Electrochemical determination of maltol in food products using a poly(L-tryptophan) modified glassy carbon electrode

ISSN 1023�1935, Russian Journal of Electrochemistry, 2012, Vol. 48, No. 8, pp. 1–7. © Pleiades Publishing, Ltd., 2014.Published in Russian in Elektrokhimiya, 2012, published in Elektrokhimiya, 2012, Vol. 48, No. 8, pp. @@@–@@@.

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1. INTRODUCTION

Maltol (3�Hydroxy�2�methyl�4H�pyran�4�one) isa naturally occurring compound that can also beformed in the pyrolysis of sucrose and the thermaldegradation of starch [1]. Although maltol is tastelessat low concentrations, it can modify or enhance theflavor of food products when added. Thus, it is widelyused as a flavor enhancer in food products such asbreads, cakes, malt beverages, beers, chocolate milk.However, maltol exhibits a dose�dependent toxicitymediated through apoptosis [2]. Furthermore, maltolcan greatly increase the oral bioavailability of galliumand the aluminum uptake in the body due to its coor�dination with these metals [3, 4]. According to EULegislation, the maximum allowed limit of maltol infood products is 200 mg kg–1 [5]. Therefore, it isimportant to develop convenient and economicalmethods for the reliable determination of maltol infood products.

Various methods have been developed for thedetermination of maltol, such as spectrophotometry[6–8], gas chromatography�mass spectrometry(GC–MS) [9], liquid chromatography�amperomet�ric detection (LC–ED) [10], high performance liquidchromatography (HPLC) [11, 12], and FIA�chemilu�minescence [13]. However, these methods suffer vari�ous drawbacks, such as low sensitivity, the requirementof expensive equipments and large amounts of organicsolvents, time�consuming procedures, which make

them considerably inconvenient for routine analysis.In recent years, electrochemical sensors have attractedwide attention due to its convenience, fastness, highsensitivity, selectivity and reproducibility. Literaturehas reported various modifier based electrodes forenhancing the sensitivity of detection of various mole�cules [14–17]. Hitherto, several voltammetric meth�ods for the determination of maltol using various elec�trodes such as carbon paste electrode [5], hangingmercury drop electrode [18], multi�walled carbonnanotubes modified electrode [19], silica sol–gelmodified electrode [20] have been reported.

Poly(amino acid) modified electrodes haveattracted great interest in electroanalytical fieldbecause of their catalytic and substrate specific char�acteristics [21–23]. Compared with polymers of non�biological origin, poly(amino acid) is more likely tointeract favorably with bio�molecules. When prepar�ing a poly(amino acid) modified electrode, electro�lytic polymerization is a commonly used techniquebecause the thickness and homogeneity of the poly�merized film on the surface of an electrode can be wellcontrolled [21, 24, 25].

In the present study, a poly(L�tryptophan) modi�fied glassy carbon electrode (Ptry/GCE) was preparedthrough electrolytic polymerization. Using the pre�pared Ptry/GCE, a fast and convenient method for thedetermination of maltol was developed.

The developed method shows good sensitivity, reli�ability and environmental friendliness. Its applicabil�ity has been demonstrated by the successful analysis ofmaltol in food samples.

Electrochemical Determination of Maltol in Food Products Using a Poly(L�tryptophan) Modified Glassy Carbon Electrode1

Mingyong Chaoz and Xinying MaDepartment of Chemistry and Chemical Engineering, Heze University, Heze 274015, P.R. China

Received April 12, 2013

Abstract—Herein, a poly(L�tryptophan) modified glassy carbon electrode (Ptry/GCE) for the determina�tion of maltol is fabricated by electrochemical polymerisation. The electrochemical behaviour of maltol atthe Ptry/GCE is studied by cyclic voltammetry (CV). The modified electrode shows excellent electrocatalyticactivity towards the oxidation of maltol and the oxidation is a one�proton�one�electron process. In pH 8.0phosphate buffer solution (PBS), the oxidation peak current of maltol shows a linear relationship with its con�centration in the range from 9.00 × 10–5 to 3.75 × 10–3 mol L–1 with a correlation coefficient of 0.9972. The limitof detection is estimated to be 8.00 × 10–6 mol L–1. The novel method shows good selectivity, recovery, repro�ducibility and great convenience and has been satisfactorily demonstrated in real food sample analysis.

