Highly Selective and Stable Electrochemical Sensor for Hydrogen Peroxide—Application in Cosmetics Quality Control

Abstract

Nowadays, electrochemical sensors have become a popular topic in cosmetics quality control. A simple and stable electrochemical sensor for hydrogen peroxide (H₂O₂) was developed on the basis of a rhodium-modified glassy carbon electrode (Rh/GCE). A quick, one-step, reproducible, and cost-effective electrodeposition procedure was applied to modify GCE with Rh nanoparticles. The sensor shows a high selectivity for H₂O₂ at a low applied potential of −0.1 V (vs. Ag/AgCl, 3 M KCl), with an excellent stability and good repeatability (RSD = 3.2%; n = 5). The modified electrode Rh/GCE demonstrates a high sensitivity of 172.24 ± 1.95 μA mM⁻¹ cm⁻² (n = 3), a linear response to H₂O₂ between 5 and 1000 µM, and a detection limit estimated to be 1.2 µM. Furthermore, Rh/GCE has been successfully used to measure H₂O₂ concentrations in hair dye and antiseptic solution, yielding satisfactory recovery rates. These findings highlight the potential of the Rh/GCE for the reliable quantitative detection of H₂O₂ in complex cosmetics matrices.

1. Introduction

Hydrogen peroxide (H₂O₂) is an active and widely used oxidizing agent with a bleaching and whitening effect, commonly found in household products and personal care items. In the fields of personal care and cosmetics, H₂O₂ is classified as an antimicrobial agent, a cosmetic biocide, an oral health care agent and an oxidizer [1]. Hydrogen peroxide is the active component in various cosmetics, such as hair dyes, hair lighteners, and oral hygiene products (mouth rinses, dentifrices, tooth-whitening products). Additionally, 3% H₂O₂ is easily available for purchase as an antiseptic solution.

For decades, H₂O₂ has been used in the hairdressing industry in hair coloring formulations. Hair bleaching methods are oxidative processes, and H₂O₂ is the most frequently used oxidant because it is an excellent decolorizing agent for melanin (natural hair pigment). Permanent hair dyes are the most common type of hair dye, producing a long-lasting effect that is resistant to washing. Briefly, the two components—“colorant” (that contains the dye precursors and an alkaline agent) and “developer” (which contains H₂O₂)—are mixed immediately before the use. Alkaline agents swell the hair cuticles, allowing the coloring molecules to penetrate deeper into the cortex. The dye precursors are oxidized by H₂O₂ to form quinone diimine intermediates, which then react with coloring couplers to give the final reaction product—large polynuclear structures that become entrapped inside the hair shaft [2,3]. The balance of H₂O₂ and dye precursors can be adjusted by manufacturers to achieve a lightening, darkening, or matching of the hair’s natural color.

The use of H₂O₂ in cosmetic products is covered by strict legislation due to its unwanted side effects, including skin irritation, burning, gum irritation, tooth sensitivity, cytotoxicity, and pain conduction in dental pulp stem cells [4], hair dye-induced dermatitis, and even hair loss [3,5]. The European Commission (EC) restricts the amount of H₂O₂ in cosmetic products—a maximum concentration of 4% H₂O₂ in products applied to the skin and 12% H₂O₂ in products applied to the hair [6]. Dyes intended for use on eyelashes are safe when they contain up to 2% H₂O₂ [6]. H₂O₂ is an aggressive oxidizer that damages human skin; therefore, a concentration of H₂O₂ higher than 20% is considered to be highly hazardous. The EC Scientific Committee on Consumer Safety, SCCS, concluded that the use of oral hygiene and tooth-whitening products containing up to 0.1% H₂O₂ does not pose a risk to consumer health. Products containing more than 0.1% H₂O₂ should be exclusively administered under the supervision of a qualified dental professional.

Thus, the rapid and reliable detection of H₂O₂ in cosmetics is of great significance for ensuring human health. For the H₂O₂ assay, various strategies have been investigated, including titration [7], HPLC [8], spectrophotometry [9], and chemiluminescence [10]. However, these methods require specific, expensive equipment, and involve time-consuming and labor-intensive sample preparation steps.

