Orthogonal analysis of functional gold nanoparticles for biomedical applications

Characterization Challenges of Self-Assembled Polymer-SPIONs Nanoparticles: Benefits of Orthogonal Methods

Size and zeta potential are critical physicochemical properties of nanoparticles (NPs), influencing their biological activity and safety profile. These are essential for further industrial upscale and clinical success. However, the characterization of polydisperse, non-spherical NPs is a challenge for traditional characterization techniques (ex., dynamic light scattering (DLS)). In this paper, superparamagnetic iron oxide nanoparticles (SPIONs) were coated with polyvinyl alcohol (PVAL) exhibiting different terminal groups at their surface, either hydroxyl (OH), carboxyl (COOH) or amino (NH₂) end groups. Size, zeta potential and concentration were characterized by orthogonal methods, namely, batch DLS, nanoparticle tracking analysis (NTA), tunable resistive pulse sensing (TRPS), transmission electron microscopy (TEM), asymmetric flow field flow fractionation (AF4) coupled to multi-angle light scattering (MALS), UV-Visible and online DLS. Finally, coated SPIONs were incubated with albumin, and size changes were monitored by AF4-MALS-UV-DLS. NTA showed the biggest mean sizes, even though DLS PVAL-COOH SPION graphs presented aggregates in the micrometer range. TRPS detected more NPs in suspension than NTA. Finally, AF4-MALS-UV-DLS could successfully resolve the different sizes of the coated SPION suspensions. The results highlight the importance of combining techniques with different principles for NPs characterization. The advantages and limitations of each method are discussed here.

Understanding the Adsorption of Peptides and Proteins onto PEGylated Gold Nanoparticles

Polyethylene glycol (PEG) surface conjugations are widely employed to render passivating properties to nanoparticles in biological applications. The benefits of surface passivation by PEG are reduced protein adsorption, diminished non-specific interactions, and improvement in pharmacokinetics. However, the limitations of PEG passivation remain an active area of research, and recent examples from the literature demonstrate how PEG passivation can fail. Here, we study the adsorption amount of biomolecules to PEGylated gold nanoparticles (AuNPs), focusing on how different protein properties influence binding. The AuNPs are PEGylated with three different sizes of conjugated PEG chains, and we examine interactions with proteins of different sizes, charges, and surface cysteine content. The experiments are carried out in vitro at physiologically relevant timescales to obtain the adsorption amounts and rates of each biomolecule on AuNP-PEGs of varying compositions. Our findings are relevant in understanding how protein size and the surface cysteine content affect binding, and our work reveals that cysteine residues can dramatically increase adsorption rates on PEGylated AuNPs. Moreover, shorter chain PEG molecules passivate the AuNP surface more effectively against all protein types.

Sizing up the Next Generation of Nanomedicines

During the past two decades the nanomedicine field has experienced significant progress. To date, over sixty nanoparticle (NP) formulations have been approved in the US and EU while many others are in clinical or preclinical development, indicating a concerted effort to translate promising bench research to commercially viable pharmaceutical products. The use of NPs as novel drug delivery systems, for example, can improve drug safety and efficacy profiles and enable access to intracellular domains of diseased cells, thus paving the way to previously intractable biological targets. However, the measurement of their physicochemical properties presents substantial challenges relative to conventional injectable formulations. In this perspective, we focus exclusively on particle size, a core property and critical quality attribute of nanomedicines. We present an overview of relevant state-of-the-art technologies for particle sizing, highlighting the main parameters that can influence the selection of techniques suitable for a specific size range or material. We consider the increasing need, and associated challenge, to measure size in physiologically relevant media. We detail the importance of standards, key to validate any measurement, and the need for suitable reference materials for processes used to characterize novel and complex NPs. This perspective highlights issues critical to achieve compliance with regulatory guidelines and to support research and manufacturing quality control.

A facile quantification of hyaluronic acid and its crosslinking using gas-phase electrophoresis

We report a facile, high-resolution approach to quantitatively characterize hyaluronic acid (HA) and study its crosslinking reaction using electrospray-differential mobility analysis (ES-DMA). Mobility size distributions, number concentrations, molecular mass distributions, and polydispersity index of HAs were obtained successfully via a rapid analysis by ES-DMA (< 30 min). The limit of detection, the limit of quantification, and the precision of the mobility size measurement achieve 2.5 nm, 4.0 nm, and 0.3 nm, respectively. Size exclusion chromatography (SEC) was employed as an orthogonal approach, showing that the averaged molecular mass and polydispersity index of HA measured by ES-DMA were close to the results of SEC on a semi-quantitative basis. The 1,4-butanediol diglycidyl ether (BDDE)-induced crosslinking of HA was also able to be successfully characterized through a time-dependent study using ES-DMA, which has shown the promise of direct analysis of solution-based reactions. Both the extent and the rate of HA crosslinking (induced by BDDE) were proportional to reaction temperature and concentration ratio of HA to BDDE. The activation energy of the reaction-limited BDDE-induced crosslinking of HA was found to be ≈ 21 kJ/mol. The prototype study demonstrates ES-DMA as a new method for a rapid quantitative characterization of HA and its derivative product and providing a capability of real-time monitoring of the HA crosslinking during formulation process. Graphical abstract.

