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Au-decorated BN nanotube as a breathalyzer for potential medical applications

Respiratory viral infections such as coronavirus (COVID-19) will cause a great mortality, especially in people who underly lung diseases such as chronic obstructive pulmonary and asthma. Very recently, the COVID-19 outbreak has exposed the lack of quick approaches for screening people who may have risen risk of pathogen contact. One proposed non-invasive potential approach to recognize the viral infection is analysis of exhaled gases. It has been indicated that the nitric oxide is one of most important biomarkers which might be emanated by respiratory epithelial cells. Using density functional theory calculations, here, we introduced a novel Au-decorated BN nanotube-based breathalyzer for probable recognition of NO gas released from the respiratory epithelial cells in the presence of interfering CO2 and H2O gases. This breathalyzer benefits from different advantages including high sensitivity (sensing response = 101.5), high selectivity, portability, short recovery time (1.8 μs at 298 K), and low cost.


Introduction
Very recently, an outbreak of unexplained pneumonia has been caused by a new coronavirus infection called COVID-19 [1]. Usual symptoms are shortness of breath, cough, and fever [2].
Other symptoms might include sore throat, loss of smell or taste, abdominal pain, sputum production, diarrhea, and muscle pain [2]. While the most of cases result in minor symptoms, some progress to multi-organ failure, severe acute respiratory syndrome, pneumonia, kidney failure, and death [3]. As of May 22, 2020, more than 335000 deaths were reported in worldwide that the overall death rate per number of diagnosed cases has been predicted to be 4.5%; ranging from 0.2% to 15% depending on the age and other health problems [4]. The COVID-19 outbreak has exposed the lack of quick non-invasive approaches for screening people who may have risen risk of pathogen contact. In contrary to the bacterial infections, in which toxins generated by the infectious agent origin much of the pathology, the symptoms of COVID-19 are due to the host response to the virus. Upon the virus infection, pro-inflammatory cytokines are released that lead to the constellation of lethargy, malaise, diffuse pain, and fatigue in addition to the early fever [5].
A usual feature of the inflammatory response in patients is the volatile organic compounds and nitric oxide (NO) production of the airway epithelium and alveolar and also, from leukocytes that penetrate the lungs [5]. It should be noted that the time course of appearance of these biomarkers nearly coincides with the time course of symptoms of disease.
J o u r n a l P r e -p r o o f 4 applied without specialized training and cannot be regarded as portable for application in public places or homes where screening of individuals is of great importance. Recently, different nanostructures have been introduced as one of the most promising candidates for developing sensing devices owing to their excellent properties such as high surface/volume ratio, short analysis time, low-temperature action, possibility of miniaturization, low-cost, good sensitivity, and fast response [8][9][10][11][12][13][14][15][16]. Nanostructure-based sensors are certainly compact enough to be simply deployed. Volatile organic compound based nao-breathalyzers have been formerly introduced and described [17][18][19]. This manuscript focuses on a novel NO-based nao-breathalyzer to be applied in the COVID-19 virus infection detection.
One-dimensional (1D) nanostructures have attracted considerable attention in manufacturing highly sensitive and portable gas sensors [19]. For 1D nanostructures, the effective adsorption places for gas molecules increase because of the relatively great specific surface area, which might simplify the surface adsorption process providing large surface charge densities [20]. Numerous 1D nanostructures have been introduced to date to be used in gas sensors, including ZnO, SnO 2 , and Al 2 O 3 nanowires, and TiO 2 , BC 3 , BC 2 N, and carbon nanotubes [21][22][23][24]. Because of the multiplicity and complexity of the environment, it is a universal need to develop gas sensing devices that can be applied in a wild environment. Boron nitride nanotubes (BNNTs) are attractive as 1D nanostructures owing to their superior intrinsic physical and chemical characteristics. They display excellent thermal and mechanical properties, and also have a high resistance to oxidation at high temperatures and great chemical stability [25]. These exceptional characteristics make BNNTs very suitable for several applications, particularly in high-temperature and hazardous environments. However, being a high stable semiconductor, BNNTs have a low tendency to interact with gas molecules [26,24]  Au-decorated BNNTs (Au@BNNTs), indicating that the reactivity and sensitivity of the BNNTs significantly changed by the Au-decoration [28]. The successful synthesis of Au@BNNTs suggests a low-cost and simple method for detecting different gas molecules.
However, it is an interesting issue that to discover how the Au atoms can influence the sensitivity of BNNTs toward the gas molecules. Density functional theory (DFT) is a powerful method which can be applied to inspect such matters, explaining the experimental observations at molecular level. Here, we performed a DFT study to investigate the probable application of pristine BNNTs and Au@BNNTs as breathalyzer to sense the exhaled NO gas in the presence of CO 2 and H 2 O gases for diagnosis of COVID-19. We present a new computational method to explore the effect of Au-decoration on the sensitivity of BNNTs according to the experimental reports.

