Article pubs.acs.org/JPCC
Effect of Graphene and Graphene Oxide Dispersions in Poloxamer-407 on the Fluorescence of Riboflavin: A Comparative Study Ana M. Díez-Pascual,* Cristina Hermosa Ferreira, María Paz San Andrés, Mercedes Valiente, and Soledad Vera Analytical Chemistry, Physical Chemistry and Chemical Engineering Department, Faculty of Biology, Environmental Sciences and Chemistry, Alcalá University, E-28871 Alcalá de Henares, Madrid, Spain S Supporting Information *
ABSTRACT: The effect of graphene (G) and graphene oxide (GO) dispersions in a triblock copolymer, Kolliphor P407 (poloxamer-407), on the fluorescence of vitamin B2 (riboflavin) has been comparatively investigated. The quality of the dispersions was assessed by measuring the thickness of their flakes and visualizing their distribution within the copolymer and degree of exfoliation via scanning and transmission electron microscopies (SEM and TEM). The influence of G or GO and copolymer concentration on the fluorescence intensity was studied, and fluorescence quenching phenomena were observed; these become more effective with increasing G or GO content for Kolliphor concentrations equal to or above the CMC, the diminution in intensity being stronger for G dispersions, which is related to the different hydrophobicity of the nanomaterials, because these modify the distribution equilibrium of the vitamin between the solution and the Kolliphor micellar aggregates. Further, for a given G or GO concentration, the intensity decreases with increasing copolymer concentration. The ratio between the fluorescence intensity in the absence and the presence of G fits to a second-order polynomial, suggesting a combined mechanism of static and dynamic quenching, while for GO dispersions it follows the Stern−Volmer linear equation in the low concentration range. The quenching observed herein could be useful in the development of optical sensors for riboflavin determination. G-based sensors are expected to have better performance in terms of sensitivity and repeatability than GO-based ones due to their stronger nanomaterial− vitamin interaction, superior quenching efficiency, and higher signal-to-noise ratio.
1. INTRODUCTION The development of nanomaterial-based composites has become a hot spot of research due to their exceptional optical, electric, mechanical, and catalytic properties.1,2 Graphene (G), which is a single layer of graphite, is among the most exciting nanomaterials being currently investigated. It is one of the strongest materials on earth,3 and possesses very high thermal4 and electrical conductivity at room temperature.5 On the other hand, graphene oxide (GO), an oxidized form of graphene laced with oxygen-containing groups, has also received great interest because of its unique properties such as good dispersity in water (and other organic solvents), lower cost, and easier processability than G.6 It contains epoxide, carbonyl, and hydroxyl groups on the basal planes and carboxylic on the edges, and can be produced by the oxidative treatment of graphite via one of the methods developed by Brodie, Hummers, or Staudenmeir.7−9 Further, G is not fluorescent, while contradictory reports have been published regarding the fluorescence of GO. Thus, Sun et al.10 and Liu et al.11 found that GO is luminescent in the visible and IR wavelength range, while Thomas et al.12 have demonstrated that the weak and wide fluorescence emission of GO is due to the remaining impurities after the oxidation treatments that are highly © XXXX American Chemical Society
photoluminescent. Both G and GO have been proved to be outstanding nanomaterials for applications in analytical chemistry,13 particularly for the design of electrochemical and optical sensors.4,14 The preparation of G and related-based materials encounters several challenges in terms of dispersion given that the large G or GO surface area and strong van der Waals forces among the flakes can lead to severe aggregation in the composite. To attain stable G or GO dispersions and tailor the microstructure of the composites, ultrasonication processes, the use of surfactants, and/or the covalent and noncovalent functionalization with polymers are typically necessary.15−17 The ultrasonication is frequently performed in organic solvents and under high ultrasonication power, which provokes defects in the G flakes that have negative effects on the properties. The exfoliation of G in aqueous media is viable using surfactants as stabilizing agents, which interact with the nanomaterial via surface adsorption, micelle formation, or π−π stacking, thus preventing the reaggregation of the sheets.16 A variety of nonionic and Received: September 28, 2016 Revised: December 20, 2016 Published: December 21, 2016 A
