Functionalization of magnetic nanoparticles with peptide dendrimers

Article abstract of DOI:10.1039/c0jm02752a

Surface functionalization of magnetic nanoparticles (MNPs) has been an exciting area of interest for researchers in biomedicine. In this paper, we introduce a new family of peptide dendritic ligands for functionalizing MNPs of superior quality. l-Lysine- and l-glutamic acid-based dendritic ligands with dopamine located at the focal points were fully designed and synthesized before the functionalization. Then ligands of different dendritic generations (G1 to G3) were immobilized on the surface of oleic-acid-coated hydrophobic MNPs via ligand-exchange method to realize phase transfer. The two series of modified MNPs were systematically studied via FTIR, TGA, XRD, TEM, DLS, VSM and zeta potential measurements. The modified MNPs exhibited an adjustable number of terminal functional groups and superior stability in aqueous solutions in a broad pH range. The surface existence of water-soluble polypeptide ligands promoted monodispersity of the particles and led to an increased hydrodynamic diameter under 30 nm from G1 to G3. After the ligand exchange process, the superparamagnetic behavior was successfully retained. The two series of modified MNPs exhibited approximate magnetization in the same generation, while the saturation magnetization of the MNPs decreased with increasing surface dendritic generation. MNPs functionalized with G1l-glutamic acid dendritic ligands had the highest saturation magnetization (55 emu g^-1), which was larger than for the initial MNPs. This novel functionalization strategy provides a potential platform for designing and preparing highly stable ultrafine MNPs with high magnetization for biomedicinal applications.

Full text of DOI:10.1039/c0jm02752a

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PAPER
Functionalization of magnetic nanoparticles with peptide dendrimers
*
Rong Zhu,† Wen Jiang,† Yuji Pu, Kui Luo, Yao Wu, Bin He and Zhongwei Gu
*
Received 20th August 2010, Accepted 2nd February 2011
DOI: 10.1039/c0jm02752a
Surface functionalization of magnetic nanoparticles (MNPs) has been an exciting area of interest for
researchers in biomedicine. In this paper, we introduce a new family of peptide dendritic ligands for
functionalizing MNPs of superior quality. L-Lysine- and L-glutamic acid-based dendritic ligands with
dopamine located at the focal points were fully designed and synthesized before the functionalization.
Then ligands of different dendritic generations (G1 to G3) were immobilized on the surface of oleic-
acid-coated hydrophobic MNPs via ligand-exchange method to realize phase transfer. The two series of
modified MNPs were systematically studied via FTIR, TGA, XRD, TEM, DLS, VSM and zeta
potential measurements. The modified MNPs exhibited an adjustable number of terminal functional
groups and superior stability in aqueous solutions in a broad pH range. The surface existence of water-
soluble polypeptide ligands promoted monodispersity of the particles and led to an increased
hydrodynamic diameter under 30 nm from G1 to G3. After the ligand exchange process, the
superparamagnetic behavior was successfully retained. The two series of modified MNPs exhibited
approximate magnetization in the same generation, while the saturation magnetization of the MNPs
decreased with increasing surface dendritic generation. MNPs functionalized with G1 L-glutamic
acid dendritic ligands had the highest saturation magnetization (55 emu gꢀ1), which was larger than
for the initial MNPs. This novel functionalization strategy provides a potential platform for designing
and preparing highly stable ultrafine MNPs with high magnetization for biomedicinal applications.
prepare hydrophilic MNPs. The surface encapsulation often
Introduction
utilizes amphiphilic polymer to form micelle-like double-layer
structures outside the hydrophobic nanoparticles.18 It has
the advantage for large-scale preparation of hydrophilic MNPs.
However, there are several bottlenecks for this approach.
First, the obtained MNPs cannot be stable under physiological
conditions because of the weak interaction between hydrophobic
double-layer structures.19 Second, the size of the amphiphilic
copolymer-encapsulated MNPs often exceeds 30 nm,19–21 and
this would result in shorter blood circulation time in vivo.22
Furthermore, it is very difficult to manipulate the number of
terminal functionalities for controllable bioconjugation due to
the uncertainty in the number functional groups in polymer.
Ligand-exchange is a novel method whereby the original
ligands on the surface can be displaced without changing the
intrinsic properties of the iron core. Compared with surface-
encapsulation, the MNPs resulting from this approach are
more stable because they form stronger coordinate bonds during
the surfactant exchange reaction,18 which overcomes the
inherent shortcomings of the surface-encapsulated MNPs.
Polyelectrolyte-23 and PEG-based hydrophilic polymers have
been employed to prepare hydrophilic MNPs by the ligand-
exchange approach.24–26 These linear polymers can preserve
agglomeration and minimize protein adsorption, but it is difficult
to control the final size of the phase-transformed MNPs to be
within the desired range.27,28
Magnetic nanoparticles (MNPs) have attracted more and more
attention for their wide range of biomedical applications, such as
targeted drug delivery,1–3 contrast agents in magnetic resonance
imaging (MRI),3–5 hyperthermia treatment,6 gene carriers7 and
protein separation.8,9 Properties of MNPs that are required for
these biomedical applications are high magnetization value,10
monodispersity and narrow size distribution, good stability and
biocompatibility.11,12 The thermal decomposition method is
commonly regarded as the best way to produce high quality
ultrafine monodisperse MNPs with controllable size less than
20 nm.13–15 However, owing to the long-chain alkane ligands on
the surface, the obtained MNPs are hydrophobic and only
disperse in nonpolar or weakly polar solvents, such as n-hexane
or chloroform. The hydrophobicity significantly restricts their
biological applications in vivo.
Surface hydrophilic functionalization has been used to
modify MNPs, leading to good dispersibility and stability in
aqueous solution.16 Two strategies, known as surface encapsu-
lation17 and ligand-exchange, are commonly carried out to
National Engineering Research Center for Biomaterials, Sichuan
University, Chengdu, 610064, People’s Republic of China. E-mail:
zwgu@scu.edu.cn; Fax: +86 28-85410653; Tel: +86 28-85414308;
wuyao@scu.edu.cn; +86 28-85410653; +86 28-85410336
† These authors contributed equally to this work.
