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Sensors and Actuators A 244 (2016) 50–55

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

j ourna l ho me page: www.elsev ier .com/ locate /sna

nhancing the quality factor of grating coupled plasmon resonance inptical recording media

. Chakrabortya,1, N. Kumawata,1, Sabiha Sultanaa, M.M. Varmaa,b,c,∗

Center for Nano Science and Engineering, Indian Institute of Science, Bangalore, 560012, IndiaDept. of Electrical Communication Engineering, Indian Institute of Science, Bangalore, 560012, IndiaRobert Bosch Center for Cyber Physical Systems, Indian Institute of Science, Bangalore, 560012, India

r t i c l e i n f o

rticle history:eceived 1 February 2016eceived in revised form 16 March 2016ccepted 9 April 2016vailable online 11 April 2016

eywords:rating coupled surface plasmon sensors

a b s t r a c t

Optical recording media such as Digital Video Discs (DVDs) or Blu-ray discs provide a readily availablesource for high quality 1D diffractive structures at the sub-wavelength scale in the visible spectrum.These are useful for several optical sensing methodologies such as Surface Plasmon Resonance (SPR)where the grating couples incident light into plasmon modes supported by the structure. The gratingsavailable in these off-the-shelf devices are not optimized for SPR based sensing. Here we describe a simplemethod to improve the quality factor, an important performance metric of SPR based sensors, of opticalrecording media based plasmonic sensors. We reduced the linewidth of SPR response from a DVD based

ptical recording mediauality factorayer-by-layer assembly

SPR sensor by an order of magnitude using a 100 nm thick polyelectrolyte multilayer film deposited usingsequential layer by layer assembly from solutions. Numerical modeling and Atomic Force Microscopystudies established the origin of quality factor improvement from the change in the topography of thegrating due to a polymer assembly. We demonstrate a limit of detection (LoD) of 7 × 10−6 RIU using themodified DVD based grating coupled SPR sensor.

© 2016 Elsevier B.V. All rights reserved.

. Introduction

Surface plasmons (SPs), which are coherent oscillations of theree-electron plasma, can be excited along interfaces where the realart of dielectric permittivity changes sign [1]. Such a situationccurs at a metal-dielectric interface where the metal possesses

large negative real part of the dielectric permittivity. SPs cane efficiently excited by matching the momentum of incidentadiation with that of the SP mode. The momentum matching isypically done using a prism based attenuated total internal reflec-ion configuration referred to as Kretschmann configuration [2,3]. Arating can also be used to excite SPs by satisfying the momentumatching condition given by Eq. (1) [1],

2�

�ndsin � + m

2�

�= ±ˇsp (1)

∗ Corresponding author at: Center for Nano Science and Engineering, Indian Insti-ute of Science, Bangalore, 560012, India.

E-mail address: mvarma@cense.iisc.ernet.in (M.M. Varma).1 These authors contributed equally to this work.

ttp://dx.doi.org/10.1016/j.sna.2016.04.025924-4247/© 2016 Elsevier B.V. All rights reserved.

where nd is the dielectric medium refractive index, m an inte-ger that represents the diffraction order, � represents the gratingperiod and ˇsp is propagation constant of surface plasmons.

Excitation of SPs results in a narrow resonant dip in the reflectedintensity from the sample either as a function of incident angle or asa function of wavelength [1]. Molecular detection is done based onthe change in the dip of the resonance peak when target moleculesbind on the receptor molecules on the gold surface [2]. SPR hasbeen exploited for a wide range of bio-molecular sensing applica-tions such as protein-ligand [3], protein–protein [4], nucleotide [5]and DNA interactions [6]. In SPR based biosensors, signal transduc-tion is based on the refractive index change when target moleculesbind to the receptor molecules immobilized on the gold surface[2–6]. Optical recording media such as CDs, DVDs and Blu-ray discsare readily available sources for diffraction gratings [7]. DVD hasgrating periodicities of 800 nm, providing a cheap source for highquality sub-wavelength gratings useful for several optical sensingapplications. In particular, by coating the grating of a DVD substrateseveral groups have demonstrated SPR sensors [8] as well as sensors

based on Surface Enhanced Raman Scattering (SERS) [9]. However,the grating parameters, such as the depth and grating profile are notoptimized for efficient coupling of incident light into the plasmon

