Research Value of Polyelectrolyte Multilayers: Layer-by-Layer Design of Interfacial Composition, Mass Transport, and Function

Introduction

Polyelectrolyte multilayers (PEMs) are often classified as a “surface coating” technology, but a more accurate description is that, through layer-by-layer (LbL) assembly, they turn interfacial composition, interlayer interpenetration, wet-state swelling, fixed charge, molecular transport pathways, and the degree of pore coverage into simultaneously tunable parameters. From a research perspective, the value of PEMs lies in the fact that they provide a framework for interfacial design in which structures can be built layer by layer, adjusted layer by layer, and then interpreted in direct connection with mass transport and functional output. This article mainly discusses polyelectrolyte multilayers formed by the alternating assembly of polycations and polyanions. Other LbL building blocks, such as proteins and nanoparticles, are mentioned only briefly to illustrate the broader scope of the LbL method and are not the focus of this article.

 

 

 

1. The research value of polyelectrolyte multilayers lies first in the layer-by-layer design of interfaces and mass-transport environments

 

The basic construction strategy of polyelectrolyte multilayers (PEMs) is to alternately adsorb polycations and polyanions onto a substrate surface, thereby building nanoscale film layers step by step. The significance of this process lies not merely in forming a surface covering, but in enabling researchers to continuously regulate film composition, thickness, interlayer interpenetration, hydration state, and surface properties through the choice of polyelectrolytes, assembly sequence, deposition conditions, and post-treatment methods. For separation membranes, catalytic interfaces, biointerfaces, and delivery systems, this layer-by-layer tunability makes PEMs more than a surface modification approach; it turns them into an experimental platform for organizing interfacial functions.

 

Another important feature of PEMs is their strong adaptability to substrate morphology. They can be applied not only to planar substrates, but also to complex objects such as particles, fibers, and porous supports. Accordingly, PEM research is usually concerned not simply with whether a film can be formed, but with whether the film can maintain controllable structure and function on different substrates. For this reason, the value of PEMs is typically reflected at three interrelated levels: whether interfacial composition can be designed layer by layer, whether the wet-state structure can be tuned with conditions, and whether molecular transport and functional output can then be controlled in a systematic way.

 

2. What layer-by-layer assembly truly changes is the mode of film growth and the internal structure

 

The formation of polyelectrolyte multilayers is a continuous construction process accompanied by surface charge reversal, interlayer interpenetration, and intrafilm rearrangement. Existing studies have shown that PEM thickness growth may be approximately linear, or it may exhibit exponential growth. Exponential growth is often associated with high mobility of at least one of the polyelectrolytes within the film, allowing it to continue entering, diffusing through, and rearranging the multilayer during subsequent deposition steps. This phenomenon deserves particular attention in some weak polyelectrolyte systems, but it should not be taken as a universal conclusion for all weak polyelectrolyte systems.

 

The number of layers alone cannot adequately represent the internal structural state of a PEM. Even when the number of bilayers is the same, different systems may differ substantially in the extent of interlayer interpenetration, segmental mobility, water content, and capacity for post-assembly rearrangement, ultimately producing completely different wet-state structures and mass-transport environments. Experimentally, the more important question is not “how many layers were deposited,” but rather “how these layers are distributed within the film, whether they are significantly interpenetrated, and whether they continue to rearrange after assembly.”

 

2.1 Key control variables affecting PEM internal structure and mass-transport behavior

 

