Nanofluidic resistive pulse sensing for characterization of extracellular vesicles.

This paper describes the development, design and characterization of a resistive pulse sensing (RPS) system for the analysis of size distributions of extracellular vesicles (EVs). The system is based on microfluidic chips fabricated using soft-lithography and operated in pressure-driven mode. This fabrication approach provided reproducible pore dimensions and the best performing chip design enabled, without calibration, sizing of both 252 nm and 460 nm test particles within 8% of theoretically calculated values, based on the size specifications provided by suppliers. The number concentration measurement had higher variations and without calibration provided estimates within an order of magnitude, for sample concentrations across 4 orders of magnitude. The RPS chips could also measure successfully EVs and other biological nanoparticles in purified samples from cell culture media and human serum. A compact, fast and inexpensive RPS system based on this design could be an attractive alternative to current gold-standard techniques for routine characterization of EV samples.

Point-of-care quantification of serum cellular fibronectin levels for stratification of ischemic stroke patients.

The abundance of cellular fibronectin (c-Fn) for ischemic stroke patients and the narrow time-window (<4.5 h) for the decision to administer the thrombolytic treatment with recombinant tissue plasminogen activator (rtPA) are challenging for the development of a point-of-care (PoC) diagnostic platform. We report a case of stratification of ischemic stroke patients based on a magnetoresistive biosensor platform that quantifies the c-Fn levels in a small volume of serum, within the clinically relevant time-window. Our PoC platform uses different ratios of biofunctionalized magnetic nanoparticles (MNPs) as immunoassay labels to adjust the sensitivity within the clinically relevant ranges for c-Fn (1-4 µg/mL). After optimizing the detection range, resolution, and sensitivity, our device was able to stratify ischemic stroke patients who developed hemorrhagic transformation, the main side-effect of rtPA, from those (both non-treated and treated with rtPA) who did not.

Crystallographic facet selective HER catalysis: exemplified in FeP and NiP2 single crystals.

How the crystal structures of ordered transition-metal phosphide catalysts affect the hydrogen-evolution reaction (HER) is investigated by measuring the anisotropic catalytic activities of selected crystallographic facets on large (mm-sized) single crystals of iron-phosphide (FeP) and monoclinic nickel-diphosphide (m-NiP2). We find that different crystallographic facets exhibit distinct HER activities, in contrast to a commonly held assumption of severe surface restructuring during catalytic activity. Moreover, density-functional-theory-based computational studies show that the observed facet activity correlates well with the H-binding energy to P atoms on specific surface terminations. Direction dependent catalytic properties of two different phosphides with different transition metals, crystal structures, and electronic properties (FeP is a metal, while m-NiP2 is a semiconductor) suggests that the anisotropy of catalytic properties is a common trend for HER phosphide catalysts. This realization opens an additional rational design for highly efficient HER phosphide catalysts, through the growth of nanocrystals with specific exposed facets. Furthermore, the agreement between theory and experimental trends indicates that screening using DFT methods can accelerate the identification of desirable facets, especially for ternary or multinary compounds. The large single-crystal nature of the phosphide electrodes with well-defined surfaces allows for determination of the catalytically important double-layer capacitance of a flat surface, C dl = 39(2) µF cm-2 for FeP, useful for an accurate calculation of the turnover frequency (TOF). X-ray photoelectron spectroscopy (XPS) studies of the catalytic crystals that were used show the formation of a thin oxide/phosphate overlayer, presumably ex situ due to air-exposure. This layer is easily removed for FeP, revealing a surface of pristine metal phosphide.

Photocatalytic Microporous Membrane against the Increasing Problem of Water Emerging Pollutants.

Emerging pollutants are an essential class of recalcitrant contaminants that are not eliminated from water after conventional treatment. Here, a photocatalytic microporous membrane based on polyvinylidene difluoride-co-trifluoroethylene (PVDF-TrFE) with immobilised TiO2 nanoparticles, prepared by solvent casting, was tested against representative emerging pollutants. The structure and composition of these polymeric membranes were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, porosimetry, and contact angle goniometry. The nanocomposites exhibited a porous structure with a uniform distribution of TiO2 nanoparticles. The addition of TiO2 did not change the structure of the polymeric matrix; however, it increased the wettability of the nanocomposite. The nanocomposites degraded 99% of methylene blue (MB), 95% of ciprofloxacin (CIP), and 48% of ibuprofen (IBP). The microporous nanocomposite exhibited no photocatalytic efficiency loss after four use cycles, corresponding to 20 h of UV irradiation. The reusability of this system confirms the promising nature of polymer nanocomposites as the basis for cost-effective and scalable treatments of emerging pollutants.

