3.2. Optimization of Sensitivity for EuPSM-LFIA
From the working principle of the sensor, it can be seen that the final detection result is directly related to the presence and depth of the fluorescence signal on the detection line, and the fluorescence signal is the signal of europium fluorescent microspheres gathered on the detection line. Therefore, the particle size of europium fluorescent microspheres is very important for the strength of the fluorescence signal. As shown in Figure 2(a), seven europium fluorescent microspheres, namely WD200, WD300, WD300+, JK191, JK288, BY200 and BY300, were respectively used in this study to carry out experiments. It was found that the detection signal-to-noise ratio (SNR) of europium fluorescent microspheres of WD300+ was superior to other microspheres in both high and low concentrations.
Figure 2.
Optimization of EuPSM-LFIA. a), EuPSM particle size screening. b), Optimization results before and after pretreatment of the conjugate pad. c), Antibody coating quality. d), The pH for EuPSM-coupled antibody. e), EuPSM@sTnI-Ab spraying concentration. The concentration of Tween-20 (f) and BSA (g). h), The pH for sample dilution solution. i), The concentration of cutting-edge membrane antibody.
Figure 2.
Optimization of EuPSM-LFIA. a), EuPSM particle size screening. b), Optimization results before and after pretreatment of the conjugate pad. c), Antibody coating quality. d), The pH for EuPSM-coupled antibody. e), EuPSM@sTnI-Ab spraying concentration. The concentration of Tween-20 (f) and BSA (g). h), The pH for sample dilution solution. i), The concentration of cutting-edge membrane antibody.
The conjugate pad is primarily composed of glass fiber, which serves as a crucial component for storing the complex of EuPSM-anti-skTnI Ab2. During the operation of the LFIA, it is essential to release this complex from the conjugate pad. To ensure full and complete release of the complex, the conjugate pad was treated y Tris-HCl solution which contains 20% sucrose and 4% S9 in this study. As shown in the Figure 2(b), the pretreated conjugate pad demonstrated a controlled-release effect on the complex of the EuPSM-anti-skTnI Ab2. This is because this pretreatment adjusts the pH and viscosity of the sample solution, and promotes the capture capability of EuPSM-anti-skTnI Ab2 to the target. This promoted more thorough binding between the antigen and the antibody, thereby enhancing the overall detection signal.
As shown in Figure 2(c), the quality of antibodies coated with EuPSMs was optimized in this study. The detection results indicated that when the antibody coating mass was within the range of 25-100 μg, the detection signal value increased with the coating mass. However, when the coating mass increased to 150 μg, there was no significant difference in the detection signal value compared to 100 μg. Considering cost-effectiveness, we selected 100 μg as the optimal antibody coating mass for EuPSMs in this study.
As shown in Figure 2(d), the optimal pH value for coupling antibodies to EuPSMs was determined in this study. The test results indicated that at pH 6 and 7, there was a noticeable improvement in the overall detection signal value of the LFIA. However, at pH 6, the distinction between low concentrations was more pronounced. So, pH 6 was selected as the optimal pH for coupling antibodies to EuPSMs.
Following the optimization of antibody coating quality and coupling pH for EuPSMs, the optimal spray concentration of EuPSM-antibody complex on the conjugate pad was further optimized in this study. The test results, as shown in Figure 2(e), revealed that the overall detection signal value of the LFIA increased with higher spray concentrations. However, analysis of low-concentration test results showed that spray concentrations of 8% and 10% exhibited poorer distinction and linearity for low concentration samples, whereas a spray concentration of 6% demonstrated better linearity and distinction. Therefore, a 6% spray concentration was selected as optimal for the EuPSM-antibody complexes.
The optimization of Tween-20 and BSA content in the sample diluent was investigated in this study. As shown in Figure 2(f), a Tween-20 content of 5% yielded significantly superior overall detection signal compared to other concentrations. Additionally, a BSA content of 0.5% (Figure 2(g)) resulted in higher distinction for low concentrations, contributing to improved detection sensitivity. Therefore, 5% Tween-20 and 0.5% BSA were identified as the optimal component concentrations for the sample diluent.
As illustrated in Figure 2(h), the optimal pH of the sample diluent was optimized in this study. The test results showed that at pH 6, the overall detection signal value of the LFIA surpassed that of other pH values, making it the preferred choice for the sample diluent.
