Study on the room temperature synthesis of highly photoluminescent and temperature-sensitive CDs/PNIPAM hybrid hydrogels and their properties

Abstract

In this present study, carbon dots (CDs) with strong fluorescence were introduced into poly(N-isopropylacrylamide) (PNIPAM) hydrogel (CDs/PNIPAM) in one pot at room temperature by atom transfer radical polymerization (ATRP). The method was simple, facile, but highly efficient and versatile. The obtained CDs/PNIPAM hybrid hydrogel was thought to combine through hydrogen bond assisted with spatial network embedding. Compared with the original CDs solution, the CDs/PNIPAM hybrid hydrogel showed temperature-sensitivity as well as strong fluorescence with a LCST of 33 °C. Around the LCST, a slight change of temperature affected the fluorescent intensity dramatically. For example, when the temperature increased from 32 °C to 33 °C, the fluorescence intensity of CDs/PNIPAM dropped sharply with a ratio of F₃₂ to F₃₃ as high as 3.5. Moreover, the fluorescence intensity recovered when the temperature fell. Through heating and cooling cycles, the fluorescence off–on phenomenon of the CDs/PNIPAM hybrid hydrogel was reversible and repeatable around the LCST region.

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1. Introduction

Carbon dots (CDs) have been an attractive fluorescent agent over the past few years due to their unique optical properties, such as high fluorescence quantum yield, excitation wavelength dependent photoluminescence (PL) behavior, emitting up-conversion fluorescence and relatively high fluorescence stability with a strong resistance to photo-degradation and photobleaching.^1,2 Based on all their advantages, CDs found their promising application in labeling,³ biomedical imaging,^4–6 sensing,^7–10 photocatalysts¹¹ and photovoltaic devices.^12,13 Also, CDs have huge potential in biomedical, biotechnological and environmental applications to replace traditional heavy-metal-based quantum dots (QDs) due to their excellent biocompatibility and low cytotoxicity.^14,15

Multifunctional hybrid nanogels have gained significant momentum since they combined several useful properties in a single nanostructure to broaden their potential applications including as drug carriers, sensors, and bioimaging labels. Poly(N-isopropylacrylamide) (PNIPAM) is a well-known temperature-responsive polymer as an active matrix transducing environmental temperature changes with a coil-to-globule transition in water.^16–18 As a promising biology matrix, it is widely used in drug delivery,¹⁹ controlled release,^20,21 sensors²² and other fields because of its good biocompatibility and lower critical solution temperature (LCST) around 32 °C near human body temperature. Fluorescent PNIPAM hydrogels are pop multifunctional materials applied to new fluorescence markers with temperature-sensitive characteristics, and various efforts have been made to prepare hybrid QD/PNIPAM hydrogels such as polymerization of NIPAM in the presence of QDs,^23,24 in situ synthesis of QDs in polymer nanospheres, or incorporation of QDs into already preformed polymer microsphere collides.²⁵ Nevertheless, the intrinsic environmental risks associated with semiconductor QDs caused some serious concerns about health and environment and make the hybrid QD/PNIPAM hydrogels unsuitable for biomedical and environmental applications. Thus, replacing QDs with low cytotoxic CDs is a feasible way to novel safe fluorescent polymer material.

Zhang et al.²⁶ reported that they had first copolymerized carbon nanodots (CNDs) with NIPAM monomers by covalent bond to produce thermo-responsive fluorescent polymeric materials in solution or the solid-state. Li et al.²⁷ crosslinked poly(acrylamide) (PAAm) in the presence of poly(N-isopropylacrylamide) (PNIPAM) and carbon nanodots (CNDs) at room temperature. To control proper fluidic properties of the hydrogel, they synthesized PNIPAM microspheres at first. Wang et al.^28,29 synthesised fluorescent carbon nanoparticles (FCNPs) and prepared [poly(NIPAM-AAm)]-FCNPs nanogels by one-pot precipitation copolymerization of NIPAM monomers with hydrogen bonded FCNP-AAm complex monomers in water at 70 °C.

In this paper, highly fluorescence CDs was introduced into temperature-sensitive PNIPAM hydrogel at room temperature by initiating atom transfer radical polymerization (ATRP) of NIPAM monomers in the presence of CDs. The CDs was supposed to combine with the PNIPAM gel steadily by hydrogen bond assisted with spatial network embedding. The method was simple, facile, but highly efficient and versatile. Compared with original CDs solution, the CDs/PNIPAM hybrid hydrogel showed not only high fluorescence but also a reversible and repeatable sensitivity to external temperature stimuli with a LCST about 33 °C. Around the LCST, a slightly change of temperature affected the fluorescent intensity dramatically. For example, when the temperature increased from 32 °C to 33 °C, the fluorescent intensity of CDs/PNIPAM dropped sharply with a ratio of F₃₂ to F₃₃ as high as 3.5. This CDs/PNIPAM hydrogel also could be excited by near infrared light and its up conversion fluorescence responses to temperature as well. The versatility of the CDs/PNIPAM hybrid hydrogels will have various potential applications in many fields, such as temperature-sensors, thermosensitive devices, temperature-controlling valves and smart fluorescent printed materials.

