Pt doped boron carbide monolayer nanosheet as a work function-type sensor for ibuprofen drug; quantum chemical study

Through density functional theory (DFT), the sensitivity of the Pt-doped and the pristine BC 3 nanosheets to ibuprofen (IBP) was scrutinized. The IBP drug does not impact the electronic properties evaluated for the pristine BC 3 . However, its sensitivity and reactivity are increased to the IBP drug to a great extent after doping it by Pt. Unlike the pristine BC 3 , the adsorption of the IBP drug decreases the HOMO-LUMO gap associated with the Pt-doped BC 3 sheet from 1.29 to 1.04 eV, which improves the electrical conductivity. In addition, the adsorption of the IBP drug will mainly impact the work function of the Pt-doped BC 3 sheet, which in turn modifies the electron emission current from its sheet. This verifies that the Pt-doped BC 3 sheet can be utilized as a work-function-type sensor to detect the IBP drug. For desorption of the IBP drug, the recovery time of the Pt-BC 3 nanosheet is short, i.e., 5.65 ms , which is another advantage of this sheet.


Introduction
Ibuprofen (IBP) is one of the commonly used nonsteroidal drugs that is prescribed as a painkiller and an antipyretic agent, which is used to treat inflammation [1]. By blocking the enzyme cyclooxygenase, IBP can inhibit prostaglandin biosynthesis and most of it is usually metabolized in liver to hydroxyl as well as carboxyl metabolites IBP metabolites, while only less than 10% is excreted in urine and bile without change [2,3]. Based on the World Health Organization, the recommended dosage for IBF is 1200 mg, and it overdosage can lead to severe side effects [4]. IBP is a common analyte in pharmaceutical analysis according to its widespread availability. Based on medicinal purposes and the pharmaceutical importance of IBP, we need a highly efficient method to detect and control this drug in various biological fluids [5][6][7][8][9].
Owing to some of the outstanding properties of nanomaterials, including flexible lifetime, considerable stability, unsubstantial size, good biocompatibility, large specific surface area, and appropriate surface modification characteristics, they are now the most ideal candidates to detect biological and chemical species [8][9][10]. Also, numerous clinical applications of such materials have already been reported owing to their controllable characteristics [13]. Compared to conventional drug detection methods, the side effects of nanomaterials have been drastically reduced since their superior properties have improved the efficiency and bioactivity of these materials [14]. In addition, they provide long-term blood circulation, as well as maintaining the biological activity of a drug [14]. Among nanomaterials, graphene and associated derivatives in their pristine form, and two-component graphene-like materials have been used successfully in medical applications as two-dimensional (2D) materials [15]. These materials are mainly provided from two major B/C components in different ratios [16][17][18] such as the boron carbide 4 nanotube shaped BCn structure having boron to carbon with atomic ratio of 1:3 that has chemical stability [17]. Similar to graphene, the formation of BC3 nanosheets is carried out via rolling a BC3 nanosheet along the chiral vector. It is worth noting that because of the insignificant stability of boron-boron bonds, there only exist carbon-boron and boron-carbon bonds in this structure [19,20]. Numerous pieces of research have been carried out into the electronic and structural properties of BC3 nanosheets [21]. In the current study, we investigated the interaction behavior and the electronic response of the Pt-doped and pristine BC3 nanosheets to the IBP drug via employing DFT.

Computational methods
By utilizing GAMESS software, full geometry as well as the optimized energy of the Pt-doped BC3 nanosheet and the pristine BC3 nanosheet were computed via implementing the B3LYP functional with the 6-31G (d) basis set [22]. B3LYP-D, as an experimental dispersion term, was introduced to the basis set to raise the exactness of the data computed, particularly related to components with noncovalent bonds. The B3LYP is a common functional employed in studies on synthesized nanostructures [23,24], through which accurate results are obtained for III-V semiconductors [25]. GaussSum software was utilized for carrying out the DOS calculations [26]. The energy of adsorption(Ead) is estimated based on the equation below: Where E (ACE/BC3) is the total adsorption energy of the ACE drug on the BC3 sheet surface; E (BC3) the total energy of the BC3 sheet; and E (BSSE) the basis set superposition.
Where Eg1 represents the adsorption energy of the bare BC3 nanosheet and Eg2 represents the 5 energy related to the adsorption of the ACE drug molecule adsorbed onto the sheet.

