A ready-to-use multifunctional coordinative system, 2,2’-dihydroxyazobenzene –aluminum (III) (DHAB–Al3+), was created to recognize several anions effectively through three channels, namely colorimetric detection, UV-Vis spectroscopy, and fluorescence spectroscopy. Under naked eye visualization, the H2PO−4 can be readily distinguished from the other anions by the DHAB–Al3+ system through a change in color from reddish-orange to light yellow.
Trang 1⃝ T¨UB˙ITAK
doi:10.3906/kim-1412-37
h t t p : / / j o u r n a l s t u b i t a k g o v t r / c h e m /
Research Article
Effective recognition of multiple anions by an azobenzene–Al3+ system
Ye ZHANG, Ping JIA, Zhangfa TONG, Hai-Bo LIU, Jing WANG∗
Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning, P.R China
Abstract: A ready-to-use multifunctional coordinative system, 2,2’-dihydroxyazobenzene –aluminum (III) (DHAB–
Al3+) , was created to recognize several anions effectively through three channels, namely colorimetric detection, UV-Vis spectroscopy, and fluorescence spectroscopy Under naked eye visualization, the H2PO−4 can be readily distinguished from the other anions by the DHAB–Al3+ system through a change in color from reddish-orange to light yellow H2PO−4 was qualitatively detected by using UV-Vis and fluorescence spectra Differentiating SO2−
3 , HPO2−
4 , and HCO−3 anions colorimetrically is difficult; thus, CO23−, SO23−, HPO24−, and HCO−3 were discriminated by adopting distinct changes
in the UV-Vis spectra CO23−, HPO24−, and HCO−3 were accurately quantified using fluorescence spectra The DHAB–
Al3+system responded to H2PO−4 and CO23− via a sensing mechanism based on a displacement approach By contrast, the system responded to SO23−, HPO24−, and HCO−3 based on a binding site–signaling subunit approach The DHAB–
Al3+system can determine the concentration ranges of H2PO−4 with the naked eye and quantify CO23−, SO23−, HPO24−, and HCO−3 through UV-Vis spectra
Key words: Multifunctional, colorimetrically, DHAB–Al3+ system, anions
1 Introduction
Anions serve important functions in chemical and physiological systems and in the environment.1 For example, phosphates (e.g., H2PO−
4 and HPO2−
4 ) are associated with a variety of fundamental processes, such as genetic information storage, energy transduction, signal processing, and membrane transport.2 Sulfite (SO2−
3 ) contents should be strictly limited in foods and beverages because of the potential toxic and harmful effects of sulfur
on hypersensitive people.3 Carbonate (CO2−
3 ) is only slightly toxic, but large doses of the anion are corrosive
to the gastrointestinal tract, causing severe abdominal pain, vomiting, diarrhea, collapse, and possible death.4 Hydrogen carbonate (HCO−
3) , a substrate in photosynthesis generated during cellular respiration from carbon dioxide, maintains the pH of biological fluids.5 Among the various methods for detecting anions, optical methods (fluorescence or UV-Vis spectroscopy) are the most prominent because of their high sensitivity and operational simplicity, while colorimetric assays are the most desirable because they are inexpensive and allow naked-eye detection Colorimetric and optical detections of anions have attracted much attention in recent years.6−11
Researchers have designed and developed various single-ion responsive organic or hybrid systems;12,13 however, the construction of multi-ion recognition systems with differential response modes remains challenging.14−17 To
∗Correspondence: wjwyj82@gxu.edu.cn
Trang 2the best of our knowledge, a single method that can discriminate between more than two anions using a single probe has remained elusive so far.18,19 Thus, we aimed to design a single system that can differentiate multiple anions via differential responses
Azobenzene, one of the most common and frequently used chromophores, has been widely used as a signal group in designing colorimetric sensors.20,21 However, applications of the azobenzene group acting as the recog-nition group have been rarely studied in the sensor field.22 The azobenzene derivative 2,2’-dihydroxyazobenzene
