Environmental Science: Nano Blog

Titania (TiO₂) nanoparticles are used in a great number of consumer products and therefore the quantity of nanoparticles and their transformation products reaching the environment is increasing. The question is, when they reach the aquatic environment, what happens to their stability?

Nanoparticles in aquatic environments are subject to different types of forces which can lead to aggregation, stabilization or deposition of the particles.

The aggregation of nanoparticles has a strong influence in determining their final fate in the environment. Julián A. Gallego-Urrea and colleagues from the University of Gothenburg have conducted a study to evaluate the effects of natural organic matter (NOM) on the aggregation kinetics of titania nanoparticles.

It is important to be able to predict nanoparticles transformation products in a physically meaningful way for near source emissions, spill situations and ecotoxicity tests, where other particles are less significant for determining ENP fate. When nanoparticles interact with NOM transformation processes can occur. For example, sorption of NOM onto the surface of engineered nanoparticles (ENP) can lead to aggregation via bridging mechanisms or stabilization.

The stability of titania nanoparticles in natural water is influenced not only by their concentration, but also by the physico-chemical characteristics of the receiving water.

In this study the stabilization of synthesized titania nanoparticles obtained at various pH levels, with a variety of electrolytes, was evaluated.

The titania nanoparticles were rapidly mixed with NOM, Sodium Alginate, Fulvic acid or Humic acid and three different electrolytes, NaCl, CaCl₂ and Na₂SO₄

The change in particle size was then monitored with time-resolved dynamic light scattering (TR-DLS) at different concentrations of three different electrolytes and different solutions of pH. By measuring the particle size it could be determined what effects the different systems actually had on the stability of the nanoparticles.

The results showed that:

• The addition of NOM increased the stability of the systems with NaCl and Na₂SO_4.
• NOM had little influence on the CaCl₂ system, suggesting that there is bridging coagulation between the NOM and calcium.

In general, it was demonstrated that a low coverage of NOM on top of bare titania particles can induce aggregation, but further coverage can protect them from aggregation – even at high ionic strengths. The degree of coverage is governed by the concentration ratio between nanoparticles and NOM.

The study showed that the ionic strength, pH, quality and quantity of NOM all have a significant influence on the aggregation behaviour of titania.

To read more about the full experiment, download a copy for free by clicking the link below.

Influence of different types of natural organic matter on titania nanoparticles stability: effects of counter ion concentration and pH

Julian Alberto Gallego-Urrea, Jenny Perez-Holmberg and Martin Hasselloev

DOI: 10.1039/C3EN00106G

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A constant critique of numerous research conducted on the fate and transformation of nanoparticles has been the inability to reach concentrations that are environmentally relevant. Finally, this problem seems to have reached the end of the tunnel. Single particle ICP-MS (spICP-MS) has come to the rescue with the potential of detection and characterization of trace level engineered nanoparticles (ENPs) in complex matrices. Not only that, the analysis is even possible in the presence of known sinks for ENPs such as dissolved organic carbon (DOC), sediments and biota. This is because the measurement is of the primary particle size instead of increasing dissolved ions in the solution.

D.M. Mitrano and the colleagues from Colarado School of Mines, USA explicitly demonstrate this measurement of particle size in their analysis of Ag ENPs dissolution in different matrices using spICP-MS. The study considers two different sized Ag ENPs; 60 and 100 nm with three different polymer coatings: citrate (CA), tannic acid (TA) and polyvinylpyrrolidone (PVP) at predicted environmentally relevant concentrations (ng/L range).  Figure 1, taken from their paper, represents the decrease in the pulse intensity over time as the dissolution of 100nm TA@Ag ENPs at 50ng/L takes place.

Figure 1

This raw intensity data is then used in computing the respective particle sizes based on calibration with dissolved standards.

The effect of water chemistry parameters under these dilute concentrations are hence investigated by quantitative evaluation of the rate and extent of particle dissolution by the using solutions with increasing complexity; deionized water, tap water, surface water and moderately hard reconstituted water to mimic the realistic environmental matrices. Comparisons have also been made between the different polymer coatings.

To read the full article click the link below:
Tracking dissolution of silver nanoparticles at environmentally relevant concentrations in laboratory, natural and processed waters using single particle ICP-MS (spICP-MS)
Denise M Mitrano, James Ranville, Anthony Bednar, Karen Kazor, Amanda S Hering and Christopher Higgins
DOI: 10.1039/C3EN00108C

Cellulose-derived structural supports can improve the performance and the environmental credentials of gold nanoparticle catalysts. This study from the Polymer Research Institute of Sichuan University in China demonstrates the potential of this approach.

Achieving the optimum performance and stability of catalysts is of paramount importance to the development of a wide range of industrial processes and the manufacture of new products and devices. Gold nanoparticles (Au NPs) are widely used as catalysts in many chemical processes. However, these often suffer from instability owing to their large active surface area which can lead to self-aggregation. They therefore require a supporting matrix, commonly using polymeric encapsulation or support on graphene oxide or silica surfaces.

