Programmed assembly of binary nanostructures in solution

Article abstract of DOI:10.1021/jp014069k

Near size-monodisperse silver nanocrystals and silica nanospheres have been prepared and surface-modified with the [2]pseudorotaxane recognition motifs, dibenzo[24]crown-8 and dibenzylammonium cation, respectively. The silver nanocrystals, 7 nm in diameter, are shown to recognize and bind to the silica nanospheres, 180 nm in diameter, via pseudorotaxane formation. The result is the templated assembly of silver nanocrystals at the surface of the larger silica nanosphere core. This composite material exhibits optical properties that are a consequence of localizing silver nanocrystals at the surface of the silica nanospheres. The templating process is also shown to be reversible. This noncovalent recognition-directed templating strategy represents a novel metallization process resulting in the formation of composite media whose collective properties can be controlled.

Full text of DOI:10.1021/jp014069k

J. Phys. Chem. B 2002, 106, 5371-5377
5371
Programmed Assembly of Binary Nanostructures in Solution
Declan Ryan, Lorraine Nagle, Håkan Rensmo,† and Donald Fitzmaurice*
Department of Chemistry, National UniVersity of Ireland - Dublin, Belfield, Dublin 4, Ireland
ReceiVed: NoVember 7, 2001; In Final Form: February 7, 2002
Near size-monodisperse silver nanocrystals and silica nanospheres have been prepared and surface-modified
with the [2]pseudorotaxane recognition motifs, dibenzo[24]crown-8 and dibenzylammonium cation, respectively.
The silver nanocrystals, 7 nm in diameter, are shown to recognize and bind to the silica nanospheres, 180 nm
in diameter, via pseudorotaxane formation. The result is the templated assembly of silver nanocrystals at the
surface of the larger silica nanosphere core. This composite material exhibits optical properties that are a
consequence of localizing silver nanocrystals at the surface of the silica nanospheres. The templating process
is also shown to be reversible. This noncovalent recognition-directed templating strategy represents a novel
metallization process resulting in the formation of composite media whose collective properties can be
controlled.
Introduction
is specific in nature, driving a templating process has not been
previously reported.
Motivated by the prediction that existing microfabrication
techniques will reach a lower size limit soon, new approaches
to arranging conducting, semiconducting and nonconducting
materials on the nanoscale are of growing interest.1,2 Recently,
size-monodisperse nanocrystals of a wide range of materials
have been prepared and their size-dependent electronic and
magnetic properties studied.3,4 However, it is the collective
electronic and magnetic properties of organized assemblies of
size-monodisperse nanocrystals that are increasingly the subjects
of investigation.5 Although many studies deal with assemblies
of single nanocrystals few studies focus on assemblies of two
(binary) or more types of nanocrystals.6-9
Recently, we have reported the preparation and characteriza-
tion of a dispersion of gold nanocrystals possessing a narrow
size distribution and stabilized by a chemisorbed monolayer of
a dodecane thiol derivative covalently linked to dibenzo[24]-
crown-8 [10]. These nanocrystals were shown to recognize, and
bind selectively in solution, a dibenzylammonium cation,
forming a [2]pseudorotaxane coated nanocrystal.
Most recently, we have demonstrated the recognition-directed
assembly of silver nanocrystals in solution via pseudorotaxane
formation.11 Specifically, a thiol modified dibenzo[24]crown-8
recognition motif was covalently linked to the surface of 7 nm
diameter silver nanocrystals. Upon addition of bis-dibenzylam-
monium dication, aggregation of nanocrystals was observed.
We further demonstrated inhibition of the aggregation phenom-
enon by addition of either an excess of dibenzo[24]crown-8 or
dibenzylammonium cation recognition motifs.
Unique to this system are the molecular components that drive
the assembly process. The recognition and selective binding of
dibenzo[24]crown-8 and dibenzylammonium salt to form a [2]-
pseudorotaxane is a well-developed supramolecular system
(Scheme 1).12,13 Accordingly, we covalently link dibenzo[24]-
crown-8 to the surface of silver nanocrystals using thiol surface
groups and a precursor of dibenzylammonium cation to the
surface of silica nanospheres using a silane coupling agent. The
dibenzylammonium cation is subsequently generated in situ by
photolysis of the precursor. These recognition motifs were
expected to interact to form a [2]pseudorotaxane between the
silica and silver surfaces resulting in the formation of a silica/
silver composite material (Scheme 2). Hence, the recognition-
directed or programmed assembly of a binary nanostructure
ought to be possible.