Keywords: maltol, L�tryptophan, modified electrode, cyclic voltammetry

DOI: 10.1134/S1023193514050048

1 The article is published in the original.z Corresponding author: [email protected] (Min�

gyong Chao).

2

RUSSIAN JOURNAL OF ELECTROCHEMISTRY Vol. 48 No. 8 2012

MINGYONG CHAO, XINYING MA

2. MATERIALS AND METHODS

2.1. Reagents

All reagents used were of analytical reagent grade orguaranteed grade and were used as such without fur�ther purification. Maltol was obtained from AladdinChemistry Co., Ltd. (Shanghai, China). L�tryp�tophan was purchased from Acros Organics (USA).

All aqueous solutions were prepared using double dis�tilled water. Maltol standard solution (1.0 × 10–3 mol L–1)was prepared by weighing 0.0063 g of maltol to50.00 mL of double distilled water. Phosphate buffersolutions (PBS) were prepared by mixing stock solu�tions of 0.2 mol L–1 Na2HPO4 and 0.1 mol L–1 citricacid.

2.2. Apparatus

Electrochemical measurements were conducted ona CHI 660C Electrochemical Workstation (Chen�hua, Shanghai, China). Scanning electron micro�scope (SEM) image was obtained using a field emis�sion SEM Sirion 200 (FEI, America). All electro�chemical experiments were carried out using a three�electrode system consisted of a working electrode(a bare GCE or a Ptry/GCE, 3 mm in diameter) acounter electrode (a platinum wire electrode), and areference electrode (a Ag/AgCl electrode). HPLCused for validating the method was a Waters instru�ment with a diode array detector using a Chemsil ODS(250 mm × 4.6 mm, 5 μm) column. Acidity was mea�sured by a PHS�3B Precision pH Meter (Shanghai,China), and all sonication was done using a KQ�100Ultrasonic Cleaner (Kunshan, China).

2.3. Fabrication of Poly (L�tryptophan) Modified Glassy Carbon Electrode

A poly(L�tryptophan) modified was fabricated byelectropolymerization of L�tryptophan on a bareglassy carbon electrode. Prior to modification, thebare glassy carbon electrode (GCE, ∅ = 3 mm) waspolished successively with a piece of grit 1000 goldsand paper and 0.05 μm alumina powder slurry to mir�ror�like smoothness, the bare GCE was rinsed thor�oughly with doubly distilled water after each polishingstep. Then, the electrode was subjected successively to1 : 1 nitric acid, absolute ethanol and doubly distilledwater in ultrasonic bath for 5 min, respectively. Afterbeing cleaned again with distilled water, using theGCE as working electrode, a Ag/AgCl electrode asreference electrode and a platinum as counter elec�trode, polymerization was carried out in a phosphatebuffer solution (pH 3.0) containing 2.0 × 10–3 mol L–1

L�tryptophan with 10 segment cyclic voltammetricsweeps in the potential range from –0.6 to 1.9 V at ascan rate of 80 mV s–1. After polymerization, theresulting modified electrode was rinsed thoroughly

with doubly distilled water to remove the physicallyadsorbed L�tryptophan, and dried in air.

2.4. Analytical Procedure

Unless otherwise stated, phosphate buffer solution(pH 8.0) as supporting electrolyte for the determina�tion of maltol. A certain volume of maltol sample solu�tion was added into a cell containing phosphate buffersolution, and then a three�electrode system wasinstalled on it. After an accumulation time of 60 s underopen�circuit, cyclic voltammograms were recordedfrom –1.0 to 1.0 V at a scan rate of 100 mV s–1. Uponcompletion of each scan, the Ptry/GCE was placedin a blank supporting electrolyte and cyclic scan wascontinued until no peak came out, then the electrodewas washed with double distilled water and dried inair for reuse.