Design variable	What it mainly changes	Common outcomes	Experimental question it is best suited to answer
Polyelectrolyte type and charge characteristics	Ion-pairing mode, fixed-charge environment, and strength of interlayer binding	Affects film compactness, stability, and separation behavior	Is stronger barrier performance needed, or is higher permeability more important?
Assembly pH	Degree of ionization of weak polyelectrolytes, chain conformation, and interlayer interpenetration	Affects thickness, swelling, surface charge, and responsiveness	Is the goal to make a pH-responsive film, or a film that remains stable under normal conditions?
Ionic strength	Charge screening, chain extension or coiling, and intrafilm rearrangement	Affects thickness, roughness, water content, and diffusion pathways	Is a looser mass-transport environment needed, or a denser separation layer?
Deposition method and flow conditions	Mass-transfer efficiency during deposition, uniformity, and the extent of assembly inside pores	Affects film formation quality and structural consistency on complex substrates	Is the target a planar thin film, or assembly within pores?
Post-treatment (crosslinking, chemical transformation, etc.)	Wet-state stability, mechanical properties, and distribution of active sites	Can further convert the assembled film into a functionally stable long-term layer	Is the goal long-term use, or stimulus responsiveness?

 

These variables are not independent of one another. The type of polyelectrolytes, pH, ionic strength, deposition process, and post-treatment ultimately determine the internal structure of PEMs through their combined effects on chain conformation, interlayer interpenetration, and wet-state swelling, and they further influence intrafilm diffusion and permeation behavior.

 

3. How wet-state structure, swelling, and fixed charge determine selective permeability and mass-transport performance

 

For PEMs, the factors that truly deserve priority attention are wet-state structure, degree of swelling, fixed-charge distribution, and the state of coverage over support pores. Together, these factors influence water permeation, size exclusion, ion rejection, and final selectivity. In 2024, Regenspurg and co-workers compared nine common PEM membrane systems under unified conditions and found that support pores can only be effectively closed when the PEM thickness and the support pore size are of the same order of magnitude. Among the comparable parameters, swelling emerged as a key integrative parameter that is relatively straightforward to characterize, and it showed correlations with water permeation, size exclusion, dielectric exclusion, and Donnan exclusion. At the same time, however, the study also pointed out that the relationship between membrane performance and polyelectrolyte properties cannot be explained by swelling alone.

 

The focus of PEM performance analysis should therefore be placed on how wet-state structure affects diffusion, permeation, and selectivity, and how assembly conditions are translated through wet-state structure into final performance.

 

3.1 Key membrane parameters that should be prioritized for different research objectives

 

Research objective	Key membrane parameters to prioritize	Core question these parameters correspond to
Dense barrier layers or highly selective permeation membranes	Swelling, fixed charge, degree of crosslinking, and matching with support pore size	Can an effective separation layer truly be formed?
pH-responsive release films or environmentally responsive films	Degree of ionization, water content, post-assembly rearrangement, and structural changes before and after post-treatment	What mainly controls the responsive behavior?
Pore-confined catalytic membranes	Site density within pores, accessibility of reactants, and residence time	Can mass transport effectively support the catalytic process?
Enzyme-immobilized membrane reactors	Retention of enzyme activity, local charge environment, and stability upon repeated use	After immobilization, can activity and stability both be maintained?
Biointerfaces or tissue-engineering coatings	Component sequence, preservation of biomolecular activity, and the mechanical and surface properties of the film	Can a suitable interfacial microenvironment be created?

 

3.2 Key characterization metrics corresponding to different performance questions

 

Core question to determine	Recommended metrics to focus on	Questions that still cannot be answered by reporting only the number of layers or dry-film thickness
Has an effective separation layer truly formed?	Wet-state thickness or swelling, support pore size, water permeability or permeation flux, rejection rate or molecular weight cut-off (MWCO)	Under operating conditions, are the pores effectively covered, and has the membrane truly formed a continuous separation layer?
Does selectivity mainly come from size or charge?	Salt rejection behavior, surface charge or fixed-charge characteristics within the membrane, response to pH and ionic strength	Does selectivity arise from size exclusion, dielectric exclusion, or Donnan exclusion?
Is pore-confined catalysis truly effective?	Conversion, linear velocity or residence time, site loading, and stability upon repeated use	Is intrapore mass transport properly matched to active-site accessibility?
Are biomolecules truly preserved in an active state?	Retained activity, changes in activity/structure before and after immobilization or crosslinking, and stability upon repeated use or release behavior	Is the function of the biomolecule truly retained and able to operate sustainably?

