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Lard detection using a tapered optical fiber sensor integrated with gold-graphene quantum dots
Sensing and Bio-Sensing Research 26 (2019) 100306
Contents lists available at ScienceDirect
Sensing and Bio-Sensing Research journal homepage: www.elsevier.com/locate/sbsr
Lard detection using a tapered optical fiber sensor integrated with goldgraphene quantum dots
T
C.N.H.C. Laha,b, N. Jamaludinc, F.Z. Rokhania,e, S.A. Rashidd,e, A.S.M. Noora,b,∗ a
Department of Computer and Communication Systems Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia Research Centre of Excellence for Wireless and Photonic Network, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia c Materials Processing and Technology Laboratory (Nanomaterials and Nanotechnology Group), Institute of Advanced Technology, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia d Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia e Halal Research and Product Institute, Universiti Putra Malaysia, Malaysia b
ARTICLE INFO
ABSTRACT
Keywords: Tapered fiber Fiber sensor Graphene quantum dots Lard Fluorescence
In this paper, we reported the detection of lard using tapered optical fiber integrated with graphene quantum dot (GQD). Two different sensors were fabricated and tested, one coated with GQD only as sensing element, the other was coated with gold (Au)-GQD to be tested with lard concentration ranging from 20% till 100%. The GQD coated sensor obtained a sensitivity of 0.034/a.u.% at fluorescence emission peak 652 nm. Meanwhile the AuGQD sensor, obtained higher sensitivity at 0.042/a.u.% with peak fluorescence emission at 680 nm. The proposed sensor shows a great potential of using sensor in detection of lard for future advancement of food technolog.
1. Introduction Lard, pork and not ritually slaughtered meats are forbidden for Muslims and Jews [1]. For this reason, several analytical methods either physical or chemical based-methods have been developed to identify lard [2–5]. Lard, one of the pig derivatives, is obtained from the rendering of adipose tissue of pig. In some countries, lard is one of the cheapest edible fats and oils; consequently, lard is deliberately added into the food products to reduce the production cost [2]. Therefore, this study was to detect lard in its pure form. The result could be used as a basis for detection of lard adulteration in products with complex composition. The deployment of tapered optical fiber as sensor by integrating nanomaterials around the sensing area are become popular due to its compactness, high flexibility and high sensitivity at a very reasonable cost [3]. Furthermore, remote and multiple sensing are now become norm as tapered optical fiber fabrication become less expensive and easy to make. Due to the pronounced quantum confinement and edge effects, GQD assume numerous novel chemical and physical properties [5,6], and had an extraordinary optical and electrical phenomena which are not obtainable in other kinds of quantum dots [7,8]. GQDs also show
considerably low toxicity, excellent solubility, high stability, stable photoluminescence, better surface grafting, high electrical conductivity and high thermal conductivity, thus making them promising potential in fields like fluorescent probe, optoelectronic devices, sensors and cell imaging [8–12]. This research has contributed to several significant improvements towards the existing knowledge in sensors using optical fiber coated with quantum dots. The development of a tapered optical fiber with sensitivity towards various concentration and the exploration of the fluorescence of GQD and Au-GQDs as a sensing element. It is expected that GQD with its unique fluorescence properties when integrated with tapered optical fiber and used to sense lard will lead to a new technology in fat detection in food technology. Two types of GQD were used, one as a standalone layer, the other one, with gold (Au) as to enhance the binding of protein in lard with sensor thus increasing its sensitivity. 2. Sensor fabrication and setup 2.1. Sample preparation Fats were extracted by rendering different batches of subcutaneous fat from the back part of pig's body which were obtained from several
∗ Corresponding author. Department of Computer and Communication Systems Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia. E-mail address: [email protected] (A.S.M. Noor).
https://doi.org/10.1016/j.sbsr.2019.100306 Received 24 June 2019; Received in revised form 5 October 2019; Accepted 7 October 2019 2214-1804/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Sensing and Bio-Sensing Research 26 (2019) 100306
C.N.H.C. Lah, et al.
local markets in Seri Kembangan, Selangor, Malaysia. Adipose tissues of pig (lard) were cut into small pieces, mixed, and melted at 90–100 °C for 2 h in conventional oven. The melted fat was strained through triple-folded muslin cloth, dried by the addition of anhydrous Na2SO4 and then centrifuged at 3000 rpm for 20 min. The fat layer was decanted, shaken well and centrifuged again before being filtered through Whatman paper containing Na2SO4 anhydrous to remove the trace of water residue. The filtered samples were directly subjected to chemical analysis or kept in tightly closed container under a nitrogen blanket in −20 °C. Before being used for analysis, all animal fats were thawed at water bath at 60 °C until they melt [2].
