Triazatruxene–Rhodamine-Based Ratiometric Fluorescent Chemosensor for the Sensitive, Rapid Detection of Trivalent Metal Ions: Aluminium (III), Iron (III) and Chromium (III)

Article abstract of DOI:10.1007/s10895-020-02491-5

We investigated the ability of a novel triazatruxene–rhodamine-based (TAT-ROD) chemosensor to detect the trivalent metal ions aluminium (Al³⁺), iron (Fe³⁺) and chromium (Cr³⁺). Operating via the through-bond energy transfer (TBET) pathway, the chemosensor exhibited low detection limits of 23.0, 25.0 and 170.0 nM for Al³⁺, Fe³⁺ and Cr³⁺, respectively, along with high sensitivity and selectivity during a brief period (<15 s). The binding ratio of the chemosensor and trivalent metal ions achieved by Job’s method was 3:1, and when we added ethylenediaminetetraacetic acid (EDTA), the sensing process reversed. Altogether, our TAT-ROD chemosensor marks the first triazatruxene-based colorimetric and fluorometric metal ion sensor reported in the literature.

Full text of DOI:10.1007/s10895-020-02491-5

Journal of Fluorescence
https://doi.org/10.1007/s10895-020-02491-5
ORIGINAL ARTICLE
Triazatruxene–Rhodamine-Based Ratiometric Fluorescent
Chemosensor for the Sensitive, Rapid Detection of Trivalent Metal
Ions: Aluminium (III), Iron (III) and Chromium (III)
Ali Enis Sadak1 & Erman Karakuş1
Received: 5 December 2019 /Accepted: 15 January 2020
#
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
We investigated the ability of a novel triazatruxene–rhodamine-based (TAT-ROD) chemosensor to detect the trivalent metal ions
aluminium (Al3+), iron (Fe3+) and chromium (Cr3+). Operating via the through-bond energy transfer (TBET) pathway, the
chemosensor exhibited low detection limits of 23.0, 25.0 and 170.0 nM for Al3+, Fe3+ and Cr3+, respectively, along with high
sensitivity and selectivity during a brief period (<15 s). The binding ratio of the chemosensor and trivalent metal ions achieved by
Job’s method was 3:1, and when we added ethylenediaminetetraacetic acid (EDTA), the sensing process reversed. Altogether, our
TAT-ROD chemosensor marks the first triazatruxene-based colorimetric and fluorometric metal ion sensor reported in the
literature.
.
.
.
.
Keywords Triazatruxene Rhodamine TBET Trivalent metal ions Ratiometric sensor
Introduction
reactions and the transport of oxygen [5, 6]. In sharp contrast
to their beneficial effects, Fe3+ ions can also cause diseases
such as Alzheimer’s, Huntington’s, Parkinson’s, anemia and
diabetes [7–11]. Yet another trivalent metal ion, chromium
(Cr3+), is an important element for human nutrition given its
influence on the metabolism of proteins, carbohydrates, fats
and nucleic acids by triggering certain enzymes and stabiliz-
ing both proteins and nucleic acids. However, excess Cr3+,
typically caused by environmental pollution from industrial
flows that threatens agriculture, can increase the possibility
of distorted glucose levels and lipid metabolism [12–14].
Therefore, to improve chemo-sensing as well as molecular
imaging, research on developing selective, sensitive signalling
frameworks for determining and visualising trivalent metal
ions remains necessary.
Given their biological and environmental properties, trivalent
metal ions play important roles in plant and animal biology.
For example, aluminium (Al3+), the third-most abundant ele-
ment in Earth’s crust, can adversely affect human health when
accumulated to excess, such as in Alzheimer’s disease,
Parkinson’s disease, osteoporosis and bone softening [1, 2].
Moreover, Al3+ concentration increases with increased levels
of acid rain, which can fatally affect living plants [3, 4].
Another trivalent metal ion, iron (Fe3+), is vital in living sys-
tems and plays an indispensable role in biochemical events at
the molecular and cellular levels, including in enzymatic
Ali Enis Sadak and Erman Karakuş contributed equally to this work.
