Stimuli-responsive polymeric materials for separation of biomolecules

pH-responsive membranes: Mechanisms, fabrications, and applications

Understanding the mechanisms of pH-responsiveness allows researchers to design and fabricate membranes with specific functionalities for various applications. The pH-responsive membranes (PRMs) are particular categories of membranes that have an amazing aptitude to change their properties such as permeability, selectivity and surface charge in response to changes in pH levels. This review provides a brief introduction to mechanisms of pH-responsiveness in polymers and categorizes the applied polymers and functional groups. After that, different techniques for fabricating pH-responsive membranes such as grafting, the blending of pH-responsive polymers/microgels/nanomaterials, novel polymers and graphene-layered PRMs are discussed. The application of PRMs in different processes such as filtration membranes, reverse osmosis, drug delivery, gas separation, pervaporation and self-cleaning/antifouling properties with perspective to the challenges and future progress are reviewed. Lastly, the development and limitations of PRM fabrications and applications are compared to provide inclusive information for the advancement of next-generation PRMs with improved separation and filtration performance.

Integrated system for temperature-controlled fast protein liquid chromatography. IV. Continuous 'one-column' 'low-salt' hydrophobic interaction chromatography

Systematic development of a temperature-controlled isocratic process for one-column low-salt hydrophobic interaction chromatography (HIC) of proteins employing a travelling cooling zone reactor (TCZR) system, is described. Batch binding and confocal scanning microscopy were employed to define process conditions for temperature-reversible binding of bovine serum albumin (BSA) which were validated in pulse-response temperature switching HIC experiments, before transferring to TCZR-HIC. A thin-walled stainless-steel column mounted with a movable assembly of copper blocks and Peltier elements (travelling cooling zone, TCZ) was used for TCZR-HIC. In pulse-response TCZR-HIC, 12 TCZ movements along the column desorbed 86.3% of the applied BSA monomers in 95.3% purity depleted >6-fold in 2-4 mers and nearly 260-fold in higher molecular weight (HMW) species. For continuous TCZR-HIC, the TCZ was moved 49-58 times during uninterrupted loading of BSA feeds at 0.25, 0.5 or 1 mg·mL-1. Each TCZ movement generated a sharp symmetrical elution peak. In the best case, (condition 1: 0.25 mg·mL-1 BSA; >17 mg BSA applied per mL of bed) the height of TCZ elution peaks approached pseudo-steady midway through the loading phase with no rise in baseline UV280 signal between peaks. Peak composition remained constant averaging 94.4% monomer, 5.6% 2-4 mers and <0.05% HMW. Monomers were recovered in quantitative yield depleted >3.1 fold in 2-4 mers and 92-fold in HMW species cf. the feed (63.6% monomers, 21.8% 2-4 mers, 14.6% HMW). However, increasing the BSA concentration to 1 mg·mL-1 (condition 2) or employing a fouled HIC column with 0.5 mg·mL-1 BSA (condition 3) compromised monomer purification performance.

In situ growth of dual-responsive polymer as coating for open tubular capillary electrochromatographic separation of epimedins

Recently, open tubular capillary electrochromatography (OT-CEC) has captured considerable interest; its efficient separation capability hinges on the interactions between analytes and polymer coatings. However, in situ growth of stimuli-responsive polymers as coatings has been rarely studied and is crucial for expanding the OT-CEC technique and its application. Herein, following poly(styrene-maleicanhydride) (PSM) chemically bonded onto the inner surface of the capillary, a dual pH/temperature stimuli-responsive block copolymer, P(SMN-COOH), was prepared by in situ polymerizing poly(N-isopropylacrylamide) carboxylic acid terminated [P(N-COOH)] in PSM. An OT-CEC protocol was first explored using the coated capillary for epimedins separation. As a proof of concept, the developed OT-CEC system facilitated hydrogen bonding and tuning the hydrophilic/hydrophobic interactions between the test analytes and the P(SMN-COOH) coating by varying buffer pH and environmental temperature. Four epimedins with similar chemical structures were baseline separated under 40 °C at pH 10.0, exhibiting dramatical improvement in separation efficiency in comparison to its performance under 25 °C at pH 4.0. In addition, the coated capillary showed good repeatability and reusability with relative standard deviations for migration time and peak area between 0.7 and 1.7% and between 2.9 and 4.6%, respectively, and no significant changes after six runs. This work introduces a paradigm for efficient OT-CEC separation of herbal medicines through adjusting the interactions between analytes and smart polymer coatings, addressing polymer coating design and OT-CEC challenges.

