Bithiophene-naphthalene chalcone as a fluorescent pigment in eco-friendly security ink formulation

Approaches to prevent document/product forgery using eco-friendly printing inks and security printing techniques on flexible substrates are two vital areas of research that demand coherent advancements. In this context, a new bithiophene-naphthalene chalcone (BTNP) was synthesized and characterized as a fluorescent pigment for use in security ink. BTNP exhibited good solid-state and solution phase fluorescence with intramolecular charge transfer confirmed using theoretical studies and emission spectra collected in THF/THF–hexane mixtures. The aggregation-induced emission of BTNP was established using solution phase studies in THF/THF–water mixtures. The strong solid-state yellow emission of BTNP prompted its use as a pigment in the preparation of an environment-friendly UV fluorescent formulation, devoid of any volatile organic compounds or hazardous air pollutants. The screen prints obtained on a UV dull paper substrate utilizing BTNP ink revealed good fluorescence, photostability, colorimetric, densitometric, and rub resistance characteristics, which showcase the potential applicability of the BTNP formulation in security printing. The low cytotoxic nature of the chalcone as observed in the MTT assay could also be exploited for the use of formulation in inkpads.


Introduction
Counterfeiting of security documents, passports, banknotes, and certificates is a significant global problem that costs billions of dollars for businesses, governments, and consumers each year (Pei et al. 2020).To prevent the forgery of documents, there is a need to improve its security features by using high-security anticounterfeiting materials with sophisticated authentication technologies (Abdollahi et al. 2020).Researchers have explored diverse classes of luminescent materials including lanthanides (Li et al. 2021), conventional organic materials (Jeon et al. 2015), plasmonic nanomaterials (Ibrar and Skrabalak 2021), quantum dots (Song et al. 2016), metal-organic frameworks (Mieno et al. 2018;Venkateswarlu et al. 2020), and carbon dots (Jiang et al. 2018) as luminous dyes/pigments for the production of coated papers and security inks in the area of counterfeit prevention.These fluorescent molecules find applications in security printing, wherein they are used as colourants to formulate inks.Recently, short-and long-wavelength UV fluorescent inks are widely applied in security printing arena including smart packaging, highlighting markers, and information storage (Bhagya et al. 2022;Ataeefard and Nourmohammadian 2015).Moreover, due to environmental concerns, there are numerous approaches to create sustainable printing inks such as water-based inks with reduced volatile organic compounds and hazardous air pollutants, rather than using solventbased inks.These water-based inks have been identified as the best viable option attributed to its environmental friendliness, high solid content, gloss, low viscosity, and so on (Zhou et al. 2015).Therefore, printing industry is now paying close attention to the development of waterbased and water-reducible ink formulations to lower the usage of organic solvents that evaporate into the environment (Żołek-Tryznowska et al. 2015).
Certain organic molecules show tunable emission based on their ability to form aggregates that can be used in certain emerging fields (Samanta and Bhattacharya 2012;Gulyani et al. 2018).Chalcones are synthesized through the reaction between aromatic aldehydes and aromatic ketones, and their basic structural framework comprises two aromatic units linked through a three-carbon, α,β-unsaturated ketone system (Ming et al. 2017).The push-pull conjugation effect is primarily responsible for the fluorescent nature of chalcones, and these materials also exhibit good thermal stability that is necessary when utilized in printing applications.The versatile chemistry of thiophene enables the preparation of a wide range of thiophene-based chalcone structures with extremely varied functionalization.Research reports on thiophene oligomers and polymers focus primarily on their semiconductor, luminescence, and sensing capabilities (Barbarella et al. 2005).There are no detailed investigations on the natural colourant bithiophene, which has two coplanar rings and a sulphur atom in each of them as a possible pigment in water-based inks.Low fluorescence emission quantum yields, both in the solution and solid states attributed to an efficient intersystem crossing of heavy sulphur atoms, aggregation-induced quenching, well-aligned packing arrangement in the solid state, and excited state relaxation favoured by inter-annular rotations restricts the usage of bithiophene in optical applications.Despite these limitations, during the past few years, numerous extremely luminous thiophene-based materials with strong fluorescence and quantum yield, as well as high absorption coefficients, have been synthesized for diverse applications (Rasmussen et al. 2015;Wakamiya et al. 2007;Kim and Son 2011);(Lee et al. 2021).Moreover, bithiophene derivatives with intense emissions spanning the entire visible spectral region have been achieved by tuning the electron-donating ability of the π-conjugated framework of bithiophene (Wakamiya et al. 2007).
In the quest for new fluorescent molecules as colourants in water-based ink compositions, naphthalene and 2,2′-bithiophene integrated chalcone (BTNP) with electronic delocalization and push-pull conjugation was synthesized.Further, the structural, optical, and cytotoxicity features of BTNP are investigated.Screen printing can deposit a thin water-based ink film with no chemical usage and thereby reduces ecofootprint (Karuppusamy et al. 2017).During screen printing, the ink is pumped into the frame and poured onto the screen, which is then pressed through the open holes of the screen by drawing it across the mesh with a rubber-bladed squeegee (Leach et al. 1988).The paper substrate is simultaneously kept in touch with the screen to allow the transfer of ink to it.Screen production is attractive for short-run work because they can be made easily and inexpensively (Leach et al. 1988).Therefore, an environmentally benign water-based formulation for screen printing was prepared using BTNP as the fluorescent pigment.Instant prints obtained with good optical and abrasion resistance characteristics may possibly be applied in a variety of anticounterfeit applications.
The open capillary method was used to check the melting point of BTNP.The FTIR, NMR, and mass spectra were obtained using ATR Shimadzu-IR Spirit analyser, 400-MHz Bruker spectrometer, and Agilent 6430 Triple Quad instrument (electrospray ionization), respectively.The absorption and emission studies were performed using 1800 Shimadzu UV-visible spectrophotometer and JASCO spectrofluorometer FP 8300 instruments.

