Copper ion sensing with fluorescent electrospun nanofibers

Article abstract of DOI:10.1016/j.saa.2012.01.042

In this work, the use of electrospun nanofibrous materials as highly responsive fluorescence quenching-based copper sensitive chemosensor is reported. Poly(methyl methacrylate) and ethyl cellulose were used as polymeric support materials. Sensing slides were fabricated by electrospinning technique. Copper sensors based on the change in the fluorescence signal intensity of fluoroionophore; N′-3-(4-(dimethylamino phenly)allylidene) isonicotinohydrazide. The sensor slides exhibited high sensitivities due to the high surface area of the nanofibrous membrane structures. The preliminary results of Stern-Volmer analysis show that the sensitivities of electrospun nanofibrous membranes to detect Cu(II) ions are 6-20-fold higher than those of the continuous thin films. By this way we obtained linear calibration plots for Cu(II) ions in the concentration range of 10^-12-10^-5 M. The response times of the sensing slides were less than 1 min. Stability of the employed ionophore in the matrix materials was excellent and when stored in the ambient air of the laboratory there was no significant drift in signal intensity after 6 months. Our stability tests are still in progress.

Full text of DOI:10.1016/j.saa.2012.01.042

Spectrochimica Acta Part A 90 (2012) 177–185
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Copper ion sensing with fluorescent electrospun nanofibers
Merve Zeyrek Onguna,∗, Kadriye Ertekinb,d, Mustafa Gocmenturka, Yavuz Ergunb, Aslıhan Suslua,c
a University of Dokuz Eylul, The Graduate School of Natural and Applied Sciences, Department of Chemistry, 35160 Izmir, Turkey
b University of Dokuz Eylul, Faculty of Arts and Sciences, Department of Chemistry, 35160 Izmir, Turkey
c University of Dokuz Eylul, Faculty Engineering, Department of Metallurgical and Materials Engineering, 35160 Izmir, Turkey
d University of Dokuz Eylul, Center for Fabrication and Application of Electronic Materials (EMUM), 35160 Izmir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, the use of electrospun nanofibrous materials as highly responsive fluorescence quenching-
based copper sensitive chemosensor is reported. Poly(methyl methacrylate) and ethyl cellulose were used
as polymeric support materials. Sensing slides were fabricated by electrospinning technique. Copper sen-
sors based on the change in the fluorescence signal intensity of fluoroionophore; Nꢀ-3-(4-(dimethylamino
phenly)allylidene)isonicotinohydrazide. The sensor slides exhibited high sensitivities due to the high sur-
face area of the nanofibrous membrane structures. The preliminary results of Stern–Volmer analysis show
that the sensitivities of electrospun nanofibrous membranes to detect Cu(II) ions are 6–20-fold higher
than those of the continuous thin films. By this way we obtained linear calibration plots for Cu(II) ions in
the concentration range of 10−12–10−5 M. The response times of the sensing slides were less than 1 min.
Stability of the employed ionophore in the matrix materials was excellent and when stored in the ambi-
ent air of the laboratory there was no significant drift in signal intensity after 6 months. Our stability tests
are still in progress.
Received 15 August 2011
Received in revised form 2 January 2012
Accepted 18 January 2012
Keywords:
Electrospinning
Electrospun nanofiber
Optical copper sensor
Fiber optic chemical sensor
Fluorescence
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
researchers mainly used the chemosensing agent in embedded
form [6–9]. In most of these designs optical chemical sensing of
Copper is the third most abundant element in the human body
and is essential in several biological pathways including electron
transport, oxygen metabolism and enzymatic catalysis. Copper is
on one side essential for life but on the other hand, depending on
the dose, it may be highly toxic for organism. A high level of Cu(II)
concentration is not necessary to produce toxicity in the human
body. Over time the heavy metal can reach toxic concentration lev-
els. Therefore, monitoring of trace amounts of Cu(II) levels in water
and food samples is quite important. In recent years a number of
chemosensors detecting Cu(II) ions either in biologically relevant
species or in aqueous solutions were proposed [1–11].
In these sensing approaches absorption and/or emission
responses of the chromophore molecules resulting from interaction
with Cu(II) ions were recorded as analytical signal.
These investigations provided valuable information for
chemosensing of copper ions. Sometimes, indicators proposed
for detection of Cu(II) exhibited weak responses to other cations
within stated tolerance limits [5]. Over the last two decades
Cu(II) ions is mainly performed with a chromoinophore immo-
bilized in polymer membranes such as polyvinylchloride (PVC)
[6–8] or ethyl cellulose (EC) [9] in thin film form. That kind of
sensing films display analyte dependent optical properties such
as absorbance, fluorescence or reflectance. Mostly, such sensors
provided enhanced stability, reversibility and sensitivity with
respect to the solution based sensing. However, development of
new technologies toward highly sensitive detection techniques is
still very important for the researchers working in the field of ana-
lytical chemistry. In Table 1 the recently published luminescence
methods for Cu(II) determination were compared in terms of their
working ranges, Limit of Detections (LODs), sensing agents and
matrix materials with the offered work.
