FIELD OF INVENTION

This invention relates to the preparation of silver quantum cluster embedded in organic-templated-boehmite-nanoarchitecture (OTBN) and its use as a colour changing sensor in the visible light for assessing the quantity of water passed through a water purification device.

BACKGROUND

The objective of providing safe and affordable drinking water is a global mission and it is eloquently articulated in United Nations Millennium Development Goal 2015, United Nations General Assembly resolutions (64/292 and 65/154) and article 47 of Indian Constitution. A major contribution to this can be made by providing an affordable safe drinking water at point-of-use, which is so far restricted largely due to un-availability of eco-conscious technology.

For the past several years, we have been actively working on developing novel materials for an affordable and all-inclusive water purifier. Such a purifier removes a broad range of contaminants: pesticides (Indian patent 200767, US patent 7968493), microorganisms (Indian patent 20070608, Indian patent applications 947/CHE/2011, 4300/CHE/2011), fluoride (Indian patent applications 2089/CHE/2009, 1529/CHE/2010, 4062/CHE/2011) and heavy metals (Indian patent applications 169/CHE/2009, 2433/CHE/2010, 2563/CHE/2010). The water purification device is further described in Indian patent applications 2892/CHE/2010 and 1522/CHE/2011.

An important aspect of a water purifier is to ensure delivery of quality output water throughout the stated life of the purifier. Usually it is quite difficult for a consumer to keep a note of volume of water passed through a water purifier. Unlike other consumer goods such as refrigerator, washing machine, etc., water purifier may still continue to function even though its performance may have shrunk significantly. The quality of output water directly relates to the health of consumer. Hence, it is necessary to ensure a reasonable check on the output water quality.

As it would be evident from prior art that such a check on the output water quality is typically enforced using a flow meter which measures the volume of water passed.

Owing to lack of actual water quality measurement at field, it is a first line of defense for output water quality. However, as it is well known that water quality across India varies significantly due to which the performance of water purification device also varies.

Hence, it is important to have a second line of defense as simultaneous measurement of volume of water passed along with the input water quality. Depending on the input water quality, the measurement of volume of water should indicate if the water purification device is exhausted. This is an important premise of the invention articulated in this application.

We articulate a novel composition based on silver quantum clusters embedded in organic-templated-boehmite-nanoarchitecture (OTBN, 1529/CHE/2010) which when used in a water purification device, undergoes a gradual visible light colour change upon passage of water. The change in colour of the material progresses steadily from pink to black, upon passage of over 250 L of water. We utilize this property to demonstrate a colour-based flow meter which can be integrated with the water purifier to indicate its status.

Considering that it is very important to ensure that users of a water purifier know the remaining useful life of a purification cartridge, this invention is an important technical improvement in a water purifier.

PRIOR ART

Prior art on quantum clusters

Quantum clusters of noble metals are a class of new materials which are less than 1 nm in core dimension, nearly equal to Fermi wavelength of an electron (~0.5 nm for silver, M. A. H. Muhammed, T. Pradeep, in Advanced fluorescence reporters in chemistry and biology II: Molecular constructions, polymers and nanoparticles, Alexander P. Demchenko (ed.), 2010, Springer, Heidelberg). These are distinctly different from nanoparticles. In them band structure breaks into discrete energy levels, they have very high confinement in electronic structure, they exhibit molecular properties such as luminescence and plasmon resonance usually found with nanoparticles is absent. Due to these properties, quantum clusters have new utility in several applications such as optical storage, biological labels, catalysis, sensors, magnetism, optical absorption tunability, etc.
Sensitivity of clusters to metal ions were reported by our group (Reactivity of Au25 clusters with Au3+, M. A. Habeeb Muhammed, T. Pradeep, Chem. Phys. Lett., 2007, 449, 186-190). Fluorescent clusters are used as sensitive and easy probes for heavy metal ions in environmental samples such as pond water and soil by fluorescent turn-on mechanism (G.-Y. Lan, C.-C. Huang, H.-T. Chang, Chem. Commun., 2010, 46, 1257-1259). A new class of water soluble silver clusters with high two-photon excitation cross-section providing tunability in excitation and emission wavelengths can be used as highly sensitive biolabels (S. A. Patel, C. I. Richards, J.-C. Hsiang, R. M. Dickson, J. Am. Chem. Soc, 2008, 130, 11602-11603). DNA sequences templated silver clusters have been synthesized which can be tuned for fluorescence emission wavelength by varying the DNA template, implying useful biological applications (J. Sharma, H.-C. Yeh, H. Yoo, James H. Werner, J. S. Martinez, Chem. Commun., 2010, 46, 3280-3282).

