Salicylaldimine-based receptor as a material for iron(III) selective optical sensing

Article abstract of DOI:10.1016/j.jphotochem.2017.06.011

α,α-Bis(salicylimino)-m-xylene (L) was prepared using both conventional and microwave-assisted procedure. The compound exhibits ability to colorimetric recognition of iron(III) ions in aqueous environment, what is manifested by significant color change fr

Full text of DOI:10.1016/j.jphotochem.2017.06.011

Accepted Manuscript
Title: Salicylaldimine-based receptor as a material for iron(III)
selective optical sensing
Authors: Natalia Łukasik, Ewa Wagner-Wysiecka
PII:
S1010-6030(17)30209-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2017.06.011
JPC 10690
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5-6-2017
10-6-2017
Please cite this article as: Natalia Łukasik, Ewa Wagner-Wysiecka,
Salicylaldimine-based receptor as material for iron(III) selective
optical sensing, Journal of Photochemistry and Photobiology A:
Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.06.011
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Salicylaldimine-based receptor as a material for iron(III) selective optical
sensing
Natalia Łukasik*, Ewa Wagner-Wysiecka*
Department of Chemistry and Technology of Functional Materials,
Faculty of Chemistry, Gdansk University of Technology,
Narutowicza Street 11/12, 80-233 Gdańsk, Poland
Corresponding authors. Tel.: +48 58 347 23 59,
fax: +48 58 341 19 49; e-mail address: natalia.lukasik@pg.gda.pl, ewa.wagner-wysiecka@pg.gda.pl

Graphical abstract
Highlights
-
-
-
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Application of microwave irradiation ensures α,α-bis(salicylimino)-m-xylene synthesis
in easy, fast, and efficient way.
The reported compounds shows ability to colorimetric iron(III) recognition in aqueous
solutions, what is shown by color change from yellow to purple.
Incorporation of the ligand into polymeric matrix enables to iron(III) detection at pH
2.9 in aqueous solution with detection limit 2.73×10-6 M.
Mechanism of ligand – iron(III) interactions on the basis of spectroscopic methods is
proposed.

Abstract:
α,α-Bis(salicylimino)-m-xylene (L) was prepared using both conventional and microwave-assisted
procedure. The compound exhibits ability to colorimetric recognition of iron(III) ions in aqueous
environment, what is manifested by significant color change from yellow to purple. In DMSO : water
(9:1 v/v) solvent system, receptor creates with iron(III) cations complexes of 2:1 stoichiometry (L:Fe3+)
with stability constant (log K) 7.54±0.21. Incorporation of ligand into polymeric matrix (cellulose
triacetate) enables iron(III) detection in 100% aqueous solution at pH 2.9 with detection limit 2.73×10-
6 M.
Keywords: Schiff base, tautomerism, iron(III) receptor, optical sensor, UV-Vis spectrophotometry
1. Introduction
Iron, one of the most essential trace element, plays a significant role in many biochemical
cellular processes. It is involved in oxygen transport and storage in diverse metalloenzymes [1].
Iron is also crucial in cellular metabolism and enzymatic reactions. Its deficiency is the most
common cause of anemia, low blood pressure, and hypoimmunity [2]. On the other hand, high
level of iron within an organism is associated with an increased risk of cancer and dysfunction
of several organs like heart or liver [3]. Therefore, reliable, sensitive and selective methods for
qualitative and quantitative iron(III) detection are needed in the field of clinical, environmental,
and industrial analysis.
In aqueous solutions iron(III) exists in several forms depending on pH i.e. at low pH
[Fe(H2O)6]3+ ions are predominant, but above pH ~ 3 hydroxo species [Fe(H2O)5OH]2+,
[Fe(H2O)4(OH)2]+, or [Fe(H2O)2(OH)4]- appear [4]. For this reason, a form of iron ions existing
in solution must be taken into account for an appropriate method development.
Due to fast and easy detection, relatively low costs, and high sensitivity optical chemosensors
gained vast popularity [5-7]. Color change occurring during recognition process between
receptor and complementary ion provides easy to measure analytical signal and even “naked-
eye” detection without a need of use of the expensive equipment. Among metal cation receptors
Schiff bases derived from salicylaldehyde are an important group of compounds. Due to the
presence of chromogenic azomethine moiety and hydroxyl group being additional binding site
they are often applied for spectrophotometric ion recognition and sensing. Velmathi et al. [8]
used salophene (N,N´-bis(salicylidine)-o-phenylenediamine) as colorimetric and fluorescent
reagent for transition metal cation detection in acetonitrile and its mixtures with water.
Spectrofluorimetric response of salophene towards copper(II) cation in aqueous solution at pH

