Many Researcher using raman spectroscopy but i dont know why. And my research is not that Advanced just need to be able to detect 2 or 3 pesticides that many people use. And maybe if i can tell the danger level of those residues.
And about the wavelength ? is it able if we use visible light wavelength or maybe there is better option to detect?
It depends on the materials, and in which environment and what concentrations you wish to detect them. A UV/vis spectrum is a good start. If they are well known compounds, the spectra may be published already. If they are fluorescent, this may be a good detection strategy. Raman is a much weaker effect, but can have much higher specificity for detection in a more complex environment.
Surface-enhanced Raman spectroscopy (SERS) provides the ultimate sensitivity: that of a single molecule detection. Although this certainly can be overkill for most applications, the inherent chemical identification power of Raman with the amplified sensitivity when combined with plasmonic nanostructures makes SERS a great choice to detect several analytes such as pesticides.
About the danger level, it is a bit more tricky with SERS cause you will need not just to be able to detect what pesticide is there but also how much of it. Once you know the quantity you can find information about the minimum lethal dose values reported in the literature for that particular pesticide(s) you're studying. The World Health Organization has some guidelines for the toxicity classification of pesticides, also including the correlation of hazard and dose if I remember well.
UV-Vis spectroscopy was widely used to detect concentrations of pesticides, but it depends on solubility of pesticide and its λmax.Also,chromatographic techniques are a good ways to detect such materials.
http://www.cwejournal.org/vol8no2/spectroscopic-methods-for-the-detection-of-organophosphate-pesticides-a-preview
While it is very fashionable to make people believe that it is easy to detect pesticide residues on food or the like, it is not currently possible. Unless your fruit is dip-coated with pesticides, there will never be enough material to give a useful spectrum with a point-and-shoot Raman system. Same for UV-VIS or NIR. Additionally, the UV-VIS spectra of pesticides, if they absorb at all, have not enough discrimatory power for pesticide identification. The only current way to identify and quantifiy pesticide residues is the old-fashioned lab-based one:
1) Extract your target substance from the base substrate
2) Up-concentrate
3) Separate, normally be using chromatography
4) Quantify, by comparing with concentration standards (here, UV-VIS detector come back into the game, magically. Not because the can discrimate pesticides, but can quantify "something" against "nothing")
Everybody claiming something else, is bragging around and/or in desperate need of your money and will not be able to deliver on his proposals.
I see a fair chance of having such detection possibilities for trace amounts with the advancement of portable mass spectrometers, as it is pioneered by 908 Devices. These are really able to detect very low levels and have the potential to also discriminate a vast amount of substances...
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