Nanotechnology application on bamboo materials: A review

: Bamboo is one of the renewable materials which can be applied in the engineering ﬁ eld widely. Previous research has shown that bamboo ’ s inherent poor durability can limit the application of bamboo mate rials. And nanotechnology has been receiving more and more attention on improving the properties of natural materials, simultaneously. This article aims to promote the application of nanotechnology on bamboo by pre senting some guides. And this article has been organized as follows: ﬁ rst, the characteristics and nanomechanical behavior of bamboo in mesoscopic and nanoscopic scale have been introduced. Then, nanomaterials for modi fying bamboo have been presented. Next, some analyses on the improvement of some properties of nano - modi ﬁ ed bamboo materials have been made. Finally, future per spectives have been discussed.


Introduction
Bamboo grows fast and has excellent mechanical properties [1,2]. The structure of bamboo consists of parenchyma cells and sclerenchyma fibers [3]. And the main chemical compositions of bamboo are hemicellulose, cellulose, and lignin [4]. The cell wall is mainly composed of cellulose (40-60%), hemicellulose (about 20%), and lignin (about 25%) [5]. Cellulose is a carbohydrate like starch, but it is a structural polysaccharide, while starch is a storage polysaccharide [6]. And cellulose has been considered as the structural and skeletal part of the bamboo cell wall [7,8]. And the cellulose separates in the secondary wall evenly [9]. Hemicellulose is an amorphous substance that fills in as a binder between fibers and microfibers [10][11][12][13]. As the most reactive biomass component, hemicellulose is hydrolyzed into oligomers and monomeric structures. Hemicellulose is similar to cellulose because it is composed of sugar, but it is more diverse [14]. Lignin is a category of hydrocarbon polymers consisting of aliphatic and aromatic structures [15]. Compared with hemicellulose and cellulose, lignin has a lower content of hydroxyl groups, better hydrophobicity, and chemical inertness [18]. Besides, lignin has been considered as a reinforcing agent for cellulose and microfiber/fibril [16]. And more lignin can be found in the narrow lamella of the structure of bamboo fiber than the broad lamella [9] around the cell lumen. In terms of nutrient content, on average, bamboo has starch (2-5%), protein (1.5-6%), glucose (2%), and fat and wax (2-3.5%) [17,18]. Compared to wood, bamboo possesses more polysaccharides and starch [19], causing bamboo to be more vulnerable to damage from mold [17,18].
"Nanotechnology refers to any technology that is implemented at the nanoscale and has applications in the real world" [57]. Nanotechnology has been applied both in wood and bamboo, while the applications in bamboo (both natural and engineered bamboo) are not adequate so far. Some nanomechanical behavior can be revealed via the nanoindentation technique to provide some guides to the further modification involving nanomaterials or other nanotechnologies. When it comes to nanomaterials used in modifying bamboo, most of the applications were involving coatings to improve the anti-microorganism capacity, hydrophobicity, wettability, thermal stability, flame-retardant property and ultra-violet (UV) resistance, and so on, and some specific functional properties can also be granted to bamboo such as conductive ability, and so on. However, the application of nanocarriers has not received enough attention which may be due to some financial factors, although nanocarriers in some studies on making drugs have been proved to be effective and the process has been optimized by artificial intelligence.
This article has been organized to introduce the main chemicals and characteristics of bamboo in the mesoscale, and the utilization of nanoindentation technique in revealing the nanomechanical properties of bamboo (especially the bamboo cell walls) has been presented later to better understand the bamboo in the nanoscopic scale so as to offer some guides in modifying bamboo via nanotechnology. The milestones of nanomaterials utilized in bamboo materials have been comprehensively presented as the main part of this article and some analyses have been made so that some guides for the further nanotechnology application on bamboo materials can be provided. Review paper on nano-modified bamboo is rare so far, although some papers have reviewed the application of nanotechnology on the wood [58][59][60].
2 Bamboo in nanoscopic and mesoscopic scale

