Interaction of graphene with excited and ground state rhodamine revealed by steady state and time resolved fluorescence

Exploration of the impact of graphene oxide, acetylenic gemini, and CTAT on the photophysical and aggregation properties of dipolar coumarin 153

Advanced spectroscopic techniques have been utilized to study the interaction between the laser dye coumarin 153 (C153) and graphene oxide (GO) nanoparticles. GO was synthesized using a modified Hummers' method and characterized by UV-vis spectroscopy, Raman laser spectroscopy, FTIR-ATR spectroscopy, FESEM, HR-TEM, and XRD techniques. The GO@C153 composite was formed by mixing two aqueous solutions of GO and C153 due to their strong interaction through stacking and hydrophobic interactions. In this case, GO acts as an effective fluorescence quencher for C153 molecules, which undergo H-type aggregation in the presence of GO. The Stern-Volmer equation and time-dependent fluorescence studies were utilized to analyse the mechanism of fluorescence quenching. According to the findings, both static and dynamic quenching processes are responsible for the reduction in fluorescence intensity. The effect of surfactants (both cetyltrimethylammonium p-toluenesulfonate (CTAT) and synthesized N,N'-dihexadecyl-N,N,N',N'-tetramethyl-N,N'-but-2-ynediyl-di-ammonium chloride (16-4-16)) on the aggregation and photophysical properties of the dye was investigated using surface tensiometry, conductometry, UV-vis absorption spectroscopy, steady-state fluorescence measurements, DLS, and time-dependent fluorescence spectroscopy. Surfactants change the microenvironment of the C153 dye, leading to spectrum shifting and a higher quantum yield, which causes a rapid rise in fluorescence intensity in the micellar medium. It has been noted that in a micellar medium rather than in an aqueous one, the luminous intramolecular charge transfer (ICT) state of C153 stabilises. Lastly, we investigated the photophysical behavior of the GO-C153-micelle ternary system and discovered that, in the presence of a micellar medium, the quenched and blue-shifted (H-type aggregation) fluorescence peak of C153 (in the presence of GO) began to intensify once more. The main goal of this work is to create an effective and fairly cost powerful fluorescence sensor. Additionally, the ternary system (GO-C153-micelle) analytical idea can be employed to identify the onset of micelle formation. In wastewater treatment analysis, the GO-C153-surfactant ternary system concept can also be used to regenerate the adsorbent (in this case, GO) from dye molecules by allowing the dye molecules to exit the adsorbent and enter the micellar medium.

Distance-Dependent Plasmon-Enhanced Fluorescence of Submonolayer Rhodamine 6G by Gold Nanoparticles

We investigate the fluorescence from submonolayer rhodamine 6G molecules near gold nanoparticles (NPs) at a well-controlled poly (methyl methacrylate) (PMMA) interval thickness from 1.5 to 21 nm. The plasmonic resonance peaks of gold NPs are tuned from 530 to 580 nm by the PMMA spacer of different thicknesses. Then, due to the plasmonic resonant excitation enhancement, the emission intensity of rhodamine 6G molecules at 562 nm is found to be enhanced and shows a decline as the PMMA spacer thickness increases. The variation of spectral intensity simulated by finite-difference time-domain method is consistent with the experimental results. Moreover, the lifetime results show the combined effects to rhodamine 6G fluorescence, which include the quenching effect, the barrier effect of PMMA as spacer layer and the attenuation effect of PMMA films.

Study of graphene dispersions in sodium dodecylsulfate by steady-state fluorescence of pyrene

HYPOTHESIS

Aqueous solutions of ionic surfactants allow the exfoliation of graphene, that can be explained considering the adsorption model of ionic surfactants to hydrophobic surfaces. For many years, pyrene has been used as a fluorescent probe because its sensitivity to the micro-environment. The study of pyrene fluorescence in the presence of different graphene dispersions in an ionic surfactant, would improve the knowledge of the graphene-surfactant interactions.

