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Radical Scavenging Activities of Biomimetic Catechol-Chitosan Films Chunhua Cao, Eunkyoung Kim, Yi Liu, Mijeong Kang, Jinyang Li, Jun-Jie Yin, Huan Liu, Xue Qu, Changsheng Liu, William E Bentley, and Gregory F. Payne Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b00809 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 24, 2018

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Biomacromolecules

RADICAL SCAVENGING ACTIVITIES OF BIOMIMETIC CATECHOL-CHITOSAN FILMS Chunhua Cao*†, Eunkyoung Kim‡§, Yi Liu‡§, Mijeong Kang‡§, Jinyang Li‡§, Jun-Jie Yin⊥, Huan Liu¶, Xue Qu¶, Changsheng Liu¶, William E. Bentley‡§, Gregory F. Payne*‡§

†

Key Laboratory of Optoelectronic Chemical Materials and Devices, Ministry of Education,

School of Chemical and Environmental Engineering, Jianghan University, Wuhan, 430056, P R China ‡

Institute for Bioscience and Biotechnology Research, University of Maryland, 4291 Fieldhouse Drive, Plant Sciences Building, College Park, Maryland 20742, USA

§

Fischell Department of Bioengineering, University of Maryland, 8228 Paint Branch Drive, 2330 Jeong H. Kim Engineering Building, College Park, Maryland 20742, USA ⊥

Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration, College Park, Maryland 20740, United States

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¶

Key Laboratory for Ultrafine Materials of Ministry of Education, The State Key Laboratory of

Bioreactor Engineering, East China University of Science and Technology, Shanghai, 200237, P R China KEYWORDS Biomimetic,

Catechol,

Chitosan,

Radical

scavenging

activity,

Redox

activity,

Spectroelectrochemistry

ABSTRACT Recent studies showed that melanin-mimetic catechol-chitosan films are redox-active and their ability to exchange electrons confers pro-oxidant activities for the sustained, in situ generation of reactive oxygen species for antimicrobial bandages. Here we electrofabricated catechol-chitosan films, demonstrate these films are redox-active, and show their ability to exchange electrons confers sustained radical scavenging activities that could be useful for protective coatings. Electrofabrication was performed in two steps: cathodic electrodeposition of a chitosan film followed by anodic grafting of catechol to chitosan. Spectroelectrochemical reverse engineering methods were used to characterize the catechol-chitosan films and demonstrate the films are redox-active and can donate electrons to quench oxidative free radicals and can accept electrons to quench reductive free radicals. Electrofabricated catechol-chitosan films that were peeled from the electrode were also shown to be capable of donating electrons to quench an oxidative free radical but this radical scavenging activity decayed upon depletion of electrons from the film (i.e., as the film became oxidized). However, the radical scavenging activity could be recovered by a regeneration step in which the films were contacted with the biological reducing agent 2 ACS Paragon Plus Environment

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ascorbic acid. These results demonstrate that catecholic materials offer important redox-based and context-dependent properties for possible applications as protective coatings.

INTRODUCTION

Melanins are ubiquitous pigments that have poorly understood biological functions. Typically, melanins are hypothesized to have beneficial (and in some cases detrimental) activities related to their antioxidant, pro-oxidant, and radical scavenging properties.1–4 Melanins have also attracted technological interest because they are reported to possess interesting electrical properties. Early studies suggested melanins are amorphous semiconductors5 while later work focused on their ionic vs electronic conducting properties6–9 as well as their potential for energy storage (e.g., for battery applications).10–13 Resolving the biologically and technologically relevant properties of melanins has been challenging despite the use of a wide variety of experimental and theoretical methods.8,14–18 We are developing electrochemical reverse engineering methodologies which aim to evaluate melanins’ redox properties by imposing redox inputs and observing the response characteristics. These studies have revealed three important features: (i) melanins are redoxactive with a redox potential in the mid physiological range;19 (ii) they can be repeatedly oxidized and reduced by a variety of oxidants and reductants thus offering electron-transfer catalytic properties; and (iii) they can quench free radicals either by donating an electron to an oxidative radical or accepting an electron from a reductive free radical.20



Scheme 1. (a) Catechol-chitosan film is prepared by 2-electrofabrication steps: the cathodic electrodeposition of chitosan, and the anodic oxidative-grafting of catechol. (b) A mediated electrochemical probing (MEP) method is employed to characterize the redox-related activities of the catechol-chitosan film. A variety of melanin-mimetic materials have also been developed21–23 and we have fabricated catechol-modified chitosan films that mimic many of melanin’s redox properties.19,24–26 These films can be electrofabricated in 2 steps as illustrated in Scheme 1a. First, chitosan can be electrodeposited onto an electrode surface by inducing this pH-responsive self-assembling polysaccharide to undergo gel formation through a cathodic neutralization mechanism.27–32 Second, catechol moieties can be grafted to the chitosan film by immersing the chitosan-coated electrode into a catechol-containing solution and biasing the electrode to an anodic voltage.33,34 Catechols can diffuse from solution through the chitosan film and be anodically oxidized at the 4 ACS Paragon Plus Environment

