Constructing nanosized CdTe nanocrystal clusters with thermo-responsive photoluminescence characteristics

Abstract

Assembling ultra-small nanoparticles into nanosized colloidal nanocrystal clusters (CNCs) to create novel collective properties still poses tremendous challenges. This work reports the fabrication of photoluminescent CdTe CNCs on the nanoscale and their thermo-responsive properties. Diblock copolymers of poly(N-(2-aminoethyl) acrylamide-b-N-isopropylacrylamide) (PNAEAM-b-PNIPAM) were synthesized and employed as self-assembling actuators of CNCs. The side chains of PNAEAM blocks act as efficient anchors to capture CdTe nanocrystals via surface ligand identification. The thermo-sensitive PNIPAM blocks serve as the protection layer of CNCs and the trigger to turn off/on the photoluminescence CNCs during heating/cooling cycles. The [HS–C₁₀mim]⁺ ligands with smart noncovalent interactions on the as-prepared nanocrystals render the CNCs rapid and reversible thermo-response performances. These make the CNCs an excellent thermo-responser, and offer a new controllable self-assembly route for designing and engineering multifunctional nanosized CNCs.

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Introduction

Colloidal nanocrystals present size-dependent electronic, optical and magnetic properties due to the quantum confinement effect, and have evoked great interest for researchers in a variety of fields, including materials, electronics, energy, physics, chemistry, biological and medical sciences. Therefore, many strategies have been developed to synthesize nanocrystals with different properties by controlling their composition and surface ligands. It has been shown that colloidal nanocrystals can be employed as artificial atoms to build colloidal nanocrystal clusters (CNCs) by bottom-up assembly routes.^1–4 The properties of CNCs depend not only on those of individual nanocrystals but also on the coupling and exchange among the parent constitutes.^5–7 The collective properties of CNCs can provide the new functions in desired materials for many practical applications in photonics,^8,9 catalysis,^10–12 separation,¹³ sensing,^14,15 diagnosis,¹⁶ and electrochemistry^17–19 etc. Although several bottom-up assembly strategies and concepts for CNCs have been developed, there are still many challenges in fine-tuning colloidal interactions and patterning of specific nanocrystals within CNCs to generate a new class of functional materials with controllable sizes and shapes.

Protection layer of clusters, surface ligand and spatial organization of nanocrystals exsert the great effects on performances of CNCs besides nanoscopic building blocks. The properties of CNCs can be tailored by taking advantage of covalent and non-covalent interactions, and the new functionalities can be generated by introducing functional components into clusters.^1,7,20 A large number of organics with different structure and composition, which usually contain thiol, amino, phosphino, and/or carboxyl groups, have been employed as surface ligands to synthesize and stabilize colloidal nanocrystals in solutions.^21–23 Among them, the ligand of 1-alkyl-3-methylimidazolium-based salts possesses several directional noncovalent interactions that can be tuned by the ionic structure and component,^24–27 and is expected to play the active multi-roles in CNCs. Block copolymers composed of two or more functional polymer chains can be synthesized by a highly efficient method of the reversible addition–fragmentation chain transfer (RAFT) polymerization^28–31 to obtain the desired compositions, molecular weights, and morphologically distinct self-assembled states, and have been reported to employ as protection layer of CNCs to render clusters great dispersity, uniformity as well as more mechanically robust coating.^32–35

The microgels of poly(N-isopropylacrylamide) (PNIPAM) in water possess the character of volume phase transition (VPT), their temperature-sensitive properties have been well evaluated based on lower critical solution temperature (LCST) that falls in physiological temperature range,^36–39 and have attracted tremendous attention from the many fields, such as drug release, diagnosis, biomedicine, bioengineering, sensor, paints, membranes, and smart materials etc.^14,40,41 During heating/cooling cycles of PNIPAM-based hydrogels, however, multiple challenges still remain in narrowing temperature-jump range, accelerating thermo-response rate, restraining gel agglomeration, along with reducing microgel size and viscosity. In this work, it is shown that a solution of those problems can be found by manipulating the interactions of ligand with diblock copolymer to self-assemble nanocrystals into temperature-responsive nanosized CNCs. The surface-functionalized CdTe nanocrystals were prepared using 1-(10-mercaptodecyl)-3-methylimidazolium bromide ([HS-C₁₀mim]Br) as the stabilizer, and poly(N-(2-aminoethyl) acrylamide-b-N-isopropylacrylamide) (PNAEAM-b-PNIPAM) was synthesized by RAFT polymerization and amidation modification. The diblock copolymer can self-assemble the CdTe nanocrystals into CNCs in aqueous solutions at ambient temperature. The resulting CNCs are the particles in nanoscale with good monodispersity, and are highly dispersible in water. When applied an external thermo-trigger at LCST, no turbidness is observed from the solutions of CNCs, but the photoluminescence (PL) intensity varies sharply with temperature. Moreover, the CdTe CNCs represent a rapid and reversible thermo-response, and are the promising candidates to monitor the temperature in microenvironment via a facile and contactless on-line way.

