Xiaoxi Chen

Bio: Xiaoxi Chen is an academic researcher from Harvard University. The author has contributed to research in topics: Self-assembled monolayer & Monolayer. The author has an hindex of 4, co-authored 5 publications receiving 1251 citations.

Zwitterionic SAMs that Resist Nonspecific Adsorption of Protein from Aqueous Buffer

Abstract: This paper describes the use of surface plasmon resonance spectroscopy and self-assembled monolayers (SAMs) of alkanethiols on gold to evaluate the ability of surfaces terminating in different combinations of charged groups to resist the nonspecific adsorption of proteins from aqueous buffer. Mixed SAMs formed from a 1:1 combination of a thiol terminated in a trimethylammonium group and a thiol terminated in a sulfonate group adsorbed less than 1% of a monolayer of two proteins with different characteristics: fibrinogen and lysozyme. Single-component SAMs formed from thiols terminating in groups combining a positively charged moiety and a negatively charged moiety were also capable of resisting the adsorption of proteins. Single-component SAMs presenting single charges adsorbed nearly a full monolayer of protein. The amount of protein that adsorbed to mixed zwitterionic SAMs did not depend on the ionic strength or the pH of the buffer in which the protein was dissolved. The amount of protein that adsorbe...

Molecular rectification in a metal-insulator-metal junction based on self-assembled monolayers

Abstract: An electrical junction formed by mechanical contact between two self-assembled monolayers (SAMs)--a SAM formed from an dialkyl disulfide with a covalently linked tetracyanoquinodimethane group that is supported by silver (or gold) and a SAM formed from an alkanethiolate SAM that is supported by mercury-rectifies current. The precursor to the SAM on silver (or gold) was bis(20-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl)) disulfide and that for the SAM on mercury was HS(CH(2))(n-1)CH(3) (n = 14, 16, 18). The electrical properties of the junctions were characterized by current-voltage measurements. The ratio of the conductivity of the junction in the forward bias (Hg cathodic) to that in the reverse bias (Hg anodic), at a potential of 1 V, was 9 +/- 2 when the SAM on mercury was derived from HS(CH(2))(15)CH(3). The ratio of the conductivity in the forward bias to that in the reverse bias increased with decreasing chain length of the alkanethiol used to form the SAM on mercury. These results demonstrate that a single redox center asymmetrically placed in a metal-insulator-metal junction can cause the rectification of current and indicate that a fixed dipole in the insulating region of a metal-insulator-metal junction is not required for rectification.

Redox Properties of Cytochrome c Adsorbed on Self-Assembled Monolayers: A Probe for Protein Conformation and Orientation

Abstract: The redox behavior of cytochrome c (cyt c) adsorbed to gold electrodes modified with self-assembled monolayers (SAMs) depends on the SAM. This paper examines SAMs generated from alkanethiols terminating in trimethylammonium (1), sulfonate (2), methyl (3), amine (4), and carboxylic acid (5) groups and from an aromatic thiol (6). The redox potentials of cyt c adsorbed on SAMs of 1 and 5 are relatively close to the formal potential of native cyt c measured in solution. The redox potentials of cyt c adsorbed on SAMs of 3, 4, and 6 are significantly shifted from the formal potential, and a reduction peak at about 0.5 V more negative than the formal potential (that is, a value corresponding to a more difficult reduction) was observed in all three cases. These observations suggest that cyt c changes its conformation significantly on adsorption on these surfaces. No redox peaks were observed for cyt c adsorbed on SAMs of 2, although surface plasmon resonance (SPR) studies indicate that the SAMs of 2 irreversibly ...

A prototype two-dimensional capillary electrophoresis system fabricated in poly(dimethylsiloxane).

Abstract: A method for carrying out 2D gel electrophoresis in a capillary format is presented. In this method, separation in the first dimension is carried out in a 1D capillary, with this system physically isolated from the capillaries that provide the separation in the second dimension. After completion of the first separation, the 1D channel is physically connected to the 2D capillaries, and a second separation is carried out in an orthogonal set of parallel capillaries. The ability of poly(dimethylsiloxane) (PDMS) to support the fabrication of 3D microfluidic systems makes it possible to produce membranes that both enclose the gel used in the first separation in a capillary and provide passages for the proteins to migrate into the array of orthogonal capillaries. The elastomeric nature of PDMS makes it possible to make reversible connections between pieces of PDMS. The feasibility of this system is demonstrated using a protein mixture containing fluorescein-conjugated carbonic anhydrase, fluorescein-conjugated BSA, and Texas Red-conjugated ovalbumin. This work suggests one type of design that might form the basis for a microfabricated device for 2D capillary electrophoresis.

Molecular electronics with a metal-insulator-metal junction based on self-assembled monolayers

Abstract: The mechanisms of electron transport in metal-insulator-metal junctions are incompletely understood. A metal-insulatormetal junction consisting of a self-assembled monolayer (SAM) supported on a mercury drop in mechanical contact with a SAM on a planar metal electrode has been developed as a test-bed with which to study electron transport through organic films. This review provides a summary of results intended to characterize this junction including: i) the determination of the electrical breakdown field of organic monolayers, ii) the determination of the tunneling decay constant for aliphatic and aromatic organic oligomers, and iii) the examination of molecular rectifier.

