Faculty Publications
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Item Dual emissive water-based flexo ink from tapioca-derived carbon dots for anti-counterfeiting applications(Elsevier B.V., 2022) Ullal, N.; Lewis, P.M.; Dhanya, D.; Kulkarni, S.D.; P.j, A.; K, U.B.Counterfeiting of high-value items is a challenging menace worldwide, and luminescent nanoparticles-based security inks have promising applications while addressing this global issue. As Carbon dots (CDs) show attractive functional properties, hydrophilic CDs were prepared through hydrothermal approach from tapioca starch as an eco-friendly precursor. CDs with bluish-green fluorescence emission under 365 nm UV light illumination was obtained using column chromatography technique. TCSPC studies indicated the presence of blue and green emitters with average lifetimes of 1.12 and 1.61 ns, respectively. The graphitic and polycrystalline nature of CDs (~9 nm) with negatively charged surface groups was confirmed through ATR-IR, XPS, RS, XRD, SAED, DLS and TEM. An eco-friendly water-based security ink for flexography printing was formulated, and the influence of the ink components on the fluorescence of CDs were studied. The prints on UV dull paper displayed good abrasion resistance, densitometry and colorimetric values. The letters written using the invisible ink exhibited good security features including excellent covertness under daylight, and a bluish-green emission under 365 nm UV light. Moreover, a yellow emission was perceived using a yellow-orange pass filter under the same light source. The surface morphology and electrical property investigations suggested the use of flexo-ink in flexible printed electronics for anti-counterfeiting. These results propose that Tapioca-derived CDs can enable unique optical features in its eco-friendly ink formulation to demonstrate promising security applications. © 2022 Elsevier B.V.Item Eco-friendly ink formulation of column purified carbon dots from GABA for anticounterfeiting applications(Elsevier B.V., 2023) Ullal, N.; Dhanya, D.; Kulkarni, S.D.; Sinha, R.K.; Anand, P.J.; Bhat K, U.K.Forgery of valuable products causes a negative impact on the society as well as the economy of the country. There is a growing demand to not only differentiate or authenticate genuine documents/products but also to protect their integrity. Carbon dots (CDs) are a class of fluorescent nanomaterials that are well-known for their facile synthesis, good photostability and less toxicity profile. The current research work focuses on the preparation of CDs via hydrothermal method using γ-aminobutyric acid (GABA) that contains amino and carboxylic groups. Column chromatography technique is adopted to purify the GABA-derived CDs from the reactants and by-products. The fourth fraction obtained after column purification containing CDs with fluorescence emission in the visible region is chosen for further studies. The presence of spherical CDs confirmed through TEM imaging are chemically characterised using SAED, EDS, DLS, FTIR, XPS, Raman and XRD spectroscopy. The blue (λem = 490 nm) and green (λem = 538 nm) emitting CDs present in the fourth fraction displayed a fluorescence lifetime of 1.90 ns and 2.02 ns. Theoretical studies are performed using B3LYP/6-31G(d,p) theory level on different plausible structures. The HOMO-LUMO band gap of 2.3 eV deduced using DFT calculation is in close agreement with the optical band gap of 2.6 eV derived from Tauc plot. A complex forming mechanism is proposed for the fluorescence quenching of CDs upon examining the EDS data of the precipitate obtained upon addition of cupric ions. Further, the CDs are used as pigments to formulate a water-based ink for flexographic printing on UV-dull paper substrate. The printed samples exhibited good colorimetric values, lightfastness, and rub resistance. The security features of the ink film include an UV-induced yellow fluorescence, which will be known to the forger and a secondary quenching of fluorescence when exposed to cupric ions, which can be used by the user to validate document/product authenticity. This stimulus responsive optical property is also explored in the design of ionochromic security paper based on cupric ion induced fluorescence quenching of CDs for data storage and decryption. Moreover, the lesser surface roughness and electrical parameter values obtained for the print proofs could be further explored for the potential application of CD-derived ink in electronic anticounterfeiting. © 2023 Elsevier B.V.Item Yellow emissive and high fluorescence quantum yield carbon dots from perylene-3,4,9,10-tetracarboxylic dianhydride for anticounterfeiting applications(Royal Society of Chemistry, 2024) Ullal, N.; Sahoo, B.; Dhanya, D.; Kulkarni, S.D.; Bhat K, U.; Anand, P.J.Forged products are widespread in the market and there is an immediate need to counter this growing menace. Anti-counterfeit techniques using fluorescent materials with covert features that appear hidden under daylight and display characteristic fluorescence upon specific source irradiation have gained popularity. Carbon dots (CDs) that can be prepared through facile synthesis from various raw materials are a class of fluorescent materials that provide tremendous opportunities to combat counterfeiting. This work focuses on the fabrication of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) derived CDs via the solvothermal approach and their subsequent purification using column chromatography. The fifth fraction obtained exhibited remarkable yellow emission (λem = 540 nm) with a high fluorescence quantum yield of 53.22% and a lifetime of 4 ns. The CDs appeared quasi-spherical during TEM imaging with an average diameter of 1-3 nm and appeared polycrystalline from the SAED pattern. The XPS and TEM-EDS results suggested carbon