Spontaneous Insertion, Helix Formation, and Hydration of Polyethylene Oxide in Carbon Nanotubes

Adaptive insertion of a hydrophobic anchor into a poly(ethylene glycol) host for programmable surface functionalization

Covalent and non-covalent molecular binding are two strategies to tailor surface properties and functions. However, the lack of responsiveness and requirement for specific binding groups makes spatiotemporal control challenging. Here, we report the adaptive insertion of a hydrophobic anchor into a poly(ethylene glycol) (PEG) host as a non-covalent binding strategy for surface functionalization. By using polycyclic aromatic hydrocarbons as the hydrophobic anchor, hydrophilic charged and non-charged functional modules were spontaneously loaded onto PEG corona in 2 min without the assistance of any catalysts and binding groups. The thermodynamically favourable insertion of the hydrophobic anchor can be reversed by pulling the functional module, enabling programmable surface functionalization. We anticipate that the adaptive molecular recognition between the hydrophobic anchor and the PEG host will challenge the hydrophilic understanding of PEG and enhance the progress in nanomedicine, advanced materials and nanotechnology.

Bistability of a helical filament confined on a cylinder

The natural configuration of an intrinsically curved and twisted filament is uniquely a helix so that it can be referred to as a helical filament. We find that confining a helical filament on a cylinder can create a bistable state. When c_{0}R=0.5, where c_{0} is the intrinsic curvature of filament and R is the radius of cylinder, the phase diagram for the stability of a helix contains three regimes. Regime I has a small intrinsic twisting rate (ITR) and exhibits a bistable state which consists of two isoenergic helices. In regime II, the filament has a moderate ITR and the bistable state consists of a metastable low-pitch helix and a stable nonhelix. In regime III, the helix is unstable, owing to a large ITR. A similar phenomenon occurs when c_{0}R∼0.5. Monte Carlo simulation confirms these conclusions and indicates further that there are bistable nonhelices in regime III. This bistable system offers a prospective green material since the wide range of parameters and distinctive configurations for bistable states favor its realization and application.

Hydrogen Bonding Sequence Directed Coil-Globule Transition in Water Soluble Thermoresponsive Polymers

The origin of the coil-globule transition for water-soluble thermoresponsive polymers frequently used in nanomaterials remains elusive. Using polypropylene oxide as an example we demonstrate by means of atomistic molecular dynamics simulations that temperature-induced increase in the sequence length of monomers that are not hydrogen bonded to water drives the coil-globule transition. Longer chains statistically exhibit longer sequences which serve as nucleation sites for hydrophobic cluster formation, facilitating chain collapse at lower temperature in agreement with experimental data.

Revisiting molecular adsorption: unconventional uptake of polymer chains from solution into sub-nanoporous media

Adsorption of polymers from the solution phase has been extensively studied to cope with many demands not only for separation technologies, but also for the development of coatings, adhesives, and biocompatible materials. Most studies hitherto focus on adsorption on flat surfaces and mesoporous adsorbents with open frameworks, plausibly because of the preconceived notion that it is unlikely for polymers to enter a pore with a diameter that is smaller than the gyration diameter of the polymer in solution; therefore, sub-nanoporous materials are rarely considered as a polymer adsorption medium. Here we report that polyethylene glycols (PEGs) are adsorbed into sub-nanometer one-dimensional (1D) pores of metal-organic frameworks (MOFs) from various solvents. Isothermal adsorption experiments reveal a unique solvent dependence, which is explained by the balance between polymer solvation propensity for each solvent and enthalpic contributions that compensate for potential entropic losses from uncoiling upon pore admission. In addition, adsorption kinetics identify a peculiar molecular weight (MW) dependence. While short PEGs are adsorbed faster than long ones in single-component adsorption experiments, the opposite trend was observed in double-component competitive experiments. A two-step insertion process consisting of (1) an enthalpy-driven recognition step followed by (2) diffusion regulated infiltration in the restricted 1D channels explains the intriguing selectivity of polymer uptake. Furthermore, liquid chromatography using the MOFs as the stationary phase resulted in significant PEG retention that depends on the MW and temperature. This study provides further insights into the mechanism and thermodynamics behind the present polymer adsorption system, rendering it as a promising method for polymer analysis and separation.

