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ZnO materials exhibit a rich family of nanostructures, which show great potential for the sensitivity improvement of optical detection systems. In this work, ∼350 nm ZnO nanowires (ZnO-NWs) were electrochemically deposited on ZnO and an indium tin oxide coated glass (ZnO-NWs/ZnO/ITO/glass) substrate. ZnO-NWs were modified with N-(3-aminopropyl)triethoxysilane (APTES) for covalent bovine serum albumin (BSA) immobilization. The studies were performed using a spectroscopic total internal reflection ellipsometry (TIRE) setup based on the Kretschmann configuration. The refractive index dispersion of ZnO-NWs in the air was obtained from the optical model applying a Bruggeman effective medium approach. It was determined that the ZnO-NWs effective layer consists of 30% ZnO and 70% void. Reflectance difference before and after ZnO-NWs modification with BSA was 6.6 times higher than in the case of the plain ZnO layer. The Δ kinetics of covalent BSA immobilization on the APTES/ZnO-NWs/ZnO/glass substrate contains two phases. Evaluation of the diffusion coefficient for BSA in PBS filled APTES/ZnO-NWs/ZnO/ITO/glass was performed using numerical calculation, and the obtained diffusion coefficient was 2.4 × 10−17 m2 s−1. Simulation of BSA immobilization on a flat ZnO layer and on a ZnO-NWs modified surface showed that 13 times higher sensitivity was observed for the substrate with ZnO-NWs. [ABSTRACT FROM AUTHOR]

Scanning electrochemical microscopy (SECM) hybridized with fast Fourier transform-based electrochemical impedance spectroscopy (FFT-EIS) seems to be a powerful variation of scanning electrochemical impedance microscopy (SEIM), wherein both state-of-the-art techniques are combined (FFT-SEIM) and can be used for the investigation and treatment of tissues at single cell level. However, in most EIS-based experiments, harmful redox mediators are applied, which affect the functioning of living cells and tissues. Therefore, the development of a redox-probe-free FFT-SEIM is still a very important challenge in electrochemistry. For this reason, in this research, we have demonstrated a redox-probe-free evaluation of conducting and non-conducting surfaces by combining scanning electrochemical microscopy with FFT-EIS. It was demonstrated that using the fast Fourier transform-based FFT-EIS technique, EIS spectra could be registered much faster compared to experiments performed using the conventional EIS equipment. An ultramicroelectrode (UME) was used as a scanning electrode to ensure high spatial resolution. We have performed FFT-SEIM measurements in a redox-probe-free mode (without any additional redox probes) and have investigated several surfaces with different conductivities. The FFT-EIS equipment and the built-in software help to avoid the influence of possible formation of hydrogen bubbles on the UME. This research opens up a new avenue for the application of FFT-SEIM in the investigation of samples that are unstable and very sensitive towards redox mediators (e.g., tissues and/or living cells). [ABSTRACT FROM AUTHOR]

In this research Al2O3/ZnO nanolaminates were evaluated for possible application in the design of optical immunosensors. Total internal reflection ellipsometry (TIRE) was utilized to study the optical response during the formation of a bovine serum albumin (BSA) based monolayer on the surface of Al2O3/ZnO nanolaminates, which were pre-modified with a N-(3-aminopropyl)triethoxysilane (APTES) layer. The influence of the thicknesses of Al2O3 and ZnO layers on the performance of Al2O3/ZnO nanolaminate based structures has been assessed. This research has shown the noticeable contribution of multiple reflections from the interfaces between Al2O3 and ZnO for the enhancement of the optical response in a total internal reflection configuration. Al2O3/ZnO nanolaminates of 200 nm total thickness based on four 50 nm thick alternating Al2O3 and ZnO layers have shown better sensitivity than the nanolaminate based on two 100 nm oxide-layers. Real-time monitoring of the ellipsometric parameters Ψ(λ) and Δ(λ) has shown that BSA was successfully covalently attached to the nanolaminate/APTES surface. The surface concentration of immobilized BSA was evaluated from the real-time data of the ellipsometric parameters Ψ(λ) and Δ(λ). The expected advantages and disadvantages of Al2O3/ZnO nanolaminates during expected application in optical immunosensors are discussed. [ABSTRACT FROM AUTHOR]

