Molecular Mechanistic Insights into the Ionic-Strength-Controlled Interfacial Behavior of Proteins on a TiO2 Surface

Holistic insights into carbon nanotubes and MXenes as a promising route to bio-sensing applications

Essential biosensor use has become increasingly important in drug discovery and recognition, biomedicine, food safety, security, and environmental research. It directly contributed to the development of specialized, reliable diagnostic instruments known as biosensors, which use biological sensing components. Traditional biosensors have poor performance, so scientists need to develop advanced biosensors with promising selectivity, sensitivity, stability, and reusability. These are all parameter modifications associated with the characteristics of the sensing material. Carbon nanotubes (CNTs) and MXenes are promising as targeted sensing agents in advanced functional materials because of their promising chemical and physical properties and limited toxic effects. Based on available data and sensing performance, MXene is better for biosensing applications than CNTs. Because of their large specific surface area (SSA), superior electrical conductivity, and adaptable surface chemistry that facilitates simple functionalization and robust interactions with biomolecules, MXenes are typically regarded as the superior option for biosensors. Additionally, because of their hydrophilic nature, they are more suited to biological settings, which increases their sensitivity and efficacy in identifying biological targets. MXenes are more suitable for biosensing applications due to their versatility and compatibility with aquatic environments, even if CNTs have demonstrated stability and muscular mechanical strength. However, MXenes offer better thermal stability, which is crucial for applications in diverse temperature environments. This study reviews and compares the biosensing capabilities, synthesis methods, unique properties, and toxicity of CNTs and MXenes. Both nanomaterials effectively detect various pollutants in food, biological substances, and human bodies, making them invaluable in environmental monitoring and medical diagnostics. In conclusion, CNTs work better for biosensors that must be strong, flexible, and long-lasting under different conditions. MXenes, on the other hand, work better when chemical flexibility and compatibility with wet environments are essential.

Diverse Role of Buffer Mediums and Protein Concentrations to Mediate the Multimodal Interaction of Phenylalanine-Functionalized Gold Nanoparticle and Lysozyme Protein at Same Nominal pH

Recently, buffer molecules have been known to affect intermolecular protein-protein interactions at physiological pH. However, the roles of buffer molecules and different monolayer protein concentrations remain elusive in controlling the interaction of gold nanoparticles (Au NPs) with protein molecules. Herein, for the first time taking phenylalanine functionalized gold nanoparticles (Au-Phe NPs) and lysozyme (Lyz) protein as model systems, we report that buffer molecules of different charges (at a particular pH) play diverse roles in protein-Au NPs interaction, particularly in protein induced Au NPs aggregation. Among different buffers, negatively charged buffer (citrate and phosphate) induces aggregation of both Au-Phe NPs and Lyz protein, whereas zwitterionic and positive buffer (HEPES, MOPS, and Tris) only cause the Au NPs aggregation. Taking the diverse role of buffer into account, we propose multimodal models for stability and protein induced aggregation mechanism of NPs at different monolayer (sub-, near-, and excess) concentrations of Lyz in different medium.

A New Approach in Lipase-Octyl-Agarose Biocatalysis of 2-Arylpropionic Acid Derivatives

The use of lipase immobilized on an octyl-agarose support to obtain the optically pure enantiomers of chiral drugs in reactions carried out in organic solvents is a great challenge for chemical and pharmaceutical sciences. Therefore, it is extremely important to develop optimal procedures to achieve a high enantioselectivity of the biocatalysts in the organic medium. Our paper describes a new approach to biocatalysis performed in an organic solvent with the use of CALB-octyl-agarose support including the application of a polypropylene reactor, an appropriate buffer for immobilization (Tris base-pH 9, 100 mM), a drying step, and then the storage of immobilized lipases in a climatic chamber or a refrigerator. An immobilized lipase B from Candida antarctica (CALB) was used in the kinetic resolution of (R,S)-flurbiprofen by enantioselective esterification with methanol, reaching a high enantiomeric excess (eep = 89.6 ± 2.0%). As part of the immobilization optimization, the influence of different buffers was investigated. The effect of the reactor material and the reaction medium on the lipase activity was also studied. Moreover, the stability of the immobilized lipases: lipase from Candida rugosa (CRL) and CALB during storage in various temperature and humidity conditions (climatic chamber and refrigerator) was tested. The application of the immobilized CALB in a polypropylene reactor allowed for receiving over 9-fold higher conversion values compared to the results achieved when conducting the reaction in a glass reactor, as well as approximately 30-fold higher conversion values in comparison with free lipase. The good stability of the CALB-octyl-agarose support was demonstrated. After 7 days of storage in a climatic chamber or refrigerator (with protection from humidity) approximately 60% higher conversion values were obtained compared to the results observed for the immobilized form that had not been stored. The new approach involving the application of the CALB-octyl-agarose support for reactions performed in organic solvents indicates a significant role of the polymer reactor material being used in achieving high catalytic activity.

