Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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24.3 ). In aqueous solution, pinacyanol chloride ( PC ) showed absorption bands at

549 and 601 nm, but these were red - shifted to 564 and 610 nm, respectively, in

NCD micelles. Such a result suggested that NCDs could act as micellar hosts and,

potentially, also as water - soluble catalysts.

Chechik, Smith, and coworkers reported the synthesis of l - lysine based den-

dron - stabilized gold nanoparticles using the modiﬁ ed one - phase Schiffrin reaction

( 5 ; Figure 24.2 ) [44] . The size of the nanoparticle core was found to decrease with

increasing dendron generation, a ﬁ nding which contrasted with data reported for

other NCD systems [39, 41, 45] . These differences suggested that the structure and

Scheme 24.3 Heck reaction (top) and Suzuki reaction

(bottom) with Pd - G - 3 NCDs. TON = turnover number;

TOF = turnover frequency. Reproduced with permission from

Scheme 24.4 Basic hydrolysis of ester - terminated NCDs to

produce unimolecular micelles. Reproduced with permission

24.2 Synthesis of Nanoparticle-Cored Dendrimers via the Direct Method, and their Properties

749

750 24 Nanoparticle-Cored Dendrimers and Hyperbranched Polymers: Synthesis, Properties, and Applications

functionality of the dendron ligands must play an important role in determining

the characteristics of the gold core. Such a size relationship could be explained in

terms of steric effects, as the much more bulky higher - generation dendritic system

would be expected to pack more efﬁ ciently around a small core, thus favoring a

smaller particle size. The thermal stability of the NCDs in solution was governed

by the extent of branching in the surrounding dendron ligands. A different order

of thermal ripening was observed at 120 ° C, which increased in size in the order

[G - 1] > [G - 2] > [G - 3].

Astruc and colleagues synthesized redox - active NCDs using the Schiffrin reac-

tion from a 1:1 mixture of dodecanethiol and nonaferrocenyl thiol dendrons

(Scheme 24.5 ) [46, 47] . However, this approach resulted in NCDs without any open

catalytic sites on the nanoparticle core. Smaller dodecanethiols could be coassem-

bled on the nanoparticle surface, so that the integrity and solubility of NCDs was

suitable for their applications in the electrochemical sensing of anions. The gold

nanoparticle core was seen to be surrounded by ∼ 360 silylferrocenyl units, such

that the general structure of these NCDs closely resembled that of large metal-

lodendrimers. These redox - active NCDs were capable of selectively recognizing

HPO

−

anions and adenosine - 5 ′ - triphosphate (ATP

2 −

) with a positive dendritic

effect, even in the presence of other anions such as Cl

−

and HSO

−

. The titration

of NCDs using anions resulted in a shift of the cyclic voltammetry ( CV ) wave to

a less positive potential. The nonaferrocenyl thiol dendron - functionalized nano-

particles could be deposited on the Pt electrode by dipping it into the NCD solu-

tion. This modiﬁ ed electrode was quite robust and was capable of recognizing

different anions. Moreover, the salt of these anions could be easily removed simply

Figure 24.3 Absorption spectra of pinacyanol chloride

(1 × 1 0

− 5

M) in water (PC) and in aqueous solutions of

(a) Au - G1(CO

Na), (b) Au - G2(CO

Na), (c) Au - G3(CO2Na),

and (d) Au - G4(CO2Na). [Au - G n (CO

Na)] were ∼ 0.1 mg ml

− 1

The American Chemical Society.

Scheme 24.5 Direct synthesis of NCDs containing the

nonaferrocenyl thiol dendron. Reproduced with permission

24.2 Synthesis of Nanoparticle-Cored Dendrimers via the Direct Method, and their Properties

751

752 24 Nanoparticle-Cored Dendrimers and Hyperbranched Polymers: Synthesis, Properties, and Applications

by dipping the electrode in CH

, the consequence being that the electrode could

be re - used many times.

Phenothiazine - terminated NCDs were prepared by Fujihara and coworkers

using the two - phase Schiffrin reaction ( 6 ; Figure 24.2 ) [48] . The gold nanoparticles

with a higher - generation dendron bearing a long chain alkanethiol at the focal

point had a smaller core size with a narrow size distribution. These NCDs under-

went the spontaneous formation of 1 - D arrays (Figure 24.4 ), and exhibited an

interesting one - electron transfer behavior. The intermolecular π – π stacking inter-

action of the thiol - terminated phenothiazine is believed to be the driving force for

this self - organization. The 1 - D assembly of redox - NCDs not only provides a good

model to study the size - dependent electronic and optical properties of metal nano-

particles, but may also play an important future role in nanoelectronics.

