Fluorescence-specific BIO-compatible zinc oxide nanoparticles for anticounterfeiting ink and its reagent free synthesis thereof

Abstract

The present invention relates to a fluorescence-specific bio-compatible zinc oxide (ZnO) nanoparticles for anticounterfeiting ink, wherein the ZnO nanoparticles show emission only for UV excitation in the range of 260-365 nm and wherein photoluminescence (PL) of the ZnO nanoparticles have a sharp near band edge emission (NBE) at 378 nm and a broad green emission at 520 nm due to deep level emission (DLE). The present invention also relates to a facile one-step reagent-free synthesis of the ZnO nanoparticles via femtosecond pulsed laser ablation in liquid (fs-PLAL) technique for anticounterfeiting inks. The fluorescence-specific bio-compatible ZnO nanoparticles of the present invention can be directly used as an anticounterfeiting fluorescent ink without adding any binder.

Description

[0001] FLUORESCENCE-SPECIFIC BIO-COMPATIBLE ZINC OXIDE NANOPARTICLES FOR ANTICOUNTERFEITING INK AND ITS REAGENT FREE SYNTHESIS

[0002] THEREOF

[0003] TECHNICAL FIELD

[0004] The present invention relates to a fluorescence-specific bio-compatible zinc oxide (ZnO) nanoparticles for anticounterfeiting ink. The present invention also relates to a facile one-step reagent-free synthesis of the ZnO nanoparticles for anticounterfeiting inks. The fluorescencespecific bio-compatible ZnO nanoparticles of present invention can be directly used as an anticounterfeiting fluorescent ink without adding any binder.

[0005] BACKGROUND

[0006] Photoluminescence (PL) study is an intrinsic phenomenon to understand and characterize the luminescence properties of materials. The large bandgap semiconductor can emit fluorescence in visible regime under the excitation of a UV source. However, tuning the fluorescence properties of pristine inorganic materials is still an active focus of scientific investigation. ZnO has a bandgap of 3.3 eV, making it a wide bandgap semiconductor and an excellent multifunctional material with well-demonstrated applications in light-emitting devices [1], UV- detectors [2], solar cells, pH sensors and biomedical applications. The presence of both intrinsic defects during the synthesis process [3] and defects engineered in the post-synthesis process can influence the luminescence property in the entire UV-visible spectrum of ZnO. To probe these defect states in ZnO nanoparticles, photoluminescence (PL) is the most straightforward and effective tool. Typically, there are two characteristic peaks: near band-edge emission (NBE), which falls under the UV regime, and deep-level emission (DLE), which can be generalized to a broadly visible regime in ZnO. Very limited work has been carried out on controlling the defect states in pristine ZnO nanoparticles. In addition to that, control over the DLE remains challenging in a single-step synthesis process.

[0007] A variety of applications require similarly sized nanoparticles having control over DLE. Importantly the anticounterfeit via fluorescent properties of nanoparticles need selective tuning of DLE. The visible emission of the ZnO nanoparticles in the PL spectra is highly influenced by defect states that can be engineered by heat treatment, adsorption of functional groups, addition of metal doping etc. [4], The formation of surface defects usually depends on the synthesis process and the growth environment. Though the shape and size of the nanoparticles influence the DLE, the surface defects can selectively control the DLE. Hence there are two control options possibilities: (a) control NBE for UV detection application (b) control DLE for fluorescent ink. To control UV emission and enhancement of NBE, several methods have been adopted in literature viz. metal-coated nanoparticles, annealing in the presence of argon and oxygen, and plasma treatment [5]-[8], The second one, the control of DLE in systematic control, only few methods have been adopted in the past, such as plasma treatment of ZnO nanoparticles by coating Au and Ar-SFe [8], [9] doping of rare-earth metals

[0010] [ll], sulfurization and desulfurization followed by annealing of nanorods

[0012] , Most of these techniques are multi-step processes with additional material added to the host by doping or gas treatment, which is either parasitic in realising further applications or could even be detrimental to the inherent biocompatibility of ZnO. This motivates the need for a single-step method that is reagent-free and residue-less, which is also capable of controlling and tuning the optical properties of deep level defects.

