Nanoparticle-Incorporated PDMS Film as an Improved Performance SPME Fiber for Analysis of Volatile Components of Eucalyptus Leaf

A new fabrication strategy was proposed to prepare polydimethylsiloxane (PDMS-) coated solid-phasemicroextraction (SPME) on inexpensive and unbreakable Cu �ber. PDMS was covalently bonded to the Cu substrate using self-assembled monolayer (SAM) of (3-mercaptopropyl)trimethoxysilane (3MPTS) as binder. To increase the performance of the �ber, the incorporation effect of some nanomaterials including silica nanoparticles (NPs), carbon nanotubes (CNTs), and carboxylated carbon nanotubes (CNT-COOH) to PDMS coating was compared.e surfacemorphology of the prepared �bers was characterized by scanning electronmicroscopy (SEM), and their applicability was evaluated through the extraction of some volatile organic compounds (VOCs) of Eucalyptus leaf in headspace mode, and parameters affecting the extraction efficiency including extraction temperature and extraction time were optimized. Extracted compounds were analyzed by �C-MS instrument. e results obtained indicated that prepared �bers have some advantages relative to previously prepared SPME �bers, such as higher thermal stability and improved performance of the �ber. Also, results showed that SPME is a fast, simple, quick, and sensitive technique for sampling and sample introduction of Eucalyptus VOCs.


SPME developed by Belardi and Pawliszyn
is a solvent free, simple, relatively fast to execute, and easily automated method for direct immersion and headspace extraction of volatile and semivolatile chemicals from solid, liquid, and gas samples and has been successfully applied in numerous environmental, food, �avor, pharmaceutical, clinical, and forensic applications [2].
e sensitivity and selectivity of SPME is strongly dependent on its coating type and thickness.Several coatings are commercially available for SPME analysis.Commercial SPME �bers have a friable silica rod as substrate coated with polymers or copolymers and must be handled with great care and thus greatly limits the service life.e commercial SPME �bers in addition to the friability of the �bers present important drawbacks such as their relatively low recommended operating temperature and stripping of the coatings [3], and �nally, their extraction efficiency is low in some cases.However, most of these �bers are generally prepared by mere physical deposition or partial crosslink of the polymer coating on the surface of the fused-silica �bers.e lack of proper interaction between the polymer coating and substrate surface and their relatively high thickness may be responsible for their low thermal and chemical stability [3].
In the past 15 years, considerable effort has been invested in the development of new SPME coatings, which has recently been subject to the literature reviews [3,4].To improve the breakage of the �bers, metal wires such as Pt [5], Ni-Ti [6], Al [7], and Cu [8] wires have been used as an alternative to fused silica, and, for �ber coating, several approaches including sol-gel technology [6,9], electrochemical procedures [10,11], and ionic liquids [12] have been  developed.Among the different approaches to stationary phase development for SPME �bers, sol-gel approach represents a promising direction in this important research with applicability in preparation of surface coatings for SPME �bers.e principle of SPME is the distribution of the analyte(s) between solid or li�uid coated �lm and the sample.In some cases, extraction capability is low because of limited adsorption sites on the �bers [13].Using nanomaterials can overcome this problem.Nanomaterials offer a signi�cant higher surface area to volume ratio that promises much greater extraction capability and efficiency compared with other materials used for SPME [14].e applications of nanomaterials in SPME coatings have shown a remarkable growth in recent years.Carbon nanotubes [15], fullerenes [16], nanoporous silica [17], nanostructured metal oxides [18,19], and Au nanoparticles [20] have been successfully used as SPME coatings.
In this study, silica sol-gel �lms chemically bonded on modi�ed copper wire were presented as a novel techni�ue for preparation of SPME �bers that have strong interaction between substrate and stationary phase.Also, to improve the extraction efficiency of prepared �bers, different nanomaterials were incorporated through the sol-gel �lm.e applicability of prepared �bers was evaluated through the extraction of volatile organic compounds (VOCs) of Eucalyptus leaf, and parameters affecting the extraction efficiency were optimized.

Instrumentation.
A Hewlett-Packard (HP, Palo Alta, USA) HP 6890 series GC equipped with a split/splitless injector and an HP 5973 mass-selective detector (MSD) system were used to evaluate the SPME �bers.e MS was operated in the EI mode (70 eV).Helium (99.999%) was employed as carrier gas, and its �ow rate was constantly adjusted to 1 mL⋅min −1 .e separation of VOCs was performed on a 30 m × 0.25 mm HP-5 MS column (0.25 m �lm thickness).e column was held at 60 ∘ C for 2 min and increased to 260 ∘ C at a rate of 7 ∘ C⋅min −1 .e injector temperature was set at 260 ∘ C, and all injections were carried out on the splitless mode.e GC-MS interface was maintained at 280 ∘ C. e scanning electron microscope images were obtained by an FE-SEM S-4160 Hitachi.