EXPERIMENTS

Different dispersions of graphene in sodium dodecylsulfate were prepared at different weight ratios 0.5, 1 and 2%. The dispersions have been studied by Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The influence of the dispersions on the pyrene fluorescence has been investigated.

FINDINGS

The graphene sheets modified by the surfactant quench the fluorescence of pyrene, which depends on the amount of graphene, the concentration of surfactant and the weight ratio. For surfactant concentrations below the critical micelle concentration, the quenching effect is higher as the weight ratio increases. Once this concentration is reached, the fluorescence increases slightly and then levels off. This behavior has been explained by the adsorption model. For a constant surfactant concentration, two straight lines can be observed in the Stern-Volmer plots whose cut-off point is approximately 20 mg L^-1 of graphene.

Comparison of Anionic, Cationic and Nonionic Surfactants as Dispersing Agents for Graphene Based on the Fluorescence of Riboflavin

Fluorescence quenching is a valuable tool to gain insight about dynamic changes of fluorophores in complex systems. Graphene (G), a single-layered 2D nanomaterial with unique properties, was dispersed in surfactant aqueous solutions of different nature: non-ionic polyoxyethylene-23-lauryl ether (Brij L23), anionic sodium dodecylsulphate (SDS), and cationic hexadecyltrimethylammonium bromide (CTAB) and dodecyltrimethylammonium bromide (DTAB). The influence of the surfactant type, chain length and concentration, G total concentration and G/surfactant weight ratio on the fluorescence intensity of vitamin B₂ (riboflavin) was investigated. The quality of the different G dispersions was assessed by scanning and transmission electron microscopies (SEM and TEM). A quenching phenomenon of the fluorescence of riboflavin was found for G dispersions in all the surfactants, which generally becomes stronger with increasing G/surfactant weight ratio. For dispersions in the ionic surfactants, the quenching is more pronounced as the surfactant concentration raises, whilst the non-ionic one remains merely unchanged for the different G/Brij L23 weight ratios. More importantly, results indicate that DTAB solutions are the optimum media for dispersing G sheets, leading to an up to 16-fold drop in the fluorescence intensity. Understanding the mechanism in fluorescence quenching of G dispersions in surfactants could be useful for several optical applications.

Graphene-Rhodamine Nanoprobe for Colorimetric and Fluorimetric Hg(2+) Ion Assay

This article reveals the first ever prospective application of Graphene-Rhodamine array (GRH) as a colorimetric and fluorimetric sensor for Hg(2+) ions. The duality of Graphene to undergo π-π and dispersive interactions with Rhodamine as well as to act as a selective adsorbent for Hg(2+) is conceptualized in this study. These interactions lead to decrease in absorbance of the dye in the presence of graphene, which is restored when kept in contact with Hg(2+) ions. The feasibility of the mechanism has been proved using EDTA as the coordinating ligand. It is noteworthy that all the optical variations occurred in the visible scale of the electromagnetic spectrum. The GRH array exhibited higher sensitivity toward the target ion with a limit of detection of 2 ppb. A perfect linear variation of absorbance at 554 nm with Hg(2+) concentration was observed in 0-1000 nM range, enabling the use of the system as a quantitative sensor for the test ion. The commendable selectivity of the array toward Hg(2+) ion has been investigated by observing the optical response in the presence of other environmentally relevant metal ions. A reversible turn off and turn on INHIBIT logic gate has been proposed which extends the scope of the designed array for the development of automated chemical systems. The fluorescence resonance energy transfer (FRET) ability of graphene paves the backbone for the fluorimetric detection. Fluorimetric strategy yielded a much lower limit of detection of 380 ppt using this probe, which makes a significant advance in trace detection of Hg(2+) ions.

