Rectification of ionic current in a nanofluidic diode.

We demonstrate rectification of ionic transport in a nanofluidic diode fabricated by introducing a surface charge discontinuity in a nanofluidic channel. Device current-voltage (I-V) characteristics agree qualitatively with a one-dimensional model at moderate to high ionic concentrations. This study illustrates ionic flow control using surface charge patterning in nanofluidic channels under high bias voltages.

Diffusion-limited patterning of molecules in nanofluidic channels.

Diffusion-limited patterning (DLP) is a new technique that enables patterning of labile molecular species in solution phase onto surfaces that are not easily accessible. This technique is self-aligning and is simple to implement for patterning multiple species. We demonstrated DLP by patterning alternating bands of fluorescently labeled and unlabeled streptavidin in biotin-functionalized nanofluidic channels with spatial resolution better than 1 microm. The methodology of DLP also enables experimental measurement of a unique parameter that relates molecular surface grafting density, concentration, diffusivity, and channel geometry.

Lithographic techniques and surface chemistries for the fabrication of PEG-passivated protein microarrays.

This article presents a new technique to fabricate patterns of functional molecules surrounded by a coating of the inert poly(ethylene glycol) (PEG) on glass slides for applications in protein microarray technology. The chief advantages of this technique are that it is based entirely on standard lithography processes, makes use of glass slides employing surface chemistries that are standard in the microarray community, and has the potential to massively scale up the density of microarray spots. It is shown that proteins and antibodies can be made to self-assemble on the functional patterns in a microarray format, with the PEG coating acting as an effective passivating agent to prevent non-specific protein adsorption. Various standard surface chemistries such as aldehyde, epoxy and amine are explored for the functional layer, and it is conclusively demonstrated that only an amine-terminated surface satisfies all the process constraints imposed by the lithography process sequence. The effectiveness of this microarray technology is demonstrated by patterning fluorescent streptavidin and a fluorescent secondary antibody using the well-known and highly specific interaction between biotin and streptavidin.

Intracellular transport dynamics of endosomes containing DNA polyplexes along the microtubule network.

We have explored the transport of DNA polyplexes enclosed in endosomes within the cellular environment by multiple particle tracking (MPT). The polyplex-loaded endosomes demonstrate enhanced diffusion at short timescales (t<7 s) with their mean-square displacement (MSD) Deltax(t)2 scaling as t1.25. For longer time intervals they exhibit subdiffusive transport and have an MSD scaling as t0.7. This crossover from an enhanced diffusion to a subdiffusive regime can be explained by considering the action of motor proteins that actively transport these endosomes along the cellular microtubule network and the thermal bending modes of the microtubule network itself.

Chemomechanics of surface stresses induced by DNA hybridization.

When biomolecular reactions occur on one surface of a microcantilever beam, changes in intermolecular forces create surface stresses that bend the cantilever. While this phenomenon has been exploited to create label-free biosensors and biomolecular actuators, the mechanisms through which chemical free energy is transduced to mechanical work in such hybrid systems are not fully understood. To gain insight into these mechanisms, we use DNA hybridization as a model reaction system. We first show that the surface grafting density of single-stranded probe DNA (sspDNA) on a surface is strongly correlated to its radius of gyration in solution, which in turn depends on its persistence length and the DNA chain length. Since the persistence length depends on ionic strength, the grafting density of sspDNA can be controlled by changing a solution's ionic strength. The surface stresses produced by the reaction of complementary single-stranded target DNA (sstDNA) to sspDNA depend on the length of DNA, the grafting density, and the hybridization efficiency. We, however, observe a remarkable trend: regardless of the length and grafting density of sspDNA, the surface stress follows an exponential scaling relation with the density of hybridized sspDNA.

Effects of biological reactions and modifications on conductance of nanofluidic channels.

Conductance characteristics of nanofluidic channels (nanochannels) fall into two regimes: at low ionic concentrations, conductance is governed by surface charge while at high ionic concentrations it is determined by nanochannel geometry and bulk ionic concentration. We used aminosilane chemistry and streptavidin-biotin binding to study the effects of surface reactions on nanochannel conductance at different ionic concentrations. Immobilization of small molecules such as aminosilane or biotin mainly changes surface charge, affecting conductance only in the low concentration regime. However, streptavidin not only modifies surface charge but also occludes part of the channel, resulting in observable conductance changes in both regimes. Our observations reflect the interplay between the competing effects of charge and size of streptavidin on nanochannel conductance.

Design of a monolithic piezoelectrically actuated microelectromechanical tunable vertical-cavity surface-emitting laser.

We report the design of a monolithic piezoelectrically actuated microelectromechanical tunable vertical-cavity surface-emitting laser (VCSEL). The main advantages of piezoelectric actuation compared with conventional capacitive techniques are improved wavelength control, reduced external and tilt losses, and lower power supply voltages. The details of the piezoelectric actuation scheme for a 980-nm VCSEL with a variable air gap are described. A tuning range of approximately 35 nm can theoretically be achieved with a 3-V power supply (2 x reduction from that of electrostatic actuation) by use of a 250-microm-long cantilever beam. The proposed actuation mechanism is insensitive to the pull-in phenomenon, therefore improving wavelength control and reducing threshold current. Drastic improvements in power efficiency make it ideal for low-power applications such as all-optical communication, chip-scale atomic clocks, and biological studies.

Characterization of grafting density and binding efficiency of DNA and proteins on gold surfaces.

The surface grafting density of biomolecules is an important factor for quantitative assays using a wide range of biological sensors. We use a fluorescent measurement technique to characterize the immobilization density of thiolated probe DNA on gold and hybridization efficiency of target DNA as a function of oligonucleotide length and salt concentration. The results indicate the dominance of osmotic and hydration forces in different regimes of salt concentration, which was used to validate previous simulations and to optimize the performance of surface-stress based microcantilever biosensors. The difference in hybridization density between complementary and mismatched target sequences was also measured to understand the response of these sensors in base-pair mismatch detection experiments. Finally, two different techniques for immobilizing proteins on gold were considered and the surface densities obtained in both cases were compared.