Patent Grant
7798164
Not Applicable
The present invention relates generally to microfluidic control techniques. In particular, the present invention provides a method of plasmon assisted optofluidics using a laser. More particularly, the present invention provides a method for optically controlling fluid in a microchannel using a plasmon resonance in fixed arrays of nanoscale metal structures to produce localized evaporation of the fluid when illuminated by a stationary, low power laser. Merely by way of example, the invention has been applied to drag the surface of the fluid, drive evaporative pumping, and provide intra-channel distillation and sample concentration, but it would be recognized that the invention has a much broader range of applicability.
Current microfluidics is realized through pumping, which is an excellent means for transport, mixing, and metering. Ideally, this and other complex functionalities would occur directly on-chip. However, the majority of microfluidic systems employ off-chip, mechanical pumps combined with valve networks to direct fluid flow. Electro-kinetic transport can be more compact and flexible, but it depends on liquid conductivity and requires large voltages and a fabrication method that integrates the fluidic and electronic circuitry. Electrowetting based devices have great utility, but are most naturally limited to discrete, droplet based devices.
Recently there has been increased interest in using optical transport methods for microfluidics. This approach uses optical beams to induce flow without connected pumps or electrical circuitry. An example is photothermal transport by resonant heating of nanoparticles in solution, which can be used to control the position of the free surface of a fluid along a complex circuit without the need for valves. Although it can be arbitrarily applied anywhere on a chip, however, this method requires that the optical beam be translated to transport the fluid. Furthermore, it may not be desirable or possible to have nanoparticles freely suspended in liquid solution, because the changing concentration of the suspended nanoparticles makes difficult for controlling the flow rate for a given laser power.
Another aspect regarding the fluid pumping in a microchannel involves interphase mass transfer. A conventional method uses a series of heaters, which are typically embedded in the channel, to produce a vapor bubble as well as a thermal gradient between the two ends of the bubble. Mass-transfer occurs as fluid on the warmer interface is vaporized and then condensed on the cooler side. In addition to pumping, vapor mass-transfer provides a simple means to separate both soluble and insoluble components of a mixture. However, although it can be applied on-chip, this method requires the high temperatures to create and to prevent the collapse of the vapor bubble and precludes many applications, especially biological ones.
From above, it is seen that there is a need in the art for an improved method and system for controlling fluid in a microchannel structure with on-chip functionality for pumping, distillation, and sample concentration based on ambient temperature interphase mass-transfer.
The present invention relates generally to microfluidic control techniques. In particular, the present invention provides a method of plasmon assisted optofluidics using a laser. More particularly, the present invention provides a method for optically controlling fluid in a microchannel using a plasmon resonance in fixed arrays of nanoscale metal structures to produce localized evaporation of the fluid when illuminated by a stationary, low power laser. Merely by way of example, the invention has been applied to drag the surface of the fluid, drive evaporative pumping, and provide intra-channel distillation and sample concentration, but it would be recognized that the invention has a much broader range of applicability.
In a specific embodiment, the present invention provides a method of microfluidic control using plasmon assisted heating. The method includes providing a microchannel structure with a base region. The microchannel structure is partially filled with a volume of liquid and a gas at an ambient temperature. The volume of liquid and the gas are separated by a liquid-gas interface region at a first position of the microchannel structure. The base region includes one or more physical structures. Additionally, the method includes supplying energy input to a portion of the one or more physical structures within the volume of liquid in a vicinity of the liquid-gas interface region to cause localized heating of the portion of the one or more physical structures. The method further includes transferring heat from the portion of the one or more physical structures to surrounding liquid in the vicinity of the liquid-gas interface region. Furthermore, the method includes generating an interphase mass transport at the liquid-gas interface region in the microchannel structure. The volume of liquid and the gas remain to be substantially at the ambient temperature during the interphase mass transport.