(DHAB), which features hydroxyl groups at the ortho position, is an important organic complexing reagent.23
DHAB can coordinate with a variety of metal ions, such as Cu2+, Zn2+, and Al3+, but the optical properties
of DHAB-related systems, especially their applicability in sensors, have not been widely studied.24,25 In our previous work, we found that high selectivity of DHAB derivatives for metal ions could be achieved by tuning the substitution groups,26 and highly selective and sensitive anion sensors could be obtained by combining DHAB ligands and metal ions DHAB–metal systems provide a unique function compared with individual DHAB or metal ions For example, we reported a DHAB–Cu2+ system used for detecting CN−,27 and the DHAB–Zn2+ system can detect H2PO−
4 28 Similar to most sensor designs, however, the DHAB–Cu2+ and DHAB–Zn2+ systems are restricted to measuring one specific anion Therefore, developing a DHAB–metal system with multifunctional capabilities that can identify more than two anions is an important undertaking The responsiveness of DHAB–metal systems toward anions can be tuned by varying the metal ions
in the complex DHAB, a fluorogenic ligand of aluminum(III) (Al3+) , has been used to quantify Al3+ by fluorometry.29−32 In spite of these studies, however, no fluorescence response of the DHAB–Al3+ system in the presence of anions was investigated before In the present work, we examined the responses of DHAB–Al3+ to seven anions, i.e SO2−
3 , NO−
2, CO2−
3 , SO2−
4 , HPO2−
4 , H2PO−
4, and HCO−
3 We report that the DHAB–Al3+
system can differentiate five anions, SO2−
3 , CO2−
3 , HPO2−
4 , H2PO−
4 , and HCO−
3 , through changes in color and absorption and fluorescence spectra The concentration range of the anions may be determined with the naked eye or through distinct variations in UV-Vis spectra, which is useful for future applications
2 Results and discussion
DHAB can chelate with Al3+ with 1:1 and 2:1 stoichiometry The 1:1 aluminium(III) chelate is fluorescent, whereas the 2:1 chelate is not fluorescent.31 It was found in earlier studies29−32 that the pH can control the
composition of DHAB and Al3+ In order to obtain the fluorescent 1:1 DHAB-Al3+ complex, the interaction between DHAB and Al3+ was examined, through UV-Vis and fluorescence spectroscopy, in EtOH-(0.1 M piperazine-N,N-bis(2-ethanesulfonic acid) (PIPES)-KOH) buffer (1:1, v/v) at pH 6.5 Figure 1a depicts the absorption spectra of DHAB in the presence of various concentrations of Al3+ Free DHAB exhibits two absorption bands at 325 nm and 395 nm, corresponding to its azo (Scheme a) and hydrazone forms (Scheme b), respectively.33 Upon addition of increasing amounts of Al3+, the band at 395 nm gradually decreased (Figure 1b) The band at 325 nm slightly decreased and exhibited a bathochromic shift to a new band at 330 nm, along with the simultaneous emergence of a new absorption at 480 nm.29−32 A well-defined isosbestic point
was noted at 425 nm, which suggests the formation of a complex between DHAB and Al3+ (DHAB–Al3+) Azo–hydrazone tautomerism occurs after Al3+ complexation, and DHAB in the DHAB–Al3+ complex exists
in azo form.34
Trang 3Wavelength (nm)
0.0
0.1
0.2
0.3
0.4
0 0.2 0.3 0.4 0.5 0.6 0.8 0.9 1.0 2.0 3.0 4.0 5.0 10.0 20.0
Concentration of Al 3+ (10 -5 M) (a)
Concentration of Al3+ (10-5 M)
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
480 nm
395 nm
325 nm
(b)
Figure 1 (a) UV-Vis spectra of DHAB (2.0 × 10 −5 M) upon gradual addition of Al3+
(0–2.0 × 10 −4 M) and (b)
effect of Al3+ (0–2.0 × 10 −4 M) on the change in the UV-Vis spectra of DHAB (2.0 × 10 −5 M) at 325 nm, 395 nm,
and 480 nm in EtOH-(0.1 M PIPES-KOH) buffer (1:1, v/v) at pH 6.5
(a)
N N
OH
OH
N H N
O
OH
(b)
Scheme Two tautomeric forms of 2,2’-dihydroxyazobenzene (DHAB): (a) azo form and (b) hydrazone form.