 

4. Pore Functionalization Best Demonstrates the Ability of PEMs to Organize Mass Transport and Reaction Interfaces

 

When polyelectrolyte multilayer (PEM) assembly extends into the interior of porous supports, the object of control expands from surface film formation to intrapore interfaces. At this point, the polyelectrolyte layers not only determine the chemical composition of the pore-wall surface, but also further influence intrapore mass-transport pathways, residence time, and the accessibility of functional sites. For PEMs, the core value of pore functionalization lies in integrating interfacial composition, intrapore diffusion, and reaction processes into a single tunable system.

 

4.1 Catalytic membranes in pores: integrating flow, mass transport, and catalytic sites into one system

In 2006, Dotzauer and co-workers reported that catalytic membranes could be created by assembling polyelectrolyte/metal nanoparticle films inside porous supports. Under a linear velocity of 0.98 cm/s, the modified alumina membrane achieved more than 99% reduction conversion of 0.4 mM 4-nitrophenol.

 

This result shows that the role of intrapore PEMs is not merely to immobilize nanoparticles, but also to simultaneously organize intrapore flow, substrate transport, and site utilization, thereby directly coupling the reaction process with mass transport inside the membrane.

 

4.2 Enzyme-immobilized membranes: combining mild immobilization with sustained reaction

In 2008, Datta and co-workers reported that by constructing polyelectrolyte functional domains within membranes and electrostatically immobilizing glucose oxidase, it was possible to establish an enzyme-catalytic membrane system with activity, stability, and reusability; the study also discussed the effects of residence time and pH on enzyme activity.

 

There are three main questions in such systems: whether enzyme activity can be retained, whether substrates can effectively reach the intrapore sites, and whether the intrapore environment can support sustained reaction. What PEMs provide here is a construction strategy that simultaneously addresses immobilization, mass transport, and regulation of the local microenvironment.

 

4.3 What pore functionalization reveals is the methodological role of PEMs

Pore functionalization is important because it advances the role of PEMs from constructing a film to constructing a tunable mass-transport–reaction interface within pores. For catalytic membranes, enzyme-immobilized membranes, and continuous-flow membrane reactors, the more critical issue is usually not whether a film has been formed, but whether the intrapore interfacial composition, mass-transport pathways, residence time, and accessibility of functional sites can be controlled in a coordinated manner.

 

5. Continuing PEM Research Today Means Focusing on the Organization of Complex Functions within Controllable Interfaces

 

Research on polyelectrolyte multilayers is no longer confined to film formation on two-dimensional surfaces. Recent reviews indicate that layer-by-layer (LbL) systems have expanded to both two-dimensional and three-dimensional objects, and continue to enter areas such as drug delivery, tissue engineering, vaccine-related systems, and biomimetic tissue models.

 

For PEMs, what matters is that their core mode of action across different research directions has not changed: by regulating interfacial composition, wet-state structure, molecular transport, and the distribution of functional sites layer by layer, they integrate complex functions into interfacial systems that can themselves be designed layer by layer. This is also the key point that distinguishes PEMs from one-time surface coverage or single-step functionalization methods.

 

6. Which Research Tasks Merit Prioritizing PEMs

 

Research task	Whether PEMs are worth prioritizing	Key basis for judgment
Constructing nanoscale, controllable functional interfaces on complex substrates	Worth prioritizing	Simultaneous adjustment of interfacial composition, thickness, and surface properties is required
Simultaneously tuning barrier properties, flux, and charge environment	Worth prioritizing	Structural parameters and mass-transport parameters need to be designed in a coupled manner
Achieving catalysis, enzyme immobilization, or continuous-flow reactions within pores	Worth prioritizing	Site construction, intrapore mass transport, and residence time need to be organized within one system
Loading or immobilizing bioactive molecules under mild conditions	Worth prioritizing	Both immobilization capability and preservation of bioactivity must be taken into account
Only needing to quickly form an inert covering layer	Usually not a priority	There is little need for layer-by-layer control or wet-state structural regulation
Placing greater emphasis on ultra-fast, large-area, low-step manufacturing	Not necessarily a priority	Spray coating, continuous assembly, or other more direct methods should also be compared

