Article pubs.acs.org/ac
Fluorescence Anisotropy as a Reliable Discrimination of LigandAsymmetric and Symmetric Mn-Doped ZnS Quantum Dots Yu Zhang,† Lin Miao,† and He-Fang Wang*,†,‡ †
Research Center for Analytical Sciences, College of Chemistry, Key Laboratory of Biosensing, Molecular Recognition, State Key Laboratory of Medicinal Chemical Biology and ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China S Supporting Information *
ABSTRACT: We presented a novel fluorescence anisotropy (FA) method for the noninvasive, effective, simple, and convenient discrimination of the symmetric and asymmetric distribution of the ligands on Mn-doped ZnS quantum dots (QDs). The symmetric or asymmetric distribution of mercaptopropionic acid (MPA) and NH2-polyethylene glycol-CH3 (PEG-m, MW 2000) was controlled by the condensation reaction of the carboxyl of MPA and the amino of PEG-m with or without the masking by the aminofunctionalized silica nanoparticles. The ligand-asymmetric Janus-QDs were obtained with the masking, whereas the ligand-symmetric PEG-QDs were gained without masking. The FA values of the QDs could not only distinguish the ligand symmetric PEG-QDs from the ligand asymmetric Janus-QDs but also discriminate the QDs with a different PEG-m amount. Besides, the FA assay also has superiority over the dynamic light scattering (DLS) and photoluminescence (PL) methods in discriminating the interaction of Janus-QDs or PEG-QDs with protamine (the sensitivity of Janus-QD-3 over PEG-QD-3 was 1.60, 1.24, and 1.11 in the FA, DLS, and PL methods, respectively).
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selective silica deposition from the side of 4-mercaptophenylacetic acid (4-MCPA) on the ligand asymmetric 4-MCPA and poly(acrylic acid) (MW = 6200) functionalized gold nanoparticles for getting the Janus Au-SiO2.21 Noteworthy, the ligand-asymmetric polyethylene glycol (PEG) and singlestranded DNA functionalized gold nanoparticles were identified by introducing Y-shaped DNA duplex to form the TEM observable dimers of gold nanoparticles.22 Although great progress has been made for identification of the ligandasymmetric Janus nanoparticles, the introduction of the third ingredients for TEM observation was somewhat complex and irreversible. Consequently, it is of vital significance to develop a noninvasive, effective, simple, and convenient method for distinguishing ligand-asymmetric Janus nanoparticles from the ligand-symmetric nanoparticles. Herein, we presented a novel fluorescence anisotropy (FA) method for distinguishing the symmetric and asymmetric distribution of the ligands on Mn-doped ZnS quantum dots (QDs). FA, a sensitive, real-time, and homogeneous analytical technology widely applied for immunoassays, enzyme assays, and host−guest interactions23,24 has the advantages of separation-free of free and bound species25−27 and immunity
anus nanoparticles, which possess dual functional ligands or consist of two jointed components with distinct properties, have attracted wider and wider attention since Gennes emphasized the concept in the Nobel Prize address.1 The asymmetry endows the nanoparticles many intriguing properties and enabled their wide applications as the surfactants, optical probes, water-repellent coatings, catalysts, drug carriers, and micromotors, etc.2,3 Various techniques, including the direct observation by transmission electron microscopy (TEM),4,5 scanning electron microscopy (SEM),6−8 and atomic force microscopy (AFM)9 and indirect evaluations via Fourier transform-infrared (FT-IR),10 infrared reflection