**1. Introduction**

With the development of nanotechnology and material science, many kinds of zero-dimensional (0D) to three-dimensional (3D) materials have been created for sensor applications [1–5], in which the one-dimensional (1D) nanomaterials exhibited promising potential [6]. It is well known that 1D architectures could provide shortened pathways for the transfer of electrons and facilitate the penetration of electrolyte along the longitudinal axis of nanofiber/nanowire [7,8], and therefore causing improved sensing performance.

Carbon nanotubes (CNTs), as one of the widely used 1D nanomaterials, have been previously utilized for the fabrication of various high-performance sensors and biosensors due to the unique mechanical, electrical, and magnetic properties of CNTs [9,10]. In addition, the high surface area and high adsorption ability towards various molecules/biomolecule of CNTs make CNTs very good candidates to fabricate chemical and biological sensors with high sensitivity and selectivity.

Besides CNTs, carbon nanofibers (CNFs) have also been widely studied due to their unique chemical and physical properties and similar structure to fullerenes and CNTs [11,12]. CNTs are hollow with a graphite layer parallel to the axis of the inner tube. The graphite layers of CNFs often form an angle with the axis of the inner tube, and the interior thereof may be hollow or solid. The diameters of CNTs are usually less than 100 nm, while the diameter of CNF is in the range of 10 to 500 nm and the length can reach 10 μm. Meanwhile, CNFs have exclusively basal graphite planes and

edge planes, exhibiting high potentials for their surface modification or functionalization to create functional hybrid CNF-based nanomaterials, which have been applied in the fields of biomedicine [13], tissue engineering [14], nanodevices [11], sensors [15,16], energy [17], and environmental science [18]. Previously, several reviews on the synthesis and applications of CNFs and CNF-based materials have been released [19–23]. For instance, Zhang et al. demonstrated the potential strategies for creating CNFs with electrospinning and then summarized their applications in biomedicine, sensor, energy, and environmental fields [19]. Feng et al. summarized the synthesis, properties, and applications of CNFs and CNF-based composites, in which the applications of CNF materials in electrical devices, batteries, and supercapacitors were introduced in detail [20]. Zhang and co-workers provided recent advances in the electrospun synthesis and electrochemical energy storage application of CNFs [21].

In this review, we focus on the preparation of CNF-based nanomaterials for sensor applications. In the second part, we introduced the synthesis strategies of CNFs via chemical vapor deposition and electrospinning techniques, and in the third part we demonstrated the design and synthesis of CNF-based nanomaterials by the functionalization of pure CNFs with metallic nanoparticles (NPs), metal oxide NPs, alloy NPs, silica, and polymers. In the fourth part, the sensor applications of CNF-based nanomaterials towards gas, strain, pressure, small molecules, and biomacromolecules are introduced and discussed. Finally, the conclusions and outlooks for the synthesis and applications of CNF-based nanomaterials are given. It is expected that this work will be helpful for the readers to understand the sensing mechanisms of CNF-based sensors and develop new CNF-based nanomaterials for the applications besides sensors.

#### **2. Synthesis of Carbon Nanofibers (CNFs)**

CNFs have large specific surface area, small number of defects, large aspect ratio, low density, high specific modulus and strength, high electrical and thermal conductivity, etc., and therefore have broad application prospects in the fields of storage, electrochemistry, adsorbent, and sensing [11,24–27]. At present, methods for preparing CNFs mainly include thermal chemical vapor deposition, plasma enhanced chemical vapor deposition, and electrospinning [28], as shown in Figure 1.

**Figure 1.** Typical method for synthesizing Carbon nanofibers (CNFs).

#### *2.1. Thermal Chemical Vapor Deposition*

The chemical vapor deposition method is a method for synthesizing CNFs by thermally decomposing a low-cost hydrocarbon compound on a metal catalyst at a constant temperature (500–1000 ◦C) [20,29]. The thermal chemical vapor deposition method can be classified into the following three types according to the manner of the catalyst added or present: The substrate method, the spray method, and the gas phase flow catalytic method.

#### 2.1.1. The Substrate Method

The substrate method utilizes ceramic or SiO2 fibers as substrate to uniformly disperse nanosized catalyst particles (such as Fe, Co, Ni, and other transition metals) on its surface. The hydrocarbon gas is pyrolyzed on the surface of the catalyst, and carbon is deposited and grown to obtain nanoscale carbon fibers. Enrique et al. fabricated high purity CNFs on the ceramic substrate by using CH4/H2 (or C2H6/H2) as the carbon source and Ni as the catalyst at 873 K, and explored the influence of conditions (different carbon sources, temperatures, etc.,) on the layer thickness, uniformity, and porosity of CNFs [30].

The substrate method can prepare CNFs of relatively high purity, but the CNFs can only grow on the substrate dispersed with catalyst particles. Since nanoscale catalyst preparation is difficult and the product and catalyst cannot be separated in time, it is difficult to achieve large-scale continuous production of CNFs.

#### 2.1.2. The Spray Method

The spraying method is to mix the catalyst in a liquid organic substance such as benzene, and then spray the mixed solution containing the catalyst into a high temperature reaction chamber to prepare CNFs. The continuous injection of the catalyst can be realized by growing the carbon fiber by the spray method, which provides favorable conditions for industrial continuous production [31,32]. However, the uneven distribution of the catalyst particles during the spraying process and the ratio of the hydrocarbon gas to the hydrocarbon gas are difficult to control, resulting in a small proportion of the CNFs produced by this method, and a certain amount of carbon black is formed.

#### 2.1.3. The Gas Phase Flow Catalytic Method

The gas phase flow catalysis method directly heats the catalyst precursor, and introduces it into the reaction chamber together with the hydrocarbon gas in the form of gas. The catalyst and the hydrocarbon gas are decomposed of different temperature zones, and the decomposed catalyst atoms are gradually aggregated into the nanoscale particles and then the CNFs produced on the nanoscale catalyst particles [33,34]. Since the catalyst particles decomposed from the organic compound can be distributed in a three-dimensional space, and the amount of volatilization of the catalyst can be directly controlled, the yield per unit time of this method is large and continuous production is possible.

#### *2.2. Plasma-Enhanced Chemical Vapor Deposition (PECVD)*

The plasma contains a large amount of high-energy electrons that can provide the activation energy required for the chemical vapor deposition process. The collision of electrons with gas phase molecules can promote the decomposition, compounding, excitation, and ionization processes of gas molecules, and generate various chemical groups with high activity [35–37]. While the plasma enhanced chemical vapor deposition method can produce aligned CNFs, the cost of this method is high, the production efficiency is low, and the process is difficult to control.
