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Article

New Life of Ancient Pigments: Application inHigh Performance Optical Sensing Materials

Sergey M Borisov, Christian Würth, Ute Resch-Genger, and Ingo KlimantAnal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac402275g • Publication Date (Web): 04 Sep 2013

Downloaded from http://pubs.acs.org on September 8, 2013

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New Life of Ancient Pigments: Application in High Performance

Optical Sensing Materials

Sergey M. Borisov,*a

Christian Würth,b Ute Resch-Genger,

b and Ingo Klimant

a

a Institute of Analytical Chemistry and Food Chemistry, Graz University of Technology,

Stremayrgasse 9, 8010, Graz, Austria

bBAM Federal Institute for Materials Research and Testing, Division 1.10 – Biophotonics,

Richard-Willstätter-Str. 11, D-12489 Berlin, Germany

Abstract: Calcium, strontium and barium copper silicates are demonstrated to possess

valuable photophysical properties which make them particularly attractive for application in

optical chemosensors. Several examples of sensing materials based on these phosphors are

provided. Particularly, broad excitation and near infrared (NIR) emission makes them ideal

candidates for the preparation of ratiometric sensors based on absorption-based indicators.

Due to their excellent chemical and photochemical stability and high brightness, these

phosphors can serve as reference for fluorescent indicators to enable ratiometric intensity or

dually lifetime referenced measurements. Finally, the moderate temperature dependence of

the luminescence decay time enables intrinsic temperature compensation of the sensing

materials at ambient temperatures. The improved sensitivity at temperatures above 100 °C

makes these new materials promising candidates for high temperature thermographic

phosphors.

Introduction

Calcium copper silicate Egyptian Blue CaCuSi4O10 and barium copper silicate Han Blue

BaCuSi4O10 are ancient pigments which were used already thousands of years ago in the Old

Kingdom in Egypt and during the Western Zhou period in China, respectively.1 Their high

chemical stability is emphasized by the fact that even after such a long time, the colors of

ancient monuments and artifacts retained their original splendor. The luminescent properties

of numerous minerals are well known for many decades, but the NIR luminescence of

Egyptian blue and Han Blue remained astonishingly undiscovered until very recently. Accorsi

et al. reported the interesting photophysical properties of Egyptian blue (absorption in the

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green-red part of the spectrum, NIR emission at about 900 nm and good brightness) and

suggested that these properties can be useful for application of this pigment in optical

materials.2 Pozza et al. indicated that Han Blue is also luminescent in the near-infrared (NIR)

region.3 Very recently Johnson-McDaniel et al. showed that Egyptian Blue nanosheets can be

produced upon prolong treatment of the phosphor with hot water, thereby preserving its

favorable luminescence properties.4 However, despite these exciting reports, the phosphors

have not yet found any application in advanced optical materials.

Optical chemosensors represent a very important and constantly growing field of

analytical chemistry.5,6

They often provide distinct advantages over conventional analytical

techniques due to the absence of electromagnetic interferences, minimal invasiveness,

suitability for miniaturization and continuous monitoring of analyte concentration as well as

their typically lower costs. Apart from an indicator (which responds to the analyte of interest

by altering its optical properties such as absorption, luminescence intensity, luminescence

decay time, anisotropy etc.) and a polymer or a sol-gel matrix, optical chemosensors often

require additional components to ensure reliable sensing. For example, fluorescence intensity

alone is a rather ambiguous parameter, which is affected also by fluctuations of the excitation

light intensity and indicator concentration, and is thus rarely used. Fluorescence lifetime

measurements are much more reliable,7 but the required instrumentation is more expensive.

Thus ratiometric wavelength referencing, requiring either two spectrally distinguishable

absorption (excitation) or emission bands, is rather popular, particularly in (microscopic)

imaging applications.8,9

Since most probes emit only at a single wavelength, typically a

second dye excitable at the same wavelength but emitting in a different part of the spectrum is

added for signal referencing. Alternatively, referencing of the fluorescence intensity can also

be realized with the help of the so-called Dual Lifetime Referencing (DLR) approach.10-13

DLR relies on the use of a phosphorescent reference which has a significantly longer decay

time than the fluorescent indicator (typically µs-ms and ns time domains, respectively). In the

frequency domain, for example, the overall phase angle is a function of the amplitudes and the

phase angles of both luminescent components. Recently, we reported a new approach for

referencing absorption-based indicators.14

In this scheme, a broadband absorbing or a

broadband emitting phosphor acts as a second luminophore, the luminescence intensity of

which is modulated by changes in the absorption of the indicator due to the inner filter effect.