An Evolutionary Review of Hemoperfusion Adsorbents: Materials, Preparation, Functionalization, and Outlook

Accumulation of pathogenic factors in the blood may cause irreversible damage and may even be life-threatening. Hemoperfusion is an effective technique for eliminating pathogenic factors, which is widely used in the treatment of various diseases including liver failure, renal failure, sepsis, and others. Hemoperfusion adsorbents are crucial in this process as they specifically bind and remove the target pathogenic factors. This review describes the development of hemoperfusion adsorbents, detailing the different properties exhibited by inorganic materials, organic polymers, and new materials. Advances in natural and synthetic polymers and novel materials manufacturing techniques have driven the expansion of hemoperfusion adsorbents in clinical applications. Stimuli-responsive (smart responsive) adsorbents with controllable molecular binding properties have many promising and environmentally friendly biomedical applications. Knowledge gaps, future research directions, and prospects for hemoperfusion adsorbents are discussed.

4D-Printed Elution-Peak-Guided Dual-Responsive Monolithic Packing for the Solid-Phase Extraction of Metal Ions

Four-dimensional printing (4DP) technologies are revolutionizing the fabrication of stimuli-responsive devices. To advance the analytical performance of conventional solid-phase extraction (SPE) devices using 4DP technology, in this study, we employed N-isopropylacrylamide (NIPAM)-incorporated photocurable resins and digital light processing three-dimensional printing to fabricate an SPE column with a [H+]/temperature dual-responsive monolithic packing stacked as interlacing cuboids to extract Mn, Co, Ni, Cu, Zn, Cd, and Pb ions. When these metal ions were eluted using 0.5% HNO3 solution as the eluent at a temperature below the lower critical solution temperature of polyNIPAM, the monolithic packing swelled owing to its hydrophilic/hydrophobic transition and electrostatic repulsion among the protonated units of polyNIPAM. These effects resulted in smaller interstitial volumes among these interlacing cuboids and improvements in the elution peak profiles of the metal ions, which, in turn, demonstrated the reduced method detection limits (MDLs; range, 0.2-7.2 ng L-1) during analysis using inductively coupled plasma mass spectrometry. We studied the effects of optimizing the elution peak profiles of the metal ions on the analytical performance of this method and validated its reliability and applicability by analyzing the metal ions in reference materials (CASS-4, SLRS-5, 1643f, and Seronorm Trace Elements Urine L-2) and performing spike analyses of seawater, groundwater, river water, and human urine samples. Our results suggest that this 4D-printed elution-peak-guided dual-responsive monolithic packing enables lower MDLs when packed in an SPE column to facilitate the analyses of the metal ions in complex real samples.

Open tubular capillary electrochromatography with dual-responsive polymer as coating for separation of chromones

Fabricating polymeric coatings that are responsive to multiple/dual stimuli is crucial and remains a major challenge in the development of highly efficient open tubular capillary electrochromatography (OT-CEC). In this study, a pH and temperature-responsive block copolymer, poly(styrene-maleic anhydride 2-dimethylamino ethyl methacrylate), P(St-MAn-DMAEMA), was designed and synthesized. Using P(St-MAn-DMAEMA) as the coating, an OT-CEC protocol was constructed for the analysis of chromones. The morphology and hydrophobicity-hydrophilicity of the polymeric coating could change via varying the environmental conditions, affecting the separation efficiency of OT-CEC. Interestingly, the best performance of OT-CEC was achieved at pH 9.7 and 45 °C via tuning the interactions between the coating and the analytes. Additionally, the proposed OT-CEC method exhibited a good linear range for the detection of the three test chromones from 10.0 to 100.0 μM, with all correlation coefficients (R2) >0.997. The coatings also had good stability and reusability. This work provides an approach for the preparation of new multiple-stimuli-responsive polymeric coatings for the establishment of OT-CEC systems.

