Experimental and calculated Raman scattering data for 4,4′-BiPy.
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",isbn:null,printIsbn:null,pdfIsbn:null,doi:null,price:0,priceEur:null,priceUsd:null,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"cc796459268324e827219d1d904e4265",bookSignature:"Prof. Moulay Tahar Lamchich",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/7196.jpg",keywords:"Induction motor, smart motor, electrical vehicles, energy generation, drives, electromechanical, hybrid transportation, smart control, high efficiency motor, variable speed drives, power electronic, energy efficiency.",numberOfDownloads:null,numberOfWosCitations:0,numberOfCrossrefCitations:0,numberOfDimensionsCitations:0,numberOfTotalCitations:0,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"July 4th 2018",dateEndSecondStepPublish:"July 25th 2018",dateEndThirdStepPublish:"September 23rd 2018",dateEndFourthStepPublish:"December 12th 2018",dateEndFifthStepPublish:"February 10th 2019",remainingDaysToSecondStep:"3 years",secondStepPassed:!0,currentStepOfPublishingProcess:5,editedByType:null,kuFlag:!1,biosketch:null,coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"21932",title:"Prof.",name:"Moulay Tahar",middleName:null,surname:"Lamchich",slug:"moulay-tahar-lamchich",fullName:"Moulay Tahar Lamchich",profilePictureURL:"https://mts.intechopen.com/storage/users/21932/images/system/21932.png",biography:"Moulay Tahar Lamchich is a Professor at the Faculty of Sciences Semlalia at Marrakech (Morocco). He completed his thesis in electromechanics in September 1991 and received his third cycle degree. Dr. Lamchich received his Ph.D. from the same university in July 2001. His main activity is based on short-circuit mechanical effects in substation structures, control of different types of machine drives, static converters, active power filters. In the last decennia, his research interests have included renewable energies, particularly the control and supervision of hybrid and multiple source systems for decentralized energy production, and intelligent management of energy. He has published more than fifty technical papers in reviews and international conferences. With IntechOpen, he has published two chapters and was editor of the books “Torque Control” and “Harmonic Analysis”. 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From chapter submission and review, to approval and revision, copyediting and design, until final publication, I work closely with authors and editors to ensure a simple and easy publishing process. I maintain constant and effective communication with authors, editors and reviewers, which allows for a level of personal support that enables contributors to fully commit and concentrate on the chapters they are writing, editing, or reviewing. I assist authors in the preparation of their full chapter submissions and track important deadlines and ensure they are met. I help to coordinate internal processes such as linguistic review, and monitor the technical aspects of the process. As an ASM I am also involved in the acquisition of editors. 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Venkateswarlu",coverURL:"https://cdn.intechopen.com/books/images_new/371.jpg",editedByType:"Edited by",editors:[{id:"58592",title:"Dr.",name:"Arun",surname:"Shanker",slug:"arun-shanker",fullName:"Arun Shanker"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"878",title:"Phytochemicals",subtitle:"A Global Perspective of Their Role in Nutrition and Health",isOpenForSubmission:!1,hash:"ec77671f63975ef2d16192897deb6835",slug:"phytochemicals-a-global-perspective-of-their-role-in-nutrition-and-health",bookSignature:"Venketeshwer Rao",coverURL:"https://cdn.intechopen.com/books/images_new/878.jpg",editedByType:"Edited by",editors:[{id:"82663",title:"Dr.",name:"Venketeshwer",surname:"Rao",slug:"venketeshwer-rao",fullName:"Venketeshwer Rao"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}}]},chapter:{item:{type:"chapter",id:"58189",title:"Nanoscale Insights into Enhanced Raman Spectroscopy",doi:"10.5772/intechopen.72284",slug:"nanoscale-insights-into-enhanced-raman-spectroscopy",body:'Raman spectroscopy is an analytical technique based on inelastic scattering of light. The light scattered by a molecule shows a wavenumber shift with respect to the excitation line. This effect was first described by C. V. Raman in 1928 and quickly became used as a powerful method for identifying molecules through their vibrational motions [1]. However, until the 1970s, Raman spectroscopy did not receive much attention of scientists working in the field of surface analysis. This is because intrinsic Raman scattering cross-sections of molecules is small, on the order of 10−32 cm2 sr−1 per molecule [2]. The observation of Raman signal enhancement of molecules adsorbed on roughened metal surfaces was a corner stone in the development of a family of enhanced vibrational spectroscopies with the surface-enhanced Raman spectroscopy (SERS) being the most widely practiced [3, 4, 5]. In SERS, Raman signals are amplified by placing a molecule in the vicinity of metal nanostructures. A large number of papers have been published over a period of 40 years on the origin of Raman signal enhancement on SERS active substrates [6, 7, 8]. Consequently, the overall enhancement of the signal has been attributed to two main mechanisms: electromagnetic enhancement (EME) and chemical enhancement (CE).
EME is explained by the enhancement of the electromagnetic field at the molecule’s position by excitation of the localized surface plasmon resonances of metallic nanostructures [9, 10]. Basic EME is now well understood and believed to be the major contribution to the enhancement of Raman signals on SERS active substrates. In the process, both the incoming and outgoing field is amplified. For a single molecule, basic EME, excluding polarization effects and tensorial nature of the Raman polarizability, scales as E4, where E is the intensity of the electromagnetic field. Polarization effects influence signal intensity but in a more complex way, depending on the symmetry of the vibrational mode. Moskovits, in his 1982 work, described Raman surface selection rules, that is, modification of band intensities for a molecule adsorbed on a flat metal surface [11]. Experimental justification of polarization effects requires Raman data obtained on flat metal surfaces and a nanoscale characterization of the environment in which a molecule is present. With the development of tip-enhanced Raman spectroscopy (TERS), we can now define the nanoscale environment of a molecule and correlate it directly with the Raman signal. It is expected that polarization studies in TERS settings, supported by calculations of the Raman polarizability tensor components of a molecule, will bring further insights into mechanism of Raman signal enhancement and surface selection rules.
CE describes various effects affecting the Raman polarizability αR with respect to the
Combining Raman spectroscopy with scanning probe microscopy into so-called TERS has made it possible to investigate CE and EME in detail with nanoscale resolution [21, 22]. TERS is an offspring of SERS, in which a “hot spot” is created between a metallic tip and a flat surface. The TERS technique eliminates the need for rough or nanostructured surfaces, allowing for investigation of the details of molecular adsorption under well-defined conditions. The biggest advantage of TERS over SERS is its capability to acquire Raman signals and nanoscale images of the molecule adsorbed on a solid substrate simultaneously.
In this chapter, readers will be introduced to the experimental and theoretical aspects of TERS based on a scanning-tunneling microscopy (STM-TERS). Subsequently, the results of TERS studies of molecules adsorbed on flat metal surfaces are summarized with an emphasis on the molecular orientation and surface selection rules. Later, the effect of chlorine activation of metal surfaces on the adsorption of organic molecules and halogen overlayer-templated growth of surface-grown metal-organic layered structures are described. At the end of the chapter, future prospects and challenges of TERS in studies of molecular adsorption on metal surfaces are discussed.
Nanoscale vibrational spectroscopy has been a longstanding dream of scientists working in various fields. In the early 1980s, a scanning tunneling microscope was invented which brought us the capability to explore surfaces with unprecedented subnanometer scale resolution [23]. Later on, images of molecules adsorbed on metal and semiconductor surfaces were obtained [24]. STM is based on a tunneling current between a conductive surface and a sharp metallic tip. In the basic mode of operation, STM does not yield vital chemical information, and formation of surface complexes cannot be confirmed. Consequently, inelastic electron tunneling spectroscopy (IETS) was developed to obtain chemical information from single molecules adsorbed on metal surfaces [25]. Despite its ultimate resolution and sensitivity, IETS has not evolved into a common nanoscale vibrational spectroscopic method. This is because IETS is a very challenging technique requiring low temperature, ultra-high vacuum conditions and ultra-low noise electronics.
From the development of the field of plasmonics and the demonstration of large electromagnetic field enhancement in SERS experiments, the idea of nanoscale vibrational spectroscopy on surfaces using Raman scattering has been brought to life [26]. Raman spectroscopy has many advantages as it is based on the optical response of the system and can be applied under ambient conditions and in water. However, Raman scattering is intrinsically a very weak process with only one in every 106−108 incident photons being scattered [27]. Theoretical reports in the late 1990s showed that the electric field at the metallic tip end is dramatically enhanced under certain polarization direction [28]. This report inspired scientists working in the field of optical microscopy and surface science to utilize the enhancement effect to develop a high-resolution molecular spectroscopic technique. The Kawata group in Japan and the Zenobi group in Switzerland simultaneously reported TER spectra from multilayers of organic molecules deposited on a glass surface using a metalized cantilever probe of an atomic force microscope (AFM) [21, 22]. Later, Pettinger et al. reported TER spectra from the monolayer of malachite green adsorbed on CN− modified gold surface, using STM tip [29]. Ultimate sensitivity and resolution of a single molecule were demonstrated in 2008 with TERS optics incorporated into ultra-high vacuum environment (UHV-TERS) [30]. Over the last decade, TERS has been adapted as an analytical tool in chemistry, biology and materials science [31].
