Recent progress in polydiacetylene mechanochromism

概要

Minireview
Recent progress in polydiacetylene mechanochromism
Bratati Das, Seiko Jo, Jianlu Zheng, Jiali Chen, Kaori Sugihara*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x

Polydiacetylenes (PDAs) are a family of mechanochromic polymers that change color from blue to red and emit fluorescence
when exposed to external stimuli, making them extremely popular materials in biosensing. Although several informative
reviews on PDA biosensing have been reported in the last few years, their mechanochromism, where external forces induce
the color transition, has not been reviewed for a long time. This minireview summarizes recent progress in PDA
mechanochromism, with a special focus on the quantitative and nanoscopic data that have emerged in recent years.

1. Introduction
Mechanochromic polymer polydiacetylenes (PDAs) have garnered
attention owing to their unique force-sensing mechanism that can be
used to detect forces impossible to quantify by the conventional
technique such as piezoresistive1, 2 or capacitive tactile sensors,3, 4
and their potential for applications in chemo/biosensing. They were
first reported by Wegner in 1969.5 Their mechanosensitive
conjugated backbone with customizable side chains reacts to a
specific type of external perturbation, which turns their original blue
color to red and causes them to fluoresce. They are frequently
fabricated using lipidic monomers, where a polar group is attached
on one side of the carbon chains, which then self-assemble into
vesicles,6, 7 supported bilayers,8, 9 and monolayers.10 Upon ultraviolet
(UV) irradiation, they polymerize into PDA (Figure 1).
Many papers have been published in the past few decades,
where researchers have attempted to use PDAs as chromic and
fluorescence sensors for the detection of temperature,11 pH
changes,12 mechanical stimuli,13 ions,14 solvents,15 light,16
surfactants,17 bacteria,18 and other biomolecules19 such as
peptides,20, 21 as well as for other non-sensing applications.22-31
Significant progress has been made to improve their specificity by
synthesizing monomers with different headgroups.32-34 For example,
a PDA with an epoxy headgroup was shown to present selectivity
toward mercury(II).32 The recent introduction of a differential
approach, in which the color change of several types of PDAs was
used as a fingerprint to identify solvents,15 has further improved their
selectivity. These promising results have paved the way for
overcoming the rather weak specificity of PDAs and the development
of a convenient colorimetric sensor.

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba Meguro-Ku,
Tokyo 153-8505, Japan
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x

The mechanochromic properties of PDAs have been extensively
characterized by the Langmuir–Blodgett technique,35 electron
paramagnetic resonance (EPR),36 X-ray crystallography,37 ultravioletvisible (UV-vis) spectroscopy,38 fluorescence,20 infrared (IR)
spectroscopy,39 Raman spectroscopy,40 nuclear magnetic resonance
(NMR),41 scanning tunneling microscopy (STM),42 atomic force
microscopy (AFM),43 fluorescence microscopy,44 and electrical
conductivity measurements.45 These experimental efforts, combined
with theoretical works,46 have provided a mechanistic model where
the torsion of the sidechains shortens the  conjugation in the
backbone, thus broadening the bandgap to cause a blueshift in the
absorption.
Despite our general understanding of their mechanism and the
large number of reports in biosensing, the lack of a quantitative
understanding of the color-changing mechanism of PDAs at the
nanoscale has hindered further development until recently, when
AFM was introduced.44, 47, 48 The recent technological advancement
of AFM could shift PDA status as a quantitative mechanosensor at
nanoscale, which may lead to another leap in the application
development. Although there have been many informative reviews
on PDAs, especially from the perspective of biosensing,49-59 their
mechanochromism, where external forces stimulate the blue-to-red
transition, has not been reviewed since a brief report by Burns et al.
in 2004.60 In this minireview, therefore, we summarize the recent
progress in PDA mechanosensing.

Figure 1: Chemical structure of 10,12-Tricosadiynoic acid (TRCDA) monomers, their
polymerization by UV light, and a 3D scheme, where the p-orbitals of the carbon atoms
(black) are colored in blue. This figure has been reproduced from ref 61 with permission
from American Chemical Society, copyright 2020.

