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. 2026 Oct 12. Online ahead of print. doi: 10.1039/d6ra04908g

Acetone sensing with a single-atom transition metal (Fe, Co, Ni, Mn, Pd, and Zn)-doped InP3 monolayer: a theoretical study

Xin Qin a, Lijuan Guo c, Weiwei Tang c, Qiulan Zhou a,✉, Li Li b,✉
PMCID: PMC13656014  PMID: 42865879

Abstract

Acetone (C3H6O) in the exhaled breath of diabetic patients serves as an important biomarker for the noninvasive diagnosis of diabetes mellitus. In this work, based on density functional theory (DFT), we theoretically examined six single-atom transition metal (Fe, Co, Ni, Mn, Pd, and Zn)-doped InP3 monolayers (denoted as M-InP3) as C3H6O gas sensors to screen suitable electrochemical materials for non-invasive diabetes detection. The adsorption and cycling performances of M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) toward the C3H6O molecule were investigated, and the calculation results indicate that single-atom transition metal (Fe, Co, Ni, Mn, Pd, and Zn) doping greatly enhances the C3H6O adsorption capacity of the InP3 monolayer. The Zn-InP3 monolayer exhibits a suitable gas response (−100%) and a short recovery time (0.1 s) at 298 K, making it potentially applicable as a room-temperature reusable acetone sensor. Fe-InP3 and Pd-InP3 deliver high gas responses (−96% and −86%, respectively) to C3H6O, positioning them as promising candidates for acetone. Pd-InP3 achieves rapid recycling within 81 s at 373 K. The long desorption times and low response values of C3H6O make the Mn-InP3, Co-InP3 and Ni-InP3 surfaces unsuitable for C3H6O detection. This work presents a theoretical study of the adsorption and recyclability of C3H6O on six single-atom transition metal (Fe, Co, Ni, Mn, Pd, and Zn)-doped InP3 monolayers, potentially offering a new class of sensing materials for acetone sensors in the non-invasive diagnosis of diabetes mellitus.

Single-atom transition-metal doping enhances C3H6O adsorption on InP3. Notably, Zn-InP3 shows a −100% response and 0.1 s recovery time at 298 K, making it a promising room-temperature reusable acetone sensor for noninvasive diabetes diagnosis.graphic file with name d6ra04908g-ga.webp

1. Introduction

Diabetes mellitus has emerged as a progressively severe global health crisis. It encompasses type I, type II, gestational diabetes, and rarer subtypes, each significantly contributing to increased morbidity and mortality.1–3 In 2021, type II diabetes constituted 97.8% of all diagnosed cases in China, contributing to approximately 178 000 diabetes-associated deaths.4 Moreover, chronic hyperglycemia is a major driver of secondary complications, which can lead to progressive renal dysfunction and life-threatening outcomes such as uremia.5–7 Exhaled acetone (C3H6O) has been identified as a potential biomarker for diabetes, with studies showing significantly elevated acetone levels in the breath of diabetic patients compared to healthy individuals.8–10 Diagnosing diabetes by detecting breath C3H6O can avoid the traditional needle-based blood sampling method, improving patient acceptance. Therefore, detecting acetone in exhaled breath can serve as a non-invasive method for diagnosing diabetes mellitus, a field that has attracted widespread attention.11–13

Owing to their compact size, low cost, and simple fabrication, electrochemical sensors hold great promise for practical applications in breath analysis.14,15 For instance, Tan et al. have developed a ZnO/CuO heterojunction sensor based on van der Waals stacking, showing a fast response toward 20 ppm C3H6O at an operating temperature of 300 °C.16 In2O3@Fe2O3 core–shell heterojunction tubes were fabricated by Li and co-workers, exhibiting an enhanced sensing response to 100 ppm C3H6O at 200 °C, along with a short recovery time.17 These devices function by detecting alterations in electrical properties, such as resistance or current, induced by the interaction of the target molecules with the active sensing layer. As the performance of electrochemical sensors is largely determined by their sensitive material, extensive research has been focused on the development and optimization of such materials to improve acetone detection.18–21

