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Hazardous gases threaten human health and the environment due to their toxicity and flammability. Managing such risks requires reliable gas detection techniques. However, accurate detection of target environmental gases remains challenging. Recent advances in materials science have enabled the combination of molecular separation membranes with chemical sensors to achieve high selectivity in the detection of target gas molecules. Herein, we discuss the underlying gas separation mechanisms for different types of membrane overlayers, including graphene oxide, metal-organic frameworks, and metal oxides, as well as their integration with sensing materials. Furthermore, we discuss the relationship between sensing performance and surface chemistry, catalytic properties, and membrane nanostructure. We conclude by highlighting future directions for selective detection of environmental gas species.

Surface-functionalized Mn–Fe oxide nanocrystals (NCs) were evaluated for single- and multi-sorbate scenarios considering As(V), Cr(VI), and U(VI) in varied water chemistries (deionized (DI), ground, and sea water) at pH 7.0. Multi-sorbate scenarios were further examined for competitive and/or cooperative effects. Precisely synthesized manganese ferrite (MnFe2O4) NCs were compared with iron oxide (Fe3O4) and manganese oxide (MnxOy) nanocrystal cores in terms of sorption capacities and colloidal stabilities. Positively charged cetyltrimethylammonium bromide (CTAB) and negatively charged oleyl phosphate (OP) were evaluated and compared as organic coatings. MnFe2O4 NCs exhibited both enhanced sorption performance and colloidal stability compared with Fe3O4 and MnxOy NC cores when functionalized with the same surfactant coating. For MnFe2O4 NCs, maximum sorption of As(V), Cr(VI), and U(VI) were observed to be 2.62, 3.43, and 4.27 mmol g−1, respectively (in DI water). Relative sorption capacity enhancement (compared with Fe3O4 and MnxOy) is due to increased surface grafting densities of MnFe2O4 NCs, providing a larger number of sorption sites (functional group) for target sorbates and higher repulsive energy (osmotic and elastic–steric interaction) for increased stability, and thus maintaining available surface area. For As(V) and Cr(VI) multi-sorbate systems, all materials evaluated preferentially adsorbed As(V) over Cr(VI). This preference was further investigated and observed using a novel quartz crystal microbalance (QCM) technique.

The application of positively charged polymers is a common treatment strategy for the sorption and separation of dissolved hexavalent chromium Cr(VI). In particular, polyethylenimine (PEI) has been demonstrated as an effective polymer for Cr treatment due to abundant amine groups and cost-effectiveness. However, PEI as a photoactive polymer has not been previously explored for Cr treatment. Toward this, we demonstrate the significant photoactive potential of PEI for Cr sorption and reduction [to Cr (III)], delineating amine structure-function relationships. Overall, Cr(VI) photoreduction is enhanced dramatically as a function of amine group density. Additionally, among the types of amine groups evaluated, tertiary amines are observed to be the most reactive followed by secondary and primary amines. When PEI is incorporated onto graphene oxide (GO) as a stabilizing scaffold, mass loading of PEI is a key variable for Cr(VI) treatment performance, which is observed to be a function of PEI molecular weight and oxidation extent of GO. Finally, when combined with magnetic nanomaterials, low energy, high efficiency separation, and reuse are demonstrated. For these, sorption-based separation processes maintained excellent performance over five cycles. Taken together, this work demonstrates that PEI can serve as highly effective, multifunctional material coating(s) not only for chromium treatment but potentially for other oxidized aqueous species as well.

