Highly sensitive plasmonic paper substrate fabricated via amphiphilic polymer self-assembly in microdroplet for detection of emerging pharmaceutical pollutants.

We report an innovative and facile approach to fabricating an ultrasensitive plasmonic paper substrate for surface-enhanced Raman spectroscopy (SERS). The approach exploits the self-assembling capability of poly(styrene-b-2-vinyl pyridine) block copolymers to form a thin film at the air-liquid interface within the single microdroplet scale for the first time and the subsequent in situ growth of silver nanoparticles (AgNPs). The concentration of the block copolymer was found to play an essential role in stabilizing the droplets during the mass transfer phase and formation of silver nanoparticles, thus influencing the SERS signals. SEM analysis of the morphology of the plasmonic paper substrates revealed the formation of spherical AgNPs evenly distributed across the surface of the formed copolymer film with a size distribution of 47.5 nm. The resultant enhancement factor was calculated to be 1.2 × 107, and the detection limit of rhodamine 6G was as low as 48.9 pM. The nanohybridized plasmonic paper was successfully applied to detect two emerging pollutants-sildenafil and flibanserin-with LODs as low as 1.48 nM and 3.45 nM, respectively. Thus, this study offers new prospects for designing an affordable and readily available, yet highly sensitive, paper-based SERS substrate with the potential for development as a lab-on-a-chip device.

Self-Assembly of Polystyrene-b-poly(2-vinylpyridine)/Chloroauric Acid at the Liquid/Liquid Interface.

The self-assembly of polystyrene-block-poly(2-vinylpyridine) at the liquid/liquid interface has been systematically investigated to develop a series of primary morphologies of the aggregates. The block copolymers self-assembled into large areas of nanodot arrays, parallel nanostrands, layered films, parallel nanobelts, honeycomb monolayers, and foams by reacting with chloroauric acid, depending on the molecular structure of the block copolymers and the amount of chloroauric acid. The formation of the first four ordered structures resulted from interfacial adsorption and self-assembly, and nucleation and epitaxial growth. The latter two structures were attributed to the water hole templating effect and spontaneous interfacial emulsification, respectively. This work provides insight into the self-assembly behavior of block copolymers at the interface and provides a facile approach for fabricating functional structures.

Large-Area Assembly of Metal-Organic Layered Ultrathin Films at the Liquid/Liquid Interface.

Two-dimensional functional metal-organic frameworks and coordination polymers have attracted much attention and have been successfully prepared in solutions and at interfaces through the coordination of ligands to metal ions. However, the preparation of large-area ultrathin ordered films is still a challenge. Here, a modified liquid/liquid interfacial epitaxial growth method has been developed. A planar liquid/liquid interface between a chloroform solution of bipyridine derivatives and pure water was constructed first, and then an aqueous solution of Eu3+ or Cu2+ ions was added dropwise into the water phase. A layered ultrathin film with the size of several hundreds of square micrometers appeared at the liquid/liquid interface after a certain time. The monitoring results showed that the formation of ultrathin films was a result of continuous epitaxial growth of the adsorbed species due to the synergistic effects of hydrophobic effects of the alkyl chains, coordination bonds between the ligands and metal ions, π-π interactions between the ligands, and the restriction of the interface on the vertical growth. This offers a way to fabricate more large-area thin films of amphiphilic molecules.

Lamellar Nanosheets of Water-Insoluble Amphiphiles via Aqueous Solution and Air/Liquid Interface Self-Assembly.

Two-dimensional (2D) lamellar nanostructures have attracted much interest due to their unique structure and properties. Various fabrication methods have been developed in recent years, including solution self-assembly, exfoliation, and Langmuir monolayer and Langmuir-Blodgett (LB) deposition. In this work, two kinds of facile methods were applied to fabricate lamellar structures of amphiphilic molecules, such as 10,12-pentacosadiynoic acid (PCDA). In method I, the amphiphilic molecules were introduced into aqueous solutions with dimethylformamide (DMF), a solvent miscible with water, through a mass transfer process across a planar liquid/liquid interface; in method II, the DMF solution of the amphiphilic molecules was added directly onto the aqueous solution surface. With the spread and diffusion of DMF, nanosheets with lamellar structures formed in the aqueous solution and at the air/liquid interface, respectively. It is very interesting that the nanosheets obtained through these two methods consist of an even number and odd number of PCDA monolayers, respectively, reflecting different fabrication mechanisms. Method I provides an approach to gently mix organic solutions with aqueous solutions, while method II can be regarded as an extension of the Langmuir monolayer technique, which combines the interfacial assembly with that in solution. These methods have been extended to a series of amphiphilic molecules, and ordered layered structures have been obtained successfully.

