Design, Self-Assembly, and Switchable Wettability in Hydrophobic, Hydrophilic, and Janus Dendritic Ligand-Gold Nanoparticle Hybrid Materials

Article abstract of DOI:10.1021/acs.chemmater.7b02928

Controlling nanoparticles' (NPs) surface polarity, colloidal stability, and self-assembly into well-defined complex architectures is of paramount importance for emergent nano- and biotechnologies, and each depends strongly on the ligand shell composition

Full text of DOI:10.1021/acs.chemmater.7b02928

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Article
Design, Self-Assembly, and Switchable Wettability in Hydrophobic,
Hydrophilic, and Janus Dendritic Ligand-Gold Nanoparticle Hybrid Materials
Katherine C. Elbert, Davit Jishkariani, Yaoting Wu, Jennifer
D. Lee, Bertrand Donnio, and Christopher B. Murray
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b02928 • Publication Date (Web): 19 Sep 2017
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Chemistry of Materials
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Design, Self-Assembly, and Switchable Wettability in Hydrophobic,
Hydrophilic, and Janus Dendritic Ligand-Gold Nanoparticle Hybrid
Materials
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Katherine C. Elbert,† Davit Jishkariani,†,‡ Yaoting Wu,† Jennifer D. Lee,† Bertrand Donnio,‡,# Christo-
pher B. Murray*,†,§
†Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, United States
‡Complex Assemblies of Soft Matter Laboratory (COMPASS), UMI 3254, CNRS-Solvay-University of Pennsylvania,
CRTB, 350 George Patterson Boulevard, Bristol, PA 19007, United States
#Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504, CNRS−Université de Strasbourg, 23 rue
du Loess, BP43, 67034 Strasbourg cedex 2, France
§Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, United States
ABSTRACT: Controlling nanoparticles’ (NPs) surface polarity, colloidal stability, and self-assembly into well-defined complex
architectures is of paramount importance for emergent nano- and biotechnologies, and each depends strongly on the ligand shell
composition and chemical nature. In this study, a series of dendritic ligands with hydrophobic, hydrophilic and Janus surface groups
were synthesized, grafted onto Au NPs, and their effects on the self-assembly behavior and the surface polarity of the corresponding
hybrid materials were investigated. The ligand synthesis utilized a generalized, flexible strategy that independently introduces den-
dritic end groups, responsible for the surface polarity and colloidal properties, and specific surface NPs binding groups, reducing the
number of synthetic steps. The dendritic ligands obtained were grafted onto NP surfaces through solution phase ligand-exchange, and
the resulting NP-dendron hybrids were studied using a variety of techniques such as transmission electron microscopy (TEM), UV-
vis, and small-angle X-ray scattering (SAXS). By controlling the solvent evaporation rate during self-assembly, these dendronized
Au hybrids self-organize into highly ordered thin films comprised of close-packed arrays of NPs, where the interparticle separation
can be varied as a function of the dendritic generation and end group chemistry. Moreover, contact angle and colloidal observations
revealed the strong dependence of the dendron end-group and generation on the NP surface polarity. Uniquely, the hybrid material
of Au NPs and the Janus dendron exhibits controlled surface wetting, where the surface polarity is dependent on solvent exposure,
revealing a surface polarity memory effect, making this material a model system for surfaces that demonstrate switchable wettability.
Introduction
The study of coordination and self-assembly behavior of lig-
ands on nanoparticle (NP) surfaces is of critical importance as
it provides the key to understanding and engineering properties
such as colloidal stability,1,2 electronic conduction,3 magnetic
and optical properties,4–6 and biological/catalytic activity.7–11
For as-synthesized NPs, the nature of the ligands coating NP
surfaces depends on the specific synthetic methods employed.
For those synthesized in organic media, capping ligands are
usually selected from commercially available alkyl-containing
molecules that have either acid, amine, phosphine, or thiol
surface binding groups.12,13 By contrast, in aqueous media, syn-
theses of NPs involve polar ligands such as ascorbic acid, citric
acid, or polyethylene glycol derived molecules.14–17 Ligands
with specific properties are often attached to the NP surface by
post-synthetic surface modification (ligand exchange) or func-
tionalization of the ligand shell.18,19 To this end, many different
types of ligands such as polymeric,20–23 dendritic,24–29 liquid
crystalline,30–35 and small molecules2,36 have been grafted on NP
Figure 1. Various examples of surface functionality en-
gineering.
surfaces to introduce solubility, steric, biological, sensing, opti-
cal, magnetic, and electronic properties.
