. The absorbance was recorded on a micro plate reader at 570 nm
and the percentage of cell viability was estimated [Cell viability (%) = Absorbance of detection
group / Absorbance of control group x 100%].
Cell imaging: The cultured HeLa cells were seeded in cultured 12 well plate and left for
overnight incubation as per standard protocol. Next, live cells were incubated with the probe 10
(25 µM and 50 µM) for 24h and 48h and after washing with PBS buffer (2 times) images were
acquired under blue and green channels. For ion interaction studies probe 10 treated cells were
incubated with Cr3+ ion (25µM) for 30 min. Similarly, cells containing 10.Cr3+ were incubated
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with PO4 anion (50 µM) for 30 min. All the cell images were taken under fluorescence
microscope.
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Results and Discussion:
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Probe Design and photophysical studies: For an efficient sensory behavior application of
homo or hetero bireceptor type dyad or triad systems are more advantageous to get significant
output signals upon interaction with an ion.39,40 The present system is like a heterobifluorophoric
format [Fluorophore1-Spacer1-Receptor1-Spacer2-Receptor2-Fluorophore2] in which we can
apply two separate excitation wavelength. One is suppose to give an ion sensitive fluorescence
(NPI unit, sensory) while the other one remain insensitive to ion concentration (ion-invariant).
The ion invariant (AN unit) component is expected to serve as an internal reference. The
designed organic supramolecular assembly is interesting because the possible photophysical
mechanism like, photoinduced electron transfer (PET) or intramolecular electron transfer (intra-
ELT) and electronic energy transfer (intra-EET) due to short range interactions, are likely to play
their important role.41
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The synthetic route adopted for the preparation of probe 10 is shown in Scheme-1. A widely
applicable “Click Chemistry” has been explored to promote cycloaddition reaction42 between
terminal alkyne containing NPI derivative, 5 and anthracene azide, 9 in presence of CuSO4 as a
catalyst in DMSO-H2O (5.0 ml, 4:1 v/v). The rigid triazole-piperazine (tertiary amine) molecular
bridge has been utilized as a suitable ionophore site to bind metal ions. In the sequence of
reactions, naphthalic anhydride was first subjected to bromination and then refluxed with
methylamine in methanol to get compound 3. Compound 3 and 4 were refluxed in pyridine to get
compound 5 in good yield. Moreover, anthracene carboxaldehyde was first reduced with NaBH4
in methanol then treated with thionyl chloride to get synthon 8 in good yield. Compound 8 was
further treated with sodium azide to get 9-azidomethylanthracene, 9. The synthesized compound
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were purified by column chromatography and characterized by H/13C NMR, FTIR and HRMS
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spectroscopic data. (Please see supporting information and Figure S1-17).
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Scheme1: (i) KOH/Br2 (ii) CH3OH/CH3NH2/60oC (iii) Pyridine/TEA/100oC (iv)
POCl3/DMF/80oC (v) MeOH/NaBH4/0oC/SOCl2/DCM/0oC (vi) NaN3/DMF/80oC (vii)
DMSO/H2O/CuSO4/sodium ascorbate.
Response probe 10 toward metal ions: First we optimized the compatibility of the probe in the
solvent of different polarities. The absorption spectrum of molecular probe 10 (5 µM) in solvent
systems such as, hexane, benzene, 1,4-dioxane, THF, methanol, chloroform, DMSO, and
acetonitrile do not show any significant modulation in structured bands (Figure S18a). In 60%
aqueous THF medium the observed low energy broad absorption band at ~ 410 nm correspond to
naphthalimide (NPI) unit while typical anthracene (AN) absorption bands appeared at ~ 387, 366
and 347 nm (Figure S18a). However, the normalized emission spectra of probe 10 in different
solvents displayed shift in ICT band toward longer wavelength e.g. λem. = 475 nm (hexane), 527
nm (MeOH), 529 nm (THF: H2O 40:60 v/v) (Figure S18b) region along with inconsistent
variation in Stokes shift and quantum yield (Table S1). This could be ascribed to interaction of
probe with solvents of different polarity and polarizability. In a preliminary experiment we first
screened the fluorogenic affinity of probe toward different metal ions in different solvents.
