Fused Silica Chip Research Articles

Microlens arrays for multichannel laser-to-waveguide coupling

An optical multichannel coupling system for coupling laser arrays to waveguide arrays is developed. Based on a microlens array, the system enables coupling of nine individual optical channels, with one aspheric microlens per channel at a lateral channel pitch of 100 µm. The design process criteria for the proposed microlenses, with 97 µm diameter and working distances from laser to lens and lens to waveguide of 150 µm and 275 µm, respectively, are described. The microlens array is fabricated on a 4mm×2mm×0.41mm fused silica chip and contains an orthogonal grid with 32×16 microlenses, of which a row of nine adjacent microlenses is used for coupling. Uniform coupling over all channels can be achieved, as well as specific coupling for each channel individually with less than −13.5dB crosstalk. The coupling system is designed for optical neural stimulators.

Millisecond timescale reactions observed via X-ray spectroscopy in a 3D microfabricated fused silica mixer.

Determination of electronic structures during chemical reactions remains challenging in studies which involve reactions in the millisecond timescale, toxic chemicals, and/or anaerobic conditions. In this study, a three-dimensionally (3D) microfabricated microfluidic mixer platform that is compatible with time-resolved X-ray absorption and emission spectroscopy (XAS and XES, respectively) is presented. This platform, to initiate reactions and study their progression, mixes a high flow rate (0.50-1.5 ml min-1) sheath stream with a low-flow-rate (5-90 µl min-1) sample stream within a monolithic fused silica chip. The chip geometry enables hydrodynamic focusing of the sample stream in3D and sample widths as small as 5 µm. The chip is also connected to a polyimide capillary downstream to enable sample stream deceleration, expansion, and X-ray detection. In this capillary, sample widths of 50 µm are demonstrated. Further, convection-diffusion-reaction models of the mixer are presented. The models are experimentally validated using confocal epifluorescence microscopy and XAS/XES measurements of a ferricyanide and ascorbic acid reaction. The models additionally enable prediction of the residence time and residence time uncertainty of reactive species as well as mixing times. Residence times (from initiation of mixing to the point of X-ray detection) during sample stream expansion as small as 2.1 ± 0.3 ms are also demonstrated. Importantly, an exploration of the mixer operational space reveals a theoretical minimum mixing time of 0.91 ms. The proposed platform is applicable to the determination of the electronic structure of conventionally inaccessible reaction intermediates.

Integration of segmented microflow chemistry and online HPLC/MS analysis on a microfluidic chip system enabling enantioselective analyses at the nanoliter scale.

In this work, we introduce an approach to merge droplet microfluidics with an HPLC/MS functionality on a single chip to analyze the contents of individual droplets. This is achieved by a mechanical rotor-stator interface that precisely positions a microstructured PEEK rotor on a microfluidic chip in a pressure-tight manner. The developed full-body fused silica chip, manufactured by selective laser-induced etching, contained a segmented microflow compartment followed by a packed HPLC channel, which were interconnected by the microfluidic PEEK rotor on the fused silica lid with hair-thin through-holes. This enabled the targeted and leakage-free transfer of 10 nL fractions of droplets as small as 25 nL from the segmented microflow channel into the HPLC compartment that operated at pressures of up to 60 bar. In a proof of concept study, this approach was successfully applied to monitor reactions at the nanoliter scale and to distinguish the formed enantiomers.

Multiple Heart-Cutting Two-Dimensional Chip-HPLC Combined with Deep-UV Fluorescence and Mass Spectrometric Detection

In this work, we introduce a new two-dimensional chip-based high-performance liquid chromatography (2D chip-HPLC) approach, which enables multiple transfers from the first dimension effluent onto the column head of the sec-ond separation dimension. By merging injection, separation, and detection features on a fused silica chip in a dead volume-free manner, all extra-column peak dispersion effects can be reduced to an absolute minimum. The application of intrinsic fluorescence detection with excitation in the deep-UV spectral region and electrospray ionization mass spectrometry after the first and second separation dimension, respectively, enables the label-free analysis of complex samples, as exemplarily shown for a pesticide mixture and a tryptic digest.

