Estimation of Size for 1−2 nm Nanoparticles Using an HPLC Electrochemical Detector of Double Layer Charging

Metal-nanocluster Science and Technology: My Personal History and Outlook

Metal nanoclusters (NCs) are among the leading targets in research of nanoscale materials, and elucidation of their properties (science) and development of control techniques (technology) have been continuously studied for...

Atomic-level separation of thiolate-protected metal clusters

Fine metal clusters have attracted much attention from the viewpoints of both basic and applied science for many years because of their unique physical/chemical properties and functions, which differ from those of bulk metals. Among these materials, thiolate (SR)-protected gold clusters (Au_n(SR)_m clusters) have been the most studied metal clusters since 2000 because of their ease of synthesis and handling. However, in the early 2000s, it was not easy to isolate these metal clusters. Therefore, high-resolution separation methods were explored, and several atomic-level separation methods, including polyacrylamide gel electrophoresis (PAGE), high-performance liquid chromatography (HPLC), and thin-layer chromatography (TLC), were successively established. These techniques have made it possible to isolate a series of Au_n(SR)_m clusters, and much knowledge has been obtained on the correlation between the chemical composition and fundamental properties such as the stability, electronic structure, and physical properties of Au_n(SR)_m clusters. In addition, these high-resolution separation techniques are now also frequently used to evaluate the distribution of the product and to track the reaction process. In this way, high-resolution separation techniques have played an essential role in the study of Au_n(SR)_m clusters. However, only a few reviews have focused on this work. This review focuses on PAGE, HPLC, and TLC separation techniques, which offer high resolution and repeatability, and summarizes previous studies on the high-resolution separation of Au_n(SR)_m and related clusters with the purpose of promoting a better understanding of the features and the utility of these techniques.

Quantitative micro-Raman analysis of micro-particles in drug delivery

Polymeric micro and nanoconstructs are emerging as promising delivery systems for therapeutics and contrast agents in microcirculation. Excellent assets associated with polymeric particulates of tunable shape, size, mechanical and chemical properties may improve the efficiency of delivery and represent the basis of personalized medicine and treatment. Nevertheless, lack of effective techniques of analysis may limit their use in biomedicine and bioengineering. In this paper, we demonstrated Raman Spectroscopy for quantitative characterization of poly lactic-co-glycolic acid (PLGA) micro-plate drug delivery systems. To do so, we (i) acquired bi-dimensional Raman maps of PLGA micro-plates loaded with curcumin at various times of release over multiple particles. We (ii) realized an exploratory analysis of data using the principal component analysis (PCA) technique to find hidden patterns in the data and reduce the dimensionality of the system. Then, we (iii) used an innovative univariate method of analysis of the reduced system to derive quantitative drug release profiles. High performance liquid chromatography (HPLC), the consolidated method of analysis of macro-sized systems, was used for comparison. We found that our system is as efficient as HPLC but, differently from HPLC, it enables quantitative analysis of systems at the single particle level.

Physicochemical study of natural fractionated biocolloid by asymmetric flow field-flow fractionation in tandem with various complementary techniques using biologically synthesized silver nanocomposites

Asymmetric flow field-flow fractionation coupled with use of ultraviolet–visible, multiangle light scattering (MALLS), and dynamic light scattering (DLS) detectors was used for separation and characterization of biologically synthesized silver composites in two liquid compositions. Moreover, to supplement the DLS/MALLS information, various complementary techniques such as transmission electron spectroscopy, Fourier transform infrared spectroscopy, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) were used. The hydrodynamic diameter and the radius of gyration of silver composites were slightly larger than the sizes obtained by transmission electron microscopy (TEM). Moreover, the TEM results revealed the presence of silver clusters and even several morphologies, including multitwinned. Additionally, MALDI-TOF MS examination showed that the particles have an uncommon cluster structure. It can be described as being composed of two or more silver clusters. The organic surface of the nanoparticles can modify their dispersion. We demonstrated that the variation of the silver surface coating directly influenced the migration rate of biologically synthesized silver composites. Moreover, this study proves that the fractionation mechanism of silver biocolloids relies not only on the particle size but also on the type and mass of the surface coatings. Because silver nanoparticles typically have size-dependent cytotoxicity, this behavior is particularly relevant for biomedical applications.

