Grafting of polymeric platforms on gold by combining the diazonium salt chemistry and the photoiniferter method

Article abstract of DOI:10.1016/j.polymer.2014.12.007

The grafting of stable and strongly attached polymeric platforms on gold is a key factor for successful applications in biology, catalysis and sensing. Here, we report on the use of a combination of the iniferter method and the diazonium salt chemistry for preparing smart polymeric platforms attached through covalent bonds on gold. For this, bifunctional molecules bearing aryl diazonium coupling agents for anchoring on gold and N,N-diethyldithiocarbamate groups for initiating the growth of polymer chains were used. These two moieties were separated by oligo(ethylene oxide) spacers of various lengths allowing a fine tuning of the hydrophilic properties of the grafted photoinitiator layers. Cross-linked copolymers of methacrylic acid (MAA) and N,N′-methylenebisacrylamide (MBAm) were then grown from the gold surfaces under UV light. The polymer films were characterized in terms of chemical composition and wettability by X-ray photoelectron spectroscopy and contact angle measurements, respectively. The grafting procedure was simple, rapid and effective in producing polymer-grafted Au surfaces at room temperature. The diethyldithiocarbamil groups remaining at the end of the growing tethered chains could then be easily exchanged by a UV-light induced radical-exchange experiment in order to obtain terminal amino moieties able to immobilize citrate-capped gold nanoparticles, through electrostatic interactions. The results obtained in the present work highlight the efficiency of the diazonium salt chemistry coupled to the photo-iniferter based surface grafting approach to spontaneously functionalize gold surfaces through covalent bonds. This strategy open new opportunities for the preparation of "smart" hybrid platforms made of pH-responsive polymers and nanoparticle assemblies.

Full text of DOI:10.1016/j.polymer.2014.12.007

Polymer 57 (2015) 12e20
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Grafting of polymeric platforms on gold by combining the diazonium
salt chemistry and the photoiniferter method
Randa Ahmad a, Adrien Mocaer a, Sarra Gam-Derouich a, Aazdine Lamouri a,
a
a
b
a, *
ꢀ ꢁ
Helene Lecoq , Philippe Decorse , Philippe Brunet , Claire Mangeney
a
ꢀ
Univ Paris Diderot, Sorbonne Paris Cite, ITODYS, UMR 7086 CNRS, 15 rue J-A de Baïf, 75205 Paris Cedex 13, France
Univ Paris Diderot, Sorbonne Paris Cite, MSC, UMR 7057 CNRS, 10 rue Alice Domon et Leonie Duquet, 75205 Paris Cedex 13, France
b
ꢀ
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
The grafting of stable and strongly attached polymeric platforms on gold is a key factor for successful
applications in biology, catalysis and sensing. Here, we report on the use of a combination of the iniferter
method and the diazonium salt chemistry for preparing smart polymeric platforms attached through
covalent bonds on gold. For this, bifunctional molecules bearing aryl diazonium coupling agents for
anchoring on gold and N,N-diethyldithiocarbamate groups for initiating the growth of polymer chains
were used. These two moieties were separated by oligo(ethylene oxide) spacers of various lengths
allowing a fine tuning of the hydrophilic properties of the grafted photoinitiator layers. Cross-linked
copolymers of methacrylic acid (MAA) and N,N0-methylenebisacrylamide (MBAm) were then grown
from the gold surfaces under UV light. The polymer films were characterized in terms of chemical
composition and wettability by X-ray photoelectron spectroscopy and contact angle measurements,
respectively. The grafting procedure was simple, rapid and effective in producing polymer-grafted Au
surfaces at room temperature. The diethyldithiocarbamil groups remaining at the end of the growing
tethered chains could then be easily exchanged by a UV-light induced radical-exchange experiment in
order to obtain terminal amino moieties able to immobilize citrate-capped gold nanoparticles, through
electrostatic interactions. The results obtained in the present work highlight the efficiency of the dia-
zonium salt chemistry coupled to the photo-iniferter based surface grafting approach to spontaneously
functionalize gold surfaces through covalent bonds. This strategy open new opportunities for the
preparation of “smart” hybrid platforms made of pH-responsive polymers and nanoparticle assemblies.
© 2014 Elsevier Ltd. All rights reserved.