Figure 2: a, (i) Schematic illustrations of PDA/dry liquid-integrated paper device. (ii) Two different settings, where compression stresses were applied using a ceramic frog ornament.
b, Hybrid mechanoresponsive polymer wire fabricated by mixing 10,12-pentacosadiynoic acid and a poly(ethylene oxide) matrix. (i) Time evolution of applied AC voltage, which
vibrated the tuning fork to produce force pulse stimulus. ii) Oscillation amplitude vs. frequency curves shows the transduction process from the blue phase to the red phase. The
inset shows bright-field and fluorescence images of the hybrid wire. Panel (a) has been adapted from ref 62 with permission from Wiley Online Library, copyright 2021. Panel (b) has
been adapted from ref 63 with permission from Wiley Online Library, copyright 2013.

2. Polydiacetylene mechanosensing
2.1 Surface pressure
Tomioka et al.10 investigated the dependence of the excitonic
absorption on the surface pressure by observing the in situ reflection
spectra of a PDA made of poly-(heptacosa-10,12-diynoic acid) at a N2
gas–water interface. A reversible color change was observed when
the PDA monolayer was compressed using a Langmuir–Blodgett
trough at pressures between 3 and 25 mN m -1. This reversible color
change was reproducible after several cycles of compression and reexpansion. The fact that thermochromically irreversible poly(heptacosa-10,12-diynoic acid) presented the reversibility in its
mechanochromism indicates that the type of stimuli or the
assembled polymer structure also affect the PDA reversibility.
2.2 Tensile strain
Reversible tensile strain (stretch)-induced phase transitions were
observed by Nallicheri et al.13 in segmented polyurethanes
containing a small fraction of PDAs made of 2,4-hexadiyne-l,6-diol
and 5,7-dodecadiyne-1,12-diol in hard-segmented structures.
Polyurethanes (PUs) are a class of segmented copolymers composed
of soft and hard segments. The diacetylene groups were linked to
hard segments via a chain extender. When the strain levels were
greater than 250%, irreversible hard-domain disruption was

observed. From visible absorption spectroscopy, it was confirmed
that during tensile elongation, stress was transmitted from the soft
segments to the hard domains, where the hard segments were
oriented perpendicular to the stretch direction. This resulted in a
tensile (or shear) stress on PDAs oriented along the stretch direction,
causing the color transformation.
2.3 Compression
Nakamitsu et al.64 prepared a highly sensitive compression stress
sensor by integrating a stimuli-responsive layered PDA and dry liquid
(DL) on a filter paper substrate to induce a “response cascade”
(Figure 2a). Macroscopic compression stresses induced the collapse
of the DLs, which were micrometer-sized particles that consisted of
liquid droplets covered by solid powders, starting the response
cascade. As a consequence, the DLs released their interior liquid, a
polyethyleneimine (PEI) solution, in response to mechanical stress.
The subsequent PEI–PDA interaction induced color changes in the
layered PDA. As the strength and duration of the compression
increased, the intensity of the red color was enhanced. This
compression stress sensor visualized weak compression stresses on
the order of 3.9 Pa–4.9 kPa, comparable to those upon the impact of
an object. The device has the potential for the visualization and
measurement of weak mechanical stresses in biomedical and
healthcare fields.

Figure 3: a, PDA microcrystal embedded in PDMS experienced drastic color and shape changes during PDMS swelling in octane at i) 0, ii) 5, iii) 10, iv) 15, and v) 23 min. b, Schematic
diagram of a microfluidic chip used for the generation of PDA-embedded PDMS microdroplets and an optical microscopic image of the microdroplets. ii) Photographs of tubes that
contain PDA-embedded PDMS microbeads upon exposure to pentane (upper left), heptane (upper right), nonane (lower left), and undecane (lower right). c, i) Schematic diagram of
alginate hydrogel-assisted PDA and ii) its colorimetric response upon water absorption. Panel (a) has been adapted from ref 65 with permission from Wiley Online Library, copyright
2014. Panel (b) has been adapted from ref 66 with permission from American Chemical Society, copyright 2015. Panels (c) has been adapted from ref 67 with permission from American
Chemical Society, copyright 2015.

