RESEARCH NEWS Inorganic Nanomaterials
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Bioinspired Hierarchical Porous Structures for Engineering Advanced Functional Inorganic Materials Mahmud Diab and Taleb Mokari* is still unrevealed. The more templates to explore, the more knowledge and technological advancement to achieve. The biotemplates present several advantages compared to the artificial ones, mainly, due to their abundance (they are accumulated in mass quantities), morphological diversity, identical structures, and large size (from a few mm to a few cm). Moreover, modification of the compositions of the biotemplates is usually facile and general. For example, formation of freestanding structures by etching the scaffold material or coating the templates with various organic or inorganic materials can be easily carried out at moderate temperatures and under mild conditions. Finally, biotemplate, contrary to the artificial templates, can be produced in large quantities through cell culturing methods. The obvious main drawbacks of using artificial methods (e.g., assembly/lithography/ printing) for such purposes are high costs, difficulty of fabrication of free-standing structures and, in some cases, limited throughput. Here, we review main advancements in the development and design of hierarchical porous structures using shells of the protist marine microorganisms, diatoms, and foraminifera, and describe their uses in various applications.
Tremendous efforts have been directed at designing functional and welldefined 3D structures in recent decades. Many approaches have been devised and are currently used to create 3D structures, including lithography, 3D printing, assembly, and template-mediated (natural or synthetic) methods. Natural scaffolds offer some unique traits, as compared to their artificial counterparts, presenting highly ordered, porous, identical, abundant, and diverse structures. Various organisms, such as viruses, bacteria, diatoms, foraminifera, and others, are used as templates to form 3D structures. Herein, advancements made in using the shell of marine microorganisms, diatoms, and foraminifera, as scaffolds for designing functional 3D structures are reported. Furthermore, a succinct overview of various synthetic methods used to coat these scaffolds with inorganic materials (i.e., metals, metal oxides, and metal sulfides) is provided. Finally, the use of such fabricated functional 3D structures in a wide range of applications, such as catalysis, sensing, drug delivery, photo-electrochemical uses, batteries, and others, is considered.
1. Introduction The rate with which the use of nanostructures in various devices has advanced in the last two decades is truly remarkable. Nonetheless, a major hindrance that limits the speed of developing prototype devices is the fabrication of nanomaterials into 3D structures, a requirement which is sometimes essential to realizing desired performance and functionalities. The formation of 3D structures from nanomaterials remains a challenge mainly due to the limited ability to form hierarchical 3D structures by a large scale, low cost, and facile process. Nature employs an enormous variety of scaffolds upon which identical 3D structures form which cannot be easily mimicked artificially.[1–4] Up to date, biotemplates have been widely used to create 3D structures. Furthermore, the achievements of very primitive organisms in terms of their synthesis and control of morphology are really remarkable and tremendous knowledge
Dr. M. Diab, Prof. T. Mokari Department of Chemistry and Ilse Katz Institute for Nanoscale Science and Technology Ben-Gurion University of the Negev Beer-Sheva 8410501, Israel E-mail:
[email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201706349.
DOI: 10.1002/adma.201706349
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2. Diatoms Diatoms are unicellular and photosynthetic microorganisms surrounded by rigid cell wall structures (called frustules), comprising hydrated silica and organic material. To date, more than one hundred thousand diatom species with unique frustule morphologies are known.[5] These organisms present sophisticated 3D surface structures that are categorized based on frustule size (1–100 µm range), shape (circular to triangular), and pattern, with examples shown in Figure 1. Diatoms differ from standard plant cells (which include a nucleus, mitochondria, chloroplasts, and cell membrane) due to the unique frustule structure which is composed of two valves connected through several silica bands, with the upper and lower parts being termed the epitheca and hypotheca, respectively.[6] Examination of the frustule surface reveals an interesting texture consisting of nanofeatures with high surface area ranging between 1.4 and 51 (m2 g−1), depending on the type of diatom.[7–13] Due to the properties of their unique shells, diatoms find use in many
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Figure 1. A–H) SEM images of unique and sophisticated morphology structure of some example of diatoms coated inorganic material. A) Fedoped Stephanopyxis turris, B) Coconeis placentula@TiO2, C) Coscinodiscus asteromphalus@Au, D) Ge-doped Pinnularia sp., E) Melosira@MnO2, F) Aulacoseira sp. @ Au, G) Synedra@Ag, and H) E. ZodiacusI@Au. A) Reproduced with permission.[23] Copyright 2017, Springer. B) Reproduced with permission.[24] Copyright 2013, Springer. C) Reproduced with permission.[25] Copyright 2012, Wiley-VCH. (D) Reproduced with permission.[26] Copyright 2008, Wiley-VCH. E) Reproduced with permission.[27] Copyright 2015, Royal Society of Chemistry. F) Reproduced with permission.[28] Copyright 2010, American Chemical Society. G) Reproduced with permission.[29] Copyright 2005, Wiley-VCH. H) Reproduced with permission.[30] Copyright 2012, Wiley-VCH.
