Platinum Nanoparticles Supported by Carbon Nanotubes: Improvement in electrochemical sensor performance for caffeine determination

Abstract

Platinum nanoparticles supported by carbon nanotubes were obtained by a simple chemical route and used for preparation of electrochemical sensor towards caffeine determination. Carbon nanotubes were used before and after an acid treatment, yielding two different materials. Morphological and structural characterization of these materials showed platinum nanoparticles (size around 12 nm) distributed randomly along carbon nanotubes. Modified electrodes were directly prepared through a dispersion of these materials. Voltammetric studies in the presence of caffeine revealed an electrocatalytic effect of platinum oxides, electrochemically produced from the chemical oxidation of the platinum nanoparticles. This behavior was explored in the development a selective method for caffeine determination based on platinum oxide reduction at a lower potential value (+0.45 V vs. Ag/AgCl). Using the best set of experimental conditions, it was shown a linear relationship for the caffeine concentration ranging from 5.0 to 25 µmol L^-1 with a sensitivity of 449 nA L µmol^-1. Limits of detection and quantification of 0.54 and 1.80 µmol L^-1 were calculated, respectively. Recovery values for real samples of caffeine pharmaceutical formulations between 98.6% and 101.0% (n = 3) were obtained using the proposed procedure. Statistical calculations showed good concordance (95% confidence level) between the added and recovery values.

Key words
Caffeine determination; carbon nanotubes; electrochemical sensors; platinum nanoparticles

INTRODUCTION

Platinum has a wide performance as catalyst, with a potential application in various areas and products such as fuel cells (Dhanasekaran et al. 2020, Nair et al. 2018, Castro et al. 2010), capacitors (Park et al. 2009, San et al. 2014) and sensors (Anuar et al. 2020, Kimuam et al. 2020, Oliveira et al. 2020). In the field of electrochemical sensors, several methods have been developed based on catalytic effect of platinum for determination of different organic and inorganic targets such as dapsone (Caetano et al. 2012) and hydrogen peroxide (Hall et al. 1997, Evans et al. 2002). Nanoparticles offer an improvement of catalytic performance due to their high surface area, which means that nanomaterials can provide an enhancement on catalytic effect.

For a better performance in real application, metallic platinum or platinum oxide nanoparticles must be anchored in appropriate substrate, such as carbon nanotubes (CNTs) (Bergamaski et al. 2006, Mu et al. 2005), graphene (Cuong et al. 2020, Kim et al. 2020, Li et al. 2020) or polymers (El-Raheem et al. 2020, Huang et al. 2020, Zhang et al. 2020). Advantages such as a control in the size, distribution and amount of the deposited particles (Sandbeck et al. 2020) can be reached. Moreover, the choice of an adequate substrate decreases the costs due to the vanishing in the amount of platinum employed (Tzitzios et al. 2006). Immobilization of metallic nanoparticles over carbon nanotubes present additional advantages, since CNTs are a conductive material (Souza et al. 2011, de Oliveira et al. 2020, Inagaki et al. 2019). In addition, it can present a large surface area, fast response time, higher electron transfer rate, easiness to immobilize molecules and particles, among others (Yang et al. 2010).

Caffeine is an alkaloid of xanthine group with chemical formula C₈H₁₀N₄O₂ and named as 1,3,7-trimethylxanthine (Figure 1) found in many foods and beverages (tea, coffee, cola and cocoa beans) daily consumed (Zhao et al. 2011). The regular consume of caffeine leads to a series of changes in the body such as the stimulation of central nervous system, relaxation of bronchial muscles, gastric acid secretion, and diuresis. Additionally, if it consumed in excess can cause cardiovascular problems, renal failure, asthma, depression, and hyperactivity (Zhao et al. 2011).

Figure 1
Molecular structure of Caffeine.

