A hydrofluoric acid-free method to dissolve and quantify silica nanoparticles in aqueous and solid matrices.

As the commercial use of synthetic amorphous silica nanomaterials (SiO2-NPs) increases, their effects on the environment and human health have still not been explored in detail. An often-insurmountable obstacle for SiO2-NP fate and hazard research is the challenging analytics of solid particulate silica species, which involves toxic and corrosive hydrofluoric acid (HF). We therefore developed and validated a set of simple hydrofluoric acid-free sample preparation methods for the quantification of amorphous SiO2 micro- and nanoparticles. To circumvent HF, we dissolved the SiO2-NPs by base-catalyzed hydrolysis at room temperature or under microwave irradiation using potassium hydroxide, replacing the stabilizing fluoride ions with OH-, and exploiting the stability of the orthosilicic acid monomer under a strongly basic pH. Inductively coupled plasma - optical emission spectroscopy (ICP-OES) or a colorimetric assay served to quantify silicon. The lowest KOH: SiO2 molar ratio to effectively dissolve and quantify SiO2-NPs was 1.2 for colloidal Stöber SiO2-NPs at a pH >12. Fumed SiO2-NPs (Aerosil®) or food grade SiO2 (E551) containing SiO2-NPs were degradable at higher KOH: SiO2 ratios >8000. Thus, hydrofluoric acid-free SiO2-NP digestion protocols based on KOH present an effective (recoveries of >84%), less hazardous, and easy to implement alternative to current methods.

Nanoparticle administration method in cell culture alters particle-cell interaction.

As a highly interdisciplinary field, working with nanoparticles in a biomedical context requires a robust understanding of soft matter physics, colloidal behaviors, nano-characterization methods, biology, and bio-nano interactions. When reporting results, it can be easy to overlook simple, seemingly trivial experimental details. In this context, we set out to understand how in vitro technique, specifically the way we administer particles in 2D culture, can influence experimental outcomes. Gold nanoparticles coated with poly(vinylpyrrolidone) were added to J774A.1 mouse monocyte/macrophage cultures as either a concentrated bolus, a bolus then mixed via aspiration, or pre-mixed in cell culture media. Particle-cell interaction was monitored via inductively coupled plasma-optical emission spectroscopy and we found that particles administered in a concentrated dose interacted more with cells compared to the pre-mixed administration method. Spectroscopy studies reveal that the initial formation of the protein corona upon introduction to cell culture media may be responsible for the differences in particle-cell interaction. Modeling of particle deposition using the in vitro sedimentation, diffusion and dosimetry model helped to clarify what particle phenomena may be occurring at the cellular interface. We found that particle administration method in vitro has an effect on particle-cell interactions (i.e. cellular adsorption and uptake). Initial introduction of particles in to complex biological media has a lasting effect on the formation of the protein corona, which in turn mediates particle-cell interaction. It is of note that a minor detail, the way in which we administer particles in cell culture, can have a significant effect on what we observe regarding particle interactions in vitro.

Carbon nanodots: Opportunities and limitations to study their biodistribution at the human lung epithelial tissue barrier.

