«Kinetic Investigations of Thiolate Protected Gold Nanoparticles: Protein Interactions, Electron Transfer, and Precursor Formation By Brian N. Turner ...»
60. Time resolved UV-Vis spectra of tetrabromoauric acid undergoing reduction by tiopronin.
Dynamic reaction profiles of Au(III)Cl4 - with tiopronin
Plots to determine the order of reaction with respect to Au(III)Cl4−
63. Plot to determine the reaction order and pseudo first order rate constant with respect to [Au(III)]
64. Plot to determine the order of reaction with respect to [RSH], n, and the overall rate constant, knet
65. Saturation behavior was suspected when kAu(III),1 lost a linear relationship with [RSH] at higher [RSH]
66. Determination of parameters a and b for the saturation kinetics rate law................ 149
67. Thermogravimetric profiles of gold hexanethiol cluster (red) and gold tiopronin cluster (black).
68. Thermal gravimetric spectrum generated on the TG-MS instrument at Oakridge.... 169
69. Selective ion scan of elemental and small molecule fragments thermally lost from AuHexanethiol nanoparticles.
70. Larger mass peaks thermally lost from AuHexanethiol nanoparticles.
71. Small mass fragments thermally lost from AuTiopronin nanoparticles................... 172
73. TEM micrograph and size distribution of AuG1S NCDs.
74. H NMR data G1 place exchanged onto AuTiopronin.
The practical focus of this dissertation is the adaptation of stable, chemically versatile, easy to synthesize thiolate protected gold nanoparticles to medicine (Chapters II and III) and molecular electronics (Chapter IV). In the course of the studies aimed at these goals, a number of questions arose about fundamental aspects of nanoparticle synthesis, their mechanism of formation, and characterization techniques used to describe them. These secondary interests came to define much of the content of this dissertation (especially Chapters I and V, and Appendices A and B).
(Note: much of this background material appears in a previously written review article by this author, Brian Huffman, and David Cliffel. It has been rewritten and truncated for clarity).1 The scientific study of colloidal metal particles dates back to Faraday in the mid-19th century.2 The synthesis and characterization, notably by electron microscope, of water “soluble” gold colloids as small as 18 nm was completed by Turkevich and co-workers in
1951.3 Monolayer protected clusters (MPCs), a term used by Royce Murray to describe small gold nanoparticles that are identical or similar to those first synthesized by Brust and coworkers,4 differ from metal colloids in that they can be repeatedly dried, isolated from, and redissolved in common solvents without decomposing or aggregating. 5 For the duration of the dissertation, the term nanoparticle will be used instead of cluster, so as not to confuse these nanomaterials with smaller cluster structures of discrete morphology.
These small gold nanoparticles are obtained using a bottom-up approach (as opposed to a top-down approach such as lithography), implying that the creation of a great variety of nanomaterials is possible from a small number of building block materials.6 Nanoparticles are created with a variety of core types and, furthermore, capping ligands, to create water or organic soluble products with desired functions. Both metallic and
non-metallic starting materials are used in the creation of nanoparticle structures, such as:
thiolate protected gold,4, 5, 7-11 organic polymers,12-15 virus-like particles,16-21 protein particles,22 colloidal particles,3, 23, 24 and semiconductor quantum dots.25 Thiolate protected gold nanoparticles have been the focus of studies in our research group because of their ease of creation, water and air stability, electrochemical and optical properties, biocompatibility, and their ability to be surface functionalized by straightforward techniques with a variety of sulfur containing ligands. Thiolate protected gold nanoparticles of the type focused on in this dissertation can range in size from 1-10 nm, containing approximately 55-1000 gold atoms which correlates with molecular weights between 20 – 200 kDa.26 Synthetic Routes The Brust reaction is a standard method to produce small gold nanoparticles that can be tuned to a number of organic solvents. Nanoparticles prepared by this method are most commonly synthesized with alkanethiolate protecting molecules, which lead to relatively stable nanoparticles. More recently, water solubility of gold nanoparticles was easily accomplished by using water-solubilizing protecting thiolate ligands in a modified Brust reaction as depicted in Scheme 1.4, 8 In the single phase Brust reaction, tetrachloroauric acid is reduced from Au(III) to Au(I) in the presence of the thiol capping ligand in a polar solvent, yielding an orange, brown, or ruby red solution that fades to clear over time. The color change has been attributed to a Au(III)-thiol complex which is reduced to a Au(I) complex over time which ultimately forms either a gold-thiol polymer,5 oligomer, or cyclic tetramer.27 Precursor formation in the context of water soluble tiopronin protected gold nanoparticles is the subject of Chapter V. Following the initial reduction, the Au(I) is further reduced to Au 0 in the presence of sodium borohydride (NaBH4), yielding a black to dark brown or purple solution. Other reducing agents have been used for other metal cores, such as lithium aluminum hydride (LiAlH 4) or lithium triethylborohydride.10, 11 Figure 1: Modified Brust reaction scheme for polar ligands.