Electroanalysis is becoming a hot topic in the field of cosmetic quality control [11,12,13,14,15,16,17,18]. Electrochemical sensors are compact, portable devices that enable the fast, precise, and low-cost quantitative detection of the target analytes. These analytical systems offer practical advantages, including simplicity, high sensitivity, extremely low limits of detection (LOD) and quantification (LOQ), wide dynamic range, and the possibility of automated measurements. Thus, the electrochemical sensors serve as a particularly suitable alternative to the traditional analytical techniques (most often titrimetric and spectrophotometric) currently applied in laboratories.

The electrochemical non-enzymatic detection of H₂O₂ is based on two-electron oxidation (Equation (1)) or reduction (Equation (2)) on the surface of an electrocatalyst:

$${\mathrm{H}}_2{\mathrm{O}}_2\to {2\mathrm{H}}^{+}+{\mathrm{O}}_2+2{\mathrm{e}}^{-}$$

(1)

$${\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{e}}^{-}+2{\mathrm{H}}^{+}\to {2\mathrm{H}}_2\mathrm{O}$$

(2)

The main disadvantage of electrochemical sensors based on the electrooxidation of H₂O₂ is the need for repeated calibration as well as accounting for the co-oxidation of electroactive species present in real samples that affect the response. Electrochemical platforms based on the electroreduction of H₂O₂ at working potentials around and below 0.0 V (vs. Ag/AgCl) represent a promising alternative, providing a practically interference-free response to target analytes. The potential range from −0.2 V to 0.0 V is optimal for electroanalytical measurements—under these conditions, the quantitative determination of H₂O₂ is highly selective, since the interference of electrochemically active compounds in real samples is minimized or completely eliminated. At this potential range, the effective electroreduction of H₂O₂ takes place without the interference of molecular oxygen (O₂), since oxygen reduction begins around −0.4 V and progressively accelerates with the application of more negative potential values.

Generally, the analytical performance of the electrode is strongly affected by the intrinsic activity and the number of accessible catalytic sites [19,20]. Therefore, sensors based on nanomaterials have demonstrated superior electrocatalytic performance compared to their conventional counterparts. According to recent comprehensive overviews on non-enzymatic catalysts designed for H₂O₂ electrochemical detection, developing innovative H₂O₂ sensing materials with controllable particle size and adjustable surface properties through simple, fast, and eco-friendly preparation processes still remains an important direction and a challenge for future research [21,22].

Since 2010, only a few articles regarding the electrochemical determination of H₂O₂ in cosmetics and personal care products were found [23,24,25,26,27,28,29,30], revealing a research gap in this field. To the best of our knowledge, there are no reports on the detection of H₂O₂ in cosmetics using Rh-modified electrodes. The proposed modified electrode Rh/GCE has significant advantages over previously reported H₂O₂ sensors, both in electrochemical behavior and in real-sample applications. The sensing platform enables single-step fabrication, demonstrates an excellent stability and good reproducibility for H₂O₂ detection, and provides the accurate measurement of H₂O₂ levels in real cosmetic and antiseptic samples. The use of Rh/GCE in the presence of dissolved oxygen holds promise for the development of portable electroanalytical devices for the in-field analysis of H₂O₂, especially for cosmetics quality control.

2. Materials and Methods

2.1. Materials

RhCl₃ × nH₂O, HCl, H₂O₂ (30%), NaOH, Na₂HPO₄.12H₂O, NaH₂PO₄.2H₂O, NaNO₃, Na₂SO₄, Na₂S₂O₃, Na-EDTA, KCl, KI, Mg(NO₃)₂, and (NH₄)₂MoO₄ were purchased from Fluka. K₃[Fe(CN)₆], K₄[Fe(CN)₆], and H₃PO₄ were purchased from Merck KGaA, Darmstadt, Germany. Salicylic acid and glycerol were purchased from Sigma-Aldrich Co. St Louis, USA. Starch was obtained from POCH (Poland). All the reagents used were of analytical grade. Double-distilled water was used to prepare aqueous solutions. Phosphate-buffered solution PBS (0.1 M, pH 7.0) was made of monobasic and dibasic sodium phosphates. The pH was adjusted by adding 0.5 M NaOH.

