Biomimetic Synthesis of Gold Nanocrystals Using a Reducing Amphiphile

full papers

1390

Crystal growth

DOI: 10.1002/smll.200701163

Biomimetic Synthesis of Gold Nanocrystals Using aReducing AmphiphileFerdinand Gonzaga, Sherdeep Singh, and Michael A. Brook*

Keywords:� biomimetic synthesis

� crystal growth

� gold

� self-assembly

� surfactants

The first synthesis of a chelating and reactive surfactant derived from citric

acid and a short silicone as hydrophobic tail is described. Aqueous solutions

of this reactive amphiphile spontaneously induce gold ion reduction,

particle nucleation, and further direct crystal growth. The process, both pH

and light dependent, occurs through lipid-directed assembly of metal ions,

their reduction and subsequent lipid-directed growth to yield ultrathin

(approximately 7 nm thick) quasi two-dimensional gold nanocrystals.

1. Introduction

In living organisms, complex interactions between ions and

biopolymers acting as templates are used to regulate

nucleation and growth of inorganic architectures. Extension

of this concept to metallic structures would require the

development of suitable precursors, able to interact both with

metal ions and the crystalline metal surface. Molecules such as

marine siderophores[1,2]–high-affinity iron(III) ligands pro-

duced by bacteria–are particularly attractive models, as they

can regulate iron acquisition through complexation, controlled

self-assembly,[3] and photoinduced reduction processes.[4] The

ability of such molecules to bind, pre-organize, and control

redox processes, if extended to other metals, would permit

materials chemists to synthesize new metallic colloidal

architectures and explore their interesting size- and shape-

dependent properties.[5]

Previous synthetic approaches to create shape-controlled

metallic structures involve chemical[6] or photochemical[7]

solution-phase methods, or reductions inside soft-

colloidal,[8–10] biomimetic,[11,12] or biological [13–18] templates.

However, the development of suitable amphiphiles[19] that can

dynamically organize metal precursors in solution, initiate

reduction processes, and effectively control crystal growth

through specific metal–head-group interactions has not been

reported. Here we describe the synthesis of such a chelating

and reactive lipid, based on citric acid, a molecule broadly

involved in biological processes[20] and in siderophore

[�] Dr. F. Gonzaga, S. Singh, Dr. M. A. Brook

Department of Chemistry, McMaster University 1280 Main Street

West Hamilton, ON L8S4M1 (Canada)

E-mail: [email protected]

: Supporting information for this article is available on the WWWunder http://www.small-journal.org or from the author.

� 2008 Wiley-VCH Ver

biochemistry. In the presence of gold cations, the lipid induces

gold ion reduction, particle nucleation, and further directs gold

crystal growth. The process, both pH and light dependent,

occurs through lipid-directed assembly and reduction of gold

cations into quasi two-dimensional (2D) gold nanocrystals. A

single molecule, an ‘‘aurophore,’’ which acts successively as a

chelating, reducing, capping, and structure-directing agent

drives the entire process, and allows an unambiguous under-

standing of how such anisotropic gold crystals are formed.

These synthetic siderophore-like reactive amphiphiles, and

their organized assemblies, represent a new biomimetic

strategy toward nanoengineered metallic architectures.

2. Results and Discussion

2.1. Synthesis of Citric Acid based Reactive Surfactant

Citrate reduction is the most popular method to prepare

aqueous spherical gold nanoparticles.[21] We reasoned that

surfactants chemically derived from citric acid could be used

both to induce structure direction and to reductively lead to

anisotropic gold crystal growth. A versatile synthetic method

permits the preparation of lipid 5 (Scheme 1) through covalent

addition of a short and highly hydrophobic silicone tag,

without affecting the carboxylic acid units. Citric acid esters

are well known but citrate ethers are extremely rare: the

methyl ether was incompletely described in the early 1900s and

a ring-opening polymerization of propylene or butylene oxide

by triethyl citrate, to ultimately give citric acid poly(oxyalk-

ene)ethers, has been reported.[22] Otherwise, little is to be

found in the literature. Extensive attempts using Williamson

etherification to prepare the key intermediate, citric acid allyl

ether 3, failed in our hands.

The detailed synthetic pathway that was successfully used

is illustrated in Scheme 1. The first step involves a classical

lag GmbH & Co. KGaA, Weinheim small 2008, 4, No. 9, 1390–1398

Biomimetic Synthesis of Gold Nanocrystals Using a Reducing Amphiphile

Scheme 1. Synthetic pathway of citric acid-based surfactant 5.