absorption spectroscopy,11 fluorescence,12,13 energy-dispersive X-ray spectroscopy,14 nuclear magnetic resonance spectra,15 UV−vis spectra,16 dynamic light scattering (DLS),17 Zeta potential,18 and X-ray diffraction19 have been used to characterize the Janus structure. Even though those techniques could confirm the asymmetric structure, the characterization of ligand-asymmetric Janus materials is still a formidable challenge. To date, the asymmetric distribution of two kinds of ligands attached onto one nanoparticle was generally confirmed by the selectively introducing some microscopy-visible ingredients from the very side of one kind of ligand. For instance, Wooley et al. confirmed the ligand-asymmetric azide- and thiolfunctionalized shell-cross-linked-knedel-like nanoparticles by labeling of the gold nanoparticles (2 nm) on the thiol side followed by TEM observation.20 Chen et al. reported the © XXXX American Chemical Society
Received: July 10, 2016 Accepted: September 3, 2016
A
DOI: 10.1021/acs.analchem.6b02614 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry to photobleaching of fluorophores and instrument parameters (for FA’s ratiometric property).28−30 For a given fluorophore, FA is dependent on the ratio of rotational diffusion time and the fluorescence lifetime, while the rotational diffusion time is related with the viscosity of the surroundings and the size and shape of the rotating fluorophore.31−41 The QDs with symmetric and asymmetric distribution of the ligands would have different fluorescence lifetime, size, and shape and thus the different FA values. To verify that hypothesis, we synthesized the Mn-doped ZnS QDs with symmetric or asymmetric distribution of the ligands and compared the FA values of those QDs (Scheme 1). The
asymmetric or symmetric distribution on the nanoparticles, which is the fundamental aspect for nanotechnology.
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EXPERIMENTAL SECTION Reagents. All reagents were at least of analytical grade. Mn(CH3COO)2·4H2O and Na2S·9H2O were purchased from the Second Chemicals Co. of Shenyang (Shenyang, China) and Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China) respectively. 3-Mercaptopropionic acid (MPA, 98%), ZnSO4·7H2O, 1-ethyl-3-(3-dimethylaminopropy) carbodiimide (EDC), N-hydroxysuccinimide (NHS), and silicon dioxide (99.5%) were from Aladdin (Shanghai, China). (3-Aminopropyl) triethoxysilane (APTES, 98%) was from Alfa Aesar (Shanghai, China). Methyl-PEG-amine (MW 2000) was from Seebio Biotech. (Shanghai, China). Protamine (MW 7000) from salmon was supplied by Sigma (Shanghai, China). The phosphate buffer (PB) consisting of NaH2PO4−Na2HPO4 (10 mM, pH 7.4) was used throughout. Apparatus. The morphology and microstructure of the QDs were characterized by high-resolution transmission electron microscopy (HRTEM) on a JEM-2100F field emission transmission electron microscope (JEOL, Japan) operating at an accelerating voltage of 200 kV. The samples for HRTEM were obtained by drying sample droplets from PB (10 mM, pH 7.4) dispersion onto a 300-mesh copper grid coated with a lacey carbon film. The FT-IR spectra (4000−400 cm−1) in KBr were recorded using a Magna-560 spectrometer (Nicolet, Madison, WI). The lifetime and fluorescence anisotropy were measured on a PTI QM/TM/NIR spectrometer (Birmingham, NJ) equipped with a xenon light source (75 W) and N2/dye laser. The DLS and Zeta potentials were measured on a Zetasizer Nano ZS (Malvern Instruments Ltd., United Kingdom) equipped with a noninvasive back scattering (NIBS) device, polystyrene