Highly Photostable and Two-Photon Active Quantum DotâPolymer

Subscriber access provided by Nottingham Trent University

Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Highly Photostable and Two-Photon Active Quantum Dot-Polymer Multicolor Hybrid Coacervate Droplets Tushar Kanti Mukherjee, and Jamuna K. Vaishnav Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01783 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Highly Photostable and Two-Photon Active Quantum Dot-Polymer Multicolor Hybrid Coacervate Droplets Jamuna K. Vaishnav and Tushar Kanti Mukherjee*

Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Khandwa Road, Indore- 453552, M.P., India.

*

Corresponding author. E-mail: [email protected]; Tel: +91-7324306779. 1 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

Abstract Fabrication and precise control of the physicochemical properties of multifunctional organicinorganic hybrid nanocomposites find great importance in various research fields. Herein, we report the fabrication of a new class of luminescent hybrid coacervate droplets from CdTe quantum dots (QDs) and poly(diallyldimethylammonium chloride) (PDADMAC) aqueous mixture. The colloidal stability of these droplets has been explored in a wide range of composition, pH, and ionic strength. While these hybrid droplets are quite stable in low ionic strength medium (6.5), they are unstable at higher ionic strength medium (>100 mM NaCl) and acidic pH (pH2.5 W, and repetition rate 80 MHz ± 1 MHz). PL lifetime decay traces were measured on a HORIBA Jobin Yvon picosecond time-correlated single photon counting (TCSPC) spectrometer (model Fluorocube-01-NL). The samples were excited at 445 nm by a picosecond diode laser (model Pico Brite-375L and DD-450L). The decay traces were collected using a photomultiplier tube (TBX-07C). The instrument response function was recorded using a Ludox

8 ACS Paragon Plus Environment

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

solution. The PL decay traces were analyzed using IBH DAS 6.0 software by the iterative reconvolution method, and the goodness of the fit was judged by reduced χ-square (χ2) value. The decays were fitted with a multiexponential function according to the earlier reported literature.31,32

Results and Discussion Characterization of MSA-Capped CdTe QDs The as-synthesized MSA-capped CdTe QDs are thoroughly characterized using UV-vis, PL, TEM, AFM, DLS, FTIR, XPS, and powder XRD. Figure 1A displays the excitonic absorption and PL spectra of synthesized QD. It is evident that the exciton band maximum appears at 490 nm. A sharp PL peak appears at 523 nm upon excitation with 445 nm wavelength.33 TEM and AFM measurements have been performed to estimate the morphology and average size of synthesized QDs. The TEM image shows the formation of spherical QDs with mean size of 2.2±0.2 nm (Figure 1B inset), which matches well with the mean size estimated from AFM height profile (Figure 1C).33 Notably, the calculated size using excitonic peak position is found to be 2.1 nm.30 The mean hydrodynamic size and ζ potential of synthesized QDs is 2.8±0.6 nm and -26.8±1.5 mV, respectively (Figure 1D). The surface functionalization of QDs with MSA ligand is established using FTIR spectroscopy (Figure 1E). The free MSA ligand shows a weak peak at 2563 cm-1, which can be attributed to the stretching frequency of the free S-H group. However, the FTIR spectrum of as-synthesized QDs shows no peak in this region, suggesting the formation of covalent bond between QD and MSA ligand through its free thiol group.28,33 Other noticeable peaks at 1638 and 1386 cm-1 can be attributed to the stretching vibration of carboxylate moieties and –C-H bending, respectively. To know the chemical composition of these MSA-capped CdTe QDs, we have performed XPS measurements.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

Figure 1. (A) Normalized absorption and PL spectra (λex = 445 nm) of green QDs. (B) TEM image of synthesized green QDs. The inset shows the size distribution histogram with mean size. (C) AFM image of green QDs. The inset shows the size distribution histogram with mean size estimated from height profile. (D) Hydrodynamic size distribution of green QD obtained from DLS measurement. The inset shows the ζ potential of green QD. (E) FTIR spectra of synthesized


Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

MSA-capped CdTe QD and free MSA ligand. (F) XPS survey spectrum of as-synthesized MSAcapped CdTe QDs along with expanded XPS spectra of Cd 3d and Te 3d. Figure 1F displays the XPS survey spectrum of as-synthesized CdTe QDs along with expanded spectra of Cd 3d and Te 3d. Survey spectrum reveals the presence of all essential elements such as cadmium (Cd), tellurium (Te), sulfur (S), carbon (C), and oxygen (O). Expanded spectrum of Cd 3d reveals the presence of 3d5/2 and 3d3/2 peaks at 402.2 and 409.1 eV, respectively. Similarly, the expanded spectrum of Te 3d shows the presence of 3d5/2 and 3d3/2 peaks at 569.8 and 580.2 eV, respectively. The expanded XPS spectra of C 1s, O 1s, and S 2p are shown in Figure S1. These QDs are further characterized using powder XRD (Figure S2). The XRD pattern shows three peaks at 2θ values of 24.9º, 42.0º, and 47.0º, corresponding to the (111), (220), and (311) crystalline planes, respectively. These data match with bulk cubic zinc blende CdTe structure (JCPDS No. 75-2086).34 Taken together, our results reveal the formation of well dispersed MSAcapped CdTe QDs. The relative PL QY of synthesized CdTe QD is 0.20.28

Initial Nanocomposite Formation between QD and PDADMAC Diluted aqueous solutions of QDs and PDADMAC were mixed in appropriate proportions to prepare different binary mixtures. The concentration of QD was kept fixed at 0.33 M, while that of PDADMAC monomer was varied in the range of 1.3-5000 M. All the binary mixtures were equilibrated for 12 h before any measurements. The binary mixtures at very low concentrations of PDADMAC (1.3-19.2 μM) remain isotropic. While the absorption peak position of QD remains unaltered upon addition of initial concentrations of PDADMAC, the absorption band gets broaden (Figure 2A).


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 30

Figure 2. (A) Absorpiton spectra, (B) PL spectra (λex = 445 nm), and (C) PL decay traces (λex = 445 nm; λem = 523 nm) of green QDs in the absence and presence of 5.2 μM PDADMAC. (D) SEM image of QD-PDADMAC solution in the C1 region. Confocal images (E) DIC, (F) fluorescence, and (C) merge of QD-PDADMAC binary mixture. In contrast, significant changes have been observed in the PL spectrum of QD in the presence of PDADMAC. The PL peak position of QD shifts from 523 to 531 nm upon addition of 5.2 μM of PDADMAC with 8 nm red shift (Figure 2B). Moreover, the PL intensity of QD quenches significantly in the presence of 5.2 μM of PDADMAC. Similar red-shift with decrease in PL intensity has been observed in the presence of different concentrations of PDADMAC in the range of 1.3-19.2 μM. In order to understand the mechanism behind this spectral changes, we have measured the PL lifetimes of QDs in the presence of different concentrations of PDADMAC. Figure 2C displays the PL lifetime decay curves of QDs (λex=445 nm; λem=523 nm) in the absence and presence of 5.2 μM of PDADMAC. In the absence of PDADMAC, the PL decay of QDs exhibits multiexponential decay kinetics and fits well with a three-exponential decay function 12 ACS Paragon Plus Environment

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(Table 1). The fitted decay of CdTe QD exhibits three lifetime components of 0.57 (19%), 2.60 (27%), and 14.0 ns (54%) with an average PL lifetime of 8.40 ns, similar to those reported recently.33 While the 2.60 ns component arrieses mainly due to the band edge exciton recombination, the 0.57 and 14.0 ns components arise due to the exciton recombination at the deep and shallow trap states, respectively. The fitted decay in the presence of 5.2 μM of PDADMAC displays three lifetime components of 0.07 (82%), 1.17 (6%), and 13.3 ns (13%) with an average PL lifetime of 1.82 ns (Table 1). Table 1. PL Lifetime Decay Parameters of Green QD in the Absence and Presence of PDADMAC

Samples

τ1 (ns)

a1

τ2 (ns)

a2

τ3 (ns)

a3

< τ > (ns)

