Air pollution and industrial emissions pose severe environmental and health hazards, with toxic gases like CO, NO, NO2, SO2, H2S, NH3, CO2, and CS2 contributing to respiratory diseases, climate change, and ecological damage. Carbon monoxide (CO), an odourless and highly toxic gas, disrupts oxygen transport in the bloodstream, leading to fatal poisoning. Nitrogen oxides (NO and NO2) are key contributors to smog and acid rain, exacerbating lung diseases and cardiovascular issues. Sulfur dioxide (SO2), released from fossil fuel combustion, causes acidification of ecosystems and severe respiratory irritation. Hydrogen sulfide (H2S), commonly found in industrial waste, is highly toxic, leading to respiratory failure at high concentrations. Ammonia (NH3), a major agricultural and industrial gas, causes severe irritation and lung damage. Carbon disulfide (CS2) is a volatile neurotoxin used in manufacturing, with exposure linked to nervous system disorders. While carbon dioxide (CO2) is non–toxic at low levels, excessive accumulation accelerates climate change and can cause asphyxiation in confined spaces [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. The development of advanced gas sensors based on metal–doped nanomaterials, metal oxides, and 2D materials is crucial for real–time detection, ensuring environmental monitoring, workplace safety, and industrial hazard prevention [[11], [12], [13], [14], [15], [16]]. DFT studies show that reduced graphene oxide–polypyrrole composites with vertically oriented polypyrrole offer strong and selective sensing of ammonia and methanol due to enhanced hydrogen bonding and charge transfer [17]. Kanan et al., summarizes advances in nanoscale metal oxide gas sensors, emphasizing enhanced sensitivity and selectivity through doping and hybrid structures under ambient conditions [18].

Metal atom clusters, ranging from a few to several hundred atoms, constitute a unique phase of matter with properties that vary significantly with size and composition. Their electronic structure deviates from bulk materials, exhibiting quantum confinement effects and discrete energy levels. These clusters have been extensively studied in both metallic and semiconductor systems, revealing size–dependent optical, catalytic, and magnetic properties, making them promising for applications in nanotechnology and advanced materials design [[19], [20], [21], [22]]. Bandyopadhyay and Sen found that Ni–doped Ge clusters (1–20 atoms) exhibit endohedral Ni configurations for clusters with more than eight Ge atoms, with enhanced stability observed in 20–valence–electron clusters, supporting the shell model for Ge–based superatoms [23]. Kumar et al. extended this analysis to Ti, Zr, and Hf–doped Ge clusters, confirming similar stability trends and a sharp ionization potential drop beyond 20 valence electrons. Infrared and Raman spectra provided further structural characterization [24]. Bandyopadhyay's study on cationic Au–doped Ge clusters (AuGen, n = 1–20) identified Ge–Au orbital hybridization as the primary stability factor, with surface adsorption dominant for n < 11 and endohedral doping emerging at n = 11 [25]. Ding et al. investigated Aun (n = 1–20) clusters on TiO2 (110) surfaces, finding that dispersion interactions lead to planar structures. These clusters lay flat, consistent with experiments, and exhibit positive net charges with odd–even oscillations for n > 12 and charge nonuniformities in even–n clusters [26].

Lithium, as the lightest metallic element with a single s valence electron, serves as an ideal system for metal clusters. Boustani et al. investigated small neutral and cationic Li clusters (n = 2–9), revealing a transition from planar to tetrahedral geometries, identifying magic numbers (Li4, Li8, Li3+, Li9+), and confirming ionization potential trends consistent with the electron–shell model [27]. Fournier et al. studied larger lithium clusters (n ≤ 20), highlighting the exceptional stability of Li7, Li8, Li19, and Li20, along with liquid–like behavior and spin alternation, aligning with the jellium model [28]. Tian et al. discovered metastable Li clusters in over–lithiated carbon cloth, demonstrating high electrochemical reversibility and improved battery performance [29]. Zhang et al. found that CO prefers bridge–site adsorption on (AuLi)n clusters, enhancing stability and facilitating charge transfer from Au–Li to CO [30]. Galitskiy et al. computed photoionization cross sections for Lin (n = 2–8) clusters, analyzing inner and outer shell trends using the single–centre method within the Hartree–Fock approximation [31]. Lee et al. used first–principles molecular dynamics to investigate Li10 and Al–doped Li clusters, showing that Al doping weakens Li–Li bonds, induces premelting and stabilizes an Al dimer in Li10Al2, delaying its melting until 800K [32].

Research on gas adsorption and catalytic properties of metal clusters has provided valuable insights into their potential applications in pollutant capture, catalysis, and hydrogen storage. Shabeeb and Maity demonstrated strong chemisorption of CO, NO, and SO on Con (n = 2–7) clusters, where bond weakening and charge transfer mechanisms indicate high catalytic efficiency for gas capture [33]. Mohammadi and Pakizeh identified Fe7 and Fe13 as magic numbers in small Fen (n < 15) clusters, exhibiting enhanced magnetic moments. Their study revealed that the adsorption of SO, O2, CO, and N2 alters bond lengths and magnetization, with Fe7 displaying a stronger response, shedding light on iron–based catalysis [34]. Kadioglu et al. explored CO and O2 adsorption on Aun, Cun, and AumCun (m = 1, 2, 3; 1 ≤ n ≤ 6) clusters, finding that Aun clusters strongly bind CO but do not adsorb O2, while Cun clusters exhibit strong binding for both gases [35].

Samanta et al. demonstrated via DFT that aluminium clusters, akin to transition metals, can effectively adsorb CO and O2, with preadsorbed O2 further enhancing CO binding, highlighting their potential in gas capture and catalysis [36]. Fielicke et al. experimentally studied CO adsorption on transition metal clusters (3–20 atoms) using vibrational spectroscopy, revealing trends in adsorption modes and geometries [37]. Ibarra–Rodríguez and Sánchez investigated lithium cluster adsorption on graphene for hydrogen storage, identifying [Li6C54H18]4H2 as the most favorable structure with the highest adsorption energy (0.31 eV), confirmed through NBO analysis of Li–H2 interactions [38]. Alkali metal doping, particularly with sodium, significantly enhances the hydrogen storage capacity of C60 fullerenes—achieving up to ∼9.5 wt%—due to strong charge transfer and electrostatic interactions [39]. Metal cluster (M3O+)–supported graphene nanoflakes show high stability, enhanced optical properties, and improved polar molecule adsorption, making them promising for sensing and optoelectronic applications [40]. These studies collectively contribute to the understanding of metal clusters in gas adsorption and catalysis, paving the way for advancements in environmental and energy applications.

This is the first manuscript that has thoroughly studied the gas sensing capabilities of lithium clusters Lin (n = 2–20) using DFT, highlighting their potential as highly sensitive and selective nano sensors for hazardous gases such as CO, CO2, NO, NO2, SO2, H2S, NH3, and CS2. The exceptional reactivity and tunable electronic properties of Li clusters make them promising candidates for next–generation sensors with enhanced performance. By analyzing key parameters such as adsorption energy, charge transfer, density of states (DOS), and charge density variations, we reveal how gas adsorption alters the electronic structure of Li clusters. Furthermore, the selectivity, sensitivity, and recovery time are critically evaluated to ensure rapid and reversible gas detection. This study paves the way for the development of Li–cluster–based sensing technologies that combine high accuracy with fast response times, offering a novel approach for real–time environmental monitoring and industrial safety applications.