Interplay of lipid and surfactant: Impact on nanoparticle structure

Liquid lipid nanoparticles (LLN) are oil-in-water nanoemulsions of great interest in the delivery of hydrophobic drug molecules. They consist of a surfactant shell and a liquid lipid core. The small size of LLNs makes them difficult to study, yet a detailed understanding of their internal structure is vital in developing stable drug delivery vehicles (DDVs). Here, we implement machine learning techniques alongside small angle neutron scattering experiments and molecular dynamics simulations to provide critical insight into the conformations and distributions of the lipid and surfactant throughout the LLN. We simulate the assembly of a single LLN composed of the lipid, triolein (GTO), and the surfactant, Brij O10. Our work shows that the addition of surfactant is pivotal in the formation of a disordered lipid core; the even coverage of Brij O10 across the LLN shields the GTO from water and so the lipids adopt conformations that reduce crystallisation. We demonstrate the superior ability of unsupervised artificial neural networks in characterising the internal structure of DDVs, when compared to more conventional geometric methods. We have identified, clustered, classified and averaged the dominant conformations of lipid and surfactant molecules within the LLN, providing a multi-scale picture of the internal structure of LLNs.

The recent advancement of low-dimensional nanostructured materials for drug delivery and drug sensing application: A brief review

In this review article, we have presented a detailed analysis of the recent advancement of quantum mechanical calculations in the applications of the low-dimensional nanomaterials (LDNs) into biomedical fields like biosensors and drug delivery systems development. Biosensors play an essential role for many communities, e.g. law enforcing agencies to sense illicit drugs, medical communities to remove overdosed medications from the human and animal body etc. Besides, drug delivery systems are theoretically being proposed for many years and experimentally found to deliver the drug to the targeted sites by reducing the harmful side effects significantly. In current COVID-19 pandemic, biosensors can play significant roles, e.g. to remove experimental drugs during the human trials if they show any unwanted adverse effect etc. where the drug delivery systems can be potentially applied to reduce the side effects. But before proceeding to these noble and expensive translational research works, advanced theoretical calculations can provide the possible outcomes with considerable accuracy. Hence in this review article, we have analyzed how theoretical calculations can be used to investigate LDNs as potential biosensor devices or drug delivery systems. We have also made a very brief discussion on the properties of biosensors or drug delivery systems which should be investigated for the biomedical applications and how to calculate them theoretically. Finally, we have made a detailed analysis of a large number of recently published research works where theoretical calculations were used to propose different LDNs for bio-sensing and drug delivery applications.

Disentangling of complex polymer dynamics under soft nanoscopic confinement

We discuss the complex interplay between host and guest dynamics for a polymer in soft confinement by a droplet-phase microemulsion. Intermediate scattering functions obtained by neutron spin echo spectroscopy are first analysed by means of an effective diffusion coefficient. From its dependence on the absolute of the scattering vector q we concluded a sophisticated model for the systems dynamics taking both polymer and microemulsion contributions into account. Global fitting of this model to the intermediate scattering functions at all measured q-values and all investigated confinement sizes eventually allows for a precise disentangling of the pure polymer dynamics in confinement from the overlaying microemulsion dynamics. Validity of our approach is further supported by numerical random walk calculations.

