Nanoscale disassembly and free radical reorganization of polydopamine in ionic liquids

Article abstract of DOI:10.1021/acs.jpcb.6b08835

Despite the growing scientific and technological relevance of polydopamine (PDA), a eumelanin-like adhesive material widely used for surface functionalization and coating, knowledge of its structural and physicochemical properties, including in particular the origin of paramagnetic behavior, is still far from being complete. Herein, we disclose the unique ability of ionic liquids (ILs) to disassemble PDA, either as a suspension or as a thin film, up to the nanoscale, and to establish specific interactions with the free radical centers exposed by deaggregation of potential investigative value. Immersion of PDA-coated glasses into four different ILs ([C1C1im][(CH3O)HPO2], [C1C1im]- [(CH3O)CH3PO2], [C1C1im][(CH3O)2PO2], [N1888][C18:1]) at room temperature caused the fast and virtually complete removal of the coating as determined by UV-visible spectroscopy and scanning electron microscopy (SEM). Transmission electron microscopy (TEM) analysis of the colored supernatants from PDA suspensions in ILs revealed the presence of nanostructures not exceeding 50 nm in diameter. Electron paramagnetic resonance (EPR) analysis indicated profound IL-dependent modifications in signal intensity, line-width, and g-factor values of PDA. These differences were interpreted in terms of a partial conversion of C-centered radicals into O-centered semiquinone-type components following destacking and interaction with the anion component in ILs. The discovery of ILs as a powerful tool to disassemble PDA under mild conditions provides a new entry both to detailed investigations of this biopolymer on the nanoscale and to mild removal of coatings from functionalized surfaces, greatly expanding the scope of PDA-based surface functionalization strategies.

Full text of DOI:10.1021/acs.jpcb.6b08835

Subscriber access provided by UNIV TORONTO
Article
Nanoscale Disassembly and Free Radical
Reorganization of Polydopamine in Ionic Liquids
Paola Manini, Piero Margari, Christian Silvio Pomelli, Paola Franchi, Gennaro Gentile,
Alessandra Napolitano, Luca Valgimigli, Cinzia Chiappe, Vincent Ball, and Marco d'Ischia
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b08835 • Publication Date (Web): 04 Nov 2016
Downloaded from http://pubs.acs.org on November 4, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry B is published by the American Chemical Society.
1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Nanoscale Disassembly and Free Radical Reorganization of Polydopamine in Ionic Liquids
†
§
§
₸
¥
Paola Manini, Piero Margari, Christian Pomelli, Paola Franchi, Gennaro Gentile, Alessandra
†
₸
§
‡ǁ
†
Napolitano, Luca Valgimigli, * Cinzia Chiappe, Vincent Ball, Marco d’Ischia *
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
†
Department of Chemical Sciences, University of Naples Federico II, via Cintia 4, Iꢀ80126 Napoli,
Italy
§
Department of Pharmacy, University of Pisa, via Bonanno Pisano 6, Iꢀ56126, Pisa, Italy
₸
Department of Chemistry “G. Ciamician”, University of Bologna, via S. Giacomo 2, Iꢀ40126
Bologna, Italy
¥
Istituto per i polimeri compositi e biomateriali, Consiglio Nazionale delle Ricerche, via Campi
Flegrei 34, Iꢀ80078 Pozzuoli, Italy
‡
Institut National de la Santé et de la Recherche Médicale, 11 rue Humann, 67085 Strasbourg
Cedex, France
ǁ
Université de Strasbourg, Faculté de Chirurgie Dentaire, 1 Place de l'Hôpital, 67000 Strasbourg,
France
Corresponding Author
*Prof. Marco d’Ischia
Phone: +39081674132
Fax: +39081674393
eꢀmail: dischia@unina.it
*
Prof. Luca Valgimigli
Phone: +390512095683
Fax: +390512095688
eꢀmail: luca.valgimigli@unibo.it
1
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 2 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ABSTRACT
Despite the growing scientific and technological relevance of polydopamine (PDA), a eumelaninꢀ
like adhesive material widely used for surface functionalization and coating, knowledge of its
structural and physicochemical properties, including in particular the origin of paramagnetic
behavior, is still far from being complete. Herein, we disclose the unique ability of ionic liquids
(ILs) to disassemble PDA, either as suspension or as thin film, up to the nanoscale, and to establish
specific interactions with the free radical centers exposed by deaggregation of potential
investigative value. Immersion of PDAꢀcoated glasses into four different ILs
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1 1 3 2 1 1 3 3 2 1 1 3 2 2
([C C im][(CH O)HPO ], [C C im][(CH O)CH PO ], [C C im][(CH O) PO ], [N1888][C18:1]) at
room temperature caused the fast and virtually complete removal of the coating as determined by
UVꢀvisible spectroscopy and scanning electron microscopy (SEM). Transmission electron
microscopy (TEM) analysis of the colored supernatants from PDA suspensions in ILs revealed the
presence of nanostructures not exceeding 50 nm in diameter . Electron paramagnetic resonance
(EPR) analysis indicated profound ILꢀdependent modifications in signal intensity, lineꢀwidth and gꢀ
factor values of PDA. These differences were interpreted in terms of a partial conversion of Cꢀ
centered radicals into Oꢀcentered semiquinoneꢀtype components following destacking and
interaction with the anion component in ILs. The discovery of ILs as a powerful tool to disassemble
PDA under mild conditions provides a new entry both to detailed investigations of this biopolymer
on the nanoscale and to mild removal of coatings from functionalized surfaces, greatly expanding
the scope of PDAꢀbased surface functionalization strategies.
2
ACS Paragon Plus Environment