2. Experiment section

2.1 Materials and reagents

α-D-Lactose and tris(hydroxymethyl) amino-methane (Tris) were analytical grade reagents purchased from Alfa Aesar and used as supplied. N-Isopropyl acrylamide (NIPAM) were purchased from Sigma. N,N′-Methylene diacrylamide (MBA), potassium persulfate (KPS) and N,N,N′,N′-tetramethylethylenediamine (TEMED) were from Beijing Solarbio Technology Co., Ltd. The water used throughout all experiments was ultrapure water which was obtained from an AWL-0502-U super pure water system (Aquapro, China).

2.2 Instruments

Fluorescence spectra were obtained by an F-4500 fluorescence spectrophotometer (Hitachi, Japan) which was coupled with a thermostatic bath to control the sample temperature with an accuracy of ±1 °C. The temperature range investigated was from 25 °C to 43 °C. If not specifically mentioned in the text, an excitation wavelength of 410 nm was used. Absorption spectra were gotten at room temperature with a UV-2450 UV-Visible spectrophotometer (Shimadzu, Japan). FTIR spectra were acquired by using a Magna-560 Fourier transform infrared spectrometer (Nicolet, USA) and freeze drying gel was used as the sample with potassium bromide (KBr) as the matrix. Thermo gravimetric analysis was conducted using a TG209 thermal analysis system (Netzsch, German), Differential scanning calorimeter was operated by a DSC1 differential scanning calorimeter (Mettler Toledo, Switzerland).

2.3 Synthesis of block CDs/PNIPAM hybrid hydrogel

The CDs were prepared as we described before³⁰ and more details were presented in ESI.† The block CDs/PNIPAM hybrid hydrogel was synthesized in a quartz cuvette as follows: 0.21 g NIPAM and 0.003 g MBA were dissolved in 2 mL water by ultrasound assisting. Then 0.5 mL CDs solution (about 30 mg mL⁻¹) was added and mixed evenly. After adding 10 μL of TEMED, the mixed solution was purged with nitrogen for 10 min to remove the oxygen. The atom transfer radical polymerization (ATRP) was initiated by 0.5 mL KPS (including 0.011 g KPS). Nitrogen was aerated for another 5 min until the cuvette was sealed with sealing film and placed in a 25 °C water bath to continue reaction for 3 h. Then the block CDs/PNIPAM hybrid hydrogel was obtained. In comparison test, a blank PNIPAM gel was synthesized by replacing CDs solution with water.

3. Results and discussion

3.1 The mechanism of block CDs/PNIPAM hybrid hydrogel

The polymerization was a free-radical reaction with MBA as the crosslinking agent and KPS as the initiator. According to our previous work,³⁰ the CDs were covered with abundance of hydroxyl groups. We supposed that the CDs could bond to the PNIPAM gel by hydrogen bond assisted with spatial network embedding, as we described in Fig. 1.

Fig. 1 Schematic diagram of block CDs/PNIPAM hybrid hydrogel synthesized at room temperature.

3.2 The optimum for the synthesis of CDs/PNIPAM hybrid hydrogel

We investigated four different NIPAM monomer concentrations at dosages of 0.15 g, 0.18 g, 0.21 g and 0.25 g. By increasing of monomer concentration, the gelation time was shorten and the thermosensitivity of gel was weaken. However, the reversibility and repeatability of gel were increased. We chose a dosage of 0.21 g to equilibrate the thermosensitivity and reversibility. We also tried several degree of crosslinking. The dosages of MBA were 0.001 g, 0.003 g, 0.004 g, 0.006 g and 0.010 g respectively (namely corresponding crosslinking were about 0.5%, 1.5%, 2%, 3% and 5%). The thermo-sensitivity of the corresponding CDs/PNIPAM gel was weaken along with an increase of crosslinking degree. When the degree of crosslinking was 0.5% or less, some of the CDs were leaked out under water. Meanwhile, when it reached 5%, the obtained gel turned even into semi-transparent and heterogeneous partially. In considering the results, we chose a crosslinking degree of 1.5%. The synthesis process was a rapid atom transfer radical polymerization. After introduction of the initiator, the NIPAM monomers were polymerized quickly. It turned from aqueous solution into gel-like just in 5 minutes and kept the gel state unchanged at 25 °C. We chose a time of 3 h to make sure the reaction was fully completed.²⁴ Time dependent photographs of the hybrid hydrogel were presented in Fig. S1.†