Sumner et al., used DsRed and the reference dye Alexa Fluor 488
in a polyacrylamide matrix and produced spherical, ratiometric,
80 nm-sized sensors. They detected copper ions, in the nanomo-
lar (ppb) regime, in biologically relevant samples [12]. Wang et al.
compared performance of thin films and nanomaterials in terms
of sensitivity, and revealed the advantage of nanoscale membranes
over continuous thin films [13]. In spite of these few examples,
usage of nanomaterials in optical chemical sensing is very rare. In
the light of the above discussion, usage of polymeric nanomaterials
is expected to offer good results in comparison to the sensing films
prepared with the same composition.
∗
Corresponding author at: University of Dokuz Eylul, Izmir Vocational School,
Department of Chemistry Technology, 35160 Izmir, Turkey. Tel.: +90 232 440 0707
179.
E-mail addresses: merve.zeyrek@deu.edu.tr, mervezeyrek@gmail.com
(M.Z. Ongun).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.01.042

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M.Z. Ongun et al. / Spectrochimica Acta Part A 90 (2012) 177–185
Table 1
Recently published emission based methods for copper ion sensing.
Ref.
Reagent or matrix
Method
Working range
LOD
[1]
[3]
Luminol derivative, alkaline aqueous media
CdS quantum dots, chitosan
Emission/410 nm
Emission/400–700 nm
0–500 nmol L−1
8.0 nmol L−1 to
3.0 ␮mol L−1
0.27 nmol L−1
1.2 nmol L−1
[5]
[6]
[7]
4,5-Dihydroxy-1,3-benzenedisulfonic acid, Water
Emission/350 nm
Absorption/420 nm
Emission/491 nm
5.0 × 10−7 to
3.83 × 10−7
M
1.0 × 10−5
M
N,N-Bis(salycilidene)-1,2-phenylenediamine, PVC
5.01 × 10−8 to
6.32 × 10−4 mol L−1
5.0 × 10−8
4.7 × 10−8 mol L−1
4-(1-Phenyl-1-methylcyclobutyl-3-yl)-2-(2-hydroxy-1-
naphtylidene) thiazole,
PVC
2.6 × 10−8
M
to1.0 × 10−4
M
[8]
[9]
1-Hydroxy-3,4-dimethyl-9H-thioxanthen-9-one, PVC
Absorption/410–480 nm 1.6 × 10−7 to
4.0 × 10−7
M
1.3 × 10−2
M
Bis(naphtho[2,1-b]furan-2-yl)methanone semicarbazone,
PVC and EC
Emission/429 and
424 nm for PVC and
EC
1.0 × 10−9 to
4.3 ppb
3.0 × 10−4
M
[10]
Terbium-[1-methyl-4-hydroxy-3-(N-2-ethyl-5-
aminothiadiazolyl-)-carbamoyl-quinoline-2-one],
Aqueous Buffer
Functional CdS nanoparticles, Aqueous Buffer
DsRed and the reference dye Alexa Fluor 488,
polyacrylamide matrix by a microemulsion polymerization
process
Emission/547 nm
10–300 nmol L−1
4.3 nmol L−1
[11]
[12]
Emission/460 nm
Ratiometric
Emission/DsRed
583 nm
0.09–27.0 ␮g L−1
Sensitive to Cu(II)
ppb levels
3.2 ng L−1
Not defined
Alexa Fluor 520 nm
Emission/535 and
490 nm for PMMA
and EC respectively
DPAINH (This work)
Nꢀ-3-(4-(dimethylamino phenly)
allylidene)isonicotinohydrazide, PMMA and/or EC
nanofibers
1.0 × 10−12 to
1.0 × 10−3 M for EC
1.0 × 10−12 to
1.0 × 10−2 M for
PMMA
3.8 × 10−14 M for
EC
1.4 × 10−13 M for
PMMA
In this report, we have successfully combined nanoscale elec-
trospun fiber materials with optical sensing technology for Cu(II)
ion detection. We conceive of making electrospun fibers from a
relatively novel solvent system containing room temperature ionic
liquids (RTILs). Here we have demonstrated that it is possible to
electrospun the EC and PMMA based composite fibers in presence
of non-volatile RTIL; EMIMBF4.