Inspired by the excellent device performance of triindole-based semiconductors in electronic and optoelectronic devices, the relationship between the solid-state organization and the charge-transporting properties of an easily accessible series of triindole derivatives is reported herein. The vacuum-evaporated organic thin-film transistors (OTFTs) exhibited a non ideal behaviour with a double slope in the saturation curves. Moreover, the treatment of the gate insulator of the OTFT device with either a self-assembled monolayer (SAM) or a polymer controls the molecular growth and the film morphology of the semiconducting layer, as shown by X-ray diffraction (XRD) analyses, atomic force microscopy (AFM) and theoretical calculations. N-Trihexyltriindole exhibited the best device performance with hole mobilities up to 0.1 cm2 V−1 s−1 at the low VG range and up to 0.01 cm2 V−1 s−1 at high VG, as well as enhanced Ion/Ioff ratios of around 106. The results suggest that the non-ideal behaviour of the here studied OTFT devices could be related to the higher interfacial disorder in comparison to that in the bulk. [ABSTRACT FROM AUTHOR]

In this research study, we investigated the morphology of polypyrrole nanostructures, which were formed during the electrochemical deposition of conducting polymer. An electrochemical quartz crystal microbalance (EQCM) cell equipped with a flow-through system was employed to exchange solutions of different compositions within the EQCM cell. When bare PBS buffer in the EQCM cell was exchanged with PBS buffer with pyrrole we observed a distinct increase in the resonance frequency Δf. This change in the resonance frequency and electrical capacitance, which was calculated from electrochemical impedance spectroscopy (EIS) data, illustrate that pyrrole on the surface of the gold electrode formed an adsorbed layer (adlayer). The formation of a pyrrole adlayer before the potential pulse that induced polymerization was investigated by QCM-based measurements. The electrochemical polymerization of this adlayer was induced by a single potential pulse and a nanostructured layer, which consisted of adsorbed polypyrrole (Ppy) nanoparticles with a diameter of 50 nm, was formed. QCM and EIS data revealed that by the next cycle of the electrochemical formation of Ppy, which was investigated after flow-through-based exchange of solutions, the initially formed Ppy surface was covered by the adlayer of pyrrole. This adlayer was desorbed when pyrrole was removed from the solution. When electrochemical polymerization was performed using 50 potential pulses, a Ppy layer, which had more complex morphology, was formed on the EQCM crystal. Scanning electron microscopy showed that the conductivity of this layer was unequally distributed. We observed that the polypyrrole layer formed by electrochemical deposition, which was performed using potential pulses, was formed out of aggregated spherical Ppy particles with a diameter of 50 nm. [ABSTRACT FROM AUTHOR]

Optical and structural experiments have been carried out on Si/ZnO thin films modified with ultra-thin gold layers of different thicknesses. ZnO was produced via Atomic Layer Deposition (ALD) and Au via Physical Vapor Deposition (sputtering). The structural properties of nanostructures were studied by XRD and AFM. Optical characterization was performed by absorbance, photoluminescence (PL) and spectroscopic ellipsometry (SE). A transition from cluster-to-thin films with the increase of Au thickness has been revealed from an analysis of optical and structural parameters. The analysis of optical features of the system has shown that slight changes of the localized plasmon absorption peaks in spectra make a significant contribution to complex refractive index of gold film and, as a result, leads to a strong enhancement of UV PL peak in the ZnO layer. The mechanism of the tailoring of ZnO optical features changes by varying the Au layer thickness was discussed. Our studies have shown that through the changes of structural properties of thin gold layer between the Si substrate and the ZnO film, we can tune the optical dispersion of each layer and hence the control of ZnO PL spectra enhancement and quenching in UV-Vis wavelengths region is possible. In order to apply the hybrid structure under consideration in various optical applications, such as LED, the dispersion of the complex refractive index of the components should be optimized taking into account a particular target. [ABSTRACT FROM AUTHOR]

