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White Light-Emitting Multi-Stimuli-Responsive Hydrogel with Lanthanides and Carbon Dots Qingdi Zhu, Lihong Zhang, Krystyn J Van Vliet, Ali Miserez, and Niels Holten-Andersen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17016 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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ACS Applied Materials & Interfaces

White Light-Emitting Multi-Stimuli-Responsive Hydrogel with Lanthanides and Carbon Dots

Qingdi Zhu1, Lihong Zhang2, Krystyn Van Vliet1, 3, 4*, Ali Miserez2, 5 *, Niels Holten-Andersen3*

1. BioSystems and Micromechanics Interdisciplinary Research Group, Singapore-MIT Alliance for Research and Technology (SMART) Centre, CREATE, Singapore 138602 2. Biological & Biomimetic Material Laboratory, School of Materials Science & Engineering, Nanyang Technological University, Singapore 637553 3. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States 4. Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States 5. School of Biological Sciences, Nanyang Technological University, Singapore, 637551

*Corresponding Authors: E-mail: [email protected] (N. Holten-Andersen) ; [email protected] (A. Miserez); [email protected] (K.J. Van Vliet).

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Abstract Polymers that confer changes in optical properties in response to chemical or mechanical cues offer diverse sensing applications, particularly if this stimuli response is accessible in humid or aqueous environments. Here, luminescent hydrogels were fabricated using a facile aqueous process by incorporating lanthanide ions and carbon dots (CD) into a network of polyacrylamide and poly(acrylic acid). White luminescence was obtained by tuning the balance of blue light-emitting CD to green and red light-emitting lanthanide ions. Exploiting the combined specific sensitivities of the different emitters, the luminescent hydrogel showed chromic responsiveness to multiple stimuli including pH, organic vapors, transition metal ions and temperature. The white light-emitting hydrogel was also stretchable with a fracture strain of 400%. We envision this photoluminescent hydrogel to be a versatile and multifunctional material for chemical and environmental sensing.

Key Words: White-light emission; lanthanides; carbon dots; stimuli-responsive hydrogel; stretchable hydrogel


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Introduction Photoluminescent (PL) hydrogels exhibit widespread potential for applications in drug delivery

1-2

, bioimaging

3-4

and chemical/biosensing

5-7

. PL gels are fabricated by

incorporating luminescent entities, such as fluorescent dyes dots

13-14

coupling

or metal ions and their complexes 4, 17

, coordination bonding

8-9

, quantum dots

10-12

, carbon

15-16

, into the gel matrix through covalent

18

, passive adsorption

19

or physical entrapment

12, 20

.

White light-emitting hydrogels are particularly appealing, because they provide more versatility for optical tunability chromic shifts

21

and higher sensitivity to stimuli-responsiveness due to

22

. A common strategy in white light-emitting hydrogel assembly is to

integrate three primary coloured (RGB) or two complementary coloured luminescence dyes into the gel network

21

. For example, Bairi et al. reported the formation of luminescent

hydrogels by co-assembly with three dyes, rhodamine B (orange to red), riboflavin (green) and 6,7-dimethoxy-2, 4[1H, 3H]-quinazolinedione (blue), within a melamine gel network 23. Upon single wavelength excitation, white light emission can be obtained via an energy transfer cascade through the three dyes with careful tuning of their stoichiometry. In contrast, Rao et al. employed a two-dye system to achieve white light emission in an amino-clay based hydrogel 24. The energy transfer from blue luminescent coronene tetra-carboxylate to yellow luminescent sulforhodamine resulted in pure white light emission. While the aforementioned strategies have proven effective, the resulting white light-emitting hydrogels are usually soft and fragile. Moreover, apart from their luminescent nature, most of these gels lack the functionality of stimuli-responsiveness, which limit their applications. Therefore, facile methods for the fabrication of mechanically robust and stimuli-responsive white luminescent hydrogels are still lacking. Lanthanide ions are ideal luminescence emitters due to their superior optical properties, such as sharp and stable emission peaks, long lifetime, intense luminescence and resistance to photo-bleaching

