pH-Responsive Coassembly of Oligo(ethylene glycol)-Coated Gold Nanoparticles with External Anionic Polymers via Hydrogen Bonding

Fair surface modification with mixed alkanethiols on gold nanoparticles through minimal unfair ligand exchange

Surface modification with functional molecules is essential for introducing various surface properties. As gold nanoparticles (AuNPs) have extraordinary chemical, physical, and optical properties, control of their surface, mainly through modification with mixed alkanethiols via Au-S interactions, has attracted much attention. However, surface modification of AuNPs with mixed alkanethiols to provide a strictly regulated composition remains challenging. Further, there are very few methods that can easily establish the nature of ligands and their replacement with similar molecules at nanoparticle surfaces, limiting precise analyses. Herein, we demonstrate an unfair ligand exchange between oligo(ethylene glycol) (OEG)-attached alkanethiols as a source of unfair surface modification utilizing programable thermo-responsive properties of OEG-alkanethiols-modified AuNPs and fair surface modification with mixed OEG-alkanethiols by minimizing this effect. OEG-alkanethiols-modified AuNPs show an assembly/disassembly behavior in response to the solution temperature. Assembly temperature (T A) changes in the presence of other OEG-alkanethiols, confirming the ligand exchange between alkanethiols in an aqueous solution. Kinetic analyses indicate that the competitive exchange reaction of these two alkanethiols results in an unfair ligand exchange, which leads to gradual changes in surface composition. As this ligand exchange between alkanethiols takes a longer time compared to that from citric acid, which initially covered the AuNPs, exact surface modification of AuNPs with OEG-alkanethiols is performed by moderate reaction conditions (25 °C, several to 24 hours). This insight regarding "more prolonged reaction is not always better" could be widely applied for surface modifications with various thiol-ligands.

Structural Determinants of Stimuli-Responsiveness in Amphiphilic Macromolecular Nano-assemblies

Stimuli-responsive nano-assemblies from amphiphilic macromolecules could undergo controlled structural transformations and generate diverse macroscopic phenomenon under stimuli. Due to the controllable responsiveness, they have been applied for broad material and biomedical applications, such as biologics delivery, sensing, imaging, and catalysis. Understanding the mechanisms of the assembly-disassembly processes and structural determinants behind the responsive properties is fundamentally important for designing the next generation of nano-assemblies with programmable responsiveness. In this review, we focus on structural determinants of assemblies from amphiphilic macromolecules and their macromolecular level alterations under stimuli, such as the disruption of hydrophilic-lipophilic balance (HLB), depolymerization, decrosslinking, and changes of molecular packing in assemblies, which eventually lead to a series of macroscopic phenomenon for practical purposes. Applications of stimuli-responsive nano-assemblies in delivery, sensing and imaging were also summarized based on their structural features. We expect this review could provide readers an overview of the structural considerations in the design and applications of nanoassemblies and incentivize more explorations in stimuli-responsive soft matters.

Hysteresis in the Thermo-Responsive Assembly of Hexa(ethylene glycol) Derivative-Modified Gold Nanodiscs as an Effect of Shape

Anisotropic gold nanodiscs (AuNDs) possess unique properties, such as large flat surfaces and dipolar plasmon modes, which are ideal constituents for the fabrication of plasmonic assemblies for novel and emergent functions. In this report, we present the thermo-responsive assembly and thermo-dynamic behavior of AuNDs functionalized with methyl-hexa(ethylene glycol) undecane-thiol as a thermo-responsive ligand. Upon heating, the temperature stimulus caused a blue shift of the plasmon peak to form a face-to-face assembly of AuNDs due to the strong hydrophobic and van der Waals interactions between their large flat surfaces. Importantly, AuNDs allowed for the incorporation of the carboxylic acid-terminated ligand while maintaining their thermo-responsive assembly ability. With regard to their reversible assembly/disassembly behavior in the thermal cycling process, significant rate-independent hysteresis, which is related to their thermo-dynamics, was observed and was shown to be dependent on the carboxylic acid content of the surface ligands. As AuNDs have not only unique plasmonic properties but also high potential for attachment due to the fact of their flat surfaces, this study paves the way for the exploitation of AuNDs in the development of novel functional materials with a wide range of applications.

