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

involves an iterative sequence of reaction steps, which lead to the addition of a

branch (or branches) to the monolayer - protected cluster ( MPC ) framework ( 12 ).

24.4.1

Nanoparticle - Cored Dendrimers by the Convergent Approach

The convergent synthesis of NCDs (Scheme 24.10 ) was accomplished by the ester

coupling reaction of 11 - mercaptoundecanoic acid - functionalized gold nanoparti-

cles with hydroxy - functionalized “ Fr é chet - type ” dendrons ([G - 1] - OH, [G2] - OH, and

[G - 3] - OH) in the presence of ester coupling reagents ( 1,3 - dicycohexylcarbodiimide

[ DCC ] and 4 - dimethylaminopyridine [ DMAP ]) [56 – 58] . This synthetic approach

provided an access to functionalized NCDs, which contain multiple reactive

(COOH) functional groups inside dendritic wedges, even after the coupling of

dendrons. These unreacted COOH groups resulted from the steric repulsion of

dendrons during the synthesis of NCDs. The appearance of a new band at ∼ 1735 cm

− 1

in FT - IR spectroscopy after the reaction indicated a formation of ester bonds due

to a successful coupling reaction between 11 - mercaptoundecanoic acids and [G - n] -

OH. The

1

H NMR results showed the same peak - broadening effect, as do monolay-

ers of alkanethiolate - protected nanoparticles generated from alkanethiols. This

peak - broadening effect also conﬁ rmed that [G - n] - OH was reacted with 11 - mercap-

toundecanoic acid - functionalized gold nanoparticles and bonded onto the surface

of nanoparticles. The increased organic fractions were also observed by TGA.

The dendritic encapsulation of a functional molecule provides a site isolation,

which mimics principles from biomaterials because dendritic scaffolds can provide

the segregation of external and internal functionality. The incorporation of elec-

troactive compounds onto the surfaces of nanoparticles in dendritic architecture

should provide an opportunity to tune their electrochemical properties. The incor-

poration of redox - active groups in NCDs was achieved by a simple coupling reac-

tion of ferrocene methanol with the remaining COOH groups inside dendritic

wedges of NCDs ([G - 2]

conv

NCDs), as shown in Scheme 24.11 [56, 58] . After an

ester coupling reaction between [G - 2]

conv

NCDs and ferrocene methanol, the IR

spectrum of the NCD - encapsulated ferrocene (Fc@NCDs) showed an increase in

the intensity of a band at 1735 cm

− 1

(ester), in addition to the strong band around

1450 cm

− 1

which indicated the presence of cyclopentadienyl groups. The NMR

spectrum of Fc@NCDs showed additional broad bands at ∼ 4.10 ppm, which was

corresponding to the signals from cyclopentadienyl groups. Cyclic voltammetry

was employed to compare electrochemical behaviors of ferrocene methanol and

Fc@NCDs. Both, ferrocene methanol and Fc@NCDs exhibited well - deﬁ ned vol-

tammetric peaks corresponding to ferrocenyl groups. From the voltammogram of

ferrocene methanol, it could be seen that there was a pair of voltammetric waves

with a peak splitting of 233 mV at 100 mV s

− 1

, indicating a quasi - reversible electron -

transfer processes. Interestingly, the peak splitting was found to be 112 mV at

100 mV s

− 1

for Fc@NCDs, which was only half of that for the ferrocene methanol;

this was most likely due to either multielectron transfer or to the adsorption of

particles onto the electrode.

24.4 Synthesis of Nanoparticle-Cored Dendrimers by Dendritic Functionalization 759

760 24 Nanoparticle-Cored Dendrimers and Hyperbranched Polymers: Synthesis, Properties, and Applications

Scheme 24.10 Synthesis of [G - n]

conv

NCDs by an ester coupling reaction.