Stable isotope ratio mass spectrometry is a conventional method used in archaeology, and medical, environmental and paleoenvironmental reconstruction studies. However new insights and applicability of the equipment often open new research areas and improve our understanding of the ongoing processes. Therefore the stable isotope ratio mass spectrometry method was applied for the stoichiometry determination of the complex polypyrrole and glucose oxidase composite (PPy-GOx composite). The enzyme glucose oxidase and conducting polymer polypyrrole were reported to form a composite, which was evaluated in time using the dynamic light scattering method. The consistent enlargement of the PPy-GOx composite and the relative decrease of the spare enzyme molecules were observed in the polymerization solution. UV-VIS spectrometry was employed to follow the polymerization process. The isotope mixing model was applied for the evaluation of the constitution of the PPy-GOx composite. According to the obtained results the determination of the PPy-GOx composite stoichiometry could be more reliably determined using the nitrogen isotope ratio approach in comparison to the carbon approach. We expect that this novel work will widen the applications of stable isotope ratio mass spectrometry in research. [ABSTRACT FROM AUTHOR]

Here, we report a novel enantioselective electrochemical biosensor for the discrimination of cystine enantiomers (D- and L-cystine) using a chiral interface for the specific recognition of D- and L-cystine. The biosensor is based on reduced graphene oxide modified by ß-cyclodextrin (rGO/ß-CD) at the GCE surface. During the preparation of rGO/ß-CD/GCE, the modified electrode surfaces were characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The electrochemical behaviours of the D- and L-cystine were investigated using the rGO/ß-CD/GCE by CV and compared to bare GCE. A clear separation between the oxidation peak potentials of D- and L-cystine was observed at 1.32 and 1.42 V, respectively. The electrochemical discrimination performance of the fabricated chiral sensor was also examined by differential pulse voltammetry (DPV) in a mixed solution of D- and L-cystine. In addition, the DPV technique was used for the determination of D- and L-cystine at low concentration values in the range of 1.0-10.0 μM. To investigate the amperometric response of rGO/ß-CD/GCE towards D- and L-cystine, the chronoamperometry technique was used in the concentration range of 10.0-100.0 μM. The interactions of the enantiomers with rGO/ß-CD were modelled by molecular docking using AutoDock Vina, and the interaction energies were predicted to be -4.8 and -5.3 kcal mol-1 for D- and L-cystine, respectively. The corresponding values of binding constants were calculated to be 3.32 x 10³ and 7.71 x 10³ M-1, respectively. The experimental and molecular docking results indicate that the rGO/ß-CD/GCE has a different affinity for each enantiomer. [ABSTRACT FROM AUTHOR]

A direct immunoassay format with human growth hormone (hGH) immobilized on a self assembled monolayer modified surface plasmon resonance (SPR) chip was chosen to detect specific antibodies (anti-hGH) using different electrochemical techniques. Atomic force microscopy imaging and SPR were used as control methods for the evaluation and confirmation of the antigen–antibody complex formation. The applicability and sensitivity of candidate electrochemical techniques to develop an accurate and sensitive electrochemical immunosensor were investigated. Four electrochemical methods for anti-hGH determination – pulse amperometry (PA), cyclic voltammetry (CV), square wave voltammetry (SWV) and differential pulse voltammetry (DPV), were compared. Higher sensitivity of the developed immunosensor was observed using PA and CV: analytical signal registered using the PA method was 2.50 times higher in comparison with CV, 16.3 times higher in comparison with SWV and 24.5 times higher in comparison with the DPV method. In the case of PA detection method, the limit of detection was lower (75 nM) than that of the CV method (108 nM). [ABSTRACT FROM AUTHOR]