A new approach to construct and modulate G-quadruplex by cationic surfactant.

HYPOTHESIS: G-quadruplex structure has raised increasing attention in supramolecular chemistry as an effective template for ordered functional materials. Thus, it is of practical significance to advance our understanding regarding G-quadruplex structures. Typically, G-quadruplex structures are formed in the presence of suitable metal ions. New methods to construct such structures need to be explored. EXPERIMENTS: The supramolecular assembly between CTAB and a guanosine derivative at different molar ratios was systematically studied, including assembly mechanisms, morphology, and macroscopic properties. Cationic surfactants with different alkyl chains were studied as control experiments. FINDINGS: A novel strategy to construct G-quadruplex with the promotion of the cationic surfactant CTAB is presented in this work. The structure-property relationships of G-quadruplex gels are characterized by rheology and shrinkage ratio experiments. MacKintosh's theory was used to rationalize the relationship between gel elasticity and water content. The transition of G-quadruplex structures could be easily enabled by modulating CTAB concentration, which promotes the phase transition from gel/sol biphase to homogeneous sol phase. This work will provide a new viewpoint for the construction and modulation of G-quadruplex structures.

Fabricating highly catalytically active block copolymer/metal nanoparticle microstructures at the liquid/liquid interface.

HYPOTHESIS: Our previous studies have shown that the metal nanoparticle/polymer composite structures fabricated at the liquid/liquid interface have good reusability but lower catalytic activity for heterogeneous reactions in aqueous solutions. This should be attributed to the poor water wettability and more compact structure of the polymer matrices. Therefore, it should be possible to improve the catalytic activity through designing and fabricating a porous composite structure with good water wettability. EXPERIMENTS: A modified liquid/liquid interface adsorption and fabrication method was used. An aqueous solution of copper acetate and a chloroform/DMF mixed solution of PS-b-PAA acted as the two phases. Through spontaneous emulsification, self-assembly of the polymer molecules with Cu2+ ions in the droplets, and adsorption of the formed spherical micelles and nanofibers to the planar liquid/liquid interface, a porous composite microstructure was formed. FINDINGS: This structure consisted of nanofiber-connected nanospheres which have a PS core and a PAA corona. Tiny and well-dispersed Cu nanoparticles were embedded in the hydrophilic corona and were adsorbed on the nanofibers surface as well. After physical cross-linking with 1,6-diaminohexane, the composite material exhibited high catalytic activity and good reusability for the reactions in aqueous solutions. For example, the rate constant for the reduction of p-nitroaniline reached 1965 s-1 g-1.

Highly selective and sensitive fluorogenic ferric probes based on aggregation-enhanced emission with -SiMe3 substituted polybenzene.

In this study, thiophene was linked to polybenzene to generate novel fluorescent probes, namely 3,4-diphenyl-2,5-di(2-thienyl)phenyl-trimethylsilane (DPTB-TMS) with a -SiMe3 substituent and 3,4-diphenyl-2,5-di(2-thienyl)phenyl (DPTB) without the -SiMe3 substituent, respectively. Both of the two compounds exhibit aggregation-enhanced emission (AEE) properties in tetrahydrofuran/water mixtures due to restricted intramolecular rotation of the peripheral groups, which make the two compounds good candidates for the detection of Fe3+ ions in aqueous-based solutions. The fluorescence intensity of the two compounds decreases immediately and obviously upon addition of a trace amount of Fe3+, and decreases continuously as the amount of Fe3+ increases. The fluorescence was quenched to 92% of its initial intensity when the amount of Fe3+ ions reached 6µmol for DPTB-TMS and to 80% for DPTB in the systems, indicating that the compound with the -SiMe3 group is a more effective probe. The detection limit was found to be 1.17µM (65ppb). The detection mechanism is proposed to be static quenching. DPTB-TMS is highly efficient for the detection of ferric ions even in the presence of other metal ions. In addition, the method is also successfully applied to the detection of ferric ions in water, blood serum, or solid films. This indicates that these polybenzene compounds can be applied as low-cost, high selectivity, and high efficiency Fe3+ probes in water or in clinical applications.

