University of Groningen Aggregation of gold clusters by ... · 1.1 Nanoparticles and clusters In nanoscience, size matters. For that reason, states of matter in diﬀerent size re-gimes

University of Groningen

Aggregation of gold clusters by complementary hydrogen bondingvan den Brom, Coenraad

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Publication date:2006

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Chapter 1

Introduction

Nanoparticles are at the heart of the scientific revolution that has become known asnanoscience. Full exploitation of their unique properties requires the development oftechniques to manipulate such particles. This thesis describes attempts to understandand control nanoparticle aggregation processes that have been brought about usingconcepts of molecular recognition. This approach enables the fabrication of two-component aggregates that are well-mixed due to specific links between the twocomponents.

This chapter begins by defining nanoparticles and clusters and describing thespecific properties of both individual nanoparticles and collectives of these. The ba-sic interactions between nanoparticles in solution will then be described, since thesedetermine the aggregation behaviour. The next section will show how hydrogenbonding can be applied in supramolecular chemistry to accomplish molecular recog-nition. The same principles may be applied to modify interparticle interactions. Anoverview of the many different ways to assemble nanoparticles into (possibly orde-red) aggregates will then be given, with some focus on techniques using molecularinteractions, in particular hydrogen bonding. Finally, the work presented in thisthesis will be outlined.

1.1 Nanoparticles and clusters

In nanoscience, size matters. For that reason, states of matter in different size re-gimes have been given different names. In order to understand the properties ofnanoparticles, they should be put in perspective (see figure 1.1): They fall in the inter-mediate region between bulk matter and single atoms (or small molecules). Withinthis so-called mesoscopic regime, it is useful to make a distinction between colloidalparticles, nanoparticles and clusters, although the borders between these categoriesare vague. The distinction is not based on mere diameter. Rather, it is derived from

1

2 CHAPTER 1. INTRODUCTION

Figure 1.1: Clusters and nanoparticles put in perspective

other characteristics, which more or less depend on size. Essentially, all of them sche-matically look like the cartoon in figure 1.2: They have a core, with (at least some)solid character. Particles may be coated with a layer which may both protect the coreand modify the particles’ behaviour. Particles are often easily manipulated when theyare dispersed in a liquid medium. Particles may also be embedded in a solid matrixor on a substrate.

A colloid is any system in which one phase, consisting of finite domains is dispersedin another, continuous phase. Most often, a solid phase dispersed in a liquid [1]is meant. The solid phase consists of particles, small enough to be (meta-)stable insolution, giving 1 µm as an approximate upper limit [2]. Though it is strictly speakingnot correct, these particles are still addressed as colloidal after removal of the solvent.Colloidal particles are discriminated from smaller particles because their intrinsicproperties are usually still dominated by a bulk-like character.

In contrast, the term nanoparticle should be reserved for particles –in particularthose consisting of semiconducting or metallic materials– whose electronic propertiesare different from the bulk, due to finite size effects. This puts ∼20 nm as their upperlimit, although this depends on the kind of material.

Figure 1.2: Schematic representation of a mesoscopic particle.

1.1. NANOPARTICLES AND CLUSTERS 3

Throughout this work, the term cluster is used only with its (strict) inorganicchemical meaning. Thus, it designates a compound, containing more than two (tran-sition) metal atoms, each of which is bound to at least two of the other metal atoms[3, 4], yet does not (only) contain a linear chain of metal atoms.1 Larger clusters(natoms � 20) are best considered a special kind of nanoparticles. The key feature ofclusters is their definite stoichiometry, meaning that (in principle) all clusters of onebatch have exactly the same composition and geometry. This discriminates them fromnanoparticles in general, since many of the latter cannot be obtained without somespread in composition and hence in particle diameter. That is, whereas nanoparticlescan be polydisperse, a cluster compound is monodisperse. The larger the particlesare, the closer the energies of formation of slightly differing particles will be, makingit more and more difficult to obtain a single phase. The largest cluster that could beisolated this way is Pd2057Phen78O1600 [5, 6].

1.1.1 Properties of nanoparticles

In the previous paragraph nanoparticles have been set apart from both atoms andbulk material. Their intermediate size gives rise to distinct properties.

When the size of a metal is reduced to a particle of a few nanometres in diameter,bulk descriptions of the electronic structure are no longer valid[7]. The effects this hason the physical properties are generically called Cluster Size Effects [8]. The onset ofsuch behaviour can be understood classically, by assuming that a nanoparticle can beapproximately described as a uniform conducting sphere. This is the so-called LiquidDrop Model (LDM) [8, 9]. Two well-known finite size effects are (1) the loweringof the melting temperature [10, 11] and (2) the divergence of electron affinity andionisation potential from the bulk work function [8]. The specific heat of metallicclusters (e.g. Pd2057Phen78O1600 and related palladium clusters) contains an electroniccontribution that is strongly size-dependent, and is lower than in the bulk, due to thelower density of states at the Fermi level [5].

