Yes it is related to solvent. Assuming your buffer is in water which may rinse out the deposited metal complex on the paper ( as it is also water soluble ) one has to use metal complexes which are not soluble in water. This can be done by changing the peripheral core of the ligand used by introducing hydrophobic groups or if the complex is ionic then one can use bulky cation ( or anion ) to precipitate out the desired complex. Introduction of such counter ion would lead the complex soluble in organic solvent but will not be soluble in water. Once this is achieved, one can get the desired solution of the complex in organic solvent and can strip of Whatman filter may be dipped into it and then dried. Now in aqueous buffer the deposited complex will not be leached out.

However, there should be some ability of the complex to interact in aqeous solvent so as to respond in a reaction once deeped into buffer. Knowing the nature of the complex and  what for it is to be used as strip in buffer a better suggestion may be provided.

about Whatman  filter paper: I do not know these methods.

It seems the problem is the solvent. Could you try a non aquous solvent?

This article maybe could help you:

Johan G. Neevel, Birgit Reißland. Bathophenanthroline Indicator Paper - Development of a New Test for Iron Ions. PapierRestaurierung Vol. 6 (2005), No. 1

Yes it is related to solvent. Assuming your buffer is in water which may rinse out the deposited metal complex on the paper ( as it is also water soluble ) one has to use metal complexes which are not soluble in water. This can be done by changing the peripheral core of the ligand used by introducing hydrophobic groups or if the complex is ionic then one can use bulky cation ( or anion ) to precipitate out the desired complex. Introduction of such counter ion would lead the complex soluble in organic solvent but will not be soluble in water. Once this is achieved, one can get the desired solution of the complex in organic solvent and can strip of Whatman filter may be dipped into it and then dried. Now in aqueous buffer the deposited complex will not be leached out.

However, there should be some ability of the complex to interact in aqeous solvent so as to respond in a reaction once deeped into buffer. Knowing the nature of the complex and  what for it is to be used as strip in buffer a better suggestion may be provided.

You could dip coat the metal ion complex onto a strip of a paper, let it dry and then simply laminate the coated region leaving an exposed paper tip.  Dipping this tip into the test solution will allow the aqueous solution to migrate up the strip by capillary action to the test reagent

A difficult  possibility but capillary action will carry the water soluble impregnated metal complex to move with the migration of the aqueous  test reagent front  and any anticipated change will be blurred. It is better to put a drop of metal complex solution in the middle of a  Whatman 1 filter paper and after drying one can place a drop of the test reagent  on it. the change can be localized with little diffusion like circular chromatgraphy but colorimetric change will be better visible.

Thank you very much. All your suggestions will be really helpful. Let me check the solvent system first. What solvent system would you suggest? Farrell suggested that lamination would help. Can you give me some idea about how to do that? I was wondering coating the paper with a polymer (PVA, PVP...) can help?

I have an idea, something similar to ligand-exchange chromatography. If you threat Whatman filter paper with acetic acid anhydride/sulphuric acid you can convert it partially to acetyl-cellulose. Acetyl group should bind Cu(II) and Co(II), and therefore immobilize them. Let try!

I would try the following

# dip into a solution with cationic surfactant or polyelectrolyte

# wash with water

# dip into solution with anionic surfactant

# wash with water

# immobilize the metal complex

You can try 0.5% or 1% PVA or Tween 20 for coating paper.

Following attachment may help you.