Fabrication of Two-Dimensional Arrays of Diameter-Tunable PS-b-P2VP Nanowires at the Air/Water Interface.

Composite thin films with well-defined and parallel nanowires were fabricated from the binary blends of a diblock copolymer polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) and several homopolystyrenes (h-PSs) at the air/liquid interface through a facile technique, which involves solution self-assembly, interface adsorption, and further self-organization processes. It was confirmed that the nanowires that appeared at the air/water interface came from the cylindrical micelles formed in solution. Interestingly, the diameters of the nanowires are uniform and can be tuned precisely from 45 to 247 nm by incorporating the h-PS molecules into the micellar core. This parallel alignment of the nanowires has potential applications in optical devices and enables the nanowires to be used as templates to prepare functional nanostructures. The extent to which h-PS molecules with different molecular weights are able to influence the diameter control of the nanowires was also systematically investigated.

Influence of gold species (AuCl4(-) and AuCl2(-)) on self-assembly of PS-b-P2VP in solutions and morphology of composite thin films fabricated at the air/liquid interfaces.

Composite thin films doped with Au species were fabricated at an air/liquid interface via a series of steps, including the mass transfer of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) across the liquid/liquid interface between a DMF/CHCl3 solution and an aqueous solution containing either AuCl4(-) or AuCl2(-), self-assembly of PS-b-P2VP in a mixed DMF-water solution, and adsorption and further self-organization of the formed aggregates at the air/liquid interface. This is a new approach for fabricating composite polymer films and can be completed within a very short time. AuCl4(-) and AuCl2(-) ions were found to significantly influence the self-assembly behavior of the block copolymer and the morphologies of the composite films, leading to the formation of nanowire arrays and a foam structure at the air/liquid interface, respectively, which originated from rod-like micelles and microcapsules that had formed in the respective solutions. The effect of the metal complex was analyzed based on the packing parameters of the amphiphilic polymer molecules in different microenvironments and the interactions between the pyridine groups and the metal chloride anions. In addition, these composite thin films exhibited stable and durable performance as heterogeneous catalysts for the hydrogenation of nitroaromatics in aqueous solutions.

A new strategy to fabricate composite thin films with tunable micro- and nanostructures via self-assembly of block copolymers.

A new and facile strategy to fabricate composite thin films with tunable morphologies via self-assembly of block copolymer molecules at the air/liquid interface is first reported. The morphologies (parallel nanowires and foams) of these freestanding thin films can be tuned by varying the molecular structure or other experimental conditions.

Emulsion-directed liquid/liquid interfacial fabrication of lanthanide ion-doped block copolymer composite thin films.

An emulsion-directed assembly and adsorption approach has been used to fabricate composite films of polystyrene-b-poly(acryl acid)-b-polystyrene (PS-b-PAA-b-PS) and Eu(3+) and La(3+) ions at the planar liquid/liquid interface of the polymer DMF/chloroform (1:1, v/v) mixed solution (lower phase) and aqueous solutions of the corresponding salts (upper phase). The lower phase gradually transformed to a water-in-oil (W/O) emulsion via spontaneous emulsification due to the "ouzo effect". Polymer molecules and the metal ions assembled around emulsion droplets that adsorbed at the planar liquid/liquid interface at last, resulting in formation of composite films. The film morphologies and structures depend on Ln(3+) ions: polymer/Eu(3+) composite films were foam films composed of microcapsules ranging in size from several hundreds of nanometers to micrometers, while polymer/La(3+) composite films were composed of hollow spheres several tens of nanometers in size. Fourier transform infrared (FTIR) spectra revealed that the coordination modes of carboxyl groups to Eu(3+) and La(3+) were bridging bidentate and ionic, respectively, in the two types of composites. These results indicate that stable microcapsules can be fabricated around droplets for polymer/Eu(3+) systems, while microcapsules of polymer/La(3+) are unstable. This leads to different film morphologies and structures. Compositions of these films were characterized using energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). In addition, foam films of polymer/Eu(3+)/2,2'-bipyridine (bpy) were fabricated using this approach, and their photoluminescence properties were investigated.

