Nonlinear Optics to Glucose Sensing: Multifunctional Nitrogen and Boron Doped Carbon Dots with Solid-State Fluorescence in Nanoporous Silica Films

Multifunctional triple color photoluminescent (PL) nitrogen–boron doped carbon quantum dots (CQDs) with high quantum yield (QY) of 58% are fabricated by one step femtosecond pulsed laser irradiation of a single precursor (2-aminopyrimidine-5-boronic acid) in solution. In situ generated non-linear and linear emissions are used to monitor CQDs formation which results in enhanced second harmonic generation, two photon absorption (2PA), and linear fluorescence; implying triple mode emission. These CQDs present blue, green, and possible red color rendering which are mostly independent to the respective excitation wavelengths (λ) with large stokes shift of 100 nm. Solid-state photoluminescence with QY of 46% is achieved by incorporating CQDs into thin transparent nanoporous silica (pSiO₂) films (thickness 50 µm) to form a CQDs-pSiO₂ composite which exhibits reverse saturable absorption at λ = 800 nm with 2PA coefficient and excited state absorption cross-section of 4.94 × 10⁻¹⁰ m W⁻¹ and 6.23 × 10⁻¹⁷ cm², respectively. CQDs-pSiO₂ is also sensitive to glucose concentration down to 1.0 mg dL⁻¹ in a wide linear range up to 100 mg dL⁻¹. This work therefore demonstrates facile, controllable, and up-scalable bottom-up fabrication of CQDs forming multifunctional solid-state CQDs-pSiO₂ with proven application in optical limiting and glucose sensing.