Ultrasensitive Competitive Lateral Flow Immunoassay with Visual Semiquantitative Inspection and Flexible Quantification Capabilities.

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.

Ratiometric Fluorescent Lateral Flow Immunoassay for Point-of-Care Testing of Acute Myocardial Infarction.

We report the development of a highly sensitive ratiometric fluorescent lateral flow immunoassay (RFLFIA) strip for rapid and accurate detection of acute myocardial infarction biomarker, namely heart-type fatty acid binding protein (H-FABP). The RFLFIA strip works in terms of ratiometric change of fluorescence signal, arising from blending of fluorescence emitted by two composite nanostructures conjugated to capture and probe antibodies and inner filter effect of gold nanoparticles. In conjunction with using custom smartphone-based analytical device and tonality analysis, quantitative detection of H-FABP was achieved with a low limit of detection at 0.21 ng mL-1 . The RFLFIA strip can generate a visually distinguishable green-to-red color change around the threshold concentration of H-FABP (6.2 ng mL-1 ), thus allowing the semi-quantitative diagnosis by the naked eye.

Hierarchical Plasmonic-Fluorescent Labels for Highly Sensitive Lateral Flow Immunoassay with Flexible Dual-Modal Switching.

Lateral flow immunoassay (LFIA), as a prominent point-of-care (POC) test platform, has been extensively adopted for rapid, on-site, and facile diagnosis of pathogen infections and disease biomarkers. Exploring novel structured optical labels of LFIA with amplified signal and complementary detection modes favors the sensitive and flexible POC diagnosis. Here, bimodal labels with both colorimetric and fluorescent readout were fabricated via a layered sequential assembly strategy based on affinity templates and hydrophobic metal-containing nanounits. High-quality colorimetric and fluorescent nanoparticles were densely incorporated into the colloidal supports and confined in separated regions, without interfering with each other. The hierarchical integration of gold nanoparticles and quantum dots with high loading density and good optical preservation realized dual readout and amplified signals from the assemblies of individual single nanoparticles. The "all-in-one" optical labels allowed both colorimetric and fluorescent detection of cystatin C (Cys C) after surface conjugation with antibodies. The LFIA strips revealed noninterfering dual signals for both visual inspection and quantitative detection of Cys C via the naked eye and portable devices, respectively. The limits of detection by colorimetric and fluorescent modes were 0.61 and 0.24 ng mL-1, respectively. The novel LFIA platform demonstrated sensitive, specific, and reproducible POC testing of biomarkers with flexible detection modes and was reliable for clinical diagnosis.

Controllable and robust dual-emissive quantum dot nanohybrids as inner filter-based ratiometric probes for visualizable melamine detection.

The ratiometric fluorescence technique is of great interest due to its visualization characteristics. The construction of a reliable fluorescent ratiometric nanoprobe for high-sensitivity visual quantification is highly sought after but it is limited by poor stability and controllability. Herein, we report a robust dual-emissive quantum dot nanohybrid with precise color tunability and demonstrate its potential as a two-signal-change ratiometric probe for visual detection. A novel assembly strategy was developed for spatially implanting hydrophobic green and red quantum dots (QDs) into a silica scaffold to form a dual-emissive hierarchical fluorescent silica nanohybrid. The fluorescence intensity ratio and color of the nanohybrid were precisely tailored by altering the amounts of green and red QDs. Particularly, after the alkylsilane-mediated phase transfer and exterior silica shell growth, the nanohybrid exhibited the well-preserved fluorescence features of the original QDs and robust optical/colloid stability. An inner filter-based ratiometric nanoprobe for the visual determination of melamine was ultimately devised by combining the spectra-overlapped two-colored fluorescent nanohybrid with analyte-specific gold nanoparticles. Furthermore, based on the reversible fluorescence signal changes in two-colored QDs induced by melamine, a logic gate strategy for melamine monitoring was constructed. The newly developed fluorescent ratiometric nanoprobe shows great prospects for the visual and quantitative determination of analytes in a complex biological matrix.

Homogeneous and high-density gold unit implanted optical labels for robust and sensitive point-of-care drug detection.

Controllable integration of gold building blocks into mesoscopic architecture produces improved optical signals with preferable stability for biological sensing. Here, we developed novel optical labels with homogeneous and high-density implanted hydrophobic gold nanoparticles (AuNPs) throughout three-dimensional silica scaffolds. The dendritic silica supports with an extra-large pore size and highly accessible central-radial channels were employed as metal-affinity templates, for anchoring with AuNPs directly from the organic phase. The nano-assemblies exhibited a high unit loading capacity while maintaining the intrinsic optical characteristics of AuNPs. After phase transfer by the alkylsilane intermediate layer and exterior silica shell encapsulation, the nanocomposites revealed an amplified plasmonic absorption signal, excellent colloidal/optical stability and convenient surface functionalization. By integrating the silica labels into the lateral flow immunoassay strip for signal enhancement, the sensitive point-of-care detection of methamphetamine in urine was established. The limit of detection achieved 0.026 ng mL-1, with a detection range from 0.023 to 375 ng mL-1 in a 10 min assay, allows both visual and on-site quantitative analysis. Encouragingly, the potential interfering drugs in the sample matrix showed a negligible influence on the results, validating the superior specificity of the current immunoassay. The newly developed gold-implanted optical labels show prospects for point-of-care testing in a complex biological matrix with the desirable stability and signal amplification.