Exposure effects of endotoxin-free titanium-based wear particles to human osteoblasts.

Titanium-based materials are widely employed by the biomedical industry in orthopedic and dental implants. However, when placed into the human body, these materials are highly susceptible to degradation processes, such as corrosion, wear, and tribocorrosion. As a consequence, metallic ions or particles (debris) may be released, and although several studies have been conducted in recent years to better understand the effects of their exposure to living cells, a consensual opinion has not yet been obtained. In this work, we produced metallic-based wear particles by tribological tests carried out on Ti-6Al-4V and Ti-15Zr-15Mo alloys. They were posteriorly physicochemically characterized according to their crystal structure, size, morphology, and chemical composition and compared to Ti-6Al-4V commercially available particles. Finally, adsorbed endotoxins were removed (by applying a specific thermal treatment) and endotoxin-free particles were used in cell experiments to evaluate effects of their exposure to human osteoblasts (MG-63 and HOb), namely cell viability/metabolism, proinflammatory cytokine production (IL-6 and PGE2), and susceptibility to internalization processes. Our results indicate that tribologically-obtained wear particles exhibit fundamental differences in terms of size (smaller) and morphology (irregular shapes and rough surfaces) when compared to the commercial ones. Consequently, both Ti-6Al-4V and Ti-15Zr-15Mo particles were able to induce more pronounced effects on cell viability (decrease) and cytokine production (increase) than did Ti-6Al-4V commercial particles. Furthermore, both types of wear particles penetrated osteoblast membranes and were internalized by the cells. Influences on cytokine production by endotoxins were also demonstrated.

Attomolar Label-Free Detection of DNA Hybridization with Electrolyte-Gated Graphene Field-Effect Transistors.

In this work, we develop a field-effect transistor with a two-dimensional channel made of a single graphene layer to achieve label-free detection of DNA hybridization down to attomolar concentration, while being able to discriminate a single nucleotide polymorphism (SNP). The SNP-level target specificity is achieved by immobilization of probe DNA on the graphene surface through a pyrene-derivative heterobifunctional linker. Biorecognition events result in a positive gate voltage shift of the graphene charge neutrality point. The graphene transistor biosensor displays a sensitivity of 24 mV/dec with a detection limit of 25 aM: the lowest target DNA concentration for which the sensor can discriminate between a perfect-match target sequence and SNP-containing one.

Template-directed self-organization of colloidal PbTe nanocrystals into pillars, conformal coatings, and self-supported membranes.

We demonstrate the formation of three morphologies relevant for integration with miniaturized devices-microscale pillars, conformal coatings, and self-supported membranes-via template-directed self-organization of lead telluride (PbTe) colloidal nanocrystals (NCs). Optimizing the self-organization process towards producing one of these morphologies typically involves adjusting the surface chemistry of the particles, as a means of controlling the particle-particle and particle-template interactions. In contrast, we have produced each of the three morphologies of close-packed NCs by adjusting only the solvent and concentration of NCs, to ensure that the high quality of the ca. 10 nm PbTe NCs produced by hot-injection colloidal synthesis, which we used as model "building blocks," remains consistent across all three configurations. For the first two morphologies, the NCs were deposited as colloidal suspensions onto micropatterned silicon substrates. The microscale cuboid pillars (1 µm × 1 µm × 0.6 µm) were formed by depositing NC dispersions in toluene onto templates patterned with resist grid motifs, followed by the resist removal after the slow evaporation of toluene and formation of the micropillars. Conformal coatings were produced by switching the solvent from toluene to a faster drying hexane and pouring NC dispersions onto silicon templates with topographically patterned microstructures. In a similar process, self-supported NC membranes were formed from NC dispersions in hexane on the surface of diethylene glycol and transferred onto the micropatterned templates. The demonstrated combination of bottom-up self-organization with top-down micropatterned templates provides a scalable route for design and fabrication of NC ensembles in morphologies and form-factors that are compatible with their integration into miniaturized devices.

Combining CXCR4-targeted and nontargeted nanoparticles for effective unassisted in vitro magnetic hyperthermia.