If the nanoparticle size is further reduced, the electronic level splitting will increasedue to space confinement and lowering of the number of electrons [7]. The (pseudo)continuous bands will split into discrete levels [8]. This discrete nature gives rise toQuantum Size Effects [8, 10, 12–14]. These QSE start to occur when the level spacingδ(ε) at the Fermi level εF exceeds the thermal energy of the electrons, according to theKubo criterion [7, 15]:

δ(εF) ≈ 32εF

NAz> kBT (1.1)

Here, NA is the Avogadro constant and z is the number of valence electrons per atom.One of the results of these QSE is that progressively smaller metal particles exhibit aSize-Induced Metal-to-Insulator Transition: below a certain threshold diameter they

1In colloid chemistry the term cluster is also used for an assembly of particles. In this work, the wordaggregate is used to designate such assemblies.


are no longer metallic [12, 13, 16]. For instance, Au55 clusters have a gap of ∼ 0.25 eV[17]. Nanoparticles with discrete electronic levels behave as if they are giant atomswith atom-like wave functions. Therefore they can be regarded as Quantum Dots [18].

Due to the small size of a cluster and the band splitting, the addition or removalof an electron (by tunnelling) may have a pronounced effect on the electronic energy.Single Electron Transitions [19] will occur if the tunnel barrier from the particle to itssurrounding is high (i.e. a large tunnel resistance) and the Coulomb charging energyEC is sufficiently large [6, 12, 18, 19]:

EC =e2

2C � kBT, (1.2)

where C is the capacitance. When a nanoparticle is placed between two electrodes,a double tunneling junction will be formed. The conductivity of such a device istherefore limited by a Coulomb blockade [20, 21]. If the bias voltage is increased, a stepmay be observed in the conductivity, every time an additional tunnelling channel isopened. Such a series of blockades is called a Coulomb staircase [12, 18, 19, 22]. If thenanoparticle under investigation is sufficiently small, these effects can be observed atroom temperature [23, 24].

Two aspects are important in the magnetic properties of nanoparticles. The dis-crete nature of the electronic structure may lead to a different source of magnetismthan the itinerant electrons that dominate bulk metal magnetism. Like in molecules,unpaired spins may give rise to a magnetic moment [25]. Notably, the low tempera-ture magnetic susceptibility of Pd clusters shows an odd-even effect as a function ofthe number of electrons on the cluster [5]. For nanoparticles of ferromagnetic metalsthe coercive energy is far smaller than the thermal energy at finite temperatures. Thisleads to superparamagnetic behaviour [25].

Nanoparticles exhibit distinct optical properties. For example, photoluminescencespectra of semiconductor particles are governed by the opening of gaps in the bandstructure and the spatial confinement of excitons inside the nanoparticle [12, 26]. Inmetallic particles, the most prominent feature is the surface plasmon excitation [27].In gold, this gives rise to an intense transition in the visible region [27]. The sensitivityof the plasmon frequency to the environment of the nanoparticle, opens the way forapplication of such particles as sensors [28, 29].

An important general property of nanoparticles is their surface-to-volume ratio.Small particles have a large proportion of their atoms at the surface. This will lead todifferent binding energies, since surface atoms will have a lower coordination numberthan bulk atoms. For example, the Au–Au binding is stronger in gold clusters thanin bulk gold, leading to a smaller bond length [30].

The reactivity of small particles differs from that of the bulk material, due to thisdifferent bonding. In combination with the large surface exposure and high numberof ‘defect sites’, this makes nanoparticles interesting as catalysts [12, 31]. Pd and Ruparticles have been used as catalysts in Heck and Suzuki coupling reactions, electron

1.1. NANOPARTICLES AND CLUSTERS 5

transfer and hydrogenation reactions [12]. Another important example is the lowtemperature oxidation of CO (carbon monoxide) by gold particles [32–34].

1.1.2 Ligand-protected nanoparticles

In many practical cases, nanoparticles are coated with a layer of material. Like withcolloidal particles, this coating serves to protect the particle core against coalescenceand ripening [1, 2].2 The most common nanoparticle capping systems can roughly begrouped into the following categories. (1) Monolayers of individual molecules (i.e.ligands) are very abundant (vide infra), but (2) polymer- [35] or dendrimer-stabilised[36, 37] particles have been reported as well. Slightly different is (3) capping withan inorganic (solid-state-like) material, like silica [38–40], yielding so-called core-shell nanoparticles. This introduction will focus on nanoparticles coated with ligandmolecules. In most cases, these ligands bind to the particle core by a specific functionalgroup. Metallic particles are typically covered with Lewis bases like amines [41],phosphines [3, 42, 43] or thiols [44–47]. The properties of clusters can be altered bythe introduction of additional functional groups in the ligands. Numerous examplesof that have been reported for gold nanoparticles, for instance [46, 48].