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Recently, dendritic macromolecules have been investigated as
ligands for surface modification of MNPs.29,30 The dendritic
ligands are multivalent macromolecules which possess highly
controlled structure, such as a single molecular weight, a large
number of controllable ‘‘peripheral’’ functionalities and nano-
scopic dimensions. PAMAM dendrimers and their derivatives
are classic examples in the modification of core–shell-type
nanoparticles.7,25,31 However, recent studies have shown that
PAMAM dendrimers are cytotoxic, particularly in the higher
generations of protonated (cationic) dendrimers with large
numbers of amine groups,32–34 which limits their biomedical
applications. Polypeptide dendrimers are considered as new bio-
materials due to their protein-like structures, multivalence, excel-
lent biocompatibility, degradability and low immunogenicity.35,36
In the previous research of our group, peptide dendrimers were
synthesized successfully as MRI probes, gene delivery and drug
delivery carriers with excellent biocompatibility and good
results.37–40
Fourier transform infrared (FTIR) spectra were performed on
PE spectrometer. The spectra were recorded in the wave number
interval between 4000 and 400 cmꢀ1 with step length 4 cmꢀ1. The
samples were compressed into KBr pellets for the measurements.
Thermal gravimetric analysis (TGA) was performed on STA
449 C Jupiter (NETZSCH). The mass loss of the dried sample
was monitored under N2 at temperatures from 35 ꢁC to 1000 ꢁC
with a heating rate of 10 K minꢀ1. The crystal structure of the
iron oxide nanoparticles was tested by powder X-ray diffraction
(XRD) pattern with angles ranging from 20ꢁ to 90ꢁ (Philips). The
transmission electron microscopy (TEM) was carried out on
a JEOL JEM-2010F instrument (Japan Electronic). Samples
were prepared by deposition of one drop of an appropriately
diluted solution onto a copper grid and dried at ambient
temperature before they were loaded into the microscope.
Dynamic light scattering (DLS) and zeta potential were
measured by a Malvern Nano-ZS instrument. The samples were
dispersed in n-hexane or Milli-Q water in glass cuvettes and
tested at 25 ꢁC for the size characterization. The magnetization of
the dried sample was measured by a vibrating sample magne-
tometer (VSM, Model BHV-525, Riken Japanese Electronics
Company) with field up from 0 to 15000 Oe at 300 K. The zeta
potentials were recorded in a solution of functionalized MNPs in
water with pH ranging from 2 to 12. The pH values were adjusted
by 10 mM HCl and 10 mM NaOH solution.
In this work, we demonstrate the feasibility of functionalizing
hydrophobic ultrafine MNPs with a new family of peptide den-
drimers through the ligand-exchange method for the first time.
Different from the PAMAM-grafted method, L-lysine- and
L-glutamic acid-based dendritic ligands of different generations
with dopamine located at the focal point have been synthesized
before grafting onto the MNP surface. It is beneficial to precisely
control the terminal functional groups and reduce the sizes of
phase-transferred MNPs to be less than 30 nm. We systemati-
cally compare the performance of the functionalized MNPs with
the initial oleic-acid-coated particles in terms of the surface
functional groups coverage, crystalline structure, size distribu-
tion, magnetic properties and surface charge at different pH
values.
Synthesis of magnetite nanoparticles (MNPs-OA)
Fe3O4 nanoparticles were prepared according to the method
reported by Sun et al.13 Briefly, Fe(acac)3 (2.0 mmol), 1,2-hex-
adecanediol (10.0 mmol), oleylamine (6.0 mmol) and oleic acid
(6.0 mmol) were dissolved in a solution of 20 mL benzyl ether.
The solution was heated at 200 ꢁC for 2 h, refluxed at 300 ꢁC for
1 h, and then cooled to room temperature. The black particles
were precipitated with ethanol and collected by centrifugation at
8000 rpm for 10 min, reprecipitated and washed with ethanol
3 times. The product was then re-dispersed in n-hexane.
Experimental methods
Materials
Iron(III) acetylacetonate, phenyl ether, benzyl ether, oleic acid
(OA) (90%), oleylamine (>70%), 1,2-hexadecanediol (90%), tri-
fluoroacetic acid (TFA) and N,N-diisopropylethylamine (DIPEA)
were purchased from Sigma-Aldrich and used as received. Na,
Ne-di-Boc-L-lysine(Boc-Lys(Boc)-OH), O-benzotriazole-N,N,N0,
Synthesis of polypeptide dendritic ligands
L-Lysine-based dendritic macromolecules with Boc protection
groups were synthesized by divergent and convergent appro-
aches, as shown in Scheme 1. L-Glutamic acid-based dendritic
macromolecules with benzyl protection groups were synthesized
by a convergent approach (Scheme 2). Compounds 1,41 7,41 5,42
943 were prepared according to the literature. The synthesis of
other compounds is described below.
N0-tetramethyluromium
hexafluorophosphate
(HBTU),
1-hydroxybenzotriazole (HOBt) were purchased from GL Bio-
chem (Shanghai) Ltd. and used as received.
Characterization methods
Characterization and structural confirmation of the dendritic
intermediates and products were performed by NMR, electro-
spray ionization time-of-flight mass spectrometry (ESI-MS) and
matrix assisted laser desorption ionization time-of-flight mass
spectrometry (MALDI-TOF MS). NMR data were obtained
using a 400 MHz Bruker Advance 400 spectrometer, and
chemical shifts were reported in ppm on the d scale relative to
TMS or solvent. Electrospray mass spectra using an electrospray
ionization mode were obtained using a Waters Q-TOF Premier
time-of-flight mass spectrometer operated in positive or negative
ion mode. MALDI-TOF analysis of modified dendritic macro-
molecules was performed on Autoflex MALDI-TOF/TOF.
Compound 2. Compound 1 (0.5 g, 1.2 mmol) was treated with
5% TFA in dichloromethane (CH2Cl2, 30 mL) at room temper-
ature for 5 h. The solvent was removed under vacuum to give an
oil, and 40 mL of ethyl ether was added. The mixture was stirred
overnight. The white solid was collected by centrifugation. The
product was used without further purification. The solid was
dissolved in 10 mL DMF. DIPEA (1.7 mL, 10.4 mmol) was
added to the mixture, followed with Na,Nb-di-Boc-L-lysine
(479.0 mg, 1.2 mmol). The solution was stirred under nitrogen
atmosphere for 10 min and cooled to 0 ꢁC. HBTU (0.455 g,
1.2 mmol) and HOBt (0.163 g, 1.2 mmol) were added to the
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Scheme 1 The synthesis of different generations of L-lysine-based dendritic ligands.
solution. The solution was stirred at room temperature for 24 h.
Ethyl acetate (EtOAc, 150 mL) was added, the organic solution
was washed with saturated NaHCO3, NaHSO4 (0.1 M), satu-
rated NaHCO3 and brine. The organic phase was dried by
anhydrous Na2SO4, filtered, and the solvent was removed under
reduced pressure. The crude product was purified by column
chromatography (silica gel, CHCl3–CH3OH ¼ 10 : 1) to give
compound 2 as a white solid. Yield ¼ 83.0% (0.56 g, 1.0 mmol).
