Chalcogen (including oxygen) materials analysis using available analytical techniques.
Analytical methods are needed to elucidate modern and complex compounds as well as to describe their physical properties. The underlying principles of chalcogen chemistry as well as the natural abundance of chalcogen elements are the base of building many biological substances, including sophisticated materials for future applications. Thus, the need for modern and state-of-the art analytical methods and techniques to characterize them, is obvious. In this chapter, challenges in analytical methods for chalcogen compounds and materials, as well as some examples of natural or synthesized materials or their combinations, including biomaterials, are discussed. Modern methods for chalcogen compounds analysis and structural determination discussed include: UV-Visible and infrared spectroscopy (UV-Vis and IR), thermo analysis, electrochemistry, magnetic analysis, chromatography, X-ray methods (mostly XRF, XRD, and EDX), high-resolution microscopy (SEM and TEM), multinuclear NMR, computational analysis, and bioassay. Also, the historical background and nature of chalcogen elements, including reactivity and magnetic properties as well as thermal behavior, common compounds of chalcogen elements: organic and inorganic materials, complex chalcogen materials, will be briefly discussed.
- analytical methods
- chalcogen materials
People living in this scientific era might not realize that chalcogen elements are everywhere in our lives. Especially oxygen which is essential for living, and then sulfur and selenium which are so close to the human life, as they are incorporated or form parts of essential and useful compounds and materials. Tellurium and polonium might not be inbound to life, but they are utilized for some purposes according to the new properties and attributes of material, as nanoselenium with their high active surface area and catalytic efficiency [1, 2]. The interest for chalcogen materials is growing rapidly, as seen through SCIFinder . Combinations with other elements make the compounds important for metabolism or for various dedications. The investigation on possibilities of chalcogen elements in reaction has been thoroughly discussed from the internal chemical properties of the elements . Availability of pair and lone-pair electrons offers so many possibilities of combination, naturally and synthetically. There has also been a description of the stability of organo-chalcogen compounds by intermolecular coordination because of their role in chiral induction .
Secondary metabolites and study on metabolomics have dominated the area of biology, food, agriculture, and also medicine, as well as pharmacy. One good example, is the chemistry of garlic, in which the sulfur-containing compounds called Allicin dominated the most beneficial properties of garlic as an antioxidant, antimicrobial and antifungal actions . More similar analysis on
The new materials in combination with metals and ligands with higher specification and sophisticated preparation are in demand due to great contribution for modern technology. Selenium, for example, can be immobilized through reaction to form selenium metal-humic ternary complexes or incorporated in carbohydrates as well . More related to life, thio-compounds cannot be uncounted for since they are all over the synthetically chemicals through the ligands or thiolates as well as selenolates . Synthetic compounds with metals like Fe by chalcogen arsenide  or chalcogen containing iron-carbonyl clusters  are already done. Chalcogen elements as dopant in silicon layers  or as organo-selenium or tellurium cations  are also analyzed by spectroscopy methods. Tellurium is a bigger chalcogen element and tends to have isotopic abundance, from which some are stable and some unstable .
There are several reports on chalcogen material synthesis with various synthetic and analytical methods, available in the Handbook of Chalcogen Chemistry ; however, with the emergence of new and complex compounds, analytical methods development is undoubtedly becoming an endless effort in the field of chalcogen chemistry. How the new materials are explored and investigated and also how the applications are being established and confirmed are both the new challenges of analytical chemistry. In the future, the chalcogen materials cannot be discussed within the scope of chemistry only, but must be broaden in the context of science in a more general knowledge.
2. Nature of chalcogen compounds and chalcogen materials
Chalcogens are elements which belong to the group VI-A (or group 16) in the periodic table, which consists of oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). Chalcogens are the basic elements of chalcogenides compounds, where chalcogens are combined with electropositive elements, organic radicals, in natural secondary metabolites and even in macromolecules such as enzymes and proteins [1, 18, 19]. All compounds with oxygen can be essentials and contribute most of the chalcogen materials. The materials can be as simple as silica, as an example of oxygen compounds was made for some important application  or macromolecule like cellulose or its derivates  which is applicable in daily life and will be so still in the future, to a more complicated complexed compounds [22, 23, 24, 25, 26] or advanced materials [27, 28, 29] which are made for the sake of science and method development. Abundant materials are naturally in existence with oxygen, and also a lot is being derived from synthesis in the laboratory [20, 21, 22, 23, 24, 25, 26, 30].