Recently, several types of fluorescent probes have been
devised to detect trivalent metal ions such as Al3+, Fe3+ and
Cr3+ [15–18]. Most of those probes operate via optical signal
changing, which involves either an increase (i.e. turn-on) or a
decrease (i.e. turn-off) in emission intensity without causing
any detectable shifts in emission wavelengths. However, en-
vironmental effects, including variations in the concentration
and excitation intensity, can easily influence the sensitivity of
measurement. As an antidote, ratiometric sensors, which af-
ford a built-in correction for environmental factors, could min-
imize problems common among intensity-based fluorescent
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10895-020-02491-5) contains supplementary
material, which is available to authorized users.
* Ali Enis Sadak
alienis.sadak@tubitak.gov.tr
* Erman Karakuş
erman.karakus@tubitak.gov.tr
1
Organic Chemistry Laboratory, Chemistry Group, National
Metrology Institute, (TUBITAK UME) Scientific and Technological
Research Council of Turkey, 41470 Gebze, Kocaeli, Turkey

J Fluoresc
probes. Nevertheless, studies on the ratiometric detection of
trivalent metal ions have been few [19–23].
(250 mL) and neutralized by the addition of adequate amount
of saturated KOH solution. The resulting brown coloured pre-
cipitate was collected by filtration and the crude product was
purified by silica gel column chromatography by using 4:1
acetone/hexane as the eluent. After crystallization from 1:1
acetone/hexane, TAT was obtained (Yield: 40%, 0.8 g) [35].
1H-NMR (600 MHz, DMSO): δ ppm 11.86 (bs, 3H), 8.67 (d,
J = 7.6 Hz, 3H), 7.73 (d, J = 7.6 Hz, 3H), 7.40–7.32 (m, 6H).
APT 13C-NMR (150 MHz, DMSO): δ ppm 139.0, 134.2,
123.0, 122.7, 120.3, 119.5, 111.4, 101.0.
In response, we constructed a molecular sensor that chem-
ically integrates two fluorophore units—triazatruxene (TAT)
and rhodamine B—with each other in order to detect trivalent
metal ions. On the one hand, given their excellent spectro-
scopic properties, including large molar extinction coeffi-
cients, high fluorescence quantum yields and both long ab-
sorption and emission wavelengths, rhodamine frameworks
are excellent candidates for constructing off- and on-type fluo-
rescent probes. On the other, TAT, containing a symmetrical
cyclotrimer of an indole derivative, is a planar, conjugated
aromatic molecule with a strong electron-donating ability
[24–26]. TAT derivatives have been extensively used in or-
ganic light-emitting diodes, semiconducting liquid crystals
and hole-transporting materials [27, 28]. Due to the superior-
ity of its weak π–π stacking backbones, the TAT complex
behaves similarly to fluorescent probes, especially in interac-
tions that decrease fluorescence [29, 30]. Despite reported
uses of TAT-based fluorescent sensors to detect nitro-
aromatic explosive vapours [31, 32] and lectin derivatives
[33, 34], our study was the first, to pursue the fluorometric
and colorimetric detection of metal ions by using fluorophores
containing TAT.
Synthesis and Characterization
of 5,10,15-Tributyl-10,15-Dihydro-
5H-Diindolo[3,2-a:3′,2′-c]Carbazole (TAT-ALKYL)
TAT (0.5 g, 1.45 mmol) and KOH (0.81 g, 14.50 mmol) was
dissolved in 100 mL of anhydrous tetrahydrofuran (THF)
heated at 70 °C for 3.5 h. After the mixture was cooled to
room temperature, 1-bromobutane (0.65 g, 4.78 mmol) was
added to mixture. The mixture was stirred magnetically at
room temperature for 1 d. After the reaction was completed,
the solvent was removed in vacuo. The crude product was
dissolved in 200 mL of ethyl acetate (EtOAc) and the organic
phase was washed with water (3 × 100 mL) and dried over
Na2SO4. The solvent was removed in vacuo. The crude prod-
uct was purified by silica gel column chromatography by
using 1:4 CH2Cl2/hexane. After crystallization over 1:1
Experimental Details
1
General Methods
CH2Cl2/acetone, obtained (Yield: 90%, 1.1 g). H-NMR
(600 MHz, CDCl3): δ ppm 8.30 (d, J = 8.0 Hz, 3H), 7.65 (d,
J = 8.0 Hz, 3H), 7.46 (t, J = 7.4 Hz, 3H), 7.35 (t, J = 7.4 Hz,
3H), 4.95 (t, J = 7.8 Hz, 6H), 2.00–1.94 (m, 6H), 1.31–1.28
(m, 6H), 0.88 (t, J = 7.4 Hz, 9H). APT 13C-NMR (150 MHz,
CDCl3): δ ppm 143.7, 141.6, 126.2, 125.2, 124.1, 122.2,
113.0105.9, 49.1, 33.7, 23.0, 16.1. HRMS: m/z: Calcd. for
(C36H39N3) [M + H+]: 514.32234; found, 514.32142.
