Influence of Temperature on the Formation of Silver Nanoparticles by using a Seed-Free Photochemical Method under Sodium-Lamp Irradiation.

Silver nanoparticles can be prepared by using a seed-free photo-assisted citrate reduction method under the irradiation of a sodium lamp. Under the same irradiation intensity, bath temperatures are crucial in influencing the reaction rate, morphologies of final products, and shape evolution of the silver nanostructures. For example, when the bath temperature is 80 °C, the product yields of silver nanoplates, nanorods, and nanodecahedra are 38±6 %, 35±10 %, and 12±8 %, respectively. However, when the bath temperature is 30 °C, the product yields of silver nanoplates, nanorods, and nanodecahedra are 6±3 %, 0 %, and 83±16 %, respectively. Time-dependent UV/Vis spectra and TEM images show that silver nanoplates were formed at the earlier reaction stage and greatly decreased in amount at the later stage when the bath temperatures are less than or equal to 40 °C. This indicates that the silver nanoplates, which can be regarded as intermediates, are kinetically favored products. They are not thermodynamically favored products at these relatively low bath temperatures. The SERS spectra of crystal violet (CV) show that all the silver colloids synthesized at various temperatures exhibit good enhancement factors and that the colloids prepared at lower bath temperatures have a higher enhancement factor.

Surface-functionalized hyperbranched poly(amido acid) magnetic nanocarriers for covalent immobilization of a bacterial γ-glutamyltranspeptidase.

In this study, we synthesized water-soluble hyperbranched poly(amido acid)s (HBPAAs) featuring multiple terminal CO2H units and internal tertiary amino and amido moieties and then used them in conjunction with an in situ Fe2+/Fe3+ co-precipitation process to prepare organic/magnetic nanocarriers comprising uniformly small magnetic iron oxide nanoparticles (NP) incorporated within the globular HBPAAs. Transmission electron microscopy revealed that the HBPAA-γ-Fe2O3 NPs had dimensions of 6-11 nm, significantly smaller than those of the pristine γ-Fe2O3 (20-30 nm). Subsequently, we covalently immobilized a bacterial γ-glutamyltranspeptidase (BlGGT) upon the HBPAA-γ-Fe2O3 nanocarriers through the formation of amide linkages in the presence of a coupling agent. Magnetization curves of the HBPAA-γ-Fe2O3/BlGGT composites measured at 300 K suggested superparamagnetic characteristics, with a saturation magnetization of 52 emu g⁻¹. The loading capacity of BlGGT on the HBPAA-γ-Fe2O3 nanocarriers was 16 mg g⁻¹ support; this sample provided a 48% recovery of the initial activity. The immobilized enzyme could be recycled 10 times with 32% retention of the initial activity; it had stability comparable with that of the free enzyme during a storage period of 63 days. The covalent immobilization and stability of the enzyme and the magnetization provided by the HBPAA-γ-Fe2O3 NPs suggests that this approach could be an economical means of depositing bioactive enzymes upon nanocarriers for BlGGT-mediated bio-catalysis.

Single-layered graphene oxide nanosheet/polyaniline hybrids fabricated through direct molecular exfoliation.

In this study, we used direct molecular exfoliation for the rapid, facile, large-scale fabrication of single-layered graphene oxide nanosheets (GOSs). Using macromolecular polyaniline (PANI) as a layered space enlarger, we readily and rapidly synthesized individual GOSs at room temperature through the in situ polymerization of aniline on the 2D GOS platform. The chemically modified GOS platelets formed unique 2D-layered GOS/PANI hybrids, with the PANI nanorods embedded between the GO interlayers and extended over the GO surface. X-ray diffraction revealed that intergallery expansion occurred in the GO basal spacing after the PANI nanorods had anchored and grown onto the surface of the GO layer. Transparent folding GOSs were, therefore, observed in transmission electron microscopy images. GOS/PANI nanohybrids possessing high conductivities and large work functions have the potential for application as electrode materials in optoelectronic devices. Our dispersion/exfoliation methodology is a facile means of preparing individual GOS platelets with high throughput, potentially expanding the applicability of nanographene oxide materials.

Non-Conventional Fluorescence and Cytotoxicity of Two Aliphatic Hyperbranched Polymer Dots Having Poly(amic acid) Structures: Implications for Labeling Nanodrug Carriers.

