Study of the morphology effect of MnO2 on Carbon dot-based nanosensor for the detection of biomolecules

Abstract

The thesis entitled “Study of the morphology effect of MnO2 on Carbon dot-based nanosensor for the detection of biomolecules” is divided into six chapters. Chapter-1. It describes the background of sensor, nanosensor, and fluorescent nanomaterials along with literature survey and scope of the work. This Chapter explains the detail information about the carbon dots (Cdots) with its synthesis approaches, fluorescence mechanism, optical properties, and effect of heteroatom doping. The different fluorescence quenching mechanisms of Cdots in the presence of quencher and different photophysical parameters were also discussed in this Chapter. The exceptional properties of MnO2 and its different morphologies available were also described in this chapter. The importance of biomolecules in living beings and their need for detection by fluorescent probe was explained. Chapter-2. A fluorescent nanosensor based on carbon dots and different-size MnO2 nanospheres has been synthesized for rapid detection of glutathione (GSH). Water-soluble and highly fluorescent Cdots were prepared by the microwave method using ascorbic acid as the precursor. MnO2 nanospheres of different sizes (large, medium, and small) were prepared by varying the concentration ratio of methionine and KMnO4 at room temperature, which was confirmed by HRTEM analysis. The different sizes of MnO2 nanospheres in Cdots result in quenching of the fluorescence intensity, quantum yields, and average lifetime values, which suggest that the fluorescence resonance energy transfer mechanism occurs between the Cdots and MnO2 nanospheres. The variations of all the photophysical parameters and fluorescence turn off properties of Cdots are significantly tuned depending on the size of the nanospheres. Moreover, detection of GSH in the presence of different-size Cdots@MnO2 systems has been explored. GSH causes the redox reaction in the presence of MnO2, which leads to transformation from MnO2 to Mn2+. As a result, fluorescence restoration (turn-on) of Cdots was observed. The large MnO2 nanospheres showed the lowest detection limit of 15 μM for GSH. The synthesized sensing system was very fast, simple, economical, and environmentally-friendly for the detection of the GSH level. Chapter-3. A facile, sensitive, and selective fluorescent nanosensor for the detection of GSH. In this protocol, Cdots with a high quantum yield were synthesized by a microwave-assisted pyrolysis technique. Moreover, different shapes of the MnO2 nanostructure were also prepared by the hydrothermal technique. A comparative photophysical study of different morphology-dependent Cdots@MnO2 nanostructure-based biosensors was explored, which showed different results for the quenching values of (“turn-off”) fluorescence intensity, quantum yields, electron transfer rate, and average lifetime. The structure, property, and performance of nanomaterials are interdependent. Therefore, the different shapes of MnO2, that is, nanoflowers (NFs), nanorods (NRs), and a mixture of NFs/NRs was prepared by the hydrothermal method owing to different specific areas (23-69 m2g-1) which put the impact on their sensing activity. It was observed that the variation in the different photophysical parameters of fluorescent Cdots such as quantum yield (𝜙), average lifetime values [τav (ns)], radiative (kr) rate constant, non-radiative (knr) rate constant, rate of electron transfer (kET), the efficiency of electron transfer (𝜙EET), FRET efficiency (E), and Förster distance (R0) were dependent on the different shapes of the MnO2 nanostructure. These results indicate that the transfer of energy occurs between the Cdots and different shapes of MnO2 nanostructures based on fluorescence resonance energy transfer at different charge-transfer rates. The recovery rate (“turn-on”) of fluorescence of Cdots with the addition of GSH was obtained best for the NF structure by conversion of MnO2 to Mn2+, and the limit of detection was obtained as ~19 μM for GSH. The developed sensing probes were rapid, easy, cheap, and eco-friendly for the determination of GSH. Chapter-4. Heteroatom doping on Cdots is