I checked for a few minutes in internet and found this:

http://pubs.rsc.org/en/content/articlelanding/1991/ft/ft9918703881#!divAbstract

http://pdf.lookchem.com/pdf/22/27f44628-7143-4f4c-b9ba-1df6600df69b.pdf

http://ijopaasat.in/yahoo_site_admin/assets/docs/1_IJPAST-277-V9N1.139102415.pdf (also explaining about the plasmon resonance)

maybe the best is:

http://www.nanoscalereslett.com/content/4/5/465, with this Figure:

http://www.nanoscalereslett.com/content/4/5/465/figure/F1

I did not quite understand your question, but the color (optical spectrum) of metallic nanoparticles is mostly determined by the localized surface plasmon resonance (SPR), see e.g. our publication [J. Opt. Soc. Am. B 30, 338 (2013)] and references therein. The optical response can be measured by e.g. standard ellipsometric techniques.

I checked for a few minutes in internet and found this:

http://pubs.rsc.org/en/content/articlelanding/1991/ft/ft9918703881#!divAbstract

http://pdf.lookchem.com/pdf/22/27f44628-7143-4f4c-b9ba-1df6600df69b.pdf

http://ijopaasat.in/yahoo_site_admin/assets/docs/1_IJPAST-277-V9N1.139102415.pdf (also explaining about the plasmon resonance)

maybe the best is:

http://www.nanoscalereslett.com/content/4/5/465, with this Figure:

http://www.nanoscalereslett.com/content/4/5/465/figure/F1

Thanks to Kamyar Khoshnevisan and Bernhard Wessling, but is there possibility to detect any substance with simple Naked eye observation like Ag and Au Np by CuNP??? Basically I am forensic Researcher I am trying to develop CuNp based sensor which gives colour change when its come in contact to my substance. Similar like Ag and Au as they agglomerate the colour changes yellow to red and red to blue respectively.........Tapio Ala-Nissila, sir this is my main problem i hope you understand what i want to convey....Thanks to you all

Do you want to use colloidal Cu (metal) as an indicator for something? but for what?

sorry, I may not easily understand what you mean: you want to detect Ag (and Au) nanoparticles by using Cu nanoparticles?

Bernhard Wessling: no but i just given that example, i want to detect drug via CuNP with colorimetric change, as Gold and Silver Np are commonly used for this purpose like,

http://www.sciencedirect.com/science/article/pii/S0956566312000449

http://pubs.rsc.org/en/Content/ArticleLanding/2012/AY/C2AY25475A#!divAbstract

http://pubs.rsc.org/en/content/articlelanding/2011/ay/c1ay05567d#!divAbstract

etc. so I just want to replace the AuNP and AgNP with CuNP as colorimetric sensor.

Is it Possible??/ Is CuNp Gives Colorimetric Change As its agglomerate, is there any change in SPR which is clearly seen by naked eye and i can use that tech as on spot recognition??? I do not found any research or paper in this regard as copper Np used as colorimetric sensor for any substance...

I have looked at the abstracts you made available, however, I can not comment on such specific topic in detail. I don't know and I can't see from the abstracts how selective this effect really is. One article detects Clenbuterol, the other Lysine (saying that the reaction was specific vs other amino acids), both by some change in particle size (agglomeration).

The 3rd one reports about a totally different effect: oxidation of Ag nanoparticles surface by oxygen in presence of cyanide.

My more general comments:

1) many effects can and will cause NP aggregation, therefore I somehow doubt (but maybe I am wrong) that the Au-NP related publications report about something really specific

2) the oxidation effect on the Ag-NP also does not seem to me to be very specific

3) even more for Cu-NP which can be more easily oxidized, and over time in water, they will anyway oxidize; you may need to passivate the Cu-NP surface before studying some sensor / indicator reaction.

Thanks Chiara Civardi for your input.....