Hi Pallavi,

Screen-printing quality depends on numerous parameters. I will list most of them here:

1) Screen type - mesh openings, stainless steel or polymer strings, angle of the strings with respect to the squeegee direction.

2) Inks - ink homogeneity,  ink purity, ink viscosity during printing, binder and substrate compatibility, choice of vehicle, solid particles size should always be smaller than the mesh openings of the screen.

3) Screen printing equipment - distance between the screen and the substrate, squeegee pressure, accuracy of layer alignment.

4) Curing process - allowing significant amount of time for surface stress relaxation, allow enough heating up of the ink to evaporate the vehicle, firing temperature profile.

All of those parameters can affect the repeatability of your printed electrodes. 

If you need more information on how these parameters can affect your electrodes let me know.

I am not screen-printing the electrodes. I utilize commercially available screen printed electrodes. I am utilizing filter paper as membrane for the electrode. The results are repeatable in the absence of membrane. However, in the presence of membrane, the results show a lot of variability.

Are the properties of the filter paper repeatable before utilizing it on the electrodes? If yes, then your problem is the way you utilize the membrane. If no, then try using another filter paper.

If I understood your dilemma, the problem with repeatability is in the filter paper. I worked with uPADs and had this same problem with the paper. The time between sample addition (electrolyte and analyte) and the measurement should be observed and the volume as well.

Thank you Marios sir for your suggestion. I wanted to enquire about how do I check if the properties of the filter paper are repeatable before utilizing it. 

Thank you Prosper sir for your suggestion. I am currently using a tape for adhesion. Can you guide me of an alternate way for adhesion? 

Thank you Ricardo sir. I agree the problem is in the filter paper. I currently give a time difference of 1-2mins between sample addition and measurement. I am not sure if that time is sufficient for accurate analysis.

If you are using a commercial filter paper, you will find its specs in the datasheet. I guess if you use a home-made filter paper, you should investigate its specs before applying it on the SPE.

Tape is not a good idea when you are dealing with liquids because the liquid might go through between the tape and and the substrate and under the filter paper instead of going through the filter. Check if you can screen print the membrane on the SPE. I have seen plenty of screen printed membranes.

I agree with Ricardo, and the only way to know for sure is to monitor the output of the SPE from the time you submerge it in the solution until you see a stable output (ideally). In reality, you might never reach a stable output but I believe that monitoring the SPE's output for about a day will give you the information you need.

Thank you so much. i will check about screen printed membranes. However, is there any alternative adhesive for the same. 

I understand that close monitoring of SPE output with time is required but any electrochemical reaction is of the order of a few minutes and hence I guess a day would turn out to be a long time and may not give any signal.

I can't think of any reliable method to utilise your membrane since it generally depends on the membrane and substrate material.

What I meant with a whole day is to continuously take readings, lets say every minute for a whole day and see how the output has changed. In that way, you will have an indication of when you should take readings.

Thank you so much. i will closely monitor the SPE output with time.

Dear Pallavi Dasgupta,

the main instability error(s) are due to the (two or more, challenging) materials in your composite (multi-) electrode, and some additional corrosion issues (ageing).

So, say, if possible, what is your cell, and the materials' composition of your screen printed (multi-) electrode ?

Hi Pallavi,

If your membrane is just to protect your sensor surface, you could consider having a look at using membranes that can be added by drop-coating as liquids that then develop membranous structure on drying.  There are plenty in the literature (nafion, cellulose acetate, etc), and which one you choose could depend on your electrode material and substrate that it is printed on.  We've found these membranes reliable in the past so perhaps worth a look at what has been published for similar materials to yours.

Thank you Adrian. I attempted drop-coating the electrode with cellulose acetate. The problem I am facing is the solubility of cellulose acetate. I tried dissolving it in PBS and DI water and Cyclohexane. But its not dissolving. I also wanted to ask if it should be dried in oven or at room temperature. 

Hi Pallavi, the group I work with use acetone for the cellulose acetate.  An example of this is in:

Gilbert, L., Browning, S., Jenkins, A.T. and Hart, J.P., 2010. Studies towards an amperometric phosphate ion biosensor for urine and water analysis. Microchimica Acta, 170(3-4), pp.331-336.

It is a simple method with drying at room temperature, but it can take a bit of practice to get the deposition consistent.

Thank you Adrian. I will try preparing the solution in acetone. 

You can get down to 10 % or better -

https://www.zimmerpeacocktech.com/2017/11/30/biosensor-wafer-mapping-and-functional-testing/