However, greener, more sustainable approaches are now being sought to improve the environmental credentials of these processes. Cellulose is the most common and abundant natural polymer.  Cellulose nanocrystals (CNs) can therefore provide a possible solution as these posses the desired properties (mechanical strength, self-assembly, high specific surface area, nanometirc dispersity and high stability) to allow them to act as an effective catalyst carrier.

However the use of these have previously involved use of toxic reducing agents such as sodium borohydride or hydrazine hydride. A cleaner synthesis is required. In this study Xiaodong Wu and co workers report the first instance of a one-step, environmentally-friendly synthesis of gold NPs (Au NPs) deposited on cellulose nanocrystals and demonstrate the technical and environmental advantages of this approach.

CNs were derived from bulk cellulose using controlled acid hydrolysis of waste cotton fabric. The Au NPs were synthesised and deposited on to CNs by hydrothermal reduction of HAuCl₄. Both Au NP-CN hybrids and unsupported Au NPs were prepared and their performance in the reduction of 4-nitrophenol was compared using UV-Vis spectroscopic analysis. The structure and properties of the resulting particles were characterized using transmission electron microscopy (TEM), X-Ray Diffraction (XRD) and X-Ray photoelectron spectrometric (XPS) techniques.

It was shown that the electron-rich –OH groups, abundant on their surface allow the CNs to provide the dual role of reductant and stabilizer in this process. The larger specific surface area and better dispersity of Au NPs supported on CNs meant these nano-hybrid catalysts displayed enhanced stability and superior catalytic activity than an unsupported Au NP catalyst for the reduction of 4-NP.

This approach offers a number of important advantages over previous traditional techniques. It has a lower overall cost and avoids the use of toxic or dangerous reducing, capping or dispersing agents. Furthermore this process utilises a renewable and biodegradable natural resource since the CNs can be obtained from various plant materials.

This investigation could therefore potentially pave the way to green production of bio-supported organic/inorganic nano-hybrid catalysts with possible further applications in the production of sensors, antibacterial materials and electronic devices.

To access the full article, download your free* copy by clicking the link below:

Green synthesis and formation mechanism of cellulose nanocrystal-supported gold nanoparticles with enhanced catalytic performance

Xiaodong Wu, Canhui Lu, Zehang Zhou, Guiping Yuan, Rui Xiong and Xinxing Zhang Environ. Sci.: Nano, 2014, 1, 71-79 DOI: 10.1039/C3EN00066D

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Virtually all environmental surfaces have microbial biofilms, which are an essential component of natural systems.  Nanoparticles are abundant in nature, but how much do we know about nanoparticle-biofilm interactions?

Kaoru Ikuma and colleagues from the University of Massachusetts Amherst have completed a study highlighting the importance of assessing small-scale biofilm surface characteristics in future nanoparticle-biofilm interaction studies. They hypothesized that the contribution of electrostatic forces in relation to other forces would be dominant in governing the deposition of bare nanoparticles onto polysaccharide-coated surfaces. The also expected biofilm surface charge to impact nanoparticle deposition.

Polysaccharides are a major extracellular component of biofilms. They are ubiquitous in the environment and occur in pure forms as well as in complexes. As polysaccharides are extracellular they are an initial point of contact for nanoparticles and play an important role in early nanoparticle-biofilm interactions. Typical characterization of biofilms often treat all polysaccharides as one entity.  This study suggests that the small-scale chemical and electrochemical identities of the polysaccharides present in the biofilms may play an important role in the initial surface attachment of nanoparticles; therefore effecting their deposition.

The significance of polysaccharide coatings on the deposition of nanoparticles was examined using in-depth characterization of surface properties. It would appear that surface charge density and distribution of the biofilms both contribute to different nanoparticle deposition behaviours.

Kelvin probe force microscopy was used as a probe for spatial variations of surface potential across two different polysaccharides (alginate and dextran sulphate). Patches of lower surface potential are observed as the areas of darker colour on the images below.

alginate (a) and dextran sulfate (b)

The results showed that even though the interactions between nanoparticles and surfaces coated with pure polysaccharides may be governed by electrostatic forces, these interactions can be altered by microscale and nanoscale differences in surface charge. Therefore, instead of treating polysaccharides as one entity, spatial characterization of biofilm surface properties is necessary to improve our understanding of nanoparticle-biofilm interactions.

To find out more, download your free* copy by following the link below.

Deposition of nanoparticles onto polysaccharide-coated surfaces: Implications for nanoparticle-biofilm interactions. Kaoru Ikuma, Andrew Madden, Alan Decho and Boris L. T. Lau 10.1039/C3EN00075C

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