Novel 1,8-naphthalimide dye for multichannel sensing of H+ and Cu2+

Article abstract of DOI:10.1007/s11164-017-3235-6

A novel 1,8-naphthalimide dye with simple structure has been produced by a facile synthetic method for colorimetric and fluorescent sensing of H⁺ and Cu²⁺. In CH₃CN/H₂O (1/1, v/v), the dye could monitor H⁺ using dual channels (ratiometric absorbance and fluorescence intensity change) from pH 6.2 to 12.0. Meanwhile, in the pH range of 1.9–5.2, the dye could also be used to detect Cu²⁺ using triple channels [ultraviolet–visible (UV–Vis) absorption, fluorescence intensity reduction, as well as fluorescence blueshift]. The detection limits for Cu²⁺ evaluated by colorimetric and fluorescent titration were 6.10?×?10^?7 and 2.62?×?10^?7?M, respectively. The dye exhibited specific selectivity and sensitivity for H⁺ and Cu²⁺ over various coexisting metal ions. Moreover, the sensing mechanism of the dye for H⁺ and Cu²⁺ was carefully examined.

Full text of DOI:10.1007/s11164-017-3235-6

Res Chem Intermed
https://doi.org/10.1007/s11164-017-3235-6
Novel 1,8‑naphthalimide dye for multichannel sensing
of H+ and Cu2+
Yufen Chen1,2 · Tengxuan Tang1,2 · Yuhua Chen3 · Dongmei Xu1,2
Received: 22 September 2017 / Accepted: 11 December 2017
© Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract A novel 1,8-naphthalimide dye with simple structure has been pro-
duced by a facile synthetic method for colorimetric and fuorescent sensing of H+
and Cu2+. In CH3CN/H2O (1/1, v/v), the dye could monitor H+ using dual chan-
nels (ratiometric absorbance and fuorescence intensity change) from pH 6.2 to 12.0.
Meanwhile, in the pH range of 1.9–5.2, the dye could also be used to detect Cu2+
using triple channels [ultraviolet–visible (UV–Vis) absorption, fuorescence inten-
sity reduction, as well as fuorescence blueshift]. The detection limits for Cu2+ eval-
uated by colorimetric and fuorescent titration were 6.10 × 10−7 and 2.62 × 10−7 M,
respectively. The dye exhibited specifc selectivity and sensitivity for H+ and Cu2+
over various coexisting metal ions. Moreover, the sensing mechanism of the dye for
H+ and Cu2+ was carefully examined.
Keywords 1,8-Naphthalimide · Colorimetric sensor · Fluorescent sensor · H+ ·
Cu2+
Electronic supplementary material The online version of this article (https://doi.org/10.1007/
s11164-017-3235-6) contains supplementary material, which is available to authorized users.
*
*
Yuhua Chen
chenyh@suda.edu.cn
Dongmei Xu
xdm.sd@163.com
1
2
3
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, No.
199 Ren-ai Road, Suzhou Industrial Park, Suzhou 215123, Jiangsu, People’s Republic of China
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Soochow
University, Suzhou 215123, Jiangsu, People’s Republic of China
College of Pre-clinical Medical and Biological Science, Soochow University, Suzhou 215123,
People’s Republic of China
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Y. Chen et al.
Introduction
H+ and Cu2+ are important species in environmental and biological systems. H+
participates in many physiological processes such as enzymatic activities and ion
transport [1], and controls pH-driven switching [2], while destruction of copper
homeostasis is implicated in a number of diseases. Copper defciency can cause
bone fragility, cardiovascular disorders, subcutaneous bleeding, and other symp-
toms, while excessive levels of copper can harm the liver and induce Wilson’s
disease or gastrointestinal disease [3, 4]. Therefore, development of easier and
faster methods for detection of H+ and Cu2+ remains an imperative task.
Spectral sensing has become a promising strategy due to its easy operation,
in situ and real-time detection ability, high selectivity, and low cost [5, 6]. Many
colorimetric and fuorescent sensors for H+ and Cu2+ have been reported. How-
ever, most of them can only detect H+ [7–12] or Cu2+ [13–19] individually. Pre-
viously, our group developed a sensor which could detect Cu2+ in CH3CN/H2O
(1/1, v/v) and Fe3+ in CH3CN/H2O (1/99, v/v) [20]. Jo et al. [21], Gupta et al.
