Elsevier

Environmental Pollution

Volume 172, January 2013, Pages 100-107
Environmental Pollution

Fluorescence quenching of fulvic acids by fullerene in water

https://doi.org/10.1016/j.envpol.2012.08.005Get rights and content

Abstract

Fullerene can be suspended in water as nanoscaled-fullerene-aggregates (nC60). However, little is known about its biogeochemical cycling in natural waters. In this paper, the interactions between nC60 and fulvic acids were investigated using fluorescence spectroscopy and fluorescence quenching titration. The results show that the intrinsic fluorescence of fulvic acids was static quenched by adding nC60. The association constants (log K) of fulvic acids and nC60 were estimated using a modified Ryan–Weber nonlinear model, and ranged from 6.76 to 7.41 l/mol. The log K was not significantly affected by the concentration levels of fulvic acids from 5.0 to 20.0 mg/l. The log K increased at low pH 3–5, but remained constant at high pH ranging from 5 to 11. The hydrophobic and π–π interactions were the likely primary mechanisms. The present observation will be helpful in understanding the environmental behavior of fullerene in natural aquatic ecosystems.

Highlights

► Fluorescence of fulvic acids can be quenched by nanoscaled-fullerene-aggregates. ► Static quenching was the main fluorescence quenching mechanism. ► Association constants were estimated with fluorescence quenching titration. ► Hydrophobic and π–π interactions control the interaction.

Introduction

Fullerene has shown promising applications in catalytic conversion, medical therapeutics, and nonlinear optics since its discovery in 1985 (Lyon et al., 2006). Fullerene is similar in structure to graphite, and its composition of stacked sheets that make it extremely hydrophobic, with a tendency to aggregate and deposit in polar solvents because of strong van der Waals forces. The solubility of fullerene is less than 10−9 mg/l in water and fullerene powder alone has no antibacterial properties (Fortner et al., 2005). However, fullerene is often required to be dispersed before potential engineering applications (Lyon et al., 2006; Yamakoshi et al., 1994). The dispersion methods commonly involve dissolving fullerene in certain kinds of solvents (e.g., dimethyl sulfoxide, polyvinylpyrrolidone, tetrahydrofuran, and toluene) that are subsequently replaced by water (Klavins and Ansone, 2010; Lyon et al., 2006). The fullerene water suspension contains nanoscaled-fullerene-aggregates (nC60), whose morphological properties differ from those of dissolved fullerene in the relevant solvent (Lyon et al., 2006). The nC60 forms are the most environmentally relevant forms of fullerene in the event of a spill of either fullerene powder or dissolved fullerene in water (Fortner et al., 2005; Lyon et al., 2006). Furthermore, the nC60 forms are toxic to micro-organisms, aquatic organisms and even animal/human cell lines (Fortner et al., 2005; Lyon et al., 2006). Therefore, it is necessary to investigate the fates and behaviors of fullerene in natural environments.

Humic substances are a poly dispersed mixture of organic macromolecules with a variety of aromatic and aliphatic blocks and functional groups occurring in water, soil and sediment (Kumke et al., 1998a,b; Wu and Xing, 2009; Zafiriou et al., 1984; Zepp et al., 1987). Humic substances can be operationally separated into fulvic acid (FA; soluble at all pH values), humic acid (soluble in alkaline media and insoluble in acidic media), and humin (insoluble at all pH values), according to the water solubility (Bai et al., 2008a,b; Wu et al., 2001; Wu and Xing, 2009). FAs play key roles in affecting speciation, toxicity and transport of inorganic and organic compounds (such as Cu2+, Hg2+, PAHs and carbamazepine) in aquatic environments due to the strong interactions between them (Cabaniss, 2011; Fang et al., 1998; Kumke et al., 1998b; Ohno et al., 2008; Wu et al., 2004). It would be a priority to examine the interactions between nC60 and FAs in cases of fullerene contaminating aquatic environments. Currently, little is known about the interaction mechanisms and possible roles of FAs in controlling the environmental and biogeochemical fate of fullerene.

Molecular fluorescence quenching titration has been widely employed to provide useful information regarding the interactions between humic substances and trace contaminants since the 1980s (Bai et al., 2008a,b; Fang et al., 1998; Gauthier et al., 1986; Kumke et al., 1998a,b; Lu and Jaffe, 2001; Ryan and Weber, 1982; Wu et al., 2001, 2004). This method was also satisfactorily used to study the interactions between fullerene and PAHs (Datta and Mukherjee, 2006) as well as fullerene and humic substances in organic solvents (Klavins and Ansone, 2010; Manciulea et al., 2009). The interactions between humic substances and nC60 in water have not yet been reported.

