Black Bioinks from Superstructured Carbonized Lignin Particles

A renewable source of carbon black is introduced by the processing of lignin from agro‐forestry residues. Lignin side streams are converted into spherical particles by direct aerosolization followed by carbonization. The obtained submicron black carbon is combined with cellulose nanofibers, which act as a binder and rheology modifier, resulting in a new type of colloidal bioink. The bioinks are tested in handwriting and direct ink writing. After consolidation, the black bioinks display total light reflectance (%R) at least three times lower than commercial black inks (reduction from 12 to 4%R). A loading of up to 20% of nanofibers positively affects the cohesion of the dried bioink (1 to 16 MPa), with no significant reduction in light reflectance. This is a result of the superstructuring of the ink components, which disrupts particle packing, intensifies colloidal interactions, introduces light absorption, and non‐reflective multiple scattering.


Introduction
Black substances are used in a broad range of applications, from textile dyeing to energy harvesting.For instance, black inks are by far the most used for printing and writing since it provides the highest contrast with common white substrates (paper, cotton, etc).Black substances comprise chemical components that change their energetic state by absorbing light energy DOI: 10.1002/adfm.202304867 with wavelengths (WL) in the visible range (380-700 nm).Conjugated polyaromatics, including those derived from carbonized biomass, exhibit short optical bandgap (ca.1-1.5 eV) enabling HOMO-to-LUMO transitions by absorbing low-energy photons, such as those in the visible electromagnetic spectrum. [1,2]Commonly used black pigments are sourced from petroleumbased coal and oil, animal bones, and iron oxides. [3,4]Yet, a vast amount of waste biomass rich in polyphenols [5] can be transformed into condensed graphitic structures upon carbonization, [6] opening an opportunity for alternative biobased pigments.
The most efficient carbon-based pigments absorb only ≈90% of the incident light.This is because of a defective polyaromatic structure that decreases the absorption efficiency, thus promoting significant light reflectance.The black color from pigments is primarily associated with their molecular structure, which can be optimized to reduce light reflection, e.g., by introducing structural features on their surface.9][10][11] Several designs have been proposed to decrease light reflectivity, including thin-films or arrays of vertically aligned carbon nanotubes (CNT), [11] carbonized low-density wood, [12] and polymer-based carbon aerogels. [7]CNT technologies are highly dependent on a metallic substrate for their growth into an array by chemical vapor deposition, whereas carbonized wood and aerogels are mechanically weak.
Particle-fiber nanonetworks can yield solid constructs that develop nanoscale structural features upon consolidation from aqueous colloidal suspensions. [13]Moreover, particle-nanofiber constructs can be engineered to maximize the cohesiveness and to induce light entrapment.In fact, we propose that such colloidal (black) inks can be made completely from plant-based building blocks, namely cellulose nanofibers, and lignin particles.Spherical lignin particles [14] can be carbonized to yield submicron black particles that evolve into a graphitic chemical structure upon pyrolysis.Constructs made only of submicron particles are brittle and depending on the consolidation conditions, they tend to self-organize into ordered lattices that can display photonic colors. [15]Superstructuring particles with cellulose nanofibers (CNF), however, disrupts the particle packing and creates an irregular and porous particle network.Herein, CNF is proposed as an alternative to current binders, for instance those based on epoxy or acrylics that yield glossy and/or greyish colors.CNF introduces cohesion by the formation of an interconnected particle-nanofiber entangled network. [13,16]This is a result of early gelation of CNF suspensions, which reduces particle mobility and prevents densification.The length scales of CNF assemblies around submicron particles is below the visible wavelength range, [13] which minimizes light scattering and reflection.The latter makes CNF an ideal binder for particles intended for photonic applications within the visible WL (CNF is considered transparent in these assemblies and does not hinder the access of light energy to the superstructured particles).This is highly relevant to the preparation of black inks as the addition of a binding component should not impact their blackness.
We introduce a versatile black bioink by combining lightabsorbing particles obtained from lignin and CNF-based transparent structural binders (Figure 1a).Consolidation of the black bioinks results in superstructured particle-nanofiber networks that couple chemical (i.e., gaphitic structures) and structural (i.e., porosity) features that achieve low light reflectance.Application of CNF as a structural binder enables an additional control over the ink viscosity and flow behavior (Figure 1b,c), thus allowing uses in a wide range of applications (Figure 1d).[19][20] We demonstrate the versatility of the black bioinks in coatings, writing, and 3D printing, but we envision these pigments in ink-jet printing, stamping, and painting.We provide further insights on the effects of composition and drying conitions by assessing the optical properties of black bioinks in the framework of finite element modeling.

