Formation and properties of molecular gradients

Recent advances in the field of self-assembly have led to the development of a plethora of new technologies based on soft lithography that enable alternative ways of creating two- and three-dimensional chemical patterns on material surfaces. Many soft lithography techniques are based on controlled deposition of self-assembled monolayers (SAMs). Various structural patterns with dimensions ranging from hundreds of nanometers to several micrometers are created on the material surface using a “pattern-transfer element” or stamp that has a three-dimensional structure moulded onto its surface.

While useful for decorating substrates with well-defined chemical patterns of various shapes and dimensions, the soft-lithography technologies always produce sharp boundaries between the distinct chemical regions on the substrate. For some applications, it is desirable that the physico-chemical characteristics, such as wetting of the substrate, change gradually. This can be accomplished by producing surfaces with a position dependent and gradually varying chemistry. In these so-called “gradient surfaces”, the gradient in surface energy is responsible for a position-bound variation in physical properties, most notably the wettability. These gradient substrates can be useful in high-throughput studies of the interfacial behavior of molecules and macromolecules (the entire behavioral spectrum can be accessed in a single experiment), can serve as templates for further processing, or be used as active elements in controlled surface transport of materials.

image of gradient

Our group has been studying the formation and properties of molecular gradients based on organosilane precursors on material substrates. We form the molecular gradients using the method developed by Chaudhury and Whitesides [Science 256, 1539 (1992)].

image of gradient prep

In their technique, the diffusing source, consisting of a mixture of the organosilane and a paraffin oil (PO), was placed on one side of the silica-covered substrate; the whole system was placed into a closed container. The concentration of the diffusing source can be conveniently adjusted by simply varying the organosilane:PO ratio. As the silane evaporated, it diffused in the vapor phase and generated a concentration gradient along the substrate. Upon impinging on the substrate, the organosilane molecules reacted with the substrate –OH functionalities on the substrate and formed a wettability gradient. Using this set up, Chaudhury and Whitesides demonstrated the ability of gradient substrates to set liquids in motion (they moved a water droplet uphill) and established the effect of the contact angle hysteresis on the droplet transport.

One of the most important properties of a molecular gradient is its wettability, which is determined primarily by the terminal group of SAM and concentration of molecules attached to the substrate at a given position along the gradient. In the figure on the right we show the contact angles of water measured on “double” molecular gradients made by vapor deposition of n-octyl trichlorosilane (OTS) (OTS:PO=1:5) from two opposite ends of the silica-covered silicon substrate. The data in the figure on the right show that at short diffusion times the two diffusion profiles stay isolated, forming a “wettability well”. However, with increasing the diffusion time of the molecules in the vapor, the two profiles start to interfere and the wettability well starts to fill up. The notion of double gradients can be further extended to form gradients of two chemically different species.

plot of gradient

In addition, we are using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy to study the formation and properties of molecular gradients formed by vapor deposition of semifluorinated (SF) mono- (F3C(CF2)8(CH2)2Si(CH3)2Cl, m-F8H2), di- (F3C(CF2)8(CH2)2Si(CH3)Cl2, d-F8H2), and tri- (F3C(CF2)8(CH2)2SiCl3, t-F8H2) organosilanes on silica-covered substrates.

plot of gradual variation

The gradual variation of the grafting density of the surface-bound molecules is expected to have profound influence on the organization of the molecules in the gradients. By studying how the gradient-forming molecules arrange across the gradient interfacial region one can learn more about the mechanisms and nature of self-assembly in organosilane SAMs. The gradient geometry offers the advantage of constraining the self-assembly growth into a given direction. This is in contrast to the classical case of “uniform” self-assembly on a substrate, where the incorporation of the molecules in the final SAM takes place at random in all directions. The ability of NEXAFS to determine the molecular orientation of the surface-bound molecules can be utilized to study the orientation of the SAMs across the gradient. In the figure on the left we plot the dependence of fraction of the F8H2 molecules on the surface (normalized by the maximum SAM coverage) (solid lines) and the variation of the average tilt of the semifluorinated part of the F8H2 molecule with respect to the surface normal, τF8, (dashed lines) as a function of the position on the silica surface for mono-, di-, and tri-functional 1H,1H,2H,2H-perfluorodecyl organosilanes, m-F8H2, d-F8H2, t-F8H2, respectively.

One of the limitations of the vapor deposition technique is that the current set up produces rather broad wettability gradients with very little tunability. Hence, our goal was to develop technologies leading to greater control over the characteristic size of the gradients. On a flexible substrate, this can be achieved by fine-tuning the grafting density of molecules by fabricating mechanically assembled monolayers (MAMs), structures that are based on synergism between self-assembly and mechanical manipulation of the grafted molecules on surfaces. The technique is based on the combination of i) the well-known grafting reaction between ω-(CH2)nSiCl3 molecules (typically, ω=-CH3, -CF3, -NH2, -CH=CH2, -CN) and -OH functionalities present on silicon-based surfaces, and ii) mechanical manipulation of the grafted ω-(CH2)nSiCl3 molecules on the substrate.

The method consists of five operational steps. First, a pristine poly(dimethyl siloxane) (PDMS) network film is prepared by casting a mixture of PDMS and a cross-linker into a thin film (thickness ca. 0.5 mm) and curing at 55 °C for about 1 h. In the second step, the cross-linked PDMS substrate is cut into small strips (ca. 1.5x5cm2) and uniaxially stretched to various strains, Δx. The stretched substrate is then exposed to an ultraviolet/ozone (UVO) treatment to produce the -OH surface functionalities (PDMS-UVO). The molecular gradient of ω-(CH2)nSiCl3 SAM is then formed following the vapor diffusion technique described above. Finally, the strain is released from the PDMS-UVO film, which returns to its original size, causing the grafted ω-(CH2)nSi- molecules to form a gradient with a steeper concentration profile as compared to non-stretched PDMS-UVO substrate.

plot of static contact angles of deionized water along gradient substrates

The figure above shows static contact angles of deionized water along gradient substrates prepared with Δx ranging from 0 to 50 %. In all cases, the exposure time of the PDMS to the UVO was 30 mins, and the vapor diffusion time from a 1:10 OTS:PO mixture was 5 min. The data show that, as expected, the gradient steepness changes with changing Δx. Specifically, the span of gradient decreases from ca. 40 mm down to 15 mm as Δx increases from 0 to 50 %, respectively. In all cases the profiles exhibit excellent Fickian-type diffusion profiles.