Fluorescence-enhancement sensing of ammonia and hydrazines via disruption of the internal hydrogen bond in a carbazolopyridinophane

Abstract

A carbazolopyridinophane has been designed and prepared, with an internal NH⋯N hydrogen bond between the carbazole and pyridine rings. This hydrogen bond causes the fluorescence of the carbazole unit to be substantially quenched. In heptane solution, addition of ammonia or simple hydrazines disrupts the internal hydrogen bond and restores fluorescence, within 1–5 min. Signal intensity varies linearly with guest concentration; concentrations of ammonia or hydrazines down to 100 ppb have been measured.

Introduction

Hydrazine, monomethylhydrazine (MMH), and 1,1-dimethylhydrazine (UDMH) are widely useful, inter alia as high-energy rocket propellants [1, pp. 820–1632]. However, not only are their vapors flammable and the neat materials detonable, but toxic effects include liver damage caused by concentrations in air far below the olfactory thresholds [1, pp. 820–1632]. Threshold limit values for these agents have been set at 10 ppb (v/v) [2].

Specifications for a useful hydrazine sensor vary with the application. To continuously monitor the safety of work areas or launch sites, one desires sensors that are ppb-sensitive, are re-usable, and respond in real time. Previously reported sensors meet only one or two of these criteria [1], [3]. Colorimetric dosimeter badges, based on vanillin and either p-dimethylaminobenzaldehyde [4] or 2,4-dinitrobenzaldehyde [5], are sensitive and react quickly, but the chemistry involves formation of hydrazones or azines, and is effectively irreversible; thus, the badges can be used only once. Cumulative response of this type is desirable for personnel dosimetry, but undesirable for continuous monitoring of workspaces. Electrochemical sensors are well-suited for analysis of aqueous hydrazines (e.g. in boiler feed waters), but electrochemical analysis of hydrazine vapors starts with dissolving them, which limits the speed of the method. (For recent work, see [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16].)

For monitoring of multiple locations remote from a central analysis station, sample handling is minimized by an array of sensors attached to the central point by a fiber-optic network. Such networks have indeed been developed; the sensors most commonly attached involve colorimetry of phosphomolybdic acid, and their chemistry is irreversible [17], [18], [19], [20]. Fiber-optic-attached sensors based on acid–base reactions of dyes offer good reversibility, but their detection limits are tens of ppm [21], [22]. There is a recent report of a fiber-optic analysis using a modified conducting polymer as cladding for the fiber, but no detection limit was stated [23]. There remains a need for improved sensor chemistry.

Fluorescent sensing would be attractive because of its exceptional sensitivity [24], but few fluorometric analyses for hydrazines have been reported. Reaction with arenedicarbaldehydes in acidic aqueous solution gives derivatives which fluoresce in the visible region, but their formation again is irreversible [25], [26], [27]. Fluorimetry of thorium ions has been used for detection in a flow-injection method [28]. Very recently, it was reported that hydrazinolysis of fluorescein ketoesters can be used to detect ppm levels of hydrazine, via fluorimetry of the fluorescein product; this reaction too is irreversible [29].

It has long been known that the fluorescence of carbazole is quenched by pyridine [30]. Since the fluorescence of N-ethylcarbazole is not quenched by pyridine [31], quenching requires a hydrogen bond between emitter and quencher. If an analyte disrupts the hydrogen bond, fluorescence might be restored, creating a basis for sensing. Herein we describe a carbazolopyridinophane (1) whose fluorescence is greatly enhanced by added ammonia or hydrazines, via disruption of the internal NH⋯N hydrogen bond [32]. Due to the reversibility of intermolecular non-covalent complexation [33], 1 may be a prototype for highly sensitive, real-time detection methods.