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electrode surface. The oxidized o-quinones are reactive and as they diffuse from the electrode into the film, they graft to the amines of the chitosan. Previous studies indicate that grafting is rapid (relative to diffusion) and thus the chitosan chains nearest the electrode are modified first with the growth of the catechol modification zone “growing into” the chitosan film as a reaction front.34,35 While alternative chemical and enzymatic methods could be used to oxidize catechol and induce grafting, anodic oxidation can be quantitatively controlled with the extent of reaction being controlled by the anodic charge transfer (qFabrication=∫i⋅dt) as illustrated in Scheme 1a.26,35 Previous studies have shown that qFabrication correlates to other physical-chemical measurements of the extent of catechol-grafting including optical absorbance,35 mass increase of the film34,36, and mechanical modulus33,34. Overall this electrofabrication method allows site-selective addressing of the melanin-mimetic film to an electrode surface while the chitosan serves as a template to guide catechol grafting (thus limiting its self-polymerization), and chitosan also allows the melanin-mimetic film to be delaminated from the electrode for non-electrode applications. We are also evaluating the redox properties of these melanin-mimetic catechol-modified films by applying electrochemical reverse engineering methods that rely on mediated electrochemical probing (MEP).37,38 Scheme 1b shows that there are three features of MEP. First, the material to be probed is localized near an electrode (i.e., electrofabrication allows the catechol-chitosan film to be coated directly onto an electrode).

Second, redox-probing is performed by

purposefully adding diffusible mediators (electron-carriers) that can establish “communication” between the electrode and film. As illustrated in Scheme 1b, mediators with reducing redox potentials can transfer electrons from the electrode to the film, while mediators with oxidizing redox potentials transfer electrons in the opposite direction - from the film to the electrode. 5 ACS Paragon Plus Environment


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Chemically, the exchange of electrons is proposed to involve the switching of the grafted catechol moieties between their oxidized (o-quinone) and reduced (catechol) states. The third feature of MEP is the use of a tailored sequence of input voltages that is imposed at the electrode. These inputs generate output response characteristics that are measured and analyzed to assess redox properties.22,36,39 In our initial studies, the output response signals were the electrochemically-measured mediator currents19,24,26,35,36,38,40,41, but more recently, we have begun extending measurement from this electrical modality to a second, complementary optical signal modality.20 To date, MEP probing has shown that the catechol-chitosan films mimic two of the three features mentioned in the first paragraph of the Introduction: they are redox-active with a redox potential in the mid physiological range (note: these films are non-conducting);26,36,38 and they can reversibly exchange electrons with various electrochemical mediators as well as with biologically-relevant of oxidants and reductants.24,38

From a technological perspective, the

catechol-chitosan films are redox-capacitors in that they can accept electrons, store these electrons (as the reduced catechol form of the grafted moieties) and donate electrons. One application under investigation for these catechol-chitosan redox-capacitors is to facilitate redoxbased communication between biology and electronics.6,37,41,42 From a signal processing perspective, this capacitor can amplify, rectify and gate redox-based signals26 and these molecular electronic properties have been used to: amplify the detection of metabolites43,44 and drugs45–47; reveal global metrics that characterize redox context48,49; and facilitate communication between an electrode and synthetic biology constructs.41,42,50 A second emerging application is to develop catechol-chitosan films as a biomimetic antimicrobial wound dressing.51–54 For instance, a catechol-chitosan bandage has been shown to enlist the redox6 ACS Paragon Plus Environment

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capacitor’s abilities to accept electrons from physiological reductants (e.g., ascorbate) and transfer them to O2 for the in situ and sustained generation of reactive oxygen species (ROS).54 The purpose of the work reported here is to demonstrate that the catechol-chitosan films mimic the third redox feature of melanin mentioned in the first paragraph of the Introduction: – they can scavenge (i.e., quench) free radicals. Potentially, the applications for such melanin-mimetic radical scavenging materials would be for protective coatings capable of both absorbing UV light energy (e.g., sunscreens) and quenching UV-induced free radicals.55–58

Materials and Methods

Chemicals. The following materials were purchased from Sigma-Aldrich: chitosan from crab shells

(85%

deacetylation,

≈200

kDa),

Ru(NH3)6Cl3

(Ru3+),

2,2’-azino-bis(3-

ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), and paraquat (PQ2+), ascorbic acid (AA). 1,1’-Ferrocenedimethanol (Fc) was purchased from Acros organics. The water (>18 MΩ) used in this study was obtained from a Super Q water system (Millipore). Chitosan solutions (1%, pH 5.5) were prepared by dissolving chitosan flakes in HCl to achieve a final pH of 5−6.29 The solutions of mediator were prepared in phosphate buffer (0.1 M; pH 7.0).

Preparation of Film-Coated Honeycomb Electrode. The honeycomb electrode (Pine research instrumentation, NC) was first cleaned with “baby piranha recipe” solution (H2SO4/3%H2O2, 3:1 v/v) for 5 min and washed thoroughly with DI water, followed by drying under N2 stream. The clean honeycomb electrode was immersed in 1% chitosan solution (pH 5.5) and connected to the power source (2400 Sourcemeter, Keithley), and the working gold 7 ACS Paragon Plus Environment


electrode in honeycomb electrode was biased to serve as the cathode (4 A/m2, 45 s) while a platinum wire served as the counter electrode. The calculation of the honey comb electrode area is described in Figure S1. After electrodeposition, the chitosan-coated honeycomb electrode was rinsed with DI water and dried in air at room temperature. To prepare catechol-chitosan film, the chitosan-coated honeycomb electrode was immersed in a catechol solution (5 mM in 0.1 M phosphate buffer, pH 7.0) and an anodic potential of +0.5 V vs Ag/AgCl was applied to the underlying electrode (typically for 5 min). After the oxidation reaction, the catechol-chitosancoated electrode was removed, washed and dried (note: no special precautions were made to preserve the as-fabricated redox state of the film).

