This research was funded by NSERC, Canada.

1. Preparation of Materials
<ol>
 <li>Microcapillaries Obtain silica capillaries from a commercial supplier. We purchase our capillaries from Polymicro Technologies . Choose a small inner diameter (~25 - 30 &mu;m) for more widely separated spectral resonances (i.e. a larger free spectral range) or a larger inner diameter (~100 &mu;m) for more closely spaced resonances with higher quality factors. A large outer diameter ensures the FCMs are durable and easily manipulated.
 <ol>
 <li>The capillaries come with a colored polyimide jacket, which must first be removed. Cut approximately 10-cm pieces of capillary from the roll, using diamond fiber cleaver. Each piece constitutes a single sample. Heat these in a tube furnace at 650 &deg;C for one hour in oxygen to burn off the coating. This process removes the jacket material, exposing the silica capillary inside. After cooling to room temperature, remove the capillaries from the heating boat.</li>
 </ol>
 </li>
 <li>Hydrogen silsesquioxane solutions Hydrogen silsesquioxane (HSQ) is reported to consist of H12Si8O12 molecules with a cage-like structure.14 This material is commercially available from Dow Corning. Purchase the HSQ in one of its FOx-series (flowable oxide) solutions, such as FOx-15. These solutions are expensive and have a limited shelf life, so careful planning is needed. Evaporation of the MIBK solvent provides an HSQ concentration of approximately 18% by weight. If the HSQ concentration is too low, quantum dots may not form in the film. If it is too high, the films may be too thick and delaminate from the capillary channel surface. In our experience, for a capillary with a 25-30 &mu;m inner diameter, a FOx solution containing ~25 wt.% HSQ is best. Thus, it may be necessary to evaporate or add more HSQ solvent (making sure that it is dry) in order to adjust the concentration. Whether a dilution or concentration is necessary is difficult to determine without trial and error; i.e. make some samples and examine the results, as discussed below.</li>
</ol>
2. Fabrication of Coated Capillaries
<ol>
 <li>Filling the capillary Take the pieces of capillary prepared in step 1.1 and dip them into the FOx solution. When a capillary is dipped into the solution, you should be able to visually follow the meniscus up the channel, as the solution is drawn into the capillary (Figure 2a).
 <ol>
 <li>When the meniscus reaches the top, remove the capillary and place it in a glass annealing crucible. It should now be completely filled with FOx solution. Repeat this for as many samples as possible, to increase the chances of success. We typically run batches of 20-30 and use two different concentrations of HSQ solutions by weight. We fill the capillaries in air, but keeping the FOx solution chilled in a glovebox is recommended if possible, in order to minimize exposure of the solution to oxygen and water vapor. Even small exposures can result in gelation of the solution.</li>
 </ol>
 </li>
 <li>Annealing Anneal the capillaries in a two stage process. Annealing evaporates the solvent and collapses the HSQ cage structure, forming an SiOx film adhering to the channel walls. Annealing at higher temperatures disproportionates the SiOx film into Si quantum dots dispersed in a silica matrix. The annealing steps include a 30-minute ramp from room temperature to 300 &deg;C, a dwell for 3 hr to evaporate the solvent, then a ramp to 1,100 &deg;C in 45 min, and a dwell for one hour to precipitate the QDs.
 <ol>
 <li>Let the capillaries slowly cool (~12 hr) back to room temperature. This helps to minimize stress-related cracking of the film deposited on the capillary wall. Other annealing protocols could probably be followed and may be more reliable, but a high-temperature anneal stage at 1,000-1,100 &deg;C is always necessary to form the QDs. At the end of this step, one should (hopefully) have 20-30 capillaries with a layer of fluorescent QDs embedded in a silica matrix coating the channel walls.</li>
 </ol>
 </li>
</ol>
3. Characterization
<ol>
 <li>Sample check The fluorescence microscope on which the capillaries are mounted must perform both imaging and spectroscopy in the 700-900 nm wavelength range. Epifluorescence or confocal setups are suitable for this purpose. Place a row of the candidate capillaries on the stage so that it is easy to move between them for quick visual analysis (Figure 2b). Excite the capillary with blue or UV radiation either in free space on the microscope stage, or directly through the objective lens using a dichroic filter, and observe the fluorescence image using the eyepieces or a color camera.
 <ol>
 <li>Observe the capillary fluorescence. If the fabrication was successful, the capillaries will exhibit a bright red fluorescence. This is the first indication of a favorable sample. Capillaries exhibiting orange-yellow fluorescence (instead of the red color associated with the QDs) in general do not have the desired optical characteristics. These samples also tend to form more often in solutions with a low HSQ concentration. Some capillaries may show no fluorescence at all; in this case the QD film did not form and the capillary can be discarded (glass sharps). This is an indication that the solution may not have been drawn into the capillary, or it may have been deficient in HSQ. Finally, some samples may show a cracked or textured film; these can also be discarded.</li>
