Preparation of Nanoparticles for ToF-SIMS and XPS Analysis

A number of different procedures for preparing nanoparticles for surface analysis are presented (drop casting, spin coating, deposition from powders, and cryofixation). We discuss the challenges, opportunities, and possible applications of each method, particularly regarding the changes in the surface properties caused by the different preparation methods.

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Because of the unique properties of the nanoparticles, the simple preparation has a significant influence on the surface analysis measurements. These methods provide several different options for sample preparation, depending on the analytical question at hand and the form in which the nanoparticles are available. The main concerns are safety and contaminations.

Since these methods can and should be modified as appropriate to the experiment, it is important to verify each step with other analytical techniques. For the preparation of nanoparticles from a suspension, weight 15 milligrams of nanoparticle powder into a 10 milliliter tube, followed by the addition of eight milliliters of ultra pure water. After capping, place the tube in a vortexer at 3000 revolutions for 15 minutes.

To perform the drop dry method, first transfer a prepared silicon wafer to the UV ozone cleaner for 30 minutes before placing the wafer into one half of the wafer holder. Place at three microliter drop of nanoparticle suspension into the center of the ring, and mount a 6.07 millimeter diameter O-ring on the wafer around the droplet taking care that the ring does not touch the droplet. Place the wafer in a vacuum desecrator at four millibars of vacuum pressure for 15 minutes to dry the wafer.

And view the wafer under light microscopy to assess the particle distribution across the surface of the wafer. Then repeat the drop drying until a homogenous layer of particles is observed. To prepare nanoparticle suspensions by spin coating, place a prepared wafer into the UV ozone cleaner for 30 minutes and program the spin coder as indicated.

At the end of the treatment, insert the wafer into the spin coder and switch on the vacuum for fixation. Deposit 80 microliters of the suspension onto the wafer and start the program. At the end of the deposition, transfer the sample to a clean wafer tray and analyze the sample by scanning electron microscopy to confirm gapless coverage of the substrate.

For deposition of the nanoparticles from a powder, fix a piece of double-sided adhesive tape to the sample holder and remove the liner. Next, collect a spatula tip of nanoparticle powder and place the sample onto the adhesive. Use the spatula to spread the sample over the adhesive before firmly pressing the sample onto the adhesive until as much of the sample is adhered as possible.

Invert and tap the sample holder, and blow a stream of gas across the sample to confirm that the powder is fixed to the tape. Alternatively, place a spatula tip of powder onto a clean surface and press the holder with the adhesive onto the powder from above. Then check that the powder is fixed on the tape as just demonstrated.

For cryofixation of the deposited nanoparticle suspension, build the main chamber of a fast freeze device with liquid nitrogen and fill a cooled fast freeze chamber with the cryogen. On the fast freeze device as cooled to its operating temperature, use a pipette to drop cast 10 to 20 microliters of nanoparticle suspension onto a clean silicon wafer. Holding the wafer with fixing tweezers, place the wafer inside the plunge freeze device and move the tweezers to the plunge position.

Press the button to drop the sample into the cryogen and wait several seconds for the sample to be completely frozen. As soon as the sample is frozen, transfer the wafer into a cooled environment and place the silicon wafer sample into the sample holder. Enhancing the surface wettability via UV or ozone treatment avoids the coffee ring effect resulting in a more homogenous distribution of the nanoparticles.

Scanning electron microscopy imaging shows significant defamation of the 60 nanometer gold silver nanoparticles after dialysis while centrifuged and redisbursed particles remained intact. Nanoparticle drop casting usually requires repeated applications to obtain a layer with full coverage. This representative film cast from a 90 milligram per milliliter suspension shows a thick and gapless multi-layer coverage, as well as notable absence of silicon peaks in the XPS spectra, making it an ideal sample for surface analysis.

This sample cast from a nine milligram per milliliter suspension shows particles and small single layer of glomerate that do not completely cover the surface, making it unsuitable for surface analysis of the nanoparticles, but ideal for analyzing, for example, particle size distribution. The sample cast from a 0.9 milligram per milliliter concentration does not provide either continuous coverage or sufficient particle density to make it suitable for either surface chemistry or particle size distribution analysis. As well as preserving larger structures within the suspension, cryo fixation avoids coffee ring effects resulting in a reduction in the signals, which can be attributed to salts or other contaminants observed using other sample preparation methods.

When performing surface analysis of the nanomaterials, remember to cross validate the method using an analytical technique to consistently prepare the method and to carefully document the results. These preparation methods are an important part of a reliable nanoparticle surface characterization, and are necessary for a detailed understanding of how nanoparticles interact with their environment.

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Journal (Chemistry)

Journal (Chemistry)

Preparation of Nanoparticles for ToF-SIMS and XPS Analysis

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