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Sequential detection of Fe3+/2+ and pyrophosphate

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Cite this: DOI: 10.1039/c7pp00354d

by a colorimetric chemosensor in a near-perfect
aqueous solution†
Ju Byeong Chae, Hyo Jung Jang and Cheal Kim *

A novel colorimetric chemosensor 1 was designed and synthesized for Fe3+/2+ and pyrophosphate.
Sensor 1 showed a selective color change toward both Fe3+ and Fe2+ from yellow to brown in a near-
perfect aqueous solution. The detection limits (0.36 μM and 0.37 μM) for Fe3+ and Fe2+ were much lower
than the guideline (5.37 μM) set by the Environmental Protection Agency (EPA) for iron in drinking water.
Received 22nd September 2017, Sensor 1 could be used to quantify Fe3+ in real water samples. Moreover, the resulting Fe3+-2·1 complex
Accepted 4th November 2017
can detect pyrophosphate selectively over various anions especially including phosphate-based anions
DOI: 10.1039/c7pp00354d through a metal-complex displacement method. Based on UV-vis titrations, Job plot and ESI-mass spec-
rsc.li/pps trometry analyses, the sensing mechanisms of Fe3+, Fe2+ and PPi were proposed.

1. Introduction Recent studies showed that chemosensors using the metal-

complex displacement method were reported as one of the suc-
Iron is one of the indispensable metal ions and plays impor- cessful strategies for detecting PPi. Some metal ions such as
tant roles in a wide range of organic and biological processes Zn2+, Cu2+, Cd2+, Al3+ and Ga3+ are used as a metal
such as oxygen-carrying, cellular metabolism, enzymatic reac- source.15,23–28 Nevertheless, only two colorimetric chemo-
tion and other various bio-syntheses.1,2 However, the deficiency sensors were reported for Fe3+ as a metal source in a mixture
or overload of iron in human body can cause various diseases of organic solvent and water until now.29,30 Therefore, it is of
such as anemia, liver damage and hemochromatosis.3,4 importance to develop colorimetric chemosensors capable of
Therefore, detecting iron with high selectivity and sensitivity operating for the system of iron and PPi in pure aqueous
has attracted a great deal of attention in various areas.5–11 solution.
Pyrophosphate (P2O74−, PPi), the product of adenosine tri- Benzofuran moiety with a unique photochemical property
phosphate (ATP) hydrolysis under cellular condition, has been is known to be a good chromophore.31–34 Julolidine moiety is
paid attention because of its important roles in many crucial reac- water-soluble and also a well-known chromophore.35–37 In this
tions, such as energy transduction, metabolic processes and DNA/ regard, we expected that the chemosensor with both benzo-
RNA polymerization.12,13 Considering its significant physiological furan and julolidine moieties might be water-soluble and have
role, therefore, there have been continual efforts to develop the distinct optical properties toward a specific metal ion.
sensors capable of detection and quantification of PPi.14–18 Herein, we report on a multiple-target colorimetric chemo-
Many analytical methods such as atomic absorption-emis- sensor 1, which could detect Fe3+ and Fe2+ by color change
sion spectrometry, electrochemical methods and inductively from yellow to brown in a near-perfect aqueous environment.
coupled plasma atomic emission spectroscopy have been used Moreover, the resulting Fe3+-2·1 complex could be used for
to detect iron ions. However, these methods need time- detection of PPi by the metal-complex displacement mecha-
consuming sample pre-treatments, laborious procedures and nism. Based on UV-vis titrations, Job plot and ESI-mass spec-
expensive equipment. On the other hands, colorimetric trometry analyses, the sensing mechanisms of Fe3+, Fe2+ and
chemosensors have been regarded as useful tools because of PPi were proposed.
their advantages, such as low cost, high sensitivity and easy
monitoring of target ions.19–22
2. Experimental section
Department of Fine Chemistry and Department of Interdisciplinary Bio IT Materials, 2.1. General information
Seoul National University of Science and Technology, Seoul 139-743, Republic of
Korea. E-mail: chealkim@seoultech.ac.kr; Fax: +82-2-973-9149; Tel: +82-2-970-6693
All solvents and reagents (analytical grade and spectroscopic
† Electronic supplementary information (ESI) available: Additional experimental grade) were purchased from Sigma-Aldrich and used without
data. See DOI: 10.1039/c7pp00354d further purification. Both 1H NMR and 13C NMR were recorded