Page 3 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
INTRODUCTION
Deposition of adhesive thin films of polydopamine (PDA), a eumelaninꢀlike insoluble biopolymer
produced by autoxidation of dopamine at pH 8.5, has emerged over the past decade as a highly
1ꢀ3
versatile coating methodology for several biomedical and technological applications, including
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
4
,5
6
7
surface modification of inorganic materials, molecular sieve membranes, polymer grafting,
8,9
10
11
multifunctional nanoparticles, cell and droplet coating, electronic devices, biointerfaces,
12ꢀ14
15,16
biosensors
and drug delivery.
PDA appears to consist of a heterogeneous mixture of
oligomeric components made up of uncyclized dopamine units, cyclized 5,6ꢀdihydroxyindole (DHI)
17ꢀ19
units and degraded pyrrolecarboxylic acid moieties.
The importance of aggregation phenomena
and physical interactions in PDA buildup and overall morphology has also been emphasized.
Studies of PDA film growth have yielded considerable insights into the mechanisms of the process
highlighting the important influence of oxidation conditions on the spectral and physicalꢀchemical
characteristics of films, coatings and particles. Depending on whether dopamine autoxidation is
carried out in Tris, bicarbonate or phosphate buffer, PDA films with different chemical composition
and properties can be obtained, reflecting in part the incorporation of Tris into the growing PDA
20
scaffolds. Even more remarkable is the role of the oxidant and pH of the medium, as indicated by
the production of superhydrophilic/superolephobic PDA thin films by oxidation of dopamine with
2
1
excess periodate in an acidic medium. Strategies for modifying PDA properties via structural
22,23
modification or copolymerization with suitable substrates have also been reported.
Along with
studies on structure and properties, increasing attention is currently being directed to elucidate the
nature and origin of PDA paramagnetic properties. This is a topic of both scientific and practical
relevance, since there is evidence that the EPR behavior of eumelanin polymers reflects the πꢀ
electron properties of the materials that underlie their both light absorption, antioxidant, redox
activity and conductivity properties. In particular, EPR spectroscopy may be a most useful approach
to disentangle the effects of intermolecular and intramolecular contributions to the electronic
properties of complex disordered insoluble systems by comparing spectral parameters on solid state
3
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 4 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
4
and deꢀaggregated samples. The majority of available information on PDA free radicals comes
20
from two recent reports. In the first, the EPR spectra of three different samples prepared in Tris,
phosphate and bicarbonate buffer were compared to gain an insight into the effects of bufferꢀ
dependent structural modifications and aggregation mechanisms on the paramagnetic centers as
probes of the πꢀelectron system. Similar single, roughly symmetric signals were determined for all
samples at a g value of ~2.004, typical of carbonꢀcentered radicals. A significantly broader signal
was however observed in the sample prepared from Tris buffer, reflecting apparently the
superposition of two main signals and interpreted as being due to the simultaneous presence of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
5
carbonꢀcentered radicals and semiquinone radicals. Moreover, normalized power saturation
profiles indicated for the PDAꢀTris sample a lower degree of molecular disorder and intermolecular
spin delocalization, due probably to a lower tendency to aggregation and stacking caused by
incorporation of Tris, hindering delocalization of the paramagnetic centers across the molecular
scaffolds.
2
6
In another report, the EPR properties of PDA samples were investigated in detail. For all samples
a single unresolved signal was observed with a slightly asymmetrical line in a broad range of
temperature. gꢀFactor measurements gave a relatively constant value as ∼2.0052 in a PDAꢀTris
sample (17% wt of H
2
O) in a broad temperature range, and ∼2.0038 in a PDAꢀphosphate sample
(8.2% wt of H O), slightly decreasing below 100 K. It was concluded that both the moisture content
2
and the type of buffer used during PDA synthesis have an influence on the spectral parameters. A
marked line broadening was apparent moreover at liquid helium temperatures, supposedly due to
enhancement of the relaxation rate.
2
5
Related studies carried out on synthetic eumelanins, showed that the water content has a strong
influence on the solidꢀstate EPR signal of eumelanins, causing a decrease in spin density as it
increases. On the other hand, the g values in the solid state are not greatly affected by changes in pH
or hydration level. Based on these results, it was concluded that: 1) the solidꢀstate EPR signal of
eumelanin may not arise from a dominant semiquinone free radical (g ≈ 2.0045 to 2.005) but rather
4
ACS Paragon Plus Environment

Page 5 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
from a dominant carbonꢀcentered free radical (g ∼ 2.0036); 2) π−π destacking, brought about by the
addition of water or increased pH may increases the proportion of semiquinone radicals via a
comproportionation reaction, leading to the exposure and partial abatement of the carbonꢀcentered
spin in the wet environment. Although these data provided direct or indirect insights into PDA
paramagnetic properties, supporting potential utilization for imaging applications, the actual nature
of the species responsible for the paramagnetic behavior of PDA and the role of
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
aggregation/stacking remain poorly understood.
On the basis of previous observations pointing to inhibition of aggregation as a useful means of
24
probing eumelanin paramagnetic properties, we set up a series of experiments aimed at exploring
for the first time the variations in the paramagnetic properties of PDA following
deaggregation/destacking under mild nonꢀdestructive conditions. This looked at the onset as a most
challenging goal considering the notorious resistance of eumelaninꢀtype materials to dissolution or
deaggregation without making recourse to harsh treatments with alkali and heating. Although it was
27
reported that treatment with aqueous ammonia can induce eumelanin destacking, the occurrence
and consequences of alkaliꢀinduced oxidative breakdown processes modifying the polymer
backbone and spin properties via oꢀquinone cleavage were not addressed. In searching for
alternative nonꢀbasic systems capable of inducing nonꢀdestructive deaggregation of PDA we turned
out attention to ionic liquids (ILs) as a promising tool to probe paramagnetic properties via
destacking and physical interaction with the paramagnetic centers. Use of ILs for studies of PDA
was suggested by the following considerations: a) ILs are a unique class of solvents so far little
explored in EPR spectroscopy, which display high polarity, extraordinarily high viscosity, and can
form nanoꢀmesoscale polar and nonpolar domains that appear to be optimally suited to probe the
effects of specific ionic interactions and diffusion/mobilityꢀcontrolled processes on free radical
28,29
properties of heterogenous systems like eumelanins;
b) ILs can promote metalꢀfree
depolymerization of phenolic polymers, thus suggesting a potential use to disentangle compact
5
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 6 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
0
PDA components; c) ILs have surface tensions suitable to induce direct exfoliation of graphite and
stabilization of exfoliated graphene via Coulombic interaction.
Aim of this study was to assess the ability of ILs to induce deaggregation of PDA and to probe the
changes occurring in PDA free radical properties upon destacking and interactions with this
peculiar ionic medium.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
EXPERIMENTAL SECTION
Synthesis of dimethylimidazolium dimethylphosphate ([C
dimethylimidazolium methyl-methylphosphonate ([C im][(CH
dimethylimidazolium methylphosphonate ([C im][(CH O)HPO
2
1
C
1
im][(CH
O)CH
]). 1ꢀMethylimidazole (1 eq)
3
O)
2
PO
2
]),
1
C
1
3
3
PO
2
]), and
1
C
1
3
and the alkylating agent (trimethyl phosphate or dimethylꢀmethyl phosphonate or dimethyl
phosphite, 1.1 eq), mixed at room temperature under argon and vigorous stirring, were heated at
1
10 °C for 24 h. Then the mixture was repeatedly washed with anhydrous diethyl ether to remove
the excess of reagent and the recovered liquid was dried under vacuum for 24ꢀ48 h at 80 °C.
Structure and purity of the recovered transparent liquids (yields 95ꢀ97%) were checked by NMR
analysis. N-methylꢀN,N-dioctylammonium oleate ([N1888][C18:1]) was synthesized by a twoꢀstep
3
1
procedure using the same experimental conditions described in literature.
SEM and TEM analysis. SEM analysis was performed by means of a FEIꢀQuanta 200ꢀFEG using
secondary electron detector and an acceleration voltage of 10ꢀ30 kV. Before the analysis the sample
was sputter coated with a 10 nmꢀthick Au/Pd layer. TEM analysis was performed in bright field
mode using a FEIꢀTecnai G12 Spirit Twin (LaB6 source) operating at 120 kV. Carbon coated TEM
grids were immersed in the PDA suspensions. Before the analysis, in order to better evidence the
PDA nanostructures, except for the sample obtained from [C
was necessary to remove ILs by washing the grids with distilled water (sample from
[C im][(CH O)CH PO ]) or ethyl acetate (samples from [C im][(CH O)HPO ] and
N8881][C18:1]).
1 1 3 3 2
C im][(CH O)CH PO ] suspension, it
1
C
1
3
3
2
1
C
1
3
2
[
6
ACS Paragon Plus Environment