applications and devices.[14–20] One of the unique characters of diatoms, as compared to other scaffolds, is the ability to culture and grow them at low costs and in large quantities, which facilitates their use in industry. Several methods were developed for culturing diatom species, such as 1) “batch culture” where the sample is confined within a vessel without replacing the medium, 2) semi-continuous in which a fresh medium culture is added to compensate the taken amount, and 3) continuous culture where new fraction medium is added automatically when the culture density reaches a predetermined set point or when a fresh medium flow continuously with specific rate. Those approaches revealed a high production yield, for example, 2 g L−1 of diatom was successfully cultured in a laboratory using the bath method.[21] Moreover, average yield of ≈130 MT (metric tons) of dry atom per hectare was recorded by Jawkai Bioengineering R&D Center (in Shenzhen, China) from July 2010 to December 2015.[22] Furthermore, shells of marine organisms (diatoms and foraminifera) provide a “friendly” scaffold of which coating and fabrication by a wide range of materials are simply achieved. This generality of chemistry is not restricted to one type of organism (diatom or foraminifera) and can be attained for diverse morphologies, sizes, and compositions.
2.1. Formation of 3D Structures Using Diatom Templates The unique 3D frustules of the diatom surface have motivated many research groups to exploit them as scaffolds for growing functional materials. Table 1 summarizes several examples of shells of marine organisms being used as templates to grow unique 3D structures, the coated materials, and fabrication
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methods used, and their potential applications. Accordingly, numerous methods and techniques have been developed for coating diatom templates, including hydrothermal growth or conversion, vapor phase transport, polymer coating or carbonization, pyrolysis, gas/solid displacement, magnesiothermic, deposition, bioprocessing, and a combination of several of these techniques. In most of those approaches, the diatoms frustule are modified, coated, or converted into other materials, all the while preserving their original structure.[31–35] We report here on four major approaches to achieve 3D functional materials.
2.1.1. Gas and Solution Phase Reactions Metal (such as Au, Ti, Ag, and others) was evaporated on the Aulacoseira sp. surface, followed by etching of the silica either with 1% hydrogen fluoride (HF) solution or 10% ammonium hydroxide, resulting in formation of 3D metal arrays.[29,36] These arrays can be easily converted into different materials via specific chemistry (such as redox reactions, oxidation, etc.). For example, Aulacoseira sp.@ Ag was easily converted into Aulacoseira sp.@ Au via a redox reaction method. Furthermore, atomic layer deposition (ALD) technique was used to coat diatom frustule with TiO2 thin film. The pore diameter of the diatom (Coscinodiscus sp. and T. eccentrica) was tuned by running multiple cycles of ALD. For instance, the size of the external and internal pores of T. eccentric was reduced from 770 ± 30 nm to 725 ± 26 nm and from 43 ± 6 to less than 5 nm, respectively.[37] Hydrothermal fabrication was utilized to design hierarchal 3D metal oxide structures by coating diatoms with TiO2, NiO, TiO2@MnO2, SnO2, MnO2, and others.[24,27,38–40] Briefly,
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Table 1. Marine templates, coated materials, fabrication method of 3D structures, and their potential applications. Marine organism scaffold
Coated material
Fabrication method
Potential applications
Reference
Aulacoseira sp.