 <li>Discard all the samples mentioned in the previous step except those that show the bright red fluorescence characteristic of silicon QDs.</li>
 </ol>
 </li>
 <li>Check for the presence of WGMs in the fluorescence spectra Ensure the image is aligned as desired on the entrance slit of the spectrometer and collect a fluorescence spectrum. Adjust the collection time so as to produce an acceptable signal-to-noise ratio. Perform wavelength and intensity calibrations as required. The QD spectra should be intense in the wavelength range from 700 to 900 nm. Spectra taken from the region corresponding to the capillary inner wall should show strong oscillations due to the presence of the cylindrical whispering gallery modes (WGMs); this is the second-to-last requirement of a successful refractometric sensor.
 <ol>
 <li>Some samples may have a bright red QD fluorescence but lack WGM oscillations in the spectrum. This is an indication the QD film cracked or delaminated from the capillary wall, which destroys the optical resonances. Discard capillaries without WGMs. At this point, only the (typically small) fraction of samples that meet the sensor requirements remain. There is one final test to be performed.</li>
 </ol>
 </li>
 <li>Refractometric analysis Attach the candidate capillaries to polyethylene, tygon, teflon, or other tubing chemically compatible with the intended analyte solutions whose inner diameter should be slightly larger than the capillary outer diameter. The choice of tubing should be made so as not to react with the solutions to be pumped into the capillary.
 <ol>
 <li>Use a good adhesive to attach the glass capillary to the tubing; otherwise the capillary-tubing interface will leak. We have reasonably good success using Norland NOA-76 or Mascot instant adhesive gel. The choice of glue depends on its adhesion to the glass capillary and the tubing, and a lack of reaction with the channel fluids. Use care to prevent the adhesive from seeping into the capillary channel and blocking it.</li>
 <li>Interface the tubing via a syringe to a micropumping system. Compounds with well-known refractive indices such as methanol, ethanol, and water can be used to determine the refractometric sensitivity of the device. This is the final test of a successful sensor. Pump each fluid, one at a time, into the capillary, making sure not to burst the adhesive seal between the capillary and the tubing (Figure 2c).</li>
 <li>Collect spectra with each liquid inside the capillary. Use an analyzer in the light path to distinguish between TE-polarized WGMs (analyzer parallel to capillary axis) and TM-polarized WGMs (analyzer perpendicular to capillary axis). There should be a shift in the wavelength of the WGM resonances, either TE or TM, with each different solution in the capillary. If there is no observable shift, the quantum-dot film is too thick and the WGMs do not sufficiently sample the channel medium. In successful samples we observe sensitivities typically between 5 and 15 nm per solution refractive index unit (RIU). Almost all the samples that do show WGMs demonstrate a measurable sensitivity; however, typically only a small fraction of prepared capillaries will show WGMs.</li>
 </ol>
 </li>
</ol>
4. Data Analysis
<ol>
 <li>Obtaining the fluorescence spectra Take a spectrum of the fluorescence of your sample. For biosensing applications, the channel surface has first to be functionalized for specific analytes. The surface of the QD film is essentially silica, so many surface modification recipes exist. Regardless of the application, the final step is the data processing and analysis.
 <ol>
 <li>Achieving low detection limits requires measuring small spectral shifts- ideally the "shift resolution" should be significantly smaller than the nominal spectrometer resolution or pitch. Care must be exercised in the spectral processing due to this. In particular, on many imaging spectrometers the spectrum may not be projected perfectly horizontally onto the CCD; thus if, between analyses, the sample image drifts vertically on the slit, false spectral shifts may be obtained. Use whatever means are necessary to ensure that this doesn't happen; e.g. use a calibration standard to determine the angle of the projected spectrum and correct for it, minimize sample drift, and ensure that the same CCD pixels are used to obtain all the spectra. 
 An example spectral image is shown in Figure 3, where the modes appear as strong oscillations at locations corresponding to the channel walls. Use a Mathematica computer code (or what your group prefers) to import the spectral images, output the 1D spectral data, and perform curve fitting and Fourier analysis of the WGM shifts, as described below.</li>
 </ol>
 </li>
 <li>Curve fitting Determine the WGM peak wavelength to measure small spectral shifts due to analytes in the FCM channel. Fit a single mode to a function that describes the spectral shape - this is a common way to obtain the peak position. In the ideal case, this will be a Lorentzian function (with proper conversion from wavelength to frequency units via &delta;&lambda; = &#9474;-c&delta;f/f2&#9474; where c is the speed of light): 
