Near-infrared aminocyanine dyes: Synthesis, optical properties, and application to the preparation of fluorescent microspheres

Article abstract of DOI:10.1007/s11164-013-1052-0

Several near-infrared (NIR) fluorescent dyes based on aminocyanine have been designed, synthesized, and used for preparation of NIR fluorescent microspheres. Their spectral properties and pH-dependent optical characteristics were investigated. It was foun

Full text of DOI:10.1007/s11164-013-1052-0

Res Chem Intermed
DOI 10.1007/s11164-013-1052-0
Near-infrared aminocyanine dyes: synthesis, optical
properties, and application to the preparation
of fluorescent microspheres
•
•
•
•
Liang Yu Tingting Li Qiuling Wang Lei Li
Ligong Chen
Received: 29 October 2012 / Accepted: 15 January 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Several near-infrared (NIR) fluorescent dyes based on aminocyanine
have been designed, synthesized, and used for preparation of NIR fluorescent
microspheres. Their spectral properties and pH-dependent optical characteristics
were investigated. It was found that different substitution at the central position of
the dyes led to diverse optical properties, with especially large effects on their
spectroscopic properties. Spectroscopic responses of the fluorescent dyes to changes
of pH were different. Under acidic conditions, the wavelengths of maximum
absorption of the N-substituted cyanine dyes were clearly red shifted. Dyes with an
N-substituted piperazidine ring were the most sensitive. The fluorescent micro-
spheres retained the properties of the parent dyes, making them suitable for appli-
cations in biological studies.
Keywords Near-infrared ꢀ pH-dependent ꢀ Fluorescent ꢀ Microsphere
Introduction
Fluorescent labeling is one of the most common bioanalytical methods [1–4].
Highly uniform fluorescent microspheres are widely used in such methods, because
of their unique properties resulting from their large surface-to-volume ratios and
small sizes [5–7]. Some fluorescent dyes, for example, rhodamine and fluorescein,
are often used to prepare fluorescent spheres [8], although such problems as limited
L. Yu ꢀ T. Li ꢀ L. Li ꢀ L. Chen (&)
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072,
People’s Republic of China
e-mail: lgchen@tju.edu.cn
Q. Wang
School of Pharmaceuticals and Biotechnology, Tianjin University, Tianjin 300072,
People’s Republic of China
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L. Yu et al.
visibility and background fluorescence, has hampered widespread application
[9, 10]. As a result, use of near-infrared fluorescent labeling reagents and
microspheres for biological applications has expanded rapidly in recent years. In
optical imaging in the NIR region (700–900 nm) absorption by intrinsic photoactive
biomolecules is low, which enables light to penetrate several centimeters into tissue,
a depth which is sufficient for practical imaging for all small animal models
[11–15]. Among organic near-infrared dyes, aminocyanines used as fluorescence
labels and sensors of biomolecules in vivo have attracted much interest because
their spectra reach the near-infrared (NIR) region [16–18]. Gabor Patonay et al. and
Peng et al. developed cyanine dyes with large Stokes shifts ([140 nm), high
photostability, and strong fluorescence which made them more suitable than
conventional cyanine dyes as fluorescence probes [19–21].
Biochemical processes frequently involve protonation and deprotonation of
biomolecules with concomitant changes in the pH of the medium [22–25]. As a
result, there has been an increasing requirement for pH-sensitive fluorescent probes
[26–29] and fluorescent microspheres [24, 25]. Only a few NIR fluorescent
microspheres have been reported [25].
Herein, we describe a series of aminocyanine dyes prepared in the hope that
introduction of different heterocyclic rings would lead to diverse optical properties.
It was also envisaged that interaction of protons with the nitrogen atom on the
central rings would result in different changes of the absorption and emission
spectra. We also wished to devise a method for preparation of NIR fluorescent
microspheres based on the cyanine dyes. All the aminocyanine dyes were
characterized by NMR spectroscopy and mass spectrometry, and their optical
properties were investigated by UV–visible and fluorescence spectroscopy. Their
absorption and emission properties over eight pH units were also studied. The
compounds had large Stokes shifts and strong fluorescence in the NIR region with
high quantum yields. These properties are suitable for in-vivo imaging.
