Chengzhi Cai

Our research can be divided into the following four intercalated themes, involving organic synthesis, dendrimers, organic thin films, surface reactions, AFM, bio-fouling, protein nanoarrays, AFM tip modification, nanoparticles, and biomaterials.

1. Precise control of the spacing between functional groups at nano-scale on organic thin films remains a challenge. The spacing between functional groups on a surface is a key parameter influencing the efficiency of these groups to fulfill their functions, e.g., as handles for attaching other large molecules or as sensor molecules for specific binding of large target molecules. While the average density of surface functional groups on self-assembled monolayers (SAMs) can be adjusted by co-deposition of a mixture of functional and inert adsorbates, aggregation of functional adsorbates due to non-random mixing has been shown to exist on many mixed SAMs even they are prepared from very similar adsorbates. We are developing a new strategy to tackle this problem, based on the use of symmetrical, multidentate absorbates having only one functional group at the center of the molecule. To demonstrate this concept, we synthesized a series of dendron molecules with a functional group at the focal point and many surface-active groups, such as ethenyl, trichlorosilyl, thiol, or amino groups, at the periphery of the dendron for chemisorption on hydrogen-terminated silicon, oxide, gold, or carboxy surfaces, respectively. Upon chemisorption, the dendron molecules flatten on the surface, exposing the focal functional groups on the film surface. The spacing between these functional groups is then defined by the size of the dendron.

2. Robust, protein-resistant monolayers on silicon surfaces are ideal platforms for silicon-based biosensors. We used surface hydrosilylation to attach a-oligo(ethylene glycol) (OEG)-w-alkenes to hydrogen-terminated silicon surfaces to form OEG-terminated monolayers that are bound to the silicon surfaces via Si-C bonds. We systematically investigated the influences of the lengths of the alkyl and OEG chains on the packing density and the protein resistance of the monolayers using a series of OEG derivatives: CH₂=CH(CH₂)_m(OCH₂CH₂)_nOCH₃ (m = 1, 3, 4, 8, 9, 16, 17; n = 3-9). We also designed an apparatus for photo-induced surface hydrosilylation in high vacuum. With our setup, only ~1 mg of sample was needed, and the deposition was rapid. The resultant monolayers were characterized by contact angle, XPS, ellipsometry, and AFM measurements. The results showed that most of the monolayers were ultra-flat, and the packing density was dependent upon the alkyl and EG chain length (m and n) as well as the deposition conditions. The protein resistance of these films was also dependent upon m and n, and increased with higher packing densities. The data contribute to the understanding of the origin of resistance of protein adsorption on OEG films. Using the lead compounds and optimized deposition conditions, high quality monolayers were routinely prepared in less than 3 h, and the procedure was as simple as the common procedure used for preparing alkanethiolate SAMs on Au(111) surfaces. These films resisted the adsorption of a wide variety of proteins to less than 0.5% monolayer.

3. Precise control of the location of individual biomolecules on bio-compatible surfaces will allow preparation of ultra-sensitive biosensors, and single molecule based model systems for studying multi-valent and/or multi-component interactions in complex biological systems. To this end, we employed conductive AFM (c-AFM) to generate nanometric templates presenting COOH groups on the above OEG-terminated monolayers. Macromolecules containing accessible amino groups, such as avidin, were selectively attached to the nano-templates, and served as handles for anchoring biotinated molecules. The feature size of the protein arrays is currently about 26 nm. To reduce the feature size, we have carried out a systematic study of the mechanism of the c-AFM process. On the basis of our proposed mechanism, it is possible to reduce the size of the oxidized spots to a few nanometers, which can accommodate no more than one focally functionalized macromolecules possessing many amino groups at the periphery of the molecule (see Research Theme 1). In this way, single molecule arrays could be fabricated and used for label-free detection of target molecules by AFM using thin, small, and OEG-coated silicon cantilever tips.

4. Modification of silicon cantilever tips with OEG derivatives is expected to greatly improve the resolution of AFM imaging and force measurement of biological samples. AFM imaging and measurement are based on the interactions between a small number of molecules at the tip apex and substrate surface. The chemistry at the tip apex, such as chemical composition, density, location, and orientation of the functional moieties, is thus paramount to many applications of AFM in biology. However, these parameters remain poorly defined. Also, commercial AFM tips have non-specific interactions with proteins and other biomolecules, thus lowering the image resolution and contrast, and causing deformation of the fragile biomolecules (for example, the protein molecules weakly bound to the nano-arrays might be removed by the tip). To reduce the non-specific interactions of biomolecules with the tip, we modified silicon AFM cantilever tips with OEG derivatives using surface hydrosilylation (see Research Theme 2). We showed that the modified tips effectively resist non-specific interactions with a variety of proteins. Importantly, the tip size remained small for high-resolution imaging. In collaboration with the Pei group at UH, we also fabricated small and thin (~ 200 nm in thickness) silicon cantilever tips on SOI wafers. These cantilever tips exert a much lower force to the sample than the commercial ones, while maintaining a high resonance frequency for fast imaging. Coating of these cantilever tips on the wafers with an OEG-terminated monolayer can be carried out in a large scale. By applying a bias voltage to the OEG-coated tips, we could selectively oxidize the molecules at the apex of the AFM tip to introduce a few COOH groups to which a variety of probe molecules could be tethered for measurement of the specific interaction with target molecules.

5. Development of materials for bone regeneration will be facilitated by gaining understanding of how the nanoscale presentation of cell adhesive ligands on a variety of materials influences cell function and fate. In collaboration with David Mooney’s group at Harvard University, we are developing methods to generate model systems with a precise control of the presentation of RGD ligands with a resolution of 20 nm, and study the cell response to these systems. We also collaborate with the groups of Gary Cheng at College of Technology, and Daniel Martinez at the Connective Tissue Physiology Laboratory, on the preparation and functionalization of coating of nanoparticles of hydroxyapatite (HAp) on titanium materials.