All reagents were purchased from commercial suppliers
(Aldrich and Merck) and used without further purification.
1H NMR and 13C NMR were measured on a Varian
VNMRJ 600 Nuclear Magnetic Resonance Spectrometer.
Mass analysis was conducted with a Thermo Q Exactive
Orbitrap device. UV absorption spectrum was obtained with
a Varian Cary UV-5000 Spectrophotometer, whereas fluores-
cence emission spectra were obtained using a Varian Cary
Eclipse Fluorescence spectrophotometer. Samples were kept
in 2.0 mL quartz cuvettes with 10.0 mm path lengths. Upon
excitation at 360 nm, the emission spectra were integrated in a
range 380 nm to 700 nm (Both excitation and emission slit
width 5 nm / 5 nm). All measurements were performed in
triplicate at least.
Synthesis and Characterization
of 5,10,15-Tributyl-10,15-Dihydro-
5H-Diindolo[3,2-a:3′,2′-c]
Carbazole-3,8,13-Tricarbaldehyde (TAT-AL)
A solution of TAT-ALKYL (1 g, 1.95 mmol) in 10 mL of
CH2Cl2 was cooled to 0 °C under nitrogen atmosphere. SnCl4
(1 g, 3.82 mmol) and dichloromethyl methyl ether (0.44 g,
3.86 mmol) were added to the reaction mixture with syringe
respectively. The mixture was permitted to warm to room
temperature within half an hour and the mixture was stirred
magnetically for 36 h at same temperature. After the reaction
was completed, black solution was transferred with methylene
chloride (3 × 30 mL) to a 250-mL separating funnel contain-
ing 30 g of crushed ice. Combined organic phases were
washed with saturated NaHCO3 (2 × 50 mL) and brine (2 ×
50 mL), dried over Na2SO4. The solvent was removed under
Synthesis and Characterization
of 10,15-Dihydro-5H-Diindolo[3,2-a:3′,2′-c]Carbazole
(TAT)
Phosphorous (V) oxychloride (POCl3) (10 mL) and 2-
oxoindole (2 g, 0.015 mol) were added into a 250 mL
round-bottomed flask and the reaction mixture was stirred at
100 °C for 8 h. The solution was cooled to room temperature
and transferred into a 500 mL beaker containing crushed ice

J Fluoresc
Scheme 1 Synthesis pathway of TAT-ROD
vacuo and the dark brown (0.55 g) was purified by silica gel
column chromatography by using 1:5 CH2Cl2/hexane. After
crystallization from 1:1 CH2Cl2/hexane, the compound TAT-
AL was obtained (Yield: 30%, 0.35 g). 1H-NMR (600 MHz,
CDCl3): δ ppm 10.16 (s, 3H), 8.79 (s, 3H), 8.01 (d, J = 7.8 Hz,
3H), 7.73 (d, J = 8.4 Hz, 3H), 4.96 (t, J = 7.8 Hz, 6H), 2.07–
2.02 (m, 6H), 1.40–1.35 (m, 6H), 0.91 (t, J = 7.4 Hz, 9H).
APT 13C-NMR (150 MHz, CDCl3): δ ppm 194.4, 147.3,
142.3, 132.2, 128.8, 126.7, 125.5, 113.4106.6, 49.7, 35.1,
22.4, 16.2. HRMS: m/z: Calcd. for (C39H39N3O3) [M + H+]:
598.30708; found, 598.31008.
