Rh and Ir-Doped PtS₂ Monolayers as Promising Sensors for Liver Disease Biomarker Detection in Exhaled Breath: A First-Principles Study

Abstract

The detection of biomarkers in exhaled breath offers an efficient approach for the early-stage identification of liver disease. In this work, first-principles calculations were employed to investigate the adsorption and sensing properties of Rh- and Ir-decorated PtS₂ monolayers toward four liver disease biomarkers (LDBs: C₂H₆O, C₃H₈O, C₃H₆O, and C₅H₈). The results reveal that pristine PtS₂ exhibits a low affinity for these biomarkers, whereas single-atom decoration with Rh or Ir significantly enhances both adsorption energy and charge transfer. These interactions were further elucidated through analyses of projected density of states, total electron density, charge density difference, and charge transfer. Furthermore, the adsorption of all four LDBs results in |ΔE _g| exceeding 16.57%, except for the C₃H₆O/Rh@PtS₂ system. Notably, the adsorption of C₅H₈ and C₃H₈O induces a pronounced semiconductor-to-metal transition in Rh@PtS₂ and Ir@PtS₂ systems, respectively. Crucially, both Rh@PtS₂ and Ir@PtS₂ show excellent selectivity, exhibiting significantly higher adsorption strengths for the target LDBs compared to common interfering molecules present in exhaled breath (H₂O, N₂, CO₂, and CH₄). Additionally, Rh@PtS₂ exhibits a suitable recovery time (τ) of 22.7 s at 298 K for C₂H₆O, along with moderate τ values of 0.27 s (C₂H₆O) and 3.94 s (C₃H₆O) at 348 K. Consequently, Rh@PtS₂ emerges as a promising reversible sensor material for the detection of C₂H₆O and C₃H₆O. This study provides a strategic blueprint for developing PtS₂-based gas sensor applications for the medical field.

1. Introduction

Chronic liver disease constitutes a significant and growing global health concern, characterized by a silent progression that frequently leads to severe and irreversible conditions such as cirrhosis and hepatocellular carcinoma. , The late-stage diagnosis of these diseases severely limits therapeutic options and contributes to high mortality rates. Traditional diagnostic methods, including liver biopsies and serological tests, are often associated with high costs, lengthy procedures, and operational risks. In recent years, the noninvasive and real-time detection of volatile organic compounds (VOCs) in the exhaled breath of individuals with liver disease has emerged as a promising approach for effective early-stage screening. Previous studies have identified specific VOCs, including ethanol (C₂H₆O), isopropanol (C₃H₈O), acetone (C₃H₆O), and isoprene (C₅H₈), as key biomarkers whose concentrations are significantly altered during liver dysfunction. − Consequently, researchers worldwide are actively pursuing the development of gas sensors to monitor these VOCs. ,, However, current gas-sensitive materials still face challenges, including weak adsorption strength, poor sensitivity, and slow response times. Therefore, the exploration of novel, rapid, and highly sensitive gas-sensing materials is essential for the early diagnosis of liver disease.

Among various sensing materials, two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant interest due to their large specific surface area and unique electronic properties. , In particular, recent studies have highlighted the substantial potential of the PtS₂ monolayer, a semiconducting TMD characterized by inherent stability and high electron mobility, for gas-sensing applications. , However, the relatively inert surface of pristine PtS₂ results in weak physisorption interactions with the VOCs. For instance, first-principles calculations by Kanungo et al. demonstrated that the pristine PtS₂ monolayer exhibits poor affinity for ethanol, acetone, and isoprene. The introduction of sulfur vacancies, however, enhances the chemical activity of PtS₂ monolayers, leading to increased adsorption energies and charge transfer for these VOCs. Additionally, single-atom metal decoration has emerged as another highly effective strategy to further improve sensing performance. , This approach introduces catalytically active sites that modulate local charge distribution and facilitate stronger, more specific analyte binding. For instance, functionalization with Nb or Ni atoms markedly improves the adsorption capacity and sensing properties of PtS₂ monolayers toward dissolved gases in transformer oil, such as CO, C₂H₂, and C₂H₄. ,

In addition to non-noble metals, noble metals like Rh and Ir are also highly attractive candidates due to their exceptional catalytic activity and distinctive electronic structures. , Their partially filled d-orbitals facilitate strong orbital hybridization and efficient charge transfer with the functional groups of the liver disease biomarkers. Thus, decorating PtS₂ with Rh or Ir atoms is expected to create high-affinity binding sites specifically tailored for certain VOCs, transforming the inert substrate into a highly responsive sensing material. To confirm this hypothesis, density functional theory (DFT) calculations were conducted to systematically investigate the adsorption and sensing mechanisms of Rh- and Ir-decorated PtS₂ monolayers (Rh@PtS₂ and Ir@PtS₂) toward four liver disease biomarkers (LDBs: C₂H₆O, C₃H₈O, C₃H₆O, and C₅H₈). The adsorption strength and corresponding electronic properties for each system, including the density of states, charge density difference, and band structure, were thoroughly analyzed. Subsequently, the sensing characteristics were evaluated by analyzing the changes in conductivity and work function of the Rh@PtS₂ and Ir@PtS₂ systems upon gas adsorption, as well as theoretical recovery time. This work aims to assess the potential of these materials for high-performance sensing and to provide a theoretical foundation for developing novel PtS₂-based chemiresistive sensors for the rapid, real-time, and noninvasive diagnosis of liver diseases.