2.5. Sample Preparation

2.5.1. Bread sample preparation. Twenty grams ofbread and 50 mL of water was added into a pulverizerand pulverized to a paste, which was then filteredunder vacuum. The isolated solid was slurried 3 timeswith water and filtered, each time with 50.00 mL ofwater. All filtrates were collected into a beaker and5.00 mL of 0.03 mol L–1 ethylenediaminetetraaceticacid (EDTA) was added. The resulting solution wastransferred into a 250.00 mL volumetric flask anddiluted to volume with double distilled water.

2.5.2. Beer and beverage samples. Fifty millilitersof beer or beverage were collected in beakers andheated to gentle boiling to remove СО2 for 10 min.Then 1.00 mL of 0.03 mol L–1 EDTA was added. Theresulting solution was transferred into a 50 mL volu�metric flask and diluted to volume with double dis�tilled water.

3. RESULTS AND DISCUSSION

3.1. Electrochemical Immobilization of L�tryptophan at the GCE Surface

Figure 1 shows the repetitive cyclic voltammo�grams of 2.0 × 10–3 mol L–1 L�tryptophan at a bareGCE. The first cycle shows a considerably large oxida�tion peak at 1.10 V and a very weak reduction peak,indicating that a large amount of electrons areinvolved in the oxidation process and L�tryptophanundergoes a chemical reaction after the initial electrontransfer. The oxidation peak current dropped signifi�cantly during the second cycle and then followed by asteady increase with the increase of scan number, indi�cating a continuous growth of the polymer film at theelectrode surface. When the modification is com�pleted, a uniform adherent blue polymer film wasformed on the GCE surface. Figure 2 shows the SEMimage of the modified electrode surface, indicatingthat L�tryprophan has been successfully polymerised


ELECTROCHEMICAL DETERMINATION OF MALTOL IN FOOD PRODUCTS 3

onto the surface of the GCE. Possible mechanism forthe polymerisation of L�tryprophan onto the GCEsurface may be as follows. According to earlier reportson the oxidation of amines at carbon�based electrode[26, 27], we ascribe the oxidation peak to the electro�chemical oxidation of amino group. The amino groupof L�tryprophan is firstly oxidized into its correspond�ing cation free radical, which is then immobilized ontothe electrode surface by carbon�nitrogen linkage.L�tryprophan has a pKa of 2.38 and exists in its deproto�nated state (СОО–) in pH 3.0 PBS. The СОО– groupinteracts with oxidised amino group radical cation andpolymerisation occurs. As a result, an electropolymer�ised poly (L�tryptophan) film with a possible struc�

ture as shown in Fig. 3 was immobilised on the GCEsurface.

3.2. Electrochemical Behavior of Maltol

The electrochemical behavior of maltol at a bareGCE and a Ptry/GCE were investigated by cyclic vol�tammetry. From Fig. 4 we can see that the intensity ofoxidation peak current at the Ptry/GCE (Fig. 4, curve 1)was sharply increased in contrast to the poor responseat the bare GCE (Fig. 4, curve 2), the oxidation peakpotential negatively shifted from 660 to 620 mV andthe oxidation peak current increased from –4.21 to⎯16.32 μA, suggesting that the poly(L�tryptophan)

1.00 µm

Fig. 2. SEM image of the electropolymerised poly (L�tryp�tophan) film on the glassy carbon electrode.

HN

CH2

C C

O

NH

HN

CH2

C C

O

HN

H2C

C C

OHN OH

HN

CH2

C C

O

NH

HN

CH2

C C

O

HN

H2C

C C

OHN OHn

mH H H

H H H

Fig. 3. A possible structure of the electropolymerizedpoly(L�tryptophan) film on the GCE.

40

0

–40

–80

1.51.00.50–0.5–1.0Е, V

i, µA

2

1

Fig. 4. Cyclic voltammograms of 1.60 × 10–4 mol L–1 mal�tol at a Ptry/GCE (1) and a bare GCE (2) in pH 8.0 PBS.Scan rate: 100 mV s–1.

