Embedded nano spin sensor for in situ probing of gas adsorption inside porous organic frameworks


Embedded nano spin sensor for in situ probing of gas adsorption inside porous organic frameworks

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ABSTRACT Spin-based sensors have attracted considerable attention owing to their high sensitivities. Herein, we developed a metallofullerene-based nano spin sensor to probe gas adsorption


within porous organic frameworks. For this, spin-active metallofullerene, Sc3C2@C80, was selected and embedded into a nanopore of a pyrene-based covalent organic framework (Py-COF). Electron


paramagnetic resonance (EPR) spectroscopy recorded the EPR signals of Sc3C2@C80 within Py-COF after adsorbing N2, CO, CH4, CO2, C3H6, and C3H8. Results indicated that the regularly changing


EPR signals of embedded Sc3C2@C80 were associated with the gas adsorption performance of Py-COF. In contrast to traditional adsorption isotherm measurements, this implantable nano spin


sensor could probe gas adsorption and desorption with in situ, real-time monitoring. The proposed nano spin sensor was also employed to probe the gas adsorption performance of a


metal–organic framework (MOF-177), demonstrating its versatility. The nano spin sensor is thus applicable for quantum sensing and precision measurements. SIMILAR CONTENT BEING VIEWED BY


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MAGNETIC RESONANCE SPECTROSCOPY Article Open access 04 February 2022 INTRODUCTION Recently, the use of electron spins for advanced science and technology has been actively pursued owing to


their genuine quantum characteristics1,2,3,4. Consequently, spin-based sensors have gained significant attention owing to their high sensitivities and broad applications in probing physical


and chemical parameters, such as electrical5 or magnetic fields3, molecule6 or protein dynamics7, and nuclei8,9 or other particles. Along with this spin-based sensing research, quantum


sensing has also emerged, causing a significant change in the paradigms of metrology. For example, spin-bearing diamonds with nitrogen-vacancy centers have been extensively studied for


quantum sensing10,11,12,13. Moreover, several other spin systems could also potentially perform unique functions, such as quantum probing, which must be explored. Among these, molecular


spins exhibit unique quantum properties, and certain proposals based on molecular spins for quantum computation and information storage have attracted considerable attention from physicists


and chemists14,15,16. In particular, molecular spin systems feature numerous advantages, including the molecular-level chemical engineering of their spins. Moreover, molecular spins have


independent nanoscale units that can realize in situ monitoring complex systems. Therefore, molecular spins demonstrate potential for quantum sensing applications. Notably, paramagnetic


endohedral metallofullerenes are nanoscale ball-like molecules with sensitive electron spins17,18,19,20. Owing to the presence of a fullerene cage, the endohedral electron spin presents high


chemical stability. Moreover, the most significant advantages of such metallofullerene molecules are their susceptible electron spins, which can be potentially employed in quantum sensing


applications. Accordingly, recent studies have explored the susceptible electron spins of metallofullerene molecules for sensing local magnetic fields, nanospace, molecule orientations, and


molecule motions20,21,22,23,24. To promote these applications, it is, however, essential to explore appropriate sensing systems. Porous organic frameworks are emerging crystalline porous


materials with broad applications in gas adsorption and separation25,26,27,28, gas storage29,30, sensing31,32,33,34,35,36, and catalysis37,38,39. Notably, covalent organic frameworks (COFs)


and metal–organic frameworks (MOFs) have been investigated for extensive applications. For example, COFs have attracted significant interest owing to their wide surface areas and


hierarchical nanopores, which are capable of encapsulating several guest species and causing chemical and physical changes. To reveal the adsorption mechanism and promote future


applications, understanding and monitoring the nanopores within COFs, specifically detecting these nanopores in situ, are crucial steps. However, effective methods for such measurements are


currently lacking. Notably, the aromatic fullerene shell of metallofullerenes endows them with lipophilicity, making them COF-nanopore-accessible. Therefore, paramagnetic endohedral


metallofullerenes with sensitive electron spins can potentially sense COF nanopores in situ. For a proof-of-concept study, we used a representative paramagnetic metallofullerene, Sc3C2@C80,


as an embedded spin probe to sense gas adsorption within a pyrene-based COF (Py-COF). Note that Sc3C2@C80 has a highly stable electron spin protected by a carbon cage, and this stability


forms the basis of its application. Moreover, Sc3C2@C80 has a highly sensitive electron spin, as reported in our previous studies18,40,41. These features make Sc3C2@C80 suitable for gas


adsorption detection within porous materials. Py-COF, prepared using self-condensing building blocks with formyl and amino groups, is a recently emerging porous organic framework material


with unique adsorption properties42. Its theoretical pore size is 1.38 nm; thus, one metallofullerene Sc3C2@C80 unit (approximately 0.8 nm in size) can enter one nanopore of Py-COF.


