Monitoring of photoluminescence decay by alkali and alkaline earth metal cations using a photoluminescent bolaamphiphile self-assembly as an optical probe

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Highlights

Abstract

Photoluminescence (PL) decay induced by the displacement of an ionic fluorescence component, Tb3+, with alkali and alkaline earth metal cations was investigated using photoluminescent spherical self-assemblies as optical probes. The photoluminescent spherical self-assembly was prepared by the self-organization of a tyrosine-containing bolaamphiphile molecule with a photosensitizer and Tb3+ ion. The lanthanide ion, Tb3+, electrically bound to the carboxyl group of the bolaamphiphile molecule, was displaced by alkali and alkaline earth metal cations that had stronger electrophilicity. The PL of the self-assembly decayed remarkably due to the substitution of lanthanide ions with alkali and alkaline earth metal cations. The PL decay showed a positive correlation with cation concentration and was sensitive to the cation valency. Generally, the PL decay was enhanced by the electrophilicity of the cations. However, Ca2+ showed greater PL decay than Mg2+ because Ca2+ could create various complexes with the carboxyl groups of the bolaamphiphile molecule. Microscopic and spectroscopic investigations were conducted to study the photon energy transfer and displacement of Tb3+ by the cation exchange. This study demonstrated that the PL decay by the displacement of the ionic fluorescent compound was applied to the detection of various cations in aqueous media and is applicable to the development of future optical sensors.

Introduction

Optical detection and quantitative analysis of alkali and alkaline earth metal cations, such as sodium and magnesium in an aqueous solution, are required for many practical uses such as water purification and ion chromatography [1], [2]. Generally, the optical quantitative analysis of the concentrations of such cations is conducted using fluorescent photo-induced electron transfer (PET), in which the fluorescence from the fluorophore is controlled by the association of the cation with a cation receptor [3], [4], [5]. Fluorophores with various receptors have been developed to enhance the receptor sensitivity since the interaction of the cation with the fluorophore is the key factor in fluorescent PET [6], [3]. In contrast to the enhanced fluorescence of the PET system, quenching (or decay) of the fluorescence by a cation is also exploited for optical sensing. For example, the fluorescence quenching of an anionic fluorescent polymer by a noble metal cation was applied for the quantitative analysis of the cation concentration, where electrical attraction of the cation was the key factor in reducing the fluorescence from the anionic polymer [7]. In a similar way, the complexation of a crown ether compound which led fluorescence quenching was also exploited for quantitative analysis of the alkali metal cation [8]. Though these approaches offered fluorescent molecular probes for selective cation detection, development of a novel optical probe to easily detect alkali and alkaline earth metal cations with visual observation is still needed.

Self-assembly of amphiphilic molecules is a facile way to prepare optical probes at a submicron scale. Generally, self-organization of bolaamphiphilic molecules associated with specific functional molecules has been exploited to prepare a solid platform with diverse physical, chemical and optical functions [9], [10]. These bolamphiphile self-assemblies have been applied as templates to produce diverse hybrid materials such as inorganic-organic nanospheres and microtubes [11], [12], [13]. Besides the use of template, association of fluorescent molecules with bolaamphipile was used to prepare optical materials [14], [15]. We recently reported a photoluminescent nanospherical self-assembly made of tyrosine-containing bolaamphiphile molecules with fluorescent Tb3+ ions [14]. The self-assembly of the bolaamphiphile molecule functioned as a host matrix enhancing the photon energy transfer from the photosensitizer to the lanthanide ions through an antenna effect of the tyrosine moiety. Whereas, the lanthanide ions were electrically bound to the carboxyl group of the bolaamphiphile molecule such that exchange of the lanthanide ion with another ion would reduce the photoluminescence (PL) emission. Based on these principles, the photoluminescent bolaamphiphile self-assembly may be applicable as an optical probe for cation sensing.

In this study, we examined PL decay by substitution of lanthanide ion with alkali metal and alkaline earth metal cations using the photoluminescent bolaamphiphile self-assembly as an optical probe. When a cation with higher electrophilicity than Tb3+ was added to the suspension of the photoluminescent self-assemblies, PL from the self-assembly was reduced by the displacement of the lanthanide ion with cations. Schematics of the PL and the chemical structure of the tyrosine-containing bolaamphiphile (Tyr-C7) are shown in Scheme 1. The correlations between cation concentration, PL decay, and excitation energy changes were investigated. The degree of PL reduction was positively proportional to the concentration, valency, and electrophilicity of the cation. These positive correlations demonstrated that optical detection of the cation concentration could be achieved using the photoluminescent bolaamphiphile self-assembly, where the displacement of the Tb3+ ion functioned as the origin of PL decay.

Section snippets

Materials

The self-assembling molecule bis(N-alpha-amido-tyrosine)-1,7-heptane dicarboxylate (Tyr-C7) was synthesized according to a previously reported protocol [11]. Terbium(III) chloride hexahydrate (TbCl3·6H2O, 99.9%, Sigma–Aldrich) and salicylic acid (SA, ACS grade, Sigma–Aldrich) were chosen as photosensitizing chemicals because they associated well with the Tyr-C7 assembly and demonstrated PL enhancement in a previous study [14]. Alkali and alkaline earth metal chloride salts were used as cation

Results and discussion

The optical probe of the spherical photoluminescent Tyr-C7 self-assembly emitted blue light under the UV excitation of λ = 330 nm. The fluorescent component emitting PL was the Tb3+ ion, while the bolaamphiphile self-assembly worked as the host matrix offering binding sites for the Tb3+ ion and photosensitizer [14]. When alkali or alkaline earth metal cations were added to the photoluminescent Tyr-C7 self-assembly, the Tb3+ ion was released such that the PL from the self-assembly decayed

Conclusion

The fluorescent Tb3+ cation associated with the photoluminescent bolaamphiphile self-assembly was exchanged with electrophilic alkali and alkaline earth metal cations, which led to decay of PL emission. As more Tb3+ ions were displaced, less photon energy could transfer from photosensitizer to Tb3+ ion such that more photon energy were remained in the photosensitizer. More Tb3+ ions were exchanged with an increase in concentration and electrophilicity of the cation, except Ca2+. Since the Ca2+

Acknowledgement

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2012R1A1A2008543).

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