50

0

–50

–100

–150

–2002.01.51.00–0.5–1.0 0.5

Е, V

i, µA

2

10

1

Fig. 1. Cyclic voltammograms of 2.0 × 10–3 mol L–1

L�tryptophan at a GCE in pH 3.0 PBS. Initial potential:⎯0.6 V; terminal potential: 1.9 V; scan rate: 80 mV s–1.

4



film imobilized on the electrode has a high electrocat�alytic activity toward the oxidation of maltol and theelectron transfer rate in the film is much faster. Theelectrocatalytic activity of the poly(L�tryptophan)film towards maltol may be attributed to the formationof hydrogen bond between hydroxyl group in maltoland the nitrogen atom in poly(L�tryptophan). Theformation of hydrogen bond can weaken the bondenergy between hydrogen and oxygen and the electrontransfer is liable to occur via the bond of N–H–O.Further, no corresponding reduction peak of maltolwas observed both at the bare GCE and thePtry/GCE, indicating that the electrochemical oxida�tion of maltol was irreversible under this experimentalcondition.

Figure 5 shows the cyclic voltammograms of 1.6 ×10–4 mol L–1 maltol at the Ptry/GCE at various scanrate, from which we can see that the oxidation peakcurrent increases linearly with the increase of scan rateand the linear regression equation can be expressed asipa = 9.22 × 10–6 + 4.64 × 10–8 v, r = 0.9984, indicat�ing that the oxidation process of maltol at thePtry/GCE is an adsorption controlled process.

For irreversible reaction of adsorbed species, thefollowing formula can be used to calculate the numberof electrons involved in a reaction [28]:

W1/2 = 62.5 mV/αnα

at 25°С, (1)

where W1/2 is peak half�width, α is the electron trans�fer coefficient, n

α is the number of electrons involved

in the reaction. The α exhibits the symmetry of energybarrier and lies between 0.3 and 0.7 in most systems,and it’s value is usually approximated as 0.5 in theabsence of actual measurements [29]. Using the peak

half�width (W1/2) of 131 mV measured from Fig. 4 andthe estimated electron transfer coefficient (α) of 0.5,the value of n

α for the irreversible oxidation of maltol

at the modified electrode was calculated in accordancewith formula 1 and the obtained value is 0.95, suggest�ing that the electrocatalytic oxidationtion of maltol isa one electron transfer mechanism.

Inset of Fig. 6 shows the dependence of peakpotential (Ера) versus pH of supporting electrolyte. Inthe pH range from 2.2 to 8.0, Ера shows a linear rela�tionship with pH and the linear regression equationcan be expressed as Epa = 1.13 – 0.061pH, r = 0.9968,with a slope close to 59 mV, indicating that the oxida�tion reaction involves proton and the number of pro�tons and electrons involved in the reaction are equal.The oxidation reaction of maltol at the Ptry/GCE wasa one�proton�one�electron process, which is consis�tent with the previous reports [10]. A possible reactionmechanism of maltol at the Ptry/GCE is shown inScheme 1.

3.3. Optimization of Conditions for the Determination of Maltol

The conditions for the determination of maltolwere optimized by investigating the effects of scan

O

OOH

O

OO–1 e

–1 H+•

Scheme 1. A possible electrochemical reaction mechanism of maltol at the Ptry/GCE.

40

0

–40

–80

1.00.80.60.20–0.2 0.4Е, V

i, µA

1

10

i, µA

v, mV s–1

2824201612

50 150 250 350 450

Fig. 5. Cyclic voltammograms of CVs of 1.60 × 10–4 mol L–1

maltol at a Ptry/GCE. Each of the numbers from 1 to 10correspond to a scan rate of 40, 80, 120, 160, 200, 240, 280,320, 360 and 400 mV s–1, respectively. Inset is the plot ofoxidation peak current of maltol versus scan rate.

