Electrochemical aspects of coinage metal nanoparticles for catalysis and spectroscopy

Down scaling bulk materials can cause colloidal systems to evolve into microscopically dispersed insoluble particles. Herein, we describe the interesting applications of coinage metal nanoparticles (MNPs) as colloid dispersions especially gold and silver. The rich plasmon bands of gold and silver in the visible range are elaborated using the plasmon resonance and redox potential values of grown metal microelectrode (GME). The gradation of their standard reduction potential values (E⁰), as evaluated from the Gibbs free energy change for bulk metal, is ascribed to the variation in their size. Also, the effect of nucleophiles in the electrolytic cell with metal nanoparticles (MNPs) is described. The nucleophile-guided reduction potential value is considered, which is applicable even for bulk noble metals. Typically, a low value (as low as E⁰ = +0.40 V) causes the oxidation of metals at the O₂ (air)/H₂O interface. Under this condition, the oxidation of noble metal particles and dissolution of the noble metal in water are demonstrated. Thus, metal dissolution as a function of the size of metal nanoparticles becomes eventful and demonstrable with the addition of a surfactant to the solution. Interestingly, the reversal of the nobility of gold (Au) and silver (Ag) microelectrodes at the water/electrode interface is confirmed from the evolution of normal and inverted ‘core–shell’ structures, exploiting visible spectrophotometry and surface-enhanced Raman scattering (SERS) analysis. Subsequently, the effect of the size, shape, and facet- and support-selective catalysis of gold nanoparticles (NPs) and the effect of incident photons on current conversion without an applied potential are briefly discussed. Finally, the synergistic effect of the emissive behaviour of gold and silver clusters is productively exploited.

For noble metal, Mⁿ⁺/M_(atom) = reduction potential (V) values are negative.     

Vascular and plaque imaging with ultrasmall superparamagnetic particles of iron oxide

Cardiovascular Magnetic Resonance (CMR) has become a primary tool for non-invasive assessment of cardiovascular anatomy, pathology and function. Existing contrast agents have been utilised for the identification of infarction, fibrosis, perfusion deficits and for angiography. Novel ultrasmall superparamagnetic particles of iron oxide (USPIO) contrast agents that are taken up by inflammatory cells can detect cellular inflammation non-invasively using CMR, potentially aiding the diagnosis of inflammatory medical conditions, guiding their treatment and giving insight into their pathophysiology. In this review we describe the utilization of USPIO as a novel contrast agent in vascular disease.     

Electrorheology of SI-ATRP-modified graphene oxide particles with poly(butyl methacrylate): effect of reduction and compatibility with silicone oil

Surface-initiated atom transfer radical polymerization (SI-ATRP) was used to modify graphene oxide (GO) particles with poly(butyl methacrylate) (PBMA) chains. This procedure facilitated the single-step fabrication of a hybrid material with tailored conductivity for the preparation of a suspension in silicone oil with enhanced sedimentation stability and improved electrorheological (ER) activity. PBMA was characterized using various techniques, such as gel permeation chromatography (GPC) and ¹H NMR spectroscopy. Thermogravimetric analysis through on-line investigation of the Fourier transform infrared spectra, together with transmission electron microscopy, X-ray photoelectron microscopy, and atomic force microscopy, were successfully used to confirm GO surface modification. The ER performance was investigated using optical microscopy images and steady shear rheometry, and the mechanism of the internal chain-like structure formation was elucidated. The dielectric properties confirmed enhanced ER performance owing to an increase in relaxation strength to 1.36 and decrease in relaxation time to 5 × 10⁻³ s. The compatibility between GO and silicone oil was significantly influenced by covalently bonded PBMA polymer brushes on the GO surface, showing enhanced compatibility with silicone oil, which resulted in the considerably improved sedimentation stability. Furthermore, a controlled degree of reduction of the GO surface ensured that the suspension had improved ER properties.

Surface-initiated atom transfer radical polymerization (SI-ATRP) was used to modify graphene oxide (GO) particles with poly(butyl methacrylate) (PBMA) chains.     

Cocaine by-product detection with metal oxide semiconductor sensor arrays

A range of n-type and p-type metal oxide semiconductor gas sensors based on SnO₂ and Cr₂O₃ materials have been modified with zeolites H-ZSM-5, Na-A and H–Y to create a gas sensor array able to successfully detect a cocaine by-product, methyl benzoate, which is commonly targeted by detection dogs. Exposure to vapours was carried out with eleven sensors. Upon data analysis, four of these that offered promising qualities for detection were subsequently selected to understand whether machine learning methods would enable successful and accurate classification of gases. The capability of discrimination of the four sensor array was assessed against nine different vapours of interest; methyl benzoate, ethane, ethanol, nitrogen dioxide, ammonia, acetone, propane, butane, and toluene. When using the polykernel function (C = 200) in the Weka software – and just five seconds into the gas injection – the model was 94.1% accurate in successfully classifying the data. Although further work is necessary to bring the sensors to a standard of detection that is competitive with that of dogs, these results are very encouraging because they show the potential of metal oxide semiconductor sensors to rapidly detect a cocaine by-product in an inexpensive way.