[0008] Anti-counterfeiting remains a significant challenge, as advancements in technology, such as reverse engineering, enable the creation of counterfeit currency and the forging of products. New materials and technology are therefore required to demonstrate this challenge from time to time. There are several methods that have been developed in the past to prepare the anti counterfeit method to print on papers. The methods are holograms based coded images in multiple frequency domain (US-6001510-A) or quantum dots (QDs), IC tag or chip based QDs, direct coating and fluorescent ink [l]-[3]. Among several methods, fluorescent ink is one of the quick techniques for encryption and security purposes in the banking sector, defense, and even day-to-day life, such as currency, checks, and barcodes. Both organic and inorganic samples have been explored in past to make this fluorescent ink for direct use as ink and labeling. The unsaturated complex organic compounds (CN111337465A, CN 11132605911,) serve as good fluorescent markers and are also biocompatible, but they are not stable or water- resistant. The organic framework makes the structure bulky and the printing size bigger. To make them stable and water-resistant, an additional layer (silane or silica) increases their size by 100-500 nm. Compounds formed by combining elements from groups II and IV are fluorescent in nature, but the majority of them (CdS, CdSe, and CdTe) are hazardous (CN103669111A, US 10822510 B2). To get rid of these people, are trying to synthesize biocompatible and cost-effective QDs of materials which can be readily used as fluorescent ink. Only limited samples are biocompatible, eco-friendly and sensitive to light, particularly UV regime. ZnO is one of the biocompatible materials that is highly fluorescent in UV- A and B light (280-365 nm). ZnO can also be synthesized with varying sizes (microns to nm) and different defects during the synthesis process. Currently, bank currency, passports, and stamps use fluorescent fibre for anti-counterfeiting (CN 102703061 A). However, using this method, one is limited to a minimum size of a few microns, but the use of nanoparticles or quantum dots with varying sizes as fluorescent ink can be used in both localized regions and large areas based on the requirement.

[0009] JP2009057568 A discloses ZnO nanoparticle for an ultraviolet light emitter, wherein the ZnO nanoparticle has an average particle diameter of 2 nm to 100 nm, and a solution in which the nanoparticle is dispersed. This document also discloses a method for producing zinc oxide nanoparticles by laser ablation of metallic zinc in an aqueous surfactant solution. JP2005264089 also relates to method of preparation of zinc oxide nanoparticles having average particle diameter of 2-100 nm by laser ablation of metal zinc in a surfactant aqueous solution.

[0010] Cheng et. al, Catalyst-Free Synthesis of Hollow-Sphere-Like ZnO and Its Photoluminescence Property, Volume 2014 | Article ID 567278, discloses hollow-sphere-like ZnO by facile combustion oxidization method without using any catalyst at the temperature of 950 °C. The morphology of the ZnO was nearly spherical with diameters ranging from 2 jim to 15 jim. Dong et. al, Fluorescent Properties of ZnO Nanostructures Fabricated by Hydrothermal Method, Nanocrystals for Electronic and Optoelectronic Applications, Volume 2012 | Article ID 251276, discloses fabrication of ZnO nanorods with a mean diameter of 200 nm on different substrates through hydrothermal at 90 °C. The fluorescent properties of the nanorods under nonlinear excitation conditions were investigated applying femtosecond pulses.

[0011] ZnO only absorbs light wavelengths below 365 nm (UV band), and it undergoes radiative emission via two distinct channels. One is sharp near band edge emission (NBE) close to excitation wavelength and other is the broad deep level emission (DLE) at central wavelength of 525 nm (green), which is far from the excitation wavelength. Anti counterfeit using fluorescent materials can be probed via a photoluminescence study, which is a reliable and fast technique. In order to get optimal contrast in a fluorescence study, there should be a minimum 0.5 eV (50-70 nm) difference between the emission and excitation wavelengths to prevent crosstalk. Importantly, the sample should not get excited in daylight or any visible wavelength (400-700 nm), which is key to anticounterfeit application.

[0012] Thus, there is a need to control green emissions, i.e., DLE for anticounterfeit application in a single-step and facile route.

[0013] OBJECTIVES

[0014] An objective of present invention is to provide a fluorescence-specific bio-compatible zinc oxide (ZnO) nanoparticles for anticounterfeiting ink which shows sharp emission at 378 nm (NBE) and a broad green emission centered at about 520 nm due to deep level emission (DLE) under UV excitation.

[0015] Another objective of the present invention is to provide a single-step synthesis of ZnO nanoparticles by fs-pulsed laser ablation in ethanol medium by varying the pulsewidth and the laser average power.

[0016] Yet another objective of the present invention is to provide anti -counterfeiting using pristine, biocompatible ZnO nanoparticles where tunable emission profile is observed between violet to green, for similarly sized nanoparticles.

[0017] Further objective of the present invention is to provide ZnO nanoparticles that can be transferred to nonfluorescent (under UV excitation) paper or suitable substrate to be used for currency for anti-counterfeiting.

[0018] Another objective of the present invention is to provide ZnO nanoparticles which can be used for fluorescent label makers for important documents, expensive liquor, electronics materials, and luxury brands.