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Liquid Chromatography Separation and Mass Spectrometry Detection of Silver-Lipoate Ag29(LA)12 Nanoclusters: Evidence of Isomerism in the Solution Phase

Evidence for the existence of condensed-phase isomers of silver-lipoate clusters, Ag₂₉(LA)₁₂, where LA = (R)-α lipoic acid, was obtained by reversed-phase ion-pair liquid chromatography with in-line UV-vis and electrospray ionization (ESI)-MS detection. All components of a raw mixture were separated according to surface chemistry and increasing size via reversed-phase gradient HPLC methods and identified by their corresponding m/z ratio by ESI in the negative ionization mode. Aqueous and methanol mobile-phase mixtures, each containing 400 mM hexafluoroisopropanol (HFIP)-15 mM triethylamine (TEA), were employed to facilitate the interaction between the clusters and stationary phase via formation of ion-pairs. TEA-HFIP (triethylammonium-hexafluoroisopropoxide) had been shown to provide superior chromatographic peak shape and mass spectral signal compared with alternative modifiers such as TEAA (triethylammonium-acetate) for analysis of oligonucleotide samples. Liquid chromatographic separation prior to mass spectrometry detection facilitated sample analysis by production of simplified mass spectra for each eluting cluster species and provided insight into the existence of at least two major solution-phase isomers of Ag₂₉(LA)₁₂. UV-vis detection in-line with ESI analysis provided independent confirmation of the existence of the isomers and their similar electronic structure as judged from their identical optical spectra in the 300-500 nm range. [Diastereomerism provides a possible interpretation for the near-equal abundance of the two forms, based on a structurally defined nonaqueous homologue.].

Cancer-Targeting Ultrasmall Silica Nanoparticles for Clinical Translation: Physicochemical Structure and Biological Property Correlations

Although a large body of literature exists on the potential use of nanoparticles for medical applications, the number of probes translated into human clinical trials is remarkably small. A major challenge of particle probe development and their translation is the elucidation of safety profiles associated with their structural complexity, not only in terms of size distribution and heterogeneities in particle composition but also their effects on biological activities and the relationship between particle structure and pharmacokinetics. Here, we report on the synthesis, characterization, and long-term stability of ultrasmall (<10 nm diameter) dual-modality (optical and positron emission tomography) and integrintargeting silica nanoparticles (cRGDY-PEG-Cy5-C' dots and ¹²⁴I-(or ¹³¹I-) cRGDY-PEG-Cy5-C'dots) and the extent to which their surface ligand density differentially modulates key in vitro and in vivo biological activities in melanoma models over a range of ligand numbers (i.e., ~6-18). Gel permeation chromatography, established as an important particle characterization tool, revealed a two-year shelf life for cRGDY-PEG-Cy5-C' dots. Radiochromatography further demonstrated the necessary radiochemical stability for clinical applications. The results of subsequent ligand density-dependent studies elucidate strong modulations in biological response, including statistically significant increases in integrin-specific targeting and particle uptake, cellular migration and adhesion, renal clearance, and tumor-to-blood ratios with increasing ligand number. We anticipate that nanoprobe characteristics and a better understanding of the structure-function relationships determined in this study will help guide identification of other lead nanoparticle candidates for in vitro and in vivo biological assessments and product translation.

Quantitatively Monitoring the Size-Focusing of Au Nanoclusters and Revealing What Promotes the Size Transformation from Au44(TBBT)28 to Au36(TBBT)24

"Size-focusing" is a well-recognized process and widely employed for the synthesis of atomically monodisperse metal nanoclusters. However, quantitatively monitoring the size-focusing of Au nanoclusters has not been achieved yet, and the in-depth understanding of the size focusing is far from completed. Herein, we introduce a facile, cheap, and powerful tool, preparative thin-layer chromatography (PTLC), to quantitatively track the size-focusing process, to reveal that mainly ∼3 nm nanoparticles promote the transformation from Au₄₄(TBBT)₂₈ to Au₃₆(TBBT)₂₄ (where TBBT is 4-tert-butylbenzenethiolate) and to improve the syntheses of Au₄₄(TBBT)₂₈ and Au₃₆(TBBT)₂₄. Our work further demonstrates the usefulness of PTLC in nanocluster research and advances one step toward understanding the "size-focusing" process of nanoclusters.

