QCM Biosensor Based on Polydopamine Surface for Real-Time Analysis of the Binding Kinetics of Protein-Protein Interactions

Covalent-Organic-Framework-Modified Quartz Crystal Microbalance Sensor for Selective Detection of Hazardous Formic Acid

Covalent organic frameworks (COFs) are a novel family of porous crystalline materials utilized in various advanced applications. However, applying COFs as a hazardous organic acid gas sensor is substantial but still challenging. Herein, a phenylenediamine-based covalent organic framework (TPDA-TPB COF) featuring excellent crystallinity, ultrastable thermal stability, and high surface area was successfully constructed. Then, the TPDA-TPB COF-modified quartz crystal microbalance (QCM) sensor is fabricated by immobilizing the TPDA-TPB COF thin film on the gold-QCM chip. The fabricated TPDA-TPB COF-modified QCM sensor demonstrates a rapid response, excellent reproducibility, high selectivity, and sensitivity to formic gas, arising from hydrogen-bonding interactions between formic acid and the outermost layer of the TPDA-TPB COF, as determined by extensive analysis and density functional theory calculations. The basic sites of the TPDA-TPB COF, which are numerous due to its high nitrogen content, and the carboxylic acid groups present in formic acid exhibit efficient interactions. The sensitivity of the TPDA-TPB COF-modified QCM sensor was found to be 7.75 Hz ppm-1 at standard room temperature and pressure conditions, with a limit of detection (LOD) of formic acid down to 1.18 ppm, which is significantly below the workplace olfactory threshold limit of 5.0 ppm established by the Occupational Safety and Health Administration. The TPDA-TPB COF-modified QCM sensor exhibits remarkable detecting capabilities, making it highly attractive for detecting organic acid vapors in diverse applications that require superior performance.

Fetuin-A adsorption to tunable polydimethylsiloxane and subsequent macrophage response

Surface modifications can be applied to biomaterials to alter the various surface properties that influence protein-material interactions and the cellular response. The plasma protein fetuin-A has been found to adsorb to many biomaterials but details of its interactions with polydimethylsiloxane (PDMS) and roles in regulating the immune response are not clear. Here, PDMS modifications are achieved by altering the ratio of PDMS formulations to control elastic modulus, and by coating PDMS with polydopamine (PDA) to attach fetuin-A. Surface characterization confirmed that altering the PDMS formulation changed the elastic modulus without affecting surface wetting properties. Surface roughness was measured using atomic force microscopy and surface chemistry was determined using X-ray photoelectron spectroscopy, with only minor changes detected on the softest samples. PDA deposition on PDMS was confirmed and contact angle measurements demonstrated an increase in hydrophilicity. Fetuin-A adsorption was influenced by the PDMS formulations, adsorption changed in a competitive plasma environment, and PDA was able to immobilize the greatest amount of fetuin-A. The inflammatory effects of fetuin-A were investigated, and data suggests that the elastic modulus influences cytokine secretion from macrophages at certain timepoints, a result likely due to varied protein amounts and orientations/conformations in response to material stiffness. The addition of a PDA layer demonstrated the potentially cytokine mitigating effect upon fetuin-A immobilization when compared to unmodified PDMS samples. The results provide new insight into the interactions of fetuin-A with PDMS and PDA, and the potential immune regulatory properties of fetuin-A modified materials.