Experimental
Chemicals and instruments
All chemicals and reagents were commercially available and used without further
purification. All solvents were carefully dried and freshly distilled by use of
1
standard procedures. H NMR and 13C NMR spectra were recorded in CDCl3 on a
Varian Inova 500-MHz NMR spectrometer using TMS as an internal standard. Mass
spectra were obtained with Bruker Daltonics Autoflex tof/tofIII. UV–visible
absorption spectra were recorded on a Perkin Elmer Lambda 750 spectrophotom-
eter. Fluorescence spectra were obtained on a Varian Eclipse fluorescence
spectrophotometer at room temperature. The environmental scanning electron
images (ESEMs) were obtained by use of a XL30 environmental scanning electron
microscope. Polystyrene microspheres with diameters of 2.81 0.03 lm was
obtained from Tianjin Baseline ChroTech Research Centre (China).
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Near-infrared aminocyanine dyes
Determination of quantum yield
The fluorescence quantum yields of the synthesized dyes were calculated relative to
that of a standard solution of rhodamine B in ethanol (u = 0.56) and were
determined by use of the equation [30]:
2
ux ¼ usðFx= FsÞðAs=AxÞðkexs=kexxÞðnx=nsÞ
where u is quantum yield, F is integrated area under the corrected emission spec-
trum, A is absorbance at the excitation wavelength, kex is the excitation wavelength,
n is the refractive index of the sample used for measurement and the subscripts
x and s denote test and the standard, respectively.
Synthesis
The structures of all the synthesized compounds are shown in Fig. 1.
2-Chloro-1-formyl-3-hydroxymethylene cyclohexene, N-methyl-2,3,3-trimethyl-
indolenium, and IR786 (1) were prepared in accordance with a literature procedure
[31] (Scheme 1).
Compound 1
Yield 66 %.1H NMR (500 MHz, CDCl3): d 1.71 (s, 12H), 1.93–1.98 (m, 2H),
2.73 (t, 4H, J = 5.97 Hz), 3.74 (s, 6H), 6.20 (d, 2H, J = 14.10 Hz), 7.18 (d, 2H,
Fig. 1 Functionalization of chlorocyanine dye 1
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L. Yu et al.
l
C
O
l
C
butanol/benzene
reflux, 4h
+
H
H
O
N
N
I
N
I
1
corresponding amines
DMF
R
N
N
I
Scheme 1 Synthetic route to aminocyanine dyes
J = 7.33 Hz),7.24 (t, 2H, J = 7.12 Hz), 7.36–7.41 (m, 4H), 8.34 (d, 2H,
J = 14.10 Hz).
Synthesis of compounds 5 and 6 is shown in Scheme 2.
General procedure for synthesis of compounds 2, 3, 4, and 7
The corresponding heterocyclic compounds (2.0 mmol) were added to compound 1
(122 mg, 0.2 mmol) dissolved in anhydrous DMF (10 mL). The mixture was stirred
at 80 °C for 2 h under a nitrogen atmosphere then concentrated. The residue was
purified by silica gel chromatography with dichloromethane and methanol, then
recrystallized from methanol and diethyl ether to afford a golden powder.
Compound 2
1
Yield 83 %. H NMR (500 MHz, CDCL3): d 1.66 (s, 12H), 1.84–1.91 (m, 2H), 2.50
(t, 4H, J = 5.97 Hz), 3.15 (br, 6H), 3.49 (s, 6H), 3.85 (br, 4H), 5.72 (d, 2H,
J = 13.14 Hz), 6.98 (d, 2H, J = 7.33 Hz),7.11 (t, 2H, J = 7.12 Hz), 7.31–7.34 (m, 4H),
N
N
H
N
N
N
I
N
Br
5
N
N
I
Br
N
4
N
N
N
6
Scheme 2 Synthetic route to NIR dyes 5 and 6
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Near-infrared aminocyanine dyes
7.55 (d, 2H, J = 13.14 Hz). 13C NMR (500 MHz, CDCl3): d 21.89, 24.54, 25.13, 28.44,
29.16, 31.2, 47.90, 57.22, 95.48, 109.14, 122.26, 123.45, 124.14, 128.68, 140.01, 140.61,
143.74, 168.58, 175.93. HRMS: calculated for [M - I-]: 532.3686, measured: 532.3685.
