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Biosensors and Bioelectronics
journal homepage:www.elsevier.com/locate/bios
Non-enzymatic electrochemical detection of glucose with a disposable paper-based sensor using a cobalt phthalocyanine – ionic liquid – graphene composite
Sudkate Chaiyo
a, Eda Mehmeti
b,c, Weena Siangproh
d, Thai Long Hoang
e, Hai Phong Nguyen
e, Orawon Chailapakul
a,f,⁎, Kurt Kalcher
b,⁎⁎aElectrochemistry and Optical Spectroscopy Center of Excellence (EOSCE), Department of Chemistry, Chulalongkorn University, 254 Phayathai Road, Patumwan, Bangkok, Thailand
bInstitute of Chemistry, Analytical Chemistry, Karl-Franzens University, Universitätsplatz 1, Graz A-8010, Austria
cUBT-Higher Education Institution, Lagjja Kalabria p.n., 10000 Prishtina, Kosovo
dDepartment of Chemistry, Srinakharinwirot University, Sukhumvit 23, Wattana, Bangkok, Thailand
eCollege of Sciences, Hue University, 77 Nguyen Hue Str., Hue, Vietnam
fNanotec-CU Center of Excellence on Food and Agriculture, Chulalongkorn University, Bangkok 10330, Thailand
A R T I C L E I N F O
Keywords:
Paper-based analytical devices Non-enzymatic glucose sensor Cobalt phthalocyanine Graphene
Ionic liquids
A B S T R A C T
We introduce for thefirst time a paper-based analytical device (PAD) for the non-enzymatic detection of glucose by modifying a screen-printed carbon electrode with cobalt phthalocyanine, graphene and an ionic liquid (CoPc/
G/IL/SPCE). The modifying composite was characterized by UV–visible spectroscopy, energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The disposable devices show excellent conductivity and fast electron transfer kinetics. The results demonstrated that the modified electrode on PADs had excellent electrocatalytic activity towards the oxidation of glucose with NaOH as supporting electrolyte (0.1 M). The oxidation potential of glucose was negatively shifted to 0.64 V vs.
the screen-printed carbon pseudo-reference electrode. The paper-based sensor comprised a wide linear con- centration range for glucose, from 0.01 to 1.3 mM and 1.3–5.0 mM for low and high concentration of glucose assay, respectively, with a detection limit of 0.67 µM (S/N = 3). Additionally, the PADs were applied to quantify glucose in honey, white wine and human serum. The disposable, efficient, sensitive and low-cost non-enzymatic PAD has great potential for the development of point-of-care testing (POCT) devices that can be applied in healthcare monitoring.
1. Introduction
Glucose detection is an important issue because diabetes mellitus is one of the leading causes of death and disability in the world (Matz et al., 2006; Shepherd and Kahn, 1999). Abnormality of the glucose level in human blood causes several disorders such as blindness, nerve degeneration and kidney failure (Garg et al., 2004; Zhu et al., 2012). As an example, about 300 million people suffer from diabetes worldwide and this number is estimated to be almost double in 2030 (King et al., 1998; Zhang et al., 2010). The diagnosis and management of diabetic patients require exact monitoring and control of the glucose level in the body. Therefore, an enormous amount of glucose assays have been proposed, such as spectrophotometric (Ali et al., 2016; Pham et al.,
2016), chromatographic (Agblevor et al., 2004; Monti et al., 2017) and electrochemical methods (Liu et al., 2008; Pla-Tolós et al., 2016).
Nevertheless, many approaches suffer from limitations of complicated pretreatment steps, equipment costs and expensive time-consuming labor resources. Apart from medicinal aspects glucose is an important analyte in foodstuffwith respect to taste and nutritional value. Ob- viously, effective analytical methods for rapid determination of glucose are still directly needed.