8.2 was also reported [9]. Amani et al. [10] described recognition ability of imines derived from
salicylaldehyde towards fluoride anions in acetonitrile-DMSO mixtures.
Among reported iron(III)-selective chemosensors majority of them cannot operate in fully
aqueous solutions due to their low binding affinity or hydrophobicity [11-15]. The necessity of
addition of organic solvent may limit potential applications of described probes. What is more,
from practical point of view operation with ligand solution is less convenient than with a ligand
incorporated into membrane (optode) that may ensure on-line monitoring [16-17].
Taking all above into consideration, we present here the synthesis and ion-binding ability of
α,α-bis(salicylimino)-m-xylene (L) (Scheme 1) towards metal cations and anions in aqueous
environment. This compound was previously reported by Maverick et al. [18]. Its complexes
with copper(II) and palladium(II) were characterized by 1H and 13C NMR as well as by X-Ray
analysis. To the best our knowledge (according to Chemical Abstracts), interactions of ligand
L with ions in aqueous solution have not been studied so far. Besides studies in solution (DMSO
: water mixture) compound L was also incorporated into polymeric matrix and adsorbed on the
test strips for easy, “naked-eye” detection of the analyte in 100% aqueous solutions.
2. Experimental
2.1 General
All chemicals of the highest available purity were purchased from commercial sources and used
without further purification. The reaction progress was monitored by TLC using aluminium
sheets covered with silica gel 60F254 (Merck). Microwave-assisted synthesis was carried out in
1
13
Plazmatronika RM 800 microwave reactor (power 300 W). H and C NMR spectra were
recorded on Varian INOVA-550 apparatus at 500 MHz and 125 MHz respectively. Chemical
shifts are reported as δ [ppm] values in relation to TMS. ATR FTIR spectra were taken on a
Nicolet iS10 apparatus. Mass spectra were recorded on a SYNAPT G2-S HDMS (Waters) (ESI)
spectrometer. Molar conductivities of the ligand and its complex with iron(III) ions were
measured in DMF (~ 10-3 M) at room temperature using Philips PW 9527 conductivity meter.
UV-Vis titrations in DMSO (Sigma-Aldrich) and its mixtures with water were carried out using
an UNICAM UV 300 spectrophotometer. For spectrophotometric measurements 1 cm quartz
cuvettes were used. In measurements carried out in aqueous solution deionized water
(conductivity < 1 µS∙cm-1, Hydrolab, POLAND) was used.
2.2 Synthesis

Schiff base (L) – α,α-bis(salicylimino)-m-xylene – was obtained using both conventional
heating (oil bath) and microwave-assisted synthetic procedures.
Conventional synthesis
To the solution of 1,3-xylylenediamine (0.75 mmol, 0.1 mL) in absolute ethanol (5 mL)
salicylaldehyde (1.50 mmol, 0.156 mL) was added stepwise. The reaction mixture was
magnetically stirred at reflux for 5 h. The resulting precipitate was filtrated off and the pure
product was obtained after crystallization from ethanol. Yield: 67% (0.171 g).
Microwave-assisted synthesis
A mixture of 1,3-xylylenediamine (0.75 mmol, 0.1 mL), salicyladehyde (1.50 mmol, 0.156
mL), and absolute ethanol (5 mL) was exposed to microwave irradiation for 10 minutes at room
temperature (300 W). Purification process was similar as describe above for conventional route
of synthesis. Yield: 97 %.
α,α-Bis(salicylimino)-m-xylene: yellow crystals, Rf = 0.93 (CHCl3:acetone, 10:1, v/v), Rf =
0.16 (chloroform), mp. 56-57°C, 1H NMR (500 MHz, DMSO-d6, δ [ppm]): 4.81 (4H, s); 6.86
(2H, d, J=7.9 Hz); 6.90 (2H, td, J=7.6 Hz); 7.26 (2H, d, J=7.7 Hz); 7.35-7.31 (3H, m); 7.38 (1H,
t, J= 7.6 Hz); 7.47 (2H, dd, J1=7.6 Hz, J2=1.5 Hz); 8.72 (2H, s); 13.41 (2H, s); FTIR (ATR)
3051; 3009; 2897; 2845; 2516; 1621; 1614; 1577; 1496; 1458; 1440; 1422; 1374; 1338; 1284;
1212; 1150; 1114; 1047; 1027; 895; 880; 796; 796; 767; 749; 703; 647 cm-1. UV-Vis (DMSO):
λ(ε) = 317 nm (8.3×103), 405 nm (2.4×102).
2.3 Ligand-ion interaction studies
Ligand-ion interactions both in solution and after immobilization of ligand into organic matrix
were investigated. Binding properties of obtained Schiff base were tested using UV-Vis
1
spectrophotometry, FTIR and H NMR spectroscopy. Spectrophotometric response of ligand
towards many metal cations (alkali, alkaline earth, heavy, and transition metal cations) in the
form of perchlorate salts as well as anions of different size and shape (as tetra-n-
butylammonium salts, TBA) was studied. Iron(II) was used as FeSO4xH2O and iron(III) as
anhydrous chloride FeCl3. All spectrophotometric titrations were carried out in the DMSO :
water (9:1 v/v) solvent mixture. On the basis of experimental data stability constant values and
stoichiometry of formed species were determined using OPIUM program [19]. In competition
studies the absorbance of ligand solution (λ = 550 nm) in the presence of iron(III) chloride at
fixed concentration (1 equivalent) was recorded before (A0) and after (A) addition of interfering

