Self-Assembled Corn-Husk-Shaped Fullerene Crystals as Excellent Acid Vapor Sensors

Low-molecular-weight acid vapors cause aging and destruction in material processing. In this paper, facile fabrication of novel corn-husk-shaped fullerene C60 crystals (CHFCs) through the dynamic liquid–liquid interfacial precipitation method is reported. The CHFCs were grown at the liquid–liquid interface between isopropyl alcohol (IPA) and a saturated solution of C60 in mesitylene under ambient temperature and pressure conditions. The average length, outer diameter, and inner diameter of CHFCs were ca. 2.88 μm, 672 nm, and 473 nm, respectively. X-ray diffraction (XRD) analysis showed the CHFCs exhibit a mixed face-centered cubic (fcc) and hexagonal-close pack (hcp) crystal phases with lattice parameters a = 1.425 nm, V = 2.899 nm3 for fcc phase and a = 2.182 nm, c = 0.936 nm, a/c ratio = 2.33, and V = 3.859 nm3 for hcp phase. The CHFCs possess mesoporous structure as confirmed by transmission electron microscopy (TEM) and nitrogen sorption analysis. The specific surface area and the pore volume were ca. 57.3 m2 g−1 and 0.149 cm3 g−1, respectively, are higher than the nonporous pristine fullerene C60. Quartz crystal microbalance (QCM) sensing results show the excellent sensing performance CHFCs sensitive to acetic acid vapors due to the enhanced diffusion via mesoporous architecture and hollow structure of the CHFCs, demonstrating the potential of the material for the development of a new sensor system for aliphatic acid vapors sensing.


Introduction
Volatile organic acids, mostly known as low-carbon number corrosive acids such as formic acid, acetic acid, are considered as detrimental factors for human diseases [1], beverage industry [2][3][4], and metal-aging protection [5,6]. Efficient, price-reasonable, sensitive sensor materials are crucial for health diagnosing [7] and environmental quality monitoring [8,9]. A selective acid detector is supposed to discriminate the aimed compound from others, which are of similar molecule weight and chemical properties like formic acid and acetic acid. However, current metal-oxide or nitride electrode sensors [10][11][12] are only possible in high-temperature environments. Carbon-based nanostructures have attracted significant attention in academia as a fabulous tool for developing gas sensors [13,14] which have good prospects for low-energy consumption and cost-effective properties.
Discovered in 1985, fullerene, an allotrope of carbon, is a cage-structured molecule with excellent redox, optoelectronic properties [15,16], which function as an ideal zerodimensional building block nanoarchitecture, and has already been broadly applied in sensing technologies [17][18][19][20][21]. Out of the demand for synthesis and fabrication of functional materials such as energy storage, environment protection, and device technology, Pristine fullerene C 60 (pC 60 ) powder (purity 99.9%) was purchased from BBS Chemicals, Chimes Drive, Houston, TX, USA. Isopropyl alcohol (IPA, purity 99.7%) and mesitylene (purity 99.8%) were purchased from the Wako Chemical Corporation, Tokyo, Japan.

Synthesis of Corn-Husk-Shaped Fullerene Crystals (CHFCs)
A saturated solution of fullerene C 60 was prepared by dissolving excess pC 60 powder (60 mg) in mesitylene (50 mL). Undissolved excessive fullerene was removed by the filtration. Using the DLLIP method, the self-assembled fullerene C 60 crystals were synthesized. Typically, isopropyl alcohol (IPA: 5 mL) was quickly added into the freshly prepared saturated solution of C 60 in mesitylene (1 mL: 0.999 mg/mL), immediately followed by a vigorous hand-shaking for about 3 s. The mixture was incubated at 25 • C for 15 min avoiding any external mechanical disturbances. The CHFCs precipitates were washed with IPA (5 mL) three times to remove the organic solvent and finally separated from the mixture by centrifugation and dried in a vacuum oven at 70 • C for 3 h.