In another specific embodiment, the present invention provides a method of plasmon resonance assisted microfluidic pumping. The method includes providing a vessel partially filled with a first volume of liquid. The first volume of liquid is separated from a gas by a first liquid-gas interface region. The vessel characterized in micrometer scale includes a base region, a width, and a height. The base region includes an array of nanometer structures associated with a plasmon resonance frequency range. Additionally, the method includes illuminating a laser beam on a portion of the array of nanometer structures within the first volume of liquid substantially near the first liquid-gas interface region. The laser beam is characterized by a power level and a determined frequency within the plasmon resonance frequency range to cause plasmon resonance excitation of the portion of the array of nanometer structures. The method further includes entrapping a gas bubble in the vessel by forming a second volume of liquid at a distance in front of the first liquid-gas interface region through evaporation and recondensation during an energy transfer facilitated by the plasmon resonance excitation. The gas bubble is bounded by the first liquid-gas interface region, surrounding inner walls of the vessel, and a second liquid-gas interface region associated with the second volume of liquid. Furthermore, the method includes generating a mass transport in the vessel across the gas bubble from first liquid-gas interface region to the second liquid-gas interface region.
In certain embodiment, generating a mass transport in the vessel across the gas bubble from first liquid-gas interface region to the second liquid-gas interface region further includes a step of illuminating the laser beam on the portion of the array of nanometer structures within the first volume of liquid near the first liquid-gas interface region; and a step of transforming heat at least partially to a latent heat of evaporation of a portion of the first volume of liquid at the first liquid-gas interface region while keeping temperature increase of the portion of the first volume of liquid less than 2 degrees of Centigrade; and a step of converting the portion of the first volume of liquid to a vapor into the gas bubble; and a step of thereafter condensing the vapor at the second liquid-gas interface region. In one embodiment, the laser beam is substantially stationary relative to the vessel and the first liquid-gas interface region. In another embodiment, the gas bubble keeps a substantially stable size defined by a spacing between the first liquid-gas interface region and the second liquid-gas interface region during the mass transport in the vessel after an earlier shrinkage within a certain amount of time of illuminating the laser beam. In yet another embodiment, the stable size of the gas bubble corresponds to a steady state pumping rate for the mass transport from the first volume of liquid to the second volume of liquid. In yet still another embodiment, the steady state pumping rate is substantially constant with time and linear with the power level of laser beam.
In an alternative embodiment, the present invention provides a method of concentrating a volume of liquid mixture in a microfluidic system. The method includes providing a vessel partially filled with a first volume of liquid mixture separated from a gas by a first liquid-gas interface region. The liquid mixture includes at least a first substance in a first concentration and a second substance in a second concentration. The first substance is characterized by a first volatility and the second substance is characterized by a second volatility. The second volatility is less than the first volatility. The vessel characterized in micrometer scale includes a base region. The base region including an array of nanometer structures associated with a plasmon resonance frequency range. Additionally, the method includes illuminating a laser beam on a portion of the array of nanometer structures within the first volume of liquid mixture substantially near the first liquid-gas interface region. The laser beam is characterized by a determined frequency within the plasmon resonance frequency range to cause plasmon resonance excitation of the portion of the array of nanometer structures. The method further includes entrapping a gas bubble in the vessel by forming a second volume of liquid mixture at a distance in front of the first liquid-gas interface region through evaporation and recondensation during an energy transfer facilitated by the plasmon resonance excitation. The gas bubble is bounded by the first liquid-gas interface region, surrounding inner walls of the vessel, and a second liquid-gas interface region associated with the second volume of liquid mixture. Moreover, the method includes illuminating the laser beam on a portion of the array of nanometer structures within the first volume of liquid mixture substantially near the first liquid-gas interface region to generate a first mass flow for the first substance with a first flow rate and a second mass flow for the second substance with a second flow rate in the vessel across the gas bubble from first volume of liquid mixture to the second volume of liquid mixture. The first flow rate is higher than the second flow rate. The method further includes concentrating the second substance in the first volume of liquid mixture while maintaining the first volume of liquid mixture substantially at an ambient state during fractional increase of the second concentration and decrease of the first concentration. Furthermore, the method includes distillating the first substance in the second volume of liquid mixture being substantially free of the second substance.