1H NMR (400 MHz, CDCl3): d 7.48–7.27 (m, 10H, C6H5), 6.88–
6.80 (m, 2H, dopamine aromatics), 6.70 (d, J 8.0, 1H, dopamine
aromatics), 6.07 (s, 1H, NH), 5.15–5.13 (m, 4H, benzyl CH2),
5.04 (s, 1H, NH), 4.54 (m, 1H, COCH(CH2)NH), 3.95 (m, 1H,
NH), 3.44 (m, 2H, NHCH2CH2–Ar), 3.07 (m, 2H, COCH(CH2)
NH), 2.72 (t, 2H, NHCH2CH2-Ar), 1.88–1.18 (m, 18H, (CH3)3C-
and 6H, CH2); 13C NMR (100 MHz, CDCl3): d (100 MHz,
CDCl3) 172.03 (CONH), 156.16, 155.74 (COOC(CH3)3), 149.08–
137.40 (dopamine aromatics), 132.14–115.40 (C6H5), 79.96–
79.10 (OC(CH3)3), 71.45–71.32 (benzyl CH2), 54.50 (COCH(R)
NH), 53.45 (COCH(R)NH), 40.98–38.38 (CH2NH), 35.31 (CH2–
C6H5), 31.15, 29.70, 29.59 (OC(CH3)3), 28.01, 27.59, 27.55,
22.95, 22.80, 22.41 (all CH2); ESI-TOF MS: m/z ¼ 662.38 (M +
H+) (calculated 661.83 for C38H51N3O7).
Compound 4 (Protected-dopamine-G2(Lys)-4NHBoc). The Boc
group on dendritic compound 2 (0.56 g, 0.85 mmol) was removed
by the same procedure as described in the synthesis of dendritic
compound 2. The deprotected compound was dissolved in 10 mL
DMF. DIPEA (1.3 mL, 7.61 mmol) and Na,Nb-di-Boc-L-lysine
(0.733 g, 2.12 mmol) were added. The solution was stirred under
nitrogen atmosphere for 10 min and cooled to 0 ꢁC. HBTU
(802 mg, 2.1 mmol) and HOBt (288 mg, 2.1 mmol) were added to
the reaction system as a solid mixture. The solution was stirred
for 48 h at room temperature. 150 mL EtOAc was added, the
organic solution was washed with saturated NaHCO3, NaHSO4
(0.1 M), saturated NaHCO3 and brine. The organic phase was
dried by anhydrous Na2SO4, filtered, and the solvent was
removed under reduced pressure. The product was purified by
column chromatography (silica gel, CHCl3–CH3OH ¼ 15 : 1) to
yield compound 4 as a white solid. Yield ¼ 83.0% (1.363 g,
0.60 mmol). 1H NMR (400 MHz, CDCl3): d 7.52–7.28 (10H, m,
C6H5), 7.15 (s, 1H, NH), 6.88–6.80 (m, 2H, dopamine
aromatics), 6.70 (d, J 8.0, 1H, dopamine aromatics), 6.50 (s, 1H,
NH), 5.90 (s, 1H, NH), 5.46 (s, 1H, NH), 5.15–5.13 (m, 4H,
benzyl CH2), 4.85 (s, 1H, NH), 4.76 (s, 1H, NH), 4.32–3.99
(m, 3H, COCH(CH2)NH), 3.48 (m, 2H, NHCH2CH2–Ar),
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Scheme 2 The synthesis of different generations of L-glutamic acid dendritic ligands.
3.19–2.86 (m, 6H, COCH(CH2)NH), 2.70 (m, 2H, NHCH2CH2-
Ar), 1.98–1.07 (m, 36H, (CH3)3C- and 18H, CH2); 13C NMR
(100 MHz, CDCl3): d 173.40, 171.78, 168.27 (CONH), 156.38
(COOC(CH3)3), 149.23, 147.77, 137.57, 137.46 (dopamine
aromatics), 132.60, 128.79, 128.55, 128.40, 127.89, 127.86,
127.54, 127.46, 121.80, 116.02, 115.72 (C6H5), 80.19, 79.98, 79.09
(OC(CH3)3), 71.68, 71.49 (benzyl CH2), 54.50, 54.39, 53.45
(COCH(R)NH), 40.98, 40.32, 40.07, 38.38 (CH2NH), 35.31
(CH2–C6H5), 32.56, 32.16, 31.68, 31.15, 29.70, 29.59, 29.01, 28.59
(OC(CH3)3), 28.55, 22.95, 22.80, 22.41 (all CH2); ESI-TOF MS:
m/z ¼ 1118.68 (M + H+) (calculated 1117.67 for C60H91N7O13).
was removed under reduced pressure. The product was purified
by column chromatography (silica gel, CHCl3–EtOAc–
CH3CH2OH ¼ 6 : 2 : 1) to yield compound 6 as a white solid.
Yield ¼ 40% (0.98 g, 0.48 mmol). 1H NMR (400 MHz, CDCl3):
d 7.78 (s, 1H, NH), 7.74 (m, 2H, NH), 7.51–7.27 (m, 10H, C6H5),
6.88–6.80 (m, 2H, dopamine aromatics), 6.70 (d, J 8.0, 1H,
dopamine aromatics), 6.06 (s, 1H, NH), 5.75 (s, 1H, NH), 5.60
(m, 2H, NH), 5.15–5.13 (m, 4H, benzyl CH2), 4.62–4.01(m, 7H,
COCH(CH2)NH), 3.43 (m, 2H, NHCH2CH2–Ar), 3.34–2.82 (m,
14H, COCH(CH2)NH), 2.69 (m, 2H, NHCH2CH2-Ar), 1.96–
1.00 (m, 72H, (CH3)3C- and 42H, CH2); 13C NMR (100 MHz,
CDCl3): d 177.43, 173.76, 173.36, 173.05, 173.03, 172.94, 172.24,
171.76, 168.03 (CONH), 158.63, 156.58 (COOC(CH3)3), 149.29,
147.86, 147.24, 137.57 (dopamine aromatics), 132.65, 131.22,
130.17, 129.13, 128.97, 128.75, 128.59, 128.11, 127.73, 127.64,
121.96, 116.07, 115.72 (C6H5), 80.51–79.28 (OC(CH3)3), 71.79,
71.65 (benzyl CH2), 68.44, 65.86 (OC(CH3)3), 55.37, 54.82, 54.52,
54.28, 53.50, 53.35 (COCH(R)NH), 40.46, 40.25, 39.01
(CH2NH), 35.36 (CH2–C6H5), 32.22, 31.72, 30.85, 30.65, 30.45,
29.99, 29.65, 29.21, 28.76 (OC(CH3)3), 28.01, 27.50, 27.02, 24.03,
23.50, 23.28, 22.98 (all CH2); MALDI-TOF MS: m/z ¼ 2054.8
(M + Na+) (calculated 2031.6 for C104H171N15O25).