The employed RTIL was chosen as polymer electrolyte due to
its high ionic conductivity, low vapor pressure, low melting point,
high thermal stability, low toxicity and recyclability [14,15]. The
successful application of RTILs in electrospinning could enhance
the number and types of materials from which the fibers can be
made.
On the other hand; electrospinnig has recently gained much
attention as a unique technique to fabricate high surface area and
highly responsive nanofibrous materials [16–20]. In electrospin-
ning, when a sufficiently large electric voltage is applied to the
precursor solution in a capillary, it becomes stretch to form a cone
because of coupled effects of the electrostatic repulsion within the
charge droplet and attraction to a grounded electrode of opposite
polarity. As the strength of the electric field is increased the charge
overcomes the surface tension and at a critical voltage fine jet is
ejected from the tip of the cone. These jets dry to form very thin
polymer fibers, which are collected on a substrate as nanofibers. The
diameters of electrospun fibers can range from several microme-
ters to nanometers depending on the nature of precursor polymer
and other experimental conditions; such as temperature, flow rate,
voltage, the distance between precursor solution and substrate
and humidity. As a result, electrospun nanofibrous membranes can
have a large surface area. It is expected that this large amount of
functional surface area has the potential to provide unusually high
sensitivity and fast response in sensing applications.
as the Cu(II) selective fluoroinophore. The electrospun nanofibers
were characterized using scanning electron microscopy (SEM) and
their average diameters were evaluated.
2. Experimental
The polymers ethyl cellulose (EC) and poly(methyl methacry-
late) (PMMA) were from Acros and Aldrich, respectively. The
plasticizer, dioctyl phthalate (DOP) was from Aldrich. The ionic
liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4)
was supplied from Fluka.
Potassium tetrakis-(4-chlorophenyl) borate (PTCPB) was
from Aldrich. Absolute ethanol (EtOH), tetrahydrofuran (THF),
dichloromethane (DCM), dimethylformamide (DMF) and
chloroform (CH3Cl) were of analytical grade. Solvents for the
spectroscopic studies were used without further purification.
Buffer components; acetic acid/acetate, phosphate and metal
salts were of reagent grade (Merck and Fluka). Millipore ultra pure
water was used throughout the studies. The 1000 ppm Atomic
Absorption Spectroscopy (AAS) certified reference standard solu-
tion of Cu(II) was from Merck. Other metal cations were prepared
from respective metal nitrates, or chlorides and diluted with
5.0 × 10−3 M buffers to demanded pH. All of the experiments were
carried out at room temperature; 25 1 ◦C utilizing reagent grade
chemicals.
Schematic structures of the exploited Cu(II) sensitive
fluoroinophore;
Nꢀ-3-(4-(dimethylamino
phenly)allylidene)isonicotinohydrazide and EMIMBF4 were
shown in Fig. 1. Dye was prepared in one step by the condensation
of isoniazide (1 mmol) and 4-dimethylamino cinnamaldehyde
(1 mmol) in the presence of glacial acetic acid in ethanol under
reflux conditions according to literature information [21]. The
following experimental data identifies chemical structure of the
fluorescent copper selective chromoionophore molecule. m.p.:
252 ◦C IR(KBr): 3302 (NH), 1664 (C O), 1595 (C N) 1H NMR
(400 MHz, CDCl3): ı 3.01 (s, 6H, N(CH3)2), 6.67 (d, 2H, J = 7.2 Hz,
ArH), 6.81–6.91 (m, 1H, CH), 7.36 (d, 2H, J = 7.4 Hz, ArH), 7.67
In this paper matrix materials of poly(methyl methacry-
late) and ethyl cellulose were used together with ionic liquids.
Optical nano-fibrous membrane chemical sensors were fabri-
cated by electrospinning technique. The Nꢀ-3-(4-(dimethylamino
phenly)allylidene)isonicotinohydrazide dye (DPAINH) was chosen

M.Z. Ongun et al. / Spectrochimica Acta Part A 90 (2012) 177–185
179
Fig. 1. Structures of the Cu2+ sensitive fluoroinophore; Nꢀ-3-(4-(dimethylamino phenly)allylidene)isonicotinohydrazide; DPAINH (I) and 1-ethyl-3-methylimidazolium
tetrafluoroborate; EMIMBF4 (II).
(d, 2H, J = 7.8 Hz, ArH), 7.75–7.82 (m, 1H, CH), 7.97–8.01 (m, 1H,
CH), 8.74 (d, 2H, J = 7.6 Hz, ArH), 8.92 (bs, 1H, NH).
the aluminum substrate. SEM images of electrospun membranes of
the EC and PMMA at various magnifications are shown in Fig. 3. It
is observed that the membrane has a 3-D structure with a random
fiber orientation that is evenly distributed on the substrate. Aver-
age fiber diameters were calculated as 536 76 and 360 82 nm
for PMMA and EC based electrospun fibers, respectively. When
conditions of the electrospinning process such as solvent composi-
tion, viscosity, concentration, temperature, humidity and working
distance were kept constant reproducibility in the preparation of
electrospun membranes was quite good.
