Application of ZnO Nanorods Based Whispering Gallery Mode Resonator in Optical Immunosensors.

In this research a whispering gallery mode (WGM) resonator based on vertically oriented ZnO nanorods, which were formed on silicon surface (silicon/ZnO-NRs), has been applied in the design of optical immunosensor that was dedicated for the determination of grapevine virus A-type (GVA) proteins. Vertically oriented ZnO-NRs were grown on silicon substrates by atmospheric pressure metal organic chemical vapor deposition (APMOCVD) and the silicon/ZnO-NRs structures formed were characterized by structural and optical methods. Optical characterization demonstrates that silicon/ZnO-NRs-based structures can act as 'whispering gallery mode' (WGM) resonator where quasi-whispering gallery modes (quasi-WGMs) are generated. These quasi-WGMs were experimentally observed in the visible and infrared ranges of the photoluminescence spectra. In order to design an immuno-sensing system the anti-GVA antibodies were immobilized on the surface of silicon/ZnO-NRs and in this way silicon/ZnO-NRs/anti-GVA structure was formed. The immobilization of anti-GVA antibodies and then the interaction of silicon/ZnO-NRs/anti-GVA structure with GVA proteins (GVA-antigens) resulted in an opposite shifts of the WGMs peaks in the visible range of the photoluminescence spectra observed as a defect-related photoluminescence emission of ZnO-NRs. Here designed silicon/ZnO-NRs/anti-GVA immuno-sensing structure demonstrates the sensitivity towards GVA-antigens in the concentration range of 1-200 ng/ml. Bioanalytical applicability of the silicon/ZnO-NRs-based structures in the WGMs registration mode is discussed.

Porous silicon based photoluminescence immunosensor for rapid and highly-sensitive detection of Ochratoxin A.

A rapid and low cost photoluminescence (PL) immunosensor for the determination of low concentrations of Ochratoxin A (OTA) has been developed. This immunosensor was based on porous silicon (PSi) and modified by antibodies against OTA (anti-OTA). PSi layer was fabricated by metal-assisted chemical etching (MACE) procedure. Main structural parameters (pore size, layer thickness, morphology and nanograins size) and composition of PSi were investigated by means of X-Ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. PL-spectroscopy of PSi was performed at room temperature and showed a wide emission band centered at 680 ± 20nm. Protein A was covalently immobilized on the surface of PSi, which in next steps was modified by anti-OTA and BSA in this way a anti-OTA/Protein-A/PSi structure sensitive towards OTA was designed. The anti-OTA/Protein-A/PSi-based immunosensors were tested in a wide range of OTA concentrations from 0.001 upto 100ng/ml. Interaction of OTA with anti-OTA/Protein-A/PSi surface resulted in the quenching of photoluminescence in comparison to bare PSi. The limit of detection (LOD) and the sensitivity range of anti-OTA/Protein-A/PSi immunosensors were estimated. Association constant and Gibbs free energy for the interaction of anti-OTA/Protein-A/PSi with OTA were calculated and analyzed using the interaction isotherms. Response time of the anti-OTA/Protein-A/PSi-based immunosensor toward OTA was in the range of 500-700s. These findings are very promising for the development of highly sensitive, and potentially portable immunosensors suitable for fast determination of OTA in food and beverages.

Interaction mechanism between TiO2 nanostructures and bovine leukemia virus proteins in photoluminescence-based immunosensors.

In this research a mechanism of interaction between a semiconducting TiO2 layer and bovine leukemia virus protein gp51, applied in the design of photoluminescence-based immunosensors, is proposed and discussed. Protein gp51 was adsorbed on the surface of a nanostructured TiO2 thin film, formed on glass substrates (TiO2/glass). A photoluminescence (PL) peak shift from 517 nm to 499 nm was observed after modification of the TiO2/glass by adsorbed gp51 (gp51/TiO2/glass). After incubation of the gp51/TiO2/glass in a solution containing anti-gp51, a new structure (anti-gp51/gp51/TiO2/glass) was formed and the PL peak shifted backwards from 499 nm to 516 nm. The above-mentioned PL shifts are attributed to the variations in the self-trapped exciton energy level, which were induced by the changes of electrostatic interaction between the adsorbed gp51 and the negatively charged TiO2 surface. The strength of the electric field affecting the photoluminescence centers, was determined from variations between the PL-spectra of TiO2/glass, gp51/TiO2/glass and anti-gp51/gp51/TiO2/glass. The principle of how these electric field variations are induced has been predicted. The highlighted origin of the changes in the photoluminescence spectra of TiO2 after its protein modification reveals an understanding of the interaction mechanism between TiO2 and proteins that is the key issue responsible for biosensor performance.

Gold coated porous silicon nanocomposite as a substrate for photoluminescence-based immunosensor suitable for the determination of Aflatoxin B1.