Figure 1. UV/Vis signature of colloidal metallic gold resulting onmixing

lipid 5 and a gold salt, as prepared and after separation of the two

constituents: green product (gold nanoleaves), supernatant (nano-

particles), and their corresponding appearance (inset).

acid-catalyzed benzylation of citric acid. In the crucial step of

the synthesis, palladium-catalyzed allyl transfer from allyl-tert-

butylcarbonate to tribenzyl citrate yielded the desired O-

allyloxy tribenzyl citrate 3 in high yield. Hydrosilylation of 3

with a hydride-terminated trisiloxane led to the benzyl

protected derivative 4 which, after hydrogenolysis over Pd/

C, gave the corresponding surfactant 5 without affecting the

siloxane moiety[23] (yield from citric acid: 78%). All synthetic

steps occur in high to quantitative yields and this methodology

is readily scaled to multigram synthesis (up to 10 g) of citrate

surfactants. It is noteworthy that the flexibility and broad

reactivity of allyl derivative 3 permits the preparation of a wide

range of surfactants, including mono-or bi-catenar surfactants,

with alkyl or silicone hydrophobic chains, as will be described

elsewhere.

2.2. Biomimetic Synthesis of Gold Nanocrystals

Gold crystals were prepared by mixing an aqueous solution

of lipid 5 with a gold salt solution (HAuCl4; tetrachloroauric

acid) under ambient conditions. The initially pale-yellow

solution undergoes a series of color changes (from pale yellow

to colorless, then light pink, and finally dark purple), indicating

the formation of colloidal metallic gold. After 48 h, centrifu-

gation reveals a deep green precipitate that, unusually, has a

continuous absorption from 512 to 900 nm in the UV/Vis and

near-infrared region (NIR) spectroscopy (Figure 1 and inset).

A representative high-magnification transmission electron

microscopy (HRTEM) image of the green product shows the

presence of fluid, round, and interconnected fractal shapes

linked to more geometric platelets (triangles, truncated

triangles, hexagons), with overall sizes of a few hundreds of

nanometers, referred to as gold nanoleaves (Figure 2a and b and

Supporting Information). The nanoleaves and fractal structures

were shown to be crystalline gold using high resolution

transmission electronic microscopy (HRTEM): Figure 2c

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shows continuous well-resolved crystal lattice fringes, even at

the crystal contours, and the presence of a diffuse layer of lipid

along the gold interface. Energy dispersive X-ray spectroscopy

(EDS) shows the presence of strong peaks for elemental silicon

and gold, confirming the chemical nature of this interfacial layer

(Supporting Information).

Scanning electron microscopy (SEM) at a lower magni-

fication shows that the green precipitate is composed of stacks

of about 200 nm in thickness (Figure 2d and Supporting

Information) of gold nanoleaves. The highest magnifications

(Figure 2e and Supporting Information) confirm that the

individual nanoleaves in the flat or rolled film are extremely

thin, typically less than 8 nm (thicknesses in the range from 6 to

12 nm were measured) as also confirmed by atomic force

microscopy (AFM; Figure 2f and g). The morphology of the

stacks, the curled ribbons shown in Figure 2d, is a consequence

of drying and delamination from the steel SEM stub.

An X-ray diffraction pattern of a thin film of the crystals

deposited on a glass side shows an overwhelming diffraction

peak at �38.2 8 assigned to the {111} facets of a face-centered

cubic (fcc) metal gold structure (Figure 2h), while diffraction

peaks of other facets are much weaker. An average thickness

of 7.7 nm is calculated from the Scherrer equation, which also

demonstrates the extremely low thickness of the nanoleaves.

Under the same conditions, citric acid only yielded roughly

spherical particles (see Supporting Information), indicating

that observed anisotropic growth is related to the surfactancy

of lipid 5 (as citric acid and the lipid only differ by the

hydrophobic silicone moiety, see below). In addition to

their very high aspect ratio, the leaves are even more

remarkable given the fact that crystal growth occurs in

solution, without the use of a preformed template: previous

attempts to produce flat and thin gold crystals (or fractal

crystals) required first the creation of a 2D interface such as a

thermally evaporated film, a Langmuir monolayer, or a

sheared lamellar phase.[23,24,25]

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full papers M. A. Brook et al.

Figure 2. Characterization of gold nanoleaves prepared with lipid 5. a, b) TEM of geometrical or fractal gold nanoleaves, respectively. c) HRTEM of a

gold nanoleaf, showing well-resolved lattice fringes (inset is the corresponding electron diffraction pattern) and the presence of an interfacial

diffuse layer of lipid 5. d) SEM image of a gold nanoleaf rolled film, seen at two different magnifications (inset). e) High-magnification SEM image of

gold nanoleaves, showing thicknesses lower than 10 nm. f, g) AFM images of a gold nanoleaf with screw dislocation, and its corresponding height

profile from 6 to 8 nm, respectively. f) X-ray diffractogram of a glass-deposited thin film of gold nanoleaves, showing an overwhelming {111} peak.