[0019] SUMMARY

[0020] An aspect of the present invention provides fluorescence-specific bio-compatible zinc oxide (ZnO) nanoparticles for anticounterfeiting ink, wherein the ZnO nanoparticles show emission only for UV excitation in the range of 260-365 nm, and wherein the photoluminescence (PL) of the ZnO nanoparticles has a sharp near band edge (NBE) emission at 378 nm and a broad green emission at 520 nm due to deep level emission (DLE). Another aspect of present invention provides a reagent-free method for synthesis of fluorescence-specific bio-compatible ZnO nanoparticles by femtosecond laser ablation in liquid (fs-PLAL), the method comprising: i) pelletizing ZnO powder followed by sintering at a temperature in the range of 700-900 °C for a time period in the range of 22-26 hours to obtain strong (unbreakable) ZnO pellets; ii) immersing the ZnO pellet in a solvent and loosely focusing ( / ’ >15 cm) the femtosecond (fs) laser pulses on the pellet to obtain the nanoparticles; iii) focusing a laser beam of wavelength centered around 800 nm (or NIR regime fs-laser i.e. 1035 nm or 1064 nm) at 1 kHz repetition rate and raster scanning the sample for a time period of 15-25 minutes to obtain the laser ablated nanoparticles; iv) adjusting the pulse width of the ablating pulse between 150-750 femtosecond (fs) and the ablation power from 0.25-1.5 W to obtain the nanoparticles; and v) centrifuging the nanoparticles at a speed in the range of 4500-5500 rpm to maintain a uniform particle size distribution followed by collection of the fluorescence-specific biocompatible ZnO nanoparticles.

[0021] Yet another aspect of present invention provides fluorescent ink comprising fluorescencespecific zinc oxide (ZnO) nanoparticles on nonfluorescent papers.

[0022] BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

[0023] A further understanding of the present subject matter can be obtained by reference to various embodiments set forth in the illustrations of the accompanying drawings. The drawings are not intended to limit the scope of the present subject matter, which is set forth with particularity in the claims as appended or as subsequently amended, but merely to clarify and exemplify the subject matter.

[0024] For a more complete understanding of the present subject matter, reference is now made to the following drawings in which:

[0025] Figure 1 shows the experimental setup for the pulse width dependent fs- pulsed laser ablation in liquid. Figure 2 shows a block diagram representing the stepwise procedure to synthesize the nanoparticles.

[0026] Figure 3 shows optical emission PL spectra of ZnO nanoparticles prepared with range of different pulse-widths a) 250 fs, b) 350 fs, c) 450 fs, d) 600 fs and e) 750 fs obtained from the fs-PLAL process. In all cases, the emission spectra were normalized at the NBE peak at 378 nm. Panel f) depicts the variation of ratio of DLE to NBE ratio as a function of the temporal width (FWHM) of ablating pulse for different ablating beam average powers from 0.75 - 1.5 W, corresponding to individual pulse energies of 0.75 - 1.5 mJ.

[0027] Figure 4 shows the room temperature PL spectra for ZnO NPs prepared at different ablation power in panel a) at 325 nm excitation power. The real time picture of fluorescence emission from those samples is shown in panel b).

[0028] Figure 5 shows the room temperature Raman spectra for ZnO nanoparticles synthesized at 0.25-1.5 W and 500 fs pulsewidth. In the plot the spectra are normalized with respect to E2lghpeak at 437 cm-1.

[0029] Figure 6 shows XRD pattern of ZnO nanoparticles prepared via pulsed laser ablation route for different ablation powers.

[0030] Figure 7 shows TEM micrographs for the ZnO nanoparticles synthesized at different ablation powers in panel (a)-(d). In panel (e), the particle size distribution for these is shown.

[0031] Figure 8: The marked region using ZnO nanoparticles on white paper under daylight is shown in panel (a) and (b) for samples prepared at 0.5 W and 1.5 W ablation power, respectively. The corresponding digital images with the exposure of UV laser (325 nm) are depicted in panels (c) and (d).

[0032] DETAILED DESCRIPTION

[0033] The disclosure will now be illustrated with the working examples, which are intended to illustrate the workings of the present subject matter and not intended to restrict or imply any limitations on the scope of the present subject matter. Other examples are also possible which are within the scope of the present subject matter.

[0034] An embodiment of present invention provides a fluorescence-specific bio-compatible zinc oxide (ZnO) nanoparticles for anticounterfeiting ink, wherein the ZnO nanoparticles show emission only for UV excitation in the range of 260-365 nm and wherein photoluminescence (PL) of the ZnO nanoparticles have a sharp near band edge emission (NBE) at 378 nm and a broad green emission at 520 nm due to deep level emission (DLE).

