I am using QCM-D (Qsense e4) to monitor building of LBL films. PEI and PSS (poly styrenesulfonate) (2 mg/ml) (pH5) were used with Au sensor. Only super small (2 Hz) change of F can be found. In papers, the change is about 50 Hz by using 0.5 mg/ml solutions. I also tried using 10 mg/ml solution, the change is still only 15 Hz.
Can anyone help me to figure it out?
Thanks
Hello,
what kind of substrate are you using? Just bare Au? I guess you cannot expect much PEI to adsorb on the Au itself. You should try to use a negatively charged surface as substrate, such as SiO2, or you could also try to build a -COOH - terminated SAM on your Au sensors first.
What flow rate have you been using? I am surprised to see the first adsorption step of 10mg/L PEI to yield such a seemingly soft/dissipative layer. Where would it stabilize if you continue water rinsing for a longer time after this adsorption step?
Best regards,
Bernhard
Hi Bernhard,
Thanks for your answer!
I just got the instrument recently so that I am totally new in this area!. I only have bare Au and PS substrates now.
I cleaned the Au with ammonia, hydrogen peroxide aqueous solution and plasma.
The flow rate was 0.15 ml/min as the instruction recommended.
I checked several random papers about LbL work. I am surprised that they all used bare Au substrates!
Could you recommend any paper about people using SiO2 or -COOH modified Au substrate to do LbL?
Thank you so much!
Ziye
Dear Zyye,
PEI should attach to your clean gold substrate in bigger amounts using the conditions that you describe. A clean Au surface if properly prepared should be negatively charged, then PEI should attach easily to it. In my opinion it is either that your Au sensor is not cleaned necessarily (do you clean it just directly before using?) or there is some type of trouble with your PEI solution. How fresh is the solution? How long did you stir it before using it?
I do not think that you need to modify your Au surface with -COOH groups. In case you want, you just deep you Au sensor in 10 mM mercaptopropionic acid for a couple of hours and then rinse well.
Just a small comment about your data, wait longer for stabilization in water before flowing your polymer through. In the data you present you have a fast increase in F just before using your polymer.
Good luck!
Hi,
I agree with Roberto that PEI should adsorb easily on gold at pH5 if the sensor is clean. However, I would add some salts (it seems that you are using pure water) to prevent PEI-PEI repulsion and therefore obtain higher amounts of the polyelectrolyte on your surface. It is also a good suggestion to wait until you get a stable baseline and to me it looks like you are probably having some issues with air bubbles.
Another thing that I would like to mention is that larger changes in frequency do not necessarily mean that you have larger amounts of PEI since the QCM also senses the water trapped within you layer. I do not know how large is your molecule but for poly-L-lysine I usually get values of approx. 10-15 HZ for the first monolayer and very low dissipation changes.
Regards,
Alejandro
Hi
Thanks for Frank, Roberto and Alejandro!!!
I just have another result with another piece of Au sensor and uploaded.
The Au was cleaned and used 1 day later.
The results from PAH/PSS part seems reasonable. (PAH Polyallylamine hydrochloride,MW 120k, pH7) The BPEI/Algnite is a little bit weird. all polymers are 2mg/ml.
In my opinion, the instrument is at least ok. Au sensor could have already been reused too many times.
Any thought?
Thanks!!!
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