Light-induced modification of silver nanoparticles with functional polymers

Article abstract of DOI:10.1039/c4cc00960f

A mild, efficient and ambient temperature photochemical approach for the synthesis of silver nanoparticle core-shell structures employing a zwitterionic polymer as well as polyethylene glycol is presented. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc00960f

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Light-induced modification of silver nanoparticles
with functional polymers†
Lukas Stolzer,ab Ishtiaq Ahmed,a Cesar Rodriguez-Emmenegger,‡bc
Vanessa Trouillet,d Pascal Bockstaller,a Christopher Barner-Kowollik*abc and
Ljiljana Fruk*a
Cite this: Chem. Commun., 2014,
50, 4430
Received 5th February 2014,
Accepted 5th March 2014
DOI: 10.1039/c4cc00960f
www.rsc.org/chemcomm
A mild, efficient and ambient temperature photochemical approach for can be prepared either via a one-pot strategy using monomers7
the synthesis of silver nanoparticle core–shell structures employing a or a polymer8 as a reducing/stabilizing agent. Alternatively, a
zwitterionic polymer as well as polyethylene glycol is presented.
grafting-from approach may be employed, which usually results
in heterogeneous particles and ill-defined polymer brushes.3,9
Polymer nanocomposites have attracted significant interest in a-Mercapto-functional polymers have been extensively used for
recent years due to their unique properties and a broad range of the functionalization of Au NPs.10,11 However, such composites
potential applications.1 Silver polymeric nanoparticles (Ag NPs) exhibit poor stability and are prone to ligand exchange with thiol-
are particularly sought after due to the combination of their containing molecules present in reaction buffers or cell media.12
optical, catalytic and antimicrobial properties.2 However, bare
With the rapid advances in nanoparticle functionalization,
Ag NPs are so far only of limited use in the biomedical and various thermally driven methods are often employed for attaching
biosensing field, since contact with the biological media results molecular species to the nanoparticle surfaces.3 Core–shell
in fouling causing either loss of colloidal stability or opsonization. polymer NPs have mainly been prepared using gold (Au)
In order to prevent such processes, the Ag NPs must be coated with NPs as core nanocomposites via thermally activated chemical
polymers able to prevent the fouling while concomitantly endowing strategies. For instance, Peng and coworkers demonstrated the
specific functions.3 By fusing the properties of Ag NPs with a corona coupling of alkyne-terminated poly(N-isopropylacrylamide)
of antifouling polymers, a plethora of new applications are acces- to azide-functionalized Au NPs through a copper-catalyzed
sible.4 In this context, it is necessary to prepare stable and Huisgen cycloaddition to form thermoresponsive core–shell
biocompatible NPs with well-defined surface coatings. Several Au NPs with high grafting densities.13 In another approach,
methods have been employed to prepare polymer-functionalized Beyer et al. employed Diels–Alder chemistry to afford a reversible
metallic NPs such as embedment in a polymer matrix5 or surface surface functionalization of Au NPs with poly(styrene),14 and
coating to form core–shell structures.6 The latter is particularly Aldeek et al. prepared highly fluorescent PEG and zwitterion-
interesting as it results in smaller, well-defined nanostructures functionalized Au nanoclusters.15 There are only limited examples
with significant biomedical potential. Such core–shell structures of Ag NP–polymer composites usually containing a poly(ethylene
glycol) (PEG) shell.16 In addition, the current thermally triggered
chemical strategies involve the use of additional reagents or
catalysts, sensitive functional groups, extensive reaction times
(up to several days), the use of elevated temperatures or a
combination of all of these, which can impair the NP stability
and induce their aggregation.
To overcome the disadvantages associated with thermally
controlled reactions, we herein apply a novel light-triggered
strategy first described by Barner-Kowollik and coworkers17 to
prepare Ag NP–polymer core–shell NPs using functional polymers.
