*2.3. Electrospinning*

Electrospinning technology first appeared in the 1930s. It has renewed interest in recent years and was used to prepare CNFs. It is also the only method to produce continuous CNFs [19,30,38–42]. In the electrospinning process, first the polymer solution or melt is charged with thousands to tens of volts of static electricity. The charged polymer forms a Taylor cone at the spinning port under the action of an electric field. When the electric force exceeds the internal tension of the spinning solution, the Taylor cone is drafted and accelerated. The moving jet is gradually drafted and thinned. Due to its extremely fast rate of motion, the fibers ultimately deposited on the collecting plate are nanoscale, forming a fibrous mat similar to a nonwoven fabric. Then, the fiber mat is pre-oxidized in air and carbonized in a nitrogen atmosphere to finally obtain CNFs.

Compared with other nanofiber manufacturing methods, the electrospinning method has the following advantages: (1) Electrospinning usually uses voltages of several thousand volts or more, but the current used is small, so that energy consumption is small; and (2) a nanofiber nonwoven fabric can be directly produced. By electrospinning, the nanofibers can be made into a nonwoven fabric in a two-dimensional expanded form, so that no further processing is required after spinning. In particular, the development of multi-nozzle spinning technology has increased the production of nanofiber nonwovens and improved production e fficiency; (3) it can be spun at room temperature. The electrospinning method allows spinning at room temperature, so that a solution containing a compound having poor thermal stability can also be spun; (4) a wide range of raw materials. Thus far, there have been reports on the use of synthetic polymers such as polyester and polyamide, and natural high molecular substances such as collagen, silk, and DNA as raw materials to prepare nanofibers by electrospinning.

#### **3. Design and Synthesis of CNF-Based Nanomaterials**

In recent years, with the rapid development of nanofabrication technology, more and more carbon-based nanomaterials have been used as sensors for detecting di fferent target molecules [43–45]. Depending on the type of material being loaded, we can classify the carbon-based nanofibers used as sensors into five types: Pure CNFs, CNFs loaded with metal NPs, CNFs loaded with metal oxides, CNFs loaded with metal alloys, and others.

#### *3.1. Pure CNFs*

Due to their high specific surface area and good electrocatalytic ability towards the oxidation of specific organic matter, pure CNFs are commonly used to detect small molecules, viruses, proteins, and nucleic acids in food quality control and clinical analysis. For example, Yue et al. reported mesoporous CNF-modified pyrolytic graphite electrode for the simultaneous determination of uric acid, ascorbic acid, and dopamine [46]. Koehn et al. prepared a vertically aligned CNF electrode array by the PECVD method, and then integrated the CNF array with the wireless instantaneous neurotransmitter sensor system to detect dopamine by fast scan cyclic voltammetry [47]. Rand and coworkers developed a biosensor based on vertically aligned CNFs for the simultaneous detection of serotonin and dopamine in the presence of excess ascorbic acid [48]. Periyakaruppan et al. reported similar CNFs based nanoelectrode arrays for label-free detecting cardiac troponin-I in the early diagnosis of myocardial infarction (Figure 2a,b) [49].

**Figure 2.** (**a**) SEM image of vertically aligned CNF array, (**b**) TEM image of a stacked cone morphology of CNFs, (**c**) SEM image of pure CNFs, and (**d**) TEM image of a single CNF. Pictures (**a**) and (**b**) were reprinted with permission from Reference [49]. Copyright American Chemical Society, 2013. Pictures (**c**) and (**d**) were reprinted with permission from Ref. [50]. Copyright Elsevier, 2011.

Tang et al. directly modified electrospun CNFs onto carbon paste electrode (CPE) for the electrochemical detection of xanthine, L-Tryptophan, L-tyrosine, and L-cysteine without any enzyme or medium, respectively (Figure 2c,d) [50,51]. The CNFs-modified CPE showed high electrocatalytic activity and fast amperometric response towards the oxidation of the xanthine and three amino acids. Guo and coworkers reported similar electrospun CNFs-modified CPE for simultaneous determination of catechol and hydroquinone in lake water samples [52].

