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Nanowires, nanofibers, nanorods

Nanowires have properties that can’t usually be found in bulk materials due to their energy levels, and that are therefore promising for the preparation of sensors or flexible transparent electrodes, but are also used for numerous other electronic applications. Nanowires, nanofibers, and nanorods also come into play in biomedical applications, where they are used to modify surfaces in order to provide better interaction with biological cells/tissues. In this section, you will find an overview of specific research examples with different measurement techniques, namely microwave synthesis and atomic force microscopy from Anton Paar. Besides the investigation of morphology and mechanical properties of nanofiber scaffolds with an AFM, you can also read about the formation of nanowires with microwave synthesis.  

Investigation of morphology and mechanical properties of nanofiber scaffolds for biomedical applications by atomic force microscopy

Introduction

Poly (ε-caprolactone) (PCL) has attracted considerable interest as a base material for biomedical applications due to its: (i) biocompatibility; (ii) biodegradability (depending on the molecular weight); (iii) approval for clinical use in humans by the US Food and Drug Administration (FDA); (iv) susceptibility to surface modification to provide better interaction with biological cells/tissues; and (v) suitability for export to countries and cultures where implantation of animal-derived products is unpopular. There is considerable interest in the preparation of biomimetic PCL substrates able to modulate cell interaction for improved substitution, restoration, or enhancement of tissue function. In this report, we use a Tosca 400 atomic force microscope to investigate scaffold properties.

Experimental

In this study, two samples were investigated: Sample A is a PCL nanofiber substrate and sample B is a PCL nanofiber substrate surface functionalized by biomolecules. The pure PCL nanofiber substrate was imaged by tapping mode using an NCR cantilever. The height images are shown in Figure 1, where the trace and retrace data at two different scan sizes are shown. For investigation of mechanical sample properties, force curves were measured at various, randomly chosen locations on the nanofibers. The Young’s modulus is obtained by applying the spherical Hertzian model to the calibrated force curve.

Results and discussion

Comparing the trace and retrace images for sample A, a slight movement of some nanofibers, caused by the tapping mode investigations, can be observed. The diameter of the nanofibers is between 80 nm and 400 nm, in accordance with the expected values. Also, some larger nanostructures with a diameter of about 500 nm were observed, possibly due to merging of the nanofibers or the presence of beads. Red squares in Figure 1 (top and bottom) indicate a movement of some nanofibers during the tapping mode investigations. For sample A and for the case shown here, the Young’s modulus is about 6.4 MPa and the maximum adhesion force is about 60 nN.

Additional information

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Source:

Gianluca Ciardelli et al: “Blends of Poly- (E-caprolactone) and Polysaccharides in Tissue Engineering Applications.” Biomacromolecules 2005, 6, 1961-1976

Application report: 

Synthesis of copper nanowires for electronic devices

Introduction

Metal nanowires are promising materials for the preparation of sensors or flexible transparent electrodes. Especially nanocopper has attracted considerable interest due to its high electrical conductivity. Various methods in aqueous media have already been investigated to efficiently generate the desired copper nanoparticles. Microwave irradiation facilitates the reductive hydrothermal approach to form the desired nanowires by utilizing inexpensive, readily available biomass as a reducing agent.

Experimental

In a beaker copper (II), chloride and octadecylamine were dissolved in 30 mL water at 65 °C and sonicated for 1 h. Glucose was dissolved in 30 mL water and added to the precursor solution under stirring. An aliquot (max. 20 mL) of the mixture was transferred to a G30 vial and subjected to microwave irradiation. After cooling, the mixture was centrifuged, and the precipitate was thoroughly washed and dispersed in chloroform for storage until purification and further use. Purification of the particles was achieved by simply extracting the chloroform dispersion with water, and centrifugation of the organic phase.

Results and discussion

Different temperature/time conditions (2-4 h at 80 °C to 120 °C) were employed to determine the highest aspect ratio of the resulting nanoparticles. Glucose acts as reducing agent whereas the alkylamine capping agent was essential for the formation of nanowires. However, nanowire formation was not observed in a reaction time under 4 h, and below 120 °C. Conventional heating requires 24 h refluxing to achieve comparable results. The resulting nanowires were used to fabricate transparent conductive electrode films, which remained stable under ambient conditions and acid treatment.

Additional information

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Source:

A. S. Hashimi et al., Curr. Appl. Phys. 2020, 20, 205–211

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Synthesis of doped hydroxyapatite nanorods with biological activities

Introduction

Porous hydroxyapatite is of considerable interest in nanomaterial research since its porosity and surface area can be easily tuned according to the desired applications in catalysis, gas adsorption, and other fields. Doping with appropriate transition metals like silver or copper leads to remarkable antibacterial activity which makes the material also suitable for the fabrication of medical devices like bone or tooth implants. Microwave irradiation provides a facile and rapid two-step process to generate valuable doped hydroxyapatite nanorods.

Experimental

Calcium nitrate was dissolved together with cetyltrimethylammonium bromide (CTAB) as a surfactant in 30 mL water. Diammonium hydrogen phosphate was separately dissolved in 30 mL water and the pH was adjusted with hydroxylamine to 10. This solution was added dropwise under stirring to the calcium solution. An aliquot (20 mL) was filled in a G30 vial and was subjected to microwave irradiation. Afterwards, the precipitate formed was centrifuged, washed with water and calcined at 550 °C.

For the next step 2 g freshly prepared hydroxyapatite nanorods were dispersed in 50 mL water. 5 mol% silver nitrate was added and the mixture was stirred at room temperature for 5 min. An aliquot (20 mL) was filled in a G30 vial and was subjected to microwave irradiation. The solid material was centrifuged, washed with water and dried at 50 °C.