24.4.2

Nanoparticle - Cored Dendrimers by the Divergent Approach

The NCDs ( 6 ) were grown in a stepwise manner from a central nanoparticle involv-

ing the amide coupling reaction with 1,2 - diaminoethane (NH

2

CH

2

CH

2

NH

2

) in the

presence of DCC and the Michael addition reaction with methyl acrylate (CH

2

CH-

CO

2

CH

3

) [11] . The repetition of the two reactions in the divergent approach was

attempted for the synthesis of higher - generation NCDs (Scheme 24.12 ) [55, 57] .

The IR spectra of 11 - mercaptoundecanoic acid ( MUA ) MPCs showed a strong

carbonyl (C = O) stretching band at ∼ 1710 cm

− 1

, the disappearance of which and the

appearance of a new band at ∼ 1650 cm

− 1

, after completion of the ﬁ rst reaction,

indicated the formation of amide bonds. This was evidence of the successful cou-

pling reaction between the COOH groups and 1,2 - diaminoethane. The excess

(10/1 = [1,2 - diaminoethane]/[COOH of MUA MPCs]) of 1,2 - diaminoethane was

used to prevent the intermolecular crosslinking of COOH groups on the clusters.

These amine - functionalized MPCs ([G - 0.5]

div

NCDs) were reacted with excess

methyl acrylate, such that the conjugate addition of the amine moiety occurred in

“ cascade ” form (2:1 molar ratio of methyl acrylate to amine functional group) to

generate [G - 1]

div

NCDs with terminal ester groups. The appearance of an ester

band at ∼ 1730 cm

− 1

was a clear indication of the successful addition reaction. The

amide bands at ∼ 1650 cm

− 1

indicated the presence of internal amide groups. Fol-

lowing the amide coupling reaction of [G - 1]

div

NCDs with 1,2 - diaminoethane, the

disappearance of the ester band at 1730 cm

− 1

was clearly observed, indicating the

Scheme 24.11 Incorporation of a ferrocene moiety in NCDs.

24.4 Synthesis of Nanoparticle-Cored Dendrimers by Dendritic Functionalization

761

762 24 Nanoparticle-Cored Dendrimers and Hyperbranched Polymers: Synthesis, Properties, and Applications

Scheme 24.12 Synthesis of [G - 1]

div

NCDs and [G - 2]

div

NCDs.

formation of amine - functionalized [G - 1.5]

div

NCDs. The reaction was again fol-

lowed by the Michael addition reaction of [G - 1.5]

div

NCDs; this resulted in the

reappearance of ester bands in the IR spectra of [G - 2]

div

NCDs. However, the rela-

tively low intensity of this band suggested that the reaction had not been com-

pleted. Further iterative reactions of [G - 2]

div

NCDs were no longer effective and

did not involve incorporation of methyl acrylate, as evidenced by an absence of

ester bands in the IR spectra of [G - 2]

div

NCDs. A TEM image of the [G - 1] NCDs

showed the presence of slightly fused domains upon evaporation of the solvent.

Additional TEM images of higher - generation NCDs prepared by the divergent

approach showed that the multistep reactions of NCDs had resulted in the forma-

tion of large aggregates. Such aggregate formation prevented or minimized

any further dendritic branching reactions for the higher - generation NCDs. This

aggregation behavior of “ Tomalia - type ” NCDs during the iterative reactions

probably resulted from the presence of strong inter - cluster hydrogen - bonding

interactions.

24.5 Synthesis of Nanoparticle-Cored Hyperbranched Polymers by Grafting on Nanoparticles 763

24.5

Synthesis of Nanoparticle - Cored Hyperbranched Polymers by

Grafting on Nanoparticles

Further studies regarding the synthesis and characterization of NCDs are impor-

tant for a fundamental understanding of nanostructures, and could also generate

interesting technological applications in areas such as catalysis and sensing.

