Colorimetric analysis of painting materials using polymer-supported polydiacetylene films

Article abstract of DOI:10.1039/c6nj02092e

Analysis of artworks and identification of their molecular components are of utmost importance for selecting proper conservation strategies and monitoring restoration. Accordingly, development of simple and readily applicable paint analysis assays is high

Full text of DOI:10.1039/c6nj02092e

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Colorimetric analysis of painting materials using
polymer-supported polydiacetylene films†
Cite this:DOI: 10.1039/c6nj02092e
Alexander Trachtenberg,a Orit Malka,a Kaviya Parambath Kootery,a Stella Beglaryan,b
Danilo Malferrari,b Paola Galletti,b Silvia Prati,*c Rocco Mazzeo,c Emilio Tagliavinib
and Raz Jelinek*a
Analysis of artworks and identification of their molecular components are of utmost importance for
selecting proper conservation strategies and monitoring restoration. Accordingly, development of simple
and readily applicable paint analysis assays is highly sought in conservation science and technology. We
prepared transparent spin-coated films comprising polydiacetylene (PDA) derivatives upon poly(methyl
methacrylate) (PMMA) substrates and employed the PDA/PMMA films for color analysis of organic
constituents in painting materials. We show that distinct color transformations were recorded upon
addition of paint compounds to different PDA films. Quantitative analysis revealed that the colorimetric
transitions were dictated by the interactions between the diacetylene headgroups and the tested
compounds. This study underscores the significance of the diacetylene headgroup for molecular
discrimination and points to the possible uses of the PMMA-supported PDA films for colorimetric
screening of organic components in painting materials.
Received (in Montpellier, France)
5th July 2016,
Accepted 9th September 2016
DOI: 10.1039/c6nj02092e
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gums, used as binders and sometimes in varnishes). This poses
additional challenges to the development of accurate and effective
Introduction
Proper restoration of old paintings and artworks often necessi- on-site identification methods.
tates identification of painting materials and diagnosis of their
Several analytical methods have been employed for paint analysis.
state of conservation. Cleaning operations of paintings are Micro-destructive methods, such as gas chromatography-mass
often necessary to remove the aged and yellowed varnish layer spectrometry (GC-MS) and high performance liquid chromato-
applied on the painting. However, the selective removal of the graphy (HPLC), enable specific identification of the organic
varnish layer may be difficult due to its thickness (10–50 mm) compounds present in a paint sample.1 While these methods
and the affinity to the painting surface. Thus, identification are very useful and informative, they cannot be applied during
of organic compounds present both in the painting and in routine restoration work because they require technical expertise
the varnish layers is of the utmost importance to identify the to carry out and analyse, and they could be expensive and time
most suitable cleaning method. Moreover, the materials used as consuming. Portable Fourier transform infrared (FTIR) and
binders in painting layers or as ingredients in the formulation of Raman spectroscopies can be used to monitor the molecular
varnishes belong to different organic classes, such as triglycerides compositions of the external painting layers.1 Development of
(i.e., siccative oil used both as painting binders and in varnishes), simple on-site analytical systems has been lacking in conserva-
terpenes (i.e., natural resins used in varnishes), proteinaceous tion science. While such applications are technically challenging,
materials (i.e., animal glue, egg, milk or casein used as binders introduction of such instruments would be of significant
and sometimes in varnishes), and polysaccharides (i.e., natural benefit in conservation and restoration work and maintaining
cultural heritage.