Therefore, the molecular spin of Sc3C2@C80 can effectively sense a nanopore of Py-COF in situ. In this study, the molecular spin of Sc3C2@C80 was employed to probe the gas adsorption


performance of Py-COF for N2, CO, CO2, CH4, C3H6, and C3H8 gases by recording the electron paramagnetic resonance (EPR) signals of the embedded Sc3C2@C80. The EPR intensities of the embedded


Sc3C2@C80 were observed to change when Py-COF adsorbed different gases. Therefore, the adsorption isotherms of Py-COF for different gases were recorded to comparatively illustrate the gas


adsorption performance. Additionally, an in situ and real-time gas sensing process of Py-COF based on embedded Sc3C2@C80 was designed, further demonstrating the application potential of this


nano spin sensor. Moreover, the mechanism and evaluation methodology of this spin sensor were studied by analyzing the EPR signal intensity and linewidth. Finally, Sc3C2@C80 was employed to


probe the gas adsorption performance of an MOF (MOF-177), illustrating the versatility of this nano spin sensor. RESULTS EPR STUDIES OF METALLOFULLERENE ENTRAPPED INSIDE PY-COF The basic


concept underlying the metallofullerene-based nano spin sensor used for probing the gas adsorption performance of Py-COF is presented in Fig. 1a. Generally, when gas molecules are adsorbed


onto COF or MOF materials, these molecules adhere to the pore frameworks and form gas layers on the frameworks43,44,45. These gas layers then restrict the motion of embedded Sc3C2@C80,


intensify spin–lattice interactions, and consequently weaken EPR signals. In this case, more gas molecules would enhance the thickness and viscosity of these gas layers and further weaken


the EPR signals of embedded Sc3C2@C80. For this testing platform, a nano spin sensor involving Sc3C2@C80 embedded in a Py-COF channel can sense gas adsorption through spin–lattice


interactions, which influence the EPR signal intensity and linewidth. Typically, Py-COF is synthesized using bifunctional monomeric 1,6-bis(4-formylphenyl)−3,8-bis(4-aminophenyl)pyrene, with


two formyl and two amino groups, through intermolecular self-condensation, forming a 2D pyrene-based imine COF42. Py-COF demonstrates good thermal stability, as revealed by


thermogravimetric analysis (Supplementary Fig. 1). In our study, Sc3C2@C80 was synthesized using the Kräschmer−Huffman arc-discharge method46, and Supplementary Fig. 2 shows the preparation


and characterization details. As presented in Fig. 1b, Sc3C2@C80 has an unpaired electron localized in the Sc3C2 cluster. A theoretically computed unit structure of Sc3C2@C80⊂Py-COF is shown


in Fig. 1c. The theoretical pore size of Py-COF is 1.38 nm; thus, a pore of Py-COF can effectively accommodate a metallofullerene Sc3C2@C80 unit (approximately 0.8 nm in size) through


host–guest interactions, including van der Waals forces and certain π–π interactions. The predicted position of Sc3C2@C80 within Py-COF pores indicates that the molecular spin of Sc3C2@C80


can sense the environment of a Py-COF nanopore in situ. In addition, the encapsulation of Sc3C2@C80 inside the channel of a Py-COF would not substantially prevent gases from infiltrating,


and ample space may be available for gas adsorption, as observed from Fig. 1c. Sc3C2@C80 was entrapped in Py-COF using the adsorption method in a toluene solution, yielding a complex of


Sc3C2@C80⊂Py-COF, as shown in Fig. 1d and Supplementary Fig. 3a. After the adsorption of Sc3C2@C80, the color of Py-COF changed from light yellow to brown, and the obtained complex was


sufficiently washed using toluene to remove Sc3C2@C80 physically adhered on the Py-COF surface. Moreover, the mass fraction of Sc3C2@C80 in Py-COF was calculated to be 1.5% (Supplementary


Fig. 3b), and the analysis process is presented in Supplementary Note 1. Next, PXRD experiments were performed to characterize the crystallinity of Py-COF and Sc3C2@C80⊂Py-COF, as shown in


