Rational design to improve the detection sensitivity of sandwich-type lateral flow immunoassays

This study presents two strategies to improve the detection sensitivity of the gold nanoparticle (Au NP)-based sandwich-type lateral flow immunoassay (LFIA) using the typical human chorionic gonadotropin (hCG) LFIA as the model. One efficient way is to adjust the size of Au NPs used as the label in the LFIA. Four kinds of Au NPs with different sizes (80, 110, 130, and 160 nm) were synthesized using a seed-mediated growth method, and the influence of nanoparticle size on the detection sensitivity of hCG was investigated. The results revealed that 110 nm Au NPs offered the highest detection sensitivity. Under the optimized conditions, the limit of detection (LOD) for hCG protein using Au NPs with a diameter around 110 nm as the signal probe was 5 mIU mL^-1. Another powerful strategy discovered accidentally involves the introduction of an additional glass cellulose pad on the MAX-line label. The MAX-line label, which features an adhesive backing and a marked line indicating the maximum sample volume, is assembled on the test strip covering the sample pad and conjugate pad. We found that the introduction of an additional glass cellulose pad with a length of 6 mm on the MAX-line label could achieve a visual LOD of 1 mIU mL^-1 for the detection of hCG. We assume that the improvement in detection sensitivity may be attributed to the increased sample carrying capacity and the extended flow time of gold nanoprobes, as the gold probes may diffuse from the conjugate pad into the enhancement pad upon rehydration. The developed LFIA also exhibited good reproducibility and stability. We believe that the developed strategies, particularly the approach of adding an enhancement pad on the MAX-line label, will serve as a general method applicable to other types of LFIAs to improve the sensitivity of target analytes.