An affinity sensor based on a capacitive micromachined ultrasound transducer (cMUT) is reported by this Communication. The cMUT micromembrane arrays modified with adsorbed bovine leukemia virus protein gp51 were applied as a biological recognition part. The cMUT-based sensor is shown to be sensitive to the antibodies against bovine leukemia virus protein gp51 (anti-gp51). Two different concentrations of anti-gp51-containing samples and one blank sample without anti-gp51 were tested. The sensitivity of cMUT-based immunosensor is comparable with the sensitivity of a quartz microbalance-based immunosensor. The cMUT array provides a multi-channel system for the measurement of analytical signal. Moreover, two different characteristics – the resonance frequency shift (Δf) and the shift of the real part of the electromechanical impedance (ΔRe) – could have been evaluated simultaneously. Both analytical signals are informative and can be applied for the estimation of immune complex formation. We found the performance of such a system being potentially superior over some other immunosensing techniques. It is more rapid than electrochemical techniques and provides two different informative parameters. [ABSTRACT FROM AUTHOR]

A direct, label-free detection method of antibodies against the human growth hormone (anti-HGH) using a surface plasmon resonance (SPR) biosensor is reported. The sensing surface of the surface plasmon resonance biosensor chip (SPR-chip) was modified by covalent coupling of the human growth hormone (HGH) to the self-assembled monolayer of 11-mercaptoundecanoic acid (MUA). HGH was immobilized viaprimary amine groups after activation of the MUA carboxyl groups with a mixture of N-hydroxysuccinimide (NHS), and N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC). The specific binding of monoclonal anti-HGH antibody on the HGH-modified surface was examined in the concentration range from 0.25 nM to 10 µM. The experimentally observed detection minimum for anti-HGH was 2.47 nM. A single immunoassay cycle could be done within 30 min including the HGH and anti-HGH association, HGH/anti-HGH complex dissociation and surface regeneration steps. The SPR biosensor response for repeatable detections of anti-HGH was highly reproducible and very stable. On the SPR-chip the formed HGH and anti-HGH complex (HGH/anti-HGH) could be gently dissociated and the sensing surface might be regenerated by 50 mM NaOH and 0.5% sodium dodecylsulfate (SDS) solution. Any changes in the original baseline level were detected during the 40 detection–regeneration cycles. This means that damage of the immobilized HGH-based sensitive layer during regeneration was minimal. It was demonstrated that the developed SPR-chip could be stored for at least 21 days before use without considerable loss of sensitivity towards anti-HGH. [ABSTRACT FROM AUTHOR]

This study describes a new, basic procedure for the tuning of some analytical parameters of enzymatic biosensors that are based on hydrogen peroxide-producing oxido-reductases. An amperometric biosensor based on glucose oxidase (GOx) (EC 1.1.3.4) from Penicillum vitale, immobilized on a carbon rod electrode by cross-linking with glutaraldehyde, was exploited as a model system for demonstration of the approach described here. Such an important analytical parameter as the upper detection limit was dramatically changed by the formation of a polypyrrole conducting polymer layer by the GOx-induced polymerization of polypyrrole (Ppy). An increase in the upper detection limits for differently modified electrodes was estimated by calculation of the apparent Michaelis–Menten constant [KM(app)]. A significant increase in the long-term stability of the GOx-based electrode modified by Ppy (GOx/Ppy) was detected compared with that of an unmodified one. Further application of this approach, based on the self-encapsulation of glucose oxidase and other oxidases, is predicted for such biosensors where extension of the detection rate as well as KM(app)are required. [ABSTRACT FROM AUTHOR]

In this research a mechanism of interaction between a semiconducting TiO 2 layer and bovine leukemia virus protein gp 51, applied in the design of photoluminescence-based immunosensors, is proposed and discussed. Protein gp 51 was adsorbed on the surface of a nanostructured TiO 2 thin film, formed on glass substrates (TiO 2 /glass). A photoluminescence (PL) peak shift from 517 nm to 499 nm was observed after modification of the TiO 2 /glass by adsorbed gp 51 ( gp 51/TiO 2 /glass). After incubation of the gp 51/TiO 2 /glass in a solution containing anti- gp 51, a new structure (anti- gp 51/ gp 51/TiO 2 /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 gp 51 and the negatively charged TiO 2 surface. The strength of the electric field affecting the photoluminescence centers, was determined from variations between the PL-spectra of TiO 2 /glass, gp 51/TiO 2 /glass and anti- gp 51/ gp 51/TiO 2 /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 TiO 2 after its protein modification reveals an understanding of the interaction mechanism between TiO 2 and proteins that is the key issue responsible for biosensor performance., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