Two-dimensional (2D) materials have generated considerable interest for use in electrochemical sensors due to their advantages of a large specific surface area, high electron mobility, and good chemical stability.22,23 Among them, graphene-like indium triphosphide (InP3), theoretically prepared via exfoliation from layered bulk InP3, is an indirect band gap semiconductor with high electron mobility and extraordinary light capturing performance.24 Moreover, atomic doping and external strain can effectively modulate its magnetic and electronic transport characteristics, endowing the InP3 monolayer with potential as a nanomaterial for application in electronic devices and optical equipment.25–27 Miao et al. have demonstrated that 2D-InP3 is a promising material for electronics and photovoltaic devices owing to its tunable magnetism and superior optical absorption.24 A layer of porous InP3 nanoflakes synthesized by Chang and co-workers was applied in flexible all-solid-state supercapacitors.28 Yang et al. theoretically demonstrated that the InP3 monolayer had high sensitivity and selectivity to HCHO and thus proposed it as a gas sensor for HCHO.29 To further improve selectivity and electronic response, transition metal (TM) decoration is considered as a promising strategy for introducing exposed active centers on InP3.30–32 Liao and co-workers demonstrated that the electronic structure of InP3 can be significantly modified by Cr doping, thereby endowing the Cr-InP3 monolayer with improved adsorption properties toward H2, CH4, and C2H2 gases.33 For C3H6O gas adsorption, theoretical studies by Ma et al. indicated that incorporating the transition metal Ce enhances the activation of surrounding Sn. As a result, 3 wt% Ce-SnO2 nanofibers exhibit good sensing performance toward C3H6O gas at 170 °C.34 A 2 wt% Zn-doped TiO2 sensor synthesized by Murugesan et al. exhibited a maximum C3H6O response of 75.5% for 5 ppm concentration under UV illumination.35 The Pd/WO3 sensor developed by Verma and co-workers showed a remarkable lower limit of detection of 0.076 ppm within the concentration range of 0.1–0.8 ppm.36 Cheng's team reported an acetone sensor based on Pd/In2O3-decorated SnO2 nanocomposites, which exhibited a high sensitivity with a response value of 2.9 at 200 ppb.37

However, the orbitals of 3d and 4d TMs differ in their spatial extension, polarizability, and energy alignment with the molecular orbitals of target analytes. The size disparity between the dopant and the native atoms of the material may also lead to distinct effects on its electrical characteristics. Here, to investigate the influence of different TMs on the adsorption and sensing behavior of C3H6O on a InP3 surface, open-shell Fe, Co, Ni, and Mn characterized by partially occupied 3d orbitals, closed-shell Zn with fully filled 3d orbitals, and Pd featuring a typical intermediate 4d electronic structure were supported on the 2D-InP3 surface. Employing density functional theory (DFT), six single-atom TM-doped InP3 (denoted as M-InP3, M = Fe, Co, Ni, Mn, Pd, and Zn) monolayers were constructed to examine the adsorption and sensing behavior toward the C3H6O molecule. The adsorption parameters, density of states, band structure, electron localization function, electrical conductivity and sensitivity response were used to evaluate the adsorption behavior and sensing performance. The recovery time at various temperatures was examined to analyze their cycling performance. It is hoped that these results can provide theoretical guidance for the further experimental development of acetone gas sensors for the non-invasive diagnosis of diabetes mellitus.

2. Computation details

The Perdew–Burke–Ernzerhof (PBE) functional with the generalized gradient approximation (GGA) was employed in this paper within the DMol3 package.38 The core and valence electrons were treated using the DFT semi-core pseudopotentials (DSPPs) and double numerical plus polarization (DNP).39,40 The Monkhorst–Pack k-point meshes of 3 × 3 × 1 and of 9 × 9 × 1 were used for structure relaxation and electronic property calculations. The Tkatchenko–Scheffler (TS) method for DFT-D was employed to better understand the van der Waals force and long-range interactions between the C3H6O molecule and substrates.41 Based on the optimized 3 × 3 × 1 supercell of the pristine InP3 monolayer, six single-atom TMs (Fe, Co, Ni, Mn, Pd, and Zn) were individually doped into the InP3 surface for acetone adsorption. A vacuum space of 15 Å was applied to avoid the interaction between the adjacent units. A global orbital cut-off radius of 5.0 Å and a self-consistent field convergence accuracy of 1 × 10−6 Ha were used to ensure the accuracy of the overall calculations.42 The energy tolerance accuracy, maximum force, and maximum displacement were 1 × 10−5 Ha, 2 × 10−3 Ha Å−1, and 5 × 10−3 Å, respectively.43 Zero-point energies, electronic energies and Gibbs free energies were obtained by vibrational frequency calculations.44