Dendrons with another focal group that is capable of metal complexation were

reported by Zheng et al. [45] . The Oct

- AuCl

in toluene was reduced by NaBH

in the presence of 4 - pyridone - functionalized dendrons ( 7 ; Figure 24.5 ) as a capping

reagent, and this resulted in stable, gold nanoparticle - cored dendrimers. There is

no known example of monolayer formations of pyridone derivatives on bulk gold

surfaces due to the chemical instability of such monolayers. Therefore, these

results suggests that the high surface curvature of the gold nanoparticles can

support the high - density packing of the capping ligands, which have only weak

metal surface - binding properties. An examination using TEM highlighted the

correlation between the particle size and the generation of dendrons, with higher -

generation dendrons producing larger particles ([G - 1], [G - 2], [G - 3] = 2.0, 3.3,

5.1 nm, respectively). However, the [G - 3] NCDs were less stable than [G - 1] and

[G - 2] NCDs, because the larger dendrons led to an increased open space between

the ligands. The resultant weaker force between the dendrons and the particle

caused a more rapid particle agglomeration of [G - 3] NCDs.

Figure 24.4 Transmission electron microscopy image of

one - dimensional arrays of phenothiazine - terminated NCDs.

Royal Society of Chemistry.

Dendrons with an ester focal point were also used for the synthesis of polyaryl

dendron - protected Pd nanoparticles ( 8 ; Figure 24.5 ) [49] . Brieﬂ y, H

PdCl

was

phase - transferred into the organic phase using tetraoctylammonium bromide. The

dendrons were then added to the reaction mixture before addition of N

reducing agent. The resultant mixture was stirred under dry argon for an addi-

tional 24 h at room temperature. The Pd nanoparticle - cored dendrimers formed

had mostly a spherical shape, and an average core size which ranged from ∼ 10 to

∼ 70 nm. The Pd core size was seen to increases with the decreasing molar ratio

of dendrons to metal ions.

24.3

Synthesis of Nanoparticle - Cored Dendrimers by Ligand Exchange Reaction, and

their Properties and Applications

Nanoparticles with protecting monolayers composed of thiolate ligands can be

functionalized by a partial ligand - replacement [1] . In this exchange reaction, the

incoming ligands replace the thiolate ligands on nanoparticles by an associative

reaction, while the displaced thiolate becomes a thiol. Ligand - exchange reactions

of dodecanethiolate - protected gold nanoparticles with triferrocenyl thiol dendrons

in dichloromethane were ﬁ rst reported by Astruc et al. (Scheme 24.6 ) [46, 47] .

Although an excess of functional thiol dendrons was used, the percentages of

dendron thiols introduced as ligands in dodecanethiolate - protected gold nanopar-

ticles were less than 5% of the overall ligands. The limit of the ligand - exchange

reaction was even greater with larger dendrons. Attempts to synthesize NCDs with

nonaferrocenyl thiol dendrons resulted in the incorporation of very small amounts

of dendrons (less than one per nanoparticle). As the rate of ligand - exchange

Figure 24.5 Structure of dendrons with other functional

groups used for the synthesis of nanoparticle - cored

dendrimers.

24.3 Synthesis of Nanoparticle-Cored Dendrimers by Ligand Exchange Reaction, and their Properties 753

754 24 Nanoparticle-Cored Dendrimers and Hyperbranched Polymers: Synthesis, Properties, and Applications

between thiols is dependent on the chain length and/or steric bulk of the initial

monolayers on nanoparticles, the bulky or very small incoming ligands are difﬁ cult

to replace the original ligands on the nanoparticle surface due to either kinetic

(e.g., steric hindrance) or thermodynamic effects, respectively. Thus, the main

short - coming of the ligand - exchange reaction, when using larger dendrons, was

in fact quite consistent with previous results on ligand - place exchange reactions

of alkanethiolate - protected nanoparticles [1, 3] . In contrast, the exchange between

ligands having different functional groups was much more efﬁ cient.

Peng et al. synthesized hydrophilic NCDs by the ligand - exchange of citrate -

capped gold nanoparticles using hydroxy - functionalized dendron thiols ( 9 ; Figure

24.6 ) [50] . The citrate reduction of HAuCl

in water led to citrate - capped gold

nanoparticles with core sizes that ranged from 10 to 150 nm [3] . Such nanoparticles

Scheme 24.6 Synthesis of NCDs using the thiol - ligand

exchange reaction. Reproduced with permission from Ref.

are typically useful when a rather loose shell of ligands is required around the gold

core for ligand - exchange [64] . Peng also performed ligand - exchange reactions of

trioctylphosphine oxide ( TOPO ) - capped CdSe nanoparticles with hydroxy - func-

tionalized dendron thiols (Scheme 24.7 ) [50, 51] . Dendron thiols can rather easily

replace the loosely bound TOPO groups, which have a weak binding property with

CdSe nanoparticles. The chemistry related to CdSe NCDs can be applied for devel-

oping photoluminescence - based labeling reagents for biomedical applications.

The photochemical, thermal, and chemical stability of both CdSe and Au NCDs

was exceptionally good compared to that of the corresponding alkanethiolate -

Figure 24.6 Structure of hydroxy - functionalized dendrons

used for the synthesis of nanoparticle - cored dendrimers.

Scheme 24.7 Schematic process for converting hydrophobic

semiconductor nanoparticles into hydrophilic and chemically

processable NCDs. Reproduced with permission from Ref.

