Selective dendritic fluorescent sensors for Zn(II)

Article abstract of DOI:10.3390/molecules13040922

A series of dendritic 8-hydroxyquinoline (8-HQ) and 5-dialkyl(aryl) aminomethyl -8-HQ derivatives were synthesized and their fluoroionophoric properties toward representative alkali, alkaline earth, group IIIA and transition metal ions were investigated. Among the selected ions, Zn(II) enhanced the fluorescence of N-di-(methoxycarbonylethyl)aminoethyl-3-[4-(8- hydroxyquinolin-5-ylmethyl)piperazin-1-yl]-propanoic amide] (7) by 31-fold, while Al(III) caused enhancement to some extent. The absence of any significant fluorescence enhancement by the other ions examined renders 7 a highly useful Zn(II)-selective fluorescent sensor.

Full text of DOI:10.3390/molecules13040922

Molecules 2008, 13, 922-930
molecules
ISSN 1420-3049
© 2008 by MDPI
www.mdpi.org/molecules
Full Paper
Selective Dendritic Fluorescent Sensors for Zn(II)
Feng Wang, Ruogu Peng and Yaowu Sha*
The Key Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology, Department of
Chemistry, Tsinghua University, Beijing 100084, P.R. China
* Author to whom correspondence should be addressed; E-mail: shayw@tsinghua.edu.cn
Received: 6 April 2008; in revised form: 17 April 2008 / Accepted: 18 April 2008 / Published: 19 April
2008
Abstract: A series of dendritic 8-hydroxyquinoline (8-HQ) and 5-dialkyl(aryl)amino-
methyl-8-HQ derivatives were synthesized and their fluoroionophoric properties toward
representative alkali, alkaline earth, group IIIA and transition metal ions were
investigated. Among the selected ions, Zn(II) enhanced the fluorescence of N-di-
(methoxycarbonylethyl)aminoethyl-3-[4-(8-hydroxyquinolin-5-ylmethyl)piperazin-1-yl]-
propanoic amide] (7) by 31-fold, while Al(III) caused enhancement to some extent. The
absence of any significant fluorescence enhancement by the other ions examined renders 7
a highly useful Zn(II)-selective fluorescent sensor.
Keywords: Dendritic, 8-hydroxyquinoline, Zn2+, fluorescence enhancement.
Introduction
Zinc, which always occurs as a divalent cation [Zn(II)] in biological systems, is the second most
abundant transition metal after iron. Its amounts in brain tissues can reach 0.1-0.5 mM [1]. The Zn2+
ion is well known to play diverse roles in biological processes [2], such as its presence in hundreds of
structural metalloproteins [3] and at the catalytic centre of human carbonic anhydrase [4] and its
involvement in the regulation of gene expression [5] and cellular apoptosis [6]. Disorders of zinc
metabolism are closely associated with many severe neurological diseases, such as Alzheimer’s
disease, amyotrophic lateral sclerosis (ALS), Guam ALS-Parkinsonism dementia, Parkinson’s disease,
hypoxia-ischemia and epilepsy [7]. There is huge scope for exploring the diverse physiological roles of

Molecules 2008, 13
923
biological zinc. Therefore, a sensitive and particularly selective chemosensor for Zn(II) is desired,
since there are also many other biologically important transition metal ions.
A fluorescent chemosensor is the first choice among various types of sensors. 8-Aminoquinoline
(8-AQ,) and 8-hydroxyquinoline (8-HQ, 1) and their derivatives have traditionally been used as
fluorogenic agents for quantitative chemical assays of Zn(II) and other metal ions. 8-HQ, 8-AQ and
their derivatives are non-fluorescent and are not influenced by changes in pH [8] due to the existence
of an intramolecular hydrogen bond between the heterocyclic nitrogen atom and the 8-substituted
group (i.e. –OH or –NH2) [9-10]. However, when these quinolines and their derivatives chelate Zn(II)
and other metal ions, they exhibit intense yellow-greenish fluorescence. Interestingly, only Zn(II) and
Ca2+ ions can interrupt hydrogen bonding in 8-AQ and its derivatives, and subsequently stimulate
fluorescence, thereby forming the basis for selective detection of these two cations. It was not until
1987 that one of the sulphonamido compounds, namely 6-methoxy-8-p-toluenesulphonamido-
quinoline (TSQ), was first applied to in vitro imaging of divalent zinc ions [11]. However, TSQ has
several limitations. The most notable among these is its poor water solubility, since this means that
TSQ stock solutions have to be prepared in dimethylsulfoxide (DMSO) or ethanol. Although some
efforts toward the development of new sensors have been made [12], TSQ remains the most widely
used compound for biological zinc staining.
Although 8-HQ is regarded as the second most important chelating agent for cation analysis after
EDTA, most indicators derived from it are not sufficiently specific for the selective detection of
particular metal ions. However, while working on the synthesis of 8-HQ metallodendrimers for
Organic Light Emitting Display (OLED), we found that, among the common transitional metal ions,
only Zn2+ could form stable complexes with the dendritic 8-HQ ligands [13]. In particular, these
ligands are easily soluble in common solvents and water. This experience inspired us to investigate the
Zn2+-selective sensing properties of dendritic 8-HQ ligands.
Results and Discussion
The synthesis of new dendritic 8-HQ ligands is shown in Schemes 1 2. First, 8-HQ (1) was
chloromethylated to give crude 5-chloromethyl-8-HQ (2). High purity products were obtained by
washing with concentrated hydrogen chloride instead of acetone [8]. The residual 8-HQ could not be
removed completely by washing with acetone and substitution reactions occurred during the
crystallization from ethanol.
Scheme 1.
Cl
NR2
R2NH
CH2Cl2
Et3N
HCHO/HCl
85%
N
3a: R=Me. 84%
3b: R=Pr. 93%
3c: R=Ph. 91%
3d: R=Morph.88%
N
HCl
N
OH
OH
OH
3a-3d
1
2

