Sodium-Selective Fluoroionophore-Based Optodes for Seawater Salinity Measurement

Article abstract of DOI:10.1021/acs.analchem.7b01373

A new fluorescent sensor for Na⁺ is presented. The sensor relies on a Na⁺ selective fluoroionophore based on a bright red-emitting BODIPY chromophore. The fluorescence of the fluoroionophore is enhanced upon binding of Na⁺-ions to the highly selective aza-crown ether receptor due to reduction of the photoinduced electron transfer (PET) quenching. Solid state sensing materials were prepared by physically embedding the fluoroionophore into water-swellable biocompatible polymer matrices (polyurethane hydrogels), thus enabling continuous measurements of aqueous samples. Despite the simple design, the sensor showed no leaching of the indicator and featured fast and reversible response. Among different polyurethane hydrogels investigated, the hydrogel D1 featuring the highest water uptake was found to be the most suitable due to the highest dynamics between "off" and "on" states. Due to little or no cross sensitivity to other ions (e.g., Mg²⁺, Ca²⁺, K⁺) and its insensitivity to potential changes in pH, this sensor is promising for use in clinical diagnostics and for biological and marine applications. Fiber-optic sensors based on referenced read-out with a compact phase fluorimeter were prepared. To demonstrate their practical applicability, the sensors were used to determine the salinity in the seawater and brackish water of the Baltic Sea.

Full text of DOI:10.1021/acs.analchem.7b01373

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Article
Sodium-Selective Fluoroionophore-based
Optodes for Seawater Salinity Measurement
Bernhard J Müller, Tanja Rappitsch, Christoph Staudinger,
Christian Rüschitz, Sergey M Borisov, and Ingo Klimant
Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017
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Analytical Chemistry
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Sodium-Selective Fluoroionophore-based Optodes for Seawater Salinity
Measurement
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Bernhard J. Müller, Tanja Rappitsch, Christoph Staudinger, Christian Rüschitz, Sergey M.
Borisov* and Ingo Klimant
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Graz University of Technology, Institute of Analytical Chemistry and Food Chemistry,
Stremayrgasse 9, 8010 Graz, Austria
E-mail: sergey.borisov@tugraz.at
Keywords
Optical Sensor, Sodium, Salinity, Photoinduced Electron Transfer, Fluoroionophore,
Hydrogel, Fluorescence
Abstract
A new fluorescent sensor for Na+ is presented. The sensor relies on a Na+ selective
fluoroionophore based on a bright red-emitting BODIPY chromophore. The fluorescence of
the fluoroionophore is enhanced upon binding of Na+-ions to the highly selective aza-crown
ether receptor due to reduction of the photoinduced electron transfer (PET) quenching.
Solid state sensing materials were prepared by physically embedding the fluoroionophore into
water-swellable biocompatible polymer matrices (polyurethane hydrogels), thus enabling
continuous measurements of aqueous samples. Despite the simple design, the sensor showed
no leaching of the indicator and featured fast and reversible response.
Among different polyurethane hydrogels investigated, the hydrogel D1 featuring the highest
water uptake was found to be the most suitable due to the highest dynamics between “off” and
“on” states. Due to little or no cross sensitivity to other ions (e.g. Mg2+, Ca2+, K+) and its
insensitivity to potential changes in pH, this sensor is promising for use in clinical diagnostics
and for biological and marine applications. Fiber optic sensors based on referenced read-out
with a compact phase fluorimeter were prepared. To demonstrate their practical applicability,
the sensors were used to determine the salinity in the seawater and brackish water of the
Baltic Sea.
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Introduction
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Optical sensors (Optodes) are promising analytical tools for in situ measurements, with
several properties making them advantageous to the conventional analytical methods. They
are non-invasive, disposable, can be miniaturized easily and when combined with other
sensors, they enable multi-analyte measurements.1 Moreover, they allow imaging of analytes
over a certain area or even in small volumes (e.g. cells).2,3 Optical sensing materials are
widely used in various formats such as mini- and microsensors 4,5,6,7, planar sensor foils 8,9
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and nanoparticles 10,11
.
There is significant interest in development of new optical sensors for a variety of analytes,
for instance oxygen, pH, carbon dioxide, reactive oxygen species or ions.12 Ions play an
essential role in all living organisms and natural processes.13 Sodium is a crucial factor for
physiological pathways in the biomedical field. It is the most abundant ion in the extracellular
medium (more than 100 mM) and is maintained at a low intracellular concentration, creating
an osmotic pressure which is crucial for the transmission of nerve impulses.14 Determination
and visualisation of the transport of ions between the inner and outer compartment of the cell
is of high interest for biological research and can be achieved by utilizing fluorescent probes.