Modi�cation of Cu �ire Surface.
Cu wires with length of 2 cm and thickness of ∼200 m were prepared and cleaned by acetone, ethanol, and water solutions, respectively.To prepare a self-assembled monolayer (SAM) of 3MPTS, the copper wire was inserted in a solution containing 3MPTS 1 mM in ethanol for 3 hrs then washed with distilled ethanol and water, respectively, to eliminate physically deposited 3MPTS [21].en it was inserted in NaOH 1 M (30 min) and HCl 0.1 M (30 min), respectively.en the 3MPTS-modi�ed surfaces were abundantly rinsed with water and then allowed to air-dry.

�eposition of Coatin� on Modi�ed
Cu �ire.e detail of this procedure has been described elsewhere [22] with some modi�cations; brie�y, the sol-gel solution was prepared in a 1.5 mL PCR tube by mixing 300 L TEOS, 180 mg OH-TSO, and 30 mg PMHS dissolved in 300 L dichloromethane, and then 200 L TFA (containing 5% water) was sequentially added to the resulting solution.en the mixture was centrifuged at 12,000 rpm, 20 ∘ C for 8 min.e Cu wire was vertically immersed (1 cm) inside the PCR tube containing the centrifuged sol-gel mixture for 30 min at room temperature.Aer that period, the �ber was carefully covered with a pile of nanomaterials: silica nanoparticles (NPs), carbon nanotubes (CNTs), or carboxylated carbon nanotubes (CNT-COOH) which were gently pressed against the sol-gel coating so as to stick the particles.e sticking of the particles was manually achieved, and its success depended on a great deal on the operator's experience gained aer trial-and-microscopic inspection.Aerwards, it was le to stand for 24 h.is coating process was repeated several times in the fresh sol solution and the same conditions until the desired �ber thickness was obtained.e prepared �bers were conditioned at 280 ∘ C under Helium for 1 h in the GC injection port.Aer removal from the injector, the �ber was cooled to room temperature in a desiccator for use.
2.4.SPME Procedure.e extractions were performed in the headspace of a 50 mL glass vial containing 0.2 g powdered Eucalyptus leaf.e glass vial was inserted into the circulator bath for establishing of temperature.e vial was sealed with an aluminum cap with a PTFE-faced septum.e outside needle of the SPME �ber assembly penetrated to the vial septum, and the coated �ber was exposed in the headspace of the sample for extraction of analytes on the stationary phase.Aer extraction, the SPME �ber was retracted into the protective sheath, removed from the headspace, and transferred into the injector of the gas chromatograph for thermal desorption of the analytes at 250 ∘ C for 4 min in splitless mode.At the same time, the GC/MS run was started.