A rapid and low cost photoluminescence (PL) immunosensor for the determination of low concentrations of Aflatoxin B1 (AFB1) has been developed. This immunosensor was based on porous silicon (PSi) covered by thin gold layer (Au) and modified by antibodies against AFB1 (anti-AFB1). PSi layer was formed on silicon substrate, then the surface of PSi was covered by 30nm layer of gold (PSi/Au) using electrochemical and chemical deposition methods and in such ways PSi/Au(El.) and PSi/Au(Chem.) structures were formed, respectively. In order to find PSi/Au the most efficiently suitable for PL-based sensor design, structure several different PSi/Au(El.) and PSi/Au(Chem.) structures were designed while using different conditions for electrochemical or chemical deposition of gold layer. It was shown that during the formation of PSi/Au structure crystalline Au nanoparticles uniformly coated the surface of the PSi pores. PL spectroscopy of PSi/Au nanocomposites was performed at room temperature and it showed a wide emission band centered at 700nm. Protein A was covalently immobilized on the surface of PSi/Au(El.) and PSi/Au(Chem.) forming PSi/Au(El.)/Protein-A and PSi/Au(Chem.)/Protein-A structures, respectively. In the next step PSi/Au(El.)/Protein-A and PSi/Au(Chem.)/Protein-A structures were modified by anti-AFB1 and in such way a structures (PSi/Au(El.)/Protein-A/anti-AFB1 and PSi/Au(Chem.)/Protein-A/anti-AFB1) sensitive towards AFB1 were designed. The PSi/Au(El.)/Protein-A/anti-AFB1- and PSi/Au(Chem.)/Protein-A/anti-AFB1-based immunosensors were tested in a wide range of AFB1 concentrations from 0.001 upon 100ng/ml. Interaction of AFB1 with PSi/Au(El.)/Protein-A/anti-AFB1- and PSi/Au(Chem.)/Protein-A/anti-AFB1-based structures resulted PL quenching. The highest sensitivity towards AFB1 was determined for PSi/Au(El.)/Protein-A/anti-AFB1-based immunosensor and it was in the range of 0.01-10ng/ml. The applicability of PSi/Au-based structures as new substrates suitable for PL-based immunosensors is discussed.

ZnO films formed by atomic layer deposition as an optical biosensor platform for the detection of Grapevine virus A-type proteins.

Novel sensitive optical biosensor for determination of Grapevine virus A-type (GVA) proteins (GVA-antigens) has been designed. This biosensor was based on thin films of Zinc Oxide (ZnO) deposited by atomic layer deposition (ALD). The ZnO-based films have demonstrated favorable surface-structural properties for the direct immobilization of antibodies against GVA-antigens in order to form a biosensitive layer sensitive to GVA-antigens. The immobilization was confirmed by intensity changes in the main near band emission (NBE) peak of ZnO and by the formation of intense photoluminescence band, discovered in the visible range around 425nm, caused by the immobilized proteins. The GVA-antigen detection was performed by the evaluation of changes and behavior of a corresponding luminescence band. The sensitivity of as-formed label-free biosensor towards the GVA-antigens was determined in the range from 1pg/ml to 10ng/ml; in addition, the selectivity of biosensor was evaluated.

Tuning of ZnO 1D nanostructures by atomic layer deposition and electrospinning for optical gas sensor applications.

We explored for the first time the ability of a three-dimensional polyacrylonitrile/ZnO material-prepared by a combination of electrospinning and atomic layer deposition (ALD) as a new material with a large surface area-to enhance the performance of optical sensors for volatile organic compound (VOC) detection. The photoluminescence (PL) peak intensity of these one-dimensional nanostructures has been enhanced by a factor of 2000 compared to a flat Si substrate. In addition, a phase transition of the ZnO ALD coating from amorphous to crystalline has been observed due to the properties of a polyacrylonitrile nanofiber template: surface strain, roughness, and an increased number of nucleation sites in comparison with a flat Si substrate. The greatly improved PL performance of these nanostructured surfaces could produce exciting materials for implantation in VOC optical sensor applications.

Photoactivation of luminescence in CdS nanocrystals.

This paper presents the results of the research on the luminescence of cadmium sulfide nanocrystals (NCs) synthesized by colloid chemistry in a gelatinous matrix. The photostimulation of the short-wavelength emission band with λmax = 480 nm has been detected. It is shown that the determining factor of the photostimulation effect is the adsorption of the water molecules on the surface of NC. The observed effect is explained by the recombination mechanism that is responsible for the short-wavelength emission band.

Evolution of microstructure and related optical properties of ZnO grown by atomic layer deposition.

A study of transmittance and photoluminescence spectra on the growth of oxygen-rich ultra-thin ZnO films prepared by atomic layer deposition is reported. The structural transition from an amorphous to a polycrystalline state is observed upon increasing the thickness. The unusual behavior of the energy gap with thickness reflected by optical properties is attributed to the improvement of the crystalline structure resulting from a decreasing concentration of point defects at the growth of grains. The spectra of UV and visible photoluminescence emissions correspond to transitions near the band-edge and defect-related transitions. Additional emissions were observed from band-tail states near the edge. A high oxygen ratio and variable optical properties could be attractive for an application of atomic layer deposition (ALD) deposited ultrathin ZnO films in optical sensors and biosensors.