2. Computational Details

Structural optimizations and electronic property calculations for all adsorption systems were conducted within the DMol³ module of Materials Studio (MS) software. The generalized gradient approximation with the Perdew–Burke–Ernzerhof functional (GGA-PBE) , was adopted to describe electron exchange-correlation effects. The integration of the DFT semicore pseudopotential (DSPP) approach with the double numerical plus polarization (DNP) orbital basis was utilized to significantly accelerate computational processes. Long-range interactions between VOCs and the PtS₂ monolayer were rigorously evaluated using Grimme’s dispersion correction (DFT-D2). Geometric optimizations were converged until the energy, maximum force, and maximum displacement were less than 10^–5 Ha, 2 × 10^–3 Ha/Å, and 5 × 10^–3 Å, respectively. The self-consistent field (SCF) energy convergence threshold was set to 10^{– 6} Ha, using the DIIS algorithm for acceleration. The Brillouin zone was sampled with a 4 × 4 × 1 k-point mesh for structural relaxations and an 8 × 8 × 1 mesh for electronic property calculations. A 20 Å vacuum layer was applied to eliminate periodic interlayer interactions with an orbital cutoff radius of 5.0 Å and a thermal smearing of 0.005 Ha.

Binding energy (E _bin) is a fundamental metric for assessing the stability of TM@PtS₂ systems and is defined as follows:

$$E_{\mathrm{bin}}=E_{\mathrm{TM}@\mathrm{Pt}{\mathrm{S}}_2}-E_{\mathrm{Pt}{\mathrm{S}}_2}-E_{\mathrm{TM}}$$ 1

where E _TM@PtS2 , E _PtS2 , and E _TM denote the total energies of the TM@PtS₂ complex, pristine PtS₂, and a single TM atom, respectively. The interaction strength between VOCs and the TM@PtS₂ monolayer is evaluated by calculating the adsorption energy (E _ads) and charge transfer (Q _T):

$$E_{\mathrm{ads}}=E_{\mathrm{VOC}/\mathrm{TM}@\mathrm{Pt}{\mathrm{S}}_2}-E_{\mathrm{TM}@\mathrm{Pt}{\mathrm{S}}_2}-E_{\mathrm{VOC}}$$ 2

$$Q_{\mathrm{T}}=Q_{\mathrm{adsorbed}‐\mathrm{VOC}}-Q_{\mathrm{free}‐\mathrm{VOC}}$$ 3

Here, E _VOC/TM@PtS2 and E _VOC represent the energies of the VOC-adsorbed TM@PtS₂ system and the isolated VOC molecule, respectively, while the Q _{adsorbed‑VOC} and Q _free‑VOC correspond to the charge of the adsorbed VOC and its free-state counterpart. A more negative Q _T value indicates stronger interactions between the VOCs and the TM@PtS₂ systems.

3. Results and Discussion

3.1. Stability Analysis of TM@PtS₂ (TM = Rh, Ir) and Adsorption of LDBs on Pristine PtS₂

A single-layer PtS₂ consists of a single plane of Pt atoms sandwiched between two planes of S atoms in an S–Pt–S trilayer arrangement, as shown in Figure a. Herein, a 4 × 4 × 1 supercell of PtS₂, containing 32 S atoms and 16 Pt atoms, was employed for all calculations. Three potential adsorption sites, including Pttop, Stop, and hollow sites, were considered for the modification of Rh and Ir atoms. The calculated binding energies of Rh and Ir atoms at these sites are listed in Figure a. It is evident that both dopants preferentially adsorb on the Pttop site, followed by the Hollow and Stop sites. The large negative binding energies (−3.66 eV for Rh and −3.72 eV for Ir) indicate that Rh and Ir atoms can be stably embedded in the PtS₂ monolayer, as shown in Figure b,c. As depicted in Figure e,f, abundant electron depletion regions are observed around the Rh and Ir atoms, while significant electron accumulation occurs on the PtS₂ surface. Consequently, the Rh and Ir atoms donate approximately 0.104 and 0.102 e, respectively, to the substrate. This substantial charge transfer further demonstrates the high stability of the Rh@PtS₂ and Ir@PtS₂ systems.

1.