#### *3.2. CNFs Modified with Metal NPs*

Since the conductivity of the metal NPs and their high electrochemical activity toward the target substance can effectively reduce the overpotential, and they can be embedded in the defect sites of the CNFs to improve the sensitivity and anti-interference ability of the sensor [53–56]. Huang et al. prepared a Pd NPs-decorated CNFs sensor for detecting H2O2 and nicotinamide adenine dinucleotide (NADH) [57] (Figure 3a,d). This Pd NPs-loaded CNFs modified electrode can also be used for simultaneously detecting dopamine, uric acid, and ascorbic acid [58]. On the other hand, Liu and coworkers modified Pd NP-loaded CNFs onto the carbon paste electrode for efficient detection of oxalic acid [59]. Claramunt et al. prepared an efficient gas sensor by modifing Au NPs onto CNFs [60]. Hu et al. developed a Rh NP-decorated CNFs sensor for the detection of hydrazine [61] (Figure 3c,f). Fu et al. modified Cu NP-loaded CNFs composite onto the glassy carbon electrode for the detection of catechol [62]. Liu et al. and Rathod et al. modified Ni and Pt NPs onto CNFs, respectively (Figure 3b,e). Additionally, the as-prepared composites can be used for non-enzymatic glucose sensing [63,64]. In addition, the loaded metal NPs can form a more sparse conductive network inside the nanocomposite, which can enhance the electrical conductivity of the CNFs, making the composite highly sensitive to stress. Hu et al. synthesized a composite material for a piezoresistive strain sensor consisting of Ag NPs-coated CNFs with an epoxy resin, which shows an extremely high sensitivity to stress changes [65].

**Figure 3.** SEM images of (**a**) Pd NPs-decorated CNFs, (**b**) Ni NPs-decorated CNFs and (**c**) Rh NPs-decorated CNFs. TEM images of (**d**) Pd NPs-decorated CNFs, (**e**) Ni NPs-decorated CNFs, and (**f**) Rh NPs-decorated CNFs. Pictures (**a**) and (**d**) were reprinted with permission from Reference [58]. Copyright Elsevier, 2008. Pictures (**b**) and (**e**) were reprinted with permission from Reference [63]. Copyright Elsevier, 2009. Pictures (**c**) and (**f**) were reprinted with permission from Reference [61]. Copyright Elsevier, 2010.

#### *3.3. CNFs Modified with Metal Oxides*

Since some acid gases and organic gases can cause changes in the electrical resistance of metal oxide-decorated CNFs, metal oxide-decorated CNFs can be used for the detection of specific acid gas and organic gas. Lee and coworkers fabricated ZnO/SnO2 nanonodules-decorated CNFs for dimethyl methylphosphonate gas detection by single nozzle co-electrospinning using two phase-separated polymer solutions [66]. Later, this group modified WO3 nanonodule to the surface of CNFs for the detection of NO2 gas using the same method, and found that the sensitivity of the WO3 nanonodule-decorated CNFs increased the amount of the decorated WO3 on the CNFs surface [67].

Hu and co-workers demonstrated the electrospun preparation of mesoporous MnO2 and Mn3O4 NPs-decorated CNFs, and found that the fabricated hybrid CNFs have a diameter of 200–300 nm with high surface area [68]. In another case, Xia and co-workers reported the general synthesis of ultrafine transition metal oxide (Zn, Mn, and Co) NPs-embedded porous CNFs via a facile electrospinning strategy, following through the calcination process [69]. As shown in Figure 4, there are abundant interconnected pores distributed in the ZnO/CNFs, MnO/CNFs, and CoO/CNFs, and the Zn, Mn, and Co elements are homogeneously distributed inside the porous CNFs, respectively.

**Figure 4.** TEM and the corresponding elemental mapping images of (**<sup>a</sup>**,**d**) ZnO/CNFs, (**b**,**<sup>e</sup>**) MnO/CNFs, and (**<sup>c</sup>**,**f**) CoO/CNFs. Reprinted with permission from Reference [69]. Copyright Wiley-VCH, 2016.

#### *3.4. CNFs Modified with Alloys*

Compared with single-metal NPs, metals alloy exhibit superior electrocatalysis due to their binary structure interface synergy, which makes metals alloy NPs-modified CNF sensors exhibit stronger anode peak potential and redox current [32]. Huang et al. prepared Ag-Pt alloy NPs by the NaBH4 reduction method and modified them onto electrospun CNFs for the selective detection of dopamine (Figure 5A) [70]. Guo and coworkers synthesized Pd-Ni alloy NP/CNFs composite by the simple method involving electrospinning of precursor polyacrylonitrile/Pd(acac)2/Ni(acac)2 and subsequent thermal process to reduce metals and carbonize polyacrylonitrile (Figure 5B). The as-prepared Pd-Ni alloy NP/CNFs composite significantly improved electrocatalytic activity for sugar oxidation, and Pd-Ni alloy NP/CNFs based electrode can be used for sugar detection in flow systems [71]. Li et al. fabricated a series of MCo (M = Fe, Cu, Mn, and Ni) alloy NPs-decorated CNFs by electrospinning and thermal treatment process, and found that the CuCo alloy NPs doped-CNFs exhibit the best detection efficiency for glucose in human serum samples [72].

