Amino and chloro derivatives of 1,10-phenanthroline as turn-off fluorescence sensors for selective and sensitive detection of Fe(II)

https://doi.org/10.1016/j.jphotochem.2020.112805Get rights and content

Highlights

  • Amino and chloro derivatives of 1,10-phenanthroline as turn-off sensors for Fe(II).

  • Fluorescence quenching by Fe(II) through 1:3 non-fluorescent complex formation.

  • Detection of Fe(II) with minimum interference from Cu(II), Ni(II) and Co(II).

  • Charge transfer spectra, temperature studies indicate static and dynamic quenching.

Abstract

Phenanthroline is a fluorophore known for its non-fluorescent complex formation with Fe(II). We present two novel turn-off fluorescence sensors developed based on 5-amino and 5-chloro derivatives of 1,10-phenanthroline for the quantification of Fe(II) with improved sensitivity and selectivity compared to pristine phenanthroline. The fluorescence intensity of 5-amino-1,10-phenanthroline at λem 512 nm when λex 280 nm was decreased remarkably mainly due to the 1:3 static quenching by Fe(II), supported by the charge transfer spectra and the temperature studies. The formation of a stable complex with Fe(II) was observed in the range of pH 4.64 to 8.21 with a detection limit of 2.74 × 10-7 M at 3.3σ and 25 °C. The 5-chloro derivative showed much higher sensitivity and selectivity towards Fe(II) in the range of pH 3.80 to 6.80 with a detection limit of 1.70 × 10-8 M at 3.3σ and 25 °C when the λex and λem were 286 nm and 379 nm, respectively. In both the fluorescence sensors, the observed interference from foreign ions was at a minimum level, indicating the sensors’ specific selectivity towards Fe(II). The interference from cations; Cu(II), Ni(II), and Co(II) was comparatively higher, while the anions showed only a little to no interference.

Introduction

Phenanthrolines are a versatile class of chelating agents that are very important in coordination chemistry [1]. 1,10-Phenanthroline is the parent compound of the class of phenanthrolines [2]. Properties like hydrophobicity, rigidity, planarity, and heteroaromaticity have led 1,10-phenanthroline to possess a unique combination of physical and chemical properties [3]. This compound has close-lying π – π* and n – π* singlet excited states and the emission originates mainly from π – π*. Excited state n – π* often decays via non-radiative pathways resulting in very low emission quantum yields. Phenanthrolines are electron deficient compounds; hence, excellent π electron acceptors. Due to the low lying π* orbitals, strong metal to ligand charge transfer (MLCT) bands in the visible spectrum and red-shifted fluorescent emissions can be seen [4].

1,10-Phenanthroline and its derivatives have been widely utilized as analytical probes [2]. The fluorescence response of the parent molecule can be tuned carefully through substitution [2,5,6]. Therefore, observation of the variation of fluorescence in the substituted phenanthrolines is a vital area of research.

Fluorescence sensor development based on fluorophores is of great attention due to its simplicity and higher sensitivity than other sensing methods such as spectrophotometry [7,8]. A typical fluorescent chemosensor has a signaling unit and a guest binding site which are separated by a spacer unit [9]. Basically, there are two types of chemosensors based on the change of fluorescence intensity of the probe; chelation enhanced fluorescence (CHEF) sensors and chelation enhanced fluorescence quenchers (CHEQ) [10,11].

Out of the trace elements present in the human body, iron (Fe) is the most abundant essential trace element [12,13]. The total iron content of the body is about 3-5 g, with most of it in the blood and the rest in the bone marrow, liver, and muscles as heme [13]. Fe is an essential element in myoglobin and other numerous enzymes such as p450, cytochrome a-c, catalases, and peroxidases [13]. The involvement of iron in physiological reactions is specific to its interconvertible multiple oxidation states, mainly ferrous / Fe(II) and ferric / Fe(III) [14,15,16]. Therefore, it is essential to selectively analyze the ionic species to understand their roles better [15]. Although there are numerous techniques such as atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES) available for the quantification of iron, none of them are capable of selective identification of the redox states separately. Therefore, fluorescence sensors have gained trust as a promising solution to address this selectivity issue.