This type of nanostructure provides higher surface area with
respect to continuous thin films.
However further enhancement of the surface area may be
achieved by changing the experimental conditions of the electro-
spinning process which results in either smaller diameter fibers or
increased porosity at the fiber surface [22].
N
2.1. Apparatus and instruments
The homogeneous PMMA or EC solutions were placed in a 10 mL
plastic syringe fitted with a metallic needle of 0.4 mm of inner
diameter. The syringe is fixed vertically on the syringe pump (Top
Syringe Pump Top-5300) and the electrode of the high voltage
power supply (Gamma High Voltage ES30) was clamped to the tip
of the metallic needle. Schematic structure of the electrospinning
apparatus was shown in Fig. 2. Absorption spectra were recorded
with a Shimadzu UV-1601 UV-visible spectrophotometer. Steady
state fluorescence emission and excitation spectra were measured
using Varian Cary Eclipse Spectrofluorometer.
2.2. Preparation of electrospun nanofibers
2.3. Thin film preparation procedures
Electrospinning was used to fabricate PMMA or EC based contin-
uous nanofibers. Experimental conditions were optimized to form
bead-free nanofibers varying concentrations of plasticizer, PMMA
or EC and RTIL in the precursor polymer solution. The concentra-
tion of RTIL was varied from 10% up to 25% (w/w) with respect to
the PMMA or EC content. We find that the presence of the RTIL in
the polymer solutions facilitates the electrospinning of bead-free
nanofibers from the lower polymer concentrations. This result can
be attributed to the ionic conductivity and proper viscosity of the
RTIL doped precursor polymer solutions.
The thin films were prepared employing the same composition
explained for the electrospun nanofibers. The resulting mixtures
were spread onto a 125 ␮m polyester support (Mylar TM type) with
a spreading device. Thickness of the films was measured using Ten-
cor Alpha Step 500 Prophylometer and found to be 5.11 0.081 ␮m
(n = 8). Each sensing film was cut to 1.2 cm diameter, fixed in the
cuvette or flow cell, and the excitation and/or emission spectra
were recorded.
Metal response measurements were carried out with fiber optic
probe (2 m long) and solid sample tip accessories constructed on the
spectrofluorometer. For instrumental control, data acquisition and
processing, the software package of the spectrofluorometer was
used. The tip of the bifurcated fiber optic probe was interfaced with
a sensing film in a buffer containing homemade 300 ␮L flow cell.
The flow cell was equipped with a four channel Ismatec Reglo Ana-
log peristaltic pump. Flow rate of the peristaltic pump was kept
at 2.4 mL min−1. Analyte solutions or buffers were transported via
tygon tubing of 2.06 mm i.d.
Analysis at the sub-nanomolar level is possible only by exploit-
ing ultra pure reagents and careful tracking of blanks. In this work
reagent grade chemicals and polyethylene or Teflon containers
were used. Stable Cu(II) standard was obtained commercially. Loss
of significant amounts of Cu(II) by adsorption onto container walls
can cause serious problems for quantification of copper in glass
containers. Therefore we treated the polyethylene vessels with dis-
tilled deionized ultrapure water to equilibrate for 2 days prior to
the solution preparation.
The resulting composites were prepared by mixing 240 mg of
polymer (PMMA or EC), 144 mg of plasticizer (DOP) and 96 mg of
RTIL. 1.5 mg of dye were dissolved in CH3Cl:EtOH (75:25) or THF
solvent systems. Then, added into the polymeric precursor solution
under magnetic stirring.
The polymer solution was taken in a hypodermic syringe and
an electric potential of 25 kV was applied between the needle
and the substrate in the form of an aluminum sheet. The distance
between the needle and the electrode was 10 cm. The solution flow
rate was maintained at 1 mL/h. When high voltage is applied the
charged polymer solution overcame the surface tension of the vis-
cous liquid and a stream of polymer jet was produced. The solvent
evaporated and very fine fibers were completely coated on the alu-
minum sheet or deposited as bulk fibers. The surface morphology
of the fabricated nanofibers was studied using the scanning elec-
tron microscope (SEM) instrument (6060-JEOL JSM). The nanofibers
deposited on aluminum substrate were cut into proper size, fixed
in the cuvette or flow cell and the excitation or emission spectra
were recorded. The electrospun fibers exhibited good adhesion to

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M.Z. Ongun et al. / Spectrochimica Acta Part A 90 (2012) 177–185
Fig. 2. Schematic structure of the electrospinning apparatus.