The most important general aspect of covering nanoparticles with ligands is theway it affects or even dominates the interaction of a nanoparticle with its surroun-ding. First, the ligands will largely determine a particle’s solubility properties. Inthis way, gold clusters have been made, that were soluble in very different media.For instance, Au55[PPh3]12Cl6 is soluble in polarisable solvents like CH2Cl2 [42, 49],whereas Au55[PPh2C6H4SO3Na] is water-soluble [50], just as Au55 clusters coatedwith cysteine or sugar-based thioethers [51]. Alkyl-chain coated nanoparticles aregenerally soluble in apolar solvents like toluene, diethyl ether or pentane [45]. Re-cently, gold nanoparticles with Zwitter-ionic ligands were found to be soluble in ionicliquids [52].

Second, ligands play an important role in the interactions with other particles[1], or with (bio)molecules [53, 54] (vide infra). Interactions with molecules may alsolead to reactions, in which the nanoparticle substrate is of influence on the reactivity[55, 56] or electrochemistry [57, 58] of its ligands.

Third, the ligands may influence the optical properties of nanoparticles. Obvious-ly, ligands change the charge density at the surface of a nanoparticle and hence thesurface plasmon resonance [27, 59]. The optical properties may also be caused bythe ligands themselves, in particular when they are chromophores [60–63]. An appli-cation of the latter is shown by nanoparticles that function as a scaffold to organisedonors and acceptors in a photovoltaic device [64].

2Coalescence designates particle growth by the merger of two particle cores. Ripening is the growth ofa particle at the expense of another (smaller) particle by migration of constituent atoms or molecules. Bothare driven by a reduction in total surface free energy [1].


1.2 Properties of nanoparticle assemblies

The desire to fabricate assemblies of nanoparticles is driven by the interesting pro-perties exhibited by such assemblies, and the possible applications these give rise to.In general, nanoparticles will start to interact when they are in each other’s vicinity.In assemblies, this will lead to collective behaviour.

The electronic properties of ‘metamaterials’ consisting of nanoparticles, dependon the level structure of the individual particles, their electron affinity and ionisationpotential, and the coupling between the particles [65]. Coupling of the electronic wavefunctions of adjacent particles, enables wave propagation, allowing electron transportby ‘super-tunnelling’ [66]. This coupling can be tuned: a Langmuir film of Agnanoparticles could be reversibly changed from insulating to metallic by compressionof the interparticle spacing [67].

In fact, other properties can be tuned as well by changing the inter-particle spacing.For example, this is the case for the magnetic behaviour of magnetite nanoparticles[68] and notably for the surface plasmon related optical properties of metallic nano-particles [67, 69].

The spacing-response has been utilised in applications where assemblies of parti-cles act as vapour sensors. The swelling or compression of the interparticle spacingcan be measured from the optical or electronic response [70–73]. Similarly, assem-blies have been prepared with photoresponsive [74] or thermosensitive spacings [75].Electrostatic modification of the spacing even allowed the use of a layer of goldnanoparticles as a mechanical actuator [76].

An ordered geometry is required in the application of nanoparticle assemblies asphotonic crystals [77–79].

The preparation of one-dimensional arrangements of nanoparticles is more ofa challenge than two- or three-dimensional assembly, since it requires anisotropy,either in the interactions leading to assembly or in a (templated) substrate [80]. Themorphology of such arrangements also leads to anisotropic properties, for instancein photonic or electronic devices [24, 80, 81].

1.3 Nanoparticle interactions

Inducing aggregation in a solution of nanoparticles requires the preparation of a stablesolution of nanoparticles and subsequent (controlled) destabilisation. The aim of thisthesis is to modify the total interparticle interaction, by the addition of molecularinterligand interactions. In this section a brief overview will be given of the otherinterparticle interactions that are relevant to such a process.

The study of colloidal interactions has become a well-developed field over thepast decades [1, 2]. Therefore, it provides a good starting point for the discussion.Basically, theories of colloidal interaction aim to explain why colloidal dispersions

1.3. NANOPARTICLE INTERACTIONS 7

are stable despite the presence of a universal interparticle attraction. This attractionis provided by London-Van der Waals dispersion interactions. Hamaker has derivedthe following relation for the potential energy of interaction VA of colloidal particleswhen the interparticledistance is small compared to theparticle size: VA = −A/12πD2,where A is the (effective) Hamaker constant and D is the interparticle separation [2,p.272]. Notice that this interaction depends on 1/D2, giving it a much longer rangethan atomic interactions, which scale as 1/D6 [82]. In the absence of stabilising forces,the interparticle distance will decrease due to Van der Waals attraction until it isbalanced (at a primary minimum) by the short range repulsion from electron overlapbetween the particles.

