Sol–Gel-Based Titania–Silica Thin Film Overlay for Long Period Fiber Grating-Based Biosensors

Abstract

An evanescent wave optical fiber biosensor based on titania-silica-coated long period grating (LPG) is presented. The chemical overlay, which increases the refractive index (RI) sensitivity of the sensor, consists of a sol-gel-based titania-silica thin film, deposited along the sensing portion of the fiber by means of the dip-coating technique. Changing both the sol viscosity and the withdrawal speed during the dip-coating made it possible to adjust the thickness of the film overlay, which is a crucial parameter for the sensor performance. After the functionalization of the fiber surface using a methacrylic acid/methacrylate copolymer, an antibody/antigen (IgG/anti-IgG) assay was carried out to assess the performance of sol-gel based titania-silica-coated LPGs as biosensors. The analyte concentration was determined from the wavelength shift at the end of the binding process and from the initial binding rate. This is the first time that a sol-gel based titania-silica-coated LPG is proposed as an effective and feasible label-free biosensor. The specificity of the sensor was validated by performing the same model assay after spiking anti-IgG into human serum. With this structured LPG, detection limits of the order of tens of micrograms per liter (10(-11) M) are attained.

Biosensing with optical fiber gratings

“…In fact, the high refractive index (HRI) coating pulls the light toward the surrounding medium extending its evanescent field, resulting in an improved sensitivity of the device to the SRI. Therefore, by carefully adjusting the overlay thickness, it is possible to tune the maximum sensitivity into the desirable range of RI [18] for both LPGs [18,19] and TFBGs [20]. The second strategy, which concerns LPGs only, consists of coupling the core mode to a high-order cladding mode (11th-14th) near its turn-around point (TAP) in its phase-matching curve [21].…”

“…As for biosensing applications, different types of nanocoatings are deposited on the fiber as improved substrates for the implementation of the selective biolayer, such as titania-silica sol-gel-derived nm-thick films [19], thin film of atactic polystyrenes (PS) [30], graphene oxide (GO) films [31], composites of GO film and single-walled carbon nanotubes (SWNTs) [32], or gold nanoparticles [33] with the purpose of improving the performance of the sensor by leading to greater sensitivities and lower detection limits. Figure 3A details the flow chart of the manufacturing steps for coating a μm-thick EFBG with GO and SWNTs, whereas Figure 3B accounts for a SEM image of the proposed sensor.…”

“…When a not uniform layer is deposited on an optical fiber, the depth of the resonance dramatically decreases, and its shape becomes asymmetric, worsening drastically the sensor performance. In addition, the use of porous coatings surely increases the surface coverage and, hence, the functionalities of the biosensor, but the sensor response time can noticeably increase [19]. Therefore, a trade-off among the film peculiarities and the used substrate should inevitably be envisaged.…”

“…Therefore, a trade-off among the film peculiarities and the used substrate should inevitably be envisaged. In general, the typical time required to perform a complete receptoranalyte binding measurement (from the binding interaction up to the washing step) is of the order of 20-60 min [19,34], which can be reduced down to 5-10 min with an alternative approach involving the initial binding rate [19,43]. Clearly, these values are not an exclusive characteristic of the OFG-based sensing systems, but are also typical features of all the other optical platforms able to monitor real-time interactions (such as those based on SPR).…”

“…As shown in Table 1, in addition to IgG/anti-IgG immunoassays [19,30,[43][44][45], the use of antibodies as BRE was selected for proteic markers (C-reactive protein (CRP) [31], thyroglobulin [50], human transferrin [49]) and for receptors present on cell membranes [47,48] or bacteria [46].…”