Finally, it is well known that the response of all optical chemosensors can be affected by

temperature as the optical properties of most indicators and fluorophores are temperature-

dependent. Although temperature can be measured independently with e.g., a thermocouple, it

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is often more attractive to use an optical probe for this purpose. In this case, both parameters,

i.e., analyte concentration and temperature, are measured at the same place which improves

the robustness of the sensor against fluctuations in temperature and enables miniaturization

(e.g., fiber-optic microsensor). A number of luminescent metal complexes15-17

and

thermographic phosphors18-21

have been employed so far in optical temperature sensors and in

dually- or multi-analyte sensing materials operating at ambient temperature.22-24

However,

high temperature thermographic phosphors are also of much interest.25

In this contribution, we will demonstrate that Egyptian blue, Han blue and strontium

copper silicate represent highly versatile materials for application in optical sensors.

Particularly, it will be shown that these phosphors are promising as references for fluorescent

indicators utilizing both ratiometric and DLR schemes as well as for absorption-based

indicators and can also serve as internal temperature probes.

Experimental part

Materials. Egyptian Blue and Han Blue were obtained from Kremer Pigmente (Aichstetten,

Germany). Ethyl cellulose (ethoxyl content 49%), m-Cresol purple, calcium carbonate,

strontium carbonate, trimethylsilyl chloride, tetraoctylammonium hydroxide solution

(TOAOH, 20 % in methanol) were obtained from Sigma-Aldrich. Silicon oxide (99.9 %, 1µm

particles), vinyl-terminated polydimethylsiloxane (viscosity 1000 cSt.), methylhydrosiloxane

dimethylsiloxane copolymer (viscosity 25-35 cSt.), tetravinyltetramethyl cyclotetrasiloxane

and platinum divinyltetramethyldisiloxane complex were purchased from ABCR (Karlsruhe,

Germany). Basic copper carbonate was obtained from Fischer Scientific. The solvents were

from VWR (Austria). Barium carbonate and the buffer salts N-cyclohexyl-2-

aminoethanesulfonic acid, 2-(N-morpholino)ethanesulfonic acid and N-cyclohexyl-3-

aminopropanesulfonic acid were supplied by Carl Roth (Germany). Polyurethane hydrogel

(HydromedTM

D4) was purchased from AdvanSource biomaterials (Wilmington, USA).

Poly(ethylene terephthalate) (PET) support Melinex 505 was from Pütz (Taunusstein,

Germany).The gases for the calibration – carbon dioxide and nitrogen were supplied by Linde

(Austria).

[5-(4-hydroxyphenyl)-3-phenyl-1H-pyrrol-2-yl]-[5-phenyl-3-phenylpyrrol-2-ylidene]amine

“aza-BODIPY” was prepared as reported previously.26

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High temperature solid state synthesis of the phosphors. The synthesis is exemplified for

calcium copper silicate (SrCuSi4O10). The phosphor was prepared via a high temperature solid

state reaction using silicon oxide, strontium carbonate and basic copper carbonate.

Stoichiometric amounts of these substances were thoroughly homogenized in an agate mortar

and sintered at 1000 °C for 24 h. The resulting material was homogenized to powder in an

agate mortar. The samples of Egyptian Blue (CaCuSi4O10) and Han Blue (BaCuSi4O10) were

prepared analogously using calcium carbonate and barium carbonate, respectively.

Lipophilization of Egyptian blue. For application in the sensing materials, Egyptian blue

was ground to microparticles (1-5 µm) in a ball mill. The surface of the particles was rendered

lipophilic via silanization using the following procedure: 1.6 g of Egyptian blue

microparticles were dispersed in 5 mL of anhydrous tetrahydrofuran and 0.5 mL of

trimethylsilyl chloride were added. The suspension was stirred for 30 min, the sediment was

separated via centrifugation, washed four times with acetone and twice with ethanol, and

dried.

