Although metal ions are sometime similar in terms of charge and size but selective sensors (fluorescence and colorimetric) seems to recognize them accurately. What can be the probable reason of so accurate sensitivity?
Let me explain it with a specific example. As we know di(2-picolyl)amine (DPA) is very well known for Zinc binding and is used as the metal selective site in various Zn(II) fluorescence sensors (http://pubs.rsc.org/en/Content/ArticleLanding/2006/CC/b606809j#!divAbstract). But it is not that DPA can only bind Zn(II) ions, there are enough evidences of DPA binding Ni(II) ions and forming stable complexes (http://www.sciencedirect.com/science/article/pii/S0277538710005851). My question is why most of the DPA based Zn(II) fluorescence sensor do not show any response in presence of Ni(II) ions, although both the ions are positively charged and has potential capability to disable PET mechanism.
This is a very general question that I am not sure has a straightforward answer. I will try to give you some thumb rules that may help.
As you stated you can have a size selective identification, this could be done for example by Calixarenes or crown ether's attached to the electrode. You can have a charge identification for example adding Self Assembled Monolayers (SAMs) that are positively charged or negatively charged. You can look for spesific interactions Cr(IV) has good interaction with pyridinium so adding that to an electrode will make it more selective to Cr. Another why is to try to make it tailor made by MIP's and it will recognize a specific molecule.
If you notice I am only talking about modification to the electrode, but there is also the electron transfer kineticks some electrodes will prefer a spesific species so they will have better interaction with it. For example L-DOPA prefers Pt over Au.
(you can read more here: http://pubs.acs.org/doi/abs/10.1021/ac401744w)
Now the final part of our puzzle the electrochemicl technique. For example Anodic stripping, you can apply potential that will cause only a spesific specie to deposit (if the other deposit at a different potential otherwise you can co-deposit) on the electrode an enrich the diffusion layer so it will give a higher signal. You have a lot more way to play with electrochemical methods but this is an easy example.
So as you can see its a mixture of electrode material, modification and electrochemical method.
Here is a nice review article to get you started:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4096685/
Seems to me it is a homework question
Someone really interested scientifically would not ask this in this general ResearchGate question of "fluorescence"
Thank you Dr. Noyhouzer and Dr Shapiro for your time.
Dr. Brink, may be my question is not well constructed. Let me explain it with a specific example. As we know di(2-picolyl)amine (DPA) is very well known for Zinc binding and is used as the metal selective site in various Zn(II) fluorescence sensors (http://pubs.rsc.org/en/Content/ArticleLanding/2006/CC/b606809j#!divAbstract). But it is not that DPA can only bind Zn(II) ions, there are enough evidences of DPA binding Ni(II) ions and forming stable complexes (http://www.sciencedirect.com/science/article/pii/S0277538710005851). My question is why most of the DPA based Zn(II) fluorescence sensor do not show any response in presence of Ni(II) ions, although both the ions are positively charged and has potential capability to disable PET mechanism.
Chirantan
You should therefore ask this and mention that your ion is zinc, in the question
Coordination Chemistry and Metal Complexes Research
here at Research Gate
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