Plant-Based Black Pigments
Lignosulfonate powder (LS), as obtained from industry, is an irregular particulate given the uncontrolled drying conditions from a typically concentrated solution.Direct carbonization of such precursor yields, non-differently, irregular particles with sizes ranging from 20 to 200 μm (Figure 2a).Spherical lignin particles with diameter ca.1.5 μm (Figure 2b) were prepared using an aerosol reactor.In this process, lignin solutions are prepared at low concentration, and then atomized into small droplets that are slowly dried under a N 2 flow. [14,21]After this the lignin particles were thermally treated at 170 °C and then carbonized at temperatures ranging from 600 to 900 0 C under N 2 atmosphere.The carbonized lignin particles (cLSP) shrunk by ca.30% during carbonization, leading to particles ranging from 900 nm to 1 μm in diameter (Figure 2c,d and Figure S1, Supporting Information).Carbonized lignosulfonate powder (cLS) and particles (cLSP) are chemically equivalent (see similar Raman profiles in Figure 2e,f).The signals in the Raman spectra refer to first-order defect-induced D band at ≈1350 cm −1 (breathing mode of A 1g symmetry), and G band at ≈1580 cm −1 (in-plane bond stretching motion of sp 2 carbon atoms of E 2g symmetry). [1]The ratio between these two bands, namely D/G ratio, as well as their peak position and shape (full width at half maximum, FWHM) are used to locate the obtained carbons within the structural scale for carbon, i.e., from (hydrogenated) disordered graphite to tetrahedral amorphous carbon (ta-C, diamond-like carbon).Following the three-stage model proposed by Ferrari and Robertson, [1] describing the amorphization trajectory ranging from graphite to ta-C, our cLS and cLSP comprise semi-nanocrystalline carbon structures with slightly increased cluster sizes as the temperature of carbonization goes from 600 to 900 0 C. Nonetheless, although the D/G ratio increases with the carbonization temperature, the range of values are rather narrow (from 0.91 to 0.97) and the wide FWHM remains nearly constant (ca. 100 cm −1 for D band and ca.60 cm −1 for G band), so that all samples display nearly the same nanocrystalline graphitic structure (Table S1, Supporting Information).This corroborates previous work on the carbonization of ligneous biomass, where distinguishable changes on the carbon structure are seen only after 1400 0 C. [6] Semi-nanocrystalline carbons, with D/G ≈1, display optical bandgap (HOMO-LUMO gap) ranging from 1 to 1.5 eV approximatively, [1,2] falling below the visible light energies (1.8-3.1 eV).Therefore, carbons obtained either from synthetic precursors or biomass are naturally black as their chemical structure enables the HOMO to LUMO transition to take place by absorbing (low) energy within the visible range.