Metal oxide semiconductor gas sensors based on SnO₂ and Cr₂O₃ were modified with zeolites H-ZSM-5, Na-A and H–Y to create a gas sensor array to detect cocaine by-product, methyl benzoate. SVMs were later used with a 4 sensor array to classify 9 gases of interest.     

Hollow silica-coated porous carbon with embedded iron oxide particles for effective methylene blue degradation

A novel catalyst, consisting of hollow silica-coated porous carbon with embedded iron oxide particles (FeO_x@C/SiO₂), was synthesized by the extended Stöber method. Iron ions were incorporated in a resorcinol–formaldehyde resin in the presence of citric acid to form a template, which was then coated with a silica layer. The iron oxide-embedded porous carbon and hollow silica were simultaneously formed during calcination under N₂ atmosphere. Through this process, silica endowed the iron oxide with low crystallinity and small size, resulting in a higher catalytic activity in the heterogeneous Fenton system for the decolorization of a methylene blue (MB) solution within 25 min. Moreover, the sample maintained 78.71% of its catalytic activity after three cycles.

A novel catalyst, hollow silica-coated porous carbon embedded with iron oxide (FeO_x@C/SiO₂), was synthesized by the extended Stöber method.     

Potentialities of bioinspired metal and metal oxide nanoparticles in biomedical sciences

To date, various reports have shown that metallic gold bhasma at the nanoscale form was used as medicine as early as 2500 B.C. in India, China, and Egypt. Owing to their unique physicochemical, biological, and electronic properties, they have broad utilities in energy, environment, agriculture and more recently, the biomedical field. The biomedical domain has been used in drug delivery, imaging, diagnostics, therapeutics, and biosensing applications. In this review, we will discuss and highlight the increasing control over metal and metal oxide nanoparticle structures as smart nanomaterials utilized in the biomedical domain to advance the role of biosynthesized nanoparticles for improving human health through wide applications in the targeted drug delivery, controlled release drug delivery, wound dressing, tissue scaffolding, and medical implants. In addition, we have discussed concerns related to the role of these types of nanoparticles as an anti-viral agent by majorly highlighting the ways to combat the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) pandemic, along with their prospects.

Bioinspired metallic nanoparticles (BMN) have revolutionized the biomedical domain and are still developing rapidly. Hence, this review on BMN elaborates the properties, biosynthesis, biomedical applications, and its role in combating the SARS-CoV-2.     

功能化纳米氧化石墨烯微粒对胶质瘤U251细胞的靶向光热作用

目的：观察在近红外线（near infrared,NIR）激光照射下功能化纳米氧化石墨烯（NGO-Tf-FITC ）微粒对脑胶质瘤 U251细胞的靶向光热作用。方法将已成功制备的单层纳米氧化石墨烯（nano-graphene oxide,NGO）偶联靶向分子转铁蛋白（transferrin,Tf）及荧光分子异硫氰酸荧光素（fluorescein isothiocyanate,FITC）的功能化NGO-Tf-FITC微粒,与U251脑胶质瘤细胞孵育培养,通过CCK8细胞活力检测判定其细胞毒性,并在808 nm NIR 照射后流式细胞仪检测对细胞杀伤效果。结果按0.1、1.0、3.0和5.0 mg/ml浓度NGO-Tf-FITC分4组与U251细胞孵育,48 h后酶标仪测吸光度值分别为0.747±0.031、0.732±0.043、0.698±0.051和0.682±0.039,空白组为0.759±0.052,各组与空白组比较差异均无统计学意义（P＞0.05）;NGO-Tf-FITC组、NGO-FITC组和空白对照组细胞凋亡和死亡指数分别为（73.6±3.41）%、（52.6±2.66）%和（51.2±2.93）%,NGO-Tf-FITC组分别与NGO-FITC组和空白组比较差异均有统计学意义（P＜0.05）,NGO-FITC组与空白组相比较差异无统计学意义（P＞0.05）。结论功能化NGO-Tf-FITC微粒细胞毒性低,且对脑胶质瘤U251细胞具有显著靶向光热杀灭作用。     

Supported discharge for patients with COPD

This article describes how an existing chronic obstructive pulmonary disease outreach service, which manages patients at home and attempts to avoid inappropriate hospital admission, could be improved by implementing an early supported discharge scheme as a collaborative effort between primary and secondary care. Consideration is given to the practicalities of successfully implementing this development using a model devised by Post (1989a, 1989b) as a framework on which to build a systematic approach. It is hypothesised that implementing such a change would provide patient-centred care and increased patient choice, as well as relieving pressures in the acute hospital sector.     

