**2. Heavy metal sensor**

With the increasing population globally, there is a demand to provide clean and safe drinking water to them. However, contaminants in water at several countries/ cities restrict the water supply. The contaminants most likely the heavy metals, occurring due to anthropogenic, agricultural, mining, industrial revolution etc. Commonly identified heavy metals are Hg2+, Pb2+, Cu2+, Cd2+, Cu2+, which are although essential for living being but these heavy metals are toxic at higher concentrations [39]. So, it is a need to find the heavy metal sensor and this chapter will specifically focus on those sensors.

A series of 2,2`-Bipyridyl functionalized Iridium(III) complexes were synthesized by Zhao et al. [40] enabled sensing behavior towards Zn2+, Cd2+, Cu2+ ions. The modification of binding sites led to conversion of intra-ligand charge transfer (ILCT) to Ligand–Ligand Charge Transfer (LLCT) and Metal–Ligand Charge Transfer (MLCT) transition upon binding with metal ions. DFT calculations using gradient corrected correlation functional PBE1PBE and 6-31G(d,p) basis set were employed to confirm the transition mechanism using HOMO and LUMO energies of the Iridium(III) complexes with heavy metal ions. Kim et al. [41] synthesized two triphenylamine-based dyes as fluorescent probes for effective sensing of Hg2+ ions. The fluorescence emission band was decreased and increased on addition of Hg2+ ions due to metal to ligand charge transfer (MLCT) mechanism. The optimization of the dye structures was carried out at DFT level with exchange correlation functional of LDA based on PWC set. The LUMO energy levels of mono-benzoxazole and di-benzoxazole were found to be −4.61 eV and −4.74 eV, respectively. The HOMO energy levels of mono-benzoxazole and di-benzoxazole were found to be −2.51 and −2.82 eV, respectively. In this way, Anand et al. [42] reported the aminoquinoline fluorescent probe for selective and sensitive detection of Pb2+ and Al3+ ions with significant fluorescence enhancement. The fluorescence signaling was due to the inhibition of photo-induced electron transfer process and restriction of C=N isomerization. The fluorescent turn-on mechanism was understood by carrying out DFT calculations at B3LYP level. The HOMO, HOMO-1 and LUMO