Wafer-scale fabrication of fused silica chips for low-noise recording of resistive pulses through nanopores

This paper presents a maskless method to manufacture fused silica chips for low-noise resistive-pulse sensing. The fabrication includes wafer-scale density modification of fused silica with a femtosecond-pulsed laser, low-pressure chemical vapor deposition (LPVCD) of silicon nitride (SiNx) and accelerated chemical wet etching of the laser-exposed regions. This procedure leads to a freestanding SiNx window, which is permanently attached to a fused silica support chip and the resulting chips are robust towards Piranha cleaning at ∼80 °C. After parallel chip manufacturing, we created a single nanopore in each chip by focused helium-ion beam or by controlled breakdown. Compared to silicon chips, the resulting fused silica nanopore chips resulted in a four-fold improvement of both the signal-to-noise ratio and the capture rate for signals from the translocation of IgG1 proteins at a recording bandwidth of 50 kHz. At a bandwidth of ∼1 MHz, the noise from the fused silica nanopore chips was three- to six-fold reduced compared to silicon chips. In contrast to silicon chips, fused silica chips showed no laser-induced current noise—a significant benefit for experiments that strive to combine nanopore-based electrical and optical measurements.

Femtosecond Laser Micromachining of Fabry-Pérot Interferometers for Magnetic Field Sensing

Fs-laser micromachining is a high precision fabrication technique that can be used to write novel three-dimensional structures, depending on the nature of light-matter interaction. In fused silica, the material modification can lead to (i) an increase of the refractive index around the focal volume, resulting in the formation of optical circuits, or (ii) an enhancement of the etch rate of the laser-affected zones relative to the pristine material, leading to a selective and anisotropic etching reaction that enables fabrication of microfluidic systems. Here, both effects are combined to fabricate a Fabry-Pérot interferometer, where optical waveguides and microfluidic channels are integrated monolithically in a fused silica chip. By filling the channel with a magnetic fluid whose refractive index changes with an external magnetic field, the device can be used as a magnetic field sensor. A linear sensitivity of -0.12 nm/mT is obtained in the 5.0±0.5 to 33.0±0.5 mT range, with the field being applied parallel to the light propagation direction.

Single Cell Isolation Using Optical Tweezers.

Optical tweezers offer a non-contact method for selecting single cells and translocating them from one microenvironment to another. We have characterized the optical tweezing of yeast S. cerevisiae and can manipulate single cells at 0.41 ± 0.06 mm/s using a 26.8 ± 0.1 mW from a 785 nm diode laser. We have fabricated and tested three cell isolation devices; a micropipette, a PDMS chip and a laser machined fused silica chip and we have isolated yeast, single bacteria and cyanobacteria cells. The most effective isolation was achieved in PDMS chips, where single yeast cells were grown and observed for 18 h without contamination. The duration of budding in S. cerevisiae was not affected by the laser parameters used, but the time from tweezing until the first budding event began increased with increasing laser energy (laser power × time). Yeast cells tweezed using 25.0 ± 0.1 mW for 1 min were viable after isolation. We have constructed a micro-consortium of yeast cells, and a co-culture of yeast and bacteria, using optical tweezers in combination with the PDMS network of channels and isolation chambers, which may impact on both industrial biotechnology and understanding pathogen dynamics.

Miniaturized 3-D Solenoid-Type Micro-Heaters in Coordination With 3-D Microfluidics

We fabricate and characterize a prototype on-chip 3-D micro-heater inside a fused silica chip, which is coordinated with a microfluidic channel. By means of enhanced femtosecond laser wet etching followed by a micro-mold injection process of a liquid metal, the solenoid-type metallic micro-heater, with a size of π × 100 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> × 1120 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , was fabricated twining outward a linear microfluidic channel. The micro-heater exhibits excellent performances, such as precisely current-temperature controllability (temperature range from 0°C to 130°C), rapid heating/cooling process (up to 16 °C/s at the beginning of the heating process and 15.8 °C/s at the beginning of cooling process), and uniform temperature distribution. This work not only provides a high-quality micro-heater for biological or chemical applications, which requires rigid temperature control capabilities, but also proposes a practical route to the fabrication of comprehensive truly-3-D lab-on-a-chip devices and microelectromechanical systems.

Fusion of Renewable Ring Resonator Lasers and Ultrafast Laser Inscribed Photonic Waveguides.