Compound 3
Yield 33 %. 1H NMR (500 MHz, CDCl3): d 1.67 (s, 12H), 1.81–1.86 (m, 2H), 2.52
(t, 4H, J = 6.43 Hz), 3.57 (s, 6H), 3.72 (t, 4H, J = 4.28 Hz), 3.97 (t, 4H,
J = 4.28 Hz), 5.86 (d, 2H, J = 13.49 Hz), 7.04 (d, 2H, J = 7.93 Hz), 7.14 (t, 2H,
J = 7.44 Hz), 7.29–7.34 (m, 4H), 7.71 (d, 2H, J = 13.49 Hz). 13C NMR
(500 MHz, CDCl3): 21.90, 25.42, 29.09, 31.81, 48.33, 55.37, 68.57, 97.46,
109.88, 122.10, 124.14, 125.43, 128.81, 140.02, 140.73, 143.46, 170.11, 172.34.
HRMS: calculated for [M - I-]: 534.3479, measured: 534.3485.
Compound 4
1
Yield 65 %. H NMR (500 MHz, CDCl3): d 1.69 (s, 12H), 1.82–1.85 (m, 2H), 2.49
(t, 4H, J = 6.29 Hz), 3.33 (br, 4H), 3.54 (s, 6H), 4.04 (br, 4H), 5.73 (d, 2H, J = 13.26 Hz),
7.00 (d, 2H, J = 7.83 Hz),7.12 (t, 2H, J = 7.42 Hz), 7.31–7.34 (m, 4H), 7.66 (d, 2H,
J = 13.26 Hz). 13C NMR (500 MHz, CDCl3): d 21.87, 25.12, 25.98, 28.72, 31.45, 46.04,
48.21, 53.07, 62.49, 97.54, 110.48, 122.63, 123.93, 124.74, 128.78, 140.71, 140.83, 143.74,
169.97, 171.32 HRMS: calculated for [M - I-]: 533.3639, measured: 533.3635.
Compound 7
1
Yield 68 %. H NMR (500 MHz, CDCl3): d 1.65 (s, 12H), 1.84–1.89 (m, 2H), 2.55
(t, 4H, J = 6.47 Hz), 3.47 (br, 4H), 3.57 (s, 6H), 3.88 (t, 4H, J = 4.41 Hz), 5.87 (d, 2H,
J = 13.46 Hz), 6.99 (t, 1H, J = 7.32 Hz), 7.04 (t, 4H, J = 7.60 Hz), 7.13 (t, 2H,
J = 7.46 Hz), 7.26–7.28 (m, 2H), 7.31–7.38 (m, 4H), 7.74 (d, 2H, J = 13. 46 Hz). 13
C
NMR (500 MHz, CDCl3): d 21.90, 25.42, 29.13, 31.83, 48.37, 50.88, 51.91, 54.81, 54.93,
97.34, 109.87, 117.37, 122.14, 124.01, 125.53, 128.89, 129.87, 140.25, 141.84, 143.46,
170.12, 172.74, HRMS: calculated for [M - I-]: 609.3952, measured: 609.3949.
Synthesis of compound 5
Allyl bromide (238 mg, 2.0 mmol) and triethylamine (40 mg, 0.4 mmol) were
added to compound 4 (132 mg, 0.2 mmol) dissolved in DMF (10 mL). The mixture
was stirred at 65 °C for 1 h under a nitrogen atmosphere. The solvent was removed
under reduced pressure, then the crude product was purified by silica gel
chromatography with dichloromethane and methanol (20/1) (v/v), and 5 was
1
obtained as a golden powder (80 mg, 57 %). H NMR (500 MHz, CDCL3): d 1.66
(s, 12H), 1.81–1.86 (m, 2H), 2.50 (t, 4H, J = 6.05 Hz), 2.97 (br, 4H), 3.42–3.47
(m, 2H), 3.56 (s, 6H), 3.90 (br, 4H), 5.34–5.43 (m, 2H), 5.80 (d, 2H, J = 13.18 Hz),
6.02 (br, 1H), 7.03 (d, 2H, J = 7.81 Hz),7.13 (t, 2H, J = 7.23 Hz), 7.31–7.35
(m, 4H), 7.64 (d, 2H, J = 13.18 Hz). 13C NMR (500 MHz, CDCl3): d 21.81, 25.14,
28.94, 31.55, 31.76, 48.17, 53.47, 54.18, 54.62, 61.30, 96.46, 109.43, 122.01,
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L. Yu et al.
