Interparticle interactions in glutathione mediated assembly of gold nanoparticles

The understanding of the detailed molecular interactions between (GSH) glutathione molecules in the assembly of metal nanoparticles is important for the exploitation of the biological reactivity. We report herein results of an investigation of the assembly of gold nanoparticles mediated by glutathione and the disassembly under controlled conditions. The interparticle interactions and reactivities were characterized by monitoring the evolution of the surface plasmon resonance band using the spectrophotometric method and the hydrodynamic sizes of the nanoparticle assemblies using the dynamic light scattering technique. The interparticle reactivity of glutathiones adsorbed on gold nanoparticles depends on the particle sizes and the ionic strength of the solution. Larger-sized particles were found to exhibit a higher degree of interparticle assembly than smaller-sized particles. The assembly-disassembly reversibility is shown to be highly dependent on pH and additives in the solution. The interactions of the negatively charged citrates surrounding the GSH monolayer on the particle surface were believed to produce more effective interparticle spatial and electrostatic isolation than the case of OH (-) groups surrounding the GSH monolayer. The results have provided new insights into the hydrogen-bonding character of the interparticle molecular interaction of glutathiones bound on gold nanoparticles. The fact that the interparticle hydrogen-bonding interactions in the assembly and disassembly processes can be finely tuned by pH and chemical means has implications to the exploitation of the glutathione-nanoparticle system in biological detection and biosensors.     

Homocysteine-mediated reactivity and assembly of gold nanoparticles

This paper reports the findings of an investigation of the reactivity and assembly of gold nanoparticles mediated by homocysteine (Hcys), a thiol-containing amino acid found in plasma. The aim is to gain insight into the interparticle interaction and reactivity, which has potential application for the detection of thiol-containing amino acids. By monitoring the evolution of the surface plasmon resonance absorption and the dynamic light scattering of gold nanoparticles in the presence of Hcys, the assembly was shown to be dependent on the nature and concentration of the electrolytes, reflecting an effective screening of the diffuse layer around the initial citrate-capped nanoparticles that decreases the barrier to the Hcys adsorption onto the surface, and around the subsequent Hcys-capped nanoparticles that facilitate the zwitterion-type electrostatic interactions between amino acid groups of Hcys bound to different nanoparticles. A key element of the finding is that the interparticle zwitterion interaction of the Hcys-Au system is much stronger than the expectation for a simple Hcys or Au solution, a new phenomenon originating from the unique nanoscale interparticle interaction. The strength and reversibility of the interparticle zwitterion-type electrostatic interactions between amino acid groups are evidenced by the slow disassembly upon increasing pH at ambient temperatures and its acceleration at elevated temperature. These findings provide new insight into the precise control of interfacial interactions and reactivities between amino acids anchored to nanoparticles and have broad implications in the development of colorimetric nanoprobes for amino acids.     

Assembly of gold nanoparticles mediated by multifunctional fullerenes

The understanding of the interparticle interactions of nanocomposite structures assembled using molecularly capped metal nanoparticles and macromolecular mediators as building blocks is essential for exploring the fine-tunable interparticle spatial and macromolecular properties. This paper reports the results of an investigation of the chemically tunable multifunctional interactions between fullerenes (1-(4-methyl)-piperazinyl fullerene, MPF) and gold nanoparticles. The interparticle spatial properties are defined by the macromolecular and multifunctional electrostatic interactions between the negatively charged nanoparticles and the positively charged fullerenes. In addition to characterization of the morphological properties, the surface plasmon resonance band, dynamic light scattering, and surface-enhanced Raman scattering (SERS) properties of the MPF-mediated assembly and disassembly processes have been determined. The change of the optical properties depends on the pH and electrolyte concentrations. The detection of the Raman-active vibration modes (Ag(2) and Hg(8)) of C60 and the determination of their particle size dependence have demonstrated that the adsorption of MPF on the nanoparticle surface in the MPF-Au nm assembly is responsible for the SERS effect. These findings provide new insights into the delineation between the interparticle interactions and the nanostructural properties for potential applications of the nanocomposite materials in spectroscopic and optical sensors and in controlled releases.     