Preparation of the pH sensors

250 mg of hydrogel D4, 0.5 mg of the pH indicator aza-BODIPY and 250 mg of Egyptian

blue lipophilic microparticles were dissolved/dispersed in 2.5 g of an isopropanol:H2O

mixture (9:1 v/v). The “cocktail” was knife-coated on a polyethylene terephthalate support to

give about 8 µm-thick sensor layers after solvent evaporation. Alternatively, the fiber-optic

sensors were prepared by coating the “cocktail” on a distal end of a PMMA waveguide (∅ 1

mm, length 1 m, Ratioplast, Lübbecke, Germany).

Preparation of the CO2 sensors

100 µL of tetraoctylammonium hydroxide were added to 2 mg of m-Cresol Purple and the

resulting solution was saturated with carbon dioxide. In the second vial, 0.2 g of ethyl

cellulose were dissolved in a mixture of 1.52 g of ethanol and 2.28 g of toluene. The content

of both vials was combined and the obtained “cocktail” was knife-coated on a polyethylene

terephthalate support and was allowed to dry for 1 h at room temperature. The thickness of the

layer after solvent evaporation was estimated to be about 3.5 µm.

A “cocktail” of silicone primers was obtained by mixing 800 mg of vinyl-terminated

polydimethylsiloxane, 32 µl of methylhydrosiloxane dimethylsiloxane copolymer, 2 µl of

tetravinyltetramethyl cyclotetrasiloxane and 800 mg of hexane. Micrometer-sized lipophilic

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particles of Egyptian Blue (400 mg) were dispersed in the “cocktail”. Finally, 4 µl of the

platinum complex catalyst were added and the composition was knife-coated onto the CO2-

sensitive layer (75 µm spacer) and was allowed to cross-link at 60 °C for 20 min.

Measurements

The absorption spectra were acquired on a Cary 50 UV-VIS spectrophotometer from Varian.

Emission spectra of individual phosphors were measured in front face geometry with a

calibrated FSP 920 fluorescence spectrometer from Edinburgh Instruments equipped with

double monochromators and a liquid nitrogen cooled NIR-sensitive InP/InGaAs

photomultiplier from Hamamatsu (R5509-72). All measurements were performed with

polarizers in the excitation (0°) and the emission channel (54.7°). All emission spectra

presented are corrected for the spectral responsivity of the instrument´s detection channel

(instrument characterization traceable to radiometric units, i.e., to the spectral radiance

scale.27,28

Luminescence excitation spectra and emission spectra of the sensing materials were

acquired on a Fluorolog 3 fluorescence spectrometer (Horiba) equipped with a

photomultiplier R2658 (Hamamatsu) optimized for the spectral range of 300-1050 nm. About

10 µm thick layers of the phosphors in polystyrene (1:1 w/w) on a glass support were used for

the measurement of the excitation spectra in order to minimize wavelength-dependent

scattering effects. The excitation spectra were corrected for the wavelength-dependent output

of the Xe arc lamp.

Planar carbon dioxide and pH optodes were read out with a Fluorolog 3 spectrometer.

A home made flow-through cell was used. In case of the carbon dioxide sensors, the gas

mixtures were adjusted using Red-y gas flow controllers (Vögtlin Instruments AG, Aesch,

Switzerland). The gas mixtures were adjusted to 100 % relative humidity. The pH of the

buffer solutions was controlled by a digital pH meter (InoLab pH/ion, WTW GmbH & Co.

KG, Germany) calibrated at 25 °C with standard buffers of pH 7.0 and pH 4.0 (WTW GmbH

& Co. KG). The buffers were adjusted to constant ionic strength (IS = 0.15 M) using sodium

chloride as the background electrolyte. The temperature was kept constant at 37 °C using a

cryostat ThermoHaake DC50 (Thermo Fisher Scientific Inc). The fiber-optic pH sensors were

read-out with a Firesting oxygen meter from Pyroscience (Aachen, Germany) which is

equipped with a read 625 nm LED and a long-pass RG 9 filter from Schott. A modulation

frequency of 1.5 kHz was used.

The carbon dioxide sensors were also read out using a dually-emitting LED (590/650

nm) from PUR-LED®

GmbH & Co. KG (Selzen, Germany). A short-pass Calflex X filter

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from Linos was used in front of the LED. The luminescence was detected at a modulation

frequency of 916 Hz using a lock-in amplifier from PreSens (Germany) equipped with a

photodiode and a long-pass RG-9 filter from Schott (Germany).