Recently, microfluidic paper-based analytical devices (μPADs) were reported by Whiteside's group as alternative devices for point-of-care testing for developing countries (Martinez et al., 2007).μPADs provide simultaneous detection of multiple analytes on the same device, in which the platform is inexpensive, easy-to-use, and portable (Agblevor
https://doi.org/10.1016/j.bios.2017.11.015
Received 7 August 2017; Received in revised form 11 October 2017; Accepted 1 November 2017
⁎Corresponding author at: Electrochemistry and Optical Spectroscopy Center of Excellence (EOSCE), Department of Chemistry, Chulalongkorn University, 254 Phayathai Road, Patumwan, Bangkok, Thailand.
⁎⁎Corresponding author.
E-mail addresses:[email protected](O. Chailapakul),[email protected](K. Kalcher).
0956-5663/ © 2017 Elsevier B.V. All rights reserved.
MARK
et al., 2004; Chaiyo et al., 2015; Dungchai et al., 2009; Martinez et al., 2007, 2008). Henry׳s group has already demonstrated the integration of electrochemical detections in paper-based analytical devices for the determination of glucose, lactate, and uric acid in biological samples using oxidase-based enzymatic reactions (Dungchai et al., 2009).
Moreover, most previous studies on the electrochemical paper-based analytical devices of glucose are related to the biocatalysis of natural enzymes (e.g., glucose oxidase, and glucose dehydrogenase) toward the zymolyte with fast, accurate, and specific responses (Amor-Gutiérrez et al., 2017; Noiphung et al., 2013). However, there is some dis- advantage for the use of enzymes originating from their sensitivity to temperature which may result in a reduced shelf lifetime by improper storage conditions (Park et al., 2006; Wang et al., 2008). Thus, fabri- cation of non-enzymatic glucose sensors has continuously been moti- vating research interests.
Metallophthalocyanines (MPcs) (e.g., cobalt(II) phthalocyanine, CoPc, copper(II) phthalocyanine, CuPc, and iron(II) phthalocyanine, FePc) are known to exhibit good electrocatalytic activities towards the oxidation of common chemical substances such as hydrogen peroxide (Foster et al., 2014), cysteine (Kuhnline et al., 2006), nitrite (Santos et al., 2006), nitric oxide (Vilakazi and Nyokong, 2001) and ascorbic acid (Agboola et al., 2009). Furthermore, metallophthalocyanine-based electrodes have been used as redox mediators for enzyme based glucose sensors (Barrera et al., 2006; Devasenathipathy et al., 2015; Özcan et al., 2008). However, MPc is not very appropriate on the electrode surface due to its low conductivity along with poor electrochemical activity (Cui et al., 2013). Recently, Agboola et al. have reported that carbon nanomaterials can combine with MPc to form a more stable composite due to the strongπinteraction between MPc and the carbon nanomaterial (Agboola et al., 2009). Graphene (G) is an important member of the family of carbon nanomaterials with hexagonal aromatic structures. With outstanding properties such as good mechanical strength, high carrier mobility and large surface area (Guo and Dong, 2011; Li and Kaner, 2008), G is a promising candidate for forming a composite with MPc for non-enzymatic based glucose sensors. Fur- thermore, ionic liquids (IL) are either organic salts or mixtures of salts.
Due to the excellent physicochemical properties such as high ionic conductivity, wide electrochemical windows, negligible vapor pressure, chemical and thermal stability, good antifouling ability, good bio- compatibility, and inherent catalytic ability (Xu et al., 2009; Zhu et al., 2012).
In the present study, for the first time, a non-enzymatic electro- chemical paper-based sensor using a cobalt(II) phthalocyanine/ionic liquid/graphene composite (CoPc/IL/G) was developed and applied for the determination of glucose.
2. Experimental
2.1. Materials and chemicals
Carbon ink was purchased from Acheson™ (California, USA).
Glucose, sucrose, galactose, ascorbic acid, fructose, dopamine, lactose,
sodium hydroxide (NaOH) and potassium chloride (KCl) were pur- chased from Merck (Darmstadt, Germany). Industrial-quality graphene was obtained from ACS Material, LLC (Medford, USA). 1-Butyl-2,3-di- methylimidazolium tetrafluoroborate (IL), cobalt(II) phthalocyanine (CoPc), Whatman chromatography paper #1 (58 cm × 60 cm), N,N- dimethylformamide (DMF), paracetamol, uric acid, dopamine and po- tassium hexacyanoferrate(III) (K3[Fe(CN)6]) were purchased from Sigma–Aldrich (Buchs, Switzerland). All reagents were of analytical grade, and were used without further purification. All solutions were prepared using ultra-purified water (> 18 MΩcm) refined by a car- tridge purification system (Millipore, UK).