Surface Plasmon Resonance Sensors for Detection of Chemical and Biological Species

Abstract: 5.1. Detection Formats 475 5.2. Food Quality and Safety Analysis 477 5.2.1. Pathogens 477 5.2.2. Toxins 479 5.2.3. Veterinary Drugs 479 5.2.4. Vitamins 480 5.2.5. Hormones 480 5.2.6. Diagnostic Antibodies 480 5.2.7. Allergens 481 5.2.8. Proteins 481 5.2.9. Chemical Contaminants 481 5.3. Medical Diagnostics 481 5.3.1. Cancer Markers 481 5.3.2. Antibodies against Viral Pathogens 482 5.3.3. Drugs and Drug-Induced Antibodies 483 5.3.4. Hormones 483 5.3.5. Allergy Markers 483 5.3.6. Heart Attack Markers 484 5.3.7. Other Molecular Biomarkers 484 5.4. Environmental Monitoring 484 5.4.1. Pesticides 484 5.4.2. 2,4,6-Trinitrotoluene (TNT) 485 5.4.3. Aromatic Hydrocarbons 485 5.4.4. Heavy Metals 485 5.4.5. Phenols 485 5.4.6. Polychlorinated Biphenyls 487 5.4.7. Dioxins 487 5.5. Summary 488 6. Conclusions 489 7. Abbreviations 489 8. Acknowledgment 489 9. References 489

Surface modification of titanium, titanium alloys, and related materials for biomedical applications

Abstract: Titanium and titanium alloys are widely used in biomedical devices and components, especially as hard tissue replacements as well as in cardiac and cardiovascular applications, because of their desirable properties, such as relatively low modulus, good fatigue strength, formability, machinability, corrosion resistance, and biocompatibility. However, titanium and its alloys cannot meet all of the clinical requirements. Therefore, in order to improve the biological, chemical, and mechanical properties, surface modification is often performed. This article reviews the various surface modification technologies pertaining to titanium and titanium alloys including mechanical treatment, thermal spraying, sol–gel, chemical and electrochemical treatment, and ion implantation from the perspective of biomedical engineering. Recent work has shown that the wear resistance, corrosion resistance, and biological properties of titanium and titanium alloys can be improved selectively using the appropriate surface treatment techniques while the desirable bulk attributes of the materials are retained. The proper surface treatment expands the use of titanium and titanium alloys in the biomedical fields. Some of the recent applications are also discussed in this paper.

Soft Lithography in Biology and Biochemistry

Abstract: ▪ Abstract Soft lithography, a set of techniques for microfabrication, is based on printing and molding using elastomeric stamps with the patterns of interest in bas-relief. As a technique for fabricating microstructures for biological applications, soft lithography overcomes many of the shortcomings of photolithography. In particular, soft lithography offers the ability to control the molecular structure of surfaces and to pattern the complex molecules relevant to biology, to fabricate channel structures appropriate for microfluidics, and to pattern and manipulate cells. For the relatively large feature sizes used in biology (≥50 μm), production of prototype patterns and structures is convenient, inexpensive, and rapid. Self-assembled monolayers of alkanethiolates on gold are particularly easy to pattern by soft lithography, and they provide exquisite control over surface biochemistry.

Antifouling coatings: recent developments in the design of surfaces that prevent fouling by proteins, bacteria, and marine organisms.

Abstract: The major strategies for designing surfaces that prevent fouling due to proteins, bacteria, and marine organisms are reviewed. Biofouling is of great concern in numerous applications ranging from biosensors to biomedical implants and devices, and from food packaging to industrial and marine equipment. The two major approaches to combat surface fouling are based on either preventing biofoulants from attaching or degrading them. One of the key strategies for imparting adhesion resistance involves the functionalization of surfaces with poly(ethylene glycol) (PEG) or oligo(ethylene glycol). Several alternatives to PEG-based coatings have also been designed over the past decade. While protein-resistant coatings may also resist bacterial attachment and subsequent biofilm formation, in order to overcome the fouling-mediated risk of bacterial infection it is highly desirable to design coatings that are bactericidal. Traditional techniques involve the design of coatings that release biocidal agents, including antibiotics, quaternary ammonium salts (QAS), and silver, into the surrounding aqueous environment. However, the emergence of antibiotic- and silver-resistant pathogenic strains has necessitated the development of alternative strategies. Therefore, other techniques based on the use of polycations, enzymes, nanomaterials, and photoactive agents are being investigated. With regard to marine antifouling coatings, restrictions on the use of biocide-releasing coatings have made the generation of nontoxic antifouling surfaces more important. While considerable progress has been made in the design of antifouling coatings, ongoing research in this area should result in the development of even better antifouling materials in the future.