Just add on to Jon's answer, you may order thiolated-aptamer (i.e. one end of the aptamer is modified with alkane thiold group). Upon reducing the S-S bond, thiold-gold interaction is pretty strong for prolonged detection. 

You may refer to one protocol here: Preparation of electrode-immobilized, redox-modified oligonucleotides for electrochemical DNA and aptamer-based sensing

Thank Jon Ashley and Lin Jin so much.  Now I use NH2- aptamer and this is my procedure:

1. Incubate SH-PEG-COOH with gold substrate: 4oC 17h => Wash 

2. Incubate EDC 16%: RT 30’ => Wash

3. Incubate NH2- aptamer  RT, 1 hour => Wash

4. Incubate cells, RT, 3’ => Wash

5. See the result in the microscope

After washing, I see the number of cells on gold is very less so I don't know what the reason is, may be the concentration of SH-PEG-COOH and configuration of aptamer.

Please let me know some comments about my protocol. 

Sincerely thanks.

I only use EDC to activate COOH group.

And I haven't tried incubating the PEG COOH at RT, may be the next time. But I read that " It has been reported that if SAM layers are formed above 25℃, incomplete SAM layers will be formed. Also, when temperature of a solution is higher than room temperature, desorption of impurities are increased and reorientation is accelerated".

So according to your experience, which is better? 

I also read that the force of bonds in SAM is pretty strong because they are covalent bonds, so are they removed easily after washing?

I am using  EDC/NHSS crosslinker to activated -COOH group, it makes stable conjugation between thiol-NH2-Aptamer

I think this article would help you

http://ac.els-cdn.com/S0956566315301160/1-s2.0-S0956566315301160-main.pdf?_tid=a81599b8-99ea-11e7-8b06-00000aab0f6b&acdnat=1505462058_c7a04135ce6defe6dd5db6ddb7df62eb

Thank you so much, Jon Ashley.