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on a Varian 400 MHz and 100 MHz spectrometer, respectively. each sensor 1 solution. Each cell had a total volume of 3 mL.
The chemical shifts (δ) were recorded in ppm relative to tetra- After stirring them for a few seconds, UV-vis spectra were taken
methylsilane Si(CH3)4. Absorption spectra were recorded at at room temperature. The same experimental procedures were
room temperature using a PerkinElmer model Lambda 2S UV/ also carried out for Fe2+ ion.
Vis spectrometer. Electrospray ionization mass spectra For PPi, sensor 1 (3.8 mg, 0.01 mmol) was dissolved in
(ESI-MS) were collected on a Thermo Finnigan (San Jose, CA, DMSO (1 mL) and 120 μL of this solution was diluted to
USA) LCQ™ Advantage MAX quadrupole ion trap instrument. 39.88 mL bis-tris buffer (10 mM, pH = 7.0) to make the final
concentration of 40 μM. Then, 40 μL of Fe3+ solution (20 mM)
2.2. Synthesis of 1 was trasferred to the sensor 1 solution (40 μM) to make Fe3+-
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3-Aminobenzofuran-2-carboxamide (0.18 g, 1.0 mmol) and 2·1 complex. 2.7, 2.4, 2.1, 1.8, 1.5, 1.2, 0.9, 0.6 and 0.3 mL of
8-hydroxyjulolidine-9-carboxaldehyde (0.22 g, 1.0 mmol) are the Fe3+-2·1 solution were transferred to quartz cells. PPi
dissolved in absolute ethanol (EtOH, 10 mL). Two drops of (0.1 mmol) was dissolved in 1 mL of bis-tris buffer (10 mM,
HCl were added into the reaction solution, which was stirred pH = 7.0) and 16 μL of the PPi solution (100 mM) was diluted
for 5 h at room temperature until an orange precipitate to 39.84 mL bis-tris buffer solution. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8,
appeared. The resulting precipitate was filtered and washed 2.1, 2.4 and 2.7 mL of the diluted PPi solution were transferred
with cold isopropyl alcohol and diethyl ether. The yield: 0.24 g to each Fe3+-2·1 solution. Each cell had a total volume of 3 mL.
(63.0%). 1H NMR (DMSO-d6, 400 MHz, ppm): δ 13.05 (s, 1H), After stirring them for a few seconds, UV-vis spectra were taken
8.69 (s, 1H), 8.42 (t, J = 8 Hz, 2H) 8.07 (s, 1H), 7.71 (d, J = 8 Hz, at room temperature.
1H), 7.63 (t, J = 8 Hz, 1H), 7.48 (m, 2H), 3.42 (m, 4H), 2.64 (m,
4H), 1.85 (m, 4H). 13C NMR (DMSO-d6, 100 MHz, ppm): δ 2.5. Competition tests
161.8, 158.6, 152.9, 135.2, 135.1, 129.1, 128.1, 127.7, 122.9, For Fe3+/2+, stock solutions of MNO3 (M = Na, K, 0.02 mmol),
122.3, 121.5, 117.1, 113.1, 112.0, 106.8, 56.4, 51.0, 50.2, 26.8, M(NO3)2 (M = Zn, Cd, Cu, Mg, Co, Ni, Ca, Mn, Pb, 0.02 mmol),
20.6, 20.1, 19.0 ESI-mass: m/z calcd, for C22H21N3O3 + H+, M(NO3)3 (M = Al, Ga, In, Fe, Cr, 0.02 mmol) and M(ClO4)2 (M =
376.17; found, 376.19. Fe, 0.02 mmol) were separately dissolved in 1 mL bis-tris
buffer (10 mM, pH = 7.0). 2.25 μL of each metal-ion solution
2.3. UV-vis titrations (20 mM) except Fe3+ was diluted to 3 mL bis-tris buffer solu-
For Fe3+/2+, sensor 1 (3.8 mg, 0.01 mmol) was dissolved in di- tion (10 mM, pH = 7.0), respectively. 2.25 μL of Fe3+ solution
methylsulfoxide (DMSO) (1 mL) and 9 μL of this solution (20 mM) was added to the solutions prepared above. Then,
(10 mM) was diluted with 2.991 mL bis-tris buffer (10 mM, 9 μL of 1 solution (10 mM) was added to the mixed solutions.
pH = 7.0) to make a final concentration of 30 μM. Fe(NO3)3 After stirring them for a few seconds, UV-vis spectra were taken
(0.02 mmol) was dissolved in bis-tris buffer solution (1 mL). at room temperature. The same experimental procedure was
Then, 0.225–2.475 μL of the Fe3+ solution (20 mM) were trans- carried out for Fe2+.
ferred to the solution of 1 (30 μM) prepared above. After stir- For PPi, tetraethylammonium salts (0.1 mmol) of CN−, F−,
ring them for a few seconds, UV-vis spectra were taken at room Cl , Br− and I−, tetrabutylammonium salts (0.1 mmol) of
−