Many colloidal particles possess a surface charge. Electrostatic repulsion thereforeis the main stabilising force in many colloids. This interaction will be moderated ifoppositely charged entities are present in solution (counter-ions). The increase inconcentration of the latter immediately around a colloidal particle gives rise to a so-called charge double layer. The total interaction of charged colloids is well describedby a classical theory developed by Deryagin, Landau, Verwey and Overbeek (DLVO-theory) [2, pp.47–53][1, pp.130–134]. The existence, position and depth of a potentialenergy minimum (as a function of interparticle separation) depends on the balancebetween particle attraction and electrolyte concentration. Further details are outsidethe scope of this introduction.

Colloids may be stabilised by molecules (often polymers) adsorbed on the surface.Their stabilising action usually depends on steric repulsion preventing particles tocome so close that Van der Waals interactions could cause aggregation [1, pp.45–50].Besides, surface molecules determine the solubility of particles in a given solvent.Solvation effects may also contribute to the stability of a colloidal dispersion. Orien-tation and osmotic effects in the confined space between particles in close proximity,may give rise to an effective pressure increase, which acts as a repelling force. Surfacemolecules can enhance these effects [1].

In comparison with colloids, it is more difficult to model the interactions for smallnanoparticles (from first principles), because their behaviour is intermediate betweenthat of small molecules and of colloids. For example, the interactions of 1.2 nmsilicotungstic acid particles in water could only be modelled straightforwardly (byadhesive hard sphere models or square well potentials) when the interactions weredrastically reduced, i.e. at very high background electrolyte concentrations (2–5MLiCl) [83].

On the other hand, it is much harder for a small particle in a non-electrolytesolution to acquire a charge than for larger colloids (see §1.1.1), unless they containeasily ionised substituents. Therefore, charge will not be taken into account in therest of this discussion, nor will magnetic interactions [25].

In particular Van der Waals interactions for nanoparticles are difficult to model,since the continuity approximations (allowing to replace summations by continuous


integrations) that are applied for colloidal particles break down for particles of smal-ler sizes [84–86]. In typical simulations, these interactions are therefore calculatedby summation of all two and three-atom interactions between two particles [84–86].Interestingly, solvation forces due to the confinement of solvent molecules betweennanoparticles can be similar in size to Van der Waals forces. As a function of inter-particle distance, they can oscillate between attractive and repulsive, due to orderingphenomena and excluded volume effects. The exact behaviour of these systems de-pends on how lyophobic or lyophilic the nanoparticle is. In many experimental cases,the nature of such solvent interactions will be determined by the capping ligands.

Finally, there may be specific ligand-related interactions. These depend heavilyon the (functional group) structure of such molecules. Ligands can actually interferewith all of the previously mentioned aspects (regarding charge, dipole moment,polarisability), but they can add more, due to the generally more localised nature ofthe interaction sites. Some types of localised intermolecular interactions may lead tomolecular recognition. More examples of the latter will be given in §§1.5.6,1.5.7, butfirst molecular recognition will be treated in a more general fashion.

1.4 Hydrogen bonding in supramolecular chemistry

The aim of this thesis is to bring about specific interactions between nanoparticles, bya method that hinges on concepts from supramolecular chemistry, or –more precisely–on molecular recognition. In this section, these two ‘concepts’ will be described andhydrogen bonding will be introduced as a means to realise supramolecular architec-tures.

Lehn defined supramolecular chemistry as follows: “Beyond molecular chemistrybased on the covalent bond there lies the field of supramolecular chemistry, whose goalit is to gain control over the intermolecular bond. It is concerned with the nextstep in increasing complexity beyond the molecule towards the supermolecule andorganized polymolecular systems, held together by non-covalent interactions.”[87,p.2]

One speaks of molecular recognition when there is a large preference for a certainmolecule (a receptor) to bind a specific other molecule (the substrate), compared toother substrates. Lehn discerns five requirements for molecules that recognise eachother [87, ch.2]: 1) steric complementarity, 2) interactional complementarity, 3) largecontact areas, 4) multiple interaction sites, and 5) strong overall binding.

Multiple hydrogenbonding is a prototypical example of a non-covalent interactionthat has proven to be very suitable to achieve molecular recognition.

1.4. HYDROGEN BONDING IN SUPRAMOLECULAR CHEMISTRY 9

Figure 1.3: Schematic representation of molecules capable of molecular recognition by thecomplementary juxtaposition of hydrogen bond donor (D) and acceptor (A) substi-tuents.

1.4.1 Hydrogen bonding

Hydrogen bonding is a bonding interaction between (molecular) functional groups,that may occur if one of them contains a hydrogen atom and the other contains alone-pair of electrons. Such an interaction, for example between a N–H, O–H or F–Hdonor group and a N, O or F acceptor group is regarded as ‘normal’ hydrogen bonding[88, ch.2].3

When a hydrogen atom is bound to an electronegative atom, part of its electrondensity is transferred to the latter. Since hydrogen contains only one electron, this des-hielding of the nucleus has a much larger effect than in the case of heavier elements.Hence, it can interact with other electron-rich groups (i.e. atoms with lone pairs).The total interaction can be considered as a sum of several contributions. In ‘normal’hydrogen bonding, the largest contribution stems from the electrostatic interactionbetween donor Dδ−–Hδ+ and acceptor Aδ− [88]. Nevertheless, the bonding will alsoexhibit some covalent character since the formation and (partial) occupation of (su-pra)molecular orbitals from atomic orbitals of the D, A and H atoms is energeticallyfavourable [82].