3.3 Characteristics of the luminescent CDs/PNIPAM hybrid hydrogel

We investigated the characteristics of high fluorescent CDs/PNIPAM gel. From the UV spectra showed in Fig. 2A, the UV absorbance of CDs/PNIPAM hybrid hydrogel (line d) was similar to the sum of absorbance of the blank PNIPAM gel and the CDs solution (line c), which suggested that the combination of CDs and PNIPAM were primarily through hydrogen bond and physical network embedding rather than covalent binding. The FTIR spectra were showed in Fig. 2B. As we can see, the CDs/PNIPAM exhibited characteristic absorption bands of N–H stretching vibrations at 3433 cm⁻¹. While C–H stretching vibrations of CH, CH₂ and CH₃ were presented at 3073 cm⁻¹, 2972 cm⁻¹ and 2876 cm⁻¹, respectively. The diagnostic stretching vibration of C=O were detected at about 1647 cm⁻¹. And we found a bending vibration peak of N–H in amide group at 1541 cm⁻¹ and an asymmetric bending vibration peak of CH₃ at 1459 cm⁻¹. We also observed two as high peaks at 1387 cm⁻¹ and 1367 cm⁻¹, indicating the presence of isopropyl.

Fig. 2 (A) UV spectra and (B) FTIR spectra of CDs/PNIPAM hybrid hydrogel.

We also investigated the thermal properties of CDs/PNIPAM hybrid hydrogel along with blank PNIPAM gel as reference. Both blank PNIPAM and CDs/PNIPAM gels were freeze drying for 48 h before TG analysis. According to the TG graph shown in Fig. 3A, the CDs/PNIPAM showed similar thermal weight loss. From room temperature to about 300 °C, there were two slightly weight loss owed to losing of free water and bound water. Since nearly 400 °C, the gels reached a sharply loss of weight caused by degradation of PNIPAM. The weight of residue from CDs/PNIPAM was 3.43%, less than 8.54% from blank PNIPAM. As we know, the CDs consisted of C, H, O and N elements may be degraded at high temperature as well, which could explain the reason of relatively low residue of the CDs/PNIPAM.

Fig. 3 (A) TG and (B) DSC of CDs/PNIPAM hybrid hydrogel.

The DSC was conducted to confirm the LCST of CDs/PNIPAM hybrid hydrogel by scanning from 10 °C to 60 °C with a rate of 5 °C min⁻¹. From DSC results shown in Fig. 3B, the CDs/PNIPAM hybrid hydrogel had an endothermic peak at 36.1 °C in temperature rise period, higher than 33 °C at which the gel turned phase transformation from colourless and transparent gel to white and muddy solid by visual method. The slight difference between the two methods may own to a hysteresis effect caused by a high rate of scanning opposite a low rate of thermal conduct in PNIPAM gel. The CDs/PNIPAM hybrid hydrogel also hold an exothermic peak at 31.3 °C in the temperature fall period, while PNIPAM gel showed an endothermic peak at 34.9 °C and an exothermic peak at 30.6 °C respectively. Since the CDs were rich in hydrophilic groups such as hydroxyl group, the improved hydrophilicity led to an increase of the LCST slightly after the combination of CDs and PNIPAM.

3.4 The fluorescence properties of the CDs/PNIPAM hybrid hydrogel and its temperature-sensitive response

We investigated the optical properties of the CDs/PNIPAM hybrid hydrogel. From the normalized fluorescent spectra shown in Fig. 4A, the CDs/PNIPAM gel showed a blue shift reference to original CDs solution with a maximum wavelength from 523 nm to 514 nm, nearly the emission changed from green to blue (can see from Fig. 5). The reason may be that the surface state of the fluorescent CDs was changed since its connection to PNIPAM by hydrogen bond. Beyond that, what caused our more interested was the highly fluorescent temperature-sensitive response of the CDs/PNIPAM gel. When the external temperature was increased from 25 °C to 43 °C, a tremendously decline of fluorescence was performed by CDs/PNIPAM gel (seen from Fig. 4B), unlike the small decrease of fluorescence by CDs solution (Fig. 4C). The fluorescence of CDs/PNIPAM at 25 °C was about 8 times higher than that at 43 °C. This phenomenon was aroused from the phase transformation of PNIPAM corresponding with temperature change, which can be observed directly from photographs exhibited in Fig. 5. At room temperature, the CDs/PNIPAM hybrid hydrogel stay the same colourless and transparent with blank PNIPAM under sunlight, which allowed light through to excite the CDs for high fluorescence emitting under UV lamp. When the external temperature was higher than the LCST, the CDs/PNIPAM gel turned into white solid state along with a sharp decrease of measured fluorescent intensity. The drop of measured fluorescent intensity was caused by series of factors. Likely, the spatial network of PNIPAM collapsed locally since the increasing of temperature enhanced its hydrophobic interaction while weakened its hydrogen bond to water, which led to an obvious self-absorption of fluorescence along with an aggregation of CDs in local position. However, the most important reason may be the poor transmittance as a result of high absorption, reflection and scattering of light caused by phase transformation of PNIPAM from transparent gel into a white and muddy solid. Therefore, while the fluorescent intensity detected by fluorescence spectrophotometer was decreased dramatically, we can still see bright fluorescence under a UV lamp (Fig. 5D).