[22], and Ozdemir [23] reported Cu2+ sensors that could also detect CN−, Al(III),
and Hg2+, respectively. Recently, Fu et al. [24] reported a colorimetric and fuo-
rescent sensor for Cu2+ and F−. Some other works have described pH sensors that
could also sense another ion, such as Zn2+, Fe(III), or Bi3+ [25–27]. However,
sensors that can respond to both H+ and Cu2+, especially using multiple channels
in the same aqueous solution, are extremely rare [28].
In this work, we used 1,8-naphthalimide as chromophore and fuorophore, and
hydrazine hydrate and furfural as receptor precursors to construct a new dye with
simple structure and facile synthetic method. Moreover, the synergistic efects of
N and O binding sites and their unique spatial locations enabled the dye to act as
a spectral sensor which is highly selective to both H+ and Cu2+ in the same aque-
ous medium using multiple signals and diferent mechanisms.
Experimental
Materials and instruments
All chemicals were obtained from commercial suppliers and used without further
purifcation. Metal ions including K+, Ca2+, Na+, Mg2+, Fe2+, Fe3+, Cr3+, Ag+,
Cd2+, Co2+, and Hg2+ were provided by their chloride salts, Zn2+, Cu2+, and Mn2+
were provided by their sulfate salts, and Pb2+ and Ni2+ were obtained from their
acetate salts. These metal salts, n-butylamine, hydrazine hydrate (NH2NH2·H2O,
85%), and furfural (≥ 98%) were purchased from Sinopharm Chemical Reagent
Co., Ltd. (Shanghai, China). 4-Bromine-1,8-naphthalic anhydride (98%) was pro-
vided by Anshan HIFI Chemical Co., Ltd.
1H and 13C nuclear magnetic resonance (NMR) were carried out on a UNITY
Inova 400 spectrometer (Varian Co., USA). Absorption spectra were obtained on
1 3

Novel 1,8‑naphthalimide dye for multichannel sensing of H+…
a U-3900 spectrophotometer (PerkinElmer Co., USA). Fluorescence measure-
ments were carried out using a F-2500 fuorescence spectrophotometer (Hitachi
Co., Japan). Mass spectra were obtained on a Bruker micro-TOF-QIII spectrom-
eter (Bruker Daltonics Co., Germany). Elemental analysis was carried out on a
Carlo-Erba EA1110CHNO-S elemental analyzer (Carlo-Erba Co., Italy). pH val-
ues were tested using precise pH paper (Tzzt, Jiangsu). IR spectra were obtained
using a MagNa-IR 550 spectrometer (Nicolet Co., USA).
Synthesis methods
The dye 2-n-butyl-6-(2-(furan-2-ylmethylene)hydrazinyl-1H-benzo[de]isoquino-
line-1,3(2H)-dione (BNFA) was prepared by three-step reaction between 4-bromo-
1,8-naphthalic anhydride, n-butylamine, hydrazine hydrate, and furfural, as shown
in Scheme 1.
Synthesis of BBN [29]
4-Bromo-1,8-naphthalic anhydride (1.00 g, 3.62 mmol) was dispersed in 15 mL eth-
anol and heated to 50 °C. Then, n-butylamine (0.75 mL) was added. The mixture
was stirred and refuxed for 24 h under nitrogen atmosphere. Subsequently, the reac-
tion liquid was cooled to room temperature and poured into ice–water. Light-yellow
precipitate appeared and was fltered and washed with water twice. The crude prod-
uct was purifed by recrystallization in ethanol to give the intermediate N-n-butyl-
4-bromine-1,8-naphthalimide (BBN) as of-white needle crystal in 75.0% yield.
Synthesis of intermediate BNN [30]
Under nitrogen atmosphere, BBN (100 mg, 0.302 mmol) was dissolved in 10 mL
ethylene glycol monomethyl ether and heated to 50 °C. Hydrazine hydrate (100 μL)
was introduced. The reaction mixture was heated under refux for 5 h, then cooled to
room temperature and poured into distilled water. A solid was obtained by fltering
and purifed by washing with water (twice) and ethanol (once) to produce the inter-
mediate 2-n-butyl-6-hydrazinyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (BNN) as
orange solid in 87.7% yield.