40

0

–40

–80

1.20.80–0.4 0.4Е, V

i, µA

17

i, µA

рН

1.0

0.9

0.8

0.7

0.62 3 4 5

1.6

6 7 8

Fig. 6. Cyclic voltammograms of 1.60 × 10–4 mol L–1 mal�tol at a Ptry/GCE in PBS of various pH. Each of the num�bers from 1 to 7 corresponds to a pH of 2.2, 3.0, 4.0, 5.0,6.0, 7.0 and 8.0, respectively. Inset is the plot of peakpotential of maltol versus pH of supporting electrolyte.



rate, pH of supporting electrolyte and sample accu�mulation time. The peak current of maltol linearlyincreases with the scan rate and the scan rate of100 mV s–1 gave the best peak shape (Fig. 5). In apH range of 2.2 to 8.0, the peak current increases withthe increase of the pH value of supporting electrolyteand the supporting electrolyte of pH 8.0 PBS gives themaximum peak current in the investigated pH range(Fig. 6). Due to the adsorption process of maltol at thePtry/GCE, stirring time should be an important factorfor the accumulation of maltol at the Ptry/GCE.Effect of accumulation time was studied by detecting1.60 × 10–4 mol L–1 maltol with various stirring timebetween 0 and 120 s. Experimental results showed thatthe peak current increased as accumulation timeincreased from 0 to 60 s, and kept almost unchangedwhen further increasing the time. Thereby, a scan rateof 100 mV s–1, a phosphate buffer solution of pH 8.0and an accumulation time of 60 s were chosen as thedetermination parameters in this study.

3.4. Linearity Range, Detection Limit and Reproducibility

Under the above�optimized conditions, the varia�tion of peak current with concentration at thePtry/GCE was studied by cyclic voltammetry. Figure 7shows cyclic voltammograms of maltol at various con�centration at a Ptry/GCE in pH 8.0 PBS. From whichwe can see that the oxidation peak current of maltol at thePtry/GCE is proportional to its concentration in a rangefrom 9.00 × 10–5 to 3.75 × 10–3 mol L–1 with a linearregression equation that can be expressed as ipc (A) =1.46 × 10–6 + 0.026c (mol L–1), r = 0.9972. The detectionlimit was estimated to be 8.00 × 10–6 mol L–1 using amethod of gradually decreasing the concentration ofmaltol. Six replicate determinations were made whendeterming each sample, the relative standard devia�tions (RSD) was found to be less than 2.5%, indicatinggood reproducibility of the method. After each mea�surement, the Ptry/GCE was regenerated by 10 cyclicsweeps in 8.0 PBS. Table 1 shows the comparisonbetween the analytical performance of the presentmethod and previous electrochemical methods for the

determination of maltol. It can be seen from the tablethat the present methods has an advantage over thenumber of analyzed samples and the linear workingrange of the present method is comparable or widerthan previous reported methods.

3.5. Interference Studies

Potential interference to the determination of mal�tol from some common species present in food prod�ucts was individually investigated by cyclic voltamme�try under the above�optimized conditions. Due tomaltol can form chelates with many different metalions, EDTA was added during determination to elim�inate interference from these metal ions. The oxida�tion peak current of 2.0 × 10–4 mol L–1 maltol in theabsence and presence of various foreign species were

60

20

–100

–140

1.00.5–0.5–1.0 0Е, V

i, µA

1

9

i, µA120100

60

20

0 1.0

80

40

02.0 3.0 4.0

–20

–60

c, mmol/L

Fig. 7. Cyclic voltammograms of maltol at various concen�tration at a Ptry/GCE in pH 8.0 PBS. Each of the numbersfrom 1 to 9 corresponds to a concentration of 9.00 × 10–5,2.00 × 10–4, 4.00 × 10–4, 6.00 × 10–4, 8.00 × 10–4, 9.00 ×10–4, 1.60 × 10–3, 2.80 × 10–3 and 3.75 × 10–3 mol L–1,respectively. Inset is the plot of oxidation peak current ofmaltol versus its concentration.

