Residual Metal Catalyst Research Articles

Uniform Polymer Microspheres by Photoinduced Metal-Free Atom Transfer Radical Precipitation Polymerization.

Herein, a photoinduced method is introduced for the synthesis of highly cross-linked and uniform polymer microspheres by atom transfer radical polymerization (ATRP) at room temperature and in the absence of stabilizers or surfactants. Uniform particles are obtained at monomer concentrations as high as 10% (by volume), with polymers being exempt from contamination by residual transition metal catalysts, thereby overcoming the two major longstanding problems associated with thermally initiated ATRP-mediated precipitation polymerization. Moreover, the obtained particles have also immobilized ATRP initiators on their surface, which directly enables the controlled growth of densely grafted polymer layers with adjustable thickness and a well-defined chemical composition. The method is then employed successfully for the synthesis of molecularly imprinted polymer microspheres.

Metal scavenging resin tethered with catechol or gallol binders via reversible addition–fragmentation chain transfer polymerisation

Resin-supported metal scavengers are interesting in the separation and purification of target organic compounds and solvents because they avail a suitable and facile material platform for the adsorption of metal ions, thus enabling successive recycling. Here, a mussel-inspired catechol-functionalised or tunicate-inspired gallol-functionalised polymer, which was tethered to the Merrifield resin containing self-reducing hydroxyl groups that could bind well with metal ions, was synthesised via reversible addition–fragmentation chain transfer polymerisation. The number of functional groups on the surface of the resin was quantitatively controlled by modulating the ratio of the monomer to the resin-attached chain transfer agent. The metal ion adsorption capacity, as well as removal ability of the resin in an aqueous solution, which increased with the increasing number of the dihydroxy and trihydroxy groups, was investigated employing silver, copper, zinc, magnesium, lead, and iron ions. The polytrihydroxystyrene-bearing resin exhibited better metal ion-scavenging performance compared with the polydihydroxystyrene-incorporated resin at the same grafting degree in an aqueous solution owing to the tridentate structure of the gallol group, which acted as a ligand in the synthesis of the metal complex. The developed resins demonstrated that the residual metal catalyst that was derived from synthesising the conjugated polymer can be successfully removed. • Polydihydroxystyrene (PDHS) or polytrihydroxystyrene (PTHS) was incorporated into microbeads. • Reversible addition-fragmentation chain transfer agents afforded mussel-inspired catechol or tunicate-inspired gallol groups. • Hydroxyl group-containing polymers were synthesised on commercial resins to avail many metal-ion-binding sites. • The successive adsorption/desorption of metals from the PDHS or PTHS-immobilised resins demonstrated its reusability. • The metal scavenger resin was used to remove the Cu catalyst after the click reaction of the block copolymers.

Synthesis of star PMMA via ARGET ATRP towards application in acrylic bone cements

AbstractIn this study, we attempted to apply a four‐arm star poly(methyl methacrylate) (4sPMMA) as a prepolymer of acrylic bone cement. The 4sPMMA with viscosity average molecular weight of about 45 kDa was synthesized via Atom Transfer Radical Polymerization method using Activators Regenerated by Electron Transfer (ARGET ATRP). In practical use, the fine powder of precipitated 4sPMMA was changed to rigid beads followed by making sheet and pulverizing. Proton nuclear magnetic resonance (HNMR), Fourier transform infrared (FTIR) spectra of 4sPMMA beads indicated the star structure of 4sPMMA with bromide end groups (C‐Br). Scanning electron microscopy (SEM) image of 4sPMMA beads showed their random shapes with size in the range of 10‐100 μm, no traces of residual metal catalysts (Cu and Sn) were detected by Energy Dispersive X‐Ray (EDX) analysizer equipped with SEM instrument. Bone cement from 4sPMMA beads had been prepared manually and curing in rectangular and cylindrical molds. The setting characterization results showed that 4sPMMA bone cements exhibited lower maximum temperature (63.9 °C), lower setting temperature (45.6 °C), than those required in ISO 5833:2002. Other properties of 4sPMMA bone cement, such as bending strength, bending modulus and setting time met the requirements of above ISO standard.

