Volume 8, Issue 25 e202300305
Research Article
Open Access

Characterization of Coating Films of Thixotropic Solvent Gel Containing Organo-modified Nanodiamonds

Yuka Hasunuma

Yuka Hasunuma

Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama, 338-8570 Japan

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Momo Maeda

Momo Maeda

Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama, 338-8570 Japan

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Yuki Mashiyama

Yuki Mashiyama

Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama, 338-8570 Japan

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Nanata Kikuchi

Nanata Kikuchi

Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama, 338-8570 Japan

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Dr. Atsuhiro Fujimori

Corresponding Author

Dr. Atsuhiro Fujimori

Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama, 338-8570 Japan

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First published: 05 July 2023
Citations: 2

Graphical Abstract

A gelation solvent containing a diamide-based thixotropic additive with two hydrocarbons and organo-modified nanodiamonds was prepared and evaluated herein as a coating film. Nanodiamonds were introduced into a solvent containing diamide-based thixotropic additive molecules with 12-hydroxystearyl chains which were uniformly dispersed due to organo-modification on the outermost layer surface.

Abstract

A gelation solvent containing a diamide-based thixotropic additive with two hydrocarbons and organo-modified nanodiamonds was prepared and evaluated herein as a coating film. Nanodiamonds were introduced into a solvent containing diamide-based thixotropic additive molecules with 12-hydroxystearyl chains which were uniformly dispersed due to organic chain modification on the outermost layer surface. In addition to common solvents including toluene, ethyl cetyl hexanoate, a cosmetic emollient, forms nanodiamond-containing gels. The helical nanofibers formed by the diamide-based thixotropic additive molecule with two hydrocarbons in the gel showed affinity for the organo-modified nanodiamonds, and the contact morphology between the nanofibers and nanodiamond particles was confirmed. Nanodiamonds exhibited a denaturation effect on biomolecules adsorbed on their surfaces. Overall, nanodiamond-containing emollient gels are expected to exhibit antibacterial properties against viruses and malodorous bacteria on skin as part of thixotropic immobilization films for cosmetic powders.

Introduction

Among surface modification techniques,1 modifying the surface of inorganic nanoparticles with organic chains of surfactant molecules2 is valuable for expanding material functionality. Most inorganic nanoparticles are hydrophilic because the outermost layer is universally modified by hydroxyl groups.3 Therefore, it is inherently difficult to disperse these particles in organic solvents and organic solids such as polymers. However, by modifying the surface of inorganic nanoparticles with organic molecular chains, a nanodispersed state can be achieved that mimics dissolution in an organic solvent.4 In contrast, it is possible to disperse them uniformly in organic solids, such as polymers, which facilitates the preparation of organic/inorganic hybrids5 and the manifestation of functions unique to inorganic components.6 For inorganic nanoparticles, surface modification with organic chains can be used to prepare solutions with organic solvents and composite materials containing organic solids (Figure 1(a)). In addition, the hydrophilicity of the inorganic nanoparticle surface and the hydrophobicity of the modified chains impart amphipathic properties to the organo-modified inorganic nanoparticles.7 Overall, organo-modified inorganic nanoparticles at the air/water interface8 can be used to prepare monolayers on water surfaces/Langmuir monolayers,9 like surfactant molecules.

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Schematic illustrations of (a) the usefulness of organic modification of the nanoparticle surface, (b) anti-drip effects of thixotropic additives in paint, and (c) antibacterial nanoparticle-containing coating gel film preparation used herein.

The excellent functions of inorganic nanoparticles have also been effectively utilized for cosmetics. Inorganic nanoparticles, which have large surface areas and can be dispersed and integrated two-dimensionally, have been used in applications including photocatalysts,10 lubricants,11 and abrasives.12 The main roles of nanoparticles in cosmetics are as UV scattering agents in sunscreens and foundation pigments. Titanium oxide (TiO2),13 zinc oxide (ZnO),14 and cerium oxide (CeO2)15 have high refractive indices that effectively scatter UV light16 and absorb it through inter-bandgap transitions to protect the skin.17 In addition, TiO2 with a particle size of approximately 200 nm exhibits high light-scattering ability and concealing ability to hide blemishes and freckles in cosmetic films.18 TiO2 particles several tens of nanometers in size can protect the skin from sunburn while maintaining the foundation transparency.19 In addition, pearl pigments can control the refraction of transmitted light by coating the mica surface with titanium oxide, resulting in a pearl-like translucent color.20

