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Printable temperature-responsive hybrid hydrogels with photoluminescent carbon nanodots
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 055603 (http://iopscience.iop.org/0957-4484/25/5/055603) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 128.135.12.127 This content was downloaded on 11/11/2014 at 20:19
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 055603 (6pp)
doi:10.1088/0957-4484/25/5/055603
Printable temperature-responsive hybrid hydrogels with photoluminescent carbon nanodots Panpan Li, Lei Huang, Youjie Lin, Leo Shen, Qi Chen and Wangzhou Shi Joint Lab with Wuhu Token for Graphene Electrical Materials and Application, Department of Physics, Shanghai Normal University, Guilin Road 100, Shanghai 200234, People’s Republic of China E-mail: [email protected] and [email protected] Received 7 October 2013, revised 16 November 2013 Accepted for publication 11 December 2013 Published 9 January 2014 Abstract
Smart ink-like hybrid hydrogels that simultaneously possess semi-interpenetrating network structure, strong photoluminescence and temperature sensitivity are successfully fabricated based on the crosslink of poly(acrylamide) (PAAm) in the presence of poly(N-isopropylacrylamide) (PNIPAM) and carbon nanodots (CNDs) at room temperature. The resulting hybrid hydrogels were highly photoluminescent. The photoluminescence was sensitive to external temperature stimuli and reversible. Moreover, the hybrid hydrogels were applied as fluorescent inks for patterning using gravure printing, which may open a door towards developing smart CND based thermosensitive photoluminescent markers and sensors. Keywords: carbon nanodot, photoluminescence, temperature-responsive hydrogel, printable ink S Online supplementary data available from stacks.iop.org/Nano/25/055603/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
with a coil-to-globule transition in water at a lower critical solution temperature (LCST) around 32 ◦ C, is a well-known temperature-responsive polymer as an active matrix transducing environmental temperature changes to embedded QDs [18, 19]. Fluorescent PNIPAM hydrogels, which endow the ‘smart’ hydrogel matrix with luminescent properties of QDs, can be applied to a new generation of fluorescence markers with temperature-sensitive characteristics. Although semiconductor QDs have been used to synthesize hybrid QD/PNIPAM hydrogels [19–24] the intrinsic environmental risks associated with semiconductor QDs make the hybrid QD/PNIPAM hydrogels unsuitable for biomedical and environmental applications [25]. Recently, Zhang et al (2012) [26] reported that they had first copolymerized CNDs with NIPAM monomers to produce thermoresponsive fluorescent polymeric materials in solution or the solid-state. Unfortunately, the viscosity of the solution is too low to provide a sol–gel platform for printing smart photoluminescent markers.
Photoluminescent (PL) carbon nanodots (CNDs) are attracting considerable attention as nascent quantum dots (QDs) owing to their favorable optical properties along with chemical inertness, low toxicity and excellent biocompatibility [1–3], which give rise to their exciting application in labeling [4], biomedical imaging [5–8], sensing [9, 10] and photovoltaic devices [11, 12]. Currently, CNDs stand to have a huge impact in biotechnological and environmental applications due to their potential as nontoxic alternatives to traditional heavy-metal-based QDs [13, 14]. Qu et al (2012) [14] have confirmed that CNDs can exist in organisms, maintaining their PL characteristics and without any damage. For such applications, embedding QDs into an appropriate matrix is an effective method of preventing their agglomeration because strong fluorescence quenching occurs in dry and aggregate states [14, 15]. Among the possible matrices, poly(N-isopropylacrylamide) (PNIPAM) [16, 17], 0957-4484/14/055603+06$33.00
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c 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 055603
P Li et al
Figure 1. Schematic diagram of the synthetic route of PAAm/PNIPAM–CND hydrogels with reversible temperature responses.
The goal of this study was to fabricate highly photoluminescent PAAm/PNIPAM–CND hybrid hydrogels with proper fluidic properties. Therefore, we directly embedded PNIPAM and CNDs into a polyacrylamide (PAAm) matrix. Then, ink-like hybrid hydrogels were obtained by controlling the proportion of PAAm. The process sequence of synthesizing the hybrid hydrogels that combine the features of semi-interpenetrating network structure and strong photoluminescence through the crosslink of PAAm in the presence of PNIPAM and CNDs at room temperature is shown in figure 1. The synthesized hybrid hydrogels show reversible, temperature-dependent on–off photoluminescent behavior at approximately 35 ◦ C. Furthermore, the ink-like hybrid hydrogels could be coated on different substrates and the printed pattern exhibits blue emission upon exciting with a UV lamp.
2.3. Synthesis of PNIPAM microspheres
PNIPAM hydrogel spheres were prepared by surfactant-free radical polymerization [28]. A 1.50 g sample of NIPAM was dissolved in 150 ml of water at room temperature and the solution obtained was purged with N2 under stirring for 30 min. A 0.068 g sample of KPS was dissolved in 30 ml of water and the resultant solution was poured into the O2 -free NIPAM solution. The polymerization was performed for 4 h at 68–70 ◦ C under N2 . After the solution was cooled to room temperature, hydrogel spheres were obtained according to the self-cross-linking effect of PNIPAM chains and purified by three repetitions of centrifugation at 18 000 rpm and re-dispersion in water. To modify the crosslinking degree of PNIPAM spheres, we added different amounts of MBA, 0.012, 0.0225, 0.045, and 0.15 g, into the monomer solutions, corresponding to weight percentages of MBA of 0.8, 1.5, 3, and 10 wt%, respectively.
