Predicting Complete Size Distribution of Nanoparticles Based on Interparticle Potential: Experiments and Simulation

Solution-based synthesis of nanoparticles does not yield monodisperse particles, but rather a well-defined particle size distribution (PSD). There is currently no simple means to anticipate or model these size distributions, which critically affect the properties of the resulting nanomaterials. We simulate the temporal evolution of the PSD in the framework of a nucleation and growth model, with the critical postulate that the coagulation efficiency between two nanoparticles is quantitatively determined by the known, interparticle potential energy. Our simulation based on this ansatz, not only <i>a priori</i> predicts experimentally obtained complete PSDs of uncoated or coated (with poly­(acrylic acid)­or dextran) iron oxide nanoparticles but also accurately captures the influence of surface coverage of a coating agent on the resulting PSD

Polyacrylic Acid-Coated Iron Oxide Nanoparticles for Targeting Drug Resistance in Mycobacteria

The emergence of drug resistance is a major problem faced in current tuberculosis (TB) therapy, representing a global health concern. Mycobacterium is naturally resistant to most drugs due to export of the latter outside bacterial cells by active efflux pumps, resulting in a low intracellular drug concentration. Thus, development of agents that can enhance the effectiveness of drugs used in TB treatment and bypass the efflux mechanism is crucial. In this study, we present a new nanoparticle-based strategy for enhancing the efficacy of existing drugs. To that end, we have developed poly­(acrylic acid) (PAA)-coated iron oxide (magnetite) nanoparticles (PAA-MNPs) as efflux inhibitors and used it together with rifampicin (a first line anti-TB drug) on <i>Mycobacterium smegmatis</i>. PAA-MNPs of mean diameter 9 nm interact with bacterial cells via surface attachment and are then internalized by cells. Although PAA-MNP alone does not inhibit cell growth, treatment of cells with a combination of PAA-MNP and rifampicin exhibits a synergistic 4-fold-higher growth inhibition compared to rifampicin alone. This is because the combination of PAA-MNP and rifampicin results in up to a 3-fold-increased accumulation of rifampicin inside the cells. This enhanced intracellular drug concentration has been explained by real-time transport studies on a common efflux pump substrate, ethidium bromide (EtBr). It is seen that PAA-MNP increases the accumulation of EtBr significantly and also minimizes the EtBr efflux in direct proportion to the PAA-MNP concentration. Our results thus illustrate that the addition of PAA-MNP with rifampicin may bypass the innate drug resistance mechanism of <i>M. smegmatis</i>. This generic strategy is also found to be successful for other anti-TB drugs, such as isoniazid and fluoroquinolones (e.g., norfloxacin), only when stabilized, coated nanoparticles (such as PAA-MNP) are used, not PAA or MNP alone. We hence establish coated nanoparticles as a new class of efflux inhibitors for potential therapeutic use

Ion Valence and Concentration Effects on the Interaction between Polystyrene Sulfonate-Modified Carbon Nanotubes in Water

We use molecular dynamics simulations and the adaptive biasing force method to evaluate the potential of mean force between two carbon nanotubes (CNTs), with each surface modified by an adsorbed sodium-polystyrene sulfonate (Na-PSS) polyanion, in aqueous electrolyte media. Changes in the electrolyte concentration and counter-ion valence can lead to qualitative changes in the interactions between polyelectrolyte-modified CNTs. We show that in the presence of monovalent NaCl salt, a long-range screened electrostatic repulsion exists between CNTs. This repulsion can be described by a generalized Derjaguin–Landau–Verwey–Overbeek interaction that accounts for anisotropy of charged cylindrical colloids. In contrast, an attraction between CNTs is observed in the presence of divalent MgCl₂ salt. The attraction is attributed to ion-pair correlations between anionic SO₃^– groups, on different PSS chains, induced by Mg²⁺ counter ions acting as bridges between the SO₃^– groups. However, in the salt-free case where divalent Mg²⁺ counter ions are considered instead of the Na⁺ counter ions, condensation of Mg²⁺ counter ions on the adsorbed PSS chain results in the neutralization of surface charge and leads to a short-range steric repulsion between the CNTs. Thus, our simulations show that qualitatively different interactions, either short-range steric repulsion, long-range repulsion or attraction, can arise between PSS-modified CNTs based on counter-ion valence and electrolyte concentration

Design of Ultrasensitive Protein Biosensor Strips for Selective Detection of Aromatic Contaminants in Environmental Wastewater

Phenol and its derivatives constitute a class of highly toxic xenobiotics that pollute both river and groundwater. Here, we use a highly stable enzyme-based in vitro biosensing scaffold to develop a chip-based environmental diagnostic for in situ accurate, direct detection of phenol with selectively down to 10 ppb. Mesoporous silica nanoparticles (MCM41) having a pore diameter of 6.5 nm was screened and found to be the optimal solid support for creation of a robust immobilized protein based sensor, which retains stability, enzyme activity, sensitivity, and selectivity at par with solution format. The sensor strip exhibits minimal cross reactivity in simulated wastewater, crowded with several common pollutants. Moreover, this design is competent towards detection of phenol content with 95% accuracy in real-time environmental samples collected from local surroundings, making it a viable candidate for commercialization. The enzyme has been further modified via evolution driven mutagenesis to generate an exclusive 2,3-dimethylphenol sensor with equivalent selectivity and sensitivity as the native phenol sensor. Thus, this approach can be extended to generate a battery of sensors for other priority aromatic pollutants, highlighting the versatility of the biosensor unit. This novel biosensor design presents promising potential for direct detection and can be integrated in a device format for on-site pollutant monitoring

Impact of the Distributions of Core Size and Grafting Density on the Self-Assembly of Polymer Grafted Nanoparticles

It is now well-accepted that hydrophilic nanoparticles (NPs) lightly grafted with polymer chains self-assemble into a variety of superstructures when placed in a hydrophobic homopolymer matrix or in a small molecule solvent. Currently, it is thought that a given NP sample should only assemble into one kind of superstructure depending on the relative balance between favorable NP core–core attractions and steric repulsion between grafted polymer chains. Surprisingly, we find that each sample shows the simultaneous formation of a variety of NP-assemblies, e.g., well-dispersed particles, strings, and aggregates. We show through the generalization of a simple geometric model that accounting for the distributions of the NP core size and the number of grafted chains on each NP (which is especially important at low coverages) allows us to quantitatively model the aggregate shape distribution. We conclude that, in contrast to molecular surfactants with well-defined chemistries, the self-assembly of these NP analogues is dominated by such fluctuation effects