Preparation of Film-Coated Gold Chips. Typically, the chitosan-coated gold chip was prepared by immersing the gold-coated silicon wafer “chip” (working electrode area ≈1.5×0.9 cm2) into the chitosan solution described above and connecting it to the power source (2400 Sourcemeter, Keithley) using alligator clips, and the gold chip was biased to serve as the cathode (4 A/m2, 50 s) while a platinum wire served as the counter electrode. After electrodeposition, the chitosan-coated gold chip was removed from the deposition solution, rinsed with DI water, and dried in air. To prepare catechol-chitosan film, the chitosan-coated gold chip was immersed in a catechol solution (5 mM in 0.1 M phosphate buffer, pH 7.0) and an anodic potential of +0.5 V was applied to the underlying electrode for 7 min, typically. After the oxidation reaction, the catechol-chitosan-coated electrode was removed, washed and dried (note: no special precautions were made to preserve the as-fabricated redox state of the film).


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Preparation of Free-standing Catechol-Chitosan Films. For fabrication of free-standing catechol-chitosan films, the chitosan-coated gold chip was prepared first by immersing the gold chip (working electrode, ≈1.5×1.0 cm2) into the chitosan solution described above and connecting it to the power source, and the gold chip was biased to serve as the cathode (4 A/m2, 1 h) while a platinum wire served as the counter electrode. After exposure to NaOH solution (1 M) for 15 min, rinsed extensively with water and dried, the resultant chitosan-coated gold chip was immersed in a catechol solution (5 mM in 0.1 M phosphate buffer, pH 7.0) and an anodic potential of +0.5 V was applied to the underlying electrode for 7 min. For the reduced form of catechol-chitosan film, the free-standing catechol-chitosan film was peeled from the gold chip after the catechol modification reaction, then, incubated in 2 mL of 50 mM ascorbate solution for 15 min, rinsed with water and dried. For the oxidized form of catechol-chitosan film, the obtained catechol-chitosan coated gold chip was immersed in 50 µM Fc solution and an anodic potential of +0.5 V was applied to the underlying electrode for 30 min. Then, the free-standing oxidized catechol-chitosan film was peeled off from the gold chip.

Spectroelectrochemical Measurements. The honeycomb electrode contains two gold electrodes. One gold electrode with holes was used as a working electrode and the other patterned gold electrode was used as the counter electrode. As described above, the working electrode in honeycomb electrode was prepared by coating chitosan or catechol-chitosan films on the electrode surface. The film-coated honeycomb electrode and Ag/AgCl reference electrode were placed into a spectroelectrochemical cell (i.e., a cuvette). The solution of mediators in the cuvette was degassed with N2 for at least 20 min before measurements, and a stream of N2 was gently blown over the surface of the solution during the experiment. All of electrodes were 9 ACS Paragon Plus Environment


connected to a CHI620E electrochemical analyzer (CH Instruments, Inc., Austin, TX) and cyclic voltammetry was performed at a scan rate of 10 mV/s. The optical absorbance was monitored over time at a fixed wavelength with a Thermo Scientific Evolution 60 spectrophotometer (before measurement, we waited 2 min to allow the optical signal to stabilize). Simultaneously, the optical (absorbance) and electrochemical (current) output responses were individually recorded over time.

Electron Paramagnetic Resonance (EPR). To check the radical scavenging properties of catechol, firstly, electrochemically oxidized ABTS+• solution was prepared by immersing the gold electrode (area = 1 cm2) in 0.1 M phosphate buffer solution (pH 7.0) containing 0.1 mM ABTS and applying a constant potential to +0.7 V vs Ag/AgCl for 30 min with stirring. Secondly, film-coated gold chips with different extents of catechol modification were prepared by first electrodepositing chitosan (1%, pH 5.5; 4 A/m2, 45 s) onto each gold chip (1.35 cm2) and then applying an anodic oxidation to each chitosan-coated gold chip at +0.5 V in 5 mM catechol solution for 0, 3, 5, 7 min, respectively. Lastly, gold chips with different extents of catechol modification were dipped into ABTS+• radical solutions (0.6 mL), respectively. After a certain time, the resulting ABTS+• solutions (50 µL) individually contacted with different film-coated gold chips were placed into a capillary EPR tube and EPR spectra were measured. All EPR measurements were performed using a Bruker EMX EPR spectrometer (Billerica, MA) at ambient temperature.

RESULTS


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Conventional Evidence for Radical Scavenging

To evaluate radical scavenging capabilities, we prepared three catechol-chitosan films with differing extents of modification (i.e., anodic grafting was performed using three different oxidative charge transfers). Specifically, chitosan was first electrodeposited onto a gold chip (1% chitosan solution, pH 5.5, 4 A/m2, 45 s) and then this chitosan-coated electrode was immersed in a catechol solution (5 mM in 0.1 M phosphate buffer, pH 7.0) for anodic grafting (0.5 V for 5 min).

To measure radical scavenging, we also used 2,2’-azino-bis(3-ethyl-

benzothiazoline-6- sulfonic acid) (ABTS) which is a somewhat standard reagent for studying free radical scavenging since a one-electron oxidation of ABTS yields the ABTS+• free radical that can be readily measured spectrophotometrically.59,60 Figure 1a illustrates that we generated the ABTS radical by anodic oxidation and the photograph shows that this oxidation converts the colorless ABTS solution (0.1 mM in 0.1 M phosphate buffer, 30 min) into the characteristic green-colored ABTS+• radical solution.