Quality control by monitoring this specie is frequently realized by chromatographic methods due its great precision and accuracy (McCusker et al. 2003). However, this methodology is expensive, has a high consume level of samples, presents low frequency of analysis, difficulty for in loco determination, high cost of reagents, beyond the need of a specialized technical to analyses (Plata et al. 2010, Sun et al. 2011, Yang et al. 2010). The development of new methodologies for caffeine determination is extremely important. In this way, electroanalytical techniques have shown as a viable alternative, because of its simplicity, low cost, high sensibility and even to allow an application in the collection site (Murray et al. 2005, Stetter et al. 2003, Sun et al. 2011, Zhao et al. 2011). Electrochemical determination of caffeine seems to be exceedingly difficult in common electrodes (metal, glassy carbon, etc.) due to the high overpotential for its oxidation. Electrooxidation of caffeine presents an irreversible peak around +1.3-1.5V (vs SCE) through a xanthine similar mechanism with formation of uric acid 4,5-diol. A secondary oxidation reaction can lead to formation of several compounds as parabenic acid, dimethyl urea, dimethyl alloxan, urea and 6,8-dimethyl Allantoin (Hansen & Dryhurst 1971, Nunes & Cabalheiro 2012, Spãtaru et al. 2002). Zhao et al. (2011) and Sun et al. (2011) reported a caffeine electrochemical sensor based on a glassy carbon electrode modified with nafion/graphene oxide by differential pulse voltammetry in potential around +1.45-1.50 V (vs Ag/AgCl). Likewise, Yang et al. (2010) reported a potential around +1.33 V (vs SCE) using a glassy carbon electrode modified with Nafion/multiwalled carbon nanotubes. Caffeine electrochemical oxidation was observed at around +1.50 V (vs SCE) using carbon fibers microelectrodes (Nunes & Cavalheiro 2012). Chemically modified electrodes have been also reported, as described by Aklilu et al. (2008) and Rezaei et al. (2014).

Systematic chemical route to prepare platinum nanoparticles over the surface of multi-walled carbon nanotubes was previously reported by some of us (Kalinke & Zarbin 2014). Here we demonstrate the electrocatalytic effect of these Pt nanoparticles supported in CNTs towards caffeine oxidation. As an alternative strategy, the present work proposes a caffeine quantification based on an Electrochemical–Chemical (EC’) catalytic mechanism (Calegari et al. 2017, Kalinke et al. 2019, Nahirny et al. 2020, Toito-Suarez et al. 2006), which allows its quantification at a lower overpotential than commonly reported (Júnior et al. 2020). Catalytic platinum oxides (eg. PtO/PtOOH) yielded at high potential values (Electrochemical step) provide a cathodic signal proportional to nanomaterial present at electrode surface (Caetano et al. 2012). However, in presence of caffeine there is chemical consumption of platinum oxides (Chemical step) leading to a decrease of cathodic peak current, which allowed indirect caffeine determination.

MATERIALS AND METHODS

Reagents and solutions

All reagents used in this work have analytical purity. Hexachloroplatinic acid hexahydrate (Riedel-de Haen), sodium borohydride (Acros), toluene (Carlo Erba), sulfuric acid (Carlo Erba), isopropanol (Carlo Erba) and caffeine (Proquimios) have been used as received. Ferrocene was purified by sublimation prior to use. Aqueous solutions were prepared using distilled water.

Preparation and treatment of carbon nanotubes

Carbon nanotubes were prepared according our previous report (Schnitzler et al. 2003), based on the decomposition of ferrocene. They are essentially multiwalled carbon nanotubes, with a medium diameter of 70 nm, and with their cavities filled by long crystals of iron-based compounds, mainly α-Fe, Fe₂O₃ or Fe₃O₄. The nanotubes were pretreated in order to increase their dispersibility in organic solvents (Moraes et al. 2011): 20 mg of CNT were placed in a round-bottom flask with 50 mL of toluene and submitted to an ultrasound bath for 1 hour. After, the dispersion was decanted for 40 minutes and centrifuged. The material was washed three more times with toluene and acetone and taken to an oven at 100 °C for 24 hours. The loss of material during all this process was approximately 10 wt%. A portion of the CNTs (15 mg) was also treated under reflux in aqueous solution of HNO₃ (3.0 mol L^-1) and H₂SO₄ (3.0 mol L^-1), for 6 hours (Matos et al. 2012, Moraes et al. 2011). After treatment, CNTs were separated from solutions by centrifugation, washed several times with deionized water to neutral pH and kept in an oven at 100 °C until completely dry.

Preparation of Pt nanoparticles supported by CNTs

The preparation of the Pt nanoparticles was carried out according our previous report, based on a chemical reduction of hexachloroplatinic acid solution in the presence of a CNTs dispersion (Kalinke & Zarbin 2014). Two different samples were prepared and employed as electrochemical sensor in this work, starting from bare CNTs or acid treated CNTs and fixing all the other synthetic parameters. Briefly, 5.0 mg of CNTs were dispersed in 10 mL of toluene under ultrasound bath for 3 minutes and dripped into 3.75 mL of an aqueous solution containing 750 μL of a solution 0.3 mol L^-1 of H₂PtCl₆ which was stirred for 10 minutes. Then, under stirring, 3.5 mL of aqueous solution containing 0.37 g of NaBH₄ was rapidly introduced to the system with a syringe, and the system was kept under stirring for 3 h. In the sequence, the organic phase was separated from the aqueous one and added 40.0 mL of ethanol; the system was maintained at -18°C for 1h. The resulting precipitate was then separated by centrifugation, washed several times with ethanol and dried in an oven at about 40°C. This material was named Pt/CNT. It was also developed a synthesis starting from the acid treated CNTs, as described before. This material will be referred here as Pt/CNT_treat.

