The analytical techniques deployed in this study each interrogate a different dimension of the antibody-nanocrystal interface, and understanding their complementary nature clarifies why the authors combined them. Fourier transform infrared spectroscopy probes molecular vibrations and can confirm the presence of sulfate ester groups on sulfuric acid-hydrolyzed cellulose nanocrystals, amine and amide bands on chitosan, and the characteristic amide I and amide II signatures of immobilized immunoglobulins. Atomic force microscopy resolves the lateral dimensions and heights of individual nanocrystals, revealing whether the rod-like morphology typical of cellulose whiskers is preserved after surface modification. Zeta-potential measurements report on the apparent surface charge in suspension, which for cellulose nanocrystals prepared by sulfuric acid hydrolysis is typically strongly negative due to grafted sulfate half-ester groups, while chitosan nanocrystals carry protonatable primary amines that confer cationic character under acidic conditions. Hydrodynamic diameter analysis by dynamic light scattering captures how particles behave in buffered media, where hydration shells and ionic screening can cause the effective size to differ substantially from the dry dimensions measured by microscopy.

The two real-time biosensing techniques at the heart of the functional evaluation deserve particular attention. Quartz crystal microbalance with dissipation monitoring measures shifts in the resonance frequency and energy dissipation of a piezoelectric quartz crystal as mass accumulates on its sensor surface. Because the dissipation parameter reports on the viscoelastic properties of the bound layer, it can distinguish between rigid, tightly coupled antibody films and softer, hydrated layers that retain coupled water and entrapped antigen. This is especially informative for polysaccharide-based coatings, which are inherently hydrated and can form swollen architectures whose apparent mass includes significant amounts of hydrodynamically coupled solvent. Multi-parameter surface plasmon resonance, by contrast, measures refractive index changes at a gold surface as molecules bind, and its multi-wavelength or multi-angle capability refines the estimation of surface coverage and layer thickness. Running both techniques in parallel allows the researchers to cross-validate binding kinetics, since each method has distinct sensitivities to hydration, viscoelasticity, and optical properties of the immobilized layer, and discrepancies between them can themselves be diagnostic of layer structure.

The carbodiimide coupling chemistry used to attach antibodies to these nanocrystals is among the most widely practiced bioconjugation strategies in biosensor fabrication. EDC activates carboxylic acid groups to form reactive O-acylisourea intermediates, which NHS then converts to more stable NHS esters that react with primary amines on lysine residues or the N-terminus of the antibody. On cellulose nanocrystals bearing sulfate half-esters and some intrinsic carboxyl groups, and on chitosan nanocrystals rich in primary amines, the direction of coupling can be engineered: carboxyl-rich surfaces present activated esters to the antibody’s amines, while amine-rich surfaces can be carboxylated first or used to capture antibodies through alternative chemistries. The density and accessibility of these functional groups, rather than their nominal abundance, ultimately determine how many antibody molecules can be loaded per unit area and in what orientation. Random coupling through lysine residues, while simple, produces heterogeneous orientation in which a fraction of antibodies present their antigen-binding Fab regions inward or sterically occluded, reducing effective capture capacity relative to the theoretical maximum.

The observation that the composite CNC/Ch support yielded less variable antigen-binding responses than either individual nanocrystal type is consistent with a growing body of literature on mixed-material interfaces. Pure cellulose nanocrystals, while mechanically robust and highly crystalline, present a strongly anionic, sulfated surface that can promote non-specific electrostatic interactions with positively charged protein patches and can induce conformational perturbations in adsorbed antibodies. Pure chitosan nanocrystals offer cationic amine chemistry but can suffer from pH-dependent charge variability, batch-to-batch differences in degree of deacetylation, and solubility transitions near the polymer’s pKa that complicate reproducible coating. Blending the two materials appears to buffer the extremes of each: the cellulose component contributes dimensional stability, high crystallinity, and a large specific surface area, while the chitosan component contributes amine functionality that can improve antibody coupling density and provide a more protein-friendly microenvironment. The result is an interface whose performance is less sensitive to small variations in preparation conditions, which is a critical attribute for manufacturing reproducibility.

The paper-based prototype component of the study connects the interfacial science to a practical deployment format. Lateral flow and related paper-based immunoassays dominate point-of-care testing because cellulose fiber networks wick aqueous samples by capillary action without external pumps or power, and detection can be read visually or with inexpensive readers. However, the porous and chemically heterogeneous nature of paper introduces challenges that flat biosensor surfaces do not face. Untreated cellulose fibers present abundant hydroxyl groups but few reactive handles, so antibodies applied directly to paper tend to adsorb physically in random orientations and can leach during sample flow. The deposition of nanocrystal coatings onto paper fibers effectively creates a high-surface-area intermediate layer that increases antibody loading capacity per unit of visible area and can reduce non-specific background by providing a more controlled chemistry than the raw fiber surface. The finding that the composite coating produced a usable signal response in this format suggests that the interfacial gains measured on model surfaces can survive translation to a porous, flow-through substrate, which is often the point where promising benchtop designs fail.