Analysis of Single-Walled Carbon Nanotubes in Estuarine Sediments by Density Gradient Ultracentrifugation Coupled to Near-Infrared Fluorescence Spectroscopy Reveals Disassociation of Residual Metal Catalyst Nanoparticles.

The continued growth of the nanotechnology industry and the incorporation of nanomaterials into consumer applications will inevitably lead to their release into environmental systems. Single-walled carbon nanotubes (SWCNTs) in particular have exhibited many attractive optical, mechanical, and electrical properties that lend themselves to new and exciting applications. Assessing their environmental impact upon release into the environment is contingent upon quantifying and characterizing SWCNTs in environmental matrixes. In this study, SWCNTs were isolated from estuarine sediments using density gradient ultracentrifugation (DGU), followed by online flow-through analysis of the density fractions via near-infrared spectroscopy. This approach yielded significant improvements in the quantitative detection limit, from 62 to 1.5 μg g-1. In addition, fractions of the density gradient were also obtained for further analysis by bulk inductively coupled plasma mass spectrometry (ICP-MS) and single-particle ICP-MS. Using fluorescent, semiconductive SWCNTs, the primary fluorescent nanotube fraction was found to be separated from the sediment matrix during DGU; however, the residual metal catalyst particles that had been assumed to be physically bound to the SWCNTs were found to form a separate band in the density gradient apart from the fluorescent SWCNTs. This result was repeated for a number of SWCNT types regardless of the metal catalyst and synthesis method, with a 0.1 g cm-3 density difference between most fractions. The apparent disconnect between the fluorescent fraction of SWCNTs and their metal-containing constituents potentially complicates CNT risk assessment as analysis techniques focusing solely on either CNT fluorescence or metal fingerprints may misrepresent exposure concentrations and their toxicological implications.

Multi-walled carbon nanotubes photochemistry: A mechanistic view of the effect of impurities and oxygen-containing surface groups

Continuous photolysis experiments and transient absorption spectroscopy were performed in combination with other techniques including HRTEM, XPS, Raman, TGA, and ESR spectroscopy, to investigate the role of residual metals and amorphous carbon on the photochemical process taking place after 350–355 nm light irradiation of as obtained commercial multi-walled carbon nanotubes, denoted as pCNT.The results indicate that 350–355 nm photolysis of pCNT leads to the oxidation of surface oxygen-containing groups and defects which in turn are eliminated leading to more graphitic –like multi-walled carbon nanotubes (MWCNT). Residual metal catalysts and oxygen containing amorphous carbon and oxidized C-functionalities of MWCNT play an important role in the generation of MWCNT photoinduced charge-separated states. The process of 350 nm excitation of pCNT leads to exciton formation followed by hole transfer to metal oxides and further oxidation of C-O functionalities. A plausible mechanism explaining the elimination of oxidized groups attached to pCNT graphene walls and amorphous carbon and leading to more graphite-like CNTs is discussed. The results presented may have implications in the nanoscale semiconductor materials for optoelectronics applications.

Origin of Nonideal Graphene-Silicon Schottky Junction

In this paper, we investigate the origin of the nonideal turn- ON characteristics of the graphene-silicon Schottky junction. Native oxide (SiO2) is proved to play a critical role in determining the behavior of a graphene-Si junction. Within the metal–oxide–semiconductor structure, the effective voltage drop across the junction degrades due to the capacitor network, which contributes to an increased ideality factor. Residual metal catalysts are detected, which act as recombination centers in the silicon and further degrade the ideality factor by enhancing the recombination current. The recombination current is found to be the dominant factor in making the junction nonideal. Forming gas annealing and the insertion of an interfacial dielectric restore the ideality of a graphene-Si junction by reducing the interface states and bulk recombination centers.

