tapered plastic optical fiber coated with graphene for uric acid detection

1704 IEEE SENSORS JOURNAL, VOL. 14, NO. 5, MAY 2014

Tapered Plastic Optical Fiber Coated WithGraphene for Uric Acid Detection

Malathy Batumalay, Sulaiman Wadi Harun, Fauzan Ahmad, Roslan Md Nor,Nurul R. Zulkepely, and Harith Ahmad

Abstract— A simple tapered plastic optical fiber (POF) sensoris proposed and demonstrated for the detection of uric acidconcentrations in deionized water. The sensor uses a taperedPOF probe coated with different concentrations of graphenein a polymer composite. The tapered fiber is fabricated usingan etching method and has a waist diameter of 0.45 mm andtapering length of 10 mm. The coating improves the sensitivityof the proposed sensor as it changes the effective refractive indexof the cladding and allows more lights to be transmitted fromthe tapered fiber. The probe is immersed in uric acid solutionand it senses the relative acid concentration using intensitymodulation technique. As the uric acid concentration varies from0 to 500 ppm, the output voltage of the sensor increases linearlyfrom 2.98 to 4.36 mV with a sensitivity of 0.0021 mV/ppm anda linearity of more than 98.88%. A more efficient and stablesensor with graphene polymer composite coating increases thesensitivity due to the effective refractive index of the depositedcladding that allows more light to be transmitted through thetapered fiber.

Index Terms— Fiber optic sensor, tapered plastic optical fiber,uric acid, sensor sensitivity, graphene polymer composite.

I. INTRODUCTION

URIC acid is a metabolite of purines, nucleic acidsand nucleoproteins which is found in biological fluids,

mainly blood, urine or serum and is excreted by the humanbody. High level of serum uric acid is also considered as arisk factor for myocardial infarction and stroke [1]. In orderto avoid diseases like gout, renal failure, hyperuricaemia,Lesh-nyhan, physiological disorder and Wilson’s disorder fromproliferating, the monitoring of uric acid level is essential [2].Therefore, the need for uric acid biosensors is pressing.

Manuscript received August 5, 2013; revised January 21, 2014; acceptedJanuary 22, 2014. Date of publication January 28, 2014; date of cur-rent version March 24, 2014. This work was supported by the Universityof Malaya Research under Grant RP008C-13AET. The associate editorcoordinating the review of this paper and approving it for publication wasProf. Sang-Seok Lee.

M. Batumalay and S. W. Harun are with the Department of ElectricalEngineering, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail:[email protected]; [email protected]).

F. Ahmad is with the Department of Electrical Engineering, University ofMalaya, Kuala Lumpur 50603, Malaysia, and also with the Department ofElectrical Engineering, Universiti Teknologi Malaysia, Kuala Lumpur 54100,Malaysia (e-mail: [email protected]).

R. M. Nor and N. R. Zulkepely are with the Department of Physics, Univer-sity of Malaya, Kuala Lumpur 50603, Malaysia (e-mail: [email protected];[email protected]).

H. Ahmad is with the Photonics Research Center, University of Malaya,Kuala Lumpur 50603, Malaysia (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2014.2302900

Several uric acid detection techniques have been intro-duced using amperometric sensor, potentiometer and zincoxide (ZnO) nanowires. Amperometric biosensors have beenused for many purposes such as for the detection of oxygenconsumption, chemiluminescense and fluoride ions [1]. Thisdetection method requires electrodes to be held at approxi-mately 0.7 V where biological electro-active molecules reactwith the surface of the electrode. On the other hand, poten-tiometer based on ZnO nanoflakes and immobilized uricasecan reduce the interferences but a limitation of ion sensitiveelectrodes (ISEs) which can only detect charged molecules [3].ZnO have unique advantages in combination with immobilizedenzymes and can enhance the direct electron transfer betweenthe enzyme’s active sites and the electrons [3]. ZnO nanowiresgrown on the surface of gold coated flexible plastic substrateresults as good uric acid biosensor [4].