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Scheme 1. Synthesis of the hydrophobic, hydrophilic, and Janus building blocksª
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RO
RO
RO
RO
Br
ii
iv
a)
2
or
OH
Cl
HO
HO
HO
RO
RO
RO
RO
O
O
O
OTs
O
O
O
O
3
6, R = C12H25
7, R = (CH2CH2O)3CH3
8, R = C12H25
9, R = (CH2CH2O)3CH3
i
RO
1
iii
RO
O
RO
4, R = C12H25
5, R = (CH2CH2O)3CH3
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10, R = C12H25
OH
11, R = (CH2CH2O)3CH3
RO
RO
RO
RO
RO
O
O
O
O
O
O
O
O
O
O
b)
O
O
O
O
O
O
O
O
Cl
or
HO
Cl
iii
RO
RO
O
RO
RO
O
O
O
8
9
O
OH
O
HO
i
RO
RO
13, R = C12H25
RO
RO
15, R = C12H25
12
14, R = (CH2CH2O)3CH3
16, R = (CH2CH2O)3CH3
c)
O
O
O
O
O
O
O
O
O
O
O
O
O
HO
O
O
O
O
Cl
O
O
O
Cl
O
O
O
8
9
v
HO
17
HO
12
i
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
iii
OH
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
18
19
°
°
ªReagents and conditions: (i) K2CO3, KI, DMF, 80 C, 12h; (ii) LiAlH4, THF, 60 C, 3h; (iii) KOH, THF/H2O/MeOH, 4h, 80 °C for 4 and
13, 4h at 50 °C for 5, 14, and 18; (iv) SOCl2, CH2CL2, 3h, rt; (v) 1 equiv. of 8, K2CO3, KI, DMF, 85 °C, 12h.
In most cases, the chemical nature of the ligands is limited to
being either hydrophilic, introducing aqueous compatibility, or
hydrophobic, giving rise to dispersibility in organic solvents of
the corresponding hybrids (Figure 1a). A limited number of lig-
ands reported to date have a dual nature, i.e. contain both hy-
drophobic and hydrophilic moieties.37 Amongst those described
in the literature are diblock copolymer based ligands (Figure 1b)
that create a double corona around the NP, where only one
block fragment is on the periphery and has the largest effect on
the final colloidal properties. There are a limited number of ex-
amples using amphiphilic block copolymers where both blocks
interact with the surrounding solvent, as these have to include a
“v-shaped” junction.38–40
A selection of these amphiphilic ligands exhibit switchable wet-
tability properties, where the polarity of the surface of the ma-
terial can be changed by exposing the system to various stimu-
lants, such as a variety of solvents.41,42 The ability to adjust the
wettability properties of a surface is of particular value due to
the applications of these materials for smart surfaces, such as
self-cleaning surfaces,43,44 and nanofabrication templates with
switchable morphologies.45
Amongst other possible ligand architectures, dendritic ligands
have unique advantages. They provide a well-controlled syn-
thetic platform where, the ligand bulkiness and the nature and
polyvalency of the endgroups can be tuned independently from
each other.46,47 These properties make them an attractive system
for NP surface functionalization and to date they have found
utility in NP synthesis48 as well as optical,26,49 magnetic,27 bio-
logical,50 colloidal,51 self-assembly,26,52 and catalytic17 property
modification.
However, in reported examples, the chemical nature of the den-
dritic ligands is limited to being either hydrophilic, or hydro-
phobic, (Figure 1c). Although, Janus dendrimers are well
known nanostructures,50 to the best of our knowledge, there is
no literature report that describes Janus dendrons as NP ligands
where both hydrophilic and hydrophobic moieties are
positioned at the terminal part of the ligand (Figure 1d), and
therefore are in direct contact with the solvent and actively con-
tribute to the hybrid surface polarity.