1392 www.small-journal.com � 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2008, 4, No. 9, 1390–1398


Having characterized the nature of the crystalline gold

nanoleaves, we wished to optimize the formation of different

structural types. The production of gold nanoleaves induced

by lipid 5 in the presence of white light was found to be

strongly pH dependent. Kinetic studies and TEM analyses

indicate that nanoleaves are already produced in 20 h at pH

6.30. Increasing the pH of the lipid solution by only 1 unit

(from 6.30 to 7.30) considerably decreases the rate of the

process (it requires 48 h at pH 7.30 to obtain the same UV/Vis

signature.

These reactions were re-examined in the absence of white

light. The reduction process still occurs at the three pH values

tested (6.30, 7.30, and 8.30), indicating that the reduction of

gold ions by lipid 5 is not exclusively a light-dependent process.

However, light was found to affect both the rate of the process

and the morphology of the colloidal gold produced. At pH

6.30, micrometer-sized gold nanoplates were obtained (Sup-

porting Information) in only 24 h. The plates aggregate and

settle down in the vial, a behavior which strongly contrasts

with the gold nanoleaves. At higher pHs, no significant

morphological differences were observed, but the rate of the

overall process was significantly impeded in the absence of

light (Supporting Information). Light effects have been

previously described in the plasmon-directed synthesis of

silver nanoprisms,[7] and a recent report by Mirkin and Xue[26]

emphasized the interplay between light and pH effects. In our

system, the decrease in the rate of formation of gold

nanoleaves in the dark supports the idea that optical attractive

forces may be involved in the growth process. However, the

fact that crystals obtained in the dark were larger than those in

the presence of visible light at pH 6.30 also indicates a more

efficient nucleation step in the presence of light: fewer

nucleation events, associated with a highly favored pH for

crystal growth, thus lead to a higher average crystal size. At

higher pH values, the process was already considerably slower

than at pH 6.30 and the absence of light reduced even more the

overall rate of crystal nucleation and growth.

A time-dependent study at pH 7.3, the pH at which the

reaction proceeded at an intermediate rate, was performed

using UV/Vis and TEM to better understand the reaction

sequence leading to the evolution of the gold nanoleaves. In

the early stages of the process (after 12 h of reaction) only

small gold clusters were observed. These particles are

embedded in membranar structures (Figure 3a) and the

strong interactions between the gold clusters are responsible

for the high wavelength-absorbance tail in the UV/Vis spectra

(Supporting Information). After 24–30 h, the sample is

dominated by long and ramified fractal dendrites. Their

lengths vary from 200 nm up to several micrometers, while

their widths are typically lower than 10 nm. However, the

fractal structures are broader at their extremities, indicating

that preferential 2D crystal growth occurs on the periphery of

the arborescent structures (Figure 3b). This observation is

even more pronounced after 36–42 h: almost all dendrites are

now terminated by flat structures, which increasingly evolve

into gold geometric shapes (Figure 3c). After 48 h of growth,

the final nanoleaves are observed (Figure 3d). The TEM

micrographs also reveal the presence of membrane-embedded

growing crystals: the membrane structures contain a high


density of gold clusters, which provides further guidance for

the mechanism of crystal growth (Figure 3e). Analysis of large

membranar structures (Figure 3f) reveals that they consist of a

regular arrangement of surfactant bilayers, with a remarkably

low value (<1 nm) for the d spacing (Figure 3g).

The time-dependent studies (TEM, UV/Vis) described

above permit us to propose a three-step mechanism to explain

the surfactant-induced synthesis of quasi 2D gold nanoplates:

complexation and self-assembly, nucleation, and membrane-

directed crystal growth.

2.3. Complexation and Self-Assembly

There is strong evidence that citrate binds to gold prior to

reduction, both in a historical context, and from our own

results. While binding constants of various metals ions to

citrate are well known, no data are currently available for the

binding constant with gold cations because, in addition to

the high affinity of citrate for gold, redox reactions between

the two species lead to metallic gold and oxidation products of

citric acid (a process used for the Turkevich synthesis of gold

nanoparticles). The two processes, chelation and reduction,

have not previously been independently observed.

On mixing the two precursor solutions, the chelating head

group strongly interacts with gold ions and a resulting citrate–

Au3þ complex is formed. Although the exact nature (e.g.,

monomeric or polymeric) of this complex is not known, UV/

Vis spectroscopy shows the presence of two new absorption

bands at 419 and 448 nm in the early stage of the process that

can only be attributed to such intermediate gold/citrate species

(Supporting Information).

