In literature varied uv absorption range of AG Nps formed are given and varied colour also. From yellow to green. But i am getting dark brown (BLACKISH) colour does this confirms the formation of Ag Nps? In my analysis i am getting UV absorption peak at 490nm. is this correct?
A typical absorbance maximum of AgNPs is in the range of 400-500 nm (some sources refer to ~380-470 nm). The optical properties of AgNPs depend on several factors, such as:
1. Size - Smaller NPs show an absorbance peak near ~400 nm, whereas larger particles exhibit an absorbance peak shift towards longer wavelengths (red-shift). Additionally, the broad area under the curve indicates the polydispersity of particles.
2. Morphology - Non-spherical and aggregated particles exhibit a red-shifted absorbance pattern.
3. Composition - The presence of various stabilizing/capping agents also may influence the absorbance maximum value of AgNPs.
The primary color of AgNP solutions varies from yellowish to brownish. In addition to the abovementioned factors, the color and its intensity may also depend on the concentration of silver ion precursor. The higher the initial concentration of silver nitrate, the darker the color of the resulting AgNPs solution. During the synthesis, you will be able to observe the shift from yellowish to more dark brown colors if the reducing agent solution is colorless. However, if you are using a colored reducing agent solution (e.g. plant extract, etc), then it may influence the color intensity and shade of the AgNPs solution due to its composition. The color of the AgNP solution may change over time depending on the NPs' stability and storage conditions.
UV-Vis is known as the primary characterization method for NPs. Dark brown color and an absorbance peak at ~490 nm may be attributed to AgNPs, but you need to apply complementary characterization methods, particularly for elemental composition and size/morphology analysis, such as AAS, ICP-MS, XRD, EDX, AAS, SEM, TEM, etc, to validate. Then you will know the reason for the red-shifted absorbance of your samples: whether it's caused by size and shape or composition of AgNPs or maybe both.
As for the blackish color, it can be caused by the presence of oxidation products, especially during drying. If you want to avoid it, I'd suggest revising the synthesis protocol and conditions.
Hope the abovementioned will be helpful. Good luck!
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