Strategy for Manufacturing a Deep-Red Ink Based on Nanocellulose

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A Strategy for Manufacturing A Deep-Red Ink Based on Nanocellulose and Reactive Red 120 Wenbo Wang, and Shiyu Fu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00256 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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ACS Sustainable Chemistry & Engineering

A Strategy for Manufacturing A Deep-Red Ink Based on Nanocellulose and Reactive Red 120 Wenbo Wang†, Shiyu Fu*,†

† State

Key Laboratory of Pulp and Paper Engineering, South China University of

Technology, Wushan Road, Tianhe District, Guangzhou City, P.R. China, Postal Code: 510641

Email Address of Shiyu Fu*: [email protected]

KEYWORDS:

Nanocellulose,

Nano-Ink,

Rheology,

Surface

tension,

Polarizing

properties

ABSTRACT. Nanocellulose (CNC) is a sustainable substrate extracted from biomass with many excellent properties and a broad spectrum of potential applications. The prepared CNC with colloid properties is supposed to fabricate a Nano-Ink for gel pens

ACS Paragon Plus Environment

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which are the most popularly used writing instruments. The new proposal method by mixing CNC, reactive red 120 (RR120) and additive surfactant in water avoids the emission of volatile organic compounds as in conventional preparation of gel inks. We found that surface tension and viscosity of the Nano-Inks are key factors for their utilization in gel pen writing because simply mixing CNC and RR120 without controlling surface tension lead to ink breakage in high CNC concentration, and both ink breakage and ink accumulation in low CNC concentration. Surfactant takes a crucial role to adjust the surface tension and viscosity of the ink to a suitable range. After optimizing color depth with various proportions of RR120 and ink viscosity with different CNC concentrations, we found that, when CNC concentration was between 1.63% and 1.82%, the dosage of surfactant OP-10 was 0.04% and the proportion of RR120 was 2.5%, the prepared new Nano-Ink can write smoothly with deep-red color. This ink also performed with anti-counterfeiting feature because the CNCs are native polarizing materials.

INTRODUCTION


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Cellulose is the most abundant renewable biopolymer resource available in nature which has been used by humans for thousands of years.1-4 Because of its attractive properties, being not petroleum based and not posing-risk for the environment and human life,5-7 it has long been considered as the most advantageous renewable source.8,

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Nanocellulose, which is mainly derived from cellulose,4,

10-15

has also been

wildly studied by researchers recent years because of its better performance than cellulose, such as higher mechanical modulus,16,

17

higher specific surface area,18

special rheological19, 20 and liquid crystalline properties.21, 22 Gel pens (rollerball pens) have gradually become the most popular writing instruments23 since they were developed in 1984.24, 25 Their suitable viscosity between that of ballpoint pen and water-based ink pen give them a great advantage when writing.26 During writing process, the rotation of the ball at the pen tip will shear the adjacent ink to decrease their viscosity and change their state rapidly from "gel-state" to "sol-state" that can allow them flow out from the pen tip to the paper surface smoothly.27,

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When writing stops, the viscosity of the gel pen ink will recovery from

"sol-state" to "gel-state" immediately and the ink become immobile again.27,

28

Both


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massive ink accumulation easy to occur in ordinary ballpoint pen29, 30 and ink leakage prone to happened in water-based ink pen31 can be avoided when writing with a gel pen that was benefited from their suitable viscosity and sensitive thixotropic property. However, the factory production of these oil-based inks used in gel pens can discharge a large amount of toxic waste gas - VOCs (volatile organic compounds) which can be inhaled by workers and damage their liver and nervous system. Besides, the preparation of gel pen inks are very complicated with dozens of additives need to be added.32 In traditional preparation of gel pen inks, pigment was mixed with a specific dispersant to grinding to a stable color paste. Several additives and thickening agents were added to endow the ink with proper viscosity.32, 33 In this process, the nano-size effect of pigment, the interaction between dispersants, thickeners and various agents would easily lead to aggregation.34 Therefore, the traditional preparation processes of gel pen inks are very difficult and complicated that limited their development. By comparing the routine gel pen inks and nanocellulose, we found that they were very similar in rheological properties4, 35-37 and thixotropy properties.35, 38 Their viscosity both can decrease to a low level rapidly when shear begins, and will recovery back to a