Compound
6 (Protected-dopamine-G3(Lys)-8NHBoc). The
crude product 3 which was obtained from compound 2 (0.8 g,
1.21 mmol) as described before, was dissolved in 50 mL DMF.
DIPEA (3.7 mL, 22.17 mmol) and compound 5 (2.17 g,
2.46 mmol) were added to the mixture. The solution was stirred
under nitrogen atmosphere for 10 min and cooled to 0 ꢁC. HBTU
(933 mg, 2.46 mmol) and HOBt (335 mg, 2.46 mmol) were added
to the solution as a solid mixture. Subsequently, the reaction
mixture was stirred at room temperature for 48 h. The solvent
was removed by rotary evaporation and the residue was dis-
solved in 100 mL of EtOAc and washed with saturated NaHCO3,
NaHSO4 (0.1 M), saturated NaHCO3 and brine. The organic
phase was dried by anhydrous Na2SO4, filtered, and the solvent
Compound 8. Compound 7 (200 mg, 0.46 mmol) was dissolved
in 20 mL DMF, H-Glu-(OBzl)2$TosOH (230 mg, 0.46 mmol)
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was added and the solution was cooled to 0 ꢁC. DIPEA (0.5 mL,
2.76 mmol), HBTU (175 mg, 0.46 mmol) and HOBt (63 mg,
0.46 mmol) were added to the reaction system. The solution was
stirred at room temperature for 24 h. The solvent was removed
using a rotary evaporator and the residue was dissolved in 45 mL
of EtOAc, and washed with saturated NaHCO3, NaHSO4
(0.1 M), saturated NaHCO3 and brine. The organic phase was
dried by anhydrous Na2SO4 and filtered. The solvent was
removed under reduced pressure and further purified by column
chromatography (silica gel, CHCl3–CH3OH ¼ 10 : 1) to give
compound 8 as a white solid. Yield ¼ 73% (250 mg, 0.34 mmol).
1H NMR (400 MHz, CDCl3): d 7.51–7.24 (m, 20H, C6H5), 6.88–
6.77 (m, 2H, dopamine aromatics), 6.62 (d, J 8.0, 1H, dopamine
aromatics), 5.72 (d, 1H, NH), 5.73 (s, 1H, NH), 5.26–5.00
(m, 8H, benzyl CH2), 4.62 (m, 1H, CH), 3.38 (m, 2H,
NHCH2CH2–Ar), 2.65 (t, 2H, NHCH2CH2-Ar), 2.56–1.60
(m, 4H, -COCH2CH2CO- and 2H, -CH2CH2CO- and 2H,
-CHCH2CH2-); 13C NMR (100 MHz, CDCl3): d 172.50, 172.24,
172.07, 171.85, 171.55 (CONH), 148.96, 147.60, 137.37, 137.27,
135.68, 135.18 (dopamine aromatics), 132.27, 128.59, 128.54,
128.44, 128.27, 128.23, 127.76, 127.74, 127.33, 127.29, 126.93,
121.56, 115.68, 115.34 (C6H5), 71.39, 71.23, 67.22, 66.47 (benzyl
CH2), 51.73 (CH), 40.74, 35.05, 31.63, 31.45, 31.38, 30.16, 29.67,
27.09 (-COCH2CH2CO-); ESI-TOF MS: m/z ¼ 742.38 (M + H+)
(calculated 742.86 for C45H46N2O8).
mmol) were added to the mixture. The solution was stirred under
a nitrogen atmosphere for 10 min and cooled to 0 ꢁC. HBTU
(468 mg, 1.24 mmol) and HOBt (169 mg, 1.24 mmol) were added
to the reaction system. The solution was stirred at room
temperature for 24 h. The solvent was removed by rotary evap-
oration and the residue was dissolved in 60 mL of EtOAc and
washed with saturated NaHCO3, 10% NaHSO4, saturated
NaHCO3 and brine. The organic phase was dried by anhydrous
Na2SO4, and filtered. The solvent was removed under reduced
pressure. The crude product was purified by column chroma-
tography (silica gel, CHCl3–EtOAc–CH3CH2OH ¼ 6 : 2 : 1) to
yield compound 12 as a white solid. Yield ¼ 32.2% (819 mg,
1
0.4 mmol). H NMR (400 MHz, CDCl3): d 8.53 (s, 1H, NH),
8.23–8.08 (m, 2H, NH), 7.94 (m, 2H, NH), 7.73 (d, J 8.8, 1H,
NH), 7.62 (d, 2H, NH), 7.54–7.16 (m, 50H, C6H5), 6.85–6.79
(m, 2H, dopamine aromatics), 6.68 (d, J 8.0, 1H, dopamine
aromatics), 5.23–4.89 (m, 20H, benzyl CH2), 4.78–4.13 (m, 7H,
CH), 3.34 (m, 2H, NHCH2CH2–Ar), 2.64 (t, J 7.2, 2H,
NHCH2CH2-Ar), 2.58–1.50 (m, 4H, -COCH2CH2CO- and 14H,
-CH2CH2CO- and 14H, -CHCH2CH2-); 13C NMR (100 MHz,
CDCl3): d 173.09, 172.89, 172.77, 172.53, 172.42, 172.12, 171.95,
171.76, 171.66, 171.36 (CONH), 148.77, 147.18, 137.96, 137.85,
136.78, 136.57 (dopamine aromatics), 136.36, 136.36, 136.28,
135.73, 128.86, 128.78, 128.48, 128.48, 128.43, 128.32, 128.25,
128.21, 128.21, 128.00, 127.89, 121.60, 116.01, 115.60, 115.25
(C6H5), 70.78, 70.66, 66.52, 66.41, 65.99 (benzyl CH2), 51.93,
51.84, 51.76, 51.52, 51.31 (CH), 40.71, 36.67, 35.20, 35.16, 33.40,
32.08, 32.06, 30.31, 30.14, 26.49, 26.40 (-COCH2CH2CO-);
MALDI-TOF MS: m/z ¼ 2081.9 (M + Na+) (calculated 2058.3
for C117H124N8O26).
