## 1 Introduction
In general, the optical signal of colorimetric assays can be read out quantitatively well with a benchtop spectrometer, but the necessity for on-site analysis of biological contamination is increasing. Over the last decade, a number of spectroscopic detection devices have been developed that allow optical on-site detection for colorimetric assays. These on-site spectrometers usually include a single or a few light emitting diodes (LEDs) as a light source and a phototransistor or light-dependent resistor (LDR) as a spectral detector. An early low-cost transportable spectrometer was realized with four different color LEDs [26]. This photometer is a pseudo-dual-beam instrument and is similar to benchtop spectrometers, but with a much lower spectral resolution. The application of an LED array, composed of more LEDs, for spectrometer construction is also widespread [27]. Within an array of eight LEDs, a simple method for discrimination of mineral waters in a flow-cell is possible. Another portable diode array spectrophotometer, requiring only two AAA-batteries for 200 h measurement by their low-power consumption, was presented [28]. However, this system lacks a reference channel. Other approaches use moveable optics to scan through the spectral range [29]. However, the use of mechanical parts increases the complexity of the system. This leads to low light throughput and slow scanning compared with benchtop devices. An interesting development represents the open source movement for free access to soft- and hardware technology, as demonstrated with an open source photometric system for enzymatic nitrate quantification with color generation over enzymatic reduction [30]. This simple detector was developed for low cost application and yields the potential to create a large network of users that will eventually improve future system optimization to reach the largest possible audience. Especially for colorimetric assays based on plasmonic nanoparticles, transportable spectrometers have been proposed. One example represents the detection of lead and aluminum ions in water by means of a narrow-band LED array with an custom-made microwell plate [31]. Another example represents a devices that monitors the wavelength-radiometric resonance Rayleigh scattering signal of the chemical sensor upon white-LED illumination [32]. Other groups developed modules that are based on digital image colorimetry (via smartphone) [33] or make use of a diffraction grating and image processing to measure the particle size and concentration [34], or use a tungsten-filament lamp with optical fibers and a commercial portable spectrometer [35]. Multichannel spectrophotometers, particularly those based on diode arrays and custom-made detection cells [36], present many advantages, such as spectrum acquisition in a very short interval of time, good resolution and accuracy in wavelength measurements, and adequate sensitivity and dynamic range for many applications. In spite of these advantages, the measurements of absorbance intensity continuously through all wavelengths in a given range may be not necessary. A variable selection is useful to reduce noise and to minimize redundancies. Moreover, an LED-based photometer can be realized at minimal costs, and has low power consumption, permitting the construction of miniaturized instruments for field measurements. In this paper, the development of a simple, hand-held, and low-cost device for carrying out colorimetric detection based on plasmonic nanoparticles and a DNA-amplification (LAMP) assay is presented, utilizing the high amplification efficiency of the LAMP method (quantity of synthesizes DNA: \(>\)10 \(\mu\)g 25 \(\mu\)L\({}^{-1}\)) [37; 38]. The device with a 3D-printed central part and dual channel (sample and reference) includes two spectrally different LEDs for the detection of the plasmon shift (color change). The LEDs are easily adaptable according to the intended colorimetric assay.