Properties of water soluble fluorescent silver clusters can be varied by adopting different synthetic routes and their stabilizing polymer ligand (H. Xu, K. S. Suslick, Adv. Mater., 2010, 22, 1078-1082). Water-soluble Ag-thioflavin T nanoclusters has been demonstrated for use in tracking of ultrasensitive biological assays both in vitro and in vivo (N. Makarava, A. Parfenov, I. V. Baskakov, Biophys. J., 2005, 89, 572-580). An important biological analyte, cysteine can be sensed at low concentration by poly(methacrylic acid) templated silver clusters with specific fluorescent quenching mechanism (L. Shang, S. Dong, Biosens. Bioelectron, 2009, 24, 1569-1573). Quantum optoelectronic logic operations can be created with electroluminescence of individual silver nanoclusters at room temperature (T.-H. Lee, J. I. Gonzalez, J. Zheng, R. M. Dickson, Ace. Chem. Res., 2005, 38, 534-541). DNA-encapsulated Ag nanoclusters exhibit high fluorescence in the near IR, enabling a single-molecule-specific bunching feature (T. Vosch, Y. Antoku, J.-C. Hsiang, C. I. Richards, J. I. Gonzalez, R. M. Dickson, PNAS, 2007, 104, 12616-12621). Metal oxide supported silver quantum clusters are used as a catalyst (A. Leelavathi, T. U. B. Rao, T. Pradeep, Nanoscale Res. Lett, 2011, 6, 123-132).

Dehydrogenation of alcohols to carbonyl compounds by supported silver clusters has also been reported (K. Shimizu, K. Sugino, K. Sawabe, A. Satsuma, Chem. Eur. J. 2009, 15, 2341-2351). Alumina supported silver clusters have been used for direct amide synthesis from aicohols and amines with high selectivity (K. Shimizu, K. Ohshima, A. Satsuma, Chem. Eur. J. 2009, 15, 9977-9980). Poly(methacrylic acid) stabilized silver nanoclusters respond to the environment by having solvatochromic and solvato-fluorochromic (i.e., absorption and emission properties) responses useful for molecular sensing (I. Diez, M. Pusa, S. Kulmala, H. Jiang, A. Walther, A. S. Goldmann, A. H. E. Muller, O. Ikkala, R. H. A. Ras, Angew. Chem. Int. Ed. 2009, 48, 2122-2125).

Poly(methacrylic acid) stabilized silver nanoclusters prepared by sonochemical method can be used for bioimaging, chemical and biosensing, single-molecule studies, and possibly catalysis (H. Xu, K. S. Suslick, ACS Nano, 2010, 4, 3209-3214). Sub-nanometer clusters are used as Raman labels to identify true chemical information about single molecules (L. P.-Capadona, J. Zheng, J. I. Gonzalez, T.-H. Lee, S. A. Patel, R. M. Dickson, Phys. Rev. Lett., 2005, 94, 058301). Silver clusters synthesized by micro-emulsion method display paramagnetic behavior (A. L.-Suarez, J. Rivas, C. F. R.-Abreu, M. J. Rodriguez, E. Pastor, A. H.-Creus, S. B. Oseroff, M. A. L.-Quintela, Angew. Chem. Int. Ed., 2007, 46, 8823-8827). Water soluble fluorescent sliver clusters have also been used for metal ion sensing (K. V. Mrudula, T. U. B. Rao, T. Pradeep, J. Mater. Chem., 2009, 19, 4335-4342; B. Adhikari, A. Banerjee, Chem. Mater., 2010, 22, 4365).

