Enhanced fluorescence effect of acridine orange sorbed on 2:1 layered clay minerals

Highlights

• Fluorescence of acridine orange on 2:1 dioctahedral clays in powder form was tested.

• Maximal fluorescence emission obtained when the dye was in monomer formation.

• Fluorescence emission was optimized at dye loading of 24–30 μmol/g on the clays.

• Higher dye loading levels resulted in significant fluorescence quenching.

• Exploration of dye-loaded clay powders for fluorescence applications is promising.

Abstract

Many dye molecules form different aggregation states in aqueous solution under different concentrations. Acridine orange (AO) is a cationic fluorescence dye and is commonly used in DNA analyses. Most measurements for light absorption and fluorescence emission of dyes sorbed on solid matrix reported in literature were performed in clay-dye suspension. Rarely, it was evaluated in solid state. In this study, the fluorescence effect of AO, after being sorbed on 2:1 layered clay minerals montmorillonite (MT), rectorite (RT), and illite (IT), was evaluated in terms of contributions by different forms of AO (monomer, dimer, H- and J-aggregates). As the amounts of AO sorbed increased, the proportion of absorption spectra made by monomer and dimer decreased, while that made by of H- and J-aggregates increased. In sorbed state, the absorbance of AO monomer and dimer was obvious while that attributed to H- and J-aggregates was higher in comparison to that in AO solution. At AO loading of 30 μmol/g on MT and 24 μmol/g on RT and IT, fluorescence emission reached maximum intensity. While the majority of the fluorescence was attributed to monomer and dimer, an unknown peak at 580–590 nm was resolved in the fluorescence spectra. Due to its red shift, it was attributed to J-aggregates on the surface of the minerals. The contribution by H-aggregates was observed in absorption spectra, but not in fluorescence spectra, which was speculated that the blue shift caused by H-aggregates overlapped with the excitation wavelength of 490 nm. The 2:1 clays could serve as a substrate for optimal dye sorption to achieve maximum fluorescence emission effect and practical applications of such dye-clay composites in powder form could be further explored.

Introduction

Interactions between clay minerals and color dye could be traced back to more than 1200 years ago when Maya blue, a mixture of indigo dyes with palygorskite, was used to color sculptures, artworks, and textiles around the Aztec area in Mexico. Modern studies of interactions between clay minerals and color dyes utilizing the absorption of color in UV and visible spectra were started in the 1980's (Cohen and Yariv, 1984; Schoonheydt et al., 1986; Grauer et al., 1987a, Grauer et al., 1987b; Cenens and Schoonheydt, 1988; Endo et al., 1988). Commonly tested dyes include cationic dyes acridine orange (AO) (Cohen and Yariv, 1984), rhodamine B (RB) (Grauer et al., 1987a), rhodamine 590 (R590) (Endo et al., 1988), proflavine (PF) (Schoonheydt et al., 1986), and methylene blue (MB) (Cenens and Schoonheydt, 1988).

MB, as a sensitive fingerprint molecule, was used to interpret the surface properties of dyes on clay minerals in terms of formation of monomer MB⁺, protonated monomer MBH²⁺, dimer (MB⁺)₂, and trimer (MB⁺)₃ with MB⁺ and MBH²⁺ being the dominant species under a low loading level, and dimers and trimers being formed at the expense of monomers as the loading increased (Cenens and Schoonheydt, 1988). In addition, metachromasy could increase photostabilization of MB adsorbed on certain types of clay minerals (Samuels et al., 2013). Similarly, the sorbed PF on different montmorillonites existed in three species: monomer PFH⁺, diprotonated monomer PFH₂²⁺, and dimer, with their relative concentrations depending on the loading and type of montmorillonite (Schoonheydt et al., 1986).

For some of the fluorescent dyes, earlier studies showed that dimeric ions of thionine and MB would not fluoresce and the self-quenching of fluorescence at higher dye concentrations was due to dimerization in aqueous solution, but not in ethanol (Rabinowitch and Epstein, 1941). In aqueous solution, dispersion force was suggested to play an important role in holding dimer molecules together against electrostatic forces (Padday, 1967).