Oxygen release from metal oxide for repeated hydrogen regeneration by proton irradiation with polyvinylpyrrolidone

In this study, we investigated the reduction of a 3D microporous NiO_x structure, used as a metal oxide catalyst, by proton irradiation with polyvinylpyrrolidone (PVP) for hydrogen regeneration. In general, the reduction process for hydrogen regeneration requires high temperatures (1000–4000 °C) to release saturated oxygen from the metal oxide catalyst. Proton irradiation with PVP could regenerate abundant oxygen vacancies by releasing the oxygen attached to NiO_x at room temperature. The 3D microporous NiO_x structure provided the maximum hydrogen generation rate of ∼4.2 μmol min⁻¹ g⁻¹ with the total amount of generated hydrogen being ∼460 μmol g⁻¹ even in the repetitive thermochemical cycle; these results are similar to the initial hydrogen generation data. Therefore, continuous regeneration of hydrogen from the oxygen-reduced 3D microporous NiO_x structure was possible. It is expected that the high thermal energy, which is the major problem associated with hydrogen regeneration through the conventional heat treatment method, would be resolved in future using such a method.

The reduction of a 3D microporous NiO_x structure, used as a metal oxide catalyst, was performed by proton irradiation with polyvinylpyrrolidone (PVP) for hydrogen regeneration.     

Graphene oxide lamellar membrane with enlarged inter-layer spacing for fast preconcentration and determination of trace metal ions

We report a graphene oxide (GO) lamellar membrane with increased inter-layer spacing for efficient permeation of water molecules and heavy metal ions through nanoporous graphene oxide. The inter-layer spacing of the GO sheets in the lamellar structure was increased by introducing poly-aminophosphonic acid (APA) in between the GO sheets. We demonstrate experimentally, the use of a prepared membrane (GO–APA) by a SPE technique for the preconcentration and extraction of heavy metal ions by chelate formation and their determination by ICP-OES. We found that this sub-micrometer-thick membrane allows unimpeded permeation of water molecules through two-dimensional capillaries formed across the pores and by closely spaced graphene sheets. Compared to the bulk GO sorbent, GO–APA membrane offers enhanced sensitivity and permeability for heavy metal ions due to relatively large inter-layer spacing and high surface area (extraction phase) with a high number of active functional groups. The potential of this technique for the preconcentration and extraction of Pb(ii), Cd(ii) and Cu(ii) is illustrated with the contaminated ground water and industrial waste water analysis. The detection limit achieved for studied ions was 1.1 ng L⁻¹, under optimized experimental conditions. The co-existing ions did not hinders the extraction of trace heavy metal ions. Accuracy of the developed method was assessed by analyzing Standard Reference Materials. The Student''s t test values were found to less than the critical Student''s t value of 4.303 at the 95% confidence level. The method shows good precision as coefficients of variation for five replicate measurements were found to be 4–5%.

A porous graphene oxide membrane with increased interlayer spacing of GO sheets was prepared by covalently introducing poly-aminophosphonic acid in between the GO sheets. The membrane was successfully employed for the extraction of heavy metal ions.     

Acid–base sites synergistic catalysis over Mg–Zr–Al mixed metal oxide toward synthesis of diethyl carbonate

In heterogeneous catalysis processes, development of high-performance acid–base sites synergistic catalysis has drawn increasing attention. In this work, we prepared Mg/Zr/Al mixed metal oxides (denoted as Mg₂Zr_xAl_1−x–MMO) derived from Mg–Zr–Al layered double hydroxides (LDHs) precursors. Their catalytic performance toward the synthesis of diethyl carbonate (DEC) from urea and ethanol was studied in detail, and the highest catalytic activity was obtained over the Mg₂Zr_0.53Al_0.47MMO catalyst (DEC yield: 37.6%). By establishing correlation between the catalytic performance and Lewis acid–base sites measured by NH₃-TPD and CO₂-TPD, it is found that both weak acid site and medium strength base site contribute to the overall yield of DEC, which demonstrates an acid–base synergistic catalysis in this reaction. In addition, in situ Fourier transform infrared spectroscopy (in situ FTIR) measurements reveal that the Lewis base site activates ethanol to give ethoxide species; while Lewis acid site facilitates the activated adsorption of urea and the intermediate ethyl carbamate (EC). Therefore, this work provides an effective method for the preparation of tunable acid–base catalysts based on LDHs precursor approach, which can be potentially used in cooperative acid–base catalysis reaction.

Mg/Zr/Al mixed metal oxides were prepared via a facile phase transformation process of hydrotalcite precursors, which showed acid–base sites synergistic catalytic performance toward the synthesis of diethyl carbonate from ethanol and urea.     