Experimental section

Chemicals

N-Isopropylacrylamide (NIPAM, 98%), 2,2′-azobisisobutyronitrile (AIBN, 99%), 4-cyano-4-(thiobenzoyl)sulfanyl pentanoic acid (CPADB, >97%), and acrylic acid (AA, 99%) were purchased from the Sigma-Aldrich Chemical Company. NIPAM and AIBN were respectively recrystallized from benzene–hexane mixture and ethanol, and AA was distilled under reduced pressure prior to use. 1,4-Dioxane (DO) and tetrahydrofuran (THF) was refluxed with CaH₂ before use. Tellurium power (5 N), sodium borohydride (>98%), cadmium chloride (99.99% metals basis), ethylenediamine (EDA, AR, 99%), tert-butanol (AR) and n-hexane (AR) were purchased from Aladdin Chemistry Co. (Shanghai, China), and were used as received without further purification. Deionized water with the electrical conductivity of <1.0 × 10⁻⁶ S cm⁻¹ was used throughout all experiments.

Synthesis of poly(acrylic acid) (PAA) macro chain transfer agents (macro-CTAs)

PAA macro-CTAs were synthesized using RAFT polymerization mediated by CPADB. Typically, AA (0.1658 g, 2.3 mmol), CPADB (0.0140 g, 0.05 mmol), AIBN (0.0018 g, 0.01 mmol), and 1,4-dioxane (1.5 mL) were added into a 50 mL three-necked flask fitted on Schlenk line under nitrogen atmosphere. The reaction mixture was completely dissolved with magnetically stirring at 25 °C for 20 minutes, and thereafter treated with three freeze–pump–thaw cycles. Then, the sealed vessel was immersed into an oil bath preheated at 70 °C for 16 h. In the reaction process, the solution turned from red to pale yellow. The polymerization was quenched by pouring into liquid nitrogen, and the solvent was removed by rotary evaporation. The residue was dissolved into 1 mL THF, and precipitated by adding into excess n-hexane. The purification was repeated at least twice to remove the unreacted monomer. Oily PAA was obtained and dried under vacuum at 35 °C for 24 h. The average degree of polymerization (DP) was estimated to be 12 through comparing integrals of methine protons (–CH₂–CH̲–) of the PAA block and aryl protons of the phenyl dithioester end-groups from the ¹H NMR signals, and the macro-CTAs are denoted as PAA₁₂. The conversion of AA was 35%.

¹H NMR (PAA₁₂, D₂O): δ/ppm = 1.8–2.5 (1H, CH₂CH̲), 1.2–1.8 (2H, CH̲₂CH).

Synthesis of poly(acrylic acid-block-N-isopropylacrylamide) (PAA-b-PNIPAM)

A small copolymer family was synthesized by employing PAA_x (x, the DP of AA) as macro-CTAs in reaction mixtures with different mass of NIPAM monomer. In a typical RAFT polymerization, PAA₁₂ (0.0663 g, 0.05 mmol), NIPAM (1.6812 g, 14.86 mmol), and AIBN (0.0025 g, 0.0152 mmol) were added in a 50 mL three-necked flask containing 4 mL 1,4-dioxane. Then, the reaction solution was sealed and degassed via three freeze–pump–thaw cycles. The polymerization was allowed to proceed at 80 °C for 20 h, after which time liquid nitrogen was poured into the vessel to stop reaction. The viscous solution was diluted with 2 mL THF, and isolated with excess n-hexane. The purification was carried out at least twice. The resulting product was dried in vacuum at room temperature overnight. The average polymerization degree of NIPAM monomers was estimated to be 290 by the integral values of the methine protons (–NHCH̲(CH₃)₂) in PNIPAM side chains and the protons in phenyl groups from the ¹H NMR spectrum, and the diblock copolymer is denoted as PAA₁₂-b-PNIPAM₂₉₀. The conversion of monomer was 90%.

