Table of contents

Nanotechnology offers immense promise for developing new technologies that are more sustainable than current technologies. All major industrial sectors have felt nanotechnology's impact, mainly from the incorporation of nanomaterials into their products. For example, nanotechnology has improved the design and performance of products in areas as diverse as electronics, medicine and medical devices, food and agriculture, cosmetics, chemicals, materials, coatings, energy, as well as many others. Moreover, the revenues from nanotechnology-enabled products are not trivial. For instance, Lux Research maintains that commercial sales in both Europe and the USA will attain revenues of over $1 trillion from nano-enabled products by 2015.

The manufacturing of the nanomaterials for these products uses many processes equivalent to chemical manufacturing processes. As a result, manufacturing nanomaterials can produce either harmful pollutants or adverse environmental impacts similar to those from chemical manufacturing. Unlike the chemical industry, however, those same processes are not ingrained in the manufacturing of nanomaterials, and the opportunity exists at the initial design stage to purposely account for and mitigate out potentially harmful environmental impacts. While prevention has not been a priority in current industries, it can become a main concern for the new and future industries that manufacture nanomaterials on a bulk commercial scale. This is where green nanotechnology comes in.

Green nanotechnology involves deliberate efforts aimed at developing meaningful and reasonable protocols for generating products and their associated production processes in a benign fashion. The goal is a conscious minimization of risks associated with the products of nanoscience. The green products of nanotechnology are those that are used in either direct or indirect environmental applications. Direct environmental applications provide benefits such as monitoring using nano-enabled sensors, remediation of hazardous waste sites with nanomaterials, or treatment of wastewater and drinking water with nanomaterials. Indirect environmental applications include, for example, the saved energy associated with either lighter nanocomposite materials in transport vehicles or reduced waste from smaller products.

The production and process aspects of green nanotechnology involve both making nanomaterials in a more environmentally benign fashion and using nanomaterials to make current chemical processes more environmentally acceptable. Examples of producing nanomaterials in a 'greener manner' could involve but are not limited to the use of supercritical CO₂, water, or ionic liquids to replace a volatile organic solvent. Either self-assembly or templating might also be used to eliminate waste in manufacturing. Renewables could be utilized as replacements for either nonrenewable and/or toxic starting materials. Microwave techniques might potentially help to conserve energy, as could both facile thermal and hydrothermal processes. Catalytic and photocatalytic reactions could also increase efficiency and decrease the formation of harmful byproducts. In addition, engineered nanomaterials themselves can be used as catalysts in current chemical processes and as separation membranes to aid in the efficiency of these operations. Furthermore, in order to be truly green, these products and processes must be considered within a lifecycle framework.

The papers in this special issue are but a small sampling of the myriad of possibilities that green nanotechnology holds. In the nascent nanotechnology industry, green nanotechnology offers the opportunity to get it right in the first place. It is not too late to take Ben Franklin's words to heart, 'an ounce of prevention is worth a pound of cure'.

Nanostructured strontium titanate visible-light-driven photocatalysts containing rhodium and ruthenium were synthesized by a modified aerogel synthesis using ruthenium chloride and rhodium nitrate as dopant precursors, and titanium isopropoxide and strontium metal as the metal sources. The well-defined crystalline SrTiO₃ structure was confirmed by means of x-ray diffraction. After calcination at 500 °C, diffuse reflectance spectroscopy shows an increase in light absorption at 370 nm due to the presence of Rh^{3 +} ; however an increase of the calcination temperature to 600 °C led to a decrease in intensity, probably due to a loss of surface area. An increase in the rhodium doping level also led to an increase in absorption at 370 nm; however, the higher amounts of dopant lowered the photocatalytic activity. The modified aerogel synthesis allows greatly enhanced H₂ production performance from an aqueous methanol solution under visible light irradiation compared with lower surface area conventional materials. We believe that this enhanced activity is due to the higher surface areas while high quality nanocrystalline materials are still obtained. Furthermore, the surface properties of these nanocrystalline aerogel materials are different, as exhibited by the higher activities in alkaline solutions, while conventional materials (obtained via high temperature solid-state synthesis methods) only exhibit reasonable hydrogen production in acidic solutions. Moreover, an aerogel synthesis approach gives the possibility of thin-film formation and ease of incorporation into practical solar devices.