Some literature exclude oxygen from the chalcogenides discussion as oxygen appears in almost all materials . Oxygen and sulfur are contained in so many secondary metabolites in living things and also in bigger molecules in tissue [6, 18, 31, 32, 33, 34]. The process of changing compounds during food treatment enables one to establish new natural resources for modern herbal medicine for cancerous treatment [7, 35, 36, 37, 38, 39]. To be able to examine the biomolecular structure and medicinal properties of such compounds in detail, new analytical methods and also high-resolution instrumentation would play important roles [40, 41, 42]. Of course that pretreatment prior to analysis will be taken into account [31, 41, 43]. Extraction process which is needed before the analysis was discussed thoroughly in the prominent book of Harborne . Many sophisticated new materials are made and dedicated for use in medicinal area [35, 38, 45] or smart electronic materials , materials from nonliving beings [3, 47, 48], nanomaterials and thin films and materials for certain applications which needs special characterizations [49, 50]. The fast development of chalcogen materials cannot be separated from the development of analytical methodologies.
3. Chalcogen bonds
Most topics on chalcogen compounds and materials are related to real chemical bonding, which are either ionic or covalent bonding in one molecular building. Fewer discussions describe the weak intermolecular forces that bind molecule or macromolecule together for certain purpose. Chalcogen bonds are important in many intermolecular interactions which in turn determine the configuration and designs of bigger biomolecules . This type of bonding can be grouped as dipole–dipole interaction. In the field of biochemistry, it has much impact due to its role in bigger molecular mobility. The chalcogen bonds also form eclipse (
The intra-molecular chalcogen bonds were obtained from the X-ray diffraction results and quantum chemical calculations, such as in thioindirubin , indicating advanced methodology in modern analysis of big molecules. Chalcogen bonds were also studied with the aid of computational programs, as the development of old theories, especially in the debates around the energetic significance and physicochemical origins of the so-called class
4. Challenges in analytical methods for chalcogen compounds and materials
Natural chalcogen compounds which are present in certain matrix in perfect blends, and separation procedure must be conducted first, followed by purification prior to analysis. There are several parameters must be taken into account for the correctness of measurements. In the extraction of sulfur-containing compounds in plants, one would normally use phytochemical procedures  by considering the separation of constituent analytes according to the similarities of the compounds.
The development of new materials from chalcogen compounds due to the functionality of the materials must be supported by a better analytical methodology. When materials are dedicated to a specific purpose, then the proof to that claim might be assigned analytically too. In this case, there are two types of analytical chemistry for the assignment of chalcogen materials. In short, the need of analytical method can cover four types of analysis:
The need of pretreatment method prior to analysis, for the natural compounds in natural matrix.
The need of methods for characterization of the chalcogen compounds, as well as new materials, in practice and theoretically.
The methods to describe the application for the new materials which include several other fields of disciplines.
The need of methods validation involving more than one method and instrumentation.
All of the human efforts in the laboratory as well as in computational analysis are based on the four types of analytical objectives listed above. The emphasis must depend on the purpose or in one segment of a longer process and all point of views can count. Therefore, any bigger steps can be started from chemistry discussion and developed into a wider investigation perspective.
5. Methods for chalcogen materials characterization
Characterization method is the backbone of chalcogen chemistry material description, as it always accompanies the explanation of material properties [54, 55, 56, 57]. There are still some divisions in chalcogen material characterization, which includes the analysis for the main material itself and characterization of the impurities. The presence of impurities will decrease the quality to some extent [58, 59]. Notwithstanding, there have been too few reports discussing the impurities aspect if not related to their main functionalities.