Table 1. Comparison of the proposed method with other electrochemical methods for analysis of maltol

ElectrodesDetection limit

(mol L–1)Linear working range

(mol L–1)Sample analyzed References

Carbon paste electrode 4.0 × 10–8 1.0 × 10–7–4.0 × 10–5 Beer 5

Hanging mercury drop electrode l.0 × 10–8 5.0 × 10–8– 3.5 × 10–7 Cake 14

MWCT modified electrode 2.0 × 10–6 5.0 × 10–6–9.0 × 10–4 Beer, cola 15

Silica sol–gel modified electrode 2.0 × 10–6 5.0 × 10–6–5.0 × 10–4 Beverage 16

Ptry/GCE 8.0 × 10–6 9.0 × 10–5–3.75 × 10–3 Beer, bread, beverage This work

6



measured. If peak current change is no more than±5% when a species is added, we assume no interfer�enc occurs. Experimental results showed that no inter�ference was observed in the presence of 50�fold con�centration of K+, Na+, Ca2+, Cu2+, Mg2+, Pb2+, Fe2+,

Zn2+, Al3+, Fe3+, Cl–, C S P citric acid,tartric acid, ethanol, ethyl acetate, sucrose, ascorbicacid etc., indicating that the Ptry/GCE is highly selec�tive towards the determination of maltol.

3.6. Analytical Application

The application of the method has been demon�strated by the analysis of maltol in bread, beer and bev�erage. First, real samples were analyzed. Then, theanalyzed samples were spiked with maltol standardsolutions and similarly analyzed. Determinationresults and calculated recoveries are listed in Table 2.From the results we can see that maltol level in theanalyzed bread is higher than EU limit. Bread manu�facturer should try to lower maltol content in bread toimprove bread quality. The proposed method was fur�ther validated by employing HPLC for maltol in vari�ous real samples and the test results of these samplesare also shown in Table 2. The results show that theamounts of maltol obtained by the proposed methodagreed well with the amount obtained by HPLCmethod, proving that the determination of maltol canbe carried out with great confidence by the proposedmethod.

4. CONCLUSIONS

A poly(L�tryptophan) modified glassy carbon elec�trode for the determination of maltol was prepared byelectrochemical polymerization. The modified elec�trode exhibits excellent electrocatalytic activitytoward the electrochemical oxidation of maltol. InPBS at pH 8.0, the oxidation peak current of maltolobtained by cyclic voltammetry at the modified elec�trode shows a linear relationship with its concentra�tion in the range of 9.00 × 105 ~ 3.75 × 103 mol L–1,with a correlation coefficient of 0.9972 and a limit ofdetection of 8.00 × 106 mol L–1. The developed

O32–

, O42–

, O4–

,

method is fast and convenient, and shows good selec�tivity, recovery, reproducibility. Its application hasbeen demonstrated in the analysis of maltol in foodproducts.

ACKNOWLEDGEMENTS

This work was financially supported by a grant fromthe National Natural Science Foundation of ShanDong Province (No. 2R2009BM003) and Heze Uni�versity Scientific Research Fund (XY12BS07).

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Table 2. Comparison between the proposed method and the HPLC method for sample analysis (n = 6)

Sample Content, mg kg–1

Added, mg kg–1

Found, mg kg–1 Recovery, % RSD, %

Bread 1 325.2a, 322.6b 300.0 621.1a, 616.0b 98.6a, 97.8b 1.50a, 1.88b

Bread 2 317.6a, 315.4b 300.0 615.2a, 610.9b 99.2a, 98.5b 1.12a, 1.79b

Beer 1 85.2a, 84.9b 100.0 186.3a, 185.6b 101.1a, 100.7b 2.33a, 2.86b

Beer 2 94.4a, 93.8b 100.0 195.0a, 193.9b 100.6a, 100.1b 1.49a, 1.92b

Beverage 1 42.7a, 42.6b 50.0 90.4a, 90.2b 95.4a, 95.2b 2.47a, 2.91b

Beverage 2 39.3a, 39.lb 50.0 87.6a, 87.1b 96.6a, 96.0b 1.97a, 2.22b

a: results obtained by the proposed methods; b: results obtained by HPLC method.



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Electrochemical determination of maltol in food products using a poly(L-tryptophan) modified glassy carbon electrode