An affinity sensor based on a capacitive micromachined ultrasound transducer (cMUT) is reported by this Communication. The cMUT micromembrane arrays modified with adsorbed bovine leukemia virus protein gp51 were applied as a biological recognition part. The cMUT-based sensor is shown to be sensitive to the antibodies against bovine leukemia virus protein gp51 (anti-gp51). Two different concentrations of anti-gp51-containing samples and one blank sample without anti-gp51 were tested. The sensitivity of cMUT-based immunosensor is comparable with the sensitivity of a quartz microbalance-based immunosensor. The cMUT array provides a multi-channel system for the measurement of analytical signal. Moreover, two different characteristics--the resonance frequency shift (Deltaf) and the shift of the real part of the electromechanical impedance (DeltaRe)--could have been evaluated simultaneously. Both analytical signals are informative and can be applied for the estimation of immune complex formation. We found the performance of such a system being potentially superior over some other immunosensing techniques. It is more rapid than electrochemical techniques and provides two different informative parameters.

A direct, label-free detection method of antibodies against the human growth hormone (anti-HGH) using a surface plasmon resonance (SPR) biosensor is reported. The sensing surface of the surface plasmon resonance biosensor chip (SPR-chip) was modified by covalent coupling of the human growth hormone (HGH) to the self-assembled monolayer of 11-mercaptoundecanoic acid (MUA). HGH was immobilized via primary amine groups after activation of the MUA carboxyl groups with a mixture of N-hydroxysuccinimide (NHS), and N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC). The specific binding of monoclonal anti-HGH antibody on the HGH-modified surface was examined in the concentration range from 0.25 nM to 10 microM. The experimentally observed detection minimum for anti-HGH was 2.47 nM. A single immunoassay cycle could be done within 30 min including the HGH and anti-HGH association, HGH/anti-HGH complex dissociation and surface regeneration steps. The SPR biosensor response for repeatable detections of anti-HGH was highly reproducible and very stable. On the SPR-chip the formed HGH and anti-HGH complex (HGH/anti-HGH) could be gently dissociated and the sensing surface might be regenerated by 50 mM NaOH and 0.5% sodium dodecylsulfate (SDS) solution. Any changes in the original baseline level were detected during the 40 detection-regeneration cycles. This means that damage of the immobilized HGH-based sensitive layer during regeneration was minimal. It was demonstrated that the developed SPR-chip could be stored for at least 21 days before use without considerable loss of sensitivity towards anti-HGH.

This study describes a new, basic procedure for the tuning of some analytical parameters of enzymatic biosensors that are based on hydrogen peroxide-producing oxido-reductases. An amperometric biosensor based on glucose oxidase (GOx) (EC 1.1.3.4) from Penicillum vitale, immobilized on a carbon rod electrode by cross-linking with glutaraldehyde, was exploited as a model system for demonstration of the approach described here. Such an important analytical parameter as the upper detection limit was dramatically changed by the formation of a polypyrrole conducting polymer layer by the GOx-induced polymerization of polypyrrole (Ppy). An increase in the upper detection limits for differently modified electrodes was estimated by calculation of the apparent Michaelis-Menten constant [K(M(app))]. A significant increase in the long-term stability of the GOx-based electrode modified by Ppy (GOx/Ppy) was detected compared with that of an unmodified one. Further application of this approach, based on the self-encapsulation of glucose oxidase and other oxidases, is predicted for such biosensors where extension of the detection rate as well as K(M(app)) are required.