Colour-tunable quantum dots/poly(NIPAM-co-AAc) hybrid microgels based on electrostatic interactions

Abstract

Monodisperse poly(NIPAM-co-AAc) microgels doped with either a monocolour photoluminescent quantum dot (QD) or multiple colour QDs have been fabricated by the charge-driven introduction of cysteamine-capped CdTe QDs and the collapse of the polymer network by adjusting the pH of the solution. The photoluminescence intensity and colour of the resulting microgels can be easily adjusted by changing the content of the CA–CdTe QDs in the hybrid microgels and using single or a mixture of different-sized QDs, respectively. In addition, the microgel-encapsulated QDs showed blue shifts, owing to surface coordination by the carboxylic acid. The combined effect of electrostatic interactions and surface chelating resulted in a reduction in microgel size. By tuning the ratio of QDs with different sizes, the photoluminescence of the hybrid hydrogels could be conveniently controlled. This work provides a facile and versatile method to fabricate colour-tunable or multifunctional hybrid hydrogels with broad potential applications.

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1. Introduction

Hybrid quantum dots (QDs)/microgels have great potential in diagnostics, bioimaging, photonics, optoelectronics, catalysis and sensors.¹ A number of methods have been developed to fabricate such hybrid materials. For example, microgels have been utilized as templates for the in situ synthesis of QDs, which enables flexible control of the QD load. However, the obtained QDs in microgels are usually polycrystalline and show broad size distributions, which results in poor control of the photoluminescence of the hybrid microgels.^2–4 In another method, microgels are loaded with pre-formed QDs through specific interactions including electrostatic interactions,^5–7 hydrogen bonding,⁸ hydrophobic forces,^9–11 covalent bonds,^12–14 entanglements,^15,16 or ligand exchange between QDs and microgels.¹⁷ Thus, the QD size and optical properties are independently controlled before being loaded into microgels.

Responsive microgels have been used to create hybrids with controlled fluorescent properties. For example, Wang et al.¹⁶ demonstrated the use of pH-sensitive poly(N-isopropylacrylamide-co-4-vinylpyridine) microgels to uptake thioglycolic acid-capped CdTe QDs. The QDs are absorbed in the swollen microgels at low pH. Afterward, the microgels collapse upon a pH increase to above the pK_a of 4-vinylpyridine groups, 5.39, and thus trap the QDs. During this process, the physical entanglement of the collapsed network and electrostatic interactions between the loaded QDs and the microgel network play an important role in the entrapment of QDs in the microgels. However, electrostatic interactions here exist only in a relatively narrow pH range from 3.53,¹⁸ pK_a of carboxylic acid from thioglycolic acid-capped CdTe QDs, to 5.39, and QDs are prone to leak out of microgels at pH beyond this range. The narrow pH range is mainly determined by the relatively small pK_a of 4-vinylpyridine group. Thus, introducing other groups with a large pK_a in microgels instead of pyridine groups is proposed to improve the stability of colour-tunable hybrid microgels.

In this paper, we demonstrate the use of poly(NIPAM-co-AAc) microgels to absorb and stabilize cysteamine-capped CdTe QDs (CA–CdTe) with different sizes and fluorescent properties, resulting in colour-tunable hybrid microgels (Fig. 1). Herein, acrylic acid (AAc) monomers are used to synthesize the copolymer microgels because it is negatively charged at pH above pK_a of AAc, ∼4.25,¹⁹ and becomes neutral below pK_a. On the other hand, the CA–CdTe QDs carry positive charges at pH lower than the pK_a (8.3) of primary amine from CA.²⁰ Such a pH-responsive behaviour allows for a convenient control of the charges and swelling behaviour of the microgels, which will be used to encapsulate and stabilize the CA–CdTe QDs based on electrostatic interactions. As to be demonstrated in the text, the QD encapsulation method is versatile for different QDs, and allows for fabrication of colour-tunable microgels by incorporating QDs with different sizes and fluorescent properties.

Fig. 1 Synthetic scheme for CdTe/poly(NIPAM-co-AAc) hybrid microgels and illustration of the interactions between CA–CdTe QDs and microgels.

