Revisiting noncovalent SO2-amine chemistry: An indicator-displacement assay for colorimetric detection of SO2

Article abstract of DOI:10.1021/ja053260v

A supramolecular approach for potential detection of SO₂ is presented, which is based on the old donor-acceptor chemistry between SO₂ and amines and includes an indicator-displacement assay.When amines were added to Zn-tetraphenylporphyrin 1 in CHCl₃, the solution changed from red to dark green. A bathochromic shift of Δλ ～10 nm was observed for the Soret band, indicating the formation of 1?amine complexes. After this, SO₂ gas was introduced, and the original red color of the solution was restored. The Soret band returned to its position for free porphyrin 1. The 1?amine complexes dissociated, and new SO₂?amine adducts formed. Porphyrin 1 thus served as an indirect colorimetric indicator for SO₂. The system discriminates between SO₂ and such typical exhaust gases as CO_X, NO_X, and H₂O. From the indicator-displacement assay, the K_assoc values between 1000 and 30000 M^-1 for SO₂?amine complexes were determined, which are comparable to those obtained by direct titration experiments between SO₂ and the amines. Spectroscopic features of SO₂?amine complexes are also presented. Copyright

Full text of DOI:10.1021/ja053260v

Published on Web 09/23/2005
Revisiting Noncovalent SO2-Amine Chemistry: An Indicator-Displacement
Assay for Colorimetric Detection of SO2
Alexander V. Leontiev and Dmitry M. Rudkevich*
Department of Chemistry and Biochemistry, The UniVersity of Texas at Arlington, Arlington, Texas 76019-0065
Received May 18, 2005; E-mail: rudkevich@uta.edu
Sulfur dioxide, or SO2, is an environmentally important gas.1 It
is formed upon burning sulfur-containing fuels, such as coal and
oil. SO2 then dissolves in water vapor to form acid and interacts
with other gases and particles in the air to generate sulfates and
other harmful products. SO2 contributes to respiratory illnesses, the
formation of acid rain, and atmospheric particles that can be
transported over long distances and deposited far from the point of
origin. SO2 is colorless, and the development of optical sensors
for this gas is ongoing. Most of these sensors are either based on
pH indicators or involve chemical reaction of SO2 and derived from
Figure 1. An indicator-displacement assay for the detection of SO2.
Porphyrin 1 is initially coordinated to amines and then displaced by SO2.
it (bi)sulfite, with various dyes.2 We present here a supramolecular
approach for potential detection of SO2. It includes a noncovalent
indicator-displacement assay and is based on the old but rather
forgotten donor-acceptor chemistry between SO2 and amines. In
the proposed sensing scheme, amines initially form colored
coordination complexes with a metalloporphyrin (Figures 1 and
2A). Once introduced, SO2 competes with the porphyrin for the
amine, which eventually leads to the release of the porphyrin and
Figure 2. (A) Visual detection of SO2 with porphyrin 1 (CHCl3 solution,
changes in the solution color/absorption. The approach is simple
left vial) upon addition of pyrrolidine and then SO2. (B and C) Absorbance
and reliable. Varieties of amines and metalloporphyrins are avail-
able, and no covalent linkages between them are required. Accord-
ingly, opportunities to tune and modify the system are very broad.
Our results may also regenerate interest in the noncovalent
chemistry of SO2 and other important gases.3
spectra of porphyrin 1 and complexes 1•pyrrolidine (B) and 1•piperidine
(C) in CHCl3 at 23 ( 1 °C ([1] ∼ 2 × 10-6 M, [amine] ∼ 10-3 M).
Bubbling SO2 results in instant hypsochromic shift of the Soret band, which
returns to the initial λmax ) 421 nm for free porphyrin 1.
Table 1. Association Constants for Complexes between Porphyrin
b
1 and Aminesa and for Amines and SO2 and [SO2]/Amine Ratios,
A series of amines were tested (Table 1). In a typical experiment,
an amine was added to a solution of Zn-tetraphenylporphyrin 1
in CHCl3, and the absorbance changes were recorded (Figure 2).
As expected, the solution changed from red to dark green; a
bathochromic shift of ∆λ ∼ 10 nm was observed for the Soret band,
indicating the formation of the corresponding 1•amine complex.
When SO2 gas was briefly bubbled through the solution, the red
color was basically restored; the Soret band returned to its position
for the free porphyrin (Figure 2). The 1•amine complex dissociated,
and the SO2•amine adduct was formed. Porphyrin 1 thus served as
an indirect colorimetric indicator for SO2. Under the same condi-
tions, CO, CO2, N2O, and H2O did not displace porphyrin 1; no
visible spectral changes were detected (see Supporting Information).
More aggressive NO2/N2O4 caused significant hypochromic changes
in the absorbance, compared to SO2, which is due to the chemical
reaction with 1.4 These gases are typically present together with
SO2 in industrial exhausts.
at which ∼50% of Porphyrin 1 is Displacedc,d
K
assoc(1
•
amine)
K
assoc(SO2
•
amine)
1
1
amine
M-
M-
[SO2]/amine
pyrrolidine
piperidine
morpholine
diethylamine
quinuclidine
1.8 × 104
1.3 × 104
1.1 × 104
7.0 × 102
7.0 × 104
2.0 × 104
3.0 × 104
1.0 × 103
2.2 × 104
1.2 × 105
2
1
30
1
2
a Direct titration. b Displacement assay. c In CHCl3 at 23 ( 1 °C. d All
experiments were performed at least in duplicate showing good reproduc-
ibility.