Co-assemblies of membranar aggregates and gold were

also observed during the time-resolved studies of formation of

the nanocrystals. Figure 3f, taken after 12 h of the reduction

process (in the early stage of the reduction) shows such an

assembly, and Figure 3g, a higher magnification of Figure 3f,

shows that those assemblies are made of stacked bilayers of the

amphiphile. The corresponding electron-diffraction pattern,

diffuse and of low intensity, indicates the presence of

polycrystalline gold from small gold clusters. The fact that

such aggregates can be observed by TEM and HRTEM,

without the use of any staining agent is by itself proof of the

presence of metallic centers within the self-assembled

structures. Indeed, noble metals (Ag, Au) as well as other

metallic ions (such as osmium or uranium) are often used as

staining agents in order to visualize surfactant self-assembled

aggregates. For all these reasons, it seems highly probable that

gold ions (which are stable in the same media, under the same

conditions, but in the absence of citrate) must complex with

the citrate-based amphiphile prior to their reduction.

The self-assembly step, which may occur in concert with

the initial complexation, involves the aggregation of these

citrate/gold species into the previously described membrane

structures. The observed bilayer units observed (Figure 3g) are

conceivably the consequence of highly favored hydrophobic

interactions between the siloxane units (the trisiloxane

backbone of 5 is estimated to be comparable, in terms of

hydrophobicity, to a C12 alkyl chain[27]) and reduced

electrostatic repulsion between the citrate polar head groups



Figure 3. Directed growth of gold nanoleaves. a–d) Representative TEM images showing the size and shape evolution of colloidal gold prepared on

mixing lipid 5 with a gold salt, at 12 (a), 24 (b), 36 (c), and 48 h (d). The structure evolves from small (<5nm) clusters to ramified dendrites to the

final gold nanoleaves. e) TEM image of growing gold dendrites, showing membrane-embedded gold clusters. f, g) TEM and HRTEM of gold–lipid 5membrane structures, respectively. Well-ordered stacked bilayers can be distinguished. Inset: corresponding electron-diffraction pattern that

appears at pH 6.30 after only 20 h, that is, a 2.4 factor decrease (Supporting Information). A further increase to pH 8.30 drastically slows down the

process, which now only yields irregular nanoparticles, and occasional ultrathin wires (Supporting Information).

1394 www.small-journal.com � 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2008, 4, No. 9, 1390–1398


following gold ion complexation, which facilitates their

assembly. Existing studies on chelating surfactants support

this proposal: self-assembled columns of stacked lipid bilayers

are obtained when copper is added to liposomes made of a

chelating surfactant.[28] Moreover, natural siderophores such

as Marinobactin E undergo a micelle to vesicle to stacked

bilayer transition through iron coordination.[3,29]

2.4. Nucleation and Membrane-Directed CrystalGrowth

The chemical process behind gold nanoparticle nucleation

is the oxidative-decarboxylation process that ultimately yields

oxidation products of citric acid and a reduced metal center

that subsequently leads to particle nucleation. The formation

of citrate membranar assemblies facilitates the gold crystal

nucleation step, due to excellent interfacial control both

before and after reduction, and local high-concentration

effects that are required for nucleation. In addition, this well-

ordered 2D reactive template can lead to oriented nuclea-

tion,[30] which directs the subsequent growth of the gold

structures into fractal structures and prismatic plates.

Two different mechanisms can be postulated for crystal

growth: an autocatalytic growth of the least-stabilized crystal-

lographic faces (i.e., along the (110) faces), or the sintering of

primary particles into larger crystals. In the first mechanism,

slow diffusion of gold cations within the organic bilayer and

their reduction at the surface of existing nuclei can explain

crystal growth at the most energetic crystal face. In the second

mechanism, aggregation-driven crystal growth occurs through

the sintering of primary gold particles in the organic bilayer

matrix, consistent with the diffusion-limited colloid aggrega-

tion model (DLCA[31]). As gold particles are formed within

the organic matrix they diffuse and adhere to the growing

crystal 2D structure.

Recently, Brust and co-workers[32] showed that 1D

sintering of small hydrophobic gold nanoparticles could be

induced in a matrix of DPPC at the air–water interface, by

compression in a Langmuir–Blodgett film. The key role of the

surfactant DPPC was highlighted, as it acts as a molecular

template, allowing sintering only in certain directions.

Oriented sintering has also been demonstrated for mono-

disperse 2-nm gold nanoparticles, yielding 2-nm diameter

nanowires.[33] Moreover, a very recent report shows how large

triangular silver single crystals can be obtained by mild

annealing of surfactant-capped 5-nm silver nanocrystal mono-

or multilayers.[34] These examples reveal that 1- or 2D

organization of nanoparticles, followed by oriented sintering,

is a powerful tool for the synthesis of new metallic

architectures.

In our case, a preferential 2D sintering could occur in a

lipid bilayer. The interaction between growing crystals and

isolated, surfactant stabilized nanoparticles in this case is

highly favored, as both the capping layer of the particles and

the lipid bilayer membranes are made of the same molecule.

