Caesalpinia sappan L., known as sappan wood, has been extensively explored by many researchers because it contains various structural types of phenolic compounds. This study objectives are to investigate the effect of pH on the flourescent color of carbon dots (CDs) from sappan wood and its application as an acid-base indicator under UV light for the first time. CDs were synthesized through the hydrothermal route in ethanol solvents. The synthesized CDs were further characterized by UV-Vis spectrophotometry, fluorescence and Fourier Transform Infrared (FT-IR) spectroscopy. Based on the results, CD solutions performed multiple flourescent colors (yellow, orange and green) under UV light at 365 nm when the pH was adjusted to be 1, 3, 5, 7, 9 and 11. These multiple colors were produced because of the instability of phenolic compounds in the sample, especially brazilin which was easily oxidized to brazilein in alkaline condition. According to the FT-IR spectra, the most prominent differences of the sample before and after the treatment process was observed in the peak region ranging from 1500-1700 cm-1. The characteristic band of carbonyl groups was identified at 1697 cm-1 in the treated sample (pH 9), but it was unobserved in the spectrum of sappan wood and the treated sample (pH 5). The presence of carbonyl group can be assumed that brazilin has been oxidized to brazilein after the alkaline treatment process and making the flourescent color was gradually shifted from yellow to green. Under normal light, the sappan wood�s extract and CD solution produced yellow and red colors in acidic and alkaline conditions, respectively. However, only CD solutions which have yellow, orange and green emissions in different pH conditions under UV light. Therefore, the synthesized CDs can be used as an acid-base indicator in both different light conditions.

Synthesis of carbon dots (c-dots) from vechile waste-oil is one of the alternatives for reducing the burden of environmental pollution. This study aimed to develop a green and economical strategy to produce C-dots by using motor oil waste. C-dots were synthesized by heating motor oil waste in the various percentage of sulfuric acid solution (95, 70, 30, and 0% H2SO4). The synthesized C-dot was characterized by UV-Vis spectrophotometry and Fourier Transform Infrared (FT-IR) spectroscopy. The result showed that the carbon dot produces blue color at 365 nm of UV light. This color was attributed to the oxidation of chemical content in motor oil waste. Therefore, four peaks were obtained at 203 nm, 204 nm, 215 nm, and 210 nm for all samples. Based on FT-IR spectra, the most prominent sample difference was observed at peak 1630 cm−1. The characteristic band of the carbonyl group was identified at the c-dot concentration (70 and 95%) but was not observed in the c-dot concentration (0 and 30%). The presence of carbonyl groups caused the fluorescent color to gradually shift from dark blue to light blue.

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