Our research group has studied silver quantum clusters from various perspectives:

synthesis (various kinds of molecular clusters), characterization and utility (sensing and catalysis). Several other applications such as metal ion sensing and cell imaging were done with gold clusters as well. A representative list for silver clusters is given herewith:

Synthesis

(i) Ag7Au6: A 13 atom alloy quantum cluster, T. U. B. Rao, Y. Sun, N. Goswami, S. K. Pal, K. Balasubramanian, T. Pradeep, Angew. Chem. Int. Ed., 2012, 51, 2155-2159

(ii) Conversion of double layer charge-stabilized Ag@citrate colloids to thiol passivated luminescent quantum clusters, L. Dhanalakshmi, T. U. B. Rao, T. Pradeep, Chem. Commun., 2012,48,859-861

(iii) A fifteen atom silver cluster confined in bovine serum albumin, A. Mathew, P. R. Sajanlal, T. Pradeep, J. Mater. Chem., 2011, 21, 11205-11212

(iv) Ag9 quantum cluster through a solid state route, T. U. B. Rao, B. Nataraju, T. Pradeep, J. Am. Chem. Soc, 2010, 132, 16304-16307

(v) Luminescent Ag7 and Ag8 Clusters by interfacial synthesis, T. U. B. Rao, T. Pradeep, Angew.

Chem. Int. Ed., 2010, 49, 3925-3929

Characterization

(i) First principle studies of two luminescent molecular quantum clusters of silver, Ag7(H2MSA)7 and Ag8(H2MSA)8 based on experimental fluorescence spectra, Y. Sun, K. Balasubramanian, T. U. B. Rao, T. Pradeep, J. Phys. Chem. C, 2011,115,.42, 20380-20387

Utility

(i) Supported quantum clusters of silver as enhanced catalysts for reduction, A. Leelavathi, T. U. B. Rao, T. Pradeep, Nanoscale Research Letters, 2011, 6, 123-132

(ii) Investigation into the reactivity of unsupported and supported Ag7 and Ag8 clusters with toxic metal ions, M. S. Bootharaju, T. Pradeep, Langmuir, 2011, 27, 8134-8143 (iii) Luminescent sub-nanometer clusters for metal ion sensing: a new direction in nanosensors, I. Chakraborty, T. U. B. Rao, T. Pradeep, J. Haz. Mater., 2012, 211-212, 396-403

Prior art on clean drinking water

An important objective of providing clean and affordable drinking water to masses is to ensure delivery of pure water at the point-of-use. Ensuring the consumption of clean drinking water would facilitate realization of the fundamental right to life, of which clean water is a recognized component. This is also an important component of the United Nations Millennium Development Goal 2015.

In order to ensure quality drinking water at point-of-use, there are two possible approaches technologically. First is to develop an affordable sensor for detection of trace concentrations of drinking water contaminants, especially microorganisms. This approach is still under development at various research laboratories across the world. Second is to integrate a flow meter with a rigorously tested water purifier having a known life. Flow meter will tell the user when the known life of the water purifier is over and consumables such as the cartridge require a change. Indeed, the first approach is more reliable; however, since the technologies are still under development, it is wise to look at flow meters till a reliable solution is ready.

It is also to be noted that gravity-fed storage water purifiers can't operate with typical flow meters due to unavailability of high pressures (P < 0.5 psi). In such cases, a few approaches have been reported for the detection of volume of water passed.

Ahmad et al. in WO 2011/013142 have reported the use of a mechanical device along with a tablet made of sparingly water soluble salts. Intent is to have the tablet slowly dissolve upon passage of pre-determined volume of water. Once the tablet is dissolved, a mechanical action is initiated which blocks the flow of the liquid.

Another attempt is reported by Jambekar et al. in WO 2007/144256, wherein the biocide used is sparingly soluble in water and upon its dissolution, a mechanical action initiates the closure of water flow.

Ehara et al. in US patent number 5458766 have utilized battery along with a LED for determination of lifetime of the filter. Williams et al. in US patent number 7249524 have used an impeller device as a sensor for determining the flow and volume of water passing through the cartridge. Larkner et al. in US patent number 6585885 have reported a water purification system containing a sensing element coupled with an electronic control for accurately indicating the volume of water. Butts et al. in US patent number 4918426 have reported an in-line filter consisting of a flow meter with no moving parts to measure the total volume of the fluid filtered. Chai et al. in US patent 7107838 have reported a water filter consisting of an electrode pair for sensing volume of the water dispensed. Guess et al. in US patent number 6613236 have used a tri-colour LED emission for indicating volume of the water passed through the filter.