Incubation of this ABTS+• radical solution with a

catechol-chitosan film coated on a gold “chip” for 30 min leads to a loss of color consistent with a quenching of the ABTS+• radical. Figure 1b shows that the attenuation of the ABTS+• color depends on the extent of catechol modification of the chitosan film. An alternative approach for measuring radical scavenging is electron paramagnetic resonance spectroscopy (EPR). The spectra in Figure 1c shows no EPR signal for the ABTS solution while a strong EPR signal is observed for the anodically generated ABTS+• radical solution. The addition of catechol-chitosan films to these ABTS solutions (50 min) was observed to attenuate the EPR signal with greater attenuation observed for films with greater extents with catechol modification (Figure 1d). Figure 1e shows the agreement between these independent 11 ACS Paragon Plus Environment


measurements and this agreement provides initial evidence that the grafting of catechol confers ABTS+• radical-scavenging activities to chitosan films.

Figure 1. Conventional evidence for radical scavenging activities of catechol-chitosan film. (a) Schematic illustrating that the green-colored ABTS+• radicals can be generated by anodic oxidation and then be quenched by catechol-chitosan film. (b) Optical-relative radical scavenging activities (attenuation of ABTS+• color) depend on the extent of catechol modification of the chitosan film (qFabrication). (c) EPR spectra of the ABTS+• radicals are 12 ACS Paragon Plus Environment

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measured 20 min after mixing with various films. (d) EPR-relative radical scavenging activities (attenuation of EPR signal) depend on the extent of catechol modification of the chitosan film (qFabrication). (e) The two independent radical scavenging activities are correlated.

Reverse Engineering Evidence for Radical Scavenging

In addition to the conventional methods in Figure 1, we used a reverse engineering spectroelectrochemical approach to demonstrate that the catechol-chitosan films can scavenge the ABTS+• radical. In this study, we used the gold honeycomb electrode shown in Figure 2a that allows the simultaneous measurement of the electrochemical current associated with the oxidation of ABTS and the optical absorbance of the ABTS+• radical that diffuses from the electrode surface into the optical path. To prepare a catechol-chitosan films on the honeycomb electrode, we first electrodeposited chitosan (1% chitosan solution, pH 5.5, 4A/m2, 45 s) and then anodically grafted catechol (5mM, 0.5V, for 30 s, 60 s, 90 s, 110 s). To measure ABTS+• radical scavenging, the honeycomb electrode was inserted into the spectroelectrochemical cell and the cell was filled with a buffered solution containing both ABTS (50 µM) and a standard electrochemical mediator (Ru3+; 50 µM).

To initiate redox probing, Figure 2b illustrates that

the electrode voltage was cycled first into an oxidizing range and then into a reducing range.



Figure 2. Spectroelectrochemical reverse engineering to detect redox-cycling and quenching of the oxidative ABTS+•• radical. (a) Images of honeycomb electrode coated with chitosan and catechol-chitosan used for spectroelectrochemical detection. (b) Schematic illustrating the thermodynamically controlled reductive and oxidative redox-cycling reactions that allow mediators to exchange electrons with the catechol-chitosan film (note: under oxidizing conditions the ABTS+• radical is generated). (c) Left: Electrical output response with catecholchitosan films show amplified currents for ABTS+• oxidation. Center: Schematic illustrating the 14 ACS Paragon Plus Environment

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calculation of the redox charge transfer (qRedox) of catechol-chitosan film from the electrical output. Right: The redox charge transfer (qRedox) for transferring electrons from the catecholchitosan film to the oxidative ABTS+• radical increases linearly with the charge transfer used for fabricating the catechol-chitosan films (qFabrication). (d) Left: Optical output response with catechol-chitosan films show attenuated absorbance of the ABTS+• radical. Right: Correlation shows linear relationship between optical radical scavenging activities and the extent of catechol modification of chitosan film. (e) The redox activity of catechol-chitosan film is correlated with its optical relative radical scavenging activity and this correlation is similar to that observed with natural melanin samples entrapped in chitosan films.20 (Scan rate = 10 mV/s for all experiments). During the oxidative portion of the cycle (0.5-1.5 min in Figure 2b), the ABTS+• radical is generated at the electrode, and this ABTS+• radical can diffuse into the catechol-chitosan film. If this radical is quenched by accepting an electron from the film, then an oxidative redoxcycling mechanism can be established such that the ABTS essentially mediates the transfer of electrons from the film to the electrode (see schematic in Figure 2b). One signature of such an oxidative redox-cycling mechanism is an amplification of the oxidative currents.

The

electrochemical output curves in Figure 2c shows a progressive amplification of oxidation currents for films modified to greater extents with catechol. Amplification of the electrical output was quantified as illustrated in Figure 2c. Specifically, the charge transfer (Q) during the oxidative portion of the cycle was determined by integration of the oxidative current. Subtracting the Q value obtained for the control chitosan film from the Q value obtained from the catechol-modified chitosan film is attributed to the number of electrons transferred from the grafted catechol moieties through redox-cycling. The plot in Figure 2c 15 ACS Paragon Plus Environment


