An antibody-sensitized microfabricated cantilever for the growth detection of Aspergillus niger spores

We demonstrate a new sensitive biosensor for detection of vital fungal spores of Aspergillus niger. The biosensor is based on silicon microfabricated cantilever arrays operated in dynamic mode. The change in resonance frequency of the sensor is a function of mass binding to the cantilever surface. For specific A. niger spore immobilization on the cantilever, each cantilever was individually coated with anti-Aspergillus niger polyclonal antibodies. We demonstrate the detection of single A. niger spores and their subsequent growth on the functionalized cantilever surface by online measurements of resonance frequency shifts. The new biosensor operating in humid air allows quantitative and qualitative detection of A. niger spores as well as detection of vital, functional spores in situ within approximately 4 h. The detection limit of the sensor is 103 CFU mL-1. Mass sensitivity of the cantilever sensor is approximately 53 pg Hz-1.

Chladni figures revisited based on nanomechanics

Chladni patterns based on nanomechanics in the microfluidic environment are presented. In contrast with the macroscopic observations in the gaseous environment, nanoparticles are found to move to the nodes, whereas micron-sized particles move to the antinodes of the vibrating interface. This opens the door to size-based sorting of particles in microfluidic systems, and to highly parallel and controlled assembly of biosensors and nanoelectronic circuits.

Investigating the Molecular Mechanisms of In-Plane Mechanochemistry on Cantilever Arrays

Free-standing cantilevers, which directly translate specific biochemical reactions into micromechanical motion, have recently attracted much attention as label-free biosensors and micro/nano robotic devices. To exploit this mechanochemical sensing technology, it is essential to develop a fundamental understanding of the origins of surface stress. Here we report a detailed study into the molecular basis of stress generation in aqueous environments focusing on the pH titration of model mercaptohexadecanoic acid self-assembled monolayers (SAMs), using in situ reference cantilevers coated with nonionizable hexadecanethiol SAMs. Semiautomated data analysis and a statistical model were developed to quantify cyclic deprotonation/protonation reactions on multiple arrays. In-plane force titrations were found to have the sensitivity to detect ionic hydrogen bond formation between protonated and nonprotonated carboxylic acid groups in the proximity of the surface pK1/2, which generated a mean tensile differential surface stress of +1.2 +/- 0.3 mN/m at pH 6.0, corresponding to 1 pN attractive force between two adjacent MHA molecules. Conversely, the magnitude of compressive differential surface stress was found to increase progressively with pH >/= 7.0, reaching a maximum of -14.5 +/- 0.5 mN/m at pH 9.0, attributed to enhanced electrostatic repulsion between deprotonated carboxylic acid groups. However, striking differences were observed in the micromechanical responses to different ionic strength and ion species present in the aqueous environment, highlighting the critical role of counter- and co-ions on surface stress. Our findings provide fundamental insights into the molecular mechanisms of in-plane mechanochemistry, which may be exploited for biosensing and nanoactuation applications.

Rapid and label-free nanomechanical detection of biomarker transcripts in human RNA

The availability of entire genome sequences has triggered the development of microarrays for clinical diagnostics that measure the expression levels of specific genes. Methods that involve labelling can achieve picomolar detection sensitivity, but they are costly, labour-intensive and time-consuming. Moreover, target amplification or biochemical labelling can influence the original signal. We have improved the biosensitivity of label-free cantilever-array sensors by orders of magnitude to detect mRNA biomarker candidates in total cellular RNA. Differential gene expression of the gene 1-8U, a potential marker for cancer progression or viral infections, has been observed in a complex background. The measurements provide results within minutes at the picomolar level without target amplification, and are sensitive to base mismatches. This qualifies the technology as a rapid method to validate biomarkers that reveal disease risk, disease progression or therapy response. We foresee cantilever arrays being used as a tool to evaluate treatment response efficacy for personalized medical diagnostics.

