Tuning Electrochemiluminescence in Multistimuli Responsive

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Tuning Electrochemiluminescence in Multi-Stimuli Responsive Hydrogel Films Haidong Li, Silvia Voci, Valerie Ravaine, and Neso Sojic J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03119 • Publication Date (Web): 31 Dec 2017 Downloaded from http://pubs.acs.org on January 2, 2018

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The Journal of Physical Chemistry Letters

Tuning Electrochemiluminescence in MultiStimuli Responsive Hydrogel Films Haidong Li, Silvia Voci, Valérie Ravaine* and Neso Sojic* Univ. Bordeaux, Bordeaux INP, ISM CNRS UMR 5255, Site ENSCBP, 33607 Pessac, France

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

Luminescent and redox properties of stimuli-responsive hydrogel materials have been modulated by different external stimuli which trigger swelling or collapse of the polymer matrix. There is a very rapid development in the field of such “smart” materials particularly combined with other sensing functionalities. Here, a poly(N-isopropylacrylamide) matrix incorporating covalently-bound phenylboronic acids as a saccharide sensing unit and a redoxactive [Ru(bpy)3]2+ luminophore was designed and exhibited multi-stimuli responsive electrochemical and luminescent switching behaviors. Redox activity of the films is reversibly changed by sequential stimuli (fructose and temperature) which control the swelling and the collapse of the films. Finally, electrogenerated chemiluminescence (ECL) is enhanced by a ~16-fold factor during the film collapse induced by the temperature whereas the swelling due to fructose provokes the decrease of the light emission. We demonstrate for the first time that ECL response correlates intrinsically with the swelling ratio and is finely modulated by both stimuli. The multi-stimuli responsive characteristics of such ECL-active hydrogels should find promising applications in biosensing, new luminescent materials, and logic gates in bioelectronic devices.

TOC GRAPHICS


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Nowadays, responsive hydrogels, which undergo conformational modifications upon changes such as pH, temperature, molecular recognition or an external field, have attracted considerable interest for many applications such as sensing, drug delivery, actuators, tissue engineering, etc.1 In particular, hydrogels made of cross-linked polymers are very attractive, since they amplify small changes at the molecular level and translate them into large volume modifications induced by swelling variations.2-3 Moreover, these swelling-deswelling changes can be finely tuned and are reversible. In electrochemistry, new opportunities have been opened by the design of electrodes modified with responsive polymeric films, whose electrochemical properties are altered upon stimulus application.4-15 This occurs through two general mechanisms: on one hand, non-redox polymeric films controls the diffusion of free redox species to the electrode though permeability changes,8, 13, 16 on the other hand, redox polymer films can promote different electron pathways by changing the local concentration of immobilized redox species.17-19 The direct application of those concepts is illustrated by the design of switchable electrodes,20-21 most of the time incorporating enzymes, for biosensing or biofuel cells, which can also ultimately be used as logic gates in bioelectronics.22 Introducing responsive polymers in electrochemiluminescent (ECL) processes23-26 is also a remarkable opportunity to manipulate the luminescence not only by the electrode potential but also by an external stimulus. Due to its interdisciplinary nature and also to its remarkable analytical performances, many efforts are focused on the development of novel ECL nanomaterials.27-28 Recently, we demonstrated that the swelling-to-collapse transition in thermoresponsive hydrogels provokes a huge amplification of the ECL emission.19,

29

This

turn-on process relates to the increase of the ECL emitters concentration in the redox film, favoring the electron-transfer process. For now, only switchable systems with ON-OFF states were reported, where temperature was used a trigger.19 In the present work, we aim at extending the concept and propose more sophisticated multi-responsive hydrogels 3 ACS Paragon Plus Environment


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incorporating biorecognition function. Saccharide sensing was chosen as a second trigger, using boronic acid chemistry to detect saccharides.30-32 Herein, we report the study of temperature- and fructose-responsive hydrogel films with electrochemical and luminescent switching behaviors.

Figure 1. (a) Chemical structure of the cross-linked pNIPAM-PBA-Ru hydrogels with the equilibrium in presence of fructose. (b) Schematic representation of the modulation of the ECL-active pNIPAMPBA-Ru hydrogel films by fructose and temperature: partially-swollen (left: without fructose, below VPTT), fully-swollen (middle: with fructose, below VPTT) and collapsed (right: with fructose, above VPTT) states. (c) Scanning electron microscopy images of hydrogel films in the partially-swollen (top) and fully-swollen (bottom) states (without and with fructose, respectively).

