Natural Organic Matter Concentration Impacts the Interaction of Functionalized Diamond Nanoparticles with Model and Actual Bacterial Membranes

Changes to nanoparticle surface charge, colloidal stability, and hydrodynamic properties induced by interaction with natural organic matter (NOM) warrant consideration in assessing the potential for these materials to adversely impact organisms in the environment. Here, we show that acquisition of a coating, or “corona”, of NOM alters the hydrodynamic and electrokinetic properties of diamond nanoparticles (DNPs) functionalized with the polycation poly­(allylamine HCl) in a manner that depends on the NOM-to-DNP concentration ratio. The NOM-induced changes to DNP properties alter subsequent interactions with model biological membranes and the Gram-negative bacterium <i>Shewanella oneidensis</i> MR-1. Suwannee River NOM induces changes to DNP hydrodynamic diameter and apparent ζ-potential in a concentration-dependent manner. At low NOM-to-DNP ratios, DNPs aggregate to a limited extent but retain a positive ζ-potential apparently due to nonuniform adsorption of NOM molecules leading to attractive electrostatic interactions between oppositely charged regions on adjacent DNP surfaces. Diamond nanoparticles at low NOM-to-DNP ratios attach to model membranes to a larger extent than in the absence of NOM (including those incorporating lipopolysaccharide, a major bacterial outer membrane component) and induce a comparable degree of membrane damage and toxicity to <i>S. oneidensis</i>. At higher NOM-to-DNP ratios, DNP charge is reversed, and DNP aggregates remain stable in suspension. This charge reversal eliminates DNP attachment to model membranes containing the highest LPS contents studied due to electrostatic repulsion and abolishes membrane damage to <i>S. oneidensis</i>. Our results demonstrate that the effects of NOM coronas on nanoparticle properties and interactions with biological surfaces can depend on the relative amounts of NOM and nanoparticles

A Citric Acid-Derived Ligand for Modular Functionalization of Metal Oxide Surfaces via “Click” Chemistry

Citric acid is a widely used surface-modifying ligand for growth and processing of a variety of nanoparticles; however, the inability to easily prepare derivatives of this molecule has restricted the development of versatile chemistries for nanoparticle surface functionalization. Here, we report the design and synthesis of a citric acid derivative bearing an alkyne group and demonstrate that this molecule provides the ability to achieve stable, multidentate carboxylate binding to metal oxide nanoparticles, while also enabling subsequent multistep chemistry via the Cu­(I)-catalyzed azide–alkyne cycloaddition (CuAAC) reaction. The broad utility of this strategy for the modular functionalization of metal oxide surfaces was demonstrated by its application in the CuAAC modification of ZnO, Fe₂O₃, TiO₂, and WO₃ nanoparticles

Quantification of Free Polyelectrolytes Present in Colloidal Suspension, Revealing a Source of Toxic Responses for Polyelectrolyte-Wrapped Gold Nanoparticles

Polyelectrolyte (PE) wrapping of colloidal nanoparticles (NPs) is a standard method to control NP surface chemistry and charge. Because excess polyelectrolytes are usually employed in the surface modification process, it is critical to evaluate different purification strategies to obtain a clean final product and thus avoid ambiguities in the source of effects on biological systems. In this work, 4 nm diameter gold nanoparticles (AuNPs) were wrapped with 15 kDa poly­(allylamine hydrochloride) (PAH), and three purification strategies were applied: (a) diafiltration or either (b) one round or (c) two rounds of centrifugation. The bacterial toxicity of each of these three PAH-AuNP samples was evaluated for the bacterium <i>Shewanella oneidensis</i> MR-1 and is quantitatively correlated with the amount of unbound PAH molecules in the AuNP suspensions, as judged by X-ray photoelectron spectroscopy, nuclear magnetic resonance experiments and quantification using fluorescent assay. Dialysis experiments show that, for a 15 kDa polyelectrolyte, a 50 kDa dialysis membrane is not sufficient to remove all PAH polymers. Together, these data showcase the importance of choosing a proper postsynthesis purification method for polyelectrolyte-wrapped NPs and reveal that apparent toxicity results may be due to unintended free wrapping agents such as polyelectrolytes

On Electronic and Charge Interference in Second Harmonic Generation Responses from Gold Metal Nanoparticles at Supported Lipid Bilayers