Top and side views of the optimized structures for (a) pristine PtS₂, (b) Ir@PtS₂, (c) Rh@PtS₂, and (d) four LDBs molecules, along with the charge density difference (CDD) of (e) Ir@PtS₂, and (f) Rh@PtS₂. The green (red) regions represent the charge accumulation (depletion).

2.

(a) Comparison of binding energy for the TM@ PtS₂, (b) projected density of states (PDOSs) of (b) Rh@PtS₂ and (c) Ir@PtS₂, along with the band structures of (d) pristine PtS₂, (e) Ir@PtS₂, and (f) Rh@PtS₂.

Figure b,c presents the projected densities of states (PDOSs) for the Rh@PtS₂ and Ir@PtS₂ monolayers. For the Rh@PtS₂, strong hybridization occurs between the S-2p orbital and the Rh-4d orbital within the energy range of −7.60–1.58 eV, with distinct hybridization peaks appearing at −6.05, −4.96, −3.86, −1.30, and −0.28 eV. This hybridization facilitates the formation of Rh–S bonds. In contrast, the interactions between the S-2s orbital and the Rh-5s, Rh-4p orbitals are significantly weaker. For Ir@PtS₂ (Figure c), the orbital hybridization between Ir and S atoms exhibits a pattern similar to that of the Rh–S system, confirming the formation of Ir–S covalent bonds between the Ir atom and its three neighboring S atoms. These findings account for the remarkable stability of the TM@PtS₂ structures. Additionally, the calculated bandgap of the pristine PtS₂ monolayer is 1.794 eV, which aligns closely with the reported value of 1.738 eV. However, doping with Rh and Ir atoms substantially reduces the bandgap to 0.623 and 0.356 eV, respectively. This suggests that Rh and Ir doping can greatly enhance the electrical conductivity of the PtS₂ monolayer, thereby improving its potential for sensing applications.

The adsorption behaviors of four LDBs on the pristine PtS₂ surface were systematically investigated. To identify the most energetically favorable structures, various adsorption sites and molecular orientations were considered, with the lowest-energy configurations shown in Figure a1–d1. The calculated adsorption strengths of these LDBs follow the order: C₅H₈ > C₃H₆O > C₃H₈O > C₂H₆O, with adsorption energies ranging from −0.27 to −0.57 eV. The corresponding equilibrium adsorption distances (d) fall within the range of 4.235 to 4.743 Å. These results indicate that the LDBs exhibit physisorption characteristics, governed primarily by van der Waals forces. This conclusion is further supported by the total electron density (TED) plots in Figure a2–d2, which reveal minimal electron sharing at the adsorption interfaces. Additionally, as shown in Figure a3–d3, the C₂H₆O, C₃H₆O, C₃H₈O, and C₅H₈ all function as electron acceptors, gaining approximately 0.058, 0.069, 0.057, and 0.060 e, respectively, from the PtS₂ substrate. The small charge transfer further confirms the weak affinity between PtS₂ and these LDBs. As shown in Figure , adsorption of C₂H₆O and C₃H₈O has a negligible effect on the PtS₂ bandgap, whereas the adsorption of C₃H₆O and C₅H₈ reduces the bandgap to 1.310 and 1.333 eV, respectively. Nevertheless, the inherently weak adsorption capacity of PtS₂ for LDBs limits its direct application as a gas sensor.

3.

Optimized adsorption structures, total electron density (TED), and CDD of (a1–a3) C₂H₆O, (b1–b3) C₃H₆O, (c1–c3) C₃H₈O, and (d1–d3) C₅H₈ adsorbed on the pristine PtS₂ monolayer. The green (red) regions represent the charge accumulation (depletion).

4.

Band structures of the adsorbed PtS₂ monolayer with (a) C₂H₆O, (b) C₃H₆O, (c) C₃H₈O, and (d) C₅H₈.

3.2. Adsorption Analysis of LDBs on the Rh@PtS₂ Monolayer

The adsorption behaviors of the four LDBs (C₂H₆O, C₃H₈O, C₃H₆O, and C₅H₈) on the Rh@PtS₂ surface were investigated, with the lowest-energy configurations depicted in Figure . The calculated adsorption energies for C₂H₆O and C₃H₈O are −0.79 and −0.97 eV, respectively, with corresponding adsorption distances of 2.282 and 2.263 Å, as shown in Figure a,c. Because these distances exceed the sum of the covalent radii of Rh and O atoms, these interactions are characteristic of physisorption. By contrast, the Rh atom forms direct chemical bonds with both C₃H₆O and C₅H₈, resulting in substantially higher adsorption energies of −0.87 and −1.72 eV, respectively. These findings demonstrate that C₃H₆O and C₅H₈ undergo chemisorption on the Rh@PtS₂ surface. To further analyze these interactions, the TED and CDD were calculated for each adsorption configuration (Figure ). As shown in Figure a, c, e, and g, all four LDBs exhibit substantial electron sharing with the PtS₂ substrate, as indicated by the blue regions at the adsorption interfaces. Consequently, the Rh dopant atom substantially enhances the adsorption capacity of the PtS₂ monolayer.

