Fluorescence Sensors for Bismuth (III) Ion from Pyreno[4,5-d]imidazole Derivatives

Article abstract of DOI:10.1111/php.13331

Three pyreno[4,5-d]imidazole derivatives are synthesized and evaluated as fluorescent sensors for bismuth (III) ion. The target compounds are prepared in 55–86% yields from a condensation reaction between pyrene-4,5-dione and aromatic aldehydes. The compound bearing a phenolic group can selectively detect bismuth (III) ion via fluorescence enhancement with a detection limit of 1.20?μm in CH₃CN-DMSO mixture and 3.40?μm in 10% pH5 aqueous in CH₃CN-DMSO mixture. The sensing mechanism involving a formation of coordination complex is investigated by UV-VIS and fluorescence titrations, ¹H-NMR and the decomplexation of the bismuth complex by sulfide ion. The application of this sensor for quantitative analysis of spiked bismuth (III) ion in real water samples from two different sources is demonstrated.

Full text of DOI:10.1111/php.13331

PROF. PAITOON RASHATASAKHON (Orcid ID : 0000-0002-7993-7968)
Article type
: Research Article
Fluorescence Sensors for Bismuth (III) ion from Pyreno[4,5-d]imidazole
Derivatives
Nichapa Chanawungmuang,a Mongkol Sukwattanasinitt,a b and Paitoon Rashatasakhon*a,b
aDepartment of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330,
Thailand
bNanotec-CU Center of Excellence on Food and Agriculture, Department of Chemistry, Faculty of
Science, Chulalongkorn University, Bangkok 10330, Thailand
*corresponding author e-mail: Paitoon.r@chula.ac.th (Paitoon Rashatasakhon)
This article has been accepted for publication and undergone full peer review but has not been
through the copyediting, typesetting, pagination and proofreading process, which may lead to
differences between this version and the Version of Record. Please cite this article as doi:
10.1111/PHP.13331
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Abstract
Three pyreno[4,5-d]imidazole derivatives are synthesized and evaluated as fluorescent sensors for
bismuth (III) ion. The target compounds are prepared in 55 to 86 % yields from a condensation
reaction between pyrene-4,5-dione and aromatic aldehydes. The compound bearing a phenolic
group can selectively detect bismuth (III) ion via fluorescence enhancement with a detection limit
of 1.20 μM in CH3CN-DMSO mixture and 3.40 μM in 10% pH5 aqueous in CH3CN-DMSO
mixture. The sensing mechanism involving a formation of coordination complex is investigated by
UV-VIS and fluorescence titrations, 1H-NMR, and the decomplexation of the bismuth complex by
sulfide ion. The application of this sensor for quantitative analysis of spiked bismuth (III) ion in
real water samples from 2 different sources is demonstrated.
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Introduction
Bismuth and its compounds have been used in the field of material chemistry as semi-conductor
gas sensor,1 alloy,2 cosmetic ingredient,3 optoelectronic,4-6 and pharmaceutical materials.7-10
Moreover, some complexes of bismuth have been used as anti-cancer11 and for CT imaging.12
Even though bismuth (III) ion has low toxicity, the prolong use of bismuth-containing drugs can
lead to negative effects on the kidney and nervous systems.13-14 Besides, the use of household
products that contain bismuth compounds can lead to water contamination and pollutions.15
Conventional methods for Bi3+ detection and quantification are inductively coupled plasma atomic
emission spectrometry (ICP-AES),16 inductively coupled plasma mass spectrometer (ICP-MS),17
resonance light scattering,18-19 atomic fluorescence spectrometry,20 and electrochemical
techniques.21-23 However, these methods require high-cost instruments and complicated equipment
set up, which makes it impossible to use these techniques for real-time monitoring. In contrast, the
fluorescence technique is more user-friendly and still can provide high sensitivity and selectivity.
There have been several reports on the design and application of bismuth fluorescent sensors
based on a metal-organic framework,24 and functionalized naphthalimide,25 rhodamine,26-27
hydroxyquinoline28 and pyrene.29
Pyreno[4,5-d]imidazole derivatives were used as the material in solar cells,30 OLEDs,31-32 and
anion sensors.33-34 They possess a rigid planar geometry and remarkable photophysical properties.
