Label-Free Optical Biochemical Sensors via Liquid-Cladding-Induced Modulation of Waveguide Modes

Abstract

We demonstrated modulation of the waveguide mode mismatch via liquid cladding of the controllable refractive index for label-free quantitative detection of concentration of chemical or biological substances. A multimode optical fiber with its core exposed was used as the sensor head with the suitable chemical modification of its surface. Injected analyte liquid itself formed the liquid cladding for the waveguide. We found that modulation of the concentration of analyte injected enables a degree of the waveguide mode mismatch to be controlled, resulting in sensitive change in optical power transmission, which was utilized for its real-time quantitative assay. We applied the device to quantitating concentration of glycerol and bovine serum albumin (BSA) solutions. We obtained experimentally the limit of detection (LOD) of glycerol concentration, 0.001% (volume ratio), corresponding to the resolvable index resolution of ∼1.02 × 10 RIU (refractive index unit). The presented sensors also exhibited reasonably good reproducibility. In BSA detection, the sensor device response was sensitive to change in the refractive indices not only of liquid bulk but also of layers just above the sensing surface with higher sensitivity, providing the LOD experimentally as ∼3.7 ng/mL (mass coverage of ∼30 pg/mm). A theoretical model was also presented to invoke both mode mismatch modulation and evanescent field absorption for understanding of the transmission change, offering a theoretical background for designing the sensor head structure for a given analyte. Interestingly, the device sensing length played little role in the important sensor characteristics such as sensitivity, unlike most of the waveguide-based sensors. This unraveled the possibility of realizing a highly simple structured label-free sensor for point-of-care testing in a real-time manner via an optical waveguide with liquid cladding. This required neither metal nor dielectric coating but still produced sensitivity comparable to those of other types of label-free sensors such as plasmonic fiber ones.

Liquid Cladding Mediated Optical Fiber Sensors for Copper Ion Detection

“…Prior to surface functionalization with APTES/chitosan-EDTA, we measured the power transmittance response of the device, consisting of core and liquid cladding (glycerol solution, Sigma-Aldrich Co.), as we injected glycerol solutions in various concentrations. Similar to a previous report [ 24 ], we obtained a linear relationship between the glycerol concentration and the output power in a low concentration range, as shown in Figure 6 a. The mechanism of NA mismatch at the interface between the liquid cladding waveguide and the solid cladding waveguide came into play for optical power increases with increasing concentrations, as depicted in Figure 6 b. Waveguide NA = , where n core and n clad , were the indices of the core and cladding, respectively.…”

“…Figure 2 a shows the FTIR spectrum for the silica surface of the fiber core. It was revealed that the Si-O-Si asymmetric stretching vibration mode was at 1074 cm −1 and the Si-O-Si bending vibration mode was at 456 cm −1 [ 24 , 25 ]. In addition, symmetric stretching vibration mode O-Si-O was exhibited at 821 cm −1 and 600 cm −1 [ 25 , 26 ], indicating the presence of all vibration-mode relevant silica.…”

“…We also observed a broadband signal in the range of 3200–3600 cm −1 for the vibration mode of an OH group, which overlapped with a stretching vibration mode of N-H [ 27 ]. The out-of-plane and in-plane bending vibrations of N-H were exhibited at 680 cm −1 and 1566 cm −1 , respectively [ 24 , 28 ]. We observed the stretching vibration mode of a carboxyl (COOH) group at 1720 cm −1 [ 29 ], and the stretching vibration mode of silanol (Si-OH mode) at 926 cm −1 , indicating the silanization of APTES on the silica surface [ 25 ].…”

“…In this work, we demonstrate a liquid-cladding modulated fiber sensor of only 1 cm in length (much shorter than the 5-cm length in Reference [ 24 ]) that still shows a similar sensitivity in the refractive index resolution, and put this sensor platform to use for detecting heavy metal ions, such as Cu 2+ , in water by additional immobilization of ligand layers. Prior to ligand layer immobilization, we used glycerol solutions of various concentrations as test mediums to check the refractive index resolution as a refractometer and obtained an index resolution of 2.48 × 10 −6 refractive index units (RIU).…”

“…Recently, label-free optical sensors with liquid-cladding modulation have been demonstrated for highly sensitive detection of analytes in a liquid phase using a multimode optical fiber without additional coatings of layers, such as plasmonic nanometer-thick metals [ 24 ]. Analyte liquid injected onto the fiber core acts as a liquid cladding of the fiber, where the original solid cladding was removed.…”