Optical bio-sensing devices based on etched fiber Bragg gratings coated with carbon nanotubes and graphene oxide along with a specific dendrimer

Sensors and Actuators B 195 (2014) 150–155

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Optical bio-sensing devices based on etched fiber Bragg gratings coated with carbon nanotubes and graphene oxide along with a specific dendrimer Sridevi S a , K.S. Vasu b , N. Jayaraman c , S. Asokan a,d , A.K. Sood b,∗ a

Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India Department of Physics, Indian Institute of Science, Bangalore 560012, India c Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India d Robert Bosch Centre for Cyber Physical Systems, Indian Institute of Science, Bangalore 560012, India b

a r t i c l e

i n f o

Article history: Received 16 September 2013 Received in revised form 26 December 2013 Accepted 27 December 2013 Available online 15 January 2014 Keywords: Fiber Bragg gratings Carbon nanotubes Graphene oxide Biosensing Dendrimer

a b s t r a c t We demonstrate that etched fiber Bragg gratings (eFBGs) coated with single walled carbon nanotubes (SWNTs) and graphene oxide (GO) are highly sensitive and accurate biochemical sensors. Here, for detecting protein concanavalin A (Con A), mannose-functionalized poly(propyl ether imine) (PETIM) dendrimers (DMs) have been attached to the SWNTs (or GO) coated on the surface modified eFBG. The dendrimers act as multivalent ligands, having specificity to detect lectin Con A. The specificity of the sensor is shown by a much weaker response (factor of ∼2500 for the SWNT and ∼2000 for the GO coated eFBG) to detect non specific lectin peanut agglutinin. DM molecules functionalized GO coated eFBG sensors showed excellent specificity to Con A even in the presence of excess amount of an interfering protein bovine serum albumin. The shift in the Bragg wavelength (B ) with respect to the B values of SWNT (or GO)-DM coated eFBG for various concentrations of lectin follows Langmuir type adsorption isotherm, giving an affinity constant of ∼4 × 107 M−1 for SWNTs coated eFBG and ∼3 × 108 M−1 for the GO coated eFBG. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Fiber Bragg grating (FBG) [1,2], a periodic variation in the refractive index of the fiber core, is an optical sensing device that has emerged as mechanical [3,4], thermal [5,6], chemical and bio-sensors [7,8]. When a broad band light propagates through the FBG, a particular wavelength known as Bragg wavelength (B ) is reflected back based on the resonance condition given by B = 2neff , where  is the grating pitch and neff is the effective refractive index depending on the refractive indices of core (ncore ) and cladding (nclad ) [2,8]. Any external perturbation such as strain, temperature, etc., which causes a shift in the Bragg wavelength of a FBG due to change in neff , can be easily measured by accurately measuring this shift. One of the important aspects in sensing using a FBG is discrimination of the temperature and strain effects, which can be achieved by different methodologies such as dual wavelength FBGs [9], Fabry Perot filter within a single FBG [10].

∗ Corresponding author. Tel.: +91 8022932271; fax: +91 8023608686. E-mail addresses: [email protected], [email protected] (A.K. Sood). 0925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.12.109

FBGs are generally less sensitive to the variations in the refractive index of the surrounding medium as the fiber core is well covered by the cladding layer. This limits the application of FBGs in chemical and bio-sensing. Therefore, Long Period Gratings (LPGs) [11,12] and etched FBGs (eFBGs) [13,14] have been utilized for chemical and bio-sensing applications. The light coupling between the cladding and the core makes LPGs sensitive to the surrounding medium, whereas in etched FBGs, the core is directly exposed to the surrounding medium. In the past, DNA hybridization and monolayer detection of biological reagents have been demonstrated using the eFBGs [15]. Recently, the covalent attachment of carbohydrates to the eFBGs has been used to study carbohydrate–protein interactions [16,17]. Single walled carbon nanotubes (SWNTs) and graphene based field effect transistors (FETs) have been used in many biological and chemical sensing application [18–20]. SWNTs and SWNT based nano-composites deposited on optical fibers using Langmuir Blodgett method have been employed as opto-chemical sensors to detect volatile organic compounds such as xylene and toluene by measuring the changes in reflectivity [21]. Recently, CO2 gas sensing experiments were carried out by using the PAA-amino CNT complex coated eFBG [22]. Also, the area selective deposition of carbon nanotubes around and at the end of the fibers [23]