In another alternative embodiment, the present invention provides a method of concentrating a substance within a volume of liquid in a microfluidic system. The method includes providing a vessel partially filled with a first volume of liquid separated from air by a first liquid-air interface region in an ambient state. The first volume of liquid includes a first concentration of a substance characterized as a plurality of suspended molecules. The vessel characterized in micrometer scale includes a base region. The base region includes an array of metal nanoparticles associated with a plasmon resonance frequency range. Additionally, the method includes illuminating a laser beam on a portion of the array of metal nanoparticles within the first volume of liquid substantially near the first liquid-air interface region. The laser beam is characterized by a determined frequency within the plasmon resonance frequency range to cause plasmon resonance excitation of the portion of the array of metal nanoparticles. The method further includes entrapping an air bubble in the vessel by forming a second volume of liquid at a distance in front of the first liquid-air interface region through liquid evaporation and recondensation during an energy transfer facilitated by the plasmon resonance excitation. The air bubble is bounded by the first liquid-air interface region, surrounding inner walls of the vessel, and a second liquid-air interface region associated with the second volume of liquid. Moreover, the method includes illuminating the laser beam on a portion of the array of metal nanoparticles within the first volume of liquid substantially near the first liquid-air interface region to generate a mass flow for the liquid in the vessel across the air bubble from the first liquid-air interface region to the second liquid-air interface region. Furthermore, the method includes concentrating the substance suspended within the first volume of liquid to increase the first concentration to a second concentration while maintaining the first volume of liquid substantially at an ambient state.
Many benefits are achieved by way of the present invention over conventional techniques. For example, the present invention provides a new class of on-chip functionality for microfluidics based on ambient temperature interphase mass-transfer. Embodiments of the present invention avoid high temperatures by using of the freedom provided by microfluidics to heat liquid in the immediate vicinity of a liquid-vapor interface. In some embodiments, only a small change in the temperature, for example less than 2 degree of Centigrade, of the fluid is required for the observed mass-transfer rates. Another advantage of the present invention lies in using plasmon assisted heating by illuminating a laser beam and is highly controllable. Certain embodiments of the present invention provide an array of nano-metal particles fixed or embedded in the base region of the microchannel structure by taking advantage of well-established soft lithography technique for easy fabrication of large-scale and quasi-ordered nanostructures. The embedded nanostructures offers a natural on-chip functionality to provide controllable plasmonic heating through plasmon resonance excitation by a laser beam. In addition, unlike other optical transport methods, it does not require translation of the laser beam. By using a novel bubble assisted interface mass-transfer method a stationary and constant powered laser beam can be used to induce plasmonic heating and produce a stable mass flow rate. Advances in microelectronic fabrication should allow for integration of microlasers on chip, and when combined with the present invention to minimize inconsistencies related to the distance of spot position and the surface of the gas bubble will allow opto-controlled microfluidic system to be successfully scaled on microchip. The present invention further provides a simple on-chip means for microfluidic pumping, distillation, and sample concentration. The technique is general and the functionality that it offers can be integrated with conventional microfluidic architectures and is believed to have a much broader range of applicability.
The present invention relates generally to microfluidic control techniques. In particular, the present invention provides a method of plasmon assisted optofluidics using a laser. More particularly, the present invention provides a method for optically controlling fluid in a microchannel using a plasmon resonance in fixed arrays of nanoscale metal structures to produce localized evaporation of the fluid when illuminated by a stationary, low power laser. Merely by way of example, the invention has been applied to drag the surface of the fluid, drive evaporative pumping, and provide intra-channel distillation and sample concentration, but it would be recognized that the invention has a much broader range of applicability.
Here we demonstrate a technique of plasmon assisted optofluidics (PAO) according to certain embodiments of the present invention. By incorporating plasmonic resonant structures into a microscale vessel channel, some embodiments show that plasmonic heating allows for dragging of the free surface of the fluid within the vessel channel using a focused, low power laser near a plasmon resonant frequency associated with the plasmonic resonant structures. Furthermore, using PAO certain embodiments of the present invention show methods for on-chip intra-channel pumping and distillation.
To prove the principles and operation of the present invention, we performed various experiments. These experiments have been used to demonstrate the invention and certain benefits associated with the invention. As experiments, they are merely examples, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. Details of these experiments are provided below.