Mesoscale characteristics
The morphology of bamboo is different from that of wood. The apparent radial-gradient distribution structure in the cross-section of bamboo, especially from the outside to the inner part, is more non-homogeneous than most wood [61]. The sclerenchyma cells (fibers) are embedded in a matrix of parenchyma cells. On average, parenchyma cells and sclerenchyma fibers take up roughly 50% and 40% of the volume of bamboo culm, respectively [61,62] (while recent research found that the fibers occupy about 25% in volume and the fraction increased from 14.6 to 39.7% when measured from the inner part to the outer [63]). Sclerenchyma fibers dominate the mechanical properties of bamboo culm, including fracture toughness [64], rigidity [65], strength [66], and so on. Parenchyma cells play a key role in resisting buckling failure and improving curve ductility [67]. The heterogeneous structure of bamboo has been shown in Figure 1 [68]. The low microfiber angle (about 10°) and high tensile modulus (about 36.7 GPa) of bamboo fibers are more than twice as those of wood fibers [69]. The chemical contents of bamboo are mainly hemicellulose, cellulose, and lignin [70]. The cellulose content increases from the bottom of the culm to the upper part. The outer surface of bamboo culm possesses a higher content of α-cellulose, holocellulose, and lignin but lower ash content and extractive content compared with the inner surface in specific cross-sections [71]. The abundant presence of hydrophilic oxhydryl groups and layered porous structures in bamboo culm can help with absorbing of water. Although the multicellular structure of both bamboo and wood is well-layered, the layered structure of the cells of bamboo is more variable [61]. There is a large variability in the number of layers, with the minimum number of layers ranging from 2-3 to a maximum of 18 layers depending on the bamboo species and site [61]. The bamboo cell wall is composed of several layers of different thicknesses alternating in sequence [9]. The thickness of these wall layers is usually small, and some are below 100 nm [72]. In addition, a special multi-wall structure of bamboo cells is formed by wall layers with unequal width, which makes it difficult to study the distribution of wall chemicals [9]. It has been recently found that the chemical composition of bamboo cell walls can be better analyzed in situ using nano-infrared techniques, which has exceeded the diffraction limit of conventional infrared spectroscopy techniques to obtain infrared spectra at the nanoscale [9]. This has been considered as an effective method to study the nano-distribution of the chemical composition of bamboo cell walls [9]. The scanning electron microscope (SEM) images of cell walls have been presented in Figure 2. The hierarchical structure of bamboo over different length scales has been proposed by Parameswaran and Liese [72,73], and is shown in Figure 3, which was reprinted by Liu et al. [74].
The pore space in bamboo is one of the main factors affecting the mechanical properties and dimensional stability of the bamboo material [4]. The International Union of Pure and Applied Chemistry classification divides pores into three categories: micropores (pore diameter <2 nm), mesopores (pore diameter between 2 and 50 nm), and macropores (pore diameter >50 nm) [75,76]. The distribution of mesopores with different sizes in volume is shown in Figure 4.

Bamboo's nanomechanical behavior measured via nanoindentation
Nanoindentation technique has been used to determine the features of materials and constituents [77,78]. And many papers have introduced this method comprehensively [79,82]. The nano-mechanical behavior of bamboo materials is mainly influenced by the complex arrangement of the solid part of the cell wall, the porosity, and the variation in the chemical composition [4]. Recent related studies were focusing on the determination of nanomechanical properties (such as modulus of elasticity (MOE), hardness, creep behavior, etc.) of bamboo materials by using the nanoindentation technique [4,[80][81][82][83].
The excellent mechanical properties of bamboo are largely derived from the sclerenchyma cell fibers [80].  [74], (b) the middle lamella is the outer-most layer, followed by the primary wall [74], (c) the short tiny fibers [74], (d) nanoscopic scale cellulose grains with orientation and other components [74]. (Reprinted from Parameswaran and Liese [72,73] by Liu et al. [74]). Guo et al. [80] performed nanoindentation tests on three different areas of the cross-section by using the continuous stiffness method. It was found that the outermost fibers had the highest MOE and a decreasing trend from the outside to the inside was observed, with the inner side having 18.1% lower MOE than the outer side, indicating that the functional gradient properties of bamboo material are not only expressed in the macroscopic but also the mesoscopic structure [80]. Fibers of Moso bamboo possess the behaviors in which the constant deformation occurred under constant load and the load kept decreasing while the deformation was constant, i.e., creep and relaxation behaviors of the material [81]. Li et al. [81] investigated the creep and relaxation behaviors of Moso bamboo fiber cell wall (BFCW) using nanoindentation. The results showed that the mechanical properties were unequal in the longitudinal and transverse directions of BFCW, and more pronounced relaxation behavior and stronger creep deformation resistance in the longitudinal direction were observed. The indentation depth and creep displacement of BFCW in the longitudinal direction were smaller than those in the transverse direction, and the differences reached 24.96 and 32.25% at the maximum indentation load of 15 mN. The relaxation capacity of BFCW longitudinal was stronger than that of transverse, and the longitudinal load relaxation was 34.58% higher than that of transverse at a loading rate of 50 nm/s. Qin et al. [82] investigated the static longitudinal nanomechanical properties of Moso bamboo cell walls by using nanoindentation and concluded that the main factors affecting the longitudinal mechanical strength of bamboo fiber cell walls were microfibril angle and lignin content, while correlations with bamboo age and height were kind of minimal (compared in one experiment). The MOE and hardness of these aforementioned research are listed below in Table 1. By comparing the literature in Table, one major factor affecting the properties of bamboo can be found as follows [71,73]. Both MOE and hardness were more sensitive to the variation in the part in cross-section than the change in height and age.
In summary, the main factors influencing the nanomechanical properties of bamboo were crystallinity, moisture content, chemical composition, density, and microfibril angle [84][85][86]. The main factors affecting the longitudinal mechanical strength of bamboo fiber cell walls were microfibril angle and lignin content, while the factors were little correlated with age and height of bamboo [83]. The improvement in creep properties can be improved in some research by increasing crystallinity and forming cross-linkages between cell wall polymers [87].