 <img src="/files/ftp_upload/50256/50256eq1.jpg" alt="Equation 1" fo:content-width="2in" fo:src="/files/ftp_upload/50256/50256eq1highres.jpg" /> 
 In Eq. 1, A is a scaling parameter and f0 is the central frequency. Unfortunately, FCMs are not an ideal case.
<ol>
 <li>In cylindrical cavities the WGMs are skewed toward higher frequencies, likely owing to the development of spiraling resonances (WGMs with a non-zero axial component of the wavevector).15 Lorentzian fits therefore perform poorly for determining the peak position. Unfortunately, there is no function that will fit a distribution of overlapping Lorentzians from the spiraling WGMs. In previous work 16 we suggested that a skewed Lorentzian 17 would give a better fit: 
 <img src="/files/ftp_upload/50256/50256eq2.jpg" alt="Equation 2" fo:content-width="5in" fo:src="/files/ftp_upload/50256/50256eq2highres.jpg" /> 
 Here, a and B are skewing parameters. As can be seen in Figure 4, Eq. 2 does give a better fit to the data than Eq. 1, but unfortunately it has no physical basis in the theory of WGMs.</li>
 </ol>
 </li>
 <li>Fourier shift analysis Alternatively, the data can be processed using a discrete Fourier transform, and the corresponding phase shifts measured from the Fourier spectrum. This method takes advantage of the periodicity of the whole spectrum, as opposed to using a single arbitrary WGM. It does not measure the peak wavelength position, but instead measures an overall shift of a given WGM spectrum with respect to an arbitrary reference spectrum.
 <ol>
 <li>Use the phase difference &Delta;&phi; for the high-power Fourier component that corresponds to the main WGM spectral oscillation to obtain the spectral shift. This corresponds to a real WGM frequency shift of: &delta;f = &Delta;&phi;(fmax - fmin)/(2&pi;k), 
 where fmin and fmax are the minimum and maximum frequency in the spectra. However, much information can be discarded if only the main component is used; additionally, truncation issues may make it difficult to determine which component is the main one. Often, the best results require some trial and error as to which Fourier components to select.</li>
 <li>The shift theorem instead uses all of the Fourier components. Accordingly, for a pure shift, each individual component is shifted proportional to k (with k being the component number). In other words, &delta;&phi;k = mk, where the proportionality m is a measure of the real shift. The total frequency shift is thus given by: &delta;f = m(fmax - fmin)/(2&pi;) . 
 This requires m to be obtained from a linear fit to the phase differences &Delta;&Phi;k for some or all of the Fourier components.</li>
 <li>For real data, the linear relation &Delta;&Phi;k = mk will have uncertainty due to noise and background signal, which can have a significant effect in the low-power Fourier components. Thus, we recommend a weighted linear fit, in which the weight for each component is proportional to its power, in order to obtain the slope of the &Delta;&Phi;k vs. k graph. The spectrum can be filtered around the main component prior to fitting, to remove both high frequencies (noise) and low frequencies (uncoupled fluorescence background). The frequency shifts from Eq. 4 are then converted into wavelength units.</li>
 <li>Unlike the case for curve fitting, a WGM "wavelength" is never obtained, but instead a wavelength shift is measured over the whole spectrum with respect to an arbitrary reference spectrum. The procedure is then repeated for every spectrum to be analyzed. The steps for this procedure are as follows:
 <ol>
 <li>Convert the spectra into frequency units to ensure a constant free spectral range.</li>
 <li>Choose the reference dataset (i.e. the first WGM fluorescence spectrum) from which all shifts will be measured.</li>
 <li>Interpolate the spectra to obtain uniform frequency spacing.18</li>
 <li>Perform a discrete Fourier transform to obtain the power and phase components of each spectrum.</li>
 <li>Find the phase differences for all k components of a given WGM spectrum for which the shift value is desired, with respect to the reference spectrum.</li>
 <li>Find the phase shift using the phase difference of either the main Fourier component only, or a weighted linear fit to the selected components. This will give the phase shift &delta;f or &delta;&lambda; between the WGM spectrum and the reference spectrum.</li>
 <li>The errors in the spectral shifts (the detection limit) can be measured by collecting repeated spectra for the same analyte. An uncertainty in the sensitivity can be obtained from the error in the weighted linear fits, if multiple components are used.</li>
 </ol>
 </li>
 </ol>
 </li>
</ol>
Repeat Steps 1-7 for every analysis. While this procedure sounds complicated, after the initial implementation the procedure is simple to automate, so that large data sets can be batch processed to find the shifts. We use a Mathematica code written specifically for this procedure, so that complete data sets can be batch processed "with the press of a button". In principle, the spectral shifts can even calculated "live", although we have not done this yet.

