Monolayer-protected Gold Clusters Research Articles

Structural Information on the Au–S Interface of Thiolate-Protected Gold Clusters: A Raman Spectroscopy Study

The Raman spectra of a series of monolayer-protected gold clusters were investigated with special emphasis on the Au–S modes below 400 cm–1. These clusters contain monomeric (SR-Au-SR) and dimeric (SR-Au-SR-Au-SR) gold–thiolate staples in their surface. In particular, the Raman spectra of [Au25(2-PET)18]0/–, Au38(2-PET)24, Au40(2-PET)24, and Au144(2-PET)60 (2-PET = 2-phenylethylthiol) were measured in order to study the influence of the cluster size and therefore the composition with respect to the monomeric and dimeric staples. Additionally, spectra of Au25(2-PET)18–2x(S-/rac-BINAS)x (BINAS = 1,1′-binaphthyl-2,2′-dithiol), Au25(CamS)18 (CamS = 1R,4S-camphorthiol), and AunBINASm were measured to identify the influence of the thiolate ligand on the Au–S vibrations. The vibrational spectrum of Au38(SCH3)24 was calculated which allows the assignment of bands to vibrational modes of the different staple motifs. The spectra are sensitive to the size of the cluster and the nature of the ligand. Au–S–C bending a...

On the Size Evolution of Monolayer-Protected Gold Clusters during Ligand Place-Exchange Reactions: The Effect of Solvents

Ligand place-exchange (LPE) reactions are extensively applied for the post-functionalization of monolayer-protected gold clusters (MPCs) by using excessive incoming ligands to displace initial ones. However, the modified MPCs are often enlarged or degraded; this results in ill-defined size-dependent properties. The growth of MPCs essentially involves an unprotected surface that is subsequently has gold atoms added or is fused with other gold cores owing to collision. Reported herein is a guideline for the selection of solvents to suppress unwanted MPC growth. Favorable solvents are those with significant affinity to gold or with low solubility for desorbed ligands because these properties retard LPE reactions and minimize the time available for unprotected gold cores. This finding provides a general and convenient approach to regulate the size of functionalized MPCs.

Temperature-Dependent Absorption and Ultrafast Luminescence Dynamics of Bi-Icosahedral Au25 Clusters

Temperature-dependent absorption and ultrafast luminescence dynamics of [Au25(PPh3)10(SC6)5Cl2 ]2+ (Au25-rod) was studied and compared with [Au25(SC6)18]− (Au25-sphere) and Au38(SC2Ph)24 (Au38-rod) to understand the influence of the crystal structure on the optical properties of monolayer protected gold clusters. The temperature-dependent absorption of Au25-rod shows a shift in the absorption maximum to high energies and a small increase in the oscillator strength with decrease in temperature. The energy shift was modeled via the O’Donnell and Chen relationship, which yielded average phonon energy of 160 ± 80 cm–1, quite smaller than the 350 cm–1 observed for Au25-sphere and Au38-rod. There is an increase in the oscillator strength with a decrease in temperature of about 40% for Au25-rod while it is nearly 250% for Au25-sphere and more than 180% for Au38-rod. The oscillator strength increase is attributed to the coupling of core-gold exciton and shell-gold phonons. The smaller increase in the oscillator strength for Au25-rod is consistent with its structure that possesses no shell-gold. Femtosecond luminescence measurements carried out on Au25-rod clusters show wavelength-independent ultrafast luminescence decay traces. The lifetimes from the analysis are consistent with the relaxation of higher energy states. In contrast, Au25-sphere and Au38-rod clusters show specific wavelength-dependent luminescence growth and decay, representing the relaxation of core-gold states to shell-gold states.