Preparation of the modified electrode

1.0 mg of each material was dispersed in a mixture of 113 µL of isopropanol, 113 µL of distilled water and 12 µL of a Nafion solution of 0.5 wt% in an ultrasound bath for 20 minutes. After this, an amount of 2 or 6 µL of this dispersion was transferred onto the surface of a glassy carbon electrode with diameter of 3 mm. The solvent was evaporated under ambient conditions and the dispersed material was obtained as a film at the electrode surface.

Preparation and analysis of caffeine sample

The proposed method was applied for caffeine determination in pharmaceutical formulation purchased from local market. Solid samples containing caffeine were adequately ground in an agate mortar and an amount of powder was dissolved in 0.10 mol L−¹ H₂SO₄ (insoluble excipients were filtered off with a filter paper before dilution 0.10 mol L−¹ H₂SO₄). No added treatment of sample was need. Standard addition method was adopted to determination of caffeine in the sample. The results obtained using the proposed sensor were compared with labeled value provides by manufacturer. Recovery studies were carried out with samples containing caffeine in the concentration range of 5.0–20 µmol L−¹.

Characterization techniques

The X-ray diffraction (XRD) data were obtained in a Shimadzu XRD 6000 equipment, at 0.02° min−¹. The Raman spectra have been collected in a Renishaw equipment, with spatial resolution of 1.0 μm, using Ar⁺ laser (λ = 514.5 nm). Spectra have been obtained from samples in the form of powder deposited on quartz substrates. The transmission electron microscopy (TEM) images were collected on a JEOL JEM 1200 equipment with voltage of 120 kV. The samples were prepared by adding a drop of the dispersion of the nanocomposite in ethanol on copper grids coated with thin carbon film.

Voltammetry measurements have been done in a potentiostat/galvanostat AUTOLAB (PGSTAT 128N) connected to a microcomputer controlled by a GPES program in a typical three electrodes electrochemical cell has been used for the measurements, composed by an Ag/AgCl (KCl 3.0 mol L^-1) as reference electrode, a platinum wire as the counter-electrode and a glassy carbon electrode (3 mm in diameter) modified with the nanocomposites as working electrode. A H₂SO₄ 0.1 mol L^-1 aqueous solution was used as supporting electrolyte. Before the measurements, the working electrode was pretreated by performing 400 voltammetric cycles at a rate of 50 mV s^-1 for the stabilization of the current. All the experiments were carried out at room temperature.The measurements were performed in a potential range between -0.2 to 1.0 V for cyclic voltammetry and between 0.0 and 1.0 V in the linear sweep (vs Ag /AgCl) for the sensor, using the cathodic peak decreased to evaluate the reactions of materials with caffeine (Caetano et al. 2012).

RESULTS AND DISCUSSION

Morphological, structural and electrochemical characterization

The XRD and Raman spectra of the carbon nanotubes, the Pt/CNT and Pt/CNT_treat materials are shown in Figure 2. The XRD pattern of the neat carbon nanotubes shows the diffraction peak at around 2θ = 26⁰ characteristic of multi-walled carbon nanotubes (Moraes et al. 2011, Matos et al. 2012) as well as peaks due the presence of α-Fe, Fe₃O₄ and Fe₃C, inherently present in the cavities of carbon nanotubes, resulting from the method of preparation (Matos et al. 2012, Cao et al. 2001) (Figure 2a). The intensity of the iron-based species peaks decreases significantly after acid-treatment, due the solubilization of these species. It was demonstrated that the iron content reduces from 43% to 14% after the acidic treatment (Kalinke & Zarbin 2014, Moraes et al. 2011). The XRD data of both the nanocomposites present, besides the CNTs-based peaks discussed before, the characteristic peaks of metallic platinum with a fcc structure (Cao et al. 2001, Kalinke & Zarbin 2014, Li et al. 2003).

Figure 2
(I) XRD patterns and (II) Raman spectra of samples: (a) CNT; (b) CNT_treat; (c) Pt/CNT; (d) Pt/CNT_treat.