25

. With their distinct red and green color emission, Eu3+ and Tb3+ are the

two major ions used in luminescence engineering, especially since the presence of an adjacent blue light-emitting “antenna” dye can provide white luminescence upon energy transfer to Eu3+ and Tb3+ 26. Moreover, these “antenna” dyes could also serve as coordination ligands of lanthanide ions, yielding luminescent metallogel networks. For example, Kim et al. synthesized blue light-emissive phenanthroline derivatives which can coordinate with Eu3+ and Tb3+ to form a white light-emitting organogel network upon careful tuning of Eu3+/Tb3+ ratio 27. Similarly, Chen et al. fabricated organo-metallogels by coordinating terpyridyl end3 ACS Paragon Plus Environment


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capped 4-arm-PEG polymer with Eu3+ and Tb3+ providing gels with different color emission, including white light emission, by tuning the molar ratio of the lanthanide ions 22. As a result of the dynamic nature of the terpyridine-lanthanide coordination bonds

28

, one attractive

property of this metallogel is its optical responsiveness to multiple stimuli, both chemical and physical. However, in most work to date and including the examples above, lanthanide-based white light-emitting gels were fabricated in organic solvents, thereby preventing their use in aqueous environments that would offer a much broader range of potential applications. To fabricate white light-emitting hydrogels with Eu3+ and Tb3+, one efficient strategy could be the incorporation of a water-soluble blue light emitter to balance the red and green lanthanide-based emissions. We identified carbon nanodots (CDs) as a promising candidate for this purpose because of their distinct blue color emission, superior resistance to photobleaching, high water solubility, overall good chemical stability and low toxicity

29-30

.

Moreover, we hypothesized that the incorporation of CDs may further endow the hydrogel with unique sensing capabilities thanks to the intrinsic responsiveness of CDs to distinct stimuli such as copper ions, ferric ions and mercury ions 31-35. Recent reports have shown that CD-incorporated hydrogels can indeed be used as effective sensing platforms for metal ions and biological species 36-38. In this study, we developed a facile strategy to fabricate white light-emitting hydrogels by incorporating Eu3+, Tb3+ and CDs into a stretchable polyacrylamide/poly(acrylic acid) hybrid hydrogel network. The lanthanide ions were stabilized within the network through coordination with the carboxylic groups of the acrylic acid, while the methacrylated CDs were covalently conjugated into the gel network through a radical polymerization process. White luminescence could be obtained by adjusting the stoichiometry between coordinated lanthanide ions and CDs. The photoluminescent properties of the hydrogels were characterized alongside a proposed energy transfer mechanism which can account for the observed behaviour. Importantly, the specific sensitivity of both lanthanide ions and carbon dots provides this new type of hydrogels with the capacity to respond to multiple chemical and physical stimuli, including pH, metal ions and temperature. To our knowledge, this is the first white light-emitting hydrogel system combining significant mechanical strain-to-failure with multi-stimuli responsiveness.


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Experimental Reagents and materials Acrylamide (AA), Acrylic acid (AAc), europium chloride hexahydrate, terbium chloride hexahydrate, 1,2-ethanediamine, glycidyl methacrylate, 2,2’:6’,2’’-terpyridine, N,N’methylenebisacrylamide

(MBAA)

and

tetramethylethylenediamine

(TEMED)

were

purchased from Sigma-Aldrich. All reagents were used as received without further purification. Deionized water, filtered through a Millipore Milli-Q purification system to a resistivity of 18 MΩ·cm, was used throughout the experiments. Synthesis of carbon dots and methacrylated carbon dots The CDs with surface amine groups were synthesized in a facile microwave-assisted pyrolysis process as previously reported

39-40

. In brief, 5.35 ml of 1,2-ethanediamine (80

mmol) and 5.49 mL of acrylic acid (80 mmol) were first dissolved in 40 mL of water under stirring. The solution was then put into a 750 W microwave and was heated for 8 min until the majority of the liquid had evaporated and a light smoke emerged. The resulting CDs were cooled down to room temperature and were re-dissolved in 20 mL of water. Finally, the CDs were lyophilized in a freeze dryer and stored in a dry box.

The methacrylated CDs (GMA-CDs) were fabricated by the reaction of glycidyl methacrylate (GMA) with amine groups on the CDs 13, 40. Typically, 5 g of dry CDs were dissolved in 20 ml of water and 10 mL of GMA was then added to the solution. The mixture was then allowed to react for 24 h at 30 ˚C. Next, the oil phase of the mixture was discarded and the water phase was washed with n-hexane five times to remove the unreacted GMA. The GMACDs were further lyophilized and stored in a dry box.