¹H NMR (PAA₁₂-b-PNIPAM₂₉₀, D₂O): δ/ppm = 0.8–1.25 (6H, –NHCH(CH̲₃)₂), 1.25–1.77 (2H, CH̲₂CH), 1.77–2.3 (1H, CH₂CH̲), 3.64–4.1 (1H, –NHCH̲(CH₃)₂).

Synthesis of [HS-C₁₀mim]⁺-capped CdTe nanocrystals

The employed functional ligand of [HS-C₁₀mim]Br was synthesized in our laboratory and reported in previous work.^24,27 80 mg NaBH₄ and 127.6 mg tellurium power were dispersed in 1 mL of N₂-saturated de-ionic water, and a colorless aqueous solution of NaHTe was formed by stirring intensively at 0 °C for 8 h. For the synthesis of CdTe nanocrystals, CdCl₂ and [HS-C₁₀mim]Br at the molar ratio of 1 : 5 were dissolved in 0.01 mol L⁻¹ of EDA aqueous solution. After the reaction mixture was purged at room temperature by bubbling N₂ for 30 min, freshly prepared NaHTe solution added with vigorously stirring until the molar ratio of Cd²⁺/HTe⁻ reached 1 : 0.2, yielding the bright-yellow precursor solution of CdTe nanocrystals. Subsequently, the precursor solution was transferred to a Teflon-lined stainless steel autoclave with a volume of 20 mL. The autoclave was heated to and maintained at 160 °C for 7 h to grow CdTe nanocrystals, and then cooled to room temperature by a hydro-cooling process.

Preparations of EDA modified PAA-b-PNIPAM copolymer and CdTe CNCs

The amidation of AA residues in the PAA-b-PNIPAM copolymers was performed in aqueous solutions of EDA at room temperature. The excess of EDA was drop-wise added to 1 mg mL⁻¹ of PAA-b-PNIPAM aqueous solution until pH reached 11.2 with continuous stirring for 5 h. The amidation reaction of EDA with carboxyl groups in the copolymer was completed at room temperature for another 24 h. The aqueous solutions of resulting diblock copolymer, poly(N-(2-aminoethyl) acrylamide-b-N-isopropylacrylamide) (PNAEAM-b-PNIPAM), was directly used to prepare CdTe CNCs without further purification.

[HS-C₁₀mim]⁺-capped CdTe nanocrystals were separated by adding 300 mL of tert-butanol into 100 mL of the as-synthesized nanocrystals solution with stirring and centrifuged at 14 000 rpm for 20 min to remove the unreacted compositions, and the collected deposition was redispersed in water before use. According to the molar ratio of 1 : 0.5, 1 : 1, 1 : 2, 1 : 3 and 1 : 3.5 for NAEAM residues in the copolymer and CdTe nanocrystals, the aqueous solutions of PNAEAM-b-PNIPAM and CdTe nanocrystals were mixed with stirring for 5 h, and further incubated overnight at room temperature to ensure the formation of CNCs. Based on their chemical composition, the resulting CNCs were respectively denoted as PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₆, PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₁₂, PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₂₄, PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₃₆, PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₄₂.

Instruments and characterization

The samples for transmission electron microscopy (TEM) were prepared by drop-casting the CNC dispersion onto a carbon film supported copper grids and allowing it to be dried at room temperature under a nitrogen stream overnight. TEM measurements were performed on a JEM-2100 microscope at an acceleration voltage of 120 kV, and high-resolution transmission electron microscopy (HRTEM) at the voltage of 300 kV. The photoluminescence spectra were recorded on an FLS980 combined steady state and time-resolved spectrometer (Edinburgh, UK) with the excitation wavelength of 400 nm. The sample temperature was controlled by a cuvette holder equipped with a magnetic stirrer and a thermoelectrical thermostat with a precision of 0.02 °C, and the sample temperature was constantly held for 45 min before measure. X-ray power diffraction (XRD) was carried out on Bruker D8Advance diffractometer using Cu Kα radiation. The sample in solid was mixed with KBr and pressed into a pellet, and Fourier transform infrared spectra were measured at room temperature on a Thermo Nicolet Nexus spectrometer with the resolution of 2 cm⁻¹. ¹H NMR spectra of samples in D₂O were respectively recorded with a Bruker Avance NMR spectrometer (400 MHz).

