Carlo A. Amadei

Revealing amphiphilic nanodomains of anti-biofouling polymer coatings.

Undesired bacterial adhesion and biofilm formation on wetted surfaces leads to significant economic and environmental costs in various industries. Amphiphilic coatings with molecular hydrophilic and hydrophobic patches can mitigate such biofouling effectively in an environmentally friendly manner. The coatings are synthesized by copolymerizing (Hydroxyethyl)methacrylate and perfluorodecylacrylate via initiated chemical vapor deposition (iCVD). In previous studies, the size of the patches was estimated to be ∼1.4-1.75 nm by fitting protein adsorption data to a theoretical model. However, no direct observations of the molecular heterogeneity exist and therefore the origin of the fouling resistance of amphiphilic coatings remains unclear. Here, the amphiphilic nature is investigated by amplitude modulation atomic force microscopy (AM-AFM). High-resolution images obtained by penetrating and oscillating the AFM tip under the naturally present water layer with sub-nanometer amplitudes reveal, for the first time, the existence of amphiphilic nanodomains (1-2 nm(2)). Compositional heterogeneity at the nanoscale is further corroborated by a statistical analysis on the data obtained with dynamic AM-AFM force spectroscopy. Variations in the long range attractive forces, responsible for water affinity, are also identified. These nanoscopic results on the polymers wettability are also confirmed by contact angle measurements (i.e., static and dynamic). The unprecedented ability to visualize the amphiphilic nanodomains as well as sub-nanometer crystalline structures provides strong evidence for the existence of previously postulated nanostructures, and sheds light on the underlying antifouling mechanism of amphiphilic chemistry.

Time dependent wettability of graphite upon ambient exposure: The role of water adsorption

We report the temporal evolution of the wettability of highly ordered pyrolytic graphite (HOPG) exposed to environmental conditions. Macroscopic wettability is investigated by static and dynamic contact angles (SCA and DCA) obtaining values comparable to the ones presented in the literature. SCA increases from ∼68° to ∼90° during the first hour of exposure after cleaving, whereas DCA is characterized by longer-scale (24 h) time evolution. We interpret these results in light of Fourier transform infrared spectroscopy, which indicates that the evolution of the HOPG wettability is due to adsorption of molecules from the surrounding atmosphere. This hypothesis is further confirmed by nanoscopic observations obtained by atomic force microscope (AFM)-based force spectroscopy, which monitor the evolution of surface properties with a spatial resolution superior to macroscopic experiments. Moreover, we observe that the results of macro- and nanoscale measurements evolve in similar fashion with time and we propose a quantitative correlation between SCA and AFM measurements. Our results suggest that the cause of the transition in the wettability of HOPG is due to the adsorption of hydrocarbon contaminations and water molecules from the environment. This is corroborated by annealing the HOPG is vacuum conditions at 150°, allowing the desorption of molecules on the surface, and thus re-establishing the initial macro and nano surface properties. Our findings can be used in the interpretation of the wettability of more complicated systems derived from HOPG (i.e., graphene).

The aging of a surface and the evolution of conservative and dissipative nanoscale interactions

A method to monitor variations in the conservative and dissipative forces in dynamic atomic force microscopy is proposed in order to investigate the effects of exposing a surface to different sets of environmental conditions for prolonged periods of time. The variations are quantified by proposing and defining two metrics, one for conservative and another for dissipative interactions. Mica and graphite are chosen as model samples because they are atomically flat and easy to cleave. It is found that long term exposure to high relative humidity (RH), i.e., 90% > RH > 70%, affects the magnitude and distance dependencies of the forces, as quantified by the respective metrics, more drastically than the actual environmental conditions at which the samples are probed. Attenuated total reflectance infrared spectroscopy experiments further indicate that accumulation of water and carbonates on the surfaces with time is responsible for the variations in force measurements. This study has implications in surface functionality, reactivity, and longevity.

