Highly selective SPR based fiber optic sensor for the detection of hydrogen peroxide

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Highlights

  • A highly selective SPR based fiber optic hydrogen peroxide sensor utilizing enzyme reaction is reported.

  • The probe is fabricated by immobilization of catalase over gold and graphene oxide coated unclad core of the fiber.

  • The graphene layer plays role in sensitivity enhancement and sensing mechanism.

  • The sensor works in the physiological range 0 – 1000 μM of hydrogen peroxide.

  • The sensor has good stability, repeatability and high selectivity.

Abstract

The present study reports the fabrication and characterization of a surface plasmon resonance (SPR) based fiber optic hydrogen peroxide sensor using enzymatic approach. The sensing probe is fabricated by depositing gold and graphene oxide (GO) layers over the unclad core of an optical fiber followed by the immobilization of catalase enzyme via EDC-NHS coupling. In this probe, gold provides stability and repeatability to the sensor because it does not oxidize in the presence of oxygen environment like silver, GO being of high refractive index enhances the sensitivity of the sensor while the catalase provides specificity to the sensor due to its very specific interaction with hydrogen peroxide. Further, GO also plays an important role in sensing mechanism by interacting with water molecules released from the enzymatic reaction which changes its effective refractive index. GO is synthesized using modified Hummer’s method and characterized by SEM, TEM and AFM. The operation of the fabricated probe is tested for the concentration of hydrogen peroxide ranging from 0–1000 μM. The sensor possesses 55 μM as the limit of detection and its performance is pH dependent. It is highly selective, stable, repeatable and fast in response. The probe being fabricated on optical fiber substrate gives additional advantages of simplicity, miniaturization, online monitoring and remote sensing of hydrogen peroxide. These advantages promise its applications in health monitoring.

Introduction

Hydrogen peroxide (H2O2) plays an important role in food, pharmaceutical, clinical, waste water treatment, surface treatment of metals and environmental analyses [[1], [2], [3], [4]]. It is also a by-product of several highly selective oxidases and hence is a crucial mediator in biological processes [5]. It is also an indicator of various diseases such as arteriosclerosis, cancer, Parkinson's, stroke, and Alzheimer's disease [6]. The abnormal range of H2O2 in the human body can cause tissue fractures and tumors. The H2O2 concentration greater than 50 μM is cytotoxic for animals, plants and bacterial cells in culture and, therefore, the monitoring range of H2O2 from 0 to 300 μM is preferred for humans, animals, and plants [7]. The use of polyphenolic beverages such as black tea, green tea, grape juice, cistus tea and wine increases H2O2 level in humans. Therefore, the monitoring of the H2O2 concentration in beverages above 300 μM is also important [8]. For the detection of H2O2, various analytical methods such as spectrophotometry [9], chemiluminescence [10], fluorescence [11] and electrochemical [12] have been reported. Most of these use bulky instruments, require tedious sample preparation, are costly and have no remote sensing facility. Therefore, a simple, low cost, portable, highly sensitive and selective sensor is required for the detection of H2O2. SPR technique along with optical fiber substrate has been widely used for the detection of various chemical and biological analytes due to simple design, portability, low cost, label free detection, and remote sensing facility [13]. Highly sensitive, selective, fast response, and miniaturized probes can be designed using this technique for clinical/medical diagnostics. SPR based fiber optic H2O2 sensors reported in the literature use silver layer for its detection [14,15] but these sensors have a lack of stability and repeatability. The mechanism of most of the H2O2 sensors reported in the literature is based on the oxidation of silver by H2O2. Hence, these sensors are not stable. In view of this some of the studies have mentioned their probe as disposable probe [15].

To overcome these problems, highly selective and stable SPR based fiber optic H2O2 sensor utilizing enzymatic reaction has been proposed in the present study. The sensor probe has been fabricated by coating layers of gold (Au) and nanosheets of graphene oxide (GO) over an unclad core of the fiber followed by the immobilization of enzyme catalase. Gold, instead of silver, has been used as plasmonic metal because gold is more stable than silver which gets oxidized due to moisture. The oxidation of silver layer can be protected by a thin uniform film of high dielectric materials such as silicon which can be uniformly coated over the silver layer by thermal evaporation method. But, in the proposed sensor, nanosheets of high index material, GO, have been coated over gold surface using dip coating method. The nanosheets of GO cannot be coated using thermal evaporation method. The coating of GO nanosheets by dip coating method results in the non-uniform film with voids at various places. Therefore, if GO is coated over silver film then these voids may help in the oxidation of silver. The second reason of not using silver as plasmonic metal is that, for the coating of GO nanosheets, the metal-coated fiber is kept in the GO/water environment. If silver is used as the plasmonic metal then it will be oxidized by water which will result in the reduction of film thickness and increase in the FWHM of the SPR signal. Therefore, due to these two reasons gold film has been used in place of silver. It may also be noted that the silver based SPR sensors have better figure of merit (FOM) than the gold based SPR sensors [16]. But, the addition of GO layer over gold or silver layer increases FWHM. Since, during the coating of GO over silver the outside medium (water) oxidizes silver layer and hence decreases its thickness. The decrease in thickness contributes to the broadening of SPR curve or further increases the FWHM of Ag/GO combination in comparison to Au/GO combination. Since FOM is defined as the ratio of the sensitivity to the FWHM, the FOM of Ag/GO combination may become smaller than the FOM of Au/GO combination. In the proposed probe GO, having high refractive index, enhances the sensitivity and is responsible for the detection of H2O2 through its interaction with the water molecules dissociated after enzymatic reaction of H2O2 by catalase. Further, GO provides a large surface area which increases the enzyme loading capacity and, therefore, the sensitivity enhancement [17]. In addition, GO has both sp2 and sp3 hybridized carbon atoms and several functional groups containing oxygen such as carboxyl (−COOH), epoxy (-O) and hydroxyl (−OH) which allow covalent immobilization of biomolecules [18]. The enhancement of sensitivity by GO layer has been shown by performing experiments on the Au coated probe and Au/GO coated probe. The operation of Au/GO/catalase probe has been tested for H2O2 concentration range from 0–1000 μM. The sensing mechanism is based on the decomposition of H2O2 in the presence of catalase and interaction of released water molecules with GO layer which alters its effective refractive index and shifts the resonance wavelength in SPR spectrum. The performance of the probe is pH dependent and is best for the pH range 6.5–7.5 pH of the sample. The selectivity, stability and repeatability of the probe have also been tested.