Inorganic-organic nanocomposites, typically as an inorganic core with surface organic coating(s), have received interest as potential platform materials for sensor, catalyst, sorbent, and environmental applications, among others. Here, we describe the critical role of organic surface coatings with regard to the colloidal stability of engineered manganese oxide nanoparticles (MnxOy NPs). Specifically, we prepared libraries of monodisperse MnxOy NPs with a serial selection of surface coatings (stearic acid (SA), oleic acid (OA), poly(maleic anhydride-alt-1-octadecene) (PMAO), linear polyethyleneimine (LPEI), and multibranched polyethyleneimine (BPEI)), which were chosen based on comparable structure(s) and functional group(s). We systematically evaluated the role of surface organic coatings via critical coagulation concentrations (CCCs), which were compared with theoretical calculations (Schulze-Hardy rule). Through a newly developed light scattering protocol, we observed that the effective density of nanoclusters can exceed NPs' primary (bulk) density depending on the open space(s) within organic coatings. Interestingly, PMAO-coated NPs were more stable at the point of zero charge (PZC) than at neutral pH (pH 7), despite the loss of effective surface charge potential. CCC was 334 mM in NaCl and 1.5 mM in CaCl2 at pH 7, compared to CCC values of 807 mM in NaCl and 210 mM in CaCl2 at PZC. This increase in stability is due to polymer (re)configuration (at PZC), which was further confirmed with a quartz crystal microbalance-based technique to evaluate surface-based polymer dynamics. Taken together, this work quantifies the role of organic coating dynamics, including structure, grafting density, and configuration on the colloidal stability of organic-coated NPs.

In this work, we develop and demonstrate highly stable organic-coated engineered superparamagnetic iron oxide nanoparticles (IONPs), which provide effective osmotic pressure without aggregation, reverse diffusion, or membrane blocking (by nanoparticles) for osmotically driven membrane systems, considering both forward osmosis (FO) and pressure-retarded osmosis (PRO). For this, we synthesized highly water stable, monodisperse 12 nm IONPs with a rational series of water stabilizing surface coatings, including sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and polyethylene glycol (PEG). We then compared the library of surface functionalized IONPs as draw solutes for osmotic pressure-driven membrane processes. As synthesized, surface (organic) coatings are compact, thin, and can have very similar surface charge as the membrane itself, which results in effective osmotic pressure in forward osmosis (FO) mode configuration. To increase the osmotic pressure further, on a per mass basis, we synthesized and demonstrated novel hollow IONPs with identical surface coatings. Finally, water flux was further enhanced for stable particle systems using an oscillating magnetic field, thus physically altering concentration gradients, as a function of particle magnetic properties.

Photoactive cetyltrimethylammonium bromide-oleic acid (CTAB-OA) bilayer coated iron oxide nanocrystals (IONCs) were precisely synthesized and demonstrated for photo-enhanced hexavalent chromium (Cr(VI)) treatment. For these materials, an outward facing, CTAB layer offers specific affinity for Cr(VI), while the core IONC (8 – 25 nm, as Fe3O4 single domain) surface participates in Fe(II)/Fe(III) redox cycling. Further, at the coating-particle interface, the OA layer alters material photoactive dynamics (near UV and visible light) via a ligand-to-metal charge transfer (LMCT) mechanism, significantly enhancing Cr(VI) reduction. To delineate process variables, we first quantified photo-catalytic Cr(VI) reduction rates under UVA irradiation (sunlight mimic intensities) as a function of IONC core size, which correlates with the amount of surface available ferrous ion (Fe(II)). It was observed that 25 nm CTAB functionalized IONC showed outstanding Cr(VI) sorption and photoreduction performance compared with other IONC explored and controls (i.e. TiO2 nanocrystals). We next systematically explored the photoreduction mechanism(s) through a matrix of control experiments, considering solution pH, dissolved gas (air, O2, and CO2), along with hydroxyl and superoxide radicals. Results indicate that Fe(II) (at the IONC interface) acts as the primary Cr(VI) reductant and that Fe(III) (oxidized from Fe(II)) (re)cycles via a photocatalyzed reaction. Taken together, this work presents a clear physical and mechanistic description of CTAB-OA bilayer IONCs which, when optimized, are highly effective for Cr(VI) treatment under UVA. In addition to potential broad application for other contaminant targets which are reactive with Fe(II), which are many, this work also highlights the need for precise interfacial understanding of nano composite materials towards process optimization.

In this work, we have developed and characterized flux-enhanced graphene oxide laminate (GOL) membranes by increasing interlayer (layer-to-layer) spacing using multibranched polyethylenimine (PEI) polymers with varied molecular weights and by controlling the graphene oxide (GO) oxidation extent. For these assemblies, water flux was demonstrated to increase by as much as ca. 30-fold compared to GO only laminate controls. PEI-embedded GOL membranes also had better methyl orange (MO) rejection performance than GO laminate controls due to the dilution effects (i.e., water is transported through the assembly much faster than MO). Further, boron removal is demonstrated via functionalized PEI with d-glucono-1,5-lactone, containing a high density of boron chelating groups, which can also be recycled/recovered with high efficiency.