In this study, we used one-pot A2 + B3 polymerizations to synthesize two aliphatic + alicyclic polymer dots (PDs) having non-conjugated hyperbranched structures, employing two types of dianhydrides as the A2 components, possessing bridged bicyclic alkene (PD-BT) and non-alkene (PD-ET) units, and Jeffamine T403 polyetheramine (T403) as the B3 components. We prepared PD-ET from commercially available ethylenediaminetetraacetic dianhydride (EDTAD, A2) and T403 (B3) and PD-BT from bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA, A2) and T403 (B3). These two types of PDs possessed non-conjugated hyperbranched poly(amic acid) structures with terminal amino functional groups. PD-BT and PD-ET exhibited non-conventional fluorescence with emissions at 435 and 438 nm, respectively, and quantum yields of 12.8 and 14.0%, respectively. The fluorescence intensity of PD-ET was influenced by the pH, but PD-BT was less affected because of its rigid aliphatic bridged bicyclic structure. In aqueous solutions, the sizes of the PD-BT and PD-ET nanoparticles were 3-5 nm, and their net charges can be adjusted by varying the pH. These PDs were non-cytotoxic toward human MCF-7 breast cancer cells and human keratinocyte HaCaT cells at concentrations of 50 µg mL-1 for PD-BT and 500 µg mL-1 for PD-ET. Confocal microscopic bioimaging revealed that the PDs were located within the cells after treatment for 6 h. These PDs were easy to prepare, highly water-soluble, and possessed a large number of peripheral functional groups for further modification. Combined with their non-conventional fluorescence, they appear to have potential uses in bioimaging and as drug-labeling carriers.

Cytotoxicity and cell imaging of six types of carbon nanodots prepared through carbonization and hydrothermal processing of natural plant materials.

In this study we prepared six types of carbon nanodots (CNDs) from natural plant materials - through carbonization of two species of bamboo (Bamboo-I, Bamboo-II) and one type of wood (Wood), and through hydrothermal processing of the stem and root of the herb Mahonia oiwakensis Hayata (MO) and of the agricultural waste of two species of pineapple root (PA, PB). The resulting CNDs were spherical with dimensions on the nanoscale (3-7 nm); furthermore, CND-Bamboo I, CND-Wood, CND-Bamboo II, CND-MO, CND-PA, and CND-PB displayed fluorescence quantum yields of 9.63, 12.34, 0.90, 10.86, 0.35, and 0.71%, respectively. X-ray diffraction revealed that the carbon nanostructures possessed somewhat ordered and disordered lattices, as evidenced by broad signals at values of 2θ between 20 and 30°. CND-Bamboo I, CND-Wood, and CND-Bamboo II were obtained in yields of 2-3%; CND-MO, CND-PA, and CND-PB were obtained in yields of 17.64, 9.36, and 22.47%, respectively. Cytotoxicity assays for mouse macrophage RAW264.7 cells treated with the six types of CNDs and a commercial sample of Ag nanoparticles (NPs) revealed that each of our CNDs provided a cell viability of 90% at 2000 µg mL-1, whereas it was only 20% after treatment with the Ag NPs at 62.5 µg mL-1. The six types of CNDs also displayed low cytotoxicity toward human keratinocyte HacaT cells, human MCF-7 breast cancer cells, and HT-29 colon adenocarcinoma cells when treated at 500 µg mL-1. Moreover, confocal microscopic cell imaging revealed that the fluorescent CND-Bamboo I particles were located on the MCF-7 cell membrane and inside the cells after treatment for 6 and 24 h, respectively. We have thoroughly investigated the photoluminescence properties and carbon nanostructures of these highly dispersed CNDs. Because of the facile green synthesis of these six types of CNDs and their sourcing from abundant natural plants, herbs, and agriculture waste, these materials provide a cost-effective method, with low cytotoxicity and stable fluorescence, for biolabeling and for developing cell nanocarriers.

Synthesis and Properties of Quinoxaline-Containing Benzoxazines and Polybenzoxazines.

The object of this work is to prepare quinoxaline-based benzoxazines and evaluate thermal properties of their thermosets. For this object, 4,4'-(quinoxaline-2,3-diyl)diphenol (QDP)/furfurylamine-based benzoxazine (QDP-fu) and 4,4',4″,4‴-([6,6'-biquinoxaline]-2,2',3,3'-tetrayl)tetraphenol (BQTP)/furfurylamine-based benzoxazine (BQTP-fu) were prepared. The structures of QDP-fu and BQTP-fu were successfully confirmed by FTIR and 1H and 13C NMR spectra. We studied the curing behavior of QDP-fu and BQTP-fu and thermal properties of their thermosets. According to DSC thermograms, QDP-fu and BQTP-fu have the attractive onset exothermic temperatures of 181 and 186 °C, respectively. The onset temperature is approximately 45 °C lower than that of a bisphenol A/furfurylamine-based benzoxazines. According to DMA TMA and TGA thermograms, the thermoset of BQTP-fu shows impressive thermal properties, with a T g value of 418 °C, a coefficient of thermal expansion of 39 ppm/°C, a 5% decomposition temperature of 430 °C, and a char yield of 72%.

