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Non-covalently functionalized graphene for the potentiometricsensing of zinc ions

Ewa Jaworska, Wiktor Lewandowski, J�ozef Mieczkowski, Krzysztof Maksymiuk and Agata Michalska*

Received 26th October 2011, Accepted 26th January 2012

DOI: 10.1039/c2an16016a

The possibility of the application of non-covalently functionalized graphene as a sensing membrane for

the potentiometric determination of zinc ions was examined. A graphene carboxylic derivative was

functionalized with 1-(2-pyridylazo)-2-naphthol, the Zn2+ ions complexing ligand, simply by

adsorption of ligand molecules due to p–p interactions. This approach has resulted in a potentiometric

sensor characterized with significant selectivity for Zn2+ ions present in solution.

Introduction

Despite the need for the direct determination of zinc cations

activity, potentiometric techniques can hardly offer a solution to

this analytical problem. Although various electrochemical

methods are available for the determination of Zn2+ content, the

activity of this element is still an interesting parameter from

a biological and/or medical point of view. Decades ago analytical

chemistry developed highly selective ligands enabling the spec-

trophotometric determination of metal ions, including Zn2+ ions

in the presence of alkali or alkali earth metals,1 e.g. 1-(2-pyri-

dylazo)-2-naphthol (PAN), Fig. 1. This compound is virtually

insoluble in water and is used for the spectrophotometric deter-

mination of metal ions. Due to its excellent Zn2+ complexing

properties in neutral pH it is of interest in the context of zinc

cations potentiometric sensors (as an alternative to solvent

polymeric membranes ones).

Recent reports on the application of nano-structured materials

for potentiometric sensing purposes2 point out the advantages of

novel structures. In the case of carbon-based materials one of the

most interesting alternatives for a traditional approach, although

not much explored yet, is the possibility of non-covalent func-

tionalization taking advantage of p–p stacking interactions

between aromatic hydrocarbon rings. These interactions are

Fig. 1 Structure of 1-(2-pyridylazo)-2-naphthol (PAN).

Department of Chemistry, Warsaw University, Pasteura 1, 02-093Warsaw, Poland

This journal is ª The Royal Society of Chemistry 2012

spontaneous and, in the case of carbon nanotubes, have led to

significant analytical advantages.3–5

Another promising carbon material is graphene, for which,

however, not many analytical applications have yet been

proposed.6 To the best of our knowledge this is the first report on

the application of graphene layers for potentiometric sensing

purposes. Nevertheless, non-covalent modification of graphene/

graphene oxide was achieved for number of molecules/

compounds, conjugated or aromatic,7 including poly(sodium 4-

styrene sulfonate),8 sulfonated polyaniline,9 porphyrin,10,11

Congo red12 and others; these clearly show the high potential of

this approach.

Herein, preliminary results on a novel approach allowing the

utilization of a highly selective zinc ion ligand in the presence of

alkali metals ions to yield potentiometric Zn2+ sensors are

reported. As a membrane material graphene (rearranged

graphite oxide) was used.

Experimental

Reagents and apparatus

All reagents used were analytical grade salts and were from

POCh (Poland) or Sigma. Doubly distilled and freshly deionised

water (resistance 18.2 MUcm, Milli-Qplus, Millipore, Austria)

was used throughout this work.

Glassy carbon (GC) electrodes of surface area 0.07 cm2 were

used, and before application of the graphene layer the electrodes

were polished using sand paper of fine grit (1000); however,

mirror smooth polishing was avoided.

In the potentiometric experiments and electrochemical

measurements, the experimental setup is as described earlier, e.g.,

as in ref. 13 (multi-channel data acquisition setup and software,

Lawson Labs. Inc. (3217 Phoenixville Pike, Malvern, PA 19355,

USA), the pump systems 700 Dosino and 711 Liquino (Met-

rohm, Herisau, Switzerland) as well as galvanostat-potentiostat

CH-Instruments model 660A (Austin, TX, USA) was used). The

recorded potential values (vs. the double junction silver/silver

Analyst, 2012, 137, 1895–1898 | 1895

Fig. 2 Potentiometric responses of electrodes with (a) graphene non-

covalently functionalized with PAN according to procedure 1 and (b)

non-functionalized graphene, recorded in (:,;,=) ZnSO4, (C) CaCl2,

(-) NaCl and (+) KCl solutions.