Page 7 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
EPR spectroscopy. XꢀBand EPR spectra were recorded at 298K on a Bruker Elexsys E500
spectrometer equipped with an ER 049 Super X MW bridge and VT 1000 variable temperature unit.
Spectra consisted of two accumulated scans set at MW power = 2.5 mW, modulation amplitude =
1
.0 G, modulation frequency = 100 kHz. Samples were placed in a 200 µL calibrated glass tube
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
sitting inside a 4 mm id quartz tube, accurately positioned inside the cavity. For sample in ILs 35
µL of solution were analyzed, so to cover the entire sensitive area in the cavity. For solid state
samples 0.2 to 2.5 mg were analyzed. Measured gꢀfactors were corrected with respect to that of a
2 4
standard sample of perylene radical cation in concentrated H SO (g = 2.00258) and checked
against the measured value for 2,2ꢀdiphenylꢀ1ꢀpicrylhydrazyl radical (DPPH•) at the solid state (g =
2.0036). For quantitative analysis of solid state samples the double integral of EPR signal was
compared to that of different samples of DPPHꢀ powder in the same calibrated tube. For PDA
samples in IL, the spectrometer response was calibrated with standard solutions of DPPHꢀ in the
same ionic liquids. For higher relative accuracy only the strongest EPR signals corresponding to
PDA samples in [N8881][C18:1] were calibrated using the double integral: spin/L values for samples
in the other ILs were determined based on samples in [N8881][C18:1] by comparing the normalized
signal intensity. Signal intensities (I) were normalized by the square of line width (W) according to
eq. 1. As an independent assessment of radical concentration, the normalized signal intensity were
normalized against the UV absorbance of the sample at 400 nm, according to eq. 2.
2
2
I(norm) = I•W /W (ref)
(1)
(2)
I/A400(norm) = I(norm)•A400(ref)/A400
RESULTS AND DISCUSSION
Synthesis of PDA and preparation of PDA thin films. PDA and PDA thin films on glass have
3
2
been prepared according to a procedure reported in the literature. Briefly, a solution of
dopaminehydrochloride has been dissolved in the appropriate buffer, that is Tris or bicarbonate
buffer (0.05 M, pH 8.5), and left under vigorous stirring for 24 h. The PDA powder has been
collected after acidification of the reaction mixture and centrifugation at 4 °C. PDA thin films have
7
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 8 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
been prepared by dipping of a glass substrate in the reaction mixture. After 2 h, the glass substrate
has been rinsed thoroughly with pure water and dried under nitrogen flux to afford a quite
homogeneous film with a thickness set in the 50ꢀ60 nm range.
Selection of ILs and PDA solubility tests. Studies of PDA destacking were carried out using in
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
four different ILs, namely 1,3ꢀdimethylimidazolium dimethylphosphate, [C
1 1 3 2 2
C im][(CH O) PO ],
1,3ꢀdimethylimidazolium methylꢀmethylphosphate [C C im][(CH O)CH PO ], 1,3ꢀ
1
1
3
3
2
dimethylimidazolium methylphosphate [C C im][(CH O)HPO ] and N-methylꢀN,N-
1
1
3
2
dioctylammonium oleate, [N8881][C18:1] (Chart 1).
+
N
+
N
N
N
O
O
P
-
P
-
CH O
CH O
3
O
3
O
CH3
OCH3
[C C im][(CH O) PO ] [C C im][(CH O)CH PO ]
1
1
3
2
2
1
1
3
3
2
O
-
O
(CH )
2 7
2 7
(CH ) CH3
+
N
N
C H
8
17
O
+
H C
N
C H
8 17
3
P
-
CH O
3
O
C H
8 17
H
[
C C im][(CH O)HPO ]
[N8881][C18:1
]
1
1
3
2
Chart 1. Structure of ILs used in this study.
Since the intercalation of ILs into polymeric materials depends on ILs’ anions and cations, being
determined by the nature of the chemical interactions that these two components are able to
establish, we decided to focalize our attention on ILs having strong hydrogen bond acceptor
33
anions. These anions should be indeed able to interact with most of the functional group present
on PDA. Halogen free and hydrolytically stable ILs based on dimethylphosphate and phosphonate
anions were selected, associating as cation the small dimethylimidazolium. ILs based on
imidazolium cations are indeed highly effective for the exfoliation of bundled singleꢀwall nanotubes
8
ACS Paragon Plus Environment