Vapor Sensing by Quartz Crystal Microbalance (QCM)
The vapor sensing property of CHFCs was studied using a QCM technique. In this study, we monitored the frequency change in the Au-resonator loaded with CHFCs as QCM electrode exposed to different guest vapors by a resonance frequency of 9 MHz (AT-cut). Note that the QCM electrode was stable in the air (±2 Hz) over a recorded period of 10 min. QCM sensor electrode was prepared as follows: CHFCs (1 mg) were dispersed in IPA (1 mL), and then the mixture was sonicated for 30 s. Since the C 60 molecule is stable in IPA, suspension of IPA will not lead to the morphology changing of the CHFCs. CHFCs suspension (2 µL) was then drop cast onto the QCM electrode (Au resonator). For the contrast group of pristine C 60 , to get obvious frequency shift on QCM, a larger volume C 60 saturated solution of mesitylene (5 µL) was drop casted onto the gold electrode. The electrodes were dried at 70 • C in a vacuum for 5 h. The resulting QCM electrode was then plugged onto the instrument and then exposed to different solvents vapors (10 mL in an opening container) at room temperature. The exposing experiments were conducted in a sealed chamber to create a saturated vapor atmosphere for absorption of respective vapors and to minimize the escaping of vapors during the frequency monitoring. After the frequency reached equilibrium, the chamber was removed for the desorption of vapors. The repeatability test of CHFCs as the QCM electrode was also performed during vapors' alternate absorption and desorption cycles by measuring the time dependence of frequency shift (∆f). Similarly, all the QCM electrodes were prepared and the mass loaded on the QCM electrodes of the CHFCs on a single electrode was normalized to 1 µg.
The mass change m (g cm −2 ) in the sample loaded on the QCM electrode can be measured by the quartz electrode's frequency shift (∆f). The frequency change linear dependence of sample amount loaded on the QCM electrode can be calculated from the Sauerbrey equation: where ƒ 0 (Hz) is the natural frequency of the quartz crystal, ∆m is mass change (g), A is area between electrodes (cm 2 ), ρ Q is quartz density (2.649 g cm −3 ), and µ Q is the shear modulus (2.947 × 1011 g cm −1 s 2 ).

Results
The surface morphology of the self-assembled CHFCs prepared by the dynamic liquid-liquid interface precipitation method (DLLIP) under the ambient conditions of pressure and temperature was studied by SEM analyses. Figure 1 shows SEM images and the corresponding size distributions of the CHFCs. The self-assembled fullerene nanostructure exhibits a corn-husk-shaped structure with a half-opening hollow structure in one end (Figure 1a  The scanning transmission electron microscopy (STEM) and TEM images revealed the hollow structure of the CHFCs (Figure 2a-d). High-resolution STEM images ( Figure  2b) and TEM images (Figure 2c,d) confirm that the CHFC has a unique thin tubular wall with a half-opening structure in one end while the other has a solid structure. As seen in the HR-TEM, the CHFCs have microporous structures with crystalline pore walls ( Figure  2e), which provide nano spaces required for the guest vapor adsorption, enhancing the gas sensing performance. Moreover, densely packed crystal lattice planes parallel to the CHFCs to the length of CHFC can be seen. The lattice spacing ca. 0.925 nm corresponds to the distance between (110) planes of the hcp crystal phase [36,37]. Previously, it has found that the precipitation from IPA and C60 mesitylene solution using static or modified LLIP method results in the formation of 1D tubular structures at both the ends with a solid core at the center of the tubes [38]. Presently, DLLIP method is used, which initially assembled into cross-linked (X or Y-shaped) structures with hollow ends (Figure 2b,c). These cross-linked assemblies disassemble due to vigorous mechanical shaking by hand and result in the formation of CHFCs. Since the nuclei formation time The scanning transmission electron microscopy (STEM) and TEM images revealed the hollow structure of the CHFCs (Figure 2a-d). High-resolution STEM images ( Figure 2b) and TEM images (Figure 2c,d) confirm that the CHFC has a unique thin tubular wall with a half-opening structure in one end while the other has a solid structure. As seen in the HR-TEM, the CHFCs have microporous structures with crystalline pore walls (Figure 2e), which provide nano spaces required for the guest vapor adsorption, enhancing the gas sensing performance. Moreover, densely packed crystal lattice planes parallel to the CHFCs to the length of CHFC can be seen. The lattice spacing ca. 0.925 nm corresponds to the distance between (110) planes of the hcp crystal phase [36,37].  (Figure 2c,d) confirm that the CHFC has a unique thin tubular wall with a half-opening structure in one end while the other has a solid structure. As seen in the HR-TEM, the CHFCs have microporous structures with crystalline pore walls ( Figure  2e), which provide nano spaces required for the guest vapor adsorption, enhancing the gas sensing performance. Moreover, densely packed crystal lattice planes parallel to the CHFCs to the length of CHFC can be seen. The lattice spacing ca. 0.925 nm corresponds to the distance between (110) planes of the hcp crystal phase [36,37]. Previously, it has found that the precipitation from IPA and C60 mesitylene solution using static or modified LLIP method results in the formation of 1D tubular structures at both the ends with a solid core at the center of the tubes [38]. Presently, DLLIP method is used, which initially assembled into cross-linked (X or Y-shaped) structures with hollow ends (Figure 2b,c). These cross-linked assemblies disassemble due to vigorous mechanical shaking by hand and result in the formation of CHFCs. Since the nuclei formation time Previously, it has found that the precipitation from IPA and C 60 mesitylene solution using static or modified LLIP method results in the formation of 1D tubular structures at both the ends with a solid core at the center of the tubes [38]. Presently, DLLIP method is used, which initially assembled into cross-linked (X or Y-shaped) structures with hollow ends (Figure 2b,c). These cross-linked assemblies disassemble due to vigorous mechanical shaking by hand and result in the formation of CHFCs. Since the nuclei formation time for C 60 precipitation is relatively short, usually within a few milliseconds, synthesis of the homogeneous CHFCs requires ultrafast addition of IPA into fullerene solution followed by vigorous handshaking to maximize the intermixing and interactions of IPA and mesitylene solution.
Structural characterization of the CHFCs was performed using the pXRD technique and compared the results with pC 60 (Figure 3a). The pXRD pattern shows a mixed facecentered cubic (fcc) and hexagonal close-pack (hcp) phase of the CHFCs. The diffraction peaks at diffraction angles of 10.7, 17.6, and 20.66 • , correspond to the (111), (022), and (024) planes of the fcc phase, which is similar to that of the pC 60 . While, the diffraction peaks at 7. for C60 precipitation is relatively short, usually within a few milliseconds, synthesis of the homogeneous CHFCs requires ultrafast addition of IPA into fullerene solution followed by vigorous handshaking to maximize the intermixing and interactions of IPA and mesitylene solution.
Structural characterization of the CHFCs was performed using the pXRD technique and compared the results with pC60 (Figure 3a). The pXRD pattern shows a mixed facecentered cubic (fcc) and hexagonal close-pack (hcp) phase of the CHFCs. The diffraction peaks at diffraction angles of 10.7, 17.6, and 20.66°, correspond to the (111), (022), and (024) planes of the fcc phase, which is similar to that of the pC60. While, the diffraction peaks at  Raman scattering spectrum of the CHFCs is similar to that of pC60 ( Figure 3b). Raman spectra showed two Ag mode (Ag(1) breathing mode of peak at 495.5 cm −1 , and Ag(2) pentagonal pinch mode of peak at 1464 cm −1 ) and six Hg vibration bands (Hg(1), Hg(2), Hg (3), Hg(4), Hg (7) and Hg (8) corresponding to the Raman peaks at 271, 431, 709.5, 771.5, 1421, 1566.5 cm −1 ). Out of these Raman active bands, peak shift of Ag(2) is a sensitive indicator of polymerization of C60 molecules and has been used as an analytical probe for the structural and optoelectronic properties of fullerene C60 molecule [39]. Note that the Ag(2) peak position of the pC60 does not shift in the CHFCs, suggesting the persistence of the free molecular rotation of C60 molecules in the self-assembled CHFCs. The Raman spectra reveal that C60 molecules make the majority of building blocks to form CHFCs nanostructure. Moreover, the laser irradiation during measurements did not cause polymerization of C60 molecules; self-assembled crystals keep in the molecular state without forming intermolecular bonding between the adjacent C60 molecules. The four significant peaks that appeared at wavenumbers of 525, 576, 1182, and 1429 cm −1 in the FTIR spectrum of the CHFCs are the characteristics of fullerene molecules (Figure 3d). The FTIR band at 2906 cm −1 corresponds to the C-H bond stretching vibrations of solvent molecules (mesitylene) remaining CHFCs (Figure 3c,d).
