The research leading to these results has received funding from NE/J010783/1. The project NanoValid has received funding from the European Union&#39;s Seventh Program for research, technological development, and demonstration under grant agreement No. 263147.

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The authors have no competing financial interest. IK and EVJ jointly conceived the study with IK&#39;s design, performed the experiments, analyzed data, and prepared the manuscript. LJE and IR carried out the TEM imaging. SA, MLM, and MC supplied the CNTs, and rest of the co-authors discussed and commented on the manuscript at all stages and RT contributed towards the editing of the manuscript.

The ultimate goal of the study is to develop a strategy that would allow the identification of optimal sonication conditions to make dispersions of a selected number of nanomaterials in water. An attempt is made here to carefully document the protocol steps and parameters during sonication in order to fulfill the gaps previously identified in reviews as well as to follow the recommendations made in the past15. The optimal dispersion conditions are identified by characterizing the dispersions after each sonication cycle and checking sample stability and uniformity. The impact of the sonication procedures and stability status is assessed based on the characteristic alterations in key physicochemical properties of nanomaterials, as determined by various analytical techniques: DLS, ELS, UV-vis, and TEM. The current protocol is an adapted methodology for the dispersion of nanomaterials from the past literature and other research projects21,22,37,38,39 with some modifications and refinements addressing the key gaps, steps, and their applicability to wider nanomaterials of similar surface profile7. However, careful adjustments are required with respect to their sonication time, strength, and type for their application to other nanomaterials. Also, further work is required to establish a correlation between sonication procedure and biological activity of nanomaterials. Six different types of nanomaterial dispersions are evaluated and compared, primarily for their stability, using an ultrasonic bath and an ultrasonic probe fitted with a vial tweeter at set time points. To maintain the suspension purity and any unintended alterations caused due to contamination, probe sonication is avoided here. In the vial tweeter, the vials can remain closed. This eliminates any cross contamination of the samples.
Calibration of sonicators is a key factor since a range of sonicators are available with different frequencies, amplitude, and powers. To determine the effective acoustic energy delivered to the suspension, calibration of sonicators is carried out using calorimetry. The acoustic power delivered for 70% amplitude setting for the vial tweeter as well as that for the 100% ultrasonic bath setting is calculated to be &#60;1 W (0.75 &#177; 0.04 W and 0.093 &#177; 0.04 W, respectively). However, the power outputs indicated by the manufacturers for vial tweeter and bath sonicator are 200 W and 80 W, respectively. This indicates that despite the high power source, most of the energy is lost during the generation of cavitational bubbles and only a small fraction is actually delivered to the dispersion under treatment26. Recent studies have highlighted the importance of cavitational measurement control as compared to the input power of the sonicator for a better dispersion control during sonication8. The methodology appears promising for the controlled dispersion of highly delicate nanomaterials such as CNTs and is recommended for future studies.
Each technique used in the study is based on different principles with limitations to all. DLS is not an ideal technique for non-spherical suspensions as well as highly polydisperse systems. In such conditions, DCS is recommended because of high resolution, accuracy, and precision40. DCS can completely separate very narrow size distribution peaks that differ by as little as 3%. TEM provides direct visual images of the nanoparticles and is a great tool for the determination of aggregation, dispersion, size, and shape of the particles, but the technique requires sample drying which may lead to artifacts41. This can be eliminated by washing the grids with ultrapure water as discussed in step 4.5.3.
Amongst others, the methodology highlights some critical steps such as the type of vials used in the protocol, immersion depth, and position of the vials in the ultrasonic bath as well as the vial tweeter. Temperature control of the system during agitation is an important parameter. Frequent water changes in the ultrasonic bath and pulsed mode run in the case of vial tweeter are recommended to avoid any heat buildup during sonication, thus avoiding any sample alterations. The pre-wetting step for hydrophobic samples such as Zinc oxide helps in the dispersion of particles but this may induce some undesired changes. The sonication time and energy should be high enough to de-agglomerate the particles but not too much that it breaks the particles. The results indicate that agglomerate breakage is dependent on particle type.