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We have nothing to disclose.

Fluorescent-core microcavities can be used as refractometric sensors. While there are isolated examples of "rolled up" microtubes that could act as microfluidic sensors,22 compared to microtubes, capillaries will be easier to integrate into microfluidic setups and have considerable practical advantages, since they are easily handled and simple to interface with an analysis setup. Using conventional Fourier analysis methods, wavelength shifts that are at least an order of magnitude smaller than the pitc...

Chemical sensing systems that require only small sample volumes and that can be incorporated into hand-held or field-operable devices could lead to the development of a wide range of new technologies. Such technologies could include field diagnostics for diseases and pathogens,1 environmental contaminants,2 and food safety.3 Several technologies are being actively explored for microfluidic chemical sensors, with devices based on the physics of surface plasmon resonances (SPR) among the most advanced.4 These sensors are now capable of detecting many specific biomolecules and have achieved commercial success, although mainly as larger-scale lab equipment.5
In recent years, optical microcavities have risen to compete with SPR-based systems. Microcavities can be amazingly sensitive, with demonstrated ability to detect single viruses6 and perhaps even single biomolecules7 (the latter remains the subject of some debate,8 however there is no doubt that the mass detection limits are small9 ). In microcavities, the detection mechanism relies upon changes in the optical resonances caused by the presence of an analyte within the electric field profile of the resonance. Typically, a given analyte will cause the resonance to change in in central frequency, visibility, or linewidth. As with SPR systems, microcavities can act as non-specific refractometric sensors, or as biosensors functionalized for a specific analysis.
Dielectric microstructures with a circular cross section (e.g. microspheres, disks, or cylinders) are characterized by electromagnetic resonances known as the whispering gallery modes, or WGMs, a term dating back to Lord Rayleigh's investigations of analogous acoustic effects.10 Essentially, an optical WGM occurs when a wave circumnavigates the circular cross section by total internal reflection, and returns to its starting point in phase. An example of an electromagnetic resonance for a silica microsphere is illustrated in Figure 1a. This resonance is characterized by one maximum in the radial direction (n = 1), while a total of 53 wavelengths fit around the equator (l = 53), only some of which are shown. The evanescent part of the field intensity extends into the medium outside the sphere boundary; thus the microsphere WGM can sense the external medium.
Capillaries are an especially interesting example of a WGM-based sensor. In a capillary, cylindrical WGMs can form around the circular cross section, similar to the case for a sphere. If the capillary wall is very thin, part of the electromagnetic field extends into the capillary channel (Figure 1b). Thus, a capillary can be a microfluidic sensor for analytes injected into the channel. This is the basis of operation of the liquid core optical ring resonator (LCORR).11 LCORRs rely on the evanescent coupling of light from a precision tuneable laser source to probe the WGMs. An important aspect of the LCORR is that the capillary walls must be thin (~1 &mu;m) to ensure that the mode samples the channel medium. This places some difficulties on their fabrication and causes them to be mechanically fragile.
In our work, we have developed an alternative structure we call a fluorescent core microcavity (FCM).12,13 To form an FCM, we coat the channel walls of a capillary with a high-refractive-index fluorophore (specifically, a layer of oxide-embedded silicon quantum dots). The high index of the film is required to confine the emitted radiation, thereby building up the WGMs (Figure 1c). In contrast to the LCORR, in an FCM the modes appear as sharp maxima in an emitted fluorescence spectrum. The thickness of the film is critically important; if it is too thick the WGM does not sample the medium in the capillary channel, and if it is too thin the optical confinement is lost and the WGMs become weak. Thus, the fabrication of an FCM is a difficult process, requiring careful preparation. This is the main topic of the current paper.

