Disposable Carbon Dots Modified Screen Printed Carbon Electrode Electrochemical Sensor Strip for Selective Detection of Ferric Ions

A disposable electrochemical sensor strip based on carbon nanodots (C-Dots) modified screen printed carbon electrode (SPCE) was fabricated for selective detection of ferric ions (Fe) in aqueous solution. C-Dots of mean diameters within the range of 1–7 nm were synthesized electrochemically from spent battery carbon rods.The analytical performance of this electrochemical sensor strip was characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).Thedeposition of C-Dots had enhanced the electron-transfer kinetics and current intensity of the SPCE remarkably by 734% as compared to that of unmodified SPCE. Under optimized conditions, the electrochemical sensor strip exhibited a linear detection range of 0.5 to 25.0 ppm Fe with a limit of detection (LOD) of 0.44 ± 0.04 ppm (at S/N ratio = 3). Validation of results by the electrochemical sensor strip was done by comparing analysis results obtained using an Atomic Absorption Spectrometer (AAS).


Introduction
Ferric ions (Fe 3+ ) are transition metal ions which play essential roles in biological activities, such as oxygen carriers in haemoglobin [1] and growth nutrients for phytoplankton [2,3].Deficiency in iron can result in anemia [4], yet high level of iron in human body may result in serious health problems, for instance, Alzheimer and Parkinson diseases [5,6].Iron may speed up the formation of reactive oxygen species in redox-active forms [7,8]; hence overdose of iron may result in diseases.Therefore it is important to monitor the level of iron in human body or in tap water supplies.
Conventionally, Atomic Absorption Spectroscopy (AAS) and Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) are being used for heavy metals analysis due to their wide range of detection and high sensitivity [9].However, these instruments are costly, time-consuming, and bulky as well as not portable for on-site testing.Furthermore, samples have to be transported from sites to laboratories, and preservation of samples is normally required [10,11].A portable, highly sensitive, and selective sensing system is highly desirable for rapid and accurate detection of heavy metals ions especially for in situ environmental monitoring.
Several ion-selective electrodes for detection of Fe 3+ have been reported.An ion-selective electrode based on -bis(tridentate) ligand was shown to be highly selective towards Fe 3+ with a limit of detection (LOD) of 0.276 ppm (evaluated as 5.0 × 10 −6 M) [12].Some researchers had proposed the use of poly(vinyl chloride) (PVC) membrane electrode incorporating 4,4  -dimethoxybenzil bisthiosemicarbazone (DBTS) and porphyrins as receptors [13,14].Fong et al. [15] had reported a fluorescence chemosensor based on carbon nanoparticles (CNP) synthesized from sodium alginate using nanoprecipitation and thermal acid carbonization method.This sensor worked by determining the fluorescence quenching of CNP in the presence of Fe 3+ and a LOD of 1.06 M was reported.However, these sensors for Fe 3+ ions still posed challenges of requiring the use of hazardous or expensive chemicals and complicated fabrication process.Therefore, a low-cost, portable, ecofriendly, and highly sensitive electrochemical sensor is highly desirable for on-site rapid detection of Fe 3+ ions.

Experimental
2.1.Reagents and Materials.All chemicals were purchased from Sigma-Aldrich Company, Merck Company, and Hamburg Company.Ultrapure water (∼18.2MΩ⋅cm, 25 ∘ C) was prepared using the Water Purifying System (ELGA Model Ultra Genetic).Hydrochloric acid (HCl) was obtained from R&M Chemicals.Carbon rods of spent EVEREADY Super Heavy Duty AA size primary battery were used for the preparation of C-Dots.Screen printed carbon electrodes (SPCE) consisting of carbon-based working and counterelectrodes and a silver/silver chloride (Ag/AgCl) reference electrode were purchased from a local vendor, Rapid Labs Sdn Bhd.Mineral water was purchased from Blue Ice Natural Mineral Water.

Electrochemical Preparation of C-Dots.
C-dots were prepared by adopting the previously reported method [16].A direct current power supply (GPR-6030D) was used as the power source.Two carbon rods (diameter = 0.48 mm) were used as both anode and cathode which were set parallel to each other and separated at a distance of 5 cm in 200 mL of ultrapure water.A constant voltage (50 V) was applied to the electrochemical cell and the electrolyte was constantly stirred for 96 hours.At the end of the process, the electrolyte turned black indicating the formation of C-Dots.The electrolyte was filtered using the quantitative filter disc Sartorius Grade 390.The filtrate dispersion was centrifuged at 13,500 RPM for 30 min to remove coarse graphite particles, and the supernatant was oven-dried at 80 ∘ C to obtain C-Dots.

Characterization of C-Dots.
The size and morphology of C-Dots were characterized using a transmission electron microscope (TEM) (JEOL JEM 1230).UV-Vis absorption spectra of the C-Dots were measured using a UV/Vis spectrophotometer (Jasco V-630).Fourier Transform Infrared (FTIR) spectra of C-Dots were obtained from KBr/sample pellets within the range of 400-4000 cm −1 using FTIR spectrometer (Thermo Scientific, Nicole iS10).

