Biomedical subjects

Yuguang Cai

Publications and source records attributed to Yuguang Cai.

8 recordsLinked to original sources

PubMed2006

High resolution non-contact AFM imaging of liquids condensed onto chemically nanopatterned surfaces.

The wetting of ethanol and octane on chemically nanopatterned surfaces has been investigated using Atomic Force Microscopy (AFM) under controlled environmental conditions. The patterns were generated on a methyl-terminated, organic monolayer using an AFM electro-oxidation process. The subsequent wetting of the organic liquids was studied using non-contact mode AFM under equilibrium conditions with the vapor. This study of condensed nanoliquids provides the first reliable measurements of sub 100 nm liquid profile shapes. The derived contact angles give an estimate of the line tension.

Ethanol↗

PubMed2005

Electro pen nanolithography.

We introduce Electro Pen Nanolithography (EPN), a nanoscale chemical patterning technique. In a single sweep of an ink-coated, biased conducting Atomic Force Microscope (AFM) probe-tip, an underlying thin organic film is oxidized, and the ink molecules are transferred directly to the oxidized regions. Results using trialkoxysilane inks with functional terminal groups are presented, where the same ink-covered tips are used to image the pattern with zero bias. This technique provides very fast--exceeding 10 microm/s--writing speeds with line-widths as small as 50 nm. Controlled multi-write operations permit the growth of an integer number of molecular layers. Multiple chemical patterns using a single probe-tip are demonstrated.

Journal Article↗

PubMed2005

Large-scale fabrication of protein nanoarrays based on nanosphere lithography.

Hexagonally patterned lysozyme nanoarrays have been assembled on silicon wafers by combining nanosphere lithography and surface silane chemistry using vapor and solution deposition processes. The patterned protein regions extend over cm sized regions, and the size of each island is approximately 120 nm for the solution-prepared template and approximately 60 nm for the vapor-prepared template. Antibody test indicates that the patterned lysozyme maintains its bioactivity on the surface. This new approach offers a fast and reliable method to fabricate protein arrays over large areas with feature sizes comparable to scanning-probe based techniques.

Biosensing Techniques↗

PubMed2005

Structures formed by the chiral assembly of racemic mixtures of enantiomers: iodination products of elaidic and oleic acids.

The self-assembled monolayer structure of the products of elaidic acid iodination (the racemic mixture of 9,10-(9S,10R)-diiodooctadecanoic acid and 9,10-(9R,10S)-diiodooctadecanoic acid) and the products of oleic acid iodination (the racemic mixture of 9,10-(9R,10R)-diiodooctadecanoic acid and 9,10-(9S,10S)-diiodooctadecanoic acid) are studied by high-resolution scanning tunneling microscopy. For the iodination products of elaidic acid, the separation of enantiomers into distinct chiral domains during the formation of the 2-D crystal on the highly ordered pyrolytic graphite (HOPG) surface is not observed. Instead, within the diiodooctadecanoic acid SAM, each row of molecules is composed of opposite racemates. The two opposite racemates pack alternately inside a row, using different faces to adsorb on the surface. The unit cell is composed of a pair of opposite racemates, forming a heterochiral structure. For the iodination products of oleic acid, the racemic mixture is observed to exhibit quasi-phase separation during the formation of the 2-D crystal on the HOPG surface. Each row is composed of homochiral acid molecules, either the 9,10-(9R,10R)-diiodooctadecanoic acid (R) or the 9,10-(9S,10S)-diiodooctadecanoic acid (S). The R row and the S row pack alternately, with a unit cell composed of four molecules. Two of the molecules in the unit cell are the 9,10-(9R,10R)-diiodooctadecanoic acid (R) molecules; two are the 9,10-(9S,10S)-diiodooctadecanoic acid (S) molecules. In the unit cell, the two molecules that have the same chirality pack antiparallel inside the homochiral row, using different faces to adsorb on the surface. These results suggest that several different types of chiral assembly are possible. Enantiomers with opposite chirality exhibit many chiral assembly patterns, forming heterochiral structures on the surface in addition to separation to form macroscopic chiral domains. By using different conformations, similar enantiomers with opposite chirality will display many chiral assembly patterns to form heterochiral structures on the surface.

Fatty Alcohols↗

PubMed2005

Scanning tunneling microscopy studies of the self-assembly of carboxylic esters on graphite: linear distortion and multiple adsorption structures.