5.

Top and side views of the optimized structures for (a) C₂H₆O, (b) C₃H₆O, (c) C₃H₈O, and C₅H₈ adsorbed on the Rh@PtS₂ monolayer.

6.

Comparisons of TED and CDD plots for different adsorption systems: (a, b) C₂H₆O/Rh@PtS₂, (c, d) C₃H₆O/Rh@PtS₂, (e, f) C₃H₈O/Rh@PtS₂, and (g, h) C₅H₈/Rh@PtS₂.

As shown in Figure b, f, h, significant charge accumulation is observed around the Rh atoms, while minimal charge is distributed around the C₂H₆O, C₃H₈O, and C₅H₈ molecules. Consequently, these molecules donate approximately 0.163, 0.119, and 0.140 e, respectively, to the Rh@PtS₂ monolayer. In contrast, the C₃H₆O molecule acts as an electron acceptor, gaining approximately 0.091 e from the Rh@PtS₂ substrate. The large charge transfer between these LDBs and Rh@PtS₂ further indicates their strong interactions. To further analyze the bonding mechanism, the PDOS for each adsorption system was calculated and is presented in Figure a–d. For the C₂H₆O and C₃H₈O adsorption systems (Figure a,c), the adsorption is primarily attributed to the hybridization of O-2p and Rh-4d orbitals within the energy range of −8.00 to 1.50 eV. Furthermore, three resonance peaks are observed at about −6.62, −4.94, and −3.33 eV for the C₂H₆O/Rh@PtS₂ system, whereas four peaks appear at −6.80, −5.20, −4.64, and −3.28 eV for the C₃H₈O/Rh@PtS₂ system. Therefore, the adsorption strength of C₃H₈O is greater than that of C₂H₆O, which matches well with the previously calculated adsorption energies.

7.

Comparisons of PDOSs and band structures for different adsorption systems: (a, e) C₂H₆O/Rh@PtS₂, (b, f) C₃H₆O/Rh@PtS₂, (c, g) C₃H₈O/Rh@PtS₂, and (d, h) C₅H₈/Rh@PtS₂.

In contrast, the orbital hybridization between O-2p and Rh-4d in the C₃H₆O adsorption system is significantly stronger than that observed in the C₂H₆O and C₃H₈O systems (Figure b). Furthermore, the C-2p orbital of C₃H₆O exhibits pronounced overlap with the Rh-4d orbital, which underpins the chemical adsorption mechanism of C₃H₆O on the Rh@PtS₂ surface. Similarly, the chemisorption of C₅H₈ involves the bonding of two carbon atoms to the active Rh atom, facilitated by a strong interaction between their C-2p orbitals and the Rh-4d orbital. Additionally, the adsorption of these molecules induces notable changes in the electronic properties of the Rh@PtS₂ substrate. Specifically, C₂H₆O adsorption increases the bandgap from 0.623 to 0.727 eV, while C₃H₆O and C₃H₈O adsorption decrease it to 0.586 and 0.409 eV, respectively, as shown in Figure e–g. Most notably, the adsorption of C₅H₈ induces a semiconductor-to-metal transition in Rh@PtS₂ (Figure h). These pronounced changes in the bandgap clearly demonstrate the high sensitivity of Rh@PtS₂ toward these specific gases.

3.3. Adsorption Analysis of LDBs on the Ir@PtS₂ Monolayer

This section explores the adsorption characteristics of four LDBs on the Ir@PtS₂ surface. The most stable adsorption configurations, along with their TED and CDD plots, are illustrated in Figures and . As shown in Figure a, c, both C₂H₆O and C₃H₈O adsorb on the Ir@PtS₂ via their O atoms approaching the active Ir sites, exhibiting the E _ads values of −1.19 and −1.35 eV, respectively, and adsorption distances of 2.252 and 2.263 Å. Obviously, the two gases do not form chemical bonds with Ir@PtS₂ and, therefore, are classified as physisorption. In contrast, one C and one O atom from C₃H₆O, as well as two C atoms from C₅H₈, form chemical bonds with the Ir atom. This results in significantly more negative E _ads values (−1.19 eV ∼ −2.24 eV) and shorter adsorption distances (2.011 Å ∼ 2.251 Å), indicating that C₃H₆O and C₅H₈ are chemisorbed on the surface. Additionally, the substantial electron sharing between these LDBs and Ir@PtS₂, as evidenced by the TED and CDD analyses, suggests their strong interactions, which is further supported by the significant charge transfer illustrated in Figure . Specifically, C₂H₆O and C₃H₈O act as electron donors, whereas C₃H₆O and C₅H₈ function as electron acceptors.