All the solutions used in the electrochemical studies were prepared immediately before the experiment. The working solution 0.03 M H₂O₂ was freshly prepared and stored in an ice bath. A solution containing 5 mM K₃[Fe(CN)₆] and 5 mM K₄[Fe(CN)₆], dissolved in 0.1 M KCl, was used in the determination of the electrochemical surface area, ECSA.

For the real sample analysis of commercial products, an antiseptic solution (3% H₂O₂) and a commercial hair dye sample (obtained from a local market) were used.

The working electrode was a disk from glassy carbon with a diameter of the working surface of 3 mm and visible surface area of ca. 7.07 mm² (Metrohm, Utrecht, The Netherlands).

2.2. Apparatus and Measurements

SEM micrographs and Energy Dispersive Spectroscopy (EDS) studies of Rh/GCE were performed with a JEOL 6390 Scanning Electron Microscope equipped with an INCA (Oxford Instruments, UK) EDS analyser for elemental analysis. The elemental analyses were carried out at a landing voltage of 5 kV.

The electrochemical measurements were performed using EmStat3 (PalmSens BV, Houten, The Netherlands) interfaced with a computer and controlled by PSTrace 5.5 software and an electrochemical workstation Vionic (Metrohm, Utrecht, The Netherlands) controlled by INTELLO software, version 1.6. A conventional three-electrode system was employed comprising a glassy carbon electrode (GCE) modified with rhodium particles as the working electrode, a platinum wire as a counter electrode, and Ag/AgCl (3 M KCl) as a reference electrode. All the electrochemical experiments were carried out under ambient conditions (22–25 °C).

Cyclic voltammetry (CV) and amperometry at a constant potential (Amp.) were used for examination of electrochemical properties of the electrode and H₂O₂ analysis procedures.

Before the determination of electrochemically accessible surface area, the unmodified glassy carbon electrode was activated by running 3–5 sets of cyclic voltammograms in a solution containing 5 mM K₃[Fe(CN)₆] and 5 mM K₄[Fe(CN)₆], dissolved in 0.1 M KCl, at scan rates varying between 50 and 316 mV s⁻¹ until reproducible CVs were obtained.

The experimental data were processed by software package “OriginPro 2015”.

2.3. Electrochemical Deposition of Rhodium onto GCE

An optimized methodology was used for the electrodeposition of rhodium [31]. Briefly, prior to the modification, the GCE surface was carefully polished with 0.05 µm alumina slurry, followed by sonication in double-distilled water for 5 min. After bath sonication, the electrode was rinsed with double-distilled water and allowed to dry at room temperature. Rhodium was electrodeposited onto GCE from electrolyte containing 2% w/v RhCl₃ (dissolved in 0.1 M HCl) by means of CV (1 cycle at a scan rate of 100 mV s⁻¹). The electrode surface was seeded with rhodium particles when starting the cycle at −0.3 V, and then the scan went up to 0.9 and returned to −0.3 V. During the electrodeposition process, an average of 8.75 ± 0.15 μg of Rh was deposited onto the GCE surface, as calculated based on the amount of charge that passed, determined from the CVs (between 29.6 and 30.5 μC). The modified Rh/GCE was subsequently rinsed with double-distilled water and employed for the electrochemical studies.

2.4. Reference Method for Determination of H₂O₂ in Hair-Care Products

The official testing of cosmetic products conducted by any laboratory (national, control, etc.) must follow the European official methods of analysis. We adhered to the Directive 82/434/EEC (page 33), which outlines the official testing requirements for cosmetic products [32].

3. Results and Discussion

3.1. Scanning Electron Microscopy Studies

In Figure 1, rhodium particles electrodeposited onto glassy carbon by means of cyclic voltammetry at different magnifications are depicted. The deposits are mostly quasi-spherical or oval, with a textured surface, and range between 80 and 250 nm in diameter. Smaller particles were ingrown between larger ones. Some structures form clusters of 2 to 6 units, most probably due to the closely placed nucleation sites that merged upon particle growth. The electrodeposits were spread evenly on the substrate surface, with no large bare regions or the formation of overgrown islands. Surface coverage varies between 25 and 30%, with a moderate particle density.

