A facile synthesis of lipid stabilized gold nanoparticles: A step towards biodegradable biosensors

Article abstract of DOI:10.1166/jnn.2011.4878

A new class of polylactone was successfully synthesized and utilized for the encapsulation and stabilization of gold nanoparticles. Core/shell nanoparticle architecture, in which a layer of this polymer surrounds the nanoparticle core have been investigated both as a means to improve the stability and surface chemistry and as a way of accessing unique physical properties that are not possible from one nano-material alone. Given the fact that only few systems has so far been developed for the encapsulation of nanoparticles, our success in using a new biodegradable biopolymer with inbuilt functionality reveals the robustness of this work. The biodegradability of this polylactone was evaluated using scanning electron microscopy (SEM). The morphology and stability of these gold-polymer hybrids were evaluated by using the transmission electron microscopy (TEM) and UV-VIS spectroscopy.

Full text of DOI:10.1166/jnn.2011.4878

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All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 11, 7027–7032, 2011
A Facile Synthesis of Lipid Stabilized
Gold Nanoparticles: A Step Towards
Biodegradable Biosensors
Sinoj Abraham1 and Suresh S. Narine2ꢀ∗
1Chemical and Materials Engineering Department, 3-021i, National Institute for Nanotechnology,
University of Alberta, 11421 Saskatchewan Drive, Edmonton, Alberta-T6G 2M9, Canada
2Trent University Biomaterials Research Program, Department of Physics & Astronomy and Chemistry,
Trent University, 1600 West Bank Drive, Peterborough, Ontario-K9J 7B8, Canada
A new class of polylactone was successfully synthesized and utilized for the encapsulation and sta-
bilization of gold nanoparticles. Core/shell nanoparticle architecture, in which a layer of this polymer
surrounds the nanoparticle core have been investigated both as a means to improve the stability
and surface chemistry and as a way of accessing unique physical properties that are not possible
from one nano-material alone. Given the fact that only few systems has so far been developed
for the encapsulation of nanoparticles, our success in using a new biodegradable biopolymer with
inbuilt functionality reveals the robustness of this work. The biodegradability of this polylactone was
evaluated using scanning electron microscopy (SEM). The morphology and stability of these gold-
polymer hybrids were evaluated by using the transmission electron microscopy (TEM) and UV-VIS
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spectroscopy.
IP: 119.93.239.208 On: Mon, 28 Mar 2016 10:10:03
Keywords: Gold Nanoparticles, Biodegradable, Polyester, Nonanolactone, Biocompatible,
Copyright: American Scientific Publishers
Hybrid.
Recently, the role of nanoparticles has substantially grown
beyond the field of pure materials science.1 The utilization
of nanoparticles in many research areas necessarily require
stabilization from agglomeration due to van-der Waals
forces, and stabilizing agents such as oligosaccharides,2
DNA,3 functionalized polymers,4 dendrimers,5 and block
copolymer micelles6 are usually used. This control on
the surface properties of nanoparticles has been a sig-
nificant theme in many research arena including micro-
electronics, biosensors, immobilization of biomolecules,
catalysis, micro fabrication and so forth.7 As polymer-
based materials played significant role in the development
of biorelated systems, surface modification of nanoparti-
cles by polymeric ligands have gained much prominence.8
Advancements in this area impose challenges on the devel-
opment of new polymeric systems with functional moi-
eties, especially those integrated with gold nanoparticles
(AuNPs).6ꢀ8ꢀ9 Surface functionalized AuNPs are promising
candidates for the construction of drug delivery devices
due to their stability, low toxicity, small size, versatile
surface properties and ability to be disguised from the
reticulo-endothelial system (RES) of the body.10 Moreover,
surface modification provide better means to alter solubil-
ity and compatibility of AuNps and render tunable optical
properties suitable for ablative therapy for cancer.11 Anti-
body conjugated functional AuNPs render high contrast
for noninvasive imaging of targeted cancer tissues.12
There are many reports in which polymers and copoly-
mers were utilized as stabilizing ligands of AuNPs.6 Bio-
compatible and biodegradable polycaprolactone and its