2.2. Instrumentation
The electrochemical experiments were performed using an Autolab electrochemical system with a potentiostat PGSTAT 128 N (EcoChemie, Utrecht, Netherlands) controlled by corresponding software (NOVA 10.1). A scanning electronic microscope (SEM) (JEOL, Ltd., Japan) was used for microscopic analysis of paper devices. Transmission electron microscopic images (TEM) were recorded with an H-7650 device (Hitachi, Japan). The screen-printing block was fabricated by Chaiyaboon Co. Ltd. (Bangkok, Thailand). Absorbance measurements were conducted by a UV–visible spectrophotometer (HP HEWLETT PACKARD 8453, UK) using a 1.0 cm path length quartz cell.
2.3. Preparation of the CoPc/IL/G casting solution
For the preparation of the CoPc/IL/G composite, 1.0 mg of the graphene was dispersed in a closed polyethylene vial in DMF (1 mL) by ultrasonication (Transsonic T700/H, 560 W, 35 kHz, water volume 6 L) for about 3 h. The temperature was not controlled and was around 37 °C after the sonication time. Then, 5.0 mg of CoPc and 20 µL of the ionic liquid solution (0.5 mg mL−1in DMF) were added to the graphene dispersion and sonicated for further 30 min.
2.4. Preparation of PADs and modified electrode
In this work, PADs were fabricated by wax-printing. First, the pat- terned paper-based sensor was designed by a graphic program (Adobe Illustrator) and then printed ontofilter paper (Whatman no. 1) using a wax printer (Xerox ColorQube 8570, Japan). Next, the printed paper- based sensor was placed on a hot plate at 175 °C for 40 s to melt the wax. The area covered with wax was hydrophobic, and the area without wax was hydrophilic. For the three electrode system on the PADs, a working electrode (WE, area = 0.196 cm2) a counter electrode (CE) and a pseudo-reference electrode (RE) were screen-printed in-house using carbon ink. The sensor was allowed to dry at 55 °C for 1 h. Then, about 2.0 µL of CoPc/IL/G composite solution was drop cast on the working electrode surface and dried at room temperature and atmo- spheric pressure for approximately 10 min as shown inScheme 1. The CoPc- and graphene-modified screen- printed carbon electrodes were prepared analogously with suspensions containing the ionic liquid plus Scheme 1.Schematic representation of the fabrica- tion, modification and analytical procedure for the glucose sensor based on the coupling of the CoPc/IL/
G SPCE with PADs.
CoPC and graphene, respectively.
2.5. Electrochemical measurements
All electrochemical experiments were carried out under aerated conditions at 25 ± 2 °C. Prior to all the electrochemical measurements, a conditioning pre-treatment of the CoPc/IL/G working electrode was performed, in which the electrode was submitted to cyclic polarization between −1.0 and 1.0 V at a scan rate of 50 mV s−1 (10 cycles) in 0.1 M NaOH to remove any possible surface contamination. Afterwards the CoPc/IL/G/SPE was rinsed with 0.1 M NaOH. CV experiments were carried out in 0.1 M NaOH over the potential range of−0.6–0.9 V with a scan rate of 50 mV s−1. EIS assays were performed in the presence of 5.0 mM Fe(CN)63-/4-
in 0.1 M KCl at a frequency range of 100 kHz to 0.01 Hz with a perturbation amplitude of 0.1 V. All measurements were carried out in triplicate. The chronoamperograms were recorded ap- plying a potential of 0.7 or 0.8 V over 60 s. Generally, 50 µL of standard or sample solution were placed onto the screen-printed electrode before starting CV or CA measurements. All potentials are referred to the carbon pseudo-reference electrode.
