CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from Italian patent application no. 102020000024382 filed on October 15, 2020, the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method for the preparation of nanoporous gold.

BACKGROUND ART

Studies on nanostructured materials are progressively increasing from year to year due to the unique properties of these materials, which are completely different from the corresponding unstructured (bulk) products.

In particular, nanoporous metals prepared with "dealloying" techniques, which show promising results in various fields of application such as for the production of actuators [1] , for the storage and energy conversion [2] , for the production of electrodes for electrocatalysis [3] , of electrochemical biosensors [4] [5] , and for substrates for Surface Enhanced Raman Scattering Spectroscopy [6] , are drawing attention. The improvement in Raman scattering has been attributed to their high surface area and their morphology consisting of ligaments, ranging from tens to hundreds of nanometers, basically made of the noblest element of the precursor alloy. The size of pores and ligaments can be adjusted by selecting a suitable alloy composition and modifying the de-alloying parameters [7] . Furthermore, previous studies have shown how the original structure of the precursor, i.e. crystalline or amorphous, influences the morphology and microstructure of the ligaments and related properties. When a crystalline alloy is de-alloyed, the nanoporous material that is formed maintains the crystalline structure of the precursor: the less noble elements dissolve in the electrolyte while the noblest element diffuses by surface diffusion forming smooth-looking ligaments [8] . In the case of an amorphous precursor, the ligaments consist of many nanocrystals [9] originated by germination during the transition from the amorphous to the crystalline structure [10] [11] : when the less noble atoms of the amorphous alloy dissolve in the electrolyte, the noblest atoms free themselves from the lateral coordination and move like adatoms by surface diffusion forming clusters that grow in size as the process proceeds, until they join together to form ligaments. As a consequence, ligaments obtained from de-alloying of an amorphous precursor are polycrystalline, with more active sites at the grain boundaries and defects. Such properties make them more attractive in electrocatalysis applications [12] [13] [14] and in Surface- enhanced Raman Spectroscopy (SERS) [15] .

SERS is a technique based on enhancement of Raman light scattering of molecules adsorbed on plasmonic metal nanostructures [16] [17] ; the nanostructures may be self- supporting or supported on oxides, as in the case of metal nanoparticles [18] . The SERS activity of different substrates can be compared based on the enhancement factor (EE) which, consequently, can influence the limit of detection (LOD) , i.e. the lowest detectable amount or concentration on the substrate for a given analyte [19] . The EE changes by several orders of magnitude depending on the interaction between substrate and molecules (chemical enhancement) and on the nanostructured surface (electromagnetic field enhancement) : the first contributes two orders of magnitude [20] [21] to the EE, while the second is generated by the resonant excitation of surface plasmons located on the metal surface and considerably increases the EE [22] .

The electromagnetic field enhancement is strongly linked to the specific features and sizes of the nanostructures, i.e. sharp edges and tips [23] , gaps between particles [24] and nanopores [25] , typically defined as "hot spots" . Thanks to this behavior, SERS represents a sensitive technique for detecting molecules of an analyte in low amounts that can be improved through the optimi zation of nanostructured metal substrates .

The need to find nanostructured metal substrates with improved properties is therefore of particular scienti fic and applicative interest .

OBJECT OF THE INVENTION

The obj ect of the present invention is thus to provide a new substrate for SERS .

Such obj ect is achieved by the present invention, as it relates to a method for the preparation of anodi zed nanoporous gold according to claim 1 , to the anodi zed nanoporous gold obtained therewith according to claim 9 , and to the uses thereof according to claims 10 and 11 .

In particular, a method for the preparation of nanoporous gold is provided, comprising the steps of : a ) de-alloying a crystalline or amorphous alloy comprising at least 5% gold atoms with a strong acid to obtain nanoporous gold, and b ) subj ecting said nanoporous gold to an anodi zation to obtain anodi zed nanoporous gold .

Anodi zation may be performed in solutions containing organic acids , for example oxalic acid, diluted sul furic acid or solutions of salts thereof , phosphoric or hydrofluoric acid at various concentrations . Mixed organic and inorganic solutions can also be used . The anodi zation treatment is carried out by applying potentials or currents capable of oxidi z ing gold, which is further nano-structured and then reduced by the ef fect of the electrolyte , returning to the metallic state . Anodi zation may be carried out in a wide solution temperature range, from above the freezing point to j ust before its boiling point , but preferably at room temperature . Treatment times may vary from a few seconds to an hour .