Nanomaterials for modifying bamboo
So far, many studies have been implemented on the nanotechnology application on wood. Related review papers [58][59][60] have indicated that the chemical properties of the surface and material properties (cell walls show molecular-scale porosity due to the partial filling of spaces between cellulose and microfibers) can be improved. And these were mainly due to the presence of pores in the wood so that nanomaterials can penetrate the material wall [58][59][60]. But the microscopic properties of bamboo are different from those of wood, which may cause changes in the penetration and distribution of nanomaterials.
Chemical methods (such as using some preservatives) to improve the durability of bamboo materials can bring good results. However, the chemical treatment process may use some chemicals that are toxic to the environment and human beings (such as alkaline copper quaternary compound [88], chromated copper arsenate [89], etc.). Therefore, chemical modification has some limits [90]. Among the studies involving the nanomaterials applied in modifying the bamboo, most studies mainly focused on the treatment towards the surface (coating treatment). And most of the nanomaterials used in bamboo modification involved metals or metal-oxides, and a lot of efforts have been carried out including impurity doping, metallization, sensitization and coating, and so on [91,92]. Nanomaterials applied in bamboo modification mainly involved: (i) coatings, (ii) nanocarriers, and (iii) nanosized compounds penetration.
Most of the research works were focused on improving the durability of the bamboo. And one solution was via reducing the interaction with moisture/water, which can make the super-repellency towards water, and special liquid sorption properties on the substrate. And this solution has become popular in determining wood conservation. And the other was improving the anti-microorganism ability. Photocatalytic microorganism disinfection depended on the interaction between microorganisms and reactive oxygen species from photocatalysts when the light is cast on them, for instance, OH − and O 2− , which can be released by the sunlight, are harmful to some microorganisms [93]. However, not all the antimicroorganism abilities were derived from the good photocatalytic performance of nanomaterials. Li et al. [94] coated Ag-TiO 2 composite films on the bamboo and excellent antifungal activity with this synergistic antifungal effect was found while it was unrelated to photoactivity.
Some other interesting properties have also been developed on bamboo. Jin et al. [95] considered that inorganic-nano-materials/polymer-composite (wood or bamboo) could be seen as a kind of portable bio-based photocatalysts for the degradation of contaminated solutions because of the merits of wood, bamboo, and cellulose materials for the immobilization and growth of nanomaterials and the environmentally friendly, easily biocompatible, easily designed, and feasibly biodegraded characteristics of those cellulose-based materials [96,97]. And the conductive ability of the bamboo surface has been realized using copper nanoparticles by Bao et al. [98].

Nanosized metal, metal, and non-metal compounds
Biomass composite involving inorganic materials possessed some functional properties such as specific electrical, magnetic, optical, and biological properties. And nano-sized metal compounds with tiny sizes, high surface-to-volume ratio, and even tunneling effect (which is [ 82] related to quantum mechanics) [99], and so on, can render biomass materials some specific values or expand the applications [100].