Flow-Induced Formation of Thin PEO Fibers in Water and Their Stability After the Strain Release

Recently, we have shown that a tensile stress applied to chains of poly(ethylene oxide) (PEO) in water reduces the solubility and leads to phase separation of PEO chains from water with the formation of a two-phase region. In this work, we further elucidate the generic mechanism behind strain-induced phase transitions in aqueous PEO solutions with concentrations of 50-60 wt % by performing all-atom molecular dynamics simulations. In particular, we study the stability of oriented PEO fibers after removing stretching forces. We found that the size of the PEO bundle increased with time, which is associated with the dissolution of PEO chains on the fiber surface due to the reformation of hydrogen bonds between the outer PEO molecules and water. For precise characterization of the fibers, the scattering patterns (small- and wide-angle X-ray spectra) for configurations taken at different relaxation times are calculated. The tendency of the oligomer chains to be peeled off from the surface of the bundle eventually might lead to a complete dissolution of the PEO fiber. We conclude that either entanglement constraints or a quick drying process are necessary to conserve the fiber structure in a quiescent state. The scattering results show that external strain induced a liquid-liquid phase separation first. On long time scales, this can be a precursor for crystallization of the fiber.

High Polymer Mass Densities at the Mouths of Carbon Nanotubes (CNTs) Control the Diffusion of Small Molecules through CNT-Based Polymer Nanocomposite Membranes

Detailed molecular dynamics (MD) simulations of model single-walled carbon nanotube (CNT) membranes based on atactic poly(methyl methacrylate) (aPMMA) indicate that PMMA chains significantly penetrate nanotubes through their faces. They predict very high-density values of the polymer in the interfacial area around the CNT mouths that can exceed by 50% the density of the bulk polymer at the same thermodynamic conditions. This dramatically decreases the diffusivity of relatively small penetrants (in our study, water molecules) in the nanocomposite membrane, because of the exceedingly long times needed by these small molecules to diffuse through such a dense interfacial layer before accessing the interior of the nanotubes where they can travel really fast. According to our simulations, the escape time of a confined water molecule from the blocked mouths of a CNT can exceed by several orders of magnitude the time needed by the same molecule to move through the CNT pore. Our work indicates the importance of completely avoiding (or at least minimizing) penetration of polymer chains into the CNT pores through the mouths of the tubes in enabling the efficient transport of small- to moderate-size molecules in model CNT-based polymer membranes, since this provides the highest resistance to their mobility through the membrane.

Hydration of diblock copolymer micelles: Effects of hydrophobicity and co-solvent

Diblock polymer micelles dispersed in an aqueous environment are being actively investigated for various applications, but there is only a qualitative understanding of the effect of the chemical structure on the micelle hydration and water dynamics as these properties are difficult to assess experimentally. Using all-atom molecular dynamics simulations, we investigate aqueous solutions of three comparable in size diblock copolymer micelles with core-forming blocks of different hydrophobicity: polybutadiene (PB), polycaprolactone (PCL), and polytetrahydrofuran (pTHF) with the same hydrophilic block, polyethylene oxide (PEO). We found that core-block hydrophobicity and ability to form hydrogen bonds with water strongly affect the water dynamics near the core: water molecules spend considerably less time in contact with the PB block than with PCL and pTHF blocks. We obtained polymer and solvent volume fraction profiles and determined that the interfacial width systematically increases with a decrease of core block hydrophobicity with water penetration into the core being negligible for PB-PEO and PCL-PEO micelles, while for pTHF-PEO micelles the interface is more diffuse and there is a noticeable penetration of water (17% by volume). For PCL-PEO micelles, which are commonly used in biomedical applications, we also investigated tetrahydrofuran (THF) penetration into the micelles from mixed THF:water solution at early stages of micelle dissolution. We found an inhomogeneous solvent distribution with a maximum of THF volume fraction in the interfacial core-corona region and partial exclusion from the PEO corona, which slows down micelle dissolution. These results can have important implications for micelle stability and use in biomedical applications.

Water/PEG Mixtures: Phase Behavior, Dynamics and Soft Confinement

Polyethylene glycol is water soluble and forms an eutectic system with water. The eutectic temperature is −19 °C for M=1500 g mol−1 and increases with molecular weight. The dielectric relaxation spectrum of the mixtures exhibits a strong loss maximum in ϵ″ (ω) similar to pure water. Relaxation time increases with the addition of PEG. Activation energies exhibit a maximum of 0.35 eV at molar fraction χp ≈0.2. This compares well with results on ethanol water mixtures. Adding PEG molecules to nanoscopic water droplets of inverse microemulsions has only small impact on the bending modulus κ of a non-ionic microemulsion. In AOT based microemulsions an increase or decrease of κ is found in dependence on the size of the droplets. This is in accordance with the variation of the dynamic percolation transition in the same systems.

