Unveiling the Effect of Dilution on the Optical Response of Folic Acid Derived Carbon Dots: Role of Surface Interactions and Inner Filter Effect

The optical properties of Carbon dot (CD) solution significantly change on dilution. Herein, the effect of dilution on the optical properties of folic acid-derived CD was meticulously analyzed using absorption and photoluminescence (PL) spectroscopy. Absorption spectra of as-prepared CD solution consist of four overlapping yet discernable absorption bands centered at ~ 249, 302, 348, and 330 nm, respectively, attributed to originate from the π-π^* transition and n-π^* transitions and/or surface states. As the CD solution is diluted from 100 to 5%, these four absorption bands become more resolved. Moreover, we observed a blueshift of ~ 30 nm for the transitions at ~ 302 nm due to surface state with dilution up to 20% CD concentration. This is attributed to the decrease in the interaction between surface states due to the increase in the interparticle distance with dilution. PL emission from the as-prepared CD solution is centered at 463 nm and is asymmetric. This can be resolved into three components centered at 446 nm (intense), 474 nm (intense) and 508 nm (weak) respectively. With dilution, the PL intensity corresponding to the 463 nm emission seems to increase up to an optimum concentration of 15% CD and then decreases. The high concentration effectively quenches the luminescence through inner filter effect which is evident from the overlapping of absorption peak with the peak in the excitation spectrum together with no notable change in the average decay time. The decrease in the percentage of overlapping area of the absorption and excitation spectra with dilution causes the reduction of inner filter effect and enhances the luminescence for diluted solutions. Furthermore, we found that the surface states become more dominant in the contribution of luminescence of CD, whose influence diminishes in extremely diluted solutions, thereby the intensity decrease below 15% dilution.