A Sulfo-Cyanine Dye as a Colorimetric Chemosensor for Metal Cation Recognition ^†

Abstract

Metal cations play important roles in several industrial and biochemical processes. However, high levels of some cations are toxic and consequently cause serious health and environmental problems. Because of these features, the search for organic molecules capable of coordinating these analytes is an increasingly studied topic in the scientific community, especially those with optical responses (optical chemosensors). Following the research group’s interest in heterocyclic optical chemosensors for various ions, a sulfo-cyanine, which absorbs and emits in the NIR region, was studied as a chemosensor for the recognition of metal cations with biological and environmental relevance. Chemosensing studies showed that this sulfo-cyanine displayed a highly sensitive colorimetric response, from blue to colorless, for Cu²⁺ and Fe³⁺ in acetonitrile solution.

1. Introduction

The development of small organic molecules as optical chemosensors for the recognition and detection of environmentally and biologically important metal ions is an essential research topic in supramolecular chemistry. In particular, chemosensors based on colorimetric changes have attracted considerable attention due to their advantages, namely cost-effectiveness, high sensitivity, and naked eye detection without the need for expensive equipment [1,2,3].

To date, various classic dyes have been applied as chemosensory signaling units, such as coumarin [4], pyrene [5], 1,8-naphthalimide [6], xanthenes [7], boron dipyrromethene difluoride (BODIPY) [8], and cyanine [9] derivatives. Among them, cyanine-based platforms have gained significant interest because of their remarkable spectral properties, including narrow absorption bands, large molar absorptivity, high sensitivity, and particularly an intense π-π* absorption which can be easily tuned from the visible to the near-infrared (NIR) region by structural modifications at the chromophore moiety [10,11,12]. For example, classic cyanine-based Cy3 dyes with three methine groups emit visible light, while pentamethine cyanine dyes Cy5 are fluorescent in the NIR region [9].

Furthermore, metal ion–cyanine interactions can produce a color change associated with the delocalization of π electrons, allowing for the easy detection of the intended analyte in solution, making them ideal candidates as colorimetric sensors for metal recognition. Therefore, there is great interest in exploring colorimetric chemosensors based on the cyanine moiety through the insertion of suitable functional groups [13,14].

Bearing these facts in mind, and considering the research group’s experience on the design, synthesis, and characterization of heterocyclic optical chemosensors [8,15,16,17], we report herein the evaluation of a sulfonated pentamethine cyanine (sulfo-Cy5) dye as a potential colorimetric chemosensor for metal cation recognition. Therefore, interaction studies of this dye in the presence of several cations were performed through a preliminary chemosensory study, followed by spectrophotometric titrations.

2. Experimental Section

2.1. Methods and Materials

The synthesis and structural characterization of sulfo-Cy5 carboxylic acid 1 has been reported by us elsewhere [18]. UV–vis absorption spectra were obtained using a Shimadzu UV/2501PC spectrophotometer (Shimadzu Europa GmbH, Duisburg, Germany) and fluorescence spectra were collected using a FluoroMax-4 spectrofluorometer (HORIBA Europe GmbH, Darmstadt, Germany) in standard quartz cuvettes.

2.2. Photophysical Characterization

The photophysical characterization of sulfo-Cy5 carboxylic acid 1 was carried out by UV–vis absorption and fluorescence spectroscopy of a 5 × 10⁻⁶ M solution in UV-grade acetonitrile. The fluorescence spectrum of 1 was obtained by excitation at the wavelength of maximum absorption. The relative fluorescence quantum yield was calculated using a solution of Nile blue in methanol as the standard (Φ_F = 0.27) [19].

2.3. Preliminary Chemosensing Studies and Spectrophotometric Titrations

An evaluation of sulfo-Cy5 carboxylic acid 1 as a colorimetric chemosensor was undertaken in the presence of several cations (Ag⁺, K⁺, Li⁺, Na⁺, Cu⁺, TBT⁺, Hg²⁺, Ca²⁺, Co²⁺, Pb²⁺, Mn²⁺, Fe²⁺, Zn²⁺, Ni²⁺, Cd²⁺, Cu²⁺, Pd²⁺, Cs²⁺, Sn²⁺, Fe³⁺, and Al³⁺). Solutions of compound 1 (1 × 10⁻⁵ M) and solutions of cations under study (ca. 1 × 10⁻² to 1 × 10⁻³ M) were prepared in UV-grade acetonitrile.

