Polyoxometalate Surfactants as Unique Molecules for Interfacial Self-Assembly

Whereas, commonly, Langmuir monolayers are structurally dominated by the aliphatic chains, we present here the first case of monolayers where the chains merely serve anchoring at the air/water interface and the organization is dictated by the hydrophilic head group self-assembling in a hexagonal lattice. These head groups are polyoxometalates known for their multifunctional potential. The chain length has been systematically varied, allowing for a general study of the impact of the chain length on the supramolecular structure. These model structures are studied here by a combination of modern techniques, the leading ones being X-ray reflectivity and grazing incidence X-ray diffraction. The quantitative structural insights offered in this Letter might represent a starting point for the rational design and study of a new class of emulsions, including an organic tail and a multifunctional inorganic polar head

Monodisperse Gold Nanotriangles: Size Control, Large-Scale Self-Assembly, and Performance in Surface-Enhanced Raman Scattering

Au nanotriangles display interesting nanoplasmonic features with potential application in various fields. However, such applications have been hindered by the lack of efficient synthetic methods yielding sufficient size and shape monodispersity, as well as by insufficient morphological stability. We present here a synthesis and purification protocol that efficiently addresses these issues. The size of the nanotriangles can be tuned within a wide range by simply changing the experimental parameters. The obtained monodispersity leads to extended self-assembly, not only on electron microscopy grids but also at the air–liquid interface, allowing transfer onto centimeter-size substrates. These extended monolayers show promising performance as surface-enhanced Raman scattering substrates, as demonstrated for thiophenol detection

Elastic Nanocomposite Structures Formed by Polyacetylene–Hemicyanine Mixed Films at the Air–Water Interface

A mixed film of 10,12-pentacosadiynoic acid (DA) and an amphiphilic cationic hemicyanine (SP) in the ratio 1:1 has been fabricated. This mixed film has been proved to be completely homogeneous due to the good compatibility between both molecules, i.e., hydrophobic regions sized and polar regions of opposite charge. This adequate balance between the vertical sections of the hydrophobic and hydrophilic groups allows the formation of an ion pair between them. In this mixed film, DA molecules organize in monolayer instead of trilayer as occurs for pure DA film. Surface pressure measurements (isotherms and compression–expansion cycles), Brewster angle microscopy, reflection spectroscopy, and PM-IRRAS prove this structure. Using BAM to reveal the structure of this mixed film, circular domains with internal anisotropy because the ordered alignment of hemicyanine groups (strong self-aggregation) are observed. Under UV irradiation a new polyacetylene (PDA) form was fabricated in a homogeneous mixed monolayer despite of the distance measured between the DA molecules by the presence of SP dye. The polymerization takes place only in the circular domains. Under expansion the domains forming during compression and after UV irradiation form a nanocomposite stable material with partially elastic outer region

Langmuir Monolayers of an Inclusion Complex Formed by a New Calixarene Derivative and Fullerene

The design of new molecules with directed interactions to functional molecules as complementary building blocks is one of the main goals of supramolecular chemistry. A new <i>p-tert</i>-butylcalix­[6]­arene monosubstituted derivative bearing only one alkyl chain with an acid group (C6A3C) has been synthesized. The C6A3C has been successfully used for building Langmuir monolayers at the air–water interface. The C6A3C molecule adopts a flatlike orientation with respect to the air–water interface. The molecular structure gives the molecule amphiphilic character, while allowing the control of both the dissociation degree and the molecular conformation at the air–water interface. The C63AC has been combined with pristine fullerene (C60) to form the supramolecular complex C6A3C:C60 in 2:1 molar ratio (CFC). The CFC complex retains the ability of C6A3C to form Langmuir monolayers at the air/water interface. The interfacial molecular arrangement of the CFC complex has been convincingly described by in situ UV–vis reflection spectroscopy and synchrotron X-ray reflectivity measurements. Computer simulations complement the experimental data, confirming a perpendicular orientation of the calixarene units of CFC with respect to the air–water interface. This orientation is stabilized by the formation of intermolecular H-bonds. The interfacial monolayer of the CFC supramolecular complex is proposed as a useful model for the well-defined self-assembly of recognition and functional building blocks

Diacetylene Mixed Langmuir Monolayers for Interfacial Polymerization

Polydiacetylene (PDA) and its derivatives are promising materials for applications in a vast number of fields, from organic electronics to biosensing. PDA is obtained through polymerization of diacetylene (DA) monomers, typically using UV irradiation. DA polymerization is a 1–4 addition reaction with both initiation and growth steps with topochemical control, leading to the “blue” polymer form as primary reaction product in bulk and at interfaces. Herein, the diacetylene monomer 10,12-pentacosadiynoic acid (DA) and the amphiphilic cationic <i>N</i>,<i>N</i>′-dioctadecylthiapentacarbocyanine (OTCC) have been used to build a mixed Langmuir monolayer. The presence of OTCC imposes a monolayer supramolecular structure instead of the typical trilayer of pure DA. Surface pressure, Brewster angle microscopy, and UV–vis reflection spectroscopy measurements, as well as computer simulations, have been used to assess in detail the supramolecular structure of the DA:OTCC Langmuir monolayer. Our experimental results indicate that the DA and OTCC molecules are sequentially arranged, with the two OTCC alkyl chains acting as spacing diacetylene units. Despite this configuration is expected to prevent photopolymerization of DA, the polymerization takes place without phase segregation, thus exclusively leading to the red polydiacetylene form. We propose a simple model for the initial formation of the “blue” or “red” PDA forms as a function of the relative orientation of the DA units. The structural insights and the proposed model concerning the supramolecular structure of the “blue” and “red” forms of the PDA are aimed at the understanding of the relation between the molecular and macroscopical features of PDAs