However, one weakness of investigations involving NCDs is that multiple reaction

steps and puriﬁ cation processes must be included in the synthesis of functional-

ized dendrons. As hyperbranched polymers may have properties and structures

which are quite similar to those of dendrimers (even though the synthetic proce-

dure requires only a direct, one - pot grafting reaction), the substitution of dendron -

capping ligands with hyperbranched polymer ligands would deﬁ nitely beneﬁ t

in realizing some of the proposed technological applications in the real world,

and the commercialization of these nanostructures. Some examples of grafting

hyperbranched polymers on either silica or zinc oxide nanoparticle surfaces have

recently been reported [59 – 61] .

The ﬁ rst example of nanoparticle - cored hyperbranched polymer s ( NCHP s) was

prepared by self - condensing vinyl polymerization via atom transfer radical polym-

erization ( ATRP ) from OH - functionalized silica nanoparticle surfaces (Scheme

24.13 ) [59] . For polymerization, the ATRP initiators were covalently attached to the

Scheme 24.13 Synthesis of nanoparticle - cored hyperbranched

polymers by self - condensing vinyl polymerization via atom

transfer radical polymerization.

764 24 Nanoparticle-Cored Dendrimers and Hyperbranched Polymers: Synthesis, Properties, and Applications

surface of silica nanoparticles. Well - deﬁ ned hyperbranched polymer chains with

multifunctional bromoester end groups were conﬁ rmed by gel permeation chro-

matography ( GPC ), GPC/viscosity, and NMR. These NCHPs can be designed to

have a fairly open structure, allowing the functional materials (such as metal ions)

to penetrate the ligands more easily than in conventional linear polymer layers.

Therefore, a well - controlled synthesis for these NCHPs might lead to the creation

of novel core – shell hybrids that are controllable on the nanoscopic scale, and have

chemically sensitive interfaces.

A similar ATRP reaction involving a surface - initiated self - condensing vinyl

polymerization of p - chloromethyl styrene was used for the preparation of NCHP

from NH

2

- functionalized silica nanoparticles [60] . The surface Br atom of the

NCHP could initiate another surface - initiated ATRP of methyl methacrylate and

produce a dendritic - graft copolymer grafted onto silica nanoparticles. The same

strategy was applied for the synthesis of ZnO nanoparticle - cored hyperbranched

polymers with multifunctional chlorobenzyl functional end groups [61] .

24.6

Conclusions and Outlook

Nanoparticle - cored dendrimers or hyperbranched polymers can be prepared using

four different synthetic strategies: (i) a direct method, using a modiﬁ ed Schiffrin

reaction; (ii) an indirect method, involving a ligand - place exchange reaction; (iii)

the dendritic functionalization of nanoparticles; and (iv) the surface grafting of a

hyperbranched polymer. The direct method is especially useful for the synthesis

of nanoparticle - cored dendrimers with a large fraction of unpassivated metal nano-

particle surface, which is ideal for catalytic reaction. The indirect method, using a

ligand - place exchange reaction, is ideal for the loading of functionalized dendrons

on the surface of nanoparticle core. Dendritic functionalization is most versatile in

terms of controlling the structure and composition of nanoparticle - cored dendrim-

ers, while surface grafting of hyperbranched polymers might be the most conven-

ient method as it can eliminate the multistep organic reactions involved in the

synthesis of dendrons. Today, the synthetic procedures developed for nanoparticle -

cored dendrimer preparation permits near - complete control over the critical molec-

ular design parameters, such as core size, dendron wedge density, surface/interior

chemistry, ﬂ exibility/rigidity, and solubility/micelle - like properties. The availability

of effective methods to prepare distinct nanoparticle - cored dendrimers and hyper-

branched polymers will allow investigators to exploit these materials in a variety of

ways, including dendritic catalysis, energy storage, sensors, and drug delivery.

Acknowledgment

These investigations were supported by a start - up grant and an SCAC grant from

California State University, Long Beach, California, USA.

