Fiber optic lead ion sensor

Abstract

The disclosure provides an integrated sensor probe 100 irradiated by an optical source 101 for detecting optical absorbance of lead ions (Pb(II)) in preference to other heavy metal ions in a sample 107 using a photodetector 106. The sensor probe 100 includes a fiber optic probe 102 with a U-bent sensing portion 103 and a photodetector 106. The U-bent sensing portion 103 includes surface immobilized gold nanoparticles (AuNP) 104 deposited with a metal-organic framework (MOF-5) 105. A method to fabricate an integrated sensor probe 200 to determine lead ions [Pb(II)] in a fluid sample in preference to other heavy metal ions is also disclosed. The probe 100 determines Pb(II) ion concentration in a range of 0.5 ppb to 50 ppm. The probe is easy to use, facile, cost- effective and the method enables efficient on-site lead ion detection.

Description

FIBER OPTIC LEAD ION SENSORCROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims priority to Indian provisional Patent Application No. 202341049904 entitled FIBER OPTIC LEAD ION SENSOR fried on 24 July, 2023.FIELD OF THE INVENTION

[0002] The present invention generally relates to sensor device and more particularly relates to fiber optic sensor for heavy metal ion detection and methods thereof.BACKGROUND OF THE RELATED ART

[0003] Lead is a highly toxic heavy metal that has detrimental effects on both human health and the environment. Mining, smelting, manufacturing, recycling, and the use of lead in various products like plumbing materials, and paints are significant contributors to environmental contamination. Lead-acid batteries for motor vehicles account for over 75% of the world's lead consumption. Exposure to lead can cause severe health problems, including damage to the brain, nervous system, and kidneys, and can even lead to death. The maximum contaminant levels (MCL) for lead ions (Pb(II)) in water can vary depending on the regulations and country. The provisional guideline value set by the World Health Organization (WHO) and the Bureau of Indian Standards (BIS) is 10 ppb (parts per billion), whereas the Environmental Protection Agency (EP A) in the United States has set an action level of 15 ppb for lead in drinking water. Therefore, precise detection of heavy metal ions in water is of paramount importance owing to its detrimental effects on human health, especially with spectroscopically silent ions such as lead ions (Pb(II)). Developing sensitive and reliable methods for detecting lead ions in various environments is urgently needed considering the strict regulatory limits and harmful effects associated with this toxic element.

[0004] Conventional analytical methods like atomic absorption spectroscopy (AAS), anodic stripping voltammetry (ASV), and inductively coupled plasma mass spectroscopy (ICP-MS) are well-established for Pb(II) ion detection and offer a desirable limit of detection with high sensitivity and selectivity. However, these methods are expensive and involve sophisticated intricate steps that are time-consuming, labour-intensive, expensive,limiting their usage in the practical scenario. The need for affordable sensors for rapid Pb(II) ion detection has gained wide attention. The scientific literature on Pb(II) ion detection reveals the development of numerous sensing schemes with the help of chemical or bioreceptors anchored to a sensitive transducer to facilitate the selective capture and detection of the spectroscopically silent Pb(II) ions from the aqueous media. The receptors explored so far can be categorized into fluorescent dyes (e.g. rhodamine fluorescein), biomolecules (e.g. proteins, DNAzymes, oligonucleotides, etc.), nanomaterials (e.g. quantum dots, carbon dots, noble metal nanostructures, reduced graphene oxide, and metal -oxides). The transduction techniques reported for Pb(II) ion sensing mostly fall under colorimetry, electrochemical, ion-selective electrodes (ISE), Field-effect Transistor (FET), surface plasmon resonance, fluorescence or luminescence.

[0005] Among a variety of advanced materials as receptors, metal-organic frameworks (MOFs) have been recently explored for selective detection of Pb(II) ions. MOFs are a promising class of porous materials that have received considerable attention in recent years for their applications in gas storage, catalysis, and sensing. MOFs are composed of metal ions or clusters coordinated to organic ligands and exhibit unique properties, such as high surface area, tunable pore size, and chemical stability, enabling them as ideal candidates for the development of sensitive and selective sensors for various analytes, including heavy metal ions like lead, chromium, mercury, etc. In recent years, a few MOFs and their composites including MIL-101, UiO-66, functionalized ZIF-8, and functionalized MOF-5 have been explored for the Pb(II) ions in water for either removal (by coagulation, filtration, and chemical precipitation) or sensing by means of fluorescence and electrochemical techniques. For example: A highly-efficient sulfonated UiO-66(Zr) optical fiber is disclosed for rapid detection of trace levels of Pb2+ (https: / / doi.org / 10.3390 / ijms22116053). In another research, Au nanoparticle incorporated CuBTC MOFs is used for detection of Pb(II) ions (https: / / doi.org / 10.3389 / fchem.2020.00803). Yet another research discloses a turn-on luminescence probe based on amino-fimctionalized metal-organic frameworks for the selective detections of Cu2+, Pb2+ and pyrophosphate (https: / / doi.Org / 10.1016 / j.saa.2020.119073). Another research discloses a fiber-based surface plasmon resonance sensor for lead ion detection in aqueous solution (DOI: 10.1007 / sl 1468-020-01160-4). The Pb(II) ion interactions with functionalizedMOFs (with iron oxides, amines, or other chelating compounds such as EDA (ethylenediamine), melamine, sulphates, etc.), may involve either direct coordination of Pb(II) ions with the functional groups attached to the metal centres or electrostatic interactions with the organic ligands. However, potential interference due to chelation with many other metal ions is possible, giving rise to poor selectivity. A study highlights use of pristine MOF-5 to form a stable Pb-BDC (Pb-benzene dicarboxylic acid) complex in the aqueous medium with a significant morphological change in the MOF-5 structure (DOI: 10.1155 / 2016 / 8095737).

[0006] Refractive index (RI) based optical transduction techniques can be effectively utilized to quantify the morphological changes in MOF-5 receptors upon their interaction with Pb(II) ions to develop highly sensitive Pb(II) ion sensors. Several Rl-based platforms including Fabry Perot interferometer, long period grating (LPG), surface plasmonic resonance (SPR), or localized surface plasmonic resonance (LSPR) have been reported for Pb(II) ion sensing by utilizing optical fiber as the waveguide. For example, Nazari et al. reported a Fabry-Perot interferometric sensor coated with a functionalized MOF (SO3H- UiO-66(Zr)) to detect Pb(II) ion at ppm level.

