The target molecule is one of the most potent contaminants humans release into the environment. 17α-ethinylestradiol is a synthetic derivative of natural estradiol, modified with an ethinyl group at the 17α position that makes it far more stable and orally bioavailable than its natural counterpart. That same stability makes it stubbornly resistant to degradation once it is excreted in urine and feces and enters sewage effluents. Conventional wastewater treatment systems were simply not designed to strip out such compounds, so the hormone persists in rivers and streams at concentrations typically between 0.1 and 50 nanograms per liter, sometimes exceeding 100 nanograms per liter near wastewater discharges. Even at these vanishingly low levels, it disrupts the hormonal systems of fish and amphibians, triggering the production of vitellogenin in males, the development of mixed reproductive organs, and reduced fertility that can collapse entire populations. The European Commission has set a protective surface-water threshold of just 0.035 nanograms per liter, among the strictest limits for any contaminant.

Detecting such a compound usually demands reversed-phase high-performance liquid chromatography, gas chromatography with mass spectrometry, or related techniques. These methods are exquisitely sensitive, but they require expensive instrumentation, complex sample preparation, large volumes of reagents, and highly trained personnel, which puts routine monitoring out of reach for many laboratories and field programs. Electrochemical sensors offer an attractive alternative because they can be miniaturized, operated in situ, and produced at low cost. The catch is that 17α-ethinylestradiol is electrochemically sluggish: its only electroactive feature is the phenolic hydroxyl group on the aromatic ring of the steroid, which oxidizes irreversibly to a phenoxy radical and then to quinone-type structures at potentials around +0.5 to +0.8 volts versus Ag/AgCl. On a bare electrode, that signal is far too weak to detect environmentally relevant concentrations, so the electrode surface must be upgraded with materials that accelerate electron transfer and concentrate the analyte.

The Brazilian team’s solution was a three-component hybrid nanostructure with an unusual twist. Gold nanoparticles are celebrated for their conductivity and catalytic properties, but most syntheses produce the familiar face-centered cubic lattice. Under specific conditions of small size, high surface energy, and particular stabilizers, gold can also adopt a hexagonal close-packed phase, a metastable arrangement with a more open atomic structure on certain facets. The hexagonal phase carries higher surface energy, more undercoordinated atoms, and more defect sites, all of which serve as highly active catalytic centers. By directly reducing gold citrate in an alcoholic solution of carbon dots, the researchers coaxed both phases to form simultaneously. The face-centered cubic particles contribute excellent conductivity and stability, while the hexagonal close-packed particles boost catalytic activity, and the two together create a dual-phase system whose properties neither phase exhibits alone.

The carbon components play equally important roles. Carbon dots are zero-dimensional carbon allotropes smaller than 10 nanometers, rich in surface functional groups and sem-filled p-orbitals that can exchange electrons. In this synthesis they acted simultaneously as reducing, passivating, and structure-directing agents, stabilizing the formation of both gold phases. The second carbon component, multi-walled carbon nanotubes, was functionalized with a 3:1 nitric and sulfuric acid treatment that opened the tube ends and grafted carboxylic and hydroxyl groups onto the sidewalls, making the normally hydrophobic nanotubes dispersible and chemically active. When the three components were combined, the carbon allotropes served as support matrices that prevented the gold nanoparticles from aggregating, increased overall conductivity, and provided π-π stacking interactions with the aromatic rings of the estrogen itself, anchoring the analyte close to the catalytic surface.

Characterization confirmed the hybrid’s structure in detail. High-resolution transmission electron microscopy showed spherical carbon dots averaging about 3 nanometers and quasi-spherical gold nanoparticles averaging 7.34 nanometers, with interplanar spacings of 0.238 nanometers corresponding to the (111) planes of face-centered cubic gold and 0.249 nanometers matching the (001) planes of the hexagonal close-packed phase. X-ray diffraction found the characteristic cubic peaks at 2θ values of 38.4, 44.5, and 64.8 degrees, alongside hexagonal peaks at higher angles, with the strongest intensity at 38.4 degrees indicating that cubic orientation remained dominant. X-ray photoelectron spectroscopy showed gold exclusively in its metallic state, with the Au 4f doublet near 84 and 87.7 electron volts and no oxidized species, while the carbon and oxygen spectra mapped the C–C, C=C, C–OH, C=O, and O–C=O environments created during synthesis and functionalization. Ultraviolet-visible spectroscopy added a surface plasmon resonance band near 545 nanometers, confirming the metallic nanoparticles had formed.