Page 9 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
and graphene, probably due to the proposed ability to adhere strongly to the πꢀconjugated graphitic
surface, most probably via cationꢀπ interactions. It is worth of note that in the presently used ILs the
small alkyl substituents present both on cation and anion practically nullify the apolar component
characterizing the threeꢀdimensional network of most ILs, generally consisting of polar and apolar
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
3
4
domains, making the eventual role of the cationꢀπ interactions dominant. However, since it has
been shown that some ILs bearing long alkyl chains are able to promote the debundling of carbon
3
5
nanotubes, we decided to investigate also an ammonium salt bearing three long alkyl chains on
cation and a long alkyl unsaturated chain on anion, [N8881][C18:1]. This IL, having oleate as anion,
associates a high hydrogen bond acceptor ability to an apolar nature.
In preliminary experiments, the effects of the ILs on PDA was investigated using thin films on
glass. Spectrophotometric analyses showed marked decrease in the absorbance at 400 nm of the
coated regions following immersion in ILs for 16 hours with respect to the untreated coated glass as
internal reference (Figure 1). Specifically, the following percent absorbance decrease values at 400
nm were determined: [C
1 1 3 2 2 1 1 3 3 2
C im][(CH O) PO ], 74%; [C C im][(CH O)CH PO ], 88%;
[
C
1
C
1
im][(CH O)HPO ], 77%; and [N8881][C18:1], 95%.
3
2
9
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 10 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Figure 1. Effect of ILs on PDA thin film detachment from glass. a) Visible absorption profiles of
PDA thin film on glass before (black trace) and after (red trace) immersion in [N8881][C18:1]. In the
inset is reported the border between the treated and untreated region of the PDA thin film on glass.
b) Percentage of thin film detachment after treatment with IL expressed as the ratio between the
absorbance at 400 nm of the treated and untreated thin films. c) Secondary electron SEM images of
the border region between the treated and untreated PDA film on glass: at high magnification the
original film morphology (upꢀright) and the almost complete removal after exposure of the glass
substrate to [N8881][C18:1] (downꢀright) is well apparent.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scanning electron microscopy (SEM) analysis revealed significant modifications in the ILꢀexposed
area with respect to the unexposed surface (Figure 1c).
In separate experiments, PDA samples produced under two different conditions, i.e. by autoxidation
of dopamine in Tris buffer or in bicarbonate buffer at pH 8.5, were suspended in the previously
desiccated ILs at a concentration of 1 mg/ml and were sonicated for 30 min in an ice bath. The
resulting suspensions were carefully centrifuged to remove a small residue which was not treated
further, and the clear supernatants, exhibiting dark colorations (Figure 2), were examined by UVꢀ
visible spectroscopy.
A
B
C
D
Figure 2. Supernatants from PDAꢀTris dispersion in ILs: From left to right:
[
[
C
1
C
1
im][(CH
3 2 1 1 3 2 2 1 1 3 3 2
O)HPO ] (A), [C C im][(CH O) PO ] (B), [C C im][(CH O)CH PO ] (C),
N8881][C18:1] (D).
10
ACS Paragon Plus Environment

Page 11 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
The results (see Supporting Information) showed in all cases almost featureless curves spanning the
entire UVꢀvisible range. Taking the absorbance at 400 nm as a reference, the relative amount of
PDA in the various ILs could be estimated assuming similar visible extinction coefficients for the
two PDA samples. PDAꢀTris proved to be slightly more soluble than PDA bicarbonate in
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
2
0
dimethylimidazoliumꢀbased ILs, in line with the smaller aggregate size of PDAꢀTris. However,
judging from absorbance values, both PDA samples proved to be slightly more dispersible in
[
N8881][C18:1], the more hydrophobic of the tested ILs.
To gain deeper insight into PDAꢀIL interactions, the supernatants from PDA suspension in ILs were
examined by transmission electron microscopy (TEM). Data in Figure 3 revealed the presence of
scattered PDA nanostructures not exceeding 50 nm in size which on closer inspection proved to
consist in turn of aggregates of much smaller components.
11
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 12 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
a)
b)
c)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
d)
Figure 3. Bright field TEM images of nanostructures obtained by sonication of PDA in ILs. a)
[C im][(CH O)HPO ], b) [C im][(CH O)CH PO ], c) [C im][(CH O) PO ], d)
N1888][C18:1].
1
C
1
3
2
1
C
1
3
3
2
1
C
1
3
2
2
[
EPR analysis. Based on these results, the EPR response of the various PDAꢀcontaining ILs was
next examined using Xꢀband spectroscopy, which had previously been successfully employed to
25
investigate the paramagnetic properties of melanins in the solid state. Data reported in Table 1
12
ACS Paragon Plus Environment

Page 13 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
showed a detectable EPR signal in all cases, with varying intensity apparently unrelated to UVꢀvis
absorbance values, suggesting different ILꢀdependent free radical properties.
Table 1. Xꢀband EPR spectral parameters recorded for PDA samples in ionic liquids and in the
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
solid state at 298 K.
a
Ionic Liquid
PDA-Tris
= 2.00400
W = 5.3 G, I = 31680
PDA-bicarbonate
= 2.00362
W = 6.3 G, I = 14630
b
[C
1
C
1
im][(CH
3
O)
2
PO
2
]
g
g
b
[
C
1
C
1
1
im][(CH
3
3
O)CH
3
PO
2
]
g
= 2.00456
W = 5.5 G, I = 36866
= 2.00443
W = 5.3 G, I = 23659
= 2.00473
W = 6.9 G, I = 107000
g
= 2.00359
W = 6.8 G, I = 16019
= 2.00365
W = 6.3 G, I = 13396
= 2.00480
W = 6.4 G, I = 126000
b
[C
1
C
im][(CH
O)HPO
2
]
g
g
b
[
N
8881][C18:1
]
g
g
c
None: powder
g
= 2.00346
g
= 2.00336
5
5
W = 4.8 G, I = 3.25×10
W = 4.6 G, I = 2.47×10
a
b
c
Neat ionic liquids had no detectable EPR signal. Recorded on 35 µL of saturated IL solution.
Recorded on 0.5 mg of solid state sample.
As a rule, PDAꢀTris displayed more intense signals than PDAꢀbicarbonate. Significantly stronger
signals for both samples were determined however in [N8881][C18:1], on account of a good
solubilization, regardless of the mode of preparation of the PDA sample, i.e. Tris or bicarbonate,
while the weakest signals are observed in [C C im][(CH O)HPO ]. This suggests that
1
1
3
2
solubilization/deaggregation effects depending on solvent polarity play an important role in ILꢀPDA
spin interactions, possibly also influencing different spin populations in the eumelaninꢀtype
materials. An increase in signal intensity was previously reported for the complexes of DOPA
3
6
melanin with gentamycin and kanamycin. Consistent with a different EPR behavior, a
comparative view of the main EPR spectral parameters, i.e. ꢀvalue and signal amplitude (W), for
g
the two PDA samples showed marked differences depending both on the sample and nature of the
IL. Overlays of spectra of the PDA samples in the various ILs, recorded at 298 K under identical
settings are given in Figure 4 along with the corresponding spectra recorded on the same PDA
samples it the solid state.
13
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 14 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. EPR spectra of PDA melanin prepared in bicarbonate (A and C) or Tris buffer (B and D)
suspended in ILs as indicated (A and B, 35 µL/sample) or in the solid state (C and D, 0.5
1 1 3 2 2
mg/sample). In A spectra in [C C im][(CH O) PO ]ꢀbased ILs have been upscaled by a factor of 2
for easier viewing.
Comparison of gꢀfactor values in Table 1 showed that the two PDA samples show similar values in
the solid state, PDAꢀTris being just slightly shifted downfield; however, they exhibit significant
differences in ILs. In particular, while PDAꢀbicarbonate displayed a modest increase of gꢀfactor in
1,3ꢀdimethylimidazoliumꢀbased ILs compared to the solid state (ca. 2.0036 vs. ca. 2.0034), PDAꢀ
Tris exhibited markedly downfield shifts in the same three solvents, with g ranging between 2.0040ꢀ
2
.0046. An even larger shift was noted however for both PDA samples in [N8881][C18:1]. A possible
20,37
conclusion from these data, based also on previous evidence
is that ILs can tip the balance of Cꢀ
14
ACS Paragon Plus Environment