Results and discussion

First, poly(acrylic acid) (PAA) oligomers were prepared by employing 4-cyano-4-(thiobenzoyl) sulfanyl pentanoic acid as the chain transfer agents (CTAs) at the fixed [CPADB] : [AIBN] molar ratio of 5 : 1, and then poly(acrylic acid-block-N-isopropylacrylamide) (PAA-b-PNIPAM) was harvested using the as-synthesized PAA as the macro CTAs to polymerize NIPAM in 1,4-dioxane with a [PAA] : [AIBN] molar ratio of 3.3 : 1. The average polymerization degree of the resulting polymers was evaluated from ¹H NMR spectra following the method as reported in literature,⁴² and respectively denoted as PAA_x and PAA_x-b-PNIPAM_y. Subsequently, PAA-b-PNIPAM was chemically modified by amidation reaction of AA residues with ethylenediamine (EDA) in water to obtain PNAEAM-b-PNIPAM copolymers (Scheme 1). Experimently, it is shown that the carboxyl functional groups in PAA-b-PNIPAM can readily react with EDA by adjusting the pH of the diblock copolymer aqueous solutions at room temperature, and the parent copolymers are quantitatively converted to the amide derivatives from the amidation. The successful modification can be verified by ¹H NMR and FT-IR spectroscopy.^38,43–47 For ¹H NMR spectrum of PNAEAM₁₂-b-PNIPAM₂₉₀ in D₂O (Fig. 1 and S1†), the peaks labelled 1 and 6 are respectively assigned to the methyl protons and the methine protons of isopropyl groups of PNIPAM blocks, the peaks 2 and 3 respectively to the overlapped signals of methylene protons and methine protons present in PNAEAM and PNIPAM backbones, and the peaks labelled 4 and 5 to the methylene protons in 2-aminoethyl side chains of PNAEAM blocks resulting from the amidation. It is observed that the unreacted free-ethylenediamine molecules represent a peak at 2.85 ppm in the ¹H NMR spectrum, which are the necessities for holding the basicity of aqueous polymer solutions to prepare photoluminescence CdTe CNCs as described in the experimental section. The IR spectral bands associated with amide vibrations in PNAEAM₁₂-b-PNIPAM₂₉₀ are marked out in Fig. 2A(b). Compared with the spectrum of PAA₁₂-b-PNIPAM₂₉₀, the new spectral signals, C=O stretching at 1577 cm⁻¹, C–H stretching at 1323 cm⁻¹ and N–H out-of-plane vibration at 822 cm⁻¹, provide further supports for the amidation of the copolymer with EDA. In addition, the N–H stretching vibration band at 3312 cm⁻¹ becomes dominant while the O–H stretching vibration band at 3499 cm⁻¹ almost disappears after amidation (Fig. 2B). The C–H stretching vibrations at 2974, 2932, and 2872 cm⁻¹ in PAA-b-PNIPAM copolymer represent as a broadening band in PNAEAM-b-PNIPAM copolymer due to the enhanced hydrogen bonds due to the introduction of 2-aminoethyl side chains. These spectral facts all verify the full modification toward the PAA blocks.

Scheme 1 RAFT synthesis of PAA₁₂-b-PNIPAM₂₉₀ diblock copolymer and the subsequent amidation modification in water at room temperature.

Fig. 1 ¹H NMR spectrum of PNAEAM₁₂-b-PNIPAM₂₉₀ in D₂O at room temperature. Insets are respectively the peak assignments and the expanded region of phenyl end group hydrogens from 7.6 to 8.4 ppm.

Fig. 2 FT-IR spectra in the region of 400–1800 cm⁻¹ (A) and 2000–4000 cm⁻¹ (B) for PAA₁₂-b-PNIPAM₂₉₀ (a), PNAEAM₁₂-b-PNIPAM₂₉₀ (b), and PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₂₄ (c).