Fe3O4
Surface functionalization
Drug carrier
[8]
Aulacoseira sp.
Au
Electroless deposition
Catalyst
[28]
Aulacoseira sp.
SnO2
Surface functionalization combined with sol–gel process
NO sensor
[39]
Aulacoseira sp.
TiO2
Gas/silica displacement reaction
Gas sensor
[40]
Aulacoseira sp.
CdS
Chemical bath deposition (CBD)
Photocatalyst
[43]
Aulacoseira sp.
TiO2
Insertion chemistry
Solar cell
[46]
Coscinodiscus
MoS2
Aerosol assisted chemical vapor deposition (AACVD)
Optoelectronic, catalytic
[10]
Coscinodiscus
TiO2@MnO2
Hydrolysis combined with displacement
Supercapacitor
[17]
Coscinodiscus
Au
Surface functionalization combined with electroless deposition, evaporation
Extraordinary optical transmission (EOT)
[25,35]
Coscinodiscus
ZnS
Sonochemistry
Nanophotonic devices
[32]
Coscinodiscus
TiO2
ALD
Photocatalysts
[37]
Coscinodiscus
NiO
Hydrothermal
Supercapacitor
[41]
Coconeis placentula
TiO2
Sol–gel
Photocatalysts
[24]
Eucampia zodiacus
Au
Layer-by-layer deposition
Surface-enhanced Raman scattering (SERS)
[30]
MnO2
Hydrothermal
Supercapacitor
[27]
Pinnularia sp.
Ge-doped
Insertion chemistry
Electroluminescence
[26,50]
Pinnularia sp
CdS
CBD
Optoelectronic sensors and
[42]
Fe-doped
Insertion chemistry
Not available
[23]
Melosira
Stephanopyxis turris Synedra
Ag, Ti
Evaporation
SERS
[29]
Thalassiosira. eccentrica
TiO2
ALD
Photocatalysts
[37]
Thalassiosira. eccentrica
Au
Evaporation
Catalyst
[35]
Crab
Si, S
Thermal infusion combined with CVD
Rechargeable lithium ion batteries
[52]
β-TCP, Zn-TCP
Hydrothermal
Bone regeneration
[61,63,64]
Sorites
Co, NiO
Thermal decomposition
Water oxidation
[62]
Sorites
Fe(OH)x
Hydrolysis
Water purification
[62]
Calcarina baculatus
coating diatoms with MO (TiO2 and NiO) was achieved by mixing metal salts (titanium tetrafluoride (TiF4) or nickel nitrate (Ni(NO3)2)) and diatom shells in water and heating the solution at 160 °C for 4 h (for TiF4) or 100 °C for 12 h (for Ni(NO3)2) inside a Teflon-lined stainless-steel autoclave followed by additional heating at 450 and 300 °C, respectively, for 2 h.[13,47] Repeating the same process using diatom@TiO2 structures (as templates) instead of plain diatoms and mixing with different metal salt (such as KMnO4 solution) produced a hierarchal structure with multiple shells structure (diatom@ TiO2@MnO2).[13] The coating of diatoms was expanded to metal sulfide inorganic materials, such as MoS2, ZnS, and CdS, with the obtained 3D structures presenting novel properties and functionalities.[14,32,42,43]
2.1.2. Molecular Functionalization An alternative strategy for forming 3D structures using diatoms scaffolds is based on the molecular functionalization of the diatom surface followed by interaction with the desired
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material.[30,33,34,44] For instance, diatoms were coated with poly 2-(dimethylamino) ethyl methacrylate-co-ethylene glycol dimethacrylate polymer that was prepared by functionalizing the diatom surface with bromide ions which act as initiators of the polymerization reaction.[29] To produce a rigid polymer, cross-linking was carried out using an ammonium persulphate and L-ascorbic acid solution without vinyl groups, and the diatoms were removed by KOH solution. Moreover, the inherent negative charge of the diatom surface (measured in solution at pH 2 to 11) can electrostatically attract a positively charged material such as poly(allylamine hydrochloride) (PAH).[44] This PAH can subsequently interact with a negatively charged material, such as surface-modified metallic nano particles, to form complex structures. This route enables the attachment of nanoparticles of desired sizes and shapes to the diatom surface.[30] Furthermore, coating of diatoms with metal nanoparticles was also attained using poly(vinylpyridine)-functionalized diatoms to anchor metal nanoparticles (10–50 nm of Au NPs). Interactions between the polymer and Au occur due to strong affinity between pyridyl groups and the metal nanoparticles.[31]