Herein, we present application of spin-coated polydiacetylene
(PDA) films for in situ colorimetric sensing of a selection of
a Department of Chemistry, Ben Gurion University of the Negev, Beer Sheva 84105,
Israel. E-mail: razj@bgu.ac.il
b University of Bologna, Chemistry Department ‘‘G. Ciamician’’, Green Chemistry
Lab, Via S. Alberto 163, 48100 Ravenna, Italy. E-mail: paola.galletti@unibo.it
c University of Bologna, Chemistry Department ‘‘G. Ciamician’’, Microchemistry and
Microscopy Art Diagnostic Laboratory (M2ADL), via Guaccimanni 42, 48100
Ravenna, Italy. E-mail: s.prati@unibo.it
organic materials present in paintings. PDAs are p-conjugated
polymers displaying unique structural and chromatic properties.2
These polymers, first synthesized in the late 1960s, have attracted
considerable interest both scientifically and as promising
sensing platforms, primarily due to their visible colour trans-
formations (generally from blue to red), induced by varied
external stimuli such as heat,3 ionic strength,4 mechanical
† Electronic supplementary information (ESI) available: Detailed synthetic
routes, 1H NMR, 13C NMR and 19F NMR spectra of TR-derivatives. See DOI:
10.1039/c6nj02092e
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pressures, and interactions with biological and chemical
molecules.5 Recently, we demonstrated that PDA films spin-coated
on poly(methyl methacrylate) (PMMA), a widely used transparent
polymer, constituted a powerful colorimetric sensing platform
for varied organic substances.6
In this study, we constructed PMMA-coated PDA films using
diacetylene monomers displaying varied headgroups exhibiting
different polarities and functional moieties, designed to tune
the interactions with the tested analytes, i.e. paint constituents.
The premise of this work is that the interactions of the organic
substances with the different headgroups would generate distinct
colorimetric transformations within the PMMA-supported PDA
films, aiding in distinguishing among the tested compounds.
In particular, different PMMA-supported PDA films were tested
on not-aged painting materials in order to make a feasibility
study and a screening of the most performing tools for the
on-site analysis of paint materials.
Scheme 1 General synthetic pathways to TR-ester derivatives 2–5, and
TR-amide derivatives 6–8.
Experimental
Materials
All solvents were purchased from Sigma-Aldrich Chemicals.
The diacetylene monomer, 10,12-tricosadiynoic acid (TRCDA),
was purchased from Aldrich (98% purity) and eventually
purified to remove the polymerized part (if present) before use.
For natural gums study, arabic, ghatti, tragacanth, guar and karaya
gums were purchased from Sigma Aldrich. 1H,1H,2H,2H,per-
fluoro-1-decanol was purchased from Fluka. Analytical thin-layer
chromatography (TLC) was conducted on Sigma-Aldrich silica gel
aluminum plates (60 Å, with fluorescent indicator) or silica gel 60
F-254 with a 0.2 mm layer thickness. For flash chromatography,
60 Å silica gel (Merck, 230–400 mesh) was employed. 1H, 13C
and 19F NMR spectra were obtained using a Varian Mercury 400
spectrometer with a 5 mm probe.
carried out at 1500 rpm for 30 seconds. The films were sub-
sequently UV irradiated (254 nm) for 0.3 minutes in a UV oven,
yielding blue-phase PDA.
Colorimetric measurements and analysis
The organic painting material samples were solubilized in different
solvents (outlined in the Results section) at a concentration of
10 mg mLÀ1 and placed upon the spin-coated PDA films and
incubated for 90 minutes. Ultraviolet (UV)-vis spectroscopy
measurements were subsequently carried out at 25 1C on a
Varioskan (Thermo, Finland) in a wavelength range of 400–800 nm.
Percentage colorimetric response (%CR) was calculated
after baseline correction of the UV-vis absorbance spectrum,
according to the equation:
Synthesis of diacetylene monomer derivatives
%CR = [(PB0 À PBi)/PB0] Â 100
Synthesis routes developed for synthesizing the TRCDA deriva-
tives 2–8 are summarized in Scheme 1. All synthesis routes
started from 10,12-tricosadiynoic acid (TRCDA) 1. Ester deriva-
tives were prepared by treating 1 with oxalylchloride and the
suitable fluorinated alcohol to obtain 2–4 or phenylboronic acid
to obtain 5. Amide derivatives 6–8 were prepared by activating 1
via its hydroxysuccinamide derivative, in the presence of EDC,
and then coupling with the corresponding amine or amino-acid
methyl ester. All diacetylene derivatives 2–8 were characterized
by standard analytical and spectroscopic techniques and proved
very stable when stored under nitrogen (or at À20 1C) in the
dark. Details on synthesis and spectroscopic data are reported in
the supporting material.
where PB = Ablue/(Ablue + Ared), Ablue = absorbance at 640 nm,
Ared = absorbance at 500 nm, PB0 corresponds to the control
film, and PBi corresponds to the sample. Reference %CR was
registered on tris buffer with pH 8.0.