Our findings highlight the importance of having a detailed dispersion protocol, as results show that key physico-chemical properties can potentially be altered during the sonication process, as governed by factors such as sonicator type, sonication duration time, and power output. Results have shown that sample integrity is potentially compromised at higher intensity agitation. Results show that CNTs are very sensitive to agitation, so breakages are highly likely to occur when sonication duration and strength are changed. Near to optimal settings for the dispersion of CNTs are between 2 - 15 min in the ultrasonic bath and only 2 min using the ultrasonic probe. However, the ultrasonication still may have caused some nanotube shortenings, which cannot be accurately quantified here. DLS may not be an ideal technique for the characterization of CNTs but it still can provide hydrodynamic diameter for nanotubes and this data could be informative of the differences in length distributions of CNTs amongst various samples16,42,43. Past studies demonstrate that the dispersion protocol of CNTs can be greatly enhanced by the addition of surfactants as the surfactant molecules are absorbed on the nanotube monolayer, thus providing a barrier to breakage due to sonication35,44. However, this cannot be compared directly to the present protocol as no surfactants are involved in this case. It is important to note that ensuring the length size distribution in the case of CNTs is very important, as the aspect ratio is often correlated with certain toxicological response. In contrast, CeO2 gave different results compared to the CNTs, in which prolonged sonication times using either ultrasonic bath or probe, lead to the formation of primary particles. The difference in findings between CNT and CeO2 cases highlights the importance to tailor dispersion protocols e.g., optimize sonication time and power output, in accordance to starting material i.e., type of nanomaterial powders. Every nanomaterial powder sample type is different, as there will be different degree of agglomeration within the powder itself. In certain instances, the de-agglomeration process has successfully resulted in the de-agglomeration up to primary particles level, as evident by the emergence of other shaped particles in the TEM images, which was not visible prior to the sonication step. The prolonged sonication resulted in the continuous breaking of cerium oxide agglomerates at different angles thus leading to multi-faceted particles.
In the case of commercially-bought aqueous sample of Ag NPs dispersions, our findings also emphasize the need for long term stability and uniformity assessment. There is a need to ensure that dispersions have been sufficiently characterized prior to use, especially in cases of long term storage. However, nanomaterials have a very short shelf life. They age with time and may behave differently after long term storage as compared to a freshly prepared dispersion.
The results here highlight the need for a harmonized strategy to identify an optimized protocol for different nanomaterials. The presented proposed strategy is to conduct different variations in the sonication method and to ensure that the dispersions at different time points are sufficiently characterized using complementary analytical methods. The importance on the use of a multi-method approach to characterize and monitor dispersion quality through time and different experimental conditions has been highlighted by past workers45. Although various methods for sonication have been presented to cater to specific nanomaterial dispersion in the study, potentially they can be used as the basis to disperse other metal and metal oxide nanomaterials (of similar surface properties) in water. However, having any change in either nanomaterial type or liquid medium requires the need to optimize the basic protocol, which can be done by careful adjustment of various factors e.g., sonication time, strength, and sonicator type. Whatever protocol is chosen and identified as optimal, there is always a need to have a detailed report on the scheme and step-wise sequence of the sonication dispersion procedure. This is important to improve interpretability and comparability. One of the applications of this protocol is to facilitate data comparability amongst other labs leading to a harmonized and standardized approach for future studies. The current methodology and control parameters can be utilized for other dispersing mediums apart from water and comparisons can be drawn on a case by case basis.