Experimental Section
Non-Photoactive Stabilizers. All compounds were character-
1
1
ized using elemental analysis and H NMR spectroscopy. H
NMR spectra were recorded using either Varian 300 MHz FT-
NMR or Varian 500 MHz FT-NMR spectrometers with either
the solvent reference or TMS as the internal standard. All
chemical shifts are quoted on the δ scale. All coupling constants
1
are expressed in Hertz. Dispersions and solutions used in H
NMR spectroscopic studies were prepared from dry samples
of the desired nanocrystal or compound by addition of the
appropriate deuterated solvent.
TEMs were obtained using a JEOL 2000 FX TEMscan (at
an acceleration voltage of 80 kV) for samples deposited on
carbon-coated copper grids. The preparation of samples for TEM
involved deposition of a few drops of a dispersion onto a carbon-
coated copper grid. UV-vis spectroscopy was performed using
a HP-8452A Spectrophotometer using LabView software written
to acquire data. All UV-vis spectra were referenced using
appropriate solution backgrounds.
In this paper, we describe, for the first time, the formation of
a binary nanostructure in solution consisting of a silica nano-
sphere that behaves as a core for the noncovalent recognition-
directed templated assembly of a silver nanocrystal shell. Recent
work has outlined the use of non-specific electrostatic effects
to assemble multilayers of nanoparticles onto polystyrene latex
templates. The use of a molecular recognition process, which
Chemicals including dodecane thiol, 1, were purchased from
Aldrich and used without further purification. Dodecane thiol-
modifed dibenzo[24]crown-8, 2, was prepared as described
* To whom correspondence should be addressed.
† Department of Physics, University of Uppsala, Box 530, 751 21
Uppsala, Sweden.
10.1021/jp014069k CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/08/2002

5372 J. Phys. Chem. B, Vol. 106, No. 21, 2002
Ryan et al.
SCHEME 1: [2]Pseudorotaxane Formation in this System is Devolved from a Recognition and Selective Binding of
Dibenzo[24]crown-8 and Dibenzylammonium Salt. This Recognition Process is the Basis of the Templated Assembly of
Silver Nanocrystals and Silica Nanospheres Discussed in this Paper.
SCHEME 2: Strategy for Assembling Templated Silver Nanocrystals on Silica Nanospheres. Each Silver Nanocrystal has
a Population of Dibenzo[24]crown-8 Recognition Motifs Covalently Linked to its Surface and Each Silica Nanosphere has
a Near-Monolayer Coverage of Dibenzylammonium Recognition Motifs at its Surface. These Recognition Motifs Interact
to Form a [2]Pseduorotaxane which Results in the Assembly of Silver Nanocrystals around the Silica Nanosphere.
previously.10 Anhydrous solvents, where used, were purchased
from Aldrich and used within 1 week.
evaporated under reduced pressure to yield 1.02 g (100%) of
4-carbomethoxybenzylidenebenzylamine as a pale yellow oil
[1H NMR (CDCl3): δ 3.83 (s, 3H), δ 4.80 (s, 2H), δ 7.33-
7.37 (m, 5H), δ 7.70-7.74 (m, 2H), δ 7.93-7.96 (d, 2H), δ
8.34 (s, 1H)] which was dissolved in a THF/MeOH (1:1, 100
mL) solution. NaBH4 (0.15 g, 4 mmol) was added with stirring
at room temperature. A further portion of NaBH4 (0.15 g, 4
mmol) was added to the reaction mixture after an additional 2
h. The solution was stirred subsequently for 20 h, before being
evaporated under reduced pressure. The residue was partitioned
between H2O (100 mL) and EtOAc (100 mL). The aqueous
phase was then further extracted with EtOAc (2 × 100 mL).
The combined organic extracts were dried (MgSO4), filtered,
and concentrated to afford 0.96 g (94%) of 4 as a pale white
oil.
1: 1H NMR (CDCl3): δ 0.88 (t, 3H, J ) 7.0 Hz); δ 1.22 (m,
16H); δ 1.36 (m, 2H); δ 1.61 (m, 2H); δ 2.52 (q, 2H, J ) 7.0
Hz, J ) 3.5 Hz); C12H26S (202.07): calcd C 71.21, H 12.95, S
15.89; found C 71.19, H 13.12, S 15.55.
