Nanoscale Surface Chemical Patterning of Soft Polyacrylamide with Elastic Modulus Similar to Soft Tissue.

Nanometer-scale control over surface functionalization of soft gels is important for a variety of applications including controlling interactions with cells for in vitro cell culture and for regenerative medicine. Recently, we have shown that it is possible to transfer a nanometer-thick precision functional polymer layer to the surface of relatively stiff polyacrylamide gels. Here, we develop a fundamental understanding of the way in which the precision polymer backbone participates in the polyacrylamide radical polymerization and cross-linking process, which enables us to generate high-efficiency transfer to much softer hydrogels (down to 5 kPa) with stiffness similar to that of soft tissue. This approach creates hydrogel surfaces with exposed nanostructured functional arrays that open the door to controlled ligand presentation on soft hydrogel surfaces.

Sub-10 µm Soft Interlayers Integrating Patterned Multivalent Biomolecular Binding Environments.

Designing surfaces that enable controlled presentation of multivalent ligand clusters (e.g., for rapid screening of biomolecular binding constants or design of artificial extracellular matrices) is a cross-cutting challenge in materials and interfacial chemistry. Existing approaches frequently rely on complex building blocks or scaffolds and are often specific to individual substrate chemistries. Thus, an interlayer chemistry that enabled efficient nanometer-scale patterning on a transferrable layer and subsequent integration with other classes of materials could substantially broaden the scope of surfaces available for sensors and wearable electronics. Recently, we have shown that it is possible to assemble nanometer-resolution chemical patterns on substrates including graphite, use diacetylene polymerization to lock the molecular pattern together, and then covalently transfer the pattern to amorphous materials (e.g., polydimethylsiloxane, PDMS), which would not natively enable high degrees of control over ligand presentation. Here, we develop a low-viscosity PDMS formulation that generates very thin films (<10 µm) with dense cross-linking, enabling high-efficiency surface functionalization with polydiacetylene arrays displaying carbohydrates and other functional groups (up to 10-fold greater than other soft materials we have used previously) on very thin films that can be integrated with other materials (e.g., glass and soft materials) to enable a highly controlled multivalent ligand display. We use swelling and other characterization methods to relate surface functionalization efficiency to the average distance between cross-links in the PDMS, developing design principles that can be used to create even thinner transfer layers. In the context of this work, we apply this approach using precision glycopolymers presenting structured arrays of N-acetyl glucosamine ligands for lectin binding assays. More broadly, this interlayer approach lays groundwork for designing surface layers for the presentation of ligand clusters on soft materials for applications including wearable electronics and artificial extracellular matrix.

Inkjet Printing of Nanoscale Functional Patterns on 2D Crystalline Materials and Transfer to Soft Materials.

Nanometer-scale control over surface functionality is important in applications ranging from nanoscale electronics to regenerative medicine. However, approaches that provide precise control over surface chemistry at the nanometer scale are often challenging to use with higher throughput and in more heterogeneous environments (e.g., complex solutions, porous interfaces) common for many applications. Here, we demonstrate a scalable inkjet-based method to generate 1 nm-wide functional patterns on 2D materials such as graphite, which can then be transferred to soft materials such as hydrogels. We examine fluid dynamics associated with the inkjet printing process for low-viscosity amphiphile inks designed to maximize ordering with limited residue and show that microscale droplet fluid dynamics influence nanoscale molecular ordering. Additionally, we show that scalable patterns generated in this way can be transferred to hydrogel materials and used to create surface chemical patterns that induce adsorption of charged particles, with effects strong enough to overcome electrostatic repulsion between a charged hydrogel and a like-charged nanoparticle.

Designing Interfacial Reactions for Nanometer-Scale Surface Patterning of PDMS with Controlled Elastic Modulus.