2. Experimental section

2.1. Materials

Cadmium chloride (CdCl₂·2.5H₂O), tellurium powder (100 mesh, 99.999%), sodium borohydride (NaBH₄, 96%), ammonium persulfate (APS) were purchased from Sinopharm Chemical Reagent Shanghai Co. Ltd. Cysteamine hydrochloride (CA, 98%), N-isopropylacrylamide (NIPAM, 98%), N,N′-methylenebis-(acrylamide) (BIS, 95%), acrylic acid (AAc, >99%) were purchased from Aladdin Reagent Company. NIPAM was purified by recrystallization from a toluene/hexane mixture (50/50, v/v). AAc was purified by distillation under reduced pressure to remove hydroquinone inhibitor before use. Deionized water was used for the synthesis and characterizations.

2.2. Preparation of sodium hydrogen telluride (NaHTe)

Sodium borohydride was used to react with tellurium with a molar ratio of 5 : 1 in water to produce sodium hydrogen telluride (NaHTe).²¹ Briefly, 2 mL of deionized water was transferred to a small flask; then 51.6 mg of tellurium powder was added. After 74.6 mg of sodium borohydride was added in the flask, the reacting system was cooled by ice. During the reaction, a small outlet connected to the flask was kept open to discharge the pressure from the resulted hydrogen. After approximately 8 h, the black tellurium powder disappeared and white sodium tetraborate precipitation appeared on the bottom of the flask instead. The resulting NaHTe in clear supernatant was used for the preparation of CA–CdTe QDs.

2.3. Synthesis of CA–CdTe QDs

Water-soluble CA–CdTe QDs were synthesized according to a previously published method.²² A series of aqueous colloidal CA–CdTe QD solutions were prepared by adding 1 mL of fresh NaHTe solution to 100 mL of 4 mmol L⁻¹ N₂-saturated CdCl₂ solutions at pH = 4.8 in the presence of CA as a stabilizing agent. The molar ratio of Cd²⁺/stabilizer/HTe⁻ was fixed at 1 : 2.4 : 0.5. The resulting mixture was refluxed to control the growth of CdTe QDs. The sizes of QDs mentioned in the text was calculated by using Peng's empirical equation and used to define QDs.

2.4. Synthesis of poly(NIPAM-co-AAc) microgels

Poly(NIPAM-co-AAc) microgels were synthesized by using a modified route.²³ Briefly, 1.5 g of NIPAM, 0.03 g of BIS, and 0.15 mL of AAc were dissolved in 45 mL of filtered, deionized water within a three-neck flask equipped with a mechanical stirrer and purged with nitrogen for 1 h. Thirty milliliter of this solution was then transferred into a dropping funnel of constant pressure. Fifteen milliliter of water and 0.1 g of NIPAM were added into the rest 15 mL of solution in the flask, and the liquid was heated to 70 °C with nitrogen purge. The polymerization was initiated by the addition of 0.02 g of APS dissolved in 2 mL of water. The solution started to become turbid in 2 min, and then was fed with the solution in the dropping funnel of constant pressure at 1 mL min⁻¹. After all solution was added, the reaction was continued for another 4 h before being cooled down rapidly in an ice bath. After cooling, the microgels were purified by at least three cycles of ultracentrifugation, decantation, and redispersion in deionized water.

2.5. Encapsulation of CA–CdTe QDs in poly(NIPAM-co-AAc) microgels

A series of samples was prepared according to the recipes in Table 1. Briefly, at room temperature, 25 mL of dispersion of microgels (the solid content is 0.7 mg mL⁻¹) were mixed with a certain amount of CA–CdTe QDs aqueous solution at pH = 5–8 under stirring. After incubation for 10 min, the pH of the mixed solution was adjusted to 4 by adding sulfuric acid solution (pH = 2). Then, the solution was centrifuged. The unloaded CA–CdTe QDs were removed and microgels loaded with CA–CdTe QDs were obtained in sulfuric acid solution of pH 4, denoted as CdTe/poly(NIPAM-co-AAc), with a solid content of 0.9 mg mL⁻¹.