Sonication is the process of generating cavitations, which involves the creation, growth, and collapse of bubbles (often called hot spots) formed in liquid due to the irradiation of high intensity ultrasound1. In a laboratory setting, the sonication method is carried out using a sonicator. There are different sonicators, all having the general function of de-agglomerating particles, which disperse in a liquid medium as individual (or primary) particles. By applying sonication, sample homogeneity can improve, potentially achieving a much narrower particle size distribution. An important aspect to consider in the dispersion process is stability of the final dispersion. Here, the stability of the suspension is defined as where the particles do not settle or sediment down in their dispersed state and the average hydrodynamic diameter measurements do not vary by more than 10% between the five repeated measurements taken during that time (around 10 min)2,3. There are several ways to measure dispersion stability. This includes estimation of zeta potential (ZP) through measurement of electrophoretic mobility of particles. Another is to measure the characteristic absorption of nanoparticles in the UV spectral range4.
In the field of nanotoxicology, the ability to have control over dispersion quality is very important, as the dispersion step will determine key physicochemical properties, such as particle size/size distribution, shape, aggregation/agglomeration, surface charge, etc. This in turn will ultimately affect the interaction of particles with test media and outcome of various in vitro and in vivo experiments, in order to deduce the potential hazards of nanomaterials.
Sonication is commonly carried out by either using a probe-type (direct) or an ultrasonic bath, or ultrasonic probe with a vial tweeter (indirect sonication). All types of sonication are available in a range of intensity and output power settings, sometimes adapted with a different type of sonotrode for specific processes or requirements, and are suitable for liquid volumes ranging from 2 to 250 mL. Although probe ultrasonication is known to perform better than bath sonication because of high localized intensity5, bath sonication is often preferred over probe-type for the preparation of toxicological test suspensions because of the possible contamination risk through the tip, erosion of titanium probe tip after prolonged use, and probe immersion depth discrepancies. Similarly, an ultrasonic probe fitted with a vial tweeter is advantageous over the direct probe due to the above-mentioned contamination risks as well as the operation friendliness of the equipment. Several vials are sonicated at the same time and at the same intensity. This not only saves time but ensures that all the samples are treated equally, which makes the results among samples more reliable and comparable. In the safety research of nanomaterials, contamination is always avoided. However, the probe sonicator does not fit this requirement and has not been tested. Probe sonicators are known to cause some unavoidable side effects such as sample contamination due to tip erosion as well as reduced energy output leading to alterations of dispersion conditions, hence compromising data reproducibility6,7,8. Moreover, samples are usually run in uncovered containers leading to liquid loss due to evaporation as well as dust deposition. In order to avoid these unintended alterations, recent studies recommend alternative indirect sonicators based on their effective energy delivery as well as suspension purity assurance6.
Non-optimized sonication can have a detrimental effect on results. Potentially, it can alter the key physical and chemical properties of the nanomaterials such as size, size distribution, morphology, and surface charge2,9. Previous literature has reported such failings to control the sonication process and the impact on particle parameters such as nano-TiO25,10,11, nano-ZnO6, and nano-copper12. Furthermore, past studies have shown that the sonication process not only alters particle characteristics but also governs the outcome of toxicological tests12,13.
To have control over the dispersion process, it is important to monitor and understand how different factors such as sonicator type, instrument power and duration, volumes, etc., can affect dispersion quality. Hence, there is a need to have a systematic procedure to analyze key physicochemical characteristics of the particles in the dispersion at different time points of the sonication process. Although such considerations have been taken into account by a few researchers, work in this area is limited. Bihari et al. have studied dispersion stability of different nanomaterial dispersions made using different ultrasound energies with various dispersion stabilisers14. A recent review by Hartmannn et al. highlighted that although work has been done to understand the different factors affecting nanomaterial dispersion quality e.g., type of sonicator used, sonication time, etc., there is still no well-defined and universally accepted sonication procedure that currently supports nanotoxicological testing and investigations7,15.
Several analytical characterization techniques are used to monitor dispersion quality. These include the use of: Dynamic Light Scattering (DLS), Disc Centrifugation, Electrophoretic Light Scattering (ELS), Ultraviolet-visible (UV-vis) spectroscopy, and Transmission Electron Microscopy (TEM), which measure particle size/size distribution, zeta potential, dispersion stability, and morphology characteristics, respectively. DLS is often used to determine the hydrodynamic diameter (Z-average) of the particles and polydispersity index (PdI) of nanomaterial dispersion. In the case of multimodal size distribution by DLS, Z-average obtained may not agree with the intensity-weighted size distribution intensity. As such, the mean of the intensity-weighted size distribution can be quoted. PdI reflects the broadness of the size distribution with a scale ranging from 0 - 1, with 0 being a monodispersed sample and 1 being a highly polydisperse sample16. Disc Centrifugation is a separation technique used to determine particle size distribution using centrifugal sedimentation in a liquid medium. The particles sediment within an optically clear and rotating disc and the amount of light scattered by the particles when they reach the edge of the disc is recorded and converted into particle size distribution using Stokes&#39; law. To resolve multi-modal particle distribution, techniques such as disc centrifuge are more