Stabilization of Gold Nanoparticles by 6-Mercaptopurine Monolayers. Effects of the Solvent Properties

Zinc-doped hydroxyapatite as a pH responsive drug delivery system for anticancer drug 6-mercaptopurine

6-Mercaptopurine (6MP) is commonly used in the treatment of acute lymphoblastic leukemia as an important agent in maintenance therapy. Despite its therapeutic benefits, 6MP has some limitations during therapy. Taking into account the disadvantages during 6MP therapy, there is a great need to create an appropriate delivery system for this drug. 6MP contains in its structure nitrogen and sulfur atoms capable of forming coordination compounds with metal ions, for example zinc. Therefore, in this work, we prepared biocompatible hydroxyapatite (HAp) doped with zinc ions, and used it as a carrier for 6MP. Doped HAp has not been used as a carrier for this drug before. The work proved that the prepared carrier-drug system has a particle size of about 130 nm, which indicates its potential for intravenous delivery. In addition, in an acidic environment (imitating cancer cells), the carrier agglomerates allow targeted release of the drug. The drug is evenly distributed, which indicates that the doses released from it will always be comparable. The release of the drug in a neutral environment is long-lasting in controlled doses, whereas in an acidic environment it is immediate. The obtained results indicate the high potential of the material in both slow-release and cancer-targeted release of 6MP.

Mesoporous Silica Modified with Polydopamine and Zinc Ions as a Potential Carrier in the Controlled Release of Mercaptopurine

Mercaptopurine is one of the drugs used in the treatment of acute lymphoblastic leukemia. A problem with mercaptopurine therapy is its low bioavailability. This problem can be solved by preparing the carrier that releases the drug in lower doses but over a longer period of time. In this work, polydopamine-modified mesoporous silica with adsorbed zinc ions was used as a drug carrier. SEM images confirm the synthesis of spherical carrier particles. The particle size is close to 200 nm, allowing for its use in intravenous delivery. The zeta potential values for the drug carrier indicate that it is not prone to agglomeration. The effectiveness of drug sorption is indicated by a decrease in the zeta potential and new bands in the FT-IR spectra. The drug was released from the carrier for 15 h, so all of the drug can be released during circulation in the bloodstream. The release of the drug from the carrier was sustained, and no 'burst release' was observed. The material also released small amounts of zinc, which are important in the treatment of the disease because these ions can prevent some of the adverse effects of chemotherapy. The results obtained are promising and have great application potential.

Behind the Optimization of the Sensor Film: Bioconjugation of Triangular Gold Nanoparticles with Hemoproteins for Sensitivity Enhancement of Enzymatic Biosensors

Electrochemical biosensors are widely used in a multitude of applications, such as medical, nutrition, research, among other fields. These sensors have been historically used and have not undergone many changes in terms of the involved electrochemical processes. In this work, we propose a new approach on the immobilization and enhancement of the electrochemical properties of the sensing layers through the control and bioconjugation of hemoproteins (hemoglobin, myoglobin, and cytochrome C) on anisotropic gold nanoparticles (gold nanotriangles (AuNTs)). The hemeproteins and the AuNTs are mixed in a solution, resulting in stable bioconjugates that are deposited onto the electrode surface to obtain the biosensors. All the systems proposed herein exhibited direct well-defined redox responses, highlighting the key role of the AuNTs acting as mediators of such electron transfers. Several protein layers surrounding the AuNTs are electroactive, as demonstrated from the charge measured by cyclic voltammetry. The retention of the stability of the hemeproteins once they are part of the bioconjugates is evidenced towards the electrocatalytic reduction of hydrogen peroxide, oxygen, and nitrite. The parameters obtained for the proposed biosensors are similar or even lower than those previously reported for similar systems based on nanomaterials, and they exhibit attractive properties that make them potential candidates for the latest developments in the field of sensing devices.