123.6,6 124.6,5 128.53, 140.01, 141.47, 143.32, 170.04, 173.2, HRMS: calculated
for [M - I-]: 573.3952, measured: 573.3955.
Synthesis of compound 6
Allyl bromide (238 mg, 2 mmol) and triethylamine (40 mg, 0.4 mmol) were added
to compound 4 (132 mg, 0.2 mmol) dissolved in DMF (10 mL). The mixture was
stirred at 65 °C for 3 h under a nitrogen atmosphere and concentrated. The residue
was then purified by silica gel chromatography with dichloromethane and methanol,
1
to afford a golden needle powder 6 (74 mg, 45 %). H NMR (500 MHz, CDCL3):
d 1.68 (s, 12H), 1.99–2.02 (m, 2H), 2.59 (t, 4H, J = 6.05 Hz), 3.47 (s, 6H),4.20 (br,
4H), 4.58 (d, 4H, J = 5.76 Hz), 4.72 (br, 4H), 5.52 (d, 2H, J = 13.09), 5.85–5.87
(m, 2H), 6.07–6.13 (m, 4H), 6.91 (d, 2H, J = 7.89 Hz), 7.08 (t, 2H, J = 7.44 Hz),
7.26–7.30 (m, 4H), 7.52 (d, 2H, J = 13.09 Hz). 13C NMR (500 MHz, CDCl3):
d 21.24, 27.12, 29.18, 31.65, 46.10, 48.12, 56.84, 58.31, 61.47, 952.4, 108.74,
122.15, 123.18, 123.26, 128.15, 131.20, 138.44, 139.96, 143.56, 168.60, 174.69.
HRMS: calculated for [M - I-]/2: 307.2169, measured: 307.2183.
Preparation of fluorescent microspheres
PS beads were dispersed in ethanol by sonication to give a completely monodisperse
microsphere system. After ultracentrifugation, the PS beads (8 mg/mL) were suspended
in phosphate-buffered saline (PBS) and a solution of the dye in ethanol was added. The
mixture was then subjected to ultrasonic irradiation for 8 h. The microspheres were then
rinsed with PBS then centrifuged (5,000 rev/min) to remove unabsorbed dye.
Results and discussion
Synthesis of fluorescent dyes
Because the purpose of this work was to investigate the effect of different nitrogen-
containing rings on the cyanine NIR dyes, first, we synthesized cyanine containing
only a simple heterocyclic ring at the middle of the chain. On the basis of the above
work, heterocyclic rings with different groups were also introduced into the dyes.
All the designed fluorescent dyes were successfully obtained in accordance with the
synthetic pathways described in Schemes 1 and 2. The structures of the synthesized
1
compounds were confirmed by H and 13C NMR and mass spectrometry.
Spectral properties of the synthesized cyanine dyes
The UV–visible and fluorescence spectra of the dyes are depicted in Figs. 2 and 3. It
is obvious there are three general features of the spectra of dyes 2, 3, 4, and 7. First,
they all have broad and fairly structureless fluorescence spectra. Second, large
Stokes shifts ([90 nm) are observed. Third, there is no mirror image relationship
between the absorption and fluorescence spectra. These features are in agreement
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Near-infrared aminocyanine dyes
with previous reports on aminocyanine dyes and result from intramolecular charge
transfer (ICT) [32–34]. Dyes 5 and 6 have two of these features only, the exception
being the Stokes shifts, which are only 34 and 44 nm, respectively.