We demonstrated the monolithic integration of reusable and wavelength reconfigurable ring resonator lasers and waveguides of arbitrary shapes to out-couple and guide laser emission on the same fused-silica chip. The ring resonator hosts were patterned by a single-mask standard lithography, whereas the waveguides were inscribed in the proximity of the ring resonator by using 3-dimensional femtosecond laser inscription technology. Reusability of the integrated ring resonator – waveguide system was examined by depositing, removing, and re-depositing dye-doped SU-8 solid polymer, SU-8 liquid polymer, and liquid solvent (toluene). The wavelength reconfigurability was validated by employing Rhodamine 6G (R6G) and 3,3′-Diethyloxacarbocyanine iodide (CY3) as exemplary gain media. In all above cases, the waveguide was able to couple out and guide the laser emission. This work opens a door to reconfigurable active and passive photonic devices for on-chip coherent light sources, optical signal processing, and the investigation of new optical phenomena.

Three-dimensional staggered herringbone mixer fabricated by femtosecond laser direct writing

We report on the fabrication of a three-dimensional (3D) staggered herringbone mixer (SHM) on a fused silica chip by femtosecond laser microfabrication, enabled by the 3D nature of femtosecond laser direct writing. We theoretically and experimentally show that a 3D SHM with patterns on not only the bottom floor but also the side walls can reduce the mixing length by almost half as compared with the conventional 2D SMH mixer.

On-chip three-dimensional high-Q microcavities fabricated by femtosecond laser direct writing

We report on the fabrication of three-dimensional (3D) high-Q whispering gallery microcavities on a fused silica chip by femtosecond laser microfabriction, enabled by the 3D nature of femtosecond laser direct writing. The processing mainly consists of formation of freestanding microdisks by femtosecond laser direct writing and subsequent wet chemical etching. CO(2) laser annealing is followed to smooth the microcavity surface. Microcavities with arbitrary tilting angle, lateral and vertical positioning are demonstrated, and the quality (Q)-factor of a typical microcavity is measured to be up to 1.07 × 10(6), which is currently limited by the low spatial resolution of the motion stage used during the laser patterning and can be improved with motion stages of higher resolutions.

Fabrication of bottle-shaped nanochannels in fused silica using a self-closing effect

The spatial control of molecular motor function, using nanostructured surfaces, is of great interest for the development of commercial devices for diagnostics and high-throughput drug screening with molecular motors as targets. In the present study we have fabricated 100-300nm wide nanochannels, completely subsurfaced on fused silica chips, with the aim to interface them with a microfluidic system. Such a system will allow for changes in the chemical environment surrounding molecular motors, with minimal influence on their directional motion. This will be achieved by changing the chemical environment in a perpendicular direction to the motor motion and allowing the chemical substances to diffuse in and out of the nanochannels via a small slit (5-10nm) on the top of the nanochannels. To create this slit, and to control its width, we here demonstrate the use of a self-closing effect based on the volume increase (2.27 times) during oxidation of silicon. The details of the fabrication steps (EBL, RIE and oxidation) are discussed.

Methacrylate monolithic stationary phases for gradient elution separations in microfluidic devices

Methacrylate monolithic stationary phases were produced in fused-silica chips by UV initiation. Poly(butyl methacrylate- co-ethylene dimethacrylate) (BMA) and poly(lauryl methacrylate- co-ethylene dimethacrylate) (LMA) monoliths containing 30, 35 and 40% monomers were evaluated for the separation of peptides under gradient conditions. The peak capacity was used as an objective tool for the evaluation of the separation performance. LMA monoliths of the highest density gave the highest peak capacities (≈40) in gradients of 15 min and all LMA monoliths gave higher peak capacities than the BMA monoliths with the same percentage of monomers. Increasing the gradient duration to 30 min did not increase the peak capacity significantly. However, running fast (5 min) gradients provides moderate peak capacities (≈20) in a short time. Due to the system dead volume of 1 μL and the low bed volume of the chip, early eluting peptides migrated over a significant part of the column during the dwell time under isocratic conditions. It was shown that this could explain an increased band broadening on the monolithic stationary phase materials used. The effect is stronger with BMA monoliths, which partly explains the inferior performance of this material with respect to peak capacity. The configuration of the connections on the chip appeared to be critical when fast analyses were performed at pressures above 20 bar.

Two-photon fluorescence excitation with a microlens fabricated on the fused silica chip by femtosecond laser micromachining

We report on the fabrication of a microlens on a fused silica chip with excellent optical performance by femtosecond laser microfabrication. We show, both in theory and experimentally, that the fabricated microlens offers a high resolution approaching the optical diffraction limit. Moreover, two-photon excitation of fluorescence with the fabricated microlens is demonstrated. The lateral and axial resolutions of fluorescence are measured to be ∼1.7 and 12.3 μm, respectively.