A preliminary study was carried out by addition of 50 equivalents of each cation to the solution (1 mL) of compound 1. Considering this preliminary result, spectrophotometric titrations of compound 1 with selected cations were performed by the sequential addition of the cation stock solution to the solution of compound 1, with the collection of absorption spectra.

3. Results and Discussion

3.1. Photophysical Characterization

The photophysical properties of the pentamethine sulfo-cyanine 1 (Figure 1), previously synthesized [18], were evaluated by UV–vis absorption and fluorescence spectroscopy in acetonitrile solution. The compound’s solution displayed a vibrant blue color and the UV–vis absorption and fluorescence data for sulfo-Cy5 carboxylic acid 1 (maximum absorption wavelength, λ_max; molar extinction coefficient, ε; maximum emission wavelength, λ_em; relative fluorescence quantum yield, Φ_F; and Stokes’ shift, ∆λ) are compiled in

Table 1. UV–vis absorption and fluorescence data of sulfo-Cy5 carboxylic acid 1 (5 × 10⁻⁶ M) in acetonitrile solution.

Compound	UV–Vis Absorption		Fluorescence
	λmax (nm)	log ε	λem (nm)	ΦF	∆λ (nm)
1	651	4.99	672	0.30	21

.

The sulfo-Cy5 carboxylic acid 1 displayed a narrow absorption band (Figure 2) and high molar absorptivity (log ε = 4.99) at 651 nm. Upon excitation at the wavelength of maximum absorption, compound 1 exhibited a sharp emission band with a wavelength of maximum fluorescence in the NIR region (λ_em = 672 nm). The narrow bands and the small Stokes’ shift are typical features of this class of compounds [20].

3.2. Preliminary Chemosensing Studies and Spectrophotometric Titrations

An evaluation of sulfo-Cy5 carboxylic acid 1 as a colorimetric chemosensor was carried out in the presence of several cations with biological and environmental relevance, through preliminary chemosensory studies, followed by spectrophotometric titrations.

Firstly, the colorimetric behavior of compound 1 was studied by the addition of 50 equivalents of each cation in acetonitrile (Figure 3). The chromogenic response of compound 1 was remarkably visible to the naked eye, with a color change from blue to colorless in the presence of Cu²⁺ and Fe³⁺ and from blue to darker blue upon interaction with Hg²⁺, Ca²⁺, and Pb²⁺.

Given the preliminary colorimetric sensing results and the most dramatic changes, spectrophotometric titrations were performed for compound 1 with Cu²⁺ and Fe³⁺ in acetonitrile solutions (Figure 4). The UV–visible spectra revealed a trend: the intensity of the longest wavelength absorption band at 651 nm decreased upon the addition of the metal cation. Moreover, the sensitivity of compound 1 towards Cu²⁺ and Fe³⁺ was evident, since the addition of only 1 and 2 equivalents, respectively, was necessary to achieve a plateau.

The very low number of Cu²⁺ and Fe³⁺ equivalents necessary for a noticeable color change is a good indicator of the sensitivity of the tested sulfo-Cy5 1 towards these cations. Among transition metal ions, copper and iron are essential trace elements in the human body, playing important roles in many fundamental physiological processes, but excessive levels of these cations can result in serious disorders such as Huntington, Parkinson’s, Alzheimer’s, Menke’s, and Wilson’s diseases [21,22]. Therefore, finding highly sensitive receptors that can detect Cu²⁺ and Fe³⁺ is of great importance.

4. Conclusions

In summary, the photophysical properties of a sulfo-cyanine containing two sulfonic acid groups were studied by UV–vis absorption and fluorescence spectroscopy in acetonitrile. Furthermore, this compound was evaluated as a colorimetric chemosensor for several cations with biological and environmental relevance. A preliminary chemosensing study showed that cyanine 1 displayed a highly colorimetric response, from blue to colorless, for Cu²⁺ and Fe³⁺ in acetonitrile solution. As a result, spectrophotometric titrations were performed for compound 1 with the selected cations, confirming the sensitivity of this compound in the presence of these analytes. These results clearly indicate that sulfo-Cy5 carboxylic acid 1 can be used to detect Cu²⁺ and Fe³⁺ with remarkable sensitivity.