Recently, graphene has attracted much interest for variousapplications in optoelectronic devices, super-capacitors, sen-sors and nanocomposite material. This is due to its uniquecharacteristics such as high surface area, excellent electricalconductivity and fast electron mobility [5]–[7]. Besides, it canbe obtained easily by chemical conversion of the inexpensivegraphite [6], [8]. The electrical properties of graphene areextremely sensitive to charge transfer and chemical dopingeffects by various molecules. Either electron-withdrawingmolecules or electron-donating molecules will interact andchange the density of the main charge carriers and changesthe conductance of graphene. This is where graphene isapplicable as electrical chemical sensors. Meanwhile, taperedoptical fibers have also garnered popularity because of theirsuperior mechanical strength and ease of manufacturing whichmake them useful especially for sensing applications [9]–[13].A tapered fiber optic is more sensitive to the environmentsince the power of its evanescent wave (EW) in the cladding ishigher. This is due to the shape and size of tapered fibers whichallow a higher portion of evanescent field to interact with theouter environment making them more reactive to variationsof external forces. Coating the tapered fiber using appropri-ate material with correct thickness will further enhances itssensitivity towards the physical quantity to measure.

In this paper, a simple fiber-optic sensor is proposed anddemonstrated for the detection of uric acid concentrationsin de-ionized water. The sensor uses a tapered plastic opti-cal fiber (POF) which is coated with different concentra-tion of graphene in a polymer composite as a probe. Thetapered fiber is fabricated using an etching method and the

1530-437X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

BATUMALAY et al.: TAPERED POF COATED WITH GRAPHENE FOR URIC ACID DETECTION 1705

Fig. 1. Photographic image of (a) solutions of graphene polymer composite at ratios of 2:20, 4:20 and 6:20 and microscope images of graphene polymercomposite at mixing ratios of (b) 2:20, (c) 4:20, and (d) 6:20 (scale: 200 µm).

graphene solution is prepared by using an electro-chemicalexfoliation process. The sensor operates based on intensitymodulation technique using the tapered POF probe, which isimmersed in the uric acid.

II. EXPERIMENT

In preparing a graphene polymer composite, the first step isto produce graphene flakes using the electrochemical exfolia-tion process. A constant voltage difference of 20 V is appliedto two electrodes (graphite rods) placed 1 cm apart in anelectrolysis cell filled with electrolyte (1% SDS in deion-ized water). During the electrochemical exfoliation process,hydroxyl and oxygen radicals are released due to electrolysisof the water at the electrode. Then oxygen radicals start to cor-rode the graphite anode. This is followed by the intercalationof anionic surfactant and finally graphene sheets are formedin the solution. In our work, black sediments (graphene)start to peel off from the anode after several minutes. Theexfoliation process is extended for another 2 h to obtain astable graphene suspension in the SDS solution. The stablegraphene suspension is centrifuged at 3000 rpm for 30 minto remove large agglomerates. The width of the graphenelayer is estimated to be around 500 nm based on the workreported by Liu [14]. Afterward, the supernatant portion of thesuspension is decanted. The concentration of the centrifugedgraphene is estimated from the weight of the suspensionused. To fabricate the composite, 1 g of polyethylene oxide(PEO) (Mw = 1 000 000 gmol-1) is dissolved in 120 ml ofdeionized water. The graphene solution obtained from theelectrochemical exfoliation is then mixed with PEO solutionat various ratios; 2:20, 4:20 and 6:20 of graphene : PEO inml respectively. Fig. 1(a) shows the photographic image ofthe prepared graphene polymer composite while Fig. 1(b)–(d)show the microscopic images (×100) of graphene polymercomposite deposited on glass slides (scale: 200µm) for mixingratios of 2:20, 4:20 and 6:20 respectively.