Recent developments on nanomaterial-based optical biosensor as potential Point-of-Care Testing (PoCT) probe in carcinoembryonic antigen detection: A review

For the past decades, several cancer biomarkers have been exploited for rapid and accurate prognosis or diagnosis purposes. In this review, the optical biosensor is targeted for carcinoembryonic antigen (CEA) detection. The CEA level is a prominent parameter currently used in clinical cases for the prognosis of cancer-related diseases. Many nanomaterial-based biosensors are invented as alternatives for the commonly used enzyme-linked immunosorbent assays (ELISA) immunoassay method in CEA detection as the traditional approach but they possess certain drawbacks such as tedious procedure, high technical demand, and costly. Nevertheless, the effort appears to be wasted as none of them are being actualised. Generally, the sensor function was carried out by converting bio-signals generated upon the interface of the receptor into light signals. These sensors were popular due to specific advantages such as sensitivity, being free from chemical and electromagnetic interferences, wide dynamic range, and being easy to be monitored. The features of PoC diagnostics are discussed and associated with the various applications of colorimetric-based and chemiluminescent-based biosensors. The roles of nanomaterials in each application were also summarised by comparing the modification, incubation period, lowest detection limit (LOD) and linear range of detection amount. The challenges and future perspectives were highlighted at the end of the review.

Data evaluation for surface-sensitive label-free methods to obtain real-time kinetic and structural information of thin films: A practical review with related software packages

Interfacial layers are important in a wide range of applications in biomedicine, biosensing, analytical chemistry and the maritime industries. Given the growing number of applications, analysis of such layers and understanding their behavior is becoming crucial. Label-free surface sensitive methods are excellent for monitoring the formation kinetics, structure and its evolution of thin layers, even at the nanoscale. In this paper, we review existing and commercially available label-free techniques and demonstrate how the experimentally obtained data can be utilized to extract kinetic and structural information during and after formation, and any subsequent adsorption/desorption processes. We outline techniques, some traditional and some novel, based on the principles of optical and mechanical transduction. Our special focus is the current possibilities of combining label-free methods, which is a powerful approach to extend the range of detected and deduced parameters. We summarize the most important theoretical considerations for obtaining reliable information from measurements taking place in liquid environments and, hence, with layers in a hydrated state. A thorough treamtmaent of the various kinetic and structural quantities obtained from evaluation of the raw label-free data are provided. Such quantities include layer thickness, refractive index, optical anisotropy (and molecular orientation derived therefrom), degree of hydration, viscoelasticity, as well as association and dissociation rate constants and occupied area of subsequently adsorbed species. To demonstrate the effect of variations in model conditions on the observed data, simulations of kinetic curves at various model settings are also included. Based on our own extensive experience with optical waveguide lightmode spectroscopy (OWLS) and the quartz crystal microbalance (QCM), we have developed dedicated software packages for data analysis, which are made available to the scientific community alongside this paper.

Electrosynthesis of polydopamine-ethanolamine films for the development of immunosensing interfaces

We report a straightforward and reproducible electrochemical approach to develop polydopamine-ethanolamine (ePDA-ETA) films to be used as immunosensing interfaces. ETA is strongly attached to polydopamine films during the potentiodynamic electropolymerization of dopamine. The great advantage of the electrochemical methods is to generate the oxidized species (quinones), which can readily react with ETA amine groups present in solution, with the subsequent incorporation of this molecule in the polymer. The presence of ETA and its effect on the electrosynthesis of polydopamine was accessed by cyclic voltammetry, ellipsometry, atomic force microscopy, FTIR and X-ray photoelectron spectroscopy. The adhesive and biocompatible films enable a facile protein linkage, are resilient to flow assays, and display intrinsic anti-fouling properties to block non-specific protein interactions, as monitored by real-time surface plasmon resonance, and confirmed by ellipsometry. Immunoglobulin G (IgG) and Anti-IgG were used in this work as model proteins for the affinity sensor. By using the one-step methodology (ePDA-ETA), the lower amount of immobilized biorecognition element, IgG, compared to that deposited on ePDA or on ETA post-modified film (ePDA/ETA), allied to the presence of ETA, improved the antibody-antigen affinity interaction. The great potential of the developed platform is its versatility to be used with any target biorecognition molecules, allowing both optical and electrochemical detection.

