Fluorescent microspheres are a novel luminescent nanomaterial proposed as an alternative probe to improve the detection sensitivity of competitive immunochromatographic assay (ICA). Quantum dot nanobeads (QBs) possess strong luminescence and resistance to matrix interference. Theoretically, large-sized QBs exhibit stronger luminescent intensity than small-sized QBs and are beneficial to ICA sensitivity. However, oversized QBs may reduce the sensitivity of competitive ICA. Thus, the relationship between the size and luminescent intensity of QBs and the competitive ICA sensitivity must be elucidated. In this study, QBs of different sizes (58, 124, 255, 365, and 598 nm) were synthesized. Ochratoxin A (OTA) was selected as the model analyte for competitive ICA. The effects of QB size on the detection performance of competitive ICA were then evaluated. The cutoff limit of QB-ICA for naked eye detection was used for qualitative analysis, and the half-maximal inhibitory concentration (IC) and LOD were employed for quantitative analysis. Results indicated that 124 nm QBs used as labeling probes for competitive ICA showed the optimal detection performance and the lowest cutoff value of 5 ng/mL for qualitative detection and IC (0.39 ng/mL) for quantitative detection. Similar to commercial ELISA, QB-ICA displayed good accuracy, specificity, reproducibility, and practicability. In summary, 124 nm QBs can be used as a new labeling probe for competitive ICA.
Herein,
we synthesized bifunctional magnetic fluorescent beads
(MFBs) with a distinct core/shell structure by encapsulating octadecylamine-coated
CdSe/ZnS QDs (OC-QDs) and oleic acid-modified iron oxide nanoparticles
(OA-IONPs) into two polymer matrixes with different hydrophobic properties.
The OC-QDs and OA-IONPs were mainly distributed in the outer layer
of MFBs. The resultant MFBs displayed ca. 226-fold stronger fluorescence
emission relative to the corresponding OC-QDs and retained ca. 45.4%
of the saturation magnetization of the OA-IONPs. The MFBs were used
to purify and enrich aflatoxin B1 (AFB1) from
dark soy sauce and then utilized as a fluorescent reporter of immunochromatographic
assay (ICA) for the sensitive detection of AFB1. Under
the optimal detection conditions, the MFB-based ICA (MFB-ICA) displayed
a dynamic linear detection of AFB1 in sauce extract over
the range of 5–150 pg/mL with a half maximal inhibitory concentration
of 27 ± 3 pg/mL (n = 3). The detection limits
for AFB1 in sauce extract and real dark soy sauce were
3 and 51 pg/mL, respectively, which are considerably better than those
of the previously reported fluorescent bead-based ICA methods. The
analytical performance of the proposed MFB-ICA in terms of selectivity
and accuracy was investigated by analyzing AFB1-spiked
dark soy sauce samples. The reliability of the proposed method was
further confirmed by ultraperformance liquid chromatography with fluorescence
detection. With the combined advantages of QDs and IONPs, the resultant
MFBs offer great potential as reporters of ICA for the sensitive detection
of trace pollutants in complex matrix samples.
The
quantitative multiplex immunochromatographic assay (mICA) has
received an increasing amount of attention in multitarget detection.
However, the quantitative results in the reported mICAs were obtained
by recording the signals on the test lines that with which various
analyte-independent factors readily interfere, resulting in inaccurate
quantitation. The ratiometric strategy using the T/C value (ratios
of signals on the test line to those of the control line) for signal
correction can effectively circumvent these issues to enable more
accurate detection. Herein, we present for the first time a novel
ratiometric mICA strip with multiple T lines for the simultaneous
quantitative detection of aflatoxin B1 (AFB1), fumonisin B1 (FB1), and ochratoxin A (OTA)
using highly luminescent quantum dot nanobeads (QBs) as enhanced signal
reporters. To achieve reliable ratiometric signal output, a biotin–streptavidin
system was introduced to replace the conventional anti-mouse IgG antibody
for reliable reference signals on the control line that are completely
independent of the signal probe and analyte. By using stable T/C values
as quantitative signals, our proposed QB–mICA method can successfully
detect three mycotoxins with concentrations as low as 1.65 pg/mL for
AFB1, 1.58 ng/mL for FB1, and 0.059 ng/mL for
OTA. The detection performance of the developed QB–mICA strip,
including precision, specificity, and reliability, was further evaluated
using artificially contaminated cereal samples. The results demonstrate
the improved accuracy and reliability of quantitative determination
by comparison with the anti-mouse IgG antibody. Thus, this work provides
a promising strategy for developing a ratiometric mICA method for
accurately quantifying multiple analytes using the biotin–SA
system, opening up a new direction in quantitative mICAs.
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