The slow kinetics of this process would highly favor growth

into prismatic or triangular flat nanocrystals: indeed, it has

previously been demonstrated that in the citric acid catalyzed

synthesis of gold nanocrystals the formation of gold nano-


triangles was kinetically controlled and highly favored at low

temperature, while spherical particles were obtained in boiling

water.[35]

The product crystals, fractal, or prismatic (Figure 3)

present a flat surface, indicating anisotropic growth, even from

the very first stages of the process. This can result from an

oriented nucleation step (which will thus induce anisotropic

growth), and a specific stabilization of the (111) facets of the

crystals by the citrate head groups such that growth occurs at

the edges. Several studies support the latter observation:

citrate adlayers have been experimentally observed at the Au

(111) surface by scanning probe microscopy. Citrate anions

form very well ordered (4� 2H3) adlayers and these ordered

adlayers were found to be extremely stable, which is in

accordance with the fact that the citrate anion is one of the best

stabilizers for metallic nanomaterials.[36] Furthermore, the

presence of chloride ions from the precursor chloroaurate

derivative is also known to promote the growth of (111)-

oriented triangular/truncated triangular particles.[35] Thus, the

driving force of the process can be seen as the formation of

highly stabilized (111) facets, along with the creation of stable

bi- or multilayers of citrate amphiphiles on those facets.

Several reports have described the synthesis of gold

nanostructures using citric acid and surfactants, in particular

CTAB: extensive work by Murphy et al. has shown that gold

nanorods could be obtained in high yield using a seeded

growth approach.[37] Following this approach, Mirkin and co-

workers reported the first chemical and aqueous procedure to

prepare triangular nanoplates in high yield.[38] Recently, a

thermal-reduction approach described the synthesis of gold

plates having three different particle size range.[39] In all these

studies, preferential growth was attributed to selective

adsorption of CTAB molecules on specific facets of the crystal.

The concentration of surfactant and the ratio of surfactant/

metal salt used in our procedures are considerably lower than

in the seeded growth procedure (less than 5 mM versus 0.1 M,

respectively), which could indicate a stronger binding of the

citrate head group compared to the alkylammonium head

group of CTAB. Moreover, the kinetics of the two processes,

CTAB versus 5, are also very different.

We believe that, as with citric acid, our amphiphile is able

to form stable adlayers at the (111) facets of the growing

crystals, and this even at the very early stage of their growth.

The presence of hydrophobic tails on the amphiphile would

certainly increase the stability of such adlayers. This has

previously been demonstrated with the alkyl chains of CTAB-

coated gold nanorods (where a bilayer of CTAB was

evidenced at the surface of the gold crystals).[36]

The presence of such a stable silicone-based lipid bilayer at

the (111) facets of the gold crystals would then favor the

growth in the direction normal to the (111) basal plane, as was

observed experimentally: the fractal crystals grow preferen-

tially from their extremities, where the amphiphile stabiliza-

tion is the weakest.

2.5. Fractal Versus Prismatic Crystalline Plates

The high-temperature synthesis of gold nanoplates

catalyzed by citric acid and CTAB, leads to fractal structures



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with a central core, an overall flat appearance and external

sheetlike structures in the very initial stages of the process (10 s

for the reaction performed at 85 8C).[40] However, unlike the

citrate stabilized crystals reported here, those structures are

not stable and rapidly evolve to more thermodynamically

stable triangular or hexagonal plates.

At the outset, in our case, growth of the 2D crystal normal

to the (111) plane occurs giving fractal structures. Initially,

gold clusters are rapidly formed and randomly sinter in a

kinetically controlled process that leads to formation of 2D

fractal ‘‘leaves’’ (Figure 2a and b), which continue to grow at

the extremities. Eventually, as formation of new particles, and

their migration to the growing crystal surface, becomes more

measured the more stable prismatic plates are nucleated and

then effectively grow: in all cases, we have observed a fractal

core followed by nucleation and growth of prismatic plates

(triangles or hexagons) at the peripheral ‘‘tips’’ of the leaves

where curvature is high. Depending on the specific reaction

conditions, it is possible to isolate purely fractal crystals,

particularly at low conversion. However, at longer reaction

times (it takes 48 h to complete the process) nucleation and

growth of prismatic structures occurs from the tips of the

fractal ‘‘leaves,’’ leading subsequently to large thermodyna-

mically favored prismatic structures at the periphery of a

fractal core: in all cases, very high aspect-ratio gold crystals are

observed.

This three-step model is consistent with the previously

postulated mechanism in which crystal nucleation and initial

growth occur within an interfacial layer such as a thin gel-like

film.[30] It further highlights that the sintering of fluid-like

metallic particles, also observed in the biological synthesis of

gold nanoprisms[18] (but for which the identity of structure-

directing agent(s) remains unknown), could also be induced by

a single molecule with self-assembly properties. The essential

roles of self-assembled lipid membrane and hydrophobic

interactions were explicitly demonstrated by adding a

destructuring agent such as THF (tetrahydrofuran) to the

reaction medium: reduction still occurs but leads now to gold

particles of ill-defined shape. Thus, the morphological control

of crystal growth due to the lipidic scaffold is lost

simultaneously with the ability of the surfactant to aggregate

in the binary solvent system (see Figure S4 in Supporting

Information). In such a situation, the system is returned to a

state similar to that with citric acid alone.

