Magdalena Dominik

Titanium oxide thin films obtained with physical and chemical vapour deposition methods for optical biosensing purposes.

This work discusses an application of titanium oxide (TiOx) thin films deposited using physical (reactive magnetron sputtering, RMS) and chemical (atomic layer deposition, ALD) vapour deposition methods as a functional coating for label-free optical biosensors. The films were applied as a coating for two types of sensors based on the localised surface plasmon resonance (LSPR) of gold nanoparticles deposited on a glass plate and on a long-period grating (LPG) induced in an optical fibre. Optical and structural properties of the TiOx thin films were investigated and discussed. It has been found that deposition method has a significant influence on optical properties and composition of the films, but negligible impact on TiOx surface silanization effectiveness. A higher content of oxygen with lower Ti content in the ALD films leads to the formation of layers with higher refractive index and slightly higher extinction coefficient than for the RMS TiOx. Moreover, application of the TiOx film independently on deposition method enables not only for tuning of the spectral response of the investigated biosensors, but also in case of LSPR for enhancing the ability for biofunctionalization, i.e., TiOx film mechanically protects the nanoparticles and induces change in the biofunctionalization procedure to the one typical for oxides. TiOx coated LSPR and LPG sensors with refractive index sensitivity of close to 30 and 3400nm/RIU, respectively, were investigated. The ability for molecular recognition was evaluated with the well-known complex formation between avidin and biotin as a model system. The shift in resonance wavelength reached 3 and 13.2nm in case of LSPR and LPG sensors, respectively. Any modification in TiOx properties resulting from the biofunctionalization process can be also clearly detected.

Specific detection of very low concentrations of DNA oligonucleotides with DNA-coated long-period grating biosensor

Label-free biosensing using optical long-period gratings (LPGs) induced in optical fiber and coated with biological compound enables for detection and monitoring the kinetics of reactions taking places on the sensors surface. The labelfree detection effect for the LPG biosensor working near the dispersion turning point (DTP) of higher order cladding modes can be observed as resonance wavelength shift induced by binding of biomolecules to fiber surface. In this study we analyzed shift of the resonance wavelength for LPG after its functionalization with DNA layer and during DNA hybridization. It has seen found that the LPG is able to be functionalized with DNA oligonucleotide, acting as a probe and after the functionalization can selectively bind specific complementary DNA oligonucleotides in a very low concentration. DNA-coated LPG can be used as a probe capturing specific DNA sequences with wide ranges of concentrations from 0.1 to 10 pM. Hybridization of the DNA on the fiber surface has also been verified with Midorii Green florescent marker.

Plasma-Based Deposition and Processing Techniques for Optical Fiber Sensing

Plasma-based techniques are widely applied for well-controlled deposition, etching or surface functionalization of a number of materials. It is difficult to imagine fabrication of novel microelectronic and optoelectronic devices without using plasma-enhanced deposition of thin films, their selective etching or functionalization of their surfaces for subsequent selective binding of chemical or biological molecules. Depending on the process parameters, i.e., generator frequency and power, composition of gases , pressure, temperature, and applied substrates, different effects of the process can be obtained. The chapter discusses current trends in application of plasma-based techniques for fabrication of novel optical sensing devices. Fabrication of materials with different structure (from amorphous to crystalline, porous, and multilayers), optical properties (absorption, refractive index ), and surface activity, as well as their processing are reviewed. Application of the plasma methods enhancing sensing properties of various optical fiber sensing structures, namely long-period gratings, intermodal interferometers based on photonic crystal fiber, sensing structures based on lossy mode resonance or stacks of nano-films are given as examples and are discussed.

Optical monitoring of thin film electro-polymerization on surface of ITO-coated lossy-mode resonance sensor

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 ITOs 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.

Refractive index sensing with Al 2 O 3 -nanocoated long-period gratings working at dispersion turning point: Temperature cross-sensitivity

In this work is discussed an effect of nanocoating long-period grating (LPG) with an aluminum oxide (Al2O3) on its refractive index (RI) and temperature (T) sensitivity. For comparison two LPGs, namely one before and after the nanocoating process were optimized towards working at dispersion turning point (DTP) of higher order cladding modes. The DTP was reached with two methods, i.e., by wet etching of LPGs cladding and by optimized deposition of Al2O3 thin overlay using atomic layer deposition (ALD) method. For both the cases we show significant increase in RI sensitivity at DTP. When Al2O3 is deposited the RI sensitivity reaches 9270 nm/RIU for traced only one resonance out of the pair and RI in range 1.33–1.34 RIU, what is over 4-fold higher than for the bare LPG. Moreover, T sensitivity for both the structures was compared and the one for Al2O3-coated sample was found to be only up to 12% higher than for the bare sample. Higher RI sensitivity and low temperature cross-sensitivity effects allow for more accurate RI measurements than in case of bare LPGs. The Al2O3-nanocoated LPGs can further find applications in label-free biosensing, due to their optimized sensitivity in RI in range close that of water.

Regeneration of titanium oxide nano-coated long-period grating biosensor

This work presents an application of sodium hydroxide (NaOH) as an effective method for regeneration of titanium oxide (TiOx) nano-coated long-period grating (LPG) biosensor. Below 100 nm in thickness TiOx coating was deposited with atomic layer deposition (ALD) method on LPGs for enhancing their refractive index sensitivity up to 2912 nm/RIU in RI range 1.33-1.36 RIU. Next, the sensors were biofunctionalized in order to immobilize receptor (biotin) on their surface and used for selective avidin detection. After successful biofunctionalization process and avidin detection the sensors were washed in NaOH and biofunctionalized again. The proposed method for recovering the sensor does not cause decrease in its functional properties. As a result of the applied procedure the biosensor was fully regenerated.

Reflection configuration of long period grating sensor working at dispersion turning point

In this work discuss an application of chemical method, i.e., Tollen’s reagent, for mirror fabrication on the end-face of the fiber with induced long-period grating (LPG). This simple and versatile technique can be used for thin silver layer deposition and formation of stable and well-reflecting mirrors for fiber-based devices. We have found that the LPGbased sensors working in reflective configuration at dispersion turning point (DTP) of higher order cladding modes allow for refractive index (RI) measurements with sensitivity reaching 4.429 nm/RIU. Such structures, after their proper biofunctionalization process, can be used as probes for label-free biosensing.

Annealing of indium tin oxide (ITO) coated optical fibers for optical and electrochemical sensing purposes

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.

Effect of oxygen plasma modification on refractive index sensing with micro-cavity in-line Mach-Zehnder interferometer

A micro-cavity in-line Mach-Zehnder interferometer (μIMZI) is an optical sensing structure fabricated in an optical fiber. Its design allows for refractive index sensing of liquid and gas in picoliter volumes, making it suitable for biochemical and medical sensing where measured material is often scarce. The fabricated structures show satisfactory levels of sensitivity, from about 400 nm/RIU in the near-water range of solutions (nD 1.336±0.003 RIU) to about 16 000 nm/RIU for solutions in approximate range from nD = 1.35 RIU to nD = 1.4 RIU. The structures were subjected to oxygen plasma, the process which was supposed to modify physical parameters of the structures, i.e., cavity surface wettability and roughness, and in consequence their sensitivity. As a result of the oxygen plasma modification we have observed a improved wettability of the structure surface, what makes it easier to introduce liquid into the cavity and simplifies the measurement process. In the case where the plasma processing is preceded by biological layer deposition, the bottom surface of the structure is smoothed and slightly deepened, causing a shift in the transmission spectrum and change in sensitivity.