Electrochemical-to-Optical Signal Transduction for Ion-Selective

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Electrochemical-to-Optical Signal Transduction for Ion-Selective Electrodes with Light-Emitting Diodes Jingying Zhai, Liyuan Yang, Xinfeng Du, and Xiaojiang Xie Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03213 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 30, 2018

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Analytical Chemistry

Electrochemical-to-Optical Signal Transduction for Ion-Selective Electrodes with Light-Emitting Diodes Jingying Zhai, Liyuan Yang, Xinfeng Du, and Xiaojiang Xie* Department of Chemistry, Southern University of Science and Technology, Shenzhen, 518055, China Email: [email protected] Abstract. Optical ion sensors normally have a relative narrow sensitive detection window. Here, based on multicolor light-emitting diodes (LEDs), we report on an electrochemical-to-optical signal transduction scheme under chronoamperometry control to convert the potentiometric response of ion-selective electrodes (ISEs) to optical output with tunable sensitivity and much wider response range. The sensing principle was demonstrated on K+, Ca2+ and Pb2+. LED light intensity was found to depend linearly on the concentration of monovalent ions. Optical signals could be captured with photomultiplier tubes or digital cameras and a visual alarming system to monitor abnormal ion concentration was also developed from super-Nernstian electrodes. could provide direct optical readout of potentiometric signals.21 The signal transduction of an electrochemical sensing event

The electrode potential of a K+ selective ISE was used to

to an easily quantifiable signal is of great importance in analytical

modulate the ECL process in linear scan voltammetry mode,

science. Electrochemiluminescence (ECL) is a one of the most

leading to ECL peak shifts with changing sample concentrations.

recognized methods to transduce electrochemical signals to an

An additional advantage of such an optical ion sensor is the clean

optical output.

1-3

Various analytical platforms have been

established based on ECL to detect both electroactive and nonelectroactive species.

4-6

ECL based optical ion sensors have also 2+ 7

2+ 8

2+ 9

2+ 10

been demonstrated including for Hg , Mg , Pb , Cd , and 2+ 11

spatial separation of the interrogation chamber from the sample to avoid optical interference from the sample. However, to obtain reliable ECL signals for quantitative analysis is not a trivial task. The electrode reaction of the

Co . Notably, methods to analyze alkali and alkali earth metal

sacrificial luminescent reagents requires rigorous control

ions based on ECL and other electrochemical-to-optical signal

including the reagent concentrations, temperature, and electrode

transductions remain very limited.

surface.1 The aqueous solution containing the luminescent

The response ranges of conventional optical ion sensors are 12

quite narrow, including fluorescent indicators, sensors,

7-11

ECL ion

the analytical device. It would be a great simplification if highly

13

robust optical reporter such as a light-emitting diode (LED) could

genetically encoded biosensors (e.g. GCaMP), and optodes.14-16

reagents further restrains the portability and miniaturization of

Ion-selective

be readily integrated into optical ion sensors. As fully integrated

electrodes (ISEs) is a well-known family of ion sensors with

electroluminescent devices themselves, LEDs are inexpensive,

detection range covering several orders of magnitude and a

physically much more robust, and have low energy consumption

number

and long lifetime.

ionophore-based

of

applications.

ion-selective

biomedical,

environmental,

and

industrial

17

Mostly, LEDs were incorporated into optical sensors as

Converting the potentiometric signal to optical readout

light source, but they were only used in few cases as reporters.

could enable the sensors as low-cost point-of-care or alarming

Previously, Diamond and co-workers presented optical sensor for

devices. Gooding and co-workers previously realized such signal

pH, Cd2+, and Pb2+ with LEDs serving as both light source and

transduction through electrochromic effect for pH and resistive-

detector.22-23 More recently, Li and Wang and co-workers

based sensors.18-19 Bakker and co-workers recently converted

incorporated LEDs into split bipolar electrochemical cells to

potentiometric signals into colorimetric readout through the

detect electroactive targets including H2O2, ascorbic acid, and

20

turnover of a redox indicator with a closed bipolar electrode. In

glucose.24 Limited by the redox rate of the reaction, the current

addition to color changes, coupling ECL to ionophore-based ISEs

passing through the LED changed with different analyte

ACS Paragon Plus Environment

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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concentration. In principle, the current could change with

