CHAPTER 3. FABRICATION OF A STANDARD RETICLE SLIDE FOR MALDI-IMS
3.6. Patterning using Surface Directed Self-assembly
66
remove the ORO was directly related to the thickness of the film, where films measured to be 40 nm by contact profilometry required 1 h for their complete removal.
Figure 3.13. (a) Optical image of the ORO patterns obtained after a PDMS stamp was peeled from the surface. A contact time of 30 min with the PDMS stamp was insufficient to remove ORO from the contacted regions; however, a contact time of 1 h completely removed ORO from the contacted regions. (b) A comparison of the photomask designed in CAD software along with the ORO pattern that was produced using diffusive lithography.
67
first step in the process was the patterning of a surface with a hydrophobic thiol. The adsorbed hydrophobic thiols then directed the deposition of the organic compound to be patterned in a second step into discrete areas. The method for surface functionalization is described below:
Surface functionalization
1. A standard gold-coated glass slide was cut into three square-shaped slides each measuring 1” x 1.”
2. A PDMS stamp was inked with the solution of hexadecanethiol (~2 mM) using a Q-tip.
The Q-tip was dipped into the solution of hexadecanethiol (HDT). The excess solvent was removed by grazing the tip against the rim. The entire surface of PDMS was gently coated with HDT.
3. 10 mL of H2O2 and 5 mL of KOH were poured into a glass Petri dish. Cut gold slides were immersed in this solution for 90 s to remove organic impurities from the gold surface. After 90 s, the slides were withdrawn from the solution, rinsed with water and ethanol, and dried in a stream of clean pressurized air.
4. After cleaning the gold slide, the stamp was brought into contact with the gold slide for 30 s to functionalize the gold surface. The edge of the stamp was first put in contact with the gold surface, and then the stamp was gradually tilted to bring it into full contact. This method minimized the possibility of trapping air bubbles.
3.6.1. Solvent Casting
After patterning a gold surface with a hydrophobic thiol by the PDMS stamp, the functionalized gold surface was used to direct the self-assembly of compounds in a spatially controlled manner. A saturated solution of the compound to be patterned was deposited on top of
68
the functionalized surface. As the solvent evaporated, crystals of the compound selectively deposited onto the bare gold regions and avoided deposition on the hydrophobic areas. As a result of this selectivity, a patterned coating of the compound was produced. The process of patterning α-cyano-4-hydroxycinnamic acid (CHCA) using this method is described below:
1. CHCA was dissolved at 10 mg/mL in Carnoy solution. Carnoy solution consists of 6:3:1 ethanol: chloroform: acetic acid
2. Water was added to this solution in a 1:1 ratio. The addition causes the solution to phase separate into organic and aqueous phases.
3. The aqueous phase was collected using a pipette and deposited onto the functionalized surface. As the solvent evaporated, the matrix crystallized and deposited in the hydrophilic regions. The solvent evaporation took about 30 min.
The directed self-assembly process is depicted in Figure 3.14. In the middle panel, the green areas represent gold regions that were stamped with the hydrophobic thiol, leaving the unstamped yellow areas more hydrophilic. A resulting pattern of CHCA produced by this process is shown in Figure 3.15.
Figure 3.14. Directed self-assembly method for patterning CHCA. PDMS stamp inked with HDT was brought into contact with the cleaned gold surface. CHCA solution was deposited onto
69
the functionalized gold surface where CHCA molecules selectively deposited into the patterned hydrophilic regions.
Figure 3.15. A patterned array of CHCA produced using self-assembly. Each circle of CHCA is about 200 µm in diameter, and the center-to-center distance is 300 µm.
Sinapinic acid was also patterned using this selective deposition process. The density of crystals could be easily controlled by varying the time of deposition. The sinapinic acid solution was pipetted onto the functionalized gold surface, and the solution was left on the slide for a specified period ranging from 1 to 11 min. As shown in Figure 3.16, the density of sinapinic acid increased as the time of deposition increased.
Figure 3.16. A microarray of sinapinic acid. The density of sinapinic acid crystal increased with the time of deposition.
70 3.6.2. Spin Coating
Selective deposition requires specific interactions between the substrate, the solvent and the solute. The process works well for relatively non-polar compounds such as sinapinic acid and CHCA. However, this method does not function for polar compounds. For example, crystal violet deposited in bulk on a functionalized gold and was not affected by the thiol pattern.
Further testing involved forming a liquid layer of dye solution on the substrate and then quickly tilting the slide to remove the bulk liquid. Only small amounts of liquid would remain on the surface, and these remnants were confined to regions of bare gold. This technique was simple but had low reproducibility and could not form complete patterns on a substrate. Deposition was much more successful when carried out in a spin coater. The centrifugal force generated by the spin coater removed the bulk dye solution. The general spin coating procedure was as follows:
the sample was spun at 600 rpm for 10 seconds during which the crystal violet solution can be deposited vertically. Typically, 300 µL of the solution was deposited. Most of the deposited solution was immediately removed from the substrate surface leaving behind a thin liquid film.
Next, the sample was accelerated to 2000 RPM, and this rate of rotation was maintained for 2 minutes. The high rotation rate reduced the thickness of the liquid film to a level that pulled away from hydrophobic regions to form a pattern.
Water, acetonitrile, acetone, ethanol, and various combinations of these solvents were attempted. Water was found to be too polar; droplets of CV in water quickly dewetted the functionalized gold surface leaving no pattern behind. It is hypothesized that the lower the surface tension of the solvent, the slower it de-wets and greater the likelihood of leaving behind dye in designed patterns. Ethanol provided the appropriate solvent for the formation of a pattern.
The solution was sonicated for 10 minutes and filtered with 0.45 µm mesh. A concentration
71
range between 2 – 20 mg/mL was attempted; 2 mg/mL generated a very thin pattern of CV whereas 20 mg/mL resulted in random blotches on the pattern. Solution with concentration of 10 mg/mL provided a good compromise between the amount of material deposited and ‘cleanliness’
of the pattern.