Label free analysis of transcription factors using microcantilever arrays

We report the measurement of protein interaction with double-stranded DNA oligonucleotides using cantilever microarray technology. We investigated two different DNA-binding proteins, the transcription factors SP1 and NF-kappaB, using cantilever arrays as they allow label-free measurement of different biomolecular interactions in parallel. Double-stranded DNA oligonucleotides containing a specific binding site for a transcription factor were sensitized on gold-coated cantilevers. The binding of the transcription factor creates a surface stress, resulting in a bending of the cantilevers. Both transcription factors could be detected independently at concentrations of 80-100 nM. A concentration dependence of the bending signal was measured using concentrations from 100 to 400 nM of NF-kappaB. The experiments show that the recognition sequence of one transcription factor can serve as a reference for the other, highlighting the sequence specificity of transcription factor binding.

Conformational change of bacteriorhodopsin quantitatively monitored by microcantilever sensors

Bacteriorhodopsin proteoliposomes were used as a model system to explore the applicability of micromechanical cantilever arrays to detect conformational changes in membrane protein patches. The three main results of our study concern: 1), reliable functionalization of micromechanical cantilever arrays with proteoliposomes using ink jet spotting; 2), successful detection of the prosthetic retinal removal (bleaching) from the bacteriorhodopsin protein by measuring the induced nanomechanical surface stress change; and 3), the quantitative response thereof, which depends linearly on the amount of removed retinal. Our results show this technique to be a potential tool to measure membrane protein-based receptor-ligand interactions and conformational changes.

Micromechanical cantilever array sensors for selective fungal immobilization and fast growth detection

We demonstrate the use of micromechanical cantilever arrays for selective immobilization and fast quantitative detection of vital fungal spores. Micro-fabricated uncoated as well as gold-coated silicon cantilevers were functionalized with concanavalin A, fibronectin or immunoglobulin G. In our experiments two major morphological fungal forms were used--the mycelial form Aspergillus niger and the unicellular yeast form Saccharomyces cerevisiae, as models to explore a new method for growth detection of eukaryotic organisms using cantilever arrays. We exploited the specific biomolecular interactions of surface grafted proteins with the molecular structures on the fungal cell surface. It was found that these proteins have different affinities and efficiencies to bind the spores. Maximum spore immobilization, germination and mycelium growth was observed on the immunoglobulin G functionalized cantilever surfaces. We show that spore immobilization and germination of the mycelial fungus A. niger and yeast S. cerevisiae led to shifts in resonance frequency within a few hours as measured by dynamically operated cantilever arrays, whereas conventional techniques would require several days. The biosensor could detect the target fungi in a range of 10(3) - 10(6) CFUml(-1). The measured shift is proportional to the mass of single fungal spores and can be used to evaluate spore contamination levels. Applications lie in the field of medical and agricultural diagnostics, food- and water-quality monitoring.

A label-free immunosensor array using single-chain antibody fragments

We report a microcantilever-based immunosensor operated in static deflection mode with a performance comparable with surface plasmon resonance, using single-chain Fv (scFv) antibody fragments as receptor molecules. As a model system scFv fragments with specificity to two different antigens were applied. We introduced a cysteine residue at the C terminus of each scFv construct to allow covalent attachment to gold-coated sensor interfaces in directed orientation. Application of an array enabled simultaneous deflection measurements of sensing and reference cantilevers. The differential deflection signal revealed specific antigen binding and was proportional to the antigen concentration in solution. Using small, oriented scFv fragments as receptor molecules we increased the sensitivity of microcantilevers to approximately 1 nM.

Micromechanical mass sensors for biomolecular detection in a physiological environment

Micromechanical cantilever arrays are used to measure time-resolved adsorption of tiny masses based on protein-ligand interactions. Here, streptavidin-biotin interactions are investigated in a physiological environment. A measurement method is introduced using higher flexural modes of a silicon cantilever in order to enhance the sensitivity of mass detection. Modeling the cantilever vibration in liquid allows the measurement of absolute mass changes. We show time-resolved mass adsorption of final 7+/-0.7 ng biotinylated latex beads. The sensitivity obtained is about 2.5 pg/Hz measuring at a center frequency of 750 kHz.