Control over the surface chemistry of elastomers such as polydimethylsiloxane (PDMS) is important for many applications. However, achieving nanostructured chemical control on amorphous material interfaces below the length scale of substrate heterogeneity is not straightforward, and can be particularly difficult to decouple from changes in network structure that are required for certain applications (e.g., variation of elastic modulus for cell culture). We have recently reported a new method for precisely structured surface functionalization of PDMS and other soft materials, which displays high densities of ligands directly on the material surface, maximizing steric accessibility. Here, we systematically examine structural factors in the PDMS components (e.g., base and cross-linker structures) that impact efficiency of the interfacial reaction that leads to surface functionalization. Applying this understanding, we demonstrate routes for generating equivalent nanometer-scale functional patterns on PDMS with elastic moduli from 0.013 to 1.4 MPa, establishing a foundation for use in applications such as cell culture.

Surface-Templated Glycopolymer Nanopatterns Transferred to Hydrogels for Designed Multivalent Carbohydrate-Lectin Interactions across Length Scales.

Multivalent interactions between carbohydrates and proteins enable a broad range of selective chemical processes of critical biological importance. Such interactions can extend from the macromolecular scale (1-10 nm) up to much larger scales across a cell or tissue, placing substantial demands on chemically patterned materials aiming to leverage similar interactions in vitro. Here, we show that diyne amphiphiles with carbohydrate headgroups can be assembled on highly oriented pyrolytic graphite (HOPG) to generate nanometer-resolution carbohydrate patterns, with individual linear carbohydrate assemblies up to nearly 1 µm, and microscale geometric patterns. These are then photopolymerized and covalently transferred to the surfaces of hydrogels. This strategy suspends carbohydrate patterns on a relatively rigid polydiacetylene (persistence length ∼ 16 nm), exposed at the top surface of the hydrogel above the bulk pore structure. Transferred patterns of appropriate carbohydrates (e.g., N-acetyl-d-glucosamine, GlcNAc) enable selective, multivalent interactions (KD ∼ 40 nM) with wheat germ agglutinin (WGA), a model lectin that exhibits multivalent binding with appropriately spaced GlcNAc moieties. WGA binding affinity can be further improved (KD ∼ 10 nM) using diacetylenes that shift the polymer backbone closer to the displayed carbohydrate, suggesting that this strategy can be used to modulate carbohydrate presentation at interfaces. Conversely, GlcNAc-patterned surfaces do not induce specific binding of concanavalin A, and surfaces patterned with glucuronic acid, or with simple carboxylic acid or hydroxyl groups, do not induce WGA binding. More broadly, this approach may have utility in designing synthetic glycan-mimetic interfaces with features from molecular to mesoscopic scales, including soft scaffolds for cells.

Nanometer-scale patterning of hard and soft interfaces: from photolithography to molecular-scale design.

Many areas of modern materials chemistry, from nanoscale electronics to regenerative medicine, require design of precisely-controlled chemical environments at near-molecular scales. Most work on high-resolution interface patterning to date has focused on hard surfaces, such as those for electronics. However, design of interfaces for biological environments increasingly requires precise control over interfaces of soft materials, which is in many cases complicated by nano-to-microscale heterogeneity in the substrate material. In this Feature Article, we describe historical approaches to nanoscale patterning on hard surfaces, challenges in extension to soft interfaces, and an approach to molecular-scale hard and soft interface design based on self-assembled molecular networks, which can be assembled noncovalently on hard surfaces to generate nanometer-scale patterns, then covalently transferred to soft materials including PDMS and hydrogels.

Nanostructured Surface Functionalization of Polyacrylamide Hydrogels Below the Length Scale of Hydrogel Heterogeneity.

Hydrogels are broadly used in applications where polymer materials must interface with biology. The hydrogel network is amorphous, with substantial heterogeneity on length scales up to hundreds of nanometers, in some cases raising challenges for applications that would benefit from highly structured interactions with biomolecules. Here, we show that it is possible to generate ordered patterns of functional groups on polyacrylamide hydrogel surfaces. We demonstrate that, when linear patterns of amines are transferred to polyacrylamide, they pattern interactions with DNA at the interface, a capability of potential importance for preconcentration in chromatographic applications, as well as for the development of nanostructured hybrid materials and supports for cell culture.