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high value immediately when shear stops. Both of their rheological properties meet the characteristics of pseudoplastic fluids.39 Inspired by some scholars using dyes instead of pigments to prepare ink-jet printing inks,40-42 in our previous study,37 we successfully prepared an ink with reactive dye grafting on nanocellulose which exhibited good writing performance. However, limited by the dye amount grafting on nanocellulose, the ink color was lighter than routine ink in gel pens which may affect its readability. In the current study, instead of grafting dyes onto nanocellulose, a new Nano-Ink directly prepared by mixing nanocellulose, reactive red 120 and additive surfactant was proposed. The problem that ink based on nanocellulose need to be highly purified by centrifugation with the addition of ammonia bicarbonate and subsequent rotary evaporation to remove the added ammonia bicarbonate before it can be used as qualified ink in our previous paper was solved by controlling the surface tension. Thus, a new Nano-Ink based on renewable nanocellulose with deep-red color and good writing performance was successful prepared.

EXPERIMENTAL SECTION


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Materials. Bleached hardwood dissolving pulp board used to prepare CNCs was provided by Yibin paper industry CO., LTD, Sichuan, China. Sulfuric acid was purchased from Guangzhou Chemical Reagent Factory, Guangzhou, China. Reactive red

120

(RR120)

was

purchased

from

Sigma-Aldrich,

China.

Octylphenol

polyoxyethylene ether-10 (OP-10) was purchased from Yixing CO., LTD, Jiangsu, China. All of the chemical reagents were used as received. The brand of gel pens used to measure the ink color depth was Deli S761. Preparation of crystal nanocelluloses (CNCs). The preparation of CNCs was carried out in accordance with our previous published paper (AH-NH4HCO3 method).37 The process was as follows: 30g pulp board pieces and 240ml 58% sulfuric acid were added into a 500ml flask to hydrolysis for 45 minutes in 55°C water bath. After the reaction, most of acid in the reaction mixture was removed by centrifugation with a refrigerated centrifuge (sigma 3K15, 25 ° C) until the fibers cannot precipitated from water. Then, NaOH was added to neutralize the reaction mixture to pH = 7. Another two centrifugations were used to remove the neutralized Na2SO4 with 3%(w/L) ammonium bicarbonate added in the neutralized mixture. The final obtained precipitation was


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diluted and purified by vacuum distillation to remove ammonium bicarbonate. The purified pulp was homogenized with a homogenizer (Noozle MINI 11007, Noozle Fluid Technology (Shanghai) Co., Ltd, China). Then the prepared CNCs was stored in refrigerator ready for use. Characterization. A Nanoscope V Multimode atomic force microscope (AFM) (Bruker Corporation, anta Barbara, USA) was used to characterize the morphology of CNCs. An ARES-G2 rheometer (TA Instruments, U.K.) was used to measure the yield stress, sheer stress and viscosity of the new Nano-Ink at 0.5 r/s and 0.01 r/s with a 50mm cone plate at room temperature (25°C). A writing robot (XY-Plotter Robot 90014, Makeblock) was used to test the writing performance of the new Nano-Inks. A DCAT 21 Tensiometer (made in Germany by Dataphysics Instruments GmbH) was used to test the surface tension of the new Nano-Inks with a PT 11 wilhelmy plate (platinum-iridium, length: 10mm, width: 19.9mm, thickness: 0.2mm). Polarizing properties of CNCs and Nano-Inks were tested by a polarizing microscope (Olympus BX-51) with or without the adding of polarizer after CNCs and the new Nano-Inks were dried on a glass slide in


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room temperature. A TecHniDYne Color-ToucH PC, CTP-ISO was used to measure the color depth of the inks in ordinary gel pens and the new Nano-Inks based on CNCs.