[0035] In another embodiment of present invention, the ZnO nanoparticles have average particle size in the range of 4-5 nm and a particle size distribution of 2 nm.

[0036] In yet another embodiment of present invention, the ZnO nanoparticles are blind to visible wavelengths of 400-700 nm.

[0037] In further embodiment of present invention, the nanoparticles have tunable emission profiles between violet to green for similarly sized nanoparticles.

[0038] Yet another embodiment of present invention provides that the colors for photoluminescence emissions can be tuned as the DLE to NBE ratio resulting in emissions ranging from violet to green.

[0039] An embodiment of present invention provides a reagentfree method for synthesis of fluorescence-specific bio-compatible ZnO nanoparticles as claimed in claims 1-5 by femtosecond laser ablation, the method comprising: i) pelletizing ZnO powder followed by sintering at a temperature in the range of 700-900 °C for a time period in the range of 22-26 hours to obtain strong (unbreakable) ZnO pellets; ii) immersing the ZnO pellet in a solvent and loosely focusing ( / ’ >15 cm) the femtosecond (fs) laser pulses on the pellet to obtain the nanoparticles; iii) focusing a laser beam of wavelength centered around 800 nm (or NIR regime fs-laser i.e. 1035 nm or 1064 nm) at 1 kHz repetition rate and raster scanning the sample for a time period of 15-25 minutes to obtain the laser ablated nanoparticles; iv) adjusting the pulse width of the ablating pulse between 150-750 femtosecond (fs) and the ablation power from 0.25-1.5 W to obtain the nanoparticles; and v) centrifuging the nanoparticles at a speed in the range of 4500-5500 rpm to maintain a uniform particle size distribution, followed by collection of the fluorescence-specific biocompatible ZnO nanoparticles. In an embodiment of present invention, the ZnO nanoparticles are obtained at an optimum pulse width of 500-600 fs.

[0040] In yet another embodiment of present invention, the solvent has polarity less than 0.7 and UV absorption cut-off of less than 250 nm. The solvent can be selected from ethanol, methanol, 2- propanol, hexane, and chloroform. Preferably, the solvent is ethanol.

[0041] In another embodiment of present invention, with pulse duration and ablation power, the deep level emission (DLE) of nanoparticles can be controlled.

[0042] In yet another embodiment of present invention, the DLE emission can be enhanced by varying the laser ablation power from 0.5 W to 1.5 W at 500-650 fs pulse duration.

[0043] An embodiment of present invention also provides fluorescent ink comprising fluorescencespecific zinc oxide (ZnO) nanoparticles as claimed in claims 1-5 on nonfluore scent papers.

[0044] The Fluorescent ink using ZnO nanoparticles on nonfluorescent papers (Optical brightening agent (OB A) free) can be easily demonstrated by direct application of colloidal solvent without adding any binder.

[0045] Femtosecond pulsed laser ablation in a liquid (fs-PLAL) is one of clean and single-step techniques to prepare nanoparticles in top-down method. The state of the art for fs-PLAL is developed using a Ti: Sapphire based laser system (Astrella, Coherent Inc.) with a central wavelength of 800 nm delivering pulses at a repetition rate of 1 kHz. The pulsewidth is varied by controlling the gratings in the compressor unit of this chirped-pulse amplification system or can be varied using external dispersive optics. The pulsewidth is measured using single-shot autocorrelator (Delta, Mini optic Inc.). As depicted in Figure 1, the energy per pulse, equivalently the average power of the ablating pulse, was controlled using a Brewster’s angle polarizer.

[0046] Thus, the setup of present invention provides for the in-situ control of the two key parameters of the ablating laser pulses i.e. (a) pulse-width and (b) pulse energy or average beam power. The target for ablation was prepared by pelletizing the ZnO powder and sintering the ZnO pellet at 800 °C for 24 hours. Initially, the pellet was submerged in ~30 ml of ethanol contained in a borosilicate glass beaker, ensuring that it remained at a distance of ~30 mm above the surface of the ablation target. Following the setup in Figure 1, a lens of focal length (f) 20 cm was used to focus pulses on target through the solvent. The beam was routed using a mirror such that the focus met with the surface of the target immersed completely in the liquid in which we intended to collect ablated nanoparticles. This type of setup retains the entire Rayleigh length of the focused laser beam in the liquid, avoiding unintended oxidation of the target during the ablation process.