The use of targeted nanoparticles for magnetic hyperthermia (MHT) increases MHT selectivity, but often at the expense of its effectiveness. Consequently, targeted MHT is typically used in combination with other treatment modalities. This work describes an implementation of a highly effective monotherapeutic in vitro MHT treatment based on two populations of magnetic particles. Cells were sequentially incubated with two populations of magnetic particles: nonfunctionalized superparamagnetic nanoparticles and anti-CXCR4-functionalized particles. After removing the excess of free particles, an alternating magnetic field (AMF) was applied to produce MHT. The induced cytotoxicity was assessed at different time-points after AMF application. Complete loss of cell viability was observed 72 h after MHT when the iron loading of the anti-CXCR4-functionalized particles was boosted by that of a nontargeted population. Additionally, induction of necrosis resulted in more efficient cell death than did induction of apoptosis. Achieving a uniquely high effectiveness in monotherapeutic MHT demonstrates the potential of this approach to achieve complete loss of viability of cancer cells while avoiding the side effects of dual-treatment strategies that use MHT only as a sensitizing therapy.

Adapting Bobbert-Vlieger model to spectroscopic ellipsometry of gold nanoparticles with bio-organic shells.

We investigate spectroscopic imaging ellipsometry for monitoring biomolecules at surfaces of nanoparticles. For the modeling of polarimetric light scattering off surface-adsorbed core-shell nanoparticles, we employ an extension of the exact solution for the scattering by particles near a substrate presented by Bobbert and Vlieger, which offers insight beyond that of the Maxwell-Garnett effective medium approximation. Varying thickness and refractive index of a model bio-organic shell results in systematic and characteristic changes in spectroscopic parameters [Formula: see text] and [Formula: see text]. The salient features and trends in modeled spectra are in qualitative agreement with experimental data for antibody immobilization and fibronectin biorecognition at surfaces of gold nanoparticles on a silicon substrate, but achieving a full quantitative agreement will require including additional effects, such as nanoparticle-substrate interactions, into the model.

Up-scaling the synthesis of Cu2O submicron particles with controlled morphologies for solar H2 evolution from water.

The synthesis of Cu2O was studied to examine the effects of up-scaling on the size and morphology of the resultant particles. As a result, a successful protocol employing an automated laboratory reactor was developed for large-scale synthesis of phase-pure Cu2O colloids with specific sizes in the submicron to micrometer range (0.2-2.6 µm). The as-synthesized products have been studied by means of powder X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, UV-Vis-NIR spectroscopy, scanning electron microscopy, and photoelectrochemical measurements. A broad range of morphologies, both equilibrium (stellated octahedrons, cubes, cuboctahedrons, truncated octahedrons, truncated cuboctahedrons) and metastable (cage-like hierarchical structures, microspheres with flower-like texture), with uniform sizes have been selectively prepared either by careful tuning of synthesis conditions. Recrystallization of primary aggregates through Ostwald ripening is proposed as the formation mechanism for these Cu2O structures. As a photocathode for photoelectrochemical H2 evolution, Cu2O submicron cubes with exposed {001} facets exhibit a high open-circuit potential of ca. 0.9 V vs. the RHE at pH 1.

Analytical protocols for separation and electron microscopy of nanoparticles interacting with bacterial cells.

An important step toward understanding interactions between nanoparticles (NPs) and bacteria is the ability to directly observe NPs interacting with bacterial cells. NP-bacteria mixtures typical in nanomedicine, however, are not yet amendable for direct imaging in solution. Instead, evidence of NP-cell interactions must be preserved in derivative (usually dried) samples to be subsequently revealed in high-resolution images, for example, via scanning electron microscopy (SEM). Here, this concept is realized for a mixed suspension of model NPs and Staphylococcus aureus bacteria. First, protocols for analyzing the relative colloidal stabilities of NPs and bacteria are developed and validated based on systematic centrifugation and comparison of colony forming unit (CFU) counting and optical density (OD) measurements. Rate-dependence of centrifugation efficiency for each component suggests differential sedimentation at a specific predicted rate as an effective method for removing free NPs after co-incubation; the remaining fraction comprises bacteria with any associated NPs and can be examined, for example, by SEM, for evidence of NP-bacteria interactions. These analytical protocols, validated by systematic control experiments and high-resolution SEM imaging, should be generally applicable for investigating NP-bacteria interactions.

