Antibiotics abuse has caused various problems threatening human health and ecological environment. Monitoring antibiotics residual levels is of great significance, yet still challenging for quantitative point-of-need testing with high-sensitivity and visual capability. Here we developed a competitive lateral flow immunoassay (CLFIA) platform with flexible readout for enrofloxacin (ENR), a regularly added antibiotic. To overcome the limitation of low sensitivity of traditional colloidal gold-based CLFIA, the three-dimensionally assembled gold nanoparticles (AuNPs) within dendritic silica scaffold were fabricated as signal reporters. The assembly structure effectively retained the intrinsic absorption features of hydrophobic AuNPs and greatly enhanced the light extinction ability of a single label for signal amplification. The obtained CLFIA strips can not only achieve qualitative screening of ENR at a very low concentration by naked eye (cutoff value: 0.125 ng/mL), but also enable ultrasensitive quantification of ENR by an optical scanner (limit of detection: 0.00195 ng/mL) or a smartphone (limit of detection: 0.0078 ng/mL). Moreover, to elaborate the visual inspection degree of CLFIA against traditional yes/no interpretation, a novel multirange gradient CLFIA strip was prepared for visually semiquantitative identification of ENR with four concentration ranges. The novel CLFIA platform demonstrated sensitive, specific, and reliable determination of ENR with flexible signal readout and provides a potential and invigorating pathway to point-of-need immunoassay of antibiotics.

Considering that hue-recognition has drawn increasing research interests for visual detection because of its high accuracy and flexible analysis modalities, the exploration of a rational hue-recognition sensing system is highly demanded. Here, we report a hue-recognition-based lateral flow immunoassay (HLFIA) by ratiometric modulation of emission from two nanoprobes, in which red-emitting ruthenium(II)-doped dendritic silica colloids and green-emitting gold nanoclusters function as reporter and reference, respectively. The HLFIA strip can generate a visually discernible green-to-orange-red color gradient alternation, enabling the naked-eye semiquantitative inspection of successive C-reactive protein (CRP) concentration and concentration intervals (0–50, 50–150, and 150–500 ng mL–1) with a visual detection limit of 10 ng mL–1. By integrating with a customized smartphone-based portable imaging device and RGB color space recognition, the precise quantification of CRP can be achieved with a linear range of 0–100 ng mL–1 and a limit of detection of 0.64 ng mL–1. The HLFIA strip is robust and validated for CRP detection in real serum, holding great promise for noninstrumental and deprofessionalized point-of-care immunoassay of biomarkers.