Surface Chemistry Provoked Strategy to Develop Sustainable Latent Fingerprints Visualization and Multiple Anti-Counterfeiting Trajectory: Aggregation-Induced Emission Mechanism Based Active Conjugated Imidazole Luminogens

Aggregation-induced emission based organic heterocyclic luminogens bearing conjugated electronic structure have paid much attention due to its excellent uorescence in aggregation state. In this communication, novel conjugated blue light emitting imidazole molecule is synthesized by one pot multicomponent reaction route is reported for the rst time. The prepared molecule exhibits a strong uorescence in aggregation state have gain much attention owing to their unique properties, namely simple synthesis, high purity, inexpensive, eco-friendly, large scale production, high photostability, etc. By considering these advantages of the prepared luminogen, a new uorescence based platform has been setup for in-situ visualization of latent ngerprints and its preservation by spray method followed by Poly(vinyl alcohol) masking. A clear and well dened uorescence ngerprint images on variety of surfaces by revealing level 1–3 ridge characteristics upon ultraviolet 365 nm light are noticed. The dual nature of binding specicity as well as excellent uorescence properties permits the visualization of latent ngerprints for longer durations (up to 365 days) with superior contrast, sensitivity, eciency, selectivity and negligible background hindrance. We further fabricated unclonable invisible security ink and make it highly suitable for various printing modes on valuable goods for protection against forging. The developed labels are displaying uniform distribution of ink and exceptional stability under various atmospheric environments. The development of long preservative forensic information as well as invisible ink opens new avenue in advanced forensic and data security applications.


Introduction
Fingerprints (FPs) are considered to be personal identity cards as well as information banks owing to their unique permanent features 1,2 . As a result, they can be utilized in unlock tools, biometric identi cation and similar authentication purposes. Most of the detectable FPs during crime investigation are latent and consequently their visualization has created new avenue in various elds, namely medical diagnostics, forensic investigation and health assessment [3][4][5] . Till date, numerous chemical as well as physical methods were attempted for visualization of latent ngerprints (LFPs). However, most of the traditional techniques suffer from severe drawbacks, namely low sensitivity, contrast, selectivity, sturdy background hindrance as well as high toxicity 6-10 . Among them, most commonly used powder dusting technique damages the FPs ridge characteristics at the time of staining and also dust is inevitably dangerous to the developers. Moreover, the chemicals used in those techniques can create a problem to the eyes, skin, DNA or mucous membranes [11][12][13][14][15] . The spraying technique is versatile for developing LFPs on various simple and complex surfaces and exhibit plentiful advantages, including simplistic, rapid, superior sensitivity and selectivity, easy-to-store, cost-effective, as well as large-area operable, which is considered to be utmost favourable technique for visualization of LFPs 16 . In image processing, digital data can be forged or easily ruined. As a result, protection of storage along with effortless retrieval technique is highly essential. It is the matter of high concern once it involves the FPs of extremely wanted criminals 17 . The digitally stored FPs can be effortlessly manipulated by many ways. Several methods have been attempted to secure the digitally stored FPs. For instance, Draper et al. 18 proposed a simple method to encode ngerprint biometrics securely for long storage, but fails in system security as well as the detailed authentication. In addition, many research communities are seriously attempted on cryptographic aspects of the problem [19][20][21] . Aforementioned techniques, encoded FPs have also stored digitally, which can be subsequently manipulated 22 . Hence, physically developed FPs which can be preserved longer duration is highly essential for crime investigations.
In recent years, counterfeiting of security documents, namely currency, certi cates, passport, pharmaceutical products have signi cantly imbalances the economic condition of the world [23][24][25] . In addition, easy accessibility of anti-counterfeiting (AC) inks and its validation for the counterfeiters are the key anxieties in this area. Hence, progress of the AC and authentication methods having multilevel securities is a key aspect to combat counterfeit 26, 27 . Till date, stimuli-chromic as well as photoluminescent materials, namely quantum dots, carbon dots, polymer nanoparticles, organic and inorganic metal complexes, supramolecular structures, polymer dots, up and down-converting nanomaterials, etc. are most reliable materials for fabrication of complex AC inks [28][29][30][31][32] . For instance, the carbon dots generally display broad emission spectra and frequently cause overlapping of spectroscopic pro les 33 . Moreover, semiconductor quantum dots prepared using hazardous raw materials can emit narrow emission peaks 34 . In addition, synthesis of metal oxide frameworks is found to be low yield and display excellent luminescent properties with superior compatibility. Hence, fabrication of novel materials to overcome from the existing limitations is still a challenging task for material scientists.