The binding energy (Eb) of single-atom TMs (Fe, Co, Ni, Mn, Pd, and Zn) on the InP3 monolayer is defined by the following equation: Eb = EM-InP3 − EInP3 − EM. Here, EM-InP3, EInP3, and EM represent the total energy of the transition metal-doped InP3 surface, pristine InP3 monolayer, and isolated metal atom, respectively. The adsorption energy (Ead) for C3H6O on the M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) surface is calculated as follows: Ead = EM-InP3/C3H6O − EM-InP3 − EC3H6O, where EM-InP3/C3H6O, EM-InP3 and EC3H6O denote the Gibbs free energies of the C3H6O-M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn), M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) and single C3H6O molecule, respectively. A more negative value of Ead corresponds to a stronger interaction between the substrate and the target molecule. The charge transfer (QT) between C3H6O and M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) monolayers was quantitatively evaluated using Mulliken population analysis and expressed as QT = Qad-C3H6O − QC3H6O, where Qad-C3H6O and QC3H6O correspond to the charges of the adsorbed and free C3H6O molecule, respective.45–47 A negative QT value indicates that the adsorbed C3H6O molecule acts as an electron acceptor, whereas a positive QT value indicates that the underlying substrate serves as an electron acceptor. The gas response (S) value was used to estimate the sensing performance of the M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) monolayers toward the C3H6O molecule. S is formulated as a combination of S = (σaf−1 − σbe−1)/σbe−1 and σ = λe(−Bg/2κT), in which σaf and σbe are the conductivity (σ) values of the system before and after adsorption, respectively. σ satisfies the formula σ = λe(−Bg/2κT), in which Bg is the band gap of the modified systems, and κ and T represent the Boltzmann constant and temperature (298 K).48,49 The band gap variation serves as a relative descriptor of the electronic response. However, the exponential response relation is derived under the idealized assumption of an intrinsic semiconductor, whereas TM doping can introduce localized impurity states and spin-dependent electronic states. Therefore, the calculated S values are interpreted as comparative theoretical indicators rather than quantitative predictions of the experimentally measured sensor responses.

3. Results and discussion

The gas response (S) value serves as an important parameter to evaluate the detection performance of sensing materials toward target molecules. Our previous work showed a poor sensitivity response (12.4%) of the pristine 2D-InP3 toward C3H6O molecule.50 To improve the detection performance of C3H6O, we constructed six M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) surfaces and theoretically examined their adsorption behaviors toward the C3H6O molecule.

The geometric configurations of the M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) monolayers were optimized by positioning single-atom TMs at an initial distance of 3.5 Å from various binding sites of the InP3 substrate, including the top of the In and P atoms and the center of the six-element rings. Fig. 1 shows the most stable M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) surfaces with the maximum binding energy (Eb). The corresponding Ebs values are −5.12, −4.79, −4.76, −4.14, and −3.24 eV for the Fe-, Mn-, Co-, Ni-, Pd-surfaces, respectively. The five single-atom TMs are situated above the six phosphorus rings of InP3 with the formation of three new M–P (M = Fe, Co, Ni, Mn, and Pd) bonds. The shortest M–P lengths for Fe-InP3, Mn-InP3, Co-InP3, Ni-InP3 and Pd-InP3 are 2.18, 2.08, 2.23, 2.22 and 2.42 Å, respectively (Table 1). In contrast, the Zn-InP3 surface exhibits the lowest binding energy (−0.75 eV) among the six modified materials, as confirmed by the longer distance (2.79 Å) between the single Zn atom and In atom of the InP3 substrate. Given the relatively lower Eb of the Zn-doped system, a short NVT ab initio molecular dynamics (AIMD) simulation was performed at 373 K to examine whether the optimized Zn-InP3 structures undergo immediate thermal disruption. A time step of 1.0 fs and a total duration of 10 ps (10 000 steps) were used.51,52 During the 10 ps trajectories, the Zn atom remained anchored at the same cavity site and maintained its structural integrity, indicating that the single-atom Zn doping exhibits robust stability (Fig. S1). Mulliken population analysis shows that the Fe, Co, Ni, Mn and Pd atoms accept 0.13, 0.22, 0.05, 0.04 and 0.24 e from the bottom surface of InP3, while Zn donates 0.20 e to the substrate after doping on the 2D-InP3 surface.

Fig. 1. (a–f) Top and side views of the most stable structures of the M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) surfaces. The distance is in Å.