The temperature sensitivity of the phosphors was investigated using a Firesting-mini

device from Pyroscience. A glass waveguide (∅ 3 mm, length 15 cm) was used as an

extender. The distal end of the waveguide was brought into direct contact with the phosphor

powder kept in a 4 mm glass vial. The vial was heated in a home-made metal block using a C-

MAG HS 7 heating plate from IKA®-Werke GmbH & Co. KG (Staufen, Germany). The

modulation frequency of 1.5 kHz was used to access the decay times.

Results and Discussion

Photophysical properties

Luminescence excitation and emission spectra of Egyptian blue, Han blue and strontium

copper silicate are shown in Fig. 1. The phosphors have a broad absorption / excitation band

located in the green-red part of the spectrum which is attributed to 2B1g →

2A1g and

2B1g →

2Eg transitions.

2 Excitation due to

2B1g →

2B2g transition occurs in the NIR part of the

spectrum. As follows from Fig. 1, the excitation bands in the visible region are very similar

for all phosphors, but a small bathochromic shift (∼ 10 nm, 280 cm-1

) of the 2B1g →

2A1g band

is observed on going from calcium copper silicate to barium copper silicate. The NIR

excitation is affected more significantly and shifts from 806 nm for calcium copper silicate to

842 nm (530 cm-1

) for barium copper silicate.

All phosphors emit in the NIR above 800 nm (Fig. 1). Similarly to the NIR excitation,

the emission associated with the 2B2g →

2B1g transition shifts bathochromically from calcium

to barium copper silicates (λmax = 909, 914 and 948 nm for calcium, strontium and barium

copper silicates, respectively). Precise determination of the luminescence quantum yields in

the NIR region, which is not possible with a spectrofluorometer in a 0°/90° or front face

measurement geometry due to size-dependent scattering of the micrometer-sized phosphor

particles and requires an integrating sphere setup, was beyond the scope of this study.27

Previous results suggest a luminescence quantum yield of 10.5 % for Egyptian blue.2

Measurements under red excitation (625 nm LED) and a photodiode as a detector revealed

that the brightnesses of the strontium copper silicate and Han blue are comparable to that of

Egyptian blue. It should be kept in mind here, however, that the brightness equals the product

of the luminescence quantum yield and the molar absorption coefficient. Considering a rather

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high amount of the chromophore in the phosphors, which is about 10-20-fold higher than for

typical phosphors (e.g., chromium(III)-activated hosts such as yttrium aluminum garnet,

yttrium aluminum borate, spinel, aluminum oxide (ruby) or manganese(IV)-activated

magnesium fluorogermanate; typical doping of 0.5-5% of the activator), the overall brightness

of our materials is excellent. This is particularly true for thin layers of the phosphor materials

since in this case a significantly more efficient absorption can compensate for lower quantum

yields. For example, excitation of a thin sensing layer of a mixture of chromium(III)-activated

gadolinium aluminum borate (Cr-GAB) and Egyptian blue (25 % w/w of each phosphor in

polystyrene) with red light (600 nm, corresponding to the excitation maxima of both

phosphors) results in about 3-fold higher signals for Egyptian blue compared to Cr-GAB.

500 600 700 800 900 1000 1100 1200

0.0

0.2

0.4

0.6

0.8

1.0

500 600 700 800 900 1000 1100 1200

0.0

0.2

0.4

0.6

0.8

1.0

Norm

. L

um

inesc

en

ce in

ten

sity

CaCuSi4O10 SrCuSi

4O10 BaCuSi

4O10

Wavelength, nm

Excitation Emission

Figure 1. Excitation (lines) and emission spectra (dashed lines) of Egyptian blue (black),

strontium copper silicate (red) and Han blue (blue line). The insert shows photographic

images of the measured µm-sized phosphor particles, i.e., the powdered pigments under

ambient light.

