Fiber-in-tube solid-phase microextraction of caffeine as a molecular tracer in wastewater by electrochemically deposited layered double hydroxide.

Modified stainless-steel wires with a layer of polyaniline conductive polymer were coated by electrochemical deposition with Zn/Al layered double hydroxide to make solid-phase microextraction fibers. The coating layer was also electrochemically deposited on the inner surface of a stainless-steel tube. Then, ten prepared fibers were put inside the inner coated tube to make a fiber-in-tube solid phase microextraction device. The device was applied for the extraction of caffeine (1,3,7-trimethylxanthine) from domestic wastewater samples. Extraction conditions including extraction and desorption times, pH and ionic strength of the sample solution, and content of the organic desorption solvent were investigated and optimized. Under the optimized conditions, the fiber-in-tube solid phase microextraction exhibited excellent extraction efficiency toward caffeine. The precision of the method was evaluated. Average relative standard deviation of 5.7% (n = 6) for intraday analysis and 8.3% (n = 5) for interday analysis was obtained. The limits of detection and limits of quantification of the method (at signal to noise ratio of 3 and 10) were obtained as 0.14 and 0.37 ng/mL, respectively. The current study can provide new prospective applications of layered double hydroxide conductive polymer fiber coatings.