# A Molecularly Imprinted Polymer for Selective Extraction of Phenolic Acids from Human Urine
## 1 Introduction  
Diets rich in fruits and vegetables are related to positive effects on health and a lower risk of diseases (cardiovascular disorders, cancer, or neurodegeneration) [1]. Flavan-3-ols are the most common group of flavonoids in the human diet and the analysis and quantification of these metabolites could report valuable information to assess their absorption mechanisms and bioavailability. Flavan-3-ols can be found in fruit, vegetables, legumes, beverages (cider, coffee, beer, tea, and wine), or chocolate, and also are added as functional ingredients in processed foods or dietary supplements. In the colon, dietary phenolic compounds can be converted into smaller phenolic acid by microorganisms and may contribute to health benefits. These benefit effects, previously related to the simple direct antioxidant hypothesis of an understanding of the complex effects on molecular targets and enzymes in specific cell types, now have undergone and the importance of the microbiome is inherently integrated with flavonoid metabolism and bioactivity [1]. In the literature, several metabolites are proposed as biomarkers for monitoring the bioavailability and human dietaryintake of polyphenol compounds after consumption, and low-molecular-weight phenolic acids, secondary metabolites present in colonic catabolism and urinary excretion, have been mostly suggested for this purpose in urine and feces [2; 3; 4; 5]. Consequently, the grand interest in knowing the bioavailability of dietary flavonoids and phenolic compounds has stimulated analytical methods development. In the last years, many methods have been published for determination of polyphenol and its hydroxylated metabolites in plants, animal, and human samples [6], but currently chromatographic techniques coupled to mass spectrometry are being widely used [2; 7]. Furthermore, other new strategies based on chemometric tools have been developed [8]. Nevertheless, the most important challenge in the analysis of polyphenols and its metabolites is the extraction and isolation of compounds from the complex matrices. Recent trends in sample preparation and purification include the development of advanced adsorbent materials and accordingly, several analytical methodologies have been proposed to improve the sensitivity and specificity in the polyphenol compound determination [9; 10; 11; 12; 13]. Molecular imprinted polymers (MIP) have attracted wide attention and attained significant applications in selective extraction of polyphenols compounds and hydroxylated metabolites in plants, natural products, foodstuffs, or biological and environmental samples. However, MIP applications in the analytical determination of polyphenols metabolites have been limited and few of us used phenolic acid as a template [10; 13; 14; 15; 16; 17; 18; 19; 20]. PCA has been used as template for the synthesis of an MIP for the extraction analogues compounds from plant in an Molecularly Imprinted Solid Phase Extraction (MISPE) procedure [13; 14] or used as High-Performance Liquid Chromatography (HPLC)stationary phase to analyze synthetic samples of red wine [15; 16]. 4-HBA-MIP has also been used in the stationary phase [16; 19] and imprinting studies were carried out [17]. The 4-HBA MISPE procedure has also been proposed for the selective extraction of polyphenols from olive mill waste waters [18]. In our knowledge, only one work has reported a MIP synthesis using 4-HPA as a template and HPLC columns application and mechanism of recognition was discussed theoretically [19; 21]. On the other hand, only one study has applied MIP for the analysis in urine. 4-methylhippuric acid has been used as a template for the synthesis of an MIP for the extraction of hippuric and 4-methylhippuric acids for the assessment exposure of toluene and xylenes [22]. Main functional monomers used in these molecular imprinting procedures were 4-Vpy, acrylamide, and methacrylic acid [10; 13; 14; 15; 16; 17; 18; 19; 20].