Fig. 1

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Table 1. Corresponding parameters of the six M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) monolayers.

Doped models E b (eV) Q T (e) D (Å) Band gap (eV) ε d
Fe-InP3 −5.12 −0.13 2.18 0.23 −1.43
Mn-InP3 −4.79 −0.04 2.08 — −1.65
Co-InP3 −4.76 −0.22 2.23 0.28 −2.26
Ni-InP3 −4.14 −0.05 2.22 0.48 −1.58
Pd-InP3 −3.24 −0.24 2.42 0.67 −2.65
Zn-InP3 −0.75 0.20 2.79 1.08 −6.13
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The calculated work functions of the M-InP3-C3H6O (M = Fe, Co, Ni, Mn, Pd, and Zn) systems before and after the adsorption of C3H6O are illustrated in Fig. 2. TM doping on the 2D-InP3 surface causes the Fermi level to shift upward with respect to the vacuum level and leads to a decrease in work function. The work function reveals the minimum energy required to release an electron from the surface.42 The decrease in the work function implies that the doped surface exhibits a stronger tendency to lose electrons, thereby promoting the adsorption of electron-acceptor-type C3H6O molecules.

Fig. 2. Work function of the M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) (pink stars) and M-InP3-C3H6O (M = Fe, Co, Ni, Mn, Pd, Zn) (green circles) systems.

Fig. 2

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In this work, the band gap of the pristine 2D-InP3 calculated at the pure DFT level is 1.10 eV.50 It should be pointed out that standard DFT generally underestimates the band gap of 2D semiconductors. Hybrid density functionals (PBE0 and HSE06) were employed to perform band-structure calculations for 2D-InP3, yielding band gaps of 1.98 and 1.26 eV, respectively.53,54 Given the tolerable deviation between our computed band gap (1.10 eV) and the reported value (1.14 eV) for the InP3 monolayer,24,26 the band gap, band structure and density of states obtained using standard DFT are adopted for the subsequent analysis.

Upon doping the InP3 surface, orbital hybridization occurs between the TM atoms and the basal atoms, significantly modifying the electronic structure. Band structure configurations were employed to analyze the electronic structure variations of 2D-InP3 upon TM doping (Fig. 3). In the Co-, Ni-, Pd- and Zn-doped systems, the symmetric majority and minority spin bands reveal non-spin-polarized semiconductor characteristics. The corresponding band gap values are 0.28, 0.48, 0.67 and 1.08 eV for the Co-, Ni-, Pd- and Zn-doped systems, respectively. For the Fe-doped system, the calculated band structure shows a spin-polarized semiconductor feature and the band gap is 0.23 eV. The reduction in band gaps indicates the enhanced electronic conductivity of these modified materials.33,50 Moreover, the change in the band gap of the Mn-InP3 surface is verified by its band structure configuration. As shown in Fig. 3, the band of the spin-up channel crosses the Fermi level, while a band gap opens in the spin-down channel, indicating the half-metallic feature of the Mn-InP3 system. Considering the strong coulombic interaction between the partially filled d-shells of the TMs, we also add the Hubbard U term for a better description of the electronic structure in the M-InP3 systems.55,56 The effective Hubbard U (Ueff = U − J) values are set as follows: 3.29, 5.9, 6.3, 5.0 and 5.0 eV for Fe-, Co-, Ni-, Mn- and Pd-InP3, respectively.57–61 The corresponding band gap values are 0.24, 0.30, 0.39, and 0.58 eV for the M-InP3 (M = Fe, Co, Ni, and Pd) surfaces. The Mn-InP3 system obtained with the Ueff correction still exhibits a half-metallic character (Fig. S4). The present results are consistent with those obtained from calculations with the pure functional.

Fig. 3. (a–f) Band structures of the Fe-InP3, Co-InP3, Ni-InP3, Mn-InP3, Pd-InP3 and Zn-InP3 systems. The black dashed line is the Fermi level.

