A localized surface plasmon resonance based immunosensor for the detection of casein in milk

In this research, a localized surface plasmon resonance (LSPR) immunosensor based on gold-capped nanoparticle substrate for detecting casein, one of the most potent allergens in milk, was developed. The fabrication of the gold-capped nanoparticle substrate involved a surface-modified silica nanoparticle layer (core) on the slide glass substrate between bottom and top gold layers (shell). The absorbance peak of the gold-capped nanoparticle substrate was observed at ～520 nm. In addition, the atomic force microscopy (AFM) images demonstrated that the nanoparticles formed a monolayer on the slide glass. After immobilizing anti-casein antibody on the surface, our device, casein immunosensor, could be applied easily for the detection of casein in the raw milk sample without a difficult pretreatment. Under the optimum conditions, the detection limit of the casein immunosensor was determined as 10 ng/mL. Our device brings several advantages to the existing LSPR-based biosensors with its easy fabrication, simple handling, low-cost, and high sensitivity.     

Label-free detection of peptide nucleic acid-DNA hybridization using localized surface plasmon resonance based optical biosensor

The development of label-free optical biosensors for DNA and other biomolecules has the potential to impact life sciences as well as screening in medical and environmental applications. In this report, we developed a localized surface plasmon resonance (LSPR) based label-free optical biosensor based on a gold-capped nanoparticle layer substrate immobilized with peptide nucleic acids (PNAs). PNA probe was designed to recognize the target DNA related to tumor necrosis factor. The nanoparticle layer was formed on a gold-deposited glass substrate by the surface modified silica nanoparticles using silane-coupling reagent. The optical properties of gold-capped nanoparticle layer substrate were characterized through monitoring the changes in the absorbance strength, as the thickness of the biomolecular layer increased with hybridization. The detection of PNA-DNA hybridization with target oligonucleotides and PCR-amplified real samples were performed with a limit of detection value of 0.677 pM target DNA. Selective discrimination against a single-base mismatch was also achieved. Our LSPR-based biosensor with the gold-capped nanoparticle layer substrate is applicable to the design of biosensors for monitoring of the interaction of other biomolecules, such as proteins, whole cells, or receptors with a massively parallel detection capability in a highly miniaturized package.     

A silver nanoparticle thin film modified glass substrate as a colourimetric sensor for hydrogen peroxide

In this work, a silver nanoparticle (AgNP) coated glass slide was developed as a device for sensing hydrogen peroxide. AgNPs were synthesised using borohydride reduction with a citrate stabiliser, resulting in a negatively charged stabilised particle surface. The particles were attached to the glass surface using the layer-by-layer (LbL) technique. Poly (diallyldimethylammonium chloride) and poly (styrene sulphonate) were used as cationic and anionic polyelectrolyte layers, respectively. The glass slide was modified with polyelectrolytes leaving a cationic layer on the top surface. The AgNPs were subsequently deposited on the slide via electrostatic interaction. As a result, a dark yellow film of AgNPs was obtained with maximum absorption at 410 nm. Film fabrication based on LbL assembly provided acceptable reproducibility (relative standard deviation = 6.5%). The fabricated film had long-term stability (>6 weeks). A very small quantity of AgNPs was used in this method. Fabrication was performed under ambient conditions. Therefore this fabrication was considered as a green method. The AgNP modified slide was developed to sense hydrogen peroxide. Detection is based upon oxidation of AgNPs by hydrogen peroxide. This results in a change in colour of the film from dark yellow to colourless. Linear calibration was obtained over the range of 1.0--100.0 mM of hydrogen peroxide. The device was successfully used for measuring hydrogen peroxide in urine.     