RESULTS AND DISCUSSION Rheological properties of ordinary ink and nanocellulose. Referring to China's national standard, gel inks should be written smoothly and continuously at the speed of 4.5 m/min.43 According to this speed, we measured the rheological properties of routine gel pen (Deli S761) inks by ARES-G2 rheometer with a cone plate (diameter: 50 mm) with the rotation speed 0.5 r/s to obtain the rheological parameters of commercial inks during writing. It can be inferred that, inks that meet these rheological parameters can also be used as gel pen inks. Figure 1a shows the shear stress evolution of ordinary gel pen ink at 0.5 r/s (0 - 1 min, simulate the normal writing process) and 0.01 r/s (1 - 2 min, simulate the unwritten state). It is clearly that, when shear rate was 0.5 r/s, the initial shear stress increased first and then decreased. The maximum shear stress was called yield stress. It was generally accepted that,44, 45 before yield stress, ink was in non-flowing gel state. After


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yield stress, ink transferred to a flowable sol state. Yield stress was the minimum value required for gel inks changing from gel state to sol state. Referring to former researchers,46-48 if yield stress exceeded the suitable range, shear stress during writing may be higher than yield stress which may cause gel ink cannot converted from gel state to sol state, ink breakage would occur. If yield stress was below the suitable range, a small force can cause gel ink transfer from gel state to sol state, ink accumulation was easy to happen.


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Figure 1. Rheological properties of commercial gel pen inks and CNCs. The concentration of CNCs was 2.46%. The shear rate was 0.5 r/s before 1 minute and changed to 0.01 r/s after 1 minute. (a) Shear stress of the ordinary ink at 0.5 r/s and 0.01 r/s. (b) Viscosity of the ordinary ink at 0.5 r/s and 0.01 r/s. (c) Shear stress of nanocellulose at 0.5 r/s and 0.01 r/s. (d) Viscosity of nanocellulose at 0.5 r/s and 0.01 r/s.


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Figure 1b shows the viscosity evolution of commercial gel ink at 0.5 r/s (0 - 1 min) and 0.01 r/s (1 - 2 min). Similar to shear stress, the maximum viscosity in the beginning was yield viscosity. According to some literatures,45, 48 if the value of yield viscosity was too high, the ink cannot convert form gel state to sol state during normal writing. If the yield viscosity was too small, it was easy to cause ink leakage even without writing. It was also clearly in Figure 1b that, when shear rate changed from 0.5 r/s to 0.01 r/s, the ink viscosity increased instantly that was a simulation of the writing complete instant with gel pens. In this brief moment, the ink state changed from flowing sol state to the nonflowing gel state. The sooner the conversion process ends, the less ink will accumulate after writing. Yield stress, yield viscosity and thixotropic recovery property are the necessary features of gel inks.48 Benefited from these special properties, the ink in gel pens was in a non-flowing state without writing, but can smoothly flow out when gel pen tip sliding on a paper with no ink accumulation. However, compared with yield stress and yield viscosity only affected the beginning and end of writing, the ink viscosity during writing was a more important parameter which directly determines the writing performance. It can be calculated that, when the


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rheometer rotates at 0.5 r/s (with a 50 mm cone plate), its corresponding maximum linear velocity is about 4.71 m/min that is similar to 4.5 m/min.43 Therefore, ink viscosity measured by rheometer at 0.5 r/s was used as an evaluation criterion to measure ink performance in current study. Inspired by previous studies on rheological properties of nanocellulose, for example, Klemm et al.49 found that nanocellulose was a pseudoplastic fluid which was the same as gel inks, the feasibility of preparing gel pen inks by mixing nanocellulose (CNCs, Figure S1) and reactive dyes (RR120) was studied. Firstly, we tested the rheological properties of CNCs using the same experimental conditions as ordinary gel pen inks. The shear stress and viscosity of CNCs at 0.5 r/s and 0.01 r/s were showed in Figure 1c and Figure 1d. It is clearly showed that the shear stress and viscosity of CNCs (concentration was 2.56%) were similar to that of ordinary gel pen ink. Both CNCs and gel pen inks had similar yield stress, yield viscosity, and thixotropic recovery properties. Therefore, we believe that it is feasible to prepare gel pen inks based on CNCs. Rheological properties and surface tension of the new Nano-Ink.