The elemental analysis (EDS) of the electrodeposits (Figure S1b–d) indicates that smaller particles (Figure S1a—blue area) consist of pure rhodium laid over the carbon substrate. The EDS spectra of larger particles (Figure S1a—green area) indicate the presence of trace amounts of oxygen (1.95 ± 0.19 wt.%) along with rhodium deposits set on the glassy carbon support. It is plausible that the presence of oxygen is caused by its chemisorption over the Rh surface, and most probably the smaller particles are also covered with surface oxides that fall below the detection limits of the equipment. On the bare substrate (Figure S1a—purple area), neither oxygen nor rhodium were detected, as elucidated by the energy dispersive spectrum (Figure S1d).

3.2. Electrochemical Characterization

The ferri/ferrocyanide redox probe in 0.1 M KCl has been employed as a model system to study the electrochemical characteristics of the bare GCE and the surface-modified electrode Rh/GCE. The cyclic voltammograms (CVs) registered in the presence of [Fe(CN)₆]^3−/4− show the characteristics of anodic and cathodic peaks of the reversible probe (Figure 2). A comparison of the measured CVs shows a higher current intensity from the electroactive probe when it was exposed to the Rh/GCE in comparison to the unmodified GCE. The peak-to-peak potential separation (ΔE_p) between the anodic and cathodic peaks was found to be 138 mV and 165 mV for the modified and activated bare electrode, respectively, demonstrating a quasi-reversible electrochemistry of the redox process for both the Rh-modified and the bare GCE. Thus, the Rh- modifier of the GCE obviously facilitates to a great extent the electron transfer. The data are fascinating because the increases in the current response and the decreases in ΔE_p suggest a remarkable improvement in the electrocatalytic activity. This result can be attributed to the increased surface area and the good conductivity of the metallic structures.

The electroactive surface area for both electrodes was evaluated by performing CVs at different scan rates from 5 to 150 mV s⁻¹ and plotting the cathodic peak current as a function of the square root of the scan rate (Figure 3). The resulting scans for Rh/GCE show that, with increasing scan rates, cathodic peaks shifted towards more positive potentials while the anodic ones went to more negative potentials, respectively (Figure 3a). Based on these results, we performed a linear fit of the anodic ($I_p^a$) and cathodic ($I_p^c$) peak currents versus the square root of the scan rate (v^1/2) (Figure 3b). The resulting linear equations were calculated to be $I_p^a$(µA) = 872.738 v^1/2 (V^1/2 s^−1/2) + 34.4649 (R² = 0.994) and $I_p^c$(µA) = −1130 v^1/2 (V^1/2 s^−1/2) − 20.2661 (R² = 0.997). These data indicate that the surface reaction at the modified electrode was a diffusion-controlled process.

The linear plot for $I_p^c$ vs. $v^{1/2}$ was fitted into the modified Randles–Sevcik Equation (3) to calculate the electroactive surface area of the electrodes:

$$I_p=0.436·{\displaystyle \frac{{(n·F)}^{3/2}}{{(R·T)}^{1/2}}}·A·C·D^{1/2}·v^{1/2}$$

(3)

where $I_p$ is the peak current (A), v is the scan rate (V s⁻¹), n = 1 (number of electrons involved in the redox reaction), A is the electrode surface area (cm²), D is the diffusion coefficient (D = 7.6 × 10⁻⁶ cm² s⁻¹), C is the bulk concentration of the redox probe (mol cm⁻³), F is the Faraday constant (F = 96,480 C mol⁻¹), R is the gas constant (R = 8.314 J mol⁻¹ K⁻¹), and T is the absolute temperature (T = 298 K). The electroactive surface areas for the bare GCE and the Rh/GCE were determined to be 0.193 ± 0.012 and 0.312 ± 0.010 cm², respectively. The electroactive surface was approximately 1.6 times higher compared to that obtained for the bare GCE.