Preparation of white light-emitting hydrogels The process for the preparation of white light-emitting hydrogel is shown in Figure 1. Firstly, 1.5 mL of precursor solution, which contains 3M acrylamide, 0.45M sodium acrylate, 10.7 mg/mL GMA-CDs and 0.75 mM MBAA was degassed. Next, 4 µL of TEMED was added to the solution, which was then immediately transferred into a glass mold (60 mm x 25 mm x 1 mm). The solution was then cured at 60 ˚C for one hour to form the polyacrylamide-co5 ACS Paragon Plus Environment


polyacrylic acid (AA/AAc) hybrid gel network. The gel was then immersed for 24 h in 50 mL of lanthanide salt solution, which contained 10 mM of EuCl3 and 10 mM of TbCl3, followed by a 48 h wash in a 200 mL water bath to remove non-coordinated lanthanide ions. Subsequently, the gel was immersed in 50 mL of 0.2 mM terpyridine solution for 24 hours, followed by another water bath as above to remove excess amount of absorbed terpyridine. The as-prepared hydrogel was stored in a custom-built wet-chamber before further use. Material Characterizations Photoluminescence spectra: The photoluminescence (PL) spectra were collected on a Horiba Fluorolog 3 fluorimeter (Kyoto, Japan). The general lifetime measurements, and the temperature-dependent spectra and lifetime of the lanthanide-centered luminescence, were performed on a Cary Eclipse fluorescence spectrophotometer with a Peltier accessory. The excitation wavelength was kept at 330 nm for all measurements. Determination of lanthanide concentration in the gels: Luminescent hydrogels were prepared with a fixed concentration of acrylic acid (3.2% w/w), which determined the amount of lanthanide ions to be incorporated. The procedures to determine lanthanide concentration inside the hydrogel are shown in Figure S1. In brief, the AAc/AA/CD gel with defined volume (1.5 mL, 60 mm x 25 mm x 1 mm, Vgel) was first immersed into 100 ml (V1) of lanthanide solution containing Eu3+ and Tb3+ for 24 h. The gel was then immersed into 200 mL (V2) of deionized water for 48 h to remove non-coordinated lanthanide ions. The lanthanide concentrations were then measured by using inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 8300, Perkin Elmer, Waltham, MA) in the following solutions (1) initial lanthanide solution (C1); (2) the lanthanide solution after gel immersion (C1’); (3) water after gel immersion (C2). The emission signals were measured at characteristic wavelengths for lanthanides (381.967 nm for Eu and 350.917 nm for Tb). The concentration of lanthanide ions (CLn) inside the gel was then determined with the equation bellow: C Ln = (V1C1 − V1C1 '− V2 C 2 ) / Vgel

Eq. 1

Mechanical properties: The tensile extension and cycling tests of the hydrogels were conducted on an Instron 5567 mechanical testing machine (Canton, MA) with a 10 N load cell. The samples were cut into a dog-bone shape using a standard cutter. The resulting hydrogel sample had a total length of 63.5 mm and a total width of 9.53 mm, and a length and 6 ACS Paragon Plus Environment

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width in the narrow section of 9.53 mm and 3.18 mm, respectively. The strain rate for both extension and cycling tests was kept at 200%/min and the hydrogel samples were coated with a thin layer of silicone oil during the entire test to minimize evaporation. As the initial stressstrain response was approximately linear, the elastic modulus was calculated from the slope of the engineering stress-strain response over the engineering strain range of 5 to 30%.

Results and Discussions Fabrication and characterization of white light-emitting hydrogel To obtain a white light-emitting hydrogel, emitters with three primary colours are required. In this work, Eu3+ and Tb3+, used as red and green emitters, respectively, were introduced into the gel network through coordination with the carboxylate group from the polymerized acrylic acid (see Figure 1). The degree of coordination of carboxylate groups to lanthanide ions in the hydrogel can be estimated from the difference between the number of water molecules coordinated to lanthanide ions (qH2O) in solution and that in the hydrogel. Based on the luminescence quenching effect of coordinated water 41, the qH2O value can be determined by measuring the lanthanide-centered luminescence lifetime in water (τH2O) and in heavy water (τD2O) using the following equation 42: qH 2O = A(1/ τ H 2O – 1/ τ D 2O − kcorr )

Eq. 2

where A is an empirical constant representing the sensitivity of lanthanides to vibronic quenching (with values of 1.2 ms for Eu3+ and 5 ms for Tb3+ constant (0.25 ms

