Facile chemical routes to mesoporous silver substrates for SERS analysis

Mesoporous silver nanoparticles were easily synthesized through the bulk reduction of crystalline silver(I) oxide and used for the preparation of highly porous surface-enhanced Raman scattering (SERS)-active substrates. An analogous procedure was successfully performed for the production of mesoporous silver films by chemical reduction of oxidized silver films. The sponge-like silver blocks with high surface area and the in-situ-prepared mesoporous silver films are efficient as both analyte adsorbents and Raman signal enhancement mediators. The efficiency of silver reduction was characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The developed substrates were applied for SERS detection of rhodamine 6G (enhancement factor of about 1–5 × 105) and an anti-ischemic mildronate drug (meldonium; enhancement factor of ≈102) that is known for its ability to increase the endurance performance of athletes.


Experimental details
Synthesis of silver (I) oxide nanocubes was performed using a procedure reported by Lyu et al. [1]. Briefly, 0.05 g of crystalline silver nitrate (Carl Roth GmbH, ≥99% , Ph.Eur., extra pure) have been dissolved in 210 mL of 0.2 M ammonium nitrate NH 4 NO 3 aqueous solution.
PVP solution was added slowly in a ratio 2:1 (PVP monomeric unit / silver atom). The corresponding concentrations of PVP were of about 5 × 10 −3 M. Next an excess of sodium hydroxide NaOH (450 mL of 0.2 M solution) was added at the first onset and then preserved in dark for 1 h under stirring. The overall molar ratios of the compounds were as the following 1 AgNO 3 : 4.2 NH 4 NO 3 : 8.5 NaOH. Varying the NH 4 NO 3 concentration in the range of 2 × 10 −3 -8 × 10 −3 M hardly influenced on the resulting micromorphology of the product ( Figure S1-a,b).
After multiple centrifugations of the precipitate and washing by ~1 liter of MilliQ pure water the product was ready for the following syntheses. The dry product could be estimated using freeze drying method that is appropriate for storage and further dispersion in water.
All the Ag particle syntheses have been performed in glass dish preliminary rinsed with 2 M nitric acid and then by an excess of distilled water to remove all possible reductants and dust. The reductants, namely, sodium borohydride (Aldrich, granular ≥98%), hydrogen peroxide (Reachim, ≥33%) and hydrazine sulfate (Reachim, granular, purum) have been kept as solids and dissolved in preliminary cooled to 4 °C aqueous media (MilliQ water) right before the experiment. Hydrogen peroxide was taken after IREA200 as a purum reagent of 37 wt % saturated aqueous solution. For each experiment 10 mL of 0.1 M preliminary sonicated aqueous suspension of Ag 2 O nanocubes was reduced using dropwise addition of the reductants. The reduction was performed with a double excess of each reducing agent, namely H 2 O 2 , N 2 H 6 SO 4 or NaBH 4 , to Ag + ions. Microstructure of the samples are given in Figure 1c,d and Figure S1c,d.
Diluted solution of hydrogen peroxide demonstrated low efficiency in Ag 2 O reduction producing incomplete reduction. In contrast, hydrazine sulfate produced highly porous silver structure which had nothing common with initial micromorphology Ag 2 O polyhydron crystallites. In the issue, as a reductant the ten-fold excess of NaBH 4 to Ag 2 O was applied as described above. This reagent is effective but less destructive for the primary microstructure than the hydrazine sulfate.
Reduction with a ten-fold excess of NaBH 4 was also carried out leading to mesoporous structures with less degradation of secondary structure of the polyhedrons. The reduction processing was performed for 15 or 40 min and then the dark-colored product was washed out and dried in air at ambient conditions. The micrographs of the corresponding products are given in Figure 1. Figure S1: SEM micrographs of silver oxide and mesoporous silver samples prepared using different ratios of silver nitrate/ammonium nitrate/sodium hydroxide: (а) Ag 2 O sedimented in 1 AgNO 3 : 4.2 NH 4 NO 3 : 8.5 NaOH reagent ratio, (b) (а) Ag 2 O sedimented in 1 AgNO 3 : 8 NH 4 NO 3 : 8.5 NaOH reagent ratio , (c) reduction product produced in 2 × 10 −3 M H 2 O 2 (less efficient), and (d) reduction product obtained in 2 × 10 −3 M N 2 H 6 SO 4 (most destructive).

S3
Mesoporous silver film was prepared in two steps using a magnetron sputtered silver films of 150 nm thick as a precursor. The primary Ag film was deposited onto thoroughly washed glass slide in argon using magnetron Quorum Technologies Q150T Turbo-Pumped Sputter Coater/Carbon Coater. A standard silver disk sputtering target (Stanford Materials, 99.99%) served as a source and the deposition rate was 2 nm/s, current 50 mA. The film was even and had less defects.
The silver film was oxidized then in vapors of concentric nitric acid (Reachim, purum).
The silver film was fixed downface in 5 cm over 10 mL of acid in a glass vessel. The film was treated for 10 min in vapor and then thoroughly washed with MilliQ pure water. The film changed in color was then immersed into fresh prepared NaBH 4 aqueous solutions of the same concentration as was taken for mesoporous polyhedrons when taken in ten-fold excess. The initial concentration of NaBH 4 was of 1 × 10 −2 M. The immersion time was 15 min and 40 min to vary process rate. To reveal the influence of PVP onto morphology of the metal silver one of S4 reductant solutions contained also 5 × 10 −3 M of PVP. The resulting plates were washed with distilled water and dried in ambient conditions for 24 h.
Scanning electron microscopy (SEM) was applied to characterize microstructure of the individual Ag and/or Ag 2 O nanostructures. The analysis has been performed in using NVision 40 microscope (Carl Zeiss) at 9 kV accelerating voltage. The instrument was equipped with an Oxford Instruments X-Max detector. Statistical analysis of micrographs was performed using ImageJ software.
XRD spectra were collected using the X-ray diffractometer RIGAKU D/max-RC with 12 kW beam gun and a rotating copper anode (Cu Kα radiation, θ−2θ Bragg−Brentano geometry, 20−60° 2θ range, 0.020° step). Phase analysis of compounds was carried out using WinXPow software using PDF2 database.
X-ray photoelectron spectroscopy analysis was performed using a setup ESCA I at the Laboratory of Industrial Chemistry of RUB equipped with the X-ray source Specs XRC 1000, UHV chamber up to 10 −6 mbar, energy analyzer power PS-EA10N). For the sample preparation the preliminary centrifuge concentrated colloid was deposited onto the carbon substrate and then evaporated for 10 times.
Specific surface area of samples was determined by low-temperature nitrogen adsorption with an ATX-06 analyzer (KATAKON) by the five-point Brunauer-Emmett-Teller (BET) method. The characteristic nitrogen adsorption-desorption isotherms are given in Figure S2.  In Figure S4 the micrographs of mp-Ag/Ag film are presented. The film was not delaminated successfully from the glass slide for additional SEM experiment (because it was too thin to keep its planar structure). The current micrographs show uniform micromorphology for two sides of the film neat the edge which formed a gather.