Received 17 September 2014
Received in revised form
5 December 2014
Accepted 8 December 2014
Available online 13 December 2014
Keywords:
Diazonium salts
Iniferter polymerization
Gold nanoparticles
1. Introduction
process can readily be controlled and confined by the selective
application of long wavelength UV light. Dithiocarbamate species,
Surface-initiated polymerization (SIP) on gold has received an
increasing interest these recent years because gold surfaces are
compatible with a large range of analytical techniques (SPR [1e3],
electrochemistry [4], SERS [5]) and present unique properties at
nanoscale. The use of photochemically activated initiators is espe-
cially convenient for grafting polymers to gold surfaces, since the
introduced as photoinitiators (or iniferter: initiator-transfer-
terminator agents) by Otsu [6e8], have the peculiarity of cleaving
upon UV irradiation, forming a radical pair constituted by a reactive
carbon radical that initiates the polymerization and a less reactive,
sulfur-centered radical that can act as a reversible capping agent
during the polymerization process. This polymerization process
addresses a wide range of economical and ecological concerns,
compared to corresponding thermal reactions, with low cost and
mild reaction conditions. Conventionally, the universal platform for
grafting iniferters on gold is based on the self-assembly of thiol
[9e12] or disulfide [13,14] molecules. However, the UV-initiated
polymerization from SAMs on Au is considered as a difficult task
because of the instability of the thioleAu bond upon UV irradiation
[15]. To overcome this limitation, Benetti et al. [13] proposed to use
Abbreviations: XPS, X-ray photoelectron spectroscopy; SEM, scanning electron
microscopy; CA, contact angles; MAA, methacrylic acid; N,N0-methyl-
enebisacrylamide, MBAm.
* Corresponding author. Tel.: þ33 1 57276878; fax: þ33 01 57277263.
E-mail addresses: mangeney@univ-paris-diderot.fr, Claire.mangeney@univ-
paris-diderot.fr (C. Mangeney).
http://dx.doi.org/10.1016/j.polymer.2014.12.007
0032-3861/© 2014 Elsevier Ltd. All rights reserved.

R. Ahmad et al. / Polymer 57 (2015) 12e20
13
lamps emitting at 300 nm coupled with a 280 nm cut-off filter to
perform the photopolymerization and avoid any degradation of the
starting monolayer. However, the AueS bond at the interface be-
tween the organic polymer coating and the gold surface remains
labile upon certain conditions (UV light or heating) which causes
instability problems for storage under light or any post-
functionalization reaction above 60 ꢀC.
Therefore, the development of versatile and efficient surface
modification iniferter strategies [16e20] able to provide strong and
stable linkages between the gold surface and the polymer coating
still remains challenging. We adress this issue in the present paper
by developping a facile methodology to surface-initiated photo-
polymerization (SIP) of vinylic monomers from gold surfaces via an
aryl diazonium salt-derived iniferter. Aryl diazonium salts have
been shown recently to be useful coupling agents for the grafting of
polymer coatings on carbon-based [21e27] and metallic [28e40]
(gold, platinum, palladium, ruthenium and titanium) planar or
nanoparticle surfaces, affording strong carbon or metalecarbon
linkages. Nevertheless, the combination of the diazonium salt
chemistry and the iniferter method, which has proved to be effi-
cient in order to grow dense polymer layers from the surface of
aluminum [41] and oxide nanoparticles [42e44] has never been
extended til now to the functionalization of gold surfaces.
In this paper, we fill this gap by exploring the propensity of aryl
diazonium coupling agents bearing N,N-diethyldithiocarbamate
(DEDTC) [9] groups to photo-initiate the growth of polymer chains
from gold surfaces, as depicted in Fig. 1. The diethyldithiocarbamil
groups remain at the end of the growing tethered chains [6,7],
which allows one to tune the chain length by variation of the
irradiation time. In addition, the use of this polymerization reaction
gives control over the end groups, which can be easily exchanged or
chemically modified. The biocompatibility of this technique should
also be underlined: no organic solvents are used and no toxic
metal/compounds are involved in the polymerization process. By
introducing oligo(ethylene oxide) spacers of various lengths be-
tween the aryl anchoring moieties and the DEDTC groups, we could
play on the hydrophilic character of the initiating layer, which is of
prime importance when the functionalization steps have to be
performed in a water environment. We demonstrate this approach
by grafting cross-linked copolymer layers of methacrylic acid
(MAA) and N,N0-methylenebisacrylamide (MBAm) on gold surfaces.