This invention reports the detection of volume of water passed through a water purification device, by use of a novel composition which undergoes change in the colour upon continuous interaction with salts usually found in drinking water. The aspect of colour change in nanomaterial, especially noble metal nanoparticles, upon interaction with ionic salts is well-studied. The conclusion from prior art is that nanoparticles undergo instant aggregation upon exposure to mild concentration of salts. This is due to the reduction in surface energy of metal nanoparticles upon interaction with the counter ion. Usually, the aggregation of metal nanoparticles, especially silver, is almost instantaneous at salt concentrations of 100 ppm and above.

Therefore, the objective of the present invention is to synthesize silver clusters in the OTBN matrix so that silver surface is well-protected from the attack of common ions found in drinking water.

Another object of the invention is to device a low cost visible sensor for the volume of water passed through the cartridge so as to detect the lifetime of the purifier.

Another object of the invention is to utilize the changes in colour in the absorbed visible light with volume of water passed, as an indicator of lifetime of the water purification cartridge.

Another object of the invention is to utilize the changes in luminescence in the absorbed UV light with volume of water passed, as an indicator of lifetime of the water purification cartridge.

SUMMARY OF THE INVENTION

The present invention describes the art of in-situ synthesis of silver quantum clusters in Oiganic-templated-boehmite-nanoarchitecture (Ag QCs-OTBN). The present invention describes the utility of OTBN matrix in protecting the silver quantum clusters. The present invention describes a method to synthesize silver quantum clusters which have long shelf life in ambient condition and do not deteriorate quickly in typical ground water. The present invention describes the utility of Ag QCs-OTBN for detecting the volume of water contacted with the material through a visible/UV colour change.

DESCRIPTION

DESCRIPTION OF THE INVENTION

The novelty of the composition reported here is in the aspect of embedding the silver clusters in a nanoarchitecture matrix which enables protection of silver surface from various ions present in ground water.

In this invention, we describe the synthesis, characterization and application of silver quantum clusters impregnated organic-templated-boehmite-nanoarchitecture (Ag QCs-OTBN). The as-synthesized Ag QCs-OTBN composition is characterized by a number of spectroscopic and microscopic techniques. We demonstrate the utility of Ag QCs-OTBN as a visible sensor of quantity of water passed through a water purification device.

The novelty of the composition reported here is that the visible sensor based on Ag QCs-OTBN not only assesses the volume of water passed as a mechanical flow meter does; it assesses the lifetime of a cartridge based on the input water quality. A measure of the input water quality can be taken as ionic strength of the input water.

In one embodiment, the present invention describes that the visible colour change of the Ag QCs-OTBN from pink to black does not happen after a defined volume of any input water is passed. The colour change will happen in a reduced volume of water if TDS of the input water is Efficiency of adsorption based removal of contaminants depends on ionic composition of input water. Interfering ions in the water are known to reduce the capacity/lifetime of the adsorption based filters. Therefore lifetime of the filter will be drastically reduced from the expected capacity if high ionic strength input water is passed. Hence, for any adsorption based filter, it is very important to have a lifetime sensor which works based on input water quality. The present invention describes such a colour changing sensor in detail.

Experimental methods Material characterization

The identification of the phase(s) of the as-prepared sample was carried out by X-ray powder diffraction (Bruker AXS, D8 Discover, USA) using Cu-Ka radiation at A = 1.5418 A. Surface morphology, elemental analysis and elemental mapping studies were done using a Scanning Electron Microscope (SEM) equipped with Energy Dispersive Analysis of X-rays (EDAX) (FEI Quanta 200). For this, sample in the gel form was re-suspended in water by sonication for 10 min and drop casted on an indium tin oxide (ITO) conducting glass and dried. High Resolution Transmission Electron Microscopy (HRTEM) was done using JEM 3010 (JEOL, Japan). The samples were spotted on amorphous carbon coated copper grids and dried at room temperature. FT-IR spectra were measured using Perkin Elmer Spectrum One instrument and KBr crystals were used as the matrix for preparing samples. Luminescence measurements were carried out by using Jobin Vyon NanoLog instrument. The band pass for excitation and emission was set as 2 nm.

The accompanying examples and figures and examples, which are incorporated in and constitute a part of this specification, illustrate several aspects and together with the description serve to explain the principles of the invention. This should, however, not be construed as limiting the scope of the invention.

