Near-infrared photothermal lens as microfluidic analyzer for determination of phosphate in food and biological samples

Abstract

In this work, photothermal lens spectrophotometer (PTLS) with crossed-beam configuration pumped by a laser diode (850 nm) was used for determination of soluble phosphorus exists mainly in the form of orthophosphate (PO43−) in nonalcoholic beverage and human blood serum samples. The determination was carried out after complexation reaction between blue molybdenum and phosphate in aqueous solution. UV–vis spectrophotometry was used by measuring absorption in the near infrared region (815 nm) for optimizing the complex formation time, concentrations of reducing agent and sulfuric acid. The UV–vis absorption measurements showed that the calibration curve was obtained in the concentration ranges of 0.5–80 mg L−1 with a linear correlation coefficient of 0.98 and a detection limit of 0.2 mg L−1. Then, PTLS at a wavelength of 850 nm after optimizing the device conditions (flow rate, excitation laser power and interrupting frequency) was used for phosphate determination. The calibration curve in the concentration range of 11–1000 µg L−1 has a correlation coefficient of 0.98 and a detection limit of 4 µg L−1. The achieved detection limit was about 50 times better than NIR absorption spectrometry using a conventional 1 cm sample cell. In PTLS with the 3 μL microfluidic cell experimental detectability amount of phosphate is 33 picogram, while in UV–vis-NIR method this amount is 1.5 µg.