This work describes an experimental study towards label-free sensing of C-reactive protein (CRP) – a protein recognized as a inflammation marker. A multimode optical fiber with a section of its core coated with indium tin oxide (ITO) thin film was used as a sensor. ITO film allows for guiding lossy modes and can simultaneously be used as a transparent electrode for electrochemical measurements. Therefore, optical and electrochemical detection based on a single sensor was possible. Such a dual-domain approach is practical, especially when the results in one of the domains are not accurate enough, which was the case in this work. A case of different functionalization methods of ITO surface was also pointed out. The proposed sensor allows for recognition as low as ng/mL.
The work discusses the possible impact of the electric charge of biological material on the properties of label-free biosensors, in particular those operating in dual domains, i.e., optical and electrochemical. Optical fiber lossy-mode resonance (LMR) sensors based on indium tin oxide (ITO) were investigated as label-free biosensors with a model biological receptor-target pair, i.e., biotin-avidin. Each of the used biological materials shows different properties, i.e., size, isoelectric point, and, therefore, also charge. The investigations were performed in two electrolytes with differently charged redox couples to better identify the possible influence of chargé of biological material on the optical readout. The obtained results clearly indicate that in designing label-free biosensing solutions, consideration of a broader range of biological materials properties than just refractive index, such as their charge, is required.
This work discusses optical fiber sensors based on lossy-mode resonance (LMR) effect and their potential for simultaneous sensing in multiple domains, i.e., optical and electrochemical. As electrically conductive materials able to guide lossy modes, two doped tin oxides, i.e., fluorine doped tin oxide (FTO) and indium tin oxide (ITO) thin films were employed. Since the ITO-LMR sensor has already been discussed broader, this work focuses on properties of the FTO-LMR sensor and brief comparison of devices based on the two materials. In optical domain the sensitivity to surrounding medium refractive index was determined by immersing the sensors in solutions of different refractive index. Both the sensors showed sensitivity of 300 nm/RIU in a refractive index range of approx. 1.33-1.39 RIU. Electrochemical measurements were performed in 0.01 M phosphate-buffered saline (PBS, pH 7.0) to identify the influence of the applied potential on the optical response of both sensors. In applied potential from -1.0 V to 1.0 V the FTO-LMR sensor reached LMR shift of 31.3 nm compared to 23.8 nm of the ITO-LMR one.
This work discuses indium tin oxide (ITO) coated optical fiber lossy-mode resonance (LMR) sensor working in an electrochemical setup for monitoring of protein binding to the sensor’s surface. The binding mechanism has been observed simultaneously in optical and electrochemical domain. The combined measurement was enabled by the electrically conductive ITO overlay. In the experiment, biotin molecules have been used to test the collective optical and electrochemical setup and to illustrate the binding effect. It has been shown that the effect is observed in the investigated domains at applied potential, and the qualitative comparison shows high resemblance of the outcome.
This work presents an optical fiber sensors based on lossy–mode resonance (LMR) phenomenon supported by indium tin oxide (ITO) thin overlay for investigation of electro-polymerization effect on ITO’s surface. The ITO overlays were deposited on core of polymer-clad silica (PCS) fibers using reactive magnetron sputtering (RMS) method. Since ITO is electrically conductive and electrochemically active it can be used as a working electrode in 3-electrode cyclic voltammetry setup. For fixed potential applied to the electrode current flow decrease with time what corresponds to polymer layer formation on the ITO surface. Since LMR phenomenon depends on optical properties in proximity of the ITO surface, polymer layer formation can be monitored optically in real time. The electrodeposition process has been performed with Isatin which is a strong endogenous neurochemical regulator in humans as it is a metabolic derivative of adrenaline. It was found that optical detection of Isatin is possible in the proposed configuration.
Glass and fiber structures with Indium Tin Oxide (ITO) coating were subjected to annealing in order to identify impact of the thermal treatment on their optical and electrochemical properties. It is shown that the annealing process significantly modifies optical properties and thickness of the films, which are crucial for performance of optical fiber sensors. Moreover, it visibly improves electrochemical activity of ITO on glass slides and thicker (∅=400 μm) ITO-coated fibers, whereas in the case of thinner fibers (∅=125 μm) it could lead to a loss of their electrochemical activity. Depending on the applied substrate and the annealing process, the investigated structures with ITO coating can be further used as fiber-based sensors with integrated opto-electrochemical readout.
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