Computational methods
All of the calculations were carried out by the software of GAMESS [29]. It has been previously specified that meta-generalized gradient approximation TPSS functional predicts the most accurate and reliable results for molecular properties of transition metal containing structures [30]. Also, they recommended the LANL2DZ (on the transition metal) + 6-31+G** mixed basis set. Accordingly, herein, the structure optimizations and natural bond orbitals (NBO), electronic, energetic, and charge analyses were performed applying the TPSS functional with the LANL2DZ basis set on the Au atoms and 6-31+G** on the others. The dispersion interaction evaluation is the drawback of the standard DFT functionals. Therefore, the TPSS functional was J o u r n a l P r e -p r o o f 6 augmented with a Grimme D3 term to predict the dispersion forces [31]. For the calculation of adsorption energies (E ad ) of an adsorbate on the surface of an adsorbent, the following equation was employed: Adsorbate may be an Au atom, O 2 or NO molecule. The adsorbent may be a pristine BNNT, Au@BNNT, or O 2 adsorbed Au@BNNT (O 2 /Au@BNNT). The E BSSE designates the basis set superposition error computed by the counterpoise method [32]. The transition state structure was predicted using the synchronous transit-guided quasi-Newton (STQN) technique [33].

The pristine BNNT
We selected a (12,0) zigzag single-walled BNNT (B 108 N 108 H 24 ) as shown in  (Table 1), respectively, creating an E g about 4.012 eV. As mentioned before, experimental works have indicated that the working mechanism of gas sensors depends on the change of the sensor resistivity (or conductivity) because of the charge transfer between the gas molecules and sensors [19]. The sensing mechanism is based on the fact that initially the O 2 molecules of the atmosphere adsorb on the sensor surface, extracting some electrons from the conduction band to J o u r n a l P r e -p r o o f 7 somewhat backs to the initial value. Here, all reactions which should be occurred on the surface of BNNT for the NO sensing are summarized as follows: Previous works have shown that the pristine BNNTs weakly adsorb the O 2 gas and the reactions of Eqs. 4 and 5 cannot be occurred [34][35][36]. Thus, the pristine BNNT cannot detect the NO gas.
To overcome this problem, we suggested the Au-decoration into the surface of BNNT.

Au@BNNT
To obtain the most stable Au@BNNT geometry, we tested some starting configurations by adding an Au atom on different sites of the BNNT, including on the B or N atom, above bridge sites of B-N bonds, and above the center of a B 3 N 3 hexagonal ring. Full structural optimization was then performed with each initial complex. It was found that the most stable Au@BNNT is that in which the Au atom adsorbed above the hollow site of the B 3 N 3 hexagon ring with Au…B interaction distances of 2.71 and 2.80 Å as shown in Fig. 1 (complex I). The E ad for Au adsorption on the BNNT is -31.7 kcal/mol in the complex I, signifying an exothermic and favorable process. The Au-decoration meaningfully reduced the E g value of BNNT from 4.012 to J o u r n a l P r e -p r o o f 8 indicate that as the Au is a metal, it tends to interact with the electron deficient sites (B atoms) compared to the electron rich sites (N atoms).
We selected the most stable Au@BNNT structure (I) and then studied the adsorption of an O 2 molecule on its surface as the first step (Eq. 3) for detection of NO gas of exhaled breath. The results of Table 2 show that the electronic properties of Au@BNNT were meaningfully perturbed by the O 2 molecule interaction. Its LUMO level intensely stabilizes by shifting from -3.029 to -3.214 eV and the HOMO energy is slightly changed. Therefore, the E g of Au@BNNT dramatically decreased from 2.125 to 1.936 eV, which will increase the electrical conductance of the Au@BNNT according to the Eq. 6: σ = AT 3/2 exp(-E g /2kT) where R 1 and R 2 are the electrical resistance of the Au@BNNT before and after the gas adsorption, respectively. Electrical conductance and resistance are inversely related to each other. Thus, we one can write: where ΔE g is the E g change after the gas adsorption on the adsorbent. Using Eq. 8, the response of Au@BNNT to NO gas is 101.5 at 298 K, demonstrating that the Au@BNNT is highly sensitive to NO gas. This indicates that the Au@BNNT may be applied as a breathalyzer for diagnosis of COVID-19.

Recovery time
The strength of an interaction is a critical parameter for sensor growth for because a strong interaction, the desorption process becomes hard. A shorter recovery time will be gotten with a smaller desorption energy (E des ) based on the following equation: where ν 0 , T, and k are the attempt frequency, the temperature, the Boltzmann's constant (~ 2.0  calculate the average E des per molecule for desorption of the produced NO 2 molecules from the surface of Au@BNNT. To this aim, we employed the following equation: where E(Au@BNNT) and E(NO 2 ) are energies of a single Au@BNNT and NO 2 , respectively. E(complex) is the energy of the final complex in which two NO 2 molecules are adsorbed on the surface of an Au@BNNT (Fig. 3). However, the E des was calculated to be 9.9 kcal/mol. Therefore, the recovery time is predicted to be 1.8 µs at 298 K and the ultra-violet light (υ ~ 10 13 s -1 ), indicating a short time for sensor recovery.

Selectivity
Finally, in order to further investigate the selectivity of Au@BNNT toward different gas molecules, we explored the interaction of two CO 2 and H 2 O gases with the O 2 /Au@BNNT because they are the most abundant gases in the exhaled breath. Our results designate that the nonpolar CO 2 gas weakly interacts with the pre-adsorbed O 2 on the Au@BNNT (Fig. 4)     Distance is in Å.