Transformation of engineered nanomaterials through the prism of silver sulfidation

Understanding the structure transformation of engineered nanomaterials (ENMs) is a grand measurement challenge, which impacts many aspects of ENMs applications, such as their efficacy, safety, and environmental consequence. To address the significant knowledge gap regarding the fundamental kinetic rate and extent of ENM transformation in the environment, we present a comprehensive and mechanistic structural investigation of the transformation, aggregation, and dissolution behavior of a polyvinylpyrrolidone-coated silver nanoparticle (AgNP) suspension upon sulfidation in moderately reduced hard water with fulvic acid and dissolved Na₂S. This reaction is among the most prevalent and industrially and environmentally relevant ENMs transformation. Using ex situ transmission electron microscopy (TEM) and both in situ and ex situ synchrotron-based small angle X-ray scattering (SAXS) and X-ray diffraction (XRD), we find that sulfidation of faceted AgNPs strongly depends on the crystallographic orientation of the facets, with nanometer-scale passivation layers developed on {111} and {100} facets and continuous nucleation and growth on {110} facets. Nanobeam electron diffraction and atomic resolution imaging show Ag and Ag₂S domains both possess a high degree of crystalline order, contradicting amorphous structures as previously reported. In situ SAXS/XRD allowed simultaneous determination of the morphological changes and extent of sulfidation of AgNPs. SAXS/XRD results strongly indicate sulfidation follows first-order reaction kinetics without any aggregation. Aided by their size monodispersity, for the first time, using direct, in situ morphology and atomic-structure probes whose results mutually corroborate, we unequivocally determined the sulfidation rate constant of AgNPs under an environmentally relevant condition (~0.013 min^-1 for 68 nm diameter AgNPs). A rigorous analysis of the long-term sulfidation product of the AgNPs under different S/Ag ratios using ex situ SAXS/XRD clearly demonstrates that the silver mass in the original AgNP and transformed Ag/Ag₂S NP is preserved. This result has important environmental implications, strongly suggesting that Ag⁺ ions, a known highly effective antimicrobial agent, are not leached into the solution during sulfidation of AgNPs. The combined nondestructive methodology can be extended to unfold the structure transformation pathway and kinetics in a broad range of ENM systems.

Thermal Stability of Metal-Organic Frameworks and Encapsulation of CuO Nanocrystals for Highly Active Catalysis

We report an aerosol-based approach to study the thermal stability of metal-organic frameworks (MOFs) for gas-phase synthesis of MOF-based hybrid nanostructures used for highly active catalysis. Temperature-programmed electrospray-differential mobility analysis (TP-ES-DMA) provides the characterization of temperature-dependent morphological change directly in the gas phase, and the results are shown to be highly correlated with the structural thermal stability of MOFs determined by the traditional measurements of porosity and crystallinity. The results show that MOFs underwent thermal decomposition via simultaneous disassembly and deaggregation. Trimeric Cr-based MIL-88B-NH₂ exhibited a higher temperature of decomposition ( T_d), 350 °C, than trimeric Fe-based MIL-88B-NH₂, 250 °C. For UiO-66, a significant decrease of T_d by ≈100 °C was observed by using amine-functionalized ligands in the MOF structure. Copper oxide nanocrystals were successfully encapsulated in the UiO-66 crystal (Cu _xO@UiO-66) by using a gas-phase evaporation-induced self-assembly approach followed by a suitable thermal treatment below T_d (i.e., determined by TP-ES-DMA). Cu _xO@UiO-66 demonstrated a very high catalytic activity and stability to CO oxidation, showing at least a 3-time increase in CO conversion compared to the bare CuO nanoparticle samples. The study demonstrates a prototype methodology (1) to determine structural thermal stability of MOFs using a gas-phase electrophoretic method (TP-ES-DMA) and (2) to gas-phase synthesize CuO nanocrystals encapsulated in MOFs.

Structure and Dynamics of Bimodal Colloidal Dispersions in a Low-Molecular-Weight Polymer Solution

We present an experimental study of the structural and dynamical properties of bimodal, micrometer-sized colloidal dispersions (size ratio ≈ 2) in an aqueous solution of low-molecular-weight polymer (polyethylene glycol 2000) using synchrotron ultra-small angle X-ray scattering (USAXS) and USAXS-based X-ray photon correlation spectroscopy. We fixed the volume fraction of the large particles at 5% and systematically increased the volume fraction of the small particles from 0 to 5% to evaluate their effects on the structure and dynamics. The bimodal dispersions were homogenous through the investigated parameter space. We found that the partial structure factors can be satisfactorily retrieved for the bimodal colloidal dispersions using a Percus-Yevick hard-sphere potential when the size distributions of the particles were taken into account. We also found that the partial structure factor between the large particles did not exhibit a significant variation with increasing volume fraction of the small particles, whereas the isothermal compressibility of the binary mixture was found to decrease with increasing volume fraction of the small particles. The dynamics of single-component large-particle dispersion obey the principles of de Gennes narrowing, where the wave vector dependence of the interparticle diffusion coefficient is inversely proportional to the interparticle structure factor. The dynamics of the bimodal dispersions demonstrate a strong dependence on the fraction of small particles. We also made a comparison between the experimental effective dynamic viscosity of the bimodal dispersion with the theoretical predictions, which suggest that the complex mutual interactions between the large and small particles have a strong effect on the dynamic behaviors of bimodal dispersions.