1
2: H NMR (CDCl3): δ 1.27 (m, 12H); δ 1.39 (m, 2H); δ
1.61 (m, 2H); δ 2.32 (t, 2H, J ) 7.0 Hz); δ 2.52 (q, 2H, J )
7.0 Hz, J ) 3.5 Hz); δ 3.82-3.85 (m, 8H); δ 3.89-3.97 (m,
8H); δ 4.12-4.24 (m, 8H); δ 5.04 (s, 2H); δ 6.85-6.95 (m,
7H); C36H54O10S (678.87): calcd C 64.25, H 7.96; found C
63.8, H 7.86.
Photoactive Molecular Stabilizer Precursors. The photo-
active molecular component was prepared according to Scheme
3. All reactions involving NVOC were carried out in flasks
protected from external light sources.
4: 4-carbomethoxydibenzylamine. Methyl aminomethyl-
benzoate hydrochloride, 3 (1.12 g, 6.0 mmol), was transformed
into the free base by washing with 5% Na2CO3 aqueous solution
(200 mL) followed by extraction using CHCl3 (3 × 100 mL).
The combined organic extracts were dried (MgSO4), filtered,
and concentrated to afford 0.60 g (67%) of methyl aminom-
ethylbenzoate. A solution of methyl aminomethylbenzoate (0.60
g, 4 mmol) and benzaldehyde (0.42 g, 4 mmol) in PhMe was
heated under reflux for 3 h, whereas the H2O discharged was
isolated with a Dean-Stark separator. The solution was
4: 1H NMR (CDCl3): δ 3.84 (s, 3H), δ 3.89 (s, 2H), δ 3.94
(s, 2H), δ 7.24-7.32 (m, 2H), δ 7.35-7.37 (m, 3H), δ 7.43-
7.49 (m, 2H), δ 8.00-8.10 (d, 2H).
5: N-(4-carbomethoxydibenzylamine)carbamate NVOC.
6-nitroveratryloxy chloroformate (NVOC) (1.06 g, 4 mmol) was
added with stirring to a solution of 4 (0.96 g, 3.8 mmol) and
NaHCO3 (0.34 g, 4 mmol) in CHCl3/Et2O (9:1, 100 mL) at 5
°C. The solution was stirred for 2 h, before being treated with
saturated aqueous NaHCO3 (1 × 100 mL) and H2O (2 × 100
mL). The organic phase was dried (MgSO4), filtered, and
concentrated to afford the crude title compound. The crude

Programmed Assembly of Binary Nanostructures
J. Phys. Chem. B, Vol. 106, No. 21, 2002 5373
SCHEME 3: Synthetic Strategy used to Obtain 6-NVOC.
Reagents and Conditions: A. (1) Na2CO3(aq) (2)
Benzaldehyde, Toluene, 140 °C, 3h (3) NaBH4, MeOH/
THF, RT, 20h. B. NVOC, NaHCO3, CHCl3/Et2O, 5 °C, 2
h. C. 3M NaOH(aq), MeOH/THF, RT, 3 h.
prepared aminopropyl functionalized silica nanospheres using
3-aminopropyldimethylethoxysilane as a silane-coupling agent.
Solid-phase organic synthetic methods were used to couple the
photoactive molecular component, 6-NVOC, to the amine-
modified nanospheres.19 Specifically, to a dispersion of amine-
modified silica (2.42 × 1015 nanospheres dm-3, 7 × 10-4 mol
NH2(surface) dm-3) in anhydrous CH2Cl2 was added 6-NVOC
(0.07 g, 1.4 × 10-4 mol) and 1-ethyl-3-(3-dimethylamino-
propyl)carbodiimide (EDAC) (0.03 g, 1.4 × 10-4 mol) and the
mixture was stirred for 24 h at ambient temperature. The
dispersion was then centrifuged (3000 rpm, 5 min; Sorvall
Instruments RT6000B using A500 rotor) and sonicated (Ultra-
wave, 15 min) consecutively from alternating MeOH (50 mL)
and CHCl3 (50 mL) washing solvents five times.
An experiment has also been performed whereby amine-
modified silica nanospheres and 6-NVOC have been mixed in
precisely the same manner as described in the manuscript but
in the absence of any coupling agent namely, EDAC. Subse-
quent centrifugation cycles and analysis by UV-vis spectros-
copy was performed to determine the absence or otherwise of
6-NVOC at the surface of the silica nanonspheres.
These modified nanospheres, referred to as SiO2-(6-NVOC)
were characterized by transmission electron microscopy (TEM)
and UV-vis spectroscopy.