compares this difference in oxidative charge transfer (qRedox: redox charge transfer) to the Q value used to modify the catechol film (qFabrication). This comparison suggests less than 5 % of the catechols oxidized during film fabrication retained redox activity for ABTS+• radical scavenging which is consistent with previous observations.35 In addition to measuring electrical outputs, the spectroelectrochemical cell allows optical signals associated with green-colored ABTS+• radical to be measured simultaneously. The optical output curves in Figure 2d show that during the oxidative portion of the cycle (0.5-1.5 min), a peak appears in optical absorbance (420 nm). The rise in absorbance occurs as the ABTS+• radical is generated at the electrode and diffuses through the catechol-chitosan film and into the optical window. A peak in absorbance is observed and then the absorbance is observed to decay when the imposed electrical voltage is cycled to become more reductive. Under these reductive conditions the ABTS+• radical is no longer being generated but can be consumed by electrochemical reduction. Also, during this time, the ABTS+• radical can diffuse out of the optical window into the bulk solution. The optical output in Figure 2d shows a progressive decrease (i.e., attenuation) in the peak optical absorbance for films modified with catechols to greater extents. Attenuation in optical absorbance is a second signature of an ABTS redox-cycling mechanism and indicates that this redox-cycling involves quenching of the ABTS+• radical. This optical attenuation was quantified as illustrated in Figure 2d which also shows the correlation between this optical attenuation based relative radical scavenging activity (i.e., optical RRSA) and the extent of catechol modification. This correlation indicates that catechol moieties confer ABTS+• radical scavenging activities to the catechol-chitosan film. Figure 2e shows a cross correlation between the redox and radical scavenging activities for these catechol-chitosan films. This 16 ACS Paragon Plus Environment

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correlation indicates that these activities are linked: the grafted catechol moieties confer both redox and radical scavenging activities. For comparison, Figure 2e also shows previouslyreported results for a set of natural melanin samples.20 In this case, we used fungal “ghosts” with cell-wall bound melanin derived from 1,8-dihydroxynaphthalene (i.e., DHN-melanin) and entrapped these samples in chitosan films at an electrode surface.20 Both the catechol-chitosan and DHN melanin samples showed a linear relationship between radical-scavenging and redoxactivities even though they have markedly different chemistries. The purpose for adding Ru3+ to the test solution is to provide a mechanism for the catecholchitosan films to be replenished with electrons that are depleted during ABTS oxidative redoxcycling. Specifically, the schematic in Figure 2b illustrates that under reducing conditions, Ru3+ can undergo a reductive redox-cycling which essentially serves to transfer electrons from the electrode to the film.

A signature of such a reductive redox-cycling mechanism is an

amplification in Ru3+ reducing currents. The electrochemical output curve in Figure 2c shows that under reducing conditions (2.5 to 3.5 min), a progressive increase in reduction currents was observed for films modified with progressively more catechol. Since neither Ru3+ nor Ru2+ has optical absorbance at 420 nm, no optical signals are observed during the reducing period of this measurement. In summary, the spectroelectrochemical results in Figure 2 support measurements from more conventional methods (Figure 1) and indicate that catechol-modified chitosan films can donate electrons to quench an oxidative free radical. Further, these results suggest that the film’s radical scavenging activities are linked to its redox activities.

Reversibility of Redox and Radical Scavenging Activities 17 ACS Paragon Plus Environment


One advantage of mediated electrochemical probing (MEP) is that it allows activities to be repeatedly probed to assess reversibilities. For instance, the input curve in Figure 3a shows that a honeycomb electrode coated with a catechol-chitosan film (qFabrication = 4.178×10-2 C) was repeatedly probed by imposing a cyclic voltage for 75 minutes (20 cycles). The electrochemical output in Figure 3a shows amplified electrical currents both for ABTS-oxidation and Ru3+reduction for this catechol-chitosan film (compared to a control chitosan coated electrode). Further, this amplified electrical output remains steady (time-invarying) indicating that the catechol-chitosan film can be repeatedly oxidized and reduced (i.e., the redox reactions are reversible). The optical output curve in Figure 3a shows sharp absorbance peaks for the control chitosan film but this optical absorbance was completely attenuated in the presence of the catechol-chitosan film. Again, this attenuation in optical absorbance occurs because the grafted catechol is donating electrons to quench the green-colored oxidative free radical, ABTS+•. Further, the optical attenuation remains steady (time-invarying) during this probing. The results in Figure 3a indicate that the redox and radical scavenging activities of the catechol-chitosan film are reversible. To further support the conclusions of reversible redox and radical scavenging activities, we imposed conditions expected to yield unsteady outputs. Specifically, if the catechol-chitosan film is purposefully depleted of electrons, it should no longer be capable of donating electrons to quench the ABTS+• radical. To impose such unsteady conditions, we again used cyclic voltage inputs but this time over a narrower voltage window. Specifically, the input curve in Figure 3b shows the imposed voltage is cycled into oxidative potentials that are sufficiently oxidative to oxidize ABTS and induce its oxidative redox-cycling. However, the imposed voltage is cycled 18 ACS Paragon Plus Environment

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to reductive potentials that are insufficiently reductive to allow Ru3+ to be reduced and thus the reductive redox-cycling mechanism cannot be engaged to transfer electrons from the electrode to the film. The use of this narrower input voltage range should result in a progressive depletion of electrons from the catechol-chitosan film. The electrochemical output in Figure 3b shows an initial amplification of the ABTS oxidation currents (vs the control chitosan film), however this amplified electrical output progressively decays over time. Similarly, the optical output in Figure 3b shows an initial high attenuation of the optical output (low absorbance) for the catechol-chitosan film (vs control chitosan film), however this attenuation progressively decays with each cycle (the absorbance progressively increases). These results indicate that when a narrower input voltage range is imposed, both the electrical and optical signals are no longer steady.

These observations suggest the catechol-chitosan film must have available (i.e.,

donatable) electrons to engage in oxidative redox-cycling that quenches the oxidative ABTS+• radical. To examine the correlation between the redox and radical scavenging activities we calculated the redox activity (Ne, number of electrons transferred from the catechol-chitosan film) and optical relative radical scavenging activities for each cycle.