24.3 Synthesis of Nanoparticle-Cored Dendrimers by Ligand Exchange Reaction, and their Properties

755

756 24 Nanoparticle-Cored Dendrimers and Hyperbranched Polymers: Synthesis, Properties, and Applications

protected nanoparticles. The stability of CdSe NCDs could be further enhanced by

a global crosslinking of the dendron ligands around a nanoparticle core [51] . Such

crosslinking of alkene - terminated CdSe NCDs was achieved via ring - closing

metathesis, and led to each nanoparticle being sealed in a dendron box. Another

strategy – dendrimer bridging – was also reported by Peng for the simultaneous for-

mation and functionalization of a biocompatible and bioaccessible dendron box

with a semiconductor nanoparticle core [52] (Scheme 24.8 ). The dendrimer bridg-

ing involved an activation of the hydroxyl terminal groups of CdSe NCDs with a

homobifunctional crosslinker, N,N - disuccinimidyl carbonate, followed by a

crosslinking reaction with [G - 2] polyamidoamine ( PAMAM ) dendrimers.

The resulting amine box nanoparticles were very stable chemically, thermally,

and photochemically. In addition, the amine groups on the surface of the box

nanoparticles provided reactive sites for the conjugation of biological entities.

Scheme 24.8 Synthesis of amine - functionalized box

nanoparticles. Reproduced with permission from Ref. [51];

Figure 24.7 Simanek (melamine) - type dendron - coated iron oxide nanoparticles.

Bioconjugation and biodetection using box nanoparticles were each successfully

demonstrated with the avidin – biotin system [52] .

Liu and Peng reported the synthesis of highly luminescent water - soluble CdSe/

CdS core – shell NCDs by ligand - exchange reactions [53] . A dendron ligand with

eight hydroxyl terminal groups and two carboxylate anchoring groups was found

to replace alkylamine ligands on CdSe/CdS nanoparticles in toluene, and to

convert the particles to water - soluble NCDs ( 10 ; Figure 24.6 ). The resultant water -

soluble NCDs retained 60% of the photoluminescence value of the alkylamine -

protected CdSe/CdS core – shell nanoparticles in toluene. Compared to the dendron

thiol - protected NCDs, the photoluminescence value of these NCDs was much

higher (about sixfold). Moreover, the UV - brightened photoluminescence could be

retained for several months.

Dendron (with a shell of Simanek) - fuctionalized superparamagnetic (iron oxide)

nanoparticles were prepared by Gao et al. using ligand - exchange methods (Figure

24.7 ) [54] . The resulting Fe

NCDs exhibited a switchable solubility in a variety

of solvents, depending on the terminal groups of the dendritic shells, and were

examined as soluble matrices for supporting magnetically recoverable homogene-

ous Pd catalysts for Suzuki crosscoupling reactions in organic solvents. For this,

Gao immobilized a Pd - triphenyl phosphine moiety to the termini of dendron -

protected iron oxide nanoparticles for catalysis. The Fe

NCDs were also inves-

tigated as potential contrast agents for magnetic resonance imaging ( MRI ) in

aqueous media.

24.3 Synthesis of Nanoparticle-Cored Dendrimers by Ligand Exchange Reaction, and their Properties 757

758 24 Nanoparticle-Cored Dendrimers and Hyperbranched Polymers: Synthesis, Properties, and Applications

24.4

Synthesis of Nanoparticle - Cored Dendrimers by Dendritic Functionalization, and

their Properties and Applications

The direct synthesis of NCDs was somewhat problematic because it required a

large excess of dendronized thiols or disulﬁ des, especially for the synthesis of

NCDs with a small and monodispersed nanoparticle core [39 – 41] . The direct

method also provided little control over the nanoparticle core dimension. NCDs

with different core size were produced, when dendrons with different sizes (or

generations) were used. In addition, the NCDs synthesized by the direct method

often contained small amounts of trapped tetraoctylammonium bromide that

could not be removed completely, even by repeated extraction with solvent. The

indirect method using the ligand - exchange reaction was also somewhat limited

considering a low exchange rate, especially for the synthesis of metal nanoparticle -

cored dendrimers, as described in Section 24.3 . A more convenient and cost - efﬁ -

cient synthetic methodology for the synthesis of NCDs with controlled particle

core sizes, generations, and dendritic wedge density has been considered highly

desirable for the basic understanding of structure – property relationships of these

nanostructures.

The present authors ’ approach is based on a strategy in which the synthesis of

monolayer - protected nanoparticles is followed by building the dendrimer architec-

ture on the nanoparticle surface, using either the convergent or divergent approach

(Scheme 24.9 ). A convergent synthesis of NCDs ( 11 ) can be accomplished by the

coupling of reactive nanoparticles with functionalized dendrons. As the reaction

only takes place at the functional groups on the surface of nanoparticles, this

approach eliminates the need for a large excess of functionalized dendrons, and

also maintains an intact core size for the synthesis of NCDs with different interior

layers (generations) and dendritic wedge densities. The divergent approach

Scheme 24.9 General reaction schemes for the synthesis of

nanoparticle - cored dendrimers by ( 11 ) convergent and ( 12 )

divergent approaches.