Liquid/Liquid interfacial fabrication of thermosensitive and catalytically active Ag nanoparticle-doped block copolymer composite foam films.

An aqueous solution of AgNO3 (upper phase) and a DMF/CHCl3 solution of polystyrene-b-poly(acryl acid)-b-polystyrene (PS-b-PAA-b-PS) or PS-b-PAA-b-PS/1,6-diaminohexane (DAH) (lower phase) constituted a planar liquid/liquid interface. The lower phase gradually transformed to a water-in-oil (W/O) emulsion via spontaneous emulsification due to the "ouzo effect". Polymer molecules, DAH molecules, and Ag(+) ions assembled into microcapsules around emulsion droplets that adsorbed at the planar liquid/liquid interface, resulting in formation of a foam film. DAH acted as a cross-linker during this process. Transmission electron microscopic observations indicated that Ag nanoclusters that were generated through reduction of Ag(+) ions by DMF were homogeneously dispersed in the walls of the foam structure. X-ray photoelectron spectroscopic investigations revealed that Ag(I) and Ag(0) coexisted in the film, and Ag(I) transformed to Ag(0) after further treatment. The film formed without DAH was not stable, while the film formed with DAH was very stable due to intermolecular attraction between PAA and DAH and formation of amides, as revealed by FTIR spectra. The film formed with DAH exhibited high and durable catalytic activity for hydrogenation of nitro compounds and, very interestingly, exhibited thermoresponsive catalytic behavior.

Fabrication of composite polymer foam films at the liquid/liquid interface through emulsion-directed assembly and adsorption processes.

The foam films of polystyrene-b-poly(acrylic acid)-b-polystyrene (PS-b-PAA-b-PS) doped with Cd(II) or Pb(II) species were fabricated at the planar liquid/liquid interfaces between a DMF/chloroform (v/v: 1/1) solution of the polymer and aqueous solutions containing cadmium acetate or lead acetate at ambient temperature. Optical microscopic observation shows the thin film is uniform on a larger length scale. Transmission electron microscopic (TEM) investigations reveal that the foam films are made up of microcapsules with the size of several hundreds of nanometers to micrometers. The walls of the microcapsules have a layered structure decorating with nanofibers and hollow nanospheres, where numerous inorganic fine nanoparticles are dispersed homogeneously. The film formation is a result of emulsion droplet-templated assembly and adsorption of the formed microcapsules at the planar liquid/liquid interface. Because of the miscibility of DMF with chloroform and water, DMF migrates to the aqueous phase while water migrates to the organic phase across the interface, resulting in the formation of a W/O emulsion, as revealed by optical microscopic observation, freeze fracture transmission electron microscopic (FF-TEM) observation, and dynamic laser scattering (DLS) investigation. The triblock copolymer molecules and the inorganic species adsorb and self-assemble around the emulsion drops, leading to the formation of the composite microcapsules. X-ray photoelectron spectroscopic (XPS) and FTIR spectroscopic results indicate that two kinds of Cd(II) or Pb(II) species, metal oxide or hydroxide, resulting from the hydrolysis of the metal ions and the coordinated metal ions to the carboxyl groups coexist in the formed thin films, which transform to metal sulfide completely after treating with hydrogen sulfide to get metal sulfide nanoparticle-doped polymer thin films.

Fabrication of composite thin films with microstructures of honeycomb, foam, and nanosphere arrays through adsorption and self-assembly of block copolymers at the liquid/liquid interface.