Figure 3c shows the results from

Figure 3a for the steady output appears as a single point (Ne =15.6 ± 2; Optical RRSA = 0.99 ± 0.003) while the results from Figure 3b for the unsteady results show a monotonically varying relationship. We should note that the shape of the curve in Figure 3c suggests a large intercept and the reason for this intercept is unclear although it presumably reflects a physical (i.e., nonredox) interaction. Possibly, the ABTS radical that is generated at the electrode physically binds to grafted catechol moieties and is less-able to diffuse into the optical window.



Figure 3. Evidence for repeated quenching of the oxidative ABTS+•• radical. (a) Schematic illustrating that the catechol-chitosan films accept electrons from the reduced mediator (Ru2+) and then donate electrons to the oxidizing radical, ABTS+•. Long-term oscillating voltage inputs are used to repeatedly probe for ABTS+• radical-scavenging by catechol-chitosan films. When a large voltage amplitude range was imposed, both the amplified electrochemical currents and the attenuated optical outputs remain steady (time-invarying) over 20 cycles. (b) When a narrow input voltage amplitude range was imposed, both the amplified electrochemical currents and the attenuated optical outputs progressively decayed over time (unsteady, time-varying response characteristics). (c) Correlation plot of the two output responses shows that redox activities are linked with radical scavenging activities. (Scan rate = 10 mV/s).


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Reductive Radical Scavenging

The above evidence indicates that the melanin-mimetic catechol-chitosan film can donate an electron to quench the oxidative ABTS+• radical. We next examined whether the catecholchitosan film could accept electrons to quench a reductive free radical. The schematic in Figure 4a shows that we tested for reductive radical scavenging using paraquat (PQ2+) because the PQ+• free radical can be readily generated by a one-electron electrochemical reduction of PQ2+, and because the PQ+• radical (but not the PQ2+ non-radical) is blue-colored and can be measured optically.

Thus, we could employ spectroelectrochemical measurements to detect a PQ2+

reductive redox-cycling mechanisms that would quench the PQ+• radical. Also illustrated in the schematic in Figure 4a is that we used the common oxidative mediator ferrocene dimethanol (Fc) to provide the oxidative redox-cycling mechanism that could remove electrons from the catechol-chitosan film. Experimentally, we electrofabricated catechol-chitosan films on the honeycomb electrode (qFabrication = 4.17×10-2 C) and immersed this film-coated electrode into buffered solutions containing both PQ2+ (50 µM) and Fc (50 µM). To initiate redox-probing we imposed cyclic input voltages. In the initial test, we probed using a single cycle as illustrated by the input curve in Figure 4a. During the initial oxidative portion of this cycle (0.5-1.5 min) the electrochemical output shows amplified Fc currents for the catechol-chitosan film (compared to the control chitosan film). This amplification in electrical output is indicative of Fc mediated oxidative redox-cycling.

The optical output shows little change in absorbance during this oxidation

portion of the cycle indicating little absorbance for either Fc or the oxidized Fc+. During the



Figure 4. Spectroelectrochemical evidence for radical scavenging activities for the reductive free radical PQ+••. (a) Schematic illustrating that PQ+• radical can be generated by the electrochemical reduction of PQ2+ and be scavenged by donating electrons to the catecholchitosan film. The oxidative mediator (Fc) is used to remove electrons from the catecholchitosan film. The voltage input and output responses show catechol-chitosan films amplify the electrochemical currents and attenuate the optical absorbance of the PQ+• radical. (b) Long-term oscillating voltage inputs are imposed to repeatedly probe for PQ+• radical scavenging. Both the 22 ACS Paragon Plus Environment

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amplified electrochemical currents and the attenuated optical outputs remain steady (timeinvarying) over 20 cycles. (c) When a narrow input voltage range was imposed, the amplified electrochemical outputs and attenuated optical outputs progressively decayed over time. (d) Correlation plot of the two output responses shows that redox activities are linked with radical scavenging activities. (Scan rate = 10 mV/s). reduction portion of the cycle (2.5-3.5 min), the electrochemical output shows amplified currents associated with PQ2+ reduction redox-cycling. Also, during the reduction portion of the cycle, the optical output curves show a strong PQ+• absorbance peak for the control chitosan, but this peak is absent (completely attenuated) for the catechol-chitosan film. Thus, the initial results in Figure 4a indicate that the catechol-chitosan film can engage in PQ2+ reductive redox-cycling that serves to quench the PQ+• radical. Analogous to experiments with ABTS, we also performed long-term cyclic experiments to evaluate the reversibility of the PQ2+ redox-cycling. When a large input potential range was used, Figure 4b shows the response for the catechol-chitosan film yielded both amplified current outputs and attenuated optical absorbance outputs, and these output responses were observed to be steady (time-invarying) over the 90 minute test (20 cycles). These steady outputs indicate that the catechol-chitosan film can repeatedly engage in the reductive redox-cycling that quenches the PQ+• radical. Figure 4c shows unsteady responses were observed when a narrower input voltage range was imposed. In this case, the imposed input voltage became sufficiently reductive to reduce PQ2+ and generate the PQ+• radical, however, the input potential was insufficiently oxidative to oxidize Fc and induce its oxidative redox-cycling. Under these conditions the catechol-chitosan 23 ACS Paragon Plus Environment