8.

Top and side views of the optimized structures for (a) C₂H₆O, (b) C₃H₆O, (c) C₃H₈O, and C₅H₈ adsorbed on the Ir@PtS₂ monolayer.

9.

Comparisons of TED and CDD plots for different adsorption systems: (a, b) C₂H₆O/Ir@PtS₂, (c, d) C₃H₆O/Ir@PtS₂, (e, f) C₃H₈O/Ir@PtS₂, and (g, h) C₅H₈/Ir@PtS₂.

Figure presents the PDOSs and band structures for various Ir@PtS₂ adsorption systems. As depicted in Figure a–d, the affinity of Ir@PtS₂ for these gases primarily stems from orbital hybridization between Ir-5d and O-2p (or C-2p) orbitals across the entire energy range. Specifically, for C₂H₆O and C₃H₈O, weak hybridization near the Fermi level and strong hybridization within the range of −7.40 to −2.20 eV are observed between Ir-5d and O-2p orbitals. In contrast, C₃H₆O and C₅H₈ exhibit significant orbital overlap at the Fermi level and pronounced hybridization spanning −8.13 to 1.50 eV, involving the Ir-5d and either the O-2p orbital of C₃H₆O or the C-2p orbital of C₅H₈, as illustrated in Figure b,d. Thus, C₃H₆O and C₅H₈ exhibit obviously stronger chemisorption on Ir@PtS₂ compared to C₂H₆O and C₃H₈O. Additionally, Figure e–h reveals that the adsorption of C₂H₆O, C₃H₆O, and C₅H₈ increases the bandgap of Ir@PtS₂ from its initial value of 0.356 eV to 0.415, 0.457, and 0.525 eV, respectively. In contrast, the adsorption of C₃H₈O induces a semiconductor-to-metal transition in the Ir@PtS₂ monolayer.

10.

Comparisons of PDOSs and band structures for different adsorption systems: (a, e) C₂H₆O/Ir@PtS₂, (b, f) C₃H₆O/Ir@PtS₂, (c, g) C₃H₈O/Ir@PtS₂, and (d, h) C₅H₈/Ir@PtS₂.

3.4. Effect of Humidity and Interfering Gases

Exhaled breath is a complex matrix, typically comprising water vapor, nitrogen, carbon dioxide, and trace amounts of other species like CH₄. A highly effective sensor must exhibit preferential affinity for the target LDBs over these potential interferents. Consequently, this section systematically investigates the adsorption behavior of H₂O, N₂, CO₂, and CH₄ on both Rh@PtS₂ and Ir@PtS₂ surfaces, with the main results summarized in Table .

1. Adsorption Energy (E _ads), Charge Transfer (Q _T), and Adsorption Distance (d) of a Single H₂O Molecule and Various Interfering Gases Adsorbed on the Rh@PtS₂ and Ir@PtS₂ Monolayers, Where the R _sum Represents the Sum of the Theoretical Radii of Two Atoms.

materials	gases	E ads, eV	Q T, e	d, Å	R sum, Å
Rh-PtS2	H2O	–0.53	0.179	2.343	2.21 (Rh–O)
	N2	–0.69	–0.064	1.985	2.29 (Rh–N)
	CH4	–0.23	0.048	2.659	2.40 (Rh–C)
	CO2	–0.15	–0.009	2.437	2.40 (Rh–C)
Ir-PtS2	H2O	–0.93	0.218	2.330	2.28 (Ir–O)
	N2	–1.06	–0.066	1.914	2.36 (Ir–N)
	CH4	–0.26	0.094	2.527	2.47 (Ir–C)
	CO2	–0.49	–0.011	2.476	2.47 (Ir–C)

For the Rh-PtS₂ system, H₂O, CH₄, and CO₂ exhibit weak physisorption, with adsorption energies of −0.53, −0.23, and −0.15 eV, respectively. These values are substantially lower than the E _ads values observed for all LDBs, indicating that the target analytes can competitively adsorb onto the Rh-PtS₂ surface, even in the presence of these interfering species. Although N₂ shows a slightly stronger interaction, with a relatively large E _ads of −0.69 eV and a short adsorption distance of 1.985 Å, its adsorption remains weaker than that of all LDBs. Therefore, Rh-PtS₂ shows good selectivity toward the target LDBs. For the Ir-PtS₂ system, the adsorption energies of H₂O, CH₄, and CO₂ are −0.93, −0.26, and −0.49 eV, with corresponding adsorption distances of 2.330 2.527, and 2.476 Å, respectively. All these adsorption distances exceed the sum of the theoretical radii of the two interacting atoms, confirming that these gases are physisorbed. In contrast, N₂ exhibits chemisorption on Ir-PtS₂, as indicated by its large E _ads of −1.06 eV and a short adsorption distance of 1.914 Å. Nevertheless, the significantly stronger binding of all four LDBs compared to H₂O, N₂, CO₂, and CH₄ underscores the high selectivity of the Ir@PtS₂ monolayer.