Laser induced fluorescence spectroscopy of various carbon nanostructures (GO, G and nanodiamond) in Rd6G solution

The effect of carbon nanostructures such as graphene (G), graphene oxide (GO) and nanodiamond (ND) on the spectral properties of Rhodamine 6G (Rd6G) emission due to the laser induced fluorescence (LIF) was investigated. It is shown that the addition of carbon nano- structures lead to sensible Red/Blue shifts which depend on the optical properties and surface functionality of nanoparticles. The current theories such as resonance energy transfer (RET), fluorescence quenching and photon propagation in scattering media support the experimental findings. Stern-Volmer curves for dynamic and static quenching of Rd6G molecules embedded with G, GO and nanodiamond are correlated with spectral shifts. Furthermore, time evolution of the spectral shift contributes to determine loading/release rates of fluorescent species with large S-parameter on the given nano-carriers.

Detection of microRNA in tumor cells using exonuclease III and graphene oxide-regulated signal amplification

In this study, we developed a label-free, ultrasensitive graphene oxide (GO)-based probe for the detection of oligonucleotides by laser desorption/ionization mass spectrometry (LDI-MS). On the basis of simple π-π stacking and electrostatic interactions between rhodamine 6G (R6G) and GO, we prepared the nanocomposite R6G-modified GO (R6G-GO). Signal intensities of R6G increased in mass spectra in the presence of single-stranded oligonucleotides under pulsed laser irradiation (355 nm) of R6G-GO. In addition, the signal intensity of R6G was stronger in the presence of short oligonucleotides. Because small oligonucleotides improve the LDI efficiency of R6G on GO, we designed an enzyme-amplified signal transduction probe system for the detection of microRNA (miRNA). After specific digestion of the probe DNA (pDNA) strand from pDNA/miRNA-hybridized complexes by exonuclease III (Exo III), the resulting small oligonucleotide fragments increased the R6G signal during LDI-MS of R6G-GO. In addition, the signal intensity of the R6G ions increased with increasing concentrations of the target miRNA. Coupling this enzyme reaction and R6G-GO with LDI-MS enabled the detection of miRNA at concentrations of the femtomolar (fM) level. We also demonstrated the analysis of miRNA in tumor cells and utilized this R6G-GO probe in the detection of a single-nucleotide polymorphism (SNP) in the Arg249Ser unit of the TP53 gene. This simple, rapid, and sensitive detection system based on the coupling of functional GO with LDI-MS appears to have great potential as a tool for the bioanalyses of oligonucleotides and proteins.

Probing electron transfer dynamics of pyranine with reduced graphene oxide

A stable reduced graphene oxide (rGO) was prepared and characterized by X-ray diffraction (XRD) and laser Raman spectroscopy. Steady state and time-resolved fluorescence quenching studies have been carried out to elucidate the process of electron transfer from excited pyranine (POH) into the rGO dispersion. POH adsorbed strongly on rGO dispersion with an apparent association constant of 33.4 (mg ml)(-1), and its fluorescence emission was quenched with an apparent association constant of 33.7 (mg ml)(-1). Picosecond lifetime measurements gave the rate constant for the electron transfer process from the excited singlet state of POH into the rGO dispersion as 8.8 × 10(9) s(-1). Laser flash photolysis studies demonstrated the formation of radicals for the evidence of electron transfer between POH and rGO.

Noncovalent binding of xanthene and phthalocyanine dyes with graphene sheets: the effect of the molecular structure revealed by a photophysical study

The fluorescence and absorption properties of several xanthene and phthalocyanine dyes were measured in the presence and absence of chemically derived graphene (CDG) sheets. The interaction of pyronine Y (PYY) with graphene sheets was compared with that of rhodamine 6G (R6G) to reveal the effect of the molecular structure. Although the presence of the perpendicular benzene moiety in a R6G or phthalocyanine molecule does cause the difficulty for forming dye-CDG complex and make CDG less efficient in quenching the fluorescence intensity and shortening the fluorescence lifetime, it does not affect the band position of charge transfer absorption, suggesting that no molecular shape change occurred in a dye molecule caused by the interaction with CDG sheets. The spectroscopic and thermodynamic data indicated that the dye-CDG binding is of charge transfer nature, while the dynamic fluorescence quenching is due to photoinduced energy and electron transfer.