The tapered POF is then prepared based on chemicaletching technique using acetone, de-ionized water and sand

paper. The POF used has an overall cladding diameter of1 mm, a numerical aperture of 0.51 and an acceptanceangle of 61°. The refractive index of the core and claddingare 1.492 and 1.402 respectively. Acetone is applied to thePOF using a cotton bud and neutralized with the de-ionizedwater. Acetone reacted with the surface of the polymer toform milky white foam on the outer cladding which is thenremoved by the sand paper. This process is repeated until thetapered fiber has a stripped region waist diameter of 0.45 mm.Beres et al. reported that tapers with waist diameters in therange of 0.40mm to 0.50mm showed good sensitivity to refrac-tive index variations whereas those with waist diameters below0.30mm and above 0.55 mm did not demonstrate substantialsensitivity [12], [15]. The total length of the tapered sectionis 10 mm. Finally, the tapered POF fiber is cleansed againusing de-ionized water. Fig. 2(a) and (b) show the microscopicimages of the original un-tapered, tapered POF, which havea cladding diameter of 1 mm and 0.45 mm respectively.Fig. 2(c) shows the tapered fiber coated with graphene polymercomposite with ratio of 6:20. The image shows the presenceof a layer of graphene polymer composite on the waist of thetapered fiber.

Fig. 3 shows the experimental setup for the proposedsensor to detect different uric acid concentration using thetapered POF coated with the composite of different graphene/PVA ratios; 2:20, 4:20, 6:20 as a sensing medium. Thesetup consists of a light source, an external mechanical chop-per, the sensor probe, a highly sensitive photo-detector, alock-in amplifier and a computer. The input and output ports ofthe tapered POF are connected to the laser source and photo-detector, respectively. The light source used in this experimentis a He-Ne laser operating at a wavelength of 633 nm withan average output power of 5.5 mW. It was chopped at afrequency of 113 Hz by a mechanical chopper to avoid theharmonics from the line frequency which is about 50 to 60 Hz.The light source was launched into the tapered POF placed ina Petri dish filled with the uric acid solution. The output lightwere sent into the silicon photo-detector (818 SL, Newport)


Fig. 2. Microscopic images of (a) un-tapered POF (with diameter of 1 mm), (b) tapered POF (with diameter of 0.45 mm), and (c) tapered POF coated withgraphene polymer composite with ratio of 6:20 (scale: 200 µm).

Fig. 3. Experimental setup for the proposed relative humidity sensor using a tapered POF without and with HEC/PVDF composite.

and the electrical signal was fed into the lock-in amplifier(SR-510, Stanford Research System) together with the refer-ence signal of the mechanical chopper. The output result fromthe lock-in amplifier was connected to a computer throughan RS232 port interface and the signal was processed usingDelphi software. The reference signal from the chopper wasmatched with the input electrical signal from the photo-diode. This allows a very sensitive detection system that willremove the noise generated by the laser source, photo-detectorand the electrical amplifier in the photo-detector [16], [17].In the experiment, the performance of the proposed sensor wasinvestigated for various uric acid concentrations. During theexperiment, the errors caused by temperature were considerednegligible and the temperature was kept constant at 25 °C.The experiment was done in room temperature where thetemperature was controlled with fluctuation around ±2° duringthe experiment.

III. RESULTS AND DISCUSSION

Fig. 4 shows the refractive index of the uric acid solution (asmeasured by using METTLER Toledo RE40D refractometer)against the uric acid concentration. As the concentration of

uric acid increases from 0 ppm to 500 ppm, the refrac-tive index of the solution also increases from 1.3330 to1.3336. The refractive index of graphene polymer compos-ite used in this experiment was also measured using thesame refractometer and the measured value was obtained at1.3342, which is slightly higher than refractive index of water(1.3330). Digital refractometer can measure the refractiveindex and other related parameters with high precision within ashort time.

Fig. 5 shows the variation of the transmitted light from thetapered POF with concentrations of uric acid solution for theprobe with various graphene polymer coating ratios of 2:20,4:20 and 6:20. The result obtained by the probe without coat-ing is also given for comparison purpose. It is observed that thetransmitted light intensity improves with the graphene coating.This is attributed to the difference in refractive index betweenthe core and cladding which slightly becomes smaller whenthe fiber is coated and thus improves the light confinementinside the core. As shown in the figure, the output voltage fromthe photo-detector, which corresponds to the transmitted lightintensity, linearly increases with the concentration of the uricacid solution. Without the graphene coating, the sensitivity of


Fig. 4. Refractive index of the uric acid solution.