15,16,17,18 Optical sensors for cations can be divided into two main groups. The first group
relies on use of hydrophobic membrane (poly(vinyl chloride) with plasticizer) containing an
analyte selective ionophore and a pH indicator. Selective extraction of the target
analyte into the lipophilic membrane triggers a release of an equivalent amount of protons out
of the sensing film, thereby changing the pH which can be measured by a lipophilic pH
indicator.19
In the second group the fluorescent reporter and the receptor are combined in a single
molecule (fluroionophore, FI). FIs operate in aqueous media and have been mostly applied as
water-soluble probes. Several FI-based probes for Na+ have been reported 20,21,22,23,24,25,26,27
primarily in the context of intracellular measurements. The sensing mechanism is based on
intramolecular quenching via photoinduced electron transfer (PET).28,29,30 The most popular
and commercially available sodium FIs are SBFI (sodium binding benzofuran isophthalate)
and SG (sodium green).14,31 They consist of a diaza-15-crown-5 ether as a receptor coupled to
a benzofuran or a fluorescein fluorophore, respectively. However, all of these indicators
possess a relatively short excitation wavelength (below 500 nm), leading to high (and
variable) levels of background fluorescence from optical components, carrier polymers, and
biological samples. Fluorescein is known for its poor photostability. Therefore, new FIs with
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longer wavelengths of absorption and emission (> 600 nm) are of particular interest as they
allow measurements in highly scattering and absorbing media (e.g. tissues) as well as in
autofluorescent media (e.g. biological samples).
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Design of FI-based ion sensors is a challenging task since the FI should not only retain its
sensitivity upon immobilization but also show no leaching out of the matrix. Thus, the matrix
should be hydrophilic enough to ensure sufficient PET efficiency (which is known to decrease
in a low polarity environment) but hydrophobic enough to be a good solvent for the non-
covalently immobilized amphiphilic FI or contain functionalities for the covalent coupling of
a hydrophilic FI. So far, only very few published indicators were immobilized in a polymer
matrix, all of them short wavelength FIs.32,33,34
In this contribution, we present a novel sensor for Na+-ions based on hydrogel-immobilized
fluoroionophore. We take advantage of the complexation properties of a crown ether receptor
that has an appropriate size to selectively bind Na+-ions. This receptor is linked to a red light
emitting BODIPY chromophore which features high brightness and excellent photostability.
The aromatic amino group in the receptor acts as efficient fluorescence quencher in the
absence of the analyte but the fluorescence is dramatically enhanced upon complexation.
Based on the correlation between Na+ concentration and seawater salinity, we demonstrate
applicability of the new optical sensor for oceanographic applications.
Materials and Methods
1H NMR and 13C spectra were recorded on a 300 MHz instrument from Bruker. MALDI-TOF
mass spectra were recorded on a Micromass TofSpec 2E in reflectron mode at an accelerating
voltage of +20 kV. Absorption measurements were performed on a Cary 50 UV-Vis
spectrophotometer from Varian. Luminescence spectra and absolute quantum yields were
measured on a Fluorolog-3 luminescence spectrometer (Horiba). Calibrations were performed
by fixing a sensor foil diagonally in a quartz cuvette filled with different buffer solutions. QY
of the indicator in solution (THF in presence of 0.3 mM trifluoroacetic acid) and in sensor
foils (at 1000 mM NaCl and 0.1 M aqueous HCl) were acquired with an integrating sphere
from Horiba. Leaching was investigated by recording the absorption spectra of a foil during
continuous pumping a TRIS-buffered (7.4) solution with 500 mM NaCl through a flow-
through cell (15 ml/min). Measurements of the referenced sensors were performed with
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FirestingO2 and FirestingGO2 phase fluorimeters (www.pyro-science.com) at a modulation
frequency of 4 kHz and a LED intensity of 80 %. Optical plastic fibers (1 mm core diameter)
were obtained from Ratioplast-Optoelectronics GmbH (www.ratioplast.eu). Polyurethane
hydrogels (Hydromed D1, D4 and D7) were purchased from AdvanSource biomaterials
(www.advbiomaterials.com). Poly(ethylene terephthalate) support Melinex 505 was obtained
from Pütz (www.puetz-folien.com). Hydrochloric acid 37% (HCl), anhydrous sodium sulfate
and all other solvents including the deuterated solvents were from VWR (www.vwr.com).
Polysulfone (Mn=16000, Mw=35000), 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ),
trifluoroacetic acid, boron trifluoride etherate (BF3·O(C2H5)2), DMF, POCl3 and water-free
dichloromethane (DCM) were purchased from Aldrich (www.sigmaaldrich.com). Buffer
substances, NaCl, KCl, CaCl2, MgCl2, K2CO3, KI, KOH and chloroform (analytical grade)
were from Roth (www.carlroth.com). Silica gel (0.04-0.063 mm) was acquired from Acros
Organics (www.fishersci.com). Carbon black was obtained from Kremer Pigments
(www.kremer-pigmente.com). Monocrystalline diamond powder was purchased from
Microdiamant (www.microdiamant.com). Silanized Egyptian blue microparticles
(trimethylsilyl form) were prepared according to a literature procedure.35 All other chemicals
were purchased from TCI Europe (www.tcichemicals.com). All chemicals were used as
received.