D-NPOE and 33.8 wt% of PVC in 1.5 mL THF. For Pb2+-

concentration polarization and electrode reaction rates, indicating

selective membrane, the membrane optimized for low detection

that for quantitative analysis, the LED brightness should only be

limits contained 0.73 mmol/kg of lead ionophore IV, 0.33

acquired at steady states or less rigorously, with the same time

mmol/kg of NaTFPB, 11.05 mmol/kg of ETH 500, 33.7 wt% of

delay. The same group also reported an excellent work where

DOS and 64.92 wt% of PVC. The cocktail solutions mentioned

they adopted LEDs as signal reporter for ion-selective field-effect

above were then poured into glass rings (22 mm in diameter)

transistor, enabling the highly sensitive and potentially portable

placed on glass slides, respectively. The cocktails were dried

+

-

+ 25

detection of H , Cl , and K .

overnight at room temperature in a dust-free environment. Small

Herein, we present for the first time a highly sensitive

disks were punched from the cast films and conditioned for 2

electrochemical-to-optical signal transduction scheme for

days in corresponding solutions (10-4 M KCl for K+- selective

potentiometric ISEs with LEDs under chronoamperometry

membrane, 5×10-9 M Pb(NO3)2 diluted by 10-4 M HNO3 for Pb2+-

control. Using K+, Pb2+, and Ca2+ as model ions, the activities of

selective membranes, 10-6 M

the ions were directly reflected on the brightness of the LEDs in

membranes). The conditioned membranes were mounted

real time. A wide linear response range was found for monovalent

separately

in

Ostec

electrode +

CaCl2 for Ca2+-selective bodies

(Ostec,

Sargans,

-4

ions, which allowed the successful determination of potassium

Switzerland). For the K ISE, 10 M KCl was used as inner filing

level in fresh water using the standard addition protocol.

solution. For the Ca2+ ISE, 10-2 M Na2EDTA, 10-4 M CaCl2,

Increasing the applied voltage in chronoamperometry mode was

adjusted to pH 7.0 with NaOH, was used as inner filling solution.

found to transform this linear response to classical Nernstian

For the Pb2+ ISE with a low detection limit, 10-2 M Na2EDTA,

response. The variation of the LED brightness also formed a

10-4 M Pb(NO3)2, adjusted to pH 7.0 with NaOH, was used as

visual alarming system to monitor abnormal ion concentrations,

inner filling solution.

which was especially the case for ISEs with super-Nernstian Instrumentation and Measurements. The AlGaInP LEDs were

response.

purchased from Thorlabs. Chronoamperometry experiments

EXPERIMENTAL SECTION Reagents.

Valinomycin

(potassium

were performed with an Autolab PGSTAT101 (Metrohm ， ionophore

I),

tert-

Switzerland). As shown in Figure 1, the working and the counter

(lead

were connected separately to the two ends of a resistor R2 (10

N,N-dicyclohexyl-N′,N′-dioctadecyl-3-

kΩ), which was in parallel with the LED and a resistor R1 (1 kΩ).

butylcalix[4]arene-tetrakis(N,N-dimethylthioacetamide) ionophore

IV),

oxapentanediamide

(calcium

potassium

The ISEs were connected to the reference channel of the

tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (KTFPB), sodium

potentiostat while a Ag/AgCl reference electrode served as an

tetrakis[3,5-bis(trifluoromethyl)phenyl]borate

(NaTFPB),

electric bridge. The LED light intensity and spectrum were

poly(vinyl chloride) (PVC), bis(2-ethylhexyl)sebacate (DOS),

recorded by a photomultiplier tube (PMT) in a fluorescence

dodecyl 2-nitrophenyl ether (D-NPOE), tetradodecylammonium

spectrometer (Fluorolog-3, Horiba Jobin Yvon). Images of the

tetrakis(4-chlorophenyl)borate

tetrahydrofuran

LEDs were captured with a digital camera (Canon EOS 5D Mark

(THF), potassium nitrate (KNO3), potassium chloride (KCl), lead

IV) and analyzed with the software ImageJ. Activity coefficients

nitrate (Pb(NO3)2), calcium chloride dihydrate (CaCl2·2H2O),

were calculated from the extended Debye–Hückel equation.

