Colorimetric polarity chemosensor based on a organometal halide perovskite functional dye

Article abstract of DOI:10.1016/j.dyepig.2016.05.049

We have investigated polarity sensing of organometal halide CH₃NH₃PbI₃ perovskite functional dye based on hydrogen bonds between alcohols and polar units in perovskite. Upon exposure of alcohol solvents, perovskite film showed a drastic visible color change with different changing time depending on the polarities of alcohols. From the kinetic study of the reaction between perovskite dye and alcohols having different polarities, efficiency of hydrogen-bonding interactions between them were clearly revealed to be the main sensing mechanism, which enables effective discrimination of polarities by naked eyes. This study suggests a novel and practical sensing platform for highly sensitive colorimetric polarity chemosensors.

Full text of DOI:10.1016/j.dyepig.2016.05.049

Dyes and Pigments 133 (2016) 73e78
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Colorimetric polarity chemosensor based on a organometal halide
perovskite functional dye
,
, *
Jong H. Kim a b, Sung-Hoon Kim c
a Department of Chemical Engineering Education, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon, 305-764, South Korea
b Graduate School of Energy Science and Technology, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon, 305-764, South Korea
c Department of Textile System Engineering, Kyungpook National University, Daegu, 702-701, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We have investigated polarity sensing of organometal halide CH3NH3PbI3 perovskite functional dye
based on hydrogen bonds between alcohols and polar units in perovskite. Upon exposure of alcohol
solvents, perovskite film showed a drastic visible color change with different changing time depending
on the polarities of alcohols. From the kinetic study of the reaction between perovskite dye and alcohols
having different polarities, efficiency of hydrogen-bonding interactions between them were clearly
revealed to be the main sensing mechanism, which enables effective discrimination of polarities by
naked eyes. This study suggests a novel and practical sensing platform for highly sensitive colorimetric
polarity chemosensors.
Received 11 April 2016
Received in revised form
24 May 2016
Accepted 25 May 2016
Available online 26 May 2016
Keywords:
Functional dye
Perovskite
© 2016 Elsevier Ltd. All rights reserved.
Solvent polarity
Chemosensor
Hydrogen bonding
1. Introduction
rather than reflective color shifts, and there still remain technical
challenges to overcome, such as limitations in marked color/
Chemosensors are sensory receptor molecules that are capable
of selective detection of interested analytes or chemical properties
of interest with a measureable optoelectronic changes. Due to their
simplicity, high sensitivity, and real-time detection for various
analytes, chemosensors have attracted considerable attention
during the past decade [1]. Among them, development of chemo-
sensor systems that have precise and rapid recognizability of po-
larity of chemical compounds is of particular importance in the
chemistry field because most chemical reactions and processes are
significantly influenced by polarity of reactant molecules. In
particular, choice of organic solvents with appropriate polarity
values plays a critical role in organic synthesis, recrystallization,
and chromatographic separation of compounds.
Recently, a large number of chemosensors detecting environ-
mentally harmful volatile organic compounds (VOCs), biologically
important anions, cations, acidity, and explosive or hazardous
chemicals have been extensively developed [2]. Polarity-sensitive
probes also have been reported, however, most of their detection
mechanisms have been focused on fluorescence emission changes
emission wavelengths shifts enough to detect with naked eyes [3].
Even worse, chemosensors or sensing techniques for discriminating
molecules having minute polarity differences by displaying distinct
outputs color changes are rarely explored. One of the reasons for
this research scarcity in the polarity-analyzing system lies in the
lack of suitable probing materials satisfying two essential issues for
the investigation; i) spontaneous interaction with polar analytes at
a molecular level and ii) quick and drastic visible color conversion
after detection.
Organometal halide-type hybrid perovskites with a structure of
ABX3 (A ¼ organic cation, B ¼ metal cation, and X ¼ halogen anion)
have emerged as a new functional dye due to their unprecedented
optoelectronic properties such as intense broad-band UV/Vis light
absorption (300e800 nm) and 10 times greater absorption coeffi-
cient (ε ¼ 1.3 ꢀ 105 cmꢁ1 at 550 nm) compared to that of the
conventional light-absorbing ruthenium-based molecular dye [4].
Moreover, as shown in Fig. 1a, ionic interactions between the
organic and inorganic portions in the structure of this hybrid
perovskite dye, for example CH3NH3PbI3, facilitate its sensitive in-
teractions with polar molecules. This unique character allows
spontaneous crystal structure conversion into CH3NH3I and PbI2 in
response to polar environments concomitant with a fast and
* Corresponding author.
E-mail address: shokim@knu.ac.kr (S.-H. Kim).
http://dx.doi.org/10.1016/j.dyepig.2016.05.049
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

