PARTICLES AS MOLECULES
C. M. S ORENSEN
3.4 NANOPARTICLES
Recently Jadzinski et al. (19) reported x-ray crystallography analysis of a thiolated gold molecular cluster with an exact stoichiometry. The cluster was made via borohy-dride reduction of gold chloride in the presence of p-thiobenzene acid, which became the ligand. The molecule contained 102 gold atoms and 44 ligands. It was shown that 79 of the gold atoms formed a truncated-decahedron inner core. Beyond that there were layers of gold atoms singly coordinated and then doubly coordinated with the ligand. The ligands not only interacted with the gold but with each other via both parallel and perpendicular ring stacking and the sulfur interacting with the ring. Most of the ligands assembled into chains extending from one pole of the roughly spherical molecule to the other. Finally the existence of this particular conformation for the molecule was ascribed to an electronic shell closing in which the 109 gold atoms donated 109 electrons, 44 of which engaged in bonding one of the 44 ligands.
This leaves 58 electrons free to pair up and occupy 29 delocalized orbitals to yield a stable zero angular momentum situation. This cluster compound is a small version of gold nanoparticles that we will discuss below.
regime is entered with bigger sizes, one finds the capping agents are more labile and surface mobile, hence implying that they are not covalently connected but rather ligated to the surface. I will use the term “ligands” to describe these surface agents.
Brus and coworkers (23) were the first to show that nanoparticle growth could be controlled and aggregation arrested by capping. They studied CdSe capped with phenylselenide. The particles, “molecular particles,” were redispersable in organic solvents and the solubility could be controlled via the capping ligand. Subsequently Brust et al. (24) invented a phase transfer method for producing gold nanoparticles via borohydride reduction in the presence of capping ligands, typically alkyl thiols.
This method has been used extensively. Capping and controlled growth were also achieved early in the development of this field by Murray, Kagan, and Bawendi (25), who used nucleation and subsequent controlled growth in the presence of ligands to produce narrowly dispersed CdSe nanoparticles. Most recently digestive ripening, discovered by our group (26–29), has proven very useful for creating ligated nanopar-ticle molecules of narrow size distribution and bulk quantities.
Early in the development of this field, Whetten’s group made quite small nano-particles that they called “gold-cluster molecules” (30–33). These nanoparticle mol-ecules, which lie at the boundary between cluster compounds and nanoparticles, were made via borohydride reduction of gold chloride in the presence of organic thiols. Depending on ligand and temperature a mix of sizes was made that could be separated into specific fractions that had molecular weights of, for example, 8, 14, 22, 30, 40, 65, and 180 kDa, corresponding to diameters of 1.1 to 3.1 nm, and 40 to 900 gold atoms. They were ligated by alkane thiols and thus soluble in organic solvents, or in one case of mole weight 10.4 kDa, glutathione, which were soluble in water (33). The fact that these fractions appeared from the rapid borohydride reduction of the gold salt implies that the sizes of these fractions have special symmetries, lower energies, or both to make them result in greater abundance than other sizes.
There is a very great variety of nanoparticle molecules known and described in the literature, and it is certainly well beyond the scope of this chapter to review them.
Good reviews do exist; see References 1 to 4. Here I classify these into four major groups.
3.4.1 Types of Nanoparticles
1. Gold. Gold nanoparticles are the most common, largely because gold is relatively inert, yet it is the most electrophillic of metallic elements and this makes ligation propitious. Figure 3.2 shows a cartoon of what one might call a canonical gold nanoparticle molecule and Table 3.1 gives some useful statistics. There is a great variety in sizes and ligands. Sizes can range from approximately 1 to 20 nm, although 4 to 7 nm, is most common. The most common ligands are the alkyl thiols; the electrophilic gold binds the lone pairs of the sulfur readily. Alkane chain lengths from C6 to C16 are typical and aromatics have been used as well. Other ligands such as amines and phos-phines have been used. Narrow sizes dispersions have been obtained through fractionation or digestive ripening (26–29).
3.4 NANOPARTICLES 43
2. Other Metals. Silver, similar to gold, copper, palladium, platinum, iron, cobalt, nickel, and various bimetallics have been synthesized. As for gold, the most common sizes range in diameters of 3 to 7 nm. Ligands include the alkyl thiols for the more “noble” of the metals, but these ligands are too reactive for the transition metals iron, cobalt, and nickel where long chain fatty acids can be used.
3. Semiconductors. Perhaps most common are the cadmium chalcogenides, CdS, CdSe, CdTe ligated with trioctylphosphine oxide, hexadecylamine, etc.
