Mr. Shukla,

The properties depend on the size of the NPs. You can use SEM in order to determine the size.

The most probable cause as the colleagues hinted is that the size of your gold particles are smaller than that would be for those absorbing at 500:600 nm wavelength as Bojidarka noted. You may try to develop your reaction to enlarge the size of the nano particles as Allan proposed.

Best wishes

Are you scanning the 190-240 nm region, also, so you know the absorbance maximum begins at 240 nm? Light scattering increases exponentially in the UV so I would make sure you are not measuring apparent absorbance from light scattering in the 240-300 nm region. See more about Rayleigh scattering vs. the Tyndall effect: https://en.wikipedia.org/wiki/Tyndall_effect

Dear Haleem,

Thanks for your response. I have synthesized gold nanoparticles by citrate reduction method and obtained deep red color by the end of reaction. Then I have taken size by using DLS and got 13nm to 76nm size ranged particles. Please let me know any alternative method to check the nanoparticles. So that I can sure about GNPs.

Dear Vikas,

The UV absorbance peak mainly depends on the size of the synthesized AuNPs, if the size as you mentioned is 13 nm and 76 nm, the absorbance peak should be in the range of 520 nm to 580 nm. To further confirm, you may use DLS. Sometimes the size distribution achieved through DLS is not exactly the one i.e. 13 nm. You may then need to use size analysis through TEM. These are the standard methods for size confirmation.

The absorption peak obtained at 240-300 nm range in Gold nanoparticles are interband absorption peak. The peak observe at 500-600 nm range are Surface plasmon resonance peak. The interband absorption is due to electron- hole pair combination which are 10 times more intense than SPR (500-600 nm ) peak. When you plot graph for 450-750 nm for the data obtained , you could see SPR peak .