Electrochemical impedance immunosensor for the detection of cardiac biomarker Myogobin (Mb) in aqueous solution

A label-free, electrochemical impedance immunosensor based on surface modified thin flat gold wire electrode is reported for the quantitative detection of cardiac biomarker Myoglobin in aqueous solution. The protein antibody, ab-Mb, was covalently immobilized through a self assembled monolayer of 11-mercaptoundecanoic acid (MUA) and 3-mercapto propionic acid (MPA) via carbodiimide coupling reaction using N-(3-dimethylaminopropyl)-N′-ethyl carbodiimide hydrochloride (EDC) and N-Hydroxy Succinamide (NHS). The immunosensor (ab-Mb/MUA-MPA/Au) was characterized by electrochemical techniques. The electrochemical performance of the immunosensor was studied by electrochemical impedance spectroscopy. The immunosensor showed an increased electrontransfer resistance on coupling with biomarker protein antigen, ag-Mb, in the presence of a redox probe [Fe (CN)₆]^3−/4−. The modified Au electrode immunosensor exhibits an electrochemical impedance response to antigen, ag-Mb concentrations in a linear range from 10 ng to 650 ng mL⁻¹ with a lowest detection limit of 5.2 ng mL⁻¹.     

Electrochemical detection of prostate-specific antigen based on gold colloids/alumina derived sol-gel film

An electrochemical immunosensor for total prostate-specific antigen (PSA) was fabricated based on anti-PSA antibody-functionalized gold colloids/alumina sol-gel film. The presence of colloidal gold provides a good microenvironment for the immobilization of biomolecules. The fabrication process of the immunosensor was characterized by cyclic voltammetry. A direct electrochemical immunoassay format was employed to detect PSA on the basis of the potential change before and after the antigen-antibody interaction. Under optimal conditions, the potential change was proportional to PSA concentration ranging from 4.0 to 13 ng mL^− 1 with a detection limit of 3.4 ng mL^− 1. In addition, the performance and factors influencing the performance of the immunosensor were investigated and optimized.     

Superhydrophobic optically transparent silica films formed with a eutectic liquid

A eutectic liquid (choline chloride and urea) that served as a templating agent in sol-gel processing was used to prepare thin silica films on glass microscope slides. Subsequent extraction of the eutectic liquid yielded a film with a rough surface. After treating the film surface with a fluoroalkyl silane, the surface became superhydrophobic with a contact angle ∼ 170° and a contact angle hysteresis < 10°. The optical transmittance of the film coated on the glass slide was comparable to that of the microscope glass slide. Atomic Force Microscopy (AFM) was used to characterize the surface structures; a tipless probe allowed measurement of the force of interaction with superhydrophobic surfaces. The interaction force between the AFM probe and the superhydrophobic surface was reduced greatly compared to that between the probe and the flat surface treated with fluoroalkyl silane.     

Langmuir-Blodgett films of polyaniline for low density lipoprotein detection

Langmuir-Blodgett (LB) films of polyaniline (PANI) were utilized for the fabrication of impedimetric immunosensor for detection of human plasma low density lipoprotein (LDL) by immobilizing anti-apolipoprotein B (AAB) via EDC-NHS coupling. The modified electrodes were characterized by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy. AAB/PANI-SA LB immunoelectrodes studied by EIS spectroscopy revealed detection of LDL in the wide range of 0.018 μM (6 mg/dl) to 0.39 μM (130 mg/dl), covering the physiological range in blood, with a sensitivity of 11.25 kΩ μM^− 1.     

Nanoporous cluster-assembled WOx films prepared by radio-frequency assisted laser ablation

The influence of substrate temperature and ambient gas pressure-composition on the characteristics of WO_x films synthesized by radio-frequency assisted pulsed laser deposition (RF-PLD) are studied with the aim to obtain nanostructured films with large surface area that appear promising for gas sensing applications. A tungsten target was ablated both in chemically reactive molecular oxygen at 5 Pa and in a mixed oxygen-helium atmosphere at 700 Pa. Corning glass was used as the substrate, at 473, 673 and 873 K. Other deposition parameters such as laser fluence (4.5 J/cm²), laser wavelength (355 nm), radio-frequency power (150 W), and target to substrate distance (4 cm) were kept fixed. The sensitivity on the deposition parameters of roughness, morphology, nanostructure and bond coordination of the deposited films were analysed by atomic force microscopy, scanning electron microscopy, transmission electron microscopy and micro-Raman spectroscopy. The role of the investigated process parameters to nanoparticle formation and to the development of an extended nanostructure is discussed.     

