How Do Proteins Associate with Nanoscale Metal-Organic Framework Surfaces?

It is well known that colloidal nanomaterials, upon exposure to a complex biological medium, acquire biomolecules on their surface to form coronas. Porous nanomaterials present an opportunity to sequester biomolecules and/or control their orientation at the surface. In this report, a metal-organic framework (MOF) shell around gold nanorods was compared to MOF nanocrystals as potential protein sponges to adsorb several common proteins (lysozyme, beta-lactoglobulin-A, and bovine serum albumin) and potentially control their orientation at the surface. Even after correction for surface area, MOF shell/gold nanorod materials adsorbed more protein than the analogous nanoMOFs. For the set of proteins and nanomaterials in this study, all protein-surface interactions were exothermic, as judged by isothermal titration calorimetry. Protein display at the surfaces was determined from limited proteolysis experiments, and it was found that protein orientation was dependent both on the nature of the nanoparticle surface and on the nature of the protein, with lysozyme and beta-lactoglobulin-A showing distinct molecular positioning.

Gold nanorod impact on mechanical properties of stretchable hydrogels.

Double-network hydrogels have attracted much attention because of their superior mechanical properties, which are more similar to rubbers and soft tissues than classic hydrogels. In this report, plasmonic gold nanorods (AuNRs) were incorporated into a stretchable double-network hydrogel, composed of alginate and acrylamide. The impact of gold nanorod concentration and surface chemistry on bulk mechanical properties such as Young's modulus and elongation at break was investigated. AuNRs with three different surface chemistries, cetyltrimethylammonium bromide, thiolated poly(ethylene glycol), and 11-mercaptoundecanoic acid were successfully dispersed into alginate/polyacrylamide hydrogels. The AuNR-loaded hydrogels could be reversibly stretched, leading to AuNR reversible alignment along the stretch direction as judged by polarized optical spectroscopy. With the proper surface chemistry, hydrogel nanorod composites were able to be stretched to more than 3000% their initial length without fracturing. These results show that plasmonic gold nanorods can be well dispersed in multi-component polymer systems, certain surface chemistries can enhance the bulk mechanical properties, and AuNR orientation can be controlled through varying strains on the matrix.

Virus-Sized Gold Nanorods: Plasmonic Particles for Biology.

Plasmons, collective oscillations of conduction-band electrons in nanoscale metals, are well-known phenomena in colloidal gold and silver nanocrystals that produce brilliant visible colors in these materials that depend on the nanocrystal size and shape. Under illumination at or near the plasmon bands, gold and silver nanocrystals exhibit properties that enable fascinating biological applications: (i) the nanocrystals elastically scatter light, providing a straightforward way to image them in complex aqueous environments; (ii) the nanocrystals produce local electric fields that enable various surface-enhanced spectroscopies for sensing, molecular diagnostics, and boosting of bound fluorophore performance; (iii) the nanocrystals produce heat, which can lead to chemical transformations at or near the nanocrystal surface and can photothermally destroy nearby cells. While all the above-mentioned applications have already been well-demonstrated in the literature, this Account focuses on several other aspects of these nanomaterials, in particular gold nanorods that are approximately the size of viruses (diameters of ∼10 nm, lengths up to 100 nm). Absolute extinction, scattering, and absorption properties are compared for gold nanorods of various absolute dimensions, and references for how to synthesize gold nanorods with four different absolute dimensions are provided. Surface chemistry strategies for coating nanocrystals with smooth or rough shells are detailed; specific examples include mesoporous silica and metal-organic framework shells for porous (rough) coatings and polyelectrolyte layer-by-layer wrapping for "smooth" shells. For self-assembled-monolayer molecular coating ligands, the smoothest shells of all, a wide range of ligand densities have been reported from many experiments, yielding values from less than 1 to nearly 10 molecules/nm2 depending on the nanocrystal size and the nature of the ligand. Systematic studies of ligand density for one particular ligand with a bulky headgroup are highlighted, showing that the highest ligand density occurs for the smallest nanocrystals, even though these ligand headgroups are the most mobile as judged by NMR relaxation studies. Biomolecular coronas form around spherical and rod-shaped nanocrystals upon immersion into biological fluids; these proteins and lipids can be quantified, and their degree of adsorption depends on the nanocrystal surface chemistry as well as the biophysical characteristics of the adsorbing biomolecule. Photothermal adsorption and desorption of proteins on nanocrystals depend on the enthalpy of protein-nanocrystal surface interactions, leading to light-triggered alteration in protein concentrations near the nanocrystals. At the cellular scale, gold nanocrystals exert genetic changes at the mRNA level, with a variety of likely mechanisms that include alteration of local biomolecular concentration gradients, changes in mechanical properties of the extracellular matrix, and physical interruption of key cellular processes-even without plasmonic effects. Microbiomes, both organismal and environmental, are the likely first point of contact of nanomaterials with natural living systems; we see a major scientific frontier in understanding, predicting, and controlling microbe-nanocrystal interactions, which may be augmented by plasmonic effects.