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has been used in mode-locking and bio-applications by monitoring the reflectivity measurements. Surface adsorption on eFBG sensors was analyzed using electrostatic layer by layer assembly of polyelectrolyte molecules to estimate the detection limit [24]. In this paper, we demonstrate that coating of functionalized SWNTs and graphene oxide (GO) on eFBG makes it a very sensitive device for biological and chemical sensing applications due to the change in refractive index of the GO or SWNT coating (which acts as a cladding layer) is more than that of silica. Here, the cladding layer of the FBG is etched using hydrofluoric acid [25] and the eFBG is treated with NaOH to modify the surface for the coating of SWNTs and GO. As a specific demonstration of the methodology, the carbohydrate–protein interaction has been probed using the mannose functionalized poly(propyl ether imine) (PETIM) dendrimers (DMs) as multivalent carbohydrate source. The dendrimers are macromolecules widely used to monitor many biologically relevant ligand–receptor interactions. Recently, we have shown that specific and non-specific carbohydrate–protein interactions can be studied by observing the changes in the electrical conductivity of the DM decorated SWNT FET device [19]. In the present work, we establish that the carbohydrate–protein interaction can be probed more sensitively by measuring the shift in the Bragg wavelength of the eFBGs coated with DM functionalized SWNTs (or GO). 2. Experimental FBGs are inscribed in high numerical aperture single mode photosensitive silicate fibers of total diameter 125 ␮m doped with germania (M/s Nufern) with the core diameter of 9 ␮m. KrF excimer UV laser of wavelength 248 nm, pulse energy 6 mJ and repetition rate of 200 Hz is used to inscribe the grating using a phase mask of 1069 nm pitch (M/s Stocker Yale Inc.). After the inscription of a uniform grating in a photosensitive fiber, the Bragg wavelength reflected from the FBG is monitored by using an FBG interrogator (Micron Optics, SM130) with a wavelength repeatability of 1 picometer (pm). The Bragg wavelength values of the FBGs used for SWNT and graphene oxide coating experiments are 1539.9524 (±0.00144) nm and 1547.3773 (±0.00247) nm respectively. The numbers in the bracket denote the value of the variance estimated for the time series of the measured Bragg wavelength. To enhance the sensitivity of the FBG, the cladding layer is removed by an etching process. The etching is carried out by dipping the grating region (∼3 mm) of the FBG in a 200 ␮L of 40% HF solution placed in a teflon block for ∼2 h. This process reduces the thickness of the clad material from about 58 ␮m to about 0.5 ␮m; this procedure blue-shifts the Bragg wavelength by 1 nm. After etching, the Bragg wavelength values are 1538.9507 (±0.00151) nm for SWNT and 1546.3760 (±0.00277) nm for graphene oxide. The etched FBG is treated with 0.2 N of NaOH solution at 40 ◦ C for 3.5 h and subsequently kept in the NaOH solution for 30 min at room temperature. The treated FBG is rinsed with deionized water for 10 min. The NaOH treatment of etched FBG surface makes it hydrophilic by creating a few –OH groups on the etched portion of the FBG. The acid treated SWNTs and as prepared graphene oxide have functional groups such as –COOH, –OH, –O– on their surfaces. These functional groups lead to the formation of hydrogen bonding between the –OH groups presented on the NaOH treated etched FBG and the nanomaterials. In our experiments, after coating the nanomaterials on the etched FBG surface, we wash them for 3 times in deionized water to remove physically adsorbed SWNT or graphene oxide. The SEM image of graphene oxide coated etched FBG sensor shown in Fig. 1b is after washing it for 3 times, which confirms that the fiber coated with SWNT and GO is stable against repeated washing with water. The SWNTs used are of average diameter ∼0.8 nm, with chirality (6, 5), purchased from M/s SouthWest NanoTechnologies and