Lignin-CNF Colloidal Black Bioinks
In our lignin-based black pigments, the chemistry-derived blackness inherent to cLS and cLSP is the same.However, when using these black pigments as bioinks for coatings or structured materials, the total light reflectance of the dried phase depends on the structure, which in turn depends on the a) dimensions and morphology of the particles, b) the binder, and the c) binder-pigment structuring.Particle superstructuring with CNF disrupts the particle packing, thus creating an irregular and porous particle-fiber network that induce multiple internal light reflections.Moreover, addition of loadings as low as 0.1 wt.% of CNF improves the colloidal stability of carbon particles suspensions (Figure S2, Supporting Information), via electrostatic interactions [22] and by creating interpenetrated fibrillar nanonetworks. [13]he consolidation of the black bioinks yielded materials with varied morphologies from the macro to the nanoscale, which depended on the black pigment used.First, we used X-ray tomography (at either 1.15 μm or 65 nm of resolution) to investigate the macroscopic features (Figure 3) of the dried phases of the cLS and cLSP based bioinks, both using CNF at 10 wt.% as binder (note: the effect of content and type of binder on the properties of the materials is shown later).The 3D visualizations in Figure 3a,b indicate that both bioinks resulted in homogenous materials with no phase separation observed at the macroscale.Only a few aggregates were found in the cLSP construct (Figure 3c).2D slices of the tomograms confirm the low packing degree and the homogenous distribution of the cLS powder and cLSP within the nanocellulose binder (Figure S3, Supporting Information).The roughness of the top surface varied depending on the black pigment used, which is observed in the color maps for surface height (Figure 3d).Overall, bigger pigments led to a wider minima-maxima height span both at the top (drying front) and bottom (in contact with the underlying substrate during drying) surfaces of the materials (Figure 3d and Figure S4, Supporting Information).In terms of surface height, the cLS materials had a much larger dimensional span of 100 μm compared to the cLSP, which was one order of magnitude lower (ca.10 μm) (Figure 3e).
Although rougher at the macroscale, the cLS materials are microscopically smoother than those materials constructed with cLSP.The size range of the cLS pigments varies from 20 to 200 μm, which can characterize a flat-like surface depending on the length scale.This is especially relevant in the context of visible light absorption, whose wavelength are below 1 μm.On the other hand, materials comprising a given concentration of cLSP and varying amounts of CNF, yielded superstructured particle constructs with submicron geometrical features (Figure 4a).The latter lower the light reflectance by favoring light reflection inward, within the structure, resulting in light trapping and near-complete absorption.These features are absent in cLS/CNF bioinks, given the size of the cLS pigments (one order of magnitude larger than that of cLSP, Figure 4a1,a2).The drastically different morphologies of the dried inks (Figure 4a) impact the cohesion of the particle-CNF network, depending mostly on content and type of binder.We investigated the efficiency of nanocellulose binders in bringing cohesion to the light-absorbing particles by using TEMPO (2,2,6,6tetramethylpiperidine-1-oxyl radical)-oxidized (TOCNF) and unmodified cellulose nanofibers (CNF).To assess the strength of the dried bioink, we prepared macroscopic cylinders using templated evaporation induced self-assembly of cLSP/cLS mixed with either TOCNF or CNF.The dried cylinders were then subjected to uniaxial compression and their stress at the yield point was compared (Figure 4b-inset).In detail, the yield point refers to transition between elastic and plastic deformation, whereby particlenanofiber networks experience crack generation and propagation.Crack propagation is rather limited in CNF-particle constructs due to the interconnectivity of the network, thus leading to a plateau in the stress-strain curve where small fractures took place but were held together by the nanofibers. [16]verall, the size of the particle dictates the topology of the nanofiber network, which has an impact on the cohesion of the material. [13]The nanocellulose binder assembled into a paperlike structure around the particles (20-200 μm in size), whereas it created an intermixed 3D particle-nanofiber network with cLSP of 900 nm.The utilization of spherical cLSP led to particlenanofiber networks two times more cohesive than those obtained by irregular cLS, which is more accentuated when using >9 wt.% binder (Figure 4b).CNF networks are more effective to reduce cracking in assemblies of submicron particles, when compared with bigger (>40 μm) particle counterparts (Figure S5, Supporting Information).The stress at yield point increases in a quasilinear fashion with the binder content.The highest strength for cLSP and cLS was observed with unmodified CNF.cLSP and cLS with 20 wt.% of CNF displayed a stress of ≈16 and ≈10 MPa at the yield point, compared to ≈13 and ≈5 MPa when using the same content of TOCNF.The branched morphology of the unmodified CNF, as opposed to the more individualized TOCNF, leads to higher entanglement and interconnectivity around the particles, leading to higher cohesion.Overall, the cohesion is induced by CNF-CNF entanglement, but there is a significant contribution coming from particle-CNF secondary interactions.Ligninbased carbon particles are partially hydrophilic, thus being able to engage in multiple hydrogen bonding and van der Waals interactions with CNF.
Besides the higher cohesion, cLSP led to remarkably lower light reflectance, i.e., darker black, when compared to cLS (Figure 4c).When averaging the reflectance in the wavelength range from 300 to 800 nm, and comparing binder content and type, we observed a clear trend of increased light reflectance at higher binder contents, but there was no clear correlation with binder type.The effect of binder content on increasing the light reflectance was more pronounced for the cLS-based dried inks, which is due to the formation of thicker paper-like assemblies on the surface of the cLS particles (20-200 μm) as opposed to the loose (and more isolated) nanofiber network observed around the 900 nm cLSP.
TOCNF-based binding led to dried inks with slightly higher light reflectance when compared to unmodified CNF used at the same loading, with clear differences observed when using up to 9 wt.% binder.This is counterintuitive considering that the negative charge of TOCNF has been shown to induce high dispersibility of carbon nanomaterials, [22] as well as films that are more transparent than those of unmodified CNF. [23]The high dispersion of the cLSP/cLS carbon particles and smaller (more transparent) nanofiber binders were expected to lower the light reflectance; however, we observed that the combined factors resulted in better packing of the particle-nanofiber network and therefore denser structures (Table S2, Supporting Information).Pore morphology and content are important factors in the design of structural blackness.Membranes prepared from biobased black inks had a porosity over 40% and peaked at 54% for the ink precursor containing 9 wt.% of unmodified CNF and cLSP (Table S2, Supporting Information).The latter was, in fact, the bioink precursor leading to the blackest pigment (Figure 4e).Using superstructured inks, without additives, we obtained reflectance values of ca.4% and 6.5% with cLSP and cLS, respectively.These values are significantly smaller than that of inks used in inkjet printing and fresh black inks, ca.12% (Figure S6, Supporting Information).Moreover, although the blackness of the plant-based bioinks is not in the superblack levels, i.e., R% < 0.4, it is comparable to those of carbonized wood cross-sections (ca.3%), [24] and it is lower than poly(lactic acid)-carbon black electro spun membranes. [25]