**Figure 5.** SEM, TEM and the elemental mapping images of (**A**) Ag-Pt alloy NPs-decorated CNFs and (**B**) Pd-Ni alloy NPs-decorated CNFs. Pictures (**A**) were reprinted with permission from Reference [70]. Copyright American Chemical Society, 2014. Pictures (**B**) were reprinted with permission from Reference [71]. Copyright American Chemical Society, 2014.

#### *3.5. CNFs Modified with Silica and Polymers*

In addition, some other materials such as silica, polyurethanes, polydimethylsiloxane, nafion, etc., are also used to modify CNFs for sensing [73]. For example, Vamvakaki et al. used biomimetically synthesized silica modified CNFs for the detection of acetylcholinesterase, and the fabricated silica/CNF composite shows an operational lifetime of more than 3.5 months under continuous polarization (Figure 6) [74]. Lu and coworkers modified hemoglobin to CNFs with the help of Nafion membrane, and the prepared CNFs-based composite can mediator-free detect H2O2 [75]. Zhu et al. prepared an elastomer/CNF strain sensing composite for detecting tensile forces [76]. Baeza and coworkers embed CNFs in cement for strain and damage detection [77]. Azhari et al. embed CNFs and carbon nanotubes in cement for piezoresisitive sensing [78]. Tallman et al. embed CNFs in polyurethane for tactile imaging and distributed strain sensing, and found that the piezoresistive response of CNFs/polyurethane nanocomposites depends strongly on the nanofiller volume fraction [79]. The sensitivity of the CNFs/polyurethane nanocomposites increased with decreasing CNFs volume fraction.

**Figure 6.** SEM images of (**a**) CNFs and (**b**) silica/CNFs composite. Reprinted with permission from Reference [74]. Copyright American Chemical Society, 2008.

#### **4. Sensor Applications of CNF-Based Nanomaterials**

The higher surface area of CNFs can adsorb relatively more target molecules. In addition, CNFs also have good electron transfer ability. These characteristics make CNFs-based nanomaterials have broad prospects in chemical sensing [80–83]. According to the type and nature of the target substances, we mainly introduce the application of CNFs-based nanomaterials as sensors in the following four aspects.

#### *4.1. Gas Sensors*

Li and coworkers prepared one-dimensional CNFs composed of graphitic nanorolls using a simple electrospinning-assisted solid-phase graphitization method, the as-prepared graphitic CNFs exhibit sensitivity to H2, CO, CH4, and ethanol gases at room temperature, and the detection limit for CO gas is as low as 50 ppm [84]. Zhang et al. reported a H2S sensor based on ZnO-CNFs composites, the as-prepared H2S sensor showed a linear response in the range of 50–102 ppm of H2S [85]. Claramunt et al. deposited metal alloy NPs-decorated CNFs on Kapton for the detection of NH3 [60]. The results show that the sensitivity of Au and Pd NPs-decorated CNFs to NH3 can be improved by controlling the percentage of Au and Pd. Moreover, the response time of the sensor is up to 5 minutes within 110–120 ◦C. However, when compared with the spectroscopic sensor such as mid-infrared sensor and quartz-enhanced photoacoustic sensor [86–90], which have the advantages of rapid detection at room temperature without any reagent, the operation temperature of Au, and Pd NPs-decorated CNFs is much higher.

In order to reduce the detection temperature, Lee et al. modified WO3 nanonodules onto the CNFs, and the prepared WO3 nanomodule-decorated CNFs not only provides a higher sensing surface area, but also WO2+ on the surface of the material can combine with the O2− of NO2, realizing the detection of NO2 gas at room temperature, and the detection limit for NO2 reach 1 ppm (Figure 7) [67].

**Figure 7.** (**a**) SEM and TEM (inset) images of the WO3 nanomodule-decorated CNFs; (**b**) High resolution transmission electron microscope (HRTEM) image of the WO3 nanomodule-decorated CNFs; and (**c**) NO2 gas detection mechanism of the WO3 nanomodule-decorated CNFs. Reprinted with permission from Reference [67]. Copyright the Royal Society Chemistry, 2013.