A single normal hydrogen bond is of intermediate strength compared to covalentbonds and intermolecular Van der Waals interactions, with ∆H◦HB ≈ 15–60 kJ/mol[88, 89]. Therefore hydrogen bonding is a highly dynamic phenomenon in solutionsat ambient conditions. The versatility of hydrogen bonding is increased by the sen-sitivity of the binding strength to the composition of the medium, predominantly bysolvation and competitive hydrogen bonding [90–92]. A more detailed introductionto medium effects on hydrogen bonding strengths will be presented in §3.2.

1.4.2 Motifs for molecular recognition

The donor/acceptor asymmetry makes hydrogen bonding a directional interaction.Therefore the complementarity requirements (1–4) for molecular recognition can ea-sily be met by properly designed molecules containing an array of H-bonding sites(see figure 1.3). Naturally, multiple hydrogen bonding is stronger than single hydro-

3The whole spectrum of hydrogen bonding is much wider and quite vaguely bordered, ranging fromweak H-bonding interactions involving C–H donors or π-electron containing acceptors for example, tostrong hydrogen bonding between ionised donors and acceptors [88].


gen bonding. Some examples of threefold and fourfold motifs are depicted in figure1.4, together with the association constant of each couple [93, 94]. There are notabledifferences between the different three-fold or four-fold association strengths, due tosecondary electrostatic interactions [95, 96]. Thus, a wide range of bonding strengthsis available for application [93, 97, 98]. Apart of bonding strength, bonding geometryis of importance. In that respect, it is useful to distinguish monotopic moieties like infigure 1.4 from ditopic (or multitopic) moieties that can bind to more than one substratesimultaneously [87], like in the barbituric acid/melamine motif depicted in figure 1.5[99].

1.4.3 Supramolecular architectures

The applicability of motifs like in figures 1.4 and 1.5 in the formation of supramole-cular structures has been widely explored. Perhaps the most intriguing ‘application’of recognition by two- and three-fold hydrogen bonding is the formation of com-plex structures of DNA and RNA that is on the basis of heredity in natural life [100].Reversible polymers, consisting of bi- or multifunctional monomers have been prepa-red, based on (monotopic) triple [97, 101] and quadruple hydrogen-bonding moieties[102, 103].

Similar hydrogen-bonding substituents play an important role in compoundsthat can gelate organic liquids [98, 104]. They have also been applied as reversiblecross-links in polyolefins [105] or to attach functionalised polymers to self-assembledmonolayers [106].

Ditopic or multitopic motifs have been applied to make linear chains [107] ororderedpatterns [108, 109] on 2Dsurfaces. Especiallymotifs basedon the combinationof derivatives of melamines and barbiturates or cyanurates like in figure 1.5 have beenwidely applied. They can form macrocycles [110], tape-like structures [99, 111], gels[104], double ’rosettes’ in solution [112, 113] and on surfaces [114, 115]. Somewhatrelated to gelation, is the formation of liquid crystals (mesogenic phases) due tomolecular recognition between molecules that are no mesogens themselves [116, 117].Three-dimensional hydrogen-bonded networks of great variety may be found incrystals of organic molecules [118].

The structure-forming power and versatility that can be achieved via hydrogenbonding has also been applied to assemble molecules with different functionalities.Examples of the latter are the organisation of dipolar chromophores on surfaces toyield structures with tunable electro-optic properties [119, 120] and the assembly ofelectron-donor and acceptor materials in organic photovoltaics [121, 122].

1.4. HYDROGEN BONDING IN SUPRAMOLECULAR CHEMISTRY 11

Figure 1.4: Selected examples of molecules that bind selectively by multiple hydrogen bonding.(Examples taken from [93].)


Figure 1.5: Formation of a tape-like structure by the ditopic barbituric acid/melamine motif. Bis diethylbarbituric acid and M is N,N’-diphenyl-melamine.

1.5 Nanoparticle aggregation and ordering

An overview of the strategies that have been employed to prepare (possibly ordered)aggregates of nanoparticles from solution will be presented in this section. For cla-rity, these techniques have been grouped into categories, corresponding to the typeof interactions used. We will begin with general non-directional forces, and move to-wards particle aggregation driven by inter-ligand molecular recognition. Therefore,after treatment of aggregates formed by for example Van der Waals forces or elec-trostatic attraction, the attention will move to ligand-related aggregation, showingthe possibilities of covalent and more ’soft’ binding. The versatility of recognisinginteractions is shown by approaches utilising either molecules with a biological back-ground or completely synthetic ligands. Extra attention is payed to methods that leadto the controlled formation of two-component assemblies of particles.