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chloride reference electrode with 1 M lithium acetate in the outer

sleeve (M€oller Glasbl€aserei, Z€urich, Switzerland)) were corrected

for the liquid junction potential calculated according to the

Henderson approximation.

Carboxy-functionalized graphene derivative (G)

The carboxy-functionalized graphene derivative has been

synthesized according to the previously reported method.14 In

short, graphite oxide was obtained via a modified Hummer’s

method15 without the use of silver nitrate and was then reacted

with N,N-dimethylacetamide dimethyl acetal (2 times the molar

equivalent of the oxygen content of graphite oxide, at 150 �C, inbis(2-methoxyethyl)ether, under inert atmosphere). The

obtained material was extensively washed and subsequently

reacted with aqueous potassium hydroxide solution to yield the

carboxy-functionalized graphene derivative (G4 in ref. 14).

Non-covalent functionalization of graphene

Procedure 1. 40 ml of an aqueous solution of G (pH¼ 7, 2.2 mg

mL�1) was applied to the electrode, while in the upside down

position, and left overnight in the laboratory atmosphere for

water evaporation. Onto the thus obtained layers portions of 1-

(2-pyridylazo)-2-naphthol (PAN) solution were drop-cast –

allowing the preceding solvent portion to evaporate before

application of the next – and in total 3 application cycles were

used each involving application of 1 ml of PAN solution (8 mM

in water/THF ¼ 1 : 1). The ratio of graphene to PAN as applied

per electrode was close to 1 mg : 10�4 mol. After evaporation of

the final portion of the solvent to remove unbound PAN mole-

cules the layers were subsequently washed 3 times, by dipping the

electrode in 1 ml of a water/THF ¼ 1 : 1 mixture for 3 min. This

procedure yields application of 0.09 mg of graphene per elec-

trode, G(1)-PAN.

Procedure 2. 1 mg of G and 1 mg (4.10�6 mol) of PAN were

dissolved in 1 ml of THF; this mixture was then stirred for 3 h

and then left for 24 h. From this mixture 100 ml (in 4 portions)

were applied per electrode, resulting in 0.1 mg of graphene per

electrode, G(2)-PAN.

For a control experiment, unmodified graphene, G, was

applied on a GC substrate, yielding 0.1 mg of graphene per

electrode.

Directly before measurements the electrodes were conditioned

in 0.1 M NaCl for 30 min.

Results and discussion

Potentiometric sensor characteristic

Fig. 2a presents the potentiometric dependences recorded for

G(1)-PAN and G layers, respectively. For graphene modified

with PAN, G(1)-PAN, a linear dependence of potential on the

logarithm of Zn2+ activity was obtained within the range from

10�4 to 10�2 M with a slope lower than Nernstian equal to 21.1 �0.1 mV/dec (R2 ¼ 0.999). A slightly lower potential change, close

to 19 mV, was obtained for the activity change from 10�2 to 10�1

M. The observed responses are most probably facilitated by the

presence of carboxylic groups in the graphene used,14 which can

1896 | Analyst, 2012, 137, 1895–1898

act as an ion-exchanger and (to some extent) compensate the

charge of the zinc–PAN complexes formed. However, it can be

expected that due to the limited number of anionic groups,

solution anions can affect the recorded potentiometric responses,

especially in the high activity range. The detection limit was equal

to 10�4.5 M, which is close to values obtained for other plastic

membrane free potentiometric systems, e.g. ones based on con-

ducting polymer layers, either doped with mobile ions,16,17 con-

taining complexing ligands (e.g.18,19) or ion-exchangers.20

For the G(2)-PAN modified electrodes, similar potentiometric

responses were obtained with slopes of dependence close to

Nernstian, within the range of experimental error, equal to

27.2 � 1.4 mV/dec (R2 ¼ 0.997) for the above given range, with

a detection limit close to 10�4.5 M. Interestingly, these close to

theoretical Nernstian responses were obtained for the system

prepared using a smaller amount of PAN, however, present in

the whole volume of receptor layer, in contrast to topical

application of the ligand on the formed graphene film.

