Jean-Rene Ella-Menye

One-step dip coating of zwitterionic sulfobetaine polymers on hydrophobic and hydrophilic surfaces.

Zwitterionic sulfobetaine polymers with a catechol chain end (DOPA-PSB) were applied to a variety of hydrophobic polymer sheets and fibers. In addition, a silica surface was tested as a representative hydrophilic substrate. The polymer-coated surfaces showed significantly lower fouling levels than uncoated controls. Because of the anti-polyelectrolyte nature of sulfobetaine zwitterionic polymers, the effect of salt concentration on the coating solutions and the quality of the polymer coating against fouling are studied. The coating method involves only water-based solutions, which is compatible with most surfaces and is environmentally friendly. To demonstrate the versatility of the reported method, we evaluated the fouling levels of the polymer coating on commonly used polymeric surfaces such as polypropylene (PP), polydimethylsiloxane (PDMS), polystyrene (PS), nylon, polyvinyl chloride (PVC), and poly(methyl methacrylate) (PMMA).

Sensitive and Fast Detection of Fructose in Complex Media via Symmetry Breaking and Signal Amplification Using Surface-Enhanced Raman Spectroscopy

A new strategy is proposed to sensitively and rapidly detect analytes with weak Raman signals in complex media using surface-enhanced Raman spectroscopy (SERS) via detecting the SERS signal changes of the immobilized probe molecules on SERS-active substrates upon binding of the analytes. In this work, 4-mercaptophenylboronic acid (4-MPBA) was selected as the probe molecule which was immobilized on the gold surface of a quasi-three-dimensional plasmonic nanostructure array (Q3D-PNA) SERS substrate to detect fructose. The molecule of 4-MPBA possesses three key functions: molecule recognition and reversible binding of the analyte via the boronic acid group, amplification of SERS signals by the phenyl group and thus shielding of the background noise of complex media, and immobilization on the surface of SERS-active substrates via the thiol group. Most importantly, the symmetry breaking of the 4-MPBA molecule upon fructose binding leads to the change of area ratio between totally symmetric 8a ring mode and nontotally symmetric 8b ring mode, which enables the detection. The detection curves were obtained in phosphate-buffered saline (PBS) and in undiluted artificial urine at clinically relevant concentrations, and the limit of detection of 0.05 mM was achieved.

Reversibly switchable polymer with cationic/zwitterionic/anionic behavior through synergistic protonation and deprotonation

A polymer capable of fully and reversibly switching throughout the entire charge regime is highly desirable for many applications such as drug and gene delivery, controlled ion and molecular transport and tunable filtration membranes. It is essential that for biologically relevant applications the polymer needs to be nonfouling. However, conventional nonfouling zwitterionic polymers have a permanently positive quaternary nitrogen center, making it impossible to switch between charges. Here, we present a rationally designed polymer with a tertiary amine and a carboxylic acid, which is capable of reversibly switching among three distinct charged states, viz., cationic, zwitterionic and anionic, and importantly maintaining the zwitterionic state under physiological pH conditions. Oppositely charged proteins adsorbed on a charged surface selectively can be completely removed by switching the surface to the zwitterionic state. We have also found that these two moieties (i.e., a tertiary amine and a carboxylate moiety) stimulate each other synergistically to achieve a strongly zwitterionic state under physiological conditions and to resist non-specific protein adsorption from undiluted blood plasma and serum when they are close to each other.

Stealth surface modification of surface-enhanced Raman scattering substrates for sensitive and accurate detection in protein solutions.

Reliable surface-enhanced Raman scattering (SERS) based biosensing in complex media is impeded by nonspecific protein adsorptions. Because of the near-field effect of SERS, it is challenging to modify SERS-active substrates using conventional nonfouling materials without introducing interference from their SERS signals. Herein, we report a stealth surface modification strategy for sensitive, specific and accurate detection of fructose in protein solutions using SERS by forming a mixed self-assembled monolayer (SAM). The SAM consists of a short zwitterionic thiol, N,N-dimethyl-cysteamine-carboxybetaine (CBT), and a fructose probe 4-mercaptophenylboronic acid (4-MPBA). The specifically designed and synthesized CBT not only resists protein fouling effectively, but also has very weak Raman activity compared to 4-MPBA. Thus, the CBT SAM provides a stealth surface modification to SERS-active substrates. The surface compositions of mixed SAMs were investigated using X-ray photoelectron spectroscopy (XPS) and SERS, and their nonfouling properties were studied with a surface plasmon resonance (SPR) biosensor. The mixed SAM with a surface composition of 94% CBT demonstrated a very low bovine serum albumin (BSA) adsorption (∼3 ng/cm(2)), and moreover, only the 4-MPBA signal appeared in the SERS spectrum. With the use of this surface-modified SERS-active substrate, quantification of fructose over clinically relevant concentrations (0.01-1 mM) was achieved. Partial least-squares regression (PLS) analysis showed that the detection sensitivity and accuracy were maintained for the measurements in 1 mg/mL BSA solutions. This stealth surface modification strategy provides a novel route to introduce nonfouling property to SERS-active substrates for SERS biosensing in complex media.