Aggregation-induced emission (AIE) based organic uorescent materials nds numerous advantages owing to its intensive uorescence, excellent contrast, simple functionalization procedure as well as intrinsic AIE properties 35,36 . In addition, these materials can also be utilized in imaging and sensing agents. Most of the uorescent materials suffers severe self-quenching in its aggregation state. From past decades, AIE based materials were utilized to visualize LFPs and AC ink, due to their less toxicity, easy development procedure, superior resolution, etc. 37,38 . On the other hand, some of the AIE based materials also have drawbacks in real-time applications, including large quantity of material required, chance of disturbing the FPs ridge characteristics at the time of staining process, inhalation of the dust particles may be harmful to the developers and typically suitable for smooth substrates 39,40 . Table S1 shows the reported AIE based materials for visualization of LFPs and AC applications [41][42][43][44][45][46][47][48][49] . Therefore, exploring the new AIE based uorescent materials are in urgent need to avoid these limitations.
To address the above issues, it is essential to fabricate alternate probes for LFPs visualization as well as AC applications by showcase the following characteristics: (i) simple fabrication and development procedure, (ii) non-toxicity, (iii) avoiding aggregation induced quenching, (iv) intensive "on − on" uorescence mode with high photostability, (v) applicable for porous/semi-porous/non-porous surfaces

Synthesis Of
uoren-3-amine(1mmol, 0.182g), ammonium acetate(1mmol, 0.229 g) were taken in an equimolar concentration and dissolved in glacial acetic acid (~ 30 ml). The mixture was subsequently ultrasonicated (~ 10-20 min) and re uxed for ~ 4-5 h. The chemical reaction was frequently supervised by a thin layer chromatography (TLC) utilizing appropriate ratio of pet ether and ethyl acetate mixture (7:3). Consequently, the resultant mixture was cool down to room temperature (RT) and transferred to ice water (~ 250 ml). The obtained solution was completely neutralized after the addition of aqueous sodium bicarbonate solution and later on puri ed through column chromatography by utilizing formerly used solvent chamber ratio to obtained desired compound. Schematic illustration for the synthesis of FDIP molecule was depicted in Fig. 1

Characterization
The Bruker made smart CCD System was utilized for recording the single crystal XRD data utilising Mo-Kα (λ ≈ 0.71073 Å) radiation. The SAINT-Plus programme was used to reduce crystal XRD data. Molecular diagrams and mean plane calculations were generated using ORTEP and PARST programs, respectively. 1 H-NMR spectra were recorded using a Jeol-delta with operating frequency of 400 MHz in deuterated chloroform (CDCl 3 ) solvent. FTIR spectral data of the prepared samples were recorded using a Nicolet 5700 FT-IR instrument with KBr pellets. Mass spectrometry (LC-MS) was performed with an 1808036-ABS 4 (0.085) Cm (4:7) mass spectrometer. The Jobin Yvon made Fluorolog-3 spectro ourimeter (Xe lamp, 450 W) was used for photoluminescence (PL) studies. High resolution FPs images are captured in a Hitachi (TM-3000) made scanning electron microscope (SEM). The aqueous solution of FDIP molecule (Ethanol: Water/ 30:70) was lled in fountain and sketch pen as an ink and written on normal paper surface to demonstrate anti-counterfeiting property of the prepared AIE based ink. Furthermore, the anti-counterfeiting patterns were also designed using obtained ink followed by simple stamping method. The encoded patterns are in situ photographed under normal as well as UV 365 nm light.

Chemistry
The FDIP molecule was prepared via simple acid catalysed to form ve membered N-heterocyclic ring.