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2.2.1. Catalysis and Photo-Electrochemical Reactions
2.1.3. Bioclastic and Shape-Preserving Inorganic Conversion (BaSIC) Approach Interestingly, further 3D structure formation was achieved by manipulating the composition of the diatom frustule during their biosynthesis or by chemical transformation into other materials via the BaSIC approach.[26,35,45,46] The BaSIC approach is based on a gas/silica displacement reaction that converts the silica shell into a new composition while preserving shell size and shape. Schoenwaelder and co-workers showed that magnesium and titanium can replace silicon to form a frustule with MgO/TiO2 composition. The magnesium was first converted to Mg(g) by heating at 900 °C and then reacted with the diatom frustule according to the following reaction: 2Mg(g) + SiO2(s) → 2MgO(s) + Mg–Si. Mg–Si is a liquid byproduct that upon cooling changes into Mg and Mg2Si.[35,40] Furthermore, Voelcker and coworkers used the magnesiothermic process to convert the silica of the diatom into silicon, followed by coating with CdS.[43] In their work, the magnesiothermic process was carried out at 650 °C, to form MgO/Si (without the presence of Mg2Si), followed by etching of the MgO using HCl solution. Then, CdS material was grown via chemical bath deposition (CBD). Moreover, for TiO2, titanium halide gas was introduced to replace Si4+ with Ti4+, according to following reaction: TiF4(g) + SiO2(s) → TiO2(s) + SiF4(g). 2.1.4. Insertion Chemistry (Cell Culture) Controlling frustule composition was also realized by inserting the desired materials/cations during diatom growth. Rorrer and co-workers demonstrated that the culture medium in which diatoms grow can be modified so as to control the size of the pore in the organism, as well as the chemical composition and shape of the frustule.[26,45,46] For instance, diatoms were grown in low concentrations of Na2SiO3 sufficient for their survival and duplication (starving conditions). When the silicon source was completely consumed, Na2SiO3 and Ge(OH)4 were added to the growth medium and the germanium was metabolically doped into the shell of Pinnularia sp., achieving an insertion of 1.6 ± 0.1 wt% Ge.[26] The same group expanded their method and inserted Ti4+ cation into the diatom shell without changing the size of the pores, as was observed in the case of Ge.[46] Recently, it was reported that iron can also be inserted into the diatom frustule, however, the amount inserted was limited and formed Fe2O3 clusters.[23] 2.2. Applications of Diatom@ Inorganic Material-Based 3D Structures The unique 3D frustules have motivated many research groups to exploit them as scaffolds for growing functional materials with special properties for catalysis, photocatalysis, supercapacitance, gas sensing, drug delivery, and others, as partially portrayed in Figure 2. We present here on a few examples of using diatom@inorganic material-based 3D structures in various applications.
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Coating Aulacoseira sp. with Au nanoparticles formed a 3D structure with high nitrophenol reducing ability (rate constant, k = 23.5 ± 1 × 10−2 min−1), such that 7.82 × 10−6 m of 4-nitrophenol was reduced within 20 min (Figure 2A).[28] This 3D structure thus offers a clear advantage over Au nanoparticles or microparticles that are not organized in a 3D manner. This augmented catalytic behavior observed was attributed to the high active surface area and also to the existence of Au clusters on the 3D surface. Furthermore, the photo-electrochemical properties of 3D diatom@inorganic materials structures were first reported by Voelcker and co-workers.[43] They showed that coating the Si structure with CdS (by CBD) increased the photocurrent density to 14 µA cm−2, as presented in Figure 2B.
