Hybrid organic-inorganic perovskite compounds for metal ion sensing applications and process of preparation thereof

Abstract

The present invention generally relates to a field of material science and chemistry. Specifically, the present invention relates to synthesis and characterization of lead-free hybrid organic-inorganic Perovskite compounds for metal ion sensing applications. More particularly, the present invention relates to lead-free low dimensional manganese (Mn)- and copper (Cu)- based hybrid organic-inorganic perovskites (HOIPs) for visual fluorometric Pb2+ ion detection. Further, the present invention relates to synthesis of lead-free low dimensional manganese (Mn)- and copper (Cu)-based hybrid organic-inorganic perovskites (HOIPs) via mechanochemical synthesis methodology.

Description

[0001] HYBRID ORGANIC-INORGANIC PEROVSKITE COMPOUNDS FOR METAL ION SENSING APPLICATIONS AND PROCESS OF PREPARATION THEREOF

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention generally relates to a field of material science and chemistry. Specifically, the present invention relates to synthesis and characterization of lead-free hybrid organic-inorganic Perovskite compounds for metal ion sensing applications. More particularly, the present invention relates to lead-free low dimensional manganese (Mn)- and copper (Cu)- based hybrid organic-inorganic perovskites (HOIPs) for visual fluorometric Pb2+ion detection. Further, the present invention relates to synthesis of lead-free low dimensional manganese (Mn)- and copper (Cu)-based hybrid organic-inorganic perovskites (HOIPs) via mechanochemical synthesis methodology.

[0004] BACKGROUND OF THE INVENTION

[0005] Hybrid organic-inorganic lead (Pb) halide perovskites (HLHPs) have been reported to perform exceptionally well in optoelectronic applications such as solar cells, LEDs, transistors, optical sensors, metal ion detection, etc. The general formula for HLHPs is APbXa where A is an organic or inorganic monovalent cation [Methyl ammonium (CH3NH3+), Formamidinum [HC(NH2)2+], Cysteammonium (C2HsN+S), Cystammonium (C4Hi3N2+S2), n-butylammonium CH3(CH2)3NH3+, and bis(phenethylammonium) CeH5(CH2)2NH3+, Cs+, Rb+] and X represents halide anion (C17Br7I ) at micro- and nano-scales. They have shown to perform at par with the other metallic systems in the thin film, single crystal, and nanocrystalline forms. These HLHPs possess several exciting properties due to the presence of a wide gamut of A, B, and X site atoms, such as tunable band gap, high carrier mobility, defect tolerance, considerable electron diffusion lengths, and easy solution processing and so on.

[0006] However, despite advantages, HLHPs suffer from numerous drawbacks due to their fragile crystal structure, which crumbles easily upon oxygen and moisture penetration, sunlight / UV exposure. The destruction of chemical structure tends to cause Pb leakage with severe human health issues and environmental hazards hindering commercialization of HLHPs in optoelectronic devices. The acceptable level of lead in bare soil given by Environmental Protection Agency (EPA) for play areas is 400 ppm and in non-play areas is 1200 ppm. However long-term use of lead-based devices can lead to lead accumulation in soil & water bodies, and in turn within human bodies.

[0007] Therefore, to overcome issues related lead toxicity there is a consequential shift in research towards lead free hybrid halide perovskites (LFHHPs). The main advantage of Pb-free perovskites, LFHHPs is their reduced toxicity and environmental impact, making them a more sustainable alternative to Pb-based perovskites; i.e. HLHPs. LFHHPs have demonstrated superior properties to their HLHP counterparts. However, Pb-free perovskites face some challenges, such as lower stability and efficiency than Pb-based perovskites. Nonetheless, significant progress has been made in addressing these challenges, and Pb-free perovskites are expected to play an increasingly important role in various applications in the future. The development of stable Mn- and Cu-based LFHHPs is significant since they change oxidation state upon exposure to moisture and oxygen and become unstable. The [MnX4]2-anions are luminous in the solid state; however, upon interaction with solvent, they lose their structural coordination and hence make the perovskite non-emissive. The Cu+in cuprous halide perovskites quickly oxidizes in the presence of moisture due to the low hydration energy of Cu+ions, which leads to the formation of Cu2+upon loss of an electron.

[0008] LFHHPs have demonstrated superior performance to HLHPs in photovoltaic, light-emitting diodes, photocatalysis, sensing, and data storage. Tin (Sn)-based perovskites have been studied extensively as efficient solar cell and gas sensing materials, owing to their excellent optoelectronic properties. Bismuth (Bi)-, Cu-, and Mn-based perovskites have been studied for their photocatalysis and data storage applications. Another exciting application for LFHHPs and HLHPs is the metal ion detection application, for which only limited studies are available in the current literature.

[0009] Heavy metal ions play a significant role in the functioning of various biological and chemical processes. However, excess accumulation of heavy metal ions is toxic to the environment thus, easy detection and monitoring have become crucial in diverse applications such as environmental monitoring, food safety, and medical diagnosis. Cortecchia Daniele et al., Inorganic chemistry 55.3 (2016), 1044-1052, “Lead-free MA2CuCl x Br4-x hybrid perovskites” disclosed lead-free Cu based hybrid perovskites for application in solar cell. The compounds were synthesized using solution-based recrystallization. Zhang, Jie, et al., Journal of Materials Chemistry A 11.36 (2023), 19427-19434, “Room temperature synthesis and pressure-induced optical properties of lead-free 2D CssffeleCh perovskite nanocrystals” disclosed lead-free hybrid perovskites synthesized using anti-solvent assisted crystallization. However, these papers do not discuss about ion detection application of the hybrid perovskites. Further, the hybrid perovskites disclosed by above papers lack in longer environmental photostability and stability under a relative humidity of 70% - 80%.

[0010] Therefore, there is a need to provide hybrid organic-inorganic perovskites (HOIPs) for visual fluorometric metal ion detection with higher stability and efficiency.

[0011] OBJECTIVES OF THE INVENTION

[0012] An objective of the present invention is to develop lead-free low-dimensional hybrid perovskite compounds for visual fluorometric metal ion detection.

[0013] Another objective of the present invention is to provide a solid-state synthesis methodology using a planetary mixer under ambient conditions for the synthesis of lead-free lowdimensional manganese (Mn) and copper (Cu) based hybrid perovskite compounds.

[0014] Another objective of the present invention is to evaluate the environmental photostability and stability under a relative humidity of 70% - 80% of the synthesized compounds.

[0015] Another objective of the present invention is to fabricate a visual sensory detector using thin films of the synthesized compounds and assess their correlation with the fluorescence titration results.

[0016] Yet another objective of the present invention is to demonstrate the potential application of the synthesized compounds as visual detectors for metal ion sensing in thin films.

[0017] SUMMARY OF THE INVENTION

[0018] The present invention generally relates to synthesis and characterization of lead-free hybrid organic-inorganic Perovskite compounds for metal ion sensing applications. More particularly, the present invention relates to lead-free low dimensional manganese (Mn)- and copper (Cu)- based hybrid organic-inorganic perovskites (HOIPs) for visual fluorometric Pb2+ion detection. The present invention also discloses synthesis of lead-free low dimensional manganese (Mn)- and copper (Cu)-based hybrid organic-inorganic perovskites (HOIPs) via mechanochemical synthesis methodology. In an aspect, the present invention relates to a hybrid organic-inorganic perovskites (HOIPs) compound of formula I for metal ion detection, the formula I is represented by:

[0019] Xm-Y-Zn Formula I wherein

[0020] X is tetraalkylammonium wherein alkyl is C1-C8 alkyl,

[0021] Y is a metal ion with at least 2+ cationic oxidation state,

[0022] Z is a ligand selected from halide, m is in range of 1-3, and n is in range of 2-6, wherein the compound of formula I is 2D material with a layered crystal structure.

[0023] In another aspect, the present invention relates to a process for the preparation of hybrid organic-inorganic perovskites (HOIPs) compound of formula I, comprising steps of: a) adding at least one organic linker being precursor of X with one or more precursor(s) of metal ion with at least 2+ cationic oxidation state separately in weight ratio in the range of 2-4: 0.5- 1.5 to obtain a blend; and b) mechanochemically mixing the blend of step a) at a temperature in the range of 25-40 °C and at speed in the range of 1400-1600 rpm for time period in the range of 20-40 minutes under mixing and degassing mode to obtain the hybrid organic-inorganic perovskites (HOIPs) compound of formula I; wherein the compound obtained after the step b) is a pure compound without impurities; and the process is done without the need of solvent.

