A novel dendritic polymer based turn- off fluorescence sensor for the selective detection of cyanide ion in aqueous medium

Article abstract of DOI:10.1016/j.reactfunctpolym.2019.01.018

Porphyrin cored polyepichlorohydrin (POR-PECH) dendritic polymer was identified as an efficient turn-off fluorescence probe for selective determination of cyanide ion in aqueous medium. The dendritic polymer showed strong red fluorescence at 656 and 718 n

Full text of DOI:10.1016/j.reactfunctpolym.2019.01.018

ReactiveandFunctionalPolymers137(2019)71–78
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
A novel dendritic polymer based turn- off fluorescence sensor for the
selective detection of cyanide ion in aqueous medium
Avudaiappan G., Anjaly Jacob K., Letcy V. Theresa, Shebitha A.M., Hiba K., Priya K. Shenoi,
Unnikrishnan V., Sreekumar K.⁎
Department of Applied Chemistry, Cochin University of Science and Technology, Kochi 682022, Kerala, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Porphyrin
POR-PECH Dendritic polymer
Fluorescence quenching
Sensor
Porphyrin cored polyepichlorohydrin (POR-PECH) dendritic polymer was identified as an efficient turn-off
fluorescence probe for selective determination of cyanide ion in aqueous medium. The dendritic polymer showed
strong red fluorescence at 656 and 718 nm which was quenched in the presence of cyanide ion. POR-PECH
showed high selectivity towards cyanide ion over other relevant anions including Cl−, Br−, F−, I−, NO2
,
−
NO3−, CH3COO−, SO32−, CO32−, SCN−, HCO3−, HPO42−, H2PO4− etc. The proposed sensor is highly selective,
sensitive which showed quick response towards cyanide ion. The limit of detection of proposed method was
found to be 1.2 μM which was admirably lower than the maximum allowed cyanide content in drinking water
(1.9 μM) as recommended by WHO. Back folding of self assembled porphyrin cored PECH dendritic chains by the
addition of cyanide ion caused the quenching of fluorescence. The time resolved fluorescence life time, FE-SEM
image and DFT studies supported the quenching mechanism.
Cyanide ion
1. Introduction
momentum [25–27].
Dendritic polymers, recognized as the fourth major class of macro-
Cyanides are fast acting poisons that can lead to death [1–3]. They
were used as chemical weapons during first World War [4,5]. Traces of
cyanide compounds are found in nature and even in some food products
such as spinach, tapioca etc. [6–8]. Cigarette smoke and vehicle exhaust
are the major sources of cyanide pollution in atmosphere [9]. Cyanide
containing compounds are used in electroplating, pesticides, plastics,
and metallurgical and dye industries [10–14]. Waste water from iron,
gold, steel production, and chemical industries can create huge cyanide
pollution in aqueous media [15–18].
Due to the aforementioned industrial activities, the concentration of
cyanide ion is increasing at an alarming rate in the atmosphere (soil,
air, water) which can adversely affect the existence of all life forms
[19]. Large dosage of cyanide causes oxygen deficiency and leads to the
cell death. The heart, central nervous system (CNS) and respiratory
system are severely affected by cyanide poisoning [20–22]. Hence the
release of poisonous cyanide compounds from the industries and other
sources into the atmosphere needs to be identified in an effective and
quantitative manner so that the hazards due to the cyanide poisoning
can be minimized to a great extent. Here the role of chemo sensor is
important [23,24]. Currently, research on dendritic polymer based
chemo sensor for versatile targets and analyte molecules is gaining
molecular architecture with controlled structure, were designed and
synthesized by Fritz Volgtle et al., in 1978. Dendritic polymers are
characterized by their unique structural characters like nanoscopic size,
spheroidal surface, high branching, cavernous interior etc. The exciting
properties like low viscosity, high solubility, high reactivity, in agree-
ment with the high functionalities of the dendritic polymers optional
that they have broad number of prospective applications in different
fields [28]. The integrated medicinal and diagnostic applications make
them an indispensible candidate for gene therapy, chemical sensors,
drug delivery systems, adhesives and coatings, light harvesting material
and catalyst, electronic applications, separating agents and many others
[29,30]. Dendritic polymers hold potential application as chemo sen-
sors and have been extensively employed in clinical fields [31–33].
The present work is focused on the development of porphyrin cored
polyepichlorohydrin (POR-PECH) dendritic polymers and evaluation of
their sensing properties for the effective detection of Cyanide ion. Due
to their vibrant photophysical and biochemical properties, porphyrins
are selected as the core of dendritic polymer. After an extensive search
for water soluble porphyrin cored dendritic polymer, it was found that
polyepichlorohydrin dendrimers were more water soluble and was easy
to be synthesized [34,35].