Step-2 A lone pair of electron on nitrogen of 2-imino-1,2-diphenylethanone reacts at imine carbon of Schiff base, which formerly obtained by the condensation of salicylaldehyde and 2-amino urene to form 2-((9H- Step-3 The 2-((9H-uoren-2-ylamino)(2-hydroxyphenyl)methylimino)-1,2-diphenylethanone undergoes intramolecular cyclization by rearrangement reaction of lone pair electrons of nitrogen (NH group) at electron de cient carbon of carbonyl group to form an intermediate. Step-4 The obtained intermediate undertakes dehydration followed by aromatization to result in nal FDIP molecule. Figure 1 (e) depicts photographic images of FDIP molecule dissolved in various solvents, such as dichloromethane (DCM), dimethylformamide (DMF), ethanol as well as tetrahydrofuran (THF) upon normal and UV 365 nm light illumination. It was evident that, FDIP molecule in ethanol shows highest emission intensity upon UV light irradiation as compared with other solvents. This result was also validated by PL emission spectra of the prepared solutions excited at λ Exc = 365 nm ( Fig. 1 (g)). The spectra exhibit a broad intense peak at ~ 450 nm, owing to the π→π* transition as well as aromatic ring rotation. The highest intensity was noticed in FDIP in ethanol solvent, and henceforth this was used for further studies. Further, the photographic images of FDIP molecule dissolved in ethanol/water solutions with various water fractions (f w ) and its corresponding PL emission spectra was shown in Fig. 1

AIE feature of FDIP molecule
As evident from the gure, no signi cant changes on PL intensity for lower water proportion up to f w = 40 %. The PL intensity reaches its maximum value at f w = 70 % and later gradually diminished. The observed phenomenon is mainly attributed to non-coplanarity in the excited state as well as intramolecular charge transfer state.  Figure S2 shows 1 H NMR spectrum of the prepared FDIP molecule. As evident from the gure, a singlet at 11.74 δ, which con rm the phenolic -OH group. The noticed large δ value of proton may be due to intramolecular hydrogen bonding between oxygen and nitrogen. The six multiplet peaks including 21 protons around at 6.38-7.82 δ, which may have assigned to aromatic ring. Further, a singlet peak at 3.98 δ have two protons is corresponding to uorene group. The obtained results clearly evidence the formation of FDIP molecule. The mass spectrum of FDIP molecule shows that the spectral value (m/z = 477.18 (M + 1 )) was well matched with theoretical value (m/z = 476.56) ( Figure S3). The single crystal FDIP molecule were obtained by solvent evaporation method at RT. Figure 1 (d) shows the ORTEP structure of FDIP molecule. It was clearly evident from the gure that the FDIP molecule have monoclinic system with space group P 21/c and volume of 2460.4(3) Å 3 . The detailed crystal data and structure re nement parameters of FDIP molecule are tabulated in Table S2.

Development and Visualization of LFPs lms using FDIP solution
From the past years, the intense research has been attempted to explore the constituents of LFPs, even though study is more complex. Generally, LFPs comprised with numerous constituents instigating from the secretory glands in the dermis, epidermis, intrinsic constituents (metabolites and traces of medications and drugs) as well as extrinsic impurities (moisturisers, dirt, cosmetics, blood, grease, hair care products and food contaminants 50 . Further, both intrinsic and extrinsic components considerably change among individuals (inter-variability) and the same individual from day to day (intra-variability) 51 . Normally, the intrinsic constituents of the LFPs are comprised of water (95-99 %) and organic-inorganic three-dimensional complex emulsions 52 . The eccrine constituents are composed of ~ 98 % water, besides inorganic and organic compounds. Moreover, up till now 20 more amino acids are quanti ed in the FPs.
