2.2. “Win-Win” Magneto-Fluorescent Performances of MANP.
The magneto-fluorescent performance of MANP is influenced by its
well-defined spatial structure, including the compact AIE core, discrete
magneto shell, and the composition ratio of its functional components.
Therefore, a rational regulation of the optical and magnetic integration
in MANP was conducted to achieve dual-retained performances. SEM and TEM
images shown in Figures 2 a–c and Figure S5demonstrate that by increasing the feeding amount of
Fe3O4 NPs@OA from 2 mg to 16 mg while
keeping the AIEgens mass at 6 mg, the MANP2:6,
MANP4:6, MANP6:6,
MANP8:6, and MANP16:6 maintain the
core–shell nanostructure. However, an increase in
Fe3O4 NPs@OA content results in gradual
surface roughness and denser magnetic inlay. The increased magnetic
feeding also leads to higher UV–vis absorbance of the synthesized MANP
due to the intrinsic optical absorption of
Fe3O4 NPs@OA (FigureS6 ). Fluorescent lifetime measurements show negligible lifetime
decay when incorporating 6 mg or 16 mg
Fe3O4 NPs@OA into MANP in comparison to
ANP6 (Figure 2 d). These results imply
the occurrence of a fluorescent IFE in MANP, which inevitably quenches
the final fluorescent emission. As validated in Figure2 e, a higher proportion of magnetic component corresponds to a
gradually decreased fluorescent signal. Additionally, Figures
2 f and S7 show that the magnetic recovery of MANP under an
external magnetic field increases with the amount of
Fe3O4 NPs@OA and reaches a plateau at a
Fe3O4 NPs@OA content of 6 mg. On the
basis of these observations, the fluorescent intensity of
MANP6:6 exhibits a much higher retention rate of 82%,
which is much higher than the traditionally reported
“magneto@fluorescence” core-shell nanostructure possessing a
fluorescent retention rate below 50%.[41] This
can be attributed to the outstanding luminescence property of AIEgens in
an aggregated state and the discrete distribution of
Fe3O4 NPs@OA that enables effective
passage of photons through the polymer shell layer. Therefore,
simultaneous high magneto-fluorescent activities can be achieved by
constructing MANP6:6 with magnetic and fluorescent of 6
mg each.
The magnetic performance of the well-emitted MANP6:6 was
evaluated. As shown in Figure 3 a, MNP6and ANP6 exhibit either magnetic or fluorescent
responsiveness, respectively, due to their single functional component.
By contrast, MANP6:6 can be readily collected and
exhibits bright fluorescence under an external magnetic field and UV
light source. The magnetization curves in Figure 3 b
quantify that MANP6:6 exhibits superparamagnetism with a
saturation magnetization of 24.8 emu g−1, which is
remarkably higher than that of MNP6 (20.9 emu
g−1). This indicates that the solid structure of
MNP6, with Fe3O4 NPs@OA
filling the interior of the nanoparticle, can suffer from a magnetic
shielding effect caused by the nonmagnetic dielectric polymer
matrix.[42] In the contrastive case of
MANP6:6, the well-defined core–shell nanostructure
allows the magnetic constituent to spread over the exterior shell while
the AIEgens occupy the inner core space. This leads to a highly retained
saturation magnetization. In other words, the rational spatial
distribution of Fe3O4 NPs@OA on the
exterior shell layer effectively guarantees the magnetic performance of
MANP6:6, curtails magnetic constituent to suppress
fluorescent quenching, and improves optical sensitivity. Thus, the
constructed MANP6:6 possesses excellent dual
functionality with simultaneous high magnetic manipulation and optical
output activity.
To further evaluate the fluorescent feasibility of the nanoparticles on
paper test strips, we also investigated the IFE of
MANP6:6 on the strip by spraying a series of particle
concentration of MANP6:6 on the NC membrane as the T
line. As presented in Figures 3 c, d, the fluorescent brightness
of MANP6:6 shows a synchronized tendency with that of
ANP6, with a limit of detection (LOD) of 42 ng
mL−1 particle
concentration. Moreover, negligible fluorescent decay of
MANP6:6 and ANP6 occurs at a high
particle concentration of 2.8 mg mL−1(~0.8 μg/strip). This indicates that the particle
stacking process on the NC membrane does not generate an obvious
fluorescent IFE owing to the reduced magnetic loading amount in
MANP6:6. The effective avoidance of IFE is thus crucial
in LFIA detection (~0.6 μg/strip in further POCT
sensors). Furthermore, the versatility of the synthesis strategy was
confirmed by individually incorporating green or yellow AIEgens
(Figures S8 a, b) into the nanoparticles. As shown inFigures S8 c–f, the obtained green, yellow, and red
MANP6:6 emit characteristic fluorescence signals at 500,
545, and 600 nm. Confocal laser scanning microscope imaging results
(Figure S8 g) demonstrate that multicolor spheres with
single-particle brightness can be visualized simultaneously under a
single excitation. This indicates that the proposed MANP holds great
potential for multicolor fluorescent imaging and high-throughput
multiplex detection.
Given the excellent combination of magnetic and fluorescent performance,
MANP6:6 shows promise as a dual-functional nanoprobe for
bioanalytical applications. The following investigations were conducted
to ensure its stability in further bioapplications. As shown inFigure 3 e, MANP6:6 exhibits slight
fluorescent decrease and hydrodynamic size increase in acidic or basic
conditions, with negligible change in magnetic recovery across pH values
ranging from 2 to 11. These results indicate that
MANP6:6 maintains good optical, magnetic, and colloid
stability over a wide pH range, making it suitable for various
biochemical applications. Furthermore, the thermal stability of
MANP6:6 was evaluated by storing the particles at 60 °C
for one week. As depicted in Figure 3 f,
MANP6:6 demonstrates excellent optical, magnetic, and
colloid stability under elevated temperatures, indicating superior
long-term stability. In addition, the stability of
MANP6:6 in biological matrices was examined by
incubating the particles in PB, human serum, and human urine for 7 days.
As shown in Figures 3 g–i, MANP6:6 exhibits
stable fluorescent signals, consistent magnetic recoveries, and high
colloid stability in PB, serum, and urine media over the course of 7
days. Taken altogether, these results demonstrate the excellent colloid,
optical, magnetic, and thermal stability of MANP6:6,
which is vital for its multifunctional bioanalytical application,
enabling simultaneous magnetic separation, enrichment, and fluorescent
signal output.