⁎ Corresponding author.
E-mail address: kskpolymer.cusat@gmail.com (K. Sreekumar).
https://doi.org/10.1016/j.reactfunctpolym.2019.01.018
Received 25 October 2018; Received in revised form 24 January 2019; Accepted 27 January 2019
Availableonline29January2019
1381-5148/©2019ElsevierB.V.Allrightsreserved.

G. Avudaiappan et al.
ReactiveandFunctionalPolymers137(2019)71–78
2. Experimental section
dropping funnel under nitrogen atmosphere. The reaction mixture was
stirred for another 3 h. 50 mL of methanol was added to the reaction
mixture and vigorously stirred for about 30 min. The crude reaction
product was evaporated to dryness. The dried residue was purified by
soxhlet with hexane in order to remove all oligomers and unreacted
monomers. The nature of POR-PECH was confirmed by UV–Vis, IR, GPC
and 1H NMR spectroscopy. Yield: 80%, IR (cm−1): 3439, 2922, 1430,
1122, 747; 1H NMR (400 MHz, CDCl3): δ = 8.93(s, 4H), 7.52 (s, 8H),
7.46 (s, 4H), 3.98 (t, 37H), 3.84(s, 140H), 3.69 (t, 961H), 3.61 (t,
764H), 1.25 (s, 133H), 0.87 (t, 51H)·13C NMR (400 MHz, CDCl3):
δ = 114.4, 112.7, 108.1, 98.8, 79, 71.5, 70.8, 69.7, 69.4, 63, 54.3, 43.6.
GPC Mass (Mn): 13335, PDI: 1.26.
2.1. Materials and methods
All the solvents were purified according to the standard procedures.
All other chemicals were used as received. TRIS buffer was purchased
from sigma Aldrich, TLC was done on silica coated alumina plates. FTIR
spectra were recorded on JASCO model 4100 FTIR spectrometer as KBr
pellets. 1H and 13C NMR spectra were recorded on Bruker 400 MHz
instrument with TMS as internal standard in DMSO/CDCl3 (SAIF,
CUSAT). Salts for interference studies were purchased from Merck and
used without further purification. Molecular mass of synthesized den-
dritic polymer was determined using i-Series Plus Integrated GPC in-
strument, with PLgel 5 μm 10E4Å column. UV–Visible spectra were
recorded on Thermo Scientific Evolution 201 UV–Visible spectro-
photometer. Fluorescence emission and Time resolved fluorescence
lifetime measurements were done using Horiba Fluorolog-3
Spectrofluorometer (Time- correlated single photon counting system) at
455 nm excited wavelength. Theoretical Band Gap of POR-PECH was
calculated using Gaussian09 suite of quantum chemical programs em-
ploying M06-2× functional with 6-31G* basis set for all atoms.
2.5. General procedure for fluorescence spectra measurements
1.5 mL of 10 μM aqueous solution of porphyrin cored PECH was
added with TRIS buffer (pH = 9), gradually titrated against standard
cyanide ion solution. The change in fluorescence intensity was recorded
periodically. The solution was equilibrated for 12 min before the
spectral measurements.
3. Results and discussion
2.2. Synthesis of 5,10,15,20-tetrakis(3,4,5-trimethoxyphenyl) porphyrin
(porphyrin 1)
The synthesis of porphyrin cored PECH dendritic polymer is illu-
strated in Scheme 1. Ring opening polymerisation of epichlorohydrin
initiated by 5,10,15,20-tetrakis(3,4,5-trihydroxyphenyl)porphyrin in
the presence of BF3 etherate gave porphyrin appended PECH. The
synthesis of 5,10,15,20-tetrakis(3,4,5-trimethoxyphenyl)porphyrin was
achieved by the two step reaction involving the preparation of
5,10,15,20-tetrakis(3,4,5-trimethoxyphenyl) porphyrin by the con-
densation of 3,4,5-trimethoxybenzaldehyde and pyrrole followed by the
demethylation of 5,10,15,20-tetrakis(3,4,5-trimethoxyphenyl)por-
phyrin to 5,10,15,20-tetrakis(3,4,5-trihydroxyphenyl)porphyrin. The
synthesized dendritic polymer was characterized by UV–Vis, IR, and 1H
and 13C NMR spectra, as well as GPC.