The experimental setup of the project is shown in Fig. 2. Ocean optic whitelight source (HL 2000) and spectrophotometer (USB 4000) were used as the input and detector respectively. The tapered fiber was placed in a fiber holder and the intensity of the light from the tapered fiber was captured. The Spectra Suite software captures and presents the data in a graph. 3. Results and discussion 3.1. Fluorescence intensity spectrum of GQDs coated TMMF The annealing process were applied in two different temperature, 70 °C and 100 °C. In Fig. 3 shows the change in the optical properties of the TMMF, before and after the GQDs were heated at different annealing. The strongest emission peak at around 652 nm is observed when GQDs annealed in 70 °C. Compared to 100 °C, the fluorescence was lower. The annealing process were applied as a surface modification or surface treatment to improve the sensor performance [16]. Under broad UV–Vis spectrum irradiation, the GQDs emit fluorescence, which attributed to surface defects of GQDs. Fig. 4 shows the thickness of coating GQDs on TMMF and annealed in 70 °C is around 16 ± 0.9 nm. In comparison with the as prepared sample and the sample annealed in 100 °C, the thickness of coating are 22 ± 0.9 nm and 6 ± 0.9 nm respectively. This can conclude that annealing also causes some partial hexagonal order in the stacking of the graphitic layers [17] and also undergo structural reordering [18]. However, GQDs annealed in 100 °C show a decrease in fluorescence intensity. Its is believed that annealing at high temperature, for thin regions, increases substrate conformation that leads to the apparent disappearance of the sheets [19]. This is also indicating that most of the functional groups have been removed at high annealing temperature [20].
2.2. Sensor fabrication Experiments were performed by using a commercially available standard multimode fiber (SMMF) with a core and cladding diameter of 62.5 μm and 125 μm respectively. SMMF were used instead of standard single mode fiber (SSMF) due to higher capability to gather light from the light source and have bigger core compare to SSMF in order to generate evanescent field [13]. Vytran glass processing workstation (GPX 3000 series) was used to taper the multimode fiber. Taper parameters are determined utilizing proprietary operating software. For this report, a taper with a waist diameter, W of 20 mm, a waist length, L of 10 mm, and a length of the stretching, of 5 mm (see Fig. 1) was fabricated. These parameters were optimized from experiments run earlier [14]. In this work, GQDs were prepared by one-pot synthesis method using Graphite nanofiber (GNF) as the precursor [15]. The as prepared GQDs were nanomaterials with size ranging from 3 to 20 nm. GQD shows a yellow-greenish fluorescence and it is agreed with the fluorescence peak which is at 555 nm of visible light spectrum. The fluorescence quantum yield was as high as 24.6% (excited at 356 nm). For each sample, 0.5 m of this fiber was used, both ends were connected to connectors, and 20 mm of the fiber cover at the center were tapered. The tapered fiber region were then coated with GQD using drop casting method. To do this, the tapered fiber was placed in room temperature for 40 min to completely dry process the tapered fiber. Then GQD was dropped on tapered fiber. These tapered fibers were then place in room temperature for drying process. The sensing mechanism in these sensor approaches relies on the optical intensity changes accomplished by the interaction of the evanescent field of the light with the sensing coating deposited on the surface of the structure. That is, some molecules can be trapped at the surface of the sensing coating, altering its optical properties.
3.2. Fluorescence intensity spectrum of Au-GQDs coated TMMF It has been reported that both the surface defects and the electronhole radiative recombination contribute to the fluorescence of carbonbased nanomaterials [21,22]. Therefore, it is observed that GQDs were adsorbed onto the surface of AuNPs and subsequently change the surface traps or electron-hole. Fig. 5 also points out an important transformation of fluorescence intensity for Au-GQDs coated TMMF that undergo different annealing treatment. From the results above, it can be observed that the Au-GQDs coated TMMF as prepared gave the highest fluorescence intensity emission
Fig. 1. Tapered fiber tip. 2
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Fig. 2. Experimental setup.
Fig. 5. Fluorescence intensity of Au-GQDs coated TMMF.
compared to Au-GQD annealed in 70 °C and 100 °C Fig. 5 also shows the different trends and behaviours compared to GQDs coated TMMF. This is due to presence of AuNPs coated on GQDs. It is speculated that GQDs were adsorbed onto the surface of AuNPs and subsequently change the surface traps or electron-hole radiative
Fig. 3. Fluorescence intensity spectrum of GQDs coated TMMF.
Fig. 4. The GQDs coated on TMMF annealing in (a) room temperature (b) 70 °C and (c) 100 °C. 3
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Fig. 6. Variation of spectra response based on the different lard concentration for GQDs coated TMMF and annealed in 70 °C.
Fig. 7. Variation of spectra response based on the different lard concentration for Au-GQDs coated TMMF and annealed in 100 °C.