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Experimental
5-Chloro-3-phenyl-1,4-dihydroindeno[1,2-b]pyrrole (6)
This synthesis was performed according to the literature procedure.36
Triethylene glycol ditosylate (3)
Triethylene glycol (4.50 g, 30.0 mmol, 1 eq) and 4-toluenesulfonyl chloride (11.80 g, 61.8
mmol, 2.1 eq) were dissolved in 100 ml dichloromethane (DCM). The solution was cooled
down to 0 °C and portions of KOH (13.60 g, 242.3 mmol, 8.1 eq) were added slowly over a
period of 15 min and then allowed to warm up to RT over 4 hours. H2O (60 ml) was added
and the reaction mixture was extracted (DCM) (2x, 30 ml). The organic phase was dried over
Na2SO4 and the solvent was removed using a rotary evaporator. The crude product was
recrystallized from methyl tert-butyl ether (MTBE) to yield a white crystalline product (10.40
g, 75 %). 1H NMR (300 MHz, CDCl3) δ 7.79 (d, J = 8.2 Hz, 4H), 7.34 (d, J = 7.9 Hz, 4H),
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4.16 – 4.11 (m, 4H), 3.67 – 3.63 (m, 4H), 3.52 (s, 4H), 2.44 (s, 6H). 13C NMR (76 MHz,
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CDCl3) δ 145.02, 130.02, 128.13, 70.87, 69.37, 68.93, 21.81.
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N,N-bis(2-Hydroxylethyl)-2-methoxyaniline (2)
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A mixture of 2-anisidine (1) (21.84 g, 0.177 mol, 1 eq), 2-chloroethanol (52.8 g, 0.655 mol, 5
eq), CaCO3 (17.75 g, 0.177 mol, 1 eq) and KI (2.27 g, 0,013 mol, 0.07 eq) was stirred in 300
ml H2O at 90 °C until TLC indicated full conversion (eluent: cyclohexane (CH) + ethyl
acetate (EA), 1+2). The reaction mixture was filtered, the filtrate extracted with DCM and the
organic phase dried over Na2SO4 before removing the solvent in vacuo. The crude product
was purified using column chromatography (eluent DCM to EA and then EA + MeOH
(90+10)) to yield the product as brown oil (15.36 g, 40.9 %). 1H NMR (300 MHz, CD2Cl2) δ
7.25 – 7.08 (m, 2H), 7.02 – 6.90 (m, 2H), 3.85 (s, 3H), 3.50 – 3.44 (m, 4H), 3.20 – 3.15 (m,
4H), 3.10 (bs, 2H). 13C NMR (76 MHz, CD2Cl2) δ 155.84, 138.90, 125.99, 125.24, 121.78,
112.08, 60.10, 57.60, 55.87.
2-Methoxyphenylaza-15-crown-5 (4)
NaH (3.243 g of 60% NaH suspension in mineral oil, 81.0 mmol, 2.5 eq) was added slowly to
anhydrous tetrahydrofuran (THF) (150 ml) under an argon counterflow. The mixture was
heated to reflux and a solution of N,N-bis(2-hydroxylethyl)-2-methoxyaniline (2) (6.719 g,
31.8 mmol, 1 eq) and triethylene glycol ditosylate (3) (14.66 g, 31.8 mmol, 1 eq) dissolved in
100 ml anhydrous THF was added dropwise over a period of 30 min and continued refluxing
for 18 hours. After cooling down, the suspension was filtered and the solvent was removed
under reduced pressure. The residue was dissolved in 20 ml MeOH, heated to reflux and
NaClO4·H2O (4.84 g, 34.45 mmol, 1.08 eq) dissolved in 10 ml MeOH was added. After 30
min, 20 ml EA was added and the solution was concentrated to a volume of 10 ml on the
rotary evaporator. This suspension was refluxed and EA was added until a total volume of
450 ml. The clear solution was then put in the fridge for 18 hours. The obtained white crystals
were filtered, dried in an oven and then dissolved in 10 ml DCM and extracted with H2O (3x
10 ml) to obtain the free crown (2.80 g, 27 %). 1H NMR (300 MHz, CDCl3) δ 7.12 (dd, J =
7.5, 2.0 Hz, 1H), 6.98 – 6.81 (m, 3H), 3.83 (s, 3H), 3.73 – 3.64 (m, 16H), 3.52 – 3.44 (m, 4H).
13C NMR (76 MHz, CDCl3) δ 152.78, 140.08, 122.12, 120.84, 120.74, 111.77, 70.97, 70.60,
70.37, 70.18, 55.40, 53.00. MS (MALDI-TOF): m/z [MNa]+ 348.1787 calcd, 348.1895 found.
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4-Formyl-2-methoxyphenylaza-15-crown-5 (5)
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2-Methoxyphenylaza-15-crown-5 (4) (2.51 g, 7.7 mmol, 1 eq) was dissolved in 4 ml DMF in
a Schlenk flask, cooled down to -8 °C and POCl3 (1.68 g, 16.1 mmol, 2.1 eq) was added
slowly so that the temperature did not raise over 0 °C. Afterwards, the reaction mixture was
allowed to warm up to RT and was stirred for 20 hours and then heated to 60 °C for 2 hours.