High-Resolution Sizing of Monolayer-Protected Gold Clusters by Differential Centrifugal Sedimentation

Differential centrifugal sedimentation (DCS) has been applied to accurately size ligand-protected gold hydrosols in the 10 to 50 nm range. A simple protocol is presented to correct for particle density variations due to the presence of the ligand shell, which is formed here by either polyethylene glycol-substituted alkane thiols (PEG-alkane thiols) of different chain length or oligopeptides. The method gives reliable data for all particle sizes investigated and lends itself to rapid routine sizing of nanoparticles. Unlike TEM, DCS is highly sensitive to small changes in the thickness of the organic ligand shell and can be applied to monitor shell thickness variations of as little as 0.1 nm on particles of a given core size.

Emergence of Large Chiroptical Responses by Ligand Exchange Cross-Linking of Monolayer-Protected Gold Clusters with Chiral Dithiol

We here present a study of cross-linking chemistry of optically inactive monothiol-protected gold clusters by chiral bidentate dithiol with two stereogenic centers, (2R,3R)-1,4-dimercapto-2,3-butanediol (L-dithiothreitol; L-DTT), and explore the impacts of the cross-linking on their chiroptical responses. The pristine protective ligand is racemic penicillamine (rac-Pen), and the products of the ligand exchange reactions include clusters containing both rac-Pen and L-DTT (partial exchange). Electrophoresis using polyacrylamide gel with a very low gel concentration (3%) can make the products separable into two components, each of which has the similar mean core diameter of 0.78 and 0.83 nm, so the difference in the relative mobility is mainly ascribed to the size of the cluster assembly. In addition, very large optical activity with the maximum anisotropy factors of about 1.0 × 10(-3) is found for the assemblies. In comparison with chiral 1,3-dithiol protection incapable of cross-linking between gold clusters, we propose that the observed optical activity is due to surface intrinsic handedness caused by a cyclic cross-linking with at least two L-DTT molecules.

Design, Synthesis, and Characterization of Novel Thiol‐Derivatized Ibuprofen Monolayer Protected Gold Clusters

A series of new thiol‐derivatized ibuprofen monolayer protected gold clusters have been prepared by amidation of ibuprofen with alkyl alcohol or aminophenol affording the carboxamides, N‐hydroxyalkyl amide 2, and N‐hydroxyphenyl amide 6, which were then tosylated with p‐toluenesulfonyl chloride at hydroxyl group to give 3 and 7. Reactions of 3 and 7 with NaSH afforded the mercapto derivatives 4 and 8. Conducting Brust’s reaction with a 3 : 1 mole ratio of thiolate ibuprofen/ yielded polydisperse thiol‐derivatized ibuprofen‐MPCs 5 and 9. All compounds have been identified by NMR, MS, UV, and IR spectroscopies. Compounds 4 and 8 and the MPCs 5 and 9 have been investigated by using the method of 1H NMR spectroscopy. The broadening of the signals from 0.8 to 2.0 ppm in 1H NMR spectrum of MPCs 5 and 9 confirmed the success of the conjugation of thiol‐containing derivatives with nanogold cluster.

Electrochemical behaviors of novel electroactive Au nanoparticles protected by self-assembled monolayers

There has been substantial recent interest in studying monolayer-protected gold clusters (MPCs) owing to their diverse applications. The present work is an electrochemical study of novel gold nanoparticles covered with a monolayer of mercapto-dodecanol ended chloro-dicyano-quinone (HS-C12O-CDQ), which was adsorbed on the electrode (CDQ-MPCs film). Our findings reveal a redox behavior for CDQ-MPCs film similar to the solution electrochemistry of dichloro-dicyano-quinone. Furthermore, a diffusion-like mechanism was found for electron transfer, which may have occurred due to proton diffusion towards or outwards the electrode through the film casted. Chronoamperometry confirmed diffusion behavior of the ET process. Finally, EIS was used to find the rate constant of ET process for the redox reaction that occurred and the contribution of MPCs in total interfacial capacitance.