ethylenediaminetetraacetic

ionophore

(ETH

acid

500),

disodium

IV),

salt

dehydrate

(Na2EDTA), nitric acid were obtained from Sigma-Aldrich. All

R1

the solutions were prepared with deionized water (Mili-Q). +

ISEs Preparation. The K - selective membrane was prepared by

E

V

reference electrode

LED

R2

PMT or Digital Camera E0

dissolving the mixture composed of 10 mmol/kg of potassium ionophore I, 1.6 mmol/kg of KTFPB, 32.5 wt% of PVC and 66.2 wt% DOS in 1.5 mL THF. The Ca2+-selective membrane was

sample solution

ion-selective electrode

prepared by dissolving the mixture composed of 16.9 mmol/kg

Figure 1. Schematic illustration of the electrochemical-to-optical

of calcium ionophore IV, 6.9 mmol/kg of NaTFPB, 64.2 wt% of

signal transduction principle under chronopotentiometry control.


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RESULTS AND DISCUSSION Figure 1 shows the signal transducing scheme. In chronoamperometry mode, a voltage (E0+∆E) was applied to a resistor R1 and an LED in series, paralleled with another resistor R2. This voltage was modulated by the potential difference (∆E) between the ISE and the reference electrode (in this case, an Ag/AgCl electrode). The ISE was connected to the reference channel of the potentiostat and the Ag/AgCl electrode only served as an electric bridge. Notice that in this configuration, there is no current flow between the reference and the ionselective electrode. The open circuit potential ∆E was only measurable in the presence of the resistor R2 due to the LED diode feature. However, current does flow through the resistors and the LED, and the light emitted from the LED can be analyzed by a photomultiplier tube (PMT), or less costly with a digital camera, and even with naked eyes. Semiconductor LEDs made from AlGaInP were used here and their characteristics have been well understood. At relatively small forward voltage, the current passing through the LED follows the Shockley diode equation as expressed in Eqn. 1,

(

i = is e

k Vf

)

−1

(1)

where i represents the current, is the saturation current, Vf the forward voltage across the LED, and k a constant related to the thermal voltage and the ideality factor of the LED.

Figure 2. (a) Theoretical response of the LED light intensity to different activities of monovalent and divalent ions, calculated from Eqn. 5. is=10-20 A, E0=1.2 V, Eθ = 0.1 V. (b) Influence of k on the linearity of the response curve (represented by R2) for monovalent (+1) and divalent ions (+2). (c) Influence of s on the linearity of the response curve (represented by R2). k=37. to LED internal resistance. Figure S1 showed a typical i-V curve and the relationship between light intensity and current pass

Electric circuit analysis from Figure 1 provided the

through the LED.

relationship between the forward voltage Vf and ∆E, as

Theoretical optical response to the activity of ions was

expressed in Eqn.2, where R1 is the ballistic resistor to limit the

simulated from Eqn. 5 and shown in Figure 2, with parameters in

V f + iR1 = E 0 + ΔE θ

ΔE = E + s log aM n+

(2)

the figure caption. Clearly, a linear response for monovalent ions

(3)

was expected which the linearity was poor for divalent ions

current passing through the LED. As expressed in Eqn. 3, the

(Figure 2a). However, it should be noted that this linearity

open circuit potential ∆E depends on the activity of the target

(assessed with R2) is in fact pseudo linearity which is only

ion Mn+ according to the Nernstian equation, where Eθis the standard potential and s is the slope (ca. 59.2 mV for monovalent

expected for a certain range of k values (Figure 2b). The common k values for semiconductor LEDs are around 20 to 40 V-1, which is in the linear range for monovalent ions. For divalent ions, the

ions and ca. 29.6 mV for divalent ions).