Molecules 2008, 13
924
Next, 2 was reacted for about 10 minutes with different secondary amines in dichloromethane at
room temperature to give 3a-3e in high yields. Triethylamine was used as the base. The use of
inorganic bases, such as sodium carbonate, sodium hydrogen carbonate and sodium hydroxide, led to
either very slow reaction rates or many unwanted byproducts.
Scheme 2.
O
O
O
Cl
N
NH
N
O
N
O
N
N
N
NH
O
30%HCl
N
HCl
CH2Cl2,Et3N
90%
CH3OH
88%
aq KOH
93%
OH
N
N
N
OH
OH
OH
5
2
3e
4
NH2
62%
H2N
CH3OH
O
N
O
O
O
NH2
N
N
H
O
N
N
O
N
H
N
O
O
CH3OH
75%
N
N
OH
OH
7
6
Compound 4 was obtained in 93% yield by treatment of 3e with 30% hydrochloric acid for about 8
hours, followed by neutralization with potassium hydroxide. Michael addition of 4 with methyl
acrylate in methanol gave 5 in 88% yield. Compound 6 was synthesized in 62% yield by amidation of
5 with ethylene diamine. Further Michael addition of 6 with methyl acrylate gave 7 in 75% yield. The
structures were identified by 1H-NMR, 13C-NMR, ESI-MS and elemental analysis.
The UV spectrum of 3a, selected as a representative compound, showed two absorption peaks at
255 and 327 nm. When equivalent Zn(II) was added, the absorption peaks shifted to 257 and 405 nm,
respectively (Figure 1). When excited at 370 nm, 3a-3e, 5 and 7 showed similar emission patterns with
maximum emission wavelengths around 406 and 432 nm.
The fluoroionophoric properties of the 8-HQ derivatives toward representative alkali ions (K+),
alkaline earth ions (Mg2+, Ca2+, Ba2+), group IIIA ions (Al3+, Ga3+) and transition metal ions (Fe3+, Fe2+,
Co2+, Ni2+, Cu2+, Ag+, Zn2+, Cd2+ Pb2+, Mn2+, Cr3+) were investigated in chloroform solutions. The
results revealed that only Zn2+ and Al3+ caused new emission peaks, while the other ions hardly had
any effects. Following addition of Zn2+ or Al3+, the maximum emission wavelengths changed to
around 500 or 530 nm, respectively (Figure 2). Furthermore, Zn2+ enhanced the fluorescence intensity

Molecules 2008, 13
925
to a greater extent than Al3+ for all the sensors examined. However, there were differences in the
sensitivities and selectivities among 3a-3e, 5 and 7. The corresponding data are summarized in Table 1.
The best sensor was 7, which showed 31-fold fluorescence enhancement for Zn2+ and 2.4-fold
enhancement for Al3+ (Figure 3).
Figure 1. UV spectra of 3a (10 μM) in chloroform, before (A) and after (B) the addition
of 1.0 equiv of Zn2+.
4
3
2
A
B
1
0
200
300
400
500
600
Wavelength(nm)
Figure 2. Fluorescence spectra of 3a (10 μM) in chloroform, before (A) and after the
addition of 1.0 equiv of Zn2+ (B) and Al3+ (C).
800000
B
600000
C
400000
200000
A
0
400
450
500
550
600
650
Wavelength(nm)