Mechanisms of carbon nanotube-induced pulmonary fibrosis: a physicochemical characteristic perspective.

Carbon nanotubes (CNTs) are engineered nanomaterials (ENMs) with numerous beneficial applications. However, they could pose a risk to human health from occupational or consumer exposures. Rodent models demonstrate that exposure to CNTs via inhalation, instillation, or aspiration results in pulmonary fibrosis. The severity of the fibrogenic response is determined by various physicochemical properties of the nanomaterial such as residual metal catalyst content, rigidity, length, aggregation status, or surface charge. CNTs are also increasingly functionalized post-synthesis with organic or inorganic agents to modify or enhance surface properties. The mechanisms of CNT-induced fibrosis involve oxidative stress, innate immune responses of macrophages, cytokine and growth factor production, epithelial cell injury and death, expansion of the pulmonary myofibroblast population, and consequent extracellular matrix accumulation. A comprehensive understanding of how physicochemical properties affect the fibrogenic potential of various types of CNTs should be considered in combination with genetic variability and gain or loss of function of specific genes encoding secreted cytokines, enzymes, or intracellular cell signaling molecules. Here, we cover the current state of the literature on mechanisms of CNT-exposed pulmonary fibrosis in rodent models with a focus on physicochemical characteristics as principal drivers of the mechanisms leading to pulmonary fibrosis. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Respiratory Disease Toxicology and Regulatory Issues in Nanomedicine > Toxicology of Nanomaterials.

Atomic layer deposition of Al2O3 catalysts for narrow diameter distributed single-walled carbon nanotube arrays growth

Taking the importance of uniform size catalysts and the unfavorable effects of residual metal catalysts into consideration, herein, we develop a facile way which utilizes the typical and facile semiconducting technique namely atomic layer deposition (ALD) to prepare non-metallic Al2O3 catalysts for narrow diameter single-walled carbon nanotube (SWNT) arrays growth. As a result, we obtained SWNT arrays with narrow-diameter distribution of 1.26 ± 0.033 nm on the a-plane sapphire substrates. Moreover, the relationship between catalysts and SWNTs were explored. These optimized conditions pave an avenue for narrow diameter even chirality distribution in SWNTs growth, which promotes the development of structure-controlled and relative applications of SWNTs.

Correlation of surface treatment, dispersion and mechanical properties of HDPE/CNT nanocomposites

The effect of carbon nanotube treatment on the mechanical property of polyethylene/carbon nanotube composite (HDPE/CNT) was investigated. CNTs were initially treated with HCl and then with H2SO4/HNO3. Nanocomposites reinforced with untreated and treated CNTs were prepared by a mechanical mixture of the molten polymer. The results demonstrated a correlation among the surface treatment, dispersion and mechanical properties of HDPE/CNT composites. Raman spectroscopy and TGA analysis showed that both acid treatments removed efficiently amorphous carbon and residual metal catalysts of CNTs. However, these treatments not only removed impurities, they also decreased the crystallinity degree of CNTs due to the addition of oxygenated functional groups to the CNTs walls, as observed by XPS analysis. SEM micrographs revealed that the functional groups improved the CNTs dispersion in the polymeric matrix, resulting in an improvement of the mechanical properties of nanocomposites.

Electrochemical Performance of Carbon Onions Fabricated by Electric Arc‐Discharge Method

AbstractArc discharged carbon onions (CNOs) were purified through successive ignition, acid wash, surfactant‐assisted dispersion, filtration and centrifugation. Cyclic voltammetry of the multistage separated products with a ferro/ferricyanide redox probe was employed to measure the heterogeneous electron transfer (HET) rate. It was found that metal (nickel) encapsulating CNOs yielded in an HET rate about 60 times higher than that of hollow CNOs. The electrochemical performances of CNOs are interfered by the residual metal catalyst and nano‐graphitic debris, and are dependent on their varying morphology and structure. This work implicates that the metal encapsulating CNOs are applicable in the field of batteries or supercapacitors.