2. Experimental section
2.4. Synthesis of PAAm/ PNIPAM–CND hydrogels
2.1. Materials
The photoluminescent PAAm/PNIPAM–CND hydrogels were prepared by radical copolymerization of AAM, PNIPAM and MBA in the presence of TEMED as catalyst, and KPS as initiator in water. Briefly, AAm (0.71 g, 0.01 mol), PNIPAM (0.98 wt%, 3 ml), CNDs (1 wt%, 3 ml ), MBA (138 mg, 0.9 mmol) and TEMED (2 µl) were dispersed in 30 ml deionized water, stirring at room temperature for 10 min. After that, KPS solution (4 ml, 28 mmol l−1 ) was added to the reaction system, then the reaction mixture was quickly moved to a quartz tube, and then kept at room temperature for 8 h.
N-isopropylacrylamide (NIPAM, 99%) was recrystallized from hexane before use. N, N 0 -Methylenebisacrylamide (MBA, 99%) and tetramethylenediamine (TEMED) were purchased from Aldrich. Acrylamide (AAm) and potassium persulfate (KPS, 99.5%) were purchased from Sinopharm Chemical Reagent. NIPAM was purified by recrystallization from hexane prior to use. Fresh chicken eggs were purchased from local markets. The egg white and yolk were well separated by an egg separator prior to use. The black papers for printing were purchased from Shanghai Geling Ltd. 2.2. Synthesis of carbon nanodots (CNDs)
2.5. Preparation of PAAm/PNIPAM–CND hydrogel coatings and patterns
The CNDs were prepared via a pyrolysis process [27]. Briefly, a quartz boat with 1 g egg white was transferred into a tube furnace, and was then pyrolyzed at 250 ◦ C for 2 h at a heating rate of 10 ◦ C min−1 under nitrogen atmosphere. The egg samples turned dark black, and were collected after they cooled down to room temperature for further use and characterization. 0.1 g of the resultant sample was dispersed in 10 ml deionized water and magnetically stirred to form a brownish-yellow solution. These solutions were centrifuged at 10 000 rpm, and the supernatant collected and further filtrated with ultra-filtration membrane (200 nm) three times to remove impurities and larger particles.
The as-synthesized ink-like hybrid hydrogels were coated on a quartz plate by simple blade coating and some simple letters were written by putting the ink-like hybrid hydrogels into a 1 ml injection syringe. The printing experiments were performed by a gravure printer (Schl¨afli Labratester) at room temperature. The prepared ink-like hybrid hydrogels were brushed on the printing plate which had engraved cells patterned on the surface. The ink-like hybrid hydrogels were transferred to the surface of a black paper substrate when the printing plate and the impression cylinder were in contact with each other. The excess ink-like hybrid hydrogels were removed with a doctor blade. 2
Nanotechnology 25 (2014) 055603
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Figure 2. (a) TEM image of synthesized CNDs; scale bar: 50 nm (inset: HRTEM image of CND; scale bar: 2 nm). (b) Raman spectrum of
CNDs. (c) XPS spectrum of CNDs and the corresponding expansion of the C 1s peak (top left in the figure). (d) UV/vis absorption and PL emission spectra (excited from 340 to 480 nm in 20 nm increments) of the CNDs in aqueous solution.
by the crosslink of PAAm (supplementary data, figure S1b available at stacks.iop.org/Nano/25/055603/mmedia) [24].
2.6. Characterization
UV–vis absorption was measured on a Cary 500 UV–visible–NIR spectrophotometer (Varian Co., USA). Photoluminescence (PL) emission measurements were performed using an Agilent Technologies Cary Eclipse Fluorescence spectrophotometer, which was coupled with a thermostatic bath to control the sample temperature with an accuracy of ±1 ◦ C. The morphology and microstructure of the CNDs were examined by high-resolution transmission electron microscopy (HRTEM) on a JEM-2100 microscope (Japan, JEOL) with an accelerating voltage of 200 kV. The sample for HRTEM was made by dropping an aqueous solution onto a 300-mesh copper grid coated with a lacey carbon film. The x-ray photoelectron spectroscopy (XPS) spectrum of the sample was measured on a PHI 5000 Versa Probe. The Raman spectrum of the samples was measured on a Raman spectroscope (JY super LabRam) using a 632.8 nm wavelength laser.
3.1. Characterization of CNDs
Herein, the blue luminescent CNDs were obtained by a pyrolysis process of chicken eggs [27]. The morphology of the CNDs was characterized using TEM. Figure 2(a) shows a TEM image of the CNDs, which have spherical shapes and are well-dispersed and range between 2 and 8 nm in diameter. The inset in figure 2(a) illustrates well-resolved lattice fringes with a lattice spacing of 0.28 nm, which is close to the (100) facet of graphite. The Raman spectrum of the CNDs (figure 1(b)) exhibits two broad peaks at around 1365 cm−1 (D band) and 1570 cm−1 (G band). The D band is associated with vibrations of carbon atoms with dangling bonds in the termination plane of disordered carbon. The G band is corresponding to the first-order scattering of photons by sp2 carbon atoms in a two-dimensional hexagonal lattice. The relative intensity ratio of the disordered D band and crystalline G band (ID /IG ) is a measure of disorder degree and average size of the sp2 domains [29, 30]. The ID /IG for the CNDs is around 0.97, indicating that they have a similar structure to graphite. By using the empirical Tuinstra–Koenig relation [31], which relates the ID /IG ratio to the sp2 cluster size, it can be concluded that the sp2 cluster size is about 4.5 nm, which is consistent with the observation of the TEM image. The chemical compositions of CNDs were characterized by XPS as shown in figure 2(c), indicating the existence of carbon (C 1s, 284 eV), nitrogen (N 1s, 399 eV) and oxygen (O 1s, 532 eV), respectively. The expanded high-resolution scan of the C 1s region shows the presence of three types
3. Results and discussion
The photoluminescent PAAm/PNIPAM–CND hydrogels were readily fabricated through the crosslink of PAAm in the presence of pre-synthesized PNIPAM and CNDs using KPS as initiator, MBA as crosslinking agent and TEMED as accelerator at room temperature. First, NIPAM monomers were initiated by KPS to form the PNIPAM microspheres (270 nm in diameter) at 68–70 ◦ C (supplementary data, figure S1a available at stacks.iop.org/Nano/25/055603/mmedia). Second, the PNIPAM microspheres swelled to form PNIPAM networks at room temperature (below the LCST), and formed the semi-interpenetrating networks with the CNDs entrapped 3
Nanotechnology 25 (2014) 055603
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Figure 3. Effects of temperature on the PL spectra of (a) the as-prepared CNDs and (b) PAAm/PNIPAM–CND hydrogels. (c) PL intensity
measured during repeated heating (55 ◦ C) and cooling (20 ◦ C) cycles of the PAAm/PNIPAM–CND hydrogels.
