Fourier Transform-Infrared Spectroscopy (FTIR) is an analytical technique used to identify organic materials. Some of the preparation techniques like KBr pellet are time-consuming and destructive extraction procedures are involved. But recent studies show an attachment of Attenuated total reflection (ATR) unit with IR spectroscopy that eliminates above drawbacks. It is a sampling technique used in conjunction with infrared spectroscopy, which enables samples to be examined directly in the solid or liquid state without further preparation. Very few studies have been done using ATR-FTIR on silk for identification of functional groups and to identify the fibre. Therefore, in this study, identification of silk materials using infrared spectral analysis (ATR-FTIR), as a tool for detection of functional groups has been carried out. This method requires minimal sample preparation by ATR sampling on a small piece of material, permits routine analysis rapidly and non-destructively, and is easy to operate also eliminates environment pollution. Raw, degummed and dyed silk show functional groups characteristic of silk and dyed silk using direct, acid and metal-complex showed amino group and hydroxyl group indicating slight change in the spectra. Spectra of commercial silk sarees resulted to show a characteristic band of silk similar to raw and degummed silk. Comparing mulberry and non-Mulberry spun silk, spectra of functional groups have similar basic protein molecules irrespective of varying sericin and amino acid molecules. Nylon showed different IR spectra that can be differentiated from Silk and differentiating wool requires additional method to reduce analyst strain.
Key Words: Silk, Mulberry, ATR-FTIR, Zari, IR spectra,
SILK fibre consists of fibroin and a soluble protein-rich gum, sericin (Robson, 1985). The major content of silk is a protein called Fibroin and it is a sequence of simple amino acids, glycine, alanine and serine. The dyes used on silk for dyeing vary widely in chemical constitution, they possess similar dyeing properties within each range. Silk offers a wide range of possible colors covering almost the entire spectrum of colors and hues, because the nature of silk makes it easily dyed. The exceptional capacity to absorb moisture from the air, the comparatively simple and orderly arrangement of fibroin molecular structure, and the abundance of hydrogen and electrostatic bonds render silk fiber, an ideally suited substrate, with a very good dye affinity. Anionic dyestuffs, namely acid and direct dyes, form a 'Dye- Fiber' complex by electrostatic and hydrogen bonds. Silk can also be dyed with basic, metal-complex and reactive dyes. Acid dyes are widely used for dyeing silk. Using this class of dyestuff, a wide range of bright colors can be obtained. These dyes are composed of sodium salts of organic acids (mostly sulphonic acid) and are applied from an acidic medium. (Sargunamani and Selvakumar 2012). Fourier Transform Infrared (FTIR) measurements were used to characterize natural dyes and traditional silk fabric dyed with plant extracts dyes avoiding the time-consuming and destructive extraction procedures necessary for the spectrophotometric and chromatographic methods previously used. Silk textiles dyed with plant extracts were then analyzed for chemical and functional group identification of their dye components and mordants (Jihye Lee et al 2013, Navarro et al., 2008). An approach to achieve a broader assessment of chemical diversity is to employ complementary spectroscopic and scattering techniques (Gheysens et al., 2011; Warwicker, 1954), which is Attenuated Total Reflection Infrared Spectroscopy (ATR-IR) for studying silks in all forms (Sander De Bruyne et al 2017, Chen et al., 2012; Gheysens et al., 2011). An advantage of FT-IR spectroscopy is its capability to identify functional groups such as C=O, C-H or N-H. Most substances show a characteristic spectrum that can be directly recognized. FT-IR spectroscopy enables measuring all types of samples: solids, liquids and gases. These functional groups are present in Silk fibre and become easy to identify using ATR-IR method. This requires minimal sample preparation can selectively probe inside and/or outside surface of silk, providing information on the local chemical composition (Boulet-Audet et al., 2014; Chen et al., 2102). It uses a property of total internal reflection resulting in an evanescent wave. A beam of infrared light passes through the ATR crystal in such a way that it reflects at least once off the internal surface in contact with the sample.
This reflection forms the evanescent wave which extends into the sample. The penetration depth into the sample is typically between 0.5 and 2 micrometres, with the exact value being determined by the wavelength of light, the angle of incidence and the indices of refraction for the ATR crystal and the medium being probed. The number of reflections may be varied by varying the angle of incidence. The beam is then collected by a detector as it exits the crystal. Most modern infrared spectrometers can be converted to characterise samples via ATR by mounting the ATR accessory in the spectrometer's sample compartment. The accessibility of ATR-FTIR has led to substantial use by the scientific community. (Sander De Bruyne et al 2017, S.G.Kazarian, K.L.A.Chan,2006), (https://en.wikipedia.org/wiki/ Attenuatedtotalreflectance). Literature cites few studies, identification of functional groups of silk cocoons using ATR-FTIR, which are pre-cocoon related. Based on Post Cocoon Technology, the Silk yarn in the form of raw, degummed and dyed yarn and fabric is almost unavailable. Therefore, to fulfil the gap, the study is taken up for analysis of silk materials and other fibres using ATR-FTIR spectroscopy for individual and comparative analysis of IR spectra. The spectral bands due to various functional groups have been reported.
Materials and methods
Raw silk and degummed silk yarn 20-22 denier, 2 ply yarn from Bengaluru is used. Silk yarn dyed with various dyes (Acid, Direct, Metal complex) using commercial methods have been used. Commercial silk sarees received from one of the societies under The Department of Handloom and Textiles, Kancheepuram having plain design, and with pure zari are considered. Non-mulberry silk such as Tassar, Muga, Eri spun yarn and other Wool and Nylon are collected from CSTRI, Bengaluru. ATR-FTIR spectroscopy (ALPHA spectrometer is a small, compact FT-IR spectrometer (Bruker make) designed for routine applications in the laboratory) is used for identification of functional groups using the OPUS software at CSTRI, Bengaluru. Samples are placed on the crystal (and after each sample cleaned thoroughly) of ATR for IR spectra analysis without any preparation and the resulting spectra of them were corrected for background air absorbance. The spectra were recorded using opus software and WordPad were measured in the region of wave number 4000 – 400 cm-1 / 4000 – 1000 cm-1 each spectrum was measured 22 times, at resolution 4. To minimize differences which were due to the baseline shifts, the spectra were baseline corrected and ATR-corrected.
