Bridging interactions of proteins with silica nanoparticles: the influence of pH, ionic strength and protein concentration

Immobilization of Thiol-Modified Horseradish Peroxidase on Gold Nanoparticles Enhances Enzyme Stability and Prevents Proteolytic Digestion

The specificity and efficiency of enzyme-mediated reactions have the potential to positively impact many biotechnologies; however, many enzymes are easily degraded. Immobilization on a solid support has recently been explored to improve enzyme stability. This study aims to gain insights and facilitate enzyme adsorption onto gold nanoparticles (AuNPs) to form a stable bioconjugate through the installation of thiol functional groups that alter the protein chemistry. In specific, the model enzyme, horseradish peroxidase (HRP), is thiolated via Traut's reagent to increase the robustness and enzymatic activity of the bioconjugate. This study compares HRP and its thiolated analog (THRP) to deduce the impact of thiolation and AuNP-immobilization on the enzyme activity and stability. HRP, THRP, and their corresponding bioconjugates, HRP-AuNP and THRP-AuNP, were analyzed via UV-vis spectrophotometry, circular dichroism, zeta potential, and enzyme-substrate kinetics assays. Our data show a 5-fold greater adsorption for THRP on the AuNP, in comparison to HRP, that translated to a 5-fold increase in the THRP-AuNP bioconjugate activity. The thiolated and immobilized HRP exhibited a substantial improvement in stability at elevated temperatures (50 °C) and storage times (1 month) relative to the native enzyme in solution. Moreover, HRP, THRP, and their bioconjugates were incubated with trypsin to assess the susceptibility to proteolytic digestion. Our results demonstrate that THRP-AuNP bioconjugates maintain full enzymatic activity after 18 h of incubation with trypsin, whereas free HRP, free THRP, and HRP-AuNP conjugates are rendered inactive by trypsin treatment. These results highlight the potential for protein modification and immobilization to substantially extend enzyme shelf life, resist protease digestion, and enhance biological function to realize enzyme-enabled biotechnologies.

Overcoming colloidal nanoparticle aggregation in biological milieu for cancer therapeutic delivery: Perspectives of materials and particle design

Nanoparticles as cancer therapeutic carrier fail in clinical translation due to complex biological environments in vivo consisting of electrolytes and proteins which render nanoparticle aggregation and unable to reach action site. This review identifies the desirable characteristics of nanoparticles and their constituent materials that prevent aggregation from site of administration (oral, lung, injection) to target site. Oral nanoparticles should ideally be 75-100 nm whereas the size of pulmonary nanoparticles minimally affects their aggregation. Nanoparticles generally should carry excess negative surface charges particularly in fasting state and exert steric hindrance through surface decoration with citrate, anionic surfactants and large polymeric chains (polyethylene glycol and polyvinylpyrrolidone) to prevent aggregation. Anionic as well as cationic nanoparticles are both predisposed to protein corona formation as a function of biological protein isoelectric points. Their nanoparticulate surface composition as such should confer hydrophilicity or steric hindrance to evade protein corona formation or its formation should translate into steric hindrance or surface negative charges to prevent further aggregation. Unexpectedly, smaller and cationic nanoparticles are less prone to aggregation at cancer cell interface favoring endocytosis whereas aggregation is essential to enable nanoparticles retention and subsequent cancer cell uptake in tumor microenvironment. Present studies are largely conducted in vitro with simplified simulated biological media. Future aggregation assessment of nanoparticles in biological fluids that mimic that of patients is imperative to address conflicting materials and designs required as a function of body sites in order to realize the future clinical benefits.

Alginate/gum arabic-based biomimetic hydrogel enriched with immobilized nerve growth factor and carnosine improves diabetic wound regeneration

Diabetic foot ulcers (DFUs) often remain untreated because they are difficult to heal, caused by reduced skin sensitivity and impaired blood vessel formation. In this study, we propose a novel approach to manage DFUs using a multifunctional hydrogel made from a combination of alginate and gum arabic. To enhance the healing properties of the hydrogel, we immobilized nerve growth factor (NGF), within specially designed mesoporous silica nanoparticles (MSN). The MSNs were then incorporated into the hydrogel along with carnosine (Car), which further improves the hydrogel's therapeutic properties. The hydrogel containing the immobilized NGF (SiNGF) could control the sustain release of NGF for >21 days, indicating that the target hydrogel (AG-Car/SiNGF) can serve as a suitable reservoir managing diabetic wound regeneration. In addition, Car was able to effectively reduce inflammation and significantly increase angiogenesis compared to the control group. Based on the histological results obtained from diabetic rats, the target hydrogel (AG-Car/SiNGF) reduced inflammation and improved re-epithelialization, angiogenesis, and collagen deposition. Specific staining also confirmed that AG-Car/SiNGF exhibited improved tissue neovascularization, transforming growth factor-beta (TGFβ) expression, and nerve neurofilament. Overall, our research suggests that this newly developed composite system holds promise as a potential treatment for non-healing diabetic wounds.

Design, characterization and applications of nanocolloidal hydrogels

Nanocolloidal gels (NCGs) are an emerging class of soft matter, in which nanoparticles act as building blocks of the colloidal network. Chemical or physical crosslinking enables NCG synthesis and assembly from a broad range of nanoparticles, polymers, and low-molecular weight molecules. The synergistic properties of NCGs are governed by nanoparticle composition, dimensions and shape, the mechanism of nanoparticle bonding, and the NCG architecture, as well as the nature of molecular crosslinkers. Nanocolloidal gels find applications in soft robotics, bioengineering, optically active coatings and sensors, optoelectronic devices, and absorbents. This review summarizes currently scattered aspects of NCG formation, properties, characterization, and applications. We describe the diversity of NCG building blocks, discuss the mechanisms of NCG formation, review characterization techniques, outline NCG fabrication and processing methods, and highlight most common NCG applications. The review is concluded with the discussion of perspectives in the design and development of NCGs.