Anticancer Drugs: Recent Strategies to Improve Stability Profile, Pharmacokinetic and Pharmacodynamic Properties

In past decades, anticancer research has led to remarkable results despite many of the approved drugs still being characterized by high systemic toxicity mainly due to the lack of tumor selectivity and present pharmacokinetic drawbacks, including low water solubility, that negatively affect the drug circulation time and bioavailability. The stability studies, performed in mild conditions during their development or under stressing exposure to high temperature, hydrolytic medium or light source, have demonstrated the sensitivity of anticancer drugs to many parameters. For this reason, the formation of degradation products is assessed both in pharmaceutical formulations and in the environment as hospital waste. To date, numerous formulations have been developed for achieving tissue-specific drug targeting and reducing toxic side effects, as well as for improving drug stability. The development of prodrugs represents a promising strategy in targeted cancer therapy for improving the selectivity, efficacy and stability of active compounds. Recent studies show that the incorporation of anticancer drugs into vesicular systems, such as polymeric micelles or cyclodextrins, or the use of nanocarriers containing chemotherapeutics that conjugate to monoclonal antibodies can improve solubility, pharmacokinetics, cellular absorption and stability. In this study, we summarize the latest advances in knowledge regarding the development of effective highly stable anticancer drugs formulated as stable prodrugs or entrapped in nanosystems.

Distinct thermoresponsive behaviour of oligo- and poly-ethylene glycol protected gold nanoparticles in concentrated salt solutions

In this work, the methoxy terminated oligo- and polyethylene glycol of different chain lengths (EGn, n = 7, 18, 45 and 136) is grafted on AuNP surfaces under conditions where they attain maximum grafting densities. These EGn-AuNPs gain stability relative to the pristine c-AuNPs in aqueous solutions and in a wide temperature interval and they form stable suspensions in solutions of high NaCl concentrations. To show the thermoresponsive properties of these EGn-AuNPs, temperature titration experiments are carried out in the presence of increasing amounts of salts. The concentrations of NaCl are chosen by checking the stability of EGn-AuNPs at room temperature and choosing the highest concentrations that allow them to form stable suspensions. The analysis of the temperature titration experiments monitored by UV-visible spectroscopy and dynamic light scattering allows us to establish the existence of transitions from individual to assembled nanoparticles, the reversibility of the temperature transitions and hysteretic behaviour in these systems. While EG7-AuNPs only show reversible temperature transitions in the presence of 5 mM NaCl, EG18-AuNPs do up to 1 M NaCl, becoming only partially reversible in 2 M NaCl. The titrations of EG45-AuNPs in 3 and 5 M NaCl show irreversible temperature transitions. Finally, EG136-AuNPs present a complex and interesting behaviour with two temperature transitions, the first one showing hysteresis and the second being reversible.

Preparation, Characterization, Pharmacokinetic, and Therapeutic Potential of Novel 6-Mercaptopurine-Loaded Oral Nanomedicines for Acute Lymphoblastic Leukemia

Background

Acute lymphoblastic leukemia (ALL) is the most common hematologic malignancy in children. It requires a long and rigorous course of chemotherapy treatments. 6-Mercaptopurine (6-MP) is one of the primary drugs used in chemotherapy. Unfortunately, its efficacy has been limited due to its insolubility, poor bioavailability and serious adverse effects. To overcome these drawbacks, we constructed 6-mercaptopurine (6-MP)-loaded nanomedicines (6-MPNs) with biodegradable poly(lactide-co-glycolide) (PLGA) to enhance the anticancer efficacy of 6-MP.

Methods

We prepared the 6-MPNs using a double-emulsion solvent evaporation method, characterizing them for the physicochemical properties. We then investigated the plasma, intestinal region and other organs in Sprague Dawley (SD) rats for pharmacokinetics. Additionally, we evaluated its anticancer efficacy in vitro on the human T leukemia cell line Jurkat and in vivo on the ALL model mice.

Results

The 6-MPNs were spherical in shape with uniform particle size and high encapsulation efficiency. The in vitro release profile showed that 6-MPNs exhibited a burst release that a sustained release phase then followed. The apoptosis assay demonstrated that 6-MPNs could improve the in vitro cytotoxicity in Jurkat cells. Pharmacokinetics profiles revealed that 6-MPNs had improved oral bioavailability. Tissue distribution experiments indicated that 6-MPNs increased the duodenum absorption of 6-MP, at the same time having a low accumulation of the toxic metabolites of 6-MP. The in vivo pharmacodynamics study revealed that 6-MPNs could prolong the survival time of the ALL model mice. The prepared 6-MPNs, therefore, have superior properties in terms of anticancer efficacy against ALL with reduced systemic toxicity.

Conclusion

Our nanomedicines provide a promising delivery strategy for 6-MP; they offer a simple preparation method and high significance for clinical translation.