[0007] However, the above-mentioned fiber optic probes nowhere disclose high RI sensitivity for the detection of minute refractive index changes in the microenvironment. The present disclosure addresses some of the drawbacks of conventional optical sensor systems and satisfies the need for a system that is simpler in construction, cheaper, more reliable, less prone to wear and tear, and can be a highly promising platform for water quality measurements on-site. Fiber optic sensor and methods are disclosed that may overcome the drawbacks in the existing art.

[0008] These and other advantages will be more readily understood by referring to the following detailed description disclosed hereinafter with reference to the accompanying drawing and which are generally applicable to other evaporators to fulfill particular applications illustrated hereinafter.SUMMARY OF THE INVENTION

[0009] According to one embodiment of the present subject matter, an integrated sensor probe irradiated by an optical source for detecting optical absorbance of lead ions (Pb(II)) in preference to other heavy metal ions in a sample using a photodetector is disclosed. Invarious embodiments, the probe includes a first distal end of a fiber optic probe configured to transmit optical input into a U-bent sensing portion. In various embodiments, the El- bent sensing portion having a sensing region with surface immobilized gold nanoparticles (AuNP) deposited with a metal-organic framework (MOF-5). In various embodiments, the sensing region is adapted to be in contact with the sample. In various embodiments, the probe includes a second distal end adapted to transmit optical output on selective binding of Pb(II) ions to MOF-5. The MOF-5 is configured to facilitate selective adsorption of Pb(II) ions from the sample thereby altering refractive index around gold nanoparticles and generating corresponding optical output.

[0010] In various embodiments, the sensing portion comprises an in-situ grown metalorganic framework (MOF-5). In various embodiments, the probe determines Pb(II) ion concentration in a range of 0.5 ppb to 50 ppm. In various embodiments, the probe has a sensitivity of 0.03 AA / log [Pb(II)] in ppb. In various embodiments, the probe yields a recovery in the range of 91% to 105%. In various embodiments, the probe has a working life of at least 3 months. In various embodiments, the AuNP surface is capped with a hydroxyl and carboxyl rich capping agent selected from tannic acid or citrate.

[0011] According to another embodiment of the present subject matter, a method to fabricate an integrated sensor probe to determine lead ions [Pb(II)] in a fluid sample in preference to other heavy metal ions is disclosed. In various embodiments, the method include the steps of providing a fiber optic probe with a first distal end, a second distal end and a pre-processed U-bent sensing portion to form a plasmonic U-FOS probe. This is followed by preparing and providing a metal-organic framework (MOF-5) growth solution. Next step includes incubating the plasmonic U-FOS probe in the MOF growth solution for 10 min to cause MOF-5 deposition thereon. This is followed by washing with deionized water, thereby forming a MOF-5-coated plasmonic U-FOS probe, wherein the MOF-5-coated plasmonic U-FOS probe is configured to selectively determine Pb(II) ion concentration within a fluid sample by immersion. In various embodiments, the fluid sample includes potable water or or sewage water or a pre-processed leachate solution.

[0012] In various embodiments, the pre-processing of U-bent sensing portion to form a plasmonic U-FOS probe includes cleaning the U-bent sensing portion by sonication in acetone followed by treating the cleaned U-bent sensing portion with piranha solution for 20 min at 60°C to oxidize organic contamination. This is followed by washing with deionized water. The next step includes dehydrating the probe at 115 °C for 1 h togenerate surface hydroxyl groups followed by surface modifying the probe to generate amine or thiol functional groups thereon for providing affinity to gold nanoparticles. The next step includes washing the probe thrice in ethanol followed by sonication for 3 mins and drying 100°C for 1 hr. Final step includes binding capped gold nanoparticles to the surface-modified probes to obtain the plasmonic U-FOS probe. In various embodiments, the optimum size of gold nanoparticles (AuNP) is 20 nm to 40 nm.

[0013] In various embodiments, preparing of MOF growth solution includes providing a transparent metal precursor by mixing zinc acetate prepared in dimethylformamide (DMF) followed by providing a transparent ligand solution by adding terephthalic acid and triethylamine (TEA) in DMF. This is followed by adding metal precursor and ligand solution in equal volumes. Next step includes mixing thoroughly to obtain a pale milky MOF growth solution. In various embodiments, the fluid sample includes potable water or sewage water or a pre-processed leachate solution.

[0014] In various embodiments, pre-processing of leachate solution includes diluting leachate to 10X with deionized water followed by adding 2% HNO3 to remove volatile substances and filtering through a syringe filter.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:

[0016] FIG. 1: illustrates a metal-organic framework (MOF-5) coated plasmonic FOS as lead (II) ion sensor probe.

[0017] FIG. 2A: a flow diagram of a method to fabricate sensor probe.

[0018] FIG. 2B: represents a method to prepare plasmonic U-FOS probe.

[0019] FIG. 2C: represents a method for MOF-5 in-situ deposition.

[0020] FIG. 3A: a graphical representation of EWA spectra for binding of 20 nm or 40 nm AuNP with either tannic acid or citrate capping agent to silanized U-FOS probes.

[0021] FIG. 3B: a graphical representation of EWA spectra for in situ MOF-5 growth over the respective AuNP coated probes.

[0022] FIG. 4A: a graphical representation of absorption spectral response from AuNP-t based PACS / MOF-5 probes exposed to sucrose solutions having different RI values from 1.333 to 1.348.

[0023] FIG. 4B: a graphical representation of Pb(II) ion detection using PACS / MOF-5 probes - Absorbance spectral response for Pb(II) ion concentrations from 250 ppb to 50 ppm.

[0024] FIG. 4C: a graphical representation showing influence of AuNP size and capping agent of PACS / MOF-5 on the Pb(II) ion detection.

[0025] FIG. 4D: a graphical representation of absorbance response from (i) bare silanized U-FOS (ii) U-FOS / MOF-5 (iii) bare PACS (iv) PACS / MOF-5 exposed to 10 ppm of Pb(II) ions.

[0026] FIG. 4E: represents an EDAX spectrum from the probes subjected to 10 ppm Pb(II) ions.

[0027] FIG. 4F: represents absorption spectra obtained from PACS probes to various sucrose solutions (RI = 1.333 to 1.348)

[0028] FIG. 5: a) graphical representation of online monitoring of the time-absorbance response during AuNP-t, MOF-5, and Pb(II) binding on the U-FOS, b) a graphical representation of EWA spectra of AuNP-t bound silanized U-FOS (with DI reference), c) a graphical representation of EWA spectra of DMF reference (solid) and subsequent PACS / MOF-5 (dotted), and d) a graphical representation of EWA spectra of DI reference (solid line) and Pb(II) bound PACS / MOF-5 (dotted line).

