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

Softer Zwitterionic Nanogels for Longer Circulation and Lower Splenic Accumulation

Zwitterionic nanogels of varying stiffness were prepared by tuning their cross-linking densities and reactant contents. <i>In vivo</i> studies of these nanogels show that softer nanogels pass through physiological barriers, especially the splenic filtration, more easily than their stiffer counterparts, consequently leading to longer circulation half-life and lower splenic accumulation. Results from this work emphasize the role of stiffness in designing long-circulating nanoparticles

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 p<i>K</i>_a 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

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

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

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, <i>N</i>,<i>N</i>-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/cm²), 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