Transmission electron microscopy (TEM) was used to obtain
information relating to the size of both silver nanocrystals and
silica nanospheres. UV-vis spectroscopy was used to confirm
the presence of 6-NVOC at the surface of silica nanospheres.
Photoactivation of Modified Silica Nanospheres. The 355
nm line of a Continuum Surelite Nd:YAG laser was used as
the light source in all photoactivation experiments. The output
from the laser was measured to be 170 mW/cm2. Briefly, 2 mL
of a chloroformic dispersion of SiO2-(6-NVOC) (2.42 × 1015
nanospheres dm-3) was placed in a glass cuvette and stirred
for a period of 1 h at room temperature while being irradiated.
The photoactivated nanospheres were then centrifuged (3000
rpm, 3 min) and washed using chloroform (20 mL) five times.
These photoactivated nanospheres, referred to as SiO2-6, were
characterized using UV-vis spectroscopy and TEM.
The dibenzylamine-modified nanospheres were converted to
their dibenzylammonium salts by addition of hexafluorophos-
phoric acid to a 2 mL methanolic dispersion of SiO2-6 (2.42 ×
1015 nanospheres dm-3) until a pH of 1 was obtained. The
dispersion was then stirred for 2 h, after which time the
nanospheres were centrifuged (3000 rpm, 3 min) and washed
using methanol (20 mL) five times followed by a further two
washings in chloroform (20 mL). After the final washing the
nanospheres were redispersed in chloroform. These dibenzyl-
ammonium-modified spheres are referred to as SiO2-6+.
Programmed Assembly of Binary Nanostructures in
Solution. Using particle size data obtained from TEM, an
estimation of the number of silver nanocrystals (approximated
as discs) required to coat a single silica nanosphere (ap-
proximated as spheres) was made. On the basis of this
estimation, a ratio of 2640 silver nanocrystals/silica nanosphere
was used in all experiments as representative of complete
coverage of silica. A typical experiment resulting in the
formation of binary nanostructures involved the addition of 100
µL of a chloroformic dispersion of Ag-2 (1.11 × 1018 nano-
crystals dm-3) to a 300 µL chloroformic dispersion of SiO2-
6+ (1.38 × 1014 nanospheres dm-3). Samples for TEM were
prepared after 24-hour equilibration. Relevant control experi-
ments were also performed: these involved mixing similar ratios
of silver nanocrystals and silica nanospheres where either one
product was chromatographed (SiO2: EtOAc/hexane, 50:50) to
yield 1.74 g (92%) of 5 as a yellow oil.
5: H NMR (CDCl3): δ 3.69 (br s, 3H), δ 3.94 (s, 3H), δ
3.97 (s, 3H), δ 4.56-4.59 (br d, 4H), δ 5.68 (br s, 2H), δ 6.87-
6.90 (br d, 1H), δ 7.21-7.39 (br m, 7H), δ 7.72 (s, 1H), δ
7.98-8.05 (d, 2H).
1
6-NVOC: N-(4-carboxydibenzylamine)carbamate NVOC.
50 mL of a 6M NaOH aqueous solution was added dropwise
to a stirred solution of 5 (1.74 g, 3.5 mmol) in THF/MeOH
(1:1, 50 mL) at ambient temperature for 3 h. The resulting
solution was evaporated under reduced pressure and the residue
was collected and partitioned between 2M HCl (100 mL) and
EtOAc (100 mL). The aqueous phase was further extracted with
EtOAc (2 × 100 mL) and the combined organic extracts were
dried (MgSO4), filtered, and concentrated. Recrystallization from
EtOH yielded 1.41 g (84%) of 6-NVOC as a fine yellow
powder.
1
6-NVOC: H NMR (CDCl3): δ 3.69 (br s, 3H), δ 3.97 (s,
3H), δ 4.56-4.61 (br d, 4H), δ 5.69 (br s, 2H), δ 6.88-6.91
(br d, 1H), δ 7.21-7.39 (br m, 7H), δ 7.72 (s, 1H), δ 7.98-
8.05 (d, 2H); C25H24N2O8 (480.47): calcd C 62.49, H 5.03, N
5.83; found C 61.82, H 4.82, N 5.74.
Preparation of Modified Silver Nanocrystals and Silica
Nanospheres. Silver nanocrystals were prepared using a modi-
fied two-phase synthesis procedure originally introduced by
Brust et al.2,14 These nanocrystals were size-selected to isolate
near size-monodisperse fractions, which were subsequently
modified with a long-chain thiol incorporating a dibenzo[24]-
crown-8 recognition motif using previously published methods.11
These silver nanocrystals are denoted as Ag-2.