The sensitivity of this model to pH should be addressed.

Metal chelation, and also carboxylate-based surfactant self-

assembly, are known to be strongly pH dependent. The

protonation state of the citric acid head group of 5 will thus

be sensitive to pH, which is reflected in the variations

observed in the rate of formation and morphologies of the

gold structures found at different pH values. Similar effects

were previously reported in the photoreduction of iron(III)–

citrate complexes to yield oxo- or hydroxy-iron(II) spe-

cies,[41] and also in the photochemistry of natural iron–

siderophore complexes.[4] However, while siderophores

manipulate the redox-chemistry of ionic iron in order to

facilitate its uptake, the gold–citrate system leads to further

reduction giving metallic structures, due to the instability of

intermediate gold oxidation states.

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3. Conclusions

The biomimetic assembly and reduction of metal ions by

aurophore 5 is a highly controlled route to exceptionally thin

gold nanoleaves. Their formation depends on a delicate

balance between nucleation rate and crystal growth in a

membrane-assisted process. The relatively slow reduction

rates of gold in the presence of silicone citrate surfactant 5,

when compared to the thermal reduction of gold by citrate

anions, allows the template to fully exert control over the

nucleation, diffusion, and growth of gold crystals. More

important, however, is the exploitation of biomimetic

hydrophobic and co-operative effects of reactive lipids that

mimic natural siderophores and direct metal reactivity and

assembly at the organic–metal interface. The flexibility in the

synthesis of citric acid based silicones, their ability to complex

a wide variety of cations, and the rich lyotropic behavior of

surfactants, including hexagonal and other packing arrange-

ments, suggests that this method may be exploited for the

synthesis of a wide range of structured materials based on gold

and other metals.

4. Experimental Section

Chemicals: Citric acid (99.5R%, Aldrich), benzyl alcohol

(Certified, Fisher Scientific), p-toluenesulfonic acid monohydrate

(pTSA, Aldrich), palladium acetate (99.98%, Aldrich), triphenyl-

phosphine (99%, Aldrich), 1,1,1,3,3,5,5-heptamethyltrisiloxane

(95%, Gelest), platinum–divinyltetramethyldisiloxane complex

(Karstedt’s catalyst) in xylene (Gelest), palladium on activated

charcoal (Degussa type E101NE/W, wet/Pd 10% dry-weight

basis, water 50%, Aldrich), Celite (Aldrich), were used as received.

Allyl-tert-butyl carbonate was prepared as previously

described.[42] Solvents were dried over activated alumina. NMR

solvents (CDCl3, CD3COCD3, and CD3OD) were obtained from

Cambridge Isotope Laboratories. All syntheses were carried out in

dry apparatus under a dry nitrogen atmosphere utilizing conven-

tional bench-top techniques. Water was purified by a Millipore

purification system (resistance¼18.1MO cm).

Spectroscopic characterization of organic compounds: 1H NMR

Fourier spectra were recorded on a Bruker AC-200 (200MHz)

spectrometer. Chemical shifts for 1H NMR spectra are reported

with respect to the following standards: residual chloroform set at

7.24 ppm, CD2HOD set at 3.30 ppm, and tetramethylsilane set at

0 ppm. J-modulated 13C NMR were performed on a Bruker AC-200

(at 50.3MHz for carbon). 13C NMR spectra are reported with

respect to the following standards: chloroform set at 77 ppm and

tetramethylsilane set at 0 ppm. Coupling constants (J) are

recorded in Hertz (Hz). The abbreviations s¼ singlet, d¼doublet,

ublet, t¼ triplet, dd¼doublet of doublets, m¼multiplet, are used

in reporting the spectra. Pneumatically assisted electrospray

ionization mass spectrometry (ESMS) was performed on a

Micromass Quattro-LC triple quadrupole mass spectrometer with

dichloromethane, dichloromethane/methanol (50:50) or metha-

nol as the mobile phase at a flow rate of 15mL minS1, with use of

a Brownlee Microgradient syringe pump. Samples were dissolved

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in dichloromethane/methanol (50:50) or pure methanol. Ammo-

nia or NH4OAc was added for analysis in the negative mode; for

analysis in the positive mode, formic acid was added. Mass

spectra were reported as percent intensity (%) versus mass/

charge (m/z) ratio.