In certain embodiments, the maximum dragging speeds are found to be sensitive to the preparation of the substrate. Substrates were rendered 1) highly hydrophobic by treating in hexamethyldisilazane (HDMS) vapor, or 2) hydrophilic by oxygen plasma cleaning. For the hydrophobic channels the maximum dragging rates were not consistent and were slow, typically 5 μm/s, regardless of channel size. This was due to entrapment of air immediately behind the laser as it scanned, which interrupted the fluid motion. The more hydrophilic substrates were able to support higher speeds. When we allowed the hydrophobic channels to age for 2-3 days, we found that their behavior began to resemble that of the hydrophilic channels, i.e. higher maximum flow rates and less occurrence of trapped air bubbles.
In other embodiments, the inherent dragging rate increases with increasing laser power used for illuminating the nanoparticle array. In another embodiment, for a given laser power the dragging rate will also be affected by the optical absorbance of the nanoparticle array, which is directly related to the particle size and the inter-particle spacing. Throughout these experiments, arrays with an average particle diameter of about 15 nm and an average inter-particle spacing of about 50 nm are used. The corresponding optical absorbance spectrum of such a typical array is shown in
In yet another embodiment, the microfluidic dragging can be combined with a microfluidic pumping process, which will be shown in more details in later sections of this specification. For example, we were also able to drag the fluid around corners using combined dragging and pumping based on the plasmon assisted microfluidics techniques according to certain embodiments of the present invention.
In another specific embodiment, the present invention is advantageous over conventional approach by using fixed arrays of nanoparticles to drive the heating instead of suspending nanoparticles randomly in a liquid solution. The advantages relies on the abilities to both spatially pattern the substrate with the nanoparticle arrays using standard lithographic techniques and combine patterning with particles of different resonances. Additional advantages of using fixed array of nanoparticles also allow the creation of a selectable y-junction for mixing where each branch is resonant at a unique wavelength and allow absorbed laser power by these fixed nanoparticles to remain constant during evaporation for achieving a controllable fluid pumping speed during microfluidic control operation. With nanoparticles in solution in some conventional approaches, there would be an increase in the particle density with evaporation and a corresponding increase in the optical absorption. These convention approach would make it difficult to have constant pumping rates for a given laser power, complicate the control of distillation, and prevent sequential distillation steps.
To gain insight into the evaporative mass-transfer mechanism, it is useful to consider a few simple numerical estimates. In one embodiment, we assume equilibrium conditions at the vapor-liquid interface, a constant pressure inside the bubble, and a constant temperature of 25° C. The power P required for evaporative transport is given by P=JΔH, where ΔH is the latent heat of vaporization, which for water at 25° C., ΔH 2.4×106 J/kg. A flow of 5 μm/s of water in a channel 30×5 μm corresponds to J=7.5×10−13 kg/s. The necessary input power P is 1.8 μW. In an embodiment, the measured absorbance A of the nanoparticle arrays at 532 nm is 0.028, and we assume that the scattering from the array of particles is small and that all of the absorbed energy is converted to heat. For 10 mW of input power, this gives 624 μW of power absorbed by the gold nanoparticles, indicating that there is sufficient laser energy available to account for the observed mass-transfer. Clearly the pumping efficiency is low. However, this estimate does not account for temperature changes that would take place in the fluid or the significant heat transfer to the glass substrate and PDMS channel. The estimation results shown here are only for illustrating that the rates of evaporative mass-transfer of the order required for our results. Certain embodiments of the present invention also demonstrate that combined with an appropriate heat transfer model, plasmon assisted evaporative mass-transfer pumping could provide a simple method for studying plasmonic heating.
As demonstrated in above examples, we have presented a method of microfluidic control with an all-optical technique using plasmon resonance heating of an array of nanoscale metal structures embedded within the fluid.
In one example, the microchannel structure or simply fluidic channels can be formed using soft lithography techniques by casting of PDMS (10:1 GE-RTV615 A:B). Replica molds are created through contact lithography of a positive photoresist (SPR 220-7, Michrochem). The fabricated microchannels had widths of 20 μm, 30 μm, 40 μm, and 60 μm, and measured heights 5 μm. The formed PDMS channels are then peeled away from the molds after curing for 30 minutes at 80° C. The PDMS chips are washed in ethanol and their surfaces are cleaned using cellophane tape (Scotch brand). Chips were placed in contact with the prepared substrate bases and examined for blockages, air bubbles, or other imperfections under 100× magnification. Chips with clean, unblocked channels were baked for at least 4 hours at 80° C. to form a strong reversible bond between the PDMS and the substrate base. The formed PDMS microchannels are optical transparent.