K. Chakraborty et al. / Sensors and Actuators A 244 (2016) 50–55 51

F age ans s, (e) sf ttach

moswfiaioctdfrWgt

F1

ig. 1. (a) Schematic representing the cross section of a DVD grating, (b) 3D AFM imhowing the polyelectrolyte multilayer and a 50 nm gold layer coated on the gratingrom the DVD gratings and (f) showing the DVD sensor in close up with a flow cell a

ode. Therefore previous reports on SPR sensors based on DVDsptimized grating profiles of commercial polycarbonate DVD sub-trates using etching of the substrate in an organic solvent [8]. Heree demonstrate an alternate method to enhance the Q factor of SPR

rom DVD substrates by depositing alternatively charged polymersn a layer-by-layer (LbL) manner on the grating surface prior to met-llization. The LbL assembly is based on self-limiting electrostaticnteractions of polyelectrolyte materials [10,11]. As the thicknessf each layer is typically around 3 nm, the profile of the grating andonsequently the plasmonic response can be precisely tuned. Fur-her, these polymers can be deposited on to the grating surface byipping the substrates in aqueous solutions permitting a benignabrication process. The grating depth and profile get modified as aesult of LbL assembly leading to an improvement in quality factor.

e performed Atomic Force Microscopy to measure the change inrating profile due to polyelectrolyte assembly. We then simulatedhe effect of modified grating profiles on SPR response using the

ig. 2. Transmitted intensity variation as a function of incident angle for different polyele00 nm thick. With increasing number of layers, the broad SPR peak of un-modified DVD

d (c) AFM height profile of the DVD gratings after removing top layer, (d) schematicchematic showing the experimental setup used to measure transmission/reflectioned.

Rigorous Coupled Wave Analysis (RCWA) method. The simulationsmatched fairly well with experimental data confirming the originof the performance improvement. Quality factor is an importantperformance metric in resonance based sensors and it is inverselyrelated to the linewidth (full width at half maximum (FWHM) of theresonance peak/dip) or sharpness of the response function. Improv-ing the Q factor or equivalently, reducing the resonance linewidthleads to an improvement in the sensor resolution and LoD [12].The best reported FWHM for angular interrogated SPR sensors isaround 2◦ [13] and the typical RI LoD of grating coupled SPR sensorsis around 10−4–10−6 RIU [13]. We reduced the FWHM of angularinterrogated DVD grating based SPR sensor by an order of magni-tude using a 100 nm thick polymer film. Further, we demonstratea limit of detection (LoD) of 7 × 10−6 RIU using the modified DVD

based SPR sensor and demonstrate the detection of the electro-static capture of Bovine Serum Albumin (BSA), a model protein, onthe polyelectrolyte layer using this technique. The performance of

ctrolyte thicknesses starting from 0 bi-layers (0 BL) to 16 bi-layers, which is abouts (0 BL case) splits into two peaks with much smaller linewidths (FWHM).

52 K. Chakraborty et al. / Sensors and Actuators A 244 (2016) 50–55

F ified Db

tbg

2

2

rhuTttckiarAai

Fd

ig. 3. Shift of peak position when water is inserted in flow cell. Polyelectrolyte modetter than that of unmodified DVDs (a).

he modified DVD grating based SPR sensor is therefore compara-le, if not better than the state of the art in angular interrogratedrating coupled SPR sensors.

