Development of a Temperature-Controlled Optical Planar Waveguide Sensor with Lossy Mode Resonance for Refractive Index Measurement

The generation of lossy mode resonances (LMR) with a metallic oxide film deposited on an optical fiber has attracted the attention of many applications. However, an LMR-based optical fiber sensor is frangible, and therefore it does not allow control of the temperature and is not suited to mass production. This paper aims to develop a temperature-controlled lossy mode resonance (TC-LMR) sensor on an optical planar waveguide with an active temperature control function in which an ITO film is not only used as the LMR resonance but also to provide the heating function to achieve the benefits of compact size and active temperature control. A simple flat model about the heat transfer mechanism is proposed to determine the heating time constant for the applied voltages. The TC-LMR sensor is evaluated experimentally for refractive index measurement using a glycerol solution. The heating temperature functions relative to the controlled voltages for water and glycerol are obtained to verify the performance of the TC-LMR sensor. The TC-LMR sensor is a valuable sensing device that can be used in clinical testing and point of care for programming heating with precise temperature control.

Subtle Application of Electrical Field-Induced Lossy Mode Resonance to Enhance Performance of Optical Planar Waveguide Biosensor

Many studies concern the generation of lossy mode resonances (LMRs) using metallic oxide thin films that are deposited on optical fiber. However, the LMR-based optical fiber sensors are frangible, do not allow easy surface modification, and are not suited to mass production. This study proposes an electrical field-induced LMR-based biosensor with an optical planar waveguide to replace surface modification and allow the mass production of protein biosensors and accelerate the speed of the analyte to decrease the detection time. Experimentally, the biosensor is evaluated using charged serum albumin molecules and characterized in terms of the LMR wavelength shift using an externally applied voltage for different durations. The externally applied voltage generates a significant electric field, which drives the non-neutralized biomolecules and increases the LMR wavelength shift. Our experimental results demonstrate that there are two different mechanisms of adsorption of serum albumin molecules for short-term and long-term observations. These are used to calculate the sensitivity of the biosensor. This electrical field-induced method is highly significant for the development and fabrication of LMR-based biosensors.

Lossy Mode Resonance-Based Glucose Sensor with High-κ Dielectric Film

In the past, high-κ dielectrics gained much attention because of the constant demand for increasingly smaller semiconductors. At the same time, in the field of optical sensing, high-κ dielectrics are key materials. This study presents the experimental investigations on a lossy mode resonance-based optical planar waveguide (LMROPW) sensor coated with a high-κdielectric of an indium tin oxide (ITO) layer. Two types of sensing structures were fabricated by coating (i) only a single-layer ITO (or bared LMROPW) and (ii) an ITO layer with glucose probes onto the optical planar waveguide (or boronic LMROPW) to detect glucose molecules. The sensing characteristics of these two types of sensors toward the surrounding analyte were determined using different concentrations of glucose solutions. It was found that the bared LMROPW sensor is only suitable for a higher concentration of glucose; the boronic LMROPW sensor with glucose probes on ITO could be applied to a lower-concentration solution to monitor glucose adsorption onto the sensing surface. Furthermore, with the advantages of a simple structure, easy alignment, and suitable production, the LMROPW sensor with a high-κ dielectric surface could be applied in clinical testing and diagnostics.

Optical Planar Waveguide Sensor with Integrated Digitally-Printed Light Coupling-in and Readout Elements

Optical planar waveguide sensors, able to detect and process information from the environment in a fast, cost-effective, and remote fashion, are of great interest currently in different application areas including security, metrology, automotive, aerospace, consumer electronics, energy, environment, or health. Integration of networks of these systems together with other optical elements, such as light sources, readout, or detection systems, in a planar waveguide geometry is greatly demanded towards more compact, portable, and versatile sensing platforms. Herein, we report an optical temperature sensor with a planar waveguide architecture integrating inkjet-printed luminescent light coupling-in and readout elements with matched emission and excitation. The first luminescent element, when illuminated with light in its absorption band, emits light that is partially coupled into the propagation modes of the planar waveguide. Remote excitation of this element can be performed without the need for special alignment of the light source. A thermoresponsive liquid crystal-based film regulates the amount of light coupled out from the planar waveguide at the sensing location. The second luminescent element partly absorbs the waveguided light that reaches its location and emits at longer wavelengths, serving as a temperature readout element through luminescence intensity measurements. Overall, the ability of inkjet technology to digitally print luminescent elements demonstrates great potential for the integration and miniaturization of light coupling-in and readout elements in optical planar waveguide sensing platforms.

Development of Novel Optical Planar Waveguide-based Biosensors using a Nanotechnology Approach

This work aims at the development of novel biosensor based on optical planar waveguide (OPW) for detection of mycotoxins, which are common contaminants in agriculture products (grains, beans, nuts, fruits) and associated food and feed. These low molecular weight toxins produced by various fungi species possess a substantial danger to human and animals, and thus are under strict legislated limits in sub-ppm (part per million) level. The detection of mycotoxins in such low concentrations is of great interest nowadays. A novel detection principle of polarization interferometry (PI) exploited in this system (which can be considered as a logical continuation of ellipsometry) in based on tracking changes in the polarization state of a laser beam passing through the waveguide and affected by immobilized in the waveguide sensing window. The key element of this sensor is a planar optical waveguide consisting of 190 nm thick silicon nitride core layer sandwiched between two thick layers of silicon dioxide; a sensing window was etched in the top silicon oxide layer to allow monitoring molecular adsorption. A 630 nm polarized light from a laser diode coupled through the slant edge of the waveguide experiences a large number of reflections (about 500 per mm) when propagating through the waveguide. The p- component of polarized light is affected by changes in refractive index in the sensing window, while s- component is less affected and thus serves as a reference. Therefore, the changes in either the medium refractive index or molecular adsorption cause the phase shift between p- and s- components. The observation of the light polarization state is enabled by a polarizer converting the changes in polarization to variations of light intensity which is then recoded with CCD linear array interfaced to PC. The refractive index sensitivity of the OPW PI sensor of about 1600 rad/RIU/mm (the highest value known for optical detection) was found by both the theoretical modelling and experimental testing. The developed experimental set-up was used for detection of mycotoxins, i.e. aflatoxin B1 (AFT B1), ochratoxin A (OTA), and zearalenone (ZEN), in direct assay with two types of bio-receptors immobilized within the sensing window: (i) antibodies electrostatically bound onto silicon nitride surface via layers of poly-allylamine hydrochloride and protein A, or (ii) aptamers covalently bound via SH groups on aminated surface of silicon nitride. The outcome of such biosensing tests was successful; all three mycotoxins were detected in a wide concentration range from10 pg/ml up to 1 g/ml in direct immunoassays with their respective antibodies. The use of specific aptamers as bioreceptors in the latest upgrade of the OPW PI set-up has resulted in much lower detected concentrations of AFT B1 and OTA down to 1pg/ml, with LDL estimated as 0.6 -0.7 pg/ml. The obtained sensitivity in sub-ppt (part per trillion) level is the highest known for optical biosensors, and it is particularly remarkable for a label-free detection of low molecular weight analyte molecules in direct assay format. The developed OPW PI biosensor is universal and can be easily adapted for detection of different analyte molecules by choosing suitable bio-receptors. It can be used equally for detection of small and large molecules, and in different assay formats, e.g. direct, sandwich, and competitive assays, and therefore can be considered as a platform biosensing technology for a wide range of applications, i.e. environmental monitoring, security, agriculture and food industry, and biomedical.