Multi-stimuli responsive hydrogel films were prepared by electrochemically-assisted radical polymerization,12,

29

resulting in pNIPAM matrix which incorporates covalently-bound

phenylboronic acids (PBA) moieties33-34 and [Ru(bpy)3]2+ centers35-36 as saccharide sensing units31,

37

and redox luminophores,19,

29, 38

respectively (Figure 1). The resulting pNIPAM-

PBA-Ru films cover homogeneously the entire electrode surface as illustrated by the scanning electron microscopy images (Figure 1c). Both images were recorded after freeze-drying the 4 ACS Paragon Plus Environment

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films which were dipped previously in solutions without or with fructose. The thickness of the films is clearly visible and is 7 ± 1 µm when let in a PBS solution without fructose at 20°C (i.e. partially-swollen state). In presence of fructose, the thickness of the films increased up to 14 ± 1 µm (i.e. fully-swollen state). The films swelled in responses to fructose because the PBA moiety reacts with the saccharide and forms a boronate ester (Figure 1a).31 The fructose complexation increases the hydrophilic character and charge density of the polymer chain which promotes hydrogel swelling as the osmotic pressure of mobile counterions increases (Donnan effect). The amplitude of swelling is consistent with previous results obtained with hydrogel particles having similar composition.39 It is a function of both saccharide concentration in solution and PBA density in the matrix33-34, 39 but also of temperature due to the presence of pNIPAM in the matrix. It should be noted that fructose was chosen as a saccharide because it has a high binding constant with PBA.30-31,

37, 40-41

With 20 mM of

fructose, the film has reached its maximal swelling stage.39 Thus this state is called the fullyswollen state, whereas the state without fructose at 20°C is partially swollen. It is important to notice that positively charged ruthenium centers interact with negatively charged boronate moieties and may reduce film swelling. However, this effect is small since the amount of ruthenium centers is very low. The hydrogels present a dominant negative charge in the conditions of the study.39 The pNIPAM matrix responded also to temperature which provokes its collapse (Figure 1). In previous works,29, 38 we demonstrated that the temperature induced a decrease of the layer thickness by a factor 2 when switching the temperature from 20°C (below the volume phase transition temperature, VPTT) to 40°C (above the VPTT). VPTT is typically 32°C for pNIPAM. However, the VPTT decreases slightly upon incorporation of PBA in the matrix, due to its hydrophobicity. It is about 25°C for pNIPAM-PBA-Ru microgels with similar composition.39 This reported size variation is also in good agreement with the changes observed for pNIPAM-Ru microgels.19



20

with fructose without fructose with fructose

15

b) Peak current (µA)

a) Current (µA)

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10% PBA 20% PBA

12

9

6 0 0.6

0.8

1.0

1.2

Potential (V) vs Ag/AgCl/KCl

1.4

with fructose

without fructose

with fructose

without fructose

Figure 2. (a) Sequence of cyclic voltammograms of the pNIPAM–PBA 10%–Ru films recorded with or without 20 mM fructose at 20°C. (b) Reversible switching of the voltammetric peak current between the fully- and partially-swollen states by the sequential changes of the solution composition with and without 20 mM fructose, respectively. Effect of PBA content (10 or 20%) contained in the films on the electrochemical responses. Experiments were performed in 100 mM PBS solution (pH 7.4) with or without 20 mM fructose. Scan rate: 0.1 V s-1.

Redox properties of the pNIPAM–PBA 10%–Ru films were characterized by cyclic voltammetry. It is noteworthy to mention that PBA 10% refers to the percentage of 3-acrylamidophenylboronic acid (AAPBA) dissolved in the solution used to prepare the films by the electrochemically-assisted radical polymerization. It does not indicate the percentage of PBA effectively incorporated in the pNIPAM–PBA 10%–Ru films (see experimental section). The signals display reversible oxidation of [Ru(bpy)3]2+ centers covalently-bound to the polymer film (Figure 2a). The shape of the signal indicates that the process is dominated by diffusion of the charge in the film. Since the redox centers are grafted to the pNIPAM matrix, charge diffusion takes place by an electron-hopping process between the [Ru(bpy)3]2+ sites. The oxidation potential was identical to those of free [Ru(bpy)3]2+ in solution. Moreover, reversibility of the film thickness and its influence on the electrochemical behavior 6 ACS Paragon Plus Environment

