▪ Abstract Soft lithography, a set of techniques for microfabrication, is based on printing and molding using elastomeric stamps with the patterns of interest in bas-relief. As a technique for fabricating microstructures for biological applications, soft lithography overcomes many of the shortcomings of photolithography. In particular, soft lithography offers the ability to control the molecular structure of surfaces and to pattern the complex molecules relevant to biology, to fabricate channel structures appropriate for microfluidics, and to pattern and manipulate cells. For the relatively large feature sizes used in biology (≥50 μm), production of prototype patterns and structures is convenient, inexpensive, and rapid. Self-assembled monolayers of alkanethiolates on gold are particularly easy to pattern by soft lithography, and they provide exquisite control over surface biochemistry.

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Soft Lithography in Biology and Biochemistry

Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

12.2.2. Four-Wave Mixing (FWM) 4849 12.2.3. Dye Aggregation 4850 12.2.4. Optoelectronic Nanodevices 4850 12.3. Sensor 4851 12.3.1. Chemical Sensor 4851 12.3.2. Biological Sensor 4851 12.4. Catalysis 4852 13. Conclusion and Perspectives 4852 14. Abbreviations 4853 15. Acknowledgements 4854 16. References 4854 * Corresponding author E-mail: tpal@chem.iitkgp.ernet.in. † Raidighi College. § Indian Institute of Technology. 4797 Chem. Rev. 2007, 107, 4797−4862

Interparticle Coupling Effect on the Surface Plasmon Resonance of Gold Nanoparticles: From Theory to Applications

Surface plasmon resonance (SPR) biosensors are optical sensors exploiting special electromagnetic waves—surface plasmon-polaritons—to probe interactions between an analyte in solution and a biomolecular recognition element immobilized on the SPR sensor surface. Major application areas include detection of biological analytes and analysis of biomolecular interactions where SPR biosensors provide benefits of label-free real-time analytical technology. This paper reviews fundamentals of SPR affinity biosensors and discusses recent advances in development and applications of SPR biosensors.

Present and future of surface plasmon resonance biosensors.

A simple method was used to assemble single-walled carbon nanotubes into indefinitely long ribbons and fibers. The processing consists of dispersing the nanotubes in surfactant solutions, recondensing the nanotubes in the flow of a polymer solution to form a nanotube mesh, and then collating this mesh to a nanotube fiber. Flow-induced alignment may lead to a preferential orientation of the nanotubes in the mesh that has the form of a ribbon. Unlike classical carbon fibers, the nanotube fibers can be strongly bent without breaking. Their obtained elastic modulus is 10 times higher than the modulus of high-quality bucky paper.

http://science.sciencemag.org/content/sci/290/5495/1331.full.pdf

Macroscopic Fibers and Ribbons of Oriented Carbon Nanotubes

Recent observations of charged colloidal particles trapped at the air-water interface revealed long-range interparticle attractive forces, not accounted for by the standard theories of colloidal interactions. We propose a mechanism for attraction which is based on nonuniform wetting causing an irregular shape of the particle meniscus. The excess water surface area created by these distortions can be minimized when two adjacent particles assume an optimum relative orientation and distance. Typically, for spheres with diameter of 1 mm at an interparticle distance of 2 mm, deviations from the ideal contact line by as little as 50 nm result in an interaction energy of the order of 10(4)kT. Roughness-induced capillarity explains the experimental findings, including the cluster dissolution caused by addition of detergent to the subphase and the formation of linear aggregates. This kind of interaction should also be of importance in particle-stabilized foams and emulsions.

Long-range attraction between colloidal spheres at the air-water interface: the consequence of an irregular meniscus

Controlled Cell Adhesion on PEG‐Based Switchable Surfaces

Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) microgel films are shown to allow controlled detachment of adsorbed cells via temperature stimuli. Cell response occurs on the timescale of several minutes, is reversible, and allows for harvesting of cells in a mild fashion. The fact that microgels are attached non-covalently allows using them on a broad variety of (charged) surfaces and is a major advantage as compared to approaches relying on covalent attachment of active films. In the following, the microgels' physico-chemical parameters in the adsorbed state and their changes upon temperature variation are studied in order to gain a deeper understanding of the involved phenomena. By means of atomic force microscopy (AFM), the water content, mechanical properties, and adhesion forces of the microgel films are studied as a function of temperature. The analysis shows that these properties change drastically when crossing the critical temperature of the polymer film, which is the basis of the fast cell response upon temperature changes. Furthermore, nanoscale mechanical analysis shows that the films posses a nanoscopic gradient in mechanical properties.

Adhesion and Mechanical Properties of PNIPAM Microgel Films and Their Potential Use as Switchable Cell Culture Substrates

A new class of lipid mols. is synthesized, based on two dipalmitoylphosphatidic mols., each extended at the lipid phosphate by a hydrophilic spacer chain of ethoxy groups of variable length, which are then coupled as a bilipid via a terminal disulfide group at the hydrophilic spacer. These anchor-bearing qthiolipidsq can attach to gold substrates by forming stable gold-sulfur bonds. In this way, the authors can couple lipid bilayers to gold surfaces, with the possibility of preserving a water layer between the support and the first monolayer. The thiolipid mols. are characterized on a Langmuir film balance using fluorescence microscopy. The mol. areas of the thiolipids on the water surface are 80-90 .ANG.2 at a fully compressed state. The thiolipid monolayers show a typical first-order phase transition on the water surface with regular, starlike domains. The formation of thiolipid-attached mono- and bilayers on gold surfaces was studied by surface plasmon resonance (SPR), impedance measurements, and cyclic voltammetry. Four different supported membrane systems are studied in detail: (1) pure thiolipid layers; (2) mixed lipid bilayers contg. a first pure thiolipid monolayer and a second one of conventional phospholipids; (3) bilayers, where the first gold-attached monolayer is composed of a mixt. of thio- and conventional phospholipids with another second phospholipid layer on top; (4) monolayers of pure 1-hexadecanethiol and layers with a second phospholipid film on top of the 1-hexadecanethiol. The electrochem. expts. reveal elec. blocking layers for all lipid systems investigated with specific resistances of 104-105 W cm2. The capacitance values for pure thiolipid bilayers are in the range of 0.5-0.7 mF/cm2 for the pure thiolipid bilayers and 0.7-0.8 mF/cm2 for the mixed thiolipid/phospholipid bilayers, which is comparable to the values found for unsupported, so-called black lipid membranes. SPR measurements confirm qual. the results of the electrochem. expts. [on SciFinder (R)]

A new class of thiolipids for the attachment of lipid bilayers on gold surfaces

We report on a non-destructive method for the separation and size-selection of carbon nanotubes and nanoparticles produced by arc discharge. A liquid-phase separation of nanotubes and nanoparticles was performed by filtering a kinetically stable colloidal dispersion consisting of the carbon material in a water/surfactant solution, allowing thus to extract the nanotubes from the suspension while leaving the nanoparticles in the filtrate. Further purification was accomplished by size-selection through controlled floculation of the dispersion. Final separation yields as high as 90 % in weight were obtained without any damage to tube tips or tube walls following the separation process.

Claus Duschl

Papers

Long-range attraction between colloidal spheres at the air-water interface: the consequence of an irregular meniscus

Controlled Cell Adhesion on PEG‐Based Switchable Surfaces

Adhesion and Mechanical Properties of PNIPAM Microgel Films and Their Potential Use as Switchable Cell Culture Substrates

A new class of thiolipids for the attachment of lipid bilayers on gold surfaces

Purification and size‐selection of carbon nanotubes