The substrate base or the base region for the PDMS microchannel can be a dielectric material that is also optical transparent. For example, the substrate is a glass slide. In a specific embodiment, the base region is simply a pre-treated glass substrate on which the one or more physical structures characterized in nanometer scale are prefabricated as an quasi-ordered Au nanoparticle array with an average diameter of about 15 nm and an average inter-particle spacing of about 50 nm. The Au nanoparticle array can be fabricated by the block copolymer lithography (BCPL) method. In one example, a mixture of 25.4 mg of the diblock copolymer [polystyrene81,000-block-poly(2-vinylpyridine)14,200 (Polymer Source, Inc.)] and 5 ml of toluene is stirred in a nitrogen purged and dark environment and stirred overnight, about 8 mg of HAuCl4H2O are added, and this solution is stirred for 90 hours. The solution is then spun on to a glass microscope slide and allowed to dry. The substrate is further treated in an oxygen plasma for 10 minutes at 75 W. The substrates are then treated in an adhesion promoting vapor (hexa-dimethyl-siloxane 100% 2 min) to render them more hydrophobic and facilitate bonding with soft-fluidic structures. As an example,
Referring to
Referring again to
We further examined the plasmonic heating of the fluid using temperature sensitive fluorescence intensity measurements. As was mentioned earlier, the dye solution is temperature sensitive. The Coumarin 4 dye is itself pH sensitive, and the Tris buffer solution has a pH with a well-known temperature dependence. By warming the fluid we decrease the pH causing a decrease in the intensity of the fluorescence, which we calibrated to the temperature. The dye was excited with a 405 nm laser. A bandpass filter inserted before the CCD passed only the fluorescence from the fluid and blocked the both the 405 nm and 532 nm lasers. To prevent the evaporative effects allowed by a bubble, we examined continuous column of fluid without a bubble, i.e., a single surface or liquid-air interface. When the beam was placed in fluid away from the free liquid-air interface, we were not able to measure any significant temperature change to within 2° C., even directly in the beam spot, and when the laser beam was placed close to the free liquid-air interface, the rapid evaporation and condensation caused the free liquid-air interface to wet-forward. These results suggest that when the laser is placed close to free liquid-air interface, a portion of the energy imparted to the fluid by the plasmonic heating goes into latent heat of vaporization.
To estimate the temperature rise of a nanoparticle by laser heating of a nanoparticle by a CW laser we consider a model where the particle temperature is ultimately determined by the incident power density and the heat transfer from the nanoparticles to the substrate. In a specific embodiment, the temperature of a spherical particle due to a power density I0 in the steady state can be shown to be:
where Kabs is the efficiency absorption factor, which can be calculated from Mie scattering theory, for a particle of radius r0 and k∞ is the coefficient of thermal conductivity of the surrounding medium at the macroscopic equilibrium temperature T∞. Due to nanoscale effects that limit the heat transfer from a nanoparticle to a solid, in one embodiment, most of the heat generated by the plasmon heating in the nanoparticles is transferred to the surrounding fluid. For example, we set k∞ to be 0.65, and we use a value Kabs=1.5. From Equation 1, the rise in the temperature of nanoparticles is less than 2° C. Of course, these numbers are all approximate and are presented to demonstrate semi-quantitatively the heat transfer results are feasible. There can be many variations, alternatives, and modifications.
Referring back to
In another specific embodiment, the present invention introduces a process of captive gas bubble into a microchannel. Unlike a vapor bubble, a gas bubble bounded by the walls of the microchannel provides two stable phase boundaries without the need for heat input to form and maintain the phase separation. At equilibrium there is no net mass-transfer between the liquid and vapor phases. However by locally heating one interface, mass-transfer can occur in the same manner as a vapor bubble but by evaporation. In one embodiment, only a slight temperature difference between the free surfaces in such a bubble is necessary to produce sufficient mass-transfer for microfluidic pumping. In another embodiment, this mechanism allows for bio-compatible intra-channel distillation and the collection of suspended solids in a mixture. This process can be referred as bubble assisted interphase mass-transfer (BAIM) through this specification.