2.4 Shear
Lee et al.68 examined the shear-induced color transition of PDA
vesicles in polymeric solutions made of 2% poly(vinyl alcohol) + 1%
sodium borate (PVA/B), 15% PVA and 1% hyaluronic acid (HA)
dissolved in water using a rheometer. In the PVA/B solution at 47 °C,
shear at 100 Pa was sufficient to induce a blue–red transition in PDA
vesicles made of 10,12-pentacosadiynoic acid (PCDA). In contrast,
there was no color change in the PVA or HA solutions. The authors
interpreted the color change as a result of the alteration of the
structure of the PDA polymeric backbone owing to perturbation.
However, the experimental setup used was not able to distinguish
the effect of the shear and the increased temperature owing to the
mixing, which made it difficult to conclude whether the shear itself
induced the color change.
2.5 Oscillation

Feng et al.63 fabricated a hybrid mechanoresponsive polymer wire by
mixing PDA made of 10,12-pentacosadiynoic acid into poly(ethylene
oxide) (PEO) as a polymer matrix (Figure 2b). The color change was
induced by high-frequency mechanical oscillations controlled by AC
voltages. The efficiency of the color transition increased by
decreasing the diameter of the wires, because smaller diameters
minimize the defect density and thus improve the mechanical
transduction. This process was characterized using Raman
spectroscopy and the resonant frequency shifts of the hybrid wire.
Such a combination of cost-effective, easily processable polymers
with PDA may be useful as a force sensor for detecting stresses and
damages at the microscale.
2.6 Swelling
Park et al.65 developed a PDA–polydimethylsiloxane (PDMS)
composite sensor, which underwent a blue-to-red colorimetric
transition during swelling upon exposure to different solvents (Figure
3a). The composite sensor was easy to fabricate by the

Figure 4: a, A hand-writable PDA sensor by using mixing–molding polymerization process. Drawings based on these pens responded to heating, cooling, and rubbing. b, (i) PDA-C6NH2-coated paper was tested against writing pressure. (ii) The intensity of the red color (Δx) vs. the applied friction force. c, (i) Sparse modeling toward the prediction of colortransition temperatures based on the sample image analysis. Panel (a) has been adapted from ref 69 with permission from Wiley Online Library, copyright 2016. Panel (b) has been
adapted from ref 70 with permission from Wiley Online Library, copyright 2018. Panel (c) has been adapted from ref 71 with permission from Royal Society of Chemistry, copyright
2020.

mixing−irradiation−curing method. The rate of swelling and the color
change depended on the alkyl chain length of the saturated aliphatic
hydrocarbons. The mechanism of this color transition is complicated.
The authors explained that the swelling of PDMS induced mechanical
strain on the embedded PDA, which exposed the unreacted
monomers present in the PDA crystals to the solvent and dissolved
them, creating voids in the PDA structures. This combined effect of
mechanical strain and void creation caused a decrease in interchain
interactions in the PDA and served as a driving force for the PDA
phase transition. The same group prepared a PDMS microbead–PDA
composite sensor.66 This sensor underwent a blue-to-red color
change in response to the hydrocarbons of the shorter alkyl chains
and enabled visual differentiation between n-pentane, n-heptane, nnonane, and n-undecane (Figure 3b). The degree of swelling of the
PDA–PDMS composite beads in these hydrocarbons was inversely
correlated with the length of the alkyl chains.
Seo et al.67 prepared a sensory system in which one-dimensional
(1D) PDA nanofibers were integrated into the three-dimensional (3D)
matrix of a hygroscopic alginate hydrogel to detect water (Figure 3c).
Alginate has a dramatic volume swelling property when it absorbs
water. This volumetric expansion induced mechanical stress on the

PDA nanofibers, which deformed the PDA backbone, leading to the
color transition.
2.7 Macroscopic scratch and rubbing
Chae et al.69 prepared a directly writable crayon-like PDA–wax
composite sensor by embedding different PDAs into paraffin wax
(melting temperature 58–62 °C) (Figure 4a). The wax mixed with 2%
PCDA showed irreversible thermochromism, whereas that with
PCDA-mBzA presented a reversible color change. A mechanically
drawn PDA image showed a colorimetric transition from blue to red
upon heating or soft rubbing. Optical microscopy showed that
diacetylene and the wax formed a complex at the single-crystal level,
where the wax molecules intercalated between the diacetylene
crystals. Upon heating, the PDA crystals underwent significant
shrinkage because of the release of unreacted diacetylene
monomers and embedded wax molecules from the crystal. The
release of the wax molecules caused distortion of the arrayed porbitals and thus the blue-to-red color transition. The same group
developed a reversible mechanochromic PDA by self-assembly of
diphenyldisulfide-containing bisdiacetylene (PCDA-4APDS).72 The
temperature of the powder remained below the thermochromic

