Recent research shows that electrostatic precipitation is a gentle method to collect airborne microorganisms and preserve their cultivability. However, the corona discharge used to charge the particles and the high electric field used to capture them are known to have a germicidal effect. The present paper investigates this paradoxical situation. Vegetative cells of E. coli and B. subtilis and spores of A. fumigatus and B. subtilis were deposited on different media and subjected to electrostatic fields of different strengths and polarities for controlled time periods. Vegetative cells are inactivated on cultivation agar plates, but remain cultivable when exposed on a stainless steel electrode and transferred afterwards onto agar plates. For the investigated conditions, spores were not affected by the corona discharge. Further experiments with a pH indicator show that chemical reactions occur when an aqueous media is exposed to the discharge. Some of these reactions are likely to create hydrogen peroxide which is known to kill a broad range of microorganisms. It is therefore highlighted that collecting electrodes in electrostatic air samplers should rather be dry conductive media.
Les fibres amyloides sont des biomateriaux prometteurs pour la bioelectronique, en particulier pour l’interfacage avec les systemes biologiques. Ces fibres, formees par l’auto-assemblage de proteines, sont aisement synthetisables et modifiables/fonctionnalisables. Elles possedent de surcroit des proprietes physiques remarquables notamment en termes de stabilite et de resistance mecanique. Nous avons etudie les mecanismes de conductions de charges dans les fibres formees par la proteine HET-s(218-289), seules fibres amyloides dont la structure atomique soit connue. Les echantillons ont ete caracterises electriquement et electrochimiquement sous la forme de films « secs ». L’influence de plusieurs parametres sur la conductivite, entre autres la temperature, l’humidite ou encore la lumiere, a ete investiguee. Nous avons montre que l’organisation de la proteine en fibres permet la mise en place de processus de transport de charges intrinseques. De plus, l’eau joue un role essentiel dans ces mecanismes et les principaux porteurs de charges sont certainement des protons. En parallele, une simulation de dynamique moleculaire appuyee notamment par des experiences de diffusions des neutrons, a mis en evidence une forte interaction entre l’eau et les fibres. Deux canaux d’eau stabilises par liaisons hydrogenes se formeraient le long des fibres. Ces derniers peuvent permettre le transport de protons par un mecanisme de type Grotthuss. Des reactions electrochimiques, en particulier l’electrolyse de l’eau, seraient la source des protons transportes grâce aux fibres. Cela conduit a l’instauration d’un courant catalytique a partir d’un seuil de tension de polarisation. Enfin, deux effets photo-electriques ont ete observes lorsque les fibres sont irradiees entre 200 et 400 nm. Le premier est un photo-courant qui serait du a la photolyse de l’eau adsorbee dans les echantillons. Le second, qualifie de « photo-courant inverse », se produit plus specifiquement a la longueur d’onde de 280nm et seulement en presence de dioxygene. Il engendre une diminution de la conductivite. Cela serait du a une reaction entre l’etat triplet des tryptophanes des fibres et le dioxygene, captant in fine des protons.
Based on their nanoscale size and morphology, as well as on their auto-assembling properties, bio-inspired nano-objects have been identified as potential interconnection and sensor materials. Their electrical properties, namely conduction mechanism and electrical contact, have to be studied in order to enable a comparison with standard microelectronic components. In this paper, a generic characterization platform for bio-inspired nano-objects is proposed, enabling electrical investigations ranging from dry electrical measurements to wet electrochemical investigation, and for bio-inspired materials morphologies from single nanowires to films. Various electrode patterns have been designed and fabricated on 200mm silicon substrates, before being diced at chip scale and embedded in easy-to-plug systems for dry and wet measurements, including climatic chamber or glove box. Fully biocompatible microelectronic fabrication processes have been selected.
Microelectronics industry aims at pushing the scaling of MOSFET devices, with a lot of challenges to solve for front-end and back-end processes. These challenges are also new opportunities, as for co-integration between silicon components and biological objects. In particular, bio-inspired nano objects such as DNA-based nanowires and protein-based nanowires have generated a huge interest based on their promising electrical properties [1, 2] and their size at the nanoscale. They could indeed bring a rich evolution in the nanotechnology community. This paper presents a common work performed by microelectronic biology and characterization teams which aims at describing the design, and the fabrication of a characterization platform dedicated to bio-inspired objects, and the first electrical measurements acquired on amyloid fibers. The design of this characterization platform takes into account biological and microelectronic constraints. Regarding biological constraints, special attention has been paid to materials choice and device fabrication in order to be biocompatible and to enable drop deposition for both wet and dry experiments. Dedicated patterns have been designed in order to measure electrical parameters such as contact resistance and resistivity of the bio-inspired objects. Regarding the fabrication step, the characterization platform has taken advantage from microelectronic technologies, especially in terms of size (nanoscale), reproducibility and robustness taken into account the specific environment. These aspects will be further discussed in this paper, as well as the first electrical measurements. To properly understand the electrical characteristics of bio-inspired objects, one critical point is the measurement of the linear conductivity of these objects, which implies the discrimination from contact resistance. Another important point is the conduction mechanism. Table 1 presents two patterns used to investigate the conduction mechanisms. To realize a complete electrical study, we have thus decided to design specific test structures as Transmission Line Matrix (TLM) and Van der Pauw patterns, structures with a variable electrode gap, and different contact areas structures. Figure 1 shows SEM images of the electrodes fabricated with this specific layout. The inter-electrode distance has been measured at 1µm and 70µm x 70µm square pads have been obtained. The figure 2 presents a cross section view of the fabricated electrodes. Electrode thickness is equal to 200nm and the width is equal to 1.7µm. Material impact and electrochemical properties of bio-inspired objects have also been investigated. Working with biological object at the nanoscale requires further restrictions. The main constraint is the use of biocompatible material to pattern all the layers in contact with the bio-inspired object. We have decided to use (i) platinum and gold because these metals are commonly used in biological measurements, and (ii) ruthenium due to its anisotropic etching properties. Another requirement relies on back gate electrode to enable the field effect investigation of bio-inspired object. To deal with wet measurement and to control the film deposition at the desirable position we have decided to engineer further cavities. Regarding packaging solutions, standard microelectronic metallic pads have been patterned to allow automated electrical characterization. Bioelectrical investigations as electrochemistry or control atmosphere measurements require the design of an “easy to plug” device enabling drop deposition and a very simple electrical connection adapted to electrical probers. This “easy to plug” device has been fabricated including a patterned silicon die assembled with a conductive glue on a Side-brazed Ceramic Dual In-inline Package (SCDIP) and wire-bonding. A SCDIP has been especially chosen to be easily carried and plugged inside biological equipment such as climatic chamber and glove box. Large package have been used in order to connect the maximum of patterns but also to provide an important protein-based nanowires density on the top of the electrodes. Figure 3 presents photography picture of the characterization platform used to perform the conduction measurements in biological equipment. The figure 4 presents the first electrical signal of the bio-inspired nano object which confirms that the characterization platform has been correctly fabricated and can be able to study the electrical properties of the protein film. This papers aims at presenting more details on (i) the design, (ii) the fabrication of silicon die in clean room (iii) the packaging adapted to the biological environment and (iv) results of electrical characterization of the bio-inspired objects at the nanoscale. References [1] D. V. Lim, M. M. Simpson, E. A. Kearns et al., Clin. Microbiol. Rev., vol. 18, no. 4, pp. 583–607, Oct. 2005. [2] F. S. Ligler and J. S. Erickson, Nature, vol. 440, no. 7081, pp. 159–160, Mar. 2006. Figure 1