Scheme 1 Synthetic route of BNFA
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Y. Chen et al.
Synthesis of BNFA
Under nitrogen atmosphere, 77 mg (0.27 mmol) BNN was dissolved in 15 mL
dichloromethane and heated to 40 °C. Furfural (100 μL, 1.2 mmol) was added drop-
wise. The reaction mixture was stirred and refuxed for 5.5 h, then cooled to room
temperature and fltered. Orange solid was obtained and purifed by washing with
water (twice) to give the dye BNFA in 67.9% yield. The structure of BNFA was fully
characterized by IR, 1H NMR, 13C NMR, elemental analysis, and electrospray ioni-
zation (ESI) mass spectrometry (MS). IR (cm−1) [Fig. S1 in Supporting Informa-
tion (SI)]: 1110.97 (C–O–C), 1317.35 (C–N), 1554.50, 1578.89, 1536.37, 1616.79
(ArH), 1635.60 (C=O), 1681.89 (C=N), 2955.72, 2932.10, 2871.79 (CH3, CH2),
1
3279.95, 3454.43 (NH). H NMR (400 MHz, DMSO-d6) (Fig. S2 in SI), δ/ppm:
0.94 (t, 3H, J = 0.4 Hz), 1.35 (m, 2H), 1.60 (m, 2H), 4.00 (t, 2H, J = 3.6 Hz), 6.68
(q, 1H), 6.95 (d, 1H, J = 3.2 Hz), 7.57 (d, 1H, J = 8.4 Hz), 7.73 (t, 1H, J = 0.4 Hz),
7.88 (s, 1H), 8.31–8.34 (t, 2H, J = 1.6 Hz), 8.42 (d, 1H, J = 7.2 Hz), 8.68 (d, 1H,
J = 8.4 Hz), 11.37 (s, 1H). 13C NMR (400 MHz, DMSO-d6) (Fig. S3 in SI), δ/ppm:
13.70, 19.79, 29.71, 39.04, 106.63, 110.90, 112.21, 112.46, 118.46, 121.86, 124.78,
127.99, 128.97, 130.64, 133.34, 133.85, 144.72, 146.07, 149.72, 162.95, 163.53.
Elemental analysis data: Found C, 69.34; H, 5.42; N, 11.42%; Molecular formula
C21H19N3O3 requires C, 69.79; H, 5.30; N, 11.63%. ESI–MS, Found (Fig. S4 in SI):
[M + H]+ 362.1541, [M + Na]+ 384.1330; Molecular formula C21H19N3O3 requires
[M + H]+ 362.1505, [M + Na]+ 384.1324.
Testing and calculation methods
General methods
Stock solution of BNFA (1 × 10−3 M) was prepared in CH3CN. Stock solutions of
metal salts (Ca2+, K+, Ni2+, Fe3+, Na+, Fe2+, Mg2+, Ag+, Mn2+, Zn2+, Hg2+, Cr3+,
Pb2+, Co2+, Cd2+, and Cu2+) and ethylenediaminetetraacetic acid (EDTA) were pre-
pared in deionized water (1 × 10−2 M). The tested solutions of BNFA, Cu2+ (or other
metal ion), and their complex were prepared by mixing 0–100 μL stock solution
of BNFA with 0–100 μL stock solution of Cu2+ (or other metal ion) in a volumet-
ric fask (10 mL) and diluting to volume. The detection medium was CH3CN/H2O
(1/1, v/v). H+ concentration (pH) was adjusted using 0.1 M HCl and 0.1 M NaOH
aqueous solutions. UV–Vis absorption and fuorescence spectra were recorded after
standing for 1.5 h at 20 °C with excitation wavelength of 461 nm and slit width of
5 nm.
Detection limit
The detection limit was calculated as 3S/K, where S is the standard deviation of fve
times of the blank measurement and K is the slope of the titration ft line [31].