Micromechanical oscillators as rapid biosensor for the detection of active growth of Escherichia coli

A rapid biosensor for the detection of bacterial growth was developed using micromechanical oscillators coated by common nutritive layers. The change in resonance frequency as a function of the increasing mass on a cantilever array forms the basis of the detection scheme. The sensor is able to detect active growth of Escherichia coli cells within 1 h which is significantly faster than any conventional plating method which requires at least 24 h. The growth of E. coli was confirmed by scanning electron microscopy. This new sensing method for the detection of active bacterial growth allows future applications in, e.g., rapid antibiotic susceptibility testing by adding antibiotics to the nutritive layer.

Enantioselective recognition of phenylalanine by a chiral amphiphilic macrocycle at the air-water interface: a copper-mediated mechanism

The synthesis of a new chiral amphiphilic calix[4]resorcinarene, tetrakis(N-methylprolyl)tetraundecylcalix[4]resorcinarene (L-RA-Pro), bearing four L-prolyl moieties at the macrocyclic upper rim and four undecyl chains at the lower rim is described. This synthesis has been carried out via a Mannich-type reaction of L-proline and formaldehyde. It has been shown by means of Langmuir balance technique that L-RA-Pro self-assemble as well-defined monomolecular layers at the air-water interface. The effect of various cations on the stability of these monolayers has been studied. The experiments reveal that while there is a slight stabilization effect of K+, Cd2+, Co2+, Mg2+, and Ni2+, there is a high decrease in the collapse pressure in the presence of Cu(II) cation, showing that monolayers of L-RA-Pro, formed at the air-water interface, have a certain selectivity for copper(II) ions with regard to other cations tested. This supramolecular complex exhibits enantioselective recognition properties vs phenylalanine; the mechanism of this interaction is discussed.

Rapid biosensor for detection of antibiotic-selective growth of Escherichia coli

A rapid biosensor for the detection of bacterial growth was developed using micromechanical oscillators coated in common nutritive layers. The change in resonance frequency as a function of the increasing mass on a cantilever array forms the basis of the detection scheme. The calculated mass sensitivity according to the mechanical properties of the cantilever sensor is approximately 50 pg/Hz; this mass corresponds to an approximate sensitivity of approximately 100 Escherichia coli cells. The sensor is able to detect active growth of E. coli cells within 1 h. The starting number of E. coli cells initially attached to the sensor cantilever was, on average, approximately 1,000 cells. Furthermore, this method allows the detection of selective growth of E. coli within only 2 h by adding antibiotics to the nutritive layers. The growth of E. coli was confirmed by scanning electron microscopy. This new sensing method for the detection of selective bacterial growth allows future applications in, e.g., rapid antibiotic susceptibility testing.

Myomesin is a molecular spring with adaptable elasticity

The M-band is a transverse structure in the center of the sarcomere, which is thought to stabilize the thick filament lattice. It was shown recently that the constitutive vertebrate M-band component myomesin can form antiparallel dimers, which might cross-link the neighboring thick filaments. Myomesin consists mainly of immunoglobulin-like (Ig) and fibronectin type III (Fn) domains, while several muscle types express the EH-myomesin splice isoform, generated by the inclusion of the unique EH-segment of about 100 amino acid residues (aa) in the center of the molecule. Here we use atomic force microscopy (AFM), transmission electron microscopy (TEM) and circular dichroism (CD) spectroscopy for the biophysical characterization of myomesin. The AFM identifies the "mechanical fingerprints" of the modules constituting the myomesin molecule. Stretching of homomeric polyproteins, constructed of Ig and Fn domains of human myomesin, produces a typical saw-tooth pattern in the force-extension curve. The domains readily refold after relaxation. In contrast, stretching of a heterogeneous polyprotein, containing several repeats of the My6-EH fragment reveals a long initial plateau corresponding to the sum of EH-segment contour lengths, followed by several My6 unfolding peaks. According to this, the EH-segment is characterized as an entropic chain with a persistence length of about 0.3nm. In TEM pictures, the EH-domain appears as a gap in the molecule, indicating a random coil conformation similar to the PEVK region of titin. CD spectroscopy measurements support this result, demonstrating a mostly non-folded conformation for the EH-segment. We suggest that similarly to titin, myomesin is a molecular spring, whose elasticity is modulated by alternative splicing. The Ig and Fn domains might function as reversible "shock absorbers" by sequential unfolding in the case of extremely high or long sustained stretching forces. These complex visco-elastic properties of myomesin might be crucial for the stability of the sarcomere.