[0047] In order to ablate a fresh spot on the target interacting with these laser pulses incessantly, the inventors mounted the container with the liquid immersed target on a two-axis motion- controlled stage. The stage was moved with a speed of 1 mm / s so that the target was raster scanned by the focus of the ablating laser beam. A complete run of this synthesis procedure typically lasted about 20 minutes for each intended ablation condition defined by the pulsewidth and average beam power. Following the ablation process, the inventors transferred the suspension containing the nanoparticles into a second container. To remove the unavoidable debris of large microparticles, the suspension was centrifuged at different rotation speeds and centrifuge time. The desired suspension of nanoparticles devoid of larger debris was carefully extracted using a pipette in repeated iterations.

[0048] The pristine, biocompatible ZnO nanoparticles of the present invention were found to have tunable emission profiles between violet to green, for similarly sized nanoparticles. Thus, the nanoparticles of present invention can be used in anti-counterfeiting applications.

[0049] The ZnO nanoparticles of present invention can be directly used as fluorescent ink.

[0050] The ZnO nanoparticles of present invention can be transferred to nonfluorescent (under UV excitation) paper or suitable substrate to be used for currency for anti-counterfeiting. These nanoparticles can also be used for fluorescent label makers for important documents, expensive liquor, electronics materials, and luxury brands.

[0051] The embodiments of the present subject matter are described in detail with reference to the accompanying drawings. However, the present subject matter is not limited to these embodiments which are only provided to explain the present invention more clearly to the ordinarily skilled in the art of the present disclosure. Examples

[0052] The present subject matter is further enumerated and substantiated by way of working examples as described hereinafter with reference to the system incorporated in the present subject matter.

[0053] Materials: Zinc Oxide powder (Sigma Aldrich 677450)

[0054] Ethanol (Hayman AR grade)

[0055] Example 1: Synthesis of ZnO nanoparticles

[0056] Figure 2 represents a stepwise flowchart for the procedure to synthesize ZnO nanoparticles. ZnO targets were prepared by palletizing the ZnO powder to a size of diameter 12.5 mm and 3 mm height and then sintered at 800 °C for 24 hours. During the fs-PLAL, the ZnO pallets were immersed in a beaker of diameter 5 cm where 30 ml of ethanol was occupied a height of 3 cm from the upper meniscus of beaker and the beam was focused on it using a lens of 20 cm focal length. The output from Ti: Sapphire laser of 800 nm at 1 kHz repetition rate was focused on the sample for 20 minutes, where the target was moving at a constant speed to avoid the overlap of pulses on the surface of the sample. The pulse width was varied from 150- 750 fs by adjusting the compressor of the fs-laser. At an optimum pulsewidth 500 fs, the nanoparticles were prepared by varying the ablation power from 0.25-1.5 W of the ablating pulses, which are collected separately in a clean bottle after every run. Later, the nanoparticles are centrifuged at 5000 revolutions per minute (rpm) to remove unwanted debris and maintain the uniform particle size distribution.

[0057] Example 2: Characterizations

[0058] 1. Photoluminescence (PL):

[0059] PL spectrum is a clear measure of defects in material systems. The ZnO nanoparticles synthesized using the fs-PLAL method have been optimized by controlling the process parameters such as the pulse width and the average power of the ablating pulses etc. The PL spectra recorded in the wavelength range from 350 nm to 625 nm, using 325 nm He-Cd laser as excitation source. As observed, PL emission has a sharp peak at 378 nm called near band edge (NBE) and a broad peak centered around 523 nm also referred as deep-level emission (DLE). To get a better understanding, the emission spectra were normalized with respect to the peak intensity of the NBE peak for comparison. The PL spectra of ZnO nanoparticles prepared with a wide range of pulse width variation starting from 150 fs to 750 fs, are recorded to obtain enhanced green-emitting nanoparticles. For comparison, the PL spectra are depicted in Figure 3 in panels (a) and (e) for pulse widths of 250 fs to 750 fs, respectively. For each pulse width, the PL spectra for the samples prepared at different pulse powers of different ablation powers from 0.75 - 1.5 W were demonstrated and normalized with respect to the NBE peak. From Figure 3 (f), it is evident that the deep-level emission (DLE) peaked at ~ 520-525 nm, increases in amplitude steadily with increasing pulse width to a maximum and then starts to decrease for each ablation power. The ratio of DLE to NBE peaks was extracted from these curves to evidence an optimal ablation pulse widths for each ablation power, e.g., pulses with temporal width of 650 fs is optimal for maximizing the DLE / NBE ratio (intensity ratio). This clearly demonstrates that the giant-green emission observed in this fs-PLAL synthesized pristine ZnO nanoparticles can be controlled by choosing a suitable pulse width. In fs-lasers, this is easily achieved by manoeuvring spacing between gratings in the amplifier, a routine control provided in generic laser systems.