2.6. Sample preparation 2.6.1. Real food samples
Commercial Austrian honey samples and white Austrian table wines (neither late harvest nor fortified) were acquired from local grocery stores in Graz. The sample solutions (wine directly, honey; 1 mL dis- solved in 10 mL supporting electrolyte) were filtered using Whatman chromatography paper #1. One hundred microliters of each solution were transferred to a volumetricflask and diluted to 10 mL with sup- porting electrolyte prior to analysis. The sample solutions had con- centrations within the linear range of the calibration graph (expected glucose concentrations: dry wines around 0.5 g L−1(Shkotova et al., 2016honey 30% m: m (Sixto and Knochen, 2009)).
2.6.2. Human blood serum sample
Blood samples (from anonymous non-diabetic patients) were ob- tained from a local hospital.
The samples were centrifuged (5000 rpm) to obtain the serum as a supernatant followed by proper dilution with the supporting electro- lyte. The analysis was carried using CA as primary mode of measure- ment (sampling the signal after 2 s) and the estimated values were confirmed valid by cross comparison with those assessed using a com- mercial glucose meter.
3. Results and discussion
3.1. Characterization of the CoPc/G/IL composite
Phthalocyanines and their derivatives exhibit typically two UV ab- sorption peaks: the B- and Q-band. The band in the 600–710 nm range (Q-band) is attributed to the allowed π-π transition for phthalocya- nines. Original CoPc exhibits an absorption peak at 650 nm as shown in Fig. 1A (b). On the other hand, the Q-band of CoPc/G/IL was sig- nificantly red-shifted to 665 nm (Fig. 1A (c)), due to the electron transfer from G to the CoPc ring. Thus, theπ-electrons of CoPc spread onto the G, resulting in the red shift of the CoPc band, which also confirms that the formation of the CoPc/G/IL composite is achieved.
Transmission electron microscopy (TEM) was employed to char- acterize the composite of CoPc/G/IL. TEM images of CoPc/G/IL com- posite (insertFig. 1C) demonstrate that CoPc on G presents a micro- crystalline structure. G still shows many of the same“wrinkles”as in the unmodified form (insertFig. 1B). However, in contrast to native G, lots of microcrystals of CoPc are uniformly distributed onto the surface of G in CoPc/G/IL composite. The average diameter of the microcrystals is approximate 100 nm with a length between 2 and 5 µm. Moreover, the
energy-dispersive x-ray spectroscopy (EDX) spectrum of CoPc/G/IL composite (Fig. 1C) shows the signals for carbon, oxygen and cobalt (copper and silicon arise from the substrate grid). The EDX spectrum of pristine G (Fig. 1B) displays the signal for carbon and the results con- firm the formation of a CoPc/G/IL composite.
The morphologies of the various modified electrodes were char- acterized by scanning electron microscopy (SEM) as shown inFig. 1D-I.
The surface of the SPCE was dominated by isolated and irregularly shaped graphiteflakes, and separated layers were seen (Fig. 2D and G).
The SEM image of G/IL/SPCE (Fig. 2E and H) shows the characteristic wrinkled, folded and sheet like morphology which reveals the suc- cessful adsorption of graphene sheets, where IL is capable of better dispersing G (which is, in fact, reduced graphene oxide with hydro- philic groups remaining from the oxidation process) on the surface and thus, can better bridge the G sheets together. In the SEM images of CoPc/G/IL/SPCE, CoPc microcrystals are uniformly decorated at both sides of the G sheets revealing the successful formation of the CoPc/G/
IL composite (Fig. 2F and I).
3.2. Electrochemical characterization of the CoPc/G/IL electrodes Cyclic voltammograms of different modified electrodes on PADs in 5.0 mM [Fe(CN)6]3-/4- containing 0.1 M KCl were recorded and are shown inFig. 2A. On the bare SPCE electrode a pair of weak redox peaks with a peak-to-peak separation (ΔEp) of 0.40 V was observed (dashed line), indicating the sluggish electron transfer rate at the in- terface. On the CoPc/SPCE, the redox peak currents of [Fe(CN)6]3-/4- increased while the peak-to-peak separation (ΔEp = 0.34 V) decreased (black line), suggesting the presence of modified CoPc on the electrode surface which would accelerate the electron transfer of ferricyanide.