In addition, the sebaceous sweat is composed of many organic constituents, among them common are lipids, namely glycerides, cholesterol, fatty acids, squalene, lipid esters as well as sterols. Figure 2 (a) depicts mechanism of LFPs visualization dynamics using FDIP solution. In order to know the main component of the LFPs, a series of solutions were prepared using the main LFPs constituents, namely glucose, glycine, urea, sodium chloride, lactic acid as well as lipids ( Fig. 2 (b)). The previously prepared FDIP solutions are soaked on the glass surface and FPs were impressed on it. Later, the FDIP solution was sprayed on the LFPs and visualized under UV 365 nm light. Well de ned clear FPs ridge information revealed on lipids soaked surface than other ones. This may be due to strong interaction between hydrophilic head of the lipids as well as hydrophobic end of FDIP molecule, which results a strong binding among lipids and FDIP molecule ( Fig. 2 (a)). The above results clearly reveal that lipids present in LFPs have greater a nity with prepared FDIP molecule, demonstrating the versatility of the FDIP solution for LFPs development. Fluorescence imaging is the prime tool in LFP development due to the high contrast as well as low background interference. In recent years, AIE based materials are considered as more e cient materials to avoid the characteristic aggregation-caused quenching (ACQ) properties of plentiful conventional uorescent materials. In addition, AIE based molecules are prepared in a binary system solution (usually acetonitrile/water or ethanol/water) exhibit lipophilicity in nature, hence are utilized as an e cient luminescent probes for LFPs visualization. Based on previous reports, development techniques for extraction of LFPs using AIE based molecules are mainly classi ed as powder dusting and solution method 53,54 . Among them, the solution technique extensively used owing to its rapid, simple operational procedure as well as least materials consumption. Herein, various binary mixture concentrations (Ethanol-water fraction) of prepared FDIP molecule for visualization of LFPs lms on glass surface upon UV 365 nm light illumination were performed ( Fig. 3 (a-j)). As can be observed from the gures, highly intensive and de ned ridge features (type 1-3) were clearly revealed in 7:3 of Ethanol-water fraction. However, visibility of the developed FPs was also retained in other concentrations, but fail to reveal all types of ridge details. Among the studied results, 7:3 of Ethanol-water fraction was found to be superior than other concentrations. Henceforth, this optimized concentration was used for further detailed investigations. The stability of the optimized FDIP solution was examined by extracting FPs lms on glass surface with a fresh and aged FDIP solution for 3 months by spraying technique (Fig. 3 (k, l, n, o)). Almost indistinguishable clear FP patterns were obtained by using the formerly prepared solutions, and no substantial variations in uorescence signal contrast. In addition, changes in the emission intensity of the FPs furrows and ridges across the yellow box was studied (Fig. 3 (m, p)). Gray scale pro les clearly show the uorescent signal indicating that the FDIP solution exactly stocked on the ridges, however, no signals on the furrows area owing to not any sweat secretions. These results demonstrate the stability of the optimized FDIP solution even for longer durability. Furthermore, the conventional reagents, namely Nile red and oil red O are unsuccessful for development of LFPs aged up to 4 weeks, owing to loss of moisture, dispersion of water-soluble components as well as fragile constituent's degradation with respect to time 56 . In the present work, we explored the effectiveness of the both development technique and chemical reagent for the visualization of aged LFPs. The developed FPs lms using FDIP solution were stored for 1, 7, 30 and 360 days on glass plate ( Fig. 4 (a-d)). As can be seen from the gure, upon UV 365 nm light nearly identical images which comprising all the ridge features were clearly observed. The PVA coating improves the physical adhesiveness of the FDIP molecule on various substrates and particularly results in superior binding of FDIP on PVA to FPs ridges owing to its intrinsic amphiphilicity of PVA. Figure 4 (a 1 -d 1 ) depicts 3dimensional interactive plots of the corresponding FPs. The obtained result evidently showing uniform distribution of the prepared solution over the FPs ridges. Furthermore, the donor's ngers were impressed on the surface of the glass substrate and subsequently aged for 1, 7, 30 and 90 days (Fig. 4 (e-h)). Later, aged LFPs were visualized by spraying FDIP solution under UV 365 nm illumination. The achieved result clearly demonstrated that, the visualization sensitivity slowly diminished with prolonged time. However, FPs aged for up to 90 days could be able to reveal characteristic features (including