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Novel 1,8‑naphthalimide dye for multichannel sensing of H+…
Binding constant
The binding constant K was calculated using the Stern–Volmer Eq. (1) [32, 33],
where F0 and F are the fuorescent intensity at 558 nm before and after adding Cu2+,
[Cu2+] is the concentration of Cu2+, and n is the binding stoichiometry for BNFA
and Cu2+.
(
)
[
]
F0 − F
lg
= n lg Cu2+ + lg K.
(1)
F
Results and discussion
Design and synthesis of dye
We used the well-known 1,8-naphthalimide as chromophore and fuorophore, and
heteroatom-containing hydrazine hydrate and furfural as receptor precursors to pre-
pare a new dye for cation sensing. All reactions in the synthesis were carried out
under very mild conditions and ofered good yields. Except for BBN (purifed by
recrystallization in ethanol), other intermediates and the dye could be obtained by
succinct washing with water or ethanol. Hence, the dye can be obtained with high
efciency.
Spectra of BNFA at diferent pH values
In view of the pH sensitivity of the NH group, UV–Vis absorption and fuorescence
spectra of BNFA in CH3CN/H2O (1/1, v/v) (initial pH 5.2) within the pH range
of 1.9–12.0 were investigated. The results showed that the spectra were negligibly
afected by pH from 1.9 to 5.2 (Fig. 1). When the pH was increased from 5.2 to 12.0,
the absorbance at 461 nm and the fuorescence of the BNFA solution reduced, while
1800
b
a
9.8 10.8
7.0 7.9
12.0
3
2
1
0
6.2
9.1
700
600
500
0.6
PH 5.5
PH 5.8
1200
600
0
6
8
10
12
6
8
10
12
pH
PH 1.9-5.2
PH 5.5
PH 5.8
pH
0.4
0.2
0.0
PH 6.2-12.0
PH 1.9-5.2
PH 5.8
PH 5.5
PH 1.9-5.2
PH 6.2-12.0
PH 6.2-12.0
500
550
600
650
700
300
400
500
600
700
Wavelength /nm
Wavelength /nm
Fig. 1 UV–Vis absorption (a) and fuorescence (b) spectra of BNFA (10 μM) at diferent pH values.
Solvent: CH3CN/H2O (1/1, v/v). λex: 461 nm, slit width: 5 nm. pH: 1.9, 3.1, 4.0, 5.2, 6.2, 7.0, 7.9, 9.1,
9.8, 10.8, 12.0. Inset: color changes, as well as the relationship between A573/A461 (or F552) and pH.
(Color fgure online)
1 3

Y. Chen et al.
the absorbance at 573 nm increased gradually, accompanied by a change in the color
of the solution from shallow purple to deep purple, which could be observed by
naked eye. Moreover, the ratios of absorbance at 573 and 461 nm (A573/A461) of
the BNFA solution increased (Fig. 1a, inset) while the corresponding fuorescence
intensity at 552 nm (F552) decreased (Fig. 1b, inset) almost linearly from pH 6.2 to
12.0. The linear equations were A573/A461 = 0.5256 × pH – 2.8137 (R = 0.9949) and
F552 = 888.2454 − 30.7614 × pH (R = 0.9926). Therefore, we selected BNFA to
detect H+ (pH) in CH3CN/H2O (1/1, v/v) from pH 6.2 to 12.0.
Sensing H+ using BNFA
Anti‑interference of BNFA for detecting H+
To investigate the disturbance of the detection of H+ due to common metal ions, one
ion from among K+, Na+, Mg2+, Ag+, Zn2+, Cr3+, Cd2+, Co2+, Ni2+, Mn2+, Fe2+,
Fe3+, Pb2+, Hg2+, Cu2+, and Ca2+ (10 eq.) was added into the solution of BNFA at
pH 6.2 and 10.8. As shown in Fig. 2, the competing ions showed trivial efects on
the A573/A461 and F552 values of the solution. Therefore, BNFA showed good anti-
interference ability when detecting H+.
Time response
The time response of BNFA to H+ is shown in Fig. 3. Upon using NaOH aque-
ous solution to change the pH of the BNFA solution from 6.2 to 10.8, A573/A461
quickly rose while F552 greatly dropped. After 33 min, they both remained almost
unchanged. Therefore, BNFA could respond to H+ with relatively fast speed.