Recently, global pandemics have become top of mind due to the spread of infectious and novel viruses. In addition to pathogenic bacteria, mold and malodorous bacteria have adverse effects on the skin and human body, with measures such as disinfection and wearing surgical masks implemented for infection control. Nanodiamonds with a particle diameter of 5 nm prepared by the detonation method21 are a type of nanocarbon material similar to fullerene,22 graphene,23 and carbon nanotubes.24 These nanodiamonds have been reported to exhibit antibacterial effects against viruses, molds, and malodorous bacteria.25 Viruses adsorbed on the nanodiamond surface are immobilized, their proliferation is suppressed, and they eventually die.26 Thus, if a small amount of nanodiamonds can be uniformly dispersed and introduced into decorative coating films, antibacterial properties, UV scattering, and pigment effects can be achieved. In particular, Figure S1 in the Supporting Information shows the changes in the circular dichroism (CD) spectra of the Cytochrome C protein before and after adsorption onto a nanodiamond surface.

Thixotropy27 has been shown to be important for decorative coatings to impart fluidity when applied, but it gels and adheres after application to prevent dripping (Figure 1(b)). Techniques for introducing thixotropic additives28 are used to artificially impart thixotropic properties to solvents. Approximately 1 wt% thixotropic agents introduced into symmetric solvents form nanofibers by hydrogen bonding, and the sponge-like structure formed by the entanglement of these nanofibers traps the solvent, leading to gelation.29-31 Reflow occurs when hydrogen bonds are broken by applying an external force.32

Herein, we attempted to immobilize a cosmetic powder and gel ethyl cetyl hexanoate/emollient33 using a diamide-based thixotropic additive molecule with two 12-hydroxystearyl chains34 (Figure 1(c)). Furthermore, we introduced nanodiamonds into the gel, prepared a thixotropic gel containing antibacterial nanoparticles, and characterized the newly prepared materials. In addition, the morphology of a simple coating of the novel solvent gel was determined along with the microstructure of the coating film35, 36 and dispersion/aggregation characteristics of the nanofibers and nanoparticles.

Results and Discussion

Preparation of the nanodiamond-containing coating gel film

Figure 2(a) shows the gelation behavior of the toluene : ethanol=9 :1 (v/v) mixed solvent. As described in the experimental section, translucent solvent gels can be prepared by adding 1.0 wt% N, N’-(hexane-1,6-diyl)bis(12-hydroxyoctadecanamide) (abbrev. 2 C18(OH)-dA), a diamide with two hydrocarbons, as a thixotropic additive. In addition, gelation was achieved by adding 0.3 wt% 2 C18(OH)-dA and 0.05 wt% organo-modified nanodiamonds. The transparency was slightly inferior to that of the solvent gel obtained using only 2 C18(OH)-dA.

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Photographs (a) showing solvent gelation containing thixotropic additives and organo-modified nanodiamonds, and (b) of coating film formation used herein and its film appearance.

Figure S2 of the Supporting Information shows the details of the solvent dispersibility and concentration of the organo-modified nanodiamonds. Figure S2(a) shows the state of dispersion in a toluene : ethanol=9 : 1 mixed solvent and hexane, which is a non-polar normal alkane. When the concentration of organo-modified nanodiamonds was fixed at 0.05 wt%, the dispersion was good in the toluene : ethanol=9 : 1 mixed solvent, but precipitation was visually confirmed in hexane.

When evaluating the concentration of the organo-modified nanodiamonds in the mixed toluene : ethanol=9 : 1 solvent in Figure S2(b), the dispersion state was acceptable at 0.05 wt% loading. However, at increased loadings of 0.10 and 0.15 wt%, significant precipitation was observed. In other words, the optimal condition was determined to be 0.05 wt% loading in the toluene : ethanol=9 : 1 mixed solvent.