temperature. The same experimental procedures were also AcO−, H2PO4−, BzO−, N3−, HP2O73− and SCN−, and sodium
carried out for Fe2+ ion. salts (0.1 mmol) of NO2−, ClO−, S2−, AMP, ADP, ATP and PPi
For PPi, sensor 1 (3.8 mg, 0.01 mmol) was dissolved in were separately dissolved in bis-tris buffer (1 mL). 16.2 μL of
DMSO (1 mL) and 9 μL of this solution (10 mM) was diluted each anion solution (100 mM) except PPi was diluted to 3 mL
with 2.991 mL bis-tris buffer (10 mM, pH = 7.0) to make the bis-tris buffer (10 mM, pH = 7.0), respectively. 16.2 μL of PPi
final concentration of 30 μM. 2.25 μL of Fe3+ solution (20 mM) solution (100 mM) was added to the solutions prepared above.
was transferred to the sensor solution (30 μM) to give 0.5 Then, 9 μL of Fe3+-2·1 complex solution (10 mM) was added to
equiv. Then, PPi (0.1 mmol) was dissolved in 1 mL of bis-tris the mixed solutions. After stirring them for a few seconds, UV-
buffer (10 mM, pH = 7.0) and 0.9–17.1 μL of this PPi solution vis spectra were taken at room temperature.
(100 mM) were transferred to Fe3+-2·1 solution (30 μM) pre-
pared above. After stirring them for a few seconds, UV-vis 2.6. pH effects
spectra were taken at room temperature. For Fe3+/2+, a series of buffers with pH values ranging from 2 to
12 were prepared by mixing sodium hydroxide solution and
2.4. Job plot measurements hydrochloric acid in bis-tris buffer solution. After the solution
For Fe3+/2+, sensor 1 (3.8 mg, 0.01 mmol) was dissolved in with a desired pH was achieved, a stock solution (10 mM) of
DMSO (1 mL) and 160 μL of this solution (10 mM) was diluted sensor 1 was prepared in DMSO (1 mL) and 9.0 μL of this solu-
to 39.84 mL bis-tris buffer (10 mM, pH = 7.0) to make the final tion was diluted to 3 mL of bis-tris buffer to make final con-
concentration of 40 μM. 2.7, 2.4, 2.1, 1.8, 1.5, 1.2, 0.9, 0.6, and centration of 30 μM. Then, 2.25 μL of Fe3+ solution (20 mM)
0.3 mL of the sensor 1 solution were transferred to quartz was transferred to each sensor 1 solution prepared above to
cells. 80 μL of an Fe3+ stock solution (20 mM) was diluted to give 0.5 equiv. After stirring them for a few seconds, UV-vis
39.92 mL bis-tris buffer solution. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, spectra were taken at room temperature. The same experi-
2.4 and 2.7 mL of the diluted Fe3+ solution were transferred to mental procedure was carried out for Fe2+.