Mechanism for sensing of H+ using BNFA
To understand the mechanism for H+ sensing by BNFA, the reversibility between
pH 6.2 and 10.8 was investigated by adding NaOH and HCl aqueous solution alter-
nately. The results showed that the detection of H+ by BNFA was reversible (Fig. 4).
Fig. 2 Efects of coexisting ions on UV–Vis absorption (a) and fuorescence (b) maxima of BNFA for
detecting H+ in CH3CN/H2O (1/1, v/v). Concentration: 10 μM for BNFA, 100 μM for 2: Fe2+, 3: Fe3+,4:
Cu2+, 5: Zn2+, 6: Hg2+,7: Cr3+, 8: Pb2+, 9: Ca2+; 10: Mg2+, 11:Ag+, 12: Na+, 13: Co2+, 14: Cd2+, 15:
Ni2+, 16: K+, 17: Mn2+. 1: without coexisting ion. λex: 461 nm, slit width: 5 nm. Red and green bar:
A573/A461 at pH 6.2 and pH 10.8. Blue and yellow bar: F552 at pH 6.2 and pH 10.8. (Color fgure online)
1 3

Novel 1,8‑naphthalimide dye for multichannel sensing of H+…
Fig. 3 Time response of BNFA
700
650
600
550
to H+. Solvent: CH3CN/H2O
2.4
(1/1, v/v). Concentration: 10 μM
for BNFA. λex: 461 nm, slit
width: 5 nm
1.8
1.2
0.6
30
40
50
60
Time /min
Fig. 4 Reversibility of ratiomet-
ric and fuorescent detection
of H+ with BNFA. Solvent:
CH3CN/H2O (1/1, v/v); 10 μM
for BNFA. λex: 461 nm, slit
width: 5 nm
3.00
2.25
1.50
0.75
720
660
600
540
pH 6.2 10.8
6.2
10.8
6.2
10.8
pH 6.2 10.8
0.0 0.5
6.2
1.0
10.8
1.5
6.2
10.8
2.5
2.0
Cycle index
1
Furthermore, H NMR spectra of BNFA and BNFA–NaOH were collected in
dimethylsulfoxide (DMSO)-d6 (Fig. 5). Upon addition of 50 μL NaOH (0.1 M) into
600 μL DMSO-d6 solution of BNFA (resulted in pH of 11.0), the peak at 11.38 ppm
1
in the H NMR spectrum of BNFA disappeared completely and the H peaks of the
naphthalene ring varied. This can be explained based on the fact that OH− trapped
the H (j) in the NH linked to the naphthalene ring, resulting in changes of charge
density and variation of the absorbance and fuorescence intensities of the BNFA
solution.
1
Based on this reversibility, the H NMR results, and literature [11, 34], we sug-
gest a mechanism for H+ sensing by BNFA in Scheme 2.
Detection of Cu2+ by BNFA
In the work described above, we used the pH sensitivity of BNFA in the range of pH
6.2–12.0 to monitor H+. Thinking in other terms, its pH insensitivity would enable this
sensor to detect analytes other than H+ in multiple media at diferent pH values. There-
fore, we further examined the spectral response of BNFA to metal ions in its insensi-
tive pH range of 1.9–5.2. When 10 eq. of various metal cations such as Na+, K+, Ca2+,
Mg2+, Fe2+, Fe3+, Mn2+,Cu2+, Zn2+, Cr3+, Hg2+, Pb2+, Ni2+, Ag+, Cd2+, and Co2+ was
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Y. Chen et al.
Fig. 5 1H NMR spectra of BNFA and BNFA–NaOH in DMSO-d6 (400 MHz)
Scheme 2 Proposed H+ sensing mechanism of BNFA
introduced into CH3CN/H2O (1/1, v/v) solution of BNFA (pH 5.2) individually, only
Cu2+ induced a 4.6-fold decrease of the absorbance at 461 nm (A461), accompanied by
a color change from yellow to colorless observable to the naked eye, as shown in the
UV–Vis absorption spectra (Fig. 6a). Similarly, only Cu2+ caused a 5.4-fold fall of the
fuorescence intensity at 558 nm (F558) accompanied by a 19-nm fuorescent blueshift
and color change from orange to green (Fig. 6b). These results indicate that BNFA can
be used as a triple-channel (UV–Vis absorption, fuorescence intensity reduction, as
well as fuorescence blueshift) sensor for Cu2+ in CH3CN/H2O (1/1, v/v).