Aldehyde formation could be seen by TLC (DCM + MeOH, 75 + 5) and staining of the TLC
with a solution containing dinitrophenylhydrazine (DNP). The solution was cooled down to
RT, poured slowly over ice and neutralized with K2CO3 to pH 7. The reaction mixture was
extracted with DCM (3x, 30 ml), the solvent was removed and the product purified by column
chromatography (gradually from DCM to DCM + MeOH, 100+5) to yield the product as
brown oil (1.12 g, 41 %). 1H NMR (300 MHz, CD2Cl2) δ 9.73 (s, 1H), 7.38 – 7.28 (m, 2H),
6.98 (d, J = 8.3 Hz, 1H), 3.84 (s, 3H), 3.72 – 3.64 (m, 4H), 3.62 – 3.55 (m, 15H). 13C NMR
(76 MHz, CD2Cl2) δ 190.49, 151.36, 146.49, 129.04, 126.71, 116.80, 110.42, 71.3, 70.74,
70.56, 70.32, 55.91, 54.05. MS (MALDI-TOF): m/z [MH]+ 354.1917 calcd, 354.1599 found.
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BODIPY Fluoroionophore (Na-FI)
4-Formyl-2-methoxyphenylaza-15-crown-5 (5) (1.10 g, 3.11 mmol, 1 eq) and 5-chloro-3-
phenyl-1,4-dihydroindeno[1,2-b]pyrrole (6) (1.69 g, 6.38 mmol, 2.05 eq) were dissolved in 5
mL of anhydrous DCM and 1 drop of trifluoroacetic acid was added. The mixture was
shielded from light and stirred at RT for 48 hours and 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) was added (0.741 g, 3.26 mmol, 1.04 eq), changing the colour of the
reaction mixture to dark blue. After stirring for another 15 min, N,N-diisopropylethylamine
(DIPEA) (4.07 g, 31.4 mmol, 10 eq) and BF3·O(C2H5)2 (6.9 g, 48.6 mmol, 15 eq) were added
under argon counterflow. After 15 minutes stirring, the mixture was extracted with H2O, the
organic phase was dried over Na2SO4 and the solvent removed in vacuo. Purification was
performed by column chromatography (2x, eluent DCM gradually to DCM + MeOH, 100+5)
yielding the product as purple crystals (211 mg, 7.4 %). UV-VIS (DCM): λmax = 538 nm, ε =
130,000 M-1cm-1. 1H NMR (300 MHz, CD2Cl2) δ 8.30 (d, J = 8.4 Hz, 2H), 7.54 – 7.45 (m,
4H), 7.05 – 6.83 (m, 10H), 6.66 – 6.58 (m, 2H), 6.39 – 6.31 (m, 1H), 3.82 – 3.45 (m, 27H).
MS (MALDI-TOF): m/z [MNa]+ 934.2782 calcd, 934.2747 found.
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Planar sensor films
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Hydrogel polymers (Hydromed D1, D4 or D7) were dissolved in THF to yield a solution of
10 wt.%. An appropriate amount of the indicator was dissolved in the hydrogel solution. The
sensor films were prepared by knife coating of these “sensor cocktails” onto dust-free
polyester foils (25 µm wet film thickness). The sensor foils were allowed to dry for several
hours at RT. Dye concentrations for calibrations and QY determination were 0.2 wt.% and for
leaching experiment 1 wt.% in respect to the polymer.
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Fiber-optic sensor for ratiometric DLR readout
The “cocktail” for the reference layer was prepared by dispersing 12 mg Egyptian blue and
100 mg polysulfon in 888 mg CHCl3. This “cocktail” was knife coated onto dust-free
polyester foils (25 µm wet film thickness) and allowed to dry in the oven at 60 °C for 2 hours.
For the first sensing layer 0.5 mg Na-FI, 100 mg diamond powder and 100 mg hydrogel D1
were dispersed/dissolved in 566 mg THF. This “cocktail” was knife coated onto the back-side
of the polyester support and allowed to dry for 2 hours at RT (75 µm wet film thickness). The
cover layer was prepared by dissolving/dispersing 0.5 mg Na-FI, 100 mg carbon black and
100 mg hydrogel D1 in 566 mg THF (15 wt.% in respect to the polymer). This “cocktail” is
knife coated onto the first sensor layer (25 µm wet film thickness) and allowed to dry for 2
hours at RT. A sensor spot was then either stamped out (2 mm diameter) and fixed with a
metal cap on a 1 m PMMA fiber or stamped out (5 mm diameter) and glued on a plastic screw
cap.
Results and Discussion
Synthesis and Photophysical Properties of the New Fluoroionophore
The synthesis of the fluoroionophore is based on a convenient approach previously
demonstrated for K+ FIs.36 The sensitivity towards the analyte is controlled by the aromatic
crown-ether receptor unit, whereas the spectral properties are determined by the BODIPY
chromophore. Condensation of the pyrrole and the aromatic aldehyde, subsequent oxidation
using DDQ and complexation with BF3-etherate yields the new fluoroionophore (Na-FI) in a
simple one-pot reaction (Figure 1).