suitable as they have a separation mechanism element integrated within the instrument. Zeta potential (&#950;-potential) of particles is defined as the electric potential at their shear or slipping plane, which is a notional boundary within the electrical double layer that separates the (bulk) liquid showing normal viscous behavior from the Stern layer, a layer that is predominantly composed of counter ions and considered to move with the particle. The zeta potential is directly related to the surface charge of particles and therefore the electrostatic interaction (i.e., repulsion/attraction) between the particles. This parameter is therefore considered a primary indicator of nanomaterial dispersion stability. By convention, zeta potential value below -25 mV and above 25 mV are considered stable17,18. The concentration and type of ions as well as the solution pH, strongly affect the zeta potential19. ELS is used to measure the electrophoretic mobility of particles in dispersion and this mobility is converted to zeta potential through the Henry equation and the Smoluchowski or H&#252;ckel models. UV-vis spectroscopy is a technique used to quantify the light that is absorbed and scattered by a sample at a particular wavelength. It is often used to monitor dispersion stability by measuring the characteristic absorption of nanomaterials in the UV region. Finally, TEM is often used to visualize and analyze the size, size distribution, agglomeration, and shape of the nanoparticles5,14,15,20.
We present a comparative study of six different nanomaterial dispersions made using ultrasonic bath and an ultrasonic probe fitted with a vial tweeter. The particle concentration, temperature, sonicator type, and settings used in the study are specified in the protocol, so that the experimental settings for similar probes and ultrasonic baths can be inferred. The following nanomaterials are used: silver (Ag), cerium oxide (CeO2), zinc oxide (ZnO, NM110-hydrophylic and NM111-hydrophobic), and carbon-based nanomaterials such as carbon nanotubes (A32 and A106, see Table of Materials).
Assessment of dispersion quality at different time points along the sonication process is made using various characterization techniques, namely DLS for particle size/size distribution, Disc Centrifugation for size distribution, ELS for zeta potential, UV-vis spectroscopy for stability, and TEM for particle shape and homogeneity. A number of different nanomaterials ranging from metal oxides to carbon-based are evaluated. For comparison, commercial aqueous suspension of silver nanoparticles (Ag NPs) stabilized with citrate capping is used in parallel, to deduce the expected long-term stability of a relevant commercially available suspension. Obviously, this Ag NPs model is not directly related to any of the dispersion procedures but solely acts to indicate the need to re-sonicate or re-stabilize the suspensions after some time of storage as changes such as re-agglomeration are bound to occur during storage. The suspension is kept in the fridge for two months. During this period, the dispersion is characterized to identify potential agglomeration of the particles. Initial results show an unstable suspension (as discussed in the Results section). Subsequently, this dispersion is further subjected to different sonication treatments, similar to the other nanomaterials used in the study. The purpose of the study is to confirm that we can de-agglomerate the suspension through the same sonication protocol. The Ag NPs model can thus be associated as the benchmark for long term studies representing re-dispersion of particles in optimized form.
The dispersion protocols presented here share similarities to those published in earlier literature and incorporates some of the few recommendations previously made by past workers7,21,22,23,24,25. In this study, a systematic and step-wise approach is used to monitor dispersion quality throughout the dispersion protocol. This approach undertakes real-time characterization of the nanomaterial dispersions, in order to identify optimal experimental dispersion conditions (Figure 1).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56074/56074fig1.jpg" /> 
Figure 1. Flowchart depicting the scheme and step-wise sequence of the dispersion protocol. <a href="//cloudflare.jove.com/files/ftp_upload/56074/56074fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table><tbody><tr><td>Cerium oxide nanopowder</td><td>Sigma-Aldrich</td><td>544841</td><td>&amp;lt;25&amp;nbsp;nm particle size (BET)</td></tr><tr><td>Zinc oxide</td><td>European Commission&#039;s Joint Research Centre (JRC)</td><td>NM110</td><td>hydrophylic</td></tr><tr><td>Zinc oxide</td><td>European Commission&#039;s Joint Research Centre (JRC)</td><td>NM111</td><td>hydrophobic</td></tr><tr><td>Multi walled carbon nanotubes</td><td>NanoMile project (Large Collaborative Project under the European Commission&#039;s 7th Framework Programme)</td><td>A32 (MWCNT1)</td><td>3.0&amp;plusmn;1.8 &amp;micro;m long, O/C ratio of 4.5%</td></tr><tr><td>Multi walledcarbon nanotubes</td><td>NanoMile project</td><td>A106 (MWCNT2)</td><td>3.3&amp;plusmn;2.4 &amp;micro;m long, O/C ratio of 7%</td></tr><tr><td>Silver dispersion</td><td>Sigma-Aldrich</td><td>730785</td><td>10&amp;nbsp;nm particle size (TEM), 0.02&amp;nbsp;mg/mL</td></tr><tr><td>Zetasizer nano</td><td>Malvern Instruments</td><td></td><td>Particle size and zeta-potential measurements&amp;nbsp;</td></tr><tr><td>Disc Centrifuge</td><td>CPS instruments Inc.</td><td>Model DC 24000</td><td>Particle size distribution by centrifugal sedimentation</td></tr><tr><td>Transmission electron microscope</td><td>JEOL USA</td><td>Jeol 1200EX TEM</td><td>Bright field images, particle size, shape, agglomeration</td></tr><tr><td>Ultrasonic probe fitted with a vial tweeter</td><td>Hielscher</td><td>UIS250V</td><td>Sonicator</td></tr><tr><td>Ultrasonic bath</td><td>Branson&amp;nbsp;</td><td>Model 1510</td><td>Sonicator</td></tr><tr><td>Eppendorf vials</td><td>Eppendorf</td><td>2236411-1</td><td>1.5ml capacity</td></tr><tr><td>UV-vis spectrophotometer</td><td>Jenson flight deck</td><td>Model 6800</td><td>SPR peaks, suspension stability</td></tr><tr><td>Disposable folded capillary cell</td><td>Malvern Instruments</td><td>DTS 1070</td><td>for the measurement of elecr</td></tr><tr><td>Zeta- potential standard</td><td>Malvern Instruments</td><td>DTS 1235</td><td></td></tr><tr><td>Quartz cuvette</td><td>Jasco</td><td>1103-0042</td><td>Rectangular quartz cell 10 x 100&lt;br/&gt; Spectrosil Quartz with lid 190 -2700 nm</td></tr></tbody></table>