This grafting procedure is simple, rapid and effective in producing
Fig. 1. General method for the functionalization of gold surfaces by bifunctional molecules bearing a diazonium moiety and an iniferter end group separated by oligo(ethylene
oxide) chains of various lengths.

14
R. Ahmad et al. / Polymer 57 (2015) 12e20
polymer-grafted Au surfaces at room temperature with controlled
thickness. Moreover, this strategy not only harnesses the advan-
tages of UV-light photoinitiated polymerizations, but also over-
comes the problem caused by the lability of the AueS bond through
the formation of stable AueC bonds at the interface between the
polymer layer and the Au surface. In order to demonstrate the
potential applications of this approach, we show that it can be used
to immobilize gold nanoparticles on the polymer layer through a
simple modification of the polymer chain extremities under UV
light.
yield. A cold solution of tert-butylnitrite in anhydrous acetonitrile
was added drop wise to a cold solution (in an ice bath) composed of
this compound, tetrafluoroboric acid and anhydrous acetonitrile.
The reaction was conducted at ꢁ20 ꢀC during 24 h. The viscous
diazonium salt was washed 3 times with ether, dissolved in acetone
and dried at room temperature.
þ
2.2.2. Synthesis of Clꢁ, 2NeC6H4-(OeCH2eCH2)n¼[1e3]-DEDTC
The synthetic strategy was similar for the three salts (with n
varying from 1 to 3) following the procedure described in a pre-
vious reference [42]. Briefly, diazonium salts were synthesized by
the diazotization of aniline derivatives. These salts were prepared
by suspending aniline derivatives in a 50/50 by volume hydro-
chloric acid and distilled water. The mixture was cooled to 3 ꢀC in
an ice bath, and then 1 equivalent of sodium nitrite was slowly
added. The mixture was stirred for 15 min. A summary of the
procedure is proposed in the form of a scheme in Fig. 2.
2. Materials and methods
2.1. Materials
Methacrylic acid (MAA), N,N0-methylenebisacrylamide (MBAm),
gold-coated silicon wafers, diethyldithiocarbamate (DEDTC),
ethylene glycol, diethylene glycol, triethylene glycol, 4-nitrophenol,
triethylamine methylsulfonyl chloride, sodium nitrite (NaNO2),
terbutylnitrite, thionyl chloride (SOCl2), 2-(4-aminophenyl), tetra-
fluorobric acid (HBF4), and tri-sodium citrate were purchased from
SigmaeAldrich and used as received. Chloroauric acid (HAuCl4) was
obtained from Alfa Aesar. All the solvents were obtained from Acros
organic and used as received. Water was deionized using a Milli-
pore purification system.
2.3. Grafting of aryl-terminated iniferters on planar gold surface
Gold-coated silicon wafers were cut into slides of 2 cm ꢂ 4 cm,
ultrasonically rinsed with water and ethanol, dried in a stream of
argon, cleaned in a UV cleaner (Boekel, Inc., Model 135500) and
rinsed with ethanol in order to remove the organic residues on the
surface. The grafting of aryl layers was conducted by immersing the
Au plates for 1, 5 and 24 h in aqueous solutions of the diazonium
salts (3.10ꢁ3 M) at room temperature. The substrates were washed
by sonication in water and ethanol during 5 min in order to remove
all the non-grafted species from the surface.
2.2. Synthesis of diazonium salts
2.2.1. Synthesis of diethyldithiocarbamyl phenyldiazonium
tetrafluoroborate (BFꢁ4 ,þ2NeC6H4eCH2eDEDTC)
This diazonium salt was prepared in three steps, as described
previously [45]. First the 4-nitrobenzyl chloride reacts with DEDTC
to give the 1-diethyldithiocarbamate-(4-nitrophenyl)methane in a
good yield. This last intermediate was hydrogenated using Raney
Nickel and led to the corresponding primary amine in quantitative
2.4. Photopolymerization from iniferters grafted on gold surface
The Au plates modified with DEDTC groups were immersed in a
polymerization mixture containing MAA as the functional mono-
mer, N,N’-methylbisacrylamide (MBAm) as the cross-linker
þ
Fig. 2. Schematic illustration of the procedure employed in order to synthesize the iniferter molecules bearing aryl diazonium end groups Clꢁ
,
2NeC6H4-(OeCH2eCH2)n¼[1e3]
-
DEDTC.