Fabrication and Characterization of C-Dots Modified SPCE Sensor Strip.
Commercial SPCE strips were modified with C-Dots using a simple drop-coating method.Simply, 10 L of C-Dots dispersion was added onto the surface of working electrode of SPCE.The SPCE was then dried in an oven at 90 ∘ C for 10 min.The C-Dots modified SPCE sensor strips were characterized by Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using a Potentiostat (Princeton Applied Research, PARSTAT 2263) in 0.01 M of HCl with a scan rate of 100 mVs −1 within a potential scan range of −0.5 to 0.5 V.The response of C-Dots modified SPCE strip was optimized by varying both the quantity of C-Dots deposited and the concentration of HCl electrolyte.10 L C-Dots dispersion of different concentrations (0.1-10.0 g) was drop-coated onto the working electrode of SPCE and oven-dried.These C-Dots modified SPCE strips were characterized by CV and EIS.HCl solutions of different concentrations (0.01-1.00 M) were purged with nitrogen gas for 5 min before use.Scheme 1 depicts the process on the fabrication of C-Dots modified SPCE working electrode.

Metal Ions Selective Study.
Stock solutions of various metal ions at concentration of 25.0 ppm were prepared.Among metal ions used for the selectivity studies included Ba 2+ , Ca 2+ , Cd 2+ , Co 2+ , Fe 3+ , Hg 2+ , Na + , K + , Sn 2+ , and Zn 2+ ions.These metal ions were more commonly found in water and associated with heavy metal pollution.10 L of these metal ions stock solutions was separately added dropwise onto the surface of C-Dots modified working electrode of SPCE, dried, and characterized by CV and EIS.Atomic Absorption Spectrophotometer (AAS) (Thermo Fisher Scientific TCE 3500) was used for determining the Fe 3+ ions concentration in real water samples.

Preparation and Characterization of C-Dots.
C-Dots of uniform size were successfully prepared by electrochemical oxidation of carbon rods of spent batteries.The preparation of carbon nanoparticles by electrochemical oxidation of graphite electrodes had been previously reported [16,17].TEM micrograph of the as-prepared C-Dots is shown in Figure 1(a).The C-Dots were observed to be spherical in shape with a size range of 1-7 nm and a mean diameter of 2.9 nm (inset Figure 1(a)).Figure 1(b) shows the UV-Vis absorption spectrum with the characteristic absorption peak of C-Dots at 225 nm which was attributed to the - transition of aromatic carbon [16,18].
Figure 2(a) shows FTIR spectrum of spent battery carbon rod.Carbon rod showed 3 distinctive peaks at 3450 cm −1 (O-H), 1635 cm −1 (C=C), and 1422 cm −1 (C-O) which were in consonance with functional groups of commercial graphite rod as reported in another work [16].Figure 2(b) shows FTIR spectrum of as-synthesized C-Dots.Absorption peaks at 1585 cm −1 and 1415 cm −1 were attributed to the COO − group [19].Absorption peaks at 3215 cm −1 and 1697 cm −1 were assigned to O-H and C=O [20], whereas peaks at 1228 cm −1 and 1089 cm −1 were attributed to C-O stretching vibration [16].As shown in the FTIR spectra, COO − groups were formed during electrochemical oxidation on the as-prepared C-Dots which therefore required no surface modification for their use in the detection of Fe 3+ ions.which could be detached easily from the SPCE.As such, the optimum 10 g of C-Dots was deposited to modify SPCE for studies on metal ions detection.

Ferric Ions (Fe 3+ ) Detection by C-Dot Modified SPCE
Sensor Strip.As shown in Figure 4(a) C-Dots-modified SPCE (curve II) exhibited a remarkable 734% higher anodic peak current intensity as compared with that of unmodified SPCE (curve IV), indicating that the deposition of C-Dots had substantially enhanced the overall electrical conductivity of SPCE.Upon addition of 10 L of 25.0 ppm Fe 3+ ions onto C-Dots-modified SPCE, the current intensity was observed to have increased substantially (curve I).However, addition of the same quantity of Fe 3+ ions onto unmodified SPCE did not result in any notable change of current intensity (curve III).As shown in Figure 4 (b), both Nyquist plots of unmodified SPCE and Fe 3+ -unmodified SPCE were nearly identical with semicircular features of similar large diameters, indicating high electron-transfer resistance ( et ).In contrast, both C-Dots-modified SPCE and Fe 3+ ions immobilized C-Dotsmodified SPCE showed semicircular features of substantially smaller diameter, indicating much lower  et .These observations were consistent with results of CV.
Figure 5(a) shows the cyclic voltammograms of C-Dots modified SPCE in the presence of various concentrations of Fe 3+ ions.The current intensity of C-Dots-modified SPCE was observed to increase linearly with increasing concentrations of Fe 3+ ions, within the range of 0.5 to 25.0 ppm (Figure 5(b)).Such increase in current intensity could be attributed to more Fe 3+ ions being bound to -COO − groups on the surfaces of C-Dots, which led to higher electrical conductivity of the C-Dots-modified SPCE.Under optimized conditions, the LOD for the detection of Fe 3+ ions by the C-Dots modified SPCE sensor strip was determined to be 0.44 ± 0.04 ppm.