Fig. 3

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The electronic characteristics are further corroborated by the density of states (DOS) and partial density of states (PDOS) of the M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) systems. Compared with the pristine 2D-InP3 surface, the DOS curves of the M-InP3 (M = Fe, Co, Ni, Mn, and Pd) surfaces significantly shifted to the left. This leftward shift may be attributed to the orbital overlap between the 3d orbit of the metal atoms and the 3p orbit of the P atom (Fig. 4). In contrast, the DOS of the Zn-InP3 system remains unchanged, which is supported by the negligible orbital hybridization between the Zn-4s and In-5p orbits at the Fermi level. The evolution of the DOS peaks near the Fermi level in the M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) surfaces indicates that the electrical conductivity undergoes corresponding modifications upon doping with TMs on the basal InP3 surface.62 The corresponding TDOS and PDOS of the M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) surfaces based on the GGA + U calculation are provided in the SI (Fig. S5).

Fig. 4. (a–f) TDOS and PDOS of the Fe-InP3, Co-InP3, Ni-InP3, Mn-InP3, Pd-InP3 and Zn-InP3 systems. The dashed line is the Fermi level.

Fig. 4

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After obtaining the stable configurations of the doped materials, the adsorption behaviors of the six M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) surfaces toward C3H6O molecule were investigated. The C3H6O molecule was initially placed at different sites on the doped InP3 surface, including the carbonyl-O atom of C3H6O facing the dopant atom, the carbonyl-O atom facing the P/In atom of the InP3 substrate, the methyl group of C3H6O facing the dopant atom, the methyl group facing the P/In atom of the InP3 substrate, and the C3H6O molecule lying parallel to the InP3 ring. The initial distance between C3H6O and the hypothetical adsorption site was set at 3.5 Å (Fig. S6–S11).

Fig. 5 displays the most stable configurations of the six modified InP3 surfaces with the adsorbed C3H6O (denoted as M-InP3-C3H6O) and the corresponding electron density difference (EDD) of the adsorption systems. In the M-InP3-C3H6O (M = Fe, Co, Ni, Mn, Pd, and Zn) systems, the C3H6O molecule interacts with the transition metal atoms through its carbonyl oxygen atom. The lengths of the M–O bonds are 1.86, 1.88, 1.94, and 1.93 Å for Fe, Co, Ni, and Mn, respectively, with the corresponding adsorption energies (Ead) of −3.49, −2.34, −2.95, and −2.90 eV (Table 2). These adsorption energies are larger than the binding affinity of pristine 2D-InP3 (−0.962 eV) and ZnO–O (−1.06 eV) toward the C3H6O molecule, suggesting that the incorporation of these TMs effectively enhances the acetone adsorption capacity of the modified M-InP3 (M = Fe, Co, Ni, and Mn) surfaces.50,63 Conversely, the Pd-InP3 and Zn-InP3 surfaces have a relatively lower Ead of −2.03 and −1.93 eV, with longer Pd–O (2.30 Å) and Zn–O (2.11 Å) bonds. The d-band centers (εd) of TM atoms in M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) were determined to examine the adsorbate–surface interactions. εd shifts closer to the Fermi level, indicating an increase in the number of empty anti-bonding states of the doped metal, which strengthens the chemical bond between the adsorbate and the substrate.64,65 The calculated results reveal that εd increases in the order of −6.13 eV (Zn-InP3), −2.65 eV (Pd-InP3), −2.26 eV (Co-InP3), −1.65 eV (Mn-InP3), and −1.58 eV (Ni-InP3) and −1.43 eV (Fe-InP3) in energy (Table 1), which is verified by a positive correlation with the order of the C3H6O adsorption energies: −1.93 eV (Zn-InP3-C3H6O) < −2.03 eV (Pd-InP3-C3H6O) < −2.34 eV (Co-InP3-C3H6O) < −2.90 eV (Mn-InP3-C3H6O) < −2.95 eV (Ni-InP3-C3H6O) < −3.49 eV (Fe-InP3-C3H6O). According to the Mulliken population analysis, the adsorbed C3H6O molecule donates 0.13, 0.15, 0.15, 0.09, 0.15 and 0.21 e to the M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) surfaces, respectively. The EDD (Fig. 5) reveals sequential and localized electron accumulation along the newly formed M–O bonds, indicating the covalent bond character and confirming the chemisorption of C3H6O on the M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) surfaces.

Fig. 5. (a–f) Most stable structures and EDD of Fe-InP3-C3H6O, Co-InP3-C3H6O, Ni-InP3-C3H6O, Mn-InP3-C3H6O, Pd-InP3-C3H6O and Zn-InP3-C3H6O. The isosurface is 0.02 e/Å3, and the distance is in Å. The magenta and green areas show electron accumulation and electron depletion, respectively.