dispersion of nanomaterials in aqueous media: towards protocol optimization

The sonication process is commonly used for de-agglomerating and dispersing nanomaterials in aqueous based media, necessary to improve homogeneity and stability of the suspension. In this study, a systematic step-wise approach is carried out to identify optimal sonication conditions in order to achieve a stable dispersion. This approach has been adopted and shown to be suitable for several nanomaterials (cerium oxide, zinc oxide, and carbon nanotubes) dispersed in deionized (DI) water. However, with any change in either the nanomaterial type or dispersing medium, there needs to be optimization of the basic protocol by adjusting various factors such as sonication time, power, and sonicator type as well as temperature rise during the process. The approach records the dispersion process in detail. This is necessary to identify the time points as well as other above-mentioned conditions during the sonication process in which there may be undesirable changes, such as damage to the particle surface thus affecting surface properties. Our goal is to offer a harmonized approach that can control the quality of the final, produced dispersion. Such a guideline is instrumental in ensuring dispersion quality repeatability in the nanoscience community, particularly in the field of nanotoxicology.

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Dispersion of Nanomaterials in Aqueous Media: Towards Protocol Optimization

Here, we present a step-wise protocol for the dispersion of nanomaterials in aqueous media with real-time characterization to identify the optimal sonication conditions, intensity, and duration for improved stability and uniformity of nanoparticle dispersions without impacting the sample integrity.

The sonication process is commonly used for de-agglomerating and dispersing nanomaterials in aqueous based media, necessary to improve homogeneity and ...

dispersion-nanomaterials-aqueous-media-towards-protocol

Research

JoVE Journal

Environment

28.3K Views.  University of Birmingham. Hier presenteren wij een stapsgewijs protocol voor de dispersie van nanomaterialen in waterige media met real-time karakterisering te identificeren van de optimale ultrasoonapparaat voorwaarden, intensiteit en duur voor verbeterde stabiliteit en uniformiteit van de nanoparticle dispersies zonder invloed op de integriteit van de steekproef.

Video: Dispersion of Nanomaterials in Aqueous Media: Towards Protocol Optimization

A 'Plug and Play' Method to Create Water-dispersible Nanoassemblies Containing an Amphiphilic Polymer, Organic Dyes and Upconverting Nanoparticles

In this protocol, we first describe a procedure to synthesize lanthanide doped upconverting nanoparticles (UCNPs). We then demonstrate how to generate amphiphilic polymers in situ, and describe a protocol to encapsulate the prepared UCNPs and different organic dye molecules (porphyrins and diarylethenes) using polymer shells to form stable water-dispersible nanoassemblies. The nanoassembly samples containing both the UCNPs and the diarylethene organic dyes have interesting photochemical and photophysical properties. Upon 365 nm UV irradiation, the diarylethene group undergoes a visual color change. When the samples are irradiated with visible light of another specific wavelength, the color fades and the samples return to the initial colorless state. The samples also emit visible light from the UCNPs upon irradiation with 980 nm near-infrared light. The emission intensity of the samples can be tuned through alternate irradiation with UV and visible light. Modulation of fluorescence can be performed for many cycles without observable degradation of the samples. This versatile encapsulation procedure allows for the transfer of hydrophobic molecules and nanoparticles from an organic solvent to an aqueous medium. The polymer helps to maintain a lipid-like microenvironment for the organic molecules to aid in preservation of their photochemical behavior in water. Thus this method is ideal to prepare water-dispersible photoresponsive systems. The use of near-infrared light to activate upconverting nanoparticles allows for lower energy light to be used to activate photoreactions instead of more harmful ultraviolet light.

Organic dye molecules and oleic acid coated upconverting nanoparticles are not water-soluble. This protocol describes a &#8216;plug and play&#8217; method that enables the transfer of organic dye molecules and upconverting particles from their initial hydrophobic solvent to water.

In this protocol, we first describe a procedure to synthesize lanthanide doped upconverting nanoparticles (UCNPs). We then demonstrate how to generate ...

Chemistry

Functionalization and Dispersion of Carbon Nanomaterials Using an Environmentally Friendly Ultrasonicated Ozonolysis Process

Functionalization of carbon nanomaterials is often a critical step that facilitates their integration into larger material systems and devices. In the as-received form, carbon nanomaterials, such as carbon nanotubes (CNTs) or graphene nanoplatelets (GNPs), may contain large agglomerates. Both agglomerates and impurities will diminish the benefits of the unique electrical and mechanical properties offered when CNTs or GNPs are incorporated into polymers or composite material systems. Whilst a variety of methods exist to functionalize carbon nanomaterials and to create stable dispersions, many the processes use harsh chemicals, organic solvents, or surfactants, which are environmentally unfriendly and may increase the processing burden when isolating the nanomaterials for subsequent use. The current research details the use of an alternative, environmentally friendly technique for functionalizing CNTs and GNPs. It produces stable, aqueous dispersions free of harmful chemicals. Both CNTs and GNPs can be added to water at concentrations up to 5 g/L and can be recirculated through a high-powered ultrasonic cell. The simultaneous injection of ozone into the cell progressively oxidizes the carbon nanomaterials, and the combined ultrasonication breaks down agglomerates and immediately exposes fresh material for functionalization. The prepared dispersions are ideally suited for the deposition of thin films onto solid substrates using electrophoretic deposition (EPD). CNTs and GNPs from the aqueous dispersions can be readily used to coat carbon- and glass-reinforcing fibers using EPD for the preparation of hierarchical composite materials.

