Magnetic Lateral Flow Strip for the Detection of Cocaine in Urine by Naked Eyes and Smart Phone Camera

Abstract

Magnetic lateral flow strip (MLFS) based on magnetic bead (MB) and smart phone camera has been developed for quantitative detection of cocaine (CC) in urine samples. CC and CC-bovine serum albumin (CC-BSA) could competitively react with MB-antibody (MB-Ab) of CC on the surface of test line of MLFS. The color of MB-Ab conjugate on the test line relates to the concentration of target in the competition immunoassay format, which can be used as a visual signal. Furthermore, the color density of the MB-Ab conjugate can be transferred into digital signal (gray value) by a smart phone, which can be used as a quantitative signal. The linear detection range for CC is 5-500 ng/mL and the relative standard deviations are under 10%. The visual limit of detection was 5 ng/mL and the whole analysis time was within 10 min. The MLFS has been successfully employed for the detection of CC in urine samples without sample pre-treatment and the result is also agreed to that of enzyme-linked immunosorbent assay (ELISA). With the popularization of smart phone cameras, the MLFS has large potential in the detection of drug residues in virtue of its stability, speediness, and low-cost.

Keywords: cocaine; magnetic bead; magnetic lateral flow strip; smart phone camera.

Scheme 1

The principle of MLFS for detection of CC based on smart phone camera. Sample of CC and MB-Ab are applied to the sample pad and migrates along the strip, CC and CC-BSA competitively react with MB-Ab on the T line. The complex of CC with MB-Ab and the excess MB-Ab migrate along the membrane and are captured by the coated goat anti-mouse IgG on the C line. The color densities of the MB-Ab conjugate on T and C lines are measured by a smart phone (color scanner, weixun lin, China).

Figure 1

Optimization of the size of MB for detection of CC sample. Different sizes of MB-Ab (20, 30, and 150 nm) are prepared to detect the blank sample. The concentrations of the MB-Ab with different sizes are all 10 mg/mL.

Figure 2

The sensitivity of MLFS for detection of CC samples in PBS. (a) Different concentrations of CC (1–1000 ng/mL) are detected by MLFS with the competitive immunoassay format. (b) The relationship of △T/C gray value with the concentration of CC is constructed. (c) The quantitative curve for detection of CC ranges from 5 to 500 ng/mL. The error bars represent the standard deviation from the three repeats (n = 3).

Figure 3

The specificity of MLFS for detection of CC samples. (a) 50 ng/mL of CC and the analogs such as Mop, Amp, Cod are detected using MLFS. The concentrations of the analogs are all 500 ng/mL. The negative sample is used as the blank. (b) The comparison of the T/C between the analogs and the CC sample. The error bars represent the standard deviation from the three repeats (n = 3).

Figure 4

Real sample analysis of MLFS for CC urine samples. (a) Eight urine samples are detected and compared with the blank sample. (b) The comParison of MLFS and ELISA for quantitative detection of CC urine samples. The red dashed line represents the concentration of CC in blank sample. The error bars represent the standard deviation from three assays (n = 3).