metal cations (as perchlorate salts) at the concentration 10 times higher than the analyte. The
influence of tested interfering ions on spectrophotometric response towards iron(III) cations is
presented as the relative response. The value of the relative response was determined as % RR
= [(A-A0)/A0]×100%.
Complex preparation and characterization
Iron(III) complex was prepared by dissolving of ligand and FeCl3 (molar ratio L : FeCl3 = 2:1)
in methanol. The respective mixture was refluxed till homogenous solution was obtained.
Sample was analyzed, after solvent evaporation under reduced pressure and crystallization from
water.
FTIR (ATR): 3377; 3050; 3021; 2899; 2844; 1611; 1541;1469; 1445; 1398; 1316; 1201; 1150;
1148; 1124;1046; 1027; 901; 752; 702; 598 cm-1.
UV-Vis: λ (ε) = 320 nm (1.2×104), 524 nm (2.2×103).
Ligand immobilization into organic matrix
Schiff base was incorporated into polymeric matrix based on cellulose triacetate. For membrane
preparation the respective quantities of ligand (0.01 g, 1.7% w/w), cellulose triacetate (0.25 g,
41.9% w/w), and triethylene glycol (0.3 mL, 56.4% w/w) in chloroform (6 mL) were dissolved.
The mixture was magnetically stirred for several hours till homogenous solution was obtained
and then poured onto a clean and dry Petri dish for solvent evaporation.
Test strips preparation
Test strips were prepared by absorption of ligand directly on a glass microfibre filter (Whatman
GF/C). Ligand solution (concentration 2, 4 or 6 mM) in methanol was poured into a
chromatographic chamber and then a strip of glass filter was placed in it, similarly as a TLC
plate. When ligand solution covered the whole the strip, it was taken out of the chamber and
left for solvent evaporation.
3. Results and discussion
3.1 Synthesis
Schiff base L was obtained as shown in Scheme 1 using salicyladehyde and 1,3-
xylylenediamine as substrates. The resulting imine is already known in literature. Its synthetic
procedure described by Maverick et al. [18] provides desired product obtainment with good
yield i.e. 90%, however carcinogenic benzene was there applied as a solvent. Proposed by us

