Solid-phase microextraction based on nickel-foam@polydopamine followed by ion mobility spectrometry for on-site detection of Sudan dyes in tomato sauce and hot-pot sample.

Sudan dyes are one kind of synthetic azo dyes that are forbidden to use in foodstuffs according to the European Union Rule. In this work, considering the structure of Sudan dyes, nickel-foam@polydopamine (NF@PDA) was prepared through the self-polymerization of dopamine on the nickel-foam surface under weak alkaline conditions and used as SPME material to selectively extract the Sudan dyes. With the feature of three dimension (3D) cross-linked grid structure, large porosity and specific surface area, Ni-foam was selected as carrier material which can increase mass adsorption capacity. The primary parameters affecting extraction efficiency such as extraction time, extraction temperature, stirring speed, pH value, ionic strength and desorption time were investigated and optimized. After the Sudan dyes were eluted from the NF@PDA, the eluate was detected by IMS with a corona discharge (CD) ionization source. Thus, an innovative and rapid screening method coupling the NF@PDA to ion mobility spectrometry (IMS) was developed. Under the optimal extraction conditions, the detection limits for Sudan dyes were in the range from 0.005 μg g to 0.25 μg g. The spiked recoveries were in the range from 81% to 91.3% by testing tomato sauce and hot-pot sample and validated by the UPLC-MS. Based on the results, the PDA coating has great ability for the extraction of Sudan dyes, mainly due to the catechol structure, electron-deficient quinones and amine groups on its surface. Thus, the developed method has great potential for on-site rapid screening of the Sudan dyes in food quarantine field.