Figure 2b shows the Raman spectra of the materials. The characteristic bands of sp²-based carbonaceous materials are detected in all spectra: one band at approximately 1350 cm^-1 (D band); one band at around 1600 cm^-1 (G band) and a third band at approximately 2700 cm^-1 (2D band) (Neto et al. 2005).

As can be seen, the presence of Pt nanoparticles in the nanocomposite has small effect on the profile of the Raman spectra of CNTs, which can be an indicative of a poor chemical interaction between the two components. Figure 3 shows TEM images of the Pt/CNT materials, which was characterized by small platinum nanoparticles (medium size around 12 nm) with heterogeneous distribution around the CNTs.

Figure
TEM images of (a) Pt/CNT and (b) Pt/CNT_treat materials.

The cyclic voltammograms of glassy carbon electrodes modified by both Pt/CNT and PtCNT_treat materials in a 0.1 mol L^-1 solution of H₂SO₄ and scan rate of 50 mV s^-1 are shown in Figure 4. Both voltammograms show characteristic voltammetric profile of platinum, presenting the region of adsorption/desorption of hydrogen between -0.15 V and 0.15 V, with peaks at -0.08 V and 0.04 V in the forward direction of potential scanning.

Figure 4
Voltammetric profile of (a) Pt/CNT; (b) Pt/CNT_treat materials. Scan rate: 50 mV s−¹, supporting electrolyte: H2SO4 0.10 mol L−¹.

These results are assigned to strong and weak hydrogen bonds with platinum (Neto et al. 2007). The voltammograms also show the characteristic area of the platinum oxide formation/reduction at 0.60 V and 0.55 V, respectively. The modified electrodes were subjected to successive scan cycles between -0.2 and 1.0 V vs. Ag/AgCl in the presence of supporting electrolyte to evaluate the number of required cycles to stabilize the current. It was noted that after 400 cycles (Figure S1) the current variations were negligible.

Electrocatalytic effect towards caffeine determination

After the stabilization, the modified electrodes were evaluated in order to construct an electrochemical sensor for caffeine determination. Figure S2 shows the voltametric profile of the proposed electrode in the absence and presence of caffeine. The proposed interaction is similar to an Electrochemical–Chemical (EC’) mechanism (Bergamini et al. 2007, 2006, Fatibello-Filho et al. 2007): initially, occurs the electrochemical oxidation of Pt to PtO/PtOOH. After that, the oxidation of caffeine is chemically performed by the platinum oxides. In the second step, the chemical oxidation of caffeine is performed by Pt-oxides, which are regenerated at positive potential value, leading to an increase in the anodic peak current and a decrease in the cathodic peak. No faradaic process was observed when an unmodified electrode or an electrode modified with only CNTs (pretreated or not) were evaluated in presence of caffeine. These results have shown that platinum oxides have an important role in the chemical caffeine oxidation and this material could be used as modifier of electrode for development of electrochemical sensors.

For analytical purposes, the use of anodic condition at high potential values to detect platinum oxides (eg. PtO/PtOOH) formation must be avoided because it occurs close to solvent/electrolyte decomposition increasing significatively the noise. So, an alternative quantification strategy was need to monitoring caffeine concentration. As can be seen in Figure 5a for the Pt/CNT_treat material, the presence of 100 µmol L^-1 of caffeine caused a significant decrease in the cathodic peak current at +0.45 V vs. Ag/AgCl, attributed to an interaction between Pt oxides and caffeine molecules. This decrease in the cathodic peak is proportional to the increase in the caffeine concentration and it was used for analytical purposes. Additionally, the Pt/CNT material has shown lower detectability when compared with the Pt/CNT_treat (∆I≅ 30.4 μA for Pt/CNT and 37.9 μA for PT/CNT_treat). Figure 5b shows a proposed mechanism for voltammetric response of the modified electrode.

Figure 5
(A) Linear sweep voltammograms obtained at 50 mV s−¹ for the Pt/CNT_treat in 0.10 mol L−¹ H₂SO₄ solution containing different caffeine concentrations (a) absence, (b) 5.0 µmol L^-1, (c) 10.0 µmol L^-1, (d) 15.0 µmol L^-1, (e) 20.0 µmol L^-1, (f) 25.0 µmol L^-1. (B) Analytical curve obtained from the cathodic peak currents.

Analytical features

In order to verify the analytical performance of the proposed sensor, analytical curves for caffeine were obtained using linear potential in the cathodic direction, between 1.0 V and 0.0 V. Before each scan, a potential of +1.2 V vs. Ag/AgCl was applied for 100s, in order to ensure the formation of oxide on the electrode surface. Figure 6 shows the linear sweep voltamograms (A) and the variation of the cathodic peak current with successive additions of caffeine (B) for the electrode modified with Pt/NTC_treat.