2.2.2. Sensing and Supercapacitance Another group of materials that was grown on diatoms shells is metal oxide (MO). MO nanostructures have been used in a wide range of applications due to their unique properties.[47–51] However, formation of MO nanoparticles in 3D configuration could further enhance their performance and encourage their use in novel and unexplored applications. 3D MO structures have already shown to have a clear advantage, in sensing and supercapacitance applications. For example, diatom@SnO2 3D structures have been used as a sensor for NO gas. Growth of SnO2 on the diatom frustule increases its time response sensing NO, as compared to reported values (Figure 2C). Presently, supercapacitors suffer from a limitation of ion accessibility within devices. Therefore, fabrication of supercapacitors with hierarchal 3D morphology can overcome this obstacle and increase the reaction rate and consequently, enhance performance. For example, diatom@NiO 3D supercapacitors presented enhanced capacitance performance with high stability over 1000 cycles, as shown in Figure 2D.
2.2.3. Drug Delivery Another promising application of these 3D structures is in drug delivery. Magnetic nanoparticles, such as Fe3O4, were electrostatically attached to the diatom surface through the negative charge of the frustule and the positively charged surface of dopamine-functionalized Fe3O4.[8,21] These magnetic structures, diatom@Fe3O4, were used as drug carrier guided by an external field. Figure 2E describes the release of indomethacin loaded in the diatom@Fe3O4; release occurs at two rates, i.e., a fast (within the first 8 h) and a slow (within 2 weeks) rate. The existence of two release rates can be attributed to release of the drug from the diatom surface and diatom pores, respectively.
2.2.4. Optoelectronic Devices Ge-doped Pinnularia sp. exhibits unique electroluminescent (EL) properties. Incorporation of Ge-doped diatom in EL devices created a new emission between 300 and 500 nm and
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Figure 2. Hierarchical diatom-based 3D structures are used in various applications such as: A) Diatom@Au as a catalyst for reduction of 4-nitrophenol, B) Diatom(Si)@CdS as a photo-electrochemical electrode, C) Diatom@SnO2 as a NO sensor, D) Diatom@NiO as a supercapacitor. E) Diatom@ Fe3O4 as a drug carrier, and F) Ge-doped diatom for enhancing the electroluminescent. A) Reproduced with permission.[28] Copyright 2010, American Chemical Society. B) Reproduced with permission.[43] Copyright 2014, Royal Society of Chemistry. C) Reproduced with permission.[39] Copyright 2007, Wiley-VCH. D) Reproduced with permission.[41] Copyright 2014, Elsevier. E) Reproduced with permission.[8] Copyright 2010, Royal Society of Chemistry. F) Reproduced with permission.[26] Copyright 2008, Wiley-VCH.
in the near IR 640 and 780 nm, as compared to plain diatoms (without Ge) that have no emission in that region, as shown in Figure 2F.[26]
3. Foraminifera The second group of microorganisms that we discuss is surrounded by shells comprising calcareous-based materials. Various organisms with highly complex calcareous shells can be found in nature, such as Strombus decorus and Strombus persicus, Homarus americanus, Ophiocoma wendtii, Echinometra mathaei, foraminifera, and others (Figure 3). The unique structures of
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these microorganisms inspired many researchers to recruit them for various applications, such as 3D printing, and as battery electrodes and microlenses.[1,52,53] Among the microorganisms that possess a calcareous shell, foraminifera are very intriguing protozoa, surrounded by shells presenting interesting properties. Indeed, foraminifera are considered as one of the most abundant microorganisms that employ calcium carbonate. To date, there are about 10 000 reported species of foraminifera, of which 40–50 are planktonic.[59] Other species are found elsewhere, such as in sand, mud, rocks, and on the ocean floor. Foraminifera possess a highly porous and complex shell structure composed of calcite, although some rely on other compositions (such as aragonite, sand, organic compounds, etc.).