Example 1

This example describes the in-situ preparation of silver quantum clusters protected by glutathione in the OTBN gel. OTBN was prepared as reported in the previous patent application (1529/CHE/2010). The filtered OTBN gel was used as a matrix for in-situ preparation of silver quantum clusters. The prepared OTBN gel was re-suspended in water, to which silver precursor (silver nitrate, silver fluoride, silver acetate, silver permanganate, silver sulfate, silver nitrite, silver salicylate or a combination thereof) was added drop-wise. The percentage of silver loading in OTBN gel was 3%. After stirring the gel for an hour, surface protecting agent (glutathione) was added drop-wise; then the solution was allowed to stir for an hour. Sodium borohydride was added drop-wise to the above solution at ice-cold condition (molar ratio of silver precursor to reducing agent ratio was 1:4). Then the solution was allowed to stir for an hour, filtered and dried at room temperature (28 °C).

Example 2

The method described in example 1 was modified to prepare the glutathione protected fluorescent silver quantum clusters in the OTBN gel material. Silver to glutathione ratio was varied from 1:1 to 1:10.

Example 3

The method described in example 1 was modified to prepare the glutathione protected fluorescent silver quantum clusters on OTBN gel material with various molar ratios of silver to sodium borohydride such as 1:4 and 1:8.

Example 4

The method described in example 1 was modified to prepare clusters with different surface protecting agents like mercaptosuccinic acid, polyvinyl pyrrolidone and trisodium citrate in OTBN gel.

Example 5

This example describes the in-situ preparation of silver quantum clusters protected with glutathione on the OTBN powder. The dried OTBN powder was crushed to a particle size of 100-150 urn. The powder was shaken in water using a shaker to which silver precursor (silver nitrate, silver fluoride, silver acetate, silver permanganate, silver sulfate, silver nitrite, silver salicylate or a combination thereof) was added drop-wise. The percentage of silver loading in OTBN powder was 3%. After shaking the dispersion for an hour, glutathione was added drop wise; then the dispersion was shaken for an hour. Sodium borohydride was added drop-wise to the above dispersion at ice-cold condition (molar ratio of silver to reducing agent ratio was 1:4). Then the dispersion was shaken for an hour, filtered and dried at room temperature (28 °C).

Example 6

This example describes the preparation of silver quantum clusters in a variety of chitosan-metal oxide/hydroxide/oxyhydroxide composite gels. The metal oxide/hydroxide/oxyhydroxide can be based on aluminum, iron, titanium, manganese, cobalt, nickel, copper, silver, zinc, lanthanum, cerium, zirconium or a combination thereof. The synthetic procedure for such a composition is as follows: the chosen salt solution was added slowly into the chitosan solution (dissolved in 1 -5 % glacial acetic acid or HCI or combination thereof) under vigorous stirring for 60 minutes and kept overnight at rest. Aqueous ammonia or NaOH solution was added slowly into the metal-chitosan solution under vigorous stirring to precipitate the metal-chitosan composites. These gels were used as matrices for the in-situ preparation of ligand protected silver quantum clusters.

Example 7

This example describes the preparation of fluorescent silver quantum clusters on magnetic materials. Superparamagnetic Fe304 was prepared by method as reported in prior art (M.T. Lopez-Lopez, J.D.G. Duran, A.V. Delgado, F. Gonzalez-Caballero, J. Colloid Interface Sci., 2005, 291, 144-151). Freshly prepared superparamagnetic particles were added to the chitosan solution, allowed to stir for 2 h, precipitated at pH 9 using NaOH or aqueous ammonia and filtered to remove the salt contents. Superparamagnetic composite was re-suspended in water, to which silver precursor (silver nitrate, silver fluoride, silver acetate, silver permanganate, silver sulfate, silver nitrite, silver salicylate or a combination thereof) was added drop-wise. The percentage of silver loading in Fe304-chitosan gel was 3%. After stirring the solution for an hour, surface protecting agent (glutathione) was added drop wise; then the solution was allowed to stir for an hour. Sodium borohydride was added drop-wise to the above gel at ice-cold condition (molar ratio of silver to reducing agent ratio was 1:4). Then the solution was allowed to stir for an hour, filtered and dried at room temperature (28 °C).