Fig. 1d. A comparison between their PXRD patterns indicated that Sc3C2@C80 did not disrupt the crystal structure of Py-COF. The detection of the Sc 2_p_ peak in Sc3C2@C80⊂Py-COF via X-ray


photoelectron spectroscopy (XPS) indicated that Sc3C2@C80 molecules were trapped inside the channels of Py-COF, as shown in Supplementary Fig. 4. Scanning electron microscopy (SEM) images of


Py-COF and Sc3C2@C80⊂Py-COF revealed that the composite material morphology and size did not change after preparation (Supplementary Fig. 5). The transmission electron microscopy (TEM)


element mapping images and energy dispersive spectroscopy results revealed that Sc3C2@C80 was uniformly distributed inside Py-COF (Supplementary Fig. 6). The EPR spectra of Sc3C2@C80⊂Py-COF


under vacuum and after adsorbing the gases are shown in Fig. 1f. The apparent distinct EPR splitting peaks indicate successful embedment and dispersion of Sc3C2@C80 molecules, as simple


physical dispersion could lead to the aggregation of Sc3C2@C80, which would present a broad single EPR peak. In addition, the EPR spectrum of Sc3C2@C80 in a CS2 solution presents 22


splitting peaks owing to the couplings between spin and three equivalent Sc nuclei, as shown in Supplementary Fig. 13. However, the EPR spectrum of Sc3C2@C80⊂Py-COF under vacuum presented


broadened EPR signals owing to the strong host–guest interaction between Sc3C2@C80 and the pores of Py-COF, which can intensify spin–lattice interactions and change the EPR signals.


Subsequently, Sc3C2@C80 was employed to probe the gas adsorption performance of Py-COF for N2, CO, CO2, CH4, C3H6, and C3H8 by recording the EPR signals. Figure 1e presents a sketch of the


experimental setup. The EPR spectra of Sc3C2@C80⊂Py-COF varied under different conditions. Briefly, after adsorbing the above gases, the EPR signal intensity of Sc3C2@C80⊂Py-COF decreased


compared to that under vacuum conditions, and the EPR signals of Sc3C2@C80⊂Py-COF changed. To further analyze the EPR signals under different conditions, integral EPR spectra were obtained,


as shown in Fig. 1g. The intensities of the EPR signals under vacuum conditions were the strongest, and the intensities under gas adsorption conditions gradually decreased in the following


order: N2, CO, CH4, CO2, C3H8, and C3H6. In addition, the difference values between the integrated EPR signal intensity (integrated EPR signal area) of Sc3C2@C80⊂Py-COF before and after gas


adsorption are plotted in Fig. 1h, which demonstrates that the difference values of the integrated EPR signal intensity gradually increased in the following order: N2, CO, CH4, CO2, C3H8,


and C3H6. As discussed above, when gas molecules adhere to porous frameworks, gas layers are formed on them. These gas layers restrict the motion of embedded Sc3C2@C80 and intensify


spin–lattice interactions. In this scenario, more gas molecules would enhance the viscosity of these gas layers and influence the rotational relaxation time of Sc3C2@C80 molecules. The


relationship between the viscosity _η_ and rotational correlation time _τ__c_ in the spin system can be expressed as47 $${\tau }_{c}=\frac{4\pi {r}^{3}\eta }{3{kT}}$$ (1) where _r_ is the


dynamic radius of a spin molecule. The spin–lattice relaxation time (T1) is expressed as follows48: $$\frac{1}{{T}_{1}}=\frac{{2\pi g\beta


}_{e}}{h}\left({B}_{x}^{2}+{B}_{y}^{2}\right)\frac{{\tau }_{c}}{1+{\tau }_{c}^{2}{\omega }_{s}^{2}}$$ (2) where \({B}_{x}^{2}+{B}_{y}^{2}\) denote the square amplitudes of fluctuating fields


along the x- and y-directions. According to Eqs. (1) and (2), a larger viscosity could result in a smaller T1. Furthermore, the double-integrated intensity (DIN) can be expressed by the


Arrhenius equation49: $${{{{{\rm{DIN}}}}}}\left(T\right)={I}_{0}\exp \left(\frac{{E}_{a}}{{kT}}\right)$$ (3) where _I_0 is a fitting parameter, and _E__a_ is the activation energy required


for dissociation of the paramagnetic spin cluster. Combining T1 with Eq. (3) 1/T1 ∝ exp(-_E__a_/_kT_)50 reveals that the integrated EPR intensity would decrease with a decrease in T1.