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Scheme 2. The synthesis of disulfide dendrons 22-26ª
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HO
S
O
R
R
9
2
ii
O
21
HO
S
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O
S
HO
SH
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R
9
2
R
2
OH
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R
10, 11, 15, 16, 19
22 - 26
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O
O
ªReagents and conditions: (i) SOCl2, CH2Cl2, rt, 3h for 10, 11, 19, and oxalyl chloride, 2,6-ditert butyl 4-methyl pyridine, CH2Cl2, rt, 3h for
15 and 16, then 20, Et3N, DMAP, CH2Cl2, -10°Càrt, 12h; (ii) I2, CH2Cl2, rt, 12h.
Of relevance to the Janus dendritic ligands are NPs that have
Janus character obtained by templating two different ligands
onto the surface of the NP.53 This strategy leads to phase sepa-
ration on the NP surface into vast regions of either hydrophobic
and hydrophilic character, where by comparison, a dendritic Ja-
nus molecule will cover the NP surface homogeneously, and the
two distinct moieties will not be able to separate into large do-
mains of different nature.
Herein, we present an efficient synthesis of a library of mono-
disperse dendritic ligands capable of tuning NP surface polarity.
Hydrophobic, hydrophilic, and Janus ligands presented here al-
low simple solution phase ligand exchange and produce uni-
form coverage where the organic shell polarity is determined by
the nature of the corresponding ligand end-group.
Hydrophobicity and hydrophilicity are engineered using
dendritic systems containing dodecyl and triethylene glycol
methyl ether moieties, respectively. The increase in the number
of these groups as a function of the generation provides subse-
quent advancement of polarity, i.e. a higher generation dendron
with polar end-groups becomes more hydrophilic and similarly
a higher generation dendron with nonpolar end-groups becomes
more hydrophobic. The Janus dendritic ligand is designed to
contain covalently linked polar and nonpolar groups positioned
at the periphery of the dendron, which enables modulation of
the polarity in between the systems with opposite polarity, that
can be further tuned through exposure to various solvents where
a solvent dependent memory effect phenomenon is revealed.
Results and Discussion
Ligand Design and Synthesis. To study the influence of den-
dritic ligands, we designed a molecular system where the sur-
face anchoring unit (disulfide-based functional group) is
located at the apex and either or both hydrophobic and hydro-
philic moieties are covalently mounted on the terminal part of
the Frechet type dendron.54 This approach gives efficient access
to the desired dendrons where the hydrophobic, hydrophilic, or
Janus moieties are present at the periphery, allowing them to
directly interact with each other as well as with the surrounding
media, precisely tuning polarity and solubility properties. To
study the effects of the ligand shell polarity as a function of the
dendritic generation, first and second generation hydrophobic
and hydrophilic dendrons were synthesized, and the Janus den-
dron (a second generation dendron) was synthesized containing
a 1:1 molar ratio of hydrophobic/hydrophilic moieties to be in-
termediate to the hydrophobic and hydrophilic ligands. The
synthesis of the targeted ligands was started by reacting readily
available methyl gallate (methyl 3,4,5-trihydroxybenzoate 1)
with dodecyl bromide or triethylene glycol methyl
benzenesulfonate using Williamson etherification under mild
basic conditions (Scheme 1a).55–57 Reduction of the methyl ester
function of 4 and 5 into benzyl alcohols 6 and 7 and subsequent
chlorination using thionyl chloride gave access to the valuable
chlorinated intermediates 8 and 9 that are required to build the
higher generation ligands. The common intermediates 4 and 5
were hydrolyzed to give acids 10 and 11 that are necessary for
building the first generation hydrophobic and hydrophilic
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a)
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200 nm
200 nm
Figure 2. Representative TEM micrographs of self-assembled NP films of (a) as-synthesized Au@Olam, (b) Au@22, (c) Au@23, (d)
Au@24, (e) Au@25, and (f) Au@26 (φAu ≈ 7.6 ±0.5 nm).
dendrons (Scheme 1a). Higher generation dendrons were syn-
thesized in very good yields (ca. 90%) by reacting methyl 3,5-
dihydroxybenzoate, 12, with two equivalents of 8 or 9 by Wil-
liamson etherification, followed by the hydrolysis of the methyl
ester function to give corresponding acids 15 and 16 with non-
polar and polar end-groups, respectively (Scheme 1b). For the
synthesis of the Janus building blocks, the introduction of both
nonpolar and polar end-groups was carried out in a stepwise
fashion to ensure the precise end-group control. Thus, etherifi-
cation of methyl 3,5-dihydroxybenzoate 12 with one equivalent
of nonpolar building block 8 gave access to the intermediary
monofunctionalized benzoate 17, which was further reacted
with one equivalent of polar building block 9 to yield the Janus
ester 18; the latter was hydrolyzed quantitatively to the corre-
sponding Janus acid 19 (Scheme 1c).