Fig. 5

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Table 2. Adsorption parameters and gas response values of the M-InP3-C3H6O (M = Fe, Co, Ni, Mn, Pd, and Zn) systems.

Adsorption systems E ad (eV) E ad (373 K, eV) Q T (e) D (Å) Band gap (eV) S (%)
Fe-InP3-C3H6O −3.49 −2.70 0.13 1.86 0.06 −96%
Ni-InP3-C3H6O −2.95 −2.13 0.15 1.94 0.44 −51%
Mn-InP3-C3H6O −2.90 −1.89 0.09 1.93 — —
Co-InP3-C3H6O −2.34 −1.55 0.15 1.88 0.24 −57%
Pd-InP3-C3H6O −2.03 −1.40 0.15 2.30 0.58 −86%
Zn-InP3-C3H6O −1.93 −1.18 0.21 2.11 0.78 −100%
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Gas adsorption can induce the redistribution of electronic states; therefore, a stronger band gap modulation contributes to a more prominent electrical signal response upon gas capture. The band structure was analyzed to evaluate the electronic structure variations in the M-InP3-C3H6O (M = Fe, Co, Ni, Mn, Pd, and Zn) systems (Fig. 6). After the adsorption of C3H6O, the band gaps decrease from 0.23, 0.28, 0.48, 0.68, and 1.08 eV to 0.06, 0.24, 0.44, 0.58 and 0.78 eV for the Fe-, Co-, Ni-, Pd- and Zn-doped systems, respectively. The corresponding band gap variation rates are 73.4%, 15.4%, 7.7%, 14.8% and 27.7%, respectively. Significant band gap narrowing in Fe-InP3-C3H6O can theoretically enhance the electrical response sensitivity, as verified by its high gas response value of −96%. The Co-InP3-C3H6O and Pd-InP3-C3H6O systems show similar band modulation capability, while Pd-InP3 yields a relatively higher gas sensitivity response (−86%). This may stem from the greater spatial extension and stronger delocalization of the Pd 4d orbitals, which facilitates stronger hybridization with the host orbitals and induces remarkable charge redistribution. The weak band gap modulation in the Ni-InP3-C3H6O system may indicate that the doping-induced electronic perturbation is insufficient, which could increase the difficulty of detecting C3H6O. This corresponds to the lower gas sensing response (51%) of the Ni-InP3-C3H6O system. As revealed by the band structure configuration, the Mn-InP3 system retains its half-metallic character after C3H6O adsorption. These results reveal that stronger adsorption is not necessarily indicative of better sensing performance, as excessively strong adsorption may result in a prolonged recovery time.

Fig. 6. (a–f) Band structures of the Fe-InP3-C3H6O, Co-InP3-C3H6O, Ni-InP3-C3H6O, Mn-InP3-C3H6O, Pd-InP3-C3H6O and Zn-InP3-C3H6O systems. The black dashed line is the Fermi level.

Fig. 6

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The reduction in band gap values correlates with enhanced electrical conductivity, as further verified by modifications in the density of states. Fig. 7 displays the DOS and PDOS of the six adsorption systems. From the DOS curves, the subtle elevation of the DOS peaks near the Fermi level indicates an increase in electrical conductivity after the C3H6O adsorption. Besides, the Zn-InP3-C3H6O system exhibits a slight leftward shift in its DOS profile compared to Zn-InP3, suggesting a small contribution to the electrical conductivity of this system. From the PDOS curves, the orbital hybridization between the 3d orbital of Fe, Co, Ni, Mn, and Pd atoms and the O-2p orbital of C3H6O is demonstrated, indicating chemisorption in the Fe-InP3, Co-InP3, Ni-InP3, Mn-InP3, and Pd-InP3 adsorption systems. In the Zn-InP3-C3H6O system, orbital hybridization occurs between the lower-energy Zn-4s orbital and O-2p orbital. The electron localization function (ELF) diagrams of the six M-InP3-C3H6O (M = Fe, Co, Ni, Mn, Pd, and Zn) systems (Fig. 8) show the relationship between C3H6O and the M-InP3 substrates. In all adsorption systems, the electron localization regions between the adsorbate and the doped substrate overlap along the M–O (M = Fe, Co, Ni, Mn, Pd, and Zn) bonds, revealing the strong orbital hybridization between C3H6O and the modified InP3 surfaces. Additionally, the blue isosurface surrounding the C3H6O molecule indicates an electron deficiency in this region, which is consistent with the electron transfer obtained from Mulliken population analysis. These results further demonstrate that the interactions between C3H6O and the six M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) substrates are chemisorption.