Synthesis and Characterization of TAT-ROD
Rhodamine B hydrazide (ROD-HDZ) was synthesized
according to literature procedure [36]. To a solution of
rhodamine B hydrazide (172 mg, 0.376 mmol) in abso-
lute ethanol (10 ml) was added TAT-AL (75 mg,
0.125 mmol) in the presence of 3 drops of acetic acid
(AcOH). The solution was stirred overnight at reflux
temperature. The reaction mixture was extracted with
dichloromethane (3 × 10 mL). Then the collected organic
layers were dried over anhydrous MgSO4, concentrated
200
TAT-ROD Only
TAT-ROD Only
a)
3+
3+
b)
TAT-ROD + Al
TAT-ROD + Fe
TAT-ROD + Al
0.03
0.02
0.01
0.00
3+
3+
3+
TAT-ROD + Fe
3+
TAT-ROD + Cr
TAT-ROD + Cr
150
100
50
0
350
400
450
500
550
600
650
700
450
500
550
600
650
700
Wavelength (nm)
Fig. 1 Emission and Absorption spectra of TAT-ROD (5 μM) and trivalent metal ions (M3+) (10 equiv.) in MeOH (λex:360 nm)
Wavelength (nm)

J Fluoresc
Na+, K+, Li+, Ag+, Ba2+
200
150
100
50
,
350
300
250
200
150
100
50
Ca2+, Ni2+, Cd2+, Co2+
,
Fe2+, Mn2+, Hg2+, Zn2+
Ce3+, Au3+, Cr6+
Cr3+
Al3+
Al3+
Fe3+
Al3+
0
0
450
500
550
600
650
700
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Fig. 4 Fluorescence intensities of TAT-ROD (5 μM) in MeOH in the
presence of 10 equiv. of the M3+ ions and 50 equiv. of the other metal ions
interest (λex:360 nm)
Fig. 2 Fluorescence spectra of TAT-ROD (5 μM) in MeOH
(λex:360 nm) in the presence of Al3+ (mole equivalents 0–10)
under vacuum, and purified by column chromatography
(hexane/EtOAc = 4/1) to give 120 mg of TAT-ROD
(50%) as a yellow-orange solid. 1H-NMR (600 MHz,
CDCl3): δ ppm 9.37 (s, 3H), 8.43 (s, 3H), 8.00 (d,
J = 7.8 Hz, 3H), 7.71 (d, J = 8.4 Hz, 3H), 7.57 (d, J =
8.4 Hz, 3H), 7.50–7.48 (m, 6H), 7.17 (d, J = 7.8 Hz,
3H), 6.60 (d, J = 8.8 Hz, 6H), 6.52–6.47 (m, 6H),
6.28–6.26 (m, 6H), 4.84 (t, J = 6.8 Hz, 6H), 3.34–3.28
(m, 24H), 1.65–1.62 (m, 6H), 1.28–1.25 (m, 6H), 1.12
(t, J = 6.8 Hz, 36H), 0.89–0.88 (m, 9H). APT 13C-NMR
(150 MHz, CDCl3): δ ppm 167.4, 156.1, 154.2, 153.3,
151.5, 144.9, 141.7, 135.6, 132.9, 130.9, 130.8, 130.1,
126.6, 125.8, 125.7, 123.7, 113.0, 110.6, 109.4, 106.3,
106.2, 100.5, 69.1, 49.3, 46.9, 34.3, 22.4, 16.5, 16.4,
15.3. HRMS: m/z: Calcd. for (C123H129N15O6) [M +
H+]: 1914.03297; found, 1914.03668.
Results and Discussion
We synthesized the TAT-ROD chemosensor by setting rho-
damine B hydrazide in reaction with an aldehyde derivative of
alkylated TAT, as shown in Scheme 1, and the structure of the
TAT-ROD was unambiguously confirmed by 1H NMR, 13
C
NMR and HRMS techniques, as detailed in the Supporting
Information (SI). We created the TAT-ROD to contain TAT
fluorophore as energy donor and rhodamine fluorophores as
energy acceptors. Thus, energy transfer was expected to occur
from TAT to rhodamine via TBET, which does not require any
spectral overlap between donors and acceptors.