47 mL of propanoic acid and 47 mL of valeric acid were taken in a
three necked RB flask. The mixture was stirred for about 30 min. 3,4,5-
trimethoxy benzaldehyde (2.354 g, 12 mmol) was dissolved in 10 mL of
propanoic acid and 10 mL of valeric acid, distilled pyrrole (0.805 g) in
3-nitrotoluene (6 mL) was added to the reaction mixture with constant
stirring to the acid solution using the dropping funnel. The reaction
mixture was refluxed at 60 °C for 2 h. The crude black colored solution
was filtered and washed with hot water and methanol. The residue was
purified by silica gel column chromatography using dichloromethane
and hexane as eluent to obtain bright purple colored crystals. Yield:
1.16 g, (40%). M.P: 128 °C, LC-MS(M−): 974.55 IR (cm−1): 3417, 2833,
1580, 1124, 722; 1H NMR (400 MHz, CDCl3): δ = 3.96(s, 24H), 4.18 (s,
12H), 7.47 (s, 8H), 8.95 (s, 8H). 13C NMR (400 MHz, CDCl3): δ = 56.4,
61.3, 112.9, 120, 137.5, 138, 151.4.
3.1. Fluorescence quenching of POR-PECH by the addition of CN− ion
The synthesized POR-PECH dendritic polymer showed strong ab-
sorption at 423 nm (soret band) and emission at 656 nm and 718 nm
which is the characteristic property of porphyrin nucleus as shown in
Fig.1 [36,37]. The water solubility of the POR-PECH can be ascribed to
the presence of large number of hydroxyl groups in its periphery. In
water, this probe depicted strong red fluorescence which was quenched
by the addition of cyanide ion as shown in the Fig.2.
2.3. Synthesis of 5,10,15,20-tetrakis(3,4,5-trihydroxyphenyl) porphyrin
(porphyrin 2)
Boron tribromide (1 mL) was added to dry distilled methylene
dichloride and the mixture was cooled in an acetone-solid CO2 bath to
−78 °C.
5,10,15,20-tetrakis(3,4,5-trimethoxyphenyl)
porphyrin
(2.925 g, 3 mmol) in the minimum volume of dry distilled di-
chloromethane was placed in a dropping funnel and was slowly added
in 20 min. The mixture was stirred for 1 h at −78 °C and then allowed
to come to room temperature with stirring over 24 h. Triethylamine was
added to neutralise the mixture which was then evaporated to dryness.
The crude product was transferred to a separating funnel and the or-
ganic layer was separate using ethyl acetate. The organic layer was
washed several times with water. The ethyl acetate layer was separated,
dried, filtered and evaporated to dryness. Purification by column
chromatography (Al2O3, CH3OH as the eluent) afforded pure TMPP as
purple solid. M.P: 115 °C, Yield: 2.29 g (95%). IR (cm−1): 3433, 2200,
2600, 1457.
3.2. Optimisation of pH
With this interesting result in hand, the effect of change in pH in the
quenching process was studied. In general, free base porphyrin shows
intense emission around 650 and 710 nm. In acidic medium, due to the
protonation of the nitrogen atoms, it forms porphyrin monocation and
porphyrin dication,(Scheme.2) which emit at 745 nm [38]. So in acidic
medium, the emission is red shifted, which is depicted in the Fig.3.
Thus, the photophysical properties of porphyrin cored PECH was pH
dependent, the emission was red shifted to 745 nm in acidic medium
[39,40]. However in acidic pH, the emission at 745 nm does not show
any characteristic response with the cyanide ion. Similarly the experi-
mental data showed that the dendritic back folding is also a pH de-
pendent process [41]. At basic pH, the backfolding of dendritic chains
takesplace which result in quenching [42]. Hence it is important to
optimize the pH for effective sensing of an analyte. For the pH study,
different buffers were tested and TRIS buffer was selected as the most
suitable buffer. The results indicated that at pH 8.8–9.2, the fluores-
cence quenching efficiency was maximum.
2.4. Synthesis of polyepichlorohydrin with porphyrin core (POR-PECH)
Epichlorohydrin (39.2 mL, 500 mmol) was added to the reaction
mixture containing dichloromethane (10 mL) and 5,10,15,20-tetrakis
(3,4,5-trihydroxyphenyl) porphyrin (4.03 g, 5 mmol). The reaction
mixture was stirred for about 15 min in order to ensure homogeneity of
the mixture. 8.5 mL of BF3–etherate (60 mmol) was added using
72

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ReactiveandFunctionalPolymers137(2019)71–78
Scheme 1. Synthesis of porphyrin cored polyepichlorohydrin dendritic polymer.
Fig. 1. Absorbance (Red) and Fluorescence (blue) spectra of POR-PECH
(10 μM) in water. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
Fig. 2. Fluorescence spectra of POR-PECH (10 μM) before and after addition of
CN− ion (10 equiv.) in water.