conventional method employs a safer solvent i.e. ethanol and allows preparation of L without
the use of catalysts with moderate yield. Application of microwave irradiation - green chemistry
approach - enables shorter reaction time (10 minutes instead of 5 hours in the conventional
procedure) and higher yield of desired product, i.e. 97 %. Shorter reaction times and higher
yields are often reported for MW assisted protocols [20], however the mechanism of reaction
acceleration and yield improvement are not always clear. One of the reason which is considered
in literature and supported by many experiments (cf. amide synthesis [21]) is the more effectual
heating of polar species under microwave irradiation. In Schiff bases' synthesis from an
aldehyde and a primary amine as substrates, the rate determining step is dehydratation of
hemiacetal, preceded by the formation of charged intermediate (see Fig. SD1, Supplementary
Data). On the other hand, the reverse reaction i.e. the hydrolysis of imine is a process which
must be taken into account, when water is not removed from reaction environment. This can
lower the final yield of Schiff base, especially when reaction is carried out for a long time.
Microwave supported preparation of L occurs in a short time, thus the probable hydrolysis of
the product can be limited. This can be an additional explanation of the more effective MW
assisted synthesis of L comparing to conventional heated reaction.
3.2 Tautomeric equilibrium
Tautomeric equilibrium can be affected by many factors, among the others by the nature of the
solvent [22]. Possible tautomers: enol (LE) and keto (LK)form and zwitterionic form (LZ) which
can be considered for L are shown in Fig. 1a. To study of keto-enol tautomerism of L solvents
of different properties and polarities were used in spectroscopic measurements: UV-Vis, 1H and
13C NMR. The comparison of the absorption UV-Vis spectra of L registered in benzene,
hexane, methanol, DMSO and the mixture DMSO:water (9:1) is shown in Fig. 1b. In nonpolar
hexane three bands at 217, 257 and 322 nm are observed. The band at 322 nm is also observable
in nonpolar benzene (bands at shorter wavelengths are overlapped by solvent). This spectral
pattern is characteristic for the enol tautomer [23], whereas the existence of the keto tautomer
in UV-Vis spectra is manifested by the presence of band over 400 nm. This band ~ 400 nm and
yellow color of solution (Fig. SD2, Supplementary data), can suggest the change of tautomeric
equilibrium towards keto form in methanol, DMSO, and mixture of DMSO with water (Fig.
1b). The intensity of this band depends on polarity of the solvent system (comparing normalized
ET(30) solvent polarity parameter [22a]): DMSO<Methanol< DMSO:water.
1
13
We compared H and C NMR spectra of L registered in solvents of different polarity (for
details see Supplementary Data). 1H NMR spectra (Fig. 1d) in all cases show signal distribution

characteristic for aromatic protons (region 6.50-7.40 ppm). The presence of the pure keto form
(LK) should change this spectral pattern due to the partial loss of the aromaticity of the system
or additional signals should be present in the case of co-existence of two tautomeric forms in
solution. In 13C NMR spectra (Fig. SD3a-c) signal characteristic for ketone carbon atom C=O,
typically seen above 180 ppm [24] here is not observed. These results prompt us to conclude
that in polar solvents the zwitterionic keto form (LZ) is present. Moreover, it is well seen from
Fig. 1c that increasing water content shifts the enol-keto tautomeric equilibrium of L towards
zwitteronionic form LZ.
3.3 Ligand- ion interactions studies
Ion complexation in solution – UV-Vis spectroscopy
Alkali, alkaline earth and heavy metal cations complexation studies were preliminary
performed in DMSO. Spectral changes were observed in the presence of copper(II), lead(II),
zinc(II), iron(III) and iron(II) salts. The use of only organic solvent limits application of
investigated ligand as possible ion sensor thus further studies were carried out in water
containing solvent mixture. In DMSO : H2O (9:1, v/v) the most significant changes in UV-Vis
spectrum of ligand L solution were observed in the presence of iron(III) salt. Upon titration
with iron(III) chloride a new band appears at ~530 nm where absorption of ligand is not
observed. This change in UV-Vis spectrum, which can be assigned to a phenolate-iron(III)
charge transfer band [25], is connected with well observable color change from yellow to
purple. Changes in absorption spectrum of L upon titration with iron(III) chloride are shown in
Fig. 2a. In Fig. 2 also color change of ligand solution in the presence of equimolar amount of
iron(III) salt in DMSO : water (9:1 v/v) solvent system can be seen.
Molar ratio and Job plots obtained from titration experiments in DMSO-water mixture point
out that under titration conditions one metal ion can participate in complex formation with two
ligand molecules (Fig. SD4). Thus we assumed that in a mixed solvent system, 2:1 (L:Fe)
complex is formed. The stability constant value was calculated with OPIUM software [19] as
(log K) 7.54±0.21.
Iron(II) also causes changes in absorption spectra of L with color change from yellow to reddish
( = 510 nm) (Fig. SD5). Absorption band of the L-Fe2+ complex is partially overlapped by the
absorption band of the used salt - iron(II) sulfate. Estimated binding constant value (log K) for
L: Fe(II) (2:1) complex is about 7 (in experiment FeSO4xH2O was used, thus stability constant