Guest Controlled Nonmonotonic Deep Cavity Cavitand Assembly State Switching

Octa-acid (OA) and tetra-endo-methyl octa-acid (TEMOA) are water-soluble, deep-cavity cavitands with nanometer-sized nonpolar pockets that readily bind complementary guests, such as n-alkanes. Experimentally, OA exhibits a progression of 1:1 to 2:2 to 2:1 host/guest complexes (X:Y where X is the number of hosts and Y is the number of guests) with increasing alkane chain length from methane to tetradecane. Differing from OA only by the addition of four methyl groups ringing the portal of the pocket, TEMOA exhibits a nonmonotonic progression of assembly states from 1:1 to 2:2 to 1:1 to 2:1 with increasing guest length. Here we present a systematic molecular simulation study to parse the molecular and thermodynamic determinants that distinguish the succession of assembly stoichiometries observed for these similar hosts. Potentials of mean force between hosts and guests, determined via umbrella sampling, are used to characterize association free energies. These free energies are subsequently used in a reaction network model to predict the equilibrium distributions of assemblies. Our models accurately reproduce the experimentally observed trends, showing that TEMOA's endo-methyl units constrict the opening of the binding pocket, limiting the conformations available to bound guests and disrupting the balance between monomeric complexes and dimeric capsules. The success of our simulations demonstrate their utility at interpreting the impact of even simple chemical modifications on supramolecular assembly and highlight their potential to aid bottom-up design.

Terminal Acetylated/Acrylated Poly(ethylene glycol) Fabricated Drug Carriers: Design, Synthesis, and Biological Evaluation

The simple acetylation or acrylation of poly(ethylene glycol) (PEG) terminus leads to the aggregation of PEG chains into spherical nanoparticles in water at room temperature and very low concentrations. The experiment results suggest that this aggregation happens by the variation of the local conformation of the O-CH₂-CH₂-O segments of PEG chains caused by the introduced acyl group, which disturbs the originally strict hydrogen bond mode between the O-CH₂-CH₂-O groups and the water molecules. The simple modified PEG nanoparticles are excellent carriers for drug delivery. As examples, the cross-linkable 1d-based drug delivery systems, cPEG@SN-38 and targeted cPEG@SN-38, are successfully established by their high drug loading content (18 wt %/wt) and enhanced anticancer efficacy both in vitro and in vivo while obviating the inherent toxicity of the employed chemotherapeutics. This strategy that revolves around the simple modification of the generally regarded as safe (GRAS) modules to fabricate drug carriers represents a new direction for the drug delivery systems with clinical potential.

Thermodynamics of Helix-Coil Transitions of Polyalanine in Open Carbon Nanotubes

Understanding structure formation in polypeptide chains and synthetic polymers encapsulated in pores is important in biology and nanotechnology. We present replica exchange molecular dynamics studies of the phase diagram for α-helix formation of capped polyalanine in nanotubes (NT) open to a water reservoir as a function of the NT diameter and hydrophobicity. A helix forms only in a narrow range of diameters, which surprisingly is comparable to the width of the ribosome tunnel. Increasing the hydrophobicity enhances helicity in the NT. Helix formation in polyalanine is driven by a small negative enthalpy and a positive entropy change at ≈300 K, in contrast to the large negative entropy change that destabilizes the helix and favors the coiled state in bulk water. There is an anticorrelation between water density inside the nanotube and structure formation. Confinement-induced helix formation depends on amino acid sequence. There is complete absence of helix in polyglutamine and polyserine confined to a open carbon nanotube.

The dynamics of solvation dictates the conformation of polyethylene oxide in aqueous, isobutyric acid and binary solutions

The dynamics of solvation dictates the conformation of polyethylene oxide in water and isobutyric acid causing a helix–coil transition in a mixed isobutyric acid/water solvent.