Compound
10
(Protected-dopamine-G2(Glu)-4COOBzl).
Compound 7 (280 mg, 0.65 mmol) was dissolved in 25 mL DMF,
then DIPEA (0.6 mL, 3.87 mmol) and compound 9 (742 mg 0.97
mmol) were added to the mixture. The solution was stirred under
nitrogen atmosphere for 10 min and cooled to 0 ꢁC. HBTU
(367 mg, 0.97 mmol) and HOBt (132 mg, 0.97 mmol) were added.
The solution was stirred at room temperature for 24 h. The
solvent was removed by rotary evaporation and the residue was
dissolved in 50 mL of EtOAc and washed with saturated
NaHCO3, NaHSO4 (0.1 M), saturated NaHCO3 and brine. The
organic phase was dried by anhydrous Na2SO4 and filtered, and
the solvent was removed under reduced pressure. The crude
product was purified by column chromatography (silica gel,
CHCl3–CH3OH ¼ 15 : 1) to yield compound 10 as a white solid.
Yield ¼ 55% (419 mg, 0.36 mmol). d 7.85 (d, 1H, NH), 7.60–7.20
(m, 30H, C6H5), 6.88–6.76 (m, 2H, dopamine aromatics), 6.68 (d,
J 8.0, 1H, dopamine aromatics), 5.27–4.94 (m, 12H, benzyl CH2),
4.80–4.30 (m, 3H, CH), 3.37 (m, 2H, NHCH2CH2–Ar), 2.64 (t,
J 7.2, 2H, NHCH2CH2-Ar), 2.54–1.62 (m, 4H, -COCH2CH2CO-
and 6H, -CH2CH2CO- and 6H,-CHCH2CH2-); 13C NMR (100
MHz, CDCl3): d 173.22, 172.96, 172.80, 172.34, 172.33, 171.94,
171.90, 171.68 (CONH), 148.99, 147.61, 137.41, 137.30, 135.76,
135.67 (dopamine aromatics), 134.99, 134.94, 132.32, 128.62,
128.61, 128.57, 128.53, 128.49, 128.45, 128.31, 128.25, 128.23,
128.18, 127.77, 127.74, 127.36, 127.32, 121.61, 115.72, 115.39
(C6H5), 71.43, 71.26, 67.71, 67.63, 66.52, 66.42 (benzyl CH2),
52.38, 51.72, 51.57 (CH), 40.71, 35.08, 32.04, 31.44, 31.34, 30.51,
30.29, 28.48, 26.68, 26.61 (-COCH2CH2CO-); ESI-TOF MS: m/z
¼ 1181.53 (M + H+) (calculated 1180.50 for C69H72N4O14).
Ligands of Gn(Lys) and Gn(Glu) (n ¼ 1, 2, 3)
100 mg (0.15 mmol) of compound 2 were added in the mixture of
CH2Cl2 (1 mL) and TFA (1 mL). The solvent was removed. The
yellow oil residue was dissolved in MeOH, and 10% Pd/C (50 mg)
was added. The resulting mixture was stirred under H2 (50 pis)
for 24 h. The Pd/C was filtered, and the filtrate was removed in
vacuum to yield G1(Lys) as a white solid. Yield ¼ 98%. 1H NMR
analysis showed that both the Bn and t-Boc protecting groups
were removed from the dendritic molecules. This compound was
used without further purification.
Dendritic ligands G2(Lys), G3(Lys), G1(Glu), G2(Glu) and G3
(Glu) were synthesized with the same procedure as described in
synthesis of G1(Lys).
Preparation of MNPs functionalized with peptide dendrimers
The solid G1(Lys) (100 mg) was dissolved in a mixture of MeOH
(40 mL) and CHCl3 (20 mL), and the MNPs-OA in n-hexane
(5 mL, 10 mg mLꢀ1) were introduced and shaken overnight to
facilitate ligand-exchange. The solvent was removed under
reduced pressure. The dendritic ligands modified MNPs were
precipitated in n-hexane and collected by centrifugation at
3500 rpm. After washing with n-hexane (30 mL) 3 times, the
product was re-dispersed in distilled water. The extra surfactants
and salts were removed by dialysis using a dialysis bag (MWCO
¼ 8000–14000) for 24 h in water to give MNPs-G1(Lys). The
Compound
12
(Protected-dopamine-G3(Glu)-8COOBzl).
Compound 7 (536 mg, 1.24 mmol) was dissolved in 25 mL DMF,
then DIPEA (1.2 mL, 7.41 mmol) and compound 11 (2.03 g, 1.24
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MNPs-G2(Lys), MNPs-G3(Lys), MNPs-G1(Glu), MNPs-G2
(Glu) and MNPs-G3(Glu) were synthesized similarly.
Results and discussion
Synthesis of dendritic ligands and surface-modified MNPs
In order to form a stable surface coating, various surface binding
groups have been explored in the past, such as carboxylate
(COOꢀ),44,45 phosphate (PO33ꢀ),29 and alcohol (OHꢀ).46 Recent
work has showed that bidentate ligands, such as dopamine,24,41,47
exhibited high affinity to the MNP surfaces and could replace the
capping ligands, oleic acid and oleyl amine. Therefore, we
utilized dopamine as a robust anchor to conjugate peptide den-
drimers with MNPs, as shown in Scheme 1–3. Polypeptide den-
drimers were synthesized through divergent and convergent
approach, and fully characterized by 1H NMR, 13C NMR, ESI-
TOF MS and MALDI-TOF MS. In the MALDI-TOF mass
spectrum of ligand 12 (M ¼ 2058.27), the most abundant peak is
observed at m/z ¼ 2081.9 and 2097.7, which correspond to the
[M + Na]+ and [M + K]+ signals, respectively, indicating that the
designed third-generation dendritic ligand with dopamine
located at the focal point has been successfully synthesized
(Fig. 1).