Figure 2(b) shows a photograph of the coating preparation where the heated gel on the spatula regained its fluidity and could be applied and smoothly solidified as a gel after application. The coating gel film applied with a spatula showed a thickness of 2–3 μm as determined using a film thickness gauge.

Figures 3(a) and (b) show the POM and AFM images, respectively, of the gel coating applied to the substrate via the process shown as following experimental section. The POM image confirmed the distribution of the 2 C18(OH)-dA nanofibers in a homogeneous gel. Particles with a diameter of approximately 500 nm, the approximate wavelength of visible light, were also confirmed.

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(a) POM and (b) AFM images of a coating gel film containing thixotropic additives and organo-modified nanodiamonds.

Furthermore, high-magnification AFM observations confirmed that the nanofibers formed by 2 C18(OH)-dA exhibited spiral growth. Previous studies have shown that 2 C18(OH)-dA with asymmetric carbons form chiral helical fibers, which efficiently incorporate solvent molecules improving gelation properties.37 In addition, it was confirmed that particulate matter with a diameter of <50 nm and hardness higher than that of the surrounding fibers were uniformly dispersed. It is highly likely that the particulate matter with diameters of <50 nm to approximately 500 nm, as confirmed by POM and AFM, are nanodiamond aggregates. Hypothetically, if this gel coating is used as an immobilization film for cosmetic powders, the large aggregated particles may promote UV scattering effects. In addition, small aggregated particles may exhibit a pigment effect owing to the structural color derived from light interference. Considering the antibacterial properties of nanodiamonds and the thixotropic properties of 2 C18(OH)-dA, these preparations are expected to be highly functional.

Evidence for the presence of nanodiamonds in thixotropic coating gel films

Figure 4 shows the Raman mapping results for the coating gel films and Raman spectra at specified positions. There are limited spectroscopic methods to directly examine nanodiamonds, but Raman spectroscopy is one of the few effective evaluation methods to detect abundant C−C bonds with excellent symmetry. Figure 4(a) shows the Raman mapping/Raman spectra for a coating gel film without organo-modified nanodiamonds. A characteristic bandwas confirmed near approximately 900 cm−1 in the spectrum obtained from the central part of the coating film. Using this band as a guide, we evaluated a gel coating containing organo-modified nanodiamonds (Figure 4(b)).

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Raman mapping and Raman spectra of the coating gel film containing (a) only thixotropic additives (b) thixotropic additives and organo-modified nanodiamonds (position at nanodiamond aggregates) and (c) thixotropic additives and organo-modified nanodiamonds (outside nanodiamond aggregates).

For the mapping, we focused on the black spots in the optical microscope image where nanodiamonds were suspected and a Raman band at approximately 1330 cm−1 derived from the nanodiamonds was confirmed.38 In Figure 4(c), the Raman band at approximately 1330 cm−1 disappeared when focused outside the black spots, as confirmed by the mapping. Therefore, thepresence of nanodiamonds in the coating film and its dispersion was confirmed by Raman mapping.

Figure 5 shows the IR spectrum measurements of the coating gel films measured before and after organo-modified nanodiamond addition. Although it is difficult to directly confirm the presence of nanodiamonds by IR, the appearance of the C=O stretching vibration (1702 cm−1) derived from stearate used for organo-modification indirectly suggests the presence of organo-modified nanodiamonds. From the above investigation, it can be concluded that nanodiamonds were present in the coating film. Figure S3 shows the IR spectrum of only organo-modified nanodiamonds. The tendency for the carbonyl band of stearic acid bound to nanodiamonds to appear at 1702 cm−1 is the same. A comparison of these results confirms the introduction of nanodiamonds into the coated film.

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IR spectra of the coating gel film containing (a) only thixotropic additives and (b) thixotropic additives and organo-modified nanodiamonds.