We conducted fluorescence titration experiments of DHAB in the presence of Al3+ under the same conditions Figure 2a shows that DHAB exhibits almost no fluorescence as a typical azo dye33 even though
the hydroxyl group at the ortho position of DHAB can prevent trans-to-cis isomerization to some extent.
Upon treatment with increasing concentrations of Al3+, a new peak at approximately 575 nm appeared and gradually increased before reaching saturation when the amount of Al3+ exceeded 2 equiv (Figure 2b) The strong fluorescence of DHAB in the presence of Al3+ may be due to blockage of the cis–trans transformation
of azobenzene as well as abolishment of intramolecular hydrogen bonding upon coordination of Al3+ with DHAB.34 The Job’s plot revealed a 1:1 stoichiometry for the binding between DHAB and Al3+ (Figures 3a, 3b, and S1 (on the journal’s website)), the association constant (Kass) of which was determined from the fluorescence titration curve to be about 6.5 × 104 M−1 (Figure S2).
The responses of the DHAB–Al3+ (2.0 × 10 −5 M/4.0 × 10 −5 M) system to SO2−
3 , NO−
2 , CO2−
3 , SO2−
4 , HPO2−
4 , H2PO−
4 , and HCO−
3 (4.0 × 10 −3 M) were investigated by monitoring changes in solution color as
well as absorption and emission spectra in EtOH-(0.1 M PIPES-KOH) buffer (1:1, v/v) at pH 6.5
Trang 4Wavelength (nm)
0
100
200
300
400
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 2.0 3.0 4.0 5.0 10.0 20.0
Concentration of Al3+ (10-5 M)
(a)
Concentration of Al 3+ (10 -5 M)
0 100 200 300
400 (b)
Figure 2 (a) Fluorescence spectra of DHAB (2.0 × 10 −5 M) upon gradual addition of Al3+ (0–2.0 × 10 −4 M) and
(b) effect of Al3+ (0–2.0 × 10 −4 M) on the change in the fluorescence spectra of DHAB (2.0 × 10 −5 M) at 575 nm in
EtOH-(0.1 M PIPES-KOH) buffer (1:1, v/v) at pH 6.5 λ ex = 330 nm
Wavelength (nm)
0
100
200
300
400
500
600
700
0 0.1 0.2 0.3 0.325 0.35 0.375 0.4 0.5 0.6 0.625 0.65 0.675 0.7 0.8 0.9 1.0
Mole fraction of Al 3+
(a)
Mole fraction of Al3+
0.0 0.2 0.4 0.6 0.8 1.0
0 100 200 300 400 500 600
700 (b)
Figure 3 (a) and (b) Job’s plot of DHAB and Al3+, λ ex = 330 nm, λ em = 575 nm The total concentration of DHAB and Al3+ ion is 0.1 mM, in EtOH-(0.1 M PIPES-KOH) buffer (1:1, v/v) at pH 6.5
We first examined color changes of the DHAB–Al3+ and DHAB system in the presence of anions (4.0 ×
10−3 M) (Figures 4a and 4b) Figure 4a shows that the reddish-orange DHAB–Al3+ solution turns light yellow when H2PO4 is added to it Anions such as SO2−
3 , HPO2−
4 , and HCO−
3 induced dramatic color changes from reddish-orange to red under identical conditions; however, discriminating between SO2−
3 , HPO2−
4 , and HCO−
3
with the naked eye was difficult The color of the DHAB–Al3+ system showed no or nearly no change in the presence of NO−
2 , CO2−
3 , and SO2−