copolymers were also employed as the ligands for the pro-
tection of AuNPs leading to drug delivery devices and sub-
cellular localization studies.10ꢀ13 Recently, we synthesized
a new biodegradable polyester, namely polynonanolac-
tone (PNL) from renewable resources, motivated by the
global interest in replacement of petroleum products due
to increased environmental concerns and its anticipated
depletion.14
In this communication, we report the thiol-
functionalization of PNL and demonstrate its ability in
passivation of AuNPs. The specific interaction of mer-
capto group with metals such as gold, silver and cadmium
has amplified the interest of thiols in biotechnology.6ꢁaꢂꢀ15
The incorporation of thiol groups in macromolecules
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 8
1533-4880/2011/11/7027/006
doi:10.1166/jnn.2011.4878
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A Facile Synthesis of Lipid Stabilized Gold Nanoparticles: A Step Towards Biodegradable Biosensors
Abraham and Narine
can be done by coupling protected thiol to an exist-
ing functionality13ꢁbꢂꢀ16 followed by deprotection or by
designing the monomers with a residual protected thiol
moeity.6ꢁaꢂ By using a functional initiator, well-defined
functional polymers can also be prepared with quantitative
functionality.17 Here we used a freshly prepared initiator
containing a protected thiol group13ꢁbꢂ for the ring-opening
polymerization (ROP) of di-nonanolactone monomers
using stannous octanoate (SnOct2ꢂ as catalyst.
The initiator, ꢃ-(2,4-dinitrophenylthiol), was prepared
by reacting mercaptoethanol (15 mmol) with Sanger
reagent (2,4-dinitrofluorobenzene) (15 mmol) in CHCl3
(18 mL) using triethylamine (4.2 mL) as catalyst.13ꢁaꢂ The
reaction mixture was allowed stir overnight at room tem-
perature and 1.0 M HCl was added drop wise until a yel-
low precipitation was observed. The pH of the liquid was
kept around 8. Thiol was recrystallized in methylene chlo-
ride and dried under vacuum.
The monomer di-nonanolactone was obtained from
canola oil by a two step process involving the synthe-
sis of 9-hydroxynonanoic acid and further its condensa-
tion reaction using hafnium chloride (HfCl4ꢂ.14 The ring
opening polymerization (ROP), carried out at an ambi-
ent temperature in toluene, generated polymers with con-
trolled molecular weight and narrow molecular weight
distributions (Scheme 1). The ROP was conducted in a
7000 and PDI of 1.2. This is in agreement with the Mn
values obtained from H NMR integration.
1
Previously we reported the comparative study on phys-
ical properties and biodegradation behavior of poly-
nonanolactone possessing various Mn values and on that
basis polynonanolactone with Mnꢁ∼ꢂ7000 was determined
for preparing the gold nanoparticle hybrids.14ꢁaꢂ The pro-
tecting group of thiol moiety of this polymer was removed
through an exchange reaction of the protected chain-end
with an excess among of 1-propanethiol in the presence of
triethylamine at pH 8. The mixture was stirred overnight
under N2 and the required thiol end capped polymer
(PNL-SH) was recovered by precipitation into heptanes.
It was then purified using methylene chloride as solvent
and cold diethyl ether as precipitant. The end groups
were confirmed by 1H NMR spectroscopy (Varian UNITY
400 NMR) (Fig. 1). The triplet split peak corresponding
to methylene (a) due to the coupling with the thiol pro-
ton is observed at 2.9 ppm and that of methylene (b) at
4.17 ppm, clearly indicates the thiol functionalization.
The thermal behavior of this thiol-polynonanolactone
was investigated by modulated differential scanning
calorimetry MDSC Q-100 (TA Instruments, New Cas-
tle, USA). This polymer demonstrates a partial crystalline
nature with a melting enthalpy (ꢄHmꢂ of 110 J/g. DSC
samples were prepared withꢀapproximately 7 mg of poly-
mer. These were held at 20 C for 3 min to reach equilib-
rium state and then heated to 125 ꢀC at a rate of 20 ꢀC/min
to erase thermal history. The sample was then cooled down
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50 mL Schlenk flask, previously flamed and purged with
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dry nitrogen fixed with a condenser equipped with a dry
Copyright: American Scientific Publishers
nitrogen inlet. The monomer (dilactone), initiator (ꢃ-(2,4-
dinitrophenylthiol)ethanol) at a molar ratio of initiator to
monomer 1:20 (mol/mol), catalyst SnOct2 (0.1 mol%) and
dry toluene (5 mL for 1 g of monomer) were added to the
flask. The reaction was refluxed for 24 h. under nitrogen.