Nanometer-Scale Precision Polymer Patterning of PDMS: Multiscale Insights into Patterning Efficiency Using Alkyldiynamines.

Most high-resolution interfacial patterning approaches are restricted to crystalline inorganic interfaces. Recently, we have shown that it is possible to generate 1 nm resolution functional patterns on soft materials, such as polydimethylsiloxane (PDMS), by creating highly structured striped patterns of functional alkyldiacetylenes on a hard crystalline surface, photopolymerizing to set the molecular pattern as a striped-phase polydiacetylene (sPDA), and then covalently transferring the sPDAs to PDMS. Transfer depends on the diacetylene polymerization, making it important to understand design principles for efficient sPDA polymerization and cross-linking to PDMS. Here, we combine single-molecule and fluorescence-based metrics for sPDA polymerization and transfer, first to characterize sPDA polymerization of amine striped phases, and then to develop a probabilistic model that describes the transfer process in terms of sPDA-PDMS cross-linking reaction efficiency and number of reactions required for transfer. We illustrate that transferred patterns of alkylamines can be used to direct both adsorption of CdSe nanocrystals with alkyl ligand shells and covalent reactions with fluorescent dyes, highlighting the utility of functional patterning of the PDMS surface.

Cow-to-cow variation in nanocrystal synthesis: learning from technical-grade oleylamine.

Many technical-grade reagents, including oleylamine, are broadly used as ligands in nanocrystal synthesis, allowing for cost-effective, and more environmentally friendly, preparation of materials in useful quantities. Impurities can represent 30% or more of these reagent blends, and have frequently emerged as substantial drivers of nanocrystal morphology, assembly, or other physical properties, making it important to understand their composition. Some functional alkyl reagents are derived from natural sources (e.g. often beef tallow, in the case of oleylamine), introducing alkyl chain structures very different than those that might be expected as side products of synthesis from pure feedstocks. Additionally, impurities can exhibit variations based on biological factors (e.g. species, diet, season). In biology, blends of alkyl chains allow for surprisingly sophisticated function of amphiphiles in the cell membrane, pointing to the possibility of similar control in synthetic materials if reagent composition were either better controlled or better understood. Here, we provide brief context on the breadth of roles technical-grade impurities have played in nanocrystal materials, followed by a perspective on oleylamine impurities, their physical properties, and their potential contributions to nanomaterial function.

Lipids: An Atomic Toolkit for the Endless Frontier.

The evolution of lipids in nanoscience exemplifies the powerful coupling of advances in science and technology. Here, we describe two waves of discovery and innovation in lipid materials: one historical and one still building. The first wave leveraged the relatively simple capability for lipids to orient at interfaces, building layers of functional groups. This simple form of building with atoms yielded a stunning range of technologies: lubricant additives that dramatically extended machine lifetimes, molecules that enabled selective ore extraction in mining, and soaps that improved human health. It also set the stage for many areas of modern nanoscience. The second wave of lipid materials, still growing, uses the more complex toolkits lipids offer for building with atoms, including controlling atomic environment to control function (e.g., pKa tuning) and the generation of more arbitrary two-dimensional and three-dimensional structures, including lipid nanoparticles for COVID-19 mRNA vaccines.

Plenty of Room at the Top: A Multi-Scale Understanding of nm-Resolution Polymer Patterning on 2D Materials.

Lamellar phases of alkyldiacetylenes in which the alkyl chains lie parallel to the substrate represent a straightforward means for scalable 1-nm-resolution interfacial patterning. This capability has the potential for substantial impacts in nanoscale electronics, energy conversion, and biomaterials design. Polymerization is required to set the 1-nm functional patterns embedded in the monolayer, making it important to understand structure-function relationships for these on-surface reactions. Polymerization can be observed for certain monomers at the single-polymer scale using scanning probe microscopy. However, substantial restrictions on the systems that can be effectively characterized have limited utility. Here, using a new multi-scale approach, we identify a large, previously unreported difference in polymerization efficiency between the two most widely used commercial diynoic acids. We further identify a core design principle for maximizing polymerization efficiency in these on-surface reactions, generating a new monomer that also exhibits enhanced polymerization efficiency.