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sonicated in 3:1 conc. H2 SO4 /conc. HNO3 for 3 h at 40 ◦ C [26]. The mixture is centrifuged and the residue is washed with water several times and dried at 100 ◦ C. For the experiments, 0.25 mg of acid treated SWNTs have been dispersed in 1 mL of water. GO suspensions (0.25 mg/mL) have been prepared as described in [27,28]. The surface roughness and hydrophilicity of the etched FBG leads to the attachment of the acid treated SWNTs and B is found to increase by ∼12 pm after the attachment of SWNTs (with respect to the B after NaOH treatment). This increase in the B is due to the increase in the effective refractive index due to the SWNTs coating. Mannose attached fourth generation PETIM dendrimer cluster glycoside, having ∼30 mannose residues at the periphery of the dendrimer was utilized in the present study [19]. Aqueous solutions in water were prepared as mentioned in [19] with a concentration of 0.5 mM. The SWNTs coated eFBG is dipped in 0.5 mM DM solution to create carbohydrate (mannose) sites. As the SWNTs are p-type, DM molecules form SWNT-DM complexes, through charge transfer interactions [19]. The formation of SWNT-DM complexes result in the decrease of B ∼ 10 pm with respect to the B of eFBG coated with SWNTs. The Bragg wavelength (0B ) of the eFBG, after the formation of the SWNT-DM complex is 1539.09 nm. Same procedure is followed to coat GO on the eFBG surface and to form the GO-DM complex on the eFBG. The solution of lectins concanavalin A (Con A) specific to mannose and non-specific peanut agglutinin (PNA), prepared in water (pH ∼ 6.6) in the concentration range of 100 pM to 5 ␮M, are used in the experiments. The Bragg wavelength values have been monitored after treating the SWNT-DM (or GO-DM) coated eFBG at different concentrations of aqueous solutions of lectin Con A ranging from 1 nM to 5 ␮M. With the increase of Con A concentration, the difference B (B − 0B ), has been found to increase.

3. Results and discussion Fig. 1a summarizes the flow chart of coating of the GO and SWNT on etched FBGs, followed by the carbohydrate and specific protein interactions. Fig. 1b shows a scanning electron microscopic image of GO coated eFBG (captured using ULTRA 55, Field Emission Scanning Electron Microscope (M/s Karl Zeiss)). GO coated eFBG shown in Fig. 1b was made by dipping the NaOH treated eFBG in 200 ␮L of GO aqueous suspension for 20 min followed by washing with deionized water. From the SEM image we observed that the two dimensional GO sheets coated on the surface of etched FBG. Fig. 2a shows the ratio B /0B as a function of concentration of the lectins Con A and PNA. The shift in B is very significant; even for 1 nM of Con A, the shift B ∼ 2 pm and after the addition of 5 ␮M Con A, it increases to ∼75 pm. A similar procedure has been carried out with the non-specific lectin, PNA. It is observed that the B value after 5 ␮M PNA treatment is only ∼5 pm. The B for 5 ␮M of PNA treatment is less than the B for 2 nM of Con A treatment, showing the high specificity (by a factor of ∼2500) of the sensor for the mannose–Con A interactions. Fig. 2a also shows the variation of B /0B as a function of concentration of the Con A without coating of SWNTs, which indicates that the eFBG coated only with DM is less sensitive when compared to the SWNT-DM coated eFBG. Experiments have also been performed by coating the GO onto the eFBG. Fig. 2b shows the change in B /0B as a function of lectin concentration for the GO coated eFBG. The Bragg wavelength of the eFBG coated with the GO-DM complex is 0B = 1546.55 nm. The change in B after addition of 1 ␮M Con A is found to be B ∼ 150 pm as compared to a B of ∼20 pm for 1 ␮M PNA. The B value for 1 ␮M PNA detection is same as the B value for 500 pM of Con A (i.e., sensitive to detect Con A by a factor of

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Fig. 1. (a) Flow chart of coating of the GO, SWNTs on etched FBGs. (b) SEM image of GO coated eFBG.

∼2000). Importantly, it is observed that for 1 ␮M concentration of Con A treatment, the B value for the GO coated eFBG is twice as compared to the B value for SWNTs coated eFBG. The enhanced sensitivity of GO coated eFBG over SWNTs coated FBG is because the surface area of GO is more than SWNTs and GO covers more surface area of eFBG, resulting in larger number of DM molecules attached to the GO coated eFBG than SWNTs coated eFBG. Also, the lowest Con A concentration sensed using GO coated eFBG is about 0.5 nM (B ∼ 9 pm) whereas it is about 1 nM (B = 2 pm) using SWNTs coated eFBG, showing the better sensitivity of the GO coated eFBG sensing devices.

The sensor readout, relative change in Bragg wavelength, as a function of lectin concentration can be explained by Langmuir type adsorption isotherm given by [19,29], B 0B

= S[log C + log(7.389KA )]

(1)

where S, called the structural factor is given by 0.58kCmax ; Cmax is the maximum surface density of receptors (i.e., mannose molecules) and k is a constant specifying the sensor characteristics [29]. C is the concentration of lectin in the solution and KA is the affinity constant. In Fig. 2a and b, Eq. (1) is fitted to extract the KA

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(a)

(b)

Fig. 3. B /0B of the GO-DM coated eFBG after dipping it in different concentrations of Con A alone (brown color squares), Con A mixed with 1.5 ␮M BSA (black color stars) and BSA alone (green color triangles).