Low-Dissipation Thermosets Derived from Oligo(2,6-Dimethyl Phenylene Oxide)-Containing Benzoxazines.

Poly(2,6-dimethyl phenyl oxide) (PPO) is known for its low dissipation factor. To achieve insulating materials with low dissipation factors for high-frequency communication applications, a telechelic oligomer-type benzoxazine (P-APPO) and a main-chain type benzoxazine polymer (BPA-APPO) were prepared from an amine end-capped oligo (2,6-dimethyl phenylene oxide) (APPO). The APPO was prepared from a nucleophilic substitution of a phenol-end capped oligo (2,6-dimethyl phenylene oxide) (a commercial product, SA 90) with fluoronitrobenzene, and followed by catalytic hydrogenation. After self-curing or curing with a dicyclopentadiene-phenol epoxy (HP 7200), thermosets with high-Tg and low-dissipation factor can be achieved. Furthermore, the resulting epoxy thermosets show better thermal and dielectric properties than those of epoxy thermoset cured from its precursor SA90, demonstrating it is a successful modification in simultaneously enhancing the thermal and dielectric properties.

Using Dicyclopentadiene-Derived Polyarylates as Epoxy Curing Agents To Achieve High T g and Low Dielectric Epoxy Thermosets.

To achieve high-T g and low-dielectric epoxy thermosets, four dicyclopentadiene-derived polyarylates (26-P, 26-M, 236-P, and 236-M) were prepared from 2,6-dimethyl (or 2,3,6-trimethyl) phenol-dicyclopentadiene adduct with terephthaloyl (or isophthaloyl) chloride by high-temperature solution polymerization. The resulting polyarylates, exhibiting active ester linkages (Ph-O-(C=O)-) are found to be reactive toward a commercial dicyclopentadiene phenol epoxy (HP7200) in the presence of some lone-pair electron-containing compounds. Five compounds including 4-dimethylaminopyridine (DMAP), imidazole, 2-methylimidazole, triphenylphosphine, and triphenylimidazole have been evaluated as a catalyst for the curing reactions. We found that DMAP, with the smallest pK b among them, is the best catalyst according to differential scanning calorimetry, infrared, and thermal analyses. The thermal and dielectric properties of the polyarylate/HP7200 thermosets are evaluated. We found that they exhibit a high T g characteristic (e.g., T g is 238 °C for DMAP-catalyzed, 236-P/HP7200 thermoset). Furthermore, because of the hydrophobic methyl and cycloaliphatic moieties, and the secondary hydroxyl-free structure, polyarylate/HP7200 thermosets show a relative low-dielectric constant of around 2.75 U. The detailed structure-properties relationship is discussed in this work.

Phosphinated Poly(aryl ether)s with Acetic/Phenyl Methacrylic/Vinylbenzyl Ether Moieties for High-T g and Low-Dielectric Thermosets.

To achieve insulating materials with a low-dielectric characteristic for high-frequency communication applications, three phosphinated poly(aryl ether)s: P1-act (with acetic moiety), P1-mma (with phenyl methacrylic moiety), and P1-vbe (with vinylbenzyl ether moiety) were modified from a phenol-functionalized phosphinated poly(aryl ether) (P1). P1-act and P1-mma, both with active ester linkages (Ph-O-(C=O)-), were reacted with three commercial epoxy resins (diglycidyl ether of bisphenol A, HP7200, and cresol novolac epoxy) to obtain secondary hydroxyl-free epoxy thermosets. Because of the secondary hydroxyl-free structure, epoxy thermosets cured by P1-act and P1-mma show an 11-15% reduction in dielectric constant than those cured by P1. P1-vbe, with reactive vinylbenzyl ether moieties, was self-cured to a high-performance thermoset with a T g value as high as 302 °C and a dielectric constant as low as 2.64U. High-T g and low-dielectric thermosets have been developed in this work.

Facile immobilization of Bacillus licheniformis γ-glutamyltranspeptidase onto graphene oxide nanosheets and its application to the biocatalytic synthesis of γ-l-glutamyl peptides.