Nanosized metals and metal compounds
Nanosized metals or metal compounds can be synthesized mainly by using chemical methods, namely: (i) solutionbased synthesis (sol-gel, sonochemical, and solvothermal) and (ii) vapor-based synthesis (combustion and chemical vapor deposition) [58][59][60]. Nano-sized metal oxide particles, such as zinc oxide (ZnO) [101], titanium dioxide (TiO 2 ) [102], cerium oxide (CeO) [103], CuO [104], Ag [105], and so on have been reported to have strong antibacterial properties. And these nano metals and their compounds have been reported to be more active than non-nanometals, which were attributed to their higher surface area and ability to exhibit unique physical and chemical properties. Inorganic materials (especially nano-sized materials [such as nanosized metal compounds]), can interact with microorganisms such as fungi, bacteria, and so on, by the electrostatic reaction which negatively affects the active external pump transport system of β-lactamase and thus, the excellent antibacterial outcome can be reached [106]. Nanosized metal particles can show bright color because of the localized surface plasmon resonance characteristics (the surface plasmon excited by light) [107]. The conduction electrons around metal nanoparticles vibrated at a certain frequency when the light interacted with the metal nanoparticles [108]. Localized surface plasmon resonance characteristics of metal nanoparticles have been widely applied in the field of modifying the surface and other fields [109,110]. And notably, plasma combined with a resin-based protective agent has been used in modifying bamboo to improve its anti-organism ability and compression strength, decrease equilibrium moisture content and wet swelling rate of natural bamboo, and besides, cracks, mold, and decay at the end part of bamboo strips and tubes can be mitigated [111].
As to Ag nanoparticles [112], they have been found to be easy to be synthesized and characterized, which possessed good chemical stability when covered by some organic stabilizer, high biocompatibility, and large atomic weight when compared to bamboo matrix [113,114]. Pandoli et al. [114] filled the bamboo matrix with silver colloids as nanofiller agents for the first time. The effectiveness of Ag countering microorganism attacks (especially fungi) has been proved due to the broad spectrum of bactericidal properties of Ag nanoparticles [113,115].
As to Cu, it is a metal with excellent electrical conductivity, and nano copper particles [116] have been proved to be a low-cost and effective reagent, which possesses good antibacterial activity [117] and excellent UV protection performance [118]. Notably, the transportation efficiency of Cu can be increased by electric field [119]. Ju et al. [110] fabricated a functional bamboo with nanosized Cu in it and the functional bamboo was realized by combining Cu nanoparticles with hemicellulose and lignin under a high voltage electric field. After the leaching test, in situ impregnation of Cu under a high voltage electric field possessed an excellent fixation performance. The functional bamboo also possessed significant antibacterial properties and excellent UV protection performance. The concentration of Cu ions reached the highest while the treatment condition was 60 kV/24 h.
As to γ-Fe 2 O 3 , it is a kind of popular metal compound in many fields due to the excellent non-toxicity, thermal stability, chemical stability, especially the superparamagnetic performance [120], and thus, it has been widely applied in environmental protection [121], biomedicine [122], microwave absorption [123], drug delivery [124], and magnetic resonance imaging [125]. Notably, the self-healing property of γ-Fe 2 O 3 particles in the oscillating magnetic field was also interesting [126].
ZnO is gaining more and more attention [127] due to its good chemical stability, non-toxic (is selectively toxic to bacteria, while it is almost harmless to human cells [128]), cost-efficient, and has good anti-microorganism ability [129]. And meanwhile, ZnO has been classified as "Generally Recognized as Safe" by the US Food and Drug Administration [130]. Compared to nanomaterials with large sizes, ZnO is smaller and possesses a higher surface-to-volume ratio, and thus, ZnO can better interact with microorganisms [131]. After further studies, it was found that ZnO-treated bamboo materials performed better when interacting with Aspergillus niger (A. niger) and Penicillium citri (P. citri)), while they were less resistant to Penicillium glabra, according to Li et al. [131].
Anatase TiO 2 , has often been used to provide roughness of surfaces due to its chemical stability and non-toxicity [132]. When used in the modification for low-surface-energy coatings, it provided superior water resistance which can separate the wetting properties from the natural properties of the substrate [133]. However, bare TiO 2 has been found to be inefficient and with a narrow photoreaction range, and besides, the agglomeration of TiO 2 nanoparticles on the surface can significantly reduce photocatalytic activity as well. Therefore, the application of bare TiO 2 in photocatalysis under sunlight [115] has been limited to some extent. While its drawbacks can be minimized by collaborating with other nanoparticles such as Ag [115] or Fe [134], and so on.
As to the modification with nanosized particles during the fabrication process of engineered bamboo, the anticorrosion and anti-mildew properties, physical and mechanical properties, excellent weather resistance, and special functions of the recombinant bamboo can be developed and improved via the presence of nano-materials. Therefore, the purpose to simultaneously improve the quality and added value of recombinant bamboo can be met [100]. Lou et al. [100] fabricated the recombinant bamboo (also known as bamboo scrimber, and parallel bamboo strand lumber) with good compressive properties, dimensional stability, mildew resistance, and last but not least, efficient electromagnetic dissipation capacity. The fabricating process is shown in Figure 5. After the experiment, they found that the bamboo bundles prepared by impregnation with 0.4 mol/L iron salts resulted in the recombinant bamboo boards possessing the best compression property, dimensional stability, and antimildew capacity. These brief fabrication methods and effects of nanomaterials have been presented in Table 2. By comparing the literature in Table 2 [100,110,114], the discussion can be made as follows. Bamboo samples with Ag particles possessed better anti-organism ability than those with Cu and Fe 3 O 4 , and thus Ag can be purposely used to resist microorganism while Cu and Fe 3 O 4 can be applied to the mixed particles planted in/on bamboo to improve the ability of fixation of particles and some physical properties of bamboo.