A comprehensive review of intelligent controlled release antimicrobial packaging in food preservation

Intelligent responsive packaging provides informative feedback or control the release of active substances like antimicrobial agents in response to stimuli in food or the environment to ensure food safety. This paper provides an overview of two types of intelligent packaging, information-responsive and intelligent controlled-release, focusing on the recent research progress of intelligent controlled-release antimicrobial packaging with enzyme, pH, relative humidity, temperature, and light as triggering factors. It also summarizes the current status of application in different food categories, as well as the challenges and future prospects. Intelligent controlled-release technology aims to optimize the antimicrobial effect and ensure the quality of food products by synchronizing the release of active substances with food preservation needs through sensing stimuli, which is an innovative and challenging packaging technology. The paper seeks to provide a reference for the research and industrial development of responsive intelligent packaging and controlled-release packaging applications in food.

Controlling the mechanical properties of hydrogels via modulating the side-chain length

Even though the toughness of hydrogels is usually adjusted by changing the cross-linking density and structure, or the polymer concentration, we have discovered a new strategy to control the toughness via modulating the side-chain length. In this study, this strategy was applied to biocompatible poly(oligo(ethylene glycol) methyl ether methacrylate) with long ethylene-oxide side chains.

On-Demand, Reversible, Ultrasensitive Polymer Membrane Based on Molecular Imprinting Polymer

The development of in vivo, longitudinal, real-time monitoring devices is an essential step toward continuous, precision health monitoring. Molecularly imprinted polymers (MIPs) are popular sensor capture agents that are more robust than antibodies and have been used for sensors, drug delivery, affinity separations, assays, and solid-phase extraction. However, MIP sensors are typically limited to one-time use due to their high binding affinity (>10⁷ M^-1) and slow-release kinetics (<10^-4 μM/sec). To overcome this challenge, current research has focused on stimuli-responsive MIPs (SR-MIPs), which undergo a conformational change induced by external stimuli to reverse molecular binding, requiring additional chemicals or outside stimuli. Here, we demonstrate fully reversible MIP sensors based on electrostatic repulsion. Once the target analyte is bound within a thin film MIP on an electrode, a small electrical potential successfully releases the bound molecules, enabling repeated, accurate measurements. We demonstrate an electrostatically refreshed dopamine sensor with a 760 pM limit of detection, linear response profile, and accuracy even after 30 sensing-release cycles. These sensors could repeatedly detect <1 nM dopamine released from PC-12 cells in vitro, demonstrating they can longitudinally measure low concentrations in complex biological environments without clogging. Our work provides a simple and effective strategy for enhancing the use of MIPs-based biosensors for all charged molecules in continuous, real-time health monitoring and other sensing applications.

DNA hydrogels as selective biomaterials for specifically capturing DNA, protein and bacteria