Characterization of the nanodiamond aggregation state in coating gel films and application to a cosmetic preparation gel

Figure 6 shows the out-of-plane XRD profiles of the coating gel films before and after the introduction of organo-modified nanodiamonds. Previously, the periodic structure of 2 C18(OH)-dA-forming nanofibers was analyzed and showed an approximately 5 nm regularly layered structure along the c-axis direction.39 In Figure 6(a), a shoulder peak at approximately 5.1 nm was confirmed in the XRD profile before the introduction of the organo-modified nanodiamonds. Previously, the periodicity was higher because the 2 C18(OH)-dA fiber films were highly ordered as Langmuir-Blodgett (LB) films.40 Herein, the diffraction intensity was slightly higher than the direct beam intensity of XRD because of the textured structure in the coating gel film. Using this signal as a guide, the coating gel film containing the organo-modified nanodiamonds was evaluated (Figure 6(b)).

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Out-of-plane XRD profile of the coating gel film containing (a) only thixotropic additives and (b) thixotropic additives and organo-modified nanodiamonds.

In addition to the above-mentioned shoulder, a diffraction peak corresponding to the layered nanodiamonds was confirmed at approximately 3.9 nm, arising from the nanodiamond aggregates 50–500 nm in size. Although this diffraction peak is sharp, its low intensity indicates it is not abundant but highly regular.

Figure S4 shows the out-of-plane XRD profiles of the LB multilayers of the organo-modified nanodiamonds. When arranged on an LB film, the layered periodicity of the organo-modified nanodiamonds was approximately 5.1 nm and the diffraction intensity was quite high,reflecting a high degree of periodicity. The long-period value was shortened from 5.1 to 3.9 nm because the nanodiamond array in the upper layer was likely stuck in the lower layer, as shown in the model presented in Figure 6(b). In this state, the distance between the nanoparticle centroids along the c-axis decreases, resulting in the change in the long-period value. Considering that the conformation of the upper and lower layers was slightly shifted owing to lateral friction during coating, this is a reasonable explanation.

In addition, as shown in Figure S5 in the Supporting Information, the in-plane XRD profile, which indicates the order of the in-plane direction of the coating film, shows a diffraction peak of the crystalline fiber film derived from 2 C18(OH)-dA, even after nanodiamond addition. This indicates that the presence of nanodiamonds does not inhibit the structural formation of thixotropic additive molecules and suggests that the gelling ability and unique properties of nanodiamonds can coexist.

Herein, we aimed to prepare a gel coating film that is similar to cosmetic formulations containing the gel cetyl ethylhexanoate, a typical emollient for cosmetics. As shown in Figure 7, a translucent emollient gel containing organo-modified nanodiamonds was prepared. When coated, it was possible to obtain a gel with a luster that complicated the photography procedure due to the glossiness of the surface and light scattering. By further controlling the aggregation structure of nanodiamonds, a coating film that provides cosmetic benefits and protection to the skin was obtained.

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(a) Photograph of the cetyl ethylhexanoate gel and coating gel film containing thixotropic additive and organo-modified nanodiamonds. (b) Viscosity measurements of the cetyl ethylhexanoate emulsion containing thixotropic additives and organo-modified nanodiamonds.

Furthermore, Figure 7(b) shows the viscosity of an emollient agent containing 2 C18(OH)-dA thixotropic additive molecules and organo-modified nanodiamonds. Initially, the solution was in a gel state as indicted by the extremely high viscosity. The viscosity gradually decreased during spindle rotation and after 120 min, when the gel disappeared completely, the viscosity began to increase slightly. This behavior after 120 min is consistent with thixotropic fluid properties41 where the viscosity increases as a function of time.

The results shown in Figure 8 summarize the findings described herein. The toluene : ethanol=9 : 1 mixed solvent and cosmetic emollients contained 0.3 wt% thixotropic additive 2 C18(OH)-dA and 0.05 wt% organo-modified nanodiamonds and were used to prepare a coatable gel. In the gel, 2 C18(OH)-dA formed helical nanofibers and the organo-modified nanodiamonds were dispersed homogeneously without phase separation. The organo-modified nanodiamonds formed agglomerated particles approximately 500 nm in size in the mesoscopic region and aggregates at approximately 50 nm in the microscopic region. The aggregated state of 2 C18(OH)-dA exhibited a higher order in the in-plane direction than in the vertical direction, and the presence of organo-modified nanodiamonds did not inhibit structure formation. In addition, the organo-modified nanodiamonds seemed to exhibit homogeneous regularity in the coating film, albeit in a relatively small abundance.

