2.3. Preparation of Nanocellulose–Nanodiamonds Hybrids
In a first attempt, NDs–ethyl cellulose mixtures were obtained for the development of screen-printed films for electron emission cathodes
[51][60]. In this case, nano-graphite, NDs, and ethyl cellulose were mixed in the proportion 2:5:6, and ethyl cellulose was used only as an organic binding material. The first hybrids containing NDs and nanocellulose were produced using chitosan (CS, deacetylation degree 85%), BC nanofibers, and 5 nm NDs and the resulting materials were studied for potential application as wound dressings in biomedicine
[52][61]. A solvent casting approach in which a chitosan solution in acetic acid was mixed with a BC gel dispersed in acetic acid and NDs dispersed in water, stirred for 24 h, sonicated until achieving a homogeneous suspension, and then poured in Petri dishes followed by evaporation of the solvent was employed for the preparation of the BC/NDs hybrids.
Drug releasing, transparent and porous cellulose-NDs hybrids were also developed as wound dressing materials
[53][62]. Prior to the preparation of the hybrids, the NDs were surface functionalized to obtain carboxylated NDs (CNDs) using a mixture of sulfuric acid and nitric acid (3:1), and thermal treatment. Further, NaOH and urea were added to the homogeneous dispersion of functionalized NDs in deionized water, and the mixture was ultrasonicated and cooled to −12.5 °C before the addition of cellulose from cotton linter. The hybrid membrane was obtained by casting the mixed solution and immersing the obtained gel-like sheet in a 5 wt% H
2SO
4 aqueous solution followed by successive washings and freeze-drying
[53][62].
Nanocellulose-NDs hybrids with a porous structure were also obtained by electrospinning. For this, a CS:BC:polyethylene oxide (PEO) (45:45:10) suspension was prepared by mixing an aqueous solution of chitosan in acetic acid with a BC gel dissolved in acetic acid, and a PEO solution in water
[54][63].
For a better dispersion of the NDs in the water suspension of nanocellulose and stronger nanodiamond–cellulose interactions, CNFs were surface modified by two chemical routes: TEMPO oxidation (TOCNFs) and glycidyltrimethylammonium chloride (GTAC) modification (QCNFs)
[55][67]. Hybrid materials containing TOCNFs or QCNFs and NDs of about 10 nm in size in different proportions (1, 2.5, 5, 10, 33, and 50 wt%) were obtained by mixing the water suspensions of the modified celluloses and NDs, followed by filtration
[55][67]. In another attempt, a water suspension of cationic cellulose nanofibers (Q-CNF) obtained using GTAC as a surface modification agent was added over a NDs aqueous suspension, and the mixture was stirred for 48 h and filtered
[48][57]. Hybrid films with a NDs content of 0.5, 1.0, 2.5 and 5 wt% were produced by drying the wet cakes obtained after filtration between metal sheets at 93 °C for 15 min and then under vacuum
[48][57].
2.4. Main Characteristics of Nanocellulose–Nanodiamonds Hybrids
2.4.1. Thermal Conductivity
Nanocellulose–nanodiamonds hybrids have been developed for obtaining materials with new or special properties or for improving the properties of nanocellulose-based materials. Heat dissipation is now a major problem for electronic devices due to the high frequency and high power density of the new systems developed to meet the needs of increased performance, integration, and miniaturization
[50][59]. A higher power density level implies a higher heat flux density, which may raise reliability and equipment lifetime issues, hindering the development of high-performance devices.
Therefore, an increased thermal conductivity for the polymeric materials used in the manufacture of electronics is a major and urgent requirement. Nanodiamonds stand out by their high thermal conductivity at room temperature (2000 Wm
−1K
−1) and remarkable electrical insulation properties, which promote them as a valuable material for electric and electronic devices
[50][59]. Several nanocellulose/NDs hybrids have been developed for this purpose. Thus, CNFs/SCND hybrids showed up to 145.6 times increase in thermal conductivity as compared to the neat nanocellulose film
[50][59]. The thermal conductivity of the hybrids was measured by a laser-flash method, which measured the temperature rise when the sample was heated on one side by a short laser pulse. The improvement in the heat dissipation coincided with an increase in the mechanical properties of the hybrid materials containing up to 30 wt% NDs due to the reinforcing effect of NDs.