IBP molecule and its relevant adsorption geometries on the surface of BC3
As can be seen in Fig. 1, the BC3 sheet possess a lattice structure that is the same as graphene, in which 6 C atoms and 2 boron atoms form each unit cell, unlike conventional BN with 2D structures. The estimated B-C bond length was 1.56, and the estimated C-C bond length was 1.42 Å. According to Fig. 1, there exist two types of hollow sites in BC3, one of which is the hexagon center C6 which is hollow and the other is the hexagon C4B2. The Eg value (Table 1) as well as the computed DOS plot indicates that the pristine sheet is considered to be a semiconductor. In addition, the optimized geometry and the HOMO and LUMO of the IBP drug are depicted in Fig.2. In the IBP molecule, the HOMO level, with energy of -5.92 eV, located above the corresponding level in all C-C atoms, and on the electronegative oxygen atom in the molecule. However, the LUMO level (0.26 eV) located on the ring carbon atoms, nearly above the carboxyl oxygen atom of IBP.
The axis of the MF drug might be parallel to the BC3 sheet surface and the tendency of the species was assessed to the adsorbent by the energies of the adsorption interaction associated with the drug that was attached to the BC3 sheets. The different frameworks that represent the interactions between the MF drug and the BC3 sheet are shown in Fig. 3 (a,b). The oxygen atom of hydroxyl groups (-OH) of the IBP drug was near the surface of the BC3 sheets in configuration I. In this configuration, the weak interactions were identified between the hydroxyl group of the IBP drug and the B atoms (side-on) and the corresponding adsorption energy was -  Table 1.
Based on Table 1, complexes that possess more stable configurations have the ability to reduce the HOMO level from -4.25 eV to -4.32 eV in the pristine sheets. Also, compared to its pristine form, this complex has decreased the Eg value by 9.56%. The change in the Eg value can be considered as a suitable factor that determines the sensitivity of an adsorbent to an adsorbate since the Eg value corresponds to the population of conduction elections as stated by Eq. 3 [27].
Where A (electrons/m 3 K 3/2 ) represents a constant, and k represents the Boltzmann's constant. Kelvin method is an important part of the process of Φ-type gas sensors. For the measurement of the work function of a sample prior to and following the adsorption of the IBP molecule by the BC3 sheet, a Kelvin probe is employed in this method. There will be a change in the gate voltage and the electrical noise will be produced for recognizing the drug in case the adsorbent work function is impacted by the IBP adsorption to a great extent [28]. Work function (Φ) is the least amount of work needed for removing an electron from the Fermi level to infinity, computed as follows: Where Vel (+∞) shows the electrostatic potential associated with an electron which is located far from the substance surface, assumed to be 0, and the Fermi level is shown by EF. According to Eq. 4, in case the electrostatic potential energy is assumed to be 0, then Φ = -EF, and the Fermi level energy will be computed as follows: Based on the Richardson-Dushman equation, when the Fermi level changes, there will be a shift in the field emissions as well since the current densities of an electron and the work function value are correlated [29]: In which A shows the Richardson constant (A/m 2 ), and T shows the absolute temperature (K).
Based on Table 1, the adsorption of IBP on the BC3 surface reduces Φ by 2.75% relative to the pristine BC3 nanosheet in configuration II. There is not a significant change in the the current 8 density of electrons that emitted from the BC3 nanosheet surface. Therefore, we can conclude that the BC3 nanosheet is not a work function-type sensor and cannot detect or adsorb the IBP drug.