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Concepts in Self - Assembly

Jeremy J. Ramsden

767

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

25

25.1

Introduction

This chapter begins with some basic deﬁ nitions, followed by a review of the early

studies conducted by Ashby and von Foerster. Some actual systems recognized as

exemplars of self - assembly are then examined, after which self - assembly as a valid

approach to nanofacture is considered; in this context it is usually known as

“ bottom - up ” manufacture. There follow some brief notes on useful ideas for

understanding the self - assembly processes. The aim is not to discuss ordering

related to phase transitions [1] (this will be detailed in a forthcoming article), or

reaction - diffusion systems leading to striation and other regular patterns, nor

phenomena such as spinodal dewetting [2] and other types of segregation (e.g., in

block copolymers).

Assembly means gathering things together and arranging them (ﬁ tting them

together) to produce an organized structure. A boy assembling a mechanism from

“ Meccano ” captures the meaning, as does an assembly line where the workers and

machines progressively build a complicated product such as a motor - car from

simpler components; or when an editor assembles a newspaper from copy pro-

vided by journalists. In all of these examples the component parts are subject to

constraints – otherwise they would not ﬁ t together – and their entropy S must nec-

essarily decrease. Since the free energy Δ G = Δ H − T Δ S must then necessarily

increase, in general the process will not, by itself, happen spontaneously. On the

contrary, a segregated arrangement will tend to become homogeneous. Hence, in

order for self - assembly to become a reality, something else must be included.

Typically, enthalpy H is lost through the formation of connections (bonds) between

the parts, and provided that | Δ H | exceeds | T Δ S | there is at least the possibility that

the process can occur spontaneously – which is presumably what is meant by “ self -

assembly. ” The ﬁ nal result is generally considered to be in some sort of

equilibrium.

As entropy is always multiplied by the temperature T in order to compute the

free energy, it should be noted that the relevant temperature is not necessarily

what is measured with a thermometer. Rather, only the (typically small number

768 25 Concepts in Self-Assembly

of) relevant degrees of freedom should be taken into account; the temperature is

the mean energy per degree of freedom.

If slightly sticky granules are thrown into a rotating drum, or stirred in a mixing

bowl, they will all eventually clump together to form a “ random ” structure (e.g.,

Figure 25.1 a), but one which is evidently less random than the initial collection of

freely ﬂ owing granules. An alternative example might be the dry ingredients of

concrete (sand and cement) when thrown into a “ cement mixer, ” to which a small

amount of water is added. Indeed the entropy is lowered, but each time the experi-

ment is repeated the result will be different in detail. Thus, if the system were

really ergodic, and if there were sufﬁ cient time to make all the repetitions, the

ergodicity would only be broken because of insufﬁ cient time.

The goal of an ideal self - assembly process is for the same structure to be formed

each time the constituent particles are mixed together (Figure 25.1 b), as was

imagined by von Foerster [3] . Of course, energy must be put into the system; in

the “ purest ” form of self - assembly, the energy is thermal (random), but it could

also be provided by an external ﬁ eld (e.g., electric or magnetic).

If we are satisﬁ ed by the constituent particles being merely joined together in a

statistically uniform fashion and, moreover, the process happens spontaneously,

then it is more appropriate to speak of “ self - joining ” or “ self - connecting. ” The

mixing of concrete referred to above is, at least at ﬁ rst sight, an example of such

a self - connecting process; more generally, one can refer to “ gelation . ” On approach-

ing the phenomenon from a practical viewpoint, it is clear that gelation is almost

always triggered by a change of external conditions imposed upon the system,

such as a change of temperature, or dehydration, and the spontaneity implied by

the preﬁ x “ self - ” is absent. The only action that can be imposed upon the system

without violating the meaning of “ self - ” is that of bringing the constituent particles

Figure 25.1 (a) The result of mixing isotropically sticky

cubelets; (b) Putative result of mixing selectively sticky

cubelets.

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

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