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L. Xing et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 203 (2018) 455–460
Scheme 1. The synthetic route of the proposed colorimetric sensor HNMAP.
in ethanol and stored at 4 °C. Phosphate buffers (PB) were prepared by
mixing 0.001 mol·L−1 H3PO4, 0.001 mol·L−1 K2HPO4, 0.001 mol·L−1
KH2PO4 or 0.001 mol·L−1 KOH aqueous solution in a proper ratio to ac-
quire the desired pH, respectively (pH = 4.0, 5.0, 6.0, 6.5, 7.0, 8.0, 9.0,
and 10.0).
of some polar solvents such as N,N-dimethyl formamide (DMF), di-
methyl sulfoxide (DMSO), acetonitrile, ethanol and water, were investi-
gated first with the same HNMAP concentration of 1.0 × 10−5 mol·L−1
(Fig. 1a). It is easy to find from Fig. 1a that HNMAP possessed almost the
same spectral profile and similar maximum absorption peak (λmax
)
FTIR spectrum of HNMAP with KBr disc was recorded using a Nicolet
NEXUS 870 FTIR spectrophotometer at room temperature from 4000 to
ranging from 442 nm to 472 nm in all the above polar solvents tested,
attributed to the huge conjugated electron transfer from ground states
to excited ones. λmax slightly shifts to longer wavelength with an in-
creasing molar absorption coefficient (εmax) in protic solvents, such as
ethanol and water, attributing to their polarity and protic characters,
which could make HNMAP in unimolecular dispersion. While in non-
protic solvents, some HNMAP molecules will congregate due to the
strong intermolecular hydrogen bond interaction, which leads to the
original conjugated system distorted or the conjugated electron cloud
density decreased to some extent [41–45]. Therefore, the mixture sol-
vent of ethanol/H2O (v/v = 4:6) was selected for the subsequent
experiments.
500 cm−1 13C NMR and 1H NMR spectra were recorded using a Bruker
.
AMX-500 spectrometer operating at 400 MHz, using tetramethyl-silane
(TMS) as the reference and DMSO d6 as solvent. Elemental analysis was
conducted using an Elemental Vario EL-III apparatus. Fe content in solu-
tion was determined using an Inductively Coupled Plasma Mass Spec-
trometer (ICP-MS) (PerkinElmer, Elan DRC Plus). UV–vis spectra were
recorded on a Lambda 35 UV/Vis spectrometer using a 1-cm square
quartz cell. pH was measured using a PHS-25 pH meter.
2.2. Preparation of HNMAP
As a protic sensing material, pH will greatly influence its existing
form, which further changes its spectral property. Accordingly, the ef-
fect of pH on the UV–vis spectra of HNMAP was investigated and re-
corded from pH 4.0 to pH 10.0 in ethanol/H2O (v/v = 4:6, Fig. 1b).
From Fig. 1b, we could find that, when pH is ≤7.0, the absorption spec-
trum is quite stable with a double-peak UV–vis spectral profile at
447 nm and 466 nm, hinting that HNMAP possesses a stable configura-
tion under neutral or acidic conditions. While pH is increasing from 8.0
to 10, a new shoulder peak at 509 nm emerges and increases gradually.
The new shoulder peak emerged maybe results from the fact that the
hydroxyl groups (\\OH) in HNMAP molecules would gradually turn
into phenol oxygen anion, which will result in the enlarged conjugated
system. pH 6.0 is selected for all next experiments.
To a 100 mL round-bottom flask, 0.344 g (2.0 mmol) 2-hydroxyl-1-
naphthaldehyde and 30.0 mL absolute ethanol were added and dis-
solved. After 1.0 mL glacial acetic acid was added and stirred for 0.5 h
at room temperature, 20.0 mL ethanol containing 0.218 g (2.0 mmol)
2-amino phenol was added into. The reaction mixture was stirred and
refluxed at 85 °C for another 4.0 h. After it was cooled to room temper-
ature, the resultant precipitate was collected and purified by recrystal-
lizing three times from ethanol to provide bright yellow crystal
HNMAP in the yield of 89.4%.