Figure gives the most stable structures of Rh@PtS₂ and Ir@PtS₂ adsorbed with different numbers of H₂O molecules to simulate the effect of high humidity on their selectivity. For the Rh@PtS₂ system, the average adsorption energies for two and three H₂O molecules are −0.63 and −0.51 eV, respectively. A similar trend is observed for the Ir@PtS₂ system, where the average adsorption energies are −0.92 eV for two H₂O molecules and −0.87 eV for three H₂O molecules. This trend indicates that as surface coverage increases, the binding affinity for additional water molecules weakens. This is likely because the first H₂O molecule strongly interacts with the active metal dopant, while subsequent molecules primarily form weaker hydrogen bonds with already adsorbed water. It is noteworthy that in both cases, the average adsorption energy is weaker than the adsorption energies calculated for all four target LDBs. This suggests that the LDBs can competitively displace adsorbed water molecules or preferentially bind to the active sites, even in a high-humidity environment. Therefore, both Rh@PtS₂ and Ir@PtS₂ are predicted to maintain high selectivity against water vapor, which is crucial for their practical application in breath analysis.

11.

Lowest-energy configurations of different numbers of H₂O molecules adsorbed on (a,b) Rh@PtS₂ and (c,d) Ir@PtS₂ monolayers, along with their corresponding average adsorption energies.

To further elucidate the microscopic interactions, H₂O and N₂ adsorption systemsboth exhibiting relatively high adsorption strengthsare selected for detailed electronic structure analysis, as shown in Figures and . From Figure a1–a4, H₂O is obliquely adsorbed on both Rh@PtS₂ and Ir@PtS₂, with adsorption distances of 2.343 and 2.330 Å, respectively. These distances exceed the sum of the theoretical radii of Rh–O or Ir–O, indicating that these adsorptions are clarified as physisorption. In contrast, N₂ is vertically adsorbed on both Rh@PtS₂ and Ir@PtS₂, with adsorption distances of 1.985 and 1.914 Å, which are smaller than the sum of the theoretical radii of Rh–N or Ir–N, suggesting the chemisorption feature for the N₂. Therefore, both materials exhibit physisorption toward H₂O and chemisorption toward N₂. Furthermore, as depicted in Figure b1–b4, the H₂O and N₂ molecules share some charges with Rh@PtS₂ and Ir@PtS₂, indicating their strong interactions. The CDD plots reveal significant charge depletion around H₂O and charge accumulation around N₂. Consequently, H₂O acts as an electron donor, while N₂ serves as an electron acceptor in these adsorption systems.

12.

(a1–a4) Top and side views of atomic structures, (b1–b4) TED, and (c1–c4) CDD plots of the (a1–c1) H₂O/Rh@PtS₂, (a2–c2) H₂O/Ir@PtS₂, (a3–c3) N₂/Rh@PtS₂ and (a4–c4) N₂/Ir@PtS₂ systems.

13.

PDOS plots of different adsorption systems: (a) H₂O/Rh@PtS₂, (b) N₂/Rh@PtS₂, (c) H₂O/Ir@PtS₂, and (d) N₂/Ir@PtS₂.

Figure presents the PDOS values for different adsorption systems. In the H₂O/Rh@PtS₂ system (Figure a), weak orbital hybridization is observed between the O-2p and Rh-4d states within the energy range of −7.50 to −3.35 eV, resulting in a weak affinity of Rh@PtS₂ for H₂O. In contrast, the orbital overlap in the H₂O/Ir@PtS₂ system is significantly stronger than that in H₂O/Rh@PtS₂, indicating a greater adsorption strength of Ir@PtS₂ for H₂O. For the N₂/Rh@PtS₂ system (Figure c), broad orbital hybridization between N-2p and Rh-4d states is observed within the energy range of −10.94 to −5.00 eV, with two resonance peaks at approximately −10.26 and −6.65 eV, indicating the formation of a Rh–N covalent bond. An even broader orbital overlap is found in the N₂/Ir@PtS₂ system, where hybridization occurs between the 2s and 2p orbitals of N and the 6s, 5p, and 5d orbitals of Ir around −11.20 eV, along with strong hybridization between N-2p and Ir-5d states in the range of −7.60 to −5.00 eV. This suggests a strong affinity of Ir@PtS₂ for N₂. Nevertheless, the interaction strengths with H₂O and N₂ remain lower than those observed for all four target biomarkers. Therefore, this electronic-level analysis confirms the high selectivity of the Rh@PtS₂ and Ir@PtS₂ monolayers, reinforcing their potential for reliable biomarker detection in complex, humid environments.

