Bhuvnesh Bharti

An environmentally benign antimicrobial nanoparticle based on a silver-infused lignin core

Silver nanoparticles have antibacterial properties, but their use has been a cause for concern because they persist in the environment. Here, we show that lignin nanoparticles infused with silver ions and coated with a cationic polyelectrolyte layer form a biodegradable and green alternative to silver nanoparticles. The polyelectrolyte layer promotes the adhesion of the particles to bacterial cell membranes and, together with silver ions, can kill a broad spectrum of bacteria, including Escherichia coli, Pseudomonas aeruginosa and quaternary-amine-resistant Ralstonia sp. Ion depletion studies have shown that the bioactivity of these nanoparticles is time-limited because of the desorption of silver ions. High-throughput bioactivity screening did not reveal increased toxicity of the particles when compared to an equivalent mass of metallic silver nanoparticles or silver nitrate solution. Our results demonstrate that the application of green chemistry principles may allow the synthesis of nanoparticles with biodegradable cores that have higher antimicrobial activity and smaller environmental impact than metallic silver nanoparticles.

Aggregation of Silica Nanoparticles Directed by Adsorption of Lysozyme

The interaction of the globular protein lysozyme with silica nanoparticles of diameter 20 nm was studied in a pH range between the isoelectric points (IEPs) of silica and the protein (pH 3-11). The adsorption affinity and capacity of lysozyme on the silica particles is increasing progressively with pH, and the adsorbed protein induces bridging aggregation of the silica particles. Structural properties of the aggregates were studied as a function of pH at a fixed protein-to-silica concentration ratio which corresponds to a surface concentration of protein well below a complete monolayer in the complete-binding regime at pH > 6. Sedimentation studies indicate the presence of compact aggregates at pH 4-6 and a loose flocculated network at pH 7-9, followed by a sharp decrease of aggregate size near the IEP of lysozyme. The structure of the bridged silica aggregates was studied by cryo-transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering. The structure factor S(q) derived from the scattering profiles displays characteristic features of particles interacting by a short-range attractive potential and can be represented by the square-well Percus-Yevick potential model, with a potential depth not exceeding 3k(B)T.

Assembly of Reconfigurable Colloidal Structures by Multidirectional Field-Induced Interactions

Field-directed colloidal assembly has shown remarkable recent progress in increasing the complexity, degree of control, and multiscale organization of the structures. This has largely been achieved by using particles of complex shapes and polarizabilites (Janus, patchy, shaped, and faceted). We review the fundamentals of the interactions leading to the directed assembly of such structures, the ways to simulate the dynamics of the process, and the effect of particle size, shape, and properties on the type of structure obtained. We discuss how directional polarization interactions induced by external electric and magnetic fields can be used to assemble complex particles or particle mixtures into lattices of tailored structure. Examples of such systems include isotropic and anisotropic shaped particles with surface patches, which form networks and crystals of unusual symmetry by dipolar, quadrupolar, and multipolar interactions in external fields. The emerging trends in making reconfigurable and dynamic structures are discussed.

Co-Assembly of Oppositely Charged Particles into Linear Clusters and Chains of Controllable Length

Colloidal particles with strongly attractive interactions snap on contact and form permanent, but disordered aggregates. In contrast, AC electric fields allow directional assembly of chains or crystals from repulsive particles by dielectrophoresis (DEP), but these structures fall apart once the field is switched off. We demonstrate how well-organized, permanent clusters and chains of micron-sized particles can be assembled by applying DEP to mixtures of oppositely charged microspheres. We found that the length of the formed chains depends on size ratio as well as the number ratio of the two species, and formulated a statistical model for this assembly mechanism, which is in excellent agreement with the experimental results. The assembly rules resulting from this study form a basis for tailoring new classes of permanent supracolloidal clusters and gels.

Bending of Responsive Hydrogel Sheets Guided by Field‐Assembled Microparticle Endoskeleton Structures

Hydrogel composites that respond to stimuli can form the basis of new classes of biomimetic actuators and soft robotic components. Common latex microspheres can be assembled and patterned by AC electric fields within a soft thermoresponsive hydrogel. The field-oriented particle chains act as endoskeletal structures, which guide the macroscopic bending pattern of the actuators.

Assembling Wormlike Micelles in Tubular Nanopores by Tuning Surfactant-Wall Interactions

Threadlike molecular assemblies are excluded from narrow pores unless attractive interactions with the confining pore walls compensate for the loss of configurational entropy. Here we show that wormlike surfactant micelles can be assembled in the 8 nm tubular nanopores of SBA-15 silica by adjusting the surfactant-pore-wall interactions. The modulation of the interactions was achieved by coadsorption of a surface modifier that also provides control over the partitioning of wormlike aggregates between the bulk solution and the pore space. We anticipate that the concept of tuning the interactions with the pore wall will be applicable to a wide variety of self-assembling molecules and pores.