Fabrication and characterization of vacuum deposited fluorescein thin films

Simple vacuum evaporation technique for deposition of dyes on various solid surfaces has been developed. The method is compatible with conventional solvent-free nanofabrication processing enabling fabrication of nanoscale optoelectronic devices. Thin films of fluorescein were deposited on glass, fluorine-tin-oxide (FTO) coated glass with and without atomically layer deposited (ALD) nanocrystalline 20 nm thick anatase TiO₂ coating. Surface topology, absorption and emission spectra of the films depend on their thickness and the material of supporting substrate. On a smooth glass surface the dye initially forms islands before merging into a uniform layer after 5 to 10 monolayers. On FTO covered glass the absorption spectra are similar to fluorescein solution in ethanol. Absorption spectra on ALD-TiO₂ is red shifted compared to the film deposited on bare FTO. The corresponding emission spectra at λ = 458 nm excitation show various thickness and substrate dependent features, while the emission of films deposited on TiO₂ is quenched due to the effective electron transfer to the semiconductor conduction band.     

Detection of phosphopeptides by localized surface plasma resonance of titania-coated gold nanoparticles immobilized on glass substrates

We herein demonstrate a new sensing method for phosphopeptides by localized surface plasmon resonance (LSPR) using titania-coated gold nanoparticles immobilized on the surface of a glass slide as the sensing substrate and using UV-visible spectrophotometry as the detection tool. Titania has been known to be an effective substrate for binding with phosphorylated species. The detection principle is the shift of wavelength of optical absorption due to SPR of the gold nanoparticles induced by binding of phosphorylated species with titania on the surface of the gold nanoparticles. The feasibility of the approach is demonstrated by detection of tryptic digest products of beta-casein and milk. Gold nanoparticles coated with thin films of titania, immobilized on a glass slide, can selectively bind traces of phosphopeptides from complex samples, resulting in a wavelength shift of the absorption band in the SPR spectrum with good reproducibility. The LSPR results are confirmed by matrix-assisted laser desorption/ionization mass spectrometry. The detection limit for the tryptic digest product of beta-casein is 50 nM.     

Thin films of tetragonal zirconia (3Y-TZ) deposited by reactive sputtering

Thin films of tetragonal zirconia (TZ), comprised of 3 mol% Y₂O₃ (3Y-TZ), were deposited onto silicon, oxide-coated silicon, slide glass and aluminum oxide substrates by reactive sputtering of metallic targets in mixtures of oxygen and argon. The texture of deposited films varied with oxygen-to-argon flow ratios with which the target surface altered between metal and oxide compound constituents. Thin films of TZP with (2 0 0) preferred orientation were obtained from sputter deposition in the metallic mode whereas (1 1 1) texture was obtained in the compound mode at ambient temperature. The film texture tends to align along the 〈1 1 1〉 direction while the substrate was heated to 300 °C during the deposition. The texture of all these films was stable upon annealing at 900 °C in air. The reasons for the texture development are discussed.     

Fabrication of discrete array of metallodielectric nanoshells and their surface plasmonic properties

In this paper we describe the fabrication of two-dimensionally periodic non-close-packed nanoshell arrays, consisting of a spherical polystyrene core coated with a thin gold shell, as well as their surface plasmonic properties. The principle of this procedure relies on stepwise integration of spin-coat-assisted colloidal self-assembly of the single layer of close-packed polystyrene nanoparticle, atmospheric pressure plasma-induced isotropic etching, and deposition of gold thin film by thermal evaporation. The plasma process converted the close-packed nanoparticle array into non-close-packed arrangement without changing their original spherical shape and periodicity. Both experimental and theoretical studies revealed that the densely packed nanoshell array with a 160 nm inner core diameter and a 20 nm thick shell strongly scattered and absorbed near infrared light, due to the interaction between primitive plasmon modes associated with the surface of the nanoparticle. Furthermore, the resultant nanoshell array was utilized for near infrared light responsible localized surface plasmon resonance based sensor. The bulk refractive index sensitivity was 220 nm RIU^− 1.     