Fig. 2. (a) B /0B of the SWNT-DM coated eFBG after dipping in different concentrations of lectins ranging from 1 nM to 5 ␮M. The data shown by the stars is the B /0B of the eFBGs coated only with DM coated eFBG without SWNTs or GO for different concentrations of Con A. (b) B /0B of the GO-DM coated eFBG after dipping it in different concentrations of lectins ranging from 100 pM to 1 ␮M. (Note: Error bars denote the variation in the average value of 3 different measurements.)

values of Con A–mannose interactions, yielding KA = 4.2 × 107 M−1 for SWNTs coated eFBG and 3.4 × 108 M−1 for GO coated eFBG. The values of the parameter S are 1.4 × 10−5 (SWNT + DM + Con A), 7.5 × 10−7 (SWNT + DM + PNA), 2.9 × 10−5 (GO + DM + Con A), 2.4 × 10−6 (GO + DM + PNA) and 4.5 × 10−6 (DM + Con A).

Enhanced sensitivity, B /0B value for DM functionalized SWNT or GO coated eFBG when compared to the eFBG without coating at a particular concentration is due to two reasons: (i) enhanced attachment of dendrimer to the SWNTs and GO due to larger surface area available and (ii) refractive index of clad material increases from 1.465 for silica to 1.7 for GO and 1.8 for SWNTs. The second point can be quantified by noting that the change in B for a given value of change in the refractive index of the clad material nclad (arising from the attachment of con A to the dendrimer) is more when nclad = 1.7 or 1.8 as compared to 1.46. It is worth noting here that the observed KA value in SWNT coated eFBG optical sensor is ∼45 times more than the KA value in the SWNT-FET sensor [19]. The sensitivity of carbohydrate–protein interactions is negligible (results are not shown) when the GO is directly coated on cladding layer of the FBG, implying that etching of the FBG is necessary to enhance the sensitivity. Further, we have used bovine serum albumin (BSA) as an interfering protein [30] to test the specificity of Con A to DM molecules available on our GO-DM coated eFBG sensors. Different concentrations of Con A were well mixed with the excessive amount of BSA protein of 1.5 ␮M. Fig. 3 shows the data on sensitivity of different concentrations of lectins Con A alone (brown color squares), Con A mixed with 1.5 ␮M BSA (black color stars) and BSA alone (green color triangles) to GO-DM complex coated eFBG. The B value for the interfering protein BSA of 1 ␮M is same as the B value for

Table 1 List of various experimental techniques to detect Con A – mannose interactions and affinity constants. Limit of detection

Affinity constant (M−1 )

Reference

GO coated eFBG SWNTs coated eFBG

500 pM 1 nM

3.4 × 10 4.2 × 107

Present work Present work

Surface plasmon resonance

1 ␮M 5 nM 5 nM 90 nM

2.3 × 108 2.4 × 107 5.6 × 106 3.9 × 106

[34] [35] [36] [37]

Localized surface plasmon resonance

50 mM 500 pM

7.7 × 106 7.4 × 106

[30] [38]

Quartz crystal monitoring

90 nM 25 nM 1.6 nM

8.7 × 105 2 × 107 2.3 × 107

[37] [39] [40]

Boron doped diamond

5 nM

2.6 × 106

[41]

1 nM 100 pM

8.5 × 10 1.7 × 105

[19] [42]

Experimental method

SWNT-FET

8

5

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more specificity to detect Con A in the presence of excess amount of BSA protein. The B /0B as a function of lectin concentration is shown to obey the Langmuir type isotherm, giving the affinity constant of 4.2 × 107 M−1 for SWNTs coated eFBG and 3.4 × 108 M−1 for GO coated eFBG. The methodology is a platform technology that can be used in different bio and chemical sensing applications by attaching specific ligands to the carbon nanotubes or graphene oxide coated on etched fiber Bragg grating. We add that by having multiple gratings in the same fiber with different Bragg wavelengths, it is possible to simultaneously detect different antigens.

Acknowledgements

Fig. 4. Change in the surrounding medium which acts as cladding of the etched FBG (for SWNTs coating – red color open circles, GO coating – black color open squares and without coating – green color open stars) as a function of concentration of Con A solution.

