A tunable detection range of ion-selective nano-optodes by controlling solvatochromic dye transducer lipophilicity

Article abstract of DOI:10.1039/c9cc06729a

A range of ionic solvatochromic dye (SD) transducers for use in ion-selective emulsified optical sensors are introduced and characterized. They share the same chromophore group, (E)-4-(4-(dimethylamino)styryl)pyridinium, but vary in their lipophilicities by grafted alkyl or ethoxy groups. The calibration curve is found to shift by a total of 2.7 orders of magnitude with the lipophilicity of the SD.

Full text of DOI:10.1039/c9cc06729a

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A tunable detection range of ion-selective
nano-optodes by controlling solvatochromic
dye transducer lipophilicity†
Cite this: Chem. Commun., 2019,
55, 12539
Received 30th August 2019,
Accepted 23rd September 2019
Lu Wang and Eric Bakker
*
DOI: 10.1039/c9cc06729a
rsc.li/chemcomm
A range of ionic solvatochromic dye (SD) transducers for use in ion- shifts the calibration curve by 1 order of magnitude to lower
selective emulsified optical sensors are introduced and characterized. concentrations, while for divalent ions the shift is 2 orders
They share the same chromophore group, (E)-4-(4-(dimethylamino)- of magnitude.20–22 Chromoionophore-based thermochromic
styryl)pyridinium, but vary in their lipophilicities by grafted alkyl or optical sensors may also be effectively tuned by the loading of
ethoxy groups. The calibration curve is found to shift by a total of surfactant.23
2.7 orders of magnitude with the lipophilicity of the SD.
Replacing the chromoionophore by an SD does bring some
drawbacks, as the lack of pH dependence now results in a
Ion-selective optodes1–9 that function on the basis of ion-exchange calibration curve that is not easily tunable.12 In theory, the
typically use a lipophilic pH indicator10 (also called a chromo- response range of such optical ion sensors depends on the ion-
ionophore) as an optical signal reporter. The target ions in the exchange equilibrium. It depends on the complex formation
sample exchange with the hydrogen ions in the organic sensing constant between analyte and ionophore as well as the partition
phase, thereby deprotonating the chromoionophore and giving coefficient of the SD between the organic and aqueous phase.17
a change in absorbance or fluorescence. As hydrogen ions Complex formation constants are related to the structure of the
actively participate in the ion-exchange process, the sensors ionophore and depend to some extent on the nature of the
give an undesired cross response to sample pH changes. This plasticizer,24 polymer and surfactant18,25 used in the sensing
was very difficult to overcome until the introduction of electrically formulation. On the other hand, the partition coefficient of the
charged solvatochromic dyes (SDs).11 The first proposed SDs12 were SD may be adjusted by controlling its hydrophobic character.
water-soluble and were therefore expelled into the bulk of the To study this, structurally different lipophilic SDs (SD-PEG4,
aqueous phase upon ion-exchange. This, however, contaminates SD-PEG2, SD-C1, SD-C4, SD-C18, see Scheme 1) were synthe-
the sample and gives signals that depend on the sample volume. sized here and incorporated into K+-selective emulsion sensors
To overcome these limitations, hydrophobic SDs have been to tune their response range.
introduced.13–16 This allows one to localize the ion-exchange
Compared to the parent molecule SD-C1 (Scheme 1),17
process, rendering the ion optodes pH-independent while SD-PEG4 and SD-PEG2 were obtained by structurally modifying
avoiding dye leakage. Based on this characteristic, a reversible the molecule with ethoxy groups, rendering them more hydro-
nylon-based optical sensor was reported.17 Recently, our group philic. In contrast, SD-C4 and SD-C18 were designed to make
also showed that hydrophobic SD-based optical emulsion sensors the molecules more hydrophobic by adding alkyl chains (–C4H9
fabricated without added surfactant exhibited improved selectivity or –C18H37) to the quaternary ammonium cation. The resulting
owing to higher complex formation constants in that phase.18
lipophilicity is expected to increase in the order: SD-PEG4 o
Traditional chromoionophore based optical sensors give a SD-PEG2 o SD-C1 o SD-C4 o SD-C18. The synthesized dyes
response range that can be tuned by adjusting the pH of the were characterized by NMR and mass spectrometry (see the
sample.19 For monovalent target ions, a 1 pH-unit increase ESI†). All five SDs share the same fluorophore and therefore
exhibit similar excitation and emission wavelengths (500 nm/
University of Geneva, 30 Quai Ernest Ansermet, 1211 Geneva 4, Switzerland.