For the practical application of Bacillus licheniformis γ-glutamyltranspeptidase (BlGGT), we illustrated a simple and efficient approach to fabricate a biocatalytic system by immobilizing the enzyme onto graphene oxide (GO) nanosheets via both non-covalent (GO-BlGGT) and covalent (GO/GA-BlGGT) bonds. The enzyme-loading capacity for the prepared GO/GA nanomaterial was 3.47 mg/mg support, corresponding to 68.7% recovery of the initial activity. Native and enzyme-bound layered GOs were characterized by X-ray diffraction, followed by Raman and Fouier transform infrared spectroscopy, elemental analysis and thermogram analysis. As compared to the free form of BlGGT, the immobilized enzymes exhibited significantly higher activity, possibly due to the beneficial effect of the layered GO carrier. The kinetic behaviors of GO-BlGGT and GO/GA-BlGGT were mostly consistent with those of free enzyme. The covalently immobilized enzyme had a comparable stability respective to free enzyme during a storage period of 30 days and could be recycled nine times with 45.3% retention of the initial activity. Besides, the biocatalytic synthesis of γ-l-glutamyl-phenylalanine and γ-l-glutamyl-leucine by immobilized enzymes resulted in the product yield of more than 31%. Taken together, these results suggest that the facile strategy is an economical means of depositing bioactive enzymes upon GO nanosheets for BlGGT-mediated biocatalysis.

Low Dielectric Polyetherimides Derived from Bis[4-(4-(4-aminophenoxy)-2-tert-butylphenoxy)phenyl] Sulfone and 4,4'-Bis[4-(4-aminophenoxy)-2-tert-butylphenoxy]perfluorobiphenyl.

Two diamines, bis[4-(4-(4-aminophenoxy)-2-tert-butylphenoxy)phenyl] sulfone (PSNH2) and 4,4'-bis[4-(4-aminophenoxy)-2-tert-butylphenoxy)perfluorobiphenyl (PFNH2), were prepared from 3-tert-butyl-4-hydroxyanisole using a four-step procedure, including two nucleophilic substitutions, demethylation, and catalytic reduction. On the basis of PSNH2 and PFNH2, two series of low dielectric polyetherimides (PEIs) were prepared. Both series of PEIs exhibit moderate-to-high thermal properties, including a glass transition temperature (T g) > 259 °C (depending on the rigidity of dianhydride), a 5 wt % decomposition temperature (T d5%) > 496 °C, and a coefficient of thermal expansion < 66 ppm/°C. Because of the hydrophobic tert-butyl phenylene oxide structure, both series of PEIs show excellent dielectric properties, with a dielectric constant as low as 2.4-2.7. The structure-property relationship of both series of PEIs is discussed in this work.

Synthesis of a Bisbenzylideneacetone-Containing Benzoxazine and Its Photo- and Thermally Cured Thermoset.

A bis(4-hydroxybenzylidene)acetone/aniline-based benzoxazine (BHBA-a) was prepared from a bisbenzylidene-containing bisphenol, bis(4-hydroxybenzylidene)acetone (BHBA), aniline, and paraformaldehyde through Mannich condensation in a cosolvent of toluene/ethanol (2:1, v/v). The structure of BHBA-a was successfully confirmed by Fourier transform infrared and 1H and 13C NMR spectra. According to the differential scanning calorimetry (DSC) thermogram of BHBA, an immediate exothermic peak after the melting peak was observed, suggesting that BHBA is thermally active. NMR data of thermally treated BHBA confirm that the immediate exothermic peak after melting of BHBA in the DSC thermogram is resulted from the curing of a double bond. UV and 1H NMR spectra of BHBA-a show that the bisbenzylideneacetone moiety underwent dimerization through the [2π + 2π] cycloaddition. Therefore, two procedures were applied to cure BHBA-a. The first one was thermal curing of the double bond of bisbenzylideneacetone and oxazine moieties. The second one was photocuring of the bisbenzylideneacetone moiety, followed by thermal curing of the oxazine moiety. The thermal properties of thermosets were evaluated based on these two procedures. Thermosets of BHBA-a exhibit T g as high as 318 °C for curing procedure 1 and 342 °C for curing procedure 2. These values are much higher than that of a traditional bisphenol/aniline-based benzoxazine thermoset. We conclude that the thermal curing of the double bond of bisbenzylideneacetone and photodimerization of bisbenzylideneacetone contributes to the good thermal properties.