Non-metal compounds
Graphene oxide (GO) has received more and more attention in the areas of interdisciplinarity such as chemistry and biology due to its excellent physical and chemical properties such as anti-microorganism activity, biocompatibility, non-toxicity, and so on [135]. As to the GO nanosheets, they possess a large surface area with a two-dimensional structure and contain different groups with oxygen. And these abundant oxygen-containing groups render GO good hydrophilic and excellent biocompatibility properties [135][136][137], and the adhesion between GO and bamboo fibers can be enhanced due to the abundance of oxygenated functional groups (such as OH and COOH groups) [138]. Recently, Wang et al. [138] incorporated the GO nanosheets with densified bamboo after heat-pressing treatment. The mechanical properties were well improved especially when compared to natural bamboo. Densified bamboo fibers, collapsed cells, and GO nanosheets (tightly attached to the surface of fibers), and the change in hydrogen bonds in bamboo can be seen in Figure 6.  [100], (c) phenolic resin-impregnating bamboo bundle [100], (d) recombination and hot-pressing treatment [100], (e) hydrophobic behavior [100], and (f) electromagnetic dissipation performance of nano-Fe 3 O 4 /bamboo bundles/phenolic resin oriented recombination ternary composite penal [100]. E. coli = Escherichia coli.
As to nano-clay, montmorillonite (MMT) is the basic raw material used in producing nano-clay. Clay nanofiller possesses a multi-layered morphology [139], and notably, MMT as one of the abundant types of clay has been widely used [140]. Rahman et al. [141] reinforced bamboo strips by taking MMT as nanosized filler and poly(ethylene-alt-maleic anhydride) (PEA) as a compatibilizer. The nano-clay layers can limit the movements of polymer chains as well as the movement of molecules in the amorphous region, while no covalent bond can be formed between this nano-clay and bamboo. And the anhydride of PEA can react with hydroxyl groups of the bamboo cell walls (a covalent bond can be formed to enhance the interfacial adhesion) [141]. Both PEA and nano-clay can enter the para-crystalline region to turn it into a crystalline region. Therefore, the MOE and MOR were improved [141]. The adhesion between polymer and bamboo can be improved due to the presence of nano-clay in the lumen, void spaces, and cell walls of bamboo so that the hydrophobicity and thermal stability can be improved [141]. Furthermore, Rahman et al. [142] found that the combination of MMT nano-clay and acrylonitrile (AN) is better than that of MMT and methyl methacrylate (MMA) as to the improvement of mechanical properties of the bamboo strip, which was mainly due to the better capability. And as to the optimization of bamboo modified by nanomaterials, Rahman et al. [143] evaluated the optimization of AN/malleated polyethylene (MAPE)/nano-clay bamboo nanocomposites via response surface methodology, and simultaneously, properties and characteristics were also investigated. And the authors predicted that this bamboo nanocomposite can have an edge over conventional nanocomposites used for indoor or outdoor construction applications [143].
To briefly sum up, compared with some expensive nanosized metals or metal compounds, GO and nanoclay might be more suitable for wide application in the engineering field. And the brief methods and effects of using particles without metal or metal compounds can be seen in Table 3. Mechanical properties of bamboo can be more largely improved via GO nanosheets than MMT nano-clay fillers which may be due to the hydrogen bonds and the robustness of nanosheets. And the combination of GO nanosheets and MMT nano-clay fillers may be a better choice to improve the mechanical performance of bamboo considering both the cost and effects.

Some combinations of nanomaterials
The cost of some nanoparticles can be high, but the combination of different nanomaterials can reduce the cost. Li et al. [94] found that compared with TiO 2 /bamboo, the Fe-doped TiO 2 /bamboo exhibited much higher photocatalytic disinfection activity to fungi in the natural environment. The electron-hole pair separation efficiency  Rahman et al. [143] can be increased by Fe dopants, and their recombination was inhibited leading to a lifetime increase in the generated electrons, and thus the antifungal activity, even under natural light, was improved. Zhang et al. [135] employed ZnO nanoparticles and GO nanosheets via a facile method to modify bamboo material. The combination of the ZnO nanoparticles and GO nanosheets gave the bamboo better antimicrobial properties compared to modify bamboo with ZnO nanoparticles or GO nanosheets alone. The antibacterial behavior of bamboo was investigated by the bacteriostatic circle method, and the results showed that ZnO/GO/bamboo materials possessed excellent antibacterial activity against E. coli (Gram-negative) and Bacillus subtilis (B. subtilis, Gram-positive) bacteria and high thermal stability. Besides, the Zn/GO bamboo composites also exhibited higher thermal stability. Liu et al. [115] incorporated Ag nanoparticles into TiO 2 nanoparticles, and thus, the photocatalytic activity in visible light was enhanced, and meanwhile, the surface plasmon absorption of silver can help extend the absorption range of bare TiO 2 to the visible region [144].

Applications in some other fields
In the electric field, the bamboo product was functionalized because metal ions can be triggered and released. Besides, ionic metals can penetrate the cell structures to combine with hemicellulose and lignin effectively and thus ionic metals can remain in the matrix of bio-materials such as wood [145] and bamboo. The direct current electric field had better antibacterial performance than alternating current and high-frequency current [146].
In the field of solar steam generation, natural bamboo has become a novel type of substrate for photothermal materials due to the merits of oriented microchannels, thermal insulation, hydrophilicity, and so on. Sheng et al. [147] applied bamboo in solar steam generation via modifying bamboo with plasmonic metal materials (as photothermal conversion materials) which can help improve the optical absorptivity of bamboo. The structure of photothermal conversion was formed by the uniform deposition of plasmonic metal nanoparticles in the oriented microchannels where water can be transported by the capillary effect. The plasmonic bamboo material displayed a high conversion efficiency of 87% under a tensun illumination (the solar flux is 10,000 W m −2 ) and excellent cycling stability with no degradation after 140 h of cycling under 5 suns (the solar flux is 5,000 W m −2 ). Some more nanometal structures involving plasmon can be found in the paper presented by Yang et al. [148], while the durability of bamboo has not been considered enough in this study. It is worth noting that this modified bamboo with optical absorptivity [147] can be a kind of potential material utilized for photovoltaic building integration [149].
As to the development of renewable catalysis system, a novel continuous-flow method was used to disperse Ag nanoparticles around the microchannels in bamboo vascular bundle evenly and tightly, aimed to reduce nitroaromatics [150]. The catalytic capillary microreactor displayed high catalytic activity and good long-term stability.