Surfactant adsorption and aggregate structure at silica nanoparticles: Effects of particle size and surface modification

The influence of particle size and a surface modifier on the self-assembly of the nonionic surfactant C12E5 at silica nanoparticles was studied by adsorption measurements and small-angle neutron scattering (SANS). Silica nanoparticles of diameter 13 to 43 nm were synthesized involving the basic amino acid lysine. A strong decrease of the limiting adsorption of C12E5 with decreasing particle diameter was found. To unveil the role of lysine as a surface modifier for the observed size dependence of surfactant adsorption, the morphology of the surfactant aggregates assembled on pure siliceous nanoparticles (Ludox-TMA, 27 nm) and their evolution with increasing lysine concentration at a fixed surfactant-to-silica ratio was studied by SANS. In the absence of lysine, the surfactant forms surface micelles at silica particles. As the concentration of lysine is increased, a gradual transition from the surface micelles to detached wormlike micelles in the bulk solution is observed. The changes in surfactant aggregate morphology cause pronounced changes of the system properties, as is demonstrated by turbidity measurements as a function of temperature. These findings are discussed in terms of particle surface curvature and surfactant binding strength, which present new insight into the delicate balance between the two properties.

Sequence-encoded colloidal origami and microbot assemblies from patchy magnetic cubes

Sequence-encoded assembly of patchy magnetic microcubes enables making self-reconfiguring colloidal origami and “microbots.” Colloidal-scale assemblies that reconfigure on demand may serve as the next generation of soft “microbots,” artificial muscles, and other biomimetic devices. This requires the precise arrangement of particles into structures that are preprogrammed to reversibly change shape when actuated by external fields. The design and making of colloidal-scale assemblies with encoded directional particle-particle interactions remain a major challenge. We show how assemblies of metallodielectric patchy microcubes can be engineered to store energy through magnetic polarization and release it on demand by microscale reconfiguration. The dynamic pattern of folding and reconfiguration of the chain-like assemblies can be encoded in the sequence of the cube orientation. The residual polarization of the metallic facets on the microcubes leads to local interactions between the neighboring particles, which is directed by the conformational restrictions of their shape after harvesting energy from external magnetic fields. These structures can also be directionally moved, steered, and maneuvered by global forces from external magnetic fields. We illustrate these capabilities by examples of assemblies of specific sequences that can be actuated, reoriented, and spatially maneuvered to perform microscale operations such as capturing and transporting live cells, acting as prototypes of microbots, micromixers, and other active microstructures.

Analysis of the field-assisted permanent assembly of oppositely charged particles.

We characterize experimentally and analyze analytically a novel electric-field-assisted process for the assembly of permanent chains of oppositely charged microparticles in an aqueous environment. Long chains of oppositely charged particles are rapidly formed when an external electric field is applied and break up into permanent linear fragments upon switching off the field. The resulting secondary chains are stabilized by attractive electrostatic and van der Waals interactions between the particles. We find that the length of the permanent chains is strongly dependent on the relative size (microsphere diameter D) of small and large particles and can be tuned by varying the particle size ratio s = Dsm/Dlg and particle number ratio r = Nsm/Nlg. Three latex microsphere systems of different particle size ratio, s = 0.9, 0.45, and 0.225, were characterized at different particle number ratios r by determining experimentally the length distribution of the permanent chains. The results are compared with statistical models based on a one-step or two-step process of forming the primary chains. We find that the one-step model is applicable to the system of similarly sized particles (s = 0.9) and the two-step chaining model is applicable to the system of dissimilarly sized particles (s = 0.225), where the large particles form chains first and the small ones serve as binders, which are later drawn in the junctions. Long permanent chains are formed only from particles of dissimilar size for which our model predicts a linear increase in the mean chain length with increasing r. On the basis of these results, we formulate a set of assembly rules for permanent colloidal chain formation by oppositely charged particles. The results make possible the precise large-scale formation of particle chains of any length, which can serve as components in new gels, biomaterials, and fluids with controlled rheology.

Multidirectional, Multicomponent Electric Field Driven Assembly of Complex Colloidal Chains

Abstract External fields (magnetic and electric) present a simple, robust and efficient route to manipulate and assemble colloidal particles. We report how biparticle dispersions can be assembled into well-defined arrays of tunable morphology using external AC electric field. Binary dispersions of strongly and weakly charged colloidal particles were arranged into linear composite chains via dipole-dipole attraction. The frequency of the applied electric field was the first control parameter for reversibly tuning the biparticle attraction from longitudinal assembly (in the direction of field) to the traverse one (perpendicular to the field). We show that in addition to frequency, spatial limitations play a key role in the assembly process and may assist in the formation of short bidirectional chain-like clusters or characteristic highly structured strings of colloidal triplets. Thus, we control the long-range organization through a combination of particle size ratio, concentration ratio and field frequency. The new strategy to reconfigure the microstructures can find application in better control of the field driven colloidal assembly processes and may be extended to the formation of more complex and precisely arranged particle networks.