Quantized Electron‐Transfer Pathways at Nanoparticle–Redox Centre Hybrids

Hexanethiolate gold monolayer-protected clusters (C6-MPCs) with an average core diameter of 1.8 nm and a capacitance of 0.6 aF are synthesised by a two-phase method. These clusters are functionalised with (6-ferrocenyl)-1-hexanethiol by a place exchange reaction at different molar ratios. The average number of ferrocene centres per cluster determined by (1)H NMR is ten, seven and four. Differential pulse voltammetry and cyclic voltammetry measurements for cluster solutions in 0.1 M TBAPF(6)/Tol:AN (2:1) clearly show the response of the Fc(+)/Fc redox couple and of quantized double layer (QDL) charging events of the gold core. A transition from single to multiple electron-transfer response for the redox couple is observed as the number of ferrocene units per cluster is increased. The distances between the redox moieties are estimated considering a homogeneous distribution of the redox sites on the nanoparticle ligand shell. In all the cases, the inter-ferrocene average separation is too large to observe self-exchange reactions and the most likely electron-transfer pathway is by fast rotational diffusion. The oxidation of the ferrocene groups results in an electrostatic switching-off of electron transfers between the electrode and the nanoparticle core.

A QM/MM approach for the study of monolayer-protected gold clusters

We report the development and implementation of hybrid methods that combine quantum mechanics (QM) with molecular mechanics (MM) to theoretically characterize thiolated gold clusters. We use, as training systems, structures such as Au25(SCH2–R)18 and Au38(SCH2–R)24, which can be readily compared with recent crystallographic data. We envision that such an approach will lead to an accurate description of key structural and electronic signatures at a fraction of the cost of a full quantum chemical treatment. As an example, we demonstrate that calculations of the 1H and 13C NMR shielding constants with our proposed QM/MM model maintain the qualitative features of a full DFT calculation, with an order-of-magnitude increase in computational efficiency.

Nonmagnetic and magnetic thiolate-protected Au25superatoms on Cu(111), Ag(111), and Au(111) surfaces

Geometry, electronic structure, and magnetic properties of methylthiolate-stabilized Au${}_{25}$L${}_{18}$ and MnAu${}_{24}$L${}_{18}$ (L = SCH${}_{3}$) clusters adsorbed on noble-metal (111) surfaces have been investigated by using spin-polarized density functional theory computations. The interaction between the cluster and the surface is found to be mediated by charge transfer mainly from or into the ligand monolayer. The electronic properties of the 13-atom metal core remain in all cases rather undisturbed as compared to the isolated clusters in gas phase. The Au${}_{25}$L${}_{18}$ cluster retains a clear highest occupied molecular orbital/lowest unoccupied molecular orbital energy gap in the range of 0.7 to 1.0 eV depending on the surface. The ligand layer is able to decouple the electronic structure of the magnetic MnAu${}_{24}$L${}_{18}$ cluster from the Au(111) surface, retaining a high local spin moment of close to 5 ${\ensuremath{\mu}}_{B}$ arising from the spin-polarized Mn($3d$) electrons. These computations imply that the thiolate monolayer-protected gold clusters may be used as promising building blocks with tunable energy gaps, tunneling barriers, and magnetic moments for applications in the area of electron and/or spin transport.

Chemical transformation of chiral monolayer-protected gold clusters: observation of ligand size effects on optical and chiroptical responses

Versatile functionalization of metal clusters is a key step in understanding the reactivity of protective monolayers. We here demonstrate that reaction of the outermost amino groups on (S)-/(R)-penicillamine-protected gold clusters with ethyl isocyanate readily modifies the chiral surface structure through carbamoylation. Interestingly, the clusters are electrophoretically separated by the size of the surface ligand, not by the size of the gold core, which is revealed by UV-vis, IR, and energy dispersive X-ray (EDX) spectroscopy as well as SAXS measurements. The ligand size (or length) is extended through additional reactions of the carbamoylated amino groups with isocyanate, while the chemical similarity in ligand structures is realized by their IR spectral similarity. Optical and chiroptical responses of the separated cluster compounds are thus overall similar to each other, but a close inspection reveals that the ligand size has a small but distinct influence on the chiroptical response of the gold clusters.