Fig. 5. The output voltage against uric acid concentrations for the proposed tapered POF based sensor without and with graphene polymer composite of2:20, 4:20, and 6:20.

the sensor is obtained at 0.0003 mV/ppm with a slope linearityof more than 97.65% and limit of detection of 96.67 ppm.

The transmitted light intensity is observed to be highest withgraphene polymer coating of 6:20 since the graphene con-centration is the highest. Compared to the sensor configuredwithout the coating, the proposed sensor produces a bettersensitivity of 0.0021 mV/ppm with a better slope linearityof more than 98.88% and a limit of detection of 7.61 ppm.The limit of detection is lower compared to the uncoatedfiber, which indicates that the system is more efficient. Itis found that as the concentration of graphene increases, thesensivity and linearity of the sensor also increases. The elec-trical properties of graphene are extremely sensitive to chargetransfer and chemical doping effects by various molecules.Either electron-withdrawing molecules or electron-donatingmolecules will interact with graphene. This will change thedensity of the main charge carriers in the graphene andchanges its conductance [15], [18], [19].

Since the cladding area of the tapered POF has beenreduced, the surrounding medium works as passive cladding

and its refractive index can influence the amount of powerloss as the signal propagates through the tapered region.The reduction of the fiber size increases the evanescent fieldpenetration of guided modes [12], [15]. When immersingthe tapered fiber into the uric acid solutions with variousconcentrations ranging from 0 ppm to 500 ppm, the refrac-tive index of the surrounding medium increases since therefractive index of uric acid is larger than water (1.333).Since the refractive index of the composite increases as theconcentration increases, the core and cladding index differencefor the proposed sensor drops with the increment of theconcentration of the acid [12]. Therefore, we observe lessleakage from the light that propagates inside the taperedregion to the surrounding, which results in the output voltageincreasing. Thus, when the concentration of uric acid solutionincreases, the output voltage also increases for both casesof the POF with and without graphene coating. However,the experimental results also indicate that the sensitivity ofthe tapered POF is enhanced when coated as the transmittedlight intensity is higher. The reason is because the polymer


Fig. 6. The reversibility of the results obtained for two different runs (concentration of uric acid).

TABLE I

PERFORMANCE OF THE PROPOSED URIC ACID DETECTION SENSOR

composite has a much higher refractive index compared towater solution. When coated with graphene polymer compos-ite, the effective cladding refractive index of the POF increasesand thus more light is allowed to be transmitted. In addition,the proposed sensor provides numerous advantages such assimplicity of design, low cost of production, higher mechan-ical strength and easier to handle compared to silica fiberoptic [19].

Reversibility of the results is another important factor inthe operation of any sensor system, so in the next study, thisparameter is tested for the reported system. The results ofthe output measurement as a function of concentration arerecorded for two different runs and the results are compared.As can be noticed from Fig. 6, the maximum differencebetween the two runs is about ±0.13mV, which is acceptablefor a full-scale output of 4.36mV.

The performance characteristic of the proposed sensor issummarized in Table I. Overall, the sensor was observed tobe sufficiently stable with standard deviations of 0.029mV and0.0.016mV for POF probe without and with graphene polymercomposite coating as recorded for a duration of 100 second.Throughout the experiment, a fix quantity of liquid solutionwas placed in the petri dish and the corresponding output

voltage was measured by a lock-in amplifier which providedaccurate measurements even though the signal was relativelyvery small compared to noise. Furthermore, a well-regulatedpower supply is used for the red He-Ne laser and thisminimizes the fluctuation of source intensity. These resultsshow that the proposed sensor is applicable and useful forthe detection of uric acid. The sensor also has the ability toprovide real time measurement.

