P.S.P. Herrmann

Atomic Force Microscopy as a Tool Applied to Nano/Biosensors

This review article discusses and documents the basic concepts and principles of nano/biosensors. More specifically, we comment on the use of Chemical Force Microscopy (CFM) to study various aspects of architectural and chemical design details of specific molecules and polymers and its influence on the control of chemical interactions between the Atomic Force Microscopy (AFM) tip and the sample. This technique is based on the fabrication of nanomechanical cantilever sensors (NCS) and microcantilever-based biosensors (MC-B), which can provide, depending on the application, rapid, sensitive, simple and low-cost in situ detection. Besides, it can provide high repeatability and reproducibility. Here, we review the applications of CFM through some application examples which should function as methodological questions to understand and transform this tool into a reliable source of data. This section is followed by a description of the theoretical principle and usage of the functionalized NCS and MC-B technique in several fields, such as agriculture, biotechnology and immunoassay. Finally, we hope this review will help the reader to appreciate how important the tools CFM, NCS and MC-B are for characterization and understanding of systems on the atomic scale.

Characterization of Langmuir-Blodgett films of parent polyaniline

Conducting Langmuir-Blodgett (LB) films have been fabricated from parent polyaniline (PAni) which was doped with functionalized acids. In order to optimize experimental conditions for the formation of stable Langmuir monolayers and their subsequent transfer onto solid substrates, PAni was dissolved in ten different combinations of chloroform solutions. Use was made of camphor sulfonic acid, dodecyl benzene sulfonic acid, and toluenesulfonic acid, and of the solvents N-methyl pyrrolidine and m-cresol as processing agents. Because acidic subphases have been employed, as-deposited LB films were already doped, which was confirmed by the appearance of a polaronic band in the UV-Vis absorption spectra. The absorbance peak increases with the number of deposited layers indicating that a suitable multilayer buildup is accomplished. When analysed by atomic force microscopy, PAni LB films show a fibrillar structure with the fibril width ranging from ≈60 to 160 nm.

Low-Cost Gas Sensors Produced by the Graphite Line-Patterning Technique Applied to Monitoring Banana Ripeness

A low-cost sensor array system for banana ripeness monitoring is presented. The sensors are constructed by employing a graphite line-patterning technique (LPT) to print interdigitated graphite electrodes on tracing paper and then coating the printed area with a thin film of polyaniline (PANI) by in-situ polymerization as the gas-sensitive layer. The PANI layers were used for the detection of volatile organic compounds (VOCs), including ethylene, emitted during ripening. The influence of the various acid dopants, hydrochloric acid (HCl), methanesulfonic acid (MSA), p-toluenesulfonic acid (TSA) and camphorsulfonic acid (CSA), on the electrical properties of the thin film of PANI adsorbed on the electrodes was also studied. The extent of doping of the films was investigated by UV-Vis absorption spectroscopy and tests showed that the type of dopant plays an important role in the performance of these low-cost sensors. The array of three sensors, without the PANI-HCl sensor, was able to produce a distinct pattern of signals, taken as a signature (fingerprint) that can be used to characterize bananas ripeness.

Thickness and size distribution of clay-sized soil particles measured through atomic force microscopy

Abstract Soil particle size distribution is a very important soil characteristic because it is related with many physical, chemical and biological processes occurring in soils. However, at the sub-micron range, where many important sorption and transformation processes occur, there are few techniques available for particle size characterization. Among the available methods are transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The classical TEM and SEM, however, are sometimes unable to clearly differentiate between agglomerates, particles and grains; and sample preparation is very difficult and tedious. To overcome these limitations, we have applied the atomic force microscopy (AFM) technique to analyze the particle size distribution of an oxisol from Sao Carlos site, in Brazil, at the sub-micron range. Thickness and diameter of soil particles deposited in freshly cleaved mica were measured for each individual particle, allowing to determine their particle size distribution. Assuming cylindrical shaped particles and a constant particle density of 2.7 g cm −3 , the mass-based soil particle distribution at the sub-micron range is obtainable.

Comparison between different conditions of the chemical polymerization of polyaniline on top of PET films

Universidade de Sao Paulo Ciencia e Engenharia de Materiais Area Interunidades, Sao Carlos, SP

Atomic force spectroscopy on poly(o-ethoxyaniline) nanostructured films: sensing nonspecific interactions.