Due to the high reactivity and short-lived nature, only a very few techniques have been reported in the selective analysis of Fe(II) in contrast to Fe(III). The first reported fluorescence probe in detecting Fe(II) is calcein. This turn-off probe has failed due to its inability to distinguish between Fe(II) and Fe(III) since Fe(II) oxidizes in binding to calcein [15]. Achieving a turn-on probe by chelation based strategy is also difficult since iron itself carries a quenching property owing to its paramagnetic effect [15]. Due to the strong chelating ability of phenanthroline to Fe(II), attempts have been taken to develop phenanthroline based fluorescence sensors for the analysis of Fe(II) [17].

Fe(II) forms a red colour 1:3 metal-ligand complex with the pristine 1,10-phenanthroline parent molecule (1) [18]. The geometry of the molecule is octahedral. The complex is non-fluorescent, enabling the quantification of Fe(II) through the reduction of the free fluorophore’s initial emission intensity [17]. This is the key concept utilized in developing the turn-off fluorescence probes throughout this study.

We present successfully developed two novel turn-off fluorescence probes based on two derivatives of 1,10-phenanthroline (1); 5-amino-1,10-phenanthroline (2) and 5-chloro-1,10-phenanthroline (3), for the selective determination of Fe(II) in different organic solvent systems enabling the comparison of the photophysical properties improved through derivatization. These new probes were experimentally found to avoid the inconsistency, irreproducibility, low selectivity, and solubility observed by pristine 1,10-phenanthroline. The improved detection limits of (2) and (3) are 2.74 × 10-7 M and 1.70 × 10-8 M, respectively.

Section snippets

Apparatus

Absorption spectra were obtained using Spectro UV-Vis Double PC 8 Auto Cell spectrophotometer using a UV quartz cuvette (3 mL). Fluorescence emission spectra were taken using a Hitachi F 7000 fluorescence spectrophotometer equipped with a 150 W xenon lamp using a 1 × 1 cm2 quartz fluorescence cuvette. Both the excitation monochromator slit width and the emission monochromator slit width were set to 10 nm. A sonicator was used to homogenize the solutions further, and the temperature studies were

Determination of the spectral properties

The main reason to measure absorption/excitation and emission spectra for the fluorophores is to identify the excitation wavelengths and to set the detector intervals for fluorescence. Fig. 1 shows the excitation and emission spectra of (2) and (3), respectively. Excitation and emission wavelengths of each derivative are summarized, and Stokes shifts are calculated in Table 1.

In the absorption studies, the fluorophores’ stock solutions were diluted enough to avoid possible aggregation of

Conclusions

The selective analysis is essential to assess the independent involvement of different redox states of iron, in chemical and biological reactions. Therefore, in this study, turn-off fluorescence sensors were successfully developed based on 5-amino and 5-chloro derivatives of 1,10-phenanthroline for the selective quantification of Fe(II). The reported LoQ values for the probes based on 5-amino and 5-chloro derivatives are 8.29 × 10-7 M and 5.16 × 10-8 M, respectively. Therefore, (2) based

CRediT authorship contribution statement

D.H. Thanippuli Arachchi: Investigation, Formal analysis, Writing - original draft, Visualization. G.I.P. Wijesekera: Investigation, Formal analysis, Writing - original draft, Visualization. M.D.P. De Costa: Conceptualization, Methodology, Supervision, Validation, Writing - review & editing. R. Senthilnithy: Conceptualization, Resources, Methodology, Supervision, Validation, Writing - review & editing.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgment

Faculty of Natural Sciences of the Open University of Sri Lanka is greatly acknowledged for the fund provided for this study.

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