High homogeneity of the sensing materials is important to ensure reproducibility of the

calibration and sensor response. This is particularly true if the phosphors or other luminescent

species are used for referencing purposes in the composite materials. Therefore, the phosphors

were ground to microparticles (∅ 1-5 µm) using a ball mill to enable a better reproducible

sensor production. Although grinding does not appreciably affect the optical properties of

these phosphors, we note a slight shortening of the decay time of the ground phosphors

compared to the larger crystals obtained after sintering. For example, the decay times of the

Egyptian Blue at 20 °C were 159 µs and 126 µs after sintering and after grinding,

respectively. For comparison, the decay time of commercially available Egyptian blue powder

(Kremer Pigmente) was measured to be 139 µs. The decrease of the decay time can be

explained by accumulation of the defects induced by mechanical stress during grinding.

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Ratiometric pH-sensing materials

The broad excitation bands in the visible part of the spectrum of the phosphors (Fig. 1) are

ideally compatible with the absorption of a large number of fluorescent indicators and very

favorable for ratiometric (2 wavelength) referencing of luminescence intensity. Since

fluorescent indicators typically possess rather small Stokes shifts, the emission of the

phosphors at 900 -1100 nm can be easily spectrally separated from that of the analyte-

responsive probe. High chemical and photochemical stability of the phosphors is of course

also advantageous. Fig. 2 shows the absorption and emission spectra of the

tetraphenylazadipyrromethene pH indicator (“aza-Bodipy”)26

and the emission spectra of the

pH-sensitive material. The sensor is prepared by dissolving/dispersing the pH indicator and

Egyptian blue micropowder in a polyurethane hydrogel D4. In order to render the surface of

the microparticles stable in aqueous media (hydrogel) end-capping with trimethylsilyl groups

was performed. The indicator and the phosphor are excitable with the red light. Although both

emit in the NIR part of the spectrum, the emissions are well separated. As expected, the

fluorescence of the pH probe is quenched as the pH increases but the luminescence of

Egyptian blue is not affected by pH. A minor increase of the luminescence intensity of the

phosphor (about 10 % for going from pH 5.2 to 10.3) is due to a change in absorption of the

pH indicator and thus, a decrease of the inner-filter effect. In fact, the absorption maxima of

the protonated and deprotonated forms of the pH indicator are located at 687 and 743 nm,

respectively (Fig. 2a), and the absorption at the excitation wavelength (625 nm) is about 3-

times more efficient for the protonated form. Evidently, the system is ideal for ratiometric

emission measurements. The apparent pKa value calculated from the respective calibration

curve for the luminescence intensity ratio is 7.53 which is adequate for measurements in the

physiological probes and for application in seawater. It should be mentioned that simple

synthetic modification allow to tune the pKa value of the indicator in a broad range.26

Considering the spectral properties of the two other phosphors (Fig. 1), they can be used for

ratiometric referencing of other indicators with even more bathochromically shifted excitation

and emission spectra.

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700 750 800 850 900 950

0

100

200

300

400

500

5 6 7 8 9 10

0.0

0.5

1.0

1.5

2.0

550 600 650 700 750 800 850

0.0

0.1

0.2

0.3

0.4

0.5

550 600 650 700 750 800 850

0.0

0.2

0.4

0.6

0.8

1.0

inte

nsi

ty,

a.u

.L

um

ines

cen

ce

Wavelength, nm

5.2 6.84

7.27 7.86

8.22 8.59

9.01 10.28

ra

tio

71

5/9

10

nm

Lu

min

escen

ce I

nte

nsi

ty

pH

pKa = 7.53

N

N

NB

FF

O

N

N

NB

FF

HO

No

rm.

Inte

nsi

ty

Ab

sorp

tio

n

Wavelength, nm

Figure 2. A: Spectral properties of the aza-Bodipy pH indicator in hydrogel D4 (black thick

line – absorption spectrum at pH 5.2; blue thin line – absorption spectrum at pH 10.3; red

dotted line – emission spectrum at pH 5.2 and λexc = 625 nm); B: pH dependence of the

emission spectra of the sensing material consisting of the pH indicator and µm-sized Egyptian

blue particles (37 °C, 150 mM ionic strength, λexc = 625 nm) dissolved/dispersed in hydrogel

D4; C: Respective calibration for the ratio of the luminescence intensities (squares) and a

sigmoidal fit (line).