[0060] Thus, it can be concluded that the PL emission shows a systematic enhancement for pulse width ranging between 450-650 fs. The inventors have chosen 500 fs pulse width and power variation starting from 0.25 W to 1.5 W, where the inventors have noticed a significant control for DLE emission. In Figure 4 (a), the PL emission spectra normalized to NBE peak for nanoparticles prepared by varying the average power ranging from 0.25 W- 1.5 W at 500 fs is shown. The ratio of DLE to NBE peaks was extracted from these curves evidence of optimal ablation pulse widths for each ablation power, e.g., pulses with temporal width of 500 fs is optimal for maximizing the DLE / NBE ratio (intensity ratio). This clearly demonstrates that the giant-green emission observed in this fs-PLAL synthesized pristine ZnO nanoparticles can be controlled by choosing a suitable pulse width and average power.

[0061] Technically, the PL measurement can be analysed to measure the defect contribution in the process. As shown in Figure 4, the NBE peak asymmetry is contributed as a sharp peak of 378 nm with a secondary emission at 393 nm [13,14,15], Furthermore, the peak centered around 525 nm can be deconvoluted into two peaks with peak positions at 510 nm and 560 nm, attributing to charged oxygen vacancies, i.e. Vo+and Vo++present in the ZnO nanoparticles [16,17].

[0062] Structural conformation: In continuing the exploration for the reasons underlying the variation and control of the optical emission (PL) spectrum with the ablating laser beam parameters, the inventors turn to Raman spectroscopy for further insights. It is well-established that ZnO with the hexagonal wurtzite structure, belonging to the space group P63mc, has two main optical phonon modes. The modes associated with the vibration of zinc sub-lattice and oxygen atoms are assigned as E2°Wand E2lgh, respectively. The inventors compared the measured Raman spectra normalized with respect to E^lghis shown for nanoparticles prepared with increasing average power from 0.25 W-l .5 W for samples at 500 fs cf. Figure 5. The broad peak in Raman spectra from 500-600 cm'1originated due to the contribution of the surface optical phonon (SOP) mode at 550 cm'1and the Ai(LO) 570 cm'1mode [18,19], These peaks arise from surface defects and oxygen vacancies in the ZnO nanoparticles, which are of particular interest for the present invention. This reveals that the underlying reason for the control of green-emission in these nanoparticles lies in the amount of defects induced by the fs-PLAL process.

[0063] The crystallinity of laser ablation assisted synthesized ZnO nanoparticles (NPs) was confirmed by X-ray powder diffraction (XRD) after drop casting on a Silicon wafer. The XRD patterns of as-prepared ZnO NPs at different laser ablation powers are shown in Figure 6. The peak positions at 31.6°, 34.2°, 36.1°, 47.4°, 56.4°, 62.8° correspond to the characteristic (100), (002), (101), (102), (110) and (103) planes of the hexagonal wurtzite structure of ZnO, space group symmetry P63mc, in good agreement with JCPDS card no- 00-036-1451.

[0064] The transmission electron microscopy (TEM) was carried out to measure the particle size distribution. The TEM micrographs for the ZnO NPs prepared at different laser ablation powers have been shown in Figure 7 (a-d). In Figure 7 (e) the size distribution has been plotted against the ablation power. The size distribution was varying from 4-5 nm for all ablation power and a full width at half maximum (FWHM) of about 2 nm. The particle distribution shows a similar distribution however the PL emissions are different.

[0065] Example 3: Fluorescent marker study on paper:

[0066] To use the ZnO nanoparticles as fluorescent ink, the nanoparticles are directly used to write on the paper or substrate. About 5 ml of ethanol has been taken using a micropipette to write on the paper with minimum fluorescence. As shown in Figure. 8, two differences are tested for observing the change in the fluorescence properties. Under the exposure of white light both looks same whereas when excited with 325 nm laser the two papers show different colours. The sample at 1.5 W ablation power turns green compared to 0.5 W sample. The inset in Figure 8 (c-d) demonstrates the measure of PL emission from the marked paper. This indicates the successful preparation of fluorescent properties of these ZnO nanoparticles for fluorescent ink.

[0067] The controlled DLE emission is achieved by optimally choosing the laser power and the pulsewidth. At 500 fs we have achieved a systematic increase in the DLE emission as we increase the laser power. Although these nanoparticles have similar size distribution due to surface defects at higher pulse power, the green emission gets enhanced. These similarly sized ZnO nanoparticles with controlled DLE lead to potential applications as these are indistinguishable in daylight and in size, which can only be tested under UV light.