Synthesis and application in asymmetric catalysis of P-stereogenic pincer–metal complexes

P-stereogenic pincer–metal complexes are one of the most interesting pincer type organometallic compounds. Many kinds of this type of complexes were synthesized and used as catalysts in asymmetric catalysis. On the basis of our work in this field, this paper reports the recent progress in P-stereogenic pincer chemistry, including the synthesis of P-stereogenic pincer ligands, the synthesis of P-stereogenic pincer–metal complexes, and the achievements in P-stereogenic pincer–metal complex catalyzed asymmetric synthesis.

P-stereogenic pincer: synthesis and application in asymmetric catalysis.     

Effectiveness of metal oxide catalysts for the degradation of 1,4-dioxane

1,4-dioxane, commonly used as a solvent stabilizer and industrial solvent, is an environmental contaminant and probable carcinogen. In this study, we explored the concept of using metal oxides to activate H₂O₂ catalytically at neutral pH in the dark for 1,4-dioxane degradation. Based on batch kinetics measurements, materials that displayed the most suitable characteristics (high 1,4-dioxane degradation activity and high H₂O₂ consumption efficiency) were ZrO₂, WO_x/ZrO₂, and CuO. In contrast, materials like TiO₂, WO₃, and aluminosilicate zeolite Y exhibited both low 1,4-dioxane degradation and H₂O₂ consumption activities. Other materials (e.g., Fe₂O₃ and CeO₂) consumed H₂O₂ rapidly, however 1,4-dioxane degradation was negligible. The supported metal oxide WO_x/ZrO₂ was the most active for 1,4-dioxane degradation and had higher H₂O₂ consumption efficiency compared to ZrO₂. In situ acetonitrile poisoning and FTIR spectroscopy results indicate different surface acid sites for 1,4-dioxane and H₂O₂ adsorption and reaction. Electron paramagnetic resonance measurements indicate that H₂O₂ forms hydroxyl radicals (˙OH) in the presence of CuO, and unusually, forms superoxide/peroxyl radicals (˙O₂⁻) in the presence of WO_x/ZrO₂. The identified material properties suggest metal oxides/H₂O₂ as a potential advanced oxidation process in the treatment of 1,4-dioxane and other recalcitrant organic compounds.

FTIR and surface poisoning results suggest 1,4-dioxane adsorbs to Lewis acid sites on metal oxide surfaces and is degraded by surface radical species.     

Curcumin-loaded metal oxide aerogels: supercritical drying and stability

Curcumin, known as a potential antioxidant and anti-inflammatory agent, has major limitations for its therapeutic use because of its lack of water solubility and relatively low bioavailability. We report for the first time the loading of different metal oxide aerogels with curcumin. The aerogels were prepared via the sol–gel process and dried under supercritical conditions. Mixing curcumin with the metal precursors prior to the formation of the solid network ensures maximum entrapment. The curcumin–network interactions stabilize the organic moiety and create hybrid aerogels as potential vehicles for curcumin in various media. The aerogels were characterized by FTIR spectroscopy, thermogravimetric analysis, electron microscopy, and fluorescence spectroscopy to confirm their hybrid nature. The stability study by fluorescence spectroscopy revealed three distinct behaviors depending on the nature of the metal oxide: (i) a minor interaction between curcumin and the solid network slightly affecting the microenvironment; (ii) a quenching phenomenon when iron is present explained by a coordination between the iron ions and curcumin; and (iii) a strong complexation of the metal ions with curcumin after gelation.

Metal oxide aerogels are investigated as encapsulation media for curcumin, a polyphenol having potential uses in medicine, probing, and sensing.     

Formulation of re-dispersible dry o/w emulsions using cellulose nanocrystals decorated with metal/metal oxide nanoparticles

This study describes for the first time the preparation of re-dispersible surfactant-free dry eicosane oil emulsion using cellulose nanocrystals (CNCs) using the freeze-drying technique. Surface properties of CNCs constitute a critical point for the stability of o/w emulsions and thus can affect both the droplet size and dispersion properties of the emulsion. Therefore, surface modification of CNCs was performed to understand its effect on the size of the obtained re-dispersible dry o/w eicosane emulsion. Decoration of the CNC surface with metal and metal oxide nanoparticles was conducted through the available alcoholic groups of glycosidic units of CNC, which played a dual role in reducing and stabilizing nanoparticles. Of these nanoparticles, silver (AgNPs), gold (AuNPs), copper oxide (CuO-NPs), and iron oxide (Fe₃O₄-NPs) nanoparticles were prepared via a facile route using alkali activated CNCs. Thorough characterizations pertaining to the as-prepared nanoparticles and their re-dispersible dry eicosane o/w emulsions were investigated using UV-vis spectroscopy, TEM, XRD, particle size, zeta potential, and STEM. Results confirmed the ability of CNCs to stabilize and/or reduce the formed nanoparticles with different sizes and shapes. These nanoparticles showed different shapes and surface charges accompanied by individual morphologies, reflecting on the stability of the re-dispersed dry eicosane emulsions with droplet sizes varying from 1.25 to 0.5 μm.