74
J.H. Kim, S.-H. Kim / Dyes and Pigments 133 (2016) 73e78
Fig. 1. (a) General (ABX3) crystal structure of organometal halide perovskite (CH3NH3PbI3). (b) Molecular electrostatic potential surface diagrams of MeOH, EtOH, n-PrOH, and n-
BuOH.
discernible color conversion, according to the following chemical
equation [5].
solvent to be dried and evaporated. Finally, the substrates were
annealed at 100 ꢂC for 15 min to complete perovskite film fabri-
cation. ~400 nm-thick perovskite layers were obtained by blade-
coating the precursor solution with a coating speed of 10 mm/s
CH3NH3PbI3 / CH3NH3I þ PbI2
(1)
with the gap between blade and substrate of 10 mm at room tem-
Taking advantages of these satisfactory optical and chemical
properties which are compatible with the investigation on the
molecule’s polarity, herein, we introduced CH3NH3PbI3 perovskite
dye as a polarity sensing probe in attempt to develop an effective
colorimetric polarity chemodosimeter. We demonstrate a simple
but very effective method to distinguish polarities of alcohol sol-
vents with minute difference in polarity values. The differences in
optical responses and kinetics of hydrogen bonding interactions
between perovskite and alcohols with different polarity provide
insights into the photophysical polarity-sensing mechanism of
perovskite.
perature (Blade-coating was performed using a knife-coating de-
vice (KP-3000H, KIPAE). UVevis absorption spectra were obtained
using an Agilent 8457 UVevis spectrophotometer. X-ray diffraction
(XRD) data were recorded using a Rigaku D/max-2500 diffrac-
tometer. Electrostatic potential surface of alcohols were calculated
using Avogadro and Jmol softwares.
3. Results and discussion
The crystal structure of perovskite dye (CH3NH3PbI3) used in
this study can be described by the general formula ABX3, as shown
in Fig. 1a. These organic-inorganic hybrid crystals are fixed together
by the interaction between organic cation (CH3NHþ3 ) and inorganic
anion (PbIꢁ3 ) component through hydrogen bonds between the
hydrogens on the NHþ3 and the iodide atoms (NeH/Iꢁ) and the
hydrogens on the CH3 and iodide atoms (CeH/Iꢁ) [4].
2. Experimental section
2.1. Materials
Chemical reagents used in this study were used without any
further purification. Methylammonium iodide (CH3NH3I) and lead
(II) iodide (PbI2) were purchased from Tokyo Chemical Industry
(TCI). Dimethyl sulfoxide (DMSO), methanol (MeOH), ethanol
(EtOH), n-propanol (n-PrOH), and n-butanol (n-BuOH) were pur-
chased from Sigma-Aldrich.
We prepared CH3NH3PbI3 films using mixed precursor
(CH3NH3I/PbI2 in DMSO) solution using a blade-coating method,
which is convenient and effective technique for large-area printing.
The wet films deposited by a blade-coater were dried under room-
temperature before annealing for crystallization at 100 ꢂC. Forma-
tion of CH3NH3PbI3 from wet film to the crystallized structure was
clearly confirmed from the gradual evolution of absorption spectra
and film color changes as shown in Fig. 2.
2.2. Film preparation and characterization
Firstly, to demonstrate the potential applications of CH3NH3PbI3
perovskite films to colorimetric probes for the polarity detection of
organic molecules, absorption spectral studies of CH3NH3PbI3 film
in the presence of alcohol solvents with different polarities were
performed. For this study, the time course of the reaction between
CH3NH3PbI3 film and EtOH was investigated by monitoring the
absorption intensities of the CH3NH3PbI3 at lmax (420 nm). As
shown in Fig. 3, it was observed that strong intensity of absorption
band at 420 nm quickly decreased to 60.2% of its initial one in less
than 5 min, accompanied by a significant color changes from dark
brown to yellow, with exposure to the EtOH solvent.
CH3NH3PbI3 perovskite precursor solution was prepared using
CH3NH3I and PbI2 following the reported method [6], and the
perovskite films in this study were prepared on the glass substrates.
The substrates were rinsed by sonication in detergent and deion-
ized water, acetone and isopropyl alcohol before film coating. Then,
uniform and large-area (10 cm2) perovskite films were prepared in
ambient condition by using a blade-coater and homogeneous
CH3NH3PbI3 precursor solution in DMSO. The wet films formed
immediately after solution blading were kept at room temperature
for 40 min before annealing to wait for the majority of DMSO