Table 1 Recipes for the synthesis of hybrid microgels

Codes^a	CdTe^b (2.5 nm)/mL	CdTe^b (2.7 nm)/mL	CdTe^b (3.2 nm)/mL	Microgels^b/mL
a HM means hybrid microgels.b The concentration of CA–CdTe QDs and microgels used here is 4 mmol L^−1 (referred to as Cd^2+ elsewhere in this article) and 0.7 mg mL^−1, respectively.
HM4-0-0	4	0	0	25
HM3-0-1	3	0	1	25
HM2-0-2	2	0	2	25
HM0-0-4	0	0	4	25
HM0-2.5-0	0	2.50	0	25
HM0-3.25-0	0	3.25	0	25
HM0-4-0	0	4.00	0	25

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2.6. Characterizations

The chemical structure of CA–CdTe QDs was characterized by using a Nicolet 6700 FT-IR spectrometer (Thermo Fisher Scientific) by scanning from 400 to 4000 cm⁻¹ at a resolution of 4 cm⁻¹. The crystal structure of QDs was characterized by using a D8 Advance X-ray diffractometer (Bruker AXS, Germany). Diluted hybrid microgel dispersions were dropped onto carbon coated copper grids and dried at room temperature for transmission electron microscopy (TEM) imaging by using a 2100 transmission electron microscope (JEOL, Japan) at 200 kV.

The energy-dispersive X-ray (EDX) was used to investigate the element compositions of CdTe/poly(NIPAM-co-AAc) microgels. The zeta potential of CA–CdTe QDs solution, and the hydrodynamic diameters (D_H) of pure and hybrid microgels dispersed in pH = 4 solutions were measured by using a Zetasizer Nano ZS dynamic light scattering (DLS) instrument (Malvern, UK) at 25 °C. The UV-Vis absorption and photoluminescent (PL) spectra were obtained by using a Lambda 950 UV-Vis spectrometer (UV-Vis) (Perkin-Elmer, USA) and a Hitachi F-4600 Fluorescence Spectrophotometer, respectively.

3. Results and discussion

3.1. Synthesis and characterization of CA–CdTe QDs

Fig. 2 shows the UV-Vis absorption and photoluminescent (PL) spectra of the obtained CA–CdTe QDs synthesized with different reflux times. Over the reflux time, the absorbance peak shifted from 507 nm to 548 nm, which indicates the growth of CA–CdTe QDs. The corresponding Tauc plots of (αhυ)² versus hυ are shown in Fig. 2B. The estimated optical band gap decreases from 2.84 eV to 2.69 eV with increasing reaction time. On the other hand, the emission peak of CA–CdTe QDs was gradually changed from 560 nm to 605 nm (Fig. 2C), with the fluorescent colour under a UV lamp changing from green (560 nm) to orange (605 nm) due to the quantum confinement effect (Fig. 2D). Moreover, the PL intensity was increased gradually owing to the decrease of defects of CA–CdTe QDs. The estimated mean particle size is from 2.5 nm to 3.2 nm according to calculations based on the absorption peak (Fig. 2A) by using Peng's empirical equation.²⁴ These values are very close to those measured by high-resolution transmission electron microscopy (HRTEM) (Fig. 3). For example, the CA–CdTe QDs prepared with refluxing for 8 h showed an average size about 3.4 nm, which is close to the calculated value (3.2 nm). HRTEM images also reveal that most of QDs have monodisperse morphology with crystalline lattice structure.

Fig. 2 The UV absorption spectra (A), Tauc plots (B) and photoluminescent (PL) emission (C) of CdTe QDs prepared with refluxing for (a → i): 10, 20, 30, 60, 120, 180, 300, 420 and 480 min. (D) Corresponding pictures of CdTe QDs excited with a UV lamp.

Fig. 3 HRTEM images (A) and an enlarged view (B) of CdTe QDs with refluxing for 8 h.

The crystal size of QDs can also be calculated from the XRD data. For example, the powders of CA–CdTe QDs obtained with refluxing for 60 min showed characteristic diffractions of a cubic zinc blende structure at 2θ = 23.9, 39.8, and 46.1° (Fig. S1†). The average crystal size of the QDs is determined by Debye–Scherrer's formula:²⁴

where D is the nanocrystal diameter, λ is the wavelength of the incident XRD, β is the FWHM, θ is the diffraction angle. Based on the XRD spectra, the reflection angle at 2θ = 23.9° is used, the estimated average crystallite diameter of the CdTe QDs is 2.7 nm, very close to the value (2.8 nm) calculated from Peng's empirical equation.