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Summarized results of this study and structural model diagram proposal.

In addition, the result of refractive index measurement of the solution and coating film as an index of dispersibility was 1.4845 in toluene : ethanol=9 : 1 mixed solvent, and the samevalue was obtained for the dispersion medium containing organo-modified nanodiamonds. At this time, the refractive index value of water at 25 °C was 1.3331. In addition, even in the gel-like coating film containing 0.3 wt% of thixotropic additive, no change in refractive index was observed before and after introducing 0.05 wt% of organo-modified nanodiamonds with a refractive index of slightly less than 2.5. These results are considered to indicate the uniform dispersibility of organo-modified nanodiamonds in the solvent and gel.

Furthermore, a toluene : ethanol=9 : 1 mixed solution containing 0.3 wt% thixotropic additive and 0.05 wt% organo-modified nanodiamonds was evaluated simply and relatively using antibacterial test paper. The solution was dropped onto a test paper containing 2,3,5-Triphenyl tetrazolium chloride (TTC) as a coloring agent, stored at 37 °C for 6 hours, and after culturing bacteria, the average number of spots was confirmed. Although it is only a relative comparison, the appearance of TTC spots is suppressed, and this result indicates the usefulness of the nanodiamond-containing medium.

Conclusions

Diamide-based thixotropic additives, which are common anti-dripping agents in automotive paints, have bio-similar amide bonds and are predicted to have high applicability in cosmetic preparations. In addition, with the recent global pandemic, a significant demand has developed for skin preparations with strong antibacterial effects. The addition of a small amount of diamide-based thixotropic additives and organo-modified nanodiamonds to gel and coating formulations created structures with controllable particle aggregation sizes. The aggregated particles in the solvent gel were clearly confirmed by POM and AFM as well as Raman and IR spectroscopy. The combined use of the two types of XRD methods enabled the clear characterization of the aggregation state of the diamide-based thixotropic additives and organically modified nanodiamonds. In the future, we hope that the antibacterial effects of nanodiamonds will be incorporated into cosmetic foundations to become widely used as highly antibacterial formulations.

Experimental Section

Synthesis of Gemini-type amphiphilic diamide derivatives

Gemini-type amphiphilic diamide derivatives were synthesized via condensation of R-12-hydroxystearic acid and hexamethylenediamine at a 2 : 1 molar ratio.34 R-12-hydroxystearic acid and hexamethylenediamine were commonly purchased from Wako Pure Chemical Industries, Ltd. Triphenylphosphite and pyridine were used as the condensing agent and catalyst, respectively, in the synthetic reaction. Triphenyl phosphite and pyridine were purchased from Tokyo Chemical Industry Co., Ltd. and Wako Pure Chemical Industries, Ltd., respectively. The resulting compound was confirmed as N, N′-(hexane-1,6-diyl)bis(12-hydroxyoctadecanamide) (abbrev. 2 C18(OH)-dA; Figure 9(a)).

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(a) Chemical structures of 2C18(OH)-dA and its raw materials. (b) Schematic diagram of the preparation method for organo-modified nanodiamonds. (c) Schematic diagram of the thixotropic coating gel film preparation.

Preparation of organo-modified inorganic nanoparticles and evaluation of denaturation behavior of absorbed biomolecules

Nanodiamonds with a diameter of 5 nm were prepared by the detonation method21 (provided by Daicel Corporation) as inorganic nanoparticles. Stearic acid (Wako Pure Chemical Industries, Ltd.) was added as an organic modifier to an aqueous solution of nanodiamonds at a 50 wt% loading. Subsequently, toluene as a water-insoluble organic solvent and methanol as an amphoteric organic solvent were added to the mixture. Toluene and methanol were purchased from Wako Pure Chemical Industries, Ltd. After stirring, the solution volume was reduced to approximately one quarter of the original amount using an evaporator (EYELA N-1110). Subsequently, methanol and toluene were added, stirred again, and repeated 9–10 times. Subsequently, unreacted substances and methanol were removed using a separating funnel to obtain a dispersion of stearic acid-modified nanodiamond (Figure 9(b)) wherein the solvent was substituted with toluene. We also investigated the denaturation behavior of biomolecules adsorbed on the nanodiamond surface using CD spectroscopy (JASCO J-600 spectrometer). Ultraviolet-visible (UV-vis) spectroscopy was used as a detector for the CD spectrum, while the UV spectrum was measured in advance (JASCO V-650 spectrometer). Cytochrome C protein (C2506-100MG, Sigma-Aldrich) from horse heart muscle was used as a representative biomolecule to evaluate denaturation behavior during nanodiamond surface adsorption.