Before adding any metal cations, we investigated the spec-
troscopic property of the TAT-ROD by way of both U/V-vis
and fluorescence spectroscopy in methanol solution. As
TAT-ROD Only
3+
TAT-ROD + Al
200
200
3+
TAT-ROD + Al + EDTA
TAT-ROD Only
3+
3+
15 sec
1 min
2 min
TAT-ROD + Al + EDTA+ Al
150
100
50
150
3 min
4 min
5 min
100
Al3+
50
0
450
500
550
600
650
700
0
Wavelength (nm)
450
500
550
600
650
700
Fig. 5 Fluorescence intensity changes of TAT-ROD (5 μM) in MeOH
(black line) after addition of 10 equiv. Al3+ (red line), 10 equiv. Al3+ + 10
equiv. EDTA (blue line) and 10 equiv. Al3+ + 10 equiv. EDTA +10 equiv.
Al3+ (green line) respectively
Wavelength (nm)
Fig. 3 Time-dependent fluorescence change of TAT-ROD (5 μM) and
Al3+ (5 equiv.) in MeOH (λex:360 nm)

J Fluoresc
Fig. 6 Job’s plot showing the 3:1
binding of Al3+ to TAT-ROD
130
110
90
70
50
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
[Al3+]/(TAT-ROD+[Al3+])
shown in Fig. 1a, the free nearly colourless TAT-ROD (5 μM,
in MeOH), exhibited a peak at 375 nm only. After we added
trivalent metal ions (i.e., Al3+, Fe3+ and Cr3+ collectively des-
ignated as M3+), a new absorption peak belonging to the open
form of the rhodamine dye appeared at 556 nm with an in-
crease in intensity, and the solution became a brilliant pink.
Under similar conditions, when the TAT-ROD (5 μM) excited
at 360 nm, it displayed green fluorescence with a single emis-
sion peak in the TAT part of the probe, with emission maxima
at 512 nm. Although no emission band appeared in the rho-
damine region due to its stable ring-closed isomeric form,
adding trivalent metal ions prompted the emergence of a
new, strong fluorescence peak cantered at 580 nm. Such an
event suggests that the rhodamine derivative primarily
remained in the ring-open form while the TAT moiety de-
creased in intensity (Fig. 1b).
We performed fluorescence titration experiments for the
TAT-ROD chemosensor with increasing concentrations of
M3+. Among the results, M3+ exhibited good linear correla-
tions in concentrations of 1–5 μM for Al3+ (Fig. 2) and Fe3+
ions and in concentrations of 5–25 μM for Cr3+ ions (Fig. S1).
Detection limits were 23.0, 25.0 and 170.0 nM for Al3+,
Fe3+ and Cr3+, respectively (S/N > 3). The spectroscopic re-
sponse of the TAT-ROD chemosensor was rapid (<15 s), and
the complete saturation of intensity occurred after 1 min
(Fig. 3 for Al3+; Fig. S3 for Fe3+ and Cr3+).
To rule out the possible effect of other reactive metals in the
sensing process, we investigated the selectivity of the TAT-
ROD chemosensor to a series of metal cations, including Na+,
K+, Li+, Ag+, Ba2+, Ca2+, Ni2+, Cd2+, Co2+, Fe2+, Mn2+, Hg2+
Zn2+, Ce3+, Au3+ and Cr6+ (Fig. 4). For all other metal cations,
we detected no changes in emission intensity for wavelengths
of both 512 and 580 nm.
To assess the possible interference of other metal ions via
competitive experiments, we measured the changes in fluores-
cence of the TAT-ROD chemosensor by treating 10 equiv. of
M3+ ions in the presence of 50 equiv. of other metal ions. As
shown in Fig. S4, the chemosensor easily detected M3+ ions in
the mixtures of the other metal ions.
To understand the reversibility of the sensing process, we
introduced an excess amount of EDTA into the colored and
fluorescent solutions containing TAT-ROD and M3+. As a
result, the fluorescence intensity of the probe solution changed
immediately, which proves the reversibility of the recognition
process (Fig. 5 for Al3+; Fig. S5 for Fe3+ and Cr3+).