Cation Chain Length of Nonhalogenated Ionic Liquids Matters in Enhancing SERS of Cytochrome c on Zr-Al-Co-O Nanotube Arrays

Surface-enhanced Raman scattering (SERS) is a remarkably powerful analytical technique enabling trace-level detection of biological molecules. The interaction of a probe molecule with the SERS substrate shows important distinctions in the SERS spectra, providing inherent fingerprint information on the probe molecule. Herein, nonhalogenated phosphonium-based ionic liquids (ILs) containing cations with varying chain lengths were used as trace additives to amplify the interaction between the cytochrome c (Cyt c) and Zr-Al-Co-O (ZACO) nanotube arrays, strengthening the SERS signals. An increased enhancement factor (EF) by 2.5-41.2 times compared with the system without ILs was achieved. The improvement of the SERS sensitivity with the introduction of these ILs is strongly dependent on the cation chain length, in which the increasing magnitude of EF is more pronounced in the system with a longer alkyl chain length on the cation. Comparing the interaction forces measured by Cyt c-grafted atomic force microscopy (AFM) probes on ZACO substrates with those predicted by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, the van der Waals forces became increasingly dominant as the chain length of the cations increased, associated with stronger Cyt c-ZACO XDLVO interaction forces. The major contributing component, van der Waals force, stems from the longer cation chains of the IL, which act as a bridge to connect Cyt c and the ZACO substrate, promoting the anchoring of the Cyt c molecules onto the substrate, thereby benefiting SERS enhancement.

Phosphonium-Based Ionic Liquid Significantly Enhances SERS of Cytochrome c on TiO2 Nanotube Arrays

Surface-enhanced Raman scattering (SERS) is an attractive technique for studying trace detection. It is of utmost importance to further improve the performance and understand the underlying mechanisms. An ionic liquid (IL), the anion of which is derived from biomass, [P6,6,6,14][FuA] was synthesized and used as a trace additive to improve the SERS performance of cytochrome c (Cyt c) on TiO2 nanotube arrays (TNAs). An increased and better enhancement factor (EF) by four to five times as compared to the system without an IL was obtained, which is better than that from using the choline-based amino acid IL previously reported by us. Dissociation of the ILs improved the ionic conductivity of the system, and the long hydrophobic tails of the [P6,6,6,14]+ cation contributed to a strong electrostatic interaction between Cyt c and the TNA surface, thereby enhancing the SERS performance. Atomic force microscopy did verify strong electrostatic interactions between the Cyt c molecules and TNAs after the addition of the IL. This work demonstrates the importance of introducing the phosphonium-based IL to enhance the SERS performance, which will stimulate further development of more effective ILs on SERS detection and other relevant applications in biology.

Selective Separation of Highly Similar Proteins on Ionic Liquid-Loaded Mesoporous TiO2

Separating proteins from their mixtures is an important process in a great variety of applications, but it faces difficult challenges as soon as the proteins are simultaneously of similar sizes and carry comparable net charges. To develop both efficient and sustainable strategies for the selective separation of similar proteins and to understand the underlying molecular mechanisms to enable the separation are crucial. In this work, we propose a novel strategy where the cholinium-based amino acid [Cho][Pro] ionic liquid (IL) is used as the trace additive and loaded physically on a mesoporous TiO₂ surface for separating two similar proteins (lysozyme and cytochrome c). The observed selective adsorption behavior is explained by the hydration properties of the [Cho][Pro] loaded on the TiO₂ surface and their partially dissociated ions under different pH conditions. As the pH is increased from 5.0 to 9.8, the degree of hydration of IL ions also increases, gradually weakening the interaction strength of the proteins with the substrates, more for lysozymes, leading to their effective separation. These findings were further used to guide the detection of the retention behavior of a binary mixture of proteins in high-performance liquid chromatography, where the introduction of ILs did effectively separate the two similar proteins. Our results should further stimulate the use of ILs in the separation of proteins with a high degree of mutual similarity.