Schematic diagram for the detailed preparation of dry eicosane o/w emulsions.     

Iron oxide and various metal oxide nanotubes engineered by one-pot double galvanic replacement based on reduction potential hierarchy of metal templates and ion precursors

A one-pot double galvanic approach was explored for the rational synthesis of metal oxide nanotubes, predictable based on the reduction potential hierarchy of templates and ion precursors (e.g., Ag nanowire substrate is oxidized by MnO₄⁻ ions and it is consecutively reduced by Fe²⁺ ions to form an Fe₂O₃ nanotube). This method generated a variety of metal oxide nanotubes via a redox potential landscape.

A one-pot double galvanic approach was explored for the rational synthesis of a variety of metal oxide nanotubes from Ag nanowire substrates, predictable based on the reduction potential hierarchy of the templates and ion precursors.

A variety of inorganic nanoparticles with different shapes and sizes have been synthesized since these parameters could be used for fine-tuning their physical and biological properties.¹ Among them, one dimensional (1D) nanostructures are pivotal due to their characteristic electrical, magnetic and chemical properties.^2,3 Free standing hollow 1D metal oxide nanoparticles represent an intriguing class of nanomaterials due to their high surface to volume ratio and low density,^4,5 leading to distinct properties in catalysis,⁶ energy storage,⁷ gas sensors,⁸ photodetectors,² and in biomedical applications such as drug delivery.⁹Previously, metal oxide nanotubes were synthesized by techniques such as template-mediated and template-free methods which include hydrothermal reaction, and chemical etching method.^4,10 In the template-mediated method, post-treatment is necessary to remove templates. Thus, it adds complexity to the synthesis process, increases frequency of structural deformation, and introduces more impurities even after undergoing a separation process for the removal of left-over template in the sample.⁷ Moreover, the size of the nanotube is also limited by the size of the template, which is often larger than 200 nm.⁵ Hydrothermal synthesis of metal oxide nanotubes has disadvantages such as difficulty of controlling structure in high aspect ratios.^11–13 The chemical etching method typically requires specialized equipment to react the precursors in high temperature, controlled atmosphere and pressure conditions, increasing production costs.¹⁴One of the most effective and versatile sacrificial template methods for synthesizing hollow metal nanoparticles with controllable composition and size is galvanic replacement.^6,15 Galvanic replacement is a corrosion process that is driven by the difference in reduction potentials between a metallic substrate and metal ions in solution.¹ The advantage of utilizing the galvanic mechanism for nanomaterial synthesis is that the resulting hollow nanoparticles can be rationally designed by using precursors that have staggered reduction potentials in their exchange reactions. Upon contact between such metal substrate and ions in solution, the one with lower reduction potential is oxidized, while the other with higher reduction potential is reduced, and as a result, metal ions from solution are plated onto the template.^1,14As these metals are exchanged, the final product typically possesses a shape similar to the original substrate, but the element of substrate is then swapped to metal ions from solution. Recently, element exchange triggered by surface energy difference has been applied to engineer various hollow metal nanoparticles,¹⁶ and Xia group has pioneered the application of galvanic exchange reactions in the synthesis of Au, Pt and Pd hollow metal nanoparticles from Ag nanoparticle substrates.^17,18 However, this approach has rarely been applied for the synthesis of 1D metal oxide hollow nanostructures starting from sacrificial silver nanowire substrates.Here, we demonstrate for the first time the synthesis of hollow iron oxide nanotubes starting from silver nanowires using one-pot, two-step galvanic replacement reactions. It should be noted that direct replacement of Ag to Fe₂O₃ is not viable considering both species need to oxidize simultaneously, and that is our motivation for developing the one-pot, double galvanic exchange reactions of nanotubes. This method can also be generalized for rational synthesis of other metal oxide nanotubes as the formation is predicted by redox potentials of involved ion species. Fig. 1 summarizes the synthesis scheme of iron oxide nanotubes starting from silver nanowires as substrate. Here, it should be noted that the reactions are broken down into two steps for the ease of understanding the reaction mechanism, while all of these reactions are done in one pot. The galvanic replacement reaction is driven by the differences in reduction potentials between the two species involved.