XPS studied the surface composition of the CHFCs. Figure 4 shows the XPS survey spectra and XPS core level C 1s and O 1s spectra with the deconvoluted peaks for CHFCs and pC60. The survey spectra XPS peaks at 284 and 532 eV indicate carbon and nitrogen as the main surface components of the CHFCs and pC60 (Figure 4a). From the peak analyses, carbon and oxygen were respectively ca. 89.6%, 10.4% for pC60, and 92.6% and 7.4% Raman scattering spectrum of the CHFCs is similar to that of pC 60 (Figure 3b). Raman spectra showed two A g mode (A g (1) breathing mode of peak at 495.5 cm −1 , and A g (2) pentagonal pinch mode of peak at 1464 cm −1 ) and six H g vibration bands (H g (1), H g (2), H g (3), H g (4), H g (7) and H g (8) corresponding to the Raman peaks at 271, 431, 709.5, 771.5, 1421, 1566.5 cm −1 ). Out of these Raman active bands, peak shift of A g (2) is a sensitive indicator of polymerization of C 60 molecules and has been used as an analytical probe for the structural and optoelectronic properties of fullerene C 60 molecule [39]. Note that the A g (2) peak position of the pC 60 does not shift in the CHFCs, suggesting the persistence of the free molecular rotation of C 60 molecules in the self-assembled CHFCs. The Raman spectra reveal that C 60 molecules make the majority of building blocks to form CHFCs nanostructure. Moreover, the laser irradiation during measurements did not cause polymerization of C 60 molecules; self-assembled crystals keep in the molecular state without forming intermolecular bonding between the adjacent C 60 molecules. The four significant peaks that appeared at wavenumbers of 525, 576, 1182, and 1429 cm −1 in the FTIR spectrum of the CHFCs are the characteristics of fullerene molecules (Figure 3d). The FTIR band at 2906 cm −1 corresponds to the C-H bond stretching vibrations of solvent molecules (mesitylene) remaining CHFCs (Figure 3c,d).
XPS studied the surface composition of the CHFCs. Figure 4 shows the XPS survey spectra and XPS core level C 1s and O 1s spectra with the deconvoluted peaks for CHFCs and pC 60 . The survey spectra XPS peaks at 284 and 532 eV indicate carbon and nitrogen as the main surface components of the CHFCs and pC 60 (Figure 4a). From the peak analyses, carbon and oxygen were respectively ca. 89.6%, 10.4% for pC 60 , and 92.6% and 7.4% for CHFCs. The presence of oxygen can be attributed to the partially oxidized surface and also due to moisture contamination. for CHFCs. The presence of oxygen can be attributed to the partially oxidized surface and also due to moisture contamination.  [39].
The surface area and porosity of the CHFCs were investigated by using nitrogen adsorption measurements. The nitrogen sorption isotherm of CHFCs suggests porous structures ( Figure 5a). While due to the lack of porosity, the nitrogen uptake of pC60 is low. The specific surface area and pore volume of the CHFCs were found to be 57.3 m 2 g −1 and 0.149 cm 3 g −1 , respectively. The pore size distribution curve obtained from the density functional theory (DFT) method ( Figure 5b) shows a pronounced peak at the half pore width of ca. 1.38 nm. Some pores with sizes in the meso length could also be seen. The pore size distribution profile The high-resolution core level C 1s XPS spectrum of the CHFCs and pC 60 could be deconvoluted into three major peaks centered at 285.5 (285.0 for pC 60 ), 286.4 (286.1 for pC 60 ), and 290.2 eV corresponding to C=C (sp 2 ), C-C (sp 3 ), and CO 3 2bonding states of carbon (Figure 4b). The high-resolution O 1s spectrum of pC 60 could be deconvoluted into two main peaks with the binding energy of 533.5 and 534.6 eV corresponding to C-OH, and C-O-C bonding states, while the O 1s spectrum of the CHFCs could be deconvoluted into three peaks with the peak position of 532.5, 533.7, and 535.1 eV corresponding to C=O, C-OH, and C-O-C bonding states (Figure 4c). The surface compositions of the CHFCs and pC 60 are similar to other fullerene assemblies [39].