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Inspired by the similarity of viscosity between CNCs and commercial gel pen inks, we prepared an ink by mixing CNCs with the common water-soluble dye Reactive Red 120 (RR120). However, it is disappointment that the new ink was unsuccessful which cannot flow out from gel pen tip. The reasons may come from too high CNC concentration and excessive dosage of RR120. Because the increase in CNC concentration49 and RR120 dosage may both lead to a higher shear stress and viscosity. Figure 2a and Figure 2b shows the evolution of shear stress and viscosity with the increase proportion of RR120. When the dosage of RR120 was less than 0.06% w/w, the ink shear stress and viscosity decreased with the addition of RR120. With the continuous increase of RR120 dosage, the shear stress and viscosity increased significantly. Based on our previous analysis, this increase may responsible for the ink breakage because viscous ink cannot flow out from gel pen tip. Therefore, we reduced the CNC concentration to offset the viscosity increase of the new ink caused by the addition of RR120. Unexpectedly, the reduction of CNC concentration cannot effectively stop ink breakage even if the viscosity of the new ink was much lower than ink in commercial gel pens, even if the writing accompanied with serious ink accumulation.


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Figure 2. The effect of RR120 dosage on shear stress (a), viscosity (b) and surface tension (c) of the new Nano-Inks. In figure 2a and figure 2b, the shear rate before 1 min was 0.5 r/s, the shear rate between 1-2 min was 0.01 r/s.


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Actually, Miyamoto in a patent of ballpoint pens45 pointed out the importance of surface tension of inks, and suggested the surface tension of ballpoint pens should be controlled in a suitable range, or the blobbing and splitting would happen. For water based ink50, if the ink had low interfacial tension to a paper surface, it can be sufficient penetrated on writing paper. We further inferred that the ink breakage may be caused by inappropriate surface tension, and then we measured the surface tension of the new ink with different RR120 amounts. It is found that the addition of RR120 did increase the surface tension of CNCs (Figure 2c). And, the surface tension of pure water was about 72 mN/m51 higher than that of routine gel pen inks (about 40 mN/m).52 The reducing of CNC concentration can reduce surface tension of the new ink (Figure S4). However, in any case, the surface tension was still much higher than 40 mN/m which was the suitable value of gel pen ink. Therefore, it is inferred that the main reason for ink breakage was the increase of surface tension caused by the addition of RR120. It is usually considered that, the reason for the formation of surface tension is that water molecules at the two-phase (water & air) interface are attracted much more by water molecules than by air molecules (Figure 3b, Fw > Fa).53 Water molecules at the


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interface tend to shrink to the interior of water, resulting the density of water molecules on the two-phase interface is much less than that inside the water. This means the distance between water molecules at the interface is longer than that between normal internal water molecules. According to the theory of intermolecular force, the increasing distance (Figure 3a) between molecules makes the intermolecular attraction stronger. The parameter surface tension (Fsf) is used to characterize this attractiveness.53


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Figure 3. Mechanism of surfactant reducing surface tension. (a) A schematic diagram of intermolecular forces. (b) Formation mechanism of water surface tension. (c) Mechanism of ions increasing surface tension and surfactant reducing surface tension. (d) Variation of the new ink's surface tension with increasing surfactant OP-10. In figure 3d, the concentration of CNCs was 2.46%, and the addition of RR120 was 0.15%.


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With the addition of RR120, the attraction between ions (Figure 3c, Fi) increased the contraction forces between molecules including the molecules at the two-phase interface that leads to an increase in surface tension.54 Generally, the surface tension of paper for writing was around 40 mN/m.52 As can be seen from Figure 2c, the surface tension of the new ink was much higher than 40 mN/m with the addition of RR120. When flowing out form the gel pen tip, the new ink cannot wet the paper sufficiently to be absorbed on it. Resulting the strange phenomenon that ink accumulation and breakage occurred at the same time during writing, when CNC concentration was relatively low. Therefore, surface tension must be adjusted to an appropriate range in the preparation of the new ink based on CNCs. Surfactants, known as " Industrial Catalysts ", can significantly reduce surface tension with small amounts.53 Normally, one end of surfactant are hydrophilic groups and the other end are hydrophobic groups.55 When added in water, surfactants are aligned at the two-phase interface, hydrophilic groups extend into water and hydrophobic groups extend into air. Hydrophobic groups tend to move away from water, resulting in forces from liquid to air which can counteract part of the hydrogen bonding forces, van der