The thio-functionalized imidazolium salt, [HS-C₁₀mim]Br, was utilized as capping agent, and the [HS-C₁₀mim]⁺-engineered CdTe nanocrystals were synthesized by the hydrothermal route in the aqueous solution of 0.01 mol L⁻¹ of EDA. In this way, EDA exerts the dual effects on the CdTe growth: creating a strongly alkaline aqueous solution (pH > 11) to ionize thiol-groups in the capping agent that provides a good advantage to stabilize the nanocrystals, and complexing with Cd²⁺ ions in precursor to restrain them from precipitation that are indispensable to prepare CdTe nanocrystals with high quality. As the aqueous precursor solution is fed at the molar ratio of NaHTe : CdCl₂ : [HS-C₁₀mim]Br of 0.2 : 1 : 5 and incubated at 160 °C for 7 h, the resulting CdTe nanocrystals appear largely as spherical shape with a mean diameter of 3.2 ± 0.2 nm (Fig. 3A). The clearly resolved lattice fringes observed from high-resolution transmission electron microscopy (HRTEM) image extend over the entire individual nanocrystal, implying that the nanoparticles are sufficiently crystalline. X-ray power diffraction (XRD) data show that the lattice parameters of as-prepared CdTe nanocrystals match the cubic structure of bulk CdTe crystal (Fig. 3B). However, a slight peak shift toward the cubic CdS phase is detected from the XRD pattern, and results mainly from the incorporation of the sulfur from [HS-C₁₀mim]Br into the growing CdTe nanocrystals, a typical phenomenon observed from aqueous synthesis of CdTe nanocrystals using thiol-ligands.^48,49 The as-prepared monodisperse CdTe nanocrystals exhibit good PL performance, and give a quantum yield of 33–40% in water.

Fig. 3 (A) TEM images of [HS-C₁₀mim]⁺-capped CdTe nanocrystals. Inset is the HRTEM image obtained from a selected individual nanocrystal. (B) XRD pattern of the [HS-C₁₀mim]⁺-capped CdTe nanocrystals.

PNAEAM₁₂-b-PNIPAM₂₉₀ can self-assemble [HS-C₁₀mim]⁺-stabilized CdTe nanocrystals into CNCs in water at ambient temperature, and the morphology of the resulting clusters depends on the content of CdTe nanocrystals as shown from the TEM images in Fig. 4. PNAEAM₁₂-b-PNIPAM₂₉₀ roughly represents as a chain consisted of polymeric nanospheres with average diameter of 7.1 ± 0.8 nm. The strong cohesion among PNAEAM and PNIPAM block supplies the driving force for self-assembly. When CdTe nanocrystals are integrated into the diblock copolymer according to the molar ratio of 0.5 : 1 for CdTe nanocrystals and NAEAM residues (CdTe NCs : NAEAM), the clustering disconnects the nanosphere chains, and makes the resultant PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₆ CNCs appear as the mixed morphologies of spheres and worms. The worm morphology is the dominant component in the CNCs, and is the combination of nanospheres of average diameter at 59 ± 5 nm. With increasing the molar ratio of CdTe : NAEAM to 2 : 1, the PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₂₄ CNCs become the compact packed nanocrystal balls with the average diameter of 92 ± 11 nm. In contrast, it is noted that the irregular morphology occurs at the molar ratio of 1 : 1, the resulting PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₁₂ CNCs is loosen and scattered CNCs, and is the intermediate state as the morphology of CNCs transitions from worms to balls. The HRTEM images of PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₆ and PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₂₄ confirm further the formation of CNCs that are built by closer packed CdTe nanocrystals (Fig. S2†). Therefore, PNAEAM-b-PNIPAM is a ideal candidate to induce self-assembly of [HS-C₁₀mim]⁺-engineered CdTe nanocrystals in water into nanosized CNCs with good uniformity.

Fig. 4 TEM images taken from PNAEAM₁₂-b-PNIPAM₂₉₀ (A), PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₆ (B), PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₁₂ (C), PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₂₄ (D).

It is known that PNIPAM blocks in the copolymer come through the changes from being solvophilic to solvophobic upon the applied temperature stimulus,^36–39 and are expected to endow the CNCs with temperature responsive characteristics that can be easily monitored by the PL signals. PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₂₄ CNCs were prepared in aqueous solution of the copolymer of 1 mg mL⁻¹, and their PL spectra were measured at different temperature with an interval of 0.2 °C (Fig. 5A). It is detected that the PL intensity at 570 nm decreases slowly with rising temperature, and sharply drops in the temperature range from 32.2 to 32.8 °C after which the PL CNCs is almost quenched. The transition temperature is in agreement with the LCST of PNIPAM, and the abrupt change of PL intensity renders CNCs the sensitive responders for the polymeric VPT. Specifically, the solution of CNCs keeps transparent after VPT rather than turbid as observed usually from PNIPAM hydrogels. In order to assess the morphology and size of the temperature-quenched CNCs, the PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₂₄ dispersion was preheated in the oven at 34 °C for 30 min, then drop-cast on a carbon film supported copper grids and dried at 34 °C for TEM measurement. It is observed that the PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₂₄ CNCs retain the intact morphology and good water dispersity after quenched (Fig. 5B), but their average size of 88 ± 12 nm is a little smaller than that determined at room temperature (Fig. 4D). Therefore, the PL quenching does not result from the aggregation of CNCs, but from the interactions of CdTe nanocrystals in a CNC induced by PNAEAM-b-PNIPAM copolymers.