The adsorption and self-organization behaviors of two kinds of block copolymers, polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and poly(4-vinylpyridine)-block-polystyrene-block-poly(4-vinylpyridine) (P4VP-b-PS-b-P4VP), at planar liquid/liquid interfaces were investigated. A gel film decorating with honeycomb-like microstructures forms at the liquid/liquid interface between PS-b-P4VP chloroform solution and chloroauric acid aqueous solution. However, foam films were developed when the chloroauric acid aqueous solution was replaced by a chloroplatinic acid solution or a silver nitrate solution. Furthermore, a free-standing film containing the ordered arrays of nanospheres appeared at the liquid/liquid interface between P4VP-b-PS-b-P4VP chloroform solution and chloroauric acid aqueous solution. The formation of these microstructures was attributed to the adsorption of polymer molecules, combining with inorganic ions and the self-assembly of the composite species at the interface. The doped metal ions and complex ions were transformed to metal nanoparticles after further treatment. This is a facile and convenient method to prepare polymer/inorganic nanoparticle composites. These results also indicate the great influences of the polymer structures and the inorganic species in the aqueous phases on the self-assembly behaviors of the polymers at the interfaces, the final morphology, and structure of the composites. In addition, the formed thin composite films doped with well-dispersed, homogeneous small noble metal nanoparticles exhibit great and durable catalytic activities for the reduction of 4-nitrophenol (4-NP) by potassium borohydride.

Honeycomb-like thin films of polystyrene-block-poly(2-vinylpyridine) embedded with gold or silver nanoparticles formed at the planer liquid/liquid interface.

Composite thin films of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) decorated with Au or Ag nanoclusters and nanoparticles were fabricated at the interfaces of chloroform solution of PS-b-P2VP and aqueous solutions of HAuCl4 or AgNO3. Transmission electron microscopy (TEM) investigations indicated that large area of a single-layer honeycomb structure was formed, which is composed of polygons (most of them are hexagons) whose walls look like spindles with the length of several hundreds of nanometers. Large amount of Au or Ag nanoparticles are embedded in the walls and the undersides of the honeycomb structures. The formation of these novel composite structures was attributed to the adsorption of block copolymer molecules and inorganic species of AuCl4(-) and Ag(+) ions at the liquid-liquid interface, the combination of the polymer molecules and the inorganic ions, and the self-assembly of the composite molecules. After UV-light irradiation and KBH4 aqueous solution treatment, the inorganic species were reduced completely, as confirmed by UV-vis spectra and X-ray photoelectron spectra. These composite films exhibited high catalytic activities for the reduction of 4-nitrophenol (4-NP) by KBH4 in aqueous solutions.

Free-standing poly(2-vinylpyridine) foam films doped with silver nanoparticles formed at the planar liquid/liquid interface.

Free-standing films of poly(2-vinylpyridine) doped with Ag(+) ions were fabricated at the planar liquid/liquid interface of an aqueous solution of AgNO(3) and a chloroform solution of the polymer through adsorption of the polymer molecules, combination with Ag(+) ions, and self-assembly of the composite species. Transmission electron microscopic (TEM) investigations indicated that the films were composed of planar thin layers decorated with separated microcapsules and foam structures of conglutinated microcapsules, and no Ag nanoparticles formed in the pristine films. After UV-light irradiation and KBH(4) aqueous solution treatment, Ag nanoparticles with the average size of 3.2 nm appeared and incorporated in the polymer matrices. X-ray photoelectron spectra (XPS) and UV-vis spectra are consistent with the TEM observations. Thermogravimetric analysis (TGA) showed good thermal stability of the composite films. The silver content was estimated to be 24.0% from the TG curve, closing to the calculated value. The catalytic performance of the composite films was evaluated by using the reduction of nitro-compounds, including nitrobenzene, 4-nitrophenol, and 4-nitrobenzoic acid by KBH(4) in aqueous solutions. The results indicated that the composite films have high and durable catalytic activity. The apparent reaction constants are related to the size of the nitro-compounds, suggesting that the Ag nanoparticles were incorporated in the matrices, and the diffusion of the reactant molecules has a great influence on the catalytic reaction.

One-step synthesis and assembly of one-dimensional parallel chains of CdS nanoparticles at the air-water interface templated by 10,12-pentacosadiynoic acid supermolecules.

