476 | Mater. Chem. Front., 2019, 3, 476--483 This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2019 Cite this: Mater. Chem. Front., 2019, 3, 476 A carbon quantum dot and rhodamine-based ratiometric fluorescent complex for the recognition of histidine in aqueous systems† Harupjit Singh, a Jagpreet Singh Sidhu, b Dhiraj Kumar Mahajan c and Narinder Singh * b Histidine is an essential a-amino acid that plays a crucial role in tissue development and helps in the transmission of metallic ions during biological events. However, an abnormal level of histidine in the body is associated with various physiological conditions such as arthritis, liver cirrhosis, kidney diseases, and asthma. Herein, a unique ratiometric fluorescence sensing system has been developed for the recognition of histidine. The sensing system was developed using carbon quantum dots (CQDs) as an energy donor and a rhodamine 6G derivative (HS30) as an energy acceptor unit. Interestingly, upon the addition of Fe(III) into the mixture of CQDs and HS30, the phenomenon of fluorescence resonance energy transfer (FRET) was observed when excited at 350 nm. The emergence of a strong emission peak at 551 nm on the addition of Fe(III) suggested the formation of a ratiometric fluorescent complex ‘‘CQDs–Fe–HS30’’. The ratiometric behavior of ‘‘CQDs–Fe–HS30’’ was studied by monitoring fluores- cence emissions at 425 nm and 551 nm with an excitation wavelength of 350 nm. Furthermore, ‘‘CQDs–Fe–HS30’’ was employed for the recognition of histidine in an aqueous system. Due to the high affinity of histidine to Fe(III), the addition of histidine to an aqueous solution of ‘‘CQDs–Fe–HS30’’ resulted in the displacement of the Fe(III) cation from the complex, and the simultaneous quenching and enhancement of the emission peaks at 551 nm and 425 nm, respectively, was observed. The developed sensing system was successfully employed for a histidine recovery experiment in human urine samples with satisfactory results. Furthermore, the mixture of CQDs and HS30 was successfully utilized to implement an inhibit logic gate with Fe(III) and histidine as inputs and emission at 551 nm as output. Introduction Histidine is an indispensable a-amino acid that plays a vital role in the biosynthesis of various proteins. 1 The structure of histidine has a highly reactive imidazole functional group that interacts with transition metal ions and controls their trans- mission in biological bases. 2,3 The expression of a low level of histidine is associated with rheumatoid arthritis, liver cirrhosis, and pulmonary dysfunctions, whereas overexpression is linked with cancer, kidney disorders, and Alzheimer’s disease. 4–9 Therefore, it is critical to develop a simple, highly selective and sensitive method for the assessment of histidine expression for the diagnosis of diseases. Advanced techniques such as gas chromatography and high- performance liquid chromatography are used along with UV-visible or fluorescence spectroscopy for histidine detection. 10 However, the typical limitations of these methods are the requirements for sophisticated instrumentation, trained instrument operators, and well-equipped laboratories. Other techniques used for the determination of histidine are electrochemical sensing, 11,12 colorimetric sensing, 13 and fluorescence spectroscopy. 14,15 Amongst all these techniques fluorescent spectroscopy has great advantages concerning its ease of handling, high sensitivity, high throughput, and real-time detection. Therefore, a wide range of single emission point fluorescent probes based on metal complexes, porphyrins, crown ethers, peptides, and polymers has been developed for the selective detection of histidine. 11,13–18 However, single emission point fluorescence measurements are severely influenced by external factors such as temperature, the source of excitation, concentration, and the solvent system. 19 Consequently, to circumvent these undesirable effects, the focus has shifted to ratiometric sensing. Ratiometric sensing employs the phenomenon of fluorescence resonance energy transfer (FRET) for precise and a Center for Biomedical Engineering, Indian Institute of Technology Ropar, Punjab 140001, India b Department of Chemistry, Indian Institute of Technology Ropar, Punjab 140001, India. E-mail: nsingh@iitrpr.ac.in c Department of Mechanical Engineering, Indian Institute of Technology Ropar, Punjab 140001, India † Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qm00554k Received 26th October 2018, Accepted 8th January 2019 DOI: 10.1039/c8qm00554k rsc.li/frontiers-materials MATERIALS CHEMISTRY FRONTIERS RESEARCH ARTICLE Published on 10 January 2019. Downloaded on 7/4/2022 7:46:42 PM. View Article Online View Journal | View Issue