IV. CONCLUSION

A simple refractive index sensor is proposed and demon-strated using a tapered POF coated with different ratio ofgraphene polymer composite for measurement of differentconcentrations of uric acid in de-ionized water. The taperedPOF is fabricated by etching method using acetone, sandpaper and de-ionized water to achieve a waist diameter of0.45 mm and tapering length of 10 mm. As the solutionconcentration of the uric acid varies from 0 ppm to 500 ppm,the ouput voltage of the sensor increases linearly with asensitivity of 0.0021 mV/% and a linearity of more than98.88%. A more efficient and stable sensor with graphenepolymer composite coating increases the sensitivity due to the


effective refractive index of the deposited cladding that allowsmore light to be transmitted through the tapered fiber. Asthe concentration of uric acid increases, the index differencebetween core and cladding of the tapered fiber reduces andthus increases the guided light. The proposed sensor providesnumerous advantages such as simplicity of design, low cost ofproduction, higher mechanical strength and ease of handlingover normal silica fiber optic.

REFERENCES

[1] D. Ivekovic, M. Japec, M. Solar, and N. Živkovic, “Amperometric uricacid biosensor with improved analytical performances based on alkaline-stable H2O2 transducer,” Int. J. Electrochem. Sci., vol. 7, pp. 3252–3264,Apr. 2012.

[2] G. Minas, J. S. Martins, J. C. Ribeiro, R. F. Wolffenbuttel, andJ. H. Correia, “Biological microsystem for measuring uric acid inbiological fluids,” Sens. Actuators A, Phys., vol. 110, nos. 1–3,pp. 33–38, Feb. 2004.

[3] S. M. U. Ali, Z. H. Ibupoto, M. Kashif, U. Hashim, and M. Willander,“A potentiometric indirect uric acid sensor based on ZnO nanoflakes andimmobilized uricase,” Sensors, vol. 12, no. 3, pp. 2787–2797, Mar. 2012.

[4] S. M. U. Ali, N. H. Alvi, Z. Ibupoto, O. Nur, M. Willander, andB. Danielsson, “Selective potentiometric determination of uric acid withuricase immobilized on ZnO nanowires,” Sens. Actuators B, Chem.,vol. 152, no. 2, pp. 241–247, Mar. 2011.

[5] A. K. Geim and K. S. Novoselov, “The rise of graphene,” Nature Mater.,vol. 6, no. 3, pp. 183–191, 2007.

[6] X. Kang, J. Wang, H. Wu, I. A. Aksay, J. Liu, and Y. Lin, “Glucoseoxidase–graphene–chitosan modified electrode for direct electrochem-istry and glucose sensing,” Biosensors Bioelectron., vol. 25, no. 4,pp. 901–905, Dec. 2009.

[7] X. Wang, L. Zhi, and K. Müllen, “Transparent, conductive grapheneelectrodes for dye-sensitized solar cells,” Nano Lett., vol. 8, no. 1,pp. 323–327, 2008.

[8] J. Xu and M. Saeys, “First principles study of the stability and theformation kinetics of subsurface and bulk carbon on a Ni catalyst,”J. Phys. Chem. C, vol. 112, no. 26, pp. 9679–9685, Jun. 2008.

[9] H. Golnabi, M. Bahar, M. Razani, M. Abrishami, and A. Asadpour,“Design and operation of an evanescent optical fiber sensor,” Opt. LasersEng., vol. 45, no. 1, pp. 12–18, 2007.

[10] M. I. Zibaii, A. Kazemi, H. Latifi, M. K. Azar, S. M. Hosseini,and M. H. Ghezelaiagh, “Measuring bacterial growth by refractiveindex tapered fiber optic biosensor,” J. Photochem. Photobiol. B, Biol.,vol. 101, no. 3, pp. 313–320, Dec. 2010.

[11] H. A. Rahman, S. W. Harun, M. Yasin, S. W. Phang, S. S. A. Damanhuri,H. Arof, et al., “Tapered plastic multimode fiber sensor for salinitydetection,” Sens. Actuators A, Phys., vol. 171, no. 2, pp. 219–222,Nov. 2011.

