Slab plasmonic platforms combined with Plastic ...

Optics and Laser Technology xxx (2018) xxx–xxx

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Slab plasmonic platforms combined with Plastic Optical Fibers and Molecularly Imprinted Polymers for chemical sensing Luigi Zeni a,⇑, Maria Pesavento b, Simone Marchetti b, Nunzio Cennamo a a b

University of Campania Luigi Vanvitelli, Department of Engineering, Via Roma 29, 81031 Aversa, Italy University of Pavia, Department of Chemistry, Via Taramelli 12, 27100 Pavia, Italy

a r t i c l e

i n f o

Article history: Received 27 April 2018 Received in revised form 7 June 2018 Accepted 12 June 2018 Available online xxxx Keywords: Surface Plasmon Resonance (SPR) Slab optical waveguides Plastic optical fibers (POFs) Molecularly Imprinted Polymer (MIP) Furan-2-carbaldehyde (2-FAL)

a b s t r a c t A novel low-cost surface plasmon resonance (SPR) platform has been tested, for the first time, to monitor the interaction between a molecularly imprinted polymer (MIP) and a small molecule as the substrate. As a proof of principle, the considered MIP was specific for furfural (2-FAL, MW = 96.4), so that the possibility of using the new device for detection of 2-FAL in aqueous media was investigated. For the sake of comparison a sensor based on the same MIP specific for 2-FAL deposited on an SPR platform in a D-shaped plastic optical fiber (POF), which has been previously demonstrated to well perform for different analytes, has been considered too. The experimental results showed good performances of the novel platform for chemical sensing based on MIP as receptor in terms of selectivity, sensitivity and figure of merit (FOM). The limit of detection (LOD) of this simple and low-cost SPR sensor system is about 0.03 ppm, completely comparable to that of previously proposed devices based on SPR in a D-shaped POF (about 0.05 ppm), but with the advantage of an easier and more reproducible preparation procedure. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Surface plasmon resonance (SPR) is a very sensitive method for determining refractive index variations at the interface between a metal layer and a dielectric medium. Thus, it is a transduction technique particularly suitable for marker free sensors, in which the dielectric is a receptor layer with refractive index depending on its interaction with a particular substrate. In the scientific literature, several review papers describe plasmonic sensor platforms and their applications [1–6]. Most often the SPR is excited by light from a high refractive index prism coated with a thin metallic layer or a glass chip coated with a thin layer of metal and bonded to the prism. In this configuration (Kretschmann configuration) the angle of the incident light, which satisfies the plasmonic resonance condition, changes when the surrounding medium refractive index changes. Instrumentation for Kretschman configuration is bulky and requires expensive optical equipment, so that the miniaturization of the different components is not easy and, in addition, the remote sensing capability may be questionable. Jorgenson et al. realized SPR sensors in optical fibers without prisms [7], in which the metal layer was directly deposited on the core of an optical fiber. The SPR sensors in optical fiber allow for remote sensing and for reduced dimension and price of the ⇑ Corresponding author. E-mail address: [email protected] (L. Zeni).

whole sensor system. At the beginning, the optical fibers employed were made of glass, but more recently plastic or special optical fibers have been used too [7–17]. The plastic optical fibers (POFs) present exceptional flexibility, simple manipulation, large numerical aperture, big diameter. Also, they are able to withstand smaller bend radii than glass fibers. Therefore, POFs are particularly advantageous for the realization of low-cost SPR sensors [16]. SPR sensors based on optical fibers show a noticeably high sensitivity, due to the fact that they are able to detect even small variations of refractive index of the medium (dielectric) in contact with the metal layer [18–21]. When biological or artificial receptors are present at the metal-dielectric interface, they selectively capture the analyte present in the sample under test, and a local variation of the dielectric’s refractive index in contact with the metal film is produced. Different structures have been proposed for surface plasmon resonance (SPR) sensors based on optical fibers [5]. D-shaped POF platforms have been successfully developed by our research group for different analytes and different receptors such as antibodies, aptamers and molecularly imprinted polymers (MIP) too [22–25]. The flat part of the D-shaped platform is suitable for an easy deposition of the receptors, in particular MIPs, and makes it possible to perform the determination in a drop, instead of requiring complex flux devices. The D-shaped POF platforms present however some challenging points, in particular the irreproducibility. Actually, they are obtained by a hand polishing procedure,

https://doi.org/10.1016/j.optlastec.2018.06.028 0030-3992/Ó 2018 Elsevier Ltd. All rights reserved.