Therefore, a corresponding relationship exists between gas adsorption and EPR signals. GAS ADSORPTION ANALYSIS OF PY-COF Next, the adsorption capacities of Py-COF for the aforementioned


gases were then measured based on adsorption isotherms, as shown in Fig. 2a and Supplementary Fig. 7. Py-COF demonstrated increasing adsorption capacity at 1 bar for the above gases in the


following order: N2, CO, CH4, CO2, C3H8, and C3H6. Notably, this order of adsorption capacity was identical to that of the changed EPR signals of Sc3C2@C80⊂Py-COF before and after adsorbing


different gases at 1 bar, as shown in Fig. 2b. Thus, in addition to the increase in the adsorption amount of Py-COF, the difference between the integrated EPR signal intensity for


Sc3C2@C80⊂Py-COF before and after adsorbing gas increased. This unique relationship indicates that embedded Sc3C2@C80 can be used as a molecular nanosensor to probe the gas adsorption


performance of Py-COF and other porous organic frameworks. In addition, differences between the change trends obtained based on the adsorption isotherm and EPR measurements were observed.


This was because the commonly used adsorption isotherm test analyzes the gas adsorption of all pores within Py-COF, whereas EPR measurements only reflect the gas adsorption near Sc3C2@C80 in


a nanoscale channel of Py-COF. As shown in Supplementary Fig. 10, Py-COF displays a combined type I dominant isotherm with a sharp uptake in the low relative pressure region51 (P/P0 < 


0.1), a characteristic of microporous materials. Although the pore size of Py-COF demonstrated a theoretical distribution centered at 1.38 nm, some sub-nanoscale cracks could be observed.


These cracks could result in a non-equivalent relationship between the amounts obtained from the adsorption isotherm and EPR measurements. Therefore, the N2 sorption analysis revealed the


average porosity of Py-COF. However, EPR measurements with a spin sensor involving Sc3C2@C80 entrapped in Py-COF could be used to probe the gas adsorption performance and adsorption and


desorption processes with in situ and real-time monitoring. In addition, we performed contrast adsorption isotherm experiments on C3H6 and C3H8 gases for Py-COF and C60⊂Py-COF (mass


fraction: 9.8%), indicating that the adsorption capacity of Py-COF did not change after fullerene encapsulation (Supplementary Fig. 8). Thus, a small amount of fullerene inside Py-COF would


not prevent the gas from infiltrating. PROBING GAS ADSORPTION OF COFS WITH A NANO SPIN SENSOR Generally, sensitivity is an essential parameter for a probe. For the proposed nano spin sensor,


detecting gas adsorption at low pressures is a significant indicator of sensitivity. To investigate the sensitivity of this spin probe, we recorded EPR signals at 0.2 bar and 0.6 bar for


N2, CO, CH4, and CO2, as shown in Fig. 3a, b. After adsorption of N2, CO, CH4, and CO2 at 0.2 bar and 0.6 bar, the EPR signal intensity of Sc3C2@C80⊂Py-COF decreased compared to that under


vacuum conditions. Moreover, integral EPR spectra were obtained, and the intensities under gas adsorption conditions of 0.2 bar and 0.6 bar decreased in the following order: N2, CO, CH4, and


CO2. The trends of the changed difference values for the integrated EPR signal areas of Sc3C2@C80⊂Py-COF before and after adsorbing different gases at 0.2 bar and 0.6 bar are shown in Fig.


3c, and the difference values for the integrated EPR signal areas appear to be significant under low pressures. In addition, the adsorption capacity of Py-COF for the gases (N2, CO, CH4, and


CO2) could still be well distinguished at 0.2 bar and 0.6 bar (Supplementary Fig. 11). Moreover, the EPR signals after gas adsorption under lower pressures were recorded, as shown in


Supplementary Fig. 9. These EPR tests were performed under the following conditions: 0.12 bar CO2, 0.03 bar C3H8, and 0.04 bar C3H6 see in Supplementary Note 3. Sc3C2@C80⊂Py-COF was able to


sense gas adsorption at extremely low pressures, revealing the high sensitivity of this molecular spin probe based on metallofullerene Sc3C2@C80. To date, relevant EPR methods to probe gas


adsorption have seldom been reported; yet there are few reports on the EPR properties of TEMPO radicals within MOF materials. For example, for a TEMPO radical and ZIF-8 complex, the


adsorption of different gases at atmospheric pressure was found to change the rotational correlation time of the TEMPO radical52. In addition, the TEMPO radical and ZIF-8 complex were