Inhalation of combustion-derived ultrafine particles (≤0.1 µm) has been found to be associated with pulmonary and cardiovascular diseases. However, correlation of the physicochemical properties of carbon-based particles such as surface charge and agglomeration state with adverse health effects has not yet been established, mainly due to limitations related to the detection of carbon particles in biological environments. The authors have therefore applied model particles as mimics of simplified particles derived from incomplete combustion, namely, carbon nanodots (CNDs) with different surface modifications and fluorescent properties. Their possible adverse cellular effects and their biodistribution pattern were assessed in a three-dimensional (3D) lung epithelial tissue model. Three different CNDs, namely, nitrogen, sulfur codoped CNDs ( N,S-CNDs) and nitrogen doped CNDs ( N-CNDs-1 and N-CNDs-2), were prepared by microwave-assisted hydrothermal carbonization using different precursors or different microwave systems. These CNDs were found to possess different chemical and photophysical properties. The surfaces of nanodots N-CNDs-1 and N-CNDs-2 were positively charged or neutral, respectively, arguably due to the presence of amine and amide groups, while the surfaces of N,S-CNDs were negatively charged, as they bear carboxylic groups in addition to amine and amide groups. Photophysical measurements showed that these three types of CNDs displayed strong photon absorption in the UV range. Both N-CNDs-1 and N,S-CNDs showed weak fluorescence emission, whereas N-CNDs-2 showed intense emission. A 3D human lung model composed of alveolar epithelial cells (A549 cell line) and two primary immune cells, i.e., macrophages and dendritic cells, was exposed to CNDs via a pseudo-air-liquid interface at a concentration of 100 µg/ml. Exposure to these particles for 24 h induced no harmful effect on the cells as assessed by cytotoxicity, cell layer integrity, cell morphology, oxidative stress, and proinflammatory cytokines release. The distribution of the CNDs in the lung model was estimated by measuring the fluorescence intensity in three different fractions, e.g., apical, intracellular, and basal, after 1, 4, and 24 h of incubation, whereby reliable results were only obtained for N-CNDs-2. It was shown that N-CNDs-2 translocate rapidly, i.e., >40% in the basal fraction within 1 h and almost 100% after 4 h, while ca. 80% of the N-CNDs-1 and N,S-CNDs were still located on the apical surface of the lung cells after 1 h. This could be attributed to the agglomeration behavior of N-CNDs-1 or N,S-CNDs. The surface properties of the N-CNDs bearing amino and amide groups likely induce greater uptake as N-CNDs could be detected intracellularly. This was less evident for N,S-CNDs, which bear carboxylic acid groups on their surface. In conclusion, CNDs have been designed as model systems for carbon-based particles; however, their small size and agglomeration behavior made their quantification by fluorescence measurement challenging. Nevertheless, it was demonstrated that the surface properties and agglomeration affected the biodistribution of the particles at the lung epithelial barrier in vitro.

Involvement of two uptake mechanisms of gold and iron oxide nanoparticles in a co-exposure scenario using mouse macrophages.

Little is known about the simultaneous uptake of different engineered nanoparticle types, as it can be expected in our daily life. In order to test such co-exposure effects, murine macrophages (J774A.1 cell line) were incubated with gold (AuNPs) and iron oxide nanoparticles (FeO x NPs) either alone or combined. Environmental scanning electron microscopy revealed that single NPs of both types bound within minutes on the cell surface but with a distinctive difference between FeO x NPs and AuNPs. Uptake analysis studies based on laser scanning microscopy, transmission electron microscopy, and inductively coupled plasma optical emission spectrometry revealed intracellular appearance of both NP types in all exposure scenarios and a time-dependent increase. This increase was higher for both AuNPs and FeO x NPs during co-exposure. Cells treated with endocytotic inhibitors recovered after co-exposure, which additionally hinted that two uptake mechanisms are involved. Cross-talk between uptake pathways is relevant for toxicological studies: Co-exposure acts as an uptake accelerant. If the goal is to maximize the cellular uptake, e.g., for the delivery of pharmaceutical agents, this can be beneficial. However, co-exposure should also be taken into account in the case of risk assessment of occupational settings. The demonstration of co-exposure-invoked pathway interactions reveals that synergetic nanoparticle effects, either positive or negative, must be considered for nanotechnology and nanomedicine in particular to develop to its full potential.

Speckle-Visibility Spectroscopy of Depolarized Dynamic Light Scattering.

We show that the statistical analysis of photon counts in depolarized dynamic light scattering experiments allows for the accurate characterization of the rotational Brownian dynamics of particles. Unlike photon correlation spectroscopy, the technique is accurate even at low temporal resolution and enables discontinuous data acquisition, which offers several advantages. To demonstrate the usefulness of the method, we present a case study in which we analyze aqueous suspensions of tunicate cellulose nanocrystals and silica particles, and discuss aspects that are specific to particle sizing.

A new angle on dynamic depolarized light scattering: number-averaged size distribution of nanoparticles in focus.

