Open Meter Duo: Low-Cost Instrument for Fluorimetric Determination of Cholinesterase Activity

Abstract

Environmental screening is essential due to the increased occurrence of harmful substances in the environment. Open Meter Duo (OMD) is an open-source field photo/fluorimeter that uses an RGB diode that imitates a color according to the selected wavelength and uses a UV LED from the security kit diode as an excitation light source. The prepared PCB shield with a 3D-printed aperture was connected to Arduino UNO R4 WiFi. This system was used for the fluorescent detection of cholinesterase activity with the indoxyl acetate method. Carbofuran-a toxic pesticide-and donepezil-a drug used to treat Alzheimer's disease-were tested as model inhibitors of cholinesterase activity. The limit of detection of indoxyl acetate was 11.6 μmol/L, and the IC50 values of the inhibitors were evaluated. This system is optimized for wireless use in field analysis with added cloud support and power source. The time of analysis was 5 min for the fluorimetric assay and 20 min for the optional photometric assay. The time of field operation was approximately 4 h of continuous measurement. This system is ready to be used as a cheap and easy control platform for portable use in drug control and point-of-care testing.

Keywords: 3D printing; Arduino; cholinesterase; fluorimetry; low cost; portable; remote sensing.

Figure 1

Overview of projections of OMD box.

Figure 2

Schematic of the remote version of the Open Meter Duo system.

Figure 3

Wiring scheme of Open Meter Duo. Pin 1 on TSL230R sensor is connected to the brown wire.

Figure 4

List of attributes needed for Open Meter Duo programming.

Figure 5

Overview of hardware (left) and system ready to use after installation (right).

Figure 6

Emission spectra of OMD light source.

Figure 7

Comparison of TSL230R sensor’s response in different light conditions. Values on Y-axes indicates the response of the sensor when RGB LED imitating color of the chosen wavelength, the excitation light source was turned ON, or both light sources were turned OFF.

Figure 8

Overview of the example dashboard made for remote version of Open Meter Duo.

Figure 9

(A) Emission spectra of various concentrations of indoxyl acetate. (B) Comparison of fluorescence stability during real-time measurement.

Figure 10

Calibration curve of indoxyl acetate. (A) Detail of end-point analysis; (B) evaluation of slope from isolated linear trends. The error bars indicate the standard error for n = 3.

Figure 11

Stability test of the “white” fluorescent by-product of hydrolysis. Two concentrations of indoxyl acetate were used. Color intensity in the green and blue RGB channel was evaluated by photogrammetry (A) using a 3D-printed cuvette holder (B) placed in a dark chamber with a UV lamp. The error bars indicate the standard error for n = 3.

Figure 12

Calibration curves of cholinesterase inhibitors. (A) Carbofuran. (B) Donepezil. The error bars indicate the standard error for n = 3.

Figure 13

Dual-signal overview of indoxyl acetate assay. (A) Photometry—6 min assay after switching from fluorimetry. A non-linear change in the signal is evident. (B) Photometry—from 6 to 20 min of analysis, the signal has a linear trend of increase, which is the desired response of the experiment.