1.5.1 Assembly and order

A distinction needs to be made between mere assembly (or aggregation) of nanopar-ticles and the formation of ordered assemblies, since the latter poses some additionalrequirements. An assembly of particles designates a state in which a number of par-ticles is kept close together in a geometrically stable (or fixed) state. Roughly, such astate can be reached by (1) confinement of the particles (by increasing their concen-tration and subsequent immobilisation), (2) by inducing a net attractive inter-particleforce. In some cases, interaction with a substrate surface plays a role. In fact, mostinter-particle forces will have a particle-surface equivalent.

An assembly is considered ordered if it exhibits (long range) translational symmetryin either two or three dimensions. To achieve order, the nanoparticle material shouldbe sufficiently monodisperse. In practice, a polydispersity smaller than 5–6% is

1.5. NANOPARTICLE AGGREGATION AND ORDERING 13

required [123, 124]. Moreover, the aggregation process should be sufficiently smooth.Therefore, the mobility of the nanoparticles (mainly by diffusion) should be largeenough compared with the sticking probability or the rate of concentration increase.Thus, particles may move until they have reached a favourable position by optimisingthe interparticle interactions [125]. In general, this implies that these interactionsbe weakly attractive, yet of sufficiently long range, thus guiding the particles totheir optimal positions. If these conditions are not met, for example by particlesaggregating due to a short range strong attraction, amorphous and open structuresresult (cf. Chapter 4).

1.5.2 Aggregation by increasing confinement

The most straightforward way to achieve aggregation by confinement, is by evapo-ration of the solvent, starting from a solution of nanoparticles. This may lead to theformation of a glassy state, but by optimising parameters like concentration, solvent-particle interactions and evaporation rate, ordered aggregates may be produced [125].This method has been widely applied [126–130]. Usually, the required attractive forcestems from the Van der Waals interaction.

This attraction may be augmented by Van der Waals interactions of protectiveligand layers. For example, the resulting interdigitation of alkyl-chains on alkylthiol-protected gold nanoparticles gives rise to several phases, depending on alkylchain length, particle size and the conditions during aggregation by slow solventevaporation [126, 130, 131].

Alternatively, colloids may be drawn together by capillary forces on a substrateslowly retracted from a colloidal solution [132, 133]. Two-dimensional order may alsobe introduced by confining particles by a Langmuir Blodgett technique [134, 135].

It should be noted that this class of interactions is not selective or directional.Thus, it is of limited applicability in the construction of multi-component nanoparticleassemblies.

1.5.3 Aggregation by induced attraction

Aggregation of dispersed nanoparticles may be induced by disrupting the balance ofinterparticle interactions described in §1.3, in such a way that the Van der Waals inter-actions become dominant. For instance, the electrostatic repulsion may be decreasedby increasing the ionic strength of the solution by raising the electrolyte concentration[1, 2]. Alternatively, the stabilisation by solvation may be lessened, either by slow ad-mixture of a non-solvent [136], or by combining two solvents with a different vapourpressure, the ’good’ solvent also being the most volatile one [137].

Charged nanoparticles can be coupled electrostatically. This can be achievedby the use of oppositely charged particles [138–140], by combining particles with apolyelectrolyte [141, 142] or by assembling particles on an ionised surface [143].


Figure 1.6: Rafts of bimodal nanoparticles forming an ordered AB2 superlattice array. Takenfrom [151].

Aggregation of nanoparticles in electric [144] or magnetic [126, 145] fields is agood example of the application of external forces. The collection and orderingof nanoparticles on liquid/liquid or liquid/air interfaces hinges on a quite differentexternal force: surface tension [146–148].

1.5.4 Binary systems

In a two-component assembly of nanoparticles, new collective properties may ari-se. Combinations of semiconducting and magnetic (metallic) particles or of electrondonating and accepting particles may be utilised in novel devices. Nanoparticle ag-gregates consisting of two nanoparticle species also are interesting from a materialsengineering point of view [149]. An example would be the production of a nanocera-mic with zero coefficient of thermal expansion by combining materials with oppositethermal expansion coefficients.

Such applications require finely inter-dispersed nanoparticles, i.e. with little or nophase separation. This may be achieved by introducing some kind of complemen-tarity between the components. Two-component assemblies of oppositely chargedparticles were already mentioned. The combination of particles with two different si-zes, may lead to the formation of ‘crystalline’ metamaterials in two [150, 151] or threedimensions [152, 153], provided the size-ratio and stoichiometric ratio are properlyadjusted. A 2D-example of a metamaterial is presented in figure 1.6.

Two-component aggregation can also be accomplished by chemical complemen-tarity (§1.5.7).