E-mail: eric.backker@unige.ch; Tel: +41 22 379 6429
600 nm, Fig. S1, ESI†). The fluorescence quantum yields of
these SDs (in EtOH) were found as 0.027, 0.031, 0.051, 0.052,
and 0.070, respectively, using fluorescein in 0.1 M aqueous
NaOH as in ref. 26 and 27. The empirically calculated para-
meters log Po/w (logarithmic partition coefficient between the water
and 1-octanol) for the five SDs are given in Table 1. If a neutral
† Electronic supplementary information (ESI) available: Experimental details,
photos of the probe, optical spectra of the SDs, calibration curves of the PVC/
DOS and DOS matrixed emulsion, lifetime calculation, partition coefficient
calculation, synthesis procedure of SDs, and related NMR and mass spectrometry
data. See DOI: 10.1039/c9cc06729a
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Scheme 2 Mechanism of the ion-exchange process.
Scheme 1 The structures of the solvatochromic dyes. SD-PEG4 and
SD-PEG2 have hydrophilic ethoxy groups grafted in the mother molecule
SD-C1. SD-C4 and SD-C18 are gained by alkyl group change in the quaternary
ammonium cation of SD-C1. The counter ion is iodide for each SD.
Fig. 1 Normalized fluorescence response of K+-selective emulsion
sensors (PU-DOS) made from the indicated SDs. The dashed lines are to
guide the eye.
Table 1 Theoretical lipophilicity of different SDs (the whole molecule),
expected calibration curve shift and practically gained shift value of the
K+-selective PU-DOS emulsion
The fluorescence response to K+ for each formulation is
shown in Fig. 1. The calibration curves are found to shift over
2.7 orders of magnitude to higher concentration with increas-
ing lipophilicity of the SD. The trend is as expected, as sensors
containing SDs with a hydrophilic PEG group exhibit a lower
measuring range compared to SDs with a hydrophobic alkyl
group because they exchange more easily with the potassium
ion. One may expect the log Po/w difference (Table 1) between
two SDs to relate to the shift in the two corresponding calibration
curves if all other parameters remain the same (see the ESI† for
calculation).
The partition coefficient P for the solvent system polymeric
sensing membrane–water is known not to be strictly identical
to the octanol–water partitioning coefficient, Po/w, and the
following relationship has been proposed for polymeric membranes
to compare the two:32
SD-PEG4
SD-PEG2
SD-C1
SD-C4
SD-C18
a
log Po/w
log P
4.44
3.95
4.63
4.10
4.80
4.24
5.65
4.92
8.14
6.91
Theoretical shift
Practical shift
0.15
0.7
0.14
0.4
0.68
0.6
1.99
1.0
a
Data from the ALOGPS 2.1 program.
molecule exhibits a log Po/w value higher than 8.56, nanoparticles
of 140 nm diameter may exhibit a loss of less than 10% during a
24 h period (see the ESI† for calculation).28,29 Structurally, each SD
has a long chain functionality (–CH2OCOC17H35) with a theoretical
lipophilicity of log Po/w = 8.73 (data from ALOGPS 2.1 program30).
In principle, therefore, all SDs exhibit the same strong linker to the
organic phase that should prevent them from leaking out to the
aqueous phase.
To standardize the results, the different SDs were incorporated
into emulsified sensors for the detection of K+ with valinomycin as
ionophore. Emulsion particles were formed with the help of the
SDs as amphiphilic molecules.18 The particles were made of an
log P = (0.8 Æ 0.1)log Po/w + (0.4 Æ 0.4)
(1)
While the sensing material used here is not identical, this
organic phase containing ion-exchanger (NaTFPB), K+ ionophore I relationship is used here as a first approximation to estimate
(valinomycin) and the SD of interest. Scheme 2 shows the the calibration curve shift for the emulsion sensors. For example,
mechanism of the ion-exchange process. The amphiphilic SD the calibration curves of SD-C1 and SD-C4 based emulsions are
acts as a surfactant and stabilizes the organic nanodroplet expected to shift by 0.7 orders of magnitude, calculated from
phase.18 When the ionic chromophore functional group of the (5.65 À 4.80) Â 0.8. This corresponds well to the observed value of
SD exchanges with the target ion K+ and moves from the organic 0.6. The shift from SD-C4 to SD-C18 based K+-selective emulsions
to the aqueous phase, the solution polarity of the fluorophore is smaller than expected, likely because the added alkyl chain
group changes, resulting in fluorescence quenching. During does not fully extract into the aqueous phase upon expulsion but
this process, the SDs remain anchored to the organic phase remains adsorbed onto the emulsion phase in analogy to the
with the help of the hydrophobic alkyl chain.13,17,18,31
C17 anchor. Perhaps not very intuitively, calculations predict
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opt
K,Na
opt
K,TBA
Table 2 Selectivity coefficients of log K
and log K
for different SD
essentially no lipophilicity decrease when introducing a PEG
spacer. Experimentally, a longer PEG chain shifts the measuring
range to lower potassium concentrations by a total of 1.1 orders
of magnitude, which may be explained by the electrostatic force
between oxygen of the PEG and the hydrogen atom of the –NH–
COO– group in the PU.