A sharp metallic tip is a central part of the TERS setup and its quality (size and shape) defines the spatial resolution of TERS and, to some extent, the magnitude of the enhanced field. Owing to its importance, several papers have discussed various tip fabrication methods [32, 33, 34]. Tips are usually made of gold or silver as these metals have plasmon resonances in the visible region of the electromagnetic spectrum. Electrochemical etching is a common method to prepare the tips. In a typical procedure, Au tips are prepared by direct current electrochemical etching in a 50:50 (v:v) mixture of concentrated HCl and ethanol. Au wire (a tip after etching) serves as the anode and a gold or platinum ring acts as the cathode. The end of the Au wire is submerged 1–2 mm into the solution at the center of the gold ring. A voltage in the range of 1.7–2.5 V is applied between the cathode and the anode. The etching reaction proceeds until the electrochemical current drops to zero. Typically tips with a tip-apex size in the range of 20–50 nm are obtained in this way.
TERS requires integration of STM/AFM with the optical components used in Raman spectroscopy. Various optical geometries have been adapted in the past 15 years to work with a variety of different samples [34]. They include a bottom-, side-, top- and a parabolic mirror illumination depicted in Figure 1. Each geometry has its own advantages and disadvantages, which are summarized hereafter.
Four common optical geometries used in TERS (a) bottom illumination, (b) side illumination, (c) top illumination, (d) parabolic mirror illumination.
The bottom illumination geometry (Figure 1a), in which the incident laser light is fed through the microscope objective placed at the bottom and the rear side of the sample, is not practiced among STM-based TERS users as it requires transparent samples. This geometry is commonly used in AFM-based TERS, in which an inverted optical microscope platform is used. The advantage of this geometry is a very high collection efficiency of scattered light as oil-immersion objectives with a high numerical aperture (NA) can be used. Recently, the Deckert group has modified the bottom illumination geometry to work with opaque samples by using a dichroic mirror and passing the incident light through the transparent side of the opaque sample [35]. The bottom illumination is well suited for biological samples.
Side illumination (Figure 1b) is the most commonly used setup in TERS built on STM. In this geometry, the incoming laser light as well as the outgoing scatter signal is collected by a long working distance microscope objective placed at an angle in the range of 45–70° relative to the tip axis that maximizes the electric light component along the tip axis. The advantage of this geometry is an easy integration with commercial STM setups. It can be used both for transparent and nontransparent samples. However, only conductive samples can be studied. The disadvantage of the side illumination geometry is relatively low collection efficiency in comparison to the bottom light illumination geometry. This is because objective lenses with a low numerical aperture are employed due to space restriction around the STM tip. Another disadvantage of the side illumination geometry is an asymmetric illumination of the tip, which leads to larger spot size and higher far-field background. In addition to tip resonances, so-called gap mode resonances can be excited in the side illumination geometry [36, 37]. They play a big role in the enhancement of the signal when the distance of the tip to the surface is below 2 nm [38]. Excitation of the gap-mode resonances improves sensitivity and resolution of the TERS technique. The side illumination geometry is widely used in the field of surface science.
Top illumination (Figure 1c) is the second most commonly used geometry in the STM-TERS community. This geometry makes focusing of the incident beam on the STM tip easier. The tip is placed at an angle to the surface with the microscope objective placed normal to the surface. Although some of the scattered light is shadowed by the tip, this geometry offers still higher excitation and collection efficiency than the side-illumination geometry does. Another advantage is a reduction in the far-field background. Both opaque and transparent samples can be studied. This geometry is ideally suited for investigating nanoscale phenomena on silicon or graphene samples.
A geometry utilizing a parabolic mirror (Figure 1d) was first demonstrated by researchers working with an STM unit operating in UHV conditions [30]. It was developed in order to increase the collection efficiency by allowing collection of light from all directions. In this geometry, a hole is made in a parabolic mirror to accommodate the STM tip. STM-TERS setups with a parabolic mirror work in a reflection mode, allowing both opaque and transparent samples to be studied. Compared to the side and top illumination geometries, the parabolic mirror geometry offers high collection efficiency. Two major disadvantages of this geometry are: it is difficult to integrate into commercial STM units and optical alignment is difficult. A small angular mismatch results in defocusing of the laser spot and loss of the signal. Thus, this geometry has not been widely used.
The origin of Raman signal enhancement in TERS is similar to that in SERS, which is due to EME and CE. In contrast to SERS, in which Raman signals are obtained from multiple hot spots across the surface, the signal in TERS originates from one central hot spot, which is created between the apex of a tip and a flat metal substrate.
Various effects, such as electrostatic lightening rod effect, excitation of localized surface plasmon polaritons (SPPs) on the tip and antenna resonances, contribute to EM field enhancement [39]. The lightening rod effect is independent of the excitation wavelength. On the other hand, excitation of SPPs is wavelength dependent with maximum field enhancement achieved when the laser energy coincides with the localized surface plasmon resonances of the tip. Finite-difference time domain (FDTD) calculations show that the magnitude of the enhancement due to plasmon excitation depends on the laser light polarization, tip radius and dielectric properties of the surrounding medium [38]. In-plane light polarization (p-polarization, parallel to the tip axis) gives much higher enhancement than the out-of-plane polarization (s-polarization) does. The maximum enhancement is predicted for tips with the apex radius of 15–20 nm [38]. Tips made of silver provide higher enhancement when visible light excitation is used. Figure 2 shows all possible effects contributing to the enhancement of the signal in TERS.
Possible CE and EME effects contributing to Raman signal enhancement in TERS.
When the tip-metal surface distance is smaller than 2 nm, additional EM field enhancement is observed. At this distance, LSP of the tip and a metal interact with each other to form hybridized modes, called gap modes [37]. The enhancement due to excitation of the gap mode resonances depends strictly on the tip-metal surface separation [40, 41]. The gap modes are efficiently excited when D/R < 1, where D is the distance of the particle from the surface and R is the radii of the tip apex. The enhancement of the scattered light intensity is found to be as high as 1012 for a 20 nm radius gold tip and tip-substrate separation of 1 nm [38]. Such small tip-substrate separations are easily controlled by the tunneling feedback function of the STM.
CE due to chemisorption, formation of a surface-complex and anion surface modification can be studied with excellent resolution using STM-TERS. These studies began in the field of surface science. A resonance enhancement of 106 has been reported by Pettinger et al. for a malachite green molecule adsorbed on an Au(111) surface [42]. Ren et al. have shown that Raman signal can be obtained from monolayers of non-resonant molecules with weak Raman cross-sections [43]. Observed frequency shifts between Au and Pt surfaces indicate that TERS is sensitive enough to identify molecular orientation and revealed details of molecule-surface interaction.
Although simultaneous observation of Raman and infrared (IR) vibrational modes are exclusive to each other in the case of centrosymmetric molecules, some TER spectra published in the literature show the presence of IR active or silent modes [44]. Polbutko explained appearance of these lines due to strong quadrupole light-molecule interactions arising from strongly inhomogeneous electromagnetic fields, which exist near rough metal surface [45].
The Dong group has recently demonstrated sub-nanometer resolution in TERS experiments [46]. As subnanometer resolution is difficult to understand in terms of the classical electromagnetic theory, these results have inspired theoreticians to work on proposing new mechanisms that could explain the results. Duan and Luo have proposed involvement of nonlinear optical processes [47]. Creation of an “atomic-scale hot spot” has also been proposed [48]. In addition, multiple elastic scattering of light between molecular dipoles adsorbed on the surface has been proposed to explain the improved signal intensity and TERS spatial resolution [49].
The ultimate goal to understand TERS from molecules adsorbed on metal surfaces is to understand how relative intensities of Raman lines depend on the molecular orientation and polarization direction of the excitation light. These studies are still challenging as there are many parameters, such as molecular binding geometry, Raman tensors and direction of local field polarization, that have to be determined. It is still not possible to formulate rules similar to the IR metal surface-selection rule (SSR). The IR SSR states that, for a molecule adsorbed on a metal surface, vibrational modes having a dipole moment perpendicular to the surface are the most enhanced [50]. Moskovits described the concept of SSR in SERS experiments for flat metal surfaces [11]. The author’s work in 1982 gave theoretical grounds for local field polarization. Recently, Ru et al. have experimentally validated Moskovits theory by studying polarization and incident angle dependences of the SERS signals [51]. Similar studies using TERS can offer more insights into the Raman SSR as the Raman signal can be directly related to the molecules present under the tip.
In order to comprehend enhancement mechanisms in detail, more sophisticated experiments and theoretical analysis are required. Emerging TERS studies under UHV and on well-defined systems can deliver more results, which should eventually bring us closer to understanding origin of signal enhancement and contribute to the development of TERS as a reliable analytical tool.
This section summarizes our studies on molecular adsorption and orientation of 4,4′-bipyridine (4,4′-BiPy) and 4,4′-bipyridine N,N′-dioxide (4,4′-BiPyO2) in monolayers formed on gold thin films deposited on muscovite mica substrates using STM-TERS supported by calculated Raman tensor polarizability components. The enhancement of the Raman signals is attributed to the formation of a chemisorbed overlayer with a standing up molecular configuration [52].