5 nM of Con A (i.e., sensitive to detect Con A by a factor of ∼200) tells us the weaker response of GO-DM coated eFBG sensor to BSA protein. We have also extracted KA values for Con A and Con A in the presence of excess of BSA protein as 1.5 (±0.3) × 108 M−1 and 1.1 (±0.2) × 108 M−1 by fitting Eq. (1) in Fig. 3, which also reveals that the excess amount of BSA protein does not affect the specificity of Con A to DM molecules attached to GO coated eFBG. Since this set of experiments have been performed using a concentration of 0.35 mM DM solution to form the GO-DM complex, the KA value observed for Con A in Fig. 3 is less than that shown in Fig. 2b. The affinity constant of Con A and mannose derivatives obtained from literature, based on various experimental methods, has been listed in Table 1. It can be clearly seen that the limit of detection and affinity constant values obtained in our method is better than those obtained from the earlier methods such as surface plasmon resonance (SPR), localized SPR, quartz crystal monitoring and FETs. The measured B after adding each concentration of lectins Con A and PNA is due to the change in refractive index of the cladding medium (SWNT (or GO)-DM complex) around the core in eFBG. Fig. 4 shows change in the refractive index of cladding layer as a function of concentration of Con A solution (for SWNTs (red color open circles), GO (black color open squares) coated eFBGs and without coating of SWNTs or GO on eFBG (green color open stars)). The change in the cladding layer (SWNT (or GO)-DM complex) refractive index can be calculated using the equation [31],



neff = nclad

n2eff − n2clad n2core − n2eff



ncore − nclad nclad





+1

(2)

The core refractive index (ncore ) is 1.471. Using nSWNT = 1.81 [32] and nGO = 1.7 [33] in Eq. (2), the estimated coated area by SWNTs is ∼55% and by GO is 65%. 4. Conclusions We have shown that an etched fiber Bragg grating coated with SWNT or GO attached with multivalent specifically functionalized dendrimer polymers is a very sensitive biosensor. As an application of the proposed methodology, we have studied the carbohydrate–protein interactions using mannose attached PETIM dendrimers as multivalent carbohydrate ligands. The change B for different concentrations of Con A are much higher than that for non-specific lectin PNA showing the specificity of the sensor. Our DM molecules functionalized GO coated eFBG sensors showed

Prof. A.K. Sood, Prof. S. Asokan and Prof. N. Jayaraman thank the Department of Science and Technology, India for financial assistance. Prof. S. Asokan also thanks the Applied Photonics Initiative and the Robert Bosch Centre for Cyber Physical Systems, Indian Institute of Science, for support.

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Biographies Sridevi S received her BE degree in Instrumentation Technology and M.Tech. degree in Biomedical signal processing and Instrumentation from Visvesvaraya Technological University, India. She is currently pursuing Ph.D. in the Department of Instrumentation and Applied Physics, Indian Institute of Science, under the supervision of Prof. S. Asokan. Her current research interest includes strain, temperature and biosensors based nanomaterials coated etched fiber Bragg gratings. K.S. Vasu received his post graduate (M.Sc.) degree in Physics from Sri Venkateswara University, India. He is currently pursuing Ph.D. with Prof. A.K. Sood in the Department of Physics, Indian Institute of Science. He is working on biosensors based on graphene oxide & SWNTs and electrical, rheological properties of graphene oxide and its derivatives. N. Jayaraman joined the Department of Organic Chemistry in December 1999, and is a professor currently. He completed B.Sc. (University of Madras), M.Sc. (Annamalai University) and doctoral research at Indian Institute of Technology, Kanpur, under the supervision of Professor S. Ranganathan. He was a postdoctoral fellow in the group of Professor Sir J.F. Stoddart, at University of Birmingham, UK and at University of California Los Angeles, USA. His research group is working in the areas of carbohydrates and dendrimers. He was honored with Shanti Swarup Bhatnagar Prize in 2009 and is a Fellow of the Indian Academy of Sciences. S. Asokan received the M.Sc. degree in Materials Science from the College of Engineering, Guindy, Anna University, Madras, India, and the Ph.D. degree in Physics from the Indian Institute of Science, Bangalore, India. He is currently a Professor at the Department of Instrumentation and Applied Physics and Chairman of the Robert Bosch Centre for Cyber Physical Systems, Indian Institute of Science. He has edited two books and published more than 180 papers in international journals/books. A.K. Sood is a Professor in the Department of Physics at Indian Institute of Science, Bangalore, India. His research interests include physics of nanosystems (e.g. nanotubes and graphene) and soft condensed matter. The former includes transport and Raman spectroscopy of nanodevices to understand basic science issues in phonon renormalization and mobility of carriers and to use them as nanosensors. He has published more than 315 papers in refereed international journals and holds several patents. His work has been recognized by many honors, awards and fellowships of the Academies in India and The World Academy of Sciences (TWAS).