Mixed Monolayer Protected Clusters Research Articles

Crystal Structure and Optical Properties of a Chiral Mixed Thiolate/Stibine-Protected Au18 Cluster.

We report the first example of a chiral mixed thiolate/stibine-protected gold cluster, formulated as Au18(S-Adm)8(SbPh3)4Br2 (where S-Adm = 1-adamantanethiolate). Single crystal X-ray crystallography reveals the origin of chirality in the cluster to be the introduction of the rotating arrangement of Au2(S-Adm)3 and Au(S-Adm)2 staple motifs on an achiral Au13 core and the subsequent capping of the remaining gold atoms by SbPh3 and Br- ligands. Interestingly, the structure and properties of this new Au18 cluster are found to be different from other reported achiral Au18 clusters and the only other stibine-protected [Au13(SbPh3)8Cl4]+ cluster. Detailed analyses on the geometric and electronic structures of the new cluster are carried out to gain insights into its optical properties as well as reactivity and stability of such mixed monolayer-protected clusters.

Engineering reversible isomerization at the nanoscale via intermolecular interactions

Engineering reversible isomerization at the nanoscale via intermolecular interactions

Tunable Aminooxy-Functionalized Monolayer-Protected Gold Clusters for Non-Polar or Aqueous Oximation Reactions.

Aminooxy (-ONH2) groups are well known for their chemoselective reactions with carbonyl compounds, specifically aldehydes and ketones. The versatility of aminooxy chemistry has proven to be an attractive feature that continues to stimulate new applications. This work describes application of aminooxy 'click chemistry' on the surface of gold nanoparticles. We present here a trifunctional amine-containing aminooxy alkane thiol ligand for use in the functionalization of gold monolayer protected clusters (Au MPCs). Diethanolamine is readily transformed into an organic-soluble aminooxy thiol (AOT) ligand using a short synthetic path. The synthesized AOT ligand was coated on ≤ 2 nm diameter hexanethiolate (C6S)-capped Au MPCs using a ligand exchange protocol to afford organic-soluble AOT/C6S (1:1 ratio) Au mixed monolayer protected clusters (MMPCs). This work describes the synthesis of Au(C6S)(AOT) MMPCs and representative oximation reactions with various types of aldehyde-containing molecules, highlighting the ease and versatility of the chemistry and how amine protonation can be used to switch solubility characteristics.

Stability of Gold Nanoparticle‐Bound DNA toward Biological, Physical, and Chemical Agents

Positively charged trimethylammonium-modified mixed monolayer protected clusters (MMPCs) interact with DNA by complementary electrostatic binding, serving as efficient DNA delivery systems. The stability of gold nanoparticle-bound DNA toward biological, physical, and chemical agents is investigated. The MMPC-bound DNA is efficiently protected from DNAse I digestion and experiences nicking/cleavage-induced morphology changes with higher concentrations of DNAse I. Significant protection of MMPC-bound DNA was also observed in a physical sonication assay. However, the MMPC-bound DNA was found to show enhanced cleavage upon exposure to chemically induced radicals. The latter may indicate that bound DNA is bent and wrapped on the surface of the cationic MMPC.

Controlled Recovery of the Transcription of Nanoparticle-Bound DNA by Intracellular Concentrations of Glutathione

Positively charged trimethylammonium-functionalized mixed monolayer protected clusters (MMPCs) bind DNA through complementary electrostatic interactions, resulting in complete inhibition of DNA transcription of T7 RNA polymerase. DNA was released from the nanoparticle by intracellular concentrations of glutathione, resulting in efficient transcription. The restoration of RNA production was dose-dependent in terms of GSH, with considerable control of the release process possible through variation in monolayer structure. This work presents a new approach to controlled release of DNA, with potential applications in the creation of transfection vectors and gene regulation systems.

Reversible Regulation of Chymotrypsin Activity Using Negatively Charged Gold Nanoparticles Featuring Malonic Acid Termini

Negatively charged gold nanoparticles featuring 2-(10-mercapto-decyl)-malonic acid were synthesized using the Murray place-displacement reaction. These water-soluble malonate gold mixed monolayer protected clusters (MMPCs) effectively bind and inhibit chymotrypsin based on complementary electrostatic surface recognition. The effect of increasing ionic strength on inhibition was also studied. It was observed that addition of high ionic strength solutions to protein-nanoparticle complexes show almost complete restoration of protein activity. The conformational change of chymotrypsin upon binding to the MMPC was investigated using fluorescence spectrometry and circular dichroism, thus correlating structural changes with enzyme activity.

Biomacromolecule Surface Recognition using Nanoparticle Receptors

Biomacromolecule surface recognition is an important factor in regulating cellular processes. Nanometer-scale mixed monolayer-protected clusters (MMPCs) provide scaffolds for creating receptors targeting biomacromolecular surfaces. Unique features of nanoparticles that make them particularly attractive resources for biomacromolecular recognition and their use in modification of structure and function of biomacromolecules are illustrated in this review.