energies of the probe were −5.18 eV, −5.67 eV and −2.29 eV, respectively. On complexation with Pb2+, the HOMO, HOMO-1, HOMO-2 and LUMO energies were −6.18 eV, −6.42 eV, −6.95 eV and −3.69 eV respectively and for Al3+, it was found to be −6.03 eV, −6.93 eV, −7.35 eV and −3.54 eV respectively. Gonzalaz et al. [43] reported the Tripodal pyrenyl-triazole probe and demonstrated citrate ion recognition with significant fluorescence enhancement. DFT calculations using B3LYP-D3/def2-TZV basis set in gas and solvent phase revealed the stable geometry of the tripodal probe as nesting type, whereas all triazole CH units bind with citrate ions. The probe also showed a remarkable chromogenic behavior from pale to deep orange seen by naked-eye which was further confirmed by DFT calculations. In 2018, Liao et al. [44] have designed and developed NiCo2O4 nanoplatelets modified glass carbon electrodes for detection of Pb2+ ions. The DFT computations were explored to calculate the adsorption bond energies of the NiCo2O4 nanoparticles with Pb2+ heavy metal ions. Pb has the largest adsorption energy of −2.78 eV with the nanoparticles and was in-line with the electrochemical experimental values. The modified glass electrodes were analyzed in real water samples and found that detection of Pb2+ ions was successful compared with detection of Hg2+ and Cu2+ ions. Particularly for Cu2+ ion sensor, Sun et al. [45] reported 1,8-diaminonaphthalene derived probe as effective fluorescence chemosensor towards Cu2+ ions. The fluorescence quenching was experienced due to the inhibition of intramolecular charge transfer (ICT) process and chelation enhanced fluorescence quenching (CHEQ) of paramagnetic Cu2+ ions. The TD-DFT calculations were performed using B3LYP/6– 31(d) and LANL2DZ basis sets produced the HOMO-LUMO band gap of copper complex as 2.15 eV and the probe as 2.82 eV. This reduction in the band gap confirms the interaction of Cu2+ ions with the receptor. The adsorption studies explored using cetyltrimethyl ammonium bromide on Poly-L-Lysine coated Ag nanoparticles were employed at B3LYP level using 6-31 g(d) and LANL2DZ basis sets reported by Moudgil et al. [46]. The binding energy of the organic core with silver nanoparticles was found to be −0.75 eV indicates the chemisorption of pi-bonding orbitals on the Ag surface. The binding energy was observed to be reduced further 1 eV while interacting with Hg2+ ion which confirms the aggregation mechanism and a charge transfer through amine group. Thiocarbazone based probe was reported by Mahajan et al. [47] as multi-ions sensor for Ni2+, Cu2+, Co2+ and Cd2+. The calculated stabilization energy by DFT and the observed result from the fluorescent spectra produced the same order for metal ions binding with probe. The order of the metal ion sensing was found to be Cu2+ > Ni2+ > Co2+ > Cd2+. Changes in the bathochromic shift in absorption spectra and fluorescence quenching for these metal ions were due to the interaction of sulfur and hydroxyl O atoms of the probe. The binding interaction was supported by TD-DFT calculations at B3LYP level using LANL2DZ basis set. The calculated HOMO-LUMO band gap values were decreased for the metal ion-probe complex than that of probe confirms the complex formation. The real time experiment was explored with river water samples. The imidazole based receptor was reported by Khan et al. [48] to sense picric acid (donoracceptor) for detection of toxic nitrobenzene through static quenching, Dexter electron transfer and Forster resonance energy transfer quenching mechanisms. The HOMO and LUMO orbital energies were obtained at B3LYP/6-31 g(d) level as −3.00 eV and −6.91 eV for the probe and the HOMO and LUMO energies were found to be −3.15 eV and −7.64 eV upon binding with nitrobenzene. These orbital energy values confirmed the transfer of electron from LUMO of the probe to the LUMO of nitrobenzene which is supported by the quenching mechanism. The charge transfer between the probe and the metal ion was confirmed by electrostatic potential calculation of the binding site. Moreover, the HOMO-LUMO energy gap of the probe-Co2+ complex was reduced to 1.86 eV well adhering to the charge transfer process. Kumar et al. [49] reported a dicarboxylic acid based receptor as turn-on fluorescent probe for detection of Hg2+ ions. The binding of Hg2+ ion takes place through the D-pi-A system by -COOH and -NH2

### *Applications of Density Functional Theory on Heavy Metal Sensor and Hydrogen Evolution… DOI: http://dx.doi.org/10.5772/intechopen.99825*

