Bimodal detection of carbon dioxide using fluorescent molecular aggregates

Article abstract of DOI:10.1039/c9cc01564g

Cyano-substituted p-phenylenevinylene (R-1) aggregates exhibiting fluorescence and Raman spectroscopic responses towards CO₂ are described. The aggregation-induced emission (AIE) as well as the aggregation-enhanced Raman scattering (AERS) of R-

Full text of DOI:10.1039/c9cc01564g

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Chem. Commun., 2019, DOI: 10.1039/C9CC01564G.
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Bimodal detection of carbon dioxide using a fluorescent molecular
aggregates
Received 00th January 20xx,
Accepted 00th January 20xx
Rakesh K. Mishra,*ab Samiyappan Vijayakumar,a Arindam Mal,ac Varsha Karunakaran,cd Jith C.
Janardhanan,a Kaustabh Kumar Maiti,cd Vakayil K. Praveen*ac and Ayyappanpillai Ajayaghosh*ac
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cyano-substituted
p-phenylenevinylene
(R-1)
aggregates moieties (Scheme 1). The receptor R-1 has been synthesized as
exhibiting fluorescence and Raman spectroscopic response per Scheme S1†. All intermediates and final compounds
1
towards CO2 is described. The aggregation-induced emission (AIE) obtained have been characterized using FT-IR, H, 13C NMR
as well as aggregation enhanced Raman scattering (AERS) of R-1 in techniques and mass spectrometry (Fig. S1-S8†).
aqueous condition get reduced in presence of small amount of CO2
that enabled its easy and fast bimodal detection in different
analytical samples.
Capture, storage, release and conversion of CO2 are issues of
high priority research.1,2 In this context, easy and fast
detection of CO2 is an important objective. Hence researchers
are looking for simple and effective sensors/receptors for
CO2.3-5 For this purpose, optical detection based on self-
assembled fluorescent molecules are considered to be viable
Scheme 1 Receptor R-1 and its interaction with CO2.
owing to their high sensitivity, fast response time, low cost,
visual signal transduction, real-time in situ responses, and
compatibility for in vivo analysis.3 A possible approach to such
The receptor R-1, is highly soluble in THF and DMSO and
forms aggregates (R-1Agg) in aqueous medium as indicated by
the observed Tyndall effect (Fig. 1a, inset). The UV/Vis
spectrum of the R-1Agg (1 × 10-4 M) in aqueous medium
showed a narrow blue shifted band (λmax = 348 nm) in
comparison to the broad spectrum in DMSO (λmax = 375 nm),
where it mainly existed as molecularly dissolved species (Fig.
1a). The aggregation of R-1 was also marked by the origin of an
intense yellow emission (λem = 550 nm), owing to AIE
properties of the chromophoric unit (Fig. 1b, inset).10 On the
contrary, the R-1 monomer in DMSO displayed a blue shifted
weak emission at 465 nm, τ = 5.84 ps (Fig. S9a, Table S1†). The
red-shift and structureless features of emission in aqueous
condition indicates the formation of an excimer like species
upon aggregation as evident from the longer average lifetime
value (τ = 5.67 ns).11 Systematic aggregation studies have been
carried out in DMSO-Water mixture using UV/Vis and
fluorescence spectroscopy revealing that R-1 starts to
aggregate in 70:30 water-DMSO mixture. The maximum
fluorescence intensity is observed in 99:1 water-DMSO,
mixture (Fig. S9† and Fig. 1b, inset). The scanning electron
microscopy (SEM) images of a drop cast sample of the R-1Agg (1
× 10-4 M, water/DMSO, 99/1 v/v) over silicon wafer showed a
particle-like morphology (Fig. 1b).
sensors depends upon analyte triggered self-assembly or
disassembly of a given molecular probe producing turn-on/off
fluorescence response.6 Moreover, the combination of more
than one detection modality is preferred in order to increase
the selectivity and sensitivity of the probe.3,6 For instance,
fluorescence probes that are Raman active, can be used as
bimodal sensors.7 This is particularly relevant by considering
the fact that many of the reported CO2 sensors rely on single
detection modality and often utilize more than one receptor
components.5
Herein, we report the fluorescent as well as Raman
assisted sensing of CO2 using
a
cyano-substituted8 p-
phenylenevinylene9 molecule bearing terminal tertiary amine
aPhotosciences and Photonics Section, Chemical Sciences and Technology Division,
CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST),
Thiruvananthapuram-695019,
aajayaghosh@niist.res.in
India.
Email:
vkpraveen@niist.res.in;
bDepartment of Sciences and Humanities, National Institute of Technology,
Uttarakhand
(NITUK),
Srinagar
(Garhwal)-246174,
India.
Email:
rkmishra@nit.uk.ac.in
cAcademy of Scientific and Innovative Research (AcSIR), Ghaziabad- 201002, India
dOrganic Chemistry Section, Chemical Sciences and Technology Division, CSIR-NIIST,
Thiruvananthapuram-695019, India.