Here, a novel method for the functionalization and stable dispersion of carbon nanomaterials in aqueous environments is described. Ozone is injected directly into an aqueous dispersion of carbon nanomaterial that is continuously recirculated through a high-powered ultrasonic cell.

Functionalization of carbon nanomaterials is often a critical step that facilitates their integration into larger material systems and devices. In the ...

Synthesis and Characterization of Supramolecular Colloids

Control over colloidal assembly is of utmost importance for the development of functional colloidal materials with tailored structural and mechanical properties for applications in photonics, drug delivery and coating technology. Here we present a new family of colloidal building blocks, coined supramolecular colloids, whose self-assembly is controlled through surface-functionalization with a benzene-1,3,5-tricarboxamide (BTA) derived supramolecular moiety. Such BTAs interact via directional, strong, yet reversible hydrogen-bonds with other identical BTAs. Herein, a protocol is presented that describes how to couple these BTAs to colloids and how to quantify the number of coupling sites, which determines the multivalency of the supramolecular colloids. Light scattering measurements show that the refractive index of the colloids is almost matched with that of the solvent, which strongly reduces the van der Waals forces between the colloids. Before photo-activation, the colloids remain well dispersed, as the BTAs are equipped with a photo-labile group that blocks the formation of hydrogen-bonds. Controlled deprotection with UV-light activates the short-range hydrogen-bonds between the BTAs, which triggers the colloidal self-assembly. The evolution from the dispersed state to the clustered state is monitored by confocal microscopy. These results are further quantified by image analysis with simple routines using ImageJ and Matlab. This merger of supramolecular chemistry and colloidal science offers a direct route towards light- and thermo-responsive colloidal assembly encoded in the surface-grafted monolayer.

A protocol for the synthesis and characterization of colloids coated with supramolecular moieties is described. These supramolecular colloids undergo self-assembly upon the activation of the hydrogen-bonds between the surface-anchored molecules by UV-light.

Control over colloidal assembly is of utmost importance for the development of functional colloidal materials with tailored structural and mechanical ...

Highly Stable, Functional Hairy Nanoparticles and Biopolymers from Wood Fibers: Towards Sustainable Nanotechnology

Nanoparticles, as one of the key materials in nanotechnology and nanomedicine, have gained significant importance during the past decade. While metal-based nanoparticles are associated with synthetic and environmental hassles, cellulose introduces a green, sustainable alternative for nanoparticle synthesis. Here, we present the chemical synthesis and separation procedures to produce new classes of hairy nanoparticles (bearing both amorphous and crystalline regions) and biopolymers based on wood fibers. Through periodate oxidation of soft wood pulp, the glucose ring of cellulose is opened at the C2-C3 bond to form 2,3-dialdehyde groups. Further heating of the partially oxidized fibers (e.g., T = 80 &#176;C) results in three products, namely fibrous oxidized cellulose, sterically stabilized nanocrystalline cellulose (SNCC), and dissolved dialdehyde modified cellulose (DAMC), which are well separated by intermittent centrifugation and co-solvent addition. The partially oxidized fibers (without heating) were used as a highly reactive intermediate to react with chlorite for converting almost all aldehyde to carboxyl groups. Co-solvent precipitation and centrifugation resulted in electrosterically stabilized nanocrystalline cellulose (ENCC) and dicarboxylated cellulose (DCC). The aldehyde content of SNCC and consequently surface charge of ENCC (carboxyl content) were precisely controlled by controlling the periodate oxidation reaction time, resulting in highly stable nanoparticles bearing more than 7 mmol functional groups per gram of nanoparticles (e.g., as compared to conventional NCC bearing &#60;&#60; 1 mmol functional group/g). Atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) attested to the rod-like morphology. Conductometric titration, Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), dynamic light scattering (DLS), electrokinetic-sonic-amplitude (ESA) and acoustic attenuation spectroscopy shed light on the superior properties of these nanomaterials.

Synthesis schemes to prepare highly stable wood fiber-based hairy nanoparticles and functional cellulose-based biopolymers have been detailed.

Nanoparticles, as one of the key materials in nanotechnology and nanomedicine, have gained significant importance during the past decade. While ...

