Generation of Zerovalent Metal Core Nanoparticles Using n-(2-aminoethyl)-3-aminosilanetriol

Podsumowanie

A novel method for metal core nanoparticle synthesis using a water stable silanol is described.

Streszczenie

In this work, a facile one-pot reaction for the formation of metal nanoparticles in a water solution through the use of n-(2-aminoethyl)-3-aminosilanetriol is presented. This compound can be used to effectively reduce and complex metal salts into metal core nanoparticles coated with the compound. By controlling the concentrations of salt and silane one is able to control reaction rates, particle size, and nanoparticle coating. The effects of these changes were characterized through transmission electron microscopy (TEM), UV-Vis spectrometry (UV-Vis), Nuclear Magnetic Resonance spectroscopy (NMR) and Fourier Transform Infrared spectroscopy (FTIR). A unique aspect to this reaction is that usually silanes hydrolyze and cross-link in water; however, in this system the silane is water-soluble and stable. It is known that silicon and amino moieties can form complexes with metal salts. The silicon is known to extend its coordination sphere to form penta- or hexa-coordinated species. Furthermore, the silanol group can undergo hydrolysis to form a Si-O-Si silica network, thereby transforming the metal nanoparticles into a functionalized nanocomposites.

Wprowadzenie

As the demand and applications of designer nanomaterials increases, so do the various methods of synthesis. The "top-down" methods, such as laser ablation or chemical etching have been employed for their excellent controllability and capability of resolving materials reliably down to the sub-micron level. These methods rely on bulk materials being processed into finer components, which typically increase the cost of production as the desired nanostructure size decreases. An alternative method of synthesis to this is the "bottom-up" approach, which controls synthesis at the molecular level and builds up to the desired nanostructure. This imparts a significant degree of control on the desired self-assembly, functionality, passivity, and stability in the generation of these nanostructured materials¹. By working from the molecular level, hybrid nanocomposites can be generated providing the benefits of both materials within the same structure.

As nanomaterials are synthesized through the bottom-up strategy, methods need to be employed to control particle size, shape, texture, hydrophobicity, porosity, charge, and functionality². In metal core nanoparticle synthesis, the initial metal salt is reduced in an autocatalytic process to generate zero-valent particles, which in turn direct the nucleation of other particle. This leads to clustering and finally nanoparticle production³. In an effort to control the size of nanoparticles created and prevent them from precipitating out of solution, stabilizers such as ligands, surfactants, ionic charge, and large polymers are exploited for their ability to block nanoparticles from further agglomeration^4-10. These materials inhibit the van der Waals attraction of the nanoparticles, either through steric hindrance due to the presence of bulky groups or through Coulombic repulsions³.

In this work, a facile, one pot, synthetic strategy for the generation of various metal core nanoparticles using the silane, n-(2-aminoethyl)-3-aminosilanetriol (2-AST) is presented (Figure 1). Ligands on this compound are capable of reducing metal precursors and stabilizing metal nanoparticles with a relatively high efficacy. The three silanol moieties present are also capable of crosslinking and this forms an interconnected network of organosilane polymer impregnated with nanoparticles within its matrix (Figure 2). Unlike most silanes, which readily undergo hydrolysis in the presence of water, this compound is stabilized in water, which is beneficial for hydrophobicity purposes, stability, and control.

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Protokół

Note: All reagents are used as is from manufacturer with no further purification. Reactions were monitored for up to one week via UV-Vis spectroscopy to ensure complete reduction. All reactions are carried out under a vent hood and appropriate safety attire is worn at all times, including gloves, eye goggles, and lab coats.

1. Synthesis of Silver Nanoparticles

1. Weigh out 0.0169 g (0.1 mmol) of silver nitrate directly into a 50 ml Erlenmeyer flask.
2. Add in 20 ml of 18.2 MΩ of ultrapure water and a magnetic stirrer bar. Cover flask with stopper to prevent evaporation.
3. Place flask in an oil bath situated on a stirrer/hot plate and ensure that temperature is maintained at 60 °C.
4. Slowly add 144 µl (0.2 mmol) of 2-AST using a precision micropipette. Flush pipette several times in solution to ensure all silane is transferred into the solution.
5. Take UV-Vis spectroscopy readings according to protocol listed in Section 5.
6. After 6 hr, remove sample from the oil bath and transfer to a 20 ml sample vial for storage, TEM, FTIR and further analysis.
   Note: Synthesis of gold and palladium nanoparticles follows the same method and stoichiometric amounts with the exception of gold nanoparticles requiring 216 µl (0.3 mmol) 2-AST. The reaction may continue to produce nanoparticles for up to 2 weeks, but the rate is not significant compared to initial rate.