Second harmonic generation (SHG) is useful for studying the properties of interfaces, including the surfaces of nanoparticles and the interaction of nanoparticles with biologically relevant surfaces. Gold nanoparticles at the biological membrane represent a particularly interesting system to be probed by SHG spectroscopy given the rich electronic structure of gold nanoparticles and the charged nature of the nano-bio interface. Here we describe the interplay between the resonant and nonresonant components of the second harmonic response as 4 and 14 nm spherical gold nanoparticles (AuNPs) wrapped in the cationic polyelectrolyte poly­(allylamine hydrochloride) (PAH) adsorb to negatively charged supported lipid bilayers. In contrast to the SHG response of 4 nm PAH-AuNPs, that we have shown previously to be dominated by resonance enhancement, the SHG response from the adsorption of the 14 nm PAH-AuNPs, with similar hydrodynamic diameters, to a 9:1 DOPC:DOTAP bilayer is dominated by the nonresonant, interfacial, potential-dependent component of the signal. We hypothesize that the difference in the SHG response is attributable to the differences in the number of PAH molecules associated with the particles and, therefore, differences in the number of positively charged ammonium groups associated with the 4 vs the 14 nm particles. For 14 nm PAH-AuNPs with larger hydrodynamic diameters, we determined two regimes in the adsorption behavior, one where the resonance enhancement from the gold core of the nanoparticle dominates the signal and a second where the nonresonant, interfacial, potential-dependent term dominates the signal. The results presented in this study provide insight into the interplay between resonant and nonresonant components of the second harmonic signal from the adsorption of charged AuNPs and are valuable for future studies with other functionalized particles and lipid systems by SHG

Direct Probes of 4 nm Diameter Gold Nanoparticles Interacting with Supported Lipid Bilayers

This work presents molecular-level investigations of how well-characterized silica-supported phospholipid bilayers formed from either pure DOPC or a 9:1 mixture of DOPC:DOTAP interact with positively and negatively charged 4 nm gold metal nanoparticles at pH 7.4 and NaCl concentrations ranging from 0.001 to 0.1 M. Second harmonic generation (SHG) charge screening measurements indicate the supported bilayers carry a negative interfacial potential. Resonantly enhanced SHG measurements probing electronic transitions within the gold core of the nanoparticles show the particles interact irreversibly with the supported bilayers at a range of concentrations. At 0.1 M NaCl, surface coverages for the particles functionalized with the negatively charged ligand mercaptopropionic acid (MPA) or wrapped in the cationic polyelectrolyte poly­(allylamine) hydrochloride (PAH) are estimated from a joint analysis of QCM-D, XPS, AFM, and ToF-SIMS to be roughly 1 × 10⁷ and 1 × 10¹¹ particles cm^–2, respectively. Results from complementary SHG charge screening experiments point to the possibility that the surface coverage of the MPA-coated particles is more limited by interparticle Coulomb repulsion due to the charges within their hydrodynamic volumes than with the PAH-wrapped particles. Yet, SHG adsorption isotherms indicate that the interaction strength per particle is independent of ionic strength and particle coating, highlighting the importance of multivalent interactions. ¹H NMR spectra of the lipids within vesicles suspended in solution show little change upon interaction with either particle type but indicate loosening of the gold-bound PAH polymer wrapping upon attachment to the vesicles. The thermodynamic, spectroscopic, and electrostatic data presented here may serve to benchmark experimental and computational studies of nanoparticle attachment processes at the nano–bio interface

Lipid Corona Formation from Nanoparticle Interactions with Bilayers and Membrane-Specific Biological Outcomes

<a></a><a>While mixing nanoparticles with certain biological molecules can result in coronas that afford some control over how engineered nanomaterials interact with living systems, corona formation mechanisms remain enigmatic. Here, we report spontaneous lipid corona formation, i.e. without active mixing, upon attachment to stationary lipid bilayer model membranes and bacterial cell envelopes, and present ribosome-specific outcomes for multi-cellular organisms. Experiments show that polycation-wrapped particles disrupt the tails of zwitterionic lipids, increase bilayer fluidity, and leave the membrane with reduced ζ-potentials. Computer simulations show contact ion pairing between the lipid headgroups and the polycations’ ammonium groups leads to the formation of stable, albeit fragmented, lipid bilayer coronas, while microscopy shows fragmented bilayers around nanoparticles after interacting with <i>Shewanella oneidensis</i>. Our mechanistic insight can be used to improve control over nano-bio interactions and to help understand why some nanomaterial/ligand combinations are detrimental to organisms, like <i>Daphnia magna</i>, while others are not. </a