More interestingly, the imidazole moiety can function as both proton and electron donors that can
facilitate the interaction with the metal ion. To this end, we designed three pyreno[4,5-
d]imidazole derivatives (P1-P3, Fig. 1) and aim to use them as specific fluorescent sensors for
bismuth (III) ion. The three compounds contain different substituents on the phenyl ring that can
provide information on the mechanism of sensing such as the Excited State Intramolecular Proton
Transfer (ESIPT) and photo-induced electron transfer (PET).
<Figure 1>
Materials and Methods
Chemicals and Instruments.
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All reagents used in reactions were analytical grade purchased from Sigma-Aldrich, Merck, or
Fluka. Solvents for extraction and chromatography such as dichloromethane (CH2Cl2), hexane,
ethyl acetate (EtOAc), and methanol (CH3OH) were commercial grade and used without further
purification. Solvents for UV-Vis and fluorescence experiments such as acetonitrile (MeCN),
tetrahydrofuran (THF), ethanol (EtOH), and dimethylsulfoxide (DMSO) were spectrophotometric
grade. Column chromatography was operated using Merck silica gel 60 (70–230 mesh) and Thin-
layer chromatography (TLC) was performed on silica gel plates (Merck F245). The target
molecules were characterized by MALDI-TOF mass spectrometer (Bruker Daltonics) using -
1
cyano-4-hydroxycinnamic acid (CCA) as a matrix. All H- spectra were acquired using Varian
400 MHz and Jeol 400 MHz, while the 13C-NMR spectra were acquired using Bruker at 100 MHz.
The UV-Vis absorption spectra were obtained from a Varian Cary 50 UV-vis spectrophotometer
and the fluorescence emission spectra were recorded on a Varian Cary Eclipse spectrofluorometer.
Synthesis and characterization.
Pyrene-4,5-diones (1). his reaction was performed per a literature procedure reported by J. Hu.35
In short, pyrene (10 mmol) was mixed with RuCl3.xH2O (0.96 mmol) in a 1:1 mixture of CH2Cl2
and CH3CN (80 mL), then NaIO4 (46.8 mmol) and H2O (50 mL) were added. The reaction was
stirred overnight at room temperature before the mixture was poured into H2O (500 mL). The
organic phased was separated and the aqueous phase was extracted with CH2Cl2 (50 ml) 3 times.
The combined organic phases were washed with H2O (200 ml) 3 times and dried over anhydrous
Na2SO4. The solvent was removed under reduced pressure to give a dark orange solid which was
purified by column chromatography using CH2Cl2. The product was obtained as an orange crystal
1
in 47%. H-NMR (400 MHz, CDCl3)  ppm 8.47 (d, J = 6.3 Hz, 2H), 8.16 (dd, J = 1.2, 8.0 Hz,
2H), 7.84 (s, 2H), 7,74 (t, J = 7.7 Hz, 2H).
Pyreno[4,5-d]imidazole derivatives (P1-P3). This reaction was performed according to a literature
procedure reported by S. Sinha.36 To a mixture of dione 1 (1 mmol) and an aromatic aldehyde (1
mmol) in CH3COOH (10 ml) was added ammonium acetate (7.5 mmol). The reaction was
refluxed for 4 to 6 h to form a yellow precipitate. The mixture was allowed to cool and poured into
an ice-H2O mixture (10 mL) to remove soluble salt and facilitate a complete precipitate. The solid
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crude product was filtrated, washed with water, and dried by suction. The yellow solid was
purified by column chromatography using 20-25% ethyl acetate in hexane.