3.4. Fluorescence spectral response to cyanide ion concentration
3.3. Optimisation of time
The effect of concentration of cyanide ion in the quenching process
was studied. 0.1 μM solution of KCN was prepared and the fluorescence
titrations were conducted. The results are displayed in the Fig.5. It re-
vealed the concentration dependent nature of the fluorescence
quenching process. The addition of cyanide ion to POR-PECH resulted
in the quenching of its fluorescence. After addition of 28 equiv. of cy-
anide ion, the fluorescence signal reached a plateau. The variation in
intensity of fluorescence went in a linear fashion with the cyanide ion
concentration enabling its service as a quantitative sensor for de-
termining the concentration of CN− in unknown samples. (Fig.6)
To analyse the time for response of the probe with cyanide ion and
the quenching process, fluorescence spectra were recorded every
minute periodically after addition of cyanide ion. The results are given
in the Fig.4. After 12 min the stable fluorescence spectrum was ob-
tained, and it was constant upto one hour. It was inferred that 12 min
incubation was needed after addition of cyanide ion. So all the fluor-
escence spectra were recorded after 12 min after the addition of cyanide
ion.
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ReactiveandFunctionalPolymers137(2019)71–78
Scheme 2. Schematic representation of porphyrin and porphyrin cations.
Fig. 3. Fluorescence spectra of POR-PECH in basic medium (red) and acidic
medium (green). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
Fig. 5. Fluorescence spectra of POR-PECH in water (10 μM, TRIS buffer, pH 9,
12 min incubation time) with various concentration of cyanide ion.
Fig. 4. Fluorescence intensity (peak at 656 nm) of POR-PECH (10 μM) before
and after addition of CN− ion (10 equiv.) in water as a function of time.
Fig. 6. Stern Volmer plot (POR-PECH 10 μM, TRIS buffer, pH 9, 12 min in-
[Linear Equation: y = 0.991 + 0.051×, R = 0.976, σ = 0.200,
k = 0.051]. The Limit of Detection(LOD) of the probe to cyanide ion
was calculated using the formula 3σ/K and was found to be 1.2 μM
[43,44]. It was much lower than the maximum permissible cyanide ion
content (1.9 μM) in drinking water recommended by WHO [45]. So this
method is suitable for quantifying the cyanide ion in real water sam-
ples.
cubation time) with various concentration of cyanide ion.
Hence life time measurement was used to investigate the stability of
excited molecule and various modes of de-excitation that occur fol-
lowing photo-excitation. Generally, one can distinguish two cases of
quenching, static and dynamic quenching. Static quenching is assumed
to result from the formation of a nonfluorescent quencher-fluorophore
complex in the ground state [46]. So there is no energy transfer in the
excited state which resulted in the unaltered fluorescence lifetimes
before and after quenching. In dynamic quenching, there is a collision
between the fluorophore and the quencher molecule in the excited
state, which resulted in the change in fluorescence lifetimes [47]. Evi-
dently in the present case the fluorescence lifetime of the POR-PECH
was decreased after the addition of cyanide ion which indicated the
3.5. Time-resolved fluorescence lifetime measurements
Fluorescence emission of porphyrin cored PECH was recorded with
photo excitation at 455 nm. Fluorescence quenching mechanisms
cannot be determined from fluorescence intensity measurements alone.
But it is possible with the help of fluorescence lifetime measurements.
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ReactiveandFunctionalPolymers137(2019)71–78
Fig. 7. Time-resolved fluorescence lifetime decay profile of POR-PECH (10 μM,
TRIS buffer, pH 9, 12 min incubation time) with various concentration of cya-
nide ion.
Fig. 10. Frontier molecular orbitals of POR-PECH obtained from the DFT cal-
culations using Gaussian 09 program.
Fig. 8. Fluorescence spectra of POR-PECH (10 μM, TRIS buffer, pH 9, 12 min
incubation time) with 100 equiv. of various anions.
Fig. 9. Fluorescence intensity response of POR-PECH (10 μM, TRIS buffer, pH 9,
12 min incubation time) before and after adding CN− ion (10 equiv.) with 100
equiv. of various anions.
dynamic nature of quenching [48]. The average life time of PECH was
8.6 ns, which was decreased by the addition of cyanide ions (Fig.7).