A TERS setup with the side-illumination geometry was used in the experiments described here. The setup consists of a commercial STM unit (Nanoscope E, Veeco Instruments Inc., USA), a spectrograph (SP-2150i, Roper Scientific, GmbH) and optical components. The STM has a modified piezo scanner head which allows to install a high numerical aperture objective lens (Mitutoyo, LWD 100×, NA = 0.7, WD = 6 mm) in front of the STM tip. The lens is placed at an angle of 60° to the surface normal with the light polarization parallel to the tip axis (p-polarization). This objective lens is used to deliver the excitation laser beam as well as to collect the backscattered light from a tip-surface junction.
The optical pathway adapted in the study is shown in Figure 3. A red, He-Ne laser beam (632.8 nm, max. Output 30 mW, CVI Melles Griot, USA) with circular polarization was used for the excitation. The laser light was allowed to pass through a band-pass filter (Sigma Koki, Japan, bandwidth = 3 nm) and a polarizer. A transmitted light was reflected by a mirror, passed through a 45° dichroic beam splitter (RazorEdge, type U, Semrock), and reflected by two other mirrors before being focused on the tip-surface junction by the objective lens.
STM-TERS setup. The inset shows a SEM picture of the Au tip, etched at a bias of 2.4 V. Adapted from Rzeznicka et al. [52]. Copyright@Elsevier B.V.
The backscattered radiation is collected by the same objective lens and reflected by two mirrors before falling on the dichroic beam splitter. The scattered signal passes through an ultra-steep long-pass edge filter (RazorEdge, type E, Semrock), and is focused by a lens (diameter = 25 mm, focal length = 100 mm) onto the slit of the spectrograph. A back-illuminated, charge-coupled device (CCD) camera (Spec-10, Princeton Instruments) cooled by liquid nitrogen was used to acquire Raman spectra. The spectrograph was installed with 300 g/mm diffraction grating. The spectral resolution of the system was 10 cm−1. All experiments were carried out in ambient conditions with the incident laser power of 0.4 mW, giving power density of 8 × 107 W/m2 in the focal region.
The geometry of the molecules and fundamental vibrational frequencies were calculated using the Gaussian 09 package. Molecular structures in the ground state were optimized by the B3LYP exchange-functional of the density functional theory and 6-31G++(d,p) basis set [53]. The optimized geometries of both molecules (4,4′-BiPy and 4,4′-BiPyO2) are nonplanar with D2 symmetry. Each vibrational mode has been ascribed to a given symmetry mode and Raman polarizability tensor components were calculated for each of the mode.
The intensities of the Raman scattering were evaluated with the matrix elements of the Raman tensor,
where ħ is the Planck constant divided by 2π, ωk and μk are the angular frequency and the reduced mass of the mode k, respectively. The molecule-fixed coordinates were defined with the principal axes of inertia, where the z axis is along the long molecular axis, and the x axis is nearly perpendicular to the rings. The principal axes of polarizability tensor coincide with the x, y and z axes, to give αxx, αyy and αzz. The vibrational analysis was performed to obtain the frequency ωk, normal mode coordinate Qk and the reduced mass μk of each mode k. The polarizability tensor components,
In order to determine orientation of the molecule, the experimental scattering intensities were compared with the scattering intensities calculated for three representative molecular orientations. The direction of the incident radiation was described in the surface-fixed coordinate system (X, Y, Z). The polarization of an incident laser beam in our TERS experiment was adjusted in a way that the electric vector, E0//Z, that is, Z axis is perpendicular to the surface. The molecules can take various orientations having various molecular Euler angles, with respect to the Au substrate plane. Three representative configurations were considered, that is, when x//Z, y//Z and z//Z. The corresponding molecular orientation for each case is shown in Figure 4. For the molecule perpendicular to the surface (“end-on” configuration), the molecular z axis is parallel to the surface-fixed Z axis (z//Z), that is, perpendicular to the surface. The molecule with the “edge-on” configuration and the “face-on” configuration are denoted as y//Z and x//Z, respectively.
Molecular coordinate system. The molecular axes are x, y and z, and the surface axes are X, Y and Z. The three cases of adsorption configuration discussed in the text are shown. E defines the electric vector of an incident radiation. Adapted from Rzeznicka et al. [52]. Copyright@Elsevier B.V.
Adsorption of 4,4′-BiPy on the surface of an Au thin film proceeded in two stages. A first adsorption stage was observed after a short immersion time (3–5 h) of the Au film into a 1 mM ethanolic solution of 4,4′-BiPy. An image of the surface at this stage is shown in Figure 5a. No well-defined overlayers were observed. The surface of Au looked very rough and dynamic. Imaging was very unstable due to apparent adsorbate-induced surface reconstruction. Surface reconstruction is associated with the ejection of gold atoms and their diffusion over the surface. Low-coordinated gold atoms are highly reactive, and they may form a complex with molecules in the solution and diffuse over the surface to stable adsorption sites. These transient species are seen in the image as whitish spots. A second stage of adsorption was observed upon a prolonged immersion time. In this stage, a well-defined overlayer was formed. An STM image of the surface immersed into the solution for 4 days is shown in Figure 5b, c. A homogeneous monolayer with pits having a depth of a single-gold-atom was observed. It looks similar to monolayers formed by alkanethiols on Au surface, indicating involvement of Au adatoms in the process of self-assembly. TER spectra for the short immersion time and the long immersion time are shown in Figure 5d. The Raman spectrum for the short immersion time has only few bands with very low intensity. The absence of low-frequency vibrational signals, which could be assigned to Au-N stretching band, indicated that molecules were only weakly adsorbed (physisorbed) on the surface. In the case of the long immersion time, intensities of the Raman signals were higher, and many vibrational bands, which were not observed in the case of the short immersion time, appeared. An intense Au-N stretching signal was detected at 185 cm−1, indicating that molecules were chemisorbed on the surface.
STM images of 4,4-BiPy adlayer formed on Au(111) after immersion of the film into a 1 mM ethanolic solution for (a) 3 hours, (b) 4 days and (c) zoom into (b). (d) TERS spectra corresponding to the layer shown in image a and b. A depth profile across the A-A’ line is shown in image (b). Adapted from Rzeznicka et al. [52]. Copyright@Elsevier B.V.
Vibrational frequencies for the two cases are summarized in Table 1. Each mode has been ascribed to a given symmetry mode, and Raman polarizability tensor components were calculated for each of the mode. The scattering intensities for the three possible molecular orientations were calculated and used to aid in determining the molecular orientation.
Experimental TERS peak position/cm−1 | Results of calculations | Modal assignment | ||||||
---|---|---|---|---|---|---|---|---|
Figure 5 | Figure 5 | |(αij)E0|(|E0| = 1)/atomic unit | ||||||
Short immersion | Long immersion | Frequency/cm−1 | Symmetry class | Mode number k | x//Z | y//Z | z//Z | |
185 s | — | — | — | — | — | — | ν(Au-N) | |
750 m | 688 | B2 | 13 | 0.25 | 0 | 0.25 | pyridyl ring deformation | |
840 s | 864 | B2 | 18 | 0.26 | 0 | 0.26 | γ(C–H) + γ(C–C) +γ(C–C)int + γ(C-N) | |
928 m | 984 | B2 | 22 | 0.01 | 0 | 0.01 | γ(C-H) | |
1016 w | 1016 w | 1014 | A | 26 | 0.44 | 0.73 | 1.94 | γ(C-H) + δ(C-C) +δ(C-N) + ν(C-C) + ν(C-N) |
1092 m | 1098 | A | 29 | 0.15 | 0.11 | 0.52 | δ(C-H) + δ(C-C) +δ(C-N) + ν(C-N) | |
1231 m | 1276 | B2 | 34 | 0.16 | 0 | 0.16 | ν(C-C) + ν(C-N) | |
1337 m | 1337 m | 1364 | B3 | 38 | 0 | 0.32 | 0.32 | δ(C-H) + ν(C-C) |
1533 s | 1492 s | 1540 | A | 42 | 0.14 | 0.47 | 0.88 | δ(C-H) + ν(C-C) +ν(C-C)int + ν(C-N) |
1543 m | 1583 | B2 | 43 | 0.07 | 0 | 0.07 | δ(C-H) + ν(C-C) + ν(C-N) | |
1624 w | 1624 m | 1645 | A | 46 | 0.22 | 0.83 | 3.43 | δ(C-H) + ν(C-C) +ν(C-C)int + ν(C-N) |
Experimental and calculated Raman scattering data for 4,4′-BiPy.
s-strong, m-medium, w-weak intensity.
The symmetry index stands on D2 class for a free molecule.
ν-stretching; δ-in-plane bending; γ-out-of-plane bending; ν(C-C)int denotes interring C-C vibration.
Adapted from Rzeznicka et al. [52]. Copyright@Elsevier B.V.
First molecular orientation of 4,4′-BiPy in the case of long immersion time is discussed.