Finally, the targeted disulfide dendrons were synthesized by an
esterification reaction between the various dendritic acids 10,
11, 15, 16, 19 and disulphide-bearing molecule 21. The synthe-
sis was carried out in a one-pot two-step process where acid
chlorides were first generated in situ from carboxylic acids and
then reacted with alcohol 21. Scheme 2 shows the synthesis of
dendrons 22-26 while all additional synthetic details are
provided in Supplementary Information. The resulting library is
designed to give the opportunity to study the effects of i) the
polarity of the ligands within identical generation, and of ii) the
dendritic generation on the colloidal stability and self-assembly
of the corresponding dendronized Au hybrids.
Ligand Exchange with Gold NPs and Self-Assembly. To
explore how various dendritic ligands affect a variety of
properties of NPs, each dendrimer was grafted onto Au NPs,
with diameters φAu ≈ 5.2 ± 0.5, 7.6 ± 0.5 and 10.2 ± 0.6 nm,
respectively, using a solution phase ligand-exchange procedure
which is known to afford uniform dendritic surface coverage26
(final NP-dendron hybrid materials will therefater be denoted
as Au@dendron). A solution of oleylamine (Olam) stabilized
as-synthesized Au NPs (Au@Olam) were mixed with an excess
of the dendritic ligand (22-26) and stirred at room temperature
to allow the exchange of the Olam ligands with the disulfide
dendrons. After 1 h, the hybrids were precipitated out using an
antisolvent (methanol for materials with ligands 22, 23, and 24,
and toluene for those with ligands 25 and 26) and the precipitate
was collected by centrifugation. This step was repeated 3 times
to ensure the complete removal of any excess unbound ligands
(dendrons and Olam).
During surface ligand exchange process, the dendritic ligands
were introduced into the organic shell without affecting the size
of the inorganic core of the material. TEM analysis of the NP
size distribution before (Au@Olam) and after ligand exchange
(Au@dendron) shows no difference in average diameter of the
core (Figure S1d), also in agreement with UV-vis measure-
ments (Figure S1b). The ligand exchange process was moni-
tored a posteriori by measuring the interparticle distance of re-
sulting hybrids in self-assembled monolayers (TEM). A pro-
longed reaction time did not yield any variation in the spacing,
leading us to believe that the NP surface had reached saturation
with the dendrons. This exchange was additionally confirmed
with Nuclear Magnetic Resonance (NMR) spectroscopy analy-
sis of Au NPs before and after the ligand exchange, as shown in
Figure S2. The disappearance of the oleylamine olefinic hydro-
gen signals at 5.3-5.4 ppm and appearance of dendron signals
at 4, 5, and 6.6 ppm clearly indicate the successful exchange.
Thermogravimetric analysis was performed on a representative
sample of the hybrid material (Au@22, φAu ≈ 5.2 ± 0.5) to de-
termine the grafting density of ligands per NP surface unit (Fig-
ure S3), which was found to be in good agreement with previous
studies using similarly sized Au NPs.26
The end-group nature and the generation of the ligands are im-
portant parameters controlling the surface polarity and colloidal
properties of the final Au@dendron hybrid materials. Ligands
with the hydrophobic end-groups 22 and 23 are expected to
have lower affinity for polar solvents compared to their PEG
based analogs 25 and 26, while the Janus ligand 24 is expected
to lie in-between. The change of surface polarity was studied
using contact angle measurements of self-assembled superlat-
tice films and solubility studies.
The Au@dendron hybrids, Au@22, Au@23, and Au@24 were
induced to self-assemble into thin films on liquid-air interface,
following a previously reported procedure.58 A diluted solution
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of Au@dendron hybrids in hexanes was deposited on a polar ligand shell. This demonstrates the precise tuning of the NP sur-
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subphase of diethylene glycol (DEG) and allowed to evaporate
slowly, resulting in floating NP films. This method only works
for hybrids whose ligand shell is dominated by nonpolar char-
acter and therefore they dissolve in hexane layer and remain on
the interface after solvent evaporation. Under these conditions,
polar hybrids such as Au@25 and Au@26, have revealed higher
affinity for the subphase and failed to remain on the interface.