Coatings
The surface of bamboo possesses very similar properties as wood: (i) porous structure and (ii) abundant hydroxyl groups. This can facilitate the attachment and growth of nanomaterials to the bamboo surface [95]. However, some researchers believed that the immobilization of inorganic nanomaterials on organic bamboo surfaces may not be easy, which was mainly due to the lack of highly reactive functional groups on the bamboo surface that can provide strong binding forces [115,151]. But notably, as mentioned before, GO nanosheets [138] can provide more opportunities for the formation of hydrogen bonds which can be a solution for the lack of functional groups on the bamboo surface. There is more research related to the use of nanotechnology treatments on wood, and there are two main ways of adding metals or metal oxides to the coating, which is adaptable to bamboo [58]: (i) solution mixing and (ii) in situ deposition/growing. The first way is mixing nanoparticles into coatings and having the surface coated by brushing or spraying or dipping. And the second method can be seen as a chemical method that can be more beneficial compared to the first one, due to the little molecules that can diffuse among nanoparticles [58]. From a review of recent research on bamboo, it was found that the main method of coating is the in situ growth method. The main ways used to seed nanomaterials on the surface of bamboo by in situ growth method are (i) hydrothermal deposition [92,115,132,[152][153][154] and (ii) sol-gel method [131,[155][156][157][158]. One of the merits of deposition in solutions is that cavities and affinity can be provided by the microstructures and hydroxyl groups on the surface of bamboo for the creation and immobilization of nanoparticles through electrostatic and hydrogenbonding interactions at the microscale [157].
Research papers related to the application of nanomaterials on bamboo materials were mostly focusing on the coating treatment. Therefore, some coating processes and effects of nanomaterials on the bamboo surface have been sorted and shown in Table 4. Different aggregate forms of the immobilized nanoparticles caused different effects. Functional properties involving anti-microorganism ability, UV-resistant properties, hydrophobicity, photocatalytic ability, etc., can be well generated or improved via metal nanomaterials in most literature, while mechanical properties of bamboo were mentioned in literature involving nano-clay [143], and SiO 2 [138] which are nonmetal nanomaterials. Notably, the good fixation ability of nanomaterials can be seen in literature [98,115,162] which can be arranged to the outmost part of a layered structure to protect the whole structural material.
As to SiO 2 , Zhou et al. [165] compared bamboo strips treated with ammonium dihydrogen phosphate + disodium octaborate tetrahydrate, ammonium dihydrogen phosphate + disodium octaborate tetrahydrate + nano-SiO 2 sol, and ammonium dihydrogen phosphate + boric acid + nano-SiO 2 sol (ABS), and found that bamboo treated with ABS possessed lower hygroscopicity, the best leaching resistance behavior, and good thermal stability. And therefore, ABS can be an optimal compound for the bamboo's retardant [165]. And besides, SiO 2 has also been coated on the GO/bamboo nanocomposites to improve the hydrophobicity and thermal stability of GO/bamboo nanocomposite which rendered excellent mechanical properties as reported by Wang et al. [138] As to ZnO, Yu et al. [156] coated the bamboo surface with a series of different ZnO films, and the results indicated that the photostability and antifungal properties of bamboo were greatly improved and highly dependent on the crystallinity and morphology of the ZnO films. ZnO with nanostructured networks with the best photostability (mainly due to higher separation efficiency of electron and hole pairs because of nanosized effects) and antibacterial ability (mainly due to the high crystallinity of this form) can be seen as the optimized one. However, longer immersing duration in colloids tended to promote the production of irregular aggregates. Yu et al. [156] found that the growth of ZnO nanonetworks was promoted within a duration of 4 h when the bamboo was immersed in the ZnO nano-sol. An aqueous solution method for growing ZnO nano-metallic films onto bamboo substrates at low temperatures has been presented by Yu et al. [158]. The formed ZnO nanosheet films consisted of randomly oriented irregular nanosheets and occasionally by nanowires/nanorods. The modified bamboo possessed photostability and antifungal and antibacterial properties at the same time. However, Li et al. [157] thought that the results were not representative. Besides, ZnO possessed the excited energy of 60 mV and a bandgap of ∼3.4 eV, and thus it can be used to improve the UV resistance [166], antibacterial activity [167], and thermal stability [168]. While a kind of recycled green ZnO/bamboo composite can also be fabricated by growing cross-linked ZnO nano-walls on the bamboo [95]. And the excellent photocatalytic ability enabled the functionalized bamboo to become a potential recyclable bio-based catalyst for the purification of pollutant solutions. The photocatalytic efficiency has been measured to be 70%, and it can be recycled 3 times at least. And notably, hydrogen bonding can be formed between bamboo and nanosized ZnO complex layer via coordination reaction between ZnCl 2 and urea [169].
The distribution of TiO 2 with different sizes is shown in Figure 7 [154]. Most sizes of TiO 2 decorated on the surface of bamboo were from 1.8 to 2.0 nm. Li et al. [134] found that when Fe was doped in the TiO 2 lattice, the optical absorption edge can be shifted to the visible region, the photocatalytic activity was improved, and the crystallinity and crystallite size of the TiO 2 products can also be significantly influenced. The variation in the morphology of the surface of bamboo can be seen in Figure 7(d) after the deposition of TiO 2 in different forms. Rao et al. [170] compared benzotriazole, benzophenone, nano-TiO 2 , and nano-ZnO which were intentionally used for absorbing UV light. And the coatings containing both organic and these inorganic UV light absorbers were investigated. They found that coatings containing benzotriazole showed the best surface photostability, and with the existence of inorganic absorbers, the degradation rate of organic absorbers decreased. And nano TiO 2 also showed good photo-degradation resistance when coated on thermally treated bamboo [171]. Both nanosized TiO2 and ZnO are considered as potential components of coating for absorbing UV [172]. Synergistic effects on UV light resistance were achieved by incorporating organic absorbers with these two inorganic ones, and Rao et al. [172] found that benzotriazole can be a better option as the organic part when compared with benzophenone.
As to the nanosized Cu, in some cases, the fungus might not recognize nano-copper. And thus, the nanoparticles can enter the cell wall of fungi, and then, the reactive oxygen species with the fungal cell can be created which can affect the erosion of wood [60]. Bao et al. [98] coated the bamboo's surface with copper which   promoted the applications of superhydrophobic and conductive bamboo products.
Some of these studies have investigated the antiorganism ability (mainly resists mold and fungi) according to the Chinese standard: GB/T18261-2013 [173] which is a test method for anti-mildew agents in controlling wood mold and stain fungi. Five grades (0-4) have been classified and defined in this standard depending on the areas infected, as shown in Table 5. And different antiinfection abilities by different nanomaterials are shown in Table 6. From the reviewed papers, it is understood that the infection of T. viride can be tough to resist. Bamboo coated by ZnO-TiO 2 [161], nano-zinc complex (U2-t2) (an ionic liquid (ZnCl 2 /urea molar ratios [n(ZnCl 2 )/n(urea) = 1:2]) for 2 h) [169], PMHS-ZnO [159], and Ag nanoparticle-decorated MT (Mesoporous anatase TiO 2 ) film [94] were well optimized. The infection values (IV) of the 3 kinds of fungi kept at 0% after 28 days or even after 90 days. The bamboo modification was provided by effective approaches to extend the service life of bamboo.
In summary, nanomaterials have been mainly utilized as coatings on bamboo, and good outcomes have been achieved with no doubt in improving the weatherability, thermal stability, anti-microorganism ability, and so on. While more attention should be paid to shrinking the cost and the recyclability of the whole bamboo materials coated by nanomaterials to facilitate their application in construction, photovoltaic building integration, or something else.