Measurements of aptamer-protein binding kinetics using graphene field-effect transistors

Quantifying interactions between biomolecules subject to various environmental conditions is essential for applications such as drug discovery and precision medicine. This paper presents an investigation of the kinetics of environmentally dependent biomolecular binding using an electrolyte-gated graphene field-effect transistor (GFET) nanosensor. In this approach, biomolecular binding occurring on and in the vicinity of a graphene surface induces a change in carrier concentration, whose resulting conductance change is measured. This allows a systematic study of the kinetic properties of the binding system. We apply this approach to the specific binding of human immunoglobulin E (IgE), an antibody involved in parasite immunity, with an aptamer at different ionic strengths (Na⁺ and Mg²⁺) and temperatures. Experimental results demonstrate increased-rate binding kinetics at higher salt-ion concentrations and temperatures. In particular, the divalent cation Mg²⁺ yields more pronounced changes in the conformational structure of the aptamer than the monovalent cation Na⁺. In addition, the dissociation of the aptamer-protein complex at room temperature is found to be characterized by large unfavorable changes in the activation enthalpy and entropy.

A Universal Photochemical Method to Prepare Carbohydrate Sensors Based on Perfluorophenylazide Modified Polydopamine for Study of Carbohydrate-Lectin Interactions by QCM Biosensor

A universal photochemical method to prepare carbohydrate sensors based on perfluorophenylazide (PFPA) modified polydopamine (PDA) for the study of carbohydrate–lectin interactions by a quartz crystal microbalance (QCM) biosensor was developed. The PFPA was immobilized on PDA-coated gold sensors via Schiff base reactions. Upon light irradiation, the underivatized carbohydrates were inserted into the sensor surface, including mannose, galactose, fucose and N-acetylglucosamine (GlcNAc). Carbohydrate sensors were evaluated for the binding to a series of plant lectins. A kinetic study of the interactions between mannose and concanavalin A (Con A), fucose and Ulex europaeus agglutinin I (UEA-I) were performed. This method can eliminate the tedious modification of carbohydrates, improve the experimental efficiency, and reduce the experimental cost, which is of great significance for the development of QCM biosensors and the study of biomolecular interactions.

Fabrication of Carbohydrate Chips Based on Polydopamine for Real-Time Determination of Carbohydrate⁻Lectin Interactions by QCM Biosensor

A novel approach for preparing carbohydrate chips based on polydopamine (PDA) surface to study carbohydrate⁻lectin interactions by quartz crystal microbalance (QCM) biosensor instrument has been developed. The amino-carbohydrates were immobilized on PDA-coated quartz crystals via Schiff base reaction and/or Michael addition reaction. The resulting carbohydrate-chips were applied to QCM biosensor instrument with flow-through system for real-time detection of lectin⁻carbohydrate interactions. A series of plant lectins, including wheat germ agglutinin (WGA), concanavalin A (Con A), Ulex europaeus agglutinin I (UEA-I), soybean agglutinin (SBA), and peanut agglutinin (PNA), were evaluated for the binding to different kinds of carbohydrate chips. Clearly, the results show that the predicted lectin selectively binds to the carbohydrates, which demonstrates the applicability of the approach. Furthermore, the kinetics of the interactions between Con A and mannose, WGA and N-Acetylglucosamine were studied, respectively. This study provides an efficient approach to preparing carbohydrate chips based on PDA for the lectin⁻carbohydrate interactions study.

Quartz Crystal Microbalance with Approximately Uniform Sensitivity Distribution

The nonuniformity of QCMs' mass sensitivity distribution is a disadvantage to practical applications. Through theoretical calculations, we found that common ring electrode QCMs could obtain approximately uniform sensitivity distribution by carefully selecting the inner and outer diameters and mass loading factor of the electrode. A series of experiments were carried out using 10 MHz ring electrode QCMs with an inner diameter of 2 mm, an outer diameter of 5 mm, and a loading factor R of 0.0044. The experimental results proved that its mass sensitivity distribution is approximately uniform. This special designed ring electrode QCMs is suitable and convenient for highly accurate measurements.