Cell culture and cell viability assay
About 1 × 10 5 cells were seeded per well in a 96-well plate and incubated for 24 h at 37 °C in humidified 5% CO 2 incubator (Sanyo, UK).The cells were treated with different concentrations of BTNP and incubated for 24 h.Further, the cells were washed, and 50 μL of MTT (2 mg/mL) was added and incubated in the dark for another 3 h.Finally, the entire medium was removed from the wells, incubated for 15 min in the dark after adding 50-μL DMSO, and the absorbance was measured at 540 nm.The percentage of viable cells was calculated using Eq. ( 1).
(1) The coatings and screen prints were obtained on UV dull paper substrates procured from Manipal Press Pvt. Ltd., Manipal, India.A 120SS mesh screen was used for printing.The photostability of the deposited BTNP ink film was studied by subjecting the screen prints for different durations under UV light of 20 W. Sutherland ink rub tester was utilized to perform the rub test by laying the printed substrate against the blank substrate on the test rubber pad (4 lbs).Xrite I1 Pro spectrophotometer was used to check the colorimetric data.The values of CIELAB colour coordinates: lightness (L*) ranging from 0 (black) to 100 (white), a* (green for negative and red for positive), and b* (blue for negative and yellow for positive) extending from -120 to 120 were explored.The colour differences (ΔE) were calculated using Eq. 2 (Żołek-Tryznowska et al. 2015;Muthamma et al. 2021). where -b * are the variations between the screen printed, and (u) is the blank/unprinted paper samples (Żołek-Tryznowska et al. 2015).The surface morphologies of the blank and screenprinted samples were imaged using INNOVA SPM atomic force (AFM) and Carl Zeiss EVO 18 analytical scanning electron microscopes (SEM). (2) Scheme 1 Synthetic route of BTNP

Results and discussion
The chalcone BTNP was synthesized via a base-catalyzed Claisen-Schmidt condensation reaction.

Structural characterization of BTNP
The spectral characterization data obtained were in accordance with the molecular structure of BTNP.The characteristic peaks for C=O and C=C in IR spectrum of the chalcone were obtained at 1651 cm −1 and 1570 cm −1 , respectively.The 1 H NMR spectrum displayed the methoxy protons at 3.8 ppm and the aromatic protons around 7.0-8.4ppm.The aliphatic carbon and the carbon of the carbonyl group appeared at 54.43 and 187.99 ppm in the 13 CNMR spectrum, and the mass spectrum was in agreement with the structure of BTNP.