"Nanoporous gold" is intended to mean a nanostructured metallic material , consisting mainly of gold, resulting from the chemical or electrochemical de-alloying of a crystalline or amorphous precursor alloy . Nanoporous gold has porosities and ligaments that can vary from a few units to several hundreds of nanometers , depending on the initial composition of the precursor and the operational conditions of dealloying such as treatment time , temperature , concentration, type and pH of the electrolyte used .

"Anodi zed nanoporous gold" is intended to mean a nanoporous gold that has undergone an anodi zation treatment by immersion in suitable solutions and application of potentials or currents for a certain period of time in order to create nanostructures on the surface which have a smaller si ze when compared to that of the ligaments .

Advantageously, the anodi zed nanoporous gold (A-NPG) exhibits a greater SERS activity being able to detect probe molecules in extremely low concentration, for example 10~ ¹⁶ M with 4-4 ’ bipyridine .

In one embodiment , the alloy comprises from 5% to 55% gold atoms .

The alloy may also comprise from 0% to 95% copper atoms and/or 0% to 95% silicon atoms and/or 0% to 95% silver atoms and/or 0% to 95% palladium atoms .

Furthermore , the alloy may also comprise , instead o f or in addition to the above elements , transition metals , rare earths , phosphorus , beryllium, boron, tin, aluminum, titanium, niobium .

In a preferred embodiment , the alloy is an amorphous alloy selected from the group consisting of Auso- _x Si2oCui8+xAg7Pd5, with x ranging from 10 to 40 , preferably Au2oSi2oCu ₄ 8Ag ₇ Pd ₅ .

The de-alloying step is carried out in the presence of a strong acid, in particular selected from the group consisting of nitric acid, hydrofluoric acid, hydrochloric acid, perchloric acid, oxalic acid, sul furic acid and mixtures thereof . In one embodiment, a mixture of nitric acid and hydrofluoric acid is used, preferably having a nitric acid/hydrof luoric acid molar ratio of between 1:0.1 and 14.4:10, more preferably 10:0.5. De-alloying may be carried out in a wide temperature range, from above the freezing point of the strong acid to just before its boiling point, but preferably at 70 °C. Treatment times may vary from a few minutes to several hours, depending on the temperature, preferably 4 hours.

In a further embodiment, the anodization process is carried out by applying a potential of between 0.5 V and 40 V, preferably between 1 and 10 V, in the presence of an organic acid or of an inorganic acid selected from the group consisting of oxalic acid, diluted sulfuric acid or solutions of salts thereof, phosphoric acid and hydrofluoric acid or mixtures thereof, preferably oxalic acid, and at room temperature .

According to a further aspect of the invention, anodized nanoporous gold obtained with the method described above is provided .

The anodized nanoporous gold can be used as a support for the SERS analysis or as an electrode for electrocatalytic applications .

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in detail with reference to the figures of the accompanying drawings, which show purely illustrative and non-limiting example embodiments, wherein:

- Figure 1 illustrates SEM images of the surface of the samples, a) and b) NPG; c) and d) A-NPG anodized for 3 minutes; e) and f) A-NPG anodized for 5 minutes; g) and h) A-NPG anodized for 7 minutes. In the inserts, a greater detail of the ligaments is shown, i) sectional view of NPG, 1) sectional view of A-NPG anodized for 5 minutes: the anodization involves only a few microns on the nanoporous structure within the thickness of the ribbon (lower arrow) ; the inner part of the ribbon, in fact, is not affected by the treatment (upper arrow) .

Figure 2 illustrates XRD patterns of rapidly solidified amorphous ribbon, NPG and A-NPG anodized for 3 minutes. The amorphous halo of the ribbon obtained by rapid solidification (a) disappears after the de-alloying treatment, while Au fee reflections appear in X-ray diffraction patterns of NPG (b) and A-NPG (c) .

- Figure 3 illustrates a) the GV scan of the 0.02 M ascorbic acid in 0.1 M KH2PO4 solution on an A-NPG electrode anodized for 3 minutes as a function of the scan rate, b) Log (p) vs log (v) graph showing the linear trend and the diffusive control process. - Figure 4 illustrates the highest SERS spectra (average of 10 accumulations) of 4 , 4 ’ -bipyridine at different concentrations on a) NPG, b) A-NPG anodized for 3 minutes, c) A-NPG anodized for 5 minutes, d) A-NPG anodized for 7 minutes, e) Raman intensity at 1616 cur ¹ , with respect to the bipyridine concentrations (in logarithmic scale) . The error bars were calculated with at least five measurements at random points on the same substrate.