R. Ahmad et al. / Polymer 57 (2015) 12e20
15
Table 1
monomer and water as the solvent. The glass vessel was degassed
by bubbling argon for 10 min. The slides were then exposed to UV
light at 365 nm at room temperature for 5 h. The slides were rinsed
with copious amounts of water and ethanol to remove the un-
reacted monomers, and dried in a nitrogen stream.
Surface atomic percentages determined by XPS for bare and functionalized Au plates
AueC6H4e[Oe(CH2)2](n¼0-3)-DEDTC obtained after 1 h incubation of the gold sub-
strate with the corresponding diazonium salt.
Materials
Au
C
O
S
N
Bare Au plate
70
42
42
48
44
25
44
43.5
37
39
5
e
e
3
2
2
1.5
AueC6H4eCH2-DEDTC
AueC6H4-[O-(CH2)2]-DEDTC
AueC6H4-[O-(CH2)2]2-DEDTC
AueC6H4-[O-(CH2)2]3-DEDTC
7
10
11
13
4
2.5
2
2.5. Immobilization of gold nanoparticles (AuNPs)
In order to immobilize AuNPs on the aryl-terminated iniferters
grafted on gold surface, the substrates were first placed in a quartz
flask containing 5 ml of 0.01 M aqueous solution of 4-amino-
2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) and were
irradiated for 25 min by UV light (365 nm) at room temperature.
The substrates were subsequently rinsed with water and ethanol
and stored under argon. In a second step, they were incubated with
gold nanoparticles suspensions during 12 h, and then washed
several times with water and ethanol.
2.5
component, while the small peak at 288 eV is due to the dithio-
carbamate SeC]S group. It is noteworthy that a shake-up satellite
p p* is observed at ca. 291 eV, which is the characteristic signa-
ꢁ
ture of the presence of aryl groups. Furthermore, the differences in
the oligo(ethylene oxide) spacer size are clearly visible in the C1s
high resolution spectra, with an enhancement of the CeO
component at 286.6 eV when the number of ethylene oxide units
increases in the spacer.
2.6. Instrumentation
The growth of the iniferter layer at the surface of gold was then
investigated by varying the time of incubation between the gold
substrate and the 2NeC6H4-[O-(CH2)2]2-DEDTC iniferter-derived
Photopolymerization was performed using the commercial ul-
traviolet processor Spectrolinker XL 1500 UV (Spectronics Corp.).
This processor was equipped with 6 tubes (8 W) with a wavelength
range of 365 nm and intensity of 17.6 mW cmꢁ2. XPS spectra were
recorded using a Thermo VG Scientific ESCALAB 250 system fitted
þ
diazonium salts.
The gold signal attenuation due to the progressive covering of
the substrate by the iniferter layer as a function of time was used to
provide an estimation of the organic layer thickness evolution. For
this, the relative attenuation of the Au 4f7/2 signal was expressed as:
with
a
microfocused, monochromatic Al
Ka X-ray source
(hn
¼ 1486.6 eV; spot size ¼ 650
mm; power ¼ 15 kV, 200 W). The
pass energy was set at 150 and 40 eV for the survey and the narrow
regions, respectively. Spectral calibration was determined by
setting the main C1s component at 285 eV. The surface composi-
tion was determined using the integrated peak areas and the cor-
responding Scofield sensitivity factors corrected for the analyzer
transmission function. Scanning electronic microscopy (SEM) im-
ages were obtained using a Zeiss SUPRA 40 FESEM equipped with a
thermal field emission gun. Images were created using SMARTSEM.
Wetting properties was characterized with a Contact Angle Digi-
drop PX 500. The contact angle measurement were performed
using the image of a sessile drop at the points of intersection be-
tween the drop contour and the projection of the surface (baseline).
Moreover, the polynomial method was used to determine the
contact angle. In the polynomial method, the parameters of a
polynomial function are fitted to the drop contour. The drop is
unsymmetrical so contact angles are measured on both sides of the
liquid drop profile, and the values are averaged.