3. Results and discussion
sputtered from the tip of the cone. In this report, we have
demonstrated that it is possible to electrospun the EC and PMMA
composite fibers in presence of non-volatile room temperature
ionic liquid; EMIMBF4. The employed RTIL was chosen as polymer
electrolyte due to its ionic conductivity, high thermal and chemical
stability, non-volatile, non-flammable and low toxicity character-
istics [23].
Room temperature ionic liquids (RTIL), specifically 1-alkyl-3-
methylimidazolium salts have been effectively used as volatile
organic solvent replacements in liquid/liquid separations of metal
ions from aqueous solutions [24]. As expected, the exploited
RTIL not only enhanced the electrospinning performance but also
3.1. Effect of RTILs on electrospinning process
To provide an electrical conductivity in polymers one must add
proper electrically conductive additive into the polymer. As men-
tioned earlier when the high electric voltage is applied on such
an electrically conductive polymer precursor, the solution at the
tip of the needle becomes stretched to form a cone because of
coupled effects of the electrostatic repulsion within the charged
droplet and attraction to a grounded electrode of opposite polar-
ity. As the strength of the electric field is increased, fine jet is
Fig. 3. SEM images of electrospun membranes at different magnifications (a and b; PMMA based, c and d; EC based nanofibers).

M.Z. Ongun et al. / Spectrochimica Acta Part A 90 (2012) 177–185
181
contributed a remarkable ability to extract the Cu(II) ions from
the aqueous phase into the polymeric materials. Additionally
the EMIMBF4 enhanced photostability of the embedded chro-
moionophore within the polymer matrix acting as a sink for
acidogenic or basic species from the ambient air of laboratory. This
result arises from the intrinsic acidic–basic characteristics of the
employed RTIL [25,26].
3.2. Effect of pH
The exploited Cu(II) sensitive fluoroionophore exhibited a cross
sensitivity to proton ions between pH 3.0 and 11.0 in solution
phase. When embedded in RTIL containing matrices the DPAINH
dye became more resistive for protonation.
We also tested the Cu(II) sensing ability of the chemosensor at
different pH values, between pH 5.0 and 9.0 in acetic acid/acetate
and phosphate buffer solutions. We obtained the highest sensitivity
in acetic acid/acetate buffer and for further studies we have chosen
this buffer as optimum working condition.
During calibration studies, we exploited 5 × 10−3 M of acetic
acid/acetate buffer solutions to avoid pH induced signal fluctu-
ations. Distribution of the Cu(II) related chemical species in the
working conditions was checked with chemical equilibrium soft-
ware program visual MINTEQ at pH 4.0 in presence of acetate
ions. The equilibrium related species in the buffer environment
were found to be as follows: acetate(aq) 15.2%; H-acetate(aq) 84.8%;
Cu2+
89.8%; Cu-acetate+
10.1%. This distribution reveals that
(aq)
(aq)
approximately 90% of the copper is in form of Cu2+
under
(aq)
these circumstances. Due to the solubility considerations acetic
acid/acetate buffered solutions of pH 4.0 was used for further stud-
ies.
Fig. 4. (I) Excitation and emission spectra of the fluoroinophore in (a) EtOH, (b) THF,
(c) DMF, (d) DCM. (II) (a) in embedded form in EC, (b) in PMMA.
3.3. Photocharacterization of the dye
potassium tetrakis-(4-chlorophenyl) borate; the DPAINH dye
becomes a Cu(II) sensitive molecular probe. In this system Cu(II)
ions are extracted into the electrospun fibers by the enforcement
of the anionic additive meanwhile two potassium ions diffuse from
the polymer into the aqueous phase via the mechanism of ion
exchange. Therefore the charge balance was maintained within the
polymer. Following equation reveals the ion exchange mechanism.
Photocharacterization of the ionophore was performed in
diluted solutions of EtOH, DCM, THF and DMF and in solid matri-
ces of EC and PMMA. Solution phase excitation and emission
spectra of the ionophore were shown in Fig. 4(I). The ionophore;
DPAINH yielded two distinct emission peaks in the exploited
solutions between 300–340 nm and 560–600 nm, depending on
the solvent. The exploited Cu(II) sensitive fluoroinophore exhibits
strong absorption and bright fluorescence due to the intramolecu-
lar charge transfer (ICT) in the excited state. The ICT is continuous
along with 9 covalent bonds throughout the molecule from electron
donating N,N-dimethylamino group to the nitrogen atoms around
carbohydrazide moiety.
DPAINH(org) + 2K+(org) + 2TpCIPB− + Cu2+ ↔ DPAINH Cu2+
(pink)
(orange)
+ 2TpCIPB− + 2K+
(1)
Immobilization of the molecule in polymers resulted with dra-
matic spectral changes. Upon immobilization the emission maxima
recorded in diluted solutions around 300 nm were disappeared.