χ2

Green QD

0.57

0.19

2.60

0.27

14.0

0.54

8.40

1.06

+ 5.2 μM PDADMAC

0.07

0.82

1.17

0.06

13.3

0.13

1.82

1.12

Here it is important to note that the relative amplitude of the deep trap state component increases remarkably from 19 to 82% in the presence of 5.2 μM PDADMAC. However, the relative amplitudes of the band edge and shallow trap states components decrease appreciably. The observed changes in relative amplitudes in the presence of PDADMAC strongly signify modification of the surface related trap states due to ligand reorganization at the surface of QDs.33 On the other hand, the observed decrease in PL lifetime of QDs in the presence of PDADMAC can be explained by considering exciton-exciton annihilation between closelypacked QDs in QD-PDADMAC nanocomposites. The FESEM image of the isotropic binary mixture at 5.2 μM of PDADMAC (C1) confirms the formation of QD-PDADMAC nanocomposites (Figure 2D). Control experiment with neutral polyvinylpyrrolidone (PVP) reveals no such changes in the PL behavior of QDs in the binary mixture, suggesting that the 13 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

initial nanocomposite formation is mainly driven by long-range electrostatic attraction between oppositely charged QD-PDADMAC pair (Figure S3). CLSM has been performed to visualized the initial QD-PDADMAC nanocomposites (C1 region). Figure 2E shows the differential interference contrast (DIC) image of the QD-PDADMAC nanocomposites prepared in the presence of 5.2 μM of PDADMAC. Distinct clusters of QDs in the PDADMAC matrix is clearly visible, which is consistent with our earlier SEM results. Confocal fluorescence image reveals green color luminescence from these polymer encapsulated clusters of QDs (Figure 2F). The merge image clearly indicates that the emission originates solely from the clusters of QDs inside QD-PDADMAC nanocomposites (Figure 2G). Coacervation and Hybrid Droplet Formation It has been observed that the phase behavior of the binary mixture changes from turbid (T1) to precipitate (P), and turbid (T2) upon further increase in the concentrations of PDADMAC in the range of 22.4-160.0 M (Figure 3A). These observations clearly indicate the formation of supramolecular nanostructures and their continuous structural rearrangements in solution. The phase behavior of the binary mixutre was explored using turbidity measurements. Figure 3B displays the turbidity profile of binary mixtures in the concentration range of 5.0-5000.0 M of PDADMAC. For the present study, we have mainly focused on the two turbid regions (T1 and T2) of the binary mixture in the presence of 25.6 and 130.0 M of PDADMAC, respectively. Notably, the binary mixtures in the T2 region are more turbid compared to those in T1 region (Figure 3B). FESEM image in the T1 region reveals distinct spherical self-assembled droplets with mean size of 383.9±17.9 nm (Figure 3C and S4A). Relatively larger-sized droplets with mean size of 590.8±23.8 nm are observed in the T2 region (Figure 3D and S4B), which account for its higher turbidity compared to T1 region. Notably, droplets in both the regions (T1 and T2) show characteristic coalescence phenomenon (Figure S5).


Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 3. (A) Photographs of QD-PDADMAC binary solution in the presence of 25.5 (T1), 52.0 (P), and 130 μM (T2) of PDADMAC. (B) Plot of turbidity against PDADMAC monomer molar concentrations. FESEM images of the binary mixtures in the (C) T1, and (D) T2 regions of the phase profile. (E) Hydrodynamic size distribution of droplets in the T1 and T2 regions. (F) ξ potentials of droplets in the T1 and T2 regions. The estimated mean hydrodynamic size of these droplets is 405 and 820 nm for T1 and T2 region, respectively (Figure 3E). In order to know the surface charge of these droplets in the T1 and T2 regions, we have measured their ζ potential. Our results reveal the presence of negatively and positively charged coacervate droplets in the T1 and T2 region, respectively. The estimated ζ potentials are -5.2 and +8.1 mV in the T1 and T2 regions, respectively (Figure 3F). 15 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

Although droplet formation is a common phenomenon for oppositely charged aqueous polyelectrolytes,12-20 observation of this phenomenon in QD-PDADMAC binary mixture is quite unique. This can be explained by considering electrostatic interactions between QD-PDADMAC nanocomposites and positively charged PDADMAC. Here negatively charged MSA-capped QDs are sandwiched between two positively charged PDADMAC chains (Scheme 2). The droplets in the T1 region show negative ζ-values due to the presence of uncomplexed free MSA ligands on QDs, whereas droplets in the T2 region contain excess of positively charged PDADMAC. Moreover, stoichiometric charge neutralization in the presence of 51.2-64.0 μM of PDADMAC accounts for the observed precipitation in the binary mixture.33 Scheme 2. Illustration of the Droplet Formation in the T1 and T2 Regions via QDPDADMAC Nanocomposite Formation


Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

In order to explore whether our organic-inorganic hybrid droplets can sequester foreign molecules as previously observed for organic coacervates,12-20 we investigated the sequestration of different solutes such as fluorescein isothiocyanate (2.0 μM), Rhodamine 6G (20 μM), Nile blue (1.0 μM), human serum albumin (59.0 μM), and bovine serum albumin (47.0 μM) by positively charged hybrid droplets (T2 region) using UV-vis spectroscopy (see Supporting Information). The estimated equilibrium partition coefficients (K) for different solutes are listed in Table S1. Significantly high value of K (137±25) has been observed for Nile blue dye, possibly due to its low water solubility (0.16 mg/mL). Moreover, the estimated K value for human and bovine serum albumins is 34±6 and 35±7, respectively. These high values of K clearly indicate preferential sequestration of different molecules irrespective of their charges, which is similar to those observed earlier for other organic coacervate droplets.12-16 More importantly, our findings indicate that both hydrophobic and electrostatic interactions are responsible for the observed sequestrations. Colloidal Stability as a Function of pH and Ionic Strength The structural and colloidal stability of these droplets have been explored as a function of pH and ionic strength of the medium. While the binary mixtures in the T1 and T2 regions remain turbid in the pH range of 6.0-10.0, they become isotropic at lower acidic pH (3.0-5.5) (Figure 4A). The plots of turbidity against pH of the medium for T1 and T2 regions are sigmoidal in nature and show a critical coacervation pH of 6.0. FESEM image of binary mixture in the T1 region clearly reveals the disassembled polymeric structures at pH 3 (Figure 4A, inset). Notably, these droplets are found to be stable even at pH 11.7 (Figure S6). The observed disassembly at lower pH is due to the protonation of carboxylate groups of MSA ligands (pKa~ 4.19 and 5.64) at the surface of QD, which weakens the electrostatic interactions with positively charged PDADMAC.33 This pH responsive assembly and disassembly makes them suitable candidates for in vivo sensing and drug delivery applications. Similarly, ionic strength of the medium has a profound effect on the present 17 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

coacervation process. The turbidity of the binary mixtures in the T1 and T2 region decreases slowly with increasing the ionic strength of the medium at lower concentrations of NaCl, followed by a steady decrease at higher NaCl concentrations indicating efficient screening of the electrostatic attraction between QD and PDADMAC (Figure 4B). The plot reveals a critical salt concentration of 120 mM for the present coacervation process.

Figure 4. (A) Changes in the turbidity of the QD-PDADMAC mixture as a function of solution pH in the T1 and T2 regions. The inset shows the FESEM image of the T1 region at pH 3. (B) Changes in the turbidity of the QD-PDADMAC mixture as a function of NaCl concentrations in the T1 and T2 regions. PL Behavior of Hybrid Droplets These hybrid droplets are expected to be inherently luminescent due to the presence of CdTe QDs. Next, we explored the luminescence properties of these hybrid droplets using confocal and epifluorescence microscopy. Figure 5 displays the CLSM images of hybrid coacervate droplets in the T1 and T2 regions of the binary mixture. As expected, distinct green color luminescent droplets have been observed in the T1 and T2 region of the binary mixture (Figure 5A,B). The 18 ACS Paragon Plus Environment

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

merge images show that the green luminescence originates exclusively from the droplets. In order to illustrate that the present approach is suitable for fabrication of multicolor luminescent droplets, we synthesized a red-emitting MSA-capped CdTe QDs (λem= 650 nm) and mixed it with aqueous PDADMAC solution (Figure S7A,C). The mean size of the red-emitting QDs is 3.9±0.2 nm (Figure S7B). The estimated ζ potential of red-emitting QDs is -10.9±0.5 mV. The confocal images of the turbid binary mixture (T1 region) of red-emitting QD and PDADMAC reveal the formation of well-dispersed droplets with distinct red luminescence (Figure 5C). To visualize the coalescence behavior of these hybrid droplets, we performed mixing experiments with green and red color coacervate droplets prepared separately either from green or red QDs, respectively (Figure 5D).