As expected for the polarization direction perpendicular to the surface only vibrational modes with A, B2 (xz) and B3 (yz) symmetry are observed. For the observed vibrational modes, only the “end-on” orientation does not have null Raman intensity values suggesting that the “end-on” orientation is the most plausible. The values are equally distributed over all symmetry modes, which imply that the molecule is tilted in all three directions of Au(111) surface. A presence of the Au-N stretching peak is another strong evidence to support the “end-on” orientation 4,4′-BiPy. Henceforward, we concluded that the 4,4′-BiPy molecules, in the case of long immersion time are adsorbed in a standing-up but tilted orientation, with one of two nitrogen ends anchored to Au.
In the case of short immersion time, many of the vibrational peaks seen in the long immersion time spectrum were missing. There was no Au-N stretching signal, and the B2-symmetry vibrational modes were not observed which rejects possibility of the “face-on” configuration. The peak intensities coincide with the y//Z-values of |(αij)E0|(|E0| = 1) in Table 1. For the missing signals, the calculated y//Z-value of |(αij)E0|(|E0| = 1) is zero or nearly zero. Henceforward, we concluded the 4,4′-BiPy has y//Z orientation, that is, the “edge-on” orientation, without the N atoms bonded to the Au substrate.
Our analysis is based on Raman polarizability tensor components calculated for a free molecule and on the assumption that a local electric field is perpendicular to the surface. As in the case of long immersion, chemisorption may change polarizability of the bonds, and Raman tensor elements may be different than the tensor elements calculated for a free molecule.
Figure 6a shows an STM image of the Au surface upon 30 min immersion into a neutral 1 mM ethanolic solution of 4,4′-BiPyO2. A two-dimensional overlayer, consisting of parallel rows, extending over a triangular terrace of the Au(111) surface was observed. A two-dimensional Fourier transform (2D–FFT) of the image, shown in Figure 6b, revealed the spacing between parallel rows to be 1.5 and 2.2 nm−1, respectively. The angle between stripes and the edges of the terrace was 30°. The overlayer is designated as (6 × 9) overlayer. Figure 6c shows TER signals from an Au thin film surface immersed for 6 h in a neutral 1 mM ethanolic solution of 4,4′-BiPyO2. The spectrum contains a peak at 850 cm−1, assigned to the in-plane ring vibrations and the N-O stretching vibrations, and a peak at 1190 cm−1, which draws its intensity mainly from the in-plane C-H bending vibrations. The position of these bands falls into the frequency region of the uncoordinated 4,4′-BiPyO2. The most intense band is at 1492 cm−1 followed by peaks at 1563 and 1614 cm−1, similarly to 4,4′-BiPy. No Au-N or Au-O stretching bands were found in TER spectrum, which indicated rather weak interaction of 4,4′-BiPyO2 with the Au substrate. Orientation of the 4,4′-BiPyO2 is deduced in the same manner, as done for 4,4′-BiPy.
(a) 100 × 70 nm constant current STM image of 4,4-BiPyO2 adlayer formed on Au(111) after immersion of the film in a neutral, 1 mM ethanolic solution for 6 h. (b) the two-dimensional Fourier-transform of the image. (c) TERS spectra of the overlayer. (d) Schematic representation of a (6 × 9) BiPyO2 adlayer. Adapted from Rzeznicka et al. [52]. Copyright@Elsevier B.V.
Vibrational frequencies, their symmetry modes and calculated Raman intensities are summarized in Table 2. TER spectrum for 4,4-BiPyO2, shown in Figure 6c contains three bands in A symmetry. Since neither Au-O nor Au-N vibrational modes were observed, it is more likely that a molecule has its molecular long axis parallel to the Au surface.
Experimental TERS peak position/cm−1 | Results of calculations | Modal assignment | ||||||
---|---|---|---|---|---|---|---|---|
Figure 6 | |(αij)E0|(|E0| = 1)/atomic unit | |||||||
TERS | Powder Raman | Frequency/cm−1 | Symmetry class | Mode number k | x//Z | y//Z | z//Z | |
850 w | 852 m | 858 | B1 | 24 | 0.22 | 0.22 | 0 | δ(C-H) + δ(C-C) + ν(C-N) + ν(N-O) |
1190 m | 1202 m | 1210 | A | 37 | 0.10 | 0.63 | 3.55 | δ(C-H) |
1253 w | 1266 | B3 | 39 | 0 | 0.51 | 0.51 | ν(C-C) + ν(C-N) | |
1326 m | 1300 m | 1319 | A | 40 | 0.23 | 0.51 | 7.16 | δ(C-H) + δ(C-C) + δ(C-N) +ν(C-C) + ν(C-C)int |
1492 s | 1512 m | 1499 | B3 | 46 | 0 | 0.23 | 0.23 | δ(C-H) + ν(C-C) |
1563 s | 1572 | B3 | 50 | 0 | 0.39 | 0.39 | δ(C-H) + ν(C-C) + ν(C-N) | |
1614 m | 1617 s | 1667 | A | 51 | 0.27 | 0.92 | 9.67 | δ(C-H) + δ(C-C) + δ(C-N) +ν(C-C) + ν(C-C)int + ν(C-N) + ν(N-O) |
Experimental and calculated Raman scattering data for 4,4′-BiPyO2.
s-strong, m-medium, w-weak intensity.
The symmetry index stands on D2 class for a free molecule.
ν-stretching; δ-in-plane bending; γ-out-of-plane bending; ν(C-C)int denotes interring C-C vibration.
Adapted from Rzeznicka et al. [52].Copyright@Elsevier B.V.
In Table 2, x//Z-values of |(αij)E0|(|E0| = 1) do not follow the real spectral intensity. The calculated x//Z-values are zero for the B3 bands. On the other hand, the values for y//Z and z//Z follow the observed frequencies, except for the peak at 850 cm−1. The value of |(αij)E0|(|E0| = 1) is zero for z//Z. The appearance of this 850 cm−1 band denies z//Z orientation. In conclusion, the Raman signal intensity supports the “edge-on” orientation.
This section describes particular surface chemistry leading to the growth of metal-organic surface crystals in the presence of halogen overlayer. The crystals were grown on an Au surface from ethanolic solutions of 4,4′-BiPy, in the presence of HCl. STM-TERS and ordinary Raman spectroscopy were used to reveal details of a crystal growth [55].
Figure 7a shows a large area STM image of the Au surface obtained after immersion of Au/mica film into a 4,4′-BiPy solution, adjusted with 0.1 M HCl to pH 3, for 2 days at room temperature. A zoom into the flat part of the image shows a periodic overlayer structure, shown in Figure 7b. The overlayer consists of bright stripes having a width of ~7.5 Å. The width is close to the length of 4,4-BiPy, which measures ~ 7.1 Å. Each stripe shows contrast modulation with periodicity of ~ 3 Å, as indicated in the figure. The overlayer has few dark vacancies (DV). The depth of dark vacancies is in the range of 1.3–1.5 Å. The overlayer was observed to grow along the crystallographic directions of the underlying Au(111) surface as shown in Figure 7c. By careful alternation of the tunneling current and scanning speed, another structure originated from the underlying layer was detected, as shown in Figure 7d (notice the transition at the bottom of the image). In this underlying layer, individual atoms are found to be arranged in a rectangular lattice with a unit cell of a = 5 Å. Figure 7(e, f) shows large area and a zoom image of the lattice. This atomic arrangement is assigned to the
STM images after immersion of Au slide into 1 mM ethanolic solution of 4,4-BiPy, acidified to pH = 3 with HCl: (a) 100 × 100 nm image showing well-defined overlayer; (b) 15 × 15 nm zoomed image into (a) showing a striped structure; (c) 30 × 30 nm image showing rotational domains; (d) 31 × 31 nm image showing a p(3 × 3)R30°-Cl overlayer structure; (e) zoomed 12 × 12 nm image of (d); (f) zoomed 4.7 × 4.7 nm image of (e) into the p(3 × 3)R30°-Cl overlayer. Possible molecular models of the striped structure; (g) growing on top of chlorine overlayer and (h) growing on top of surface chloride. Adapted from Rzeznicka et al. Copyright@Elsevier B.V [55].
Figure 8a shows an STM image of Au surface after prolonged immersion of Au/mica film into the acidic solution of 4,4′-BiPy. A new overlayer with a long-range order was observed as shown in Figure 8b. The overlayer consists of bright stripes with a periodicity of ~ 10 Å. A growth of the next top layers can be seen at the left side of Figure 8c. The top layer, is rotated in respect to the bottom layer, at an angle of 120°, indicating a three-dimensional growth with the Au(111) surface registry. The stripes of the top layer consist of bright protrusions with a height of 1.5–1.8 Å. A TER spectrum taken on this surface is shown in Figure 8f. In-plane vibrational modes are observed above 1000 cm−1. Six vibrational peaks are found in the spectrum: peaks at 1606, 1503, 1293, 1225, 1071 and 1017 cm−1. The observed vibrational frequencies correspond to protonated form of 4,4′-BiPy [57]. No out-of-plane modes are observed, suggesting the “edge-on” molecular orientation. Below 1000 cm−1, only a small peak at ~255 cm−1 is observed. Pettinger et al. assigned vibration at this frequency to the metal-halogen vibration of a surface complex containing metal adatom, halogen ions and pyridine [58].
