Leveraging the excellent performance of the low-cost Si PIN photodiode, we then integrated it into a portable fluorescence reading setup. The optical path is similar to an inverted fluorescence microscope setup (Figure ). A single-color LED (565 nm) excites the SWCNTs, and the fluorescence passes through a filter cube before reaching the photodiode (Figure ). To amplify the nA signal, a transimpedance amplifier (TIA) with a gain of 10 7 V/A and a cut-off frequency of 8 kHz (Figure ) was used. For further amplification and to suppress noise, we used a lock-in amplifier (LIA), which consisted of a small integrated circuit (Figure ). We modulated the LED power using a sine wave with a frequency of 4 kHz and an oscillating amplitude between 0 V and 5 V (Figure ). The output of the LIA produced a clean DC signal that is directly proportional to the amplitude of the oscillating photodiode signal. For the biosensor barcode, we used a well plate with wells aligned in one direction, serving as spots for the SWCNTs. Each well was filled with 150 µL of 1 nM SWCNTs with different surface chemistry ((GT)10, (CT)15, (GA)15, Aptamer, C30, PEG-PL, (GT)15). For background subtraction, an additional well was filled with pure phosphate-buffered saline (PBS). The stage moved across these wells with a constant velocity of 2 mm/s, while the output signal of the LIA was continuously measured using an oscilloscope. We observed voltage peaks in the range of 1 V to 2 V, depending on the SWCNT surface chemistry (Figure ). The background noise signal was recorded at approximately 180 ± 1 mV, resulting in an SNR between 6.4 dB ± 0.6 dB (PEG-PL-SWCNTs) and 9.4 dB ± 0.4 dB ((GT)15-SWCNTs), which is comparable to the SNR (6.4 dB ± 0.4 dB to 7.0 dB ± 0.1 dB) measured at the microscope. After the addition of 100 µM levodopa, the intensity change of the SWCNTs shifts the voltage. We repeated this measurement with levodopa concentrations ranging from 1 nM to 100 µM and subsequently analyzed the peaks of the barcode. The mean values of the voltage changed across the entire barcode are illustrated in a heatmap (Figure ). The aptamer-and C30-SWCNTs showed the greatest response, achieving a 15 % change at 100 µM. In contrast, PEG-PL-SWCNTs show a slightly negative change of 6 %. The data was again fitted and showed a dynamic range between 100 nM and 10 µM (Figure and Table ). Concentrations below 100 nM yielded no traceable response, while responses above 10 µM reached saturation. Compared to the Si PIN photodiode directly attached to a microscope, the response of the (GT)10 was approximately two times smaller. This discrepancy can be attributed to the more complex and expensive optical system used in the microscope setup, as well as the different excitation source (white light LED).