The surface chemistry of the CA–CdTe QDs was investigated by using FTIR. Fig. 4 shows the IR spectra of CA and CA–CdTe QDs. For cysteamine hydrochloride, the absorbance at 2600–2500 cm⁻¹ is assigned to S–H stretching vibration. The bands at 3500–3000 cm⁻¹ are related to stretching vibration of O–H (H₂O) and N–H (–NH₂). The band at 1608 cm⁻¹ is attributed to N–H deformation vibration. In particular, no absorption related to S–H stretching vibration at 2600–2500 cm⁻¹ was observed for the CA-QDs, suggesting that –SH in cysteamine hydrochloride is covalently bound on the QDs surface. Moreover, the broad bands at 3300–3000 cm⁻¹ in the QDs almost disappear. More –NH₂ exposed onto the QDs surface could be illustrated by much narrower absorption band around 3400 cm⁻¹, a decreased absorption at 1636 cm⁻¹, and a blue-shift from 1608 cm⁻¹ to 1636 cm⁻¹. The appearance of 1038 to 1142 cm⁻¹ band indicates the presence of C–N on the CA–CdTe QDs. These results confirm the surface modification of QDs with CA molecules.

Fig. 4 FTIR spectra of cysteamine hydrochloride and the CA–CdTe QDs.

Such surface modification of CA molecules entitles positive surface charges on the QDs, as confirmed by the zeta potential measurements, which demonstrates a surface potential of about 5.01 mV for the CA–CdTe QDs with refluxing time for 8 h in water. Hereafter, the positive surface charges will be used to composite with negatively charged microgels through electrostatic interactions.

3.2. Synthesis and characterization of poly(NIPAM-co-AAc) microgels and QD/microgel composites

The microgels used in this work were prepared by a semi-batch method.^23,25–28 Optical micrograph of microgels in Fig. 5 reveals an average size of 1.4 μm with narrow polydispersity. The chemical structures were qualitatively characterized by FTIR (Fig. S2, ESI†). Typical bands of amide groups (1648 cm⁻¹, C=O stretching; 1549 cm⁻¹, N–H in-plane bending vibration), ester carboxyl group (1713 cm⁻¹), and symmetrical bending vibration and coupling split originating from dimethyl of isopropyl group (1389 and 1369 cm⁻¹, respectively) have been featured on the spectrum, indicating the formation of copolymers.

Fig. 5 Optical micrograph of poly(NIPAM-co-AAc) microgels.

The AAc moieties, a weak acid, of microgels render the microgels pH sensitivity. As the pH is higher than the apparent pK_a of AAc, ∼4.25,¹⁹ the AAc groups of poly(NIPAM-co-AAc) microgels carry negative charges due to deprotonation, leading to internal electrostatic repulsion between the charged AAc groups, and thus causing a swelling of the poly(NIPAM-co-AAc) microgels. Otherwise, the AAc groups are less ionized and the polymer–polymer interactions become dominant, leading to a collapse of poly(NIPAM-co-AAc) microgels.

In order to load CA–CdTe QDs into poly(NIPAM-co-AAc) microgels, the CdTe QDs of 2.7 nm were mixed with microgels and incubated at pH = 7 for 30 min. Subsequently, the pH was reduced to 4 in order to collapse the microgels. Thus, the QDs are trapped in the microgels (Fig. 1). The volume ratio of CA–CdTe QDs dispersion to poly(NIPAM-co-AAc) microgel dispersion was varied from 0.10 to 0.16. In the following part, the volume ratio of 0.16 was used to prepare photoluminescent nanocomposite microgels unless otherwise specified. The obtained suspension was ultracentrifuged and the supernatant was decanted. The sediment emitted yellow fluorescence under UV irradiation, indicating successful loading of CA–CdTe QDs in the poly(NIPAM-co-AAc) microgels.

As the microgels were composited with QDs, the UV absorbance and the photoluminescence (PL) of the QDs were blue shifted (Fig. 6). The 2.7 nm CA–CdTe QDs showed an absorbance at 520 nm. As incorporated into microgels, the peak band shifted to 514, 510, and 509 nm with decreasing QD content (Fig. 6A). Meanwhile, the PL peak shifted from 569 nm for CA–CdTe to 559, 557, and 556 nm for hybrid microgels with decreasing QD content (Fig. 6B), while the PL intensity was decreased (data not shown).

Fig. 6 The absorption (A) and photoluminescent spectra (B) of hybrid microgels under different preparation conditions.

Such blue-shifted photoluminescence is likely caused by the changed chemical environment of the QDs. Within the microgel, the AAc groups chemically etched the surface of CdTe QDs, leading to the formation of a “CdO” layer as Cd was coordinated with the carboxyl groups (Fig. 1). In addition to the electrostatic interactions and hydrogen bonds between the QDs and the microgels, it is likely that AAc/Cd coordination may be another important tie of the poly(NIPAM-co-AAc) chains to the CA–CdTe QDs. This coordinated polymer chains from microgels changed the refractive index surrounding as well as the surface states of the QDs, leading to the blue shift of emission peak. Thus, more polymer chains with lower Cd/AAc ratio could promote this influence, resulting in lower values of emission peaks.