STM images after prolonged immersion of an Au slide into 1 mM ethanolic solution of 4,4-BiPy, acidified to pH = 3 with HCl: (a) a 70 × 70 nm image showing a chain structure; (b) a 20 × 20 nm zoomed image of the chain structure; (c) a 14 × 14 nm zoomed image of the chain structure showing development of the next top layers; (d) a bright-field microscope image of the Au slide showing surface-grown large 3D crystals; (e) a possible molecular model of the chain structure and (f) Raman spectra. Adapted from Rzeznicka . Copyright@Elsevier B.V [55].
An optical microscopic image of the sample after a prolonged immersion into the solution is shown in Figure 8d. Rectangular shaped, 3D islands of different sizes are found on the surface. Depression defects are always seen near the islands. We speculate that these defects act as a supply of Au adatoms that are further incorporated into the crystal. Figure 8f shows a Raman spectrum taken within the area of a large 3D island using a confocal Raman unit. The spectrum above 1000 cm−1 is consistent with the Raman spectrum of a solid BiPyH2Cl2 [59]. The spectrum is similar to the TER spectrum but bands are more intense.
In contrast to TER spectrum, out-of-plane modes are also observed suggesting that molecules with “flat-on” orientation are also present. A very weak Raman signals were also observed at 2460 and 3450 cm−1. They were assigned to the N-H+…Cl−–4,4′-BiPy stretching, and free N-H stretching vibrations, respectively [60, 61]. At very low frequencies, two strong peaks at ~88 and 116 cm−1 with the shoulder at 134 cm−1 were observed. Similarly, low-frequency Raman peaks are observed in dinuclear Au complexes containing Cl, and in the case of pyridine adsorption on Ag electrodes. In dimethylgold halides, Au-Au vibrations are found at ~74 cm−1 [62]. Thus, these two peaks were assigned to Au-Au and Au-Cl stretching vibrations, respectively. The assignment was supported by the results of secondary mass ion spectrometry (SIMS) which yields information on the surface species. A highest intensity gain was observed for m/z = 465 corresponding to Au2Cl2 species [55].
In this chapter, we have reviewed the principles of STM-based TERS and discussed how molecular binding and halogen overlayer influence the intensity of the Raman signals. These two effects contribute to CE, which is system specific, that is, its magnitude depends on the metal-molecule system and experimental conditions of sample preparation. Further studies on the effects of molecular orientation on signal enhancement under more well-defined conditions, such as those provided by UHV environment, can bring us more knowledge on the mechanism of TERS.
Combined studies using cryogenic, polarized UHV-TERS and nanolithographically fabricated model nanostructures, supported by the state-of-the-art calculations to determine the Raman polarizability tensor components of a molecule-metal can lead to the formulation of TERS surface selection rules [46, 63, 64]. Home built STM-TERS systems in the Duyene and the Wang group are making first steps in this direction. We have collaborated with the company Unisoku in Japan in the development of a commercial UHV-TERS and have shown its capability to obtain relatively strong Raman signals from organic molecules adsorbed on a metal surface. Cryogenic cooling has been found to resolve issues of spectral fluctuations, as shown in Figure 9.
UHV-TER spectra of 1, 2-di-(4-pyridyl)-ethylene (BPE) at room (300 K) (left) and liquid nitrogen temperature (78 K)(right). Adapted from http://www.unisoku.com/products.
Fukumura et al. have proposed that single molecule sensitivity could be facilitated by employing vibrational excitation of molecules using inelastic scattering of tunneling electrons synchronized with the laser excitation to the excited states [65]. The technical challenge with this approach lies in the synchronization of the laser pulse with the electric pulse. The Duyene group has just started incorporating ultra-short laser pulses with UHV-TERS [66]. Apart from a purely academic interest, STM-TERS could contribute to understanding surface chemistry under ambient or solution conditions and aid in the development of large-scale metal protective organic layers. Moreover, metal leads are also important in electrical applications. It is a challenging task to minimize Ohmic losses for metal electrodes covered with thin organic films. As demonstrated in this chapter, halogen-modified surfaces could act as templates for the subsequent growth of metal-organic framework structures directly on the surfaces of metals.
Studies using electrochemical STM-TERS (EC STM-TERS) could assist in the fabrication of conductive metal/organic molecule thin films by utilizing anion-overlayers as templates for formation of well-defined organic thin films, as demonstrated here. Such organic thin films are increasingly important in the field of sensing, molecular electronics and optoelectronics. A challenge in Raman spectroscopy of organic molecules adsorbed on metal surfaces is detection of low frequency Raman signals, which give information on the chemical state of the molecule and possible metal–organic surface complex formation. Utilizing ultra-narrowband notch filters and a pinhole in front of the spectrograph slit, we recently observed signals down to 15 cm−1. Improvement in the optical density of the filters would allow for detection of Raman signal from organic molecules that have weak Raman scattering cross-sections.
In the field of lithium-ion batteries, growth of conductive metal-organic interfaces with small contact resistances and catalytic functions is very attractive but remains very challenging. New experimental setups based on the TERS idea could allow for the study of interfacial processes during battery operation. A challenge in this case is a strong fluorescence signal from various battery components such as organic electrolytes, additives, binders, and so on. In this respect, systems based on near-infrared excitation would offer elimination of the fluorescence signal. Another advantage of this approach based on hyper Raman phenomena is that Raman signals originate from the small focal volume, which allow for distinguishing the interface signals from the signals originating from the bulk.
In summary, the demand for chemical analysis with nanoscale resolution makes SPM-based TERS attractive in many fields of science and engineering. We expect that, in the next 10 years, we will witness further developments in this technique and obtain more system-specific information, which will expand our knowledge of surface chemistry and the interactions of molecules with light in confined fields. Understanding system-specific chemical enhancement will advance the field of molecular plasmonics, which is an emerging field of science exploiting the molecule-plasmon interactions to harness light at the nanoscale for nanophotonic devices.
The preservation and conservation of the environment are of great significance for healthy living. However, efforts to conserve the environment have been futile due to escalated pollution from biogenic and anthropogenic sources, which constantly release pollutants to the environment [1]. In the recent past, increased industrial and agricultural activities have immensely contributed to the pollution of aquatic environments such as rivers and streams, which pose major detrimental environmental problems to humans [2]. It is evident that industrial development has generated a myriad of new chemicals produced and applied in daily activity, which is becoming a major concern for citizens, the research community, and authorities [3]. Among the pollutant chemicals that have been introduced into the environment are polybrominated diphenyl ethers (PBDEs). PBDEs are toxic, lipophilic, hydrophobic, and persistent artificial chemicals characterized by high physical and chemical stability [4]. They are commonly applied as flame retardants in polymer products such as electronics, plastics, textiles, and building materials [5, 6]. PBDEs have become a growing concern over the last two decades due to their ubiquity, persistence and accumulation capacity in the environment, as well as their potential risks to human health and wildlife [7, 8]. PBDEs are normally additive compounds, meaning they are not covalently bound to the polymeric products [9]. Therefore, they may leach out into the surrounding environment during their production, usage, disposal, or recycling process [10]. PBDEs can be transported away from their sources for long-ranges through aqueous and/or terrestrial environmental compartments [11, 12]. In this context, monitoring and assessment of environmental pollution by these compounds are very important.
Their determination involves a series of steps from sample pre-treatment to quantification of analytes using various detection systems. Different sample preparation strategies that range from conventional to advanced strategies have been applied for the determination of PBDEs in environmental samples. Some of the conventional sample enrichment methods include Soxhlet extraction [13, 14] and liquid-liquid extraction (LLE) [15]. More recently, ultrasound-assisted extraction (UAE) [16, 17], pressurized liquid extraction (PLE) [18, 19], microwave-assisted extraction (MAE), solid-phase extraction (SPE), and solid-phase microextraction (SPME) have exhibited successful extraction of PBDEs from environmental samples [20, 21]. The application of SPE and SPME has advanced from conventional adsorbent formats to the most improved formats which allow easy transfer of analytes from their complex matrices. This has been achieved by using novel adsorbent materials to replace conventional silica-based adsorbents which exhibit low selectivity towards targeted analytes [22]. Similarly, analytical techniques for the qualitative and quantitative determination of PBDEs have advanced from well-known gas chromatography-electron capture detection (GC-ECD) to sensor-based techniques that are more advantageous in terms of excellent selectivity, with opportunities for in-situ application. The following sections provide detailed information on PBDEs, advances in sample pre-treatment methods and detection techniques with a view of providing the current state-of-the-art as far as their monitoring is concerned.
PBDEs comprise of two halogenated aromatic rings bonded by an ester bond and are classified in relation to the number and position of bromine atoms in a particular molecule [23]. They have a general molecular formula of C12H(10 - x) BrxO, where x is the number of bromine atoms in a molecule with numerical values [x = 1, 2, 3, …, 10 = m + n] (Figure 1). Substitution of bromine atoms can take place at 10 possible positions on the two benzene rings resulting in 209 possible congeners [24].