The ability to selectively capture biomacromolecules and other components from solution has many important applications in biotechnology. However, capturing targets from solution while minimizing interference with the sample solution is still challenging. Here, we describe the design and assembly of a group of DNA hydrogels consisting of long single-stranded DNA produced by rolling amplification reaction (RCA) and crosslinked by DNA duplexes. The developed DNA hydrogels can selectively capture and separate oligonucleotides, proteins and bacteria from solution in situ without complex separation processes. Since such DNA hydrogels can capture their targets in the solution independently, multiple DNA hydrogels that target different compounds can be employed to separate different compounds in the solution at the same time. The work not only expands the application of DNA hydrogels, but also paves the way for developing novel selective biomaterials. STATEMENT OF SIGNIFICANCE: Biomaterials capable of selectively capturing various components have great potential in the field of biotechnology. Here, we proposed a new class of hydrogel composed of crosslinked long DNA strands for selectively capturing DNA, protein and bacteria. Unlike traditional polymeric hydrogels that have small meshes and limit macromolecule diffusion owing to the short distance between two adjacent crosslinks, the described DNA hydrogel has a much larger distance between its crosslinks because of the sequence designability of DNA, which allows easy diffusion of biomacromolecules through its networks and greatly expand its specific surface area. Moreover, the developed DNA hydrogel can also easily combine different aptamers to target different components via the Watson-Crick base pairing without making significant changes in its original design.

Thermoresponsive interfaces obtained using poly(N-isopropylacrylamide)-based copolymer for bioseparation and tissue engineering applications

Poly(N-isopropylacrylamide) (PNIPAAm) is the most well-known and widely used stimuli-responsive polymer in the biomedical field owing to its ability to undergo temperature-dependent hydration and dehydration with temperature variations, causing hydrophilic and hydrophobic alterations. This temperature-dependent property of PNIPAAm provides functionality to interfaces containing PNIPAAm. Notably, the hydrophilic and hydrophobic alterations caused by the change in the temperature-responsive property of PNIPAAm-modified interfaces induce temperature-modulated interactions with biomolecules, proteins, and cells. This intrinsic property of PNIPAAm can be effectively used in various biomedical applications, particularly in bioseparation and tissue engineering applications, owing to the functionality of PNIPAAm-modified interfaces based on the temperature modulation of the interaction between PNIPAAm-modified interfaces and biomolecules and cells. This review focuses on PNIPAAm-modified interfaces in terms of preparation method, properties, and their applications. Advances in PNIPAAm-modified interfaces for existing and developing applications are also summarized.

Cationic Covalent Organic Framework as an Ion Exchange Material for Efficient Adsorptive Separation of Biomolecules

Although covalent organic frameworks (COFs) have earned significant interest in separation applications, the use of COFs in biomolecule separation remains unexplored. We examined the ionic COF Py-BPy²⁺-COF as an ion exchange material for biomolecule separation. After characterizing the properties of the synthesized COF with a variety of techniques, binding experiments with both large and small biomolecules were performed. High adsorption capacities of amino acids with different hydrophobicity and charge, as well as proteins of different isoelectric points and molecular weights, were determined in batch equilibrium experiments. Desorption experiments with mixtures of model proteins demonstrated an ability to successfully separate one protein from another with the selectivity hypothesized to be a combination of the isoelectric point, hydrophobicity, and ability to penetrate the crystalline material. Overall, the results demonstrated that Py-BPy²⁺-COF can be exploited as a robust crystalline anion exchange biomolecule separation material.

Informative and corrective responsive packaging: Advances in farm-to-fork monitoring and remediation of food quality and safety

Microbial growth and fluctuations in environmental conditions have been shown to cause microbial contamination and deterioration of food. Thus, it is paramount to develop reliable strategies to effectively prevent the sale and consumption of contaminated or spoiled food. Responsive packaging systems are designed to react to specific stimuli in the food or environment, such as microorganisms or temperature, then implement an informational or corrective response. Informative responsive packaging is aimed at continuously monitoring the changes in food or environmental conditions and conveys this information to the users in real time. Meanwhile, packaging systems with the capacity to control contamination or deterioration are also of great interest. Encouragingly, corrective responsive packaging attempting to mitigate the adverse effects of condition fluctuations on food has been investigated. This packaging exerts its effects through the triggered release of active agents by environmental stimuli. In this review, informative and corrective responsive packaging is conceptualized clearly and concisely. The mechanism and characteristics of each type of packaging are discussed in depth. This review also summarized the latest research progress of responsive packaging and objectively appraised their advantages. Evidently, the mechanism through which packaging systems respond to microbial contamination and associated environmental factors was also highlighted. Moreover, risk concerns, related legislation, and consumer perspective in the application of responsive packaging are discussed as well. Broadly, this comprehensive review covering the latest information on responsive packaging aims to provide a timely reference for scientific research and offer guidance for presenting their applications in food industry.