Quantum Mechanical Modeling of the Interactions between Noble Metal (Ag and Au) Nanoclusters and Water with the Effective Fragment Potential Method

Explicit solvent interactions can significantly alter the physical and chemical properties of noble metal (e.g., gold and silver) nanoclusters. In order to compute these solvent interactions at a reasonable computational cost, a quantum mechanical (QM)/molecular mechanics (MM) approach, where the metal nanocluster is treated with full QM and the water molecules are treated with a MM force field, can be used. However, classical MM force fields were typically parameterized using molecules containing main group elements as the reference. The accuracy of noble metal-solvent interactions obtained with these force fields therefore remains unpredictable. The effective fragment potential (EFP) force field, designed to model explicitly solvated systems, represents an attractive method to simulate solvated noble metal nanoclusters because it is derived from first principles and contains few or no fitted parameters, depending on implementation. At the density functional theory-optimized geometries, good correlation is obtained between the nanocluster-water interaction energies computed with EFP and those computed with the reference coupled cluster singles, doubles, and perturbative triples method. It is shown that the EFP method gives qualitatively accurate interaction energies at medium-large intermolecular distances for various molecular configurations. In order to achieve higher quantitative accuracy, the first solvation shell should be treated with full QM, if possible. EFP is therefore a promising method for the QM modeling of explicitly solvated silver and gold nanoclusters.

Theoretical Investigation of 6-Mercaptopurine Isomers' Adsorption on the Au(001) Surface: Revealing the Fate of Different Isomers

Density functional theory (DFT) calculations were performed for the adsorption of different isomers of 6-mercaptopurine on the Au(001) surface. All of the configurations of four thione and two thiol isomers were considered. The results show that the thione isomers adsorbed more strongly on the Au(001) surface compared with the thiol ones. In all of the configurations, the calculated binding energy of ma-8 is the largest, in which the S atom of 6-mercaptopurine binds strongly with one Au atom on the monodentate sites and 6-mercaptopurine retains a flat geometry, predominantly with an approximately 30° orientation between the C-S bond and the Au-Au bond of the catalyst. Additionally, the 6-mercaptopurines in ma-2, mb-5, and mc-3 also bind more strongly onto the surface, which show relatively higher stability on the surfaces, indicating a high preference for adsorption. Charge density differences and TDOS analyses for the four configurations also show that the electronic charges are accumulated between Au and S atoms in the Au-S bonds, indicating occurrence of adsorption and chemical-bond formation.

Effective replacement of cetyltrimethylammonium bromide (CTAB) by mercaptoalkanoic acids on gold nanorod (AuNR) surfaces in aqueous solutions

The highly packed cetyltrimethylammonium bromide (CTAB) bilayer built up on the surface of gold nanorods (AuNRs) when synthesized by the seed-mediated procedure hampers the complete ligand exchange under experimental conditions that preserves the stability of the dispersions. In the present work, a ligand exchange protocol by using carboxy-terminated alkanethiols of different chain lengths by means of a green approach that uses only aqueous solutions is presented. The protocol is based on the knowledge of the stability in the aqueous solution of both the starting CTAB-AuNRs and the final products that help in the choice of the experimental conditions used for ligand exchange. The characterization of the CTAB protective layer as well as the study of its colloidal stability in solution has helped us to design an appropriate methodology. Cyclic voltammetry of CTAB-AuNRs demonstrates the high stability of the bilayer showing the existence of a two-dimensional phase transition from a highly ordered to a less organized phase. Other techniques such as XPS, FT-IR and Raman spectroscopy provide information about the structure of the layer and UV-visible-NIR spectroscopy establishes the stability conditions in aqueous solution. We have chosen an exchange procedure for 11-mercaptoundecanoic acid (MUA) and 16-mercaptohexadecanoic acid (MHDA) based on a one-pot methodology under conditions where all the species involved are stable. The protocol, however, can be extended to different chemical functionalities that are considered useful to be applied in living systems. Under these conditions the complete exchange of CTAB by the mercaptoderivatives was successful as demonstrated by the different characterization techniques used: UV-visible-NIR, FT-IR, Raman, XPS spectroscopy, cyclic voltammetry and transmission electron microscopy (TEM).