Tunable Reactivation of Nanoparticle-Inhibited β-Galactosidase by Glutathione at Intracellular Concentrations

Positively charged trimethylammonium-functionalized mixed monolayer protected clusters (MMPCs) of different chain lengths (C(8) and C(11)) have been used to bind beta-galactosidase through complementary electrostatic interactions, resulting in complete enzyme inhibition. This inhibition can be reversed in vitro by intracellular concentrations of glutathione (GSH), the main thiol component of the cell. The restoration of activity depends on the chain length of the monolayer. The activity of enzyme bound to particles with C(8) monolayer was completely restored by intracellular concentrations (1-10 mM) of GSH; however, little or no release was observed at extracellular GSH concentrations. In contrast, no restoration was observed for enzyme bound to the C(11) particles at any of the concentrations studied. Taken together, these studies demonstrate that the GSH-mediated release of enzymes bound to MMPCs can be tuned through the structure of the monolayer, a significant tool for protein and drug delivery applications.

Recognition and stabilization of peptide alpha-helices using templatable nanoparticle receptors.

alpha-Helices are important structural elements in proteins. To provide a scaffold for the facial recognition of peptides, we have explored the interaction of cationic mixed monolayer protected clusters (MMPCs) with a tetra-aspartate peptide in water. In these studies, substantial enhancement of peptide helicity was observed upon addition of the MMPC. Significantly, this stabilization increased with time, demonstrating templation of the monolayer to the peptide helix.

Effect of Ionic Strength on the Binding of α-Chymotrypsin to Nanoparticle Receptors

Negatively charged carboxylate-functionalized mixed monolayer protected clusters (MMPCs) effectively bind and inhibit alpha-chymotrypsin based on complementary electrostatic surface recognition. We demonstrate that this binding can be disrupted by varying the ionic strength of the medium. Enzyme activity in the presence of MMPCs increases from 5% to 97% of native activity as salt concentration is increased from 0 to 1.5 M. Variation of ionic strengths after complete binding over 13 h results only in a modest restoration of enzymatic activity (< 35%). The conformation of chymotrypsin was characterized using circular dichroism and fluorescence spectroscopy, correlating structure with enzymatic activity. This work provides a useful insight of the electrostatic influence on protein--MMPC interactions and can be applied toward MMPC-based controlled release of proteins in vivo.

Synthesis of Mixed Monolayer-Protected Gold Clusters from Thiol Mixtures: Variation in the Tail Group, Chain Length, and Solvent

We present a systematic study on the synthesis of mixed monolayer-protected clusters (MMPCs) from mixtures of alkanethiols. Our results show that solvation-driven thermodynamic preferential adsorption of precursor ligands governs monolayer composition of MMPCs. In the mixed system HO(CH2)nSH/ CH3(CH2)mSH (n = m or n ≠ m) in toluene, adsorption of the polar component is largely favored because of the poorer solvation of polar tail groups in toluene compared to tetrahydrofuran (THF). When MMPCs are generated from the mixed system HO(CH2)nSH/CH3(CH2)mSH (n = m) in THF, the tail group effect is much less. The MMPC synthesis in THF solutions containing HO(CH2)nSH/CH3(CH2)mSH (n ≠ m) shows large thermodynamic control, which promotes a preferential adsorption of the thiols with a longer alkyl chain onto the surface of clusters. Transmission electron microscopy (TEM) and UV−vis spectroscopy data suggest that the average core dimension of MPCs generated from alkanethiols in THF is slightly smaller than that of MPC...

Controlled interparticle spacing through self-assembly of Au nanoparticles and poly(amidoamine) dendrimers.

Control of particle-particle spacing is a key determinant of optical, electronic, and magnetic properties of nanocomposite materials. We have used poly(amidoamine) (PAMAM) dendrimers to assemble carboxylic acid-functionalized mixed monolayer protected clusters (MMPCs) through acid/base chemistry between the particle and dendrimer. Small angle X-ray scattering was then used to establish average inter-MMPC distances. Five generations of PAMAM dendrimer (0, 1, 2, 4, 6) were investigated, with a monotonic increase in interparticle spacing from 4.1 to 6.1 nm observed with increasing generation.

Dendrimer Mediated ‘Bricks and Mortar’ Self-Assembly of Nanoparticles

ABSTRACTControl of particle-particle spacing is a key determinant of optical, electronic, and magnetic properties of nanocomposite materials. We have used poly(amidoamine) (PAMAM) dendrimers to assemble carboxylic acid-functionalized mixed monolayer protected clusters (MMPCs) through acid/base chemistry between particle and polymer. IR spectroscopy and selective dendrimer staining, observed by Transmission Electron Microscopy (TEM), establish that the PAMAM dendrimers are the mortar in the assembly and act to space the MMPCs in the resulting aggregates. Small angle X-ray scattering (SAXS) was then used to establish average interparti cle distances; five generations of PAMAM dendrimer (0, 1, 2, 4, 6) were investigated and monotonic increase in interparticle spacing from 4.1 nm to 6.1 nm was observed.Initial studies involving the application of this methodology to control the magnetic properties of 3-iron oxide nanoparticles have been completed. γ-Iron oxide nanoparticles (6.5 nm in diameter) have been assembled with PAMAM dendrimers generations 2.5, 4.5, and 6.5. The resulting aggregates were characterized with SAXS and magnetization obtained on a super conducting quantum interference devise (SQUID). An observed correlation between the blocking temperature (TB) and the average interparticle spacing suggests that our methodology could be used to tailor the magnetic profile of the nanoparticles.