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Waals forces and ion attractions forces between cations and anions. Because these two opposing forces can counteract each other, the shrinkage tendency of the molecules on the two-phase interface towards internal water was reduced which can decrease surface tension. The mechanism was shown in Figure 3c. Figure 3d shows the variation of the new ink's surface tension with the addition of surfactant OP-10 (octylphenol polyoxyethylene ether-10). It clearly showed that, surface tension of the new ink decreased significantly when OP-10 was added. However, when the proportion of OP-10 was 0.02%, the surface tension decreased to a stable value. When excessive OP-10 was added, the surface tension no longer decreased significantly. In order to further verify the effect of surfactant OP-10 on surface tension reduction, we measured the surface tension of CNCs with different concentrations before and after adding surfactant OP-10. Results showed that OP-10 can significantly reduce the surface tension of CNCs with different concentrations. (Figure S4)


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Figure 4. The effect of OP-10 dosage on shear stress (a) and viscosity (b) of the new ink. The CNC concentration was 2.46% and RR120 usage was 1.0%.

The effects of surface tension on shear stress and viscosity of the new Nano-Ink were also studied (Figure 4). Results showed that, shear stress and viscosity can also be significantly reduced by the addition of surfactant. However, similar to the surface tension, shear stress and viscosity did not decrease with the excessive addition of OP10 when they were reduced to a stable value. As can be seen from Figure 4a and Figure 4b, when the dosage of OP-10 was 0.04%, it was enough to reduce the viscosity and shear stress to a stable value. Although excessive OP-10 did not adversely affect surface tension, shear stress and viscosity, however, it is known that bubbles were


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prone to occur during agitation with excessive surfactants. Therefore, after ensuring surface tension, shear stress and viscosity were reduced to a suitable range, the dosage of OP-10 should be controlled at a low level as much as possible. According to the above analysis, we finally choose 0.04% as the suitable proportion of OP-10 used in the new Nano-Ink. Color depth and viscosity of the new Nano-Ink. In our previous paper, we prepared an ink by grafting RR120 onto nanocellulose.37 However, all unreacted RR120 must be washed away completely because the existence of ionic RR120 can increase the ink's surface tension which may cause ink breakage occurred during writing. Limited by the amount of grafting RR120 on nanocellulose, the color depth of that ink is lighter than commercial inks in our previous paper. A qualified ink requires not only can flow smoothly from the gel pen tip to paper surface, but also need sufficient color depth to make the handwriting clear. The color depth of the new Nano-Ink is directly determined by the amount of RR120. Therefore, we also need to optimize the amount of RR120 used in the new Nano-Ink. In this experiment, commercial gel pen inks (Deli S761) and different concentrations of RR120 were adsorbed on filter paper. After drying, a residual


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ink detector was used to measure their color depth. Results showed that the color luminosity of these filter papers decreased with the increase of RR120 concentration (Figure 5a, Figure 5b). However, when the concentration of RR120 was higher than 2.5%, the decrease of luminosity was no longer significant. Under the same conditions, the luminosity of commercial gel pen inks (Deli S761) was 33.18, not much different from that of the filter paper immerged in 2.5% RR120. Considering the economic cost and the writing effect, in the current study, the proportion 2.5% was chosen as the appropriate amount of RR120 used in the new Nano-Ink. Compared to our previous paper, the maximum grafting amount of RR120 on nanocellulose was 280 mg/ml, and the concentration of nanocellulose used for ink was 5.96%,37 the calculated proportion of RR120 in that ink was 1.67%. It can be calculated form Figure 5a, the corresponding luminosity was about 44, much higher than the luminosity of commercial gel pen inks 33.18. Therefore, the ink prepared in our current study has great advantage in luminosity.


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Figure 5. The influence of RR120 dosage on ink color depth and the influence of CNC concentration on ink viscosity and writing performance. (a) The influence of RR120 dosage on color depth of the new Nano-Ink measured by residual ink tester. (b) Filtration paper with different color depths prepared with different RR120 dosages corresponding to (a). (c) Evolution of the new Nano-Ink’s viscosity with the increasing CNC concentration. RR120 usage was 2.5%, and OP-10 dosage was 0.04%. (d)


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Comparison of the writing performance of the new Nano-Inks with different CNC concentrations.