Direct vs Mediated Coupling of Antibodies to Gold Nanoparticles: The Case of Salivary Cortisol Detection by Lateral Flow Immunoassay

Stable and efficient conjugates between antibodies and gold nanoparticles (GNP-Ab) are sought to develop highly sensitive and robust biosensors with applications in medicine, toxicology, food safety controls, and targeted drug delivery. Several strategies have been proposed for directing the antibody attachment to GNPs thus preserving antibody activity, including covalently coupling the antibody to a polymer grafted on GNP surface and exploiting the high affinity of bioreceptors as mediators for the binding. Both approaches also allow for shielding GNPs with a protective layer that guarantees the robustness of the conjugate. Notwithstanding, antibodies freely adsorb to GNP with high binding efficiency. The nonspecific adsorption is far more simple, fast, and inexpensive than any mediated coupling. Therefore, it is preferred for most applications, although it is considered to produce GNP-Ab with a limited activity. In this work, we compared three strategies for producing GNP-Ab, such as (i) covalent coupling mediated by a chemical layer, (ii) affinity-based binding mediated by a biomolecular layer composed of Staphylococcal protein A, and (iii) direct attachment via adsorption. The so-prepared GNP-Ab were employed as probes in a colorimetric lateral flow immunoassay (LFIA) for measuring salivary cortisol as a model biosensor that relies on the use of active GNP-Ab conjugates. Unexpectedly, the biosensors fabricated using the three probes were completely comparable in terms of their ability to measure salivary cortisol. Furthermore, we observed that the sensitivity of the LFIA primarily depended on the amount of the antibody bound to GNPs rather than on the method by which it was bound. The probes prepared using both the direct adsorption approach and mediated coupling via the biochemical mediator enabled development of point-of-care devices for the fast, sensitive, and reliable measurement of human salivary cortisol.

Effects of amendments on heavy metal immobilization and uptake by Rhizoma chuanxiong on copper and cadmium contaminated soil

An improved method was applied for remediating cadmium and copper co-contaminated soil and reducing the metal concentration in Rhizoma chuanxiong. Pot experiments were conducted with six amendments (composed with bentonite, phosphate, humic acid, biochar, sepiolite powder, etc.). The results showed that soil pH, biological activities (soil enzymatic activities and microbial counts) and R. chuanxiong biomass were greatly improved with the addition of amendments in all treatments, especially in T3 and T6. Also, amendments effectively decreased the concentration of malondialdehyde and H₂O₂ in R. chuanxiong. In the T3 treatment, the bio-available Cd and Cu in soil were significantly decreased by 0.53 and 0.41 mg kg^-1, respectively. Meanwhile, the amendment in T3 reduced Cd and Cu accumulation in R. chuanxiong about 45.83 and 39.37%, respectively, compared to T0. Moreover, the Fourier transform infrared spectroscopy spectra showed the surface functional groups of every amendment. To conclude, this study offers an effective and environmental method to reduce metal accumulation in R. chuanxiong on heavy metal co-contaminated soil.

Electronic Structure of a Self-Assembled Monolayer with Two Surface Anchors: 6-Mercaptopurine on Au(111)

The electronic structure of aromatic and aliphatic thiols on Au(111) has been extensively studied in relation to possible applications in molecular electronics. In this work, the effect on the electronic structure of an additional anchor to the S-Au bond using 6-mercaptopurine as a model system has been investigated. Results from X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and density functional theory (DFT) confirm that this molecule adsorbs on Au(111) with S-Au and iminic N-Au bonds. Combined ultraviolet photoelectron spectroscopy and DFT data reveal that formation of the 6MP self-assembled monolayer generates a molecular dipole perpendicular to the surface, with negative charges residing at the metal/monolayer interface and positive charges at the monolayer/vacuum interface, which lowers the substrate work function. Scanning tunneling microscopy shows two surface molecular domains: a well-ordered rectangular lattice where molecules are tilted on average 30° with respect to the substrate and aligned 6MP islands where molecules are standing upright. Finally, we found a new electronic state located at -1.7 eV with respect to the Fermi level that corresponds to a localized π molecular state, while the state corresponding to the N-Au bond is hybridized with Au d electrons and stabilized at much lower energies (-3 eV).