The electrocatalytic properties of the Rh/GCE were examined in 0.1 M PBS (pH 7.0) at a scan rate of 50 mV s⁻¹ using cyclic voltammetry. Figure 4a shows CVs recorded in the supporting electrolyte and in the presence of H₂O₂. The addition of H₂O₂ leads to a dramatic change in the voltammetric curve, and an increased cathodic peak was registered. When H₂O₂ concentration was doubled in the electrolyte, the reduction peak of the analyte became more significant, confirming that the cathodic peak is associated with the H₂O₂ reduction. These results clearly showed that Rh/GCE can effectively catalyze the H₂O₂ electrochemical reduction and can be used for quantitative applications.

The amperometric sensing of H₂O₂ was performed using the hydrodynamic amperometry technique (Figure 4b, inset). We have examined the amperometric response of Rh/GCE to the successive addition of different concentrations of H₂O₂ at a constant applied potential of −0.1 V. The background current stabilized within 10 s after switching to the detection potential, and the current value was very stable for more than 8 min. As shown in the inset of Figure 4b, a typical stepwise increase in cathodic current was observed in response to H₂O₂ injections. The increase in the reduction current is linear with the concentration within a wide range between 5 and 1000 µM, and the linear regression equation is I (μM) = 0.05374 (±0.6074 × 10⁻⁴) C (μM) + 0.868 (R² = 0.9987). From the corresponding calibration curve, a sensitivity of 172.24 ± 1.95 μA mM⁻¹ cm⁻² (n = 3) was calculated. The limit of detection (LOD) and limit of quantification (LOQ) were calculated to be 1.2 μM and 4 μM (LOD = 3σ/S and LOQ = 10σ/S, where S is the slope of the calibration curve and σ is the standard deviation of the blank solution).

3.3. Interference Study

The anti-interference capability is an essential factor in assessing the analytical performance of the sensing platform and its practical applicability for cosmetic products. Therefore, to evaluate the specificity and detection capability of the Rh/GCE, we have systematically studied the interference of substances labeled by the manufacturer as components of the hair dye oxidant. According to the information provided, the oxidant contains H₂O₂, glycerol, salicylic acid, Na-EDTA, and H₃PO₄.

At a constant potential of −0.1 V two injections of H₂O₂ stock solution were consequently introduced into the electrochemical cell at regular intervals, after which the following were injected in sequence: salicylic acid, Na-EDTA, glycerol, and H₃PO₄, respectively. The concentrations of these substances in the electrolyte exceeded the concentration of the target analyte (H₂O₂) by tenfold. Finally, two portions of the H₂O₂ solution were injected. From the authentic record (Figure 5a) it is evident that the tested substances do not undergo electrochemical processes on Rh/GCE under the given experimental conditions. There was no noticeable current change after the addition of these species; the current response to H₂O₂ was unaffected and stable, confirming the exceptional selectivity of the proposed platform in the electroreduction of H₂O₂. Additionally, some common inorganic ions such as K⁺, Na⁺, Mg²⁺, Cl⁻, NO₃⁻, and SO₄²⁻ were also tested, and no alteration of the current intensity was observed (Figure 5b). These mentioned molecules/ions did not interfere with the current response of H₂O₂. Based on these observations, the fabricated modified electrode can be considered as a highly selective platform toward H₂O₂ measurement in hair dye oxidant samples.

3.4. Reproducibility, Repeatability, and Stability

In order to examine the reproducibility of the sensor fabrication procedure, three identical electrodes were prepared to detect H₂O₂ at the same concentration (0.5 mM). The relative standard deviation (RSD) of the current responses for the sensing of H₂O₂ by the three identically prepared sensors was found to be 4.1%.

The repeatability of amperometric signal of one and the same Rh/GCE was investigated in the presence of 0.5 mM H₂O₂. The RSD for five independent measurements was calculated to be 3.2%.