[0024] In an aspect, the present invention provides a method of detection of a metal ion in a sample comprises reacting and treating the sample containing a metal oleate with the hybrid organic- inorganic perovskites (HOIPs) compound of formula I, by a fluorescence titration method or a visual sensory test; wherein the metal ion is selected from transition metal ion, lanthanide metal ion and heavy metal ion.

[0025] In an embodiment, the fluorescence titration method comprises: a. preparing the sample by dissolving the metal oleate in a solvent to form a solution A having concentration of the metal ion in the range of 0.25 x 10’7M to 2.0 x 10’7M; and b. treating and reacting the solution A of step a) with the hybrid organic-inorganic perovskites (HOIPs) compound of formula I under sonication for time period in the range of 2-5 seconds followed by optical measurement to confirm the presence of the metal ion.

[0026] In an embodiment, the concentration of HOIP compound of formula I used in said titration method is in the range of 0.25xl0-7M to 2xl0-7M.

[0027] In an embodiment, the visual sensory test comprises: a. dipping a spin coated film of the hybrid organic-inorganic perovskites (HOIPs) compound of formula I in a solution containing the sample with metal ion concentration in the range of 0.25 x 10’7M to 3.0 x 10’7M for time period in the range of 2-5 seconds to obtain a dipped film; and b. analyzing the dipped film of step a) under UV-lamp to check the emission change in the film confirming the presence of said metal ion.

[0028] In an embodiment, the emission change is from blue colour to green colour with initial emission before step a) or step b) is around 330 nm, and after step a) or b), the emission change is around 520 nm (refer, figure 22).

[0029] BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Fig. 1 illustrates (a) The precursor combinations (i, ii, iii and iv) used for the synthesis of compounds 1, 2, 3, and 4, respectively; (b) The actual images of the reaction vessel and its subsequent mounting in the KaKuHunter made planetary mixer instrument; (c) The pictures of the compounds under ambient condition; (d) The emission on the respective compounds under UV light, images for compounds 1 and 2 are recorded under 365 nm light and for compounds 3 and 4 images are recorded under 254 nm.

[0031] Fig. 2 illlustrates pXRD patterns of as-synthesized compounds compared with the reported compounds’ simulated single crystal pXRD patterns; pXRD pattern for (a) compounds 1, 2 and 3, and (b) compound 4.

[0032] Fig. 3 illustrates FTIR spectra for (a) compounds 1 and 3 matched with the precursor salt of tetraethylammonium bromide; (b) compounds 2 and 4 matched with the precursor salt of tetrabutylammonium bromide. Fig.4 illustrates high-resolution deconvoluted XPS spectra for Mn 2p core levels for compounds (a) 1 and (b) 2 powders, toluene dispersions are drop casted on a Si substrate.

[0033] Fig. 5 illustrates high-resolution deconvoluted XPS spectra for Cu 2p core levels for compounds (a) 1 and (b) 2 powders, toluene dispersions are drop casted on a Si substrate.

[0034] Fig. 6 illustrates high-resolution deconvoluted XPS spectra for Br 3d core levels for compounds (a) 1, (b) 2 (c) 3 and (d) 4 powders; Toluene dispersions are drop casted on a Si substrate for sample preparation.

[0035] Fig. 7 illustrates FESEM images of powder samples for compound (a) 1, (b) 2, (c) 3 and (d) 4. (Scale = 50 pm).

[0036] Fig. 8 illustrates FESEM analysis for compound 1 powder samples (a) ED AX spectra with elemental ratio the bold numbers are indicative of the Mn: Br ratio, and (b) The elemental mapping, the colour codes used are, yellow for Bromine, red for Carbon, gray for Manganese and green for Nitrogen elements.

[0037] Fig. 9 illustrates FESEM analysis for compound 2 powder samples (a) ED AX spectra with elemental ratio the bold numbers are indicative of the Cu: Br ratio, and (b) The elemental mapping, the colour codes used are, yellow for Bromine, red for Carbon, blue for Copper and green for Nitrogen elements.

[0038] Fig. 10 illustrates FESEM analysis for compound 3 powder samples (a) ED AX spectra with elemental ratio the bold numbers are indicative of the Mn: Br ratio, and (b) The elemental mapping, the colour codes used are, yellow for Bromine, red for Carbon, gray for Manganese and green for Nitrogen elements.

[0039] Fig. 11 illustrates FESEM analysis for compound 4 powder samples (a) ED AX spectra with elemental ratio the bold numbers are indicative of the Cu: Br ratio, and (b) The elemental mapping, the colour codes used are, yellow for Bromine, red for Carbon, blue for Copper and green for Nitrogen elements.

[0040] Fig. 12 illustrates TGA and DSC spectra for compounds (a) 1, (b) 2 (c) 3 and (d) 4. Recorded in the range of 0-1000 °C, using a ramp rate of 5 °C min1.

[0041] Fig. 13 illustrates (a) The diffuse reflectance spectra (DRS), and (b) PL spectra for compounds 1, 2, 3 and 4 [excitation wavelength, Ax = 375 nm (1, 2) and 275 nm (3, 4)]. All spectra were recorded for solid state compounds. Fig. 14 illustrates Tauc’s plot calculated from the DRS spectra, at the maximum intensity edge state.

[0042] Fig. 15 illustrates UV and PL spectra for solid as-synthesized compounds (a) 1, (b) 2, (c) 3 and (d) 4 indicating high stokes shift for compounds in solid -state.

[0043] Fig. 16 illustrates PL spectra for compound (a) 1, and (b) 3 at under 70-80% humidity conditions in solid state.

[0044] Fig. 17 illustrates UV and PL spectra for toluene dispersion of compound 1.

[0045] Fig. 18 illustrates digital photographs showing as prepared solutions of different metal-oleates of transition metalloids and lanthanides in toluene used for the metal ion detection experiments. Fig. 19 illustrates digital photographs of solutions at concentration of 0.25 mg / mL of compounds (a) 1, and (b) 2 in toluene; ambient light images (i), (iii) and images under UV light (365 nm) (ii), (iv) respectively.

[0046] Fig. 20 illustrates HRTEM image for as synthesized solid compound 1.

[0047] Fig. 21 illustrates compound dispersions: (a) Fluorescence titration spectra for compound 1; (b) Fluorescence titration spectra for compound 2; (c) Absorption spectra for compound 1 upon addition of Pb2+ions; (d) Fluorescence Intensity v / s concentration of Pb2+ions for compound 1; Fluorescence results in blank and presence of various metal cations: (e) Emission intensity spectra; (f) Selectivity at X peak position, 553 nm.

[0048] Fig. 22 illustrates As-synthesized compound 1 bulk crystallites (a) HRTEM image at 100 nm scale, inset shows SAED pattern and PL emission spectra; Toluene dispersion of compound 1 bulk crystallites after ultra-sonication (b) HRTEM image at 100 nm scale, inset shows SAED pattern and PL emission spectra; (c) Post Pb2+ion detection application studies for compound 1; (i) HRTEM image at 20 nm scale, inset shows SAED pattern and PL emission spectra, (ii) HAADF-STEM image, and elemental mapping of (iii) Pb, (iv) Mn, (v) Br, (vi) N and (vii) C; (d) shows the XRD data for compound 1 in the presence of various concentrations of Pb2+along with the possible formation of tetraethylammonium lead bromide; the arrows indicate change in peak intensities; (e) Emission and excitation spectra for the tetraethylammonium lead bromide; (f) graphical representation of scheme demonstrates the proposed mechanism for selective detection of Pb2+by compound 1; (g) Fabrication of a preliminary visual sensor of compound 1, (i-iii) shows digital photographs of thin film compound 1 thin film , under X = 245 nm, (i) thin film of compound 1 as casted on glass slide by spin coating method; (ii) 1 / 3 portion of as cast film into Pb2+solution with 2.0 x 10’7M for 2-4 seconds; (iii) complete dipping of as cast film into Pb2+solution of 2.0 x 10’7molarity Pb2+solution.

[0049] Fig. 23 illustrates HRTEM images for toluene dispersion of compound 1, (a) particle size distribution, & (b) d-spacing.

[0050] Fig. 24 illustrates HRTEM analysis for compound 1 toluene dispersion after Pb2+ion detection experiment, (a) d-spacing; (b) shows the ED AX elemental ratios of various elements.

[0051] DETAILED DESCRIPTION OF THE INVENTION

[0052] The meaning of the degassing mode is that the gasses if any generated during the course of the reaction are removed during the synthesis process itself.

[0053] In an aspect, the present invention relates to a hybrid organic-inorganic perovskites (HOIPs) compound of formula I for metal ion detection, the formula I is represented by:

[0054] Xm-Y-Zn Formula I wherein

[0055] X is tetraalkylammonium wherein alkyl is Cl-C8alkyl,

[0056] Y is a metal ion with at least 2+ cationic oxidation state,

[0057] Z is a ligand selected from halide, m is in range of 1-3, and n is in range of 2-6, wherein the compound of formula I is 2D material with a layered crystal structure.

