P. Chinapang et al. / Dyes and Pigments 112 (2015) 236e238
237
2.3. Selectivity screening
The selectivity of 1 and 2 towards various metal ions were
investigated by monitoring their fluorescent signals in the presence
of Ag(I), Al(III), Au(I), Au(III), Ba(II), Bi(III), Ca(II), Cd(II), Co(II), Cr(II),
Cu(II), Fe(III), Ga(III), Hg(I), Hg(II), Mn(II), Ni(II), Pb(II), Pd(II), Sr(II),
Zn(II), Li(I) and Na(I). In the case of 1, no significant change in
fluorescent signal was found (Figure S2), whereas the signal of 2
was enhanced to approximately 9 and 3 folds of its original in-
tensity in the presence of Au(III) and Au(I), respectively (Fig. 2). This
selectivity towards gold ions is attributed to the special alkyno-
13
philicity of the gold ions toward the triple bond. From the C NMR
spectrum of 2 in CD CN (Figure S8a), the two acetylenic carbons
3
resonated at 90.3 and 85.8 ppm. Upon addition of Au(III) to the
same NMR sample, these signal disappeared and could not be
observed even after more scans (Figure S8b). This data is a strong
evidence for the presumed coordination of Au(III) to the acetylene
moiety. In addition, our calculation using density functional theory
Fig. 1. Structures of target compound 1 and 2.
(B3LYP/LANL2DZ) indicated that the HOMO energy level of the
ferrocenyl imidophenlacetylene moiety in 2 is at ꢀ5.54 eV, which is
higher than that of the naphthalimide pendant in the same mole-
cule (ꢀ6.47 eV) [7]. The coordination of the gold ion at the triple
bond could disturb this conjugation and prohibit the charge
transfer, thus enhancing the fluorescent signal.
naphthalimide 5 in 80% yield. The installation of a hydrophilic tri-
ethylene glycol group into 3 was carried out by O-alkylation of the
phenolic eOH group using 1-iodo-2-(2-(2-methoxyethoxy)ethoxy)
ethane under basic conditions. Meanwhile, a nucleophilic aromatic
substitution of the bromine in 5 by triethylene glycol monomethyl
ether in the presence of NaH afforded 6 in 42% yield. Finally, a
Sonogashira coupling with ethynyl ferrocene transformed 4 and 6
into target compound 1 and 2 in excellent yields.
2.4. Optimization of sensing conditions
Next, the enhanced fluorescence intensity and the time required
to reach a constant intensity (response time) were investigated by
varying the water content in CH
that when the aqueous fraction was at 0.6, the highest fluorescent
enhancement by Au(III) (F/F
time of 15 min (see Figure S4 for more data). These conditions were
then chosen for further studies on sensing system optimization.
The pH of the medium could also affect intensities of enhanced
fluorescence signal (Figure S5). We thus carried out a series of ex-
periments using solutions of 2 in 60/40 mixture of aqueous buffer
3
CN. Results from Figure S3 showed
2
.2. Photophysical properties
The photophysical properties of 1 and 2 were investigated using
0
¼ 9.1) was obtained in a practicable
their solutions in aqueous MeCN (1:1). The results are tabulated in
Table 1 and the normalized absorption and emission spectra are
shown in Figure S1. For the absorption, 1 and 2 showed a maximum
absorption wavelength at 380 and 369 nm, respectively, with a
relatively similar molar extinction coefficient. The maximum
emission wavelengths for 1 and 2 are at 405 and 453 nm, respec-
tively. The longer emission wavelength in 2 is a typical character-
istic for 1,8-naphthalimide with an electron-donating group at the
3
at various pH and CH CN (acetate buffer for pH 4 to 5, phosphate
buffer for pH 6.0 to 8.0, and triseHCl buffer for pH 9.0). Upon
addition of Au(III), the fluorescent signals were enhanced to a
saturated level within 15 min under each of the applied conditions,
but the levels of enhancement depended on the pH of the buffer.
Results indicated that the best sensitivity of the Au(III) detection by
4
-position [6]. As expected, both 1 and 2 exhibit unusually low
quantum efficiencies for 1,8-naphthalimide derivatives. Since one
2
þ
of the cyclopentadienyl ligands for Fe in 1 is in conjugation with
the ethynyl naphthalimide moiety, it is possible that either MLCT or
LMCT is responsible for the low emissive behaviour. In addition,
this cyclopentadienyl group which is relatively electron-rich is in
conjugation with the carbonyl group of the naphthalimide. This
structural arrangement should lead to the internal-charge transfer
upon excitation, which also weakens the fluorescence intensity.
The fluorescence emission of 2 cannot be completely quenched by
the ferrocene owing to a longer distance between naphthalimide
and ferrocene moiety. However, this quenching property of ferro-
cene could be visually observed during the transformation of the
strongly fluorescent 6 into weakly emissive 2.
2
could be achieved at pH 7 to 8.
2.5. Selectivity and sensitivity determination
The interference of other metal ions in Au(III) detection was
examined using solutions of 2 (5
mM), Au(III) (20 mM), and each of
the interfering ion (200 M) (Figure S6). With the exception of
m
Fe(III), other interfering ions did not cause significant differences in
fluorescence response in the presence of Au(III). The interference by
Fe(III) may result from the filtering effect as the ferric ion possesses
a significant absorption in the UV range [8]. In order to determine
the detection limit, we constructed a plot between fluorescent
signal and concentration of Au(III) (Figure S7). From the linearity
obtained in the concentration range between 0.5 and 3.0
mM,
Table 1
detection limit of three-times-noise was determined to be
Photophysical properties of 1 and 2 in CH
3
CNeH
2
O (v:v ¼ 1:1).
Emission
max (nm)
4
4
.8 ꢁ 10ꢀ
mM or 95 ppb.
Compound
Absorption
max (nm)
ꢀ
1
ꢀ1
a
l
Log ε (M cm
)
l
F
F
3. Conclusions
1
2
380
369
3.95
4.23
405
453
NAb
0.01
Two novel derivatives of 1,8ꢀnaphthalimide were successfully
a
synthesized
from
the
commercially
available
Quinine sulphate in 0.1 M H
Cannot be determined due to insufficient fluorescent intensities.
2
SO
4
(
F
¼ 0.54) was the reference.
b
4ꢀbromoꢀ1,8ꢀnaphthalic anhydride and ethynylferrocene. The