10-Phenyl-9H-pyreno[4,5-d]imidazole (P1) was obtained as a yellow solid in 75% yield. 1H-NMR
(400 MHz, DMSO-d6)  ppm 13.84 (s, br, NH), 8.85 (d, J = 7.5 Hz, 2H), 8.40 (d, J = 7.5 Hz, 2H),
8.27 (d, J = 7.4 Hz, 2H), 8.21 (s, 2H), 8.17 (t, J = 7.6 Hz, 2H), 7.65 (t, J = 7.6 Hz, 2H), 7.54 (t, J =
13
7.3 Hz, 1H); C-NMR (400 MHz, DMSO-d6)  149.41, 133.5, 130.35, 129.32, 128.96, 126.22,
124.13, 121.77, 118.97; MS (MALDI-TOF) calcd C23H14N2 [M+] 318.116 found 318.027.
1
2-(9H-Pyreno[4,5-d]imidazol-10-yl)phenol (P2) was obtained as yellow solid in 55% yield. H-
NMR (400 MHz, DMSO-d6)  ppm 13.99(s, OH), 13.21 (s, NH), 8.87 (d, J = 7.6 Hz, 2H), 8.78 (d,
J = 7.6 Hz, 2H), 8.32 (d, 1H), 8.23 (s, 2H), 8.19 (t, 2H), 7.44 (t, J = 7.7 Hz, 1H), 7.16-7.11 (m,
13
2H); C-NMR (400 MHz, DMSO-d6)  157.44, 149.56, 131.51, 131.18, 127.67, 126.46, 125.89,
124.62, 121.83, 119.13, 117.22, 112.98; MS (ESI) calcd C23H14N2O [M+H] 335.12 found 335.02.
10-(2-Methoxyphenyl)-9H-pyreno[4,5-d]imidazole (P3) was obtained as yellow solid in 86
1
%yield. H-NMR (400 MHz, DMSO-d6)  ppm 13.07 (s, br, NH), 8.91 (d, J = 7.5 Hz, 2H), 8.31
(d, J = 6.8 Hz, 1H), 8.25 (d, J = 7.6 Hz, 2H), 8.21 (s, 2H), 8.13 (t, J = 7.6 Hz, 2H), 7.54 (t, J = 7.2
13
Hz, 1H), 7.31 (d, J = 8.3 Hz, 1H), 7.20 (t, J = 7.4 Hz, 1H), 4.08 (s, 3H); C-NMR (400 MHz,
DMSO-d6)  156.79, 147.39, 131.65, 131.13, 130.31, 127.80, 126.33, 124.26, 121.92, 121.02,
119.41, 112.12, 56.11; MS (MALDI-TOF) calcd C24H16N2O [M+] 348.126 found 348.716.
Spectrophotometric Measurement.
The stock solutions of P1, P2, and P3 were prepared in CH3CN and the stock solutions of metal
ions were prepared in Milli-Q water. They were pipetted and diluted to the desired concentration
before spectrophotometric analysis.
Results and Discussion
Synthesis
The synthesis of target compounds started with the oxidation of pyrene using NaIO4 under
catalysis of RuCl3. The dione 1 was produced in 47% as an orange solid with consistent character
according to a literature report.35 An amination-condensation of this dione with aromatic
aldehydes afforded that the target compound P1, P2, and P3 in moderated to good yields.
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<Scheme 1>
Photophysical properties
The photophysical properties were summarized in Table 1 and Figure S10. All target molecules
showed two maximum absorption peaks around 290 and 330 nm. which resembles the
characteristic peaks for pyrene derivatives. For the emission property studies, these compounds
were excited at their longer maximum absorption wavelengths. The maximum emission
wavelengths were observed at 407, 448, and 389 nm for compound P1-P3, respectively. In
comparison to the other two compounds, P2 which contains a hydroxy group on the phenyl ring
exhibited a longer maximum emission wavelength and lower fluorescent quantum yield. These
may cause by the Excited State Intramolecular Proton Transfer (ESIPT) initiated by the
intramolecular hydrogen bonding.
<Table 1>
Solvent effect
The fluorescence spectra of P1-P3 in solvents such as CH3CN, THF, and C2H5OH are shown in
Figure S11. The fluorescence properties of P1 and P3 are independent of the types of solvent
(high polar, low polar, protic), while the fluorescence intensity of P2 in CH3CN was very weak.