Pt-doped BC3 nanosheet
The Pt atom was replaced by the B atom for scrutinizing the impacts upon the electronic and structural characteristics of the BC3 surface. Fig.4 b depicts the optimized structure of heteroatoms that was doped with Pt in the BC3 nanosheet. Doping the Pt atom, because of its larger size than B, will reduce the repulsion and stress in the BC3 structure. Compared to the length of the B-C bond in the pristine BC3, the Pt-C bond length was significantly longer, which was calculated to be 1.92 Å. Also, compared to the C-B-C bond angle in the pristine BC3 (117°), the C-Pt-C bond angle in the Pt-doped sheet is larger, which was calculated to be 121°.
In the Pt-doped BC3 nanosheet, the Eg value was computed to be approximately 1.29 eV, being smaller compared to the pure BC3 nanosheet. At the doped sites, the Si atom will act as affinity center for the ACE drug to be chemisorbed.
Afterward, by putting the species above the Pt atom with the IBP oxygen atom of carboxyl groups (-C=O), above the Pt in the doped BC3, the interaction between the Pt-doped BC3 and the IBP drug was inspected, as depicted in Fig. During  with the IBP drug. In comparison with the pristine BC3 nanosheet, the Pt-doped BC3 sheet exhibited enhanced performance because of its higher charge transfer value of 0.45 e, more negative energy of adsorption, and stronger adsorption between the Pt-doped BC3 and the IBP drug. According to the interaction mechanism, it can be suggested that the oxygen atom, a region rich in electrons, binds to the exposed Pt, a region poor in electrons.
According to the results, Pt doping is very effective in creating a more favorable sensor for the IBP drug, unlike the pristine sample. In addition, the adsorption energy of the IBP drug in the Pt-doped BC3 nanosheet was computed to be -23.56 kcal/mol. The electronic properties will be altered dramatically by the IBP drug reaction with Pt-doped BC3 and after the the complex is formed based on DOS plot in Fig.4 c and data in Table 2. Unsurprisingly, in the complex, the LUMO energy associated with the nitrogen atom was stabilized by the adsorption process. Since Pt interacts with the oxygen atom of carboxyl groups of electron lone pairs, this change is perfectly acceptable, resulting in significant changes in the LUMO level in the Pt-doped BC3.
Also, the considerable stabilization of the LUMO energy is considered to be the cause of the substantial decrease in the adsorption energy of the Pt-doped BC3, which was reduced from 1.29 to 1.04.
According to Eq. 3, this decrease will enhance the electrical conductivity of the Pt-BC3.
The change in the electrical conductivity corresponds to the electrical signal, indicating the presence of the IBP drug in the urine and bile mediums. Hence, Pt-BC3 can be considered as a cheap and good electronic sensor to detect the IBP drug. The Pt-BC3 work function value changed dramatically from 4.88 eV in Zn-BC3 to 3.89 eV in the IBP complex. Therefore, with the adsorption of the IBP drug, the electron current density that emitted from the Pt-BC3 nanosheet is reduced, enabling the detection of the IBP adsorbents. As a Φ -type sensor, the IBP drug can be detected using Pt-BC3.
In sensor applications, one of the preconditions for an acceptable sensor is a facilitated desorption, and because of this reason, chemical interactions that are too strong are not favored.
In a strict sense, too strong adsorptions lead to longer recovery times, not advantageous for sensor applications. Based on the transition-state theory, longer recovery time is expected in strong adsorptions as follows [30]: Where the attempt frequency is represented by ν0. In case of employing the vacuum ultra-violet light (υ~10 12 s -1 ), it will take approximately 5.65 ms for the IBP drug to recover from the surface of the Pt-doped BC3 sheet at room temperature. It should be noted that at different temperatures, this time can be decreased to some extent. The associated recovery time for the Pt-doped is short, being adequate for sensor usability in IBP.

Conclusions
In this paper, the impact of         Figure 1 Optimized structure of BC3 nanosheet and DOS plots.   Optimized structure of (a) Pt@BC3+ IBP, (b) Pt doped BC3 and (c) comparative DOS plots.