film is expected to become progressively reduced while no mechanism exists to re-oxidize the film. The output curves in Figure 4c show unsteady current amplifications and absorbance attenuations although these responses are less dramatic compared to those observed with ABTS. Finally, Figure 4d shows the correlation between radical scavenging and redox activities. Results from the steady output appear as a single point (Ne = 11.40 ± 2; Optical RRSA= 0.93 ± 0.02), while results from the unsteady output show a monotonically varying relationship. As observed for the case of ABTS (Figure 3c), the shape of the curve in Figure 4d suggests a large intercept which again is possibly due to a physical (non redox) mechanism (e.g., binding of the PQ+• radical to the grafted catechol moieties). One final reverse engineering test was performed in which the ability of the catechol-chitosan film was probed in a single experiment to observe quenching of both an oxidative and a reductive free radical. In this experiment, the film-coated honeycomb electrode was immersed in a solution containing both ABTS (50 µM) and PQ2+ (50 µM). The schematic in Figure 5a shows that under oxidative voltages, ABTS is electrochemically oxidized to generate the ABTS+• radical, while under reductive conditions PQ2+ is electrochemically reduced to generate the PQ+• radical. Both the ABTS+• radical and PQ+• radical can be monitored optically by measuring the absorbance at 394 nm. The results in Figure 5a for a single cycle show the catechol-chitosan film yields amplifications in the electrical outputs for both ABTS oxidation and PQ2+ reduction. Further, the results in Figure 5a show complete attenuation in the optical outputs associated with quenching of the ABTS+• and PQ+• radicals. Long term tests using cyclic voltage inputs over a broad potential range are shown in Figure 5b and these results show that the amplified electrical outputs and attenuated optical outputs are steady (time-invarying).


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In sum, these reverse engineering studies indicate that the grafting of catechol onto chitosan films confers redox and radical-scavenging activities and these activities are linked.

Figure 5. Reversibility of redox and radical scavenging activities of catechol-chitosan film. (a) Schematic illustrating that reductive radical PQ+• and oxidative radical ABTS+• can be electrochemically generated and quenched by the catechol-chitosan films. Input/output curves show that the scavenging processes for both the PQ+• and ABTS+• radicals are associated with redox-cycling reactions, which result in amplified electrochemical current outputs and attenuated optical outputs. (b) Steady (time-invarying) output responses show that catechol-chitosan film can repeatedly scavenge radicals by sequentially accepting electrons (from PQ+•) and donating electrons (to ABTS+•).



Electrofabrication of Catechol-Chitosan Films for Radical Scavenging

The above results indicate that the electrofabricated catechol-chitosan films are redox-active and could provide protection from free radical exposure. To further evaluate these radical scavenging activities, Figure 6a shows that we prepared two catechol-chitosan films on a goldcoated wafer. One film was exposed to conditions expected to reduce the film: incubation in 2 mL of 50 mM ascorbate solution (15 min).54 The other film was exposed to conditions expected to oxidize the film: incubation with 50 µM Fc with an imposed oxidative voltage of 0.5 V (30 min) to induce oxidative redox-cycling. Both films were then rinsed with water, dried and immersed in a solution containing the ABTS+• radical. Figure 6b shows photographs of these solutions before and after 30 min incubation. As can be seen, the catechol-chitosan film that was reduced by ascorbate (R-Cat) could scavenge the ABTS+• radical, while the oxidized film (OCat) and the chitosan control film (Chit) offered considerably less ABTS+• radical-scavenging activities. These radical-scavenging activities were further evaluated by incubating films in ABTS+• solutions and measuring the absorbance at 420 nm. Radical scavenging was quantified as an attenuation in absorbance ((Abs0 - Abst)/Abs0, where Abs0 is the initial absorbance of ABTS+• solution and Abst is the ABTS+• absorbance at t minutes). Figure 6c shows that the control chitosan film and the oxidized catechol-chitosan film possess some radical scavenging activities, while the catechol-chitosan film that is purposefully reduced by prior-incubation with ascorbate has considerably higher scavenging activities. Figure 6c shows this reduced form of the catechol-chitosan could scavenge 98% of the initial ABTS+• radicals during the 30 min incubation.


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Figure 6. Ascorbate can provide electrons to the catechol-chitosan film for radical scavenging activity. (a) Schematic illustrating the experimental approach to generate oxidized and reduced films. (b) Photographs indicate the ascorbate-reduced catechol-chitosan film can scavenge more ABTS+•-radical compared with other films. (c) Quantitation of ABTS+•-radical scavenging activities over time show the ascorbate-reduced catechol-chitosan can scavenge 98% of the initial ABTS+•-radicals during a 30 min incubation.



Finally, we checked the radical-scavenging activities of free-standing catechol-chitosan films. Specifically, we electrofabricated catechol-chitosan films and then peeled each film from the electrode by treating it with 1 M NaOH for 15 min. To convert one of these free-standing catechol-chitosan films to its reduced form, we incubated it with 50 mM ascorbate solution for 15 min. An oxidized catechol-chitosan film was prepared by first performing oxidative redoxcycling before peeling the film from the electrode (as described in Figure 6a). The photographs in Figure 7a show results for 1-minute incubation of these films in the ABTS+• radical solution and these results indicate that the catechol-chitosan film that was first reduced by incubation with ascorbate possesses high ABTS+• radical scavenging activities. A final experiment was performed to demonstrate the importance of the ascorbate reduction treatment and the ability of the film to repeatedly scavenge the oxidative ABTS+• free radical. In this experiment, triplicate catechol-chitosan films were electrofabricated and peeled from the electrode. As illustrated in Figure 7b, each film was initially reduced by placing it in an ascorbate solution (2 mL of 50 mM ascorbate acid for 10 min), rinsed with water, dried and then contacted with a 0.1 mM ABTS+• solution (1 minute) after which the absorbance (420 nm) was measured. The attenuation in absorbance was used as a measure of radical scavenging activity. This procedure of treating the film with ascorbate (to reduce the film) followed by washing and immersion in a 0.1 mM ABTS+• solution was repeated to test the film’s ability to repeatedly scavenge the oxidative ABTS+•-radical. The results in Figure 7c show that for the first 5 cycles, the films show high ABTS+•-radical scavenging activities.