cell (1 cm × 1 cm for DLS), and folded capillary sample cell (for Zeta potentials). For DLS, the sample solution was filtered through a membrane with a 0.22 μm pore size and stablized for 120 s before each measurement. Preparation of SiO2−NH2 Nanoparticles. The SiO2− NH2 nanoparticles were prepared by the hydrolysis of APTES on nano-SiO2. A total of 80 mg of SiO2 was added into 60 mL of 5% APTES ethanol solution. The mixture was stirred for 24 h at room temperature. Then the SiO2−NH2 nanoparticles were harvested by centrifugation at 5000 rpm, washed three times with ethanol, and dried in vacuum. Synthesis of QDs. The as-prepared MPA-capped-Mndoped-ZnS QD was synthesized as described previously.42 The Janus-QDs were prepared as follows. First, 25 mg of the asprepared QD and 12.5 mg of SiO2−NH2 (the mass ratio has been optimized to get the maximum adsorption of as-prepared QD onto SiO2−NH2, Figure S1, Supporting Information) were dispersed in 10 mL of PB and stirred vigorously for 20 min. Then the mixtures were centrifuged at 5000 rpm, and the resultant solid (QD−SiO2−NH2) was washed with PB four times (the solid had the orange emission under excitation of 302 nm, Figure S2, Supporting Information). The harvested QD−SiO2−NH2 was redispersed in 2 mL of PB and EDC (10 mg), NHS (5 mg), and PEG-m (1, 2, 3, 4, and 5 μmol) were added in sequence. The mixture was stirred gently in an ice bath for 10 h and then centrifuged at 5000 rpm. The resultant solid was washed three times with PB and redispersed in 2 mL of PB. Then the dispersion was adjusted to pH 12 and ultrasonicated for 2 min to detach the Janus-QDs from SiO2− NH2. After removal of SiO2−NH2 by centrifugation at 5000
Scheme 1. Synthesis of Mn-Doped ZnS QDs with Symmetric (PEG-QDs) or Asymmetric (Janus-QDs) Distribution of the Ligandsa
a The QD detached from the binding of SiO2−NH2 without reaction with PEG-m was marked as MPA-QD.
symmetric or asymmetric distribution of mercaptopropionic acid (MPA) and NH2−PEG−CH3 (PEG-m, MW 2000) was controlled by condensation reaction of carboxyl of MPA and amino of PEG-m with or without the masking by the aminofunctionalized silica (SiO2−NH2) nanoparticles. First, the asprepared MPA-capped-Mn-doped-ZnS QDs with negative charges were adsorbed onto the surface of positively charged SiO2−NH2 through electrostatic interaction. Then the PEG-m was attached onto the QDs through the condensation reaction with or without the masking by SiO2−NH2 (Scheme 1). The Janus-QDs were obtained with the masking (first condensation and then desorption), whereas the PEG-QDs were gained without masking (first desorption and then condensation). The QD detached from the binding of SiO2−NH2 without the reaction with PEG-m was marked as MPA-QD for control and comparison. The comparison of the FA values of PEG-QDs and Janus-QDs demonstrated that FA values could not only distinguish the ligand symmetric PEG-QDs from the ligand asymmetric Janus-QDs but also discriminate the QDs with different PEG-m amounts. To further examine the features of the QDs resulting from the ligand symmetric or asymmetric distribution, we also explored the interactions of those QDs with protamine, an arginine-rich polycationic protein. The results demonstrated the superiority of FA measurements over other methods such as DLS and photoluminescence (PL) in distinguishing the interactions of PEG-QDs or Janus-QDs toward protamine. To the best of our knowledge, this work was the first exploration of FA as the reliable noninvasive examination of the ligands’ B