of carbon bonds, corresponding to sp2 (C=C) at 284.8 eV, C–OH/C–O–C at 286.0 eV, and C=O at 289.2 eV. The UV–visible absorption and emission spectra are presented in figure 2(d). The CNDs show broad UV absorption at 275 nm, which is consistent with carbon dots synthesized by plasma-induced pyrolysis of eggs [27]. In addition, the CNDs exhibit excitation-wavelength-dependent PL behavior; the PL maximum shifts from ∼410 to ∼500 nm when the excitation wavelength is varied from 340 to 480 nm. The strongest fluorescence emission band located at 420 nm is observed under 360 nm excitation. It is well known that variously sized CNDs prepared via different approaches can emit PL with different colors [1, 2, 32]. However, the exact PL mechanism of the CNDs remains elusive. Typically, the luminescence mechanism may derive from recombination of localized electron–hole pairs [32] and defect state emission [13]. Herein, it has been hypothesized that it is associated with passivated surface defects of the core carbon particles. As evident from XPS, the synthesized CNDs have various functional groups like C–OH, C–O–C and C=O on the surface which may result in the PL emission [33].
Moreover, it is demonstrated that the reproducibility of the PAAm/PNIPAM–CND hydrogels’ fluorescence intensity corresponding to the temperature variation as shown in figure 3(c). After subjecting the sample to five heating (at 55 ◦ C) and cooling (at 20 ◦ C) cycles, the PAAm/PNIPAM–CND hydrogels show a conspicuous temperature-dependent on–off PL and the intensity can almost revert back to the original value, indicating the connection between the volume phase transition of the hydrogels and the fluorescence properties. From the repeated heating and cooling process, it can be concluded the CNDs in the PAAm/PNIPAM gels had little effect on the polymer characteristics of the temperature-induced phase transition. Therefore, the reversible temperature-sensitive PL properties make the hybrid hydrogels promising in printing thermoresponsive photoluminescent markers for labeling some biological products, whose storage temperature need to be strictly controlled. Figure 4(a) presents the photographs of PAAm/PNIPAM–CND hydrogels taken before and after the phase transition under visible and UV light. When the temperature increased from 20 ◦ C (below the LCST) to 40 ◦ C (above the LCST) the hydrogel became opaque and had small volume change (volume expansion ∼ 1.4%); the PL intensity of the hydrogels under UV light was decreased. Finally, these PAAm/PNIPAM–CND hydrogels were utilized in inks for printing patterns. A commercial black paper (paper without background UV fluorescence) was chosen as the printing paper. The viscosity (the most important factor of inks) of PAAm/PNIPAM–CND hydrogels was controlled by varying the proportion of PAA. The optimal viscosity was measured to be 360 cP, so that the ink-like hybrid hydrogels could be used with pen-on-paper (figure 4(b)) or easily coated quartz plate (figure 4(c)). Gravure printing is a unique approach for creating highly defined patterns, which can be used for conductive circuits, flexible electronics and sensors [35]. In this study, gravure printing was also used for making such fluorescent patterns. The prepared ink-like PAAm/PNIPAM–CND hybrid hydrogels were brushed on the printing plate which had engraved cells patterned on the surface. The ink-like hybrid hydrogels were transferred to the surface of the black paper as a strip luminescent pattern when the printing plate and the impression cylinder were in contact with each other. Upon exciting with a UV lamp, the prepared
3.2. Characterization of PAAm/PNIPAM–CND hydrogels
We adopted fluorescence spectroscopy to monitor the optical changes arising from the swelling and collapsing of the hybrid hydrogels at different temperatures. The fluorescence spectra of PAAm/PNIPAM–CND hydrogels were investigated according to the temperature change from 20 to 55 ◦ C as shown in figure 3. It can be seen from figure 3(a) that the PL intensities of the original CND solution have a small change when the temperature is increased from 20 to 55 ◦ C. Interestingly, the PL intensity of the PAAm/PNIPAM–CND hydrogels is also sensitive to the external temperature stimuli. As shown in figure 3(b), the PL intensities of the PAAm/PNIPAM–CND hydrogels decrease with increasing temperature and decrease sharply above the LCST of around 35 ◦ C. When the temperature is increased to above the LCST, the microspheres have a polymer volume transition so that the PNIPAM can act as a strong scattering center [34]. Scattering caused by a polymer volume transition and reduction of PL efficiency of CNDs upon heating could decrease the PL intensities. 4
Nanotechnology 25 (2014) 055603
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Figure 4. (a) Photographs of the PAAm/PNIPAM–CND hydrogels at 20 ◦ C (left) and 40 ◦ C (right) both under visible and UV light,
respectively. (b) Patterning of PAAm/PNIPAM–CND hydrogels on black paper under UV light. (c) PAAm/PNIPAM–CND hydrogel coating on a quartz plate under visible and UV light, respectively. (d) Effects of temperature on the optical transmittance spectra of PAAm/PNIPAM–CND coating on quartz.