Page 15 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
centered and Oꢀcentered radicals toward the latter, which would be rescued from aggregation
25
dependent quenching effects. Most likely, the ionic nature of the medium and especially the
basicity of the anion promotes the generation of charged semiquinone radical species in a similar
manner as an increase in pH, i.e. by shifting the quinone/hydroquinone to semiquinone
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
2
5
38
equilibrium, or by promoting quinoneꢀlike radical cations and anions. Comparing the relative
basicity of the anions in the various ionic liquids (pK [(CH O) PO ]H = 1.29, [(CH O)CH PO ]H =
a
3
2
2
3
3
2
2
,31, [(CH O)HPO ]H = 2.05. [C18:1]H = 4.78) it can be argued that the marked shift in the
3 2
resonant field in [N8881][C18:1] is due to the stronger basicity of the carboxylate anion compared to
phosphate/phosphonate anions. On the other hand, the peculiar structural features of this IL,
characterized by large apolar hydrocarbon domains interconnected in a bicontinuous, spongeꢀlike
nanostructure, could also affect this parameter. The higher g values of PDAꢀTris relative to PDAꢀ
bicarbonate in the solid state as well as in all tested ILs may reflect the effect of the Tris moieties
incorporated into the PDA scaffolds, which would favor the generation of semiquinoneꢀtype free
20
radical species as previously observed in the solid state.
Data in Table 1 revealed for PDA/bicarbonate and, to a lesser extent PDAꢀTris, a marked
broadening of the EPR signal (W) when recorded in ILs with respect to the solid state. In principle,
signal broadening in the ILs compared to the solid state might reflect a higher degree of spin
delocalization in the eumelanin components as freed from the constraints of the solid state. When
viewed in the light of the gꢀfactor values, W data in 1,3ꢀdimethylimidazolium based ILs, but not
[N8881][C18:1], revealed an opposite behavior for the two PDA samples: significant gꢀvalue shifts
were related to more modest broadening, while more modest g factor shift was associated to the
larger broadening.
Table 2. EPR signal intensity normalized on line width and on UVꢀVis absorbance and
corresponding quantitative radical concentration for PDA melanins suspended in ionic liquids
a
b
c
Melanin / IL
I/W(norm) I/A400(norm) N×spins/L
15
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 16 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
PDA-Tris
ꢀ5
[
[
[
C
1
C
1
im][(CH
3
O)
2
PO
2
]
29525
19683
37000
10500
56339
5.91×10
6.30×10
5.53×10
ꢀ
5
5
C C im][(CH O)CH PO ] 37000
1
1
1
1
3
3
3
2
ꢀ
C
C
im][(CH
O)HPO
2
]
22050
169020
ꢀ4
[N8881][C18:1
PDA-bicarbonate
]
1.31×10
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ꢀ5
[
[
C C im][(CH O) PO ]
19266
] 24576
17641
19266
24576
10377
68493
5.38×10
5.66×10
5.30×10
1.33×10
1
1
3
2
2
ꢀ5
ꢀ5
ꢀ4
C
1
C
1
im][(CH
3
3
O)CH PO
2
[C C im][(CH O)HPO ]
1
1
3
2
[
N
8881][C18:1
]
171232
a
2
2
b
I/W(norm) = EPR signal intensity normalized on EPR line width (I•W /W (ref)). I/A400(norm) =
c
EPR signal intensity normalized both on EPR line width and on UV absorbance at 400nm. N ×
spins/L = concentration of radicals (unpaired electrons) in the IL suspension (corresponding to
molar concentration in solution).
Since linewidth (W) in EPR spectra of melanin samples varied significantly and intensity (I) is
known to be reversely proportional to the square of the linewidth, in order to compare EPR signal
2
intensities for the different samples, all spectra were normalized against W . Absolute values of
radical concentration in suspension were also estimated upon calibration of the spectrometer
response, by comparison of the double integral of the signal intensity with that recorded for
standard solutions of persistent 2,2ꢀdiphenylꢀ1ꢀpicrylhydrazyl radical (DPPH•) in the same ionic
liquid. The result expressed as Avogadro's N × spins / L would provide the apparent molar
concentration of radicals (or unpaired electrons) in the different ionic liquids (Table 2). Of interest,
1
9
this quantity clusters around 3.5×10 spins / L in the three 1,3ꢀdimethylimidazoliumꢀbased ILs, and
is more than twice as large in [N8881][C18:1], irrespective of the mode of synthesis (Tris or
bicarbonate) of the PDA sample. For comparison, the number of spins per mg of sample was also
estimated for solid state PDA samples upon calibration of the spetrometer response with solid state
DPPH• radical. The results indicate that our PDAꢀTris and and PDAꢀbicarbonate samples had
1
5
15
respectively 3.5×10 and 2.2×10 spins/mg, which are in excellent agreement with the values in
1
5
15
the range 2.4×10 to 3.8×10 spins/mg recently reported for solid state PDAꢀTris, respectively by
2
6
Lꢀband and Xꢀband EPR spectroscopy.
16
ACS Paragon Plus Environment