involved in investigations on high-pressure induced amorphization53. We consider this molecular spin probe based on metallofullerene Sc3C2@C80 for the gas adsorption measurement of porous


organic frameworks to be closer to sensing application. As mentioned, EPR measurements using a nano spin sensor involving Sc3C2@C80 entrapped in a Py-COF could be used to probe the


adsorption and desorption processes. To illustrate this concept, we designed a group of EPR experiments in which C3H6 gas was selected as a detection object. As shown in Fig. 4a, four states


of Sc3C2@C80⊂Py-COF were selected to record the EPR spectra, with 20 repeated measurements performed for each state to avoid instrumental errors. Sc3C2@C80⊂Py-COF under vacuum, State I,


presented high-intensity integrated EPR signals (Fig. 4b and Supplementary Fig. 12). However, after absorbing N2, State II, the integrated EPR signals of Sc3C2@C80⊂Py-COF decreased slightly


in intensity. Notably, after absorbing C3H6, State III, the integrated EPR signals of Sc3C2@C80⊂Py-COF decreased significantly in intensity. Finally, after another vacuum treatment, State


IV, Sc3C2@C80⊂Py-COF could recover its high-intensity EPR signals. These results demonstrate that the nano spin sensor involving Sc3C2@C80 entrapped in a Py-COF could detect the existence or


leakage of C3H6 gas, an essential chemical material. In addition, we compared the EPR signals of different gases at the same adsorption capacity and discovered that the integrated EPR


intensity variations were similar, as shown in Supplementary Fig. 9. This indicates that the EPR signals of Sc3C2@C80 entrapped within porous materials mainly depend on the gas amounts


around the spin probe. Therefore, this nano spin sensor is suitable for probing and evaluating the gas adsorption performance of porous materials. Further, EPR signal linewidths were


analyzed for the nano spin sensor involving Sc3C2@C80 entrapped in Py-COF to reveal the probing mechanism. Notably, compared with the 22 hyperfine coupling splitting peaks of Sc3C2@C80 in a


solution, the free rotation of Sc3C2@C80 embedded in Py-COF is restricted owing to strong host–guest interactions, resulting in significant linewidth broadening, as shown in Fig. 1f.


According to Kivelson’s theory54, the linewidth (_ΔH_) of EPR spectra depends on the quantum number (_M_I) and can be expressed as $$\triangle H=\alpha+\beta {M}_{{{{{{\rm{I}}}}}}}+\gamma


{M}_{{{{{{\rm{I}}}}}}}^{2}+\delta {M}_{{{{{{\rm{I}}}}}}}^{4}$$ (4) where the _α_, _β_, _γ_, and _δ_ parameters depend on the rotational correlation time τc47. From Eq. (4), it can be


inferred that the linewidth was enlarged at both the wing parts of the EPR spectra, and at certain times, a high value of _M_I could cause partial disappearance of the side EPR signals55.


Different EPR signal linewidths of Sc3C2@C80⊂Py-COF under vacuum and after adsorbing N2 and C3H6 at 1 bar successively are shown in Fig. 5. After adsorbing N2, Sc3C2@C80⊂Py-COF demonstrated


slightly increased linewidth compared with Sc3C2@C80⊂Py-COF under vacuum conditions, revealing that adsorbed N2 could further restrict the rotation of embedded Sc3C2@C80. Furthermore, after


adsorbing C3H6 successively, Sc3C2@C80⊂Py-COF had a significantly increased linewidth, revealing the severely restricted rotation of embedded Sc3C2@C80 after adsorbing C3H6. Thus, adsorbed


gases in Py-COF can result in a longer rotational correlation time τc and a larger linewidth for the molecular spin probe involving Sc3C2@C80. EPR STUDIES OF METALLOFULLERENE ENTRAPPED


INSIDE MOF-177 To explore the versatility of this metallofullerene-based nano spin sensor, Sc3C2@C80 was employed to probe the gas adsorption performance of an MOF (MOF-177), as shown in


Fig. 6. For this, Sc3C2@C80 was entrapped in MOF-177 using an adsorption method in a toluene solution, yielding a complex of Sc3C2@C80⊂MOF-177. The mass fraction of Sc3C2@C80 in MOF-177 was


calculated to be 3.78%, and the analysis process is presented in Supplementary Note 4 and Supplementary Fig. 14. The MOF-177 and Sc3C2@C80⊂MOF-177 complexes were characterized using SEM,


which revealed that their morphologies were similar (Supplementary Fig. 15). Moreover, TEM element mapping indicated that Sc3C2@C80 molecules were dispersed in the MOF-177 (Supplementary