Fig. 2 FTIR spectra of the MNPs: (A) MNPs-OA; (B) MNPs-G1(Lys);
(C) MNPs-G2(Lys); (D) MNPs-G3(Lys); (E) G3(Lys); (F) MNPs-G1
(Glu); (G) MNPs-G2(Glu); (H) MNPs-G3(Glu); (I) G3(Glu).
the particles remained in a colloidal state. It presumed that
a generally good integrity of polymer shell on the nanoparticles
was obtained during the ligand-exchange reaction.48
Moreover, the ligand-exchange reaction has been optimized
from typical procedures, involving the solvent used in the
grafting experiments and the amount of dendritic ligands coated
on the MNPs. In a typical grafting and phase transfer procedure,
DMF or aqueous solution was usually used. In order to make the
solution homogeneous, three solvents (CH3OH, CHCl3 and
n-hexane) were introduced in our system. In such a medium, all
Surface coverage and functional groups of the modified MNPs
The surface properties of the MNPs modified with peptide den-
drimers were confirmed by FTIR spectra as shown in Fig. 2. The
oleic-acid-coated MNPs (Fig. 2A) showed strong bands at 2922
and 2852 cmꢀ1, which are known as the characteristic peaks of
CH2 chain in oleic acid. Alkene stretches at 3009 and 1638 cmꢀ1
,
and two carboxylate stretches at 1538 and 1415 cmꢀ1 further
confirmed that oleic acid was chemisorbed on the surface of the
nanoparticles.49 After the ligand-exchange reaction, several
differences were observed in the spectra. In the series of L-lysine-
based dendrimers functionalized MNPs (Fig. 2B–D), the bands
at 3200–3600 cmꢀ1 were attributed to the bending vibration of
the –NH2 group. The strength of –CH2 stretching at around 2922
and 2852 cmꢀ1 dramatically decreased compared with the initial
MNPs, indicating the disappearance of oleic acid. However,
Fig. 2B–D showed that the bands at 2922–2931 cmꢀ1 greatly
increased with increasing dendritic generation, due to the
different number of alkane chains in polypeptide dendrimers.
Compared with FTIR spectra of lysine-based dendrimers, such
as G3(Lys) (Fig. 2E), two prominent bands at 1650 and 1547
cmꢀ1 were observed in the corresponding modified MNPs
(Fig. 2B–D), which was attributed to the presence of –CO–NH–
bands. A characteristic C–N peak was observed at about
1130 cmꢀ1. The strong bands at about 600 cmꢀ1 were the Fe–O
vibrations related to the ferrite core.49 The results indicated that
oleic acid was successfully replaced by lysine-based dendritic
ligands.
Scheme 3 The schematic route to MNPs unctionalized with peptide
dendrimers.
In another series of MNPs modified with L-glutamic acid
dendrimers (Fig. 2F–H), the O–H stretching frequency appeared
in the range 3200–3600 cmꢀ1. Similar to the L-lysine-based series
in Fig. 2B–D, the two characteristic –CH2 stretches of oleic acid
were also significantly reduced, but the bands at 2922–2931 cmꢀ1
noticeably increased from MNPs-G1(Glu) to MNPs-G3(Glu).
Fig. 1 The MALDI-TOF MS spectra of dendritic ligand 12.
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Compared with FTIR spectra of L-glutamic acid dendrimers,
such as G3(Glu) (Fig. 2I), the bands at ꢂ1730 cmꢀ1 were observed
in the functionalized MNPs due to the carbonyl vibration in
carboxyl groups. Two bands at around 1650 and 1540 cmꢀ1 also
represented the –CO–NH– bands. Similarly, strong Fe–O
vibrations were observed at about 600 cmꢀ1. In both series of
dendrimer-modified MNPs, the positions of the amide bands
were shifted, probably resulting from conformational changes
during the ligand-exchange process.
stage from 460 ꢁC to 900 ꢁC. This could be explained by the
breaking of the covalent band between the Fe3O4 and dopamine.50
The amount of polypeptide dendritic ligands covering the
MNPs could be calculated using the TGA results. Table 1
summarizes the weight loss and the ligand coverage, which was
calculated via the formula reported in the literature.49 A similar
phenomenon was observed in the MNPs modified with both
L-lysine and L-glutamic acid dendrimers. The ligands per particle
decreased with increasing generation due to the steric hindrance
between the polypeptide dendritic ligands and the magnetic core
in the ligand-exchange reaction, but the total amount of the
terminal amino or carboxyl groups still increased with increasing
generation. Therefore, the modification of higher-generation
dendritic ligands introduced more well-organized functional
groups on the surface, which could serve as anchor points for
further attachment of biological molecules, such as target
ligands, drugs or proteins, to achieve targeted delivery or specific
biological functions.
TGA measurement was employed to quantify the polypeptide
dendritic ligands grafted onto the surface of the MNPs. For
peptide dendritic ligands, the weight losses were only observed at
one stage within the temperature range 150–450 ꢁC. For
example, the decomposition of dendritic ligands G3(Lys) and G3
(Glu) started at 285 ꢁC and 295 ꢁC, respectively, as shown in
Fig. 3A and E. Fig. 3B–D and F–H show the weight losses of
L-lysine- and L-glutamic acid-based dendritic ligands modified
MNPs separately. The decomposition process was divided into
three stages within the temperature range 35–1000 ꢁC, which was
similar to the initial oleic-acid-coated MNPs (Fig. 3I). The first
stage was from 35 ꢁC to 200 ꢁC, within which the decrease in
weight loss was below 10%. This is assigned to the evaporation of
physically adsorbed water. The second weight loss step from
200 ꢁC to 460 ꢁC was attributed to the decomposition of the
polypeptide dendritic ligands immobilized on the surfaces of
MNPs. The TGA curves then suddenly leveled off in the third
Dispersibility and stability of the functionalized MNPs in water
The dispersibility and stability of the MNPs in water is very
important for biological applications. Fig. 4 shows that MNPs-
OA coated with a hydrophobic layer of oleic acid dissolved very
well in n-hexane. After the ligand-exchange reaction, the
hydrophilicity of particles greatly improved, enabling phase
transfer. Both of L-lysine and L-glutamic acid dendritic ligands
modified MNPs exhibited excellent water dispersibility due to the
water soluble peptide dendrimers. No precipitates were observed
even after one month. Moreover, the polypeptide dendrimers
modified MNPs could be dispersed in various organic solvents to
form stable magnetic colloids, such as ethanol, methanol and
DMSO. This phenomenon should be partly ascribed to the
amphiphilic peptide dendritic ligands, which are encouraging for
the development of MNPs modified with biocompatible poly-
peptide dendrimers and their analogs as novel biomaterials.
Crystalline structure and size of the functionalized MNPs
The crystal structure of the MNPs was determined by XRD,51,52
as shown in Fig. 5. The position and relative intensity of all
diffraction signals matched well with the characteristic peaks of
magnetite crystals obtained from standard Fe3O4 powder
diffraction data.13 The results revealed that the ligand-exchange
reaction did not change the structure of the MNPs. The average
crystallite sizes calculated from Scherrer’s formula53 were
consistent with those measured by TEM, as shown in Table 1,
though there was a little deviation. This could be ascribed to the
different mechanism of the two techniques for the measurements
of the particle sizes.