As shown in Figure 2(i), the concentration of the detecting antibody on the test line was optimized in this study. The detection results demonstrated that an increase in the concentration of membrane-coated antibodies corresponded to an increase in the detection signal value of the LFIA. This increase was attributed to the capture of more EuPSM-antibody complexes at the test line, resulting in an enhanced detection signal and improved detection sensitivity. However, a continued increase in the concentration of the detecting antibody posed a risk of heightened non-specific adsorption at the test line, leading to false positive results, potentially leading to false positive results. Additionally, considering both performance and cost-effectiveness, a concentration of 1.5 mg/mL was selected as the optimal coating concentration for the detecting antibody on the detection line.
3.3. Detection Feasibility and Performance of Serum-Spiked Samples
3.3.1. Detection Feasibility of Serum-Spiked Samples
To verify the feasibility and accuracy of the highly sensitive muscle injury immediate detection device optimized in the above experiments in clinical testing, negative human serum samples and sample dilutions were selected as the detection matrix. SkTnI antigen standards were added in sequence, and gradient concentrations for 100, 50, 20, 10, 5, 2, 1, 0.5, 0.2, 0 ng/mL of serum-spiked were prepared. The optimized EuPSM-LFIA was then employed to detect the prepared skTnI serum-spiked samples, with the test results shown in
Figure 3(a). Notably, when the concentrations of the skTnI serum-spiked samples were 0 and 0.2 ng/mL respectively, there was no significant difference in signal intensity observed by naked eye. However, when the concentration of skTnI serum-spiked sample reached 0.5 ng/mL, there was a noticeable difference in signal intensity compared to the 0 ng/mL sample, indicating a positive result. This suggests that the optimized device can achieve a detection sensitivity of 0.5 ng/mL for the skTnI serum-spiked samples, with a concentration of 0.5 ng/mL serving as the exclusion threshold for clinical judgment of muscle injury[
13,
16]. Therefore, the optimized EuPSM-LFIA can effectively determine whether a subject has muscle injury, enabling instant detection.
Figure 3.
Detection of skTnI-spiked serum samples by EuPSM-LFIA. a), Optimized biosensor detection results for serum addition samples. b), Quantitative standard curves for portable muscle injury instant detection sensors.
Figure 3.
Detection of skTnI-spiked serum samples by EuPSM-LFIA. a), Optimized biosensor detection results for serum addition samples. b), Quantitative standard curves for portable muscle injury instant detection sensors.
3.3.2. Evaluation of Detect Linearity
Using the concentration of skTnI in serum-spiked samples (100, 50, 20, 10, 5, 2, 1, 0.5, 0.2, 0 ng/mL) as the x-axis, and the mean the detection signal intensity after three repeated measurements of each concentration as the y-axis, the quantitative curve for the detection of skTnI serum-spiked samples by the optimized highly sensitive EuPSM-LFIA was plotted, as shown in Figure 3(b). The four-parameter fitting relationship of the quantitative curve was y=2.57374/[1+(x/71.53607)^(-1.0543)]+0.00694, indicating that the detection signal intensity of the device was positively correlated with the concentration of the target in the sample, meaning that the detection signal intensity increases with the concentration of the target substance in the sample. The correlation coefficient R2=0.999, demonstrating a very strong quantitative relationship between the detection signal intensity and the concentration of the target substance in the serum-spiked sample. Therefore, the optimized highly sensitive EuPSM-LFIA meets the standard detection range.
3.3.3. Evaluation of Detection Limit
After conducting 20 repeated measurements on the zero-concentration calibrator of skTnI serum-spiked samples, the M and SD of the detection signal intensity of the device were calculated to be M=0.012544 and SD=0.002881, and M + 3*SD=0.021187, respectively. According to the statistical determination index of a 95% confidence interval, M + 3*SD was substituted as y into the above quantitative curve y=2.57374/[1+(x/71.53607)^(-1.0543)]+0.00694, yielding x=0.520405. Therefore, the limit of detection (LOD) of the optimized highly sensitive EuPSM-LFIA was approximately 0.5 ng/mL.
3.3.4. Evaluation of Detection Precision
In this study, the detection precision of a highly sensitive EuPSM-LFIA was evaluated through the coefficient of variation (CV). The skTnI serum-spiked samples with calibrated concentrations of 30, 50, and 75 ng/mL were measured three times. The average detection signal intensity was found to be 1.46231, 1.29458, and 0.90577 respectively. The variation of the above three concentrations was calculated to be 2.99%, 7.16%, and 10.38% respectively. The average CV for these three concentrations was 6.84%, indicating that the optimized highly sensitive EuPSM-LFIA had a low CV and good detection precision, which meets the requirement of CV<15% specified in the detection standard.