1.5.5 Aggregation by chemical linking

The simplest form of chemical linking of nanoparticles is the use of (equi-)bifunctionalbridging molecules. Dithiols have been used to covalently couple gold or silver


particles [154–156]. Similarly, diamines were used to couple Pd and Pt clusters [157].As a next step, multifunctional molecules like tri- and tetradentate thiol-derivativeswere used [158–161], or even functionalised dendrimers [162, 163].

A variety of coupling reactions has been used to link molecules that were alreadyattached to nanoparticles. For example, gold clusters functionalised with mercapto-aniline were coupled by peptide formation upon the addition of succinyl chloride (adiacid chloride) [155]. Photo-coupling of thymine-functionalised gold nanoparticleswas carried out by irradiation with UV light [164]. Organometallic complexation re-actions were used, either with a bridging ligand (alkyl-aluminium bearing Pt clusterswith diols [165]) or with a bridging transition metal ion [166–169]. The latter methodhas proven to be very suitable for the detection of such ions [170].

In general, covalent coupling is irreversible. Therefore it is unlikely that orderedaggregates of nanoparticles can be manufactured by such an approach. Weaker che-mical interactions would be better candidates. The most important weak interactionis hydrogen bonding. Indeed, this interaction is crucial to induce nanoparticle ag-gregation both in many biological and in purely synthetic systems. Many of thesesystems apply the concepts of molecular recognition discussed in §1.4

1.5.6 Bio-inspired aggregation

Nanoparticles have been used as scaffolds for many different forms of biologicalrecognition [54], involving DNA, proteins and receptor-ligand pairing. The latterwas exploited in the aggregation of nanoparticles functionalised with biotin, uponthe addition of streptavidin or analogous compounds [171–175]. Protein-proteinrecognition was used in aggregation of nanoparticles upon coiled-coil formation [176].

The most widely studied form of biological recognition on nanoparticles is the re-combination of DNA or RNA. The first successful attempts to induce DNA-mediatedaggregation of nanoparticles was simultaneously reported by Mirkin [177] and Ali-visatos [178]. Many variations or improvements to the original approach have beenreported [177–188] and have recently been reviewed [189]. The highly detectible chan-ges in electrical [190] or optical [191] properties have made this approach very usefulfor the detection of (specific) DNA [181]. The advantage of using DNA to accomplishnanoparticle binding is the precision which can be achieved, due to the high bindingselectivity of complementary DNA strands [180]. A possible disadvantage in themanufacturing of three-dimensional aggregates is the large size of the DNA-ligandscompared to the nanoparticles themselves.

1.5.7 Aggregation by synthetic H-bonding ligands

In this section, examples of hydrogen bonding on particle surfaces will bediscussed, guided by the following triad of increasing complexity: interacti-on/recognition/assembly. It will be shown how ligands with an increasing number


Figure 1.7: Three categories of molecular recognition involving nanoparticles. 1. Recognitionof molecules by particle-bound receptors. 2. Mediated particle-particle recognition.3. particle-particle recognition.

Figure 1.8

of hydrogen-bonding groups can be used to achieve increasingly complex architec-tures. All examples can be grouped into three categories, as depicted in figure 1.7: 1)Particles bearing a receptor molecule binding a ‘free’ molecule. 2) Particles bearinga receptor binding a mediator molecule that binds a receptor on another particle. 3)Particles binding together by complementary receptors.

The application of singly hydrogen-bonding moieties using donor-acceptor com-plementarity was successful in the layer-by-layer assembly of CdSe nanoparticlescarrying 4-mercapto-benzoic acid (MBA) and polyvinylpyridine (PVP, figure 1.8)[192, 193]. There have not been many attempts to directly bind particles together bysingle hydrogen bonds, though some aggregation effects in drying gold nanoparti-cle solutions carrying e.g. 4-amino-benzenethiol (H2NC6H4SH) might be ascribed tohydrogen bonding [194].

Double hydrogen bonding has been studied using carboxylic acid derivatives.Alkane thiol-protected gold nanoparticles could be assembled into networks on elec-trode surfaces, by partial ligand exchange with mercapto-undecanoic acid (MUA,


Figure 1.9

figure 1.9) [195]. These layers show pH-dependent behaviour [195–197], since thedegree of protonation determines the H-bonding capacity of the particles. Similarly,Rotello and coworkers have reported the formation of (disordered) three-dimensionalaggregates of nanoparticles with a shell consisting of octanethiol and MUA (1:1) bylowering the pH [198]. The same approach was effective with different mercaptocar-boxylic acids on Ag, Au and Pd particles too [199].

Kimura and coworkers prepared 3.5 nm gold particles coated with mercaptosuc-cinic acid (MSA, figure 1.9), that could be used to form highly ordered monolayers atair-water interfaces or three-dimensional colloidal crystals [200, 201]. Again, aggre-gation and ordering were induced by lowering the pH. Presumably, bridging watermolecules play a decisive role in the formation of these crystals [202]. The formationof aggregates of these carboxylic acid-terminated particles can also be mediated bymore specific hydrogen-bonding molecules [199, 203], by proteins [204, 205] or byamine-functionalised dendrimers [36].