Example 8

This example describes the visible sensor for volume of water passed through a column using silver quantum clusters in organic-templated-boehmite-nanoarchitecture (Ag QCs-OTBN). A known quantity of Ag QCs-OTBN was packed as a disk of diameter anywhere between 35 mm to 55 mm, in a column. Challenge water having ionic concentration as prescribed by US NSF for testing contaminant removal was used in the study. The output water from a standard carbon block was passed through Ag QCs-OTBN disk at 60 to 120 mL/min flow rate. At periodic intervals, colour of the disk was photographed and emission spectra of the material were collected. The change in colour from pink to black was observed after the passage of 250 L of water. The material was collected, dried and analyzed using various techniques. Experiment was conducted with the carbon block at the output of the AgQCs-OTBN disk as well.

Example 9

This example describes the visible sensor based on fluorescence quenching of Ag QCs-OTBN to quantify volume of water passed through a column. A known quantity of Ag QCs-OTBN was packed in the form of a disk of diameter anywhere between 35 mm to 55 mm. The feed water was passed through this disk at a flow rate of 80 mL/min. At periodic intervals, colour of the disk was photographed and emission spectra of the material were collected. The change in colour from pink to black was observed after the passage of 250 L of water. The black material was collected, dried and analyzed using XRD and EDAX.

DESCRIPTION WITH REFERENCE TO DRAWINGS

Figure 1. Luminescence of glutathione protected Ag QCs embedded in OTBN under UV-lamp (preparation detailed in example 1). Image shown is in colour.

Figure 1 depicts that the Ag QCs-OTBN is highly luminescent under UV light and luminescence can be observed even under low UV intensity (8 W low pressure Hg lamp).

The pink luminescence of Ag QCs-OTBN under UV light is shown in figure 1. 20 g of glutathione-Ag QCs-OTBN was taken in a petri dish and kept under an 8 W low pressure Hg UV lamp. The composition shown here was stable and it exhibited pink luminescence intensity even after a few months of storage under ambient conditions. This is in contrast to other monolayer protected Ag clusters reported in the literature as they exhibit poor stability under ambient conditions. The stability of Ag QCs in OTBN is due to the presence of highly protective OTBN environment around the quantum cluster. The role of OTBN matrix in stabilizing nanoparticles has already been demonstrated in our previous patent application (947/CHE/2011). It was shown that the presence of OTBN matrix ensures the stability of silver nanoparticles in real water conditions and can be used successfully for water treatment applications. AgQCs prepared in other matrices as described in Example 6, especially those of titanium, zinc, cerium, and zirconium were also luminescent.

Figure 2. Colour change observed during the passage of real water through Ag QCs embedded in OTBN (first row: photographs of disc in visible light, second row: photographs of disc in UV light). Image shown is in colour.

Ag QCs embedded in OTBN is used as a sensor for detecting the volume of water that can be filtered by a water filtration unit. As lifetime of any water purifier depends on the input water quality, the Ag QCs-OTBN sensor should indicate the volume of water that can be passed through a filter and also should indicate whether the water purification device is exhausted or not. To achieve this, the output water from the water filtration unit is passed through the sensor material and collected in the storage container. After the passage of water, the colour of the Ag QCs-OTBN changes as shown in Figure 2. First row in Figure 2 shows colour of Ag QCs-OTBN disc in visible light and second row shows luminescence of Ag QCs-OTBN disc in UV light. Prior to the passage of water, the material is pink in colour and exhibits high luminescence. Upon passage of water, the material undergoes gradual change and finally turns black with quenching in luminescence. A blank trial with OTBN matrix alone indicated that OTBN matrix does not contribute to the colour change upon passage of water. This confirms that the change in colour of the material is due to silver quantum clusters. Similar colour change was seen in AgQCs prepared in matrices containing titanium, zinc, cerium, and zirconium.

Figure 3. (a) TEM image of Ag QCs embedded in OTBN matrix (b) TEM image of Ag QCs-OTBN, upon electron beam irradiation for 20 minutes.

Figure 3a shows the TEM image of Ag QCs embedded in OTBN. Clusters in OTBN are not observable in TEM images. This is due to sub-nanometer size of the Ag QC. In our earlier report, we observed the formation of large size silver nanoparticles upon electron exposure on naked glutathione protected silver clusters (T. U. B. Rao, B. Nataraju, T. Pradeep, J. Am. Chem. Soc, 2010, 132, 16304-16307). Unlike naked clusters, Ag QCs in OTBN described in this invention was stable under the electron beam (Figure 3b). The stability of Ag QCs in OTBN under electron beam confirms that Ag cluster is highly protected by the OTBN matrix. Here, the electron beam induced aggregation of silver clusters did not happen as the clusters were embedded inside the OTBN matrix.