[0058] In another aspect, the present invention relates to a hybrid organic-inorganic perovskites (HOIPs) compound of formula I for metal ion detection, the formula I is represented by:

[0059] Xm-Y-Zn Formula I wherein

[0060] X is tetraalkylammonium wherein alkyl is Cl-C8alkyl,

[0061] Y is a metal ion with at least 2+ cationic oxidation state,

[0062] Z is a ligand selected from halide, m is in range of 1-3, and n is in range of 2-6, wherein the compound of formula I is prepared by a process comprising: a) adding at least one organic linker being precursor of X with one or more precursor(s) of metal ion with at least 2+ cationic oxidation state separately in weight ratio in the range of 2-4: 0.5- 1.5 to obtain a blend; and b) mechanochemically mixing the blend of step a) at a temperature in the range of 25-40 °C and at speed in the range of 1400-1600 rpm for time period in the range of 20-40 minutes under mixing and degassing mode to obtain the hybrid organic-inorganic perovskites (HOIPs) compound of formula I; wherein the compound obtained after the step b) is a pure compound without impurities; and the process is done without the need of solvent.

[0063] In an embodiment, the compound of formula I is lead (Pb)-free compound.

[0064] In an embodiment, the compound of formula I is phase pure, and shows stable solid-state emission at room temperature.

[0065] In an embodiment, the compound of formula I is environmentally photostable for over a month, and has a stability under a relative humidity of 70% - 80%.

[0066] In an embodiment, the compound of formula I shows sheet like structure which is converted to nanoparticles of at least 5 nm, increasing the rate of reaction for metal ion detection drastically and thereby facilitates easy detection of metal (e.g. Pb2+) ions.

[0067] In specific embodiment, the well-dispersed nanoparticles of compound of formula I is having the particle size in the range of ~ 5-10 nm.

[0068] In an embodiment, the tetraalkylammonium is selected from tetramethylammonium (TMA), tetrapropylammonium, tetraethylammonium (TEA), and tetrabutylammonium (TBA).

[0069] In an embodiment, the metal ion with at least 2+ cationic oxidation state is copper and / or manganese.

[0070] In an embodiment, the halide being ligand is selected from chloro, bromo, iodo and fluoro.

[0071] In an embodiment, the compound is selected from compound 1 (TEA)2MnBr4, compound 2 (TBA)2MnBr4, compound 3 (TEA)2CuBr4 and compound 4 (TBA)CuBr2.

[0072] In an embodiment, the HOIPs compound of formula I is in the form of 2D perovskites.

[0073] In an embodiment, the compound of formula I comprises formation of low dimensional perovskites as the size of the cation (Y) exceeds the size of the halide (Z) cavity.

[0074] In an embodiment, the compound of formula I comprises an electronic quantum-well like structure. In an embodiment, the crystal structure of the compound shows orientated growth on the (110) plane probably due to change in organic cations.

[0075] In an embodiment, the weight ratio of Y and Z of compound of formula I is in the range of 0.5- 1.5: 1.5-4.5.

[0076] In an embodiment, the weight ratio of Y and Z of compound of formula I is in the range of 0.5- 1.5: 2-4.

[0077] In an embodiment, the compound of formula I is stable at temperature of at least 220 °C.

[0078] In an embodiment, the compound of formula I is stable at temperature in the range of 220-300 °C.

[0079] In an embodiment, the metal Y of the compound of formula I acts as deep trap centers for metal (or heavy metal) ion detection / analysis.

[0080] In an embodiment, the compound of formula I is resistant to moisture based degradation considering metal Y has an ionic radius which makes the bond stronger between metal Y and halide Z, wherein the ionic radius is in the range of 4.95 - 7.5 Angstrom.

[0081] In specific aspect, the present invention relates to one or more lead (Pb)-free manganese (Mn)- and copper (Cu)-based hybrid organic-inorganic perovskites (HOIPs) compounds for visual fluorometric Pb2+ion detection having general chemical formula as Xm-Y-Zn, where X is tetraalkylammonium wherein alkyl is Cl-C8alkyl; where Y is any metal ion with 2+ cationic oxidation state; wherein Z is being a ligand selected from halide; wherein m is in range of 1- 3; and wherein n is in range of 2-6.

[0082] Specifically, the X is Tetraethylammonium (TEA) or Tetrabutylammonium (TBA). Specifically, the Y is Copper or Manganese.

[0083] The halide is selected from chloro, bromo, iodo and fluoro.

[0084] Specifically, m is 2 and n is 4.

[0085] In various embodiments, the one or more lead (Pb)-free manganese (Mn)- and copper (Cu)- based hybrid organic-inorganic perovskites (HOIPs) compounds having general chemical formula as X2-Y-Z2 includes compound 1 (TEA)2MnBr4, compound 2 (TBA)2MnBr4, compound 3 (TEA)2CuBr4 and compound 4 (TBA)CuBr2.

[0086] In another aspect, the present invention relates to a process for the preparation of hybrid organic-inorganic perovskites (HOIPs) compound of formula I, comprising steps of: a) adding at least one organic linker being precursor of X with one or more precursor(s) of metal ion with at least 2+ cationic oxidation state separately in weight ratio in the range of 2-4: 0.5- 1.5 to obtain a blend; and b) mechanochemically mixing the blend of step a) at a temperature in the range of 25-40 °C and at speed in the range of 1400-1600 rpm for time period in the range of 20-40 minutes under mixing and degassing mode to obtain the hybrid organic-inorganic perovskites (HOIPs) compound of formula I; wherein the compound obtained after the step b) is a pure compound without impurities; and the process is done without the need of solvent.

[0087] The pXRD pattern as shown in figure 2 does not show any impure sample peaks, confirming the compound of formula I is pure compound without impurities.

[0088] In an embodiment, the at least one organic linker is selected from tetramethylammonium (TMA) bromide, tetrapropylammonium bromide, tetraethylammonium (TEA) bromide, and tetrabutylammonium (TBA) bromide.

[0089] In an embodiment, the one or more precursor(s) of metal ion with at least 2+ cationic oxidation state is selected from copper bromide and manganese bromide.

[0090] In various embodiment, the synthesized compounds are used directly for further characterizations without any need of compound purification since the as-synthesised compounds use the exact ratio of the precursors and no impurity peaks are detected during characterization.

[0091] In certain embodiments, Tetraethylammonium bromide (TEABr) as the organic linker is mixed with Manganese bromide (MnBn) to obtain the compound 1 (TEA)2MnBr4.

[0092] In certain embodiments, Tetrabutylammonium bromide (TBABr) as the organic linker is mixed with Manganese bromide (MnBn) to obtain the compound 2 (TBA)2MnBr4.

[0093] In various embodiments, Tetraethylammonium bromide (TEABr) as the organic linker is mixed with Copper bromide (CuBr) to obtain the compound 3 (TEA)2CuBr4.

[0094] In certain embodiments, Tetrabutylammonium bromide (TBABr) as the organic linker is mixed with Copper bromide (CuBr) to obtain the compound 4 (TBA)CuBr2.

[0095] In various embodiments, mixing at the step b) is carried out using KaKuHunter made planetary mixer instrument (model no. SK-300II). In various embodiments, the process for synthesis of the lead (Pb)-free manganese (Mn)- and copper (Cu)-based hybrid organic-inorganic perovskites (HO IPs) compounds is carried out at 30°C.

[0096] In certain embodiments, all the as-synthesized compounds are phase pure and demonstrate a stable solid-state emission (green for Mn-based compounds 1, 2; cyan and blue for Cu -based compounds 3, 4, respectively) at room temperature.

[0097] In various embodiments, a fascinating difference in the PL spectra between the pair of Mn- and Cu-based HOIPs with organic cations as TEA+and TBA+is also explained. The structural characterizations indicated that all the compounds are crystalized in 2D, except for compound 4, (TBA)CuBr2, which has crystallized in the OD lattice, supported by Tauc’s plot. In certain embodiments, the Mn-based HOIPs compound 1 and 2 showed excellent environmental photostability for over a month and reasonable stability under a relative humidity of 70% - 80%, compared to the copper-based HOIPs.

[0098] In an aspect, the present invention provides a method of detection of a metal ion in a sample comprises reacting and treating the sample containing a metal oleate with the hybrid organic- inorganic perovskites (HOIPs) compound of formula I, by a fluorescence titration method or a visual sensory test; wherein the metal ion is selected from transition metal ion, lanthanide metal ion and heavy metal ion.