Therefore, the self-assembled monolayers of these polar sys-
tems were produced on a solid-air interface, where colloidal so-
lutions in chloroform were dropcast on TEM grids and dried
slowly in a partially enclosed chamber. In all cases, the thick-
ness of the self-assembled film was controlled by the concen-
tration of NPs and the amount of the initial solution. This set of
ligands allow NPs to self-assemble into highly ordered colloidal
crystalline films. Figure 2 shows TEM images of ordered mon-
olayers obtained from Au@Olam 7.6 ± 0.5 nm NPs, as well as
films obtained from same batch of NPs coated with dendritic
ligands 22-26.
face polarity and wettability by adapting the chemical nature of
the ligand end-groups and dendritic generation.
Interestingly, the contact angle measurements done after expo-
sure to various solvents revealed a surface polarity memory ef-
fect that is present only in case of the hybrid material containing
the Janus ligand. As shown in Figure 3, the hybrid materials
with either hydrophobic or hydrophilic character, Au@23 and
Au@26, show no changes in the contact angle after exposure to
solvents with very different polarity such as MeOH and hex-
anes. Conversely, Au@24 exhibits hysteresis, where the polar-
ity of the surface can be controlled by exposing the film to either
one of these solvents, as seen by the contact angle changing
from 76 ± 3° to 60 ± 3°, respectively, shown in Figure 3 and
Table 1. The original contact angle of 65 ± 2° is consistent with
our findings, as the Janus material in that case was exposed to
CHCl3, which has an intermediate polarity compared to hexanes
and MeOH. This switchable wettability property is due to the
presence of both a hydrophilic and hydrophobic arm on the den-
dron, where the presence of a polar solvent will change the
structural conformation of the material, possibly by backfolding
the nonpolar dodecyl based arms, allowing the surface to be
dominated by the polar triethylene glycol arms similar to the
phenomenon observed previously on V-shaped polymers.40,41
The opposite of this is true when nonpolar solvent such as hex-
anes is used. This unique property makes the Au@24 hybrid
material a model system for controlled surface wetting. While
this hybrid material was tested as a smooth surface, we expect
that this property would be amplified if the material was depos-
ited onto a rough surface.
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c)
b)
e)
h)
a)
d)
g)
f)
i)
a)
b)
c)
Hexanes
Exposure
CHCl3
MeOH
Exposure
hexanes
water
Exposure
Figure 3. Contact angle measurements: images showing water
droplets cast onto films of (a, b, and c) Au@23, (d, e, and f)
Au@26, (g, h, and i) Au@24 (φAu ≈ 7.6±0.5 nm) after treatment
with hexanes (a, d, g), CHCl3 (b, e, and h), or MeOH (c, f, and i).
d)
e)
f)
The wettability of the hybrids was evaluated by contact angle
measurements, using the static sessile drop method, performed
with a water droplet cast on Au@dendron films, deposited on a
glass substrate. These films were multiple layers thick, as they
were also used for SAXS and UV-vis studies, negating any sub-
strate surface effects. Hydrophobic hybrids, Au@22 and
Au@23, show large contact angles (79 ± 5° and 83 ± 6°, respec-
tively, Figure 3, S4b, Table 1), whereas for their hydrophilic
analogs, Au@25 and Au@26, much smaller angles (57 ± 3° and
50 ± 5°) were observed. This is in good agreement with what
should be expected, as high surface polarity should result in in-
creased wettability and therefore evidenced by the spreading of
the water droplet on the films, and thus, with smaller contact
angles. The Janus dendron based-hybrid system, Au@24,
shows a contact angle of 65 ± 2° that is intermediate compared
to the polar and nonpolar systems, reflecting the 1:1 molar ratio
of hydrophobic and hydrophilic moieties present at the periph-
ery of the dendritic molecule, and therefore the homogeneous
distribution of both polar and nonpolar segments within the NP
water
chloroform
Figure 4. Dispersibility of Au@dendron hybrids in (top row) hex-
ane/water, and (bottom row) water/chloroform mixtures: (a and d)
Au@22, (b and e) Au@24, (c and f) Au@25 (φAu ≈ 7.6 ±0.5 nm).
