Fig 1

(a) Diagram of a wordless book. (b) Structural diagram of CNDs printed on paper fibers. (c) Fluorescence spectrum and lifetime decay curve of CNDs@fiber. (d) Phosphorescence spectrum and lifetime decay curve of CNDs@fiber. (e) Time-dependent phosphorescence spectra of CNDs@fiber. (f) Images of the standard testing card and the CND-inks-printed test card. The pixel analysis of high-resolution printing with 150 μm and 200 μm."

Fig 2

(a) TEM image of the CNDs, the insert are the HRTEM image and size distribution of the CNDs. (b) Fluorescence spectrum of CND inks, the insert image is the CND inks under UV light. (c) Fluorescence lifetime of CND inks. (d) SEM image of fiber with and without CNDs, and the corresponding EDS mapping images of C, O, N, and P elements. (e) Time-resolved emission and phosphorescence spectra of CNDs@fiber. (f) The phosphorescence intensity of the CNDs@fiber as a function of the cycle number of UV light activation. (g) The phosphorescence intensity stability of the CNDs@fiber after repeated washing cycles. (h) The phosphorescence intensity of the CNDs@fiber versus time under illumination of UV light (365 nm) for 8 days."

Fig 3

(a) Phosphorescence excitation-emission contours of the CNDs@fiber. (b) The temperature-dependent spectra of the CNDs@fiber. (c) Temperature-dependent phosphorescent decay curves of the CNDs@fiber. (d) The plot of phosphorescence intensity versus temperature. (e) The fluorescence (blue curve) and phosphorescence (green curve) spectra of CNDs@fiber under excitation at 365 nm at 77 K. (f) Schematic diagram of the luminescence mechanism of phosphorescent CND inks printed on paper fibers. (g) Printed images using phosphorescent CND inks on different types of paper fibers."

Fig 4

(a) 256 square blocks with different gray levels were fabricated using CND inks. (b) Pseudo-color images of 256 blocks (a) with different gray levels. (c) The Chinese character "Fu" was written on paper using CND inks, along with the corresponding pseudo-color image. (d) The images of circles of varying radii were printed using CND inks. (e) The color blocks were printed using CND inks of different concentrations, followed by pseudo-color analysis. (f) Different thickness lines were printed using CND inks. (g) An image of a feather was printed using CND inks. (h) Pixel analysis was conducted on different areas of the feather. (i) Images of a 200-page wordless book in daylight and dark conditions. (j) Analysis of the fidelity of millions of pixels printed with CND inks (the left is the printed image, and the right is the original image)."