The overall goal of this procedure is to develop a microfluidic sensor technology. This is accomplished by first fabricating potential devices by coding capillaries with the silicon quantum dot layer. The second step is to determine which of the samples meet the basic requirements for use as a sensor.
Next, a sensor device is connected to a micro pumping system to allow the introduction of analytes. The final step is to collect fluorescent spectra and analyze the results. Ultimately, fluorescence microscopy is used to show refrac metric sensing of fluids in a capillary channel.
The main advantages of this device included its small size, its microfluidic compatibility, and the fact that one can readily envision a complete device being built from a set of fairly inexpensive components. Because of the microfluidic channel, any fluid can be pumped inside the capillary. The hardest part is to functionalize the surface in order to bind to specific analytes.
This device has a wide range of potential applications, such as food safety, groundwater analysis, or even healthcare. The hardest part about this device is that the sample making preparation is very finicky, and there's a very low success rate because the smallest thing can lead to failure in the sample preparation. A visual demonstration of this technique helps to increase the success rate.
Begin with the preparation of micro capillaries. Obtain silica capillaries from a commercial supplier. Choose an inner diameter of approximately 25 to 30 micrometers for more widely separated spectral resonances, or an inner diameter of about 100 micrometers.
For more closely spaced resonances with higher quality factors, large outer diameters will ensure more durable and easily manipulated micro cavities. Next, use a diamond fiber cleaver to cut approximately 10 centimeter pieces of capillary from the roll. Each piece constitutes a single sample.
Heat the samples in a tube furnace at 650 degrees Celsius for one hour in oxygen. This process removes the colored polyamide jacket to expose the silica capillary inside. Wait for the samples to return to room temperature before proceeding.
Obtain hydrogen YLS OX or HSQ in one of its flowable oxide solutions and adjust its concentration to an appropriate range for quantum dot formation. This will require some trial and error for a capillary. With a 25 to 30 micrometer in a radius, a solution of about 25%by weight is a good target.
With the preparatory steps completed, produce a coated capillary by dipping the prepared capillaries into the HSQ and flowable oxide solution. The meniscus should be visible as the solution is drawn up the channel. When the meniscus reaches the top, remove the capillary and place it in a glass, a kneeling crucible.
Repeat this for 20 to 30 capillaries using two different concentrations of HSQ solutions. By weight, while the capillaries are filled in the air, the flowable oxide solution is kept chilled in a glove box of possible to minimize exposure of the solution to oxygen and water vapor. The quantum dots are formed in the silica matrix.
During the annealing process, place the capillaries in a furnace that is set to increase the temperature from room temperature to 300 degrees Celsius over 30 minutes. Dwell at that temperature for three hours, then increase to 1100 degrees Celsius in 45 minutes and stay at that temperature for one hour. Let the capillary slowly cool back to room temperature over about 12 hours.
This process results in 20 to 30 capillaries whose walls are coated with a layer of fluorescent quantum dots embedded in a silica matrix. To characterize the samples, use a fluorescence microscope that can perform both imaging and spectroscopy in the 700 to 900 nanometer wavelength range. Place a row of the candidate capillaries on the stage so that it is easy to move between them.
For quick visual analysis, excite each capillary with blue or ultraviolet radiation, either in free space on the microscope stage or directly through the objective lens. Using a dichroic filter, observe the fluorescence of the capillaries using the eyepiece or a color camera. If the fabrication was successful, the capillaries will exhibit a bright red fluorescence capillaries exhibiting a yellow orange fluorescence, or none at all, indicate a low HSQ concentration during fabrication.
These should be discarded along with those with a cracked or textured film. Quantum dot spectra should be intense in the 700 to 900 nanometer range. Spectra taken from the inner capillary wall should show strong oscillations due to the presence of a whispering gallery mode, discard any samples that do not show evidence of this mode.
Next, prepare samples for refractory metric analysis. Use an appropriate adhesive to attach each end of a candidate capillary to polyethylene or similar tubing to carry fluids into and away from the capillary. After gluing the first tube wait for it to cure before attempting to glue the other side.
The inner diameter of the tubes should be just slightly larger than the outer diameter of the capillary. Use care to prevent adhesive from seeping into the capillary channel and blocking it. Next, connect the tubing with the syringe to a micro pumping system.
Be prepared to pump compounds with well known refractive indices such as methanol, ethanol, and water. Pump each fluid one at a time into the capillary to determine the refractive metric sensitivity of the device. Collect spectra with each liquid inside the capillary.
Use an analyzer in the light path to measure shifts in the Whispering Gallery modes with each different solution in the capillary. If there is no observable shift, the quantum film is too thick and the capillary should be discarded. Successful samples typically have a sensitivity of five to 15 nanometers per solution.
Refractive index unit data is collected using the refractive metric setup. Begin by taking a reference spectrum of a sensor capillary for biosensing applications. This should be done after the channel surface is functionalized.
For specificly to achieve low detection limits care must be taken to minimize sample drift. Also use a calibration standard to correct the spectrometer for wavelength and intensity. Finally, collect spectral data with the analyte introduced into the sensor capillary.
Analyze the data with the methods described in the manuscript. The setup can be programmed to take spectra continuously as analytes are pumped into the capillary channel. At the top is a set of spectra taken as methanol, then water, then ethanol were pumped into the capillary.
The spectra were taken sequentially from red to blue. This figure shows the forer power spectrum of each fluorescent spectrum. The 40th component represents the main observable whispering gallery mode oscillation.
The phase differences for the 40th component were converted to wavelength shifts and plotted as a function of time. As seen here. The error bars represent one standard deviation of the peak shift for 60 measurements.
The inset shows the average sensitivity over the refractive index range from methanol to ethanol. Note that analysis of the continuous time series data reveals a bump in the shift data between water and methanol. This is consistent with the presence of a small mixing region with a higher refractive index than either pure phase Once mastered.
This technique can take a few hours to perform, but as many as a few days to complete.