Dually lifetime referenced pH sensors

Dual Lifetime Referencing (DLR) represents another technique for referencing the

fluorescence of indicators. Here, an inert analyte-insensitive luminescent material is dispersed

in the polymer matrix along with the analyte-responsive fluorescent indicator. A reference

suitable for this application should be chemically and photochemically inert and should

possess a relatively long luminescence decay time (typically in µs – ms time domain). This

technique can be used both in the time domain12

and in the frequency domain.11,29

In the

frequency domain, the overall phase shift is measured; both the fast-decaying fluorescence of

the indicator and the long-lived luminescence of the reference contribute to this value. The

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most frequently used DLR references are hence metal complexes .30,31

Due to their

disadvantageous liability to quenching by oxygen, these dyes, however, have to be

encapsulated in gas-blocking polymers (e.g., polyacrylonitrile)11,30

which may be challenging.

Application of phosphors as DLR-reference materials is rather rare.32

As we demonstrate

here, Egyptian Blue and other copper silicate phosphors represent excellent DLR references

since they combine bright long-lived luminescence, excellent (photo)stability, and inertness to

oxygen. Fig. 3 shows the calibration curve for a fiber-optic pH sensor. The material used to

coat the plastic PMMA fiber is identical to that employed in the planar optodes with

ratiometric read-out. The fluorescence amplitude of the indicator is proportional to the

difference of the cotangents of the overall phase shift and the phase shift of the reference:30

ref

indref

A

A

sin

1cotΦcotΦ ⋅

Φ=−

ref

where Φ and Φref are the phase angles of the overall signal and of the luminescent reference,

respectively; Aind and Aref – are the amplitudes of the fluorescent indicator and of the

luminescent reference, respectively. As follows from Fig. 3, the apparent pKa value obtained

for the DLR-sensor (7.47) is very close to that calculated from the ratiometric read-out. The

individual sensors have slightly varying ratios of the fluorescent dye and the reference and are

not calibration-free (Fig. 3, phase shift plots). Thus, a two point calibration is recommended

(low pH and high pH buffer). The same refers also to the ratiometric sensor.

5 6 7 8 9 10

10

20

30

40

50

5 6 7 8 9 10

0.0

0.2

0.4

0.6

0.8

1.0

Norm

ali

zed

Cot ΦΦ ΦΦ

-Co

t ΦΦ ΦΦ0

Ph

ase

sh

ift ΦΦ ΦΦ

, °

pH

pKa = 7.47

Figure 3. Calibration plots for the fiber-optic DLR-pH sensor (37 °C, 150 mM ionic strength)

based on an aza-Bodipy pH indicator and Egyptian blue in hydrogel D4. Symbols represent

experimental data and lines sigmoidal fits. The phase shifts for three individual sensors are

shown. The values CotΦ-CotΦref were normalized to enable the comparison of all individual

sensors. The inserts show the photographic images of a fiber-optic sensor without LED

illumination and during the readout.

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Ratiometric inner filter effect sensors

Very recently, we presented a new scheme for the referencing of absorption-based

indicators.14

Here, a reference acts a secondary emitter, the luminescence intensity of which is

modulated by an indicator with an analyte-responsive absorption via inner filter effects (in

absorption and /or emission). A suitable reference for this application should feature either a

broad excitation spectrum or a broad emission spectrum which overlap with at least one

absorbing form of the indicator. Such a scheme allows also a ratiometric readout. In addition

to the general versatility of this approach, which can utilize all types of broad available

colorimetric indicators and probes, and its inertness to fluorescence quenchers, the use of

phosphors with long luminescence decay times as reference allows complete elimination of

autofluorescence originating from the sample, indicator and/or optical components of the

detection system. As follows from Fig. 1, the optical properties of Egyptian blue and other

copper silicate phosphors make them ideal references for this new sensing scheme. Currently

the number of available NIR colorimetric probes is rather limited, particularly those absorbing

above 850 nm, but the situation is likely to change in the future. We therefore demonstrate

here only the applicability of the phosphors for this versatile sensing scheme using an

indicator-modulated inner filter effect in excitation, see Fig. 4. The broad luminescence

excitation spectra of the phosphors almost perfectly overlap with the absorption of the

deprotonated form of triphenylmethane indicators applied as pH, carbon dioxide, and

ammonia probes. A two layer carbon dioxide sensor was realized, with the first layer

containing m-Cresol purple and tetraoctylammonium hydroxide dissolved in ethylcellulose

and the second layer Egyptian blue microparticles dispersed in silicone rubber (Fig. 4a). In

this sensor design, silicone rubber acts not only as a matrix for the immobilization of the

reference phosphor but also as a permeation-selective membrane which protects the sensitive

layer from protons and other ions. As can be observed in Fig. 4c, the excitation spectrum of

the sensing material is dependent on pCO2, thereby reflecting the modulation of the excitation

light intensity by the absorption of m-Cresol purple (Fig. 4b) as change in the emission

intensity of the phosphor. This also demonstrates that ratiometric readout in luminescence

excitation is possible.