Ligand polarity is also reflected in the colloidal properties of
the hybrid systems, as seen in Figure 4 and Table S1. Shells of
nonpolar 1st and 2nd generation ligands 22 and 23 behave simi-
larly, and allow dispersibility of the corresponding hydrophobic
hybrids in nonpolar organic solvents such as hexanes and tolu-
ene. Conversely, ethylene glycol based end-groups introduce a
polar shell, and thus the corresponding hybrid systems gain dis-
persibility in polar solvents such as methanol and water. Inter-
estingly, all ligands synthesized here have affinity towards po-
lar aprotic solvents with medium dielectric constants like THF
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Table 1. Structural, plasmonic, and polarity properties of the Au@dendron hybrids.
1
Edge-to-edge separation (nm) measured on
different size NPs
2
3
λmax (nm)
Contact Angle (º), measured after
exposure to different solvents*
(films)*
4
5
6
Hexanes
CHCl3
MeOH
5.2 ± 0.5 nm
7.6 ± 0.5 nm
10.2 ± 0.6 nm
NPs
NPs
NPs
7
8
9
Au@Olam
Au@24
3.3**
4.7
2.4
4.4
1.3
3.9
596
550
-
-
71 ± 2
65 ± 2
76 ± 3
60 ± 3
10
11
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22
23
24
25
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Au@22
Au@23
Au@25
Au@26
4.3
5.4
-
3.5
5.6
-
2.5
5.2
-
547
544
-
-
-
79 ± 5
83 ± 6
57 ± 3
50 ± 5
79 ± 4
80 ± 2
-
-
-
5.1
-
557
48 ± 1
48 ± 2
*Measurements taken using samples φAu ≈ 7.6±0.5 nm, **Measurement taken from as-synthesized NPs 5.6 ± 0.5 nm in size.
a)
b)
c)
d)
e)
f)
100 nm
g)
100 nm
h)
100 nm
i)
100 nm
k)
100 nm
l)
100 nm
j)
100 nm
100 nm
100 nm
m)
200 nm
n)
200 nm
100 nm
100 nm
o)
200 nm
400 nm
400 nm
Figure 5. Superlattice monolayers of various dendron coated NPs: Au@22 a) 5.2 ± 0.5 nm, b) 7.6 ± 0.5 nm, and c) 10.2 ± 0.6 nm Au NPs;
Au@23 d) 5.2 ± 0.5 nm, e) 7.6 ± 0.5 nm, and f) 10.2 ± 0.6 nm Au NPs; Janus Au@24 g) 5.2 ± 0.5 nm, h) 7.6 ± 0.5 nm, and i) 10.2 ± 0.6
nm Au NPs and Au@Olam j) 5.6 ± 0.5 nm, k) 7.6 ± 0.5 nm, and l) 10.2 ± 0.6 nm Au NPs. A low m) and high resolution n) hexagonal dou-
ble layer and o) fcc assembly of Au@23 coated 10.2 ± 0.6 nm Au NPs.
and EtOAc, and all hybrid systems tested here including
Au@Olam are dispersible in CHCl3 (Table S1). When the hy-
brids are suspended on an interface of the immiscible solvent
combination of water and chloroform, they do not always prefer
to be suspended into the solvent of matching polarity. For ex-
ample, hybrids with polar ligands, Au@25 and Au@26, are
readily water soluble, but remain in chloroform when dispersed
in water/chloroform mixture, an example of which is shown in
Figure 4. This can be attributed to the presence of triethylene
glycol methyl ether units that appear to be sufficient for wa-
ter solubility, but the presence of nonpolar parts such as linear
alkyl (undecyl) chain and benzene ring would have undoubt-
edly increased the overall lipophilicity of the system to support
its higher affinity towards chloroform. This can be especially
important for applications where liquid-liquid extraction is a
purification or an isolation step.