Catalytic hydrodechlorination (HDC) is an effective means of detoxifying chlorinated waste. Gold nanoparticles supported on Fe₃O₄ have been tested in the gas phase (1 atm, 423 K) HDC of 2,4-dichlorophenol. Two 1%  w/w supported gold catalysts have been prepared by: (i) stepwise deposition of Au on α-Fe₂O₃ with subsequent temperature-programmed reduction at 673 K (Au/Fe₃O₄-step); (ii) direct deposition of Au on Fe₃O₄ (Au/Fe₃O₄-dir). TEM analysis has established the presence of Au at the nano-scale with a greater mean diameter (7.6 nm) on Au/Fe₃O₄-dir relative to Au/Fe₃O₄-step (4.5 nm). We account for this difference in terms of stronger (electrostatic) precursor/support interactions in the latter that can be associated with the lower pH point of zero charge (with respect to the final deposition pH) for Fe₂O₃. Both catalysts promoted the preferential removal of the ortho-Cl substituent in 2,4-dichlorophenol, generating 4-chlorophenol and phenol as products of partial and total HDC, respectively, where Au/Fe₃O₄-step delivered a two-fold higher rate (2 × 10⁻⁴ mol_Cl h⁻¹ m_Au⁻²) when compared with Au/Fe₃O₄-dir. This unprecedented selectivity response is attributed to activation of the ortho-C–Cl bond via interaction with electron-deficient Au nanoparticles. The results demonstrate the feasibility of a controlled recovery/recycling of chlorophenol waste using nano-structured Au catalysts.

We have demonstrated heterogeneous photocatalytic degradation of microcystin-LR (MC-LR) by visible light activated carbon doped TiO₂ (C–TiO₂) nanoparticles, synthesized by a modified sol–gel route based on the self-assembly technique exploiting oleic acid as a pore directing agent and carbon source. The C–TiO₂ nanoparticles crystallize in anatase phase despite the low calcination temperature of 350 °C and exhibit a highly porous structure that can be optimized by tuning the concentration of the oleic acid surfactant. The carbon modified nanomaterials exhibited enhanced absorption in the broad visible light region together with an apparent red shift in the optical absorption edge by 0.5 eV (2.69 eV), compared to the 3.18 eV of reference anatase TiO₂. Carbon species were identified by x-ray photoelectron spectroscopy analysis through the formation of both Ti–C and C–O bonds, indicative of substitution of carbon for oxygen atoms and the formation of carbonates, respectively. Electron paramagnetic resonance spectroscopy revealed the formation of two carbon related paramagnetic centers in C–TiO₂, whose intensity was markedly enhanced under visible light illumination, pointing to the formation of localized states within the anatase band gap, following carbon doping. The photocatalytic activity of C–TiO₂ nanomaterials was evaluated for the degradation of MC-LR at pH 3.0 under visible light (λ > 420 nm) irradiation. The doped materials showed a higher MC-LR degradation rate than reference TiO₂, behavior that is attributed to the incorporation of carbon into the titania lattice.

Palladium (Pd) nanoparticle catalysts were successfully synthesized within an aqueous phase using sodium carboxymethyl cellulose (CMC) as a capping ligand which offers a green alternative to conventional nanoparticle synthesis techniques. The CMC-stabilized Pd nanoparticles were subsequently dispersed within support materials using the incipient wetness impregnation technique for utilization in heterogeneous catalyst systems. The unsupported and supported (both calcined and uncalcined) Pd nanoparticle catalysts were characterized using transmission electron microscopy, energy dispersive x-ray spectrometry, x-ray diffraction, and Brunauer–Emmett–Teller surface area measurement and their catalytic activity toward the hydrodechlorination of trichloroethylene (TCE) in aqueous media was examined using homogeneous and heterogeneous catalyst systems, respectively. The unsupported Pd nanoparticles showed considerable activity toward the degradation of TCE, as demonstrated by the reaction kinetics. Although the supported Pd nanoparticle catalysts had a lower catalytic activity than the unsupported particles that were homogeneously dispersed in the aqueous solutions, the supported catalysts retained sufficient activity toward the degradation of TCE. In addition, the use of the hydrophilic Al₂O₃ support material induced a mass transfer resistance to TCE that affected the initial hydrodechlorination rate. This paper demonstrates that supported Pd catalysts can be applied to the heterogeneous catalytic hydrodechlorination of TCE.