Page 17 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
As an independent assessment of the relative concentration of suspended radicals, the normalized
EPR signal intensities were normalized again against the visible absorbance values of the IL
solutions, that is, the EPR signal intensity/A400 ratio was taken as an arbitrary index of unpaired
electron concentration in ILs. Again it was found that the highest radical concentration can be
detected in [N8881][C18:1] regardless of the mode of preparation of the PDA sample, i.e. Tris or
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
1 1 3 2
bicarbonate, while the lowest is observed in [C C im][(CH O)HPO ]. This calibration, however,
offered interesting insights concerning the role of ILs in affecting the paramagnetic properties of
PDA. A plot of EPR normalized signal intensity versus I/A400 normalized signal shows a roughly
linear dependence in 1,3ꢀdimethylimidazoliumꢀbased ILs, with values in [N8881][C18:1] clearly
escaping such correlation (Figure 5). This suggests that the ionic liquid affects the EPR signal in
different ways. Namely, 1,3ꢀdimethylimidazoliumꢀbased ILs have slightly different ability in the
solubilization/deaggregation of PDA, which reflects in different concentration of both PDA and its
radicals in the solution. In addition to this behaviour, [N8881][C18:1] appears also to affect the
paramagnetic properties of suspended PDA, by favoring the formation of radical species.
PDA – Tris
PDA – bicarbonate
Figure 5. Correlation of EPR signal intensity (nomalized on linewidth) versus the corresponding
A
400 normalized values for PDAꢀTris and PDAꢀbicarbonate samples.
A model for PDA paramagnetic behavior based on interaction with ILs. As per the simplified
2
5
model previously proposed for synthetic eumelanins, PDA paramagnetism would be due to the
17
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 18 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
contribution of two different free radical populations, Cꢀcentered (g = 2.0032) and Oꢀcentered (g =
2
.0045), the latter of which would predominate as pH increases, due to comproportionation. Based
on this model, it could be argued that for PDAꢀTris the passage from the solid state to dispersion in
the ILs is marked by a substantial conversion of the Cꢀcentered free radicals into Oꢀcentered
species, a process which is likely assisted by the nitrogenous Trisꢀderived moieties. A less
pronounced conversion seems to occur in the case of PDAꢀbicarbonate, for which a similar process
would become more marked in the relatively basic [N8881][C18:1]. However, signal broadening is the
most relevant spectral change accompanying suspension of PDAꢀbicarbonate in all ILs, including
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
[
N8881][C18:1]. This behavior of PDA is different from that reported for synthetic 5,6ꢀ
dihydroxyindole eumelanins, which did not display line broadening in solution compared to solid
2
4
state conditions. It should be noted, however, that in the case of 5,6ꢀdihydroxyindoles the term
“
eumelanin solutions” referred to eumelanins generated in the presence of polyvinylalcohol, and
not to postꢀsynthetic deaggregation, as in the present case. Several hypotheses can be made to
account for the marked signal broadening in ILs observed herein. A most plausible explanation
would call into play an increase in spin delocalization across the molecular scaffolds consequent to
destacking. Although difficult to verify based on available data, this conclusion would point to the
operation of strong intermolecular interactions in the solid state which appear to “limit” spin
delocalization within aggregated components preventing also extensive conversion of Cꢀcentered
free radical populations. An alternate view, which would not be exclusive of the former, would
consider line width broadening as the result of specific interactions between the charged
components of ILs and PDA scaffolds, as depicted in Figure 6.
18
ACS Paragon Plus Environment

Page 19 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
.
HO
O
N
H
H N+
O
3
HO
O
HO
+NH3
OH
H N+
3
.
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
HO
OH
O
H
N
NH2
O
O
SOLID STATE PDA
(
mainly C-centered spins)
N8881][C18:1]
[
N+
-O
COO-
.O
N
H
H N
O
2
HO
-O
O-OOC
NH2
N+
PDA IN [N8881][C18:1]
increased O-centered spins)
(
Figure 6. Schematic illustration of the effect of ILs on PDA spin properties.
The detailed nature of free radical species in PDA is unknown, but there is ample evidence that they
cannot be effectively represented only by oversimplified schemes based on monomer semiquinones
and their equilibria with catechol and quinone species. Wrapping of PDA components within the
hydrophobic chains of [N8881][C18:1] might lead to efficient proton transfer from delocalized
semiquinones to carboxylate groups in the IL followed by electrostatic stabilization of the resulting
radical anions, showing largely Oꢀcentered spins. In line with our model, a recent DFT study
indicates that gꢀvalues in the range 2.004ꢀ2.005 are not exclusive indicators of neutral semiquinone
3
8
free radicals as they are also compatible with quinoneꢀlike anionic and cationic radicals, which are
likely favored in highly polar ionic liquids.
In the proposed scheme, proton transfer to the IL basic groups may occur because of the expectedly
high acidity of delocalized semiquinones, exceeding that of the monomers. It is also worth noting
19
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 20 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
that pKa of semiquinones is much lower for catechol units (4.2) than for DHI (7.2), suggesting a
different response of uncyclized versus cyclized structural moieties. In line with this view, the
limited line broadening with dissolution in 1,3ꢀdimethylimidazoliumꢀbased ILs may be due to the
3 2 3 3 2 3 2 2
lower basicity of the [(CH O)HPO ], [(CH O)CH PO ] and [(CH O) PO ] anionic components.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
CONCLUSIONS
The discovery of the unique ability of ILs to disassemble polymer particles and aggregates to the
nanoscale in a structureꢀdependent manner may pave the way to novel promising developments in
16,39
PDA chemistry and technology. Besides some previous approach based on oxidative treatments,
no method has been reported so far, to the best of our knowledge, for the mild nonꢀoxidative and
nonꢀaqueous disassembly and removal of PDA coatings. An important outcome of this study is yet
the first insight and chemical probing of PDA free radical properties made possible by disassembly
to the nanoscale level in ILs. The key finding of the study is the marked alteration of EPR
parameters in an IL structureꢀdependent manner. A general shift of gꢀfactors toward high values
was observed with destacking, which would reflect the effects of removal of the aggregationꢀ
dependent constraints to spin delocalization, as well as the effects of the ionic medium on the
resulting PDA nanostructures. In some instances, a marked line broadening was also apparent in IL,
which would be another consequence of the above effects on spin delocalization. The marked
variations of the EPR response of PDA samples with IL structure revealed specific effects on
exposed electronic spins of potential usefulness for structural investigation. The different EPR
response of PDA samples prepared in Tris and bicarbonate buffers to ILs confirmed moreover the
effect of the medium on PDA structure and properties. An attractive perspective raised by this
study, which is currently under assessment in our laboratories, is the possibility of exploiting a
battery of ILs as chemical probes for comparative EPR scanning of nanoparticles to develop for the
first time a rational picture of IL structureꢀPDA signal modification relationships as a new approach
to the origin of paramagnetism in melanins. The relevance of this study is related also to the fact
20
ACS Paragon Plus Environment