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Figure 3. Optical and SEM images of: A,F) Strombus decorus persicus shell structure, B,G) exoskeletons of crustaceans, C,H) O. wendtii, D,I) sponge structures of sea urchin after coated with polypyrrole, and E,J) scylla serrata crab, respectively. A,F) Reproduced with permission.[54] Copyright 2003, Royal Society of Chemistry. B,G) Reproduced with permission.[55] Copyright 2009 Wiley-VCH. C,H) Reproduced with permission.[56] Copyright 2005, Royal Society of Chemistry. D,I) Reproduced with permission.[57] Copyright 2011, American Chemical Society. E,J) Reproduced with permission.[57] Copyright 2010, Royal Society of Chemistry.
The size of the foraminiferal shell can vary from 0.1 to 200 mm. Noteworthy, ≈50% of the annual precipitation of CaCO3 in the ocean originates from foraminifera and is estimated at 43 million tons.
minutes to form Sorites@Fe(OH)x, as shown in Figure 4B. Furthermore, the growth of other inorganic materials was achieved via a thermal decomposition process of single source precursor,
3.1. Formation of 3D Structures Using Foraminifera Templates Unlike the diatom case, the use of foraminifera as scaffolds is not widely explored, yet. We describe here on the main two examples of forming foraminifera@inorganic materialbased 3D structures. Hydrothermal, hydrolysis, and thermal decomposition: Otsuka and co-workers used the shell of Calcarina baculatus as a carbonate precursor to obtain tricalcium phosphate (β-TCP) and Zn-TCP via a hydrothermal process in which carbonate and calcium are replaced by phosphate and zinc ions, respectively, as shown in Figure 4A.[60,61] Recently, we reported the fabrication of various types of hierarchical 3D structures consisting of nanofeatures using foraminiferal shell as scaffolds, as shown in Figure 4.[62] In that work, we chose the Sorites structure as template in a case study addressing the growth of hierarchical 3D structures of inorganic materials. Optical images of functional 3D structures such as Sorites@Co, Sorites@ NiO, and Sorites@Fe(OH)x are shown in Figure 4B–D. The coating process of Fe(OH)x was carried out via a hydrolysis process in which Sorites shells were placed in iron aqueous solution and heated at 90 °C for few
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Figure 4. A) SEM image of Calcarina baculatus after conversion to tricalicum phosphate (β-TCP). Reproduced with permission.[61] Copyright 2013, Wiley-VCH. B–D) Optical images of Sorites coated with three different inorganic materials (Fe(OH)x, Co, and NiO, respectively. B–D) Reproduced with permission.[62] Copyright 2018, Wiley-VCH.
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in which Sorites templates were placed in solution of M(acetate)2 (M: Co, Ni) dissolved in hexadecylamine and heated to 270–300 °C.
3.2. Applications of Foraminifera@inorganic Material-based 3D Structures Surprisingly, there are only a few published papers that discuss the use of foraminiferal shells as scaffolds in various applications.[61–67]
3.2.1. Drug Delivery Otsuka and co-workers showed that the Zn-TCP material offers significant improvement in accelerating bone regeneration and restoration.[63,64] Bone regeneration is a lengthy process, therefore, controlled drug release improves and supports restoration. Figure 5A,B shows that when simvastatin was injected directly into mice, muscle was severely damaged. While simvastatin was injected by β-TCP, no harmful effect was observed.