Self-assembled monolayers of carboxylic esters (stearic acid palmityl ester, lauric acid palmityl ester, and lauric acid behenyl ester) on graphite were investigated using scanning tunneling microscopy. All three esters, which are bent at the carboxylic group in the gas phase, are distorted into a straight-chain shape upon self-assembly on graphite. This results from optimizing the adsorption energy by matching the adsorbate molecular chain with the graphite substrate lattice periodicity. In all the formed lamellae, the long alkyl chain of the ester always aligns with the long chain of the adjacent molecule. Steric repulsion of the carbonyl group pointing perpendicularly to the neighboring molecule weakens the interaction of the ester molecule with the substrate. The ester molecules then easily self-assemble into multilamellae with molecular chain-trough angles of 73, 61, and 49 degrees in addition to the 90 degrees angle typical of n-alkane monolayers. This results from a shifting of 1/2, 1, or 3/2 units from the adjacent molecule in a lamella. The relatively weak interaction between ester molecules and substrate lattice also results in the formation of zigzag patterns with domain-domain angles of 145, 133, and 122 degrees , respectively. The structures of esters adsorbed on HOPG indicate, contrary to what might be expected, that physisorbed molecular adsorbates do not necessarily have the same geometry as in the gas phase.

Journal Article↗

PubMed2004

Automatic and subsequent dissolution and precipitation process in inorganic macroionic solutions.

We report an interesting phenomenon in the NaCl-containing aqueous solution of {Mo72Fe30} macroions, where dissolution and precipitation processes of hydrophilic macroions automatically and subsequently occur without changing external conditions or chemical reactions. Our previous work indicates that {Mo72Fe30} macroions tend to slowly self-assemble into single-layer, vesicle-like "blackberries". Such macroions have two solute states in solutions: the entropy-favored general state (homogeneous distribution) and the free-energy favored second solute state (blackberries). With additional salts, the originally stable blackberries become less stable due to their shortened screening length, and they tend to further aggregate and precipitate at much lower concentrations. Therefore, in such a solution, we can observe a subsequent process: crystal solids --> homogeneous single macroion solution --> homogeneous blackberry solution --> precipitates containing noncrystalline solids. In other words, we observed the behaviors of both soluble inorganic ions and colloids in the same solution due to the unique features of the macroions. Static and dynamic laser light scattering, as well as AFM measurements, were used to characterize both the macroionic solutions and the precipitates.

Journal Article↗

PubMed2004

Adsorption-induced asymmetric assembly from an achiral adsorbate.

Symmetry breaking in the self-assembled monolayer (SAM) structure of 1-octadecanol on highly ordered pyrolytic graphite (HOPG) is observed. Due to the slight mismatch of the octadecanol molecule with the graphite lattice, the alkane chain undergoes distortion upon adsorption on the surface. The asymmetric distortion of the octadecanol SAM unit cell pair is observed by scanning tunneling microscopy at the liquid/solid interface. Asymmetric distortion is due to the requirement for planarity of the hydrogen bond connecting the two octadecanol molecules in the chevron-shaped unit cell. This very simple structure provides the first example of an adsorption-induced distortion to form a supramolecular asymmetric structure, which is formed by achiral molecules adsorbed on an achiral surface. What makes this system interesting and different from other examples of adsorption-induced chirality is that the adsorbate itself undergoes asymmetric distortion due to the existence of the substrate and the adsorbate conformation is different from the molecule in solution.

Journal Article↗

PubMed2003

Chiral pair monolayer adsorption of iodine-substituted octadecanol molecules on graphite.

High-resolution scanning tunneling microscopy has been used to examine the adsorbate structures formed when a racemic mixture of (9R,10R)-9,10-diiodooctadecan-1-ol and (9S,10S)-9,10-diiodooctadecan-1-ol is adsorbed at the basal plane of highly ordered pyrolytic graphite. The herringbone structure characteristic of the adsorption of long-chain molecules on graphite is observed. Close examination of the micrographs indicates a unique structure in which the chiral molecules adsorb in pairs, with one enantiomer filling half of the unit cell, and the other enantiomer filling the other half. Instead of forming separate chiral domains, as is sometimes observed when a racemic mixture adsorbs on an achiral surface, chiral pairs are formed and the pairs form an ordered monolayer, exposing opposite faces of the same molecule. An achiral racemic mixture is observed to form a chiral structure on an achiral surface in the regions of the surface examined here.

Journal Article↗