should be taken as approximate value) and it is comparable with stability constant value for
iron(III) complex.
The effect of other heavy metal cations: copper(II), zinc(II), and lead(II) on iron(III) binding
by investigated Schiff base L was also studied. From spectra registered during
spectrophotometric titrations of ligand solution in DMSO : water solvent system (9:1 v/v) with
copper(II), zinc(II) or lead(II) perchlorate solutions (Fig. 3a-c) it can be seen that absorption
bands of formed complexes of ligand L with tested metal cations are not in the same region as
band of L-Fe3+. Analysis of spectrophotometric data revealed that under measurement
conditions 2:1 (L:M) complexes are formed with stability constant values (log K) 9.160.03
and 6.960.08 for L-Cu2+ and L-Zn2+ respectively. Evaluation of reliable value of binding
constant for L-Pb2+ was not possible due to relatively small changes in absorption spectra.
Application of tested metal cations salts in concentration 10-fold higher than iron(III) salt
revealed that the most significant effect on spectrophotometric response of ligand L towards
iron(III) have copper(II) cations. The influence of interfering ions on iron(III) determination
expressed as %RR is shown in Fig. 3d (see also Fig. SD6).
Schiff base L due to the presence of O-H and azomethine C-H groups in the structure can be a
potential anion receptor. Ability of ligand to anions of different size and shape complexation
was tested in DMSO and DMSO : water (9:1 v/v) solvent system. Among studied ions changes
in UV-Vis spectrum of ligand solution only in the presence of fluoride and hydroxide (tested
as tetra-n-butylammonium salts) were noticed. Large batochromic shift and similarities in
changes caused by those strongly basic anions suggest that under measurement conditions the
ligand deprotonation is a dominant process (Fig. SD7).
Iron(III)-L complex characterization
The nature of iron(III) binding by tested here ligand was analyzed using spectroscopic methods.
Comparison of FTIR (ATR) spectra of Schiff base L and its iron(III) complex is shown in Fig.
4. In free imine spectrum a broad band c.a. 2560 cm-1, which can be assigned to O-H…N
intramolecular hydrogen bond, is observed [26-27]. This and the presence of strong bands at
1621 and 1284 cm-1 indicate that in a solid state ligand L exists in a phenol-imine tautomeric
form. The most significant differences between L and iron(III) complex are observed in
absorption region of (C=N) and (C-C)ar bands, which in free ligand are present as a purely
resolved dublet at 1621 and 1614 cm-1. In the complex those bands, seen as a singlet, are shifted

towards lower frequencies (1611 cm-1), what may indicate that metal ion is coordinated through
the nitrogen atom of azometine group. In N, O donor Schiff bases metal cation is usually
coordinated also by oxygen atom of deprotonated OH group in ortho position to azometine
residue [28-29]. In IR spectra of the reported here free ligand L the well observable (OH)
broad band signal ~ 2600 cm-1 is absent in the spectrum of complex. Concurrently, the (C-O)
band seen in the imine spectrum at 1284 cm-1 in the complex spectrum is broaden and less
intense (however, its position is nearly the same). Changes of intensity and position of (C-C)ar
and δ(Car-H) bands are also observed. It may suggest that in metal coordination phenolic group
is involved.
In the mass spectrum of the obtained complex a base peak at m/z = 813.14 is observed. Peaks
at m/z: 345.16 and 398.07 of intensity of 26% and 32% respectively are also seen, which
correspond to ligand ion [M + H]+ and an adduct of the deprotonated ligand and iron(III) ion
(Fig. SD8). Molar conductivities of L-Fe3+ complex and free L in DMF are 37.7 and 1.7 S∙cm2
∙mol-1 respectively, what indicates their nonelectrolyte properties. This means that in solution
complex exists mostly in neutral form and there are no anions in the coordination sphere [30-
32]. Additionally, test for chloride ions with the use of silver nitrate excluded its participation
in the complex formation.
Taking into account all the obtained results it may be concluded that one iron(III) ion is
coordinated by 2 molecules of ligand through two nitrogen atoms and two oxygen atoms from
each molecule of the compound. It is possible that complex is stabilized by 4 molecules of water
interacting with azomethine CH protons of the ligand molecules via hydrogen bonds. The base
peak observed in the mass spectrum at m/z = 813.14 may thus correspond to [M + H]+ ion.
The reversibility of complexation process is an important feature in regard to usage of the ligand
as an analytical reagent. Performed studies reveal that anionic species have an impact on
stability of L-Fe3+ complex. Strongly basic anions namely: fluoride, hydroxide, acetate,
benzoate, and dihydrophosphate ions (in the form of tetra-n-butylammonium salts) cause the
reversibility of complexation process in L-Fe3+ system. During spectrophotometric titration of
L-Fe3+ solution in DMSO : water (9:1 v/v) with one of the mentioned above salt the band ~ 550
nm, characteristic for complex of the Schiff base with iron(III), disappears. Addition of other
tested anions: chlorides, bromides, iodides, nitrates(V), monohydrogen sulfates, thiocyanates,
p-toluenesulfonates, and perchlorates does not influence on the complex stability.