Interestingly, the emission feature of probe 10 in solvents like, THF, MeOH and DMSO showed
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inexplicable behavior with different metal ions. However, selectivity of probe 10 for Cr3+ was
relatively high (Figure S19). Knowing the fact that chromium metal interacts with water
molecule to form corresponding hydrates and hydroxides and reduces the pH of the medium
below ≤5.1 To avoid the extent of hydrolysis of Cr3+ and to inhibit pH drop photophysical
behavior of the probe in the absence and presence of metal ions were examined in the Tris buffer
solution (5.0 µM; 60% aqueous THF, v/v = 4:6; pH, 7.2) (Figure 1).
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The electronic transition spectrum of probe 10 showed high energy anthracene (AN) absorption
bands at 352 nm, 367 nm and 387 nm while the low energy naphthalimide (NPI) broad
absorption band appeared at ~ 410 nm (Figure 1, S20a). Upon providing excitation energy
corresponding to AN band (at 387 nm) probe 10 (5.0 μM) displayed a weak broad emission band
at 529 nm (Ф10= 0.03) along with very weak AN emission band at 410 nm. This could be
attributed to photoinduced electron transfer (PET) and/or intramolecular electronic energy
transfer (EET) process (Figure S21). Upon interaction with different metal ions probe 10
revealed relatively high affinity for Cr3+ ion wherein, the low energy absorption bands
disappeared and the molar absorptivity of high energy band increased. Similarly, upon
interaction with Cr3+ (5.0 equiv) the weak emission band of probe 10 centered, at 529 nm
displayed enhanced emission, “turn-on” with a blue shift of 14 nm and the quantum yield of the
probe (Ф10= 0.03) increased by ~11 fold (Ф10 = 0.34). And a bright green color fluorescence
(switched – On) appeared in the medium. However, the other tested metal ions such as, Na+, K+,
Ca2+, Mg2+, Co2+, Ni2+, Zn2+, Fe2+, Pb2+, Cd2+, Fe3+ and Hg2+ failed to exhibit any considerable
change in the color and photophysical behavior of the probe (Figure 1). The observed
considerable enhancement in the relative fluorescence intensity of probe 10 with Cr3+ may be
attributed to suppression of PET process. It is noteworthy to mention that PET process generally
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involves Off-On or On-Off switching behavior without any considerable wavelength shift. In
probe 10 AN unit (ion-invariant) was chosen as first fluorophore (Fl1) in which triazole (Rec1)
and piperazine (Rec2) receptor fragments are connected through methylene spacers (Spr1 and
Spr2). The second fluorophore NPI was considered as a sensory unit. The tertiary amine function
(connected with triazole) of piperazine unit is involved in intramolecular PET whereas the 4-
amino function (tertiary) of NPI unit is responsible for internal charge transfer (ICT) process
through the lowest energy singlet excited state. Therefore, the observed blue shift of 14 nm, upon
interaction with Cr3+ ion, is attributed to inhibition in ICT process. Moreover, the normalized
emission and absorption spectra revealed that emission of AN unit (at λex. = 386 nm) is
completely merged within the absorption bands of NPI derivative 5 (Figure S20b). The
excitation spectra (at λex.= 529 nm) of probe 10 was found almost similar to absorption spectra
and upon interaction with Cr3+ (Figure S22a) or below pH, ≤6 (Figure S22c) the broad band of
NPI unit diminished while the intensity of AN unit enhanced successively. Further, the excitation
spectra at 410 nm also suggested about the energy transfer from AN to NPI moiety (Figure
S22b). Thus, it is noteworthy to mention that due to some extent of rigidity around molecular
bridge electronic energy transfer from S1 state of AN to NPI either through space or bond
interaction, is very much possible via intra-EET process along with conventional PET in which,
naphthalimide emission dominate.39-41,43
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Figure 1: Emission and in inset absorption spectra of probe 10 (5.0 µM) upon interaction with
different metal ions in THF/Tris-HCl buffer (THF:H2O, v/v 4:6; pH, 7.2). Image: Naked-eye
sensitive fluorogenic response of probe 10 in the presence of tested metal ions (under UV light;
at 365 nm).
The high selectivity of probe 10 for Cr3+ ion was assessed by interference studies. Upon addition
of competitive metal ions (in excess) to a solution of a probable complex, 10.Cr3+ or reversibly,
addition of Cr3+ to the solution of 10.Mn+ insignificant change was observed in the both
absorption and emission spectra (Figure S23, S24). Metal ions like Cu2+ and Cr3+ are known to
induce fluorescence quenching however, present structural motif displayed “turn–on” emission
with Cr3+ even in the presence Cu2+ ion in the medium (Figure S25). Further, to examine the
reversibility in complexation the complex, 10.Cr3+ was treated with a strong chelating ligand,
EDTA. Upon addition of excess of EDTA (20 equiv) the intensity of the complex, 10.Cr3+
diminished. Further addition of Cr3+ to the same solution regained the intensity of probe. We
could repeat this process for about eight cycles, and found that the emission behavior of probe
remained consistent without any significant loss in relative fluorescence intensity (Figure S26).