Selection and Identification of Molecular Gold Clusters at the Nano(gram) Scale: Reversed Phase HPLC-ESI-MS of a Mixture of Au-Peth MPCs

Recent advances in cluster synthesis make it possible to produce an enormous variety molecule-like MPCs of size, composition, shape, and surface-chemical combinations. In contrast to the significant growth in the synthetic capability to generate these materials, progress in establishing the physicochemical basis for their observed properties has remained limited. The main reason for this has been the lack of the analytical capability to generate and measure samples of suitably high (molecular) purity; such capability is also essential to support therapeutic and diagnostic MPC development. In order for MPC products to get to market, especially those products that are medical-field related, characterization is required to identify and quantify all components present in a material mixture. Here, we show results from analysis of several synthetic mixtures of gold MPCs by nonaqueous reversed-phase chromatography coupled with mass spectrometry detection. The additional or hidden components, revealed to be present in these mixtures, provide novel insights into their comparative stability and interactions.

Electronic Processes within Quantum Dot-Molecule Complexes

The subject of this review is the colloidal quantum dot (QD) and specifically the interaction of the QD with proximate molecules. It covers various functions of these molecules, including (i) ligands for the QDs, coupled electronically or vibrationally to localized surface states or to the delocalized states of the QD core, (ii) energy or electron donors or acceptors for the QDs, and (iii) structural components of QD assemblies that dictate QD-QD or QD-molecule interactions. Research on interactions of ligands with colloidal QDs has revealed that ligands determine not only the excited state dynamics of the QD but also, in some cases, its ground state electronic structure. Specifically, the article discusses (i) measurement of the electronic structure of colloidal QDs and the influence of their surface chemistry, in particular, dipolar ligands and exciton-delocalizing ligands, on their electronic energies; (ii) the role of molecules in interfacial electron and energy transfer processes involving QDs, including electron-to-vibrational energy transfer and the use of the ligand shell of a QD as a semipermeable membrane that gates its redox activity; and (iii) a particular application of colloidal QDs, photoredox catalysis, which exploits the combination of the electronic structure of the QD core and the chemistry at its surface to use the energy of the QD excited state to drive chemical reactions.

Simultaneous analysis of nanoparticles and small molecules by high-performance liquid chromatography using a silica monolithic column

A high-performance liquid chromatography method using a commercially available silica monolithic column for the simultaneous analysis of nanoparticles and small molecules was developed. The method uses the micrometer-sized flow-through pores and nanometer-sized mesopores of the monolithic column for separation: first, size separation of nanoparticles was performed by the micrometer-sized pores using the hydrodynamic mode, and then small molecules were separated by the nanometer-sized pores using the normal-phase mode. The method was used to evaluate and compare three existing methods for purifying nanoparticles and to analyse nanoparticle stability. The bimodal structure of the monolithic column is promising for the simultaneous separation of nanoparticles and small molecules.

High-performance liquid chromatographic analysis of as-synthesised N,N'-dimethylformamide-stabilised gold nanoclusters product

Reverse-phase high-performance liquid chromatographic (RP-HPLC) separation and analysis of polydisperse water-soluble gold nanoclusters (AuNCs) stabilised with N,N'-dimethylformamide (DMF) were investigated. Under optimal elution gradient conditions, the separation of DMF-AuNCs was monitored by absorption and fluorescence spectroscopy. The UV-vis spectral characteristics of the separated DMF-AuNCs have been captured and they do not possess distinct surface plasmon resonance bands, indicating that all DMF-AuNCs are small AuNCs. The photoluminescence emission spectra of the separated DMF-AuNCs are in the blue-light region. Moreover, cationic DMF-AuNCs are for the first time identified by ion chromatography. Our proposed RP-HPLC methodology has been successfully applied to separate AuNCs of various Au atoms as well as DMF-stabilised ligands. Finally, the composition of the separated DMF-AuNCs was confirmed by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry and electrospray ionisation mass spectrometry, proving that the as-synthesised DMF-AuNCs product consists of Au₁₀⁺, Au₁₀, Au₁₁, Au₁₂, Au₁₃, and Au₁₄ NCs stabilised with various numbers of DMF ligands.