Silica nanospheres, 180 nm in diameter, were synthesized
according to methods originally developed by Sto¨ber et al.15
and subsequently modified by van Blaaderen and Vrij.16
Modification of silica surfaces using alkoxysilanes is a well-
established method for preparing hydrophobic silica. Here, we

5374 J. Phys. Chem. B, Vol. 106, No. 21, 2002
Ryan et al.
Figure 1. (a) Size monodisperse silver nanocrystals, Ag-2, where the stabilizing ligands constitute dodecane thiol incorporating a dibenzo[24]-
crown-8 motif (15%) and dodecane thiol (85%). The nanocrystals crystallize to hexagonally close-packed arrays and have a polydispersity number
of 1.09. (b) Size monodisperse silica nanospheres, SiO2-(6-NVOC). These nanospheres are readily dispersed in organic solvents and have a
polydispersity number of 1.08.
or both recognition motifs were absent. To support the assertion
that a2pseudorotaxane is being formed at the interface between
silver nanocrystals and silica nanospheres, an excess of dibenzo24-
crown-8 (0.02 g, 1.12 × 10-4 mol) was added to a 300 µL
chloroformic dispersion of SiO2-6+ (1.38 × 1014 nanospheres
dm-3, approximately 1.12 × 10-8 mol 6+) thereby saturating
all dibenzylammonium binding sites at the surface of the
nanospheres. Within minutes, 100 µL of a chloroformic disper-
sion of Ag-2 (1.11 × 1018 nanocrystals dm-3) was added to
the sample. These conditions are expected to inhibit binding of
Ag-2 to SiO2-6+.
All binary nanostructures were characterized using TEM and
UV-vis spectroscopy. Prior to characterization, each sample
was centrifuged (3000 rpm, 3 min) and the precipitate redis-
persed in 400 µL of CHCl3. Analysis was performed on this
isolated fraction only. UV-vis spectra are presented as normal-
ized spectra (uncorrected for scattering).
Reversal of Programmed Assembly of Binary Nanostruc-
tures in Solution. Realizing that the [2]pseudorotaxane used
in this study is formed between a dibenzylammonium salt and
dibenzo[24]crown-8, it should be possible to reverse the
assembly of silver nanocrystals on silica nanospheres by addition
of a base, in this case triethylamine. This is expected to
deprotonate the dibenzylammonium salt and convert it into a
dibenzylamine moiety which has no interaction with dibenzo-
[24]crown-8. To demonstrate this feature, an excess of triethyl-
amine (0.02 g, 1.98 × 10-4 mol) was added to the dispersion,
described above, of Ag-2 and SiO2-6+ and was left to
equilibrate for 24 h.
Reversal of the templated assembly of binary nanostructures
was characterized using TEM and UV-vis spectroscopy. Again,
prior to characterization the sample was centrifuged (3000 rpm,
3 min) and redispersed in 400 µL of CHCl3 for analysis. UV-
vis spectra are presented as normalized spectra (uncorrected for
scattering).
Results and Discussion
Hydrophobic monodisperse silica nanospheres have been
prepared using traditional methods.16 It is noted that the synthesis
of silica nanospheres does not permit complete coverage of
amine groups at the surface of the nanospheres.17,18 However,
when coupling 6-NVOC to amine-modified silica nanospheres
monolayer coverage of amine termini was assumed. Established
techniques from solid-phase synthesis have been used to couple
the surface amine termini with photoprotected dibenzylamine
moieties, 6-NVOC. Using UV-vis spectroscopy, we have been
able to quantify the number of 6-NVOC molecules at the surface
of the silica nanospheres and this was determined to be 1.61 ×
104 6-NVOC per nanosphere. Both the coupling strategy
employed and the photoprotecting group used are known and
practiced in the literature.19,20
To determine if the coupling strategy used in this work was
effective, an attempted synthesis whereby the coupling agent,
EDAC, was absent was performed. The absence of any
absorption trace representing 6-NVOC at the surface of the silica
nanospheres indicates that a coupling agent is necessary to
couple 6-NVOC covalently to the surface of the nanospheres
and no non-specific interactions lead to the UV-vis spectra
presented in Figure 2a.