Thus, suggesting about the extraction of Cr3+ from its complex, 10.Cr3+.
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Further to understand exact stoichiometry for interaction between probe 10 and Cr3+ ion titration
experiment was performed. Upon a gradual increment of Cr3+ ions (0-5 equiv) to a solution of
probe 10 the relative intensity centered, at 529 nm (switched–Off state) enhanced, “turn-on”
significantly (~ 93–94%) with a blue shift of ~ 14 nm (Figure 2) and a bright green color
fluorescence appeared (switched–On) in the medium. Similarly, the absorption spectra of probe
10 displayed rise in molar extinction coefficient upon increasing the concentration of Cr3+ ion
(Figure S27).
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Figure 2: Emission titration spectra of probe 10 (5.0 µM) with Cr3+ (5 equiv) in THF/Tris-HCl
buffer (THF:H2O, v/v 4:6; pH 7.2). Inset: Job’s plot based on emission titration data.
Job’s plot analysis based on emission spectral data suggested about a 1:1 stoichiometry (Figure
2, inset) for which the binding constant was estimated using Benesi-Hildebrand (B-H) method44
and was found to be Kass= 5.87x105 M-1 (Figure S28). In order to estimate the limit of detection
(LOD) for Cr3+ a calibration curve was obtained. An almost straight line calibration curve with
standard deviation (σ) 0.03203 suggested about a linear correlation between intensity and
concentration of probe 10 (Figure S29a). The slope of fluorescence curve gave the calibration
sensitivity (m) 59.436 (Figure S29b) in the aforementioned range. The estimated limit of
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detection (LOD = 3/m) 5.567x10-8 M was found comparable to some other reported methods
(Table S2).
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pH sensitivity: The probe 10 contains piperazine unit and is sensitive to pH of the medium.
Secondly, the availability of Cr(III) ions in the medium is also pH dependent (pH, 5.2–8.3).25,47
Therefore, we examined the pH dependent photophysical behavior of probe 10. Notably, the
probe 10 displayed insensitive behavior between pH 7 to 14. However, under acidic condition
the molar absorptivity corresponding to anthracene bands increased while the NPI broad band
gets diminished (Figure S30). However, the intensity of probe enhanced significantly (~33%
between pH ~ 5.0 to 6.0, and ~ 91% between pH ~ 4.0 to 1.0) (Figure 3, and S22c) due to the
protonation of the N atoms of piperazine unit (pH < 5.0) consequently, suppressing the PET
(PET-Off) from piperazine to naphthalimide unit. Further, the pH-dependent emission spectra of
probable complex, 10+Cr3+ remained insensitive to pH variations (1 to 14) (Figure 3, inset).
Since, below pH ≤5.0 quaternary ammonium salt is formed therefore no coordination sites are
available to tether Cr3+ ion. Thus, the distribution of probe 10 in a broad pH range has suggested
applicability of probe to detect Cr3+ under physiological pH (pH 7.0) condition and biological
medium.
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Figure 3: Change in emission spectra of probe 10 (5μM) at different pH in THF/Tris-HCl buffer
(THF:H2O, v/v 4:6; pH, 7.2). Inset: Shows change in emission behavior of probe 10 its complex
10.Cr3+.
Response of probe 10 toward Anions: In order to understand the affinity of probe 10 for
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different class of anions such as, F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, CN⁻, N3 , CO3 , HSO4⁻, NO3⁻ and PO4 ⁻
the absorption and emission spectra were acquired under similar experimental condition.
Notably, the probe 10 showed insignificant behavior toward the tested anions (Figure S31). Next,
we examined the affinity of a probable complex 10.Cr3+ with tested anions (~20.0 equiv).