- Figure 5 illustrates the highest SERS spectra (average of 10 accumulations) of ascorbic acid at different concentrations on A-NPG anodized for 3 minutes. The spectra were acquired randomly on the surface.

PREFERRED EMBODIMENT OF THE INVENTION

Further features of the present invention will result from the following description of some purely illustrative and non-limiting examples.

EXAMPLE

Pure elements (Au: 99.99%, Si: 99.9995%, Ag, Cu, Pd: 99.99%) were arc melted in an argon atmosphere, after further purification from oxygen by melting Ti blocks which would absorb any oxygen still present in the melting chamber, to obtain an ingot of master alloy of composition Au2oSi2oCu4sAg7Pd5 (at.%) . The ingot was then placed in a quartz crucible, melted by induction, and rapidly cooled with the technique of rapid solidi fication by melt-spinning; the rapid solidi fication process consists in the hardening of the induction molten alloy on a copper wheel rotating at a speed of 25 m/ s . The heat of the molten alloy is rapidly dissipated by the wheel and the solidi fication takes place in the form of ribbons , exploiting a cooling rate such as to avoid the nucleation and growth of crystalline phases , and thus stabili zing the amorphous phase at room temperature . The ribbons thus obtained are 2 mm wide , 20-25 pm thick, and appear fully amorphous by X-ray di f fraction (XRD) analysis .

De-alloying of the ribbons was carried out with chemical means in a mixture of 10 M HNO3 + 0 . 5 M HF at 70 ° C for 4 h; HF was added to avoid SiCh formation when the silicon contained in the alloy precursor is oxidi zed .

Pieces of NPG ribbon, 3 cm long, were subsequently anodi zed in 0 . 3 M oxalic acid solution applying a potential of 8 V for 3 , 5 and 7 minutes . The samples were used as working electrode in a cell consisting of an Ag/AgCl reference electrode with a double bridge configuration and a Pt grid counter electrode .

NPG and anodi zed samples (A-NPG) were properly characteri zed . Microstructure and structure of the dealloyed samples were studied by scanning electron microscopy

( SEM) with energy dispersive X-ray spectroscopy (EDS , after calibration with Co, and X-ray diffraction (XRD) in Bragg- Brentano geometry with Cu-Ka radiation. The sizes of the ligaments were measured at their narrowest neck using a Leica software [27] . All samples were rinsed thoroughly with ultrapure water to remove excess acid solution inside the pores, and then air-dried prior to the electrochemical or SERS experiments.

Electrocatalytic properties towards oxidation of ascorbic acid (AA) were studied using the same experimental set-up applied for anodization. A 0.1 M KH2PO4 buffer solution with 0.02 M ascorbic acid was used as electrolyte. Current densities were normalized using the electrochemical active surface area of the electrode [28] [29] .

Micro-Raman measurements were performed with a Renishaw inVia Raman microscope using a 785 nm laser line with an acquisition time of 20 s, accumulating 10 spectra, 0.05% power to the sample and a 50x objective; 4 , 4 ’ -bipyridine was selected as the probe molecule for SERS experiments .

NRG and A-NPG samples were immersed in a solution of 4 , 4 ’ -bipyridine in ethanol with a concentration from 10~ ¹⁶ M to 10~ ¹² M overnight, allowing the probe molecules to be adsorbed on the surface. Measurements were performed on the sample after air-drying, acquiring random points on the surface or maps in contiguous areas. The image, having a 20 x 24 pm ² area, of the SERS intensity mapping with a 2 pm step length was collected using a 4.4 ’ -bipyridine concentration of 10~ ¹² M by monitoring the characteristic peak of the probe molecule at 1619 err ¹ . All solutions were prepared with chemical grade reagents and deionized water. The spectrometer was calibrated prior to the measurements using the Raman band of a silicon wafer at 520 err ¹ .

Results and Discussion

NRG was obtained by free de-alloying of the amorphous precursor in 10 M HNO3 + 0.5 M HF at 70 °C for 4 h, then it was anodized in a 0.3 M oxalic acid solution applying a potential of 8 V (vs. Ag/AgCl) for a different time.