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For PPi, a series of buffers with pH values ranging from 2 to

12 were prepared by mixing sodium hydroxide solution and
hydrochloric acid in bis-tris buffer. After the solution with a
desired pH was achieved, a stock solution (10 mM) of sensor 1
was prepared in DMSO (1 mL) and 9.0 μL of this solution was
diluted to 3 mL of bis-tris buffer to make final concentration
of 30 μM. 2.25 μL of Fe3+ solution (20 mM) was transferred to
each sensor 1 solution prepared above to give 0.5 equiv. Then,
16.2 μL of a PPi stock solution (100 mM) was transferred to
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each Fe3+-2·1 solution prepared above to give 18 equiv. After

stirring them for a few seconds, UV-vis spectra were taken at
room temperature.

2.7. Determination of Fe3+ in water samples

UV-vis spectral measurements of water samples (tap and drink-
ing water) containing Fe3+ were performed by adding 12 μL of
10 mM stock solution of 1 and 0.3 mL of 100 mM bis-tris
buffer stock solution to 2.688 mL sample solutions. After stir-
ring them for a few seconds, UV-vis spectra were taken at room
Fig. 1 (a) UV-vis absorption and (b) color changes of 1 (30 μM) upon
temperature.
addition of 0.5 equiv. of different metal ions in bis-tris buffer (10 mM,
pH = 7.0).

3. Results and discussion

Sensor 1 was synthesized by the condensation reaction of
8-hydroxyjulolidine-9-carboxaldehyde with 3-aminobenzo-
furan-2-carboxamide in absolute EtOH at room temperature
(Scheme 1), and characterized by 1H and 13C NMR and ESI-
mass spectrometry analysis.

3.1. Colorimetric sensing of 1 toward Fe2+ and Fe3+

The UV-vis spectral changes of 1 were conducted with various
metal ions such as Al3+, Ga3+, In3+, Zn2+, Cd2+, Cu2+, Fe2+, Fe3+,
Mg2+, Cr3+, Co2+, Ni2+, Na+, K+, Ca2+, Mn2+ and Pb2+ in bis-tris
buffer solution (10 mM, pH = 7.0). As shown in Fig. 1, there
were no spectral and color changes in the absence and pres-
ence of most metal ions. In contrast, Fe3+ and Fe2+ ions caused
both distinct spectral and color changes from yellow to brown.
Hg2+ precipitated out with sensor 1 under the conditions.
These results indicated that 1 could be used as a colorimetric
chemosensor for detection of Fe3+ and Fe2+ ions via “naked-
eye” in a near-perfect aqueous solution.
In order to investigate the binding property of 1 with Fe3+
Fig. 2 UV-vis absorption change of 1 (30 μM) with Fe3+ ions (0–0.55
ions, UV-vis titration experiments were conducted (Fig. 2). equiv.) in bis-tris buffer (10 mM, pH = 7.0). Inset: Plot of the absorbance
Upon the addition of Fe3+, the absorbance band at 430 nm at 490 nm as a function of Fe3+ concentration.

Scheme 1 Synthetic procedure of 1.