Pluronic Induced Interparticle Attraction and Re-entrant Liquid-Liquid Phase Separation in Charged Silica Nanoparticle Suspensions

Tuning surface properties of nanoparticles by introducing charge, surface functionalization, or polymer grafting is central to their stability and applications. Here, we show that introducing non-DLVO forces like steric and hydrophobic effects in charged silica nanoparticle suspensions through interaction with a nonionic surfactant brings about interesting modulations in their interparticle interaction and phase behavior. The Ludox TM-40 negatively charged silica suspensions thus exhibit liquid-liquid phase separation driven by the onset of interparticle attraction in the system in the presence of the triblock copolymer Pluronic P123. The observed phase separations are thermoresponsive in nature, as they are associated with lower consolute temperatures and a re-entrant behavior as a function of temperature. The nanoparticle-Pluronic system thus undergoes transformation from one-phase to two-phase and then back to one-phase with monotonic increase in temperature. Evolution of the interparticle interaction in the composite system is investigated by dynamic light scattering (DLS), small angle neutron scattering (SANS), zeta potential, rheological, and fluorescence spectroscopy studies. Zeta potential studies show that the charge interaction in the system is partially mitigated through adsorption of a Pluronic micellar layer on the nanoparticle surfaces. Contrast-matching SANS studies suggest that hydrophobic interactions between the adsorbed micellar layer bring about the onset of interparticle attraction in the system. The results are unique and not reported hitherto in charged silica nanoparticle systems.

Synergistic Role of Temperature and Salinity in Aggregation of Nonionic Surfactant-Coated Silica Nanoparticles

The adsorption of nonionic surfactants onto hydrophilic nanoparticles (NPs) is anticipated to increase their stability in aqueous medium. While nonionic surfactants show salinity- and temperature-dependent bulk phase behavior in water, the effects of these two solvent parameters on surfactant adsorption and self-assembly onto NPs are poorly understood. In this study, we combine adsorption isotherms, dispersion transmittance, and small-angle neutron scattering (SANS) to investigate the effects of salinity and temperature on the adsorption of pentaethylene glycol monododecyl ether (C₁₂E₅) surfactant on silica NPs. We find an increase in the amount of surfactant adsorbed onto the NPs with increasing temperature and salinity. Based on SANS measurements and corresponding analysis using computational reverse-engineering analysis of scattering experiments (CREASE), we show that the increase in salinity and temperature results in the aggregation of silica NPs. We further demonstrate the non-monotonic changes in viscosity for the C₁₂E₅-silica NP mixture with increasing temperature and salinity and correlate the observations to the aggregated state of NPs. The study provides a fundamental understanding of the configuration and phase transition of the surfactant-coated NPs and presents a strategy to manipulate the viscosity of such dispersion using temperature as a stimulus.

Insight into the Lysozyme-Induced Aggregation of Aromatic Amino Acid-Functionalized Gold Nanoparticles: Impact of the Protein Conjugation and Lipid Corona on the Aggregation Phenomena

The aggregation and subsequent precipitation of gold nanoparticles (Au NPs) in the presence of protein molecules restrict the usefulness of NPs in biomedical applications. Till now, the influence of different properties of Au NPs (size, surface charge, surface coatings) and proteins (surface charge, chemical modification, folded and unfolded states) and pH and ionic strength of the solution on the aggregation of both Au NPs and proteins has been thoroughly discussed in the literature. However, the underlying different mechanistic pathways of the protein concentration-dependent aggregation of both Au NPs and proteins are poorly understood. The impact of the lipid corona on the protein-induced Au NP aggregation has remained an unresolved issue. In this context, we investigate the interaction of the negatively charged aromatic amino acid (phenylalanine and tyrosine)-functionalized gold nanoparticles (Au-AA NPs) with the positively charged globular protein lysozyme at different protein concentrations and compare the results with those of conventional citrate-functionalized Au NPs (Au-Cit NPs). Next, we conjugate lipids and proteins to Au NPs to impede the aggregation of Au NPs induced by the lysozyme. Our results reveal that the aggregation mechanism of the Au-AA NPs is distinctly different at low and high protein concentrations with the uniqueness of the Au-AA NPs over the Au-Cit NPs. Furthermore, we find that human serum albumin (HSA) protein-conjugated Au-AA and Au-Cit NPs are more effective in preventing the lysozyme-induced Au NP aggregation than bovine serum albumin (BSA)-conjugated Au NPs. For the first time, we also report the significant role of "hard" and "soft" lipid coronas in the aggregation of amino acid (phenylalanine)-functionalized gold nanoparticles in the presence of lysozyme protein.

Role of the Protein Corona in the Colloidal Behavior of Microplastics

Microparticles of polyethylene and polypropylene are largely found in aquatic environments because they are the most produced and persistent plastic materials. Once in biological media, they are covered by a layer of molecules, the so-called corona, mostly composed of proteins. A yeast protein extract from Saccharomyces cerevisiae was used as a protein system to observe interactions in complex biological media. Proteins, acting as surfactants and providing hydrophilic surfaces, allow the dispersion of highly hydrophobic particles in water and stabilize them. After 24 h, the microplastic quantity was up to 1 × 10¹¹ particles per liter, whereas without protein, no particles remained in solution. Label-free imaging of the protein corona by synchrotron radiation deep UV fluorescence microscopy (SR-DUV) was performed. In situ images of the protein corona were obtained, and the adsorbed protein quantity, the coverage rate, and the corona heterogeneity were determined. The stability kinetics of the microplastic suspensions were measured by light transmission using a Turbiscan analyzer. Together, the microscopic and kinetics results demonstrate that the protein corona can very efficiently stabilize microplastics in solution provided that the protein corona quality is sufficient. Microplastic stability depends on different parameters such as the particle's intrinsic properties (size, density, hydrophobicity) and the protein corona formation that changes the particle wettability, electrostatic charge, and steric hindrance. By controlling these parameters with proteins, it becomes possible to keep microplastics in and out of solution, paving the way for applications in the field of microplastic pollution control and remediation.