<table border="1">
<tbody>
 <tr>
 <td>Table of Materials</td>
 <td>Company</td>
 <td>Catalog #</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>silica microcapillaries</td>
 <td></td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>flexible microbore tubing</td>
 <td></td>
 <td></td>
 <td>polyethylene, tygon, etc</td>
 </tr>
 <tr>
 <td>adhesive</td>
 <td>Mascot, Norland NOA</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>HSQ dissolved in MIBK</td>
 <td></td>
 <td></td>
 <td>e.g., FOx-15</td>
 </tr>
 <tr>
 <td>methanol</td>
 <td></td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>ethanol</td>
 <td></td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>distilled water</td>
 <td></td>
 <td></td>
 <td></td>
 </tr>
 </tbody>
</table>
Table 1. List of materials used.

synthesis and operation of fluorescent-core microcavities for refractometric sensing

This paper discusses fluorescent core microcavity-based sensors that can operate in a microfluidic analysis setup. These structures are based on the formation of a fluorescent quantum-dot (QD) coating on the channel surface of a conventional microcapillary. Silicon QDs are especially attractive for this application, owing in part to their negligible toxicity compared to the II-VI and II-VI compound QDs, which are legislatively controlled substances in many countries. While the ensemble emission spectrum is broad and featureless, an Si-QD film on the channel wall of a capillary features a set of sharp, narrow peaks in the fluorescence spectrum, corresponding to the electromagnetic resonances for light trapped within the film. The peak wavelength of these resonances is sensitive to the external medium, thus permitting the device to function as a refractometric sensor in which the QDs never come into physical contact with the analyte. The experimental methods associated with the fabrication of the fluorescent-core microcapillaries are discussed in detail, as well as the analysis methods. Finally, a comparison is made between these structures and the more widely investigated liquid-core optical ring resonators, in terms of microfluidic sensing capabilities.

Small deviations in the capillary fabrication procedure can lead to significant changes in the sample success rate. In Figure 5(a-d), we show representative examples of failed capillaries as well as a successful one. Generally, the visual indication of a successful sample is a red fluorescence combined with a high intensity at the capillary walls and a featureless interior. The fluorescence spectrum also clearly indicates the difference between success and failure (Figure 5e). A ...

Watch this Scientific Journal Video about Synthesis and Operation of Fluorescent-core Microcavities for Refractometric Sensing at JoVE.com

Synthesis and Operation of Fluorescent-core Microcavities for Refractometric Sensing

Fluorescent-core microcavity sensors employ a high-index quantum-dot coating in the channel of silica microcapillaries. Changes in the refractive index of fluids pumped into the capillary channel cause shifts in the microcavity fluorescence spectrum that can be used to analyze the channel medium.

This paper discusses  fluorescent core microcavity-based sensors that can operate in a microfluidic  analysis setup. These structures are based on the ...

synthesis-operation-fluorescent-core-microcavities-for-refractometric

Research

JoVE Journal

Engineering

12.8K Aufrufe.  University of Alberta. Fluoreszenzlichtbild-Kern Mikrokavität Sensoren verwenden eine hochbrechende Quantenpunkt-Beschichtung in dem Kanal von Siliciumdioxid Mikrokapillaren. Änderungen im Brechungsindex der Flüssigkeit in den kapillaren Kanal gepumpt verschiebt Ursache in der Mikrokavität Fluoreszenzspektrum, mit dem der Kanal Medium analysiert werden können.

Serie: Synthesis and Operation of Fluorescent-core Microcavities for Refractometric Sensing

Fabrication, Operation and Flow Visualization in Surface-acoustic-wave-driven Acoustic-counterflow Microfluidics

Surface acoustic waves (SAWs) can be used to drive liquids in portable microfluidic chips via the acoustic counterflow phenomenon. In this video we present the fabrication protocol for a multilayered SAW acoustic counterflow device. The device is fabricated starting from a lithium niobate (LN) substrate onto which two interdigital transducers (IDTs) and appropriate markers are patterned. A polydimethylsiloxane (PDMS) channel cast on an SU8 master mold is finally bonded on the patterned substrate. Following the fabrication procedure, we show the techniques that allow the characterization and operation of the acoustic counterflow device in order to pump fluids through the PDMS channel grid. We finally present the procedure to visualize liquid flow in the channels. The protocol is used to show on-chip fluid pumping under different flow regimes such as laminar flow and more complicated dynamics characterized by vortices and particle accumulation domains.

In this video we first describe fabrication and operation procedures of a surface acoustic wave (SAW) acoustic counterflow device. We then demonstrate an experimental setup that allows for both qualitative flow visualization and quantitative analysis of complex flows within the SAW pumping device.

Surface acoustic waves (SAWs) can be used to drive liquids in portable microfluidic chips via the acoustic counterflow phenomenon. In this video we ...