The mixture was then cooled to room temperature and dis-
solved in chloroform. The polymer was precipitated from
excess methanol. The filtered polymer was re-dissolved in
toluene and re-precipitated in methanol for the completely
removal of unreacted monomer, initiator and the catalyst.
The polymer was then finally filtered, washed and dried
under vacuum. The gel permeation chromatography (GPC-
Agilent G1311A quaternary pump and G1362A refrac-
tive index detector) results shows that polylactones were
obtained with number-average molecular weight (Mnꢂ of
ꢀ
ꢀ
to −75 C at a constant rate of 5 C/min and kept at this
temperature for 3 min for the completion of crystalliza-
ꢀ
tion. ꢀFinally it was heated to 125 C at a constant rate
of 3 C/min. No noticeable glass transition was observed,
which might be because the heat capacity change over the
glass transition region is too small to be detected. The ther-
mal degradation was also evaluated using thermo gravi-
metric analysis (TGA-Q50 (TA Instruments, New Castle,
USA)) by determining the weight loss with temperature.
ꢀ
The samples were heated from 25 to 600 C under dry
nitrogen at a constant heating rate of 10 ꢀC/min. This poly-
mer exhibited a three step degradation process similar to
a few previous reports of certain polyesters.18 The weight
losses can be attributed to random ester bond degradation
followed by the decomposition of low mass fragments and
Scheme 1. Synthesis of thiol functionalized polynonanolactone by (a) ROP using SnOct2 as catalyst and (b) deprotection by 1-propane thiol and
triethylamine.
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Abraham and Narine
A Facile Synthesis of Lipid Stabilized Gold Nanoparticles: A Step Towards Biodegradable Biosensors
washed with distilled water and then vacuum dried at room
temperature before being subjected to analyses. The spec-
imens were weighed and surface morphology was exam-
ined by the scanning electron microscopy (SEM- Philips
XL30 ESEM). The SEM images exhibit the corroded sur-
face of polymer films (Fig. 3).
The weight loss due to hydrolytic degradation of poly-
mer films was determined at intervals of 24 h for 8 con-
secutive days as shown in the plot depicted in Figure 4.
A modest decrease of the film weight (0.05 wt%) was
detected after 48 h of hydrolytic degradation and it was
observed upto 0.36 wt% after 8 consecutive days. The per-
centage weight loss with time is included in Figure 4.
Fig. 1. 1H NMR of polynonanolactone containing thiol and hydroxyl
end groups.
An advantage of thiol chemistry is the suite of
thiol functional molecules available for functionalization
of materials. The capability of this biocompatible and
biodegradable polyester to qualify the surface of metal
nanoparticles is evaluated by anchoring them on the
AuNPs surface by utilizing the thiol chemistry. Thiol
groups possess a high affinity towards metal surfaces,
thereby, forming strong covalent bonds. Scheme 2 shows
the organization of polymer molecules on the gold surface.
A solution of AuNP colloid solution (20 mL; 20 nm in
diameter, 7×1011 particles mL−1, TedPella Inc.) was con-
centrated to 250 mL by centrifugation at 14000 rpm for
30 min. the concentrated solution was diluted with DMF
(750 ꢅL). 50 ꢅL of this solution was mixed with poly-
finally by depolymerization of chains. The TGA profile is
included in Figure 2.
In order to evaluate the biocompatibility due to
biodegradation of this polyester, the hydrolytic degrada-
tion of the film samples was conducted. The biodegrada-
tion experiments were carried out with a mixer/shaker at
ꢀ
200 cycles/min in a thermo-stated oven at 25 C. Poly-
mer films for this experiment were prepared by solution
casting on glass plate. 1 wt% polymer solution in chlo-
roform were cast onto glass plates and slowly allowed to
ꢀ
dry at room temperature and further at 50 C for three
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mer solution (8 mg/mL in DMF:EtOH:H2O 2:1:1). The
days under vacuum to remove the residual solvents. The
Copyright: American Scientific Publishers
ꢀ
mixture was heated to 110 C for 2 h, and then cooled
at a rate of 1 ꢀC min−1 by using TA thermal cycler.