Tremendous interest exists towards synthesizing nanoassemblies for dye-sensitized solar cells (DSSCs) using earth-abundant and -friendly materials with green synthetic approaches. In this work, high surface area TiO₂ nanowire arrays were grown on the surface of functionalized graphene sheets (FGSs) containing −COOH functionalities acting as a template by using a sol–gel method in the green solvent, supercritical carbon dioxide (scCO₂). The effect of scCO₂ pressure (1500, 3000 and 5000 psi), temperature (40, 60 and 80 °C), acetic acid/titanium isopropoxide monomer ratios (HAc/TIP = 2, 4 and 6), functionalized graphene sheets (FGSs)/TIP weight ratios (1:20, 1:40 and 1:60 w/w) and solvents (EtOH, hexane) were investigated. Increasing the HAc/TIPweight ratio from 4 to 6 in scCO₂ resulted in increasing the TiO₂ nanowire diameter from 10 to 40 nm. Raman and high resolution XPS showed the interaction of TiO₂ with the −COOH groups on the surface of the graphene sheets, indicating that graphene acted as a template for polycondensation growth. UV–vis diffuse reflectance and photoluminescence spectroscopy showed a reduction in titania's bandgap and also a significant reduction in electron–hole recombination compared to bare TiO₂ nanowires. Photocurrent measurements showed that the TiO₂nanowire/graphene composites prepared in scCO₂ gave a 5× enhancement in photoefficiency compared to bare TiO₂ nanowires.

Silicon nanoparticles can be considered a green material, especially when prepared via a microwave-assisted method without the use of highly reactive reducing agents or hydrofluoric acid. A simple solution synthesis of hydrogen-terminated Si- and Mn-doped Si nanoparticles via microwave-assisted synthesis is demonstrated. The reaction of the Zintl salt, Na₄Si₄, or Mn-doped Na₄Si₄, Na₄Si_4(Mn), with ammonium bromide, NH₄Br, produces small dispersible nanoparticles along with larger particles that precipitate. Allylamine and 1-amino-10-undecene were reacted with the hydrogen-terminated Si nanoparticles to provide water solubility and stability. A one-pot, single-reaction process and a one-pot, two-step reaction process were investigated. Details of the microwave-assisted process are provided, with the optimal synthesis being the one-pot, two-step reaction procedure and a total time of about 15 min. The nanoparticles were characterized by transmission electron microscopy (TEM), x-ray diffraction, and fluorescence spectroscopies. The microwave-assisted method reliably produces a narrow size distribution of Si nanoparticles in solution.

Mercury (Hg) is one of the most pervasive and bio-accumulative metals in the environment. Yet, effective in situ remediation technologies have been lacking. This study investigated the effectiveness of a class of soil-deliverable FeS nanoparticles for in situ immobilization of Hg in two field-contaminated soils from a New Jersey site and one sediment from an Alabama site. The nanoparticles were prepared using sodium carboxymethyl cellulose (CMC) as a stabilizer. Transmission electron microscopy measurements revealed a particle size of 34.3 ± 8.3 nm (standard deviation), whereas dynamic light scattering gave a hydrodynamic diameter of 222.5 ± 3.2 nm. Batch tests showed that at an FeS-to-Hg molar ratio of 28:1–118:1, the nanoparticles reduced water-leachable Hg by 79%–96% and the TCLP (toxicity characteristic leaching procedure) based leachability by 26%–96%. Column breakthrough tests indicated that the nanoparticles were deliverable in the sediment/soil columns under moderate injection pressure. However, once the external pressure was removed, the delivered nanoparticles remained virtually mobile under typical groundwater flow conditions. When the Hg-contaminated soil and sediment were treated with 52–95 pore volumes of a 500 mg l⁻¹ FeS nanoparticle suspension, water-leachable Hg was reduced by 90%–93% and TCLP-leachable Hg was reduced by 65%–91%. The results warrant further field demonstration of this promising in situ remediation technology.