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decreased gradually, while the absorbance at 350 nm and water.43 Thus, sensor 1 could be a powerful tool for the detec-
490 nm increased with a clear isosbestic point at 382 nm. The tion of Fe3+ ion in drinking water. To check the selectivity of 1
isosbestic point indicated the clean conversion of the free towards Fe3+ ions over the various metal ions, competitive
sensor 1 to an iron complex. The peak at 490 nm with high studies were carried out (Fig. 4). There was no significant inter-
molar extinction coefficient, 1.0 × 104 M−1 cm−1 (ε490 nm), is ference in the detection of Fe3+. These results indicated that
too large to be Fe-based d–d transitions. Thus, the new peak sensor 1 could be efficiently used for selective detection of Fe3+.
might be attributed to a metal-to-ligand charge-transfer For the practical application, the effect of pH on the absor-
(MLCT),38–40 resulting in the color change of the solution. bance change of 1 to Fe3+ was studied in a series of pH
To examine the binding stoichiometry of 1 with Fe3+, the ranging from 2 to 12 (Fig. S4†). The result showed that 1 can
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Job plot analysis was conducted (Fig. S1†). A maximum absor- act as a colorimetric chemosensor for Fe3+ between pH 6 and
bance appeared at the molar fraction of 0.3, which indicated a 8. This result warranted its application under environmental
2 : 1 binding mode between 1 and Fe3+. To further confirm the conditions, without any change in detection of Fe3+.
binding mode between 1 and Fe3+, ESI-mass spectrometry ana- Moreover, the real sample analysis was conducted for quanti-
lysis was carried out (Fig. 3). The peak at m/z = 804.32 was tative measurement of Fe3+. As shown in Fig. 5, a good calibration
assignable to 2·1-2H+ + Fe3+ [calcd, m/z = 804.23]. Based on curve was obtained between 1 and Fe3+. Then, 1 was applied for
UV-vis titration, Job plot and ESI-mass analyses, the proposed the determination of Fe3+ in both tap and drinking water samples
structure of Fe3+-2·1 complex is shown in Scheme 2. (Table 1). Appropriate recoveries and R.S.D values were obtained
On the basis of the Li’s equation,41 the association constant for the water samples. These results indicated that sensor 1 can
(K) of 1 with Fe3+ was calculated as 5.0 × 109 M−2 (Fig. S2†). be used for the determination of Fe3+ levels in real samples.
The detection limit of sensor 1 for Fe3+ was calculated to be
0.36 μM on the basis of 3σ per slope (Fig. S3†).42 The value is
much lower than the guideline (5.37 μM) set by the
Environmental Protection Agency (EPA) for iron in drinking

Fig. 4 (a) UV-vis absorption (at 490 nm) and (b) color changes of 1
Fig. 3 Positive-ion ESI-mass spectrum of 1 (100 μM) upon addition of (30 μM) upon addition of Fe3+ (0.5 equiv.) in the absence and presence
0.5 equiv. of Fe3+. of other metal ions (0.5 equiv.).

Scheme 2 Proposed structure of Fe3+-2·1 complex.

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sensor 1 with Fe3+/2+ were evaluated (Fig. S9†). The formation

time of Fe3+-2·1 obtained from the reaction of 1 with Fe2+ was
about 2 times slower (240 s) than that obtained from the reac-
tion of 1 with Fe3+ (120 s). This observation further suggested
the oxidation of Fe2+ in the Fe2+-2·1 complex into Fe3+ by
oxygen molecule. Furthermore, we carried out electron para-
magnetic resonance (EPR) study to verify the oxidation state of
the iron ion in the Fe2+-2·1 complex formed from the reaction
of 1 and Fe2+. It showed the typical high-spin (S = 3/2) Fe3+
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with the value of g = 4.2 (Fig. S10†). These results clearly