High-Temperature Magnetism as a Probe for Structural and Compositional Uniformity in Ligand-Capped Magnetite Nanoparticles.

To investigate magnetostructural relationships in colloidal magnetite (Fe3O4) nanoparticles (NPs) at high temperature (300-900 K), we measured the temperature dependence of magnetization (M) of oleate-capped magnetite NPs ca. 20 nm in size. Magnetometry revealed an unusual irreversible high-temperature dependence of M for these NPs, with dip and loop features observed during heating-cooling cycles. Detailed characterizations of as-synthesized and annealed Fe3O4 NPs as well as reference ligand-free Fe3O4 NPs indicate that both types of features in M(T) are related to thermal decomposition of the capping ligands. The ligand decomposition upon the initial heating induces a reduction of Fe3+ to Fe2+ and the associated dip in M, leading to more structurally and compositionally uniform magnetite NPs. Having lost the protective ligands, the NPs continually sinter during subsequent heating cycles, resulting in divergent M curves featuring loops. The increase in M with sintering proceeds not only through elimination of a magnetically dead layer on the particle surface, as a result of a decrease in specific surface area with increasing size, but also through an uncommonly invoked effect resulting from a significant change in Fe3+/Fe2+ ratio with heat treatment. The interpretation of irreversible features in M(T) indicates that reversible M(T) behavior, conversely, can be expected only for ligand-free, structurally and compositionally uniform magnetite NPs, suggesting a general applicability of high-temperature M(T) measurements as an analytical method for probing the structure and composition of magnetic nanomaterials.

Residue-dependent adsorption of model oligopeptides on gold.

The adsorption to gold surfaces in aqueous solutions has been systematically evaluated for a series of model oligopeptides. The series includes GG-X-GG "host-guest" sequences, where the central X residue is one of 19 proteinogenic amino acids, and water-soluble X5 and X10 homo-oligopeptides. Irreversible adsorption on gold of GG-X-GG peptides, which lack significant secondary structure, was quantitatively analyzed by X-ray photoelectron spectroscopy (XPS). The broad range of the quasi-equilibrium surface densities measured by XPS corroborates the hypothesis that surface interactions of GG-X-GG peptides are dominated by their central X residues. The highest surface density was produced by GGCGG, followed by sequences with hydrophobic, charged, and polar central residues. Neither electrostatic nor hydrophobic interactions dominate the adsorption of GG-X-GG peptides: for charged and polar central residues, surface densities correlate with the size of the side chains but not with the sign of the charges, while for hydrophobic residues, the surface densities are uncorrelated with side-chain hydrophobicity. An intriguing result is the disparity in surface adsorption of structural isomers of Leu and Val, which exhibit a correlation between the position of the branched carbon in the side chain and the interaction of the peptide backbone with the surface. The surface density produced by the adsorption of GG-X-GG peptides overall was low; however, adsorption tended to increase as the number of X residues increased (GG-X-GG < X5 < X10), suggesting that cooperative binding is important for surface attachment of proteins that readily adsorb on inorganic surfaces. The Leu and Val isomer investigation and trends revealed by our analysis show how the methodology and results described here provide a fundamental reference for future experimental and computational studies and for rational design of peptides that exhibit predictable adsorption behaviors on a given surface.

Nickel foam supported mesoporous MnO2 nanosheet arrays with superior lithium storage performance.

Mesoporous MnO2 nanosheet arrays have been directly grown on nickel foam current collectors and exhibited a reversible capacity as high as 1690 mA h g(-1) even after one hundred cycles at 100 mA g(-1). They also reveal good rate capability and excellent cycling stability.

Circular dichroism analysis of cyclic ß-helical peptides adsorbed on planar fused quartz.