IR (KBr), υ (cm−1): 3334 (\\OH), 3033 (Ar-H), 1629, 1593, 148,
1512 (Ar ring), 1210 cm−1 (C\\O). 1H NMR (400 MHz, DMSO-d6) δ:
15.70 (s, 1H, H\\C_N), 10.31 (s, 1H,\\OH), 9.49 (s, 1H,\\OH), 8.38
(d, J = 7.8 Hz, 1H, Ar-H), 7.93 (d, J = 7.8 Hz, 1H, Ar-H), 7.80 (d, J =
7.9 Hz, 1H, Ar-H), 7.68 (d, J = 7.9 Hz, 1H, Ar-H), 7.48 (t, J = 7.5 Hz,
1H, Ar-H), 7.26 (t, J = 7.5 Hz, 1H, Ar-H), 7.10 (t, J = 7.4 Hz, 1H, Ar-H),
7.00 (t, J = 7.8 Hz, 1H, Ar-H), 6.95 (t, J = 7.5 Hz, 1H, Ar-H), 6.78 (d, J
= 7.4 Hz, 1H, Ar-H). 13C NMR (400 MHz, DMSO-d6) δ: 109.2, 115.8,
118.0, 119.6, 120.8, 122.4, 123.3, 127.7, 128.4, 128.9, 129.3, 133.7,
134.6, 143.0, 149.8, 155.2, 176.2. Anal. Calcd for C17H13NO2 (%): C
77.55, H 4.98, N 5.32, O 12.15; Found: C 77.50, H 5.08, N 5.21, O 12.21.
3.2. Special Colorimetric Response of HNMAP to Fe2+ and Fe3+
To make sure of the possibility of HNMAP to be applied in practice as
what we expected, UV–vis spectra of HNMAP were recorded in the
presence of some common environment-relative metal ions, i.e., Na+
,
K+, Ca2+, Mg2+, Ag+, Zn2+, Hg2+, Ni2+, Pb2+, Mn2+, Cr3+, Co3+, Cu2+
,
Fe3+ and Fe2+ in 1.0 × 10−5 M, respectively, and the results were
given in Fig. 2a.
2.3. UV–Vis Spectra Analytical Procedure
From Fig. 2a, we could find that in ethanol/H2O (4/6 v/v, pH 6.0),
HNMAP possesses a strong double absorption peak at 447 nm and
466 nm with ε = 4.78 × 104 L·mol−1·cm−1 and 4.49 × 104 L·mol−1·-
cm−1, respectively. Upon addition of all the tested metal ions above ex-
cept Fe2+ and Fe3+, no obvious change in the absorption intensity or
peak could be observed. All the changes in the absorption intensities
at 447 nm, 466 nm and 520 nm are less than 5% comparing with these
taken in the presence of Fe2+ or Fe3+. Importantly, the color of the pro-
posed sensing system exclusively turns from yellow to colorless upon
the addition of Fe3+, while to red-brown in the presence of Fe2+, re-
spectively (shown as the insert in Fig. 2a). HNMAP will be a potential
multi-wavelength colorimetric probe for differentiating Fe2+ from
Fe3+, even in naked-eyes.
Generally, 1.0 mL 1.0 × 10−4 mol·L−1 HNMAP, 3.0 mL ethanol, 1.0 mL
PB buffer solution (pH 6.0) without or in the presence of 1.0 mL metal ion
to be tested with different concentrations were transferred into a 10 mL
volumetric flask. After the mixture was diluted to 10 mL with doubly de-
ionized water, its absorption spectrum was recorded from 300 nm to
600 nm. The multiple wavelength absorption intensities at 520 nm
(A520), 466 nm (A466) and 447 nm (A447) were utilized for the quantita-
tive analyses of different metal ions, respectively.
3. Results and Discussion
3.1. UV–Vis Spectral Property of HNMAP
To express the response speed of the proposed sensing system to
Fe2+ and Fe3+, the absorption intensity at 466 nm was recorded as
soon as the addition of Fe2+ or Fe3+, respectively (Fig. 2b). From
Fig. 2b, it is easy to find that HNMAP displays quite different response
Being a strong polar molecule, the UV–vis spectral property of
HNMAP will be great influenced by solvent polarity. Hence, the effect