The dissipation of heat in highly integrated circuits is currently an important issue because most of the composite materials dissipate heat well only in one direction, being anisotropic materials. An increase of only 10 °C in the temperature of a light-emitting diode (LED) device leads to a reduced luminous efficiency and a halving of the LED’s lifetime
[56][72]. Materials showing a high bidirectional thermal conductivity are required in these applications for an efficient in-plane and through-plane spreading of heat in advanced electronic devices. Incorporating microdiamonds (M) in anisotropic nanofibrillated cellulose (CNF)–graphene nanoplatelets (G) films can enhance the thermal conductivity in both in-plane and through-plane directions by constructing three-dimensional heat dissipating pathways and reducing phonon scattering
[57][66]. Thus, more balanced in-plain (k
‖)/through-plain (k
⊥) thermal conductivity results were obtained for CNF/G/M hybrids due to the addition of M of different sizes
[57][66].
2.4.2. Mechanical Properties
The mechanical properties of the nanocellulose/nanodiamonds hybrids depend primarily on the dispersion of the ND nanoparticles in the hybrids and the NC/NDs interface strength as well as on the ratio between the components in the formulations and the preparation method
[22]. In particular, CNFs/NDs hybrid films obtained by the vacuum filtration of the CNFs/NDs mixtures with a NDs content of 0.5–10 wt% showed a decrease in the tensile strength of up to 25% as compared to the tensile strength of the NC film
[58][64]. The tensile properties were determined through dynamic thermomechanical analysis using a Q800 from TA Instruments (USA). The CNFs/0.5 wt% NDs hybrid film was proposed as a lateral heat spreader for portable electronic devices due to its good thermal conductivity, satisfactory tensile strength, good toughness, as well as its electrical insulation and optical transparency properties
[58][64].
Nanocellulose/nanodiamonds hybrids with improved tensile strength and modulus and high heat dissipation capacity were obtained by using up to 30 wt% SCND flakes. An important role in the improvement of the thermal and mechanical properties of these hybrids was attributed to the hydrogen bonds between the CNFs and SCND and the ordered layered structure of the SCND flakes. The CNFs/70 wt% SCND hybrid showed excellent flexibility and mechanical strength, a tensile strength of 113.4 MPa and a tensile modulus of 10.8 GPa that were proven by the easy bending and folding of the film
[50][59].
A remarkable reinforcing effect of the NDs was observed for cationic cellulose nanofibers/NDs hybrids
[48][57]. In this case, the ionic interactions between the electropositive charges on the nanocellulose surface and the electronegative NDs had a significant role in increasing the strength and preserving the ductility of the hybrid material. Thus, the Young’s modulus increased from 9.8 to 16.6 GPa with an increase in the NDs content from 0 to 5 wt%
[48][57]. Strong interactions between NCs and NDs were obtained by the surface functionalization of nanocellulose by TEMPO-oxidation
[55][67].
2.4.3. Optical Properties
The optical transparency of the nanocellulose film, also known as nanopaper, is an important property when the application of cellulose as a substrate for electronics is pursued
[59][73]. Although characterized by high optical transparency, NDs may change the optical transparency of the NC hybrid films. Thus, the transparency in the visible region of CNFs/NDs hybrid films with a thickness of 50–60 μm decreased when the concentration of the ND particles in the hybrids increased from 0 to 10 wt%
[58][64]. As a measure of the transparency of the hybrid films, the light transmittance at 550 nm wavelength decreased from more than 75% for the unmodified CNFs film, to about 20% for the CNFs/10 wt% ND hybrid film. This was due to the light scattering created by the ND particles and possible agglomerations when their concentration in the hybrids increased
[58][64].
2.5. Applications of Nanocellulose–Nanodiamond Hybrids
Materials more appropriate for the new electronic devices and, in particular, for power electronics are in great demand. These materials must have very good heat dissipation properties to be used in applications such as portable electronic devices or energy storage systems. Nanodiamonds stand out due to their remarkable thermal conductivity, of 2000 Wm
−1K
−1 at room temperature, which makes them the correct choice when an increase in the conductivity of various polymers is desired. NC/NDs hybrids benefit from the properties of both materials, combining good mechanical properties, high thermal conductivity, and electrical insulation, with biodegradability, and lack of toxicity. Thus, CNFs/NDs hybrid films with a low content of NDs showing good thermal conductivity and mechanical properties as well as optical transparency were developed for the lateral heat spreader in portable electronic devices
[58][64].
