A general strategy toward pH-controlled aggregation-dispersion of gold nanoparticles and single-walled carbon nanotubes

Non-autonomous zinc-methylimidazole oscillator and the formation of layered precipitation structures in a hydrogel

Oscillations are one of the intrinsic features of many animate and inanimate systems. The oscillations manifest in the temporal periodic change of one or several physical quantities describing the systems. In chemistry and biology, this physical quantity is the concentration of the chemical species. In most chemical oscillatory systems operating in batch or open reactors, the oscillations persist because of the sophisticated chemical reaction networks incorporating autocatalysis and negative feedback. However, similar oscillations can be generated by periodically changing the environment providing non-autonomous oscillatory systems. Here we present a new strategy for designing a non-autonomous chemical oscillatory system for the zinc-methylimidazole. The oscillations manifested in the periodic change of the turbidity utilizing the precipitation reaction between the zinc ions and 2-methylimidazole (2-met) followed by a partial dissolution of the formed precipitate due to a synergetic effect governed by the ratio of the 2-met in the system. Extending our idea spatiotemporally, we also show that these precipitation and dissolution phenomena can be utilized to create layered precipitation structures in a solid agarose hydrogel.

DNA Assembly-Based Stimuli-Responsive Systems

Stimuli-responsive designs with exogenous stimuli enable remote and reversible control of DNA nanostructures, which break many limitations of static nanostructures and inspired development of dynamic DNA nanotechnology. Moreover, the introduction of various types of organic molecules, polymers, chemical bonds, and chemical reactions with stimuli-responsive properties development has greatly expand the application scope of dynamic DNA nanotechnology. Here, DNA assembly-based stimuli-responsive systems are reviewed, with the focus on response units and mechanisms that depend on different exogenous stimuli (DNA strand, pH, light, temperature, electricity, metal ions, etc.), and their applications in fields of nanofabrication (DNA architectures, hybrid architectures, nanomachines, and constitutional dynamic networks) and biomedical research (biosensing, bioimaging, therapeutics, and theranostics) are discussed. Finally, the opportunities and challenges for DNA assembly-based stimuli-responsive systems are overviewed and discussed.

Stimuli-Responsive DNA Self-Assembly: From Principles to Applications

Stimuli-responsive DNA self-assembly shares the advantages of both designed stimuli-responsiveness and the molecular programmability of DNA structures, offering great opportunities for basic and applied research in dynamic DNA nanotechnology. In this minireview, we summarize the most recent progress in this rapidly developing field. The trigger mechanisms of the responsive DNA systems are first divided into six categories, which are then explained with illustrative examples following this classification. Subsequently, proof-of-concept applications in terms of biosensing, in vivo pH-mapping, drug delivery, and therapy are discussed. Finally, we provide some remarks on the challenges and opportunities of this highly promising research direction in DNA nanotechnology.

Water-Mediated Assembly of Gold Nanoparticles into Aligned One-Dimensional Superstructures

This Article shows that water in ethanol colloids of gold nanoparticles enhances the formation of linear clusters and, more important for applications in electronics, determines their assembly on surfaces. We show by dynamic light scattering that ethanol colloids contain mainly monomers and dimers and that wormlike superstructures are mostly absent, despite UV-vis evidence of aggregation. Water added to the colloid as a cosolvent was found to enhance the number of clusters as well as their average size, confirming its role in linear self-assembly, on the scale of a few particles. Water adsorbed from the atmosphere during coating was also found to be a powerful lever to tune self-assembly on surfaces. By varying the relative humidity, a sharp transition from branched to linear superstructures was observed, showing the importance of water as a cosolvent in the formation of cluster superstructures. We show that one-dimensional superstructures may form due to long-range mobility of precursor clusters on wet surfaces, allowing their rearrangement. The understanding of the phenomenon allows us to statistically align both clusters and resulting superstructures on patterned substrates, opening the way to rapid screening in molecular electronics.

Overcoming the Coupling Dilemma in DNA-Programmable Nanoparticle Assemblies by "Ag+ Soldering"

Strong coupling between nanoparticles is critical for facilitating charge and energy transfers. Despite the great success of DNA-programmable nanoparticle assemblies, the very weak interparticle coupling represents a key barrier to various applications. Here, an extremely simple, fast, and highly efficient process combining DNA-programming and molecular/ionic bonding is developed to address this challenge, which exhibits a seamless fusion with DNA nanotechnology.