Acknowledgments
coating and pattern exhibit bright blue emission and they remain emissive over three months under ambient conditions. This result has important implications for the use of such hybrid hydrogels on luminescent patterns using printing techniques. Figure 4(d) displays optical transmittance spectra of PAAm/PNIPAM–CND films on a quartz plate at different temperatures. It is found that the optical transmittance of the film decreases with increasing the testing temperature and decreases significantly at above 35 ◦ C, which is consistent with the temperature dependence properties of PNIPAM, and further confirmed the above conclusion that such reversible PL as a result of the thermoresponse enables the PAAm/PNIPAM–CND hydrogels to have potential in various applications for printing smart photoluminescent markers.
This research was supported by the National Natural Science Foundation of China (No. 50972091), the Shanghai Municipal Science & Technology Commission key supporting project (No. 12120502900), and the industrial research fund from Wuhu Token Sciences Co., Ltd. Professor Lei Huang appreciates the support of The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. References [1] Baker S N and Baker G A 2010 Luminescent carbon nanodots: emergent nanolights Angew. Chem. Int. Edn. 49 6726–44 [2] Sun Y P et al 2006 Quantum-sized carbon dots for bright and colorful photoluminescence J. Am. Chem. Soc. 128 7756–7 [3] Zheng L, Chi Y, Dong Y, Lin J and Wang B 2009 Electrochemiluminescence of water-soluble carbon nanocrystals released electrochemically from graphite J. Am. Chem. Soc. 131 4564–5 [4] Genger U R, Grabolle M, Jaricot S C, Nitschke R and Nann T 2008 Quantum dots versus organic dyes as fluorescent labels Nature Methods 5 763–75 [5] Cao L et al 2007 Carbon dots for multiphoton bioimaging J. Am. Chem. Soc. 129 11318–9 [6] Yang S T et al 2009 Carbon dots for optical imaging in vivo J. Am. Chem. Soc. 131 11308–9 [7] Liu R, Wu D, Liu S, Koynov K, Knoll W and Li Q 2009 An aqueous route to multicolor photoluminescent carbon dots using silica spheres as carriers Angew. Chem. 121 4668–71 [8] Nair A, Shen J, Thevenot P, Zou L, Cai T, Hu Z and Tang L P 2008 Enhanced intratumoral uptake of quantum dots
4. Conclusion
In summary, we have successfully designed and synthesized ink-like hybrid hydrogels with semi-interpenetrating networks and reversible photoluminescent thermoresponses. The PL intensity and the optical transmittance of the hydrogels are reversibly sensitive to external temperature stimuli in the temperature range of 20–55 ◦ C, which is attributed to the responsive behavior of the PNIPAM matrix. Furthermore, the prepared hydrogels can be coated on various substrates and the gravure printed pattern exhibits blue emission upon exciting with a UV lamp, showing promise as a smart fluorescent ink. All of the above imply that the versatility of the hybrid hydrogels will have potential in applications in many fields, such as thermosensitive devices, printed smart photoluminescent labeling and sensors. 5
Nanotechnology 25 (2014) 055603
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[22] Salcher A, Nikolic M S, Casado S, V´elez M, Weller H and Ju´arez B H 2010 CdSe/CdS nanoparticles immobilized on pNIPAm-based microspheres J. Mater. Chem. 20 1367–74 [23] Ghosh Mitra S, Diercks D R, Mills N C, Hynds D L and Ghosh S 2011 Excellent biocompatibility of semiconductor quantum dots encased in multifunctional poly(N-isopropylacrylamide) nanoreservoirs and nuclear specific labeling of growing neurons Appl. Phys. Lett. 98 103702 [24] Chang C Y, Peng J, Zhang L and Pang D W 2009 Strongly fluorescent hydrogels with quantum dots embedded in cellulose matrices J. Mater. Chem. 19 7771–6 [25] Ja´nczewski D, Tomczak N, Han M Y and Vancso G J 2011 Synthesis of functionalized amphiphilic polymers for coating quantum dots Nature Prot. 6 1546–53 [26] Zhang P, Li W, Zhai X, Liu C, Dai L and Liu W 2012 A facile and versatile approach to biocompatible ‘fluorescent polymers’ from polymerizable carbon nanodots Chem. Commun. 48 10431–3 [27] Wang J, Wang C F and Chen S 2012 Amphiphilic egg-derived carbon dots: rapid plasma fabrication, pyrolysis process, and multicolor printing patterns Angew. Chem. 124 9431–5 [28] Gao J and Frisken B J 2003 Cross-linker-free N-isopropylacrylamide gel nanospheres Langmuir 19 5212–6 [29] Ferrari A C and Robertson J 2000 Interpretation of Raman spectra of disordered and amorphous carbon Phys. Rev. B 61 14095–107 [30] Huang L, Liu Y, Ji L C, Xie Y Q, Wang T and Shi W Z 2011 Pulsed laser assisted reduction of graphene oxide Carbon 49 2431–6 [31] Tuinstra F and Koenig J L 1970 Raman spectrum of graphite J. Chem. Phys. 53 1126–30 [32] Liu Y, Liu C Y and Zhang Z Y 2013 Graphitized carbon dots emitting strong green photoluminescence J. Mater. Chem. C 1 4902–7 [33] Hu S L, Niu K Y, Sun J, Yang J, Zhao Z Q and Du X W 2008 One-step synthesis of fluorescent carbon nanoparticles by laser irradiation J. Mater. Chem. 19 484–8 [34] Wang Y Q, Zhang Y Y, Zhang F and Li W Y 2011 One-pot synthesis of thermal responsive QDs–PNIPAM hybrid fluorescent microspheres by controlling the polymerization temperature at two different polymerization stages J. Mater. Chem. 21 6556–62 [35] Voigt M M et al 2010 Polymer field-effect transistors fabricated by the sequential gravure printing of polythiophene, two insulator layers, and a metal ink gate Adv. Funct. Mater. 20 239–46