This might be the ground-state stabilization effect of the polar compound (P2) by the high polar
solvent. Since we were interested in the turn-on sensing mode, the selectivity screening would
then be conducted in CH3CN.
Metal ion sensing studies
The selectivity of sensor P1-P3 was tested using their CH3CN solution at 10 μM with various
metal ions (10 eq.) such as Li+, Na+, K+, Ag+, Mg2+, Ca2+, Ba2+, Co2+, Cd2+, Zn2+, Pb2+, Ni2+, Cu2+,
Hg2+, Fe2+, Fe3+, Al3+, Bi3+, and Cr3+. The result in Fig. 2 shows no exclusive selectivity for all
sensors in CH3CN. However, their maximum emission wavelengths were red-shifted to around
450 nm by the trivalent metal ions like Fe3+, Al3+, Bi3+, and Cr3+ (Figure S12). Based on the
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fluorescence enhancement ratio of around 30-40 folds at 450 nm, P2 appeared to be most sensitive
towards those four metal ions. This bathochromic spectral shift may result from the complex
formation between metal ions and the sensor molecules.
To improve the selectivity for P2, we opted to incorporate another organic solvent to the system.
The ability of DMSO to coordinate with trivalent metal ions may affect the selectivity in our
case.37 To our delight, the use of 20% of DMSO in CH3CN could improve the selectivity toward
Bi3+ ion. Unfortunately, the selectivity of P1 and P3 could not be improved by this solvent system
(Figures S13 and S14).
<Figure 2>
<Figure 3>
Fluorescence titration
The sensitivity of P2 in the detection of Bi3+ was studied by fluorescence titration experiment. The
fluorescence intensity gradually increased upon the addition of Bi3+ from 1 to 500 μM (Figure
S15a), with an apparent upper limit for signal enhancement around 500 μM of Bi3+ (Figure S15b).
A linear plot was constructed for the concentration range of Bi3+ between 0 to 20 μM (Fig. 4). The
three-time-noise detection limit was estimated to 1.20 μM.
<Figure 4>
Interference studies
The interfering effect of other metal ions on the detection of Bi3+ was investigated using P2, Bi3+,
and other metal ions at a ratio of 1:10:10 molar equivalent. Figure 5 shows that only Fe3+ can
slightly interfered to the detection of Bi3+. The increasing signal may results from the competitive
coordination with P2 with Fe3+.
<Figure 5>
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Sensing mechanism studies
The interaction between P2 and Bi3+ was first investigated using UV-Vis spectroscopy. Upon the
addition of 0.1 to 50 eq. of Bi3+, the absorption band at 278 nm was gradually increased (Fig. 6).
This new absorption band might correspond to the coordination complex between P2 and Bi3+.
However, the attempt to use this technique to monitor a decomplexation process by Na2S was
unsuccessful, probably due to the formation of the insoluble Bi2S3 that could scatter the incident
light and result in an upward absorption baseline shift (Figure S16). However, the emission
spectra in Fig. 7 suggest that the fluorescence enhancement was caused by a complex formation
between P2 and Bi3+, and the addition of S2- ion could lead to decomplexation and a descending
fluorescence signal. The binding stoichiometry of 1:1 was determined by a Job plot (Fig. 8).
The mechanistic investigation was also performed by 1H-NMR on P2 in deuterated methanol (Fig.
9). Upon the addition of Bi3+(prepared in D2O), most of the proton signals were shifted
downfield, except for the Hl which shifted slightly upfield. This data suggests that there is a
coordination complex between P2 and Bi3+ which may also lead to a deprotonation of the phenolic
proton. Therefore, the enhanced fluorescence signal should also involve the inhibition of the
Excited-State Intramolecular Proton Transfer (ESIPT) process.
<Figure 6>
<Figure 7>
<Figure 8>
<Figure 9>
Effect of water and pH
Before the use P2 as a sensor for Bi3+ in the aqueous sample, the investigation on the pH effect
towards the fluorescence property was necessary since the structure of P2 contains both acidic
phenol and basic imidazole groups. However, the hydrophobic nature of P2 has limited the
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amount of water used in these experiments. In this study, the stock solution of P2 was diluted with
9:1 (v/v) mixture of organic solvent (20% of DMSO in CH3CN) and an aqueous solution of
various pHs. Data in Fig. 10a showed that the fluorescent intensity of P2 was quite persistent
under the pH range from 5 to 11, while the addition of Bi3+ could enhance those signals at all pHs.