I=I0 ¼ expð ꢁ d=l sin qÞ;
(1)
where d is the layer thickness,
substrate-specific photoelectron in the organic layer,
l
the mean free path of the
q the analysis
take-off angle relative to the surface, and I/I0 the ratio of the Au4f7/2
peak intensities for the modified (I) and bare Au surface (I0). Details
of this calculation are reported in the Supporting Information. Fig. 4
plots the variations of the thickness of the iniferter layer as a
function of time, estimated from XPS data, as well as the (C þ O)/Au
atomic ratios, representative of the chemical composition of the
organic coating.
From these results, it appears that, contrary to alkylthiols mol-
ecules which tend to form monolayers on surfaces [46], the
diazonium-derived iniferter layers grow progressively with time,
forming functional poly(phenylene) multilayers as previously re-
ported in the literature [47]. The thickness of these iniferter layers
varies from less than 1 nm after 15 min of incubation to almost
20 nm after 24 h of incubation.
3. Results and discussion
3.1. Grafting of aryl-terminated iniferters on planar gold surface
A comprehensive study of the chemical nature of the interface
was performed by closely examining the high resolution spectra of
all the elements present in the XPS survey spectrum at low iniferter
layer thickness (after incubation time of 15 min). Three types of
chemical bonding between the Au substrate and the iniferter
grafted groups were considered: (i) the interaction of Au with the
sulfur atoms from DEDTC units; (ii) the direct reaction of diazonium
salts with the gold surface leading to AueN interfacial bonds, as
already observed by G. Deniau et al. [48], or (iii) the presence of
AueC bonds explained by the grafting of aryl cations or radicals on
the surface, arising from dediazoniation.
The high resolution spectra of sulfur S2p and nitrogen N1s are
displayed in Fig. 5. The S2p signal, at 163.8 eV is characteristic of
DEDTC units which do not interact with Au. This highlights the
absence of AueS interaction (no visible peak at ca. 162 eV). The N1s
signal, at 400.3 eV is also characteristic of DEDTC units. The absence
of any signal at lower binding energy (around 397.5 eV) indicates
that the grafting of the iniferter molecules is not based on AueN
The spontaneous grafting of the diazonium-derived iniferter
layer at the surface of gold was performed by simply incubating the
substrates with the corresponding diazonium salts in water at room
temperature. Strong modifications in surface chemical composition
were observed by XPS after functionalization of the gold substrates.
Indeed, the gold signal appears strongly attenuated while the car-
bon and oxygen content increase and new peaks assigned to sulfur
and nitrogen appear, indicating the covering of gold by the iniferter
layers (see Table 1). Depending on the size of the oligo(ethylene
oxide) spacers between the diazonium anchoring moieties and the
iniferter groups, the proportion of oxygen changes, increasing for
longer spacers.
The peak fitting of the C1s signal displayed in Fig. 3 clearly
reveals all the components contained in the grafted iniferter
layers. The intense peak at 285 eV is assigned to CeC and C]C
environments, the one at 286.6 eV corresponds to the CeO

16
R. Ahmad et al. / Polymer 57 (2015) 12e20
Fig. 3. High resolution C1s spectra of the functionalized gold substrates (a) AueC6H4eCH2-DEDTC; (b) AueC6H4e[Oe(CH2)2]1-DEDTC; (c) AueC6H4e[Oe(CH2)2]2-DEDTC;
(d) AueC6H4e[Oe(CH2)2]3-DEDTC.
interfacial bonds. Concerning the presence of AueC interfacial
bonds, XPS could not bring a direct experimental proof, due to the
low difference in electronegativity between carbon and gold atoms
salts using SERS combined to modeling [49,50], one could antici-
pate that the grafting of the iniferter layers is based on AueC co-
valent bonds.