Fig. 7. (a–f) TDOS and PDOS of the Fe-InP3-C3H6O system, Co-InP3-C3H6O system, Ni-InP3-C3H6O system, Mn-InP3-C3H6O system, Pd-InP3-C3H6O system and Zn-InP3-C3H6O system. The dashed line is the Fermi level.

Fig. 7

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Fig. 8. (a–f) ELF of Fe-InP3-C3H6O, Co-InP3-C3H6O, Ni-InP3-C3H6O, Mn-InP3-C3H6O, Pd-InP3-C3H6O and Zn-InP3-C3H6O. The red and blue areas show electron accumulation and electron depletion, respectively.

Fig. 8

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The electronic interaction between the M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) monolayers and the C3H6O molecule is further analyzed using the molecule orbital theory.66Fig. 9 shows the distributions of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of the six modified surfaces before and after the C3H6O adsorption. In Pd-InP3, Zn-InP3, Co-InP3 and Ni-InP3 adsorption systems, the electron density is predominantly localized around the M–O bonds in the HOMOs. In contrast, for the Fe-InP3 and Mn-InP3 systems, both HOMOs and LUMOs exhibit electron density distributions across the chemical bonds. The change in the HOMO–LUMO energy gap (Eg) correlates with the stability of the chemical bond, which also has an influence on the electrical conductivity of the systems.48 The calculation formula is as follows: (|EM-InPg3-C3H6O − EM-InPg3|)/EM-InPg3 × 100%, where EM-InPg3 and EM-InPg3-C3H6O are the HOMO–LUMO energy gaps of the six M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) surfaces and M-InP3-C3H6O (M = Fe, Co, Ni, Mn, Pd, and Zn) systems. The corresponding values are 3.78%, 6.45%, 6.06%, 6.41%, 13.39% and 22.32%, respectively, revealing that Pd-InP3 and Zn-InP3 exhibit more pronounced conductivity variations compared to the other four M-InP3 (M = Fe, Co, Ni, Mn) surfaces. This result also indicates that these two modified materials exhibit stronger gas sensitivity responses to the C3H6O molecule, which is in agreement with the previously obtained analytical results.

Fig. 9. HOMO and LUMO (a-c) of the Fe-InP3 and Ni-InP3 systems, Co-InP3 and Mn-InP3 systems, Pd-InP3 and Zn-InP3 systems before and after C3H6O adsorption.

Fig. 9

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Since the carbonyl group of C3H6O possesses a substantial molecular dipole, the changes in the HOMO and LUMO levels can be partly attributed to the dipole effect of the adsorbed C3H6O molecule.67 Moreover, Mulliken analysis shows that the coordination of C3H6O with the metal dopant results in charge transfer from the target adsorbate to the doped substrate, which leads to a modification of the local electrostatic potential at the adsorption interface. Consequently, the Fermi level is shifted further upward with respect to the vacuum level, and this shift dominates the subsequent reduction of the work function of the adsorption system (Fig. 2).

The recovery time (τ) serves as a key parameter for estimating both the desorption ability of the C3H6O molecule from the six M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) surfaces and the recycling potential of the modified InP3 materials.68,69 The van't Hoff-Arrhenius expression τ = A−1e(−Ea/κT) is applied here, where A represents the attempt frequency (1012 s−1), T is the temperature and κ denotes the Boltzmann constant. Ea is the energy required for gas desorption, which is numerically equivalent to Ead in this model. As the Ead value increases, a higher temperature is required for gas desorption. Fig. 10 gives the recovery time bar chat for the six M-InP3-C3H6O (Fe, Co, Ni, Mn, Pd, and Zn) adsorption systems at three temperatures (273 K, 298 K and 373 K). For the Fe- and Pd-InP3 surfaces, the suitable gas response (−96% and −86%) allow the application of the Fe-InP3 and Pd-InP3 monolayers as promising acetone adsorbents. Meanwhile, the Pd-InP3 monolayer achieves rapid cyclic utilization in 81 s upon heating to 373 K. Moreover, the satisfactory gas response (−100%) and short recovery time (0.1 s) at 298 K make Zn-InP3 a potential sensor material for acetone detection under normal conditions. Although the recovery time gradually shortens with increasing temperature, it can be observed that at 373 K, the desorption of C3H6O from the Co-InP3, Ni-InP3 and Mn-InP3 surfaces still requires a considerable amount of time. This indicates that the three materials are unsuitable for the detection of the C3H6O molecule.