The maximum enhancement of around 5.5 folds was observed at pH 5 (Fig. 10b). Therefore, the
aqueous Bi3+ solution was adjusted to this pH using HCl and NaOH for the subsequent
experiments. This level of acidity is also necessary since it can prevent the hydrolysis of Bi3+ in
water.
<Figure 10>
Quantitative analysis of Bi3+ in real water samples
In the quantitative analysis of Bi3+ in water samples, a calibration curve for the system containing
10% (v/v) of pH 5 aqueous solution in mixed organic solvents (CH3CN-DMSO (8:2, v/v)) was
established (Fig. 11). The three-time-noise detection limit in this solvent system was estimated at
3.40 μM. The water samples from two different sources were adjusted to pH 5, then spiked with
Bi3+ at 10.00 and 15.00 μM. The data in Table 2 showed nearly completed recovery of Bi3+ in each
water sample. The formation of Bi3+ compounds with anions such as sulfate, sulfide, or phosphate
in these water samples might be the cause of these incomplete recovery percentages.
<Figure 11>
<Table 2>
Conclusion
We successfully synthesized three derivatives of pyreno[4,5-d]imidazole (P1-P3) from pyrene-
4,5-dione and aromatic aldehydes in moderated to good yields. In acetonitrile, all three
compounds exhibit a fluorescent enhancement at 450 nm by trivalent cations such as Bi3+, Fe3+,
Cr3+, and Al3+. The selectivity of the compound with a phenolic group (P2) could be improved
towards Bi3+ using a CH3CN-DMSO mixture (80:20 %v/v) as the solvent. The three-time-noise
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1
detection limit for Bi3+ was estimated at 1.20 μM. The investigation by UV-Vis titration, H-
NMR, and the decomplexation study using Na2S suggested that the fluorescence enhancement was
involved in a complex formation between the sensor (P2) and Bi3+.
In the system with 10% (v/v) of aqueous solution in mixed organic solvents (CH3CN-DMSO (8:2,
v/v)), the optimal condition which yielded the highest signal enhancement ratio upon addition of
Bi3+ was at pH 5 with a detection limit at 3.40 μM. The application of P2 in the quantitative
determination of Bi3+ in two real water samples resulted in acceptable recovery percentages.
According to a literature survey (Table 3), the performance of P2 is comparable to other
previously reported fluorescent sensors for Bi3+.
<Table 3>
Acknowledgements
This work was supported by the National Nanotechnology Center (NANOTEC), NSTDA,
Ministry of Science and Technology, Thailand, through its program on Research Network of
Nanotec and the Thailand Research Fund (RTA6180007). NC thanks to the Development and
Promotion of Science and Technology Talent Project (DPST) for her scholarship.
Supporting Information
Additional supporting information may be found online in the Supporting Information section at
the end of the article:
Figure S1. 1H-NMR of P1 in DMSO-d6.
Figure S2. 13C-NMR of P1 in DMSO-d6.
Figure S3. MALDI-ToF spectrum of P1.
Figure S4. 1H-NMR of P2 in DMSO-d6.
Figure S5. 13C-NMR of P2 in DMSO-d6.
Figure S6. MALDI-ToF spectrum of P2.
Figure S7. 1H-NMR of P3 in DMSO-d6.
Figure S8. 13C-NMR of P3 in DMSO-d6.
Figure S9. MALDI-ToF spectrum of P3.
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Figure S10. Normalized absorption and emission spectra of P1-P3 in EtOH.
Figure S11. The fluorescence spectra of P1-P3 in solvents such as CH3CN, THF, and C2H5OH.
Figure S12. Fluorescence spectra of a) P1, b) P2 and c) P3 (10 μM) in CH3CN before and after
addition 10 eq of metal ions.
