Solid-State Fluorescent Carbon Nanodots: Synthesis Strategies, Emission Mechanisms, and Emerging Applications A Systematic Review

Abstract

Fluorescent carbon nanodots (CDs) have emerged over the past two decades as a biocompatible, low-cost, and structurally tunable alternative to semiconductor quantum dots and organic dyes. While CDs display strong photoluminescence in dilute solution, most formulations suffer from aggregation-caused quenching (ACQ) once concentrated into films, powders, or polymer matrices a limitation that has historically restricted their use in light-emitting diodes (LEDs), anti-counterfeiting tags, and solid dosage forms. This systematic review synthesizes recent primary literature (2022–2025) on strategies for achieving and stabilizing solid-state fluorescence in CDs, and on the industrial and biological applications this capability enables. Following a PRISMA-informed protocol, five peer-reviewed studies meeting predefined eligibility criteria were retained for narrative synthesis: a femtosecond-laser bottom-up synthesis elucidating the molecular-fluorophore origin of CD emission; a one-step, gram-scale, room-temperature synthesis exploiting J-aggregation to produce red-emitting electroluminescent CDs; a metal–organic-framework (MOF) encapsulation strategy yielding multicolor solid-state phosphors for LEDs; a Maillard-reaction-derived route generating CDs from thermally processed spices for food and nutraceutical applications; and an industrial upcycling route converting steel-plant coal-tar waste into CDs for fiber-reinforced polymer (FRP) composite tagging. Thematic synthesis identifies convergent solutions to ACQ supramolecular J-aggregation, rigid host-matrix immobilization (MOF, polymer, mineral), and dilute-matrix dispersion alongside a persistent unresolved debate over whether solid-state and even solution-phase CD emission originates from molecular fluorophores, graphitic-core quantum confinement, or crystallization-induced emission enhancement. Across the reviewed studies, quantum yields in the solid state ranged from 15% to over 80%, and demonstrated applications spanned electroluminescent displays, tunable white-light LEDs, forensic and industrial tagging, and food-derived nutraceutical carriers. The review concludes that mechanistic consensus remains elusive because CD photophysics are precursor- and route-dependent, and that future work should prioritize standardized characterization protocols, life-cycle and toxicological assessment of waste-derived CDs, and scale-up of J-aggregation and MOF-encapsulation strategies for commercial optoelectronic and anti-counterfeiting applications.