Controlled Assembly of Plasmonic Nanoparticles: From Static to Dynamic Nanostructures

The spatial arrangement of plasmonic nanoparticles can dramatically affect their interaction with electromagnetic waves, which offers an effective approach to systematically control their optical properties and manifest new phenomena. To this end, significant efforts were made to develop methodologies by which the assembly structure of metal nanoparticles can be controlled with high precision. Herein, recent advances in bottom-up chemical strategies toward the well-controlled assembly of plasmonic nanoparticles, including multicomponent and multifunctional systems are reviewed. Further, it is discussed how the progress in this area has paved the way toward the construction of smart dynamic nanostructures capable of on-demand, reversible structural changes that alter their properties in a predictable and reproducible manner. Finally, this review provides insight into the challenges, future directions, and perspectives in the field of controlled plasmonic assemblies.

Morphology control in crystalline nanoparticle-polymer aggregates

Self‐assembly of nanoparticles can be mediated by polymers, but has so far led almost exclusively to nanoparticle aggregates that are amorphous. Here, we employed Coulombic interactions to generate a range of composite materials from mixtures of charged nanoparticles and oppositely charged polymers. The assembly behavior of these nanoparticle/polymer composites depends on their order of addition: polymers added to nanoparticles give rise to stable aggregates, but nanoparticles added to polymers disassemble the initially formed aggregates. The amorphous aggregates were transformed into crystalline ones by transiently increasing the ionic strength of the solution. The morphology of the resulting crystals depended on the length of the polymer: short polymer chains mediated the self‐assembly of nanoparticles into strongly faceted crystals, whereas long chains led to pseudospherical nanoparticle/polymer assemblies, within which the crystalline order of nanoparticles was retained.

Molecular configuration-mediated thermo-responsiveness in oligo(ethylene glycol) derivatives attached on gold nanoparticles

Biomolecular systems actively control their local environment on a sub-nm scale via changes in molecular configuration from their flexible structures and derive emergent functions. Although this functional emergence based on local environmental control is attracting a great deal of attention in chemistry, it remains challenging to realize this artificially. Herein, we report the tuning of the thermo-responsive properties of oligo(ethylene glycol) (OEG) derivatives attached on gold nanoparticles via local environmental control not only by the hydrophobic moiety at their terminus but also by their molecular configuration. OEG-attached alkane thiol-modified AuNPs showed thermo-responsive assembly/disassembly in water through the hydration/dehydration of the OEG portions in a manner dependent both on the hydrophobicity at their terminus and the surface curvature of the core nanoparticles. Further, the assembly temperature (T _A) was also tuned by ligand mixing with a non-thermo-responsive ligand with a shorter OEG length. Molecular dynamics simulations show that the distribution of the hydrophobic terminus in the normal direction along the gold surface varied in accordance with the surface curvature, indicating variations in molecular configuration. It is expected that a bent configuration could accelerate the thermo-responsiveness of OEG by allowing them greater accessibility to the hydrophobic terminus. Experimental and simulation results support the notion that local OEG density tuning by surface curvature or ligand mixing with a different OEG length leads to different degrees of accessibility to the hydrophobic terminus via changes in molecular configuration, promoting local environmental control-directed assembly temperature tuning.

Aptamer-Functionalized Micro- and Nanocarriers for Controlled Release

Sequence-specific nucleic acids recognizing low-molecular-weight ligands or macromolecules (aptamers) have found growing interest for biomedical applications. The present review article summarizes recent applications of aptamers as stimuli-responsive gating units of drug (or dye)-loaded nano- or microcarriers for controlled and targeted drug release. In the presence of cellular biomarkers, the nano-/microcarriers are unlocked by forming aptamer-ligand complexes. Different aptamer-functinalized nano-/microcarriers are presented, including inorganic nanomaterials, metal-organic framework nanoparticles, and soft materials. The chemistries associated with the preparation of the carriers and the mechanisms to unlock the carriers are discussed. Stimuli-responsive gated drug-loaded micro-/nanocarriers hold great promise as functional sense-and-treat materials for the targeted and selective release of drugs.

Stimuli-Responsive Plasmonic Assemblies and Their Biomedical Applications

Among the diverse development of stimuli-responsive assemblies, plasmonic nanoparticle (NP) assemblies functionalized with responsive molecules are of a major interest. In this review, we outline a comprehensive and up-to-date overview of recently reported studies on in vitro and in vivo assembly/disassembly and biomedical applications of plasmonic NPs, wherein stimuli such as enzymes, light, pH, redox potential, temperature, metal ions, magnetic or electric field, and/or multi-stimuli were involved. Stimuli-responsive assemblies have been applied in various biomedical fields including biosensors, surfaced-enhanced Raman scattering (SERS), photoacoustic (PA) imaging, multimodal imaging, photo-activated therapy, enhanced X-ray therapy, drug release, stimuli-responsive aggregation-induced cancer therapy, and so on. The perspectives on the use of stimuli-responsive plasmonic assemblies are discussed by addressing future scientific challenges involving assembly/disassembly strategies and applications.