recombination of GQDs. Therefore, the fluorescence decreases especially when annealed. Additionally, it was revealed that the fluorescence emission peak of Au-GQDs is around 660 nm for Au-GQDs coated TMMF as prepared and annealed in 70 °C and in 100 °C
Table 1 Sensitivity of GQDs (a) as prepared (b) annealed in 70 °C and (c) annealed in 100 °C. GQDs
Annealing Treatment Standard deviation Sensitivity Linearity
3.3. Spectra response based on the different lard concentration
Io = Ecl I
(4.5)
f
100 °C ± 0.0190 0.007 27%
Table 1 show the sensitivity for each coated TMMF in different annealing processed that were tested on different concentrations of lard. The standard deviation for GQDs coated TMMF as prepared, annealed in 70 °C and 100 °C are ± 0.0650, ± 0.0027 and ± 0.0190 respectively. GQDs coated TMMF annealed in 70 °C show the lowest standard deviation among others. The sensitivity of the GQDs coated TMMF and annealed in 70 °C is shown to be 0.047/a.u.% and a linearity of 81%. By comparing to GQDs coated TMMF as prepared and 100 °C, the sensitivity and linearity were 0.045/a.u.% and 66%, and 0.007/a.u. % and 27% respectively. Sensitivity and linearity for the annealing process in 70 °C shows a higher value than the other two annealing treatments. Therefore, in can be concluded that the GQDs coated TMMF in 70 °C is the most sensitive in detecting different concentration of lard from 20% to 100%. The reason for GQD annealed at 70 °C to have better sensitivity is due to the thickness of the GQD itself. Extensive work has been performed on probing the effect of high-temperature heat treatment on the properties of carbon fibers and carbon/carbon composites [17]. It has been shown that, as for graphitizable carbon materials, structural reordering occurs and some properties improve after annealing, such as an increase in the crystallite size, and better frictional properties [18]. Results on the thickness of GQDs coated on TMMF in different annealing conditions are discussed. From Table 2, the fluorescence intensity of the Au-GQDs coated TMMF also decreased when exposed to different concentrations of lard. From Table 2 also show the standard deviation. The standard deviation for Au-GQDs coated TMMF as prepared, annealed in 70 °C and 100 °C are ± 0.0034, ± 0.0041 and ± 0.0031 respectively. Au-GQDs coated
where I is intensity of transmitted light, Io is intensity of incident light, E is molecular extinction coefficient, c is concentration of lard and l is path length of sample. Based on the equation below, diluted samples affected the molecular extinction coefficient that affects the final fluorescence intensity:
FI = I° (2.303 E cl)
70 °C ± 0.0027 0.047 81%
4. Sensitivity
Since the main reason are to detect the different lard concentration, the GQDs coated TMMF section were exposed to the different levels of lard concentrations in the range of 20%–100% of lard. Fluorescence intensity spectrum of the light travelling through the TMMF was recorded as in Fig. 6. Fig. 6 represents the fluorescence intensity spectrum response of GQDs coated TMMF annealed in 70 °C towards various concentrations of lard. It can be observed from these results that the fluorescence intensity of the light received at the spectrophotometer decreases as detecting the lard concentration increases from 20% to 100% for all coating condition. As noted before, fluorescence is the phenomenon of some atoms and molecules absorbing light at a particular wavelength and to subsequently emit light of a longer wavelength. According to Beer-Lambert Law below:
log10
As prepared ± 0.0650 0.045 66%
(4.6)
Therefore, the intensity of fluorescence emission is directly proportional to the intensity of the incident radiation [24]. The fluorescence intensity will decrease with increasing concentration of the sample. Then, Fig. 7 shows the interaction between the Au-GQDs on the TMMF with different concentrations of lard changes the optical characteristic of the Au-GQDs and annealed in 10 °C. It can be observed from these results that the fluorescence intensity of the light received at the spectrophotometer decreases as the lard concentration increases. The fluorescence emission peak also shifted to a higher wavelength when the Au-GQDs coated TMMF expose to the sample. Multifluorescence components in the sample tested [25] are the reasons the shift in the fluorescence peak happened.
Table 2 Sensitivity of Au-GQDs (a) as prepared, (b) annealed in 70 °C and (c) annealed in 100 °C. Au-GQDs
Annealing Treatment Standard deviation Sensitivity Linearity
4
As prepared ± 0.0034 0.022 80%
70 °C ± 0.0041 0.045 63%
100 °C ± 0.0031 0.048 98%
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TMMF annealed in 100 °C show the lowest standard deviation among others. Additionally, the Au-GQDs coating on the TMMF annealed in 100 °C also showed high sensitivity compared to Au-GQDs coated as prepared, and annealed in 70 °C and 100 °C. It can be observed that the Au-GQDs annealed in 100 °C showed higher sensitivity. The sensitivity of the Au-GQDs coated TMMF, annealed in 100 °C is shown to be 0.048/ a.u.% lard concentration compare to coated by as prepared and annealed in 70 °C, the sensitivity were 0.022/a.u.% and 0.044/a.u.% respectively. From Table 2, lard concentration detection shows the highest linearity also on the Au-GQDs coated TMMF in 100 °C, which is 98% compared to Au-GQDs coated by as prepared and annealed in 70 °C. The linearity of lard concentration detection for Au-GQDs coated TMMF by as prepared and annealed in 70 °C were 80% and 63% respectively.
fluorescence sensor probe have been presented. It is suggested that tapered optical fiber dip coating process are used. This is because the resulting probe is more sensitive as well as enhances the interaction between the light and the coated material. Then, the concentration of GQDs and Au-GQDs were fixed in this experiment. It is believed that the concentration of sensing element will give different responses on fluorescence emission. The investigation on using different concentration of GQDs and Au-GQDs as sensing elements are needed in enhancing the sensor performance. In food production, different fats were blended to reduce the cost of production. Mixed fats such as chicken fats, tallow, beef and vegetables shortening are suggested as a sample to detect. Analysing these differences in fluorescence spectra could give insights on the protein substance existed in lard that enabling the identification of lard against other animal fats. The aim of this research is to design and fabricate tapered optical sensors for lard concentration detection. Therefore, there is limitation that should be considered which is other types of fat are not discussed in this research. In order to focus on the detection of different concentration of lard and the different sensing element used. The visibility of this work was focus on quantum dots coated on TMMF to detect the concentration of lard.