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the masking by SiO2−NH2 (condensation of PEG-m into QDsSiO2−NH2 first and then desorption of the QDs from SiO2− NH2 at pH 12). The PEG-QDs were gained without masking (first desorption and removal of SiO2−NH2 and then condensation of PEG-m onto the detached QDs). All the prepared MPA-QD, Janus-QDs, and PEG-QDs displayed the fine morphology (Figure 1a) and good PL
rpm, the Janus-QDs were precipitated by ethanol, washed with ethanol, and dried in vacuum. The QDs with the addition amount of PEG-m 1−5 μmol were named as Janus-QD-1, Janus-QD-2, Janus-QD-3, Janus-QD-4, Janus-QD-5, respectively. The QD subjected to the same adsorption−desorption step but without PEG-m condensation marked as MPA-QD were also prepared in parallel for control and comparison. The preparation of PEG-QDs was similar as the procedure of Janus-QDs but with desorption at pH 12 first and then condensation (Scheme 1). The 2 mL dispersion of QD-SiO2− NH2 in PB was adjusted to pH 12, ultrasonicated for 2 min, and then centrifuged at 5000 rpm. The supernatant was adjusted to pH 7.4 with hydrochloric acid and EDC (10 mg), NHS (5 mg), PEG-m (1, 2, 3, 4, 5, and 10 μmol) were added in sequence. The mixture was stirred gently in an ice bath for 10 h. Finally, the PEG-QDs were precipitated with absolute ethanol, washed with ethanol, and dried in vacuum. The QDs with the addition amount of PEG-m 1−10 μmol were named as PEG-QD-1, PEG-QD-2, PEG-QD-3, PEG-QD-4, PEG-QD-5, PEG-QD-6, respectively. FA Measurements. FA measurements were taken in the time-based polarization mode with the excitation wavelength of 290 nm and emission wavelength of 595 nm, the slit width both set at 4 nm. A pair of quartz cuvettes and an optical long-pass emission filter (400 nm) were used throughout. For FA measurements of QDs and their interaction with protamine in PB, 2 mL of PB buffer was for the background, while 2 mL of QDs (200 mg L−1) in PB buffer in the absence or presence of protamine was the samples. The FA value (r) was calculated by the equation of r = (IVV − GIVH)/(IVV + 2GIVH), where IVV is the intensity measured with the vertical excitation and emission polarizers, and IVH is the intensity measured with the vertical excitation and horizontal emission polarizers. G is the instrumental corrector which is equal to the ratio of IHV to IHH. PL Measurements. Typically, the PL measurements were taken in the phosphorescence mode of the spectrophotometer with the excitation wavelength of 290 nm. The slit width was both 10 nm for excitation and emission, and the photomultiplier tube (PMT) voltage was set at −950 V. For examining the interaction of QDs with protamine, the QDs solution in PB (200 mg L−1, 2 mL) was put into in 4 mL quartz cell, and then different amounts of protamine were added under gentle stirring of 5 min, followed by the PL measurement. FA Measurements of QDs in the Presence of Protamine in Human Serum. The human serum was supplied by a local hospital. To prevent serum metamorphism, frozen sample first was moved to the upper refrigerator at 4 °C to ice out and then transferred to the room temperature with soft shaking to avoid a sediment. Similar with the measurements in PB, 2 mL of PB buffer containing 20 μL of serum was for background, while 2 mL of QDs solutions (200 mg L−1) in the same media as the background was samples.
Figure 1. (a) HRTEM image of Janus-QD-3, (b) PL spectra, (c) FTIR spectra, and (d) Zeta potentials of MPA-QD, Janus-QD-3, and PEG-QD-3. The concentration of QDs at 200 mg L−1 in pH 7.4 PB buffer was used for Zeta potential and PL measurement.