level 1-3). The corresponding gray scale pro les of each FP images display distinguishable uctuations of the gray values between FP ridges and furrows. The above results clearly authenticate the prolonged durability of the developed method and prepared FDIP solution for LFPs visualization. The FPs constituents varies after impression and is affected by donor factors, transfer conditions and environmental factors (light exposure, atmospheric pollution, air circulation, dust, friction, humidity, temperature, etc.) 57 . Most of the FPs on surfaces of the substrates available at the crime spot are undergone severe environmental conditions, which intern affect the detection sensitivity. Among mentioned conditions, light exposure is found to affect much on visibility of FPs. For instance, various ndings authenticated that the decomposition of squalene happens quickly upon UV irradiation. Jones et. al. 58 studied that the variations of lipid components over the period under various environments and also fatty acids were quickly vanished in dark environments. Herein, the photo stability of FDIP solution for visualization of LFPs on glass surface was examined upon prolonged UV 365 nm illumination up to ~ 6 h ( Fig. 5 (a-g)). It is apparent that, no obvious uorescence quenching was noticed. Further, the prepared solution was utilized to visualize the LFPs on the glass surface followed by spray method under UV 365 nm light. The obtained results clearly showed well de ned ridge details, indicating that UV exposure will not in uence much on the visualization ability and its corresponding grayscale images are shown in Figure S4. The present results validate the outstanding photo stability of the prepared solution. Generally, the rate of water loss in LFPs was more prominent in temperature and humid environment. The high temperature atmosphere of LFPs results in more degradation of amino acids as compared those at RT. Sampson et.
al. 59 investigated the optimum temperature to develop successful LFPs by utilizing amino acid reagents is found to be ~ 20-35°C. The LFPs were treated with heat for longer time period, visualization would be considerably more challenging, apparently owing to amino acid degradation. However, acid salts are signi cantly resistant for higher temperatures and LFPs are still visible, even after heating at ~ 70°C for 72 h. To validate the sensitivity of the prepared FPs lm, a series of experiments were performed by maintaining different temperatures (30, 50, 100 and 150°C) for 30 min (Fig. 5 (h-k)). The obtained results clearly indicate that the developed FPs lms are highly thermal stable. Besides, variation of the emission intensity between the FP ridges and furrows which exhibit an excellent contrast. It was worth mentioning that the obtained LFPs details clearly withstand even for higher temperatures. The effect of humidity on LFPs development has been attempted under various weather as well as time durations (May, temperature ~ 45°C, humidity 47 %; September, temperature ~ 30°C, humidity 81 %; January, temperature 15°C, humidity 40 %) under UV 365 nm light (Fig. 5 (l-n)). It was presided that no noticeable variations in the clarity of images, which enabling de ned level 1-3 minutiae ridge information. To study the versatility of the developed FPs lms on different porous (paper with different background, magazine covers), non-porous (glass, stainless steel, plastic and ceramics) and semi-porous surfaces (leather, cardboard, wood and lter paper) under UV 365 nm irradiation ( Fig. 6 (a-l) and corresponding grayscale images are shown in Figure S5). High contrast and resolution FPs images were clearly noticed. The visualized FPs on different surfaces clearly exhibit level 1-3 details with high sensitivity and selectivity. In addition, 3D interactive plots were also studied ( Fig. 6 (m-o)). As evident from the gure, prepared FDIP solution staked exactly on the FPs ridges, whereas no signature of uorescence in furrows regions. The clear uorescence on the ridges even after drying indicating that the sprayed FDIP solution is not endured any outward capillary ow from the center of the liquid towards the edge, showing the suppression of well-known "coffee-ring effect". Furthermore, series of experiments were performed on FPs lms after physical abrasion (grayscale images are shown in Figure S6) and photographic images under UV 365 nm light irradiation (Fig. 7 (a-h)). As evident from the gure, well de ned FPs images were photographed even after physical abrasion (8 times). Hence, the present development technique and material are more convenient in storing FPs lms for longer duration without any effect in the existence of various physical abrasions. SEM images of the part of the developed FPs lms is taken and shown in Fig. 7 (i-n). A clear distribution of FDIP solution over the ridges and reveals level 2 details, which enabling lawful and trustworthy evidence for individualization.
