## 2 Materials and Methods
### Design of the Sensing System Prototype
#### 2.4.1 Optical Subsystem  
In a portable instrument, the focus is on light sources based on semiconductor devices, such as LEDs, and lasers. Currently, advances in nanotechnology have enabled the manufacture of LEDs emitting in the far UV, which was unthinkable until a few years ago. The optical features of the LED must fit the spectral characteristics of the sensing phase shown in Figure 3. A high-intensity blue emission LED (UVTOP275-FW-TO39, Roithner LaserTechnik GmbH, Vienna, Austria) was chosen. It has an emission peak centered at 275 nm, which perfectly matches the band of excitation of the Mn:ZnS QDs. However, due to the novelty of this type of LED, it presents some problems that consist mainly in the form of parasite emissions [19; 20]. For this reason, it is necessary to use an optical filter since these parasite emissions match the emission spectrum of the sensing material. Hence, to reduce these interferences, a bandpass filter centered around 550 nm was used. This filter was set on the photomultiplier tube's (PMT) window. The light detection can be made using a PMT, avalanche photodiodes (APDs), or a Si PIN photodiode. In applications where there is a weak signal, a PMT is the best option [21]. APDs also have the advantage of having a high gain, but being semiconductors, their signal-to-noise ratio is low and highly temperature-dependent. In fact, in previous studies, both options had been evaluated, but it was concluded that to be able to use APD detectors, it must be cooled with a Peltier cell, resulting in a more complex and expensive device [22; 23].  
Because the QDs have an emission peak centered at 590 nm, the spectral response of the PMT must include this wavelength within the bandwidth. PMTs are very sensitive in that range of the spectrum and can provide a great amplification that retains an excellent signal-to-noise ratio. They provide an output signal in the form of a current that is proportional to the intensity of the incident light. In this work, a H7827-001 photosensor module (Hamamatsu Photonics, Hamamatsu, Japan) was selected. This module incorporates a 19 mm (3/4\") diameter head-on PMT tube, a high-voltage power supply circuit, and a low-noise current-to-voltage amplifier. This module has a higher resistance to vibration and shock, making it suitable for portable measurement equipment. This high level of integration simplifies the design and minimizes the electric noise, though it increases the price of the instrument. The spectral response of this photosensor module ranges from 300 nm to 650 nm, its gain can vary between \(10^{3}\) and \(10^{7}\), and uses a digital potentiometer (10 k\(\Omega\)) that is adjusted via the touch  
Figure 2: Block diagram of the proposed instrument system.  
screen liquid-crystal display (LCD). To achieve good sensitivity, the PMT gain must be chosen to be as large as possible but ensuring the output voltage of the PMT is not saturated.  
The flow cell that houses the sol-gel sensing material was constructed to avoid the influence of ambient light. However, considering the high sensitivity of the system to light, to ensure an accurate quantitative measurement, it is highly recommended that the system is calibrated every time a measurement is made. The use of a microcontroller device will allow this calibration to be performed automatically, as discussed in the next subsection.