Inorganic–organic nanoscale materials have recently received significant interest as tunable, next generation, sorbents for the separation of metal and metalloid contaminants, including arsenic (As) and chromium (Cr), among others. In this work, we have designed and synthesized IONCs coated with specific functionalized organic materials with the goal of variable explicit evaluation. Specifically, single domain, superparamagnetic, monodisperse IONCs were synthesized and transferred into water via surface functionalization by ligand exchange and encapsulation methods. The as synthesized hybrid materials showed high performance for both As(V) and Cr(VI) sorption when nanocrystals are coated with positively charged organic surface coatings such as polyethyleneimine (PEI) and cetyltrimethylammonium bromide (CTAB). IONC cores coated with negatively charged organic coating materials (polyethyleneglycol (PEG), oleic acid (OA), sodium dodecyl sulfate (SDS)) and silica (SiO2) demonstrated significantly lower sorption capacities. When silica coated IONCs (Fe3O4@SiO2, core–shell materials) were surface coated with PEI, sorption capacities for As(V) and Cr(VI) of Fe3O4@SiO2@PEI are comparable to Fe3O4@PEI, underscoring the importance of surface coating functionality. To complement these studies, real-time sorption behavior of As(V) and Cr(VI) with PEI was explored by quartz crystal microbalance with dissipation (QCM-D).

Herein, we describe engineered superparamagnetic iron oxide nanoparticles (IONPs) as platform materials for enhanced uranyl (UO22+) sorption and separation processes under environmentally relevant conditions. Specifically, monodispersed 8–25 nm iron oxide (magnetite, Fe3O4) nanoparticles with tailored organic acid bilayered coatings have been systematically evaluated and optimized to bind, and thus remove, uranium from water. The combined nonhydrolytic synthesis and bilayer phase transfer material preparation methods yield highly uniform and surface tailorable IONPs, which allow for direct evaluation of the size-dependent and coating-dependent sorption capacities of IONPs. Optimized materials demonstrate ultrahigh sorption capacities (>50% by wt/wt) at pH 5.6 for 8 nm oleic acid (OA) bilayer and sodium monododecyl phosphate (SDP) surface-stabilized IONPs. Synchrotron-based X-ray absorption spectroscopy shows that iron oxide core particle size and stabilizing surface functional group(s) substantially affec...

Developments in nanoscale engineering now allow for molecular scale optimization of reactivity, sorption, and magnetism, among other properties, for advanced, material-based environmental applications, including sorption, separation, and sensing of radionuclides. Herein, we describe novel, monodisperse nanoscale manganese ferrite crystals (MnFe2O4) for ultra-high capacity environmental sorption and subsequent separation of uranyl in water. System optimization was explored as a function of nanocrystal (core) composition, surface coating(s), and water chemistry. 11 nm MnFe2O4 nanocrystals, which were colloidally stabilized via engineered oleyl-based surface bilayers, demonstrate extreme, yet specific, uranium binding capacities while remaining monomerically stable under environmentally relevant conditions (water chemistries), which are key for application. In particular, MnFe2O4 cores with oleyl phosphate (as the outer facing layer) bilayers demonstrate preferential uranium binding of >150% (uranium weight)/(particle system weight) while being highly water stable in elevated ionic strengths/types and pH (up to 235.4 ppm (10.24 mM) of NaCl and 51.3 ppm (1.28 mM) of CaCl2, in addition to 60 ppm of uranyl, pH 5–9). Further, when normalized for size and surface coatings, MnFe2O4 nanocrystals had significantly enhanced sorption capacities compared to Mn2FeO4, Fe3O4 and manganese oxide core analogs. Mechanistically, we demonstrate that observed uranium sorption enhancement is due not only to thermodynamically favorable interfacial interactions (for both particle and selected bilayer coatings), but also due to significant uranyl reduction at the particle interface itself. Uranium sorption capacities for optimized systems described are the highest of any material reported to date.