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Figure 1. Synthesis of the Na+ fluoroionophore (Na-FI).
As a receptor, we chose a methoxy-substituted N-phenyl-aza-crown ether due to its optimal
binding constant towards Na+ ions.37 This crown ether was already used as the recognition
unit in Na+ FIs based on naphthalimide 34 and triazole–coumarin derivates.33 Based on our
experience with K+ FIs, a rigid tetraaryl-BODIPY dye was selected since the rigidification
and planarization of the chromophore results in a bathochromic shift of absorption and
emission and ensures excellent fluorescence brightness.36 Indeed, the FI shows efficient
absorption and emission in the red/NIR region of the electromagnetic spectrum (Figure 2).
The absorption and emission bands are very sharp which is typical for BODIPY dyes.25 It also
features excellent luminescence brightness due to high molar absorption coefficient (130,000
M-1cm-1) and a quantum yield of 69 % in the “on” state (fully protonated in THF using
trifluoroacetic acid). The introduction of chlorine favourably affects the PET efficiency, as the
electron withdrawing character enhances the reduction potential of the fluoroionophore in the
excited state.36 Therefore, the fluorescence is almost completely “switched off” when no
analyte is present.
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Figure 2. Normalized absorption and emission spectra of the fluoroionophore dissolved in dichloromethane.
Molar absorption coefficient was determined in dichloromethane and the quantum yield in tetrahydrofuran with
0.3 mM trifluoroacetic acid to ensure full protonation of the crown ether.
Solid State Sensing Materials
To enable continuous measurement, it is necessary to immobilize the FI into a polymer
matrix. Although covalent coupling represents the most efficient strategy to eliminate
indicator migration and leaching, several additional synthetic steps would be necessary to
introduce the required functionalities which in turn may negatively affect the photophysical
properties of the FI. Therefore, we preferred non-covalent immobilisation.
This can be achieved by simply dissolving the indicator dye and the polymer in an organic
solvent and knife-coating of the solution onto a transparent and inert poly(ethylene-
terephthalate) foil, which acts as sensor support. The polymer matrix acts as a solvent for the
indicator dye and therefore cannot be too hydrophobic (due to poor PET in hydrophobic
environment) or too hydrophilic, as the FI can either leach out of the matrix or aggregate.
Therefore, we chose commercially available water swellable hydrogels (known under the
name Hydromed D1, D4 and D7) which are ether-based polyurethanes with hydrophobic and
hydrophilic blocks. Depending on the ratio of the blocks, they show different swelling (water
uptake) behaviour: swollen hydrogels D1, D4 and D7 contain 70 %, 50 % and 30 % water,
respectively.38 Importantly, no leaching or aggregation of the immobilized FI was observed
upon continuous rinsing of the sensor foil with a 500 mM NaCl solution for several hours
even in case of the most hydrophilic hydrogel D1 and high dye concentration (1 wt.%).
(Figure S1 and S2, Supporting Information).
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Influence of the Polymer Matrix
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As expected, the immobilized FIs show low fluorescence intensity in the absence of the
analyte (Figure 3). The fluorescence intensity of the indicator dye is enhanced in the presence
of Na+ due to the decrease of the PET effect. The sensing behaviour of the FI is strongly
influenced by the hydrophilicity of the hydrogel used. The fluorescence enhancement factor is
the highest (12-fold at 1000 mM Na+) in more hydrophilic hydrogel D1. In case of the most
hydrophobic hydrogel D7, fluorescence is enhanced only 4-fold in the same conditions.
Hydrogel D4 occupies intermediate position (Table 1, Figure S3, Supporting Information).
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In the absence of Na+, fluorescence quantum yields are very similar for all the hydrogels
(Table 1). However, at high Na+ concentration (1000 mM), the quantum yields are very
different: D1 > D4 > D7. The same trend is observed for fluorescence enhancement in acidic
media. pH calibration curves show, that the FI is completely “switched on” only in case of
hydrogel D1 and D4 (QY of 86 % and 84 % respectively in 0.1 M HCl) (Figure S4,
Supporting Information). In case of D7, a QY of 50 % was determined in 0.1 M HCl.
However, it was not possible to fully protonate the FI with even highly concentrated HCl
solutions (Figure S4, Supporting Information). The dye appears to be located in more
hydrophobic domains, which are not fully accessible to Na+-ions or protons. This difference
in analyte accessibility is indicated by the decrease of the apparent pKa value and decrease of
QYs in 1000 mM Na+ with increase in the hydrogel hydrophobicity (Table 1). Among the
investigated hydrogels, D1 has the highest degree of swelling and therefore best accessibility
of the FI to Na+. Even though the QY in hydrogel D1 in presence of 1000 mM Na+ is 23 %,
the overall brightness is very good considering high molar absorption coefficients. High
brightness allows reduction of the thickness of the sensing layer which is beneficial for the
development of sensors with fast response time. An LOD of 1.3 mM Na+ was calculated when
using hydrogel D1 (3 x standard deviation of the blank). Due to the best performance, the
sensing material based on hydrogel D1 was chosen for further experiments.