property (Figure 1b). Typically, Janus-QD-3 and PEG-QD-3 were used as the example. The MPA-QD, Janus-QD-3, and PEG-QD-3 displayed very similar HRTEM images as shown in Figure 1a due to the soft nature of the ligands. The PL intensity of Janus-QD-3 and PEG-QD-3 was slightly decreased compared to that of MPA-QD (Figure 1b), which was most probably due to the decreased amount and protection of the MPA ligand.28 To confirm the successful grafting of PEG-m onto the prepared Janus-QDs and PEG-QDs, FT-IR spectra of the MPA-QD, Janus-QD-3, and PEG-QD-3 were recorded (Figure 1c). Compared with MPA-QD, both the Janus-QD-3 and PEGQD-3 presented the characteristic peaks of PEG-m, including 1557 cm−1 (the N−H bending motions), 1397 cm−1 (symmetrical bending vibration of CH3), and 2930 cm−1 (aliphatic C−H stretching band). The successful conjugation of PEG-m was also proved by the difference of Zeta potentials (Figure 1d). The MPA-QD displayed a negative Zeta potential of −(39.95 ± 1.34) mV, while Janus-QD-3 and PEG-QD-3 exhibited the decreased Zeta potentials of −(14.05 ± 0.64) and −(12.75 ± 1.31) mV, respectively. Nearly the same Zeta potentials of Janus-QD-3 and PEG-QD-3 revealed nearly the same grafting amount of PEG-m on the Janus-QD-3 and PEGQD-3, which was the basis of the subsequent discussion that the difference between the Janus-QDs and corresponding PEGQDs was only because of the ligand asymmetric or symmetric distribution of the ligands. To further examine the asymmetric and symmetric distribution of the ligands on Janus-QDs and PEG-QDs, hydrodynamic diameters (by DLS) and FA values of the QDs were measured (Figure 2). As shown in Figure 2a, the grafting of PEG-m onto the QDs resulted in the obvious increase of
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RESULTS AND DISCUSSION Synthesis and Characterization of Janus-QDs and PEG-QDs. The synthesis procedure was illustrated in Scheme 1. The masking step was the key difference for the synthesis of Janus-QDs and PEG-QDs. To ensure the effective masking by SiO2−NH2, the relative amount ratios of the as-prepared QDs and SiO2−NH2 were optimized to saturate the adsorption of SiO2−NH2 toward the as-prepared QDs (Figure S1), and the excess free QDs were completely removed (as stated in the Experimental Section). The Janus-QDs were obtained through C
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series, the gradually decreased lifetime was the main reason for the increased FA values with the increased feeding amount of PEG-m. Interaction of the QDs with Protamine. The asymmetric Janus nanoparticles would lead to their unique applications in analytical chemistry, such as the Janus-gold-nanoparticles-based colorimetric sensors with the greatly broadened dynamic range of detection22 and the Janus-Mg/Au-micromotors-based strategy for simultaneous degradation and detection of diphenyl phthalate.3 To further examine the property resulted from the ligand symmetric or asymmetric distribution of the Mn-doped ZnS QDs and potential application of the Janus Mn-doped ZnS QDs, the simple electrostatic interaction between the QDs and protamine, an arginine-rich polycationic protein with isoelectric point at pH 13.8, was invested by different methods, namely, PL, DLS, and FA. First, the PL spectra of Janus-QD-3 and PEG-QD-3 in the presence of various amount of protamine were recorded, and MPA-QD was also involved for comparison (Figure 3a−c). To quantitatively describe the PL response of Figure 2. (a) Hydrodynamic diameters and (b) the FA (r) values of MPA-QD (black square), Janus-QDs (red dots), and PEG-QDs (blue triangles). The QDs concentration at 100 mg L−1 and 200 mg L−1 in 10 mM PB (pH 7.4) was used for DLS and FA measurements, respectively.
hydrodynamic diameters, and the symmetric distribution of PEG-m (PEG-QDs) led to the much larger hydrodynamic diameters than the corresponding asymmetric distribution of PEG-m (Janus-QDs). For the Janus-QDs, the hydrodynamic diameter was gradually increased as the feeding amount of PEG-m was increased; however, for the PEG-QDs synthesized with the increased feeding amount of PEG-m, the hydrodynamic diameters were nearly in the same range. In contrast, the FA values in Figure 2b could not only distinguish the ligand-symmetric PEG-QDs from the ligand-asymmetric JanusQDs but also discriminate the QDs with different feeding amount of PEG-m. The PEG-QDs had much larger FA values than the corresponding Janus-QDs. Besides, either PEG-QDs or Janus-QDs displayed the regularly increased FA values against the increasing feeding amount of PEG-m. The above data demonstrated the obvious superiority of FA over DLS in distinguishing the ligand asymmetric or symmetric distribution on the QDs. The reason behind this could be explained by the Perrin equation:
Figure 3. PL spectra of (a) MPA-QD, (b) Janus-QD-3, and (c) PEGQD-3 in the absence and presence of protamine and (d) the ΔP/P0 of MPA-QD (black squares), Janus-QD-3 (red dots), and PEG-QD-3 (blue triangles) at 595 nm versus Cprotamine. All the QDs at 200 mg L−1 in 10 mM PB buffer (pH 7.4) were used for all measurements; ΔP = P − P0, where P and P0 were the PL intensity in the presence and absence of protamine.