-1

for Eu

3+

and 0.06 ms

-1

42

) and kcorr is a correction

3+

for Tb ). With τH2O and τD2O measured to be

3+

0.120 ms and 2.10 ms, respectively, for Eu , and 0.410 ms and 2.70 ms, respectively, for Tb3+, we calculated qH2O values of 9.1 for Eu3+ and 10.0 for Tb3+, with an error of ± 0.5 originating from the uncertainty in the estimated empirical constant A

41, 43

. These estimated

qH2O values indicate that nine to ten water molecules coordinate with lanthanide ions in free aqueous solution, in agreement with previously reported data

41-42

. The values of τH2O in an

AA/AAc hydrogel with only one type of lanthanide ions were found to be 0.212 ms and 0.640 ms for Eu3+ and Tb3+, respectively (Table S1, Figure S2 in Supporting Information). These increased lifetimes indicate that qH2O for Eu3+ and Tb3+ in the hydrogels have dropped to 4.8 and 5.5, respectively (Table S1), which suggests an exchange of four to five water 7 ACS Paragon Plus Environment


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molecules by the carboxylate groups in the first coordination sphere of the lanthanide ions. When the antenna molecule terpyridine was added to the hydrogel, the calculated qH2O further dropped to 2.9 and 3.5 for Eu3+ and Tb3+ respectively (Table S1), indicating an additional replacement of two to three water molecules. As terpyridine is a tri-dentate ligand, this result suggests that terpyridine coordinates with lanthanide ions in a 1:1 stoichiometry in the aqueous hydrogel matrix, distinct from the 3:1 stoichiometry reported in organic solvents

28,

44

. The quantum yield values of the 1:1 complex of terpyridine with lanthanide ions were

determined to be 0.37% for Eu3+ and 1.83% for Tb3+ using a standard slope method with sulfate quinine (quantum yield = 54.6%) as the reference material (Figure S3). To balance the red and green luminescence from lanthanide ions and thereby obtain white light emission, blue luminescent CDs were introduced into the hydrogel. The aminefunctionalized CDs can be readily modified with GMA to allow efficient polymer coupling 13. The FT-IR spectra of CDs before and after modification of GMA are shown in Figure S4a. While both CDs and GMA-CDs have characteristic peaks at 3600 to 3200 cm-1 (-OH stretching and –NH stretching), 3000 to 2800 cm-1 (-CH stretching), 1645 cm-1 (-C=O stretching, secondary amide) and 1550 cm-1 (-NH bending), the distinct peak at 1715 cm-1 (C=O stretching, α,β-unsaturated ester) for GMA-CDs indicates successful modification of the CD surface. Moreover, GMA-CDs share photoluminescence properties with CDs such as maximum absorption wavelength, maximum emission wavelength and tunable emission under different excitations (Figure S4b-d). The quantum yield of GMA-CDs was determined to be 15.8% (Figure S2). CD-doped hydrogels were fabricated through co-polymerization of GMA-CDs with acrylamide and acrylic acid (see Figure 1). We hypothesized that it should be possible to tune the emission color of the hydrogel by controlling the ratio of CDs to lanthanide ions. To test this hypothesis, a series of hydrogels were prepared with a fixed concentration of acrylic acid (3.2% w/w), which determined the amount of lanthanide ions to be incorporated, and varied concentrations of CDs. As shown in Figure 2a, the emission color of the hydrogel changed from blue to yellow with decreasing content of CDs, i.e. increasing the acrylic acid/CD ratio (weight ratio of acrylic acid to CD from 1:1 to 6:1). The luminescence spectra of the hydrogels (Figure 2b) supported this trend with an increased intensity of the lanthanide-centered luminescence at 490 nm (Tb3+, 5D4→7F6), 544 nm (Tb3+, 5D4→7F5), 580 nm (Tb3+, 5D4→7F5), 590 nm (Eu3+, 5D0→7F1), and 616-620 nm (combined emission peaks of 8 ACS Paragon Plus Environment