[0099] In an embodiment, the transition metal ion is based on a metal selected from scandium, titanium, vanadium, chromium, manganese, cobalt, nickel, zinc, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, cadmium, hafnium, tantalum, tungsten, rhenium, osmium, iridium and mercury.

[0100] In an embodiment, the lanthanide metal ion is based on a metal selected from lanthanum (Ln), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu).

[0101] In an embodiment, the heavy metal ion is based on a metal selected from iron, copper, tin, silver, gold, and platinum.

[0102] In an embodiment, the metal oleate is selected from scandium oleate, titanium oleate, vanadium oleate, chromium oleate, manganese oleate, cobalt oleate, nickel oleate, zinc oleate, zirconium oleate, niobium oleate, molybdenum oleate, ruthenium oleate, rhodium oleate, palladium oleate, cadmium oleate, hafnium oleate, tantalum oleate, tungsten oleate, rhenium oleate, osmium oleate, iridium oleate, mercury oleate, lanthanum oleate, cerium oleate, praseodymium oleate, neodymium oleate, promethium oleate, samarium oleate, europium oleate, gadolinium oleate, terbium oleate, dysprosium oleate, erbium oleate, thulium oleate, ytterbium oleate, lutetium (Lu), iron oleate, copper oleate, tin oleate, silver oleate, gold oleate, and platinum oleate.

[0103] In an embodiment, the fluorescence titration method comprises: a. preparing the sample by dissolving the metal oleate in a solvent to form a solution A having concentration of the metal ion in the range of 0.25 x 10’7M to 2.0 x 10’7M; and b. treating and reacting the solution A of step a) with the hybrid organic-inorganic perovskites (HOIPs) compound of formula I under sonication for time period in the range of 2-5 seconds followed by optical measurement to confirm the presence of the metal ion.

[0104] In an embodiment, the visual sensory test comprises: a. dipping a spin coated film of the hybrid organic-inorganic perovskites (HOIPs) compound of formula I in a solution containing the sample with metal ion concentration in the range of 0.25 x 10’7M to 3.0 x 10’7M for time period in the range of 2-5 seconds to obtain a dipped film; and b. analyzing the dipped film of step a) under UV-lamp to check the emission change in the film confirming the presence of said metal ion.

[0105] In an embodiment, the solvent used for the preparation of sample in the fluorescence titration method is selected from but not limited to toluene, chloroform, toluidine, iso-propyl alcohol and other antisolvent for HOIPs.

[0106] In an embodiment, the sheet like structure of (TEA)2PbBr4 is converted to nanoparticles of approximately few nanometers which increases the rate of reaction for metal ion detection drastically and thereby facilitates easy detection of Pb2+ions.

[0107] In specific embodiment, the well-dispersed nanoparticles of compound of formula I with the size range of - 5-10 nm were useful for direct metal ion sensing application. Interestingly, the photoluminescence of the compound appears to have dramatically shifted from -513 nm (2D bulk, green) to -383 nm (NPs, blue), which was attributed to the quantum size effect. In an embodiment, the optical measurements in the fluorescence titration method is done by photoluminesce spectroscopy.

[0108] In an embodiment, the spin coated film of the hybrid organic-inorganic perovskites (HO IPs) compound of formula I is prepared by: i) dissolving the HOIPs compound of formula I in a dimethylformamide as solvent to obtain a solution; ii) spin coating the solution of step i) using a spin coater machine; iii) film casting the solution in said spin coated machine by two processing sub-steps: iii-a) spinning the coater machine at 800-1200 RPM for 50-70 seconds, and iii-b) then, at 3000-5000 rpm for 20-40 seconds; and iv) annealing the casted film of step iii) at temperature in the range of 70-90°C for 20 minutes to obtain the spin coated film of the compound of formula I.

[0109] Specifically, the (TEA)2MnBr4 exhibited exceptional fluorescence switching performance upon titration with Pb2+, achieving a detection limit of 2.43 x 108M, making it suitable for the development of a Pb2+colorimetric sensor. To further explore its potential, a preliminary thin- film-based sensor utilizing nanoparticles (NPs) of compound 1 was fabricated, demonstrating a visible emission shift from blue to green, consistent with observations in NP solutions during Pb2+sensing. The sensitivity of the NPs toward Pb2+was attributed to an intriguing in situ ionexchange mechanism, which was experimentally confirmed.

[0110] In summary, the present invention reported a novel, accessible, and energy -efficient one-step mechanochemical synthesis methodology to synthesize hybrid organic-inorganic perovskites compounds using a KaKuHunter-made planetary mixer under ambient conditions for the first time, avoiding multi-steps temperature-driven solution-based synthesis under inert atmosphere and further product purification. Four different Pb-free manganese- and copper-based HOIPs, namely, compound 1 (TEA)2MnBr4, compound 2 (TBA)2MnBr4, compound 3 (TEA)2CuBr4, and compound 4 (TBA)CuBr2 were synthesized. The synthesized compounds were characterized by XRD, UV-Vis, PL, IR, TGA, XPS, SEM, HRTEM, HAADF, and EDS analyses. All the as-synthesized compounds are phase pure and demonstrate a stable solid-state emission (green for Mn-based compounds 1, 2; cyan and blue for Cu-based compounds 3, 4, respectively) at room temperature. A fascinating difference in the PL spectra between the pair of Mn- and Cu-based HOIPs with organic cations as TEA+and TBA+was also explained. The structural characterizations indicated that all the compounds are crystalized in 2D, except for compound 4, (TB A)CuBr2, which has crystallized in the 0D lattice, supported by Tauc’s plot. Furthermore, the stability of all four Mn- and Cu -based HOIPs were studied under a relative humidity of 70% - 80% for a relatively long period of 45 days. The Mn- based HOIPs compound 1 and 2 showed excellent environmental photostability for over a month and reasonable stability under a relative humidity of 70% - 80%, compared to the copper-based HOIPs. Therefore, compounds 1 and 2 were used further for application-based studies.

[0111] For direct metal ion sensing application, the as-synthesized 2D (TEA)2MnBr4, compound 1, was solvated in toluene, further sonicated to obtain well -dispersed NPs of compound 1 in the size range of - 5-10 nm. Interestingly, the PL of compound 1 appears to have dramatically shifted from -513 nm (2D bulk, green) to -383 nm (NPs, blue), which was attributed to the quantum size effect. Further, the well-dispersed NPs of (TEA)2MnBr4, compound 1 was used for fluorescence titration studies for the detection of heavy metal ions using various heavy metal cations, including transition metals and lanthanides (e.g., Y3+, Nb3+, Co2+, Ni2+, Ag+, Zn2+, Cd2+, Pb2+, Gd3+, and Dy3+), where it showed exceptional sensitivity towards Pb2+ion. A steady decrease of PL peak for NPs of compound 1 around - 380 nm and concomitant rise of a new PL peak -553 nm causes the change in NPs solution emission color from blue to green during Pb2+titration, with Pb2+solutions concentration as low as 0.25 x 10’7M. Thus, the dispersed NPs solnof compound 1 showed excellent controlled fluorescence switching (blue — green) performance with Pb2+titration with high sensitivity, limit of detection (LOD) of 0.24 nM (2.43 x 10’8M); including ultrafast reaction time (< 1 minute), and reasonable selectivity. Hence, the NPs of compound 1 can be used to fabricate a Pb2+calorimetric sensor. A preliminary visual thin film-based sensor using NPs of compound 1 was fabricated to test the change in emission (blue — green) in correlation with the results obtained in NPs solution during Pb2+detection. An exciting in-situ ion exchange-based mechanism for the sensitivity of NPs of compound 1 is proposed, where an in-situ ion exchange reaction between Pb2+, partially replacing Mn2+present within the lattice in NPs of compound 1 , (TEA)2MnBr4 leading to the formation of tetraethylammonium lead bromide, to form a biphasic material, which causes the change in NPs solnemission color from blue to green. The proposed mechanism was confirmed through XRD, PL, and HRTEM studies.

[0112] EXAMPLES Materials used: Copper bromide (CuBr, Sigma-Aldrich, 99%), Manganese bromide (MnBr2, Sigma-Aldrich, 99%), Tetraethylammonium bromide (TEABr, Sigma-Alderich, 99.9%), Tetrabutylammonium bromide (TBABr, Sigma- Aldrich, 99.9%), Lead nitrate (Sigma- Aldrich, 99%), Silver Nitrate (Sigma-Aldrich, 99%), Yttrium Nitrate (Sigma-Aldrich, 99%), Neodymium Nitrate Hexahydrate (Sigma- Aldrich, 99%), Cobalt nitrate (Sigma- Aldrich, 99%), Nickel nitrate (Sigma-Aldrich, 99%), Zinc nitrate (Sigma-Aldrich, 99%), Cadminum nitrate (Sigma-Aldrich, 99%), Gadolinium nitrate (Sigma-Aldrich, 99%), sodium oleate (Sigma Aldrich, 99.9%) and Anhydrous toluene (Sigma-Aldrich, 99%).