Hemoglobin becomes electroactive upon interaction with surface-protected Au nanoparticles

In this work, we report on the electrochemical behavior of bioconjugates prepared with gold nanoparticles (AuNP) capped with three different molecular layers (citrate anions, 6-mercaptopurine and ω-mercaptoundecanoic acid) and the protein hemoglobin (Hb). Freshly formed bioconjugates are deposited on a glassy carbon electrode and assayed for electroactivity. A pair of redox peaks with formal potential at -0.37V is obtained, in contrast with the free Hb protein that is inactive on the glassy carbon substrate. The redox response is typical for quasi-reversible processes allowing the determination of the electron transfer rate constant for the three bioconjugates. Additional evidence of the structural integrity of protein upon forming the bioconjugate is obtained by monitoring the electrochemical response of the Hb heme Fe(III)/Fe(II) redox couple as a function of solution pH. Moreover, the Hb forming the protein corona around the AuNPs show good electrocatalytic activity for the reduction of hydrogen peroxide and oxygen. It has been found that only the first layer of Hb surrounding the AuNPs are electroactive, although some part of the second layer also contribute, pointing to the role of the AuNP in the electrochemical response.

Hemoglobin bioconjugates with surface-protected gold nanoparticles in aqueous media: The stability depends on solution pH and protein properties

The identification of the factors that dictate the formation and physicochemical properties of protein-nanomaterial bioconjugates are important to understand their behavior in biological systems. The present work deals with the formation and characterization of bioconjugates made of the protein hemoglobin (Hb) and gold nanoparticles (AuNP) capped with three different molecular layers (citrate anions (c), 6-mercaptopurine (MP) and ω-mercaptoundecanoic acid (MUA)). The main focus is on the behavior of the bioconjugates in aqueous buffered solutions in a wide pH range. The stability of the bioconjugates have been studied by UV-visible spectroscopy by following the changes in the localized surface resonance plasmon band (LSRP), Dynamic light scattering (DLS) and zeta-potential pH titrations. It has been found that they are stable in neutral and alkaline solutions and, at pH lower than the protein isoelectric point, aggregation takes place. Although the surface chemical properties of the AuNPs confer different properties in respect to colloidal stability, once the bioconjugates are formed their properties are dictated by the Hb protein corona. The protein secondary structure, as analyzed by Attenuated total reflectance infrared (ATR-IR) spectroscopy, seems to be maintained under the conditions of colloidal stability but some small changes in protein conformation take place when the bioconjugates aggregate. These findings highlight the importance to keep the protein structure upon interaction with nanomaterials to drive the stability of the bioconjugates.

Controlling the reproducibility of Coulomb blockade phenomena for gold nanoparticles on an organic monolayer/silicon system

Two types of highly ordered organic layers were prepared on silicon modified with an amine termination for binding gold nanoparticles (AuNPs). These two grafted organic monolayers (GOMs), consisting of alkyl chains with seven or 11 carbon atoms, were grafted on oxide-free Si(111) surfaces as tunnel barriers between the silicon electrode and the AuNPs. Three kinds of colloidal AuNPs were prepared by reducing HAuCl4 with three different reactants: citrate (Turkevich synthesis, diameter ∼16 nm), ascorbic acid (diameter ∼9 nm), or NaBH4 (Natan synthesis, diameter ∼7 nm). Scanning tunnel spectroscopy (STS) was performed in a UHV STM at 40 K, and Coulomb blockade behaviour was observed. The reproducibility of the Coulomb behavior was analysed as a function of several chemical and physical parameters: size, crystallinity of the AuNPs, influence of surrounding surfactant molecules, and quality of the GOM/Si interface (degree of oxidation after the full processing). Samples were characterized with scanning tunneling microscope, STS, atomic force microscope, Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy (XPS), and high resolution transmission electronic microscope. We show that the reproducibility in observing Coulomb behavior can be as high as ∼80% with the Natan synthesis of AuNPs and GOMs with short alkyl chains.

Purine-stabilized green fluorescent gold nanoclusters for cell nuclei imaging applications

We report facile one-pot synthesis of water-soluble green fluorescent gold nanoclusters (AuNCs), capped with 8-mercapto-9-propyladenine. The synthesized AuNCs were characterized by Fourier transform infrared (FTIR), powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), (1)H NMR, and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. These nanoclusters show high photostability and biocompatibility. We observed that AuNCs stain cell nuclei with high specificity, where the mechanism of AuNC uptake was established through pathway-specific uptake inhibitors. These studies revealed that cell internalization of AuNCs occurs via a macropinocytosis pathway.

