Cyanine-based Fluorescent Probe for Cyanide Ion Detection

Article abstract of DOI:10.1007/s10895-021-02771-8

Cyanine-based probe-possessing indolium iodide and indole unit were synthesized in two-step with easy available raw material: a potential probe for the cyanide ion detection. The detecting ability of the probe was investigated and confirmed by a visual and instrumental approach. A noticeable color change from orange to colorless obtained only for cyanide ions and other added ions does not impart any changes visually and through UV and Fluorescence technique. To confirm the mechanism of sensing 1H-NMR recorded. From the result, the peak belonging to?N-methyl displayed an upfield shift from 4.01 δ ppm to 2.74 δ ppm due to the disappearance of indolium iodide ion and the olefin protons peaks were shifted from 7.19 to 6.17 and 8.70 to 7.20 δ ppm confirms the nucleophilic addition of cyanide ion to the probe. Test kit from filter paper prepared for the real-time monitoring cyanide ion. The prepared strip is effective in detecting cyanide ion with a visual color change.

Full text of DOI:10.1007/s10895-021-02771-8

Journal of Fluorescence
https://doi.org/10.1007/s10895-021-02771-8
ORIGINAL ARTICLE
Cyanine‑based Fluorescent Probe for Cyanide Ion Detection
1
1
1
Mahesh Gosi · Nagaraju Marepu · Yeturu Sunandamma
Received: 30 March 2021 / Accepted: 30 June 2021
©
The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
Cyanine-based probe-possessing indolium iodide and indole unit were synthesized in two-step with easy available raw mate-
rial: a potential probe for the cyanide ion detection. The detecting ability of the probe was investigated and conꢀrmed by a
visual and instrumental approach. A noticeable color change from orange to colorless obtained only for cyanide ions and other
added ions does not impart any changes visually and through UV and Fluorescence technique. To conꢀrm the mechanism
of sensing 1H-NMR recorded. From the result, the peak belonging to N-methyl displayed an upꢀeld shift from 4.01 δ ppm
to 2.74 δ ppm due to the disappearance of indolium iodide ion and the oleꢀn protons peaks were shifted from 7.19 to 6.17
and 8.70 to 7.20 δ ppm conꢀrms the nucleophilic addition of cyanide ion to the probe. Test kit from ꢀlter paper prepared
for the real-time monitoring cyanide ion. The prepared strip is eꢁective in detecting cyanide ion with a visual color change.
Keywords Orange to colorless · Nucleophilic addition · Test strip · Fluorimetric probe · Cyanide ion
Introduction
were reported in the literature, such as hydrogen bonding,
nucleophilic addition (chemodosimeter), and supramolecular
self-assembly [18–24]. The maximum of developed probes
suꢁered by challenging synthetic procedures, inadequate
color response, lack of selectivity, and sensing limits in a
complete aqueous medium. However, some of the recep-
−
Cyanide (CN ) is the primary pollutant found in our water
surface due to its extensive industrial usage. It is involved
in several chemical procedures such as silver extraction and
gold extraction, metallurgy, tanning, plastic and medicine
manufacturing, electroplating, etc. [1–5]. Besides, cyanide
anion containing gases used in eliminating pests and insects
and is used as a warfare chemical agent [6, 7]. The world
−
tors overcome these drawbacks to detect CN ions reported
in the literature [25–27]. Indolium moieties are also identi-
−
ꢀed CN ions [28–30]. Thus, the progress of developing
−
health organization set a bearable amount of CN ions in
a useful probe, which could be proꢀcient at recognizing
cyanide ions in a maximum water-consuming medium, is
getting extreme care. Considering these points, we have
synthesized a simple cyanine-based probe (PI) having the
indolium iodide and indole moieties connected with an ole-
ꢀn bridge. Hence, an intramolecular charge transfer tran-
sition (ICT) will take place between the electron-deꢀcient
indolium iodide and electron-rich indole moiety via oleꢀn
bridge. Therefore, it showed brilliant recognizing properties.