5. Conclusion In this research, quantum dots were introduced in fluorescence sensing application using TMMF. Quantum dots were used to enhance the sensing process. GQDs, the nano-size particles were synthesized using biochar, are an organic material. Then, GQDs were coated with purple solution, which is gold nanoparticles functionalized with cysteamine. The bonding reaction between N–H and C–N from carboxyl and amine group produces the GQDs coated with Au NPs. Different annealing treatments was tested to identify the sensitivity of the sensor. There are four different annealing treatments applied to GQDs and Au-GQDs in this study. The different annealing treatments performed resulted in different sensitivity. Basically, an annealing process acted as fastener between TMMF and sensing element. Annealing treatment will change the thickness of coating of GQDs and Au-GQDs respectively. Due to the reaction between the evanescence wave and fluorophore on the sensing layer of TMMF, fluorescence was emitted at the surface of the optical sensors to detect the binding events. The fluorescence emission detected by the spectrometer that collected the signals of wavelength between 200 nm and 850 nm. The study on the performance of the TMMF coated by GQDs and AuGQDs were discussed in detail. The sensitivity and linearity of fluorescence intensity emission were measured and compared. The sensitivity of different lard concentration was detected. The different lard concentrations were synthesized by adding the different volumes of hexane in different volumes of lard. Hexane was known as a solvent for fats. Therefore, five different lard concentrations were successfully produced and to be detected. This study proves that the GQDs and Au-GQDs enhancement can detect different concentrations of lard. The result from this study shows that TMMF coated with GQDs by annealing in 70 °C gave the strongest fluorescence emission peak at 652 nm. Moreover, excellent sensitivity and linearity of lard detection can be obtained using TMMF coated GQDs that underwent 70 °C annealing treatment. Their sensitivity was 0.047/a.u.% and the linearity was 81%. Meanwhile, for TMMF coated with Au-GQDs as prepared show a better fluorescence intensity emission compared to others. Lard detection can be obtained using TMMF coated Au-GQDs and annealed in 100 °C because it is gave the highest sensitivity and linearity of detection which were 0.048/a.u.% and 98% respectively. In conclusion, all the objectives stated in this thesis were achieved. This study has contributed to several significant improvements towards the existing knowledge in sensors using optical fiber coated with Quantum Dots. The findings of this study are the development of a tapered optical fiber with sensitivity towards various concentrations of lard. Besides, this study shows the exploration of the fluorescence of GQDs and Au-GQDs as a sensing element. It also investigation on coating process of quantum dots in four different conditions on tapered multimode fiber that contribute to different intensity spectrum of sensing layer. In this study, improvements in detection of lard concentration via
Conflicts of interests The authors declare that they have no competing interests. Acknowledgment The work reported in this paper supported by the Universiti Putra Malaysia's Putra Grant - Putra Graduate Initiative (IPS) (Vote Number 9499000). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.sbsr.2019.100306. References [1] K. Bonne, W. Verbeke, “Muslim consumer trust in halal meat status and control in Belgium,”, Meat Sci. 79 (1) (2008) 113–123. [2] A. Rohman, K. Triyana, Sismindari, Y. Erwanto, “Differentiation of lard and other animal fats based on triacylglycerols composition and principal component analysis,”, Int. Food Res. J. 19 (2) (2012) 475–479. [3] S.H. Girei, A.A. Shabaneh, P.T. Arasu, S. Painam, M.H. Yaacob, “Tapered multimode fiber sensor for ethanol sensing application,”, no. 2, 4th Int. Conf. Photonics, ICP 2013 - Conf. Proceeding, 2013, pp. 275–277. [4] K.L.C. Francisco J, Yanjing LIU. ARREGUI, Ignacio R. MAT´IAS, O. Richard, CLAUS, “Optical fiber humidity sensor with a fast response time using the ionic self-assembly method,”, IEICE Trans. Electron. E83–C (3) (2000) 360–365. [5] X. Wu, Y. Zhang, T. Han, H. Wu, S. Guo, J. Zhang, “Composite of graphene quantum dots and Fe3O4 nanoparticles: peroxidase activity and application in phenolic compound removal,”, RSC Adv. 4 (7) (2014) 3299. [6] D. Pan, J. Zhang, Z. Li, M. Wu, “Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots,”, Adv. Mater. 