Two fillers with different geometry were used in NC hybrids to ensure a high bidirectional thermal conductivity for the development of thermal interface materials. Thus, new materials based on NC, graphene nanoplatelets, and micro or nanodiamonds were designed for this purpose
[57][66]. Similarly, hybrids containing NC, BNNS and micro/nanodiamonds were fabricated and tested with good results in the heat dissipation system of a main processor
[60][65]. Reportedly, the temperature of the processor recorded during 100% utilization was lower than the limit temperature for stable operation (53 °C) when the CNFs/BNNS/NDs hybrid used as a thermal interface material in the computer contained both nano and microdiamonds
[60][65]. An even lower difference between the in-plane and through-plane thermal conductivity was reported for a CNFs/BNNS/NDs hybrid obtained via an electrostatic self-assembly route
[61][75].
CNCs/NDs hybrid materials were also tested for humidity detection and humidity level measurement
[62][78]. The detection and control of humidity are vital in several fields such as food and drug storage, medical health, industry, and agriculture. A humidity sensor should measure humidity over a large range, with high sensitivity and accuracy and it must show cyclic responsiveness
[63][79]. The CNCs/NDs hybrid acting as an adsorption layer was combined with a quartz crystal microbalance (QCM) and its humidity-sensing properties were tested for a wide range of humidity, from 11.3% to 97.3%
[62][78]. The encouraging results, fast response, high sensitivity (54.1 Hz/%RH), and small humidity hysteresis (3.2%RH) recommended this hybrid as a humidity sensor in breath monitoring
[62][78].
The favorable chemical stability, mechanical strength, and biocompatibility recommend NDs for biomedical applications such as wound dressing, wound healing, skin tissue engineering, bone tissue regeneration, and drug delivery
[52][53][54][64][65][61,62,63,68,80].
CNCs/NDs conjugates obtained by covalently bonding silylated CNCs and oxidized NDs were designed as a biointerface material for bone tissue regeneration
[64][68]. In vitro biocompatibility investigations were conducted by studying the proliferation and differentiation of human fetal osteoblastic cells (hFOB) using the CNCs/NDs conjugates as cell culture substrates. The cytotoxicity tests showed that the CNCs/NDs conjugates induce cell growth and showed no cytotoxic effects after 7 days of culture
[64][68].
Chitosan/bacterial cellulose/nanodiamonds (CS/BC/NDs) hybrid films were recommended as wound dressings
[52][61]. The hybrid films containing 2 wt% NDs showed enhanced mechanical properties and only a slight decrease in transparency as compared to the CS/BC film without NDs. Moreover, the mouse skin fibroblast cells’ (L929) viability on this hybrid film (CS/BC/2 wt% NDs), evaluated using the MTT assay, was higher than 90% after 24 h incubation and exceeded 75% after 48 h incubation, showing a good biocompatibility
[52][61].
Nanocellulose/NDs hybrids were also tested for the delivery of drugs, using doxorubicin as a model drug
[53][62]. The addition of CNDs to cellulose greatly improved the modulus of elasticity and tensile strength of the hybrid films without significantly modifying their elongation at break and transparency. This improvement in properties was explained by the good dispersion of the CND particles as well as the strong interactions established between the hydroxyl groups of nanocellulose and the carboxylic groups of carboxylated NDs
[53][62]. The doxorubicin loading was investigated by immersing the hybrid membranes in a doxorubicin solution overnight. A higher amount of NDs in the hybrid material led to a higher content of doxorubicin loaded in the porous films while the drug release profile was influenced by the pH. The cytotoxicity of the hybrid membrane was evaluated by the MTT assay using HeLa cells. The results showed very good cell viability (>95%) in the absence of doxorubicin after 48 h. These results were considered strong evidence of the feasibility of these hybrid materials as systems for the loading and release of bioactive compounds
[53][62].