For the first time, this work develops and demonstrates precisely engineered manganese oxide nanoscale particles for the sorption of uranium, as uranyl, in water. Size controlled monodisperse nanocrystalline manganese oxides (12 to 28 nm) were systematically synthesized via thermal decomposition of manganese oleate and phase-transferred into water by ligand exchange and bilayer stabilization methods. Resulting monodisperse suspensions demonstrate significantly enhanced uranyl adsorption as a function of size, surface coating chemistries, and solution pH. In particular, 12 nm particles coated with the unsaturated–unsaturated carbon chains linked bilayers, (e.g. oleic acid-oleyl phosphate linked bilayer coatings) have binding capacities well over 600 mg U per g of Mn, which is the highest reported uranium sorption capacity for any manganese based sorbent to date. Further, we spectrally identify significant uranyl reduction as part of the adsorption mechanism(s) for high capacity materials. Last, oleyl-based (phosphate and carboxylic) functionalized bilayered nanocrystals were extremely stable in the presence of high ionic strength/type (>800 mM); calcium (>19 mM), including the presence of uranyl cations (from 0.1 to 60 ppm). Taken together, these data demonstrate the potential for engineered monodisperse manganese oxide nanocrystals as ultra-high capacity platform sorbent materials for uranium separation at environmentally relevant ionic strengths and pH.

Here, we evaluate H2 generation, via NaBH4 hydrolysis, for a rationally designed library of cobalt based metal oxide nanocrystals. Specifically, we developed highly surface active wusite cobalt oxide nanorods (wz-CoO-NRs) with varied dimensions through the thermal decomposition of cobalt oleate (Co-OL) and cobalt stearate (Co-ST) – yielding multiple ‘nail’ structures grown along (002) direction. The length of wz-CoO-NRs (from 30 to 200 nm) was controlled by the ratio of Co-OL to Co-ST, temperature ramp rate, and reaction time. Further, by adjusting oleic acid concentration (as a surfactant) and/or (other) metal co-precursors (e.g. iron oleate (Fe-OL) and nickel oleate (Ni-OL)), shape (e.g. nanorod to urchin-like, irregular cube, flower, or sphere), crystal structure(s), and composition (from wz-CoO to cubic type CoO (cu-CoO) or ferrite metal oxides (Fe3O4, CoFe2O4, and Co0.5Ni0.5Fe2O4)) were serially tuned. The library of cobalt based metal oxide nanocrystals was compared, as catalysts, for H2 generation via NaBH4 decomposition, and performance was observed to be a function of crystal length for wz-CoO-NRs. A maximum hydrogen production rate of 10,367 mL min−1 g−1 at 293 K with an activation energy of 27.4 kJ mol-1 was observed for 200 nm wz-CoO-NRs, which is among the lowest reported activation energies for this reaction. Enhanced reactivity is likely due to increased favorable surface chemistries (multiple catalytically active (002) sites on the edge of multiple nails) and lowered charge transfer energy for longer (e.g. 200 nm) wz-CoO-NRs.

The sorption properties of TiO2 nanoparticles (NPs) on sand were investigated in the presence of Suwannee River humic acid (SRHA) as a surrogate for natural organic matter. SRHA drastically reduced the sorption preference regardless of the initial pH and ionic strength of the solution. Despite the higher SRHA loading onto sand versus the TiO2 NPs, the consequent zeta (ζ)-potential drop was more significant in the TiO2 NPs, indicating that the decreased sorption should be primarily ascribed to the fraction of SRHA sorbed onto the TiO2 NP surfaces. The hindering effect of SRHA on the sorption increased with SRHA concentration, but stabilized at the concentration equivalent to full surface coverage of the TiO2 NPs and sand in the reaction system. Interaction force measurements indicated that SRHA inhibits the approach of the TiO2 NPs to the sand surface, as expected by the Derjaguin–Landau–Verwey–Overbeek calculation. However, the adhesion force of TiO2 NPs to sand was not critically affected by the presence of SRHA. TiO2 NP sorption to natural sand was limited as to the acid-treated sand in the presence of SRHA, implying that the organics dissolved from natural sand might facilitate the transport of TiO2 NPs in aquifers by inhibiting TiO2 NP sorption to aquifer materials.

