Environmental Effects of Nanotechnology

By Dr Priyanka Battacharya

Table of Contents

Introduction

Recent advances in material science and nanotechnology have given rise to a myriad of developments, which have lead to calls for research into the impacts of nanomaterials on the environment and human health. Over the past decade, there has been growing concern over the potentially adverse environmental and health impacts of nanomaterials.¹ At the same time, nanotechnology has provided improved environmental solutions, especially in the field of water quality.² Environmental problems are a complicated mosaic of multiple phenomena that require multidimensional analysis and solutions. We as physicists try to understand the fundamental physical interactions between nanomaterials and the ecosystem, and have developed several facile schemes for doing so using the principles and techniques of physics, materials, and physical chemistry. The focus of this article is the key contributions our lab has made to the field of drinking water remediation using nanomaterials.

Nanomaterials for Water Treatment

Worldwide, 1.1 billion people lack access to sufficient amounts of safe water.³ Adequate supplies of decontaminated water with high throughput at a low cost are a growing challenge around the world. Current water purification methods in wide use employ chemically intensive treatment that is relatively expensive, harmful to the environment, and is not adaptable to the non-industrialized world. Nanomaterial-based technologies, adsorbents and catalysts could create novel, environmentally benign solutions for water treatment. There are three main applications where nanomaterials show promise – sensing and detection of pollutants, treatment and remediation of contaminants, and finally, prevention of pollution. Nanomaterials are also being used to enhance membrane separation processes, leading to greater selectivity and lower costs. However, successful applications of these technologies require high degree of control of nanoparticle (NP) mobility, reactivity, and ideally, specificity for the contaminant of interest.

The unknown ecological effects, environmental stability, fouling properties, low detection limits, high costs, and concerns over their regeneration and environmental deposition limits the large scale applications of many commonly used nanomaterials for water treatment, such as nano zero valent iron, titanium dioxide nanoparticles, carbon nanotubes and zeolites. Advances in macromolecular chemistry such as the synthesis of dendritic polymers have provided great opportunities for improving and developing effective filtration processes for water purification to eliminate different organic solutes and inorganic anions. Dendritic polymers which include hyperbranched and dendrigraft polymers, dendrons and dendrimers are highly synthetic, nanoscale branched structures with a high degree of surface functionalities, monodispersity, controlled composition, and architecture which display interesting physicochemical behavior due to their shape, size and multiple functionalities.⁴

Environmental Applications of Dendritic Polymers

A dendritic polymer may be regarded as a ‘soft’ nanoparticle, consisting of three main components – a core, interior branch cells, and a terminal branch cell. The size of a dendritic polymer is characterized by its ‘generation’ (G), or number of branches emanating from the central core (Figure 1). Dendrimers have a high hosting capacity for toxic metal ions, radionuclides, inorganic anions, organic solutes and polycyclic aromatic hydrocarbons (PAHs).^5,6 Moreover, the pH-dependent amphiphilic property of dendritic polymers allows them to capture various chemical species in diverse environments such as aqueous and organic solutions and oil-water interfaces, and then release these chemical species in a controlled manner by changing the pH in situ without having to resort to intensive and expensive regeneration.

Figure 1. Exemplary structure of generation 0 (G0) and generation 2 (G2) polyamidoamine (PAMAM) dendrimers with amino and amidoethanol surface groups. Image courtesy of Sigma Aldrich.

The following unique physicochemical properties of dendrimers make them particularly attractive as functional materials for water treatment.

1. Hosting capacity and recyclability

A high degree of flexibility in the synthesis of nearly monodisperse nanoscale (size ranges between 1-20 nm) dendrimers with well-defined molecular composition, size and shape, variable functionalization, and hydrophobic cavities afford them with flexible but rigid scaffolding. Dendrimers have much smaller intrinsic viscosities than linear polymers with the same molar mass because of their globular shape.⁴ They also have a much large surface area than bulk particles of the same mass. Thus, unlike reverse osmosis (RO) and nanofiltration membranes whose operations require high pressure, dendrimer-enhanced ultrafiltration (UF) membranes operate at lower pressures (200–700 kPa) and can capture both low and high molecular weight contaminants, unlike unmodified UF membranes which can only remove dissolved organic and inorganic compounds of >3 kDa. Moreover, it has been demonstrated that dendritic polymers can be integrated into existing, commercial UF membrane separation processes.^7,8

In a proof-of-concept study, our group showed that a trifunctional G4-tris PAMAM dendrimer displays exceptional and selective hosting capacity towards major chemical species of environmental relevance, namely – 64 cationic copper (Cu(II)) ions per dendrimer through ligand-to-metal charge transfer (LMCT) complex formation at pH 10, 32 anionic nitrate (NO^3-) ions through electrostatic interactions at pH 2, and 10 PAH phenanthrene (PN) molecules through hydrophobic interactions at pH 7 (Figure 2).⁹ Remarkably, when the pH was lowered to 2, both PN and Cu(II) were released from the dendrimer interior while NO^3- ions were released when the pH was raised to 10.