Quantum sized, thiolate-protected gold nanoclusters

The scientific study of gold nanoparticles (typically 1-100 nm) has spanned more than 150 years since Faraday's time and will apparently last longer. This review will focus on a special type of ultrasmall (<2 nm) yet robust gold nanoparticles that are protected by thiolates, so-called gold thiolate nanoclusters, denoted as Au(n)(SR)(m) (where, n and m represent the number of gold atoms and thiolate ligands, respectively). Despite the past fifteen years' intense work on Au(n)(SR)(m) nanoclusters, there is still a tremendous amount of science that is not yet understood, which is mainly hampered by the unavailability of atomically precise Au(n)(SR)(m) clusters and by their unknown structures. Nonetheless, recent research advances have opened an avenue to achieving the precise control of Au(n)(SR)(m) nanoclusters at the ultimate atomic level. The successful structural determination of Au(102)(SPhCOOH)(44) and [Au(25)(SCH(2)CH(2)Ph)(18)](q) (q = -1, 0) by X-ray crystallography has shed some light on the unique atomic packing structure adopted in these gold thiolate nanoclusters, and has also permitted a precise correlation of their structure with properties, including electronic, optical and magnetic properties. Some exciting research is anticipated to take place in the next few years and may stimulate a long-lasting and wider scientific and technological interest in this special type of Au nanoparticles.

Arylthiolate-protected silver quantum dots

This paper describes a new, organic-soluble 4-tert-butylbenzyl mercaptan (BBT) monolayer-protected silver cluster (AgBBT MPC) as the first example of a dissolved silver nanoparticle that exhibits quantized one-electron double layer charging (QDL) voltammetry. Polydisperse AgBBT MPCs made by two different synthetic protocols, but with similar average core diameters (2.1 nm), exhibit sharply differing electrochemistry and optical absorbance spectra. A two-phase procedure (organic/aqueous, termed Prep A-AgBBT) produced MPCs exhibiting a 475 nm surface plasmon absorbance and QDL voltammetry. Neither property was seen for MPCs made by a single-phase procedure, termed Prep B-AgBBT. The difference is thought to reflect poor passivation to oxide formation in the latter Prep B procedure, which is supported by X-ray photoelectron spectroscopy results. Thermogravimetry, mass spectra, and electrochemistry results suggest an average stoichiometric formula of Ag140BBT53, but transmission electron microscopy shows that the products are also polydisperse and include polycrystalline aggregates. Dry, cast films of both Ag MPC preparations on interdigitated array electrodes exhibit low electron hopping conductivity, compared to Au MPCs.

NIR Luminescence Intensities Increase Linearly with Proportion of Polar Thiolate Ligands in Protecting Monolayers of Au38and Au140Quantum Dots

The near-infrared photoluminescence of monolayer-protected Au38 and Au140 clusters (MPCs) is intensified with exchange of nonpolar ligands by more polar thiolate ligands. The effect is general and includes as more polar in-coming ligands: thiophenolates with a variety of p-substituents; alkanethiolates omega-terminated by alcohol, acid, or quaternary ammonium groups; and thio-amino acids. Remarkably, place exchanges of the initial phenylethanethiolates on Au38 MPCs by p-substituted thiophenolates and thio-amino acids and of hexanethiolates on Au140 MPCs by omega-quaternary ammonium terminated undecylthiolates result in increases in the near-infrared (NIR) luminescence intensities that are linear with the number of new polar ligands. The increased intensities are systematically larger for thiophenolate ligands having more electron-withdrawing substituents. Analogous effects on intensities are observed in the NIR emission of Au140 MPCs upon place exchange of alkanethiolates with thiolates having short connecting alkanethiolate chains to quaternary ammonium and to omega-carboxylic acid termini, and with oxidative charging of the Au cores. The observations are consistent with sensitivity of the luminescence mechanism to any factor that enhances the electronic polarization of the bonds between the Au core atoms and their thiolate ligands. The luminescence is discussed in terms of a surface electronic excitation, as opposed to a core volume excitation.

