Ethanol as an alternative to formaldehyde for the enhancement of manganese(IV) chemiluminescence detection

Highlights

• Formaldehyde is often used to enhance chemiluminescence reactions with manganese(IV).

• Many other potential enhancers have been found to be much less effective.

• Ethanol offers a simple, safe and inexpensive alternative to formaldehyde.

• Ethanol is a particularly effective enhancer for the detection of thiols/disulfides.

Abstract

Previous applications of manganese(IV) as a chemiluminescence reagent have required the use of formaldehyde to enhance the emission intensity to analytically useful levels. However, this known human carcinogen (by inhalation) is not ideal for routine application. A wide range of alternative enhancers have been examined but to date none have been found to provide the dramatic increase in chemiluminescence intensities obtained using formaldehyde. Herein, we demonstrate that ethanol offers a simple, safe and inexpensive alternative to the use of formaldehyde for manganese(IV) chemiluminescence detection, without compromising signal intensity or sensitivity. For example, chemiluminescence signals for opiate alkaloids using 50–100% ethanol were 0.8–1.6-fold those using 2 M formaldehyde. This innocuous alternative enhancer is shown to be a particularly effective for the direct detection of thiols and disulfides by manganese(IV) chemiluminescence, which we have applied to a simple HPLC procedure to determine a series of biomarkers of oxidative stress.

Introduction

The ‘soluble’ (colloidal) manganese(IV) reagent prepared by adding freshly precipitated manganese dioxide to 3 M orthophosphoric acid [1], [2] has been employed for chemiluminescence detection of a variety of inorganic and organic compounds [3], [4], [5]. The emitting species in these reactions has been identified as manganese(II) [4], [6]. This emitter is also formed in reactions with acidic potassium permanganate [7], [8], but these two chemiluminescence oxidants exhibit considerably different selectivity [4], [5], [8], where manganese(IV) produces light with a much wider range of analytes. Recent notable applications of this chemiluminescence reagent include a simple HPLC procedure to determine the ratio of glutathione to its disulfide in biological fluids to assess oxidative stress (without the need for pre-column analyte derivatization or disulfide bond reduction) [9], and a rapid flow injection analysis (FIA) approach to establish total phenolic antioxidants in plant derived samples [10], [11].

The analytical application of manganese(IV) chemiluminescence has to date required the use of formaldehyde (between 0.2 M and 3.0 M), which increases the chemiluminescence intensity by orders of magnitude [3], [4]. However, formaldehyde is a carcinogen (by inhalation) and not ideal for routine use. Aside from the obvious concerns for the user׳s safety, such classification also leads to practical issues involving ordering, storage and disposal. Consequently, a variety of other compounds have been examined as potential enhancers (including other aldehydes, formic acid, sodium polyphosphates, surfactants, β-cyclodextrin, sodium thiosulfate, sodium sulfite and quinine [12], [13], [14]), but these species have generally given only poor enhancement of the light-producing reactions with manganese(IV).

Considering that, in aqueous solution at room temperature, formaldehyde is largely hydrated to form methanediol [15], we have explored a series of simple alcohols and related compounds as possible enhancers of manganese(IV) chemiluminescence. This has revealed that the addition of ethanol can provide superior emission intensities to that of the conventional formaldehyde enhancer for manganese(IV) chemiluminescence detection.