Carbon nanotubes (CNTs) have shown great promise as high performance materials for adsorbing priority pollutants from water and wastewater. This study compared uptake of two contaminants of interest in drinking water treatment (atrazine and trichloroethylene) by nine different types of carbonaceous adsorbents: three different types of single walled carbon nanotubes (SWNTs), three different sized multi-walled nanotubes (MWNTs), two granular activated carbons (GACs) and a powdered activated carbon (PAC). On a mass basis, the activated carbons exhibited the highest uptake, followed by SWNTs and MWNTs. However, metallic impurities in SWNTs and multiple walls in MWNTs contribute to adsorbent mass but do not contribute commensurate adsorption sites. Therefore, when uptake was normalized by purity (carbon content) and surface area (instead of mass), the isotherms collapsed and much of the CNT data was comparable to the activated carbons, indicating that these two characteristics drive much of the observed differences between activated carbons and CNT materials. For the limited data set here, the Raman D:G ratio as a measure of disordered non-nanotube graphitic components was not a good predictor of adsorption from solution. Uptake of atrazine by MWNTs having a range of lengths and diameters was comparable and their Freundlich isotherms were statistically similar, and we found no impact of solution pH on the adsorption of either atrazine or trichloroethylene in the range of naturally occurring surface water (pH = 5.7–8.3). Experiments were performed using a suite of model aromatic compounds having a range of π-electron energy to investigate the role of π–π electron donor–acceptor interactions on organic compound uptake by SWNTs. For the compounds studied, hydrophobic interactions were the dominant mechanism in the uptake by both SWNTs and activated carbon. However, comparing the uptake of naphthalene and phenanthrene by activated carbon and SWNTs, size exclusion effects appear to be more pronounced with activated carbon materials, perhaps due to smaller pore sizes or larger adsorption surface areas in small pores.

In the advancement of green syntheses and sustainable reactions, enzymatic biocatalysis offers extremely high reaction rates and selectivity that goes far beyond the reach of chemical catalysts; however, these enzymes suffer from typical environmental constraints, e.g. operational temperature, pH and tolerance to oxidative environments. A common hydrolase enzyme, diisopropylfluorophosphatase (DFPase, EC 3.1.8.2), has demonstrated a pronounced efficacy for the hydrolysis of a variety of substrates for potential toxin remediation, but suffers from the aforementioned limitations. As a means to enhance DFPase's stability in oxidative environments, enzymatic covalent immobilization within the polymeric matrix of poly(propylene sulfide) (PPS) nanoparticles was performed. By modifying the enzyme's exposed lysine residues via thiolation, DFPase is utilized as a comonomer/crosslinker in a mild emulsion polymerization. The resultant polymeric polysulfide shell acts as a 'sacrificial barrier' by first oxidizing to polysulfoxides and polysulfones, rendering DFPase in an active state. DFPase–PPS nanoparticles thus retain activity upon exposure to as high as 50 parts per million (ppm) of hypochlorous acid (HOCl), while native DFPase is observed as inactive at 500 parts per billion (ppb). This trend is also confirmed by enzyme-generated (chloroperoxidase (CPO), EC 1.11.1.10) reactive oxygen species (ROS) including both HOCl (3 ppm) and ClO₂ (100 ppm).

Stable gold/mesoporous silica nanocomposites (with Au nanoparticles intercalated in the walls of mesoporous silica) were successfully synthesized by the hydrothermal method and applied as catalysts. A challenging issue associated with intercalation and the use of coordinating agents is the effect of the coordinating agent on the mesoporous silica structure and periodicity. This investigation is targeted at elaborating the effect of the coordinating agent on the resulting mesoporous structure. The amount of Au coordinating agent bis[3-(triethoxysilyl)propyl]-tetrasulfide (TESPTS) was systematically altered to synthesize a range of materials with varying Au loadings and morphologies. These materials were characterized by N₂ adsorption–desorption, x-ray diffraction, transmission electron microscopy and UV–visible spectroscopy. The structures of the catalysts were found to range from mesoporous to vesical- and foam-like upon varying the TESPTS/polymer template (P123) ratio. Additionally, the sizes of Au nanoparticles increased by increasing the amount of TESPTS. The catalytic properties of the resulting materials were examined using oxidation of benzyl alcohol and reduction of 4-nitrophenol as probe reactions. The intercalated systems demonstrated high activity and more importantly were robust and readily reusable. This approach to imparting stability to nanoscale materials may be much more broadly applicable and expand the types of environments in which they can be utilized.

