Nanoporous chromium thin film for active detection of toxic heavy metals traces using surface-enhanced Raman spectroscopy

A uniform PAA template was fabricated by using a two-step anodization technique. The 1st and 2nd anodizations were carried out for 5 h and 8 min, respectively, in 0.3 M C₂H₂O₄ at 42 V and 9 °C. The applied voltage was decreased from 42 to 12 V with a rate of 1 v min⁻¹ and then maintained at 12 V for 12 min to thin the barrier layer of PAA. The pore widening was performed at room temperature in 6 wt% H₃PO₄ for 50 min.

The Cr nanoporous thin film was formed bythe electrodeposition at a very high current density (J = 4000 mA cm⁻²) for 45 min@ 60 °C. The PAA substrate was used as a cathode while the Pt sheet acted as an anode. Chromic acid (25.0 g CrO₃) and sulfuric acid (0.1 ml H₂SO₄) were dissolved in 100 ml distilled water and used as an electrolytic bath.

Morphological studies of the PAA template and Cr nanostructures are scanned using field emission (FE)- scanning electron microscope (ZEISS Gemini SEM). Energy dispersive x-ray (EDX) spectroscopy (Oxford ISIS 300) was used to observe the chemical compositional of nanostructures. X-ray diffraction (XRD, PANalytical Philips X'Pert Pro MRD) was used to study the crystal structure of the fabricated nanostructured films.

A small drop (0.5 μl) of ion-contaminated water (Pb²⁺, Hg²⁺, and Cd²⁺) was injected into the top surface of the nanoporous Cr substrate at the room temperature. Raman scattering spectroscopy was used to detect heavy metals ions. The SERS measurements were performed using Enwave Raman microscopy provided with a 532 nm laser of spot 1 μm. For each run, 532 nm laser beam with power P = 100 mW is incident on the ion-contaminated water positioned above the surface of the substrate and data collected for 20 s.

Figure 1 shows the images of the PAA template anodized in oxalic acid for 8 min and pore widened for 0.84 h. These images display nanopores arrays with hexagonal distribution and aligned vertically on the Al substrate. The nanopores of PAA have a high uniformity in their shape, height, diameter, and density. The pore diameter is approximate 51.85 nm, the interpore distance is 92.63 nm and the height of PAA about h = 576.48 nm. There were six protuberant nanodots in a hexagonal arrangement around each nanopore as seen in the cross-sectional FE-SEM image (red circle), which was compatible with the hexagonal packing structure of the pores.

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The XRD and EDX techniques were used for the analysis phase structure and chemical composition of the PAA. The XRD diffraction chart in figure 2(A) shows that the crystal orientation of PAA forms an aluminum oxide with a cubic structure and a lattice constant a = b = c = 0.4 nm. The sharp peak suggests that the well crystallized PAA template can be easily produced by using two steps anodization method. The (400) plane is the preferred orientation which appeared in the chart at 2θ = 44.75° (card No. 77-0403). The diffraction peak at 2θ = 38.52° corresponds to (311) crystal plane. There are no extra traces peaks corresponding to Al or C. The EDX pattern of the PAA illustrates that signals from O and Al only observed (figure 2(B)). The quantitative analysis for the alumina is 37.84% O and 62.25% Al. No impurities of S, Cr or C are detected by the EDX pattern. This is indicating that the PAA is pure Al₂O₃.

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Cr electrodeposition on the PAA/Al template was carried out at a 4000 mA cm⁻² and 60 °C for deposition time 0.75 h. The Cr film formed on the Al substrate in two steps. Firstly, the electrochemical etching of the pore walls of the PAA and the barrier layer at the bottom of some pores due to the presence of H₂SO₄ in the electrolyte. Secondly, the deposition of Cr nanoparticles on the Al substrate as a consequence of the full etching of the barrier layer of PAA. Figures 3(A)–(D) shows four different magnifications of the electrodeposited nanoporous Cr film on the Al after 0.75 h.

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The surface of the substrate was totally covered by continuous Cr film, which indicates the homogeneous deposition of this film. The Cr film is composed of many aggregates and accumulates of Cr micro/nanoparticles randomly distributed on the Al substrate. These Cr nano/microparticles have semispherical shapes with different diameters. The diameters of Cr particles are ranged from 5.32 to 10.74 μm. The high magnification FE-SEM images show that the Cr particles have a rough surface which contains very small nanopores with irregular shapes. The larger number of Cr particles with nanopores represents highly surface area which is an advantage for sensor enhancements. Figure 3(D) shows that the surface of the Cr nanoporous film is covered with ultrafine Cr nanoparticles (average diameter 2.77 nm). These nanoparticles can act as hot-spots during the detection of heavy metal ions by Raman measurements.

The nanoporous Cr film is very thick. It has a thickness of about 30 μm as shown in the cross-sectional FE-SEM image (figure 3(E)). This is due to the electrodeposition occurs at very high current density (4000 mA cm⁻²) and high temperature (60 °C) for a long time (45 min).