[0113] Example 1: Material and Methods

[0114] A) Synthesis of hybrid organic-inorganic perovskites (HOIPs) (TEAhMnBn Compound 11 and (TEA)2CuBr4 [Compound 31:

[0115] Solid state synthesis is carried out, wherein 3 mmol of TEABr and 1 mmol of CuBr and Mn Bn is added to the reaction vessel, separately with the organic linker. Further this vessel is mounted in KaKuHunter made planetary mixer instrument (model no. SK-300II), at 1500 rpm for 30 minutes, under mixing and degassing mode. The temperature of the reaction vessel reached during the synthesis is 30°C. The sample was then collected from the reaction vessel and used directly for further characterizations. Please note no compound purification step is carried out since the as-synthesised compounds use the exact ratio of the precursors and no impurity peaks are detected during characterization.

[0116] B) Synthesis of hybrid organic-inorganic perovskites (HOIPs) (TBA )MnB [Compound 2] and (TBA)CuBr [Compound 4]:

[0117] The synthesis procedure for compounds 2 and 4 are similar as done in above example A), except TBABr is used as an organic linker.

[0118] The crystalline compounds of 1 (TEAhMnB , 2 (TBA)2MnBr4, 3 (TEA)2CuBr4, and 4 (TB A)CuBr2 were prepared by a straightforward and effortless method by using the elementary principle of molar ratio control. The dimensionality can be tuned easily by controlling the process of synthesis, molar ratios, alkylamine chain length, and functional groups. Hence, the molar ratios of the precursors of manganese (II) bromide (MnBn), copper (I) bromide (CuBr), tetraethylammonium bromide (TEAB), and tetrabutylammonium bromide (TBAB) are fixed at 1: 3 (metal halide: organic precursor) [Figure 1 (a)]. Table 1 enlists the concentration precursors powders (in mmol) used to form the respective compounds for more clarity. After measuring the appropriate amount of precursors, they are hand shaken in a reaction vessel for 2-3 minutes, followed by mounting the vessel inside a KaKuHunter made planetary mixer instrument [Figure 1(b)]. After this, the machine is set to rotate at 1500 rpm for 30 minutes, ultimately synthesizing lead-free low dimensional HOIP compounds [Figure 1 (c)]. The compounds 1 and 2 when viewed under 365 nm UV light show excellent green emission, and compounds 3 and 4 when viewed under 254 nm UV light showed strong cyan emission indicating the formation of optically active compounds [Figure 1 (d)].

[0119] Table 1: The concentration of precursors used to form the respective compounds.

[0120] C) Nanoparticles (NPs) solutions preparation:

[0121] 0.25 mg / mL of each sample (compound 1, 2, 3 or 4) is dispersed in 30 mL of toluene to make the respective NPs solution, followed by ultra-sonication (Elmasonic P 120H, frequency = 80 Hz, power =100 W) for 10-15 minutes to make a well dispersed NPs solution.

[0122] D) Materials Characterization:

[0123] 1) XRD Analysis: The phase identification of the as-synthesized samples was performed by Powder X ray diffraction (PXRD), recorded on a Panalytical X’pert PRO powder X-ray diffractometer with Cu Ka radiation. Upon matching the XRD patterns of the obtained products with the simulated SCXRD patterns of (TEA)2 MnBr4 compound, it is found that the compounds 1, 2 and 3 crystallized as 2D perovskites [Figure 2 (a)]. The XRD patterns of compound 4 also compared with the reported SCXRD for the 0D structure of (TBA)CuBr2 as shown in Figure 1(b). The ionic radii (nm) for (TEA)+and (TBA)+are 0.337 and 0.413 respectively. This causes the formation of low dimensional perovskite systems as the size of the cation exceeds the size of the A-site cavity hence pushing the [MX4]2" units away from each other and leading to the creation of an electronic quantum-well like structure. It is also noticed that crystal structure of compound 2 shows orientated growth on (110) plane probably due to change in organic cations.

[0124] 2) Morphological Analysis using HRTEM and FESEM: The morphologies, ED AX elemental ratio analysis and elemental mapping of the as-synthesized products were investigated using high-resolution TEM (HRTEM) (FEI, Tecnai 20 ST with an accelerating voltage of 200 kV) and scanning electron microscope (FEI, FESEM Quanta 200-3D). Figure 7(a-d) shows the FESEM images for all the compounds. It can be seen that compound 1 has a comparatively smooth and compact surface structure, thereby the number of grain boundary and the defect arising thereafter would be less. Compounds 2, 3 and 4 have a comparatively rough surface morphology. FESEM-EDAX and elemental mapping for all the samples are also carried out to confirm the presence of all elements in the equitable abundance. The ratios for Mn: Br for compounds 1 and 3 as realized from their structural formula should be 1 : 4 which is reflected from the atomic% ratio as obtained from the FESEM elemental ratios [Figures 8(a) and 10 (a)] and verified by the elemental mapping [Figures 8(b) and 10 (b)]. Also, the Cu: Br ratios from the structural formula for compound 2 is 1: 4 and for compound 4 should be 1: 2. These matched well with the FESEM-EDAX atomic % ratios [Figures 9(a) and 11(a)] and also reflected from the elemental mapping micrographs [Figures 9(b) and 11(b)].

[0125] 3) TGA Analysis: Thermo-Gravimetric Analysis (TGA) was performed using SDT Q600 DSC-TGA instrument in Argon atmosphere at ramping rate 10 °C / min. To elucidate the thermal stability of the compounds thermo-gravimetric (TGA-DSC) analysis is carried out. All the compounds are stable upto ~ 220 °C. The weight loss for compounds 1 and 2 are shown in Figure 12(a), which starts around 200°C with the evaporation of surface water molecules. Both 1 and 2 show a similar weight loss characteristic, with 70.81% weight loss up to 400°C. The TEABr melting peak is observed at ~ 286 °C as indicated in Figure 12(a). The organic moiety TBABr has a melting point of 133 °C as seen in Figure 12(b, d). This is followed by the final melting of MnBr2 at 698 °C for both 1 and 2 which marks the complete dissociation of the structure. Figure 12(c,d) shows the TGA spectra for compounds 3 and 4. The first step in the TGA graphs for hybrid organic-inorganic perovskites (HOIPs) mark the evaporation of organic moieties within the framework and the second step is generally assigned to the breaking of octahedral bonds. Figure 12(c) shows phase change steps for compound 3 wherein, 44.39% degradation occurs up to 450°C indicating slow evaporation of organic moiety however the second phase change starts after the complete melting of TEA Br at 286 °C causing a weight loss of 31.22%. Compound 4 shows first change up to 250°C and a weight loss of 57.59%. The second change causes weight loss of 25.58%. Finally, structure crumbles at 504 °C which indicates onset for CuBr melting [Figure 12(d)].

[0126] 4) XPS Analysis: The surface properties of the as-synthesized samples were characterized by X-ray Photoelectron Spectra (XPS) using Thermo Scientific K-Alpha+ X-ray photoelectron spectrometer analyzer chamber operating at 2xl0-7mbar pressure. To further validate the authenticity of the ambient one-step mechanochemical synthesis methodology for formulating luminous Pb-free halide perovskites, XPS analysis is carried out and it was found that no oxidation of the Mn2+ / Cu+species occurred during the ambient synthesis (Figure 4 and 5). The XPS peak maxima for Mn 2p3 / 2 is at 645.64 eV for compound 1 [Figure 4(a)] and 645.51 eV for compound 2 [Figure 4(b)]. Also, the Cu 2p spectra shows characteristic peaks at 952.98 eV and 933.07 eV corresponding to Cu 2pi / 2 and Cu 2p3 / 2 core levels, respectively for compound 3 [Figure 5 (a)]. Similarly, the peak positions for Cu 2pi / 2 and Cu 2p3 / 2 core levels for compound 4 are 953.47 eV and 933.92 eV, respectively [Figure 5 (b)]. The bromine XPS peak positions for Br 3d spectra are also recorded for all the compounds and are shown in Figure 6 (a-d). The slight changes in the XPS peak binding energy values is related to the change in the crystal structure and orientation of the inorganic layer with respect to the organic layers, as previously reported by our group.