proved that Fe2+ of the 2·1-Fe2+ complex might be rapidly oxi-
dized to Fe3+ by air. Based on the Job plot, ESI-mass spec-
trometry analyses, degassed condition experiment, time-
dependence experiment and EPR study, we proposed the
sensing mechanism of Fe2+ by 1 as shown in Scheme 2.
On the basis of the Li’s equation,40 the association con-
Fig. 5 UV-vis absorbance (at 490 nm) of 1 as a function of Fe3+ con-
stants (K) of 1 with Fe2+ ion was calculated as 3.0 × 109 M−2
centration. [1] = 40 μM and [Fe3+] = 0–8 μM in bis-tris buffer solution
(10 mM, pH = 7.0).
(Fig. S11†). The detection limit of sensor 1 for Fe2+ was calcu-
lated as 0.37 μM (Fig. S12†) using the 3σ per slope,41 which
was nearly identical to that for Fe3+.
Competitive studies also showed a similar tendency to
Table 1 Determination of Fe3+ in water samplesa
those of Fe3+ (Fig. S13†). For the practical application, the
Fe(III) added Fe(III) found Recovery R.S.D effect of pH on the absorbance change of 1 to Fe2+ was studied
Sample (μmol L−1) (μmol L−1) (%) (n = 3) (%) in a series of pH ranging from 2 to 12 (Fig. S14†). The result
showed that 1 can act as a colorimetric chemosensor for Fe2+
Tap water 0.00 0.00 — —
4.00 3.85 96.3 1.8 between pH 6 and 8.
Drink water 0.00 0.00 — —
4.00 3.88 97.0 0.5 3.2. Colorimetric and spectral responses of Fe3+-2·1 complex
toward PPi
a
Conditions: [1] = 40 μmol L−1 in bis-tris buffer (10 mM, pH 7.0).
In recent years, some metal complexes showed the selectivity
toward specific anions as shown in the systems such as Cu–S,
Next, UV-vis titration experiments of 1 with Fe2+ were con- Ni–CN, and Al–F.46–50 Therefore, we also examined the selecti-
ducted to understand their binding property (Fig. S5†). The vity of Fe3+-2·1 complex toward various anions such as CN−,
results were nearly identical to those obtained from the reac- AcO−, F−, Cl−, Br−, I−, H2PO4−, BzO−, N3−, SCN−, NO2−, ClO−,
tion of 1 with Fe3+. Upon the addition of Fe2+, the absorbance S2−, HP2O73−, AMP, ADP, ATP and PPi in bis-tris buffer solu-
band at 430 nm decreased gradually, while the absorbance at tion (Fig. 6). Upon addition of 18 equiv. of each anion to Fe3+-
350 nm and 490 nm increased with two clear isosbestic points 2·1, there were no spectral changes in absorption bands in the
at 382 nm and 468 nm. presence of CN−, AcO−, F−, Cl−, Br−, I−, H2PO4−, BzO−, N3−,
The binding stoichiometry between 1 and Fe2+ was deter- SCN−, NO2−, ClO−, S2−, AMP, ADP and ATP. In contrast, the
mined by the Job plot analysis. The result also showed a 2 : 1 addition of PPi into Fe3+-2·1 complex showed immediate spec-
ratio as shown in case of the Fe3+-2·1 complex (Fig. S6†). The tral and color changes from brown to yellow. These results
2 : 1 stoichiometry was supported by ESI-mass spectrometry indicated that Fe3+-2·1 complex can serve as a “naked eye”
analysis (Fig. S7†). The peak at m/z = 804.29 was assignable to chemosensor for PPi in a near-perfect aqueous solution.
2·1-2H+ + Fe3+ [calcd, m/z = 804.23]. These results led us to Importantly, this is the first example that a chemosensor could
propose that Fe3+ ion was generated by the rapid oxidation of sequentially detect Fe3+ and PPi in a near-perfect aqueous
Fe2+ ion in the Fe2+-2·1 complex by oxygen molecule.39,44,45 solution, to the best of our knowledge (Table S1†).
To verify our proposal, we examined the spectral changes of The binding properties of Fe3+-2·1 with PPi were studied by
sensor 1 with Fe2+ under the degassed conditions (Fig. S8†). UV-vis titration experiments (Fig. 7). When PPi was added to
Upon the addition of Fe2+ into a solution of 1 under an anaero- Fe3+-2·1 solution, the absorption band at 430 nm gradually
bic condition, there was no significant change in spectrum of increased, while the absorbance at 350 nm and 490 nm gradu-
1. However, when Fe2+-2·1 complex solution was exposed to ally decreased and reached a minimum at 18 equiv. of PPi. An
air, its spectrum was remarkably changed and nearly identical isosbestic point was observed at 396 nm, indicating that only
to that of Fe3+-2·1 complex. These results indicated that the one product was generated from the interaction of Fe3+-2·1
Fe2+-2·1 complex formed under the degassed conditions might with PPi.
be oxidized to the Fe3+-2·1 complex by oxygen molecule. To examine the binding mode between Fe3+-2·1 and PPi,
Additionally, the time-dependent changes for the reactions of the Job plot analysis was carried out. The result showed a 1 : 1

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stoichiometric ratio between the Fe3+-2·1 and PPi (Fig. S15†).