4 DHAB–Al3+ displays two main peaks centered at 330 nm and 480 nm as illustrated in Figure 5 Upon treatment with H2PO−
4, the band at 480 nm disappeared, and the band at 330 nm showed hypsochromic ( ∆λ
= 5 nm) and hyperchromic (1.25-fold) shifts with concomitant development of a new absorption band at 395 nm When CO2−
3 was added to the DHAB–Al3+ system, the intensity of the absorption band at 480 nm decreased with the concomitant formation of a new blue-shifted band at approximately 325 nm and the appearance of a
Trang 5new shoulder band at approximately 600 nm The absorbance of DHAB–Al3+ at 480 nm red-shifted to about
505 nm by addition of SO2−
3 , HPO2−
4 , and HCO−
3 ; however, recognizing 100 equiv of SO2−
3 , HPO2−
4 , and HCO−
3 through UV-Vis spectroscopy was difficult In the presence of NO−
2 and SO2−
4 , the position of the absorption peak was maintained with increases (NO−
2) or decreases (SO2−
4 ) in the absorbance of the DHAB–
Al3+ system at 480 nm The emission of the DHAB–Al3+ system at 575 nm was completely quenched by all
of the investigated anions except for NO−
2 and SO2−
4 (Figure 6)
DHAB
(b) (a)
Figure 4 Colorimetric changes in (a) DHAB–Al3+ (2.0 × 10 −5 M/4.0 × 10 −5 M) and (b) DHAB (2.0× 10 −5 M)
when different anions are added (4.0× 10 −3 M, 100 equiv of Al3+, 200 equiv of DHAB) in EtOH-(0.1 M PIPES-KOH) buffer (1:1, v/v) at pH 6.5 Anions from left to right: SO23−, NO−2 , CO23−, SO24−, HPO24−, H2PO−4, HCO−3 , and blank
Wavelength (nm)
0.0
0.1
0.2
0.3
0.4
DHAB-Al 3+
SO 3
2-NO2
-CO 3
2-SO4 2-HPO 4
2-H2PO4 -HCO 3
-Wavelength (nm)
0 100 200 300
400
DHAB-Al3+
SO 3 2-NO2
-CO 3
2-SO 4 2-HPO 4 2-H2PO 4 -HCO 3
-Figure 5 UV-Vis spectra of DHAB–Al3+ (2.0 × 10 −5
M/4.0 × 10 −5 M) in the presence of anions (4.0 × 10 −3
M, 100 equiv of Al3+) in EtOH-(0.1 M PIPES-KOH)
buffer (1:1, v/v) at pH 6.5
Figure 6 Fluorescence spectra of DHAB–Al3+ (2.0 ×
10−5 M/4.0 × 10 −5 M) in the presence of anions (4.0
× 10 −3 M, 100 equiv of Al3+
) in EtOH-(0.1 M
PIPES-KOH) buffer (1:1, v/v) at pH 6.5 λ ex = 330 nm
These results indicate that the DHAB–Al3+ system might exhibit specific colorimetric selectivity for
H2PO−
4 and could recognize H2PO−
4 and CO2−
3 in the UV-Vis channel We thus hypothesized that the DHAB–Al3+ system responds to SO2−
3 , HPO2−
4 , and HCO−
3 with differential modes even though 100 equiv
of SO2−
3 , HPO2−
4 , and HCO−
3 cannot be discriminated visually or optically We conducted detailed titration experiments to demonstrate the above assumption and explored the possible mechanisms
Trang 62.3 Sensitivity of the DHAB–Al 3+ system to anions and mechanism studies
Changes in the color and UV-Vis and fluorescence spectra of the DHAB (2.0 × 10 −5 M) and DHAB–Al3+
(2.0 × 10 −5 M/4.0 × 10 −5 M) system upon titration of anions are shown in Figures 7–10 and Figures S3–S9.