Effect of Polymeric Additive on the Performance of Black Bioinks
We investigated the effect of polymeric additive on the cohesion and light reflectance of dry inks.Both natural and synthetic polymers have been used to manage light interactions in materials.Tannic acid (TA) and condensed tannins (CT), i.e., plant-based polyphenols, have been incorporated in packaging materials to enable UV blocking properties [26][27][28] as well as to strengthen the material via crosslinking. [29][32] Herein, we fixed the CNF binder content at 10%, and varied the polymer fraction from 1% to 10% (both with respect to the mass of the cLSP black pigment).
The addition of TA and CT at 1% resulted in weaker materials when compared to those obtained with only CNF (Figure 5a).The strength of particle-nanofiber network is primarily associated with fibril-fibril interactions and, to a lesser extent, to fibrilparticle interfacial coupling.The presence of macromolecules in the CNF binder suspension disrupted the cohesive fibril-fibril interactions due to the strong adsorption of tannins on the surface of cellulose. [28]The addition of PPy and PEDOT:PSS at the same fraction, however, led to stronger materials when compared to the reference CNF and tannin-added binder (Figure 5a).PPy and PE-DOT:PSS are not soluble in water, and form stable dispersions in aqueous media (10-50 nm polymeric particles).Particles in this range could accommodate in the interstitial spaces of the network of cLSP black pigments, as well as co-assemble with cellulose nanofibers around the pigment particles (Figure 5e,f).Such physical interactions do not disrupt cellulose fibril-fibril interactions and add to the cohesion due to the chemical compatibility of PPy/PEDOT:PSS with carbon particles.The mechanical strength of the PPy/PEDOT:PSS-containing inks increased with increased fractions of both polymers (Figure 5b), reaching over 9 MPa at the highest content, which is two times higher than that when only CNF is used.We emphasize that the CNF is the primary component bringing cohesion to the system, and the polymer additive boosts such cohesion.The polymer alone is not sufficient to create cohesive inks, to the extent that it is not possible to measure the strength.The strongest consolidated materials were originated by combining 1:1 CNF and polymer additive, both at 10% of the pigment mass fraction in the ink.The dry ink contained 83.3 wt.% of cLSP, 8.3 wt.% CNF, and 8.3% polymeric additive.
The addition of polymers with aromatic and conjugated features, which induce HOMO-LUMO transitions with low energy input, lowered further the overall reflectance of the consolidated black bioinks (Figure 5c,d).The condensed molecular structure of CT led to high absorption at short WLs (Figure S7, Supporting Information), leading to an overall low reflectance (Figure 5c).Tannic acid led to relatively higher absorption of WLs close to the UV range (Figure S7, Supporting Information), whereas PE-DOT:PSS addition induced high scattering at the edges of the tested WLs (Figure S8, Supporting Information).This explains the high standard deviation after averaging the reflectance values obtained across the 300-800 nm WLs.PPy led to very consistent reflectance across the measured range, which, coupled to its high cohesion, makes it an ideal polymeric additive for black bioinks.Moreover, increasing the PPy content from 1% to 10% (based on the mass of black pigments), we observed a decreased light reflectance, from 4.2% to 3%.Higher content of PPy would not result in lower reflectance values (Figure S8, Supporting Information), therefore setting a lower limit for light reflectance at ca. 3% for the composited bioinks.This is not only related to chemical-derived blackness but also to new structural features introduced by the polymer (Figure 5e,f), which were absent in the tannin-added inks (Figure 5g).The presence of small nanoparticles between the submicron particles, and within the nanocellulose network, created a porosity with length scales <10 nm that enhanced multiple internal reflections, but also reduced the flatness of the top surface of the consolidated ink.Flat, featureless, surfaces are known to decrease light absorption due to the principles of (specular) light reflectance, i.e.,  i =  r , where light reflects ( r ) at the same angle () of incidence ( i ).Our consolidated inks are superstructures of particles-nanofibers (added polymers) with a multiscale architecture that is efficient to reduce the reflection up to four-fold when compared to commercial pigments (Figure S6, Supporting Information).