Anti‑interference of BNFA for detecting Cu2+
To investigate the disturbance from other coexisting metal ions on the detection of
Cu2+ by BNFA, Cu2+ was added to CH3CH/H2O (1/1, v/v) solution of BNFA with
1 3

Novel 1,8‑naphthalimide dye for multichannel sensing of H+…
a
b 1200
800
0.6
0.4
0.2
0.0
K+,Ag+,Ca2+,None,
Pb2+,Zn2+,Cr3+
Na+,Cd2+,Fe2+
BNFA
+Cu
BNFA
2+
,
,
Ni2+,Hg2+,Mn2+
,
Ni2+,Mn2+,Na+,Ca2+
K+,Ag+,Mg2+,Cr3+
Cd2+,Zn2+,Fe2+,Co2+
None,Pb2+,Hg2+,Fe3+
,
Mg2+,Co2+,Fe3+
,
400
,
Cu2+
Cu2+
0
300
400
500
600
480
560
640
720
Wavelength /nm
Wavelength /nm
Fig. 6 UV–Vis absorption (a) and fuorescence (b) spectra as well as color change of BNFA in absence
and presence of metal ions. Solvent: CH3CN/H2O (1/1, v/v), concentration: 10 μM for BNFA, 100 μM
for metal ions. λex: 461 nm, slit width: 5 nm
one metal ion from among K+, Na+, Mg2+, Ag+, Zn2+, Cr3+, Cd2+, Co2+, Ni2+, Mn2+,
Fe2+, Fe3+, Pb2+, Hg2+, and Ca2+. As shown in Fig. 7, Cu2+ could still decrease the
UV–Vis absorption (a) and fuorescence (b) maxima remarkably. Therefore, BNFA
exhibited good anti-interference performance in detection of Cu2+.
Dependence of BNFA’s spectra on Cu2+ concentration
To investigate whether BNFA could detect Cu2+ quantitatively, titration experi-
ments were performed. As shown in Fig. 8, A461 of BNFA in CH3CN/H2O (1/1, v/v)
gradually decreased with increase of the Cu2+ concentration ([Cu2+]). When [Cu2+]
reached 30 μM, A461 exhibited a stable trend. Moreover, A461 and [Cu2+] exhibited
a good linear relationship with concentration in the range of 0–6 μM (A461 = − 0.0
2493 × [Cu2+] + 0.2475, R = 0.9713). The fuorescence spectra of BNFA exhibited
behavior similar to the UV–Vis absorption spectra, with F558 = − 144.5071 × [Cu
2+] + 1280.6929 (R = 0.9625). The detection limits evaluated by colorimetric and
fuorescent titration were 6.10 × 10−7 and 2.62 × 10−7 M, respectively.
Fig. 7 Efects of coexisting ions (Mn+) on UV–Vis absorption (a) and fuorescence (b) maxima of
BNFA–Cu2+ in CH3CN/H2O (1/1, v/v). Concentration: 10 μM for BNFA, 100 μM for 1: Cu2+, 4: Zn2+
5: Hg2+, 6: Cr3+, 8: Ca2+; 9: Mg2+, 10: Ag+, 11: Na+, 12: Co2+, 13: Cd2+, 14: Ni2+, 15: K+, 16: Mn2+
,
.
50 μM for 2: Fe2+, 3: Fe3+, 7: Pb2+. Red, green, and yellow bar: A461 of BNFA, BNFA–Mn+, and BNFA–
M
n+–Cu2+, respectively; Purple, orange, and blue bar: F558 of BNFA, BNFA–Mn+, and BNFA–Mn+
Cu2+, respectively. λex: 461 nm, slit width: 5 nm. (Color fgure online)
–
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Y. Chen et al.