Temperature-Dependent Optical Absorption Properties of Monolayer-Protected Au25 and Au38 Clusters

The temperature dependence of electronic absorption is reported for quantum-sized monolayer-protected gold clusters (MPCs). The investigations were carried out on Au25L18 (L = SC6H13) and Au38L24 (L = SC2H4Ph) clusters, which show discrete absorption bands in the visible and near-infrared region at room temperature and with a decrease in temperature: (i) the optical absorption peaks become sharper with the appearance of vibronic structure, (ii) the absorption maximum is shifted to higher energies, and (iii) the oscillator strengths of transitions increased. Smaller temperature dependence of absorption is observed for plasmonic gold nanoparticles. The results of the band gap shifts are analyzed by incorporating electron–phonon interactions using the O’Donnell–Chen model. An average phonon energy of ∼400 cm–1 is determined, and is attributed to the phonons of semiring gold. The unique property of decreasing oscillator strength with increasing temperature is modeled in the Debye–Waller equation, which relates oscillator strength to the exciton–phonon interaction.

Synthesis, Assembly, and Characterization of Monolayer Protected Gold Nanoparticle Films for Protein Monolayer Electrochemistry

Colloidal gold nanoparticles protected with alkanethiolate ligands called monolayer protected gold clusters (MPCs) are synthesized and subsequently incorporated into film assemblies that serve as adsorption platforms for protein monolayer electrochemistry (PME). PME is utilized as the model system for studying electrochemical properties of redox proteins by confining them to an adsorption platform at a modified electrode, which also serves as a redox partner for electron transfer (ET) reactions. Studies have shown that gold nanoparticle film assemblies of this nature provide for a more homogeneous protein adsorption environment and promote ET without distance dependence compared to the more traditional systems modified with alkanethiol self-assembled monolayers (SAM). In this paper, MPCs functionalized with hexanethiolate ligands are synthesized using a modified Brust reaction and characterized with ultraviolet visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), and proton (¹H) nuclear magnetic resonance (NMR). MPC films are assembled on SAM modified gold electrode interfaces by using a "dip cycle" method of alternating MPC layers and dithiol linking molecules. Film growth at gold electrode is tracked electrochemically by measuring changes to the double layer charging current of the system. Analogous films assembled on silane modified glass slides allow for optical monitoring of film growth and cross-sectional TEM analysis provides an estimated film thickness. During film assembly, manipulation of the MPC ligand protection as well as the interparticle linkage mechanism allow for networked films, that are readily adaptable, to interface with redox protein having different adsorption mechanism. For example, Pseudomonas aeruginosa azurin (AZ) can be adsorbed hydrophobically to dithiol-linked films of hexanethiolate MPCs and cytochrome c (cyt c) can be immobilized electrostatically at a carboxylic acid modified MPC interfacial layer. In this report, we focus on the film protocol for the AZ system exclusively. Investigations involving the adsorption of proteins on MPC modified synthetic platforms could further the understanding of interactions between biomolecules and man-made materials, and consequently aid the development of biosensor schemes, ET modeling systems, and synthetic biocompatible materials.

Synthesis, Assembly, and Characterization of Monolayer Protected Gold Nanoparticle Films for Protein Monolayer Electrochemistry