[0068] Example 4: Comparative Data

[0069] The comparison of known method of synthesis of fluorescent ZnO nanoparticles vis-a-vis the method of present invention is provided below in Table 1 :

[0070] Table 1

[0071] Further, the parameters required for synthesizing ZnO nanoparticles via Laser ablation in liquid are also compared in below table 2:

[0072] Table 2

[0073] From Table 1 and Table 2, it is clear that the present invention provides a greener approach for synthesis of fluorescent nanoparticles as the method is reagent free. Further, the deep level emission (DLE) of the synthesized ZnO can be selectively tuned by fs-PLAL techniques. The synthesised ZnO nanoparticles can be used as fluorescent ink directly without addition of any binder.

[0074] References:

[0075] [1] M. Willander et al., “Luminescence from zinc oxide nanostructures and polymers and their hybrid devices,” Materials (Basel)., vol. 3, no. 4, pp. 2643-2667, 2010, doi:

[0076] 10.3390 / ma3042643.

[0077] [2] A. J. Gimenez, J. M. Yanez-Limon, and J. M. Seminario, “ZnO-paper based photoconductive UV sensor,” J. Phys. Chem. C, vol. 115, no. 1, pp. 282-287, 2011, doi: 10.1021 / jpl07812w. [3] A. Janotti and C. G. Van De Walle, “Native point defects in ZnO,” no. October, pp. 1-

[0078] 22, 2007, doi: 10.1103 / PhysRevB.76.165202.

[0079] [4] M. Scepanovic, M. Grujic, S. Bernik, and T. Sreckovic, “Raman study of structural disorder in ZnO nanopowders " cepanovi vol. 2009, no. November 2009, pp. 914-921, 2010, doi: 10.1002 / jrs.2546. [5] A. Sahoo et al., “Femtosecond pulse ablation assisted mg-ZnO nanoparticles for uv- only emission,” Nanomaterials, vol. 10, no. 7, pp. 1-11, 2020, doi: 10.3390 / nanol0071326.

[0080] [6] M. Mahanti and D. Basak, “Highly enhanced UV emission due to surface plasmon resonance in Ag-ZnO nanorods,” Chem. Phys. Lett., vol. 542, pp. 110-116, 2012, doi: 10.1016 / j.cplett.2012.06.004.

[0081] [7] Y. Zhou et al. , “Direct writing of single germanium vacancy center arrays in diamond,” New J. Phys., vol. 20, no. 12, pp. 2-7, 2018, doi: 10.1088 / 1367-2630 / aaf2ac.

[0082] [8] T. Dixit, I. A. Palani, and V. Singh, “Role of Surface Plasmon Decay Mediated Hot Carriers toward the Photoluminescence Tuning of Metal-Coated ZnO Nanorods,” J. Phys. Chem. C, vol. 121, no. 6, pp. 3540-3548, 2017, doi: 10.1021 / acs.jpcc.6bll526.

[0083] [9] B. Panigrahy and D. D. Sarma, “Enhanced photocatalytic efficiency of AuPd nanoalloy decorated ZnO-reduced graphene oxide nanocomposites,” RSCAdv., vol. 5, no. 12, pp. 8918— 8928, 2015, doi: 10.1039 / C4RA13245A.

[0084]

[0010] K. Fabitha and M. S. Ramachandra Rao, “HoA3+-doped ZnO nano phosphor for low- threshold sharp red light emission at elevated temperatures,” J. Opt. Soc. Am. B, vol. 34, no.

[0085] 12, p. 2485, 2017, doi: 10.1364 / josab.34.002485.

[0086]

[0011] B. Dierre, “Cathodoluminescence study of ZnO bulk and nanostructures,” no. February 2010, 2014.

[0087]

[0012] Y. Zhou, S. Chen, X. Pan, and Z. Ye, “Photoluminescence enhancement in non-polar ZnO films through metallodi electric mediated Al surface plasmons,” Opt. Lett., vol. 43, no. 10, p. 2288, 2018, doi: 10.1364 / ol.43.002288.

[0088]

[0013] S. A. M. Lima, F. A. Sigoli, M. J. Jr, and M. R. Davolos, “Luminescent properties and lattice defects correlation on zinc oxide q,” Int. J. Inorg. Mater., vol. 3, pp. 749-754, 2001.

[0089]

[0014] B. Cao, W. Cai, and H. Zeng, “Temperature-dependent shifts of three emission bands for nanoneedle arrays,” vol. 88, no. 161101, pp. 1-3, 2006, doi: 10.1063 / 1.2195694.

[0090]

[0015] C. H. Ahn et al., “A comparative analysis of deep level emission in ZnO layers deposited by various methods A comparative analysis of deep level emission in ZnO layers deposited by,” J. Appl. Phys., vol. 013502, no. 105, pp. 1-6, 2009, doi: 10.1063 / 1.3054175.