Page 21 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
that polydopamine films are highly unsoluble in most organic solvents and in water unless at
alkaline pHs (>10). The discovery that solvents like ionic liquids are able to disassemble such films
to form a stable suspension of particles is of major importance not only in colloid science but also
for applications in organic synthesis because ionic liquids are more and more considered as ideal
reaction media where polydopamine may play the role of a catalyst for different reactions like
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
4
0
aldolations. The potential technological utility of ILs for deꢀcoating applications warrants further
investigation. The potential technological utility of ILs for deꢀcoating applications is also worthy of
further work.
Supporting Information
The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.
Experimental details and UVꢀvisible spectra registered on the supernatants obtained from the
treatment of PDA powders with ionic liquids are reported.
ACKNOWLEDGMENT
This work was supported in part by MIUR, PRIN 2010ꢀ11 “PROxi” project, and by European
Commission, PolyMed project in the FP7ꢀPEOPLEꢀ2013ꢀIRSES frame (PIRSESꢀGAꢀ2013ꢀ
6
12538).
REFERENCES
1
)
Liu, Y.; Ai, K.; Lu, L. Polydopamine and Its Derivative Materials: Synthesis and Promising
Applications in Energy, Environmental, and Biomedical Fields. Chem. Rev. 2014, 114, 5057–
5115.
2)
Kang, S. M.; Hwang, N. S.; Yeom, J.; Park, S. Y.; Messersmith, P. B.; Choi, I. S.; Langer, R.;
Anderson, D. G.; Lee, H. OneꢀStep Multipurpose Surface Functionalization by Adhesive
Catecholamine. Adv. Funct. Mater. 2012, 22, 2949–2955.
21
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 22 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3)
d'Ischia, M.; Napolitano, A.; Ball, V.; Chen, C.ꢀT.; Buehler, M.J. Polydopamine and
Eumelanin: From StructureꢀProperty Relationships to a Unified Tailoring Strategy. Acc.
Chem. Res. 2014, 47, 3541ꢀ35.
4
)
)
Nam, H. J.; Kim, B.; Ko, M. J.; Jin, M.; Kim, J. M.; Jung, D. Y. A New MusselꢀInspired
Polydopamine Sensitizer for DyeꢀSensitized Solar Cells: Controlled Synthesis and Charge
Transfer. Chem. Eur. J. 2012, 18,14000ꢀ14007.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
Neto, A. I.; Vasconcelos, N. L.; Oliveira, S. M.; RuizꢀMolina, D.; Mano, J. F. Highꢀ
Throughput Topographic, Mechanical, and Biological Screening of Multilayer Films
Containing MusselꢀInspired Biopolymers. Adv. Funct. Mater. 2016, 26, 2745ꢀ2755.
Liu, Q.; Wang, N. Y.; Caro, J.; Huang, A. S. BioꢀInspired Polydopamine: A Versatile and
Powerful Platform for Covalent Synthesis of Molecular Sieve Membranes. J. Am. Chem. Soc.
6)
2013, 135, 17679−17682.
7
)
)
PopꢀGeorgievski, O.; Popelka, S.; Houska, M.; Chvostova, D.; Proks, V.; Rypacek, F.
Poly(ethylene oxide) Layers Grafted to Dopamine Melanin Anchoring Layer: Stability and
Resistance to Protein Adsorption. Biomacromolecules 2011, 12, 3232−3242.
Ju, K.ꢀY.; Lee, Y.; Lee, S.; Park, S. B. Lee, J.ꢀK. Bioinspired Polymerization of Dopamine to
Generate MelaninꢀLike Nanoparticles Having an Excellent FreeꢀRadicalꢀScavenging Property
Biomacromolecules 2011, 12, 625ꢀ632.
8
9)
Rodriguez, C.; Rebeca, A.; SaizꢀPoseu, J.; GarciaꢀPardo, J.; Garcia, B.; Lorenzo, J.; Ojeaꢀ
Jimenez, I.; Komilis, D.; Sedo, J.; Busque, F. et al. RSC Advances 2016, 6, 40058ꢀ40066.
1
0) Quignard, S.; Chen, Y.; d’Ischia M.; Fattaccioli, J. UVꢀInduced Fluorescence of
PolydopamineꢀCoated Emulsion Droplets. ChemPlusChe 2014, 79, 1254ꢀ1257.
m
1
1) Kang, K.; Choi, I. S.; Nam, Y. A Biofunctionalization Scheme for Neural Interfaces Using
Polydopamine Polymer. Biomaterials 2011, 32, 6374−6380.
22
ACS Paragon Plus Environment

Page 23 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
12) Yildirim, A.; Bayindir, M. Turnꢀon Fluorescent Dopamine Sensing Based on in Situ
Formationof Visible Light Emitting Polydopamine Nanoparticles. Anal. Chem. 2014, 86,
5
508−5512.
1
1
3) Lin, J.ꢀH.; Yu, C.ꢀJ.; Yanga, Y.ꢀC.; Tseng, W.ꢀL. Formation of Fluorescent Polydopamine
Dots from Hydroxyl RadicalꢀInduced Degradation of Polydopamine Nanoparticles.
Phys.Chem.Chem.Phys.,2015, 17, 15124ꢀ151ꢀ30.
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
4) Zheng, L.; Xiong, L.; Zheng, D.; Li, Y.; Liu, Q.; Han, K.; Liu, W.; Tao, K.; Yang, S.; Xia, J.
Bilayer Lipid Membrane Biosensor with Enhanced Stability for Amperometric Determination
of Hydrogen Peroxide. Talanta, 2011, 85, 43–48.
15) Cui, J.; Yan, Y.; Such, G.K.; Liang, K.; Ochs, C.J.; Postma, A.; Caruso, F. Immobilization
and Intracellular Delivery of an Anticancer Drug Using MusselꢀInspired Polydopamine
Capsules. Biomacromolecules 2012, 13, 2225ꢀ2228.
1
6) Chen, X.; Yan, Y.; Müllner, M.; van Koeverden, M.P.; Noi, K.F.; Zhu, W.; Caruso, F.
Engineering Fluorescent Poly(dopamine) Capsules. Langmuir 2014, 30, 2921ꢀ2925.
17) Della Vecchia, N. F.; Avolio, R.; Alfè, M.; Errico, M. E.; Napolitano, A.; d'Ischia, M.
BuildingꢀBlock Diversity in Polydopamine Underpins a Multifunctional EumelaninꢀType
Platform Tunable Through a Quinone Control Point. Adv. Funct. Mater. 2013, 23, 1331ꢀ1340.
1
8) Liebscher, J.; Mrowczynski, R.; Scheidt, H. A.; Filip, C.; Hadade N. D.; Turcu, R.; Bende, A.;
Beck S. Structure of Polydopamine: A NeverꢀEnding Story? Langmuir 2013, 29,
10539−10548.
1
2
9) Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski,C. W. Perspectives on
Poly(dopamine). Chem. Sci. 2013, 4, 3796−3802.
0) Della Vecchia, N.F.; Luchini, A.; Napolitano, A.; D'Errico, G.; Vitiello, G.; Szekely, N.;
d'Ischia, M.; Paduano, L. Tris Buffer Modulates Polydopamine Growth, Aggregation, and
Paramagnetic Properties. Langmuir 2014,30, 9811ꢀ9818.
23
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 24 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
21) Ponzio, F.; Barthès, J.; Bour, J.; Michel, M.; Bertani, P.; Hemmerlé, J.; d’Ischia, M.; Ball, V.
Oxidant Control of Polydopamine Surface Chemistry in Acids: A MechanismꢀBased Entry to
SuperhydrophilicꢀSuperoleophobic Coatings. Chem. Mater. 2016, 28, 4697–4705.
2
2) Yang, Y.; Qi, P.; Ding, Y.; Maitz, M.F.; Yang, Z.; Tu, Q.; Xiong, K.; Leng, Y.; Huang, N. A
Biocompatible and Functional Adhesive AmineꢀRich Coating Based on Dopamine
Polymerization. J. Mater. Chem. B, 2015, 3, 72–81.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
3) Ambrico, M.; Ambrico, P.F.; Cardone, A; Della Vecchia, N.F.; Ligonzo, T.; Cicco, S.R.;
Talamo, M.M.; Napolitano, A.; Augelli, A.; Farinola, G.M. et al. Engineering Polydopamine
Films with Tailored Behaviour for NextꢀGeneration EumelaninꢀRelated Hybrid Devices. J.
Mat. Chem C, 2013, 1 ,1018ꢀ1028.
2
4) Panzella, L.; Gentile, G.; D'Errico, G.; Della Vecchia, N.F.; Errico, M.E.; Napolitano, A.;
Carfagna, C.; d'Ischia, M. Atypical Structural and πꢀElectron Features of a Melanin Polymer
That Lead to Superior FreeꢀRadicalꢀScavenging Properties. Angew. Chem. Int. Ed. 2013,
52,12684ꢀ12687.
25) Mostert, A. B.; Hanson, G. R.; Sarna, T.; Gentle, I. R.; Powell, B. J.; Meredith, P. Hydrationꢀ
Controlled XꢀBand EPR Spectroscopy: A Tool for Unravelling the Complexities of the Solidꢀ
State Free Radical in Eumelanin. J. Phys. Chem. B 2013, 117, 4965−4972.
2
6) Mrówczyński, R.; Emerson Coy, L.; Scheibe, B.; Czechowski, T.; AugustyniakꢀJabłokow,
M.; Jurga, S.; Tadyszak, K. Electron Paramagnetic Resonance Imaging and Spectroscopy of
Polydopamine Radicals J. Phys. Chem. B 2015, 119, 10341−10347.
2
7) Vogel, R.; Dobe, C.; Whittaker, A.; Edwards, G.; Riches, J. D.; Harvey, M.; Trau,
M.;Meredith, P. Postsynthesis Stabilization of FreeꢀStanding Mesoporous Silica Films.
Langmuir 2004, 20, 2908ꢀ2914.
28) Wakasa, M. The Magnetic Field Effects on Photochemical Reactions in Ionic Liquids J. Phys.
Chem. B 2007, 111, 9434– 9436.
24
ACS Paragon Plus Environment