†Electronic Supplementary Information (ESI) available: Experimental details,
synthesis procedures and characterization data of the compounds, and additional
figures. See DOI: 10.1039/x0xx00000x
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instrument response function. (d) Fluorescence response (λex = 370 nm) of the R-1Agg
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towards CO2 after purging with different gases (3.0 mL) in aqueous medium; from left
DOI: 10.1039/C9CC01564G
to right: R-1Agg, CO2, O2, H2, Ar, N2 and Air.
This observation is also supported by dynamic light
scattering (DLS) studies. The average particle diameter of R-
1Agg (1 × 10-4 M) as measured by DLS was found to be 156±10
nm. Upon purging a small amount of CO2, the size of the
aggregates was gradually decreased as inferred from the
reduction in the average particle diameter to 102±10 nm
which was further reduced to 83±5 nm in excess amount (0.33
mL) of CO2 (Fig. S11†). Furthermore, a reduction in the Tyndall
effect of R-1Agg in the presence of CO2 (0.33 mL) was also
noticed (Fig. S11c†). Results of the fluorescence titration
studies, size distribution, Tyndall effect as well as CO2
chemistry with amine moieties in aqueous medium suggest
that R-1Agg gets protonated in presence of CO2 gas, leading to a
change in molecular character from hydrophobic to
hydrophilic (Scheme 1). Thus, the large R-1Agg get converted
into a mixture of monomers and smaller aggregates of the
positively charged R-1.12 These processes are indicated by the
simultaneous red-shift of the absorption maximum and the
reduction in AIE property of fluorophore as corroborated by
the pH dependent fluorescence titration studies (Table S2 and
Fig. S12†) yielding a pKa value for R-1 as 6.98 (±0.007).
To further evaluate the selectivity of R-1Agg towards CO2,
we carried out fluorescence titration experiment with different
gases in excess amount (3.0 mL), including N2, O2, Ar, H2, and
air under identical conditions (Fig. 2d). It was clearly observed
that CO2 produced the most significant effect on the
fluorescence properties of R-1Agg. The experimental results
also indicated that the detection of CO2 using R-1Agg is less
interfered with other common gases present in air. In addition,
CO2 binding to R-1Agg is found to be reversible (Scheme 1). The
less emissive R-1Agg-CO2 adduct is readily converted back into
highly fluorescent R-1Agg by purging N2/Ar or by adding small
amount of K2CO3 at room temperature. As shown in Fig. S13†,
upon repeated purging of CO2 and addition of 0.1 mM K2CO3
solution, the emission of R-1Agg showed multiple cycles of
reversible on-off changes.
Fig. 1 (a) Absorption spectra of R-1 (1 × 10-4 M) in DMSO and aqueous medium
(water/DMSO, 99/1 v/v). Inset shows the photographs of Tyndall effect of the
corresponding solutions indicating aggregation behaviour of R-1 in aqueous medium (i).
(b) SEM image of R-1agg (1 × 10-4 M, water/DMSO, 99/1 v/v) drop cast over silicon
wafer. Inset shows change in fluorescence of R-1 in DMSO (A) and in 99:1 water/DMSO
mixture (B). Photographs have taken under 365 nm UV light illumination.
Having these results in hand, we tested the CO2 sensing ability
of R-1Agg (1 × 10-4 M, water/DMSO, 99/1 v/v) upon purging
CO2 into R-1Agg while monitoring the corresponding changes in
absorption and emission properties. It was observed that in
the presence of CO2, the sharp absorption band at 348 nm of
R-1Agg broadened and red shifted to 364 nm (Fig. 2a).
Interestingly, a gradual quenching in the emission of R-1Agg
was observed up on purging of CO2 (Fig. 2b and Fig. S10†).
Fluorescence titration experiments revealed that the yellow
emission of the aggregates got reduced by 25-fold with 0.33
mL of CO2. Fig. 2c shows the fluorescence decay profiles of R-
1Agg (probed at 550 nm) upon purging of CO2. R-1Agg exhibited
triexponential decay with an average lifetime of 5.67 ns,
whereas in the presence of CO2 a faster decay with an average
lifetime of 0.33 ns was observed (Table S1†). The changes in
absorption, steady-state and time-resolved emission
characteristics indicate the transformation of R-1Agg into a
new-type of species having monomer-like absorption and
emission features.
Fluorescence microscopy imaging was carried out to
explore the capability of R-1Agg as a probe for monitoring the
variation of external CO2 concentration inside the living cells.