Please cite this article in press as: L. Zeni et al., Slab plasmonic platforms combined with Plastic Optical Fibers and Molecularly Imprinted Polymers for chemical sensing, Opt. Laser Technol. (2018), https://doi.org/10.1016/j.optlastec.2018.06.028

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L. Zeni et al. / Optics and Laser Technology xxx (2018) xxx–xxx

which could lead to a somewhat irreproducible morphology of the D-shaped region (roughness and total depth). Previous investigations showed that this could strongly influence the performance of the platform [26,27]. It has been shown that MIPs are easily produced over gold by in-situ polymerization, forming thin and firm receptor layers, which are suitable for SPR detection by the D-shaped POF platforms [23,25]. MIPs are synthetic receptors with many favorable aspects with respect to bio-receptors, such as an easier and faster preparation, the possibility of application outside the laboratory, for example under environmental conditions, a longer durability [28,29]. The advantage of MIPs is that they can be directly formed on a flat gold surface by depositing a drop of prepolymeric mixture directly over gold, spinning in a spin coater machine [23,25], and in situ polymerization without modifying the surface (functionalization and passivation), as needed for the bio-receptors [22,24]. Besides, their refractive index can in principle be modulated in order to be suitable for the SPR transduction. Taguchi et al. presented a different interesting approach for Bisphenol A (BPA) detection, exploiting molecularly imprinted nanoparticles combined with a slab optical waveguide [30]. This approach is similar to that used in the Kretschmann configuration but exploiting optical fibers combined with a slab waveguide. In this work, an innovative SPR platform [31], based on a slab waveguide and optical fibers, much simpler than the Kretschmann configurations and similar to Taguchi’s platform [30], has been considered for developing an MIP based sensor. The optical platform, which has been previously described as a refractive index sensor [31], is a removable slab waveguide based on a commercially available polymethyl methacrylate (PMMA) chip covered by a gold thin film. It is tightly held by a holder specifically designed to assure a reproducible positioning of the slab. A distinctive feature of the proposed experimental set-up is the way for launching the light in the slab, which occurs through an air trench. It allows a large number of light modes to penetrate the slab, some of which are able to excite the SPR in the considered refractive index range. Two POFs connect the special holder with a light source and a spectrometer. The figure of merit of this platform has been found to be very near to that of the D-shaped platform [16,31], but some benefits in terms of reproducibility are expected. Namely, this novel platform doesn’t require the polishing procedure of the POFs, while maintaining the flat shape of the sensing surface and the use of optical fibers, which is expected to give a better reproducibility of the sensing interface and the possibility of an easy replacement of the chip. As a proof of principle, a specific MIP for 2-FAL (furan-2carbaldehyde) is deposited on the PMMA-gold chip. This particular MIP has been considered since it is well known, from previous investigation [32], as a strong and selective receptor of 2-FAL from mineral oil, producing a good SPR signal. Here the response of the MIP-SPR sensor, based on the newly proposed slab SPR platform, to the concentration of 2-FAL in water is investigated in order to test its effectiveness. For the sake of comparison, a non-imprinted polymer (NIP) on the same slab configuration is presented. In addition, the same MIP specific for 2-FAL but applied on an SPR D-shaped POF platform, i.e. that which has been previously demonstrated to well perform combined with MIP receptors [23,25,32], will be presented too.

2. Material and methods 2.1. Chemicals Divynilbenzene [1321-74-0] (DVB), methacrylic acid (MAA) [79-41-4] (Sigma–Aldrich cod. M0782) and 2,20 -azobisisobutyroni

