Ran An

Electroosmotic flow can generate ion current rectification in nano- and micropores.

This paper introduces a strategy for generating ion current rectification through nano- and micropores. This method generates ion current rectification by electroosmotic-driven flow of liquids of varying viscosity (and hence varying conductance) into or out of the narrowest constriction of a pore. The magnitude of current rectification was described by a rectification factor, R(f), which is defined by the ratio of the current measured at a positive voltage divided by the current measured at a negative voltage. This method achieved rectification factors in the range of 5-15 using pores with diameters ranging from 10 nm to 2.2 microm. These R(f) values are similar to the rectification factors reported in other nanopore-based methods that did not employ segmented surface charges. Interestingly, this work showed that in cylindrical nanopores with diameters of 10 nm and a length of at least 275 nm, electroosmotic flow was present and could generate ion current rectification. Unlike previous methods for generating ion current rectification that require nanopores with diameters comparable to the Debye length, this work demonstrated ion current rectification in micropores with diameters 500 times larger than the Debye length. Thus this method extends the concept of fluidic diodes to the micropore range. Several experiments designed to alter or remove electroosmotic flow through the pore demonstrated that electroosmotic flow was required for the mode of ion current rectification reported here. Consequently, the magnitude of current rectification could be used to indicate the presence of electroosmotic flow and the breakdown of electroosmotic flow with decreasing ionic strength and hence increasing electric double layer overlap inside nanopores.

Simultaneous multi-microhole drilling of soda-lime glass by water-assisted ablation with femtosecond laser pulses

Longitudinal and transverse microholes are drilled in soda-lime glass by water-assisted ablation with femtosecond laser pulses. True three-dimensional microchannels consisting of longitudinal and transverse microholes are presented. At low incident pulse energy, only one transverse microhole is observed. At high incident pulse energy, multiple transverse microholes can be simultaneously drilled. Using a focusing lens with numerical aperture of 0.5, two, three and four transverse microholes are fabricated at 3.2, 4.9 and 9.3microJ/pulse, which is qualitatively explained by the multiple foci process.

Ultrafast laser fabrication of submicrometer pores in borosilicate glass

We demonstrate rapid fabrication of submicrometer-diameter pores in borosilicate glass using femtosecond laser machining and subsequent wet-etch techniques. This approach allows direct and repeatable fabrication of high-quality pores with diameters of 400-800 nm. Such small pores coupled with the desirable electrical and chemical properties of glass enable sensitive resistive-pulse analysis to determine the size and concentration of macromolecules and nanoparticles. Plasma-enhanced chemical vapor deposition allows further reduction of pore diameters to below 300 nm.

Permanent computer-generated holograms embedded in silica glass by femtosecond laser pulses.

We present a novel technique to directly fabricate permanent computer-generated holograms inside silica glass with femtosecond laser pulses. The Fourier transform of an object is performed using a computer and the complex amplitude distribution is encoded by the detour phase method. The resulted cell-oriented hologram is directly written inside a bulk of silica glass by femtosecond laser-induced microexplosion. The image is then reconstructed with a collimated He-Ne laser beam.

Liquid glass electrodes for nanofluidics

Nanofluidic devices exploit molecular-level forces and phenomena to increase their density, speed and accuracy1. However, fabrication is challenging because dissimilar materials need to be integrated in three dimensions with nanoscale precision. Here we report a three-dimensional nanoscale liquid glass electrode (NLGE) made from monolithic substrates without conductive materials by femtosecond laser nanomachining. The electrode consists of a nanochannel terminating at a nanoscale glass tip that becomes a conductor in the presence of high electric fields through dielectric breakdown, and returns to an insulator when this field is removed. This reversibility relies on control of nanoampere breakdown currents and extremely fast heat dissipation at nanoscale volumes. We use the NLGE to fabricate a nano-injector that includes an electrokinetic pump, 4 µm across with 0.6 µm channels, and capable of well-controlled flow rates below 1 fL/s. The electrode can be easily integrated into other nanodevices and fluidic systems, including actuators and sensors.

Hyperspectral fluorescence lifetime imaging for optical biopsy

Abstract. A hyperspectral fluorescence lifetime imaging (FLIM) instrument is developed to study endogenous fluorophores in biological tissue as an optical biopsy tool. This instrument is able to spectrally, temporally, and spatially resolve fluorescence signal, thus providing multidimensional information to assist clinical tissue diagnosis. An acousto-optic tunable filter (AOTF) is used to realize rapid wavelength switch, and a photomultiplier tube and a high-speed digitizer are used to collect the time-resolved fluorescence decay at each wavelength in real time. The performance of this instrument has been characterized and validated on fluorescence tissue phantoms and fresh porcine skin specimens. This dual-arm AOTF design achieves high spectral throughput while allowing microsecond nonsequential, random wavelength switching, which is highly desirable for time-critical applications. In the results reported here, a motorized scanning stage is used to realize spatial scanning for two-dimensional images, while a rapid beam steering technique is feasible and being developed in an ongoing project.

Water-assisted femtosecond laser machining of electrospray nozzles on glass microfluidic devices

Using water-assisted femtosecond laser machining, we fabricated electrospray nozzles on glass coverslips and on assembled microfluidic devices. Machining the nozzles after device assembly facilitated alignment of the nozzles over the microchannels. The basic nozzle design is a through-hole in the coverslip to pass liquids and a trough machined around the through-hole to confine the electrospray and prevent liquid from wicking across the glass surface. Electrospray from the nozzles was stable with and without pressure-driven flow applied and was evaluated using mass spectra of the peptide bradykinin.

Porcine cortical bone ablation by ultrashort pulsed laser irradiation.

Ultrashort pulsed lasers in bone ablation show promise for many orthopedic applications. To minimize collateral tissue damage and control the ablation process, the ablation threshold fluence must be well characterized. Using an amplified femtosecond laser (170 fs, 800 nm, 1 kHz), the ablation threshold on unaltered porcine cortical bone was measured using the D(2) method at multiple incident pulse numbers ranging from 25 to 1000 pulses per spot. The lowered threshold at greater pulse numbers indicated an incubation effect. Using a power law model, the incubation coefficient of unaltered porcine cortical bone was found to be 0.89 ± 0.03. Through extrapolation, the single-pulse ablation threshold was found to be 3.29 ± 0.14 J/cm(2).

Ultrafast laser ablation and machining large-size structures on porcine bone

Abstract. When using ultrafast laser ablation in some orthopedic applications where precise cutting/drilling is required with minimal damage to collateral tissue, it is challenging to produce large-sized and deep holes using a tightly focused laser beam. The feasibility of producing deep, millimeter-size structures under different ablation strategies is investigated. X-ray computed microtomography was employed to analyze the morphology of these structures. Our results demonstrated the feasibility of producing holes with sizes required in clinical applications using concentric and helical ablation protocols.

Computer-Generated Holograms Written inside Silica Glass by Femtosecond Laser Pulses

An image is Fourier transformed by a computer, the complex amplitude distribution is encoded and the resulted binary hologram is directly written inside silica glass by femtosecond laser-induced microexplosion.