Abstract
Biosensors based on surface plasmons have shown to be reliable and highly sensitive platforms. Here we investigate the feasibility and the potential of an alternative platform for biological sensing based on Bloch surface waves sustained by a one-dimensional photonic crystal made of dielectric materials.
Introduction
Surface plasmon resonance (SPR) optical biosensors have been established as a mature technology for simple and fast label free biodetection [1]. In commercial surface plasmon resonance (SPR) platforms [2], surface plasmon polaritons (SPPs) are used to sense the refractive index changes at the sensor surface, mainly in real-time conditions.
Recently, electromagnetic modes propagating at the interface between a homogeneous medium and a finite one-dimensional photonic crystal (1DPC), also named Bloch Surface Waves (BSWs), have been proposed as an alternative to SPPs [3–5]. BSWs offer several possible advantages as compared to SPPs. Their dispersion can be designed at almost any wavelength by properly choosing both the refractive index and thickness of the layers constituting the 1DPC. Since dielectrics are characterized by much lower extinction coefficients in their transmission window than metals, BSW resonances appear much narrower than those observed for SPPs, leading to an increase of the expected performances. In addition, it has been shown the possibility of fabricating temperature-insensitive BSW-sensors [6], while this would not be possible with SPP devices.
In this contribution, we investigate the feasibility of using BSWs on dielectric multilayer structures for the biological sensing as a sensitive and robust alternative to SPP-based schemes. Firstly, we experimentally compare the performances of BSWs and SPPs in terms of sensitivity by using a commercial SPR platform. A figure of merit for the direct comparison of the two approaches is suggested. Secondly, we show an example of a time resolved sensing experiment in which the binding of an antibody/anti-antibody mediated by means of BSW is demonstrated.
Sensitivity Comparison of BSW- and SPP-Based Biosensors
We experimentally report on the direct comparison of the sensitivity of BSW and SPP based biosensors. The experiments were carried out by using an SPR platform [2], operating in a focused-beam Kretschman-configuration. In order to compare BSW and SPP performances, we used either a 1DPC or a 45-nm thick gold layer deposited on a common glass slide, respectively. The 1DPC supporting BSWs consists of a six-period stack of alternate high and low index materials, based on different silicon nitride stoichiometry. The thicknesses of the high index (nH = 2.44 at 804 nm) and low index layer (nL = 1.76 at 804 nm) are 123 and 185 nm, respectively. With the purpose of evaluating the sensitivity of the two substrates as a function of the refractive index change at the surface, we spotted droplets of glucose solutions at different concentration, ranging from 0.01 % to 10 %. The resonance shift was then recorded and compared with the position of the resonance as measured for the bare substrate.
In Fig. 18.1 we show the reflectance shifts at different glucose concentration for BSWs (a) and SPPs (b). In the inset are the sensitivities of the two sensors as defined by Homola et al. (1999) as a function of the shift of the reflectance curve minimum. Here, the shift is estimated in terms of CCD pixels. The SPP modes appear to be much broader compared to BSW modes and show a greater shift of the minimum. By using the formula Δn = βC relating the refractive index variation to the concentration C (β = 1.527 · 10−3 [7]), we can express the sensitivity for the BSW and SPP sensors as SBSW = 26.3 deg/RIU and SSPP = 79.3 deg/RIU. According to the standard sensitivity analysis, SPP sensors are thus to be considered to perform better as compared to BSW sensors. However, width and depth of the resonance curves must be taken into account. In literature, a figure of merit (FOM) is given that includes the full-width half-maximum (FWHM) [8]. As a fair comparison between the sensors, we suggest a modified FOM that takes also into account the resonance depth, as follows: FOM = D · S/W, where D is the curve reflectance depth, S is the sensitivity and W is the FWHM. According to the suggested method, the FOMs for the SPP and the BSW sensor are FOMSPP = 38.6 RIU−1 and FOMBSW = 51.7 RIU−1, respectively. By taking the ratio of the above FOM estimates, we find that FOMBSW/FOMSPP = 1.3.
Polymer-Functionalized 1DPC for Biosensing
We investigate the feasibility of a biosensor based on a 1DPC sustaining BSWs whose surface has been functionalized through plasma deposition of 30-nm polymerized PolyAcrylic Acid [9]. The PPAA functionalization results in about 106 -COOH groups to be exposed on the 1DPC surface, providing binding sites for the anti-Angiopoietin-1 antibody that was subsequently incubated.
The biodetection consisted in the real-time monitoring of the BSW-resonance amplitude at the flex point while subsequently injecting VEGF (negative antibody) and Angiopoietin-1 (positive cancer marker) at a concentration of 9 ng/mL. The photonic crystal was designed to work in PBS using a laser emitting at 1,550 nm. Figure 18.2 shows that the VEGF negative antibody does not specifically bind to the surface, as the refractive index of the resonance amplitude returns to its original level. Instead, the specific recognition and binding of the antibody Ang-1 is demonstrated by the persistent amplitude variation observed.
Conclusions
We demonstrated the feasibility of a biosensor exploiting the properties of BSW propagating at the surface of a 1DPC. The performances of such a sensor were compared against a commercial SPR-based device. It was concluded that a BSW-based sensor may show higher sensitivity in terms of RIU−1 if both resonance FWHM and depth are considered for estimating a comparative figure of merit. The specific binding dynamics of a cancer marker onto a properly PPAA-functionalized 1DPC surface were shown in a real-time experiment.
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Ballarini, M. et al. (2014). Bloch Surface Waves on Dielectric Photonic Crystals for Biological Sensing. In: Baldini, F., et al. Sensors. Lecture Notes in Electrical Engineering, vol 162. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-3860-1_18
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DOI: https://doi.org/10.1007/978-1-4614-3860-1_18
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