Interestingly, the excitation wavelength was shifted to the visible
end of the electromagnetic spectrum and the emission efficiency
was only appeared at longer wavelengths; around 500 nm (see
Table 2 and Fig. 4(II)). When doped in EC and PMMA the Stoke’s
shifts of DPAINH exhibited an enhancement with respect to the
solution phase (see Table 2). Therefore when immobilized, the
DPAINH dye could be excited at further wavelengths with respect
to the solution phase. This result can be attributed to the restricted
vibrational rotational motions in solid states.
Figs. 5 and 6 show the variations in fluorescence spectra of the
thin film and electrospun materials upon exposure to Cu(II) ions for
PMMA and EC matrices respectively. The emission maxima approx-
imately 20 nm and 10 nm red shifted in form of nanofiber with
respect to continuous thin films for PMMA and EC respectively (see
Figs. 5 and 6). In all cases, response to Cu(II) ions was in direction
of decrease in signal intensity. In form of nanomaterials the rela-
tive signal changes were approximately three times and two times
better compared to the continuous thin films. In PMMA, the rela-
tive signal drops were reported as 20% and 62.5% for thin film and
nanofiber forms, respectively. Similarly 40% and 83% relative signal
drops were calculated for EC based matrices for thin film and nano-
materials. The insets of Figs. 5 and 6 reveal intensity based linear
response ranges of the ionophore to Cu(II) ions in different forms in
terms of molar concentration. In PMMA based matrices the working
range was shifted from 10−12–10−5 M to 10−12–10−2 M. Similarly
in EC based sensing agents the enhancement in working range was
more distinct and enlarged from 10−9–10−4 M to 10−12–10−3 M.
The detection limits (LOD) were calculated exploiting concentra-
tion of the metal ion giving a signal equal to average of the blank
3.4. Copper ion sensing studies: Cu(II) uptake into the membrane
and fluorescence based response
The optical sensors are mainly composed of a polymer compati-
ble ion-selective indicator dye (chromoionophore), and a lipophilic
anionic additive dissolved in polymer matrix material. When
doped into polymeric matrices together with the anionic additive;

182
M.Z. Ongun et al. / Spectrochimica Acta Part A 90 (2012) 177–185
Table 2
The excitation–emission spectra related characteristics of the DPAINH in diluted solutions of DCM, THF, and DMF and in solid matrices of EC and PMMA.
ex
em
max 1
em
max 2
Compound
Matrix
ꢀ
(Excitation wavelength)
ꢀ
(Emission wavelength)
ꢀ
(Emission wavelength)
ꢁꢀST (Stoke’s shift)
max
DPAINH
DPAINH
DPAINH
DPAINH
DPAINH
DPAINH
EtOH
THF
DMF
DCM
EC
285
275
285
265
405
395
315
315
340
305
560
605
565
560–605
490
30
40
55
40
85
PMMA
535
140
signal (n = 20) plus three standard deviation and were found to
be 3.8 × 10−14 M for EC and 1.4 × 10−13 M for PMMA respectively.
The attained LOD values exploiting nano-materials for the offered
design is an evident of the enhanced sensitivity (see Table 1).
The intensity drops is known to be due to the quenching of the
DPAINH dye by Cu(II) ions.
Quenching can occur by different mechanisms. In collisional
quenching, excited state fluorophore is deactivated upon contact
with the ion in solution which is called the quencher. However,
sometimes the fluorophore can be quenched both by collisions and
by complex formation. The intensity related data (I0/I) exhibiting
an upward-concave curvature toward the y-axis is the evidence of
the combined quenching both by collisions (dynamic quenching)
and by complex formation (static quenching) by the same quencher
[27]. In case of dynamic quenching because the collisions between
the quencher and the fluorophore effects only the excited state
of the molecule, no changes in absorption or excitation spectrum
are expected. Contrarily, the formation of ground state complex
in static quenching will influence the absorption spectra of the
fluorophore. Therefore, by a careful evaluation of the absorption
spectrum, mechanism of the quenching can easily be distinguished.
Fig. 7 shows the absorption spectra of the fluoroionophore in the
absence and presence of the quencher. From the absorption spectral
data, it can be concluded that the quencher affects only the excited
state of the fluorophore which indicates dynamic quenching. When
the linear shape of the fluorescence intensity based response and
absorption spectroscopy related data were considered together,
the quenching mechanism between DPAINH and Cu(II) can be con-
cluded as dynamic quenching.