Basic spectroscopy methods, especially X-ray methods, are the main tool for material characterization, including chalcogen materials. The methods are based on incoming X-ray beam that undergoes some natural phenomena like absorption, emission, fluorescence, and diffraction, then scattering with many possibilities to explore the chemical composition and properties of the sample. The crystallinity of materials can be derived from the X-ray diffraction patterns and the crystal database from instrument companies. In this case, the X-ray penetrates through the materials, and a number of particles can be expected to be oriented in such a way as to fulfill the Bragg’s law. Almost all crystalline compounds analysis rely on XRD spectra, such as analysis of metal complexes of metal-thiourea and metal phenyl-thiourea [60, 61] after several steps of synthesis, to determine the coordination sphere on the metal Zn(II), Co(II) and Cu(II) and the possible crystal structures. More study of the spectra confirmed the shape of crystalline compounds together with UV-Visible and infrared as well as magnetic susceptibility measurement. In other synthesis the X-ray spectra were used to calculate reactive tendency as well as shape of molecules [23, 24, 25] besides characterization. Another X-ray technique is the energy dispersive analysis by X-rays (EDX) which intensity is proportional to the amount of the elements. The method is commonly combined with scanning electron microscopy (SEM) to get pre-experimental data, before the variables are given [22, 24, 26] or to characterize and give elemental confirmation .
Nuclear magnetic resonance for solid sample can be powerful to characterize chalcogen materials. It is based on the impact of radiofrequency irradiation on specific nuclei in certain field strength of the magnet (FT-NMR), causing the nuclei to spin resulting in resonance frequency which is an indication of the atom (e.g., 77Se and 125Te) . NMR for chemist such as 1H or 13C NMR is usually the most important method for chemical structure elucidation. However, material scientists need solid-state NMR with its magic angle spinning (MAS). Moreover, when other nuclei of resonance are used, one must swing the magnetic field according to selected NMR active probe nuclei, such as 77Se and 125Te NMR [28, 29] to describe and confirm newly synthesized octahedral coordination compounds. NMR method assisted the description of how chalcogen elements (Se and Te) that can replace halogen as inner ligands in forming cluster cores in octahedral cluster complexes. This action reduces the symmetry and makes distortion on the metallic cluster as well as their isomers and of course changes the properties of the whole materials. Proton and (1H) carbon (13C) NMR of complex protein molecules were employed to investigate
Thermogravimetric analysis is a method in which changes in physical and chemical properties of materials are measured as a function of increasing temperature (with constant heating rate) or as a function of time (with constant temperature and mass loss). The important method of differential thermal analysis (DTA) one in which the mass changes is related to specific heat capacity. The increase of temperature is programmed to be linear and the heat flow to both sample and reference. Solid decomposition will occur due to phase changes, and this process can be endothermic or exothermic. One example of the methods on chalcogen compound was reported in the discussion of TGA of zinc and cadmium thiolate and selenolate complexes that showed the formation of metal sulfide (MS) and metal selenides (MSe) [M = Zn, Cd], while the mercury complexes showed complete weight loss in this temperature range . TGA is mentioned as a good method to characterize metal chalcogenides  and has become a key analytical information generating technique together with X-ray diffraction data to validate the formation on chalcogen arsenide clusters in the iron with carbonyl functional groups . In studying the degradation of palladium thiolate and selenolate, TGA was also used, to confirm the formation of Pd4S and Pd15Se17 which was then characterized by XRD and EDX .
Optical microscopy is used after the synthesis steps of chalcogen materials, which aids the visual characterization of materials. This method is improved continuously and became the earlier stage of today’s electron microscopy. Reflection mode of the instruments is preferable, and this method is called episcopic light differential interference contrast (DIC) microscopy, which enables imaging of polymer, glasses, semiconductor, metals and minerals sample with various reflective properties. DIC microscopy also has its limitation, as it gives experimental uncertainties during measurement, which is discussed in several numerical methods to minimize them .