 

7. Product Navigation Table for Polyelectrolyte Multilayer Research (Choose by Table 1–Table 4)

 

Research or experimental goal	Recommended table to consult first	Why this table should be consulted first	Suggested related table(s) to consult	Navigation note
Want to establish a basic PEM film-construction route first and determine which class of classical polyelectrolyte systems to start with	Table 1	Table 1 focuses on classical synthetic polyelectrolytes such as PAA, PAH, PEI, PSS, PMAA, and PDADMAC, making it the most suitable starting point for deciding whether to begin with strong/weak polyelectrolyte systems, dense-film systems, or responsive systems	Then consult Table 3	First determine “which pair of polyelectrolytes will serve as the main system,” then use Table 3 to supplement pH, ionic strength, and crosslinking/coupling conditions; this makes it easier to establish a stable basic film-construction route
Want to prepare biocompatible multilayers, or introduce natural polysaccharides, polypeptides, or anticoagulant components into the interface	Table 2	Table 2 focuses on chitosan, alginate, hyaluronic acid, heparin-related components, and polypeptide-based components, making it most suitable for designing cell-compatible surfaces, mild assembly systems, and biofunctional interfaces	Then consult Table 3	First determine the natural/biologically derived film-forming components, then supplement with crosslinking and coupling conditions; this is more suitable for improving film stability under aqueous and biological conditions
Want to optimize multilayer thickness, swelling, flux, rejection, or surface charge, and determine which condition variables should be adjusted first	Table 3	Table 3 focuses on acids, bases, salts, buffers, and crosslinking/coupling components, and therefore corresponds directly to the most critical control variables in PEMs: pH, ionic strength, and post-treatment	Then consult Table 1	First clarify the logic of condition control, then return to Table 1 to select a more suitable polyelectrolyte combination; this is more suitable for systematic screening of film thickness, compactness, and selective permeability
Want to work on separation membranes, composite membranes, or nanofiltration/ultrafiltration, and first determine how the support layer and separation layer should be matched	Table 4	Table 4 focuses on support materials such as cellulose acetate, PVDF, polysulfone, and PAN, making it the most suitable starting point for establishing the material framework of a “support membrane–multilayer separation layer” system	Then consult Tables 1 and 3	First select a reliable support, then use Table 1 to choose film-forming polyelectrolytes and Table 3 to tune compactness and flux; this better reflects the practical sequence of composite membrane development
Want to construct catalytic membranes, antibacterial membranes, or metal nanoparticle functional layers on porous substrates or within pores	Table 4	Table 4 focuses on alumina spheres, Pd, Ag, and Au precursors, as well as NaBH4, and also includes common membrane support materials, making it most suitable for first establishing routes for pore functionalization and in situ metallization	Then consult Table 1	First determine the functionalized substrate and metal system, then return to Table 1 to choose polyelectrolyte systems better suited for supporting metals or immobilizing functional layers
Want to create enzyme-immobilized membranes, membrane reactors, or online protein digestion interfaces	Table 2	Table 2 includes recombinant trypsin as well as mild-assembly polysaccharide/polypeptide components, making it most suitable for first determining the type of biocompatible interface in which the enzyme should be placed	Then consult Tables 3 and 4	First choose film-forming components suitable for preserving enzyme activity, then supplement with coupling/crosslinking conditions and support substrates; this is more suitable for developing reusable membrane reactors
Want to create antibacterial, antifouling, or blood-contacting interfaces, and determine which class of functional layers should be prioritized	Tables 2 and 4	Table 2 is more suitable for anticoagulant, hydrophilic, and biocompatible interfaces; Table 4 is more suitable for Ag, Au, and other functional particles or for surface functionalization on porous supports	Then consult Table 3	First determine whether the functional direction is more oriented toward bioactive interfaces or inorganic functional layers, then use Table 3 to supplement stabilization and surface immobilization conditions; this is more convenient for making the functional layer truly effective
The research question is still unclear, and only a comparable, extensible basic PEM system is needed as a starting point	Tables 1 and 3	Table 1 provides the “main film-forming system,” while Table 3 provides the “control variables”; together they are best suited for establishing a general system that can later be extended to separation, biointerfaces, or pore functionalization	Then consult Tables 2 and 4	First establish the basic system, then branch according to subsequent tasks into natural biological systems or porous functionalization systems; this is more suitable for systematic methodological screening