3.5. Evaluation of Sensing Behavior

Sensitivity is a critical parameter in evaluating the gas sensing performance of a material, as it directly determines its practical applicability. Gas adsorption induces changes in surface charge distribution, leading to dynamic shifts in the bandgap, which in turn affect carrier concentration and electrical conductivity. Therefore, the change in the band gap (ΔE _g) of a material upon gas adsorption can be used as an indicator of its sensitivity to a specific gas. Alternatively, the change in the work function (ΔΦ) resulting from gas adsorption can also serve as a metric for assessing the intrinsic sensitivity of a material. The conductivity (σ) and work function (Φ) can be calculated as follows , :

$$\sigma =\lambda ·\exp (-E_{\mathrm{g}}/2k_{\mathrm{B}}T)$$ 4

$$\mathrm{\Phi }={\mathrm{\Phi }}_{\mathrm{vacuum}}-{\mathrm{\Phi }}_{\mathrm{Fermi}}$$ 5

Here, λ, k _B, and T represent a material-specific constant, the Boltzmann constant, and the absolute temperature, respectively, while Φ_vacuum and Φ_Fermi represent the vacuum energy level and Fermi level of the material, respectively.

Figure illustrates the ΔE _g and ΔΦ of pristine PtS₂ and TM@PtS₂ monolayers before and after LDB adsorption. As shown in Figure a, the PtS₂ monolayer exhibits high sensitivity to C₃H₆O and C₅H₈ with ΔE _g values of −25.70 and −27.00%. However, its poor adsorption capacity restricts its real-world applicability. In contrast, the adsorption of C₂H₆O, C₃H₈O, and C₅H₈ on the Rh@PtS₂ monolayer induces ΔE _g values of 16.69, −34.35, and −100.00%, respectively, demonstrating its excellent sensitivity to these three LDBs. More remarkably, the Ir@PtS₂ monolayer displays high responsiveness to all four LDBs, with |ΔE _g| ranging from 16.57 to 100%, indicating its potential as a resistive gas sensor for these analytes. Regarding work function variation (Figure b), the adsorption of C₂H₆O, C₃H₈O, and C₅H₈ yields ΔΦ values of −16.92, −18.45, and −14.36% for Rh@PtS₂, and 9.24, −19.49, and −18.45% for Ir@PtS₂, respectively. These results indicate that both Rh@PtS₂ and Ir@PtS₂ monolayers are promising candidates for work function-based sensors for the detection of C₂H₆O, C₃H₈O, and C₅H₈ molecules.

14.

Changes in the (a) bandgap and (b) work functions for various materials induced by the adsorption of the LDBs.

In addition to its high sensitivity, the practical utility of a gas sensor is fundamentally determined by its reusability. An appropriate recovery time is essential for real-time continuous monitoring. According to transition state theory, the recovery time (τ) can be calculated using the van’t Hoff-Arrhenius equation , :

$$\tau ={\nu }_0^{-1}\exp (-E_{\mathrm{ads}}/k_{\mathrm{B}}T)$$ 6

where ν₀ represents the attempt frequency (10¹² s^{– 1}), E _ads is the adsorption energy, k _B is the Boltzmann constant, and T is the absolute temperature. The calculated recovery times for LDBs desorption from Rh@PtS₂ and Ir@PtS₂ surfaces at various temperatures are presented in Figure .

15.

Recovery times for the desorption of various LDBs from (a) Rh@PtS₂ and (b) Ir@PtS₂ surfaces.

As illustrated in Figure a, C₂H₆O exhibits a reasonable recovery time of 22.7 s at 298 K. Furthermore, both C₂H₆O and C₃H₆O molecules can rapidly desorb from the Rh@PtS₂ surface at 348 K, with τ values of 0.27 and 3.94 s, respectively. These results confirm the suitability of Rh@PtS₂ for the reversible detection of these two biomarkers. In contrast, C₃H₈O and C₅H₈ molecules are bound so strongly that their theoretical recovery times extend to 2.5 × 10⁴ s and 1.2 × 10¹⁷ s at 298 K, respectively, rendering desorption practically impossible at room temperature. When the temperature is increased to 398 K, the recovery time for C₃H₈O desorption from the Rh@PtS₂ surface becomes acceptable (1.9 s). However, the desorption kinetics for the Ir@PtS₂ monolayer, as shown in Figure b, present a more significant challenge. At 298 K, the Ir@PtS₂ exhibits prohibitively long recovery times for all the four (1.3 × 10⁸ s ∼ 7.4 × 10²⁵ s). Even at 398 K, the recovery times remain excessively long, indicating that effective recovery is still impractical for these gases. Consequently, the application of Ir@PtS₂ would likely be limited to single-use detection.