linearity was quite poor (with a k value of 37). The result of our

i = is × e

k (Eθ +E 0 +s log a

M n+

)

calculation indicated that the k values should be around 75 V-1 to

(4)

Combining the Eqn. 1, 2 and 3 resulted in Eqn. 4 to express

achieve linear optical response for divalent ions. Figure 2c shows

the current passing through the LED when its magnitude is small.

that for a LED with k of 37, the linearity changes with the number

Therefore, the input power of the LED (P) can be expressed in

of s. A good linear response was expected for s values around 60

Eqn. 5. The light intensity is proportional to input power below

mV, which is close to the Nernstian slope (59.2 mV) for

the saturation current while at higher forward bias, the levelling

monovalent ions at 298 K.

off of the LED i-V curve renders the relationship more linear due

P = is (E θ + E 0 + s log aM n+ )e

k (Eθ +E 0 +s log a

M n+

)

(5)



Figure 4. (a) Light intensity at 625 nm of the red LED monitored with PMT where the sample K+ concentrations increased as indicated. (b) The linear calibration of the light intensity against K+ activity. E0: 1.4 V. analytical platform reported here is suitable for continuous monitoring of concentration changes because the light emitted from LEDs remains highly stable in time. The light intensity variation is essentially dictated by the electrode potential and the response time depends on how fast the ISEs respond to sample changes. For potentiometric ISEs, the response time could readily +

Figure 3. (a) Emission spectra of the LEDs coupled to K selective ISEs at different K+ concentrations as indicated. (b) Plot of the emission maxima of the LEDs as a function of logarithmic K+ activity. (c) Images of the LEDs captured with digital camera at different K+ concentrations. E0 for the red, orange, and green LEDs were chosen at 1.5, 1.6, and 2.0 V, respectively.

reach a few milliseconds.17 In contrast, ECL is closely related to the mass transport of the luminescence reagents and the reaction rate at the electrode surface, which could complicate the time evolution of the light intensity. Previous reported ion-selective chronopotentiometric methods could also differentiate small ion

The optical response toward monovalent target ions was

concentration changes with a linear response.26-27 However,

experimentally evaluated with K+ selective ISE containing

continuous monitoring with chronopotentiometry could be

valinomycin as the ionophore. LEDs with three different colors

limited by the sampling time (typically more than a few seconds).

(centered at 625, 590, and 525 nm, respectively) were integrated

The linear optical response allowed us to determine the K+

+

into the circuit, respectively. The K concentration in the aqueous sample was varied from 10-6 M to 10-1 M (within the Nernstian response range). Figure 3a shows the emission spectra of the LEDs recorded at various K

+

concentrations. Indeed, an

exponential increase of the emitted light intensity was observed. As shown in Figure 3b, the response curve from the LEDs of the three colors exhibited high consistence, which also confirmed similar ideality of the semiconductor LEDs. Figure 3c shows the pictures of the LEDs at K+ different concentrations. The light intensity changes were readily observable by naked eyes. Figure 4 shows the kinetic mode recording of the LED light intensity at 625 nm where the K+ concentration in the sample

concentration in fresh water by following the standard addition analytical protocol. The experiments were performed with four slightly different E0 values (1.23, 1.29, 1.35, 1.41 V). The measured K+ concentration (0.106±0.002 mM) was highly reproducible and consistent with the results found in direct potentiometry (0.106±0.001 mM, Figure S4 and Table S1). When operating at much higher input voltage E0 (e.g. >2 V), the i-V curve of typical semiconductor LEDs starts to follow the Ohm’s law. As shown in Figure 5, a response similar to the Nernstian relationship was observed for K+ between the light intensity and the logarithm of the sample ion concentration. Therefore, the selection of the E0 value became quite important.