The Cr/Al film was analyzed by EDX as shown in figure 4 to verify the chemical composition of the fabricated nanostructure. The EDX chart indicates the signals of Cr, Al, and O elements. There are four peaks relevant to Cr. The quantitative results were 89.94% Cr, 3.01% Al, and 7.05% O. This shows the high purity of the manufactured nanostructures.

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Raman spectroscopy is an optical technique to study the vibrational, rotational, and other low-frequency modes in the adsorbed molecules yielding that give structural information on the molecules and their local interactions [26]. The SERS is a surface-sensitive method characterized by increasing the Raman intensity signal by the molecules that adsorbed on roughened nanostructured noble metallic surfaces.

To perform SERS measurements, the Raman scattering was scanned by using a Raman microscope of a green laser source (532 nm). The Cr/Al nanopores film was loaded with 0.5 μl of water contaminated with Cd²⁺, Hg²⁺, and Pb²⁺.

The proposed sensor established high selectivity at low concentrations between different heavy metal ions. Figure 5 displays Raman spectra of Hg, Cd, and Pb solutions on Cr/Al substrate at 4 ppm concentration. In this case, the accumulation of various analyte molecules is revealed in the response of Pb²⁺ with the highest Raman intensity whereas Hg²⁺ shows the medium intensity and Cd²⁺ shows the lowest Raman intensity. There are three peaks appear for each metal in the range from 700 to 1200 cm⁻¹ as seen in figure. The highest peak (peak II) is located at 1054, 1052, and 1050 cm⁻¹ for Cd²⁺, Hg²⁺, and Pb²⁺, respectively. This may be due to the peak position depends upon the vibrational energy states which related to the chemical structure of the molecules responsible for the scattering [27, 28]. Raman spectrum of Cr/Al uploaded with 0.5 μl after mixing the three heavy metal ions (Hg²⁺, Pb²⁺, and Cd²⁺ solutions) at concentration 4 ppm is shown in figure 5(B). As seen in this figure, the peak II is located at 1050 cm⁻¹ which matches to the same peak for Pb²⁺.

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The sensitivity of the Cr/Al sensor is illustrated in figure 6, whereas Pb²⁺ ions with different concentrations from 1 to 10 ppm were applied. Figure 6(A) displays Raman spectra of Pb²⁺ solution on Cr/Al substrate. A significant increase in the Raman peak with increasing concentration can be seen in this figure. The trapping of adsorbed heavy metals on the Cr/Al rough surface improves the scattering performance due to an increase in the number of analyte molecules in SERS-active hot-spots within the detection volume as well as the interference and excitation of light waves within this volume. The SERS enhancement is ascribed to two mechanisms: electromagnetic and chemical enhancement [29, 30]. The fabricated Cr nanostructures have smaller nanopores which lead to strong coupling and resonance (electromagnetic enhancement). Moreover, the attached Cr nanoparticles on the nanoporous film's surface (figure 3(D)) were led to the creation of extra hot-spots that causes the accumulation of the analyte molecules (chemical enhancement). This reveals the dependence of the vibrational energy mode on the molecule's structure and the high sensitivity of the manufactured Cr/Al substrate. The intensity of the detected signals is therefore increased.

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Figure 6 (B) displays the variation of the logarithmic intensity of peaks I, II and III with different Pb²⁺ concentrations. According to the logarithmic fitting of the experimental data, solid lines of figure 6(B), the logarithmic of peaks I, II and III intensities are given by the empirical equations;.

$$\begin{eqnarray*}&&\mathrm{Log}\,\left({\rm{intensity}}\,{\rm{of}}\,{\rm{peak}}\,{\rm{I}}\right)=0.0416\,{{\rm{C}}}_{{{\rm{Pb}}}^{2+}}+2.2420,\,({{\rm{R}}}^{2}=0.983)\end{eqnarray*}$$

$$\begin{eqnarray*}&&\mathrm{Log}\,\left({\rm{intensity}}\,{\rm{of}}\,{\rm{peak}}\,{\rm{II}}\right)=0.0380\,{{\rm{C}}}_{{{\rm{Pb}}}^{2+}}+2.4331,\,({{\rm{R}}}^{2}=0.988)\end{eqnarray*}$$

$$\begin{eqnarray*}&&\mathrm{Log}\,\left({\rm{intensity}}\,{\rm{of}}\,{\rm{peak}}\,{\rm{III}}\right)=0.0427\,{{\rm{C}}}_{{{\rm{Pb}}}^{2+}}+2.3682,\,({{\rm{R}}}^{2}=0.994)\end{eqnarray*}$$

Where ${{\rm{C}}}_{{{\rm{Pb}}}^{2+}}$ is the ${{\rm{Pb}}}^{2+}$ ions concentration in ppm and ${{\rm{R}}}^{2}$ is the correlation coefficient between the experimental and the fitting data.

Based on the fitting equations, the relationship between the logarithm of Raman peak strength and Pb²⁺ ion concentration can be used to determine the Pb²⁺ ion concentration in drinking water by using the intensity of Raman peak. The slope of the line and the value of the correlation coefficient (R²) for peak III are higher than the other peaks (I and II). Therefore, this peak provided the highest sensitivity with the best fitting.