Based on the above discussions, Rh@PtS₂ exhibits a good balance of sensitivity, selectivity, and recovery, making it a strong candidate for a reusable sensor. In contrast, Ir@PtS₂ is better suited for use as a high-performance, disposable sensor. While it displays superior sensitivity, evidenced by the higher adsorption energies and more significant electronic changes, its strong binding to the analytes results in prohibitively long recovery times, rendering it effectively irreversible under practical conditions. In summary, both Rh- and Ir-modified PtS₂ monolayers are highly promising for detecting LDBs. The choice between them depends on the specific application requirements: Rh@PtS₂ for reusable sensing applications, and Ir@PtS₂ for single-use diagnostic tools.

Finally, the adsorption and sensing performance of C₂H₆O and C₃H₆O on the Rh@PtS₂ surface are systematically compared with previously reported 2D materials (Table ). For the C₂H₆O, materials such as Pt-MoSe₂ and Au-MoSe₂ exhibit excessively strong adsorption strength, with E _ads values of −1.56 and −1.52 eV, respectively, resulting in theoretical recovery times on the order of 10¹⁰ s at 348 K. Such prolonged recovery durations render these materials impractical for reversible sensing applications. Conversely, Ag-WTe₂ exhibits overly rapid recovery due to its weaker adsorption energy (−0.57 eV). In contrast, the Rh@PtS₂ monolayer achieves a balanced adsorption energy (−0.79 eV), which induces substantial bandgap modulation (ΔE _g = 16.69%) while enabling rapid recovery (0.27 s at 348 K). For C₃H₆O, Pd-SnS₂ and Ag-WTe₂ similarly suffer from insufficient adsorption strength, resulting in undetectable signals due to excessively short recovery times. Meanwhile, Ru-PtTe₂ and Al-MoSe₂ exhibit overly high affinity with C₃H₆O, severely compromising their reusability. Overall, the favorable adsorption energy and moderate recovery of Rh@PtS₂ position it as a highly promising material for the development of reliable, reusable C₂H₆O and C₃H₆O sensors.

2. Comparative Analysis of Ethanol (C₂H₆O) and Acetone (C₃H₆O) Adsorption and Sensing on Rh@PtS₂ Versus Other 2D Materials.

target gases	materials	E ads, eV	△E g, %	△Φ, %	τ, s (348 K)	ref.
C2H6O	Ag-WTe2	–0.57	100	–0.79	1.79 × 10–4
	Pt-MoSe2	–1.56			3.85 × 1010
	Au-MoSe2	–1.52			1.01 × 1010
	Rh@PtS2	–0.79	16.69	–16.92	0.27	this work
C3H6O	Pd-SnS2	–0.53	–25.96		4.71 × 10–5
	Ag-WTe2	–0.40		–5.88	6.18 × 10–7
	Ru-PtTe2	–1.12	0.16		1.64 × 104
	Al-MoSe2	–1.79	–23.51		8.22 × 1013
	Rh@PtS2	–0.87	–5.94	7.18	3.94	this work

4. Conclusions

Employing DFT calculations, this work systemically investigates the adsorption properties of four LDBs on the Rh@PtS₂ and Ir@PtS₂ monolayers, with a detailed analysis of their electronic structures through TED, CDD, and DOS. The main conclusions are as follows:

• 1.The pristine PtS₂ monolayer exhibits weak physisorption interactions with the four LDBs, characterized by low adsorption energies (−0.27 to −0.57 eV) and minimal charge transfer. However, decoration with a single Rh or Ir atom creates highly active sites that dramatically enhance its adsorption capabilities.
• 2.The enhanced adsorption capacity of TM@PtS₂ (TM = Rh, Ir) systems is attributed to strong orbital hybridization between the Rh-4d/Ir-5d orbitals of the dopant atoms and the O-2p/C-2p orbitals of the LDBs. As a result, C₂H₆O and C₃H₈O are physisorbed onto both substrates, whereas C₃H₆O and C₅H₈ undergo chemisorption.
• 3.Both Rh@PtS₂ and Ir@PtS₂ exhibit remarkable sensitivity, as evidenced by substantial changes in their band structures and work functions upon LDBs adsorption. Notably, the adsorption of C₅H₈ on Rh@PtS₂ and C₃H₈O on Ir@PtS₂ induces a semiconductor-to-metal transition.
• 4.The evaluation of selectivity against common exhaled breath components (H₂O, N₂, CO₂, and CH₄) demonstrates that both Rh@PtS₂ and Ir@PtS₂ possess a pronounced preferential affinity for the target LDBs. This high selectivity is crucial for reliable detection in realistic and complex gaseous environments.
• 5.Rh@PtS₂ exhibits a reasonable recovery time (22.7 s) for the C₂H₆O at 298 K. It also exhibits suitable recovery time for C₃H₆O at 348 K and for C₃H₈O at 398 K, suggesting its potential as a reusable sensor for these three gases. In contrast, while Ir@PtS₂ displays superior sensitivity, it binds the analytes too strongly, making it more suitable as a disposable sensor.

The authors declare no competing financial interest.