Among the seven anions investigated, the DHAB–Al3+ system displayed no or almost no response to SO2−
4
(Figure S8) and NO−
2 (Figure S9)
Wavelength (nm)
0.00 0.05 0.10 0.15 0.20 0.25 0.30
DHAB
SO3
2-NO 2
-CO 3
2-SO4 2-HPO4
2-H2PO4 -HCO 3
-Figure 7 UV-Vis spectra of DHAB (2.0 × 10 −5 M) in the presence of anions (4.0 × 10 −3 M, 200 equiv of DHAB)
in EtOH-(0.1 M PIPES-KOH) buffer (1:1, v/v) at pH 6.5
2 10-3 M
1 10-3 M
1 10-4 M
2 10-4 M
2 10-5 M
4 10-4 M
5 10-5 M
8 10-4 M
DHAB-Al 3+
HCO 3
-
H 2 PO 4
-
HPO 4 2-
SO 4 2-
CO 3
2-NO 2
SO 3 2-
×
×
×
×
×
×
×
×
Figure 8. Photographs showing the color change of the DHAB–Al3+ system (2 × 10 −5 M/4 × 10 −5 M) in the
presence of different concentrations of anions (from 0.5 equiv to 50 equiv of Al3+) in EtOH-(0.1 M PIPES-KOH) buffer (1:1, v/v) at pH 6.5
Trang 7Wavelength (nm)
0.0
0.1
0.2
0.3
0.4
0 1.0 2.0 5.0 8.0 10.0 20.0 40.0 80.0 100.0 200.0
Concentration of H2PO4(10 -5 M)
(a)
Wavelength (nm)
0.0 0.1 0.2 0.3
0.4
0 0.1
0.5
1.0
5.0
10.0 20.0
80.0 100.0 200.0
Concentration of CO32- (10 -5 M) (b)
Figure 9. Absorption spectra of DHAB–Al3+ (2.0 × 10 −5 M/4.0 × 10 −5 M) upon progressive addition of (a)
H2PO−4 (0–2.0 × 10 −3 M, 0–50 equiv of Al3+) and (b) CO23− (0–2.0 × 10 −3 M, 0–50 equiv of Al3+) in EtOH-(0.1
M PIPES-KOH) buffer (1:1, v/v, at pH 6.5)
Wavelength (nm)
0.0
0.1
0.2
0.3
0.4
0 0.1 0.5 0.8 2.0 8.0 10.0 40.0 100.0 200.0
Concentration of SO32- (10 -5 M)
(a)
Wavelength (nm)
0.0 0.1 0.2 0.3
0.4
0 0.1 0.2 0.8 1.0 5.0 10.0 20.0 80.0 100.0 200.0
Concentration of HPO42- (10 -5 M)
(b)
Wavelength (nm)
0.0 0.1 0.2 0.3
0.4
0 0.1 0.2 0.8 2.0 5.0 10.0 40.0 80.0 100.0 200.0
Concentration of HCO3(10 -5 M)
(c)
Figure 10 Absorption spectra of DHAB–Al3+ (2.0× 10 −5 M/4.0 × 10 −5 M) upon progressive addition of (a) SO2−
3
(0–2.0 × 10 −3 M, 0–50 equiv of Al3+
) , (b) HPO24− (0–2.0 × 10 −3 M, 0–50 equiv of Al3+
) and (c) HCO−3 (0–2.0 ×
10−3 M, 0–50 equiv of Al3+) in EtOH-(0.1 M PIPES-KOH) buffer (1:1, v/v, at pH 6.5)
Trang 82.3.1 Displacement approach: H 2 PO−
3
4
DHAB, which features absorption peaks at 325 nm and 395 nm, showed no response to H2PO−
4 (Figures 4b and 7) The DHAB and DHAB–Al3+ system exhibited light yellow (Figure 4b) and reddish-orange (Figures 4a and 8) coloration, respectively The color, absorption, and emission spectra of DHAB gradually recovered upon titrating H2PO−
4 with the DHAB–Al3+ (2.0 × 10 −5 M/4.0 × 10 −5 M) system.