Isothermal modelling of protein adsorption to thermo-responsive polymer grafted Sepharose Fast Flow sorbents

In this study, five adsorption isotherm models, that is, the Langmuir, Freundlich, Langmuir-Freundlich, Temkin and Brunauer-Emmett-Teller isotherms, were utilized for the analysis of the experimental adsorption data for six classes of poly(N-isopropylacrylamide)-based thermo-responsive copolymer-grafted Sepharose Fast Flow sorbents of different copolymer compositions with two structurally related proteins, namely bovine holo-lactoferrin and bovine holo-transferrin at 20 and 50°C. The experimental data for bovine holo-lactoferrin could be mathematically fitted to the Freundlich and Temkin isotherms when the protein feed concentrations were in the range of 1-40 mg/mL at both 20 and 50°C. Similar analysis of the binding of the homologous protein, bovine holo-transferrin, to the same thermo-responsive copolymer-grafted sorbents revealed that the experimental data could be fitted to the Langmuir, Freundlich and Temkin isotherms with coefficients of determination value over 0.90.

Adsorption of a Humanized Monoclonal Antibody onto Thermoresponsive Copolymer-Grafted Sepharose Fast Flow Sorbents

The batch adsorption behavior of a humanized monoclonal antibody (hIgG2 mAb) with thermoresponsive polymer (TRP)-modified Sepharose Fast Flow sorbents with different compositions of grafted copolymers is described. At high protein loadings, the adsorption with negatively charged copolymer-modified sorbents exhibited S-shaped isotherms in most cases, indicative of unrestricted multilayer adsorption. The adsorption capacity of the negatively charged copolymer-modified sorbents increased with an increase in the applied environmental temperature due to increased protein-sorbent surface hydrophobic and electrostatic interactions. The affinity of the hIgG2 mAb for a positively charged copolymer-grafted sorbent was much lower than that found for the negatively charged copolymer-grafted sorbents at both 20 and 50 °C due to electrostatic repulsive effects. This study has documented that the molecular functionalities of the grafted copolymer can significantly affect the adsorption behavior of this humanized mAb at both 20 and 50 °C with the isothermal dependencies revealing subtle effects due to copolymer composition.

Thermoresponsive Poly(vinyl methyl ether) (PVME) Retained by 3-Aminopropyltriethoxysilane (APTES) Network

Thermoresponsive polymers (TRP)s have been widely used for various applications from controlling membrane fouling in separation to cell/cell sheet harvesting in regenerative medicine. While poly(N-isopropylacrylamide) (pNIPAAm) is the most commonly used TRP, less expensive and easily processed poly(vinyl methyl ether) (PVME) also shows a hydrophilic to hydrophobic transition at 32-35 °C, near physiological conditions. In this study, we investigated the processing conditions for retaining a stable layer of PVME thin film on silica surfaces via entrapment in a 3-aminopropyltriethoxysilane (APTES) network. In addition, the thermoresponsive behaviors (TRB) of the retained PVME films were evaluated. Blend thin films of PVME/APTES with 90:10 and 50:50 mass ratios were spin-coated from their solutions in ethanol under ambient conditions and then annealed in a vacuum oven at 40, 60, 80, or 120 °C for 1, 2, or 3 days. The annealed films were then thoroughly rinsed with room temperature water and then soaked in water for 3 days. Our results showed that annealing at a temperature of ≥40 °C was necessary for retaining a PVME film on the surface. The higher annealing temperature led to greater film retention, probably due to the formation of a tighter APTES network. Regardless of processing conditions, all retained PVME films showed TRB, determined by water contact angles below and above the transition temperature of PVME. Additionally, particle attachment and protein adsorption on retained PVME films showed lower attachment or adsorption at room temperature as compared to that at 37 °C, and a greater difference was observed for the 90:10 blend where more PVME was consisted. Furthermore, human mesenchymal stem cells attached and proliferated on the retained PVME surfaces at 37 °C and rapidly detached at room temperature. These results illustrated the potential applications of PVME surfaces as thermoresponsive supports for low-fouling applications and noninvasive cell harvesting.