. Materials and methods

.1. DVD grating substrates

We used commercially available Digital Video Discs (DVDs) as aeadily available source of gratings suitable for SPR sensing. DVDsave a grating fabricated on a polymeric substrate and coveredsing a top layer for mechanical protection as shown in Fig. 1(a).he grating is usually coated with a metal layer to enhance reflec-ion. In the DVDs used for the experiments described in this paper,he grating layer is sandwiched between two 0.5 mm thick poly-arbonate layers. The grating layer was separated by using a sharpnife and used as the substrate. The grating substrate was keptn methanol solution for about 2 min followed by DI water rinsend nitrogen drying to remove a dye layer which is used for optical

ecording and any other coatings which may be present on the DVD.fter removing the top layer and the cleaning procedure describedbove, Atomic Force Microscopy (AFM) revealed a high quality grat-ng strucure with a period of about 700 nm and grating depth of

ig. 4. (a) RI sensitivity of the modified DVD grating SPR sensor probed using NaCl:DI waetermined to be 7.1 × 10−6 RIU. (b) Measurement of electrostatic adsorption of BSA on t

VDs (b) produced a shift relative to the linewidth which was an order of magnitude

about 180 nm (Fig. 1(b) and (c)). LbL assembly of polyelectrolyteswith different thicknesses were performed on these substrates fol-lowing which a gold layer of approximately 50 nm thickness wasdeposited on the polyelelctrolyte modified grating substrates usingDC sputtering to form a final structure as shown in Fig. 1(d).

2.2. LbL assembly of polyelectrolytes

Polyelectrolytes (PAH—Poly-aniline hydrochloride andPAA—Poly-acrylic acid) (Sigma-Aldrich) were used for mod-ification of the grating profile using LbL deposition process.Polyelectrolytes are charged polymers, and in this case, PAH iscationic and PAA is anionic. The polyelectrolytes were depositedin a layer-by-layer (LbL) fashion by alternately dipping the sub-strate in aqueous solutions of these polymers in concentrationof 1 mg/ml. The LbL deposition is possible on any substrate suchas metals, glass, plastic and even transparency sheets [10,11].Thickness of one bilayer is approximately 3–4 nm [10,11]. On theDVD grating substrates 2, 4, 6, 8, 10, 12, 14 and 16 bilayers (one

cationic and one anionic polyelectrolyte together form one bilayer)were deposited. A 50 nm thick gold layer was deposited on top ofthe polymer layer as described in the previous section forming thefinal sensor as shown in Fig. 1(d).

ter mixtures in different molarities. Based on this data, the RI limit of detection ishe polyelectrolyte layer.

K. Chakraborty et al. / Sensors and Actuators A 244 (2016) 50–55 53

F oatingb

2

Fa(salmopsffat

3

sFtDomsabtgottslop

ig. 5. DVD grating depth profile before polymer coating (a) and after polymer cecome smoother compared to the unmodified DVD grating.

.3. SPR measurement setup

The setup used for SPR measurements, shown schematically inig. 1(e), consisted of a 633 nm, 5 mW diode laser source (Coherent),

polarizer (Thorlabs), a mechanical chopper and lock-in ampliferSRS 830 DSP, Stanford research systems). The grating SPR sen-ors were mounted on a rotation stage (PRM 1/M, Thorlabs) and

photo-detector (DET100A/MThorlabs) to collect the transmittedight through the sensor. The mechanical chopper was used to

odulate the intensity of the incident laser beam at a frequencyf around 1.5 kHz. The transmitted light was detected using thehoto-detector and sent to the lock-in amplifier. The transmittedignal intensity through the grating substrate was acquired as aunction of incident angle using a GPIB card and a lab-view inter-ace on a desktop computer. A home-made PDMS based flow cell,s shown in Fig. 1(f) was attached to the DVD sensor to facilitatehe flow of protein solutions and other analytes into the probe area.