Figure 5: a, Schematic diagram of a molecular design of TzDA. b, AFM morphology images of the (A) monomer, (B) polymer, and (C) polymer after mechanical stimulation. (D)
Fluorescence image of poly-TzDA after the application of various forces. c, Plot showing the recovered fluorescence signal vs. applied vertical force. This figure has been adapted
from ref 48 with permission from Royal Society of Chemistry, copyright 2019.

transition temperature (80 °C) of the PDA during grinding, which
proves that the color change was cause by the mechanical stress. The
mechanical energy produced from grinding transferred to the alkyl
chain of the PDA, which caused partial distortion of the conjugated
ene–yne backbone, shortened the effective conjugation length of the
PDA, and thus induced the color transition.
Ishijima et al.73 prepared an organic-layered material using PDA
with the intercalation of guest organic amines, that is, alkyl amines
and alkyl diamines. The amine-intercalated PDA showed a tunable
temperature (46–106 °C) and a mechanoresponsive color change.
Rubbing gradually changed the PDA color, which indicated that force
was the reason for the color change rather than heat during rubbing.
The same group fabricated a paper-based friction detector based on
the amine-intercalated PDA (Figure 4b).70 The composite was formed
through self-organization followed by polymerization, which was
then homogeneously coated on a sheet of paper. The application of
friction force by a cage showed a force-dependent color change with
a detectable range of 7.6–23.0 N. The properties were tuned by using
various types of guest ions and amines without complex synthesis.
The measurement of writing pressure was demonstrated using a
paper device. Weak, moderate, and strong writing forces in the range
of 8.80–31.1 N were visualized and quantitatively detected by the
friction force. The group further developed a layered PDA/PEI
composite-coated paper device to detect weak force (Figure 4c).71 In
this work, tooth-brushing force in the range of 0.91–6.60 N was used
as a model to create weak forces. Soft, normal, and hard brushing
forces were applied to the paper-based PDA device, which showed
the blue-to-red color change. Sparse modeling was used to analyze
the PDA color images for selecting the best interlayer guest.

2.8 Atomic force microscopy
PDA studies by AFM were pioneered by Burns et al.74 They observed
an irreversible blue-to-red transformation by applying shear forces
with an AFM tip. Out-of-plane rotations of the side chains caused by
the tip–PDA interactions disrupted the π-orbital overlap, causing the
mechanochromic transition. Although this work marked an
important step toward PDA mechanochromism research at the
nanoscale, the shear force used was shown only qualitatively as
standard AFM can only quantify forces vertical to the substrate. The
same group also prepared ultrathin PDA films using the horizontal
Langmuir deposition technique from two diacetylene monomers,
PCDA(I) and N-(2-ethanol)-10,12-pentacosadiynamide (PCEA) (II).60
AFM or near-field scanning optical microscopy (NSOM) tips were
used to apply qualitative shear forces. Their data revealed that the
friction depended on the angle between the polymer backbone and
the scanning direction; the maximum occurred when the scanning
direction was perpendicular to the backbones. PCEA in particular
exhibited threefold friction anisotropy depending on the scanning
direction relative to the polymer backbone. These works revealed
the anisotropic friction of PDA for the first time based on scanning
microscopy.
Polachhi et al.48 prepared a PDA derivative with an amplified
fluorescence response by covalently linking a tetrazine fluorophore
to diacetylene (poly-TzDA, Figure 5). The fluorescence emission
wavelength from tetrazine matched with the absorption of blue PDA,
causing an energy transfer only when the system was in the blue
phase. Therefore, in the monomer state, the sample presented
fluorescence from tetrazine, which was quenched during the
polymerization owing to the increased amount of blue PDA.

Figure 6: Characterization of 5,7-docosadiynoic acid (DCDA) Langmuir−Blodgett films by lateral force microscopy coupled with fluorescence microscopy. a, Scheme of the experiment,
a bright-field microscopy image, an AFM height image, Δfluorescence before and after scratching, lateral force map, and vertical force map. The scale bars show 10 μm. b, Chemical
structure of the used monomers with different color transition temperature (DCDA: 50 °C, TRCDA: 55 °C, PCDA: 65 °C). c, Fluorescence increase vs. lateral force. This figure has been
adapted from ref 47 with permission from American Chemical Society, copyright 2021.

