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.
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.
In this manuscript we have studied the photophysics of 7-(N,N'-diethylamino)coumarin-3-carboxylic acid (7-DCCA) in water/AOT/isooctane reverse micelles. For this purpose we have used steady state absorption and fluorescence emission spectroscopy and time resolved fluorescence spectroscopy. We have studied the spectral behaviour of 7-DCCA inside the water/AOT/isooctane reverse micelles with variation of excitation wavelength. We have studied the dynamics of solvent and rotational relaxation by using two different excitation wavelengths (λ(exi) = 375 nm and λ(exi) = 405 nm). We have observed the excitation wavelength dependent dynamics of 7-DCCA in the reverse micelles. The fluorescence quantum yield, decay time, solvent relaxation time and rotational relaxation time of 7-DCCA in reverse micelles vary with the excitation wavelength. A two step and wobbling-in-cone model was used to interpret the rotational relaxation dynamics of 7-DCCA in reverse micelles.
Herein, eco-friendly, water-soluble, and fluorescent carbon quantum dots (CQDs) with an average size of 8.3 nm were synthesized from rice husk (RH) using the hydrothermal method, and the CQDs were labeled as rice husk CQDs (RH-CQDs). The composition and surface functionalities were studied using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. A study on the impact of pH and irradiation time on fluorescence affirmed the stability of RH-CQDs. The assynthesized nanosensor has high selectivity and sensitivity for Fe 3+ ions. Several photophysical studies were performed to investigate the interaction between RH-CQDs and Fe 3+ . Using the time-correlated single-photon technique, it is determined that the average lifetime value of RH-CQDs significantly decreases in the presence of Fe 3+ , which supports a dynamic quenching mechanism. The developed sensor exhibited excellent sensitivity with a detection limit in the nanomolar range (149 nM) with a wide linear range of 0−1300 nM for Fe 3+ ions. The prepared nanosensor was also used to detect Fe 3+ in a tablet supplement with high recoveries. Moreover, the RH-CQD nanoprobe was used to detect other analytes (fluoroquinolones) using the fluorescence enhancement technique. It showed high selectivity and sensitivity toward ofloxacin (OFX) and ciprofloxacin (CPX). The detection limits calculated were 150 nM and 127 nM with a linearity range of 50−1150 nM for OFX and CPX, respectively. The enhancement of the average lifetime value and quantum yield in the presence of OFX and CPX favors the increased fluorescence property of RH-CQDs through hydrogen bonding and charge transfer. In this work, the integration of two different mechanisms (fluorescence quenching and fluorescence enhancement) was followed to construct a single sensing platform for accurate quantification of dual-mode nanosensors for the detection of metal ions and fluoroquinolones by the excited-state electron transfer and hydrogen bonding mechanism, respectively. This strategy also stimulates the detection of more than one analyte.
The photophysics of a hydrophilic molecule, 7-(diethylamino)-coumarin-3-carboxylic acid (7-DCCA), was studied in the presence of two macrocycles, (2-hydroxypropyl)-γ-cyclodextrin and cucurbituril. We have used steadystate absorption, fluorescence, and time-resolved fluorescence emission spectroscopy; Fourier transform infrared (FTIR) spectroscopy; 1 H NMR spectroscopy; and isothermal titration calorimetry (ITC) to confirm the supramolecular host−guest complex formation. The spectral properties of 7-DCCA were modulated in the presence of both macrocycles. It was assigned that 7-DCCA forms a 1:2 complex with (2-hydroxypropyl)-γ-cyclodextrin and cucurbituril. The large modulation of the emission properties of 7-DCCA in the presence of the macrocycles indicates the formation of supramolecular complexes. A significant shift in the bond vibration frequencies in the FTIR studies showed encapsulation of the dyes in the hydrophobic cavity of the macrocycles. This is further substantiated by the 1 H NMR studies, in which the upfield and downfield shifts of the protons were observed in both the aliphatic and aromatic region in the presence of macrocycles. The time-resolved anisotropy measurements further reinforce the conception of host−guest supramolecular complex formation because, in both cases, the rotational relaxation time increases significantly compared to that in water. A deeper understanding between the differences in interaction of an anionic molecule with cucurbituril and (2-hydroxypropyl)-γ-cyclodextrin will be achieved through this work. From the ITC measurement, we have formulated the forces due to complex formation.
In this study, we have reported the binding interaction and photophysics of a nonsteroidal anti-inflammatory drug (NSAID) indomethacin (IMC) in the presence of different micelles. We have used several spectroscopic techniques such as UV-vis absorption, steady state fluorescence and time-resolved fluorescence emission spectroscopy. The spectral properties of IMC were modulated in the presence of micelles compared to that in neat water. The weak emitting drug molecule (IMC) becomes highly fluorescent after binding with the micelles. The fluorescence quantum yield and fluorescence lifetime increase in the presence of micelles compared to those in neat water. The isothermal titration calorimetry (ITC) method was used to study the binding interaction of IMC with different micelles. The thermodynamic parameters and the nature of binding between IMC and different micelles have been estimated. Moreover, addition of KCl salt in the respective micelles releases IMC molecule from the micelles to the aqueous medium. This study will help elicidate the binding behavior of IMC in the presence of different micelles for possible use as potential drug delivery systems.
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