Construction of simple gold nanoparticle aggregates with controlled plasmon–plasmon interactions

We have developed a colloidal assembly for the study of plasmon–plasmon interactions between gold nanoparticles. Colloidal aggregates of controlled size and interparticle spacing were synthesized on silica nanoparticle substrates. Following the immobilization of isolated gold nanoparticles onto silica nanoparticles, the surfaces of the adsorbed gold nanoparticles were functionalized with 4-aminobenzenethiol. This molecular linker attached additional gold nanoparticles to the ‘parent' gold nanoparticle, forming small nanoparticle aggregates. The optical absorption spectrum of these clusters differed from that of gold colloid in a manner consistent with plasmon–plasmon interactions between the gold nanoparticles.     

Coupling between molecular and plasmonic resonances in freestanding dye-gold nanorod hybrid nanostructures

Hybrid nanostructures of organic dyes and inorganic gold nanorods are constructed using the layer-by-layer assembly method via electrostatic interactions. Strong coupling is observed between the molecular resonance of dyes and the plasmonic resonance of gold nanorods when their spectra overlap strongly. The coupling strength is tuned by choosing gold nanorods with longitudinal plasmon wavelengths varying from 570 to 870 nm. The resonance coupling-induced plasmon shift is found to be strongly dependent on the dye concentration and the spacing between the dye and nanorod. Moreover, the resonance coupling can be switched off by laser illumination to decompose adsorbed dyes. We believe this is the first time that the coupling between molecular and plasmonic resonances is observed for freestanding hybrid nanostructures constructed out of dyes and colloidal gold nanorods. These results will be helpful in understanding the fundamental interactions between molecular and plasmonic resonances and useful for the design of resonance coupling-based chemical and biological sensors.     

Monodispersed core-shell Fe3O4@Au nanoparticles

The ability to synthesize and assemble monodispersed core-shell nanoparticles is important for exploring the unique properties of nanoscale core, shell, or their combinations in technological applications. This paper describes findings of an investigation of the synthesis and assembly of core (Fe(3)O(4))-shell (Au) nanoparticles with high monodispersity. Fe(3)O(4) nanoparticles of selected sizes were used as seeding materials for the reduction of gold precursors to produce gold-coated Fe(3)O(4) nanoparticles (Fe(3)O(4)@Au). Experimental data from both physical and chemical determinations of the changes in particle size, surface plasmon resonance optical band, core-shell composition, surface reactivity, and magnetic properties have confirmed the formation of the core-shell nanostructure. The interfacial reactivity of a combination of ligand-exchanging and interparticle cross-linking was exploited for molecularly mediated thin film assembly of the core-shell nanoparticles. The SQUID data reveal a decrease in magnetization and blocking temperature and an increase in coercivity for Fe(3)O(4)@Au, reflecting the decreased coupling of the magnetic moments as a result of the increased interparticle spacing by both gold and capping shells. Implications of the findings to the design of interfacial reactivities via core-shell nanocomposites for magnetic, catalytic, and biological applications are also briefly discussed.     

Collective and individual plasmon resonances in nanoparticle films obtained by spin-assisted layer-by-layer assembly

Nanoscale uniform films containing gold nanoparticle and polyelectrolyte multilayer structures were fabricated by the using spin-assembly or spin-assisted layer-by-layer (SA-LbL) deposition technique. These SA-LbL films with a general formula [Au/(PAH-PSS)nPAH]m possessed a well-organized microstructure with uniform surface morphology and high surface quality at a large scale (tens of micrometers across). Plasmon resonance peaks from isolated nanoparticles and interparticle interactions were revealed in the UV-visible extinction spectra of the SA-LbL films. All films showed the strong extinction peak in the region of 510-550 nm, which is due to the plasmon resonance of the individual gold nanoparticles redshifted because of a local dielectric environment. For films with sufficient density of gold nanoparticles within the layers, the second strong peak was consistently observed between 620 and 660 nm, which is the collective plasmon resonance from intralayer interparticle coupling. Finally, we suggested that, for certain film designs, interlayer interparticle resonance might be revealed as an independent contribution at 800 nm in UV-visible spectra. The observation of independent and concurrent individual, intralayer, and interlayer plasmon resonances can be critical for sensing applications, which involve monitoring of optomechanical properties of ultrathin optically active compliant membranes.     