[0029] FIG. 6: a) SEM micrograph of the sensor probes- silanized U-FOS with 1 arb unit of AuNP-t / 20 (after wash), b) SEM micrograph of bare PACS probe exposed to MOF-5 growth solution for 10 min and washed, and c) SEM micrograph of PACS / MOF-5 probe exposed to 10 ppm of Pb(II) ion solution.

[0030] FIG. 7A: a graphical representation of temporal sensor absorbance response obtained from the PACS / MOF-5 probes at 570 nm for 10 min.

[0031] FIG. 7B: a graphical representation of dose-response curve obtained from the absorbance response from the sensor probes exposed to Pb(II) ion solutions of the concentrations of 0, 0.5, 1, 10, 100, 500, Ik, 5k, 10k, 25k, and 50k ppb prepared in DI water (for n > 3).

[0032] FIG. 8A: a graphical representation showing selectivity of PACS / MOF-5 probes at 1 ppm against 11 potential interfering metal ions at 50 ppm, quantified in terms of (AA565nm).

[0033] FIG. 8B: a graphical representation showing spectral absorption responses from PACS / MOF-5 probes subjected to 50 ppm of potential interfering metal ions and 1 ppm of Pb(II) ions.

[0034] FIG. 9: a graphical representation showing shelf-life evaluation studies for PACS / MOF-5 probes over 12 weeks by comparison of the probe sensitivity to 1, 10, and 100 ppb of Pb(II) ion concentrations.

[0035] FIG. 10A: showing UV-vis spectra of MOF-5 growth solution.

[0036] FIG. 10B: showing UV-vis spectra of [100 ppm] Pb (II) ion solution using UV- visible spectrophotometer.

[0037] Throughout the drawings, like numbers indicate like parts.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

[0039] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of "a", "an", and "the" include plural references. The meaning of "in" includes "in" and "on." Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

[0040] The present disclosure provides for an integrated sensor probe for detection of lead ions (Pb(II)) in preference to other heavy metal ions. The probe includes a photo detector with a U-bent sensing portion. Also, a method to fabricate an integrated sensor probe to determine lead ions [Pb(II)] in a fluid sample in preference to other heavy metal ions is disclosed.

[0041] In various embodiments, the invention includes an integrated sensor probe 100 for detection of lead ions (Pb(II)) in preference to other heavy metal ions. The probe is irradiated by an optical source 101 for detecting optical absorbance of lead ions (Pb(II)) in preference to other heavy metal ions in a sample 107 using a photodetector 106. The probe100 as shown in FIG. 1 includes, a fiber optic probe 102 with a U-bent sensing portion 103.

[0042] In various embodiments, the optical source 101 is configured to generate optical input to the sensor. In one embodiment, the optical sensor may be a LED. In various embodiments, the fiber optic probe 102 is configured to receive optical input from the optical source 101.

[0043] In various embodiments, the fiber optic probe 102 may have a U-bent shape. In various embodiments, the fiber optic probe 102 may include a U-bent sensing portion positioned between a first distal end 102A a second distal end 102B. In various embodiments, the first distal end 102A may be in optical communication with the source 101. The first distal end 102A may be configured to receive optical input from the optical source 101 and transmit optical input into the U-bent sensing portion 103.

[0044] In various embodiments, the U-bent sensing portion 103 is configured to be in contact with a sample 107. In various embodiments, the U-bent sensing portion 103 may include a sensing portion adapted to be in contact with the sample 107. In various embodiments, the sensing portion 103 may include an in-situ grown metal-organic framework (MOF-5). In various embodiments, the U-bent sensing portion 103 may include surface immobilized gold nanoparticles (AuNP) 104 deposited with a metalorganic framework (MOF-5) 105 . In various embodiments, the AuNP surface 104 is capped with a hydroxyl and carboxyl rich capping agent selected from tannic acid or citrate. In various embodiments, the MOF-5 105 may facilitate selective adsorption of Pb(II) ions from the sample 107. The selective absorption of Pb(II) ions from the sample enables alteration of refractive index around gold nanoparticles 104. In various embodiments, optical input from the first distal end 102A may be configured to excite the surface immobilized gold nanoparticles (AuNP) 104 and generation of corresponding optical output through the second distal end 102B.

[0045] In various embodiments, the second distal end 102B of the fiber optic probe 102 may be adapted to transmit optical output on selective binding of Pb(II) ions to MOF-5. In various embodiments, the second distal end is coupled to a photodetector 106. In various embodiments, the photodetector 106 may be configured to provide optical absorbance up on the selective binding of Pb(II) ions to MOF-5.

[0046] In various embodiments, the integrated sensor probe 100 may determine Pb(II) ion concentration in a range of 0.5 ppb to 50 ppm. In various embodiments, the probe 100 has a sensitivity of 0.03 AA / log [Pb(II)] in ppb. In various embodiments, the probe 100 yields a recovery in the range of 91% to 105%. In various embodiments, the probe 100 has a working life of at least 3 months.

[0047] In various aspects, the subject matter is a method to fabricate an integrated sensor probe 200 to determine lead ions [Pb(II)] in a fluid sample in preference to other heavy metal ions. The method as shown in FIG. 2A includes providing a fiber optic probe 210 with a first distal end, a second distal end and a pre-processed U-bent sensing portion to form a plasmonic U-FOS probe. Step 220 includes preparing and providing a metalorganic framework (MOF-5) growth solution. Incubating the plasmonic U-FOS probe in the MOF growth solution for 10 min to cause MOF-5 deposition thereon is included in step 230. In various embodiments, step 240 includes washing with deionized water, thereby forming a MOF-5-coated plasmonic U-FOS probe. In various embodiments, the MOF-5-coated plasmonic U-FOS probe is configured to selectively determine Pb(II) ion concentration within a fluid sample by immersion.

[0048] FIG. 2B represents the step of pre-processing 210 of U-bent sensing portion to form a plasmonic U-FOS probe. The pre-processing 210 of U-bent sensing portion includes the steps of cleaning 211 of the U-bent sensing portion by sonication in acetone followed by treating 212 the cleaned U-bent sensing portion with piranha solution for 20 min at 60°C to oxidize organic contamination. The next step 213 includes washing with deionized water. This is followed by dehydrating 214 the probe at 115 °C for 1 h to generate surface hydroxyl groups. Surface modification of the probe to generate amine or thiol functional groups thereon for providing affinity to gold nanoparticles is involved in step 215. Further, in step 216 washing the probe thrice in ethanol is followed by sonication for 3 mins and drying at 100°C for 1 hr. The pre-processing then involves binding of capped gold nanoparticles to the surface -modified probes to obtain the plasmonic U-FOS probe in step 218. In one embodiment, optimum size of gold nanoparticles (AuNP) is 20 nm to 40 nm.