Metal cation recognition by ligand incorporated into organic matrix
A remarkable color change of ligand L solution upon iron(III) coordination from yellow to
purple makes the proposed here imine an interesting candidate for an optical sensor (optode).
This is why Schiff base L was incorporated into polymeric matrix i.e. cellulose triacetate and
its ability to iron(III) recognition in fully aqueous solution was examined. The use of triacetate
cellulose material based on natural polymer - instead of popular PCV matrix is the advantage
of our system as such materials seem nowadays to be biodegradable [33]. Similarly to UV-Vis
spectrum of ligand solution in DMSO : water solvent system, in the optode spectrum a new
band at ~ 530 nm upon iron(III) chloride addition was observed, what was accompanied by
optode color change (Fig. 5).
Iron(III) due to its small size and relatively high charge reveals strong tendency to hydrolyze in
aqueous solution [4]. This is why, iron(III) determination needs to be carried out at fixed and
low value of pH. The influence of pH value on absorbance of the optode doped with ligand L
was tested (Fig. 6a). At low pH ~ 2 complete decrease of membrane absorbance at the analytical
wavelength is observed. In such conditions hydrolysis of imine may take place and the prepared
optode cannot operate. Keeping constant pH value at around 3 during all measurements is
preferable from stability of both the optode, and iron ions point of view.
One of the important features of chemical sensor is its response time. Figure 6b shows the time-
dependent response characteristics of reported here optode immersed in solution containing 1.6
mM of FeCl3. The optical membrane gives stable response after short time i.e. 50 seconds. This
response is proportional to iron(III) concentration in whole tested concentration range: from
8.1×10-4 M to 1.5×10-2 M with correlation factor R2=0.998. The detection limit, estimated as a
concentration of analyte generating a signal equal to three times of standard deviation of blank
membrane [34], is 2.73×10-6 M. In order to determine the sensor selectivity the influence of
several metal cations on the membrane absorbance was tested. In competition studies the
absorbance of optode in the presence of iron(III) chloride in hydrochloric acid solution (10-3 M)
at fixed concentration (csalt = 1.60×10-3 M) was recorded before (A0) and after (A) addition of
interfering metal cations at the concentration 10 times higher than the analyte (λ=550 nm). The
influence of tested interfering ions on spectrophotometric determination of iron(III) cations is
presented as the relative response (Fig. 7a). The most significant influence on the optode
response towards iron(III) chloride have calcium and copper(II) ions. Sodium, potassium,
magnesium, and lead(II) cations cause negligible effect on absorbance changes.