Molecules 2008, 13
926
Table 1. Fluorescence enhancement of 3a-3d, 4, 5 and 7 after addition of 1.0 equiv of Zn2+ or Al3+
Compd
.
λ
0max(nm
)
Enhancing
percent. of Zn2+ percent. of Al3+
Enhancing
λ
( Al3+)max(nm)
λ
( Zn2+)max(nm)
3a
3b
3c
3d
4
406, 432
407, 431
406, 432
407, 431
407, 431
406, 432
405, 433
531
530
529
530
530
529
529
507
506
505
506
506
505
505
3
4
2
2
3
2
2
3
3
4
4
5
5
8
7
31
The fluorescence enhancement data and selectivity of sensor 7 are shown in Figure 3. Zn2+ caused
the highest level of fluorescence enhancement, while Al3+ enhanced the fluorescence to some extent.
The other ions examined showed little fluorescence enhancement.
Figure 3. Relative fluorescence intensity (If/I0) of 7 (10 μM) in the presence of 1.0 equiv.
of various metal ions (10 μM) in chloroform.
30
O O
25
O
N
O
N
N
H
N
O
20
15
10
5
N
OH
0
+
2+ 2+ 2+ 3+ 3+ 2+ 2+ 3+ 2+ 2+ 2+
+
2+ 2+ 2+ 3+
K Mg Ca Ba Al Ga Mn Fe Fe Co Ni Cu Ag Zn Cd Pb Cr
In order to gain further insight into the analytical possibilities, fluorescence titration of 7 with Zn2+
was performed and the obtained curves are shown in Figure 4. The fluorescence enhancement was
very effective and 31-fold enhancement was obtained at the ratio of 1:1. The break around 0.5 equiv of
Zn2+ suggested a 1:1 stoichiometry for the 7-Zn2+ complex system.

Molecules 2008, 13
927
Figure 4. Fluorescence titration curves of 7 (10 μM) by addition of 1μM Zn2+ each time
in chloroform.The inset shows a plot of the fluorescence intensities against [Zn2+].
140000
30
20
120000
10
100000
0
0
2
4
6
8
10
Zn2+ ( μM )
80000
60000
40000
20000
0
2+
Zn
400
450
500
550
600
650
Wavelength (nm)
Conclusions
A series of novel Zn2+ fluorescent sensors based on 8-HQ were synthesized and their
fluoroionophoric properties toward K+, Mg2+, Ca2+, Ba2+, Al3+, Ga3+ Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Ag+,
Zn2+, Cd2+, Pb2+, Mn2+ and Cr3+ were investigated in chloroform solutions. Only Zn2+ caused
considerable enhancement of the fluorescence emission, and the other metal ions examined showed
little interference with the detection of Zn2+. Among these potential metal ion fluorescent sensors,
dendritic ligand 7 showed the best efficiency and selectivity toward Zn2+. These results are consistent
with our earlier discovery.
Experimental
General
All reagents were purchased from Aldrich and used without further purification. 1H-NMR and 13C-
NMR spectra were recorded in CDCl3 on a JEOL JNM-ECA300 spectrometer operating at 300 and 75
MHz, respectively. Chemical shifts (δ) are reported in parts per million (ppm) relative to
tetramethylsilane (TMS) as an internal standard, and coupling constants (J) are given in hertz (Hz). All
ESI-MS experiments were undertaken on a Bruker ESQUIRE-LC. Fluorescence intensity was
measured using a HORABA Fluoromax 3.
General procedure for the preparation of 5-chloromethyl-8-HQ hydrochloride (2)
Into a solution of 8-HQ (1, 0.1 mol) and formaldehyde (20 mL, 37%) in 37% hydrochloric acid (50
mL) was blown a stream of hydrogen chloride gas for 8 hours at 25°C. After filtration, the product was
washed with 37% hydrochloric acid and dried to afford 2 in 85% yield.