The surface area and porosity of the CHFCs were investigated by using nitrogen adsorption measurements. The nitrogen sorption isotherm of CHFCs suggests porous structures (Figure 5a). While due to the lack of porosity, the nitrogen uptake of pC 60 is low. The specific surface area and pore volume of the CHFCs were found to be 57.3 m 2 g −1 and 0.149 cm 3 g −1 , respectively. for CHFCs. The presence of oxygen can be attributed to the partially oxidized surface and also due to moisture contamination.  (Figure 4c). The surface compositions of the CHFCs and pC60 are similar to other fullerene assemblies [39].
The surface area and porosity of the CHFCs were investigated by using nitrogen adsorption measurements. The nitrogen sorption isotherm of CHFCs suggests porous structures (Figure 5a). While due to the lack of porosity, the nitrogen uptake of pC60 is low. The specific surface area and pore volume of the CHFCs were found to be 57.3 m 2 g −1 and 0.149 cm 3 g −1 , respectively. The pore size distribution curve obtained from the density functional theory (DFT) method ( Figure 5b) shows a pronounced peak at the half pore width of ca. 1.38 nm. Some pores with sizes in the meso length could also be seen. The pore size distribution profile The pore size distribution curve obtained from the density functional theory (DFT) method ( Figure 5b) shows a pronounced peak at the half pore width of ca. 1.38 nm. Some pores with sizes in the meso length could also be seen. The pore size distribution profile obtained from the Barrett-Joyner-Halenda (BJH) model (Figure 5c) indicates the mesopore structure in the CHFCs with an average pore radius of 2.11 nm. Considering the acetic acid molecular size of 4 Å, the well-defined pore structures of the CHFCs are suitable for adsorption of various volatile acid vapors [40].
Self-assembled fullerene nanomaterials are highly hydrophobic and thus present strong van der Waals and hydrophobic interactions with guest molecules. Inspired by the unique corn-husk morphology with porous structures, the CHFCs were employed as sensor material to detect toxic VOCs such as benzene, toluene, formic acid, and acetic acid with quartz crystal microbalance (QCM) techniques. Figure 6a shows a typical example of the time-dependent frequency shifts (∆f) of the CHFCs modified QCM electrode upon exposure to different volatile solvents vapors: acetic acid, ethanol, toluene, and formic acid. The frequency shifts swiftly upon exposure to the volatile vapors. At the same time, the frequency returns to the initial frequency when the solvent vapors are removed from the chamber. Thus, the CHFCs exhibited a rapid absorption and desorption process of VOC.