Regardless of the method of graphene modification, the

recorded dependences were stable over at least a few repeated

calibrations recorded. Neither the slope nor the detection limit

was significantly altered, clearly indicating the stability of this

non-covalent functionalization of graphene with PAN mole-

cules. The within-day stability of potential values recorded was

higher for layers prepared according to procedure 1, i.e. more

topical application of PAN, or by procedure 2 and, regardless of

the activity of the Zn2+ solution, was in the range of mV. Slightly

This journal is ª The Royal Society of Chemistry 2012

Fig. 3 Voltammetric curves recorded for electrodes with graphene

modified with PAN, according to procedure 2, in 0.1 M KCl solution, in

the absence (black line) and presence (red line) of 5 mM Fe(CN)64� ions.

Scan rate: 50 mV s�1.

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higher deviation was observed for graphene layer modification

yielding to introduction of PAN molecules into the whole gra-

phene layer volume.

On the other hand, as can be seen in Fig. 2b, unmodified

graphene, G, shows only a slight potential change when the zinc

cations activity in solution is altered, with a slope equal to 7.4� 1

mV/dec, within the range from 10�4 to 10�2 M. The clear

difference in the potentiometric response pattern in Zn2+ solu-

tions between non-covalently PAN-functionalized and unfunc-

tionalized graphene layers can be attributed to the effective

presence of PAN on the graphene which modifies the electrode

substrate.

The beneficial effect of graphene modification with PAN is

clearly seen as an increase of selectivity for Zn2+ compared to the

model interfering cations tested, Fig. 2a and 2b. In the case of

graphene layers modified with PAN, G(1)-PAN (Fig. 2a),

potentials recorded in Zn2+ ions solution were significantly higher

compared to values recorded in the presence of interferent (K+,

Na+, Ca2+) solutions of similar activities. This effect is attributed

to the properties of the applied zinc complexing ligand, similarly

as observed earlier, e.g. for conducting polymers,16,19,20 where

doping with complexing ligands led to increased potentiometric

selectivity. On the other hand, for graphene layers unmodified

with PAN, in the presence of tested interferents, significantly

higher potential values were obtained compared to those recor-

ded in Zn2+ ions solutions (Fig. 2b), pointing to the lack of

selectivity; however, proving cation exchanging properties of

graphene layers. The logarithms of selectivity coefficients for the

PAN-modified graphene layer G(1)-PAN (calculated using the

Nernstian slope value and potentials recorded for activities from

10�1 to 10�4 M, separate solution method) were equal to �4.9 �0.05, �3.7 � 0.5, �2.6 � 0.3 for K+, Na+, Ca2+, respectively,

whereas for the graphene modified according to procedure 2

(G(2)-PAN) calculated log KZn,J (for potentials recorded under

the same experimental conditions) were equal to �2.0 � 0.1,

�2,0 � 0.1, �1.5 � 0.1 for K+, Na+, Ca2+, respectively. The

difference in recorded selectivities clearly points out the superi-

ority of the sensor preparation procedure 1 as yielding potentio-

metric selectivity for Zn2+. The comparison of potentiometric

selectivity for Zn2+ ions obtained for unmodified and modified

graphene clearly proves the effectiveness of non-covalent func-

tionalization with PAN.

Due to the presence of carboxylic groups on the graphene

sheets, the potentiometric responses of both unmodified and

modified graphene layers for pH change are possible. Indeed the

change of pH of the solution within the range from 3.6 to 9.2 has

resulted in potential change with a slope close to 35 mV/dec,

regardless o whether or not the tested layer was modified with

PAN. It should be also stressed that the overall change of the

solution pH accompanying the change of Zn2+ ions activity from

0.1 M to 10�5 M was equal to 0.7 units. Thus, if there is a small

contribution of this effect to the recorded potentiometric

responses (similarly as observed for e.g. conducting poly-

mers16,21), the overall potential change observed for Zn2+ ions

solutions of altered activity cannot be attributed solely to the

change of the solution pH.