(
c
c ¼ 2.55 and
c
Au ¼ 2.54 using the Pauling scale). Nevertheless,
The influence of the oligo(ethylene glycol) chains upon the
chemical properties (hydrophilic/hydrophobic) of the iniferter
layers was investigated by measuring the water contact angles (CA)
of the bare and modified samples (see Fig. 6). Bare gold substrates
have typical water contact angles around 65ꢀ. After grafting of the
iniferter layers, the CA values decrease progressively as the number
of ethylene oxide units increases in the diazonium-derived
coupling agent. The CA value changes from 70ꢀ for
AueC6H4eCH2-DEDTC, which does not contain any hydrophilic
ethylene oxide units to 47ꢀ for AueC6H4-[O-(CH2)2]3-DEDTC, in
agreement with an enhanced hydrophilic character.
based on the absence of any interfacial AueN or AueS bonds and on
recently reported papers demonstrating the presence of AueC
interfacial bonds on gold nanoparticles modified by diazonium
3.2. Photopolymerization from iniferters grafted on gold surface
The iniferter pendant groups connected to the gold substrates
through oligo(ethylene oxide) chains of various lengths were then
used to photo-initiate the polymerization of methacrylic acid
(MAA) cross-linked with N,N0-methylbisacrylamide (MBAm), as
illustrated in Fig. 7. The gold substrates obtained after 6 h of poly-
merization were characterized by XPS. Strong variations in the
surface chemical composition of the samples were detected, as can
be observed in Table 2. The gold signal disappears almost
completely, screened by the organic overlayers while the carbon
and oxygen contents increase sharply.
Fig. 4. Variations of the (C þ O)/Au atomic ratios versus incubation time between the
gold substrate and þ2NeC6H4e[Oe(CH2)2]2-DEDTC (black curve, circles) and evolution
of the iniferter layer thickness (d), as estimated by XPS (red curve, up triangles). NB: It
is to note that due to the strong attenuation of the gold signal, almost invisible after
24 h (1440 min), it was not possible to calculate the corresponding (C þ O)/Au atomic
ratio. (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
The high resolution C1s signal also changes significantly upon
polymerization becoming very close to that of PMAA with a main
peak at 285 eV due to CeC/CeH, a second one at 285.8 eV due to the
central CeCeC and a new intense component at 289.2 eV due to the

R. Ahmad et al. / Polymer 57 (2015) 12e20
17
Fig. 5. XPS high resolution spectra of the functionalized gold substrate AueC6H4e[Oe(CH2)2]1-DEDTC (a) S2p and (b) N1s.
Table 2
Surface atomic percentage determined by XPS for bare and PMAA-grafted Au plates.
Materials
Au
C
O
S
N
Bare Au plate
70
0
6
1
2
25
72
67
71
70
5
e
0
1
1
1
e
2
6
3
4
AueC6H4eCH2-PMAA-DEDTC
AueC6H4-[Oe(CH2)2]-PMAA-DEDTC
AueC6H4-[Oe(CH2)2]2-PMAA-DEDTC
AueC6H4-[Oe(CH2)2]3-PMAA-DEDTC
26
20
24
23
It is noteworthy that while the Au surface is almost completely
screened by the polymer chains, the S2p signal is still detectable after
6 h of polymerization, confirming the living character of the poly-
merization process, leaving the DEDTC moieties at the chain ex-
tremities. From the gold signal attenuation, using Equation (1), an
estimation of the polymer layer thickness could be obtained. The
thickness values were found around 10e15 nm for all the types of
iniferter-grafted Au surfaces, which confirms the efficient grafting of
poly(MAA-co-MBAm) layers from the modified gold surface what-
ever is the diazonium salt used initially. The iniferter method is
usually based on benzyl initiating radicals. However, in the iniferter
molecules reported in this paper, the presence of the hydrophilic
spacers located between the diethyl dithiocarbamate moieties and
the aryl groups grafted on gold generates alkyl initiating free radicals.
Despite this difference, the polymerization process appears to occur
efficiently, whatever the length of the oligo(ethylene oxide) chains.
Fig. 6. Water contact angles (ꢀ) as a function of the number of ethylene oxide units in
the iniferter molecules grafted on gold.
OeC]O environments of methacrylic acid units, as displayed in
Fig. 8. A fourth component of low intensity is also visible at
287.5 eV, assigned to the amide groups presents in the bis-
acrylamide cross-linker.
Fig. 7. General scheme of the chemical procedure for the substrate functionalization by surface-initiated iniferter photo-polymerization.