This process facilitates the controlled precipitation of poly-
mer, thus inducing molecular self-assembly and resulting
in a stable micelle solution. To remove excess reactants
and free polymers, a 5 mL batch of this solution was
diluted with water (2 mL) and centrifuged at 10000 rpm
for 30 min. This step was repeated twice more and a con-
centrated polymer-gold conjugate solution was obtained.
The polyester-AuNP hybrid compound was characterized
by transmission electron microscope (TEM) analysis. The
nanoparticles appeared as dark spots with a white covering
of polymer shells (Fig. 5). It is clear from the image that
AuNPs (20 nm) are covered by a polymer shell with a total
diameter of about 60 nm and uniform thickness. These
hybrids appear to be well-defined and contain singly occu-
pied AuNPs with apparent stability. These AuNP-polymer
conjugates are stable without any additional crosslinking
reactions. Repeated centrifugation did not result in defor-
mation or fusion of the spherical shape and no ellipsoidal
deformations was observed.
glass substrates were previously weighed in order to get
the tare weight of polymer. The films attached to the glass
substrates were placed in vials and filled with buffer solu-
tion (0.1 M, pH = 7). Polymer films attached to the glass
substrates were placed in vials and filled with 5 mL of
buffer solution. The solution was changed every 24 h. At
selected degradation times, the specimens were taken out,
UV-Vis spectroscopy was used to evaluate the surface
plasmon resonance (SPR) of the polyester-gold nanopar-
ticle hybrids. Figure 6 shows UV-Vis spectrum of gold
nanoparticles and with polyester shell. The UV absorp-
tion band which appears at ∼523 nm assigns the sur-
face plasmon resonance (SPR) of the gold nanoparticles.
Fig. 2. TGA and DTGA data of thiol functionalized polynonanolactone.
J. Nanosci. Nanotechnol. 11, 7027–7032, 2011
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A Facile Synthesis of Lipid Stabilized Gold Nanoparticles: A Step Towards Biodegradable Biosensors
Abraham and Narine
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Copyright: American Scientific Publishers
Fig. 3. SEM images of polynonanolactone after (a) two days (b) four days (c) six days and (d) eight days of hydrolytic degradation.
While conjugating with polynonanolactone, a red-shift
(∼534 nm) occurs, which corresponds to the SPR absorp-
tion of gold nanoparticles within 20 nm size.19 This
information reflects the stability of individual particles
from mutual aggregation. Additionally, these nanoparticle
hybrids are stable at room temperature for a prolonged
time. These biocompatible AuNPs are not be completely
soluble in water due to the hydrophobic PNL shell. The
Scheme 2. Encapsulation of gold nanoparticles by the polynonanolac-
Fig. 4. Percentage weight loss of polymers due to hydrolytic
tone assisted by thiol functionalization.
degradation.
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Abraham and Narine
A Facile Synthesis of Lipid Stabilized Gold Nanoparticles: A Step Towards Biodegradable Biosensors
engineering towards bio-nanotechnology in an environ-
mentally friendly manner. The free chain end hydroxyl
groups of this polymer are still available for functional-
izing with biomolecules or drugs. Moreover, noncovalent
incorporation of drugs into the hydrophobic pockets in
the monolayer provides an alternative delivery strategy.
We also anticipate that the current investigation can be
reasonably extended to the development of biodegradable
biosensors.
Acknowledgment: This work was financially supported
by Bunge Oils, Alberta Crop Industry Development Fund,
Alberta Agriculture Research Institute, Alberta Canola
Producers Commission, The University of Alberta, and
the Natural Sciences and Engineering Research Council
(NSERC) of Canada.
Fig. 5. TEM images of polynonanolactone-gold nanoparticle conjugate.
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Copyright: American Scientific Publishers
Received: 9 September 2010. Accepted: 20 February 2011.
7032
J. Nanosci. Nanotechnol. 11, 7027–7032, 2011