2. Transmission Electron Microscope (TEM) Sample Preparation

1. Ensure that sample has cooled to RT.
2. Place a 200 carbon-mesh formvar-coated copper grid onto a clean piece of filter paper.
3. Using a 1 ml plastic Pasteur pipette, cast-drop approximately 60 µl of the nanoparticle sample directly onto the grid.
4. Allow grid to dry for 24 hr before imaging.
5. Take high-resolution TEM images with the following conditions: 10 µA current and 100 kV accelerating voltage²².

3. Nuclear Magnetic Resonance (NMR) Sample Preparation

Note: Perform NMR at RT. At high temperatures signals may coalesce, which degrades the quality of spectra obtained.

1. Using a precision pipette, pipette 50 µl of deuterium dioxide (D₂O) into a clean NMR tube.
2. With another clean precision pipette, pipette 400 µl of nanoparticle sample into the same NMR tube.
   1. As samples may adhere to the inner walls of the NMR tube, slowly add solutions into the NMR tube. If sample does adhere, cap the tube and shake the top of the tube to force the solution to the bottom.
3. Mix the sample by shaking and repeatedly inverting the NMR tube.
4. Place sample tube into the NMR following directions set by NMR protocol provided by manufacturer. An upwards of 1,000 scans may be necessary for proper resolution in a ¹H proton NMR pulse program.
   Note: NMR tube walls should be clean. It is recommended that the outer wall of the tube is wiped with a microfiber or lint free cloth prior to analysis for spectra clarity.
5. Discard sample when finished. Do not return sample to parent solution.

4. Fourier Transform Infrared (FTIR) Spectroscopy Sample Preparation

1. Place 2 ml of nanoparticle sample into a small glass container. A 3 ml tube or 1 dram glass vial works well.
2. Dry the samples by placing the glass container in a vacuum desiccator fitted with a stopcock.
3. Attach desiccator to vacuum pump apparatus. Drying of samples may take a few hours depending on vacuum strength. Consider samples dry after there is no visible liquid in container.
4. Scrape down the sample using a clean spatula and collect solid materials.
5. Place solid material onto ATR-FTIR spectroscope fitted with a ZnSe crystal diode laser.
6. Obtain FTIR spectra integrating 32 scans between 4,000-500 cm^-1 with a spectral resolution of 2.0. Use the air background²³.

5. UV-Vis Spectroscopy Sample Preparation

1. Conduct UV-Vis spectroscopy on nanoparticle samples that are in a one to ten dilution of nanoparticle sample to water so that saturation does not occur in spectrometer analysis.
2. Remove nanoparticle samples for UV-Vis while reaction is running at half hour intervals.
3. Using a precision pipette, remove 100 µl of nanoparticle material and place into a plastic cuvette.
4. Add 1 ml of ultrapure water to the same cuvette and mix thoroughly by flushing the pipette several times.
5. Record UV-Vis absorbance spectrum between 250-800 nm.
6. After analysis, do not return sample to reaction. Dispose of analyte in an appropriate manner.

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Wyniki

The reaction was monitored via UV-Vis spectrometry as nanoparticle formation should produce peaks characteristic for each individual metal nanoparticle. The final analysis of synthesized materials was accomplished through TEM and FTIR. The FTIR spectra was obtained from dried powder of samples. The particle size analysis can be accomplished by measuring nanoparticle diameter from images obtained via TEM and averaging results.

Co...

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Dyskusje

Salts reported in this paper are the only salts that were tested of that metal. As a result, it is uncertain that this reaction strategy would work with all salts of the metals, particularly gold. The solubility of these salts in water may also affect the outcome of the reaction in terms of reaction time, morphology, and yields. In all reactions, the silane was added to an already dissolved metal salt solution.

It is worth noting that care must be taken to ensure accuracy for these reactions r...

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Materiały

Name	Company	Catalog Number	Comments
n-(2-aminoethyl)-3-aminosilanetriol (2-AST)	Gelest	SIA0590.0	25% in H2O
Silver nitrate	Sigma Aldrich	S6506
Gold(III) chloride trihydrate	Sigma Aldrich	520918
Palladium(II) Nitrate	Alfa Aesar	11035
Deuterium Dioxide	Cambridge Isotope Laboratories	DLM-4-100