A novel oligothiophene-based colorimetric and fluorescent “turn on” sensor for highly selective and sensitive detection of cyanide in aqueous media and its practical applications in water and food samples

Highlights

• A new oligothiophene-based naked-eye colorimetric and fluorescent turn-on sensor 3TT was developed.

• 3TT exhibited high selectivity and sensitivity for rapid detection of CN⁻.

• The low detection limit of sensor 3TT for CN⁻ is 39.9 nM.

• 3TT-based test strips were used to efficiently detect CN⁻ in water by naked eyes.

• 3TT was successfully applied to detect trace CN⁻ in water and food samples.

Abstract

A novel oligothiophene-based naked-eye colorimetric and fluorescent turn-on sensor 3TT with a typical donor-π-acceptor structure has been successfully developed, which displayed excellent selectivity and sensitivity for the dual-channel detection of toxic cyanide (CN⁻) in 70% aqueous THF media due to the nucleophilic addition of CN⁻ to the β-conjugated position of the barbituric acid moiety of the sensor 3TT by hampering intramolecular charge transfer (ICT) process. The cyanide sensing mechanism was confirmed by optical spectral studies, FT-IR spectra, ¹H NMR titration, HRMS spectra and DFT calculations. Sensor 3TT for the naked-eye detection of CN⁻ in aqueous media exhibited high anti-interference ability, immediate response (20 s), extremely low detection limit (39.9 nM) as well as wide pH response range. Moreover, sensor 3TT displays excellent sensing performance in the solid state, and the 3TT-based test filter papers strips were used to conveniently and efficiently detect CN⁻ in water by naked eyes. Furthermore, the sensor 3TT was successfully applied to the rapid detection of cyanide in real water samples, bitter seeds and food samples, demonstrating its great potential for practical applications in our daily life and environment.

Introduction

Among various anions, cyanide is well known as one of the most highly toxic species, and is extremely harmful to mammals attributed to decrease oxidative metabolism utilization of oxygen and destruct the mitochondrial electron–transport chain and causes serious damages on the central nervous system [1], [2]. Nevertheless, cyanide anions as versatile reagents play a vital role in many industrial processes such as chemical synthesis, gold and silver extraction, tanning, metallurgy, electroplating and plastic production, and thus inevitably causing accidental release of CN⁻ into the environment [3], [4], [5], [6], [7]. Once human intake of cyanide above 0.05 mg/kg body weight can lead to eventual death. By taking into account the hazards of cyanide, the World Health Organization (WHO) has recommended the permissible maximum safety concentration level CN⁻ in drinking water to be 1.9 μM [8], [9]. Focus on the detecting of hazardous cyanide in food and water sources have increased dramatically in the past few years. Therefore, developing simple and efficient methods for cyanide detection in vivo and environment is of great importance to protect human health.

Compared with various traditional detection methods for cyanide, colorimetric and fluorescent sensors for cyanide are regarded as the most preferable approach owing to their convenient use, inexpensive instrumentation, fast detection time, high sensitivity and tenability, and possibility for naked eye detection. Various cyanide sensing methods including nucleophilic attacks on carbonyl carbon, hydrogen bonding mechanism, and cyanide addition on alkenes are the few approaches reported [10], [11], [12], [13], [14], [15]. However, some of them showed some drawbacks with respect to sophisticated structure, low sensitivity, weak anti-interference ability, high temperature, long response time, short wavelength emission and poor solubility in aqueous media, to some extent which limited their practical applications [16], [17], [18], [19], [20], [21], [22], [23]. Hence, the development of simple and efficient naked-eye colorimetric and fluorescent turn-on sensors for the rapid detection of CN⁻ in aqueous media under mild condition is a very meaningful and challenging work.

For the time being, thiophene-based oligomer has been exploited as an effective sensor to detect CN⁻ [24], Fe³⁺ and Hg²⁺ [25], [26], [27], [28] in terms of its well-defined structure, strong electron-donating properties, high photostability and excellent photophysical properties [28], [29], [30], [31], [32]. Barbituric acid derivative with strong electron-accepting properties and hydrophilicity has been utilized as an important structure constructed unit for organic functional materials. Currently, the typical donor-π-acceptor (D-π-A) type sensor with intramolecular charge transfer (ICT) behavior has been developed to detect trace ions, which exhibited high selectivity, excellent sensitivity and remarkable fluorescence enhancement attributed to blocking of the ICT process [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]. This type of sensor molecule is quite interesting and appealing, which caused much concern by many researchers, because it exhibits strong absorption and displays obvious color in the visible region ascribed to the occurrence of an ICT transition from a donor moiety to an acceptor moiety via the π-conjugated reactive subunit [43], [44]. And nucleophilic addition reaction at the subunits like CC and CN efficiently inhibits the ICT process and consequently causes distinct color and spectral changes. Considering the unique nucleophilicity of cyanide ions, the D-π-A-type sensor can act as a colorimetric and fluorimetric sensor for detection of cyanide ions.

Herein, we reported a novel ICT-based D-π-A-type sensor 3TT with oligothiophene as an electron-donating group and barbituric acid as an electron-withdrawing group (Scheme 1), which exhibited highly sensitive and selective detection for CN⁻ based on colorimetric and fluorescent dual-channel in 70% aqueous THF media. The CN⁻ sensing mechanism is based on the nucleophilic addition reaction, which disrupts the π-conjugation and breaks off the ICT process to produce a sensitive colorimetric and fluorescent turn-on response. In addition, sensor 3TT for CN⁻ detection exhibited high specificity, fast response time and low detection limit of 39.9 nM. Moreover, 3TT-based test filter paper strips showed excellent sensitivities for on-site and real-time detection of CN⁻ in water. Furthermore, the sensor 3TT could successfully detect CN⁻ in environmental water, bitter seeds and the sprouting potatoes, indicating the practical applications in our lives and environment.