Absorbance titration data supported the 1:1 stoichiometry
between amine and SO2, and the association constant values were
in the range of 104 M-1. The complexation process was also
followed by 1H and 13C NMR spectroscopy in CDCl3. In particular,
1
upon addition of SO2, the H NMR signals of the amine R-CH
That SO2 and secondary or tertiary amines form stable, 1:1
charge-transfer complexes has been known for decades.5-8 The
structure of such complexes has been a subject of crystallographic6
and computational7 papers. Several complexes have been studied
in solution.8 For this project, we first revisited spectroscopic features
of SO2•amine complexes in apolar solution. Stepwise addition of
piperidine or pyrrolidine to a solution of SO2 in CHCl3 at room
temperature resulted in disappearance of the free SO2 absorption8
at λmax ∼ 288 nm (Figure 3).
protons notably shifted downfield with ∆δ ∼ 0.2-0.3 ppm. The
SO2•amine complexes were also obtained on a preparative scale
by adding the corresponding amine to liquid SO2 at -30 °C; they
were characterized by UV-vis and NMR spectroscopy.
Amines strongly interact with metalloporphyrins. Used in these
studies, Zn-porphyrin 1 forms stable 1•amine complexes in
CHCl3.9 The amine nitrogen is coordinated to the metal center.
Stepwise addition of an amine to a solution of porphyrin 1 resulted
in a bathochromic shift of ∆λ ∼ 10 nm of the Soret band. For
9
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10.1021/ja053260v CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
respectively, and the [SO2]/diethylamine ratio is 1. Morpholine is
less basic than other tested amines (pKa ) 8.3 compared to pKa g
11 for others) and, as a consequence, binds to SO2 weaker than to
the porphyrin. Accordingly, 30 equiv of SO2 is required to displace
50% of porphyrin 1. Considering that 102-103-fold excess of
amines versus 1 is necessary in the displacement assays, these
calculations set the SO2 detection limit at a low millimolar range
and also can be used in the design of more sensitive systems.12
In conclusion, it is now possible to detect SO2 utilizing its
noncovalent chemistry with amines. The indicator-displacement
approach once again proved to be useful. While there are obvious
UV changes simply upon addition of SO2 to an amine, incorporating
the porphyrin in the assay brings the response into the visible region
of the spectrum. The proposed system discriminates between SO2
and such exhaust gases as COX, NOX, and moisture. For this
preliminary report, commercially available amines and a porphyrin
were used; however, synthetic modification of both is possible to
achieve more colorful responses. It would also be possible to modify
the system for SO2 detection in aqueous solutions and at the gas-
solid interface.13 We are working toward these goals and further
testing the system selectivity with respect to other gases and
volatiles.
Figure 3. Titrations of SO2 (λmax ∼ 288 nm) with piperidine (A) and
pyrrolidine (B) in CHCl3 at 23 ( 1 °C ([SO2] ) 4 × 10-3 M, [amine] )
4 × 10-4 - 7 × 10-3 M).
Acknowledgment. Financial support is acknowledged from the
NSF (CHE-0350958) and the A. P. Sloan Foundation.
Supporting Information Available: Experimental procedures and
spectra. This material is available free of charge via Internet at http://
pubs.acs.org.
References
Figure 4. Absorbance spectra of complex 1•quinuclidine (A) and 1•pyr-
rolidine (B) upon addition of SO2 in CHCl3 at 23 ( 1 °C. Determination
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binding algorithm; see refs 10 and 11.
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In the indicator-displacement assay,10 ∼102-103-fold excess of
amines versus 1 was used to ensure the quantitative formation of
1•amine complexes at UV-vis concentrations (2 × 10-6 M).
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1 and the corresponding amine in CHCl3, and the Soret absorbance
was monitored. Hypsochromic shifts of the Soret band were
observed, indicating the liberation of 1 and thus the formation of
SO2•amine adducts (Figure 4). The isosbestic points imply that clean
transformation from the 1•amine complexes to free porphyrin 1
takes place.
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rable to those obtained by direct titration experiments between SO2
and the amines. For example, the Kassoc values for pyrrolidine and
piperidine, obtained by direct titration, are 1.3 × 104 and 3.3 ×
104 M-1, respectively.
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(13) For visual detection of SO2 at the air-solid interface, a test strip was
prepared by depositing red solution of 1 in CHCl3 on a filter paper and
drying it in air. Addition of pyrrolidine in CHCl3 changed the color of
the strip to green. After drying, this test strip was exposed to SO2, and its
color returned to the original red. See Supporting Information.
To estimate the sensitivity, the [SO2]/amine ratios were deter-
mined in the displacement experiments, at which the concentrations
of 1•amine complexes and free 1 are roughly equal (Table 1). When
binding constants between amines and porphyrin 1 are smaller than
or comparable to those between amines and SO2, the [SO2]/amine
ratios are small. For example, the Kassoc values for 1•diethylamine
and SO2•diethylamine are 7.0 × 102 and 2.2 × 104 M-1
,
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