For the electrodes with a graphene layer, similarly as for other

conducting materials that have been applied as potentiometric

sensing membranes,5,16,18 the change of the sample solution redox

This journal is ª The Royal Society of Chemistry 2012

potential is expected to result in a change of the recoded poten-

tial. Indeed, regardless of the modified (either procedure 1 or 2)

or unmodified graphene layer applied, a change of the

Fe(CN)63�/Fe(CN)6

4� concentration ratio from 0.1 to 10 (with

the lower concentration of each reaching 0.01 M, while main-

taining constant total ionic strength due to addition of KCl to

concentration of 1 M) resulted in Nernstian behavior within the

range of experimental error, slope of potential dependence on

logarithm of the ratio of oxidized and reduced form concentra-

tions, as expected for a conducting membrane present on the

substrate electrode.

Voltammetric and electrochemical impedance studies

The electroactivity of the above described electrodes covered by

graphene was tested under voltammetric conditions. Fig. 3

presents curves recorded for the electrode with graphene modi-

fied with PAN (procedure 2), in 0.1 M KCl solutions, in the

potential range from 0 to 0.5 V. In the absence of added redox

reactant a quasi-capacitive response was recorded for potentials

lower than 0.3 V, whereas in the presence of 5 mM K4Fe(CN)6a pair of peaks appeared. The potential peak difference is slightly

over 0.1 V and its higher than Nernstian value can point to the

influence of ohmic resistance of the modifying layer. Assuming

a fast charge transfer reaction and taking into account the peak

current and either cathodic or anodic peak shift due to ohmic

effect in the range 20–30 mV, the approximate calculated resis-

tance is close to 1 kU. Similar results were obtained for the

electrode with graphene but in the absence of PAN (results not

shown). The observed electroactivity explains the high influence

of redox reactants on the potentiometric responses, as described

above.

The graphene modified electrodes were also studied by elec-

trochemical impedance spectroscopy. Spectra were recorded in

0.1 M KCl, in the frequency range from 0.01 to 105 Hz, using an

amplitude of 20 mV at a potential of 0.3V. Fig. 4 presents Bode

plots for the electrodes with graphene in the absence and pres-

ence of PAN (procedure 2). In both cases a systematic decrease of

impedance with increasing frequency can be observed, with

higher impedance at lower frequency for the PAN-free graphene.

Analyst, 2012, 137, 1895–1898 | 1897

Fig. 4 Bode plots recorded for electrodes with graphene (red lines) and

graphene modified with PAN, according to procedure 2 (black lines).

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On the other hand, the phase angle changes non-monotonically

with a minimum at around 103 Hz, where the phase angle is not

far from 0�. At this frequency the recorded impedance can be

close to the ohmic resistance of the layer, approximately 1 kU,

being consistent with the voltammetric data. A high impedance

at low frequencies probably results from the presence of

a blocked interface (electronic conductor/ionically conducting

solution), whereas a decreasing impedance and phase angle shift

to more negative values for highest frequencies points to the

influence of capacitance. Extrapolating the results to higher

frequencies, above 105 Hz, suggests a capacitance,C (C¼ 1/(uZ),

where Z is the impedance) close to 10�6 F, i.e. a typical double

layer capacitance for carbon electrodes.

Conclusions

The unique properties of graphene are, among many other,

interesting for potentiometric applications. One of the attractive

1898 | Analyst, 2012, 137, 1895–1898

opportunities to explore is potentiometric sensing

modification taking advantage of adsorption through non-

covalent interactions, mainly due to p–p interaction with an

aromatic moiety containing ligands. For the first time, we have

used this approach to develop a simple, low-cost chemical

sensor for the potentiometric, selective detection of Zn2+

cations.

Acknowledgements

Financial support from National Centre of Science (Poland),

project No. N204 247640, in the years 2011–2014, is gratefully

acknowledged. This work was partially founded

through Foundation for Polish Science MPD Programme

and co-financed by the EU European Regional Development

Fund.

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