In addition, studies on the fluorescence properties of some fluorescent dyes after being sorbed on the surface or in the interlayer of clay minerals were also extensively reported. Addition of laponite to dye solution could control the dimer to monomer ratio by decreasing the dimer formation (Bhattacharjee et al., 2013). The fluorescence of the PF dimers was detected at 580 nm and the fluorescence intensity could be quenched by Fe³⁺, by protonation to diprotonated monomer and by dimerization; and in the absence of Fe³⁺, the fluorescence emission intensities decreased significantly with increasing PF loading (Schoonheydt et al., 1986). For RB the intensity of fluorescence emission decreased as the dye loading on MT increased and the saturation point to cause fluorescence quenching was 0.45 mmol/g of RB on MT (Grauer et al., 1987a). In addition, the maximum wavelength of emission for RB was 575 nm in aqueous solution and it increased to 592 nm in the presence of MT or laponite (Grauer et al., 1987a). The fluorescence intensity of R590 reached maximum at about 0.08 mmol/g loading level on a saponite, about 1/10 of its CEC value, and the fluorescence emission wavelength increased from 560 to 580 nm as the R590 loading increased (Endo et al., 1988).

AO is a fluorescent cationic dye that is commonly used to probe DNA structure and interactions between DNA and drug or protein due to its nucleic acid selectivity (Darzynkiewicz, 1990). The intense auto-fluorscence background of AO in solution could be minimized through a π–π stacking interaction with single-wall carbon nanotubes before probing DNA (Meng et al., 2012). Similar quenching effects occurred predominantly via formation of H-type aggregates governed by electrostatic and π–π stacking interactions between AO and graphene oxide nanosheets (Hansda et al., 2016). Many organic color dyes including AO could have extensive applications as tunable fluorescent materials or sensors due to their attractive stimuli-responsiveness and reversibility through non-covalent interactions with polymers or surfactants (Wang et al., 2017). After the addition of surfactant cetyltrimethylammonium bromide into AO-polystyrensulfonate solutions, fluorescence intensities were distinctly enhanced in association with a decrease of dimer/monomer absorbance ratio due to the dissociation of AO dimers adsorbed on polystyrensulfonate chains into monomers and the replacement of AO on the polymer chains by long-chain surfactant molecules, which demonstrated binding of surfactant molecules to polymer chains prior to the arrival of the critical micelle concentration (Mondek et al., 2014). As such AO can be considered a sensor for monitoring polyelectrolyte-surfactant interactions. Because of their extensive use, many studies were devoted to the removal of color dyes including AO from water using novel materials such as layered double hydroxides (LDHs) (Khan et al., 2016), magnetic nano γ-Fe₂O₃ particles (Qadri et al., 2009), and clays (Lv et al., 2011, Lv et al., 2014). In addition, AO-loaded LDHs were tested for its detection of Hg²⁺ in aqueous solution (Liu et al., 2017).

Studies on fluorescence properties of AO after sorbed on solid matrix were limited only to investigate the effect in particle suspensions or colloidal dispersions, with results indicating significant influences on aggregation state and spectral structures of AO molecules by the surface area, layer charge (surface charge density), and interlayer expandability and interaction of clays (Cohen and Yariv, 1984; Schoonheydt et al., 1986; Garfinkel-Shweky and Yariv, 1997a, Garfinkel-Shweky and Yariv, 1997b; Bujdák and Iyi, 2002). Limited studies were reported to investigate the absorption and fluorescence properties of the dyes sorbed on solid matrix in particulate forms. Montmorillonite, rectorite (regular mixed layers of illite and montmorillonite), and illite belong to the 2:1 layered clay minerals that make up a significant portion of the surface deposits under different geological processes and typically have distinct surface charge densities, surface areas, and interlayer properties. In this study, we uploaded different amounts of AO onto these three types of 2:1 layered clay minerals and investigated the UV–Vis absorption and fluorescence emission properties of the AO-loaded clay minerals in powder form. In such a case, the influence of dissolved AO on light absorption and fluorescence emission would be minimized and additional aspects of application may be explored, utilizing clay powders with potentially enhanced thermostability and photostability (Valandro et al., 2017).