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Figure 3.a: Exemplar normalized emission spectra of Na-FI in hydrogel D1 at different Na+ concentrations (20
mM TRIS buffer pH 7.4) b: F/F0 calibration curves for the indicator immobilized in hydrogel D1, D4 and D7.
The values of F and F0 were taken at λ = 645 nm.
Table 1. Fluorescence intensity ratios F/F0 at 645 nm and fluorescence quantum yields for Na-FI in different
hydrogel matrices.
F/F0 at
1000 mM
NaCl
12.4
QY at 0
QY at 1000
mM NaCl
QY at 0.1 M
HCl
Apparent pKa
value
mM NaCl
Hydrogel D1
Hydrogel D4
Hydrogel D7
0.01
0.01
0.02
0.23
0.16
0.09
0.86
0.84
0.50
3.03
2.60
1.46*
8.3
4.0
*pKa was estimated assuming a QY of 0.85 in the fully protonated state to enable a sigmoidal fit.
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Cross-Sensitivity of the Sensing Material to Cations and Protons
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Figure 4.a: Cross-sensitivity of Na-FI/D1 to common cations at pH 7.4 (20 mM TRIS buffer). b: Fluorescence
response detected in the kinetic mode of the fluorimeter. A foil of Na-FI in hydrogel D1 was fixed in a flow-cell
and solutions with different Na+, Mg2+, Ca2+ concentrations or pH values were pumped through it. The insert
shows a zoom of the response from 0 mM to 100 mM NaCl (t90 < 20 sec).
The selectivity of the indicator dye is mostly determined by the size of the recognition unit
(crown ether). The cross-talk to cations depends on the size and charge of the ions. The aza-
15-crown-5 used in this study is well known for complexing Na+ and shows good selectivity
against other alkali ions. Figure 4a shows the selectivity of the FI/ D1sensor. Some cross-talk
to K+ is visible at very high concentrations. However, this interference is not problematic for
envisaged applications as very low K+ is present (e.g. ∼ 5 mM in blood and 10 mM in
seawater) whereas the concentration of Na+ is much higher (∼150 mM and 500 mM,
respectively). In fact, addition of 5 or 10 mM KCl to a solution of 150 mM NaCl does not
result in a statistically significant increase of the fluorescence intensity. Addition of as much
as 150 mM K+ shows some enhancement of the signal that would correspond to an increase in
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Na+ concentration from 150 to ~160 mM. Additionally, the ionic strength of the solution is
doubled in this test which may also contribute to the observed changes.
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Notably, virtually no cross-talk to Ca2+ and Mg2+ is observed (Figure 4). Interestingly,
BODIPYs with aza-15-crown-5 without side arms were reported to have rather high cross
sensitivity to Ca2+ and Mg2+.39,40,41 Therefore, the side arm of the receptor is likely to be
necessary for elimination of this cross sensitivity.
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Inertness to pH changes is of major importance for applications of optical ion sensors. Aza-
crown ethers based on aromatic amines are advantageous to those containing aliphatic amines
due to lower pKa value (∼ 5), which further decreases upon immobilization in the hydrogels
(Table 1, Figure S4 Supporting Information) which enables measurements in the
physiological pH range without any influence from pH. Indeed, no cross-sensitivity to pH in
the range 6.5-8.5 is observed (Figure 4b). As can be seen, the response to Na+ is fully
reversible without hysteresis or drift. The dynamic response of about 2.5 µm-thick sensing
foil was very fast (t90 < 20 seconds).
Miniaturized Referenced Na+ Optode
All measurements discussed above are intensity based. Fluorescence intensity is affected by
many parameters which makes it unsuitable for practical applications. To overcome this
limitation dual lifetime referencing (DLR) ratiometric scheme was applied.42 This method
requires a phosphorescent reference dye featuring a long decay time, spectral properties
similar to those of the indicator and inertness towards the analyte and other substances. The
DLR scheme can be applied both in frequency and in time domain.42 In the case of the former,
the overall phase shift measured by a phase-fluorimeter is determined only by the ratio of the
amplitudes of the FI (Aind) and the reference material (Aref) (Eq. 1). For instance, if the
fluorescence intensity of the indicator is low, mostly the luminescence intensity of the
reference material can be detected, and a low cotangent of the overall phase shift (ꢀ) is
measured. As a reference material, we utilized Egyptian blue, an inorganic phosphor excitable
in the green-red part of the electromagnetic spectrum and emitting in the NIR. A good
quantum yield of 10 %,43 a luminescence lifetime of ∼107 µs and its high photostability make
it an almost ideal reference material for DLR.35
ꢁ
ꢂꢃꢄ
ꢅꢆꢇ
= ꢈꢉꢊꢀ ∙ ꢈꢉꢊꢋꢌꢍꢎ − ꢈꢉꢊꢀꢌꢍꢎ (Eq. 1)
ꢁ
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Figure 5. Miniaturized phase fluorimeter FirestingGO2 from Pyro Science and the cross-section of the sensor for
referenced read-out using a phase fluorimeter. a: Hydrogel D1 with immobilized indicator dye and carbon black
for optical isolation. b: Hydrogel D1 with immobilized indicator dye and diamond powder for enhancing signal
by light scattering. c: sensor support. d: Egyptian blue microparticles immobilized in polysulfone.