Results and discussion

Various organic surfactants have been used to study the influence on the morphology of the resulting nanoparticles. The crystal structure of the products was analyzed by powder X-ray diffraction. Solely cationic CTAB as a surfactant already showed good results, but the highest surface area and pore diameter were obtained when a mixture of cationic and anionic surfactants was employed. The doped nanosized hydroxyapatite nanorods generated were evaluated for adsorption properties towards dye and metal ions and antibacterial activity against E. coli. For the latter, a 5 % silver doping showed the highest effectiveness after 24 h of incubation.

Additional information

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Source:

M. Sharma et al., J. Nanosci. Nanotechnol. 2018, 18, 623-633

Application Database Entry:

Formation of GeSn nanowires for optoelectronic devices

Introduction

Semiconductor nanowires have attracted interest in recent years as potential material for optoelectronic devices. Incorporation of dopants to the base semiconductor allows for easy tuning of the electronic properties. Especially nanoalloys of tin and germanium in various constitutions show promising effects for LEDs or biological sensors. Microwave heating drastically shortens the reaction time for the formation of desired nanowires and thus allows for simply varying the tin content in the prepared nanomaterial.

Experimental

In a G10 vial bis[bis(trimethylsilyl)]tin and 4 equivalents bis[bis(trimethylsilyl)]germanium were suspended under inert atmosphere in dodecylamine. The mixture was heated at 100 °C under stirring in a glove box overnight. After the pretreatment, the vial was sealed and subjected to microwave irradiation.

After cooling, the mixture was diluted with toluene and centrifuged. The decanted nanowires were redispersed and centrifuged (2x in toluene, 3x in ethanol, 3x in toluene) and finally stored in toluene.

Before further use, the nanowires were annealed under helium atmosphere at 250 °C for 60 min.

Results and discussion

Although rather time-consuming, the pretreatment of the reaction mixture at 100 °C was essential, as otherwise no nanowires would have formed. The nanostructures obtained were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction (see Fig. 1). Following the described procedure, the resulting nanowires had a high tin content of approx. 20 %. The material was thoroughly evaluated for ohmic behavior by deposition on silicon substrates and revealed strong semiconductor conditions with increasing resistivity with decreasing temperatures.

Figure 1: SEM image (a) and STEM-EDX analysis of GeSn nanowires

Additional information

Instruments:

Source:

M. Sistani et al., Nanoscale 2018, 10, 19443-19449

Application Database Entry:

Synthesis of functionalized nanorods for electronic devices

Introduction

Creating monolayers can be effectively achieved by self-assembly of appropriate substrates, such as functionalized nanorods. Beneficial are self-assembly techniques resulting in thin monolayers, which can be applied in electronic and photonic devices. Here, a microwave-mediated method to efficiently generate metal oxide nanorods, wrapped by reduced graphene oxide, for further self-assembly studies, is presented.

Experimental

In a beaker, graphene oxide sheets were dispersed in 20 mL water and sonicated for 1 h. Anatase powder was added and the mixture was placed on a shaker overnight. The dispersion was then added to 30 mL of a 10M NaOH solution. The resulting mixture was evenly distributed to four 100 mL PTFE liners. The reaction vessels were sealed and placed in the rotor accordingly and subjected to microwave irradiation.

Results and discussion

The parallel microwave approach was chosen to generate sufficient nanomaterial for further investigations in a single experiment. The nanorods were suspended in deionized water (10 mg/mL) and subsequently several coating methods, such as dip-coating, drop-casting or blade coating, were investigated. With the different methods to deposit the as-prepared graphene coated sodium titanate nanorods on various substrates, the accessible thickness of the resulting layers was investigated. The deposited films were characterized by applying the 2D Fourier transformation to scanning electron microscopy images.

Additional information

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Source:

M. H. Modarres et al., Adv. Mater. Interf. 2019, 6, 1900219

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Synthesis of core/shell silver-tin oxide nanowires for fabrics

Introduction

Silver nanowires with their remarkable infrared reflectance are of considerable interest as thermal management products for textiles. Current research is focusing on improvements of the nanomaterial stability. Passivation of the nanowires with a nanosized tin oxide layer has potential to enhance the environmental stability while retaining its IR reflectance. Microwave irradiation facilitates a straightforward hydrothermal process to generate core/shell silver-tin oxide nanowires.

Experimental

In a beaker, 3 mL of an ethanolic dispersion of silver nanowires was diluted with water to 100 mL, and sodium citrate (1 wt% in water) was added under stirring. Sodium stannate trihydrate (0.25 wt% in water) was added and the mixture was evenly distributed to eight 100 mL PTFE liners. The reaction vessels were placed in the rotor, which was closed with the lid and subjected to microwave heating at 250 W. The temperature was held at 100 °C for 15 min before further heating to a target temperature of 150 °C.  

Results and discussion

The final nanocomposite was fully characterized by scanning transmission electron microscopy, transmission electron microscopy, electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and UV/Vis spectroscopy. The tin oxide coating improved the air stability of the nanowires, since no degradation could be observed even after 4 months (see Figure 1 below). The nanocomposite was used to produce fabrics for para-aramid textiles to save energy and maintain warmth in the human body.

Figure 1: STEM images of SnO2 decorated nanowires: as synthesized (a), after 3 weeks (b), after 4 months (c)

Additional information

Instruments:

Source:

A. Baranowska-Korczyc et al., RSC Adv. 2021, 11, 4174–4185

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