#### *4.2. Strain*/*Pressure Sensors.*

Conventional micro-electro mechanical system (MEMS) pressure sensors such as silicon piezoresistive pressure sensor and silicon capacitive pressure sensor have the advantages of high measurement accuracy, low power consumption, and low cost, but perform poorly in high-intensity piezoresistive measurements. Due to its low cost, electrical conductivity, and potentially enhanced mechanical properties such as fracture toughness and strain capacity, CNFs are also commonly used for material structure health monitoring [91–95]. Zhu and coworkers used vistamaxx 6202FL (ethylene content 15 wt%, propylene 85%) as the hosting polymer matrix to fabricate conductive polymer nanocomposites reinforced with CNFs via the solvent-assisted casting method. The as-prepared electrically conductive polymer nanocomposite can be utilized as strain sensors with large mechanical

deformation (Figure 8a,b). The resistivity is reversibly changed by 102–103 times upon stretching to 120% strain and recovery to 40% strain (Figure 8c) [76].

**Figure 8.** (**a**) SEM image of the 5 wt% CNFs/Vistamaxx 6202FL polymer nanocomposite; (**b**) real permittivity of the CNFs/Vistamaxx 6202FL polymer nanocomposite in the frequency range of 20–1000; and (**c**) cyclic strain applied to specimen and the instantaneous response of resistivity with strain of the 5 wt% CNFs/Vistamaxx 6202FL polymer nanocomposite. Reprinted with permission from Reference [76]. Copyright American Chemical Society, 2011.

Azhari et al. prepared a conductive cement-based piezoresistive sensor by mixing 15% CNFs and 1% carbon nanotubes. The sensor is more accurate and repeatable than traditional cement-based sensors, with load amplitudes up to 30 kN and the gauge factor is about 445 [78]. Bazea et al. synthesized a CNF and cement composite to measure strains on the surface of a structural element, and found that the CNF cement-based composite with a gauge factor of 190 can be obtained by adding 2 wt% CNFs to cement [77]. Hu and coworkers fabricated a resistance-type strain sensor by using Ag-coated CNFs and epoxy. The as-prepared Ag-coated CNFs/epoxy composite shows higher strain sensitivity and better conductivity than that of CNFs without Ag-coating, and has a gauge factor of 155, this value is ~80 times higher than that in a metal-foil strain gauge [65]. In the application of CNFs/polyurethane nanocomposite for tactile imaging and distributed strain sensing, Tallman et al. found that the piezoresistive response is most sensitive to strain changes when the CNFs filling volume fraction is 12.5%–15%. When the CNFs filling volume fraction is 7.5%, there is a region in which the conductivity changes the most in the tactile imaging [79]. Yan and coworkers fabricated a flexible strain sensor by using carbon/graphene composites nanofiber yarn/thermoplastic polyurethane, this strain sensor shows a high level of stability during 300 stretching relaxation, and the average gauge factor value is more than 1700 under an applied strain of 2% [94].

#### *4.3. Sensors of Small Molecules*

CNFs-based nanomaterials can not only be used to detect gas molecules and strain sensing, but can also to detect small molecules [96]. Table 1 lists the CNF-based nanomaterials for detecting different small molecules and their properties. Huang et al. loaded palladium NPs on CNFs to prepare a Pd/CNFs modified carbon paste electrode for the detection of dopamine (DA), uric acid (UA), and ascorbic acid (AA) [57]. After being modified with Pd NPs-loaded CNFs (Pd/CNFs), the oxidation overpotentials of DA, UA, and AA were significantly reduced when compared to the bare carbon paste electrode. The detection limits of Pd/CNFs modified carbon paste electrodes for DA, UA and AA were 0.2 μM, 0.7 μM, and 15 μM, respectively, and the linear range was 0.5–160 μM (DA), 2–200 mM (UA), and 0.05–4 mM (AA). Liu et al. reported another Pd NPs-loaded CNFs modified carbon paste electrode for oxalic acid detection, the detection limit of the as-prepared sensor for oxalic acid as low as 0.2 mM, and shows a linear range from 0.2 to 45 nM [59]. Liu et al. also prepared a Ni/CNFs composite electrode by electrospinning for glucose detection [63]. The as-prepared Ni/CNFs hybrid shows higher sensitivity towards glucose due to the electrocatalytic activity of the Ni NPs and the stability of the carbon electrode. In the absence of chloride poisoning, the detection limit of the Ni/CNFs composite

electrode for glucose is 1 μM, with a linear range of 2 μM–2.5 mM (R = 0.9997). Li and coworkers synthesized a magnetic composite through one-pot polymerization of dopamine, laccase, and Ni NPs loaded CNFs (Figure 9). The as-prepared magnetic composite exhibited high selectivity towards catechol, and showed a linear range from 1 to 9100 μM, with a detection limit of 0.69 μM for catechol in water samples [56].