The referenced sensor consists of three layers coated onto a transparent polyester support
(Figure 5). Silanized Egyptian blue particles were immobilized in polysulfone (d) and coated
on the support (c). To prevent potential interaction between the reference layer and the
sensing layer, we chose to coat the latter on the opposite side, in order to have the polyester
foil as a barrier. The other side of the support foil has a layer of hydrogel D1 incorporating the
indicator dye and diamond powder for signal enhancement caused by light scattering (b). A
further hydrogel D1 layer consisting of Na-FI and carbon black was used as the final layer
(a). Carbon black acts as optical isolation in order to prevent interference from ambient light.
The dye was added in the same concentration as in layer b in order to prevent migration from
one layer to the other, thus changing the signal intensities. As both “cocktails” are dissolved
in THF, applying the second layer yield to some mixing of the layers. However, as only one
type of polymer and the same amount of indicator dye is used in both, this mixing of layers
will not cause any disturbing effects. Notably, diamond powder and carbon black are not
necessary in order to obtain a referenced sensor, but they improve the sensors performance by
elimination of potential interferences caused by background fluorescence and ambient light
(which may saturate the photodetector). A sensor spot was stamped out from this material and
was mounted either directly onto PMMA fibers using a metal cap, or glued on a plastic screw-
cap which could be fixed directly onto the read-out device. A commercially available phase-
fluorimeter (FirestingGO2 from Pyro Science) was used for measurement of the phase shift
(Figure 5).
Application of Na+ Optode for Measurement of Seawater Salinity
The salinity of water is an important measure of water quality. The survival of different
species of animal and plant life depends on the salinity range within their habitat. The most
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abundant cations in seawater are Na+, Mg2+, Ca2+ and K+, while Cl-, SO4 , Br- and HCO3 are
the predominant anions (Table 2).44 The sum of masses of these ionic species is referred to as
salinity; it is expressed as practical salinity unit (PSU) which is the mass fraction of salts in
water (grams salt per kilogram solution).45,46,47 The average practical salinity in the open
ocean is 35, and typically varies between 33 and 37 PSU, while much lower salinities are
found in brackish waters where the seawater is diluted by a high quantity of freshwater
(between 0-30 PSU).44,48 Since the only change occurring to seawater is a change in the entire
concentration of dissolved ions (e.g. dilution with water), the ratio of the different ion
concentrations is not affected. Consequently, the composition stays the same, enabling the
determination of salinity by simply measuring one component of the seawater.49
2-
-
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Seawater salinity is typically measured with a conductivity sensor. An optical sensor for
determination of salinity via measurement of Cl- concentration has been reported.49 Here we
propose an alternative method based on quantification of Na+ concentration.
The dynamic range of the new sensing material based on Na-FI/D1 matches very well the
concentration of Na+ present in seawater. As discussed above, the minor cross-sensitivity of
the sensor to K+ is not critical for seawater measurement since the concentration of K+ is too
low to be detected by the sensor (Table 2). Advantageously, comparably high concentrations
of Mg2+ occurring in seawater do not trigger any fluorescence response.
Table 2. Summary of mean composition of seawater with salinity 35 at 25 °C.49
Cations
Na+
g/kg
mM
479
Anions
Cl-
g/kg
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mM
559
28.9
10.77
1.30
Mg2+
Ca2+
K+
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10.4
0.09
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2-
2-
0.409
0.388
0.010
CO3
0.0026 2.33
0.065 0.86
0.0013 0.075
Br-
F-
Sr2+
Determination of temperature cross-talk of the optical sensor is essential for in situ monitoring
of seawater salinity. A high dependency towards temperature was observed within the
relevant concentrations (Figure 6). This can be attributed to the influence of temperature on
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the stability constant of the crown ether-Na+ complex. At higher temperatures, the
complexation of the analyte is less favourable, and therefore less fluorescence emission is
6
observed (lower cotangent φ). In addition to the complexation equilibrium, Egyptian blue
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contributes to the temperature dependency, as the emission intensity and lifetime decrease
with increasing temperature.35 This dependency can be compensated for with simultaneous
temperature measurements and with the introduction of the temperature coefficient into the
calibration equation (Eq. 2). A is the upper limit of the fit (PSU 45), B the minimum value
(PSU 0) and k describes the increment of the fit. AT, BT and kT are linear temperature
coefficients for the three parameters and T represents temperature.
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ꢏ
ꢑ
ꢏꢗꢘꢙꢚꢙ ∙ ꢔꢗꢜꢝ ꢞ∙ꢟꢠꢑ
ꢛ
ꢏ
ꢑ
ꢏ
ꢑ
ꢏ ꢑ
ꢑ ꢓ ꢏꢡ ꢓ ꢡꢔ ∙ ꢕ − 20 ꢑ (Eq. 2)
cot ꢐꢀ = ꢏꢒ ꢓ ꢒꢔ ∙ ꢕ − 20 ꢑ ∙ ꢏ1 − ꢖ
Figure 6. Temperature-dependent calibration of the salinity optode based on Na+ transduction performed with a
phase fluorimeter and an optical plastic fiber with a sensor spot attached to the distal end of the fiber.