Compositional and Structuring Factors in Colloidal Bioinks
So far, we have demonstrated that colloidal inks result in dry porous constructs (schematically represented in Figure 6a) that display lower light reflectance when compared to monolithic (macroscale) black surfaces (Figure 4c).We experimentally assessed the optical properties of air-dried black surfaces generated by inks containing 900 nm lignin-based carbon pigments and nanocellulose binders.Next, we modeled the optical properties of randomly packed particles (of various sizes), to discuss the effect of pigment size and porosity (affected by the drying dynamics) [33] of the consolidated system on its light reflectance.Finite element (FE) software COMSOL Multiphysics (Electromagnetic Waves, Frequency Domain in Wave Optics Module), in the framework of continuum description of condensed matter, was used to model the optical properties of several particle constructs (Figure 6b and Figures S9-S15, Supporting Information).More details of the modeling framework are found in the Experimental Section and Section S2 (Supporting Information).The overall reflectance values obtained in the modeling are in a slightly higher regime that those observed experimentally.This discrepancy derives from the bulk porosity and optical properties of the model material (a graphite pellet, Section S2, Supporting Information), which may not match perfectly with those from the lignin-based carbon particles.Biobased carbon typically displays micro-meso porosity, [34,35] which may also contribute to decreasing the light reflectance.
Overall, in the modeled carbon-based particulate constructswithin the size range tested-the porosity played a bigger role in the averaged light reflectance (300-800 nm) (Figure 6c).Reflectance values decrease from ca. 15% to 6%, i.e., by a factor of ca.2.5, when increasing the porosity of the model graphite from a nearly monolithic material to an aerogel-like structure displaying porosity over 80%. [36]The effect of porosity becomes evident in the finite element modeling maps of the distribution of the electric field norm for  = 300 nm, obtained from a particle construct displaying porosities of 90% (top) and 50% (bottom) (Figure 6d).Higher porosity, within a given size of the primary building blocks, leads independently to a more open path for light to penetrate deeper into the light-absorbing carbon structure, thus intensifying multiple internal reflections (MIR) mechanism that increases the light-substrate interactions and reduces overall the outward reflection.In particle-based materials or coatings, the porosity can be adjusted by following different drying methods and controlling their dynamics.For instance, Cusola et al. have shown that the packing density of lignin particle coatings produced by evaporation induced self-assembly can be controlled by simply controlling the drying temperature (from 25 to 100 °C). [33]On the other hand, particle-based materials with porosity over 70% require more specialized techniques, such as freeze drying of foamed particle precursors. [37]he particle size of the primary elements used in the model constructs, however, had a minor impact on their overall visible light reflectance.Although there is a remarkable difference between a flat, featureless graphitic surface when compared to surfaces of nano-micro particle assemblies (Figure 6f), there is not a significant difference among particles sizes (in the range of 5 μm to 200 nm) as far as the total light reflectance (Figure S15, Supporting Information).In fact, light reflectance is reduced when using particles in the upper submicron sizes (500-900 nm).Assemblies of submicron particles create interparticle spaces with length scales in the submicron regime as well, which enables light with wavelength in the visible range to penetrate the structure and promote MIR (Figure S11 and Figure S12, Supporting Information).Elementary particles in the range of 200 nm-especially in denser structures-tend to create tighter particle networks enabling substrate-light interaction mostly at their top surface (Figure S13, Supporting Information).Experimental measurements of the light reflectance of dried bioinks made from 200 nm lignin-based carbon particles and CNF, confirmed the FE simulation (Figure S16, Supporting Information).In constructs made from bigger particles (ca.5-50 μm), their primary elements can already be considered as bulk, monolithic structures thus exposing hundreds of squared microns with low radii of curvature (Figure S10, Supporting Information) at the top surface (exposed to light) thus leading to light reflectance outward the coating or material.