22 (6) (2010) 734–738. [7] X. Li, X. Wang, L. Zhang, S. Lee, H. Dai, “Chemically derived ultramooth graphene nanoribbon semiconductors,”, Sci. Magna 319 (5867) (2008) 1229–1232. [8] L.A. Ponomarenko, et al., “Chaotic Dirac billiard in graphene quantum dots,”, Science 320 (5874) (2008) 356–358. [9] S. Zhu, et al., “A general route to make non-conjugated linear polymers luminescent,”, Chem. Commun. 48 (88) (2012) 10889–10891. [10] X. Li, et al., “Self-assembled graphene quantum dots induced by cytochrome c: a novel biosensor for trypsin with remarkable fluorescence enhancement.,”, Nanoscale 5 (17) (2013) 7776–7779. [11] J. Zhao, G. Chen, L. Zhu, G. Li, “Graphene quantum dots-based platform for the fabrication of electrochemical biosensors,”, Electrochem. Commun. 13 (1) (2011) 31–33. [12] S. Zhu, et al., “Strongly green-photoluminescent graphene quantum dots for bioimaging applications.,”, Chem. Commun. 47 (24) (2011) 6858–6860. [13] T. Irujo, “Multimode or single-mode optical fiber,”, (2017). [14] B. Musa, Y. Mustapha Kamil, M.H. Abu Bakar, A.S.M. Noor, A. Ismail, M.A. Mahdi, “Effects of taper parameters on free spectral range of non-adiabatic tapered optical fibers for sensing applications,”, Microw. Opt. Technol. Lett. 58 (4) (Apr. 2016)
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C.N.H.C. Lah, et al. 798–803. [15] S. Abdul Rashid, S.A. Mohd Zobir, S. Krishnan, M.M. Hassan, H.N. Lim, “One-pot synthesis of graphene oxide sheets and graphene oxide quantum dots from graphite nanofibers,”, J. Nanoparticle Res. 17 (5) (2015) 1–11. [16] C. Li, Y. Yue, “Fluorescence spectroscopy of graphene quantum dots: temperature effect at different excitation wavelengths,”, Nanotechnology 25 (43) (2014). [17] M. Tortello, et al., “Effect of thermal annealing on the heat transfer properties of reduced graphite oxide fl akes : a nanoscale characterization via scanning thermal microscopy,”, 109 (2016) 390–401. [18] R. Andrews, D. Jacques, D. Qian, E.C. Dickey, “Purification and structural annealing of multiwalled carbon nanotubes at graphitization temperatures,”, Carbon N. Y. 39 (11) (2001) 1681–1687. [19] M. Alyobi, C. Barnett, R. Cobley, “Effects of thermal annealing on the properties of mechanically exfoliated suspended and on-substrate few-layer graphene,”, Crystals
7 (11) (2017) 349. [20] C.M. Chen, et al., “Annealing a graphene oxide film to produce a free standing high conductive graphene film,”, Carbon N. Y. 50 (2) (2012) 659–667. [21] S. Huang, L. Wang, C. Huang, B. Hu, W. Su, Q. Xiao, “Graphene quantum dot coupled with gold nanoparticle based ‘off-on’ fluorescent probe for sensitive and selective detection of L-cysteine,”, Microchim. Acta 183 (6) (2016) 1855–1864. [22] J. Hou, et al., “Sensitive detection of biothiols and histidine based on the recovered fluorescence of the carbon quantum dots-Hg(II) system,”, Anal. Chim. Acta 859 (2015) 72–78. [24] J.L. Robeson, R.D. Tilton, “Effect of concentration quenching on fluorescence recovery after photobleaching measurements,”, Biophys. J. 68 (5) (1995) 2145–2155. [25] X. Fei, et al., “One-pot green synthesis of flower-liked Au NP@GQDs nanocomposites for surface-enhanced Raman scattering,”, J. Alloy. Comp. 725 (2017) 1084–1090.
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Contents lists available at ScienceDirect
Sensing and Bio-Sensing Research journal homepage: www.elsevier.com/locate/sbsr
Lard detection using a tapered optical fiber sensor integrated with goldgraphene quantum dots
T
C.N.H.C. Laha,b, N. Jamaludinc, F.Z. Rokhania,e, S.A. Rashidd,e, A.S.M. Noora,b,∗ a
Department of Computer and Communication Systems Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia Research Centre of Excellence for Wireless and Photonic Network, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia c Materials Processing and Technology Laboratory (Nanomaterials and Nanotechnology Group), Institute of Advanced Technology, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia d Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia e Halal Research and Product Institute, Universiti Putra Malaysia, Malaysia b
ARTICLE INFO
ABSTRACT
Keywords: Tapered fiber Fiber sensor Graphene quantum dots Lard Fluorescence
In this paper, we reported the detection of lard using tapered optical fiber integrated with graphene quantum dot (GQD). Two different sensors were fabricated and tested, one coated with GQD only as sensing element, the other was coated with gold (Au)-GQD to be tested with lard concentration ranging from 20% till 100%. The GQD coated sensor obtained a sensitivity of 0.034/a.u.% at fluorescence emission peak 652 nm. Meanwhile the AuGQD sensor, obtained higher sensitivity at 0.042/a.u.% with peak fluorescence emission at 680 nm. The proposed sensor shows a great potential of using sensor in detection of lard for future advancement of food technolog.