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Eu3+,5D0→7F1, and Tb3+, 5D4→7F4) and a decreased intensity of CD-centered luminescence (broad emission peak at 414 nm). Particularly, when the acrylic acid/CD ratio was 3:1, white light emission could be observed from the hydrogel with CIE1931 coordinates of (0.27, 0.35) (Figure 2c). ICP-OES analysis indicated lanthanide ion concentrations in the white lightemitting gel of Eu3+ and Tb3+ as 164 mM and 94 mM, respectively, corresponding to weight ratios between the CDs, Eu3+ and Tb3+ of 1:2.3:1.4. The direct control over the ratio of the three primary emitters provides a facile yet effective strategy to tune the luminescence of the hydrogel, in principle covering the entire color spectrum (Figure S5). For example, we could also obtain a hydrogel with nearly ideal white-emission CIE1931 color coordinates (0.32, 0.33) (Figure S6). It is noted that the UV-excited white light-emitting hydrogel displays weaker emission peaks for both lanthanide- and CD-centered luminescence compared to gels that only contain individual lanthanide ion(s) or CDs (Figure S5b). This is most likely due to an enhanced “inner filter effect” caused by the overall increase of gel UV absorbance upon addition of CDs, thereby resulting in more shallow UV penetration and decreased excitation of both CD and terpyridine. Moreover, the energy transfer process between lanthanide ions and CDs may also contribute to this phenomenon. Photoluminescent measurements of hydrogels with varied combinations of emitters indicated a decrease in Tb3+-centered luminescence lifetime (τ544nm) in the presence of either Eu3+ or CDs, while Eu3+-centered luminescence lifetime (τ616nm) did not change in the presence of CDs (Table 1, Figure S2). It can thus be deduced that the energy transfer direction is from Tb3+ to both Eu3+ and CDs (Scheme 1) with energy transfer efficiencies (e) calculated to be 5.9% and 22.8%, respectively, using the following equation 45 : e = (τ 0 – τ ’) / τ 0

Eq. 3

where τ0 and τ’ are the lifetime values of the donor (Tb3+) in the absence and in the presence of the acceptor (Eu3+ and CDs), respectively. These results support the concept that the energy transfer process could contribute partially to the decreased intensity of the Tb3+centered luminescence in the white light-emitting hydrogels. The mechanical behavior of the white light-emitting hydrogel was investigated by uniaxial tensile testing. As shown in Figure S7a, the hydrogel displayed an ultimate strain-to-failure of 400%. Such high strain capacity has not been reported in previous white light-emitting 9 ACS Paragon Plus Environment


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hydrogel systems and can be attributed to the the loosely-crosslinked polyacrylamide network 46

. Figure S7b shows the results from a cycling tensile test with a maximum strain of 300%.

Mechanical hysteresis (indicating elastic energy dissipation) was observed during the first tensile cycle, concurrent with significant permanent deformation. The subsequent tensile cycles did not display significant hysteresis and no further permanent deformation was observed. These observations may indicate that internal energy dissipation occurs via breakage of the non-covalent coordination bonds between the carboxylate groups and lanthanide ions, which may suggest a load-bearing role of these coordination bonds. The higher elastic modulus of the gels after introduction of Ln ions into the gel network (Young’s modulus E = 27.4 kPa for Eu3+-incorporated gel, E = 24.9 kPa for Tb3+-incorporated gel), as compared to Ln ions-free gels (E = 16.9 kPa), also corroborated this conclusion (Figure S7c). In addition, the stiffness of the hydrogel was also enhanced by the introduction of CDs ( E= 63.7 kPa, Figure S7c). This stiffening of the gel strongly suggests that the GMA-CDs act as effective network crosslinkers, in line with the classical scaling law between modulus and crosslink density. Compared with recently reported white light-emitting hydrogels

23-24

, the

hydrogel reported herein is stiffer (E= 63.7 kPa) and stronger (tensile strength of 239 kPa) due to the dual-crosslinked network design

46-47

. These relatively robust mechanical

properties make the present white light-emitting hydrogel easier to store, handle, and process.

Multi-stimuli responsiveness of the white light-emitting hydrogel The unique physical-chemical properties of both lanthanide ions and CDs have resulted in their functional, but separate, integration in previously reported stimuli-responsive materials utilized in sensing applications

30, 48

. Given the successful co-integration of lanthanide ions

and CDs, we expected a unique combination of stimuli responsive properties from our white light-emitting hydrogels. We first investigated the pH response of the white light-emitting hydrogels. As shown in Figure 3a, the hydrogel displayed both colorimetric and volumetric changes at different pH conditions. At pH 7 the hydrogel retained its white luminescence with a minimal (~ 6%) volume change as compared to the as-prepared initial volume (Figure 3b). At both pH < 7 and pH > 7, the hydrogel displayed similar white-to-blue color shifts, but these shifts were caused by different mechanisms in the acidic and basic regimes. At pH < 7 (acidic), the protonation of the terpyridine (pKa2 = 3.57, pKa3 = 4.54

