In order to uniformly coat the inner surface of the pipette tips composed of polypropylene (Diamed Lab Supplies Inc.), they were placed onto a cylinder-shaped apparatus which acted as a holding device. The apparatus with tips were oxygen plasma treated and silanized with trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane (TPFS) using the CVD method. This allows the hydrophilic terminal (trichlorosilane) of the fluorosilane molecule to bind to the plasma induced hydroxyls on the surface of the tips, resulting in self-assembled monolayers (SAMs) of fluorosilane (FS) with an umbrella-shaped structure in a way that the fluorine terminals are exposed on the surface (Fig. 1). The fluorinated tips were then lubricated by simply pipetting in and out a fluorinated lubricant called perfluoroperhydrophenanthrene (PFPP).
To analyze the changes in the chemical composition of the pipette tip surface after oxygen plasma treatment and after CVD modification, fourier-transform infrared spectroscopy (FT-IR) was performed on 1 mL fluorosilanized and untreated pipette tips (Fig. 2a). The spectra of both fluorosilanized and untreated tips had a number of peaks in the absorption band between 800 to 1200 cm−1, specifically at 850 cm−1, 1000 cm−1, and 1200 cm−1 vibrations that are characteristic for isotactic polypropylene. These absorption bands can be interpreted as vibrations of C-H, CH2 and CH3 groups in the polymer chain 30. In the fluorosilanized tips, however, a broad absorbance band is present around the 3700-3200 cm−1. This is believed to correspond to the Si-OH group bonds formed as a result of the hydrolysis of TPFS molecules. It should be mentioned that 3700-3200 cm−1 is the range of absorbance bands that commonly corresponds to alcohols and phenols, including bonds to silicone compounds 30. This pattern is notably absent when looking at the untreated tip sample spectra, which produces a flat line over the 4000-3000 cm−1 region (Fig. 2a).
To examine the relative hydrophobicity/hydrophilicity of the treated and untreated pipette tips, contact angle measurements were performed using a 5 µL droplet of deionized water. The static contact angle measurements of the untreated tips, fluorosilanized tips, and lubricant infused tips are shown in Fig. 2b. Untreated tips demonstrated a lower contact angle (θst= 75 ± 11°) implying low levels of hydrophobicity. It is worth noting that the standard deviation value of the untreated samples is relatively high due to the conditions of the untreated tip surface. Upon CVD treatment and before lubricant addition, the contact angle increased to 89 ± 3°. Finally, after addition of PFPP lubricant to the fluorosilanized tips, contact angles of water were demonstrated to increase to 101.7 ± 7°.
Due to the surface curvature of the pipette tip, measuring the contact angle of a liquid droplet on the tip using a tensiometer presents complications and might result in slight inaccuracy. As such, the contact angle of water, tween 20, and isopropanol as a low surface tension liquid were measured on treated and standard pipette tips using capillary rise method. Capillary rise can be utilized to compute the contact angle of the rising/dropping liquid using hydrostatics principles. Due to the close resemblance of the tips to a truncated cone shape, the capillary rise equation was modified to account for such capillary shape (equation 1):
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where θ is the contact angle, ρ is the density of the liquid, g is the gravitational constant, h is the rise/drop height, R is the diameter of tube at the three-phase region, γ is the surface tension of the liquid, and β is the conical angle of tip.
As such, the contact angles of liquids with various surface tensions on the fluorosilanized and untreated tips were acquired to test the effect of surface modification on omniphobic properties of the tips (Fig. 2c). The contact angles of all three liquids were higher on the fluorosilanized tips compared to untreated tips. This is especially evident for the lower surface tension liquids such as the tween 20 and isopropanol. The increase in the contact angles of the treated samples highlight the higher surface repellency induced by the treatment. The contact angle of tween 20 was 30° higher on the fluorosilanized surface as compared to the standard while that of isopropanol was 28° higher. Since the tips exhibited hydrophobic properties even prior to surface treatment due to the properties of the material used to manufacture the tips, there was a smaller difference in contact angle of water on the treated surface with 6.9° increase using the capillary rise method and 40° increase using the optical tensiometer. Notable, the hydrophobic nature of the intact pipette tips was not fully appreciated in Fig. 2b which could be due to the fact that the surface properties of the tips somewhat changed when the tips were cut and flattened for the purpose of the contact angle measurement.
In order to study the efficacy of our surface medication in preventing the physical (non-specific) adsorption of biomolecules and reagents, we first performed a visual test through pipetting in and out a food coloring red dye diluted in water containing 0.05% tween 20. As it can be seen in Fig. 3a, the untreated 200 μL pipette tip showed a great amount of dye adsorption onto the tip, whereas the lubricant infused tip maintained significantly lower amount of dye mostly at the bottom end of the tip after pipetting out the dye. In the next experiment, we used lubricant infused tips at different sizes of 10 μL, 200 μL, and 1 mL to pipette in and out recalcified citrated human blood. As it is demonstrated in Fig. 3b, the lubricant infused tips in all sizes could effectively suppress blood adhesion and clot formation inside the tip in comparison with the untreated tips. SEM images in Fig. 3c also exhibits the presence of blood cells and clot on the surface of the untreated tips, while the lubricant infused modification significantly reduced the cell attachment and clot formation.
To further quantify the effects of lubricant infused technology on the functionality of the pipette tips, 10-fold serial dilutions of 1 mg mL−1 dye were performed in water containing 0.05% tween 20. The same pipette tip was used to perform all the dilution steps to better illustrate the effect of dye residue inside the tip during the serial dilutions. Using absorbance measurements (at the wavelength of 512 nm), we quantified the results obtained from lubricant infused and untreated pipette tips in two sizes of 10 µL and 200 µL shown in Fig. 4a and b, respectively. The results of the dilutions indicate that the untreated tips result in have significantly higher carryover residue; as a result, the dilutions did not occur efficiently. Thus, the more amount of dye that can be seen at lower concentrations could be due to the dye residue that had remained onto the tip surface from the initial high concentrations and was carried to the low concentrations.
As can be seen from the Fig. 4a and b, the lubricant infused tips show a steeper decrease in absorbance per dilution number followed by a sooner levelling off to the diluent’s absorbance value (with zero concentration of the dye) that is indicated by dashed line in Fig. 4a. On the other hand, the untreated tips reveal higher absorbance levels even for higher dilution numbers which is a result of the sample adhering to the tip’s wall even after performing several dilutions. In addition, there is significantly higher variation observed with untreated tips compared to treated ones as indicated by large error bars shown in Fig. 4a and b. In Fig. 4c, the colour difference in the well-plate after performing the serial dilutions via the lubricant infused tips and untreated tips could be observed.