Conformational changes of three cyclic ß-helical peptides upon adsorption onto planar fused-quartz substrates were detected and analyzed by far-ultraviolet (UV) circular dichroism (CD) spectroscopy. In trifluoroethanol (TFE), hydrophobic peptides, Leu ß and Val ß, form left- and right-handed helices, respectively, and water-soluble peptide WS ß forms a left-handed helix. Upon adsorption, CD spectra showed a mixture of folded and unfolded conformations for Leu ß and Val ß and predominantly unfolded conformations for WS ß. X-ray photoelectron spectroscopy (XPS) provided insight about the molecular mechanisms governing the conformational changes, revealing that ca. 40% of backbone amides in Leu ß and Val ß were interacting with the hydrophilic substrate, while only ca. 15% of the amines/amides in WS ß showed similar interactions. In their folded ß-helical conformations, Leu ß and Val ß present only hydrophobic groups to their surroundings; hydrophilic surface groups can only interact with backbone amides if the peptides change their conformation. Conversely, as a ß helix, WS ß presents hydrophilic side chains to its surroundings that could, in principle, interact with hydrophilic surface groups, with the peptide retaining its folded structure. Instead, the observed unfolded surface conformation for WS ß and the relatively small percentage of surface-bound amides (15 versus 40% for Leu ß and Val ß) suggest that hydrophilic surface groups induce unfolding. Upon this surface-induced unfolding, WS ß interacts with the surface preferentially via hydrophilic side chains rather than backbone amides. In contrast, the unfolded ß-hairpin-like form of WS ß does not irreversibly adsorb on fused quartz from water, highlighting that solvation effects can be more important than initial conformation in governing peptide adsorption. Both label-free methods demonstrated in this work are, in general, applicable to structural analysis of a broad range of biomolecules adsorbed on transparent planar substrates, the surface properties of which could be customized.

Evaluating protocols and analytical methods for peptide adsorption experiments.

This paper evaluates analytical techniques that are relevant for performing reliable quantitative analysis of peptide adsorption on surfaces. Two salient problems are addressed: determining the solution concentrations of model GG-X-GG, X5, and X10 oligopeptides (G = glycine, X = a natural amino acid), and quantitative analysis of these peptides following adsorption on surfaces. To establish a uniform methodology for measuring peptide concentrations in water across the entire GG-X-GG and X n series, three methods were assessed: UV spectroscopy of peptides having a C-terminal tyrosine, the bicinchoninic acid (BCA) protein assay, and amino acid (AA) analysis. Due to shortcomings or caveats associated with each of the different methods, none were effective at measuring concentrations across the entire range of representative model peptides. In general, reliable measurements were within 30% of the nominal concentration based on the weight of as-received lyophilized peptide. In quantitative analysis of model peptides adsorbed on surfaces, X-ray photoelectron spectroscopy (XPS) data for a series of lysine-based peptides (GGKGG, K5, and K10) on Au substrates, and for controls incubated in buffer in the absence of peptides, suggested a significant presence of aliphatic carbon species. Detailed analysis indicated that this carbonaceous contamination adsorbed from the atmosphere after the peptide deposition. The inferred adventitious nature of the observed aliphatic carbon was supported by control experiments in which substrates were sputter-cleaned by Ar(+) ions under ultra-high vacuum (UHV) then re-exposed to ambient air. In contrast to carbon contamination, no adventitious nitrogen species were detected on the controls; therefore, the relative surface densities of irreversibly-adsorbed peptides were calculated by normalizing the N/Au ratios by the average number of nitrogen atoms per residue.

Divalent-anion salt effects in polyelectrolyte multilayer depositions.

We systematically investigate the effects of divalent anions on the assembly of polyelectrolyte multilayers by fabricating polystyrene sulfonate (PSS)/polyallylamine hydrochloride (PAH) multilayer films from aqueous solutions containing SO(4)(2-), HPO(4)(2-), or organic dicarboxylate dianions. The chosen concentrations of these anions (i.e., ≤0.05 M) allow us to isolate their effects on the assembly process from those of the polyelectrolyte solubility or solution ionic strength (maintained constant at µ = 1.00 M by added NaCl). Compared to a control film prepared from solutions containing only Cl(-) anions, stratified multilayers deposited in the presence of dianions exhibit increased UV absorbance, thickness, and roughness. From the dependence of film properties on the solution concentration of SO(4)(2-) and number of polyelectrolyte layers deposited, we derive a generic model for the PSS/PAH multilayer formation that involves adsorption of PAH aggregates formed in solution via electrostatic interactions of PAH with bridging dianions. Experiments using HPO(4)(2-) and organic dicarboxylate species of varying structure indicate that the separation, rigidity, and angle between the discrete negatively charged sites in the dianion govern the formation of the PAH aggregates, and therefore also the properties of the multilayer film. A universal linear relationship between film UV absorbance and thickness is observed among all dianion types or concentrations, consistent with the model.

Impact of DNA-surface interactions on the stability of DNA hybrids.