Measurement of Salinity in the Baltic Sea
The applicability of this novel sensor was demonstrated in in situ measurements of the salinity
in the Baltic Sea. We used two different read-out devices for three different application areas.
A commercially available handheld phase fluorimeter (FirestingGO2, Pyro Science)
incorporating a PMMA fibre with the sensing material coated on the tip was used for quick in
situ salinity measurements. Additionally, we conducted salinity profiling measurements up to
a depth of 25 m and surface salinity measurements. For this purpose, the stand alone phase
fluorimeter was embedded into a polyoxymethylene (POM) pressure housing to enable
autonomous long term measurements. The module is also equipped with an internal
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temperature sensor, and the Na+ sensitive material is mounted on the optical feedthrough with
a screw-cap system.
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Figure 7. Measured salinity in the Baltic Sea compared to reference conductivity measurements from the CTD
rosette. a: Measurement performed in surface water for one day (Kiel, 54.3301°N , 10.1498°E). b: Profiling
measurement with a speed of 0.15 m/s and stops of 10 min at 24, 22 and 20 m acquired at Boknis Eck (54.5° N,
10.0° E). c: Measurement points and salinity values determined with the sensor material and a handheld phase
fluorimeter as the read-out device.
Figure 7a shows the 24 h measurement of salinity in surface water. The temperature varied
between 18.5 °C and 20 °C during the measurement time. The sensor was calibrated at room
temperature before deployment with appropriate salinity solutions containing Na+ ranging
from 0 to 1000 mM. Our data is in good agreement with the reference data obtained via
conductivity measurements (CTD). The sensor responds sufficiently fast to detect small
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changes in salinity. Figure 7b shows profiling measurements up to 25 m depth. Contrary to
the surface measurement, the first measured point of the CTD at the surface was used as a
reference calibration point for the optode, as it showed an offset of around 4 PSU. The trend
of salinity measured by the sensor is in good agreement with the reference measurement.
However, there is a disparity beyond a depth of 5 m. A possible explanation for this is the
reduction in response time at lower temperatures. A combination of the high speed of
profiling (0.15 m/s) and the change in temperature at greater depths (from 19.4 °C at 5 m to
11.3 °C at 25 m) could cause this variation to the reference. Preparing thinner sensor foils
featuring faster response may overcome this issue.
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Several point measurements along the mouth of river Schwentine (Kiel) have also been
performed (Figure 7c). The lowest salinity (2.2 PSU) was measured in the river before a
bridge. There was a gradient in the altitude of about 1-1.5 m with water flowing in the
direction of the sea, which prevents movement of the salt water upwards to the river. As the
river gets wider, measurements at locations closer to the harbour indicated an increase in the
salinity. The last point of measurement is virtually a fjord and the widest location of all
measurements, with the highest salinity of 20.7 PSU. This is in good agreement with
commonly measured salinity values in the harbour. Using this optode, it was possible to
perform in situ salinity measurements along a river in minimal time, effort and equipment.
Conclusions
A highly selective optical sensor material for the measurement of Na+-ions in aqueous media
was presented. The new fluoroionophore is based on a aza-crown ether receptor unit coupled
to a BODIPY chromophore which shows emission in the red region of the spectrum and good
fluorescence brightness. By incorporating this fluoroionophore into a polymer matrix a robust
and fast responding sensor is obtained. Among different polyurethane hydrogels tested as
matrices the most hydrophilic D1 hydrogel proved to be the most suitable. The material
showed the highest dynamics between the “off” and “on” state and the highest fluorescence
quantum yield in the “on” state. The sensor displays high selectivity over other competing
ions (e.g. Mg2+, Ca2+, K+) and is not affected by pH changes in the relevant range. As all
chemosensors, the sensor shows cross-talk to temperature which can be easily compensated
for.
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A referenced sensing material has also been prepared to enable measurements with a
commercially available compact phase fluorimeter. These sensors were used to determine the
salinity of seawater and brackish water in the Baltic Sea, and show good correlation with
reference measurements. They demonstrate the potential of optical technology for measuring
of this very important parameter. It should be emphasized that the new indicator and sensing
materials can also be used for a variety of other applications, such as clinical diagnostics or
biological imaging.
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Acknowledgment
We thank Prof. Eric Achterberg and the GEOMAR team for their assistance in field
measurements. Financial support from the European Union FP7 Projects SenseOcean (Grant
Agreement Number 614141) and SCHeMA (Grant Agreement Number 614002) is gratefully
acknowledged.
Associated content
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI:
Leaching experiments, additional emission spectra, MS spectra.
Author Information
Corresponding Author
*E-mail: sergey.borisov@tugraz.at
ORCID
Sergey M. Borisov: 0000-0001-9318-8273
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