Regeneration of bulk optode by the use of EDTA solution (0.1 M) was provided. After 2
minutes of immersion of membrane in EDTA solution iron(III) ions were efficiently removed
and the membrane absorbance returned to its initial value. Before next iron(III) determination
the optode was kept for 1 minutes in water. The same membrane can be used 2 times without
significant loss of effectiveness. In the third cycle the response decreases to 50% (Fig. 7b).
For comparative purpose linear range, detection limit and working media of the presented here
sensor and several other colorimetric reagents for iron(III) recognition are summarized in Table
1. Most of iron(III)-selective sensors reported in [5-7, 11] work in solutions containing organic
solvents whereas the proposed optode operates in 100% aqueous environments. Other
important feature of the proposed sensor is that the ligand can be obtained in fast way with very
high yield form commercially available, inexpensive reagents, what makes it a cheap and
effective tool for iron(III) recognition.
Metal cation recognition by ligand adsorbed on test strips
For fast, qualitative experiments the test strips are a convenient tool. In presented here studies,
the effect of ligand concentration (cL: 2.20×10-3, 4.10×10-3, and 6.00×10-3 M) on iron(III)
“naked-eye” detection in aqueous solution was examined. As described above, the strongest
influence on iron(III) recognition by optode with ligand L has copper(II) ion (Fig. 7a). For this
reason response of the test strip towards iron(III) in the presence of copper(II) ions (10-fold
molar excess to iron(III) concentration) was tested. As an optimum ligand concentration
4.10×10-3 M was chosen (because of color change) (Fig. 8). A characteristic, for iron(III)
recognition, change of color of the test strip even in the presence of interfering ion was
observed. The limit for “naked-eye” detection for test strips is on the level of 10-3 M FeCl3.
4. Conclusions
Ligand L - α,α-bis(salicylimino)-m-xylene is obtained in eco-friendly synthetic route (etanol,
microwave heating) with a high yield (97%) from simple, commercially available substrates. L
exhibits ability to iron(III) recognition in water containing systems. In DMSO : water solvent
mixture (9:1 v/v) the imine interacts with iron(III) creating ML2 complexes with stability
constant (log K) 7.54±0.21 (it can act as a chromogenic reagent for metal ion detection and
determination). The presence of iron(III) chloride causes the appearance of a new band at
around 530 nm in UV-Vis spectrum of ligand solution, what is accompanied by remarkable

color change from yellow to purple. Incorporation of ligand into polymeric matrix based on
cellulose triacetate, considered as a biodegradable material, provides iron(III) detection in
100% aqueous solution at pH 2.9 in concentration range from 8.1×10-4 M to 1.5×10-2 M with
detection limit 2.73×10-6 M. It is worth to point out, that the determined detection limit is lower
than the one recommended by World Health Organization in drinking water (5.36×10-6 M) [35].
Acknowledgments
Authors kindly acknowledge financial support from the Gdansk University of Technology Grant No. 031841T004.
Authors also thank Prof. E. Luboch for valuable discussion and students: Julita Meger, Paulina Oller, Małgorzata
Krupa, and Marta Ołów for their experimental contribution. Authors thank anonymous Reviewers for the careful
review, which helped to improve the quality of above manuscript.
Appendix A. Supplementary data
Supplementary data associated with this article can be found , in the online version, at
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a)
+
+
N
N
N
N
N
N
HC
H
O
H
O
CH
HC
H
O
H
O
CH
HC
H
O
CH
H
O
_
_
LZ
LK
LE
c)
1.0
b)
2.0
1.5
1.0
0.5
0.0
0
4
%
%
benzene
hexane
DMSO
methanol
DMSO:water(48%)
0.8
0.6
0.4
0.2
0.0
12 %
21 %
30 %
40 %
48 %
250
300
350
400
450
300
400
wavelength [nm]
500
600
wavelength [nm]
Fig. 1. (a) Possible: enol (LE) and keto (LK) tautomers and zwiterrionic (LZ) form of L; (b) the comparison of UV-
Vis spectra of L (~4×10-5 M) in different solvents ; (c) changes in UV-Vis spectrum of ligand L (9.90×10-5 M) in
1
the presence of different amounts of water; (d) partial H NMR spectra of L registered in solvents of different
polarity.

1.0
0.8
0.6
0.4
0.2
0.0
increase of iron(III)
salt concentration
ligand, L
500
450
550
600
650
700
wavelength [nm]
Fig. 2. Photo: color change of ligand solution in the presence of equimolar amount of iron(III) chloride in mixed
DMSO : water (9:1, v/v) solvent system. Spectrophotometric titration (spectral range 450-700 nm) of ligand
solution (6.69×10-4 M) with iron(III) chloride (0-8.0×10-4 M) in mixed DMSO : water (9:1, v/v) solvent system.

1.2
1.0
0.8
0.6
0.4
0.2
0.0
1,2
1,0
0,8
0,6
0,4
0,2
0,0
a)
b)
2+
2+
Cu
Pb
300
400
500
600
700
300
400
500
600
wavelength[nm]
wavelength [nm]
1,2
c)
d)
20
10
1,0
0,8
0,6
0,4
0,2
0,0
Pb2+
Zn2+
0
Cu2+
-10
-20
-30
-40
2+
Zn
300
400
500
600
wavelength [nm]
Fig. 3. Changes in UV-Vis spectra of ligand solution (cL = 1.40×10-4 M) upon addition of (a) copper(II) perchlorate
(cCu = 0-2.16×10-4 M); (b) lead(II) perchlorate (cPb = 0-3.38×10-3 M); (c) zinc(II) perchlorate (cZn= 0-6.85×10-4 M);
(d) the influence of metal cation (10-fold molar excess) on spectrophotometric response of ligand solution (cL =
2.10×10-4 M) towards iron(III) chloride (cFe = 5.17×10-5 M) in DMSO : water (9:1) solvent system (λ= 550 nm).