Fiber Preparation and Characterization. Sol-gel technology is based on the hydrolysis and simultaneous condensation of metal alkoxides; this technique has been widely
used to prepare SPME �bers.e �lms prepared by sol-gel technology do not have good adherence to the metal �ber surfaces and will be cracked and �acked off aer deposition; therefore, commercial SPME �bers commonly have been prepared on friable silica �bers.To solve the problem of weak interaction of sol-gel �lms to metal substrates, there are some ways; at the �rst of them, the surface of metal oxide substrates should be activated (hydroxylated) through the NaOH treatment [23].Another strategy to solve the problem is using an intermediate layer such as 3APTES [24] that can bind to the metal surface and, to the sol-gel �lm, on the other hand.
Sinapi et al. studies [21] show that 3MPTS chemisorbs on Cu surfaces through strong thiolate bonds forming a wellorganized self-assembled monolayer (SAM) and is able to decrease the corrosion of the underlying copper substrate.In the present study, SAM of 3MPTS was used as intermediate layer to covalently bind the PDMS �lm to the copper substrate.
Figure 1 schematically shows the procedure of chemical bonding of PDMS coating on copper substrate and incorporation of nanomaterials in coating.At the presence of NaOH and HCl, the 3MPTS molecules that formed selfassembled monolayer on copper surface will be hydrolyzed and condensed to form a 2D network with some - functional groups as active sites to stick the sol-gel �lm.
e extraction e�ciency of prepared �bers was evaluated through extraction of volatile organic compounds (VOCs) of Eucalyptus leaf.Table 1 shows VOCs of Eucalyptus leaf extracted by SPME-GC-MS.Five components of VOCs including alpha-pinene, 1,8-cineole, isopinocarveol, aromadendrene, and neoalloocimene were selected as model compounds.
To increase the extraction capability of prepared �ber, the effect of incorporation of different nanomaterials including CNT, CNT-COOH, and silica NPs in PDMS �lm was studied.As can be seen at Figure 2, the extraction capability of PDMS is increased at the presence of nanomaterials.e extraction capability of PDMS/CNT-COOH �ber is higher than PDMS/CNT, due to higher polarity of CNT-COOH to extract semipolar volatile components of Eucalyptus leaf.Also, the extraction capability of PDMS at the presence of silica NPs is higher than CNT and CNT-COOH-incorporated �ber, due to the higher surface area of NPs relative to the nanotubes.
e surface characteristics of Cu wire at different steps of coating were investigated by scanning electron micrograph (SEM) technique.e SEM image (Figure 3(b)) shows that no observable change was caused by SAM of 3MPTS on Cu wire surface, except smoother surface of Cu modi�ed by SAM.As can be seen from Figures 3(c) and 3(d), the sol-gel �lm is coated on Cu substrate.Also, Figure 3(d) shows that silica NPs-incorporated sol-gel coating possesses a porous structure, which resulted in larger surface areas and higher extractive capability than PDMS coating without silica NPs.e coating thicknesses of each coating were estimated by SEM and found to be about 5 m and 7 m for the PDMS and silica NPs/PDMS �ber, respectively.Commercial SPME �bers have low operating temperatures, due to weak interaction between the substrate and coating [3].e thermal properties of the silica NPs/PDMS �ber were assessed by its T�A curve.Results show that the mass loss step begins above 320 ∘ C, which can be assigned to the maximum practicable temperature of the sol-gel coating.e higher thermal stability of prepared PDMS �ber relative to commercial PDMS �ber is due to strong chemical bonding of coating to the Cu substrate provided by SAM of 3MPTS.Such high thermal stability can expand the SPME application range toward higher boiling-point compounds.
e performance of the developed �bers was also evaluated in terms of �ber-to-�ber (reproducibility) and batchto-batch (repeatability) relative standard deviations (Table 2): RSDs lower than 10.8% were obtained for repeatability of sampling procedure and reproducibility of �ber preparation.e obtained results allowed assessing the feasibility of the proposed coating procedure in the development of thermally stable and efficient �bers.e effect of the temperature in the present study was studied in the range of 30-70 ∘ C. As shown in Figure 4, peak areas of all analytes reach the highest values at 60 ∘ C, and hence, the extraction temperature was �xed at 60 ∘ C for subsequent experiments.

Optimization of Extraction
SPME is an equilibrium-based technique, and the highest extraction efficiency is usually reached aer equilibrium.To �nd the optimum extraction time, a time range from 5 to 30 min was investigated.Figure 5 shows the effect of extraction time on the extraction efficiency for �ve selected Eucalyptus leaf VOCs.As shown in Figure 5, for all components, the equilibrium was obtained aer 20 min.Hence, 20 min was chosen as the optimum extraction time.Relatively short extraction times show that SPME of Eucalyptus volatile components using prepared �ber is a simple and quick sample preparation and sample introduction technique.

Conclusions
PDMS sol-gel �lm was chemically bonded to copper wire surface through self-assembled monolayer of 3MPTS as an intermediate.e thermogravimetric analysis showed that prepared �ber has higher thermal stability relative to commercial PDMS �ber.It was demonstrated that different nanomaterials including silica NPs, CNT, and CNT-COOH could be strongly attached to an inexpensive and unbreakable Cu wire by making use of a sol-gel layer, allowing a novel way to prepare more efficient SPME �bers.e prepared silica NPs/PDMS �ber has higher extracting capability relative to CNT-incorporated PDMS �bers, due to the higher surface area to volume ratio of silica NPs.Also, results showed that SPME-based analytical method is simple, inexpensive, environmental friendly sampling, and sample preparation technique for analysis of volatile compounds in Eucalyptus leaf.

F 2 :
e effect of different nanomaterials incorporation on extraction capability of prepared �ber.

F 3 :
Scanning electron microscopy of (a) Cu wire, (b) Cu wire modi�ed by self-assembled monolayer of 3MPTS, (c) PDMS coating on modi�ed Cu wire, and (d) silica NPs-incorporated PDMS on modi�ed Cu wire.

F 5 :
Optimization of extraction time, 0.2 g powdered Eucalyptus leaf, and 60 ∘ C extraction temperature.
Volatile organic compounds (VOCs) of Eucalyptus leaf extracted by SPME-GC-MS.
Parameters.To achieve the best extraction efficiency, effects of extracting parameters, including extraction temperature and extraction time, were systematically studied.It is generally accepted that temperature has adverse effects on the extraction; on one hand, at elevated temperatures, the higher mobility of molecules increases the extraction rate; on the other hand, it would decrease the distribution coefficient of analytes between the coating and sample solution, because extraction is an exothermic process.erefore, the selection of a proper temperature is necessary for the extraction process.