Comment on "Multimodal coupling of optical transitions and plasmonic oscillations in rhodamine B modified gold nanoparticles" by M. Stobiecka and M. Hepel, Phys. Chem. Chem. Phys. 2011, 13, 1131

An article recently published in this journal claimed that a resonance enhanced light scattering (RELS) peak for 22 nm gold nanoparticles was observed at 653 nm, which was about 130 nm higher than the surface plasmon resonance maximum. They also claimed to observe RELS from dilute solutions of Rhodamine B, under conditions where it is expected to be in its monomeric form. This comment shows that the position of the RELS peak for the gold nanoparticles is an artefact of measurement and the RELS from Rhodamine B is simply fluorescence. These findings are likely to have a significant impact on the interpretation of the results in terms of interactions of dyes with gold nanoparticles as well as aggregation of gold nanoparticles, which has been reported elsewhere by the same authors.     

Reversible association of thermoresponsive gold nanoparticles: polyelectrolyte effect on the lower critical solution temperature of poly(vinyl methyl ether)

Thermoresponsive gold nanoparticles (GNPs) have been prepared by the borohydride reduction of gold salt in the presence of water-soluble polymer, poly(vinyl methyl ether) (PVME). The PVME-coated GNPs (PVME-GNPs) have been assembled into large aggregates in the presence of polyelectrolytes, viz., poly(sodium-4 styrene sulfonate) and sodium salt of carboxymethylcellulose at low pH by raising the solution temperature from 20 to 40 degrees C. Increase of temperature triggers the interparticle association due to hydrophobic interaction of pendent methyl group of the surface adsorbed PVME. This assembly process is reversible with respect to temperature and pH of the medium and was studied by monitoring the change in surface plasmon resonance band of PVME-GNPs. Three-dimensional assemblies of various architectures, depending on the concentration of polyelectrolytes, were observed through transmission electron microscopy. A mechanistic model has been suggested for the reversible assembly formation that suits well with the experimental observations. The changes in optical properties of the PVME-GNPs due to their aggregation/disaggregation enabled us to use it as an effective tool to monitor the change in lower critical solution temperature (LCST) of PVME in the presence of polyelectrolytes due to interpolymer complexation at low pH. This result agrees well with the variation of LCST of pure aqueous PVME solution in the presence of polyelectrolytes measured by conventional turbidimetric technique.     

Kinetic and thermodynamic assessments of the mediator-template assembly of nanoparticles

The understanding of kinetic and thermodynamic factors governing the assembly of nanoparticles is important for the design and control of functional nanostructures. This paper describes a study of the kinetic and thermodynamic factors governing the mediator-template assembly of gold nanoparticles into spherical assemblies in solutions. The study is based on spectrophotometric measurements of the surface plasmon (SP) resonance optical property. Gold nanoparticle cores ( approximately 5 nm) encapsulated with tetraoctylammonium bromide shells were studied as a model system. The mediator-template assembly involves a thioether-based multidentate ligand (e.g., MeSi(CH2SMe)3) which functions as a mediator, whereas the tetraoctylammonium bromide capping molecules function as template agents. On the basis of the temperature dependence of the SP optical property in the mediator-template assembly process, the kinetic and thermodynamic parameters such as the reaction rate constant and reaction enthalpy have been determined. The results led to two important findings. First, the mediator-template assembly of nanoparticles is an enthalpy-driven process. Second, the enthalpy change (-1.3 kcal/mol) is close to the magnitude of the van der Waals interaction energy for alkyl chains and the condensation energy of hydrocarbons. Implications of the findings to the understanding of the interparticle interactions have also been discussed.     

Fluorescence of dyes adsorbed on highly organized,nanostructured gold surfaces

It is shown that fluorescent dyes can be adsorbed selectively on gold nanoparticles which are immobilized on a glass substrate and that the fluorescence originating from the adsorbed dyes exhibits significantly less quenching when compared to dyes adsorbed on bulk gold. Self-assembled monolayers of lissamine sulfide molecules have been studied both on bulk gold and on glass surfaces bearing gold nanoparticles. Gold nanoparticles have been arranged in ordered, two-dimensional patterns, with periodicity in the microm range and used as substrate for the fluorescent dyes. Optical resolution of the fluorescence originating from the pattern has been achieved with laser-scanning confocal microscopy.     