Figure 6
(A) Linear sweep voltammograms obtained at 50 mV s−¹ for the Pt/CNTtreat in 0.10 mol L−¹ H2SO4 solution containing different caffeine concentrations (a) absence, (b) 5.0 µmol L^-1, (c) 10.0 µmol L^-1, (d) 15.0 µmol L^-1, (e) 20.0 µmol L^-1, (f) 25.0 µmol L^-1. (B) Analytical curve obtained from the cathodic peak currents.

Analytical curve was linear over the concentration range of caffeine between 5.0 µmol L^-1 and 25 µmol L^-1 of caffeine (-∆I _pc(μA) = -1.40 + 0.449 × C_CAF (µmol L^-1)). Limit of detection (LOD) of 0.54 µmol L^-1 and limit of quantification (LOQ) of 1.80 µmol L^-1 were calculated as three and ten times standard deviation of blank (n=3) divided by slope of calibration plot, respectively (Miller & Miller 2005). As described before, caffeine is an electroactive compound that usually oxidizes at around 1.50 V. In this work the oxidation occurred at around 1.0 V and it was indirectly monitored at +0.45 V based on platinum oxide reduction. The potential value used in this work is lower than those commonly found in the literature and it promotes an improvement in selectivity of the method.

Electrode modified with Pt/CNT_treat nanocomposite has higher sensitivity and lower detection limit in all analyzed situations when compared with the electrode modified with Pt/CNT. This result has been attributed to the presence of functional groups (carboxyl and hydroxyl) on the surface of acid treated CNTs (in the Pt/CNT_treat nanocomposite), which increase the number of defects in the material and make it more reactive. These defects generated by the functional groups have high reactivity and easiness of oxidation at low potential (Kalinke & Zarbin 2014). During the electrochemical measurements, the groups containing oxygen provide continuous defects to the CNTs to maintain the defects that are being consumed (Chen et al. 2006).

Recovery studies were performed to investigate the possibility of applying the proposed sensor in the determination of caffeine in real samples of pharmaceutical formulations. Values between 98.6% and 101.0% (n = 3) of caffeine were obtained using the proposed procedure. Statistical calculations showed good concordance (at 95% confidence level) between the added and recovery values. Voltametric measurements (n =5) were recorded using the same electrode (repeatability) as well as different electrodes (reproducibility) in absence and presence of 10.0 μmol L−¹ of caffeine. Relative standard deviations (RSD) of 3.8% and 5.3% were found for repeatability and reproducibility, respectively. Successive voltametric scan (n= 30) shown only a slight decrease of less than 10% on response, suggesting no leaching of nanomaterial from electrode neither significative poisoning of surface of sensor.

CONCLUSIONS

This work demonstrates a simple and feasible route to synthesize platinum nanoparticles supported by carbon nanotubes as well as their utilization as electrochemical sensor to caffeine. In presence of caffeine, modified electrode acted following a well-knowledge Electrochemical–Chemical (EC´) mechanism against target oxidation. Electrodes modified with the nanocomposites showed a decrease on cathodic peak observed at low potential values (~0.45V), which was proportional to the increase in the caffeine concentration used for analytical purposes. Nanocomposites prepared with carbon nanotubes previously treated with acid showed better analytical response, due the presence of functional groups on the surface of these materials. Linear responses for caffeine in concentrations have been found, between 5.0 µmol L^-1 and 25 µmol L^-1 with limits of detection and quantification of 0.54 and 1.80 µmol L^-1, respectively. In addition, potential value used in this work is lower than those commonly found in the literature and it promotes an improvement in selectivity of the method. In addition, disposable and/or miniaturized systems can be build using stable dispersion of CNT decorated with Pt nanoparticles, which can be easily combined to commercial or even lab made potentiostats. Typically, lab made apparatus are simple and have the advantage of being fully portable and managed by cell phone.

ACKNOWLEDGMENTS

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brazil (CAPES - Finance Code 001) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). Authors also acknowledge the financial support from UFPR, Fundação Araucária and National Institute of Science and Technology of Carbon Nanomaterials (INCT-Nanocarbon) and National Institute of Science and Technology of Nanomaterials for Life (INCT NanoVida).

SUPPLEMENTARY MATERIAL

Figues S1, S2.

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Publication Dates

• Publication in this collection
  22 Apr 2024
• Date of issue
  2024

History

• Received
  19 Jan 2023
• Accepted
  13 June 2023