Facile Synthesis of Worm-like Micelles by Visible Light Mediated Dispersion Polymerization Using Photoredox Catalyst

Presented herein is a protocol for the facile synthesis of worm-like micelles by visible light mediated dispersion polymerization. This approach begins with the synthesis of a hydrophilic poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) homopolymer using reversible addition-fragmentation chain-transfer (RAFT) polymerization. Under mild visible light irradiation (&#955; = 460 nm, 0.7 mW/cm2), this macro-chain transfer agent (macro-CTA) in the presence of a ruthenium based photoredox catalyst, Ru(bpy)3Cl2 can be chain extended with a second monomer to form a well-defined block copolymer in a process known as Photoinduced Electron Transfer RAFT (PET-RAFT). When PET-RAFT is used to chain extend POEGMA with benzyl methacrylate (BzMA) in ethanol (EtOH), polymeric nanoparticles with different morphologies are formed in situ according to a polymerization-induced self-assembly (PISA) mechanism. Self-assembly into nanoparticles presenting POEGMA chains at the corona and poly(benzyl methacrylate) (PBzMA) chains in the core occurs in situ due to the growing insolubility of the PBzMA block in ethanol. Interestingly, the formation of highly pure worm-like micelles can be readily monitored by observing the onset of a highly viscous gel in situ due to nanoparticle entanglements occurring during the polymerization. This process thereby allows for a more reproducible synthesis of worm-like micelles simply by monitoring the solution viscosity during the course of the polymerization. In addition, the light stimulus can be intermittently applied in an ON/OFF manner demonstrating temporal control over the nanoparticle morphology.

This article describes a process for producing polymeric self-assembled nanoparticles using visible light mediated dispersion polymerization. Using low energy visible light to control the polymerization allows for the reproducible formation of self-assembled worm-like micelles at high solids content.

Presented herein is a protocol for the facile synthesis of worm-like micelles by visible light mediated dispersion polymerization. This approach begins ...

Preparation of Liquid-exfoliated Transition Metal Dichalcogenide Nanosheets with Controlled Size and Thickness: A State of the Art Protocol

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. Layered crystals of molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are exfoliated in aqueous surfactant solution by sonication. This yields highly polydisperse mixtures containing nanosheets with broad size and thickness distributions. However, for most purposes, specific sizes (in terms of both lateral dimension and thickness) are required. For example, large and thin nanosheets are desired for (opto) electronic applications, while laterally small nanosheets are interesting for catalytic applications. Therefore, post-exfoliation size selection is an important step that we address here. We provide a detailed protocol on the efficient size selection in large quantities by liquid cascade centrifugation and the size and thickness quantification by statistical microscopic analysis (atomic force microscopy and transmission electron microscopy). The comparison of MoS2 and WS2 shows that both materials are size-selected in a similar way by the same procedure. Importantly, the dispersions of size-selected nanosheets show systematic changes in their optical extinction spectra with size due to edge and confinement effects. We show how these optical changes are related quantitatively to the nanosheets dimensions and describe how mean nanosheets length and layer number can be extracted reliably from the extinction spectra. The exfoliation and size selection protocol can be applied to a broad range of layered crystals as we have previously demonstrated for graphene, gallium sulphide (GaS) and black phosphorus.

A protocol for the liquid exfoliation of layered materials to nanosheets, their size selection and size measurement by microscopic and spectroscopic techniques is presented.

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. ...

Engineering

Preparation of Nanoparticles for ToF-SIMS and XPS Analysis

Nanoparticles have gained increasing attention in recent years due to their potential and application in different fields including medicine, cosmetics, chemistry, and their potential to enable advanced materials. To effectively understand and regulate the physico-chemical properties and potential adverse effects of nanoparticles, validated measurement procedures for the various properties of nanoparticles need to be developed. While procedures for measuring nanoparticle size and size distribution are already established, standardized methods for analysis of their surface chemistry are not yet in place, although the influence of the surface chemistry on nanoparticle properties is undisputed. In particular, storage and preparation of nanoparticles for surface analysis strongly influences the analytical results from various methods, and in order to obtain consistent results, sample preparation must be both optimized and standardized. In this contribution, we present, in detail, some standard procedures for preparing nanoparticles for surface analytics. In principle, nanoparticles can be deposited on a suitable substrate from suspension or as a powder. Silicon (Si) wafers are commonly used as substrate, however, their cleaning is critical to the process. For sample preparation from suspension, we will discuss drop-casting and spin-coating, where not only the cleanliness of the substrate and purity of the suspension but also its concentration play important roles for the success of the preparation methodology. For nanoparticles with sensitive ligand shells or coatings, deposition as powders is more suitable, although this method requires particular care in fixing the sample.

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.

Nanoparticles have gained increasing attention in recent years due to their potential and application in different fields including medicine, cosmetics, ...