Electromagnetic interactions in plasmonic nanoparticle arrays

Single two-dimensional planar silver arrays and one-dimensional linear gold chains of nanoparticles were investigated by dark-field surface plasmon spectroscopy and studied as a function of interparticle distance, particle size, and number of particles. In agreement with recent theoretical predictions, a red shift of the surface plasmon resonance occurring in two-dimensional arrays was found for lattice spacings below 200 nm. This red shift is associated with a significant broadening of the resonance and is attributed to the onset of near-field interactions. We found that the relative contributions of the long-range and short-range interactions in two-dimensional arrays of particles are fundamentally different to those occurring in individual linear chains.     

Gold nanoparticle networks with photoresponsive interparticle spacings

Photoresponsive gold nanoparticle networks were prepared by functionalizing them with azobenzene derivatives. A network can be formed when a linker molecule constituting the azobenzene moiety suitably derivatized on either side with gold surface sensitive groups such as thiols and amines is added to the nanoparticle solution. It is shown that the interparticle spacing in the networks could be controlled by the reversible trans-cis isomerization of the azobenzene moiety induced by UV and visible light, respectively. The photoinduced variation in the interparticle spacings is inferred by the changes in the optical spectra of the gold nanoparticles which display a red or blue shift in the surface plasmon resonance peak depending on a decrease or increase in the interparticle spacing, respectively. Transmission electron microscopy images are in consonance with the evidence from the optical spectra.     

Interfacial phase transition of an environmentally responsive elastin biopolymer adsorbed on functionalized gold nanoparticles studied by colloidal surface plasmon resonance

The change in optical properties of colloidal gold upon aggregation has been used to develop an experimentally convenient colorimetric method to study the interfacial phase transition of an elastin-like polypeptide (ELP), a thermally responsive biopolymer. Gold nanoparticles, functionalized with a self-assembled monolayer (SAM) of mercaptoundecanoic acid onto which an ELP was adsorbed, exhibit a characteristic red color due to the surface plasmon resonance (SPR) of individual colloids. Raising the solution temperature from 10 degrees C to 40 degrees C thermally triggered the hydrophilic-to-hydrophobic phase transition of the adsorbed ELP resulting in formation of large aggregates due to interparticle hydrophobic interaction. Formation of large aggregates caused a change in color of the colloidal suspension from red to violet due to coupling of surface plasmons in aggregated colloids. The surface phase transition of the ELP was reversible, as seen from the reversible change in color upon cooling the suspension to 10 degrees C. The formation of colloidal aggregates due to the interfacial phase transition of adsorbed ELP was independently verified by dynamic light scattering of ELP-modified gold colloids as a function of temperature. Colloidal SPR provides a simple and convenient colorimetric method to study the influence of the solution environment, interfacial properties, and grafting method on the transition properties of ELPs and other environmentally responsive polymers at the solid-water interface.     

Multivalent host-guest interactions between beta-cyclodextrin self-assembled monolayers and poly(isobutene-alt-maleic acid)s modified with hydrophobic guest moieties

Poly(isobutene-alt-maleic acid)s modified with p-tert-butylphenyl or adamantyl groups interact with beta-cyclodextrin self-assembled monolayers (beta-CD SAMs) by inclusion of the hydrophobic substituents in the beta-cyclodextrin cavities. The adsorption was shown to be strong, specific, and irreversible. Even with a monovalent competitor in solution, adsorption to the beta-CD SAMs was observed, and desorption proved impossible. The adsorbed polymer layer was very thin as evidenced by surface plasmon resonance spectroscopy and AFM. Apparently, all or most hydrophobic groups of the polymers were employed efficiently in multivalent binding, as was further supported by the absence of specific binding of beta-CD-modified gold nanoparticles to the polymer surface assemblies. Supramolecular microcontact printing of the polymers onto the beta-CD SAMs led to assembly formation in the targeted areas of the substrates.     

Luminescence Resonance Energy Transfer Sensors Based on the Assemblies of Oppositely Charged Lanthanide/Gold Nanoparticles in Aqueous Solution

This work demonstrates luminescence resonance energy transfer (LRET) sensors based on lanthanide‐doped nanoparticles as donors (D) and gold nanoparticles as acceptors (A), combined through electrostatic interactions between the oppositely charged nanoparticles. Negatively charged lanthanide‐doped nanoparticles, YVO₄:Eu and LaPO₄:Ce,Tb, with high luminescence quantum yield and good water‐solubility, are synthesized through a polymer‐assisted hydrothermal method. Positively charged polyhedral and spherical gold nanoparticles exhibit surface plasmon resonance (SPR) bands centered at 623 and 535 nm, respectively. These bands overlap well with the emission of the Eu³⁺ and Tb³⁺ ions within the lanthanide nanoparticles. Herein, the gold nanoparticles are synthesized through a seed‐mediated cetyltrimethylammonium bromide (CTAB)‐assisted method. The assemblies of the oppositely charged donors and acceptors are developed into LRET‐based sensors exhibiting a donor quenching efficiency close to 100 %.     