3
Notably, the emission intensity of the probe 10 revived upon interaction with PO4 ⁻ anion while
remained silent toward other tested anions (Figure 7c, and S32b). This may be attributed to
displacement of Cr3+ from its probable complex, 10.Cr3+. To know the equivalent of PO4 ⁻ anion
3
required to displace Cr3+ from its complex titration experiment was performed (Figure S33a). It
3
is interesting to mention that upon a gradual addition of PO4 ⁻ (0-20equiv) the enhanced
emission of complex, 10.Cr3+ diminished gradually probably due to release of Cr3+ ion in the
medium and color of the solution reached to a switched–Off state. Further addition of Cr3+ to a
solution of 10.Cr3++PO4 the emission intensity regained and was almost similar to the intensity
3-
of a complex 10.Cr3+. On repetition of this process for about eight cycles we observed
simultaneous decrease and increase in the relative intensity of the probe (switch–Off–On) without
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any considerable loss in intensity and color of the solution (Figure S33b). This clearly indicated
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about the reversible mode of sensing with Cr3+ and PO4 ions. To make sure about high
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selectivity of the complex, 10.Cr3+ for PO4 ion studies have been performed with ATP, ADP,
9
and PPi. The emission behavior of 10.Cr3+ revealed insignificant sensitivity for ATP and ADP
while about a small fluorescence quenching (9%) occurred with PPi (Figure S34). Moreover, the
estimated limit of detection of the complex, 10.Cr3+ for PO43- anion was found to be 3.90 x 10-7
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M (Figure S29c). In order to understand the extent of fluorescence quenching with PO4 anion,
quenching constant (Ksv) was estimated from an almost linear Stern-Volmer plot (R2 = 0.9979)
and was found to be 5.87x105 M-1 (Figure S35). Thus, the probe 10 could serve as a potential
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reusable “PET–Off –On” chemosensor for Cr3+ and PO4 ions in the studied medium.
Furthermore, the fluorescence life time (τ) studies have been performed to rationalize the sensing
mechanism (Figure S36). Interestingly, upon interaction with chromium (10.Cr3+) the faster
decay components (A1) of probe 10 (31.71%) decreased (24.81%) while the longer decay
component value (A2) increased (75.19%) (Table S3). Similarly, upon interaction of 10.Cr3+
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with PO4 the faster and longer decay components of 10.Cr3+ were respectively increased
(36.12%) and decreased (63.88%). Moreover, the decay time of probe 10 (4.7184 ns), based on
double exponential decay fittings, increase and decrease for the species 10.Cr3+ (5.6164ns) and
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10.Cr3++PO4 (4.3330ns) respectively. Thus, TRPL studies also corroborate fluorescence “turn-
On-Off” sensing mechanism with respective ions.
Mechanism of Interaction with Cr3+:
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Figure 4: Stacked 1H NMR spectra of probe 10 (2.0 x 10-2 M) upon addition of Cr3+ ions (0-2.0
equiv.) in CDCl3.
1H NMR titration studies: To gain a deep insight about the actual mode of interaction between
probe 10 and Cr3+ ion 1H NMR titration experiments were performed in CDCl3. Upon interaction
with Cr3+ (0-2.0 equiv) the resonances corresponding to piperazine and methylene protons, Ha (δ
3.18, s), Hb (δ 2.73, s) and Hc (δ 3.52, s) broadened and exhibited downfield shift (∆δ = 0.21, ∆δ
= 0.08 and ∆δ = 0.15 ppm) respectively, while Hd proton (δ 8.31, merged with AN protons) of
triazole ring exhibited a narrow upfield shift (Figure 4). Moreover, in the FT-IR spectra the
typical C-N function stretching vibration bands (1286, 1231, 1039, 1020 cm-1) upon interaction
with Cr3+ shifted upfield to appear at 1283, 1229, 1039, 1006 cm-1 respectively (Figure S12).
Additionally, in HRMS the molecular ion peak of probe 10 appeared at m/z 567.2485 (10+H+;
calc. 566.2430) while upon complexation with Cr3+ a new peak appeared at m/z 806.1810 (calc.
804.1470) (Figure S13-14). Thus, 1HNMR, FTIR and HRMS studies clearly supported about the
binding of probe with Cr3+ through N atoms of piperazine-triazole molecular bridge in 1:1
stoichiometry.
