# Fluorescent Imprinted Nanoparticles for Sensing of Chlorogenic Acid in Coffee Extracts
## 1 Introduction  
Chlorogenic acids (CGAs) are polyphenols widely present in the plant kingdom, and they are secondary metabolites involved in the defense mechanism against environmental stress of plants. Green coffee beans are particularly rich in these compounds, which are considered as indicators of the quality of coffee, since they affect the flavor and the nutritional value of the final product. Although the highest concentration of CGAs is in coffee, they can also be found in tea, berry fruits, cocoa, citrus fruits, apples, and pears [1]. CGAs possess several biological activities, such as being antioxidants, antibacterials, hepatocytes, cardioprotectives, anti-inflammatory agents, antipyretics, neuroprotective, anti-obesity agents, antivirals, anti-microbials, anti-hypertension agents, free radical scavengers, and central nervous system (CNS) stimulators, providing health benefits upon consumption [2]. Chemically, CGAs are a class of esters of (-)-quinic acid with different cinnamic acids, and due to the different hydroxyl groups present in the quinic acid structure and the different cinnamic acids present in nature (caffe acid, ferulic acid, \(p\)-coumaric acid, and sinapic acid), it is possible to form different monoesters, as well as diesters or triesters. A total number of 76 different CGAs has been recognized in the last years in coffee, but the most abundant is 5-O-caffeoylquinic acid (5-CQA) **1**, which is simply named chlorogenic acid. (Figure 1) The total content of CGAs in coffee depends on the coffee species (_Coffea arnica_4-8% and _Coffea canephora_ 7-14% on the dry matter basis), but also on the degree of roasting, the agricultural practices, as well as the soil composition. The identification and quantification of CGAs in coffee is usually performed by HPLC coupled with mass spectrometry (LC-MS), and the different types of CGAs are recognized by the different fragmentation patterns according to their corresponding base ions [3]. In the literature, the characterization of CGAs was achieved by LC-ion trap-MSn, using fragment-targeted MSn, without the need of isolation of the products. Other analytical methods have been used, such as infrared spectroscopy, nuclear magnetic resonance (NMR), ultraviolet spectroscopy (UV-Vis), or chemiluminescence, but these methods are expensive and time consuming. In recent years, attention has been paid to the determination of CGAs by molecularly imprinted polymers (MIP), which offer the possibility of specific molecular recognition. MIPs are synthetic materials able to specifically and selectively recognize a target compound, and their preparation starts from the formation of a complex between a target molecule, that acts as a template, and one or more functional monomers, in which are present acryloyl- or vinyl-groups that allow polymerization and which are chosen based on their ability of interacting with the functional groups of the template (analyte). The synthesis is carried out in the presence of a cross-linking agent and a radical initiator in a suitable porogenic solvent, leading to the formation of a rigid three-dimensional network around the template. After polymerization, the template is removed from the matrix, creating specific recognition sites that are complementary in shape, size, and chemical functionality to the target, conferring to the polymer a kind of permanent memory that will allow it to selectively recognize the analyte from mixtures of similar compounds. They are usually robust and stable in relation to drastic changes of pH, temperature, and solvents, and they are easy to prepare. MIPs can be used as sorbents for solid phase extraction or can constitute the recognition element in the development of a sensor, which needs a transducer, such as electrochemical, calorimetric, piezoelectric, or optical, to transform the binding process into a measurable output signal. Biosensors are chemical sensors where the recognition is based on biochemical or biological elements.