Organic synthesis: carbohydrates, peptides, dendrimers, oligo(ethylene glycol) (OEG) derivatives, and bioconjugation via copper-catalyzed alkyne-azide cycloaddition (CuAAC, a “click” reaction) under physiological conditions.

Thin films and surface functionalization: highly protein-resistant and stable monolayers on silicon and perfluorocarbon substrates, oxidation of polymer surfaces with UV/ozone and O2/CO2 plasma, dendrimer monolayers, and bioconjugation via click reactions on thin films, (bio)polymers, nanoparticles, tissue skeletons and cells.

Nanochemistry: micro- and nanopatterning of biomolecules on protein-resistant monolayers, single molecule patterns of monofunctionalized dendrimers, and modification of silicon AFM tips, liposomes, gold nanoparticles, quantum dots, and (bio)polymer nanoparticles.

While developing the above Chemistry centered on Synthesis and Characterization, we apply the knowledge, techniques and tools to solve suitable and significant biological and biomedical problems through collaborations, taking advantage of the excellent environment for biomedical research in Houston.

1) Carbohydrate surfaces for growing biofilms of benign E. coli

Greater than 30 million urinary catheters are used by 5 million patients each year in the US. Everyday, 3-10% of the patients acquire catheter associated urinary track infection (CAUTI), rendering it the second largest hospital acquired infection. Standard antimicrobial approaches are ineffective at preventing CAUTI due to the formation of biofilms of pathogens that are highly resistant to antimicrobial agents and host defenses. A new strategy is to grow a living, protective biofilm of benign E. coli, currently under clinical trial by our collaborator, Dr. Barbara Trautner’s group at the Baylor College of Medicine. We hypothesized that modifying catheter surfaces with carbohydrate ligands that specifically bind the benign E. coli will promote the growth of the benign biofilms and greatly enhance their interferences with the colonization of pathogenic bacteria.

This project involves the design, synthesis and optimization of carbohydrate ligands, development of the immobilization chemistry on model surfaces and on activated catheter surfaces, and the study of the adhesion and biofilm formation in response to the surface presentation of the carbohydrate ligands. Come back to check our publications!

2) Immobilized antimicrobial peptides

Antimicrobial peptides (AMPs) are the first line of defense in immune systems against invading pathogens in all living organisms. They exhibit a broad spectrum of antimicrobial activities and low susceptibility to development of bacterial resistance. The mechanistic action of AMPs has been the subject of numerous studies. In this project, we immobilize AMPs on model surfaces with a control on the density and micro- and nanoscale patterns. In addition, the flexibility of the molecules is varied by immobilization methods, including physisorption, random attachment via amidation, and site-specific attachment via click chemistry. We study how the surface presentation of AMPs affects their antimicrobial activity and cytotoxicity. The results of the study have provided new insights to the antimicrobial activities of AMPs in vivo. Furthermore, we have applied what we have learnt to modify contact lenses with AMPs, leading to high antibacterial efficiency and low cytotoxicity to human corneal epithelial cells. This work was in collaboration with Dr. Alison McDermott at the College of Optometry at UH.

3) Efficient catalysts of CuAAC reactions for bioconjugation

CuAAC reactions are highly specific and compatible to nearly all biomolecules and physiological conditions. However, using this click reaction for bioconjugation on live cells has not been successful due to the toxicity of Cu(I) catalysts to the cells. We are developing water-soluble copper catalysts to greatly improve the efficiency of this reaction, lowering the reaction time from hours to minutes, and copper concentration from milimolar to micromolar. We hope that further improvement of the catalytic efficiency will allow the reactions to be performed on live cells. The method has been used for modification of liposomes with ligands and imaging agents, in collaboration with Dr. King Li’s group at the Methodist Hospital Research Institute. We are also developing catalysts and conditions for click reactions on thin films, (bio)polymers, nanoparticles, and tissue skeletons.

4) Coating on Hap-Ti implants for bone regeneration

Titanium alloys have been implanted into patients with bone deficiency. To improve the biocompatibility of the metal, nanoparticles of hydroxyapatite (HAp), a bioactive ceramic, is coated with a gradient on Ti6Al4V using a pulsed laser by our collaborator, Dr. Gary Cheng at Purdue University. In addition to the preparation of HAp nanoparticles, we also modify the HAp surfaces with adhesion ligands and growth factors to promote the attachment, growth and differentiation of bone cells, in collaboration with Dr. Daniel Martinez at the Connective Tissue Physiology Laboratory at UH.

5) Monofunctionalized dendritic adsorbates

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-randomized 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. Come back to check our publications!

6) Highly protein-resistant, stable, and “clickable” monolayers on silicon

Robust, protein-resistant monolayers on silicon surfaces are ideal platforms for silicon-based biosensors. We use surface hydrosilylation to attach oligo(ethylene glycol) (OEG)-alkenes to hydrogen-terminated silicon surfaces to form OEG-terminated monolayers that are bound to the silicon surfaces via Si-C bonds. Using our optimized deposition conditions, high quality monolayers are routinely prepared in less than 3 h. These films reduce the adsorption of a wide variety of proteins to less than 0.5% monolayer and remained protein-resistant in PBS buffer at 37ºC for a month. We have also incorporated alkynyl groups to the films surface, allowing them to be functionalized with biomolecules via click reactions. The long-term goal of this project is the development of ideal interfaces for the electronic-ionic interactions between the implanted silicon microelectronic devices and neurons.

XPS of films derived from EG3, EG6, and EG9 on Si(111): C 1s region before (a) and N 1s region after (b) immersion in protein solution. (Chem. Commun, 2004, 21, 2510-2511)

7) Single molecule patterns

Positioning and subsequent derivatizing of individual single molecules with a precision of nanometers on surfaces represents an ultimate challenge on surface nanochemistry. The availability of this tool will allow for creating single molecule based model systems for studying multivalent and/or multi-component interactions in complex biological systems. Our approach is a combination of top-down nanopatterning using conductive AFM (c-AFM) and a bottom up synthesis of dendrimers as a nanocarrier of a defined number of functional moieties. We have used c-AFM to generate nanometric templates presenting COOH groups on the OEG-terminated monolayers. Avidin (a protein with many amino groups) 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 25 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 dendrimers possessing many amino groups at the periphery of the molecule (see below).

(Top) Illustration of c-AFM lithography on an OEG monolayer andthe sequential attachment of proteins to the generated nanotemplate. eight (a, 4 °— 4 μm2, 10 nm contrast) and friction (b, 0.2 Vcontrast) (Bottom) AFM images of an EG7 film on Si(111) after c-AFM lithographyof the same area upon treatment with succinic anhydride, DMAP, and pyridine. (J. Am. Chem. Soc, 2004, 126, 8098-8099)

8) Single molecule AFM tip

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 above). 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. 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.

Tapping-mode AFM images of a Nioprobe tip calibration sample, obtained with the same AFM tip before (a) and after (b) cleaning with Piranha solution, etching in HF solution (c), and then reaction with the OEG-alkene 1 (d). The images were acquired with a MultiMode IIIa AFM (Digital Instrument) under identical ambient conditions. Image size: 400 nm; z-scale (contrast): 10 nm; scan rate: 1 Hz. ( J. Am. Chem. Soc, 2003, 125, 7498-7499)