[0091]

[0016] K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tailant, and J. A. Voigt, “Correlation between photoluminescence and oxygen vacancies in ZnO phosphors,” Appl. Phys. Lett., vol. 403, no. 1996, p. 403, 1995, doi: 10.1063 / 1.116699.

[0092]

[0017] K. Vanheusden, W. L. Warren, C. H. Seager, D. R. Tailant, J. A. Voigt, and B. E. Gnade, “Mechanisms behind green photoluminescence in ZnO phosphor powders,” J. Appl. Phys., vol. 79, no. 10, pp. 7983-7990, 1996, doi: 10.1063 / 1.362349.

[0018] U. O. Hadic Morkoc, inc Oxide Fundamentals, Materials and Device Technology . Wiley-VCH, 2007.

[0093]

[0019] M. Chandran, B. Tiwari, C. R. Kumaran, S. K. Samji, S. S. Bhattacharya, and M. S. R. Rao, “Integration of perovskite PZT thin films on diamond substrate without buffer layer,” J. Phys. D. Appl. Phys., vol. 45, no. 20, 2012, doi: 10.1088 / 0022-3727 / 45 / 20 / 202001.

[0094] It is to be understood that the above-described embodiments are merely illustrative principles of the present invention and that many variations may be devised by those skilled in the art without departing from the scope of the present invention. It is, therefore, intended that such variations be included with the scope of the claims.

Claims

WE CLAIM:

1. A fluorescence-specific bio-compatible zinc oxide (ZnO) nanoparticles for anticounterfeiting ink, wherein the ZnO nanoparticles show emission only for UV excitation in the range of 260-365 nm, wherein photoluminescence (PL) of the ZnO nanoparticles have a sharp near band edge (NBE) emission at 378 nm and a broad green emission at 520 nm due to deep level emission (DLE), and wherein the ZnO nanoparticles are blind to visible wavelength of 400-700 nm.

2. The fluorescence-specific bio-compatible ZnO nanoparticles as claimed in claim 1, wherein the ZnO nanoparticles have average particle size in the range of 4-5 nm and a particle size distribution of 2 nm.

3. The fluorescence-specific bio-compatible ZnO nanoparticles as claimed in claim 1, wherein the nanoparticles have tunable emission profile between violet to green for similarly sized nanoparticles.

4. The fluorescence-specific bio-compatible ZnO nanoparticles as claimed in claim 3, wherein colors for photoluminescence emissions can be tuned as the DLE to NBE ratio, resulting in emissions ranging from violet to green.

5. A reagent free method for synthesis of fluorescence-specific bio-compatible ZnO nanoparticles as claimed in claims 1-4 by femtosecond laser ablation in liquid (fs- PLAL), the method comprising: i) pelletizing ZnO powder followed by sintering at a temperature in the range of 700-900 °C for a time period in the range of 22-26 hours to obtain strong (unbreakable) ZnO pellets; ii) immersing the ZnO pellets in a solvent and loosely focusing ( / >15 cm) the femtosecond (fs) laser pulses on the pellet to obtain the nanoparticles; iii) focusing a laser beam of wavelength centered around 800 nm (or NIR regime fs-laser i.e. 1035 nm or 1064 nm) at 1 kHz repetition rate and raster scanning the sample for a time period of 15-25 minutes to obtain the laser ablated nanoparticles;iv) adjusting the pulse width of the ablating pulse between 150-750 femtosecond (fs) and the ablation power from 0.25-1.5 W to obtain the nanoparticles; and v) centrifuging the nanoparticles at a speed in the range of 4500-5500 rpm to maintain a uniform particle size distribution followed by collection of the fluorescence-specific bio-compatible ZnO nanoparticles.

6. The method as claimed in claim 5, wherein, the ZnO nanoparticles are obtained at an optimum pulse width of 500-650 fs.

7. The method as claimed in claim 5, wherein the solvent has polarity less than 0.7 and UV absorption cut-off of less than 250 nm.

8. The method as claimed in claim 7, wherein the solvent is selected from the group consisting of ethanol, methanol, 2-propanol hexane, and chloroform and preferably ethanol.

9. The method as claimed in claim 5, wherein with pulse duration and ablation power, the deep level emission (DLE) of nanoparticles can be controlled.

10. The method as claimed in claim 9, wherein the DLE emission can be enhanced by varying the laser ablation power from 0.5 W to 1.5 W at 500-600 fs pulse duration.

11. The fluorescence-specific ZnO nanoparticles as claimed in claims 1-4 could be used as a fluorescent anticounterfeiting ink on nonfluorescent papers.

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