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DFT/TD-DFT Studies: The DFT optimized minimum energy structure (Figure S37) also
revealed that probe interacts with Cr3+ through the N atoms of piperazine-triazole bridge with
bond lengths 2.09, 2.10 and 2.08 Å. The distance between NPI and AN units is found to be
10.47Å. The minimum energy structure revealed chair conformation for piperazine in which NPI
and triazole linked AN units occupy equatorial (1,4) positions. The vertically placed two
chromophoric units are almost perpendicular to each other and provide a virtual “tub” shape
structure. Upon complexation with Cr3+ the piperazine unit rearranges to attain boat
conformation wherein both triazole and piperazine units come close to each other. Thus, a cavity
like structure generated to tether Cr3+ ion successfully, while the distance between the two
chromophores increased to ~ 12.84Å. The frontier molecular orbital diagram suggested that the
electron density in HOMO is mainly localized over NPI and piperazine units while in LUMO
electrons are mainly localized on AN moiety, with energy difference (∆E) about 1.8912 eV
(Figure S38). Whereas, in case of 10.Cr3+ the maximum electron density resides on HOMO of
NPI and LUMO of AN moieties, with energy difference (∆E) 0.0239eV. In order to understand
the PET mechanism the TD-DFT calculations were performed and the relevant MOs of 10 and
its complex 10.Cr3+ were analyzed (Figure 5 and S39). It has been found that the energy (-
5.49996eV) associated with HOMO of 10 reduced (-13.29167eV) significantly upon
complexation with Cr3+ consequently, restricting the PET. It is important to mention that, for 10
most favorable electronic transition (Table S4) occurred from HOMO to LUMO+1 (∆E,
3.43027eV) in which the electron density of piperazine unit is partially transferred to NPI unit
(PET–ON). In contrast, for a complex 10.Cr3+ the most favorable transition occurred from
HOMO-4 to LUMO+1 (∆E, 4.09857eV) wherein, the electron density of NPI-piperazine
fragment remained toward metal centre (PET-Off) (Figure 5). A significant decrease in the
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energy of HOMO (∆E, ~8.95336eV) also suggested about the involvement of EET process either
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Figure 5: Excited state DFT optimized HOMO-LUMO energy distribution for 10 and its
complex, 10.Cr3+.
Studies of Probe 10 in biological medium:
Encouraged by the observed high affinity of probe 10 to detect Cr3+ ion selectively in a buffer
solution (pH, 7.2) we further intended to examine the photophysical behavior of probe and its
affinity for Cr3+ ion in biological medium. In the protein medium (bovine serum albumin, BSA)
upon providing high excitation energy (λex = 278 nm due to tryptophan residues of BSA) both
probe 10 and its complex, 10.Cr3+ could not able to display any considerable change and
remained independent to even high concentration of BSA (3μM) (Figure S40). Moreover, probe
10 in the presence of BSA (10.0 equiv.) showed high affinity for Cr3+ with enhanced, “turn–On”
emission, almost similar to the emission observed without BSA. It clearly indicated about
negligible chance of possible nonbonding, electrostatic and H-bonding interactions and even
with electrolytes present in blood serum.
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Live cell imaging: Prior to live cell imaging the effect of probe on the viable HeLa cells was
examined by tetrazolium (MTT) assay based on the action of mitochondrial dehydrogenase
enzyme and calorimetric measurement.37 The MTT assay suggested that probe can easily
permeate into HeLa cells with good cytocompatibility (Figure S41) as the 70 to 80% cells were
viable at 100µM to 200µM concentration of probe. For cell imaging the probe 10 (25 µM and 50
µM) incubated HeLa cells were visualized under blue and green channels. However,
insignificant intracellular fluorescence was observed (Figure 6b and S42b). Whereas when the
probe incubated HeLa cells were incubated with Cr3+ (25 µM) for 30 min a bright green
fluorescence, “turn–On” appeared (Figure 6c). This clearly shows the compatibility of probe to
monitor the Cr3+ ions intracellularly. It is important to mention that in solution around pH 5 the
relative intensity of probe marginally increases (10-15%) while the probe incubated cells do not
show any considerable intracellular fluorescence. A long term retention studies have been
performed to assess the extent of intracellular retention of probe. The images of probe (25 µM)
loaded HeLa cells showed insignificant fluorescence even after 24h and 48h incubation (Figure
S43). Thus, the probability of interaction with the lysosomes was almost negligible and we could
observe the green fluorescence only in the presence of Cr3+. The cell imaging experiments were
repeated to understand the fluorescence “On–Off” switching behavior in the presence of Cr3+ and
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PO4 ions, intracellularly. Interestingly the observed bright green fluorescence of 10.Cr3+
diminished (switched–Off) after interaction with PO43- ions (Figure 6d).
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Figure 6: Fluorescence imaging in HeLa cells (a) Control and with (b) probe 10 (25 µM) (c)
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probe 10+Cr3+ (25 µM) (d) 10.Cr3+ and PO4 (50 µM).