Gold Nanoparticle Synthesis

Gold nanoparticles (Au nanoparticles) that are ~12 nm in diameter were synthesized by rapidly injecting a solution of 150 mg (0.15 mmol) of tetrachloroauric acid in 3.0 g (3.7 mmol, 3.6 mL) of oleylamine (technical grade) and 3.0 mL of toluene into a boiling solution of 5.1 g (6.4 mmol, 8.7 mL) of oleylamine in 147 mL of toluene. While boiling and mixing the reaction solution for 2 hours, the color of the reaction mixture changed from clear, to light yellow, to light pink, and then slowly to dark red. The heat was then turned off, and the solution was allowed to gradually cool down to room temperature for 1 hour. The gold nanoparticles were then collected and separated from the solution using a centrifuge and washed three times; by vortexing and dispersing the gold nanoparticles in 10 mL portions of toluene, and then precipitating the gold nanoparticles by adding 40 mL portions of methanol and spinning them in a centrifuge. The solution was then decanted to remove any remaining byproducts and unreacted starting materials. Drying the gold nanoparticles in a vacuum environment produced a solid black pellet; which could be stored for long periods of time (up to one year) for later use, and then redissolved in organic solvents such as toluene.

A protocol for synthesizing ~12 nm diameter gold nanoparticles (Au nanoparticles) in an organic solvent is presented. The gold nanoparticles are capped with oleylamine ligands to prevent agglomeration. The gold nanoparticles are soluble in organic solvents such as toluene.

Gold nanoparticles (Au nanoparticles) that are ~12 nm in diameter were synthesized by rapidly injecting a solution of 150 mg (0.15 mmol) of ...

Triplet Fusion Upconversion Nanocapsule Synthesis

Triplet fusion upconversion (UC) allows for the generation of one high energy photon from two low energy input photons. This well-studied process has significant implications for producing high energy light beyond a material's surface. However, the deployment of UC materials has been stymied due to poor material solubility, high concentration requirements, and oxygen sensitivity, ultimately resulting in reduced light output. Toward this end, nanoencapsulation has been a popular motif to circumvent these challenges, but durability has remained elusive in organic solvents. Recently, a nanoencapsulation technique was engineered to tackle each of these challenges, whereupon an oleic acid nanodroplet containing upconversion materials was encapsulated with a silica shell. Ultimately, these nanocapsules (NCs) were durable enough to enable triplet fusion upconversion-facilitated volumetric three-dimensional (3D) printing. By encapsulating upconversion materials with silica and dispersing them in a 3D printing resin, photopatterning beyond the surface of the printing vat was made possible. Here, video protocols for the synthesis of upconversion NCs are presented for both small-scale and large-scale batches. The outlined protocols serve as a starting point for adapting this encapsulation scheme to multiple upconversion schemes for use in volumetric 3D printing applications.

This protocol details the synthesis of upconversion nanocapsules for subsequent use in photopolymerizable resins for triplet fusion upconversion-facilitated volumetric 3D printing.

Triplet fusion upconversion (UC) allows for the generation of one high energy photon from two low energy input photons. This well-studied process has ...

Flash NanoPrecipitation for the Encapsulation of Hydrophobic and Hydrophilic Compounds in Polymeric Nanoparticles

The formulation of a therapeutic compound into nanoparticles (NPs) can impart unique properties. For poorly water-soluble drugs, NP formulations can improve bioavailability and modify drug distribution within the body. For hydrophilic drugs like peptides or proteins, encapsulation within NPs can also provide protection from natural clearance mechanisms. There are few techniques for the production of polymeric NPs that are scalable. Flash NanoPrecipitation (FNP) is a process that uses engineered mixing geometries to produce NPs with narrow size distributions and tunable sizes between 30 and 400 nm. This protocol provides instructions on the laboratory-scale production of core-shell polymeric nanoparticles of a target size using FNP. The protocol can be implemented to encapsulate either hydrophilic or hydrophobic compounds with only minor modifications. The technique can be readily employed in the laboratory at milligram scale to screen formulations. Lead hits can directly be scaled up to gram- and kilogram-scale. As a continuous process, scale-up involves longer mixing process run time rather than translation to new process vessels. NPs produced by FNP are highly loaded with therapeutic, feature a dense stabilizing polymer brush, and have a size reproducibility of &#177; 6%.

Flash NanoPrecipitation (FNP) is a scalable approach to produce polymeric core-shell nanoparticles. Lab-scale formulations for the encapsulation of hydrophobic or hydrophilic therapeutics are described.

The formulation of a therapeutic compound into nanoparticles (NPs) can impart unique properties. For poorly water-soluble drugs, NP formulations can ...

Dispersion of Nanomaterials in Aqueous Media: Towards Protocol Optimization

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