Nanocarriers
The polymeric carriers are considered as a delivery system at nanoscale or microscale which can provide a container for some nanoparticles by physical encapsulation or chemical bonding connection [60]. So far, few studies were focusing on carriers. Nanocarriers have been used in wood modification for some interesting properties in improving the permeability of wood preservatives [59].
The nanocarriers can provide a way for organic biocides or nanoparticles so as to be immobilized in or on bamboo. And it has been proved to be beneficial for those biocides or nanoparticles being hard to adhere to bamboo material. Some synthetic polymers employed in the preparation of nanoparticles for the delivery of antimicrobial contents have been presented by Spizzirri et al. [174]. And some functionalization of photosensitive nanocarriers has been sorted and presented by Fernández et al. [175]. It is worth noting that Abd-algaleel et al. [176] presented a process of drug loading optimization that may be a guide to the application of nanocarriers in the modification of bamboo which involved artificial intelligence and machine learning, as shown in Figure 8.
The hydrogel can be seen as a carrier for some nanoparticles. Huang et al. [177] prepared loaded nano/ZnO composite hydrogels, as shown in Figure 9. The hydrogels which were sensitive to the temperature helped control the release of the drugs or nanoparticles. Nano-ZnO/poly(Nisopropylacrylamide) hydrogel was synthesized by the addition of nano-ZnO dispersion to realize the smaller particle, the more even dispersion, and the higher stability by Zhang et al. [178], when compared to the hydrogel with the addition of nano-ZnO particles with dispersant sodium dodecylbenzene sulfonate, nano-ZnO particles with dispersant polyvinylpyrrolidone, and nano-ZnO particles with dispersant hexadecyltrimethylammonium bromide.
And the nano-Ag was also embedded in hydrogels which helped the anti-fungal properties by Wei et al. [179]. Yu et al. [180] designed Ag-loaded thermal-sensitive nanogels to resist microorganisms at ambient temperature, and the optimal impregnation process was revealed. While research on the nanocarriers applied in the bamboo modification is rare by far which may be due to the higher cost, MMT nano-clay as a nanomaterial costing less has been used in a drug carrier system [181]. And the MMT composite can be produced via utilizing anionic, cationic, and nonionic surfactants to have the basal spacing enhanced so that organoclay can be used in drug loading and drug release [182].
Some nanotubes have been applied in modifying bamboo. Zhang et al. [183] utilized halloysite, which is a natural mineral with a nanotube structure, and iodopropynyl butycarbamate was loaded on it to achieve good anti-mildew activity (resist mold and blue-stain fungi). And the effects of utilizing nanocarriers on bamboo, which were mainly relevant to the anti-microorganism ability, are presented in Table 7.
In summary, the purpose of lengthening the service life of bamboo has been met via nanotechnology and some specific functions have also been granted to bamboo.  Bamboo timber refers to the bamboo material without inner and outer parts [153].
The majority of the studies were focusing on the coatings using nanosized metals or metal compounds, while insufficient attention has been paid to using nanocarriers which was mainly focusing on hydrogels that are sensitive to temperature by far, and the nanocarriers in modifying bamboo need more exploration in the future.