Scanning electron microscopy (SEM), which is a technique used for a better description of materials’ surface textures up to nanometer scale, is a good way of visualizing chemistry by secondary electrons. The ability to focus the extremely small incident wavelength of the energetic electrons to resolve object in extraordinary spatial resolution, makes the method popular for nanotechnology’s purposes. Electrons are scattered very intensively compared to X-rays in both elastic and inelastic ways for both organic and inorganic materials, in dimensions less than 1 nm. While transmission electron microscopy (TEM) is a similar microscopy electron, but the image is formed from the passage of some electrons passing through thin sliced samples together with the elastic and inelastic scattering of the electrons. Thicker samples result in decreasing energy of the electron beams and increasing the scattering as well as complexity from the bigger distribution of energy and at the end declined resolution is obtained. Most of the synthetic chalcogen materials or metal chalcogenides are firstly being visualized with the aid of SEM and TEM before any other theoretical modeling or use of methods for analysis [15, 20, 24, 50, 56, 62, 68, 69, 70].
Dispersive infrared (IR) spectroscopy has been in use and became more popular with the modern nondispersive Fourier transform-infrared (FT-IR) systems to probe the presence of certain functional groups. From the energy point of view, vibrational frequencies are the base of most analyses, and rotational frequencies also count. Raman spectroscopy, on the other hand, from very different principles of spectroscopy, the scattered intensity of the absorbed energy informs the same energy absorbed by vibration. Sulfur is observable by infrared spectroscopy [66, 71, 72] by their vibrational modes, especially stretching and bending vibrational modes in solid, liquid, or gaseous phases. Fingerprint region is also important. Bulk characterization by IR was employed to analyze synthetic compounds to prove the presence of thionyl vibrational mode (
Surface characterization modes use additional probes such as attenuated total reflectance (ATR) or diffuse reflectance infrared Fourier Transformed spectroscopy (DRIFT)  or reflection absorption infrared spectroscopy (RAIRS). Sorption study of selenium(IV) solution on natural zeolites was done by infrared spectroscopy . In this case, pH and concentration of sodium selenite solution onto shabazite, analcime, stilbite, mesolite from volcanic fields were studied. Some new absorption bands from Se-O as well as Se-O-Se bridges were observed, different from original infrared spectra recorded before. The strongest changes due to the highest pH of sodium selenite were the shifted absorption of tetrahedral Al-Si-O of the natural zeolite framework downfield in alkaline situation and another band appeared that confirms the absorption state of the ions. The partial desilylation of zeolite in alkaline medium as well as dealumination of zeolites occurring in acidic solution were observed in the infrared spectrum. In thin film form materials, near infrared (NIR) analysis was used to compare the photo-response of silicon doped with Se and Te via laser irradiation . In this case, surface morphology and optical properties were accessed by NIR spectroscopy, as well as the stability of Si-chalcogen interaction. In other discussion, the vibration-rotation spectrum informs the bond length of the molecules being investigated  and process chemistry can be followed. A very minute detail of absorption energy can be useful, making this method valuable from time to time during synthesis, for calculating and determining the crystal building of the structure. Moreover, some workers have used IR spectroscopy to complement the computational calculation of new inorganic complex cluster with chalcogen elements .
Most of organic and inorganic compound or ions adsorb radiation in the ultraviolet and visible region (UV-Vis) (180–750 nm). Part of chalcogen materials also produces electronic spectra that show shape of molecules or crystal as a result of the frequency absorption bands from ligands, especially for the bands near the visible region as expected [60, 61]. Furthermore, the electronic transition in
One other important analytical method for both characterization as well as application of chalcogen materials is electrochemistry . The role of electrochemistry in synthesis, development, as well as characterization, up to applications, is obvious. This method is based on electron transfer in chemical reactions, in which metals have the most possible elements for electron storage systems. In photo-electrochemical systems, in which electron from the reaction is to be stored as energy or used for the next reaction.