Key examples of thiolate ligands that have been used to produce water soluble, long-term air and solvent stable clusters are tiopronin, 8 glutathione,28 4-mercaptobenzoic acid,291thio--D-glucose,30 and N,N,N-trimethyl(mercaptoundecyl)ammonium (TMA), 31 as shown in Figure 2.
Figure 2: Examples of thiolate ligands used in the synthesis of water soluble Au MPCs.
(A) tiopronin, (B) glutathione, (C) 4-mercaptobenzoic acid, (D) 1-thio-β-D-glucose, (E) N,N,N-trimethyl(mercaptoundecyl)ammonium.
In Chapter III, tiopronin (3-mercaptopropionyl glycine, Figure 2(A)) protected gold nanoparticles will be used as a scaffold to present synthetic peptide epitopes to a commercial antibody immobilized on a novel immunosensor platform. Tiopronin has been popular in our lab as it produces high yield, long-lasting nanoparticles in a straightforward synthesis with robust results.
Functionalization of Thiolate Protected Gold Nanoparticles Transformation of these gold nanoparticles into materials with relevance to biology and engineering has been accomplished by a variety of synthetic functionalization strategies, out of which, the most widely used and thoroughly studied method is the thiol placeexchange reaction depicted in Figure 3.
Figure 3: Scheme of the solution phase place-exchange reaction. The stoichiometry of incoming to exiting ligand is 1:1.
Place-exchange involves an incoming ligand, such as a thiol containing biomolecule, taking the place of one of the original capping ligands in a 1:1 ratio. Place-exchange was first described by Murray and co-workers,32 using alkanethiolate-clusters with functionalized thiols in toluene. This reaction has since been expanded to aqueous solutions9 and can be carried out in aqueous buffer solutions. Multiple research groups have studied the dynamics by which place-exchange occurs in ligand solutions.
According to Murray’s work, the rate of ligand exchange depends both upon the concentration of incoming and exiting ligands, implying an associative (S n2-like) mechanism.33 Lennox and co-workers,34 on the other hand, report the reaction is zeroorder with respect to the incoming lingand. Zerbetto’s lab found that the associative mechanism is accurate, but that the interactions of the newly introduced chains interact with multiple chains on the cluster, causing the kinetics to change as the reaction proceeds.35 The reaction rate increases with smaller sized entering ligands and shorter chain length of the protecting ligand.33 Consequently, it is favorable to place-exchange a large biomolecule (such as a peptide) with a small protecting ligand (such as tiopronin), although the process will proceed at a slower rate. It is still practical to place exchange similar sized ligands in shorter time courses, as will be shown later with small molecule wire-like molecules and small alkanethiols, but the extent of exchange is expected to be lower in these cases. Furthermore, it is important to consider that subtle differences in the structure of the incoming ligand, such as branching, can have a significant effect on both the rate of place-exchange, and the stability of the monolayer. 36 Additionally, it should be noted that the reaction proceeds more favorably at different sites on the core;
vertex sites edge sites near-edge sites terrace sites 33 as shown in Figure 4.
Figure 4: Chart of the different relative rates at which place-exchange and (possibly) migration occur. Copyright 1999 of the American Chemical Society.