To further understand the binding mode of Fe3+-2·1 and PPi,
ESI-mass spectrometry analysis was conducted (Fig. S16†). The
positive ion mass spectrum showed that a peak at m/z = 476.24
was assignable to 1 + Na+ + DMSO [calcd, m/z = 476.16], indi-
cating that sensor 1 might be released from Fe3+-2·1 complex
by the metal-complex displacement method (Scheme 3).51–53
Based on the result of UV-vis titration, the association con-
stant between Fe3+-2·1 and PPi was calculated as 4.0 × 103 M−1
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from Benesi–Hildebrand equation (Fig. S17†).54 The detection

limit for PPi was determined to be 14.16 μM on the basis of 3σ
per slope (Fig. S18†).42
In order to check practical applicability of Fe3+-2·1 as a PPi-
selective sensor, we conducted competition experiments
(Fig. 8). When Fe3+-2·1 complex was treated with 18 equiv. of
PPi and 18 equiv. of other coexistent anions such as CN−,
AcO−, F−, Cl−, Br−, I−, H2PO4−, BzO−, N3−, SCN−, NO2−, ClO−,

Fig. 6 (a) Absorption spectral changes of Fe3+-2·1 complex (30 μM) in

the presence of 18 equiv. of different anions in bis-tris buffer solution
(10 mM, pH = 7.0). (b) The color changes of Fe3+-2·1 complex (30 μM)
upon addition of various anions (18 equiv.) in bis-tris buffer solution.

Fig. 7 UV-vis absorption changes of Fe3+-2·1 complex (30 μM) in the

presence of different concentrations of PPi (0–19 equiv.) in bis-tris Fig. 8 (a) UV-vis absorbance (at 490 nm) and (b) color changes of
buffer solution (10 mM, pH = 7.0) at room temperature. Inset: Fe3+-2·1 (30 μM) upon addition of PPi (18 equiv.) in the absence and
Absorption at 490 nm versus the number of equiv. of PPi added. presence of other anions (18 equiv.).

Scheme 3 Proposed sensing mechanism of PPi by Fe3+-2·1.

