Novel red-emitting CDs@LaCaAl3O7:Eu3+ nanocomposites: A sustainable breakthrough for optical thermometry, indoor plant growth and intelligent security labels

Abstract

A series of melilite LaCaAl₃O₇:xEu³⁺ (x = 1–11 mol%) nanoparticles (LCAO:xEu³⁺ NPs) are synthesized using a solution combustion method with spinach extract as a bio-fuel. When carbon dots (CDs) derived from papaya seeds via microwave-assisted synthesis are introduced into LCAO:7Eu³⁺ NPs, the resulting bright red-emitting CDs(y)@LCAO:7Eu³⁺ (y = 2, 4, 6, 8 wt%) nanocomposites (NCs) exhibited a wide excitation band peaking at approximately 395 nm, aligning well with the emission range (550–750 nm) of near-ultraviolet (n-UV) LED chips. Remarkably, the CDs (4 wt%)@LCAO:7Eu³⁺ NCs demonstrated a 20-fold increase in PL intensity compared to LCAO:7Eu³⁺ alone, attributed to Förster Resonance Energy Transfer (FRET) between CDs and Eu³⁺ ions. This enhancement likely arises from the CDs capturing electrons and transferring energy to Eu³⁺ ions, offering a simple and eco-friendly strategy to improve luminescent properties. Furthermore, the material retained 91.5 % of its luminescence intensity at 420 K, with an activation energy of 0.39 eV, showcasing exceptional thermal stability. With an internal quantum efficiency (I_QE) of 89.58 % and impressive color purity (99.7 %), the composite achieved a relative sensitivity of 3.10 % K⁻¹ at 300 K, highlighting its suitability for non-contact optical thermometry. The material exhibited minimal chromaticity shifts even at high temperatures, ensuring excellent chromaticity stability. A white light-emitting diodes (w-LEDs) fabricated using commercial blue and green phosphors combined with CDs (4 wt%)@LCAO:7Eu³⁺ NCs on a 395 nm near ultra violet (n-UV) LED chip achieved a color rendering index (CRI) exceeding 90 under varying currents. Additionally, the NPs demonstrated remarkable potential for indoor agriculture, promoting chilli plant growth by increasing height, stem diameter, and leaf width by 6.17 %, 21.42 %, and 10.65 %, respectively, and boosting total chlorophyll content by 19.50 % compared to one grown under natural light. Furthermore, luminescence-based binary and quick response (QR) code encryption showcased its advanced applications in optical security. These findings emphasize the material's versatility for sustainable indoor plant cultivation and anticounterfeiting (AC) technologies.