Colloidal gold nanoparticles protected with alkanethiolate ligands called monolayer protected gold clusters (MPCs) are synthesized and subsequently incorporated into film assemblies that serve as adsorption platforms for protein monolayer electrochemistry (PME). PME is utilized as the model system for studying electrochemical properties of redox proteins by confining them to an adsorption platform at a modified electrode, which also serves as a redox partner for electron transfer (ET) reactions. Studies have shown that gold nanoparticle film assemblies of this nature provide for a more homogeneous protein adsorption environment and promote ET without distance dependence compared to the more traditional systems modified with alkanethiol self-assembled monolayers (SAM).1-3 In this paper, MPCs functionalized with hexanethiolate ligands are synthesized using a modified Brust reaction4 and characterized with ultraviolet visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), and proton (1H) nuclear magnetic resonance (NMR). MPC films are assembled on SAM modified gold electrode interfaces by using a "dip cycle" method of alternating MPC layers and dithiol linking molecules. Film growth at gold electrode is tracked electrochemically by measuring changes to the double layer charging current of the system. Analogous films assembled on silane modified glass slides allow for optical monitoring of film growth and cross-sectional TEM analysis provides an estimated film thickness. During film assembly, manipulation of the MPC ligand protection as well as the interparticle linkage mechanism allow for networked films, that are readily adaptable, to interface with redox protein having different adsorption mechanism. For example, Pseudomonas aeruginosa azurin (AZ) can be adsorbed hydrophobically to dithiol-linked films of hexanethiolate MPCs and cytochrome c (cyt c) can be immobilized electrostatically at a carboxylic acid modified MPC interfacial layer. In this report, we focus on the film protocol for the AZ system exclusively. Investigations involving the adsorption of proteins on MPC modified synthetic platforms could further the understanding of interactions between biomolecules and man-made materials, and consequently aid the development of biosensor schemes, ET modeling systems, and synthetic biocompatible materials.5-8

Optical and electrochemical properties of multilayer polyelectrolyte thin films incorporating spherical, gold colloid nanomaterials

Polyelectrolyte multilayer (PEM) films incorporating various types of spherical, gold nanomaterials (NMs) were investigated to assess the existence of electrochemical and/or optical signal enhancement effects directly attributable to embedded NMs and the relationship of these effects to film structure and composition. Specifically, electrostatically assembled films of cationic poly-l-lysine (PLL) and anionic poly(4-styrene sulfonate) (PSS) incorporating one of four types of spherical, gold colloid NMs were constructed on 3-(aminopropyl)trimethoxysilane (3-APTMS)-modified glass substrates for optical studies or 11-mercaptoundecanoic (MUA)-modified gold electrodes for electrochemical studies. The NMs inserted into the PEM films include citrate-stabilized gold nanoparticles, thioctic acid-stabilized gold nanoparticles (TAS-NPs), MUA-modified monolayer protected gold clusters, and hollow gold nanoshells (Au-NSs). Optical sensitivity of the NM-embedded films, in terms of absorbance, surface plasmon band shifts, and the dependence of these optical responses on film thickness, varied depending on the type of NM within the film (e.g., TAS-NPs versus Au-NSs) but exhibited no corresponding electrochemical effects in the diffusional voltammetry of a ferricyanide redox probe. While not correlated to optical responses, the increased Faradaic current achieved during voltammetry at NM-embedded PEM films suggested that electrochemical effects of NMs were less dependent on the type of NMs and were, instead, more related to their location within the film and the electrostatic interactions built into the interfacial chemistry of the films. These results should prove useful for developing strategies constructing thin films with NMs that are specifically designed for optical or electrochemical sensing, taking full advantage of the signal enhancements provided by individual types of NMs.

Reaction Mechanism Governing Formation of 1,3-Bis(diphenylphosphino)propane-Protected Gold Nanoclusters