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were also investigated by changing the composition of the solution (Figure 2a). We started the experiments with films incubated with PBS solution containing fructose (in the fully-swollen state, blue plain line in Figure 2a). Then, the films were dipped in a solution without fructose and voltammetric signal increased with the gel collapse to the partially-swollen state (black line in Figure 2a). When fructose was added again (blue dashed line in Figure 2a), it decays to its initial value. Voltammetric signals were reversibly cycled between low and high currents upon the film transition provoked by the fructose stimulus (Figure 2). The decrease of the current reflects the facts that a lower number of [Ru(bpy)3]2+ sites is electrochemically accessible. Indeed, the swelling occurring in presence of fructose provokes the increase of the average distance between adjacent [Ru(bpy)3]2+ centers in the films and therefore the electron-hopping process becomes less efficient and less [Ru(bpy)3]2+ sites are reduced and oxidized. To study further the impact of the swelling on the redox properties of the film, we varied the amount of PBA in the films (Figure 2b). By doubling the amount of AAPBA in the solution used during the electrochemically-assisted radical polymerization (i.e. 20% instead of 10%), a higher amount of PBA was incorporated in the resulting films named pNIPAM–PBA 20%–Ru. One can observe that the amplitude of the current variations was increased from 4.3

µA to 7.4 µA and that the phenomenon remained reversible. It shows that the higher amplitude of the film swelling modulated the current and the redox behavior in the films. Therefore, further experiments we performed with films containing 20% PBA.



30

without fructose at 20°C with fructose at 20°C with fructose at 40°C

25

Current (µA)

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20 15 10 5 0 0.6

0.8

1.0

1.2

1.4


Figure 3. (a) Influence of fructose and of temperature on the voltammetric responses of the pNIPAM– PBA 20%–Ru films. The blue and the red arrows indicate the addition of fructose at 20°C and the increase of temperature up to 40°C, respectively. Experiments were performed in 100 mM PBS solution (pH 7.4) with or without 20 mM fructose. Scan rate: 0.1 V s-1.

Electrochemical properties of the hydrogel films were further analyzed upon sequential stimuli, fructose and then temperature (Figure 3). As mentioned above, addition of fructose induced the gel swelling and the decrease of the [Ru(bpy)3]2+ oxidation current. Raising the temperature above the critical VPTT provoked the film collapse and the current increased. Notice that the current measured in the resulting collapsed state at 40°C is even higher than in the partially-swollen state (i.e. without fructose at 20°C). It is important to keep in mind that the thickness of the films are 2-fold thinner in the collapsed state provoked by the temperature than in the partially-swollen state. Since the current intensity reflects the number of oxidized redox centers, it shows that a higher number of [Ru(bpy)3]2+ sites are electrochemicallyaccessible by electron-hopping process in the collapsed state of the film induced by temperature increase in comparison to the partially-swollen state. It is well-known that the distance between adjacent redox centers governs the efficiency of the electron-hopping 8 ACS Paragon Plus Environment

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process. Electron-transferring collisions are effective in hydrogels when the distance between the redox sites is in the 1−3 nm range.18 In other words, more [Ru(bpy)3]2+ sites are oxidized in the collapsed state due to the smaller distance between adjacent redox sites. The measured currents correlate with the collapse degree of the hydrogel films depending on the experimental conditions and it is the key parameter governing the redox behavior of the films. In brief, the electrochemical response of the pNIPAM–PBA–Ru film is modulated by both fructose and temperature stimuli.

200

150

b)

without fructose at 20°C with fructose at 20°C with fructose at 40°C

ECL intensity (a.u.)

a)

Current (µA)

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100

50

3.0 2.5 2.0 1.5 1.0 0.5

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.0

0.2


0.4

0.6

0.8

1.0

1.2

1.4


Figure 4. (a) Modulation of the electrochemical (a) and ECL (b) signals of the pNIPAM–PBA 20%– Ru films by both stimuli: fructose and temperature. Experiments were performed in 100 mM PBS solution (pH 7.4) containing 100 mM TPrA with or without 20 mM fructose. Scan rate: 0.1 V s-1.