Results and discussion
Figure 1 shows IR spectra of mulberry raw silk, degummed silk and dyed silk (Acid dyed - Arakku). To evaluate the chemical variability in silk, only the strongest, best defined and most frequently occurring peaks, which occurred consistently, have been considered. The figure confirms the presence of functional groups assigned is indicated for raw and degummed silk by 3300 cm-1 for N-H hydrogen bond, 3080 cm-1 for N-H stretching vibrations, 3000 cm-1 for C-H and CH2 , 2950 cm-1 for CH3, 1650 cm-1 for C=O stretching vibrations, 1530cm-1 for N-H deformation, 2350-3333 cm-1 for OH, 1240 cm-1 for C-N Stretching Vibrations and 1170 cm-1 for C-C linkages. The study results are evidenced with similar functional groups of silk using ATR-FTIR work done by Signe Vahur et al. (2016) in which, it is reported spectral collection of over 150 IR spectra of materials measured in the extended region of 4000-80 cm–1 (mid-IR and far-IR region).
Figure 2 shows IR spectra of dyed silk yarn using Acid violet. It can be observed that change in spectra band at 2400 cm-1 indicating decreased OH groups may be due to dye molecule interaction. Spectra of dyed silk using Direct dye – Mustard and Metal Complex dyes are shown in Figure 3 and 4 respectively. The spectra of Direct dye silk show little difference while metal-complex dyed silk has shown changes in spectra band at 2400 cm-1 indicating decreased OH groups may be due to dye molecule interaction. The structural conformational analysis evaluated for silk using FTIR for the range 4000 -500 cm-1 and it is reported that the degumming does not alter the secondary and crystalline structure of fibroin,(T Asakura et al 1985; X.Chen et al 2001). The dyeing of silk depends on free amino and carboxyl groups and any other OH group. Silk is slightly cationic in the character having an isoelectric point of pH is > 5 and therefore many dyes such as acid, direct, metal-complex etc are used in this study show very little difference. It can be observed that, though IR spectra is broadened for amino groups in the region 3300-3500 cm-1 and 2400 – 3550 cm-1 of OH group which is less for direct, acid and metal complex colours indicating dye interaction effect.
In the dyeing of silk with acid dyes, the dye anion bonds with NH3+ radicals produced from NH2 groups, which are available at the side chains and terminals of the silk molecules and the bulk of the dye adsorption occurs at the side chains Masaru Mitsuishi and Hiroshi Kato . Metal-Complex dyes having sulphonyl groups combine with protonated group of silk by electrostatic forces. Coordinate bonds may also form between chromium atom and suitable groups (Hydroxy, Amino etc) of Silk. It may be reasoned that the dye molecules adsorbed at amino groups have little effect on resulting the spectra so that slight change in the spectra. Jihye Lee et al (2013) reported that FTIR Technique was used to characterize commercial natural dyes on silk fabric dyed with plant extract dyes. In the study, results show that the similar trend spectrum is obtained for all dyed silk.
Figure 5 shows spectra of commercial silk sarees indicating functional groups as discussed above for silk yarn of amino groups in the region from 3300-3500 cm-1 and OH group from 2400 – 3550 cm-1 observed that all the dyed sarees except a different spectra at the bottom of silk containing zari content (Metal like Gold and Silver).
Figure 6 shows, Spectra of Spun silk yarn made up of Mulberry and Non-mulberry silk (Tassar, Muga and Eri). Table 1 shows Peaks assigned for various functional groups of various samples. It can be seen that silk spectra of various functional groups showing the very less difference, though the basic protein molecules irrespective of varying sericin content and other amino acid vary to different extent.
Figure 7 shows spectra of zari thread used in Silk sarees indicating spectra to be different almost functional groups could not be seen since it is made up of metal (gold, silver and copper).
Figure 8 shows, spectra of Wool, Silk and Nylon. It can be seen that, wave number from 2300 – 3400 cm-1 of change in spectra NH and OH groups may be observed for Nylon and Wool functional groups peaks and the band looks narrowed in wool than Silk.
ATR-FTIR method of identifying functional groups of fibres is much useful since it is non-destructive and quick method of measuring samples that customers need. This preliminary study indicates that possibility of identifying Silk from other fibres and repeatability of IR spectra of the same sample is difficult, that depends on various factors such as IR Light absorbed, Silk sample chosen to cut and place containing crystalline and amorphous region at different places. Differentiating silk type (mulberry and non-mulberry spun) spectra is very little/partially possible, since basic protein molecules and functional groups, irrespective of varying sericin content and amino acid molecule are same for all type of silk. Spectra of yarn dyed using direct, acid and metal-complex has shown change in spectra amino groups and hydroxy groups. Commercial silk sarees dyed have resulted to show functional groups which are the characteristic bands of silk, further a detail in depth research is essential for confirming the repeatability of IR spectra and standardizing dyed silk. Zari content of the saree has shown different spectra as it contains metal. Comparing wool, nylon and silk spectra, results showed that silk can be differentiated.
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D.Sargunamani, K.Raghu, Subhas V Naik
Silk Conditioning & Testing House, Central Silk Technological Research Institute, Central Silk Board, Kancheepuram-631502, India