. Results

The transmitted intensity response from DVD grating SPR sen-ors described above was acquired as a function of incident angle.ig. 2(a), 0 BL (bi-layer) curve shows the transmitted intensity forhe unmodified DVD grating sensor. It is observed that unmodifiedVD gratings produced a broad SPR peak with an angular FWHMf about 20◦. This broad SPR peak shifts by about 35◦ when theedium above the grating is changed from air to water. The sen-

itivity here are about 106◦/RIU which compares well with otherngular interrogated grating based methods reporting sensitivitiesetween 70◦/RIU and 230◦/RIU [14,15]. It is seen that even thoughhe sensitivity of unmodified DVDs is comparable to that of otherrating based SPR sensors, the FWHM of 20◦ is almost an orderf magnitude worse. It was seen that deposition of progressivelyhicker polyelectrolyte layers caused the emergence of features inhe SPR curve which had much smaller FWHM. Fig. 2(a) and (b)

how the change in SPR curve as a function of the number of bi-ayers deposited (Each bi-layer is about 5–6 nm thick). The FWHMf the resonant feature near 10◦ was about 2◦ with a 16 bi-layerolyelectrolyte film, which is roughly about 110 nm thick. This

(b). Polymer coating reduces the grating depth and causes the grating profile to

FWHM is now comparable to the best FWHMs reported in literature[13].

In order to verify if the features with enhanced Q factors wouldproduce a similar SPR shift as the unmodified DVDs, we input DIwater into the flow cell containing the modified DVD grating SPRsensors. When, water was injected inside the flow cell, the SPR peakshifted to 34.8◦ from 10◦. The shift in this case was 25◦ instead of 35◦

as before. Although the modified DVDs produced lesser SPR shiftscompared to unmodified ones, it should be noted that this shift asa multiple of the FWHM was 10 times better with the modifiedDVDs compared to unmodified ones as shown in Fig. 3(a) and (b),i.e. the shift relative to the linewidth was improved by an order ofmagnitude using polyelectrolyte coating.

We measured the refractive index (RI) sensitivity of the modifiedDVDSPR sensors using a ratiometric scheme [16] to eliminate com-mon mode noise and signal drifts. The sample flow cell was keptfixed at a position of 35◦, and two photodetectors collected the firstand zeroth order light intensities. NaCl solutions in DI water withconcentrations from 0.1 M to 0.5 M in steps of 0.1 M were injectedinto the flow cell following the establishment of baseline signalwith pure DI water (0 M). After the injection of the 0.5 M NaCl solu-tion, pure DI water was injected into the flow cell to check therecovery of the signal back to the baseline value. The results of RIsensitivity measurements are shown in Fig. 4(a). Based on thesemeasurements the RI LoD was 7 × 10−6 RIU. In Fig. 4(a) each stepcorresponds to a refractive index change of about 1 × 10−3 RIU anda noise equivalent signal change of ∼142, that gives us a limit ofdetection of 7 × 10−6 RIU.

To demonstrate that surface binding measurements of biolog-ical samples can be done using modified DVD SPR sensors, theelectrostatic binding of bovine serum albumin (BSA) of concen-trations 1 �g/ml and 10 ng/ml on PAH coated sensors was done.The measurements were started with buffer solutions inside theflow cell and a constant intensity value was observed in the ratiochannel. After that a layer of PAH (a positive polyelectrolyte) wasinjected inside the flow cell. The PAH layer will bind BSA based on

electrostatic interaction [10]. BSA of concentration 10 ng/ml wasinjected inside the flow cell after which the intensity value startedincreasing due to binding of BSA and reached saturation after sometime. Once the intensity value saturated the flow cell was flushed

54 K. Chakraborty et al. / Sensors and Actuators A 244 (2016) 50–55

Fig. 6. (a) Schematic of the grating model used for RCWA simulations (b) Simulated transmitted intensities as a function of incident angle for different grating depths. isshowing how the normalized transmitted light is changing and as well as FWHM with variation of height. (c) How the normalized transmitted light varies with incidencea d anotl

wswmcctscpo5l

sosbaptbafiiuiat

ngle for same height but for different shape of top gold layer. One is for rectangle anight and FWHM is changing with variation of height.