Optical features of BTNP
To get insights into the optical characteristics of the chalcone, its electronic spectral traces were examined.The absorption spectrum of BTNP dissolved in THF (1 × 10 -5 M) as presented in Fig. S5 exhibited two absorption peaks at 329 and 406 nm owing to the transitions of the conjugated structural framework as well as π-π * and n-π * transitions (Al-Ansari 2016; Omar et al. 2018).The emission spectrum of solid BTNP was recorded (Fig. 1a) to comprehend the fluorescence features for its use as a colourant in water-based ink.BTNP strongly emits at 545 nm, which corresponds to yellowish-green fluorescence as observed from the chromaticity diagram presented in Fig. 1b (λ ex = 490 nm).The sensitivity of BTNP to general solvents helps in analysing its solution phase emission characteristics.The emission spectrum of BTNP dissolved in THF (1 × 10 -4 M) was recorded upon excitation at 360 and 440 nm and is portrayed in Fig. 1c, which shows two distinct peaks at around 490 When fluorophores are subjected to change in solvent polarity, emission peaks may shift mainly due to the chemical interactions not only between the reactive domains induced in the surrounding solvent and the dipole moment of the fluorophore, but also between one or more solvent molecules and the fluorophore (Lakowicz 1983).Therefore, to determine the impact of solvent polarity on the fluorescence of BTNP, the emission characteristics in THF/THF-hexane (1 × 10 -4 M) mixtures were studied.The respective spectrum as shown in Fig. 2a displays a reduction in the fluorescence intensity together with a hypsochromic shift from 490 to 460 nm, indicating that the molecule under study undergoes an intramolecular charge transfer phenomenon (Sowmiya et al. 2011).The density functional theoretical (DFT) studies were executed to generate the optimal molecular geometry (Fig. 2b), highest occupied (HOMO: Fig. 2c), and lowest unoccupied molecular orbitals (LUMO: Fig. 2d).It is observed that the electron density in HOMO is localized in the bithiophene moiety, whereas the electron cloud is present exclusively on the spacer and thiophene moiety in the LUMO.The energy values calculated for HOMO and LUMO are − 5.294 eV and − 2.281 eV with an optical band gap of 3.013 eV.
The conformation of chalcone with extended π-conjugation is generally responsible for the origin of aggregation-induced emission (AIE) that can result in intensified solid-state emission, which have garnered research focus due to their wide potential applications (Kagatikar and Sunil 2021).Therefore, AIE property of BTNP was studied by recording the emission spectra in THF/THF-water mixtures (1 × 10 -4 M) with increasing water percentages from 0 to 99%.The samples were excited at 360 nm, and it is observed from the spectra (Fig. 3a) that 0% water fraction has emission at 490 (major peak) and 530 nm (minor peak).With the increase in water content, fluorescence intensity increases and reaches a maximum at 60% water fraction, whereas the fluorescence peak from 530 nm is shifted to 547 nm with high intensity (emission corresponding to solid-state emission), indicating the aggregate formation.The photographs of the respective solutions in daylight and under 365-nm UV illumination are presented in Fig. 3b.

Cytotoxicity studies
The biocompatibility of BTNP was inspected in Vero cells.The MTT assay demonstrated a low toxicity profile of BTNP in Vero cells at low concentrations.The cell viability plot against the studied BTNP concentration is shown in Fig. 3c.

Printing ink features and assessment of screen print properties
Screen printing is mainly used in the printing of fluorescent posters, stickers, display showcards, water-slide transfers, etc. UV fluorescent BTNP was used to prepare a water-based screen-printing ink (Fig. S6), which demonstrated particle size below 10 µm.The carrier/solvent of the ink was primarily water along with other additives, while the pigment BTNP served as the fluorescent colourant as mentioned in the experimental part.The ink displayed bright yellow fluorescence upon illumination with a 365 nm UV light source.Surface interactions between printing ink and the cellulose substrate are vital to obtain an adherent and uniform print.The UV dull paper substrate was initially manually coated with BTNP ink using a zero number bar coater to check the colour mixing and print properties.Further, the fluorescence spectrum (Fig. 4a) collected for the ink-coated paper sample displayed an emission at 543 nm.The chromaticity diagram as portrayed in Fig. 4b indicates that the fluorescence emission from the coated paper sample corresponds to the solid-state fluorescence of BTNP.Moreover, the spectra also provide evidence for the increase in fluorescence emission intensity with rising film thickness (Fig. 4a) upon coating the substrate twice and thrice with the ink.The screen prints obtained using the BTNP formulation on UV dull paper substrate dried immediately at room temperature and showed intense yellowish-green fluorescence under 365 nm UV illumination as presented in Fig. 4c.Further, the corresponding emission was perceived at 543 nm as depicted in Fig. 4d.In addition, the lightfastness of the deposited ink film was probed, wherein the printed samples were exposed to UV light of 20 W for 10-50 min.The emission spectra recorded as depicted in Fig. 4e indicate that the fluorescence intensity of the samples slightly decreased till 30 min of UV exposure and did not show any significant reduction in intensity even up to 50 min of UV exposure, suggesting good lightfastness of the BTNP ink film.The corresponding chromaticity plots (Fig. 4f) also direct towards the good UV stability of the water-based formulation on the flexible porous substrate.