trile [78-67-1] (AIBN), were obtained from Sigma–Aldrich. MAA and DVB were purified with molecular sieves (Sigma–Aldrich cod 208604) prior to use in order to remove stabilizers. 2-furaldehyde (2-FAL) was obtained from Sigma-Aldrich, and was used as received. All other chemicals and solvents were of analytical reagent grade. Stock solutions were prepared by weighing and dissolving 2-FAL in water. Standard solutions were prepared by dilution from the stock solution. 2.2. Prepolymeric mixture: preparation and deposition The prepolymeric mixture for MIP was prepared according to the procedure reported in [32]. Divinylbenzene (DVB), the crosslinker, was at the same time the solvent in which the functional monomer (that is, methacrylic acid, MAA), and the template, furfural (2-FAL), were dissolved. The reagents were at molar ratio 1 (2-FAL):4 (MAA):40 (DVB). The mixture was uniformly dispersed by sonication (visually homogeneous solution) and de-aerated with nitrogen for 10 min. Then the radical initiator AIBN (molar ratio: 1 (2-FAL):0.4 (AIBN)) was added to the mixture. The non-imprinted polymer (NIP) composition and its polymerization method was that described for MIP but without the template. The same procedure for the MIP deposition has been used for the sensor obtained on slab waveguide and for the sensor on D-shaped POF. These two sensors are indicated respectively as SPR-Slab-MIP (the sensor obtained on PMMA slab) and SPR-POF-MIP (the sensor on POF). Before the MIP deposition the gold planar surface (SPR active surface) was washed with ethanol and then dried in a thermostatic oven at 60 °C. 50 lL of the prepolymeric mixture were dropped over the sensing region (gold film) and spun for 45 s at 800 rpm. Thermal polymerization was then carried out for 16 h at 80 °C [32]. The template was extracted from the polymer by ten washings with 96% ethanol [32]. In each washing one drop of ethanol was contacted with the platform for a few seconds, and then fluxed away with fresh ethanol. The sensors were finally dried in an oven at 60 °C. Fig. 1 shows a schematic view of the cross section of the sensor with the deposited MIP receptor. The NIP layer was deposited on the SPR-slab platform with the same procedure used for the MIP layer. 2.3. Optical platform based on slab waveguide and POFs The optical sensor platform with the MIP receptor is shown in Fig. 2. It is composed of a removable multilayer chip (a PMMA chip with a gold film on the top), two plastic optical fibers and a special holder [31]. The removable chip is a PMMA slide (0.5 mm thick and 10 mm 10 mm), with a thin gold film on the top (60 nm thick) deposited by sputtering. The MIP is formed over the gold layer by the procedure described in Section 2.2. The special holder is composed by two aluminum blocks: the first one accommodates the PMMA-gold chip and the two POFs while the second one is a cover, equipped with a hole and an o-ring to retain the liquid samples. Fig. 2 shows a schematic view of the light path in the SPR sensor platform with the MIP receptor layer in contact with the gold film (sensing area). The exciting light is introduced in the slab waveguide by a trench (size: 1 mm 1 mm and 10 mm long), realized directly in the holder, illuminated by a POF (core of PMMA and cladding of fluorinated polymer, with 1 mm total diameter). Another similar POF was placed at the end of the slab waveguide, at 90° with respect to the trench, to carry the output light to a spectrometer. The trench has been introduced because a large incident angle is required for the surface plasmon resonance excitation [31].



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Fig. 1. Cross section of an MIP receptor layer deposited on a gold film.

Fig. 2. Schematic view of the light path in the SPR sensor with an MIP receptor on the gold film. Top and cross section view of the sensor system and outline of the experimental setup with picture of the equipments.

The experimental setup is shown in Fig. 2. It is composed of a light source, a halogen lamp (HL–2000–LL, Ocean Optics) exhibiting a wavelength emission range from 360 nm to 1700 nm, the SPR sensor system and a spectrometer (FLAME-S-VIS-NIR-ES, Ocean Optics) connected to a computer [31]. The spectrum analyzer detection range is from 350 nm to 1023 nm. 2.4. Optical platform based on a D-shaped POF The SPR platform in a D-shaped POF was prepared according to a procedure which has been widely described in previous biochemical applications [22–25,32], as schematized in Fig. 3. In par-

ticular, this SPR sensing platform was realized by removing the cladding and part of the core of a multimode plastic optical fiber along half circumference by manual side-polishing with polishing papers, (step 1 in Fig. 3), then depositing an optical buffer layer of Microposit S1813 photoresist on the exposed core by spinning (step 2 in Fig. 3) and sputtering a thin gold film (step 3 in Fig. 3) [16]. Finally, an MIP layer specific for the considered analyte (step 4 in Fig. 3) is deposited over the flat SPR surface, with a spin coater machine, as described in the previous section. To obtain the signal from this optical sensor we have used the same equipment (halogen lamp and spectrometer) of the slab waveguide and POFs configuration, previously described.