The use of DNA molecular beacons as nanoscale temperature probes for microchip-based biosensors

Today's biosensors and drug delivery devices are increasingly incorporating lithographically patterned circuitry that is placed within microns of the biological molecules to be detected or released. Elevated temperatures due to Joule heating from the underlying circuitry cannot only reduce device performance, but also alter the biological activity of such molecules (i.e. binding, enzymatic, folding). As a consequence, biochip design and characterization will increasingly require local measurements of the temperature and temperature gradients on the biofunctionalized surface. We have developed a technique to address this challenge based on the use of DNA molecular beacons as a nanoscale temperature probe. The surface of fused-silica chips with lithographically patterned, current-carrying gold rings have been functionalized with a layer of molecular beacons. We utilize the temperature dependence of the molecular beacons to calibrate the temperature at the center of the rings as a function of applied current from 25 to 50 °C. The fluorescent images of the rings reveal the extent of heating to the surrounding chip due to the applied current while resolving temperature gradients over length scales of less than 500 nm. Finite element analysis and analytic calculations of the distribution of heat in the vicinity of the current-carrying rings agree well with the experimental results. Thus, molecular beacons are shown to be a viable tool for temperature calibration of micron-sized circuitry and the visualization of submicron temperature gradients.

Deep UV Laser-Induced Fluorescence Detection of Unlabeled Drugs and Proteins in Microchip Electrophoresis

Deep UV fluorescence detection at 266-nm excitation wavelength has been realized for sensitive detection in microchip electrophoresis. For this purpose, an epifluorescence setup was developed enabling the coupling of a deep UV laser into a commercial fluorescence microscope. Deep UV laser excitation utilizing a frequency quadrupled pulsed laser operating at 266 nm shows an impressive performance for native fluorescence detection of various compounds in fused-silica microfluidic devices. Aromatic low molecular weight compounds such as serotonin, propranolol, a diol, and tryptophan could be detected at low-micromolar concentrations. Deep UV fluorescence detection was also successfully employed for the detection of unlabeled basic proteins. For this purpose, fused-silica chips dynamically coated with hydroxypropylmethyl cellulose were employed to suppress analyte adsorption. Utilizing fused-silica chips permanently coated with poly(vinyl alcohol), it was also possible to separate and detect egg white chicken proteins. These data show that deep UV fluorescence detection significantly widens the application range of fluorescence detection in chip-based analysis techniques.

On-chip hydrodynamic chromatography separation and detection of nanoparticles and biomolecules.

For the first time, on-chip planar hydrodynamic chromatography is combined with UV absorption detection. This technique is suitable for size characterization of synthetic polymers, biopolymers, and particles. Possible advantages of an on-chip hydrodynamic chromatography system over conventional techniques, such as size exclusion chromatography, and field-flow fractionation are fast analysis, high efficiency, reduced solvent consumption, and easy temperature control. The hydrodynamic separations are performed in a planar configuration realized in fused silica using a mixture of fluorescent and nonfluorescent polystyrene particles with sizes ranging from 26 to 155 nm. The planar chip configuration consists of a 1-microm-high, 0.5-mm-wide, and 69-mm-long channel, an integrated 150-pL injection structure, and a 30-microm-deep and 30-microm-wide detection cell, suitable for UV absorption detection. By combination of the separation data obtained in the new fused-silica chip with those obtained using a previously presented planar hydrodynamic chromatography chip, which was realized using silicon and glass microtechnology, a description of the retention and dispersion behavior of planar hydrodynamic chromatography is obtained. Especially the influence of the sidewalls on the dispersion is investigated. Furthermore a hydrodynamic separation within 70 s of several biopolymers is shown in the glass-silicon chip.

Conduct-as-cast polymer monoliths as separation media for capillary electrochromatography.

We have developed porous polymer monoliths (PPMs) that are versatile and robust reversed-phase chromatography media. The PPMs are cast-to-shape, UV-cured polymers that form uniform packings within pretreated glass capillaries and fused-silica chips. No applied pressure is ever needed to flush the PPMs since they support electroosmotic flow as cast. Such characteristics make the PPMs useful for chip-based devices. Our results show efficiencies greater than or equal to 150,000 plates/m for both capillary and chip-based separations of polycyclic aromatic hydrocarbons. By changing the monomers, the hydrophobicity of the polymers, and the direction of the electroosmotic flow can be altered without degrading chromatographic performance. We describe here the development of these acrylate-based materials along with both physical and chromatographic characterization.