Cx 551 ph meter

UV–VIS analysis was carried out using an Agilent 8453 diode array spectrophotometer [44 (link)]. Fluorescence study was carried out using a Jasco FP-750 spectrofluorometer (JASCO, Tokio, Japan). Quantum yield was determined by the comparative method described in our previous article [44 (link)]. The study PL-pH dependence was carried out at pH = 4, 5, 6, 7, 8, 9. The pH was adjusted using an Elmetron CX-551 pH-meter (combined glass pH electrode, Elmetron, Zabrze, Poland). The solvatochromism study was carried out using the following solvents: water, acetone, acetonitrile, tetrahydrofuran, and methanol. Infrared analysis was performed using a Nexus 470 diamond crystal ATR FTIR spectrophotometer (Thermo Fisher Scientific (Waltham, MA, USA).

The UV–Vis spectra were collected using Agilent 8453 Diode array spectrophotometer [3 (link)], whereas fluorescence spectra were collected with the use of Jasco FP-750 spectrofluorometer. The fluorescence quantum yield (QY) was determined according to the previously described method using quinine sulfate solution as in reference [3 (link)] according to Equation (1): $Q{Y}_s = Q_r\left(\frac{A_r}{A_s}\right)\left(\frac{E_s}{E_r}\right){\left(\frac{{\eta }_s}{{\eta }_r}\right)}^2,$
QY = fluorescence quantum yield;
ŋ = refractive index of the solvent;
A = absorbance of the solution;
E = integrated fluorescence intensity of the emitted light.
Subscripts ‘r’ and ‘s’ refer to the quinine sulfate (reference) and sample.
Solutions’ pH was determined with Elmetron CX-551 pH-meter. The pH-sensitivity tests were carried out in the range between 4 and 10.
Photostability was determined by placing CQDs solutions in quartz cuvettes which were exposed to the continuous irradiation with the mercury lamp (λ = 365 nm, power = 20 W). The QY was calculated after 7 and 30 days using Equation (1). The sensing ability studies were carried out using CQDs solutions with concentration of 0.20 mg/mL.

Carbon quantum dots were synthesized in a stainless-steel hydrothermal reactor (50 mL volume, 30 bar) (Toption, Shanxi, China) with a Teflon reaction vessel. To the starting material in the amount of 0.5 g (glucose, glucosamine, cellulose and chitosan) 0.5 mL of the hydrochloric acid and 0.1 g of the modifying agent (urea, urotropin) (Sigma-Aldrich, Poznań, Poland) was added. The reactors were placed in oven at 180 °C for 12 h. After the carbonization process suspensions were sonificated using Emmi-20 HC ultrasound bath (EMAG Polska, Juszczyn, Poland) and filtered by using membrane filters (VWR, Gdańsk, Poland) with a 0.22 µm pores diameter. All samples were neutralized by using 5% NaOH solution by using an Elmetron CX-551 pH meter (Elmetron, Zabrze, Poland) and the solutions were dialyzed by using dialysis tubing (MWCO 500–1000 Da) and water (VWR, Gdańsk, Poland) as the purifying agent for 4 days to remove small molecular weight compounds and inorganic ions. Prepared CQDs solutions were diluted for spectroscopic analysis. The reaction parameters and samples composition are given in Table 1.