DOI: 10.1021/acs.jpcc.6b09800 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Riboflavin (vitamin B2) is an important biological molecule that also acts as an antioxidant within the body. This watersoluble vitamin is responsible for maintaining healthy blood cells, helps to boost energy levels, prevents free radical damage, contributes to growth, protecting skin and eye health, and participates in the metabolism of other vitamins, fats, carbohydrates, and proteins, and hence plays a key role in the human diet.35 The lack of riboflavin can create a number of serious side effects, such as hematological, cardiovascular, and gastrointestinal illnesses, and can be a threat factor for some cancers. For instance, the increase in the ingestion of this vitamin can prevent breast and uterine cancer.36 Considering the high nutritional value of riboflavin and its potential for medical utilization, it is of great importance to develop a variety of methods to analyze this vitamin. Techniques for quantification of riboflavin include fluorometry, high performance liquid chromatography (HPLC), microbiological assays, immunoassays, and biosensors.37−39 Among these procedures, fluorometry is the most widely employed due to its straightforwardness, excellent sensitivity and selectivity, large range of applicability, noninvasiveness, and quickness. The sensitivity of the fluorescence measurements can be further improved in the presence of surfactants. Thus, very small amounts of riboflavin have been determined via synchronous fluorescence spectroscopy (SFS) in the presence of an anionic surfactant, bis2-ethylhexysulfosuccinate sodium salt (AOT), with very good repeatability and sensitivity.37 In a preceding study, a quenching phenomenon of vitamin B2 fluorescence was observed in the presence of G dispersions in polyethylene glycol (PEG).38 The current work is dedicated to gain insight into the effect of G and GO dispersions in Kolliphor P407 on the fluorescence of riboflavin, and to investigate the potential existence of quenching phenomena. The influence of G, GO, and copolymer concentration on the fluorescence intensity has been evaluated. To the best of our knowledge, there is no previous study that comparatively investigates the quenching behavior of these two nanomaterials on the fluorescence emission of a vitamin.
ionic surfactants have been explored, and the former have been proven to be especially promising.18 On the other hand, the covalent functionalization is based on the reaction between the functional groups on the GO surface and specific functional groups on the polymer.4 Alternatively, the noncovalent functionalization, which relies on van der Waals forces, hydrogen bonding, electrostatic, or π−π stacking interactions, provides an effective means to tailor the properties and solubility of the nanosheets without altering their chemical structure.17 Poloxamer 407, also known as Kolliphor P407, is a triblock copolymer consisting of a central hydrophobic block of poly(propylene oxide) (PPO) flanked by two hydrophilic blocks of poly(ethylene oxide) (PEO), Scheme 1, with a PPO content of Scheme 1. Structure of Kolliphor P407
about 30 wt %. This biodegradable, biocompatible, and lowcost macromolecule belongs to the general class of copolymers known as poloxamers.19 In aqueous solutions and at low concentrations, poloxamers exist as monomers. Upon warming and/or increasing the polymer concentration above the critical micelle concentration (CMC), micelle aggregates are formed. These micelles are spherical and consist of a PPO core poorly water-soluble with a PEO shell highly soluble in aqueous solvent. This type of amphiphilic block copolymers with surfactant character can be used as steric stabilizers of G or GO suspensions in water.20,21 They noncovalently bind to the nanomaterial surface and minimize the aggregates due to nanosheet−nanosheet interactions. It has been demonstrated that high G concentrations can be dispersed in water using Pluronic block copolymers, and the dispersion efficiency depends on the length of the hydrophilic and hydrophobic domains.22 The hydrophobic PPO block strongly interacts with the aromatic rings of the G basal plane and the hydrophilic PEO blocks interact with the water molecules, thus stabilizing the nanomaterial dispersion. Fluorescence quenching is an issue that has received considerable attention given that it can provide valuable information about biochemical systems.23 Even though a number of recent studies have been published on the fluorescence quenching of organic dyes by G,11,24−28 similar reports related to GO are scarce.29−31 Recently, the interaction between phtalocyanines and G, GO, and single-wall carbon nanotubes (SWCNTs) has been comparatively investigated using fluorescence,32 and it was found to be stronger for G than for the other nanomaterials. The quenching effect takes place mainly via π−π stacking between the aromatic rings of G or GO and those of the fluorescent molecules, although electrostatic and hydrophobic interactions as well as covalent bonding can also contribute to the quenching phenomenon.33 For instance, it has been demonstrated that cationic charged dyes like Rhodamine physically adsorb onto GO sheets and interact via π−π and electrostatic cooperative interactions.34 Nonetheless, the quenching behavior for both G and GO has seldom been addressed; hence the research in this direction is highly interesting.
2. EXPERIMENTAL SECTION Reagents. Riboflavin (C17H20N4O6, Mw = 376.36 g/mol) and the poloxamer (Kolliphor P407, HO(C2 H 4 O) 101 (C3H6O)56(C2H4O)101H, Mn ≈ 12 000 g/mol) were provided by Sigma (Spain). Pristine graphene (av-PRIST) consisting of layers with a lateral size in the range of 100−500 μm, and an oxygen content