Here, a photoactive group based on ortho-quinodimethane (caged
diene) is allowed to react with dienophiles such as maleimide
groups under UV irradiation (Fig. 1). Light has already been
employed for the preparation of tailored nanomaterials,18,19
however, herein the first example of employing light-triggered
a Center for Functional Nanostructures, Karlsruhe Institute of Technology (KIT),
Wolfgang-Gaede-Str.1a, 76131 Karlsruhe, Germany. E-mail: ljiljana.fruk@kit.edu
b
¨
Preparative Macromolecular Chemistry, Institut fur Technische Chemie und
Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstr. 18,
76128 Karlsruhe, Germany
c
¨
¨
Institut fur Biologische Grenzflachen, Karlsruhe Institute of Technology (KIT),
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
E-mail: christopher.barner-kowollik@kit.edu
d Institute for Applied Materials (IAM-ESS) and Karlsruhe Nano Micro Facility
(KNMF), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1,
76344 Eggenstein-Leopoldshafen, Germany
† Electronic supplementary information (ESI) available: Materials and methods,
experimental and synthetic procedures, ESI-MS spectra and TEM images. See
DOI: 10.1039/c4cc00960f
‡ Current address: Institute of Macromolecular Chemistry, Academy of Science of
the Czech Republic, v.v.i. Heyrovsky Sq. 2, Prague, 162 06 Czech Republic.
4430 | Chem. Commun., 2014, 50, 4430--4433
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inside betaine brushes has already been exploited as an effective
antibacterial interface.5,23
Initially, a novel bifunctional linker 1 containing benzotriazole
(BT) as the Ag NP anchoring group and a caged diene (photoenol)
moiety for light-induced cycloaddition were prepared (Fig. 1).
Benzotriazoles have previously been shown to bind strongly to
Ag surfaces and have been employed for the modification of Ag
NPs.16,24 To assess the reactivity of the linker, a model reaction
between BT–photoenol 1 and maleimide-functionalized PEG 3
(Mn B 1300 g molÀ1) was carried out (Fig. 2a). Low molecular
weight maleimide-PEG was chosen due to its excellent dienophile
reactivity and the ease of precise ESI-MS characterization of the
adduct. A reaction mixture of 1 and 3 was irradiated using a
320 nm light source for 15 min in DMSO. Subsequent analysis by
ESI-MS evidenced that a new set of peaks is formed, which was
assigned to the product of the light-induced Diels–Alder reaction
indicating the complete conversion to photoadduct 6 (Fig. 2a).
Subsequently, the successful model reaction was adapted to
modify the surface of Ag NPs. First, we explored the one-pot
synthesis in which AgNO3 is reduced using sodium borhydride or
ascorbic acid in the presence of linker 1 to prepare photoenol-
decorated Ag NPs. However, the use of such reducing agents leads
to the reduction of the aldehyde group to the hydroxyl group in 1,
which results in the loss of reactivity (data not shown). To
Fig. 1 Light-induced functionalization of Ag NPs with polymers. The novel
photoactivable linker 1 and the inert linker 2 are attached to the Ag NP surface.
1 enables the light-induced cycloaddition with either PEG-Mal 3 or pCBMA 4
upon UV-irradiation resulting in cycloadduct 5.
cycloaddition for the mild, efficient, fast and catalyst-free Ag NP circumvent this drawback, a milder ligand exchange methodology
modification with antifouling and biocompatible polymers (i.e. was employed in which the linker is added to the citrate-capped
poly(carboxybetaine methacrylate) (pCBMA) and poly(ethylene Ag NPs (10 000 eq. per NP). To minimize the surface crowding
glycol) (PEG)) enabling temporal reaction control is presented. effect, an inert linker 2, which does not contain the photoenol
PEG is one of the most widely used water soluble, biocompatible moiety, was used in a 1 : 10 ratio (linker 1 to linker 2). The
polymers, which can enhance the stability of NPs in aqueous combination of reactive and inert linkers improved the colloidal
solutions or cell media.20 Zwitterionic pCBMA, on the other hand, stability of the nanoparticles and enabled control over the density
outperforms PEG in terms of resistance to non-specific protein (and availability) of reactive groups as previously reported.25 After
adsorption from blood serum and plasma21,22 as well as to marine incubation in DMSO over night at ambient temperature, unbound
fouling and bacteria. In addition, the in situ generation of Ag NPs ligands were removed by centrifugation. The concentration of
Fig. 2 (a) ESI-MS spectra of PEG maleimide 3 and photoadduct 6 after the light-induced cycloaddition with the bifunctional linker 1, (b) C 1s XP spectra
of untreated citrate-capped Ag NPs (top), modified Ag NPs-1 (2nd row), PEG-functionalized Ag NPs-3 (3rd row) and pCBMA-functionalized Ag NPs-4
(bottom) evidenced successful ligand exchange and photoreactions.