Computational Reverse-Engineering Analysis for Scattering Experiments for Form Factor and Structure Factor Determination ("P(q) and S(q) CREASE")

In this paper, we present an open-source machine learning (ML)-accelerated computational method to analyze small-angle scattering profiles [I(q) vs q] from concentrated macromolecular solutions to simultaneously obtain the form factor P(q) (e.g., dimensions of a micelle) and the structure factor S(q) (e.g., spatial arrangement of the micelles) without relying on analytical models. This method builds on our recent work on Computational Reverse-Engineering Analysis for Scattering Experiments (CREASE) that has either been applied to obtain P(q) from dilute macromolecular solutions (where S(q) ∼1) or to obtain S(q) from concentrated particle solutions when P(q) is known (e.g., sphere form factor). This paper's newly developed CREASE that calculates P(q) and S(q), termed as "P(q) and S(q) CREASE", is validated by taking as input I(q) vs q from in silico structures of known polydisperse core(A)-shell(B) micelles in solutions at varying concentrations and micelle-micelle aggregation. We demonstrate how "P(q) and S(q) CREASE" performs if given two or three of the relevant scattering profiles-I _total(q), I _A(q), and I _B(q)-as inputs; this demonstration is meant to guide experimentalists who may choose to do small-angle X-ray scattering (for total scattering from the micelles) and/or small-angle neutron scattering with appropriate contrast matching to get scattering solely from one or the other component (A or B). After validation of "P(q) and S(q) CREASE" on in silico structures, we present our results analyzing small-angle neutron scattering profiles from a solution of core-shell type surfactant-coated nanoparticles with varying extents of aggregation.

Nanoparticle protein corona: from structure and function to therapeutic targeting

Nanoparticle (NP)-based therapeutics have ushered in a new era in translational medicine. However, despite the clinical success of NP technology, it is not well-understood how NPs fundamentally change in biological environments. When introduced into physiological fluids, NPs are coated by proteins, forming a protein corona (PC). The PC has the potential to endow NPs with a new identity and alter their bioactivity, stability, and destination. Additionally, the conformation of proteins is sensitive to their physical and chemical surroundings. Therefore, biological factors and protein-NP-interactions can induce changes in the conformation and orientation of proteins in vivo. Since the function of a protein is closely connected to its folded structure, slight differences in the surrounding environment as well as the surface characteristics of the NP materials may cause proteins to lose or gain a function. As a result, this can alter the downstream functionality of the NPs. This review introduces the main biological factors affecting the conformation of proteins associated with the PC. Then, four types of NPs with extensive utility in biomedical applications are described in greater detail, focusing on the conformation and orientation of adsorbed proteins. This is followed by a discussion on the instances in which the conformation of adsorbed proteins can be leveraged for therapeutic purposes, such as controlling protein conformation in assembled matrices in tissue, as well as controlling the PC conformation for modulating immune responses. The review concludes with a perspective on the remaining challenges and unexplored areas at the interface of PC and NP research.

Electric, magnetic, and shear field-directed assembly of inorganic nanoparticles

Ordered assemblies of inorganic nanoparticles (NPs) have shown tremendous potential for wide applications due to their unique collective properties, which differ from those of individual NPs. Various assembly methods, such as external field-directed assembly, interfacial assembly, template assembly, biomolecular recognition-mediated assembly, confined assembly, and others, have been employed to generate ordered inorganic NP assemblies with hierarchical structures. Among them, the external field-directed assembly method is particularly fascinating, as it can remotely assemble NPs into well-ordered superstructures. Moreover, external fields (e.g., electric, magnetic, and shear fields) can introduce a local and/or global field intensity gradient, resulting in an additional force on NPs to drive their rotation and/or translation. Therefore, the external field-directed assembly of NPs becomes a robust method to fabricate well-defined functional materials with the desired optical, electronic, and magnetic properties, which have various applications in catalysis, sensing, disease diagnosis, energy conversion/storage, photonics, nano-floating-gate memory, and others. In this review, the effects of an electric field, magnetic field, and shear field on the organization of inorganic NPs are highlighted. The methods for controlling the well-ordered organization of inorganic NPs at different scales and their advantages are reviewed. Finally, future challenges and perspectives in this field are discussed.

Competitive protein adsorption on charge regulating silica-like surfaces: the role of protonation equilibrium

We develop a molecular thermodynamic theory to study the interaction of some proteins with a charge regulating silica-like surface under a wide range of conditions, including pH, salt concentration and protein concentration. Proteins are modeled using their three dimensional structure from crystallographic data and the average experimental pKa of amino acid residues. As model systems, we study single-protein and binary solutions of cytochrome c, green fluorescent protein, lysozyme and myoglobin. Our results show that protonation equilibrium plays a critical role in the interactions of proteins with these type of surfaces. The terminal hydroxyl groups on the surface display considerable extent of charge regulation; protein residues with titratable side chains increase protonation according to changes in the local environment and the drop in pH near the surface. This behavior defines protein-surface interactions and leads to the emergence of several phenomena: (i) a complex non-ideal surface charge behavior; (ii) a non-monotonic adsorption of proteins as a function of pH; and (iii) the presence of two spatial regions, a protein-rich and a protein-depleted layer, that occur simultaneously at different distances from the surface when pH is slightly above the isoelectric point of the protein. In binary mixtures, protein adsorption and surface-protein interactions cannot be predicted from single-protein solution considerations.

Nanoparticle-Protein Interaction: Demystifying the Correlation between Protein Corona and Aggregation Phenomena

Protein corona formation and nanoparticles' aggregation have been heavily discussed over the past years since the lack of fine-mapping of these two combined effects has hindered the targeted delivery evolution and the personalized nanomedicine development. We present a multitechnique approach that combines dynamic light and small-angle X-ray scattering techniques with cryotransmission electron microscopy in a given fashion that efficiently distinguishes protein corona from aggregates formation. This methodology was tested using ∼25 nm model silica nanoparticles incubated with either model proteins or biologically relevant proteomes (such as fetal bovine serum and human plasma) in low and high ionic strength buffers to precisely tune particle-to-protein interactions. In this work, we were able to differentiate protein corona, small aggregates formation, and massive aggregation, as well as obtain fractal information on the aggregates reliably and straightforwardly. The strategy presented here can be expanded to other particle-to-protein mixtures and might be employed as a quality control platform for samples that undergo biological tests.