The long-term stability of the proposed modified electrode was also evaluated. Two electrodes of Rh/GCE were stored for 3 months. The residual catalytic activity was monitored by the measurement of the signal in the presence of 0.5 mM H₂O₂, performed once every 10 days (Figure 6). When the electrode was stored in air at room temperature, a gradual decrease in the signal to 87% of its initial value was recorded within the first 2 weeks, after which the electrode activity remained practically unchanged. When the electrode was stored in double-distilled water, we found a lower decrease in activity. After two weeks of storage, a residual activity of 94% was registered, and it remained virtually unchanged (93%) over the next 3 months. The observed phenomenon is due to the recrystallization processes of the deposited metal phase. The more significant loss of catalytic activity, which registered when the electrode material was stored in air, is most likely caused by the specific adsorption of atmospheric gases, leading to a partial blocking of the active sites on the electrode surface. These data indicate the excellent stability of the rhodium deposits and a high residual activity of the electrode material.

3.5. Application of Rh/GCE for Determination of H₂O₂ Content in Cosmetic and Medical Products

For the validation of the suitability of the proposed electroanalytical method based on Rh/GCE, the target analyte was determined in the hair dye oxidant sample and pharmaceutical solution sample. First, the modified Rh/GCE was used for the selective quantitative determination of the H₂O₂ content in a hair dye oxidant. The sample pretreatment included only a dilution: 10 g of the product was dissolved in PBS (pH 7.0) to a total volume of 250 mL before measurement.

The working cell contained 30 mL of supporting electrolyte PBS (pH 7.0). A constant potential of −0.1 V was set on the working electrode (Rh/GCE), and, after establishing a background current, two consecutive equal portions of 60 µL of the sample solution were introduced into the cell. Figure 7a presents the I–t curve recorded in this study. The sensor response after the injection of the real sample is fast and reproducible, proving that the other components of the hair dye oxidant do not affect the signal. The experiment was performed in triplicate (Figure S2). The concentration of H₂O₂ was found by substituting the current value in the first step in the regression equation obtained for the standard H₂O₂ solution (Figure 4b). The content of H₂O₂ in the tested sample was calculated to be 5.91 ± 0.05% (n = 3). Using the same real sample, the electrochemical method was compared to the standard titrimetric method routinely used in laboratory practice. The results of this study using the standard titrimetric method (n = 3) showed that the H₂O₂ content in the tested sample was 6.11 ± 0.18%. The relative error (RE) was calculated from the ratio as follows:

RE = [100 × (amperometric method − reference method)]/reference method

(4)

Relative error RE = −3.27% indicates an excellent agreement between the two methods. These results highlight the potential of Rh/GCE for the rapid, efficient, and accurate detection of H₂O₂ in complex cosmetics matrices.

The second real sample was a 3.0% medical solution purchased from the local pharmacy. This sample was subjected to dilution as follows: 230 µL was diluted with PBS (pH = 7.0) to a total solution volume of 10 mL. After establishing a background current, portions of the prepared solution were injected into the working cell (30 mL) every 40 s as follows: 150 µL, 400 µL, and 400 µL. Figure 7b presents the authentic record of the electrode signal. From the average value of the registered current in the first step of three independent measurements, and using the calibration curve, we calculated 2.96 ± 0.06% H₂O₂ content in the studied sample.

Table 1. Comparison of the analytical parameters of electrochemical non-enzymatic sensors for the determination of H₂O₂ in cosmetics and pharmaceutical and cleaning products.