After surface modification with 6-NVOC, the UV-vis
spectrum of SiO2-(6-NVOC) reveals the presence of 6-NVOC
at the surface of the nanospheres (Figure 2a). Furthermore, after
photolysis of this precursor, using laser light, it is equally clear
that the photoactive component has been removed, whereas the
dibenzylamine group still resides at the surface of the nano-
spheres (Figure 2b). Care has been taken here not to use
wavelengths below 355 nm as these may lead to degradation
of the dibenzylamine groups. Protonation of SiO2-6 converts
the free dibenzylamine to its corresponding hexafluorophosphate
dibenzylammonium salt. Salts of this nature are typically found
to be insoluble in chloroformic solvents; however, we have
found that when 6+ is chemisorbed on the surface of silica
nanospheres in SiO2-6+ the dispersion remains stable.
Thermal Processing of Templated Binary Nanostructures.
To fabricate robust architecture from nanocrystal assemblies it
will be important to develop facile methods to fix the assembled
components and thus eliminate the possibility for disassembly.
In this context, a sample of the binary nanostructure obtained
from mixing Ag-2 and SiO2-6+ was placed in an oven and
heated (Carbolite furnace fitted with a Eurotherm controller)
from 150 °C to 300 °C in 50 °C increments over a period of 20
min. The resultant assembly was characterized using TEM.
The [2]pseudorotaxane recognition-directed templated as-
sembly of binary nanostructures is discussed in the paragraphs
that follow.
Previously published data has established the formation of
pseudorotaxanes at the surface of metal nanocrystals and their
subsequent aggregation.10,11 These data was supported in both

Programmed Assembly of Binary Nanostructures
J. Phys. Chem. B, Vol. 106, No. 21, 2002 5375
Figure 2. (a) Shown is the UV-vis spectrum obtained for SiO2-(6-NVOC) nanospheres dispersed in chloroform. Also shown is the UV-vis
spectrum of 6-NVOC in chloroform. (b) Shown is the UV-vis spectrum obtained for SiO2-(6-NVOC) dispersed in chloroform before and after
irradiation.
Figure 3. (a) Typical result obtained from mixing a dispersion of Ag-2 and SiO2-6+. Control experiments involving the addition of silver nanocrystals
to silica nanospheres where (b) 2, (c) 6+, and (d) both are absent are also included. Scale bar is 100 nm in each case.
1
cases using high resolution H NMR and, in the latter case,
dynamic light scattering. Realizing that high resolution 1H NMR
is not suitable in this case, careful controls have been performed
that preclude any alternative to [2]pseudorotaxane formation
driving the assembly process and this is further supported by
the selective reversal of assembly upon addition of a weak base
outlined below.
same time period. Each of these controls represents a situation
where either one or both of the recognition elements are absent.
A further control was performed: here, dibenzo[24]crown-8,
in excess, was added to a chloroformic dispersion of SiO2-6+
and allowed to equilibrate. Further addition of Ag-2 to this
sample did not result in precipitation after a 24 h period. These
controls are strong evidence to support our assertion that
pseudorotaxane formation is the driving force for the templated
assembly of silver nanocrystals on silica nanospheres.
It is evident from TEM analysis (Figure 3) that only when
Ag-2 is mixed with SiO2-6+ is templated nanocrystal assembly
observed (Figure 3a). Further addition of Ag-2 did not improve
the density of nanocrystal coverage at the surface of SiO2-6+.
The addition of Ag-2, i.e., silver nanocrystals modified with
dibenzo[24]crown-8 recognition motifs, to SiO2-6+ yielded a
precipitate within 24 h. This result was not observed in either
of the following cases: addition of Ag-1, i.e., silver nanocrystals
capped with dodecane thiol, to SiO2-6+; addition of Ag-1 to
SiO2-(6-NVOC); addition of Ag-2 to SiO2-(6-NVOC) over the

5376 J. Phys. Chem. B, Vol. 106, No. 21, 2002
Ryan et al.
dibenzo[24]crown-8 and SiO2-6+. This further confirms our
assertion that [2]pseudorotaxane formation is the mechanism
for the templated assembly of silver nanocrystals on silica
nanospheres. The addition of dibenzo[24]crown-8 has blocked
all 6+ sites on the silica nanospheres and thereby inhibits
recognition and binding of Ag-2 by SiO2-6+.