Lysozyme amyloid fibril: Regulation, application, hazard analysis, and future perspectives

Self-assembly of misfolded proteins into ordered fibrillar aggregates known as amyloid results in various human diseases. However, more and more proteins, whether in human body or in food, have been found to be able to form amyloid fibrils with in-depth researches. As a model protein for amyloid research, lysozyme has always been the focus of research in various fields. Firstly, the formation mechanisms of amyloid fibrils are discussed concisely. Researches on the regulation of lysozyme amyloid fibrils are helpful to find suitable therapeutic drugs and unfriendly substances. And this review article summarizes a number of exogenous substances including small molecules, nanoparticles, macromolecules, and polymers. Small molecules are mainly connected to lysozyme through hydrophobic interaction, electrostatic interaction, π-π interaction, van der Waals force and hydrogen bond. Nanoparticles inhibit the formation of amyloid fibers by stabilizing lysozyme and fixing β-sheet. Besides, the applications of lysozyme amyloid fibrils in food-related fields are considered furtherly due to outstanding physical and mechanical properties. Nevertheless, the potential health threats are still worthy of our attention. Finally, we also give suggestions and opinions on the future research direction of lysozyme amyloid fibrils.

pH Shock-promoted lysozyme corona for efficient pathogenic infections treatment: Effects of surface chemistry of mesoporous silica nanoparticles and loading method

The emergence of antibiotic resistant bacteria because of the antibiotics abusement was the motivation to develop the effective alternatives to traditional antibiotics. Hence, various lysozyme corona were prepared through the physical and covalent attachment of lysozyme molecules onto either the bare or carboxyl-functionalized mesoporous silica particles. The prepared samples were characterized by STEM, TGA/DTA, zeta potential, FTIR, UV-vis and CD spectroscopic methods. All the prepared lysozyme-coated particles exhibited an efficient antibacterial activity against Listeria monocytogenes, as a case study, in vitro with no cytotoxicity. The minimal inhibition concentration (MIC) of the lysozyme-physically adsorbed bare and carboxyl-functionalized mesoporous silica nanoparticles (L-MS and L-ads-CMS, respectively) and the lysozyme-covalently attached carboxyl-functionalized MS particles (L-cov-CMS) was 2, 5.3 and 1.7 folds lower than that of the free lysozyme, respectively. Additionally, for the first time, it was reported that the pretreatment of lysozyme corona of L-ads-CMS through inducing a pH-shock can lead to the enhancement of antibacterial properties thereof. This behavior was associated to the controlled release of the immobilized lysozyme molecules and their conformational stability. These natural antibacterial lysozyme-coated silica nanoparticles showing the "pH-shock enhanced activity" could be of utmost interest for design of the highly active enzyme-modified nanoparticles.

Inside the Protein Corona: From Binding Parameters to Unstained Hard and Soft Coronas Visualization

Proteins spontaneously adsorb on nanoparticle surfaces when injected into the bloodstream. It drastically modifies the nanoparticle's fate and how they interact with organs and cells. Although this protein layer (protein corona) has been widely studied, the robustness of the most employed characterization methods and the visualization of its unstained fractions remain open questions. Here, synchrotron-based small-angle X-ray scattering was used to follow the corona formation and estimate binding parameters. At the same time, transmission electron microscopy under cryogenic conditions associated with cross-correlation image processing and energy-filtered transmission electron microscopy allowed to determine protein corona morphology and thickness together with the visualization of its unstained hard and soft fractions. The above-presented strategy shows tremendous potential for deciphering fundamental protein corona aspects and can contribute to rational medical nanoparticle engineering.

High-Affinity Points of Interaction on Antibody Allow Synthesis of Stable and Highly Functional Antibody-Gold Nanoparticle Conjugates

Many emerging nanobiotechnologies rely on the proper function of proteins immobilized on gold nanoparticles. Often, the surface chemistry of the AuNP is engineered to control the orientation, surface coverage, and structure of the adsorbed protein to maximize conjugate function. Here, we chemically modified antibody to investigate the effect of protein surface chemistries on adsorption to AuNPs. A monoclonal anti-horseradish peroxidase IgG antibody (anti-HRP) was reacted with N-succinimidyl acrylate (NSA) or reduced dithiobissuccinimidyl propionate (DSP) to modify lysine residues. Zeta potential measurements confirmed that both chemical modifications reduced the localized regions of positive charge on the protein surface, while the DSP modification incorporated additional free thiols. Dynamic light scattering confirmed that native and chemically modified antibodies adsorbed onto AuNPs to form bioconjugates; however, adsorption kinetics revealed that the NSA-modified antibody required significantly more time to allow for the formation of a hard corona. Moreover, conjugates formed with the NSA-modified antibody lost antigen-binding function, whereas unmodified and DSP-modified antibodies adsorbed onto AuNPs to form functional conjugates. These results indicate that high-affinity functional groups are required to prevent protein unfolding and loss of function when adsorbed on the AuNP surface. The reduced protein charge and high-affinity thiol groups on the DSP-modified antibody enabled pH-dependent control of protein orientation and the formation of highly active conjugates at solution pHs (<7.5) that are inaccessible with unmodified antibody due to conjugate aggregation. This study establishes parameters for protein modification to facilitate the formation of highly functional and stable protein-AuNP conjugates.

Biomimetic and bioinspired surface topographies as a green strategy for combating biofouling: a review

Biofouling refers to the adverse attachment and colonization of fouling organisms, including macromolecules, bacteria, and sessile invertebrates, on the surfaces of materials submerged in aquatic environments. Almost all structures working in watery surroundings, from marine infrastructures to healthcare facilities, are affected by this sticky problem, resulting in massive direct and indirect economic loss and enormous cost every year in protective maintenance and remedial cleaning. Traditional approaches to preventing marine biofouling primarily rely on the application of biocide-contained paints, which certainly impose adverse effects on the ocean environment and marine ecology. Biomimicry offers an efficient shortcut to developing environmentally friendly antifouling techniques and has yielded encouraging and promising results. The antifouling strategies learned from nature can be broadly classified into two categories according to the nature of the cues applied for biofouling control. One is the chemical antifouling techniques, which are dedicated to extracting the effective antifoulant compounds from marine organisms and synthesizing chemicals mimicking natural antifoulants. In contrast, the physical biomimetic (BM) antifouling practices focus on the emulation and optimization of the physical cues such as micro and nanoscale surface topographies learned from naturally occurring surfaces for better antifouling efficacy. In this review, a synopsis of the techniques for manufacturing the BM and bioinspired (BI) antifouling surface topographies is introduced, followed by the bioassay to assess the antifouling performance of the structured surfaces. Then, the BM and BI surface topographies that were reported to possess enhanced antifouling competence are introduced, followed by a summary of theoretical modeling. The whole paper is concluded by summarizing the studies' deficiencies so far and outlooking the research directions in the future.