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Figure 7. Repeated radical scavenging activities of free-standing catechol-chitosan films requires repeated ascorbate-reduction. (a) Upper photographs show free-standing catecholchitosan films peeled from the electrode while lower photographs show the ascorbate-reduced film can scavenge the ABTS+•-radical. (b) Schematic illustrating the experimental approach for a multi-cycle study with intermediate ascorbate reduction (cycles 1-5 and 11-15) or without intermediate ascorbate treatment (cycles 6-10). (c) Experimental results show ascorbate 29 ACS Paragon Plus Environment


treatment is necessary to retain the ABTS+•-radical scavenging activities of the catechol-chitosan film (experiment performed with triplicate films). After these first 5 cycles, we used the same films and tested their radical scavenging activities without performing intermediate ascorbate treatments. The expectation is that the films will become progressively depleted of electrons (i.e., oxidized) by donating electrons to the ABTS+•radical, and this depletion of electrons should result in a decay in the films’ abilities to continue scavenging the ABTS+•-radical. Consistent with this expectation, Figure 7c shows the radical scavenging activities for cycles 5-10 progressively decays. In the last few cycles in Figure 7c (cycles 10-15) the films were again exposed to an intermediate ascorbate treatment and the results indicate that they recover their high ABTS+•-radical scavenging activities. In summary, these results demonstrate that the free-standing catechol-chitosan films can scavenge the oxidative ABTS+•-radical, although this radical-scavenging activity is contextdependent. Specifically, the film must be in a reduced state to have donate-able electrons to scavenge this oxidative free radical. Importantly, these results indicate that the catechol-chitosan film can be readily “recharged” with electrons by accepting them from the common biological reductant ascorbate and this observation is consistent with previous results.54

CONCLUSIONS There are three broad conclusions from this work. First, we used electrochemistry to fabricate a functional hydrogel film. Electrochemistry uses electrical energy and when it can be used for chemical synthesis it often offers the advantages of being simple, rapid, quantitatively


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controllable and often reagentless.61–66 Here, we provide another example of the extension of electrosynthesis to the electrofabrication of functional hydrogel-based materials.54,67 Second, we characterized our electrofabricated films using spectroelectrochemical reverse engineering. This approach differs from conventional materials characterization approaches in that the focus is on functional properties (i.e., redox-based properties) rather than chemical/structural characterization.14,20,68,25,26,35,37,39–41,48

In this approach, complex voltage

inputs are imposed to generate: electrical output responses that characterize redox activities; and optical output responses that characterize radical scavenging activities. The observed response characteristics indicate that the electrofabricated catechol-chitosan films: (i) are redox-active; (ii) can donate electrons to quench oxidizing free radicals; (iii) can accept electrons to quench reducing free radicals; and (iv) the film’s redox and radical scavenging activities are linked. Finally, we report that catechol confers important redox-based properties to chitosan films. Catecholic materials are ubiquitous in nature (e.g., melanin) and their biological activities 16,69–72 and technological possibilities8,11–13 continue to stimulate research. Synthetic catechol-based materials (e.g., polydopamine) have also emerged as important technological materials that offer diverse properties (e.g., surface coating and metal chelation).73–81 Our focus is on the redox properties of catechol-based materials.

Previously, we showed that the redox-activity of a

catechol-chitosan film conferred pro-oxidant activities to an antimicrobial bandage by enabling it to “catalyze” the transfer of electrons from the physiological reductant ascorbate to O2 for the in situ generation of reactive oxygen species.54 Here we show that the redox-activity of a catecholchitosan film confers protective radical-scavenging activities. Importantly, in both examples the pro-oxidant and radical-scavenging activities were context-dependent: the films could only donate electrons for ROS-generation or for oxidative-radical scavenging if they were in a 31 ACS Paragon Plus Environment


reduced state. Also important, the catechol-chitosan film can accept electrons from a wide range of reductants (including biologically-relevant reductants).24,25,38 Thus, this work illustrates the potential of that this catechol-chitosan film can engage biological systems in redox-based molecular communication.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Supporting Information illustrates the calculation of the honey comb electrode area using an electrochemical method. (PDF)

AUTHOR INFORMATION Corresponding Author Chunhua Cao e-mail: [email protected] Phone : 86-278-422-6806 Fax : 86-278-422-5198

Gregory F. Payne e-mail: [email protected] 32 ACS Paragon Plus Environment

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Phone:

301-405-8389

FAX:

301-314-9075

ORCID Gregory F. Payne: 0000-0001-6638-9459 Eunkyoung Kim: 0000-0003-2566-4041 Jun-Jie Yin: 0000-0003-0532-2116 William E. Bentley: 0000-0002-4855-7866

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work has been supported by NSF (DMREF-1435957) and DTRA (HDTRA1-13-1- 0037). This paper is not an official U.S. FDA guidance or policy statement. No official support or endorsement by the U.S. FDA is intended or should be inferred.



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