Fibre optic absorbance meter with low limit of detection for waterborne cations

Abstract

We report an evanescent wave based fibre optic absorbance meter that enables the colorimetric detection of waterborne cations with water insoluble chromoionophoric sensitisers. This establishes an alternative to the PVC membrane based transducers that are conventionally used for this purpose. Here, a water insoluble sensitiser is coated as a thin film on an unclad section of a multimode optical fibre to overlap with the evanescent field of a light beam propagating along the fibre core. The colorimetric response of the sensitiser when in contact with waterborne cation leads to increased absorption of virtual photons associated with the evanescent field. The resulting intensity loss of the propagating beam is detected by a bespoke newly designed self-referenced evanescent wave absorbance meter with beam intensity modulation and Lock-in amplification. We validate our transducer with the well characterised water insoluble sensitizer, 1-(2-pyridylazo)-2-naphthol (PAN), for the detection of aqueous Zn 2+ cations. We find a limit of detection (LoD) of 54 nM Zn 2+ , 28 times lower compared to a PVC membrane based sensor using same sensitiser for same cation (Albero et al., Journal of Pharmaceutical and Biomedical Analysis 29 (2002), 779). Our evanescent wave absorbance meter can easily be adapted to other colorimetric sensitisers, including chromoionophoric complex forming macrocycles.

Adaptive and sensitive fibre-optic fluorimetric transducer for air- and water-borne analytes

“…The fibre was cleaved, polished, and a 1 cm long section of the silica core was exposed by stripping off the polymer cladding. The exposed core was cleaned and dried, as described previously in [6,17] but in this case with the exposed section near the end of the fibre instead of the middle to adapt to fluorescence-rather than absorbance-based sensing. A 1 mg/mL solution of MDMO-PPV (Sigma Aldrich) in dichloromethane was prepared by stirring for an hour at 50 o C. To sensitise fibres, the MDMO-PPV solution was then sprayed onto the stripped section of the fibre using a WilTec airbrush (WilTec, Königsbenden 28, DE-52249 Eschweiler, sourced via ManoMano; nozzle diameter 0.3 mm, air pressure 1.5 bar) and left to dry under ambient air before using it.…”

“…Vout was recorded vs. time on a PC running a bespoke LabVIEW routine. The setup is adapted from the fibre optic absorbance meter described in more detail in [6,17], omitting the beam splitter and reference signal path required for absorbance measurement.…”

“…Such more direct use of fibre optics is favourable because of easy handling and fabrication, low cost and attenuation, flexibility, and can lead to high sensitivity [4,5]. Fibres may detect the absorption of propagating fibre modes via their evanescent waves in the sensitised section [6,7], or collect fluorescence of the sensitiser that has been excited by absorption of evanescent waves [1,8,9,10,11]. However, despite efforts such as fibre tapering and longer sensitised sections [4,9,10,12], evanescent wave excited fluorometric sensors often suffer from low intensity.…”

“…In this study, we adapt a lock-in based light detector that we have developed for an evanescent wave absorption sensor [6] as the transducer and systematically compare different fluorophoresensitised fibre optic sensor operation modes. As an example analyte, we used the nitroaromatic explosive dinitrotoluene (DNT) both in vapour-and aqueous phase.…”

‘Rough guide’ evanescent wave optrode for colorimetric metalloporphyrine sensors

“…This may be because dedicated equipment to detect the intensity loss of the propagating wave is not commercially available, and researchers used conventional spectrometers or simple photodiode circuits. Some of us have therefore designed a bespoke LED-driven self-referenced 'light balance' fibre optic absorbance meter with Lock-in detection specifically for evanescent wave absorbance measurement [11]. Assembled only from generic and affordable electronic and fibre-optic components, this instrument enables highly sensitive evanescent wave absorption measurement, which we demonstrated on a generic chromoionophore [12] to detect waterborne cations.…”