concealed within hydrogel nanoparticles Nanotechnology 19 485102 Zhu A, Qu Q, Shao X, Kong B and Tian Y 2012 Carbon-dot-based dual-emission nanohybrid produces a ratiometric fluorescent sensor for in vivo imaging of cellular copper ions Angew. Chem. 124 7297–301 Li H et al 2010 Water-soluble fluorescent carbon quantum dots and photocatalyst design Angew. Chem. 122 4532–6 Yan X, Cui X, Li B and Li L S 2010 Large, solution-processable graphene quantum dots as light absorbers for photovoltaics Nano Lett. 10 1869–73 Li Y, Hu Y, Zhao Y, Shi G, Hou Y and Qu L 2011 An electrochemical avenue to green-luminescent graphene quantum dots as potential electron-acceptors for photovoltaics Adv. Mater. 23 776–80 Esteves da Silva J C G and Goncalves H M R 2011 Analytical and bioanalytical applications of carbon dots Trends Anal. Chem. 30 1327–36 Qu S, Wang X, Lu Q, Liu X and Wang L 2012 A biocompatible fluorescent ink based on water-soluble luminescent carbon nanodots Angew. Chem. Int. Edn 51 12215–8 Pan D, Zhang J, Li Z, Zhang Z, Guo L and Wu M 2011 Blue fluorescent carbon thin films fabricated from dodecylamine-capped carbon nanoparticles J. Mater. Chem. 21 3565–7 Otake K, Inomata H, Konno M and Saito S 1990 Thermal analysis of the volume phase transition with N-isopropylacrylamide gels Macromolecules 23 283–9 Kubota K, Fujishige S and Ando I 1990 Single-chain transition of poly(N-isopropylacrylamide) in water J. Phys. Chem. 94 5154–8 Li J, Hong X, Liu Y, Li D, Wang Y, Li J, Bai Y and Li T 2005 Highly photoluminescent CdTe/poly(N-isopropylacrylamide) temperature-sensitive gels Adv. Mater. 17 163–6 Ja´nczewski D, Tomczak N, Song J, Long H, Han M Y and Vancso G J 2011 Fabrication and responsive behaviour of quantum dot/PNIPAM micropatterns obtained by template copolymerization in water J. Mater. Chem. 21 6487–90 Lv S, Liu L and Yang W 2010 Preparation of soft hydrogel nanoparticles with PNIPAm hair and characterization of their temperature-induced aggregation Langmuir 26 2076–82 Gong Y, Gao M, Wang D and M¨ohwald H 2005 Incorporating fluorescent CdTe nanocrystals into a hydrogel via hydrogen bonding: toward fluorescent microspheres with temperature-responsive properties Chem. Mater. 17 2648–53
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My IOPscience
Printable temperature-responsive hybrid hydrogels with photoluminescent carbon nanodots
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 055603 (http://iopscience.iop.org/0957-4484/25/5/055603) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 128.135.12.127 This content was downloaded on 11/11/2014 at 20:19
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 055603 (6pp)
doi:10.1088/0957-4484/25/5/055603
Printable temperature-responsive hybrid hydrogels with photoluminescent carbon nanodots Panpan Li, Lei Huang, Youjie Lin, Leo Shen, Qi Chen and Wangzhou Shi Joint Lab with Wuhu Token for Graphene Electrical Materials and Application, Department of Physics, Shanghai Normal University, Guilin Road 100, Shanghai 200234, People’s Republic of China E-mail: [email protected] and [email protected] Received 7 October 2013, revised 16 November 2013 Accepted for publication 11 December 2013 Published 9 January 2014 Abstract
Smart ink-like hybrid hydrogels that simultaneously possess semi-interpenetrating network structure, strong photoluminescence and temperature sensitivity are successfully fabricated based on the crosslink of poly(acrylamide) (PAAm) in the presence of poly(N-isopropylacrylamide) (PNIPAM) and carbon nanodots (CNDs) at room temperature. The resulting hybrid hydrogels were highly photoluminescent. The photoluminescence was sensitive to external temperature stimuli and reversible. Moreover, the hybrid hydrogels were applied as fluorescent inks for patterning using gravure printing, which may open a door towards developing smart CND based thermosensitive photoluminescent markers and sensors. Keywords: carbon nanodot, photoluminescence, temperature-responsive hydrogel, printable ink S Online supplementary data available from stacks.iop.org/Nano/25/055603/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
with a coil-to-globule transition in water at a lower critical solution temperature (LCST) around 32 ◦ C, is a well-known temperature-responsive polymer as an active matrix transducing environmental temperature changes to embedded QDs [18, 19]. Fluorescent PNIPAM hydrogels, which endow the ‘smart’ hydrogel matrix with luminescent properties of QDs, can be applied to a new generation of fluorescence markers with temperature-sensitive characteristics. Although semiconductor QDs have been used to synthesize hybrid QD/PNIPAM hydrogels [19–24] the intrinsic environmental risks associated with semiconductor QDs make the hybrid QD/PNIPAM hydrogels unsuitable for biomedical and environmental applications [25]. Recently, Zhang et al (2012) [26] reported that they had first copolymerized CNDs with NIPAM monomers to produce thermoresponsive fluorescent polymeric materials in solution or the solid-state. Unfortunately, the viscosity of the solution is too low to provide a sol–gel platform for printing smart photoluminescent markers.