[0127] 5) FTIR Analysis: Fourier transform infrared (FT-IR) spectra were recorded using Perkin Elmer 2000 FTIR spectrometer in the 400-4000 cm-1region. No presence of unreacted precursors was noted as seen in the pXRD pattern and confirmed by the FTIR data (Figure 3). It can be seen from Figure 3(a) that the final products, i.e. compounds 1 and 3, resulting from reactions between metal halides and TEABr salt possess all the characteristic precursor salt peaks. The prominent peaks in the spectra were assigned to N-H stretching at 3411.48 cm1, C- H stretching at 2875.82 cm1, C-H2 wagging and twisting vibrations at 1459.86 cm1, C-H3 out- of-plane vibrations at 1389.38 cm1, C-N+stretch and aliphatic amine stretching at 1005.97 cm'1and 793.82 cm1. Presence of a very sharp peak at 3411.48 cm'1shows the characteristic N- H stretching, which can occur due to partial hydrogen bonding with the atmospheric humidity but the absence of any precursor peaks in XRD shows this bond is present only on the surface and does not affect the perovskite structure and its properties. Also, interestingly the intensity of the N-H stretching vibrations has reduced drastically from the TEABr salt as the compounds 1 and 3 are synthesized indicating increased interactions of the free N+species with the [MnBr4]2-and [CuBr4]2-units and less availability of free N+for moisture attack. Figure 3(b) shows the IR spectra for compounds 2 and 4, all the characteristic peaks are similar to the aforementioned peak positions due to similarity in the TEABr and TBABr salts. However, slight changes in the wavenumbers are noticed and are as follows, C-H stretching vibrations are noted at 2963.93 cm'1and 2875.82 cm1, C-H3 out-of-plane bending vibrations are at 1459.86 cm1, C-H2 wagging & twisting vibrations at 1389.38 cm1, C-N+stretch aliphatic amine stretching at 1364.28 cm'1and 734.61 cm1. The spectra show the successful formation of the perovskite structure using our method of synthesis.

[0128] 6) Optical absorption studies: Optical absorption measurements are carried out by a Shimadzu UV-Vis-IR (UV-3600 Plus) spectrophotometer. The Diffuse Reflectance Spectra (DRS) spectra was recorded for each of the compounds [Figure 13(a)]. It is observed a strong absorption edge at around 290 nm for all the compounds. This is the characteristic peak for formation of the 2D perovskite structure. It can be noted that the Mn-compounds (1, 2) display several absorption bands at ~ 377 nm (i), 416 nm (ii), 529 nm (iii) and, 619 nm (iv). These absorption peaks are associated with electronic transition (from the Ai ground state to different excited states) of Mn2+ions in the tetrahedral [MnB ]2-crystal-field. The Cu-compounds (3, 4) show a single edge sharp absorption at about 290 nm with weak tail states upto 500 nm. All the materials show luminesce under UV lamp as shown in figure 1. The normalized PL spectra shown in Figure 13(b) demonstrate that the PL emissions are in order with the visible emissions of the compounds from figure 1. The compounds 1 and 2 show emission peak at centered at 513 nm and compounds 3 and 4 show peaks centered at 440 nm. Noted a very interesting difference between the pair of compounds with TEA+and TBA+, which can arise due to difference in the size of the organic cations. The PL spectra for compounds 1 and 3 are sharp, whereas the PL spectra for compounds 2 and 4 are broad. The increase in size of the organic cations from TEA+to TBA+has resulted in peak broadening due to increase in Vander Waals interactions and steric hindrances between large cations in 2D hybrid perovskite compounds. The Tauc’s plot for all the compounds is shown in Figure 14. The as calculated band gaps for compounds 1, 2, 3 and 4 are in the range of 3.25, 3.50, 3.77 and 3.80 eV, respectively as shown in Table 2. The absorption and emission spectra for all the compounds is shown Figure 15. Noted a very high stokes shift for compounds 1, 2, 3, and 4 are 1.19 eV, 1.08 eV, 1.06 eV and 0.99 eV, respectively [Figure 15(a-d)]. This can be attributed to trap mediated PL emission in 2D HOIPs. The transition elements i.e. Mn2+and Cu+respectively are acting as deep trap centers. It is predicted that when the electron-hole pairs in the compound are PL excited the electrons form a bound exciton on the transition metal sites. Especially for compounds 1 and 2 the band edge is not sharp in Figure 13(a), thus the Mn2+center acts like a deep trap which facilitates decay, hence the observed large stokes shift.

[0129] Table 2: Calculated band gap values using Tauc’s plot.

[0130] 7) Photoluminescence (PL) Spectroscopy Analysis: Steady-state PL measurements (both liquid and thin film) are performed utilizing a spectrofluorometer FS5 (Edinburgh Instruments). To elucidate the difference in stability of the smaller molecule TEA+cationbased compounds 1 and 3, PE measurements are carried out at 70-80% humidity (Figure 16). Noted a drastic change in PL for compound 3 indicating a complete loss of the perovskite structure [Figure 16(b)]. However, comparatively very low material degradation is noted for compound 1 [Figure 16(a)]. This is likely due to the fact that Mn2+has a larger ionic radius than Cu2+, which means that it can form stronger bonds with the surrounding ligands in the perovskite structure. This results in a more stable crystal lattice, making the Mn-based 2D perovskites less prone to degradation in the presence of moisture. Furthermore, the stable electronic configuration for Mn+2, electronic configuration [Ar]4s°,3d5, configuration where d- block is half filled making the center more stable as opposed to Cu+center in compound 3. These stronger metal-ligand bonds help to stabilize the crystal lattice and prevent moisture from attacking the perovskite. In contrast, Cu-based 2D perovskites are more susceptible to degradation in the presence of moisture due to weaker metal-ligand. This is due to the fact that Cu+tends to undergoes oxidation faster in the presence of moisture, which can further destabilize the crystal lattice and lead to degradation. Therefore, the combination of a larger ionic radius and higher oxidation state of manganese compared to copper results in higher moisture stability in Mn-based 2D perovskites in HOIPs.

[0131] Example 2: Analysis of the prepared hybrid organic-inorganic perovskites (HOIPs) in metal ion detection:

[0132] A) Synthesis of metal-oleate solution:

[0133] Metal oleate (Mx-oleate) was prepared by the method reported by Y. Liu et al., for preparing metal ion (J. Mater. Chem. C 2018, 6, pages 4793-4799), which is used as analyte for metal ion detection. In the typical synthesis of metal-oleate, 0.1 mmole of metal sulfate / nitrate was added with 0.5 mL water followed by the ultrasonication for 10 min until it becomes transparent. In another 10 mL vial, 0.25 mmol sodium oleate was added with water: ethanol (1: 1 by volume) solution and kept for ultra-sonication for 10 to 15 min until a homogeneous solution appears. The above metal salt solution was further mixed with the sodium oleate solution and ultra-sonicated (Elmasonic P 120H) for 15 min. The as-prepared mixture was further centrifuged and washed several times with ethanol and vacuum dried overnight. Inventors have chosen several transitions metals and lanthanide ions such as, Y3+, Nb3+, Co2+, Ni2+, Ag+, Zn2+, Cd2+, Pb2+, Gd3+, and Dy3+under “metal” in the metal oleate preparation, for fluorescence titration studies. Metal-oleate solutions with concentrations ranging from 0.25 x 10’7M - 2.0 x 10’7M were prepared by dissolving the metal-oleate salt in toluene by sonication.

[0134] B) Metal Ion Detection Applications:

[0135] (a) Dispersion: Fluorescence titration studies were carried out in the presence of different Pb2+concentrations ranging from 0.25 x 10’7M - 2.0 x 10’7M. The as synthesized metal-oleates at a fixed concentration were added to a 0.25 mg / mL toluene dispersion of Compound 1, and 2, respectively. After addition of the respective metal-oleate solution the mixture was sonicated for 2-3 seconds followed by optical measurements. All the metal-oleates of Y3+, Nb3+, Co2+, Ni2+, Pb2+, Ag+, Zn2+, Cd2+, Gd3+, and Dy3+ions were analysed under identical conditions. (b) Thin film preparation: Compound 1 was solubilized at concentration of 1 mg / mL in DMF solvent. The solution was spin coated using HOLMARK spin coater (Model No. H0-TH-05). A two-step programme at 1000 RPM for 60 seconds, and second step at 4000 RPM for 30 seconds was pre-set for film casting. This was followed by annealing at 80°C for 20 minutes.

[0136] (c) Visual Sensory Pb2+test: A calorimetric sensitivity test was performed by dipping the spin coated films of compound 1 in toluene solution containing 2.0 x 10’7M Pb-oleate in toluene for 2-4 seconds and the emission change was observed under a UV-lamp at A = 253 nm.