When introducing G on the electrode surface (G/SPCE, blue line), a pair of well-defined peaks appeared with a peak potential separationΔEp of 0.32 V. The presence of G on the electrode surface not only improves the conductivity of the interface, but also offers a large surface for the redox reaction of [Fe(CN)6]3-/4-. The presence of the ionic liquid in the IL/G/SPCE (green line) causes well-defined and enhanced redox peaks with a smallΔEp of 0.30 V, which can be ascribed to the high polarity of the modifier by which the affinity of the surface for hydrophilic substances is significantly increased. After addition of CoPc to the IL/G/
SPCE, the anodic and cathodic peak currents showed aΔEp of 0.31 V, indicating that the IL in the CoPc/G/IL/SPCE (red line), compared to the CoPc/G/SPCE (without IL, ΔEp = 0.33 V, brown line), slightly overcompensates and synergistically improves its otherwise negative influence. Hence, it can be concluded that the presence of IL and CoPc in combination provided a significant synergistic increase of the elec- trochemical signals. According to the Randles–Sevcik equation:
= × C v
Ipc (2.69 10 )n5 3/2AD1/2 *1/2
(1) where Ipcis the reduction peak current (A), n is the electron transfer number, A is the apparent electrode area (cm2), D is the diffusion coefficient of the K3[Fe(CN)6] in the solution (cm2/s), C* is the bulk concentration of the K3[Fe(CN)6] (mol/cm3) and v is the scan rate (V/
s), the effective area of different modified electrodes can be calculated.
By exploring the redox peak current with the scan rate, the average effective area of the bare SPCE and CoPc/G/IL/SPCE was calculated as 0.21 cm2 and 0.45 cm2, respectively. Furthermore, these results give evidence that the CoPc/G/IL/SPCE greatly improves the effective sur- face area, accompanied with concomitant improvement of electro- chemical responses.
There are reports indicating that G promotes the electron transfer rate of electrochemical reactions (Fu et al., 2017). We assumed that the introduction of G can initially improve the properties of CoPc. Con- cerning insight into this speculation, electrochemical impedance spec- troscopy (EIS) was employed to explore the modified electrodes. The semicircle portion at high frequencies is attributed to the charge
transfer resistance (Rct) in EIS, which reflects the electron transfer ability of the electrode. The radius of the semicircle corresponds to this resistance. As shown in Fig. 2B (black dots), the large well-defined semicircle of the screen printed carbon electrode shows a considerable Rct. It refers to the limited conductivity of carbon on SPCE surface. As seen from the CV results inFig. 2A (black dashed line), the response signal on the bare SPCE is extremely weak. A very small semicircle can be observed for the G/IL/SPCE inFig. 2B (green dots), indicating a fast electron transfer of G and IL. This phenomenon is consistent with the electrochemical properties of G. As shown inFig. 2B (red dots), also only a small semicircle is observed with the CoPc/G/IL/SPCE, due to the presence of G and IL which improve the electron transfer rate of CoPc.
3.3. Electrochemical detection of glucose on PADs
Cyclic voltammetry was initially employed to investigate the
electrochemical behavior of glucose on the CoPc/G/IL/SPCE coupled with PADs. The cyclic voltammograms (CVs) in the absence and pre- sence of 2.0 mM glucose, recorded at the bare SPCE, CoPc/SPCE, CoPc/
G/SPCE and CoPc/G/IL/SPCE, corresponding to the background with a scan rate of 50 mV s−1are shown inFig. 2C. The anodic peak of glucose did not appear at the bare SPCE as shown inFig. 2C (red line), sup- porting the observation that the unmodified electrode has no electro- chemical activity toward glucose.