At H2PO−
4 concentrations at or above 5.0 × 10 −5 M, the color of the DHAB–Al3+ system changed from reddish-orange to light yellow (Figure 8) When H2PO−
4 was added to this system, the intensity of the absorption and emission bands of the DHAB–Al3+ system at λ max,abs = 480 nm (Figure 9a) and λ em =
575 nm (Figure S3a) rapidly and significantly decreased A new absorption band at 395 nm also appeared
and gradually increased, and moderate hypsochromic ( ∆λ = 5 nm) and hypochromic shifts were observed for
the band centered at 330 nm DHAB recovery was attributed to the larger association constant35 between
Al3+ and H2PO−
4 than that of the complex of DHAB–Al3+,36 i.e H2PO−
4 bonded with the Al3+ from the DHAB–Al3+ system, DHAB-Al3+ dissociated, and DHAB was released from the system
Various concentration ranges of H2PO−
4 (0–2.0 × 10 −5 M, 2.0 × 10 −5–5.0 × 10 −5 M, and above
5.0 × 10 −5 M) could be determined with the naked eye by utilizing the DHAB–Al3+ system (Figure 8) The fluorescence intensity at 575 nm also showed a linear relationship (R = 0.997, Figure S4a) with H2PO−
4
concentration, which indicates that the DHAB–Al3+ system is particularly sensitive to the detection of H2PO−
4
3
When 200 equiv of CO2−
3 was added to the DHAB solution, the absorption of DHAB at 325 nm decreased as the peak at 395 nm disappeared, as illustrated in Figure 7 A new peak at 485 nm and a shoulder-like absorption peak at about 600 nm appeared, with the solution color changing from light yellow to reddish-orange (Figure 4b)
The UV-Vis and fluorescence spectra of the DHAB–Al3+ system exhibited almost no change in the presence of 0–2.0 × 10 −5 M CO2−
3 , as shown in Figures 9b and S3b In the concentration range of 5.0 ×
10−5–4.0 × 10 −4 M, the absorbance of the DHAB–Al3+ system at 480 nm decreased and gradually red-shifted to 502 nm The fluorescence at 575 nm gradually decreased, which may be caused by equilibration among DHAB, Al3+, and CO2−
3 When the CO2−
3 concentration was above 8.0 × 10 −4 M, a new absorption
peak at approximately 600 nm appeared and the absorption peak at 330 nm blue-shifted by 5 nm (325 nm) and decreased The absorption peak at 480 nm red-shifted by 5 nm (485 nm) (Figure 9b) and fluorescence at 575
nm was completely quenched (Figure S3b); these results are attributed to the interaction between DHAB and
CO2−
3 (Figure 7) The concentration range of CO2−
3 cannot be determined by color changes (Figure 8) and must instead by monitored via changes in UV-Vis spectra (Figure 9b)
The fluorescence at 575 nm showed an interesting “turn-off” optical response when 5.0 × 10 −5–8.0 ×
10−4 M of CO2−
3 was added, as shown in Figure S3b Varying the concentrations of CO2−
3 resulted in a particularly linear response (R = 0.992) over the concentration range of 5.0 × 10 −5–1.0 × 10 −4 M (Figure
S4b)
Trang 92.3.2 Binding site–signaling subunit approach: detection of SO 2−
3
The responses of the DHAB–Al3+ system to SO2−
3 , HPO2−
4 , and HCO−
3 are shown in Figures 8, 10, and S5–S9 When the concentration of the three anions was in the range of 0–1.0 × 10 −4 M, SO2−