¹⁵ In Fig. 1a, Ag is dissolved into solution and electrons through Ag oxidation reduce MnO₄⁻ ions in solution to Mn₃O₄. The replacement to Mn₃O₄ in nanotubes takes place due to the redox potential landscape between Ag and MnO₄⁻ (Fig. 1c) since the absolute value of reduction potential of MnO₄⁻ (1.47 V) is larger than the one for Ag (−0.8 V) as shown in Fig. 1c.¹⁹ This first galvanic replacement reaction was initiated by mixing silver nanowires (5 mg mL⁻¹, 3 mL) of 20 nm in diameter dispersed in water with an aqueous solution of potassium permanganate (1 mM, 18.95 mL) and subsequently heating the reaction mixture to 100 °C for 40 minutes.¹⁹ At this point, the shape of the original nanowire (Fig. 2a) has already transformed into a hollow nanotube (Fig. 2b).Open in a separate windowFig. 1General concept of one-pot double galvanic exchange reactions for the synthesis of iron oxide nanotubes. (a) The first galvanic exchange reaction to generate Mn₃O₄ nanotubes from Ag nanowire substrate. (b) The second galvanic exchange reaction to replace Mn to Fe nanotubes. (c) Reduction potential landscape for iron oxide and other possible metal oxide in nanotube formation (half-reactions and their electric reduction potentials are also shown in ESI Table S-1^†). When the reduction potential of one element is larger than the other, the element with lower reduction potential (e.g., Fe) can donate its electrons to reduce Mn₃O₄, which has a larger reduction potential (see blue arrows for the direction of electron flow). A variety of metal oxide nanotubes in addition to iron oxide can be formed when the final element of the nanotube is oxidized due to the large reduction potential of the intermediate Mn₃O₄ nanotube.Open in a separate windowFig. 2TEM images of (a) Ag nanowire substrates, (b) Mn₃O₄ nanotubes, (c) Fe₂O₃ nanotubes, (d) a section of Mn₃O₄ nanotubes where pinholes were observed on the surface (red circles), (e) pinholes in (d) with a higher magnification.In the second galvanic exchange reaction (Fig. 1b), Mn₃O₄ nanotubes from galvanic reaction 1 are transformed to Fe₂O₃ nanotubes as Mn ions are replaced by Fe ions. The redox potential difference is aligned favourably for the oxidation of Fe ions (−0.77 V) via the reduction of Mn ions (1.82 V) (Fig. 1c) during this process. Fe₂O₃ nanotubes were generated when Mn₃O₄ nanotubes were mixed with iron(ii) perchlorate solution (1 mg mL⁻¹, 12 mL) at 80 °C for 2 hours. The resulting sample is then centrifuged at 3000g for 10 minutes, and the pellet is resuspended in deionized water for analysis. Transmission electron microscopic (TEM) image of Fe₂O₃ nanotubes (Fig. 2c) reveals that the diameter of nanotubes increases from 27.0 ± 2.0 nm in Mn₃O₄ nanotubes to 40.0 ± 3.0 nm in Fe₂O₃ nanotubes. The elemental mapping of energy-dispersive X-ray spectroscopy (EDXS) was also applied to analyze the elemental replacement in the process of one-pot galvanic reaction in Fig. 1. When intermediate products of nanotubes after galvanic reaction 1 (Fig. 3a) were probed by EDXS mapping with scanning transmission electron microscopy (STEM), the composition was mostly Mn (Fig. 3c) and O (Fig. 3d) with minimal leftover Ag substrate (Fig. 3b). The second galvanic exchange reaction generated Fe₂O₃ nanotubes (Fig. 3e) in high yield and Mn ions were successfully exchanged by Fe ions (Fig. 3i), and the resulting nanotubes were almost free from Ag (Fig. 3f) and Mn (Fig. 3g).Open in a separate windowFig. 3(a) Scanning transmission electron microscopic (STEM) image of Mn₃O₄ nanotubes and corresponding elemental mapping in (b) Ag, (c) Mn, and (d) O with EDXS. (e) STEM image of Fe₂O₃ nanotubes and corresponding elemental mapping in (f) Ag, (g) Mn, (h) O, and (i) Fe with EDXS.When the surface structure of Mn₃O₄ nanotubes was examined by high resolution TEM (HRTEM), pinholes on the sidewalls of Mn₃O₄ nanotubes were revealed where the dark contrast of remaining Ag inside make these holes more visible (inside red circles in Fig. 2d). These pinholes are even more evident in a HRTEM image in Fig. 2e. This observation led to the hypothesis that the hollowing process of template nanowires could occur through the pinhole dissolution mechanism. Pinhole corrosion is the process where pinholes serve as paths of material transport during the dissolution of the core of template nanoparticles in the course of the redox reactions.¹⁵ In this mechanism, the stoichiometry of redox reaction is important to determine the degree of atomic replacement and hollowing process which in turn affects the nanotube''s composition and sidewall thickness.²⁰ In the hollowing process of Ag nanowires, the Ag → Mn replacement reaction is;16H_(aq)⁺ + 3MnO_4(aq)⁻ + 13Ag_(s) → Mn₃O_4(s) + 13Ag_(aq)⁺ + 8H₂O_(l)In this reaction, one Mn₃O₄ is produced at the expense of 13Ag atoms based on the stoichiometric ratio above. Such high turnover of Ag replacement through the efficient pinhole dissolution paths should guide the redox reaction to nearly complete replacement, which is difficult to be accomplished solely by room temperature Kirkendall effect (i.e., the diffusion of Ag from template nanowires is slow at relatively low reaction temperature).