Extended friction elucidates the breakdown of fast water transport in graphene oxide membranes

The understanding of water transport in graphene oxide (GO) membranes stands out as a major theoretical problem in graphene research. Notwithstanding the intense efforts devoted to the subject in the recent years, a consolidated picture of water transport in GO membranes is yet to emerge. By performing mesoscale simulations of water transport in ultrathin GO membranes, we show that even small amounts of oxygen functionalities can lead to a dramatic drop of the GO permeability, in line with experimental findings. The coexistence of bulk viscous dissipation and spatially extended molecular friction results in a major decrease of both slip and bulk flow, thereby suppressing the fast water transport regime observed in pristine graphene nanochannels. Inspection of the flow structure reveals an inverted curvature in the near-wall region, which connects smoothly with a parabolic profile in the bulk region. Such inverted curvature is a distinctive signature of the coexistence between single-particle Langevin friction and collective hydrodynamics. The present mesoscopic model with spatially extended friction may offer a computationally efficient tool for future simulations of water transport in nanomaterials.

Role of Oxygen Functionalities in Graphene Oxide Architectural Laminate Subnanometer Spacing and Water Transport

Active research in nanotechnology contemplates the use of nanomaterials for environmental engineering applications. However, a primary challenge is understanding the effects of nanomaterial properties on industrial device performance and translating unique nanoscale properties to the macroscale. One emerging example consists of graphene oxide (GO) membranes for separation processes. Thus, here we investigate how individual GO properties can impact GO membrane characteristics and water permeability. GO chemistry and morphology were controlled with easy-to-implement photoreduction and sonication techniques and were quantitatively correlated, offering a valuable tool for accelerating characterization. Chemical GO modification allows for fine control of GO oxidation state, allowing control of GO architectural laminate (GOAL) spacing and permeability. Water permeability was measured for eight GOALs characterized by different GOAL chemistry and morphology and indicates that GOAL nanochannel height dictates water transport. The experimental outputs were corroborated with mesoscale water transport simulations of relatively large domains (thousands of square nanometers) and indicate a no-slip Darcy-like behavior inside the GOAL nanochannels. The experimental and simulation evidence presented in this study helps create a clearer picture of water transport in GOAL and can be used to rationally design more effective and efficient GO membranes.

Elucidation of the wettability of graphene through a multi-length-scale investigation approach

Univocal conclusions around the wettability of graphene exposed to environmental conditions remain elusive despite the recent efforts of several research groups. The main discrepancy rests on the question of whether a graphene monolayer (GML) is transparent or not to water and more generally what the role is that the substrate plays in determining the degree of wetting of the GML. In this work, we investigate the water transparency of GML by means of a multi-length-scale approach. We complement traditional static contact angle measurements and environmental scanning electron microscopy experiments with atomic force microscopy based force spectroscopy to assess the role that intermolecular interactions play in determining the wetting of GML. To gain deeper insight into the wetting transparency issue, we perform experiments on inert metals, such as gold and platinum, covered or not covered by GML. The comparison of the results obtained for different systems (i.e. GML covered and uncovered inert metals), provides unambiguous evidence that supports the non-wetting transparency theory of GML. This work aims to assist the development of technologies based on graphene–water interaction, such as graphitic membranes for water separation processes.

The Mendeleev–Meyer force project

Here we present the Mendeleev-Meyer Force Project which aims at tabulating all materials and substances in a fashion similar to the periodic table. The goal is to group and tabulate substances using nanoscale force footprints rather than atomic number or electronic configuration as in the periodic table. The process is divided into: (1) acquiring nanoscale force data from materials, (2) parameterizing the raw data into standardized input features to generate a library, (3) feeding the standardized library into an algorithm to generate, enhance or exploit a model to identify a material or property. We propose producing databases mimicking the Materials Genome Initiative, the Medical Literature Analysis and Retrieval System Online (MEDLARS) or the PRoteomics IDEntifications database (PRIDE) and making these searchable online via search engines mimicking Pubmed or the PRIDE web interface. A prototype exploiting deep learning algorithms, i.e. multilayer neural networks, is presented.