Power Law Behavior in Protein Desorption Kinetics Originating from Sequential Binding and Unbinding

Tracking Molecular Signatures at ppb Sensitivity Using Fluctuational Kinetics in Metal-Organic Frameworks

Biological systems achieve parts-per-billion (ppb) sensitivity in gas detection by tracking molecular fluctuations over time─a level of precision that remains difficult to replicate in engineered sensors. Conventional sensing relies on adsorption processes that require activation energies (Ea) ∼10 kBT, resulting in exponentially long equilibration times and limited selectivity due to small differences in Ea among analytes. Here, we show that volatile organics interacting with a ∼200 nm-thick nanoporous metal-organic framework (MOF), when subjected to shear-induced strain via a quartz crystal microbalance (QCM), exhibit a secondary fluctuational adsorption time scale distinct from the steady-state response. This emergent kinetic signature allows for reliable molecular discrimination at sensitivities down to ∼100 ppb. Our approach introduces a new selectivity metric based on dynamic adsorption kinetics, opening avenues for real-time molecular identification in environmental monitoring, portable diagnostics, and selective detection in chemically complex settings.

Shear-induced rotation enhances protein adsorption

Theories predicted that shear promotes desorption, but due to the presence of factors such as aggregation effects, it is difficult to observe how shear influences the adsorption and desorption of individual protein molecules. In this study, we employed high-throughput single-molecule tracking and molecular dynamics simulations to investigate how shear flow affects the adsorption kinetics of plasma proteins (including human serum albumin, immunoglobulin G, and fibrinogen) at solid-liquid interfaces. Over the studied shear rate range of 0 - 103 s-1, shear stress did not trigger the protein desorption. Notably, we observed a significant increase, up to two orders of magnitude, in the adsorption rate constants ka, in the dilute limit at solid-liquid interfaces. However, this shear-induced increase in ka diminished with increasing the protein concentrations. At least in the scenarios studied, these trends were consistent across all three types of proteins and two types of surfaces investigated. Through a systematic analysis combining control experiments, coarse-grained, and all-atom molecular dynamics simulations, we identified that the shear-induced increase in ka could be attributed to enhanced protein rotational diffusion, thereby increasing the likelihood of favorable surface proximity for adsorption.

Highly Stable Biotemplated InP/ZnSe/ZnS Quantum Dots for In Situ Bacterial Monitoring

Despite their unique optical and electrical characteristics, traditional semiconductor quantum dots (QDs) made of heavy metals or carbon are not ideally suited for biomedical applications. Cytotoxicity and environmental concerns are key limiting factors affecting the adoption of QDs from laboratory research to real-world medical applications. Recently, advanced InP/ZnSe/ZnS QDs have emerged as alternatives to traditional QDs due to their low toxicity and optical properties; however, bioconjugation has remained a challenge due to surface chemistry limitations that can lead to instability in aqueous environments. Here, we report water-soluble, biotemplated InP/ZnSe/ZnS-aptamer quantum dots (QDAPTs) with long-term stability and high selectivity for targeting bacterial membrane proteins. QDAPTs show fast binding reaction kinetics (less than 5 min), high brightness, and high levels of stability (3 months) after biotemplating in aqueous solvents. We use these materials to demonstrate the detection of bacterial membrane proteins on common surfaces using a hand-held imaging device, which attests to the potential of this system for biomedical applications.

Protein Adsorption on Solid Surfaces: Data Mining, Database, Molecular Surface-Derived Properties, and Semiempirical Relationships