Photoinduced and Electrochemical Applications of Carbon-Based Nanoparticles from Spent Coffee Grounds

The interest of the scientific community toward carbon-based nanostructures is justified by the wide range of applications of such multifaceted structures. In the present work, carbon nanoparticles (CNPs) were synthesized, in mild conditions, by using spent coffee grounds as the carbon source. Green-emitting CNPs of about 40 nm were obtained, and they were characterized by X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, fluorescence spectroscopy, and Fourier transform infrared spectroscopy, highlighting that they are naturally doped with N, K, Mg, and P. Such intrinsic doping promotes the characteristic green emission; furthermore, the N- and P-doping prompted us to evaluate their ability to photoproduce 1O2 for PDT applications. The presence of heteroatoms in the CNP structure was also used to electrochemically promote the oxygen reduction reaction and hydrogen evolution reaction catalysis

A General Method for Solvent Exchange of Plasmonic Nanoparticles and Self-Assembly into SERS-Active Monolayers

We present a general route for the transfer of Au and Ag nanoparticles of different shapes and sizes, from water into various organic solvents. The experimental conditions for each type of nanoparticles were optimized by using a combination of thiolated poly­(ethylene glycol) and a hydrophobic capping agent, such as dodecanethiol. The functionalized nanoparticles were readily transferred into organic dispersions with long-term stability (months). Such organic dispersions efficiently spread out on water, leading to self-assembly at the air/liquid interface into extended nanoparticle arrays which could in turn be transferred onto solid substrates. The dense close packing in the obtained nanoparticle monolayers results in extensive plasmon coupling, rendering them efficient substrates for surface-enhanced Raman scattering spectroscopy

Interfacial Activity of Gold Nanoparticles Coated with a Polymeric Patchy Shell and the Role of Spreading Agents

Gold patchy nanoparticles (PPs) were prepared under surfactant-free conditions by functionalization with a binary ligand mixture of polystyrene and poly­(ethylene glycol) (PEG) as hydrophobic and hydrophilic ligands, respectively. The interfacial activity of PPs was compared to that of homogeneous hydrophilic nanoparticles (HPs), fully functionalized with PEG, by means of pendant drop tensiometry at water/air and water/decane interfaces. We compared interfacial activities in three different spreading agents: water, water/chloroform, and pure chloroform. We found that the interfacial activity of PPs was close to zero (∼2 mN/m) when the spreading agent was water and increased to ∼14 mN/m when the spreading agent was water/chloroform. When the nanoparticles were deposited with pure chloroform, the interfacial activity reached up to 60 mN/m by compression. In all cases, PPs exhibited higher interfacial activity than HPs, which were not interfacially active, regardless of the spreading agent. The interfacial activity at the water/decane interface was found to be significantly lower than that at the water/air interface because PPs aggregate in decane. Interfacial dilatational rheology showed that PPs form a stronger elastic shell at the pendant drop interface, compared to HPs. The significantly high interfacial activity obtained with PPs in this study highlights the importance of the polymeric patchy shell and the spreading agent

Au Nanoparticles–Mesoporous TiO₂ Thin Films Composites as SERS Sensors: A Systematic Performance Analysis

The combination of plasmonic nanoparticles and mesoporous materials is of much interest in applications such as sensing or catalysis. The production of such hybrid materials can be done in various ways, leading to different architectures. We present a comparative study of the SERS performance of different nanocomposite architectures comprising mesoporous TiO₂ thin films and Au nanoparticles (NPs). The selection of TiO₂ as mesoporous support material was based on its high chemical and mechanical stability. Au NPs of different sizes and shapes were placed at different locations of the composite and used as a plasmonic material compatible with the synthesis conditions of the mesoporous films, displaying a high chemical stability. Using <i>p</i>-nitrothiophenol as a molecular probe, we evaluated the performance toward surface-enhanced Raman scattering (SERS) sensing, on the basis of minimum acquisition time, spot-to-spot reproducibility, and limit of detection. The obtained results indicate that each platform features different sensing capabilities. While systems comprising Au NPs within the mesopores allow working with low acquisition times and present high signal uniformity, only a detection limit of micromolar was achieved. On the other hand, those systems made of branched Au NPs covered with mesoporous films require low acquisition times and can achieve detection limits as low as 10 pM, but signal uniformity is compromised. We propose that careful comparison of different SERS platforms based on Au NPs and mesoporous thin films will facilitate selecting an appropriate configuration for any desired application