²¹ In addition, the atomic% ratio of Ag versus Mn of the Mn₃O₄ nanotubes is 0.1 : 1.0 by the STEM-EDX mappings of Fig. 3b and c, showing efficient Ag atom replacement. This result also supports that the transformation from Ag nanowire to Mn₃O₄ nanotubes is mainly driven by the pinhole dissolution mechanism and that the Kirkendall effect may play only a minor role in the replacement reaction.¹⁵ Based on TEM images, the wall thickness of Mn₃O₄ nanotubes is 5.1 ± 2.0 nm (n = 23) (Fig. 2b) after Ag nanowire templates with diameters of 20 ± 2.0 nm (Fig. 2a) are consumed, which is consistent with the previous observations about the degree of sidewall thickness with respect to the diameter of templates with similar stoichiometry in their replacement reactions.^20,22 Further atomic replacement from Mn to Fe occurs by the reaction;8H_(aq)⁺ + Mn₃O_4(s) + 2Fe_(aq)²⁺ → 3Mn_(aq)²⁺ + 4H₂O_(l) + 2Fe_(s)³⁺where two Fe ions are produced by replacing one Mn₃O₄. Since numerous lattice vacancies would be formed on the sidewall of Mn₃O₄ nanotubes (due to high atomic replacement ratio of Ag-to-Mn in 13 : 1) and the possibility of these vacancies to coalesce to form pinholes,²³ the replacement from Mn to Fe is efficient through pinholes. Such processes will further deplete the amount of Ag in the resulting Fe₂O₃ nanotubes, thus, it is reasonable to observe almost complete replacement of Ag in Fe₂O₃ nanotubes with atomic% ratio of Ag versus Fe of 0.006 : 1 from Fig. 3f and i. Moreover, due to the atomic ratio of this replacement, it is plausible to observe the wall thickness of Fe₂O₃ nanotubes to increase to 11.2 ± 3.2 nm (n = 23) (Fig. 2c).Another supporting observation for the pinhole dissolution mechanism is that the volume of voids inside Ag nanowires increases during the transformation to Mn₃O₄ nanotubes with the concentration of Mn precursor. When various concentrations of KMnO₄ were reacted with Ag nanowire solution, a series of TEM images in Fig. S1^† showed that void formation is expanded by increasing the amount of KMnO₄. Since more Mn ions consume Ag ions with efficient atomic replacement through pinholes, these TEM images support the pinhole dissolution mechanism as a major pathway. In summary, all evidence here supports that pinhole dissolution is the main mechanism for the synthesis of Fe₂O₃ nanotubes from Ag nanowire templates.Since manganese possesses a very high reduction potential of 1.82 V, it is feasible to be utilized as a template for a variety of metal oxide nanotubes. Thus, this enables the design of a variety of metal oxide nanotube syntheses, as long as the replacing metal ions can be oxidized and have reduction potentials smaller than 1.82 V (in absolute values). Due to the high reduction potential of Mn ions, there are many metal ions that align with this reduction potential hierarchy for the second galvanic replacement as shown in Fig. 1c. To test the generality of this one-pot, double galvanic method for metal oxide nanotube synthesis, we examined the synthesis of NiO₂, CuO, and SnO₂ nanotubes from Ag nanowire substrates. In terms of the redox potential landscape, all of the reduction potentials are appropriate for one-pot double galvanic exchange as the reduction potential of NiO₂/Ni²⁺, SnO₂/Sn²⁺ and Cu²⁺/Cu⁺ is 1.68 V, −0.094 V, and 0.161 V, respectively.^24,25 In theory, these ions can reduce Mn₃O₄ (1.82 V) during the second galvanic replacement reaction because their reduction potentials in absolute values are lower than Mn₃O₄. In this trial, tin(ii) chloride, copper(i) chloride, and nickel(ii) chloride were used as precursor solutions to synthesize SnO₂, CuO, and NiO₂ nanotubes. TEM images (Fig. 4a, d and g) indicate that all of these nanotubes possess a hollow structure with an average diameter of 40 nm, suggesting that their size was controlled by the size of Ag nanowire substrate. The elemental transformation of template nanowires to target nanotubes was confirmed by EDXS maps in Fig. 4b, c, e, f, h and i.Open in a separate windowFig. 4(a) TEM image of SnO₂ nanotubes and elemental mapping of (b) Sn and (c) O with EDXS. (d) TEM image of CuO nanotubes and elemental mapping of (e) Cu and (f) O with EDXS. (g) TEM image of NiO₂ nanotubes and elemental mapping of (h) Ni and (i) O with EDXS.In conclusion, we combined two galvanic reactions in one-pot for the synthesis of iron oxide nanotubes from commercially available Ag nanowires. The large reduction potential of the Mn₃O₄ nanotube intermediate opens this method to final exchange by various metal ions to form many different metal oxide nanotubes. We have demonstrated it by the synthesis of other metal oxide nanotubes such as SnO₂, CuO, and NiO₂, which have favorable staggered redox potential arrangement with Mn₃O₄ reduction. This generalized double galvanic replacement approach will offer robust, economical, and scale-up engineering of a variety of hollow one-dimensional metal oxide nanostructures.     