ith buffer solution as marked by the arrow in Fig. 4(b). The inten-ity value remained constant even after the flow cell was flushedith buffer solution. Therefore, this signal change is because of aonolayer of protein bound at the gold surface. After this, a higher

oncentration of BSA, namely, 1 �g/ml, was injected inside the flowell to see the concentration dependent behavior of the binding athe sample substrate. Intensity value started increasing and gotaturated at a higher intensity value compared to the 10 ng/mlase. The expected concentration dependence of the signal furtherroved that the observed signal results from the surface bindingf BSA. Based on the baseline noise level, we estimated a LoD of00 pg/ml for the detection of surface binding of BSA, assuming

inear working regime.To understand the change in SPR response of the DVD grating

ensors due to LbL deposition of polyelectrolytes, we used a Rig-rous Couple Wave Analytical (RCWA) model implemented in theoftware package RSoft DiffractMod [17]. The AFM images takenefore and after coating 16 bilayers of polyelectrolytes and met-llization on top of the DVD grating substrates revealed that theolyelectrolyte coating altered the grating depth and the profile ofhe DVD substrates. From the AFM images shown in Fig. 5, it cane seen that polyelectrolyte coating decreased the grating depth tobout 80 nm from the initial value of 180 nm. Also, the grating pro-le became smoother. We used the RCWA model to understand the

mpact of these modifications on the SPR curve. The RCWA model

sed for simulation and the results are shown in Fig. 6. The grat-

ng depth was varied and the transmitted intensity was calculateds a function of incident angle. The incident wavelength was seto 633 nm under TM polarization. The refractive index of the DVD

her one is trapezoid. (d) Experimental data is showing how normalized transmitted

substrate was taken as 1.55 and the refractive index of gold wastaken from the database provided in RSoft.

The simulated transmitted intensities and the experimentallyobserved transmitted intensities are shown in Fig. 6(b) and (d)respectively. There is a good quantitative agreement between theexperimental and the simulated data. In order to make a propercomparison, both simulated and experimental data were normal-ized with respect to the transmission at normal incidence. Fromthese results, it can be concluded that the reduction in the grat-ing depth leads to the emergence of the narrow linewidth featureobserved in the experimental data. However, polyelectrolyte depo-sition leads to a smoothening of the grating profile in addition to achange in grating depth. In order to investigate the contribution ofgrating profile change on the transmitted intensities, we comparedthe rectangular grating profile with a trapezoidal profile as shownin Fig. 6(c). It was seen that the emergence of the narrow linewidthfeature happens in the case of trapezoidal profile also. Therefore,the change in grating depth is the dominant mechanism for theemergence of narrow linewidth features from the broad SPR peakexhibited by unmodified DVD gratings.

In summary, we have demonstrated a method to modify off theshelf DVD gratings to enhance the quality factor of SPR responseby coating the DVD substartes using a simple, benign coating pro-cess. The linewidth was reduced by an order of magnitude usingthis approach and competetive performance, in terms of RI detec-

tion limit, was obtained with the modified DVD grating based SPRsensors.

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Manoj M. Varma is currently an Associate Professor at theCenter for Nano Science and Engineering (CeNSE),IndianInstitute of Science, Bangalore. His interests span a varietyof topics related to label-free bio-molecular sensing.

K. Chakraborty et al. / Sensors

cknowledgement

We thank the Department of Science and Technology, Sci-nce and Engineering Research Board (DST-SERB) for provindingnancial support for this work through the research grantB/S3/EECE/084/2013.

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ctuators A 244 (2016) 50–55 55

Biographies

Krishnendu Chkaraborty is a doctoral student in theCenter for Nano Science and Engineering at the IndianInstitute of Science, Bangalore. He is interested in prob-lems related to optical bio-sensing, particularly plasmonicsensing.

Nityanand Kumawat is a research associate in the Cen-ter for Nano Science and Engineering at the IndianInstitute of Science, Bangalore. He is interested inproblems broadly related to optical bio-sensing andmicro-fabricated devices.

Sabiha Sultana is a technical staff member at the Centerfor Nano Science and Engineering at the Indian Instituteof Science, Banglaore. Her interest is in nanofabricationmethods.