drinking water and is nearly 500 μM for short-term and 70
μM for long-term exposures [8]. Several methods are cur-
−
rently reported to recognize CN ions, such as titrimetric,
potentiometric, chromatographic, voltammetry, and elec-
trochemical approaches [9–12]. However, many of these
approaches are time consuming, expensive, and diꢂcult
for on-site recognition. Hence, it is essential to produce
extremely selective, sensitive, and suitable methods for rec-
−
−
ognizing CN ions in real samples. The simple, economi-
Further, the CN ion reacts with probes’ indolium iodide site
cal, and quick executions of optical sensors (colorimetric/
ꢃuorimetric) have been getting much attention recently
by the nucleophilic attack that interrupts the π-conjugation
and thus blocked the ICT from indole to indolium iodide
group follow-on a substantial ꢃuorescence and absorption
variation can occur.
[
13–17]. In the optical chemosensors, various mechanisms
*
Yeturu Sunandamma
yeturuasnu7@gmail.com
1
Department of Chemistry, Acharya Nagarjuna University,
Nagarjunanagar Guntur-522510 Andhra Pradesh, India
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Journal of Fluorescence
Scheme 1 Synthesis of probe
PI)
(
Experimental
Experimental Procedure for the Synthesis
of 1,2,3,3‑tetramethyl‑3H‑indolium Iodide (2)
Materials and Methods
Compound 1 (0.5 g, 4.399 mmol) was dissolved in tolu-
ene (10 mL) and iodomethane (0.85 mL, 13.207 mmol) was
added at room temperature and the reaction was reꢃuxed
for 6 h. during the reaction the solid precipitate was formed.
The reaction mixture was cooled to room temperature and
the solid was ꢀltered and washed with diethyl ether (20 mL).
The resulting white crystalline solid was used for next step
2
,3,3-Trimethyl-3H-indole, 1H-indole-3-carbaldehyde,
iodomethane, anions in the form of tetrabutylammonium
salts were purchased from Sigma Aldrich, Piperidine pur-
chased from TCI chemicals. Toluene and ethanol HPLC
1
13
grade solvents received from Rankem. H NMR and
C
NMR spectra were reordered to conꢀrm the compounds.
Bruker AVANCE III spectrometer was used and operated
without puriꢀcation. Yield: (0.734 g, 96%), mp 258 – 260
1
13
1
at 600 MHz ( H NMR), and 150 MHz ( C NMR) at ambi-
℃, H NMR (DMSO-d , 600 MHz) δ 7.94 – 7.92 (m, 1H),
6
ent temperature, DMSO-d is used as a solvent. Bruker-
7.86 – 7.83 (m, 1H), 7.64 – 7.61 (m, 2H), 3.99 (s, 3H), 2.80
6
1
3
micrOTOF QII mass spectrometer was used to record the
HRMS. Agilent 8453 spectrophotometer was used to obtain
UV–Vis absorption spectra and RF-5301PC spectroꢃuoro-
photometer was used for ꢃuorescence emission spectra. For
the ꢃuorescence measurements, the excitation wavelength
was ꢀxed as 290 nm, and both the excitation and emission
slit widths were 3 nm.
(s, 3H), 1.54 (s, 6H); C NMR (DMSO-d , 150 MHz) δ
6
196.4, 142.5, 142.0, 129.7, 129.2, 123.7, 115.6, 54.4, 35.3,
+
+
22.2. HRMS m/z calculated for C H N (M) : 174.1277,
1
2
16
found: 174.1280.