4


Fig. 3. SPR-POF platform with MIP receptor and schematic view of the production steps.

2.5. Experimental procedure The flat shape of these platforms makes it possible to perform the SPR measurements in a drop, which is a very convenient aspect of the proposed sensors. A small amount of solution, about 50 lL, was dropped over the MIP layer and the spectrum recorded after ten minutes incubation. The SPR curves along with data values were displayed online on the computer screen and saved with the help of advanced software provided by Ocean Optics. The SPR transmission spectra were normalized to the reference spectrum by Matlab software. In particular, for both the SPR platforms, the transmission spectra have been normalized to the spectra obtained in air before MIP deposition, in which no plasmon resonance is excited [31,32]. The dose-response curves were obtained by plotting the resonance wavelength variation (Dk) as a function of the 2-FAL concentration (ppm).

3. Results and discussion 3.1. Determination of 2-FAL concentration by SPR-slab platform The normalized transmission spectra obtained with the SPRslab-MIP sensor exhibit a large band with several transmission minima in the considered wavelength range, as shown in Fig. 4. Nevertheless, the minimum at around 690 nm is much better defined than the other minima, and, moreover, it depends on the 2-FAL concentration, as seen in the inset of Fig. 4. There is a noticeable difference between the shape of the resonance peak, in partic-

Fig. 4. Normalized SPR spectra acquired by the SPR-slab-MIP sensor for different furfural concentrations (in ppm) in water. Inset: zoom of resonance wavelengths at 690 nm.

ular the width at half height, of the platform exploiting liquid dielectric [31] and the one with polymeric dielectric, as MIP is. This could be tentatively ascribed to the presence of several resonances due to the not perfect homogeneity of the polymeric layer over gold, in contrast to what happens with the liquid layer, and/or to the presence of resonant nanostructures produced during the spinning procedure or the polymerization at high temperature [6]. The resonance wavelength is shifted to higher values (red shift) when the 2-FAL concentration increases. Fig. 5 shows the variation of the resonance wavelength with respect to the blank (Dk, nm) versus the analyte concentration (ppm), in a semi-log scale, and


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L. Zeni et al. / Optics and Laser Technology xxx (2018) xxx–xxx Table 2 Chemical parameters for 2-FAL detection in water by a SPR-Slab-MIP sensor. Sensor

Detection of 2-FAL in water solutions

SPR-slab-MIP

Dkc ¼

Fig. 5. Resonance wavelength variation (with respect to blank 0 ppm) versus 2-FAL concentrations, in semi-logarithmic axes, for SPR-Slab-MIP and SPR-Slab-NIP. When MIP is present the Hill fitting of the experimental values is also shown.

the fitting by the Hill equation [33] reported below (Eq. (1)), which is satisfactory. Each experimental point is the average of 5 subsequent measurements and the error bars are the respective standard deviations (see Fig. 5).

Dkc ¼ Dkmax

cn K n þ cn

ð1Þ

where Dkmax is the maximum resonance wavelength variation at increasing concentration of 2-FAL, i.e. the wavelength at the plateau value obtained at high 2-FAL concentration:

Dkmax ¼ kmax k0

ð2Þ

The symbol c indicates the concentration of the analyte and kc the resonance wavelength at c2-FAL concentrations. The resonance at 0 ppm concentration is indicated as k0. The dependent variable considered was Dkc = kc k0, i.e. the shift of the resonance wavelength at increasing concentration of 2-FAL. The two parameters K and n are here considered simply as descriptors of the standardization curve (Eq. (1), but they can also have a physical meaning [33], as it will be discussed later. The fitting was performed by Hill fitting equation (OriginPro 8.5, Origin Lab. Corp., Northampton, MA, USA). The parameters obtained and the associated standard errors are listed in Table 1. The ‘‘START” value (Dk0) should be not significantly different from 0, since it indicates the shift of the resonance wavelength with the analyte at 0 concentration, as it actually is. The value of n in the Langmuir sorption model should be 1, and it is different in case of cooperative effects. Here it can’t be considered as significantly different from 1. From Eq. (1), it is possible to notice that, if n 1 and at low concentration, i.e. at c much lower than K, the dose-response curve is linear, with sensitivity Dkmax/K, defined as the ‘‘sensitivity at low concentration”, as shown in Eq. (3):

Parameters

Value

Kaff [ppm1 ] Sensitivity at low c [nm/ppm] LOD [ppm] (3*standard deviation of blank/sensitivity at low c)