Figure 4. FTIR spectra of (a) OTBN, (b) Ag QCs embedded in OTBN and (c) Ag QCs embedded in OTBN, after passage of 250 L of real water.

FTIR spectra of (a) OTBN, (b) Ag QCs embedded in OTBN and (c) Ag QCs embedded in OTBN after passage of 250 L of real water are shown in Figure 4. Impregnation of Ag QCs in OTBN leads to change in the N-H stretching band around 1402 cm'1 (shown in curve b). After passage of 250 L real water, N-H band resembles the same as of OTBN.

The features present in the region of 2000-500 cm"1 confirm the presence of glutathione (M. A. Habeeb Muhammed, S. Ramesh, S. S. Sinha, S. K. Pal and T. Pradeep, Nano Res., 2008, 1, 333-340). The spectra show a strong band at 3450 cm"1 due to hydrated water.
Figure 5. Luminescence spectra of (a) Ag QCs embedded in OTBN and those after the passage of (b) 50 L, (c) 150 L and (d) 250 L of water, excited at 450 nm.

Luminescence spectra of (a) Ag QCs embedded in OTBN and those after the passage of (b) 50 L, (c) 150 L and (d) 250 L of water are shown in Figure 5. The excitation spectrum was measured at 450 nm whereas corresponding emission spectrum was measured around 650 nm. It can be observed that the luminescence of Ag QCs-OTBN gradually decreases upon passage of real water. After the passage of 250 L, emission has fully quenched. It is to be noted that peaks observed at A = 400 nm and 475 nm are impurity lines of the excitation source.

Figure 6. X-ray diffractogram of (a) AIOOH (JCPDS PDF #832384), (b) chitosan, (c) OTBN, (d) silver quantum clusters embedded in OTBN, (e) silver clusters embedded in OTBN after the passage of 250 L of synthetic challenge water and (f) silver sulfide (JCPDS PDF #893840). The peaks attributed to Ag2S are marked in (e) as (). These are the prominent and distinguishable lines in (f).

X-ray diffractogram of (a) AIOOH (JCPDS PDF #832384), (b) chitosan, (c) OTBN, (d) silver quantum clusters embedded in OTBN, (e) silver clusters embedded in OTBN after the passage of 250 L of synthetic challenge water and (f) JCPDS PDF #893840 of silver sulfide are shown in Figure 6. The peaks attributed to Ag2S are marked in (e). The XRD of as-synthesized OTBN showed peaks corresponding to (120), (013), (051), (151), (200), (231) and (251) planes (Figure 6c). All these peaks can be indexed to orthorhombic-AIOOH (JCPDS PDF #832384) (Figure 6a). The broadened XRD peaks imply that the OTBN crystallite size is very small. The mean crystallite size calculated from the Scherrer formula shows that nanocrystals are of ~3.5 nm. The presence of organic template (chitosan) is also clear from the XRD data. The peaks corresponding to 29 (in degrees) 18.7°, 20.6°, 41.2° in Figure 6c are attributed to the presence of the organic template. XRD of Ag QCs-OTBN (Figure 6d) is not different from OTBN (Figure 6c). This is due to the fact that clusters are composed of very few atoms and is also smaller than wavelength of X-ray used. Figure 6e shows that after the passage of 250 liters of water, new peaks appeared corresponding to silver sulfide. The new peaks are indexed based on the pattern of standard silver sulfide (JCPDS PDF #893840) (Figure 6f). The labeled peaks (marked with () are designated as (-121) and (-112) respectively.

Figure 7. EDAX spectrum of Ag QCs embedded in OTBN. Inset: elemental X-ray images of Al Ka, O Ka, C Ka, Ag La and S Ka of the sample. The corresponding SEM image is also shown in the inset. Image shown is in colour.

Figure 8. EDAX spectrum of Ag QCs embedded in OTBN after the passage of 250 L of water. Inset: elemental X-ray images of Al Ka, 0 Ka, C Ka, Ag La, Si Ka, Ca Ka, CI Ka and S Ka of the sample. The corresponding SEM image is also shown in the inset. Image shown is in colour.