18
R. Ahmad et al. / Polymer 57 (2015) 12e20
As expected, the CA values reveal a transition between a nearly
hydrophobic character (with CA~72ꢀ) to a hydrophilic one (with
CA~18ꢀ) as the pH changes from acidic to basic. This behavior is in
agreement with the protonation of MAA units at pH < pKa(PMAA)
and their ionization at pH > pKa(PMAA)
.
3.3. Immobilization of gold nanoparticles
Chemical modification of the diethyldithiocarbamyl groups
remaining at the end of the iniferter grafted layers and the polymer
chain extremities can be achieved by radical exchange with e.g.
stable 2,2,6,6-tetramethylpiperidyl-1-oxyl (TEMPO) radicals. In a
simple experiment (schematized in Fig. 10), the iniferter layers and
tethered polymer chains were irradiated in the presence of
4-amino-TEMPO radicals that irreversibly couple at room temper-
ature with the in-situ formed macroradicals. In the next step,
citrate-capped gold nanoparticles were reacted with the primary
amine moieties of the exchanged groups.
Fig. 8. Peak-fitted high resolution C1s spectrum of AueC6H4e[Oe(CH2)2]2-PMAA-
SEM images of the iniferter-coated gold substrates AueC6H4-[O-
(CH2)2]2-DEDTC and the polymer-coated ones AueC6H4-[O-
(CH2)2]2-PMAA-DEDTC recorded after this two-step procedure (see
Fig. 11), evidence the efficient immobilization of a relatively uni-
form coverage of gold nanoparticles on the amine-coated surfaces.
These gold nanoparticle assemblies were found to be strongly
attached and remained at the surface even after a cleaning proce-
dure involving various solvents (water and ethanol) under ultra-
sounds. This chain-terminal-group-exchange strategy therefore
offers an easy way to immobilize nanoparticles at the end of
tethered polymer layers.
DEDTC.
4. Conclusion
In conclusion, we have developed diazonium-derived photo-
iniferters for surface-tethered polymerization on gold through
stable AueC anchoring linkages. This process offers a suitable
alternative to self-assembled monolayers of iniferters based on
thiols or disulfides which are known to be unstable under UV
light or heating. In order to finely tune the hydrophilic/hydro-
phobic properties of the initiator layers, oligo(ethylene oxide)
spacers were introduced between the diethyl dithiocarbamate
initiating groups and the aryl anchoring moieties. Depending on
the length of the spacers (containing a number of ethylene oxide
units varying from 0 to 3), the measured water contact angles
changed from 70ꢀ to 46ꢀ evidencing an enhancement of the
hydrophilic character of the iniferter-grafted substrates as the
number of ethylene oxide units increases. Nevertheless, the
presence of the hydrophilic spacers had no influence on the
Fig. 9. Water contact angles for AueC6H4e[Oe(CH2)2]2-PMAA-DEDTC at (a) pH 5.5
and (b) pH 13.
PMAA is known to behave as a pH-responsive polymer, which
offers promising potentialities for pH sensing applications (such
as biomedical or environmental pH-sensors [51e53]. Therefore,
the pH response of the PMAA grafted polymer layers was studied
by water contact angle measurements at pH 5,5 and pH 13 (see
Fig. 9).
Fig. 10. Scheme of the exchange procedure of the PMAA end groups with the stable 4-amino-TEMPO radicals and the subsequent immobilization of gold nanoparticles.

R. Ahmad et al. / Polymer 57 (2015) 12e20
19
Fig. 11. SEM images of the iniferter-coated gold substrates AueC6H4e[Oe(CH2)2]2-DEDTC, (a) before and (b) after immobilization of gold NPs; and the polymer-coated ones
AueC6H4e[Oe(CH2)2]2-PMAA-DEDTC (c) before and (d) after immobilization of gold NPs.
polymerization process with dense polymer layers obtained after
6 h of UV irradiation, whatever the nature of the iniferter layers.
The PMAA coating demonstrated a pH-responsive behavior, as
expected, with water contact angle measurements switching
from 72ꢀ at acidic pH to 18ꢀ at basic pH. Furthermore, this
method enables control of chain ends and immobilization of
nanoparticles, as was proven by a radical-exchange experiment
followed by incubation with citrate-capped gold nanoparticles.
All these characteristics make this grafting technique a promising
tool for producing “smart”, pH-responsive platforms, with
potential for the chemical control of the chain-terminating
groups.
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