Black Bioink Applications Prospects
The complex network of particles and nanofibers led to a light reflectance of ca.3-4%, which is below the blackness found in most pigmentation-centered applications.Nevertheless, the use of nanocellulose as a structural binder enables adherence to a variety of surfaces, including polyester, paper, and wood (Figure 7 and Figure S17, Supporting Information).This cor-roborates to the ability of cellulose nanomaterials to act as adhesive. [13,38]The ink adheres very strongly to the substrates, resisting several mechanical stresses related to folding, rolling, and crumpling without any apparent damage (Figure S18, Supporting Information).Given the hydrophilic character of the cellulosic binder, the current formulation should be sensitive to water, thus requiring further development or considerations on using existing water-resistant additives.The viscosity of the ink is controlled by the nanocellulose content (Figure S19, Supporting Information), which allows to match the required flow behavior of the application.The high aspect ratio of the binding nanofibers leads to shear thinning (Figure S19a, Supporting Information), whereas the interconnectivity of the CNF network results in elastic structures over a wide range of shear stresses (Figure S19b, Supporting Information).The viscosity of the black bioinks can be adjusted to cover a range over two orders of magnitude, within the same shear stress, by varying the binder content between 2-25% based on the carbon particle (i.e., pigment) mass fraction in the ink.Therefore, using the same components it is possible to produce low viscosity formulations for coatings (Figure 7a), writing (Figure 7b), stamping and processes such as casting (Figure 7c).Higher viscosity inks, i.e., with higher nanocellulose content, may find applications in brush painting, e.g. to gain control over thixotropic behavior of the ink. [39,40]Nevertheless, the shear thinning and elastic behavior (in a wide range of shear stress, Figure S19b, Supporting Information) of nanocelluloses make the black inks suitable for printing technologies, such as 3D printing (Figure 7d).3D printing is facilitated by replacing the unmodified CNF by TOCNF, which are smaller in size and facilitates extrusion through the nozzles.This also applies to inkjet printing, where smaller carbon pigments (≈200 nm) should induce better printability of smaller droplets (pL volumes) and higher resolution of the printed pattern.
The black bioinks could be utilized as precursors for the development of several materials that rely on carbon properties, including carbon capture, [34] electrocatalysis, [41] filtration, [42] electromagnetic shielding, [43] and heat-induced solar steam generation. [44]Such wide variety of applications is a result of the versatility of our materials as far as formulation, and because the nanocellulose binder does not hinder access to the surface and functionality of the particles.As a demonstration, herein we coated a cross-section of balsa wood with a low viscosity black bioink (containing ca. 5 wt.% of nanocellulose).The darkened surface was then exposed to simulated sunlight while the bottom of the sample was immersed in water.We follow the evaporation rate (kg h −1 m −2 ), and compared to a fully carbonized wood scaffold, a structure that has been often used for such purposes.In fact, the coated wood surface performed similarly to the fully carbonized material, 2.5 kg h −1 m −2 compared to 3 kg h −1 m −2 (Figure S20, Supporting Information) with the advantage of displaying the mechanical strength inherent to natural wood.This is not the case of carbonized wood-a very brittle material.Nevertheless, with the presence of either PPy or PEDOT:PSS (10% over the particle content) the consolidated ink was electrically conduc-tive, which we demonstrated by creating a simple circuit on a wood veneer (Figure S21, Supporting Information).