This report outlines the determination of a reaction mechanism that can be manipulated to develop directed syntheses of gold monolayer-protected clusters (MPCs) prepared by reduction of solutions containing 1,3-bis(diphenylphosphino)propane (L(3)) ligand and Au(PPh(3))Cl. Nanocluster synthesis was initiated by reduction of two-coordinate phosphine-ligated [Au(I)LL'](+) complexes (L, L' = PPh(3), L(3)), resulting in free radical complexes. The [Au(0)LL'](•) free radicals nucleated, forming a broad size distribution of ligated clusters. Timed UV-vis spectroscopy and electrospray ionization mass spectrometry monitored the ligated Au(x), 6 ≤ x ≤ 13, clusters, which comprise reaction intermediates and final products. By employing different solvents and reducing agents, reaction conditions were varied to highlight the largest portion of the reaction mechanism. We identified several solution-phase reaction classes, including dissolution of the gold precursor, reduction, continuous nucleation/core growth, ligand exchange, ion-molecule reactions, and etching of colloids and larger clusters. Simple theories can account for the reaction intermediates and final products. The initial distribution of the nucleation products contains mainly neutral clusters. However, the rate of reduction controls the amount of reaction overlap occurring in the system, allowing a clear distinction between reduction/nucleation and subsequent solution-phase processing. During solution-phase processing, the complexes undergo core etching and core growth reactions, including reactions that convert neutral clusters to cations, in a cyclic process that promotes formation of stable clusters of specific metal nuclearity. These processes comprise "size-selective" processing that can narrow a broad distribution into specific nuclearities, enabling development of tunable syntheses.

Sweep, step, pulse, and frequency-based techniques applied to protein monolayer electrochemistry at nanoparticle interfaces

Protein monolayer electrochemistry (PME), a strategy using synthetic platforms to study the electron transfer (ET) properties of adsorbed proteins, has been successfully applied to proteins adsorbed at monolayer-protected gold cluster (MPCs) assembled films, an adsorption interface shown to be an effective alternative, compared to traditional self-assembled monolayer (SAM) films, for the immobilization and study of ET proteins. Within PME studies, cyclic voltammetry (CV) remains the most commonly applied electrochemical technique in spite of several limitations that occur when the sweep technique is used at either platform. In particular, CV for PME at MPC films results in analysis complications stemming from the increased charging current inherent to electrochemical interfaces incorporating MPCs with capacitive properties. In this study, multiple electroanalytical techniques, involving step (chronocoulometry, CC), pulse (square wave voltammetry, SWV), and frequency-based impedance (electrochemical impedance spectroscopy, EIS) measurements, are applied to monolayers of adsorbed Pseudomonas aeruginosa azurin and horse heart cytochrome c at both MPC film assemblies as well as traditional SAMs. Electrochemical parameters (formal potential, electroactive surface coverage, double-layer capacitance, and ET rate constant) measured from these various techniques are directly compared and offer insight into the performance and reliability of each technique’s effectiveness in PME. While certain techniques result in measurements indistinguishable from CV, others offer distinct differences. Moreover, the application of alternative techniques reveals systemic limitations and complications within the electrochemical analysis that we further explore, including strategies for applying fast scanning techniques like SWV as well as the construction of MPC platforms with controlled levels of charging current that enable successful impedance analysis. The application of more advanced electrochemical techniques to developing electrochemical interfaces such as MPC film assemblies allows for a greater understanding of not only PME but also the applicability and effectiveness of these techniques to optimize the measurement of specific electrochemical parameters.

Preparation and photocatalytic activity of TiO2 nanocomposites coated with monolayer-protected gold clusters

We report the preparation and photocatalytic activity of TiO2 nanocomposites modified with monolayer-protected gold clusters (MPCs). Highly monodisperse tiopronin-coated MPCs with core diameters of 2.2 ± 0.2 nm were pre-prepared and then anchored onto a TiO2 surface using the bifunctional tiopronin linker. In this method, the gold core size was preserved after calcination at 260 °C as well as the anchoring process, and the gold loading on TiO2 could be precisely controlled by the experimental condition. The photocatalytic degradation of Rhodamine 590 (Rh-590) was carried out with thus prepared MPC–TiO2 composites. Mechanistic study of the photocatalytic reactions revealed that the degradation of Rh-590 occurs via the oxidative pathway by photogenerated holes. The photocatalytic activity of the MPC–TiO2 composites was found to increase significantly upon calcination at 260 °C, whereas the size of gold particles remained at their initial size. The photocatalytic activity of the composites, however, drastically decreased when the composites were calcined at 400 °C. X-ray photoelectron spectroscopy analysis of the calcined composites was conducted to understand the vastly different calcination results. A significant amount of oxidized sulfur remained in the composites after calcination at 400 °C, which appears to be responsible for the drastic decrease in the photocatalytic activity.