Fig. 16). Subsequently, EPR measurements were employed to probe the gas adsorption performance of MOF-177 for N2, H2, CO2, CH4, C3H6, and C3H8. The EPR spectra of Sc3C2@C80⊂MOF-177 under


vacuum and after adsorbing the foregoing gases are shown in Supplementary Fig. 16a, and the corresponding integral EPR spectra are depicted in Fig. 6b. After adsorbing these gases, the EPR


signal intensity of Sc3C2@C80⊂MOF-177 decreased compared to that under vacuum conditions. In addition, the difference values between the integrated EPR signal intensity of Sc3C2@C80⊂MOF-177


before and after adsorbing gases are illustrated in Fig. 6c, demonstrating the following increasing order for the difference values of the integrated EPR signal intensity: N2, H2, CH4, CO2,


C3H8, and C3H6. The adsorption capacity of MOF-177 for the same gases was then measured based on adsorption isotherms, as shown in Supplementary Fig. 16b. Notably, MOF-177 presents


increasing adsorption capacity at 1 bar for the gases in the following order: N2, H2, CH4, CO2, C3H8, and C3H6. As shown in Fig. 6d and Supplementary Fig. 17, a corresponding relationship


exists between the measured adsorption amounts of MOF-177 and changed EPR signals of Sc3C2@C80⊂MOF-177 before and after adsorbing different gases at 1 bar. This distinct relationship reveals


that Sc3C2@C80 embedded in MOF-177 can be employed as a nano spin sensor to probe the gas adsorption performance of MOF-177. In addition, differences still persist between the change trends


obtained from the adsorption isotherm and EPR measurements because the EPR measurement using a spin sensor involving Sc3C2@C80 entrapped in MOF-177 is capable of probing gas with in situ


and real-time monitoring. These results demonstrate that Sc3C2@C80 embedded in porous organic frameworks can act as a nano spin sensor to probe the gas adsorption performance and adsorption


process. DISCUSSION Sc3C2@C80 was embedded into the pores of a Py-COF, and its molecular spin was employed to probe the gas adsorption performance of Py-COF for N2, CO, CO2, CH4, C3H6, and


C3H8. When combined with traditional adsorption isotherm measurements, Sc3C2@C80 exhibited considerably decreased EPR signals upon gas adsorption. The adsorbed gases in Py-COF could restrict


the rotation of embedded Sc3C2@C80 and result in a longer rotational correlation time, a larger linewidth, and decreased intensity for the molecular Sc3C2@C80 nanosensor. We investigated


the sensitivity of this spin probe by recording EPR signals after adsorbing N2, CO, CH4, and CO2 at 0.2 bar and 0.6 bar; the difference values for the integrated EPR signal intensities were


significant, revealing the high sensitivity of the spin probe at low pressures. This high sensitivity was further verified by testing CO2, C3H8, and C3H6 at 0.12, 0.03, and 0.04 bar,


respectively, where these gases at lower pressures were similarly detected. Furthermore, the proposed Sc3C2@C80⊂Py-COF spin sensor was used to probe the adsorption and desorption processes


of C3H6, which revealed that Sc3C2@C80⊂Py-COF could be used to detect the leakage of C3H6 gas, an important and hazardous chemical. The versatility of this Sc3C2@C80 spin sensor was explored


by probing the gas adsorption performance of MOF-177 for N2, H2, CO2, CH4, C3H6, and C3H8. These results demonstrated that Sc3C2@C80 embedded in other porous organic frameworks can also act


as a nano spin sensor to probe the gas adsorption performance, gas adsorption and desorption processes, and gas leakage. The changed difference values for the integrated EPR signal areas


(ΔEPR) of Sc3C2@C80 entrapped within porous materials before and after gas adsorption are dependent on the amount of entrapped Sc3C2@C80, as well as the interaction between Sc3C2@C80 and


nanopores. A larger amount of embedded Sc3C2@C80 would result in a larger ΔEPR; however, the interactions between Sc3C2@C80 and nanopores would influence the EPR signal and ΔEPR. Therefore,


a comparative analysis of ΔEPR could be employed for one type of complex. For example, the amount of Sc3C2@C80 in Py-COF is low (mass fraction: 1.5%); hence, the integrated EPR signal area


and ΔEPR are relatively small compared to those of Sc3C2@C80⊂MOF-177, which has a higher amount of Sc3C2@C80 in MOF-177 (mass fraction: 3.78%). In addition, the different interactions