Specifically, Figure S1 in the Supporting Information shows the changes in the CD spectra of the Cytochrome C protein before and after adsorption on the nanodiamond surface. The intensity of the distinct negative Cotton effect at approximately 200 nm, confirmed before nanodiamond adsorption, diminished, and the Soret band at approximately 400 nm also changed. These changes strongly suggest denaturation of the second-order structure of Cytochrome C via adsorption on the nanodiamond surface.

Coating film formation

First, 0.05 wt% organo-modified nanodiamonds were dispersed in a toluene: ethanol (9 : 1, v/v) mixed solution and heated to 100 °C to dissolve 1.0 wt% of the thixotropic additive. A thixotropic solvent gel coating containing organo-modified nanodiamonds was formed by coating with a spatula heated to approximately 50 °C (Figure 9(c)). A similar coating film was formed on hexadecyl 2-ethylhexanoate, an emollient agent for cosmetics, with a thixotropic additive loading of 0.3 wt%.

Characterization of coating gel films for thixotropic solvents containing organo-modified nanodiamonds

The surface morphology of the coating film was characterized using a polarized optical microscope (POM; OLYMPUS BX51) and an atomic force microscope (AFM; Bruker AXS Multimode 8, Si tip, spring constant 26 N ⋅ m−1, Tapping Mode). The hydrogen bonding states and various functional groups were evaluated using infrared (IR) spectroscopy (Jasco FT/IR-4200). Raman spectra and mapping (Renishaw inVia Microscope-Laser Raman spectrophotometer; measurement laser wavelength: 532 nm; grating: 1800 gr/mm) were used for spectroscopic analyses to determine the presence of nanodiamonds in the films. In addition, the structure of the paint film was evaluated by X-ray diffraction (XRD). In-plane XRD was performed using a Bruker AXS MXP-BX instrument with a multilayer mirror, CuKα radiation, 40 kV, 40 mA,35, 36 while out-of-plane XRD was performed using a Rikgaku Rint-Ultima III (CuKα radiation, 40 kV, 40 mA).

Viscosity measurements to observe changes in thixotropic viscosity over time were performed using a B-type analog viscometer (Brookfield, LVT), where SC4-18 and SC-13(P) were used for the spindle and sample chamber, respectively. The rotation speed was 6 rpm, sample volume was 6.7 mL, and the mixture was stirred for 2 h. The refractive indices of the solutions and coating films were measured with an Abbe refractometer (1T) manufactured by Atago Co., Ltd. attached to the digital thermometer. Simple and relative antibacterial properties of solutions containing nanodiamonds were evaluated by dropping them onto general bacterial test paper manufactured by Shibata Scientific Co., Ltd.

Supporting Information Summary

Comparison of CD spectra of the cytochrome C protein before and after adsorption on the surface of organo-modified nanodiamonds and corresponding UV spectra. Dispersion tests in solvents containing organo-modified nanodiamonds: solvent species dependence and concentration dependence. IR spectra of organo-modified nanodiamond. Out-of-plane XRD profiles of LB multilayers of organo-modified nanodiamonds. In-plane XRD profiles of the coating gel film containing thixotropic additives and organo-modified nanodiamonds.

Acknowledgments

This study was supported by JSPS KAKENHI Grant Number (C) JP 21K05180. This research was also supported by Tokuzo Ito Castor Research Fund.

    Conflict of interest

    The authors declare no conflict of interest.

    Data Availability Statement

    The data that support the findings of this study are available from the corresponding author upon reasonable request.