Analytical techniques
Scanning electron microscopy (SEM). SEM images were
recorded on a JEOL JSM-7400F (Tokyo, Japan) scanning electron
microscope. Images were taken after sputtering a thin film of
Au (B15 nm thickness) over the substrates for better contrast
and minimum charging.
Raman scattering. The Raman system comprised a Jobin-Yvon
LabRam HR 800 micro-Raman system, equipped with a synapse
CCD detector. The excitation sources were an Argon laser (514.5 nm)
Film preparation
40 mg of TRCDA and TR derivatives (mole ratio 3 : 1) were and a He–Ne laser (633 nm), both exhibiting 3 mW power.
dissolved in a solvent mixture comprising 500 mL CH2Cl2 and The full laser power was reduced by 1000 using ND filters to
500 mL THF. 40 mL of this solution was placed upon a PMMA prevent affecting the measurements. The laser was focused
disk (0.5 cm diameter). After 30 seconds, spin coating was with a Â100 objective to a spot of around 1 mm2. The grating
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used was a 600 g mmÀ1 and the confocal hole of 100 mm with
exposure times of 10–60 seconds.
Results
Fig. 1 outlines the structural features of the diacetylene mono-
mer derivatives used in this study (prepared according to
Scheme 1). In particular, we designed and synthetized different
diacetylene headgroups in order to examine both the distinct
molecular interactions with the compounds tested, as well as
modulate the overall structural organization and corresponding
colorimetric properties of the sensor films. In particular,
the diacetylene monomers examined displayed carboxylic acid
(e.g., the commercial product TRCDA, compound 1), fluorine-
containing headgroups with different fluorine chain lengths
designed to enhance non polar interaction (compounds 2, 3
and 4), phenyl boronic acid (compound 5)7 designed to establish
polar/ionic interactions and eventually form boronic esters with
diols-containing materials with the aim of exhibiting affinity to
carbohydrates,8 ammonium residue (compound 6)9 aimed to
promote ionic interactions with negative compounds, and amino-
acid ester derivatives (compounds 7 and 8) for attaining
protein-like hydrogen bonding with polar targets.
The colour sensor films were assembled through spin-coating
mixtures comprising the starting monomer 10,12-tricosadiynoic
acid (TRCDA, 1) and the different functionalized-headgroup
derivatives at a 1 : 1 mole ratio (TRCDA: derivative) upon PMMA
disks.6 Notably, we found that films comprising pure diacetylene
derivatives exhibit lower stability and/or were too sensitive to
buffer/solvents, thereby rendering very high background signals.
Following deposition and drying of the organic solvent, the films
were subjected to UV (254 nm) irradiation to generate the blue
polymerized polydiacetylene (PDA) phase.
Fig. 2 Scanning electron microscopy (SEM) of PMMA-supported diacetylene
films. The films comprised the indicated derivatives and the TRCDA (1)
monomer at 1 : 1 mole ratio. Scale bars correspond to 10 mm.
PDA derivatives, and attest the pronounced effects of diacetylene
headgroups upon film organization. In particular, the cooperative
film properties, determined to a large extent by the distinct
headgroups, might have generated different morphologies
during the dewetting accompanying the spin coating process.
