Surface functionalization allowing repetitive use of optical sensors for real-time detection of antibody-bacteria interaction.

In this study, sensor surface functionalization allowing the repetitive use of a sensing device was evaluated for antibody-based detection of living bacteria using an optical planar Bragg grating sensor. To achieve regenerable immobilization of bacteria specific antibodies, the heterobifunctional cross-linker N-succinimidyl 3-(2-pyridyldithio) propionate (SPDP) was linked to an aminosilanized sensor surface and subsequently reduced to expose sulfhydryl groups enabling the covalent conjugation of SPDP-activated antibodies via disulfide bonds. The immobilization of a capture antibody specific for Staphylococcus aureus on the sensor surface as well as specific binding of S. aureus could be monitored, highlighting the applicability of optical sensors for the specific detection of large biological structures. Reusability of bacteria saturated sensors was successfully demonstrated by cleaving the antibody along with bound bacteria through reduction of disulfide bonds and subsequent re-functionalization with activated antibody, resulting in comparable sensitivity towards S. aureus.

Influence of salt type and ionic strength on self-assembly of dextran sulfate-ciprofloxacin nanoplexes.

We evaluated an analytical setup to identify optimal preparation conditions for nanoplex formation of small molecule drugs and polyelectrolytes using ciprofloxacin (CIP) and dextran sulfate (DS) as model compounds. The suitability of isothermal titration calorimetry (ITC) as a screening tool for rational formulation optimization was assessed. Besides ITC, static and dynamic light scattering, zeta potential measurements and scanning electron microscopy were applied to analyze the influence of different salt types and ionic strengths on CIP/DS nanoplex formation. The addition of low amounts of salt, especially 0.1M NaCl, improved the formation of CIP/DS nanoplexes. The presence of low amounts of salt led to smaller and more numerous particles of higher uniformity but had no influence on the release of CIP from nanoplexes. Furthermore, the molar range, within which efficient complexation was achieved, was broader in the presence of 0.1M NaCl than in the absence of salt with overall comparable complexation efficiency. Importantly, binding affinity correlated with particle shape and morphology, potentially enabling optimization of critical quality attributes based on ITC data. Altogether, ITC along with supplemental methods is a versatile screening tool for the evaluation of nanoplex formulation conditions regarding mixing ratio, salt type and ionic strength.

Deciphering the mechanism of protein interaction with silk fibroin for drug delivery systems.

Silk fibroin (SF) is an exceptional drug delivery carrier with respect to stabilizing, protecting, and delivering sensitive biologics. A synopsis of thermodynamic, static light scattering, hydrophobicity probing, and nanoparticle tracking analyses served as a basis to decipher the mechanism of interaction between SF and two model proteins, protamine and polylysine. The impact of salts aiding (chaotropic), not affecting (neutral), or opposing (cosmotropic) SF unfolding was a major determinant, ranging from complete abolishment to maximal interaction efficacy. Evidence is provided, that the underlying mechanism of the remarkable ability to tailor drug/SF interaction throughout such large ranges and by appropriate salt selection is the control of structural breakdown of SF micelles as present in pure SF ad initium. This study provides a mechanistically justified and hypothesis driven blueprint for future experimental designs addressing the controlled interaction of biologics and SF.

Fluorescence single particle tracking for the characterization of submicron protein aggregates in biological fluids and complex formulations.

PURPOSE: To evaluate the potential of fluorescence single particle tracking (fSPT) for the characterization of submicron protein aggregates in human serum, plasma and formulations containing human serum albumin (HSA). METHODS: A monoclonal IgG was covalently labeled with a fluorescent dye and cross-linked with glutaraldehyde. IgG aggregates and fluorescent beads of 0.1 µm (control) were diluted in buffer, serum and plasma, and their size distributions were analyzed by fSPT and nanoparticle tracking analysis (NTA). In a separate experiment, IgG and HSA, fluorescently labeled with different dyes, were mixed and subjected to heat stress. The stressed sample was analyzed by fSPT using a dual color mode and by NTA. RESULTS: The accuracy and precision of fSPT proved to be comparable to NTA. fSPT was able to successfully measure all the samples in buffer, serum and plasma. The average size of the cross-linked protein aggregates showed a slight increase in biological fluids. Moreover, fSPT analysis showed that a significant proportion of the aggregates formed by subjecting an IgG/HSA mixture to heat stress were composed of both proteins. CONCLUSION: fSPT is a powerful technique for the characterization of submicron protein aggregates in biological fluids and complex formulations.