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S2−, AMP, ADP and ATP in bis-tris buffer solution, there was no 2 H. J. Jung, N. Singh and D. O. Jang, Highly Fe3+ selective
interference for the detection of PPi by Fe3+-2·1. These results ratiometric fluorescent probe based on imine-linked benzi-
indicated that Fe3+-2·1 could be an excellent chromogenic midazole, Tetrahedron Lett., 2008, 49, 2960–2964.
chemosensor with high selectivity for PPi over competing 3 N. Narayanaswamy and T. Govindaraju, Aldazine-based col-
anions and even phosphate-based ones. In order to investigate orimetric sensors for Cu2+ and Fe3+, Sens. Actuators, B,
pH dependence of Fe3+-2·1 with PPi, the pH test was carried 2012, 161, 304–310.
out in a wide range of pH (Fig. S19†). The optimal range for 4 V. M. Cardenas, Z. D. Mulla, M. Ortiz and D. Y. Graham,
the colorimetric sensing of PPi by Fe3+-2·1 was turned out to Iron deficiency and Helicobacter pylori infection in the
be between pH 6 and 8. Thus, it is able to detect PPi by Fe3+- United States, Am. J. Epidemiol., 2006, 163, 127–134.
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2·1 complex under environmental conditions without any 5 S. Das, K. Aich, S. Goswami, C. K. Quah and H. K. Fun,
change in detection of PPi. FRET-based fluorescence ratiometric and colorimetric
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4. Conclusion 6 V. K. Gupta, O. Moradi, I. Tyagi, S. Agarwal, H. Sadegh,
R. Shahryari-Ghoshekandi, A. S. H. Makhlouf, M. Goodarzi
We presented a simple, selective and efficient sensor 1 for the and A. Garshasbi, Study on the removal of heavy metal ions
sequential detection of Fe3+/2+ and PPi by naked-eye color from industry waste by carbon nanotubes: effect of the
change. Senor 1 could selectively bind with Fe3+/2+ among surface modification: a review, Crit. Rev. Environ. Sci.
other competitive metal ions, and detect lower concentrations Technol., 2016, 46, 93–118.
of Fe3+ (0.36 μM) and Fe2+ (0.37 μM) than EPA drinking guide- 7 M. Zhu, C. Shi and X. Xu, Near-infrared cyanine-based
line (5.37 μM). Based on the ESI-mass spectroscopy analysis, sensor for Fe3+ with high sensitivity: its intracellular
degassed condition experiments and EPR study, we demon- imaging application in colorectal cancer cells, RSC Adv.,
strated that Fe2+ of the Fe2+-2·1 complex formed from the reac- 2016, 6, 100759–100764.
tion of Fe2+ and 1 was rapidly oxidized to Fe3+ in air. Sensor 1 8 N. V. Ghule, R. S. Bhosale, A. L. Puyad, S. V. Bhosale and
could successfully quantify Fe3+ in real water samples. S. V. Bhosale, Naphthalenediimide amphiphile based col-
Moreover, the resulting Fe3+-2·1 complex could selectively orimetric probe for recognition of Cu2+ and Fe3+ ions, Sens.
detect PPi in the presence of other various anions, especially Actuators, B, 2016, 227, 17–23.
including phosphate-based anions. The detection mechanism 9 T. Nandhini, P. Kaleeswaran and K. Pitchumani, A highly
for PPi was proposed to be a metal-complex displacement selective, sensitive and “turn-on” fluorescent sensor for the
method, based on spectroscopic studies. Importantly, this is paramagnetic Fe3+ ion, Sens. Actuators, B, 2016, 230, 199–205.
the first example that a chemosensor could sequentially detect 10 C. J. Hua, H. Zheng, K. Zhang, M. Xin, J. R. Gao and
Fe3+ and PPi in a near-perfect aqueous solution. Therefore, we Y. J. Li, A novel turn off fluorescent sensor for Fe(III) and
believe that these results may contribute to development of a pH environment based on coumarin derivatives: the fluo-
new type of chemosensors to sequentially detect Fe3+ and PPi rescence characteristics and theoretical study, Tetrahedron,
in aqueous solution. 2016, 72, 8365–8372.
11 A. Luo, H. Wang, Y. Wang, Q. Huang and Q. Zhang, A novel
colorimetric and turn-on fluorescent chemosensor for
Conflicts of interest iron(III) ion detection and its application to cellular
imaging, Spectrochim. Acta, Part A, 2016, 168, 37–44.
There are no conflicts to declare.
12 M. Hosseini, M. R. Ganjali, M. Tavakoli, P. Norouzi,
F. Faridbod, H. Goldooz and A. Badiei, Pyrophosphate
selective recognition in aqueous solution based on fluo-
Acknowledgements rescence enhancement of a new aluminium complex,
Basic Science Research Program through the National J. Fluoresc., 2011, 21, 1509–1513.
Research Foundation of Korea (NRF) funded by the Ministry of 13 S. Goswami, S. Paul and A. Manna, Selective “naked eye”
Education, Science and Technology (NRF-2014R1A2A1- detection of Al(III) and PPi in aqueous media on a rhoda-
A11051794 and NRF-2016M3D3A1A01913239 for the Korea C1 mine–isatin hybrid moiety, RSC Adv., 2013, 3, 10639–10643.
Gas Refinery R&D Center) are gratefully acknowledged. 14 L. M. Mesquita, V. André, C. V. Esteves, T. Palmeira,
M. N. Berberan-Santos, P. Mateus and R. Delgado,
Dinuclear Zinc(II) Macrocyclic Complex as Receptor for
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