We have directly imaged n-alkane layers adsorbed at the liquid/graphite interface using a scanning tunneling microscope. The layers possessed a high degree of two-dimensional ordering. The adsorbate was observed to enhance the tunneling current, and the atomic structure of the images was dominated by features associated with the substrate. These systems are excellent vehicles for studies concerning the imaging mechanism of adsorbed organic layers because of their stability and simplicity.

Short-period fiber Bragg gratings with weakly tilted grating planes generate multiple strong resonances in transmission. Our experimental results show that the wavelength separation between selected resonances allows the measurement of the refractive index of the medium surrounding the fiber for values between 1.25 and 1.44 with an accuracy approaching 1x10(-4). The sensor element is 10 mm long and made from standard single-mode telecommunication grade optical fiber by ultraviolet light irradiation through a phase mask.

Nanostructures are becoming increasingly important for technology and basic science. 1 Fabrication techniques currently employed for material deposition include low-pressure chemical vapor deposition (LPCVD), 2 laser-assisted chemical vapor deposition (LCVD), 3 plasma-enhanced chemical vapor deposition (PECVD), 4,5 ultraviolet stereo lithography, 6 spinning, 7 dipping, 8 spraying, 9 and electrodeposition. [10][11][12][13][14][15] Electrodeposition methods have many advantages over the other stated techniques and are attractive, as they are simple, inexpensive, reproducible, and damage-free. In addition, many materials can be deposited using electrodeposition, including metals, metal alloys, conducting polymers, and semiconductors with essentially no limitations on the size of the substrate or on the thickness of the deposited material. 16 Scanning probe microscopy (SPM) such as scanning tunneling microscopy (STM), 17 atomic force microscopy (AFM), 18 and scanning electrochemical microscopy (SECM) 19 has been widely used as a tool for surface imaging with atomic resolution. Furthermore, creation of structures using SPM has lately attracted considerable attention. 20-23 Using SPM for surface modification has advantages in that the modification process can be followed in real time and submicrometer resolution can be achieved. 24-25 SPM-based nanofabrication has potential uses in applications such as high-density information storage, high-resolution lithography, and production of nanoscale integrated chemical systems and electronic devices.Several groups have employed SPM to deposit metal and polymer microstructures. [28][29][30][31] Bard et al. 32 used the SECM to deposit different metals (e.g., Cu, Ag, Au, Pd) on polymer-coated substrates, whereas Shahat and Mandler et al. used the same technique to deposit Ni(OH) 2 structures 33 from aqueous solutions by changing the pH locally on the substrate and gold patterns by the controlled dissolution of a gold ultramicroelectrode (UME) tip. 34 Wipf and Zhou 35 used the "microreagent" SECM mode to deposit conducting polyaniline patterns on different substrates. Lagraff and Gewirth 36 employed the tip of an AFM to direct the growth of nanoscopic copper protrusions, whereas Madden and Hunter used a tip-directed scheme to deposit several micrometer-scale nickel structures. 25 In tip-directed localized deposition, 32 a faradaic current flows through the solution between a UME tip and a metal substrate electrode all immersed in an ionically conducting electrolyte when a bias voltage is applied between these two electrodes. If reducible metal ions are present in the electrolyte (e.g., Cu 2ϩ ions) and the substrate electrode potential is negative with respect to the tip electrode, then the passage of the faradaic current results in the deposition of metal on the substrate and an oxidation process at the tip. The magnitude of the faradaic current is kept constant by means of a conventional feedback control that monitors the current and adjusts the interelectrode spacing according...

Immune cell migration is a fundamental process that enables immunosurveillance and immune responses. Understanding the mechanism of immune cell migration is not only of importance to the biology of cells, but also has high relevance to cell trafficking mediated physiological processes and diseases such as embryogenesis, wound healing, autoimmune diseases and cancers. In addition to the well-known chemical concentration gradient based guiding mechanism (i.e. chemotaxis), recent studies have shown that lymphocytes can respond to applied physiologically relevant direct current (DC) electric fields by migrating toward the cathode of the fields (i.e. electrotaxis) in both in vitro and in vivo settings. In the present study, we employed two microfluidic devices allowing controlled application of electric fields inside the microfluidic channel for quantitative studies of lymphocyte electrotaxis in vitro at the single cell level. The first device is fabricated by soft-lithography and the second device is made in glass with integrated on-chip electrodes. Using both devices, we for the first time showed that anti-CD3/CD28 antibodies activated human blood T cells migrate to the cathode of the applied DC electric field. This finding is consistent with previous electrotaxis studies on other lymphocyte subsets suggesting electrotaxis is a novel guiding mechanism for immune cell migration. Furthermore, the characteristics of electrotaxis and chemotaxis of activated T cells in PDMS microfluidic devices are compared.

In biomedical applications ranging from the study of pathogen invasion to drug efficacy assays, there is a growing need to develop minimally invasive techniques for single-cell analysis. This has inspired researchers to develop optical, electrical, microelectromechanical and microfluidic devices for exploring phenomena at the single-cell level. In this work, we demonstrate an electrical approach for single-cell analysis wherein a 1.6 GHz microwave interferometer detects the capacitance changes (DeltaC) produced by single cells flowing past a coplanar interdigitated electrode pair. The experimental and simulated capacitance changes generated by yeast cells are in close agreement. By using the capacitance changes of uniform polystyrene spheres (diameter = 5.7 microm) for calibration purposes, we demonstrate a 0.65 aF sensitivity in a 10 ms response time. Using an RC circuit, a low frequency sinusoidal potential is simultaneously superimposed on the electrode pair to generate a dielectrophoretic force that translates cells. Specifically, when yeast cells suspended in a solution of 90 ppm NaCl in deionized water are exposed to 10 kHz and 3 MHz potentials (ranging from 1-3 V(pp)), they experience negative and positive dielectrophoresis, respectively. The corresponding changes in cell elevation above the interdigitated electrodes are detected using the asymmetry of the capacitance signature produced by the cell. Cell elevation changes can be detected in less than 80 ms. The minimum detectable change in elevation is estimated to be 0.22 microm. This approach will have applications in rapid single-cell dielectrophoretic analysis, and may also prove useful in conjunction with impedance spectroscopy.