The morphology and size of the modified MNPs was deter-
mined by TEM. Fig. 6 shows the TEM micrographs of the MNPs
before and after ligand-exchange with different generations of
peptide dendritic ligands (G1–G3). The diameter and poly-
dispersity calculated by averaging more than 50 MNPs are
summarized in Table 1. The oleic-acid-stabilized MNPs were
nearly spherical with an average size of 8.8 nm, as shown in
Fig. 6A. Fig. 6B–G indicate that the peptide dendrimer-modified
MNPs were nearly monodisperse in water after the phase
Fig. 3 TGA curves of polypeptide dendritic ligands and modified
MNPs: (A) G3(Lys); (B) MNPs-G1(Lys); (C) MNPs-G2(Lys); (D)
MNPs-G3(Lys); (E) G3(Glu); (F) MNPs-G1(Glu); (G) MNPs-G2(Glu);
(H) MNPs-G3(Glu); (I) MNPs-OA.
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Table 1 The calculated numbers of ligands on each functionalized MNP (data were obtained from three independent experiments), and particle
diameters and magnetic properties for the MNPs
TGA
Size (nm)
Ms (emu gꢀ1
)
Sample
Weight loss (u%) Ligands/particle –NH2 /particle –COOH /particle XRD TEM
DLS
VSM TGAa
MNPs-OA
35
—
—
—
—
—
—
2166
2276
2936
9.03
12.49 9.62 ꢃ 0.81 12.3 ꢃ 3.8 53.4
9.25
9.49 ꢃ 0.83 15.5 ꢃ 4.3 50.3
8.82 ꢃ 1.09 9.2 ꢃ 1.8
44.8
—
52
49
42
54
52
48
MNPs-G1(Lys) 24.39
MNPs-G2(Lys) 28.85
MNPs-G3(Lys) 37.92
MNPs-G1(Glu) 20.92
MNPs-G2(Glu) 24.31
MNPs-G3(Glu) 30.31
984
644
496
1083
569
367
1968
2576
3968
—
—
—
10.43 9.74 ꢃ 0.87 25.0 ꢃ 8.5 41.6
8.89
9.80
9.62 ꢃ 1.20 13.4 ꢃ 4.3 55.0
9.18 ꢃ 0.97 16.1 ꢃ 4.0 50.1
10.13 9.22 ꢃ 0.94 26.2 ꢃ 9.2 42.9
a
Calculated from magnetic content determined via TGA in Table 1.
transfer. Compared with the TEM image of the initial MNPs, the
morphology of the modified particles changed from sphere-like
to cube-like, indicating slight Fe3O4 surface corrosion during the
exchange. This phenomenon was in agreement with the previous
report.24 TEM measurement only provides the information for
the inorganic ferrite core, since organic materials are transparent
by TEM. Thus, the sizes for all the functionalized MNPs given in
Table 1 are approximate.
DLS was used to determine the hydrodynamic diameter of the
MNPs, including the peptide dendritic ligands coating and the
ferric core. In Fig. 7, only one narrow peak in the size distribu-
tion was observed before and after ligand-exchange indicating
excellent monodispersity. After modification, the hydrodynamic
diameter of nanoparticles increased with the dendritic genera-
tions increasing, which were around 12.3, 15.5, 25.0 nm for the
MNPs functionalized with G1(Lys), G2(Lys), G3(Lys) (Fig. 7a)
and 13.4, 16.1, 26.2 nm for the ones with G1(Glu), G2(Glu), G3
(Glu) (Fig. 7b), respectively. Additionally, the size distribution of
the modified MNPs became broader with the dendritic genera-
tion increasing. This phenomenon mainly attributed to increased
molecular weight and size for higher generation,35,36 and partly
owing to the enhanced hydration properties resulting from more
peripheral functional groups. The DLS measurement confirmed
the feasibility to produce high quality ultrafine monodisperse
MNPs with controllable sizes via peptide dendrimers. To our
knowledge, this is the first synthesis of this kind of hydrophilic
MNP with polypeptide dendrimers less than 30 nm in size by the
ligand-exchange method. The ultra-small hydrodynamic diameter
should be effective at overcoming biological defense systems and
vascular barriers through enhanced permeability and retention
(EPR) effects to the target tissues in the field of biomedicine.22
Fig. 4 The dispersibility of the functionalized MNPs varied from n-
hexane to water. (A) MNPs-G1(Lys); (B) MNPs-G2(Lys); (C) MNPs-G3
(Lys); (D) MNPs-G1(Glu); (E) MNPs-G2(Glu); (F) MNPs-G3(Glu).
Magnetic properties of the MNPs
To study the magnetic behavior before and after the peptide
dendritic ligand-exchange, VSM was employed to measure the
magnetization (Fig. 8). The magnetization curve for oleic-acid-
coated MNPs showed no hysteresis and was completely revers-
ible at room temperature (Fig. 8A). Neither coercivity nor
remanence was observed, indicating a typical superparamagnetic
behavior.9,47,49,50 After the ligand-exchange process, the hyster-
esis loops of the modified MNPs still past 0 Oe (Fig. 8B–G).
Therefore the superparamagnetic properties were successfully
Fig. 5 XRD patterns of (A) MNPs-OA; (B) MNPs-G1(Lys); (C) MNPs-
G2(Lys); (D) MNPs-G3(Lys); (E) MNPs-G1(Glu); (F) MNPs-G2(Glu);
(G) MNPs-G3(Glu).
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Fig. 6 TEM images of the MNPs: (A) MNPs-OA; (B) MNPs-G1(Lys); (C) MNPs-G2(Lys); (D) MNPs-G3(Lys); (E) MNPs-G1(Glu); (F) MNPs-G2
(Glu); (G) MNPs-G3(Glu). A was in n-hexane; B–G were in water.
Fig. 7 The size distribution of the dendrimer-functionalized MNPs by
DLS: (A) MNPs-OA; (B) MNPs-G1(Lys); (C) MNPs-G2(Lys); (D)
MNPs-G3(Lys); (E) MNPs-G1(Glu); (F) MNPs-G2(Glu); (G) MNPs-
G3(Glu).
Fig. 8 Hysteresis loops for the MNPs at room temperature: (A) MNPs-
OA; (B) MNPs-G1(Lys); (C) MNPs-G2(Lys); (D) MNPs-G3(Lys); (E)
MNPs-G1(Glu); (F) MNPs-G2(Glu); (G) MNPs-G3(Glu).