Chemosensors 2022, 10, x FOR PEER REVIEW 7 of 11 obtained from the Barrett-Joyner-Halenda (BJH) model (Figure 5c) indicates the mesopore structure in the CHFCs with an average pore radius of 2.11 nm. Considering the acetic acid molecular size of 4 Å , the well-defined pore structures of the CHFCs are suitable for adsorption of various volatile acid vapors [40]. Self-assembled fullerene nanomaterials are highly hydrophobic and thus present strong van der Waals and hydrophobic interactions with guest molecules. Inspired by the unique corn-husk morphology with porous structures, the CHFCs were employed as sensor material to detect toxic VOCs such as benzene, toluene, formic acid, and acetic acid with quartz crystal microbalance (QCM) techniques. Figure 6a shows a typical example of the time-dependent frequency shifts (Δf) of the CHFCs modified QCM electrode upon exposure to different volatile solvents vapors: acetic acid, ethanol, toluene, and formic acid. The frequency shifts swiftly upon exposure to the volatile vapors. At the same time, the frequency returns to the initial frequency when the solvent vapors are removed from the chamber. Thus, the CHFCs exhibited a rapid absorption and desorption process of VOC. Previous investigations have shown that the fullerene nanomaterials are sensitive to aromatic vapors due to the strong π-π interactions [18,41]. Therefore, the value of frequency shift (Δf) greatly depends on the interactions between the guest vapor molecules and the nanostructure, which explains why aliphatic vapors lead to a reasonably slight shift in the frequency. As for the self-assembled CHFCs', the frequency shift caused by the adsorption of aromatic vapors are ca. 85 Hz (benzene), 80 Hz (toluene), 56 Hz (aniline), while the aliphatic vapors are 10 Hz (hexane), 15 Hz (cyclohexane), and hydroxyl compound vapors are 10 Hz (2-propanol), 210 Hz (ethanol), 114 Hz (methanol), 143 Hz (water). Interestingly, the CHFCs are especially sensitive to volatile acid vapor 680 Hz (formic acid), 2460 Hz (acetic acid) (Figure 6b). The sensitivity of the CHFCs modified QCM electrode decreases as follows: acetic acid > formic acid > ethanol > water > acetone > pyridine Previous investigations have shown that the fullerene nanomaterials are sensitive to aromatic vapors due to the strong π-π interactions [18,41]. Therefore, the value of frequency shift (∆f ) greatly depends on the interactions between the guest vapor molecules and the nanostructure, which explains why aliphatic vapors lead to a reasonably slight shift in the frequency. As for the self-assembled CHFCs', the frequency shift caused by the adsorption of aromatic vapors are ca. 85 Hz (benzene), 80 Hz (toluene), 56 Hz (aniline), while the aliphatic vapors are 10 Hz (hexane), 15 Hz (cyclohexane), and hydroxyl compound vapors are 10 Hz (2-propanol), 210 Hz (ethanol), 114 Hz (methanol), 143 Hz (water). Interestingly, the CHFCs are especially sensitive to volatile acid vapor 680 Hz (formic acid), 2460 Hz (acetic acid) (Figure 6b). The sensitivity of the CHFCs modified QCM electrode decreases as follows: acetic acid > formic acid > ethanol > water > acetone > pyridine > formaldehyde > methanol > toluene > aniline > cyclohexane > hexane > 2-propanol. The VOC sensing of the pC 60 was also investigated and compared the performance with that of the CHFCs (Figure 6b). Compared to the pC 60 , the self-assembled CHFCs show extraordinary sensing performance. Due to the unique morphology (hollow structure) with porous architectures, the CHFCs improve the possibility of volatile vapors' selective adsorption. Figure 6c shows the repeatability test results of the QCM electrode recorded upon alternate exposure and removal of acetic acid vapors. The electrode shows sensitive sensing performance with reliable repeatability and reproducibility. Table 1 shows a table comparing the gas-sensing performance of fullerene assemblies as the QCM sensor materials.

Discussion
Molecular sensing of toxic VOCs has received attention and has been deeply investigated to deal with the continuously growing safety concern of the environment and workstation of indoor environments. Low molecular weight acids such as formic acid and acetic acid have been known as essential constituents for environmental contamination, metal corrosion, and food storage. The acetate and formate-based contamination can be traced back to being released from wooden materials [42]. Notably, a high concentration of acetic acid has been monitored to be released from the wooden building materials such as cases, cabinet, draws, shelving, etc. The mechanism is considered to be that the efflorescent salts within the pores or surface of the objects emit acetic acid. Thus, acetic acid is thought to be a threatening component for the indoor environment and ambient air.