solution increased from 1 to 5 mM. Normally optical ion sensors

The dependence of the optical response curves on E0 values

have relative narrow sensitive detection range. Here, the linear

was evaluated with Pb2+ ISEs. As shown in Figure 6, upon

response range was found to be much wider, as shown in Figure

increasing the input voltage E0 from 1.1 V to 1.6 V, the response

S2. The sensitivity region could be easily tuned from 1~5 mM to

curve indeed underwent an S shape transition where the R2 value

0.1~0.5 mM by changing the input voltage E0 (Figure S3). The


Page 4 of 8

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Figure 5. Optical response of the K+ selective system to various K+ concentrations at different applied E0 values. (a): 1.5 V, (b): 2.0 V represented the linearity (light intensity versus logarithmic activity). At high E0, Nernstian responses were observed. The Nernstian response gradually disappeared by lowering the E

0

value, resulting in exponential responses where on the other hand, the sensitivity increased. Practically, it is important to select appropriate E0 values such that enough sensitivity is reached for the intended application.

Figure 7. (a) Pictures of the LEDs for the ion sensor containing Pb2+ selective ISEs at various Pb2+ activities as indicated. (b) Relative brightness of the LEDs in (a) analyzed with ImageJ. (c) Images of the LED in the sensor containing Ca2+ selective ISEs with super-Nernstian response at various Ca2+ activities as indicated. (d) Relative brightness of the LEDs in (c) analyzed with ImageJ. Besides recording the LED intensity with PMT, digital cameras also could be used for signal analysis. The use of digital camera

(even on mobile phones) combined with quantitative

image analysis could greatly reduce the overall setup cost.28 Here, pictures of the LEDs were taken in the dark to reduce background. As shown in Figure 7a, the light intensity difference at various Pb2+ concentrations (from 10 nM to 100µM) was clearly observable. The intensity of the pictures was readily extracted with the software ImageJ to obtain the calibration curve (Figure 7b). For Pb2+ ions, a vivid red color appeared observable by naked eyes at above 0.1 µM Pb2+ concentrations, making this a potentially useful alarming platform for Pb2+. Admittedly, to act as an alarming system, it is more desirable if the LED could abruptly light up once a certain concentration threshold was exceeded. In that case, ISEs with super-Nernstian response become even more compatible with the reported methodology.29 For electrodes with super-Nernstian response, a large potential jump could occur at a certain concentration range. Previously,

Figure 6. (a) Optical calibration curves of a Pb2+ selective sensor with E0 value from 1.1 V to 1.6 V as indicated. (b) A function of the R2 value (obtained from linear data fitting in (a)) against the applied E0 values.

Pretsch and co-workers demonstrated that property adjustment of the electrode membrane composition and the inner solution could lead to large potential step at a specific sample activity.29 As shown in Figure 7c and 7d, when calcium electrodes with super-



Nernstian response were used, a much higher contrast in the LED -6

-5

intensity was observed between 10 M and 10 M of Ca

2+

ions.

2004, 104, 3003-3036. 2.

Zhai, Q.; Li, J.; Wang, E., Recent Advances Based on

The Ca2+ ISE was obtained by using calcium ionophore IV as the

Nanomaterials as Electrochemiluminescence Probes for the

ionophore and EDTA in the inner filling solution (see

Fabrication of Sensors. ChemElectroChem 2017, 4, 1639-1650.

experimental section).

3.

Valenti, G.; Rampazzo, E.; Kesarkar, S.; Genovese, D.;

Fiorani, A.; Zanut, A.; Palomba, F.; Marcaccio, M.; Paolucci, F.;

CONCLUSIONS

Prodi, L., Electrogenerated chemiluminescence from metal

To summarize, a methodology to convert potentiometric ISE responses to optical signals with LEDs was presented. The

complexes-based nanoparticles for highly sensitive sensors applications. Coord. Chem. Rev. 2018, 367, 65-81.

forward voltage of the LEDs was modulated by the electrode

4.

potential in chronoamperometry control mode. The LED

Nanopore-Based Electrochemiluminescence for Detectionof

intensity showed high sensitivity in real time upon ion

MicroRNAs via Duplex-Specific Nuclease-Assisted Target

concentration changes, where the most sensitive region could be

Recycling. ACS Appl. Mater. Interfaces 2017, 9, 33360-33367.

readily adjusted with input voltage E0. The optical response

5.

exhibited linear dependence on the activity of monovalent ions,

Forster, R. J.; Rusling, J. F., Electrochemiluminescent Array to

enabling the use of standard addition for quantitative analysis.