With knowledge of previous studies on [2]pseudorotaxane
formation detailing the dis-assembly of the supramolecular
structure,23 it was proposed that the process of templated
assembly of silver nanocrystals on silica nanospheres may be
reversed. The addition of triethylamine has been shown to
deprotonate dibenzylammonium salts and result in subsequent
destruction of any interaction that favors [2]pseudorotaxane
formation. An analogous experiment was performed in this
study. The TEM data presented in Figure 5a represents an almost
complete removal of Ag-2 from the surface of SiO2-6+ (now
SiO2-6), whereas the corresponding UV-vis spectra (Figure 5b)
yields the expected blue-shift in the surface plasmon band back
to 430 nm. This represents an elegant demonstration of
molecular control over the collective optical properties of a
composite material and is further evidence that2pseudorotaxane
formation is driving the templated assembly of binary nano-
structures.
Figure 4. UV-vis spectrum of Ag-2 and SiO2-6+ obtained 24 h after
mixing. Shown also are the spectra obtained for the relevant controls
where one or both recognition motifs are absent. Evident from this is
the presence of a large absorption band representative of a change in
the dielectric environment of silver nanocrystals when these nanocrystals
are templated at the surface of silica nanospheres. UV-vis spectra are
presented as normalized spectra (uncorrected for scattering).
Furthermore, we can rule out any interaction between silver
nanocrystals and silica nanospheres when these materials do
not both contain 2 or 6+ chemisorbed on their respective
surfaces. For example, in Figure 3b, SiO2-(6-NVOC) is observed
on top of a close-packed region of Ag-2. The ordering of these
nanocrystals is seen to extend beneath the surface of the silica
nanospheres without disruption of any nature.
The UV-Visible spectra (Figure 4) further support the TEM
data. It is noted that the absorption of 6-NVOC relative to the
surface plasmon resonance of silver nanocrystals is very weak
and will not appear in these spectra. In the case where Ag-2 is
added to SiO2-6+, a shift in the surface plasmon band of silver
nanocrystals from their solution value, 422 nm, to 452 nm is
evident. This red-shift is most likely due to an increase in the
effective dielectric environment of the silver nanocrystals.21,22
In all the remaining control spectra, the surface plasmon band
of silver remains at its free solution value. This is indicative of
no interaction occurring between silver nanocrystals and silica
nanospheres in the absence of both 2 and 6+ chemisorbed at
their respective surfaces. Furthermore, no shift in the surface
plasmon band is observed for Ag-2 in the presence of excess
One focus of this work is to fabricate useful nanoscale
architecture that can be used in potential photonic or electronic
applications. To achieve such a goal it is important to
demonstrate structural integrity and robustness of material.24 To
this end, we have heated the templated binary nanostructure to
investigate how structurally sound the composite material is.
Figure 6 shows the results of TEM analysis of thermal
processing of the binary nanostructure. It is our belief that,
although the silver nanocrystals have altered to form larger
crystals, their position at the surface of the silica nanosphere
remains intact as is their templated structure. It is noted that
the spectroscopic analysis of this material was not performed
due to difficulties in obtaining such analysis from a TEM
sample. Furthermore, spectroscopic analysis was not performed
on isolated samples on a more suitable substrate (e.g., quartz)
as this would not allow TEM imaging of the templated
assembly. Overall, this indicates a useful metallization process
involving initial recognition-directed assembly of binary com-
ponents followed by thermal processing that fixes the structure.
Such a result points toward the eventual fabrication of functional
Figure 5. (a) TEM image of a mixture of Ag-2 and SiO2-6+ (i) before and (ii) after addition of an excess of triethylamine followed by centrifugation.
(b) Corresponding UV-vis spectrum showing (i) Ag-2, (ii) Ag-2 on SiO2-6+, and (iii) as for (ii) after addition of triethylamine.

Programmed Assembly of Binary Nanostructures
J. Phys. Chem. B, Vol. 106, No. 21, 2002 5377
to acknowledge the Swedish Foundation for International
Cooperation in Research and Higher Education (STINT) for
their kind support. This work was supported in part by a
Strategic Research Grant from Enterprise Ireland co-sponsored
by Loctite (Ireland), and in part by a grant from the Petroleum
Research Fund (#32879-ACS).
References and Notes
(1) Vossmeyer, T.; DeIonno, E.; Heath, J. R. Angew. Chem., Int. Ed.
Engl. 1997, 36, 1080.
(2) Korgel, B. A.; Fitzmaurice, D. AdV. Mater. 1998, 10, 661.
(3) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. ReV. Phys. Chem.
1998, 49, 371.
(4) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270,
1335.