Molecules 2008, 13
928
General procedure for the preparation of 5-dialkyl(aryl)aminomethyl-8-HQ compounds 3a-3e
To a solution of dialkyl(aryl)amine (10 mmol) and triethylamine (30 mmol for dimethylamine
hydrochloride and 20 mmol for other secondary amines) in dichloromethane (50 mL) was added
dropwise a solution of 2 (10 mmol) in dichloromethane at room temperature over 10 minutes. After a
further 15 minutes of reaction, the solution was washed thoroughly with brine, dried over sodium
sulfate and concentrated to afford 3a-3e without further purification (yields are listed in Schemes 1 and
2).
1
5-Dimethylaminomethyl-quinolin-8-ol (3a). H-NMR δ 8.77 (dd, 1H, J1=4.4, J2=1.4), 8.61 (dd, 1H,
J1=8.6, J2=1.4), 7.48 (dd, 1H, J1=8.6, J2=4.4), 7.30 (d, 1H, J=8.3), 7.07 (d, 1H, J=8.3), 3.72 (s, 2H),
2.25 (s, 6H); 13C-NMR δ 153.4, 148.2, 137.4, 132.6, 132.2, 127.4, 122.6, 115.8, 111.1, 56.6, 42.2; ESI-
MS: calcd. for (M+H)/z: 203.1. Found: (M+H)/z: 203.1; Elemental analysis: C11H11N2O calcd. (%) C,
70.57; H, 5.92; N, 14.96. Found (%) C, 70.49; H, 6.04; N, 14.64.
1
5-Dipropylaminomethyl-quinolin-8-ol (3b). H-NMR δ 8.75 (m, 2H), 7.44 (dd, 1H, J1=6.5, J2=4.4),
7.34 (d, 1H, J=7.56), 7.06 (d, 1H, J=7.56), 3.85 (s, 2H), 2.38 (t, 4H, J=7.2), 1.49 (m, 4H), 0.79 (t, 6H,
J=7.2); 13C-NMR δ 151.6, 147.5, 138.8, 134.4, 128.7, 128.0, 126.4, 121.2, 108.8, 57.3, 55.8, 19.9, 12.1;
ESI-MS: calcd. for (M+H)/z: 259.2. Found: (M+H)/z: 259.2; Elemental analysis: C16H22N2O calcd. (%)
C, 74.38; H, 8.58; N, 10.84. Found (%) C, 74.49; H, 8.44; N, 11.01.
1
5-(Diphenylaminomethyl)quinoline-8-ol (3c). H-NMR δ 8.80 (dd, 1H, J1=4.1, J2=1.0), 8.36 (dd, 1H,
J1=8.5, J2=1.0), 7.46 (m, 2H), 7.23 (m, 4H), 7.05(m, 5H), 6.95 (t, 2H, J=8.5), 5.30 (s, 2H); 13C-NMR δ
151.4, 148.1, 147.5, 138.7, 131.9, 129.4, 126.5, 126.4, 123.7, 121.8, 121.7, 121.0, 109.5, 53.3; ESI-MS:
calcd. for (M+H)/z: 327.2. Found: (M+H)/z: 327.2; Elemental analysis: C22H18N2O calcd. (%) C, 80.96;
H, 5.56; N, 8.58. Found (%) C, 80.61; H, 5.94; N, 8.39
1
5-(Morpholin-4-ylmethyl)-quinolin-8-ol (3d). H-NMR δ 8.78 (dd, 1H, J1=4.2, J2=1.3), 8.67 (dd, 1H,
J1=8.6, J2=1.3), 7.45 (dd, 1H, J1=8.6, J2=4.2), 7.32 (d, 1H, J=7.9), 7.06 (d, 1H, J=7.9), 3.79 (s, 2H),
3.66 (m, 4H), 2.45 (m, 4H); 13C-NMR δ 152.1, 147.7, 138.8, 134.2, 129.2, 127.9, 124.0, 121.6, 108.7,
67.1, 61.1, 53.6; ESI-MS: calcd for (M+H)/z: 245.1. Found: (M+H)/z: 245.1; Elemental analysis:
C14H16N2O2 calcd. (%) C, 68.83; H, 6.60; N, 11.47. Found (%) C, 68.91; H, 6.35; N, 11.65.
1
5-(4′-Benzoylpiperazin-1-ylmethy)l-quinolin-8-ol (3e). H-NMR δ 8.78 (dd, 1H, J1=4.2, J2=1.3), 8.66
(dd, 1H, J1=8.6, J2=1.3), 7.48 (dd, 1H, J1=8.6, J2=4.2), 7.39 (m, 5H), 7.32 (d, 1H, J=7.9), 7.06 (d, 1H,
J=7.9), 3. 83 (s, 2H), 3.76 (m, 2H), 3.37 (m, 2H), 2.57 (m, 2H), 2.39 (m, 2H);13C- NMR δ 170.2, 152.0,
147.6, 138.7, 135.8, 133.9, 129.6, 129.1, 128.4, 128.0, 127.7, 127.0, 121.5, 108.6, 67.9, 60.5, 52.2;
ESI-MS: calcd for (M+H)/z: 347.2. Found: (M+H)/z: 347.2.
1
5-(Piperazin-1-ylmethyl)-quinolin-8-ol (4). H-NMR δ 8.77 (dd, 1H, J1=4.2, J2=1.3), 8.67 (dd, 1H,
J1=8.6, J2=1.3), 7.46 (dd, 1H, J1=8.6, J2=4.2), 7.32 (d, 1H, J=7.9), 7.06 (d, 1H, J=7.9), 3.77 (s, 2H),
2.83 (m, 4H), 2.43 (m, 4H); 13C-NMR δ 151.9, 147.6, 138.8, 134.2, 129.0, 128.0, 124.6, 121.4, 108.8,