3 , HPO2−
4 , and HCO−
3
could be discriminated by the naked eye (Figure 8) The DHAB–Al3+ system did not exhibit color changes for SO2−
3 The system underwent three color changes: from reddish orange (2.0 × 10 −5 M) to light yellow
(5.0 × 10 −5 M)-light red (1.0 × 10 −4 M) upon coordination with HPO2−
4 , and from reddish orange (0–5.0
× 10 −4 M) to red (1.0 × 10 −4 M) upon addition of HCO−
3 Selective visual recognition of SO2−
3 , HPO2−
4 , and HCO−
3 (Figure 8) was difficult at concentrations above 2.0 × 10 −4 M UV-Vis spectra and fluorescence
spectra were measured in detailed titration experiments to further investigate the selectivity and sensitivity of the DHAB–Al3+ system to SO2−
3 , HPO2−
4 , and HCO−
3
Al3+ system did not initially exhibit any detectable change in the presence of 0–1.0 × 10 −4 M SO2−
3 When
SO2−
3 concentrations at or above 2.0 × 10 −4 M were added to the system, however, the absorption spectra
displayed an 8 nm red-shift with a new peak appearing at approximately 505 nm (Figure 10a) The fluorescence intensity at 575 nm was significantly quenched (Figure S5), which was attributed to the interaction between
SO2−
3 and Al3+ in the DHAB–Al3+ system because DHAB is unreactive to SO2−
3 (Figures 4b and 7) The DHAB–Al3+–SO2−
3 system exhibited nearly constant absorbance at 480 nm (0–1.0 × 10 −4 M) and 505 nm
(above 2.0 × 10 −4 M of SO2−
3 ) Thus, SO2−
3 can be qualitatively recognized but not quantitated using the proposed DHAB–Al3+ system
× 10 −5 M of HPO2−
4 was added to the DHAB–Al3+ system (Figure 10b) The absorption spectra of the DHAB–Al3+ system underwent a blue-shift from 480 nm to 474 nm when the concentration of HPO2−
4 was 5.0 × 10 −5–1.0 × 10 −4 M and a red-shift from 474 nm to 500 nm when the concentration of HPO2−
4 was 1.0
× 10 −4–4.0 × 10 −4 M After HPO2−
4 concentrations of up to 4.0 × 10 −4 M were added to the system, the
absorbance at 500 nm decreased with the appearance of a new band at approximately 395 nm and generation
of an isosbestic point at 425 nm, which indicates equilibrium between DHAB and DHAB–HPO2−
4 (Figure 7) The fluorescence intensity of the DHAB–Al3+ system at 575 nm gradually decreased (Figure S6a) Data analysis showed that a linear relationship exists between normalized fluorescence intensities at 575 nm and HPO2−
4 concentrations in the range of 2.0 × 10 −5–1.0× 10 −4 M (Figure S6b) Regrettably, consecutive blue
(5.0 × 10 −5–1.0 × 10 −4 M) and red shifts (1.0 × 10 −4–4.0 × 10 −4 M) in absorbance depending on the
concentrations of HPO2−
4 were unclear at this stage
of HCO−
3 was added to it, as shown in Figures 10c and S7a As the amount of HCO−
3 increased to 2.0 ×
10−4 M, the absorbance red-shifted to 490 nm and the fluorescence at 575 nm decreased significantly In the
range of 2.0 × 10 −4–2.0 × 10 −3 M, the absorbance at 490 nm increased and gradually red-shifted to 504 nm
when the concentration of HCO−
3 was increased (Figure 10c) The fluorescence at 575 nm was also drastically quenched during titration (Figure S7a) because of the interaction between HCO−