Figure 5. Confocal images (DIC, fluorescence, and merge) of coacervate droplets prepared with green QDs in the (A) T1, and (B) T2 region along with droplets prepared from red QDs in the (C) T1 region. (D) Confocal image showing coalescence of green and red color coacervate droplets prepared separately from green and red color QDs with PDADMAC aqueous solution. Yellow color apears due to the coalescence of green and red color droplets.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

The mixture was equilibrated for 12 h before confocal imaging. It is evident that green and red color droplets undergo spontaneous coalescence to yield yellow color mixed droplets along with individual populations of green and red color droplets (Figure 5D). These observations clearly indicate that coalescence is a spontaneous and characteristic phenomenon for mixed populations of these hybrid droplets and may find tremendous importance in various enzymatic and catalytic reactions. More specifically, the characteristic coalescence phenomenon of these membrane-free luminescent droplets can be utilized in various chemical, enzymatic, and nanocatalysis reactions as well as for studying interactions between diverse encapsulated materials. Moreover, our findings reveal that the present approach can be easily extended for the fabrication of multicolor luminescent droplets for various bioimaging and sensing applications. Here it is important to mention that earlier reports on coacervate droplets have utilized external organic fluorophores such as fluorescein, cyanine 5, and green fluorescence protein for visualization under the fluorescence microscope, which is time consuming and inefficient.9,12,13 In contrast, our hybrid droplets are unique in the sense that they are inherently luminescent and do not require external fluorophore labelling. However, a common limiting factor for any bioimaging applications is the photostability of fluorophores. To explore the photostability of these hybrid droplets, we performed epifluorescence video microscopy in the T1 and T2 region of the binary mixture (see Supporting Information). Epifluorescence images reveal localized bright luminescent spots from coacervate droplets (Figure 6A,B), similar to those observed in confocal images. We analyzed the intensity trajectories of individual droplets and compared with those of bare QDs on PVA coated coverslip. Individual QD on PVA coated coverslip shows PL blinking or intermittence with single step photobleaching, which is a characteristic phenomenon of single emitters (Figure 6C).33 In contrast, intensity trajectories of luminescent droplets in both T1 and T2 region reveal absence of any PL blinking (Figure 6C). This is expected as these droplets are composed of several QDs 20 ACS Paragon Plus Environment

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

which are electrostatically associated with the polymer networks. More importantly, these luminescent droplets are extremely photostable and don’t show any photobleaching even after several minutes (observation time ~7 minutes) of laser exposure.

Figure 6. Epifluorescence images of droplets in the (A) T1, and (B) T2 regions. (C) Representative intensity trajectories of individual coacervate drolpets in the T1 and T2 regions of the binary mixture along with green QD. This unusual photostability arises likely due to the efficient capping of QDs by the polymeric chains of PDADMAC inside these droplets. The slight fluctuations observed in the intensity profiles of droplets arise due to the PL blinking of QDs (individual/cluster) inside the coacervate droplets (Movie S1). The present finding of photobleaching resistant and nonblinking


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

luminescence from coacervate droplets is unprecedented in the literature and may have significant impact on various bioimaging applications. Fabrication of Mixed Two-Color and Two-Photon Active Hybrid Droplets With the aim towards designing new multicolor and multifunctional hybrid materials, we fabricated mixed two-color luminescent droplets by simultaneously mixing (1:1) green and red emitting QDs with PDADMAC aqueous solution (Figure 7A). These droplets were characterized using confocal microscopy with appropriate green (490-550 nm) and red (570-670 nm) emission filter sets. Phase contrast image of this binary mixture (T1 region) confirms the formation of coacervate droplets (Figure 7B), similar to those observed earlier. Confocal fluorescence images in the green and red channels clearly reveal the appearance of both green and red luminescence from these mixed droplets, respectively (Figure 7B).


Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 7. (A) Illustration of the mixed two-color droplet formation from green and red QDs mixture. (B) Confocal images in the green and red channels along with their DIC and merge images showing the formation of mixed two-color droplets from green and red QDs. (C) Schematic representation of the 2P excitation and emission of mixed two-color droplets upon 800 nm laser irradiation. (D) 2P confocal images of mixed two-color droplets in the green (490-550 nm) and red (570-670 nm) channels along with their merge image. More importantly, the merged image of the green and red channels reveals distinct yellow color luminescent droplets, which unambiguously confirms the presence of both green and red QDs in each coacervate droplet (Figure 7B). Notably, absence of any individual population of green or red color luminescent droplets indicates that the initial QD-PDADMAC nanocomposite formation is purely electrostatic in nature and does not depends on the nature of QDs. The present strategy for the fabrication of mixed two-color hybrid droplets can be extended further with different combinations of inorganic nanoparticles (NPs) for a wide range of applications. Finally, we explored the applicability of these hybrid droplets in two-photon (2P) PL imaging using 2P confocal microscopy. Importantly, 2P excitation provides high signal to noise ratio with enhanced penetration depth into biological specimens, which significantly reduces the phototoxicity.35 These properties find particular importance in tissue engineering and in vivo bioimaging and sensing applications. Two-photon imaging experiments were performed using two-color hybrid droplets prepared using 1:1 mixture of green and red-emitting QDs with PDADMAC aqueous solution (Figure 7C). Samples were excited with a 800 nm pulsed laser and images were collected in the green and red channels separately. 2P confocal images reveal both green and red luminescence from mixed two-color droplets with intense two-photon yellow luminescence and very low background noise (Figure 7D). These findings clearly highlight the importance of these droplets as two-photon imaging probes for various bioimaging applications.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

Conclusions In summary, we present a facile approach for the fabrication of highly photostable and 2P active organic-inorganic hybrid coacervate droplets in aqueous medium. Our findings demonstrate that the self-assembly process is triggered by the formation of initial QD-PDADMAC nanocomposites, which subsequently associate with positively charged PDADMAC to form spherical coacervate droplets in aqueous medium. These hybrids droplets are found to be stable in a broad range of composition, pH, and ionic strength of the medium. The strategy for the fabrication of mixed two-color luminescent droplets presented here may open up a new avenue for the designing of other multifunctional hybrid materials for nanocatalysis and nanosensing applications. Moreover, the unique luminescence properties such as high photostability and multicolor 2P active luminescence observed for these droplets may have profound implications in various applications such as bioimaging, sensing, and light-emitting devices. Further investigations are underway to explore the inherent physicochemical properties of these hybrid luminescent droplets for various applications. Associated Content Supporting information Expanded XPS spectra of C 1s, O 1s, and S 2p, Powder XRD of CdTe QDs, Change in the PL intensity ratio of green QD in the presence of different concentrations of PVP, Size distribution histograms estimated from FESEM images of droplets in the T1, and T2 regions, FESEM images of droplets in the T1, and T2 regions showing coalescence phenomenon, FESEM image of droplets at pH 11.7, AFM image and cross section analysis of synthesized red QD along with normalized absorption and PL (λex = 445 nm) spectra of red QDs. Notes The authors declare no competing financial interest.


Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Acknowledgments Authors acknowledge IIT Indore for providing the infrastructure, experimental facilities, and financial support. Authors thank SIC, IIT Indore for instrumental facilities. Authors also thank Prof. Anindya Datta and Dr. Soumyadipta Rakshit from IIT Bombay for DLS measurements. Authors thank IISER Pune for TEM measurement. Authors acknowledge Central Surface Analytical Facility (ESCA), Department of Physics, IIT Bombay for XPS measurements.

References (1) Liu, Y.; He, J.; Yang, K.; Yi, C.; Liu, Y.; Nie, L.; Khashab, N. M.; Chen, X.; Nie, Z. Folding Up of Gold Nanoparticle Strings into Plasmonic Vesicles for Enhanced Photoacoustic Imaging. Angew. Chem. Int. Ed. 2015, 54, 15809-15812. (2) Discher, B. M.; Won, Y.-Y.; Ege, D. S.; Lee, J. C-M.; Bates, F. S.; Discher, D. E.; Hammer, D. A. Polymersomes: Tough Vesicles Made from Diblock Copolymers. Science 1999, 284, 1143-1146. (3) Yuk, H.; Lu, B.; Zhao, X. Hydrogel Bioelectronics Chem. Soc. Rev. 2019, 48, 1642-1667. (4) Yan, X.; Delgado, M.; Fu, A.; Alcouffe, P.; Gouin, S. G.; Fleury, E.; Katz, J. L.; Ganachaud, F.; Bernard, J. Simple but Precise Engineering of Functional Nanocapsules through Nanoprecipitation. Angew. Chem. Int. Ed. 2014, 53, 6910-6913. (5) Keating, C. D. Aqueous Phase Separation as a Possible Route to Compartmentalization of Biological Molecules. Acc. Chem. Res. 2012, 45, 2114-2124. (6) Zhang, H.; Liu, Y.; Yao, D.; Yang, B. Hybridization of Inorganic Nanoparticles and Polymers to Create Regular and Reversible Self-Assembly Architectures. Chem. Soc. Rev. 2012, 41, 6066-6088. (7) Li, Y.; Wang, Y.; Huang, G.; Gao, J. Cooperativity Principles in Self-Assembled Nanomedicine. Chem. Rev. 2018, 118, 5359-5391.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