Molecules 2008, 13
929
61.4, 54.4, 46.2; ESI-MS: calcd. for (M+H)/z: 244.1. Found: (M+H)/z: 244.1; Elemental analysis:
C14H17N3O calcd. (%) C, 69.11; H, 7.04; N, 17.27. Found (%) C, 69.43; H, 7.11; N, 17.04.
The compounds 5,6 and 7 were obtained by a reported method [13].
1
3-[4′-(8-Hydroxyquinolin-5-ylmethyl)-piperazin-1′-yl]-propionic acid methyl ester (5). H-NMR δ
8.77 (d, 1H, J=4.1), 8.65 (d, 1H, 8.2), 7.46 (dd, 1H, J1=8.2, J2=4.1), 7.33 (d, 1H, J=7.9), 7.06 (d, 1H,
J=7.9), 3.79 (s, 2H), 3.67 (s, 3H), 2.67 (m, 2H), 2.48 (m, 10H); 13C-NMR δ 171.6, 153.4, 148.2, 139.8,
132.2, 129.3, 128.4, 124.6, 121.8, 110.4, 60.2, 54.4, 51.7, 48.7, 46.2, 34.2; ESI-MS: calcd. for
(M+H)/z: 330.2. Found: (M+H)/z: 330.2; Elemental analysis: C18H23N3O3 calcd. (%) C, 65.63; H, 7.04;
N, 12.76. Found (%) C, 65.85; H, 6.98; N, 12.73.
1
N-Aminoethyl-3-[4-(8-hydroxyquinolin-5-ylmethyl)piperazin-1-yl]propanamide (6). H-NMR δ 8.77
(d, 1H, J=4.1), 8.65 (d, 1H, 8.2), 7.46 (dd, 1H, J1=8.2, J2=4.1), 7.33 (d, 1H, J=7.9), 7.06 (d, 1H, J=7.9),
3.77 (s, 2H), 3.66 (t, 2H), 2.75 (t, 2H), 2.69 (m, 2H), 2.46 (m, 10H); 13C-NMR δ 171.6, 153.4, 148.2,
139.8, 132.2, 129.3, 128.4, 124.6, 121.8, 110.4, 60.2, 54.4, 48.7, 46.2, 40.6, 39.8, 34.2; ESI-MS: calcd.
for (M+H)/z: 357.2. Found: (M+H)/z: 357.2; Elemental Analysis: C19H27N5O2 calcd. (%) C, 63.84; H,
7.61; N, 19.59; O, 8.95. Found (%) C, 65.85; H, 7.64; N, 19.57; O, 8.93.
N-di(Methoxycarbonylethyl)aminoethyl-3-[4-(8-hydroxyquinolin-5-ylmethyl)piperazin-1-yl]propanoic
amide] (7). 1H-NMR δ 8.85 (d, 1H, J=3.0), 8.62 (d, 1H, J=7.9), 7.57 (dd, 1H, J1=7.9, J2=3.0), 7.32 (d,
1H, J=7.9), 7.00 (d, 1H, J=7.9), 3.74 (s, 2H), 3.61 (s, 6H), 3.05 (t, 2H, J=6.5), 2.46-2.55 (m, 10H),
2.37 (m, 10H), 2.22 (t, 2H, J=6.9); 13C-NMR δ 172.4, 172,0, 171.3, 153.4, 147.9, 139.4, 132.1, 129.4,
128.0, 124.7, 121.9, 111.2, 60.8, 54.4, 53.2, 51.8, 51.7, 48.7, 46.2, 37.1, 34.2, 33.5; ESI-MS: calcd. for
(M+H)/z: 530.3. Found: (M+H)/z: 530.3; Elemental analysis: C27H39N5O6 calcd. (%) C, 61.23; H, 7.42;
N, 13.22. Found (%) C, 61.55; H, 7.38; N, 13.04.
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Sample Availability: Compound 3e is available from the authors.
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