3.6. Selectivity and interference studies
To analyse the selectivity of the POR-PECH dendritic polymer, the
Fig. 11. Optimized structure of POR-PECH obtained from the DFT calculations
fluorescence spectrum of POR-PECH dendritic polymer with the addi-
using Gaussian 09 program.
tion of different anions including F−, Cl−, Br−, I−, NO2−, NO3−, N3
,
,
−
−
SCN−, CO32−, HCO3−, SO42−, HSO4−, H2PO4−, HPO42−, ClO4
CH3COO– (100 equiv. of each ion) were recorded, and it was found that
no anions other than cyanide quenched the fluorescence spectrum of
75

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ReactiveandFunctionalPolymers137(2019)71–78
Fig. 12. Proposed mechanism of sensing.
Fig. 13. FE-SEM image of POR-PECH; (A) before addition of CN− ion (B) FE-SEM image of POR-PECH after addition of CN− ion.
Table 1
Determination of cyanide ion concentration in real water samples.
Sl. No.
Sample name
Amount of CN− spiked (μM)
Amount of CN− detected by POR-PECH (μM)
Recovery (%)
1
2
3
4
5
Sample 1
Sample 2
Sample 3
1.5
2
25
0
1.41
1.85
25.21
6.39
0
94
92.5
100.8
–
Waste water (from electro plating industry)
Distilled water
0
–
POR-PECH 10 μM, TRIS buffer, pH 9, 12 min incubation time.
POR-PECH dendritic polymer. Even SCN− ion did not quench the
fluorescence of the dendritic polymer. The results are shown in Fig.8.
This result underlined the selective nature of the interaction of den-
dritic polymer towards cyanide ion. For the interference studies the
fluorescence spectrum of POR-PECH + CN− was recorded with the
addition of all the above anions individually and in a mixture. No
characteristic interference was observed even under this condition. The
results supported the selectivity of the porphyrin cored PECH for the
detection of cyanide ion (Fig.9).
the POR-PECH are given in the Figs.10 and 11.
3.8. Mechanism of sensing process
On the basis of previous reports [49–52], the sensing mechanism of
the dendritic polymers was proposed. With the addition of cyanide ion
to POR-PECH, a negative charge interface was developed on the surface
of the POR-PECH, owing to the interaction of cyanide ion with the
hydroxyl group of the long polymer chain. This leads to the back
folding of dendritic chains, resulting in interrupting the electronic flow
of the porphyrin system which is the root cause for quenching of
emission. (See Fig. 12) The back folding is taking place in the exited
state, so that there is no reasonable change in the absorbance spectra.
The excited state interaction was further confirmed by the change in
fluorescence lifetime (Fig. 7). The back folding leads to the change in
orderly arranged morphology of the POR-PECH, which was confirmed
by the FE-SEM. The FE-SEM images of POR-PECH shows a particle
3.7. DFT studies
The geometry optimization of synthesized dendritic polymer was
performed using Gaussian09 suite of quantum chemical codes em-
ploying M06-2× functional with 6-31G* basis set for all atoms. The
HOMO and LUMO of the POR-PECH was calculated. The band gap
turned out to be 4.1 eV. The HOMO, LUMO and optimized structure of
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ReactiveandFunctionalPolymers137(2019)71–78
arrangement, which was destructed by the addition of cyanide ion as
given in Fig. 13(A) and (B).
The interaction of cyanide ion with the hydroxyl group of the POR-
PECH was confirmed by the methylation of hydroxyl group, as the
methylated POR-PECH did not show any quenching effect with cyanide
ion.
Similar experiments were conducted using glycerol cored PECH and
5,10,15,20-tetrakis(3,4,5-trihydroxyphenyl)porphyrin as probes, but
both of them failed to sense the target analyte. These results confirmed
the significance of porphyrin cored dendritic chain in the quenching
mechanism.
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3.9. Application to quantify CN− ion in real water sample
The porphyrin cored PECH sensor was applied to detect CN− ion in
NaCN spiked real water samples and waste water from electroplating
industry. Each sample was tested three times, and the estimated con-
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4. Conclusion
A novel, porphyrin cored polyepichlorohydrin (POR-PECH) based
fluorescence sensor for selective detection of cyanide ion in aqueous
solution was reported. It offered high selectivity due to the unique host-
guest interaction between cyanide ion and POR-PECH. The back folding
of dendritic chain was confirmed by FE-SEM image, UV–Vis spectro-
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for the POR-PECH was found to be 1.2 μM, which is lower than the
maximum allowed cyanide content in drinking water (1.9 μM) and this
highlights the merit of the prepared sensor. This method was found to
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The authors gratefully acknowledge University Grand commission
(UGC), New Delhi, India, Kerala State Council for Science, Technology
and Environment (KSCSTE), Kerala, India and Cochin University of
Science and Technology, Kerala, India for the financial support.
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