A Smartphone-based Diffusometric Immunoassay for Detecting C-Reactive Protein

Abstract

In this study, we developed a portable smartphone-based diffusometry for analyzing the C-reactive protein (CRP) concentration. An optimized fluorescence microscopic add-on system for a smartphone was used to image the 300 nm fluorescent beads. Sequential nanobead images were recorded for a period and the image data were used for fluorescence correlation spectrometric (FCS) analysis. Through the analysis, the nanobeads' diffusion coefficient was obtained. Further, the diffusion coefficients of the anti-CRP-coated nanobeads, which were suspended in the samples with various CRP concentrations, were estimated using smartphone-based diffusometry. After 10 min of reaction, the anti-CRP-coated nanobeads in a higher CRP concentration solution led to a lower diffusion coefficient. Based on the experiments, a linear sensing range of 1~8 µg/mL was found.

Figure 1

(a) Smartphone add-on system for analyzing diffusion coefficient of nanobeads. (b) The anti-CRP modified nanobeads interacted with CRP in 10 min in a micro-chamber, and the nanobeads’ diffusion coefficient can be carried out in real-time.

Figure 2

(a) Schematic optical system of the high-resolution fluorescent microscope. (b) Setup of the optical system. (c) The ray tracing results compared to the achromatic lens set with simple dual convex lens. Schematics of the sensing mechanism.

Figure 3

Preparation procedures for nanobeads with anti-CRPs.

Figure 4

(a) Smartphone-based microscope for imaging 300 nm fluorescent beads (b) A video of the Brownian motion of the nanobeads. The interrogation window is shown as a dashed-line circle. (c) The intensity variation, I(t), in the interrogation window. (d) The autocorrelation function, G(τ), in the interrogation window.

Figure 5

(a) The anti-CRP-coated nanobeads’ diffusion coefficient measured by smartphone-based diffusometry in different CRP solutions. (b) After repeating the analysis three times, the values of the nanobeads’ diffusion coefficients in an equilibrium state showed a linear decrease in the concentration range of 1~8 µg/ml (n = 3).