General structural formula of PBDEs.
Different congeners are easily identified by their corresponding IUPAC numbers ranging from 1 to 209. In this case, 2,2′,4,4′-tetrabromodiphenyl ether is BDE-47, with bromine atoms in ortho and para positions on the first and second benzene rings, respectively (Figure 2).
Chemical structure of 2,2′,4,4′-tetrabromodiphenyl ether (BDE 47).
Molecules with one to four bromine atoms are classified as low molecular mass PBDEs, whereas the ones with five to ten bromine atoms are categorized as high molecular mass PBDEs. Less brominated PBDEs are more persistent and toxic than highly brominated diphenyl ethers [25]. The substitution pattern also affects the physicochemical properties of PBDEs, whereby the solubility of PBDEs decreases significantly with an increase in bromine substitution. The aqueous solubility (SW) of low molecular mass PBDEs at room temperature ranges from 6.57 × 10−7 to 7.82 × 10−11 mol L−1 while those of high molecular mass have aqueous solubility values lower than 7.82 × 10−11 mol L−1 [26]. A wide range of PBDE congeners exhibit high lipophilic capacity and high resistance to degradation; a property that makes them bioaccumulate and magnify in biota [7]. PBDEs are also associated with high octanol-air partition coefficients (KOA) with values ranging between 9.3 and 12.0 from BDE-17 to -126, which is approximately 1 to 2 orders of magnitude greater than PCBs [27]. Therefore, PBDEs are easily transported through air from one point to another, increasing their chances of exposure to humans. Dissolved organic matter has shown a high tendency to interact with hydrophobic compounds such as PBDEs, which hinders their mobility and degradation in the environment [28]. Reported binding coefficients of PBDEs (log KDOC) towards organic matter range from 5.1 to 7.14, which implicates the high capability of PBDEs to adsorb and partition on organic matter [29].
PBDEs were commercially produced in three technical mixtures, typically known as pentaBDE, octaBDE, and decaBDE, basing on the number of bromine atoms [10]. By early 2000, the global production of commercial PBDE formulations was approximately 67,000 tons in the ratio 1:1.98:14.8 for octa-BDE, penta-BDE, and deca-BDE respectively, of which the United States production was approximately 50% of the global production [30]. Several governmental regulations and international environmental agencies have restricted and completely banned the use and production of some PBDE congeners [31]. In 2004, the European Union phased out the use and production of penta-BDE and octa-BDE. Consequently, in December 2004, Great Lakes Chemical Corporation, a sole manufacturer of penta-BDE and octa-BDE in North America, voluntarily phased out the production of these BDE formulations [32]. These efforts were boosted by the Stockholm Convention in 2012 when it listed commercial octa-BDE and penta-BDE among persistent organic pollutants that need to be eliminated. Despite the ban in the production of most PBDEs, they are still reported in air, soil and aquatic environments, which is attributed to their stability and subsequent release from techno-ecosystems, and production of deca-BDE, which still continues to be produced in some countries [33, 34].
There are diverse pathways by which PBDEs enter the environment. Major environmental sources of PBDE pollution comprise of leakage from consumer products and industrial facilities that synthesize PBDEs or PBDE-containing products [5]. Besides, PBDEs may enter the aquatic environment from illegal disposal of obsolete electrical appliances and electronic devices flame-retarded with PBDEs or other PBDEs-containing products [7]. They can also enter the aquatic environment through raw sewage and into the surrounding air through volatilization from products containing PBDEs and toxic fumes from e-waste recycling plants [35]. Since the first discovery of PBDEs in the aquatic environment on the West coast of Sweden in 1981, several studies have reported the presence of PBDEs in the environment [36]. This is despite the strict regulatory measures imposed by some governments and international environmental agencies to phase out some PBDE congeners and subsequent reduction in the production of particular PBDEs. BDE-47, 99, 100, and 153 are the ones that are frequently investigated because they are primary components of commercial mixtures, therefore, their ratios in the environment are expected to be significantly high. Moreover, less substituted BDE congeners such as BDE-28 and 47 are more toxic and non-biodegradable, hence their investigation in the environment and biota is of great significance in the monitoring of these pollutants [37]. Soil and sediment harbour higher concentrations of PBDEs, which is attributed to the organic carbon content, which makes them a sink for most organic pollutants [38]. Elevated levels of PBDEs have since been reported in agricultural soils after the application of sewage sludge at a concentration of 21 to 690 ng g−1 dry weight (dw) [39]. From statistics, human beings spend more than 70% of their lifetime indoors, in occupational offices, homes, learning institutions, and transport vehicles, and are therefore exposed to an array of contaminants from indoor dust [40]. The highest levels of PBDEs in dust samples have been reported in major industrialized cities in China and Europe at a concentration of 397–40,236 ng g−1 and 950–54,000 ng g−1, respectively [41, 42], with comparably lower levels of 1710 ng g−1 in African regions [43]. Table 1 presents a summary of reported PBDE levels in selected environmental matrices.
Country | Sample matrix | Concentration | Reference |
---|---|---|---|
South Africa | River water | 2.60–4.83 ng L−1 | [44] |
North America | River water | 0.00013–0.01 ng L−1 | [45] |
Great Britain | Indoor dust | 950–54,000 ng g−1 | [46] |
South Africa | Home dust | 1710 ng g−1 | [43] |
Office dust | 1520 ng g−1 | ||
Nigeria | Indoor dust | 3700–19,000 ng g−1 | [47] |
China | Indoor dust | 397–40,236 ng g−1 | [41] |
Uganda | Air | 0.00340–0.00984 ng m−3 | [48] |
Kenya | Soil | 0.19–35.64 ng g−1 | [49] |
China | Soil | 4.8–533 ng g−1 | [50] |
China | Sediment | 0.03- 5.22 ng g−1 | [51] |
China | Sediment | 0.13–1.98 ng g−1 | [52] |
Sweden | Sewage sludge | nd-450 ng g−1 | [53] |
Spain | Sewage sludge | 197–1185 ng g−1 | [39] |
Kuwait | Sewage sludge | 52.5–377* ng g−1 | [54] |
USA | Serum | 5.0–27.9 | [55] |
South Africa | Tigerfish | 5.8 | [56] |
Uganda | Breast milk | 0.59–8.11 | [57] |
Levels of PBDEs reported in the environment and biota from different locales worldwide.
Mean concentration.
nd, not detected.
The principal route for PBDE exposure to humans was thought to be through food consumption [58]. However, inhalation of contaminated indoor and outdoor dust is also a significant pathway via which human beings may be exposed to PBDEs [46, 59]. Dermal absorption is another potential route for PBDE exposure [60]. Numerous studies have reported levels up to 160.3 ng g−1 of PBDEs in human samples, such as serum and milk. Increased application of PBDEs in electronics has significantly aroused more research work on the concentration of these pollutants in the blood of workers in e-waste processing plants and other exposed populations [61]. BDE 47, 153, and 209 are the most predominant congeners reported in human serum and milk [55, 62]. The toxicity of PBDEs is backed up by numerous epidemiological studies. Scientific research has linked PBDE exposure to an array of adverse health effects [63]. To mention a few, penta- and octa-BDEs at a concentration of 10,000 ng g−1 have been associated with disruption of thyroid hormone homeostasis [7]. Moreover, penta- and tetra-BDEs, within the range of 8000–18,000 ng g−1, have been reported to affect the neurodevelopment of mice [64]. Exposure to high levels of deca-BDEs is likely to cause breast cancer [7]. PBDEs have been linked to developmental neurotoxicity and hence leading to severe effects on cognitive ability, behaviour, and health of both animals and humans [65, 66]. Several studies have also linked PBDEs with adverse effects on the human reproductive system. In particular, BDE-47, BDE-153, and BDE-154 in the range of 0.2–1.6 ng g−1 have been confirmed to have negative impacts on testosterone, luteinizing hormone, and estradiol [67]. Therefore, there is a need to have robust, accurate and reproducible methods to quantify PBDEs in different environmental matrices. The sections that follow will discuss these aspects with a particular focus on aquatic media.
Sample pre-treatment steps such as pre-concentration and clean-up are paramount before instrumental analysis [2, 68]. These steps ensure that analytes are enriched and converted into the right form/state to achieve their detection and any matrix that may interfere with the determination of the analytes is removed [69]. The choice of sample pre-treatment step is dependent on the physicochemical properties of the targeted analytes, their concentration in the environment, and the complexity of matrix interference [70, 71]. Soxhlet extraction, a traditional liquid-solid extraction method, has been used for decades in the extraction of analytes from their complex solid matrices. With the combination of polar and non-polar solvents, the Soxhlet extraction strategy has been proved to be efficacious, achieving extraction efficiencies greater than 70% [72, 73]. However, this method is hindered by several factors such as long extraction duration, excessive solvent consumption, and the need for subsequent clean-up steps [74]. With increasing demand for economical and fast sample extraction strategies with high enrichment factors, coupled with SPE clean-up procedures, techniques such as UAE, PLE, MAE, and supercritical fluid extraction (SFE) have been adopted in enrichment of analytes from solid matrices.