**Table 1.** Different CNF-based nanomaterials for small molecules detection 1.

1 CNFs: carbon nanofibers; DA: dopamine; Pd/CNFs: palladium nanoparticle-loaded CNFs; NADH: nicotinamide adenine dinucleotide; UA: uric acid; AA: ascorbic acid; Ni/CNFs: Ni NP-loaded CNFs; PANI-IL-CNF: polyaniline-ionic liquid-CNF; OA: oxalic acid; Pt/CNFs: platinum NP-loaded CNFs; Rh/CNFs: rhodium NP-loaded CNFs; DMMP: dimethyl methylphosphonate; ZnO/CNFs: ZnO decorated CNFs; GNPs/CNF/Au: gold electrode modified with CNFs and gold NPs; CC: catechol; HQ: quinone; MCNF/PGE: mesoporous CNF-modified pyrolytic graphite electrode; Trp: L-tryptophan; Tyr: L-tyrosine; Cys: L-cysteine; VACNFs: vertically aligned CNFs; 5-HT: serotonin; CuO/rGO/CNFs: CuO nanoneedle/reduced graphene oxide/CNFs; Pd-HCNF: palladium-helical CNF hybrid; HRP-CNFs: CNFs modified with horseradish peroxidase; PtNP-CNF: platinum NP-decorated CNF; Ag-Pt/pCNFs: nanoporous CNFs decorated with Ag-Pt bimetallic NPs; Cu/CNFs: copper/carbon composite nanofibers; PDA-Lac-NiCNFs: polydopamine-laccase-nickel NP loaded CNFs; Pd-Ni/CNFs: Pd-Ni alloy NP/CNF composites; CuCo-CNFs: bimetallic CuCo NPs anchored and embedded in CNFs.

Lee et al. fabricated a ZnO/CNFs composite for the detection of DMMP, and ZnO NPs decorated on CNFs increased the specific surface area of the sensor and its affinity for DMMP [66]. The detection limit of ZnO/CNFs composite for DMMP is 0.1 ppb, with a linear range of 0.1–1000 ppb. Huang et al. modified glass carbon electrode using electrospun CNFs loaded with Ag-Pt alloy NPs [70]. The as-prepared composite electrode can detect DA in the presence of UA and AA, and the detection limit for DA is 0.11 μm, and the linear range is 10–500 μm. Tang et al. directly modified CNFs onto carbon paste electrode for determining amino acids [51]. The detection limit for the L-tryptophan (Trp), L-tyrosine (Tyr), and L-cysteine (Cys) was 0.1 μm, with linear ranges of 0.1–119 μM for Trp, 0.2–107 μM for Tyr, and 0.15–64 μM for Cys. Li et al. prepared CuCo alloy NPs-decorated CNFs by electrospinning [72]. The as-prepared CuCo/CNFs composite exhibits high sensitivity to glucose in human urine. The response time for glucose is 2 s and the linear range is 0.02–11 mM.

**Figure 9.** Synthetic route of magnetic Polydopamine-Laccase-Ni NP loaded CNFs composite and its catalytic oxidation of catechol on the electrode. Reprinted with permission from Reference [56]. Copyright American Chemical Society, 2014.

#### *4.4. Sensors of Biomacromolecules*

The high surface area and large number of active sites of CNFs can not only provide the grounds for the adsorption of proteins and enzymes, but CNFs can also provide the direct electron transfer and stabilize enzyme activity [103]. Therefore, CNFs are the most promising substrates for the development of biosensors [104–106]. Periyaruppan and coworkers developed a CNF-based nanoelectrode array for cardiac troponin-I (cTnI) detection in the early diagnosis of myocardial infraction [49]. After being modified with the anti-cTnI, the as-prepared biosensor showed high selectivity and sensitivity to cTnI, it could detect as low as 0.2 ng/mL of cTnI, and showed linear concentration relationships in the ranges of 0.25–1.0 and 5.0–100 mg/mL.