J.H. Kim, S.-H. Kim / Dyes and Pigments 133 (2016) 73e78
75
Fig. 4. XRD patterns of (a) a pristine CH3NH3PbI3 film and (b) a CH3NH3PbI3 film after
5 min exposure to EtOH.
Fig. 2. (a) Schematic diagram of blade-coating process. (b) Evolution of UV/Vis ab-
sorption spectra from bladed wet film to annealed crystallized film.
peak of PbI2 [8]. These results reveal that the color or absorption
spectral changes originated from the structural conversion of
CH3NH3PbI3 into PbI2, which responds very sensitively to polar
EtOH exposure.
It has been reported that CH3NH3PbI3 makes complexes with
polar species such as H2O by forming both monohydrate
CH3NH3PbI3$H2O and dihydrate (CH3NH3)4PbI6$2H2O following
the reaction below.
4CH3NH3PbI3 þ 4H2O 4 4
[CH3NH3PbI3$H2O] 4 (CH3NH3)4PbI6$2H2O þ 3PbI2 þ 2H2O (2)
The main driving force of this hydration (moisture intercalation
into the perovskite lattice) is the formation of hydrogen bonding
interaction between the hydrogen atoms of the H2O and the iodide
atoms (OeH/Iꢁ), and that between the oxygen atoms of the H2O
and the hydrogen atoms of NHþ3 (NeH/O). Based on the same
principle, polar alcohols containing OH group will form similar
types of hydrogen bonding interactions with CH3NH3PbI3 perov-
skite crystal lattices (Fig. 5 shows the proposed mechanism of
perovskite crystal structure conversion via hydrogen bonding in-
teractions between EtOH and ionic components of the CH3NH3PbI3
(OeH/Iꢁ, NeH/O)) [9].
Furthermore, according to the theoretical calculation, the sta-
bilization energy of the NeH/O hydrogen bond is much stronger
than those of the NeH/Iꢁ and CeH/Iꢁ in the CH3NH3PbI3 [10]. This
evidence supports that new hydrogen bonds between CH3NHþ3 and
alcohols are much stronger than those between CH3NHþ3 and PbI3ꢁ,
which leads to the conversion of crystal structure and absorption
spectra changes, and consequently the drastic visible color changes
Fig. 3. UV/Vis absorption spectra change of CH3NH3PbI3 upon exposure to the EtOH
with time (0e270 s).
After then X-ray diffraction (XRD) measurements were per-
formed on the CH3NH3PbI3 film in order to investigate the origin of
these color changes. Fig. 4 compares XRD spectra of CH3NH3PbI3
perovskite films before and after EtOH exposure. Before exposure,
the film shows intense XRD peaks corresponding to crystalline
perovskite at 2
q
¼ 14.08 and 28.38ꢂ, assigned to (110) and (220)
planes of the CH3NH3PbI3 crystal, indicating an orthorhombic
crystal structure of organometal halide perovkskite [7]. However,
after 5 min exposure to EtOH solvent, the XRD peaks from perov-
skite crystal totally disappeared, and we observed a new strong
signal at 2
q
¼ 12.63ꢂ which can be assigned to the (001) diffraction