Characterization of magnetic properties is also important in the study of chalcogen materials. Before, vibrating sample magnetometer was used to get information about the magnetization of samples when vibrated in a uniform magnetizing field. Magnetization is therefore induced, the product of magnetic susceptibility and the applied magnetic field provide chemical information of the materials. The specific techniques include: magnetic separation, magnetic spectroscopy, magnetic susceptibility measurement, magneto-relaxometry, magnetic particle spectroscopy, and rotating magnetic field. Some magnetic properties can be changed due to chalcogen substitution to metal iron complexed compounds [55, 75].
Dynamics in chalcogen materials is also trending in the field since it is crucial to describe the desired properties of the materials. In addition, the dynamics of materials are now core in understanding conductivity and diffusivity of the materials . Materials with ion dynamics of different substructures enable phonon scattering process in their solid state. Actually, NMR relaxation and diffusion experiments are powerful tools used to describe molecular mobility, no matter what the nuclei probe is [77, 78]. The same method can be applied to characterize chalcogen materials as well as metal chalcogenides.
Since computer is involved in most of the modern chemical analysis, the chalcogen bonds in protein are one example in this field . The computational analysis needs some unfamiliar tools and methods; however, it provides a lot of information about how molecules bind together naturally. Sulfur, selenium and tellurium are the probes of energetically favorable trends in the synthesis of chalcogen complex structures , following modeling by computational analysis. In addition, while there are more types of interaction occurring together in the biomolecules, which one cannot resolve them one by one, nonionic and noncovalent bonds are usually resolved by computational calculation. The intramolecular forces between sulfur and oxygen was also reported as the chalcogen bonds which is responsible for many bond formations in bigger molecules  is often being modeled by means of computation.
6. Methods for testing the applications of chalcogen materials
The method of applications will depend on the field of applications. The difficult part of it is to find a probe or indicator for the desired properties needed to be performed by the materials. This includes more analytical chemistry, with biological capacity or computer calculations. Suitable characterization is also essential to correlate the application and the properties of the materials.
Application of many types of chalcogen materials for environmental purposes employs infrared sensing for chalcogenides fibers  and also extended to other signal in infrared region can be utilized for environmental sensors. The manufacturing and testing of optical fiber sensors made from transparent chalcogen compounds for environmental can also be an alternative device for use in infrared spectroscopy .
For agricultural or medicinal application, bioassay is mostly used. New materials for antimicrobial properties are tested using qualitative or quantitative microbial assay [60, 61], in which pathogenic microbials were used to test the biological potentials of the compounds synthesized, as seen from disc diffusion method. The cultured microorganism in petri dishes would give clear inhibition zones around a spot of medium impregnated with stock solution of the synthesized complexes during incubation under certain conditions. Potential antibacterial activity can be further traced quantitatively. Usually, several methods are used together for the specific area of applications. Moreover, many methods can be compared one to other since some treatments are meant to be complementary to others. Method validation between more than one approach would be important. Table 1 presents some available analytical methods for characterization and testing the applications.
|Uv visible||[60, 61]||Characterization of compounds structures.|
|||For the analysis of colored compound when probing SOx and NH3 reduction.|
|IR||[13, 15, 24, 66, 68, 73]||For the characterization of functional groups after reactions, characterization of chalcogen dopants on silicon. Sorption study of|
selenium(IV) solution on natural zeolites
|||Investigation on fabrication of silicon based|
new material by near infrared analysis.
|Luminescence||||Characterization of chalcogen compounds by|
spectral luminescence study.