1. Introduction Lard, pork and not ritually slaughtered meats are forbidden for Muslims and Jews [1]. For this reason, several analytical methods either physical or chemical based-methods have been developed to identify lard [2–5]. Lard, one of the pig derivatives, is obtained from the rendering of adipose tissue of pig. In some countries, lard is one of the cheapest edible fats and oils; consequently, lard is deliberately added into the food products to reduce the production cost [2]. Therefore, this study was to detect lard in its pure form. The result could be used as a basis for detection of lard adulteration in products with complex composition. The deployment of tapered optical fiber as sensor by integrating nanomaterials around the sensing area are become popular due to its compactness, high flexibility and high sensitivity at a very reasonable cost [3]. Furthermore, remote and multiple sensing are now become norm as tapered optical fiber fabrication become less expensive and easy to make. Due to the pronounced quantum confinement and edge effects, GQD assume numerous novel chemical and physical properties [5,6], and had an extraordinary optical and electrical phenomena which are not obtainable in other kinds of quantum dots [7,8]. GQDs also show
considerably low toxicity, excellent solubility, high stability, stable photoluminescence, better surface grafting, high electrical conductivity and high thermal conductivity, thus making them promising potential in fields like fluorescent probe, optoelectronic devices, sensors and cell imaging [8–12]. This research has contributed to several significant improvements towards the existing knowledge in sensors using optical fiber coated with quantum dots. The development of a tapered optical fiber with sensitivity towards various concentration and the exploration of the fluorescence of GQD and Au-GQDs as a sensing element. It is expected that GQD with its unique fluorescence properties when integrated with tapered optical fiber and used to sense lard will lead to a new technology in fat detection in food technology. Two types of GQD were used, one as a standalone layer, the other one, with gold (Au) as to enhance the binding of protein in lard with sensor thus increasing its sensitivity. 2. Sensor fabrication and setup 2.1. Sample preparation Fats were extracted by rendering different batches of subcutaneous fat from the back part of pig's body which were obtained from several
∗ Corresponding author. Department of Computer and Communication Systems Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia. E-mail address: [email protected] (A.S.M. Noor).
https://doi.org/10.1016/j.sbsr.2019.100306 Received 24 June 2019; Received in revised form 5 October 2019; Accepted 7 October 2019 2214-1804/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Sensing and Bio-Sensing Research 26 (2019) 100306
C.N.H.C. Lah, et al.
local markets in Seri Kembangan, Selangor, Malaysia. Adipose tissues of pig (lard) were cut into small pieces, mixed, and melted at 90–100 °C for 2 h in conventional oven. The melted fat was strained through triple-folded muslin cloth, dried by the addition of anhydrous Na2SO4 and then centrifuged at 3000 rpm for 20 min. The fat layer was decanted, shaken well and centrifuged again before being filtered through Whatman paper containing Na2SO4 anhydrous to remove the trace of water residue. The filtered samples were directly subjected to chemical analysis or kept in tightly closed container under a nitrogen blanket in −20 °C. Before being used for analysis, all animal fats were thawed at water bath at 60 °C until they melt [2].
The experimental setup of the project is shown in Fig. 2. Ocean optic whitelight source (HL 2000) and spectrophotometer (USB 4000) were used as the input and detector respectively. The tapered fiber was placed in a fiber holder and the intensity of the light from the tapered fiber was captured. The Spectra Suite software captures and presents the data in a graph. 3. Results and discussion 3.1. Fluorescence intensity spectrum of GQDs coated TMMF The annealing process were applied in two different temperature, 70 °C and 100 °C. In Fig. 3 shows the change in the optical properties of the TMMF, before and after the GQDs were heated at different annealing. The strongest emission peak at around 652 nm is observed when GQDs annealed in 70 °C. Compared to 100 °C, the fluorescence was lower. The annealing process were applied as a surface modification or surface treatment to improve the sensor performance [16]. Under broad UV–Vis spectrum irradiation, the GQDs emit fluorescence, which attributed to surface defects of GQDs. Fig. 4 shows the thickness of coating GQDs on TMMF and annealed in 70 °C is around 16 ± 0.9 nm. In comparison with the as prepared sample and the sample annealed in 100 °C, the thickness of coating are 22 ± 0.9 nm and 6 ± 0.9 nm respectively. This can conclude that annealing also causes some partial hexagonal order in the stacking of the graphitic layers [17] and also undergo structural reordering [18]. However, GQDs annealed in 100 °C show a decrease in fluorescence intensity. Its is believed that annealing at high temperature, for thin regions, increases substrate conformation that leads to the apparent disappearance of the sheets [19]. This is also indicating that most of the functional groups have been removed at high annealing temperature [20].
2.2. Sensor fabrication Experiments were performed by using a commercially available standard multimode fiber (SMMF) with a core and cladding diameter of 62.5 μm and 125 μm respectively. SMMF were used instead of standard single mode fiber (SSMF) due to higher capability to gather light from the light source and have bigger core compare to SSMF in order to generate evanescent field [13]. Vytran glass processing workstation (GPX 3000 series) was used to taper the multimode fiber. Taper parameters are determined utilizing proprietary operating software. For this report, a taper with a waist diameter, W of 20 mm, a waist length, L of 10 mm, and a length of the stretching, of 5 mm (see Fig. 1) was fabricated. These parameters were optimized from experiments run earlier [14]. In this work, GQDs were prepared by one-pot synthesis method using Graphite nanofiber (GNF) as the precursor [15]. The as prepared GQDs were nanomaterials with size ranging from 3 to 20 nm. GQD shows a yellow-greenish fluorescence and it is agreed with the fluorescence peak which is at 555 nm of visible light spectrum. The fluorescence quantum yield was as high as 24.6% (excited at 356 nm). For each sample, 0.5 m of this fiber was used, both ends were connected to connectors, and 20 mm of the fiber cover at the center were tapered. The tapered fiber region were then coated with GQD using drop casting method. To do this, the tapered fiber was placed in room temperature for 40 min to completely dry process the tapered fiber. Then GQD was dropped on tapered fiber. These tapered fibers were then place in room temperature for drying process. The sensing mechanism in these sensor approaches relies on the optical intensity changes accomplished by the interaction of the evanescent field of the light with the sensing coating deposited on the surface of the structure. That is, some molecules can be trapped at the surface of the sensing coating, altering its optical properties.