10.59 27.66 0.029

Dkmax c K

ð3Þ

Standardization curves like that reported in Fig. 5, described by Eq. (1), and with the Hill parameters listed in Table 1, are commonly used for chemo and biosensors, and their physical meaning can be related to the fact that the absorption takes place by the combination of the substrate at specific sites, when the number of receptor sites available for the combination with the substrate is limited. In that case the adsorption takes place according to the Langmuir absorption isotherm, as previously reported for different MIP based SPR sensors [23,25,32]. The parameter K of the Hill equation (Eqs. (1) and (3) corresponds to the reciprocal of the affinity constant of the specific sites of the Langmuir model [33]. The affinity constant (Kaff), the sensitivity at low concentration and the lower detection limits (LOD) of 2-FAL in water, for this new SPR-slab-MIP sensor, are reported in Table 2. The LOD can be calculated as the ratio of three times the standard deviation of the blank (reported in Table 1 as ‘‘START”) and the sensitivity at low concentration (Dkmax/K) [33]. Finally, a sensor based on a non-imprinted polymer (NIP) layer on the same SPR-Slab platform was examined by a similar procedure. The resonance shifts relative to the NIP-Slab sensor were compared to those of the previous test in Fig. 5. When an NIP layer is present on the SPR-Slab platform, the resonance wavelength is the same at different 2-FAL concentrations (the shift is around 0.5 nm), indicating that 2-FAL is not adsorbed by the NIP, or if it is, it does not produce any refractive index variation of the polymer layer. Thus aspecific adsorptions, i.e. those not involving the combination with the specific imprinted sites, are not relevant for the signal formation.

3.2. Reference sensor platform: MIP on SPR D-shaped POF For the sake of comparison, the same specific MIP for 2-FAL was deposited on an SPR platform in a D-shaped POF, as already illustrated in the previous investigations where the solvent was a transformer oil [32]. The experimental results, obtained with the classic SPR-POF-MIP sensor for different concentrations of 2-FAL in water, are reported in Fig. 6. The SPR curves are reported in Fig. 6(a) and the dose-response curve in Fig. 6(b), respectively. As shown in Fig. 6(a), in this case the width at half height seems to be somewhat smaller, around 300 nm. Even if this may not have any particular meaning, it could be worth to notice that another difference is the depth of the resonance which is higher in SPRSlab-MIP than in the SPR-POF-MIP. The spectra for the D-shaped platform reported in Fig. 6(a) are very similar to those reported in Fig. 4 for the SPR-Slab-MIP sensor. When the 2-FAL

Table 1 Hill parameters of 2-FAL detection in water by SPR-Slab-MIP sensor. START (Dk0)

END (Dkmax)

K

Value

Standard error

Value

Standard error

Value

Standard error

Value

Standard error

Reduced Chi-Sqr

Adj. R-square

0.398

0.266

3.008

0.145

0.094

0.017

2.22

1.01

1.40

0.96

n

Statistics


6


Fig. 6. (a) Normalized SPR spectra acquired by the SPR-POF-MIP sensor for different 2-FAL concentrations (in ppm) in water. Inset: zoom of resonance wavelengths. (b) Resonance wavelength versus the 2-FAl concentrations, by SPR-POF-MIP sensor, with the Hill fitting of the experimental values.

concentration increases the resonance wavelength at around 690 nm is shifted to higher values (red shifted), as seen in the case of SPR-Slab-MIP too. When compared to the spectra obtained with the SPR-Slab-MIP sensor (Fig. 4), those with the SPR-POF-MIP sensor (Fig. 6a) present a shape difference only around 450–550 nm, far from the resonance wavelength which is at about 690 nm. The shape difference and the opposite dip intensity variation with the 2-FAL concentrations, could be caused by the photoresist buffer layer which is present between the gold film and the POF’s core in the D-shaped POF platform but not in the SPR-slab-MIP one. This photoresist layer, apart from improving the SPR performances and the adhesion of the gold film, could be the reason for a different intensity variation with the 2-FAL concentrations. Furthermore, the photoresist gives rise to nanostructures on the

gold surface which could be responsible for the other observed resonance peaks. In Fig. 6(b) the dose-response curve obtained with the SPR-POFMIP sensor, with the Hill fitting of the data, is reported, whereas the Hill parameters obtained and the associated standard errors are listed in Table 3. The affinity constant (Kaff), the sensitivity at low concentration and the LOD of 2-FAL in water, for the SPRPOF-MIP sensor, are reported in Table 4. The LOD is calculated as the ratio of three times the standard deviation of the blank (reported in Table 3 as ‘‘START”) and the sensitivity at low concentration (Dkmax/K) [33]. Table 4 clearly shows that the same performance obtained with the SPR-POF-MIP sensor, in which the POF acts directly as the waveguide, is obtained by this new SPR-MIP sensor in which the PMMA slab is the waveguide (see Table 2).

