Atomic force microscopy as a biophysical tool for nanoscale forensic investigations

The atomic force microscope (AFM) has found its way to the arsenal of tools available to the forensic practitioner for the analysis of samples at the nano and microscales. As a non-destructive probing tool that requires minimal sample preparation, the AFM is very attractive, particularly in the case of minimal or precious sample. To date, the use of the AFM has primarily been in the arena of imaging where it has been complementary to other microscopic examination tools. Forensic applications in the visual examination of evidence such as blood stains, questioned documents, and hair samples have been reported. While a number of reviews have focused on the use of AFM as an imaging tool for forensic analyses, here we not only discuss these works, but also point to a versatile enhancement in the capabilities of this nanoscale tool - namely its use for force spectroscopy. In this mode, the AFM can determine elastic moduli, adhesion forces, energy dissipation, and the interaction forces between cognate ligands, that can be spatially mapped to provide a unique spatial visualization of properties. Our goals in this review are to provide a context for this capability of the AFM, explain its workings, cover some exemplary works pertaining to forensic sciences, and present a critical analysis on the advantages and disadvantages of this modality. Equipped with this high-resolution tool, imaging and biophysical analysis by the AFM can provide a unique complement to other tools available to the researcher for the analysis and characterization of forensic evidence.

The interaction between BSA and DOTAP at the air-buffer interface

In this article, the interaction between bovine serum albumin (BSA) and the cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) at the air-buffer interface was investigated at different subphase's pH values (pH = 3, 5 and 10). Surface pressure measurements (π - A) and penetration kinetics process (π - t) were carried out to reveal the interaction mechanism and the dynamical behavior. The data showed that π - A isotherms moved towards larger mean molecular area when the concentration of BSA ([BSA]) increased, the amount of BSA adsorbed onto DOTAP monolayer reached a threshold value at a [BSA] of 5 × 10^-8 M, and BSA desorbed from the lipid monolayer as time goes by. The results revealed that the association of BSA with DOTAP at the air-buffer interface was affected by the subphase's pH value. When pH = 10, the interaction mechanism between them was a combination of hydrophobic interaction and electrostatic attraction, so BSA molecules could be well separated and purified from complex mixtures. AFM images demonstrated that pH value and [BSA] could affect the morphology feature of DOTAP monolayer and the adsorption and desorption processes of BSA. So the study provides an important experimental basis and theoretical support for learning the interaction mechanism among biomolecules in separation and purification of biomolecules and biosensor.

Tribology and stability of organic monolayers on CrN: a comparison among silane, phosphonate, alkene, and alkyne chemistries

The fabrication of chemically and mechanically stable monolayers on the surfaces of various inorganic hard materials is crucial to the development of biomedical/electronic devices. In this Article, monolayers based on the reactivity of silane, phosphonate, 1-alkene, and 1-alkyne moieties were obtained on the hydroxyl-terminated chromium nitride surface. Their chemical stability and tribology were systematically investigated. The chemical stability of the modified CrN surfaces was tested in aqueous media at 60 °C at pH 3, 7, and 11 and monitored by static water contact angle measurements, X-ray photoelectron spectroscopy (XPS), ellipsometry, and Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). The tribological properties of the resulting organic monolayers with different end groups (fluorinated or nonfluorinated) were studied using atomic force microscopy (AFM). It was found that the fluorinated monolayers exhibit a dramatic reduction of adhesion and friction force as well as excellent wear resistance compared to those of nonfluorinated coatings and bare CrN substrates. The combination of remarkable chemical stability and superior tribological properties makes these fluorinated monolayers promising candidates for the development of robust high-performance devices.

Quasi in situ scanning force microscope with an automatic operated reaction chamber

We describe the design and performance of a quasi in situ scanning force microscope with an automatic operated reaction chamber. The design provides a repetitive hermetically sealed sample environment for successive processing. The reaction chamber is based on a combination of a flexure-guided cover, a piezo-positioning system and a force applicator system. An axial force seals the cover against the reactor enabling flow-through applications at low pressure, ambient pressure, or elevated pressure. The position stability of the sample relative to the probe is characterized and a full automated operation of the instrument is explored by the alignment of an ABC terblock copolymer thin film undergoing solvent vapor annealing in the presence of a high electric field. Due to the high electric field strength and the sharp scanning force microscope tip it is impossible to perform in situ scanning in the presence of the electric field.

Sampling protein form and function with the atomic force microscope

To study the structure, function, and interactions of proteins, a plethora of techniques is available. Many techniques sample such parameters in non-physiological environments (e.g. in air, ice, or vacuum). Atomic force microscopy (AFM), however, is a powerful biophysical technique that can probe these parameters under physiological buffer conditions. With the atomic force microscope operating under such conditions, it is possible to obtain images of biological structures without requiring labeling and to follow dynamic processes in real time. Furthermore, by operating in force spectroscopy mode, it can probe intramolecular interactions and binding strengths. In structural biology, it has proven its ability to image proteins and protein conformational changes at submolecular resolution, and in proteomics, it is developing as a tool to map surface proteomes and to study protein function by force spectroscopy methods. The power of AFM to combine studies of protein form and protein function enables bridging various research fields to come to a comprehensive, molecular level picture of biological processes. We review the use of AFM imaging and force spectroscopy techniques and discuss the major advances of these experiments in further understanding form and function of proteins at the nanoscale in physiologically relevant environments.

