Alexandre Kisner

Bioelectrochemical systems with oleylamine-stabilized gold nanostructures and horseradish peroxidase for hydrogen peroxide sensor

This paper describes ultrathin gold nanowires (NWs) and nanoparticles (NPs) prepared by oleylamine (OA) synthesis and their assembly with horseradish peroxidase enzyme (HRP) for bioelectrochemical sensing of hydrogen peroxide for the first time. The immobilization of oxidoreductase enzyme HRP on the electrodes modified with OA gold nanostructures (OANSs) is discussed. The HRP-sensor characteristics, namely sensitivity, working concentration range, sensor-to-sensor and measurement-to-measurement reproducibility as well as long-term stability, are improved significantly compared to the planar thin-film sensors by using OANSs. The thin-film gold electrodes modified with OANWs and OANPs exhibit a catalytic activity towards oxidation of hydrogen peroxide with a working concentration range from 20 µM to 500 µM, a sensitivity of 0.031 A M(-1) cm(-2) (RSD 0.046) and 0.027 A M(-1) cm(-2) (RSD 0.045), and a detection limit of 5 µM and 8 µM, respectively (RSD near the detection limits was 9-12%). Our study shows that ultrathin gold nanowires and nanoparticles prepared by oleylamine synthesis are prospective materials to assemble biomolecules into functional nanoarchitectures for enzyme-based bioelectrochemical sensors, metalloprotein bioelectronics, and energy research.

Features of Transport in Ultrathin Gold Nanowire Structures

The origin of the interface formation appearing due to the realization of contacts to ultrathin gold nanowire devices is revealed. Such interfaces play an important role in transport mechanisms in nanowire structures and can determine the electrical and operating parameters of a nanodevice. Based on experimental results, the specific electrical properties of bundles of ultrathin gold nanowires fabricated by wet chemical synthesis and subsequently assembled and contacted with gold electrodes are reported. It is demonstrated that these properties are strongly affected by the monolayers of organic molecules inevitably present on the surface of the nanowires due to synthetic conditions. In particular, such layers form a potential barrier to tunneling of the electrons from contacts to the nanowires. The electric transport behavior of the investigated nanowire structures in the temperature range from 500 mK to 300 K obeys the model of thermal fluctuation-induced tunneling conduction through the nanowire-metal electrode molecular junction. Application of this model allows calculation of the parameters of the molecular potential barrier. The formation of such a molecular barrier is verified by scanning tunneling microscope (STM) and transmission electron microscope (TEM) measurements performed using a supporting graphene layer. These findings are important for designing novel nanodevices for molecular electronics on the basis of ultrathin nanowires.

The Role of Oxidative Etching in the Synthesis of Ultrathin Single‐Crystalline Au Nanowires

The fabrication of ultrathin single-crystal Au nanowires with high aspect ratio and that are stable in air is challenging. Recently, a simple wet-chemical approach using oleylamine has been reported for the synthesis of Au nanowires with micrometer length and 2 nm in diameter. Despite efforts to understand the mechanism of the reaction, an ultimate question about the role of oxygen (O(2)) during the synthesis remained unclear. Here we report that the synthesis of ultrathin Au nanowires employing oleylamine is strongly affected by the amount of O(2) absorbed in the reaction solution. Saturating the solution with O(2) leads to both a high-yield production of nanowires and an increase in their length. Nanowires with diameters of about 2 nm and lengths of 8 μm, which corresponds to an aspect ratio of approximately 4000, were produced. The role of oxygen is attributed to the enhanced oxidation of twin defects on Au nanoparticles formed in the first stage of the reaction. Understanding the role of oxidative etching is crucial to significantly increasing the yield and the length of ultrathin Au nanowires.

Probing the effect of surface chemistry on the electrical properties of ultrathin gold nanowire sensors.

Ultrathin metal nanowires are ultimately analytical tools that can be used to survey the interfacial properties of the functional groups of organic molecules immobilized on nanoelectrodes. The high ratio of surface to bulk atoms makes such ultrathin nanowires extremely electrically sensitive to adsorbates and their charge and/or polarity, although little is known about the nature of surface chemistry interactions on metallic ultrathin nanowires. Here we report the first studies about the effect of functional groups of short-chain alkanethiol molecules on the electrical resistance of ultrathin gold nanowires. We fabricated ultrathin nanowire electrical sensors based on chemiresistors using conventional microfabrication techniques, so that the contact areas were passivated to leave only the surface of the nanowires exposed to the environment. By immobilizing alkanethiol molecules with head groups such as -CH3, -NH2 and -COOH on gold nanowires, we examined how the charge proximity due to protonation/deprotonation of the functional groups affects the resistance of the sensors. Electrical measurements in air and in water only indicate that beyond the gold-sulfur moiety interactions, the interfacial charge due to the acid-base chemistry of the functional groups of the molecules has a significant impact on the electrical resistance of the wires. Our data demonstrate that the degree of dissociation of the corresponding functional groups plays a major role in enhancing the surface-sensitive resistivity of the nanowires. These results stress the importance of recognizing the effect of protonation/deprotonation of the surface chemistry on the resulting electrical sensitivity of ultrathin metal nanowires and the applicability of such sensors for studying interfacial properties using electrodes of comparable size to the electrochemical double layer.

