Extraction and high-performance liquid chromatographic analysis of C₆₀, C₇₀, and [6,6]-phenyl C₆₁-butyric acid methyl ester in synthetic and natural waters

Abstract

Studies have shown that C₆₀ fullerene can form stable colloidal suspensions in water that result in C₆₀ aqueous concentrations many orders of magnitude above C₆₀'s aqueous solubility; however, quantitative methods for the analysis of C₆₀ and other fullerenes in environmental media are scarce. Using a 80/20 v/v toluene–acetonitrile mobile phase and a 4.6 mm × 150 mm Cosmosil 5 μ PYE column, C₆₀, C₇₀, and PCBM ([6,6]-phenyl C₆₁-butyric acid methyl ester) were fully resolved. Selectivity factors (α) for C₆₀ relative to PCBM and C₇₀ relative to C₆₀ were 3.18 and 2.19, respectively. The best analytical wavelengths for the fullerenes were determined to be 330, 333, and 333 nm with log molar absorption coefficients (log ɛ) of 4.63, 4.82, and 4.60 for PCBM, C₆₀, C₇₀, respectively. Extraction and quantitation of all three fullerenes in aqueous suspensions over a range of pH (4–10) and ionic strengths were very good. Whole-method quantification limits for ground and surface suspensions were 2.87, 2.48, and 6.54 μg/L for PCBM, C₆₀, and C₇₀, respectively.

Introduction

The development, production, and use of nanomaterials are expected to grow rapidly over the next decade. It is estimated that there are currently close to 500 consumer products on the market that contain nanoscale materials [1]. Recognizing the need for further research on the environmental and human health effects of nanomaterials, the United States federal departments and agencies that participate in the National Nanotechnology Initiative, with input from industry liaison groups, produced a report identifying the key environmental, health, and safety research needs for engineered nanomaterials [2]. Some of the key research needs identified in this report were the development of sampling and analytical methodologies for nanomaterials in environmental matrices.

Fullerenes as a class of nanomaterials were selected for this study because of their growing use in commercial products: the Woodrow Wilson International Center for Scholar's Project on Emerging Nanotechnologies database indicates that fullerenes and other carbon-based nanomaterials use in consumer products is second only to that of nano-sized silver [1]. In addition, fullerenes were studied due to their unique colloidal properties in water which can affect both their behavior in the environment as well as their partitioning behavior in the extraction process in ways not predictable solely by their molecular solubilities. Therefore, the goal of this work was to develop analytical methodologies for extracting and quantifying fullerenes from aqueous colloidal suspensions that are representative of the type of suspensions that could form in the environment as the industrial and consumer use of these materials increases.

Studies have indicated that C₆₀ can form stable colloidal suspensions in water when C₆₀ is introduced to the aqueous phase through solvent exchange, sonication, extended mixing, or some combination of these techniques [3], [4], [5]. Most studies to date have utilized the solvent exchange technique; however, the solvents used may themselves be retained in the cluster structure of C₆₀ aggregates, which may influence the surface properties of the colloidal suspension [3]. Extended mixing with water without the aid of any organic solvents, the technique used in this study, produces colloidal fullerene suspensions which are more representative of the type of fullerene suspensions likely to occur in natural waters. All of these techniques for creating fullerene suspensions yield colloidal suspensions with effective aqueous phase concentrations many orders of magnitude above the aqueous solubilities of molecular fullerenes. For C₆₀, aqueous colloidal suspensions with concentrations as high as 235 mg/L have been reported [6], which is 10¹⁶ times higher than C₆₀'s estimated water solubility [7].

Methods for analysis of aqueous C₆₀ suspensions have been reported [8], [9], [10], [11], as have methods for extraction and analysis of C₆₀ in biological samples [8], [10], [12]; however, quantitative methods for the analysis of C₆₀ and other fullerenes in actual environmental media or under varying solution conditions are scarce [13]. Sampling and analyzing fullerene suspensions pose several challenges. For example, it has been observed that C₆₀ adsorbs to glassware from pure methanol, being essentially 100% adsorbed in 4 h [14]. Also, despite molecular C₆₀'s very high solubility in toluene relative to water, C₆₀ will not readily partition into toluene from water when C₆₀ is present as stable colloidal aggregates in aqueous suspension (often referred to as nano-C₆₀) without the presence of an oxidizing agent or salt to disrupt the stability of the colloidal aggregates and enhance C₆₀ partitioning into the toluene phase [8], [9].

Many analytical, and most environmental, studies on fullerenes in aqueous suspensions have focused on the analysis and colloidal properties of C₆₀. For this study, three fullerenes, C₆₀, C₇₀, and PCBM ([6,6]-phenyl C₆₁-butyric acid methyl ester) were selected for investigation. Specifically, C₆₀ and C₇₀ fullerenes were selected because they are common constituents in commercial fullerene mixtures – additionally, C₆₀ was selected as it is a common impurity in nanotube formulations and to allow for comparison to prior studies. The C₆₀ fullerene derivative PCBM was selected as it has been used in many nanomaterial research and engineering applications, particularly in polymer electronic applications where PCBM is the most commonly used n-type semiconductor in organic photovoltaics [15], [16]. Since pH and ionic strength are often the primary solution chemistry properties governing colloid size and surface charge in aqueous media, this study examined the extractability and analysis of fullerenes from aqueous suspensions at varying pH and ionic strengths using laboratory prepared background solutions. In addition, fullerene suspensions were also extracted and analyzed from ground water and from surface water collected from a small first-order stream (a perennial stream without any perennial tributaries).