UAE encompasses the introduction of a finely divided sample contained in a sample holder in an ultrasonic bath with solvent and subjected to ultrasonic radiation. UAE is a vital technique in achieving sustainable green chemistry and is primarily employed in the extraction of analytes from solid sample matrices [75, 76]. This technique can achieve complete extraction with high reproducibility within a short duration. Moreover, small quantities of extraction solvents are used as compared to conventional Soxhlet extraction [77]. Methanol, acetonitrile, ethanol, and acetone are typical extractants used in this method in minimal volume. UAE based on ultrasound assisted-dispersive solid phase extraction (UAE-DSPE) coupled to GC-MS has been reported to achieve exemplary limits of detection and extraction efficiencies for 7 BDE congeners from dust samples collected from air conditioning filters in the range of 1.4–8.4 ng g−1 and 90–102%, respectively [78]. Some of the benefits of UAE include faster kinetics and an increase in extraction yield. Ultrasound can also reduce the operating temperature allowing the extraction of thermally labile compounds [79].
Unlike traditional Soxhlet extraction that consumes a large volume of solvent, PLE, also referred to as pressurized solvent extraction, has been of great interest due to its extraction effectiveness. Extraction of analytes from their environmental matrices is achieved via a synergistic mechanism that proceeds through liquid solvents at elevated temperature and pressure, which altogether enhance extraction throughput as compared with other techniques performed at ordinary atmospheric conditions [80]. PLE is viewed as another \'green\' option for traditional sample extraction methods. High temperature accomplishes a higher dispersion rate, while high pressure keeps the extraction solvent below its boiling point. During the determination of brominated flame retardants in e-waste samples, PLE and UAE were evaluated in regard to extraction efficiencies. PLE demonstrated high extraction efficiencies of 95–100% as compared to 10–50% for UAE [81]. When contrasted with the conventional methods, PLE shows a decrease in extraction time and a significant decrease in the overall consumption of organic solvents [82].
Another type of extraction technique that enables a three-fold reduction in extraction time and solvent is MAE. This is a sample extraction method that employs microwave energy to extract analytes from solid sample matrices in contact with extraction solvents. Microwave energy directly generates heat which initiates molecular motion of the analytes in the solid-solvent complex mixture, hence facilitating the mass transfer of the target analyte from the solid matrix to the extracting solvent [83, 84]. MAE has been reported to achieve good recoveries of 80–106%, 72.4–108.4%, and 80–110% in the extraction/pre-concentration of PBDEs from airborne particulate matter [85], e-waste materials [86], and sewage sludge samples [87], respectively. Compared with Soxhlet extraction, MAE achieves better recoveries and uses small amounts of solvents (30 mL versus 200 mL for Soxhlet extraction), at the same time allowing control of extraction parameters, such as extraction time and temperature [88]. However, MAE has some shortcomings, whereby the extracted sample usually contains some matrix interferences, such as lipids and lipophilic compounds, therefore, filtration and clean-up steps are required, which subsequently consume extra organic solvents.
Supercritical fluid extraction (SFE) is another method employed to extract PBDEs from solid matrices. Supercritical CO2 is often used as an extracting solvent, which has the capability of attaining recoveries above 97%. Moreover, the extraction efficiency of SFE can be further improved by the use of modifiers such as acetonitrile, toluene, and tetrahydrofuran [89]. A successful application of SFE in the extraction of PBDEs from polymeric materials was reported by Peng et al. [90]. The authors used supercritical CO2 as a solvent and SFE operating parameters such as temperature and pressure were optimized at 65°C and 20 MPa, respectively, achieving 97.6% extraction efficiency. This technique is a greener alternative to other techniques that use a large volume of solvents.
Numerous methodologies have been adopted in the determination of PBDE pollutants in liquid matrices. SPE and conventional LLE have been embraced as routine extraction techniques for PBDEs in liquid samples. The extractive capability of LLE is based on the transfer of analytes from an aqueous polar phase to a non-polar organic phase [91]. LLE coupled with GC-MS has been applied in the determination of 13 PBDEs and their metabolites in water, with recoveries of 77%-102% [92]. LLE has also been a desirable extraction method in the preparation of biota samples for the determination of PBDEs. Recently, a study aimed at assessing in utero exposure of 24 tri- to deca- BDE congeners on primiparous mothers in Kampala, Uganda reported a successful application of LLE, with appreciable recoveries of 81–91% [93]. However, LLE has some shortcomings; it suffers from low recovery, poor selectivity, high matrix interference in chromatographic analysis and increased sample loads [94]. In addition, the extraction of PBDEs from water samples requires extremely large volumes of solvents due to their hydrophobic character and low concentration in water, thus limiting its applications [95]. To overcome these challenges, different configurations of SPE have been adopted in sample enrichment strategies. SPE is a modern sample pre-treatment technique employed to concentrate analytes from liquid samples and to remove matrix interferents during the clean-up step, achieving exemplary recoveries and reproducible results over LLE [96, 97]. SPE protocols are usually performed by the use of a small column or separation cartridge packed with an appropriate sorbent material [98, 99]. Target analytes are adsorbed by the sorbent materials and later eluted with a solvent that has a greater affinity for the analytes. The chemistry behind this separation is based on intermolecular forces between the analytes, active sites of the adsorbent, and the liquid phase of the matrix [100]. SPE can be performed through an on-line or off-line approach. The on-line SPE configuration, which may enable automation, is directly coupled with specific analytical systems such as gas chromatography (GC) or high-performance liquid chromatography (HPLC). Whereas in the off-line protocol, a pre-concentration step is done separately using cartridges and further eluting the adsorbed analyte with an appropriate solvent for eventual chromatographic analysis [101]. Because of its robustness and flexibility, SPE has been widely employed in different analytical procedures in pre-concentration and clean-up steps in the determination of PBDEs [96, 102].
While SPE continues to be used because of its affordability and ease of use, other formats that offer high enrichment factors and shorter extraction times, such as SPME, stir-bar sorptive extraction (SBSE) and dispersive solid-phase extraction (DSPE), have been introduced [103]. SPME is an innovation and improvement of conventional SPE. Its stationary phase comprises of fused-silica fibers coated with a polydimethylsiloxane (PDMS) layer which are reusable. With this new formulation, the application of SPE has become versatile such that it can accommodate small volumes of samples. Furthermore, SPME has been considered an almost solvent-free extraction technique and can be easily automated as compared to conventional SPE [104, 105]. A miniaturized SPME has been applied in the extraction of PBDEs in environmental water samples followed by GC-MS quantitation, with low limits of detection and appreciable recoveries of 76.5–125.4% [106]. SBSE is a similar technique to SPME that has been adopted in the enrichment of PBDEs in liquid samples due to its improved extraction efficiency. The stir bars are coated with a thinner PDMS layer, as opposed to a thicker layer in SPME, a factor that allows improved enrichment efficiency [107, 108]. DSPE is another format of SPE based on the dispersion of solid sorbent materials in liquid samples to facilitate the isolation and extraction of target analytes from the complex sample matrix. In this process, matrix interferences remain embedded in the supernatant, which is later discarded while the target analyte is bound to the sorbent material and which is eventually eluted with a viable solvent [109]. DSPE has been employed in the enrichment and determination of PBDEs with recoveries within the range of 60–140% [110].
Complexity and matrix interferences encountered during sample preparation steps have attracted the invention of more selective sorbents to replace conventional silica sorbents that are associated with a number of drawbacks, such as instability at extreme pHs and low extraction efficiencies [111]. The new sorbents that include, nanocomposite materials, metal-organic frameworks, and molecularly imprinted polymers, among others, are characterized with high sensitivity and selectivity towards various environmental organic pollutants. They achieve fast dispersion and efficient recycling when applied in complex sample matrices [112, 113]. Reported nanocomposite sorbents in SPE for PBDE-containing samples include carbon nanotubes, graphene oxide (GO) [114, 115], and magnetic nanocomposite materials [113]. However, nanocomposite sorbents in classical SPE schemes have been associated with various drawbacks. A few of these challenges have been described in flow as well as batch systems, which originate from a slow flow rate of the sample through the packed SPE column and difficulty in separating the sorbent from the large volume of aqueous sample [113].
Other sorbent materials with fascinating properties are metal-organic frameworks (MOFs). These are hybrids of organic and inorganic materials characterized by a porous structure, large surface area, uniform nanoscale cavities, high adsorption capacity, and high thermal and chemical stability. Due to these advantageous properties, this class of materials has recently attracted enormous attention in the field of sample preparation [116]. The development of MOF adsorbents is still at its infancy stages, therefore, a limited number of studies have reported their application particularly in enrichment and determination of environmental PBDEs. A zirconium-based metal-organic framework material (UiO-66-OH) is a good example of a MOF. It has been synthesized and successfully applied as an adsorbent in SPME for enrichment and detection of 5 BDE congeners in milk samples using GC-MS, with low limits of detection in the range of 0.15–0.35 ng L−1 and excellent recoveries of 74.7%–118.0% [117]. A contrast study using silica-based sorbents in SPE for determination of 12 PBDEs in human serum, achieved mean recoveries of 64–95% and limits of detection in the range of 0.1–4.0 ng g−1 by using GC-MS [102], an evidence that MOF sorbents offer promising analytical results as compared with conventional sorbents.
