A new form of organizing nanostructures generates Raman signals that are amplified hundreds of thousands of times, this is for the benefit of identifying and characterizing different molecules
![Detection of extremely low concentrations of a test substance molecule using silicon nanowires to which silver nanoparticles are anchored [courtesy of VS Vendamani]](https://www.hayadan.org.il/images/content3/2018/05/180102114213_1_540x3601-500x282.jpg)
[Translation by Dr. Nachmani Moshe]
As the fields of medicine and pharmacy focus more and more on the processes that occur at the nanometer level, the identification and characterization of different molecules becomes an important task. Raman spectroscopy, which is based on the dispersion of a laser beam to identify molecules, is limited in its ability to identify molecules in mixed samples due to receiving signals that are too weak, but researchers in India have succeeded in improving molecular identification at low concentrations by organizing silver nanoparticles on silicon nanowires.
Surface Enhanced Raman Spectroscopy (SERS) uses electromagnetic fields to improve the Raman scattering and increase the sensitivity of the method using standard dyes such as Rhodamine 6G up to a billion times. The research team combined varying concentrations of silver nanoparticles on vertical silicon nanowires to improve the three-dimensional structure of the array. Their results, published long ago in the scientific journal Journal of Applied Physics, show that the innovative device is capable of amplifying the Raman signals of materials on the order of a hundred thousand. "The beauty of our method is that we are able to increase the density of these nanowires through simple chemistry," said the lead researcher.
"If there is a high density of nanowires, you can incorporate many more silver nanoparticles into the substrate and thus increase the sensitivity of the device." And yet, incorporating the required nanostructures into SERS spectroscopy devices remains a challenge. Designing these three-dimensional structures with silicon nanowires has generated much attention due to their high surface area and preferred performance level, however, silicon nanowires are still expensive to manufacture. Instead, the research team was able to find a cheaper way to make silicon nanowires and use a special digestion method to produce a wide variety of nanowires. The researchers "decorated" these nanowires with silver nanoparticles of variable and controlled density, an array that led to an increase in the surface area of the nanowires used for detection.
"The optimization of these vertical structures required considerable effort at the beginning," explains the researcher. After the researchers improved their system so that it could detect a rhodamine dye at a nanomolar concentration, the researchers were able to obtain ten thousand to one hundred thousand times higher sensitivity. The system was able to detect cytosine, a nucleic acid that makes up DNA, as well as the substance ammonium perchlorate, a molecule capable of hinting at the presence of explosives, in very minimal concentrations of 50 and 10 micromolar, respectively. These results encouraged the researchers to believe that in a short time it would be possible to detect compounds at even lower concentrations, at nanomolar or even picomolar levels. The work of the team of researchers paves the way for several future trends in research, from experiments with different nanoparticles (for example, gold) to increasing the efficiency of the nanowires or testing these devices with different molecules.
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One response
Ladies and gentlemen, there are quite a few mistakes and misrepresentations in this publication. The two main ones are listed below:
First, the researchers talk about signal amplification using the SERS method up to a billion-fold signal amplification; However, in the professional literature, it is known that the SERS structure increases (already today!) the detection sensitivity 10 times to the 10th power and up to 10 times to the 11th power, which means that there are already devices on the market today that can amplify the signal 100 times more than what is reported in this publication. If so - what is special about this publication? It does not come close to the heels of the technology that exists now! The researchers who posted here reach too low an amplification, so they really have no reason to get excited.
Secondly, from the drawing of the system it appears that the silver nanoparticles on the surface of the silicon wires are immersed in the analyte solution and maintain a state of pre-concentration, that is, they extract the required molecules from the solution and thus increase their signal. It is completely unrelated and similar to the SERS sensing form! Here it is the increase of the two-dimensional concentration of the molecules through adsorption, which will surely increase the signal that will be received according to Bar-Lambert's law! And yet, as I mentioned in section 1, they reach at best about 1% of the sensitivity to Raman signals obtained from the "real" SERS that already exists today!
In conclusion, I would suggest that these researchers continue with their existing system but measure the signals using electrochemistry (which, by the way, has an amplification of about 100,000 times due to the Faraday constant whose exact value is 96,485 coulombs per mole). This device can also be referred to as a FET (Field Effect Transistor) device, due to the dependence of the electrical conduction of the silicon wires on their immediate environment - such as changes in the ratio between the number of silver particles that remain exposed and the number of silver particles that are bound to the organic molecules being studied. The enthusiasm of these researchers in measuring Raman in the way described in this publication is really shameful... ???