After de-alloying, the nanoporous structure appears as a network of ligaments and pores spread throughout the thickness of the ribbon (Fig. la, lb and li) . The average size of the ligaments was evaluated to be 60 nm, measured at the neck of the ligament. The ligaments consist of crystals having sizes of the order of a few nanometer units, with grain boundaries that join adjacent crystals, as evidenced by the dotted lines in the insert of Fig. lb; such morphology is obtained following the de-alloying of an amorphous precursor [11] , where the dissolution mechanism of the less noble elements and the diffusion of the noble one leads to the nucleation of gold crystals which, growing at the expense of the amorphous phase, reach touching each other forming ligaments consisting therefore of many nanocrystals. The ligaments are therefore characterized by the presence of changes in curvature and defects. After anodization for 3 minutes, the A-NPG sample shows (Fig.lc, Id and insert) a greater roughness consisting of unevenness of less than 10 nm, changes in curvature and defects that are randomly formed on the surface during the treatment.

In the sample anodized for 5 minutes (5 min A-NPG) the roughness evolves in several pointed regions and tips of about ten nanometers (Fig.le, If and insert) . By increasing the anodizing time to 7 minutes, the roughness appears to be reduced in smooth particles ranging from 20-40 nm in size (Fig.lg, Ih and insert) .

The cross-sectional view for the anodized samples (Fig.11) shows in the outer part of the ribbon a morphology influenced by the anodization treatment (see lower arrow) , while moving towards the inside of the section thickness the NPG morphology is maintained (see upper arrow) . EDS analyses performed on NPG and A-NPG samples show that the ligaments are made of nearly pure gold with impurities of copper, silver, and palladium as residues of the de-alloying process. During anodization, the NPG surface undergoes electrooxidation [30] ; however, this gold oxide is easily reduced to zero valence gold by oxalic acid, as reported in other publications [31] .

XRD patterns of the rapidly solidified ribbon, of NPG and A-NPG anodized for 3 min (3 min A-NPG) were reported in Fig. 2. The amorphous halo of the precursor disappeared after complete de-alloying of the ribbon, while Au fee reflexes appeared in the NPG diffraction pattern. As expected, the pattern of the 3 min A-NPG sample showed no significant differences with respect to the previous one.

As regards the mechanical stability and the electrocatalytic properties, NPG and A-NPG have unique characteristics with respect to the SERS substrates currently on the market (Ocean Optics [32] , Hamamatsu [33] ) .

Commercial substrates are divided into two groups: patterned metal nanostructures formed by nanoimpression and laser technology, and metal nanoparticles (Au, Ag) supported by oxides. Both are assembled on a support plate, having a larger size than the active area of the substrate, that limits the versatility of use thereof.

NPG and A-NPG do not require the use of a support since they are mechanically stable and flexible when handled with laboratory tweezers; this makes unnecessary to integrate the substrate into a support plate, thus expanding their application in different operating conditions. In fact, the ribbon shape makes them extremely practical for measurements in solution, in air after incubation, and in a cuvette. As an electrode, NPG and A-NPG can be incorporated into sensors and small devices by exploiting their mechanical stability. Another important aspect concerns the possibility of reusing the substrate, which is not possible with the substrates currently on the market. In fact, NPG can be reused up to twenty times [4] and A-NPG twice, maintaining their sensitivity unchanged after washing with piranha solution (70% sulfuric acid, 30% hydrogen peroxide) and copious rinsing with deionized water up to neutrality .

In the case of the electrochemical oxidation of ascorbic acid (AA) , the electrocatalytic properties were investigated using A-NPG as a working electrode in a cell consisting of a Pt counter-electrode and a saturated Ag/AgCl reference electrode in a double bridge configuration. A 0.1 M KH2PO4 buffer solution with 0.02 M ascorbic acid was used as electrolyte .

Fig. 3 shows GV scans at different scan rates. The oxidative current of AA increases as a function of the scan rate due to the system heterogeneity. Plotting the intensity of the current density with respect to the scan rate, both in logarithmic scale, a linear trend is observed (Fig. 3b) which suggests that the process is under diffusive control [34] . Similar results were obtained with NPG, under the same experimental conditions.

Furthermore, in that case, a calibration line was obtained by scanning the electrode in the same buffer solution but at different concentrations of AA.