28

) will cause dissociation of the


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terpyridine-lanthanide complex, hence weakening the “antenna effect” and resulting in a decrease of the lanthanide-centered green and red emission peaks (Figure 3c). This prediction was validated by the increased amount of terpyridine released from the gel as the pH was lowered, which was determined by absorbance measurements of the solution after gel immersion (Figure S8). At the same time the hydrogels swelled, with swelling ratios (V/V0, volume after swell/initial volume) decreasing with decreasing pH and increased acidity (Figure 3b). We attribute the reduced swelling ratio at lower pH to the increasing protonation of free carboxylic groups (from acrylic acid) in the gel network, which would result in a decreased overall ionic charge repulsion between the poly(acrylic acid) chains. At pH > 7 (basic), the formation of insoluble lanthanide hydroxide should lead to both dissociation of the terpyridine-lanthanide complex and precipitation of lanthanide ions out of the gel network. Accordingly, ICP-OES results showed that the amount of lanthanide ions released from the hydrogels dramatically increased at pH 9 and pH 11 compared to only trace amounts at pH 7 (Figure S9). Hence, a decrease or even complete loss of lanthanide-centered luminescence was observed (Figure 3d). The loss of lanthanide ions from the gel network also caused strong charge repulsion between non-coordinated carboxylate groups, which could explain the significant swelling of the gel network observed especially at pH ≥ 9 (Figure 3b). Regardless of the influence on the absolute luminescence intensity due to volumetric changes, the ratio between blue and green emissions (I414/I544, insets, Figure 3c-d) increased both above and below pH 7. This ratio could thus possibly be used as a quantitative index of pH response for the white light-emitting hydrogel in sensing applications. We next investigated the response of the white light-emitting hydrogel to different vapors. Consideration of this cue was motivated by our previous demonstration of vapo-chromism of lanthanide-ligand complex in a white light-emitting organogel system

22

. As illustrated in

Figure 4a, white-to-blue luminescence shift was observed when the hydrogel was exposed to either HCl or NH3 vapor. With no volumetric change after vapor exposure, the hydrogel showed a decrease of the lanthanide-centered luminescence in response to HCl and a complete quenching upon exposure to NH3 (Figure 4b). These responses likely arose from the same mechanisms that caused the pH response in aqueous solutions, i.e. dissociation of terpyridine-lanthanide complexes in acidic environment and precipitation of lanthanide ions in basic environment. On the other hand, when exposed to acetone vapor, the hydrogel responded with a greater increase in the lanthanide-centered luminescence peaks (Figure 4ab). Further investigations showed that drastic increase in lanthanide-centered luminescence 11 ACS Paragon Plus Environment


only occurred when the gel was exposed to water-miscible organic solvent vapors, such as acetone and ethanol, while the exposure to water immiscible organic solvent vapors, including chloroform, hexane and toluene, did not lead to significant change in hydrogel luminescence (Figure S10). Therefore, this response can be attributed to a reduced vibrational quenching of the lanthanide-centered emission, driven by the replacement of water molecules with water-miscible organic solvent inside the gel. Subsequently, we explored the responsiveness of the white light-emitting hydrogel to various metal ions, from monovalent to trivalent and from alkali to transition metals. Figure 5a shows the luminescence of the white light-emitting hydrogels after incubation with 10 mM of metal ions for 30 min. No colorimetric response to alkali or alkaline earth metal ions was detected regardless of their valence, which is indicative of a stronger coordination of the carboxylate ligands to lanthanide ions than to these ions. In contrast, transition metal ions could effectively replace lanthanide ions in the gel network, which resulted in the complete quenching of the lanthanide-centered luminescence (Figure 5b). This observation is in agreement with the more stable directional and covalent-like coordination bonding of the poly(acrylic acid) carboxylic groups with transition metal ions, compared to the more ionic and non-directional interaction with lanthanide ions

49

. In addition, as transition metal ions

form more stable complexes with terpyridine (with a stability constant β > 16 50 as compared to that of the lanthanide-terpyridine complex with β ~ 7