Atomic force spectroscopy (AFS) was used to measure interaction forces between the tip and nanostructured layers of poly(o-ethoxyaniline) (POEA) in pure water and CuSO4 solutions. When the tip approach and retraction were carried out at low speeds, POEA chains could be physisorbed onto the Si3N4 tip via nonspecific interactions. We conjecture that while detaching, POEA chains were stretched and the estimated chain lengths were consistent with the expected values from the measured POEA molecular weight. The effects from POEA doping could be investigated directly by performing AFS measurements in a liquid cell, with the POEA film exposed to liquids of distinct pH values. For pH > or = 6.0, the force curves normally displayed an attractive region for POEA, but at lower pH values-where POEA is protonated-the repulsive double-layer forces dominated. Measurements in the liquid cell could be further exploited to investigate how the film morphology and the force curve are affected when impurities are deliberately introduced in the liquid. The shape of the force curves and the film morphology depended on the concentration of heavy metal in the liquid cell. AFS may therefore be used to study the interaction between film and analyte, with important implications for the understanding of mechanisms governing the sensing ability of taste sensors.

Microcantilever Sensors Coated With Doped Polyaniline for the Detection of Water Vapor

In the present work, PANI (polyaniline) emeraldine salt (doped) and base (dedoped) were used as the sensitive layer of a silicon microcantilever, and the mechanical response (deflection) of the bimaterial (coated microcantilever) was investigated under the influence of humidity. PANI in the emeraldine base oxidation state was obtained by interfacial synthesis and was deposited on the microcantilever surface by spin-coating (dedoped). Next, the conducting polymer was doped with 1 M HCl (hydrochloric acid). A four-quadrant AFM head with an integrated laser and a position-sensitive detector (AFM Veeco Dimension V) was used to measure the optical deflection of the coated microcantilever. The deflection of the coated (doped and undoped PANI) and uncoated microcantilever was measured under different humidities (in triplicate) at room pressure and temperature in a closed chamber to evaluate the sensors sensitivity. The relative humidity (RH) in the chamber was varied from 20% to 70% using dry nitrogen as a carrier gas, which was passed through a bubbler containing water to generate humidity. The results showed that microcantilevers coated with sensitive layers of doped and undoped PANI films were sensitive (12,717 ± 6% and 6,939 ± 8%, respectively) and provided good repeatability (98.6 ± 0.015% and 99 ± 0.01%, respectively) after several cycles of exposure to RH. The microcantilever sensor without a PANI coating (uncoated) was not sensitive to humidity. The strong effect of doping on the sensitivity of the sensor was attributed to an increased adsorption of water molecules dissociated at imine nitrogen centers, which improves the performance of the coated microcantilever sensor. Moreover, microcantilever sensors coated with a sensitive layer provided good results in several cycles of exposure to RH (%).

Doping in poly(o-ethoxyaniline) nanostructured films studied with atomic force spectroscopy (AFS).

The study of intermolecular interactions at interfaces is essential for a number of applications, in addition to the understanding of mechanisms involved in sensing and biosensing with liquid samples. There are, however, only a few methods to probe such interfacial phenomena, one of which is the atomic force spectroscopy (AFS) where the force between an atomic force microscope tip and the sample surface is measured. In this study, we used AFS to estimate adhesion forces for a nanostructured film of poly(o-ethoxyaniline) (POEA) doped with various acids, in measurements performed in air. The adhesion force was lower for POEA doped with inorganic acids, such as HCl and H(2)SO(4), than with organic acids, because the counterions were screened by the ethoxy groups. Significantly, the morphology of POEA both in the film and in solution depends on the doping acid. Using small-angle X-ray scattering (SAXS) we observed that POEA dissolved in a mixture of dimethyl acetamide exhibits a more extended coil-like conformation, with smaller radius of gyration, than for POEA in water, as in the latter POEA solubility is lower. In AFS measurements in a liquid cell, the force curves for a POEA layer displayed an attractive region for pH>or=5 due to van der Waals interactions, with no contribution from a double-layer since POEA was dedoped. In contrast, for pH<or=3, POEA was doped and the repulsive double-layer force dominated. With AFS one is therefore able to correlate molecular-level interactions with doping and morphology of semiconducting polymers.

Adhesion Forces for Mica and Silicon Oxide Surfaces Studied by Atomic Force Spectroscopy (AFS)

The possibility of analyzing surfaces at the nanoscale provided by atomic force microscopy [1] (AFM) has been explored for various materials, including polymers [2], biological materials [3] and clays [4]. Further uses of AFMs involved nanomanipulation [5] and measurements of interaction forces, where the latter has been referred to as atomic force spectroscopy (AFS) [6]. Measurements of surface-surface interactions at the nanoscale are important because many materials have their properties changed at this range [7]. For samples in air, the interactions with the tip are a superimposition of van der Waals, electrostatic and capillary forces. A number of surface features can now be monitored with AFS, such as adsorption processes and contamination from the environment. Many implications exist for soil sciences and other areas, because quantitative knowledge of particle adhesion is vital for understanding technological processes, including particle aggregation in mineral processing, quality of ceramics and adhesives. In this paper, we employ AFS to measure adhesion (pull-off force) between the AFM tip and two types of substrate. Adhesion maps are used to illustrate sample regions that had been contaminated with organic compounds.