the QDs to protamine, the ΔP/P0 was plotted against the concentration of protamine (Cprotamine) (in μM), where ΔP = P − P0, and P and P0 were the PL intensity at 595 nm of QDs in the presence and absence of protamine (Figure 3d and Table S2 in the Supporting Information). Because of the aggregation of the QDs induced by the electrostatic interaction of protamine, the PL of the three QDs was all enhanced. However, the MPA-QD with more negative carboxyl groups displayed the more sensitive PL enhancement against protamine (with the slope of 7.87 ± 0.48), while the PEG-QD-3 and Janus-QD-3 with less negative carboxyl groups exhibited less sensitive PL enhancement (the slope of Janus-QD-3 and PEG-QD-3 was 1.10 ± 0.014 and 0.99 ± 0.027 respectively, Table S2). On the other hand, the PL intensity at 595 nm of MPA-QD had much narrow linear response range than that of PEG-QD-3 and Janus-QD-3 (Table S2). For the Janus-QD-3 and PEG-QD-3, the sensitivity of PL enhancement against Cprotamine was very similar (the sensitivity of Janus-QD-3 over
1 1 τRT = + r r0 r0ηV
where r0 is fundamental anisotropy, τ is the fluorescence lifetime, R is the gas constant, T is the temperature in K, η is the viscosity, and V is the volume of the rotating unit. Except the rotating volume (reflected by the hydrodynamic diameter here), the FA values are also dependent on the fluorescence lifetime. The QDs with different ligands and ligand distributions have quite different lifetime (Table S1). Typically, take Janus-QD-3 and PEG-QD-3 with the same amount of PEG-m as the example. The Janus-QD-3 had the much prolonged lifetime (771 ± 15.5 μs) over MPA-QD (514.5 ± 6.0 μs), while PEG-QD-3 had the slightly longer lifetime (583.3 ± 2.1 μs) over MPA-QD (Table S1). Consequently, the much larger rotating volume and the much shorter lifetime of PEGQDs over Janus-QDs resulted in the much higher FA values of PEG-QDs over Janus-QDs. For the Janus-QDs or PEG-QDs D
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presence of protamine and the resultant most sensitive PL enhancement and DLS diameter enlargement. For the JanusQD-3 and PEG-QD-3, the condensation of PEG-m led to the decrease of negative charges but the enlarged diameters, and thus the milder aggregation in the presence of protamine and the resultant less sensitive PL enhancement and DLS diameter enlargement. Compared with the PEG-QD-3, the Janus-QD-3 had slightly higher sensitivity against protamine in both PL and DLS methods, probably because of the lower steric hindrance of the ligand-asymmetric Janus-QD-3. The interaction of the QDs and protamine resulted in the PL enhancement and hydrodynamic diameters enlargement; however, the FA values of the three QDs were decreased with the increase of Cprotamine (Figure 4b). According to Perrin equation, the FA values should increase as the rotating volume increased. This abnormal result was because of the greatly prolonged lifetime of the QDs in the presence of protamine (Figure 5 and Table S3). As lifetime increased much more obvious than volume, the value of τ/V was increased with Cprotamine, finally led to the decreased FA against Cprotamine.