between Sc3C2@C80 and nanopores of Py-COF and MOF-177 would influence the initial EPR signals. Therefore, when the proposed nano spin sensor is employed to probe the gas adsorption


performance, ΔEPR after adsorbing one gas should be compared with the counterpart under vacuum conditions for one type of complex. Notably, the spin sensor involving Sc3C2@C80 embedded in


porous organic frameworks can probe gas adsorption and desorption processes with in situ and real-time monitoring. Traditional adsorption isotherm measurements often indicate the average


porosity of porous organic frameworks. Therefore, differences are often noted between the change trends obtained from adsorption isotherms and EPR measurements. Compared with the adsorption


amounts measured using adsorption isotherms, the EPR signals have more significant difference values for each gas adsorption (Figs. 2a and 6d). The reason for this is that the commonly used


adsorption isotherm method quantifies the gas adsorption of all pores in porous organic frameworks, whereas EPR measurements only reflect gas adsorption near Sc3C2@C80 in the nanoscale


channel of porous organic frameworks. Therefore, the Sc3C2@C80 nano spin sensor embedded in porous organic frameworks can probe gas adsorption with high sensitivity. Another advantage of


this nano spin sensor is that the required amounts of porous organic frameworks are small. In this study, a milligram mass of porous organic frameworks (ca. 1 mg) was required for adsorption


evaluation with this spin sensor of metallofullerene; however, traditional adsorption isotherm measurements often require significantly more sample amounts (ca. 100 mg). The minimal amount


of sample required indicates that this nano spin sensor has promising applications in the field of trace sensing, wherein a small amount of porous material can facilitate numerous gas


adsorption evaluations. This study revealed the significant interaction between the metallofullerene spin and gas molecules in nanospace. Thus, the molecular spins of metallofullerene can be


employed to probe gas adsorption inside the pores of porous organic frameworks based on their quantum characteristics, demonstrating that metallofullerene-based molecular spins are


applicable for quantum sensing applications. METHODS PREPARATION OF SC3C2@C80 Metallofullerene Sc3C2@C80 was synthesized based on the Kräschmer−Huffman arc-discharge method using an


arc-discharge generator. All reagents were obtained commercially and used without further purification unless otherwise specified. The products were extracted from carbon ash via ultrasound


in an ortho-dichlorobenzene (_o_-DCB) solvent. The extracts were separated through multistage high-performance liquid chromatography with a Buckyprep column (20 × 250 mm) and a Buckyprep-M


column (20 × 250 mm) using toluene as the eluent. Sc3C2@C80 was detected using a matrix-assisted laser desorption/ionization time-of-flight mass spectrometer (AXIMA Assurance, Shimadzu).


SOURCE OF POROUS ORGANIC FRAMEWORKS Py-COF was purchased from Chemsoom Co., Ltd. MOF-177 was prepared using a method reported in the literature56. Briefly, Zn(NO3)2·6H2O (1.8 g) and


1,3,5-Tri(4-carboxyphenyl)benzene (H3BTB, 120 mg) were dissolved in N, N-Dimethylformamide (DMF, 36 ml) and sonicated for 1 h. The mixed solution was then sealed in a glass tube and heated


at 85 °C for 3500 min. The products were washed three times with DMF and three times with toluene. PREPARATION OF SC3C2@C80⊂PY-COF AND SC3C2@C80⊂MOF-177 First, Py-COF and MOF-177 samples


with large sizes and high degrees of crystallinity were selected for preparation. Then, Py-COF (2 mg) powder was immersed in the toluene solution containing Sc3C2@C80 (4 × 10−5 mol·L−1) for


one week to ensure that the Sc3C2@C80 molecules completely entered the channels of Py-COF. The resulting brown powder was washed with toluene and vacuum dried at 80 °C for 12 h to obtain the


Sc3C2@C80⊂Py-COF complex. Sc3C2@C80⊂MOF-177 was prepared according to the method above, and related details are described in Supplementary Note 4. XPS MEASUREMENT The XPS analysis of


Sc3C2@C80⊂Py-COF was performed using the VG Scientific ESCALab220i-XL electron spectrometer with 300 W Al Kα radiation. As is typical, the hydrocarbon C1s line at 284.8 eV from adventitious


carbon was used for energy referencing. SEM AND TEM MEASUREMENT The morphologies and microstructures of the samples were observed using a field emission scanning electron microscope