Protein-nanoparticle interactions and a new insight

The study of protein-nanoparticle interactions provides knowledge about the bio-reactivity of nanoparticles, and creates a database of nanoparticles for applications in nanomedicine, nanodiagnosis, and nanotherapy. The problem arises when nanoparticles come in contact with physiological fluids such as plasma or serum, wherein they interact with the proteins (or other biomolecules). This interaction leads to the coating of proteins on the nanoparticle surface, mostly due to the electrostatic interaction, called 'corona'. These proteins are usually partially unfolded. The protein corona can deter nanoparticles from their targeted functionalities, such as drug/DNA delivery at the site and fluorescence tagging of diseased tissues. The protein corona also has many repercussions on cellular intake, inflammation, accumulation, degradation, and clearance of the nanoparticles from the body depending on the exposed part of the proteins. Hence, the protein-nanoparticle interaction and the configuration of the bound-proteins on the nanosurface need thorough investigation and understanding. Several techniques such as DLS and zeta potential measurement, UV-vis spectroscopy, fluorescence spectroscopy, circular dichroism, FTIR, and DSC provide valuable information in the protein-nanoparticle interaction study. Besides, theoretical simulations also provide additional understanding. Despite a lot of research publications, the fundamental question remained unresolved. Can we aim for the application of functional nanoparticles in medicine? A new insight, given by us, in this article assumes a reasonable solution to this crucial question.

Probing the Mechanism of Antibody-Triggered Aggregation of Gold Nanoparticles

The unique physicochemical properties of gold nanoparticles (AuNPs) provide many opportunities to develop novel biomedical technologies. The surface chemistry of AuNPs can be engineered to perform a variety of functions, including targeted binding, cellular uptake, or stealthlike properties through the immobilization of biomolecules, such as proteins. It is well established that proteins can spontaneously adsorb onto AuNPs, to form a stable and functional bioconjugate; however, the protein-AuNP interaction may result in the formation of less desirable protein-AuNP aggregates. Therefore, it is imperative to investigate the protein-AuNP interaction and elucidate the mechanism by which protein triggers AuNP aggregation. Herein, we systematically investigated the interaction of immunoglobulin G (IgG) antibody with citrate-capped AuNPs as a function of solution pH. We found that the addition of antibody triggers the aggregation of AuNPs for pH < 7.5, whereas a monolayer of antibody adsorbs onto the AuNP to form a stable bioconjugate when the antibody is added to AuNPs at pH ≥ 7.5. Our data identifies electrostatic bridging between the antibody and the negatively charged AuNPs as the mechanism by which aggregation occurs and rules out protein unfolding and surface charge depletion as potential causes. Furthermore, we found that the electrostatic bridging of AuNPs is reversible within the first few hours of interaction, but the protein-AuNP interactions strengthen over 24 h, after which the protein-AuNP aggregate is irreversibly formed. From this data, we developed a straightforward approach to acrylate the basic residues on the antibody to prevent protein-induced aggregation of AuNP over a wide pH range. The results of this study provide additional insight into antibody-nanoparticle interactions and provide a pathway to control the interaction with the potential to enhance the conjugate function.

Adsorption of Myoglobin and Corona Formation on Silica Nanoparticles

The adsorption of proteins from aqueous medium leads to the formation of protein corona on nanoparticles. The formation of protein corona is governed by a complex interplay of protein-particle and protein-protein interactions, such as electrostatics, van der Waals, hydrophobic, hydrogen bonding, and solvation. The experimental parameters influencing these interactions, and thus governing the protein corona formation on nanoparticles, are currently poorly understood. This lack of understanding is due to the complexity in the surface charge distribution and anisotropic shape of the protein molecules. Here, we investigate the effect of pH and salinity on the characteristics of corona formed by myoglobin on silica nanoparticles. We experimentally measure and theoretically model the adsorption isotherms of myoglobin binding to silica nanoparticles. By combining adsorption studies with surface electrostatic mapping of myoglobin, we demonstrate that a monolayered hard corona is formed in low salinity dispersions, which transforms into a multilayered hard + soft corona upon the addition of salt. We attribute the observed changes in protein adsorption behavior with increasing pH and salinity to the change in electrostatic interactions and surface charge regulation effects. This study provides insights into the mechanism of protein adsorption and corona formation on nanoparticles, which would guide future studies on optimizing nanoparticle design for maximum functional benefits and minimum toxicity.

Complex Formation of Silica Nanoparticles with Collagen: Effects of the Conformation of Collagen

Negatively charged Ludox silica nanoparticles (SiNPs) form a complex with atelocollagen (AC) in acidic buffers (pH = 4 or 3). AC is a low-immunogenic derivative of collagen obtained by the removal of N- and C-terminal telopeptide components. Mixed solutions of negatively charged SiNPs and AC were turbid, while positively charged SiNPs (Ludox CL) did not form a complex with AC in pH 4 buffer, indicating that electrostatic attraction is the dominant force to form the complex. Small-angle X-ray scattering (SAXS) and circular dichroism (CD) measurements were made for AC and Ludox LS (or CL) solutions in acetate buffer (pH 4.0) and citrate buffer (pH 3.0). The CD data showed that the stability of the triple helical structure of AC in the buffers is not affected by the complexation. The resulting complex consisting of triple helical AC and SiNPs did not influence the SAXS profile except for the lowest q region investigated. On the contrary, different scattering profiles were observed for the single chain AC and SiNP mixture indicating densely packed SiNPs in the complex. This scattering behavior was fairly explained in terms of the sticky hard sphere model (SHSM). This AC conformation-dependent complexation may be because of the hydrogen bonding interaction between the single chain AC and SiNPs. The temperature-induced change of the complex formation can be applied for thermoresponsive hybrid materials.