Table 3 Hill parameters of 2-FAL detection in water by SPR-POF-MIP sensor. START (Dk0)

END (Dkmax)

K

Value

Standard error

Value

Standard error

Value

Standard error

Value

Standard error

Reduced Chi-Sqr

Adj. R-square

0.253

0.476

2.647

0.219

0.087

0.045

1.011

0.503

3.500

0.968

n

Statistics


L. Zeni et al. / Optics and Laser Technology xxx (2018) xxx–xxx Table 4 Chemical parameters for 2-FAL detection in water by a SPR-POF-MIP sensor. Sensor

SPR-POF-MIP

Detection of 2-FAL in water solutions Parameters

Value

Kaff [ppm1 ] Sensitivity at low c [nm/ppm] LOD [ppm] (3*standard deviation of blank/sensitivity at low c)

11.49 27.49 0.052

4. Conclusions The experimental results demonstrate that the newly proposed SPR platform is suitable for the realization of a chemical sensor based on MIPs as selective receptors. It has been applied to the selective determination of 2-FAL in aqueous samples even at the low concentrations usually found, for example, in food or in drinking waters. For comparison, we have also tested a sensor based on a non-imprinted polymer (NIP), which demonstrated that not any signal is obtained by the non-specific adsorption on the NIP, either because such adsorption would not take place or it does not produce any variation in the refractive index of the polymer. The detection of 2-FAL in water solutions is possible because the adsorption of 2-FAL on an MIP receptor produces a refractive index variation which is measurable with the SPR platform based on the PMMA slab illuminated through a trench in air here proposed, with the same performances obtained by another low-cost SPR-POF-MIP sensor in which the resonance is induced by light coming directly from the optical fiber itself. Several sensors for 2-FAL detection have been tested to assess the tolerance against the fabrication process and the repeatability of the performances, as well. The results showed a good tolerance and repeatability. In fact, one of the advantages of this novel SPR approach is the possibility of exploiting the input and output POFs to connect the measuring equipment with the SPR sensor chip, with a better reproducibility from platform to platform with respect to the D-shaped one, due to the better controlled thickness and roughness of the PMMA slab and to the lower number of steps required for the preparation. Just to mention one, the new platform does not require a manual grinding of the fiber. Furthermore, the measurement itself is more repeatable since the connection of the input and output optical fibers is much more stable because of the holder. In the future, we will test this new SPR sensor system for specific applications in real matrices, as for example, the determination of 2-FAL in beverages as wine [34] and beer [35], which can have a high practical interest on one hand for its toxic and carcinogenic effects on human beings and, on the other hand, for its impact on the aroma. References [1] J. Homola, Present and future of surface plasmon resonance biosensors, Anal. Bioanal. Chem. 377 (2003) 528–539. [2] A. Leung, P.M. Shankar, R. Mutharasan, A review of fiber-optic biosensors, Sens. Act. B: Chem. 125 (2007) 688–703. [3] K. Anuj, R.J. Sharma, B.D. Gupta, Fiber-optic sensors based on surface Plasmon resonance: a comprehensive review, IEEE Sens. J. 7 (2007) 1118–1129. [4] J. Homola, S.S. Yee, G. Gauglitz, Surface plasmon resonance sensors: review, Sens. Act. B: Chem. 54 (1999) 3–15. [5] C. Caucheteur, T. Guo, J. Albert, Review of plasmonic fiber optic biochemical sensors: improving the limit of detection, Anal. Bioanal. Chem. 407 (2015) 3883–3897. [6] M.C. Estevez, M.A. Otte, B. Sepulveda, L.M. Lechuga, Trends and challenges of refractometric nanoplasmonic biosensors: a review, Anal. Chim. Acta 806 (2014) 55–73.

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