(A) Schematic of an optical cube. The optical excitation module
has an LED, a linear polarizer, and a focusing lens. The emission light is
measured by a pair of detector sets, each consisting of a lens, a polarization
filter, and a photodiode (PD). (B) Circuit diagram. The on-board
computer controls the entire system and communicates with a smartphone. To
enhance the signal-to-noise ratio (SNR), the system uses the optical lock-in
detection scheme. The intensity of the excitation light is amplitude-modulated,
and the resulting emission intensities are mixed with the carrier frequency.
The dotted box indicates an optical cube. BPF, band-pass filter; LPF, low-pass
filter; AMP, amplifier. (C) The lock-in method significantly
improved the signal-to-noise ratio (630 times, 28 dB). (D) The
accuracy of the PAD was benchmarked against a commercial plate reader. The
measured values show excellent agreements (R2 =
99%). Experiments were performed in triplicate, and the data were displayed as
means ± SD. Horizontal and vertical error bars were from the plate
reader and the PAD measurements, respectively.