The structure and stability of single- and double-stranded DNA hybrids immobilized on gold are strongly affected by nucleotide-surface interactions. To systematically analyze the effects of these interactions, a set of model DNA hybrids was prepared in conformations that ranged from end-tethered double-stranded to directly adsorbed single-stranded (hairpins) and characterized by surface plasmon resonance (SPR) imaging, X-ray photoelectron spectroscopy (XPS), fluorescence microscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The stabilities of these hybrids were evaluated by exposure to a series of stringency rinses in solutions of successively lower ionic strength and by competitive hybridization experiments. In all cases, directly adsorbed DNA hybrids are found to be significantly less stable than either free or end-tethered hybrids. The surface-induced weakening and the associated asymmetry in hybridization responses of the two strands forming hairpin stems are most pronounced for single-stranded hairpins containing blocks of m adenine (A) nucleotides and n thymine (T) nucleotides, which have high and low affinity for gold surfaces, respectively. The results allow a qualitative scale of relative stabilities to be developed for DNA hybrids on surfaces. Additionally, the results suggest a route for selectively weakening portions of immobilized DNA hybrids and for introducing asymmetric hybridization responses by using sequence design to control nucleotide-surface interactions--a strategy that may be used in advanced biosensors and in switches or other active elements in DNA-based nanotechnology.

Vibrational circular-dichroism spectroscopy of homologous cyclic peptides designed to fold into ß helices of opposite chirality.

Cyclic ß-helical peptides have been developed as model structured biomolecules for examining peptide adsorption and conformation on surfaces. As a key prerequisite to circular-dichroism (CD) analysis of these model peptides on surfaces, their conformations and the corresponding vibrational spectra in the 1400-1800 cm⁻¹ range were analyzed by vibrational circular-dichroism (VCD) spectroscopy in solution. The two model peptides ("ß Leu and ß Val") were examined in chloroform, where they each fold into a homogeneous well-defined antiparallel double-stranded ß-helical species, as determined previously by NMR and electronic CD spectroscopy. Because the ß-helical conformations of ß Leu and ß Val are well characterized, the VCD spectra of these peptides can be unambiguously correlated with their structures. In addition, these two ß-helical peptides differ from one another in two key respects that make them uniquely advantageous for CD analysis--first, while their backbone conformations are topologically similar, ß Leu and ß Val form helices of opposite chiralities; second, the two peptides differ in their sequences, i.e., composition of the side chains attached to the backbone. The observed VCD spectra for ß Leu and ß Val are roughly mirror images of each other, indicating that the VCD features are dominated by the chirality and conformation of the peptide backbone rather than by the peptide sequence. Accordingly, spectra similarly characteristic of peptide secondary structure can be expected for peptides designed to be structural analogs of ß Leu and ß Val while incorporating a variety of side chains for studies of surface adsorption from organic and aqueous solvents.

Surface composition, chemistry, and structure of polystyrene modified by electron-beam-generated plasma.

Polystyrene (PS) surfaces were treated by electron-beam-generated plasmas in argon/oxygen, argon/nitrogen, and argon/sulfur hexafluoride environments. The resulting modifications of the polymer surface energy, morphology, and chemical composition were analyzed by a suite of complementary analytical techniques: contact angle goniometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and reflection electron energy loss spectroscopy (REELS). The plasma treatments produced only minimal increases in the surface roughness while introducing the expected chemical modifications: oxygen-based after Ar/O(2) plasma, oxygen- and nitrogen-based after Ar/N(2) plasma, and fluorine-based after Ar/SF(6) plasma. Fluorinated PS surfaces became hydrophobic and did not significantly change their properties over time. In contrast, polymer treated in Ar/O(2) and Ar/N(2) plasmas initially became hydrophilic but underwent hydrophobic recovery after 28 days of aging. The aromatic carbon chemistry in the top 1 nm of these aged surfaces clearly indicated that the hydrophobic recovery was produced by reorientation/diffusion of undamaged aromatic polymer fragments from the bulk rather than by contamination. Nondestructive depth profiles of aged plasma-treated PS films were reconstructed from parallel angle-resolved XPS (ARXPS) measurements using a maximum-entropy algorithm. The salient features of reconstructed profiles were confirmed by sputter profiles obtained with 200 eV Ar ions. Both types of depth profiles showed that the electron-beam-generated plasma modifications are confined to the topmost 3-4 nm of the polymer surface, while valence band measurements and unsaturated carbon signatures in ARXPS and REELS data indicated that much of the PS structure was preserved below 9 nm.