3.2. Fluorescence intensity spectrum of Au-GQDs coated TMMF It has been reported that both the surface defects and the electronhole radiative recombination contribute to the fluorescence of carbonbased nanomaterials [21,22]. Therefore, it is observed that GQDs were adsorbed onto the surface of AuNPs and subsequently change the surface traps or electron-hole. Fig. 5 also points out an important transformation of fluorescence intensity for Au-GQDs coated TMMF that undergo different annealing treatment. From the results above, it can be observed that the Au-GQDs coated TMMF as prepared gave the highest fluorescence intensity emission
Fig. 1. Tapered fiber tip. 2
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Fig. 2. Experimental setup.
Fig. 5. Fluorescence intensity of Au-GQDs coated TMMF.
compared to Au-GQD annealed in 70 °C and 100 °C Fig. 5 also shows the different trends and behaviours compared to GQDs coated TMMF. This is due to presence of AuNPs coated on GQDs. It is speculated that GQDs were adsorbed onto the surface of AuNPs and subsequently change the surface traps or electron-hole radiative
Fig. 3. Fluorescence intensity spectrum of GQDs coated TMMF.
Fig. 4. The GQDs coated on TMMF annealing in (a) room temperature (b) 70 °C and (c) 100 °C. 3
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Fig. 6. Variation of spectra response based on the different lard concentration for GQDs coated TMMF and annealed in 70 °C.
Fig. 7. Variation of spectra response based on the different lard concentration for Au-GQDs coated TMMF and annealed in 100 °C.
recombination of GQDs. Therefore, the fluorescence decreases especially when annealed. Additionally, it was revealed that the fluorescence emission peak of Au-GQDs is around 660 nm for Au-GQDs coated TMMF as prepared and annealed in 70 °C and in 100 °C
Table 1 Sensitivity of GQDs (a) as prepared (b) annealed in 70 °C and (c) annealed in 100 °C. GQDs
Annealing Treatment Standard deviation Sensitivity Linearity
3.3. Spectra response based on the different lard concentration
Io = Ecl I
(4.5)
f
100 °C ± 0.0190 0.007 27%
Table 1 show the sensitivity for each coated TMMF in different annealing processed that were tested on different concentrations of lard. The standard deviation for GQDs coated TMMF as prepared, annealed in 70 °C and 100 °C are ± 0.0650, ± 0.0027 and ± 0.0190 respectively. GQDs coated TMMF annealed in 70 °C show the lowest standard deviation among others. The sensitivity of the GQDs coated TMMF and annealed in 70 °C is shown to be 0.047/a.u.% and a linearity of 81%. By comparing to GQDs coated TMMF as prepared and 100 °C, the sensitivity and linearity were 0.045/a.u.% and 66%, and 0.007/a.u. % and 27% respectively. Sensitivity and linearity for the annealing process in 70 °C shows a higher value than the other two annealing treatments. Therefore, in can be concluded that the GQDs coated TMMF in 70 °C is the most sensitive in detecting different concentration of lard from 20% to 100%. The reason for GQD annealed at 70 °C to have better sensitivity is due to the thickness of the GQD itself. Extensive work has been performed on probing the effect of high-temperature heat treatment on the properties of carbon fibers and carbon/carbon composites [17]. It has been shown that, as for graphitizable carbon materials, structural reordering occurs and some properties improve after annealing, such as an increase in the crystallite size, and better frictional properties [18]. Results on the thickness of GQDs coated on TMMF in different annealing conditions are discussed. From Table 2, the fluorescence intensity of the Au-GQDs coated TMMF also decreased when exposed to different concentrations of lard. From Table 2 also show the standard deviation. The standard deviation for Au-GQDs coated TMMF as prepared, annealed in 70 °C and 100 °C are ± 0.0034, ± 0.0041 and ± 0.0031 respectively. Au-GQDs coated
where I is intensity of transmitted light, Io is intensity of incident light, E is molecular extinction coefficient, c is concentration of lard and l is path length of sample. Based on the equation below, diluted samples affected the molecular extinction coefficient that affects the final fluorescence intensity:
FI = I° (2.303 E cl)
70 °C ± 0.0027 0.047 81%
4. Sensitivity
Since the main reason are to detect the different lard concentration, the GQDs coated TMMF section were exposed to the different levels of lard concentrations in the range of 20%–100% of lard. Fluorescence intensity spectrum of the light travelling through the TMMF was recorded as in Fig. 6. Fig. 6 represents the fluorescence intensity spectrum response of GQDs coated TMMF annealed in 70 °C towards various concentrations of lard. It can be observed from these results that the fluorescence intensity of the light received at the spectrophotometer decreases as detecting the lard concentration increases from 20% to 100% for all coating condition. As noted before, fluorescence is the phenomenon of some atoms and molecules absorbing light at a particular wavelength and to subsequently emit light of a longer wavelength. According to Beer-Lambert Law below:
log10
As prepared ± 0.0650 0.045 66%
(4.6)
Therefore, the intensity of fluorescence emission is directly proportional to the intensity of the incident radiation [24]. The fluorescence intensity will decrease with increasing concentration of the sample. Then, Fig. 7 shows the interaction between the Au-GQDs on the TMMF with different concentrations of lard changes the optical characteristic of the Au-GQDs and annealed in 10 °C. It can be observed from these results that the fluorescence intensity of the light received at the spectrophotometer decreases as the lard concentration increases. The fluorescence emission peak also shifted to a higher wavelength when the Au-GQDs coated TMMF expose to the sample. Multifluorescence components in the sample tested [25] are the reasons the shift in the fluorescence peak happened.