Dually emitting LEDs are available matching both the excitation wavelength of the

phosphors and the indicator absorption. Thus, a very compact, simple, and inexpensive setup

for the ratiometric readout of the optical sensors can be realized (Fig. 4a), using a dually

emitting LED, two optical filters, and a silicon photodiode. Fig. 4d shows the calibration plot

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obtained using a dually emitting red-orange (650/590 nm) LED. Relatively high standard

deviations are observed since the thickness of both layers can vary significantly for individual

sensors. Thus, the sensors realized according to the above scheme are not calibration-free, and

2 point calibration is required before the measurement. Nevertheless, the combination of

Egyptian blue (or other copper silicate phosphors) with absorption-based indicators represent

an excellent way to realize simple and robust optical sensors for various analytes. Indeed, the

pH and ammonia sensors based on triphenylmethane dyes can be designed in a similar

manner. Referencing of irreversible colorimetric probes can also be performed. Notably, very

large effective shift between excitation and emission of the phosphors (> 200 nm) favors

elimination of potential interferences such as autofluorescence and Raman scattering. Further

simplification can include a photodiode with an integrated daylight filter (e.g. BPW 34 FAS

available from Osram), rendering other optical filters unnecessary. Alternatively, the filter-

less set-up can be realized by eliminating potential interferences with time-resolved

measurement.

It is interesting to compare the properties of the sensing materials based on the new

phosphors which employ three different read-out schemes presented above (Table 1).

Generally, the material relying on the inner filter effect-modulated excitation read-out is

significantly more robust towards potential interferences (fluorescence quenchers, background

fluorescence from the sample) which can be even further minimized in the time-resolved

measurement. Along with the DLR scheme this method requires only a very simple

instrumentation with few optical components. Indeed, only a simple long-pass filter is

necessary for the inner filter read-out, a long-pass and a short-pass filter for the DLR method,

and at least three filters (short-pass for the excitation, a long-pass and a band pass for the

emission, and an additional dichroic mirror) for the ratiometric scheme. On the other hand, the

variation in the layer thickness only minor affects the calibration in the DLR and ratiometric

scheme, but more severely in the inner-effect read-out.

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Figure 4. A: Cross-section of the carbon dioxide sensor and optical setup for the readout of

the sensor with a dually emitting LED; B: Absorption spectra of m-Cresol purple (in ethyl

cellulose) in the deprotonated (low pCO2, 1) and protonated (high pCO2, 2) forms and the

excitation spectrum of Egyptian blue (3); C: Excitation spectra (λem = 900 nm) of the carbon

dioxide sensor at varying pCO2 (25 °C) and the emission spectra of the dually emitting LED

(590/650 nm); D: calibration plot obtained for the excitation of the carbon dioxide sensor with

a dually emitting LED, the emission monitored at λ > 730 nm (RG 9 filter) with a photodiode

(25 °C) Average values for 3 individual sensors are shown. The chemical structure of the

indicator used here is shown in Fig. 4D.

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Table 1. Comparison between different read-out schemes used in this work.

Sensing scheme

Property

Ratiometric

(2-λ) emission

Dual Lifetime

Referencing

Inner filter effect-

modulated

excitation

Influence of the fluorescence

quenchers on the calibration

strong strong no influence

Influence of the thickness of

the sensing layer on the

calibration

minor minor large

Influence of background

fluorescence on the calibration

minor minor negligible

Possibility of the time-

resolved measurements

not possible not possible possible

Versatility of formats (planar

optodes, paints, microsensors

etc.)

high high low

Complexity of optical set-up moderate low very low

Intrinsic optical temperature

compensation

possible (more

complex

calibration

algorithms)

possible (more

complex

calibration

algorithms)

possible

(calibration is not

affected)