Tuning the Photothermal Effect of Carboxylated-Coated Silver Nanoparticles through pH-Induced Reversible Aggregation

The photothermal response of mercaptoundecanoic acid (MUA)-coated Ag nanoparticles (Ag@MUA NPs) in both aqueous dispersions and paper substrates was determined as a function of pH when irradiated with a green laser or a blue LED source. Aqueous dispersions of Ag@MUA NPs showed an aggregation behavior by acidification that was used for the formation of NPs clusters of variable sizes. Aggregation was induced by changing the pH across the apparent pK_a of the acid, higher than the pK_a of the free acid. Formation of these aggregates was completely reversible allowing the return to the well-dispersed initial state by simply increasing the pH by the addition of a base. Aggregation produced a shift of the plasmon band that changed the spectra of the dispersions and their ability to be remotely heated when irradiated with visible light. These aggregates could be transferred to paper by simple impregnation of the substrates with the dispersion. On the solid substrate, a higher photothermal response than in the liquid medium was observed. A high local increase of up to 75 °C could be recorded on paper after only 30 s of irradiation with a green laser, whereas a blue LED array was enough for inducing the melting of a solid paraffin (T_m = 36-38 °C) deposited on it. This work demonstrates that photothermal heating can be controlled by the reversible aggregation of NPs to induce different thermal responses in liquid and solid media.

Effect of Light Intensity on the Aggregation Behavior of Primary Particles during In Situ Photochemical Synthesis of Gold/Polymer Nanocomposites

Metal/polymer nanocomposites have attracted much attention in recent years due to their exceptional properties and wide range of potential applications. A key challenge to obtain these materials is to stabilize the metal nanoparticles in the matrix, avoiding uncontrolled aggregation processes driven by the high surface free energy of nanosized particles. Here, we investigate the aggregation mechanism of primary particles in gold-epoxy nanocomposites prepared via light-assisted in situ synthesis, under different irradiation conditions. The growth and aggregation of gold nanoparticles were monitored in situ by time-resolved small-angle X-ray scattering experiments, whereas spectroscopic measurements were performed to interpret how matrix polymerization influences the aggregation process. It was found that light intensity has a greater influence on the reduction rate than on the polymerization rate. Under irradiation, gold nanostructures evolve through five time-defined stages: nuclei-mass fractals-surface fractals-spherical nanoparticles-aggregates. If the maximum in the polymerization rate is reached before the aggregation step, individual primary nanoparticles will be preserved in the polymer matrix due to diffusional constraints imposed by the reaction medium. Because the light intensity has a different influence on the reduction rate than on the polymerization rate, this parameter can be used as a versatile tool to avoid aggregation of gold nanoparticles into the polymer matrix.

Reversible changes in the orientation of gold nanorod arrays on polymer brushes

Nanoparticles exhibit a number of unique properties such as localized surface plasmon resonance (LSPR). As this LSPR is sensitive to geometrical or spatial conditions, the arrangement of nanoparticles, in particular the active arrangement of plasmonic structures, is an important issue. In this study, gold nanorod (GNR) arrays were prepared by GNR attachment on anionic polymer (DNA) brushes via electrostatic interactions and their stimuli-responsive changes in orientation were investigated. As a result, the orientation of GNR arrays on DNA brushes reversibly changed by the modulation of electrostatic interactions between GNRs and polymers via changes in the solution pH. As these extensive GNR arrays are prepared via easy bottom-up processes, GNR surface properties are easily tuned by simple modification, and DNAs could be replaced with various synthetic polymers, we believe that this study will lead to the development of next-generation materials and devices with actively tunable structures.

Surface Coating Structure and Its Interaction with Cytochrome c in EG6-Coated Nanoparticles Varies with Surface Curvature

The composition, orientation, and conformation of proteins in biomolecular coronas acquired by nanoparticles in biological media contribute to how they are identified by a cell. While numerous studies have investigated protein composition in biomolecular coronas, relatively little detail is known about how the nanoparticle surface influences the orientation and conformation of the proteins associated with them. We previously showed that the peripheral membrane protein cytochrome c adopts preferred poses relative to negatively charged 3-mercaptopropionic acid (MPA)-gold nanoparticles (AuNPs). Here, we employ molecular dynamics simulations and complementary experiments to establish that cytochrome c also assumes preferred poses upon association with nanoparticles functionalized with an uncharged ligand, specifically ω-(1-mercaptounde-11-cyl)hexa(ethylene glycol) (EG₆). We find that the display of the EG₆ ligands is sensitive to the curvature of the surface-and, consequently, the effective diameter of the nearly spherical nanoparticle core-which in turn affects the preferred poses of cytochrome c.