It was already shown that true molecular recognition becomes feasible when tri-ply hydrogen-bonding ligands are used. Nanoparticles have been used as “scaffolds”for molecular recognition in various ways [170]. The work of the Rotello group andthe Fitzmaurice group is noteworthy, comprising examples of all categories depictedin figure 1.7. Diamidopyridine-bearing gold nanoparticles were shown to selective-

Figure 1.10: Schematic representation of nanoparticle aggregation by the ‘Brick-and-mortar’method (taken from [206]).


Figu

re1.

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atin

gsy

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1.6. SCOPE AND OUTLINE OF THIS THESIS 19

ly bind flavin [57, 209, 210]. Conversely, dissolved diaminopyridines can bind tothymine derivatives bound to gold nanoparticles too [211].

Mediated assembly of nanoparticle aggregates has been demonstrated with brid-ging molecules [207], with the system depicted in figure 1.11.a. Alternatively, com-plementary functionalised polymers were used by Rotello and coworkers to bind na-noparticles together by a ‘brick-and-mortar’ approach [212–214] (figure 1.10.). Func-tionalised silsequioxanes are more elaborate mediators, relying on both hydrogenbonding and silsequioxane stacking [215].

Fitzmaurice and coworkers have investigated the formation of nanoparticle ag-gregates, by direct particle-particle recognition based on diamidopyridine and uracilgroups: Thiol derivatives of these were attached to 5.2 nm silver and 2.6 nm goldparticles respectively [207, 208, 211] (figure 1.11.b.). More details of this work will becontrasted with our own approach in Chapter 3.

1.6 Scope and outline of this thesis

In this thesis a method is presented to produce two-component aggregates of Au55

clusters, based on complementarily hydrogen-bonding ligands. We designed a modelsystem, taking into account that it should ultimately be possible to form orderedaggregates by the optimisation of the process conditions. The latter requires thatthe nanoparticles be monodisperse and the aggregation proceed along a reversiblepathway.

The design of the model system and the preparation and functionalisation of Au55

clusters is discussed in Chapter 2. It is shown how ligand exchange reactions can beused to functionalise Au55[PPh3]12Cl6 with thiol-bearing hydrogen-bonding moieties.The hydrogen-bonding properties of these ligands are also described. The structureand stoichiometry of Au55 clusters have been subject of debate. Therefore attentionis payed to the assessment of the cluster stoichiometry and its monodispersity.

Chapter 3 describes the solution-based formation of three-dimensional aggregatesof the functionalised clusters. Transmission electron microscopy revealed that theaggregate morphology was determined by solvent parameters. In an ideal mediumfor aggregation, each of the individual cluster components should be soluble, whereasaggregation should occur upon combination of both components. In practice, it isnecessary to find a balance between the cluster solubility and (complete) screeningof the inter-cluster hydrogen-bonding interactions. Mixtures of dimethylsulfoxide(DMSO) and 1,4-dioxane are shown to provide this balance. In such mixtures, thetwo-component aggregate morphology is dominated by strand-like structures. Somecontrol over this morphology is possible by tuning of the solvent composition.

Knowledge of the dynamics of the aggregation process is necessary to explainthis solvent dependence. Therefore Chapter 4 reports how the process can be moni-tored in situ by dynamic light scattering (DLS). The resulting kinetic information is


discussed in the perspective of colloidal aggregation phenomena. It turns out thatin DMSO/dioxane mixtures the time development of the aggregation process bearsgreat resemblance with the kinetics of Reaction-Limited Colloidal Aggregation. Thisconfirms that the aggregation process ultimately is irreversible under the conditionsexplored. However, the reaction-limited character indicates that it may be possible toachieve reversible aggregation by further modification of the aggregation conditions,that are beyond the scope of this chapter.

The versatility of cluster binding by hydrogen bonding is shown in Chapter 5: Thesame complementary molecules that give rise to three-dimensional aggregates canalso be applied to immobilise Au55 clusters on gold substrates bearing functionalisedself-assembledmonolayers (SAMs). In this setup, it is exactly knownwhich functionalgroup is on the cluster and which is on the substrate. Therefore it is possible to provethat only complementary hydrogen-bonding interactions lead to the immobilisationof clusters. Thus, it confirms the importance of the complementarity in the case ofthree-dimensional aggregation, where such evidence could only be obtained fromcontrol experiments. As before, the immobilisation can be controlled by solventcomposition, but also by the degree of functionalisation of the SAMs.

Altogether, a versatile method to achieve selective binding of functionalised goldclusters is presented. It can be tuned by modifying the strength of the hydrogen-bonding interactions by changing the medium or the degree of functionalisation.

REFERENCES 21

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