Previous reports also underscored the significant effect of
headgroup interactions in determining macro-scale organiza-
tion of PDA assemblies.10 While the SEM results in Fig. 2 reveal
distinct macro-scale organizations of the spin-coated films,
representative UV-vis spectra and Raman scattering data in
Fig. 3 indicate that the PDA backbones within the different films
exhibit the same chromatic properties, specifically the presence
of the well-known blue and red PDA phases.11 Fig. 3A depicts the
UV-vis spectra of a spin-coated PDA film in the blue phase (after
polymerization), and after transformation to the red phase
induced by placing the PMMA-supported film at 50 1C.
Importantly, similar spectral features corresponding to the
blue-red transitions were recorded for the other PDA derivatives,
indicating that the chromatic properties of the conjugated
polymer backbone were retained in all films. Indeed, the Raman
scattering data in Fig. 3B corroborate this interpretation.
The Raman spectral region corresponding to the conjugated
Fig. 2 presents the scanning electron microscopy (SEM)
images of the PMMA-coated PDA films. The SEM experiments
demonstrate significantly different surface morphologies among the
Fig. 1 Structures of TR-derivatives and PDA polymers and possible intermolecular interactions between PDA and painting material components.
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terpenic compounds (mastic, dammar, colophony, and sandarac),
and polysaccharide gums (arabic gum, gum karaya, ghatti gum,
guar gum and gum tragacanth). Detailed compositions of the
materials are provided in Table S1 of the ESI.†12
Fig. 4 presents the colorimetric responses of the PDA films
comprising the different headgroup derivatives upon incuba-
tion with the compounds outlined in Table 1.
In particular, the bar diagrams in Fig. 4A–D depict the
percentage colorimetric response (%CR)13 after subtraction of
the buffer recorded upon incubation of the painting materials
with the PDA films, calculated through comparison of the
relative intensities of the UV-vis peaks at 500 nm (i.e., ‘‘red’’
peak in the UV-vis spectrum) and 640 nm (‘‘blue’’ peak), see
Experimental section.14 Essentially, the %CR values reflect
the degrees of blue-red transformations with respect to the
reference solution (here a tri buffer pH 8.0). In particular, high
%CRs correspond to more pronounced red appearance,
whereas lower %CR values reflect less significant blue-red
transformations (e.g., more blue PDA films).15 Negative %CR
values are occasionally recorded, likely reflecting the surface
interactions of target analytes, which stabilize the PDA headgroups
in comparison with the effects of buffer/solvents. Notably, the
colorimetric data in Fig. 4 were grouped according to the
solvent used, making sure the comparison of %CR is reliable
in terms of compounds dissolution.
Fig. 4 demonstrates significant variations both among
the colorimetric responses of the PDA films to the different
painting materials, as well as among the diacetylene derivatives
used in the PMMA-supported films. Among the painting materials
dissolved in acetonitrile (Fig. 4A, terpenic compounds), films
comprising PDA units displaying fluorinated residues (i.e.,
monomers 2 and 3) successfully distinguished between dammar,
mastic, and sandarac resins. Derivative 2, in particular, gave rise
to the most significant blue-red transformations (%CR of 15, 40,
and 50 induced by dammar, mastic, and sandarac, respectively,
Fig. 4A). This specificity is likely ascribed to the terpenic apolar
backbone interacting with the apolar terminus of fluorinated
derivatives. The dammar and mastic are triterpenic resins with
very similar composition, whereas sandarac is partially polymer-
ized diterpenic materials.16 Among the diacetylene derivatives,
monomer 2 exhibited the most versatile interactions, both polar
through the ester moiety, as well as apolar affinity via the
fluorinated group. Indeed, the significance of the fluorine-
containing PDA headgroups is supported by the observation of
small %CR and no distinction among the terpenic compounds
apparent in the films comprising the parent polymerized
TRCDA (monomer 1, Fig. 4A). Similarly, polar or charged
residues (i.e., derivatives 5 and 6) exhibited small %CR, which
was similar to the parent diacetylene 1, reflecting insignificant
interactions with terpenes.
Fig. 3 Spectroscopic characterization of PMMA-supported PDA films.