[0137] The superior stability of Mn-LFHHPs lead to investigate fluorescence titration studies on compounds 1 and 2 for detecting metal cations using a signal transduction pathway by titration. Heavy metal ion detection is essential in many fields such as medical diagnosis, environmental monitoring, and industrial quality control. LFHHPs offer a unique solution to this challenge due to their selective binding to specific metal ions, high sensitivity, cost-effectiveness, rapid response, and versatility. Despite the several advantages of these LFHHPs there applicability for heavy metal ion detection is limited. These studies are carried out in toluene solvent in order to minimize solvent related degradation pathways and allow uninterrupted interactions with heavy metal ions to facilitate their detection.

[0138] Metal oleate compounds were prepared as mentioned above. Transitions metals and lanthanide ions such as, Y3+, Nb3+, Co2+, Ni2+, Ag+, Zn2+, Cd2+, Pb2+, Gd3+, and Dy3+respectively, are chosen to test the efficacy of as synthesized Mn-LFHHPs for fluorescence titration studies. The toluene solutions of the respective metal-oleates are shown in Figure 18. The compounds were dispersed in toluene at a concentration of 0.25 mg / mL by ultra-sonicating for 10-15 minutes. Figures 19(a) and 19(b) show dispersions of compounds 1, and 2 and their respective emissions in dark under UV-lamp. This shift in emission along with the drastic reduction in stokes shift from 1.19 eV (solid) to 0.21 eV (dispersion) (see Figure 17) indicates probable formation of nanoparticles. It arises from fact that as solid goes to its nano-regime; free- excitonic electron emission will arise at band edge instead of trap mediated transitions in bulk. Fluorescence emissive performance of compounds 1 and 2 was carried out for detecting various above mentioned heavy metal ions. These investigations were carried out using a signal transduction pathway by titration. Fluorescence titration spectra were carried out in the presence of different Pb2+concentrations ranging from 0.25 x 10’7M - 2.0 x 10’7M. The compounds 1 and 2 show bright fluorescence in absence of Pb2+ions [Figure 21 (a, b)]. However, upon sequential addition of Pb2+ions (concentration range 0.25 x 10’7M - 2.0 x 10’7M) the peak intensity at ~ 383 nm shows quenching behavior for both the compounds. Surprisingly, noted a sequential occurrence of a new emission peak at 553 nm for compound 1 and at 599 nm for compound 2.

[0139] Interestingly, the peak strength at 553 nm for compound 1 increases monotonically with maxima at 2.0 x 10’7M concentration of Pb2+with a simultaneous quenching and peak minima for 383 nm emission at the same concentration [Figure 21(a)]. However, upon comparison of the initial results for compounds 1 and 2 [Figure 21 (a, b)] it can be found that compound 1 is more sensitive towards the Pb2+ion detection. Hence further analysis is carried out on compound 1 as Figure 21(a) shows a uniform reduction in PL intensity at of 383 nm which is the initial peak. UV-vis absorbance spectroscopy [Figure 21(c)] is carried out to co-relate the changes in the emission spectra as identified by the appearance of a new peak at around 553 nm. It is noted a similar dual peak nature in the absorbance spectra. The peak initially at about 280 nm retained as the concentration of Pb2+ions increased along with the occurrence of a new peak around 350 nm. This can point towards the possibility of formation of a biphasic material within the matrix. Figure 21(d) plots the fluorescence intensity at 383 nm for compound 1 as a function of the Pb2+ions concentrations, the inset shows the Pb2+concentration vs. emission intensity, demonstrating a linear relationship. The titration method, which is superior to previous methods for detecting Pb2+in solutions, was found to have a detection range as low as 0.25 x 10’7M. Also, because all of the fluorescence data were captured during the Pb2+titration process, they implied a quick response, which was advantageous for quickly and quantitatively detecting Pb2+. The limit of detection (LOD) was determined to be 2.43 x 10’8M (0.24 nM) using the equation,

[0140] LOD = 3S / s where 5 is the standard deviation of the signals and s is the slope of the linear calibration plot. Selectivity towards various metal cations was tested under identical conditions while taking into consideration the interference of other metal cations. The fluorescence intensity rose by 100% at 2.0 x 10’7M Pb2+at 553 nm peak position, as shown in Figure 21(e), therefore this concentration was chosen to test compound l's selectivity with the other 10 metal cations. Figure 21(e) shows how different metal cations have distinct effects on the fluorescence intensity at 383 nm. It is noted note that upon incorporation of various metal ions the initial peak at 383 nm has substantially broadened along with appearance of multiple peaks. This could point to the possible degradation of compound 1. However, a change in the emission color was observed upon addition of Pb2+ions along with evolution of two distinct peaks. This indicated that the compound 1 is unstable in the presence of other heavy metal ions used in this study, whereas the selectivity of compound 1 is extremely high towards Pb2+metal ions indicated by a change in its emissive properties. The formula for determining a sample's selectivity to a certain metal cation (SM) is,

[0141] SM= (Fo - F) / Fo x 100% where F and Fo are the fluorescence intensities in the presence and absence of the metal ions, respectively.35Figure 21(f) illustrates the specific selectivity of compound 1 for other metal ions. With an Spb(ii) value of 80.41%, the selectivity towards Pb2+ions is significantly higher than that of other heavy metal ions.

[0142] Example 3: Characterization analysis of the compound after testing in metal ion detection (to check stability and reusability): In order to understand the correlation between the photophysical properties on compound 1 morphology in solid and dispersed states inventors have carried out HRTEM studies. Figure 22(a) shows the HRTEM image of the as- synthesized bulk sample of compound 1 showing a 2D layered structure as designed with a d- spacing of 0.29 nm (Figure 20). Fig. 22(a) inset shows the PL spectrum of the compound 1 powder sample recorded by sandwiching the powder between two glass slides. It shows a prominent emission at around 513 nm which has shifted far away from the band edge for the bulk sample indicating a very large Stoke shift. There is a definitive absence of near band edge excitonic emission which would represent a free exciton. The PL around 513 nm can thus be attributed to the transition metal mediated deep trap state related bound excitonic transition because it does relate to the Mn-related d-d transition in the absorbance located at the same spectral location. Figure 22(b) shows the HRTEM data for the same powder compound 1 after the bulk was solvated in toluene, sonicated and dispensed on the HRTEM lacey carbon grid. The high energy input due to sonication clearly leads to the conversion of the bulk powder into nanoparticles of the dimension of ~ 5-10 nm [Figure 23(a)]. The d-spacing values (0.27 nm) of the nanoparticles [Figure 23(b)] are closely matching with the d-spacing value of as- synthesized bulk sample of compound 1 [Figure 20]. Very interestingly, the PL of the nanoparticles as shown in inset of Figure 22(b) appears to have dramatically shifted to about 383 nm (along with a hump near 370 nm) which is now closer to the band edge. This can be attributed to the quantum size effect mediated significant broadening of the wave function with concomitant enhancement of the band gap, which presumably weakens the effect of the role of TM trap on binding of the wave function. This would facilitate the free excitonic emissions near band edge. Figure 22(c) shows HRTEM, HAADF-STEM image, and elemental mapping for compound 1 after Pb2+ion detection experiment at a Pb2+ concentration of 2.0 x 10-7 M. Figure 22 (c-i) shows the retention of the nanoparticle structure upon addition of Pb2+ions to the solution of compound 1 with morphology as shows in Figure 22(b). The d-spacing value remains at 0.25 nm even after Pb2+detection, which further confirms the no deterioration of the nanoparticle of compound 1 even after ultrafast in situ Pb2+ion exchange reaction [Figure 24(a)]. The high-angle annular electron microscopy (HAADF-STEM) images and elemental mappings of nanoparticles of compound 1 after Pb2+detection revealed the Pb, Mn, Br, N, and C presence [Figure 8 c(ii-vii)], supporting the Mn2+replacement by Pb2+in the compound 1 nanostructure and the uniform distribution of Pb and Mn in the nanoparticles after Pb2+detection. The EDS elemental analysis of the nanoparticles of compound 1 after Pb2+detection further confirmed the presence of Mn and Pb, where the atomic ratio of Mn was found to be substantially lower than Pb [Figure 24 (b)]. Figure 22(d) shows the evolution of the XRD as a function of addition of Pb-oleate to the toluene -based dispersion of nanoparticulate matter of compound 1. It was found that this leads to the progressive formation of (TEA)2PbBr4 phase with increasing concentration of Pb-oleate clearly implying Mn2+replacement by Pb2+. In order to further explore the formation of (TEA)2PbBr4 phase, we separately synthesized this material and recorded its PL emission spectra as shown in Figure 22(e). It shows the primary broad signature at over a range from 540 nm to 620 nm. Turning to the concentration dependence of Pb2+addition into the toluene-based dispersion of compound 1, it can be seen in Figure 21(a), a progressive decrease in the intensity of the peak near 383 nm and concomitant rise of a broad PL peak in the same region as shown in Figure 22(e). Thus mechanism of Pb2+sensing appears to be via displacement reaction involving Mn2+replacement by Pb2+. The conclusions drawn from HRTEM, XRD and PL studies lead to a proposed mechanism for the selective detection of Pb2+by compound 1 is depicted in Figure 22(f). There is an in-situ ion exchange reaction between Pb2+, replacing Mn2+present in compound 1, leading to the formation of tetraethylammonium lead bromide, which causes change in emission color and formation of new peak (-553 nm) at green wavelength. The Pb2+detection by fluorescence titration is observed to be very fast, as ion exchange reaction occurs within fraction of minutes. The compound 1 shows remarkable ultrafast sensitivity towards Pb+2ion detection with LOD as 2.43 x 10’8M. A preliminary visual sensor for detection of Pb2+by compound 1 is also fabricated and noticed change in emission color of as casted thin film under 365 nm UV lamp upon emersion in 2.0 x 10’7M Pb-oleate solution for 2-4 seconds [Figure 22(g)],

[0143] ADVANTAGES OF THE INVENTION:

[0144] 1. This is the first report for synthesis of highly crystalline and phase pure Pb-free hybrid organic-organic perovskites by mechanochemical method using a very easy to use planetary mixer.