Normally  Figure  S7). High magni ed uorescence images of ridge features, namely whorl, ridge end, bifurcation, ridge termination, hook, scar, trifurcation, lake and short ridge are clearly distinguishable. As can be seen from the gure that, the number, position, width of the ridges as well as distribution of characteristic sweat pores on the FPs are effortlessly distinguished. The 3D interactive plot of the portion of the FP clearly show the distribution of sweat pores on the ridges of the FP (yellow markings) ( Fig. 8 (b)). Furthermore, uctuations in the gray value over the FP evidences that the uniform distribution of FDIP solution on the FP ridges (Fig. 8 (c)). Herein, AIE based FDIP solution demonstrates signi cant competence on long preservative FPs lms through "on-on" uorescence mode by following spraying method upon UV 365 nm illumination. The present FDIP solution is considered to be excellent uorescent probe for LFPs development and analysis as compared with previous literature as follows; (i) simple preparation method followed by four-step reactions process, (ii) easy to develop preservative FPs lms via simply spraying on the LFPs surfaces, (iii) versatile for many surfaces, namely non-ltrating substrates, (iv) high contrast, superior sensitivity and low background hindrance due to the "on − on" characteristic uorescent probe and (v) reveal of unprecedented microscopic details of such as sweat pores, shape of the ridge edge and the width of the ridges. Herein, we believe that the "on − on" characteristic uorescent probe can excel effortlessly in the long preservative FPs development procedure, providing an alternate platform for the forensic scientists.

Application to anti-counterfeiting labels
Exploring real-time applications of the novel materials like FDIP molecule is always signi cant, due to its exceptional properties, such as high and steady uorescence emission as well as superior transparency in the visible region, which makes them appropriate candidates for uorescent ink applications. The characteristic uorescent "on − on" probe of the optimized FDIP solution made us to attempt anticounterfeiting labels by following simple stamping method. Figure 9 (a) shows schematic illustration of different input anti-counterfeiting patterns developed on paper surface by following stamping method. The photographs of the word "PRINCIPAL INVESTIGATOR IUAC (UGC)", "PRINCIPAL INVESTIGATOR ISRO RESPOND" and "PRINCIPAL INVESTIGATOR Naval Research Board (DRDO)" stamped by using FDIP solution on paper and leather surfaces were shown in Fig. 9 (b). As evident from the gure, stamped labels did not observe on these surfaces under normal light. However, clear images with uniform distribution of the ink was noticed on the encoded patterns upon UV 365 nm irradiation. In addition, a series of experiments were performed by varying stamping time (15, 30 and 45 s) on the paper surfaces ( Fig. 9 (c)). It was clearly demonstrated that the emission intensity of the encoded patterns upon 365 nm excitation can be easily engineered by varying the stamping time. Besides, con dential anti-counterfeiting water-mark were developed using optimized FDIP solution on the complex paper surface by stamped method (Fig. 9 (d)). Under normal light, there was no evidence of encoded water-mark on the paper, although highly intensive bright encoded water-mark "PRINCIPAL INVESTIGATOR, ISRO RESPOND" on the complex paper surface was clearly noticed upon UV 365 nm irradiation. The obtained results clearly demonstrated that the developed stamping method is quite simple and can be used for protecting the con dentiality of the documents. Further, anti-counterfeiting labels are developed via a hero, marker and sketch pen mode upon loading with FDIP solution and used for handwritten of English and Kannada characters on the normal writing paper ( Fig. 10 (a)). As can be seen from the gures that the texted information is undoubtedly visible under UV 365 nm light, however under normal light the clear blank paper was observed. Besides, the uniform distribution and emission intensity of the texted information on the normal paper is still reliable even after lengthened writings and can be retained even after 6 months when stored under RT ( Fig. 10 (b)). The photostability of the encoded information using FDIP solution upon UV 365 nm light irradiation was also studied (Fig. 10 (c)). The developed QR code information was placed at different UV irradiation time period (0, 30, 60 and 120 min) and their corresponding images are photographed. The obtained photographs clearly found that the uorescence property gradually diminished and became ambiguous above a certain time. This is mainly due to the un-decorated portion exhibit intensive uorescence upon UV light exposure, although intensity of the decorated QR code reached its maximum