Nanoparticle coatings for stir bar sorptive extraction, synthesis, characterization and application

Highlights

• This article reviews all available literature on application of nano-coatings as media for microextraction by stir bars.

• Non-functionalized (metallic/metallic oxide, graphene based) and functionalized coatings/decorated coatings are discussed.

• Various applications, advantages and disadvantages of each nano-coated prepared stir bar are discussed in detail.

Abstract

During the last ten years, number of articles published on synthesis and applications of nano-particles (NPs) have been increased by a factor of 40. One of the most interesting applications of NPs is their using as high capacity, robust and highly selective coatings for stir bar sorption extraction (SBSE). Utilizing NPs greatly promoted applications of SBSE and has gained importance on green sample preparation. In this article, all available literature on nano-coatings as media for microextraction by stir bar is reviewed. This includes non-functionalized NPs (mostly, metallic/metallic oxide and graphene based), functionalized coatings, and decorated coatings (both mono- and multifunctional). Various applications, advantages and disadvantages of each nano-coated prepared stir bar are discussed in detail along with critical evaluation of currently available methods.

Introduction

One of the main parts of an analytical process is its sample preparation step which is often necessary to isolate the components of interest from complicated matrices of the samples and at the same time to purify and concentrate target analytes [[1], [2], [3], [4]]. Despite extensive advances occurred in instrumentation and microcomputer technology, still majority of routine sample preparation techniques are based on techniques developed around a century ago. These extraction and enrichment protocols including liquid-liquid extraction and solid phase extraction often consume high volume of toxic organic solvents which cause health and environmental problems. Moreover, they need large volume of sample [[5], [6], [7], [8]]. Several attempts to improve these techniques resulted to invent solventless microextraction (ME) techniques such as solid phase microextraction (SPME), liquid phase microextraction, and stir bar sorptive extraction (SBSE) which have become some of the most important extraction techniques nowadays [[9], [10], [11]]. Utilizing a stir bar, coated with poly-dimethylsiloxane (PDMS), for extraction was first reported in 1999 by Sandra and colleagues and named stir bar sorptive extraction (SBSE) [12,13]. SBSE is based upon sorption which creates analytes partition between retaining polymer and liquid or vapor phase of sample; so, originating bulk retention. SBSE operation is very straight forward (Fig. 1 [14]). Stir bar simply immerges in sample and stirs for a pre-determined time. After extraction of the desired analytes, SB is removed from the sample solution and dipped in a vial containing proper volume of the elution solvent and stirs again. Finally, the elution solvent is introduced into a suitable instrument for analysis. Sampling is performed by direct insertion of the SBSE into liquid sample or its headspace (HS). In HS mode, analytes desorption is performed in static mode by immerging the SBSE into thermal desorption unit coupled on-line to a gas chromatograph (GC) or a gas chromatograph with mass spectrometer detector (GC-MS) [12,13,15,16].

This technique has the same protocol similar to solid phase microextraction (SPME). However, the volume of PDMS coating the stir bar typically is 25–125 μL which is 50–250 times higher than that of SPME fiber, achieving better enrichment, extraction efficiency and sample capacity [13,15,16]. Stir bar with PDMS coating is the most widely polymeric phase which is applied for SBSE so far and is known worldwide as Twister® trademark, commercialized by GERSTEL GmbH & Co KG (Mülheim, Germany) [17]. Because PDMS has been in use for many years as sorbent on commercial SBSEs, the large majority of applications used this coating. However, PDMS coating can be used only for the extraction and preconcentration of non-polar compounds. As a result, a new coating, ethylene glycol-PDMS copolymer (EG-silicone) has been introduced to the market recently which shows high recovery for both polar and non-polar analytes, because of the polar nature of EG and non-polarity of its silicone base [18]. However, both PDMS and EG-silicone coatings may have not enough capacity and suitable selectivity, especially for trace amounts of target compounds in complex matrix [12].

The crucial issues associated for development of new coatings are achievement of more stable and reproducible coatings. That's not only due to consecutive usage of coated SBSE and impact with high temperature, but also necessitate of better selectivity and capacity. That's why at the early stages of SBSE, coatings such as molecular imprinted polymers (MIPs), were regarded as successful coatings and used for the extraction and preconcentration of a variety of analytes such as copper [19], naphthalene sulfonates [14,20], and chorophenols [18] and diclofenac [16]. MIPs have high selectivity and large capacity and are robust. Octadecyl silane [21] and graphene oxide (GO) are other examples of coatings which are of high interest [22,23].

As in many other technological sections, nanomaterials (NMs) have demonstrated their appropriateness for various applications in chemical analysis, including sorbent materials for effective extraction and enrichment [24].

Nanoparticles (NPs) are NMs with a size in the range of 1–100 nm. Utilizing nano-sized materials results in the development of new technologies for extraction, separation and preconcentration. Materials reduced to the nanoscale indicate special properties, compared to their micro-scale size, because of two effects [25]: first, there are more atoms in their surface compared to their volume, which also results availability of more surface free energy. Moreover, having larger surface area increases the rate of their chemical reaction. Second, because of having higher frequency and higher energy a blue shift of atoms for optical absorption spectra occurs; and if the material is smaller than the magnetic domain in a particle, super paramagnetic properties may be observed [26].

Up to now, nanoparticles has been applied as sorbent for preconcentration of many analyes including heavy metals [15], organophosphorous pesticides [35], pseudoephedrine [27], amphetamines [27], thiabendazole [28], carbendazim [28], triazines [29], clonazepam [30], flavor enhancers [31], carbofuran [32], propoxur insecticides [32], losartan and valsartan [33], naproxen [34], chlorpyrifos, ultraviolet filters [36], phenols [37]; and other polar, and non-polar compounds [38,39] NPs also found variety of applications in microextraction (ME) methodologies [9,10] The main advantage of NP as sorbents is having a large active surface area for a given mass of material which leads to extremely large adsorption capacity toward many analytes. Moreover, since the surface atoms are unsaturated, they are subject to couple to compounds with static electricity; so, the adsorption process is fast. The other advantage of using NPs is the potential of their chemical modification to achieve higher selectivity [15,[40], [41], [42]].

In this review, NMs which have been applied as a coating for SBSE include nanoparticles, nanotubes, nanospheres and nanosheets have been summarized, together with their methods of synthesis and analytical applications. For better clarification, these coatings are divided into two categories according to if they are non-functionalized particles (metallic/metallic oxide, graphene based) or functionalized (both mono- and multifunctional). In each section, advantages and disadvantages of each coating is discussed in details.