Subsequently, the electrode ability to detect the analyte in a real sample was verified; one Cebion Vitamin C orange effervescent tablet including 1000 mg AA (Bracco) was directly dissolved in a suitable amount of buffer solution. The oxidative current density recorded in such solution was reported in terms of concentration, and therefore grams per tablet, of the analyte thanks to the previous calibration line, providing a recovery concentration and a content in grams in good agreement with the information leaflet.

The SERS activity of NPG and A-NPG was measured by immersing the samples in a 4-4 ’ -bipyridine solution in ethanol having a diluted concentration (i.e. 10~ ¹² M, 10~ ¹⁴ M, ICr ¹⁶ M) . Figs. 4a) to d) show the highest spectrum collected for NPG, 3 min A-NPG, 5 min A-NPG, and 7 min A-NPG, respectively. The spectra show the main signals attributed to the molecule in accordance with the literature [35] [36] . The signal improvement is observed in all samples, but it is strongly improved especially at 10~ ¹⁶ M for the 3 min A-NPG sample : the average signal intensity as a function of the 4 -

4 ' bipyridine concentration was 11 times higher than that of the NPG sample .

This value is extremely interesting, considering that for the SERS substrates currently on the market ( i . e . supported Au/Ag nanoparticles , lithographed Au) a detection limit in the ppm-ppb range is reported with the probe molecules , therefore no more than 10~ ⁹ M and five orders of magnitude less than 10~ ¹⁶ M obtained with a 3 min A-NPG s amp 1 e .

In the literature, a similar detection limit is reported with rhodamine 6G, as a probe molecule , on complex structures consisting of heterostructured zinc oxide/ silicon nanoarrays decorated with silver nanoparticles [ 37 ] or three- dimensional sunflower-like nanoarrays decorated with Ag nanoparticles [ 38 ] . However, the costly and time-consuming preparation methods are a drawback for large-scale applications .

The higher sensitivity of A-NPGs is due to the finer double-nanostructured morphology of the samples : 60 nm ligaments with smaller features are obtained with the anodi zation treatment . The literature reports that the plasmonic ef fect of the nanostructured metal surfaces is divided into two contributions : the chemical enhancement of the signal due to the charge trans fer between the adsorbed molecules and the substrate , and the electromagnetic field enhancement due to the morphology and to the microstructure of the substrate . Considering that the former contributes two orders of magnitude to the overall enhancement , the latter contributes considerably, thus demonstrating the greater sensitivity of the A-NPG .

The detection of molecules of interest was success fully proven for NPG with melamine , approaching a detection limit of 10~ ⁶ M [ 40 ] . Fig . 5 shows the SERS activity of 3 min A- NPG tested in an aqueous solution of ascorbic acid; a LOD of 10~ ³ M was measured for the anodi zed sample, while NPG showed no interaction with the analyte and the spectrum, not reported, is a flat line . This further confirms the increased sensitivity of the A-NPG sample .

Conclusions

Nanoporous gold obtained by free de-alloying of an amorphous precursor was success fully anodi zed in a 0 . 3 M oxalic acid solution at 8 V (versus Ag/AgCl ) for di f ferent amounts of time . The morphology of the ligament evolved going from the NPG sample to the A-NPG one , increasing the roughness and forming asperities and tips as a function of time : after three minutes of anodization, roughness and 5-

10 nm features were formed on the ligaments , whereas when the treatment is extended to 7 minutes a smoothing ef fect prevails and smooth rounded particles of 20-40 nm are formed .

The NPGs and A-NPGs samples result in a sel f-supporting material that is easy to handle , suitable as electrode for the electrocatalysis and as substrate for SERS .

The electrocatalytic performance was demonstrated by studying the electro-oxidation of ascorbic acid in a buf fer solution : the GV scan plots show a signal associated with the oxidative current density of the analyte , that increases as a function of the scan rate and suggests that the process is under di f fusive control .

SERS activity was first proved with a probe molecule , 4-4 ' bipyridine , reaching a LOD of 10~ ¹⁶ M, and for a 3 min A-NPG sample an intensity 11 times higher than that of NPG . A greater activity of 3 min A-NPG was attributed to the enhanced electromagnetic fields located in correspondence of the nanometer-si zed ligaments and to the features obtained after anodi zation . A test with ascorbic acid in aqueous solution showed a SERS signal of the molecule up to 10~ ³ M . These results underline that the anodi zed nanoporous gold of the invention can be success fully used as an ultra-sensitive substrate-sensor for SERS , and as electrode for catalysis or chemical and biological analyses . BIBLIOGRAPHIC REFERENCES

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