51

), the quenching effect could also

be attributed to the dissociation of the lanthanide-terpyridine complex. Interestingly, after the replacement of the lanthanide ions, the blue emission from CDs dropped in the presence of Cu2+ and was completely quenched when Fe3+ was introduced (inset, Figure 5b). The partial quenching of CD luminescence by Cu2+ could be attributed to the formation of UV-absorbing cupric amine on the surface of the CD, causing an inner filter effect i.e. less UV absorption by the CD itself

31

. In contrast, the complete quenching of CD luminescence by Fe3+ may

result from a non-radiative electron transfer from CDs to surface-coordinated Fe3+, most likely through carboxylic groups

52-53

. This phenomenon usually occurs when the LUMO

energy level of CD couples well with the energy level of an unfilled orbit of Fe3+ 29, 53. In addition to chemical stimuli, the responsiveness of the white light-emitting hydrogel to temperature was investigated. As shown in Figure 6a-b, a significant increase in the overall luminescence intensity, both for lanthanide- and CD-centered emission, was observed upon cooling to -196˚C. This can be explained by much weaker vibrational quenching of lanthanide-centered luminescence by water molecules upon formation of ice, as well as 12 ACS Paragon Plus Environment

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reduced thermal de-population for all emitters in low temperature environment. Interestingly, the hydrogel exhibited a white-to-red luminescent color shift when the temperature was raised from 20˚C to 70˚C (Figure 6a and 6d). The luminescence spectra (Figure 6c) revealed a significant drop of the CD- and Tb3+-centered luminescence with little change in the intensity of Eu3+-centered luminescence as the temperature was elevated. The drop of CD luminescence could be due to the thermal activation of the non-radiative process of the emission. Assuming this thermally activated non-radiative process follows a classical Arrhenius-type law, the relationship between luminescence intensity and temperature can be expressed as:

I (T1 ) / I (T2 ) =

( a + b * exp ( c / T ) ) / ( a + b * exp ( c / T ) ) 2

1

Eq. 4

where I is the luminescence intensity of CDs, T1 and T2 are the absolute temperature, and a, b and c are fitting constants which can be determined by least square regression fitting. The detailed derivation process of Eq. 4 is given in the Supporting Information. As shown in Figure S11, the proposed function is well-fitted (R2 = 99%) with an Arrhenius plot showing logarithm of normalized peak intensity of CDs (at 414 nm) as a function of inverse temperature, which corroborates that the drop of CD luminescence at elevated temperature is caused by a single thermally driven, non-radiative process. This process most likely occurs through thermal activation of non-radiative channels, such as surface/defects/ ionic impurities, as previously observed in other CD-based systems 54,55. On the other hand, the ratio between the green and red luminescence, as demonstrated by the luminescence intensity ratio at 544 nm to 616 nm (I544/I616), exhibited a linear relationship with temperature (Figure 6e) from 20 ˚C to 70 ˚C that could be fitted as:

T = 84.97 – 17.45 I 544 / I 616

Eq. 5

This linear relationship suggests that the white light-emitting hydrogel could be used as a “soft” thermometer

in this temperature range. A temperature cycling test further

demonstrated reversibility of this thermo-chromism (Figure 6f). To better understand the observed linear thermal dependence of this chromism, the lifetime of lanthanide-centered luminescence was measured at different temperatures (Figure S12). For the white lightemitting hydrogel, the lifetime of Tb3+-centered luminescence (544 nm, Figure S12a) dropped linearly with increasing temperature, while the lifetime of Eu3+-centered luminescence (616 nm, Figure S12b) did not change significantly. Similarly, in hydrogels 13 ACS Paragon Plus Environment


with only one lanthanide emitter, a linear lifetime-temperature relationship was observed when Tb3+ was present, but not for Eu3+-loaded gels (Figure S12c-d). These results suggest that the thermo-chromism effect could be mainly attributed to the difference in thermodependent energy transfer efficiency between the antenna ligand (terpyridine) and different lanthanide ions. As Tb3+ exhibits a much higher accepter energy level (5D4, 20500 cm-1) than Eu3+ (5D0, 17200 cm-1) 56, the energy gap between the Tb3+ and the antenna triplet state could be much smaller than for Eu3+, thus making Tb3+ more susceptible to metal-to-ligand energy back transfer, especially at elevated temperature

57

. It is worth noting that this proposed

mechanism differs from most previously reported lanthanide based thermometers, which instead relied on the thermally induced change of energy transfer efficiency between Tb3+ and Eu3+

56, 58

. Although thermo-responsive hydrogels have previously been reported, they

usually display an “on-off” response when a threshold transition temperature is reached 59, 60. In contrast, the present white light-emitting hydrogel is able to continuously monitor temperature changes owing to the well-resolved linear thermo-chromism, which provides superior performance as a soft thermometer.