Optical properties and sensing applications of lithium iron phosphate thin films

A composite optical waveguide sensor, consisting of lithium iron phosphate (LiFePO₄, LFP) as the sensing material, was constructed and utilized for the detection of volatile organic compound gases. Nano-LFP powder was prepared via the hydrothermal method and was subsequently utilized in a dip-coating procedure for the fabrication of LFP thin films. The effect of heat treating temperature on the refractive index of the thin films was studied. A glass optical waveguide gas sensor was fabricated by coating an LFP thin film on the surface of single-mode tin-diffused glass optical waveguide. The sensor was found to exhibit a linear response to xylene in the range of 50-1000 ppm, with response times of less than 5 s.     

Electrical conductivity of copper nanoparticle thin films annealed at low temperature

Copper nanoparticles with a mean diameter of 20 nm were used to prepare electrical conductive films at low temperature. After dispersal in an organic solvent, the copper nanoparticle pastes were coated onto a glass substrate, which was then annealed under various conditions to investigate the effects of various atmospheric conditions, such as air, nitrogen gas or hydrogen gas, as well as different annealing temperatures. Two-step annealing, which first involves oxidation in air followed by reduction, is effective in the preparation of high electrical conductive copper nanoparticle films. The copper nanoparticle films that were calcined in air for 1 h and then hydrogen gas for 1 h at a low temperature of 200 °C showed a low resistivity of 2 × 10^-5 Ω cm.     

Effect of inserting a buffer layer on the characteristics of transparent conducting impurity-doped ZnO thin films prepared by dc magnetron sputtering

In transparent conducting impurity-doped ZnO thin films prepared on glass substrates by a dc magnetron sputtering (dc-MS) deposition, the obtainable lowest resistivity and the spatial resistivity distribution on the substrate surface were improved by a newly developed MS deposition method. The decrease of obtainable lowest resistivity as well as the improvement of spatial resistivity distribution on the substrate surface in Al- or Ga-doped ZnO (AZO or GZO) thin films were successfully achieved by inserting a very thin buffer layer, prepared using the same MS apparatus with the same target, between the thin film and the glass substrate. The deposition of the buffer layer required a more strongly oxidized target surface than possible to attain during a conventional dc-MS deposition. The optimal thickness of the buffer layer was found to be about 10 nm for both GZO and AZO thin films. The resistivity decrease is mainly attributed to an increase of Hall mobility rather than carrier concentration, resulting from an improvement of crystallinity coming from insertion of the buffer layer. Resistivities of 3 × 10^− 4 and 4 × 10^− 4Ω cm were obtained in 100 nm-thick-GZO and AZO thin films, respectively, incorporating a 10 nm-thick-buffer layer prepared at a substrate temperature around 200 °C.     

Immunosensor for detection of Yersinia enterocolitica based on imaging ellipsometry

An immunosensor for the detection of pathogens was developed using imaging ellipsometry (IE) as a detection method. Yersinia enterocolitica was selected as the target pathogen in this study. A gold surface deposited with a self-assembled layer of 11-mercaptoundecanoic acid (11-MUA) was used as a substrate. For the fabrication of the immunosensor, protein G spots were made on the substrate using an inkjet-type microarrayer, and monoclonal antibody (Mab) was adsorbed onto the protein G spots. Deposition of each layer onto the substrate was confirmed by the measurement of surface plasmon resonance. The ellipsometric image of the protein G spot and the Mab-adsorbed protein G spot were acquired using an off-null ellipsometry type of imaging ellipsometry system. By measuring the ellipsometric angles of the protein layers, the surface concentration of each protein layer was calculated. The change in the mean optical intensity of the protein spot to the various concentrations of Y.enterocolitica was estimated. The immunosensor using imaging ellipsometry could successfully detect Y. enterocolitica in concentrations varying from 10(3) to 10(7) cfu/mL. The proposed immunosensor system has the advantage of allowing label-free detection, high sensitivity, and operational simplicity.     