Regular one-dimensional (1D) parallel chains composed of CdS nanoparticles with cubic zinc blende crystal structure were prepared at the air/water interface via one-step synthesis and assembly process. These nanostructures were produced through an interfacial reaction between Cd(2+) ions in the aqueous solution of cadmium acetate and H(2)S in the gaseous phase under Langmuir monolayers of 10,12-pentacosadiynoic acid (PDA). It was demonstrated that PDA molecules self-assembled into parallelly aligned linear supermolecules at the air/water interface with the aid of π-π interactions and acted as templates for the formation of the superstructures. The experimental conditions including temperature and reaction time have great influences on the superstructure formation and the parameters of the parallel chains.

Formation of Ag nanoparticle-doped foam-like polymer films at the liquid-liquid interface.

The composite poly(2-vinylpyridine) (P2VP)-Ag(+) foam-like thin films were prepared at the interface between AgNO(3) aqueous solution and polymer chloroform solution at 25 °C. An X-ray photoelectron spectroscopy (XPS) investigation indicated that Ag(+) ions in the composite films were partially transformed to Ag atoms after irradiated by UV-light and completely transformed to Ag atoms after being treated with KBH(4) aqueous solution. Ag nanoparticles with the average sizes of 2.71 ± 0.82 and 3.28 ± 1.20 nm were generated in these two transferred films with different treatments, respectively. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images showed clearly that the composite films were composed of microcapsules whose walls had multilayer structures, and the nanoparticles were incorporated in the walls. The formation of the composite films at the liquid-liquid interface was attributed to the adsorption of the polymer molecules at the interface, coordination between the pyridine groups and Ag(+) ions, and self-assembly of the composite molecules. Furthermore, the catalytic activity of the composite films was evaluated using the reduction of 4-nitrophenol (4-NP) by KBH(4). The results demonstrated that the composite thin films have high and durable catalytic activity.

Synthesis and assembly of gold nanoparticle-doped polymer solid foam films at the liquid/liquid interface and their catalytic properties.

Gold nanoparticle-doped poly(2-vinylpyridine) (P2VP) microcapsules and foam films were synthesized and assembled at the P2VP chloroform solution/HAuCl(4) aqueous solution interface at 25 °C. It was found that Au nanoparticles with the average diameter of 2.1 nm were homogeneously embedded in and adsorbed on the walls of the capsules and foams, the nanoparticles were composed of Au(0) and Au(III) with the molar ratio of about 75/25, and the mass percent of Au elements was measured to be 19.65%. The formation of the nanostructures was attributed to the self-assembly of P2VP at the liquid/liquid interface, the simultaneous reduction of AuCl(4)(-) ions by a small amount of ethanol in the chloroform and adsorption of AuCl(4)(-) ions. After irradiated by UV-light for 1h, the average diameter of the nanoparticles was found to be 2.2 nm, and the AuCl(4)(-) ions were transformed to Au(0) completely. The catalytic performance of these composite nanostructures were evaluated by using the reduction of 4-nitrophenol (4-NP) by potassium borohydride in aqueous solutions. The catalytic activity was very high in the first cycle, decreased rapidly and slightly in the second and third cycles, respectively, due to the aggregation of some nanoparticles, and stabilized after the third cycle.

One-step synthesis and assembly of two-dimensional arrays of mercury sulfide nanocrystal aggregates at the air/water interface.

Ordered two-dimensional (2D) arrays of ß-HgS nanocrystal aggregates were prepared successfully at the air/water interface through the interfacial reaction between Hg(2+) ions in the subphase and H(2)S in the gaseous phase under the direction of liquid-expanded monolayers of arachidic acid (AA). These 2D arrays are composed of hexagonal or quasi-hexagonal aggregates with the size of several hundreds of nanometers that consist of several tens of HgS nanocrystals with the size of several nanometers. The formed HgS nanocrystals together with AA molecules self-assembled into round aggregates due to the interactions between the species, and the aggregates self-assembled into 2D arrays further due to the attractions between them. During the self-assembly process, the soft round aggregates transformed into hexagonal or quasi-hexagonal ones. The experimental conditions, especially the phase states of the AA monolayers and temperature, have great influences on the formation of the 2D arrays. To the best of our knowledge, this is the first case to get 2D ordered arrays at the air/water interface through a one-step synthesis and assembly process.