Figure S13. a) Fluorescence spectra and b) fluorescence enchantment ratio of P1 (10 μM) in
CH3CNDMSO (8:2, v/v) before and after addition 10 eq of metal ions.
Figure S14. a) Fluorescence spectra and b) fluorescence enchantment ratio of P3 (10 μM) in
CH3CNDMSO (8:2, v/v) before and after addition 10 eq of metal ions.
Figure S15. Fluorescence titration of P2 (10 μM) by Bi3+ (1 to 500 μM) in CH3CN-DMSO (8:2,
v/v).
Figure S16. UV-Vis spectra of P2 before and after the addition of Bi3+, and upon a subsequent
addition of Na2S
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Figure Captions
Scheme 1. Synthesis of pyreno[4,5-d]imidazole derivatives
Figure 1. Target molecules P1-P3
Figure 2. Fluorescence enhancement ratio of P1, P2, and P3 (10 μM) in CH3CN upon addition of
various metal ions (10 eq).
Figure 3. (a) Fluorescence spectra and (b) fluorescence enhancement ratio of P2 (10 μM) in
CH3CN-DMSO (8:2, v/v) before and after addition 10 eq of metal ions.
Figure 4. Linear Plot between the concentration of Bi3+ and fluorescence enhancement ratio of P2
in CH3CN-DMSO (8:2 v/v).
Figure 5. Interference studies of P2 (10 μM) and Bi3+ (10 eq) using foreign metal ions (10 eq).
Figure 6. UV-Vis titration of P2 (10 μM) upon the addition of various concentrations of Bi3+.
Figure 7. Fluorescence spectra and photographs (inset) of P2 solution before and after the
addition of Bi3+, and upon subsequent addition of Na2S.
Figure 8. Job plot for determination of binding stoichiometry between P2 and Bi3+.
Figure 9. 1H-NMR of P2 (in MeOH-d4) before and after the addition of Bi3+ (in D2O).
Figure 10. (a) Fluorescence intensity of P2 (10 μM ) with and without Bi3+ (50 μM) and (b)
fluorescence enhancement ratio in 10% (v/v) of aqueous solution at various pH in mixed organic
solvents (CH3CN-DMSO (8:2, v/v)).
Figure 11 Linear Plot between the concentration of Bi3+ and fluorescence enhancement ratio of P2
in 10% (v/v) of pH 5 aqueous solution in mixed organic solvents (CH3CN-DMSO (8:2, v/v)).
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Table 1. Photophysical properties of molecules
Absorptiona
Emissiona
max
ex
max

cpd
P1
P2
P3
b
(nm)
(M-1cm-1)
(nm)
(nm)
290
331
9900
8500
331
335
333
407
448
389
0.75
0.41
0.78
289
335
13000
11300
290
333
11300
10400
b
a solutions in ethanol; Anthracene in ethanol (quantum yield 0.27) was used as the reference
compound.
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Table 2. Analytical results of Bi3+ in water samples (n = 3).
Water source
Tab water
Spiked (μM)
10.00
Found (μM)
9.72  0.10
13.77  0.09
9.61  0.08
Recovery (%)
97.20  1.00
91.80  0.60
96.10  0.80
15.00
Drinking water
10.00
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Table 3. Comparison of Bi3+ fluorescent sensors.
Sensing LODa
Compound
Solvent(s)
Ref
Mode
(μM)
0.008 38
pH 6 aqueous solution
Off-on
Rhodamine functionalized Zr-based
MOFs
1,8-Naphthalimide derivative
Off-on
2.78
25
-Cyclodextrin
aqueous
solution
Rhodamine derivative
Tiron
Ethanol-MeCN
DI water
Off-on
On-off
Off-on
2.69
0.24
0.12
26
39
29
Pyren-1-ylmethylene-hydrazide
DMSO/water
Pyreno[4,5-d]imidazole derivative
Pyreno[4,5-d]imidazole derivative
a Limit of detection.
DMSO/MeCN
Off-on
Off-on
1.20
3.40
This work
This work
Water/DMSO/MeCN
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