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J.H. Kim, S.-H. Kim / Dyes and Pigments 133 (2016) 73e78
Fig. 6. Photographs showing the different time ((a): 0, (b): 5 min, (c): 15 min, and (d):
30 min) for visible color shifts of the CH3NH3PbI3 films depending on the exposed
alcohols.
exhibited rapid color conversion in 2 min, while elapsed time for n-
Fig. 5. Hydrogen bonding interactions (a) between CH3NHþ3 and PbI3ꢁ in CH3NH3PbI3,
(b) between CH3NH3PbI3 and one EtOH molecule, and (c) between CH3NH3PbI3 and
two EtOH molecules.
BuOH, relatively less polar alcohol (ET ¼ 50.2 kcal molꢁ1
,
m
¼ 1.63 D),
was 30 min. Based on these observations we could infer that
minute difference in polarity values significantly influences
hydrogen bonding interaction between CH3NH3PbI3 and alcohols
(dielectric constant, polarity and dipole moment values of MeOH,
EtOH, n-PrOH, and n-BuOH are summarized in Table 1).
In order to further understand the recognition of CH3NH3PbI3
towards alcohol’s polarity, we examined the kinetic profiles of the
CH3NH3PbI3 perovskite’s reaction under first-order reaction with
four different alcohols (MeOH, EtOH, n-PrOH, and n-BuOH). The
first-order rate constant (k) can be calculated according to the
following equation:
of perovskite films after EtOH exposure.
Motivated by this excellent perovskite’s reactivity with polar
molecules accompanied by sensitive optical signal changes, we
have turned our attention to CH3NH3PbI3 for the application of
colorimetric polarity sensors for four different alcohols (MeOH,
EtOH, n-PrOH, and n-BuOH).
CH3NH3PbI3 perovskite films exhibit clear color change from
dark brown to yellow in response to the exposure of four alcohol
solvents. We recorded photographs at regular intervals (i.e., 0, 1, 10,
and 30 min) as shown in Fig. 6. Interestingly, each sample shows
noticeable difference in color conversion time depending on the
kinds of alcohols with different polarities (ET) or dipole moments
ln(At ꢁ A∞)/(Ai ꢁ A∞) ¼ kt
(3)
where Ai is the absorbance at 420 nm, and At is the absorbance at
420 nm at time t after exposure to the alcohol. A∞ and k refer to
(m
). Most polar alcohol, MeOH (ET ¼ 55.4 kcal molꢁ1
,
m
¼ 1.70 D)