PEG-QD-3 was 1.11 times); however, the Janus-QD-3 have much wider linear range over PEG-QD-3 (Table S2, 0.01−1.00 μM (100 times) for Janus-QD-3 and 0.08−1.32 μM (16.5 times) for PEG-QD-3). The hydrodynamic diameters of the QDs in the absence or presence of protamine were measured by DLS (Figure 4a). For
Figure 4. Interaction of the QDs (MPA-QD, Janus-QD-3, and PEGQD-3) with protamine measured by (a) DLS and (b) FA. The concentration of QDs for DLS and FA measurements was 100 and 200 mg L−1 in 10 mM PB buffer (pH 7.4), respectively.
all the QDs tested, the hydrodynamic diameters were gradually enlarged with the increase of Cprotamine. Similarly, the ΔD/D0 was plotted against Cprotamine (in micromolar), where ΔD = D − D0, and D and D0 were the hydrodynamic diameters of QDs in the presence and absence of protamine. For MPA-QD, the increase of protamine resulted in more sensitive increase of the diameters (with the slope of 35.5 ± 4.1); however, for JanusQD-3 and PEG-QD-3, the sensitivity of ΔD/D0 against Cprotamine had a very tiny difference (with the slope of 15.5 ± 1.1 for Janus-QD-3, 12.5 ± 1.4 for PEG-QD-3, and the slope ratio of 1.24). In contrast to the above-mentioned PL and DLS assay, FA has exhibited the advantages in distinguishing the Janus-QD-3 and PEG-QD-3 in the interaction with protamine. As shown in Figure 4b, the significant decrease of FA value was observed with the continuous increasing of Cprotamine. In the plot of Δr/r0 against Cprotamine (Cprotamine in μM, Δr = r − r0, r and r0 were the FA values of QDs in the presence and absence of protamine), the three lines corresponding to the three QDs were distinguished well from each other (Figure 4b). It was worth nothing that among the three methods (PL, DLS, and FA), the FA assay was the only technique that could remarkably discriminate the ligand asymmetric Janus-QD-3 and symmetric PEG-QD-3 in interacting with protamine (the sensitivity of Janus-QD-3 over PEG-QD-3 was 1.60 times). The ligands and their distributions on the surface of QDs play an important role for their characteristic features, such as the dispersity, charge state, hydrodynamic diameter, PL lifetime, and interaction with other molecules. For MPA-QD, the relatively abundant MPA ligands and the small size of MPA led to the most negative charges and the smallest diameter of MPA-QD, and thus the most dramatic aggregation in the
Figure 5. Lifetime of MPA-QD, Janus-QD-3, and PEG-QD-3 in the absence and presence of protamine.
FA Assay of the Interaction of QDs with Protamine in Human Serum. To further demonstrate the superiority of FA assay in distinguishing the ligand-asymmetric and symmetric QDs, the interaction between the QDs and protamine in human serum was explored. Owing to the slower rotating of the QDs in the higher viscosity media, FA values of the three QDs were sharply increased in the serum samples (Table S4). However, the tendency of FA value against Cprotamine was very similar to that in PB buffer, and the three lines corresponding to the three QDs could also be remarkably discriminated (Figure 6). This result further announced the super capability of FA assay for discriminating the ligand-asymmetric and symmetric QDs.
Figure 6. FA assay of the interaction of the QDs (MPA-QD, JanusQD-3, and PEG-QD-3) with protamine in human serum buffered by PB (10 mM, pH 7.4). E
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CONCLUSION We presented here a novel noninvasive FA method for discriminating the ligand asymmetric and symmetric Mndoped ZnS QDs. The FA values of the QDs could remarkably discriminate the ligand-asymmetric Janus-QDs from the ligandsymmetric PEG-QDs. Besides, the FA assay also has superiority over the DLS and PL methods in discriminating the interaction of those QDs with protamine. This contribution offered a new method for the simple and noninvasive judgment of the asymmetric or symmetric distributions of the ligands on the photoluminescent nanoparticles.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b02614. Additional experimental data and results (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21435001, 21575070, and 21175073) and the Tianjin Natural Science Foundation (Grant No. 13JCYBJC17000).
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REFERENCES
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DOI: 10.1021/acs.analchem.6b02614 Anal. Chem. XXXX, XXX, XXX−XXX