Photoluminescent (PL) carbon nanodots (CNDs) are attracting considerable attention as nascent quantum dots (QDs) owing to their favorable optical properties along with chemical inertness, low toxicity and excellent biocompatibility [1–3], which give rise to their exciting application in labeling [4], biomedical imaging [5–8], sensing [9, 10] and photovoltaic devices [11, 12]. Currently, CNDs stand to have a huge impact in biotechnological and environmental applications due to their potential as nontoxic alternatives to traditional heavy-metal-based QDs [13, 14]. Qu et al (2012) [14] have confirmed that CNDs can exist in organisms, maintaining their PL characteristics and without any damage. For such applications, embedding QDs into an appropriate matrix is an effective method of preventing their agglomeration because strong fluorescence quenching occurs in dry and aggregate states [14, 15]. Among the possible matrices, poly(N-isopropylacrylamide) (PNIPAM) [16, 17], 0957-4484/14/055603+06$33.00
1
c 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 055603
P Li et al
Figure 1. Schematic diagram of the synthetic route of PAAm/PNIPAM–CND hydrogels with reversible temperature responses.
The goal of this study was to fabricate highly photoluminescent PAAm/PNIPAM–CND hybrid hydrogels with proper fluidic properties. Therefore, we directly embedded PNIPAM and CNDs into a polyacrylamide (PAAm) matrix. Then, ink-like hybrid hydrogels were obtained by controlling the proportion of PAAm. The process sequence of synthesizing the hybrid hydrogels that combine the features of semi-interpenetrating network structure and strong photoluminescence through the crosslink of PAAm in the presence of PNIPAM and CNDs at room temperature is shown in figure 1. The synthesized hybrid hydrogels show reversible, temperature-dependent on–off photoluminescent behavior at approximately 35 ◦ C. Furthermore, the ink-like hybrid hydrogels could be coated on different substrates and the printed pattern exhibits blue emission upon exciting with a UV lamp.
2.3. Synthesis of PNIPAM microspheres
PNIPAM hydrogel spheres were prepared by surfactant-free radical polymerization [28]. A 1.50 g sample of NIPAM was dissolved in 150 ml of water at room temperature and the solution obtained was purged with N2 under stirring for 30 min. A 0.068 g sample of KPS was dissolved in 30 ml of water and the resultant solution was poured into the O2 -free NIPAM solution. The polymerization was performed for 4 h at 68–70 ◦ C under N2 . After the solution was cooled to room temperature, hydrogel spheres were obtained according to the self-cross-linking effect of PNIPAM chains and purified by three repetitions of centrifugation at 18 000 rpm and re-dispersion in water. To modify the crosslinking degree of PNIPAM spheres, we added different amounts of MBA, 0.012, 0.0225, 0.045, and 0.15 g, into the monomer solutions, corresponding to weight percentages of MBA of 0.8, 1.5, 3, and 10 wt%, respectively.
2. Experimental section
2.4. Synthesis of PAAm/ PNIPAM–CND hydrogels
2.1. Materials
The photoluminescent PAAm/PNIPAM–CND hydrogels were prepared by radical copolymerization of AAM, PNIPAM and MBA in the presence of TEMED as catalyst, and KPS as initiator in water. Briefly, AAm (0.71 g, 0.01 mol), PNIPAM (0.98 wt%, 3 ml), CNDs (1 wt%, 3 ml ), MBA (138 mg, 0.9 mmol) and TEMED (2 µl) were dispersed in 30 ml deionized water, stirring at room temperature for 10 min. After that, KPS solution (4 ml, 28 mmol l−1 ) was added to the reaction system, then the reaction mixture was quickly moved to a quartz tube, and then kept at room temperature for 8 h.
N-isopropylacrylamide (NIPAM, 99%) was recrystallized from hexane before use. N, N 0 -Methylenebisacrylamide (MBA, 99%) and tetramethylenediamine (TEMED) were purchased from Aldrich. Acrylamide (AAm) and potassium persulfate (KPS, 99.5%) were purchased from Sinopharm Chemical Reagent. NIPAM was purified by recrystallization from hexane prior to use. Fresh chicken eggs were purchased from local markets. The egg white and yolk were well separated by an egg separator prior to use. The black papers for printing were purchased from Shanghai Geling Ltd. 2.2. Synthesis of carbon nanodots (CNDs)
2.5. Preparation of PAAm/PNIPAM–CND hydrogel coatings and patterns
The CNDs were prepared via a pyrolysis process [27]. Briefly, a quartz boat with 1 g egg white was transferred into a tube furnace, and was then pyrolyzed at 250 ◦ C for 2 h at a heating rate of 10 ◦ C min−1 under nitrogen atmosphere. The egg samples turned dark black, and were collected after they cooled down to room temperature for further use and characterization. 0.1 g of the resultant sample was dispersed in 10 ml deionized water and magnetically stirred to form a brownish-yellow solution. These solutions were centrifuged at 10 000 rpm, and the supernatant collected and further filtrated with ultra-filtration membrane (200 nm) three times to remove impurities and larger particles.
The as-synthesized ink-like hybrid hydrogels were coated on a quartz plate by simple blade coating and some simple letters were written by putting the ink-like hybrid hydrogels into a 1 ml injection syringe. The printing experiments were performed by a gravure printer (Schl¨afli Labratester) at room temperature. The prepared ink-like hybrid hydrogels were brushed on the printing plate which had engraved cells patterned on the surface. The ink-like hybrid hydrogels were transferred to the surface of a black paper substrate when the printing plate and the impression cylinder were in contact with each other. The excess ink-like hybrid hydrogels were removed with a doctor blade. 2
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Figure 2. (a) TEM image of synthesized CNDs; scale bar: 50 nm (inset: HRTEM image of CND; scale bar: 2 nm). (b) Raman spectrum of
CNDs. (c) XPS spectrum of CNDs and the corresponding expansion of the C 1s peak (top left in the figure). (d) UV/vis absorption and PL emission spectra (excited from 340 to 480 nm in 20 nm increments) of the CNDs in aqueous solution.
by the crosslink of PAAm (supplementary data, figure S1b available at stacks.iop.org/Nano/25/055603/mmedia) [24].