Experimental Procedure
for the Synthesis of 2‑(2‑(1H‑indol‑3‑yl)
vinyl)‑1,3,3‑trimethyl‑3H‑indol‑1‑ium Iodide (PI)
Synthesis
Compound 2 (0.5 g, 2.871 mmol) and indole-3-carbaldehyde
(0.41 g, 2.871 mmol) was dissolved in ethanol (15 mL) and
piperidine (3 µL, 0.12 mmol) was added and reꢃuxed for 3 h
under nitrogen atmosphere. The orange solid was obtained,
ꢀltered and washed with diethyl ether (20 mL) to produce a
Fluorescent probe (PI) was easily synthesized in two steps,
by using simple and inexpensive starting materials. In the
ꢀ
rst step, methylation of compound (1) with iodomethane
gives compound (2) excellent yields. In the second step,
compound (2) undergoes a simple aldol condensation reac-
tion with indole aldehyde to provide a ꢃuorescent probe (PI)
in good yield. The synthesized compounds were conꢀrmed
1
bright solid. Yield: (0.95 g, 78%), mp 285 – 287 ℃, H NMR
(DMSO-d , 600 MHz) δ 12.76 (brs, 1H), 8.70 (d, J=15.8 Hz,
6
1H), 8.69 (s, 1H), 8.28 (dd, J=5.8, 3.0 Hz, 1H), 7.81 (d,
J=7.3 Hz, 1H), 7.76 (d, J=7.9 Hz, 1H), 7.62 (dd, J=5.8,
3.0 Hz, 1H), 7.57 (t, J=7.5 Hz, 1H), 7.50 (t, J=7.5 Hz, 1H),
1
13
by NMR ( H and C) and HRMS. Scheme 1 Synthesis of
probe (PI).
Fig. 1 Color changes of PI with
various anions A in day light,
B under UV lamp (λ=365 nm)
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Journal of Fluorescence
colorimetric appreciation performance was exhibited in
Fig. 1A. The synthesized probe PI display deep orange color
which immediately changed to colorless with cyanide addi-
tion while the other added ions not generating such color
variation. Similarly, the changes obtained were checked
under UV lamp the probe and the added ion except cyanide
ion showed little yellow ꢃuorescence means no substan-
tial change in the probe solution and with cyanide ion, it is
showing blue ꢃuorescence under UV light Fig. 1B. This pri-
mary examination exposed that the probes PI can selectively
−
recognize CN ion with a discrete color variation.
Absorption Studies
The absorption modiꢀcation of probe PI was examined with
the existence of several anions as mentioned directly above.
As seen from Fig. 2, the probe PI displayed a band pointed
at 489 nm, this might be because of the ICT throughout the
probes, and this transition is accountable for the color of
probes. Further, the added ions did not produce the least
alteration in the bands while the adding of cyanide ion
showed a fresh band at 289 nm array with the disappearance
of a band at 489 nm. The blue shift observed in the probe
Fig. 2 Absorption spectral changes of PI with several anions
7
.39 (dd, J=6.0, 3.0 Hz, 1H), 7.19 (d, J=15.8 Hz, 1H), 4.01
1
3
(
s, 3H), 1.81 (s, 6H); C NMR (DMSO-d , 150 MHz) δ
6
1
1
1
80.6, 149.3, 142.9, 142.5, 138.8, 129.7, 129.2, 129.1, 127.9,
25.0, 124.8, 123.6, 123.1, 121.9, 116.3, 115.5, 114.0, 113.9,
+
05.5, 54.3, 51.3, 22.1. HRMS m/z calculated for C H N
21
21
2
+
(
M) : 301.1699, found: 301.1704.
−
with CN ion interaction indicates that there was a strong
binding between the probe and cyanide ion. To know the
–
5
binding capability of probe PI (1×10 M), a titration was
Results and discussion
−
conducted with an increase in the addition of CN ion to the
−
probe solution (Fig. 3A). With the gradual quantities of CN
Visual Reaction of Probe PI
–
5
ions (0–1×10 M) added, the fresh band placed on 289 nm
detected with the growing intensity of the band, which is
attributed by clear color transformation, which could be seen
−
5
The binding interaction of the probe PI (10 × 10 M) with
−
5
−
−
2−
3
−
4
−
various anions (10 × 10 M) AcO , SCN , CO , SCN ,
−
−
−
−
−
−
−
−
−
2−
4
above. The limit of detection of PI for CN was deliberated
PF , CNO , HSO , H PO , CL , Br , NO , CIO , F , SO
6
4
2
4
3
–
5
−
3
as 1.73×10 M, based on 3δ/k (Fig. 3B).