Sub-Nanometer Au Monolayer-Protected Clusters Exhibiting Molecule-like Electronic Behavior: Quantitative High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy and Electrochemical Characterization of Clusters with Precise Atomic Stoichiometry

The synthesis and characterization of the clusters Au13[PPh3]4[S(CH2)11CH3]2Cl2 (1) and Au13[PPh3]4[S(CH2)11CH3]4 (2) are described. These mixed-ligand, sub-nanometer clusters, prepared via exchange of dodecanethiol onto phosphine-halide gold clusters, show enhanced stability relative to the parent. The characterization of these clusters features the precise determination of the number of gold atoms in the cluster cores using high-angle annular dark-field scanning transmission electron microscopy, allowing the assignment of 13 gold atoms (+/-3 atoms) to the composition of both cluster molecules. Electrochemical and optical measurements reveal discrete molecular orbital levels and apparent energy gaps of 1.6-1.7 eV for the two cluster molecules. The electrochemical measurements further indicate that the Au13[PPh3]4[S(CH2)11CH3]2Cl2 cluster undergoes an overall two-electron reduction. The electrochemical and spectroscopic properties of the two Au13 cluster molecules are compared with those of a secondary synthetic product, which proved to be larger Au thiolate-derivatized monolayer-protected clusters with an average core of Au180. The latter shows behavior fully consistent with the adoption of metallic-like properties.

Chromatographic Isolation of “Missing” Au55 Clusters Protected by Alkanethiolates

We report on the first synthesis of alkanethiolate-protected Au55 (11 kDa), which has been a "missing" counterpart of Schmid's Au55(PR3)12Cl6. Au:SCx clusters (x = 12, 18) were prepared by the reaction of alkanethiol (CxSH) with polymer-stabilized Au clusters ( approximately 1.3 nm) and subsequently incubated in neat CxSH. The resulting clusters were successfully fractionated by recycling gel permeation chromatography into Au approximately 38:SCx and Au approximately 55:SCx and identified by laser-desorption ionization mass spectrometry. The Au approximately 55:SCx clusters exhibited structured optical spectra, suggesting molecular-like properties. The thiolate monolayers were found to be liquid-like on the basis of the IR spectrum and the monolayer thickness, which was estimated from the hydrodynamic diameter.

Mercaptoethane sulfonate protected, water-soluble gold and silver nanoparticles: Syntheses, characterization and their building multilayer films with polyaniline via ion–dipole interactions

Mercaptoethane sulfonate protected, water-soluble gold and silver nanoparticles (Au-MES and Ag-MES) are synthesized by one-phase method and characterized by TEM, TGA and XPS techniques, UV-vis and FTIR spectra. Both Au-MES and Ag-MES nanoparticles are soluble in the water up to 2.0 mg/ml and the stability of Au-MES is much better than that of Ag-MES. When dissolved in the water, they behave like a polyanion and can be used to build multilayer films with polyaniline (PANI) by way of layer-by-layer. A new approach is presented to fabricate the multilayer films of Au-MES/PANI and Ag-MES/PANI. The assembly mechanism of these multilayer films is also discussed. We anticipate highly conducting PANI films can be obtained by doping with these nanoparticles.

The use of nanoparticles in electroanalysis: a review

Nanoparticles can display four unique advantages over macroelectrodes when used for electroanalysis: enhancement of mass transport, catalysis, high effective surface area and control over electrode microenvironment. Therefore, much work has been carried out into their formation, characterisation and employment for the detection of many electroactive species. This paper aims to give an overview of the investigations carried out in this field. Particular attention is paid to examples of the advantages and disadvantages nanoparticles show when compared to macroelectrodes and the advantages of one nanoparticle modification over another. Most work has been carried out using gold, silver and platinum metals. However, iron, nickel and copper are also reviewed with some examples of other metals such as iridium, ruthenium, cobalt, chromium and palladium. Some bimetallic nanoparticle modifications are also mentioned because they can cause unique catalysis through the mixing of the properties of both metals.