Colorimetric studies of the screen-printed paper samples
The colour coordinates can be determined using basic colorimetric measurements.The International Commission on Illumination (CIE) provides an approach to achieve numbers that measure the sample colour as observed under a standard illumination source.The L*, a*, and b* values for the coated and screen-printed samples specify that with an increase in the number of coatings (C1-C3), the b* value increases and is greater than a* indicating the dominance of yellow colour.The increasing ΔE value for the coated samples indicates that the colour becomes brighter.The colour alterations of the screen prints after UV exposure and rub test were measured using CIELAB colour space and are listed in Table 1.Although the b* value is greater than a*, with increasing UV exposure, there is a decrease in b* and ΔE values, which The introduction of 6-methoxynaphthalene moiety into the bithiophene chalcone framework rather than 4-fluorophenyl (BTCF) as in our earlier report (Muthamma et al. 2023) has brought in more intense emission due to extension in the delocalization of electrons.The screen print proofs obtained using BTNP ink displayed superior features compared to the BTCF ink film obtained on the paper substrate using flexography.Though the print properties of the ink films obtained through screen and flexo printing processes cannot be directly compared, generally the BTNP ink film was found to be superior in terms of lightfastness and rub resistance.

Surface topographies of the screen-printed paper samples
The surface topographical characteristics of both blank and screen-printed papers were examined, and the scanning electron microscopic images captured are shown in Fig. 5.The blank paper displayed a fibrous and porous texture (Fig. 5a), but the print demonstrated an even and smoother surface (Fig. 5b) because the screen print develops a consistent and uniform ink film on the porous substrate.The formation of a uniform homogenous ink film is further supported by the AFM images for blank (Fig. 5c) and printed papers (Fig. 5c) with R q , R a , and R max values of 158 nm, 124 nm, and 2003 nm for the unprinted and 119 nm, 97 nm, and 735 nm for the screen-printed paper samples.

Security applications
The BTNP ink which shows an intense yellowish-green UV fluorescence could find potential applications in anticounterfeiting, automatic identification, and data security.Moreover, the biocompatibility of the chalcone-derived water-based ink enables its usage as an alternative to the traditional inks in thumbprint inkpads and stamp pads, which is an effective and simple technique to allow the safety of security documents against fraud.The BTNP ink on cost-effective, recyclable, eco-friendly, and flexible paper substrates could also be explored for printed technologies in electronic anticounterfeiting including RFIDs, wherein genuine products/items can be identified and tracked.The water-based ink can also be used to add more security features as labels or in packaging against fraudulent replication.

Conclusion
The preparation of a bithiophene-naphthalene chalcone BTNP for potential usage as a colourant in the formulation of a water-based UV fluorescent screen-printing ink, which could expand future practical applications is reported in the present study.The pigment synthesized using a simple aldol condensation reaction showed aggregation-induced emission and intramolecular charge transfer features.The UV fluorescent screen prints exhibited fast drying, good adhesion, light stability, and rub resistance on porous paper substrate.The eco-friendly ink can find potential applications in diverse fields including data security, inkpads, and in electronic anticounterfeiting for product tracking and identification.Moreover, the BTNP ink being environment-friendly and less toxic can be used in the labelling and packaging industry.

Fig. 1
Fig. 1 BTNP in solid state: a excitation and emission spectra and b corresponding CIE plot.BTNP in THF (1 × 10 -4 M): c emission and excitation spectra and d corresponding CIE plot

Fig. 2 a
Fig. 2 a Fluorescence spectra of BTNP in THF-hexane mixtures with varying hexane fractions (1 × 10 -4 M) at ex = 360 nm, b optimized geometry, c HOMO, and d LUMO energy levels of BTNP

Fig. 3 a
Fig. 3 a AIE phenomenon of BTNP in THF-water mixtures with varying water fractions (1 × 10 -4 M) at ex = 360 nm, b photographs of BTNP in THF/THF-water mixtures (from 0 to 99%) in daylight and illumination with 365-nm UV light, and C) MTT assay plot of BTNP

Fig. 4 a
Fig. 4 a Emission spectra and b respective chromaticity diagram of ink-coated paper sample, c photographs of the screen-printed sample under daylight and UV light, d emission spectra of screen-printed

Fig. 5
Fig. 5 SEM images of a blank and b screen-printed and AFM images of c blank and d screen-printed UV dull paper samples

Table 1
Comparison of colorimetric values of coated and screen-