2.6. Characterization
UV–vis absorption was measured on a Cary 500 UV–visible–NIR spectrophotometer (Varian Co., USA). Photoluminescence (PL) emission measurements were performed using an Agilent Technologies Cary Eclipse Fluorescence spectrophotometer, which was coupled with a thermostatic bath to control the sample temperature with an accuracy of ±1 ◦ C. The morphology and microstructure of the CNDs were examined by high-resolution transmission electron microscopy (HRTEM) on a JEM-2100 microscope (Japan, JEOL) with an accelerating voltage of 200 kV. The sample for HRTEM was made by dropping an aqueous solution onto a 300-mesh copper grid coated with a lacey carbon film. The x-ray photoelectron spectroscopy (XPS) spectrum of the sample was measured on a PHI 5000 Versa Probe. The Raman spectrum of the samples was measured on a Raman spectroscope (JY super LabRam) using a 632.8 nm wavelength laser.
3.1. Characterization of CNDs
Herein, the blue luminescent CNDs were obtained by a pyrolysis process of chicken eggs [27]. The morphology of the CNDs was characterized using TEM. Figure 2(a) shows a TEM image of the CNDs, which have spherical shapes and are well-dispersed and range between 2 and 8 nm in diameter. The inset in figure 2(a) illustrates well-resolved lattice fringes with a lattice spacing of 0.28 nm, which is close to the (100) facet of graphite. The Raman spectrum of the CNDs (figure 1(b)) exhibits two broad peaks at around 1365 cm−1 (D band) and 1570 cm−1 (G band). The D band is associated with vibrations of carbon atoms with dangling bonds in the termination plane of disordered carbon. The G band is corresponding to the first-order scattering of photons by sp2 carbon atoms in a two-dimensional hexagonal lattice. The relative intensity ratio of the disordered D band and crystalline G band (ID /IG ) is a measure of disorder degree and average size of the sp2 domains [29, 30]. The ID /IG for the CNDs is around 0.97, indicating that they have a similar structure to graphite. By using the empirical Tuinstra–Koenig relation [31], which relates the ID /IG ratio to the sp2 cluster size, it can be concluded that the sp2 cluster size is about 4.5 nm, which is consistent with the observation of the TEM image. The chemical compositions of CNDs were characterized by XPS as shown in figure 2(c), indicating the existence of carbon (C 1s, 284 eV), nitrogen (N 1s, 399 eV) and oxygen (O 1s, 532 eV), respectively. The expanded high-resolution scan of the C 1s region shows the presence of three types
3. Results and discussion
The photoluminescent PAAm/PNIPAM–CND hydrogels were readily fabricated through the crosslink of PAAm in the presence of pre-synthesized PNIPAM and CNDs using KPS as initiator, MBA as crosslinking agent and TEMED as accelerator at room temperature. First, NIPAM monomers were initiated by KPS to form the PNIPAM microspheres (270 nm in diameter) at 68–70 ◦ C (supplementary data, figure S1a available at stacks.iop.org/Nano/25/055603/mmedia). Second, the PNIPAM microspheres swelled to form PNIPAM networks at room temperature (below the LCST), and formed the semi-interpenetrating networks with the CNDs entrapped 3
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Figure 3. Effects of temperature on the PL spectra of (a) the as-prepared CNDs and (b) PAAm/PNIPAM–CND hydrogels. (c) PL intensity
measured during repeated heating (55 ◦ C) and cooling (20 ◦ C) cycles of the PAAm/PNIPAM–CND hydrogels.
of carbon bonds, corresponding to sp2 (C=C) at 284.8 eV, C–OH/C–O–C at 286.0 eV, and C=O at 289.2 eV. The UV–visible absorption and emission spectra are presented in figure 2(d). The CNDs show broad UV absorption at 275 nm, which is consistent with carbon dots synthesized by plasma-induced pyrolysis of eggs [27]. In addition, the CNDs exhibit excitation-wavelength-dependent PL behavior; the PL maximum shifts from ∼410 to ∼500 nm when the excitation wavelength is varied from 340 to 480 nm. The strongest fluorescence emission band located at 420 nm is observed under 360 nm excitation. It is well known that variously sized CNDs prepared via different approaches can emit PL with different colors [1, 2, 32]. However, the exact PL mechanism of the CNDs remains elusive. Typically, the luminescence mechanism may derive from recombination of localized electron–hole pairs [32] and defect state emission [13]. Herein, it has been hypothesized that it is associated with passivated surface defects of the core carbon particles. As evident from XPS, the synthesized CNDs have various functional groups like C–OH, C–O–C and C=O on the surface which may result in the PL emission [33].