Role of pH controlled DNA secondary structures in the reversible dispersion/precipitation and separation of metallic and semiconducting single-walled carbon nanotubes

Single-stranded DNA (ss-DNA) oligomers (dA20, d[(C3TA2)3C3] or dT20) are able to disperse single-walled carbon nanotubes (SWNTs) in water at pH 7 through non-covalent wrapping on the nanotube surface. At lower pH, an alteration of the DNA secondary structure leads to precipitation of the SWNTs from the dispersion. The structural change of dA20 takes place from the single-stranded to the A-motif form at pH 3.5 while in case of d[(C3TA2)3C3] the change occurs from the single-stranded to the i-motif form at pH 5. Due to this structural change, the DNA is no longer able to bind the nanotube and hence the SWNT precipitates from its well-dispersed state. However, this could be reversed on restoring the pH to 7, where the DNA again relaxes in the single-stranded form. In this way the dispersion and precipitation process could be repeated over and over again. Variable temperature UV-Vis-NIR and CD spectroscopy studies showed that the DNA-SWNT complexes were thermally stable even at ∼90 °C at pH 7. Broadband NIR laser (1064 nm) irradiation also demonstrated the stability of the DNA-SWNT complex against local heating introduced through excitation of the carbon nanotubes. Electrophoretic mobility shift assay confirmed the formation of a stable DNA-SWNT complex at pH 7 and also the generation of DNA secondary structures (A/i-motif) upon acidification. The interactions of ss-DNA with SWNTs cause debundling of the nanotubes from its assembly. Selective affinity of the semiconducting SWNTs towards DNA than the metallic ones enables separation of the two as evident from spectroscopic as well as electrical conductivity studies.

Differential adsorption of gold nanoparticles to gold/palladium and platinum surfaces

Integration of molecule-capped gold nanoparticles (AuNP) into nanoelectronic devices requires detailed knowledge about the AuNP-electrode interface. Here, we report the pH-dependent adsorption of amine or carboxylic acid-terminated gold nanoparticles on platinum or gold/palladium (30% Pd) alloy, respectively. We synthesized amine-terminated AuNP, applying a new solid phase supported approach, as well as AuNP exhibiting carboxylic acid as terminal groups. The pH-induced agglomeration of the synthesized AuNP was investigated by UV-vis, DLS, and ζ-potential measurements. Depending on the pH and the ionic strength of the AuNP solution a preferential adsorption on the different metals occurred. Thereby, we demonstrate that by choosing the appropriate functional group and adjusting the pH as well as the ionic strength a directed binding can be achieved, which is an essential prerequisite for applications of these particles in nanoelectronics. These findings will pave the way for a controlled designing of the interface between molecule-capped AuNP and metallic electrodes for applications in nanoelectronics.

One at a time: counting single-nanoparticle/electrode collisions for accurate particle sizing by overcoming the instability of gold nanoparticles under electrolytic conditions

In response to an increasing demand for understanding electrochemical processes on the nanometer scale, it now becomes possible to monitor electron transfer reactions at the single-nanoparticle level, namely particle collision electrochemistry. This technique has great potential in the development of research tools towards single-particle electrocatalysis and selective and multiplexed particle sizing. However, one existing problem that may discourage these applications is the relatively weak colloidal stability of nanoparticles in an electrolytic solution. Here we report on a facile but efficient way to achieve a good stability of gold nanoparticles in an acidic media so that 'zero-aggregation' collisions can be achieved at a carbon ultramicroelectrode. This allows us to obtain anodic dissolution currents from individual nanoparticles in a 'one particle at a time' manner, based on which accurate particle sizing with a resolution of 1-2 nm can be achieved. Our work strongly suggests that to maintain a well dispersed nanoparticle solution during a particle impact electrochemical experiment is critically important for accurate particle sizing, as well as other applications that require information to be extracted from individual nanoparticles (not their aggregates).

Converging hazard assessment of gold nanoparticles to aquatic organisms

The gold nanoparticles (Au-NPs) are being increasingly used because of their huge diversity of applications, and consequently, elevated levels in the environment are expected. However, due to their physico-chemical properties and functionalization a high variety of Au-NPs can be found, and complete toxicological information for each type of Au-NPs still lacks, and even, the toxicological information for the same species is sometimes contradictory. Therefore, hazard assessment should be done case by case. Hence, the objective of this study was to obtain ecotoxicological information of the same Au-NPs in aquatic organisms and to find a rationale for Au-NPs toxicity. For such a purpose, bare and hyaluronic acid capped Au-NPs (12.5 nm) along with Au-NPs bulk material were tested on freshwater algae, Daphnia and zebrafish. Results showed that while gold nanoparticles were found to be harmless to the tested organisms, the soluble gold showed to be toxic to algae and Daphnia, with an LC50 between 1 and 2 mg L(-1). Comparing our results with those gathered in the literature, it appears that a common hazard assessment of Au-NPs on the studied organisms can be elucidated.

Natural organic matter concentration and hydrochemistry influence aggregation kinetics of functionalized engineered nanoparticles

Understanding the colloidal stability of functionalized engineered nanoparticles (FENPs) in aquatic environments is of paramount importance in order to assess the risk related to FENPs. In this study, gold nanoparticles (GNPs) of 68 and 43 nm diameter, coated with citrate and 11-mercaptoundecanoic acid (MUA) respectively, were used as models of FENPs. Time-resolved dynamic light scattering was employed to investigate the aggregation kinetics of two types of GNPs. The results show that without Suwannee river natural organic matter (SRNOM), MUA coating resulted in greater stability than citrate coating for GNPs. Cations have a destabilizing effect on both GNPs following the order Ca(2+) ≈ Mg(2+) >> Na(+); different anions (Cl(-) and SO4(2-)) showed no difference in effects. In the fast aggregation regime, adding SRNOM enhanced the stability of MUA-coated GNPs in both Ca(2+) and Mg(2+) solutions. However citrate-coated GNPs were only stabilized in Mg(2+) solution but enhanced aggregation occurred in high Ca(2+) concentration due to interparticle bridging. For the investigated GNPs and in the presence of SRNOM, Ca(2+) does not always act as a strong coagulant. This indicates that for the new materials emerging from the application of nanotechnology the well-described aggregation mechanisms of colloids in the environment require a detailed re-examination.

