1. Introduction
A huge number of indispensable functions in human body are realized with the support of proteins, for which amino acids form the rudimentary building block. The distribution of amino acids in protein also regulates the grade and freshness of numerous foods. A notable operation that results in the emission of malicious aura in the outer atmosphere is anaerobic digestion, which is achieved by microbes on amino acids. Many compounds are produced as a result of amino acid catabolism [1], out of which we have concentrated on the two products – Volatile Fatty Acid (VFA) and amines. The deduction behind the catabolic reduction of amino acid to VFA and amines are accounted to be deamination and decarboxylation, respectively. Rappert and Muller [2] reported in their review article, which galvanized us to explore these three products – amino acid, VFA and amines. To be precise, the chief molecules chosen for the current work are asparagine (amino acid), lactate (VFA) and putrescine (amine). Asparagine is a non-essential amino acid, which is demanded by humans for the growth of brain. Some of the animal sources of asparagine are beef, eggs, poultry, diary, fish, whey and so on. Moreover, this amino acid is found to exist in potatoes, soybean, asparagus, corn, rye, maize, etc., [3, 4]. In concordance with the expected structure of amino acid, asparagine is reported to have an alpha-carboxylic acid group, alpha-amino group and a unique carboxamide sidechain (CONH2). The molecular formula of asparagine, comprising the above-said groups is C4H8N2O3and the vapor pressure is computed to be 4.8 x 10-8 mm Hg at 25 oC. Owing to the supreme nitrogen to carbon ratio, asparagine is accounted to be a productive choice for nitrogen transport and storage mechanism [5]. The possible sensing strategies previously employed for detecting asparagine amino acid are High Performance Liquid Chromatography (HPLC) [6] and MALDI/TOF-mass spectrometry (MS) [7]. The anaerobic deamination of the amino acid (present in vegetables and animals) lead to an intermediate product, VFAs which in general is wielded in a variety of industries manufacturing plastics, paints, fuels, textiles, pharmaceuticals and so on [8]. The VFA – lactate (C3H6O3) is regarded to be a bio-indicator owing to the physiological roles played by it in human body. The presence of lactate in blood, sweat, plasma and urine is utilized to witness the changes in the health of human body viz., sepsis [9], muscle fatigue, wound repair [10], acidosis, decubitus ulcers [11], etc. Besides, 0.5 to 2.0 mM is recorded to be the regular concentration range of lactate in the blood of human beings [12]. Moreover, higher the concentration of lactate in human body, higher is the risk associated with the human beings [13]. The formerly reported detection methods of lactate are HPLC, electrochemical-based, potentiometer-based and electro-chemiluminescent detection [14]. Microbial decarboxylation of amino acids leads to the release of a polyamine known as putrescine [15], which is noted to be a food quality indicator owing to the harmful impacts induced by it on the health of human beings upon the amplification of its level [16]. The polyamine with the molecular formula C4H12N2 possesses hydrophobic methylene group and cationic demeanor. It is observed to be useful for the growth and proliferation of cells, bacteria and eukaryotes. In addition, certain physiological functions like collaboration of ribosomal subunits, alteration in the chromatin state, sustentation of tRNA structure, etc, are fulfilled using putrescine. To detect putrescine, several well-known techniques like HPLC, ion-exchange chromatography, capillary zone electrophoresis, colorimetric-based and capillary GC are used [17, 18]. The mode of sensing handled by us is of chemi-resistive type, which wields two-dimensional nanomaterials (2DNMs) as a fundamental component. The logic behind the preference of 2DNMs for the detection of amino acid, VFA and polyamine is that 2DNMs enjoys the advantage of extravagant surface area, which aids in intense physisorption of the bio-molecules. In addition, many marvellous attributes like malleability, optical transparency and pint-size of 2DNMs make them desirable. Moreover, the literary evidences recommend the exploitation of 2DNMs to identify the availability of bio-molecules – amino acid, VFA and polyamine. Some of the attestations include DNA nucleotide adsorption on graphene nanoribbons [19]; tyrosine and phenylalanine adsorption on graphene, graphene oxide and boron nitride [20]; histidine, aspartic acid, arginine, glycine, lysine, alanine, proline, glutamic acid, phenylalanine, tyrosine on graphene [21]; glycine, phenylalanine, histidine and glutamic acid on graphdiyne/graphene [22]; tryptophan, histidine, phenylalanine, tyrosine on graphene and single walled carbon nanotubes [23]. These documentations actuated us to find out a capable 2DNM to sense the presence of the amino acid – asparagine, VFA – lactate and polyamine – putrescine. One of the attractive 2DNM, which is known for its level-constrained band gap is phosphorene, a single layer of group-VA phosphorus (P) atoms. The tri-coordination of P atom together with itssp3 type hybridization makes this 2DNM – phosphorene to have various kinds of conformations. Initially, α (black), β (blue), γ and δ-type of phosphorene were reported, followed by a variety of allotropes namely ζ, ε, λ (green), η, violet phosphorene and Kagome-form. Experimental realization of β-phosphorene and violet phosphorene by molecular beam epitaxy [24] and exfoliation [25] correspondingly encouraged the scientific community to persist many researches on this 2DNM [26-30]. Among the wide range of available allotropes, we have selected Kagome-form of Phosphorene (Kagome-P) as a fundamental component due to the spectacular characteristics exhibited by the material. The vigor demeanor of the 2DNM (Kagome-P) is observed owing to the geometry of the material, which follows both the α-P and β-P conformations. By perceiving the novel Kagome-P from side sight, α-P conformation is reckoned which is attributed to the interaction of a single P atom with two P atoms of the same level and one P atom of the second level. Also, every P atom in β-P conformation is being superseded by a triangle of P atoms. Experimental realization of Kagome-P can be carried out through polymerization mechanism [31] and compressive in-plane strain-based derivation mechanism (from β-P) [32]. Moreover, appealing attributes of Kagome-P like desirable absorption coefficient (105 cm-1), wide-ranging band gap and strain-based band gap transition prepare the 2DNM to be operated in many optoelectronic devices. We studied the sensing properties of phosphorene nanotubes and nanosheets to vapors [33-37]. The ability of the 2DNM to take various forms like nanosheet, nanotube, nanoribbon, etc, without losing its stable 2D nature [38] along with the above-stated fascinating points impelled us to investigate the interaction of the bio-molecules – amino acid (asparagine), VFA (lactate) and polyamine (putrescine) on the Kagome-form of phosphorene nanotube (Kagome-PNT).