Normal and shear forces generated during the ordering (directed assembly) of confined straight and curved nanowires

The effects of shape on nanowire interactions and shear-induced ordering were studied. Both the normal and lateral forces were sensitive to the particles' curvature. (i) No adhesion was observed between the confining surfaces, and the force profiles were short-ranged and mostly reversible for straight wires but longer-ranged and irreversible for curved wires. (ii) Structural phase transitions can occur in confined nanoparticle films. (iii) During shearing, straight wires order or align more easily than curved wires.

Future aspects of bioprocess monitoring

Nature has the impressive ability to efficiently and precisely control biological processes by applying highly evolved principles and using minimal space and relatively simple building blocks. The challenge is to transfer these principles into technically applicable and precise analytical systems that can be used for many applications. This article summarizes some of the new approaches in sensor technology and control strategies for different bioprocesses such as fermentations, biotransformations, and downstream processes. It focuses on bio- and chemosensors, optical sensors, DNA and protein chip technology, software sensors, and modern aspects of data evaluation for improved process monitoring and control.

Design of a scanning probe microscope with advanced sample treatment capabilities: An atomic force microscope combined with a miniaturized inductively coupled plasma source

We describe the design and performance of an atomic force microscope (AFM) combined with a miniaturized inductively coupled plasma source working at a radio frequency of 27.12 MHz. State-of-the-art scanning probe microscopes (SPMs) have limited in situ sample treatment capabilities. Aggressive treatments such as plasma etching or harsh treatments such as etching in aggressive liquids typically require the removal of the sample from the microscope. Consequently, time consuming procedures are required if the same sample spot has to be imaged after successive processing steps. We have developed a first prototype of a SPM which features a quasi in situ sample treatment using a modified commercial atomic force microscope. A sample holder is positioned in a special reactor chamber; the AFM tip can be retracted by several millimeters so that the chamber can be closed for a treatment procedure. Most importantly, after the treatment, the tip is moved back to the sample with a lateral drift per process step in the 20 nm regime. The performance of the prototype is characterized by consecutive plasma etching of a nanostructured polymer film.

Self-assembled monolayers of dipolar nonlinear optical nickel(II) molecules on the Si(100) surface with nanoscale uniformity

The synthesis and structure of a dipolar nonlinear optical bis(salicylaldiminato)Ni(II)-derivatized Schiff base complex chemisorbed on H-terminated Si(100) surfaces is reported. The existence of a monolayer of the derivatized complex chemisorbed on the Si(100) surface is unambiguously confirmed by high-resolution core-level XPS and AFM/SNOM analyses. The comparison between the optical SNOM images highlights the contribution of the monolayer to the local reflectivity of the sample. Angle-resolved XPS data indicate the presence of chlorine head atoms on the monolayer surface. Altogether, XPS and AFM/SNOM data suggest the formation of a nanoscale uniform, homogeneous, complete, ordered monolayer self-assembled on the Si(100) surface.

Chain-Length Effects on Molecular Conformation in and Chirality of Self-Assembled Monolayers of Alkoxylated Benzo[c]cinnoline Derivatives on Highly Oriented Pyrolytic Graphite

Self-assembled structures of alkoxylated benzo[c]cinnoline derivatives prepared on highly oriented pyrolytic graphite at room temperature from their solutions in solvents such as 1-phenyloctane, toluene, and 1-octanol were studied by scanning tunneling microscopy. The alkoxy chain length markedly affected the molecular conformations in 2-dimensional assemblies of these derivatives. Long-chain derivatives adopted the trans conformations more often than cis, whereas short-chain derivatives took exclusively the cis conformations in the self-assembled monolayers (SAMs). For the derivatives of intermediate chain lengths, polymorphism existed, with four molecular conformations identified experimentally. Experimental evidence substantiated the formation of chiral SAM structures at the surface, which can be explained by the conformations of the molecules. The chirality was also affected by the chain length of the molecules. A simple method analyzing the angles between different domains in the SAMs was used to identify the molecular conformations and to predict their relative structures.

Patterning of cantilevers with inverted dip-pen nanolithography: efforts toward combinatorial AFM

We report patterning of AFM cantilevers by inverted dip-pen nanolithography, thereby markedly enhancing the development of combinatorial AFM as a high-throughput force-measuring instrument capable of determining interactions between opposing libraries of biomolecules.

Nanoscale wear and machining behavior of nanolayer interfaces

An atomic force microscope was used to subnanometer incise a nanomultilayer to consequently expose individual nanolayers and interfaces on which sliding and scanning nanowear/machining have been performed. The letter reports the first observation on the nanoscale where (i) atomic debris forms in a collective manner, most-likely by deformation and rupture of atomic bonds, and (ii) the nanolayer interfaces possess a much higher wear resistance (desired for nanomachines) or lower machinability (not desired for nanomachining) than the layers.

Ultra-Sensitive Imaging and Interfacial Analysis of Patterned Hydrophilic SAM Surfaces Using Energy Dissipation Chemical Force Microscopy

Energy dissipation chemical force microscopy has been used to analyze the dissipative properties of chemically similar regions of hydroxyl- and carboxyl-terminated SAMs on gold with a hydroxyl-terminated tip. Energy dissipation imaging quantitatively isolates dissipative interfacial interactions from topography, producing a significantly more informative image than phase imaging. Also, energy dissipation force curves probed the rheological properties of the tip-sample interaction. Viscosity of the confined water increased slightly over that of the bulk, and SAM deformation was found to have a longer retardation time than restructuring of interfacial ions and solvent during tip-sample contact.