3 and Al3+ in DHAB–Al3+
Trang 10given that DHAB does not respond to HCO−
3, as shown in Figures 4b and 7 A relationship with R2 = 0.993 (linear range: 5.0 × 10 −5–2.0 × 10 −4 M) allows quantification of HCO−
3 (Figure S7b)
The DHAB–Al3+ system allowed differentiation of SO2−
3 , CO2−
3 , HPO2−
4 , H2PO−
4 , and HCO−
3 using dual channels via colorimetric determination and UV-Vis spectra CO2−
3 , HPO2−
4 , H2PO−
4 , and HCO−
3 could
be sensitively detected by detecting changes in the fluorescence spectra of the DHAB–Al3+ system
3 Conclusions
A three-channel system, including colorimetric, ultraviolet, and fluorescent systems, based on DHAB–Al3+ has been presented to identify SO2−
3 , CO2−
3 , HPO2−
4 , H2PO−
4 , and HCO−
3 qualitatively and detect CO2−
3 (5.0
× 10 −5–1.0 × 10 −4 M), HPO2−
4 (2.0 × 10 −5–1.0 × 10 −4 M), H
2PO−
4, and HCO−
3 (2.0 × 10 −5–1.0 ×
10−4 M) quantitatively H2PO−
4 and CO2−
3 were detected using the displacement approach, while effective detection of SO2−
3 , HPO2−
4 , and HCO−
3 was achieved using the binding site–signaling subunit approach The DHAB–Al3+ system allowed determination of the concentration ranges of SO2−
3 , HPO2−
4 , CO2−
3 ,
H2PO−
4 , and HCO−
3 with the naked eye (colorimetric detection) or through the UV-Vis spectra The DHAB–
Al3+ system can discriminate SO2−
3 , CO2−
3 , and H2PO−
4 from SO2−
4 , HCO−
3, and HPO2−
4 , respectively, with the naked eye or by UV-Vis spectroscopy in EtOH-(0.1 M PIPES-KOH) buffer (1:1, v/v) at pH 6.5
4 Experimental
4.1 Materials and instrumentations
All the chemicals used were of analytical reagent grade and purchased from Sigma-Aldrich Chemical Company UV-Vis absorption spectra were recorded using a TU-1900 spectrophotometer Fluorescence spectra measure-ments were performed on a 960 MC fluorescence spectrophotometer The excitation and emission slit width was kept constant at 5 nm
4.2 Preparation of stock solutions
First, 0.0214 g of DHAB (4.0 × 10 −4 M) and 0.0240 g of AlCl3 (2.0 × 10 −3 M) were dissolved in 250.0 mL
and 50.0 mL EtOH-(0.1 M PIPES-KOH) buffer (1:1, v/v), respectively A total of 0.0126 g of Na2SO3 (2.0 ×
10−3 M), 0.0069 g of NaNO
2(2.0 × 10 −3 M), 0.0106 g of Na
2CO3 (2.0 × 10 −3 M), 0.0084 g of NaHCO
3
(2.0 × 10 −3 M), 0.0142 g of Na
2SO4 (2.0 × 10 −3 M), 0.0358 g of Na
2HPO4 (2.0 × 10 −3 M), and 0.0156 g
of NaH2PO4 (2.0 × 10 −3 M) were separately dissolved in 50.0 mL of EtOH-(0.1 M PIPES-KOH) buffer (1:1,
v/v)
4.3 Preparation of anion titration solutions
A stock solution of the DHAB–Al3+ (4.0 × 10 −5 M/8.0 × 10 −5 M) system was obtained by adding AlCl
3
(1.6 × 10 −4 M, 50 mL) to the solution of DHAB (8.0 × 10 −5 M, 50 mL) in EtOH-(0.1 M PIPES-KOH)
buffer (1:1, v/v) Test solutions were prepared by placing 2.0 mL of the DHAB–Al3+ stock solution in a test tube, adding an appropriate aliquot of each anion stock, and diluting the solution to 4.0 mL with EtOH-(0.1
M PIPES-KOH) buffer (1:1, v/v) Excitation was performed at 330 nm for all measurements Both excitation and emission slit widths were 5 nm