Sensitivity of the quenching process and consequently sensor
dynamics can be manipulated by controlling the quencher diffu-
sion rate to fluorophores encapsulated in the polymer phase via the
microstructural properties of the sensing agent. The offered nano-
structure was quite beneficial for both the sensitivity and dynamic
characteristics of the sensor due to its large specific area, which has
high number of active sites for diffusion of Cu(II) ions towards the
solid phase with respect to the thin films made up of same mate-
rial. In these structures the possibility of interaction of the analyte
Fig. 5. (I) Fluorescence response of the dye doped PMMA thin film to Cu(II) ions at pH 4.0. (a) Cu-free buffer, (b) 10−12 M, (c) 10−11, (d) 10−10, (e) 10−9, (f) 10−8, (g) 10−7
,
(h) 10−6, (i) 10−5, (j) 10−4, (k) 10−3 M Cu(II). Inset: Calibration plot for the concentration range of 10−12–10−5 M Cu(II). (II) Response of the PMMA based nanofiber to Cu(II)
ions at pH 4.0. (a) Cu-free buffer, (b) 10−12 M, (c) 10−11, (d) 10−10, (e) 10−9, (f) 10−8, (g) 10−7, (h) 10−6, (i) 10−5, (j) 10−4, (k) 10−3, (l) 10−2, (m) 10−1 M Cu(II). Inset: Linearized
calibration plot for the concentration range of 10−12–10−2 M Cu (II).

M.Z. Ongun et al. / Spectrochimica Acta Part A 90 (2012) 177–185
183
Fig. 6. (I) Fluorescence response of the dye doped EC thin film to Cu(II) ions at pH 4.0. (a) Cu-free buffer, (b) 10−12 M, (c) 10−11, (d) 10−10, (e) 10−9, (f) 10−8, (g) 10−7, (h) 10−6
(i) 10−5, (j) 10−4, (k) 10−3 M Cu(II). Inset: Calibration plot for the concentration range of 10−9–10−4 M Cu(II). (II) Response of the EC based nanofiber to Cu(II) ions at pH 4.0. (a)
Cu-free buffer, (b) 10−12 M, (c) 10−11, (d) 10−10, (e) 10−9, (f) 10−8, (g) 10−7, (h) 10−6, (i) 10−5, (j) 10−4, (k) 10−3 M Cu(II). Inset: Linearized calibration plot for the concentration
range of 10−12–10−3 M Cu (II).
ions with the Cu(II) selective chromoionophore molecules is higher
than that of in conventional thin films.
quencher diffusion rate to fluorophores employing electrospinning
technique. The Stern–Volmer plots for quenching by Cu(II) in both
cases, exhibited excellent linearity indicating that most of the total
emission may be accessible to quenching. Additionally the recorded
response time for the electrospun nanofibers is less than 1 min
(ꢂ90 = 0.83 0.16 min, n = 5). This value is at least five times bet-
ter than that of the response times measured for continuous thin
films (ꢂ90 = 4.27 0.8 min, n = 5).
3.5. Stern–Volmer analysis
The employed fluoroionophore is sensitive to quenching by
Cu(II) ions. In a homogeneous medium, such as in solution, the
quantitative measure of fluorescence quenching is described by the
Stern–Volmer constant, Ksv
,
3.6. Recovery, selectivity and regeneration studies
I0
= 1 + Ksv[Q]
(2)
I
The applicability of the sensing system for real samples was
tested for the tap water.
In Eq. (2), I0 and I are the intensities of fluorescence in the absence
and in the presence of the quencher, respectively. According to the
equation, I0/I increases directly proportional to the concentration
of the quencher. When all other variables are kept constant, the
higher the Ksv, the lower the concentration of quencher required
to quench the fluorescence. In a heterogeneous medium, such as in
polymer films, a negative deviation from the linear Stern–Volmer
equation occurs at high quencher concentration [28,29]. The data
obtained by gathering Stern–Volmer analysis for each electrospun
and continuous thin film are shown in Fig. 8(I) and (II). For quencher
concentrations in the range of 10−12–10−5 M, linear plots between
concentration of quencher and I0/I are obtained showing an excel-
lent Stern–Volmer relationship.
Stern–Volmer constants (Ksv) of the electrospun films, calcu-
lated from slopes of the plots were found to be 0.23 (M−1) and
0.07 (M−1) for EC and PMMA nanofibers respectively. These values
are 23- and 6-fold greater than that obtained from the continuous
thin film sensor slides of EC and PMMA, respectively (see Table 3).
The higher Ksv values reveal important practical consequences. The
sensitivity of the quenching process is enhanced by controlling the
Fig. 7. Absorption spectra of the fluoroionophore in the absence (a) and presence
(b) of the quencher; Cu (II).