Superlattice assembly by interpolymer complexation

We present a coarse grained model for a system where nanocrystals are functionalized with a polymer that is a hydrogen bond acceptor, such as polyethylene glycol (PEG), and are dispersed in a solution with a polymer whose monomers consist of a hydrogen bond donor, such as polyacrylic acid (PAA) at low pH (interpolymer complexation). We determine the minimum concentration of the polymer donor to induce aggregation and the structure and dynamics of the induced (fcc) superlattice. Our results are compared to previous and new experiments.

Nanoscale uniformity in the active tuning of a plasmonic array by polymer gel volume change

Active plasmonic tuning is an attractive but challenging research subject, leading to various promising applications. As one of the approaches, nanostructures are placed in or on soft matter, such as elastomers and gels, and their gap distances are tuned by the mechanical extension or volume change of the supporting matrices. As hydrogels possess various types of stimuli-responsiveness with large volume change and biocompatibility, they are good candidates as supporting materials for active nanostructure tuning. However, it remains unclear how accurately we can control their nanogap distance changes using polymer gels with a low deviation due to major difficulties in the precise observation of nanostructures on the gels. Here, we prepared gold arrays with sub-100 nm dots on silicon substrates by electron beam lithography and transferred them onto the hydrogel surface. Then, their nanopattern was actively tuned by the changes in gel size in water and their structural changes were confirmed by optical microscopy, microspectroscopy, and atomic force microscopy (AFM). Further, we successfully prepared ionic liquid (IL) gels with various degrees of swelling via solvent exchange. Scanning electron microscopy (SEM) observation of the IL gels provided clear pictures at nanoscale resolution. Finally, we calculated the plasmonic spectra using a finite difference time domain (FDTD) simulation based on the SEM images and compared them with the measured spectra. The results in this study totally support the notion that active changes in plasmonic nanodot patterns via volume changes in the hydrogel are quite homogenous on a several nanometer scale, making them ideal for precise active surface plasmon tuning.

Size-Defined Cracked Vesicle Formation via Self-Assembly of Gold Nanoparticles Covered with Carboxylic Acid-Terminated Surface Ligands

The self-assembly of gold nanoparticles (GNPs) into a defined structure, particularly hollow capsule structures, provides great potential for applications in materials science and medicine. However, the complexity of the parameters for the preparation of those structures through self-assembly has limited access to critical mechanistic questions. With this in mind, we have studied GNP vesicle (GNV) formation through self-assembly by the surface modification of GNPs with low-molecular-weight ligands. Here, we successfully prepared GNVs composed of GNPs with a diameter of 30 nm by surface modification with carboxylic acid-terminated fluorinated oligo(ethylene glycol) ligands (CFLs). As the carboxylic acid has two states (protonated and deprotonated), the balance of the attraction and repulsion between GNPs covered with CFLs is tunable. Sodium carboxylate-terminated fluorinated oligo(ethylene glycol) ligands (SCFLs) provided smaller GNVs than did CFLs at 0.8 × 10¹¹ NPs/mL. Time-course study revealed that CFL-covered GNPs quickly form small aggregates and gradually grow to larger GNVs (ca. 200 nm), but no gradual growth was observed for SCFL-covered GNPs. This result indicated that the electrostatic repulsion inhibits fusion of the small GNVs. The size of the GNVs formed with the aid of CFLs was independent of the initial GNP concentration, but the extinction spectra were concentration-dependent. Electron microscopy imaging and simulations supported the defect formation in the assemblies. These results provided new insights into the vesicle formation mechanism.

External-Stimuli-Assisted Control over Assemblies of Plasmonic Metals

Assembly of plasmonic nanoparticles (NPs) in suspensions is a promising approach for the control of optical and sensing properties that depend on the assembled states of plasmonic NPs. This review focuses on the controlling methods to assemble the NP via external stimuli such as pH, temperature, light, magnetic field, and electric field. External stimuli are introduced as powerful tools to assemble the NPs because of various operational factors, such as the intensity, application time, and frequency, which can be employed. In addition to a summary of recent studies on the controlling methods, a future study on the reversible control over assembled states of the plasmonic NPs via external stimuli is proposed.