Probing Distance‐Dependent Plasmon‐Enhanced Near‐Infrared Fluorescence Using Polyelectrolyte Multilayers as Dielectric Spacers

Owing to their applications in biodetection and molecular bioimaging, near‐infrared (NIR) fluorescent dyes are being extensively investigated. Most of the existing NIR dyes exhibit poor quantum yield, which hinders their translation to preclinical and clinical settings. Plasmonic nanostructures are known to act as tiny antennae for efficiently focusing the electromagnetic field into nanoscale volumes. The fluorescence emission from NIR dyes can be enhanced by more than thousand times by precisely placing them in proximity to gold nanorods. We have employed polyelectrolyte multilayers fabricated using layer‐by‐layer assembly as dielectric spacers for precisely tuning the distance between gold nanorods and NIR dyes. The aspect ratio of the gold nanorods was tuned to match the longitudinal localized surface plasmon resonance wavelength with the absorption maximum of the NIR dye to maximize the plasmonically enhanced fluorescence. The design criteria derived from this study lays the groundwork for ultrabright fluorescence bullets for in vitro and in vivo molecular bioimaging.     

Spatially controlled SERS patterning using photoinduced disassembly of gelated gold nanoparticle aggregates

Controlling the assembly of the nanoparticles is important because the optical properties of noble metal nanoparticles, such as the surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS), are critically dependent on interparticle distances. Among many approaches available, light-induced disassembly is particularly attractive because it enables spatial modification of the optical properties of nanoparticle assemblies. In this study, we prepare gold nanoparticle (AuNP) aggregates in a gel matrix. Irradiation of the gelated AuNP aggregates at 532 nm leads to the disassembly of the aggregates, changing the color (SPR) from dark blue to red and extinguishing the SERS signal along the irradiated pattern, which opens the possibility of facile fabrication of spatially controlled SERS-generating microstructures. The photoinduced disassembly of the AuNP aggregates in solution is also investigated using UV-vis spectroscopy and transmission electron microscopy.     

Two-dimensional self-organization of polystyrene-capped gold nanoparticles

Thiol end-functionalized polystyrene chains have been introduced onto the surface of gold nanoparticles via a two-step grafting-to method. This simple grafting procedure is demonstrated to be efficient for gold nanoparticles of different sizes and for particles initially dispersed in either aqueous or organic media. The method has been applied successfully for a relatively large range of polystyrene chain lengths. Grafting densities, as determined by thermogravimetric analysis, are found to decrease with increasing chain length. In all cases, the grafting density indicates a dense brush conformation for the tethered chains. The resulting functionalized nanoparticles self-organize into hexagonally ordered monolayers when cast onto solid substrates from chloroform solution. Furthermore, the distance between the gold cores in the dried monolayer is controlled by the molecular weight of the grafted polystyrene. Optical absorption spectra recorded for the organized monolayers show the characteristic plasmon absorption of the gold particles. Importantly, the plasmon resonance frequency exhibits a distinct dependence on interparticle separation that can be attributed to plasmon coupling between neighboring gold cores.     

Supramolecular layer-by-layer assembly: alternating adsorptions of guest- and host-functionalized molecules and particles using multivalent supramolecular interactions

The stepwise construction of a novel kind of self-assembled organic/inorganic multilayers based on multivalent supramolecular interactions between guest-functionalized dendrimers and host-modified gold nanoparticles has been developed, yielding supramolecular layer-by-layer assembly. The deposition process was monitored by surface plasmon resonance spectroscopy. Further characterization of the multilayer films was performed by means of UV/vis absorption spectroscopy, which showed a linear increase in absorption with the number of bilayers. The growth of the gold nanoparticle plasmon absorption band corresponded to approximately a dense monolayer of gold nanoparticles per bilayer. Ellipsometry and atomic force microscopy (AFM) scratching experiments were used to measure the development of the film thickness with the number of bilayers, confirming linear growth and a thickness increase of approximately 2 nm/bilayer.