Gas-phase self-assembly of uniform silica nanostructures decorated and doped with silver nanoparticles

We report a systematic study of the controlled gas-phase synthesis of silver-silica hybrid nanostructures (Ag-SiO₂ NP) using the concept of evaporation-induced self-assembly. The approach includes the use of a direct gas-phase electrophoresis for size classification and in situ characterization of mobility size. Transmission electron microscopy and ultraviolet-visible light spectroscopy were employed complementarily to determine the morphology and surface plasmon resonance of Ag-SiO₂ NP. Results show that two types of Ag-SiO₂ NPs were successfully synthesized: (1) AgNPs decorated on a SiO₂-NP (Ag-T-SiO₂ NP), and (2) AgNPs doped in a cluster of SiO₂-NPs (Ag-C-SiO₂ NP). The physical size, morphology, and compositions of Ag-SiO₂ NPs were tunable through the adjustments of precursor concentrations and the selected mobility sizes. The results also show that SPR performance, colloidal stability, and dispersibility of AgNPs enhanced significantly in an aqueous environment after the hybridization with SiO₂-NP (especially for Ag-C-SiO₂ NP). The results and corresponding methodology summarized here provide the proof of concept to fabricate high-purity AgNP-based hybrid nanostructures through gas-phase evaporation-induced self-assembly for future biomedical applications (e.g., hyperthermal therapy, targeted drug delivery, and antibacterial applications).

Surface PEGylation of Silver Nanoparticles: Kinetics of Simultaneous Surface Dissolution and Molecular Desorption

A quantitative study of the stability of silver nanoparticles (AgNPs) conjugated with thiolated polyethylene glycol (SH-PEG) was conducted using gas-phase ion-mobility and mass analyses. The extents of aggregation and surface dissolution of AgNPs, as well as the amount of SH-PEG adsorption and desorption, were able to be characterized simultaneously for the kinetic study. The results show that the SH-PEG with a molecular mass of 6 kg/mol (SH-PEG6K) was able to adsorb to the surface of AgNP to form PEG6K-HS-AgNP conjugates, with the maximum surface adsorbate density of ∼0.10 nm(-2). The equilibrium binding constant for SH-PEG6K on AgNPs was calculated as ∼(4.4 ± 0.9) × 10(5) L/mol, suggesting a strong affinity due to thiol bonding to the AgNP surface. The formation of SH-PEG6K corona prevented PEG6K-HS-AgNP conjugates from aggregation under the acidic environment (pH 1.5), but dissolution of core AgNPs occurred following a first-order reaction. The rate constant of Ag dissolution from PEG6K-HS-AgNP was independent of the starting surface packing density of SH-PEG6K on AgNP (σ0), indicating that the interactions of H(+) with core AgNP were not interfered by the presence of SH-PEG6K corona. The surface packing density of SH-PEG6K decreased simultaneously following a first-order reaction, and the desorption rate constant of SH-PEG6K from the conjugates was proportional to σ0. Our work presents the first quantitative study to illustrate the complex mechanism that involves simultaneous aggregation and dissolution of core AgNPs in combination with adsorption and desorption of SH-PEG. This work also provides a prototype method of coupled experimental scheme to quantify the change of particle mass versus the corresponding surface density of functional molecular species on nanoparticles.

Assembly of functional gold nanoparticle on silica microsphere

We demonstrate a controlled synthesis of silica microsphere with the surface-decorated functional gold nanoparticles. Surface of silica microsphere was modified by 3-aminopropypltriethoxysilane and 3-aminopropyldimethylethoxysilane to generate a positive electric field, by which the gold nanoparticles with the negative charges (unconjugated, thiolated polyethylene glycol functionalized with the traceable packing density and conformation) were able to be attracted to the silica microsphere. Results show that both the molecular conjugation on gold nanoparticle and the uniformity in the amino-silanization of silica microsphere influenced the loading and the homogeneity of gold nanoparticles on silica microsphere. The 3-aminopropyldimethylethoxysilane-functionalized silica microsphere provided an uniform field to attract gold nanoparticles. Increasing the ethanol content in aminosilane solution significantly improved the homogeneity and the loading of gold nanoparticles on the surface of silica microsphere. For the gold nanoparticle, increasing the molecular mass of polyethylene glycol yielded a greater homogeneity but a lower loading on silica microsphere. Bovine serum albumin induced the desorption of gold nanoparticles from silica microsphere, where the extent of desorption was suppressed by the presence of high-molecular mass polyethylene glycol on gold nanoparticles. This work provides the fundamental understanding for the synthesis of gold nanoparticle-silica microsphere constructs useful to the applications in chemo-radioactive therapeutics.