Fig. 4 Normalized fluorescent spectra of CDs solution and CDs/PNIPAM hybrid hydrogel at 25 °C (A) and relative fluorescent intensity at 25 °C and 43 °C of CDs/PNIPAM hybrid hydrogel (B) and CDs solution (C).

Fig. 5 Photographs of PNIPAM and CDs/PNIPAM gels: (A) at room temperature under sunlight, (B) at room temperature under a 365 UV lamp, (C) at 43 °C under sunlight and (D) at 43 °C under a 365 UV lamp (inset: photographs of CDs solution).

Using water bath to control the temperature at 25 °C, 28 °C, 31 °C, 32 °C, 33 °C, 34 °C, 35 °C, 36 °C, 37 °C, 40 °C and 43 °C, respectively, the fluorescent spectra of CDs/PNIPAM hybrid hydrogel at various temperatures were gained (see Fig. 6A, some of the spectra were not shown) and the fluorescent intensity at indicated temperature were also shown in Fig. 6B. We noticed that the decrease of fluorescent intensity dependent on temperature was nonlinear. Around the LCST, a slightly change of temperature affected the fluorescent intensity dramatically. For example, when the temperature increased from 32 °C to 33 °C, the fluorescent intensity of CDs/PNIPAM dropped sharply with a ratio of F₃₂ to F₃₃ as high as 3.5 (F₃₂ and F₃₃ means the fluorescent intensity at 32 °C and 33 °C respectively), while the ratio of F₃₁ to F₃₄ was 4.9. In Fig. 6C, the two curves from heating and cooling process of the CDs/PNIPAM gel were in a good coincidence, declaring an excellent temperature sensitivity and reversibility. The temperature dependent off–on fluorescence properties of CDs/PNIPAM hybrid hydrogel were repeatable while we chose 25 °C and 43 °C as representatives of its two phase (see Fig. 6D), and the temperature-sensitive response of CDs/PNIPAM hydrogel was alike in its up-conversion fluorescence (shown in Fig. 7).

Fig. 6 (A) Fluorescent spectra and (B) fluorescent intensity of CDs/PNIPAM gel at indicated temperatures, (C) fluorescence reversibility of CDs/PNIPAM gel correlatively to temperature sensitivity and (D) fluorescence repeatability of CDs/PNIPAM gel in heating-cooling circles.

Fig. 7 Up conversion fluorescence spectra (A) and its repeatability (B) of CDs/PNIPAM gel in heating-cooling circles.

The influence of NaCl on fluorescent intensity of the CDs/PNIPAM gel was also investigated. Three test gels were immerged into water, 0.5 M NaCl and 1.0 M NaCl respectively with stirring. The fluorescent intensity of test gels at different time were measured refering as F while the fluorescent intensity of a reference gel kept in room temperature in air refering as F₀. According to the results shown in Fig. S2,† the fluorescent intensity of the CDs/PNIPAM gel was affected scarcely by low concentration of salt like 0.5 M NaCl. Nevertheless, the fluorescence markedly dropped after long-term immersion into high-salt solution as the gel turned phase transformation into white solid state partially and it could be recovered by washing with water again. Since there were much more hydrogen bond between PNIPAM and water than that between PNIPAM and CDs, this may explain why the high salt affected PNIPAM primarily into phase transformation rather than changed the fluorescent emitting wavelength.

4. Conclusion

In summary, this paper provides a convenient route to introduce CDs into PNIPAM gel at room temperature in simple step. The method was simple, facile, but highly efficient and versatile, and it lays a foundation to prepare multifunctional materials based on CDs and PNIPAM through hydrogel bond. The obtained CDs/PNIPAM hydrogel shows not only high fluorescence but also a reversible and repeatable sensitivity to external temperature stimuli. The versatility of the CDs/PNIPAM hybrid hydrogels will have various potential applications in many fields, such as temperature-sensors, thermosensitive devices, temperature-controlling valves and smart fluorescent printed materials.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program) (No. 2011CB707703 and 2012CB910601) and the National Natural Science Foundation of China (No. 21475069 and 21275078).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11217f

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