Boron-doped silicon nanowires (SiNWs) were used to create highly sensitive, real-time electrically based sensors for biological and chemical species. Amine- and oxide-functionalized SiNWs exhibit pH-dependent conductance that was linear over a large dynamic range and could be understood in terms of the change in surface charge during protonation and deprotonation. Biotin-modified SiNWs were used to detect streptavidin down to at least a picomolar concentration range. In addition, antigen-functionalized SiNWs show reversible antibody binding and concentration-dependent detection in real time. Lastly, detection of the reversible binding of the metabolic indicator Ca2+ was demonstrated. The small size and capability of these semiconductor nanowires for sensitive, label-free, real-time detection of a wide range of chemical and biological species could be exploited in array-based screening and in vivo diagnostics.

/pdf/nanowire-nanosensors-for-highly-sensitive-and-selective-4f3aqhtkwe.pdf

Nanowire Nanosensors for Highly Sensitive and Selective Detection of Biological and Chemical Species

http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

Friction, wear and lubrication between materials in contact are of fundamental importance in many pure and applied sciences. Owing to the development of experimental and computer-simulation techniques for studying these phenomena at the atomic scale, an understanding is beginning to emerge of the molecular mechanisms of tribology in thin films and at surfaces.

Nanotribology: friction, wear and lubrication at the atomic scale

Carbon nanotubes combine a range of properties that make them well suited for use as probe tips in applications such as atomic force microscopy (AFM)1,2,3. Their high aspect ratio, for example, opens up the possibility of probing the deep crevices4 that occur in microelectronic circuits, and the small effective radius of nanotube tips significantly improves the lateral resolution beyond what can be achieved using commercial silicon tips5. Another characteristic feature of nanotubes is their ability to buckle elastically4,6, which makes them very robust while limiting the maximum force that is applied to delicate organic and biological samples. Earlier investigations into the performance of nanotubes as scanning probe microscopy tips have focused on topographical imaging, but a potentially more significant issue is the question of whether nanotubes can be modified to create probes that can sense and manipulate matter at the molecular level7. Here we demonstrate that nanotube tips with the capability of chemical and biological discrimination can be created with acidic functionality and by coupling basic or hydrophobic functionalities or biomolecular probes to the carboxyl groups that are present at the open tip ends. We have used these modified nanotubes as AFM tips to titrate the acid and base groups, to image patterned samples based on molecular interactions, and to measure the binding force between single protein–ligand pairs. As carboxyl groups are readily derivatized by a variety of reactions8, the preparation of a wide range of functionalized nanotube tips should be possible, thus creating molecular probes with potential applications in many areas of chemistry and biology.

/pdf/covalently-functionalized-nanotubes-as-nanometre-sized-4dlq75blea.pdf

Covalently functionalized nanotubes as nanometre- sized probes in chemistry and biology

Biomedical surface science: Foundations to frontiers

Mapping the spatial arrangement of chemical functional groups and their interactions is of significant importance to problems ranging from lubrication and adhesion to recognition in biological systems. A force microscope has been used to measure the adhesive and friction forces between molecularly modified probe tips and organic monolayers terminating in a lithographically defined pattern of distinct functional groups. The adhesive interactions between simple CH(3)/CH(3), CH(3)/COOH, and COOH/COOH functional groups correlate directly with friction images of sample surfaces patterned with these groups. Thus, by monitoring the friction between a specifically functionalized tip and sample, one can produce friction images that display predictable contrast and correspond to the spatial distribution of functional groups on the sample surface. Applications of this chemically sensitive imaging technique are discussed.

https://science.sciencemag.org/content/sci/265/5181/2071.full.pdf

Functional Group Imaging by Chemical Force Microscopy

Chemical force microscopy (CFM) has been used to measure adhesion and friction forces between probe tips and substrates covalently modified with self-assembled monolayers (SAMs) that terminate in distinct functional groups. Robe tips have been modified with SAMs using a procedure that involves coating commercial Si3N4 cantilevedtip assemblies with a thin layer of polycrystalline Au followed by immersion in a solution of a functionalized thiol. This methodology provides a reproducible means for endowing the probe with different chemical functional groups. The spring constants and radii of the chemically modified cantilevedtip assemblies have been characterized to allow for quantitative friction and adhesion measurements. Au-coated Si and Si substrates have been treated with functionalized thiols and silanes, respectively, to produce SAM coated substrates terminating with different functional groups. A force microscope has been used to characterize the adhesive interactions between probe tips and substrates that have been modified with SAMs which terminate with COOH, CH3, and N H 2 functional groups in EtOH and HzO solvents. Force vs distance curves recorded under EtOH show that the interaction between functional groups decreases as follows: COOWCOOH > CHdCH3 > COOWCH3. The measured adhesive forces were found to agree well with predictions of the Johnson, Kendall, and Roberts (JKR) theory of adhesive contact and thus show that the observed adhesion forces correlate with the surface free energy of the molecular groups in EtOH. Electrostatic contributions to adhesive forces have also been studied using a COO-/NH3+ tip/surface in aqueous solution. Force vs distance curves recorded as a function of ionic strength show that the observed adhesive interaction decreases with increasing ionic strength. These results have been interpreted in terms of contact and noncontact contributions to the experimentally measured adhesive force. The friction forces between tips and samples modified with COOH and CH3 groups have also been measured as a function of applied load. The magnitude of the friction force was found to decrease in the following manner with different tip/sample functionalities: COOWCOOH > CH3/CH3 > COOWCH3. Friction forces between different chemical functional groups thus correlate directly with the adhesion forces between these same groups. Specifically, high friction is observed between groups that adhere strongly, while low friction is observed between weakly interacting functional groups. The dependence of friction forces on the tip and sample functionality is shown to be the basis for chemical force microscopy in which lateral force images are interpreted in terms of the strength of both adhesive groups.

Chemical force microscopy: Exploiting chemically-modified tips to quantify adhesion, friction, and functional group distributions in molecular assemblies

Chemical force microscopy (CFM) was used to probe interactions between ionizable and neutral functional groups in aqueous solutions. Force microscope probe tips and sample substrates have been cova...

http://cml.harvard.edu/assets/JACS_119_2006.pdf

Force Titrations and Ionization State Sensitive Imaging of Functional Groups in Aqueous Solutions by Chemical Force Microscopy

Selective Electrochemical Deposition of Polyaniline via Photopatterning of a Monolayer-Modified Substrate

A photosensitive self-assembled monolayer (SAM) is selectively irradiated to fabricate a pattern on an Au electrode, and a thin film of aniline or 3-methylthiophene is deposited on it by electropol...

Lawrence F. Rozsnyai

Papers

Functional Group Imaging by Chemical Force Microscopy

Chemical force microscopy: Exploiting chemically-modified tips to quantify adhesion, friction, and functional group distributions in molecular assemblies

Force Titrations and Ionization State Sensitive Imaging of Functional Groups in Aqueous Solutions by Chemical Force Microscopy

Selective Electrochemical Deposition of Polyaniline via Photopatterning of a Monolayer-Modified Substrate

Controlling the Adhesion of Conducting Polymer Films with Patterned Self-Assembled Monolayers