100
80
60
40
20
0
2560
1621
1615
1284
1611
1500
L
L + Fe3+
2500
1000
Wavenumber [cm-1]
Fig. 4. Comparison of FTIR (ATR) spectra of Schiff base L and its iron(III) complex (L : Fe3+ = 2:1, w/w).

b)
1,0
0,8
0,6
0,4
0,2
0,0
a)
0,8
0,6
0,4
0,2
0,0
L + Fe3+
L
Iron(III) concentration
0,000
0,004
0,008
0,012
0,016
450
500
550
600
650
700
750
800
Iron(III) salt concentration
wavelength [nm]
Fig. 5. a) Changes in UV-Visible spectrum of optode with ligand L in the presence of FeCl3 (c = 0 - 1.48×10-2 M)
in HCl (10-3 M) at pH = 2.9; b) Absorbance of optode with ligand L vs. iron(III) chloride concentration (λ = 550
nm).

a)
b)
0,30
0,25
0,20
0,15
0,10
0,05
0,00
0,070
0,065
0,060
0,055
0,050
0,045
0,040
2
4
6
8
10
12
0
50
100
150
200
250
300
pH
time [s]
Fig. 6. (a) pH Effect on optode absorbance (λ=400 nm); (b) Changes in absorbance of optode in the presence of
FeCl3 (c = 1.60×10-3 M) vs. time at pH= 2.9 (λ= 550 nm).

b)
100
80
60
40
20
0
a)
metal cation
30
20
10
0
Cd2+ Pb2+
Cu2+
Zn2+
Mg2+
NH4+
Ca2+
Na+
K+
-10
1
2
3
experiment no.
Fig. 7. (a) Interferences from several metal cations (10-fold molar excess) to the spectrophotometric response of
optode with ligand L (λ= 550 nm) towards FeCl3 (csalt = 1.60×10-3 M) in HCl solution (pH=2.9); (b) Changes in
optode response towards iron(III) before (A0) and after (AI) regeneration in 0.01 M EDTA solution and water
(λ= 550 nm).

Fig. 8. Changes of color of ligand (cL= 4.10×10-3 M) adsorbed on the test strip in the presence of different
concentration of FeCl3 (from left) and mixture of FeCl3/CuCl2 (cFe=0.1 M, cCu=1 M) in aqueous solution (pH=2.9).
From right: color of test strip without ligand in the presence of FeCl3/CuCl2 solution.

O
H
absolute EtOH,
reflux, 5h
N
N
Yield:
OH
HC
CH
+
conventional heating: 67%
MW assisted reaction: 97%
or MW, rt,
10 min
2
OH HO
NH2
NH2
L
Scheme 1. Conventional or microwave-assisted (MW) synthesis of ligand L. Yield was added in scheme.

Tab. 1 The comparison of the proposed sensor with some iron(III)-selective colorimetric reagents found in the
literature
Colorimetric sensor Linear range [M]
Detection limit [M] Working media
Ref. [5]
n.d.
1.7×10-6
EtOH
:
Tris-HCl
(1:1, v/v); pH 7
CH3CN : H2O
Ref. [6]
Ref. [7]
n.d.
5.03×10-8
1.9×10-7
1×10-6 - 1×10-5
aqueous
solution
containing DMSO;
pH 7
Ref. [11]
n.d.
5.6×10-7
EtOH : THF (99:1,
v/v)
Ref. [12]
Ref. [13]
1×10-5 - 4×10-4
n.d.
6.8×10-6
1.18×10-6
DMF
aqueous
solution
containing DMF; pH
7
Ref. [14]
Ref. [15]
n.d.
9.5×10-7
6.2×10-10
DMSO
1×10-9 - 1×10-3
aqueous solution; pH
4.7
The proposed sensor 8.1×10-4 - 1.5×10-2
2.73×10-6
aqueous solution; pH
2.9
n.d. not discussed