High-Tg, Low-Dielectric Epoxy Thermosets Derived from Methacrylate-Containing Polyimides.

Three methacrylate-containing polyimides (Px⁻MMA; x = 1⁻3) were prepared from the esterification of hydroxyl-containing polyimides (Px⁻OH; x = 1⁻3) with methacrylic anhydride. Px⁻MMA exhibits active ester linkages (Ph⁻O⁻C(=O)⁻) that can react with epoxy in the presence of 4-dimethylaminopyridine (DMAP), so Px⁻MMA acted as a curing agent for a dicyclopentadiene-phenol epoxy (HP7200) to prepare epoxy thermosets (Px⁻MMA/HP7200; x = 1⁻3) thermosets. For property comparisons, P1⁻OH/HP7200 thermosets were also prepared. The reaction between active ester and epoxy results in an ester linkage, which is less polar than secondary alcohol resulting from the reaction between phenolic OH and epoxy, so P1⁻MMA/HP7200 are more hydrophobic and exhibit better dielectric properties than P1⁻OH/HP7200. The double bond of methacrylate can cure at higher temperatures, leading to epoxy thermosets with a high-Tg and moderate-to-low dielectric properties.

Organic/Metallic nanohybrids based on amphiphilic dumbbell-shaped dendrimers.

In this study, we synthesized a series of amphiphilic dumbbell-shaped dendrimers through the addition reactions of a hydrophilic poly(oxyalkylene) with hydrophobic dendrons based on 4-isocyanate-4'-(3,3-dimethyl-2,4-dioxo-azetidine)diphenylmethane with different numbers of branching generations. The addition reaction of azetidine-2,4-diones of dendrons to amines of poly(oxyalkylene) was proceeded by stirring the reactants in dry tetrahydrofuran (THF) under nitrogen at 60 °C. In aqueous media, the dumbbell-shaped dendrimers self-assembled into micelles with their hydrophobic dendrons in the core and their hydrophilic poly(oxyalkylene) segments forming loops in the corona shell. Employing the unique self-assembled micelle structures as templates for subsequent chemical reduction of the Ag(+) ions, we generated new types of organic/metallic [silver nanoparticle (AgNP)] nanohybrid clusters. The long poly(oxyalkylene) loops that extended into the aqueous phase complexed with the Ag(+) ions, providing the suspension with steric stabilization to prevent the AgNPs from collision and flocculation. After reduction, the AgNPs were present in a homogeneous distribution in the round dendrimer micelle-stabilized nanoclusters. The diameter of each AgNP was less than 10 nm; the diameter of each round nanocluster was in the range of 50-200 nm. The encapsulation efficiency of the AgNPs in micelles was about 54-69% for the dumbbell-shaped dendrimer based organic/AgNP nanohybrid.

Orderly arranged NLO materials based on chromophore-containing dendrons on exfoliated layered templates.

Three chromophore-containing dendrons were intercalated into montmorillonite layered silicates via an ion-exchange process. Enlarged d spacings ranging from 50 to 126 A were achieved for these novel organoclays. After the organoclays were blended with a polyimide, the steric bulkiness of the dendrons and the interaction between dendron and polyimide resulted in an ordered morphology. The orderly arranged nanocomposites were characterized by a UV-visible spectrophotometer, a variable-temperature infrared spectrometer, and electro-optical modulation. The dendrons in layered silicates were capable of undergoing a critical conformational change into an ordered structure, indicated by the drastic changes of interlayer distances at certain packing densities. Electro-optical coefficients increased sharply from 0 to 6 pm/V while the conformational change occurred. Furthermore, the addition of a polyimide capable of interaction-induced orientation was found to exert an enhancing effect on the degree of the noncentrosymmetric alignment.

Preparation of protein-silicate hybrids from polyamine intercalation of layered montmorillonite.

Hybrids of the model BSA protein and layered silicate clay with d spacing of approximately 62 A were prepared from either direct or stepwise intercalation. The pristine montmorilloinite (Na+-MMT) was first modified by poly(oxyalkylene)-amine salts (POP- and POE-amine) of 2000 g/mol Mw to a gallery-expanded silicate (d spacing=53 and 18 A, respectively), which became accessible for BSA protein embedding. Subsequent BSA substitution allowed the embedding of the protein into the layered clay galleries in an uncompressed conformation. The stepwise process of embedding large molecules into the silicate gallery provides a new method for synthesizing biomaterial/clay hybrids potentially useful in drug delivery or biomedical design.