In recent years, a variety of biosensors based on manganese dioxide (MnO 2 ) nanostructures were used for the detection of different biomolecules. Structural versatility and different oxidation states of Mn present in MnO 2 nanostructures significantly enhance their biosensing applications. In this review, we have mainly focused on different morphologies of MnO 2 nanostructures and their role in different types of biosensors. We are elaborately discussing the electrochemical and optical-based biosensors on different morphologies of MnO 2 and their sensing techniques with proposed mechanisms. In electrochemical biosensors, various electrodes are available but there is a need for cheap, flexible, portable, and nontoxic electrodes. Therefore, we have discussed the fabrication of MnO 2 nanostructures on the surfaces of different types of electrodes and their real-time applications. MnO 2 nanomaterials are used in various optical biosensors due to their excellent light absorption properties, and they act as strong quenching agents. MnO 2 nanomaterials are termed "alternative natural enzymes" due to their significant enzyme-like activity. The study of various morphology-dependent sensing properties of MnO 2 nanostructures provides an insight into the development of many electrochemical and optical biosensors. Recently, MnO 2 based biosensors have earned great popularity as an alternative method for fast and easy detection of many biomolecules with very low detection limits.

Herein, a fluorescent nanosensor based on carbon dots (Cdots) 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.

Graphene quantum dots (GQDs) are carbon-based fluorescent nanomaterials having various applications due to attractive properties.

Heteroatom doping of graphene quantum dots (GQDs) leads to modify their intrinsic properties and used as a fluorescent sensor for the metal ions sensing. Here, Nitrogen-doped GQDs (N-GQDs) was synthesized...

Herein, we develop a facile, sensitive, and selective fluorescent nanosensor for the detection of glutathione (GSH). In this protocol, carbon dots (Cdots) with a fairly high quantum yield were synthesized by a microwave-assisted pyrolysis technique. Moreover, different shapes of the MnO 2 nanostructure were also prepared by the hydrothermal technique. A comparative photophysical study of different morphology-dependent Cdots@MnO 2 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 MnO 2 , that is, nanoflowers (NFs), nanorods (NRs), and a mixture of NFs/NRs was prepared by the hydrothermal method owing to different specific surface areas (23−69 m 2 g −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 [τ av (ns)], radiative (k r ) rate constant, nonradiative (k nr ) rate constant, rate of electron transfer (k ET ), the efficiency of electron transfer (Φ EET ), FRET efficiency (E), and Forster distance (R 0 ) were dependent on the different shapes of the MnO 2 nanostructure. These results indicate that the transfer of energy occurs between the Cdots and different shapes of MnO 2 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 MnO 2 to Mn 2+ , 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.

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Structural versatility of MnO 2 nanostructures plays a significant role in biosensing applications. So, we have prepared simple and selective "turn-off−on" sensing probes for the detection of glutathione (GSH), based on nitrogen, sulfur codoped carbon dots (N, S-Cdots) and different morphologies of one-dimensional (1-D) MnO 2 nanostructures. N, S-Cdots with a high fluorescence quantum yield (73.42%) were prepared by a green approach through hightemperature pyrolysis in just 5 min. The different morphologies of 1-D MnO 2 nanostructures (nanowires with varying aspect ratios and nanorods) were synthesized through a hydrothermal method by varying the reaction period (8, 10, and 12 h). MnO 2 nanowires prepared at 8 h showed a high specific surface area (34 m 2 g −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-MnO 2 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 costeffective sensing system can be further used for real-time biosensing and bioimaging application.

Environmental worsening, energy crises, and various other factors have enhanced the demand for facile, cheap, and green approaches for creating emerging materials from bioresources. In recent times, graphene quantum dots (GQDs) are the most developing zero-dimensional nanomaterials owing to exclusive electronic and optical properties. Biomass-derived GQDs have been considered an advanced material and have gained complete attention because of their green, inexpensive, nonsustainable, environmental-friendly, and recyclable nature. This review elucidates the current challenges and the use of bioprecursors (e.g., rice husk, plant leaves, honey, coffee) and the methodologies (top-down and bottom-up) of their conversion into ecofriendly GQDs. Biomass resources are converted into ecofriendly GQDs and a facile, low-cost, scalable synthesis. Biomass-derived GQDs are in great demand due to attractive properties like large surface area, low toxicity, and good biocompatibility. Various parameters like absorption, surface and edge states, and quantum confinement affecting the physical, chemical, and electrochemical properties of GQDs are discussed. This review also focuses further on the result of heteroatom doping, pH, and solvent on the photoluminescence (PL) emission of GQDs. The optical and electrochemical sensors based on biomass-derived GQDs are explored in detail. Biomass-derived GQDs have tremendous performance in the biomedical field and energy applications due to their very low toxicity and biocompatibility. This review addresses the future approaches and possible research directions in biomass-derived GQDs. GQDs display minimal cytotoxic reactivity, superior biocompatibility, and chemical insensitivity. To enhance the optoelectronic and physicochemical characteristics of GQDs, their properties can be tuned via surface/edge functionalization or doping. Monitoring the concentrations of pollution gases severely harming the biosphere requires developing more precise and sensitive sensors. Doped GQDs can significantly increase their capacity to adsorb. The reliable gas sensors based on doped GQDs could be a viable replacement for the present sensors available in the market.