Ultra-wideband self-powered photodetector based on suspended reduced graphene oxide with asymmetric metal contacts

The ultraviolet to terahertz band forms the main focus of optoelectronics research, while light detection in different bands generally requires the use of different materials and processing methods. However, researchers are aiming to realize multi-band detection simultaneously in the same device in certain specific application scenarios and ultra-wideband photoelectric detectors can also realize multi-function and multi-system integration. Therefore, the research and development work on ultra-wideband photoelectric detectors has important practical application value. Here, we produced self-powered suspended Pd-reduced graphene oxide-Ti (Pd-rGO-Ti) photodetectors. We varied the properties of the rGO films by using different annealing temperatures and achieved p-doping and n-doping of the films by evaporating palladium films and titanium films, respectively, thus enabling preparation of photothermoelectric (PTE) photodetectors based on rGO films. The resulting detectors have excellent photoelectric responses over a wideband illumination wavelength range from 375 nm to 118.8 μm (2.52 THz). At the same time, we determined the best experimental conditions and device structure by varying the channel width, the laser spot irradiation position and the experimental atmospheric pressure. The maximum responsivity obtained from our detectors is 142.08 mV W⁻¹, the response time is approximately 100–200 ms and the devices have high detection sensitivity. Based on this work, we assumed in the subsequent experiments that detectors with higher performance can be obtained by reducing the channel width and atmospheric pressure. With advantages that include simple fabrication, low cost, large-scale production potential and ultra-wideband responses, these Pd-rGO-Ti photodetectors have broad application prospects in high-performance integrated optoelectronics.

An ultra-wideband self-powered photodetector based on suspended reduced graphene oxide with asymmetric metal contacts is reported.     

Synergistic effect of adsorption coupled with catalysis based on graphene-supported MOF hybrid aerogel for promoted removal of dyes

A three-dimensional MIL-100(Fe)/graphene hybrid aerogel (MG-HA) was fabricated via in situ decoration of graphene oxide with MIL-100(Fe) nanoparticles. The resulting MG-HA with interconnected pore structure was applied as both adsorbent and catalyst for the removal of methylene blue (MB) from aqueous solutions. The result shows that the saturation adsorption capacity of the MG-HA was as high as 333.33 mg g⁻¹, exceeding that of both the corresponding pristine graphene aerogel and MIL-100(Fe) nanoparticles. In the presence of hydrogen peroxide, MG-HA further exhibited catalytic degradation ability. The dual functions achieved a synergistic effect leading to the quick and complete removal of MB. The benefit was revealed in the treatment of high concentration of pollutants without leaving secondary pollution. The merit was intuitively demonstrated in the instant removal of MB through a model separation device in comparison with a series of common adsorbents. A feasible mathematic model was built based on the synergistic adsorption/catalysis process, which perfectly fitted the experimental data. A pseudo-second-order adsorption process and pseudo-first-order catalytic degradation kinetics were revealed. Additionally, the MG-HA was able to retain 93.4% of its initial removal efficiency after 5 cycles of application. The macro-material body can be easily separated and reused without a time-consuming and high-cost recycling process.

A three-dimensional MIL-100(Fe)/graphene hybrid aerogel was fabricated for highly efficient removal of dye pollutants via synergistic adsorption and degradation.     

动脉粥样硬化斑块OxLDL靶向的MR分子影像学实验研究

目的 研究氧化低密度脂蛋白(oxLDL)靶向的超微超顺磁性纳米氧化铁(USPIO)探针在载脂蛋白E基因敲除小鼠(ApoE-/-)颈动脉动脉粥样硬化MR成像中的作用.方法 将聚乙二醇(PEG)包被的USPIO与抗小鼠oxLDL抗体结合,构建anti-oxLDL-USPIO靶向探针.以非特异性IgG-USPIO及单纯USP...