Fabrication and Operation of a Nano-Optical Conveyor Belt

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological and physical sciences over the past few decades. The progress made in this field invites further study of even smaller systems and at a larger scale, with tools that could be distributed more easily and made more widely available. Unfortunately, the fundamental laws of diffraction limit the minimum size of the focal spot of a laser beam, which makes particles smaller than a half-wavelength in diameter hard to trap and generally prevents an operator from discriminating between particles which are closer together than one half-wavelength. This precludes the optical manipulation of many closely-spaced nanoparticles and&#160;limits the&#160;resolution of optical-mechanical systems. Furthermore, manipulation using focused beams requires beam-forming or steering optics, which can be very bulky and expensive. To address these limitations in the system scalability of conventional optical trapping our lab has devised an alternative technique which utilizes near-field optics to move particles across a chip. Instead of focusing laser beams in the far-field, the optical near field of plasmonic resonators produces the necessary local optical intensity enhancement to overcome the restrictions of diffraction and manipulate particles at higher resolution. Closely-spaced resonators produce strong optical traps which can be addressed to mediate the hand-off of particles from one to the next in a conveyor-belt-like fashion. Here, we describe how to design and produce a conveyor belt using a gold surface patterned with plasmonic C-shaped resonators and how to operate it with polarized laser light to achieve super-resolution nanoparticle manipulation and transport. The nano-optical conveyor belt chip can be produced using lithography techniques and easily packaged and distributed.

The scalability and resolution of conventional optical manipulation techniques are limited by diffraction. We circumvent the diffraction limit and describe a method of optically transporting nanoparticles across a chip using a gold surface patterned with a path of closely spaced C-shaped plasmonic resonators.

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological ...

Fabrication and Operation of Acoustofluidic Devices Supporting Bulk Acoustic Standing Waves for Sheathless Focusing of Particles

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must be smaller than the incident wavelength of sound and the width of the fluidic channels are typically tens to hundreds of micrometers across, acoustofluidic devices typically use ultrasonic waves generated from a piezoelectric transducer pulsating at high frequencies (in the megahertz range). At characteristic frequencies that depend on the geometry of the device, it is possible to induce the formation of standing waves that can focus particles along desired fluidic streamlines within a bulk flow. Here, we describe a method for the fabrication of acoustophoretic devices from common materials and clean room equipment. We show representative results for the focusing of particles with positive or negative acoustic contrast factors, which move towards the pressure nodes or antinodes of the standing waves, respectively. These devices offer enormous practical utility for precisely positioning large numbers of microscopic entities (e.g., cells) in stationary or flowing fluids for applications ranging from cytometry to assembly.

Acoustofluidic devices use ultrasonic waves within microfluidic channels to manipulate, concentrate and isolate suspended micro and nanoscopic entities. This protocol describes the fabrication and operation of such a device supporting bulk acoustic standing waves to focus particles in a central streamline without the aid of sheath fluids.

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must ...

Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic nanoscale patch antennas. This includes a detailed procedure for the fabrication of thin films with a well-controlled thickness over macroscopic areas using layer-by-layer deposition of polyelectrolyte polymers, namely poly(allylamine) hydrochloride (PAH) and polystyrene sulfonate (PSS). These polyelectrolyte spacer layers serve as a dielectric gap in between silver nanocubes and a gold film. By controlling the size of the nanocubes or the gap thickness, the plasmon resonance can be tuned from about 500 nm to 700 nm. Next, we demonstrate how to incorporate organic sulfo-cyanine5 carboxylic acid (Cy5) dye molecules into the dielectric polymer gap region of the nanopatch antennas. Finally, we show greatly enhanced fluorescence of the Cy5 dyes by spectrally matching the plasmon resonance with the excitation energy and the Cy5 absorption peak. The method presented here enables the fabrication of plasmonic nanopatch antennas with well-controlled dimensions utilizing colloidal synthesis and a layer-by-layer dip-coating process with the potential for low cost and large-scale production. These nanopatch antennas hold great promise for practical applications, for example in sensing, ultrafast optoelectronic devices and for high-efficiency photodetectors.

A protocol for the colloidal synthesis of silver nanocubes and fabrication of plasmonic nanoscale patch antennas with sub-10 nm gaps is presented.

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic ...

Construction and Operation of a Light-driven Gold Nanorod Rotary Motor System

The possibility to generate and measure rotation and torque at the nanoscale is of fundamental interest to the study and application of biological and artificial nanomotors and may provide new routes towards single cell analysis, studies of non-equilibrium thermodynamics, and mechanical actuation of nanoscale systems. A facile way to drive rotation is to use focused circularly polarized laser light in optical tweezers. Using this approach, metallic nanoparticles can be operated as highly efficient scattering-driven rotary motors spinning at unprecedented rotation frequencies in water.
In this protocol, we outline the construction and operation of circularly-polarized optical tweezers for nanoparticle rotation and describe the instrumentation needed for recording the Brownian dynamics and Rayleigh scattering of the trapped particle. The rotational motion and the scattering spectra provides independent information on the properties of the nanoparticle and its immediate environment. The experimental platform has proven useful as a nanoscopic gauge of viscosity and local temperature, for tracking morphological changes of nanorods and molecular coatings, and as a transducer and probe of photothermal and thermodynamic processes.