Phosphors as optical temperature probes

The response of almost every optical sensor is temperature dependent. Hence, the temperature

of the sensor environment should be known or be kept constant in order to obtain reliable

data. To circumvent this additional source of measurement uncertainties, either a constant

monitoring of the ambient temperature is required or a tool that enables an intrinsic

temperature compensation of the optical sensor. As follows from Fig. 5, which

shows the temperature dependence of the luminescence decay time (a) and luminescence

intensity (c) of the phosphors, our NIR-emissive phosphors are also excellently suited for this

task. Evidently, both parameters decrease with temperature for all materials. The temperature

sensitivity of all phosphors (Fig. 5b) is moderate at ambient temperature (dτ/dT 0.25, 0.24 and

0.21 %/K at 25 °C for Han blue, strontium copper silicate and Egyptian blue, respectively).

These values are significantly lower than those for recently reported chromium(III)-activated

yttrium aluminum borate (0.8%/K)21

but are comparable to e.g. Ruby (∼0.22 %/K)20

or

manganese(IV)-activated magnesium fluorogermanate (∼0.25 %/K)33

which are used as

thermographic phopshors. Interestingly, the dynamic range of the phosphors is significantly

different: Han blue operates at lower temperatures than Egyptian blue and strontium copper

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silicate. The luminescence of Han blue is almost completely quenched at 220 °C but the other

phosphors are still luminescent at temperatures above 300 °C. Importantly, the sensitivity

significantly improves at higher temperatures (Fig. 5c). For example, the sensitivities as high

as 1 %/K are achieved for Han blue, strontium copper silicate and Egyptian blue at 154, 283

and 303 °C, respectively. About 30-34 % of the original luminescence intensity is retained at

these temperatures which insures good signal-to-noise ratios. These observations underline

the excellent suitability of the investigated materials as thermographic phosphors operating at

100-400 °C. Importantly, no hysteresis of the calibration was observed and the plots were

identical for the cycles of increasing and decreasing temperature. Despite moderate

sensitivities at ambient temperatures, the phosphors enable the temperature compensation of

optical sensors (e.g. pO2, pH, pCO2 etc.). They are particularly attractive as temperature

probes for ratiometric sensing relying on absorption-based indicators since variations of the

luminescence intensity due to temperature-induced quenching are not relevant in this sensing

scheme.

0

50

100

150

200

0

1

2

3

4

-50 0 50 100 150 200 250 300 350 4000.0

0.2

0.4

0.6

0.8

1.0

Dec

ay

tim

e, µ

s (a)

(c)

(b)

dττ ττ/

dT

, %

/K

Han Blue

SrCuSi4O

10

Egyptian Blue

Norm

. In

ten

sity

, a

.u.

Temperature, °C

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Figure 5. Temperature dependence of the luminescence decay time (a) and luminescence

intensity (c) and the temperature sensitivity of the luminescence decay time (b) for the

phosphors.

Conclusions

We have demonstrated that calcium/strontium/barium copper silicates represent very

promising phosphors for application in optical sensing materials. Particularly, they can be

used as inert references for ratiometric and DLR readouts in combination with NIR

fluorescent indicators and as secondary emitters for the ratiometric luminescence readout of

absorption-based indicators. The latter scheme provides unique possibility of using very

simple and inexpensive instrumentation for the sensor readout. The phosphors favorably

compare to other materials used as references in optical sensors due to the combination of

several unique features such as broad excitation spectra in green-red part of the

electromagnetic spectrum, strong NIR emission, long luminescence decay times, high

chemical and photochemical stability, and inertness to oxygen. It is also important to note that

all the phosphors can be easily prepared from low cost materials and Egyptian blue and Han

blue are also commercially available. The phosphors can be additionally used to optically

access temperature for such sensors (albeit with moderate sensitivity at ambient temperatures)

or as high temperature thermographic phosphors if used without other additives. It can be

concluded that the ancient pigments finally found a new life as components of high

performance sensing materials. Other potential applications of these exciting phosphors in

optical materials are yet to be discovered.

AUTHOR INFORMATION

Corresponding author. Tel.: +43 316 873 32516. Fax: +43 316 873 32502. E-mail:

sergey.borisov@tugraz.at

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Graphical Table of Content Figure

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