Short-Chain PEG Mixed Monolayer Protected Gold Clusters Increase Clearance and Red Blood Cell Counts

Monolayer-protected gold nanoparticles have great potential as novel building blocks for the design of new drugs and therapeutics based on the easy ability to multifunctionalize them for biological targeting and drug activity. In order to create nanoparticles that are biocompatible in vivo, polyethylene glycol functional groups have been added to many previous multifunctionalized particles to eliminate nonspecific binding. Recently, monolayer-protected gold nanoparticles with mercaptoglycine functionalities were shown to elicit deleterious effects on the kidney in vivo that were eliminated by incorporating a long-chain, mercapto-undecyl-tetraethylene glycol at very high loadings into a mixed monolayer. These long-chain PEGs induced an immune response to the particle presumably generating an anti-PEG antibody as seen in other long-chain PEG-ylated nanoparticles in vivo. In the present work, we explore the in vivo effects of high and low percent ratios of a shorter chain, mercapto-tetraethylene glycol within the monolayer using simple place-exchange reactions. The shorter chain PEG MPCs were expected to have better water solubility due to elimination of the alkyl chain, no toxicity, and long-term circulation in vivo. Shorter chain lengths at lower concentrations should not trigger the immune system to create an anti-PEG antibody. We found that a 10% molar exchange of this short-chain PEG within the monolayer met three of the desired goals: high water solubility, no toxicity, and no immune response as measured by white blood cell counts. However, none of the short-chain PEG mixed monolayer compositions enabled the nanoparticles to have a long circulation time within the blood as compared to mercapto-undecyl-ethylene glycol, which had a residence time of 4 weeks. We also compared the effects of a hydroxyl versus a carboxylic acid terminal functional group on the end of the PEG thiol on both clearance and immune response. The results indicate that short-chain-length PEGs, regardless of termini, increase clearance rates compared to the previous long-chain PEG studies, while carboxylated termini increase red blood cell counts at high loadings. Given these findings, short-chain, alcohol-terminated PEG, exchanged at 10%, was identified as a potential nanoparticle for further in vivo applications requiring short circulation lifetimes with desired features of no toxicity, no immune response, and high water solubility.

Electrochemical properties of ruthenium metallopolymer: Monolayer-protected gold cluster nanocomposites

Nanocomposite polymer films of [Ru(bpy) 2(PVP) 10] 2+ metallopolymer and gold monolayer-protected clusters (MPCs) have been formed. Cyclic voltammetry of the films has been performed in both acidic (0.1 M H 2SO 4) and neutral (0.1 M Na 2SO 4) pH electrolyte solution. In acidic electrolyte, the protecting monolayers is desorbed as the gold surface is oxidized. Transmission electron microscopy of the nanocomposite following electrochemical cycling reveals aggregates of “naked” MPCs. In sharp contrast, in neutral pH electrolyte, the MPCs are stable towards voltammetric cycling and no gold oxide formation is evident in the voltammetry. The homogeneous charge-transfer diffusion coefficient, D CT , was determined for a range of nanocomposite films as the MPC mole fraction is systematically varied. When acidic electrolyte is used, D CT decreases from 2.8 ± 0.5 × 10 −10 to 1.5 ± 0.1 × 10 −11 cm 2 s −1 as the MPC mole fraction is increased from 0 to 0.45. In contrast, the D CT determined in neutral electrolyte is universally lower than that determined in acidic electrolyte, it increases by over two orders of magnitude from 2.6 ± 0.9 × 10 −15 to 4.0 ± 0.5 × 10 −13 cm 2 s −1 as MPC mole fraction increases from 0.05 to 0.5.