The ECL properties of the films were examined using tri-n-propylamine (TPrA) as a sacrificial coreactant. Figure 4a shows first the cyclic voltammograms of pNIPAM–PBA 20%–Ru in the presence of 100 mM TPrA in different experimental conditions. In this set of experiments, we recorded much higher current values than in PBS solution (Figure 3) because the current is now mainly due to the oxidation of the TPrA coreactant. Therefore it corresponds to a different configuration with the oxidation of the freely-diffusing TPrA 9 ACS Paragon Plus Environment


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coreactant (Figure 4a) unlike the covalently-bound [Ru(bpy)3]2+ sites (Figures 2 and 3). When we switched from the partially-swollen state to the fully-swollen state (i.e. at 20°C without and with fructose, respectively), the TPrA oxidation current increases because the diffusion of free TPrA species is much easier in the swollen hydrogels. When heating the solution above the VPTT, the oxidation current decreased even more than in the partially-swollen state. It is directly related to the collapse of the pNIPAM films which hinders the diffusion of the TPrA into the film where it is oxidized.42-44 Even if the electron-hopping process is more efficient in the collapsed state, the blocked diffusion of TPrA is the main factor limiting the oxidation current.

Whatever, both stimuli caused drastic effects also on the ECL response of the hydrogel films (Figure 4b). ECL is emitted at 1.3 V where the [Ru(bpy)3]2+ sites are oxidized but no ECL emission was observed at the oxidation potential of TPrA. It shows that the ECL mechanism requires the oxidation of the [Ru(bpy)3]2+ centers to generate light emission. Addition of fructose which provoked hydrogel swelling induced a significant decrease of the ECL intensity by a ~4.5-fold factor. To check the effects of fructose on the redox and ECL properties, we performed control experiments by adding fructose in solutions with free [Ru(bpy)3]2+ and at bare GC electrodes (i.e. without pNIPAM–PBA–Ru films). In this case, fructose has not modified the current neither the ECL intensity (data not shown). Thus we can conclude that fructose is not electroactive in the explored potential window and it does not influence directly the photophysical properties of the luminophore. It demonstrates that the observed effects on both current and ECL intensity are related only to the swelling/collapse of the hydrogel films. In control experiments performed without TPrA, negligible ECL signals similar to the background were recorded on such pNIPAM–PBA–Ru films (data not shown). Then, when the temperature was raised above the VPTT in presence of fructose, ECL intensity is remarkably increased by a ~16-fold factor with the resulting hydrogel collapse. 10 ACS Paragon Plus Environment

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Such an ECL enhancement is consistent with the behavior observed on thermo-responsive microgels.19 Considering the different effects occurring in the films during the swelling/collapse

transition

(hindered

hydrophilic/hydrophobic change),29,

42

diffusion,

electron-hopping

process,

it shows that the main factor governing the ECL

behavior is the electron-hopping process leading to excited state generation which depends on the distance between adjacent [Ru(bpy)3]2+ sites. Smaller distances in the collapsed or partially-swollen states increase exponentially the rate of the annihilation reaction which produces the excited state and the resulting ECL emission.19, 29 In addition, since the number of [Ru(bpy)3]2+ sites closer to the electrode is more important in these cases, the effects related to the higher efficiency of the electron-transfer reactions could also contribute to the enhanced ECL signal. Both fructose and temperature stimuli controlled the thickness of the films and thus modulated the ECL response.

In summary, we have demonstrated that the electrochemical and ECL properties of stimuliresponsive hydrogel films can be reversibly controlled by sequential external stimuli such as temperature and fructose. On one hand, swelling of the film provoked by the reaction of PBA moieties with fructose decreased the number of electro-active sites and of ECL intensity. On the other hand, film collapse induced by temperature increased the voltammetric signal and enhanced remarkably the ECL emission by a ~16-fold factor. Therefore, electrochemical responses and ECL light emission correlate with the swelling ratio and are finely modulated by both stimuli which operate in opposite directions. We established that the correlation between swelling and ECL properties of luminophore-covalently bound hydrogels is a general process that can be applied to any stimulus. The fast growing availability of responsive hydrogels opens the possibility to build a broad range of smart electrode materials for biosensing. Enzyme inclusion may provide additional differentiated states through the modulation of bioelectrocatalytic systems that can lead to sophisticated responses. The 11 ACS Paragon Plus Environment


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process is also applicable to nanogels dispersed in solution as local sensors, which may also be valuable in biology, medical imaging or diagnostics. This letter presents a valuable contribution to the rapidly developing field of stimuli-responsive materials combined with other functionalities such as electrochemical sensing and light emission.