Analysis of metal catalyst content in magnetically filtered SWCNTs by SQUID magnetometry

Removal of the residual magnetic metal catalyst from the single-wall carbon nanotubes (SWCNTs) is important prerequisite for many further applications. We present here a facile analysis method enabling direct control of the removed fraction of the catalyst nanoparticles (NPs) after purification. Determination of distribution of the magnetic moments attributed to the catalyst NPs enables proper interpretation of the efficiency and mechanism of the used purification process. The study has been performed on the SWCNTs containing magnetic metal NPs, exposed to sonication and magnetic filtration. Two different SWCNT precursors (HiPco and laser ablation SWCNTs), three solvents and multiple filtration steps, respectively, have been tested. Magnetic property measurements are supported by the results of thermal decomposition and Raman spectroscopy.

Surface modification of multiwall carbon nanotubes determines the pro-inflammatory outcome in macrophage

Carbon nanotubes (CNTs) are widely used in industry and biomedicine. While several studies have focused on biological matters, attempts to systematically elucidate the toxicity mechanisms of CNTs are limited. The aim of the present study was to evaluate and compare the cytotoxicity of raw multi-walled carbon nanotubes (MWCNTs) and MWCNTs functionalized with carboxylation (MWCNTs–COOH) or polyethylene glycol (MWCNTs–PEG) in murine macrophages. Our results show that only MWCNTs–COOH and raw MWCNTs alter the oxidative potential of macrophages by increasing reactive oxygen species and the expression of pro-inflammatory factors in both a concentration- and surface coating-dependent manner. The data suggest that compare with raw MWCNTs and MWCNTs–PEG, the MWCNTs–COOH produces a significant increase in ROS generation, interruption of ATP synthesis, and activation of the MAPK and NF-κB signaling pathways, which in turn upregulates IL-1β, IL-6, TNF-α, and iNOS to trigger cell death. These findings suggest that contributory cellar uptake caused by physicochemical factors rather than residual metal catalysts plays a role in ROS-mediated pro-inflammatory responses in vitro.

Purification of Carbon Nanotube Sheets

Large volumes of carbon nanotube (CNT) sheets and yarns are required to enable practical applications such as structural reinforcements, electrical conductors, electromagnetic interference shielding, and energy storage. Although these macroscopic assemblages of CNTs are commercially available, reports on their handling and processing have been quite limited. In this article, we report a systematic study of the purification of commercially available CNT sheets produced directly from as‐grown nanotubes. These CNT sheets contain residual metal catalysts encapsulated in graphitic carbon nanoparticles known to contaminate common powdery pristine CNT materials. Although many purification methods effective for powdery CNTs are known, they cannot be directly applied to CNT sheets because these processes often involve sonication and/or reflux, which are too aggressive and deleterious to the integrity of the sheets. Therefore, alternative physically benign processes were chosen and evaluated in detail. Assessment of the purification method included characterization of parameters such as residual metal content, weight retention, and changes in the Raman D‐to‐G intensity ratio. Importantly, the purification methodology was also placed in the context of the targeted application as a structural reinforcement in advanced composites. In this application, the mechanical properties of the CNT sheets are crucial, thus specific strength and modulus were evaluated and compared for the purified CNT sheets and the nanocomposites produced therefrom. Several important processing parameters were identified that resulted in purified CNT sheets with not only low catalyst content but also reasonable retention of mechanical properties. In addition, the scalability of the purification method and a procedure to prevent CNT sheet wrinkling after purification were demonstrated. Assessment criteria of CNT sheet purification techniques toward other potential applications are also discussed.