Dendronized NPs were found to assemble into close-packed
colloidal crystalline film structures where the interparticle spac-
ings are controlled by the generation of the ligand, as previously
demonstrated,26,59 but also by the amphiphilic nature of the end-
groups, as well as the size of the inorganic core. To explore this
phenomenon further, we studied every possible combination
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Chemistry of Materials
to the different ligand architecture reported here, a direct com-
made of three different sizes (φAu ~ 5.2 ± 0.5 nm, 7.6 ± 0.5 nm
1
parison to the literature is difficult.
and 10.2 ± 0.6 nm) of Au NPs with three different ligands 22-
24, as well as with Olam as reference compounds. Information
about interparticle spacings was derived from TEM images and
further confirmed by small-angle selected area electron diffrac-
tion (SAED) patterns of well-ordered superlattices assembled
on liquid-air interface (Figure 5a-l and S5). From the small-an-
gle SAED patterns (Figure S5), the distance from the center of
an outer spot to the center of the opposite outer spot was
measured, and divided by 2 to provide the radius, r, and this
distance was converted from pixels to nanometers. Then, using
2
3
4
5
6
7
8
9
The crystalline nature and the modulation of interparticle spac-
ing of the self-assembled films in selected examples were fur-
ther confirmed by the small-angle X-ray scattering (SAXS) and
UV-Vis absorption, shown in Figure S1. In SAXS (Figure S1c),
performed on films made from dendron coated 7.6 ± 0.5 nm
NPs, the maximum of the first reflection peak of the NP-
dendron hybrids shifted from q = 0.0705 Å-1 for the as-synthe-
a)
b)
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23
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60
the relationship ꢀ = ꢁꢃꢂ, where λ is the electron wavelength and
L is the distance of the sample from the screen, the interplanar
distance, d, was calculated and converted into interparticle dis-
tance.
200 nm
c)
200 nm
d)
Figure 5a-l shows the superlattice films of all 12 combinations.
The resulting interparticle separations are reported in Table 1.
For every hybrid tested, the edge-to-edge separation increases
as expected with the ligand size, but this separation is the largest
when they are placed on the smallest NPs (5.2 ± 0.5 nm) and
the smallest when they are placed on the largest NPs (10.2 ± 0.6
nm). This trend is clearly visible when plotting edge-to-edge
separations against ligand molecular weights (Figure 6a) for the
3 NPs diameters. Additionally, the larger ligand (23) seems to
counterbalance this tendency more effectively than their
smaller analogs and appears less sensitive to NP diameter vari-
ations as seen when plotting edge-to-edge separations against
NP core diameters (Figure 6b). This seems to indicate that dur-
ing superlattice films formation, the soft and deformable ligand
shell is slightly compressed by interparticle attractive forces
(Au cores are incompressible), which are stronger for larger
particles.60,61 The softness of the ligand shell is dependent on
the coverage density which varies as a function of NP diameter
(number of ligands per nm2 of the NP surface). Due to the larger
curvature (c = 2/φAu) of the small NPs, larger free volume is
generated, allowing ligands to accommodate more easily at the
NP surface, yielding thus a more efficient ligand packing. Hy-
brids with ligands 22 and 24 follow this trend as well, but the
slope is less pronounced as for Olam, confirming a slight coars-
ening of the ligand shell upon ligand size increase.
100 nm
e)
200 nm
f)
100 nm
200 nm
Figure 7. Various Moiré patterns in dendron 23 coated 10.2 ± 0.6
nm (a-c and e) and 5.2 ± 0.5 nm (d and f) Au NPs.
-sized Au@Olam to q = 0.0585 Å-1 for Au@24, indicating an
overall increase in separation from 1.7 nm to 3.7 nm, respec-
tively. The slight discrepancies with the values reported in Ta-
ble 1 are possibly due to the thickness of the NP films, as it has
been previously reported that an increase in film thickness can
result in a change in the superlattice structure, which requires
further analysis outside the scope of this study.63 All the sam-
ples, except for Au@25, showed an additional diffuse reflec-
tion, and together, these two diffraction signals correspond to
the reflections of a hexagonal lattice. In the case of Au@25,
self-assembly was not observed in TEM, which is consistent
with the absence of any reflection peaks from SAXS.
In solution phase UV-Vis measurements (Figure S1b), all the
NPs with the various ligands on their surface have the same
maximum absorption wavelength (λmax = 540 nm), which indi-
cates that the ligand exchange does not alter the size and shape
uniformity of the NPs. This is in good agreement with the size
distributions of the same NP samples measured from their TEM
images (Figure S1d). When the samples are assembled into thin
films, the plasmon resonance shows a red shift from the solution
spectra. The shift is maximum for as-synthesized particles and
decreases as a function of increasing interparticle separation
reaching a minimum value in case of the largest ligand 23 (Ta-
ble 1). The observed trend in this shift is caused by changing
the average dielectric constant of the surroundings of the NPs,
and the distance dependent interparticle coupling which is
consistent to the trend observed previously.26
a)
b)
Figure 6. Edge-to-edge separation dependence on a) ligand mo-
lecular weight, and b) NP core size, where dotted lines represent
calculations from SAED patterns.