|XRD||[13, 20, 21, 23, 24, 51, 52, 53, 60, 61, 73]||Determination of crystallinity of materials and its combination, for characterization of intermolecular interactions, the effect of chalcogen substitution|
|NMR||[2, 5, 13, 27, 28, 61, 62, 63, 64]||Characterization of hydrogen and carbon- containing groups, also Se and Te NMR for chalcogen elements in the molecules|
|[81, 82, 83, 84, 85]||For the mobility of small molecules in porous oxide materials, testing of chalcogen material application|
|Mass spectrometer||||Characterization of compounds synthesized|
|||For the determination of volatile compounds released from the silica oxides materials, testing of chalcogen materials application|
|Atomic spectroscopy||||Quantitative measurement of selenium content in soil using HG atomic fluorescence spectroscopy|
|Optical microscopy||[18, 24, 85]||Surface texture characterization, as well as structural components of chalcogen materials.|
|Chromatography||[20, 86]||Separation of plant pigments on surface of silica materials, as one application for chalcogen compounds|
|Thermoanalysis||[3, 13, 64, 65, 80]||Characterization of chalcogen compounds, thermal clusters on chalcogen materials, formation of Zinc and cadmium sulfide and selenide, the study of degradation of palladium thiolate and selenolate|
|Magnetic analysis||[56, 60, 61, 75]||Characterization of magnetic nanoparticle of chalcogen materials, the effect of chalcogen substitution, suggestions for shape of complex metal chalcogenides together with electronic transition results.|
|Electrochemistry||||Characterizing molecules based on reactions of chalcogen compounds|
|[49, 69, 74]||Voltammetric measurement for application chalcogen doped Mo as electrode, photoelectrochemical solar cells with chalcogen materials|
|SEM||[18, 24, 48, 54, 60, 66, 67, 68]||Describing porous texture of the surface|
|EDX||[24, 64]||Describing the composition of elements of materials, characterization of Pd4S and|
|Bioassay||[60, 61]||Testing bioactivity of the chalcogen materials (thiourea).|
|Computation methods||[52, 53]||Calculation on S···O chalcogen bonds and modeling|
In modern information technology, new challenges come from big data handling too. Most recent development in the handling of chemistry data, qualitative and quantitative can provide more information about the materials. Chemometrics has also become an important analytical chemistry tool in many disciplines including chemistry and other applied chemistry fields (e.g., biochemistry and bioinorganic chemistry). The data analysis gives trends, and this hypothetical analysis creates continuity in the investigation. Thus, the challenges of analytical chemistry are real, yet exalting!
7. More accounts on development of chemistry now and for the future
It is clear that there is an increasing demand for modern materials for various industry applications . Therefore, modern analysis is needed to ensure that the synthesis of these newly made materials complies with quality attributes and satisfy their purposes. With the emergence of new devices, information technology and materials for big data handling, demand in the field of health and the pharmaceutical sector, as well as materials for application in science and the environment, are all factors bound to accelerate research to produce more types of new materials. At this point in time, chemistry is the key in technological and engineering developments, as everything can be manipulated from molecular  to structural levels. Chemistry education has the perspective of chemistry contextualization to socio-scientific orientation . Awareness of chemistry concepts and also biochemistry is crucial in chemistry teaching, since the chemistry content alone is not enough to shape up the scientific attitude. When technology reigns, without good attitude of the chemist behind that, then the society as well as the future as a whole is in danger. So, it is clear here, that chemistry education is important for the right technology for the benefit of mankind.
Last but not the least, material science is a new fascinating area of interest attracting more and more scientists around the world. The need of raw materials leading to earth exploration and exploitation especially in mining has shifted the natural equilibrium to some extent and in turn will move as a self-reorganization phenomenon which in the context of human and other living being is categorized as a disaster. In this case, scientist must work within the scope of ethics since any changes from structural level, from chemistry level can develop up to environmental level. The idea of green chemistry has been developed due to new awareness of imbalancing nature by human activity. Chemistry education has to be more “eco-reflexive”  and technology must develop the environment with responsibility.
One of the main problems analytical chemist has to face is the lack of compositional and structural information concerning chalcogens and related chemical compounds and materials. The development of efficient procedures for the synthesis, extraction, and characterization or structural determination of this class of compounds is bottleneck of each analysis, for both characterization as well as optimization and applications. Thus, the analytical protocol for such analysis usually performed with the use of a wide range of techniques, both single and hyphenated, should be designed on the basis of the need to provide required knowledge about translocation of the metal and character of its interactions with examined chalcogen materials or compounds toward establishing goal oriented method. Method development as well as validation is, therefore, crucial in shaping future technology development and application of chalcogen and chalcogenides. Analytical chemistry premised on new ideas out of human creativity is set to shape future technology for chalcogen materials.
Conflict of interest
We declare that this chapter has no “conflict of interest.”