Figure 2. (a) Structure of a generation 1 poly(amidoamine)-tris(hydroxymethyl)amidomethane dendrimer (Tris-dendrimer), the building block of the generation 4 Tris-dendrimer used in the present study. Red: oxygen; Green: secondary amine; Blue: tertiary amine. (b) Scheme of the dendrimer absorbing chemical species at different pH.⁹

In addition, we have demonstrated efficient removal of dissolved humic acid (HA) using PAMAM dendrimers.¹⁰ HA is an extremely complex molecule consisting of several anionic chemical groups. The high degree of surface functionalities on PAMAM dendrimers allowed them to behave as a “nanosponge” in adsorbing such complex molecular species. Central to this method was complex formation resulting from electrostatic interactions between the cationic dendrimers and the anionic HA at neutral pH. The PAMAM dendrimers demonstrated double the capacity of commonly used polymeric adsorbents for HA, once charge neutralization was reached. However, loading of additional dendrimers re-stabilized and re-suspended the aggregates via electrostatic repulsion.

We also developed a novel optical scheme based on the surface plasmon resonance of a gold nanowire (Au-NW) to selectively detect Cu(II) in aqueous solutions, down to the nM range by PAMAM dendrimers electrostatically immobilized on the Au-NW substrate.¹¹ Such a detection limit is by far the lowest and most feasible amongst commonly used analytical schemes for metal ion detection.

Furthermore, we characterized these soft, environmentally benign nanomaterials for mitigating potentially harmful discharged nanoparticles from the aqueous environment. Here fullerenols were used as a model nanomaterial, and their interactions with dendrimers of two different generations, G1 and G4 were studied using spectrophotometry and thermodynamic methods. Specifically, we found that each fullerenol bound with two primary amines per dendrimer (both G1 and G4) through ionic bonding, and the formation of large aggregates due to inter-cluster interactions facilitated by hydrogen bonding and hydrophobic interactions were evident (Figure 3). Apparently, such inter-cluster formation can be controlled by adjusting the molar ratio of dendrimer to fullerenol. In addition, the formation of dendrimer-fullerenol assemblies at maximum loading capacity was energetically favorable and thermodynamically spontaneous.¹² Such inter-cluster interactions between dendrimer-fullerenol complexes are deemed desirable for mitigating the accidental release of nanomaterials in the environment; however they should be minimized for the drug delivery of fullerene derivatives by a dendrimer - in light of their diffusion in the bloodstream and eventual cell uptake. Based on this study, we recommend a G4/fullerenol loading ratio of 0.005-0.02 for drug delivery (the range below precipitation), and a G4/fullerenol loading ratio of above 0.02 for environmental remediation. Furthermore, for both nanomedicinal and environmental applications, the scope of this study may be extended to that of branched/hyperbranched polymers and nanoparticles of opposite charge.

Figure 3. Illustration of the self-assembly of a G4-PAMAM dendrimer (red) and fullerenols (silver). The primary amines of the dendrimer are indicated in blue.12

Taken together, these laboratory-scale studies above demonstrated that PAMAM dendrimers enable a more thermodynamically spontaneous adsorption process for various chemical species and environmental pollutants than other conventional water purification procedures such as RO, which require energy to drive the process to completion. Such properties of PAMAM dendrimers should appeal to future generations of water treatment devices.

2. Biocompatibility

Dendrimer-related toxicity has been observed only for G7 and larger and even then, only minimally.¹³ Several studies on the use of dendrimers for DNA transfection, metal ion contrast agent carriers for MRI, targeted drug and therapeutic agent delivery vehicles, and viral inhibitors suggested that hyperbranched polymers and PAMAM dendrimers of G5 and below are non-toxic and biodegradable.^14,15 Moreover, dendrimers do not leave behind any potentially harmful by-products.

3. Degradability

PAMAM dendrimers show measurable degradation only in the third year of storage at 5°C, and a shelf life of ~6-9 months at ambient temperature, based on retro-Michael reaction. Such a long lifetime ensures the stability and effectiveness of dendritic polymers for the practice of water treatment. Furthermore, addition of enzyme-degradable bonds (e.g., amides in PAMAM) can be used such that intracellular or extracellular hydrolytic enzymes can break the polymer chains within one. This property could become relevant upon accidental uptake of dendritic polymers during water consumption. A third mechanism for dendrimer degradation is by water hydrolysis acting on ester bonds in the polymers. Such mechanisms may be utilized to selectively break down dendritic polymers post their usage in water treatment.

Thus, the high and versatile hosting capacities, energy efficiency, regenerability, selectivity, biocompatibility, and environmentally benign nature make dendritic polymers a desirable nanomaterial for environmental applications. It is therefore our effort to exploit the physico-chemical behavior of dendritic polymers for environmental remediation.

Future Directions

We are developing strategies for extending the scope of dendritic polymers for environmental remediation. One of our recent studies explored the ability of these dendritic polymers to disperse spilled oil,¹⁶ a huge environmental hazard associated with the offshore operation of the petroleum industry. Energetically, the hydrophobic interior of these polymers at ambient water pH provide for ample space for hydrophobic oil molecules to partition in.

While over 70% of the earth’s surface is covered by water, only about 3% of it is available for human consumption. Even worse, in developing countries, 80% of illnesses are water related. In addition to providing technical solutions to the staggering challenge of providing clean drinking water, regulatory and public acceptance to using nanotechnology for drinking water treatment must be established. In addition, life cycle assessments of the risks and benefits of these nanomaterials are crucially necessary.

References

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