The target compound, DAQZ, was synthesized through the
corresponding 5,5¢-(ethyne-1,2-diyl)diquinolin-8-amine interme-
diate 3, which was obtained by coupling 5-iodoquinolin-8-
amine (1) with 5-ethynylquinolin-8-amine (2) by the Castro–
Stephens/Sonogashira protocol (ESI, Scheme S1†).13
The [DAQZ@2Zn2+] complex was prepared by titrating Zn2+
with a solution of DAQZ in a tris-HCl solution (ethanol–water,
1 : 9, v/v). The result was characterized by UV-vis absorption
spectra and fluorescence emission spectra. As shown in Fig. S1,†
DAQZ has a main absorption peak centered at 375 nm in ethanol–
water (1 : 9, v/v) solution, which is longer than AQZ over 50 nm.
While adding Zn2+ to the solution of DAZQ, a new absorption
peak at 426 nm appeared, and the peak at 375 nm decreased, with
an isosbestic point at 400 nm (ESI, Fig. S1†).
The emission spectrum of free DAQZ displays a broad band
with a maximum at 470 nm in an aqueous tris-HCl buffer solution
(Fig. 1). When Zn2+ was added to the solution of DAQZ, a 42 nm
red-shifted band was observed with a significant emission decrease
and increase at 470 nm and 512 nm, respectively, and a clear
isoemission point at 487 nm, which was attributed to the formation
of a [DAQZ@2Zn2+] complex (Fig. 1). The inset in Fig. 1 exhibits
the dependence of the intensity ratios of emission at 512 nm and
470 nm (I512 nm/I470 nm) on Zn2+, which indicates the formation of
a [DAQZ@2Zn2+] adduct of 1 : 2 stoichiometry. Moreover, a Job’s
plot, which exhibits a maximum at 0.667 fraction of Zn2+, further
indicates that only a 1 : 2 complex is formed (ESI, Fig. S2†). In
order to determine a reliable association constant, a more diluted
DAQZ (0.2 mM) solution was titrated with Zn2+ to get a smoother
titration curve. And the association constant is determined to be
Ks1 = 1.9 ¥ 105, Ks2 = 9.2 ¥ 105 from this titration curve by a
non-linear least-squares analysis (ESI, Fig. S3†). The UF values
of free DAQZ and the [DAQZ@2Zn2+] complex are 0.18 and 0.21,
respectively.14
investigated and the result is shown in the ESI, Fig. S4†. The
emission intensity of the [DAQZ@2Zn2+] complex had no obvious
change in the range of pH 6.0–8.0. Therefore, all the detections
of various mono- and dicarboxylic acids were evaluated in the
tris-HCl buffer solutions (10 mM, ethanol–water, 1 : 9, v/v) at pH
7.02.
The [DAQZ@2Zn2+] complex was prepared by adding 6.0 mM
Zn2+ to a tris-HCl solution of DAQZ (3 mM) for the detection of
oxalic acid. Fig. 2 displays the changes profile of emission spectra
of the [DAQZ@2Zn2+] complex with oxalic acid concentration
at pH = 7.02 (10 mM, tris-HCl). With the increasing of oxalic
acid concentration in a solution of the [DAQZ@2Zn2+] complex,
the fluorescence intensity of the [DAQZ@2Zn2+] complex sharply
decreased with a 20 nm blue shift in wavelength. The fluorescent
intensity ratio at 467 nm and 513 nm (I467 nm/I513 nm) increased in
a linear fashion with the concentration of 0–120 mM of oxalic
acid (linearly dependent coefficient: R2 = 0.9987, Fig. 3a). This
indicated that the [DAQZ@2Zn2+] complex can be potentially
used to quantitatively detect oxalic acid concentration. The
Job’s plot indicates that a 1 : 2 complex is formed between the
[DAQZ@2Zn2+] complex and oxalic acid (ESI, Fig. S5†). The
binding constant for oxalic acid was calculated to be Ks1 = 2.6 ¥
104, Ks2 = 2.5 ¥ 103 through a least-squares analysis of titration
profiles (ESI, Fig. S6†). 1H-NMR studies also provided evidence
for the interaction between the [DAQZ@2Zn2+] complex and
oxalate (ESI, Fig.S7–S9†). Upon addition of 2 equiv. of Zn2+
in the DAQZ, the peaks (Fig. S7,† from 2.8 ppm to 3.7 ppm)
assigned to methylene protons of the alkoxyethylamino chain
shifted significantly downfield and broadened (Fig. S8,† from
3.4 ppm to 4.3 ppm). Obviously, N–Zn(II) and O–Zn(II) (N,O-
alkoxyethylamino chain) complexation lowers the electron density
of the N/O and the deshielding effect is responsible for these
NMR spectral changes. Then, after addition of 2 equiv. of oxalate
in the [DAZQ@2Zn2+] complex, the peaks shifted moderately
upfield (Fig. S9,† from 3.3 ppm to 4.2 ppm). It indicates that
the interaction of N–Zn(II) and O–Zn(II) are weakened and the
shielding effect is recovered a little bit after oxalate binding the
Fig. 1 Emission spectra of a solution of DAQZ (10 mM) in the presence
of increasing Zn2+ concentration (0–4 equiv.) in tris-HCl (0.01 M) solution
(ethanol–water, 1 : 9, pH = 6.02) Inset: ratiometric calibration curve
I
512 nm/I470 nm as a function of Zn2+ concentration (lex = 400 nm).
Fig. 2 Oxalic acid titration induced the fluorescence spectra changes
of the [DAQZ@2Zn2+] complex (3 mM) in tris-HCl (0.01 M) solution
(ethanol–water, 1 : 9, pH = 7.02, lex = 400 nm). Inset: images were
taken under UV irradiation. Left: DAQZ, middle: [DAQZ@2Zn2+], right:
[DAQZ@2Zn2+@oxalic acid].
As emphasized above, it is of particular interest to explore
the possibility of the [DAQZ@2Zn2+] complex for effectively
recognizing the presence of oxalic acid. The influence of pH on
the fluorescence intensity of the [DAQZ@2Zn2+] complex was
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 1034–1037 | 1035
©
He, Chunsheng