Particle Detection and Characterization for Biopharmaceutical Applications: Current Principles of Established and Alternative Techniques

Detection and characterization of particles in the visible and subvisible size range is critical in many fields of industrial research. Commercial particle analysis systems have proliferated over the last decade. Despite that growth, most systems continue to be based on well-established principles, and only a handful of new approaches have emerged. Identifying the right particle-analysis approach remains a challenge in research and development. The choice depends on each individual application, the sample, and the information the operator needs to obtain. In biopharmaceutical applications, particle analysis decisions must take product safety, product quality, and regulatory requirements into account. Biopharmaceutical process samples and formulations are dynamic, polydisperse, and very susceptible to chemical and physical degradation: improperly handled product can degrade, becoming inactive or in specific cases immunogenic. This article reviews current methods for detecting, analyzing, and characterizing particles in the biopharmaceutical context. The first part of our article represents an overview about current particle detection and characterization principles, which are in part the base of the emerging techniques. It is very important to understand the measuring principle, in order to be adequately able to judge the outcome of the used assay. Typical principles used in all application fields, including particle–light interactions, the Coulter principle, suspended microchannel resonators, sedimentation processes, and further separation principles, are summarized to illustrate their potentials and limitations considering the investigated samples. In the second part, we describe potential technical approaches for biopharmaceutical particle analysis as some promising techniques, such as nanoparticle tracking analysis (NTA), micro flow imaging (MFI), tunable resistive pulse sensing (TRPS), flow cytometry, and the space- and time-resolved extinction profile (STEP^®) technology.

Modeling of Cell-Mediated Self-Assembled Colloidal Scaffolds

A critical step in tissue engineering is the design and synthesis of 3D biocompatible matrices (scaffolds) to support and guide the proliferation of cells and tissue growth. The most existing techniques rely on the processing of scaffolds under controlled conditions and then implanting them in vivo, with questions related to biocompatibility and implantation that are still challenging. As an alternative, it was proposed to assemble the scaffolds in loco through the self-organization of colloidal particles mediated by cells. To overcome the difficulty to test experimentally all the relevant parameters, we propose the use of large-scale numerical simulation as a tool to reach useful predictive information and to interpret experimental results. Thus, in this study, we combine experiments, particle-based simulations, and mean-field calculations to show that, in general, the size of the self-assembled scaffold scales with the cell-to-particle ratio. However, we have found an optimal value of this ratio, for which the size of the scaffold is maximal when the cell-cell adhesion is suppressed. These results suggest that the size and structure of the self-assembled scaffolds may be designed by tuning the adhesion between cells in the colloidal suspension.

Smoluchowski equations for linker-mediated irreversible aggregation

We developed a generalized Smoluchowski framework to study linker-mediated aggregation, where linkers and particles are explicitly taken into account. We assume that the bonds between linkers and particles are irreversible, and that clustering occurs through limited diffusion aggregation. The kernel is chosen by analogy with single-component diffusive aggregation but the clusters are distinguished by their number of particles and linkers. We found that the dynamics depends on three relevant factors, all tunable experimentally: (i) the ratio of the diffusion coefficients of particles and linkers; (ii) the relative number of particles and linkers; and (iii) the maximum number of linkers that may bond to a single particle. To solve the Smoluchoski equations analytically we employ a scaling hypothesis that renders the fraction of bondable sites of a cluster independent of the size of the cluster, at each instant. We perform numerical simulations of the corresponding lattice model to test this hypothesis. We obtain results for the asymptotic limit, and the time evolution of the bonding probabilities and the size distribution of the clusters. These findings are in agreement with experimental results reported in the literature and shed light on unexplained experimental observations.

Interactions between functionalised silica nanoparticles and Pseudomonas fluorescens biofilm matrix: A focus on the protein corona

Biofilms are microbial communities embedded in an extracellular polymeric matrix and display an enhanced tolerance to the action of antimicrobials. The emergence of novel functionalised nanoparticles is considered a promising avenue for the development of biofilm-specific antimicrobial technologies. However, there is a gap in the understanding of interactions between nanoparticles and the biofilm matrix. Particularly, questions are raised on how nanoparticle charge and surface groups play a role in aggregation when in contact with biofilm components. Herein we present the synthesis of four types of silica nanoparticles and undertake an analysis of their interactions with Pseudomonas fluorescens biofilm matrix. The effect of the biofilm matrix components on the charge and aggregation of the nanoparticles was assessed. Additionally, the study focused on the role of matrix proteins, with the in-depth characterisation of the protein corona of each nanoparticle by Liquid Chromatography with Tandem Mass Spectrometry experiments. The protein corona composition is dependent on the nanoparticle type; non-functionalised nanoparticles show less protein selectivity, whereas carboxylate-functionalised nanoparticles prefer proteins with a higher isoelectric point. These outcomes provide insights into the field of biofilm-nanoparticle interactions that can be valuable for the design of new nano-based targeting systems in future anti-biofilm applications.

From Protein Corona to Colloidal Self-Assembly: The Importance of Protein Size in Protein-Nanoparticle Interactions

Protein adsorption on nanoparticles is an important field of study, particularly with regard to nanomedicine and nanotoxicology. Many factors can influence the composition and structure of the layer(s) of adsorbed proteins, the so-called protein corona. However, the role of protein size has not been specifically investigated, although some evidence has indicated its potential important role in corona composition and structure. To assess the role of protein size, we studied the interactions of hemoproteins (spanning a large size range) with monodisperse silica nanoparticles. We combined various techniques-adsorption isotherms, isothermal titration calorimetry, circular dichroism, and transmission electron cryomicroscopy-to address this issue. Overall, the results show that small proteins behaved as typical model proteins, forming homogeneous monolayers on the nanoparticle surface (protein corona). Their adsorption is purely enthalpy-driven, with subtle structural changes. In contrast, large proteins interact with nanoparticles via entropy-driven mechanisms. Their structure is completely preserved during adsorption, and any given protein can directly bind to several nanoparticles, forming bridges in these newly formed protein-nanoparticle assemblies. Protein size is clearly an overlooked factor that should be integrated into proteomics and toxicological studies.

Optimal number of linkers per monomer in linker-mediated aggregation

We study the dynamics of diffusion-limited irreversible aggregation of monomers, where bonds are mediated by linkers. We combine kinetic Monte Carlo simulations of a lattice model with a mean-field theory to study the dynamics when the diffusion of aggregates is negligible and only monomers diffuse. We find two values of the number of linkers per monomer which maximize the size of the largest aggregate. We explain the existence of the two maxima based on the distribution of linkers per monomer. This observation is well described by a simple mean-field model. We also show that a relevant parameter is the ratio of the diffusion coefficients of monomers and linkers. In particular, when this ratio is close to ten, the two maxima merge at a single maximum.