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retained and not affected by the introduction of different series of
peptide dendritic ligands.
attributed to the presence of amino groups on the particle
surface, and those groups were in their protonated form.54 The
pIs of these MNPs were all at pH > 9, indicating the loss of
protonated amino groups. Under such basic conditions, the
abundant OHꢀ resulted in the surface being negatively charged.
In contrast, the MNPs functionalized with L-glutamic acid den-
drimers (Fig. 9 D–F) were negatively charged almost over the
entire pH range due to the abundance of peripheral carboxyl
groups. The pIs were observed at around pH ¼ 3. Slight differ-
ences of pIs were also observed for both series of MNPs modified
with different generations (G1–G3) of dendritic ligands, sepa-
rately. It is noteworthy that all the functionalized MNPs tended
to aggregate when the pH was close to the pI due to the small
amount of electrostatic repulsion and the attractive van der
Waals force. The MNPs modified with higher dendritic genera-
tions (G3) showed greater stability in aqueous solution than
those modified with lower dendritic generations (G1) in both
series, because of the steric interactions of dendritic ligands and
the electrostatic repulsions. Similar steric interactions have also
been observed in the case of PEG-modified MNPs.49 For the
MNPs modified with both L-lysine and L-glutamic acid den-
drimers, no aggregation was observed in the pH range 2–9 and 5–
12, respectively. This implies that the functionalized MNPs are
potentially suitable for biomedical applications in different
media.
Table 1 summarizes the saturation magnetization (Ms) of the
MNPs. For oleic-acid-coated MNPs, the saturation magnetization
was ꢂ44.8 emu gꢀ1 (Fig. 8A). After the ligand-exchange reaction
via L-lysine-based dendritic ligands (Fig. 8B–D), The saturation
magnetization significantly increased to 53.4 emu gꢀ1 for MNPs-
G1(Lys) and decreased to 41.6 emu gꢀ1 for MNPs-G3(Lys).
Similarly, for L-glutamic acid-functionalized MNPs (G1–G3), the
saturation magnetization fell significantly from 55.0 to 42.9 emu
gꢀ1. According to the previous studies, the saturation magnetiza-
tion is in direct proportion to the particle size.13 However, the two
series of modified MNPs had extremely similar-sized iron cores, as
observed in the TEM images in Fig. 6. The main reason for the
variation of magnetization should therefore be ascribed to the
introduction of a nonmagnetic mass onto the nanoparticles’
surface and the different magnetic content per gram of particles
(Table 1). The largest saturation magnetization for G1(Glu)-
functionalized MNPs (55.0 emu gꢀ1) was ascribed to the lowest
amount of organic compounds on each particle surface (20.92%).
In contrast, a great loss of magnetization per gram of particles was
observed when G3(Lys) and G3(Glu) were used for ligand-
exchange because of the surface coverage of each particle increased
to 37.92% and 30.31% separately. The Ms value could also be
estimated on the basis of TGA data using eqn (1) (see also Table
1)49 where MsMNPs-OA is the saturation magnetization of oleic-acid-
coated MNPs (44.8 emu gꢀ1) and u is the mass loss (%) determined
by TGA. The saturation magnetization values measured via VSM
were consistent with those calculated by TGA.
Positively charged nanoparticles show a higher rate of cellular
uptake by the phagocytic system compared with the neutral or
negatively charged formulations.55 Thus, the MNPs modified
with L-lysine-based dendritic ligands might be available as
multifunctional drugs or gene delivery carriers due to their
positive charge.7,39,56,57 On the other hand, the polypeptide den-
drimers can also be utilized to functionalize other hydrophobic
nanoparticles, such as quantum dots or Au nanoparticles, merely
by altering the connecting ligands. The biomedical applications
of nanoparticles functionalized with polypeptide dendritic
ligands are currently being explored in our group.
MsMNPs-OAð1 ꢀ uÞ
Ms ¼
(1)
1 ꢀ uMNPs-OA
Surface charge of polypeptide dendritic ligands modified MNPs
The zeta potential provides an important parameter to determine
the surface charge of nanoparticles and calculate the isoelectric
point (pI). Fig. 9 shows a plot of the zeta potential vs. the pH for
the two series of functionalized MNPs. In the case of MNPs
modified with L-lysine-based dendrimers (Fig. 9A–C), the parti-
cles were positively charged under acidic conditions, with
a surface potential higher than +30 mV at pH < 3. This can be
Conclusions
In this paper, a new family of MNPs surface-functionalized with
polypeptide dendrimers were successfully designed and fabri-
cated by the ligand-exchange method without changing the
crystalline structure. Two series of MNPs modified with L-lysine
and L-glutamic acid dendrimers showed controllable peripheral
functional groups, excellent dispersibility and long-term colloidal
stability in aqueous solution over a broad pH range. The
hydrodynamic diameters of the functionalized MNPs were all
less than 30 nm and increased gradually with increasing gener-
ation. The superparamagnetism of the MNPs was not affected by
the ligand-exchange process. Modified MNPs with the same
generation number of peptide dendritic ligands exhibited similar
magnetization. MNPs-G1(Lys) and MNPs-G1(Glu) possessed
the largest saturation magnetization values, 53.4 and 55.0 emu
gꢀ1 in their respective series, which is larger than the oleic-
acid-coated MNPs. The saturation magnetization of modified
MNPs reduced from ꢂ55 to ꢂ42 emu gꢀ1 with increasing
dendritic generation because of an increased nonmagnetic mass
on the surface. The peptide dendritic ligands not only afforded
phase-transferred MNPs high water-solubility, but could also
Fig. 9 Zeta potential vs. pH of dendrimer-functionalized MNPs: (A)
MNPs-G1(Lys); (B) MNPs-G2(Lys); (C) MNPs-G3(Lys); (D) MNPs-G1
(Glu); (E) MNPs-G2(Glu); (F) MNPs-G3(Glu).
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27 J. Qin, Y. S. Jo and M. Muhammed, Angew. Chem., Int. Ed., 2009, 48,
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serve as potential anchor points for the attachment of biological
molecules. Their functionalization with peptide dendrimers to
achieve the synthesis of high quality MNPs provides a unique
approach to design and explore new nanobiomaterials.
28 M. Talelli, C. J. F. Rijcken, T. Lammers, P. R. Seevinck, G. Storm,
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The authors thank P. C. Deng, X. M. Jia and X. Y. Wang
(Analytical & Testing Center, Sichuan University) for testing
samples in NMR. This research work was supported by the
National Basic Research Program of China (National 973
program, No. 2011CB606206), the National Natural Science
Foundation of China (31070849, 50830105), the Department of
Science and Technology of Sichuan Province (2009HH0001) and
the Ministry of Science and Technology (2010DFA51550).
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