According to certain embodiments of the present invention, Localized heating is key to this process. In one specific embodiment, we present a microfluidic system where heating is provided by a stationary, low power laser. Evaporation, unlike boiling, is a surface phenomena, and microfluidics is naturally suited for accessing the liquid in the immediate vicinity of a free surface or liquid-vapor interface of a bubble. In this experiment, energy is added near the liquid-vapor interface, some of which goes directly into the latent heat of vaporization. The process is not exclusive to photothermal heating, for example, it may be replaced by resistive heating or magnetic resonance heating, however, as will be discussed later there are certain advantages of this heating technique.
In this experiment, the channels are cast in poly-dimethylsiloxane (PDMS) and sealed to a glass substrate coated with an array of Au nanoparticles, which is created by block-copolymer lithography. The average particle diameter is 14.5 nm with an average spacing of 46 nm. The channels range in width from 20 to 40 μm and the heights are all 5 μm. Unless noted otherwise, de-ionized water is used exclusively as the working fluid. A 532 nm laser, which is close to plasmon resonant frequency of the gold nanoparticle arrays, is focused through the glass substrate onto the gold nanoparticle layer. The power at the sample is 14 mW and the diameter of the beam spot is about 10 μm. A schematic side view of the microchannel system (simplified as a channel 120 over a base 100) is illustrated in
Gas bubbles can be formed in the liquid by trapping gas in the partially filled channel. In one embodiment, we placed the laser spot near the free surface of the liquid, causing local accelerated evaporation of the free surface and vapor recondenses on the channel walls at about 10-30 μm away from the surface. In one embodiment, the vapor selectively recondenses in the areas where there is already a nucleated water droplet. The droplets on the wall tend to grew together to form a continuous liquid plug, trapping a gas bubble with a width of 10-20 μm between the original free liquid-gas interface and the plug.
In certain experiments we examine the mass-transfer rates of the liquid by digitizing images of the channel using a color video camera. In particular, the position of the ‘free-surface’, i.e. the leading liquid-gas interface of the fluid column, for example the one located far right of the bubble in
In one embodiment, the steady state rate of bubble assisted interphase mass-transfer (BAIM) for a given laser power is constant with time.
A summary of the values of the mean free surface velocity
J=ρνA, where ρ is the density of water, ν is the measured velocity of the free surface and A is the cross sectional area of the channel. We expect higher values of
In another specific embodiment, the pumping rate monotonically increases with laser power.
In yet another specific embodiment, the pumping rate decreases with increasing distance between the position of the laser spot and the edge of the gas bubble.
In an alternative embodiment, the evaporative mass transfer through the bubble can also serve as method for distillation in microfluidic system. Distillation is an important and widely used application of interphase mass-transfer, but its use in microfluidics, especially with biological systems, is limited by the association with the relatively high temperatures used to create the vapor phase. Certain embodiments of the present invention provide a method for ambient temperature distillation in microfluidic system.
In yet another alternative embodiment, bubble assisted interphase mass-transfer (BAIM) induced by Plasmon resonance excitation using a laser can be applied to concentrate insoluble (suspended) components in liquid mixture, in particular for sample concentration. Conventional methods for sample-concentration include using membranes and electrokinetic trapping. Here we show that embodiments of the BAIM method is applicable to concentration over a large range of molecule or particle sizes: we are able to concentrate solids ranging from microns to nanometers, and it does not require that the solids be charged.
Additionally, embodiments of the present invention is also applicable to much smaller insoluble components, such as short strands of DNA.
The method 1300 further includes a process (1320) of illuminating a laser beam on a portion of the array of nanometer structures within the first volume of liquid mixture substantially near the first liquid-gas interface region. The laser beam is characterized by a determined frequency within the plasmon resonance frequency range to cause plasmon resonance excitation and accelerated heating of the portion of the array of nanometer structures. In a specific embodiment, a laser beam with 14 mW power and 532 nm in wavelength is illuminated and focused onto an array of gold nanoparticles coated on the base region of the micrometer scaled vessel. The laser wavelength is selected to be within the plasmon resonance absorption band corresponding to the array of gold nanoparticles with an average diameter of about 15 nm and an average inter-particle spacing of about 50 nm. Thus, the laser beam, which is displaced within 10 microns of the first liquid-gas interface region, can induce accelerated photo-absorption and subsequently causes localized heating of a portion of the array of gold nanoparticles under illumination of the laser beam.