[0049] FIG. 2C represents a method for MOF-5 in-situ deposition 220. The method 220 includes the step of providing 221 a transparent metal precursor by mixing zinc acetate prepared in dimethylformamide (DMF). In the next step 222, a transparent ligand solutionby adding terephthalic acid and triethylamine (TEA) in DMF is provided. This is followed by adding metal precursor and ligand solution in equal volumes in step 223. The in- situ deposition then involves mixing thoroughly to obtain a pale milky MOF growth solution in step 224.

[0050] In various embodiments, the fluid sample in step 240 includes potable water or or sewage water a pre-processed leachate solution. In various embodiments, the preprocessing of leachate solution includes diluting leachate to 10X with deionized water followed by adding 2% HNO3 to remove volatile substances. Next step includes filtering through a syringe filter.

[0051] Further, the integrated sensor probe 100 of the present invention is a high sensitive plasmonic absorption-based chemical sensor (PACS) for Pb(II) ion detection by exploiting the selective MOF-5 as a receptor over the plasmonically active U-FOS probes. The probe 100 of the present invention provides detection limits 10 times better than the permissible limit of 10 ppb and a wide dynamic range of 0.01 - 50 ppm. The probe 100 exhibits an excellent sensitivity suitable for on-site environmental monitoring. Further, the probe 100 demonstrates exceptional reproducibility with negligible loss in sensitivity over 12 weeks when stored under vacuum at room temperature. The probe 100 of the present invention manifest a high selectivity to Pb(II) ions over potential interferents at 50 times higher concentrations. The method 200 of the present invention has been experimentally validated and has demonstrated production of an extremely low-cost sensor probe 100 and lead detection. Moreover, with a simple optical source- photodetector based optoelectronic instrumentation, a facile and scalable deposition process, efficient sensor performance parameters, and shelf-life, the probe 100 and method 200 of the present invention offer great promise for on-site lead ion detection.

[0052] Although the detailed description contains many specifics, these should not be construed as limiting the scope of the invention but merely as illustrating different examples and aspects of the invention. It should be appreciated that the scope of the invention includes other embodiments not discussed herein. Various other modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the system and method of the present invention disclosed herein without departing from the scope of the invention as described here and as set forth in the claims attached herewith.EXAMPLESEXAMPLE 1: FABRICATION OF AN INTEGRATED SENSOR PROBE FOR DETECTING LEAD IONS (Pb(II)) IN PREFERENCE TO OTHER HEAVY METAL IONS

[0053] 1.1. Material And Equipment: Step-index multimode silica optical fibers (FT200UMT) of 200 pm core diameter and 0.39 numerical aperture, a silicon PIN photodiode (S150C, 350 - 1100 nm, 5 mW) with a USB console (PM100) for data acquisition were procured from Thorlabs Inc. USA. Lead nitrate, zinc acetate dihydrate, terephthalic acid, cobalt nitrate hexahydrate, lithium chloride, ferrous chloride, copper sulphate, potassium dichromate, calcium chloride, potassium permanganate, potassium chloride, cadmium chloride, nickel sulphate, magnesium sulphate, mercuric sulphate, and sucrose were procured from Sigma-Aldrich (Merck, India). Sulphuric acid (98%), hydrogen peroxide (30%), (3 -aminopropyl) trimethoxy silane (APTMS), and triethylamine (TEA) were procured from Fischer chemicals (Fischer Scientific, USA). Reagents including acetone, ethanol, and hydrogen chloride were of analytical grade. Gold nanoparticles (20 and 40 nm, each optical density = 1) capped with tannic acid and citrate were obtained from Ted Pella, Inc., USA, and BBI solutions, UK, respectively. Milli-Q deionized water (DI, resistivity 18.2 MQ at 25°C, pH 6.9) was used for all experiments. A tungsten-halogen light source (HL2000, 360 - 2400 nm) and a fiber optic spectrometer (USB4000-XR1-ES) were procured from Ocean Optics Inc. (now Ocean Insight), USA. A narrow band LED source driven by a constant current circuit (Roithner LED570-03), 570 nm, FWHM of 15 nm, 0.75 mW) was procured from ChemBioSens Pvt. Ltd. India.

[0054] 1.2. Fabrication of U-bent Fiber Optic Sensor (U-FOS) Probe: A 25 cm long optical fiber was cut from a fiber spool and the buffer over 1 cm in the middle of the fiber was removed mechanically using an optical fiber stripper. The U-FOS probes were fabricated using a CO2 laser-based automated bending machine. Briefly, the optical fiber on either side of the decladded portion was tightly held onto an articulating mechanism. A CO2 laser was fired to heat and soften the optical fiber core and the articulating arms were rotated inwards to bend the fiber and obtain a U-shape. Then the tightly held optical fiber was released from the articulating arms to retrieve the U-FOS probes. Thus, U-FOS probes with a bend diameter of 0.7 mm were fabricated. Subsequently, the bent region was exposed to a butane flame to deflagrate the remaining cladding (800 °C) and thensonicated in an acetone bath to wipe off any debris over the decladded bent region. Lastly, fiber polishing sheets with grits of 5 pm, 1 pm, and 0.3 pm were used to polish the distal ends of the probes to get smooth and flat ends for efficient optical coupling.

[0055] 1.3. Experimental Set-Up: For optical absorption spectral investigations, the U- FOS probes were connected between the tungsten-halogen light source (HL2000) and a fiber optic spectrometer (USB4000-XR1-ES). The absorbance spectra were recorded using Spectrasuite® software with an integration time of 20 ms and with a scan-to-average of 50 spectra. Further, to demonstrate Pb(II) ion sensing application, the sensor probes were coupled to a pair of an amber LED (at 570 nm) and a photodetector (S150C). The photodetector response was recorded with the help of the optical power meter utility software (version 5.9). The distal ends of the U-FOS probes were securely held using bare fiber adapters and multimode SMA905 connectors (Thorlabs Inc., USA).