Page 25 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
29) Takahashi, K.; Tezuka, H.; Kitamura, S.; Satoh, T.; Katoh, R. Reactions of ExcitedꢀState
Benzophenone Ketyl Radical in a RoomꢀTemperature Ionic Liquid. Phys.Chem.Chem.Phys.
2010, 12, 1963–1970.
3
0) Yang, Y; Fan, H.; Song, J; Meng, Q.; Zhou, H.; Wu, L.; Yang, G; Han, B. Free Radical
Reaction Promoted by Ionic Liquid: a Route for MetalꢀFree Oxidation Depolymerization of
Lignin Model Compound and Lignin. Chem. Commun. 2015, 51, 4028ꢀ4031.
1) Parmentier, D.; Metz, S.J.; Kroon, M.C. Homogeneous LiquidꢀLiquid Extraction of Rare
Earths with the BetaineꢀBetainium Bis(trifluoromethylsulfonyl)imide Ionic Liquid System.
Green Chem., 2013, 15, 205ꢀ209.
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
3
32) Ponzio, F.; Barthes, J.; Bour, J.; Michel, M.; Bertani, P.; Hemmerle,́ J.; d’Ischia, M.; Ball, V.
Oxidant Control of Polydopamine Surface Chemistry in Acids: A MechanismꢀBased Entry to
SuperhydrophilicꢀSuperoleophobic Coatings. Chem. Mater. 2016, 28, 4697−4705
3) Palgunadi, J.; Yun Hong, S.; Kyu Lee, J.; Lee, H.; Deuk Lee, S.; Cheong, M., Sik Kim, H.
Correlation between Hydrogen Bond Basicity and Acetylene Solubility in Room Temperature
Ionic Liquids. J. Phys. Chem. B 2011, 115, 1067–1074.
3
3
3
3
4) Hayes, R.; Warr, G.G.; Atkin , R. Structure and Nanostructure in Ionic Liquids. Chem. Rev.
2015, 115, 6357−6426.
5) Acik, M.; Dreyer, D.R.; Bielawski, C.W.; Chabal, Y.J. Impact of Ionic Liquids on the
Exfoliation of Graphite Oxide J. Phys. Chem. C 2012, 116, 7867−7873.
6) Pilawa, B.; Buszman, E.; Wrześniok, D.; Latocha, M.; Wilczok, T. Application of EPR
Spectroscopy to Examination of Gentamicin and Kanamycin Binding to DOPAꢀMelanin.
Appl. Magn. Reson. 2002, 23, 181–192.
3
7) Meredith, P.; Sarna, T. Pigment Cell Res. 2006, 19, 572ꢀ594.
38) BataginꢀNeto, A.; Soares BronzeꢀUhle, E.; de Oliveira Graeff, C.F. Electronic Structure
Calculations of ESR Parameters of Melanin Units. Phys. Chem. Chem. Phys. 2015,17, 7264ꢀ
7274.
25
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 26 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
39) Ball, V.; Del Frari, D.; Toniazzo, V.; Ruch, D. Kinetics of Polydopamine Film Deposition as
a Function of pH and Dopamine Concentration: Insights in the Polydopamine Deposition
Mechanism. J. Colloid Interf. Sci. 2012, 386, 366ꢀ372.
4
0) Mrówczyński, R.; Bunge, A.; Liebscher, J. Polydopamineꢀan Organocatalyst Rather Than an
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Innocent Polymer. Chem. Eur. J. 2014, 20, 8647ꢀ8653.
26
ACS Paragon Plus Environment

Page 27 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Table of Contents
Ionically Radical: Sonication of polydopamine (PDA) in different ionic liquids (ILs) resulted in
the generation of nanostructures with marked alterations of the electron paramagnetic resonance
properties, suggesting destackingꢀrelated free radical reorganization driven by ionic interactions.
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
27
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 28 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
+
+
N
N
N
N
O
O
P
-
P
-
CH O
CH O
3
O
3
O
OCH3
CH3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
[C C im][(CH O) PO ] [C C im][(CH O)CH PO ]
1
1
3
2
2
1
1
3
3
2
O
-
O
(CH )
(CH ) CH
2 7 3
+
N
2 7
N
C H
8
17
O
+
H C
N
C H
17
3
8
P
-
CH O
3
O
C H
8 17
H
[
C C im][(CH O)HPO ]
[N8881][C18:1]
1
1
3
2
ACS Paragon Plus Environment

Page 29 of 34
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
a)
b)
100
8
6
4
2
0
0
0
0
0
0
.0
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4
00
500
600
700
Wavelenght (nm)
c)
ACS Paragon Plus Environment