Fig. 10. Recovery times of the (a) Fe/Co/Ni-InP3 and (b) Mn/Zn/Pb-InP3 monolayers for C3H6O desorption.

Fig. 10

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It is worth noting that the adsorption energy Ead is generally not equivalent to the desorption activation barrier for chemisorption. The precise energy required for C3H6O desorption from M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) surfaces will need to be determined via CI-NEB transition state calculations. Moreover, the acceptable recovery time under experimental conditions depends on various factors, including the intended application, measurement cycle, gas concentration, device architecture, operating temperature, and required temporal resolution. Therefore, the recovery times reported here should be viewed as rough estimates rather than quantitative experimental predictions.

In summary, this work theoretically proposes three TM-doped InP3 materials for the non-invasive diagnosis of diabetes mellitus through acetone markers. The Zn-InP3 monolayer can function as a promising reusable sensor for sensitive acetone detection at room temperature. Although the Fe-InP3 and Pd-InP3 surfaces have the potential to serve as adsorbents toward acetone, and Pd-InP3 requires a higher operating temperature of 373 K for recycling. Nevertheless, it is important to recognize that various practical factors may influence the acetone-sensing performance of the modified InP3 materials. Our calculated results provide theoretical indicators for comparative screening among the investigated systems. Comprehensive experimental investigations are essential to explore their practical applicability in the non-invasive diagnosis of diabetes mellitus.

4. Conclusion

In this work, we provide theoretical insights into six single-atom transition metal (Fe, Co, Ni, Mn, Pd, and Zn)-doped InP3 monolayers as gas sensors for the C3H6O molecule, aiming to screen suitable electrochemical materials for the non-invasive detection of diabetes mellitus. The adsorption performance of the six modified InP3 surfaces toward C3H6O is ranked as follows: Fe-InP3 > Ni-InP3 > Mn-InP3 > Co-InP3 > Pd-InP3 > Zn-InP3. The appropriate sensing response and desirable desorption property make Zn-InP3 a promising candidate for a recyclable electrochemical sensor of C3H6O at room temperature. Fe-InP3 and Pd-InP3 show potential as the adsorbent for the C3H6O molecule, and recycling of Pd-InP3 can be achieved by heating at 373 K. Owing to their lower gas response and extended recovery times, Co-InP3, and Ni-InP3 and Mn-InP3 are unsuitable for the detection of C3H6O.

Author contributions

Qin, X. and Zhou, Q. conceived and supervised the overall project. Qin, X. performed the theoretical calculations and analyzed the data. Guo, L. and Tang, W. prepared the figures. Zhou, Q. and Li, L. wrote the manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Supplementary Material

RA-OLF-D6RA04908G-s001

Acknowledgments

This work was supported by the Natural Science Foundation of Hunan Province (Grant No. 2026JJ81262, 2026JJ60134, and 2026JJ81263), the Scientific Research Foundation of Hunan Provincial Education Department, China (Grant No. 23B0870, 25A0801), and the Open Fund Project of Hunan Provincial Key Laboratory of the Research and Development of Novel Pharmaceutical Preparations (Grant No. 2023CYY007).

Data availability

All data generated during this study are fully available within the article.

Supplementary information (SI): AIMD simulations of the Zn-InP3 and Pd-InP3 surfaces, band structures, TDOS and PDOS of M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) based on GGA + U calculations, and all computational models of M-InP3-C3H6O (M = Fe, Co, Ni, Mn, Pd, and Zn) adsorption systems (PDF). See DOI: https://doi.org/10.1039/d6ra04908g.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-OLF-D6RA04908G-s001
RA-OLF-D6RA04908G-s001.pdf (13.8MB, pdf)

Data Availability Statement

All data generated during this study are fully available within the article.

Supplementary information (SI): AIMD simulations of the Zn-InP3 and Pd-InP3 surfaces, band structures, TDOS and PDOS of M-InP3 (M = Fe, Co, Ni, Mn, Pd, and Zn) based on GGA + U calculations, and all computational models of M-InP3-C3H6O (M = Fe, Co, Ni, Mn, Pd, and Zn) adsorption systems (PDF). See DOI: https://doi.org/10.1039/d6ra04908g.

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