Sensing small neurotransmitter–enzyme interaction with nanoporous gated ion-sensitive field effect transistors

Ion-sensitive field effect transistors with gates having a high density of nanopores were fabricated and employed to sense the neurotransmitter dopamine with high selectivity and detectability at micromolar range. The nanoporous structure of the gates was produced by applying a relatively simple anodizing process, which yielded a porous alumina layer with pores exhibiting a mean diameter ranging from 20 to 35 nm. Gate-source voltages of the transistors demonstrated a pH-dependence that was linear over a wide range and could be understood as changes in surface charges during protonation and deprotonation. The large surface area provided by the pores allowed the physical immobilization of tyrosinase, which is an enzyme that oxidizes dopamine, on the gates of the transistors, and thus, changes the acid-base behavior on their surfaces. Concentration-dependent dopamine interacting with immobilized tyrosinase showed a linear dependence into a physiological range of interest for dopamine concentration in the changes of gate-source voltages. In comparison with previous approaches, a response time relatively fast for detecting dopamine was obtained. Additionally, selectivity assays for other neurotransmitters that are abundantly found in the brain were examined. These results demonstrate that the nanoporous structure of ion-sensitive field effect transistors can easily be used to immobilize specific enzyme that can readily and selectively detect small neurotransmitter molecule based on its acid-base interaction with the receptor. Therefore, it could serve as a technology platform for molecular studies of neurotransmitter-enzyme binding and drugs screening.

In situ fabrication of ultrathin porous alumina and its application for nanopatterning Au nanocrystals on the surface of ion-sensitive field-effect transistors

In situ fabrication in a single step of thin films of alumina exhibiting a thickness of less than 100 nm and nanopores with a highly regular diameter distribution in order to pattern nanostructures over field-effect devices is a critical issue and has not previously been demonstrated. Here we report the fabrication in situ of 50 nm thick ultrathin nanoporous alumina membranes with a regular pore size directly over metal-free gate ion-sensitive field-effect transistors. Depositing thin films of aluminum by an electron beam at a relatively low rate of deposition on top of chips containing the transistors and using a conventional single-step anodization process permits the production of a well-adhering nanoporous ultrathin layer of alumina on the surface of the devices. The anodization process does not substantially affect the electrical properties of the transistors. The small thickness and pore size of ultrathin alumina membranes allow them to be sequentially employed as masks for patterning Au nanocrystals grown by an electroless approach directly on the top of the transistors. The patterning process using a wet chemical approach enables the size of the patterned crystals to be controlled not only by the dimensions of the pores of alumina, but also by the concentration of the reactants employed. Surface modification of these nanocrystals with alkanethiol molecules demonstrates that the electrostatic charge of the functional groups of the molecules can modulate the electrical characteristics of the transistors. These results represent substantial progress towards the development of novel nanostructured arrays on top of field-effect devices that can be applied for chemical sensing or non-volatile memories.

Biosensors for Antioxidant Evaluation in Biological Systems

The prevention of oxidative reactions in a biological medium as well as the role of reactive oxygen species (ROS) in chronic degenerative diseases are questions that continue to be investigated. Electrochemical biosensors have shown attractive features to evaluate the oxidative stress condition at a level comparable to chromatographic and spectroscopic techniques. The biosensors developed so far are based on direct analysis of specific indicators such as biomarkers of oxidative stress on the monitoring of reactive oxygen species the free radicals in cells or tissues, aiming to obtain a correlation between the index obtained from these indicators with the oxidative stress levels in cells. In this review we will provide an overview of the development of electrochemical biosensors to evaluate the content of antioxidants and reactive oxygen species in physiological systems. Some discussion regarding the analysis of antioxidant capacity at the single cell level is also presented.

Microfluidic anodization of aluminum films for the fabrication of nanoporous lipid bilayer support structures

Summary Solid state nanoporous membranes show great potential as support structures for biointerfaces. In this paper, we present a technique for fabricating nanoporous alumina membranes under constant-flow conditions in a microfluidic environment. This approach allows the direct integration of the fabrication process into a microfluidic setup for performing biological experiments without the need to transfer the brittle nanoporous material. We demonstrate this technique by using the same microfluidic system for membrane fabrication and subsequent liposome fusion onto the nanoporous support structure. The resulting bilayer formation is monitored by impedance spectroscopy across the nanoporous alumina membrane in real-time. Our approach offers a simple and efficient methodology to investigate the activity of transmembrane proteins or ion diffusion across membrane bilayers.

Synthesis and Structural Characterization of Ultra-thin Flexible Au Nanowires

Au nanowires (AuNWs) were produced by electroless reduction of HAuCl 4 in a micellar structure formed by oleylamine and investigated by means of high-resolution transmission electron microscopy (HRTEM). Micrometer long ultra-thin flexible AuNWs with 1—2 nm diameter and AuNWs with about 12 nm diameter and a few hundred nm length were produced. Their extremities show a characteristic bulging. In contradiction with previous work, the bodies of the 12 nm nanowires are defect-free along the axial direction, their extremities, however, show the presence of twin boundaries. Ultra-thin AuNWs were often found as bundles presenting lengths of few micrometers. Although they are stable in solution for months, they were found to be quite sensitive to electron beam irradiation during HRTEM experiments, with a tendency to break up into face centered cubic (fcc) Au droplets. It is proposed that the micellar configuration of oleylamine plays a fundamental role in the atomic arrangement of nanowires. Finally, we anticipate our results to be a starting point for a more realistic experimental investigation of surface effects on the mechanical properties of ultra-thin nanowires with high aspect ratio, which have been only widely exploited theoretically.