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Fig. 3 HRTEM images of (a) PEG-coated Ag-NPs-3 and (b) pCBMA-coated Ag NPs-4. (c) An EDXS line scan (orange) of pCBMA-coated Ag NPs. (d) C and
Ag peak intensities along the EDXS line scan showing the Ag-pCBMA core–shell structure. The counts of C and Ag are plotted against the position. The
core is close to 20 nm and the shell approx. 4 nm in size.
modified Ag NPs was determined by UV-Vis spectroscopy. The additionally ascertained by an EDXS line scan indicating the
efficient ligand exchange was evidenced via a change in the zeta corresponding Ag and C contents (Fig. 3c and d). It should be
potential (negative charge of citrate Ag NPs decreases, ESI,† noted that control samples – into which both polymers were
Table S2) and X-ray photoelectron spectroscopy (XPS) measure- added without irradiation – did not show any increase in the C
ments (Fig. 2b). The C 1s XP spectrum of the modified Ag NPs content via EDX line scans (ESI,† Fig. S26 and S27).
shows a strong decrease of the peak intensity assigned to
In conclusion, we introduce a novel light-induced route for
OQC–O bonds at 288.5 eV, stemming from the replaced citrate the covalent functionalization of photoenol-modified Ag NPs with
groups. Once the ligand exchange was confirmed, maleimide- poly(ethylene glycol) and poly(carboxybetaine methacrylate). The
PEG 3 was added to the dispersion of modified Ag NPs, purged novel bifunctional linker containing benzotriazole as the Ag
with nitrogen and irradiated for 15 min at 320 nm (for experi- anchoring group and a caged diene (photoenol) moiety enabled
mental details please refer to the ESI†). After performing wash- photo-grafting of o-maleimide polymers to Ag NPs as demon-
ing steps, UV-Vis spectroscopy was employed to evidence that the strated by XPS, HRTEM and EDXS. Such a light-induced approach
NPs remain stable and no aggregation occurs as indicated by the for preparation of the polymer Ag NPs expands the synthetic
unchanged characteristic plasmonic resonance of Ag NPs at toolbox for the tailoring of nanomaterials and can be translated
420 nm (ESI,† Fig. S16). XPS analysis of Ag NPs-3 shows a strong to the other molecular species. In addition to the potential that Ag
increase of the peak intensity at 286.6 eV in the C 1s spectrum, NPs could have in biosensing/biomedicine, photoenol-modified
which is assigned to C–O bonds, stemming from the ethylene Ag NPs could be used for light-induced pattering onto a range of
glycol unit. The formation of a corona of PEG was evidenced by surfaces – a concept that is the subject of our ongoing studies.
high-resolution transmission electron microscopy (HRTEM). As
The study was supported by the EU grant Single Molecule
depicted in Fig. 3a, a 1–2 nm PEG shell can be observed around Activation and Computing (FOCUS, no 270483) and DFG-CFN
the 20 nm Ag core. To further prove the formation of the corona projects A5.7 and E2.6. C.B.-K. is additionally grateful for
and its composition, energy-dispersive X-ray spectroscopy continued support from the KIT as well as the BioInterfaces
(EDXS) was carried out (ESI,† Fig. S22). An EDXS line scan along program of the Helmholtz association. C. R.-E. acknowledges
the core–shell structure recorded a high C content in the shell support by the Alexander von Humboldt Foundation and
resulting from the attached PEG layer and a high Ag content at the Grant Agency of the Czech Republic under Contract No
the core resulting from Ag NPs.
P106121451.
Once the successful light-induced modification of Ag NPs
with PEG was demonstrated, the more challenging zwitterionic,
non-fouling pCBMA 4 (Mn B 21 700 g molÀ1) was also grafted
onto the Ag NPs. A longer irradiation time was used for
the coupling of pCBMA (1 h vs. 15 min for PEG 3) to compensate
for the steric hindrance effects of the longer, highly swollen
polymer chain. The successful photografting was unambiguously
confirmed by XPS and EDXS analyses and HRTEM measure-
ments. Namely, the C 1s XP spectrum of Ag NPs 4 shows a strong
increase of the peak intensity at 288.5 eV assigned to the OQC–O
bonds, stemming from the carboxylic acid and ester groups of
pCBMA. In addition, the N 1s XPS spectrum shows a new peak
assigned to the high-energy quaternary ammonium groups
(ESI,† Fig. S25). HRTEM measurements also demonstrated
the formation of the core–shell structure consisting of a
20 nm Ag core and a 4 nm pCBMA shell (Fig. 3b), which was
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