Reaction of Au(55)(PPh(3))(12)Cl(6) with thiols yields thiolate monolayer protected Au(75) clusters

This paper describes the reaction of the phosphine-protected Au nanoparticle Au(55)(PPh(3))(12)Cl(6) (1, "Au55") with hexanethiol (2) and other thiols. The voltammetry of the reaction product 2 displays a well-defined pattern of peaks qualitatively reminiscent of Au(38) nanoparticles, but with quite different spacing (0.74 +/- 0.01 V) between the potentials of initial oxidation and reduction steps (electrochemical gap). Correction of this "molecule-like" gap for charging energy indicates a HOMO-LUMO gap energy of about 0.47 V. Voltammetry of the products (3 and 4) of reaction of 1 with C(3)H(7)SH and PhC(2)H(4)SH, respectively, is similar. Laser desorption/ionization mass spectrometry (LDI-MS) shows that 2 contains a high proportion of a core mass in the 14-15 kDa range, which is proposed to be Au(75). UV-vis spectra of 2-4 are relatively featureless, similar to previous reports of thiolate-protected Au(75) nanoparticles. HPLC analysis of 2 shows a Au(75) content of ca. 73%; the electrochemical purity estimate is also high, about 55%. Combining the mass spectrometric result with thermogravimetric analysis of 2 leads to a preliminary formulation Au(75)(SC(6)H(13))(40). This Au(75) synthesis complements a previous Brust-type synthesis and is unusual in the apparent provocation in the reaction of an increase in core size.

Preparative Manipulation of Gold Nanoparticles by Reversible Binding to a Polymeric Solid Support

A preparative scheme is presented for controlled modification of gold nanoparticles (NPs) by using reversible binding to a polymeric solid support through boronic acid chemistry. Octanethiol-capped Au NPs were bound to a boronic acid functionalized resin by custom-synthesized bifunctional linker molecules. The NPs were chemically released from the resin to the solution, with one (or a few) linker molecules embedded in their capping layer. This was confirmed by rebinding the linker-derivatized NPs to a boronic resin, exploiting the reversibility of the boronic acid/diol chemistry. The same scheme was employed to demonstrate a new method for affinity separation of NPs by means of a solid-phase reaction. The use of boronic acid provides versatility and chemical reversibility, while the polymeric solid support affords the separation and preparative aspects. The method presented here may be useful in various facets of NP handling, manipulation, and separation.

The Detection of Nitrate Using in-situ Copper Nanoparticle Deposition at a Boron Doped Diamond Electrode

Electrochemical deposition from a 0.1 M sodium sulphate solution, containing Cu2+ (adjusted to pH 3 with hydrochloric acid) produced a well defined copper nanoparticle deposit on the surface of a boron doped diamond electrode. Changing conditions such as potential (-0.8, -1.0 and -1.2 V), time (5, 2 and 0.5 s) and concentration of Cu2+ (500, 250 and 100 microM) was found to give copper nanoparticles of varying size and particle density. The electrocatalytic properties of the copper surface towards nitrate reduction were explored. An in-situ copper nanoparticle production method was developed for the detection of nitrate; this involves electrodeposition, followed by linear sweep voltammetry for the reduction of nitrate and then application of a stripping potential to renew the electrode surface. The linear sweep was discovered to have homogenised the size of the nanoparticles but their number density was still dependant on the initial conditions of deposition. Some particles were still present at the surface after the stripping potential had been applied but repetitions of the procedure showed these did not have an effect on subsequent deposits. Optimisation of the method lead to applying a deposition potential of -0.8 V, at a BDD electrode for 5 s in a 0.1 M sodium sulphate solution (pH 3) containing 100 microM Cu2+ followed by a linear sweep at 1 V/s; this yielded a limit of detection of 1.5 microM nitrate. The analytical applicability of the technique was evaluated for nitrate detection in a natural mineral water sample and was found to agree well with that stated by the manufacturer.