To achieve an ultra-thin silane coating with DBCO-C6-trimethoxysilane, you can consider several strategies to reduce the thickness of your coating:

1. Concentration Adjustment

• Dilution: Significantly dilute the silane solution to decrease the deposition rate. For ultra-thin coatings, you might need to work with very low concentrations, possibly in the range of 0.01-0.1% silane by volume.
• Solvent Choice: Ensure that the solvents (DMSO, DCM, acetone) are compatible and do not cause aggregation or rapid hydrolysis of the silane. Sometimes, using a single solvent or a different combination can help in achieving thinner layers.

2. Deposition Technique

• Spin Coating: This technique can help achieve uniform and ultra-thin coatings. Adjust the spin speed and time to control the thickness. Higher speeds and longer spin times generally result in thinner layers.
• Dip Coating: Control the withdrawal speed from the silane solution. Slower withdrawal speeds can result in thinner coatings.

3. Reaction Conditions

• Reaction Time: Reduce the reaction time. A shorter exposure (e.g., a few minutes) might be sufficient for forming a monolayer.
• Controlled Humidity: Conduct the reaction in a controlled humidity environment to slow down the hydrolysis and condensation reactions, potentially resulting in thinner layers.
• Temperature: Conduct the reaction at a lower temperature to slow down the reaction kinetics.

4. Post-Treatment

• Rinsing: After the initial deposition, rinse the substrate thoroughly with an appropriate solvent to remove any loosely bound or excess silane molecules.
• Annealing: Gentle annealing at a low temperature might help in stabilizing the monolayer without adding significant thickness.

5. Surface Preparation

• Pre-treatment: Ensure the silicone wafer is thoroughly cleaned and activated. Plasma treatment or UV-ozone cleaning can increase the number of active sites on the surface, allowing for more controlled and possibly thinner silane layer formation.

Example Protocol

Monitoring and Characterization

• AFM: Use Atomic Force Microscopy (AFM) to measure the thickness of the coating and ensure it meets your ultra-thin criteria.
• Contact Angle Measurement: Assess the hydrophobicity of the surface using contact angle measurements to confirm the surface is hydrophobic.

By carefully optimizing these parameters, you should be able to achieve an ultra-thin and uniform silane coating that meets the requirements of your application.

Mohammed's suggestions from earlier are all very excellent ideas for generating high-quality surface coatings of siloxanes. It may be possible to produce highly repeatable layers using those methodologies but you will need to very precisely control process conditions every time.

The central issue you have is that your choice of silane has three reactive groups that tend to crosslink and create multilayers. To create a true monolayer with silanes, monofunctional silanes are the optimal choice for silicon oxide surfaces. If you can find DBCO-C6-dimethylmethoxysilane (or the Me2SiCl- analog) then it will be self-limited to only react with surface silanols. Any cross-condensation of siloxane can be washed away with solvent. To increase graft density, exposure to acid cleaning (simple piranha acid recipe is good) then extensive DI rinse, dry then immediate exposure to the silane in solvent at 60C for 30 min to 1 hr should work best. Alternatively it is reported that 24 hr/85C hydrothermal treatment can significantly increase silanol content.

If you cannot find the aforementioned derivate, a two-step approach of first reacting the surface with 3-aminopropyldimethylmethoxysilane then coupling DBCO-C6-NHS ester to that surface could work.

If all else fails, try to find a vinyl-functional DBCO-C6 derivative then HF etch the oxide away to generate Si-H at the surface and use hydrosilylation chemistry to attach the DBCO-C6

Good luck!