Mechanical Force-Induced Color-Variable Luminescence of Carbon Dots in Boric Acid Matrix

Abstract

Mechano-luminescent materials that exhibit distinct luminescence responses to force stimuli are urgently anticipated in view of application needs in the fields of sensing, anti-counterfeiting, optoelectronic devices, etc. However, most of the reported materials normally exhibit force-induced changes in luminescent intensity, whereas materials that possess force-induced color-variable luminescence remain rarely reported. Herein, for the first time, a novel mechanical force-induced color-variable luminescence material from carbon dots (CDs) in boric acid (CD@BA) is reported. At low CDs concentration, the luminescence of CD@BA exhibits a grinding-induced color variable from white to blue. This grinding-induced color variable can be switched to yellow-to-white changing by increasing the CDs concentration in BA. The grinding-induced color-variable luminescence originates from dynamic variation in emission ratio of fluorescence and room temperature phosphorescence, due to the influence of oxygen and water vapor in the air. At high CDs concentration, short-wavelength fluorescence undergoes more severe reabsorption compared to room temperature phosphorescence, leading to grinding-induced color-variable switching from white-to-blue to yellow-to-white. Based on the unique properties of CD@BA powder, the applications of recognizing and visualizing fingerprints on the surfaces of various of materials are demonstrated.

Keywords: carbon dots; fluorescence; force-induced color-variable luminescence; room temperature phosphorescence.

Figure 1

Schematic illustration of the preparation procedure for the CDs and CD@BA bulk and powder.

Figure 2

(a) TEM image of the CD@BA and inset, corresponding HRTEM image; (b) FTIR spectra of CDs and CD@BA powder; (c) High-resolution B 1 s XPS spectrum of CD@BA powder; (d) UV-visible absorption spectrum of CDs aqueous solution; (e) PL emission spectrum of the CDs aqueous solution, PL and RTP emission spectra of the CD@BA bulk; (f) RTP emission spectra of CD@BA bulk at 298 and 77 K, under 365 nm excitation.

Figure 3

(a) PL emission spectra and digital picture of the CD@BA bulk and CD@BA powder at low CDs concentration under 365 nm lamp; (b) PL emission spectra and digital picture of the CD@BA bulk and CD@BA powder at high CDs concentration under 365 nm lamp; (c) XRD spectra of the CD@BA bulk and CD@BA powder; (d) The excitation spectra (λ_em = 530 nm) of CD@BA with different CDs concentrations and FL emission spectrum of the CDs aqueous solution (blue line).

Figure 4

Schematic of the proposed force-induced color-variable luminescence mechanism of CD@BA composites. Abs., absorption; FL., fluorescence; RTP, room temperature phosphorescence.

Figure 5

(a) fingerprint identification process of fingerprint morphology; (b) transparent, (c) opaque, and (d) fluorescent materials under different light conditions.