 

Table 1 | Core Synthetic Polyelectrolyte Film-Forming Components

(Commonly used in classical PEM/LbL film construction, regulation of weak/strong polyelectrolyte films, selective permeation layers, and nanofiltration/ion-exchange membrane systems)

 

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Weak anionic synthetic polyelectrolyte	9003-01-4	P661414	Poly(acrylic acid) (PAA)	Viscosity ≤2000 cP (25°C)	A classical weak polyanionic layer, often alternately assembled with PAH, PEI, and related components to form films with tunable swelling and charge density; suitable for pH-responsive membranes, nanofiltration separation layers, antifouling surfaces, and subsequent carboxyl-coupling platforms.
Weak cationic synthetic polyelectrolyte	71550-12-4	P433385	Poly(allylamine hydrochloride) (PAH)	Average Mw 50,000	A classical weak polycationic layer, commonly deposited alternately with PSS, PAA, and PMAA; suitable for constructing multilayer films with tunable thickness and permeability, and also widely used for ion-selective separation and surface functionalization primer layers.
Branched cationic polyelectrolyte / primer-layer component	9002-98-6	P434400	Polyethylenimine, branched (PEI)	Average Mw ~25,000 (by LS), average Mn ~10,000 (by GPC), branched	A cationic polyelectrolyte with high amine-group density. It can serve as an initiating first layer/primer to enhance substrate surface charge, and can also be combined with PAA to construct crosslinked selective-permeation layers; suitable for ion-exchange membranes, nanofiltration membranes, and surface modification of particles and pores.
Strong anionic synthetic polyelectrolyte	25704-18-1	P432468	Poly(sodium 4-styrenesulfonate) (PSS)	Average Mw ~1,000,000, powder	A classical strong polyanionic layer, often paired with PAH, PEI, and PDADMAC to form stable multilayer films; suitable for studying layer-by-layer assembly kinetics, membrane-thickness growth, ion selectivity, and intrapore immobilization.
Weak anionic synthetic polyelectrolyte	54193-36-1	P580750	Poly(methacrylic acid, sodium salt) solution	Viscosity 5000–8000 mPa·s, 30 wt.% in H2O	A typical alternative weak polyanionic system that can be combined with PAH and related components to build multilayer films with more pronounced pH responsiveness; suitable for tunable-swelling membranes, stimulus-responsive release layers, and comparative systems for membrane flux/rejection performance.
Strong cationic synthetic polyelectrolyte	26062-79-3	P109720	Poly(diallyldimethylammonium chloride) (PDADMAC)	Mw 200,000–350,000, 20 wt.% in water, 250–500 cP (25°C)	A strong cationic polyelectrolyte, commonly combined with PSS to form multilayer films with high charge density and good stability; suitable for ion-exchange layers, antifouling surfaces, dense barrier layers, and film-construction studies under high-salt conditions.
Sulfonated strong synthetic polyanion	27119-07-9	P478257	Poly(2-acrylamido-2-methyl-1-propanesulfonic acid) solution	Average Mw 2,000,000, 15 wt.% in H₂O	A highly hydrophilic sulfonated polyanion that can serve as an alternative anionic film-forming component to PSS in PEM assembly; suitable for alternate deposition with polycations such as PAH and PEI to construct highly hydrated multilayers with pronounced fixed negative charge, and useful for comparative studies of wet-state swelling, ion transport, surface wettability, and selective permeation behavior.