Table 2 Sensitivity of Au-GQDs (a) as prepared, (b) annealed in 70 °C and (c) annealed in 100 °C. Au-GQDs
Annealing Treatment Standard deviation Sensitivity Linearity
4
As prepared ± 0.0034 0.022 80%
70 °C ± 0.0041 0.045 63%
100 °C ± 0.0031 0.048 98%
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TMMF annealed in 100 °C show the lowest standard deviation among others. Additionally, the Au-GQDs coating on the TMMF annealed in 100 °C also showed high sensitivity compared to Au-GQDs coated as prepared, and annealed in 70 °C and 100 °C. It can be observed that the Au-GQDs annealed in 100 °C showed higher sensitivity. The sensitivity of the Au-GQDs coated TMMF, annealed in 100 °C is shown to be 0.048/ a.u.% lard concentration compare to coated by as prepared and annealed in 70 °C, the sensitivity were 0.022/a.u.% and 0.044/a.u.% respectively. From Table 2, lard concentration detection shows the highest linearity also on the Au-GQDs coated TMMF in 100 °C, which is 98% compared to Au-GQDs coated by as prepared and annealed in 70 °C. The linearity of lard concentration detection for Au-GQDs coated TMMF by as prepared and annealed in 70 °C were 80% and 63% respectively.
fluorescence sensor probe have been presented. It is suggested that tapered optical fiber dip coating process are used. This is because the resulting probe is more sensitive as well as enhances the interaction between the light and the coated material. Then, the concentration of GQDs and Au-GQDs were fixed in this experiment. It is believed that the concentration of sensing element will give different responses on fluorescence emission. The investigation on using different concentration of GQDs and Au-GQDs as sensing elements are needed in enhancing the sensor performance. In food production, different fats were blended to reduce the cost of production. Mixed fats such as chicken fats, tallow, beef and vegetables shortening are suggested as a sample to detect. Analysing these differences in fluorescence spectra could give insights on the protein substance existed in lard that enabling the identification of lard against other animal fats. The aim of this research is to design and fabricate tapered optical sensors for lard concentration detection. Therefore, there is limitation that should be considered which is other types of fat are not discussed in this research. In order to focus on the detection of different concentration of lard and the different sensing element used. The visibility of this work was focus on quantum dots coated on TMMF to detect the concentration of lard.
5. Conclusion In this research, quantum dots were introduced in fluorescence sensing application using TMMF. Quantum dots were used to enhance the sensing process. GQDs, the nano-size particles were synthesized using biochar, are an organic material. Then, GQDs were coated with purple solution, which is gold nanoparticles functionalized with cysteamine. The bonding reaction between N–H and C–N from carboxyl and amine group produces the GQDs coated with Au NPs. Different annealing treatments was tested to identify the sensitivity of the sensor. There are four different annealing treatments applied to GQDs and Au-GQDs in this study. The different annealing treatments performed resulted in different sensitivity. Basically, an annealing process acted as fastener between TMMF and sensing element. Annealing treatment will change the thickness of coating of GQDs and Au-GQDs respectively. Due to the reaction between the evanescence wave and fluorophore on the sensing layer of TMMF, fluorescence was emitted at the surface of the optical sensors to detect the binding events. The fluorescence emission detected by the spectrometer that collected the signals of wavelength between 200 nm and 850 nm. The study on the performance of the TMMF coated by GQDs and AuGQDs were discussed in detail. The sensitivity and linearity of fluorescence intensity emission were measured and compared. The sensitivity of different lard concentration was detected. The different lard concentrations were synthesized by adding the different volumes of hexane in different volumes of lard. Hexane was known as a solvent for fats. Therefore, five different lard concentrations were successfully produced and to be detected. This study proves that the GQDs and Au-GQDs enhancement can detect different concentrations of lard. The result from this study shows that TMMF coated with GQDs by annealing in 70 °C gave the strongest fluorescence emission peak at 652 nm. Moreover, excellent sensitivity and linearity of lard detection can be obtained using TMMF coated GQDs that underwent 70 °C annealing treatment. Their sensitivity was 0.047/a.u.% and the linearity was 81%. Meanwhile, for TMMF coated with Au-GQDs as prepared show a better fluorescence intensity emission compared to others. Lard detection can be obtained using TMMF coated Au-GQDs and annealed in 100 °C because it is gave the highest sensitivity and linearity of detection which were 0.048/a.u.% and 98% respectively. In conclusion, all the objectives stated in this thesis were achieved. This study has contributed to several significant improvements towards the existing knowledge in sensors using optical fiber coated with Quantum Dots. The findings of this study are the development of a tapered optical fiber with sensitivity towards various concentrations of lard. Besides, this study shows the exploration of the fluorescence of GQDs and Au-GQDs as a sensing element. It also investigation on coating process of quantum dots in four different conditions on tapered multimode fiber that contribute to different intensity spectrum of sensing layer. In this study, improvements in detection of lard concentration via
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