Moreover, the air contaminated with acetic acid or formic acid is detrimental to the human respiratory tract, affecting the equilibrium of the interior milieu and the health of organisms. Therefore, based on these fundamental problems, developing novel sensor materials with high sensitivity towards these toxic acid vapors has attracted much attention. Previous investigations have shown that the sensing performance (selectivity and sensitivity) of a sensor depends on the structure and properties of the sensor material. Porous materials offer better sensitivity due to the easy diffusion of guest vapor molecules that can be accommodated in the pore structures. Therefore, the VOC sensing material design needs to engineer the pore architecture of the nanomaterials. Generally, the higher the surface area the better the adsorption of the guest vapors. Therefore, to solve the existing problem of toxic acid vapor contaminations, one needs to design novel nanomaterials with high surface areas and well-defined pore structures essential for high-performance sensing [43][44][45][46][47][48][49].
It has been found that fullerene, due to its fused π-conjugated structure, offers strong van der Walls or π-π interactions and shows high sensitivity to the aromatic guest vapors. Furthermore, the insertion of micro-mesopore architectures in the host materials promotes the diffusion of the guest vapors and enhances the sensing system for the vapor phase. For example, hierarchically-structured fullerene C 70 cubes consisting of mesoporous nanorods having crystalline pore walls showed excellent sensing performance for aromatic guest vapors due to the strong π-π interactions with the sp 2 carbon-rich pores, and easy and fast diffusion through the mesoporous architecture [18]. As a result, the frequency shift caused by the adsorption of aromatic solvents' vapor such as toluene (721 Hz) and pyridine (538 Hz) is much higher than the frequency shift caused by the adsorption of aliphatic hydrocarbon vapors hexane (40 Hz) and cyclohexane (70 Hz). The sensing sensitivity decreases as toluene > pyridine > ethanol > acetone > cyclohexane > hexane > water. Nevertheless, regardless of the generally accepted trend, it is found in this study that the sensitivity of the CHFCs QCM sensor is higher for acetic acid compared to the aromatic vapors despite the higher saturated vapor pressure of the aromatic solvents. It is anticipated that the hydrogen bonds that hold together acetic acid in the vapor phase in the dimeric species strongly interact with electron-deficient fullerene molecules, enhancing the vapor adsorption in the CHFCs. The sensing sensitivity of the CHFCs sensor decreases as acetic acid > formic acid > ethanol > water > acetone > pyridine > formaldehyde > methanol > toluene > aniline > cyclohexane > hexane > 2-propanol. This result demonstrates a strong interaction between fullerene and acetic acid vapor, which is promoted by the enhanced diffusion of the acetic acid vapor due to the unique thin tubular wall with a half-opening structure in one end of the CHFCs. Furthermore, the tubular wall comprises a microporous structure with crystalline pore walls (Figure 2e), which provides nano spaces required for the adsorption of acetic acid guest vapors, enhancing the gas sensing performance. On the other hand, due to the lack of well-defined hollow and micro/mesoporous structures, the VOC sensing performance of the pC 60 is poor than the self-assembled novel CHFCs.

Conclusions
In conclusion, the formation of novel corn-husk-shaped fullerene C 60 crystals (CHFCs) at the liquid-liquid interface between isopropyl alcohol (IPA) and a saturated solution of C 60 in mesitylene under ambient temperature and pressure conditions is successfully demonstrated. The CHFCs exhibit a mixed face-centered cubic (fcc) and hexagonal-close pack (hcp) crystal phases with lattice parameters a = 1.425 nm, V = 2.899 nm 3 for fcc and a = 2.182 nm, c = 0.936 nm, a/c ratio = 2.33, V = 3.859 nm 3 for hcp phase, respectively. In addition to the hollow structure in one end, the CHFCs possess porous structures with well-defined micro-and mesopore structures. Quartz crystal microbalance (QCM) sensing results show the excellent sensing performance of the CHFCs modified QCM electrode sensitive to acetic acid vapors due to the enhanced diffusion via mesoporous architecture and hollow structure of the CHFCs. Thus, these results demonstrate that in addition to the conventional nanomaterials and molecular nanocarbon such as graphene and carbon nanotubes [50][51][52], the corn-husk-shaped fullerene C 60 crystals have the considerable possibility of developing new functional sensor systems for VOC sensing selective to the aliphatic acid vapors.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.