 

Table 2 | Natural/Biologically Derived Polyelectrolytes and Biofunctional Components

(Commonly used in biocompatible PEMs, cell/protein interfaces, anticoagulant surfaces, enzyme immobilization, and mild assembly systems)

 

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Natural cationic polysaccharide polyelectrolyte	9012-76-4	C105802	Chitosan	Medium viscosity, 200–400 mPa·s	A commonly used natural cationic layer, suitable for alternate assembly with anionic components such as alginate, hyaluronic acid, and heparin-related species; can be used for biocompatible coatings, antibacterial surfaces, drug-loading layers, and coating of porous substrates.
Natural anionic polysaccharide polyelectrolyte	9005-38-3	A434499	Alginic acid sodium salt from brown algae	Medium viscosity	A commonly used natural anionic layer that readily forms mild assembly systems with chitosan, PEI, and related components; suitable for constructing hydrophilic film layers, enzyme-immobilization carrier layers, antifouling interfaces, and biomedical coatings.
Natural anionic polysaccharide polyelectrolyte / extracellular matrix-mimicking component	9067-32-7	S774025	Sodium Hyaluronate	European Pharmacopoeia (Ph.Eur.)	Highly hydrophilic and strongly characteristic of extracellular matrix environments, suitable for constructing low-protein-adsorption, biocompatible, or cell-regulatory multilayer films; also suitable for forming soft interfaces with cationic polypeptides/polysaccharides.
Sulfated polysaccharide anionic polyelectrolyte	9011-18-1	D110733	Dextran sulfate sodium salt (DSS)	Molecular weight 500,000, DNase/RNase/protease-free	A highly charged anionic polysaccharide suitable for constructing strongly anionic surfaces, studying protein adsorption/repulsion behavior, and serving as an anionic functional layer in biointerfaces and drug-loading layers.
Cationic polypeptide polyelectrolyte	25988-63-0	P303255	Poly-L-lysine hydrobromide	Molecular weight: 30,000–70,000	A classical cationic polypeptide layer suitable for constructing cell-adhesive coatings, protein/nucleic-acid adsorption layers, and biocompatible multilayer films; also commonly used for pre-modification of glass slides, particles, and membrane surfaces.
Anticoagulant surface functional component / sulfated glycosaminoglycan	9041-08-1	H284091	Heparin sodium	Moligand™, anti-Xa potency 110–210 IU/mg	A highly anionic bioactive component suitable for surface functionalization of blood-contacting interfaces, anticoagulant surfaces, and biomedical PEM top layers; it can also be introduced through post-coupling or layer-by-layer adsorption to impart hydrophilic antifouling properties.
Anionic polypeptide polyelectrolyte	26247-79-0	P477797	Poly-L-glutamic acid sodium salt	Average MW 7500	A degradable anionic polypeptide suitable for constructing multilayer films with cationic polypeptides or polysaccharides in environments closer to biological macromolecular systems; commonly used in drug/protein loading and the design of cell-compatible surfaces.
Cationic polypeptide polyelectrolyte	26982-20-7	P489891	Poly(L-arginine hydrochloride)	—	A cationic polypeptide layer suitable for constructing multilayer interfaces with strong cell interaction, and also useful as an adsorption and delivery carrier layer for proteins, nucleic acids, and negatively charged biomolecules.
Functional molecule for enzyme immobilization	9002-07-7	R1505825	Recombinant Trypsin	Animal-free, carrier-free, biologically active, recombinant, PharmPure™, ActiBioPure™, high performance, EnzymoPure™, β-trypsin ≥70%, α-trypsin ≤20%; ≥2500 U/mg enzyme powder; ≥3800 U/mg protein	Suitable for immobilization on multilayer-film surfaces or inside pores to construct online protein-digestion membranes, membrane reactors, and biocatalytic interfaces; its animal-free nature is advantageous for bioanalysis and process control in preparation workflows.