EDAX spectrum of as-synthesized QCs embedded in OTBN is shown in figure 7. This confirms the presence all expected elements such as Ag, S, C and O. The inset shows SEM and its elemental mapping before the passage of water. EDAX spectrum after the passage of 250 L of real water is shown in figure 8 and it confirms the presence of all the expected elements such as Al, O K, C K, Ag L, Si K, Ca K, CI K and S K. Ca, Si and CI are from water. The inset shows the SEM and elemental maps of the material after the passage of water. The presence of Ca, Si and CI on Ag QCs-OTBN indicates that the quenching in luminescence and change in colour is due to salt induced aggregation of silver quantum clusters.

The described aspects are illustrative of the invention and not restrictive. It is therefore obvious that any modifications described in this invention, employing the principles of this invention without departing from its spirit or essential characteristics, still fall within the scope of the invention. Consequently, modifications of design, methods, structure, sequence, materials and the like would be apparent to those skilled in the art, yet still fall within the scope of the invention.

We Claim:

1: A method of preparing composition having quantum clusters embedded in organic-templated-nanometal oxyhydroxide, wherein the composition is used as a colour changing sensor in visible light for assessing the quantity of contaminated water passed through a water purification device.

2. The method according to claim 1, wherein the organic-templated-nanometal oxyhydroxide is organic-templated-boehmite nanoarchitecture (OTBN).

3. The method according to claim 2, wherein the quantum clusters is silver quantum clusters.

4. A method as claimed in claim 3 wherein silver quantum clusters are embedded in OTBN through impregnation of silver ions with OTBN in the gel state, reducing the silver ions to zerovalent state by the use of a reducing agent and protecting by a surface protecting agent.

5. A method as claimed in claim 3 wherein silver quantum clusters are embedded in OTBN by contacting externally prepared silver quantum clusters with OTBN in the gel state.

6. A method as claimed in claim 3 wherein silver quantum clusters are embedded in OTBN by contacting externally prepared silver quantum clusters with OTBN in the solid
state.

7. A method as claimed in claim 3 wherein embedding procedure involves drop-wise addition of silver ion or silver quantum clusters to OTBN and consequently soaking for a duration of 30 minutes to 12 hours and preferably for a period of 1 hour.

8. A method as claimed in claim 1 wherein the organic template is chitosan.

9. A method as explained in claim 1 wherein templates such as chitosan, banana silk and cellulose is used individually or in combination, with varying ratios.

10. A method as claimed in claim 4 wherein the reducing agent is sodium borohydride.

11. A method as claimed in claim 3 wherein the silver precursor used for the preparation of silver quantum clusters is chosen from silver nitrate, silver fluoride, silver acetate, silver sulfate or silver nitrite or a combination thereof.

12. A method as claimed in claim 3 wherein the weight ratio of silver quantum cluster to OTBN is in the range of 0.01% to 10%, preferably in the range of 0.1% to 5%.

13. A method as claimed in claim 4 wherein the concentration of the reducing agent is in the range of 0.005 M to 1 M.

14. A method as explained in claim 1 wherein quantum clusters is based on silver, gold, copper, iron, nickel, platinum, palladium or a combination thereof.

15. A method as explained in claim 1 wherein the metal is aluminum, iron, titanium, manganese, cobalt, nickel, copper, silver, zinc, lanthanum, cerium, zirconium or a combination thereof.

16. A water flow meter device based on silver quantum clusters embedded OTBN composition as explained in claim 3, wherein the composition is packed in the form of granules inside a transparent casing for observation having an opening for water inlet and another opening for water outlet.

17. A gravity fed water purification device comprising a water flow meter device based on silver quantum clusters embedded OTBN composition as explained in claim 16, wherein the input water passes through a particulate filter followed by contacting the water purification medium followed by passing through the water flow meter device prior to being retained as clean water in the output water chamber.

18. A water flow meter device as explained in claim 16, wherein the particle size of the granules is between 0.3 mm to 5 mm and preferably between 0.3 mm to 1 mm.

19. A water flow meter device as explained in claim 16, wherein the composition is packed in a transparent pipe with an inlet and an outlet for passage of water, so as to provide a visual indication of amount of water passed through the device.