During the blue-to-red transition, the energy transfer was again
weakened and the tetrazine fluorescence was efficiently restored.
Poly-TzDA was further characterized using AFM coupled with
fluorescence microscopy. A vertical force in the range of 20–500 nN
was applied by the tip with a scanning speed of 6.1 m s-1.
Fluorescence images showed that poly-TzDA restored its
fluorescence emission locally from the scratched part of the film.
Such a fluorescence resonance energy transfer (FRET)-based
enhancement of PDA fluorescence may be used to improve its
sensitivity toward sensing applications.
Although the abovementioned works showed that PDA is
mechanochromic at the nanoscale and that shear force is the key to
inducing the blue-to-red transition, they failed to quantify the shear
forces required to activate PDAs. This is because standard AFM can
measure and manipulate only the forces vertical to the substrate. In
2021, we overcame this bottleneck by utilizing quantitative friction
force microscopy, which measures lateral forces (Figure 6).47 Friction
force microscopy is an AFM-based technique that enables the
quantification of forces lateral to the substrate by calibrating lateral

laser deflection into forces.75, 76 The use of this experimental
technique was partially enabled by our recent identification of an
error source in the wedge calibration method over the nanonewton
range.77 Quantitative friction force microscopy combined with
fluorescence microscopy confirmed that PDA reacts only to lateral
forces, F//, as the lateral force presents perfect correlation with
fluorescence, whereas vertical force does not (Figure 6a). The setup
also disproved the previously claimed hypothesis that the edges of
the polymer crystals exhibit higher force sensitivity than the rest of
the crystal. This was accomplished by correlating fluorescence and
lateral forces at the edge and the bulk separately. In addition, we
reported a link between mechanochromism and thermochromism,
which can be attributed to the fact that both work and heat are
different means of providing the same transition energy (Figure 6b,
c).47 These data provide the first insight into quantitative, anisotropic
PDA mechanochromism at the nanoscale, where crystal-toamorphous transition of PDA structure seems to play an important
role. The friction force microscopy combined with fluorescence
microscopy can be also used to characterize other mechanosensitive
polymers78-80 and mechanophores81 in future.

Figure 7: Categories of ligand-PDA interactions in biosensing.

3. Conclusions and future perspective
This minireview provides an overview of the recent progress in PDA
mechanochromism, where various forces can be used as stimuli to
induce the color change. The quantitative and anisotropic force–
fluorescence correlation at the nanoscale obtained in recent years
has just begun to provide a new perspective on its mechanism. In
biosensing, bound ligands are expected to exert forces on PDAs, as
in the case of an AFM cantilever, yet how each molecule does that is
unclear in many cases. Ligands are often surface-bound,82 deepbound (where the receptor is embedded within PDAs),83 inserted,84
or aggregated85 to initiate the color transition (Figure 7; also see the
categorization shown in Table S1). Nevertheless, when binding to a
receptor exposed on a PDA headgroup (surface-bound), there is no
obvious reason why the ligand should apply force to the PDA except
that some receptors undergo a conformational change after binding
that induces stress on the PDA; alternatively, the ligands may
eventually inserted into the PDA matrix, where the role of the
binding was to elevate the local ligand concentration right above the
PDA surfaces. Some binding events may not be interpreted as
applying forces. For example, we previously reported that
antimicrobial peptides are “partially melting PDAs” (inducing the
solid-to-liquid phase transition of side chains) by increasing the
disorder in the system, which turns their blue color to red.61 In such
a case, although the peptide–PDA mechanical interaction must
occur, it is difficult to attribute a certain amount of force to the
change in the color. As a future perspective, learning from the
quantitative information obtained in mechanochromism studies and
applying them in biosensing research to understand the detailed
mechanism of how each ligand induces the color change will be

useful for improving the sensitivity and selectivity, and thus for the
further development of applications.

Author Contributions
K. S. has initiated the entire work and wrote the article together with
B. D. The rest of S. J., J. Z., and J. C. all contributed to the literature
search, the investigation of the current trends and making the Table
S1 in SI.

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
Part of the research leading to these results has received funding
from Shiseido Female Researcher Science Grant,UTEC-UTokyo FSI
Research Grant Program, the FY 2020 University of Tokyo Excellent
Young Researcher, Japan Society for the Promotion of Science
(JP20K22324), Takeda Science Foundation, the Mitsubishi Foundation,
Inoue Foundation for Science, the Naito Foundation and the Kanamori
Foundation.

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