## 1 Introduction  
Body fluids are complex biological fluids that contain proteins and lipids, which are highly susceptible to oxidation processes. When body fluids age, they can undergo a variety of oxidation reactions which can lead to changes in their chemical structure and physical properties. Oxidation reactions are known to occur during ageing, food processing, and storage and are associated with a number of diseases [1, 2, 3]. In the forensic field, several studies have been conducted to investigate the oxidative degradation of fingermark residues [4, 5, 6, 7, 8, 9] and to develop methods that rely on oxidation reactions to determine the time of deposition of biological evidence. [PERSON] et al. developed an age estimation method for blood stains, based on the quantification of oxidation products of hemoglobin using reflection spectroscopy [10]. [PERSON] et al. monitored protein-lipid reactions over time and used protein fluorescence and fluorescent oxidation products(FOX) signatures to determine the age of fingermarks [11]. [PERSON] et al. showed that this fluorescence-based method is also applicable to other protein/lipid-containing traces, such as semen stains [12]. In foods, the oxidative reaction of proteins and lipids have been widely studied. The production of fluorescence due to protein-lipid interactions can occur through different pathways. Well-known oxidation reactions that induce fluorescent changes include the formation of protein carbonyls, dityrosine, and advanced glycation end-products (AGEs) [13; 14; 15]. Carbonyls can be formed due to protein-lipid reactions, whereby the lipid oxidation degradation products react directly with amino groups of amino acids leading to fluorescent compounds [16]. Dityrosine, the oxidized form of tyrosine, is a highly fluorescent molecule which is formed when a cross-link between two tyrosine residues is produced by a radical reaction. Fluorescence can also be produced in a non-radical reaction in which aldehydes react directly with amino groups of amino acids. Aldehydes, such as malondialdehydedehyde (MDA) and 4-hydroxynonenal (HNE), play an important role in the degradation of amino acids and are considered important markers for lipid peroxidation (LP) [17]. Furthermore, MDA and HNE can lead to the formation of AGEs by a Maillard reaction. The Maillard reaction occurs when amino acids react with the carbonyl group of reducing sugars, such as glucose or aldehydes, and form Schiff bases. The base can rearrange itself to a stable Amadori product, which can undergo further rearrangement, cleavage, and covalent binding reactions forming AGEs [18]. It is thought that, as body fluids age, the presence of oxygen causes the unsaturated lipids to oxidize and form reactive oxidation products, which react with proteins to form FOX [11; 12]. This ageing process shows similarities to the oxidation reactions that have been widely studied as they play a role in food quality assessment. We expect similar reactions to be involved in the formation of fluorescent protein-lipid complexes in ageing body fluids. Knowledge about the chemical reaction can improve the current protein-lipid model system upon which fluorescence-based age estimation methods rely. Therefore, the aim of the present study is to investigate if these lipid and protein oxidation markers can be measured in semen, saliva, and urine, over the course of time. The biomarkers will be measured in body fluid samples aged for 0, 2, 4, 7, 14, and 21 days, with a focus on the detection of protein carbonyls, dityrosine, AGEs, MDA, and HNE as oxidation biomarkers using surface plasmon resonance imaging (SPRi), enzyme-linked immunosorbent assay (ELISA), and fluorescence spectroscopy. SPRi is a detection technique used to monitor biomolecular interactions in real time without the need of labels.