(A) UV-vis spectra of blue (i) and red (ii) phase; (B) Raman scattering
spectra of the blue (i) and red (ii) PDA film.
alkyne–alkene groups initially featured the typical blue-phase
signals at around 1450 cmÀ1 and 2090 cmÀ1 (Fig. 3B, i). Distinct
peaks corresponding to red-phase PDA appeared at approxi-
mately 1520 cmÀ1 and 2125 cmÀ1 following heating (Fig. 3B, ii).
Similar to the UV-vis results in Fig. 3A, Raman signals presented
in Fig. 3B were recorded for all PDA derivative films prepared,
confirming the formation of the conjugated PDA networks.
Table 1 outlines the paint materials tested in this study. The selected
substances belong to different classes of molecules employed as
painting binders or in varnish formulation. In particular, the
screened substances include polymerized triglycerides (linseed oil),
proteinaceous compounds (fish glue, rabbit glue, and ovalbumine),
Table 1 Paint substances used in the study
Paint material
Linseed oil
Family
Triglycerides
Main composition
Polymerized tryglicerides, low
chain carboxylic fatty acids
Ovalbumin
Proteinaceous
Fig. 4B depicts the colorimetric data recorded for colophony
and linseed oil, substances that were only dissolved in trifluoro-
ethanol (TFE). The bar diagram in Fig. 4B indicates that apolar
fluorinated derivatives 3–4 produced low %CR, whereas polar
derivatives generated more pronounced %CR. Notably, only films
comprising ionic positively-charged headgroup, PDA derivative 6,
Fish glue, rabbit glue Proteinaceous Denaturated collagen
Sandarac
Terpenes
Diterpenic compounds, with
a polymerized component
Triterpenic compounds
Diterpenic compounds
Mastic, dammar
Colophony
Terpenes
Terpenes
Arabic, karaya, ghatti, Polysaccharides Carbohydrates,
guar, tragacanth gums
monosaccharides % in ESI
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Fig. 4 Colorimetric responses of PMMA-supported PDA films comprising different derivatives to paint substances. %CR values after subtraction of the
buffer divided according to the solvent used for dissolving the material. (A) Materials dissolved in acetonitrile (terpenic compounds); (B) materials dissolved in
trifluoroethanol TFE (triglycerides, colophony); (C) materials dissolved in cold water (gums); (D) materials dissolved in hot water (proteinaceous materials).
could distinguish between the two substances, possibly
reflecting interactions of the amine residues with carboxylic
Discussion
acid functional groups in colophony.
This study explores the use of PMMA-supported PDA films as
Fig. 4C depicts the colorimetric transformations induced colorimetric sensors for common constituents in artwork paint.
by paint substances solubilized in cold water, specifically gum In particular, we synthesized novel diacetylene derivatives dis-
derivatives with different saccharide compositions. Among playing headgroups, exhibiting different molecular properties
these compounds, gum arabic and gum karaya induced the and functionalities, and prepared spin-coated films comprising
most pronounced blue-red transformations. Interestingly, the polymerized diacetylene derivatives. In particular, different
Fig. 4C reveals that both the gums did not induce a colour PMMA-supported PDA films were tested on not-aged painting
change in the films comprising diacetylene 6, which exhibits an materials in order to evaluate the PDA platform for paint
ionic headgroup. Moreover, a clear distinction between the screening and analysis. The PMMA-supported PDA films featured
two gums was accomplished by films containing the parent distinct morphologies and particularly important, different
diacetylene monomer 1 and derivatives 7–8. Compound 8, in fact, colorimetric response upon interaction with paint materials.
appears the most promising for screening other gums (Fig. 4C).
