Eva Hedborg

Some studies of molecularly-imprinted polymer membranes in combination with field-effect devices

Investigations of the usefulness of molecularly-imprinted polymers in sensor applications were undertaken. Thin polymer membranes containing molecular imprints against l-phenylalanine anilide were prepared and applied as sensing layer in field-effect capacitors. In this report some experimental results obtained for C-V measurements when the polymer membranes were exposed to l-phenylalanine anilide (FAA), tyrosinanilide (TA), and phenylalaninol (FA) in ethanol are presented. Despite some difficulties, it was shown that in ethanol solution the compounds FAA and TA can be measured and distinguished from FA using a imprinted polymer membrane.

Differentiation of human serum samples by surface plasmon resonance monitoring of the integral glycoprotein interaction with a lectin panel

Bacterial infection and inflammation result in massive changes in serum glycoproteins. These changes were investigated by the interaction of the saccharide glycoprotein moiety with lectins. A panel of eight lectins (Canavalia ensiformis, Bandeiraea simplicifolia BS-I, Arachis hypogaea, Phytolacca americana, Phaseolus vulgaris, Artocarpus integrifolia, Triticum vulgaris and Pisum sativum) was used to differentiate human serum glycoproteins obtained from patients with various bacterial infections. Lectin functionalised sensing layers were created on gold-coated wafers and lectin-glycoprotein interactions were monitored by surface plasmon resonance. The interaction of the lectin panel with serum glycoproteins produces unique patterns. Principal component analysis (PCA) was used to analyse the patterns. The actual panel of eight lectins enabled discrimination between sera obtained from patients sick with bacterial infection and healthy patients. Extended lectin panels have the potential to distinguish between types of bacterial infection and identify specific disease state. (Less)

Visual images of gas mixtures produced with field-effect structures

The details of a recently demonstrated possibility of making two-dimensional response maps of a gas mixture are discussed. It is described how maps of work-function changes due to the interaction between molecules like ethanol and ammonia and thin catalytic metal films as gates on field-effect structures can be created with a scanning light-pulse technique. Simple image processing is shown to enhance the differences in the response patterns created by different molecules.

Electronic Noses Based on Field Effect Structures

In this chapter we first review the properties of a discrete array of field effect devices to understand some of their selectivity properties. A scanning light pulse technique applied for gas sensing will then be described. The use of the technique to produce maps of the gas sensitivity of large catalytic metal areas is demonstrated. The metal area consisted of overlapping bands of platinum, iridium and palladium which were kept “cold” (≈ 110 °C) at one end and “hot” (≈ 180 °C) at the other end. It is shown how, for example, ammonia, ethanol and hydrogen in air produce distinctly different maps which can be further processed to yield images identifying the gas. These first “olfactory images” serve as an illustration to what can be achieved with a continuous sensing surface or a large chemical sensor matrix. The similarity between such images and pattern formation in the olfactory systems is also pointed out.

Charge migration on hydrophobic and hydrophilic silicon dioxide

Abstract Experimental results are presented which shed light on the properties of thin discontinuous metal films as chemical sensing elements. They demonstrate further some interesting differences between hydrophilic and hydrophobic sensing structures. Two different methods are used to study the migration of charges out onto the oxide surface outside the metal gate of metal–oxide–semiconductor capacitors after treatment of the surface with hydrochloric acid. The charge migration is observed either as a time dependent increase of the inversion capacitance or as a possibility to generate a photocapacitive current by a chopped light beam hitting the oxide surface at a distance from the contact. It is concluded that the charge migration occurs only if the surface was hydrophilic before the HCl-treatment. For a hydrophobic surface neither the inversion capacitance nor the photocapacitive current changed upon ion-treatment. An explanation for an observed increase of the capacitance level of hydrophilic structures immersed in electrolytes is also given.

Air pockets in thin porous platinum films studied by spectroscopic ellipsometry

Abstract Optical characterization of thin porous platinum films has been carried out by spectroscopic ellipsometry in air, in water and in methanol. From the measurements in air, the thicknesses and porosity of the films were determined. The measurements in water showed that air pockets are present at the interface between water and a platinum film if the sample surface is made hydrophobic by silanization. In methanol such platinum films are wetting and the air pockets vanish. In the optical analysis it was necessary to include a thin organic layer on the hydrophobic surfaces. However, no organic layer or air pockets were found in water on as-prepared hydrophillic surfaces. Furthermore, upon storage in laboratory air, an organic layer formed spontaneously on the hydrophillic surface. The ellipsometric results support the suggestion that, by means of the air pockets, hydrophobic thin platinum films act as gas permeable structures in electrolytes which leads to a possibility to develop chemical sensors for molecular species in solutions.

Influence of wettability on the properties of thin porous platinum films as gates of metal-oxide-semiconductor devices in electrolytes

Thin discontinuous (4 and 35 nm thick) platinum films were used as the gate metal of metal-oxide-semiconductor capacitors. It was observed that the metal film acts as a gas permeable structure in electrolyte solutions when the surface was hydrophobic and sufficiently rough. The probable explanation to this is that the electrolyte does not penetrate into the voids of the metal film. The geometry of the surface was investigated by transsion electron microscopy and profilometer measurements. Surfaces with different contact angles were studied in order to determine whether the electrolyte penetrates into the voids in the platinum film when the surface is sufficiently hydrophilic.

Evaluation of gas sensing surfaces with a scanned light pulse technique

Abstract The scanned light pulse technique applied to gas sensing surfaces is evaluated for some applications. The basis of the technique is briefly described, and also some advantages. It is further described how the thickness of a thin (10–15 nm) metal layer can be determined. It is also shown how the technique can be used for investigations of the mobility of hydrogen atoms on a palladium surface and the use of large sensing surfaces to decide the direction and distribution of a molecular flow. Finally it is shown how the scanned light pulse technique can be used for discrimination of different alcohols. Some of the possibilities for processing the obtained data with a simple image processing software (for Macintosh) are also shown.

NEW OBSERVATION ON THIN DISCONTINUOUS METAL FILMS

It is shown from the capacitance‐voltage curves of metal‐insulator‐semiconductor structures that molecular species in an electrolyte interact with thin discontinuous, hydrophobic platinum films in a way similar to that in a gas phase. It is suggested that this is due to the presence of air volumes in the cracks of the discontinuous metal. This suggestion is supported by theoretical models for the wetting of thin films. The observation may have implications for the use of thin metal layers as the sensing material in chemical sensors for molecular species in solutions.

Polymer membranes for modification of the selectivity of field-effect gas sensors

Abstract Polymer membranes are used to increase the selectivity to certain gases of metal-silicon dioxide-semiconductor (MOS) structures. Other parameters which influence the selectivity of MOS structures are the type of gate metal, its microstructure (dense or porous) and the operating temperature of the device. Photoresists as membranes can be patterned by photolithographic methods. Membranes, 1–2 μm thick, of positive and negative photoresist are applied on MOS capacitors with 6 nm iridium as the gate metal, operated at 150 °C. The influence of the membranes on the response to three gases, hydrogen, ammonia and ethanol, has been investigated. The hydrogen response decreases by about half with the use of a photoresist membrane. The ammonia response shows a characteristic change in the kinetics, while the ethanol response almost disappears. Positive and negative resist influence the gas response in similar ways, in spite of their different molecular structures.