[0056] 1.4. Preparation of Plasmonic U-FOS Probes: The U-FOS probes were cleaned by sonication in acetone (15 min, 1000-watt, 28 kHz) followed by piranha treatment (20 min, 60 °C) to oxidize any organic contamination and generate hydroxyl groups on the sensor surface. Then, the fiber probes were washed with deionized (DI) water and dehydrated for 1 h at 115 °C. For amino-silanization, the fiber probes were dipped in a 1% solution of APTMS in a 5:2 (v / v) mixture of ethanol and acetic acid (10 min). Finally, the fiber probes were washed thrice in ethanol, sonicated (3 min), and dried (100 °C, 1 h). To obtain LSPR active probes, the amino-silanized probes were dipped in 50 pL of gold nanoparticle solution and the evanescent wave absorbance (EWA) spectral response from the probes was monitored in real-time using the halogen lamp-spectrometer setup. The AuNP were allowed to adsorb to the amine functional groups on the probe surface until EWA at the plasmonic peak wavelength reaches 1.0 arb unit, as shown in FIG. 3A.

[0057] 1.5. In situ MOF-5 Deposition on Plasmonic U-FOS Probes: For the in situ MOF deposition onto the plasmonic U-FOS probes at room temperature, a modified method of MOF-5 synthesis using TEA was adopted. Zinc acetate (200 pM) prepared in dimethyl formamide (DMF) was used as the metal precursor. The ligand solution was prepared by the addition of terephthalic acid (668 mg) to 7 mL of DMF followed by 170 pL of TEA. The MOF growth solution (200 pL) was obtained by adding the transparent metal precursor and ligand solutions in equal volumes, followed by a thorough mixing resulting in a pale milky solution. The probes were immediately dipped into the MOFgrowth solution (pH of 8.9) and incubated for 10 min. Then the probes were washed in DI water to remove any loosely bound MOF and solvents, prior to exposing them to Pb(II) ion solutions. A transformation in the visual appearance of plasmonic probes from pink to a scaly thin-white shade was observed after MOF-5 deposition.

[0058] 1.6. Lead (II) Solutions and Pre-treatment of Field Samples: Stock solutions for lead and interfering ions at 100 ppm concentration were prepared in DI water. For example, 100 ppm of Pb(II) stock was prepared by adding 16 mg lead nitrate to -100 mL of DI water. For the dose-response studies, Pb(II) ion concentrations in the range of 1- 100000 ppb were prepared by serial dilution. The probes were dipped in a PCR tube containing a sample volume of 0.2 mL for all the experiments. MOF-5 coated U-FOS probes were also subjected to the leachate samples collected from a few dumpsites in southern India. The leachate samples were pre-processed using the following steps: (i) dilution (lOx) with DI water, (ii) filtration through 0.22 pm syringe filters, (iii) acidification by adding 2% cone. HN03 to remove any volatile substances. The samples were analyzed using ICP-MS (Argon plasma source; flow rate = 18 L / min, calibrated with multi-element standard solutions) to obtain Pb(II) ion concentrations for reference.

[0059] 1.7. Characterization Studies: The surface morphology of the MOF-5 layer on the fiber surface was studied using high-resolution scanning electron microscopy (HR- SEM) (Hitachi S4800, Japan) with an accelerating voltage of 5 kV. The bare plasmonic probes, MOF-5 coated probes with and without exposure to Pb(II) ions were washed in DI water and dried in a hot air oven for 10 min at 100 °C. The U-bent region of the prepared probes was adhered to a carbon tape over a stud. The samples over the stud were sputtered with gold (120s, Hitachi E-1010 ion sputter coater unit) to get a conductive layer. The energy-dispersive X-ray technique (EDAX) (Hitachi S4800 attached with an Oxford instrument X-Max 80 mm2 detector) was used to analyze the elemental composition to confirm the presence of Pb(II) ions in the MOF matrix. Average measurements were taken at three different points on three sensor probes. Optical absorption spectra of metal precursor and ligand solutions were recorded using a UV-vis spectrophotometer (Analytik Jena Specord Plus).

[0060] EXAMPLE 2: PERFORMANCE OF THE FABRICATED SENSOR PROBE: In order to design and develop a sensitive, dip-type optical absorbance-based sensor for Pb(II) ion detection, neither the Pb(II) ions nor the MOF-5 receptor chosen for theirselective entrapment - exhibit any considerable optical extinction in the UV-visible-NIR region (FIG. 10A and FIG. 10B). Thus, AuNP were chosen as an intermediate optically active nanomaterial to exploit its plasmonic absorbance properties for the detection of lead ions by means of the refractive index changes in its surrounding medium. To maximize the refractive index changes within the evanescent field of the surface plasmons of AuNP and thus the LSPR-based optical absorbance changes, the Pb(II) ion binding was confined to the vicinity of the AuNP surface by forming stable thin MOF-5 chemoreceptor films over the AuNP surface.

[0061] 2.1. PACS / MOF-5 Probe Development and Plasmonic Characterization of Probes: A novel technique for an in situ growth of MOF-5 confined to the surface of AuNP bound to a U-FOS probe (but not the silica surface of the optical fiber) for a sensitive detection of Pb(II) ions was developed. Given the affinity of MOF-5 to the surface carboxyl groups, a strategy to exploit tannic acid-capped AuNP (AuNP-t) as opposed to the conventional citrate-capped AuNP (AuNP-c) was adopted. In addition, the optimum size of AuNP was also investigated to achieve a maximum possible sensitivity. For this, AuNP of 20 and 40 nm with either citrate or tannic acid as capping agents were chosen to prepare PACS probes. Thus, four different kinds of AuNP were explored to realize plasmonic probes and assess their suitability for an in situ MOF-5 growth for selective detection of lead ions.

[0062] Amine functionalized U-FOS probes were incubated in each of the above- mentioned AuNP suspensions, and the EWA-based spectral response due to AuNP binding to the U-FOS probe surface was monitored in real-time using a halogen lamp - spectrometer-based set-up. The AuNP were allowed to bind to the probe until the peak absorbance response reached 1.0 arb units. The ESPR characteristic peak wavelength of the plasmonic absorption for 20 and 40 nm AuNP-t was observed at 526 and 533 nm respectively, while it was 529 and 536 nm for 20 and 40 nm AuNP-c respectively (FIG.3A). Table 1 lists the characteristic peak shift and full width at three-fourth maximum (FWTM) for PACS probes in comparison to that of the AuNP suspension (FIG.3B). Table 1 provides ESPR spectral characteristics including peak wavelength, full width at three-fourth maximum (FWTM), and red-shift obtained from AuNP suspensions, bare PACS probes, and MOF-5 coated PACS probes. No considerable deviation in the plasmonic characteristics of AuNP was observed, except for the LSPR peak shift (~3-5nm) which is atributed to the chemisorption of AuNP to the probe surface. The AuNP on the probe surface were uniformly coated without any aggregation as confirmed by the SEM images. Subsequently, these four different PACS probes were subjected to the freshly prepared MOF-5 growth solution in DMF for in situ deposition of MOF-5.Table 1: LSPR Spectral Characteristics obtained from AuNP suspensions, Bare PACS Probes, and MOF-5 Coated PACS Probes

