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Taylor Dispersion of Nanoparticles Published in "Journal of Nanoparticle Research 19 (8): 287, 2017" which should be cited to refer to this work. Taylor dispersion of nanoparticles Sandor Balog & Dominic A. Urban & Ana M. Milosevic & Federica Crippa & Barbara Rothen-Rutishauser & Alke Petri-Fink Abstract The ability to detect and accurately character- sizes determined by transmission electron microscopy ize particles is required by many fields of nanotechnol- confirms that our approach is model-independent, non- ogy, including materials science, nanotoxicology, and parametric, and of general validity that provides an nanomedicine. Among the most relevant physicochem- accurate account of size polydispersity—independently ical properties of nanoparticles, size and the related on the shape of the size distribution and without any surface-to-volume ratio are fundamental ones. Taylor assumption required a priori. dispersion combines three independent phenomena to determine particle size: optical extinction, translational Keywords Nanoparticle . Nanoparticle size diffusion, and sheer-enhanced dispersion of nanoparti- characterization . Nanopowder. Polydispersity. Taylor cles subjected to a steady laminar flow. The interplay of dispersion these defines the apparent size. Considering that parti- cles in fact are never truly uniform nor monodisperse, we rigorously address particle polydispersity and calcu- Introduction late the apparent particle size measured by Taylor dis- persion analysis. We conducted case studies addressing Let it be beneficial or adverse, most often it is the size aqueous suspensions of model particles and large-scale- that grants unique physicochemical properties to nano- produced Bindustrial^ particles of both academic and particles (NPs). Accurate characterization is necessary, commercial interest of various core materials and sizes, and over the last decade many methods have been ranging from 15 to 100 nm. A comparison with particle applied for characterizing the size of NPs (Allen 1990; Borsali and Pecora 2008), including dynamic light scat- http://doc.rero.ch Sandor Balog and Dominic A. Urban contributed equally. tering (DLS), fluorescence correlation spectroscopy, UV-vis spectroscopy, electron microscopy, particle tracking, atomic force microscopy, field-flow-fraction- ation, and analytical ultracentrifugation. Most laborato- ries are familiar with the majority of these techniques S. Balog (*) : D. A. Urban : A. M. Milosevic : F. Crippa : B. Rothen-Rutishauser : A. Petri-Fink and their merit is indisputable. However, depending on Adolphe Merkle Institute, University of Fribourg, Chemin des the material, each method can reach its limits in a Verdiers 4, 1700 Fribourg, Switzerland number of situations, such as the case of a highly e-mail: [email protected] heterogeneous sample or the presence of a complex A. Petri-Fink multicomponent matrix (Laborda et al. 2016; Bolea Chemistry Department, University of Fribourg, Chemin du Musée et al. 2016; Moore et al. 2015) For example, fluores- 9, 1700 Fribourg, Switzerland cence correlation spectroscopy requires dye labeling 1 and UV-vis spectroscopy is essentially limited to plas- Cipelletti et al. 2014, Biron et al. 2014; Cipelletti et al. monic NPs. Standard electron microscopy is inherently 2015; Biron et al. 2015; Cotett 2010;d’Orlye et al. an ex situ approach and generally cannot describe Bsoft^ 2008; Varenne et al. 2008, Goodall 2010;Haweetal. features—such as a polymer shell—due to either poor 2011; Hulse et al. 2011; Hulse and Forbes 2011; contrast or simply the lack of a liquid environment. Oukacine et al. 2015; Morel et al. 2015; Pyell et al. Particle tracking analysis, similarly to electron micros- 2015; Jalil et al. 2015; Saux and Cottet 2008; Wuelfing copy, is an alternative that is able to provide number- et al. 1999; Templeton et al. 1999). Lately TDA analysis averaged particle size distribution, but unfortunately, it has been used to characterize moderately polydisperse is unable to reliably detect and quantify particles smaller polymers and their mixtures (Cipelletti et al. 2014; than 10–15 nm—even with strong scattering contrast, Biron et al. 2014; Cipelletti et al. 2015; Biron et al. such as the one offered by gold (Filipe et al. 2010;Hawe 2015). Particles—and especially inorganic NPs—are et al. 2010). While DLS does have the sensitivity that nonetheless quite different from small molecules, pro- particle tracking and electron microscopy cannot offer, it teins, and polymers in several aspects, which actually is well known that this very sensitivity becomes a seri- renders them to be a quite remarkable class of materials ous shortcoming when samples are heterogeneous, for (Giner-Casares et al. 2016; Henriksen-Lacey et al. the capacity of measuring the