A highly efficient ligand exchange reaction on gold nanoparticles: preserving their size, shape and colloidal stability

Almost complete ligand exchange and excellent colloidal stability of gold nanoparticles can be achieved by a new solvent extraction based method.

Targeted magnetic hyperthermia

Many nanotechnologies, which enable unique approaches to treat cancer, have been developed based upon non-toxic organic and inorganic materials to improve current cancer treatments. The use of inorganic materials to form magnetic nanoparticles for hyperthermia therapy is of great interest for localized treatment of cancers without effecting adjacent healthy tissue. Extensive clinical trials have begun using magnetic hyperthermia in animal models. The purpose of this article is to address different factors that affect targeting, heating and biodistribution to safely control the therapeutic efficacy of targeted magnetic hyperthermia. This method involves accumulation of magnetic nanoparticles at a tumor site and then manipulating the magnetic properties of the nanoparticles to heat the targeted tissues.

Determination of 6-mercaptopurine in the presence of uric acid using modified multiwall carbon nanotubes-TiO2 as a voltammetric sensor

In this work, a multiwall carbon nanotubes modified electrode (prepared by incorporating TiO(2) nanoparticles with p-aminophenol as a mediator) was used as voltammetric sensor for the determination of 6-mercaptopurine (6-MP) in the presence of uric acid (UA). The voltammograms of 6-MP and UA in a mixture can be separated from each other by differential pulse voltammetry with a potential difference of 380 mV at a scan rate of 10 mV s(-1). These conditions are sufficient to allow for the determination of 6-MP and UA both individually and simultaneously. The electrocatalytic currents increase linearly with 6-MP concentration in the ranges of 0.09-350 µmol L(-1) (two linear segments with different slopes). The detection limit for 6-MP was 0.065 µmol L(-1) . The RSD% for 1.0 and 15.0 µmol L(-1) 6-MP were 0.7%, and 1.2%, respectively. The kinetic parameters of the system were determined using electrochemical approaches. The method was successfully applied for the determination of 6-MP in drug sample, and 6-MP plus UA in urine samples.

Thiol with an unusual adsorption-desorption behavior: 6-mercaptopurine on Au(111)

A multitechnique study of 6-mercaptopurine (6MP) adsorption on Au(111) is presented. The molecule adsorbs on Au(111), originating short-range ordered domains and irregular nanosized aggregates with a total surface coverage by chemisorbed species smaller than those found for alkanethiol SAMs, as derived from scanning tunneling microscopy (STM) and electrochemical results. X-ray photoelectron spectroscopy (XPS) results show the presence of a thiolate bond, whereas density functional theory (DFT) data indicate strong chemisorption via a S-Au bond and additional binding to the surface via a N-Au bond. From DFT data, the positive charge on the Au topmost surface atoms is markedly smaller than that found for Au atoms in alkanethiolate SAMs. The adsorption of 6MP originates Au atom removal from step edges but no vacancy island formation at (111) terraces. The small coverage of Au islands after 6MP desorption strongly suggests the presence of only a small population of Au adatom-thiolate complexes. We propose that the absence of the Au-S interface reconstruction results from the lack of significant repulsive forces acting at the Au surface atoms.

Mechanistic insights into seeded growth processes of gold nanoparticles

A facile approach for the synthesis of monodisperse gold nanoparticles with radii in the range of 7 to 20 nm is presented. Starting from monodisperse seeds with radii of 7 nm, produced in the first step, the addition of a defined amount of additional precursor material permits distinct size regulation and the realization of predicted nanoparticle sizes. These information were derived from ex- and in situ investigations by comprehensive small angle X-ray scattering (SAXS), X-ray absorption near edge structure (XANES) and UV-Vis data to obtain information on the physicochemical mechanisms. The obtained mechanisms can be transferred to other seeded growth processes. Compared to similar approaches, the presented synthesis route circumvents the use of different reducing or stabilizing agents. The size of resulting nanoparticles can be varied over a large size range presented for the first time without a measurable change in the shape, polydispersity or surface chemistry. Thus, the resulting nanoparticles are ideal candidates for size dependence investigations.