(Regulus8100, HITACHI, Japan) operated at a voltage of 10 kV, along with a TEM (JEM-2100F, JEOL, Japan). Element mapping was characterized on a TEM equipped with Oxford detection. ELECTRON


PARAMAGNETIC RESONANCE SPECTRUM MEASUREMENTS EPR spectra were recorded on an EPR spectrometer (CIQTEK EPR200-Plus) with a continuous-wave X-band frequency (~9.6 GHz). A small quantity of


complex (0.86 mg for Sc3C2@C80⊂Py-COF and 5.0 mg for Sc3C2@C80⊂MOF-177) was placed at the bottom of a quartz tube. The modulating field strength, frequency, and microwave power were 2 G, 100


 kHz, and 2 mW (20 dB), respectively. To probe the gas adsorption of Py-COF using the molecular spin of Sc3C2@C80, the EPR signals of Sc3C2@C80⊂Py-COF were recorded after adsorbing N2, CO,


CO2, CH4, C3H6, and C3H8. Sc3C2@C80⊂Py-COF (0.86 mg) was placed in a quartz tube connected to a vacuum pump and gas bag (at 1 bar). Sc3C2@C80⊂Py-COF in the quartz tube was evacuated for 30 


min to remove air from the pores before recording EPR signals under vacuum. Then, gases were filled into the quartz tube; after adsorbing for 30 min, the EPR signals were collected. The


detailed test procedure is described in Supplementary Note 2. PXRD MEASUREMENTS The solid samples were loaded onto a glass sample holder to record the PXRD spectra on a Rigaku D/max-2500n


diffractometer with Cu Kα radiation (λ  =  1.541 Å) at 40 kV and 200 mA. The scanning range was from 2_θ_ = 2° to 80° with a scan speed of 2° min−1. GAS ADSORPTION ISOTHERM MEASUREMENTS Gas


adsorption isotherms were recorded on a BeiShiDe Instrument volumetric adsorption analyzer. Prior to the measurements, 100 mg samples were degassed for over 300 min at 300 °C, and the


equilibration time was 120 s. Ultra-high purity N2, CO, H2, CO2, CH4, C3H6, and C3H8 were used for measurements. Oil-free vacuum pumps and oil-free pressure regulators were used for


measurements to prevent contamination of the samples during the degassing process and isotherm measurement. A liquid N2 bath was used for the adsorption measurements at 77 K. Before


measurement, the samples were degassed in vacuum at 150 °C for 300 min. To provide high accuracy and precision in determining P/P0, the saturation pressure P0 was measured throughout the N2


analyses using a dedicated saturation pressure transducer, which allowed us to monitor the vapor pressure at each data point. STATISTICAL INFORMATION Statistics were performed either with


OriginPro 2021 software. The bar chart data in Fig. 5 represents the mean ± SD (_n_ = 20). DATA AVAILABILITY The data that support the findings of this study are available in the online


version of this paper and the accompanying Supplementary Information, or available from the corresponding authors on request. Source data are provided with this paper. CHANGE HISTORY * _ 04


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ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (52022098, 51972309) and the Youth Innovation Promotion Association of Chinese Academy of


Sciences (Y201910). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Beijing National Laboratory for Molecular Sciences, Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute


of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Beijing, 100190, China Jie Zhang, Linshan Liu, Chaofeng Zheng, Wang Li, Chunru Wang & Taishan Wang *


University of Chinese Academy of Sciences, Beijing, 100049, China Jie Zhang & Chaofeng Zheng Authors * Jie Zhang View author publications You can also search for this author inPubMed 


Google Scholar * Linshan Liu View author publications You can also search for this author inPubMed Google Scholar * Chaofeng Zheng View author publications You can also search for this


author inPubMed Google Scholar * Wang Li View author publications You can also search for this author inPubMed Google Scholar * Chunru Wang View author publications You can also search for


this author inPubMed Google Scholar * Taishan Wang View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS T.W. and C.W. conceived the research and


supervised the project. J.Z., W.L. and C.Z. performed the material preparations and EPR measurements. L.L. performed the DFT calculations. T.W. and J.Z. wrote the paper. All authors


discussed the results and commented on the manuscript. CORRESPONDING AUTHOR Correspondence to Taishan Wang. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing interest.


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probing of gas adsorption inside porous organic frameworks. _Nat Commun_ 14, 4922 (2023). https://doi.org/10.1038/s41467-023-40683-2 Download citation * Received: 11 January 2023 * Accepted:


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