We also gauged the binding stoichiometry of M3+ ions
association with the TAT-ROD chemosensor by applying
Job’s method with fluorescence data. Among the results, the
Fig. 7 Benesi-Hildebrand plots
from fluorescence titration spectra
data of TAT-ROD with Al3+
3
y = 2.8674x + 38.017
2.5
2
R² = 0.9601
1.5
1
0.5
0
-13.2 -13.1 -13 -12.9 -12.8 -12.7 -12.6 -12.5 -12.4 -12.3
ln [Al3+]

J Fluoresc
Fig. 8 Proposed binding mechanism and color change under sunlight and UV-light of TAT-ROD and TAT-ROD / M3+ complex
plot revealed that M3+ bound to TAT-ROD in a 3:1 ratio, as
we expected (Fig. 6 for Al3+; Fig. S6 for Fe3+ and Cr3+).
The association constants of TAT-ROD towards M3+ ions
were 5.91 × 104 M−2, 5.97 × 104 M−2 and 4.57 × 104 M−2 for
Al3+, Fe3+ and Cr3+ respectively. The Benesi-Hildebrand co-
efficient was approximately ~3, which strongly supports the
3:1 binding mode of M3+ with the TAT-ROD molecule (Fig. 7
for Al3+; Fig. S7 for Fe3+ and Cr3+). The coordination property
of TAT-ROD towards Al3+ was also supported by the results
containing TAT and rhodamine cores. After that, a ring-
opening reaction of rhodamine dyes occurred, and the solution
became pink and fluorescent orange.
Last, to investigate the practical applicability of the
TAT-ROD chemosensor, we prepared test strips by im-
mersing filter papers into the MeOH solution containing
the TAT-ROD probe (0.1 mM), air-dried them and pre-
pared Al3+ solutions in five concentrations (i.e. 2.0, 1.0,
0.2, 0.1 and 0.02 mM). As shown in Fig. 9, when we
added the test strips to the solutions, the previously
colourless mixture became increasingly pink as the con-
centration of Al3+ increased. We obtained similar results
for Fe3+ and Cr3+ ions as well. Other metal ions, save
trivalent ones, did not cause any changes in colour in
the test strips. Developing such test strips can be a
ready means to qualitatively detect trivalent metal ions
in environmental samples with the naked eye and with-
out the need for other instruments.
1
of a H-NMR titration study (Fig. S8). The proton signals
ranging from 6 to 9 ppm, dramatically shifted to higher fields,
which is clear proof of a structural reorganization and of a
ring-opening process.
The proposed mechanism for the change in fluorescence
(Fig. 8) could be that the TAT-ROD chemosensor was
colourless and green-emissive before we added M3+. After
adding M3+ ions to the solution, we observed that M3+ coor-
dinated with electron-rich nitrogen and oxygen atoms
Fig. 9 Colorimetric test strips of TAT-ROD (0.1 mM) with Al3+ (in water) (a) 2.0 × 103− M, (b) 1.0 × 103− M (c) 2.0 × 104− M, (d) 1.0 × 104− M, (e)
2.0 × 105− M (f) TAT-ROD (0.1 mM) only

J Fluoresc
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Conclusion
In sum, we have determined the design, synthesis and spectral
properties of a TAT-ROD chemosensor as a novel fluorescent
probe for detecting Al3+, Fe3+ and Cr3+ over other metal ions
via the TBET pathway. The probe demonstrated high sensi-
tivity at the nano-molar level and rapid response (<15 s). To
the best of our knowledge, the probe is the first TAT-based
colorimetric and fluorometric metal ion sensor reported in the
literature. Accordingly, we believe that our findings can aid
the efficient development of new TAT-based probes with high
specificities towards any anion or cation species.
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11.074
Acknowledgements The authors gratefully acknowledge TUBİTAK-
UME for financial support and thank to Muhiddin CERGEL and İlker
ÜN for NMR analysis and Gökhan BİLSEL for HRMS analysis.
15. Dey S, Sarkar S, Maity D, Roy P (2017) Rhodamine based
chemosensor for trivalent cations: synthesis, spectral properties,
secondary complex as sensor for arsenate and molecular logic
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