EXPERIMENTAL SECTION

Chemicals.

All the reagents were purchased from Sigma-Aldrich unless otherwise noted. Nisopropylacrylamide (NIPAM) was recrystallized from hexane (ICS) and dried under vacuum prior to use. The cross-linker N,N’-methylenebis(acrylamide) (BIS) and the initiator potassium persulfate were used as received. Ruthenium(II) (4-vinyl-4′-methyl2,2′-bipyridine)bis(2,2′-bipyridine)bis(hexafluorophosphate) [Ru(bpy) monomer)] was synthesized according to the procedure described by Spiro and co-workers.45 3-acrylamidophenylboronic acid (AAPBA) was achieved adapting the procedure previously described by Kitano and co-workers.46 Electrode modification by pNIPAM-PBA-Ru films Glassy carbon (GC) electrodes were polished with wet alumina powder on a polishing cloth, and then washed ultrasonically with ethanol and water, respectively. In a typical process, the electrode was immersed in an aqueous solution containing 70 mM NIPAM, 1.75 mM BIS (2.5 mol % against NIPAM), 0.14 mM of Ru(bpy) monomer (0.2 mol % of Ru monomer against NIPAM), AAPBA (10 or 20% mol against NIPAM) and 1.5 mM K2S2O8 in 0.2 M KNO3 solution. Prior to electrochemically assisted polymerization, the reaction solution was


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deoxygenated by bubbling nitrogen gas for 30 min at room temperature. The potential was applied by cycling between -0.35 and -1.35 V at 100 mV s-1 for 60 cycles. Free-radical polymerization8,

47-48

of NIPAM, BIS, AAPBA and Ru(bpy)3 monomer was initiated by

reduction of potassium persulfate which produces radicals at the electrode surface. The current decreased over the cycles, due to the growth of the resulting pNIPAM-PBA-Ru films which are insulating in this potential window. After the polymerization, the electrodes modified with pNIPAM-PBA-Ru films were thoroughly washed with double-distilled water to remove the monomers and stored in ultrapure water. pNIPAM-PBA 10%-Ru and pNIPAMPBA 20%-Ru films were fabricated by electrochemically-assisted radical polymerization in the solutions described above that contain 10 or 20% mol of AAPBA against NIPAM, respectively. Since the films were not soluble and were attached to the electrode, we could not perform any quantitative characterization. However, in previous work using microgels with similar composition, the ruthenium and boron contents were determined by ICP-AES. The incorporated amounts of Ru were slightly below than that of the feed, whereas the amount of PBA was slightly higher than the feed.39 Scanning electron microscopy (SEM). A flat glassy carbon plate was used as working electrode and modified using the same procedure as described on the macroscopic electrode. After rinsing with distilled water, it was immersed in a water bath and frozen rapidly in liquid nitrogen. After freeze drying, the electrode was sputtered with gold using a sputter-coater (Emitech K550X) and observed in SEM (Hitachi TM3000). Electrochemical and ECL measurements. Cyclic voltammetry and ECL experiments were performed with a µ-Autolab Type III potentiostat. ECL intensity was measured by using a Hamamatsu photomultiplier tube R4632. The signal was amplified by a Keithley Picoammeter before acquisition via the second input 13 ACS Paragon Plus Environment


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channel of the µ-Autolab. The three-electrode system consisted in a glassy carbon (GC) electrode as a working electrode, Ag wire as a pseudo-reference and a platinum wire as a counter-electrode. The temperature of the sample was controlled using a thermostatically controlled cell compartment. The measurements were taken under equilibrium conditions after holding the sample for 15 min at the required temperature.

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was supported by the Agence Nationale de la Recherche (NEOCASTIP ANR-15CE09-0015-03). HL acknowledges the China Scholarship Council for his PhD fellowship.

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