With such strong interactions, the incorporation of QDs reduced the hydrodynamic diameters of the microgels. Dynamic light scattering (DLS) results demonstrated a decrease in hydrodynamic diameters (D_H) of the microgels from about 2400 nm to about 1530 nm after inclusion of QDs (ESI Fig. S3†). The CA–CdTe QDs may serve as local crosslinkers through electrostatic interactions, hydrogen bonding, and the coordination of Cd²⁺ on QDs surface with the carboxylic groups.¹⁸

TEM was used to image the CdTe/poly(NIPAM-co-AAc) microgels. The microgel diameter was about 920 nm (Fig. 7A), significantly smaller than the DLS value, probably due to the shrinkage of hybrid microgels after dehydration. The CA–CdTe QDs are well distributed in the microgel matrix (Fig. 7B). The QDs were found as aggregates with average size about 20 nm, larger than the single CA–CdTe QDs, which may be caused by the collapse of microgels during pH change. In order to further identify the presence of CdTe QDs, electron dispersive spectroscopy (EDS) was performed on the microgels, showing abundant presence of Cd and Te atoms (Fig. S4†).

Fig. 7 TEM image (A), enlarged TEM view (B) of the CdTe/poly(NIPAM-co-AAc) microgels.

Apparently, such a facile and versatile method to incorporate QDs into microgels could be applied for QDs with different sizes, as well as different photoluminescence. This versatility has been demonstrated by incorporating CA–CdTe QDs with different sizes into the microgels. For example, CdTe QDs of 2.5 nm and 3.2 nm (Fig. 8A) were separately incorporated into the microgels. These nanocomposite microgels appeared green (Fig. 8B) and yellow (Fig. 8E) under UV irradiation, respectively. Moreover, pure microgels were incubated with a mixture of 2.5 (green) and 3.2 nm (yellow) CdTe QDs. After removing excess CdTe QDs, distinctive emission colours of the resulted hybrid microgels are obtained, showing a single photoluminescent peak, instead of two overlapping ones (Fig. 8C and D). The peak position is dependent on the ratio of the QDs with different sizes. Thus, it is convenient to control the photoluminescence of the hybrid microgels by adjusting the ratio of QDs with different sizes (Fig. 8F). These results suggest an excellent preparation strategy for colour-tunable hybrid microgels. Based on such a versatile synthesis of QDs/microgel hybrid, it is natural to anticipate the preparation of similar functional hybrid microgels by smart combination of functional moieties with responsive microgels.

Fig. 8 The fluorescence spectra of CdTe of 2.5 and 3.2 nm (A), HM4-0-0 (B), HM3-0-1 (C), HM2-0-2 (D) and HM0-0-4 (E), respectively. Insets show the corresponding fluorescence images of the QDs and hybrid microgels. (F) The value of photoluminescent peak as a function of the ratio of CdTe of 3.2 nm to all added QDs.

4. Conclusions

Responsive poly(NIPAM-co-AAc) microgels doped either with a single luminescence colour or with multiple colours have been fabricated based on electrostatic interactions. The poly(NIPAM-co-AAc) microgels carry negative charges and the cysteamine-capped CdTe QDs carry positive charges within broad pH range. Besides, pH-triggered swelling and collapse of polymer network were utilized to encapsulate the CA–CdTe QDs. This method has been demonstrated versatile for encapsulation of QDs with different sizes or photoluminescent colours at the same time, and can be easily used to fabricate photoluminescent microgels with tunable colours. Such colour-tunable-tagged microgels may find application to multiplexed optical coding of biomolecules and biological cells, and for the creation of encoding combinatorial libraries.

Acknowledgements

This work was supported by the Ministry of Science and Technology and China (2016YFC1101902), Natural Science Foundation of China (21574145), the Zhejiang Natural Science Foundation of China (LR13B040001, LY17E030011), and the Program for Ningbo Innovative Research Team (2012B82019).

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Footnote

† Electronic supplementary information (ESI) available: The XRD pattern of CA–CdTe QDs, FTIR spectra of pure microgels and DLS histograms of pure and hybrid microgels. See DOI: 10.1039/c6ra19659d

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