In order to protect the protein from protease attack, Vamvakaki et al. used biomimetically synthesized silica to encapsulate the CNFs-immobilized enzyme acetylcholine esterase [74]. The obtained silica/CNF architecture improves enzyme stabilization against thermal denaturation and avoids protease attack, and exhibits an operational lifetime of more than 3.5 months under continuous polarization. Arumugam and coworkers fabricated a 3 × 3-array biosensor using nanopatterned vertically aligned CNF arrays (VACNFs) for E. Coli O157:H7 detection, the as-prepared patterned

array showed nanoelectrode behavior and produced reliable electrochemical responses with high signal-to-noise (>3) [107]. Gupta et al. also reported a nanoelectrode array based on vertically aligned CNFs, and found that the decrease in redox current and the increase in charge transfer resistance are proportional to the concentration of the C-reactive protein [108]. The detection limit of this biosensor for C-reactive protein reaches 90 pM, which is in the clinically relevant range. Later, Swisher and coworkers fabricated another nanoelectrode arrays using VACNFs for measuring the activity of proteases [109]. As shown in Figure 10, legumain and cathepsin B are covalently attached to the exposed VACNFs tip, with a ferrocene moiety linked at the distal end. The enhanced AC voltammetry properties enable the kinetic measurements of proteolytic cleavage of the surface-attached tetrapeptides by proteases, and the "specificity constant" kcat/Km of the VACNF nanoelectrode arrays for cathepsin B and legumain is (4.3 ± 0.8) × 10<sup>4</sup> and (1.13 ± 0.38) × 10<sup>4</sup> M−<sup>1</sup> s<sup>−</sup>1, respectively. These values are about two times that measured by a fluorescence assay.

**Figure 10.** (**a**) Vertically aligned carbon nanofiber (VACNF) array embedded in the SiO2 matrix and (**b**) electron transfer from appended ferrocene at the distal end of the peptide to the underlying metal film electrode through the VACNFs and the loss of the electrochemical signal from ferrocene due to the cleavage of the peptide at specific sites. Reprinted with permission from Reference [109]. Copyright American Chemical Society, 2013.

#### **5. Conclusions and Outlooks**

Based on the above introduction and discussion on the synthesis and sensor applications of CNF-based functional nanomaterials, it can be concluded that CNFs play important roles for the fabrication of various sensors for gas, pressure, strain, small molecules, macromolecules, and other analytes. The using of CNF-based materials for sensor applications has a few advantages, for instance, the mesoporous of CNFs and nano/micro porous structures of CNF-based materials improved the surface area of electrode materials, the modification of CNFs with various NPs, polymers, and biomolecules enhanced the sensing performance, and the physical and chemical interactions between analytes and CNFs increased the sensing sensitivity of the fabricated sensors. Moreover, CNFs can be continuously prepared by electrospinning and raw materials polyvinylpyrrolidone (PVP) and polyacrylonitrile (PAN) are inexpensive. CNFs-based sensors generally exhibit high stability and selectivity to target molecules due to the high mechanical strength and chemical inertness of CNFs, and its ability to significantly reduce the oxidation overpotential. It is believed that this work will be valuable for readers to develop novel CNF-based functional materials through various fabrication techniques, and explore other potential applications in energy, catalysis, and environmental science.

While the synthesis and applications of CNFs and CNF-based materials have been widely studied in the last years, in our opinion, more efforts could be done in the following aspects. First, new synthesis methods for CNFs could be developed. Currently, chemical vapor deposition and electrospinning are the main strategies for creating CNFs. Other methods like template-based synthesis, self-assembly, and chemical hydrothermal methods could also be considered to achieve in the synthesis of CNFs with high efficiency. Second, the biological modification of CNFs for subsequent biomedical applications including biosensors, anti-bacterial materials, bone tissue engineering, and others could be further explored. Third, it is possible to fabricate CNF-based of 2D membranes and 3D aerogels/scaffolds for water purification applications. In addition, more attentions could be paid to the design and fabrication of high-performance energy storage materials/devices such as batteries, supercapacitors, solar cells, and fuel cells by introducing suitable functional nanoscale building blocks into the CNF systems.

**Author Contributions:** G.W. proposed and organized the contents of this review. Z.W., S.W., J.W., A.Y. and G.W. wrote the paper. G.W.made revisions of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Zhuqing Wang thanks the financial support from Education department and Science and Technology of Anhui province (gxyqZD2019045, 1808085MB42). Gang Wei acknowledges the support from the Deutsche Forschungsgemeinschaft (DFG) under grants WE 5837/1-1 (GW).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