Synthesis of lipid 5: 3-Hydroxy-pentanedioic acid dibenzyl 3-

benzyloxycarbonyl ester (tribenzyl citrate) (2): Citric acid (1)

(9.00 g, 46.8mmol), benzyl alcohol (29.1mL, 280.8mmol), and

toluene (250mL) were placed in a 500mL round-bottomed flask. A

catalytic amount of pTSA (0.20 g) was added and the reaction

mixture was then stirred under reflux with azeotropic removal of

the water produced during the ester formation (Dean-Stark). After

18 h, the mixture was allowed to cool to room temperature. The

solvent and unreacted benzyl alcohol were removed in vacuo. The

residue was dissolved in EtOAc, and washed twice with saturated

aqueous NaHCO3, twice with water, and finally twice with brine.

The organic phase was dried over Na2SO4, and the solvent was

removed under reduced pressure, leaving an oily substance that

solidifies slowly at room temperature. Recrystallization in hexanes/

EtOAc (95:5) afforded 21.6 g (quantitative) of the crystalline title

compound (white needles).1H NMR (acetone-d6, 200MHz): d 2.83 (br s, 1H); 2.90, 3.04

(2d, 4H, J¼15.5 Hz); 5.07 (s, 6H); 7.30–7.35 (m, 15H). 13C NMR

(acetone-d6, 200MHz): d 43.89; 66.70; 67.76; 74.16; 128.83;

129.11; 136.53; 136.88; 169.96; 173.52. IR (cm�1): 3493, 3066,

3034, 2955, 1738, 1498, 1456, 1382, 1341, 1214, 1186, 1174,

1079, 972, 750, 697. MS: ES-positive mode: (m/z): 463.3 (MþHþ)

(calculated: M¼ 462.50). 480.4 (MþNH4þ), 485.3 (MþNaþ).

3-Allyloxy-3-benzyloxycarbonyl-pentanedioic acid dibenzyl es-

ter (3): Under a nitrogen atmosphere, compound (2) (10.32 g, 22.3

mmol) was introduced in a round-bottomed flask, followed by allyl-

tert-butylcarbonate (5.29g, 33.5 mmol) and 110mL of dry toluene.

Palladium acetate (30mg, 0.13 mmol) and triphenylphosphine

(0.31 g, 1.18 mmol) were then added, and this mixture was refluxed

overnight. After being cooled to room temperature, the reaction

medium was washed with dilute aqueous NaHCO3, water, then

brine. The organic phase was dried over Na2SO4, and the volatiles

removed in vacuo. The residue was purified by chromatography over

a silica gel column, eluting with increasing amounts of EtOAc in

hexanes (9:1 to 4:1), to afford 8.97 g of the title compound (93%) as

a clear oil.1H NMR (acetone-d6, 200MHz): d 3.109, 3.27 (2d, 4H,

J¼15.7Hz); 4.03 (d, 2H, J¼5.2 Hz); 4.90–5.20 (m, 8H); 5.60–

5.90 (m, 1H); 7.34 (br.s, 15H). 13C NMR (acetone-d6, 200MHz): d

39.86; 66.20; 66.73; 67.58; 79.21; 116.36; 128.78; 128.83;

129.15; 135.24; 136.55; 136.89; 169.91; 170.64. IR (cm�1):

3454, 3091, 3067, 3035, 2954, 2892, 1957, 1878, 1739, 1499,

1456, 1383, 1346, 1280, 1213, 1170, 1062, 996, 924, 751, 698,

583. MS: ES-positive mode: (m/z) 503.4 (MþHþ), calculated:

M¼502.57; 520.4 (MþNH4þ), 525.3 (MþNaþ).

3-(1,1,1,3,3,5,5-Heptamethyltrisiloxane)-propyloxy-3-benzy-

loxy-carbonyl-pentanedioic acid dibenzyl ester (4): In a round-

bottom flask was introduced (3) (5.03 g, 10 mmol) in 25mL of dry

toluene, followed by 1,1,1,3,3,5,5-heptamethyltrisiloxane (2.90 g,

13 mmol) in of dry toluene (10mL). Karstedt’s platinum

hydrosilylation catalyst (platinum–divinyltetramethyldisiloxane

complex, solution in xylenes: Karstedt’s catalyst, 0.02mL) was

added, and the mixture was stirred at room temperature in a dry


atmosphere for 24 h. The volatiles were then removed in vacuo

without heating, and the residue was purified by chromatography

over a silica gel column, eluting with hexanes/ethyl acetate (15:1

to 4:1) to afford 6.08 g (84%) of the title compound as a colorless

oil.1H NMR (CDCl3, 200MHz): d 0.02 (s, 6H); 0.05 (s, 6H); 0.09 (s,

9H); 0.37 (m, 2H); 1.47 (m, 2H); 3.10, 3.27 (2d, 4H, J¼15.7 Hz);

3.34 (t, 2H, J¼6.8 Hz); 5.08 (s, 4H); 5.12 (s, 2H); 7.32 (br.s, 15H).13C NMR (CDCl3, 200MHz): d 0.82; 2.03; 2.57; 14.68; 24.32;

39.82; 67.18; 67.99; 68.16; 77.13; 128.95; 129.11; 129.25;

136.06; 136.35; 170.43; 171.20. IR (cm�1): 3092, 3068, 3036,

2958, 2899, 1742, 1499, 1456, 1383, 1346, 1258, 1214, 1167,

1138, 1047, 842, 796, 751, 697, 582, 491. MS: ES-positive

mode: (m/z): 725.3 (MþHþ) (calculated: M¼725.08).