(8) Crosby, J.; Treadwell, T.; Hammerton, M.; Vasilakis, K.; Crump, M. P.; Williams, D. S.; Mann, S. Stabilization and Enhanced Reactivity of Actinorhodin Polyketide Synthase Minimal Complex in Polymer-Nucleotide Coacervate Droplets. Chem. Commun. 2012, 48, 11832-11834. (9) Mason, A. F.; Buddingh, B. C.; Williams, D. S.; van Hest, J. C. M. Hierarchical SelfAssembly of a Copolymer-Stabilized Coacervate Protocell. J. Am. Chem. Soc. 2017, 139, 17309-17312. (10) Drobot, B.; Iglesias-Artola, J. M.; Vay, K. L.; Mayr, V.; Kar, M.; Kreysing, M.; Mutschler, H.; Tang, T.-Y. D. Compartmentalised RNA Catalysis in Membrane-Free Coacervate Protocells. Nat. Commun. 2018, 9, 3643. (11) Strulson, C. A.; Molden, R. C.; Keating, C. D.; Bevilacqua, P. C. RNA Catalysis through Compartmentalization. Nat. Chem. 2012, 4, 941-946. (12) Douliez, J.-P.; Martin, N.; Gaillard, C.; Beneyton, T.; Baret, J.-C.; Mann, S.; Beven, L. Catanionic Coacervate Droplets as a Surfactant-Based Membrane-Free Protocell Model. Angew. Chem. Int. Ed. 2017, 56, 13689-13693. (13) van Swaay, D.; Tang, T.-Y. D.; Mann, S.; de Mello, A. Microfluidic Formation of Membrane-Free Aqueous Coacervate Droplets in Water. Angew. Chem. Int. Ed. 2015, 54, 8398-8401. (14) Williams, D. S.; Koga, S.; Hak, C. R. C.; Majrekar, A.; Patil, A. J.; Perriman, A. W.; Mann, S. Polymer/Nucleotide Droplets as Bio-Inspired Functional Micro-Compartments. Soft Matter 2012, 8, 6004-6014. (15) Koga, S.; Williams, D. S.; Perriman, A. W.; Mann, S. Peptide-Nucleotide Microdroplets as a Step towards a Membrane-Free Protocell Model. Nat. Chem. 2011, 3, 720-724. (16) Martin, N.; Li, M.; Mann, S. Selective Uptake and Refolding of Globular Proteins in Coacervate Microdroplets. Langmuir 2016, 32, 5881-5889.


Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(17) Black, K. A.; Priftis, D.; Perry, S. L.; Yip, J.; Byun, W. Y.; Tirrell, M. Protein Encapsulation via Polypeptide Complex Coacervation. ACS Macro Lett. 2014, 3, 10881091. (18) Priftis, D.; Leon, L.; Song, Z.; Perry, S. L.; Margossian, K. O.; Tropnikova, A.; Cheng, J.; Tirrell, M. Self-Assembly of α-Helical Polypeptides Driven by Complex Coacervation. Angew. Chem. Int. Ed. 2015, 54, 11128-11132. (19) Priftis, D.; Xia, X.; Margossian, K. O.; Perry, S. L.; Leon, L.; Qin, J.; de Pablo, J. J.; Tirrell, M. Ternary, Tunable Polyelectrolyte Complex Fluids Driven by Complex Coacervation. Macromolecules 2014, 47, 3076-3085. (20) Priftis, D.; Tirrell, M. Phase Behaviour and Complex Coacervation of Aqueous Polypeptide Solutions. Soft Matter 2012, 8, 9396-9405. (21) Cruz, M. A.; Morris, D. L.; Swanson, J. P.; Kundu, M.; Mankoci, S. G.; Leeper, T. C.; Joy, A. Efficient Protein Encapsulation within Thermoresponsive Coacervate-Forming Biodegradable Polyesters. ACS Macro Lett. 2018, 7, 477-481. (22) Garenne, D.; Beven, L.; Navailles, L.; Nallet, F.; Dufourc, E. J.; Douliez, J. P. Sequestration of Proteins by Fatty Acid Coacervates for Their Encapsulation within Vesicles. Angew. Chem. Int. Ed. 2016, 55, 13475-13479. (23) Kim, S.; Huang, J.; Lee, Y.; Dutta, S.; Yoo, H. Y.; Jung, Y. M.; Jho, Y. S.; Zeng, H.; Hwang, D. S. Complexation and Coacervation of Like-Charged Polyelectrolytes Inspired by Mussels. Proc. Natl. Acad. Sci. USA 2016, 113, E847-E853. (24) Long, M. S.; Jones, C. D.; Helfrich, M. R.; Mangeney-Slavin, L. K.; Keating, C. D. Dynamic Microcompartmentation in Synthetic Cells. Proc. Natl. Acad. Sci. USA 2005, 102, 5920-5925. (25) Deng, N.-N.; Huck, W. T. S. Microfluidic Formation of Monodisperse Coacervate Organelles in Liposomes. Angew. Chem. Int. Ed. 2017, 56, 9736-9740.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

(26) Dewey, D. C.; Strulson, C. A.; Cacace, D. N.; Bevilacqua, P. C.; Keating, C. D. Bioreactor Droplets from Liposome-Stabilized All-Aqueous Emulsions. Nat. Commun. 2014, 5, 4670. (27) Tang, T.-Y. D.; Hak, C. R. C.; Thompson, A. J.; Kuimova, M. K.; Williams, D. S.; Perriman, A. W.; Mann, S. Fatty Acid Membrane Assembly on Coacervate Microdroplets as a Step towards a Hybrid Protocell Model. Nat. Chem. 2014, 6, 527-533. (28) Vaishnav, J. K.; Mukherjee, T. K. Long-Range Resonance Coupling-Induced Surface Energy Transfer from CdTe Quantum Dot to Plasmonic Nanoparticle. J. Phys. Chem. C 2018, 122, 28324-28336. (29) Zhang, L.; Zou, X.; Ying, E.; Dong, S. Quantum Dot Electrochemiluminescence in Aqueous Solution at Lower Potential and its Sensing Application. J. Phys. Chem. C 2008, 112, 4451-4454. (30) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem. Mater. 2003, 15, 2854-2860. (31) Chatterjee, S.; Mukherjee, T. K. Thermal Luminescence Quenching of AmineFunctionalized Silicon Quantum Dots: A pH and Wavelength-Dependent Study. Phys. Chem. Chem. Phys. 2015, 17, 24078-24085. (32) Chatterjee, S.; Mukherjee, T. K. Size-Dependent Differential Interaction of AllylamineCapped

Silicon

Quantum

Dots

with

Surfactant

Assemblies

Studied

using

Photoluminescence Spectroscopy and Imaging Technique. J. Phys. Chem. C 2013, 117, 10799-10808. (33) Vaishnav, J. K.; Mukherjee, T. K. Surfactant-Induced Self-Assembly of CdTe Quantum Dots into Multicolor Luminescent Hybrid Vesicles. Langmuir 2019, 35, 6409-6420.


Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(34) Tenório, D. P. L. A.; Andrade, C. G.; Filho, P. E. C.; Sabino, C. P.; Kato, I. T.; Jr. Carvalho, L. B.; Jr. Alves, S.; Ribeiro, M. S.; Fontes, A.; Santos, B. S. CdTe Quantum Dots Conjugated to Concanavalin A as Potential Fluorescent Molecular Probes for Saccharides Detection in Candida Albicans. J. Photochem. Photobiol. B 2015, 142, 237-243. (35) Kim, H. M.; Cho, B. R. Small-Molecule Two-Photon Probes for Bioimaging Applications. Chem. Rev. 2015, 115, 5014-5055.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

“TOC Graphic”


Highly Photostable and Two-Photon Active Quantum DotâPolymer

Recommend Documents

Highly Photostable and Two-Photon Active Quantum DotâPolymer