Solvent-dependent switching of graphene oxide (GO) as fluorescence quencher or enhancer was observed. In some solvents, GO increases the fluorescence yield of a hydrophilic molecule 7-(diethylamino)-coumarin-3-carboxylic acid (7-DCA), and in some solvents GO act as a quencher of fluorescence.The intangibility between the carbon-based nanomaterials and biomolecules is an interesting area of research in biomedical imaging, biotechnology, material science, and so on. [1][2][3] To investigate the potential applications, utilization and biocompatibility of these carbon-based nanomaterials in biological systems, a proper understanding of the interaction between these carbon-based nanomaterials and biological systems are necessary. [4,5] One of the carbon-based nanomaterial is graphene oxide (GO); it took immense attention of the scientist in modern days research. Though GO took lots of attention to the scientist in different fields of researches, but the interaction of GO with the biologically active molecules like coumarin dyes has not studied extensively. So there is a scope to study the GO-dye molecule interaction to invent the role of GO on the photophysics of the dye molecules. Since GO has a large extent of hydrogen bonding ability so one should expect that GO can easily interact with organic dyes/drug molecules and modulate their spectral properties. Vovushaet al. studied the interaction of nucleobases and aromatic amino acids with GO and graphene flakes by using density functional theory, and they found that GO complexes are stabilized by hydrogen bonding interaction whereas, graphene complexes are stabilized by π-π stacking interaction. [6] The active functional groups which are present in the edges of GO bind the drug covalently, and the localized π electrons in the nanosheet are stabilized the drug through π-π interaction, and it is used for targeted drug delivery. [7,8] GO not only used for drug delivery but Kim et al. also reported that GO useful for gene delivery. [9] Lu et al. reported that single-stranded DNA adsorbed easily with the GO as compared to the double-stranded DNA because double-stranded DNA does not give the scope to bind DNA bases into the GO surface. [10] Kuchlyanet al. reported that fluorescence of tryptophan moiety of BSA gradually quenched upon the addition of GO, the reason behind it is π-π interaction between GO sheet and the indole structure of the tryptophan. [11] Nowadays, GO has been utilized with great interest in the field of near-infrared photothermal treatment of cancer, Alzheimer diseases etc.. [12][13][14] In recent years several researchers have investigated the adsorption phenomenon of biomolecules on GO surface. [6,[15][16] It is deeply-rooted that fluorescence of the organic fluorophore, [17,18] and biomolecules [19,20] were quenched in the presence of graphene oxide. Electron transfer, fluorescence resonance energy transfer (FRET), non-radiative dipole-dipole interaction are commonly responsible for this quenching phenomena. [21,22] Leblanc and co-workers stated that GO...

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Fluorescence enhancement of fluorophores in neat solvents media in the presence of graphene oxide (GO) is less known. It is necessary to re-examine the role of GO from fundamental scientifc...

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Several applications of graphene oxide (GO) have been established over the years, and it has the potential to be used as a biomedical material. Studying the effect of GO on protein–ligand (small molecules/drugs) complex systems are vital as the mechanisms involved are not well understood. The interaction of GO on the protein–ligand binding is also vital for the preparation of an effective drug carrier in the bloodstream. In this work, we have tried to explore in details the effect of GO on the interaction between a hydrophilic molecule, namely, 7-(N,N′-diethylamino) coumarin-3-carboxylic acid (7-DCA) with human serum albumin (HSA) by employing multispectroscopic, microscopic, calorimetric, and molecular docking studies. We find out that protein–ligand complexes were placed on the GO surface, and GO gives stability to the protein–ligand complex via hydrogen bonding, electrostatic interactions, hydrophobic interactions, and so forth. Due to the presence of a large surface area in GO, it offers a hydrophobic environment, and as a result, the emission maxima of 7-DCA in the ternary complex is more blue-shifted, and the average lifetime becomes higher compared to the binary system. Circular dichroism spectral studies give information about the conformational changes of HSA in the absence and presence of GO when it forms complex with 7-DCA. The fluorescence lifetime imaging study shows the presence of the 7-DCA/HSA complex on the GO sheet. Molecular docking simulation shows that the closest distance between 7-DCA and HSA is 11.9 Å, and the protein interacted with the ligand through hydrogen bonding, hydrophobic interaction, and so forth.