## 1 Introduction  
In the field of pathogen diagnostics, innovative point-of-care compatible methods such as colorimetry have become increasingly important in recent years. Precious noble-metal nanoparticles with distinct optical properties, induced by the localized surface plasmon resonance (LSPR), are of growing interest for diagnostics. This optical effect is based on the oscillation of the conduction electrons in the nanoscale noble-metal particles. The intensity and position of their LSPR band are strongly dependent on the shape, size, material, dielectric environment, and the distance between the colloidal nanoparticles [1]. In case of plasmonic nanoparticles, the extinction coefficient and the absorption cross section are several orders of magnitude higher compared with common dyes [2]. Therefore, colorimetric assays using plasmonic particles have an outstanding detection capability [3]. The absorption spectrum of plasmonic nanoparticles is usually in the visible range [4], which enables simple optical detection for the detection of various analytes. Colorimetric assays [5, 6] are based on an effect caused by a changing distance between the particles; the resonance coupling on neighboring nanoparticles results in their spectral shift. The nanoparticle solution color can change owing to salt-induced coalescence and the resulting coupling of the resonances. Colorimetric assays can be realized in different ways [7, 8]. On the one hand, particle could be linked by specific binding of attached (bio-) molecules to bridging target molecules. This so-called interparticle cross-linking is usually reversible, both for antigen-antibody and DNA-DNA interactions [9]. On the other hand, non-crosslinking aggregation describes a controlled gold nanoparticles (AuNPs) interaction using the ability of noble-metalnanoparticles to interact without actual bond formation [10]. Other studies investigate anti-aggregation or de-aggregation assays [11]. To perform colorimetric assays, the colloidal noble-metal nanoparticles have to exhibit a specific binding capability [12]. For the detection of proteins, for example, in immunoassayassays, antibodies are usually used as receptors [13]. Enzyme activity can be detected using peptides [14]. Further, the in situ formation of protein corona can be detected using nanoparticles optical signal [15]. Carbohydrate functionalized nanoparticles allow the detection of toxins such as cholera toxin (CTB), which binds to the lactose derivative on particles and induces their aggregation [16]. Gold nanoparticles-based colorimetric aptasensors are currently used for a wide variety of analytes [17; 18]. Further studies focus on the nanoparticle-based detection of metal ions [19; 20]. DNA-oligonucleotides can be used as recognition elements for the specific detection of pathogen DNA. The DNA to be examined hybridizes with single-stranded oligonucleotides on the particles. Several rapid assays based on the amplification of nucleic acids have previously been presented to detect both environmental and clinical samples, but these assays are expensive, more complex, or of lower sensitivity, and often require sophisticated instrumentation [21; 22]. Different approaches with DNA-functionalized gold nanoparticles have been published, such as the detection of white spot syndrome virus in combination with loop-mediated isothermal amplification (LAMP) [23], the detection of viral DNA with mono- and bifunctional gold nanoparticles conjugated oligonucleotide probes [24], the use of a single Au-nanoprobe for detection of two distinct targets (pathogens), and one-step detection [25]. In principle, the sensor response can be qualitatively read out with the naked eye. However, for quantification or higher sensitivity, a quantification instrument is needed. UV/vis photometry is widely used for measuring a variety of sample properties, typically the concentration of an analyte in a liquid sample.