The experimental data demonstrate that the extent of blue-
This result points to hydrogen bonding and interactions between red transformations of the PDA films was closely related to
the polar headgroup of 7 (Scheme 1) and hydroxyl units in the affinity between the headgroup and tested analytes. In
the carbohydrate gums, rather than boronate ester formation, particular, the results point to the significance of polar, non-
as the most prominent factors affecting the colorimetric polar, and electrostatic interactions between the PDA head-
transformations.
group and paint constituents as fundamental parameters
The colorimetric results recorded upon interactions of the affecting the colour transitions. As such, this study points to
proteinaceous substances dissolved in hot water with the PDA/ the potential of the PDA/PMMA system as a viable tool for
PMMA films (Fig. 4D) underscore low %CR values, likely distinguishing among paint substances. Further studies will be
reflecting weak interactions between the tested compounds performed to evaluate the response on aged and real paintings.
and the PDA headgroups. Films comprising PDA derivatives
Polar interactions affecting enhanced colour transformations are
2, 7, and 8 generated %CR values that were somewhat higher manifested, for example, in the more pronounced transformations
than from the background, although the %CR signals could involving the fluorine-displaying headgroups, particularly derivatives
not be used for distinguishing among the tested substances. 2 and 3. These films gave rise to stronger blue-red transformations
Nevertheless, identification of generic proteinaceous substances upon addition of different paint material families (e.g. Fig. 4A–C).
rather than the precise protein type may be sufficient for selecting The three fluorine-based PDA derivatives will be further tested to
a suitable paint restoration treatment.
monitor the conservation state of terpenic varnishes considering
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that during ageing, they change their polarity and solubility. Thus, Application of the colorimetric polydiacetylene film assay is simple
we expect a change in their response to the three derivatives at and analysis can be readily carried out by the use of visible spectro-
different level of apolarity compared with the non-aged materials.
photometry. This study shows that the polydiacetylene technology
While polar or apolar interactions (as well as combined polar/ might open new analytical avenues in molecular analysis, in general,
apolar effects involving different parts of the PDA framework) painting restoration and conversation science, in particular.
contributed to the colorimetric transformations and aided in
distinguishing among paint substances, our results additionally
illuminate other chemical effects modulating the colour response
Acknowledgements
of the PDA films. Specifically, Fig. 4 revealed that hydrogen
bonding and ionic interactions played significant roles in certain
instances. These observations point to the possibility to employ
different diacetylene derivatives in a comprehensive ‘‘colorimetric
toolbox’’ for distinguishing paint analytes.
It should be emphasized that the experimental results pre-
sented in Fig. 4 suggest that ionic PDA films may be suitable to
monitor the polarity changes of oil painting constituents. Indeed,
siccative oils adopt three-dimensional networks upon drying, in
which low molecular weight compounds formed during the
polymerization are entrapped. In particular, dicarboxylic fatty
acids are produced during the photo-polymerization of siccative
oils and tend to increase in concentration due to ageing.16 In
addition, degradation processes are responsible for the formation
of free fatty acids and metal carboxylates, which further contribute
to overall polarity changes in paintings.
PDA-derivatives capable to distinguish different types of gums
may find applications not only in conservation science, but also in
other disciplines, such as food production, in which gum formula-
tion may be required.17 The colorimetric results obtained for the
proteinaceous substances, for example, provided %CR values that
were sufficiently different from the background colorimetric signals,
even if it is not possible yet to clearly distinguish among the different
materials. However, the PDA films might be applied to develop
readily applicable tools for in situ identification of generic protein-
aceous substances. Indeed, exact determination of the type of
protein may not be necessary to define a suitable restoration
treatment. Further studies will be performed to evaluate the
ageing effect on the painting materials and to verify the applic-
ability of the PDA-based sensors to other proteinaceous materials
used in conservation. One example of application may be related
to the evaluation of residues after the bio-cleaning procedures.
Biotechnological researches have proposed the use of enzymes and
bacteria as a cleaning method for the selective removal of specific
class of compounds.18 When applied, conservators have to verify
that any residue of the biological treatments were removed from
the surface to avoid any interaction with the clean surface.
We acknowledge the University of Bologna (RFO Ricerca
Fondamentale Orientata) and the Regione Emilia Romagna
(POR-FESR) for funding.
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