Plasmonic gold nanorods can be trapped in liquids and rotated at kHz frequencies using circularly-polarized optical tweezers. Introducing tools for Brownian dynamics analysis and light scatteringspectroscopy leads to a powerful system for research and application in numerous fields of science.

The possibility to generate and measure rotation and torque at the nanoscale is of fundamental interest to the study and application of biological and ...

Femtosecond Laser Filaments for Use in Sub-Diffraction-Limited Imaging and Remote Sensing

Probing remote matter with laser light is a ubiquitous technique used in circumstances as diverse as laser-induced breakdown spectroscopy and barcode scanners. In classical optics, the intensity that can be brought to bear on a remote target is limited by the spot size of the laser at the distance of the target. This spot size has a lower bound determined by the diffraction limit of classical optics. However, amplified femtosecond laser pulses generate intensity sufficient to modify the refractive index of the ambient air and undergo self-focusing. This self-focusing effect leads to the generation of highly intense laser filaments which maintain their intensity and small sub-millimeter diameter size at distances well beyond the classical Rayleigh length. Such intensity provides the capability of remote scanning, imaging, sensing, and spectroscopy with enhanced spatial resolution. We describe a technique for generating filaments with a femtosecond regenerative chirped-pulse amplifier, and for using the resulting filament to conduct imaging and spectroscopic measurements at remote distances of at least several meters.

High-intensity femtosecond pulses of laser light can undergo cycles of Kerr self-focusing and plasma defocusing, propagating an intense sub-millimeter-diameter beam over long distances. We describe a technique for generating and using these filaments to perform remote imaging and sensing beyond the classical diffraction limits of linear optics.

Probing remote matter with laser light is a ubiquitous technique used in circumstances as diverse as laser-induced breakdown spectroscopy and barcode ...

Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing

We present a protocol for performing gas spectroscopy using infrared degenerated four-wave mixing (IR-DFWM), for the quantitative detection of gas species in the ppm-to-single-percent range. The main purpose of the method is the spatially resolved detection of low-concentration species, which have no transitions in the visible or near-IR spectral range that could be used for detection. IR-DFWM is a nonintrusive method, which is a great advantage in combustion research, as inserting a probe into a flame can change it drastically. The IR-DFWM is combined with upconversion detection. This detection scheme uses sum-frequency generation to move the IR-DFWM signal from the mid-IR to the near-IR region, to take advantage of the superior noise characteristics of silicon-based detectors. This process also rejects most of the thermal background radiation. The focus of the protocol presented here is on the proper alignment of the IR-DFWM optics and on how to align an intracavity upconversion detection system.

Here, we present a protocol to perform sensitive, spatially resolved gas spectroscopy in the mid-infrared region, using degenerate four-wave mixing combined with upconversion detection.

We present a protocol for performing gas spectroscopy using infrared degenerated four-wave mixing (IR-DFWM), for the quantitative detection of gas species ...

Synthesis, Functionalization, and Characterization of Fusogenic Porous Silicon Nanoparticles for Oligonucleotide Delivery

With the advent of gene therapy, the development of an effective in vivo nucleotide-payload delivery system has become of parallel import. Fusogenic porous silicon nanoparticles (F-pSiNPs) have recently demonstrated high in vivo gene silencing efficacy due to its high oligonucleotide loading capacity and unique cellular uptake pathway that avoids endocytosis. The synthesis of F-pSiNPs is a multi-step process that includes: (1) loading and sealing of oligonucleotide payloads in the silicon pores; (2) simultaneous coating and sizing of fusogenic lipids around the porous silicon cores; and (3) conjugation of targeting peptides and washing to remove excess oligonucleotide, silicon debris, and peptide. The particle&#8217;s size uniformity is characterized by dynamic light scattering, and its core-shell structure may be verified by transmission electron microscopy. The fusogenic uptake is validated by loading a lipophilic dye, 1,1&#39;-dioctadecyl-3,3,3&#39;,3&#39;-tetramethylindocarbocyanine perchlorate (DiI), into the fusogenic lipid bilayer and treating it to cells in vitro to observe for plasma membrane staining versus endocytic localizations. The targeting and in vivo gene silencing efficacies were previously quantified in a mouse model of Staphylococcus aureus pneumonia, in which the targeting peptide is expected to help the F-pSiNPs to home to the site of infection. Beyond its application in S. aureus infection, the F-pSiNP system may be used to deliver any oligonucleotide for gene therapy of a wide range of diseases, including viral infections, cancer, and autoimmune diseases.