[0145] 2. The planetary mixer-based synthesis methodology allows for easy scaling up of the synthesis process, making it suitable for larger production quantities.

[0146] 3. Compared to traditional ball milling or mortar pestle methods, the planetary mixerbased synthesis method significantly reduces the reaction times required for the synthesis of the compounds.

[0147] 4. The synthesis process using the planetary mixer minimizes or eliminates the generation of byproducts, reducing waste and increasing the efficiency of the synthesis.

[0148] 5. Unlike solvothermal protocols, the synthesized compounds obtained using the planetary mixer-based method do not require a final washing step, saving time and resources.

[0149] 6. The choice of quaternary alkyl ammonium salts as cations in the synthesis process improves the moisture tolerance of the resulting perovskite compounds, making them more stable and suitable for various applications.

[0150] 7. The synthesized compounds exhibit reasonable environmental photostability for up to 45 days, indicating their potential for long-term stability in practical applications.

[0151] 8. The synthesized compounds show high sensitivity towards heavy metal cations, including lead (Pb) ions, making them promising candidates for metal ion detection and sensing applications. The ability of the synthesized compounds to undergo a change in emission from blue to green enables their use as visual detectors in thin films, providing a convenient and easily observable indication of metal ion presence. It provides the efficient, ultrafast, and quantitative Pb2+ ion detection ability of as-synthesized NPs of Pb-free HOIPs compounds.

Claims

We Claim:

1. A hybrid organic-inorganic perovskites (HOIPs) compound of formula I for metal ion detection, the formula I is represented by:Xm-Y-ZnFormula I whereinX is tetraalkylammonium wherein alkyl is C1-C8 alkyl,Y is a metal ion with at least 2+ cationic oxidation state,Z is a ligand selected from halide, m is in range of 1-3, and n is in range of 2-6, wherein the compound of formula I is 2D material with a layered crystal structure.

2. A hybrid organic-inorganic perovskites (HOIPs) compound of formula I for metal ion detection, the formula I is represented by:Xm-Y-Zn Formula I whereinX is tetraalkylammonium wherein alkyl is Cl-C8alkyl,Y is a metal ion with at least 2+ cationic oxidation state,Z is a ligand selected from halide, m is in range of 1-3, and n is in range of 2-6, wherein the compound of formula I is prepared by a process comprising: a) adding at least one organic linker being precursor of X with one or more precursor(s) of metal ion with at least 2+ cationic oxidation state separately in weight ratio in the range of 2-4: 0.5- 1.5 to obtain a blend; and b) mechanochemically mixing the blend of step a) at a temperature in the range of 25-40 °C and at speed in the range of 1400-1600 rpm for time period in the range of 20-40 minutes under mixing and degassing mode to obtain the hybrid organic-inorganic perovskites (HOIPs) compound of formula I; wherein the compound obtained after the step b) is a pure compound without impurities; and the process is done without the need of solvent.

3. The hybrid organic-inorganic perovskites (HOIPs) compound as claimed in claim 1 or 2, wherein the tetraalkylammonium is selected from tetramethylammonium (TMA), tetrapropylammonium, tetraethylammonium (TEA), and tetrabutylammonium (TBA).

4. The hybrid organic-inorganic perovskites (HOIPs) compound as claimed in claim 1 or 2 wherein the metal ion with at least 2+ cationic oxidation state is copper and / or manganese.

5. The hybrid organic-inorganic perovskites (HOIPs) compound as claimed in claim 1 or 2 wherein the halide being ligand is selected from chloro, bromo, iodo and fluoro.

6. The hybrid organic-inorganic perovskites (HOIPs) compound as claimed in claim 1 or 2, wherein the compound is selected from compound 1 (TEA)2MnBr4, compound 2 (TBA)2MnBr4, compound 3 (TEA)2CuBr4 and compound 4 (TBA)CuBr2.

7. A process for the preparation of hybrid organic-inorganic perovskites (HOIPs) compound of formula I as claimed in claim 1 or 2, comprising the steps of: a) adding at least one organic linker being precursor of X with one or more precursor(s) of metal ion with at least 2+ cationic oxidation state separately in weight ratio in the range of 2-4: 0.5- 1.5 to obtain a blend; and b) mechanochemically mixing the blend of step a) at a temperature in the range of 25-40 °C and at speed in the range of 1400-1600 rpm for time period in the range of 20-40 minutes under mixing and degassing mode to obtain the hybrid organic-inorganic perovskites (HOIPs) compound of formula I; wherein the process is purification free and solvent free.

8. The process as claimed in claim 7, wherein the at least one organic linker is selected from tetramethylammonium (TMA) bromide, tetrapropylammonium bromide, tetraethylammonium (TEA) bromide, and tetrabutylammonium (TBA) bromide.

9. The process as claimed in claim 7, wherein the one or more precursor(s) of metal ion with at least 2+ cationic oxidation state is selected from copper bromide and manganese bromide.

10. A method of detection of a metal ion in a sample comprises reacting and treating the sample containing a metal oleate with the hybrid organic-inorganic perovskites (HOIPs) compound of formula I as claimed in claim 1 or 2, by a fluorescence titration method or a visual sensory test; wherein the metal ion is selected from transition metal ion, lanthanide metal ion and heavy metal ion.

11. The method as claimed in claim 10, wherein i. the transition metal ion is based on a metal selected from scandium, titanium, vanadium, chromium, manganese, cobalt, nickel, zinc, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, cadmium, hafnium, tantalum, tungsten, rhenium, osmium, iridium and mercury; wherein the lanthanide metal ion is based on a metal selected from lanthanum (Ln), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu); and wherein the heavy metal ion is based on a metal selected from iron, copper, tin, silver, gold, and platinum; and ii. the metal oleate is selected from scandium oleate, titanium oleate, vanadium oleate, chromium oleate, manganese oleate, cobalt oleate, nickel oleate, zinc oleate, zirconium oleate, niobium oleate, molybdenum oleate, ruthenium oleate, rhodium oleate, palladium oleate, cadmium oleate, hafnium oleate, tantalum oleate, tungsten oleate, rhenium oleate, osmium oleate, iridium oleate, mercury oleate, lanthanum oleate, cerium oleate, praseodymium oleate, neodymium oleate, promethium oleate, samarium oleate, europium oleate, gadolinium oleate, terbium oleate, dysprosium oleate, erbium oleate, thulium oleate, ytterbium oleate, lutetium (Lu), iron oleate, copper oleate, tin oleate, silver oleate, gold oleate, and platinum oleate.

12. The method as claimed in claim 10, wherein the fluorescence titration method comprises: a. preparing the sample by dissolving the metal oleate in a solvent to form a solutionA having concentration of the metal ion in the range of 0.25 x 10’7M to 2.0 x 10’7M; andb. treating and reacting the solution A of step a) with the hybrid organic-inorganic perovskites (HOIPs) compound of formula I under sonication for time period in the range of 2-5 seconds followed by optical measurement to confirm the presence of the metal ion.

13. The method as claimed in claim 10, wherein the visual sensory test comprises: a. dipping a spin coated film of the hybrid organic-inorganic perovskites (HOIPs) compound of formula I in a solution containing the sample with metal ion concentration in the range of 0.25 x 10’7M to 3.0 x 10’7M for time period in the range of 2-5 seconds to obtain a dipped film; and b. analyzing the dipped film of step a) under UV-lamp to check the emission change in the film confirming the presence of said metal ion.

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