The overall goal of the following experiment is to perform thermal measurements for sensitive detection in microfluidic devices. Three different fabricated microfluidic devices will be used to demonstrate different thermal measurement techniques in analytical microfluidic devices. As a second step, resistance based temperature detectors are coupled to the devices and the setup is connected to a computer.
Next, the samples are loaded into the devices and thermal detection is activated by the controller. The results show the extended detection capabilities of thermal measurement in microfluidic devices. The main advantage of this technique over existing methods like chole metric and electrochemical detections, is that this technique reduces the complexity of the measurement while the range of detection is extended.
Begin by using micro fabricating a thin film silicon nitride membrane with an integrated temperature sensor using the technique described by vota et al. Rinse the completed device with deionized water and dry the device using nitrogen gas. Next, fabricate an SU eight mold with micro channels using a previously published technique, tailor the length and width of the micro channels to fit the range of particle sizes to be detected.
Make the PDMS by mixing 30 milliliters of the base with three milliliters of the curing agent. Once mixed, remove any bubbles by exposing it to a vacuum for five to 10 minutes. Then pour the PDMS onto the mold.
Then place the mold onto a hot plate at 70 degrees Celsius for two hours. Once cured, peel off the PDMS very carefully so as not to damage the microchannels. Using a manual punch punch a one millimeter hole at one end of the microchannel for the PTFE tubing.
Use a two millimeter punch at the other end to make a reservoir. Next, place the punched microchannel on top of the device and align the resistance based temperature detector at the center of the microchannel with the aid of a microscope in the electrical interface. Connect the electrical pins at the contact pad positions and tighten up the locking screws.
Make sure the height adjustable pins sit at the correct electrode pads on the device. Next, prepare the polystyrene beads by diluting them one to 10 into 100 microliters of DI water in a 1.5 milliliter tube. To ensure the PS beads remain neutrally buoyant at 2.7 microliters of glycerol to the mixture and mix by pipetting up and down.
Fill a one milliliter glass syringe with 0.5 milliliters of DI water. Connect the glass syringe to one end of the PTFE tube and attach the other end of the tubing to the microfluidic channel. Place the DI water filled syringe on a computer controlled syringe pump and set the flow rate to between five and 20 microliters per minute.
Fill the whole channel with fluid all the way to the reservoir. Next, load 10 microliters of the balanced bead solution to the reservoir, and introduce the bead solution to the microchannel by changing the flow direction on the syringe pump. Finally, turn on the resistance based temperature detector by biasing one milliamp of DC current through the computer controlled meter while measuring the resistance and sorting the measured data.
To begin, fabricate the on-chip calorimeter device and microfluidic layer as described elsewhere. Assemble the device by placing the micro calorimeter device in the device holder and aligning the device to the microfluidic inlets and outlets along with the holder fittings of the PDMS seal layer. Next, install the electrical connection pins on the device holder and lock the holder screws in place.
Then connect the PTFE tubes to both inlets and the outlet. Connect one inlet to a sample loaded syringe pump and close the other one as the enthalpy is not measured in this case. Then load a 300 microliter sample into the glass syringe and place it on the syringe pump.
Use very slow constant flow rates for high viscosity samples such as glycerol and ionic liquids. For thermal diffusivity measurements. Connect the measurement setup as shown here.
Load the glycerol sample to the micro calorimeter chamber and run a modified computer controlled program. For heat penetration time measurement. Use the calibrated heat penetration equation shown here to calculate thermal diffusivity from the measured heat penetration time.
For specific heat measurements, use the thermal wave analysis measurement setup as shown here. This time, load the ionic liquid in the chamber and use the same sample loading program. Run the thermal wave analysis program to get the amplitude of the AC temperature fluctuations and calculate the specific heat for each ionic liquid sample.
This process is described in more detail elsewhere. For calori metric detection, micro fabricate a 40 to 50 nanometer thick nickel film resistance temperature detector as previously described. Next, use a knife plotter to cut L-shaped paper.
Microfluidic channels that are three millimeters wide and 10 millimeters long on the larger leg and three millimeters long on the short leg. Use clean tweezers to add a five micron thick layer of double-sided acrylic adhesive to the sensor and place the paper channel on top. Use a clean blade to push the paper to the device and remove any air bubbles.
Add one milligram of glucose oxidase enzyme to one milliliter of sodium acetate buffer. To make a one milligram per milliliter solution, adjust the pH of the solution to 5.1 with sodium acetate buffer if necessary. Next, bias the resistance temperature detector with one milliamp of DC current.
To activate the detector and start measuring the resistance continuously, introduce two microliters of the prepared glucose oxidase solution to the center of a mobilization site of the paper microchannel. The detected temperature will start to decrease to measure the glucose concentration, introduce glucose standards to the channel's inlet, and measure the resistance change caused by the reaction. Repeat this experiment with all different glucose control solutions and save the resistance data.
Finally, convert the resistance change to glucose concentration using the equations listed in the accompanying text protocol and elsewhere. The polystyrene bead shown here is passing over the resistance temperature detector with a flow rate of five microliters per minute. As it passes the sensor, the resistance spikes slightly and the size of the particle can be detected based on this change.
The typical output from a paper-based calor metric sensor is shown here. Once the glucose oxidase is added, the temperature begins to quickly decrease until the enzyme meets the glucose solution. Once it does, a reaction takes place and the temperature spikes and then begins to cool off again.
This temperature change can be converted into glucose concentration to accurately measure glucose levels in samples bowls While attempting this procedure, it's important to remember device design and structure plays a key role in thermal measurements. The thermal parameters, thermal mass, and terminal resistance, and the sensor parameters are well described in reference articles.