true value, and the repro- 2016). First, unlike molecules and proteins, NPs are ducibility are both prone to severely suffer from non- never truly uniform in shape or monodisperse in size. purified samples. However, nowadays, to reach an un- Second, the optical extinction of a given NP is a func- biased verdict about the definition of poorly purified is tion of its size, and depending on material and the far from trivial. Given the ever-growing complexity of wavelength of observation, optical extinction may be suspensions of NPs—which by now come in nearly any dominated by either absorption or scattering. sizes and shapes, with nontrivial composition and multi- To the best of our knowledge, the literature nei- functionalities—there are countless situations where ther provides theoretical basis nor describes any samples cannot be fractionated or purified further— TDA experiment taking into account the polydisper- even if it is desired for the subsequent characterization. sity of inorganic NPs. Therefore, here we focus on In fact, many materials from academic and industrial deriving an analytical approach that interprets and labs are characterized in the very state received, prefer- analyses TDA spectra from polydisperse NPs. Our ably by using a non-destructive method. primary interest is a model-independent and non- Taylor dispersion analysis (TDA) is an in situ and parametric description of general validity. Indepen- non-destructive experimental technique determining the dently of the true shape of the size distribution—and translational diffusion coefficient from a very small without any assumption required a priori—our ap- sample volume (from nanoliters to microliters), and proach provides an accurate account of size polydis- the equivalent hydrodynamic radius is calculated via persity and quantifies the apparent average size de- the Stokes-Einstein equation (Einstein 1905, 1906, termined by TDA. 1911). Although TDA is not new, the phenomenon After validating the approach by determining the size http://doc.rero.ch and the related method were described already in the of a well-known protein, we investigated aqueous sus- 1950s (Aris 1956;Taylor1953, 1954), it has been pensions of dielectric and metallic NPs of various sizes. recently reintroduced to measuring the size of NPs The selected NPs are model nanoparticles of academic (Cipelletti et al. 2014; Biron et al. 2014; Cipelletti interest (superparamagnetic iron oxide NPs (SPIONs), et al. 2015; Biron et al. 2015; d’Orlye et al. 2008; gold (Au) NPs, and silica (SiO2) NPs synthesized by us) Varenne et al. 2008; Hawe et al. 2011; Hulse et al. and particles with considerable commercial interest and 2011; Oukacine et al. 2015; Morel et al. 2015; Pyell produced on industrial scale (zinc oxide (ZnO), and et al. 2015; Jalil et al. 2015; Wuelfing et al. 1999; titania (TiO2) nanopowders, and copper (Cu) suspen- Templeton et al. 1999). Traditionally, TDA was applied sions). While Cu suspensions are used, e.g., for wood to observe chemical reactions (Chamieh et al. 2015; impregnation, SPIONs and Au NPs are model particles Biron et al. 2015; Cotett 2010) and characterize small studied in relation to biomedical applications, and SiO2 molecules, proteins, and macro molecules (Bello et al. NPs are widely found in food and agricultural technol- 1994; Rezzonico et al. 1994; Belongia and Baygents ogies. TiO2 is the most widely used white pigment, for 1997; Chamieh et al. 2012; Oukacine et al. 2012; example in toothpastes, and it sees a growing demand as 2 a photocatalyst. The most known application of ZnO— dispersion in MilliQ water (Millipore purification sys- produced also in multi-ton volumes—is perhaps sun tem, Bedford, MA, USA). protection in cosmetic products. SiO2 NPs The SiO2 NPs were synthesized by following a modified procedure of the Stöber synthesis, de- Materials and methods scribed by Giesche et al. (1994). A mixture of ethanol, MilliQ water, and ammonia (VWR, SPIONs synthesis The iron oxide nanoparticles were Dietikon, Switzerland) were transferred to a round synthetized by thermal decomposition, based on the bottom flask, heated with the help of an oil bath, methoddescribedbyParketal.(2004) (An et al. and stirred with a magnetic stirrer. TEOS was trans- 2004). Iron oleate complex—obtained by reacting iron ferred rapidly to the round bottom flask and con- chloride (FeCl3∙6H2O) (Sigma-Aldrich, St. Louis, USA) tinuously stirred for several hours. Next, the etha- and sodium oleate (Sigma-Aldrich, Sl. Louis, USA)— nol was removed from the suspension by evapora- was heated to and kept at 320 °C for 30 min in the tion under reduced atmosphere. The particles were presence of oleic acid (Sigma-Aldrich, St. Louis, dialyzed in water for 5 days. USA). After the formation of particles, the
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