I have been using QCM-D to study binding affinity. I pre-coated the chips with cysteamine to present amine groups on the surface and then use it to bind -NHS end of NHS-PEG-maleimide. The maleimide end then is expected to bind with cysteine presented in the terminus of a peptide. Do you think these types of SAM layers ( with chemical reaction) can be used for QCM ( I am getting low frequency shifts~ 10)? Also I have observed that the chips are changed in color and turned into black after coating with cysteamine! (10 mM Cysteamine solution for 2 hours under nitrogen flow in a dark) what can be the reason?
The metal film coating of QCM sensor is that, the first layer is chromium, and then the second is gold. From your experiment, I can feel that the sensor you used has been damaged. Gold layer has come off and the black color is chromium layer. So take a new QCM sensor chip to study binding affinity for your experiment.
Dear Hadi Hezaveh,
Regarding your first question, as long as the QCM substrates you are using have gold electrodes, then the formation of cysteamine SAMs is feasible [1].
Regarding the colour change of QCM substrates, the gold electrode peel-off seems unlikely, as it would require strong oxidizers to do so [2]. To the best of my knowledge, silver electrodes have been known to peel off QCM substrates during cell detection [3]. Perhaps the paper of Lee et al. [2] may give you some answers about the structure of cysteamine SAMs with respect to process variables such as the period of immersion and the type of solvent.
[1] Lee, S.Y., Noh, J., Ito, E., Lee, H., Hara, M., "Solvent Effect on Formation of Cysteamine Self-Assembled Monolayers on Au(111)", Japanese Journal of Applied Physics 42 (2003) 236.
[2] http://www.microchemicals.eu/technical_information/gold_etching.pdf
[3] Chou, H.-C., Yan, T.-R., Chen, K.-S., "Detecting Cells on the Surface of a Silver Electrode Quartz Crystal Microbalance Using Plasma Treatment and Graft Polymerization", Colloids and Surfaces B: Biointerfaces 73 (2009) 244-249.
I believe you might have to give some additional information regarding your experiments. In which solvent do you dilute the Cysteamine? You're working with gold coated chips, correct?
To measure affinity and kinetics a would recommend you use a system that is designed for that, e.g. the QCM-system from Attana. In that case you can obtain different kind of surface and a fluidic system optimized for these applications.
The black stain you observe is probably black colored metal-sulfide originating from your cysteamine preparation (You also find similar black sulfide is you boil egg too hard). Be careful, the S affinity to gold is not very strong, only half of the binding stength compared to C-C (Carbon - carbon)
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