Fig. 5 (A) PL spectra of PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₂₄ CNCs in water under different temperatures with an interval of 0.2 °C. (B) TEM images of PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₂₄ CNCs prepared at 34 °C.

In addition to the cluster uniformity and high sensitivity toward temperature trigger, PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₂₄ CNCs represent rapid response rate and robust reversibility in repeatedly heating–cooling cycles. When the PL CNCs at 31 °C is subjected to rise suddenly in temperature to 34 °C, the relative PL intensity (I/I₀), where I₀ and I respectively are the PL intensities of PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe NC₂₄ CNCs at 31 and 34 °C, attenuates with temporal evolution, and reaches the minimum after 6 min (Fig. 6A). As a burst of dropping in temperature from 34 to 31 °C, on the contrary, the quenched CNCs go firstly through an induction period (t₀) of 3 min, and then the I/I₀ ascends rapidly to the maximum after more 3 min. Within the given temperature-jump range, the processes for the turning off/on the PL CNCs can reversibly evolve by heating/cooling at least five cycles (Fig. S3†), and demonstrated that PNAEAM-b-PNIPAM protection layer renders CNCs mechanically robust against thermo-stimuli.

Fig. 6 (A) dependence of I/I₀ on time in the temperature-jump range between 31 and 34 °C for turning off (red line) and on (blue line) the PL PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₂₄ CNCs in aqueous solution. (B) I/I₀ as a function of temperature for CNCs of PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₆ (▼), PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₁₂ (○), PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₂₄ (▲), PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₃₆ (■), PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₄₂ (●).

PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN_n (n = 6, 12, 24, 36, 42) CNCs were respectively prepared according to the molar ratio of CdTe : NAEAM in diblock copolymer, and their dependences of I/I₀ on temperature were determined under exactly the same experimental conditions (Fig. 6B). With the exception of PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₁₂ CNCs that have VPT temperature between 31.0 to 32.0 °C, the rest all demonstrate roughly the same VPT temperature as that of PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₂₄ CNCs within a margin of experimental error. Following the same RAFT synthesis procedures (Scheme 1), a small family of PNAEAM_x-b-PNIPAM_y (x = 4, 8, 12; y = 290, 440, 580) copolymers were synthesized by varying the mass of AA and NIPAM in reaction mixtures, and then used to self-assemble CdTe nanocrystals into CNCs with various CdTe contents. The experimental results show that the NAEAM : CdTe molar ratio in the CNCs exerts little effect on the VPT temperature except the situation in which the NAEAM : CdTe molar ratio is 1 : 2. For a given molar ratio of CdTe : NAEAM in CNCs, the VPT temperatures, which fall within 32.2–32.8 °C, are roughly independent of the average degrees of polymerizations of NAEAM and NIPAM in the di-blocks copolymers and the concentration of the copolymer solution (Fig. S4 and S5†). On grounds of these facts, it is safe to say that the CNCs can straightly reflect the polymeric natural instinct of VPT, and are different from the nanocomposite hydrogels with composition-dependent VPT temperatures in which the nanocrystals are entrapped within the polymer network by noncovalent or covalent interactions.^50,51 Obviously, the thermo-response sensitivity, including narrow temperature-jump range and fast response rate, depends definitively on the compact stacked structures and interactions of CdTe nanocrystals in the CNCs, not on the CdTe content and composition of PNAEAM-b-PNIPAM copolymer to some content.