Achieving One-Step Surface Coating of Highly Hydrophilic Poly(Carboxybetaine Methacrylate) Polymers on Hydrophobic and Hydrophilic Surfaces

It is highly desirable to develop a universal nonfouling coating via a simple one-step dip-coating method. Developing such a universal coating method for a hydrophilic polymer onto a variety of surfaces with hydrophobic and hydrophilic properties is very challenging. This work demonstrates a versatile and simple method to attach zwitterionic poly(carboxybetaine methacrylate) (PCB), one of the most hydrophilic polymers, onto both hydrophobic and hydrophilic surfaces to render them nonfouling. This is achieved by the coating of a catechol chain end carboxybetaine methacrylate polymer (DOPA-PCB) assisted by dopamine. The coating process was carried out in water. Water miscible solvents such as methanol and tetrahydrofuran (THF) are added to the coatings if surface wettability is an issue, as for certain hydrophobic surfaces. This versatile coating method was applied to several types of surfaces such as polypropylene (PP), polydimethyl siloxane (PDMS), Teflon, polystyrene (PS), polymethylmethacrylate (PMMA), polyvinyl chloride (PVC) and also on metal oxides such as silicon dioxide.

Engineering buffering and hydrolytic or photolabile charge shifting in a polycarboxybetaine ester gene delivery platform.

Polycarboxybetaine esters (PCB-esters) can condense plasmid DNA into nanosized polyplexes for highly effective gene delivery with low toxicity. The design and characterization of tertiary CB-ester monomers and PCB-ester polymers are presented here to study the effects of molecular variation on functions important to nonviral gene transfer. Both buffering capacity and charge-shifting behavior can be tuned by modifying the distance between the charged groups and the ester size or type. A carbon spacer length (CSL) of one was found to bring the pKa of the tertiary amine into the optimal range for proton buffering. Ester hydrolytic degradation switches this polymer from cationic (DNA binding) to zwitterionic (DNA releasing) form while conferring nontoxicity. To allow rapid and externally controlled degradation, the effect of this charge-switching behavior on DNA release from polyplexes was directly studied with a novel photolabile PCB-nitrobenzyl ester (PCB-NBE). Photoinitiated ester degradation precipitated the rapid release of 72±5% of complexed DNA from PCB-NBE polyplexes. These insights reveal the key parameters important for the PCB-ester platform and the significance of charge switching to an effective and nontoxic nonviral gene delivery platform.

Single nonfouling hydrogels with mechanical and chemical functionality gradients.

Hydrogels are widely studied as tissue engineering scaffolds, but the biological tissues they are designed to mimic are often complex tissues with non-uniform chemical and mechanical profiles. This work reports a new strategy to create hydrogels composed of a continuous sheet of a single nonfouling but functionalizable material with mechanical and/or chemical functionality gradients. By using different combinations of functionalizable or nonfunctionalizable versions of nonfouling carboxybetaine methacrylate (CBMA) and carboxybetaine dimethacrylate crosslinker (CBMAX), various hydrogels with gradients of crosslinking densities and/or functionalizable groups can be created. In this work, we demonstrate this concept with two nonfouling hydrogels, both with a mechanical gradient: one with uniform functionalizability and the other with a gradient in chemical functionalizability. With this versatile system, hydrogels with built-in gradient profiles of various types can be controlled at will for a given application.

High viability of cells encapsulated in degradable poly(carboxybetaine) hydrogels.

In this study, we report a degradable poly(carboxybetaine) (pCB) hydrogel, produced via a thiol-disulfide exchange reaction for cell encapsulation. A pCB dithiol was synthesized as a cross-linker and reacted with a pyridyl dithiol-containing CB copolymer to form a hydrogel. We evaluated the biocompatibility of the pCB-based hydrogel via encapsulation of three cell types, including NIH3T3 fibroblasts, MG63 osteoblast-like cells, and HepG2 hepatocarcinoma cells. Up to 90% of cells retained their viability in the pCB hydrogel even at low cell-seeding densities under serum-free conditions after a 9-day culture. Results are compared with a degradable poly(ethylene glycol) methacrylate (PEGMA) hydrogel, which showed very low cell viability under serum-free condition after a 3-day culture. We incorporated an RGD peptide into the CB hydrogel using a cysteine-terminated cross-linker, which was shown to promote cell proliferation.

Free energy of solvated salt bridges: a simulation and experimental study.

Charged amino acids are the most common on surfaces of proteins and understanding the interactions between these charged amino acids, salt bridging, is crucial for understanding protein-protein interactions. Previous simulations have been limited to implicit solvent or fixed binding geometry due to the sampling required for converged free energies. Using well-tempered metadynamics, we have calculated salt bridge free energy surfaces in water and confirmed the results with NMR experiments. The simulations give binding free energies, quantitative ranking of salt bridging strength, and insights into the hydration of the salt bridges. The arginine-aspartate salt bridge was found to be the weakest and arginine-glutamate the strongest, showing that arginine can discriminate between aspartate and glutamate, whereas the salt bridges with lysine are indistinguishable in their free energy. The salt bridging hydration is found to be complementary to salt bridge orientation with arginine having specific orientations.

Stable and Functionalizable Quantum Dots with a Thin Zwitterionic Carboxybetaine Layer

A new type of ligand chemistry, with a zwitterionic carboxybetaine headgroup and a bidentate thiol end group (CBSS), is introduced to promote the stability of quantum dots (QDs) with targeting capability. Results show that QDs are stable over a broad range of pH values after surface modification. Surface binding assays and cellular internalization studies show that QDs capped with CBSS exhibit low nonspecific adsorption. The CBSS ligand also allows the conjugation of highly specific targeting ligands while effectively maintaining the nonfouling background. This QD chemistry offers a unique approach to presenting abundant functional groups for ligand immobilization in a thin layer with an ultralow background and holds significant potential for imaging applications.