CV was utilized to assess the electrocatalytic response of glucose with different types of modified electrodes, as shown inFig. 2D. With a CoPc/SPCE, in the absence of glucose, the CoPc/SPCE (black line) showed a pair of well-defined redox peak with a potential of−0.33 V and 0.26 V in a 0.1 M of NaOH solution, corresponding to Co3+/Co2+
and Co2+/Co3+couples in CoPc. However, in the presence of 2.0 mM glucose, a sensitive oxidation peak appeared at a potential of 0.65 V (blue line). This effect shows that the CoPc/SPCE has a strong elec- trocatalytic activity towards the oxidation of glucose. An obvious Fig. 1.(A) UV–visible spectra of G(a), CoPc (b) and CoPc/G/IL composite (c). (B) EDX and TEM (insert) of G and (C) EDX and TEM (insert) of CoPc/G/IL composite. SEM images of the bare SPCE on PADs (D, G), G/IL/SPCE (E, G), and CoPeG/IL/SPCE composite (F, I) at 2000X (D, E, F) and 5000X (G, H, I) magnification.
increase in oxidation currents of glucose could be detected upon using CoPc/G/SPCE (green line) at a potential of approximately 0.68 V vs.
SPCE, an improvement that can be attributed to the surface area in- crease and electric conductivity of graphene. A further increase of the oxidation peak current of glucose could be observed with the CoPc/G/
IL/SPCE while the oxidation peak potential was shifted cathodically (red line). The anodic peak potential was decreased by 40 mV com- pared to the CoPc/G/SPCE, and by 20 mV compared to the CoPc/SPCE.
This effect demonstrates that the CoPc/G/IL-modified electrode has a strong electrocatalytic effect and the high ionic conductivity and hy- drophilic properties of the IL improve the oxidation behavior of glu- cose. On the CoPc/G/IL/SPCE coupled with PADs, the oxidation of glucose in 0.1 M NaOH occurs as a two-step electrocatalytic process.
As a first step, glucose forms a complex with Co(II)Pc (Eq.(2)).
When polarizing the potential in anodic direction, the oxidation of free
Co(II)Pc is still observable (Eq.3a) at around 0.3 V, whereas the oxi- dation of the glucose complex generates the signal at around 0.6 V which could be catalytically assisted by free Co(III)Pc.
Co(II)Pc·H2O + glucose→Co(II)PC·glucose + H2O (2)
Co(II)Pc→Co(III)Pc + e- (3a)
Co(II)Pc·glucose +Co(III)Pc→2Co(II)Pc + gluconolactone + 2H++ e- (3b) The most important feature of this reaction is that the anodic peak current for the glucose oxidation is generated and enhanced, and the peak potential shifted negatively due to the presence of Co(II)phtha- locyanine. The results show that the CoPc/G/IL composite-modified electrode shows a synergistic effect of all involved modifier components on the electrocatalytic oxidation to glucose.
Fig. 2.(A) Cyclic voltammograms for 5.0 mM [Fe(CN)6]3−/4−in 0.1 M KCl obtained with a bare SPCE (black dashed line, a), CoPc/SPCE (black line, b), G/SPCE (blue line, c), G/IL/SPCE (green line, d), CoPc/G/SPCE (brown line, e) and CoPc/G/IL/SPCE (red line, f) coupled with PADs at a scan rate of 50 mV s−1. (B) EIS of a bare SPCE (black dots, c), G/IL/SPCE (green dots, a) and CoPc/G/IL/SPCE (red dots, b) in 5 mM Fe(CN)63−/4−containing 0.1 M KCl solution. Inset: equivalent circuit applied to model EIS data. Rs: the electrolyte solution resistance;
Cdl: the constant phase angle element; Rct: the interfacial electron transfer resistance; W: the Warburg impedance introduced by the diffusion of ions. (C) Cyclic voltammograms obtained at the bare SPCE in the absence (a) and presence (b) of 2.0 mM glucose in 0.1 M NaOH at a scan rate of 50 mV s−1. (D) Cyclic voltammograms obtained at a SPCE (black line, a, in the absence of glucose), CoPc/SPCE (blue line, b), CoPc/G/SPCE (green line, c), and CoPc/G/IL/SPCE (red line, d) coupled with PADs in the presence of 2.0 mM glucose in 0.1 M NaOH at a scan rate of 50 mV s−1. (E) The cyclic voltammetry response of CoPc/G/IL/SPCE on PADs in 0.1 M NaOH containing 1.0 mM glucose at different scan rates from 20 to 100 mV s-1, (inset) plot of Ipavs. scan rate.
