Fig. 5. (A) Optical images of the wells of cellulose papers added with 10 μL of BCA assay and treated with D.I water (Group 3) and bleach (Group 4). Four groups of the cellulose samples were studied. Group 1: unheated pristine filter paper, Group 2: heated on a hot plate at 200 °C for 60 min, Group 3: heated and washed in D.I. water, and Group 4: heated and treated with bleach. Both the front side and the backside were shown for each group. (B) A graph showing the blue intensity of the wells. The lower blue intensity indicated the darker color of the well, suggesting the presence of the aldehydes in the well that produced the BCA complex via reduction of Cu²⁺ (n = 4).