 

Table 3 | Film-Formation Condition Control, Buffers, and Post-Crosslinking/Coupling Components

(Mainly used for controlling the ionization degree of weak polyelectrolytes, regulating ionic strength, carboxyl–amine coupling, and stabilizing multilayer films)

 

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Acidic dissolution / protonation-control component	7647-01-0	H485680	Hydrochloric acid fuming 37%	Guaranteed reagent, suitable for analysis, max. 0.001 ppm Hg	Used to lower the pH of assembly solutions and increase the protonation degree of weak polycations; also suitable for acid-assisted dissolution of amino polysaccharides such as chitosan and for pretreatment prior to multilayer-film assembly.
Weakly acidic medium / chitosan-dissolution component	64-19-7	A116166	Acetic acid	Guaranteed reagent, ≥99.5%	A commonly used dissolution medium for chitosan that can gently adjust the acidity of assembly solutions; suitable for layer-by-layer adsorption in natural polysaccharide systems, construction of flexible thin films, and establishment of biocompatible assembly conditions.
Basic deprotonation / assembly-pH control component	1310-73-2	S111498	Sodium hydroxide	Guaranteed reagent, ≥96%	Used to raise the pH of assembly solutions and regulate the ionization degree of weak polyanions; can also be used in post-treatment to alter multilayer-film charge state, swelling behavior, and surface hydrophilicity/hydrophobicity.
Ionic-strength regulating salt	7647-14-5	S433744	Sodium chloride	Anhydrous, high purity, reagent grade, ≥99%	A classical ionic-strength regulator that can alter polyelectrolyte chain conformation, interlayer interpenetration, and membrane-thickness growth; commonly used to optimize multilayer-film compactness, flux, and rejection performance.
Buffer component / carboxyl-activation reaction medium	4432-31-9	M108951	MES	Molecular biology grade, ≥99.5% (T)	Suitable for aqueous EDC/NHS coupling systems, and also commonly used to maintain mild pH conditions during layer-by-layer assembly and post-modification of biomolecules, thereby reducing fluctuations during film construction and coupling processes.
Natural crosslinker	6902-77-8	G101204	Genipin	Moligand™, ≥98%	Commonly used for mild crosslinking of chitosan, lysine-containing, or other amino-group-containing multilayer films; can significantly improve wet-state stability, swelling resistance, and long-term operational stability.
Carboxyl–amine coupling activator	25952-53-8	E106172	N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride	≥98%	Commonly used for covalent coupling between carboxyl-containing components such as PAA, hyaluronic acid, and heparin and amino-containing layers; can enhance multilayer-film resistance to washing/elution and is also used for immobilizing proteins, polypeptides, and anticoagulant components.
Active ester promoter	6066-82-6	H109330	N-Hydroxysuccinimide (NHS)	≥98%	Used together with EDC to improve carboxyl-activation efficiency and coupling stability; suitable for multilayer-film crosslinking, surface grafting, and subsequent introduction of biomolecules.
Water-soluble active ester promoter	106627-54-7	H109337	N-Hydroxysulfosuccinimide sodium salt	≥98%	Offers better water solubility and is suitable for surface coupling in pure water or buffer systems; commonly used for the mild immobilization of proteins, polypeptides, and polysaccharides on multilayer films.

