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.