In this work, we report the formation of well-defined silver nanodendrites via biomineralization under mild conditions in a single step, in the presence of the plant phytohormone chicoric acid (CA), a well-known HIV-I integrase inhibitor. CA played a dual role as reductant as well as directed the growth of the nanodendrites, which were found to grow primarily in the [111] and [200] directions. In addition to the formation of highly ordered hierarchical structures, the formed Ag nanodendrites were found to exhibit luminescence, as observed by confocal microscopy. This study not only demonstrates a new method for the preparation of luminescent silver nanodendrites using a simple, environmentally friendly biological method, but also indicates the ability of CA, a potent HIV-integrase inhibitor, to interact with silver ions which may shed light on its potential for additional biomedical and biosensor applications.

Nanostructured highly porous TiO₂/WO₃/Fe³⁺ aerogel composite photocatalysts are prepared, characterized and tested for model photocatalytic reactions. The catalyst structure is tailored to capture environmental pollutants and enable their decomposition in situ under both ultraviolet (UV) and visible light through oxidation to smaller benign molecules.

A novel and green method is utilized to prepare the unique surfactant-templated aerogel composite photocatalyst that has a highly accessible porous nanostructure with high surface area and tailored pore size distribution. The sol–gel process is combined with supercritical extraction and drying. Supercritical drying with heat treatment results in titanium dioxide with anatase crystal form. Templates used further enable retention and tuning of the nanopore structure and surface properties.

The synthesized catalysts were characterized using SEM, FIB, XRD and porosimetry prior to post-evaluation in model reactions. The bandgap of the catalyst particles was also determined using diffuse reflectance.

The resulting aerogel TiO₂/WO₃/Fe³⁺ has similar photocatalytic capability compared to highly optimized commercial Degussa P25 under UV exposure and offers much superior photocatalytic capability under visible light exposure. The model reaction utilized employed methylene blue (MB) photooxidation under visible and UV light.

The power densities of microbial fuel cells with yeast cells as the anode catalyst were significantly increased by immobilizing the yeast in electrically conductive alginate electrodes. The peak power densities measured as a function of the electrical conductivity of the immobilized electrodes show that although power increases with rising electrical conductivity, it tends to saturate beyond a certain point. Changing the pH of the anode compartment at that point seems to further increase the power density, suggesting that proton transport limitations and not electrical conductivity will limit the power density from electrically conductive immobilized anodes.

Nanocrystalline (1.7 ± 0.3 nm) zinc sulfide with a specific surface area up to 360 m² g⁻¹ was prepared from the thermal decomposition of a single-source precursor, zinc ethylxanthate. Zinc ethylxanthate decomposes to cubic zinc sulfide upon exposure to temperatures greater than or equal to 125 °C. The resulting zinc sulfide was tested as a water impurity extractant. The target impurities used in this study were As⁵⁺, As³⁺, and Pb²⁺. The reaction of the nanocrystalline ZnS with Pb²⁺ proceeds as a replacement reaction where solid PbS is formed and Zn²⁺ is released into the aqueous system. Removal of lead to a level of less than two parts per billion is achievable. The results of a detailed kinetics experiment between the ZnS and Pb²⁺ are included in this study. Unlike the instance of lead, both As⁵⁺ and As³⁺ adsorb on the surface of the ZnS extractant as opposed to an ion-exchange process. An uptake capacity of  > 25 mg g⁻¹ for the removal of As⁵⁺ is possible. The uptake of As³⁺ appears to proceed by a slower process than that of the As⁵⁺ with a capacity of nearly 20 mg g⁻¹. The nanocrystalline zinc sulfide was extremely successful for the removal of arsenic and lead from simulated oil sand tailing pond water.