A Highly Integrated and Diminutive Fluorescence Detector for Point-of-Care Testing: Dual Negative Feedback Light-Emitting Diode (LED) Drive and Photoelectric Processing Circuits Design and Implementation

Abstract

As an important detection tool in biochemistry, fluorescence detection has wide applications. Quantitative detection can be achieved by detecting fluorescence signals excited by excitation light at a specific wavelength range. Therefore, the key to fluorescence detection is the stable control of the excitation light and the accurate acquisition of weak photoelectric signals. Moreover, to improve portability and instantaneity, devices are developing in miniaturization and integration. As the core of such devices, fluorescence detectors should also have these features. Under this circumstance, we designed a highly integrated and diminutive fluorescence detector and focused on its excitation light driving and photoelectric signal processing. A current-light dual negative feedback light-emitting diode (LED) driving circuit was proposed to obtain constant current and luminance. In addition, a silicon photodiode (PD) was used to receive and convert the fluorescence signal to an electric signal. Then, amplifying, filtering, and analog-to-digital (A/D) converting were applied to make the detection of weak fluorescence signals possible. The test results showed that the designed circuit has wonderful performance, and the detector shows good linearity (R² = 0.9967) and sensitivity (LOD = 0.077 nM) in the detection of fluorescein sodium solution. Finally, a real-time fluorescence polymerase chain reaction (real-time PCR) of Legionella pneumophila was carried out on a homemade platform equipped with this detector, indicating that the detector met the requirements of real-time PCR detection.

Keywords: LED drive circuit; fluorescence detection; photoelectric processing circuit; point-of-care testing; real-time fluorescence PCR.

Figure 1

Overall structure of the detector: (A) a functional block diagram of the fluorescence detector, including optical components and drive circuit; (B) a schematic diagram of the optical path—blue indicates the excitation light, and green indicates the fluorescence emitted by sample; (C) a printed circuit board (PCB) picture of the fluorescence detector.

Figure 2

Structure and assembly of the fluorescence detector: (A) an exploded view of the three-dimensional (3D) structure; (B) a photo of the internal structure of the detector; (C) a photo of the detector with the metal box.

Figure 3

Overall circuit of the fluorescence detector: (A) a principle block diagram of the drive circuit; (B) the current–light dual negative feedback LED driving circuit schematic, including three parts: (1) current feedback, (2) light intensity feedback, and (3) LED selection signal; (C) photoelectric processing circuit schematic, including: transimpedance amplifier (TIA), filter, and analog-to-digital (A/D) conversion.

Figure 4

Photo of the test platform based on the fluorescence detector in this paper.

Figure 5

LED stability test result: (A) Multisim simulation of voltage value across the LED current sampling resistor; (B) oscilloscope waveform of voltage value across the LED current sampling resistor.

Figure 6

Comparison of the luminous power of LEDs driven by two different LED driving circuits: the LED fluctuation in 5 s (A) and 1 h (B) driven by the dual negative feedback circuit built in this paper, and the LED fluctuation in 5 s (C) and 1 h (D) driven by a LED driver (MAX16836).

Figure 7

The fluorescence curves of water and sodium fluorescein solutions with concentrations of 0.05 nM and 0.5 nM obtained by the detector.

Figure 8

Linearity of the fluorescence signal with the concentration of fluorescein sodium solution: (A) the results from the detector in this paper; (B) the results from the spectrofluorophotometer.

Figure 9

Real-time fluorescence PCR curves for detection of Legionella pneumophila and negative control from our detector and StepOnePlus system.