The maximum absorption wavelengths (kmax) of these dyes are in the range
666–769 nm (Table 1). When the central chlorine is replaced by piperazine,
morpholine, and mono-substituted piperazine, obvious bathochromic shifts occur in
the wavelengths of maximum absorption (kmax) (from 22 to 103 nm) and the
wavelengths of maximum emission (kem) (from approx. 3 to 32 nm) compared with
dye 2. The molar extinction coefficient and fluorescence quantum yield of dyes 3, 4,
5, and 7 are similar to those of dye 2, whereas that of dye 6 is larger. Dyes 5 and 6,
which are synthesized from dye 4, have unique spectral properties. When the first
allyl group is introduced into dye 4, an interesting change is observed in the
absorption spectrum—the absorption peak at the longer wavelength becomes
stronger than those for dyes 3 and 7, and is the main absorption peak. After
introduction of the second allyl group, an obvious increase is found in the
absorbance intensity of dye 6, and the extinction coefficient become 2.03 9 105
(M-1 cm-1), compared with 0.75 9 105 (M-1 cm-1) for dye 2. Also, a difference
appears in the emission spectra; the wavelength of maximum emission increases to
813 nm. These results indicate that substitution of the central chlorine with different
nitrogen heterocyclic rings strongly affects the wavelength of maximum absorption
and the absorbance intensity, and contributes substantially to the emission spectra.
Details of the optical properties of the dyes are summarized in Table 1.
pH-dependent optical properties
To reveal the importance of medium pH to the optical properties of these dyes, their
spectra were recorded in 50:50 (v/v) water–ethanol from pH 2.0 to 12.0, adjusted by
addition of 1 M HCl or NaOH. The results are summarized in Figs. 4, 5, 6, and 7.
Fig. 2 Absorption spectra of the dyes (10-5 mol L-1 in methanol)
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L. Yu et al.
Fig. 3 Emission spectra of the dyes (10-5 mol L-1 in methanol)
It is obvious that these dyes can be divided into two types on the basis of trends in
pH-dependent changes of their spectral properties. The first group is dyes 2, 3, and
7; their absorption intensity becomes weaker with decreasing pH and their
wavelengths of maximum emission and fluorescence intensity hardly change on
protonation or deprotonation (Fig. 4). The difference between them is the
sensitivity. For the other group, dyes 4, 5, and 6, the trend is completely different.
The absorption intensity becomes stronger with decreasing pH. At pH 6.24, dye 4
has a broad absorption band at 710 nm and a smaller band at 765 nm (Fig. 5). At
higher pH, the wavelength of maximum absorption blue shifts to 687 nm. Above
pH 7.39, no further changes in the absorption band can be observed, except for a
slight decrease of absorption intensity with increasing pH. Under acid conditions,
the absorption red shifts to 769 nm and intensity increases substantially. The trend
with changing pH is the same for compound 5, but it is not as sensitive as dye 4.
Only below pH 4.72 does the absorption band at 710 nm appear (Fig. 6). For dye 5
intensity changes only are observed. With regard to fluorescence properties,
intensity for dyes 4, 5, and 6 become stronger with increasing pH. Dye 5 fluoresces
weakly at 805 nm (Fig. 6) below pH 4.72. At pH 6.02, the fluorescence becomes
Table 1 UV–visible fluorescent data of the dyes
Dye
k
max (nm) Extinction coefficient (M-1 cm-1
)
kex (nm) kem (nm) Stokes shifts (nm)
u
2
3
4
5
6
7
666
693
688
750
769
697
0.75 9 105
0.94 9 105
0.87 9 105
0.94 9 105
2.03 9 105
0.95 9 105
666
693
688
695
700
697
781
791
787
784
813
787
115
98
99
34
44
90
0.18
0.13
0.16
0.11
0.08
0.14
Conc. 10-5 mol L-1 in methanol
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Near-infrared aminocyanine dyes
Fig. 4 Absorption (left) and emission (right) spectra of 3 at different pH
Fig. 5 Absorption (left) and emission (right) spectra of 4 at different pH
Fig. 6 Absorption (left) and emission (right) spectra of 5 at different pH
much stronger and blue-shifts to 790 nm. Further increasing pH does not increase
the emission intensity. The trend is the same for dyes 4 and 6.