Two-Step Assembly of Thermoresponsive Gold Nanorods Coated with a Single Kind of Ligand

Gold nanorods (GNRs) coated with a single kind of ligand show thermoreponsive two-step assembly to provide a hierarchical structure. The GNRs (33 nm in length × 14 nm in diameter) coated with a hexa(ethylene glycol) (HEG) derivative form side-by-side assemblies at 30 °C (T_A1 ) as a steady state through dehydration. By further heating to over 40 °C (T_A2 ), larger assemblies, which are composed of the side-by-side assembled units, are formed as hierarchical structures. The dehydration temperature of the HEG derivative varies depending on the free volume of the HEG unit, which corresponds to the curvature of the GNRs. Upon heating, dehydration first occurs from the ligands on the side portions with a lower curvature, and then from the ligands on the edge portions with a higher curvature. The different sized GNRs (33 × 8 and 54 × 15 nm) also show two-step assembly. Both the T_A1 and T_A2 are dependent on the diameter of the GNRs, but independent of their length. This result supports that the dehydration is dependent on the free volume, which corresponds to the curvature. Anisotropic assembly focusing on differences in curvature provides new guidelines for the fabrication of hierarchical structures.

Environment-stimulated nanocarriers enabling multi-active sites for high drug encapsulation as an “on demand” drug release system

Limited active sites in polyesters hinder fabrication of multifunctional biodegradable nanocarriers for successful clinical applications.

Amphiphilic N-Heterocyclic Carbene-Stabilized Gold Nanoparticles and Their Self-Assembly in Polar Solvents

Soft matter-directed self-assembly of amphiphilic inorganic nanoparticles (NPs) has recently emerged as a promising approach to access NP ensembles with superior collective properties. While thiol-terminated molecules are primarily employed to tether the amphiphilic ligand to the metal, concerns remain regarding the stabilities of the resulting NPs and their corresponding aggregates. As an alternative, we report amphiphilic N-heterocyclic carbene (NHC)-functionalized gold nanoparticles (AuNPs). To accomplish this, an amphiphilic NHC-Au^I complex based on an asymmetric triethylene glycol-/dodecyl-functionalized benzimidazole was first synthesized and used to prepare the corresponding stable amphiphilic NHC-decorated AuNPs. The resulting NPs were comprehensively characterized using both solution- and solid-state-based techniques such as proton nuclear magnetic resonance spectroscopy, dynamic light scattering, transmission electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. By optimizing the self-assembly behavior of these amphiphilic AuNPs in deionized water, ethanol, and their mixtures, we were able to fine-tune the plasmonic properties of the AuNPs in the wide range of 525-640 nm. Furthermore, when treated with thiols, the ensembles showed greater stability compared to their parent discrete AuNP counterparts at room temperature.

Layer-by-layer nanoparticles co-loading gemcitabine and platinum (IV) prodrugs for synergistic combination therapy of lung cancer

Purpose

Cisplatin plus gemcitabine (GEM) is a standard regimen for the first-line treatment of advanced non-small cell lung cancer. The aim of this study was to prepare biocompatible and biodegradable polymeric prodrugs and construct nanoparticles (NPs) with layer-by-layer (LbL) technique.

Methods

Platinum (Pt) (IV) complex with a carboxyl group was conjugated to the amino group of chitosan (CH), resulting in a CH-Pt conjugation with positive charge. GEM with amino group was conjugated to the carboxyl group of hyaluronic acid (HA), resulting in a HA-GEM conjugation with negative charge. Novel LbL NPs consisting of the CH-Pt core and the HA-GEM layer, named as HA-GEM/CH-Pt NPs, were constructed. The physicochemical properties of the HA-GEM/CH-Pt NPs were investigated. In vitro cytotoxicity against human non-small lung cancer cells (NCl-H460 cells) was investigated, and in vivo antitumor efficiency was evaluated on mice bearing NCl-H460 cells xenografts.

Results

HA-GEM/CH-Pt NPs have a size of about 187 nm, a zeta potential value of −21 mV and high drug encapsulation efficiency of 90%. The drug release of HA-GEM/CH-Pt NPs exhibited a sustained behavior. HA-GEM/CH-Pt NPs could significantly enhance in vitro cytotoxicity and in vivo antitumor effect against lung cancer animal model compared to the single-drug-loaded NPs and free drug solutions.

Conclusion

The results demonstrated that the HA-GEM/CH-Pt NPs might be a promising system for the synergetic treatment of lung carcinoma.