Conclusion
We produced black bioinks using light-absorbing particles formed by lignin precursors and a "transparent" binder, namely, cellulose nanofibers.The cohesion of the particle-nanofiber networks, i.e., the dried bioinks, is easily tuned by the nanofiber loading (with only 10 wt.% nanocellulose addition a highly robust ink is produced).The blackness of the bioinks, which is higher than traditional fossil-based black pigments, is explained from the coupling of chemical and structural features.For instance, while the graphitic structure of the lignin black pigments enables light absorption in the visible range via low-energy HOMO-LUMO transitions, the particulated nature of the introduced constructs enables multiple internal reflections and light trapping, thus reducing the overall light reflectance.The configuration and structure of the particle-nanofiber network, especially its porosity, plays a major role in light reflection.We assessed the influence of morphology on the blackness of the substrate using finite element modeling of the optical properties of idealized particle beds.
The blackness of the bioinks could be lowered down with the addition of polymeric additives, such as PPy, but this must be rationalized in terms of performance-renewability.The purely plant-based bioinks display not only a high performance but are sustainable alternative to traditional black pigments, e.g., those based on fossil carbon or metals.Given their graphitic nature, the bioinks can be used in a variety of platforms (light energy and electroactive materials, among others).More specific and application-centered characterizations, e.g., thixotropic behavior, are required to further develop these black bioinks.The formulation and the tailorable rheology open additional opportunities in the fabrication of carbon-based materials with a broad window of practical applicability.
in random directions, Floquet periodic boundary conditions were used on the vertical sides of the modeling domain.Quadratic discretization was used for electric field.More details can be found in Section S2 (Supporting Information).