Study of the mechanical properties of myomesin proteins using dynamic force spectroscopy

Myomesin is the most prominent structural component of the sarcomeric M-Band that is expressed in mammalian heart and skeletal muscles. Like titin, this protein is an intracellular member of the Ig-fibronectin superfamily, which has a flexible filamentous structure and which is largely composed of two types of domain that are similar to immunoglobulin (Ig)-like and fibronectin type III (FNIII) domains. Several myomesin isoforms have been identified, and their expression patterns are highly regulated both spatially and temporally. Particularly, alternative splicing in the central part of the molecule gives rise to an isoform, EH (embryonic heart)-myomesin, containing a serine and proline-rich insertion with no well-defined secondary structure, the EH segment. EH-myomesin represents the major myomesin isoform at embryonic stages of mammalian heart and is rapidly down-regulated around birth, but it is re-expressed in the heart of patients suffering from dilated cardio-myopathy. Here, in order to facilitate a better understanding of the physiological, and possibly pathological, functions of myomesin proteins, we explore the mechanical stability, elasticity and force-driven structural changes of human myomesin's sub-molecular segments using single-molecule force spectroscopy and protein engineering. We find that human myomesin molecules are composed of modules (Ig and FNIII), that are designed to withstand force and we demonstrate that the human cardiac EH segment functions like an additional elastic stretch in the middle part of the EH-myomesin and behaves like a random coil. Consequently myomesin isoforms (proteins with or without the EH segment) have different elastic properties, the EH-myomesin being the more compliant one. These findings imply that the compliance of the M-band increases with the amount of EH-myomesin it contains. So, we provide the evidence that not only titin but also other sarcomeric proteins have complicated visco-elastic properties depending on the contractile parameters in different muscle types.

Inkjet deposition of alkanethiolate monolayers and DNA oligonucleotides on gold: evaluation of spot uniformity by wet etching

Inkjet printing allows localized, contact-free deposition of liquids onto arbitrary substrates. In this article we demonstrate the fast formation of high-quality self-assembled monolayers (SAMs) on gold surfaces. Using a selective etch process, we verify the uniformity of the deposited spots. A direct comparison with microcontact-printed SAMs on Au revealed similar resist quality as inkjet-deposited alkanethiolate SAMs. Likewise, inkjet printing of thiol-functionalized and non-thiolated single-stranded DNA oligomers formed molecular layers protecting Au from etchants. For all compounds used, we achieved etched patterns that were homogeneous and free of defects. These results indicate that an inkjet is a convenient tool for surface functionalization and the direct writing of molecular films and resists.

Topological structure and chemical composition of inner surfaces of borosilicate vials

The use of atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) is described to characterize the inner surfaces of pharmaceutical vials. The two type I borosilicate glasses included in this study slightly differ in their amounts of alkaline oxides. The topography and chemistry of the inner surfaces of vials are predominantly caused by the forming process. A structural and chemical modification of the inner surface of vials was also observed when exposing the surface to different pH conditions and special treatment like washing and sterilization, which are routine operation steps during galenical manufacturing.

Mechanics and imaging of single DNA molecules

We review recent experiments that have revealed mechanical properties of single DNA molecules using advanced manipulation and force sensing techniques(scanning force microscopy (SFM), optical or magnetic tweezers, microneedles). From such measurements, intrinsic relevant parameters (persistence length, stretch modulus) as well as their dependence on external parameters (non-physiological conditions, coating with binding agents or proteins) are obtained on a single-molecule level. In addition, imaging of DNA molecules using SFM is presented.