Dual Refractive Index and Viscosity Sensing Using Polymeric Nanofibers Optical Structures

Abstract

Porous materials have demonstrated to be ideal candidates for the creation of optical sensors with very high sensitivities. This is due both to the possibility of infiltrating the target substances into them and to their notable surfaceto-volume ratio that provides a larger biosensing area. Among porous structures, polymeric nanofibers (NFs) layers fabricated by electrospinning have emerged as a very promising alternative for the creation of low-cost and easy-to-produce high performance optical sensors, for example, based on Fabry-Pérot (FP) interferometers. However, the sensing performance of these polymeric NFs sensors is limited by the low refractive index contrast between the NFs porous structure and the target medium when performing in-liquid sensing experiments, which determines a very low amplitude of the FP interference fringes appearing in the spectrum. This problem has been solved with the deposition of a thin metal layer (∼ 3 nm) over the NFs sensing layer. We have successfully used these metal-coated FP NFs sensors to perform several real-time and in-flow refractive index sensing experiments. From these sensing experiments, we have also determined that the sponge-like structure of the NFs layer suffers an expansion/compression process that is dependent of the viscosity of the analyzed sample, what thus gives the possibility to perform a simultaneous dual sensing of refractive index and viscosity of a fluid.

Label-Free Optical Biosensing Using Low-Cost Electrospun Polymeric Nanofibers

“…According to the smoothed data in Figure 4, a spectral shift of the maximum peak of 1.22 ± 0.01 nm toward longer wavelengths was obtained for the 2 × 10 −3 RIU (refractive index unit) change. Since the linear response of this type of NF layer in the range from 2 × 10 −3 to 2.5 × 10 −3 RIU has been previously demonstrated [14], sensitivity can be directly estimated to be ca. 610 ± 5 nm/RIU from the spectral shift observed.…”

“…Regarding the stability of the NF layer, the fact that the maximum peak position stayed centered at a given wavelength when both DIW and 1× PBS were flowed together with the absence of a drift in the measurement demonstrate that the structure is not swelling, and hence, that it was properly stabilized [14,15]. To start the in-flow experiment, the microfluidic chamber was assembled on the NF layer and deionized water (DIW) was flowed to characterize its reflectivity spectrum in an aqueous medium.…”

“…The outlet tube was connected to a syringe pump working in withdraw mode at 20 µL/min (see Figure 2). In order to improve the optical response of the resulting NF layers, a 3-nm-thick gold layer was deposited over them by sputtering for 3 min to increase the amplitude of the FP fringes in aqueous media [14]. Afterward, in order to stabilize their structure by welding the local contact network at the NF mesh, the NF layers were heated at 190 • C for 3 h under a pressure of approximately 500 g/cm 2 , allowed to cool down at room temperature, and immersed in water for 1 h [15].…”

“…In the end, the fact that the maximum peak reached a stable final spectral position (i.e., a position centered at a given wavelength) when 0.1 M MES was flowed demonstrates that the RI change provoked by the binding of AGp to the surface of the NF layer was stable and thus, its desorption did not occur. The subsequent step, after the AGp layer formation over the NFs, was flowing a 2.5 mg/mL casein solution in 1× PBS to block the remaining voids between the AGp molecules and avoid the Regarding the stability of the NF layer, the fact that the maximum peak position stayed centered at a given wavelength when both DIW and 1X PBS were flowed together with the absence of a drift in the measurement demonstrate that the structure is not swelling, and hence, that it was properly stabilized [14,15].…”

Simultaneous Refractive Index Sensing Using an Array of Suspended Porous Silicon Membranes

“…We chose this structure because closely-spaced fringes are formed in the NIR region, where the equipment used during the experiments recorded the spectra. The position of each fringe is given by the next equation [40], where λ is the wavelength position, n the RI, h the thickness and m an integer sorting all the fringes: = 2ℎ / .…”