The method 1300, referring to
Referring again to
Moreover, the method 1300 includes a process (1350) of concentrating the second substance in the first volume of liquid mixture while maintaining the first volume of liquid mixture substantially at an ambient state during fractional increase of the second concentration and decrease of the first concentration. furthermore, the method further includes distillating the first substance in the second volume of liquid mixture being substantially free of the second substance. In certain embodiments, the method 1300 has been demonstrated to be applicable in experiments shown in
For the experiments shown above, fluid was injected into the microchannels using a syringe and a length of Tygon tubing (Cole-Palmer ID 0.092 inches). Channels were partially filled so the air-liquid interface was near the center of the device. The distillation studies were performed using a mixture of 0.1 M Coumarin 4 dye (peak emission 420 nm) in pure ethanol, with a temperature dependent buffer of HCL and tris (hydroxymethyl) aminomethane (Tris buffer). The pH of this mixture was adjusted via titration with added buffer solution to the point of maximum sensitivity with temperature. The dye was excited using a 405 nm solid state laser (5 mW) focused to the approximate field of view of the camera. Fluorescence images were recorded through a band pass filter centered around 420 nm (Semrock) with an exposure of 15 s. The maximum temperature sensitivity was calibrated using a thermocouple and a Peltier cooler, and was determined to be around 2° C. Fluorescence quenching was linearly proportional to temperature over a range of 25-55° C. We did not observe significant photo-bleaching of the solution.
For the pumping measurements, edge detection techniques were implemented into Matlab to determine the position of the leading fluid edge in still frames captured every 5 seconds. Linear fits were constructed using linear fitting algorithms, which are built into Matlab. In the distillation studies, Matlab was used to compare the fluorescence intensities between images by taking the mean of identical regions of pixels in each image and using only the blue channel of the CCD image.
For vapor pumping and distillation measurements, images of the channel were captured every 5 s during pumping. The position of the free surface with time was determined from the images using Matlab's edge detection techniques and built in linear fitting algorithms. In the distillation, fluorescence intensity was compared between images by taking the mean of identical regions of pixels in each image, using only the blue channel of the image. We found that the flow-rate due to BAIM pumping was sensitive to variations in the location of the beam focus, as well as variations in input energy density. To minimize these effects during the pumping studies, we examined the change in flow rate due to changes in input power for a constant beam location. After forming a stable bubble, the laser was switched off, and the beam position was adjusted to be approximately 20 μm behind the air bubble, on the fluid filled channel. We ran the laser at full power for 1 minute and then introduced a neutral density filter without stopping the laser. We allowed the flow to proceed for another minute at the reduced power.
For DNA concentration measurements, solutions of oligomer were prepared from a lyophilized sample provided by Alpha DNA Inc. The supplied oligomers were 20 bases long, and were prepared with a 5′ modification of APC Cy5.5 dye (Glen Research). A concentrated stock solution was prepared by suspending the lyophilized DNA in TE buffer (pH 8.0). A working solution was prepared from the stock solution by addition of an annealing buffer (pH 8.0) to a final concentration of 160 nM. The working solution was injected in to a 30 μm wide microchannel. The fluorescence excitation source was a multimode He—Ne laser passed through a 633 nm bandpass filter (Edmund Optics). The power of the laser after the filter was measured at 10.7 mW. The laser spot was brought from beneath the sample directly onto the microchannel. The excitation flux through the channel was approximately 1×106 W/m2. Fluorescence measurements were performed by imaging the channel through a microscope with a 10× objective, using a monochrome video camera (Sony XC-710). A long-pass wavelength filter was inserted into the optical system before the camera to reduce the excitation light recorded (685 nm cut-off filter, Melles Griot). To avoid excessive photobleaching, fluorescence images were captured both prior to and immediately after the evaporation process only. An air bubble was formed using the 532 nm laser in the manner described in the text, and a small quantity of liquid was transported across the bubble (50 μm). An initial image was captured before further evaporative transport was performed, using an exposure time of 2 s. The excitation light was manually un-shuttered during exposure, and then re-shuttered while evaporative transport was resumed. The evaporative transport was performed for 5 minutes, after which the 532 nm laser was shuttered and another fluorescence exposure was captured. The fluorescence images were analyzed using Matlab.