[0063] The LSPR characteristics of the probes were exploited to carefully study the extent of MOF-5 deposition on the PACS probes and its dependence on the AuNP size and capping agent, as shown in FIG. 5. FIG. 5 a) shows change in optical absorbance through AuNP deposition, MOF-5 in situ growth and Pb(II) ion binding. Prior to MOF-5 deposition, these plasmonic probes in DI water were transferred into DMF, and their LSPR characteristic spectra were recorded with DI water as a reference FIG. 5 c). A change in the refractive index in the bulk environment (DI water (1.33 RI units) to DMF (1.43 RI units)) of the plasmonic probes resulted in LSPR peaks around 550-552 nm irrespective of the AuNP size (Table 1). A significantly large redshift over a wide range of 14 to 26 nm was observed due to the solvent effect. The probes coated with AuNP-t show a slightly higher redshift in comparison to that of AuNP-c as much as 3 and 5 nm in both DI water and DMF media respectively. This could be attributed to the higher molecular weight of tannic acid in comparison to citrate and hence a refractive index changes in the microenvironment around the AuNP surface.

[0064] Subsequently, the PACS probes were subjected to freshly prepared MOF-5 growth solution, and the LSPR characteristic absorption spectra at the end of 10 min were recorded with DMF as reference. FIG. 3B reveals the highest plasmonic absorbance due to MOF-5 deposition for 20 nm AuNP-t coated probes, followed by 40 nm AuNP-t and lowest for citrated capped AuNP. The SEM image of PACS with AuNP-t / 20 confirms the MOF-5 deposition. This result clearly demonstrates the importance of carboxylic acidgroups on the capping agent for MOF growth and successfully establishes the optimum conditions for MOF growth over the plasmonic probes. However, the absolute redshifts in the LSPR peak wavelengths for the four types of PACS probes after MOF-5 deposition were in a narrow range of 21 to 24 nm, indicating a minimal influence of the size of the AuNP over the redshifts. After the in situ deposition, when the MOF-5 coated probes were washed in DMF and transferred to DI water medium, no desorption was found from these probes during the washing steps, revealing the stable adhesion of MOF-5 to the AuNP. Interestingly, the PACS / MOF-5 probes were found to regain their LSPR absorption spectral characteristics that were matching with bare PACS probes (FIG. 5 b) and FIG. 5 d)), however with the corresponding absolute LSPR peak shifts due to MOF-5 deposition as given in Table 1.

[0065] 2.2. PACS / MOF-5 Probes for Pb(II) ion Sensing: Selective detection of Pb(II) ions using MOF-5 coated PACS based on refractive index changes at the AuNP surface was evaluated. Initial studies were carried out with PACS coated with AuNP-t / 20. Subsequently, the performance of all four types of PACS was compared to identify the optimum AuNP size and capping agent for MOF-5 based Pb(II) ion detection.

[0066] 2.2.1, Evaluation of RI Selectivity: The RI sensitivity of PACS / MOF-5 was investigated for its ability to detect the entrapment of Pb(II) ions in the MOF-5 matrix. To evaluate the LSPR-based RI sensitivity, the bare and MOF-5 coated PACS were subjected to sucrose solutions with increasing RI values (micro-RI solutions from 1.333 to 1.348), and the corresponding absorbance spectra were recorded (with DI water as reference). PACS / MOF-5 with AuNP-t / 20 shows a consistent increase in the optical absorbance at its plasmonic peak wavelength of 549 nm proportional to the RI value of the sucrose solutions FIG. 4A. An enhanced RI sensitivity of 21.92 A549nm / RIU for PACS / MOF-5 probes in comparison to the bare PACS (13.31 A526nm / RIU was found as shown in FIG. 4F. This result confirmed the ability of the PACS / MOF-5 to detect the RI changes in the vicinity of the AuNP, pointing to their potential for Pb(II) ion detection.

[0067] 2.2.2. Evaluation of LSPR Spectral Characteristics: LSPR spectral characteristics of PACS / MOF-5 prepared with AuNP-t / 20 were investigated in response to Pb(II) ion binding by subjecting them to various concentrations in the range of 0.25 to 50 ppm FIG. 4B. A further redshift in the characteristic LSPR peak wavelength to 565 nm was observed upon exposure to Pb(II) ions. This indicates a significant change in the RI ofthe microenvironment of AuNP, confirming the binding of Pb(II) ions to the MOF-5 matrix. The sensor probes show an increase in the absorbance at 565 nm proportional to Pb(II) ion concentration.

[0068] 2.2.3. Evaluation of Optimum AuNP Size And Capping Agent: To determine the optimum AuNP size and the capping agent for Pb(II) ion sensing, PACS / MOF-5 obtained using all the four types of AuNP including t / 20, t / 40, c / 20, and c / 40 were tested with Pb(II) ions between 1 and 10 ppm. The sensor responses illustrated in FIG. 4C show that AuNP-t / 20 based PACS provided the maximum sensitivity, which is almost 2-fold higher than the other AuNP types. This trend corroborates well with the LSPR spectral characteristics of these probes obtained after MOF-5 in situ deposition. Thus, AuNP-t / 20 based PACS / MOF-5 were chosen for all the subsequent studies.

[0069] 2.2.4. Determining Key Roles of AuNP And MOF-5 In Pb(II) Ion Sensing: To further establish the key roles of AuNP and MOF-5 in Pb(II) ion sensing, a control study was carried out with bare and MOF-5 coated U-FOS probes as well as bare and PACS probes. All the probes were subjected to a 10 ppm Pb(II) ion concentration for 10 min and the sensor response from each of these control probes was compared with PACS / MOF-5, as shown in FIG. 4D. No considerable absorbance response was observed from the control probes in comparison to PACS / MOF-5 probes. While some reports point to AuNP-based selective sensing of Pb(II) ions using either AuNP suspension or AuNP decorated sensor substrates, 13 we failed to observe any Pb(II) ion binding or desorption of AuNP from the PACS surface. This may be attributed to the fact that the AuNP are strongly bound to the U-FOS, where the influence of the positively charged Pb(II) ions capable of shifting the isoelectric point of AuNP suspension and thereby inducing aggregation or desorption is negligible.