Detect Oxidative Damage in ds- DNA Using [Os(bpy)2(phen-

The potassium level in fresh water was successfully determined.

benz-COOH)]2+/Nafion/Graphene Films. ACS Sens. 2016, 1,

The light from the LEDs could be captured with digital cameras

272-278.

and readily analyzed from image processing software such as

6.

ImageJ. A visually observable alarming system was also built on

Q.; Pang, X., Sandwich-Type Electrochemiluminescence Sensor

calcium selective electrodes with super-Nernstian response.

for Detection of NT-proBNP by Using High Efficiency Quench

ACKNOWLEDGEMENTS

Strategy of Fe3O4@PDA toward Ru(bpy)32+ Coordinated with

The authors thank the Chinese Thousand Talent program and the Shenzhen government for financial support. We acknowledge Prof. Fangyi Chen for discussion on the circuit analysis.

Huo, X.-L.; Yang, H.; Zhao, W.; Xu, J.-J.; Chen, H.-Y.,

Bist, I.; Song, B.; Mosa, I. M.; Keyes, T. E.; Martin, A.;

Shi, L.; Li, X.; Zhu, W.; Wang, Y.; Du, B.; Cao, W.; Wei,

Silver Oxalate. ACS Sens. 2017, 2, 1774-1778. 7.

Jiang, X.; Wang, H.; Wang, H.; Yuan, R.; Chai, Y., Signal-

Switchable Electrochemiluminescence System Coupled with Target Recycling Amplification Strategy for Sensitive Mercury

ASSOCIATED CONTENT

Ion and Mucin 1 Assay. Anal. Chem. 2016, 88, 9243-9250.

Supporting Information

8.

Additional information as noted in the text include: determination

Biosensing of Mg2+ Dependent DNAzyme Triggered Ratiometric

of K+ concentration in real fresh sample, LED L-i-V curves and

Electrochemiluminescence. Anal. Chem. 2014, 86, 5158-5163.

supplementary figures. This material is available free of charge

9.

via the Internet at http://pubs.acs.org.

J., Colorimetric and Electrochemiluminescence Dual-Mode

AUTHOR INFORMATION

Sensing of Lead Ion Based on Integrated Lab-on-Paper Device.

Corresponding Author

ACS Appl. Mater. Interfaces 2018, 10, 3431-3440.

* Email: [email protected]

10.

ORCID

Electrogenerated chemiluminescence behavior of Tb complex

Xiaojiang Xie: 0000-0003-2629-8362

and its application in sensitive sensing Cd2+. Electrochim. Acta

Notes

2017, 228, 1-8.

The authors declare no competing financial interest.

11.

Cheng, Y.; Huang, Y.; Lei, J.; Zhang, L.; Ju, H., Design and

Xu, J.; Zhang, Y.; Li, L.; Kong, Q.; Zhang, L.; Ge, S.; Yu,

Zhu, Y.; Zhao, M.; Hu, X.; Wang, X.; Wang, L.,

Chen, H.; Li, W.; Wang, Q.; Jin, X.; Nie, Z.; Yao, S.,

Nitrogen doped graphene quantum dots based single-luminophor

ACKNOWLEDGEMENTS The authors thank the start fund of SUSTC and the Thousand Talents Program of China for financial support.

generated dual-potential electrochemiluminescence system for ratiometric sensing of Co2+ ion. Electrochim. Acta 2016, 214, 94102. 12.

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Electrochemical-to-Optical Signal Transduction for Ion-Selective

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