Section snippets

Materials and reagents

Step index silica core/polymer clad/nylon jacketed multimode optical fiber of core diameter 600 μm and NA 0.37 was procured from Fiberguide Industries, USA. Its trade name is Anhydroguide™ VIS-IR (Low OH): 300 nm–2400 nm. Potassium permanganate (KMnO4) and hydrochloric acid were bought from Fisher Scientific, India. Sodium dihydrogen phosphate dihydrated (NaH2PO4.2H2O), disodium hydrogen phosphate dihydrated (Na2HPO4.2H2O), urea, sucrose, ethanol, acetone and concentrated sulphuric acid (98 %)

Refractive index sensitivity of Au and Au/GO coated SPR probe : importance of GO layer

To observe the effect of GO layer over Au layer in the probe, the RI sensing capability of the probes with Au layer and Au/GO layers were first studied. For this, various sample solutions of RI varying from 1.335 to 1.360 were prepared by dissolving different amounts of urea in DI water. Fig. 7(a) shows the SPR spectra of Au coated probe for different RI of the sample. As observed, the SPR wavelength shifts towards the higher wavelength with increasing RI of the sample around the probe. A shift

Conclusion

To summarize, a simple, highly sensitive, selective and repeatable SPR based optical fiber H2O2 sensor has been reported. The probe has been fabricated by coating layers of gold and graphene oxide over unclad core of the fiber. To sense H2O2, enzyme catalase has been immobilized over GO layer. The sensing is based on the decomposition of H2O2 by catalase into water and oxygen. The released water molecules interact with GO layer and changes its effective refractive index which results in the

CRediT authorship contribution statement

Vivek Semwal: Conceptualization, Data curation, Methodology, Validation, Formal analysis, Investigation, Writing - original draft. Banshi D. Gupta: Validation, Investigation, Resources, Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work in this paper.

Vivek Semwal received his M.Sc. degree in Physics in 2013 from H.N.B. Garhwal University, Uttarakhand (India). Since January 2015, Mr. Semwal is a full time Ph.D. student at the Physics Department, Indian Institute of Technology Delhi. Mr. Semwal is a student member of OSA (The optical society).

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Vivek Semwal received his M.Sc. degree in Physics in 2013 from H.N.B. Garhwal University, Uttarakhand (India). Since January 2015, Mr. Semwal is a full time Ph.D. student at the Physics Department, Indian Institute of Technology Delhi. Mr. Semwal is a student member of OSA (The optical society).

Banshi D. Gupta received his M.Sc. degree in physics (1975) from Aligarh Muslim University (India) and a Ph.D. degree in physics (1979) from the Indian Institute of Technology Delhi (India). In 1978, he joined the Indian Institute of Technology Delhi, where he is currently a professor of physics. In addition, Prof. Gupta has worked at the University of Guelph (Canada) in 1982–1983, the University of Toronto (Canada) in 1985, the Florida State University (USA) in 1988, the University of Strathclyde (UK) in 1993 and the University of Birmingham (UK) in 2010. In 1992, he was awarded the ICTP Associateship by the International Centre for Theoretical Physics, Trieste (Italy), which he held for 8 consecutive years. In this capacity, he visited ICTP (Italy) in 1994 and 1996. Prof. Gupta is a recipient of the 1991 Gowri Memorial Award of the Institution of Electronics and Telecommunication Engineers (India). He has published more than 185 research papers in international journals and 110 papers in conferences. Prof. Gupta has authored books entitled Fiber Optic Sensors: Principles and Applications (NIPA New Delhi, 2006), Fiber Optic Sensors based on Plasmonics (World Scientific, 2015), and Optical Sensors for Biomedical Diagnostics and Environmental Monitoring (CRC Press USA, 2017). He is a Co-Editor of the Proceedings of SPIE (USA) Vol. 3666 (1998) and Vol. 8173 (2010). Prof. Gupta has delivered invited talks on plasmonics based sensors in various international conferences held in USA, China, Australia, Korea, Norway and India. His current areas of interest are plasmonic biosensors, fiber optic sensors, and nanotechnology.

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