[0070] 2.2.5. Evaluation Of Surface Morphology And Elemental Composition: Further, the surface morphology and elemental composition of the bare PACS prepared with AuNP-t / 20, after MOF-5 in situ deposition and after exposure to Pb(II) ions were analyzed using HR-SEM and EDAX techniques. FIG. 6 a) illustrates AuNP-t / 20 uniformly chemisorbed on the U-FOS probe surface without any aggregation. Similarly, the FIG. 6 b) demonstrates the in situ growth and deposition of MOF-5 mostly confined to the AuNP surface. Cubic structures of MOF-5 observed over the AuNP surface are in agreement with morphological studies reported elsewhere.41. However, a noticeablevariation in the size of the MOF-5 crystals was evident from the SEM images (FIG. 6 c)). This heterogeneity could be attributed to several factors including AuNP size and its crystalline phases, the density of the tannic acid capping agent, formation of MOF-5 crystals on an AuNP surface instead of solution-phase. The PACS subjected to Pb(II) ions show a significant change in the morphology of the MOF-5 structures FIG. 4E, mostly due to the formation of Pb-BDC complexes within the MOF-5 structures. The ED AX analysis of the MOF-coated PACS exposed to 50 ppm of Pb(II) ion concentration confirms the presence of Pb in its elemental composition FIG. 4E.

[0071] 2.2.6. Selectivity of PACS / MOF-5 Probe against Potential Interfering Metal Ions: The selectivity of PACS / MOF-5 was investigated with the help of 11 potential interfering heavy metal ions at 50 ppm. The respective absorption spectral responses were recorded FIG. 8B and the change in absorbance at 565 nm (AA565nm) was compared to evaluate the selectivity of the sensor. FIG. 8A illustrates the ability of the PACS / MOF-5 to selectively capture Pb(II) and give rise to a significant response. While the probes show only a minimal response to Cu(II), Mn(VII), As(II) and Li(I) at a higher concentration of 50 ppm (12.7, 6.8, 6.3, and 3.9 %, respectively with respect to [Pb(II)] at 1 ppm), these were immune to other important heavy metal ions tested here.

[0072] 2.3. Sensor Dose-Response and Field Sample Analysis: To demonstrate a highly sensitive PACS / MOF-5 based Pb(II) ion sensor with a limit of detection below the permissible limits in drinking water by WHO (10 ppb), a compact setup consisting of a narrowband LED (peak at 570 nm) and photodetector was utilized for the further experiments. The sensor response to Pb(II) ion concentration in the range of 0.5 ppb to 50 ppm in DI water was obtained in terms of optical intensity changes at 570 nm over 10 min. The temporal absorbance response for each of the above Pb(II) ion concentration solutions was calculated with respect to DI water reference and shown in FIG. 7A. The final absorbance value at the end of 10 min was marked for obtaining the sensor dose-response curve. The mean absorbance response (m) for 0 ppb of Pb(II) ion concentration was 0.000169 ± 0.000965 (n > 10). A variant (namely, the curve for 50 ppm) in the sigmoidal binding kinetics among the other Pb(II) ion concentrations was observed. The spectral FIG. 4B as well as the temporal absorbance response FIG. 7A obtained in the presence of 50 ppm of Pb(II) ion concentration articulates an alternative binding mechanism, as noted in the case of allosteric binding kinetics.44 This additionally boosted the PACS / MOF-5matrix’s capability to adhere more Pb(II) ions even at higher concentrations after the Pb- BDC complex formation. FIG. 7B depicts the dose-response of the PACS / MOF-5. Given the sigmoidal nature of the dose-response curve, two linear regression fits, one each in the [Pb(II)] range of [0.5 - 50 ppb] and [0.05 - 50 ppm] were applied to determine the sensor parameters including sensitivity and detection limits. From the linear regression fit for the lower range of Pb(II) ion concentrations, the sensitivity (S) was found to be 0.031 AA / log ([Pb(II)]) in ppb. The limit of detection (LoD) and limit of quantification (LoQ) calculations, based on 3 o / S and 10 o / S criteria (o = standard deviation of the sensor response to the control sample), were obtained as 0.09 and 0.31 ppb respectively. The experimental evidence to detect Pb(II) ion concentration down to 0.5 ppb is demonstrated in FIG. 7B, which is 20-fold lower than the permissible limit of 10 ppb.

[0073] Leachate Sample Testing Using Fabricated PACS / MOF-5 Probe: Performance After the successful realization of a highly sensitive MOF-5 coated PACS for Pb(II) ion detection, the PACS / MOF-5 probes were challenged with leachate samples collected from three different industrial locations. The samples were pre-treated and tested with the probes as described in Section 1.6. The estimated Pb(II) ion concentration values were compared with the standard measurements obtained from ICP-MS. In addition, the probes were also subjected to the samples spiked with 10 ppb of Pb(II) ions. Table 2 shows the Pb(II) ion concentrations obtained from ICP-MS and estimated from the calibration curve FIG. 7B along with the recovery rates (n>3). The over-estimated responses from the 2nd sample site may be due to the interference from coloured compounds in the sample (dyes, coloured metal species other than those used in this study, etc.). The mean recoveries in the range of 95.4 - 104.75 % suggest that the developed PACS / MOF-5 probes could be highly promising for on-site Pb(II) ion detection.Table 2: Leachate Sample Testing using PACS / MOF-5 Probes| Sample 3 - spiked | 10 | - | 17,72 ± 0,035 | 96,1 |’Drinking water collected from RO-purifier

[0074] 2.4. Shelf-Life Studies: The sensor probes were tested for their shelf-life for ~3 months. The PACS / MOF-5 probes were stored in a vacuum desiccator under moisture- free conditions and tested at an interval of 2 weeks. The stored sensor probes were exposed to 1, 10, and 100 ppb of Pb(II) ion solutions for 10 min. The sensor responses were recorded, and the sensitivity of the probes was calculated and compared with that of the freshly prepared probes. The stored probes exhibit no significant variation or loss in the sensitivity over 12 weeks as shown in FIG. 9. However, a relatively appreciable standard deviation (0.3 - 4%) in the sensitivity was observed. This is attributed to the plasmonic nature of the PACS platform combined with the multiple steps involved during U-FOS preparation, thereby resulting in the noted variations. Overall, these results demonstrated the reliable and stable nature of the PACS / MOF-5 probes for consistent sensor performance.