184
M.Z. Ongun et al. / Spectrochimica Acta Part A 90 (2012) 177–185
Table 3
The Stern–Volmer plot related calibration characteristics of PMMA and EC based electrospun nanofibers and thin films.
Indicator dye
Matrix/form
Linear regression equation
Regression coefficient (R2)
Ksv constant
DPAINH
DPAINH
DPAINH
DPAINH
EC/electrospun nanofiber
PMMA/electrospun nanofiber
EC/thin film
y = 0.2323x + 0.733
y = 0.0695x + 0.99
y = 0.0124x + 0.9806
y = 0.0133x + 0.992
0.9827
0.9962
0.6148
0.9498
0.2323
0.0678
0.0124
0.0133
PMMA/thin film
The content of Cu2+ in the diluted certified reference mate-
rial of Merck (SRM from NIST Cu(NO3)2 in HNO3 0.5 mol L−1
1000 mg L−1 Cu CertiPUR®) were determined by the prepared elec-
trospun nanofibers. The tap water samples were freshly collected
from the laboratory in polyethylene vials, after allowing the water
to flow for 5 min. All water samples were filtered through a 0.45 ␮m
membrane filter and analyzed as soon as possible after sampling.
The recovery experiments were carried out adding certified refer-
ence material into the tap water. The resulting concentration of the
Cu2+ was 10−9 M.
The average recovery performance was found to be 103 3.7%
for five measurements. The recovery test results reveal the capa-
bility of the offered sensing design for the determination of copper
ions in real samples without considerable error.
In order to examine the cross sensitivities of the DPAINH dye to
possible interferents the sensing agents were treated with 10−5 M
concentrations of Ag+, Al3+, Ca2+, Co2+, Cu2+, Fe3+, Hg2+, Mn2+
,
Mg2+, Ni2+, Pb2+, Sn2+ and Zn2+ cations in acetic acid/acetate buffer
solutions at pH 4.0. Fig. 9 reveals that the sensing agent is capa-
ble of determining Cu(II) ions with a high selectivity over other
ions. The fluorescence was distinctly quenched in the presence of
Cu(II) at 535 nm and 490 nm for PMMA and EC based sensing slides
respectively. The interference effects of the conventional anions of
NO3−, ClO4−, SO42−, Cl−, HCO3− and HPO43− were also tested. Rel-
ative signal changes of less than 5% were obtained for most of the
tested potential interferents. Only Fe3+ and HCO3 ions exhibited
−
Fig. 9. (I) Response of DPAINH to 10−5 M of different metal cations in acetic
acid/acetate buffer solutions at pH 4.0. (II) Response of the dye to 10−5 M of the
major anions in near neutral waters.
remarkable response in terms of signal intensity. However these
problems can be overcome tuning the pH of the working conditions.
3.7. Sensor dynamics and analytical figures of merit
The major requirements of an ideal sensor are high sensitivity,
good LOD values, wide linear working range, selectivity and fast
response time. The working ranges of the sensing agents calculated
from emission profiles were broad enough and extends from 10−12
to 10−3 M and from 10−12 to 10−2 M for EC and PMMA respectively.
The quadratic fit is more convenient for such kind of large working
ranges. However we obtained excellent linearity with nanomate-
rials for both of the matrices within the whole working range. The
regression coefficients of 0.9827 and 0.9962 for ethyl cellulose and
poly(methyl methacrylate) are evidences of the attained linear-
ity. The measured equilibrium response times of less than 1 min
and high analytical signal values upon exposure to Cu(II) ions with
electrospun nanofibers are very promising.
4. Conclusion
Electrospinning looks like a promising, simple, and an effec-
tive method for fabricating nanomaterials. On the other hand
such kind of materials should be coupled with optical chem-
ical sensor devices. With respect to continuous thin films,
Fig. 8. Gathered Stern–Volmer plots of the electrospun nano-fibers and continuous
thin films in PMMA (I) and in EC (II).

M.Z. Ongun et al. / Spectrochimica Acta Part A 90 (2012) 177–185
185
electrospun films can offer enhanced sensitivity and reactivity
in sensing applications. In this work, we performed coupling of
electrospun nanomaterials with fluorescence based measurement
techniques without scattering and other side effects. We per-
formed to measure the Cu(II) concentrations as low as 10−12 M
exploiting nanofibrous materials. Our nano-material based sens-
ing approach resulted with large linear working ranges extending
to 10−12–10−3 mol L−1 and 10−12–10−2 mol L−1 Cu(II) for EC and
PMMA respectively. To our knowledge these are the largest
reported working ranges and best LOD values among similar emis-
sion based sensing methods. Further efforts will focus on exploring
new sensing materials and polymer compositions, controlling the
nanoscale size of the electrospun membranes, and optimizing the
sensitivities for the detection of a variety of analytes.
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