Oleylamine Impurities Regulate Temperature-Dependent Hierarchical Assembly of Ultranarrow Gold Nanowires on Biotemplated Interfaces.

Nanocrystals are often synthesized using technical grade reagents such as oleylamine (OLAm), which contains a blend of 9-cis-octadeceneamine with trans-unsaturated and saturated amines. Here, we show that gold nanowires (AuNWs) synthesized with OLAm ligands undergo thermal transitions in interfacial assembly (ribbon vs. nematic); transition temperatures vary widely with the batch of OLAm used for synthesis. Mass spectra reveal that higher-temperature AuNW assembly transitions are correlated with an increased abundance of trans and saturated chains in certain blends. DSC thermograms show that both pure (synthesized) and technical-grade OLAm have primary melting transitions near -5 °C (20-30 °C lower than the literature melting temperature range of OLAm). A second, broader melting transition (in the previous reported melting range) appears in technical grade blends; its temperature varies with the abundance of trans and saturated chains. Our findings illustrate that, similar to biological membranes, blends of alkyl chains can be used to generate mesoscopic hierarchical nanocrystal assembly, particularly at interfaces that further modulate transition temperatures.

One Nanometer Wide Functional Patterns with a Sub-10 Nanometer Pitch Transferred to an Amorphous Elastomeric Material.

Decades of work in surface science have established the ability to functionalize clean inorganic surfaces with sub-nm precision, but for many applications, it would be useful to provide similar control over the surface chemistry of amorphous materials such as elastomers. Here, we show that striped monolayers of diyne amphiphiles, assembled on graphite and photopolymerized, can be covalently transferred to polydimethylsiloxane (PDMS), an elastomer common in applications including microfluidics, soft robotics, wearable electronics, and cell culture. This process creates precision polymer films <1 nm thick, with 1 nm wide functional patterns, which control interfacial wetting and reactivity, and template adsorption of flexible, ultranarrow Au nanowires. The polydiacetylenes exhibit polarized fluorescence emission, revealing polymer location, orientation, and environment, and resist engulfment, a common problem in PDMS functionalization. These findings illustrate a route for patterning surface chemistry below the length scale of heterogeneity in an amorphous material.

Large-Scale Noncovalent Functionalization of 2D Materials through Thermally Controlled Rotary Langmuir-Schaefer Conversion.

As two-dimensional (2D) materials are more broadly utilized as components of hybrid materials, controlling their surface chemistry over large areas through noncovalent functionalization becomes increasingly important. Here, we demonstrate a thermally controlled rotary transfer stage that allows areas of a 2D material to be continuously cycled into contact with a Langmuir film. This approach enables functionalization of large areas of the 2D material and simultaneously improves long-range ordering, achieving ordered domain areas up to nearly 10 000 µm2. To highlight the layer-by-layer processing capability of the rotary transfer stage, large-area noncovalently adsorbed monolayer films from an initial rotary cycle were used as templates to assemble ultranarrow gold nanowires from solution. The process we demonstrate would be readily extensible to roll-to-roll processing, addressing a longstanding challenge in scaling Langmuir-Schaefer transfer for practical applications.

Hierarchically patterned striped phases of polymerized lipids: toward controlled carbohydrate presentation at interfaces.

Complex biomolecules, including carbohydrates, frequently have molecular surface footprints larger than those in broadly utilized standing phase alkanethiol self-assembled monolayers, yet would benefit from structured orientation and clustering interactions promoted by ordered monolayer lattices. Striped phase monolayers, in which alkyl chains extend across the substrate, have larger, more complex lattices: nm-wide stripes of headgroups with 0.5 or 1 nm lateral periodicity along the row, separated by wider (∼5 nm) stripes of exposed alkyl chains. These anisotropic interfacial patterns provide a potential route to controlled clustering of complex functional groups such as carbohydrates. Although the monolayers are not covalently bound to the substrate, assembly of functional alkanes containing an internal diyne allows such monolayers to be photopolymerized, increasing robustness. Here, we demonstrate that, with appropriate modifications, microcontact printing can be used to generate well-defined microscopic areas of striped phases of both single-chain and dual-chain amphiphiles (phospholipids), including one (phosphoinositol) with a carbohydrate in the headgroup. This approach generates hierarchical molecular-scale and microscale interfacial clustering of functional ligands, prototyping a strategy of potential relevance for glycobiology.