Electrode Material	Method (Potential)	Electrolyte	Linear Range (µM)	LOD (µM)	Stability	Real Sample	Ref. (Year)
SPAgE-Binano	CV	PBS pH 7.0	100–5000	56.59	–	Hair dyes	[23] (2011)
PLL-GA/PW/GCE	Amp. (−0.3 V *)	0.1 M H2SO4	2.5–6850	–	– (2 weeks)	Antiseptic solution, soft-contact lens cleaning solution	[24] (2011)
La0.7Sr0.3Mn0.75Co0.25O3/CPE	Amp. (0.6 V *)	0.1 M NaOH	0.5–1000	0.17	91.8% (1 month)	Toothpaste, medical solution	[25] (2015)
RGO/CuFe2O4/CPE	Amp. (−0.15 V *)	PBS pH 5.0	2–200	0.52	95.8% (2 weeks)	Hair dye, mouthwash solution	[26] (2017)
	DPV		2–10 10–1000	0.064
3DGrE/PB	Amp. (0.0 V *)	PBS pH 7.4	1–700	0.11	–	Mouthwash solution	[27] (2019)
CuO/g-C3N4/GCE	DPV	PBS pH 7.0	0.5–50	0.31	92% (2 months)	Makeup remover	[28] (2021)
CuO/Cu wire	Amp. (−0.7 V *)	0.1 M NaOH	10–1800	1.34	95% (1 month)	Listerine mouthwash	[29] (2022)
POM–CNT/Au	Amp. (−0.7 V *)	0.1 M PBS pH 5.0	300–1350	0.5	90% (9 weeks)	Hand sanitizer, surface cleaner, contact lens preservation solution	[30] (2022)
Rh/GCE	Amp. (−0.1 V *)	PBS pH 7.0	5–1000	1.2	93% (3 months)	Hair dye, medical solution	This work

* Reference electrode, Ag/AgCl; SPAgE-Bi^nano, three-in-one screen-printed electrode assembly containing nano bismuth species-deposited silver as working, pre-oxidized silver as reference, and unmodified silver as counter electrodes; PLL-GA-PW, phosphotungstate-doped glutaraldehyde-cross-linked poly-l-lysine film electrode; 3DGrE/PB, 3D-printed graphene electrode with Prussian blue; CuO/g-C₃N₄/GCE, copper oxide/graphitic carbon nitride composite/glassy carbon electrode; POM–CNT, polyoxometalate-CNT nanohybrid; DPV, differential pulse voltammetry; CV, cyclic voltammetry; Amp., amperometry.

summarizes the previously reported electrochemical sensors applied for the analysis of cosmetics and the operational parameters for detecting H₂O₂. As evident from the data, the analytical characteristics reported in the present study are comparable to, or even superior to, those of the existing methods in terms of dynamic range, long-term stability, cost-effectiveness, and the use of a monometallic catalytically active phase instead of more complex composite materials. Moreover, some of these systems involve the detection of H₂O₂ in highly acidic media such as H₂SO₄ [24] or alkaline electrolytes such as NaOH [25,29]. Relative to all previous studies, the preparation process of the modified electrode–catalyst Rh/GCE is much simpler. In contrast to some works [25,28,30], the electrode preparation procedure applied here does not use the drop-casting technique. It is well known that the evaporation of a droplet containing suspended nanoparticles forms a ring-like pattern (“coffee ring effect” phenomenon). The uniform distribution of the drop-cast particles across the electrode area negatively affects the sensing performance and reproducibility [33]. Although drop-casting is a simple and quick method for surface modification, it is better suited for proof-of-concept studies. Some researchers have used carbon paste electrodes (CPEs) [25,26]. However, the reproducibility of CPEs is also a challenge. Batch-to-batch variations in the paste (particle size and distribution) can affect the electrochemical behavior and reproducibility between different electrodes. The penetration of the analyte and other components of real samples into the electrode paste causes a progressive loss of linearity and lack of signal repeatability between measurements. In contrast, the proposed fast, single-step electrodeposition procedure has a promising future as a fabrication approach in the field of electrochemical sensors.

4. Conclusions

In this study, a novel stable and precise electrochemical sensing platform was developed by modifying GCE with electrodeposited rhodium particles for the sensitive and selective detection of H₂O₂ in cosmetics and pharmaceutical samples, surpassing the performance of several previously reported sensors. Among the various types of modified electrodes, Rh/GCE is one of the most attractive sensing materials because of its good electrocatalytic activity, well-defined robust structure, and excellent stability. The fabricated sensor demonstrated an excellent specificity against various interferences, short response time, good reproducibility, accuracy, repeatability, and high stability of the electrode material. Moreover, the electrode preparation procedure is facile, quick, cost-effective, and reproducible. The sensor was applied to real samples containing H₂O₂, and the obtained results were in excellent agreement with the standard method. Thus, the constructed sensor is easily applicable and offers an economically feasible alternative to the conventional analytical methods, being an attractive strategy for analytical laboratories with minimal research infrastructure.