Formation of a 1,8-octanedithiol self-assembled monolayer on Au(111) prepared in a lyotropic liquid-crystalline medium

A characterization of the 1,8-octanedithiol (ODT) self-assembled monolayer (SAM) formed from a Triton X-100 lyotropic medium has been conducted by electrochemical techniques. It is found that an ODT layer of standing-up molecules is obtained at short modification time without removing oxygen from the medium. The electrochemical study shows that the ODT layer formed after 15 min of modification time has similar electron-transfer blocking properties to the layers formed from organic solvents at much longer modification times. On the basis of XPS data, it is demonstrated that the inability to bind gold nanoparticles (AuNPs) is due to the presence of extra ODT molecules either interdigited or on top of the layer. Treatment consisting of an acid washing step following the formation of the ODT-Au(111) SAM produces a layer that is able to attach AuNPs as demonstrated by electrochemical techniques and atomic force microscopy (AFM) images.

Stability of polydimethylsiloxane-magnetite nanoparticle dispersions against flocculation: interparticle interactions of polydisperse materials

The colloidal stability of dispersions comprised of magnetite nanoparticles coated with polydimethylsiloxane (PDMS) oligomers was investigated theoretically and experimentally. Particle-particle interaction potentials in a theta solvent and in a good solvent for the PDMS were predicted by calculating van der Waals, electrostatic, steric, and magnetic forces as functions of interparticle separation distances. A variety of nanoparticle sizes and size distributions were considered. Calculations of the interparticle potential in dilute suspensions indicated that flocculation was likely for the largest 1% of the population of particles. Finally, the rheology of these complexes over time in the absence of a solvent was measured to probe their stabilities against flocculation as neat fluids. An increase in viscosity was observed upon aging, suggesting that some agglomeration occurs with time. However, the effects of aging could be removed by exposing the sample to high shear, indicating that the magnetic fluids were not irreversibly flocculated.

Gold nanoparticles enhance the anti-leukemia action of a 6-mercaptopurine chemotherapeutic agent

6-mercaptopurine and its riboside derivatives are some of the most widely utilized anti-leukemic and anti-inflammatory drugs. Their short biological half-life and severe side effects limit their use. A new delivery method for these drugs based on 4-5 nm gold nanoparticles can potentially resolve these issues. We have found substantial enhancement of the antiproliferative effect against K-562 leukemia cells of Au nanoparticles bearing 6-mercaptopurine-9-beta-d-ribofuranoside compared to the same drug in typically administered free form. The improvement was attributed to enhanced intracellular transport followed by the subsequent release in lysosomes. Enhanced activity and nanoparticle carriers will make possible the reduction of the overall concentration of the drug, renal clearance, and, thus, side effects. The nanoparticles with mercaptopurine also showed excellent stability over 1 year without loss of inhibitory activity.

Influence of the solution pH in the 6-mercaptopurine self-assembled monolayer (6MP-SAM) on a Au(111) single-crystal electrode

Self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) have been prepared on a Au(111) single-crystal electrode by immersion of the metal surface in a 100 microM 6MP and 0.01 M HClO4 solution. The 6MP-SAM Au(111) single-crystal electrodes were transferred to the cell and allowed to equilibrate with the different aqueous working solutions before the electrochemical experiments. The influence of the solution pH was studied by cyclic voltammetry, double layer capacitance curves, and electrochemical impedance spectroscopy. The electrochemical behavior of the 6MP-SAM in acetic acid at pH 4 presents important differences in comparison to that obtained in 0.1 M KOH solutions. Cyclic voltammograms for the reductive desorption process in acid medium are broad and show some features that can be explained by a phase transition between a chemisorbed and a physisorbed state of the 6MP molecules. The low solubility of these molecules in acid medium could explain this phenomenon and the readsorption of the complete monolayer when the potential is scanned in the positive direction. The variation of the double-layer capacitance values in the potential range of monolayer stability with the pH suggests that the acid-base chemistry of the 6MP molecules is playing a role. This fact has been studied by following the variations of the electron-transfer rate constant of the highly charged redox probes as are Fe(CN)(6)-3/-4 and Ru(NH3)(6)+3/+2 as a function of solution pH. The apparent surface pKa value for the 6MP-SAM (pKa approximately 8) is explained by the total conversion of the different 6MP tautomers that exist in solution to the thiol species in the adsorbed state.