This report presents a study of electrokinetic transport in a series of integrated macro- to nano-fluidic chips that allow for controlled injection of molecular mixtures into high-density arrays of nanochannels. The high-aspect-ratio nanochannels were fabricated on a Si wafer using interferometric lithography and standard semiconductor industry processes, and are capped with a transparent Pyrex cover slip to allow for experimental observations. Confocal laser scanning microscopy was used to examine the electrokinetic transport of a negatively charged dye (Alexa 488) and a neutral dye (rhodamine B) within nanochannels that varied in width from 35 to 200 nm with electric field strengths equal to or below 2000 V m-1. In the negatively charged channels, nanoconfinement and interactions between the respective solutes and channel walls give rise to higher electroosmotic velocities for the negatively charged dye than for the neutral dye, towards the negative electrode, resulting in an anomalous separation that occurs over a relatively short distance (<1 mm). Increasing the channel widths leads to a switch in the electroosmotic transport behavior observed in microscale channels, where neutral molecules move faster because the negatively charged molecules are slowed by the electrophoretic drag. Thus a clear distinction between "nano-" and "microfluidic" regimes is established. We present an analytical model that accounts for the electrokinetic transport and adsorption (of the neutral dye) at the channel walls, and is in good agreement with the experimental data. The observed effects have potential for use in new nano-separation technologies.

This article presents a summary of theory, experimental studies, and results for the electrokinetic transport in small fluidic nanochannels. The main focus is on the effect of the electric double layer on the EOF, electric current, and electrophoresis of charged analytes. The double layer thickness can be of the same order as the width of the nanochannels, which has an impact on the transport by shaping the fluid velocity profile, local distributions of the electrolytes, and charged analytes. Our theoretical consideration is limited to continuum analysis where the equations of classical hydrodynamics and electrodynamics still apply. We show that small channels may lead to qualitatively new effects like selective ionic transport based on charge number as well as different modes for molecular separation. These new possibilities together with the rapid development of nanofabrication capabilities lead to an extensive experimental effort to utilize nanochannels for a variety of applications, which are also discussed and analyzed in this review.

The fabrication of nanoscale structures with dimensions approaching the scale of biological molecules offers approaches to the study of fluid dynamics and biomolecular transport. Ultimately, a parallel lithographic approach will be necessary if devices based on these nanofluidics are to achieve widespread availability and acceptance. We report on a flexible, all-optical lithography alternative that is amenable to large-scale production. We use interferometric lithography (IL) and anisotropic etching to produce large areas of parallel, nanofluidic channels with widths of ∼100 nm and depths of up to 500 nm. We also use standard optical lithography to create interfacing microchannels, such that the range of spatial scales on one chip varies by 104 (from mm scale reservoirs to 100 nm nanochannels). We provide initial demonstrations of capillary action and electrophoretic motion of fluorescent dye solutions.

The interactions between charged molecules and channel surfaces are expected to significantly influence the electrokinetic transport of molecules and their separations in nanochannels. This study reports the effect of wall-molecule interactions on flow control of negatively charged Alexa 488 and positively charged Rhodamine B dye molecules in an array of nanochannels (100 nm wx 500 nm dx 14 mm l) embedded in fluidic field effect transistors (FETs). For FET flow control, a third electrical potential, known as a gate bias, is applied to the channel walls to manipulate their zeta-potential. Electroosmotic flow of charged dye molecules is accelerated or reversed according to the polarity and magnitude of the gate bias. During FET flow control, we monitor how the electrostatic interaction between charged dye molecules and channel walls affects the apparent velocity of molecules, using laser-scanning confocal fluorescence microscopy. We observe that the changes in flow speed and direction of negatively charged Alexa 488 is much more pronounced than that of positively charged Rhodamine B in response to the gate bias that causes either repulsive or attractive electrostatic interactions. This observation is supported by calculations of concentration-weighted velocity profiles of the two dye molecules during FET flow control. The velocity profile of negatively charged Alexa 488 is much more pronounced at the center of each nanochannel than near its walls since Alexa 488 molecules are repelled from negatively charged channel walls. This pronounced center velocity further responds to the gate bias, increasing the average velocity by as much as 23% when -30 V is applied to the gate (zeta-potential = -80.6 mV). In contrast, the velocity profile of positively charged Rhodamine B is dispersed over the entire channel width due to dye-wall attraction and adsorption. Our experimental observations and calculations support the hypothesis that valence-charge-dependent electrostatic interaction and its manipulation by the gate bias would enhance molecular separations of differentially charged molecules in nanofluidic FETs.

Harnessing hot carriers from photoexcited metallic nanoparticles for catalysis is very challenging because these carriers have extremely short lifetimes. Here, we demonstrate that smaller particles have higher surface-to-volume ratios that allow hot carriers to diffuse to particle surfaces with a higher probability and thereby exhibit higher photocatalytic activities as quantified by quantum yields. The measured photocatalytic activities for photoinduced etching of gold nanospheres by FeCl3, and the previously unreported aqueous hydrogenation of styrene using sodium borohydride under interband excitation show perfect dependence on the reciprocal of particle size. The size-dependent photocatalytic activity for photoinduced etching of gold nanospheres by FeCl3 under plasmon excitation, however, slightly deviates from this scaling law and may be influenced by other factors such as the surface field enhancement effect. This scaling law is expected to apply to other nanomaterial-based photocatalysts that rely on hot carrier diffusion to a surface for catalysis. Future design of nanomaterials for the harnessing of hot carriers for catalysis should take this scaling law into account.