Selectivity Analysis of C-Dots Modified SPCE Sensor Strip.
The selectivity of C-Dots-modified SPCE sensor strip was investigated with a wide range of metal ions including Ba 2+ , Ca 2+ , Cd 2+ , Co 2+ , Hg 2+ , Na + , K + , Sn 2+ , and Zn 2+ ions which were commonly associated with heavy metal pollution in water.Selectivity tests were conducted under experimental conditions optimized in this study.As shown in Figure 6, the current intensity of C-Dots-modified SPCE sensor strip was the highest in the presence of Fe 3+ ions among all metal ions evaluated.Addition of 10 L of 25.0 ppm of Fe 3+ ions had led to increase in the current intensity of C-Dots-modified SPCE sensor strip by 259% as compared to the blank signal.In contrast, all other metal ions were observed to have negligible effects on the current intensity of C-Dots-modified sensor strip.Hence, the C-Dots-modified SPCE sensor strip was observed to exhibit high selectivity towards Fe 3+ ions.
In addition, the interference study was conducted in order to evaluate the selectivity of C-Dots-modified SPCE sensor strip for the detection of Fe 3+ ions in the presence of other Hence, the addition of Fe 3+ ions onto the working electrode of the sensor strip would result in higher current intensity which in turn gave rise to higher relative error in the Fe 3+ ions concentration determination.The selectivity of C-Dots-modified SPCE sensor strip was attributed to the presence of the carboxylate groups (-COO − ) on the surfaces of C-Dots with strong coordination affinity towards Fe 3+ ions [14,24].The formation of Fe 3+ -COO − complexes could have given rise to higher electrical conductivity of the sensor strip.

Real Sample Analysis.
The potential application of C-Dots modified SPCE sensor strip for the detection and quantification of Fe 3+ ions in real samples was evaluated by using samples of tap water, reverse osmosis (RO) water, and commercial mineral drinking water.Water samples were filtered with filter paper and then spiked to prepare water samples containing 10.0 ppm of Fe 3+ ions.As shown in Table 2, the % recovery of Fe 3+ ions from these samples as determined by using the C-Dots modified SPCE sensor strip ranged between 92.3% and 97.5% with relative standard deviation (RSD) of 5.1% to 6.8%.These analysis results were further validated against those obtained by AAS.Analysis results for all real samples obtained by both AAS and C-Dots modified SPCE sensor strips were consistent and comparable.The C-Dots modified SPCE sensor strip was shown to be sensitive and selective for the detection of Fe 3+ ions in aqueous samples.

Conclusion
A disposable C-Dots modified SPCE sensor strip for sensitive and selective detection of Fe 3+ ions in aqueous samples had been fabricated.C-Dots were prepared from carbon rods of spent batteries using a green electrochemical method.Under optimized conditions, the LOD for Fe 3+ ions by C-Dots modified SPCE sensor strip was determined to be 0.44 ± 0.04 ppm.High percentage recovery of Fe 3+ ions from various water samples with low % RSD and low % relative error in the presence of other metal ions showed that the C-Dots modified SPCE sensor strip could potentially be used for sensitive and selective detection of 3+ ions in real samples.The detection limit can further be improved in the future by doping the C-Dots with other elements such as nitrogen (N) and sulfur (S) for more sensitive detection and application in water quality studies.

Scheme 1 :
Scheme 1: Schematic diagram of the fabrication of C-Dots modified SPCE sensor strip and its use for the detection of Fe 3+ ions.

Figure 1 :− 1 )Figure 2 :
Figure 1: (a) TEM micrograph of C-Dots electrochemically synthesized from carbon rods of spent batteries.(b) UV-Vis absorption spectrum of C-Dots.Inset in (a) shows the particle size distribution of C-Dots.

3. 2 .Figure 3 :Figure 4 :
Figure 3: Effect of (a) concentration of HCl and (b) concentration of C-Dots on the current intensity of C-Dots modified SPCE sensor strip.(Error bars were calculated from the mean value, / = 3.)

Figure 5 :Figure 6 :
Figure 5: (a) Cyclic voltammogram of C-Dots modified SPCE with various concentrations of Fe 3+ ions and (b) relationship between current intensity and Fe 3+ ions concentration within the range of 0.5 to 25.0 ppm.(Error bars were calculated from the mean value, / = 3.)

Table 1 :
Relative error in the determination of Fe 3+ ions concentration in the presence of other metal ions at 10 ppm.The relative errors of Fe 3+ ions concentration determined by the sensor strip due to interferences of other metal ions were calculated as shown in Table1.Interferences from Sn 2+ and Na + were observed to cause comparatively higher errors of 18.68% and 14.85% in the determination of Fe 3+ ions concentration.The presence of Sn 2+ ions on C-Dots modified SPCE sensor strip which exhibited higher current intensity than that of C-Dots modified SPCE sensor strip alone.

Table 2 :
Percentage recovery of spiked Fe 3+ ions for various water samples using C-Dots modified SPCE sensor strips and AAS.