value. When the un-decorated portion attains its higher uorescence intensity like QR code, the difference between the QR code as well as un-decorated portion will not be distinguishable. The obtained results clearly demonstrated that the prepared FDIP solution based counterfeiting labels are used for UV-light sensitive smart labels. Photographic images of encoded information on various surfaces, namely currency, marble, cardboard and leaf under normal light and UV 365 nm light excitation ( Fig. 11 (a-j)). It was evident that the encoded information was clearly decoded under UV light, however, it is invisible under normal light. This indicates that the developed AC ink quite useful for most of the industrial applications to combat counterfeiting of the products. The effect of time period and temperature on the encoded pattern was exclusively studied, as shown in Fig. 11 (k-n). The developed patterns on the glassy ceramic tile can withstand its uorescence up to 30 days at RT. When the patterns are treated with below 90°C for 24 hrs showed any drastic alterations of the uorescent patterns. Normally, the uorescent patterns are obliterating under different conditions. In the present work, the developed patterns are heat treated at ~ 100°C for 30 min results in complete encoded information is erased. Figure 11 (o-q) depicts the reliability of encoded patterns on the paper surface by treated with water, oil and acetone for ~ 12 h. The encoded uorescence patterns are highly stable without much difference, these outstanding advantages offers the prepared FDIP solution based labels to combat counterfeiting in packaging, food and pharmaceutical industries. These results highlighted the potentiality of the material for smart package material for temperature-sensitive goods.

Mechanistic study and ink-free printing
The relief template technique was employed to develop anti-counterfeiting labels by simple procedure as follows; a commercially purchased PVA (~ 50 ml) and FDIP molecule (~ 250 mg) were taken in a beaker and irradiated by probe sonicator for ~ 1 h by maintained at a frequency of 22 kHz to obtain a clear and transparent solution. The used PVA medium offers excellent dispersion for FDIP molecule without any clusters which resulting superior viscous nature by providing more adhesive property on the various surfaces of the substrates. Later, the resulted mixture was drop-casted onto a surface of the pre-designed shapes, followed by the evaporation of solvents, which results a exible transparent lm. Then the lm was peeled slowly from the surface, and pre-designed shapes was exhibited on the lm. This developed lm was photographed and analysed upon UV 365 nm irradiation. Figure 12 (a) depicts the schematic representation of simple ink-free relief template method. The relief printing procedure for "lord Ganesha" by using previously prepared mixture as ink-free patterned substrates were shown in Fig. 12 (b). The obtained result clearly displays uorescent Ganesha pattern on the lm, where intensive uorescence was noticed in the edge areas of the pattern. Figure 12 (c) depicts the process underlying in intaglio printing by using prepared mixture as ink-free patterned substrates. The prepared mixture was poured on the intaglio xylograph, followed by solvent evaporation. Then lm was peeled slowly from the surface resulting in a exible transparent lm. The obtained lm comprises with well-developed "numericals" pro les upon UV 365 nm excitation.

Conclusions
In conclusion, an AIE-based conjugated imidazole molecule is synthesized via one pot multicomponent reaction route. The prepared molecule shows strong blue uorescence by overwhelming aggregation induced self-quenching. Thanks to the "on−on" property of FDIP molecule, in situ uorescence visualization of LFPs and invisible security ink was effectively executed. The visualized LFPs images are developed by spraying method followed by PVA masking, which is advantageous for long term preservation with high resolution. The developed FPs reveal level 1−3 ridge features, which are reliable evidences for personal identity. Besides, nanoscopic level-3 ridge information, including ridge end shape, sweat pores distribution and ridge width can be revealed in SEM images. The masked lms comprised with FPs acting as a new "information kit" used for protection of data and its effortless retrieval. In addition, invisible AC labels are developed by both with and without ink-free techniques. Emission intensity of the encoded patterns upon 365 nm excitation can be easily engineered by varying the stamping time. The encoded AC labels are easy-reading upon UV 365 nm light illumination and stable under various conditions. The developed AC techniques are quite useful to combating counterfeiting in various elds.

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download.