functional groups through intra-molecular charge transfer (ICT) mechanism. DFT calculations confirmed the ICT mechanism by analyzing the HOMO-LUMO energy differences. The band gap was found to be decreased for the Hg2+ ion complex rather than the unbound probe which confirms the host-guest interaction. Chiral CdSe nanoplatelets – L/D-cysteine as sensitive optical probe for recognition of Pb2+ ions was reported by Wang et al. [50]. The TD-DFT calculations at B3LYP level using LANL2DZ basis set was carried out. The results of frontier molecule orbital calculations of the HOMO and LUMO orbitals showed overlapping between the cysteine capped CdSe nanocluster and the organic core. Moreover, the reason for the transformation into chirality was observed to be the orbital coupling effect in the nanocluster. The Pb2+ was recognized by L-Cysteine capped CdSe nanoplatelets. The merocyanine dye based receptor [51] crystallized in monoclinic P21/n space group showed pi-pi stacking between merocyanine units. The B3LYP method was employed to optimize the geometries of the free receptor and mercuric complex. For optimization, 6-31 g(d) and LANL2DZ basis sets were used for non-metal and metal atoms, respectively. The change in the HOMO-LUMO band gap for the mercury complex than the receptor confirms the selective sensing of Hg2+ ion with phenolic and methoxy oxygen moities. Modified calix[4] arene based probe was reported by Anandababu et al. [52] selectively detected Zn2+ ions with 240-fold fluorescence enhancement due to the imine C=N isomerization. In turn, Cu2+ and Hg2+ ions showed fluorescence quenching due to its paramagnetic and spin-orbit coupling properties. DFT studies revealed the HOMO-LUMO energy difference of unbound probe as 1.07 eV and [probe-Zn2+] has a decrement to 0.15 eV, confirming the strong complexation. The luminescent metal organic frameworks (LMOFs) as nanocages to recognize Fe3+ and Cu2+ ions in trace quantity was designed and synthesized by Li et al. [53]. The fluorescent quenching activity of the nanocages was due to their weak interaction with the metal ions at N-rich sites. DFT calculations confirmed the uncoordinated N atoms in the nanocage interact weakly with the metal ions and hence the fluorescence was quenched. The other chemosensor based on Nitrogenous carbon dots impregnated on natural microcline nanostructures demonstrated a dual fluorometric response towards Fe3+ and Cr6+ metal ions was reported by Bardhan et al. [54]. The fluorescence turn OFF response was attributed by photoinduced electron transfer mechanism and was confirmed by TD-DFT calculations. The B3LYP level and 6-31 g(d,p) with RIJCOSX basis sets were utilized and revealed that the microcline structure holds the carbon dots which in turn bind with the metal ions. The possible binding site is the nitrogen atoms present in the C-dots deduced by its electrostatic potential surface calculation. The HOMO-LUMO band gap was reduced by 2.49 eV in the metal ion complexes. Carbohydrazide based colorimetric sensors for detection of multiple ions were reported by Tekuri et al. [55]. The receptors showed selectivity towards Cd2+, Hg2+, Pb2+, Cu2+ and AsO2 − ions with low detection limit. Using B3LYP functional and non-relativistic effective core potential (ECP) with LANL2DZ and 6-31 g(d,p) basis sets confirmed the binding of receptor with different metal ions as per hard and soft (Lewis) acids and bases (HSAB) theory. The hard -OH group and soft C=N group binds with complementary metal ions. The HOMO-LUMO band gap for the [receptor(s)-metal ion(s)] complexes showed a reduction in their values confirmed the binding of the respective metal ions. The novel silsesquioxane cage – porphyrin nanocomposites reportedly [56] a dual chemosensor towards fluoride and cyanide ions particularly these ions bind with the Si atoms in the cage resulted in turn on fluorescence. The microporous surface of the polymeric cage with extended conjugation was confirmed by non-local density functional theory using slit shape modeling. The extended conjugation gives a red shift in the absorption band and was confirmed by DFT calculation specifically the dropping in band gap value of the nanocomposites. In turn, the fluorescence emission was quenched in the presence of Hg2+ and Cu2+ ions due to the restricted photo-induced electron transfer process. An eco-friendly garlic

extracted alliin based silver nanoparticles was synthesized by Paw et al. [57]. This AgNPs selectively detects Hg2+ and Sn2+ ions with easily detectable color change from brown to colorless. The structure was optimized by M06-2X level with LANL2DZ and 6-31 g(d) basis sets. Ag0 with alliin stabilizes the nanocluster with the stabilization energy of 18.4 kcal mol−1. On interaction with Hg2+ and Sn2+ ions, alliin separated out from AgNPs with stabilization energies of 7.6 kcal mol−1 and 8.3 kcal mol−1, respectively. A novel N-doped diaza derivative of ovalene as nanographenes was reported by Jin et al. [58]. The N atom in the zigzag edges of ovalene favored the interaction with protons, Cu2+ and Fe2+ ions. The HOMO-LUMO band gap with 2.20 eV and 2.13 eV for diaza and ovalene nanographenes were calculated at DFT level of theory in gas phase. The N-bearing diaza nanographene showed fluorescence quenching in the presence of Cu2+ and Fe2+ ions due to intermolecular charge transfer. The aromaticity was reported by nucleus-independent chemical shift (NICS) calculations using GIAO at B3LYP/6- 31G(d) level. The result shows that the NMR chemical shift has also been computed and confirmed.