For this purpose, the lung adenocarcinoma cells A549 were
incubated with 100 µM of R-1Agg for 1 h under normal
atmospheric condition (0.038% CO2) at 25 °C as we observed
the mentioned concentration is giving the optimum
fluorescence intensity with minimal cytotoxicity around 3 h
(Fig. S14†). Also Initial localization of the R-1Agg was observed
inside the nucleus in 1 h (Fig. S15†) but eventually, it spreads
throughout the cells. As shown in Fig. 3, a yellow fluorescence
signal could be observed, confirming the internalization of the
aggregates inside the intercellular milieu. Thereafter, the cells
were kept in the incubation chamber with 5% CO2 for 2 h and
the fluorescence was monitored against time. The emission
intensity of the yellow fluorescence was reduced and almost
no fluorescence was observed, while the sample cells kept
Fig. 2 (a) Absorption spectra of R-1Agg (1 × 10-4 M) in aqueous medium (water/DMSO,
99/1 v/v) after purging with excess amount of CO2 (0.33 mL). (b) Emission spectra of R-
1Agg (1 × 10-4 M, λex = 370 nm) in the presence of different concentrations of CO2 (0,
0.016, 0.032, 0.048, 0.064, 0.080, 0.096, and 0.33 mL). Inset shows the fluorescence
change of R-1Agg under UV illumination (λex = 365 nm) before and after purging with
CO2. (c) Fluorescence lifetime decay profile of R-1Agg (λex = 370 nm, λem = 550 nm) in the
presence of increasing CO2 concentrations (0, 0.016, 0.048 and 0.33 mL). IRF =
2 | Chem. Commun., 2019, 00, 1-3
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under ambient condition still showed the corresponding enhanced sensitivity compared to normal Raman scattering.
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DOI: 10.1039/C9CC01564G
emission (Fig. S16†). These observations suggest that the Raman spectrum of R-1Agg was recorded using spherical
increase of external CO2 concentration causes the reduction in nanogold (Au-NPs) as SERS substrate. The R-1Agg was mixed
the fluorescence emission intensity and thus revealing the with as prepared Au-NPs (size ca. 40 nm) in 1:9 ratio (v/v) and
capability of R-1Agg to detect CO2 variations in cellular the mixture was incubated for 10 min in order to get maximum
conditions. Even though R-1 is showing slight toxicity upon adsorption. The SERS spectra were recorded as described
higher incubation time, the probe is efficiently sensing the CO2 above, and ca. 25-fold enhancement in the intensity was
inside the cellular environment.
observed when compared with normal Raman intensity
obtained for R-1Agg (Fig. 4). In comparison to the spectrum
obtained from AERS, the SERS assisted spectrum showed well
resolved peaks along with a unique signal at 2213 cm-1
resulting from the –CN group, which can be monitored easily
for the CO2 sensing experiment.
In conclusion, the reported a cyano substituted p-
phenylenevinylene derivative R-1 that undergoes aggregation
in aqueous medium exhibiting AIE as well as AERS phenomena,
which have been used for the detection of CO2 gas among
different specimens of neutral gases. The AIE-based CO2-assay
described here is useful for detecting CO2 in biological
specimen. The novelty of the present system is that a
combination of fluorescence and AERS or SERS can be used for
sensing of CO2. However, for ratiometric sensing and in vivo
application further improvement over the present design is
required and such studies are in progress.
Fig. 3 (a) Fluorescence and (b) bright field images of R-1Agg in A549 lung cancer
cells incubated with 100 µM R-1Agg. (c) Fluorescence image of A549 cells further
incubated for 2 h at 25 °C and 5% CO2 and (d) corresponding bright field image.
Keywords: Self-assembly · Carbon dioxide · Bimodal probe ·
Aggregation induced emission · AERS/SERS
Aggregation-enhanced Raman scattering (AERS) of R-1Agg
was exploited as an additional modality for chemosensing
purposes.7,13 First, the Raman spectrum of the R-1Agg (4 × 10-4
M, water/THF, 99/1 v/v) was recorded with a Raman
microscope using 633 nm laser excitation.13 The spectra were
collected using the CCD detector with an integration time of
0.5 sec and 10 accumulations. A tiny droplet of the R-1Agg was
placed on a glass slide to obtain the corresponding Raman
fingerprint, which was observed with high signal to noise ratio.
The Raman spectrum of R-1Agg showed two peaks at 1584 cm-1
(strong, C-C aromatic ring chain vibrations) and 1180-1208 cm-
Acknowledgements
R.K.M. is thankful to DST, Govt. of India, for an INSPIRE Faculty
Award (IFA-12 CH-50). V.K.P. and K.K.M. acknowledge CSIR
mission mode project, nano-biosensor and microfluidics for
healthcare (HCP-0012) and fast-track translation project,
cellular sensor (MLP0027), Government of India for financial
support. A.A. is grateful to the DST-SERB, Govt. of India, for a J.
C. Bose National Fellowship (SB/S2/JCB-11/2014).
1
(weak, C-O-C asym). On the other hand, the droplets
examined after purging the aggregate solution with the CO2
showed a reduction in peak intensity to almost baseline and
thus by monitoring the Raman active peaks of R-1Agg it was
possible to detect CO2 gas (Fig. 4a).
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Fig. 4 (a) Raman spectra of R-1Agg (4 x 10-4 M) in aqueous medium (water/THF,
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Table of Content
Fluorescent aggregates of a cyano-substituted phenylenevinylene derivative (R-1) have been used as a bimodal probe for an easy and fast
detection of CO2.