and N were inspected in DMF: water (2:8, v/v) through
–
5
−
–5
Fig. 3 A Absorption changes of PI (1×10 M) with CN ion (0–1×10 M) addition in 2:8 v/v DMF: Water, B Plot for limit of detection cal-
culation
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Journal of Fluorescence
Fig. 4 Fluorescence spectral changes of PI with various anions
Fig. 6 The pH eꢁect of probe PI and its cyanide complex
–
5
Fluorescence Spectral Studies
ion (0–1 × 10 M). The color variations of PI with the
added cyanide ion under UV lamp was shown in Fig. 5A
−
Similarly, the recognizing capability of PI with chosen ani-
ons has been inspected by emission spectral technique when
exiting at 289 nm. As seen from Fig. 4, the probe PI exited
at 289 nm. Then the emission peak appeared at 373 nm and
(inset). With the added CN ion, the probes turned their
less ꢃuorescent yellow color to deep blue ꢃuorescent. From
the obtained data from the ꢃuorescence spectra, the binding
constant can be calculated using the equation for cyanide
−
−
5
−1
was enhanced with the added CN ion, whereas no changes
ion (F —F )/(F —F )=1/K[CN ] it is 1.17×10 M (Fig.
α 0 x 0
in the spectra were spotted with the other chosen anions.
Thus, the above observation states that there is an interaction
among the probes with cyanide ions.
S7). The calculated detection limit by emission proꢀle was
–
8
found to be 1.8×10 M for PI by means of 3σ/k (Fig. 5B).
−
The ‘On–Oꢁ’ response of ꢃuorescence is eꢁectively mon-
pH Effect of Probe Towards CN Ion
−
–5
itored with the steady addition of CN ion (0–1×10 M) to
the solution of probes. Figure 5A displayed that the probes
shown its emission at 373 nm for PI were in progress to
The recognizing aptitude of PI for cyanide ion was further
checked by the eꢁect of pH by absorption study. As seen in
Fig. 6, in acidic and neutral pH (2–7), the free probes exhib-
ited absorbance at 489 nm while the absorbance is reduced
−
increase progressively with the CN ion addition. The emis-
sion peak of the probe enhanced with the added cyanide
–
5
−
–5
Fig. 5 A Fluorescence spectral changes of PI (1×10 M) with CN ion (0–1×10 M) addition in 2:8 v/v DMF: Water, B Plot for limit of detec-
tion calculation
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Journal of Fluorescence
Anti‑interference Study of Probes
Moreover, to inspect the anti-interference study of probe PI
−
with CN ion in the existence of added interfering anions, a
competitive investigation has been carried out (Fig. 7). The
–
5
emission of probe PI (1×10 M) with several chosen ions
–
5
−
(
1×10 M) excepting CN were initially monitored. After-
wards, the emission was measured for a second time with the
–
5
−
same samples with the addition of 1×10 M of CN ions.
The ꢀgure pointed out that there is no or weak emission
intensity for free probes and probes with the chosen ions, but
the peak intensity increased with the added cyanide ion to
the probe with other ion. These observations suggested that
the anti-interference of the probes for cyanide ion possibly
would not be inclined by chosen competing ions. Hence,
these probes conꢀrm the exceptional selectivity for cyanide
ions.