Figure 1 .
Figure 1.Aqueous suspensions of light-interacting plant biocolloids are used to prepare versatile black bioinks.a) Carbonized lignin particles are used as light absorbing components, whereas cellulose nanofibers assemble into partially visibly transparent structures are used as colloidal binders.b) The combination of both plant biocolloids enables nanostructures with tuned rheology, with c) viscosity increasing with the increase of nanofiber content.d) The various formulations enable a wide variety of applications for these precursors, from various pigmentation methods to several techniques used for materials assembly.

Figure 2 .
Figure 2. Structural properties of the carbonized lignin materials.Scanning electron microscope images of a) carbonized lignosulfonate powder (cLS), b) lignin particles (LSP) used as precursors for obtaining c) carbonized lignin particles (cLSP).d) Size distribution of lignin particles before and after carbonization at 900 °C, showing a decrease in particle size after thermal treatment.Raman spectra of e) lignosulfonate (cLS) and f) lignin particles (cLSP) carbonized at temperatures ranging from 600 to 900 °C.Respective D/G ratio is shown next to each spectrum.

Figure 3 .
Figure 3. Macroscopic morphology of the dried bioinks.3D visualization, built from microCT scans, of a) cLS(CNF) and b) cLSP(CNF) materials.c) 3D visualization of cLSP(CNF) built from nanoCT scans with a pixel size of 65 nm.Color maps for surface height variations at the top surface of d 1 ) cLS(CNF) and d 2 ) cLSP(CNF) materials.e) Numerical results for surface height of the top and bottom surfaces of the cLS(CNF) and cLSP(CNF) materials.Note: 3D visualizations contain perspective and might induce minor deviations in the accuracy of the scale bars.We used orthographic projection to minimize such deviations.

Figure 4 .
Figure 4. Cohesion and light reflectance properties of ink precursors consolidated into particle-nanofiber macroscopic constructs.Scanning electron microscope images of a 1 ) cLS and a 2 ) cLSP membranes assembled using 9 wt.% of CNF.b) Stress at yield point as a function of binder content and type, as well as particle type (inset graph show the yield point). c) UV-visible spectra obtained in reflectance mode of cLS and cLSP membranes assembled with 9 wt.%CNF content.Averaged reflectance (300-800 nm) of d) cLS and e) cLSP membranes consolidated with TOCNF and CNF at various mass fractions.

Figure 5 .
Figure 5. Cohesion and light reflectance properties of macroscopic constructs prepared from polymer-added black bioinks.a) Stress at yield point as a function of polymer additive (CNF 10% and polymer 15, related to the mass of black pigments), and b) stress at yield point for PPy and PEDOT:PSS at increased concentrations.c) Averaged reflectance (300-800 nm) of cLSP membranes consolidated with the addition of 1% of polymers, and d) reflectance spectra for materials containing 10% CNF and increased content of PPy.Scanning electron microscopy images of cLSP-CNF membranes (10% of binder) added of e) PPy, f) PEDOT:PSS, and g) CT, all at 10% related to the mass of particles.

Figure 6 .
Figure 6.a) Schematic representation of the particulate construct obtained after the drying of the black bioinks.Substrate-light interactions are dominated by particle size and porosity.b) Simplified scheme (more details in Supporting Information) of the finite element modeling (FEM) of opticalproperties of particles beds used to further discuss the substrate light interactions taking place in consolidated black bioinks.c) 3D plot of modeled averaged light reflectance values as a function of porosity and particle size.d) Distribution maps of the electric field norm for  = 300 nm, obtained from a particle construct displaying porosities of 90% (top) and 50% (bottom) and particle size of 0.5 μm.FEM results for averaged reflectance (300 to 800 nm) for particle constructs with e) increased porosity and f) various particles sizes.

Figure 7 .
Figure 7. Demonstration of applications for black bioinks in pigments and as precursors for the fabrication of carbon materials.Plant-based black bioinks were used to a) coat polyester fabric, b) as inks in dipping pens, c) coat the cross section of wood to enable solar-powered stem generation, and d) 3D print scaffolds that can find several final applications.