We demonstrate the synthesis of fusogenic porous silicon nanoparticles for effective in vitro and in vivo oligonucleotide delivery. Porous silicon nanoparticles are loaded with siRNA to form the core, which is coated by fusogenic lipids through extrusion to form the shell. Targeting moiety functionalization and particle characterization are included.

With the advent of gene therapy, the development of an effective in vivo nucleotide-payload delivery system has become of parallel import. Fusogenic ...

Robotic Sensing and Stimuli Provision for Guided Plant Growth

Robot systems are actively researched for manipulation of natural plants, typically restricted to agricultural automation activities such as harvest, irrigation, and mechanical weed control. Extending this research, we introduce here a novel methodology to manipulate the directional growth of plants via their natural mechanisms for signaling and hormone distribution. An effective methodology of robotic stimuli provision can open up possibilities for new experimentation with later developmental phases in plants, or for new biotechnology applications such as shaping plants for green walls. Interaction with plants presents several robotic challenges, including short-range sensing of small and variable plant organs, and the controlled actuation of plant responses that are impacted by the environment in addition to the provided stimuli. In order to steer plant growth, we develop a group of immobile robots with sensors to detect the proximity of growing tips, and with diodes to provide light stimuli that actuate phototropism. The robots are tested with the climbing common bean, Phaseolus vulgaris, in experiments having durations up to five weeks in a controlled environment. With robots sequentially emitting blue light-peak emission at wavelength 465 nm-plant growth is successfully steered through successive binary decisions along mechanical supports to reach target positions. Growth patterns are tested in a setup up to 180 cm in height, with plant stems grown up to roughly 250 cm in cumulative length over a period of approximately seven weeks. The robots coordinate themselves and operate fully autonomously. They detect approaching plant tips by infrared proximity sensors and communicate via radio to switch between blue light stimuli and dormant status, as required. Overall, the obtained results support the effectiveness of combining robot and plant experiment methodologies, for the study of potentially complex interactions between natural and engineered autonomous systems.

Distributed robot nodes provide sequences of blue light stimuli to steer the growth trajectories of climbing plants. By activating natural phototropism, the robots guide the plants through binary left-right decisions, growing them into predefined patterns that by contrast are not possible when the robots are dormant.

Robot systems are actively researched for manipulation of natural plants, typically restricted to agricultural automation activities such as harvest, ...

Operation of the Collaborative Composite Manufacturing (CCM) System

The automated tape placement and the automated fiber placement (AFP) machines provide a safer working environment and reduce the labor intensity of workers than the traditional manual fiber placement does. Thus, the production accuracy, repeatability and efficiency of composite manufacturing are significantly improved. However, the current AFP systems can only produce the composite components with large open surface or simple revolution parts, which cannot meet the growing interest in small complex or closed structures from industry.
In this research, by employing a 1-degree of freedom (DoF) rotational stage, a 6-RSS parallel robot, and a 6-DoF serial robot, the dexterity of the AFP system can be significantly improved for manufacturing complex composite parts. The rotational stage mounted on the parallel robot is utilized to hold the mandrel and the serial robot carries the placement head to mimic two human hands that have enough dexterity to lay the fiber to the mandrel with complex contour.
Although the CCM system increases the flexibility of composite manufacturing, it is quite time-consuming or even impossible to generate the feasible off-line path, which ensures uniform lay-up of subsequent fibers considering the constraints like singularities, collisions between the fiber placement head and mandrel, smooth fiber direction change and keeping the fiber placement head along the norm of the part&#39;s surface, etc. Moreover, due to the existing positioning error of the robots, the on-line path correction is needed. Therefore, the on-line pose correction algorithm is proposed to correct the paths of both parallel and serial robots, and to keep the relative path between the two robots unchanged through the visual feedback when the constraint or singularity problems in the off-line path planning occur. The experimental results demonstrate the designed CCM system can fulfill the movement needed for manufacturing a composite structure with Y-shape.

Operation of the Collaborative Composite Manufacturing CCM System

A collaborative composite manufacturing system is developed for robotic lay-up of composite laminates using the prepreg tape. The proposed system allows the production of composite laminates with high levels of geometrical complexity. The issues in the path planning, coordination of the robots and control are addressed in the proposed method.

The automated tape placement and the automated fiber placement (AFP) machines provide a safer working environment and reduce the labor intensity of ...