Fig. 7 Competitive study of probe PI with cyanide ion in presence of
diꢁerent anions
−
1
with CN ions. At basic pH (8–10), the probes absorbance
Probable Mechanism by H NMR Study
gently weakened and touched the lowermost ꢀ (pH 10),
max
−
whereas no absorbance was detected with the added CN
In the synthetic scheme, N-methylation step (step-1) was
1
ions. The changes in the absorbance peak in basic pH are
conꢀrmed by the appearance of sharp singlet at 3.99 ( H
−
13
primarily because of the OH ion interference, which binds
NMR) and 54.4 δ ( C NMR) ppm, which corresponds to the
intensely with the probes.
methyl protons. These protons are more deshielded due to the
1
Fig. 8 The partial H-NMR spectra of A PI and B PI-CN in DMSO-d
6
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Journal of Fluorescence
Fig. 9 Color change of test
−
strips of PI with CN ion A day
light, B UV light
formation of indolium iodide ion (strong electron-withdrawing
Conclusions
+
ability (N -CH ). The HRMS spectra also further proved the
3
N-methylation. In the second step, the ꢂuorescent probe (PI)
To conclude, we have developed a cyanine-based con-
jugated system of probes for the recognition of cyanide
ions by nucleophilic addition mechanism. The recognition
method of cyanide ion is visualized by a notable color var-
iation to colorless from orange color. The lowermost limit
was conꢁrmed by the disappearance of a sharp singlet peak of
1
13
methyl protons at 2.80 ( H NMR) and 35.3 δ ( C NMR) ppm,
and the appearance of indole protons as well as oleꢁn bridge
1
protons in the H NMR spectrum. The stereochemistry of the
−
double bond (trans or “Entagagen”) was also conꢁrmed by the
of detection for CN ion was proven by both ꢂuorimetric
1
H NMR coupling constant of oleꢁn protons (J=15.8 Hz).
approaches and thus below the tolerable limit set by WHO.
−
1
This probe PI is used for the detection of CN anion. The
The H-NMR investigation was established the proposed
−
nucleophilic addition of CN anion to the C=N bond in the
mechanism of probes with cyanide ion. Additionally, the
−
indolium iodide ring was also conꢁrmed by the in situ analysis
prepared strips were useful for the recognition of CN ions
(NMR) of the sample.
short of resorting to additional equipment.
In the probe PI, the chemical shift values of N-methyl
protons appeared at 4.01 (HX) δ ppm and the oleꢁn protons
were observed at 7.19 (HY) 8.70 (HZ) δ ppm. In the next
step, we dissolved this probe PI and TBACN in DMSO-
Supplementary Information The online version contains supplemen-
tary material available at https://doi.org/10.1007/s10895-021-02771-8.
Acknowledgements The authors thank Acharya Nagarjuna University
1
−
d solvent and recorded H NMR spectra. After CN anion
6
for providing lab facilities to conduct research work.
attack, the chemical shift values were shifted to upꢁeld
1
region on the H NMR scale (Fig. 8). Moreover, a remark-
Author’s Contributions All authors (Mahesh Gosi, Nagaraju Marepu,
and Yeturua Sunandamma) made substantial contribution in preparing
the manuscript.
able diꢀerence was observed in the chemical shift values of
HX, HY and HZ. The peak belonging to N-methyl displayed
an upꢁeld shift from 4.01 δ ppm to 2.74 δ ppm due to the
disappearance of indolium iodide ion. Further, the oleꢁn
protons peaks were shifted from 7.19 to 6.17 (HY) and 8.70
to 7.20 (HZ) δ ppm. From the above data, it is conꢁrmed
Funding No funding received from any source.
Data Availability All data available.
−
that the PI molecule underwent CN nucleophilic addition
Declarations
reaction that cleanly converted the indolium iodide ion to
indoline-2-carbonitrile (PI-CN).
Consent to Participate Informed consent obtained from all individual
participants included in the study.
Practical Application
Competing Interests The authors declare that they have no conꢂict
of interest.
The probes’ practical use was monitored by making test strips
−
for the immediate recognition of CN ions in aqueous solutions.
–3
Whatman paper was immersed into DMF solution (1×10 M)
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