Surfactant 5: 3-(1,1,1,3,3,5,5-heptamethyltrisiloxane)-propyl-

oxy-3-carboxy-pentanedioic acid: Benzyl-protected silicone-surfac-

tant 4 was dissolved in THF/MeOH (9:1, typically 25mL g�1). Then,

a 5% molar amount of 10% Pd/C was added, and hydrogen was

bubbled into the mixture under stirring at rt. Completion of the

reaction was checked by 1H NMR, following the disappearance of

the benzylic protons. After completion, the catalyst was removed

by filtration over a 0.45mm Teflon filter. Activated carbon was

added to the filtrate, and the mixture was stirred for more 3 h,

before being filtrated again. Volatiles were removed by flushing

nitrogen through the solution, until ca. 95% was removed, then in

vacuo without heating to afford the corresponding silicone-

carboxylic acid surfactant in a quantitative or almost quantitative

yield.1H NMR (CD3OD, 200MHz): d 0.02 (s, 6H); 0.07 (s, 6H); 0.09

(s, 9H); 0.54 (m, 2H); 1.58 (m, 2H); 2.97, 3.13 (2d, 4H,

J¼15.8 Hz); 3.45 (t, 2H, J¼6.8Hz). 13C NMR (CDCl3, 200MHz):

d �1.81; 0.75; 1.26; 14.34; 24.14; 38.98; 67.25; 78.65; 172.88;

173.47. MS: IR (cm�1): 3670–2153 (broad), 2959, 2617, 1721,

1426, 1400, 1293, 1258, 1225, 1204, 1147, 1050, 841, 796,

689, 643, 620. Elemental analysis: C: 42.36% H: 7.53%

(Calculated: C: 42.26% H: 7.54%). MS: ES-positive mode: (m/z):

472.1870 (MþNH4þ) (calculated: MþNH4

þ¼472.1854).

Synthesis of gold nanoleaves: Synthesis of the different metal

nanostructures involves initially the preparation of surfactant 5

aqueous solutions, typically 50mL of 5.0mM stock solutions. The

following procedure was used: in a beaker, compound 5 (0.114 g;

0.25 mmol) was partially dissolved in about 35mL of water; the

initial pH value was about 2.75. The pH was adjusted to the

required value by slowly adding aliquots of a 1 N aqueous NaOH

solution, while monitoring with a color digital pH meter. Complete

dissolution was achieved using brief sonication. The solution was

then carefully transferred to a 50mL volumetric flask, and water

was added to the mark. The pH of the total volume solution was

checked again. Stock solutions were always used immediately

after preparation.

In a typical experiment, 1mL of a freshly prepared tetrachlor-

oauric(III) acid aqueous solution (2.5mM) was added to 2mL of a

surfactant 5 aqueous solution (pH 7.30; 5.0mM), in a glass

scintillation vial. The resulting mixture was briefly hand shaken

and left at room temperature under ambient and static conditions

(no stirring). Light was provided by two classical fluorescent tubes

(white light type) located approximately 2 m above the vial. The

mixture undergoes a series of color changes, from pale yellow



1398

initially, to colorless, then light pink, purple, and finally dark

purple due to the formation of the green products, which slowly

starts to settle down after ca. 48 h.

Successive centrifugations (4000 rpm; 15min) followed by

redispersion in water were used in order to separate the gold

nanoleaves from a small amount of gold nanoparticles also

produced. The purification process was improved by allowing the

gold nanoleaves to slowly settle down in the vials (after 5 days):

the pink upper layer of gold nanoparticles was then easily

removed by a Pasteur pipette. Redispersion in water followed by a

single centrifugation cycle led to pure gold nanoleaves.

For the dark experiments, the two solutions were mixed in the

dark, and the sample vials were double-wrapped in aluminum foil

to prevent any exposure to light.

Acknowledgements

We thank the Natural Sciences and Engineering Research

Council of Canada for financial support of this research. We are

grateful to the Canadian Centre for Electron Microscopy

(McMaster University) and to Marcia West and the McMaster

Hospital Electron microscopy facility staff for extensive support

with TEM. W. Gong (Brockhouse Institute for Materials

Research, McMaster University) is acknowledged for X-Ray

analysis and helpful discussions.

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Received: November 24, 2007Revised: February 20, 2008

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Biomimetic Synthesis of Gold Nanocrystals Using a Reducing Amphiphile

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