4. Metal Oxides. Iron, cobalt, nickel oxides, and some ferrites ligated with long chain fatty acids.
3.4.2 The Capping Ligand
The surface capping ligands keep the nanoparticles from irreversibly aggregating and largely determine their solution and superlattice properties. With regard to gold and thiols, the properties of self-assembled monolayers, SAMs, of organic thiols adsorbed on bulk gold surfaces have been studied for some time, and this has been Figure 3.2 Schematic of a nanoparticle of radius R capped with alkyl thiol ligands of length L.
TABLE 3.1 Gold Nanoparticle Statistics
Gold atomic volume 16.9 10224cc¼ 16.9 1023nm3
Number of gold atoms in a spherical gold nanoparticle
31 d3(nm)
Number of thiols on the surface of a spherical gold nanoparticle
14.7 d2(nm)
PARTICLES AS MOLECULES 44
used to understand the ligation of thiols at the curved surfaces of the nanoparticles.
For example, the footprint of a SAM alkylthiol on gold is 0.214 nm2and this value is used for the nanoparticle as well. Leff et al. (34) have shown that the size of gold nanoparticles prepared via the Brust – Schiffrin method depends on the ligand to gold ratio and gave surface energy arguments to explain this. A considerable amount of work from our laboratory has shown how the ligand is very active in digestive ripening, a process whereby the nanoparticle size distribution can be nar-rowed significantly by digesting solutions under reflux in the presence of the ligand (26–29). We found size dependence on the nonmetal of the ligand and a slight dependence on the chain length. Comparing alkyl thiol ligands to alkyl ammonium ligands showed a remarkable reversible change between spherical particles and flat plates (35).
The lability of the ligands allows for their controlled exchange. So-called “place exchange” reactions have been described (22, 36). With this method, one can change the ligand end group (for a thiol, opposite the sulfur) functionality. For example, one can change the solubility from organic to aqueous by place exchanging ligands terminated with methylene groups with alcohol or carboxylic acid groups.
In one application, so-called mixed monolayer protected clusters involved gold capped with a mixture of octylthiol and 11-thioundecanoic acid (37). These could be made to aggregate at low pH due to hydrogen bonding between COOH groups and dis-perse at high pH due to the negatively charged COO2terminal groups. Another use is to put chemically active groups that can bind with complementary groups on other nanoparticles.
Control of the ligand capping can also allow formation of amphiphilic nanoparticle molecules by capping with both hydrophilic and hydrophobic ligands. Such mixed coatings have been reported in the literature (38–40) and phase equilibria of such nanoparticle molecules have been studied with simulations to yield a potentially rich phase diagram (41). Mixed coatings portend the prospect to have anisotropic interactions between nanoparticles.
Experimental and simulation studies have shown that the ligand chains can undergo an order to disorder transition with increasing temperature. The transition temperature increases with increasing chain length. The ordered state is one of chain bundling, often in a zigzag all-trans conformation and often extending radially outward from nanoparticle faces. These assemblies are called 3d SAMs to distinguish them from their 2d analogues. The transition can have a latent heat (42) but has also been seen to evolve continuously with temperature (21).
Structuring of the ligands has been demonstrated by coating NPs with two different ligands that in the bulk would phase separate (43). Octanethiol (OT) and mercaptopro-panoic (MPA) acid were used as capping agents for 3.2-nm gold particles. These ligands phase separated on the surface of the NPs into parallel bands of “latitude”
as small as 0.5 nm in width; see Figure 3.3. Changes in core size, ligand lengths, and molar ratios modified the patterns. This nano-phase separation implies that the ligands are mobile on the surface of the nanoparticle.
The nanoparticle molecules can be made chemically reactive by placing appro-priate functional groups at opposite ends of the capping ligands. For example,
3.4 NANOPARTICLES 45
alpha-omega dithiols can link to two metal nanoparticles (20, 44). Then nanoparticle molecules can bind together into roughly spherical clumps, as reported in Reference 44. With such dithiol linking, Sidhaye et al. (45) were able to reversibly expand and contract the spacing between linked nanoparticles with an optically induced cis/ trans conformation change in the linking molecule. Another tack is to put matching functional groups on the nonligating ends of the ligands, for example, hydrogen bond donors and acceptors (46). Gold nanoparticles have been ligated with mercap-toalkyl oligonucleotides that can detect, via binding, complementary nucleotides bound to other gold nanoparticles (47).