The overall goal of this filter-based Surface Enhanced Raman Spectroscopy method is to detect various target molecules at trace levels in liquid samples. Two advantages of this technique are the reduced costs in sample preparation time, more importantly, this technique provides higher sensitivity by using larger sample volumes, thus lowering the limit of detection. Generally, individuals new to this method will struggle because nano-particles may not be trapped in the pores of the membranewithout a proper aggregation step or they may not be uniformly distributed on the membraneTo begin, prepare 100 milliliters of fresh silver nitrate solution and heat it to boiling on a 350 degree Celsius hotplate with magnetic stirring at 700 RPM.
Once boiling, immediately add one milliliter of freshly prepared sodium citrate solution and let the solution boil for 25 minutes. As the silver nano-particles form, the solution will turn greenish-brown. After 25 minutes, put the flask on an unheated stir plate and let it stir overnight at 700 RPM.
In the morning, the mixture should be at a stable state, with a constant color and transparency. If desired, measure the absorbence of the nano-particles. Proceed by diluting the mixture with ultra-pure water to 100 milliliters.
Then, use a zetasizer to measure the size of the silver nano-particles. For storage, transfer the silver nano-particles to a sealed container. Protect it from light by a foil wrap.
The colloid will remain good for up to two months at four degrees Celsius. To make a SERS active filter membranefirst add one milliliter of five millimolar sodium chloride to one milliliter of prepared colloid. And mix them using a nutator at 20 RPM for 10 minutes.
As the silver nano-clusters form, prepare a PVDF membranewith a zero point one micron pores. It is critical that the filter holder be completely dry and that the filter is in the correct orientation. After 10 minutes of mixing, filter the two milliliters of nano-clusters by pushing them through the membraneat a rate of about one drop per second.
Push the solution through continuously without breaks. The result is a SERS active filter membraneCarefully detach the active filter membraneusing tweezers. Then, set it up to air dry for three minutes.
Once dry transfer the SERS active membraneto a glass slide. Now, for Raman detection of the SERS substrate, set the instrument to a 780 nano-meter laser operating at five miliwatts. Set the exposure time to one second and the exposure strength to two.
Using a 10 X subjective, focus on the sample. Make sure the software is also set to a 10 X subjective. Now, randomly select eight to 10 spots on the membraneand use the instrument to collect data from them.
Then, open the spectral data in the manufacturer's software for analysis. To begin, make a serial dilution of 100 parts per million ferbam solution to obtain five milliliters of 10 parts per billion ferbam solution and 50%acitre-nitrile. Now, position the active membraneinto a filter holder with the nano-particle coating facing up.
Then, load the five milliliter sample into a new syringe and attach the active filter. Now, push the sample through the filter at a continuous flow of about one drop per second. Once the solution is filtered, carefully detach the membraneand allow it to air dry for three minutes.
Immediately proceed with Raman detection once the membraneis dry. Perform the data collection as before. Various volumes of silver nano-particles were tested using this method.
The volume of silver nano-particles solution used to coat the membranethat provided the maximized sensitivity was one milliliter. Measuring the Raman shift shows that the SERS spectrum of 10 parts per billion of ferbam was detectable. The peak, at 1386, is due to the mixed vibration of cyanide and carbon sulfur double bond stretching and from symmetric methyl group deformation.
The peak at 1516, is associated with methyl group and cyanide stretching and the peak at 561, is generated by sulfur bond stretching. A spectrum from one parts per million of ampicillin was also clearly detected. The peaks as 1594 and 1447 are from carbon double bond stretching and methylin ethyl group deformation respectively.
The strong peak at 1001, is from the benzine ring vibration. The peak at 852 is associated with symmetric CNC stretching. By increasing the sample volume, the detection limit increased.
The advantages of this filter based method include an adjustable volume and adjustable limit of detection. After watching this video you should have a good understanding of how to fabricate a uniform sensitive nanosubstrate using a filter membraneOnce mastered this technique can be done in 30 minutes if it is performed properly, following this procedure other methods like SEM or TEM can be performed in order to answer additional questions like the distribution of silver nano-particles on the membraneAfter its development, this technique paved the way for asserters in field of analytical chemistry to explore trace chemical targets in various food and environmental samples. Don't forget that working with pesticides and biological toxins can be extremely hazardous.
A lab coat, respirator and safety goggles should always be worn while performing this procedure.