When the hybrid was drop-cast onto disposable carbon screen-printed electrodes, the electrochemical performance improved dramatically. Differential pulse voltammetry in Britton-Robinson buffer at pH 6 shifted the oxidation potential of the estrogen to a less positive 0.52 volts and produced a much larger current than the bare electrode or electrodes modified with the carbon components alone. The electroactive surface area grew from 0.02 square centimeters on the bare electrode to 1.02 square centimeters after modification. Electrochemical impedance spectroscopy told the same story: charge-transfer resistance fell from 465.8 ohms on the unmodified electrode to 49.84 ohms on the coated one, a nearly tenfold reduction that reflects far faster electron transfer at the electrode-solution interface. The calibration curve was linear from 2.5 times 10 to the minus 8 up to 2.5 times 10 to the minus 6 moles per liter, with a correlation coefficient of 0.99 and a detection limit of 67 nanomoles per liter calculated by IUPAC criteria.

Mechanistic experiments revealed how the oxidation proceeds. The peak potential shifted linearly with pH at roughly 53 millivolts per pH unit, close to the theoretical Nernstian value of 59.2 millivolts, indicating that protons and electrons participate in equal numbers. Randles-Ševčík analysis gave a transfer coefficient of 0.50 and an electron number of approximately two, establishing a two-electron, two-proton oxidation of the phenolic group. Peak current scaled linearly with the square root of scan rate, showing that the process is mainly diffusion-controlled, with a minor adsorption contribution from the high surface area and π-π interactions of the carbon materials. The functional groups on the carbon dots and nanotubes also engage the hormone through hydrogen bonding and London dispersion forces, and the two gold phases participate directly in the electrooxidation, their crystallographic orientation, shape, and size shaping the electronic and catalytic character of the surface.

Real-world testing and interference studies rounded out the picture. In tap water and in urine from a woman taking contraceptive pills, the sensor recovered spiked estrogen at 95 to 106 percent and 94 to 102 percent respectively, using the standard addition method without any prior sample treatment. When the electrode was challenged with ascorbic acid, dopamine, progesterone, uric acid, and glucose at concentrations 100 times higher than the hormone, the peak current dropped only about 4 percent and the oxidation potential shifted modestly to 0.6 volts, changes the authors attribute to steric hindrance and an enlarged electrical double layer rather than true chemical interference. Common ions such as chloride, sulfate, nitrate, calcium, potassium, and magnesium at 250 milligrams per liter caused no significant signal change. Perhaps most impressively, 60 electrodes produced over one month all remained functional, with a relative standard deviation of just 3.5 percent, demonstrating that the fabrication is reproducible enough for routine use. The researchers suggest the platform could underpin field-deployable testing for a synthetic estrogen whose environmental threshold is among the lowest ever set, offering water utilities and health laboratories a tool that costs a fraction of a chromatography system and fits in a shirt pocket.

Subject of Research: Development of a dual-phase gold nanoparticle and carbon allotrope hybrid nanostructure electrochemical sensor for detecting 17α-ethinylestradiol in water and urine

Article Title: Electrochemical sensor based on face-centered cubic and hexagonal closed-packed AuNPs/carbon allotrope hybrid nanostructures to determine 17α-ethinylestradiol in water and urine

Article References: Torres, Y. F., Ferreira, J. H. A., Peres, R. M., & Canevari, T. C. (2026). Electrochemical sensor based on face-centered cubic and hexagonal closed-packed AuNPs/carbon allotrope hybrid nanostructures to determine 17α-ethinylestradiol in water and urine. Journal of Nanoparticle Research, 28(9), Article 237. https://doi.org/10.1007/s11051-026-06768-x

Image Credits: AI Generated

DOI: 10.1007/s11051-026-06768-x

Keywords: electrochemical sensor, gold nanoparticles, carbon dots, carbon nanotubes, 17α-ethinylestradiol, endocrine disruptor, water contamination, differential pulse voltammetry, hybrid nanostructures, screen-printed electrode, emerging pollutants, nanosensors