Many benefits are achieved by way of the present invention over conventional techniques. For example, the present invention provides a new class of on-chip functionality for microfluidics based on ambient temperature interphase mass-transfer. Excessive temperatures as high as about 60° C. in some conventional techniques are a concern for bio applications. Embodiments of the present invention avoid high temperatures by using of the freedom provided by microfluidics to heat liquid in the immediate vicinity of a liquid-vapor interface. In some embodiments, we have shown by means of experiment and a simple model that only a small change in the temperature of the fluid is required for the observed mass-transfer rates. According to Equation 1, we would not expect a high temperature increase for our system for the following reasons: 1) The measured absorption for the arrays is low, which is in consistent with the calculated value of Kabs for a gold nanoparticle of diameter of 15 nm at 532 nm wavelength. Values of Kabs for a strongly absorbing gold nanoparticle for this wavelength are nearly a factor of three larger. 2) The radius r0 of the nanoparticles in the array is smaller by more than a factor of six than the particle size reported in a conventional suspension liquid. In one embodiment, the effect of the particle radius on the optical absorption is taken into account by parameter Kabs, and r0 in Equation 1 is only related to the heat transfer from the particle to the surrounding medium.
The optical absorption Kabs of a spherical nanoparticle in an array is not only related to the particle size but also the inter-particle spacing. As mentioned earlier, throughout these experiments arrays with an average particle diameter of about 14.5 nm and an average inter-particle spacing of about 46 nm were used. By decreasing the inter-particle spacing it should be possible to increase the total absorption for a given r0. This would presumably increase the pumping rates for a given laser power at the expense of having the particles obtain higher temperatures. For the arrays having smaller, less absorptive particles, correspondingly more particles are needed to achieve the necessary heating for a given laser power.
The photothermal properties of the array, i.e. particle size and spacing, could be tailored to maximize mass-transfer for a given laser power while maintaining the temperature of the particles below acceptable levels. Wider channels and correspondingly wider laser spot i.e. a line source would allow a larger area and therefore an increase in mass flow. There are other factors that affect the pumping efficiency. The rate of the evaporative mass-transfer will be affected by the materials of the channel. PDMS is gas permeable, and eventually the gas in bubble will diffuse into the walls of the channel. Heat loss is also a consideration as the thermal conductivity of supporting glass is high and much of the heat imparted to the liquid from the nanoparticles is lost to the support. The evaporation process is not limited to plasmonic heating, and a light absorbing surface such as carbon black or even resistive heaters could in principle be used as a heat source. However, plasmonic heating has the advantage of an optical frequency dependence and does not limit the optical access at off-resonance frequencies. This is potentially useful for simultaneous application of other optical techniques such as fluorescence spectroscopy, which is widely used for studying biological systems and was demonstrated here.
Another advantage of the present invention lies in using plasmon assisted heating by illuminating a laser beam and is highly controllable. Unlike other optical transport methods, it does not require translation of the beam. The present invention provides a method for performing a microfluidic control on chip, though the price to pay for driving the process optically requires an external laser. However, advances in microelectronic fabrication allow for integration of microlasers on chip, and such an approach would minimize inconsistencies related to the distance of spot position and the surface of the gas bubble and would allow the technique to be scaled on-chip. The present invention have successfully demonstrated that the approach affords a simple on-chip means for pumping, distillation, and sample concentration. The technique is general and the functionality that it offers can be integrated with conventional microfluidic architectures and is believed to have a much broader range of applicability.
It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.
This application claims priority of U.S. Patent Application No. 60/897,743, and titled “PLASMON ASSISTED CONTROL OF OPTOFLUIDICS,” filed by Adleman et al. at Jan. 26, 2007 and claims priority to U.S. Patent Application No. 60/966,402, and titled “METHOD FOR MICROFLUIDIC DISTILLATION AND SAMPLE CONCENTRATION,” filed by Adleman et al. at Aug. 28, 2007 commonly assigned, and each of which is incorporated by reference in its entirety.
The U.S. Government has certain rights in this invention pursuant to Grant No. HR0011-04-1-003267 awarded by DARPA and Grant No. N00014-06-1-0454 awarded by the Office of Naval Research.
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