The control experiments demonstrate that the PNIPAM homopolymer is not able to induce the formation of CdTe CNCs, and the introduction of NAEAM blocks are essential to provide handles for anchoring the nanocrystals. On average, each of NAEAM comonomers can immobilize at most 3.5 nanocrystals, and one nanocrystal requires at least 7 NIPAM comonomers because they coat only on the surfaces of CNCs. It is shown that the FT-IR spectrum of PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₂₄ is roughly similar to that of PNAEAM₁₂-b-PNIPAM₂₉₀ (Fig. 2), but represents very differences in the band relative intensities, indicating the significant changes in conformations as CdTe nanocrystals are incorporated into the copolymers. Therefore, it is assured that the self-assembly performances of PNAEAM-b-PNIPAM copolymer toward the CdTe nanocrystals result from the synergistic interactions of PNAEAM and PNIPAM blocks with [HS-C₁₀mim]⁺ ligands. In order to reveal the roles played by each block in manipulating the assembly of primary nanocrystals, ¹H NMR measurements of samples are carried out in D₂O at different temperatures. For ¹H spectrum of PNAEAM₁₂-b-PNIPAM₂₉₀ copolymer at 31 °C (Fig. 7A), the protons of the methyl group on side chains represent an asymmetric triplet peak at 1.07–1.14 ppm, and the peak converts into a doublet at 1.10–1.14 ppm as PNIPAM blocks come into VPT at 34 °C. The similar ¹H signals of methyl group are respectively observed from PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe₂₄ CNCs at 31 and 34 °C, and demonstrate that the PNIPAM blocks stick readily to the surfaces of CNCs and the incorporation of CdTe nanocrystals into the copolymer hardly affects on the PNIPAM VPT behaviour. In contrast, the triplet peaks of methylene group on 2-aminoethyl side chains of PNAEAM₁₂-b-PNIPAM₂₉₀ copolymer at 2.92 and 3.20 ppm shift respectively to 2.95 and 3.23 ppm as the temperature changes from 31 to 34 °C (Fig. 7B). Because of the strong interactions of PNAEAM blocks with [HS-C₁₀mim]⁺ ligands on CdTe nanocrystals, the further shifts are detected due to the formation of PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₂₄ CNCs, and occur most significantly in 34 °C at which the chemical shifts respectively are 3.03 and 3.29 ppm. These experimental results reveal that the PNAEAM blocks act as the potential scaffolds of CdTe nanocrystals via the interactions of 2-aminoethyl side chains with nanocrystal ligands, and the PNIPAM blocks as protection layer to bring CNCs the temperature-response property. Presumably, the VPT of PNIPAM blocks triggers the volume change of CNCs during heating/cooling cycles, and leads to turn off/on the PL CdTe nanocrystals. The contracting volume of CNCs at 34 °C causes the overlapping of CdTe nanocrystals and mutual interpenetration of ligand shells. The PL quenching process results most likely from the electron-transfer from the photoexcited nanocrystals to the electron-accepting imidazolium cations that insert them into another ligand shell.^52,53 It is expected that the volume dilation of CNCs at 31 °C is relatively rapid owing to electrostatic repulsion among imidazolium cations, and is accompanied by the rebound of PL intensity of CdTe CNCs. Evidently, it is the noncovalent interactions that render the CNCs good reversibility to thermo-stimulus and the capability to continuous cycle-use.

Fig. 7 ¹H NMR spectra of PNAEAM₁₂-b-PNIPAM₂₉₀ (a and b) and PNAEAM₁₂-b-PNIPAM₂₉₀-CdTe CN₂₄ (c and d) in D₂O recorded respectively at 31 and 34 °C.

Conclusions

The chemically modified PNAEAM-b-PNIPAM diblock copolymers and [HS-C₁₀mim]⁺-functionalized nanocrystals were successfully synthesized in the work. They can synergistically self-organize into the photoluminescence CNCs via a facile way, and behavior several remarkable advantages in the generation of CNCs: (i) self-assembling nanocrystals into good mono-dispersive nanosized CNCs in aqueous solutions at room temperature; (ii) producing robust protection layer for CNCs with good reversibility during heating/cooling cycles; (iii) bringing high thermo-sensitivity to CNCs with rapid response rate; (iv) rendering CNCs high dispersity and low viscosity in water as compared with the counterparts of temperature sensitive micro-gels. This provides an attractive way for the architecture and design of smart CNCs as multifunctional materials.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (grant no. 21073055 and 21573059).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: NMR spectrum, HRTEM images, and data from PL spectra of CNCs. See DOI: 10.1039/c5ra20111j

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