Off-On switching behavior and Logic implication: Numerous chemically driven molecular
switches can be derived from the unique modulation in optical signature of chemosensors incur
upon interaction with important guest species. The present molecular assembly remains in
switched–On state in the presence of Cr3+ (output; green fluorescence) while in switched–Off
3
state (diminished emission) in the absence of Cr3+ and upon interaction of 10+Cr3+ with PO4 ⁻
anion. Thus, the observed emission behavior correlates the “Off-On-Off” switching mechanism
and has been realized to construct a logic gate by applying two chemical inputs as, ln1 (Cr3+) and
3
ln2 (PO4 ⁻). According to the truth table (Figure 7b) sequential logic operations 10+Cr3+
displayed high output emission (threshold value ~70%; at 515 a.u.) whereas, low output emission
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was obtained when the applied logic inputs were 10, 10+PO4 ⁻, 10+Cr3++PO4 ⁻ respectively.
Thus, the truth table corresponds to a combinational (consisting of a NOT and an AND gate)
INHIBIT logic gate (Figure 7a).
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Figure 7: (a) INHIBIT logic gate with two inputs, In1 (Cr3+) and In2 (PO4 ) (b) Truth table (1 =
On; 0 = Off states) (c) Emission spectra and Bar diagram show output signals of probe 10
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corresponding to inputs, 10.Cr3+, 10.PO4 and 10.Cr3++PO4 .
Application of probe in Press-Jet Printing: An attempt was made to explore the real time
fluorescence response of probe 10 as a potential press-jet printing material. The printing papers
were prepared by simply dipping cellulose paper (WhatmanTM) into a solution of probe 10
(2mM) in THF for 5 min and then washed with PBS buffer and dried in air. A very weak
fluorescence (at 365nm) suggested the adsorption of probe 10 on cellulose paper (Figure 8a). A
secret code, “BHU” was written with Cr3+ solution as a “marker ink” (2 × 10−3 M). Upon
exposure of UV light (at 365 nm) a high-definition image was appeared (Figure 8c) while in
normal light (control) no such images were observed (Figure 8b). The stability of image on the
paper was good and no photo-bleaching occurred for more than 14 days at room temperature.
Thus, the probe loaded papers afford great promise in hiding and sending the information
secretly. Moreover, to know the reversibility and reusability of tested paper washing was done
with phosphate buffer (2x10-2 M) for several times. Insignificant loss in the intensity of image of
secret code shows one time use of such material.
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Figure 8: Image of press-jet printing sheet with Probe 10. Decoding of Secret code, “BHU”
written with Cr3+ containing “marker ink” under (b) visible light and (c) UV light (at 365 nm).
Conclusion:
In summary, we have developed a new efficient PET based molecular probe for a selective
detection of Cr3+ in solution and biological medium. The probe 10 upon interaction with
chromium ion selectively, displayed enhanced emission, fluorescence “turn–On” response. The
bright fluorescent green color appeared in the medium was sensitive to the naked-eyes. The
molecular probe has shown good detection limit and stability in wide pH range. The excellent
biocompatibility of probe holds great promise for selective detection of Cr3+ in biological
medium and environment. Moreover, the fluorescence “Off–On–Off ” switching response of
probe 10 in the presence of two chemical inputs, in the form of Cr3+ and PO43- ions, has been
utilized to construct an INHIBIT logic gate. Also probe 10 has been utilized to print secret letters
through Cr3+ solution ink that can be decoded under UV light. Overall, the experimental
observation further strengthens the designing of good sensor motif for the detection of Cr3+ in the
solution, on printing paper and as well as in live cells.
ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
10.1021/acs.analchem. Synthetic route, photophysical data, and different spectral data (PDF)
AUTHOR INFORMATION
ORCID
Arvind Misra: 0000-0003-2716-4139
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Notes
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The authors declare no competing financial interest.
Authors contribution: Priya Singh and Biplob Koch have performed live cell imaging
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experiments.
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Acknowledgments:
The authors are thankful to the Council of Scientific and Industrial Research
(CSIR02(0199)/14/EMR-II), and University Grant Commission (UGC) New Delhi India for
financial support and fellowships (to SKD, RCG) to carry out research works. We are also
thankful to the Head, Department of Chemistry for their valuable support (through DST-FIST,
DST-PURSE, CAS, UGC-UPE programs) and encouragements.
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