 

Table 4 | Porous Substrates, Membrane Supports, and Pore Functionalization Components

(Mainly used for separation-membrane support layers, porous particle/porous membrane substrates, in situ generation of metal nanoparticles, and pore-confined catalysis)

 

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Porous inorganic carrier / particulate substrate	1344-28-1	A1507067	Activated alumina spheres	Used as catalyst carrier, particle size 5–7 mm	The porous inorganic surface is suitable for adsorbing polyelectrolyte layers and loading metal-active components, and can be used for particle-catalyst coating, adsorption-interface construction, and stabilization studies of multilayer films on inorganic supports.
Porous membrane support material	9004-35-7	C106244	Cellulose acetate	Acetyl content 39.8 wt%, hydroxyl content 3.5 wt%	A classical hydrophilic membrane material that can serve as a flat-sheet or porous support layer for PEM separation layers; suitable for constructing ultrafiltration/nanofiltration composite membranes and low-pressure separation membranes.
Membrane support material	24937-79-9	P1492342	Poly(vinylidene fluoride) (PVDF)	Melt viscosity (K Poise): 23.5–29.5, powder	Offers good chemical stability and mechanical strength and is commonly used to prepare porous membrane supports; after surface activation it is suitable for supporting PEM separation layers, antifouling layers, and functionalized pore layers.
Membrane support material	25608-63-3	P478438	Poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene)	Melt index 6 g/10 min (380°C/2.16 kg)	Can be used to prepare heat- and chemical-resistant porous support membranes suitable for carrying layer-by-layer-assembled separation layers; commonly used in membrane systems that require both mechanical strength and long-term operational stability.
Membrane support material	25135-51-7	P707031	Polysulfone	Mw ~75,000	A common ultrafiltration/nanofiltration support material suitable for supporting thin PEM layers to form high-flux composite separation membranes; also frequently used in studies of surface modification, antifouling behavior, and ion-selective layers.
Membrane support material	25014-41-9	P303200	Polyacrylonitrile (PAN)	Average Mw 85,000	Commonly used as a substrate for porous support membranes and enzyme-immobilized membrane reactors; its surface is readily further modified, making it suitable for carrying multilayer films and introducing proteases or ion-exchange functional sites.
Metal nanoparticle precursor	7647-10-1	P433731	Palladium(II) chloride	Reagent grade, high purity, ≥99%	Suitable for in situ conversion into Pd nanoparticles within multilayer films or porous substrates, enabling the construction of catalytic membranes, pore reaction interfaces, and recyclable catalytic layers.
Metal nanoparticle precursor	7761-88-8	S433976	Silver nitrate	European Pharmacopoeia (Ph.Eur.), suitable for analysis, ACS, premium grade	Suitable for introducing Ag or AgCl functional phases for antibacterial surfaces, antibiofouling membranes, and antibacterial modification of multilayer-film top layers.
Metal nanoparticle precursor	16961-25-4	G141105	Gold chloride trihydrate	≥99.9% metals basis	A classical Au precursor suitable for in situ formation of gold nanoparticles within multilayer films or pores; commonly used in catalytic membranes, surface-enhanced interfaces, and studies of functionalized porous supports.
Reducing agent for metal precursors	16940-66-2	S432207	Sodium borohydride	purum p.a., ≥96% (gas-volumetric)	Commonly used for in situ reduction of Au, Pd, Ag, and related precursors into nanoparticles within multilayer films or pores; suitable for preparing catalytic membranes, antibacterial layers, and metal–polyelectrolyte composite layers.

 

Note: The above are representative Aladdin products. For more product specifications, please refer to the product list at the end of the article or search the Aladdin website using the “product name/CAS/catalog number.”

 

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[6] Regenspurg JA, Jonkers WA, Junker MA, et al. Polyelectrolyte Multilayer Membranes: An Experimental Review. Desalination. 2024;583:117693. doi:10.1016/j.desal.2024.117693.

 

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[9] Borges J, Zeng J, Liu X, et al. Recent Developments in Layer-by-Layer Assembly for Drug Delivery and Tissue Engineering Applications. Advanced Healthcare Materials. 2024;13(8):2302713. doi:10.1002/adhm.202302713.

 

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