Fig. 2E shows the CVs of 1.0 mM glucose in 0.1 M NaOH at different scan rates. Both the oxidation peak current and oxidation peak poten- tial were observed to increase with increasing scan rate from 20 to 100 mV s−1. with a correlation coefficient of 0.9974. The result further confirms that the oxidation of glucose at the surface of the modified electrode is a surface reaction-controlled process which is underlined by the reaction model shown above.
3.4. Optimization of operation conditions
Concentrations of the composite components in the casting solution and were optimized with respect to the amperometric glucose signal;
best results were obtained with the following concentrations:
1.0 mg mL−1of G, 0.5 mg mL−1of IL and 5.0 mg mL−1of CoPc. The optimized supporting electrolyte had a concentration of 0.1 mg mL−1 NaOH. Detailed information and discussion can be found in the sup- plementary material.
3.5. Chronoamperometric detection of glucose at CoPc/G/IL/SPCEs on PADs
Amperometry is a very convenient method for the quantitative de- termination of glucose. Chronoamperometry is a simple and fast tech- nique that can be easily implemented in portable devices. The response signal can be read much faster than with equilibrium-based techniques.
Thus, the ability of CoPc/G/IL/SPCEs on paper-based devices to detect glucose by chronoamperometry was studied in detail. Generally the applied potential has an effect on the sensing performance of an electrochemical sensor, so the applied potential was optimized in the range of the voltammetric signal (Fig. S3). The currents increased as the detection potential increased, but the background current also in- creased due to gradual oxidation of the electrode material yielding a higher detection limit. Considering sensitivity, the amperometric signal-to-background ratios (S/B) were investigated instead of currents only. Typical response curves are shown inFig. 3. The S/B ratio eval- uated at 0.7 V and 0.8 V provided a maximum value for both, low and high concentrations of glucose. Therefore, operating potentials of 0.7 V and 0.8 V vs. SPCE were chosen for further chronoamperometric in- vestigations.
Under experimental conditions as described above the chron- oamperometric currents of glucose provided linear relationships with the glucose concentration over wide ranges. For a working potential of 0.7 V a linear dynamic range of 0.01–1.3 mM glucose (R2= 0.9979) was found (Fig. 3A). For higher concentrations an operating potential of 0.8 V yielded best results with respect to linearity (1.3–5.0 mM, R2= 0.9964, Fig. 3B). The benefit of the latter is that the method can be directly applied to higher concentrations of the analyte without dilution whereas for low concentrations the lower potential seems preferable.
The limit of detection (LOD) for a working potential of 0.7 V with a signal-to-noise ratio 3 (S/N = 3) was 0.64μM glucose and the re- peatability on the same sensor was < 4.6% RSD (n=5, 20μM glucose).
A comparison this sensors with some other paper-based devices for the determination of glucose is summarized in Table 1. The sensor pre- sented in this work demonstrates for the first time the use of paper- based sensing for non-enzymatic glucose analysis. Moreover, the de- tection limit is lower and the linear range of the proposed electrode is comparable with or even better than those obtained by other methods.
3.6. Selectivity of the sensor
Selectivity of the paper-based sensor is essential for practical ap- plications. Electroactive compounds which can easily oxidized at the positive potential, such as dopamine, ascorbic acid, uric acid and other carbohydrates, usually co-exist with glucose in human blood and other bodyfluids. In order to estimate the selectivity of the fabricated CoPc/
G/IL glucose paper-based sensors, the chronoamperometric response of
other potentially active compounds was studied. Fig. S4 shows the current response of 1.0 mM glucose and of 1.0 mM interfering species (sucrose, galactose, uric acid, fructose, ascorbic acid, paracetamol, dopamine and lactose). A well-defined chronoamperometric response was obtained for 1.0 mM glucose whereas interfering species showed a much lesser and practically negligible signal. The result confirmed that the CoPc/G/IL modified electrode on paper-based sensor is selective for the detection of glucose in the presence of potentially active interfering compounds.