as the major element along with oxygen and nitrogen as the other heteroatoms. The water-based ecofriendly ink formulated using the CDs was printed on UV dull paper using the flexography technique. The print-proof paper samples appeared pale pink under daylight and fluorescent yellow upon 365 nm UV illumination. Moreover, the stability of the print was confirmed upon exposure to strong UV radiation cycles and abrasion resistance. Besides, the fluorescence emission remained unaltered even after 5 months of storage under room temperature conditions. The ink was used to print on PVC sheets and FBB boards with good stability against scuffing, suggesting its applicability in the packaging industry. The CDs could also serve as fluorescent markers for identifying post-consumer plastic packaging for a circular economy. © 2024 The Royal Society of Chemistry.Item Pyrene carbaldehyde derived carbon dots for detecting water in alcohol and security printing(Elsevier B.V., 2024) Ullal, N.; Sahoo, B.; Dhanya, D.; Kulkarni, S.D.; Bhat K, U.; AnandThis study focuses on preparing Carbon dots (CDs) from Pyrene-1-carbaldehyde (PCA) using a solvothermal method and further purification using column chromatography. The aggregation-induced emission (AIE) of CDs was systematically investigated in a THF/water medium. The CDs showed red shifts in their photoluminescence (PL) spectra upon increase in water content. Scanning electron microscopic (SEM) images revealed the formation of aggregates, while X-ray diffraction (XRD) confirmed that the d-spacing values remains unchanged. The NMR spectrum of the CDs displayed peaks corresponding to aromatic carbon, which disappeared upon addition of water due to ?-? stacking, indicating aggregate formation. Based on the aggregation-induced fluorescence emission mechanism, detection of water content in alcohol is demonstrated. Moreover, the synthesized CDs were used as fluorescent colorant in screen inks along with polyvinyl alcohol (PVA) and hydroxyethyl cellulose (HEC) as binders. The print proofs obtained on UV-dull paper using PVA-based screen ink exhibited fluorescence emission at longer wavelengths and showcased desirable photostability under prolonged UV exposure compared to the prints obtained using HEC-based ink. Moreover, though the PVA based print appeared blue or cyan fluorescent, the actual yellow emissions from the CDs can be visualised using UV block filter. Such features, masked to the forger, but known to the user can be utilised in checking the authenticity of the print. © 2024 The AuthorsItem Investigation of column purified dye derived carbon nanomaterials for security printing and supercapacitor applications(Elsevier B.V., 2024) Mehta, R.; Ullal, N.; Sahoo, B.; Dhanya, D.; Kulkarni, S.D.; Sudhakar, Y.N.; Abdul Salam, A.A.; K, U.B.; Anand, P.J.Literature evidence reveals versatile applications of carbon dots (CDs), but generally mixtures of various types of carbon nanomaterials, molecular intermediates as well as side products are obtained upon hydrothermal treatment of the precursor material. This demands isolation of pure components and their complete characterization before these nano carbonaceous materials are chosen for suitable applications. In the present study, perylenetetracarboxylic dianhydride (PTCDA) is subjected to hydrothermal treatment and the mother liquor obtained is separated using column chromatography technique using dichloromethane-methanol solvent system to isolate fractions of various fluorescent carbonaceous nanostructures. The TEM imaging of nano carbonaceous particles of all five fractions indicated that the first and third fractions were composed of nanoribbons, while the latter two fractions largely contained quasi-spherical nanoparticles of both lesser (carbon quantum dots) and greater (carbon nanodots) than 10 nm dimensions. The XPS results of all the fractions suggested separation based on polarity difference. The ID/IG ratios obtained from Raman spectra implied the presence of several defects on the CDs. The time resolved fluorescence spectra of third, fourth and fifth fractions revealed mono-exponential decay of fluorophores with excitation independent average lifetime values. The fifth fraction exhibited good biocompatibility and the highest absolute fluorescence quantum yield of 58.47 % among all the isolated samples. As these CDs displayed a remarkable rise in the quantum yield to 88.60 % when dispersed in water, a water-based flexo-ink was formulated. The photostable pale yellow flexo print proofs obtained on UV dull paper exhibited a green fluorescence under 365 nm illumination, whereas a yellow glow when shined with blue light, which can serve as an authentication feature for security documents and currency notes. Moreover, as the third fraction constitutes mainly of carbon nanoribbons (CNRs), an optimized polymer electrolyte was prepared along with sodium alginate (SA), and MgCl2 to understand their potential use in energy storage application. A supercapacitor was fabricated and tested for its electrochemical performance such as cyclic voltammtery (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (GCD). An enhanced current window was observed in the CV of SA/CNRs compared to pure SA and SA/CNRs/Mg films, which indicated a structural interaction of CNRs with SA. The conductivity of SA/CNRs/Mg was lesser than SA/CNRs in EIS studies due to the presence of Mg ions, while pure SA showed lesser conductivity. The dual ionic interaction of Na and Mg along with enhanced structural stability due to doped CNRs favors its convenient supercapacitor application. The fabricated eco-friendly supercapacitor showed a specific capacitance of 84 F/g. The GCD of the device displayed pseudocapacitance behaviour and was quite stable for 2000 cycles with coulombic efficiency of 96 %. © 2024 The Author(s)