The differences in reactivity due to thermodynamics and kinetics originate from the electron density differences37 and steric accessibility differences38 of these sites. These unique properties of nanoparticles lead to some degree of predictability, and therefore control, of where place-exchanged functional groups will anchor on the gold core. The rate of exchange is also increased by oxidative electronic charging of the core by electrochemical means39 or in the presence of dioxygen.40 The extent of reaction can be enhanced by increasing the incoming ligand concentration, but it should be noted that the extent of exchange rarely approaches 100%, due to the difficulty of exchange at terrace sites.33 It is also important to note that the rate of ligand place-exchange on MPCs becomes slower as the particles age, probably due to a prolonged rearrangement of the ligands on the surface.41 To date, no kinetic or mechanistic study of place-exchange has considered new findings about the presence of gold thiolate tetramer rings on the surface of MPCs as reported by our group using mass spectrometry27 and Häkkinen and co-workers in a theoretical paper.42 These nuances in surface structure could help to better explain the complexities of the place-exchange mechanism.
More recently, a number of crystal structures of thiolate protected nanoparticles 43, 44 have been published, revealing that terrace, vertex, and edge sites, as described earlier, may not be as important as ring and staple45 structures that project out of a tightly packed, central core (termed the “grand core”). It should be noted, however, that the available crystal structures describe smaller cores than those commonly used, and the vertex, edge, terrace description may hold more relevance for these materials.
It is difficult to use first principles to predict the extent and location of place exchange.
In practice, a place exchange study in sufficient excess of incoming ligand to the protecting monolayer (e.g. 10-50 fold in excess of protecting groups in solution) over a long reaction time (e.g. 3 to 7 days) is used. This allows for determination of the “feed ratio” that is required for a given exchange percentage, and conditions can be tuned either by varying the reaction time or ligand concentration. Once exchanged, there is not a straightforward technique to determine the location of the exchanged ligands. Recent efforts by Harkness and coworkers have, however, led to the development of a method for the determination of ligand microstructure on the surface of a polyfunctionalized nanoparticle.46 As an alternative to the solution phase place-exchange discussed above, Huo and coworkers have studied solid phase place-exchange reactions. In their original report of a solid phase place-exchange reaction, they employ a polystyrene Wang resin with acetyl protected 6-mercaptohexanoic acid attached via an ester bond. 47 The thiol groups are deprotected and allowed to undergo place-exchange with butanethiolate-protected gold nanoparticles, followed by washing away of unexchanged product, and cleaving of the exchanged particles. The same group has compared this solid phase approach to the solution phase approach and found the solid phase approach to be advantageous in terms of controlling the number of ligands attached per cluster and preserving the order of ligands on the surface.48 Recently, the same group has reported a solid phase approach using a non-covalent interaction of the incoming ligand with silica gel49 (see Figure 5).
The solid phase approach employs milder reaction conditions, thus making it amenable to a wider class of molecules, such as large biologically relevant functional groups.
Figure 5: The non-covalent interaction based place-exchange reaction.49 Reprinted with permission of John Wiley & Sons.
While this strategy has not been employed in the current research, there are certainly situations where it would be useful. For instance, the future of Chapter IV involves studying how small an amount of a functional ligand it takes to achieve an electronic effect. Solid phase exchnage would be a more effective way to reach the minimum exchange (possibly one ligand per particle) than constantly reducing the concentration and stirring time of a solution phase place exchange reaction.
Macromolecules often contain thiols, for example, cysteine residues in proteins or adenosyl phosphothiolate residues in DNA oligonucleotides, 50 and are readily adaptable to the place-exchange reaction. Strategies to introduce thiol groups into macromolecules include, but are certainly not limited to, the use of Traut’s reagent (2-iminothiolane)51 in the case of proteins, the inclusion of terminal cysteine residues during the synthesis of peptides, and the conversion of phosphates to phosphorothioates using 3H-1,2benzodithiole-3-one 1,1-dioxide.52 It is also possible to introduce ligands into the MPC monolayer which undergo electrostatic interactions with biomolecules, for example, the use of biotin-streptavidin interaction or biotin-anti-biotin interaction.53 All of these routes provide straightforward methods to ready a group for place-exchange and create functional nanoparticles.