Progress on Cytotoxicity <i>In Vitro</i> of Carbon Nanotubes

According to recent references, the progress on cytotoxicity in vitro of carbon nanotubes (CNTs) including the factors such as size , residual metal catalysts,and detection methods influencing cytotoxicity of CNTs are summarized. The results shown that residual metal catalysts must be removed as much as possible and ensure CNTs in some concentration range before using them.Biocompatibility of CNTs must be considered and chemical modification of CNTs to improve their use safety and advantages is indispensable.

Stability, metal leaching, photoactivity and toxicity in freshwater systems of commercial single wall carbon nanotubes

Carbon nanotubes (CNTs) are exciting new materials that have been intensively researched and are becoming increasingly used in consumer products. With rapid growth in production and use of CNTs in many applications, there is the potential for emissions to the environment and thus research is needed to assess the risks associated with CNTs in the environment. Here we show that commercial CNTs differ in their stability, photoactivity, metal leachate, and toxicity to freshwater algae. The behavior between raw and purified variants of the CNTs differs considerably; for example purified CNTs are generally more photoactive, producing singlet oxygen and superoxide, while raw CNTs show little or no photoactivity. Residual metal catalysts differ based on synthesis method used to prepare CNTs and thus may be comprised of elements with varying degrees of toxic potential. Influenced by pH and other constituents of the natural waters, our work shows that metals can leach out from all the commercial CNTs studied, even purified versions, albeit at different levels in many natural waters. As much as 10% of the total residual nickel leached from a purified CNT after 72 h. Aqueous concentrations of molybdenum leached from a different purified CNT were nearly 0.060 mg L−1 after 72 h. With little sample preparation, CNTs are dispersible in most freshwaters and stable for several days. Not all tested CNTs were toxic; for those CNTs that did induce toxicity we show that photoactivity, not metal leaching, contributes to the toxicity of commercial CNTs to freshwater algae, with growth rates significantly reduced by as much as 200%.

Structure and magnetic response of a residual metal catalyst in highly purified single walled carbon nanotubes

This article presents methods for detailed physical analysis of partial steps leading to the removal of residual metal catalyst nanoparticles (NPs) from single walled carbon nanotubes (SWCNTs) and options for detecting negligible amounts of metal in samples possessing diamagnetic response. Based on the previous knowledge of the composition, structure and magnetic properties of NPs included in the commercial HiPco_raw and HiPco_SP SWCNTs, the properties of remaining NPs after the multi-step purification (oxidation followed by mild acid treatment) and annealing both under static and dynamic vacuum have been investigated. Thermogravimetry, X-ray diffraction, static and dynamic magnetic property measurements and the Extended X-ray Absorption Fine Structure (EXAFS) experiments have been performed. The data provide information about the nature of the residual NPs in purified SWCNTs, which is crucial for further understanding of the purification processes and their improvement. It has been demonstrated that even if all macroscopic methods indicate a high purity of the treated sample, a non-negligible amount of the metal may still be present and the metal content has to be examined using local and element sensitive probes such as EXAFS.

Impact of Synthesis Methods on the Transport of Single Walled Carbon Nanotubes in the Aquatic Environment

In this study, a systematic approach has been followed to investigate the fate and transport of single walled carbon nanotubes (SWCNTs) from synthesis to environmentally relevant conditions. Three widely used SWCNT synthesis methods have been investigated in this study including high pressure carbon monoxide (HiPco), SWeNT CoMoCat, and electric arc discharge technique (EA). This study relates the transport of three SWCNTs (HiPco-D, SG65-D, and P2-D) with different synthesis methods and residual catalyst content revealing their influence on the subsequent fate of the nanotubes. To minimize nanotube bundling and aggregation, the SWCNTs were dispersed using the biocompatible triblock copolymer Pluronic, which allowed the comparison in the transport trends among these SWCNTs. After purification, the residual metal catalyst between the SWCNTs follows the trend: HiPco-D > SG65-D > P2-D. The electrophoretic mobility (EPM) and hydrodynamic diameter of SWCNTs remained insensitive to SWCNT type, pH, and presence of natural organic matter (NOM); but were affected by ionic strength (IS) and ion valence (K(+), Ca(2+)). In monovalent ions, the hydrodynamic diameter of SWCNTs was not influenced by IS, whereas larger aggregation was observed for HiPco-D with IS than P2-D and SG65-D in the presence of Ca(2+). Transport of HiPco-D in the porous media was significantly higher than SG65-D followed by P2-D. Release of HiPco-D from porous media was higher than SG65-D followed by P2-D, though negligible amount of all types of SWCNTs (<5%) was released. Both transport and release patterns follow a similar trend to what was observed for residual metal catalysts in SWCNTs. Addition of NOM increased the transport of all SWCNTs primarily due to electrosteric repulsion. HiPco-D was notably more acidic than SG65-D followed by P2-D, which is similar to the transport trend. Overall, it was observed that the synthesis methods resulted in distinctive breakthrough trends, which were correlated to metal content. These findings will facilitate the safe design of environmental friendly SWCNTs by minimizing mobility in aquatic environments.