Application of stimuli-responsive materials for extraction purposes

Stimuli-responsive materials, frequently designated as "smart/intelligent materials", can modify their structure or properties by either a biological, physical, or chemical stimulus which, if properly controlled, could be used for specific applications. Such materials have been studied and exploited in several fields, like electronics, photonics, controlled drugs administration, imaging and medical diagnosis, among others, as well as in Analytical Chemistry where they have been used as chromatographic stationary phases, as part of sensors and for extraction purposes. This review article pretends to provide an overview of the most recent applications of these materials (mostly polymeric materials) in sample preparation for extraction purposes, as well as to provide a general vision of the current state-of-the-art of this field, their potential use and future applications.

Selective Targeted Drug Delivery Mechanism via Molecular Imprinted Polymers in Cancer Therapeutics

Artificial receptor-like structures such as molecular imprinted polymers (MIPs) are biomimetic molecules are used to replicate target specific antibody-antigen mechanism. In MIPs, selective binding of template molecule can be significantly correlated with lock and key mechanism, which play a major role in the drug delivery mechanism. The MIPs are biocompatible with high efficiency and are considered in several drug delivery and biosensor applications besides continuous and controlled drug release leading to better therapeutics. There is a need to explore the potential synthetic methods to improve MIPs with respect to the imprinting capacity in cancer therapeutics. In this review, we focus on MIPs as drug delivery mechanism in cancer and the challenges related to their synthesis and applications.

Imaging Switchable Protein Interactions with an Active Porous Polymer Support

Mechanistic details about how local physicochemistry of porous interfaces drives protein transport mechanisms are necessary to optimize biomaterial applications. Cross-linked hydrogels made of stimuli-responsive polymers have potential for active protein capture and release through tunable steric and chemical transformations. Simultaneous monitoring of dynamic changes in both protein transport and interfacial polymer structure is an experimental challenge. We use single-particle tracking (SPT) and fluorescence correlation spectroscopy Super-resolution Optical Fluctuation Imaging (fcsSOFI) to relate the switchable changes in size and structure of a pH-responsive hydrogel to the interfacial transport properties of a model protein, lysozyme. SPT analysis reveals the reversible switching of protein transport dynamics in and at the hydrogel polymer in response to pH changes. fcsSOFI allows us to relate tunable heterogeneity of the hydrogels and pores to reversible changes in the distribution of confined diffusion and adsorption/desorption. We find that physicochemical heterogeneity of the hydrogels dictates protein confinement and desorption dynamics, particularly at pH conditions in which the hydrogels are swollen.

Nanoparticle-supported temperature responsive polymer brushes for affinity separation of histidine-tagged recombinant proteins