a
0.27
0.18
0.09
b
1500
1000
500
0
0.27
1500
1000
500
0
0.18
0.09
0.00
0
30
60
90
0
30
60
90
[Cu2+]/10-6 M
[Cu2+]/10-6 M
500
600
700
300
400
500
600
Wavelength/nm
Wavelength /nm
Fig. 8 UV–Vis absorption (a) and fuorescence (b) spectra of BNFA (10 μM) with various concentra-
tion of Cu2+. Solvent: CH3CN/H2O (1/1, v/v). λex: 461 nm, slit width: 5 nm. From top to bottom, concen-
tration of Cu2+: 0, 1, 2, 3, 4, 5, 6, 8, 10, 15, 20, 30, 50, 75, 100 μM. Inset: relationship between A461 (or
F558) and concentration of Cu2+
Efect of pH
We also studied the efect of pH on the UV–Vis absorption and fuorescence spec-
tra of BNFA–Cu2+ solution in the pH range of 1.9–5.2 (Fig. 9). Compared with
that of BNFA solution (data from Fig. 1a), A461 and F558 of BNFA–Cu2+ solution
were at very low level, and the absorbance (or fuorescence intensity) deviation
between BNFA–Cu2+ and BNFA solutions was remarkable and stable. Therefore,
BNFA could detect Cu2+ sensitively and reliably in the pH range of 1.9–5.2.
Time response
With the extension of time, A461 and F558 gradually reduced, then remained
almost stable after about 60 min (Fig. 10). Therefore, BNFA could test Cu2+ with
acceptable speed and good reliability.
Fig. 9 Efect of pH on UV–
Vis absorption at 461 nm and
fuorescence intensity at 558 nm
of BNFA and BNFA–Cu2+ in
CH3CN/H2O (1/1, v/v). Con-
centration: 10 μM for BNFA,
100 μM for Cu2+. pH: 1.9, 3.1,
4.0, 5.2
0.4
0.3
0.2
0.1
1600
1200
800
BNFA
BNFA-Cu2+
BNFA
BNFA-Cu2+
400
2
3
4
5
pH
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Novel 1,8‑naphthalimide dye for multichannel sensing of H+…
Fig. 10 Efect of time on
UV–Vis absorption at 461 nm
and fuorescence intensity at
558 nm of BNFA (10 μM)–
Cu2+(100 μM) in CH3CN/H2O
(1/1, v/v)
0.25
0.20
0.15
0.10
0.05
1200
900
600
300
0
10
40
70
100
130
Time /min
Table 1 Recovery of Cu2+ in pond water and tap water (n = 3)a
Sample
Cu2+ added
Cu2+ found
Recovery (%)
RSDa (%)
(10−6 mol/L)
(10−6 mol/L)
Pond water
Tap water
4
6
4
6
3.78
5.86
3.94
5.86
94.5
97.7
98.5
97.7
4.4
0.9
0.8
0.9
aSolvent: CH3CN/H2O (1/1, v/v), BNFA: 10 μM, RSD: relative standard deviation
Practicability
Further, diferent concentrations of Cu2+ were added to tap water and pool water to
examine the practicability of BNFA using our proposed fuorescence assay method.
The data in Table 1 show that the concentrations of Cu2+ measured and added were
indeed similar, with recovery values between 94.5 and 98.5%. The relative standard
deviation (RSD) of three measurements was less than 4.4%. Therefore, BNFA can
be used to detect Cu2+ in environmental water samples efectively.
Mechanism for Cu2+ sensing by BNFA
To explore the mechanism of Cu2+ sensing by BNFA, a Job’s plot experiment was
performed (Fig. 11). The total concentration of BNFA and Cu2+ was kept constant at
50 μM, while the molar fraction of Cu2+ was changed from 0.0 to 1.0. The maximal
increment of the fuorescence intensity between BNFA and BNFA–Cu2+ appeared
at 0.5 (Cu2+ molar fraction), indicating 1:1 stoichiometry between BNFA and Cu2+.
Based on this 1:1 binding mode and the fuorescence titration results, the binding
constant calculated using the Stern–Volmer equation was 3.27 × 104 M−1.
Furthermore, reversibility was studied by adding EDTA into BNFA–Cu2+ solu-
tion. As shown in Fig. 12, the absorbance and fuorescence of BNFA–Cu2+ in
CH3CN/H2O (1/1, v/v) could not be recovered by addition of excess EDTA, reveal-
ing that the reaction of BNFA with Cu2+ was irreversible.