based K+-selective emulsion sensors
SD-PEG4
SD-PEG2
SD-C1
SD-C4
SD-C18
opt
log K
log K
BÀ5.3
À1.5
À5.6
À1.5
À5.4
À4.3
À2.3
À1.5
K,Na
opt
BÀ0.5
BÀ1.3
K,TBA
The calibration shift is also related to the matrix. Sensors
prepared with PU-DOS have a larger shift and better repeat-
ability than the sensors with PVC-DOS or DOS alone (see Fig. S2
and Table S1, ESI†). Emulsion sensors containing SD-PEG4 give
a higher measuring range than the one with SD-PEG2 when
prepared with PVC-DOS or DOS, which runs counter to the
lipophilicity difference. Generally, PU emulsions give a higher
response range compared to PVC-DOS or DOS based emulsions.
This is attributed to the urethane groups. The improved entrapment
of SDs might be due to interactions between the cross-linked PU
matrix and additional covalent bonds from amine functional groups
of the carbamate structure (–NH–COO–). UV-vis and NMR spectra
(Fig. S3, ESI†) confirm that the PU used here contains benzene rings
Scheme 3 Illustration of alginate gel encapsulation.
that might additionally retain aromatic dyes through p–p interac- Emulsion sensors with more lipophilic SDs give better selectivity
tions. In electrochemistry, it has been reported that the ion transfer over the interfering ion Na+. SD-C1 based K+-selective emulsion
peak position of cyclic voltammograms for membranes containing sensors show poor selectivity over TBA+. However, the other
PU are at more positive potentials than the ones with PVC,33 emulsified sensors exhibit similar logarithmic selectivity coeffi-
suggesting stronger interactions with the matrix. This is consistent cients over TBA+, at about À1.5.
with the optical sensors studied here.
The stability of the K+-selective sensors was studied by
Table S2 (ESI†) shows that the diameters of the different encapsulating the emulsion in alginate gel spherical shells
emulsion particles are all around 130 nm to 140 nm with z-potentials (Scheme 3). The pore size of the hydrated alginate gel has been
at about À40 to À50 mV. Standard deviations are calculated reported to range from 6 to 17 nm,35,36 only allowing free molecules
from three independent experiments.
to pass through while blocking emulsion particles. Alginate micro-
Fig. 2 characterizes the selectivity of the emulsified sensors encapsulation is frequently used for cell culturing37 and drug
based on different SDs, with the corresponding selectivity delivery38 owing to its biocompatibility and size selectivity.
coefficients from the horizontal distance between two calibration Here, five alginate particles (dia. ca. 4 mm) were prepared for
curves of the primary ion and interfering ion shown in Table 2.34 each kind of emulsion sensor, transferring them to 20 mL of
Fig. 2 Normalized fluorescence response of K+-selective emulsion sensor selectivity over Na+ and TBA+ with (a) SD-PEG4, (b) SD-PEG2, (c) SD-C1,
(d) SD-C4, and (e) SD-C18. Error bars: standard deviations from triplicates. The dashed curves are drawn to guide the eye.
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Conflicts of interest
There are no conflicts to declare.
Notes and references
Fig. 3 Fluorescence microscope images of alginate particles containing
SD-C1 based emulsion sensors stored in Milli-Q water for (a) 2 days and
(b) 8 days. 8 day old alginate particle in contact with (c) 10À2 M KCl and
(d) 10À2 M NaCl for one hour. Optical filter used: LP590. The number
below each image is the average fluorescence intensity of the 5 observed
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