developed as an efficient approach to modify its optical and electronic properties. Herein, nitrogen-sulfur doped Cdots (N, S-Cdots)-MnO2 nanospheres was used as “on-off-on” biosensor for the selective detection of glutathione (GSH) and also for intracellular imaging. N, S-Cdots have been synthesized through high-temperature treatment for 5 min. The value of quantum yield was evaluated to be high for the N, S-Cdots i.e., 73.42 %. N, S-Cdots showed very high fluorescence quenching efficiency in the presence of MnO2 nanospheres due to the inner filter effect (IFE) and also showed the highest fluorescence recovery rate after the addition of GSH in the prepared system. N, S-Cdots-MnO2 nanospheres as a sensing system satisfy the Parker equation to confirm the IFE as a quenching mechanism. No variation in the average lifetime values of N, S-Cdots after the addition of MnO2 nanospheres confirms the IFE. MnO2 nanospheres and GSH involves a redox reaction between them which leads to the removal of Cdots from MnO2 nanosphere’s surface and further, the fluorescence restoration of the Cdots was achieved. The limit of detection (LOD) for GSH was determined to be 80 nM. The heteroatom doping in the structure of Cdots has a significant role in the sensitive detection of GSH. The prepared-sensor is rapid, economical, less toxic, and highly applicable in diagnosing diseases. Chapter-5. Structural versatility of MnO2 nanostructures plays a significant role in biosensing applications. So, we have prepared simple and selective “turn-off-on” sensing probes for the detection of GSH, based on nitrogen, sulfur co-doped carbon dots (N, S-Cdots) and different morphologies of one dimensional (1-D) MnO2 nanostructures. N, S-Cdots with a high fluorescence quantum yield (73.42 %) were prepared by a green approach through high-temperature pyrolysis in just 5 min. The different morphologies of 1-D MnO2 nanostructures (nanowires with varying aspect ratios and nanorods) were synthesized through a hydrothermal method by varying the reaction period (8, 10, and 12 h). MnO2 nanowires prepared at 8 h showed a high specific surface area (34 m2g-1) with a large aspect ratio. They showed significant fluorescence quenching, Stern-Volmer constants, and binding constants in the presence of N, S-Cdots. Further, ultraviolet-visible absorption, zeta potential, and time decay studies showed that the quenching mechanism of the developed sensing system was the inner filter effect, which was further confirmed by using the Parker equation. The N, S-Cdots-MnO2 nanowire (with a high aspect ratio) sensing system showed the best limit of detection, i.e., 28.5 μM for GSH. This fast, simple, eco-friendly, and cost-effective sensing system can be further used for real-time biosensing and bio-imaging application. Chapter-6. MnO2 nanostructure has a unique light absorption ability and redox feature, due to which it is mainly used as a fluorescence quencher. However, its quantum dots can also act as a self-fluorescent nanomaterial with exclusive properties for sensing applications. Therefore, a cheap and straightforward route for fabricating citric acid-functionalized MnO2 quantum dots (CA-MnO2 QDs) was proposed through one-step ultrasonication of bulk MnO2 in the presence of citric acid as acidifying and stabilizing agent. The prepared CA-MnO2 QDs exhibited high photostability, good water solubility, high photoluminescence (PL) stability from pH range 5 to 9, and significant fluorescence quantum yield value, i.e., 13.3 %. Further, CA-MnO2 QDs was used for the detection of Fe3+ ions with a detection limit of 43 nM, based on dynamic quenching. The fluorescence quenching mechanism was confirmed by determining different photophysical parameters such as fluorescence quantum yield, average lifetime values, radiative constants (kr), non-radiative rate constants (knr), rate of electron transfer (kET), and electron transfer efficiency (EET). The CA-MnO2 QDs in the presence of Fe3+ was used for the detection of ascorbic acid with a detection limit of 90 nM based on the redox reaction between Fe3+ ions and AA. Furthermore, the developed sensing probe was also used to detect Fe3+ ions and AA in real samples, i.e., iron and AA supplements, respectively.