Spatially Controlled Noncovalent Functionalization of 2D Materials Based on Molecular Architecture.

Polymerizable amphiphiles can be assembled into lying-down phases on 2D materials such as graphite and graphene to create chemically orthogonal surface patterns at 5-10 nm scales, locally modulating functionality of the 2D basal plane. Functionalization can be carried out through Langmuir-Schaefer conversion, in which a subset of molecules is transferred out of a standing phase film on water onto the 2D substrate. Here, we leverage differences in molecular structure to spatially control transfer at both nanoscopic and microscopic scales. We compare transfer properties of five different single- and dual-chain amphiphiles, demonstrating that those with strong lateral interactions (e.g., hydrogen-bonding networks) exhibit the lowest transfer efficiencies. Since molecular structures also influence microscopic domain morphologies in Langmuir films, we show that it is possible to transfer such microscale patterns, taking advantage of variations in the local transfer rates based on the structural heterogeneity in Langmuir films. Nanoscale domain morphologies also vary in ways that are consistent with predicted relative transfer and diffusion rates. These results suggest strategies to tailor noncovalent functionalization of 2D substrates through controlled LS transfer.

Hierarchically Patterned Noncovalent Functionalization of 2D Materials by Controlled Langmuir-Schaefer Conversion.

Noncovalent monolayer chemistries are often used to functionalize 2D materials. Nanoscopic ligand ordering has been widely demonstrated (e.g., lying-down lamellar phases of functional alkanes); however, combining this control with micro- and macroscopic patterning for practical applications remains a significant challenge. A few reports have demonstrated that standing phase Langmuir films on water can be converted into nanoscopic lying-down molecular domains on 2D substrates (e.g., graphite), using horizontal dipping (Langmuir-Schaefer, LS, transfer). Molecular patterns are known to form at scales up to millimeters in Langmuir films, suggesting the possibility of transforming such structures into functional patterns on 2D materials. However, to our knowledge, this approach has not been investigated, and the rules governing LS conversion are not well understood. In part, this is because the conversion process is mechanistically very different from classic LS transfer of standing phases; challenges also arise due to the need to characterize structure in noncovalently adsorbed ligand layers <0.5 nm thick, at scales ranging from millimeters to nanometers. Here, we show that scanning electron microscopy enables diynoic acid lying-down phases to be imaged across this range of scales; using this structural information, we establish conditions for LS conversion to create hierarchical microscopic and nanoscopic functional patterns. Such control opens the door to tailoring noncovalent surface chemistry of 2D materials to pattern local interactions with the environment.

Multimicrometer Noncovalent Monolayer Domains on Layered Materials through Thermally Controlled Langmuir-Schaefer Conversion for Noncovalent 2D Functionalization.

As functionalized 2D materials are incorporated into hybrid materials, ensuring large-area structural control in noncovalently adsorbed films becomes increasingly important. Noncovalent functionalization avoids disrupting electronic structure in 2D materials; however, relatively weak molecular interactions in such monolayers typically reduce stability toward solution processing and other common material handling conditions. Here, we find that controlling substrate temperature during Langmuir-Schaefer conversion of a standing phase monolayer of diynoic amphiphiles on water to a horizontally oriented monolayer on a 2D substrate routinely produces multimicrometer domains, at least an order of magnitude larger than those typically achieved through drop-casting. Following polymerization, these highly ordered monolayers retain their structures during vigorous washing with solvents including water, ethanol, tetrahydrofuran, and toluene. These findings point to a convenient and broadly applicable strategy for noncovalent functionalization of 2D materials in applications that require large-area structural control, for instance, to minimize desorption at defects during subsequent solution processing.