Figure 2

Characterization of the GQDs and the GQD-modified strip. (A) Absorbance (black line) and photoluminescent (red line) spectra. Iinset, a picture of the GQDs under UV (right) and visible (left) light, respectively. (B) FT-IR spectrum of citric acid (black line) and GQDs (red line), respectively. (C) TEM image (upper and bottom scale bars are 5 nm, respectively). Inset is the plot of particles size distribution. GQDs lattice planes are denoted by yellow circles. (D) Optimization of the GQDs-modified paper-based sensing platform in terms of fluorescence intensity and homogeneity of the sensing area. (1) Concentration of GQDs solution (a) 6.25, (b) 12.5, (c) 25 and (d) 50 mg/ml. (2) Volume of GQDs solution, (a) 1, (b) 2, (c) 3, and (d) 4 µL. (3) GQDs pH value, (a) 4, (b) 6, (c) 8, (d) 13. (4) Number of additions of a volume of 4 µL, (a) 1, (b) 2, (c) 3, and (d) 4 times. All of them placed into the wax-patterned areas from the nitrocellulose support. The concentration of GQDs solution is always 50 mg/ml except in (a). (5) 0 mg/ml GQDs (from a to d) used as control to discart other possible interferences. Experiments were performed in quadruplicate and represented in each row (a–d).