Separation and size characterization of zinc oxide nanoparticles in environmental waters using asymmetrical flow field-flow fractionation

Highlights

• AF4 based method for separation and size characterization of ZnO-NPs was developed.

• The method performed in precise and reliable way.

• The method can be employed as an alternate option for size characterization.

• The technique demonstrated good applicability on real samples.

Abstract

There are few studies on separation and size characterization of zinc oxide nanoparticles (ZnO-NPs), which have wide applications in several science and technology areas, in the environment. In this work, we report a method for the separation and size characterization of ZnO-NPs by asymmetrical flow field-flow fractionation (AF4) coupled to UV-vis detector. Experimental conditions such as composition of the carrier solution, focus time, crossflow, detector flow rate and injection volume were systematically studied in terms of NPs separation, recovery, and repeatability. Size characterization was achieved using polystyrene nanoparticles as a size standard and a mixture of < 35 nm (NP-A) and < 100 nm (NP-B) ZnO-NPs were separated and size characterized posterior preconcentration using ultracentrifugation. The method was also employed to characterize the size of homemade ZnO-NPs, and the results were in concordance with dynamic light scattering (DLS) analysis and thus, the method can be used as an alternate method. Upon application on environmental water samples, the two ZnO-NPs, NP-A and NP-B, have been separated and size characterized. The estimated hydrodynamic sizes of the NP-A and NP-B were found to be in the range of 83–97 nm and 188–202 nm, respectively, with good precision (RSD, <11%), suggesting that the current method can satisfactorily separate and generate information about sizes of the NPs in samples with a complex matrix. Therefore, the developed technique can be used as a baseline to investigate size related environmental processes of the NPs in environmental water samples.

Introduction

Recent advances in science and technology are based on the applications of nanomaterials, objects having a diameter (at least in one dimension) between 1 and 100 nm [1], [2]. As one class of the extensively consumed nanomaterials [2], metal oxide nanoparticles (MeO-NPs) are becoming a common ingredient in various products such as personal care, clothing, and cosmetics. Along with various metal and MeO-NPs (e.g. silver, titanium dioxide and silicon dioxide NPs), zinc oxide nanoparticles (ZnO-NPs) are one of the materials commonly used in consumer products [3], [4]. Due to their ability to scatter and reflect UVA/UVB rays [5], [6], and antimicrobial and photocatalytic properties [7], [8], ZnO-NPs have been predominantly comprised in mineral sunscreens, sprays and paints [4], [6], [9]. Furthermore, ZnO-NPs have a myriad of applications and have been used as a potential material in environmental cleanup [10] and analysis [11], [12], food packaging [13], textiles [14], optoelectronic devices and sensors [15], [16], catalysis [10], and others. Application of the NPs in these activities inevitably leading to their release to the environmental systems [2], [17], [18], [19], [20], [21] with unidentified consequences, even though limited information exists, currently, on the released quantity [22]. In the environment, the NPs are subjected to various physicochemical processes that will drive them away from their pristine state toward different products having distinct properties from their parent source [23], [24].

On the other hand, toxicity of ZnO-NPs towards the biota have been reported [24], [25], [26], [27], [28], [29], [30], and their deleterious-effect may be attributed to the formation of various hazardous entities [24], [31] due to their transformations in the environmental system. Besides, the toxicity of the NPs is found to be dependent, strongly, on their size [24], [32]. Therefore, sizes of the NPs in the environment should be known to exactly delineate their fate, behavior, transformations and effects [33].

Analytical procedures for a reliable study of the environmental processes of MeO-NPs are a precondition to assess potential risks arising from the material production, application and disposal. The available state-of-the-art techniques have limitations to fulfill this demand as they are incapable of separating the NPs according to their size, to track their behavior and transformation pathways, and to separate the target NPs from the matrix [33]. Furthermore, due to their uneven behavior under various conditions, separation and analysis of MeO-NPs are a challenging work and hence, should be monitored, systematically. There are recent reports on the extraction and determination of ZnO-NPs [34], [35], but, they are insufficient to provide information regarding size of the materials, which do have a strong effect on their physicochemical behaviors. For example, the various transformations that the ZnO-NPs can undergo in the environment, their bioavailability and toxic-effects are strictly affected with their size [24], [36]. Therefore, besides to their quantitative determination, fractionation according to their size is very important.

Techniques such as hydrodynamic chromatography [37], size exclusion chromatography [38], and field-flow fractionation (FFF) [4], [39], [40] have been proposed to achieve the demand of fractionation. Hydrodynamic chromatography is underdeveloped and size exclusion chromatography has limitations such as high affinity of the analyte particles to the stationary phase resulting in low recoveries, and the upper size limit of the columns (due to the small pore sizes). Among the available methods, FFF is perhaps the most broadly adaptable fractionation method for nano- to micro-scale particles [41] due to its ability to provide reliable information about the size of the materials and its excellent fractionating potential in complex samples.

Asymmetrical flow FFF (AF4), has proven to be the most promising, universal and frequently used of all FFF techniques [41], [42], [43]. In AF4, the separation is made according to size of the materials within a separation channel under the influence of perpendicularly applied force to the parabolic flowing carrier solution without using stationary phase [41], [42], [43]. The field is created by a crossflow which pushes the particles to the bottom of the channel covered with an ultrafiltration membrane which retains the analyte of interest. The analytes diffusion within the channel can counter the effect of the crossflow on the particles and since smaller particles exhibit a higher diffusion coefficient than the larger particles, they form a cloud with a concentration profile that extends further back into the channel than that for the larger particles. Hence, particles of different sizes experience different flow velocities due to the parabolic flow profile of the carrier solution and separated [41], [42], [43]. In the literature, AF4 has been widely employed for the fractionation of exosomes [44], [45], macromolecules [42], [46] and nanomaterials [42], [43], [47], [48], [49], [50], [51], [52]. In recent, Cuddy et al. [53] and Bocca et al. [4] used FFF with inductively coupled plasma mass spectrometry (ICP-MS) and determined ZnO-NPs in sunscreens, and albeit size of the NPs was determined, fractionation of the NPs according to their size was not performed in the reports.

Therefore, the present study proposes a method for separation and size characterization of ZnO-NPs based on AF4 coupled to UV-vis detector as schematically represented in Fig. 1. Even though AF4 has been considered as a powerful technique for size characterization of nanomaterials of different nature, there is a lack of studies dealing with separation of ZnO-NPs according to its size. In this study, different conditions affecting the stability of the ZnO-NPs suspensions and parameters which can affect the separation strategy have been investigated. This report constitutes the application of separation and size characterization of ZnO-NPs by AF4 and thus, environmentalists can employ the technique and easily investigate the environmental processes of the materials.