Microchip capillary electrophoresis (CE) with integrated four-electrode capacitively coupled conductivity detection is presented. Conductivity detection is a universal detection technique that is relatively independent on the detection pathlength and, especially important for chip-based analysis, is compatible with miniaturization and on-chip integration. The glass microchip structure consists of a 6 cm etched channel (20 microm x 70 microm cross section) with silicon nitride covered walls. In the channel, a 30 nm thick silicon carbide layer covers the electrodes to enable capacitive coupling with the liquid inside the channel as well as to prevent interference of the applied separation field. The detector response was found to be linear over the concentration range from 20 microM up to 2 mM. Detection limits were at the low microM level. Separation of two short peptides with a pI of respectively 5.38 and 4.87 at the 1 mM level demonstrates the applicability for biochemical analysis. At a relatively low separation field strength (50 V/cm) plate numbers in the order of 3500 were achieved. Results obtained with the microdevice compared well with those obtained in a bench scale CE instrument using UV detection under similar conditions.

Metal clusters deposited on a substrate and positioned at a nanometric distance from a wave-reflecting layer act as nanoresonators able to receive, store and transmit energy within the visible and infrared range of the spectrum. Among the unique effects of these metal nanocluster assemblies are high local field enhancement and nanoscale resonant behaviour driving optical absorption in the visible and infrared range of the spectrum. In these types of devices and sensors the precise nanometric assembly coupling the local field surrounding a cluster is critical for allowing resonance with other elements interacting with this field. In particular, the cluster–mirror distance or the cluster–fluorophore distance gives rise to a variety of enhancement phenomena (e.g. resonant-enhanced fluorescence, REF). Depending on the desired application this ‘resonance’ distance is tuned from 5 up to 500 nm. High-throughput transducers using metal cluster resonance technology are based on surface enhancement of light absorption by metal clusters (surface-enhanced absorption, SEA). These devices can be used for detection of biorecognition binding as well as structural changes in nucleic acids, proteins or any polymer. The optical property made use of in the analytical application of metal cluster films is so-called anomalous absorption. An absorbing film of clusters is positioned 10–400 nm from an electromagnetic wave-reflecting layer. At a well-defined mirror–cluster distance the reflected electromagnetic field has the same phase at the position of the absorbing cluster as the incident field. This feedback mechanism strongly enhances the effective cluster absorption coefficient. These systems are characterized by a narrow reflection minimum whose spectral position shifts sensitively with interlayer thickness, because a given cluster–mirror distance and wavelength defines the optimum phase.

Electrospraying in a stable cone-jet mode at <400 microm above a substrate is shown to be a powerful technique to produce arrays of identical micrometer-sized spots consisting of biologically active substances. Aqueous solutions with a surface tension of 0.04 N m(-1) and conductivities ranging from 0.04 to 2.2 S m(-1) were sprayed at ultralow flow rates ranging from 100 to 300 pL s(-1). The charged jet that emanates from the cone tip breaks up into a spray of charged droplets that are deposited in the form of a uniform spot of 130-350 microm in diameter by spraying during 0.5-3 s at 220-400 microm above a substrate, respectively. After a spot was deposited, spraying was stopped instantaneously by increasing the distance between the capillary tip and the substrate by an additional 100 microm using a computer-controlled x-y-z table. This was immediately followed by a rapid shift of the substrate 400 microm sideways and 100 microm upward, thus causing spraying to resume instantaneously because of the increased electric field strength, which resulted in the deposition of the next spot. It is shown here that spraying of lactate dehydrogenase (LDH), glucose-6-phosphate dehydrogenase (G6P-DH), and pyruvate kinase (PK) on a liquid layer resulted in the complete preservation of their activities despite the high solution conductivity of 3.3 S m(-1) and high currents ranging from 300 to 500 nA. LDH and PK activities were fully preserved after spraying onto dry aluminum by adding 0.05 M buffer and 0.5 and 1 wt % of trehalose, respectively, to the spray solutions. Electrospraying allows for accurate dispensing of liquid volumes as small as 50 pL. Enzymatic activities of LDH and PK are fully preserved after spraying.

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