The barcode reader is based on an inverted fluorescence setup. The excitation light comes from a single-color LED with a center wavelength of 565 nm (M565L3, Thorlabs), which was driven by an LED controller (LEDD1B, Thorlabs). The LED light was collimated by an aspheric lens with a 20.1 mm focal length (SM1U25-A, Thorlabs). To suppress NIR light of the LED, we placed a 700 nm shortpass filter (FESH0700, Thorlabs) in front of it. The light was reflected at the 805 nm longpass dichroic mirror (DMLP805R, Thorlabs) and focused onto the sample by an aspheric lens (ACL25416U-B, Thorlabs). The emission light passed the dichroic mirror and was filtered by an additional 775 nm longpass filter (#86-061, Edmund Optics). A 15 mm convex lens (LA1540-B, Thorlabs) focused the emission light onto the PIN photodiode (S2386-18K, Hamamatsu Photonics). For further processing of the signal, the photocurrent was converted to a voltage by a transimpedance amplifier (TIA) circuit (VOLTCON, sglux GmbH). We customized the feedback resistor and capacitor to the values 𝑅 ! = 10 𝑀𝛺 and 𝐶 ! = 1 𝑝𝐹 (approx. cutoff frequency of 8 kHz). The amplifier was powered by a 9 V block battery and encased in an aluminum housing for electromagnetic shielding (Figure ). Further analog signal processing was done by a lock-in amplifier (LIA) integrated circuit (AD630 lock-in amplifier, Douk Audio). The LIA was powered by a dual supply consisting of two power supplies with a DC output of 15 V. For the measurements, the LIA and the LED modulation were driven by a sine wave (0 -5 V, 4 kHz) generated with the analog function generator output of an oscilloscope (DHO924S, Rigol Technologies). The same oscilloscope was also used for signal acquisition. For scanning the samples, a motorized stage was used (ASR100B120B-T3A, Zaber Technologies).