It has been reported that amidation of aminocyanine dyes can induce a substantial red-
shift in absorption [28], which suggests that when protonation occurs at the two nitrogen
atoms of such aminocyanines, a red shift of the absorption peak will occur owing to a
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L. Yu et al.
Fig. 7 Absorption (left) and emission (right) spectra of 6 at different pH
Fig. 8 Dependence of fluorescence intensity of microspheres on dye-to-microsphere ratio (dye 7)
decrease of the electron-donating ability of the amine. This behavior is observed for dyes
4 and 5. Dye 6 is a modification of 5. After introduction of the second allyl group,
protonation is unable to occur at the two nitrogen atoms. The presence of phenyl in
compound 7 may be the reason it differs from this trend. Phenyl makes the nitrogen atom
of the piperazine less basic by conjugation, and it is more rigid than the allyl group.
Preparation and spectral properties of fluorescent microspheres
After investigation of their optical properties the dyes were used for preparation of
fluorescent microspheres. Dye 7 was chosen as the first to be loaded on to the PS
bead, because there is a phenyl group on the piperazidine ring which gave it greater
affinity for the PS bead. The absorption method was chosen instead of copolymer-
ization, because of the instability of cyanine dyes during the copolymerization
process. This method has advantages over others because bead size and shape and
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Near-infrared aminocyanine dyes
Fig. 9 ESEM image of fluorescent microsphere based on dye 7
Fig. 10 Emission spectra of different microspheres
Table 2 Optical characteristics
Dye kem1 (nm) (dye) kem2 (nm) (PS bead) kem2 - kem1 (nm)
of the dyes and fluorescent
microspheres
2
3
4
5
7
781
791
787
784
787
796
800
795
800
797
15
9
8
16
10
fluorescence quenching can be controlled [35]. To achieve the highest fluorescence
intensity, fluorescent microspheres with different ratios of dye 7 and PS bead were
prepared. As can be seen from Fig. 8, the fluorescence intensity reached a
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L. Yu et al.
maximum when the ratio was 0.022 %. When the ratio decreased from 0.66 to
0.022 %, the fluorescence intensity at 800 nm increased from 42 to 870, and the
intensity decreased with the continuous decrease of the ratio. The ESEM image
(Fig. 9) showed that the microspheres were highly monodispersed. We therefore
chose the ratio 0.022 %. The other fluorescent microspheres were prepared by use
of the same method. The emission spectra are depicted in Fig. 10.
All the microspheres were obtained successfully and their spectral properties
were studied. Investigations were performed in PBS, an aqueous medium often used
in biological studies. As can be seen from Fig. 10, the microspheres had
fluorescence profiles very similar to those of the parent dyes in methanol (Fig. 3),
except for slight red shifts, which are listed in Table 2. The emission wavelengths
were all approximately 800 nm, which were in the NIR range. Fortunately, the
fluorescent microspheres retained the property of pH-sensitivity in aqueous media
[36]. The emission intensity increased sharply as medium pH increased. Fluores-
cence intensity varied with the changes of pH, making it suitable for applications in
biological studies.
Conclusion
On the basis of this investigation of the absorption and fluorescence properties of
these cyanine dyes, and their optical response to different pH, we can conclude that
substitution of the chloro group with simple heterocyclic structures can lead to
diverse optical properties. When piperidine is substituted by piperazine and
morpholine, a red-shift is observed in the wavelength of maximum absorption
(kmax), with increased absorption and a decrease in fluorescence quantum yield.
Modification of the heterocycles at the middle position also has a large effect on the
optical properties of cyanine dyes. Furthermore, substituted piperazine incorporated
into the fluorophore resulted in a wealth of pH-dependent optical properties, the
absorption and fluorescence characteristic of the new dyes depending substantially
on their molecular structures.
A method for preparation of NIR cyanine fluorescent microspheres is reported.
Some of the synthesized dyes were used for preparation of fluorescent microspheres.
The microspheres obtained had good optical properties. Most important, the
aminocyanine-loaded microspheres were highly sensitive to changes in the pH of
aqueous solutions, making them suitable for applications in biological studies.
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