In-vitro permeability of neutral polystyrene particles via buccal mucosa

Drugs can be absorbed well in the oral cavity, which eliminates problems related to intestinal and hepatic first-pass metabolism. Although it is well-established that nanoparticles are small enough to penetrate/permeate epithelial barriers, there is no clear understanding of how they interact with the buccal mucosa. This work provides useful information regarding particle properties with regard to mucosal uptake and can be used for the rational design of nanocarriers. In the buccal mucosa, the uptake of neutral polystyrene nanoparticles (PP) is size-dependent. Compared to 25 and 50 nm particles, 200 nm PP particles penetrate into deeper regions of the mucosa. This is attributed to the structure of the buccal mucosa, i.e., mucus layer and microplicae. The particles permeate the mucus layer and deposit in ridge-like folds of superficial buccal cells. Thus, the effects of thermodynamic driving forces and/or interparticle electrostatic repulsion are enhanced and cellular uptake might be reduced for smaller particle sizes.

Logical regulations of the aggregation/dispersion of gold nanoparticles via programmed chemical interactions

Nanoparticles that respond to various chemical and physical stimuli form the basis for various conceivable applications including sensors, chemical logic, biomedical imaging, and therapies. In this work, we demonstrate that the electrostatic and chemical (complexing and gold-thiol bonding) interactions existing in a gold nanoparticle/Zn(2+)/dithiothreitol-based ternary chemical system is "programmable" and can be utilized to regulate the aggregation and dispersion of nanoparticles via XOR and INHIBIT logics. The resulting solutions alter their colors according to different input combinations because of the well-controlled aggregation or dispersion of plasmonic gold nanoparticles, opening up new possibilities for the developments of advanced sensors and nanobiomedical devices based on the coupling, gating, and signaling of different chemical stimuli.

Role of salt in the spontaneous assembly of charged gold nanoparticles in ethanol

This paper investigates the role of salt in the spontaneous linear assembly of charged gold nanoparticles in ethanol and attempts to clear up a misunderstanding on the role of ethanol in this process. Many prior reports have noted that the addition of ethanol to an aqueous solution of gold nanoparticles causes their aggregation into linear assemblies. It was therefore believed that ethanol plays the determining role during the assembly process. In this work, we carried out systematic studies which indicate that residual salt in conjunction with ethanol, instead of ethanol itself, induces the assembly of gold nanoparticles in ethanol. In the absence of salt, gold nanoparticles can be well dispersed in an ethanol solution. Furthermore, we find that the chainlike assemblies can disassemble upon dilution of the salt or the evaporation of ethanol if the gold nanoparticles are protected with a sufficiently strong ligand.

DNA-SWNT hybrid hydrogel

In this communication, we report the preparation of DNA-SWNT hybrid hydrogel which is pH responsive and strength tunable.

Wrapping nanotubes with micelles, hemimicelles, and cylindrical micelles

This work uses a simple model based on hydrophobic and hydrophilic forces to investigate the molecular dynamics that lead to the supramolecular self-assembly of surfactants around carbon nanotubes (CNTs). The effects of the concentration and the structure of surfactants are explored. The bead-based mesoscopic description spontaneously develops the several micellar morphologies that are known to wrap and solvate CNTs.

A general procedure to functionalize agglomerating nanoparticles demonstrated on nanodiamond

Upon reduction of particle size to the nanometer range, one has to deal with the general issue of spontaneous agglomeration, which often obstructs postsynthesis modification of nanoparticle surfaces. A technique to cope with this phenomenon is required to realize a wide variety of applications using nanoparticles in solvents or as refined assemblies. In this article, we report on a new technique to facilitate surface chemistry of nanoparticles in a conventional glassware system. A beads-assisted sonication (BASD) process was examined to break up persistent agglomerates of nanodiamonds in two different reactions for simultaneous surface functionalization. The chosen reactions are the silanization with an acrylate-modified silane and the arylation using diazonium salts. The BASD process can be successfully applied even where the original material is not dispersible in the reaction solvent at all, as the formation of ever smaller, increasingly functionalized agglomerates is improving their solubility. We have confirmed that the presence of ceramic beads enables functionalization of each primary particle, while conventional magnetic stirring or beadless sonication can reach primary particles only when agglomeration is loose. Additionally, mechanical surface modification of nanodiamond was found to take place by BASD with high energy density, leading to sp(2)-hybridized surface patches on nanodiamond. This allowed for the efficient grafting of aryl groups to the surface of primary diamond nanoparticles. Stable, homogeneously functionalized nanodiamond particles in colloidal solution can be obtained by this method.