Through the above analysis, the optimum dosage of surfactant OP-10 and dye RR120 were determined. Based on these two optimum dosages, we further studied the influence of different CNC concentration on the new Nano-Ink writing performance. Figure 5c shows the evolution of the new Nano-Ink’s viscosity with the increasing CNC concentration, when the dosage of OP-10 was 0.04% and the amount of RR120 was 2.5%. It clearly shows that, viscosity of the new Nano-Ink increased gradually with the increase of CNC concentration. In order to find the appropriate range of the new Nano-Ink’s viscosity and the corresponding CNC concentration, a writing robot was used to test the writing performance of the new Nano-Inks with different viscosity (Video. S1). These handwriting effects were shown in Figure 5d. When CNC concentration was 1.45%, there would be ink accumulation occurred. And when CNC concentration was 2.00%, there would be obvious ink breakage happened. However, when CNC concentration


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was 1.63% and 1.82%, the handwriting was clear, with no ink breakage or ink accumulation occurred. So, when CNC concentration was between 1.63% and 1.82%, the dosage of surfactant OP-10 was 0.04% and the usage amount of dye RR120 was 2.5%, the new Nano-Ink can meet the requirements of commercial inks used in gel pens. It can also be inferred from Figure 5c that the optimum viscosity of the new NanoInk for writing was between 85.10 and 156.13 mPa·s. Polarizing properties of the new Nano-Ink. The new Nano-Inks not only inherited the special viscosity of CNCs, but also inherited their polarization properties which can be used for anti-counterfeiting.56 Unlike our previous paper,37 in the current study, the dried handwriting of the new Nano-Ink was detected, not liquid ink. The preparation process was shown in Figure S3. The results showed that the dry handwriting of the new NanoInk exhibited a clearer polarization. Figure 6a and Figure 6b were the photos of original CNCs taken with a polarizing microscope without or with polarizer. Figure 6c and Figure 6d were the photos of the new Nano-Inks without or with polarizer. It can be easily figure out in these pictures that glass slide was bright when illuminated by natural light (Figure 6a and Figure 6c), but turned to dark in polarized light (Figure 6b and Figure


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6d). However, unlike glass slide, the CNCs and the new Nano-Inks were still bright whether the polarizer was added or not. Therefore, it can be inferred that, the new Nano-Ink inherited the polarizability of CNCs which can be used for anti-counterfeiting.

Figure 6. Polarization properties of CNCs and the new Nano-Ink. (a) CNCs and glass slide under natural light. (b) CNCs and glass slide under polarized light (Same part of the same sample as (a)). (c) The new Nano-Ink and glass slide under natural light. (b)


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The new Nano-Ink and glass slide under polarized light (Same part of the same sample as (c)).

CONCLUSIONS A qualified Nano-Ink based on CNCs for gel pens was successfully manufactured by adjusting surface tension and viscosity of ink with surfactants, while it cannot be produced just by mixing CNC colloid suspension and RR120. During the manufacture process, the shear stress, viscosity and surface tension of CNC suspension increased significantly with the addition of RR120. The addition of surfactants effectively reduced surface tension, shear stress and viscosity to a suitable range for gel ink. The color depth of the new Nano-Inks was optimized by changing the Proportion of RR120. With the optimum dosage of surfactant OP-10 and RR120, we determined the optimum CNC proportion in the new Nano-Ink by comparing the handwriting effect with different CNC concentrations. In addition, the polarizing property of the new Nano-Inks has been proved and can be used for anti-counterfeiting.


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ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: Morphological characterization and length distribution of CNCs prepared by AHNH4HCO3 method. The writing process video of a writing robot with a gel pen refilled with the new Nano-Ink based on nanocellulose. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID Wenbo Wang: 0000-0002-5176-1526 Shiyu Fu: 0000-0001-5680-7493 Notes


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The authors declare no competing financial interest.

ACKNOWLEDGEMENT

This work was supported by the National Natural Science Foundation of China (31570569), the Science and Technology Program of Guangzhou (201704020038), National Key R&D Program of China (2017YFD0601003), and the foundation of the State Key Laboratory of Pulp and Paper Engineering (2018PY01).

REFERENCES

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TOC/ABSTRACT GRAPHIC

Synopsis A new deep-red gel pen ink based on renewable nanocellulose was prepared in a simple, green and non-polluting process which can avoid the emission of volatile organic compounds in factory preparation of gel inks.


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Strategy for Manufacturing a Deep-Red Ink Based on Nanocellulose

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