Multifunctional Cu, N -doped Carbon Dots as Fluorescent Sensor for Dopamine with Additional Catalytic and Antibacterial Applications

Abstract

This work reports the synthesis of nitrogen doped carbon quantum dots (N/CQDs) and copper, nitrogen co-doped carbon quantum dots (Cu/N/CQDs) prepared by hydrothermal method. The size and morphology of Cu/N/CQDs was confIrmed by TEM. TEM analysis depicted the spherical shape of Cu/N/CQDs with an average size of 12nm. Structural confirmation of N/CQDs and Cu/N/CQDs was obtained by XRD and FTIR studies. The X-ray diffraction (XRD) pattern depicted amorphous nature ofN/CQDs and the crystalline nature of Cu/N/CQDs with copper peaks corresponding to 020, 021 , Ill, 200, 120,211 planes. The FTIR spectrum of N/CQDs showed peaks at 3450cm-I, 1706 cm-l and 898 cm-1 corresponding to O-H, C=O and N-H functional groups, while FTIR spectrum ofCu/N/CQDs show peaks at 3300cm-l , 1406cm-l , 980cm-1 and 450cm-l which corresponds to O-H, C-N-C, N-Cu-N and Cu-O functionalities, respectively. UV-visible spectral analysis ofthe N/CQDs exhibit absorption peaks at 228 nm which corresponds to 7t~7t * transition of C=C of aromatic structure, whereas the UV -visible spectrum of Cu/N/CQDs show absorption peaks at 225 nm and 352 nm corresponding to 7t~7t * and n~7t* of Sp2 carbon and C=OIC=N, respectively. Cu/N/CQDs was selected to be used as fluorescence sensor for dopamine due to its greater photoluminescence intensity and selectivity towards dopamine as determined by photoluminescence spectroscopy. Dopamine (DA) is the simplest catecholamine and belongs to the family of neurotransmitters which governs memory, mood, etc. Steady state photoluminescence spectroscopy (SSPL) and time resolved photoluminescence spectroscopy (TRPL) were used to compare the fluorescence behavior of synthesized sensing materials as probe in the presence of dopamine. The copper, nitrogen co-doped quantum dots show excellent fluorescence quenching in the presence of DA. The effect of pH on the performance of fluorescence sensor (Cu/N/CQDs) was also optimized. Stem-volmer graph along with TRPL was used to study the mechanism of quenching which was found to be static. Under optimal conditions, the designed sensor shows an appreciable linear range of 50nM to 700 mM for DA, limit of detection (LOD) came out to be 15nM, and limit of quantification (LOQ) was 49.9nM. Moreover, the sensor also displayed excellent selectivity for DA in the presence of interfering agents and good stability for a significant period of time. Density functional calculations were also performed to study interaction between analyte and ii Cu/N/CQDs, with a binding energy of -3.0xl03kJ/mol. The CulNCQDs also exhibited excellent catalytic activity in reducing 4-nitrophenol and 4- nitro aniline within an efficient time frame of 6 and 8 minutes, respectively. This catalytic activity was studied using UV visible spectroscopy. The copper, nitrogen co-doped CQDs also showed anti-bacterial activity towards Escherichia Coli, Klebsiella Pneumonia, Bacillus Subliltis and Staphylococcus Aureus, with best results recorded against Klebsiella Pneumonia strain.