Conclusion In summary, we have presented a new strategy to fabricate optical multi-stimuli-responsive hydrogels with widely-tunable luminescence by facile incorporation of luminescent lanthanide ions and CDs through coordination bonding and radical co-polymerization, respectively. With simple adjustment of the ratio of RGB emitters, a white light-emitting hydrogel was obtained. Photoluminescence characterization revealed energy transfer mechanisms between emitters, i.e. from Tb3+ to Eu3+, and more uniquely, from Tb3+ to CDs. Moreover, taking advantage of the distinct combined responsiveness of lanthanides and CDs, the white light-emitting hydrogel showed optical response to multiple chemical stimuli such as pH, vapors and metal ions as well as thermal stimulus. Investigation of the hydrogel thermo-chromism uncovered a linear correlation between temperature and the green-to-red emission ratios in the 20-70 ˚C temperature range. To the best of our knowledge, luminescent “soft” thermometers based on stretchable hydrogels have previously not been reported. The multi-stimuli responsiveness, facile fabrication process and robust mechanical properties of our white light-emitting hydrogel, suggest a versatile material platform for environmental sensing. 14 ACS Paragon Plus Environment

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Acknowledgements This research was funded by a seed grant from the BioSystems and Micromechanics thrust of the Singapore-MIT Alliance for Research and Technology (BioSym-SMART). We thank Ms. Maria Chong from the School of Civil and Environmental Engineering at NTU for her help with ICP-OES measurements.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI xxxx.

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Figures

Figure 1. Fabrication process of white luminescence hydrogel.



Figure 2. White-light-emitting hydrogel. (a) Photograph of luminescent hydrogels with different ratios (w/w) between acrylic acid and carbon dots (CDs) under UV light (330 nm). (b) Luminescence spectra of luminescent hydrogels in (a) with excitation at 330 nm. (c) CIE (Commission International de L’Eclairage) coordinates of the luminescent hydrogels in (a).

Table 1. Lifetime of lanthanide-centered luminescence in hydrogels with different luminescence emitters. τ544 nm (ms) --0.883 0.831 --0.682 0.651

Type of Gel Eu/Terpy Tb/Terpy Eu/Tb/Terpy CD/Eu/Terpy CD/Tb/Terpy CD/Eu/Tb/Terpy

τ616 nm (ms) 0.322 ----0.322 -----

Scheme 1. Proposed energy transfer mechanism in the white light-emitting hydrogel.


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Figure 3. pH responsiveness of the white light-emitting hydrogel. (a) Photograph of white light-emitting hydrogels under different pH conditions. (b) Swelling ratio (V/V0) of white light-emitting hydrogel under different pH conditions. V0 is the volume of the as-prepared hydrogel. Luminescence spectra of white light-emitting hydrogels in (c) acidic pH 3 to pH 7 and (d) basic pH 7 to pH 11. Insets: the I414/I544 ratio under different pH conditions.

Figure 4. Vapor responsiveness of the white light-emitting hydrogel. (a) Photographs of white light-emitting hydrogels in different vapors. (b) The luminescence spectra of white light-emitting hydrogels in different vapors.



Figure 5. Responsiveness of the white light-emitting hydrogel to metal ions. (a) Photograph of white light-emitting hydrogels after incubation with different metal ions. (b) The luminescence spectra of white light-emitting hydrogels after incubation with transition metal ions. Inset: the luminescence spectrum of hydrogel after incubation with Fe3+.


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Figure 6. Temperature responsiveness of the white light-emitting hydrogel. (a) Photograph of white light-emitting hydrogels at low temperature (-196˚C) and at elevated temperature (70˚C). (b) The luminescence spectra at RT and at (-196˚C). (c) Temperature-dependent emission spectra from 20 to 70˚ C. (d) Thermo-chromism as reflected by the changing coordinates on a CIE diagram. (e) Relationship between the temperature and green-to-red emission ratio (I544/I616). (f) Change of I544/I616 at 20˚ C and 70˚ C during five consecutive heating and cooling cycles.



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