The possible fluorescence quenching mechanisms include dynamic, static, and a combination of dynamic and static quenching (Kumke et al., 1998a; Pan et al., 2012). The dynamic quenching agent provides a non-radiative route for the loss of the excited state energy during the lifetime of an excited state of fluorescer (Pan et al., 2012). Static quenching however is a process where non-fluorescent complexes are formed. Both mechanisms were reported for different compounds quenched by fullerene (Datta and Mukherjee, 2006; Klavins and Ansone, 2010; Manciulea et al., 2009). For example, dynamic quenching is the main mechanism involved when anthracene is quenched by fullerene (Datta and Mukherjee, 2006), while the static quenching process is involved for humic substances quenched by fullerene in organic mediums (Klavins and Ansone, 2010). Therefore, the discussion on the quenching mechanism involved in water is still open. For mono-dispersal systems, the linear Stern–Volmer plot could easily be indicative of a single mechanism of fluorophores with equal accessibility to the quencher, and a combination of two quenching mechanisms typically produces a nonlinear plot (Bai et al., 2008a,b; Klavins and Ansone, 2010). Diffusion rates and dynamic collision rates increase with increasing temperature. In contrast, complex formation strength tends to decrease with increasing temperature. Thus, fluorescence quenching increases under dynamic quenching, but decreases under static quenching as temperature increases. The temperature-dependant method can simply be used to investigate the quenching mechanisms, by comparing the quenching extent at various temperatures. Dynamic quenching reduces the average lifetime of the fluorophore while static quenching does not have the same effect. Therefore, the fluorescence lifetime measurement is the most commonly used method to identify the quenching mechanism (Marwani et al., 2007, 2009). Thus, the quenching mechanisms can be confirmed by: (a) exploring the curvature with a Stern–Volmer plot, (b) comparing the association constant with an extremely efficient quencher, (c) investigating temperature dependence of fluorescence titration, and (d) fluorescence lifetime measurement (Bai et al., 2008a,b; Marwani et al., 2007, 2009). Because of the complicated composition and structure of humic substances, as well as the unknown process of their interactions with nC60, all four methods were performed to investigate the main quenching mechanisms.

The objective of the present study was to investigate the interactions between FAs and nC60 and its affecting factors, e.g., FA concentration levels, pH values and temperatures. Using the fluorescence quenching method, temperature-dependent experiments, and fluorescence lifetime measurements, association constants between FAs and nC60 were carried out and estimated, and the interaction mechanisms between fulvic acid and nC60 are discussed.

Section snippets

Reagents and materials

Fullerene with a purity of 99.9% was obtained from Puyang Yongxin Fullerene Technology Co., Ltd. (Puyang City, China). The nC60 suspension was produced using the method recommended by Andrievsky et al. (1995) and Lyon et al. (2006). Using this method, about 5 ml of 1.0 g/l fullerene in toluene was added to about 500 ml pure water. The layered mixture was sonicated with a sonifier cell disruptor (Ultrasonic Processor, Auto Science, Canada) at 70–90 W for 15 min intervals (allowing the machine to

Results and discussion

The three-dimensional excitation–emission matrixes indicated that the major fluorescence peaks occurred at excitation/emission wavelengths of 280–400 nm/380–500 nm for both AFA and BFA. Both peaks are referred to as humic-like fluorescence (Kumke et al., 1998a,b; Zafiriou et al., 1984; Zepp, 1978; Zepp and Schlotzhauer, 1983; Zepp et al., 1985). The maximal fluorescence intensities measured with emission fluorescence spectra showed a significant linear relationship (p < 0.01) with FA

Conclusion

Overall, the intrinsic fluorescence of FAs could be significantly quenched by nC60, while the location of main fluorescence peaks remained constant with nC60 added, indicating the unchanged conformation and rigidity of FAs during titration. The static quenching mechanism proved to be the predominant mechanism for FA quenching with nC60 by obtaining the significant linear coefficient of the Stern–Volmer model, higher log K compared with extremely efficient quencher, temperature-dependent

Acknowledgments

The authors are grateful for the financial support from China's National Basic Research Program (2008CB418200) and the National Natural Science Foundation of China (40903036 and 41173084), KYZX (2010KYYW04), BARD (IS-4353-10), and USDA Hatch program (MAS00978).

References (41)

  • K. Yang et al.

    Adsorption of fulvic acid by carbon nanotubes from water

    Environmental Pollution

    (2009)
  • C. Zhang et al.

    Study of the contact charge transfer behavior between cryptophanes (A and E) and fullerene by absorption, fluorescence and 1H NMR spectroscopy

    Analytica Chimica Acta

    (2009)
  • D. Zhou et al.

    Photoluminescence and fluorescence quenchings of C60-pyrrolidine derivatives at room temperature

    Journal of Photochemistry and Photobiology A: Chemistry

    (1996)
  • G.V. Andrievsky et al.

    On the production of an aqueous colloidal solution of fullerenes

    Chemical Communications

    (1995)
  • Y.C. Bai et al.

    Interaction between carbamazepine and humic substances: a fluorescence spectroscopy study

    Environmental Toxicology & Chemistry

    (2008)
  • S.E. Cabaniss

    Forward modeling of metal complexation by NOM: II. Prediction of binding site properties

    Environmental Science & Technology

    (2011)
  • J.D. Fortner et al.

    C60 in water: nanocrystal formation and microbial response

    Environmental Science & Technology

    (2005)
  • R. Foster

    Organic Charge Transfer Complexes

    (1969)
  • T.D. Gauthier et al.

    Fluorescence quenching method for determining equilibrium constants for polycyclic aromatic hydrocarbons binding to dissolved humic materials

    Environmental Science & Technology

    (1986)
  • S. Ghosh et al.

    Behavior of aluminum oxide nanoparticles as affected by pH and natural organic matter

    Langmuir

    (2008)
  • Cited by (0)

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