J.H. Kim, S.-H. Kim / Dyes and Pigments 133 (2016) 73e78
77
Table 1
Summarized physical characteristics of alcohols (polarity, dielectric constant, dipole moment, rate constant).
Alcohol
Polarity (ET, kcal/mol)
Dielectric constant
Dipole moment (
m, D)
k (secꢁ1
)
MeOH
EtOH
n-PrOH
n-BuOH
55.4
51.9
50.7
50.2
33
30
20
18
1.70
1.69
1.68
1.63
1.13 ꢀ 10ꢁ1
9.78 ꢀ 10ꢁ3
1.14 ꢀ 10ꢁ3
3.76 ꢀ 10ꢁ4
Fig. 7. (a) First-order kinetic plots of CH3NH3PbI3 in the presence of MeOH, EtOH, n-PrOH, and n-BuOH and (b) plots showing relationship between polarity of alcohols and reaction
rate.
absorbance at 420 nm after full conversion and first-order rate
constant, respectively [11]. During the color changing reaction, the
kinetic analysis predicts the logarithm of the difference between
More importantly, enlarged neutral portion in the alcohol mole-
cules is observed with increased alkyl chain lengths due to the
increased portion of non-polar hydrocarbons. Decreased negative
potential on the oxygen atom, therefore, results in weaker
hydrogen bonds between polar units (CH3NH3þ and Iꢁ) in
CH3NH3PbI3 and OH in alcohols and subsequently delays inter-
molecular interaction rate and color changes. Moreover, both linear
steric hindrance and hydrophobic effects from longer alkyl chain
also hamper the hydrogen bonding interaction and lower the
interaction rate.
A
∞ and At at time t to be linear with time, where the slopes provide
the color change rate constant, k. First-order plots according to Eq
(3) are presented in Fig. 7a. The first reaction rate constants for
MeOH, EtOH, n-PrOH, and n-BuOH were determined as
k ¼ 1.13 ꢀ 10ꢁ1, 9.78 ꢀ 10ꢁ3, 1.14 ꢀ 10ꢁ3, and 3.76 ꢀ 10ꢁ4 secꢁ1
,
respectively. These results indicate that the hydrogen bonding
interaction occurs more readily in the following order:
MeOH [ EtOH > n-PrOH > n-BuOH, which is in a good accordance
with the order of their polarity values as plotted in Fig. 7b.
The reaction rate greatly decreases as alkyl chain length or
number of hydrocarbon attached to oxygen atom increases from 1
(MeOH) to 4 (n-BuOH). Considering that difference in electroneg-
4. Conclusions
In conclusion, we developed a simple but very sensitive che-
mosensor for distinguishing polarity of molecules using highly
chromogenic CH3NH3PbI3 perovskite functional dye. Polar alcohols
were clearly detected from color changes from dark brown to yel-
low which was visible with naked eyes, and moreover difference in
polarity of alcohols was also precisely observed from color change
time or reaction rate. Based on these results we found that the
hydrogen bonding interaction between perovskite dye and alcohols
is the driving force for the polarity detection and the strength of the
hydrogen bonds had a significant effect on the practical recognition
of the degrees of polarity.
ativity (c) between carbon (2.55) and oxygen (3.44) in a CeO bond
plays a crucial role in generating polarity of the alcohols, this
observation means that longer alkyl chain will weaken the negative
charge of the oxygen atom due to the increased portion of non-
polar hydrocarbons.
Molecular electrostatic potential (MEP) surface is a useful pro-
tocol to interpret and compare relative reactivity or reactive sites
for hydrogen bonding interactions, electrophilic/nucleophilic re-
actions, and molecular cluster and crystal behavior by mapping
electrostatic potential on the molecule surfaces [12]. Fig. 1b pre-
sents MEPs of MeOH, EtOH, n-PrOH, and n-BuOH, where red and
blue colors indicate electron-rich regions with partial negative
charge and electron-deficient regions with partial positive charge,
while yellow and green colors indicate slightly electron-rich and
neutral regions, respectively.
Acknowledgements
This research was supported by Basic Science Research Program
through the National Research Foundation (NRF) funded by the
Ministry
of
Science,
ICT
and
Future
Planning
MEPs in Fig. 1b shows that electron-deficient sites of alcohols
are located over oxygen atoms of hydroxyl groups and electron-rich
sites are present over hydrogen atoms attached to hydroxyl group.
(2015R1C1A1A01053241). This work was supported by the Na-
tional Research Foundation of Korea (NRF) grant funded by the
Korea government (MSIP) (No. 2014002985).

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J.H. Kim, S.-H. Kim / Dyes and Pigments 133 (2016) 73e78
properties. Dyes Pigments 2016;129:1e8.
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