[0075] 2.5. Comparative Study Of Sensing Performance Of Fabricate Probe With Other Reported And Commercial Pb(II) Ion Sensors: The sensing performance of the developed Pb(II) ion sensor was compared with the recently reported optical sensing strategies and commercially available ones. The commercial Pb(II) ion sensors have been thoroughly reviewed by Dore, et al. 2021 Table 3. These sensors work on either colorimetry, fluorescence, or electrochemical (anodic stripping voltammetry) techniques. While the dynamic range (from 2, up to 500 ppb) and response times are comparable to the current study, the sample volumes (5 to 100 mL) and number of sample processing steps (3 to 20) are higher compared to 0.2 mL and a single filtration step required for PACS based detection.Table 3: Commercially Available Field Sensors for Lead Ion Detection (Dore et. al 2020)* https: / / doi.org / 10.1080 / 10643389.2020.1782654

[0076] Table 5 compares the performance of the PACS / MOF-5 sensor with the various optical sensing strategies for Pb(II) ion detection reported in the literature, especially those that have utilized (i) a variety of receptors including glutathione, black phosphorus, MOFs, and its composites, noble metal nanoparticles, carbon nanomaterials, etc., or (ii) optical fibers as the transducer, prioritizing the studies that demonstrated better LoDs. A detailed comparison of the studies with respect to the selectivity and the ease of fabrication of these sensors is given in Table 5. The sequence of references present in Table-4 and Table-5 is the same.Table 4. Comparison of The Selectivity Efficiency And Ease Of PACS / MOF-5 Fabrication With That Of The Reported Optical Techniques (In Table 5)Selectivity: <10= +++, <50 = ++ , >50 = +

[0077] While the DNAzyme receptor-based sensors exhibit excellent sensitivity and LoDs down to the ppt level, the LoD in our study is below the MCL and meets the requirements for reliable water quality monitoring. The selectivity score for PACS / MOF-5 obtained based on the ratio of sensor responses from heavy metal interferents to Pb(II) ions is significantly lower among the reported studies. On the other hand, the U-FOS probes used to realize the PACS / MOF-5 offer outstanding advantages over other optical waveguide transducers such as fiber gratings, including microfluidics-free and scalable fabrication processes and dip-type sensing. In addition, the PACS / MOF-5 based Pb(II) ions sensing involves a simpler and less expensive LED-PD based instrumentation as opposed to the optical spectrum analyzer, spectrophotometer, or fluorimeter required for the reported studies.Table 4. Comparison With Other Previously Reported Optical Pb(II) Ion Sensors

Claims

We claim:

1. An integrated sensor probe (100) irradiated by an optical source (101) for detecting optical absorbance of lead ions (Pb(II)) in preference to other heavy metal ions in a sample using a photodetector (106), the probe comprising: a first distal end (102A) of a fiber optic probe (102) , configured to transmit optical input into a U-bent sensing portion (103); the U-bent sensing portion (103) having a sensing region with surface immobilized gold nanoparticles (AuNP) (104) deposited with a metal-organic framework (MOF-5) (105), the sensing region adapted to be in contact with the sample (107), ; a second distal end (102B) adapted to transmit optical output on selective binding of Pb(II) ions to MOF-5; wherein, the MOF-5 (105) is configured to facilitate selective adsorption of Pb(II) ions from the sample (107), thereby altering refractive index around gold nanoparticles (104) and generating corresponding optical output.

2. The sensor probe (100) as claimed in claim 1, wherein the sensing portion comprises an in-situ grown metal-organic framework (MOF-5).

3. The sensor probe (100) as claimed in claim 1, wherein the probe determines Pb(II) ion concentration in a range of 0.5 ppb to 50 ppm.

4. The sensor probe (100) as claimed in claim 1, wherein the probe has a sensitivity of 0.03 AA / log [Pb(II)] in ppb.

5. The sensor probe (100) as claimed in claimed 1, wherein the probe yields a recovery in the range of 91% to 105%.

6. The sensor probe (100) as claimed in claim 1, wherein the probe has a working life of at least 3 months.

7. The sensor probe (100) as claimed in claim 1, wherein the AuNP surface (104) is capped with a hydroxyl and carboxyl rich capping agent selected from tannic acid or citrate.

8. A method to fabricate an integrated sensor probe (200) to determine lead ions [Pb(II)] in a fluid sample in preference to other heavy metal ions, the method comprising the steps of: providing a fiber optic probe (210) with a first distal end, a second distal end and a pre- processed U-bent sensing portion to form a plasmonic U-FOS probe; preparing and providing a metal-organic framework (MOF-5) growth solution (220); incubating (230) the plasmonic U-FOS probe in the MOF growth solution for 10 min to cause MOF-5 deposition thereon; and washing (240) with deionized water, thereby forming a MOF-5-coated plasmonic U-FOS probe, wherein the MOF-5-coated plasmonic U-FOS probe is configured to selectively determine Pb(II) ion concentration within a fluid sample by immersion.

9. The method as claimed in claim 8, wherein pre-processing (210) of U-bent sensing portion to form a plasmonic U-FOS probe includes: cleaning (211) the U-bent sensing portion by sonication in acetone; treating (212) the cleaned U-bent sensing portion with piranha solution for 20 min at 60°C to oxidize organic contamination; washing (213) with deionized water; dehydrating (214) the probe at 115 °C for 1 h to generate surface hydroxyl groups; surface modifying the probe (215) to generate amine or thiol functional groups thereon for providing affinity to gold nanoparticles; washing (216) the probe thrice in ethanol followed by sonication for 3 mins and drying 100°C for 1 hr; and binding (218) capped gold nanoparticles to the surface-modified probes to obtain the plasmonic U-FOS probe.

10. The method as claimed in claim 9, wherein optimum size of gold nanoparticles (AuNP) is 20 nm to 40 nm.

11. The method as claimed in claim 9, wherein preparing (220) of MOF growth solution includes: providing (221) a transparent metal precursor by mixing zinc acetate prepared in dimethylformamide (DMF); providing (222) a transparent ligand solution by adding terephthalic acid and triethylamine (TEA) in DMF; adding (223) metal precursor and ligand solution in equal volumes; and mixing (224) thoroughly to obtain a pale milky MOF growth solution.

12. The method as claimed in claim 8, wherein the fluid sample in step (240) includes potable water or sewage water or a pre-processed leachate solution.

13. The method as claimed in claim 13, wherein pre-processing of leachate solution includes: diluting leachate to 10X with deionized water; adding 2% HNO3 to remove volatile substances; and filtering through a syringe fdter.

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