With growing interest in sorbents that offer extraordinary extractive capability in SPE, molecularly imprinted polymers (MIPs) have been extensively explored as attractive options due to their robustness and selectivity towards particular target analytes providing exemplary substitute sorbents in sample clean-up and pre-concentration steps, especially in SPE and SPME [118]. MIPs are synthesized through molecular imprinting technology that involves polymerizing functional and cross-linking monomers in the presence of a target analyte, followed by the removal of the analyte to leave behind analyte-specific cavities. Their selectivity enables substantive removal of matrix interferents during the sample pre-treatment step [119]. MIP-based sorbents are readily available substitutes to silica-based adsorbents, which are reported to suffer from matrix interference, low selectivity, and sensitivity towards organic pollutants and may involve multiple steps that are labour-intensive for complete removal of interferences [120]. For example, commercial molecularly imprinted solid-phase extraction (MISPE) cartridges alongside alkaline extraction have been applied in aqueous enrichment and quantitation of PBDEs using GC-MS [121]. The extraction of PBDEs using MISPE gave recoveries above 60% compared to alkaline extraction which was below 60%. This confirms the selectivity capability of MIPs towards PBDEs from a complex environmental matrix. A more recent study has also reported recoveries of 60–87% in clean-up of soil and sediment samples using dummy molecularly imprinted polymers as SPE sorbent materials during determination of BDE-47 and BDE-99 [122].
However, a wide range of limitations still exist in MIPs, especially their poor water compatibility. Consequently, since MIPs and target analytes mainly interact through hydrogen-bonding, their recognition capability would be easily disturbed by polar solvents such as water. Therefore, the adsorption process is normally performed in non-polar or low-polar solvents such as dichloromethane and n-hexane rather than polar solvents. Additionally, polar solvents have a tendency to occupy binding sites, which affects the recognition capacity for the target analytes. In this context, it is necessary to continually invent new synthesis strategies for water-compatible MIPs [123, 124]. A summary of some of the sample pre-concentation strategies and their extraction efficiencies is presented in Table 2.
Sample preparation technique | PBDE congeners | Sample analyzed | Analytical technique | % Recoveries | Reference |
---|---|---|---|---|---|
SPE | BDE-28, 47, 49, 66, 85, 99, 100, 138, 153, 154, 183 & 209 | Human serum | GC-ECD | 64–95 | [102] |
BDE-47 and 99 | Soil and bottom sediment | GC-MS | 60–87 | [125] | |
PLE | BDE-28, 47, 99, 100, 153, 154 & 183 | Soil | GC-MS | 95 ± 9 | [68] |
BDE-28, 47, 99, 100, 154, 155 & 183 | Soil and sediment | GC-MS | 84–103 | [92] | |
LLE | BDE-17, 28, 47, 66, 71, 85, 99, 100, 138, 153, 154, 183 & 190 | Soil and sediment | GC-MS | 85–103 | [92] |
Soxhlet extraction | 42 mono- to deca-BDEs | Indoor dust sample | GC-MS | ≥ 70 | [72] |
UAE | BDE-1, 3, 7, 8, 28 & 47 | Industrial effluent | HPLC | 98.7 | [126] |
SPME | BDE-49, 99, 100, 153 & 154 | Milk and water | GC-ECD | 90–119 | [127] |
MAE | BDE-47, 99, 100, 138, 153, 154, 184 & 209 | Sewage sludge | GC-MS | 80–110 | [87] |
Examples of sample preparation strategies.
Sample pre-treatment steps are followed by quantification of the analytes using various detection systems. The choice of detection system depends on the physicochemical properties of the target analyte and the required detection levels. Detection techniques for quantification of PBDEs have evolved from liquid chromatography to gas chromatography and recently, miniaturized systems that involve the use of sensors. For chromatographic techniques, it’s important to optimize the operational parameters to actualize reliable instrumental results. It is highly recommended to use a sample injector with programmed temperature vaporization (PTV) to avoid degradation of thermally labile BDE congeners. Additionally, the temperature of injection should be accurately defined, especially when using a split/splitless injector, which minimizes chances of thermal degradation of higher BDEs congers as well as discrimination of lower brominated congeners [95, 128]. The choice of a column is another important aspect in the analysis of PBDEs where lower brominated congeners are well separated on longer columns, whereas higher brominated congeners are well separated on shorter columns. In the case of a mixture comprising of a wide range of BDE congeners, a short column is highly recommended, which well separates nona- and deca-BDEs [129]. HPLC coupled with mass spectrometry (MS), is one of the chromatographic techniques which has rarely been applied in the quantification of some PBDE congeners. The HPLC separation is hindered by several factors such as poor solubility of highly brominated diphenyl ethers in the polar solvents of the mobile phase, especially in reversed-phase, and, thus, requiring the sample to be enriched with an organic modifier. Normal phase HPLC has offered better separation of some PBDEs though it still results in incomplete separation, especially when an electrospray ionization detector is incorporated [130]. One group used an automated on-line sample preconcentration device coupled with HPLC-MS to determine decabrodiphenyl ether in human serum samples. This method achieved detection limits of 26.0 ng L−1 [130]. Otherwise, better detection limits of 0.2-25 ng L−1 were tenable when similar samples were analyzed for 12 PBDEs including decabromodiphenyl ether using gas chromatography-electron capture detection (GC-ECD) [102]. However, GC-ECD exhibits low selectivity and suffers from matrix interferences originating especially from halogenated species, as compared to GC-MS, which overcomes these challenges [131]. Fontana et al. [16] employed a coupled system, ultrasound-assisted emulsification microextraction-GC-MS (UAEMA-GC-MS) to determine PBDEs in water samples, with appreciably low detection limits of 1–2 ng L−1. Moreover, lower limits of detection are achievable when tandem-mass spectrometry (MS2) is utilized. For example, GC-MS2 has been reported to achieve detection limits within the range of 0.002–0.0136 ng g−1 lipid weight (lw) in the determination of PBDEs in breast milk and serum samples [132].
With the recent technological revolution, a more sensitive mass spectrometer, a high-resolution mass spectrometer (HRMS), has been found to be a promising alternative to a conventional mass spectrometer as it identifies the analyte without mass fragmentation and at the lowest mass unit [133]. With this new format of detection, very low detection limits of 0.000262–0.046 ng g−1 for 23 PBDEs in dust samples were achieved [134]. However, GC-HRMS is more expensive than conventional GC-MS, compelling researchers to often rely on GC-MS since it is less expensive and readily available. Besides, the demand for techniques that provide rapid results at minimal cost has resulted in the introduction of sensor technology in the determination of PBDEs. In this context, various detection systems have been fabricated and shown a discerning capability in the detection of PBDEs. For instance, an immunoassay detection system based on graphene oxide-polydimethylsiloxane has demonstrated desirable limits of detection of 0.018 ng g−1 for PBDEs in a standard solution and environmental water samples [135]. Similarly, a novel electrochemical immunoassay sensor used for the detection of BDE-28, 47, 99, 100, 153, and 154 in food samples, achieved a detection limit of 0.00018 ng L−1 [136]. These limits are comparable with those obtained by HPLC, GC-MS, or GC-HRMS. A surface-enhanced Raman scattering-based sensor is another detection system that has been successfully applied for rapid detection of BDE 47 in aqueous media, with detection limits of 0.0364 ng L−1 [137]. The use of sensory techniques is cheaper and a low concentration of contaminants can be detected. Moreover, the analysis duration is reduced from 10 minutes to 3 minutes. Thus, these sensor methods offer scope for further evaluation.
This chapter has discussed PBDEs as emerging environmental pollutants, their sources, and toxicological implications on humans and their determination in the environment. Sample pre-concentration methods for PBDE-containing samples that include UAE, PLE, UAME, PLE, SFE, SPE, SPME, SBSE, and DSPE have been critically reviewed as preferred alternatives to LLE and Soxhlet extraction due to their enhanced extraction efficiency. Novel SPE and SPME sorbents that provide the desired selectivity in the determination of PBDEs have also been discussed. Though these sorbents are promising, their application in MISPE in the determination of PBDEs has been scantly employed and its dynamics are still at its infancy stages. Therefore, there is room for continuous introduction of highly selective materials for the quantification of PBDEs in the environment. Alongside the evolution of sample pre-treatment techniques for the detection of PBDEs, rapid sensor-based techniques that achieve the desired figures of merit similar to traditional instrumentation techniques have demonstrated great potential.
ENN is grateful to the FLAIR fellowship programme, which is a partnership between the African Academy of Sciences and the Royal Society, funded by the UK Government’s Global Challenges Research Fund (GCRF), for financial support. BSM and VON thank the National Research Foundation (NRF) of South Africa, the University of KwaZulu-Natal (UKZN) and the UKZN Nanotechnology Platform for research support. BSM is also grateful for support from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 734522, and for funding from NAS, USAID and DST (South Africa), under the PEER program cooperative agreement number: No. AID-OAA-A-11-00012.
The authors declare no conflict of interest.
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