Engineered, superparamagnetic, iron oxide nanoparticles (IONPs) have significant potential as platform materials for environmental sensing, imaging and remediation due to their unique size, physicochemical and magnetic properties. To this end, controlling the size and surface chemistry of the materials is crucial for such applications in the aqueous phase, and in particular, for porous matrixes with particle-surface interaction considerations. In this study, superparamagnetic, highly monodispersed 8 nm IONPs were synthesized and transferred into water as stable suspensions (remaining monodispersed) by way of an interfacial oleic acid bilayer surface. Once stabilized and characterized, particle-particle and model surface interactions (deposition and release) were quantitatively investigated and described systematically as a function of ionic strength (IS) and type with time-resolved dynamic light scattering (DLS), zeta potential, and real-time quartz crystal microbalance with dissipation monitoring (QCM-D) measurements. The critical coagulation concentration (CCC) for oleic acid bilayer coated iron oxide nanoparticles (OA-IONPs) were determined to be 710 mM for NaCl (matching DLVO predictions) and 10.6 mM for CaCl2, respectively. For all conditions tested, surface deposition kinetics showed stronger, more favorable interactions between OA-IONPs and polystyrene surfaces compared to silica, which is hypothesized to be due to increased particle-surface hydrophobic interactions (when compared to silica surfaces).

The physical and chemical factors affecting boron solute flux behavior and membrane transport mechanisms in forward osmosis (FO) have been systematically investigated. Boron solute flux behavior in FO was further compared with that in reverse osmosis (RO) by employing identical plate-and-frame cells and membranes under the same filtration conditions. The influence of draw solution pH, draw solution type, and membrane orientation on boron solute flux was examined for FO, and the effects of water flux, cross-flow velocity, feed water boron concentration, and solution pH on boron solute flux were examined for both FO and RO. Results show that reverse salt diffusion, a unique feature of FO, is a key mechanism governing boron solute flux in FO. Boron solute flux through the FO membrane was inversely proportional to the degree of reverse salt diffusion by draw solution. The higher boron rejection observed in FO compared to RO is also attributed to reverse salt diffusion in FO. It is also shown that membrane orientation in FO plays an important role, affecting boron solute flux due to different degrees of internal concentration polarization. In both FO and RO, boron solute flux increased with increasing water flux. However, the influence of water flux on boron solute flux was less significant in FO than RO. Furthermore, boron solute flux decreased with increasing feed water pH due to the conversion of the neutral boric acid to borate anions. The findings provide new insight into the mechanisms and factors controlling boron solute transport in FO. © 2012 Elsevier B.V.

Feasibility of osmotic backwashing for cleaning fouled membranes during forward osmosis (FO) process was investigated focusing on the mechanisms and factors involved. Alginate and humic acids were used as model organic foulants; and colloidal silica particles with different sizes were used as model inorganic particulate foulants. Results showed that noticeable flux recovery was achieved by osmotic backwashing through the instantaneous replacement of the draw solution with the dilute solution that has much less osmotic pressure than that of the feed solution. The switch of water flow direction through the membrane from feed-to-draw to draw-to-feed allows the effective detachment of foulants from the membrane surface. It was found that the efficiency of osmotic backwashing was affected by several factors including foulant type, membrane orientation and backwashing conditions (i.e. initial flux and duration). In addition, concentration polarization was found to play an important role in determining ...

Soft phase magnetic Fe-B nanoparticles were prepared by coprecipitation and the hard phase magnetic Nd-Fe-B nanoparticles were obtained from mechanical ball mill of the bulk Nd-Fe-B with a shaker mill. Also mechanical ball mill technique was applied to the mixture of Nd-Fe-B and Fe-B nanoparticles to build up an exchange-coupled nanocomposite magnet. It was loaded in electronic furnace and annealed under a flow of hydrogen at 953 K for 30 min. The exchange coupled interaction of isotropic magnets was characterized by smooth hysteresis loop with a high value of remanence (Mr/Ms > 0.5).

High temperature solution phase decomposition of Fe(CO)5 and reduction of Pt(acac)2 in the presence of stabilizers of oleic acid and oleyl amine are employed to produce Fe1–xPtx alloy nanoparticles. The Fe and Pt composition of the nanosized materials can be tuned by adjusting the molar ratio of Fe(CO)5 to Pt(acac)2, and the compositions ranging from Fe40Pt60 to Fe80Pt20 are obtained. This study is to investigate the influence of the composition on Curie temperature (Tc) of Fe1–xPtx nanoparticles monitored by thermomagnetic analysis (TMA).