A health concern regarding the protein corona, aggregation and disaggregation

Nanoparticle (NP)-protein complexes exhibit the "correct identity" of NP in biological media. Therefore, protein-NP interactions should be closely explored to understand and modulate the nature of NPs in medical implementations. This review focuses mainly on the physicochemical parameters such as dimension, surface chemistry, morphology of NPs, and influence of pH on the formation of protein corona and conformational changes of adsorbed proteins by different kinds of techniques. Also, the impact of protein corona on the colloidal stability of NPs is discussed. Uncontrolled protein attachment on NPs may bring unwanted impacts such as protein denaturation and aggregation. In contrast, controlled protein adsorption by optimal concentration, size, pH, and surface modification of NPs may result in potential implementation of NPs as therapeutic agents especially for disaggregation of amyloid fibrils. Also, the effect of NPs-protein corona on reducing the cytotoxicity and clinical implications such as drug delivery, cancer therapy, imaging and diagnosis will be discussed. Validated correlative physicochemical parameters for NP-protein corona formation frequently derived from protein corona fingerprints of NPs which are more valid than the parameters obtained only on the base of NP features. This review may provide useful information regarding the potency as well as the adverse effects of NPs to predict their behavior in vivo.

pH-Induced reorientation of cytochrome c on silica nanoparticles

The orientation of cytochrome c molecules at the surface of silica nanoparticles was studied in a wide pH range by combining small-angle neutron scattering, adsorption measurements, and molecular dynamics simulations. The results indicate a reorientation of the ellipsoidal protein from head-on to side-on as the pH is increased. This is attributed to changes in the surface charge distribution of both the protein and the nanoparticles.

Mechanism of silica-lysozyme composite formation unravelled by in situ fast SAXS

A quantitative understanding of aggregation mechanisms leading to the formation of composites of inorganic nanoparticles (NPs) and proteins in aqueous media is of paramount interest for colloid chemistry. In particular, the interactions between silica (SiO2) NPs and lysozyme (LZM) have attracted attention, because LZM is well-known to adsorb strongly to silica NPs, while at the same time preserving its enzymatic activity. The inherent nature of the aggregation processes leading to NP-LZM composites involves structural changes at length scales from few to at least hundreds of nanometres but also time scales much smaller than one second. To unravel these we used in situ synchrotron-based small-angle X-ray scattering (SAXS) and followed the subtle interparticle interactions in solution at a time resolution of 50 ms/frame (20 fps). We show that if the size of silica NPs (ca. 5 nm diameter) is matched by the dimensions of LZM, the evolving scattering patterns contain a unique structure-factor contribution originating from the presence of LZM. We developed a scattering model and applied it to analyse this structure function, which allowed us to extract structural information on the deformation of lysozyme molecules during aggregation, as well as to derive the mechanisms of composite formation.

Dual Size-Dependent Effect of Fe₃O₄ Magnetic Nanoparticles Upon Interaction with Lysozyme Amyloid Fibrils: Disintegration and Adsorption

Nanomedicine compounds containing nanoparticles, such as iron oxides and gold, have been demonstrated to be effective in promoting different magnitudes of interaction with amyloid β fibrils, of which disintegrating or inhibiting effects are of great importance to treating fibrillary aggregation-induced neurological disorders such as Alzheimer’s disease. This research herein studies the interaction between lysozyme amyloid fibrils, a type of fibers derived from hen egg white lysozyme, and Fe₃O₄ magnetic nanoparticles (MNPs) of an assorted diameter sizes of 5 nm, 10 nm and 20 nm, using atomic force microscopy (AFM). Specifically, the effects of the sizes of negatively charged MNPs on the resultant amyloid fibrillary mixture was investigated. Our results of AFM images indicated that the interaction between MNPs and the fibrils commences immediately after adding MNPs to the fibril solution, and the actions of such MNPs-doped fibrillary interplay, either integration or segmentation, is strongly dependent on the size and volume concentration of MNPs. In the cases of 5 nm and 20 nm particles of equivalent volume concentration, the adsorption and agglomeration of MNPs onto the fibrillary surfaces was observed, whereas, interestingly, MNPs with diameter size of 10 nm enables segmentation of the slender fibrils into debris when a proper implemented volume concentration was found, which signifies utter destruction of the amyloid fibrillary structure.

Binding of Lignin Nanoparticles at Oil-Water Interfaces: An Ecofriendly Alternative to Oil Spill Recovery

Synthetic amphiphiles used for managing large-scale oil spills have a toxic impact on the environment and marine life. Developing new oil spill recovery technologies is critical to minimize the environmental and ecological impact of such disasters. Here, we show that a mixture of lignin nanoparticles and 1-pentanol forms a biocompatible alternative to nondegradable, synthetic amphiphiles used for oil spill recovery. The pentanol in the mixture generates initial Marangoni flow and confines the spilled oil into a thick slick on the surface of water. While the alcohol solubilizes, lignin nanoparticles irreversibly adsorb onto the oil-water interface. We find that the lignin nanoparticle adsorption to the oil-water interface is governed by a combination of electrostatic, van der Waals, and hydrophobic interactions between the particles and the interface. These interactions, combined with interparticle electrostatic repulsion between nanoparticles adsorbed at the oil-water interface, drive the formation of a submonolayer. The submonolayer transforms into a film of jammed nanoparticles due to compressive stress acting on the interface upon the solubilization of pentanol. This interfacial layer of lignin nanoparticles restricts oil from respreading and locks the oil in its confined state. The herded state of the oil with the interfacial layer of nanoparticles facilitates safe removal of the spilled oil using mechanical methods. The study presents a new principle of using a mixture of heavy alcohol and biocompatible nanoparticles for oil herding applications, thus providing an ecofriendly alternative to oil spill recovery.

