Fluorescent Carbon Dot Hydrogel Waveguides

Fluorescent carbon dots were synthesized from citric acid with hydrothermal method following previously reported procedures^{31 (link), 32 (link)} and the obtained solution was used for waveguide fabrication. To fabricate the fluorescent waveguide, the hydrogel precursor was synthesized by mixing degassed aqueous solution of 40% w/v PEGDA (700 Da, Sigma-Aldrich), cardon dots (0–12 × 10⁻⁵ % w/v) and 2% w/v 2-hydroxy-2-methyl-propiophenone (Sigma-Aldrich) in deionized water. Afterwards, the precursor was injected in a rectangular glass mold through a syringe and polymerized under UV irradiation (365 nm, 5 mW cm⁻²) for 5 min. The waveguide was then taken out after removing the cover slide of the mold. For light coupling, a silica multimode fiber (core/clad: 200/215 μm) was pigtailed to the waveguide by embedding a short section (~5 mm) of the fiber tip into the precursor and aligned to its center before photo-crosslinking. Additional epoxy resin was used to reinforce the joint. Optical losses of the waveguides at various thicknesses were measured by integrating the intensity at the width direction from the scattering pattern. To measure the refractive index of the carbon dots incorporated hydrogel, 300 μL of the precursor solution were placed on the prism of a digital refractometer (Atago) and the refractive index was measured after photo-crosslinking.