Effect of sublayer surface treatments on ZnO transparent conductive oxides using dc magnetron sputtering

Novel sublayer surface treatments were investigated to improve the conductivity of aluminum-doped zinc oxide (ZnO:Al) fabricated by using dc magnetron sputtering on a glass substrate. Introducing artificial minute flaws on the surface of glass substrates enhanced the crystallinity of ZnO:Al films and decreased the resistivity accompanying the increase of electron mobility. Combination of the surface treatment and sputter beam control, i.e., interruption of high-energy oxygen with shadow masks, further reduced the resistivity of the film to 3.7 × 10^− 4 Ω cm (thickness 70 nm).     

Sintering effect on the optoelectronic characteristics of HgSe nanoparticle films on plastic substrates

The optoelectronic characteristics of HgSe nanoparticle films spin-coated on flexible plastic substrates are investigated under the illumination of 1.3 μm wavelength light. The sintering process improves the optoelectronic characteristics of the HgSe nanoparticle films. The photocurrent of the sintered HgSe nanoparticle films under the illumination of 1.3 μm wavelength light is approximately 20 times larger in magnitude than that of the non-sintered films in air at room temperature. Moreover, the endurance of the flexible optoelectronic device investigated by the continuous substrate bending test reveals that the photocurrent efficiency changes negligibly up to 250 cycles.     

A comparative study on the NO2 gas sensing properties of ZnO thin films, nanowires and nanorods

ZnO thin films were fabricated using the spin coating method, ZnO nanowires by cathodically induced sol-gel deposition by the means of an anodic aluminum oxide (AAO) template, and ZnO nanorods with the hydrothermal technique. For thin film preparation, a clear, homogeneous and stable ZnO solution was prepared by the sol-gel method using zinc acetate (ZnAc) precursor which was then coated on a glass substrate with a spin coater. Vertically aligned ZnO nanowires which were approximately 65 nm in diameter and 10 μm in length were grown in an AAO template by applying a cathodic voltage in aqueous zinc nitrate solution at room temperature. For fabrication of the ZnO nanorods, the sol-gel ZnO solution was coated on glass substrate by spin coating as a seed layer. Then ZnO nanorods were grown in zinc nitrate and hexamthylenetetramine aqueous solution. The ZnO nanorods are approximately 30 nm in diameter and 500 nm in length. The ZnO thin film, ZnO nanowires and nanorods were characterized by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). The NO₂ gas sensing properties of ZnO thin films, nanowires and nanorods were investigated in a dark chamber at 200 °C in the concentration range of 100 ppb-10 ppm. It was found that the response times of both ZnO thin films and ZnO nanorods were approximately 30 s, and the sensor response was depended on shape and size of ZnO nanostructures and electrode configurations.     

Effects of substrate temperature on tin-doped indium oxide films deposited by dc arc discharge ion plating

Indium tin oxide (ITO) films were deposited on glass substrate at temperatures ranging from room temperature to 120 °C by the dc arc discharge ion plating technique. The electrical properties and crystallinity of ITO films were investigated. The resistivity of ITO films decreased with the increase of the substrate temperature in deposition, mostly due to increase in Hall mobility above 90 °C. The resistivity of ITO film obtained at temperature 120 °C was 1.33×10⁻⁴ Ω cm. The ITO films crystallized at the substrate temperature higher than 90 °C and the grain size estimated from the (2 2 2) peak in the direction parallel to the surface of the substrate became large with the increase of the substrate temperature. That the Hall mobility increased with the increase of the substrate temperature was speculated to be due to the increase of the grain size in the direction parallel to the surface.