[12] C. Beres, F. V. B. de Nazaré, N. C. C. de Souza, M. A. L. Miguel, andM. M. Werneck, “Tapered plastic optical fiber-based biosensor—Testsand application,” Biosensors Bioelectron., vol. 30, no. 1, pp. 328–332,Dec. 2011.

[13] Y. Huang, X. Dong, Y. Shi, C. M. Li, L. J. Li, and P. Chen, “Nano-electronic biosensor based on CVD grown graphene,” Nanoscale, vol. 2,no. 8, pp. 1485–1488, Jun. 2010.

[14] N. Liu, F. Luo, H. Wu, Y. Liu, C. Zhang, and J. Chen, “One-step ionic-liquid-assisted electrochemical synthesis of ionic-liquid-functionalizedgraphene sheets directly from graphite,” Adv. Funct. Mater., vol. 18,no. 10, pp. 1518–1525, May 2008.

[15] X. Dong, Q. Long, A. Wei, W. Zhang, L. J. Li, P. Chen, et al.,“The electrical properties of graphene modified by bromophenylgroups derived from a diazonium compound,” Carbon, vol. 50, no. 4,pp. 1517–1522, Apr. 2012.

[16] S. Harun, M. Batumalay, A. Lokman, H. Arof, H. Ahmad, and F. Ahmad,“Tapered plastic optical fiber coated with HEC/PVDF for measurementof relative humidity,” IEEE Sensors J., vol. 13, no. 12, pp. 4702–4705,Dec. 2013.

[17] M. Batumalay, F. Ahmad, A. Lokman, A. A. Jasim, S. W. Harun, andH. Ahmad, “Tapered plastic optical fiber coated with single wall carbonnanotubes polyethylene oxide composite for measurement of uric acidconcentration,” Sensor Rev., vol. 34, no. 1, pp. 75–79, 2014.

[18] H. Lian, Z. Sun, X. Sun, and B. Liu, “Graphene doped molecularlyimprinted electrochemical sensor for uric acid,” Anal. Lett., vol. 45,no. 18, pp. 2717–2727, Nov. 2012.

[19] R. J. Bartlett, R. Philip-Chandy, P. Eldridge, D. F. Merchant, R. Morgan,and P. J. Scully, “Plastic optical fiber sensors and devices,” Trans. Inst.Meas. Control, vol. 22, no. 5, pp. 431–457, Dec. 2000.

Malathy Batumalay received the B.E. degree in electrical and electronicsfrom the University Tun Hussion Onn in 2004, and the M.Eng. degree inelectrical, electronic and communication from the University of Malaya in2010, where she is currently pursuing the Ph.D. degree in photonics.

Her current research interests include tapered plastic fiber based opticalsensors and coating of sensitive materials.

Sulaiman Wadi Harun received the B.E. degree in electrical and electronicssystem engineering from the Nagaoka University of Technology, Nagaoka,Japan, in 1996, and the M.Sc. and Ph.D. degrees in photonics from the Uni-versity of Malaya, Kuala Lumpur, Malaysia, in 2001 and 2004, respectively.

He is currently a Full Professor with the Faculty of Engineering, Universityof Malaya. His current research interests include fiber optics, active andpassive devices.

Fauzan Ahmad, photograph and biography not available at the time ofpublication.

Roslan Md Nor received the B.Sc. degree in physics from the University ofMalaya in 1981, the M.Sc. degree in ionization physics from the Universityof Wales in 1985, and the Ph.D. degree from Queen’s University of Belfastin 1995.

He is currently an Associate Professor with the University of Malaya. Hiscurrent research interests include carbon and oxide nanomaterial, chemicalsensors, and radiation dosimetry.

Nurul R. Zulkepely, photograph and biography not available at the time ofpublication.

Harith Ahmad received the Ph.D. degree in laser technology from theUniversity of Swansea, Swansea, U.K. in 1983.

He is a Full Professor with the Photonics Research Center, University ofMalaya, Kuala Lumpur, Malaysia.

tapered plastic optical fiber coated with graphene for uric acid detection

Documents