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Y. Chen et al.
Fig. 11 Job’s plot for Cu2+
versus BNFA. Total concentra-
tion ([BNFA] + [Cu2+]): 50 μM.
F0 and F: fuorescence maxima
at 558 nm before and after addi-
tion of Cu2+, respectively. λex:
461 nm, slit width: 5 nm
1800
1200
600
0
0.0
0.2
0.4
0.6
0.8
1.0
[Cu2+] / ([Cu2+]+[BNFA])
On the basis of the Job’s plot, reversibility experiments, and literature [18, 35],
a possible model for binding of BNFA with Cu2+ is proposed in Scheme 3. When
Cu2+ meets BNFA in CH3CN/H2O (1/1, v/v), it frst bonds via the N and O atoms of
the moieties linked in C4 of the naphthalene ring (BNFA1), inducing intramolecular
charge transfer (ICT), which results in cleavage of the C4–N bond to form BNFA2.
To validate the mechanism, liquid chromatography (LC)–MS spectra of
BNFA–Cu2+ were recorded 30 min and 2 h after Cu2+ was added (Fig. 13). The
peaks at m/z values of 447.4988, 362.1410, and 276.1903 in Fig. 13a can be ascribed
to [BNFA1 + Na]+, [BNFA + H]+, and [BNFA2 + Na]+, respectively. Meanwhile,
Fig. 13b shows m/z peaks at 254.1100, 362.1499, and 384.1314, which correspond
to [BNFA2 + H]+, [BNFA + H]+, and [BNFA + Na]+, respectively. These results
basically support the mechanism proposed above.
To further verify the generation of BNFA2 in the sensing process, 2 mg BNFA
was reacted with 2 mg CuSO4·5H2O in 10 mL CH3CN/H2O (1/1, v/v) at 20 °C for
4 h. The resulting of-white precipitate was isolated by fltration, and its LC–MS
spectrum was collected. As shown in Fig. 14, the peaks at m/z values of 254.1174
1500
a
0.3
b
BNFA+EDTA
BNFA Only
BNFA Only
1000
500
0
BNFA+EDTA
0.2
0.1
0.0
BNFA+Cu2++EDTA
BNFA+Cu2+
BNFA+Cu2++EDTA
BNFA+Cu2+
350
450
550
650
500
550
600
650
700
Wavelength /nm
Wavelength /nm
Fig. 12 Reversibility of absorbance (a) and fuorescent (b) detection of Cu2+ (100 μM) with BNFA
(10 μM) in CH3CN/H2O (1/1, v/v). EDTA: 500 μM. λex: 461 nm, slit width: 5 nm
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Novel 1,8‑naphthalimide dye for multichannel sensing of H+…
Scheme 3 Proposed Cu2+ sensing mechanism of BNFA
Fig. 13 LC–MS of BNFA at 30 min (a) and 2 h (b) after adding Cu2+
and 276.0994 exactly match with the [M + H]+ and [M + Na]+ molecular ion peaks
of BNFA2, respectively. This result clearly confrms the validity of the sensing
mechanism suggested above.
Conclusions
We report a novel, multifunction, multichannel 1,8-naphthalimide-based spectral
sensor with simple structure and facile synthetic method. In CH3CN/H2O (1/1, v/v),
BNFA was highly selective and sensitive for Cu2+ using triple channels in the pH
range of 1.9–5.2, while it responded efectively to H+ using dual channels within
the pH range of 6.2–12.0. Its detection performance remained almost undisturbed
by diverse coexistent ions. The H+ sensing results from reversible transformation
between N− and NH at C-4 position of naphthalene ring. In contrast, the Cu2+ sens-
ing can be explained based on the fact that Cu2+ frst bonds to the two N atoms and
one O atom of the receptor located in C-4 position of naphthalene ring, followed by
irreversible cleavage of the complex.
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Y. Chen et al.
Fig. 14 LC–MS of product of reaction of BNFA and CuSO4·5H2O
Acknowledgements This work was supported by the National Natural Science Foundation of China
(21074085), Priority Academic Program Development of Jiangsu Higher Education Institutions, and
State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials.
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