Future scopes
Nanoindentation technique has been widely applied in determining some nanomechanical behaviors of bamboo such as MOE, hardness, creep behavior, and so on, and many investigations have been implemented. Further  research involving both nanoindentation technique and nanomaterials may focus on the determination of the improvements in nanomechanical behavior of some nanomodified bamboo materials. As to nanoparticles, (1) bamboo materials' properties can be well improved via merging nanomaterials with bamboo. More attention may be paid to the cost of the modification process, via combining the nanotechnology with chemical treatments or heat treatment, or the concentration of nanomaterials used so that a compromise can be made between the cost and practical use. And this aim may be well-realized via artificial intelligence.
(2) More research may be needed on the selective distribution of different nanoparticles in the pores or vascular bundles of bamboo considering some impacts on the deposition and distribution of the nanomaterials by the variation in the number of circles. (3) Nanoparticles in different forms were subject to agglomeration, and perhaps electromagnetic fields can help solve this problem in addition to adding other nanoparticles. And more experiments for comparisons are needed, considering the variations in bamboo as a natural biomass material. (4) Combining plasma and nanomaterials may help develop some more efficient plasma-synthesized nanomaterials which can be granted with some specific functions for modifying bamboo. (5) Some studies on giving bamboo with some properties related to the field of electron-wave or solar power system were refreshing while insufficient attention has been paid to the durability at the same time.
As to coatings, anti-organism ability has been well improved via coatings. While more attention may be paid to the cost and the recyclability of the whole bamboo materials coated by nanomaterials to facilitate their application in construction, photovoltaic building integration, or some other aspects.
As to nanocarriers, Attention granted to the utilization of nanocarriers in modifying bamboo has not been enough so far. Some carriers that can be made from non-polymeric materials may collaborate with some other polymeric materials to obtain better outcomes. MMT nano-clay has been used as carriers for drugs and thus, it may be applied in modifying bamboo in future work.
Bamboo materials mentioned in this article were mainly some isolated small bamboo strips, while bamboo cylinders and other engineered bamboo materials have not received enough attention. And some other parameters may be considered more such as residual stress during or after the modification processes, and so on.

niger)
Yu et al.

[183]
Sustained-release property (increased with the higher amount of iodopropynyl butycarbamate)

Conclusion
This article has reviewed the application of nanotechnology in modifying bamboo materials. The mesoscopic characteristics and nanomechanical properties (via nanoindentation technique) of bamboo has been presented. The utilization of nanomaterials in modifying bamboo materials (mainly involving the main part of bamboo materials instead of cellulose fibers, etc.) has been reviewed to promote the application of bamboo nanocomposites in the engineering field. And the effects of different nanomaterials have been summarized in Sections 3.1, 3.2, and 3.3. The mentioned experimental studies have contributed to a better understanding of the mechanisms of the effects brought by nanomaterials in/on bamboo. Nanomechanical properties, like MOE and hardness, were more sensitive to the variation in the part in cross-section than the change in height and age. Bamboo samples with Ag particles possessed better anti-organism ability than those with Cu and Fe 3 O 4 . Functional properties involving anti-microorganism ability, UV-resistant properties, hydrophobicity, photocatalytic ability, etc., can be well generated or improved via metal nanomaterials in most literature. And some non-metal nanomaterials can help improve the mechanical properties of bamboo well. Mechanical properties can be more largely improved via GO nanosheets than MMT nano-clay fillers which may be due to the hydrogen bonds and the robustness of nanosheets. And the combination of GO nanosheets and MMT nano-clay fillers may be a better choice to improve the mechanical performance of bamboo considering both the cost and effects. So far, research involving carriers in modifying bamboo materials mainly aimed at improving the anti-microorganism ability. And the carriers for carrying nanoparticles were mainly hydrogels that are sensitive to temperature. Research work on nanotube carriers was rare in modifying bamboo and it was mainly used to carry chemical anti-mildew agents. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: David Hui, who is the co-author of this article, is a current Editorial Board member of Nanotechnology Reviews. This fact did not affect the peer-review process. The authors declare no other conflict of interest.