We developed a modular approach for the preparation of nanoparticle-supported polymer brushes carrying repeating iminodiacetate units for affinity separation of histidine-tagged recombinant proteins. The nanoparticle-supported polymer brushes were prepared via the combination of surface-initiated atom transfer radical polymerization with Cu(I)-catalyzed azide-alkyne cycloaddition reaction. The nanocomposite materials were characterized to determine the particle size, morphology, organic content, densities of polymer chains and the affinity ligand. Protein binding assay illustrated that the iminodiacetate-rich polymer brushes enable to selectively bind histidine-tagged recombinant proteins in the presence of abundant interfering proteins. More importantly, the protein binding capacity can be tuned by adjusting the environmental temperature. STATEMENT OF SIGNIFICANCE: The nanoparticle core-polymer brush structure enables selective binding of histidine-tagged recombinant proteins via multiple metal-coordination interactions. The soft and flexible structure of the polymer brushes was found beneficial for lowering the steric hindrance in protein binding. Taking advantage of the conformational changes of the polymer brushes at different temperatures, it is possible to modulate the protein binding on the nanocomposite by adjusting the environmental temperature. In general, the iminodiacetate-rich core-brush nano adsorbents are attractive for purifying histidine-tagged recombinant proteins practically. The synthetic approach reported here may be expanded to develop other advanced functional materials for applications in various biomedical fields ranging from biosensors to drug delivery.

A Temperature-Responsive Boronate Core Cross-Linked Star (CCS) Polymer for Fast and Highly Efficient Enrichment of Glycoproteins

Fast and highly efficient enrichment and separation of glycoproteins is essential in many biological applications, but the lack of materials with high capture capacity, fast, and efficient enrichment/separation makes it a challenge. Here, a temperature-responsive core cross-linked star (CCS) polymer with boronate affinity is reported for fast and efficient enriching and separating of glycoproteins from biological samples. The temperature-responsive CCS polymers containing boronic acid in its polymeric arms and poly(N-isopropyl acrylamide) in its cross-linked core are prepared using reversible addition-fragmentation chain transfer polymerization via an "arm-first" methodology. The soluble boronate polymeric arms of the CCS polymers provide a homogeneous reaction system and facilitate interactions between boronic acid and glycoproteins, which leads to a fast binding/desorption speed and high capture capacity. Maximum binding capacity of the prepared CCS polymer for horseradish peroxidase is determined to be 210 mg g^-1 , which can be achieved within 20 min. More interestingly, the temperature-responsive CCS polymers exhibit rapid reversible thermal-induced volume phase transition by increasing the temperature from 15 to 30 °C, resulting in a facile and convenient sample collection and recovery for the target glycoproteins. Finally, the temperature-responsive CCS polymer is successfully applied to enrichment of low abundant glycoproteins.

Mixed Polymer Brushes for the Selective Capture and Release of Proteins

Selective protein adsorption is a key challenge for the development of biosensors, separation technologies, and smart materials for medicine and biotechnologies. In this work, a strategy was developed for selective protein adsorption, based on the use of mixed polymer brushes composed of poly(ethylene oxide) (PEO), a protein-repellent polymer, and poly(acrylic acid) (PAA), a weak polyacid whose conformation changes according to the pH and ionic strength of the surrounding medium. A mixture of lysozyme (Lyz), human serum albumin (HSA), and human fibrinogen (Fb) was used to demonstrate the success of this strategy. Polymer brush formation and protein adsorption were monitored by quartz crystal microbalance, whereas protein identification after adsorption from the mixture was performed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) with principal component analysis and gel electrophoresis with silver staining. For the ToF-SIMS measurements, adsorption was first performed from single-protein solutions in order to identify characteristic peaks of each protein. Next, adsorption was performed from the mixture of the three proteins. Proteins were also desorbed from the brushes and analyzed by gel electrophoresis with silver staining for further identification. Selective adsorption of Lyz from a mixture of Lyz/HSA/Fb was successfully achieved at pH 9.0 and ionic strength of 10^-3 M, while Lyz and HSA, but not Fb, were adsorbed at ionic strength 10^-2 M and pH 9.0. The results demonstrate that by controlling the ionic strength, selective adsorption can be achieved from protein mixtures on PEO/PAA mixed brushes, predominantly because of the resulting control on electrostatic interactions. In well-chosen conditions, the selectively adsorbed proteins can also be fully recovered from the brushes by a simple ionic strength stimulus. The developed systems will find applications as responsive biointerfaces in the fields of separation technologies, biosensing, drug delivery, and nanomedicine.