3.2.2. Water Oxidation and Purification We demonstrated the use of foraminifera@ metal oxide as electrocatalysts in water oxidation and as filters in water purification Figure 5. A,B) Optical images of mice after direct injection of simvastatin, and releasing processes.[62] As mentioned above, one of through β-TCP, respectively. Reproduced with permission.[61] Copyright 2013, Wiley-VCH. the drawbacks of using artificially fabricated B) J–V curve using Sorites@Co (red), Sorites@NiO (blue), and silver paint (black) anodes for templates is their small size. For most of water oxidation. C) Schematic2+illustration of the 2+purification process,2+and optical image of the filter. D) Concentration of Pb (black bars), Cd (orange bars), Cu (pink bars) before and the reported values characterizing electro- after filtration using pure Sorites and Sorites@Fe(OH) , respectively. C–E) Reproduced with x catalysts, the actual size of the active area is permission.[62] Copyright 2018, Wiley-VCH. tiny (a few micrometers) and they are usually normalized to a large area (i.e., cm2). based on untreated and Fe(OH)x-modified Sorites as the active Such normalization sometimes does not accurately reflect the true performance of the larger area. It is worth mentioning that material were assessed. These filters presented superior pernormalization is commonly used in many nanocrystal-based formance, with the concentrations of Pb2+, Cd2+, and Cu2+ conapplications due to the low quantities of synthesis outcome taminates being reduced after filtration by 99.98%, 99.9%, and and difficulties in incorporating nanostructures into defect-free 99.99%, respectively (Figure 5E). large scale 3D structures. Using the Sorites shell as a scaffold to Table 2 summarizes the unique physical properties of diagrow inorganic functional 3D materials offers a simple solution toms scaffolds compared to the foraminifera ones. It is clearly to these obstacles. Figure 5 shows the potential of using 3D MO structures in Table 2. Diatom versus foraminifera. water oxidation and purification processes. Sorites@Co and Sorites@NiO electrodes were prepared as electrocatalysts for Diatom Foraminifera an oxygen evolution reaction. These exhibited excellent per2 −1 Surface area Few m2 g−1 Few tens of m g formance, exceeding current state-of-the-art Co and NiO-based Size Few micrometers 0.1–200 mm elctrocatalysts, with the 3D MO structures presenting values of 154.6 and 73.5 mA cm−2 at 1 V versus Ag/AgCl, respectively, Pore size Few nm Few micrometers and an onset potential of ≈0.55 V versus Ag/AgCl (Figure 5C). Free-standing structures Strong acid (HF), strong base Mild acid solution (0.1 m) Sorites and Sorites@Fe(OH)x have also been used for removing Morphological diversity Ten thousands Ten thousands inorganic (metal ion) contaminates from water (a schematic Culturing Simple Limited illustration is shown in Figure 5D). The performances of filters
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seen that the surface area of diatom scaffolds is higher and pore size is smaller than the foraminiferal templates. Obviously, the difference in the surface area and the pores size is a real advantage for using diatoms in sensing, catalysis, or any other application that involves using high surface area materials. However, using calcite templates has a clear advantage in terms of the simplicity of etching the template under mild conditions. This advantage becomes more prominent when the presence of the template interferes with the performance of the devices. Moreover, foraminiferal scaffolds are commonly larger in size (from a few mm to a few cm) than diatoms, which facilitates their use in many technological applications due to the ability to easily fabricate the device using only a few species. Usually, fabrication of small templates in a large scale process is limited and requires more efforts.
4. Conclusions and Future Perspectives Despite the amazing progress made in controlling the size, morphology, composition, and assembly of nanostructures, the design of functional 3D porous structures presenting desired nanofeatures remains a challenge. Nature includes numerous examples in which the shape and properties of materials can be controlled. Inspired by the fascinating natural structures, we can design functional 3D structures that cannot be easily mimicked artificially. This paper provided a concise overview of the use of shells of the marine organisms diatoms and foraminifera as scaffolds for forming functional 3D structures. These shells possess unique, abundant, identical, and highly complex structures. A combination of natural scaffolds with facile, cheap, and general coating methods paves the way for forming hierarchical 3D structures with superior properties and performances. For example, we showed that coating calcareous foraminiferal shells with metal oxide nanostructures opened new perspectives for catalytic and water purification applications. These findings have implications for a wide range of other potential applications, such as batteries, photonic crystals, cell culturing, etc.
Acknowledgements The authors thank Prof. Sigal Abramovich and Prof. Uri Abdu for helpful discussion. M.D. also acknowledges a Ministry of Science, Technology and Space scholarship.
Conflict of Interest The authors declare no conflict of interest.
Keywords 3D structures, diatoms, microorganism scaffolds
foraminifera,
inorganic
nanomaterials,
Received: November 1, 2017 Revised: March 14, 2018 Published online: June 19, 2018
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