Browsing by Author "Sinha, R.K."

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5,10,15,20-Tetrakis(p-tolyl)porphyrin derived carbon dots as colorant in flexo and screen inks with multi-level covert features for security printing
(Elsevier B.V., 2024) Ullal, N.; Sahoo, B.; Dhanya, D.; Kulkarni, S.D.; Sinha, R.K.; Anand, P.J.; Udaya, B.K.
Fluorescent inks with covert security features, which make it difficult for forgers to replicate, are highly sought after among the array of anti-counterfeit technologies. The present study focuses on the synthesis of Carbon dots (CDs) from 5,10,15,20-tetrakis(p-tolyl)porphyrin (TTP) via a solvothermal approach. The prepared CDs exhibit attractive emissive characteristics with an absolute fluorescence quantum yield of 38.85 %. Time-resolved fluorescence studies reveal the multiple exponential decay nature of the emitters, which could be due to the diverse chemical environment on the surface of the CDs. XPS analysis proves oxygen as the major element, along with carbon, sulfur, and nitrogen as minor elements. DFT studies on four optimized structures of CDs along with their respective UV and FT-IR plots are conducted. The theoretically derived photoluminescence plots corresponding to the 25-30th excited states coincide with the experimental emission peak. CDs demonstrate characteristic variations in fluorescence in the presence of water, attributed to ?-? stacking induced aggregation. Aggregate formation is also influenced by pH changes, affecting the surface functional groups in proximity to the solvent medium. CDs are used as colorants in the fabrication of both flexo and screen inks. Flexo ink retains the fluorescence features of the CDs, while screen ink offers multilevel security features. Exposing UV-dull paper to 365 nm UV light allows the ink inscribed letters to appear fluorescent blue, whereas the use of UV/green optical filters facilitates the visualization of green/yellow emissions. The introduction of multilevel features makes the duplication of the print difficult, protecting the integrity of the product and could be used for real-life applications. © 2024 The Author(s)
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.
New benzisoxazole derivative: A potential corrosion inhibitor for mild steel in 0.5 M hydrochloric acid medium -insights from electrochemical and density functional theory studies
(Elsevier Ltd, 2023) Kumari P, P.P.; G, A.; Mishma, J.N.C.; Sinha, R.K.; Suvarna, A.S.; Gaonkar, S.L.
6-fluoro-3-(4-piperidinyl)-1,2-benzisoxazole. HCl (FPBH), a substituted benzisoxazole derivative, was prepared from isonipecotic acid and characterized using various spectroscopic techniques. Using electrochemical examinations such as potentiodynamic polarisation (PDP) and electrochemical impedance spectroscopic (EIS) technique, the corrosion mitigation capabilities of this compound for mild steel (MS) in 0.5 M HCl medium were investigated. Theoretical studies were performed using quantum chemical calculations and density functional theory (DFT). PDP results exhibited the mixed-type behavior of FPBH and showed a maximum efficiency of 94.5 % at 1 × 10−3 M. The development of a protective adsorbed layer of FPBH decreases the corrosion current density (icorr) and corrosion rate (CR). The EIS technique revealed that the rise in the charge transfer resistance (Rct) values and reduction in the thickness of the double-layer capacitance (Cdl) reflected the drop in corrosion rate. The adsorption of FPBH took place through physisorption by conforming Langmuir's isotherm. The DFT method was performed on the optimized structure of FPBH to get additional evidence on the action mode of FPBH with the metal surface. © 2023
NUMERICAL AND EXPERIMENTAL INVESTIGATION INTO THE EFFECT OF LEADING-EDGE PROTUBERANCES ON THE AERODYNAMIC PERFORMANCE OF WIND TURBINE
(Begell House Inc., 2025) Sathyabhama, A.; Sinha, R.K.; Reddy, C.J.
In this paper, the numerical and experimental analysis of the effect of leading-edge protuberances on the performance of small horizontal axis wind turbines (SHAWT) at low Reynolds number was carried out. The wind turbine blades were designed using the blade element momentum theory (BEMT) with wake rotation. The E216 profile was chosen over other airfoils because, in low Reynolds number flow conditions, it gives a high lift-to-drag ratio. The tubercle shapes employed for the study are slot, triangular, and sinusoidal, and their effects on the performance of wind turbine were compared with baseline turbine as well as among themselves. The flow behavior and the influence of pitch angle on the performance of baseline wind turbine were investigated. The numerical simulations were conducted in ANSYS FLUENT R2021, and the experiments carried out in a low-speed wind tunnel were used to validate the results. The numerical equations were solved using a three-dimensional Reynolds-averaged Navier-Stokes equation with a shear stress turbulence (SST) k-? turbulence model. The output power, torque, and coefficient of power (CP) values for the baseline turbine increased up to 25° pitch angle and afterwards, a decline was seen. The optimum tip-speed ratio (TSR) was also investigated and found to be 2.67. The pitch angle 25° provides the greatest improvement among all pitch angles examined for the same blade profile. Hence, for the study of different-shaped tubercles (triangular, sinusoidal, and rectangular slot) pitch angle of 25° was considered. Sinusoidal tubercles show a greater lift-to-drag (CL /CD) ratio than baseline wind turbines, although there is no substantial difference in CP. Furthermore, the CL /CD for triangular and slotted tubercles is more significant than that of the baseline wind turbine, as is the CP. When all three tubercles are compared, the slot has the highest CP, while the sinusoidal wind turbine has the highest CL /CD. © 2025 by Begell House, Inc.