Moreover, it is demonstrated that the reproducibility of the PAAm/PNIPAM–CND hydrogels’ fluorescence intensity corresponding to the temperature variation as shown in figure 3(c). After subjecting the sample to five heating (at 55 ◦ C) and cooling (at 20 ◦ C) cycles, the PAAm/PNIPAM–CND hydrogels show a conspicuous temperature-dependent on–off PL and the intensity can almost revert back to the original value, indicating the connection between the volume phase transition of the hydrogels and the fluorescence properties. From the repeated heating and cooling process, it can be concluded the CNDs in the PAAm/PNIPAM gels had little effect on the polymer characteristics of the temperature-induced phase transition. Therefore, the reversible temperature-sensitive PL properties make the hybrid hydrogels promising in printing thermoresponsive photoluminescent markers for labeling some biological products, whose storage temperature need to be strictly controlled. Figure 4(a) presents the photographs of PAAm/PNIPAM–CND hydrogels taken before and after the phase transition under visible and UV light. When the temperature increased from 20 ◦ C (below the LCST) to 40 ◦ C (above the LCST) the hydrogel became opaque and had small volume change (volume expansion ∼ 1.4%); the PL intensity of the hydrogels under UV light was decreased. Finally, these PAAm/PNIPAM–CND hydrogels were utilized in inks for printing patterns. A commercial black paper (paper without background UV fluorescence) was chosen as the printing paper. The viscosity (the most important factor of inks) of PAAm/PNIPAM–CND hydrogels was controlled by varying the proportion of PAA. The optimal viscosity was measured to be 360 cP, so that the ink-like hybrid hydrogels could be used with pen-on-paper (figure 4(b)) or easily coated quartz plate (figure 4(c)). Gravure printing is a unique approach for creating highly defined patterns, which can be used for conductive circuits, flexible electronics and sensors [35]. In this study, gravure printing was also used for making such fluorescent patterns. The prepared ink-like PAAm/PNIPAM–CND hybrid hydrogels were brushed on the printing plate which had engraved cells patterned on the surface. The ink-like hybrid hydrogels were transferred to the surface of the black paper as a strip luminescent pattern when the printing plate and the impression cylinder were in contact with each other. Upon exciting with a UV lamp, the prepared
3.2. Characterization of PAAm/PNIPAM–CND hydrogels
We adopted fluorescence spectroscopy to monitor the optical changes arising from the swelling and collapsing of the hybrid hydrogels at different temperatures. The fluorescence spectra of PAAm/PNIPAM–CND hydrogels were investigated according to the temperature change from 20 to 55 ◦ C as shown in figure 3. It can be seen from figure 3(a) that the PL intensities of the original CND solution have a small change when the temperature is increased from 20 to 55 ◦ C. Interestingly, the PL intensity of the PAAm/PNIPAM–CND hydrogels is also sensitive to the external temperature stimuli. As shown in figure 3(b), the PL intensities of the PAAm/PNIPAM–CND hydrogels decrease with increasing temperature and decrease sharply above the LCST of around 35 ◦ C. When the temperature is increased to above the LCST, the microspheres have a polymer volume transition so that the PNIPAM can act as a strong scattering center [34]. Scattering caused by a polymer volume transition and reduction of PL efficiency of CNDs upon heating could decrease the PL intensities. 4
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Figure 4. (a) Photographs of the PAAm/PNIPAM–CND hydrogels at 20 ◦ C (left) and 40 ◦ C (right) both under visible and UV light,
respectively. (b) Patterning of PAAm/PNIPAM–CND hydrogels on black paper under UV light. (c) PAAm/PNIPAM–CND hydrogel coating on a quartz plate under visible and UV light, respectively. (d) Effects of temperature on the optical transmittance spectra of PAAm/PNIPAM–CND coating on quartz.
Acknowledgments
coating and pattern exhibit bright blue emission and they remain emissive over three months under ambient conditions. This result has important implications for the use of such hybrid hydrogels on luminescent patterns using printing techniques. Figure 4(d) displays optical transmittance spectra of PAAm/PNIPAM–CND films on a quartz plate at different temperatures. It is found that the optical transmittance of the film decreases with increasing the testing temperature and decreases significantly at above 35 ◦ C, which is consistent with the temperature dependence properties of PNIPAM, and further confirmed the above conclusion that such reversible PL as a result of the thermoresponse enables the PAAm/PNIPAM–CND hydrogels to have potential in various applications for printing smart photoluminescent markers.
This research was supported by the National Natural Science Foundation of China (No. 50972091), the Shanghai Municipal Science & Technology Commission key supporting project (No. 12120502900), and the industrial research fund from Wuhu Token Sciences Co., Ltd. Professor Lei Huang appreciates the support of The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. References [1] Baker S N and Baker G A 2010 Luminescent carbon nanodots: emergent nanolights Angew. Chem. Int. Edn. 49 6726–44 [2] Sun Y P et al 2006 Quantum-sized carbon dots for bright and colorful photoluminescence J. Am. Chem. Soc. 128 7756–7 [3] Zheng L, Chi Y, Dong Y, Lin J and Wang B 2009 Electrochemiluminescence of water-soluble carbon nanocrystals released electrochemically from graphite J. Am. Chem. Soc. 131 4564–5 [4] Genger U R, Grabolle M, Jaricot S C, Nitschke R and Nann T 2008 Quantum dots versus organic dyes as fluorescent labels Nature Methods 5 763–75 [5] Cao L et al 2007 Carbon dots for multiphoton bioimaging J. Am. Chem. Soc. 129 11318–9 [6] Yang S T et al 2009 Carbon dots for optical imaging in vivo J. Am. Chem. Soc. 131 11308–9 [7] Liu R, Wu D, Liu S, Koynov K, Knoll W and Li Q 2009 An aqueous route to multicolor photoluminescent carbon dots using silica spheres as carriers Angew. Chem. 121 4668–71 [8] Nair A, Shen J, Thevenot P, Zou L, Cai T, Hu Z and Tang L P 2008 Enhanced intratumoral uptake of quantum dots
4. Conclusion
In summary, we have successfully designed and synthesized ink-like hybrid hydrogels with semi-interpenetrating networks and reversible photoluminescent thermoresponses. The PL intensity and the optical transmittance of the hydrogels are reversibly sensitive to external temperature stimuli in the temperature range of 20–55 ◦ C, which is attributed to the responsive behavior of the PNIPAM matrix. Furthermore, the prepared hydrogels can be coated on various substrates and the gravure printed pattern exhibits blue emission upon exciting with a UV lamp, showing promise as a smart fluorescent ink. All of the above imply that the versatility of the hybrid hydrogels will have potential in applications in many fields, such as thermosensitive devices, printed smart photoluminescent labeling and sensors. 5
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