Protein adsorption on solid surfaces is a process relevant to biological, medical, industrial, and environmental applications. Despite this wide interest and advancement in measurement techniques, the complexity of protein adsorption has frustrated its accurate prediction. To address this challenge, here, data regarding protein adsorption reported in the last four decades was collected, checked for completeness and correctness, organized, and archived in an upgraded, freely accessible Biomolecular Adsorption Database, which is equivalent to a large-scale, ad hoc, crowd-sourced multifactorial experiment. The shape and physicochemical properties of the proteins present in the database were quantified on their molecular surfaces using an in-house program (ProMS) operating as an add-on to the PyMol software. Machine learning-based analysis indicated that protein adsorption on hydrophobic and hydrophilic surfaces is modulated by different sets of operational, structural, and molecular surface-based physicochemical parameters. Separately, the adsorption data regarding four "benchmark" proteins, i.e., lysozyme, albumin, IgG, and fibrinogen, was processed by piecewise linear regression with the protein monolayer acting as breakpoint, using the linearization of the Langmuir isotherm formalism, resulting in semiempirical relationships predicting protein adsorption. These relationships, derived separately for hydrophilic and hydrophobic surfaces, described well the protein concentration on the surface as a function of the protein concentration in solution, adsorbing surface contact angle, ionic strength, pH, and temperature of the carrying fluid, and the difference between pH and the isoelectric point of the protein. When applying the semiempirical relationships derived for benchmark proteins to two other "test" proteins with known PDB structure, i.e., β-lactoglobulin and α-lactalbumin, the errors of this extrapolation were found to be in a linear relationship with the dissimilarity between the benchmark and the test proteins. The work presented here can be used for the estimation of operational parameters modulating protein adsorption for various applications such as diagnostic devices, pharmaceuticals, biomaterials, or the food industry.

Tracking of Protein Adsorption on Poly(l-lactic acid) Film Surfaces: The Role of Molar Mass

Poly(l-lactic acid) (PLLA) has been extensively utilized as a biomaterial for various biomedical applications. The first and one of the most critical steps upon contact with biological fluids is the adsorption of proteins on the material's surface. Understanding the behavior of protein adsorption is vital for guiding the synthesis and preparation of PLLA for biomedical purposes. In this study, total internal reflection fluorescence microscopy was employed to investigate the adsorption of human serum albumin (HSA) on PLLA films with different molar masses. We found that molar mass affects HSA adsorption in such a way that it affects only the adsorption rate constants, but not the desorption rate constants. Additionally, we observed that HSA adsorption is spatially heterogeneous and exhibits many strong binding sites regardless of the molar mass of the PLLA films. We found that the free volume of PLLA plays a crucial role in determining its water uptake capacity and surface hydration, consequently impacting the adsorption of HSA.

Single-Molecule Imaging in Commercial Stationary Phase Particles Using Highly Inclined and Laminated Optical Sheet Microscopy

We resolve the three-dimensional, nanoscale locations of single-molecule analytes within commercial stationary phase materials using highly inclined and laminated optical sheet (HILO) microscopy. Single-molecule fluorescence microscopy of chromatography can reveal the molecular heterogeneities that lead to peak broadening, but past work has focused on surfaces designed to mimic stationary phases, which have different physical and chemical properties than the three-dimensional materials used in real columns and membranes. To extend single-molecule measurements to commercial stationary phases, we immobilize individual stationary phase particles and modify our microscope for imaging at further depths with HILO, a method which was originally developed to resolve single molecules in cells of comparable size to column packing materials (∼5-10 μm). We describe and characterize how to change the angle of incidence to achieve HILO so that other researchers can easily incorporate this method onto their existing epi- or total internal reflection fluorescence microscopes. We show improvements up to a 32% in signal-to-background ratio and 118% in the number of single molecules detected within stationary phase particles when using HILO compared to epifluorescence. By controlling the objective position relative to the sample, we produce three-dimensional maps of molecule locations throughout entire stationary phase particles at nanoscale lateral and axial resolutions. The number of localized molecules remains constant axially throughout isolated stationary phase particles and between different particles, indicating that heterogeneity in a separation would not be caused by such affinity differences at microscales but instead kinetic differences at nanoscales on identifiable and distinct adsorption sites.

Protein Desorption Kinetics Depends on the Timescale of Observation

The presence of so-called reversible and irreversible protein adsorption on solid surfaces is well documented in the literature and represents the basis for the development of nanoparticles and implant materials to control interactions in physiological environments. Here, using a series of complementary single-molecule tracking approaches appropriate for different timescales, we show that protein desorption kinetics is much more complex than the traditional reversible-irreversible binary picture. Instead, we find that the surface residence time distribution of adsorbed proteins transitions from power law to exponential behavior when measured over a large range of timescales (10^-2-10⁶ s). A comparison with macroscopic results obtained using a quartz crystal microbalance suggested that macroscopic measurements have generally failed to observe such nonequilibrium phenomena because they are obscured by ensemble-averaging effects. These findings provide new insights into the complex phenomena associated with protein adsorption and desorption.