Small-angle SAED of ligands 22 and 23 (Figure S5), the largest
and smallest synthesized ligands, was employed to confirm this
trend and the results are included in Figure 6b. This observation
indicates that large bulky ligands counterbalance the interparti-
cle attraction forces more than smaller ligands, and is in good
agreement with earlier work where the ligand linear contraction
and deformation are found to be the smallest for higher genera-
tion dendrons (reinforcement of the rigidity).62 Although, due
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Page 8 of 11
Additionally, 7.6 ± 0.5 nm Au NPs with 22 on the surface were are monodisperse in nature that rules out any ligand size related
1
2
3
4
5
6
7
8
mixed with Au@23, as well as Au@24, to obtain binary mix-
tures, which are shown in Figure S7. These mixtures also form
superlattice films, where the edge-to-edge spacings of 4.4 nm
for Au@22 mixed with Au@23, and 3.7 for Au@22 mixed with
Au@24, are intermediate compared to the single component su-
perlattices.
During liquid-air interface self-assembly, the film thickness is
controlled by the amount and the concentration of NP contain-
ing solution. While interparticle spacings are measured on sin-
gle layered areas, well-ordered multilayers can also be obtained.
In Figures 5m,n we show TEM images of low and high resolu-
tion of hexagonal close-packed hcp bilayer superlattices which,
together with single layered domains, dominate the entire sam-
ple. However, in some rare cases, other structures such as fcc
(Figure 5o) can also be found.
discrepancies when conducting in vitro or in vivo experiments.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website. Experimental details, full characterization of
organic compounds, additional microscopy, and supporting spec-
troscopy can be found in the Supporting Information.
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AUTHOR INFORMATION
Corresponding Author
Further TEM analysis of Au@dendron hybrid films reveal a
range of different Moiré patterns that arise from misalignment
of ordered layers.64 Figures 7a-f shows TEM images of bilayer
or trilayer films where at least one layer is rotated by a certain
degree in relation to another. Thus, repeating hexagonal motifs
are revealed, the size of which has an inverse relationship with
the misalignment degree.65 Figures 7a,b show two ordered lay-
ers of dendron 23 coated 10.2 ± 0.6 nm Au NPs offset by ~5°
and ~9° resulting in hexagonal motifs that have a center to cen-
ter distance of 160 and 80 nm. Larger rotation angles cause the
motifs to decrease in size and at ~30° it starts to resemble 12-
fold order found in quasicrystals (Figures 7c,d and Figure
S6).60,65–67 Three layered misaligned structures are more com-
plex, as seen with examples of dendron 23 coated 10.2 ± 0.6 nm
(Figure 7e) and dendron 23 coated 5.2 ± 0.5 nm Au NPs (Figure
7f), where both are thought to arise from ~20° rotation of one
layer with respect to hcp bilayer. In some cases, the variety of
Moiré patterns could be found in a close proximity to each other
allowing them to be imaged in a single TEM image (Figure S8).
Interestingly, these well-defined patterns were more prominent
in samples where largest dendritic ligand 23 was used however,
more research is needed to identify the ligand size dependence
on layer to layer misalignment.
* cbmurray@sas.upenn.edu
Notes
The authors declare the following competing financial interest:
Aspects of this work have been included in a U.S. provisional pa-
tent filing.
ACKNOWLEDGMENT
We acknowledge Emmabeth Parrish and Professor Russell J. Com-
posto for assistance with contact angle measurements and helpful
discussions. This work was primarily supported by the CNRS-
UPENN-SOLVAY through the Complex Assemblies of Soft Mat-
ter Laboratory (COMPASS), in partnership with the University of
Pennsylvania’s NSF MRSEC under Award No. DMR-112090. We
acknowledge secondary support from French National Agency of
Research (ANR) to the project REACT through the grant ANR15-
PIRE-0001-06. C.B.M. acknowledges the Richard Perry University
Professorship at the University of Pennsylvania. D.J. acknowledges
Dr. Mirna El Khatib for helpful discussions.
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