3.7. Practical application of the sensor
The above results indicate that the paper-based CoPc/G/IL modified electrode is feasible for the determination of glucose in foods and human blood.
Before analysis, the samples were diluted 100-fold (honey, wine) and 10-fold (human serum), respectively. Comparative results were obtained by a commercial glucose meter (Ascensia ENTRUST). The results are summarized inTable 2. Recoveries of glucose were found in the range of 95.2–104.9% with RSDs from 2.6–5.2%. The results are in good agreement with those obtained with a commercial glucometer; the pairedt-test at the 95% confidence level did not show any significant difference (the calculated t value of 1.833 is significantly below the critical t of 2.444 with 9 degrees of freedom). The test also did not show any significant difference for each type of sample. Hence, the CoPc/G/
IL modified electrode can be used as a suitable material for the detec- tion of glucose in foods and human serum samples.
Fig. 3.(A) Chronoamperograms of glucose determination at (A) 0.7 V and (B) 0.8 vs.
SPCE on proposed devices. The calibration plot of the currents at 2 s of sampling time for the determination of glucose is shown in the insert. Relative standard deviations (RSD) = 2.6–4.3%.
4. Conclusions
In this study, a novel and portable PAD based on modification of a screen-printed carbon electrode modified with a cobalt phthalocyanine, graphene and ionic liquid composite (CoPc/G/IL/SPCE) was devolved for enzyme-free glucose detection. Experimental results showed that the modified electrode provides an increased surface area, improved con- ductivity and a good catalytic effect towards the oxidation of glucose, whose oxidation overpotential was significantly shifted towards less positive values. Under optimum conditions, a sensitive PAD was pro- posed to determine glucose with wide linear dynamic ranges (0.01–1.3 mM and 1.3–5.0 mM) and a low detection limit (0.67 µM).
The sensor was suitable for the determination of glucose in honey, white wine and human serum. This novel non-enzymatic glucose PADs not only exhibited satisfactory sensitivity and specificity, but also showed many advantages including low-cost, disposability, time- and sample-saving which favors it for the development of POCT devices.
Acknowledgements
This research was funded by the Ratchadapiseksompoch Endowment Fund (2016), Chulalongkorn University (CU-59-012-FW).
S. Chaiyo is grateful to the Ratchadaphiseksomphot Endowment Fund, Chulalongkorn University, for a Postdoctoral Fellowship. The authors highly appreciate mobility funding by ASEA UNINET. O. Chailapakul is grateful to Thailand National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network.
Novelty statement
This work demonstrates for thefirst time the use of non-enzymatic electrochemical detection of glucose in paper-based analytical devices (PADs) as rapid, easy to use, inexpensive, and portable sensors for point-of-care monitoring. The assay is based on a carbon electrode screen-printed on paper and modified with cobalt phthalocyanine, graphene and an ionic liquid (CoPc/G/IL/SPCE). The results show that the modified electrode provides an increased surface area, improved conductivity and a good catalytic effect towards the oxidation of glu- cose. Due to the catalytic action of cobalt phthalocyanine oxidation of glucose occurs at less positive values. Under optimum conditions, glu- cose can be determined within wide linear dynamic ranges (0.01–1.3 mM and 1.3–5.0 mM) with a detection limit of 0.67 µM.
Appendix A. Supplementary material
Supplementary data associated with this article can be found in the online version athttp://dx.doi.org/10.1016/j.bios.2017.11.015.
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Paper-based sensors system Method Linear range (mM) LOD (µM) Ref
Zinc oxide nanowires electrode enzyme CA 5.12−15.12 94 (Li et al., 2015)
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CA: Chronoamperometric method, AM: Amperometric method, CM: colorimetric method.
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