Size polydispersity is a common phenomenon that strongly influences the physicochemical properties of nanoparticles (NPs). We present an analytical approach that is universally applicable to characterizing optically anisotropic round NPs and determines directly the number-averaged size distribution and polydispersity via depolarized dynamic light scattering (DDLS). To demonstrate, we use aqueous suspensions of Au NPs of different sizes and surface functionalization.

Plasmonic nanoparticles and their characterization in physiological fluids.

Nanoparticles possess unique properties beyond that of classical materials, and while these properties can be used for designing a dedicated functionality, they may also pose a problem to living organisms, to human health and the environment. The specific primary routes by which nanoparticles may interact with the human body include inhalation, injection, ingestion and application to the skin. Independent of the entry route, the particles inevitably encounter a complex physiological fluid populated with e.g. proteins, vitamins, lipids and salts/ions. Different consequences of such an encounter may include formation of a surface-bound protein layer, particle dissolution or aggregation, which are expected to have a crucial impact on cellular interaction. Understanding cellular responses to nanoparticle interactions starts with understanding particle behavior in physiological fluids. Nanoparticles are now available in practically any size, shape and functionalization, to promote distinct optical, magnetic, and physico-chemical properties, making the prediction of their behavior, in physiological fluids, not a trivial task. Characterization has therefore become of paramount importance. In this review, we give an overview about the diversity of physiological fluids as well as present an inventory of the most relevant experimental techniques used to study plasmonic nanoparticles.

Characterization of hydrophilic coated gold nanoparticles via capillary electrophoresis and Taylor dispersion analysis. Part II: Determination of the hydrodynamic radius distribution - Comparison with asymmetric flow field-flow fractionation.

In the first paper of this series we have shown for hydrophilic coated Au nanoparticles that capillary electrophoresis in combination with Taylor dispersion analysis in fused silica capillaries with an inner diameter of 75 µm allows for the unbiased precise determination of the number-weighted mean hydrodynamic diameter, the zeta potential and the effective charge number, although mobility corrected double layer polarization has to be taken into account. In this second paper we investigate whether the modified approximate analytic expression developed by Ohshima (2001) permits the calculation of calibration lines and the concomitant conversion of electropherograms into number-weighted particle radius distributions. We show that with the method developed size distributions are obtained which are independent of the measurement conditions. These size distributions are much narrower than those obtained via dynamic light scattering and data evaluation by the CONTIN algorithm. Capillary electrophoresis together with the proposed data evaluation method reveals that the analyzed nanoparticle populations have very narrow size distributions with a width of 2-4 nm. The hydrodynamic radius distributions of the coated NPs are only slightly broader than the solid particle radius distribution of the Au-NP cores. The presence of monomodal/bimodal size distributions is confirmed by asymmetric flow field-flow fractionation.

Nanoparticle colloidal stability in cell culture media and impact on cellular interactions.

Nanomaterials are finding increasing use for biomedical applications such as imaging, diagnostics, and drug delivery. While it is well understood that nanoparticle (NP) physico-chemical properties can dictate biological responses and interactions, it has been difficult to outline a unifying framework to directly link NP properties to expected in vitro and in vivo outcomes. When introduced to complex biological media containing electrolytes, proteins, lipids, etc., nanoparticles (NPs) are subjected to a range of forces which determine their behavior in this environment. One aspect of NP behavior in biological systems that is often understated or overlooked is aggregation. NP aggregation will significantly alter in vitro behavior (dosimetry, NP uptake, cytotoxicity), as well as in vivo fate (pharmacokinetics, toxicity, biodistribution). Thus, understanding the factors driving NP colloidal stability and aggregation is paramount. Furthermore, studying biological interactions with NPs at the nanoscale level requires an interdisciplinary effort with a robust understanding of multiple characterization techniques. This review examines the factors that determine NP colloidal stability, the various efforts to stabilize NP in biological media, the methods to characterize NP colloidal stability in situ, and provides a discussion regarding NP interactions with cells.