(5) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath,
J. R. Science 1997, 277, 1978.
Figure 6. TEM image of templated binary nanostructure after thermal
processing. Evident is the presence of larger silver nanocrystals that
remain templated at the surface of the silica nanospheres.
(6) Kiely, C. J.; Fink, J.; Zheng, J. G.; Brust, M.; Bethell, D.; Schiffrin,
D. J. AdV. Mater. 2000, 12, 640.
(7) Mitchell, G. P.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc.
1999, 121, 8122.
nanoscale materials whereby the molecular recognition force
that assembles the solid phase materials is eliminated without
drastically altering the structural integrity of the resulting
assembly.
(8) Caruso, F.; Susha, A. S.; Giersig, M.; Mohwald, H. AdV. Mater.
1999, 11, 950.
(9) Vossmeyer, T.; Jia, S.; Delonno, E.; Diehl, M. R.; Kim, S. H.; Peng,
X.; Alivisatos, A. P.; Heath, J. R. J. Appl. Phys. 1998, 84, 3664.
(10) Fitzmaurice, D.; Rao, S. N.; Preece, J. A.; Stoddart, J. F.; Wenger,
S.; Zaccheroni, N. Angew. Chem., Int. Ed. Engl. 1999, 38, 1147.
(11) Ryan, D.; Rao, S. N.; Rensmo, H.; Fitzmaurice, D.; Preece, J. A.;
Wenger, S.; Stoddart, J. F.; Zaccheroni, N. J. Am. Chem. Soc. 2000, 122,
6252.
(12) Fyfe, M. C. T.; Stoddart, J. F. Acc. Chem. Res. 1997, 30, 393.
(13) Ashton, P. R.; Chrystal, E. J. T.; Glink, P. T.; Menzer, S.; Schiavo,
C.; Spencer, N.; Stoddart, J. F.; Tasker, P. A.; White, A. J. P.; Williams,
D. J. Chem. Eur. J. 1996, 2, 709.
(14) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R.
J. Chem. Soc., Chem. Commun. 1994, 801.
(15) Stober, W.; Fink, A. J. Colloid Interface Sci. 1968, 26, 62.
(16) van Blaaderen, A.; Vrij, A. J. Colloid Interface Sci. 1993, 156, 1.
(17) van Blaaderen, A.; van Geest, J.; Vrij, A. J. Colloid Interface Sci.
1992, 154, 481.
(18) Badley, R. D.; Ford, W. T.; McEnroe, F. J.; Assink, R. A. Langmuir
1990, 6, 792.
(19) Pillai, V. N. R. Synthesis 1980, 1.
(20) Stewart, J. M.; Young, J. D.Solid-Phase Peptide Synthesis; W. H.
Freeman and Company: New York, 1969.
(21) Mulvaney, P. Langmuir 1996, 12, 788.
(22) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem.
Phys. Lett. 1998, 288, 243.
Conclusion
The recognition-directed templated assembly of a binary
nanostructure consisting of dibenzo[24]crown-8-modified silver
nanocrystals and dibenzylammonium-modified silica nano-
spheres has been reported. It has been shown that the dibenz-
ylammonium cation may be generated in situ, at the surface of
a silica nanosphere, by photolysis and subsequent protonation
of a suitable precursor. It has also been shown that the assembly
of this binary nanostructure is due to the formation of a [2]-
pseudorotaxane between the interfaces of silver nanocrystals
and silica nanospheres. Furthermore, we have demonstrated the
ability to reverse the assembly process or, in contrast, the ability
to fix the structure into an irreversible state. These results present
an insight into the optical properties of a binary nanostructure
while also presenting novel control over the structural integrity
of the templated assembly.
(23) Ashton, P. R.; Ballardini, R.; Balzani, V.; Baxter, I.; Credi, A.;
Fyfe, M. C. T.; Gandolfi, M. T.; Gomez-Lopez, M.; Martinez-Diaz, M. V.;
Piersanti, A.; Spencer, N.; Stoddart, J. F.; Venturi, M.; White, A. J. P.;
Williams, D. J. J. Am. Chem. Soc. 1998, 120, 11 932.
(24) Fullam, S.; Cottell, D.; Rensmo, H.; Fitzmaurice, D. AdV. Mater.
2000, 12, 1430.
Acknowledgment. The Authors acknowledge useful dis-
cussions with Drs. S. Nagaraja Rao, K. Nikitin, and S. Wenger
and the services provided by D. Cottell at the Electron
Microscopy Centre, University College Dublin. H.R. would like