Bioactive effects of silica nanoparticles on bone cells are size, surface, and composition dependent

Silica based nanoparticles have been demonstrated to have intrinsic biologic activity towards the skeleton and to function by promoting the differentiation of bone forming osteoblasts while inhibiting the differentiation of bone resorbing osteoclasts. The excitement surrounding nanomedicine in part revolves around the almost unlimited possibilities for varying the physicochemical properties including size, composition, and surface charge. To date few studies have attempted to manipulate these characteristics in concert to optimize a complex biologic outcome. Towards this end, spherical silica nanoparticles of various sizes (50-450 nm), of different surface properties (OH, CO₂H, NR₄+, mNH₂), and of different composition (silica, gold, and polystyrene) were synthesized and evaluated for biological activity toward skeletal cells. Osteoblast activity was most influenced by composition and size variables, whereas osteoclasts were most affected by surface property variation. The study also establishes nanoparticle mediated suppression of Nfatc1, a key transcriptional regulator for osteoclast differentiation, identifying a novel mechanism of action. Collectively, the study highlights how during the design of bioactive nanoparticles, it is vital to consider not only the myriad of physical properties that can be manipulated, but also that the characteristics of the target cell plays an equally integral role in determining biological outcome. STATEMENT OF SIGNIFICANCE: Silica nanomaterials represent a promising biomaterial for beneficial effects on bone mass and quality as well as regenerative tissue engineering and are currently being investigated for intrinsic bioactivity towards the primary cells responsible for skeletal homeostasis; osteoblasts and osteoclasts. The goal of the current study was to assess the physical properties of silica nanoparticles that impart intrinsic bioactivity by evaluating size, surface charge, and composition. Results reveal differential influences of the physical properties of nanoparticles towards osteoblasts and osteoclasts. This study provides new insights into the design of nanoparticles to specifically target different aspects of bone metabolism and highlights the opportunities provided by nanotechnology to modulate a range of cell specific biological responses for therapeutic benefit.

Lysozyme as a flocculant-inducing agent improving the silica removal from aqueous solutions - A turbidimetric study

In this paper, the lysozyme (LSZ) adsorption impact on the silica suspension stability was established. In other words, the stabilization/destabilization mechanism of the SiO₂/LSZ system was explained based on the adsorption, electrokinetic and stability measurement results. Lysozyme adsorbs on the silica surface in the whole pH range. This process contributes to the changes in silica surface charge and zeta potential values. The lysozyme addition influences the system stability too. At pH 7.6 and 9, a large decrease in the silica suspension stability was found. It is connected with the neutralization of solid negative charge by the positively charged macromolecules. As a result, large aggregates can be formed, which is highly desirable in the silica removal procedure.

Nanoparticle self-assembly: from interactions in suspension to polymer nanocomposites

Recent experimental results using in particular small-angle scattering to characterize the self-assembly of mainly hard spherical nanoparticles into higher ordered structures ranging from fractal aggregates to ordered assemblies are reviewed. The crucial control of interparticle interactions is discussed, from chemical surface-modification, or the action of additives like depletion agents, to the generation of directional patches and the use of external fields. It is shown how the properties of interparticle interactions have been used to allow inducing and possibly controlling aggregation, opening the road to the generation of colloidal molecules or potentially metamaterials. In the last part, studies of the microstructure of polymer nanocomposites as an application of volume-spanning and stress-carrying aggregates are discussed.

Structure and Interaction of Nanoparticle-Protein Complexes

The integration of nanoparticles with proteins is of high scientific interest due to the amazing potential displayed by their complexes, combining the nanoscale properties of nanoparticles with the specific architectures and functions of the protein molecules. The nanoparticle-protein complexes, in particular, are useful in the emerging field of nanobiotechnology (nanomedicine, drug delivery, and biosensors) as the nanoparticles having sizes comparable to that of living cells can access and operate within the cell. The understanding of nanoparticle interaction with different protein molecules is a prerequisite for such applications. The interaction of the two components has been shown to result in conformational changes in proteins and to affect the surface properties and colloidal stability of the nanoparticles. In this feature article, our recent studies exploring the driving interactions in nanoparticle-protein systems and resultant structures are presented. The anionic colloidal silica nanoparticles and two globular charged proteins [lysozyme and bovine serum albumin (BSA)] have been investigated as model systems. The adsorption behavior of the two proteins on nanoparticles is found to be completely different, but they both give rise to similar phase transformation from one phase to two phase in respective nanoparticle-protein systems. The presence of protein induces the short-range and long-range attraction between the nanoparticles with lysozyme and BSA, respectively. The observed phase behavior and its dependence on various physiochemical parameters (e.g., nanoparticle size, ionic strength, and solution pH) have been explained in terms of underlying interactions.

Aggregate Formation of Surface-Modified Nanoparticles in Solvents and Polymer Nanocomposites

A new method based on the combination of small-angle scattering, reverse Monte Carlo simulations, and an aggregate recognition algorithm is proposed to characterize the structure of nanoparticle suspensions in solvents and polymer nanocomposites, allowing detailed studies of the impact of different nanoparticle surface modifications. Experimental small-angle scattering is reproduced using simulated annealing of configurations of polydisperse particles in a simulation box compatible with the lowest experimental q-vector. Then, properties of interest like aggregation states are extracted from these configurations and averaged. This approach has been applied to silane surface-modified silica nanoparticles with different grafting groups, in solvents and after casting into polymer matrices. It is shown that the chemistry of the silane function, in particular mono- or trifunctionality possibly related to patch formation, affects the dispersion state in a given medium, in spite of an unchanged alkyl-chain length. Our approach may be applied to study any dispersion or aggregation state of nanoparticles. Concerning nanocomposites, the method has potential impact on the design of new formulations allowing controlled tuning of nanoparticle dispersion.

Understanding the curvature effect of silica nanoparticles on lysozyme adsorption orientation and conformation: a mesoscopic coarse-grained simulation study

In nanobiotechnology applications, curvature of nanoparticles has a significant effect on protein activities. In this work, lysozyme adsorption on different-sized silica nanoparticles (SNPs) was simulated at the microsecond timescale by using mesoscopic coarse-grained molecular dynamics simulations. It is found that, with the increase of nanoparticle size, which indicates a decrease of surface curvature, adsorbed lysozyme shows a narrower orientation distribution and a greater conformation change, as the electrostatic attraction dominates lysozyme adsorption, and this trend is more pronounced on larger SNPs. Interestingly, the effect induced by different SNP surface curvatures is not related to the direct contact area between lysozyme and SNPs, but to the interfacial hydration layer above the silica surface, since a smaller curvature can lead to a stronger interfacial hydration and make the distribution of interfacial water molecules more ordered. Besides, at higher ionic strength, lysozyme conformation is less affected by strongly negatively charged SNPs, especially for larger nanoparticles. This work might shed some light on how to prepare protein coronas with higher bioactivities in nanobiotechnology.