Amorphous Carbon Impurities Play an Active Role in Redox Processes of Carbon Nanotubes

Carbon nanotubes (CNTs) produced via chemical vapor deposition (CVD) generally contain significant amounts of impurities ranging from residual metallic catalysts to nanographitic and amorphous carbon debris. Here, we show that in addition to the graphitic portion of impurities, also the amorphous carbon impurities significantly alter the electrochemical behavior of CNTs. We use a particular type of amorphous carbon black to specifically simulate amorphous carbon impurities and cyclic voltammetry with a ferro/ferricyanide redox probe to measure the heterogeneous electron transfer (HET) rate. The presence of about 30 wt % of amorphous carbon impurities resulted in an HET rate increased about 16 times compared to that of impurity-free CNTs. Electrochemical investigations of CNTs should, therefore, be performed only after a careful and complete purification from residual metal catalysts, nanographitic, and also amorphous carbon impurities. Our findings have direct implications for the construction of CNT-based electrochemical devices, such as batteries, supercapacitors, and sensors.

Magnetic Properties of Iron Catalyst Particles in HiPco Single Wall Carbon Nanotubes

We have investigated the magnetic response of residual metal catalyst in the raw and super purified HiPco single wall carbon nanotubes (HiPco_raw and HiPco_SP SWCNTs). It has been shown that the residual metal catalyst is in the form of nanoparticles, even in the HiPco_SP SWCNTs that should contain a minimal amount of the metal. Mössbauer spectroscopy of the HiPco_raw SWCNTs proved the catalyst nanoparticles are in the form of Fe3C. Analysis of the synchrotron X-ray diffraction data provided an average diameter of nanoparticles about 1.9 nm. Magnetic studies by means of temperature dependence of magnetization, magnetization isotherms and susceptibility suggested that the nanoparticles obey the behavior of weakly interacting superparamagnetic systems in both samples. Further analysis of the data revealed a core–shell structure of the nanoparticles in the HiPco_raw nanotubes, with a magnetically oriented core and a paramagnetic shell, which is almost removed in the case of the HiPco_SP catalyst nanoparticles.

Rapid purification/oxidation of multi-walled carbon nanotubes under 300 kHz-ultrasound and microwave irradiation

The use of ultrasound (US) and microwaves (MW) in the oxidation and purification of multi-walled carbon nanotubes (MWCNTs) was investigated. These techniques, in particular US at a frequency of 300 kHz, strongly accelerate the process and avoid the heavy structural damage, observed at the 20–35 kHz classic range, even at low power. Due to the residual metal catalyst on the head of MWCNTs, MW heating is strongly absorbed, causing the rupture of the tip and the loss of the metal. All our chemico-physical treatment types were performed by suspending the CNTs in a 3 ∶ 1 H2SO4/HNO3 mixture. The resulting samples were investigated by TEM microscopy, TGA analyses and Raman spectroscopy, while the degree of oxidation was estimated by colourimetric analyses.