The choice of Whatman Grade 41 filter paper as the substrate is also worth contextualizing. Cellulose filter papers differ substantially in fiber length, porosity, flow rate, and wet strength, and these parameters directly influence assay performance by controlling residence time of the sample at the capture zone and the accessibility of immobilized probes. Grade 41 is a relatively fast-flowing, ashless qualitative paper, which facilitates rapid capillary transport but may reduce contact time between antigen and immobilized antibody. Optimizing the match between nanocrystal coating density, paper porosity, and flow rate is identified by the authors as a priority for future work, and this is indeed where much of the remaining engineering challenge lies for translating composite-coated paper diagnostics into validated products.

The broader significance of establishing structure-function relationships among renewable biopolymer nanocrystals extends beyond cancer diagnostics. Cellulose and chitosan are among the most abundant biopolymers on Earth, derived from plant biomass and crustacean shell waste respectively, and their nanocrystalline forms can be produced at scale with comparatively low environmental burden relative to synthetic polymer nanoparticles or gold nanoparticles commonly used in biosensing. Demonstrating that these renewable materials can serve as high-performance antibody immobilization supports aligns with sustainability goals in diagnostics manufacturing, where single-use plastic cartridges and precious-metal reagents contribute to waste streams. The use of an ionic liquid, 1-butyl-3-methylimidazolium hydrogen sulfate, in the preparation workflow also reflects ongoing efforts to develop greener processing routes for nanocellulose, since ionic liquids can dissolve or hydrolyze cellulose under conditions milder than mineral acid hydrolysis and can be recycled in closed loops.

The authors’ candid identification of remaining limitations provides a realistic roadmap for the field. Specificity and selectivity limits are paramount for clinical utility: CEACAM5 shares structural homology with other members of the CEA family, such as CEACAM1 and CEACAM6, and cross-reactivity with related glycoproteins or with heterophilic antibodies in patient sera can produce false positives. Complex biological matrices such as serum, whole blood, or saliva introduce proteins, lipids, and competing antigens that can foul the capture surface or alter binding kinetics relative to buffered standards. The impact of these carrier matrices on the composite-coated paper format remains to be systematically quantified. Additionally, the relationship between the interfacial characterization performed on planar sensor surfaces and the behavior of the same coatings within the three-dimensional, tortuous pore network of paper is not fully resolved, and bridging this gap between model surface studies and porous substrate performance is a recognized challenge in the paper diagnostics literature.

Looking forward, the comparative methodology demonstrated here, in which multiple nanocrystal chemistries are evaluated side by side under identical assay conditions with orthogonal characterization techniques, offers a template that can be extended to other biomarker-antibody pairs and other renewable nanomaterials such as chitin nanocrystals, starch nanocrystals, or nanocrystalline variants of algal and bacterial cellulose. As regulatory pathways for paper-based diagnostics mature and manufacturing scale-up processes for nanocellulose and nanochitosan become more economical, composite biopolymer nanocrystal capture layers of the type characterized in this study may find application not only in cancer biomarker monitoring but also in infectious disease testing, environmental monitoring, and food safety, wherever robust, low-cost, antibody-based detection at the point of need is required.

Subject of Research: Comparison of antibody immobilization on renewable nanocrystals for paper-based detection

Article Title: Comparison of antibody immobilization on renewable nanocrystals for paper-based detection

Article References: Vazquez-Hernandez, K., Herrera, B. M., Dourado Fernandes, C., Effanga, E.-J., Farkhondehnia, H., Shibu, R. P., Shamshina, J. L., Turner, C., Abidi, N., & Gomez-Maldonado, D. (2026). Comparison of antibody immobilization on renewable nanocrystals for paper-based detection. Frontiers in Biomaterials Science, 5, Article 1815047. https://doi.org/10.3389/fbiom.2026.1815047

Image Credits: AI Generated

DOI: 10.3389/fbiom.2026.1815047

Keywords: cellulose nanocrystals, chitosan nanocrystals, antibody immobilization, paper-based diagnostics, CEACAM5, biosensors, QCM-D, MP-SPR, cancer biomarkers, point-of-care testing, Comparison, antibody